Crucial role for Sodium Hydrogen Exchangers in SGLT2 inhibitor-induced arterial relaxations

Introduction

Sodium dependent glucose transporter 2 (SGLT2 or SLC5A2) inhibitors effectively lower blood glucose and are also approved treatments for heart failure independent of raised glucose. One component of the cardioprotective effect is reduced cardiac afterload but the mechanisms underlying peripheral relaxation are ill defined and variable. We speculated that SGLT2 inhibitors promoted arterial relaxation via the release of the potent vasodilator calcitonin gene-related peptide (CGRP) from sensory nerves independent of glucose transport.

Experimental approach

The functional effects of SGLT2 inhibitors (dapagliflozin, empagliflozin, ertugliflozin) and the sodium/hydrogen exchanger 1 (NHE1) blocker cariporide were determined on pre-contracted mesenteric and renal arteries from male Wistar rats using Wire-Myography. SGLT2, NHE1, CGRP and TRPV1 expression in both arteries was determined by Western blot and immunohistochemistry. Kv7.4/5/KCNE4 and TRPV1 currents were measured in the presence and absence of dapagliflozin and empagliflozin.

Results

All SGLT2 inhibitors produced a concentration dependent relaxation (1µM-100µM) of mesenteric arteries that was considerably greater than in renal arteries. Cariporide relaxed mesenteric arteries but not renal arteries. Immunohistochemistry with TRPV1 and CGRP antibodies revealed a dense innervation of sensory nerves in mesenteric arteries that was absent in renal arteries. Consistent with a greater sensory nerve component, the TRPV1 agonist capsaicin produced significantly greater relaxations in mesenteric arteries compared to renal arteries. Relaxations to dapagliflozin, empagliflozin and cariporide were attenuated by incubation with the CGRP receptor antagonist BIBN-4096, the Kv7 blocker linopirdine and the TRPV1 antagonist AMG-517 as well as by depletion of neuronal CGRP. Neither dapagliflozin nor empagliflozin directly activated heterologously expressed TRPV1 channels or Kv7 channels. Strikingly, only NHE1 colocalised with TRPV1 in sensory nerves, and cariporide pre-application prevented the relaxant response to SGLT2 inhibitors.

Conclusions

SGLT2 inhibitors relax mesenteric arteries by a novel mechanism involving the release of CGRP from sensory nerves following inhibition of the Na + /H + exchanger.

KCNQ5 activation by tannins mediates vasorelaxant effects of barks used in Native American botanical medicine

Tree and shrub barks have been used as folk medicine by numerous cultures across the globe for millennia, for a variety of indications, including as vasorelaxants and antispasmodics. Here, using electrophysiology and myography, we discovered that the KCNQ5 voltage-gated potassium channel mediates vascular smooth muscle relaxant effects of barks used in Native American folk medicine. Bark extracts (1%) from Birch, Cramp Bark, Slippery Elm, White Oak, Red Willow, White Willow, and Wild Cherry each strongly activated KCNQ5 expressed in Xenopus oocytes. Testing of a subset including both the most and the least efficacious extracts revealed that Red Willow, White Willow, and White Oak KCNQ-dependently relaxed rat mesenteric arteries; in contrast, Black Haw bark neither activated KCNQ5 nor induced vasorelaxation. Two compounds common to the active barks (gallic acid and tannic acid) had similarly potent and efficacious effects on both KCNQ5 activation and vascular relaxation, and this together with KCNQ5 modulation by other tannins provides a molecular basis for smooth muscle relaxation effects of Native American folk medicine bark extracts.

KCNQ Potassium Channels as Targets of Botanical Folk Medicines

Since prehistory, human species have depended on plants for both food and medicine. Even in countries with ready access to modern medicines, alternative treatments are still highly regarded and commonly used. Unlike modern pharmaceuticals, many botanical medicines are in widespread use despite a lack of safety and efficacy data derived from controlled clinical trials and often unclear mechanisms of action. Contributing to this are the complex and undefined composition and likely multifactorial mechanisms of action and multiple targets of many botanical medicines. Here, we review the newfound importance of the ubiquitous KCNQ subfamily of voltage-gated potassium channels as targets for botanical medicines, including basil, capers, cilantro, lavender, fennel, chamomile, ginger, and Camellia, Sophora, and Mallotus species. We discuss the implications for the traditional use of these plants for disorders such as seizures, hypertension, and diabetes and the molecular mechanisms of plant secondary metabolite effects on KCNQ channels.

KCNQ and KCNE Isoform-Dependent Pharmacology Rationalizes Native American Dual Use of Specific Plants as Both Analgesics and Gastrointestinal Therapeutics

Indigenous peoples of the Americas are proficient in botanical medicine. KCNQ family voltage-gated potassium (Kv) channels are sensitive to a variety of ligands, including plant metabolites. Here, we screened methanolic extracts prepared from 40 Californian coastal redwood forest plants for effects on Kv current and membrane potential in Xenopus oocytes heterologously expressing KCNQ2/3, which regulates excitability of neurons, including those that sense pain. Extracts from 9 of the 40 plant species increased KCNQ2/3 current at –60 mV by ≥threefold (maximally, 15-fold by Urtica dioica) and/or hyperpolarized membrane potential by ≥-3 mV (maximally, –11 mV by Arctostaphylos glandulosa). All nine plants have traditionally been used as both analgesics and gastrointestinal therapeutics. Of two extracts tested, both acted as KCNQ-dependent analgesics in mice. KCNQ2/3 activation at physiologically relevant, subthreshold membrane potentials by tannic acid, gallic acid and quercetin provided molecular correlates for analgesic action of several of the plants. While tannic acid also activated KCNQ1 and KCNQ1-KCNE1 at hyperpolarized, negative membrane potentials, it inhibited KCNQ1-KCNE3 at both negative and positive membrane potentials, mechanistically rationalizing historical use of tannic acid-containing plants as gastrointestinal therapeutics. KCNE dependence of KCNQ channel modulation by plant metabolites therefore provides a molecular mechanistic basis for Native American use of specific plants as both analgesics and gastrointestinal aids.

KCNQ5 Potassium Channel Activation Underlies Vasodilation by Tea

Background/Aims:

Tea, produced from the evergreen Camellia sinensis, has reported therapeutic properties against multiple pathologies, including hypertension. Although some studies validate the health benefits of tea, few have investigated the molecular mechanisms of action. The KCNQ5 voltage-gated potassium channel contributes to vascular smooth muscle tone and neuronal M-current regulation.

Methods:

We applied electrophysiology, myography, mass spectrometry and in silico docking to determine effects and their underlying molecular mechanisms of tea and its components on KCNQ channels and arterial tone.

Results:

A 1% green tea extract (GTE) hyperpolarized cells by augmenting KCNQ5 activity >20-fold at resting potential; similar effects of black tea were inhibited by milk. In contrast, GTE had lesser effects on KCNQ2/Q3 and inhibited KCNQ1/E1. Tea polyphenols epicatechin gallate (ECG) and epigallocatechin-3-gallate (EGCG), but not epicatechin or epigallocatechin, isoform-selectively hyperpolarized KCNQ5 activation voltage dependence. In silico docking and mutagenesis revealed that activation by ECG requires KCNQ5-R212, at the voltage sensor foot. Strikingly, ECG and EGCG but not epicatechin KCNQ-dependently relaxed rat mesenteric arteries.

Conclusion:

KCNQ5 activation contributes to vasodilation by tea; ECG and EGCG are candidates for future anti-hypertensive drug development.

The ubiquitous flavonoid quercetin is an atypical KCNQ potassium channel activator

Many commonly consumed plants are used as folk medicines, often with unclear molecular mechanisms. Recent studies uncovered the ubiquitous and influential KCNQ family of voltage-gated potassium (Kv) channels as a therapeutic target for several medicinal plant compounds. Capers - immature flower buds of Capparis spinosa - have been consumed for food and medicinal purposes for millennia. Here, we show that caper extract hyperpolarizes cells expressing KCNQ1 or KCNQ2/3 Kv channels. Capers are the richest known natural source of quercetin, the most consumed dietary flavonoid. Quercetin potentiated KCNQ1/KCNE1, KCNQ2/3 and KCNQ4 currents but, unusually, not KCNQ5. Strikingly, quercetin augmented both activation and inactivation of KCNQ1, via a unique KCNQ activation mechanism involving sites atop the voltage sensor and in the pore. The findings uncover a novel potential molecular basis for therapeutic effects of quercetin-rich foods and a new chemical space for atypical modes of KCNQ channel modulation.

Kaitlyn E. Redford and Geoffrey W. Abbott show that quercetin, a flavonoid highly expressed in capers, potentiates KCNQ currents to varying degrees depending on the subunit composition of the channel complex. By combining in silico docking, mutagenesis, and electrophysiology they show that this flavonoid can bind KCNQ channels atop the voltage sensor and within the pore module, highlighting an atypical mode of channel modulation.

Survival of Anaerobic Fe2+ Stress Requires the ClpXP Protease

Shewanella oneidensis strain MR-1 is a versatile bacterium capable of respiring extracellular, insoluble ferric oxide minerals under anaerobic conditions. The respiration of iron minerals results in the production of soluble ferrous ions, which at high concentrations are toxic to living organisms. It is not fully understood how Fe2+ is toxic to cells anaerobically, nor is it fully understood how S. oneidensis is able to resist high levels of Fe2+ Here we describe the results of a transposon mutant screen and subsequent deletion of the genes clpX and clpP in S. oneidensis, which demonstrate that the protease ClpXP is required for anaerobic Fe2+ resistance. Many cellular processes are known to be regulated by ClpXP, including entry into stationary phase, envelope stress response, and turnover of stalled ribosomes. However, none of these processes appears to be responsible for mediating anaerobic Fe2+ resistance in S. oneidensis Protein trapping studies were performed to identify ClpXP targets in S. oneidensis under Fe2+ stress, implicating a wide variety of protein targets. Escherichia coli strains lacking clpX or clpP also display increased sensitivity to Fe2+ anaerobically, indicating Fe2+ resistance may be a conserved role for the ClpXP protease system. Hypotheses regarding the potential role(s) of ClpXP during periods of high Fe2+ are discussed. We speculate that metal-containing proteins are misfolded under conditions of high Fe2+ and that the ClpXP protease system is necessary for their turnover.IMPORTANCE Prior to the evolution of cyanobacteria and oxygenic photosynthesis, life arose and flourished in iron-rich oceans. Today, aqueous iron-rich environments are less common, constrained to low-pH conditions and anaerobic systems such as stratified lakes and seas, digestive tracts, subsurface environments, and sediments. The latter two ecosystems often favor dissimilatory metal reduction, a process that produces soluble Fe2+ from iron oxide minerals. Dissimilatory metal-reducing bacteria must therefore have mechanisms to tolerate anaerobic Fe2+ stress, and studying resistance in these organisms may help elucidate the basis of toxicity. Shewanella oneidensis is a model dissimilatory metal-reducing bacterium isolated from metal-rich sediments. Here we demonstrate a role for ClpXP, a protease system widely conserved in bacteria, in anaerobic Fe2+ resistance in both S. oneidensis and Escherichia coli.

MgtE Homolog FicI Acts as a Secondary Ferrous Iron Importer in Shewanella oneidensis Strain MR-1

The transport of metals into and out of cells is necessary for the maintenance of appropriate intracellular concentrations. Metals are needed for incorporation into metalloproteins but become toxic at higher concentrations. Many metal transport proteins have been discovered in bacteria, including the Mg2+ transporter E (MgtE) family of passive Mg2+/Co2+ cation-selective channels. Low sequence identity exists between members of the MgtE family, indicating that substrate specificity may differ among MgtE transporters. Under anoxic conditions, dissimilatory metal-reducing bacteria, such as Shewanella and Geobacter species, are exposed to high levels of soluble metals, including Fe2+ and Mn2+ Here we characterize SO_3966, which encodes an MgtE homolog in Shewanella oneidensis that we name FicI (ferrous iron and cobalt importer) based on its role in maintaining metal homeostasis. A SO_3966 deletion mutant exhibits enhanced growth over that of the wild type under conditions with high Fe2+ or Co2+ concentrations but exhibits wild-type Mg2+ transport and retention phenotypes. Conversely, deletion of feoB, which encodes an energy-dependent Fe2+ importer, causes a growth defect under conditions of low Fe2+ concentrations but not high Fe2+ concentrations. We propose that FicI represents a secondary, less energy-dependent mechanism for iron uptake by S. oneidensis under high Fe2+ concentrations.IMPORTANCEShewanella oneidensis MR-1 is a target of microbial engineering for potential uses in biotechnology and the bioremediation of heavy-metal-contaminated environments. A full understanding of the ways in which S. oneidensis interacts with metals, including the means by which it transports metal ions, is important for optimal genetic engineering of this and other organisms for biotechnology purposes such as biosorption. The MgtE family of metal importers has been described previously as Mg2+ and Co2+ transporters. This work broadens that designation with the discovery of an MgtE homolog in S. oneidensis that imports Fe2+ but not Mg2+ The research presented here also expands our knowledge of the means by which microorganisms have adapted to take up essential nutrients such as iron under various conditions.