The action of Semaphorin7A on thalamocortical axon branching.

Developing axons form extensive branches to make synaptic contacts with their target cells. Despite the important role of axon branching in neural circuit formation, its underlying molecular mechanism is still largely unknown. In this study, we investigated the involvement of Semaphorin7A (Sema7A) in thalamocortical (TC) axon branching. In situ hybridization demonstrated that sema7a was expressed specifically in layer 4, the TC recipient layer, when TC axons form extensive arbors. A similar protein expression pattern was observed by immunohistochemistry with an anti-Sema7A antibody. The effect of Sema7A on axon branching was investigated in dissociated cell cultures from embryonic rat thalamus. TC axon branching increased dramatically on Sema7A-coated dishes. We further studied the activity of Sema7A in vivo using loss- and gain-of-function analyses. The number of vesicular glutamate transporter 2-positive puncta was markedly reduced in the Sema7A-deficient cortex. In contrast, their number increased significantly when Sema7A was over-expressed in layer 4 cells by in utero electroporation. Taken together, these findings suggest that Sema7A acts as a positive regulator for TC axon branching and/or pre-synaptic puncta formation.