Involvement of GPR37L1 in murine blood pressure regulation and human cardiac disease pathophysiology.

Multiple mouse lines lacking the orphan G protein-coupled receptor, GPR37L1, have elicited disparate cardiovascular phenotypes. The first Gpr37l1 knockout mice study to be published reported a marked elevation in systolic blood pressure (SBP; ∼60 mmHg), revealing a potential therapeutic opportunity. The phenotype differed from our own independently generated knockout line, where male mice exhibited equivalent baseline blood pressure to wild type. Here, we attempted to reproduce the first study by characterizing the cardiovascular phenotype of both the original knockout and transgenic lines alongside a C57BL/6J control line, using the same method of blood pressure measurement. The present study supports the findings from our independently developed Gpr37l1 knockout line, finding that SBP and diastolic blood pressure (DBP) are not different in the original Gpr37l1 knockout male mice (SBP: 130.9 ± 5.3 mmHg; DBP: 90.7 ± 3.0 mmHg) compared with C57BL/6J mice (SBP: 123.1 ± 4.1 mmHg; DBP: 87.0 ± 2.7 mmHg). Instead, we attribute the apparent hypertension of the knockout line originally described to comparison with a seemingly hypotensive transgenic line (SBP 103.7 ± 5.0 mmHg; DBP 71.9 ± 3.7 mmHg). Additionally, we quantified myocardial GPR37L1 transcript in humans, which was suggested to be downregulated in cardiovascular disease. We found that GPR37L1 has very low native transcript levels in human myocardium and that expression is not different in tissue samples from patients with heart failure compared with sex-matched healthy control tissue. These findings indicate that cardiac GPR37L1 expression is unlikely to contribute to the pathophysiology of human heart failure.NEW & NOTEWORTHY This study characterizes systolic blood pressure (SBP) in a Gpr37l1 knockout mouse line, which was previously reported to have ∼60 mmHg higher SBP compared with a transgenic line. We observed only a ∼27 mmHg SBP difference between the lines. However, when compared with C57BL/6J mice, knockout mice showed no difference in SBP. We also investigated GPR37L1 mRNA abundance in human hearts and observed no difference between healthy and failing heart samples.

Type 2 diabetes influences intraepithelial corneal nerve parameters and corneal stromal-epithelial nerve penetration sites.

INTRODUCTION: The quantification of intraepithelial corneal basal nerve parameters by in vivo confocal microscopy represents a promising modality to identify the earliest manifestations of diabetic peripheral neuropathy. However, its diagnostic accuracy is hampered by its dependence on neuron length, with minimal consideration for other parameters, including the origin of these nerves, the corneal stromal-epithelial nerve penetration sites. This study sought to utilize high-resolution images of murine corneal nerves to analyze comprehensively the morphological changes associated with type 2 diabetes progression. MATERIALS AND METHODS: ßIII-Tubulin immunostained corneas from prediabetic and type 2 diabetic mice and their respective controls were imaged by scanning confocal microscopy and analyzed automatically for nerve parameters. Additionally, the number and distribution of penetration sites was manually ascertained and the average length of the axons exiting them was computed. RESULTS: The earliest detectable changes included a significant increase in nerve density (6.06 ± 0.41% vs 8.98 ± 1.99%, P = 0.03) and branching (2867.8 ± 271.3/mm2 vs 4912.1 ± 1475.3/mm2 , P = 0.03), and in the number of penetration sites (258.80 ± 20.87 vs 422.60 ± 63.76, P = 0.0002) at 8 weeks of age. At 16 weeks, corneal innervation decreased, most notably in the periphery. The number of penetration sites remained significantly elevated relative to controls throughout the monitoring period. Similarly, prediabetic mice exhibited an increased number of penetration sites (242.2 ± 13.55 vs 305.6 ± 30.96, P = 0.003) without significant changes to the nerves. CONCLUSIONS: Our data suggest that diabetic peripheral neuropathy may be preceded by a phase of neuron growth rather than regression, and that the peripheral cornea is more sensitive than the center for detecting changes in innervation.

Metabolic Profiling of Mice with Deletion of the Orphan G Protein-Coupled Receptor, GPR37L1.

Understanding the neurogenic causes of obesity may reveal novel drug targets to counter the obesity crisis and associated sequelae. Here, we investigate whether the deletion of GPR37L1, an astrocyte-specific orphan G protein-coupled receptor, affects whole-body energy homeostasis in mice. We subjected male Gpr37l1-/- mice and littermate wildtype (Gpr37l1+/+, C57BL/6J background) controls to either 12 weeks of high-fat diet (HFD) or chow feeding, or to 1 year of chow diet, with body composition quantified by EchoMRI, glucose handling by glucose tolerance test and metabolic rate by indirect calorimetry. Following an HFD, Gpr37l1-/- mice had similar glucose handling, body weight and fat mass compared with wildtype controls. Interestingly, we observed a significantly elevated respiratory exchange ratio in HFD- and chow-fed Gpr37l1-/- mice during daylight hours. After 1 year of chow feeding, we again saw no differences in glucose and insulin tolerance or body weight between genotypes, nor in energy expenditure or respiratory exchange ratio. However, there was significantly lower fat mass accumulation, and higher ambulatory activity in the Gpr37l1-/- mice during night hours. Overall, these results indicate that while GPR37L1 may play a minor role in whole-body metabolism, it is not a viable clinical target for the treatment of obesity.

Deletion of Orphan G Protein-Coupled Receptor GPR37L1 in Mice Alters Cardiovascular Homeostasis in a Sex-Specific Manner.

GPR37L1 is a family A orphan G protein-coupled receptor (GPCR) with a putative role in blood pressure regulation and cardioprotection. In mice, genetic ablation of Gpr37l1 causes sex-dependent effects; female mice lacking Gpr37l1 (GPR37L1-/-) have a modest but significant elevation in blood pressure, while male GPR37L1-/- mice are more susceptible to cardiovascular dysfunction following angiotensin II-induced hypertension. Given that this receptor is highly expressed in the brain, we hypothesize that the cardiovascular phenotype of GPR37L1-/- mice is due to changes in autonomic regulation of blood pressure and heart rate. To investigate this, radiotelemetry was employed to characterize baseline cardiovascular variables in GPR37L1-/- mice of both sexes compared to wildtype controls, followed by power spectral analysis to quantify short-term fluctuations in blood pressure and heart rate attributable to alterations in autonomic homeostatic mechanisms. Additionally, pharmacological ganglionic blockade was performed to determine vasomotor tone, and environmental stress tests were used to assess whether cardiovascular reactivity was altered in GPR37L1-/- mice. We observed that mean arterial pressure was significantly lower in female GPR37L1-/- mice compared to wildtype counterparts, but was unchanged in male GPR37L1-/- mice. GPR37L1-/- genotype had a statistically significant positive chronotropic effect on heart rate across both sexes when analyzed by two-way ANOVA. Power spectral analysis of these data revealed a reduction in power in the heart rate spectrum between 0.5 and 3 Hz in female GPR37L1-/- mice during the diurnal active period, which indicates that GPR37L1-/- mice may have impaired cardiac vagal drive. GPR37L1-/- mice of both sexes also exhibited attenuated depressor responses to ganglionic blockade with pentolinium, indicating that GPR37L1 is involved in maintaining sympathetic vasomotor tone. Interestingly, when these mice were subjected to aversive and appetitive behavioral stressors, the female GPR37L1-/- mice exhibited an attenuation of cardiovascular reactivity to aversive, but not appetitive, environmental stimuli. Together, these results suggest that loss of GPR37L1 affects autonomic maintenance of blood pressure, giving rise to sex-specific cardiovascular changes in GPR37L1-/- mice.

GPCRs in context: sexual dimorphism in the cardiovascular system.

Cardiovascular disease (CVD) remains the largest cause of mortality worldwide, and there is a clear gender gap in disease occurrence, with men being predisposed to earlier onset of CVD, including atherosclerosis and hypertension, relative to women. Oestrogen may be a driving factor for female-specific cardioprotection, though androgens and sex chromosomes are also likely to contribute to sexual dimorphism in the cardiovascular system (CVS). Many GPCR-mediated processes are involved in cardiovascular homeostasis, and some exhibit clear sex divergence. Here, we focus on the G protein-coupled oestrogen receptor, endothelin receptors ETA and ETB and the eicosanoid G protein-coupled receptors (GPCRs), discussing the evidence and potential mechanisms leading to gender dimorphic responses in the vasculature. The use of animal models and pharmacological tools has been essential to understanding the role of these receptors in the CVS and will be key to further delineating their sex-specific effects. Ultimately, this may illuminate wider sex differences in cardiovascular pathology and physiology. LINKED ARTICLES: This article is part of a themed section on Molecular Pharmacology of GPCRs. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.21/issuetoc.

Orphan receptor GPR37L1 contributes to the sexual dimorphism of central cardiovascular control.

BACKGROUND: Over 100 mammalian G protein-coupled receptors are yet to be matched with endogenous ligands; these so-called orphans are prospective drug targets for the treatment of disease. GPR37L1 is one such orphan, abundant in the brain and detectable as mRNA in the heart and kidney. GPR37L1 ablation was reported to cause hypertension and left ventricular hypertrophy, and thus, we sought to further define the role of GPR37L1 in blood pressure homeostasis. METHODS: We investigated the cardiovascular effects of GPR37L1 using wild-type (GPR37L1wt/wt) and null (GPR37L1KO/KO) mice established on a C57BL/6J background, both under baseline conditions and during AngII infusion. We profiled GPR37L1 tissue expression, examining the endogenous receptor by immunoblotting and a ß-galactosidase reporter mouse by immunohistochemistry. RESULTS: GPR37L1 protein was abundant in the brain but not detectable in the heart and kidney. We measured blood pressure in GPR37L1wt/wt and GPR37L1KO/KO mice and found that deletion of GPR37L1 causes a female-specific increase in systolic, diastolic, and mean arterial pressures. When challenged with short-term AngII infusion, only male GPR37L1KO/KO mice developed exacerbated left ventricular hypertrophy and evidence of heart failure, while the female GPR37L1KO/KO mice were protected from cardiac fibrosis. CONCLUSIONS: Despite its absence in the heart and kidney, GPR37L1 regulates baseline blood pressure in female mice and is crucial for cardiovascular compensatory responses in males. The expression of GPR37L1 in the brain, yet absence from peripheral cardiovascular tissues, suggests this orphan receptor is a hitherto unknown contributor to central cardiovascular control.