Project Summary During embryogenesis and in adult, the delicate balance between proliferation and differentiation of the diverse cellular populations in the Salivary Gland (SG) must be tightly regulated to ensure normal tissue development and homeostasis. A better understanding of the regulatory mechanisms driving the cell fate decisions and lineage choices that anchor salivary gland morphogenesis will help identify pathways that are critical for tissue regeneration following injury, damage, or during diseased states. Xbp-1, specifically the spliced and transcriptionally active Xbp-1, isoform, has been implicated in Unfolded Protein Response (UPR), differentiation and a host of other biologically important cellular processes. Our knowledge of Xbp-1 in context of the SG is limited to scRNA-seq data showing Xbp-1 to be highly expressed in pro-acinar cells (the precursors to acinar cells) as well as a previously uncharacterized acinar cell phenotype observed in Xbp-1 null mice. The highly enriched expression of Xbp-1 in the SG and its association with super-enhancers as revealed by our analysis of genomic and epigenomics datasets, has led us to hypothesize that Xbp-1 plays an important and molecularly deterministic role in regulating cellular identity and function in the SG. However, the molecular mechanisms through which Xbp-1, dictates cell fate decisions and controls specific gene expression programs during SG development remains unexplored. To address these knowledge gaps, we will utilize conditional Xbp-1 knockout mouse models and sophisticated genomic/transcriptomic approaches to study three independent areas of interest. First, we will identify the spatiotemporal expression pattern of Xbp-1 in diverse SG cell types and measure the relative abundance of unspliced and spliced isoforms through developmental stages and during adult gland maintenance utilizing immunostainings and qRT-PCR (Aim 1). Second, we will use a conditional knockout (cKO) mouse model to determine the role of Xbp-1, during SG development and adult homeostasis (Aim 2). Such studies are necessary as they will identify, for the first time, the in vivo functional role of Xbp-1 in the SG. Finally, we will perform mechanistic studies to identify critical target genes and pathways that are governed by Xbp-1 and determine how the loss of Xbp-1 alters the gene expression program of the SG. Importantly, by implementing scRNA-seq studies, we will evaluate effects of loss of Xbp-1 on cell fate trajectories specifically during acinar development and maturation (Aim 3). Collectively, these studies will further our understanding of the Xbp1-dependent transcriptional and cellular networks important for the biological function of the SG particularly as it pertains to secretory acinar cell differentiation. Advances in our understanding of the underlying mechanisms driving SG development can provide a paradigm for regeneration, identify potential therapeutic targets to increase salivary flow in patients suffering from hyposalivation, and better inform therapies to treat patients with developmental dysfunctions and diseases of the salivary gland.

Project Narrative The development, differentiation and functional maturation of the salivary gland is a tightly regulated process that requires the coordinated activity of important transcriptional networks and signaling pathways. This application focuses on examining the gene-regulatory mechanisms, and in particular the role of the transcription factor Xbp-1, in directing morphogenesis, cellular identity and fate. Such studies will lead to an improved understanding of the inner workings of the salivary gland and can provide molecular insights into regeneration processes that can help patients with salivary gland dysfunction.