Gone with the Wnts

A reoccurring problem in the field of the actin cytoskeleton is that careful study of a protein by one approach will sometimes give results that are different from those acquired by another approach. This is the case for proteins of the Ena/VASP family, which are implicated in cellular actin dynamics. The Ena/VASP family has at least three members in metazoans – VASP, Evl and Mena. These proteins comprise three subdomains: an EVH1 domain that permits docking to a subclass of proline-rich proteins such as zyxin and ActA; a central glycine- and proline-rich domain that binds to profilin, followed by an EVH2 domain that might be required for homodimerization and possibly binding to F-actin. These properties, as well as the location of Ena/VASP proteins at actinrich sites in cells suggests that they are regulators of actin cytoskeleton dynamics. But what exactly is their role? Bear and colleagues1 propose a negative role for Ena/VASP proteins after examination of Mena function in cell-motility assays. They found that increased levels of Mena expression in fibroblasts correlated with a reduction in the rate of cell movement. They then displaced Ena/VASP proteins from focal contacts and the lamellipodia to ask what effect this would have upon cell motility. Under these conditions, cells moved faster, suggesting

that the presence of Ena/VASP proteins at actin-rich sites inhibits actin dynamics. In support of this observation, when Ena/VASP proteins were forced to the plasma membrane, cells moved slower than those in which Ena/VASP proteins were not displaced. In another test, using a cell line derived from mice in which the genes encoding VASP and Mena were abrogated, complementation of these cells with a green-fluorescent protein (GFP) variant of Mena reduced cell motility to speeds found in wild-type cells. These data were consistent with an inhibitory role for Ena/VASP in cell motility and possibly actin polymerization. As the authors state, one would have expected a stimulatory effect of Ena/VASP proteins – and there are many good reasons for believing so. VASP enhances the rate of motility of Listeria monocytogenes, a bacterium that requires actin polymerization to move. Furthermore, zyxin requires Ena/VASP proteins in cell-spreading experiments and is able to generate actin-filled structures in an Ena/VASPdependent manner, suggesting a positive effect by Ena/VASP upon actin dynamics rather than a negative effect. Finally, in cultured cell lines, VASP concentration at the leading edge of the lamellipodia correlates positively with actin-dependent membrane protrusion.

HEADLINES

Placing Ena/VASP proteins onto the circle of actin dynamics These differences in the outcomes of Ena/VASP-stimulated processes might still suggest a common role. Cell motility is the output of the effects of actin dynamics upon movement and adhesion; therefore, reducing adhesion would increase motility. Similarly, actin dynamics reflect a system at equilibrium. An example of the effects of changing the equilibrium has been given by the study of Listeria movement in a reconstitution assay. Loisel and colleagues2 have shown that all proteins tested for actin-polymeriation properties have inhibitory and stimulatory effects, depending upon their concentration. Stating that a protein has both positive and negative effects might be less satisfying than choosing one, but it is probably more correct when describing the role of proteins in dynamic systems such as the actin cytoskeleton.

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Bear, J.E. et al. (2000) Negative regulation of fibroblast motility by Ena/VASP proteins. Cell 101, 717–728 Loisel, T.P. et al. (1999) Reconstitution of actin-based motility of Listeria and Shigella using pure proteins. Nature 401, 613–616

Gone with the Wnts Wnts regulate cell differentiation through receptors of the Frizzled family, involving translocation of bcatenin to the nucleus, where it binds to TCF/LEF transcription factors that regulate Wnt target genes. Ross et al.1 investigated how Wnts and associated signals affect adipogenesis using pre-adipocytes 3T3-L1 and 3T3-F442A and myogenesis using myoblasts C2C12 and G8. They first induced 3T3-L1 adipogenesis in vitro and found that treating these cells with Wnt-1 or Wnt signalling activators lithium or b-catS33Y could inhibit adipogenesis. Next, they attempted in vivo experiments by injecting control and Wnt-1-infected 3T3-F442A subcuta-

neously into athymic mice. This resulted in control mice having fat pads, and Wnt-1 mice having fibroblast pads. They also found that Wnt-1-expressing cells lacked the adipocyte differentiation markers C/EBPa and PPARg, and adipogenesis was rescued when C/EBPa and PPARg were overexpressed. Furthermore, endogenous Wnt signalling was probed with dominant-negative TCF4 (dnTCF4) that interfered TCF/LEF actions. As expected, dnTCF4-expressing cells, as well as cells treated with the Wnt inhibitor Axin underwent adipogenesis. Finally, when they expressed dnTCF4 in C2C12 and G8 cells, they differentiated into adipocytes. Wnt-5a and

trends in CELL BIOLOGY (Vol. 10) November 2000

Wnt-10b were later identified as the endogenous Wnts in 3T3-L1 cells. Together, these data showed that Wnts are involved in an adipogenic switch. Considering that myoblasts could undergo terminal differentiation, one might then ask how the presence of Wnts affects the terminal fate of muscles.

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Ross, S.E. et al. (2000) Inhibition of adipogenesis by Wnt signaling. Science 289, 950–953

This month’s headlines were contributed by Roy Golsteyn, Volker Haucke, Chung Lau, Wallace Marshall, Robin May and Jonathan Weitzman.

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