The research performed in our laboratory aims at understanding the normal and pathological function of the heart. Our research has evolved around the main functions of the cardiomyocyte: electrical activity, excitation-contraction coupling, contractility and energetics, and their integration in the whole heart. During the last years, this functional approach has been enlarged by exploring vascular tissue and enriched by biochemical, pharmacological, and molecular approaches, and in vivo experiments in the whole animal. Nowadays, it encompasses cardiovascular physiology, from molecular to integrated level.
At the cellular level, the contraction of cardiac muscle is a consequence of a transitory rise in intracellular free Ca2+ concentration ([Ca]i) during the course of the cardiac action potential. The rise in [Ca]i originates at the opening of membrane L-type Ca2+ channels. It is then amplified by the sarcoplasmic reticulum, and this triggers the mechanical response through the activity of contractile proteins. The force of contraction depends on the amount of Ca2+ in the cytoplasm, the Ca2+ sensitivity of the contractile proteins, and on the energetic metabolism. At the level of the organ, myocardial function depends not only on the activity of cardiac myocytes but also on signals generated by other cellular types. Together with calcium, second messengers such as cyclic nucleotides, and enzymatic cascades, these signals modulate in a coordinated manner the activity of different cellular compartments – sarcolemmal membrane, sarcoplasmic reticulum, mitochondria, myofilaments and nucleus – in response to the command of the organism. On more fundamental grounds, our activity is directed towards the functional study of these cellular compartments, the communications that they establish, and their integration in the whole organ.
We have shown that these compartments are interconnected at the energetic level. The specific binding of kinases (such as creatine kinase) to these compartments warrants a perfect balance between production, transfer and utilization of energy within the cell. This efficient functioning is achieved by high-energy bounds present in the vicinity of key sites of excitation-contraction (EC) coupling, and by energy synthesis, forming dynamic microcompartments. Any perturbation of this energetic balance is turned into a change in the force of contraction. The origin of the metabolic failure of the failing heart has been depicted and the alteration of the mitochondrial biogenesis transcription cascade has been shown to be involved in the pathogenesis of heart failure (HF). The project of Team 1 is aimed at understanding the alterations of the energetic balance in heart failure (HF) and the energy signalling pathways involved.
Dynamic microcompartments are also responsible for the specificity of action of hormonal signals on cardiac function. It is well established that different membrane receptors (b1-, b2-adrenergic, glucagon, histamine H2, PGE1, etc.) mediate different functional effects, even though they act via a common cAMP-signalling pathway. We have shown that these differences in their effects might partly result from dynamic cAMP compartments, resulting in cAMP-dependent kinase (PKA) phosphorylation of different molecular targets. Phosphodiesterases (PDE) play a determinant role in this subtle subcellular organization of cAMP, as well as cGMP concentrations. The project of Team 2 is to test the hypothesis that a disruption of such an organization induces a loss in targeting of cAMP, and contributes to the alteration of cardiac and vascular function in HF, as well as to find solution to restore it.
But cAMP is not acting exclusively via PKA. Team 3, which was led by Frank Lezoualc'h from 2006 to the end of 2010, has identified a new signalling cascade implicated in HF, which involves small G proteins and Epac as a cAMP-sensitive guanine nucleotide exchange factor.
Defective Ca2+ handling is a key contributor to the pathophysiology of heart failure (HF), not only in weakening heart contraction but also in the erratic heart beats causing sudden death. One of the cellular mechanisms for those life-threatening arrhythmias is triggered activity caused by either early or delayed after-depolarizations, both of which are commonly associated with intracellular Ca2+ mishandling. After the departure of Frank Lezoualc'h from the unit, a new Team 3, led by Ana Maria Gomez, has joined our laboratory in January 2011 to explore the role of Ca2+ in EC coupling as well as in arrhythmias and in activating transcription factors (excitation-transcription coupling).
Our three teams share common animal and cellular models (rat and mouse models of HF (aortic constriction, doxorubicin-induced cardiotoxicity; isolated cardiomyocytes from adult rats and mice; rat neonatal cardiac myocytes in primary culture; vascular smooth muscle cells) or cardiac muscle human biopsies, as well as experimental techniques (patch-clamp and fluorescence imaging on isolated cells; mechanical, energetic and biochemical measurements on skinned isolated cells and fibres; cardiac imaging of small animals using echocardiography; ECG recording in conscious small rodents by Holter telemetry; molecular biology and biochemistry).
Altogether, our projects aim at understanding physiological stimuli acting on cardiac function through the activity of membrane receptors, ion channels, cyclic nucleotides, energetic metabolism and intracellular compartments, both structural (contractile proteins, sarcoplasmic reticulum, mitochondria, nucleus) and dynamic. The characterization of the underlining signaling cascades is necessary for the identification of new therapeutic targets, which are explored within the the LabEx LERMIT. This project has both fundamental and therapeutic grounds, as it addresses the molecular and cellular mechanisms involved and their modifications during adaptations or pathological situations.