Sema7A/PlxnCl signaling triggers activity-dependent olfactory synapse formation

Journal: Nature Communication

Summary:In mammals, neural circuits are formed based on a genetic program and further refined by neuronal activity during the neonatal period. They report that in the mouse olfactory system, the glomerular map is not merely refined but newly connected to second-order neurons by odorant-receptor- derived neuronal activity. To determine what kinds of molecules are essential for initiating the synapse formation in glomeruli, they searched for a receptor and ligand pair each of which was expressed by OSN  axons  or M/T-cell dendrites. They examined 26 different  genes  of axon guidance, cell adhesion, and signaling molecules for their expression in the OE and OB. To examine the activity dependency of Sema7A expression,  they performed uni-lateral naris occlusion at postnatal day 0–6 (P0-6). When one naris was occluded, Sema7A expression in the OE was markedly reduced on the occluded side. In the next step, they studied the candidate receptors for Sema7A in M/T cells. Both in the immune and in central nervous systems, PlxnC1 and Integrin β1 are known to serve as receptors for Sema7A. After evaluation, they then examined whether Sema7A in OSN axons indeed interacts with PlxnC1 in M/T-cell dendrites by alkaline phosphatase (AP) staining. PlxnC1 and Sema7A were each fused to AP, and resulting fusion proteins were used as affinity probes. Serial OB sections were treated with AP-fusion proteins and developed with AP substrate. To examine the in vivo function of Sema7A signaling, they analyzed the Sema7A KO14 after crossing with TgMOR29B or MOR28 knock-in mice. They also analyzed the PlxnC1 KO mice. The M/T-cell-specific conditional KO (cKO) was generated for PlxnC1 using the Pcdh21-Cre mouse as a driver. It was confirmed that PlxnC1 was absent in the OB of the KO by immunostaining, although the MCL was normally formed. They studied synapse formation and dendrite selection of M/T cells. In the cKO, M/T-cell layers were normally formed and dendrite extension took place. It is interesting that in the Sema7Ahigh-type glomeruli (e.g., for MOR29B),  synapse  formation  and dendrite selection occur a few days earlier  than in the Sema7Alow-type (e.g., forMOR29A).

In addition to in vivo analyses with KOs, they also studied how the Sema7A–PlxnC1 interaction induces  synapse  formation  using  the  heterologous  system. They introduced the secret type

of Sema7A and myc-tagged PlxnC1 genes into HEK293 cells in culture. In this system, the secreted