Inhibitors of protein phosphatase-2A: topography and subcellular localization

Abstract

The mRNA and protein expressions of I 1 PP2A and I 2 PP2A, the two inhibitors of protein phosphatase 2A (PP2A) were investigated in adult rat brain. The rat brain and human brain inhibitors showed similar molecular weights by Western blots. The cDNA probes for human brain I 1 PP2A and I 2 PP2A readily hybridized with the corresponding mRNAs of rat brain inhibitors in Northern blots. We detected 3.7 and 2.1 kb transcripts of I 1 PP2A and 2.9 and 2.0 kb transcripts of I 2 PP2A in rat brain. In situ hybridization revealed that the mRNAs of the two inhibitors were mainly localized in neurons. Strong expression of both I 1 PP2A and I 2 PP2A mRNAs were observed in the olfactory bulb, hippocampal pyramidal and dentate granule cell layers, and cerebellar Purkinje cell, granular and molecular layers. Moderate expression of I 1 PP2A and I 2 PP2A mRNAs were observed in the cerebral cortex, caudate putamen, thalamus, hypothalamus, amygdala and pontine nucleus. The expression of I 1 PP2A and I 2 PP2A and as well as of PP-2A was also investigated by immunohistochemistry using antibodies to each protein. The distribution patterns of the two inhibitor proteins were similar to those of their corresponding mRNAs and to the expression of PP-2A. While PP-2A was localized to neuronal perikarya, I 1 PP2A was observed both in the neuronal cytoplasm and the nucleus. I 2 PP2A had mainly nuclear localization but it could also be seen in the neuronal cytoplasm. All three proteins were also expressed in the neuropil. These studies suggest that PP-2A activity is probably regulated by I 1 PP2A and I 2 PP2A in the adult mammalian central nervous system, and that these inhibitors are conserved between rat and human brains.