Influence of phosphodiesterases and cGMP on cAMP generation and on phosphorylation of phospholamban and troponin I by 5-HT4 receptor activation in porcine left atrium

Cardiac Roles of Serotonin (5-HT) and 5-HT-Receptors in Health and Disease

Serotonin acts solely via 5-HT₄-receptors to control human cardiac contractile function. The effects of serotonin via 5-HT₄-receptors lead to positive inotropic and chronotropic effects, as well as arrhythmias, in the human heart. In addition, 5-HT₄-receptors may play a role in sepsis, ischaemia, and reperfusion. These presumptive effects of 5-HT₄-receptors are the focus of the present review. We also discuss the formation and inactivation of serotonin in the body, namely, in the heart. We identify cardiovascular diseases where serotonin might play a causative or additional role. We address the mechanisms which 5-HT₄-receptors can use for cardiac signal transduction and their possible roles in cardiac diseases. We define areas where further research in this regard should be directed in the future, and identify animal models that might be generated to this end. Finally, we discuss in what regard 5-HT₄-receptor agonists or antagonists might be useful drugs that could enter clinical practice. Serotonin has been the target of many studies for decades; thus, we found it timely to summarise our current knowledge here.

Phosphodiesterases and Compartmentation of cAMP and cGMP Signaling in Regulation of Cardiac Contractility in Normal and Failing Hearts

Cardiac contractility is regulated by several neural, hormonal, paracrine, and autocrine factors. Amongst these, signaling through β-adrenergic and serotonin receptors generates the second messenger cyclic AMP (cAMP), whereas activation of natriuretic peptide receptors and soluble guanylyl cyclases generates cyclic GMP (cGMP). Both cyclic nucleotides regulate cardiac contractility through several mechanisms. Phosphodiesterases (PDEs) are enzymes that degrade cAMP and cGMP and therefore determine the dynamics of their downstream effects. In addition, the intracellular localization of the different PDEs may contribute to regulation of compartmented signaling of cAMP and cGMP. In this review, we will focus on the role of PDEs in regulating contractility and evaluate changes in heart failure.

Cardiovascular effects of cisapride and prucalopride on human 5-HT4 receptors in transgenic mice

Cisapride and prucalopride act as 5-HT₄ receptor agonists. As a part of our ongoing effort to study the utility of a transgenic (TG) mouse model overexpressing cardiac 5-HT₄ receptors, we assessed the extent to which we could recapitulate cisapride and prucalopride agonists. Contractile studies were performed using isolated left and right atrial preparations of TG mice showing cardiac-specific human 5-HT_4a receptor expression and those of their wild-type (WT) littermates. 5-Hydroxytryptamine (5-HT), cisapride, and prucalopride exerted concentration-dependent positive inotropic effects in the left atrial preparations of TG mice. Moreover, 5-HT induced concentration-dependent arrhythmias in the right atrial preparations of TG mice starting from 10-nM concentration. However, cisapride induced arrhythmias not only in the right atrial preparations of TG mice but also in the right atrial preparations of WT mice. For instance, 10 μM cisapride induced arrhythmias in the right atrial preparations of TG and WT mice to the same extent. Prucalopride did not exert concentration-dependent proarrhythmic effects in the isolated atrial preparations (left or right, WT or TG). Furthermore, cisapride and prucalopride increased the contractility and beating rate in vivo in TG mice, as assessed by performing echocardiography and surface electrocardiography. In summary, our results indicate that cisapride and prucalopride increase contractility and beating rate in the isolated atrial preparations of TG mice or in intact TG mice. Moreover, 5-HT induced arrhythmias in the isolated right atrial preparations of TG mice in a concentration-dependent manner. Furthermore, cisapride induced arrhythmias in the isolated right atrial preparations of both TG and WT mice. In contrast, prucalopride did not induce arrhythmias in the atrial preparations (left or right) of both WT and TG mice. We suggest that the present TG mouse model might be useful to predict at least some important cardiac effects of 5-HT₄ receptor agonists in the human heart.

The PDE4 cAMP-Specific Phosphodiesterases: Targets for Drugs with Antidepressant and Memory-Enhancing Action

The PDE4 cyclic nucleotide phosphodiesterases are essential regulators of cAMP abundance in the CNS through their ability to regulate PKA activity, the phosphorylation of CREB, and other important elements of signal transduction. In pre-clinical models and in early-stage clinical trials, PDE4 inhibitors have been shown to have antidepressant and memory-enhancing activity. However, the development of clinically-useful PDE4 inhibitors for CNS disorders has been limited by variable efficacy and significant side effects. Recent structural studies have greatly enhanced our understanding of the molecular configuration of PDE4 enzymes, especially the "long" PDE4 isoforms that are abundant in the CNS. The new structural data provide a rationale for the development of a new generation of PDE4 inhibitors that specifically act on long PDE4 isoforms. These next generation PDE4 inhibitors may also be capable of targeting the interactions of select long forms with their "partner" proteins, such as RACK1, β-arrestin, and DISC1. They would therefore have the ability to affect cAMP levels in specific cellular compartments and target localized cellular functions, such as synaptic plasticity. These new agents might also be able to target PDE4 populations in select regions of the CNS that are implicated in learning and memory, affect, and cognition. Potential therapeutic uses of these agents could include affective disorders, memory enhancement, and neurogenesis.

Phosphodiesterase 4 interacts with the 5-HT4(b) receptor to regulate cAMP signaling

Phosphodiesterase (PDE) 3 and PDE4, which degrade cyclic adenosine monophosphate (cAMP), are important regulators of 5-hydroxytryptamine (5-HT) 4 receptor signaling in cardiac tissue. Therefore, we investigated whether they interact with the 5-HT4(b) receptor, and whether A-kinase anchoring proteins (AKAPs), scaffolding proteins that bind to the regulatory subunit of protein kinase A (PKA) and contribute to the spacial-temporal control of cAMP signaling, are involved in the regulation of 5-HT4(b) receptor signaling. By measuring PKA activity in the absence and presence of PDE3 and PDE4 inhibitiors, we found that constitutive signaling of the overexpressed HA-tagged 5-HT4(b) receptor in HEK293 cells is regulated predominantly by PDE4, with a secondary role for PDE3 that is unmasked in the presence of PDE4 inhibition. Overexpressed PDE4D3 and PDE3A1, and to a smaller extent PDE4D5 co-immunoprecipitate constitutively with the 5-HT4(b) receptor. PDE activity measurements in immunoprecipitates of the 5-HT4(b) receptor confirm the association of PDE4D3 with the receptor and provide evidence that the activity of this PDE may be increased upon receptor stimulation with 5-HT. A possible involvement of AKAPs in 5-HT4(b) receptor signaling was uncovered in experiments using the St-Ht31 inhibitor peptide, which disrupts the interaction of AKAPs with PKA. However, St-Ht31 did not influence 5-HT4(b) receptor-stimulated PKA activity, and endogenous AKAP79 and gravin were not found in immunoprecipitates of the 5-HT4(b) receptor. In conclusion, we found that both PDE3A1 and PDE4D3 are integrated into complexes that contain the 5-HT4(b) receptor and may thereby regulate 5-HT4(b) receptor-mediated signaling.

Differential regulation of C-type natriuretic peptide-induced cGMP and functional responses by PDE2 and PDE3 in failing myocardium

Recently, we showed C-type natriuretic peptide (CNP)-induced negative inotropic (NIR) and positive lusitropic response (LR) in failing rat heart. We wanted to study whether, and if so, how phosphodiesterases (PDEs) regulate CNP-induced cyclic 3',5'-guanosine monophosphate (cGMP) elevation and functional responses. Inotropic and lusitropic responses were measured in left ventricular muscle strips and cyclic nucleotide levels, PDE activity and phospholamban (PLB) and troponin I (TnI) phosphorylation were measured in ventricular cardiomyocytes from Wistar rats with heart failure 6 weeks after myocardial infarction. CNP-mediated increase in global cGMP was mainly regulated by PDE2, as reflected by a marked amplification of the cGMP increase during PDE2 inhibition and by a high PDE2 activity in cardiomyocytes. PDE3 inhibition, on the other hand, caused no significant cGMP increase by CNP. The functional consequences did not correspond to the changes of cGMP. PDE3 inhibition increased the potency of the CNP-induced NIR and LR, while PDE2 inhibition desensitized the CNP-induced NIR, but not LR. A role for PDE2 on the maximal LR and PDE5 on the maximal NIR to CNP was revealed in the presence of PDE3 inhibition. CNP increased PLB phosphorylation about 25- to 30-fold and tended to increase TnI phosphorylation about twofold. As a whole, CNP-induced functional responses were only modestly regulated by PDEs compared to the cAMP-mediated functional responses to β1-adrenoceptor stimulation, which are highly regulated by PDEs. There is a mismatch between the CNP-induced cGMP increase and functional responses. Global cGMP levels are mainly regulated by PDE2 after CNP stimulation, whereas the functional responses are modestly regulated by both PDE2 and PDE3, indicating cGMP compartmentation by PDEs affecting CNP-induced responses in failing hearts.

Cardiac PDEs and crosstalk between cAMP and cGMP signalling pathways in the regulation of contractility

Elucidation of cAMP and cGMP signalling in the heart remains a hot topic, and new regulatory mechanisms continue to appear. Studying the influence of phosphodiesterases on 5-HT4 receptor signalling in porcine atrium, a paper from this issue of the journal expands findings of a crosstalk between cardiac cGMP and cAMP signalling recently discovered in failing rat ventricle to a different species and cardiac region. The overall data suggest that cGMP, produced following stimulation of the NPR-B receptor for C-type natriuretic peptide (CNP), inhibits cAMP degradation by phosphodiesterase 3 and thereby enhances cAMP-mediated signalling from β-adrenoceptors and 5-HT4 receptors to inotropic effects. In porcine atrium, this effect can be seen both as an increase in inotropic effect and as a reduced fade of the inotropic effect with time. Thus, accumulating evidence brings together several active fields of research, including cardiac phosphodiesterases, compartmentation of cyclic nucleotide signalling and the field of natriuretic peptides. If present in human hearts, this effect of CNP may have clinical implications.