1. ABSTRACT

Microglia respond rapidly to injury of peripheral nerve axons (axotomy). This response is integrated into the responses of the injured neurons, i.e. processes for neuron survival, axon regeneration and restoration of target contact. The microglial response is also integrated in changes in presynaptic terminals on axotomized motor or autonomic neurons and in changes in the central terminals of peripherally axotomized sensory neurons. Microglia also has an established role in interacting with astrocytes to shape their response to peripheral axotomy. Axotomy models in mice have demonstrated a role for microglia in regulating the entry of lymphocytes into motor nuclei or sensory areas following peripheral axotomy. Whether this is a universal component of peripheral nerve injury remains to be determined. Under certain circumstances, microglia activated by axotomy are major contributors to CNS pathology, e.g. in models of neuropathic pain. However, the general roles played by microglia after peripheral nerve injury are still incompletely understood. Early proposals that the microglial reaction to peripheral nerve injury is preparatory for the eventuality of neuron degeneration may still have relevance.

2. INTRODUCTION

The aim of this review is to describe and discuss the response of microglia and its functional implications after peripheral axons are interrupted by crush or transection (axotomy) of peripheral nerves in adult mammals. Following these injuries the affected neurons undergo a marked shift in gene expression (reviewed in 1, 2). In simple terms, this shift brings the neurons from a "transmitting" to a "growing" mode, a shift which provides the neuron-intrinsic structural and molecular basis for regeneration of the injured axon and restoration of peripheral target contact. This process is associated with alterations in the synaptic network of the axotomized neurons, as well as with prominent changes in adjacent astrocytes and microglial cells, and under certain circumstances, an infiltration of hematogenous cells.

Based on experimental and clinical research during the recent two decades microglia is now considered to have a central and dual role in most neurodegenerative disorders, including traumatic brain injury, stroke, Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. Dual in the way that microglia may help tissue repair through targeted phagocytosis and release of growth factors, whereas other aspects of microglia directly or indirectly may drive the neurodegenerative process (3). Compared to the disorders mentioned the microglial responses to peripheral nerve injury are unique in the sense that they occur at a considerable distance from the injury.

Historically, responses by microglia to peripheral nerve injury were first recognized in the vicinity of axotomized motor neurons. Later studies showed that that injury to peripheral sensory axons also led to a prompt microglial response in the central projection territories of peripherally axotomized sensory ganglion cells. Motor axon injuries in experimental animals have served and still serve as important models for exploring general mechanisms and functional implications of interactions between neurons and non-neuronal cells. Understanding the role of microglia is obviously relevant in the context of achieving efficient and optimal functional recovery following peripheral nerve injury. This objective has recently received particular attention in relation to sensory nerve injury, since there is strong evidence that microglia play a pivotal role in the emergence and maintenance of injury-induced neuropathic pain. These aspects have been discussed in a number of recent reviews (see e.g. ref 4-8) and will not be in the focus here.

In order to understand the implications of microglial responses following peripheral nerve injury the overall context of these responses will first be briefly described.

3. PERIPHERAL NERVE AXOTOMY

3.1. The neuron-glia network and the dynamic functions of brain and spinal cord

In the intact CNS, nerve cell bodies and dendrites are covered by synaptic terminals and intervening thin astroglial lamellae, a structural arrangement which serves to regulate local chemical homeostasis and secure optimal synaptic function. Activity in neuronal circuits is also modified by signals carried through local networks of astrocytes that communicate via gap junctions or through extracellular mediators (9). Microglia in the intact CNS are highly ramified and continuously extend and retract their processes (10, 11), sweeping them close to synapses in a manner which appears to be activity dependent (12) and then most likely also along astroglial cell surfaces. In view of these close interrelationships in the intact CNS, it should be anticipated that interruption of normal impulse propagation from motor neurons to muscle and sensory end organ to the CNS, respectively, will have a major impact on glial cells surrounding axotomized neurons.

3.2. Neuronal responses to peripheral axon injury - implications for glial activation

The intrinsic neuronal responses to peripheral axotomy are initiated and maintained through several independent mechanisms, which are likely to be fundamentally the same in motor and sensory neurons (2, 13, 14). The early changes in gene expression appear to be induced by the electrical discharge caused by the injury ("injury potential"). These are followed by i) influx at the injury site of extracellular mediators and their retrograde transport to the nerve cell body, ii) depletion of trophic factors produced in the target tissue, which are normally conveyed by retrograde transport to the nerve cell body, and iii) retrograde transport of post-translationally modified molecules from the end of the proximal axon stump.

Peripheral axotomy will have markedly different consequences on motor and sensory neurons, which relate to their distinct differences in morphology, location and function. Furthermore, motor and sensory neurons at the spinal and brainstem level have different developmental origin, and at both sites there are multiple morphologically and functionally different subtypes. Thus, there may be details in the neuronal response to axotomy among subpopulations of motor and sensory neurons that will have an impact on the associated microglial response.

3.3. Central responses to peripheral axotomy - general aspects

The microglial response to peripheral axotomy has an early phase which includes immediate changes in their motility, shortly followed by their proliferation, migration and activation. From a functional point of view, the activation phase is clearly the most important one, and the one which most strongly correlates with the severity of the injury: crushing the nerve results in a less intense and more time-limited microglial response compared to transection. In the activation phase, microglia will also be engaged in complex interactions with astrocytes, and, at least under certain circumstances, with infiltrating hematogenous cells that will influence the outcome in terms of neuronal survival and functional repair.

Our current information on neuronal and glial cell responses to peripheral axotomy, particularly as it comes to their molecular aspects and outcome, emanate almost exclusively from studies on rat and mouse. The basic morphological features from these studies appear to be reproducible in larger animal species, including humans (15), but important differences occur. E.g., observations in the cat indicate that motor nerve axotomy and synaptic stripping are not necessarily associated with microglial proliferation (16-19). Furthermore, extensive infiltration of T-cells following motor nerve axotomy is limited or absent in the rat, but a characteristic and functionally important feature in the mouse (20). In the absence of detailed comparative studies at the molecular level some caution is warranted in translating these findings to humans.

4. MOTOR NEURON AXOTOMY

4.1. Anatomical aspects on models of motor neuron axotomy

The predominating experimental models for studies on microglial response around motor neurons are the facial motor nucleus following injury to the facial nerve, and the lateral motor column at the lumbar spinal level following sciatic nerve or ventral root injury. To a lesser extent, the hypoglossal or the preganglionic parasympathetic dorsal motor nucleus of the vagus nerve has been used. Facial nerve, hypoglossal nerve and ventral root injuries are pure motor axotomies, still different in the sense that the facial nerve is injured outside, the ventral root within the subarachnoid space, thereby influencing e.g. the local properties of the cerebrospinal fluid.

Sciatic nerve and vagus nerve injury affect not only the motor components of these nerves, but also sensory input to the motor neurons indirectly via interneurons in the spinal cord dorsal horn and nucleus of the solitary tract, respectively. Sciatic nerve injury also directly affects a small fraction of the synaptic input to motor neurons by injuring muscle spindle afferent axons with monosynaptic motor neuron connections. A distinct feature of the spinal motor neuron circuitry in contrast to the brainstem motor nuclei mentioned above is the presence of γ-motor neurons and the recurrent inhibition by Renshaw cells, which are driven by the activity of the α-motor neurons themselves.

Given these anatomical and functional differences, some caution is necessary in extrapolating information between different peripheral motor systems.

4.2. Cellular and molecular changes in axotomized motor neurons

In parallel with up-regulation of the expression of a set of growth-associated proteins, prominent molecular and structural modifications occur at the motor neuron interface with surrounding glia. Components of neurotransmitter receptors for e.g. glutamate and glycine are down-regulated (21, 22). Down-regulation of microtubule-associated protein (MAP)-2, a major structural protein of dendritic microtubules (23) is accompanied by extensive shrinkage of the dendritic tree (24, 25). Down-regulation of drebrin (26), a spine-asosciated protein, is likely to further impair normal dendritic functionality. In the other pole of the axotomized neuron, down-regulation of neurofilament proteins, the major structural proteins of myelinated axons is accompanied by extensive reduction of axon caliber, myelin sheath thickness and conduction velocity proximal to the injury (27). Thus, not only is the normal communication between motor neuron and target muscles interrupted, the conditions for local motor neuron network operations are also markedly altered.

These intrinsic events in the injured neurons are accompanied by the removal of a large number of synaptic terminals located on soma and dendrites, a process often referred to as "synaptic stripping" (28-30), an increased perineuronal occupation by fine lamellar processes from neighboring astrocytes, and proliferation, migration and molecular phenotypic transformation of microglia (31, 32).

4.3. Microglial responses to motor neuron axotomy

The mechanisms that initiate the microglial response after motor neuron axotomy are incompletely understood. Critical roles are probably played by the massive efflux of potassium ions from the injury discharge, closely followed or accompanied by the release of ATP from the microvasculature and perineuronal astrocytes. This notion is strengthened by previous findings implicating potassium channels (33-36) and purine receptors (37) in various stages of microglial activation. The sensitivity of microglia to alterations in CNS homeostasis is evidenced by their response along waves of spreading depression, a phenomenon of neuronal depolarization, which can be induced in intact brain regions as a result of distant functional pathology (38-40). In vivo imaging studies have shown that cortical injury induces rapid and targeted extensions of microglial processes towards the lesion site (10, 11). Likewise, hypoxia prolongs the time periods of contact between sweeping microglial processes and synapses (12).

Following this initial response, microglia withdraw their processes, proliferate, and migrate towards the axotomized motor neuron cell bodies This activation is easily recognized by labeling for the complement receptor 3 (CD11b) (Figure 1). Concomitantly, microglia begin to express a range of immune system related molecules, including complement components (41, 42), trombospondin (43), interleukin (IL)-1beta, IL-6, tumor necrosis factor (TNF)-alpha, interferon-gamma, the co-stimulatory factor B7.2, as well as major histocompatibility complex glycoproteins (44-46). Activated microglia has the potential also to release and respond to a wide range of other cytokines and chemokines, depending on the state of activation (47, 48).

In rodents and mice microglial cells begin to proliferate within one-two days after peripheral nerve injury with a peak around four days and reaching baseline levels around seven to ten days after injury (49, 50). The signal(s) initiating this response is not yet precisely identified. Results from studies in vitro indicate that microglial proliferation can occur by autocrine factors (50). One of these factors is likely to be colony-stimulating factor (CSF)-1. CSF-1 can be produced by microglia and mice with a mutation in the gene for macrophage colony-stimulating factor (CSF)-1 (op/op mice) show a markedly reduced proliferation (52).

Microglia migrate towards axotomized motor neurons while still proliferating. An interesting feature of this migration is that although dendrites provide the overwhelming part of the motor neuron surface, the nerve cell body appears to be the main target for this process (cf. Figure 2). The precise structural interrelationship between microglia and axotomized motor neurons has been the subject of some controversy. Although microglial cells and their processes are in the immediate vicinity of these motor neurons, electron microscopic images often show the presence of fine astroglial processes intervening between microglia and the neuronal membrane (53, 54). Proliferating microglia occur also outside the motor neuron nucleus proper. Thus, proliferating microglia were observed along the intramedullary portion of the hypoglossal nerve, presumably in the vicinity of nodes of Ranvier (50).

The possibility for microglia to move into a close relationship with the neurons may depend on microglia mediated down-regulation of tenascin-R in the perineuronal nets (55, 56). Deletion of cathepsin-S, a lysosomal/endosomal protease in cells of the mononuclear lineage, reduces migratory capacity of microglia towards axotomized facial motor neurons (56). Migration of microglia is associated with induction of Annexin III, which translocates to the actin cytoskeleton at the microglial cell membrane (57). These intracellular modifications of microglia cytoskeleton are accompanied by changes in the expression of integrin family cell adhesion molecules (58) and in the expression level of the non-integrin laminin receptor 37-LRP (59). Recent in vitro findings indicate that glutamate exerts a chemotaxic effect on microglia (60). Thus, migration of microglia after peripheral nerve injury is likely to be mediated by the combination of cell-cell interactions and diffusible molecules. Among the latter, synaptic signaling molecules may require more interest than has hitherto been the case.

The rate, extent and persistence of the early microglial responses are influenced by chemokines. Already six hours after facial or hypoglossal nerve injury in the mouse, affected motor neurons increase their synthesis of chemokine monocyte chemoattractant protein (MCP)-1 (CCL2) (61). Its corresponding receptor CCR2 is expressed by microglia and other cells of the monocyte lineage, and MCP-1 is critical for recruiting these cells to sites of injury. Mice with deletion of MCP-1 display a delayed microglial response in the lateral geniculate nucleus following visual cortex lesion (62). Surprisingly, mice with deletion of CCR2 showed similar microglia response as wildtype mice. The chemokine fractalkine (CX3CL3) is produced and secreted by or shed from the surface of axotomized neurons, binds to its receptor CX3CR1 on microglia and induces cytokine production (63). However, fractalkine appears to be dispensable for axotomy-induced proliferation, migration and phenotypic alteration of microglia (64). The chemokine CCL21 is neuronally transported released from injured neurons and appears to have a critical role in several brain pathologies (65), but the possible implication of CCL21 in the context of peripheral axotomy is presently unknown.

4.4. Functional consequences of microglial responses to motor neuron axotomy

The microglial responses to motor neuron axotomy have since long been viewed in the context of motor neuron survival, motor axon regeneration, and displacement of presynaptic terminals. More recently, microglia has also been associated with influencing the properties of astrocytes and with promoting infiltration of T-cells into the axotomized motor nucleus.

4.4.1. Role of microglia for survival of axotomized motor neurons

It is now well established that activated microglia can take on a predominantly cytoprotective or cytotoxic role. In both situations, microglia are able to clear the tissue from degenerating elements, but in the first instance microglia produce and secrete growth supporting factors, whereas in the second instance agents that contribute to cell death are released. The complex mechanisms underlying these distinctive consequences of microglial activation are demonstrated by experiments on one hand showing that the presence of activated microglia is deleterious for neuron survival, on the other hand showing that elimination of microglial activation promotes neuron survival (3).

The extent of motor neuron degeneration after axotomy is highly dependent on the type of injury. Injuries with favorable conditions for rapid regeneration show no or minimal loss of motor neurons, whereas injuries in which contact between proximal and distal nerve stumps is prevented in general are associated with significant motor neuron degeneration (66, 67). The latter situation is typically associated with a greater and more prolonged microglial response. Studies on the influence of activated microglia in brain disorder have reported divergent results. E.g., selective ablation of proliferating microglia was found to exacerbate ischemic brain injury (68), whereas interfering with receptor-mediated microglial activation improved neuron survival (69, 70).

Earlier studies in the rat indicated that elimination of microglia from the axotomized rat hypoglossal nucleus did not reduce motor neuron survival (71). Thus, communication between the injured neuron and the environment in the distal stump of the injured peripheral nerve appears to be the main determinant for the extent of motor neuron degeneration. In this context, the more prominent microglial response after nerve transection-resection may just be secondary to the more extensive neurodegeneration. Such an interpretation would be in line with previous observations of a correlation between the microglial response and neuron phagocytosis (72) and the extensive neuron loss and concomitant microglia response, including their transformation to cells positive for the phagocytosis marker ED1 following motor neuron axotomy in neonatal compared to adult animals (73). Moreover, modifying the microglial immune properties did not influence the extent of motor neuron degeneration in the axotomized facial motor nucleus (74)

However, results from recent studies in genetically manipulated mice have shown that interfering with microglial response after motor neuron axotomy may compromise motor neuron survival. Deletion of cathepsin-S limits microglial migration and spreading on axotomized facial motor neurons and reduces motor neuron survival (56). The chemokine receptor CCR5 is up-regulated in microglia, and its ligands regulated on activation normal T-cell-expressed and secreted (RANTES/CCL5), macrophage inflammatory protein (MIP)-1 in motor neurons after axotomy. CCR5-/- mice show increased motor neuron death after hypoglossal nerve injury (75). Microglia-mediated infiltration of Th2 cells into the facial nucleus protects loss of axotomized facial motor neuron in the mouse (76, 77). Taken together, these findings may reflect important species differences or a situation where functionally deficient microglial cells provide a more unfavorable environment for axotomized motor neurons than a complete absence of microglia. This interpretation may be in line with genetic analysis of rats and mice with diffent extent of motor neuron loss following ventral root avulsion. Data from these studies indicate that variations in genes associated with inflammatory mechanisms determine susceptibility to neurodegeneration (78, 79).

4.4.2. Role of microglia for axon regeneration

A role for activated microglia in axon regeneration could be expected since they are a potential source of growth factors such as brain-derived neurotrophic factor, insulin-like growth factor and transforming growth factor-beta, all of which are known to have beneficial effects on motor neurons in vitro and in vivo. Furthermore, activated macrophages which could potentially be derived from microglia are able to promote growth of adjacent intraspinal axons, albeit at the expense of simultaneously increased neurotoxicity (80). Complete elimination of microglia from the axotomized hypoglossal nucleus did not affect the rate or extent of motor axon regeneration and tongue muscle re-innervation (81). However, growth of injured axons from intrinsic CNS neurons into a peripheral nerve graft were found to be correlated with a perineuronal microglial response (82), indicating that microglia are able to promote axon regeneration, although this may not be required when conditions for this process are anyway favorable, such as after peripheral nerve crush.

4.4.3. Role of microglia for synapse removal

Synaptic stripping was first described on axotomized rat facial motor neurons and found to occur in the vicinity of activated microglia (28). Synaptic stripping has since been shown to be a general feature after motor neuron axotomy, in some instances affecting as many as 70% of the presynaptic terminals, preferentially those located on motor neuron dendrites (29, 30) and predominantly those of excitatory synapses. After muscle reinnervation, only a partial restoration of presynaptic terminals takes place. Thus axtomy-induced synapse remodeling occurs on a large scale and most likely has significant consequences for post-injury recovery of motor function. In fact, recent findings indicate that reducing the amount of synaptic stripping on motor neurons promotes functional recovery after sciatic nerve injury (83).

Based on the morphological coincidence between synapse removal and the perineuronal location of microglia, a causal relationship between these two events was suggested (28). Observations in later studies showed that astrocytes rather than microglia displayed the closest structural relationship with axotomized motor neurons during the process of synapse elimination (53, 54, 84). Using infrared gradient contrast live microscopy of slices containing axotomized facial motor neurons, microglial cells were found to move closely along motor neuron dendrites (85). These and observations in vivo showing that processes of microglia are sweeping close to synapses in the intact CNS and increasing this activity after injury (12) are compatible with an active role for microglia in synapse removal after motor neuron axotomy. However, synapse elimination was unaffected in the rat axotomized hypoglossal nucleus after complete elimination of microglia (54), indicating that microglia at least is not necessary for this process.

Microglia has a well established role in neural development as effectors of targeted phagocytosis of apoptotic cells and clearance of inappropriate synapses (86). Recently, the MHC and complement systems emerged as important regulators of developmental synaptic plasticity and synaptic stripping of axotomized motor neurons. Mice with impaired surface expression of MHCI showed a significant and selective increased removal of presynaptic terminals from axotomized spinal motor neurons (87, 88). Complement C3 -/- mice showed a significantly reduced removal of presynaptic terminals, a reduction which preferentially included terminals of excitatory synapses (83). These remarkable effects were accompanied by evidence for a decreased and an increased rate of functional recovery in functionally MHCI deficient and C3 -/- mice, respectively.

To what extent if at all, microglia is operative in these situations are unknown. MHCI is expressed on neurons and astrocytes, and although an up-regulation of C3 has been shown in microglia in the axotomized hypoglossal nucleus of the rat (41), C3 expression in the mouse spinal cord ventral horn appeared to be largely associated with astrocytes (79). Results from other studies have also emphasized a correlation between astroglial reactivity and the extent of presynaptic terminal removal (84, 89). Furthermore, the pronounced axotomy-induced changes in postsynaptic receptor expression and function (21, 22, 90, 91), the extensive shrinkage of the dendritic tree (29, 30), changes in the expression and conformation of synaptic adhesion molecules (92, 93), and in the architecture of postsynaptic complexes (94) are also likely to have a direct influence on the distribution of motor neuron presynaptic terminals.

4.4.4. Role of microglia for astroglial reactivity

Astroglial responses are commonly assessed by the level of glial fibrillary acidic protein (GFAP) expression, the unique astroglial intermediate filament in the CNS. However, GFAP is part of the cytoskeletal core of astrocytes, whereas the actual functions of astrocytes are carried out in the distal lamellar processes, from which GFAP is absent. The levels of GFAP expression may therefore not be a sensitive measure of the functional activity of astrocytes.

In the absence of microglia, up-regulation of GFAP mRNA and protein are attenuated in the axotomized hypoglossal nucleus (95). Similarly, in mice with a genetic deletion of interleukin-6 (IL-6), presumably derived from microglia, up-regulation of GFAP does not occur in the axotomized facial nucleus (96). Furthermore, the functional properties of astrocytes are modified by a number of other inflammatory cytokines (97). At the same time, astrocytes have a central role in buffering extracellular potassium, are highly sensitive to ATP and adenosine, are responsible for the main part of glutamate and GABA uptake, in addition to being directly responsive to various neurotransmitters. The way these factors contribute to altered astroglial reactivity and function after motor neuron axotomy is still largely unknown.

4.4.5. Role of microglia for lymphocyte infiltration

Although there is a report of T-lymphocytes entering the axotomized rat facial nucleus (98), this phenomenon appears not to be typical for axotomized motor neurons in this species. However, facial nerve axotomy in the mouse produces extensive infiltration of lymphocytes (20). Initially, this invasion was considered to correlate with the extensive delayed motor neuron loss that occurs in the mouse compared to the rat following axotomy. However, results from recent studies indicate that T-cell infiltration may benefit motor neuron survival (99). These findings may lead to identification of survival promoting factors that can be helpful in other conditions of neuron degeneration and in other species. It is important to explore lymphocyte infiltration into the CNS after motor neuron axotomy in other mammals, including humans, to evaluate the generality of this phenomenon.

5. SENSORY NEURON AXOTOMY

5.1. Anatomical aspects on sensory neuron axotomy

Sensory neurons in dorsal root and cranial nerve ganglia are heterogeneous in terms of morphology, molecular phenotype, modality, impulse propagation properties and central projections. On stimulation they probably all release glutamate from their central terminals. In addition, many sensory neurons contain one or more peptides, e.g. substance P (SP) and calcitonin gene-related peptide (CGRP) which are also released following stimulation.

Here, we will discuss the role of microglia in the same injury models as for motor neuron axotomy, crush or transection with or without permitted peripheral regeneration. These experiments have been made on spinal nerves or the trigeminal nerve, with the sciatic nerve as by far the most popular one. Microglial proliferation and activation in the dorsal horn following sensory nerve injury were first demonstrated in the spinal cord dorsal horn following spinal nerve injury (100-102). Later studies have revealed that this is a general response following peripheral sensory axon injury (see. e.g. 103). Microglial responses in sensory areas of the dorsal horn and trigeminal nucleus have been the subject of intense research within the context of injury-induced neuropathic pain. Peripheral nerve injuries such as partial chronic constriction (CCI), spinal nerve ligation (SNL), spared nerve injury (SNI) and toxin-induced diabetic neuropathy results in sensory hypersensitivity and/or allodynia in experimental animals, which are interpreted as reflecting aspects of injury-induced neuropathic pain in humans. The central role of microglia in these conditions has been extensively reviewed (4-8)).

5.2. Cellular and molecular changes in axotomized sensory neurons

The expression pattern of regeneration-associated genes in the cell bodies of peripherally axotomized sensory neurons in dorsal root or cranial nerve ganglia is analogous to those occurring in axotomized motor neurons. These changes are primarily directed towards rebuilding the peripheral axon and restoring target contact. Associated with these changes are changes in the molecular phenotype of many sensory neurons. E.g., substance P and CGRP are down-regulated in the axotomized dorsal root ganglion cells, whereas e.g. galanin, pituitary adenylase cyclase activating polypeptide (PACAP) and neuropeptide Y (NPY) are up-regulated (2).

Furthermore, central terminals of peripherally axotomized sensory neurons display signs of degeneration or are lost by an unknown process (104-106), leading to a substantial deafferentation of postsynaptic neurons. Concomitantly, existing connections are modified and novel connections may appear (107). The loss of terminals from peripherally injured sensory neurons has some analogy to the synaptic stripping from axotomized motor neurons; in both cases leading to altered input to the neuron which is postsynaptic to the lost/retracted terminals. At the same time, evidence of regenerative processes occurs, as evidenced by an increase in the growth-associated protein GAP-43 in central primary sensory terminals, and the formation of novel synaptic connections (108, 109). Thus, the functional properties of the circuitries in the dorsal horn and cranial nerve sensory nuclei are dramatically altered following peripheral sensory axotomy.

5.3. Microglial responses to sensory neuron axotomy and their functional consequences

The microglial response to sensory neuron axotomy clearly resembles that around motor neurons in terms of their proliferation and activation as determined by their molecular expression pattern, including up-regulation of CD11b (Figure 3). As in motor neuron axotomy the immediate release of potassium, ATP and neurotransmitters are likely to underlie the initial activation of the local microglial population. In the subsequent enhancement and maintenance of the microglial response, chemokine CCL2 (MIP-1)/CCR2 and CXCL3 (fraktalkine)/CXCR3 signaling between neurons and microglia appear to be involved (4-8). However, it is unclear whether there is a distinct wave of migration after sensory neuron atotomy. Although, no detailed studies appear to exist on this issue, microglial cells appear to proliferate and become activated essentially at the same site. Electron microscopic images show these cells to be interspersed in the neuropil of the dorsal horn after spinal nerve injury and in the trigeminal sensory nuclei after trigeminal nerve injury without distinct relationship to nerve cell bodies (100, 110, 111).

Engulfment and presumed phagocytosis of degenerating axons or axon terminals were described after sensory neuron axotomy in the dorsal horn (99) and trigeminal nuclei (110, 111). However, the majority of microglia in sensory projection areas do not associate with neural degeneration. E.g., numerous markedly abnormal axons without any association with microglia are found in the gracile nucleus following sciatic nerve injury in the rat (112). Thus, apart from the reasonable assumption that an extended monitoring of the local CNS environment is called for, there is no defined physiological role for activated microglia after sensory neuron axotomy. The recent findings on the role of the MHC and complement systems for synapse elimination on axotomized motor neurons should prompt studies on whether related mechanisms are involved in synapse loss in sensory projection areas after sensory neuron axotomy.

Since the original reports of microglial proliferation and activation in the dorsal horn following sensory nerve injury microglial responses in sensory areas of the dorsal horn and trigeminal nucleus have been the subject of intense research within the context of injury-induced neuropathic pain. This research exploits somewhat different models than those commonly used for issues of neuron-glial interactions in nerve regeneration or neuron degeneration. Partial chronic constriction (CCI), spinal nerve ligation (SNL), spared nerve injury (SNI) and various forms of inflammation or toxin-induced nerve injury results in sensory hypersensitivity and/or allodynia in experimental animals, which are interpreted as reflecting relevant aspects of injury-induced neuropathic pain in humans. The central role of microglia in these conditions has been extensively documented and the subject of numerous recent reviews (see e.g. 4-8).

Intuitively, one would like to view the microglial response to peripheral nerve injury, a presumably evolutionary well conserved response pattern, as a potentially beneficial response, or at least not harmful. An enigmatic issue in relation to apparently critical role of microglia in the development of neuropathic pain is why microglia takes on these properties in certain types of peripheral nerve injuries or after peripheral nerve injury in certain individuals. Genetic predisposition appears to play a major role, but its links to neuropathic pain behavior is still largely unclear (113).

6. SUMMARY AND PERSPECTIVES

Microglial proliferation, migration and phenotypic activation is a hallmark of peripheral nerve injury (axotomy), both in association with axotomized motor and autonomic neuron cell bodies and in association with central terminals of peripherally axotomized sensory neurons. Recent studies with live imaging and on transgenic mice have provided important and novel information on the mechanisms underlying microglial responses following injury. Studies using injury models of neuropathic pain have highlighted a central role for activated microglia in this condition. However, there are still substantial gaps in our understanding of the significance and implications of these responses for survival of axotomized neurons, and their ability to regenerate the axon and restore function. Studies using conditional gene regulation and in vivo RNA silencing should help to clarify these issues. Studies with an evolutionary perspective should be helpful in our understanding possible evolutionary modifications and their functional significance in the microglial response to peripheral nerve injury. The limited information available on this issue indicates that the prime role of microglia is to promote tissue repair and functional restoration (114). Finally, translational research to mammalian models closer to humans is necessary in order to verify the generality and validity of the data obtained in mouse and rat models.

7. ACKNOWLEDGEMENTS

The author's research was sponsored by the Swedish Research Council, proj 5420.

8. REFERENCES

1. Gennadij Raivich and Milan Makwana: The making of successful axonal regeneration: genes, molecules and signal transduction pathways. Brain Res Rev 53, 287-311 (2007)
doi:10.1016/j.brainresrev.2006.09.005
PMid:17079020

2. Peter Richardson, Tizong Miao, Dongsheng Wu, Yi Zhang, John Yeh and Xuenong Bo: Responses of the nerve cell body to axotomy. Neurosurgery 65, 74-79 (2009)
doi:10.1227/01.NEU.0000352378.26755.C3
PMid:19927082

3. Lisa Walter and Harald Neumann: Role of microglia in neuronal degeneration and regeneration. Semin Immunopathol 31, 513-525 (2009)
doi:10.1007/s00281-009-0180-5
PMid:19763574

4. Erin Milligan and Linda Watkins: Pathological and protective roles of glia in chronic pain. Nat Rev Neurosci 10, 23-36 (2009)
doi:10.1038/nrn2533
PMid:19096368    PMCid:2752436

5. Ru-Rong Ji, Robert Gereau, Marzia Malcangio and Gary Strichartz: MAP kinase and pain. Brain Res Rev 60, 135-148 (2009)
doi:10.1016/j.brainresrev.2008.12.011
PMid:19150373    PMCid:2666786

6. Kazuhide Inoue and Makato Tsuda: Microglia and neuropathic pain. Glia 57, 1469-1479 (2009)

7. Yong-Jing Gao and Ru-Rong Ji: Chemokines, neuronal-glial interactions, and central processing of neuropathic pain. Pharmacol Ther 126: 56-68 (2010)
doi:10.1016/j.pharmthera.2010.01.002
PMid:20495595

8. Howard Smith: Activated microglia in nociception. Pain Physician 13, 295-304 (2010)
PMid:20148679

9. Michael Halassa and Philip Haydon: Integrated brain circuits: astrocytic networks modulate neuronal activity and behavior. Annu Rev Physiol 72, 335-255 (2010)
doi:10.1146/annurev-physiol-021909-135843
PMid:15895084

10. Dimitrios Davalos, Jaime Grutzendler, Guang Yang, Jiyun Kim, Yi Zuo, Steffen Jung, Dan Littman, Michael Dustin and Wen-Biao Gan: ATP mediates rapid microglial response to local brain injury in vivo. Nat Neurosci 8, 752-758 (2005)
doi:10.1038/nn1472
PMid:15831717

11. Axel Nimmerjahn, Frank Kirchhoff and Fritjof Helmchen: Resting microglial cells are highly dynamic surveillants of brain parenchyma in vivo Science 308, 1314-1318 (2005)
doi:10.1126/science.1110647
PMid:19339593

12. Hiroaki Wake, Andrew Moorhouse,Shozo Jinno, Shinichi Kohsaka and Junichi Nabekura: Resting microglia directly monitor the functional state of synapses in vivo and determine the fate of ischemic terminals. J Neurosci 29, 3974-3980 (2009)
doi:10.1523/JNEUROSCI.4363-08.2009

13. Hana Lin H, Jianxin Bao, Ying-Ju Sung, Edgar Walters and Richard Ambron: Rapid electrical and delayed molecular signals regulate the serum response element after nerve injury: convergence of injury and learning signals. J Neurobiol 57, 204-220 (2003) Erratum in: J Neurobiol 59, 259 (2004)
doi:10.1002/neu.20052
PMid:16899067

14. Shlomit Hanz and Mike Fainzilbe: Retrograde signaling in injured nerve--the axon reaction revisited. J Neurochem 99, 13-19 (2006)
doi:10.1111/j.1471-4159.2006.04089.x
PMid:8213072

15. Manuel Graeber, Karl Bise and Parviz Mehraein: Synaptic stripping in the human facial nucleus. Acta Neuropathol 86, 179-181 (1993)
doi:10.1007/BF00334886
PMid:6476403

16. John Cova and Hakan Aldskogius: Effect of nerve section on perineuronal glial cells in the CNS of rat and cat. Anat Embryol (Berl) 169, 303-307 (1984)
doi:10.1007/BF00315635
PMid:3980778

17. John Cova and Hakan Aldskogius: A morphological study of glial cells in the hypoglossal nucleus of the cat during nerve regeneration. J Comp Neurol 233:421-428 (1985)
doi:10.1002/cne.902330402

18. John Cova and Hakan Aldskogius: Effect of axotomy on perineuronal glial cells in the hypoglossal and dorsal motor vagal nuclei of the cat. Exp Neurol 93, 662-667 (1986)
doi:10.1016/0014-4886(86)90187-1
PMid:1812171

19. Mikael Svensson, Eva Tornqvist, Hakan Aldskogius and John Cova: Synaptic detachment from hypoglossal neurons after different types of nerve injury in the cat. J Hirnforsch 32, 547-552 (1991)
PMid:9671668

20. Gennadij Raivich, Leonard Jones, Christian Kloss, Alexander Werner, Harald Neumann and Georg Kreutzberg: Immune surveillance in the injured nervous system: T-lymphocytes invade the axotomized mouse facial motor nucleus and aggregate around sites of neuronal degeneration. J Neurosci 18, 5804-5816 (1998)
PMid:15733085

21. Lyndell Eleore, Isabelle Vassias, Pierre-Paul Vidal, Antoine Triller and Catherine de Waele: Modulation of glycine receptor subunits and gephyrin expression in the rat facial nucleus after axotomy. Eur J Neurosci 21, 669-678 (2005)
doi:10.1111/j.1460-9568.2005.03887.x
PMid:16182453

22. Lyndell Eleore, Isabelle Vassias, Pierre-Paul Vidal and Catherine de Waele: Modulation of the glutamatergic receptors (AMPA and NMDA) and of glutamate vesicular transporter 2 in the rat facial nucleus after axotomy. Neurosci 136, 147-160 (2005)
doi:10.1016/j.neuroscience.2005.06.026
PMid:1560254

23. Mikael Svensson and Hakan Aldskogius: The effect of axon injury on microtubule-associated proteins MAP2, 3 and 5 in the hypoglossal nucleus of the adult rat. J Neurocytol 21, 222-231 (1992)
doi:10.1007/BF01194980
PMid:1578011

24. Thomas Brannstrom, Leif Havton and Jan-Olof Kellerth: Changes in size and dendritic arborization patterns of adult cat spinal alpha-motoneurons following permanent axotomy. J Comp Neurol 318, 439-451 (1992)
doi:10.1002/cne.903180408
PMid:1578012

25. Thomas Brannstrom, Leif Havton and Jan-Olof Kellerth: Restorative effects of reinnervation on the size and dendritic arborization patterns of axotomized cat spinal alpha-motoneurons. J Comp Neurol 318, 452-461 (1992)
doi:10.1002/cne.903180409

26. Satoshi Kobayashi, Tomoaki Shirao and Tomio Sasaki: Drebrin expression is increased in spinal motoneurons of rats after axotomy. Neurosci Lett 311, 165-168 (2001)
doi:10.1016/S0304-3940(01)02155-3
PMid:6204997

27. Paul Hoffman, John Griffin and Donald Price: Control of axonal caliber by neurofilament transport. J Cell Biol 99, 705-714 (1984)
doi:10.1083/jcb.99.2.705

28. Karl Blinzinger and Georg Kreutzberg: Displacement of synaptic terminals from regenerating motoneurons by microglial cells. Z Zellforsch Mikrosk Anat 85, 145-157 (1968)
doi:10.1007/BF00325030

29. Thomas Brannstrom and Jan-Olof Kellerth: Changes in synaptology of adult cat spinal alpha-motoneurons after axotomy. Exp Brain Res 118, 1-13 (1998)
doi:10.1007/s002210050249

30. Thomas Brannstrom and Jan-Olof Kellerth: Recovery of synapses in axotomized adult cat spinal motoneurons after reinnervation into muscle. Exp Brain Res 125, 19-27 (1999)
doi:10.1007/s002210050653

31. Hakan Aldskogius and Elena Kozlova: Central neuron-glial and glial-glial interactions following axon injury. Prog Neurobiol 55, 1-26 (1998)
doi:10.1016/S0301-0082(97)00093-2

32. Linda Moran and Manuel Graeber: The facial nerve axotomy model. Brain Res Brain Res Rev 44, 154-178 (2004)
doi:10.1016/j.brainresrev.2003.11.004
PMid:15003391

33. Rajesh Khanna, Lipi Roy, Xiaoping Zhu and Lyanne Schlichter: K+ channels and the microglial respiratory burst, Am J Physiol Cell Physiol 280, C796-C806 (2001)

34. Christopher Fordyce, Ravi Jagasia and Xiaoping Zhu and Lyanne Schlichter: Microglia Kv1.3 channels contribute to their ability to kill neurons, J Neurosci 25, 7139-7149 (2005)
doi:10.1523/JNEUROSCI.1251-05.2005
PMid:16079396

35. Fan Li F, Jia Lu, Chun-Yun Wu, Charanjit Kaur, Viswanathan Sivakumar, Jun Sun, Shuqing Li and Eng-Ang Ling: Expression of Kv1.2 in microglia and its putative roles in modulating production of proinflammatory cytokines and reactive oxygen species. J Neurochem 106, 2093-2105 (2008)
doi:10.1111/j.1471-4159.2008.05559.x
PMid:18627436

36. Chun-Yun Wu, Charanjit Kaur, Viswanathan Sivakumar, Jia Lu and Eng-Ang Ling: Kv1.1 expression in microglia regulates production and release of proinflammatory cytokines, endothelins and nitric oxide. Neuroscience 158, 1500-1508 (2009)
doi:10.1016/j.neuroscience.2008.11.043
PMid:19118603

37. Kazuhide Inoue K: Purinergic systems in microglia. Cell Mol Life Sci 65, 3074-3080 (2008)
doi:10.1007/s00018-008-8210-3
PMid:18563292

38. Jochum Gehrmann, Günter Mies, Bonnekoh P, Roger Banati, Iijima T, Georg Kreutzberg and Konstatin-Alexander Hossmann : Microglial reaction in the rat cerebral cortex induced by cortical spreading depression. Brain Pathol 3, 11-17 (1993)
doi:10.1111/j.1750-3639.1993.tb00720.x
PMid:8269080

39. Sebastian Jander, Michael Schroeter, Oliver Peters, Otto Witte and Guido Stoll: Cortical spreading depression induces proinflammatory cytokine gene expression in the rat brain. J Cereb Blood Flow Metab 21, 218-225 (2001)
doi:10.1097/00004647-200103000-00005

40. Yilong Cui, Tadajuki Takashima, Misato Takashima-Hirano, Yuasuhiro Wada, Shukuri Miho, Yasuhisa Tamura, Hisashi Doi, Hitotaka Onoe, Yosky Kataoka and Yasuoshi Watanabe: 11C-PK11195 PET for the in vivo evaluation of neuroinflammation in the rat brain after cortical spreading depression. J Nucl Med 50, 1904-1911 (2009)
doi:10.2967/jnumed.109.066498
PMid:19837755

41. Mikael Svensson and Hakan Aldskogius: Evidence for activation of the complement cascade in the hypoglossal nucleus following peripheral nerve injury. J Neuroimmunol 40, 99-109 (1992)
doi:10.1016/0165-5728(92)90217-9

42. Mikael Svensson, Li Liu, Per Mattsson, Paul Morgan and Hakan Aldskogius: Evidence for activation of the terminal pathway of complement and upregulation of sulfated glycoprotein (SGP)-2 in the hypoglossal nucleus following peripheral nerve injury. Mol Chem Neuropathol 24, 53-68 (1995)
doi:10.1007/BF03160112
PMid:7755847

43. Carsten Moller, Michael Klein, Stefan Haas, Leonard Jones, Georg Kreutzberg and Gennadij Raivich: Regulation of thrombospondin in the regenerating mouse facial motor nucleus. Glia 17, 121-132 (1996)
PMid:11553283

44. Matthias Galiano, Zhi Qiang Liu, Roger Kalla, Marion Bohatsche, Andrea Koppius, Andreas Gschwendtner, ShengLi Xu, Alexander Werner, Christian Kloss, Leonard Jones, Horst Bluethmann and Gennadij Raivich: Interleukin-6 (IL6) and cellular response to facial nerve injury: effects on lymphocyte recruitment, early microglial activation and axonal outgrowth in IL6-deficient mice. Eur J Neurosci 14, 327-341 (2001)
doi:10.1046/j.0953-816x.2001.01647.x
PMid:11438923

45. Roger Kalla, Zhi Liu, ShengLi Xu, Andrea Koppius, Yoshinori Imai, Christian Kloss, Shinichi Kohsaka, Andreas Gschwendtner, Carsten Moller, Alexander Werner and Gennadiaj Raivich: Microglia and the early phase of immune surveillance in the axotomized facial motor nucleus: impaired microglial activation and lymphocyte recruitment but no effect on neuronal survival or axonal regeneration in macrophage-colony stimulating factor-deficient mice. J Comp Neurol 436, 182-201 (2001)
doi:10.1002/cne.1060
PMid:15465604

46. Marion Bohatschek, Christian Kloss, Klaus Pfeffer, Horst Bluethmann and Gennadij Raivich: B7.2 on activated and phagocytic microglia in the facial axotomy model: regulation by interleukin-1 receptor type 1, tumor necrosis factor receptors 1 and 2 and endotoxin. J Neuroimmunol 156, 132-145 (2004)
doi:10.1016/j.jneuroim.2004.07.018
PMid:8776579

47. Uwe-Karsten Hanisch: Microglia as a source and target of cytokines. Glia 40, 140-155 (2002)
PMid:12580336

48. Gareth John, Sunhee Lee and Celia Brosnan: Cytokines: Powerful regulators of glial cell activation. Neuroscientist 9, 10-22 (2003)
doi:10.1177/1073858402239587

49. Manuel Graeber, Wolfram Tetzlaff, Wolfgan Streit and Georg Kreutzberg: Microglial cells but not astrocytes undergo mitosis following rat facial nerve axotomy. Neurosci Lett 85, 317-321 (1988)
doi:10.1016/0304-3940(88)90585-X
PMid:7649789    PMCid:1166660

50. Mikael Svensson, Per Mattsson and Hakan Aldskogius: A bromodeoxyuridine labelling study of proliferating cells in the brainstem following hypoglossal nerve transection. J Anat 185, 537-542 (1994)
PMid:16673407

51. Kazuyuki Nakajima, Manuel Graeber, Maya Sonoda, Yoko Tohyama, Shinichi Kohsaka and Tadashi Kurihara: In vitro proliferation of axotomized rat facial nucleus-derived activated microglia in an autocrine fashion. J Neurosci Res 84, 348-359 (2006)
doi:10.1002/jnr.20882
PMid:7850025

52. Gennadij Raivich, Teresa Moreno-Flores, Carsten Moller and Georg Kreutzberg: Inhibition of posttraumatic microglial proliferation in a genetic model of macrophage colony-stimulating factor deficiency in the mouse. Eur J Neurosci 6, 1615-1618 (1994)
doi:10.1111/j.1460-9568.1994.tb00552.x
PMid:6490973

53. Ingrid Reisert, G Wildemann, D Grab and Christoph Pilgrim: The glial reaction in the course of axon regeneration: a stereological study of the rat hypoglossal nucleus. J Comp Neurol 229, 121-128 (1984)
doi:10.1002/cne.902290109
PMid:8477825

54. Mikael Svensson and Hakan Aldskogius: Synaptic density of axotomized hypoglossal motorneurons following pharmacological blockade of the microglial cell proliferation. Exp Neurol 120, 123-131 (1993)
doi:10.1006/exnr.1993.1046
PMid:9698315

55. Doychin Angelov, Michael Walther, Michael Streppel, Orlando Guntinas-Lichius, Wolfram Neiss, Rainer Probstmeier and Penka Pesheva: Tenascin-R is antiadhesive for activated microglia that induce downregulation of the protein after peripheral nerve injury: a new role in neuronal protection. J Neurosci 18, 6218-6229 (1998)
PMid:17539023

56. Hai Peng Hao, Katsumi Doh-Ura and Hiroshi Nakanishi: Impairment of microglial responses to facial nerve axotomy in cathepsin S-deficient mice. J Neurosci Res 85, 196-206 (2007)
doi:10.1002/jnr.21357

57. Hiroyuki Konishi, Kazuhiko Namikawa and Hiroshi Kiyama: Annexin III is implicated in the microglial response to motor nerve injury. Glia 53, 723-732 (2006)
PMid:19196078

58. Christian Kloss, Alexander Werner, Michael Klein, Jun Shen, Karen Menuz, Christoph Probst, Georg Kreutzberg, Gennadij Raivich: Integrin family of cell adhesion molecules in the injured brain: regulation and cellular localization in the normal and regenerating mouse facial motor nucleus. J Comp Neurol 411, 162-178 (1999)
doi:10.1002/(SICI)1096-9861(19990816)411:1<162::AID-CNE12>3.0.CO;2-W
PMid:19302147

59. Hasna Baloui, Olivier Stettler, Stefan Weiss, Fatiha Nothias, Ysander von Boxberg: Upregulation in rat spinal cord microglia of the nonintegrin laminin receptor 37 kDa-LRP following activation by a traumatic lesion or peripheral injury. J Neurotrauma 26, 195-207 (2009)
doi:10.1089/neu.2008.0677

60. Guo Jun Liu, Rajini Nagarajah, Richard Banati and Max Bennett: Glutamate induces directed chemotaxis of microglia. Eur J Neurosci 29, 1108-1118 (2009)
doi:10.1111/j.1460-9568.2009.06659.x

61. Alexander Flügel, Gerhard Hager, Andrea Horvat, Christoph Spitzer, Gamal Singer, Manuel Graeber, Georg Kreutzberg and Franz-Werner Schwaiger: Neuronal MCP-1 expression in response to remote nerve injury. J Cereb Blood Flow Metab 21, 69-76 (2001)
doi:10.1097/00004647-200101000-00009

62. Michelle Muessel, Robert Klein, Angela Wilson and Nancy Berman: Ablation of the chemokine monocyte chemoattractant protein-1 delays retrograde neuronal degeneration, attenuates microglial activation, and alters expression of cell death molecules. Brain Res Mol Brain Res 103, 12-27 (2002)
doi:10.1016/S0169-328X(02)00158-4
PMid:10805752    PMCid:85780

63. Jeffrey Harrison, Yan Jiang, Shizong Chen, Yiyang Xia, Dominique Maciejewski, Robert McNamara, Wofgang Streit, Mina Salafranca, Soumya Adhikari, Darren Thompson, Paolo Botti, Kevin Bacon and Lili Feng: Role for neuronally derived fractalkine in mediating interactions between neurons and CX3CR1-expressing microglia. Proc Natl Acad Sci U S A 95, 10896-10901 (1998)
doi:10.1073/pnas.95.18.10896
PMid:18538419

64. Steffen Jung, Julio Aliberti, Petra Graemmel, Mary Jean Sunshine, Georg Kreutzberg, Alan Sher, Dan Littman: Analysis of fractalkine receptor CX(3)CR1 function by targeted deletion and green fluorescent protein reporter gene insertion. Mol Cell Biol 20, 4106-4114 (2000)
doi:10.1128/MCB.20.11.4106-4114.2000

65. Knut Biber, J Vinet and Erik Boddeke: Neuron-microglia signaling: Chemokines as versatile messengers. J Neuroimmunol 198, 69-74 (2008)
doi:10.1016/j.jneuroim.2008.04.012
PMid:18538865    PMCid:2577374

66. Orlando Guntinas-Lichius, Wolfram Neiss, Andreas Gunkel and Eberhard Stennert: Differences in glial, synaptic and motoneuron responses in the facial nucleus of the rat brainstem following facial nerve resection and nerve suture reanastomosis. Eur Arch Otorhinolaryngol 25, 410-417 (1994)
PMid:17344397

67. Grace Ha, Shivani Parikh, Zhi Huang and John Petitto: Influence of injury severity on the rate and magnitude of the T lymphocyte and neuronal response to facial nerve axotomy. J Neuroimmunol 199, 18-23 (2008)
doi:10.1016/j.jneuroim.2008.04.027
PMid:18486483

68. Mélanie Lalancette-Hebert, Geneviève Gowing, Alain Wimard, Yuan Cheng Weng and Jasna Kriz: Selective ablation of proliferating microglia exacerbates ischemic injury in the brain. J Neurosci 27:2596-2605 (2007)
doi:10.1523/JNEUROSCI.5360-06.2007
PMid:20305648

69. Urban Kilic, Ertugrui Kilic, Christian Matter, Claudio Bassetti and Duirk Hermann: TLR-4 deficiency protects against focal cerebral ischemia and axotomy-induced neurodegeneration. Neurobiol Dis 31, 33-40 (2008)
doi:10.1016/j.nbd.2008.03.002
PMid:12379902

70. Martin Fuhrmann, Tobias Bittner, Christian Jung, Steffen Burgold, Richard Page, Gerda Mitteregger, Christian Haas, Frank LaFerla, Hans Kretschmer and Jochen Herms: Microglial CX3CR1 knockout prevents neuron loss in a mouse model of Alzheimer's disease. Nat Neurosci 13, 411-413 (2010)
doi:10.1038/nn.2511
PMid:16506224

71. Mikael Svensson and Hakan Aldskogius: Infusion of cytosine-arabinoside into the cerebrospinal fluid of the rat brain inhibits the microglial cell proliferation after hypoglossal nerve injury. Glia 7, 286-298 (1993)
PMid:10800205

72. Doychin Angelov, Claudia Krebs, Michael Walther, Francisco Martinez-Portillo, Andreas Gunkel, Lay CH, Michael Streppel, Orlando Guntinas-Lichius, Eberhard Stennert, Wolfram Neiss: Altered expression of immune-related antigens by neuronophages does not improve neuronal survival after severe lesion of the facial nerve in rats. Glia 24, 155-171 (1998)
PMid:19005063

73. Doychin Angelov, Andres Gunkel, Eberhard Stennert E, Wolfram Neiss: Phagocytic microglia during delayed neuronal loss in the facial nucleus of the rat: time course of the neuronofugal migration of brain macrophages. Glia 13, 113-129 (1995)

74. Ran Tao and Hakan Aldskogius: Glial cell responses, complement and apolipoprotein J expression following axon injury in the neonatal rat. J Neurocytol 28, 559-570 (1999)
doi:10.1023/A:1007067305837

75. Kazushige Gamo, Sumiko Kiryu-Seo, Hiroyuki Konishi, Shunsuke Aoki, Kouji Matsushima, Keiji Wada and Hiroshi Kiyama: G-protein-coupled receptor screen reveals a role for chemokine receptor CCR5 in suppressing microglial neurotoxicity. J Neurosci 28, 11980-11988 (2008)
doi:10.1523/JNEUROSCI.2920-08.2008
PMid:16934480

76. Derek Wainwright, Junping Xin, Nicole Mesnard, Taylor Beahrs, Christine Politis, Virgnia Sanders and Kathryn Jones: Exacerbation of facial motoneuron loss after facial nerve axotomy in CCR3-deficient mice. ASN Neuro 1, e00024 (2009)
PMid:19525015

77. Derek Wainwright, Nicole Mesnard, Junping Xin, Virginia Sanders and Kathryn Jones: Effects of facial nerve axotomy on Th2-associated and Th1-associated chemokine mRNA expression in the facial motor nucleus of wild-type and presymptomatic SOD1. J Neurodegen Regen 2, 39-44 (2009)
PMid:19321792    PMCid:2693768

78. Maria Swanberg, Kristina Duvefelt, Margarita Diez, Jan Hillert, Tomas Olsson, Fredrik Piehl and Olle Lidman: Genetically determined susceptibility to neurodegeneration is associated with expression of inflammatory genes. Neurobiol Dis 24, :67-88 (2006)
doi:10.1016/j.nbd.2006.05.016
PMid:8261093

79. Margarita Diez, Nada Abdelmagid, KarinHarnesk, Mikael Strom, Olle Lidman, Maria Swanberg, Rickard Lindblom, Faiez Al-Nimer, Maja Jagodic, Tomas Olsson and Fredrik Piehl: Identification of gene regions regulating inflammatory microglial response in the rat CNS after nerve injury. J Neuroimmunol 212, 82-92 (2009)
doi:10.1016/j.jneuroim.2009.05.004
PMid:20137064    PMCid:2829570

80. John Gensel, Satoshi Nakamura, Zhen Guan, Nico van Rooijen, Daniel Ankeny and Philip Popovich: Macrophages promote axon regeneration with concurrent neurotoxicity. J Neurosci 29, 3956-3968 (2009)
doi:10.1523/JNEUROSCI.3992-08.2009
PMid:624794

81. Mikael Svensson and Hakan Aldskogius: Regeneration of hypoglossal nerve axons following blockade of the axotomy-induced microglial cell reaction in the rat. Eur J Neurosci 5, 85-94 (1993)
doi:10.1111/j.1460-9568.1993.tb00208.x
PMid:10767097

82. Bahman Shokouhi, Bernadette Wong, Samir Siddiqui, Robert Lieberman, Gregor Campbell, Koujiro Tohoyama and Patrick Anderson: Microglial responses around intrinsic CNS neurons are correlated with axonal regeneration. BMC Neuroscience 11, 13 (2010)
doi:10.1186/1471-2202-11-13
PMid:15591351    PMCid:539738

83. Johan Zelano: Adhesion molecules and synapse remodeling during motoneuron regeneration. Thesis, Karolinska institutet, ISBN: 978-91-7409-623-1 (2010)

84. David Chen: Qualitative and quantitative study of synaptic displacement in chromatolyzed spinal motoneurons of the cat. J Comp Neurol 177, 635-664 (1978)
doi:10.1002/cne.901770407
PMid:17764750

85. J Schiefer, K Kampe, HU Dodt, Walter Zieglgansberger, Georg Kreutzberg: Microglial motility in the rat facial nucleus following peripheral axotomy. J Neurocytol 28, 439-453 (1999)
doi:10.1023/A:1007048903862
PMid:16714003

86. Alain Bessis, Catherine Béchade, Delphine Bernard and Anne Roumier: Microglial control of neuronal death and synaptic properties. Glia 55, 233-238 (2007)
PMid:15994018

87. Alexandre Oliveira, Sebastian Thams, Olle Lidman, Fredrik Piehl, Tomas Hokfelt, Klas Karre, Hans Linda and Staffan Cullheim: A role for MHC class I molecules in synaptic plasticity and regeneration of neurons after axotomy. Proc Natl Acad Sci U S A 101, 17843-17848 (2004)
doi:10.1073/pnas.0408154101
PMid:17492651

88. Sebastian Thams, Alexandre Oliviera and Staffan Cullheim: MHC class I expression and synaptic plasticity after nerve lesion. Brain Res Rev 57, 256-269 (2008)
doi:10.1016/j.brainresrev.2007.06.016
PMid:19827159

89. Amanda Emirandetti, Renata Graciele Zanon, Mario Sabha Jr and Alexandre de Oliveira: Astrocyte reactivity influences the number of presynaptic terminals apposed to spinal motoneurons after axotomy. Brain Res 1095, 35-42 (2006)
doi:10.1016/j.brainres.2006.04.021

90. Ryo Ikeda and Fusao Kato: Early and transient increase in spontaneous synaptic inputs to the rat facial motoneurons after axotomy in isolated brainstem slices of rats. Neuroscience 134, 889-899 (2005)
doi:10.1016/j.neuroscience.2005.05.002
PMid:8360946

91. Jun Yamada, Yoshinori Hayashi, Shozo Jinno, Zhou Wu, Kazunide Inoue, Shinichi Kohsaka and Hiroshi Nakanishi: Reduced synaptic activity precedes synaptic stripping in vagal motoneurons after axotomy. Glia 56, 1448-1462 (2008)
PMid:8320000

92. Johan Zelano, Wilhelm Wallquist, Nils Hailer and Staffan Cullheim: Down-regulation of mRNAs for synaptic adhesion molecules neuroligin-2 and -3 and synCAM1 in spinal motoneurons after axotomy. J Comp Neurol 503, 308-318 (2007)
doi:10.1002/cne.21382
PMid:9728762

93. Johan Zelano, Alexander Berg, Sebastian Thams, Nils Hailer and Staffan Cullheim: SynCAM1 Expression correlates with restoration of central synapses on spinal motoneurons after two different models of peripheral nerve injury. J Comp Neurol 517, 670-682 (2009)
doi:10.1002/cne.22186
PMid:7649615

94. Yong Ho Che, Michio Tamatani and Masaya Tohyama: Changes in mRNA for post-synaptic density-95 (PSD-95) and carboxy-terminal PDZ ligand of neuronal nitric oxide synthase following facial nerve transection. Brain Res Mol Brain Res 76, 325-335 (2000)
doi:10.1016/S0169-328X(00)00013-9
PMid:17106878

95. Mikael Svensson, Petri Eriksson, Hakan Aldskogius H. Evidence for activation of astrocytes via reactive microglial cells following hypoglossal nerve transection. J Neurosci Res 35, 373-381 (1993)
doi:10.1002/jnr.490350404
PMid:18512252

96. Michael Klein, Carsten Moller, Leonard Jones, Horst Bluethmann, Georg Kreutzberg and Gennadij Raivich: Impaired neuroglial activation in interleukin-6 deficient mice. Glia 19, 227-233 (1997)
PMid:9063729

97. Daniela Rossi and Andrea Volterra: Astrocytic dysfunction: insights on the role in neurodegeneration. Brain Res Bull 80, 224-232 (2009)
doi:10.1016/j.brainresbull.2009.07.012
PMid:19631259

98. Tomas Olsson, Per Diener, Ake Ljungdahl, Bo Hojeberg, Peter van der Meide and Krister Kristensson: Facial nerve transection causes expansion of myelin autoreactive T cells in regional lymph nodes and T cell homing to the facial nucleus. Autoimmunity 13, 117-126 (1992)
doi:10.3109/08916939209001912
PMid:1281678

99. Zhi Huang, Grace Ha, John Petitto: IL-15 and IL15R alpha gene3 deletion: effects on T lymphocyte trafficking and the microglial and neuronal responses to facial nerve axotomy. Neurosci Lett 417, 160-164 (2007)
doi:10.1016/j.neulet.2007.01.086
PMCid:1903346

100. John Cova, Hakan Aldskogius, Jan Arvidsson and Carl Molander: Changes in microglial cell numbers in the spinal cord dorsal horn following brachial plexus transection in the adult rat. Exp Brain Res 73, 61-66 (1988)
doi:10.1007/BF00279661

101. Petri Eriksson, Jonas Persson, Mikael Svensson, Jan Arvidsson, Carl Molander and Hakan Aldskogius: A quantitative analysis of the microglial cell reaction in central primary sensory projection territories following peripheral nerve injury in the adult rat. Exp Brain Res 96, 19-27 (1993)
PMid:8243580

102 Li Liu, Eva Tornqvist, Per Mattsson, Petri Eriksson, Jonas Persson, Morgan BP, Hakan Aldskogius and Mikael Svensson: Complement and clusterin in the spinal cord dorsal horn and gracile nucleus following sciatic nerve injury in the adult rat. Neuroscience 68, 167-179 (1995)
doi:10.1016/0306-4522(95)00103-P

103. Antonio Campos Torres, Pierre-Paul Vidal and Catherine de Waele: Evidence for a microglial reaction within the vestibular and cochlear nuclei following inner ear lesion in the rat. Neuroscience 92, 1475-1490 (1999)
doi:10.1016/S0306-4522(99)00078-0

104. Hakan Aldskogius, Jan Arvidsson and Gunnar Grant: The reaction of primary sensory neurons to peripheral nerve injury with particular emphasis on transganglionic changes. Brain Res 357, 27-46 (1985)
PMid:2412661

105. José Castro-Lopes JM, Antonio Coimbra, Gunnar Grant and Jan Arvidsson: Ultrastructural changes of the central scalloped (C1) primary afferent endings of synaptic glomeruli in the substantia gelatinosa Rolandi of the rat after peripheral neurotomy. J Neurocytol 19, 329-337 (1990)
doi:10.1007/BF01188402
PMid:2391537

106. Gábor Jancsó: Pathobiological reactions of C-fibre primary sensory neurones to peripheral nerve injury. Exp Physiol 77, 405-431 (1992)

107. Robert Chen, Leonard Cohen and Mark Hallett: Nervous system reorganization following injury. Neuroscience 111, 761-773 (2002)
doi:10.1016/S0306-4522(02)00025-8

108. Clifford Woolf, Mary Reynolds, Carl Molander, Claire O'Brien, Ronald Lindsay and Larry Benowitz: The growth-associated protein GAP-43 appears in dorsal root ganglion cells and in the dorsal horn of the rat spinal cord following peripheral nerve injury. Neuroscience 465-478 (1990)
doi:10.1016/0306-4522(90)90155-W

109. Richard Koerber, Karoly Mirnics and Jeffrey Lawson: Synaptic plasticity in the adult spinal dorsal horn: the appearance of new functional connections following peripheral nerve regeneration. Exp Neurol 200, 468-479 (2006)
doi:10.1016/j.expneurol.2006.03.003
PMid:16696973

110. Jan Arvidsson: Transganglionic degeneration in vibrissae innervating primary sensory neurons of the rat: a light and electron microscopic study. J Comp Neurol 249, 392-403 (1986)
doi:10.1002/cne.902490306
PMid:3734162

111. Katarina Bjelke, Hakan Aldskogius, Jan Arvidsson: Short- and long-term transganglionic changes in the central terminations of transected vibrissal afferents in the rat. Exp Brain Res 112, 268-276 (1996)
doi:10.1007/BF00227645

112.Jonas Persson, Hakan Aldskogius, Jan Arvidsson and Anders Holmberg: Ultrastructural changes in the gracile nucleus of the rat after sciatic nerve transection. Anat Embryol (Berl) 184, 591-604 (1991)
doi:10.1007/BF00942581
PMid:1776705

113. Cecilia Dominguez, Olle Lidman, Jing-Xia Hao, Margarita Diez, Jonatan Tuncel, Tomas Olsson, Zsuzsanna Wiesenfeld-Hallin, Fredrik Piehl and Xiao-Jun Xu: Genetic analysis of neuropathic pain-like behavior following peripheral nerve injury suggests a role of the major histocompatibility complex in development of allodynia. Pain 136, 313-319 (2008)
PMid:16047551

114. Yuanli Duan, Joseph Panoff, Brian Burrell, Christie Sahley and Kenneth Muller: Repair and Regeneration of Functional Synaptic Connections: Cellular and Molecular Interactions in the Leech. Cell Mol Neurobiol 25, 441-450 (2005)
doi:10.1007/s10571-005-3152-x

Key Words: Neuron degeneration, Neuron regeneration, Motor neuron, Sensory neuron, Synaptic plasticity, Astroglia, Immune system, Review