Cerebroglycan is a glycosylphosphatidylinositol-linked integral membrane heparan sulfate proteoglycan found exclusively in the developing nervous system. In the rodent, cerebroglycan mRNA first appears in regions containing newly generated neurons and typically disappears 1 to several days later (Stipp et al., 1994, J. Cell Biol. 124:149-160). To gain insight into the roles that cerebroglycan plays in the developing nervous system, monospecific antibodies were prepared and used to localize cerebroglycan protein. In the rat, cerebroglycan was prominantly expressed on axon tracts throughout the developing brain and spinal cord, where it was found at times when axons are actively growing, but generally not after axons have reached their targets. Cerebroglycan was also found on neuronal growth cones both in vivo and in vitro. Interestingly, cerebroglycan immunoreactivity was rarely seen in or around neuronal cell bodies. Indeed, by examining the hippocampus at a late stage in development-when most neurons no longer express cerebroglycan but newly generated granule neurons do-evidence was obtained that cerebroglycan is strongly polarized to the axonal, and excluded from the somatodendritic, compartment of neurons. The timing and pattern of cerebroglycan expression are consistent with a role for this cell-surface heparan sulfate proteoglycan in regulating the growth or guidance of axons.

ARH-77 cells do not adhere to type I collagen and readily invade into collagen gels, but following expression of the transmembrane heparan sulfate proteoglycan syndecan-1, they bind collagen and fail to invade. We now show that cells transfected with syndecan-2 or syndecan-4 also bind collagen and are non-invasive. In contrast, cells transfected with the glycosylphosphatidylinositol-anchored proteoglycan glypican-1 do not bind to collagen and remain invasive, even though glypican- and syndecan-expressing cells have similar surface levels of heparan sulfate, and their proteoglycans have similar affinities for collagen. Analysis of cells expressing syndecan-1-glypican-1 chimeric proteoglycans reveals that inhibition of invasion requires the extracellular domain of syndecan but not its transmembrane or cytoplasmic domain. Surprisingly, cells bearing a chimera composed of the glypican extracellular domain fused to the syndecan transmembrane and cytoplasmic domains bind to collagen but remain invasive, implying that adhesion to collagen is not by itself sufficient to inhibit invasion. Apparently, the extracellular domain of syndecan-1, presumably by interacting with cell-surface signal transducing molecules, directly regulates complex cell behaviors such as motility and invasiveness. These results also show for the first time that syndecans and glypicans can have distinct functions, even when expressed by the same cell type.

The nonreceptor tyrosine kinase encoded by the c-Abl gene has the unique feature of an F-actin binding domain (FABD). Purified c-Abl tyrosine kinase is inhibited by F-actin, and this inhibition can be relieved through mutation of its FABD. The c-Abl kinase is activated by physiological signals that also regulate the actin cytoskeleton. We show here that c-Abl stimulated the formation of actin microspikes in fibroblasts spreading on fibronectin. This function of c-Abl is dependent on kinase activity and is not shared by c-Src tyrosine kinase. The Abl-dependent F-actin microspikes occurred under conditions where the Rho-family GTPases were inhibited. The FABD-mutated c-Abl, which is active in detached fibroblasts, stimulated F-actin microspikes independent of cell attachment. Moreover, FABD-mutated c-Abl stimulated the formation of F-actin branches in neurites of rat embryonic cortical neurons. The reciprocal regulation between F-actin and the c-Abl tyrosine kinase may provide a self-limiting mechanism in the control of actin cytoskeleton dynamics.