Abstract P203: Nuclear PKA Compartmentation Manages Hypertrophic Responses to β-Adrenergic Signaling

Abstract

Protein kinase A (PKA) directly mediates several cardiac behaviors in response to β-adrenergic stimulation. However, it remains unclear how PKA makes context-dependent decisions to select between contractile and hypertrophic β-adrenergic signaling responses. We have previously shown PKA activation by isoproterenol is spatially heterogeneous in neonatal rat cardiac myocytes, indicating subcellular β-adrenergic signaling compartmentation. We hypothesize hypertrophic β-adrenergic signaling responses are specifically regulated by nuclear compartmentation of PKA activity. We expressed spatially targeted FRET reporters for PKA activity and found PKA responses to isoproterenol to be slower and less sensitive in the nucleus than in the cytosol. To investigate mechanisms underlying these differences, we constructed a biochemically mechanistic computational model of cytosolic and nuclear PKA activity in rat cardiac myocytes. Using this model, we found that nuclear PKA dynamics are limited by diffusional transport barriers, whereas nuclear PKA sensitivity to isoproterenol is limited by protein kinase inhibitor (PKI) inhibition. Moreover, our model predicts these differences in compartmented PKA activity may help PKA select between different substrates, sensitive to the temporal characteristics of β-adrenergic stimulation. In the model, short, transient isoproterenol treatment fully phosphorylates phospholamban while CREB is only partially phosphorylated. In contrast, long, sustained isoproterenol treatment fully phosphorylates both phospholamban and CREB. To test the model prediction that compartmented PKA can regulate different cardiac behaviors, we over-expressed PKA targeted to the cytosol and nucleus. Myocytes over-expressing PKA in the nucleus were 24.8% larger than control (n = 908 cells, p = 0.0002), while myocytes over-expressing PKA in the cytosol were the same size as control cells (n = 987 cells, p = 0.9971). Together, these results highlight an important role for nuclear PKA compartmentation in selecting hypertrophic responses to β-adrenergic stimulation.