APOPTOSIS IN THE DEVELOPING CEREBELLUM OF THE THYROID HORMONE DEFICIENT RAT

Qianxun Xiao and Vera M. Nikodem

Mechanism of Gene Regulation Section, Genetics and Biochemistry Branch, National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases, 10 Center Dr. MSC 1766, Bethesda, MD 20892-1766

Received 8/20/98 Accepted 9/7/98

2. INTRODUCTION

Programmed cell death (PCD) plays a major role in the differentiation and development of central nervous system (CNS) (1,2,3). Cell death is often controlled by survival promoting signals from other cells, but is executed in a cell autonomous manner (4). Apoptotic cells exhibit morphological and biochemical changes, such as, chromatin condensation, internecleosomal DNA fragmentation, cytoplasmic vacuolation, membrane blebbing and cell shrinkage (5, 6). Numerous experiments suggest that genes expression is required for neuronal death (7, 8). Most of the cell cycle regulators are involved in apoptosis, for example, cyclin D1 has been identified as an essential mediator of apoptosis of neuronal cell death (9). On the other hand, neurotrophic factors are presumably the limiting survival factors for neuronal cell types in vivo (10). Programmed death of supernumerary neurons occurring around the time when the neurons are making functional connections, abrogated by mRNA and protein synthesis inhibitors indicative of an activation of a specific genetic program (11).

Thyroid hormone (TH) influences gene expression, either positively or negatively, through binding to nuclear thyroid hormone receptors the members of steroid/thyroid hormone receptor superfamily (12, 13). Certain genes expressed in the brain have been shown to be under thyroid hormone control. These include myelin genes (14), the Purkinje cell specific gene, PCP2, (15) the transcription factor NGFI-A (16), and neuron specific enolase (NSE) (17). The lack of TH in early life has a marked effect on the development of the rat cerebellum (18, 19). In humans, the lack of adequate levels of TH during critical periods of development, results in cretinism, a syndrome of severe mental retardation often accompanied by growth retardation and/or neurological deficits (20).

The relative simplicity, structural homogeneity and postnatal development of cerebellum compared with that of the cerebral cortex makes the former ideally suited for the study of TH dependent regulation of neurogenesis. Nearly all cerebellum cells are formed postnatally originating from the external germinal layer (EGL). Following the extensive mitotic activity, granular cells migrate towards the molecular layer (ML) or descend further to the internal granular layer (IGL), leaving their axons (parallel fibers) in the ML to establish connections with the Purkinje cells (21). Legrand and others studied the effect of TH on the development of the rat cerebellum evaluating mostly anatomical and histological changes in TH deficient neonatal rats (18, 22, 19). In the present study, we have applied a sensitive apoptotic detection method to compare the degree of apoptosis in the developing cerebellum of euthyroid and hypothyroid rats. We have tried to understand whether the reduction of cell number in the hypothyroid cerebellum during development is due to apoptosis and whether the deficiency of hormone may exacerbate or extend the apoptosis.