Loss of voltage-gated proton channel Hv1 leads to diet-induced obesity in mice

Voltage-Gated Proton Channels in the Tree of Life

With a single gene encoding H_V1 channel, proton channel diversity is particularly low in mammals compared to other members of the superfamily of voltage-gated ion channels. Nonetheless, mammalian H_V1 channels are expressed in many different tissues and cell types where they exert various functions. In the first part of this review, we regard novel aspects of the functional expression of H_V1 channels in mammals by differentially comparing their involvement in (1) close conjunction with the NADPH oxidase complex responsible for the respiratory burst of phagocytes, and (2) in respiratory burst independent functions such as pH homeostasis or acid extrusion. In the second part, we dissect expression of H_V channels within the eukaryotic tree of life, revealing the immense diversity of the channel in other phylae, such as mollusks or dinoflagellates, where several genes encoding H_V channels can be found within a single species. In the last part, a comprehensive overview of the biophysical properties of a set of twenty different H_V channels characterized electrophysiologically, from Mammalia to unicellular protists, is given.

Role of the voltage-gated proton channel Hv1 in insulin secretion, glucose homeostasis, and obesity

Diabetes is characterized by an absolutely inadequate insulin secretion (type 1 diabetes mellitus) or a relative deficit in insulin secretion due to insulin resistance (type 2 diabetes mellitus), both of which result in elevated blood glucose. Understanding the molecular mechanisms underlying the pathophysiology of diabetes could lead to the development of new therapeutic approaches. The voltage-gated proton channel Hv1 is an ion channel with specific selectivity for protons, which is regulated by membrane potential and intracellular pH. Recently, our studies showed that Hv1 is expressed in β cells of the endocrine pancreas. Knockout of Hv1 reduces insulin secretion and results in hyperglycemia and glucose intolerance, but not insulin resistance. Furthermore, knockout of Hv1 leads to diet-induced obesity due to inflammation and hepatic steatosis. Increasing evidence suggests that Hv1 plays a pivotal role in glucose homeostasis and lipid metabolism. This review aims to summarize advances made so far in our understanding of the roles of Hv1 in the regulation of insulin secretion in β cells, glucose homeostasis, and obesity.

Mice lacking the proton channel Hv1 exhibit sex-specific differences in glucose homeostasis

Sex as a physiologic factor has a strong association with the features of metabolic syndrome. Our previous study showed that loss of the voltage-gated proton channel Hv1 inhibits insulin secretion and leads to hyperglycemia and glucose intolerance in male mice. However, there are significant differences in blood glucose between male and female Hv1-knockout (KO) mice. Here, we investigated the differences in glucose metabolism and insulin sensitivity between male and female KO mice and how sex steroids contribute to these differences. We found that the fasting blood glucose in female KO mice was visibly lower than that in male KO mice, which was accompanied by hypotestosteronemia. KO mice in both sexes exhibited higher expression of gluconeogenesis-related genes in liver compared with WT mice. Also, the livers from KO males displayed a decrease in glycolysis-related gene expression and an increase in gluconeogenesis-related gene expression compared with KO females. Furthermore, exogenous testosterone supplementation decreased blood glucose levels in male KO mice, as well as enhancing insulin signaling. Taken together, our data demonstrate that knockout of Hv1 results in higher blood glucose levels in male than female mice, despite a decreased insulin secretion in both sexes. This sex-related difference in glucose homeostasis is associated with the glucose metabolism in liver tissue, likely due to the physiological levels of testosterone in KO male mice.

Deficiency of voltage-gated proton channel Hv1 aggravates ovalbumin-induced allergic lung asthma in mice

Asthma is a chronic airway inflammation that caused by many factors. The voltage-gated proton channel Hv1 has been proposed to extrude excessive protons produced by NADPH oxidase (NOX) from cytosol to maintain its activity during respiratory bursts. Here, we showed that loss of Hv1 aggravates ovalbumin (OVA)-induced allergic lung asthma in mice. The numbers of total cells, eosinophils and neutrophils in bronchoalveolar lavage fluid (BALF) of Hv1-deficiency (KO) mice are obviously increased after OVA challenge compared with that of wild-type (WT) mice. Histopathological staining reveals that Hv1-deficiency aggravates OVA-induced inflammatory cell infiltration and goblet cell hyperplasia in lung tissues. The expression of IL-4, IL-5 and IL-13 are markedly increased in lung tissues of OVA-challenged KO mice compared with that of WT mice. Furthermore, the expression levels of NOX2, NOX4 and DUOX1 are dramatically increased, while the expression levels of SOD2 and catalase are significantly reduced in lung tissues of OVA-challenged KO mice compared with that of WT mice. The production of ROS in lung tissues of KO mice is significantly higher than that of WT mice after OVA challenge. Our data suggest that Hv1-deficiency might aggravate the development of allergic asthma through increasing ROS production.

Functions and Mechanisms of the Voltage-Gated Proton Channel Hv1 in Brain and Spinal Cord Injury

The voltage-gated proton channel Hv1 is a newly discovered ion channel that is highly conserved among species. It is known that Hv1 is not only expressed in peripheral immune cells but also one of the major ion channels expressed in tissue-resident microglia of the central nervous systems (CNS). One key role for Hv1 is its interaction with NADPH oxidase 2 (NOX2) to regulate reactive oxygen species (ROS) and cytosolic pH. Emerging data suggest that excessive ROS production increases and requires proton currents through Hv1 in the injured CNS, and manipulations that ablate Hv1 expression or induce loss of function may provide neuroprotection in CNS injury models including stroke, traumatic brain injury, and spinal cord injury. Recent data demonstrating microglial Hv1-mediated signaling in the pathophysiology of the CNS injury further supports the idea that Hv1 channel may function as a key mechanism in posttraumatic neuroinflammation and neurodegeneration. In this review, we summarize the main findings of Hv1, including its expression pattern, cellular mechanism, role in aging, and animal models of CNS injury and disease pathology. We also discuss the potential of Hv1 as a therapeutic target for CNS injury.