Semaphorin3A enhances endocytosis at sites of receptor-F-actin colocalization during growth cone collapse

Axonal growth cone collapse is accompanied by a reduction in filopodial F-a ctin, We demonstrate here that semaphorin 3A (Sema3A) induces a coordinated rearrangement of Sema3A receptors and F-actin during growth cone collapse. Differential interference contrast microscopy reveals that some sites of S ema3A-induced F-actin reorganization correlate with discrete vacuoles, stru ctures involved in endocytosis. Endocytosis of FITC-dextran by the growth c one is enhanced during Sema3A treatment, and sites of dextran accumulation colocalize with actin-rich vacuoles and ridges of membrane. Furthermore, th e Sema3A receptor proteins, neuropilin-1 and plexin, and the Sema3A signali ng molecule, rad, also reorganize to vacuoles and membrane ridges after Sem a3A treatment,These data support a model whereby Sema3A stimulates endocyto sis by focal and coordinated rearrangement of receptor and cytoskeletal ele ments. Dextran accumulation is also increased in retinal ganglion cell (RGC ) growth cones, in response to ephrin A5, and in RGC and DRG growth cones, in response to myelin and phorbol-ester. Therefore, enhanced endocytosis ma y be a general principle of physiologic growth cone collapse. We suggest th at growth cone collapse is mediated by both actin filament rearrangements a nd alterations in membrane dynamics.