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RESEARCH INTERESTS
Characterization of PIP2 lipid signaling in C. elegans
PUBLICATIONS
Cell membranes contain a complex mixture of different lipid types. These lipids are responsible for much of a cells function: providing structure, maintaining appropriate membrane permeability, trafficking proteins, and organizing protein complexes. However, many of the details for how membrane, and membrane lipids contribute to cellular function are not known.
We study a specific lipid called phosphatidylinositol 4, 5 bisphosphate (PIP2). PIP2 is only a fraction of the total cellular lipid but is important for multiple processes including second messenger signaling, recruiting protein complexes to membranes, cytoskeleton function and regulating activity of enzymes and ion channels. PIP2 is primarily made by the type 1 PIP kinase which phosphorylates PI(4)P at the 5 position of the inositol ring to make PI(4, 5) P2. PIP2 is degraded by two main phosphatase proteins: synaptojanin, at the plasma membrane, and OCRL, at the Golgi and endosomes.
Defects in PIP2 metabolism lead to inherited disorders in humans. Specifically, high levels of PIP2, caused by mutations in the OCRL (oculocerebrorenal of Lowe) gene, lead to a disease known as Lowe Syndrome. Individuals with Lowe Syndrome have multiple symptoms including cataracts, renal dysfunction, and mental retardation. Thus, maintaining proper levels of PIP2 is important for human health.
We are using the nematode C. elegans as a model to dissect the role of PIP2 in cellular processes. Specifically, we are increasing cellular PIP2 levels by increasing production of the lipid, or by preventing degradation of the lipid. To increase production, we have overexpressed the type 1 kinase (PPK-1) in neurons of C. elegans. These animals show an age dependent disintegration of neuron structure. Some neurons show dramatic membrane overgrowth, while others degenerate suggesting that the cytoskeleton and possibly ion permeability are disrupted in these animals.
Increased local levels of PIP2 are caused by mutating the synaptojanin and OCRL phosphatases. In synaptojanin mutants there is a defect in synaptic vesicle endocytosis, leading to a severe depletion of synaptic vesicles at the synapse. OCRL mutant animals exhibit multiple phenotypes including molting defects, structural abnormalities in muscles, and uncoordinated locomotion possibly due to the hypodermal or body wall muscle defects. Animals that make it to the adult stage are sterile possibly due to a failure in yolk protein uptake or other gonad morphology defects. As animals age they become increasingly sick with vacuoles forming in many tissues. These phenotypes suggest that the absence of OCRL protein has a devastating effect on C. elegans development.
We propose that in PPK-1 overexpression, synaptojanin, and OCRL mutants, elevated PIP2 disrupts different cellular processes by activating different effector proteins. Mutating proteins inappropriately activated by PIP2 should alleviate the defects. We are screening for second site suppressors that alleviate the membrane overgrowth and degeneration phenotype in animals overexpressing PPK-1, and the synaptic vesicle recycling defects in synaptojanin mutants in order to identify proteins that function downstream of PIP2 in these cellular processes. In addition, we are characterizing the defects in ocrl-1 mutants with the goal of gaining insight into the cellular role of OCRL.
Selected publications
Weinkove, D*, Bastiani, M*, Chessa, T., Joshi, D., Hauth, L., Cooke, F. T., Divecha, N., Schuske, K. Overexpression of PPK-1, the C. elegans Type 1 PIP kinase, inhibits growth cone collapse in the developing nervous system and causes axonal degeneration in adults. Developmental Biology (In press).
Jospin, M., Watanabe, S., Joshi, D., Young, S., Hamming, K., Thacker, C., Snutch, T. P., Jorgensen, E. M., Schuske, K. (2007) UNC-80 and the NCA Ion Channels Contribute to Endocytosis Defects in Synaptojanin Mutants. Current Biology, 2007 17: 1595-1600.
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