Na+/H+ Exchanger Regulatory Factor 1 (NHERF1) Directly Regulates Osteogenesis

Li Liu,Veronica Alonso,Lida Guo,Irina Tourkova,Sarah E Henderson,Alejandro J Almarza,Peter A Friedman,Harry C Blair

doi:10.1074/jbc.m112.422766

Abstract

Bone formation requires synthesis, secretion, and mineralization of matrix. Deficiencies in these processes produce bone defects. The absence of the PDZ domain protein Na(+)/H(+) exchange regulatory factor 1 (NHERF1) in mice, or its mutation in humans, causes osteomalacia believed to reflect renal phosphate wasting. We show that NHERF1 is expressed by mineralizing osteoblasts and organizes Na(+)/H(+) exchangers (NHEs) and the PTH receptor. NHERF1-null mice display reduced bone formation and wide mineralizing fronts despite elimination of phosphate wasting by dietary supplementation. Bone mass was normal, reflecting coordinated reduction of bone resorption and formation. NHERF1-null bone had decreased strength, consistent with compromised matrix quality. Mesenchymal stem cells from NHERF1-null mice showed limited osteoblast differentiation but enhanced adipocyte differentiation. PTH signaling and Na(+)/H(+) exchange were dysregulated in these cells. Osteoclast differentiation from monocytes was unaffected. Thus, NHERF1 is required for normal osteoblast differentiation and matrix synthesis. In its absence, compensatory mechanisms maintain bone mass, but bone strength is reduced.

Highlights

The bone phenotype of Na؉/H؉ exchange regulatory factor 1 (NHERF1)-null mice was ascribed to indirect actions
In whole bone mRNA from wild-type mice, NHERF1 mRNA was confirmed by PCR (Fig. 1B), and NHERF1 mRNA was present in human osteoblasts
Because Na؉/H؉ exchangers (NHEs) mediates acid elimination in mineralizing osteoblasts [13] and renal phosphate wasting per se is insufficient to account for the bone phenotype of NHERF1-deficient mice or of humans harboring NHERF1 mutations, we hypothesized that NHERF1 directly regulates bone mineralization

Summary

Background

The bone phenotype of NHERF1-null mice was ascribed to indirect actions. Results: With dietary supplementation to maintain normal serum phosphate, NHERF1-deficient mice showed aberrant bone mineralization and decreased bone quality. Mice deficient in NHERF1 exhibit prominent renal phosphate wasting and skeletal abnormalities [11], as do patients harboring NHERF1 mutations [14] In both cases the bone pathology, which is characterized by severe osteomalacia with accumulation of nonmineralized osteoid, was interpreted as a secondary consequence of hypophosphatemia. Mice lacking sodium phosphate cotransporter 2a exhibit even greater losses of phosphate but have minimal bone changes [15,16,17] This discrepancy raised the issue that the skeletal phenotype of NHERF1-null mice might reflect direct consequences of the absence of NHERF1 in bone. To determine whether NHERF1 directly regulates mineralization, we maintained NHERF1-null mice on a diet supplemented with phosphorous to exclude secondary skeletal effects caused by renal phosphate wasting. Direct Effect of NHERF1 on Osteogenesis studies using mesenchymal stem cells showed that these deficiencies reflect abnormal differentiation and function of osteoblasts in the absence of NHERF1

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION