A B56 regulatory subunit of protein phosphatase 2A localizes to nuclear speckles in cardiomyocytes

1 Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, Maryland; and 2 Department of Pathology, Medical School, and 3 Department for Molecular Genetics, Research Institute for Microbial Diseases, Osaka University, Suita, Osaka, Japan Submitted 22 December 2004 ; accepted in final form 7 March 2005 Protein phosphatase 2A (PP2A) is widely distributed in heart tissues, yet its precise cellular functions are poorly understood. This study is based on the notion that PP2A action is governed by interactions of the core enzyme with B targeting/regulatory subunits. The subcellular localizations of two B subunits, B56 and B56 1, were assessed using adenovirus-driven expression of epitope-tagged (hemagglutinin, HA) in cultured neonatal and adult rat ventricular myocytes. Confocal imaging revealed that HA-B56 was excluded from the nucleus and decorated striated structures, whereas HA-B56 1 was principally found in the nucleus. Precise immunolabeling studies showed that B56 1 was concentrated in intranuclear structures known as nuclear speckles, macromolecular structures that accumulate transcription and splicing factors. Western blot analyses revealed that overexpression of either B subunit had no effect on the levels of other PP2A subunits in cultured neonatal cardiac cells. However, overexpression of only B56 1 increased whole cell PP2A activity by 40% when measured in cell extracts. Finally, B56 1 did not alter global gene expression or expression of hypertrophic gene markers such as -skeletal actin. However, morphometric analyses of confocal images revealed that B56 1 alters the dynamic assembly/disassembly process of nuclear speckles in heart cells. These studies provide new insight into mechanisms of PP2A targeting in the subnuclear architecture in cardiomyocytes and into the role of this phosphatase in nuclear signaling. Address for reprint requests and other correspondence: T. B. Rogers, Dept. of Biochemistry and Molecular Biology, Univ. of Maryland School of Medicine, 108 N. Greene St., Baltimore, MD 21201 (E-mail: trogers{at}som.umaryland.edu ) Related articles in AJP - Heart: Corrigendum AJP - Heart 2005 289: H2753. [Full Text]