Reverse Signaling: Nervous System Development. Figure 1.

Bi-directional signaling mediated by two ligand-receptor systems. Binding of ephrin and semaphorin activates Eph and Plexin forward signaling, respectively. Binding of Eph and Plexin can also activate ephrin and transmembrane semaphorin to trigger reverse signaling, respectively. The identity of cell surface protein (colored in blue) that activates semaphorin reverse signaling during neural development is unknown.