Abstract
The super-conserved receptors expressed in the brain (SREB) constitute a family of orphan G protein-coupled receptors that include GPR27 (SREB1), GPR85 (SREB2) and GPR173 (SREB3). Their sequences are highly conserved in vertebrates, and they are almost exclusively expressed in the central nervous system. This family of receptors has attracted much attention due to their putative physiological functions and their potential as novel drug targets. The SREB family has been postulated to play important roles in a wide range of different diseases, including pancreatic β-cell insulin secretion and regulation, schizophrenia, autism and atherosclerosis. This review intends to provide a comprehensive overview of the SREB family and its recent advances in biology and medicinal chemistry.
Plain language summary
In recent years, the super-conserved receptors expressed in the brain called GPR27, GPR85 and GPR173 have attracted much interest in the field of medicinal science. They have one important feature in common: they are all almost entirely found in the brain. Researchers have investigated their functions in the body in various animal models, as well as their utility in future drug development. GPR27 has been found to be involved in insulin and blood sugar processes in the body and therefore may be important for diabetes treatment. GPR85 is thought to be linked to brain diseases such as schizophrenia and autism. GPR173 is linked to many different illnesses, including atherosclerosis (the buildup of fats, cholesterol and other substances in arteries) and Type 2 diabetes.
Papers of special note have been highlighted as: • of interest
References
- 1. . The role of GPCRs in neurodegenerative diseases: avenues for therapeutic intervention. Curr. Opin. Pharmacol. 32, 96–110 (2017).
- 2. IUPHAR-DB: the IUPHAR database of G protein-coupled receptors and ion channels. Nucleic Acids Res. 37(Suppl. 1), S680–S685 (2009).
- 3. . G-protein-coupled receptors in CNS: a potential therapeutic target for intervention in neurodegenerative disorders and associated cognitive deficits. Cells 9(2), 506 (2020).
- 4. Characterization of the G protein-coupled receptor family SREB across fish evolution. Sci. Rep. 11(1), 12066 (2021).
- 5. . The repertoire of G-protein-coupled receptors in fully sequenced genomes. Mol. Pharmacol. 67(5), 1414–1425 (2005).
- 6. . Trends in GPCR drug discovery: new agents, targets and indications. Nat. Rev. Drug Discov. 16(12), 829–842 (2017).
- 7. . Deletion of Gpr27 in vivo reduces insulin mRNA but does not result in diabetes. Sci. Rep. 10(1), 5629 (2020). • This publication investigated the role of GPR27 in glucose homeostasis.
- 8. Structure-activity relationships of agonists for the orphan G protein-coupled receptor GPR27. Eur. J. Med. Chem. 225, 113777 (2021). • The powerful agonists of the first identified class of GPR27 agonists were found to be soluble and free of cellular toxicity, according to the structure–activity relationships.
- 9. G protein-coupled receptors: structure- and function-based drug discovery. Signal Transduct. Target. Ther. 6(1), 7 (2021).
- 10. . Orphan GPCR research. Br. J. Pharmacol. 153(Suppl. 1), S339–S346 (2008).
- 11. . The G protein-coupled receptors deorphanization landscape. Biochem. Pharmacol. 153, 62–74 (2018).
- 12. An evolutionarily conserved G-protein coupled receptor family, SREB, expressed in the central nervous system. Biochem. Biophys. Res. Commun. 272(2), 576–582 (2000).
- 13. . Anatomical profiling of G protein-coupled receptor expression. Cell 135(3), 561–571 (2008).
- 14. . Gpr85, a novel member of the G-protein coupled receptor family, prominently expressed in the developing mouse cerebral cortex. Brain Res. Gene Expr. Patterns 1(1), 13–16 (2001).
- 15. . Characterizing ovarian gene expression during oocyte growth in Atlantic cod (Gadus morhua). Comp. Biochem. Physiol. D 9, 1–10 (2014).
- 16. Research resource: RNA-seq reveals unique features of the pancreatic β-cell transcriptome. Mol. Endocrinol. 26(10), 1783–1792 (2012).
- 17. . An siRNA screen in pancreatic beta cells reveals a role for Gpr27 in insulin production. PLoS Genet. 8(1), e1002449 (2012).
- 18. A conserved mRNA expression profile of SREB2 (GPR85) in adult human, monkey, and rat forebrain. Brain Res. Mol. Brain Res. 138(1), 58–69 (2005).
- 19. Probing cell type-specific functions of Gi in vivo identifies GPCR regulators of insulin secretion. J. Clin. Invest. 117(12), 4034–4043 (2007).
- 20. Genetic deletion of gpr27 alters acylcarnitine metabolism, insulin sensitivity, and glucose homeostasis in zebrafish. FASEB J. 34(1), 1546–1557 (2020).
- 21. . Constitutive activity among orphan class-A G protein coupled receptors. PLoS One 10(9), e0138463 (2015).
- 22. Identification and molecular docking studies for novel inverse agonists of SREB, super conserved receptor expressed in brain. Genes Cells 21(7), 717–727 (2016). • The discovery of new non-selective inverse agonists for the super-conserved receptors expressed in the brain family.
- 23. Activation of the orphan G protein–coupled receptor GPR27 by surrogate ligands promotes β-arrestin 2 recruitment. Mol. Pharmacol. 91(6), 595–608 (2017). • The first GPR27 agonists were discovered in this study, and they are selective versus the closely related receptors GPR85 and GPR173, respectively.
- 24. . Constitutive G protein coupling profiles of understudied orphan GPCRs. PLoS One 16(4), e0247743 (2021).
- 25. Defining a novel role for the Pdx1 transcription factor in islet β-cell maturation and proliferation during weaning. Diabetes 66(11), 2830–2839 (2017).
- 26. . Chronic activation of a designer G(q)-coupled receptor improves β cell function. J. Clin. Invest. 123(4), 1750–1762 (2013).
- 27. High-throughput discovery of novel developmental phenotypes. Nature 537(7621), 508–514 (2016).
- 28. . Islet G-protein coupled receptors: therapeutic potential for diabetes. Curr. Opin. Pharmacol. 37, 24–28 (2017).
- 29. . Neuronal orphan G-protein coupled receptor proteins mediate plasmalogens-induced activation of ERK and Akt signaling. PLoS One 11(3), e0150846 (2016).
- 30. The activation of GPR27 increases cytosolic L-lactate in 3T3 embryonic cells and astrocytes. Cells 11(6), 1009 (2022).
- 31. Development of novel potent ligands for GPR85, an orphan G protein-coupled receptor expressed in the brain. Genes Cells
doi:10.1111/gtc.12931 (2022). • The GPR85 inverse agonist was investigated for its role in potassium channel opening. - 32. . The brain-specific G-protein coupled receptor GPR85 with identical protein sequence in man and mouse maps to human chromosome 7q31. Biochim. Biophys. Acta 1493(1–2), 269–272 (2000).
- 33. . Cloning and localization of rgpr85 encoding rat G-protein-coupled receptor. Biochem. Biophys. Res. Commun. 298(4), 613–618 (2002).
- 34. . Adult neurogenesis: from precursors to network and physiology. Physiol. Rev. 85(2), 523–569 (2005).
- 35. . Computational influence of adult neurogenesis on memory encoding. Neuron 61(2), 187–202 (2009).
- 36. . Neurogenesis in the adult hippocampus. Cell Tissue Res. 331(1), 243–250 (2008).
- 37. . MR-based in vivo hippocampal volumetrics: 2. Findings in neuropsychiatric disorders. Mol. Psychiatry 10(2), 160–184 (2005).
- 38. Stress, depression and hippocampal apoptosis. CNS Neurol. Disord. Drug Targets 5(5), 531–546 (2006).
- 39. A heterogeneity-based genome search meta-analysis for autism-spectrum disorders. Mol. Psychiatry 11(1), 29–36 (2006).
- 40. Transcriptomic analysis of autistic brain reveals convergent molecular pathology. Nature 474(7351), 380–384 (2011).
- 41. Conduct disorder and ADHD: evaluation of conduct problems as a categorical and quantitative trait in the international multicentre ADHD genetics study. Am. J. Med. Genet. B Neuropsychiatr. Genet. 147b(8), 1369–1378 (2008).
- 42. . Translocation breakpoint at 7q31 associated with tics: further evidence for IMMP2L as a candidate gene for Tourette syndrome. Eur. J. Hum. Genet. 19(6), 634–639 (2011).
- 43. . PDZ domains – common players in the cell signaling. Acta Biochim. Pol. 50(4), 985–1017 (2003).
- 44. The association of GPR85 with PSD-95–neuroligin complex and autism spectrum disorder: a molecular analysis. Mol. Autism 6, 17 (2015).
- 45. Effect of schizophrenia risk-associated alleles in SREB2 (GPR85) on functional MRI phenotypes in healthy volunteers. Neuropsychopharmacology 38(2), 341–349 (2013).
- 46. . Sex differences in the responses of the human amygdala. Neuroscientist 11(4), 288–293 (2005).
- 47. . The effect of anticipation and the specificity of sex differences for amygdala and hippocampus function in emotional memory. Proc. Natl Acad. Sci. USA 103(38), 14200–14205 (2006).
- 48. . Sex influences on the neurobiology of learning and memory. Learn. Mem. 16(4), 248–266 (2009).
- 49. . Sex differences in schizophrenia. Int. Rev. Psychiatry 22(5), 417–428 (2010).
- 50. SREB2/GPR85, a schizophrenia risk factor, negatively regulates hippocampal adult neurogenesis and neurogenesis-dependent learning and memory. Eur. J. Neurosci. 36(5), 2597–2608 (2012).
- 51. The evolutionarily conserved G protein-coupled receptor SREB2/GPR85 influences brain size, behavior, and vulnerability to schizophrenia. Proc. Natl. Acad. Sci. USA. 105(16), 6133–6138 (2008). • This study revealed that SREB2 has a role in regulating brain size, influencing a variety of behaviors and perhaps predicting schizophrenia vulnerability.
- 52. Phoenixin-20 prevents ox-LDL-induced attachment of monocytes to human aortic endothelial cells (HAECs): a protective implication in atherosclerosis. ACS Chem. Neurosci. 12(6), 990–997 (2021). • This study revealed PNX-20′s potential role as an anti-atherosclerotic treatment.
- 53. . The protective effects of phoenixin-14 against lipopolysaccharide-induced inflammation and inflammasome activation in astrocytes. Inflamm. Res. 69(8), 779–787 (2020).
- 54. . Phoenixin 20 promotes neuronal mitochondrial biogenesis via CREB–PGC-1α pathway. J. Mol. Histol. 51(2), 173–181 (2020).
- 55. . Phoenixin-20 suppresses lipopolysaccharide-induced inflammation in dental pulp cells. Chem. Biol. Interact. 318, 108971 (2020).
- 56. Effect of the neuropeptide phoenixin and its receptor GPR173 during folliculogenesis. Reproduction 158(1), 25–34 (2019).
- 57. . Phoenixin– a pleiotropic gut-brain peptide. Int. J. Mol. Sci. 19(6), 1726 (2018). • This study has proven GPR173 to be linked to mediating hypothalamus activity and being the putative receptor for the pleiotropic peptide phoenixin.
- 58. . Phoenixin-20 stimulates mRNAs encoding hypothalamo-pituitary-gonadal hormones, is pro-vitellogenic, and promotes oocyte maturation in zebrafish. Sci. Rep. 10(1), 6264 (2020).
- 59. A novel reproductive peptide, phoenixin. J. Neuroendocrinol. 25(2), 206–215 (2013).
- 60. Phoenixin: a novel peptide in rodent sensory ganglia. Neuroscience 250, 622–631 (2013).
- 61. . Brain–gut–microbiota axis – mood, metabolism and behaviour. Nat. Rev. Gastroenterol. Hepatol. 14(2), 69–70 (2017).
- 62. . Phoenixin activates immortalized GnRH and kisspeptin neurons through the novel receptor GPR173. Mol. Endocrinol. 30(8), 872–888 (2016).
- 63. Hypothalamic action of phoenixin to control reproductive hormone secretion in females: importance of the orphan G protein-coupled receptor Gpr173. Am. J. Physiol. Regul. Integr. Comp. Physiol. 311(3), R489–R496 (2016).
- 64. . Phoenixin(smim20), a gene coding for a novel reproductive ligand, is expressed in the brain of adult zebrafish. Gene Expr. Patterns 39, 119164 (2021).
- 65. Levels of the neuropeptide phoenixin-14 and its receptor GRP173 in the hypothalamus, ovary and periovarian adipose tissue in rat model of polycystic ovary syndrome. Biochem. Biophys. Res. Commun. 528(4), 628–635 (2020).
- 66. Phoenixin as a new target in the development of strategies for endometriosis diagnosis and treatment. Biomedicines 9(10), 1427 (2021).
- 67. Novel regulator of vasopressin secretion: phoenixin. Am J Physiol Regul Integr Comp Physiol 314(4), R623–R628 (2018).
- 68. . A novel regulator of thirst behavior: phoenixin. Am J Physiol Regul Integr Comp Physiol 318(6), R1027–R1035 (2020).
- 69. . GPR173 agonist phoenixin 20 promotes osteoblastic differentiation of MC3T3-E1 cells. Aging (Albany NY) 13(4), 4976–4985 (2020).
- 70. . Pre-metazoan origin of neuropeptide signalling. Mol. Biol. Evol.
doi:10.1093/molbev/msac051 (2022). - 71. . Epidemiology of atherosclerosis and the potential to reduce the global burden of atherothrombotic disease. Circ. Res. 118(4), 535–546 (2016).
- 72. . Oxidative stress and lectin-like ox-LDL-receptor LOX-1 in atherogenesis and tumorigenesis. Antioxid. Redox Signal 15(8), 2301–2333 (2011).
- 73. . Blood and aqueous humor phoenixin, endocan and spexin in patients with diabetes mellitus and cataract with and without diabetic retinopathy. Peptides 150, 170728 (2022).