Pharmacological and Genetic Blockade of Trpm7 in the Carotid Body Treats Obesity-Induced Hypertension

Carotid body denervation improves hyperglycemia in obese mice

The carotid bodies (CBs) have been implicated in glucose abnormalities in obesity via elevation of activity of the sympathetic nervous system. Obesity-induced hypertension is mediated by insulin receptor (INSR) signaling and by leptin, which binds to the leptin receptor (LEPRb) in CB and activates transient receptor potential channel subfamily M member 7 (TRPM7). We hypothesize that in mice with diet-induced obesity, hyperglycemia, glucose intolerance, and insulin resistance will be attenuated by the CB denervation (carotid sinus nerve dissection, CSND) and by knockdown of Leprb, Trpm7, and Insr gene expression in CB. In series of experiments in 75 male diet-induced obese (DIO) mice, we performed either CSND (vs. sham) surgeries or shRNA-induced suppression of Leprb, Trpm7, or Insr gene expression in CB, followed by blood pressure telemetry, intraperitoneal glucose tolerance and insulin tolerance tests, and measurements of fasting plasma insulin, leptin, corticosterone, glucagon and free fatty acids (FFAs) levels, hepatic expression of gluconeogenesis enzymes phosphoenolpyruvate carboxykinase (PEPCK) and glucose 6-phosphatase (G-6-Pase) mRNA and liver glycogen levels. CSND decreased blood pressure, fasting blood glucose levels and improved glucose tolerance without any effect on insulin resistance. CSND did not affect any hormone levels and gluconeogenesis enzymes, but increased liver glycogen level. Genetic knockdown of CB Leprb, Trpm7, and Insr had no effect on glucose metabolism. We conclude that CB contributes to hyperglycemia of obesity, probably by modulation of the glycogen-glucose equilibrium. Diabetogenic effects of obesity on CB in mice do not occur via activation of CB Leprb, Trpm7, and Insr.NEW & NOTEWORTHY This paper provides first evidence that carotid body denervation abolishes hypertension and improves fasting blood glucose levels and glucose tolerance in mice with diet-induced obesity. Furthermore, we have shown that this phenomenon is associated with increased liver glycogen content, whereas insulin sensitivity and enzymes of gluconeogenesis were not affected.

Mechanisms and treatment of obesity-related hypertension-Part 1: Mechanisms

The prevalence of obesity has tripled over the past five decades. Obesity, especially visceral obesity, is closely related to hypertension, increasing the risk of primary (essential) hypertension by 65%-75%. Hypertension is a major risk factor for cardiovascular disease, the leading cause of death worldwide, and its prevalence is rapidly increasing following the pandemic rise in obesity. Although the causal relationship between obesity and high blood pressure (BP) is well established, the detailed mechanisms for such association are still under research. For more than 30 years sympathetic nervous system (SNS) and kidney sodium reabsorption activation, secondary to insulin resistance and compensatory hyperinsulinemia, have been considered as primary mediators of elevated BP in obesity. However, experimental and clinical data show that severe insulin resistance and hyperinsulinemia can occur in the absence of elevated BP, challenging the causal relationship between insulin resistance and hyperinsulinemia as the key factor linking obesity to hypertension. The purpose of Part 1 of this review is to summarize the available data on recently emerging mechanisms believed to contribute to obesity-related hypertension through increased sodium reabsorption and volume expansion, such as: physical compression of the kidney by perirenal/intrarenal fat and overactivation of the systemic/renal SNS and the renin-angiotensin-aldosterone system. The role of hyperleptinemia, impaired chemoreceptor and baroreceptor reflexes, and increased perivascular fat is also discussed. Specifically targeting these mechanisms may pave the way for a new therapeutic intervention in the treatment of obesity-related hypertension in the context of 'precision medicine' principles, which will be discussed in Part 2.

Recent Advances in Understanding the Mechanistic Role of Transient Receptor Potential Ion Channels in Patients With Hypertension

The transient receptor potential (TRP) channel superfamily is a group of nonselective cation channels that function as cellular sensors for a wide range of physical, chemical, and environmental stimuli. According to sequence homology, TRP channels are categorized into 6 subfamilies: TRP canonical, TRP vanilloid, TRP melastatin, TRP ankyrin, TRP mucolipin, and TRP polycystin. They are widely expressed in different cell types and tissues and have essential roles in various physiological and pathological processes by regulating the concentration of ions (Ca2+, Mg2+, Na+, and K+) and influencing intracellular signalling pathways. Human data and experimental models indicate the importance of TRP channels in vascular homeostasis and hypertension. Furthermore, TRP channels have emerged as key players in oxidative stress and inflammation, important in the pathophysiology of cardiovascular diseases, including hypertension. In this review, we present an overview of the TRP channels with a focus on their role in hypertension. In particular, we highlight mechanisms activated by TRP channels in vascular smooth muscle and endothelial cells and discuss their contribution to processes underlying vascular dysfunction in hypertension.

Carotid body: an emerging target for cardiometabolic co-morbidities

NEW FINDINGS

What is the topic of this review? Regarding the global metabolic syndrome crisis, this review focuses on common mechanisms for high blood sugar and high blood pressure. Connections are made between the homeostatic regulation of blood pressure and blood sugar and their dysregulation to reveal signalling mechanisms converging on the carotid body. What advances does it highlight? The carotid body plays a major part in the generation of excessive sympathetic activity in diabetes and also underpins diabetic hypertension. As treatment of diabetic hypertension is notoriously difficult, we propose that novel receptors within the carotid body may provide a novel treatment strategy.

ABSTRACT

The maintenance of glucose homeostasis is obligatory for health and survival. It relies on peripheral glucose sensing and signalling between the brain and peripheral organs via hormonal and neural responses that restore euglycaemia. Failure of these mechanisms causes hyperglycaemia or diabetes. Current anti-diabetic medications control blood glucose but many patients remain with hyperglycemic condition. Diabetes is often associated with hypertension; the latter is more difficult to control in hyperglycaemic conditions. We ask whether a better understanding of the regulatory mechanisms of glucose control could improve treatment of both diabetes and hypertension when they co-exist. With the involvement of the carotid body (CB) in glucose sensing, metabolic regulation and control of sympathetic nerve activity, we consider the CB as a potential treatment target for both diabetes and hypertension. We provide an update on the role of the CB in glucose sensing and glucose homeostasis. Physiologically, hypoglycaemia stimulates the release of hormones such as glucagon and adrenaline, which mobilize or synthesize glucose; however, these counter-regulatory responses were markedly attenuated after denervation of the CBs in animals. Also, CB denervation prevents and reverses insulin resistance and glucose intolerance. We discuss the CB as a metabolic regulator (not just a sensor of blood gases) and consider recent evidence of novel 'metabolic' receptors within the CB and putative signalling peptides that may control glucose homeostasis via modulation of the sympathetic nervous system. The evidence presented may inform future clinical strategies in the treatment of patients with both diabetes and hypertension, which may include the CB.

Adipose-specific deletion of the cation channel TRPM7 inhibits TAK1 kinase-dependent inflammation and obesity in male mice

Chronic inflammation of white adipose tissue is a key link between obesity and the associated metabolic syndrome. Transient receptor potential melastatin-like 7 (TRPM7) is known to be related to inflammation; however, the role of TRPM7 in adipocyte phenotype and function in obesity remains unclear. Here, we observe that the activation of adipocyte TRPM7 plays an essential role in pro-inflammatory responses. Adult male mice are used in our experiments. Adipocyte-specific deficiency in TRPM7 attenuates the pro-inflammatory phenotype, improves glucose homeostasis, and suppresses weight gain in mice fed a high-fat diet. Mechanistically, the pro-inflammatory effect of TRPM7 is dependent on Ca²⁺ signaling. Ca²⁺ influx initiated by TRPM7 enhances transforming growth factor-β activated kinase 1 activation via the co-regulation of calcium/calmodulin-dependent protein kinase II and tumor necrosis factor receptor-associated factor 6, leading to exacerbated nuclear factor kappa B signaling. Additionally, obese mice treated with TRPM7 inhibitor are protected against obesity and insulin resistance. Our results demonstrate TRPM7 as a factor in the development of adipose inflammation that regulates insulin sensitivity in obesity.

Carotid Body Dysfunction and Mechanisms of Disease

Emerging evidence shows that the carotid body (CB) dysfunction is implicated in various physiological and pathophysiological conditions. It has been revealed that the CB structure and neurochemical profile alter in certain human sympathetic-related and cardiometabolic diseases. Specifically, a tiny CB with a decrease of glomus cells and their dense-cored vesicles has been seen in subjects with sleep disordered breathing such as sudden infant death syndrome and obstructive sleep apnea patients and people with congenital central hypoventilation syndrome. Moreover, the CB degranulation is accompanied by significantly elevated levels of catecholamines and proinflammatory cytokines in such patients. The intermittent hypoxia stimulates the CB, eliciting augmented chemoreflex drive and enhanced cardiorespiratory and sympathetic responses. High CB excitability due to blood flow restrictions, oxidative stress, alterations in neurotransmitter gases and disruptions of local mediators is also observed in congestive heart failure conditions. On the other hand, the morpho-chemical changes in hypertension include an increase in the CB volume due to vasodilation, altered transmitter phenotype of chemoreceptor cells and elevated production of neurotrophic factors. Accordingly, in both humans and animal models CB denervation prevents the breathing instability and lowers blood pressure. Knowledge of the morphofunctional aspects of the CB, a better understanding of its role in disease and recent advances in human CB translational research would contribute to the development of new therapeutic strategies.

TRPM7 channels regulate breathing during sleep in obesity by acting peripherally in the carotid bodies

Sleep-disordered breathing (SDB) affects over 50% of obese individuals. Exaggerated hypoxic chemoreflex is a cardinal trait of SDB in obesity. We have shown that leptin acts in the carotid bodies (CB) to augment chemoreflex and that leptin activates the transient receptor potential melastatin 7 (TRPM7) channel. However, the effect of leptin-TRPM7 signalling in CB on breathing and SDB has not been characterized in diet-induced obesity (DIO). We hypothesized that leptin acts via TRPM7 in the CB to increase chemoreflex leading to SDB in obesity. DIO mice were implanted with EEG/EMG electrodes and transfected with Leprb short hairpin RNA (shRNA) or Trpm7 shRNA vs. control shRNA in the CB area bilaterally. Mice underwent a full-polysomnography and metabolic studies at baseline and after transfection. Ventilatory responses to hypoxia and hypercapnia were assessed during wakefulness. Leprb and Trpm7 were upregulated and their promoters were demethylated in the CB of DIO mice. Leprb knockdown in the CB did not significantly affect ventilation. Trpm7 knockdown in the CB stimulated breathing during sleep in normoxia. These effects were not driven by changes in CB chemosensitivity or metabolism. Under sustained hypoxia, Trpm7 shRNA in the CB augmented ventilation during sleep, but decreased oxyhaemoglobin saturation. We conclude that the suppression of TRPM7 in the CB improved sleep-related hypoventilation and that the respiratory effects of CB TRPM7 channels in obesity are independent of leptin. TRPM7 signalling in the CB could be a therapeutic target for the treatment of obesity-related SDB. KEY POINTS: The leptin-TRPM7 axis in the carotid bodies may play an important role in the pathogenesis of sleep-disordered breathing. TRPM7 channels regulate breathing during sleep by acting peripherally in the carotid bodies. Suppression of TRPM7 signalling in the carotid bodies improves the obesity-induced hypoventilation in mice. Pharmacological blockade of TRPM7 channels in the carotid bodies could be a therapy for sleep-disordered breathing in obesity.

Circ_0021155 can participate in the phenotypic transformation of human vascular smooth muscle cells via the miR-4459/TRPM7 axis

The phenotypic transformation of vascular smooth muscle cells (VSMCs) plays a key role in the pathological process of atherosclerosis (AS), and TRPM7 is involved in this process. In this study, we verified whether circRNAs participate in the phenotypic transformation of VSMCs by regulating TRPM7 in AS. The RNA-sequencing data of atherosclerosis were downloaded and analysed from the GEO database. Only hsa_circ_0021155 related to TRPM7 was differentially expressed in AS. circRNA distribution and expression were observed via FISH and PCR. CCK8, scratch test and Transwell assay were used to observe the proliferation and migration of cells. Western blot was performed to examine changes in α-actin, calponin, SMMHC and TRPM7 proteins. The expression of hsa_circ_0021155 against has-miR-4459/miR-3689c was verified via PCR. The ceRNA relationship of TPRM7-miR4459-circ0021155 was verified via dual luciferase assay, and the effects of miR4459 mimic/inhibitor on the proliferation of cells were further observed. The expression of hsa_circ_0021155 and OX-LDL was increased in VSMCs. hsa_circ_0021155 promoted the expression of TRPM7 and inhibited the protein expression of α-actin, calponin and SMMHC. In addition, it promoted the proliferation and migration of cells and inhibited the expression of miR-3689c and miR-4459 but did not affect miR-4756-5p. The dual luciferase assay showed that circ0021155-miR4459-TRPM7 mRNA was highly compatible and could be mutually regulated by a ceRNA network. In conclusion, hsa_circ_0021155 regulates the proliferation, migration and phenotype transformation of VSMCs induced by OX-LDL via the miR-4459/TRPM7 axis. hsa_circ_0021155 and TRPM7 may offer novel therapeutic targets for atherosclerosis.

Are Multiple Mitochondrial Related Signalling Pathways Involved in Carotid Body Oxygen Sensing?

It is generally acknowledged that the carotid body (CB) type I cell mitochondria are unique, being inhibited by relatively small falls in PaO2 well above those known to inhibit electron transport in other cell types. This feature is suggested to allow for the CB to function as an acute O2 sensor, being stimulated and activating systemic protective reflexes before the metabolism of other cells becomes compromised. What is less clear is precisely how a fall in mitochondrial activity links to type I cell depolarisation, a process that is required for initiation of the chemotransduction cascade and post-synaptic action potential generation. Multiple mitochondrial/metabolic signalling mechanisms have been proposed including local generation of mitochondrial reactive oxygen species (mitoROS), a change in mitochondrial/cellular redox status, a fall in MgATP and an increase in lactate. Although each mechanism is based on compelling experimental evidence, they are all not without question. The current review aims to explore the importance of each of these signalling pathways in mediating the overall CB response to hypoxia. We suggest that there is unlikely to be a single mechanism, but instead multiple mitochondrial related signalling pathways are recruited at different PaO2s during hypoxia. Furthermore, it still remains to be determined if mitochondrial signalling acts independently or in partnership with extra-mitochondrial O2-sensors.

Leptin Receptor Blockade Attenuates Hypertension, but Does Not Affect Ventilatory Response to Hypoxia in a Model of Polygenic Obesity

Background

Obesity can cause hypertension and exacerbates sleep-disordered breathing (SDB). Leptin is an adipocyte-produced hormone, which increases metabolic rate, suppresses appetite, modulates control of breathing, and increases blood pressure. Obese individuals with high circulating levels of leptin are resistant to metabolic and respiratory effects of leptin, but they appear to be sensitive to hypertensive effects of this hormone. Obesity-induced hypertension has been associated with hyperleptinemia. New Zealand obese (NZO) mice, a model of polygenic obesity, have high levels of circulating leptin and hypertension, and are prone to develop SDB, similarly to human obesity. We hypothesize that systemic leptin receptor blocker Allo-aca will treat hypertension in NZO mice without any effect on body weight, food intake, or breathing.

Methods

Male NZO mice, 12–13 weeks of age, were treated with Allo-aca (n = 6) or a control peptide Gly11 (n = 12) for 8 consecutive days. Doses of 0.2 mg/kg were administered subcutaneously 2×/day, at 10 AM and 6 PM. Blood pressure was measured by telemetry for 48 h before and during peptide infusion. Ventilation was assessed by whole-body barometric plethysmography, control of breathing was examined by assessing the hypoxic ventilatory response (HVR), and polysomnography was performed during light-phase at baseline and during treatment. Heart rate variability analyses were performed to estimate the cardiac autonomic balance.

Results

Systemic leptin receptor blockade with Allo-aca did not affect body weight, body temperature, and food intake in NZO mice. Plasma levels of leptin did not change after the treatment with either Allo-aca or the control peptide Gy11. NZO mice were hypertensive at baseline and leptin receptor blocker Allo-aca significantly reduced the mean arterial pressure from 134.9 ± 3.1 to 124.9 ± 5.7 mmHg during the light phase (P < 0.05), whereas the control peptide had no effect. Leptin receptor blockade did not change the heart rate or cardiac autonomic balance. Allo-aca did not affect minute ventilation under normoxic or hypoxic conditions and HVR. Ventilation, apnea index, and oxygen desaturation during NREM and REM sleep did not change with leptin receptor blockade.

Conclusion

Systemic leptin receptor blockade attenuates hypertension in NZO mice, but does not exacerbate obesity and SDB. Thus, leptin receptor blockade represents a potential pharmacotherapy for obesity-associated hypertension.