Alexandra Evagelidis

Molecular Determinants of Anion Selectivity in the Cystic Fibrosis Transmembrane Conductance Regulator Chloride Channel Pore

Ionic selectivity in many cation channels is achieved over a short region of the pore known as the selectivity filter, the molecular determinants of which have been identified in Ca(2+), Na(+), and K(+) channels. However, a filter controlling selectivity among different anions has not previously been identified in any Cl(-) channel. In fact, because Cl(-) channels are only weakly selective among small anions, and because their selectivity has proved so resistant to site-directed mutagenesis, the very existence of a discrete anion selectivity filter has been called into question. Here we show that mutation of a putative pore-lining phenylalanine residue, F337, in the sixth membrane-spanning region of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel, dramatically alters the relative permeabilities of different anions in the channel. Specifically, mutations that reduce the size of the amino acid side chain present at this position virtually abolish the relationship between anion permeability and hydration energy, a relationship that characterizes the anion selectivity not only of wild-type CFTR, but of most classes of Cl(-) channels. These results suggest that the pore of CFTR may indeed contain a specialized region, analogous to the selectivity filter of cation channels, at which discrimination between different permeant anions takes place. Because F337 is adjacent to another amino acid residue, T338, which also affects anion selectivity in CFTR, we suggest that selectivity is predominantly determined over a physically discrete region of the pore located near these important residues.

Stimulatory and inhibitory protein kinase C consensus sequences regulate the cystic fibrosis transmembrane conductance regulator.

Protein kinase C (PKC) phosphorylation stimulates the cystic fibrosis transmembrane conductance regulator (CFTR) channel and enhances its activation by protein kinase A (PKA) through mechanisms that remain poorly understood. We have examined the effects of mutating consensus sequences for PKC phosphorylation and report here evidence for both stimulatory and inhibitory sites. Sequences were mutated in subsets and the mutants characterized by patch clamping. Activation of a 4CA mutant (S707A/S790A/T791A/S809A) by PKA was similar to that of wild-type CFTR and was enhanced by PKC, whereas responses of 3CA (T582A/T604A/S641A) and 2CA (T682A/S686A) channels to PKA were both drastically reduced (>90%). When each mutation in the 3CA and 2CA constructs was studied individually in a wild-type background, T582, T604, and S686 were found to be essential for PKA activation. Responses were restored when these three residues were reintroduced simultaneously into a 9CA mutant lacking all nine PKC consensus sequences (R6CA revertant); however, PKC phosphorylation was not required for this rescue. Nevertheless, two of the sites (T604 and S686) were phosphorylated in vitro, and PKC alone partially activated wild-type CFTR, the 4CA mutant, and the point mutants T582A and T604A, but not S686A channels, indicating that PKC does act at S686. The region encompassing S641 and T682 is inhibitory, because S641A enhanced activation by PKA, and T682A channels had 4-fold larger responses to PKC compared to wild-type channels. These results identify functionally important PKC consensus sequences on CFTR and will facilitate studies of its convergent regulation by PKC and PKA.

Regulation of the CFTR channel by phosphorylation

Abstract. Cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels are regulated tightly by protein kinases and phosphatases. The regulatory domain of CFTR has about 20 potential sites for phosphorylation by protein kinases A (PKA) and C (PKC). The reason for this large number of sites is not known, however their conservation from fish to humans implies that they play important roles in vivo. PKA is an important activator, and its stimulation of CFTR is enhanced by PKC via mechanisms which are not fully understood. The physiological stimuli of CFTR are not known for some epithelia, and it appears likely that other serine/threonine and even tyrosine kinases also regulate CFTR in particular tissues. Phosphatases that deactivate CFTR have yet to be identified definitively at the molecular level, however CFTR is regulated by a membrane-bound form of protein phosphatase-2C (PP2C) in several cell types. Patch-clamp studies of channel rundown, co-immunoprecipitation, chemical cross-linking studies, and pull-down assays all indicate that CFTR and PP2C are closely associated within a stable regulatory complex. Understanding the regulation of CFTR by PP2C is a priority due to its potential as a target for pharmacotherapies in the treatment of cystic fibrosis.

Association of Cystic Fibrosis Transmembrane Conductance Regulator and Protein Phosphatase 2C

Cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels are rapidly deactivated by a membrane-bound phosphatase activity. The efficiency of this regulation suggests CFTR and protein phosphatases may be associated within a regulatory complex. In this paper we test that possibility using co-immunoprecipitation and cross-linking experiments. A monoclonal anti-CFTR antibody co-precipitated type 2C protein phosphatase (PP2C) from baby hamster kidney cells stably expressing CFTR but did not co-precipitate PP1, PP2A, or PP2B. Conversely, a polyclonal anti-PP2C antibody co-precipitated CFTR from baby hamster kidney membrane extracts. Exposing baby hamster kidney cell lysates to dithiobis (sulfosuccinimidyl propionate) caused the cross-linking of histidine-tagged CFTR (CFTRHis10) and PP2C into high molecular weight complexes that were isolated by chromatography on Ni2+-nitrilotriacetic acid-agarose. Chemical cross-linking was specific for PP2C, because PP1, PP2A, and PP2B did not co-purify with CFTRHis10 after dithiobis (sulfosuccinimidyl propionate) exposure. These results suggest CFTR and PP2C exist in a stable complex that facilitates regulation of the channel.

Phosphorylation of CFTR by PKA promotes binding of the regulatory domain

The unphosphorylated regulatory (R) domain of the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) is often viewed as an inhibitor that is released by phosphorylation. To test this notion, we studied domain interactions using CFTR channels assembled from three polypeptides. Nucleotides encoding the R domain (aa 635–836) were replaced with an internal ribosome entry sequence so that amino‐ and carboxyl‐terminal half‐molecules would be translated from the same mRNA transcript. Although only core glycosylation was detected on SplitΔR, biotinylation, immunostaining, and functional studies clearly demonstrated its trafficking to the plasma membrane. SplitΔR generated a constitutive halide permeability, which became responsive to cAMP when the missing R domain was coexpressed. Each half‐molecule was co‐precipitated by antibody against the other half. Contrary to expectations, GST‐R domain was pulled down only if prephosphorylated by protein kinase A, and coexpressed R domain was precipitated with SplitΔR much more efficiently when cells were stimulated with cAMP. These results indicate that phosphorylation regulates CFTR by promoting association of the R domain with other domains rather than by causing its dissociation from an inhibitory site.

Enhanced Ca2+ entry due to Orai1 plasma membrane insertion increases IL-8 secretion by cystic fibrosis airways

Cystic fibrosis (CF) is caused by mutations in the gene encoding the CF transmembrane conductance regulator (CFTR). The most common mutation, ΔF508, causes retention of CFTR in the endoplasmic reticulum (ER). Some CF abnormalities can be explained by altered Ca2+ homeostasis, although it remains unknown how CFTR influences calcium signaling. This study examined the novel hypothesis that store‐operated calcium entry (SOCE) through Orai1 is abnormal in CF. The significance of Orai1‐mediated SOCE for increased interleukin‐8 (IL‐8) expression in CF was also investigated. CF and non‐CF human airway epithelial cell line and primary cells (obtained at lung transplantation) were used in Ca2+ imaging, electrophysiology, and fluorescence imaging experiments to explore differences in Orai1 function in CF vs. non‐CF cells. Protein expression and localization was assessed by Western blots, cell surface biotinylation, ELISA, and image correlation spectroscopy (ICS). We show here that store‐operated Ca2+ entry (SOCE) is elevated in CF human airway epithelial cells (hAECs; ~1.8‐ and ~2.5‐fold for total Ca2+i increase and Ca2+ influx rate, respectively, and ~2‐fold increase in the ICRAC current) and is caused by increased exocytotic insertion (~2‐fold) of Orai1 channels into the plasma membrane, which is normalized by rescue of ΔF508‐CFTR trafficking to the cell surface. Augmented SOCE in CF cells is a major factor leading to increased IL‐8 secretion (~2‐fold). CFTR normally down‐regulates the Orai1/stromal interaction molecule 1 (STIM1) complex, and loss of this inhibition due to the absence of CFTR at the plasma membrane helps to explain the potentiated inflammatory response in CF cells.—Balghi, H., Robert, R., Rappaz, B., Zhang, X., Wohlhuter‐Haddad, A., Evagelidis, A., Luo, Y., Goepp, J., Ferraro, P., Roméo, P., Trebak, M., Wiseman, P. W., Thomas, D. Y., Hanrahan, J. W. Enhanced Ca2+ entry due to Orai1 plasma membrane insertion increases IL‐8 secretion by cystic fibrosis airways. FASEB J. 25, 4274–4291 (2011). www.fasebj.org

PKC phosphorylation modulates PKA-dependent binding of the R domain to other domains of CFTR

Activity of the CFTR channel is regulated by phosphorylation of its regulatory domain (RD). In a previous study, we developed a bicistronic construct called DeltaR-Split CFTR, which encodes the front and back halves of CFTR as separate polypeptides without the RD. These fragments assemble to form a constitutively active CFTR channel. Coexpression of the third fragment corresponding to the missing RD restores regulation by PKA, and this is associated with dramatically enhanced binding of the phosphorylated RD. In the present study, we examined the effect of PKC phosphorylation on this PKA-induced interaction. We report here that PKC alone enhanced association of the RD with DeltaR-Split CFTR and that binding was further enhanced when the RD was phosphorylated by both kinases. Mutation of all seven PKC consensus sequences on the RD (7CA-RD) did not affect its association under basal (unphosphorylated) conditions but abolished phosphorylation-induced binding by both kinases. Iodide efflux responses provided further support for the essential role of RD binding in channel regulation. The basal activity of DeltaR-Split/7CA-RD channels was similar to that of DeltaR-Split/wild type (WT)-RD channels, whereas cAMP-stimulated iodide efflux was greatly diminished by removal of the PKC sites, indicating that 7CA-RD binding maintains channels in an inactive state that is unresponsive to PKA. These results suggest a novel mechanism for CFTR regulation in which PKC modulates PKA-induced domain-domain interactions.

Basolateral chloride loading by the anion exchanger type 2: role in fluid secretion by the human airway epithelial cell line Calu‐3

•  The companion paper provided evidence for basolateral anion exchange during cAMP stimulation of chloride and fluid secretion by Calu‐3 monolayers; however, the molecular basis of this transport was not identified. •  To test the role of AE2, an anion exchanger expressed at the basolateral membrane of Calu‐3 and many other epithelial cells, we used lentivirus‐mediated RNA interference to generate a stable Calu‐3 AE2 knock‐down cell line and characterized its fluid and anion transport properties. •  AE2 knock‐down suppressed fluid secretion and increased the fraction of cAMP‐stimulated anion secretion that was sensitive to bumetanide inhibition. •  Basolateral Cl−/HCO3− exchange was nearly abolished in AE2 knock‐down cells. •  We conclude that AE2 is active during forskolin‐stimulated fluid secretion and mediates chloride uptake and bicarbonate recycling at the basolateral membrane.