Proteomic identification of molecules involved in neuronal polarity formation and analysis of their intracellular molecular networks

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The basic function of neurons is to receive, integrate and transmit signals. To do so, most neurons develop polarity by forming a single axon and multiple dendrites. However, little is known about the molecular basis of neuronal polarization. To approach this problem, we performed proteome analyses of cultured hippocampal neurons using highly sensitive large-gel two-dimensional electrophoresis (2-DE). In this project, we identified 94 proteins up-regulated during the initial step of neuronal polarization and 92 proteins enriched in axons, and analyzed the functions and molecular networks of novel proteins, shootin1 and singar.Shootin1 became up-regulated during polarization of hippocampal neurons and began fluctuating accumulation among multiple neurites. Eventually shootin1 accumulated asymmetrically in a single neurite, which led to axon induction for polarization. Shootin1 was transported anterogradely to the growth cones and diffused back to the soma ; inhibiting this transport prevented its asymmetric accumulation in neurons. Disturbing the asymmetric organization of shootin1 by excess shootin1 disrupted polarization, while repressing shootin1 expression delayed polarization. Functional analysis suggested that shootin1 regulates neuronal polarity through the PI 3-kinase pathway. These results suggest that shootin1 is involved in the generation of internal asymmetric signals required for neuronal polarization.Singar becomes up-regulated during polarization of cultured hippocampal neurons and remained at high levels thereafter. Knockdown of singar expression by RNA interference led to an increase in the population of neurons bearing surplus axons. Conversely, overexpression of singar suppressed the formation of surplus axons induced by excess level of shootin1. These results suggest that singar ensures the robustness of neuronal polarity by maintaining a single axon