GPRASP1 loss-of-function links to arteriovenous malformations by endothelial activating GPR4 signals

Local DIO2 Elevation Is an Adaption in Malformed Cerebrovasculature

BACKGROUND

Cerebrovascular malformations are a pivotal cause of hemorrhage and neurological disability alongside lacking effective medication. Thyroid hormones (THs), including thyroxine and triiodothyronine, are essential for vascular development, yet whether they participate in malformed cerebrovascular pathology remains elusive.

METHODS

Single-cell transcriptome analysis characterized human cerebral cavernous malformations and brain arteriovenous malformations, 2 typical cerebrovascular malformation diseases. Adeno-associated virus-mediated Dio2 (iodothyronine deiodinase 2; an enzyme that converts thyroxine to active triiodothyronine) overexpression/knockdown or triiodothyronine/methimazole (an antithyroid drug) treatment was applied to mouse models of cerebral cavernous malformations (endothelial-specific Pdcd10 knockout mice, Pdcd10 endothelial-specific knockout [KO]) and brain arteriovenous malformations (endothelial-specific KrasG12D mutant mice, KrasG12D) to evaluate the involvement of DIO2 and TH signaling in cerebrovascular malformations.

RESULTS

TH signaling was markedly activated in fibroblasts of human cerebral cavernous malformation and arteriovenous malformation single-cell samples, accompanied by elevated DIO2 expression. Similar DIO2 upregulation was observed in cerebrovascular fibroblasts of Pdcd10 KO/KrasG12D mice and patient brain sections. Exogenous Dio2 or triiodothyronine replenishment effectively reduced brain hemorrhage, excessive ECM (extracellular matrix) remodeling, and vascular leakage in juvenile and adult male and female Pdcd10 KO/KrasG12D mice. In contrast, Dio2 silencing or TH inhibition deteriorated vascular anomalies. Mechanistically, transcription factor FOXK1 (forkhead box K1) was determined to interact with the DIO2 promoter region. The activation of fibroblast PI3K (phosphoinositide 3-kinase)-Akt (protein kinase B)-mTOR (mammalian target of rapamycin) signaling in Pdcd10 KO/KrasG12D mice triggered Foxk1 nuclear translocation to promote Dio2 transcription. Triiodothyronine treatment mitigated inflammatory infiltration, normalized mitochondrial morphology, and restored mitochondrial biogenesis in malformed brain vessels by activating the Pgc1a (peroxisome proliferator-activated receptor gamma coactivator 1-alpha)-Sod2 (superoxide dismutase 2)/Prdx3 (peroxiredoxin 3)/Gpx1 (glutathione peroxidase 1) axis to reduce reactive oxygen species accumulation. We also determined that the vascular repair effects of triiodothyronine were Pgc1a-dependent.

CONCLUSIONS

We delineate a novel DIO2-mediated adaption in malformed cerebrovasculature and conclude that targeting TH signaling may represent a potential therapy for cerebrovascular disorders.

Proton perception and activation of a proton-sensing GPCR

Maintaining pH at cellular, tissular, and systemic levels is essential for human health. Proton-sensing GPCRs regulate physiological and pathological processes by sensing the extracellular acidity. However, the molecular mechanism of proton sensing and activation of these receptors remains elusive. Here, we present cryoelectron microscopy (cryo-EM) structures of human GPR4, a prototypical proton-sensing GPCR, in its inactive and active states. Our studies reveal that three extracellular histidine residues are crucial for proton sensing of human GPR4. The binding of protons induces substantial conformational changes in GPR4's ECLs, particularly in ECL2, which transforms from a helix-loop to a β-turn-β configuration. This transformation leads to the rearrangements of H-bond network and hydrophobic packing, relayed by non-canonical motifs to accommodate G proteins. Furthermore, the antagonist NE52-QQ57 hinders human GPR4 activation by preventing hydrophobic stacking rearrangement. Our findings provide a molecular framework for understanding the activation mechanism of a human proton-sensing GPCR, aiding future drug discovery.

Inhibition of acid-sensing receptor GPR4 attenuates neuronal ferroptosis via RhoA/YAP signaling in a rat model of subarachnoid hemorrhage

BACKGROUND AND PURPOSE

Subarachnoid hemorrhage (SAH) is a devastating stroke, in which acidosis is one of detrimental complications. The extracellular pH reduction can activate G protein-coupled receptor 4 (GPR4) in the brain. Yet, the extent to which proton-activated GPR4 contributes to the early brain injury (EBI) post-SAH remains largely unexplored. Ferroptosis, iron-dependent programmed cell death, has recently been shown to contribute to EBI. We aimed to investigate the effects of GPR4 inhibition on neurological deficits and neuronal ferroptosis after SAH in rats.

METHODS

A total 253 Sprague Dawley (SD) male rats (weighing 275-330g) were utilized in this study. SAH was induced by endovascular perforation. NE-52-QQ57 (NE), a selective antagonist of GPR4 was administered intraperitoneally 1-h post-SAH. To explore the mechanisms, RhoA activator U-46619 and YAP activator PY-60 were delivered intracerebroventricularly. Short- and long-term neurobehavior, SAH grading, Western blot assay, ELISA assay, immunofluorescence staining, and transmission electron microscopy was performed post-SAH.

RESULTS

Following SAH, there was an upregulation of GPR4 expression in neurons. GPR4 inhibition by NE improved both short-term and long-term neurological outcomes post-SAH. NE also reduced neuronal ferroptosis, as evidenced by decreased lipid peroxidation products 4HNE and MDA levels in brain tissues, and reduced mitochondrial shrinkage, increased mitochondria crista and decreased membrane density. The application of either U-46619 or PY-60 partially offset the neuroprotective effects of NE on neuronal ferroptosis in SAH rats.

CONCLUSIONS

This study demonstrated that acid-sensing receptor GPR4 contributed to neuronal ferroptosis after SAH via RhoA/YAP pathway, and NE may be a potential therapeutic strategy to attenuate GPR4 mediated neuronal ferroptosis and EBI after SAH.

The Roles of Proton-Sensing G-Protein-Coupled Receptors in Inflammation and Cancer

The precise regulation of pH homeostasis is crucial for normal physiology. However, in tissue microenvironments, it can be impacted by pathological conditions such as inflammation and cancer. Due to the overproduction and accumulation of acids (protons), the extracellular pH is characteristically more acidic in inflamed tissues and tumors in comparison to normal tissues. A family of proton-sensing G-protein-coupled receptors (GPCRs) has been identified as molecular sensors for cells responding to acidic tissue microenvironments. Herein, we review the current research progress pertaining to these proton-sensing GPCRs, including GPR4, GPR65 (TDAG8), and GPR68 (OGR1), in inflammation and cancer. Growing evidence suggests that GPR4 and GPR68 are mainly pro-inflammatory, whereas GPR65 is primarily anti-inflammatory, in various inflammatory disorders. Both anti- and pro-tumorigenic effects have been reported for this family of receptors. Moreover, antagonists and agonists targeting proton-sensing GPCRs have been developed and evaluated in preclinical models. Further research is warranted to better understand the roles of these proton-sensing GPCRs in pathophysiology and is required in order to exploit them as potential therapeutic targets for disease treatment.

Immunomodulation of Proton-activated G Protein-coupled Receptors in Inflammation

Proton-activated G protein-coupled receptors (GPCRs), initially discovered by Ludwig in 2003, are widely distributed in various tissues. These receptors have been found to modulate the immune system in several inflammatory diseases, including inflammatory bowel disease, atopic dermatitis, and asthma. Proton-activated GPCRs belong to the G protein-coupled receptor family and can detect alternations in extracellular pH. This detection triggers downstream signaling pathways within the cells, ultimately influencing the function of immune cells. In this review, we specifically focused on investigating the immune response of proton-activated GPCRs under inflammatory conditions.

Recent advances in acid sensing by G protein coupled receptors

Changes in extracellular proton concentrations occur in a variety of tissues over a range of timescales under physiological conditions and also accompany virtually all pathologies, notably cancers, stroke, inflammation and trauma. Proton-activated, G protein coupled receptors are already partially active at physiological extracellular proton concentrations and their activity increases with rising proton concentrations. Their ability to monitor and report changes in extracellular proton concentrations and hence extracellular pH appears to be involved in a variety of processes, and it is likely to mirror and in some cases promote disease progression. Unsurprisingly, therefore, these pH-sensing receptors (pHR) receive increasing attention from researchers working in an expanding range of research areas, from cellular neurophysiology to systemic inflammatory processes. This review is looking at progress made in the field of pHRs over the past few years and also highlights outstanding issues.

Expression of endogenous epitope-tagged GPR4 in the mouse brain

GPR4 is a proton-sensing G protein-coupled receptor implicated in many peripheral and central physiological processes. GPR4 expression has previously been assessed only via detection of the cognate transcript or indirectly, by use of fluorescent reporters. In this work, CRISPR/Cas9 knock-in technology was used to encode a hemagglutinin (HA) epitope tag within the endogenous locus of Gpr4 and visualize GPR4-HA in the mouse central nervous system using a specific, well characterized HA antibody; GPR4 expression was further verified by complementary Gpr4 mRNA detection. HA immunoreactivity was found in a limited set of brain regions, including in the retrotrapezoid nucleus (RTN), serotonergic raphe nuclei, medial habenula, lateral septum, and several thalamic nuclei. GPR4 expression was not restricted to cells of a specific neurochemical identity as it was observed in excitatory, inhibitory, and aminergic neuronal cell groups. HA immunoreactivity was not detected in brain vascular endothelium, despite clear expression of Gpr4 mRNA in endothelial cells. In the RTN, GPR4 expression was detected at the soma and in proximal dendrites along blood vessels and the ventral surface of the brainstem; HA immunoreactivity was not detected in RTN projections to two known target regions. This localization of GPR4 protein in mouse brain neurons corroborates putative sites of expression where its function has been previously implicated (e.g., CO2-regulated breathing by RTN), and provides a guide for where GPR4 could contribute to other CO2/H+ modulated brain functions. Finally, GPR4-HA animals provide a useful reagent for further study of GPR4 in other physiological processes outside of the brain.Significance Statement GPR4 is a proton-sensing G-protein coupled receptor whose expression is necessary for a number of diverse physiological processes including acid-base sensing in the kidney, immune function, and cancer progression. In the brain, GPR4 has been implicated in the hypercapnic ventilatory response mediated by brainstem neurons. While knockout studies in animals have clearly demonstrated its necessity for normal physiology, descriptions of GPR4 expression have been limited due to a lack of specific antibodies for use in mouse models. In this paper, we implemented a CRISPR/Cas9 knock-in approach to incorporate the coding sequence for a small epitope tag into the locus of GPR4. Using these mice, we were able to describe GPR4 protein expression directly for the first time.