Brain Advance Access originally published online on October 10, 2007
Brain 2007 130(12):3275-3284; doi:10.1093/brain/awm247
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Mechanisms underlying intranuclear rod formation
1Institute for Neuromuscular Research, 2Oncology Research Unit, Children's Hospital at Westmead, 3Discipline of Paediatrics and Child Health, University of Sydney, 4Muscle Development Unit, Children's Medical Research Institute, Sydney, NSW 2145, Australia and 5Department of Biochemistry and Molecular Biology, Colorado State University, Fort Collins, Colorado 80523-1870, USA
Correspondence to: Prof. Kathryn N. North, Children's Hospital at Westmead, Locked Bag 4001, Westmead, NSW 2145, Australia E-mail: kathryn{at}chw.edu.au
Specific mutations within the 
-skeletal actin gene (ACTA1) result in intranuclear rod myopathy (IRM), characterized by rod-like aggregates containing actin and -actinin-2 inside the nucleus of muscle cells. The mechanism leading to formation of intranuclear aggregates containing sarcomeric proteins and their impact on cell function and contribution to disease pathogenesis is unknown. In this study, we transfected muscle and non-muscle cells with mutants of -skeletal actin (Val163Leu, Val163Met) associated with intranuclear rod myopathy. By live-cell imaging we demonstrate that nuclear aggregates of actin form within the nuclear compartment, rather than entering the nucleus after formation in the cytoplasm, and are highly motile and dynamic structures. Thus, the nuclear environment supports the polymerization of actin and the movement and coalescence of the polymerized actin into larger structures. We show that the organization of actin within these aggregates is influenced by the binding of -actinin, and that -actinin is normally present in the nucleus of muscle and non-muscle cells. Furthermore, we demonstrate that, under conditions of cell stress (cytoskeletal disruption and ATP depletion), WT skeletal actin forms aggregates within the nucleus that are similar in morphology to those formed by the mutant actin, suggesting a common pathogenic mechanism for aggregate formation. Finally, we show that the presence of intranuclear actin aggregates significantly decreases the mitotic index and hence impacts on the function of the cell. Intranuclear aggregates thus likely contribute to the pathogenesis of muscle weakness in intranuclear rod myopathy.
Key Words:
intranuclear rod myopathy; nuclear aggregates; -skeletal actin; -actinin
Abbreviations:
ACTA1, (-skeletal actin gene; CFTD, congenital fibre-type disproportion; IRM, intranuclear rod myopathy; LMB, leptomycin B; NES, nuclear export sequences; NLS, nuclear localization sequence.
Received May 24, 2007. Revised August 14, 2007. Accepted September 17, 2007.