MAPKAPK-2 modulates p38-MAPK localization and small heat shock protein phosphorylation but does not mediate the injury associated with p38-MAPK activation during myocardial ischemia

MAPKAPK-2 (MK2) is a protein kinase activated downstream of p38-MAPK which phosphorylates the small heat shock proteins HSP27 and alpha B crystallin and modulates p38-MAPK cellular distribution. p38-MAPK activation is thought to contribute to myocardial ischemic injury; therefore, we investigated MK2 effects on ischemic injury and p38 cellular localization using MK2-deficient mice (KO). Immunoblotting of extracts from Langendorff-perfused hearts subjected to aerobic perfusion or global ischemia or reperfusion showed that the total and phosphorylated p38 levels were significantly lower in MK2(-/-) compared to MK2(+/+) hearts at baseline, but the ratio of phosphorylated/total p38 was similar. These results were confirmed by cellular fractionation and immunoblotting for both cytosolic and nuclear compartments. Furthermore, HSP27 and alpha B crsytallin phosphorylation were reduced to baseline in MK2(-/-) hearts. On semiquantitative immunofluorescence laser confocal microscopy of hearts during aerobic perfusion, the mean total p38 fluorescence was significantly higher in the nuclear compared to extranuclear (cytoplasmic, sarcomeric, and sarcolemmal compartments) in MK2(+/+) hearts. However, although the increase in phosphorylated p38 fluorescence intensity in all compartments following ischemia in MK2(+/+) hearts was lost in MK2(-/-) hearts, it was basally elevated in nuclei of MK2(-/-) hearts and was similar to that seen during ischemia in MK2(+/+) hearts. Despite these differences, similar infarct volumes were recorded in wild-type MK2(+/+) and MK2(-/-) hearts, which were decreased by the p38 inhibitor SB203580 (1 mu M) in both genotypes. In conclusion, p38 MAPK-induced myocardial ischemic injury is not modulated by MK2. However, the absence of MK2 perturbs the cellular distribution of p38. The preserved nuclear distribution of active p38 MAPK in MK2(-/-) hearts and the conserved response to SB203580 suggests that activation of p38 MAPK may contribute to injury independently of MK2.