Postnatal shifts in ischemic tolerance and cell survival signaling in murine myocardium

The immature heart is known to be resistant to ischemia-reperfusion (I/R) injury; however, key proteins engaged in phospho-dependent signaling pathways crucial to cell survival are not yet defined. Our goal was to determine the postnatal changes in myocardial tolerance to I/R, including baseline expression of key proteins governing I/R tolerance and their phosphorylation during I/R. Hearts from male C57Bl/6 mice (neonates, 2, 4, 8, and 12 wk of age, n = 6/group) were assayed for survival signaling/effectors [Akt, p38MAPK, glycogen synthase kinase-3β (GSK-3β), heat shock protein 27 (HSP27), connexin-43, hypoxia-inducible factor-1α (HIF-1α), and caveolin-3] and regulators of apoptosis (Bax and Bcl-2) and autophagy (LC3B, Parkin, and Beclin1). The effect of I/R on ventricular function was measured in isolated perfused hearts from immature (4 wk) and adult (12 wk) mice. The neonatal myocardium exhibits a large pool of inactive Akt; high phospho-activation of p38MAPK, HSP27 and connexin-43; phospho-inhibition of GSK-3β; and high expression of caveolin-3, HIF-1α, LC3B, Beclin1, Bax, and Bcl-2. Immature hearts sustained less dysfunction and infarction following I/R than adults. Emergence of I/R intolerance in adult vs. immature hearts was associated with complex proteomic changes: decreased expression of Akt, Bax, and Bcl-2; increased GSK-3β, connexin-43, HIF-1α, LC3B, and Bax:Bcl-2; enhanced postischemic HIF-1α, caveolin-3, Bax, and Bcl-2; and greater postischemic GSK-3β and HSP27 phosphorylation. Neonatal myocardial stress resistance reflects high expression of prosurvival and autophagy proteins and apoptotic regulators. Notably, there is high phosphorylation of GSK-3β, p38MAPK, and HSP27 and low phosphorylation of Akt (high Akt "reserve"). Subsequent maturation-related reductions in I/R tolerance are associated with reductions in Akt, Bcl-2, LC3B, and Beclin1, despite increased expression and reduced phospho-inhibition of GSK-3β.