Disease-associated mutations prevent GPR56-collagen III interaction.
ABSTRACT: GPR56 is a member of the adhesion G protein-coupled receptor (GPCR) family. Mutations in GPR56 cause a devastating human brain malformation called bilateral frontoparietal polymicrogyria (BFPP). Using the N-terminal fragment of GPR56 (GPR56(N)) as a probe, we have recently demonstrated that collagen III is the ligand of GPR56 in the developing brain. In this report, we discover a new functional domain in GPR56(N), the ligand binding domain. This domain contains four disease-associated mutations and two N-glycosylation sites. Our study reveals that although glycosylation is not required for ligand binding, each of the four disease-associated mutations completely abolish the ligand binding ability of GPR56. Our data indicates that these four single missense mutations cause BFPP mostly by abolishing the ability of GPR56 to bind to its ligand, collagen III, in addition to affecting GPR56 protein surface expression as previously shown.
Project description:GPR56 is a member of the adhesion G protein-coupled receptor (GPCR) family. Despite the importance of GPR56 in brain development, where mutations cause a devastating human brain malformation called bilateral frontoparietal polymicrogyria (BFPP), the signaling mechanism(s) remain largely unknown. Like many other adhesion GPCRs, GPR56 is cleaved via a GPCR autoproteolysis-inducing (GAIN) domain into N- and C-terminal fragments (GPR56N and GPR56C); however, the biological significance of this cleavage is elusive. Taking advantage of the recent identification of a GPR56 ligand and the presence of BFPP-associated mutations, we investigated the molecular mechanism of GPR56 signaling. We demonstrate that ligand binding releases GPR56N from the membrane-bound GPR56C and triggers the association of GPR56C with lipid rafts and RhoA activation. Furthermore, one of the BFPP-associated mutations, L640R, does not affect collagen III-induced lipid raft association of GPR56. Instead, it specifically abolishes collagen III-mediated RhoA activation. Together, these findings reveal a novel signaling mechanism that may apply to other members of the adhesion GPCR family.
Project description:GPR56 is a member of the family of adhesion G-protein-coupled receptors that have a large extracellular region containing a GPS (G-protein proteolytic site) domain. Loss-of-function mutations in the GPR56 gene cause a specific human brain malformation called bilateral frontoparietal polymicrogyria (BFPP). BFPP is a radiological diagnosis and its histopathology remains unclear. This study demonstrates that loss of the mouse Gpr56 gene leads to neuronal ectopia in the cerebral cortex, a cobblestone-like cortical malformation. There are four crucial events in the development of cobblestone cortex, namely defective pial basement membrane (BM), abnormal anchorage of radial glial endfeet, mislocalized Cajal-Retzius cells, and neuronal overmigration. By detailed time course analysis, we reveal that the leading causal events are likely the breaches in the pial BM. We show further that GPR56 is present in abundance in radial glial endfeet. Furthermore, a putative ligand of GPR56 is localized in the marginal zone or overlying extracellular matrix. These observations provide compelling evidence that GPR56 functions in regulating pial BM integrity during cortical development.
Project description:PURPOSE:Bilateral frontoparietal polymicrogyria (BFPP) has been reported in sporadic patients and in recessive pedigrees. Eleven mutations in GPR56, a gene encoding an evolutionarily dynamic G-protein-coupled receptor, have been identified in 29 patients from 18 families. The clinical features of BFPP include severe mental retardation, motor and language impairment, and epilepsy. No detailed description of the epilepsy is available for the patients reported to date. We report three consanguineous families in which four affected individuals with BFPP and GPR56 mutations had Lennox-Gastaut syndrome. METHODS:Family studies, brain magnetic resonance imaging (MRI), electroencephalography (EEG)-video recordings, and mutation analysis. RESULTS:In Family 1, with one affected proband, we found an R565W change in the second extracellular loop of GPR56, involving a highly conserved aminoacidic residue. In Family 2, with one affected proband, we found an R79X change affecting the protein N-terminus and predicted to cause a premature truncation with loss of the G-protein-coupled receptor proteolytic site. In family 3, with two affected siblings, we found an R33P substitution in the protein N-terminus, involving a highly conserved aminoacidic residue. Epilepsy, present in all four patients, had started between ages 1 and 8 years, with infantile spasms in one patient and with de novo Lennox-Gastaut syndrome in the remaining three. All patients had Lennox-Gastaut syndrome when last observed, at ages 13 to 32 years. DISCUSSION:Several genes, when mutated, can cause malformations of cortical development that have been associated with the Lennox-Gastaut syndrome. BFPP caused by GPR56 mutations represents an additional, although rare, genetically determined cause of Lennox-Gastaut syndrome.
Project description:Loss of function mutations in GPR56, which encodes a G protein-coupled receptor, cause a specific human brain malformation called bilateral frontoparietal polymicrogyria (BFPP). Studies from BFPP postmortem brain tissue and Gpr56 knockout mice have previously showed that GPR56 deletion leads to breaches in the pial basement membrane (BM) and neuronal ectopias during cerebral cortical development. Since ?3?1 integrin also plays a role in pial BM assembly and maintenance, we evaluated whether it functions together with GPR56 in regulating the same developmental process. We reveal that loss of ?3 integrin enhances the cortical phenotype associated with Gpr56 deletion, and that neuronal overmigration through a breached pial BM occurs earlier in double knockout than in Gpr56 single knockout mice. These observations provide compelling evidence of the synergism of GPR56 and ?3?1 integrin in regulating the development of cerebral cortex.
Project description:Mutations in GPR56, an orphan G-protein-coupled receptor (GPCR), cause bilateral frontoparietal polymicrogyria (BFPP), a disorder characterized by mental retardation, seizures, motor developmental delay, and ataxia. BFPP patients have structural abnormalities of the cerebral cortex, cerebellum, and pons. To shed light on the function of GPR56 and the anatomical and behavioral defects underlying BFPP, we analyzed the cerebellum of mice lacking this GPCR. Gpr56(-/-) mice display a severe malformation of the rostral cerebellum that develops perinatally. Defects involve fusion of adjacent lobules, disrupted layering of neurons and glia, and fragmentation of the pial basement membrane. At the age of defect onset, GPR56 expression is restricted specifically to developing granule cells in the rostral cerebellum, suggesting that GPR56 regulates properties of these cells. Indeed, granule cells from the rostral region of perinatal Gpr56(-/-) cerebella show loss of adhesion to extracellular matrix molecules of the pial basement membrane. Interference RNA-mediated knockdown of GPR56 recapitulates the loss of adhesion seen in knock-outs, and reexpression of GPR56 rescues the adhesion defect in knock-out granule cells. Loss of GPR56 does not affect cell proliferation, migration, or neurite outgrowth. These studies establish a novel role for GPR56 in the adhesion of developing neurons to basal lamina molecules and suggest that this adhesion is critical for maintenance of the pia and proper cerebellar morphogenesis.
Project description:Mutations in GPR56, a member of the adhesion G protein-coupled receptor family, cause a human brain malformation called bilateral frontoparietal polymicrogyria (BFPP). Magnetic resonance imaging (MRI) of BFPP brains reveals myelination defects in addition to brain malformation. However, the cellular role of GPR56 in oligodendrocyte development remains unknown. Here, we demonstrate that loss of Gpr56 leads to hypomyelination of the central nervous system in mice. GPR56 levels are abundant throughout early stages of oligodendrocyte development, but are downregulated in myelinating oligodendrocytes. Gpr56-knockout mice manifest with decreased oligodendrocyte precursor cell (OPC) proliferation and diminished levels of active RhoA, leading to fewer mature oligodendrocytes and a reduced number of myelinated axons in the corpus callosum and optic nerves. Conditional ablation of Gpr56 in OPCs leads to a reduced number of mature oligodendrocytes as seen in constitutive knockout of Gpr56. Together, our data define GPR56 as a cell-autonomous regulator of oligodendrocyte development.
Project description:Bilateral frontoparietal polymicrogyria (BFPP, MIM 606854) is a heterogeneous autosomal recessive disorder of abnormal cortical lamination, leading to moderate-to-severe intellectual disability (ID), seizure disorder, and motor difficulties, and caused by mutations in the G protein-coupled receptor 56 ( GPR56 ) gene. Twenty-eight mutations in 40 different families have been reported in the literature. The clinical and neuroimaging phenotype is consistent in these cases. The BFPP cortex consists of numerous small gyral cells, with scalloping of the cortical-white matter junction. There are also associated white matter, brain stem, and cerebellar changes. GPR56 is a member of an adhesion G protein-coupled receptor family with a very long N-terminal stalk and seven transmembrane domains. In this study, we identified three families from Pakistan, ascertained primarily for ID, with overlapping approximately 1 Mb region (chr16:56,973,335-57,942,866) of homozygosity by descent, including 24 RefSeq genes. We found three GPR56 homozygous mutations, using next-generation sequencing. These mutations include a substitutional variant, c.1460T?>?C; p.L487P, (chr16:57693480 T?>?C), a 13-bp insertion causing the frameshift and truncating mutation, p.Leu269Hisfs*21 (NM_005682.6:c.803_804insCCATGGAGGTGCT; Chr16: 57689345_57689346insCCATGGAGGTGCT), and a truncating mutation c.1426C?>?T; p.Arg476* (Chr16:57693446C?>?T). These mutations fully segregated with ID in these families and were absent in the Exome Aggregation Consortium database that has approximately 8,000 control samples of South Asian origin. Two of these mutations have been reported in ClinVar database, and the third one has not been reported before. Three families from Pakistan with GPR56 mutations have been reported before. With the addition of our findings, the total number of mutations reported in Pakistani patients now is six. These results increase our knowledge regarding the mutational spectrum of the GPR56 gene causing BFPP/ID.
Project description:Bilateral frontoparietal polymicrogyria is an autosomal recessive inherited human brain malformation with abnormal cortical lamination. The affected cortex appears to consist of numerous small gyri, with scalloping of the cortical-white matter junction. There are associated white matter, brain stem, and cerebellar changes. Affected individuals manifest mental retardation, language impairment, motor developmental delay, and seizure disorder. GPR56 is the causative gene. Here we report a novel missense mutation of GPR56, E496K, identified in a consanguineous pedigree with bilateral frontoparietal polymicrogyria. GPR56 protein is cleaved at the G-protein-coupled receptor proteolytic site into an N- and a C-terminal fragment, named GPR56(N) and GPR56(C), respectively. E496K is located in GPR56(C). Further biochemical studies reveal that this mutation affects GPR56(C) cell surface expression similar to the effect of a previously reported mutation, R565W. These results provide further insights into how GPR56 mutation causes neurologic disease.
Project description:AIMS:G-protein coupled receptor 56 (GPR56) is the most abundant islet-expressed G-protein coupled receptor, suggesting a potential role in islet function. This study evaluated islet expression of GPR56 and its endogenous ligand collagen III, and their effects on ?-cell function. METHODS:GPR56 and collagen III expression in mouse and human pancreas sections was determined by fluorescence immunohistochemistry. Effects of collagen III on ?-cell proliferation, apoptosis, intracellular calcium ([Ca2+]i) and insulin secretion were determined by cellular BrdU incorporation, caspase 3/7 activities, microfluorimetry and radioimmunoassay, respectively. The role of GPR56 in islet vascularisation and innervation was evaluated by immunohistochemical staining for CD31 and TUJ1, respectively, in pancreases from wildtype (WT) and Gpr56-/- mice, and the requirement of GPR56 for normal glucose homeostasis was determined by glucose tolerance tests in WT and Gpr56-/- mice. RESULTS:Immunostaining of mouse and human pancreases revealed that GPR56 was expressed by islet ?-cells while collagen III was confined to the peri-islet basement membrane and islet capillaries. Collagen III protected ?-cells from cytokine-induced apoptosis, triggered increases in [Ca2+]i and potentiated glucose-induced insulin secretion from WT islets but not from Gpr56-/- islets. Deletion of GPR56 did not affect glucose-induced insulin secretion in vitro and it did not impair glucose tolerance in adult mice. GPR56 was not required for normal islet vascularisation or innervation. CONCLUSION:We have demonstrated that collagen III improves islet function by increasing insulin secretion and protecting against apoptosis. Our data suggest that collagen III may be effective in optimising islet function to improve islet transplantation outcomes, and GPR56 may be a target for the treatment of type 2 diabetes.
Project description:Polymicrogyria is a cerebral cortical malformation that is grossly characterized by excessive cortical folding and microscopically characterized by abnormal cortical layering. Although polymicrogyria appears to have one or more genetic causes, no polymicrogyria loci have been identified. Here we describe the clinical and radiographic features of a new genetic form of polymicrogyria and localize the responsible gene. We studied two consanguineous Palestinian pedigrees with an autosomal recessive form of bilateral frontoparietal polymicrogyria (BFPP), using linkage analysis. Five affected children had moderate-to-severe mental retardation, developmental delay, and esotropia, and four of the five affected children developed seizures. Brain magnetic-resonance imaging revealed polymicrogyria that was most prominent in the frontal and parietal lobes but involved other cortical areas as well. A genomewide linkage screen revealed a single locus that was identical by descent in affected children in both families and showed a single disease-associated haplotype, suggesting a common founder mutation. The locus for BFPP maps to chromosome 16q12.2-21, with a minimal interval of 17 cM. For D16S514, the maximal pooled two-point LOD score was 3.98, and the maximal multipoint LOD score was 4.57. This study provides the first genetic evidence that BFPP is an autosomal recessive disorder and serves as a starting point for the identification of the responsible gene.