Studies on the role of Coronin 1 and the actin cytoskeleton in T cell signaling and survival

Calcium ions (Ca2+) function as universal second messengers in most if not all eukaryotic cells, including the cells of the immunesystem. Ca2+ signals are required for the proper activation of lymphocytes, such as T-lymphocytes, their proliferation, differentiation and effector functions. In lymphocytes store-operated calcium entry (SOCE) through calcium release activated calcium (CRAC) channels in the plasma membrane is the major mechanism to increase cytosolic Ca2+ concentrations and is essential for the activation of T and B cells as well as induction of their cytokine gene production. How exactly store operated calcium entry operates in T cells has remained unclear. While one model, based on results obtained using a variety of F-actin-modulating drugs, involves the cortical actin cytoskeleton, the function of F-actin in SOCE has remained controversial during the last decade, since independent studies have found no effects of the same drugs on SOCE in other model systems such as the rat basophile leukemia (RBL) cell line. The first part of this thesis aims at defining the role of the actin cytoskeleton during SOCE in human T cells.
The data presented in this thesis clearly demonstrate that the F-actin cytoskeleton in human T cells has no role in SOCE. These results therefore contribute to our understanding of calcium signaling in cells of the immunesystem.
The second part of this thesis focuses on coronin 1, a member of the conserved WD repeat containing protein family which is encoded in mice (and human) by the gene coro1a. Coronin 1, which is specifically expressed in leukocytes, was originally identified as a protein that is maintained around phagosomes containing live mycobacteria, thereby preventing the fusion of the mycobacterial phagosome with lysosomes and mycobacterial destruction.
The aim of this part of the thesis was to define the function of coronin 1 in immune cells by characterizing coronin 1 deficient mice with a special focus on T cell development, T cell receptor signaling, migration and survival as well as the proposed regulatory role of coronin 1 in F-actin dependent processes. We found, in contrast to the long held dogma of coronin 1 being a major regulator of the F-actin cytoskeleton, that coronin 1 regulates cellular signaling rather that F-actin modulation.
Coronin 1 was shown to interact with phospholipase C γ1 (PLC γ1) thereby being an important regulator of inositol-1,4,5-trisphosphate (IP3) generation from phosphatidylinositol-4,5-bisphosphate (PIP2). The absence of coronin 1, although not affecting T cell development, resulted in a profound defect in Ca2+ mobilization, interleukin-2 (IL-2) production, T cell proliferation and T cell survival in naïve T-cells.
Finally in the last part of this thesis we provide data showing that coronin 1 maintains the balance between survival and apoptosis in naive T cells independent of F-Actin via a calcium/calcineurin dependent pathway.
The data presented in part two and three of this thesis establish the leukocyte specific protein coronin 1 as an essential regulator of T cell receptor signaling as well as naïve T cell homeostasis and survival. This work refutes the proposed role of coronin 1 in the regulation of the F-actin cytoskeleton, instead providing evidence for coronin 1 being a central regulator of Ca2+-dependent signaling in T cells. The work described here further offers new possibilities for the development of compounds for the treatment of T cell mediated disorders of the immunesystem.