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 RARα and RARγ have opposing effects on K5⁺ cell cycle progression and cell distribution. RARα negatively regulates K5⁺ cells in both whole organ explants and in isolated epithelial rudiments. In contrast, RARγ is necessary but not sufficient to positively maintain K5⁺ cells, as agonism of RARγ 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, RARα agonism and RARγ 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 RARα and RARγ 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 RARα may be more effective than pan-RAR inhibitors for regenerative therapies that seek to expand the K5⁺ progenitor cell pool. Summary statement: RARα and RARγ 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.

了解器官发生过程中基底祖细胞群的受控扩张和分化机制对于开发靶向再生疗法至关重要。由于唾液腺中的细胞角蛋白 5 阳性 (K5 ⁺ ) 基底上皮细胞群受到视黄酸信号传导的调节，因此我们研究了亚型特异性视黄酸受体 (RAR) 信号传导如何在唾液腺器官发生过程中影响 K5 ⁺细胞群，以确定RAR 亚型特异性机制可用于未来的再生疗法。在这项研究中，我们将 RAR 异构体特异性抑制剂和激动剂用于小鼠颌下唾液腺器官外植体。我们确定 RARα 和 RARγ 对 K5 ⁺细胞周期进程和细胞分布具有相反的影响。 RARα 对整个器官外植体和分离的上皮雏形中的 K5 ⁺细胞具有负调节作用。相反，RARγ 对于积极维持 K5 ⁺细胞是必要的，但不足以充分，因为单独的 RARγ 激动剂无法显着扩大细胞群。尽管已知类视黄醇可刺激分化，但 K5 水平与分化的导管细胞角蛋白并不呈负相关。相反，RARα 激动和 RARγ 抑制（与 K5 减少相对应）导致过早的管腔化，如 prominin-1 所示。通过谱系追踪，我们证明 K5 ⁺细胞有能力成为 prominin-1 ⁺细胞。 我们得出结论，RARα和RARγ以细胞周期依赖性方式内源性地控制发育中的下颌下唾液上皮中的K5 ⁺祖细胞，独立于角化分化控制管腔化。基于这些数据，对于寻求扩大 K5 ⁺祖细胞库的再生疗法来说，异构体特异性靶向 RARα 可能比泛 RAR 抑制剂更有效。摘要：RARα 和 RARγ 以细胞周期依赖性方式相互控制发育中的下颌下唾液上皮中 K5 ⁺祖细胞的增殖和分布，同时独立于角化分化调节管腔化。