*4.4. Activation of Cyclic Guanosine Monophosphate (cGMP) and PKG Decreases Titin Passive Tension and Represents a Viable Therapy for Treatment of Heart Failure*

In addition to metformin and NRG-1, sildenafil has been proposed as a therapy for diastolic dysfunction due to its effects on phosphorylation of titin. Sildenafil inhibits phosphodiesterase-5 leading to increased nitric oxide (NO) and increased cGMP signaling. cGMP is known to activate PKG, which phosphorylates the N2Bus element of titin resulting in softening of titin and decreased myocardial stiffness [88]. In a dog model of diastolic dysfunction, treatment with sildenafil increased phosphorylation of the N2Bus element via increased activation of cGMP resulting in improved diastolic function compared to untreated controls [89]. Based on these results, sildenafil was studied in patients with diastolic dysfunction to determine whether cGMP dependent PKG phosphorylation of titin can improve clinical outcomes. Unfortunately, in this placebo controlled, randomized trial, treatment with sildenafil did not significantly improve clinical, laboratory, or echocardiographic outcomes in patients [90]. Interestingly patients who received sildenafil showed no significant increase in plasma cGMP levels suggesting that the medication did not have the desired therapeutic effect on altering PKG dependent phosphorylation. Activating cGMP to improve heart failure has been pursued further with novel therapies including vericiguat which is an oral soluble guanylate cyclase stimulator that generates cGMP. This compound was studied in patients with systolic dysfunction in a high quality randomized clinical trial. Compared to placebo, treatment with vericiguat reduced hospitalizations and cardiovascular death among patients with systolic dysfunction [91]. This represents a significant breakthrough for clinical management of heart failure with a medication that utilizes a novel mechanism of action. Although increasing levels of cGMP is likely to have many effects for improving heart failure, the effects on phosphorylation of the titin N2Bus element is likely to be significant. Further study is warranted to determine mechanistically how vericiguat alters cardiomyocytes and how it affects titin. Based on the mechanism of action, vericiguat is an appealing medication to treat diastolic dysfunction due to activation of PKG, phosphorylation of the TTN N2Bus element and softening of the titin protein. Given the clinical burden of diastolic dysfunction, and the lack of effective therapeutic medications, vericiguat should be studied in patients with diastolic dysfunction. Indeed, there are at least two clinical trials of vericiguat ongoing in patients with diastolic dysfunction [92,93].

Post-translational modifications of titin including phosphorylation and dephosphorylation of specific elements contribute to development of heart failure and represent targets for therapies to treat heart failure. Further study of post-translational modifications is needed to better understand and treat heart failure. In addition to phosphorylation, there are likely to be many other post-translational modifications of titin that exist and should be studied.
