ASARM peptides: PHEX-dependent and -independent regulation of serum phosphate

Abstract

Increased acidic serine aspartate-rich MEPE-associated motif (ASARM) peptides cause mineralization defects in X-linked hypophosphatemic rickets mice (HYP) and "directly" inhibit renal phosphate uptake in vitro. However, ASARM peptides also bind to phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) and are a physiological substrate for this bone-expressed, phosphateregulating enzyme. We therefore tested the hypothesis that circulating ASARM peptides also "indirectly" contribute to a bone-renal PHEXdependent hypophosphatemia in normal mice. Male mice (n ϭ 5; 12 wk) were fed for 8 wk with a normal phosphorus and vitamin D 3 diet (1% P i diet) or a reduced phosphorus and vitamin D3 diet (0.1% Pi diet). For the final 4 wk, transplantation of mini-osmotic pumps supplied a continuous infusion of either ASARM peptide (5 mg·day Ϫ1 ·kg Ϫ1 ) or vehicle. HYP, autosomal recessive hypophosphatemic rickets (ARHR), and normal mice (no pumps or ASARM infusion; 0.4% P i diet) were used in a separate experiment designed to measure and compare circulating ASARM peptides in disease and health. ASARM treatment decreased serum phosphate concentration and renal phosphate cotransporter (NPT2A) mRNA with the 1% P i diet. This was accompanied by a twofold increase in serum ASARM and 1,25-dihydroxy vitamin D 3 [1,25 (OH)2D3] levels without changes in parathyroid hormone. For both diets, ASARM-treated mice showed significant increases in serum fibroblast growth factor 23 (FGF23; ϩ50%) and reduced serum osteocalcin (Ϫ30%) and osteopontin (Ϫ25%). Circulating ASARM peptides showed a significant inverse correlation with serum P i and a significant positive correlation with fractional excretion of phosphate. We conclude that constitutive overexpression of ASARM peptides plays a "component" PHEXindependent part in the HYP and ARHR hypophosphatemia. In contrast, with wild-type mice, ASARM peptides likely play a bone PHEX-dependent role in renal phosphate regulation and FGF23 expression. They may also coordinate FGF23 expression by competitively modulating PHEX/DMP1 interactions and thus bone-renal mineral regulation. matrix extracellular phosphoglycoprotein; dentin matrix protein-1; mineralization; osteomalacia; SIBLING proteins; fibroblast growth factor 23; X-linked hypophosphatemic rickets; 1,25 dihydroxy vitamin D3SIBLING PROTEINS MAP to a tightly clustered region on chromosome 4q in humans and chromosome 5 in mice (50). Members include matrix extracellular phosphoglycoprotein (MEPE), dentin matrix protein-1 (DMP1), osteopontin (OPN), bone sialoprotein, enamelin, dentin sialo phosphoprotein, and statherin (19,50). Key features of several SIBLINGs are an acidic serine aspartate-rich MEPE-associated motif (ASARM motif) (50) involved in bone and teeth mineralization (49, 50). Among these proteins, MEPE was cloned from a tumor resected from a patient with tumor-induced osteomalacia (TIO) and hypophosphatemia (50). Increased expression of MEPE occurs in TIO, X-linked hypophosphatemic rickets (HYP), and autosomal r...

Amelogenesis Imperfecta and Other Biomineralization Defects in Fam20a and Fam20c Null Mice

“…14,67 In addition, increased acidic serine aspartate-rich MEPEassociated motif (ASARM) peptides cause the mineralization defects in HYP mice. 20,74 Thus, inactivating mutations of either Phex or Dmp1 produce similar intrinsic bone mineralization defects as well as increased FGF23 expression by osteocytes. Comparative analysis of the Phex-mutant HYP mouse and Dmp1 null mouse showed that PHEX and DMP1 both regulate bone mineralization by inhibiting FGF23 production.…”

Regulation and Function of the FGF23/Klotho Endocrine Pathways

“…The ASARM peptide, a motif in MEPE and DMP1, is a substrate for PHEX in vitro (1). Additional data suggest that accumulation of ASARM as a consequence of inactivation of Phex can impair mineralization (129) and phosphate homeostasis (29). We have shown that ASARM binds to and inhibits PHEX activity against a synthetic substrate in vitro (115), but it is unlikely that ASARM from MEPE is responsible for stimulating FGF23 in Hyp, since ablation of MEPE fails to alter FGF23 expression in Hyp mice (30, 112).…”

Do ASARM peptides play a role in nephrogenic systemic fibrosis?

“…Male wild-type (WT) or mutant X-linked hypophosphatemic rickets mice (Hyp) were used for the study (n ϭ 6) and maintained on a 1% phosphorus and 2.41 U/g vitamin-D 3 diet (Harlan Teklad Rodent Diet 8604, Indianapolis, IN) (18). As reported previously, Hyp mice had major increases in circulating ASARM peptide compared with WT (6,13,17,18).…”