fluid secretion pathway unmasked by acinar-specific Tmem16A gene ablation in the adult mouse salivary gland

• Published in: Proceedings of the National Academy of Sciences - PNAS 2015-02, Vol.112 (7), p.2263-2268
• Main Authors: Catalán, Marcelo A., Kondo, Yusuke, Peña-Munzenmayer, Gaspar, Jaramillo, Yasna, Liu, Frances, Choi, Sooji, Crandall, Edward, Borok, Zea, Flodby, Per, Shull, Gary E., Melvin, James E.
• Format: Article
• Language: English
• Subjects: Acinar Cells - secretion; Animals; Anoctamin-1; Biological Sciences; Body fluids; Cells; Chloride Channels - genetics; Exocrine glands; Genes; Mice; Mice, Knockout; Receptors, Adrenergic, beta - physiology; Rodents; Saliva - secretion; Salivary Glands - secretion
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1415739112

Activation of an apical Ca ²⁺-activated Cl ⁻ channel (CaCC) triggers the secretion of saliva. It was previously demonstrated that CaCC-mediated Cl ⁻ current and Cl ⁻ efflux are absent in the acinar cells of systemic Tmem16A (Tmem16A Cl ⁻ channel) null mice, but salivation was not assessed in fully developed glands because Tmem16A null mice die within a few days after birth. To test the role of Tmem16A in adult salivary glands, we generated conditional knockout mice lacking Tmem16A in acinar cells ( Tmem16A ⁻/⁻). Ca ²⁺-dependent salivation was abolished in Tmem16A ⁻/⁻ mice, demonstrating that Tmem16A is obligatory for Ca ²⁺-mediated fluid secretion. However, the amount of saliva secreted by Tmem16A ⁻/⁻ mice in response to the β-adrenergic receptor agonist isoproterenol (IPR) was comparable to that seen in controls, indicating that Tmem16A does not significantly contribute to cAMP-induced secretion. Furthermore, IPR-stimulated secretion was unaffected in mice lacking Cftr (Cftr F⁵⁰⁸/F⁵⁰⁸) or ClC-2 (Clcn2 ⁻/⁻) Cl ⁻ channels. The time course for activation of IPR-stimulated fluid secretion closely correlated with that of the IPR-induced cell volume increase, suggesting that acinar swelling may activate a volume-sensitive Cl ⁻ channel. Indeed, Cl ⁻ channel blockers abolished fluid secretion, indicating that Cl ⁻ channel activity is critical for IPR-stimulated secretion. These data suggest that β-adrenergic–induced, cAMP-dependent fluid secretion involves a volume-regulated anion channel. In summary, our results using acinar-specific Tmem16A ⁻/⁻ mice identify Tmem16A as the Cl ⁻ channel essential for muscarinic, Ca ²⁺-dependent fluid secretion in adult mouse salivary glands. Significance There are no effective treatments for dry mouth, a common problem that affects millions of people in the United States. Salivary gland hyposecretion is caused by multiple diseases, including Sjáñögren's syndrome, as well as by radiation therapy and medications. The resulting dry mouth is often associated with dysphagia, reduced taste sensation, and opportunistic infections. The mammalian fluid secretion model predicts that Ca ²⁺-activated Tmem16A and/or cAMP-dependent Cftr Cl ⁻ channels are critical in this process. We demonstrate that activation of the Tmem16A channel is required for muscarinic, Ca ²⁺-dependent salivation, but that β-adrenergic receptor-mediated salivation is independent of Tmem16A, Cftr, and ClC-2 Cl ⁻ channels. A better understanding of the β-adrenergic st