Age dependent regulation of bone-mass and renal function by the MEPE ASARM-motif

Bone. 2015 Oct;79:131-42. doi: 10.1016/j.bone.2015.05.030. Epub 2015 Jun 4.

Abstract

Context: 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.

Design: 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.

Keywords: ASARM-peptide; FGF23; MEPE; Mineralization; Rickets; Sclerostin.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Absorptiometry, Photon
  • Animals
  • Bone and Bones / metabolism*
  • Disease Models, Animal
  • Extracellular Matrix Proteins / genetics
  • Extracellular Matrix Proteins / metabolism*
  • Fibroblast Growth Factor-23
  • Glycoproteins / genetics
  • Glycoproteins / metabolism*
  • Immunoblotting
  • Immunohistochemistry
  • Kidney / metabolism*
  • Kidney Function Tests
  • Mice
  • Mice, Inbred C57BL
  • Mice, Transgenic
  • Osteoporosis / genetics
  • Osteoporosis / metabolism*
  • Phosphoproteins / genetics
  • Phosphoproteins / metabolism*
  • Polymerase Chain Reaction

Substances

  • Extracellular Matrix Proteins
  • Fgf23 protein, mouse
  • Glycoproteins
  • Mepe protein, mouse
  • Phosphoproteins
  • Fibroblast Growth Factor-23