Neurons typically show polarized morphology, with a single axon and several dendrites. The neuronal polarization is required for the formation of highly ordered and complex neuronal networks. Recently, we identified a novel protein, shootin1, that becomes up-regulated during polarization of hippocampal neurons and accumulated in the growth cone. We previously reported that shootin1 is involved in axon formation and neuronal polarization of cultured hippocampal neurons. Furthermore, the axonal outgrowth activity of shootin1 in cultured neurons is enhanced by netrin-1, an attractive axonal guidance cue. However, the roles of shootin1 in vivo are unknown.
To investigate the functions of shootin1 in brain development, we produced and analyzed shootin knockout (shootin KO) mice, which are deleted shootin1 and its splicing variant, shootin2. Shootin KO mice exhibited defects in the corpus callosum, anterior commissure, hippocampal commissure and anterior white commissure of the spinal cord where prominent axon bundles are formed. These phenotypes are consistent with the function of shootin1 in axon formation in vitro. Unexpectedly, shootin KO mice also exhibited defects in several regions in the forebrain not relevant to axon bundles. These results suggest a possibility that shootin1 and/or shootin2 are also involved in forebrain morphogenesis. To confirm whether shootin1 and/or shootin2 are involved in forebrain morphogenesis, we analyzed the expression of shootin1 and shootin2 during brain development. As a result, we revealed that shootin1 and shootin2 were expressed from early developmental stage. Additionally, we found that shootin KO mice exhibit developmental anomaly of the forebrain at early stage. These results suggest that shootin1 and/or shootin2 have important roles not only in axon formation, but also in early development of the forebrain.