A Role for Glutamate Receptor Interacting Protein 1 in Platelets and T Cells

Thesis (Ph.D.)--University of Rochester. School of Medicine & Dentistry. Dept. of Pharmacology & Physiology, 2015.PDZ molecular scaffolding proteins have been most studied in neurons, but are present in other cells. PDZ proteins typically bind membrane proteins to regulate receptor surface expression and to support junctional complexes. GRIP1 is a 7-PDZ domain scaffolding protein that has roles in neurons to facilitate glutamate receptor trafficking. However, GRIP1 has very different functions in other tissues. GRIP1 is needed to form a junctional barrier between the skin dermis and epidermis. We have found that GRIP1 is expressed in platelets and T cells and that GRIP1 has a functional role in each cell type independent of glutamate receptor signaling. We hypothesized that platelet GRIP1 regulates platelet function and thrombus formation and that T cell GRIP1 regulates T cell activation as evidenced by in vitro studies and using a cardiac transplant model. Platelet-specific GRIP1-/- mice were found to have decreased hemostasis and delayed thrombosis in vivo, but no change in agonist-induced activation in vitro. Using mass spectrometry analysis of proteins that immunoprecipitated with GRIP1, we identified GRIP1 interactions with the GPIb-IX complex. The GPIb-IX complex is necessary for platelet adhesion. We confirmed that GPIb-IX complex protein interactions are different in WT and GRIP1-/- platelets and may cause the in vivo defects. We also investigated GRIP1 function in T cells. WT and T cell-specific GRIP1-/- mice received MHC mismatched cardiac allografts and T cell GRIP1-/- mice had prolonged transplant survival in multiple rejection models. GRIP1-/- T cells had inhibited proliferation and activation following in vitro stimulation. We found that GRIP1-/- T cells had increased surface expression of the strong inhibitory receptor CTLA-4. Blocking CTLA-4 in GRIP1-/- mice rescued transplant rejection. Overall, we have found that GRIP1, a molecular scaffolding protein, has important cell regulatory effects in platelets and T cells that markedly differ in various cell types. We have revealed important functions for GRIP1 in mediating platelet thrombosis and in T cell-mediated transplant rejection