Natriuretic PEPtide SIgnalling in the HEART: Mechanisms, compartmentation and mitochondrial effects (PEPSI-HEART)

In patients with heart failure, the heart is unable to meet the circulatory requirements of the body, resulting in symptoms at different levels of activity, depending on the degree of heart failure. Heart failure is frequently caused by insufficient pumping function of the heart, but in about half of heart failure patients the problem is rather that the heart is stiff, resulting in impeded cardiac filling. This is called heart failure with preserved ejection fraction (HFpEF) and was previously also called diastolic heart failure. High blood pressure, metabolic syndrome (overweight and diabetes) and high age are risk factors, and drug therapy that improves the life expectancy for these patients is limited to one class of drug. The heart produces hormones called natriuretic peptides, of which there are three types, ANP, BNP and CNP, and which are produced in increasing amounts in heart failure. In addition to effects in the kidneys (increased sodium excretion) and blood vessels (relaxation of smooth muscle causing vasodilation), these hormones also affect the heart itself. They all increase the signalling molecule cyclic GMP (cGMP), but we have found that they still have different effects in the heart, where CNP, but not ANP or BNP, reduces the stiffness of cardiac muscle cells. This may alleviate the increased cardiac stiffness and thereby ameliorate HFpEF. In the project we will study the mechanisms of the selective effects of CNP compared to ANP and BNP. The aim is to understand how cGMP from different sources is localised and acts in the heart muscle cells. This may form the basis for novel future therapy of HFpEF. In the project, we have so far successfully developed novel FRET-based biosensors for cGMP and targeted these biosensors to selected intracellular proteins and mitochondria. We have started to study compartmented effects of CNP vs. ANP/BNP using these targeted biosensors, and the first results of these studies have been submitted for publication and are under revision.

Better treatment for heart failure with preserved ejection fraction (HFpEF; diastolic heart failure: DHF) is needed. HFpEF, which is more common in women and elderly and caused by e.g. hypertension and diabetes, results from increased cardiac stiffness impairing filling of the heart and thus congestion and reduced cardiac output. We found that the natriuretic peptide CNP, unlike the related ANP and BNP, enhances relaxation and reduces stiffness of the cardiac myocytes through changes in calcium handling and phosphorylation of the cardiac protein titin, a molecular spring regulating cardiac compliance. CNP activates natriuretic peptide receptor B (NPR-B), generating cyclic GMP (cGMP) that mediates these cardiac effects. In this project, we seek to understand the mechanisms of the unique effects of CNP, compared to other cGMP-enhancing stimuli like ANP, BNP or nitric oxide. We will develop fluorescent sensors to visualise cGMP at specific intracellular locations, combined with methods to measure cardiac stiffness both in the whole heart, isolated heart muscle and isolated cardiac myocytes. Using localized cGMP sensors we also found that CNP increases cGMP in the cardiac mitochondria and will examine the mechanisms and functional consequences of this CNP-mediated increase in cardiac mitochondrial cGMP. The project should clarify the molecular and subcellular mechanisms for the selective effects of CNP on cardiac relaxation as well as the effects of CNP-mediated increase in mitochondrial cGMP.