Degradation of MEPE, DMP1, and release of SIBLING ASARM-peptides (minhibins): ASARM-peptide(s) are directly responsible for defective mineralization in HYP.
Mutations in PHEX (phosphate-regulating gene with homologies to endopeptidases on the X chromosome) and DMP1 (dentin matrix protein 1) result in X-linked hypophosphatemic rickets (HYP) and autosomal-recessive hypophosphatemic-rickets (ARHR), respectively. Specific binding of PHEX to matrix extracellular phosphoglycoprotein (MEPE) regulates the release of small protease-resistant MEPE peptides [acidic serine- and aspartate-rich MEPE-associated motif (ASARM) peptides]. ASARM peptides are potent inhibitors of mineralization (minhibins) that also occur in DMP1 [MEPE-related small integrin-binding ligand, N-linked glycoprotein (SIBLING) protein]. It is not known whether these peptides are directly responsible for the mineralization defect. We therefore used a bone marrow stromal cell (BMSC) coculture model, ASARM peptides, anti-ASARM antibodies, and a small synthetic PHEX peptide (SPR4; 4.2 kDa) to examine this. Surface plasmon resonance (SPR) and two-dimensional (1)H/(15)N nuclear magnetic resonance demonstrated specific binding of SPR4 peptide to ASARM peptide. When cultured individually for 21 d, HYP BMSCs displayed reduced mineralization compared with wild type (WT) (-87%, P < 0.05). When cocultured, both HYP and WT cells failed to mineralize. However, cocultures (HYP and WT) or monocultures of HYP BMSCs treated with SPR4 peptide or anti-ASARM neutralizing antibodies mineralized normally. WT BMSCs treated with ASARM peptide also failed to mineralize properly without SPR4 peptide or anti-ASARM neutralizing antibodies. ASARM peptide treatment decreased PHEX mRNA and protein (-80%, P < 0.05) and SPR4 peptide cotreatment reversed this by binding ASARM peptide. SPR4 peptide also reversed ASARM peptide-mediated changes in expression of key osteoclast and osteoblast differentiation genes. Western blots of HYP calvariae and BMSCs revealed massive degradation of both MEPE and DMP1 protein compared with the WT. We conclude that degradation of MEPE and DMP-1 and release of ASARM peptides are chiefly responsible for the HYP mineralization defect and changes in osteoblast-osteoclast differentiation.
Martin
Aline
A
Department of Internal Medicine, Division of Nephrology and Hypertension, The Kidney Institute, Kansas University Medical Center, 3901 Rainbow Boulevard, Kansas City, KS 66160, USA.
David
Valentin
V
Laurence
Jennifer S
JS
Schwarz
Patricia M
PM
Lafer
Eileen M
EM
Hedge
Anne-Marie
AM
Rowe
Peter S N
PS
D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D002113:Calcification, Physiologic; D016326:Extracellular Matrix Proteins / Q000378:metabolism*; D053098:Familial Hypophosphatemic Rickets / Q000378:metabolism*; D000089703:Fibroblast Growth Factor-23; D005346:Fibroblast Growth Factors / Q000235:genetics; D040181:Genetic Diseases, X-Linked; D006023:Glycoproteins / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D051379:Mice; D008969:Molecular Sequence Data; D010010:Osteoclasts / Q000502:physiology; D053402:PHEX Phosphate Regulating Neutral Endopeptidase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism*; D020349:Surface Plasmon Resonance
C513308:Dmp1 protein, mouse; D016326:Extracellular Matrix Proteins; D006023:Glycoproteins; C518716:Mepe protein, mouse; D010750:Phosphoproteins; D005346:Fibroblast Growth Factors; D000089703:Fibroblast Growth Factor-23; D053402:PHEX Phosphate Regulating Neutral Endopeptidase; C506232:Phex protein, mouse