Cellular migration is fundamental to a variety of biological processes that include innate and adaptive immunity. Analysis of cell migration in diverse model systems has revealed that this process is accomplished by chemoattractant receptors signaling the spatially-restricted activation of Rho GTPase family members. Specifically, chemoattractant-bound receptors signal through Gαi and downstream guanine nucleotide exchange factors (GEFs), P-Rex and PIXα, to activate Rac and Cdc42 at the leading edge and ultimately leads to cellular protrusion, or assembly of lamellipodia. In contrast, chemoattractant receptor signaling by Gα12/13 activate RhoA at the trailing edge and promote cytoskeletal contraction. However, the GEF(s) that activate RhoA as a consequence of chemoattractant receptor signaling through Gα12/13 have not been identified. Lsc is a hematopoietic-restricted RhoA-specific GEF that functions as an effector of Gα12/13-associated G-protein coupled receptors. In the initial studies of this thesis, we demonstrate that oligomerization via the carboxyl-terminal region of Lsc negatively regulates its activity. We further demonstrate that during cell migration Lsc localizes toward the trailing edge where it likely activates RhoA and is required for the release of integrin-mediated cell adhesion.

In the immune system, G protein-coupled receptors have also been suggested to regulate lymphocyte antigen receptor signaling, in addition to controling cell migration. In the second section of this study, we have evaluated how B cell antigen receptor signaling is influenced by G proteins and lysophosphatidic acid (LPA), a bioactive lysophospholipid that signals via G protein-coupled receptors, and the underlying molecular mechanisms of this regulation. These data not only reveal that Gαq regulates antigen receptor-mediated extracellular calcium influx but, also, that LPA receptor signaling via Gα12 and Gα13 inhibits both antigen receptor stimulated calcium release from intracellular stores and extracellular calcium influx. Moreover, we further demonstrate that the Lsc RGS domain is critical for this regulation.

Together, these findings identify Lsc as the GEF that activates RhoA at the trailing edge of migrating cells upon signaling via Gα12/13-associated chemoattractant receptors and establish that Gα12/13-associated lysophospholipid chemoattractant receptors are able to regulate antigen receptor-dependent signal transduction.