Regulatory mechanisms of ion transporters expressed in ruffled-border of osteoclasts

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We studied what kind of ion transport had been activated in ruffled-border of resorbing osteoclasts. First, the ionic currents of resorbing rat osteoclast with resorption lacuna formed on calcium phosphate courted cover glass were compared with that of non-resorbing osteoclast cultured on the glass cover glass by using whole-cell patch clamp technique. Then, an outward rectifying Cl-current had been remarkably activated in the resorbing osteoclasts. Next, we investigated the Cl-transporter had been expressed in mouse resorbing osteoclasts using RT-PCR method to clarify the Cl-transport activated during bone resorption ClC3 and ClC7 (outward rectifying Cl-channel) and KCC1 (K^+/Cl^-cotransporter) were significantly expressed in osteoclasts. The existences of ClC7 and KCC1 in the cell membrane of mouse osteoclasts were confirmed by immunochemical staining and Western blot analysis. On the other hand, ClC3 was mainly localized in intracellular organelle. KCC inhibitors and Cr-channels blockers increased [Cl^-]_i and [H^+]_i in resorbing osteoclasts, suggesting that the suppression of Cl-extrusion via KCC and Cl-channels leads to reduced H^+ extrusion activity. The combination of both inhibitors greatly suppressed these extrusion activities. KCC inhibitors and Cl-channel blockers also decreased osteoclastic bone resorption in our pit area essay. Furthermore, KCC1 antisense oligo-nucleotides and siRNA suppressed osteoclastic pit formation as well as treatment of ClC7 inhibitors. These results indicate that ClC7 channel and K^+/Cl^- cotransporter-1 expressed in mouse osteoclasts acts as Cl-extruder and plays an important role for H^+ extrusion during bone resorption.