Astrocytic αVβ3 Integrin Inhibits Neurite Outgrowth and Promotes Retraction of Neuronal Processes by Clustering Thy-1

Thy-1 is a membrane glycoprotein suggested to stabilize or inhibit growth of neuronal processes. However, its precise function has remained obscure, because its endogenous ligand is unknown. We previously showed that Thy-1 binds directly to alpha(V)beta(3) integrin in trans eliciting responses in astrocytes. Nonetheless, whether alpha(V)beta(3) integrin might also serve as a Thy-1-ligand triggering a neuronal response has not been explored. Thus, utilizing primary neurons and a neuron-derived cell line CAD, Thy-1-mediated effects of alpha(V)beta(3) integrin on growth and retraction of neuronal processes were tested. In astrocyte-neuron co-cultures, endogenous alpha(V)beta(3) integrin restricted neurite outgrowth. Likewise, alpha(V)beta(3)-Fc was sufficient to suppress neurite extension in Thy-1(+), but not in Thy-1(2) CAD cells. In differentiating primary neurons exposed to alpha(V)beta 3-Fc, fewer and shorter dendrites were detected. This effect was abolished by cleavage of Thy-1 from the neuronal surface using phosphoinositide-specific phospholipase C (PI-PLC). Moreover, alpha(V)beta(3)-Fc also induced retraction of already extended Thy1(+)-axon-like neurites in differentiated CAD cells as well as of axonal terminals in differentiated primary neurons. Axonal retraction occurred when redistribution and clustering of Thy-1 molecules in the plasma membrane was induced by alpha(V)beta(3) integrin. Binding of alpha(V)beta(3)-Fc was detected in Thy-1 clusters during axon retraction of primary neurons. Moreover, alpha(V)beta(3)-Fc-induced Thy-1 clustering correlated in time and space with redistribution and inactivation of Src kinase. Thus, our data indicates that alpha(V)beta(3) integrin is a ligand for Thy-1 that upon binding not only restricts the growth of neurites, but also induces retraction of already existing processes by inducing Thy-1 clustering. We propose that these events participate in bidirectional astrocyte-neuron communication relevant to axonal repair after neuronal damage.