The protein tyrosine phosphatase superfamily, all of which contain a highly conserved active site motif, Cys- X5-Arg (CX5R) are key mediators of a wide variety of cellular processes, including growth, metabolism, differentiation, motility, and programmed cell death. Our laboratory has demonstrated that some phosphatases harboring CX5R motifs utilize phosphoinositides instead of phosphoproteins as their physiological substrates. This includes the myotubularin (MTM) subfamily that removes the 3-phosphate from phosphatidylinositol 3-phosphate (PI(3)P) and phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2. Mutations in MTM family members have been shown to cause the human neuromuscular disorders myotubular myopathy and Charcot-Marie-Tooth disease (CMT). This proposal will continue to study the regulation of the MTM family of PTPs as well as another novel phosphatase known as PTPMT1. In the next five years, we plan to develop a better understanding of how levels of PI(3)P transmit signals in the cell. First, we will explore the roles of MTMR2 and MTMR13 with respect to a new protein known as Frabin.which senses changes in PI(3)P levels resulting in the modulation of the activity of CDC42 and other Rho-like GTPases. Specifically, we will examine Frabin's role in mediating PI(3)P signal transduction events in Schwann cells, cells that produce myelin for insulating axons. We will then determine if this signal transduction pathway is widely used in other cell types. Second, we have demonstrated that PTPMT1 regulates insulin secretion by decreasing ATP production in INS-1 cells. In order to address PTPMTI's mechanism of action, we genetically engineered mice in which the PTPMT1 gene has been ablated. We are characterizing these mice phenotypically and will combine a series of systemic and biochemical approaches to study the role of PTPMT1 in cellular and whole animal contexts.
Work on MTMR2, MTMR13, and Frabin will further our understanding of the neurodegenerative disorder, CMT. CMT is the most frequent class of inherited disorders of the peripheral nervous system. Work on PTPMT1 will give us a better understanding of how the cell regulates insulin secretion which is an important parameter in Type II diabetes.
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