Sema4D deficiency results in an increase in the number of oligodendrocytes in healthy and injured mouse brains

Semaphorins, a family of secreted and membrane‐bound proteins, are known to function as repulsive axon guidance molecules. Sema4D, a class 4 transmembrane‐type semaphorin, is expressed by oligodendrocytes in the central nervous system, but its role is unknown. In this study, the effects of Sema4D deficiency on oligodendrocytes were studied in intact and ischemic brains of adult mice. As observed in previous studies, Sema4D marked by β‐galactosidase in Sema4D mutant mice was localized exclusively on myelin‐associated glycoprotein (MAG)‐positive oligodendrocytes but not on NG2‐positive oligodendrocyte progenitor cells (OPCs). Although there was no difference in the number of the latter cells between Sema4D‐deficient and wild‐type mice, the number of MAG‐positive cells was significantly increased in the cerebral cortex of both nonischemic and postischemic brains of Sema4D‐deficient mice. Cell proliferation, observed by using bromodeoxyuridine incorporation, was evident in the MAG‐positive cells that developed after cerebral ischemia. These data indicate that Sema4D is involved in oligodendrogenesis during development and during recovery from ischemic injury. © 2009 Wiley‐Liss, Inc.