Age-associated failure of efficient cardiac proteostatic adaptations to chronic cAMP-stress is associated with accelerated heart aging

Dysregulated proteostasis, leading to accumulation of misfolded proteins, electron-dense aggregates (lipofuscin, LF), preamyloid oligomers (PAOs), and proteotoxic stress is a hallmark of aging. We investigated how efficiently proteostatic adaptations to chronic cardiac cyclic adenosine monophosphate (cAMP)-dependent stress change with aging in mice harboring marked, cardiac-specific over-expression of adenylyl cyclase VIII (TG ^AC8 ). We assessed protein quality control (PQC) mechanisms: ubiquitin proteasome system (UPS), autophagic flux via macroautophagy, and mitophagy in left ventricles (LVs) of TG ^AC8 and wild type littermates (WT) at 3-4 months and at 17-21 months of age. At 3-4 months of age TG ^AC8 mice exhibited markers of increased autophagic flux, measured by levels of microtubule-associated protein 1 light chain 3 (LC3), p62, and their phospho-forms in TG ^AC8 LV; cathepsin L1 activity was also significantly increased. In addition, canonical mitophagy signaling was enhanced, as receptors PARKIN, p62 ^S405 and p62 ^S351 were all upregulated, confirming a more efficient proteostasis in TG ^AC8 at 3-4 months vs WT. In advanced age, however, the PQC mechanisms were overwhelmed by proteotoxic stress, manifested in insufficient proteasome activity and an unbalanced autophagic flux (accelerated for markers such as LC3A in the context of a slower overall flux), leading to an increase in the accumulation of protein aggregates (increased ratio of insoluble/soluble protein fractions). Although both canonical (PARKIN, p62 ^S405 and p62 ^S351 receptors) and non-canonical (FKBP8 receptor) mitophagy signaling were upregulated in advanced age in TG ^AC8 , mitophagy was markedly impaired and mitochondrial dysfunction increased. Accumulation of LF bodies, of brownish-to-black pigments, and of LC3 ⁺ and p62 ⁺ -inclusions of aberrant sizes, of desmin cardiac preamyloid oligomers (PAOs) and of cleaved desmin, tagged for ubiquitination, were all increased in TG ^AC8 compared to young TG ^AC8 . In contrast, the rate of protein synthesis and levels of soluble aggregates were reduced in aged vs young TG ^AC8 , a sign of “normal” aging. Thus, increased proteostatic mechanisms maintain cardiac health in TG ^AC8 in youth (3-4 months), but long-term exposure to chronic cardiac stress, imposed by sustained activation of the AC/cAMP/PKA/Ca ²⁺ signaling axis, results in severely dysregulated proteostasis in TG ^AC8 vs WT mice, associated with proteostatic insufficiency and increased cardiomyopathy that leads to accelerated cardiac aging.