Data from the initial funding period of the current award indicate that increased cardiac AC type 6 (AC6), a dominant AC isoform expressed in mammalian cardiac myocytes, has protean beneficial effects on the failing left ventricle (LV). These effects, so consistent in a variety of species and pathophysiological models, must be reconciled with the dire consequences on the heart of 2-adrenergic receptor stimulation and elevations in intracellular cAMP. Logic would dictate that either: a) cAMP is not bad for the heart after all-that it is something else which leads to poor outcomes when cAMP levels are increased in the failing heart;or b) increased AC6 has beneficial cardiac effects independent of cAMP, which counterbalance its expected deleterious effects. Using pharmacological inhibitors and other approaches, our data suggest that many of the beneficial effects of increased cardiac AC6 expression do not require increased cAMP generation. Because of the inherent limitations of studies using pharmacological inhibition, we generated a catalytically inactive AC6 mutant (AC6mut) molecule by substitution of Ala for Asp at position 426 in the catalytic core. This AC6mut does not generate cAMP, but retains the cellular distribution pattern and favorable signaling effects associated with AC6. These data indicate that the beneficial effects of AC6, at least in part, are independent of cAMP. We propose now to conduct a series of experiments, both in vitro and in vivo, to establish unassailable mechanisms for the beneficial effects. The ideal inotrope would increase cardiac contractile function in the absence of 2AR stimulation or cAMP generation, improve Ca2+ handling, reduce adverse remodeling and apoptosis, and have favorable electrophysiological properties. With the exception of its effects on cAMP generation, increased cardiac AC6 expression accomplishes many of these features, and the FDA has approved an NHLBI-funded trial of AC6 gene transfer in patients with severe CHF (ClinicalTrials.gov NCT00787059). However, there are four reasons to conduct studies of the cAMP-incompetent AC6 mutant: 1) to obtain definitive mechanistic data on whether AC6's beneficial cardiac effects are unrelated to cAMP generation;2) to conduct structure-activity studies of the AC6mut molecule by generating a variety of AC6mut fragment molecules and to determine the effects of subcellular targeting of the AC6mut fragments on Ca2+ handling and Akt activation;3) to determine the cardiac effects of AC6mut in vivo and 4) to generate a suitable long term regulated expression vector encoding the optimal AC6mut fragment, and conduct preclinical gene transfer studies to determine its efficacy and safety in improving function of the failing heart. The studies are designed to uncover mechanisms, but also to have clinical relevance. For example, in clinical heart failure, agents that increase cardiac contractile function by increasing cAMP levels have failed to prolong survival. The AC6mut, based on preliminary data, preserves the beneficial effects seen with AC6, but does so in the absence of increased cAMP production, thus fulfilling features of a potentially ideal therapeutic inotrope.
We have modified adenylyl cyclase type 6 (AC6), a molecule that regulates cardiac function. This modified AC6 (AC6mut) preserves the beneficial effects on the heart that are seen with AC6, but does so in the absence of increased cAMP production. In patients with heart failure, drugs that improve heart function by increasing cAMP levels have failed to reduce mortality. Our studies will determine if this modified AC6 molecule will be a suitable heart failure treatment.
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