Research

Stem cells are embedded in many of our tissues and organs, and in that context, their function is controlled by signaling cues emanating from the niche they occupy. In the absence of injury or otherwise the need for new tissue cells, stem cells are retained in a quiescent state. Quiescent stem cells display a very low metabolic rate and rarely cycle. Stem cell quiescence is perturbed upon injury to the tissue which disrupts stem cell-niche interactions (Activation). Activated stem cells are highly glycolytic in their metabolism, rapidly increasing their size and numbers. Ultimately, activated stem cells commit to differentiate into a defined cell type (Commitment), and undergo differentiation to regenerate functional tissue (Differentiation). A subset of activated stem cells returned to quiescence and reoccupy the new niche (Self-renew). Our lab studies signaling and transcriptional control of stem cell function at each of these four stages. Specifically, we are investigating how environmental cues such as nutrition, hormone, and growth factors control the passage of stem cells through each of these phases. 

We have recently described a new signaling pathway that connects multiple differentiation signaling inputs with the epigenetic complexes to drive myogenesis, providing a potential connecting link between environmental cues and transcription. In this pathway, Per-Arnt-Sim (PAS) kinase (PASK) phosphorylates Wdr5, a member of multi-protein enzyme complexes. Wdr5 containing MLL complex deposits H3K4me3 marks for transcriptional activation. Our data shows that PASK-Wdr5 signaling activates the Myog transcription via H3K4me3 modification and MyoD recruitment at the Myog promoter. This establishes a committed myoblast population that ultimately fuses to form multi-nucleated myofibers during regenerative myogenesis. Our subsequent work shows that the PASK-Wdr5 signaling pathway is a downstream target of insulin and amino-acid activated mechanistic Target of Rapamycin (mTORC1) signaling pathway. We believe PASK is a long-sought missing effector of the mTOR signaling pathway that regulates the commitment to differentiate during myogenesis. We are continuing to study the role the mTORC1-PASK-Wdr5 pathway plays during regenerative myogenesis. 

PASK consists of a sensory PAS domain (sensory domains found in proteins such as Period, Clock, HIF-1alpha) in addition to a kinase domain. We have previously described the mechanism of substrate phosphorylation by the kinase domain by solving the X-ray crystallographic structure of PASK. Our studies showed that PASK assumes a stable and active configuration in cells. These results suggest that the catalytic function of PASK should be regulated in atypical manner. We are investigating how is PASK activity is regulated in distinct stem cell stages such as quiescent, activated and committed state. Our crystal structure of the kinase domain of PASK revealed a unique structural motif in the N-terminal lobe (anti-parallel β-sheets ) which might play important role in maintaining active, open conformation of the kinase domain. We are also employing structure-based modeling to identify functionally important residues within the kinase domain. 

Alteration in metabolism is both a necessity and a liability for human cancers. In order to satisfy the demand for accelerated cellular growth, cancer cells rewire their metabolism to divert metabolites into generating biosynthetic precursors, sometimes at the expense of net ATP generation. Amongst many approaches cancer cells employ to achieve metabolic rewiring, we are interested in the study of the altered subcellular distribution of signaling and metabolic proteins in cancers. We are pursuing a hypothesis that by changing the subcellular distribution of signaling and metabolic proteins, new pathways and new metabolite pools are being generated to fuel metabolic rewriting. We have shown that nuclear translocation of a metabolic protein kinase, 3’-Phosphoinositide Dependent Kinase-1 (PDK-1) triggers an oncogenic transformation of fibroblasts and that nuclear PDK1 is accumulated in human prostate cancers. PDK-1 is typically a cytoplasmic protein but accumulates in the nucleus when oncogenic pathways are activated, or upon prolonged insulin stimulation. We are continuing to investigate the mechanistic underpinning of the PDK-1 nuclear translocation process and subsequent cellular transformation due to nuclear PDK-1.