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First published online July 12, 2005
doi: 10.1242/10.1242/jcs.02446
Research Article |
1 Centro de Estudios Moleculares de la Célula, Instituto de Ciencias Biomédicas, Facultad de Medicina, Universidad de Chile, Independencia 1027, Santiago 7, Chile
2 Institut Fédératif de Neurobiologie Alfred Fessard, Laboratoire de Neurobiologie Cellulaire et Moléculaire, UPR 9040 CNRS, 91198 Gif-sur-Yvette CEDEX, France
3 Laboratorio de Fisiología de La Conducta, Facultad de Medicina, Universidad de Los Andes, Mérida 5101, Venezuela
4 Department of Physiology, Stritch School of Medicine, Loyola University Chicago, Maywood, Illinois 60153, USA
* Author for correspondence (e-mail: ejaimovi{at}med.uchile.cl)
Accepted 20 April 2005
Several lines of evidence indicate that increases in nuclear Ca2+ have specific biological effects that differ from those of cytosolic Ca2+, suggesting that they occur independently. The mechanisms involved in controlling nuclear Ca2+ signaling are both controversial and still poorly understood. Using hypotonic shock combined with mechanical disruption, we obtained and characterized a fraction of purified nuclei from cultured rat skeletal myotubes. Both immunoblot studies and radiolabeled inositol 1,4,5-trisphosphate [IP3] binding revealed an important concentration of IP3 receptors in the nuclear fraction. Immunofluorescence and immunoelectron microscopy studies localized type-1 and type-3 IP3 receptors in the nucleus with type-1 receptors preferentially localized in the inner nuclear membrane. Type-2 IP3 receptor was confined to the sarcoplasmic reticulum. Isolated nuclei responded to IP3 with rapid and transient Ca2+ concentration elevations, which were inhibited by known blockers of IP3 signals. Similar results were obtained with isolated nuclei from the 1B5 cell line, which does not express ryanodine receptors but releases nuclear Ca2+ in an IP3-dependent manner. Nuclear Ca2+ increases triggered by IP3 evoked phosphorylation of cAMP response element binding protein with kinetics compatible with sequential activation. These results support the idea that Ca2+ signals, mediated by nuclear IP3 receptors in muscle cells, are part of a distinct Ca2+ release component that originates in the nucleus and probably participates in gene regulation mediated by cAMP response element binding protein.
Key words: Skeletal muscle, Myonuclei, Inositol 1, 4, 5-trisphosphate receptors, Nuclear envelope, Transcription factors, Gene expression
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