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Purpose: Cardiac Sympathetic Neurons (cSNs) densely innervate the myocardium and operate physiologically both as short term enhancers of cardiac function, and as constitutive regulator of cardiomyocyte (CM) size, by controlling proteolysis through the β2-AR/FOXO3/MuRF-1 dependent signaling axis (1). The density of cSNs decreases progressively from the epi- to the endocardium. We here tested the hypothesis that such innervation pattern would reflect on the degree of constitutive β2-AR input to the different regions of the heart and thus heterogenously control myocardial proteolysis and cell size.Methods: IF and morphometric analyses were performed on heart slices from neonatal and adult normal and KO hearts. Heart denervation was induced by 6-OH-DOPA (100mg/kg). Surgical dissection of the subepi- and subendocardium was set up.Results: CM size is heterogenous throughout the mouse myocardium, with the cells in the outer layers (sEPI hereafter) being significantly larger than the similarly-oriented ones in the innermost (sENDO) layers (sEPI: 8590±2121 vs sENDO 4697±1433, in μm3; sEPI/sENDO CM size: 1.83±0.43). Such size differences, that parallel the ‘EPI-to-ENDO’; gradient in cSN density (cSNs/CM: sEPI 0.45±0.06 vs sENDO 0.15±0.02), are completely ablated upon cSN denervation (sEPI/sENDO CM size: 0.83±0.26), and are absent in both β2-AR-/- and MuRF1-/- mice. Interestingly, chronic treatment with either the β2-AR agonist clenbuterol (3mg/kg), or the β2-AR antagonist ICI118,551 (1mg/kg), which diffuse throughout the myocardium resulting in hypertrophic or atrophic remodeling, respectively, also abolish the sEPI-to-sENDO CM size difference, supporting the notion that SN control over CM morphology is operated regionally. In further support of the causal role of cSNs in controlling myocardial heterogeneity, sEPI/sENDO CMs size differences are not present immediately after birth and start to establish at P7, in parallel with the development of myocardial sympathetic innervation and, consistently, never establish in mice permanently denervated at P1. Furthermore, we demonstrated that proteolysis is differentially regulated in the sEPI and sENDO, as sympathetic denervation upregulated the muscle specific atrogenes MuRF1 and Atrogin-1 significantly more in the former than in the latter regions.Conclusions: Constitutive activity of the cardiac SNs is required for the establishment and maintenance of the correct trophic CM properties. This is the first demonstration of an otherwise homogenous tissue that is shaped by a superimposed pattern of cSNs.1. Zaglia et al. Cardiovasc Res, 2013 [ABSTRACT FROM PUBLISHER]

Purpose: Sympathetic ganglia neurons (SGNs) innervate the myocardium and tune heart rate and contractility. Development and viability of cardiac neurons depend on neurotrophins that are released in low amounts by the myocardium. This study aims to understand whether the direct cell-to-cell contact plays a role in NGF-mediated signaling between cardiomyocytes (CMs) and SGNs.Methods and results: Electron microscopy and immunofluorescence on mouse heart slices and rat SGN/CM co-cultures showed a close association between SGNs and CMs, neurotransmitter vesicle accumulation, increased membrane protein density and enrichment of the NGF receptor (TrkA) at the contact site. These data support that specialized and locally organized signaling domains exist (neuro-cardiac junction, NCJ, hereafter).We tested the functional role of the NCJ in NGF-mediated prosurvival signaling. NGF expression by CMs was assessed by western blot analysis and its silencing in co-cultures caused a 66% decrease of neuronal density, suggesting that neurons depend on NGF released by CMs. NGF binding to its receptor triggers TrkA retrograde transport to neuronal soma that was monitored using fluorescently labeled TrkA. TrkA-RFP retrograde movements were faster in processes contacting CMs than other cardiac cells, further supporting that neuronal NGF signaling is activated by the coupled CMs. SGNs cultured on NGF-silenced CMs showed a 20% decrease in the NCJ area when compared to those on wild type CMs of the same culture. Moreover, NGF uptake was observed only in processes contacting NGF overexpressing CMs, supporting that neurotrophin mediated signaling is involved at the SGN/CM interaction. Consistently, cultured SGNs in contact with CMs survived NGF withdrawal, whereas neurons alone treated with CM-conditioned medium did not survive because of the very low NGF concentration (<5pg/mL).An anti-NGF antibody, a TrkA antagonist (c(92-96)) and inhibitor (k252a) were used to antagonize receptor activation by NGF. Only the small membrane permeable k252a reduced neuronal density, suggesting that the NCJ is an isolated microdomain. K252a was used to estimate NGF concentration at the contact site, which resulted about 1.5nM, four orders of magnitude higher than that in CM-conditioned medium, supporting that the NCJ allows amplification of intercellular NGF signaling.Conclusions: Taken together, our results suggest that the NGF-dependent pro-survival signal to the SGN needs a direct interaction with the CM that facilitates NGF activation of TrkA thanks to the development of an isolated microdomain characterized by a high NGF concentration. [ABSTRACT FROM AUTHOR]

Purpose: Control of cardiomyocyte (CM) proteostasis operated by the ubiquitin/proteasome (UPS) and autophagy/lysosome systems is fundamental for heart adaptation to both physiologic and pathologic stresses. Reduced efficiency of either UPS or autophagy/lysosome system occurs during ageing and has been associated to cardiomyopathies. Atrogin-1 is a muscle specific ubiquitin ligase, targeting for degradation signalling proteins involved in cardiac hypertrophy. However, the role of Atrogin-1 in CM biology and its involvement in the molecular mechanism of cardiac dysfunction, are largely unexplored.Methods: We analyzed hearts from Atrogin-1 knock-out (KO) mice from 6 mo. onwards until death, and compared it to that of age- and sex-matched controls. Functional, immunofluorescence and electron microscopy analyses were performed. Markers of ER/SR stress and autophagy/lysosome systems were investigated by RTqPCR and WB. In vivo pulsed SILAC proteomics and bioinformatics, Co-IP, in vitro assays and in vivo viral silencing were performed to identify novel targets of Atrogin-1.Results: By using in vivo and in vitro assays we identified a novel target of Atrogin-1, the ESCRTIII protein CHMP2B, that plays a fundamental role in autophagy. Failure to degrade CHMP2B in Atrogin-1 KO mice caused autophagy impairment, accumulation of intracellular protein aggregates, activation of the unfolded protein response and subsequent CM apoptosis, all of which increased progressively during ageing. The alterations in cellular proteostasis resulted in cardiomyopathy with a restrictive pattern, characterized by myocardial remodelling with interstitial fibrosis, diastolic dysfunction (Edt, KO: 20.7±2.7 vs WT: 27.8±5.7, in msec), arrhythmias and secondary LA and ventricular remodelling, as well as CM hypertrophy (CM area, KO: 289.34±2.23 vs WT: 236.77±1.64, in μm2). Aged Atrogin-1 KO mice had reduced tolerance to treadmill exercise compared to controls, and shortened life span (KO: 17±1 vs WT: 24±1.2, in mo.). In vivo reduction of CHMP2B protein level in the KO mice restored normal autophagy and protected CMs from cell death resulting from CHMP2B proteotoxicity.Conclusions: Our data highlight the importance of regulated proteolysis in the heart and show that the loss of Atrogin-1 per se is sufficient to cause cardiac damage, which evolves into cardiomyopathy when protein quality control becomes less efficient, as occurring in aging. Such cardiomyopathy represents a novel model of proteotoxic myocardial remodelling and will be useful to determine the mechanism of impaired proteostasis to CM damage. [ABSTRACT FROM PUBLISHER]

In mammals, the physiological activation of the glucocorticoid receptor (GR) by glucocorticoids (GCs) promotes the maturation of cardiomyocytes during late gestation, but the effect on postnatal cardiac growth and regenerative plasticity is unclear. Here we demonstrate that the GC-GR axis restrains cardiomyocyte proliferation during postnatal development. Cardiomyocyte-specific GR ablation in conditional knockout (cKO) mice delayed the postnatal cardiomyocyte cell cycle exit, hypertrophic growth and cytoarchitectural maturation. GR-cKO hearts showed increased expression of genes involved in glucose catabolism and reduced expression of genes promoting fatty acid oxidation and mitochondrial respiration. Accordingly, oxygen consumption in GR-cKO cardiomyocytes was less dependent on fatty acid oxidation, and glycolysis inhibition reverted GR-cKO effects on cardiomyocyte proliferation. GR ablation or transient pharmacological inhibition after myocardial infarction in juvenile and/or adult mice facilitated cardiomyocyte survival, cell cycle re-entry and division, leading to cardiac muscle regeneration along with reduced scar formation. Thus, GR restrains heart regeneration and may represent a therapeutic target., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

It is well appreciated that, differently from skeletal muscles, the heart contracts independently from neurogenic inputs. However, the speed and force of heartbeats are finely modulated during stresses, emotions, and daily activities, by the autonomic neurons (both parasympathetic and sympathetic) which highly innervate the myocardium. Despite this aspect of cardiac physiology has been known for long, research has only recently shed light on the biophysical mechanisms underlying the meticulous adaptation of heart activity to the needs of the organism. A conceptual advancement in this regard has come from the use of optogenetics, a revolutionary methodology which allows to control the activity of a given excitable cell type, with high specificity, temporal and spatial resolution, within intact tissues and organisms. The method, widely affirmed in the field of neuroscience, has more recently been exploited also in research on heart physiology and pathology, including the study of the mechanisms regulating heart rhythm. The last point is the object of this book chapter which, starting from the description of the physiology of heart rhythm automaticity and the neurogenic modulation of heart rate, makes an excursus on the theoretical basis of such biotechnology (with its advantages and limitations), and presents a series of examples in cardiac and neuro-cardiac optogenetics., (© 2022. The Author(s), under exclusive license to Springer Science+Business Media, LLC, part of Springer Nature.)

Key Points: The heart is innervated by a dense sympathetic neuron network which, in the short term, controls chronotropy and inotropy and, in the long term, regulates cardiomyocyte size. Acute neurogenic control of heart rate is achieved locally through direct neuro-cardiac coupling at specific junctional sites (neuro-cardiac junctions). The ventricular sympathetic network topology is well-defined and characteristic for each mammalian species. In the present study, we used cell size regulation to determine whether long-term modulation of cardiac structure is achieved via direct sympatho-cardiac coupling. Local density of cardiac innervation correlated with cell size throughout the myocardial walls in all mammalian species analysed, including humans. The data obtained suggest that constitutive neurogenic control of cardiomyocyte trophism occurs through direct intercellular signalling at neuro-cardiac junctions., Abstract: It is widely appreciated that sympathetic stimulation of the heart involves a sharp increase in beating rate and significant enhancement of contractility. We have previously shown that, in addition to these evident functions, sympathetic neurons (SNs) also provide trophic input to cardiomyocytes (CMs), regulating cell and organ size. More recently, we have demonstrated that cardiac neurons establish direct interactions with CMs, allowing neuro-cardiac communication to occur locally, with a 'quasi-synaptic' mechanism. Based on the evidence that cardiac SNs are unevenly distributed throughout the myocardial walls, we investigated the hypothesis that CM size distribution reflects the topology of neuronal density. In vitro analyses of SN/CM co-cultures, ex vivo confocal and multiphoton imaging in clarified hearts, and biochemical and molecular approaches were employed, in both rodent and human heart biopsies. In line with the trophic effect of SNs, and with local neuro-cardiac communication, CMs, directly contacted by SNs in co-cultures, were larger than the non-targeted ones. This property reflects the distribution of CM size throughout the ventricles of intact mouse heart, in which cells in the outer myocardial layers, which were contacted by more neuronal processes, were larger than those in the less innervated subendocardial region. Such differences disappeared upon genetic or pharmacological interference with the trophic SN/CM signalling axis. Remarkably, CM size followed the SN distribution pattern in other mammals, including humans. Our data suggest that both the acute and chronic influence of SNs on cardiac function and structure is enacted as a result of the establishment of specific intercellular neuro-cardiac junctions., (© 2019 The Authors. The Journal of Physiology © 2019 The Physiological Society.)

Background: Anthracyclines, such as doxorubicin (DOX), are potent anticancer agents for the treatment of solid tumors and hematologic malignancies. However, their clinical use is hampered by cardiotoxicity. This study sought to investigate the role of phosphoinositide 3-kinase γ (PI3Kγ) in DOX-induced cardiotoxicity and the potential cardioprotective and anticancer effects of PI3Kγ inhibition., Methods: Mice expressing a kinase-inactive PI3Kγ or receiving PI3Kγ-selective inhibitors were subjected to chronic DOX treatment. Cardiac function was analyzed by echocardiography, and DOX-mediated signaling was assessed in whole hearts or isolated cardiomyocytes. The dual cardioprotective and antitumor action of PI3Kγ inhibition was assessed in mouse mammary tumor models., Results: PI3Kγ kinase-dead mice showed preserved cardiac function after chronic low-dose DOX treatment and were protected against DOX-induced cardiotoxicity. The beneficial effects of PI3Kγ inhibition were causally linked to enhanced autophagic disposal of DOX-damaged mitochondria. Consistently, either pharmacological or genetic blockade of autophagy in vivo abrogated the resistance of PI3Kγ kinase-dead mice to DOX cardiotoxicity. Mechanistically, PI3Kγ was triggered in DOX-treated hearts, downstream of Toll-like receptor 9, by the mitochondrial DNA released by injured organelles and contained in autolysosomes. This autolysosomal PI3Kγ/Akt/mTOR/Ulk1 signaling provided maladaptive feedback inhibition of autophagy. PI3Kγ blockade in models of mammary gland tumors prevented DOX-induced cardiac dysfunction and concomitantly synergized with the antitumor action of DOX by unleashing anticancer immunity., Conclusions: Blockade of PI3Kγ may provide a dual therapeutic advantage in cancer therapy by simultaneously preventing anthracyclines cardiotoxicity and reducing tumor growth.

Key Points: The present study demonstrates, by in vitro and in vivo analyses, the novel concept that signal transmission between sympathetic neurons and the heart, underlying the physiological regulation of cardiac function, operates in a quasi-synaptic fashion. This is a result of the direct coupling between neurotransmitter releasing sites and effector cardiomyocyte membranes., Abstract: Cardiac sympathetic neurons (SNs) finely tune the rate and strength of heart contractions to match blood demand, both at rest and during acute stress, through the release of noradrenaline (NE). Junctional sites at the interface between the two cell types have been observed, although whether direct neurocardiac coupling has a role in heart physiology has not been clearly demonstrated to date. We investigated the dynamics of SN/cardiomyocyte intercellular signalling, both by fluorescence resonance energy transfer-based imaging of cAMP in co-cultures, as a readout of cardiac β-adrenergic receptor activation, and in vivo, using optogenetics in transgenic mice with SN-specific expression of Channelrhodopsin-2. We demonstrate that SNs and cardiomyocytes interact at specific sites in the human and rodent heart, as well as in co-cultures. Accordingly, neuronal activation elicited intracellular cAMP increases only in directly contacted myocytes and cell-cell coupling utilized a junctional extracellular signalling domain with an elevated NE concentration. In the living mouse, optogenetic activation of cardiac SNs innervating the sino-atrial node resulted in an instantaneous chronotropic effect, which shortened the heartbeat interval with single beat precision. Remarkably, inhibition of the optogenetically elicited chronotropic responses required a high dose of propranolol (20-50 mg kg -1 ), suggesting that sympathetic neurotransmission in the heart occurs at a locally elevated NE concentration. Our in vitro and in vivo data suggest that the control of cardiac function by SNs occurs via direct intercellular coupling as a result of the establishment of a specific junctional site., (© 2018 The Authors. The Journal of Physiology © 2018 The Physiological Society.)

So far, opposing outcomes have been reported following neonatal apex resection in mice, questioning the validity of this injury model to investigate regenerative mechanisms. We performed a systematic evaluation, up to 180 days after surgery, of the pathophysiological events activated upon apex resection. In response to cardiac injury, we observed increased cardiomyocyte proliferation in remote and apex regions, neovascularization, and local fibrosis. In adulthood, resected hearts remain consistently shorter and display permanent fibrotic tissue deposition in the center of the resection plane, indicating limited apex regrowth. However, thickening of the left ventricle wall, explained by an upsurge in cardiomyocyte proliferation during the initial response to injury, compensated cardiomyocyte loss and supported normal systolic function. Thus, apex resection triggers both regenerative and reparative mechanisms, endorsing this injury model for studies aimed at promoting cardiomyocyte proliferation and/or downplaying fibrosis., (Copyright © 2018 The Author(s). Published by Elsevier Inc. All rights reserved.)

Optogenetics is a technique exploded in the last 10 years, which revolutionized several areas of biological research. The brightest side of this technology is the use of light to modulate non-invasively, with high spatial resolution and millisecond time scale, excitable cells genetically modified to express light-sensitive microbial ion channels (opsins). Neuroscience has first benefited from such fascinating strategy, in intact organisms. By shining light to specific neuronal subpopulations, optogenetics allowed unearth the mechanisms involved in cell-to-cell communication within the context of intact organs, such as the brain, formed by complex neuronal circuits. More recently, scientists looked at optogenetics as a tool to answer some of the questions, remained in the dark, of cardiovascular physiology. In this review, we focus on the application of optogenetics in the study of the heart, a complex multicellular organ, homing different populations of excitable cells, spatially and functionally interconnected. Moving from the first proof-of-principle works, published in 2010, to the present time, we discuss the in vitro and in vivo applications of optogenetics for the study of electrophysiology of the different cardiac cell types, and for the dissection of cellular mechanisms underlying arrhythmias. We also present how molecular biology and technology foster the evolution of cardiac optogenetics, with the aim to further our understanding of fundamental questions in cardiac physiology and pathology. Finally, we confer about the therapeutic potential of such biotechnological strategy for the treatment of heart rhythm disturbances (e.g. cardiac pacing, cardioversion)., (Copyright © 2016 Elsevier Inc. All rights reserved.)

Extrasystoles lead to several consequences, ranging from uneventful palpitations to lethal ventricular arrhythmias, in the presence of pathologies, such as myocardial ischemia. The role of working versus conducting cardiomyocytes, as well as the tissue requirements (minimal cell number) for the generation of extrasystoles, and the properties leading ectopies to become arrhythmia triggers (topology), in the normal and diseased heart, have not been determined directly in vivo. Here, we used optogenetics in transgenic mice expressing ChannelRhodopsin-2 selectively in either cardiomyocytes or the conduction system to achieve cell type-specific, noninvasive control of heart activity with high spatial and temporal resolution. By combining measurement of optogenetic tissue activation in vivo and epicardial voltage mapping in Langendorff-perfused hearts, we demonstrated that focal ectopies require, in the normal mouse heart, the simultaneous depolarization of at least 1,300-1,800 working cardiomyocytes or 90-160 Purkinje fibers. The optogenetic assay identified specific areas in the heart that were highly susceptible to forming extrasystolic foci, and such properties were correlated to the local organization of the Purkinje fiber network, which was imaged in three dimensions using optical projection tomography. Interestingly, during the acute phase of myocardial ischemia, focal ectopies arising from this location, and including both Purkinje fibers and the surrounding working cardiomyocytes, have the highest propensity to trigger sustained arrhythmias. In conclusion, we used cell-specific optogenetics to determine with high spatial resolution and cell type specificity the requirements for the generation of extrasystoles and the factors causing ectopies to be arrhythmia triggers during myocardial ischemia.

Cardiomyocyte proteostasis is mediated by the ubiquitin/proteasome system (UPS) and autophagy/lysosome system and is fundamental for cardiac adaptation to both physiologic (e.g., exercise) and pathologic (e.g., pressure overload) stresses. Both the UPS and autophagy/lysosome system exhibit reduced efficiency as a consequence of aging, and dysfunction in these systems is associated with cardiomyopathies. The muscle-specific ubiquitin ligase atrogin-1 targets signaling proteins involved in cardiac hypertrophy for degradation. Here, using atrogin-1 KO mice in combination with in vivo pulsed stable isotope labeling of amino acids in cell culture proteomics and biochemical and cellular analyses, we identified charged multivesicular body protein 2B (CHMP2B), which is part of an endosomal sorting complex (ESCRT) required for autophagy, as a target of atrogin-1-mediated degradation. Mice lacking atrogin-1 failed to degrade CHMP2B, resulting in autophagy impairment, intracellular protein aggregate accumulation, unfolded protein response activation, and subsequent cardiomyocyte apoptosis, all of which increased progressively with age. Cellular proteostasis alterations resulted in cardiomyopathy characterized by myocardial remodeling with interstitial fibrosis, with reduced diastolic function and arrhythmias. CHMP2B downregulation in atrogin-1 KO mice restored autophagy and decreased proteotoxicity, thereby preventing cell death. These data indicate that atrogin-1 promotes cardiomyocyte health through mediating the interplay between UPS and autophagy/lysosome system and its alteration promotes development of cardiomyopathies.

Aims: Increased cardiac sympathetic neuron (SN) activity has been associated with pathologies such as heart failure and hypertrophy, suggesting that cardiac innervation regulates cardiomyocyte trophism. Whether continuous input from the SNs is required for the maintenance of the cardiomyocyte size has not been determined thus far., Methods and Results: To address the role of cardiac innervation in cardiomyocyte size regulation, we monitored the effect of pharmacological sympathetic denervation in mice on cardiac structure, function, and signalling from 24 h to 30 days in the absence of other pathological stimuli. SN ablation caused an immediate reduction in the cardiomyocyte size with minimal consequences on the resting contractile function. Atrophic remodelling was mediated by the ubiquitin-proteasome system through FOXO-dependent early induction of the muscle-specific E3 ubiquitin ligases Atrogin-1/MAFbx and MuRF1, which was followed by activation of the autophagy-lysosome system. MuRF1 was found to be determinant in denervation atrophy as remodelling did not develop in denervated MuRF1 knock-out (KO) hearts. These effects were caused by decreased basal stimulation of cardiomyocyte β2-adrenoceptor (AR), as atrophy was prevented by treatment of denervated mice with the β2-AR agonist clenbuterol. Consistent with these data, we also observed that β2-AR KO mice showed cardiac atrophy at rest., Conclusion: Cardiac SNs are strong regulators of the cardiomyocyte size via β2-AR-dependent repression of proteolysis, demonstrating that the neuro-cardiac axis operates constitutively for the determination of the physiological cardiomyocyte size. These results are of great clinical relevance given the role of β-AR in cardiovascular diseases and their modulation in therapy.