[S32]: Roles of semaphorins and their receptors in laminar projection of hippocampal mossy fibers

The hippocampus is one of the good experimental models to elucidate cellular and molecular mechanisms that govern lamina-restricted termination of axons, because pyramidal cells, principal neurons of the cornu ammonis (CA), receive inputs from a variety of source in a lamina-specific fashion. For example, fibers from the dentate gyrus (mossy fibers) project preferentially to the proximal-most lamina of the suprapyramidal region of CA3. Molecular mechanisms that govern the laminal projection of the hippocampal afferents, however, have been unknown. We generated protein-null mutant mice for two semaphorin receptors, plexin-A2 and plexin-A4, and examined projection patterns of mossy fibers in the mutant animals. We here show that the deficiency of plexin-A2 causes the shift of mossy fibers from the suprapyramidal region to the infraand intra-pyramidal regions, while the deficiency of plexin-A4 induces wide-spreading of mossy fibers within CA3. Immunohistochemical analyses reveal that the plexin-A4 proteins are distributed in mossy fibers. On the other hand, the plexin-A2 proteins are distributed in the suprapyramidal region of CA3 in a proximal-distal gradient. In vitro studies show that plexin-A4-deficient mossy fibers are unresponsive to the repulsive activities of Sema6A expressing in CA3. In addition, we show that the plexin-A2 loss-of-function phenotype is genetically suppressed by Sema6A loss-offunction. Based on these results, we propose a model for the lamina-restricted projection of mossy fibers; The expression of plexin-A4 on mossy fibers prevents them from entering the Sema6A-expressing suprapyramidal region of CA3 and restrict them to the proximal-most part where Sema6A repulsive activity is attenuated by plexin-A2.