Leptin Activates Trpm7 Channels in the Carotid Body As a Mechanism of Obesity-Related Hypertension

More than 100 million individuals in the United States suffer from hypertension, and its worldwide incidence is at least 1.3 billion.^1,2 It is a leading contributor to stroke, myocardial infarction, and other adverse cardiovascular events.³ Although many families of medications are prescribed to treat hypertension, a significant subpopulation of patients cannot achieve blood pressure control. Indeed, 10% to 20% of individuals with resistant hypertension exhibit the persistence of elevated blood pressure despite the concurrent administration of at least three classes of antihypertensive medications. In these patients, the risk of death from cardiovascular disease is nearly 50% higher compared with their nonresistant hypertensive counterparts.⁴ Accordingly, it is important to identify the pathogenic mechanisms that underlie the development and maintenance of resistant hypertension to design improved medications to alleviate it.

In recent years, a growing body of evidence has suggested that obesity and insulin resistance contribute substantially to the rising incidence of resistant hypertension.^5,6 Among the diverse and complex signaling pathways that apparently are responsible for the persistence of elevated blood pressure despite clinical management, high plasma levels of leptin have emerged as an important feature. The leptin hormone is synthesized by adipose cells and enterocytes in the small intestine and regulates body weight by increasing metabolic rate and suppressing appetite. Although an acute injection of leptin only minimally affects blood pressure due to its dual pressor and depressor effects, a chronic infusion of leptin or hyperleptinemia can elevate blood pressure by increasing sympathetic nerve activity and aldosterone secretion, among other proposed mechanisms.⁷ Obese individuals exhibit high circulating levels of leptin, which one would assume is metabolically beneficial, but instead they are resistant to leptin’s metabolic effects. In these individuals, leptin seems to act centrally and peripherally to contribute to blood pressure elevation.

In this issue of Circulation Research, Polotsky et al⁸ build on their earlier studies designed to identify the peripheral mechanisms by which leptin contributes to hypertension. This research team previously reported that leptin increases the activity of the carotid sinus nerve to elevate blood pressure. They further noted that leptin-induced activity of the carotid sinus nerve, which transmits chemosensory input from the oxygen-sensitive glomus cells of the carotid bodies (CBs) to the medullary centers, was abolished by nonselective blockers of Trp (transient receptor potential) channels.⁹ Although this finding helped to narrow the search for potential downstream targets of leptin in CB glomus cells, the Trp gene superfamily includes an array of subfamilies of cation-conducting channels with highly diverse biophysical and biological properties, leaving uncertainty as to the precise ion channel target of leptin. Thus, before studies in the current article, the authors searched the mouse transcriptome database and identified Trpm7 (transient receptor potential melastatin 7) as the most abundant Trp superfamily gene in the CB and a potential target of leptin signaling. Logically, the current study focuses on the link between leptin, Trpm7 channels in CB glomus cells, and blood pressure elevation.

One strong feature of the article by Polotsky et al⁸ is the vertically integrated and complementary methods used to test the authors’ hypothesis that leptin induces hypertension by activating Trpm7 channels in CB glomus cells. Additionally, extensive supplementary data provide control studies to validate experimental interventions. For example, the author’s initial experiments compare levels of Trpm7 transcript between CB of wild-type C57BL/6J mice, leptin receptor (Lepr^b)-deficient db/db mice, and leptin deficient ob/ob mice. The results confirmed a 15- and 5-fold loss of Trpm7 in db/db and ob/ob mice, respectively, which correlated with increased methylation of the Trpm7 promoter. Immunofluorescence staining in CB of Lepr^b-EGFP mice corroborated the gene expression studies and suggested localization of Trpm7 with Lepr^b in glomus cells of CB. Antibody specificity was confirmed by multiple control studies. The authors concluded that leptin positively regulates the expression of Trpm7 transcript in CB glomus cells.

Next, Polotsky et al⁸ observed that leptin activates Trpm7 channels in single CB glomus cells. Using the perforated patch-clamp method that retains cytosolic signaling constituents, they observed that leptin increases an outwardly rectifying cation current, which was reversed by FTY720 and NS8593, two small molecule antagonists of Trpm7 channels. Recognizing that FTY720 also modulates the S1PR (sphingosine-1-phosphate receptor) as an off-target effect,¹⁰ the authors further demonstrated that a FTY720 analogue that acts on S1PR but lacks affinity for Trpm7 failed to inhibit the leptin-elicited cation current.

They subsequently demonstrated that leptin-induced elevation of blood pressure in C57BL/6J mice is abolished by CB denervation, implicating the CB as a necessary participant in leptin’s pressor effect. Notably, the Trpm7 channel antagonist, FTY720, applied locally to the CBs also ameliorated leptin-induced hypertension, as did local application of Trpm7 shRNA. However, it should be noted again that FTY720 modulates S1PR, which may affect diverse second messenger pathways in vivo.¹⁰ Additionally, NS8593, the other Trpm7 channel antagonist used in the authors’ patch-clamp studies, also lacks selectivity for Trpm7 channels; it blocks small conductance Ca²⁺-activated K⁺ channels with higher potency than Trpm7.¹¹ Concerns related to the off-target effects of Trpm7 antagonists are mitigated by the parallel use of Trpm7 shRNA to knockdown Trpm7 channels in CBs, an intervention that also attenuated leptin-induced hypertension.

In a final set of studies, Polotsky et al⁸ used adenoviral vectors to express Lepr^b (Ad-Lepr^b) in CB of Lepr^b-deficient db/db mice, an intervention that resulted in demethylation of the Trpm7 promoter and a marked increase in Trpm7 transcript compared with an Ad-LacZ control. In other nonglomus cell types, Trpm7 expression was unaffected or increased mildly to less than wild-type levels by Ad-Lepr^b infection. Biotelemetry revealed that adenoviral-mediated Lepr^b expression in CB of db/db mice caused a rise in resting mean arterial pressure of 9.4 mm Hg by 24 hours after infection, whereas control virus infection did not affect blood pressure.

Collectively, Polotsky et al⁸ provide compelling data to argue that leptin activation of Trpm7 channels in CB glomus cells contributes to leptin-induced hypertension. However, the precise signaling pathway by which leptin activates Trpm7 channels remains uncertain. Trpm7 channels are unique among members of the Trp superfamily in two aspects: these channels are protein kinases and they have low permeability to monovalent cations at negative membrane potentials. Trpm7 channels reportedly are activated by G-protein coupled receptors via the phospholipase C signaling pathway,¹² although this finding is controversial.¹³ Alternatively, it is possible that leptin opens Trpm7 channels via the PKA-cAMP signaling pathway, which depends on the endogenous kinase activity of Trpm7.¹⁴ Thus, future studies are needed to elucidate how leptin communicates with Trpm7 channels. The exact mechanism by which Trpm7 channel-mediated current in CB glomus cells elevates blood pressure also requires an explanation. One reason that the in vivo role of Trpm7 channels is not fully elucidated is because genetic knockout of Trpm7 is lethal at embryonic stages. Some of the better recognized roles of Trpm7 channels include managing cellular magnesium homeostasis and cell adhesion, roles which have not been directly linked to resistant hypertension. Given that Trpm7 channels are more permeable to divalent transitional metals such as zinc and nickel than calcium at resting membrane potential,¹⁵ the role of these transitional metals in regulating the function of CB glomus cells also may be of interest. Finally, as acknowledged by the authors, the inherent assumption that increased Trpm7 channel activity in CBs drives a rise in sympathetic nerve activity as the cause of elevated blood pressure is not directly documented in the study. Thus, the findings of the article raise important questions for this research team and others to pursue.

As summarized in the Figure, the main finding of the article that leptin-induced hypertension is mediated by acute activation and increased expression of Trpm7 channels is new and may have broad implications. For example, it provides further evidence for the authors’ hypothesis that CB glomus cells act as metabolic monitors in obesity by linking plasma leptin levels to blood pressure elevation.⁹ The findings add to our growing knowledge of the molecular mechanisms that contribute to resistant hypertension in obesity, which is associated with an increased incidence and severity of other cardiovascular pathologies. Finally, the findings of Polotsky et al⁸ suggest that Trpm7 channels may represent a potential therapeutic target to lower blood pressure in cases of resistant hypertension involving elevated plasma levels of leptin.

Figure. Elevated levels of plasma leptin during obesity are associated with increased carotid sinus afferent (CSA) activity and elevated blood pressure (BP).

Polotsky et al⁸ provide evidence that leptin activates and increases the abundance of cation-conducting Trpm7 (transient receptor potential melastatin 7) channels in the glomus cells of the carotid body (CB) as one mechanism of obesity-related hypertension.