Targeting Troponin I Phosphorylation for Precision Treatment of Diastolic Dysfunction

BackgroundDefective cardiac relaxation (diastolic dysfunction) is prevalent in heart failure, particularly in the context of diabetes, obesity, hypertension, and ageing, and is associated with increased mortality. Yet no effective therapy directly targets this abnormal relaxation. Phosphorylation of troponin I by cAMP-dependent protein kinase A (PKA) promotes relaxation, but global PKA activation has widespread, undesirable effects. Here we investigated whether selectively enhancing troponin I phosphorylation could improve relaxation without engaging the broader PKA signalling network. MethodsTo resolve cAMP signalling at subcellular scales, we combined real-time cAMP measurements with FRET-based genetically encoded reporters targeted to defined nanodomains, alongside biochemical and genetic approaches. Protein-protein interaction surfaces were mapped using peptide-array technology, which guided the design of a disruptor peptide to displace specific interactions. The peptides effects on cardiomyocyte function were assessed in vitro using rodent and human cardiomyocytes with biochemical assays, real-time imaging, and work-loop analysis. In vivo effects were evaluated through echocardiography and haemodynamic measurements. ResultsWe identified a previously unrecognized regulatory mechanism in which the cAMP-hydrolyzing enzyme PDE4D9 interacts with troponin I and restricts local cAMP levels within a nanometre-scale domain, thereby selectively controlling troponin I phosphorylation. We found that PDE4D9-troponin I association is markedly increased in cardiac disease in both rodents and humans. A peptide that displaces PDE4D9 from troponin I selectively enhances troponin I phosphorylation, accelerates cardiomyocyte relaxation, and prevents diastolic dysfunction in a mouse model of heart failure. ConclusionsThese findings reveal a highly specific regulatory nanodomain governing troponin I phosphorylation and points to a first-in-class, mechanism-based therapeutic strategy with the potential to address a major unmet contributor to heart failure. Clinical PerspectiveO_ST_ABSWhat Is New?C_ST_ABSO_LIWe identify a previously unrecognized, nanometre-scale regulatory mechanism in which PDE4D9 binds troponin I to locally suppress cAMP and limit troponin I phosphorylation. C_LIO_LIWe demonstrates that the PDE4D9-troponin I interaction is markedly upregulated in failing hearts in both rodents and humans, which can mechanistically explain the impaired cardiac relaxation. C_LIO_LIWe show that this interaction is druggable: a rationally designed peptide can selectively displace PDE4D9, restore localized cAMP-PKA signalling, and normalize myocardial relaxation without global cAMP perturbation. C_LI What Are the Clinical Implications?O_LIDiastolic dysfunction, a major cause of heart failure with no targeted treatments, may be amenable to therapies that modulate cAMP signalling with subcellular precision rather than global pathway modulation. C_LIO_LISelective disruption of PDE4D9-troponin I binding offers a mechanism-based approach with potential to restore diastolic function while minimizing off-target effects typical of GPCR-directed therapies. C_LIO_LIMore broadly, this work establishes a platform for targeting localized signalling domains, a concept that may extend beyond cardiovascular disease to other pathological conditions in which dysregulated cAMP contributes to dysfunction. C_LI