RAR and RAR reciprocally control K5+ progenitor cell expansion in developing salivary glands

Understanding the mechanisms of controlled expansion and differentiation of basal progenitor cell populations during organogenesis is essential for developing targeted regenerative therapies. Since the cytokeratin 5-positive (K5(+)) basal epithelial cell population in the salivary gland is regulated by retinoic acid signaling, we interrogated how isoform-specific retinoic acid receptor (RAR) signaling impacts the K5(+) cell population during salivary gland organogenesis to identify RAR isoform-specific mechanisms that could be exploited in future regenerative therapies. In this study, we utilized RAR isoform-specific inhibitors and agonists with murine submandibular salivary gland organ explants. We determined that RAR and RAR have opposing effects on K5(+) cell cycle progression and cell distribution. RAR negatively regulates K5(+) cells in both whole organ explants and in isolated epithelial rudiments. In contrast, RAR is necessary but not sufficient to positively maintain K5(+) cells, as agonism of RAR alone failed to significantly expand the population. Although retinoids are known to stimulate differentiation, K5 levels were not inversely correlated with differentiated ductal cytokeratins. Instead, RAR agonism and RAR inhibition, corresponding with reduced K5, resulted in premature lumenization, as marked by prominin-1. With lineage tracing, we demonstrated that K5(+) cells have the capacity to become prominin-1(+) cells. We conclude that RAR and RAR reciprocally control K5(+) progenitor cells endogenously in the developing submandibular salivary epithelium, in a cell cycle-dependent manner, controlling lumenization independently of keratinizing differentiation. Based on these data, isoform-specific targeting RAR may be more effective than pan-RAR inhibitors for regenerative therapies that seek to expand the K5(+) progenitor cell pool. Summary statement: RAR and RAR reciprocally control K5(+) progenitor cell proliferation and distribution in the developing submandibular salivary epithelium in a cell cycle-dependent manner while regulating lumenization independently of keratinizing differentiation.