Louise P. Cramer

The Randall Institute, Kings College London, 26-29 Drury Lane, London WC2B 5RL, UK.

Received 5/21/97; Accepted 5/26/97

3. STRUCTURAL ORGANIZATION OF ACTIN FILAMENTS IN LAMELLIPODIA

Determining the structural organization of actin filaments in motile cells is crucial for solving the molecular mechanism of any type of actin-dependent cell motility (recently reviewed and discussed in detail (14). In lamellipodia and other leading edge structures the general organization of actin filaments is well known. Here, I will focus on certain details, that may turn out to be relevant for determining the precise source of actin filament organization that is responsible for driving retrograde flow of a particular type of particle.

Leading edge structures of motile cells are highly dynamic and are filled with dense arrays of actin filaments. Actin filaments, in general, where it has been possible to study, are organized with their barbed ends (fast growing, or plus ends) oriented preferentially in the direction of protrusion (15-19) (Fig 4A). One issue is whether there is a difference in the polarity of the actin network between the ventral and dorsal surfaces in these leading edge structures (Fig 4B). Such a difference has been reported for lamellipodia of growth cones of certain mammalian neurons (17). On the ventral surface of these growth cones, actin filaments are long and bundled and have expected uniform barbed end polarity facing the direction of protrusion. In contrast, actin filaments associated with the dorsal growth cone surface are shorter and apparently have more mixed polarity. Although it is unclear where these measurements were precisely made in the growth cone, information on the polarity of actin filaments associated with the dorsal surface of lamellipodia in other motile cell types is likely still missing. This is because experimental procedures in most studies of polarity involve extracting with detergent. Since the dorsal surface is more exposed than the ventral surface, detergent is more likely to disrupt an actin organization associated with the dorsal surface. Indeed in lamellipodia of keratocytes, detergent is thought to remove most of the dorsal-associated actin filaments (18). A distinct type of actin organization, similar to muscle sarcomeres (alternating polarity actin filament bundles; (19) has been identified within 0.1-1 mm of the cell surface in locomoting heart fibroblasts. Since some of these bundles are associated with the dorsal surface (but not ventral surface) of the front of the lamella, the possibility remains that they are also associated with the dorsal surface at the back of the lamellipodium (Fig 4B). If alternating polarity actin bundles are associated with the dorsal surface of other motile cell types, under experimental conditions, less-than-optimal for preserving the dorsal surface, they might appear to have more random polarity.

Fig 4. Actin filament organization in leading edge structures. (A, top view) In nearly every case, studies show that almost all the barbed ends of detected actin filaments face the front of the leading edge structure (facing the direction of protrusion). This is a type of uniform polarity. Filopodia and microspikes contain a tight bundle of long actin filaments. Lamellipodia contain an orthogonal, crosslinked, network, of actin filaments oriented at approximately 45^o to the direction of protrusion (18) The length of actin filaments in lamellipodia has been debated (14). (B, side view) Additional types of actin filament organization also seen in a few studies: short actin filaments of more mixed polarity under the dorsal surface of certain mammalian growth cones [17]; alternating polarity bundles, comprised of short actin filaments, under the dorsal surface of the front of the lamella/back of the lamellipodium (19). It is not known if these structures are present in lamellipodia of other motile cell types. It is possible they are preferentially extracted during experimental procedures. Since the front of a lamellipodium, from the ventral to dorsal surface, is at most only about 0.5 mm thick (equivalent to roughly 50 actin filaments stacked on top of each other), these additional actin organizations are likely to be a minor component of lamellipodia.