My laboratory uses transgenic and gene knockout (KO) mouse models to study lymphocyte homing, T cell homeostasis, and their involvement in development of autoimmune diseases, protective immune responses and industrial immunosuppressant drugs. Specifically, we are interested in two types of genes, heterotrimeric G proteins and a NF-kB-regulated anti-apoptotic gene IEX-1 (Immediate Early Response gene X-1).
Heterotrimeric G proteins are coupled to seven transmembrane receptors named GPCRs (G protein coupled receptors) to which an extensive array of hormones and chemokines bind, controlling lymphocyte trafficking and differentiation. Gai2-deficient mice develop both an inflammatory bowel disease (IBD) and colon cancer. Many questions arise from the mice: for instance, why does deletion of Gai2 cause cancer and a hyper-immune response in colon only? Is there any causal link between inflammation and cancer development or do these two diseases result from a common defect in TGF-b signaling in the mouse? These fundamental questions will be addressed by biochemical, genetic and In-vivo tissue imaging approaches.
IEX-1 can be rapidly induced in cells under stress and protects these cells from apoptosis by modulating the production of reactive oxygen species (ROS). Mice with targeted IEX-1 expression to lymphocytes develop a lupus-like autoimmune disease and T cell lymphoma. Moreover, IEX-1-KO mice have hypertension, but without accompanying with hypertrophy or infiltration of inflammatory cells in the hypertensive arterial walls. These In-vivo studies stress an important physiological role of IEX-1. Investigation of how IEX-1 regulates ROS homeostasis is the key to understand mechanisms whereby overexpression or deletion of IEX-1 causes a disease.
Current projects:
1. To study the complex interplay between Gai2 and Gai3 proteins. Heterotrimeric Gai proteins play a vital role in cell trafficking, angiogenesis, differentiation and proliferation and have been studied for decades in variety of areas. Our current view about functions of Gai proteins is derived mainly from investigations using pertussis toxin that however cannot distinguish the role of individual members of the Gai protein family. We are investigating, with two strains of Gai-KO mice, whether Gai2 and Gai3 are functionally overlapping, distinct, antagonizing, or synergistic in guiding lymphocyte migration in response to a specific stimulus. The interplay between Gai2 and Gai3 proteins introduces another level of complexity in chemokine receptor-mediated signaling. The complex regulation of GPCR signaling at multiple levels is crucial in temporal and spatial regulation of cell migration.
2. To address molecular basis for unresponsiveness of Gai2-/- T cells to TGF-beta. Deletion of the G-protein ai2 subunit in mice produces a diffuse colitis that mimics the human IBD. T cells in the animal not only display a Th1-skewed hyperimmune response, but also do not respond to TGF-b-mediated inhibition of T cell proliferation and cytokine production. Gai2-deficient T cells appear to express more Smad phosphatase activity than wild type T cells, resulting in diminished phosphorylation of Smad2 and Smad3. We are isolating and characterizing the Smad phosphatase using biochemical and molecular approaches.
3. To elucidate the mechanism underlying IEX-1-mediated cell survival. IEX-1 is a stress-inducible gene and its overexpression can either suppress or enhance apoptosis dependent of the nature of a stress. IEX-1 transcription can be regulated either positively or negatively by NF- kB/rel, Sp1, c/EBP, AP-4, p300, c-Myc, and p53. The broad effects implicate that IEX-1 is involved in a widely conserved cell-signaling pathway. In support of this, we show that IEX-1 modulates the production of ROS in mitochondria, which correlates well with its ability to protect cells from undergoing apoptosis. We are investigating how IEX-1 is involved in regulation of mitochondrial oxygen phosphorylation and ROS production in cells under stress.
4. To investigate abnormal immune responses in IEX-1-Tg mice and IEX-1 KO mice. Our study shows that constitutive expression of IEX-1 prolongs the lifespan of Ag-responding T cells, resulting in a delay in their depletion. A too long life of Ag-responding T cells can however cause autoimmune diseases as shown by a dozen of mouse models including our IEX-1-Tg mice. We postulate that uncontrolled expression of IEX-1 in activated T cells may take place in autoimmune diseases, since the diseases are typically characterized by a high level of pro-inflammatory cytokines. These proinflammatory cytokines can induce IEX-1 expression in T cells in local immune microenvironments. The cytokine-induced expression of IEX-1 would extend the life expectancy of infiltrating T cells, which would, in turn, increase the secretion of various pro-inflammatory cytokines. Such a positive feedback may be a key factor for persistent autoimmunity and inflammation. In contrast to the prolonged lifespan in T cells overexpressing IEX-1, T cells from IEX-1 KO mice are expected to have a shortened lifespan, contributing to a lack of hypertrophy or infiltration of inflammatory cells in hypertensive arterial walls in IEX-1-KO mice. |
