Background: Sodium/glucose co-transporter 2 (SGLT2) inhibitors exert robust cardioprotective effects against heart failure in diabetes patients and there is intense interest to identify the underlying molecular mechanisms that afford this protection. As the induction of the late component of the cardiac sodium channel current (late-I Na ) is involved in the etiology of heart failure, we investigated whether these drugs inhibit late-I Na . Methods: Electrophysiological, in silico molecular docking, molecular, calcium imaging and whole heart perfusion techniques were employed to address this question. Results: The SGLT2 inhibitor empagliflozin reduced late-I Na in cardiomyocytes from mice with heart failure and in cardiac Nav1.5 sodium channels containing the LQT3 mutations R1623Q or ∆KPQ. Empagliflozin, dapagliflozin and canagliflozin are all potent and selective inhibitors of H 2 O 2 -induced late-I Na (IC 50s = 0.79, 0.58 and 1.26 µM respectively) with little effect on peak-I Na . In mouse cardiomyocytes, empagliflozin reduced the incidence of spontaneous calcium transients induced by the late-I Na activator veratridine in a similar manner to tetrodotoxin, ranolazine and lidocaine. The putative binding sites for empagliflozin within Nav1.5 were investigated by simulations of empagliflozin docking to a 3D homology model of human Nav1.5 and point mutagenic approaches. Our results indicate that empagliflozin binds to Nav1.5 in the same region as local anaesthetics and ranolazine. In an acute model of myocardial injury, perfusion of isolated mouse hearts with empagliflozin or tetrodotoxin prevented activation of the cardiac NLRP3 inflammasome and improved functional recovery after ischemia. Conclusions: Our results provide evidence that late-I Na may be an important molecular target in the heart for the SGLT2 inhibitors, contributing to their unexpected cardioprotective effects.

Cl(-) and HCO(3)(-) share a common transport pathway in CFTR, and selectivity between Cl(-) and HCO(3)(-) is independent of ionic conditions. The mechanism of transport is therefore effectively identical for both ions. We suggest that mutations in CFTR that cause cystic fibrosis by selectively disrupting HCO(3)(-) transport do not impair direct CFTR-mediated HCO(3)(-) transport, but may predominantly alter CFTR regulation of other HCO(3)(-) transport pathways.

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We have used site-directed mutagenesis and functional analysis to identify positively charged amino acid residues in the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel that interact with extracellular anions. Mutation of two positively charged arginine residues in the first extracellular loop (ECL) of CFTR, R104, and R117, as well as lysine residue K335 in the sixth transmembrane region, leads to inward rectification of the current-voltage relationship and decreased single channel conductance. These effects are dependent on the charge of the substituted side chain and on the Cl(-) concentration, suggesting that these positive charges normally act to concentrate extracellular Cl(-) ions near the outer mouth of the pore. Side chain charge-dependent effects are mimicked by manipulating charge in situ by mutating these amino acids to cysteine followed by covalent modification with charged cysteine-reactive reagents, confirming the location of these side chains within the pore outer vestibule. State-independent modification of R104C and R117C suggests that these residues are located at the outermost part of the pore. We suggest that ECL1 contributes to the CFTR pore external vestibule and that positively charged amino acid side chains in this region act to attract Cl(-) ions into the pore. In contrast, we find no evidence that fixed positive charges in other ECLs contribute to the permeation properties of the pore.

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl ؊ channel is gated by intracellular factors; however, conformational changes in the channel pore associated with channel activation have not been identified. We have used patch clamp recording to investigate the state-dependent accessibility of substituted cysteine residues in the CFTR channel pore to a range of cysteine-reactive reagents applied to the extracellular side of the membrane. Using functional modification of the channel current-voltage relationship as a marker of modification, we find that several positively charged reagents are able to penetrate deeply into the pore from the outside irrespective of whether or not the channels have been activated. In contrast, access of three anionic cysteine-reactive reagents, the methanesulfonate sodium (2-sulfonatoethyl)methanesulfonate, the organic mercurial p-chloromercuriphenylsulfonic acid, and the permeant anion Au(CN) 2 ؊ , to several different sites in the pore is strictly limited prior to channel activation. This suggests that in nonactivated channels some ion selectivity mechanism exists to exclude anions yet permit cations into the channel pore from the extracellular solution. We suggest that activation of CFTR channels involves a conformational change in the pore that removes a strong selectivity against anion entry from the extracellular solution. We propose further that this conformational change occurs in advance of channel opening, suggesting that multiple distinct closed pore conformations exist.

Transient receptor potential (TRP) channels are regulated by diverse stimuli comprising thermal, chemical, and mechanical modalities. They are also commonly regulated by phosphatidylinositol-4,5-bisphosphate (PIP2), with underlying mechanisms largely unknown. We here revealed an intramolecular interaction of the TRPP3 N and C termini (N-C) that is functionally essential. The interaction was mediated by aromatic Trp81 in pre-S1 domain and cationic Lys568 in TRP-like domain. Structure-function analyses revealed similar N-C interaction in TRPP2 as well as TRPM8/-V1/-C4 via highly conserved tryptophan and lysine/arginine residues. PIP2 bound to cationic residues in TRPP3, including K568, thereby disrupting the N-C interaction and negatively regulating TRPP3. PIP2 had similar negative effects on TRPP2. Interestingly, we found that PIP2 facilitates the N-C interaction in TRPM8/-V1, resulting in channel potentiation. The intramolecular N-C interaction might represent a shared mechanism underlying the gating and PIP2 regulation of TRP channels.

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