Chronic β1-adrenergic blockade enhances myocardial β3-adrenergic coupling with nitric oxide-cGMP signaling in a canine model of chronic volume overload: new insight into mechanisms of cardiac benefit with selective β1-blocker therapy

Age and sex mediated effects of estrogen and Β3-adrenergic receptor on cardiovascular pathophysiology

Sex differences are consistently identified in determining the prevalence, manifestation, and response to therapies in several systemic disorders, including those affecting the cardiovascular (CV), skeletal muscle, and nervous system. Interestingly, such differences are often more noticeable as we age. For example, premenopausal women experience a lower risk of CV disease than men of the same age. While at an advanced age, with menopause, the risk of cardiovascular diseases and adverse outcomes increases exponentially in women, exceeding that of men. However, this effect appears to be reversed in diseases such as pulmonary hypertension, where women are up to seven times more likely than men to develop an idiopathic form of the disease with symptoms developing ten years earlier than their male counterparts. Explaining this is a complex question. However, several factors and mechanisms have been identified in recent decades, including a role for sex hormones, particularly estrogens and their related receptors. Furthermore, an emerging role in these sex differences has also been suggested for β-adrenergic receptors (βARs), which are essential regulators of mammalian physiology. It has in fact been shown that βARs interact with estrogen receptors (ER), providing further demonstration of their involvement in determining sexual differences. Based on these premises, this review article focused on the β3AR subtype, which shows important activities in adipose tissue but with new and interesting roles in regulating the function of cardiomyocytes and vascular cells. In detail, we examined how β3AR and ER signaling are intertwined and whether there would be sex- and age-dependent specific effects of these receptor systems.

Modulation of cAMP/cGMP signaling as prevention of congenital heart defects in Pde2A deficient embryos: a matter of oxidative stress

Phosphodiesterase 2A (Pde2A) is a dual-specific PDE that breaks down both cAMP and cGMP cyclic nucleotides. We recently highlighted a direct relationship between Pde2A impairment, a consequent increase of cAMP, and the appearance of mouse congenital heart defects (CHDs). Here we aimed to characterize the pathways involved in the development of CHDs and in their prevention by pharmacological approaches targeting cAMP and cGMP signaling. Transcriptome analysis revealed a modulation of more than 500 genes affecting biological processes involved in the immune system, cardiomyocyte development and contractility, angiogenesis, transcription, and oxidative stress in hearts from Pde2A-/- embryos. Metoprolol and H89 pharmacological administration prevented heart dilatation and hypertabeculation in Pde2A-/- embryos. Metoprolol was also able to partially impede heart septum defect and oxidative stress at tissue and molecular levels. Amelioration of cardiac defects was also observed by using the antioxidant NAC, indicating oxidative stress as one of the molecular mechanisms underpinning the CHDs. In addition, Sildenafil treatment recovered cardiac defects suggesting the requirement of cAMP/cGMP nucleotides balance for the correct heart development.

Inside the Biology of the β3-Adrenoceptor

Since the first discovery in 1989, the β3-adrenoceptor (β3-AR) has gained great attention because it showed the ability to regulate many physiologic and metabolic activities, such as thermogenesis and lipolysis in brown and white adipose tissue, respectively (BAT, WAT), negative inotropic effects in cardiomyocytes, and relaxation of the blood vessels and the urinary bladder. The β3-AR has been suggested as a potential target for cancer treatment, both in adult and pediatric tumors, since under hypoxia its upregulation in the tumor microenvironment (TME) regulates stromal cell differentiation, tumor growth and metastases, signifying that its agonism/antagonism could be useful for clinical benefits. Promising results in cancer research have proposed the β3-AR being targeted for the treatment of many conditions, with some drugs, at present, undergoing phase II and III clinical trials. In this review, we report the scientific journey followed by the research from the β3-Ars' discovery, with focus on the β3-Ars' role in cancer initiation and progression that elects it an intriguing target for novel antineoplastic approaches. The overview highlights the great potential of the β3-AR, both in physiologic and pathologic conditions, with the intention to display the possible benefits of β3-AR modulation in cancer reality.

Continuous short-term acclimation to moderate cold elicits cardioprotection in rats, and alters β-adrenergic signaling and immune status

Moderate cold acclimation (MCA) is a non-invasive intervention mitigating effects of various pathological conditions including myocardial infarction. We aim to determine the shortest cardioprotective regimen of MCA and the response of β1/2/3-adrenoceptors (β-AR), its downstream signaling, and inflammatory status, which play a role in cell-survival during myocardial infarction. Adult male Wistar rats were acclimated (9 °C, 1-3-10 days). Infarct size, echocardiography, western blotting, ELISA, mitochondrial respirometry, receptor binding assay, and quantitative immunofluorescence microscopy were carried out on left ventricular myocardium and brown adipose tissue (BAT). MultiPlex analysis of cytokines and chemokines in serum was accomplished. We found that short-term MCA reduced myocardial infarction, improved resistance of mitochondria to Ca2+-overload, and downregulated β1-ARs. The β2-ARs/protein kinase B/Akt were attenuated while β3-ARs translocated on the T-tubular system suggesting its activation. Protein kinase G (PKG) translocated to sarcoplasmic reticulum and phosphorylation of AMPKThr172 increased after 10 days. Principal component analysis revealed a significant shift in cytokine/chemokine serum levels on day 10 of acclimation, which corresponds to maturation of BAT. In conclusion, short-term MCA increases heart resilience to ischemia without any negative side effects such as hypertension or hypertrophy. Cold-elicited cardioprotection is accompanied by β1/2-AR desensitization, activation of the β3-AR/PKG/AMPK pathways, and an immunomodulatory effect.

Nitric Oxide: Physiological Functions, Delivery, and Biomedical Applications

Nitric oxide (NO) is a gaseous molecule that has a central role in signaling pathways involved in numerous physiological processes (e.g., vasodilation, neurotransmission, inflammation, apoptosis, and tumor growth). Due to its gaseous form, NO has a short half-life, and its physiology role is concentration dependent, often restricting its function to a target site. Providing NO from an external source is beneficial in promoting cellular functions and treatment of different pathological conditions. Hence, the multifaceted role of NO in physiology and pathology has garnered massive interest in developing strategies to deliver exogenous NO for the treatment of various regenerative and biomedical complexities. NO-releasing platforms or donors capable of delivering NO in a controlled and sustained manner to target tissues or organs have advanced in the past few decades. This review article discusses in detail the generation of NO via the enzymatic functions of NO synthase as well as from NO donors and the multiple biological and pathological processes that NO modulates. The methods for incorporating of NO donors into diverse biomaterials including physical, chemical, or supramolecular techniques are summarized. Then, these NO-releasing platforms are highlighted in terms of advancing treatment strategies for various medical problems.

Status of β1-Adrenoceptor Signal Transduction System in Cardiac Hypertrophy and Heart Failure

Although β 1-adrenoceptor ( β 1-AR) signal transduction, which maintains cardiac function, is downregulated in failing hearts, the mechanisms for such a defect in heart failure are not fully understood. Since cardiac hypertrophy is invariably associated with heart failure, it is possible that the loss of β 1-AR mechanisms in failing heart occurs due to hypertrophic process. In this regard, we have reviewed the information from a rat model of adaptive cardiac hypertrophy and maladaptive hypertrophy at 4 and 24 weeks after inducing pressure overload as well as adaptive cardiac hypertrophy and heart failure at 4 and 24 weeks after inducing volume overload, respectively. Varying degrees of alterations in β 1-AR density as well as isoproterenol-induced increases in cardiac function, intracellular Ca 2 + -concentration in cardiomyocytes and adenylyl cyclase activity in crude membranes have been reported under these hypertrophic conditions. Adaptive hypertrophy at 4 weeks of pressure or volume overload showed unaltered or augmented increases in the activities of different components of β 1-AR signaling. On the other hand, maladaptive hypertrophy due to pressure overload and heart failure due to volume overload at 24 weeks revealed depressions in the activities of β 1-AR signal transduction pathway. These observations provide evidence that β 1-AR signal system is either unaltered or upregulated in adaptive cardiac hypertrophy and downregulated in maladaptive cardiac hypertrophy or heart failure. Furthermore, the information presented in this article supports the concept that downregulation of β 1-AR mechanisms in heart failure or maladaptive cardiac hypertrophy is not due to hypertrophic process per se. It is suggested that a complex mechanism involving the autonomic imbalance may be of a critical importance in determining differential alterations in non-failing and failing hearts.

Driving force of deteriorated cellular environment in heart failure: Metabolic remodeling

Heart Failure (HF) has been one of the leading causes of death worldwide. Though its latent mechanism and therapeutic manipulation are updated and developed ceaselessly, there remain great gaps in the cognition of heart failure. High morbidity and readmission rates among HF patients are waiting to be addressed. Recent studies have found that myocardial energy metabolism was closely related to heart failure, in which substrate utilization, as well as intermediate metabolism disorders, insulin resistance, oxidative stress, and mitochondrial dysfunction, might underlie systolic dysfunction and progression of HF. This article centers on the changes and counteraction of cardiac energy metabolism in the failing heart. Therefore, targeting impaired energy provision is of great potential in the treatment of HF. And shifting the objective from traditional neurohormones to improving the cellular environment is expected to further optimize the management of HF.

β3AR-Dependent Brain-Derived Neurotrophic Factor (BDNF) Generation Limits Chronic Postischemic Heart Failure

BACKGROUND

Loss of brain-derived neurotrophic factor (BDNF)/TrkB (tropomyosin kinase receptor B) signaling accounts for brain and cardiac disorders. In neurons, β-adrenergic receptor stimulation enhances local BDNF expression. It is unclear if this occurs in a pathophysiological relevant manner in the heart, especially in the β-adrenergic receptor-desensitized postischemic myocardium. Nor is it fully understood whether and how TrkB agonists counter chronic postischemic left ventricle (LV) decompensation, a significant unmet clinical milestone.

METHODS

We conducted in vitro studies using neonatal rat and adult murine cardiomyocytes, SH-SY5Y neuronal cells, and umbilical vein endothelial cells. We assessed myocardial ischemia (MI) impact in wild type, β3AR knockout, or myocyte-selective BDNF knockout (myoBDNF KO) mice in vivo (via coronary ligation [MI]) or in isolated hearts with global ischemia-reperfusion (I/R).

RESULTS

In wild type hearts, BDNF levels rose early after MI (<24 hours), plummeting at 4 weeks when LV dysfunction, adrenergic denervation, and impaired angiogenesis ensued. The TrkB agonist, LM22A-4, countered all these adverse effects. Compared with wild type, isolated myoBDNF KO hearts displayed worse infarct size/LV dysfunction after I/R injury and modest benefits from LM22A-4. In vitro, LM22A-4 promoted neurite outgrowth and neovascularization, boosting myocyte function, effects reproduced by 7,8-dihydroxyflavone, a chemically unrelated TrkB agonist. Superfusing myocytes with the β3AR-agonist, BRL-37344, increased myocyte BDNF content, while β3AR signaling underscored BDNF generation/protection in post-MI hearts. Accordingly, the β1AR blocker, metoprolol, via upregulated β3ARs, improved chronic post-MI LV dysfunction, enriching the myocardium with BDNF. Last, BRL-37344-imparted benefits were nearly abolished in isolated I/R injured myoBDNF KO hearts.

CONCLUSIONS

BDNF loss underscores chronic postischemic heart failure. TrkB agonists can improve ischemic LV dysfunction via replenished myocardial BDNF content. Direct cardiac β3AR stimulation, or β-blockers (via upregulated β3AR), is another BDNF-based means to fend off chronic postischemic heart failure.

The β3 adrenoceptor in proliferative retinopathies: "Cinderella" steps out of its family shadow

In the retina, hypoxic condition leads to overgrowing leaky vessels resulting in altered metabolic supply that may cause impaired visual function. Hypoxia-inducible factor-1 (HIF-1) is a central regulator of the retinal response to hypoxia by activating the transcription of numerous target genes, including vascular endothelium growth factor, which acts as a major player in retinal angiogenesis. In the present review, oxygen urge by the retina and its oxygen sensing systems including HIF-1 are discussed in respect to the role of the beta-adrenergic receptors (β-ARs) and their pharmacologic manipulation in the vascular response to hypoxia. In the β-AR family, β1- and β2-AR have long been attracting attention because their pharmacology is intensely used for human health, while β3-AR, the third and last cloned receptor is no longer increasingly emerging as an attractive target for drug discovery. Here, β3-AR, a main character in several organs including the heart, the adipose tissue and the urinary bladder, but so far a supporting actor in the retina, has been thoroughly examined in respect to its function in retinal response to hypoxia. In particular, its oxygen dependence has been taken as a key indicator of β3-AR involvement in HIF-1-mediated responses to oxygen. Hence, the possibility of β3-AR transcription by HIF-1 has been discussed from early circumstantial evidence to the recent demonstration that β3-AR acts as a novel HIF-1 target gene by playing like a putative intermediary between oxygen levels and retinal vessel proliferation. Thus, targeting β3-AR may implement the therapeutic armamentarium against neovascular pathologies of the eye.

Sex/Gender- and Age-Related Differences in β-Adrenergic Receptor Signaling in Cardiovascular Diseases

Sex differences in cardiovascular disease (CVD) are often recognized from experimental and clinical studies examining the prevalence, manifestations, and response to therapies. Compared to age-matched men, women tend to have reduced CV risk and a better prognosis in the premenopausal period. However, with menopause, this risk increases exponentially, surpassing that of men. Although several mechanisms have been provided, including sex hormones, an emerging role in these sex differences has been suggested for β-adrenergic receptor (β-AR) signaling. Importantly, β-ARs are the most important G protein-coupled receptors (GPCRs), expressed in almost all the cell types of the CV system, and involved in physiological and pathophysiological processes. Consistent with their role, for decades, βARs have been considered the first targets for rational drug design to fight CVDs. Of note, β-ARs are seemingly associated with different CV outcomes in females compared with males. In addition, even if there is a critical inverse correlation between β-AR responsiveness and aging, it has been reported that gender is crucially involved in this age-related effect. This review will discuss how β-ARs impact the CV risk and response to anti-CVD therapies, also concerning sex and age. Further, we will explore how estrogens impact β-AR signaling in women.

The Endothelial Dysfunction Could Be a Cause of Heart Failure with Preserved Ejection Fraction Development in a Rat Model

50% of patients with heart failure have a preserved ejection fraction (HFpEF). Numerous studies have investigated the pathophysiological mechanisms of HFpEF and have shown that endothelial dysfunction plays an important role in HFpEF. Yet no studies answered whether endothelial dysfunction could be the cause or is the consequence of HFpEF. Recently, we have shown that the endothelial overexpression of human β3-adrenoreceptor (Tgβ3) in rats leads to the slow development of diastolic dysfunction over ageing. The aim of the study is to decipher the involvement of endothelial dysfunction in the HFpEF development. For that, we investigated endothelial and cardiac function in 15-, 30-, and 45-week-old wild-type (WT) and Tgβ3 rats. The aortic expression of •NO synthase (NOS) isoforms was evaluated by Western blot. Finally, electron paramagnetic resonance measurements were performed on aortas to evaluate •NO and O2•- production. Vascular reactivity was altered as early as 15 weeks of age in response to isoproterenol in Tgβ3 aortas and mesenteric arteries. NOS1 (neuronal NOS) expression was higher in the Tgβ3 aorta at 30 and 45 weeks of age (30 weeks: WT: $1.00\pm 0.21$ ; Tgβ3: $6.08\pm 2.30$ ; 45 weeks: WT: $1.00\pm 0.12$ ; Tgβ3: $1.55\pm 0.17$ ; $p<0.05$ ). Interestingly, the endothelial NOS (NOS3) monomer form is increased in Tgβ3 rats at 45 weeks of age (ratio NOS3 dimer/NOS3 monomer; WT: $1.00\pm 0.37$ ; Tgβ3: $0.13\pm 0.05$ ; $p<0.05$ ). Aortic •NO production was increased by NOS2 (inducible NOS) at 15 weeks of age in Tgβ3 rats (+52% vs. WT). Aortic O2•- production was increased in Tgβ3 rats at 30 and 45 weeks of age (+75% and+76%, respectively, vs. WT, $p<0.05$ ). We have shown that endothelial dysfunction and oxidative stress are present as early as 15 weeks of age and therefore conclude that endothelial dysfunction could be a cause of HFpEF development.

Inhibition of eNOS Partially Blunts the Beneficial Effects of Nebivolol on Angiotensin II-Induced Signaling in H9c2 Cardiomyoblasts

We have recently illustrated that nebivolol can inhibit angiotensin II (Ang II)-mediated signaling in cardiomyoblasts; however, to date, the detailed mechanism for the beneficial effects of nebivolol has not been studied. Here, we investigated whether the inhibition of NO bioavailability by blocking eNOS (endothelial nitric oxide synthase) using L-NG-nitroarginine methyl ester (L-NAME) would attenuate nebivolol-mediated favorable effects on Ang II-evoked signaling in H9c2 cardiomyoblasts. Our data reveal that the nebivolol-mediated antagonistic effects on Ang II-induced oxidative stress were retreated by concurrent pretreatment with L-NAME and nebivolol. Similarly, the expressions of pro-inflammatory markers TNF-α and iNOS stimulated by Ang II were not decreased with the combination of nebivolol plus L-NAME. In contrast, the nebivolol-induced reduction in the Ang II-triggered mTORC1 pathway and the mRNA levels of hypertrophic markers ANP, BNP, and β-MHC were not reversed with the addition of L-NAME to nebivolol. In compliance with these data, the inhibition of eNOS by L-N⁵-(1-Iminoethyl) ornithine (LNIO) and its upstream regulator AMP-activated kinase (AMPK) with compound C in the presence of nebivolol showed effects similar to those of the L-NAME plus nebivolol combination on Ang II-mediated signaling. Pretreatment with either compound C plus nebivolol or LNIO plus nebivolol showed similar effects to those of the L-NAME plus nebivolol combination on Ang II-mediated signaling. In conclusion, our data indicate that the rise in NO bioavailability caused by nebivolol via the stimulation of AMPK/eNOS signaling is key for its anti-inflammatory and antioxidant properties but not for its antihypertrophic response upon Ang II stimulation.

Management of Chronic Congestive Heart Failure Caused by Myxomatous Mitral Valve Disease in Dogs: A Narrative Review from 1970 to 2020

Simple Summary

Myxomatous mitral valve disease (MMVD) is the most common acquired cardiovascular disease in dogs. The progression of the disease and the increasing severity of valvular regurgitation cause a volume overload of the left heart, leading to left atrial and ventricular remodeling and congestive heart failure (CHF). The treatment of chronic CHF secondary to MMVD in dogs has not always been the same over time. In the last fifty years, the drugs utilized have considerably changed, as well as the therapeutic protocols. Some drugs have also changed their intended use. An analysis of the literature concerning the therapy of chronic heart failure in dogs affected by this widespread degenerative disease is not available; a synthesis of the published literature on this topic and a description of its current state of art are needed. To the authors’ knowledge, a review of this topic has never been published in veterinary medicine; therefore, the aim of this study is to overview the treatments of chronic CHF secondary to MMVD in dogs from 1970 to 2020 using the general framework of narrative reviews.

Abstract

The treatment of chronic congestive heart failure (CHF), secondary to myxomatous mitral valve disease (MMVD) in dogs, has considerably changed in the last fifty years. An analysis of the literature concerning the therapy of chronic CHF in dogs affected by MMVD is not available, and it is needed. Narrative reviews (NRs) are aimed at identifying and summarizing what has been previously published, avoiding duplications, and seeking new study areas that have not yet been addressed. The most accessible open-access databases, PubMed, Embase, and Google Scholar, were chosen, and the searching time frame was set in five decades, from 1970 to 2020. The 384 selected studies were classified into categories depending on the aim of the study, the population target, the pathogenesis of MMVD (natural/induced), and the resulting CHF. Over the years, the types of studies have increased considerably in veterinary medicine. In particular, there have been 43 (24.29%) clinical trials, 41 (23.16%) randomized controlled trials, 10 (5.65%) cross-over trials, 40 (22.60%) reviews, 5 (2.82%) comparative studies, 17 (9.60%) case-control studies, 2 (1.13%) cohort studies, 2 (1.13%) experimental studies, 2 (1.13%) questionnaires, 6 (3.40%) case-reports, 7 (3.95%) retrospective studies, and 2 (1.13%) guidelines. The experimental studies on dogs with an induced form of the disease were less numerous (49–27.68%) than the studies on dogs affected by spontaneous MMVD (128–72.32%). The therapy of chronic CHF in dogs has considerably changed in the last fifty years: in the last century, some of the currently prescribed drugs did not exist yet, while others had different indications.

Redox Switches Controlling Nitric Oxide Signaling in the Resistance Vasculature and Implications for Blood Pressure Regulation: Mid-Career Award for Research Excellence 2020

The arterial resistance vasculature modulates blood pressure and flow to match oxygen delivery to tissue metabolic demand. As such, resistance arteries and arterioles have evolved a series of highly orchestrated cell-cell communication mechanisms between endothelial cells and vascular smooth muscle cells to regulate vascular tone. In response to neurohormonal agonists, release of several intracellular molecules, including nitric oxide, evokes changes in vascular tone. We and others have uncovered novel redox switches in the walls of resistance arteries that govern nitric oxide compartmentalization and diffusion. In this review, we discuss our current understanding of redox switches controlling nitric oxide signaling in endothelial and vascular smooth muscle cells, focusing on new mechanistic insights, physiological and pathophysiological implications, and advances in therapeutic strategies for hypertension and other diseases.

Targeting Adrenergic Receptors in Metabolic Therapies for Heart Failure

The heart has a reduced capacity to generate sufficient energy when failing, resulting in an energy-starved condition with diminished functions. Studies have identified numerous changes in metabolic pathways in the failing heart that result in reduced oxidation of both glucose and fatty acid substrates, defects in mitochondrial functions and oxidative phosphorylation, and inefficient substrate utilization for the ATP that is produced. Recent early-phase clinical studies indicate that inhibitors of fatty acid oxidation and antioxidants that target the mitochondria may improve heart function during failure by increasing compensatory glucose oxidation. Adrenergic receptors (α1 and β) are a key sympathetic nervous system regulator that controls cardiac function. β-AR blockers are an established treatment for heart failure and α1A-AR agonists have potential therapeutic benefit. Besides regulating inotropy and chronotropy, α1- and β-adrenergic receptors also regulate metabolic functions in the heart that underlie many cardiac benefits. This review will highlight recent studies that describe how adrenergic receptor-mediated metabolic pathways may be able to restore cardiac energetics to non-failing levels that may offer promising therapeutic strategies.

Effect of β1 /β2 -adrenoceptor blockade on β3 -adrenoceptor activity in the rat cremaster muscle artery

BACKGROUND AND PURPOSE

The physiological role of vascular β₃ -adrenoceptors is not fully understood. Recent evidence suggests cardiac β₃ -adrenoceptors are functionally effective after down-regulation of β₁ /β₂ -adrenoceptors. The functional interaction between the β₃ -adrenoceptor and other β-adrenoceptor subtypes in rat striated muscle arteries was investigated.

EXPERIMENTAL APPROACH

Studies were performed in cremaster muscle arteries isolated from male Sprague-Dawley rats. β-adrenoceptor expression was assessed through RT-PCR and immunofluorescence. Functional effects of β₃ -adrenoceptor agonists and antagonists and other β-adrenoceptor ligands were measured using pressure myography.

KEY RESULTS

All three β-adrenoceptor subtypes were present in the endothelium of the cremaster muscle artery. The β₃ -adrenoceptor agonists mirabegron and CL 316,243 had no effect on the diameter of pressurized (70 mmHg) cremaster muscle arterioles with myogenic tone, while the β₃ -adrenoceptor agonist SR 58611A and the nonselective β-adrenoceptor agonist isoprenaline caused concentration-dependent dilation. In the presence of β_1/2 -adrenoceptor antagonists nadolol (10 μM), atenolol (1 μM) and ICI 118,551 (0.1 μM) both mirabegron and CL 316,243 were effective in causing vasodilation and the potency of SR 58611A was enhanced, while responses to isoprenaline were inhibited. The β₃ -adrenoceptor antagonist L 748,337 (1 μM) inhibited vasodilation caused by β₃ -adrenoceptor agonists (in the presence of β_1/2 -adrenoceptor blockade), but L 748,337 had no effect on isoprenaline-induced vasodilation.

CONCLUSION AND IMPLICATIONS

All three β-adrenoceptor subtypes were present in the endothelium of the rat cremaster muscle artery, but β₃ -adrenoceptor mediated vasodilation was only evident after blockade of β_1/2 -adrenoceptors. This suggests constitutive β_1/2 -adrenoceptor activity inhibits β₃ -adrenoceptor function in the endothelium of skeletal muscle resistance arteries.

Interaction of the Joining Region in Junctophilin-2 With the L-Type Ca2+ Channel Is Pivotal for Cardiac Dyad Assembly and Intracellular Ca2+ Dynamics

RATIONALE

Ca2+-induced Ca2+ release (CICR) in normal hearts requires close approximation of L-type calcium channels (LTCCs) within the transverse tubules (T-tubules) and RyR (ryanodine receptors) within the junctional sarcoplasmic reticulum. CICR is disrupted in cardiac hypertrophy and heart failure, which is associated with loss of T-tubules and disruption of cardiac dyads. In these conditions, LTCCs are redistributed from the T-tubules to disrupt CICR. The molecular mechanism responsible for LTCCs recruitment to and from the T-tubules is not well known. JPH (junctophilin) 2 enables close association between T-tubules and the junctional sarcoplasmic reticulum to ensure efficient CICR. JPH2 has a so-called joining region that is located near domains that interact with T-tubular plasma membrane, where LTCCs are housed. The idea that this joining region directly interacts with LTCCs and contributes to LTCC recruitment to T-tubules is unknown.

OBJECTIVE

To determine if the joining region in JPH2 recruits LTCCs to T-tubules through direct molecular interaction in cardiomyocytes to enable efficient CICR.

METHODS AND RESULTS

Modified abundance of JPH2 and redistribution of LTCC were studied in left ventricular hypertrophy in vivo and in cultured adult feline and rat ventricular myocytes. Protein-protein interaction studies showed that the joining region in JPH2 interacts with LTCC-α1C subunit and causes LTCCs distribution to the dyads, where they colocalize with RyRs. A JPH2 with induced mutations in the joining region (mutPG1JPH2) caused T-tubule remodeling and dyad loss, showing that an interaction between LTCC and JPH2 is crucial for T-tubule stabilization. mutPG1JPH2 caused asynchronous Ca2+-release with impaired excitation-contraction coupling after β-adrenergic stimulation. The disturbed Ca2+ regulation in mutPG1JPH2 overexpressing myocytes caused calcium/calmodulin-dependent kinase II activation and altered myocyte bioenergetics.

CONCLUSIONS

The interaction between LTCC and the joining region in JPH2 facilitates dyad assembly and maintains normal CICR in cardiomyocytes.

The Beta3 Adrenergic Receptor in Healthy and Pathological Cardiovascular Tissues

The third isotype of beta-adrenoreceptors (β3-AR) has recently come (back) into focus after the observation of its expression in white and beige human adipocytes and its implication in metabolic regulation. This coincides with the recent development and marketing of agonists at the human receptor with superior specificity. Twenty years ago, however, we and others described the expression of β3-AR in human myocardium and its regulation of contractility and cardiac remodeling. Subsequent work from many laboratories has since expanded the characterization of β3-AR involvement in many aspects of cardiovascular physio(patho)logy, justifying the present effort to update current paradigms under the light of the most recent evidence.

Overexpression of endothelial β3 -adrenergic receptor induces diastolic dysfunction in rats

AIMS

Diastolic dysfunction is common in cardiovascular diseases, particularly in the case of heart failure with preserved ejection fraction. The challenge is to develop adequate animal models to envision human therapies in the future. It has been hypothesized that this diastolic dysfunction is linked to alterations in the nitric oxide (• NO) pathway. To investigate this issue further, we investigated the cardiac functions of a transgenic rat model (Tgβ3 ) that overexpresses the human β3 -adrenoceptor (hβ3 -AR) in the endothelium with the underlying rationale that the • NO pathway should be stimulated in the endothelium.

METHODS AND RESULTS

Transgenic rats (Tgβ3 ) that express hβ3 -AR under the control of intercellular adhesion molecule 2 promoter were developed for a specific expression in endothelial cells. Transcriptomic analyses were performed on left ventricular tissue from 45-week-old rats. Among all altered genes, we focus on • NO synthase expression and endothelial function with arterial reactivity and evaluation of • NO and O2 •- production. Cardiac function was characterized by echocardiography, invasive haemodynamic studies, and working heart studies. Transcriptome analyses illustrate that several key genes are regulated by the hβ3 -AR overexpression. Overexpression of hβ3 -AR leads to a reduction of Nos3 mRNA expression (-72%; P < 0.05) associated with a decrease in protein expression (-19%; P < 0.05). Concentration-dependent vasodilation to isoproterenol was significantly reduced in Tgβ3 aorta (-10%; P < 0.05), while • NO and O2 •- production was increased. In the same time, Tgβ3 rats display progressively increasing diastolic dysfunction with age, as shown by an increase in the E/A filing ratio [1.15 ± 0.01 (wild type, WT) vs. 1.33 ± 0.04 (Tgβ3 ); P < 0.05] and in left ventricular end-diastolic pressure [5.57 ± 1.23 mmHg (WT) vs. 11.68 ± 1.11 mmHg (Tgβ3 ); P < 0.05]. In isolated working hearts, diastolic stress using increasing preload levels led to a 20% decrease in aortic flow [55.4 ± 1.9 mL/min (WT) vs. 45.8 ± 2.5 mL/min (Tgβ3 ); P < 0.05].

CONCLUSIONS

The Tgβ3 rat model displays the expected increase in • NO production upon ageing and develops diastolic dysfunction. These findings provide a further link between endothelial and cardiac dysfunction. This rat model should be valuable for future preclinical evaluation of candidate drugs aimed at correcting diastolic dysfunction.

Muscarinic-dependent phosphorylation of the cardiac ryanodine receptor by protein kinase G is mediated by PI3K-AKT-nNOS signaling

Post-translational modifications of proteins involved in calcium handling in myocytes, such as the cardiac ryanodine receptor (RyR2), critically regulate cardiac contractility. Recent studies have suggested that phosphorylation of RyR2 by protein kinase G (PKG) might contribute to the cardioprotective effects of cholinergic stimulation. However, the specific mechanisms underlying these effects remain unclear. Here, using murine ventricular myocytes, immunoblotting, proximity ligation as-says, and nitric oxide imaging, we report that phosphorylation of Ser-2808 in RyR2 induced by the muscarinic receptor agonist carbachol is mediated by a signaling axis comprising phosphoinositide 3-phosphate kinase, Akt Ser/Thr kinase, nitric oxide synthase 1, nitric oxide, soluble guanylate cyclase, cyclic GMP (cGMP), and PKG. We found that this signaling pathway is compartmentalized in myocytes, as it was distinct from atrial natriuretic peptide receptor-cGMP-PKG-RyR2 Ser-2808 signaling and independent of muscarinic-induced phosphorylation of Ser-239 in vasodilator-stimulated phosphoprotein. These results provide detailed insights into muscarinic-induced PKG signaling and the mediators that regulate cardiac RyR2 phosphorylation critical for cardiovascular function.

cGMP signalling in cardiomyocyte microdomains

3',5'-Cyclic guanosine monophosphate (cGMP) is one of the major second messengers critically involved in the regulation of cardiac electrophysiology, hypertrophy, and contractility. Recent molecular and cellular studies have significantly advanced our understanding of the cGMP signalling cascade, its local microdomain-specific regulation and its role in protecting the heart from pathological stress. Here, we summarise recent findings on cardiac cGMP microdomain regulation and discuss their potential clinical significance.

Left ventricular remodelling in chronic primary mitral regurgitation: implications for medical therapy

Surgical repair or replacement of the mitral valve is currently the only recommended therapy for severe primary mitral regurgitation. The chronic elevation of wall stress caused by the resulting volume overload leads to structural remodelling of the muscular, vascular and extracellular matrix components of the myocardium. These changes are initially compensatory but in the long term have detrimental effects, which ultimately result in heart failure. Understanding the changes that occur in the myocardium due to volume overload at the molecular and cellular level may lead to medical interventions, which potentially could delay or prevent the adverse left ventricular remodelling associated with primary mitral regurgitation. The pathophysiological changes involved in left ventricular remodelling in response to chronic primary mitral regurgitation and the evidence for potential medical therapy, in particular beta-adrenergic blockers, are the focus of this review.

Rationale and design of a multicentre, randomized, placebo-controlled trial of mirabegron, a Beta3-adrenergic receptor agonist on left ventricular mass and diastolic function in patients with structural heart disease Beta3-left ventricular hypertrophy (Beta3-LVH)

Aims

Progressive left ventricular (LV) remodelling with cardiac myocyte hypertrophy, myocardial fibrosis, and endothelial dysfunction plays a key role in the onset and progression of heart failure with preserved ejection fraction. The Beta3‐LVH trial will test the hypothesis that the β₃ adrenergic receptor agonist mirabegron will improve LV hypertrophy and diastolic function in patients with hypertensive structural heart disease at high risk for developing heart failure with preserved ejection fraction.

Methods and results

Beta3‐LVH is a randomized, placebo‐controlled, double‐blind, two‐armed, multicentre, European, parallel group study. A total of 296 patients will be randomly assigned to receive either mirabegron 50 mg daily or placebo over 12 months. The main inclusion criterion is the presence of LV hypertrophy, that is, increased LV mass index (LVMi) or increased wall thickening by echocardiography. The co‐primary endpoints are a change in LVMi by cardiac magnetic resonance imaging and a change in LV diastolic function (assessed by the E/e′ ratio). Secondary endpoints include mirabegron's effects on cardiac fibrosis, left atrial volume index, maximal exercise capacity, and laboratory markers. Two substudies will evaluate mirabegron's effect on endothelial function by pulse amplitude tonometry and brown fat activity by positron emission tomography using 17F‐fluorodeoxyglucose. Morbidity and mortality as well as safety aspects will also be assessed.

Conclusions

Beta3‐LVH is the first large‐scale clinical trial to evaluate the effects of mirabegron on LVMi and diastolic function in patients with LVH. Beta3‐LVH will provide important information about the clinical course of this condition and may have significant impact on treatment strategies and future trials in these patients.

Aucubin protects against pressure overload-induced cardiac remodelling via the β3 -adrenoceptor-neuronal NOS cascades

BACKGROUND AND PURPOSE

Aucubin, the predominant component of Eucommia ulmoides Oliv., has been shown to have profound effects on oxidative stress. As oxidative stress has previously been demonstrated to contribute to acute and chronic myocardial injury, we tested the effects of aucubin on cardiac remodelling and heart failure.

EXPERIMENTAL APPROACH

Initially, H9c2 cardiomyocytes and neonatal rat cardiomyocytes pretreated with aucubin (1, 3, 10, 25 and 50 μM) were challenged with phenylephrine. Secondly, the transverse aorta was constricted in C57/B6 and neuronal NOS (nNOS)-knockout mice, then aucubin (1 or 5 mg·kg^-1 body weight day^-1 ) was injected i.p. for 25 days. Hypertrophy was evaluated by assessing morphological changes, echocardiographic parameters, histological analyses and hypertrophic markers. Oxidative stress was evaluated by examining ROS generation, oxidase activity and NO generation. NOS expression was determined by Western blotting.

KEY RESULTS

Aucubin effectively suppressed cardiac remodelling; in mice, aucubin substantially inhibited pressure overload-induced cardiac hypertrophy, fibrosis and inflammation, whereas knocking out nNOS abolished these cardioprotective effects of aucubin. Blocking or knocking down the β₃ -adrenoceptor abolished the protective effects of aucubin in vitro. Furthermore, aucubin enhanced the protective effects of a β₃ -adrenoceptor agonist in vitro by increasing cellular cAMP levels, whereas treatment with an adenylate cyclase (AC) inhibitor abolished the cardioprotective effects of aucubin.

CONCLUSIONS AND IMPLICATIONS

Aucubin suppresses oxidative stress during cardiac remodelling by increasing the expression of nNOS in a process that requires activation of the β₃ -adrenoceptor/AC/cAMP pathway. These findings suggest that aucubin could have potential as a treatment for cardiac remodelling and heart failure.

Redox regulation of soluble guanylyl cyclase

The nitric oxide/soluble guanylyl cyclase (NO-sGC) signaling pathway regulates the cardiovascular, neuronal, and gastrointestinal systems. Impaired sGC signaling can result in disease and system-wide organ failure. This review seeks to examine the redox control of sGC through heme and cysteine regulation while discussing therapeutic drugs that target various conditions. Heme regulation involves mechanisms of insertion of the heme moiety into the sGC protein, the molecules and proteins that control switching between the oxidized (Fe³⁺) and reduced states (Fe²⁺), and the activity of heme degradation. Modifications to cysteine residues by S-nitrosation on the α1 and β1 subunits of sGC have been shown to be important in sGC signaling. Moreover, redox balance and localization of sGC is thought to control downstream effects. In response to altered sGC activity due to changes in the redox state, many therapeutic drugs have been developed to target decreased NO-sGC signaling. The importance and relevance of sGC continues to grow as sGC dysregulation leads to numerous disease conditions.

Nitric oxide signalling in cardiovascular health and disease

Nitric oxide (NO) signalling has pleiotropic roles in biology and a crucial function in cardiovascular homeostasis. Tremendous knowledge has been accumulated on the mechanisms of the nitric oxide synthase (NOS)-NO pathway, but how this highly reactive, free radical gas signals to specific targets for precise regulation of cardiovascular function remains the focus of much intense research. In this Review, we summarize the updated paradigms on NOS regulation, NO interaction with reactive oxidant species in specific subcellular compartments, and downstream effects of NO in target cardiovascular tissues, while emphasizing the latest developments of molecular tools and biomarkers to modulate and monitor NO production and bioavailability.

Molecular pathways of oestrogen receptors and β-adrenergic receptors in cardiac cells: Recognition of their similarities, interactions and therapeutic value

Oestrogen receptors (ERs) and β-adrenergic receptors (βARs) play important roles in the cardiovascular system. Moreover, these receptors are expressed in cardiac myocytes and vascular tissues. Numerous experimental observations support the hypothesis that similarities and interactions exist between the signalling pathways of ERs (ERα, ERβ and GPR30) and βARs (β₁ AR, β₂ AR and β₃ AR). The recently discovered oestrogen receptor GPR30 shares structural features with the βARs, and this forms the basis for the interactions and functional overlap. GPR30 possesses protein kinase A (PKA) phosphorylation sites and PDZ binding motifs and interacts with A-kinase anchoring protein 5 (AKAP5), all of which enable its interaction with the βAR pathways. The interactions between ERs and βARs occur downstream of the G-protein-coupled receptor, through the G_αs and G_αi proteins. This review presents an up-to-date description of ERs and βARs and demonstrates functional synergism and interactions among these receptors in cardiac cells. We explore their signalling cascades and the mechanisms that orchestrate their interactions and propose new perspectives on the signalling patterns for the GPR30 based on its structural resemblance to the βARs. In addition, we explore the relevance of these interactions to cell physiology, drugs (especially β-blockers and calcium channel blockers) and cardioprotection. Furthermore, a receptor-independent mechanism for oestrogen and its influence on the expression of βARs and calcium-handling proteins are discussed. Finally, we highlight promising therapeutic avenues that can be derived from the shared pathways, especially the phosphatidylinositol-3-OH kinase (PI3K/Akt) pathway.

Targeting β3-Adrenergic Receptors in the Heart: Selective Agonism and β-Blockade

Cardiac diseases, such as heart failure, remain leading causes of morbidity and mortality worldwide, with myocardial infarction as the most common etiology. HF is characterized by β-adrenergic receptor (βAR) dysregulation that is primarily due to the upregulation of G protein–coupled receptor kinases that leads to overdesensitization of β₁ and β₂ARs, and this clinically manifests as a loss of inotropic reserve. Interestingly, the “minor” βAR isoform, the β₃AR, found in the heart, lacks G protein–coupled receptor kinases recognition sites, and is not subject to desensitization, and as a consequence of this, in human failing myocardium, the levels of this receptor remain unchanged or are even increased. In different preclinical studies, it has been shown that β₃ARs can activate different signaling pathways that can protect the heart. The clinical relevance of this is also supported by the effects of β-blockers which are well known for their proangiogenic and cardioprotective effects, and data are emerging showing that these are mediated, at least in part, by enhancement of β₃AR activity. In this regard, targeting of β₃ARs could represent a novel potential strategy to improve cardiac metabolism, function, and remodeling.

Novel sGC Stimulators and sGC Activators for the Treatment of Heart Failure

The burden of heart failure (HF) increases worldwide with an aging population, and there is a high unmet medical need in both, heart failure with reduced ejection fraction (HFrEF) and with preserved ejection fraction (HFpEF). The nitric oxide (NO) pathway is a key regulator in the cardiovascular system and modulates vascular tone and myocardial performance. Disruption of the NO-cyclic guanosine monophosphate (cGMP) signaling axis and impaired cGMP formation by endothelial dysfunction could lead to vasotone dysregulation, vascular and ventricular stiffening, fibrosis, and hypertrophy resulting in a decline of heart as well as kidney function. Therefore, the NO-cGMP pathway is a treatment target in heart failure. Exogenous NO donors such as nitrates have long been used for treatment of cardiovascular diseases but turned out to be limited by increased oxidative stress and tolerance. More recently, novel classes of drugs were discovered which enhance cGMP production by targeting the NO receptor soluble guanylate cyclase (sGC). These compounds, the so-called sGC stimulators and sGC activators, are able to increase the enzymatic activity of sGC to generate cGMP independently of NO and have been developed to target this important signaling cascade in the cardiovascular system.This review will focus on the role of sGC in cardiovascular (CV) physiology and disease and the pharmacological potential of sGC stimulators and sGC activators therein. Preclinical data will be reviewed and summarized, and available clinical data with riociguat and vericiguat, novel direct sGC stimulators, will be presented. Vericiguat is currently being studied in a Phase III clinical program for the treatment of heart failure with reduced ejection fraction (HFrEF).

β1-Blockade Prevents Post-Ischemic Myocardial Decompensation Via β3AR-Dependent Protective Sphingosine-1 Phosphate Signaling

BACKGROUND

Although β-blockers increase survival in patients with heart failure (HF), the mechanisms behind this protection are not fully understood, and not all patients with HF respond favorably to them. We recently showed that, in cardiomyocytes, a reciprocal down-regulation occurs between β₁-adrenergic receptors (ARs) and the cardioprotective sphingosine-1-phosphate (S1P) receptor-₁ (S1PR₁).

OBJECTIVES

The authors hypothesized that, in addition to salutary actions due to direct β₁AR-blockade, agents such as metoprolol (Meto) may improve post-myocardial infarction (MI) structural and functional outcomes via restored S1PR₁ signaling, and sought to determine mechanisms accounting for this effect.

METHODS

We tested the in vitro effects of Meto in HEK293 cells and in ventricular cardiomyocytes isolated from neonatal rats. In vivo, we assessed the effects of Meto in MI wild-type and β₃AR knockout mice.

RESULTS

Here we report that, in vitro, Meto prevents catecholamine-induced down-regulation of S1PR₁, a major cardiac protective signaling pathway. In vivo, we show that Meto arrests post-MI HF progression in mice as much as chronic S1P treatment. Importantly, human HF subjects receiving β₁AR-blockers display elevated circulating S1P levels, confirming that Meto promotes S1P secretion/signaling. Mechanistically, we found that Meto-induced S1P secretion is β₃AR-dependent because Meto infusion in β₃AR knockout mice does not elevate circulating S1P levels, nor does it ameliorate post-MI dysfunction, as in wild-type mice.

CONCLUSIONS

Our study uncovers a previously unrecognized mechanism by which β₁-blockers prevent HF progression in patients with ischemia, suggesting that β₃AR dysfunction may account for limited/null efficacy in β₁AR-blocker-insensitive HF subjects.

GRK2 as negative modulator of NO bioavailability: Implications for cardiovascular disease

Nitric oxide (NO), initially identified as endothelium-derived relaxing factor (EDRF), is a gaso-transmitter with important regulatory roles in the cardiovascular, nervous and immune systems. In the former, this diatomic molecule and free radical gas controls vascular tone and cardiac mechanics, among others. In the cardiovascular system, it is now understood that β-adrenergic receptor (βAR) activation is a key modulator of NO generation. Therefore, it is not surprising that the up-regulation of G protein-coupled receptor kinases (GRKs), in particular GRK2, that restrains βAR activity contributes to impaired cardiovascular functions via alteration of NO bioavailability. This review, will explore the specific interrelation between βARs, GRK2 and NO in the cardiovascular system and their inter-relationship for the pathogenesis of the onset of disease. Last, we will update the readers on the current status of GRK2 inhibitors as a potential therapeutic strategy for heart failure with an emphasis on their ability of rescuing NO bioavailability.

New and Emerging Therapies and Targets: Beta-3 Agonists

While crucial for the acute physiologic response to stress, the adrenergic system may become maladaptive upon prolonged stimulation in the course of development of heart failure. This has been the basis for the development of beta-blocking therapies, targeting mainly beta1-2 adrenoreceptors (B1-2AR). The third isotype, B3AR, was more recently identified in cardiac myocytes and endothelial cells from human (and many other animal species), where its distinctive coupling to nitric oxide and antioxidant pathways suggested potential protective properties that were unexploited so far. The observation of beneficial effects of B3AR expression/activation on myocardial remodeling and the availability of specific agonists for clinical use now open the way for directly testing the hypothesis in heart failure patients. We will briefly review the specificities of B3AR signaling in the context of the cardiovascular adrenergic system, the evidence supporting its beneficial effects and outline an ongoing clinical trial using the B3AR agonist, mirabegron in patients with/at risk of developing heart failure with preserved ejection fraction (HFpEF).

Cardiac salvage by tweaking with beta-3-adrenergic receptors

Overstimulation of the orthosympathetic system leads to cardiovascular cell and tissue damage through prolonged activation of β-1-2 adrenergic receptors (BARs). The more recent identification of the third isotype of BAR (B3AR) in cardiac myocytes and endothelial cells with a distinctive coupling and effect on cardiac function and remodelling introduced a new facet to this paradigm. In particular, B3AR is up-regulated in cardiac disease and less prone to homologous desensitization, which may reinforce its influence on the diseased myocardium. Mice with transgenic cardiac-specific expression of the human B3AR are protected from cardiac hypertrophy and fibrosis in response to neurohormonal stimulation. B3AR has also been implicated in cardiac protection after ischaemia-reperfusion and the benefits of exercise on the heart. Many of these salvage mechanisms are mediated by B3AR coupling to nitric oxide synthase (eNOS and nNOS) and downstream cGMP/protein kinase G signalling. Notably, B3AR exerts antioxidant protective effects on these and other signalling elements, which may subserve its protective properties in the setting of chronic heart failure. Additional vasorelaxing properties and paracrine NO-mediated signalling by B3AR in endothelium, together with systemic metabolic effects on beige/brown fat complete the pleiotropic protective properties of this new therapeutic target.

Novel Perspectives in Redox Biology and Pathophysiology of Failing Myocytes: Modulation of the Intramyocardial Redox Milieu for Therapeutic Interventions-A Review Article from the Working Group of Cardiac Cell Biology, Italian Society of Cardiology

The prevalence of heart failure (HF) is still increasing worldwide, with enormous human, social, and economic costs, in spite of huge efforts in understanding pathogenetic mechanisms and in developing effective therapies that have transformed this syndrome into a chronic disease. Myocardial redox imbalance is a hallmark of this syndrome, since excessive reactive oxygen and nitrogen species can behave as signaling molecules in the pathogenesis of hypertrophy and heart failure, leading to dysregulation of cellular calcium handling, of the contractile machinery, of myocardial energetics and metabolism, and of extracellular matrix deposition. Recently, following new interesting advances in understanding myocardial ROS and RNS signaling pathways, new promising therapeutical approaches with antioxidant properties are being developed, keeping in mind that scavenging ROS and RNS tout court is detrimental as well, since these molecules also play a role in physiological myocardial homeostasis.

Neuronal nitric oxide synthase localizes to utrophin expressing intercalated discs and stabilizes their structural integrity

The neuronal nitric-oxide synthase (nNOS) splice variant nNOSµ is essential for skeletal muscle function. Its localization is dependent on dystrophin, which stabilizes the dystrophin glycoprotein complex (DGC) at the sarcolemma of skeletal muscle fibers. In Duchenne muscular dystrophy (DMD) dystrophin is absent and sarcolemmal nNOS is lost. This leads to functional ischemia due to a decrease in contraction-induced vasodilation. In cardiomyocytes, nNOSµ is believed to be the predominant NOS isoform. However, the association of nNOS with the DGC in the heart is unclear. Here, we report nNOS localization at the intercalated discs (IDs) of cardiomyocytes, where utrophin is highly expressed. In mdx, mdx:utr, nNOSµ knock-out (KO), and mdx:nNOSµ KO mice, we observed a gradual reduction of nNOS at IDs and disrupted ID morphology, compared to wild-type. In mdx:nNOSµ KO mice, but not in mdx or nNOSµ KO mice, we also observed an early development of cardiac fibrosis. These findings suggest that nNOS localization in the heart may not depend exclusively on the presence of dystrophin. Additionally, the β1 subunit of soluble guanylyl cyclase (sGC), responsible for the production of cGMP through nitric oxide (NO) signaling, was also detected at the IDs. Together, our results suggest a new role of nNOS at the IDs for the cGMP-dependent NO pathway and the maintenance of ID morphology.