Brain expression of the calcineurin inhibitor RCAN1 (Adapt78)

RCAN1 (Adapt78) is an endogenous inhibitor of calcineurin, an important intracellular phosphatase that mediates many cellular responses to calcium. RCAN1 is expressed in multiple organs, especially heart, skeletal muscle and brain. In brain, it is thought to be important due to its strong expression, developmental regulation, abundance of target protein (calcineurin), and putative links to multiple brain-related disorders. Surprisingly, however, few studies have examined RCAN1 protein expression here. This has led to some confusion in the field over the exact nature and cell-type expression of isoform 4, the more studied of the two major RCAN1 protein isoforms, in brain. Here we characterize RCAN1 brain isoforms in more detail by assessing their size and distribution under conditions of calcium elevation, a hallmark of the isoform 4 response, and using rodent models to allow for more expanded analyses. We find that the 25-29 kDa version of this protein, reported in many non-brain studies, is indeed also present in neurons, and most observable after calcium induction. We also observe that expression of isoform 4 is not specific to neurons, as both microglia and astrocyte cells in culture exhibit a strong induction of isoform 4 protein following calcium stress that is not observable in non-stressed tissue sections. Isoform 1 expression is also observable in a primary glial cell-type (rat microglia). Finally, our observations confirm previous reports of low or non-detectable constitutive isoform expression in non-stressed glia, and of a larger sized, RCAN1 antibody-interacting species. These studies extend and complement previous studies on RCAN isoforms toward better understanding the role of RCAN1 in brain function and as a potential new target for treating calcineurin-related brain disorders.