Astrogliosis induced by brain injury is regulated by sema4B phosphorylation

Injury to the CNS induces astrogliosis, an astrocyte-mediated response that has both beneficial and detrimental impacts on surrounding neural and non-neural cells. The precise signaling events underlying astrogliosis are not fully characterized. Here, we show that astrocyte activation was altered and proliferation was reduced in Semaphorin 4B (Sema4B)-deficient mice following injury. Proliferation of cultured Sema4B^–/– astrocytes was also significantly reduced. In contrast to its expected role as a ligand, the Sema4B ectodomain was not able to rescue Sema4B^–/– astrocyte proliferation but instead acted as an antagonist against Sema4B^+/– astrocytes. Furthermore, the effects of Sema4B on astrocyte proliferation were dependent on phosphorylation of the intracellular domain at Ser825. Our results suggest that Sema4B functions as an astrocyte receptor, defining a novel signaling pathway that regulates astrogliosis after CNS injury.