Ascl3 Transcription Factor Functions in Salivary Gland Maintenance

Objective: The transcription factor, Ascl3, has been identified as a marker for multipotent progenitor cells in the mammalian salivary gland. Ascl3 expression is limited to a subset of cells in the ducts of all three major salivary glands. We hypothesize that Ascl3 marks a population of specialized progenitor cells that function to regenerate multiple cell types in the aging salivary gland. These studies were performed to elucidate the role that both the cells and the Ascl3 transcription factor play in the development and maintenance of the salivary gland. Methods: BrdU labeling and pulse-chase studies were conducted to examine the proliferative activity of Ascl3-expressing progenitor cells. Ascl3-/- mutants were generated for in vivo phenotypic analysis and immunohistochemistry. Results: Consistent with their presumed role as progenitors, Ascl3-expressing cells proliferate during the postnatal stage of salivary gland development as shown by BrdU-labeling. However, longer chase times reveal that Ascl3 positive cells eventually lose the BrdU label, indicating that they are not label-retaining stem cells, but rather intermediate progenitors or transit amplifying cells. Ascl3-/- mice develop normal, albeit smaller, salivary glands, consistent with the finding that overall cellular proliferation is significantly reduced in adult mutants. The Ascl3-expressing duct cells are uniquely distinguished by the expression of Nkcc1, a sodium-potassium-chloride cotransporter, normally localized to the basal surface of secretory acinar cells. Specific loss of Nkcc1 expression occurs in the duct cells of Ascl3-/- mice. Conclusion: Ascl3 labels a subset of proliferating duct cells in the salivary gland that function as progenitors of both acinar and duct cell types, but are not stem cells. These cells are distinguished by the expression of the cotransporter Nkcc1, which appears to be regulated by Ascl3. We conclude that the Ascl3 transcription factor is required to specify the progenitor cell subset. (NIH RO1 1R01DE018896-01A1)