Cardiac overexpression of the human 5-HT₄receptor in mice

Abstract

Serotonin (5-HT) exerts pleiotropic effects in the human cardiovascular system. Some of the effects are thought to be mediated via 5-HT4 receptors, which are expressed in the human atrium and in ventricular tissue. However, a true animal model to study these receptors in more detail has been hitherto lacking. Therefore, we generated, for the first time, a transgenic (TG) mouse with cardiac myocyte-specific expression of the human 5-HT4 receptor. RT-PCR and immunohistochemistry revealed expression of the receptor at the mRNA and protein levels. Stimulation of isolated cardiac preparations by isoproterenol increased phospholamban phosphorylation at Ser 16 and Thr 17 sites. 5-HT increased phosphorylation only in TG mice but not in wild-type (WT) mice. Furthermore, 5-HT increased contractility in isolated perfused hearts from TG mice but not WT mice. These effects of 5-HT could be blocked by the 5-HT 4 receptor-selective antagonist GR-125487. An intravenous infusion of 5-HT increased left ventricular contractility in TG mice but not in WT mice. Similarly, the increase in contractility by 5-HT in isolated cardiomyocytes from TG mice was accompanied by and probably mediated through an increase in L-type Ca 2ϩ channel current and in Ca 2ϩ transients. In intact animals, echocardiography revealed an inotropic and chronotropic effect of subcutaneously injected 5-HT in TG mice but not in WT mice. In isolated hearts from TG mice, spontaneous polymorphic atrial arrhythmias were noted. These findings demonstrate the functional expression of 5-HT4 receptors in the heart of TG mice, and a potential proarrhythmic effect in the atrium. Therefore, 5-HT4 receptorexpressing mice might be a useful model to mimic the human heart, where 5-HT 4 receptors are present and functional in the atrium and ventricle of the healthy and failing heart, and to investigate the influence of 5-HT in the development of cardiac arrhythmias and heart failure. serotonin; arrhythmia; transgenic mice; signal transduction MOST OF THE SEROTONIN (5-HT) in the blood originates from enterochromaffine cells of the gastrointestinal tract (53). 5-HT is released by these cells and is avidly taken up by platelets. Platelets seem to be the main source of 5-HT that influences the cardiovascular system. These influences include vasoconstriction, an increase in platelet aggregation, apoptosis of cardiac cells, augmentation in beating rate, the generation of arrhythmias (27), valvular heart disease (49), and positive inotropic and relaxant effects (for an overview, see Ref. 29).At present, seven groups of 5-HT-receptors have been distinguished (5-HT 1 -5-HT 7 ) (29). The 5-HT 4 receptor mediates the positive inotropic effect in humans (8,31,33,51). In the human atrium and ventricle, mRNAs of several splice variants of the 5-HT 4 receptor have been found (6, 2, 9).In isolated multicellular preparations from human atria, 5-HT exerts a positive inotropic effect and a relaxant (or lusitropic) effect (31, 33). These effects were accompanied by increases in cAMP content and ele...

Surprises from a cardiac 5-HT4 TG mouse: spontaneous atrial arrhythmias by endogenous 5-HT of atrial origin? Different mechanism of arrhythmias through 5-HT4 receptors and β-adrenoceptors?

“…In addition, they also demonstrated phosphorylation of PLB at Thr17, not a substrate for PKA but a substrate for calcium calmodulin kinase II (CaMKII). In hearts of 5-HT 4 TG mice, but not WT mice, 5-HT also increased the phosphorylation of PLB both at Ser16 and Thr 17 (Gergs et al 2010). Furthermore, basal phosphorylation of PLB at Thr17 in the absence of exogenous 5-HT in TG mice was increased in the ventricle of perfused hearts from TG mice and reduced by a 5-HT 4 receptor antagonist to levels of WT mice.…”

“…5-HT, but hardly isoprenaline, also caused arrhythmic Ca 2+ transients in venricular myocytes from TG hearts. Interesting, in isolated hearts from these TG hearts Gergs et al (2010) demonstrated episodes of spontaneous polymorphic atrial arrhythmias in the absence of exogenous 5-HT that resemble shortlasting episodes of atrial fibrillation.In an additional report of Gergs et al (2013) in the present issue of Naunyn-Schmiedeberg's Archives of Pharmacology, the authors studied the effects of 5-HT on paced left atria and spontaneously beating right atria from their TG mouse. As expected, 5-HT produced sinoatrial tachycardia and increases in left atrial force, left atrial arrhythmias, phosphorylation of PLB at Ser16 and arrhythmias, which were antagonised by 5-HT 4 receptor blockers.…”

“…5-HT, but hardly isoprenaline, also caused arrhythmic Ca 2+ transients in venricular myocytes from TG hearts. Interesting, in isolated hearts from these TG hearts Gergs et al (2010) demonstrated episodes of spontaneous polymorphic atrial arrhythmias in the absence of exogenous 5-HT that resemble shortlasting episodes of atrial fibrillation.…”

“…Rat ventricular 5-HT 4 receptor expression and function is induced in rats with ischaemic heart failure but not in normal rats (Qvigstad et al 2005), plausibly by reactivating a phenotype resembling late foetal development (Brattelid et al 2012). Gergs et al (2010) overexpressed human 5-HT 4a receptors in the heart of mice that normally do not express functional cardiac 5-HT 4 receptors. In these transgenic mice 5-HT produced sinoatrial tachycardia, increases in ventricular I Ca,L and Ca 2+ transients, positive inotropic and lusitropic effects in ventricle, associated with phosphorylation of phospholamban (PLB) at Ser16 in ventricular myocytes and Thr 17 in perfused working hearts.…”

“…An alternative mechanism for the spontaneously arrhythmias in both left and right atria of the TG mice (Gergs et al 2010(Gergs et al , 2013, which cannot completely be discarded, is that the conformational change of the overexpressed 5-HT 4 receptors allows spontaneous coupling of the receptors to biochemical cascades responsible for the arrhythmias. Constitutive 5-HT 4 receptor activity has been demonstrated at a recombinant 5-HT 4 receptor splice variant, transfected into HeLa cell, and GR113808 was an inverse agonist (Vilaró et al 2002).…”