EphB1 controls proper long-range cortical axon guidance through a cell non-autonomous role in GABAergic cells

EphB1 is required for proper guidance of cortical axon projections during brain development, but how EphB1 regulates this process remains unclear. We show here that EphB 1 conditional knockout (cKO) in GABAergic cells ( Vgat -Cre or Dlx1/2 -Cre), but not in cortical excitatory neurons ( Emx1 -Cre), reproduced the cortical axon guidance defects observed in global EphB1 KO mice. Interestingly, in EphB1 cKOVgat mice, the misguided axon bundles contained comingled striatal GABAergic and somatosensory cortical glutamatergic axons. In wildtype mice, somatosensory axons also co-fasciculated with striatal axons notably in the globus pallidus, suggesting that a subset of glutamatergic cortical axons normally follows long-range GABAergic axons to reach their targets. Surprisingly, the ectopic axons in EphB1 KO mice were juxtaposed to major blood vessels. However, conditional loss of EphB1 in endothelial cells (Tie2-Cre), or in mural and oligodendrocyte precursor cells (Cspg4-Cre) did not produce the axon guidance defects, suggesting that EphB1 in GABAergic neurons normally promotes avoidance of these ectopic axons from following the developing vasculature. Together, our data reveal a new role for EphB1 in GABAergic neurons to influence proper cortical glutamatergic axon guidance during brain development. ### Competing Interest Statement The authors have declared no competing interest.