CLCN7 POINT MUTANT INHIBITED Cl-EXTRUSION, RESUTLING IN OSTEOPETROSIS

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ClC7 Cl^- channels are crucial for osteoclastic bone resorption since they play an important role for acid secretion. Mice with a deficient ClC7 gene (CLCN7) have impaired osteoclastic Cl extrusion during bone resorption, resulting in severe osteopetrosis. It also has been reported that autosomal osteopetrosis type II (ADO II) results from heterozygous mutations in CLCN7. Although extracellular acidification is known to induce ClC7 Cl^- current in CLCN7 transfected oocytes, other characteristics of this acid-induced Cl^- current as well as effects of mutant CLCN7 in ADO II on osteoclastic Cl^- extrusion activity are unknown. The aim of present study was to characterize the ClC7 Cl^- currents cloned from mammalian osteoclasts and to clarify the effect of point mutations found in ADO II patients on Cl^- extrusion activity. Extracellular acidification (pH 6.0) evoked small native Cl^- currents in mouse osteoclasts. Transient expression of wild type human CLC7 in HEK293 cells induced significant Cl^- currents compared to mock-transfected cells. These acid-activated Cl^- currents were independent of intracellular acidification or [Ca^<2+>]i increase. In contrast, CLCN7 mutant gene expression in HEK293 cells did not display these acid-activated Cl currents. Furthermore, RAW 264.7 with the CLCN7 mutation at G215R demonstrated no change in osteoclast differentiation potential but a significant decrease in bone resorption activity compared to wild type CLCN7 expression. In summary, our results indicate that a CLCN7 mutation at the G215R locus associated with ADO II results in suppression of Cl extrusion activity during osteoclastic bone resorption.