Genetic and Biochemical Approaches to Tyrosine Kinase Fu

Human Genome Research

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Abstract

Our laboratory studies signal transduction involving tyrosine phosphorylation and non-receptor tyrosine kinases, molecules required for intracellular signaling pathways involved in normal cellular growth and differentiation as well as the abnormal growth and development involved in the formation and progression of cancer. Using a combination of mouse genetics, cell biology and protein biochemistry, our work has concentrated on how these molecules contribute to normal function of cells of the immune system and the skeletal system. Through these studies we hope to understand how manipulation of these pathways can be utlilized to develop therapeutics for diseases affecting these systems. In recent years our work has concentrated on studies of the Tec family of tyrosine kinases, the prototypical member of which, Btk, is required for normal function of B cells. Mutation of Btk is responsible for the human genetic disorder X-linked agammmaglobulimemia. We have previously shown that mutation of Tec family kinases expressed in T cells can severely impair T lymphocyte function in mice, thereby establishing for the first time a role for these kinases in T cell mediated immune responses. We have further shown that these kinases participate in the signaling pathways involved in T helper cell differentiation, a critical regulatory component of immune responses that helps determine whether an individual mounts a cellular (Th1) or antibody based, (Th2) immune response. We have found that mutation of Tec kinases can alter activation and repression of transcription factors involved in Th2 CD4+ T helper cell differentiation. Our results suggest that impairment of TCR responses may not only reduce the ability of an animal to respond to antigen, but may also alter the type of immune response generated. In the last year, we have concentrated on the biochemical defects associated with mutation of the Tec kinases in T lymphocytes. We have found that mutation of these kinases impairs actin cytoskeletal reorganization and activation of WASP, the protein mutated in Wiskott-Aldrich Syndrome, a syndrome associated with defective cytoskeleton organization. Our results place the Tec kinases as critical regulators of WASP and suggest that cytoskeletal defects may contribute to the phenotypes associated with Tec kinase deficiency. As an extension of these studies, we began examining other signaling molecules potentially involved in T helper cell differentiation including SAP, which is mutated in the genetic disorder X-linked proliferative syndrome (XLP). We have generated mice deficient in SAP and have found that upon challenge with infectious agents, these mice recapitulated features of XLP, including increased T cell activation and IFN-g production, and decreased antibody production. Splenocytes from uninfected SAP- mice produced increased IFN-g and decreased IL-4, suggesting that T helper cell misregulation may contribute to phenotypes associated with (XLP). In the last year, we have focused on the impaired antibody response in these mice. The ability to develop a sustained antibody response is a hallmark of productive immunity and a standard for successful vaccine development. Thus, understanding the cellular interactions and signals leading to productive immunization is of high importance. In collaboration with R. Ahmed (Emory University Vaccine Research Center), we have found that initial B cell responses are only mildly affected in SAP-deficient mice but germinal center formation and the generation of memory and long-lived plasma (antibody secreting) B cells is severely impaired post-infection with LCMV. Using adoptive cell transfer experiments we found that the defect in antibody production is intrinsic to CD4+ T cells, ie SAP deficient T cells fail to provide an essential signal to B cells for generating long-term antibody responses, a critical step for the development of successful immunization and immune responses.