Signal transduction, Apoptosis, Infectious diseases, Immune Tolerance,Genomics.
Apoptosis is essential in elimination of lymphocytes that recognize self-proteins but also plays a crucial role in many aspects of biology. We are interested in the apoptotic signal transduction events. All apoptotic processes are believed to converge at the downstream caspases, which execute the final demise of a cell. In T cells, signals from the T-cell receptor (TCR) complex can lead to different outcomes: apoptosis, proliferation or differentiation. We'd like to understand the underlying mechanisms of the decision between life and death.
Fas and a subset of its tumor necrosis factor (TNF) receptor family members (TNF-RI, DR3, TRAIL receptors) contain a death domain at their cytoplasmic tails, which initiate apoptosis through an adapter protein called FADD. FADD recruits caspase 8, which activation leads to a cascade of caspases and rapid cell death. FADD-/- cells are resistant to FasL-, TNF- and TRAIL-killing, suggesting that FADD is a universal adapter for many receptor-mediated death. In addition to its function in apoptosis, FADD surprisingly has a role in proliferation. FADD-/- T cells are defective in TCR-stimulated proliferation and exhibit abnormal regulation of cell cycle proteins like CDK4 or cyclinD3. FADD is phosphorylated at the G2/M stage of the cell cycle and its regulation is crucial for the function of FADD in cell cycle progression. Current projects include biochemical characterization of FADD-associated protein during proliferation and how FADD balances proliferation and apoptosis.
TRAIL (TNF- related apoptosis inducing ligand) is a cytokine that can kill a variety of tumor cells but leave normal cells largely intact. Several hypotheses have been put forward to explain this differential sensitivity of apoptosis between normal and cancerous cells. These include the presence of decoy receptors in normal cells, loss of an anti-apoptotic molecule in tumor cells and selective mutation of p53 and p19 in cancerous cells that somehow confer sensitivity to the TRAIL apoptotic inducing activity. Experiments are in progress to distinguish these possibilities and to understand the role of TRAIL in normal physiology by challenging mutant mice with a variety of pathogens.
The orphan steroid nuclear receptor Nur77 and its family member Nor-1 are two of the genes that are rapidly induced during TCR induced apoptosis. A dominant negative Nur77 mutant can inhibit TCR-mediated apoptosis. Constitutive expression of Nur77 or Nor1 leads to massive apoptosis in immature Tcells. Recent data indicate that Nur77 might translocate to mitochondria to initiate apoptosis. Current projects include generation of Nor-1 deficient mice, identification of the downstream target genes by DNA microarray analysis and understanding Nur77 in mitochondria apoptosis biology.
To understand how Nur77 is regulated by TCR signaling, we are working on the signal transduction pathway between TCR and Nur77. We showed that Nur77 transcription is mediated through two MEF2 elements. However, MEF2 protein level or phosphorylation does not change when T cells are stimulated. Instead, we found a MAP kinase ERK5, which contains two domains important for the regulation of MEF2 activity: a MEF2-interacting domain and a transcriptional activation domain. Interaction between ERK5 and MEF2 depends on activation in T cells, and expression of ERK5 lacking its activation domain inhibits MEF2-dependent transcription. Up-regulation of Nur77 through kinase-dependent co-activation of MEF2 by ERK5 constitutes a novel mechanism of MAP kinase function. Current projects are aimed at understanding ERK5 regulation in vitro and in vivo.