Molecular Mechanisms of Ion Transport by the SMG

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Abnormal function of salivary gland occurs in radiation, drug therapies, Sjogren's syndrome and Cystic Fibrosis. Saliva is formed by secretion of proteins and fluid by acinar cells. The ductal system absorbs the Na+ and Cl- and secretes K+ and HCO3- to form the final saliva. Over the last few years, we came to appreciate the central role of CFTR-regulated HCO3- transport in epithelial fluid and electrolyte secretion, including that by salivary glands. CFTR supports Cl--dependent HCO3- transport, and CF-causing mutations with normal C1- channel activity have aberrant HCO3- transport. In the preliminary data we show that SMG express several members of the new SLC26 family of Cl-/HCO3- exchangers-DRA, PDS, SMCBT-and BTR1, that function in HCO3- secretion. SMG also express splice variants on the NBCn1 family that function in HCO3- salvage. The proteins of both families are regulated by CFTR. Based on these findings, we developed a new hypothesis to propose that transcellular HCO3- transport is central to salivary gland function and that CFTR coordinates HCO3- transport at rest and during stimulation. The hypothesis will be tested by following four aims. In aim 1 we will study the reciprocal regulatory interaction between CFTR and mDRA. Preliminary data shows that CFTR markedly stimulates mDRA activity and that mDRA may affect anion selectivity of CFTR. We will use several CFTR and DRA mutants to a) characterize the mechanism by which CFTR activates mDRA b) study how mDRA affects CFTR channel properties and c) study the reciprocal regulation in vivo in WT and deltaF mice. In aim 2 we will probe interactions between CFTR and the SLC6 family members Pendrin and SLC26A6 that are expressed at high levels in SMG ducts. After basic characterization of SLC26A6 C1-/HCO3- exchange activity, we will probe whether CFTR regulates PDS and SMCBT in vitro and in vivo as it controls mDRA. In aim 3 we will study the role of BTR1 in SMG function. BTR1 is the first member of a new family of HCO3- transporters that is expressed in SMG duct and acinar cells. We propose to characterize Cl- and HCO3- transport by BTR1 as the potential HCO3- transporter or channel in the LM of the SMG that generates the final 140 mM HCO3- in saliva. In aim 4 we will characterize regulation of the NBCn1 splice variants by CFTR and their role in HCO3- salvage by the SMG. We will characterize the activity of individual and combinations of NBCn1 isoforms and the mechanism of their inhibition by CFTR in vitro and in vivo using the deltaF mouse. Successful completion of the experiments should considerably clarify the role of CFTR in regulating HCO3- homeostasis in the resting and stimulated states. The studies may also shift the emphasis from efforts to correct Cl- transport to efforts to correct Cl- and HCO3- transport in diseases of secretory epithelia such as CF and Sjogren's syndrome.