Axonal branching in lateral olfactory tract is promoted by Nogo signaling

Mitral cells are major projection neurons of the olfactory bulb (OB) that form an axonal bundle known as the lateral olfactory tract (LOT). After axonal bundle formation, collateral branches sprout from primary axons of the LOT. Recently, we identified LOT usher substance (LOTUS) as an endogenous Nogo receptor-1 (NgR1) antagonist and demonstrated that LOTUS contributes to the formation of the LOT axonal bundle. Immunoblots revealed that the expression level of Nogo-A in the OB developmentally increased during axonal collateral formation. Next, we found that the axonal collateral branches were increased in cultured OB neurons from LOTUS-knockout (KO) mice, whereas they were decreased in cultured OB neurons from NgR1-KO mice. Knockdown of Nogo-A in cultured OB neurons reduced the number of axonal collateral branches, suggesting that endogenous Nogo-A induces axonal branching. Finally, the collateral branches of the LOT were increased in LOTUS-KO mice, whereas those in NgR1-KO mice were decreased. Moreover, the abnormal increase of axonal branching observed in LOTUS-KO mice was rescued in the double mutant of LOTUS- and NgR1-KO mice. These findings suggest that Nogo-A and NgR1 interactions may contribute to axonal branching in LOT development.