title: Age dependent regulation of bone-mass and renal function by the MEPE ASARM-motif.
abstract: BACKGROUND
  Mice with null mutations in matrix extracellular phosphoglycoprotein (MEPE) have increased bone mass, increased trabecular density and abnormal cancellous bone (MN-mice). These defects worsen with age and MEPE overexpression induces opposite effects. Also, genome wide association studies show that MEPE plays a major role in bone mass. We hypothesized that the conserved C-terminal MEPE ASARM-motif is chiefly responsible for regulating bone mass and trabecular structure.
  
  
  METHODS
  To test our theory we overexpressed C-terminal ASARM-peptide in MN-mice using the Col1α1 promoter (MNAt-mice). We then compared the bone and renal phenotypes of the MNAt-mouse with the MN-mouse and the X-linked hypophosphatemic rickets mouse (HYP). The HYP mouse overexpresses ASARM-peptides and is defective for the PHEX gene.
  
  
  RESULTS
  The MN-mouse developed increased bone mass, bone strength and trabecular abnormalities that worsened markedly with age. Defects in bone formation were chiefly responsible with suppressed sclerostin and increased active β-catenin. Increased uric acid levels also suggested that abnormalities in purine-metabolism and a reduced fractional excretion of uric acid signaled additional renal transport changes. The MN mouse developed a worsening hyperphosphatemia and reduced FGF23 with age. An increase in the fractional excretion of phosphate (FEP) despite the hyperphosphatemia confirms an imbalance in kidney-intestinal phosphate regulation. Also, the MN mice showed an increased creatinine clearance suggesting hyperfiltration. A reversal of the MN bone-renal phenotype changes occurred with the MNAt mice including the apparent hyperfiltration. The MNAt mice also developed localized hypomineralization, hypophosphatemia and increased FGF23.
  
  
  CONCLUSIONS
  The C-terminal ASARM-motif plays a major role in regulating bone-mass and cancellous structure as mice age. In healthy mice, the processing and release of free ASARM-peptide are chiefly responsible for preserving normal bone and renal function. Free ASARM-peptide also affects renal mineral phosphate handling by influencing FGF23 expression. These findings have implications for understanding age-dependent osteoporosis, unraveling drug-targets and developing treatments.
authors.affiliation: The Kidney Institute, Kansas University Medical Center, Kansas City, KS, USA.
authors.forename: Lesya V
authors.initials: LV
authors.lastname: Zelenchuk
authors.affiliation: The Kidney Institute, Kansas University Medical Center, Kansas City, KS, USA.
authors.forename: Anne-Marie
authors.initials: AM
authors.lastname: Hedge
authors.affiliation: The Kidney Institute, Kansas University Medical Center, Kansas City, KS, USA. Electronic address: prowe@kumc.edu.
authors.forename: Peter S N
authors.initials: PS
authors.lastname: Rowe
mesh_terms: D015502:Absorptiometry, Photon; D000818:Animals; D001842:Bone and Bones / Q000378:metabolism*; D004195:Disease Models, Animal; D016326:Extracellular Matrix Proteins / Q000235:genetics / Q000378:metabolism*; D000089703:Fibroblast Growth Factor-23; D006023:Glycoproteins / Q000235:genetics / Q000378:metabolism*; D015151:Immunoblotting; D007150:Immunohistochemistry; D007668:Kidney / Q000378:metabolism*; D007677:Kidney Function Tests; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D010024:Osteoporosis / Q000235:genetics / Q000378:metabolism*; D010750:Phosphoproteins / Q000235:genetics / Q000378:metabolism*; D016133:Polymerase Chain Reaction
chemical_list: D016326:Extracellular Matrix Proteins; C000717428:Fgf23 protein, mouse; D006023:Glycoproteins; C518716:Mepe protein, mouse; D010750:Phosphoproteins; D000089703:Fibroblast Growth Factor-23
keywords: ASARM-peptide; FGF23; MEPE; Mineralization; Rickets; Sclerostin