Investigating the molecular mechanisms of early neuronal morphogenesis

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SUMMARY Cellular morphology is one of the most distinctive features of somatic cells in multicellular organisms and is intimately linked with cellular function. How neurons and other polarized cells commit to their morphologies is poorly understood, but spontaneous morphogenesis of dissociated cells in culture suggest that the basic instructions for morphology are often intrinsically encoded. We previously identified a sequence-specific RNA- binding protein, Unkempt, as a factor that is essential for the establishment of the early neuronal morphology and as a protein that is capable of endowing a similar shape to cells of nonneuronal origin. Unkempt recognizes a unique binding motif predominantly within coding regions mRNAs the translation of which it suppresses. It is unclear how Unkempt regulates translation, and whether translation or another functional modality of Unkempt is critical to its induction of cell polarization. Here, we seek to solve this problem by deciphering the mechanistic basis of Unkempt-driven remodeling of cellular shape. Our preliminary studies indicate that in the broad protein-protein interaction network of Unkempt, the interaction between Unkempt’s low-complexity domain (LCD) and the CCR4-NOT complex is exquisitely required for the induction of cell morphogenesis. We propose three specific aims to investigate the molecular and cellular roles of the CCR4- NOT complex as a critical effector of Unkempt-controlled cell morphogenesis. First, we will investigate the nature of Unkempt – CCR4-NOT interactions and their impact on the fate of the targeted messages, focusing in particular on their poly(A) tail length, stability, and translation. Second, we will interrogate the recruitment and function of CCR4-NOT in Unkempt-induced cell morphogenesis. Third, we will determine the effect of post-translational modifications of Unkempt on its interactions with the CCR4-NOT complex and RNA, as well as its control of local protein translation. This study will shed light on the molecular underpinnings of the early neuronal morphogenesis and contribute to our general understanding of the cues that control cellular morphology in development and disease.