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

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, B56alpha and B56gamma1, were assessed using adenovirus-driven expression of epitope-tagged (hemagglutinin, HA) in cultured neonatal and adult rat ventricular myocytes. Confocal imaging revealed that HA-B56alpha was excluded from the nucleus and decorated striated structures, whereas HA-B56gamma1 was principally found in the nucleus. Precise immunolabeling studies showed that B56gamma1 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 B56gamma1 increased whole cell PP2A activity by 40% when measured in cell extracts. Finally, B56gamma1 did not alter global gene expression or expression of hypertrophic gene markers such as alpha-skeletal actin. However, morphometric analyses of confocal images revealed that B56gamma1 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.