Pulmonary hypertension associated with chronic hypoxia: just ASIC-ness?

Abstract

Chronic hypoxia (CH) associated with lung diseases such as chronic obstructive pulmonary disease often results in structural and functional changes in the pulmonary vasculature leading to pulmonary hypertension (PH). Studies in animal models have revealed that CH induces PH by causing an array of changes in the phenotype of pulmonary artery (PA) endothelial and smooth muscle cells (PASMCs) which promote arterial remodelling and vasoconstriction. One of these, an increased cytosolic [Ca2+] in PASMCs, is at least partly due to enhanced Ca2+ influx through L-type voltage-gated Ca2+ channels resulting from a depolarization of the resting membrane potential (Em), which has long been ascribed to the decreased expression and activity of voltage-gated K+ (KV) channels. The rise in PASMC [Ca2+]cyt is also associated with increased expression of TRPC6 and TRPV4 channels, which mediate vasoconstrictor-induced Ca2+ influx, and of STIM/Orai and TRPC1 channels, which cause store-operated Ca2+ entry (SOCE) (Reyes et al. 2018). Nikki Jernigan and colleagues at the University of New Mexico School of Medicine in Albuquerque have previously demonstrated that CH also acts on PASMC to promote the membrane localization and increased activity of ASIC1a, one of a family of homoor heterotrimeric acid-sensing ion channels formed from subunits encoded by four genes (ASIC1–4). ASICs are non-selective cation channels (NSCCs) which are gated by extracellular protons. Nitta et al. (2014) reported that activation of ASIC1a contributes to the increased SOCE and receptor-mediated Ca2+ influx evoked by CH in PASMCs. This is possible because some ASICs, including ASIC1a homomers and ASIC1a/2b heteromers are Ca2+-permeable. Since ASIC1 is a NSCC, it can be predicted that its increased activity should additionally cause Na+ influx and membrane depolarization. In an article in the current issue of The Journal of Physiology, Jernigan et al. (2021) report that enhanced ASIC1a activity does indeed contribute significantly to CH-induced PASMC depolarization. The authors used whole cell patch clamping to evaluate the contribution of ASIC1 to the NSCCs present in freshly dispersed PASMCs from rats and mice exposed to normoxia or sustained hypobaric hypoxia (CH) for 4 weeks. They also measured Em in isolated small PAs, which were cannulated to allow measurement of Em at different levels of internal pressure. As previously reported by many labs, the authors observed that the KV current was smaller in PASMCs from CH animals. In the absence of K+ and extracellular Ca2+, with Na+ and Cs+ as the predominant extracellular and intracellular cations, respectively, they observed a small quasi-linear non-inactivating currentwhich reversed at 0 mV. The current density was markedly increased by CH. This current was unaffected by Cl– channel antagonists but was greatly diminished by removal of extracellular Na+, especially in the CH cells, indicating that it was due to one or more types of NSCC. Applying psalmotoxin-1, which blocks ASIC1a andASIC1a/2b channels, effectively abolished the NSCC current in cells from CHbut not control rats, implying that hypoxia had induced the de novo development of ASIC1a channel activity. Measurements of Em in PA showed that PASMCs from CH rats were depolarized by ∼16–18 mV compared to controls. Importantly, this difference in Em was abolished by psalmotoxin-1. Knockout of ASIC1 in mice produced effects resembling those caused by psalmotoxin-1 in rats. A small NSCC current was present in PASMC from the ASIC–/– animals and their wild-type littermates which had been maintained under normoxic conditions. This current was greatly enhanced by CH in the ASIC1+/+ mice but not in the knockouts. In isolated PA from the wild-types, the resting Em was depolarized in the CH group compared to the normoxic controls, but this difference in Em was absent in the knockouts. Further experiments showed that increasing the internal pressure within the PA from both rats and mice caused a membrane depolarization. This was blocked by replacing extracellular Na+ with N-methyl-d-glucamine and by La3+, suggesting that it was due to opening of stretch-activated NSCCs. This depolarization occurred in both the wild-type and ASIC1a knockouts regardless of whether they had beenmaintained under normoxic or hypoxic conditions, indicating that ASIC1a was not the NSCC responsible for stretch-induced depolarization. These provocative results suggest that as well as promoting voltage-independent Ca2+ influx, channels incorporating ASIC1 may contribute to CH-induced depolarization of PA, and therefore could play a crucial role in the development of PH in this widely used animal model of human group 3 PH. However, this study also highlights the fact that very little is known about the function and regulation of ASICs in pulmonary (or other) arteries. The regulation of these channels is remarkably complex and only imperfectly understood (Gründer & Pusch 2015). Since they desensitize rapidly following activation, it is unclear how they can give rise to a sustained current, and indeed the nature of the stimuli inducing their opening and how their activation is coupled to SOCE or receptor-mediated Ca2+ influx remain unknown. There is also evidence that ASIC1 itself generates insufficient Ca2+ influx to raise [Ca2+]cyt under physiological conditions (Samways et al. 2009), raising questions about how it can mediate SOCE. The relative contributions of KV channel downregulation vs. ASIC upregulation to CH-induced PA depolarization also requires investigation. Nonetheless, this new paper from the Albuquerque group, who have heretofore been alone in looking at ASICs in PA, demonstrates that these