Synergy of cAMP and calcium signaling pathways in CFTR regulation

Abstract

SignificanceCystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that encodes a chloride channel located in the apical membrane of epithelia cells. The cAMP signaling pathway and protein phosphorylation are known to be primary controlling mechanisms for channel function. In this study, we present an alternative activation pathway that involves calcium-activated calmodulin binding of the intrinsically disordered regulatory (R) region of CFTR. Beyond their potential therapeutic value, these data provide insights into the intersection of calcium signaling with control of ion homeostasis and the ways in which the local CFTR microdomain organizes itself.

NM23 proteins: innocent bystanders or local energy boosters for CFTR?

“…Thus, CFTR does not act alone and a recent review of the data suggests that calmodulin is attracted to CFTR by this R region. 12 This R region may lie to the side of the structure based on atomic maps. 38 Interestingly, recent evidence from three-dimensional structures of both zebrafish (Danio rerio) and human CFTR from cryo-electron microscopy at a medium resolution of 3.7 Å in the absence of ATP reveals (a) a funnel-shaped anion conduction pathway lined by positive charged residues, (b) a single gate at the extracellular surface, and with (c) the dephosphorylated regulatory domain located between (and not to the side of) the two NBDs preventing dimerization and channel opening.…”

“…Channels do not open in the absence of ATP and the open channels close within 1 s of removal of ATP. 12,30,47,48 The binding of ATP to the NBDs of phosphorylated CFTR promotes the initiation of the channel-gating cycle. 49 In most epithelial cells, the normal ATP concentration of~2 mM is thought to be sufficient to ensure channel activation.…”

“…This is especially interesting given that the transition between the monomeric and multimeric states of CK2 is dependent on ionic strength. It is tempting to speculate that just as the R region of CFTR has been proposed to be a trap for proteins such as calmodulin, 12 perhaps multiple C-terminal interacting proteins whose expression might differ in different cell types 65 might also increase the functional tethering of CFTR in different cell contexts. Perhaps consideration should be given to the hierarchy of potential steric restrictions when multiple protein partners try to bind to the tail of CFTR.…”

“…[8][9][10][11] Cell calcium plays a critical role in exocytotic mucus release and recently, CFTR has been shown to bind calmodulin, which might link anion transport to the viscid mucus found in CF airways. 12 The CFTR anion channel also recycles using endosomes to and fro from the apical surface of many epithelial cells and is a key hub that generates a macromolecular complex including both sodium and potassium channels, anion exchangers, transporters that export cAMP, and other regulatory molecules. 13 We will return to this theme of CFTR as an input and output hub later in the review when we consider the molecular events leading to embryonic tracheogenesis where calciumdependent exocytosis is used (in fetal life) to build a tracheal lumen.…”

“…CFTR is driven by one ATP-binding site acting as a non-hydrolyzing 'degenerate' ATP holding 'bridgelike' domain with the other site permitting free hydrolysis that enables domain shifts to facilitate pore access. The relationship between ATP binding and channel gating is a very complex process as reviewed recently by Zwick et al 7 However, these models take no account of the binding of CFTR-associated proteins that drive a cAMP or calciumdependent CFTR activation 12,18 or to the possibility that CFTR might also become a (protease) split molecule that could recombine its N-and C-terminal fragments 28 in the membrane to create an array of different CFTR halves that add to the diversity of CFTR molecules in the membrane. It is accepted that hydrolysis of clamped ATP drives conformational changes in the translocator 'blades' structure, fueling the transport of biological substrates across the cellular membranes.…”