Phosphoinositide signaling, axonal degeneration and neurological disease
The overall goal of our research is to determine how disturbances in phosphoinositide signaling lead to cellular dysfunction and human disease. Mutations in phosphoinositide (PI) phosphatases cause a number of human neurological diseases, including conditions affecting the central and peripheral nervous systems. For example, mutations in several members of the myotubularin family of PI 3-phosphatases lead to Charcot-Marie-Tooth (CMT) peripheral neuropathy, one of most common inherited neurological disorders, which affects about 1 in 2500 worldwide. CMT causes progressive degeneration of muscles in the extremities as well as loss of sensory function.
Inositol lipids, a component of cell membranes, are at the crossroads of critical membrane-based cell signaling events. In eukaryotic cells, including neurons and glia, phosphatidylinositol (PtdIns) can be phosphorylated on the D3, D4 and D5 positions of the inositol ring, leading to the formation of seven distinct phosphatidylinositol polyphosphates, usually called phosphoinositides (PIs). As membrane-tethered signaling molecules, PIs regulate membrane trafficking, cell growth and survival, cell division and cellular motility. PIs also play key roles in the regulation of ion channels, often acting as direct activators.
We are investigating how mutations in members of the myotubularin family of PI 3-phosphatases disrupt PI signaling, and thus cause alterations in endosomal-lysosomal membrane trafficking that lead to peripheral neuropathy. In addition, we are investigating the functions of phosphoinositide signaling in the central nervous system. Myotubularin PI 3-phosphatases specifically dephosphorylate PtdIns3P and PtdIns(3,5)P2, two PIs that regulate membrane traffic within the endosomal-lysosomal pathway. Mutations in the genes for either myotubularin related protein 2 (MTMR2) or MTMR13 cause type 4B CMT (CMT4B), a severe form of the disease characterized by abnormal myelin sheaths and secondary axonal degeneration. We demonstrated that the MTMR2 and MTMR13 phosphatases form a membrane-associated complex that is capable of regulating 3-phosphoinositides (Figure 1). Notably, the MTMR13 protein lacks phosphatase activity. As loss of either MTMR2 or MTMR13 is sufficient to cause CMT4B, MTMR13 is likely an essential, non-catalytic regulator of MTMR2. To further probe the relationship between MTMR2 and MTMR13, we recently generated Mtmr13-deficient mice, thus establishing a valuable mouse model of human CMT4B2 disease (Figure 2).
Our work will provide novel insights into how dysregulation of PtdIns3P and PtdIns(3,5)P2 leads to cellular dysfunction and neurological disease. This work also has the potential to lead to therapies for peripheral neuropathies and other diseases of myelin. Understanding how the Schwann cell endosomal-lysosomal pathway is altered by the dysregulation of 3-phosphoinositides is critical to the design of an appropriate pharmacological intervention in CMT4B. In addition, the identification of downstream targets of Mtmr2-Mtmr13 may allow us to define novel signaling pathways, both in the peripheral and central nervous system.
Ng AA, Logan AM, Schmidt EJ, Robinson FL (2013) The CMT4B disease-causing phosphatases Mtmr2 and Mtmr13 localize to the Schwann cell cytoplasm and endomembrane compartments, where they depend upon each other to achieve wild-type levels of protein expression. Hum Mol Genet 22:1493-1506.
Jean S, Cox S, Schmidt EJ, Robinson FL, Kiger A (2012) Sbf/MTMR13 coordinates PI(3)P and Rab21 regulation in endocytic control of cellular remodeling. Mol Biol Cell 23:2723-2740.
Robinson, F. L., Niesman, I. R., Beiswenger, K. and Dixon, J. E. (2008). Loss of the inactive myotubularin-related phosphatase Mtmr13 leads to a Charcot-Marie-Tooth 4B2-like peripheral neuropathy in mice. Proc Natl Acad Sci U S A. 105:4916-4921.
Robinson, F. L. and Dixon, J. E. (2006). Myotubularin phosphatases: Policing 3-phosphoinositides. Trends Cell Biol. 16:403-412.
Robinson, F. L. and Dixon, J. E. (2005). The phosphoinositide-3-phosphatase MTMR2 associates with MTMR13, a membrane-associated pseudophosphatase also mutated in type 4B Charcot-Marie-Tooth disease. J. Biol. Chem. 2005:31699-31707.
Dr. Robinson joined the Jungers Center as an Assistant Scientist and Assistant Professor of Neurology in August 2009. Dr. Robinson received an undergraduate degree at Oregon State University in biology and genetics, and a master’s degree in biological sciences from the University of Nebraska-Lincoln. After graduate training in Biochemistry at UT Southwestern with Melanie Cobb, he joined the laboratory of Jack Dixon at the University of California, San Diego in 2002. Dr. Robinson is a recipient of a prestigious NIH Pathway to Independence Award, which will facilitate the development of his laboratory in the Jungers Center.
BS, University of California, Davis
I received my BS in Microbiology from University of California, Davis where I did research as an undergraduate in Dr. Angela Gelli’s lab studying the human fungal pathogen Cryptococcus neoformans. After graduating, I moved to San Francisco and joined Dr. Allan Basbaum’s lab at UCSF as a research associate where I studied the modulation of pain by the drug Sumatriptan. I feel very fortunate that I ended up working in such a great lab, and I am especially fortunate because this is where I discovered my passion for neuroscience. After two years, I decided to pursue a Ph.D. and came to the Neuroscience Graduate Program at OHSU. In the Robinson lab, I am studying the phospholipid phosphatases MTMR2 and MTMR13, which are mutated in the peripheral neuropathy Charcot-Marie-Tooth disease. I am interested in how membrane trafficking might be disrupted in Schwann cells which lack functional MTMR2 or MTMR13 proteins.