Abstract

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 aVb3 integrin in trans eliciting responses in astrocytes. Nonetheless, whether aVb3 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 aVb3 integrin on growth and retraction of neuronal processes were tested. In astrocyteneuron co-cultures, endogenous aVb3 integrin restricted neurite outgrowth. Likewise, aVb3-Fc was sufficient to suppress neurite extension in Thy-1(+), but not in Thy-1(2) CAD cells. In differentiating primary neurons exposed to aVb3-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, aVb3-Fc also induced retraction of already extended Thy- 1(+)-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 aVb3 integrin. Binding of aVb3-Fc was detected in Thy-1 clusters during axon retraction of primary neurons. Moreover, aVb3-Fcinduced Thy-1 clustering correlated in time and space with redistribution and inactivation of Src kinase. Thus, our data indicates that aVb3 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 thes