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Transcriptional Activity of Nuclei in Multinucleated Osteoclasts and Its Modulation by Calcitonin

Laboratoire de Biologie Moléculaire et Cellulaire de l’Ecole Normale Supérieure de Lyon (P.B., F.S., P.J., I.M.-G.), Unité Mixte de Recherche 5665 Centre National de la Recherche Scientifique/Ecole Normale Supérieure, Institut National de la Recherche Agronomique (INRA) 913, 69364 Lyon Cédex 07, France; and Laboratorie BCM (C.B.), INRA Domaine de Vilvert, 78352 Jouy-en-Josas cedex, France

Address all correspondence and requests for reprints to: Irma Machuca-Gayet, Laboratoire de Biologie Moléculaire et Cellulaire de l’Ecole Normale Supérieure de Lyon, Unité Mixte de Recherche 5665, Centre National de la Recherche Scientifique/Ecole Normale Supérieure, Institut National de la Recherche Agronomique 913, 46 allée d’Italie, 69364 Lyon Cedex 07, France. E-mail: . Irma.Machuca{at}ens-lyon.fr

The function of osteoclasts is to digest the calcified bone matrix. Osteoclasts, together with myotubes, are among the rare examples of multinucleated cells found in higher vertebrates, resulting from the fusion of mononucleated progenitors belonging to the monocyte/macrophage lineage. So far, no information is available about function and transcriptional activity of multiple nuclei in osteoclasts. We have used a run-on technique to visualize RNA synthesis in individual nucleus. We provide the first evidence that nuclei of resorbing osteoclasts, isolated from chick embryo long bones, or differentiated in vitro from murine spleen cells in presence of RANKL and macrophage-colony stimulating factor, are all transcriptionally active. Nevertheless, if transcriptional activity is the same for all the nuclei within a cell, its level varies between osteoclasts: osteoclasts with highly active nuclei are always associated with resorption pits. We found that global transcription activity of resorbing osteoclasts seeded on calcified matrix is down-regulated after 5-h treatment with calcitonin, which transiently blocks resorption. This effect is reversible because calcitonin removal led to nuclear transcription activation. These results indicate a strong correlation between transcription and resorption. Finally, our data indicate that the resorption pit surface is linearly related to the nuclei number per osteoclast, strongly suggesting that functional advantage of osteoclast multinucleation is to improve resorption efficiency.

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