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1. Introduction

G protein-coupled receptor 124 (GPR124) belongs to the B2 class of adhesive trans-membrane signal transduction proteins and is involved in a number of physiological and biochemical reactions in the human body, such as regulating endothelial cell (EC) function, participating in inflammation and angiogenesis (1-4). Cardiovascular and cerebrovascular diseases are general terms of cardiovascular and cerebrovascular diseases, which generally refer to ischemic or hemorrhagic diseases of the heart, brain and whole body tissues caused by hyperlipidemia, blood viscosity, atherosclerosis (AS) and hypertension (HTN) (5). There is evidence that GPR124 is closely related to ischemic stroke, AS, HTN, cancer and other diseases. In this paper, the biological characteristics of GPR124 were summarized, the main research progress in its related diseases was introduced and its potential signal transduction and associated pathways were highlighted.

2. Overview of GPR124

Biological characteristics of GPR124

The guanosine-binding protein coupled receptors (GPCRs) are the largest family of transmembrane receptors in eukaryotes and the most abundant protein family in the genome (6). All GPCRs have a similar structure, i.e., consisting of an extracellular N terminus, an intracellular C terminus and a secondary trans-membrane helix, TM-7, formed by connecting the extracellular and intracellular loops (7,8). They transmit signals in response to a variety of physical and chemical stimuli, including neurotransmitters, hormones, local mediators, metabolism or olfaction. GPCR has regulatory functions in almost all organ systems and the dysregulation of GPCR signaling is related to the pathogenesis of numerous diseases (9).

GPCRs can be divided into five categories: A, B1, B2, C and F according to their genetic sequence and structural similarity (10). GPR124 belongs to the B2 family of adhesive GPCR. This receptor-related gene is located in the 16q12.4 region of the rat chromosome and 8p11.22 in the human chromosome, with 84% homology, encoding a protein of 1,331 amino acids (11). The extracellular N-terminus of GPR124 has a leucine-rich repeat (LRR), immunoglobulin domain, hormone binding domain (12). The C-terminal tail in the cell membrane interacts with the Gβ1γ2 subunit and connects to the molecular scaffold of the engulfment and cell motility-dedicator of cytokinesis family and guanine nucleotide exchange factor, which has a potentially extensive role in the signal transduction of GPR124(13). GPR124 is a transmembrane signal transduction protein, also known as tumor endothelial marker 5 (TEM5). As it was first found in the neovascular ECs of human colorectal cancer, it is closely related to tumor and vascular biology research (14,15). At present, the research on the orphan receptor GPR124 at home and abroad is still in the initial stage and most studies have focused on the central nervous system (CNS), cardiovascular and cerebrovascular diseases and tumors.

Association between GPR124 and CNS development

GPR124 is an endothelial-specific receptor necessary for normal pre-cerebrovascular formation and blood-brain barrier (BBB) function in mouse embryos, mediating CNS angiogenesis (16,17). Studies have shown that GPR124 has a cellular autonomic function in the brain ECs, regulating the embryonic forebrain and neural tube angiogenesis (14,18-20). A clear lethal CNS-specific phenotype associated with GPR124 deletion is observed during embryonic development. This phenotype is marked by impaired angiogenesis, hemorrhagic glomeruloid malformations, developmental BBB defects and loss of expression of the BBB marker glucose transporter 1 (Glut1, also referred to as Slc2a1) (20). The result was also confirmed in experiments on zebrafish (21). Similar developmental abnormalities were seen in ECs after deleting β-catenin or dual-deleting Wnt7a/Wnt7b, indicating a potential connection between GPR124 and canonical WNT signaling (22,23). Wnt ligands participate in a variety of signaling pathways that are active throughout different stages of disease, during development and during tissue homeostasis maintenance. Wnt family member 1 (WNT1)- and WNT7B-mediated synergistic Wnt signaling requires frizzled class receptor 5 (FZD5), FZD8 and LDL receptor-related protein 6 (LRP6), as well as the WNT7B co-receptors GPR124 and reversion inducing cysteine-rich protein with Kazal motifs (RECK) (24). Further studies, such as genetic analyses, have shown that GPR124 typically stimulates β-catenin signaling to cooperate with WNT7 to regulate angiogenesis and is of great significance in the specific angiogenesis of the CNS and the formation of the blood-brain barrier (25). GPR124 and Reck are part of the cell surface protein complex and enable brain ECs to selectively respond to Wnt7 (26,27). Dishevelled increases the local concentrations of Wnt7 that are available for Frizzled signaling by recruiting GPR124 and the related Reck-bound Wnt7 into dynamic Wnt/Frizzled/Lrp5/6 signalosomes by polymerization (26). Studies have demonstrated that GPR124 coactivates canonical Wnt signaling induced by Wnt7a and Wnt7b via a Lrp coreceptor and Frizzled receptor, and that GPR124-stimulated signaling coordinates with Norrin/Frizzled4 signaling to regulate CNS vascular development (28-30). Therefore, GPR124 is an important regulatory factor for the development of the neurovascular system.

3. GPR124 and related diseases

Ischemic stroke

The expression of GPR124 is induced by small GTPase Rac and mediates contact inhibition of EC proliferation during angiogenesis (31). A layer of cells in the arterial lumen known as vascular ECs (VECs) is crucial in the development of vascular disorders. They can release a variety of proteins that govern thrombosis, smooth muscle cell (SMC) migration and proliferation, vascular wall inflammation and cell adhesion and migration-all of which are critical for maintaining vascular homeostasis. VECs exhibit an active phenotype or endothelial dysfunction, in response to adverse stimuli (32). Abnormal EC proliferation and angiogenesis may lead to the formation of cerebral arteriovenous malformation, increasing the risk of ischemic stroke. GPR124 conditional knockout in the endothelium of adult mice did not affect steady-state BBB integrity, but resulted in BBB disruption and microvascular bleeding in a mouse model of ischemic stroke, accompanied by decreased cerebrovascular classical WNT/β-catenin signaling (20). During ischemia, morphological analysis showed that GPR124 was localized in focal adhesion, directly bound to actin and promoted the formation and directed migration of vascular pericytes (33). Studies have shown that the Wnt7/Gpr124/Reck signaling pathway regulates EC function by regulating VEGF expression (34,35). Statistical analysis suggested that periodontitis was a risk factor for ischemic stroke (36,37). The inflammatory markers caspase-1 and IL-6 have not only been identified as biomarkers of periodontitis, but also as indicators of increased stroke risk. Both can be detected in the bloodstream, which suggests a possible worsening of the stroke process (36). This further confirmed the role of GPR124 in ischemic stroke.

Studies revealed that TGF-β increases the expression of GPR124 and that GPR124 ablation causes abnormal TGF-β pathway activation, indicating that GPR124 has a function in regulating TGF-β signaling (38). During CNS angiogenesis, pericytes and astrocytes have been identified as cellular sources of TGF-β (26). TGF-β signaling in ECs has previously been shown to be associated with CNS angiogenesis, as its absence leads to abnormal blood vessel germination and bleeding (39). This suggests a potential association of GPR124 with TGF-β in ischemic stroke (Fig. 1).

Figure 1 GPR124 promotes the process of ischemic stroke-associated periodontitis. GPR124, G protein-coupled receptor.

AS

AS is considered to be a chronic inflammatory disease that results in arterial plaque formation and vessel bed stenosis, and is the major cause of cardiovascular disease (40). Every stage of atherosclerosis is influenced by vascular SMCs (VSMCs), and lineage tracing research has shown that at least 30% of the cells in atherosclerotic lesions are produced from VSMCs (41). It is thought that the proliferation and migration of VSMCs are of great significance for the formation of AS, among which inhibiting VSMC proliferation and migration has a protective effect on arteries (42). GPR124 showed neuronal and non-neuronal expression profiles (19). Overexpression of GPR124 in ECs activated by inflammation or other microenvironmental stimuli can induce increased expression of α-SM actin, a marker of SMCs, suggesting that SMCs at the root of the aorta proliferate and migrate to the intima after EC dysfunction, thereby aggravating atherosclerotic plaques (43). GPR124 not only increases serum cholesterol and lipid deposition in the aortic sinus, leading to EC dysfunction, but also exacerbates the proliferation of VSMCs. After 16 weeks of high-fat diet in apolipoprotein E knockout mice, mouse aortic SMCs showed obvious signs of inflammatory activation and dedifferentiation (44). At the same time, the expression of GPR124 in aortic SMCs increased significantly. Lipopolysaccharide (LPS) is one of the inflammatory substances that aggravate the progression of AS and LPS can lead to the dedifferentiation of VSMCs. Inflammatory activation in a septicemia model constructed by intraperitoneal injection of LPS led to characteristic changes in the GPCR library, in which dedifferentiated SMC upregulated GPR124(45). Cell experiments have shown that in vitro culture of primary aortic SMCs not only leads to the upregulation of genes of a dedifferentiated SMC phenotype, such as intercellular adhesion molecule 1, vascular cell adhesion molecule 1, proliferation marker Ki-67 or IL-6, but also leads to upregulation of genes of the dedifferentiated SMC phenotype. Furthermore, the expression frequency of GPR124 increased significantly. This confirms that GPR124 is involved in the mechanism of VSMC dedifferentiation during atherosclerosis. However, studies have also shown that GPR124 is lower in atherosclerotic aortic dedifferentiated SMCs than in healthy aorta. Based on the theory that oxidized low-density lipoprotein (mox-LDL) can induce the transformation of VSMC from a ‘contractile’ phenotype to a ‘migration, proliferation and synthesis’ phenotype, which is the core of intimal hyperplasia and atherosclerosis formation, researchers found that VSMCs are transformed after mox-LDL action and the expression of GPR124 is downregulated (46).

Studies have demonstrated the potential importance of molecular and cellular mechanisms associated with NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome signaling in the disease process (47). Patients with atherosclerosis had higher expression of the NLRP3 inflammasome in their plaque and peripheral blood mononuclear cells, suggesting a connection between the two conditions (48). At the molecular level, abnormal activation of NLRP3 inflammasome and its resulting recruitment of high circulating levels of IL-1β and IL-18 and macrophages to aortic wall lesions promote the phenotypic transformation of VSMCs, inducing foam-cell formation and increasing plaque instability (49,50). Numerous studies have suggested that the inflammasome NLRP3/Caspase-1/IL-1β pathway triggers an inflammatory response in the blood vessel wall, leading to the progression of atherosclerosis (51). However, the induced endothelium-specific overexpression of GPR124 can promote atherosclerosis by activating nitrosation, a process of converting organic compounds into nitroso derivatives and NLRP3 inflammasome signaling (43,52). The expression of GPR124 in AS mice was increased at the same time as the level of oxidized low-density lipoprotein (53). The progression of AS is closely associated with several inflammatory diseases, one of which is periodontitis (54). In the pathological process of periodontitis, NLRP3 inflammasome recruitment and trigger activation of Caspase-1, pro-inflammatory factor IL-1β, IL-18 through chemotaxis to increase local neutrophils, thus further destroying the periodontal membrane and forming an environment suitable for the proliferation of periodontal pathogens (55-58). It will also activate osteoclasts and eventually lead to alveolar bone resorption (59) (Fig. 2). Therefore, the relationship between GPR124 and NLRP3/Caspase-1/IL-1β pathways may provide a new treatment strategy for AS and improve the prognosis.

Figure 2 GPR124 promotes the process of atherosclerosis. GPR124, G protein-coupled receptor; SMC, smooth muscle cell; NLRP3, NOD-, LRR- and pyrin domain-containing protein 3; GSDMD, gasdermin D.

HTN

HTN is defined as systolic and diastolic blood pressure greater than or equal to 140/90 mmHg (60). Consistent with the above-mentioned, GPR124 is expressed in the heart, aorta, cerebrovascular system, kidney and other organs (61,62). GPR124 expression was significantly increased in the aorta of HTN mice aged 6-8 weeks, and significantly increased in the kidneys and left atrium at 10-12 weeks. Overall, GPR124 expression was upregulated in the left ventricle and left atrium (63). This suggests that GPR124 may be overexpressed after HTN progression to reduce heart damage caused by HTN. VSMCs participate in the tissue structure of the vascular wall and serve as one of the cells that mainly maintain vascular tension (15). The proliferation and hypertrophy of SMCs and the contractile changes of their functions after stimulation will lead to the occurrence of hypertension. GPR124 is expressed in aortic SMCs and skeletal SMCs, which not only increases the expression during the differentiation of skeletal SMCs, but also inhibits the inflammatory factors in them. This suggests that GPR124 has a regulatory effect on blood pressure. GPR124 also has a promoting effect on angiogenesis, which is also related to the development of HTN. Poor periodontal health was associated with an increased prevalence of hypertension, and after periodontal treatment, the mean systolic blood pressure of hypertensive patients with periodontitis was significantly reduced, suggesting that periodontitis has an impact on blood pressure control (64,65). At the same time, it has also been suggested that HTN is related to peridental microecological imbalance, suggesting that the influence of HTN leads to microcirculation changes and subsequent periodontal tissue ischemia, which leads to the progression and deterioration of periodontitis (66). Further research is needed on the potential association of GPR124 in hypertension with periodontitis.

Cancer

GPR124 is expressed in ECs and pericytes during angiogenesis. Also, this receptor plays a role in EC migration and differentiation without impacting cell proliferation. ECs create the vasculature of the developing brain and express this receptor at high levels (24). The importance of GPR124 during development suggests that its abnormal expression may be involved in tumor growth (67). Research has shown that GPR124 silencing in human ECs inhibits angiogenesis and tumor growth in mouse xenograft tumors (33). Thus, it is suggested that GPR124 plays a crucial role in the neovascularization of cancer. The first indication of GPR124's overexpression came from tumor endothelial capillaries in human colorectal malignancies (68).

Brain metastasis of cancer causes high mortality, but the exact mechanisms underlying the metastasis remain unclear. Brain metastasis of lung adenocarcinoma is caused by GPR124 activation of Wnt7-β-catenin signal transduction to enhance the ability of pericytes to migrate across the endothelium for trans-endothelial migration (69). Similarly, patients with lung adenocarcinoma brain metastasis with GPR124 mutations have an unfavorable prognosis (70). In addition, by downregulating the protein levels of GPR124, VEGF, MMP-3 and MMP-9 in U-2OS cells with drugs, blood metastasis and lymph node metastasis of bone tumors can be inhibited (71). A study used transcriptome datasets for a weighted gene co-expression network analysis to identify networks and hub genes related to the prognosis of gastric cancer. The results showed that in gastric cancer, high expression of GPR124 was associated with poor prognosis (72).

Other diseases

After menopause and in old age, osteoporosis is common in women. Osteoporotic fractures are easy to produce and can end in death or severe disability. Biomarker expression during osteogenic differentiation in bone marrow mesenchymal stem cells (BMSCs) was measured using western blotting and reverse transcription-quantitative PCR. The findings demonstrated that during the osteogenic differentiation of BMSCs, there was a considerable increase in the expression of GPR124(73). The researchers further proposed that GPR124 could promote the osteogenic differentiation of BMSCs through the Wnt/β-catenin pathway, given the importance of the typical Wnt/βcatenin signaling pathway in promoting osteoblast differentiation and the association of GPR124 with the typical Wnt/βcatenin signaling pathway (73,74). BMSCs are important for the recovery of bone defects after periodontitis (75). Future studies on GPR124 regulation of BMSCs through the Wnt/βcatenin signaling pathway to promote bone tissue regeneration have great prospects. GPR124, also known as TEM5, is expressed in the forebrain and spinal cord (76). It is the first basic endothelial receptor as a regulator of cerebral angiogenesis. GPR124 is one of the key molecules in the differentiation and maturation of the BBB. BBB dysfunction is associated with a variety of neurological diseases, including stroke, multiple sclerosis and brain tumors. Genetic deletion of GPR124 has been implicated in brain-specific regulation of transforming growth factor-β (TGF-β) signaling. TGF-β signaling pathway in ECs has been identified in the angiogenesis of the CNS. A lack of the TGF-β pathway leads to vascular germination and bleeding (19) (Fig. 3).

Figure 3 Dysregulation of GPR124 has been linked to a range of diseases. GPR124, G protein-coupled receptor.

4. Conclusion

In conclusion, GPR124 is involved in the process of cardiovascular and cerebrovascular diseases and cancers by affecting EC function, regulating angiogenesis and mediating inflammation, and the possible mechanism of GPR124 in systemic diseases accompanied by periodontitis has been proposed. This makes GPR124 a potential therapeutic target, particularly in cancer and other vascular-related diseases. At the same time, GPR124 may also affect neural development by regulating the TGF-β signaling pathway, which is of great significance for the treatment and research of nervous system diseases. However, the current research on GPR124 and its mediated signaling pathway is still in the preliminary stage, so it is of great significance to further analyze and explore GPR124 and its related signaling pathways for the study of related diseases.

Acknowledgements

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Funding

Funding: This research was funded by the National Natural Science Foundation of China (grant no. 82201080) and High-level Talents Project of Hainan Natural Science Foundation (grant no. 821RC687).

Availability of data and materials

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Authors' contributions

WYL, YLD and DS: Investigation, Writing. YL: Investigation and editing. ZLG: Funding acquisition, project administration, supervision, writing-review & editing. Data authentication is not applicable. All authors have read and approved the final manuscript.

Ethics approval and consent to participate

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Patient consent for publication

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Competing interests

All authors declare that they have no competing interests.

References