Transcriptional Regulatory Mechanisms of Salivary Gland Branching Morphogenesis

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Project Summary During embryonic development, the delicate balance between proliferation and differentiation of stem/progenitor (SP) cells of the salivary gland must be tightly regulated to maintain a homeostatic balance, which is critical for normal tissue development and regeneration. Therefore, a better understanding of the regulatory mechanisms driving SP cell function will provide fundamental insights into salivary gland morphogenesis and help identify pathways that are critical for tissue regeneration following injury, damage, or during diseased states. The transcription factor p63, specifically the ΔNp63 isoform, is highly expressed in the SP cell populations of epithelial rich tissues including those of the skin and glandular organs such as the salivary gland (SG). The critical importance of p63 in the SG is highlighted by the dramatic phenotype of p63-null mice, which display a complete block in tissue morphogenesis. Although some aspects of p63 function in SG development have been reported, our current knowledge regarding the role of p63 in the early developmental stages of SG morphogenesis and differentiation, including branching morphogenesis, is rather limited. In particular, the molecular mechanisms through which p63 dictates cell fate decisions and how it controls specific gene expression programs during early SG branching, is lacking. Therefore, identifying the ΔNp63-driven regulatory networks, particularly in the context of branching morphogenesis, will serve as an essential first step in our understanding of SG stem/progenitor cell biology with the long-term goals of developing new strategies for regeneration and for re-engineering the complex branching architecture of epithelial rich organs like the SG. To address these knowledge gaps, we will utilize mouse models and sophisticated genomic approaches to study three independent areas of interest. First, we will use conditional knockout mouse models to determine the role of ΔNp63 in salivary gland branching morphogenesis (Aim1). Such studies are much needed as they will identify for the first time, the functional role of p63 in SG branching morphogenesis. Second, we will perform mechanistic studies to identify critical target genes and pathways that are governed by ΔNp63 and determine how the loss of ΔNp63 alters the global gene expression program and affects cell fate trajectories during branching morphogenesis at single cell resolution (Aim2). Finally, we will utilize ex vivo tissue explants to examine the role of the WNT signaling pathway, and specifically Sfrp1, in mediating the function of ΔNp63 and test whether Sfrp1 is able to restore the branching defects observed upon the loss of p63. Collectively, these studies have the potential to further our understanding of the ΔNp63-dependent transcriptional and cellular networks important for the biological function of the SG particularly as it pertains to branching. Advances in our understanding of the underlying mechanisms driving SG branching and development can provide a paradigm for regeneration of both mouse and human SGs and better inform therapies to treat patients with developmental dysfunctions and diseases of the SG.