Growth hormone-releasing hormone receptor antagonist MIA-602 attenuates cardiopulmonary injury induced by BSL-2 rVSV-SARS-CoV-2 in hACE2 mice

COVID-19 pneumonia causes acute lung injury and acute respiratory distress syndrome (ALI/ARDS) characterized by early pulmonary endothelial and epithelial injuries with altered pulmonary diffusing capacity and obstructive or restrictive physiology. Growth hormone-releasing hormone receptor (GHRH-R) is expressed in the lung and heart. GHRH-R antagonist, MIA-602, has been reported to modulate immune responses to bleomycin lung injury and inflammation in granulomatous sarcoidosis. We hypothesized that MIA-602 would attenuate rVSV-SARS-CoV-2-induced pulmonary dysfunction and heart injury in a BSL-2 mouse model. Male and female K18-hACE2tg mice were inoculated with SARS-CoV-2/USA-WA1/2020, BSL-2-compliant recombinant VSV-eGFP-SARS-CoV-2-Spike (rVSV-SARS-CoV-2), or PBS, and lung viral load, weight loss, histopathology, and gene expression were compared. K18-hACE2tg mice infected with rVSV-SARS-CoV-2 were treated daily with subcutaneous MIA-602 or vehicle and conscious, unrestrained plethysmography performed on days 0, 3, and 5 (n = 7 to 8). Five days after infection mice were killed, and blood and tissues collected for histopathology and protein/gene expression. Both native SARS-CoV-2 and rVSV-SARS-CoV-2 presented similar patterns of weight loss, infectivity (~60%), and histopathologic changes. Daily treatment with MIA-602 conferred weight recovery, reduced lung perivascular inflammation/pneumonia, and decreased lung/heart ICAM-1 expression compared to vehicle. MIA-602 rescued altered respiratory rate, increased expiratory parameters (Te, PEF, EEP), and normalized airflow parameters (Penh and Rpef) compared to vehicle, consistent with decreased airway inflammation. RNASeq followed by protein analysis revealed heightened levels of inflammation and end-stage necroptosis markers, including ZBP1 and pMLKL induced by rVSV-SARS-CoV-2, that were normalized by MIA-602 treatment, consistent with an anti-inflammatory and pro-survival mechanism of action in this preclinical model of COVID-19 pneumonia.

TSC2 S1365A mutation potently regulates CD8+ T cell function and differentiation and improves adoptive cellular cancer therapy

MTORC1 integrates signaling from the immune microenvironment to regulate T cell activation, differentiation, and function. TSC2 in the tuberous sclerosis complex tightly regulates mTORC1 activation. CD8+ T cells lacking TSC2 have constitutively enhanced mTORC1 activity and generate robust effector T cells; however, sustained mTORC1 activation prevents generation of long-lived memory CD8+ T cells. Here we show that manipulating TSC2 at Ser1365 potently regulated activated but not basal mTORC1 signaling in CD8+ T cells. Unlike nonstimulated TSC2-KO cells, CD8+ T cells expressing a phosphosilencing mutant TSC2-S1365A (TSC2-SA) retained normal basal mTORC1 activity. PKC and T cell receptor (TCR) stimulation induced TSC2 S1365 phosphorylation, and preventing this with the SA mutation markedly increased mTORC1 activation and T cell effector function. Consequently, SA CD8+ T cells displayed greater effector responses while retaining their capacity to become long-lived memory T cells. SA CD8+ T cells also displayed enhanced effector function under hypoxic and acidic conditions. In murine and human solid-tumor models, SA CD8+ T cells used as adoptive cell therapy displayed greater antitumor immunity than WT CD8+ T cells. These findings reveal an upstream mechanism to regulate mTORC1 activity in T cells. The TSC2-SA mutation enhanced both T cell effector function and long-term persistence/memory formation, supporting an approach to engineer better CAR-T cells for treating cancer.

Transient receptor potential canonical type 6 (TRPC6) O-GlcNAcylation at Threonine-221 plays potent role in channel regulation

Transient receptor potential canonical type 6 (TRPC6) is a non-voltage-gated channel that principally conducts calcium. Elevated channel activation contributes to fibrosis, hypertrophy, and proteinuria, often coupled to stimulation of nuclear factor of activated T-cells (NFAT). TRPC6 is post-translationally regulated, but a role for O-linked β-N-acetyl glucosamine (O-GlcNAcylation) as elevated by diabetes, is unknown. Here we show TRPC6 is constitutively O-GlcNAcylated at Ser14, Thr70, and Thr221 in the N-terminus ankryn-4 (AR4) and linker (LH1) domains. Mutagenesis to alanine reveals T221 as a critical controller of resting TRPC6 conductance, and associated NFAT activity and pro-hypertrophic signaling. T→A mutations at sites homologous in closely related TRPC3 and TRPC7 also increases their activity. Molecular modeling predicts interactions between Thr221-O-GlcNAc and Ser199, Glu200, and Glu246, and combined alanine substitutions of the latter similarly elevates resting NFAT activity. Thus, O-GlcNAcylated T221 and interactions with coordinating residues is required for normal TRPC6 channel conductance and NFAT activation.

Single serine on TSC2 exerts biased control over mTORC1 activation mediated by ERK1/2 but not Akt

Both ERK1/2 and Akt kinases activate mTORC1, but only the former is bidirectionally regulated by the status of serine S1364 on TSC2 that confers input-selective mTORC1 amplification or attenuation.

Tuberous sclerosis complex-2 (TSC2) negatively regulates mammalian target of rapamycin complex 1 (mTORC1), and its activity is reduced by protein kinase B (Akt) and extracellular response kinase (ERK1/2) phosphorylation to activate mTORC1. Serine 1364 (human) on TSC2 bidirectionally modifies mTORC1 activation by pathological growth factors or hemodynamic stress but has no impact on resting activity. We now show this modification biases to ERK1/2 but not Akt-dependent TSC2-mTORC1 activation. Endothelin-1–stimulated mTORC1 requires ERK1/2 activation and is bidirectionally modified by phospho-mimetic (S1364E) or phospho-silenced (S1364A) mutations. However, mTORC1 activation by Akt-dependent stimuli (insulin or PDGF) is unaltered by S1364 modification. Thrombin stimulates both pathways, yet only the ERK1/2 component is modulated by S1364. S1364 also has negligible impact on mTORC1 regulation by energy or nutrient status. In vivo, diet-induced obesity, diabetes, and fatty liver couple to Akt activation and are also unaltered by TSC2 S1364 mutations. This contrasts to prior reports showing a marked impact of both on pathological pressure-stress. Thus, S1364 provides ERK1/2-selective mTORC1 control and a genetic means to modify pathological versus physiological mTOR stimuli.

Abstract MP240: Site-specific O-Glcnacylation Of Transient Receptor Potential Canonical Type 6 (trpc6) Channel Is Required For Normal Structure And Function

O-GlcNAcylation is a dynamic, reversible posttranslational modification (PTM) that regulates a multitude of biological processes. Fluctuations in O-GlcNAC of various calcium handling proteins impact their functionality in cardiomyocytes. Here, we show for the first time that TRPC6, a nonselective receptor-operated cation channel and mediator of hypertrophy and fibrosis, is constitutively O-GlcNAcylated in the ankyrin repeat domain (AR4), at Ser 14, Thr 70, and Thr 221 within the N-terminal cytoplasmic segment. Of these, only substitution of Thr 221 with alanine (T221A) results in a change of function, notably a hyperactive TRPC6 channel with 5X greater increase in consequent NFAT promoter activity, which is a marker of TRPC6 calcium signaling. Patch-clamp analysis of T221A mutant channels found a 75-80% increased conductance compared to WT. Myocardial injection of T221A in homozygous TRPC6 KO mice by AAV-9 mediated gene transfer results in systolic dysfunction, hypertrophy, and cardiac fibrosis, by loss of OGlcNAc modification at site T221. T221 is highly conserved across species and found in the AR4 domain, which forms the core structure of TRPC6 intracellular domain. Mutating the site in its closest homologs, TRPC3 and TRPC7, also activates channel activity. T221 O-GlcNAcylation also protects the nascent protein from premature proteasomal degradation. Molecular modeling from the crystal structure of human TRPC6 predicts that OGlcNACylation stabilizes electrostatic interactions with the 193-203 loop near AR4, and loop connecting AR4 to the linker helix 1 (LH1) at S199, E200, and E246. Mutating these sites to alanine also increases TRPC6-NFAT signaling similar to what was observed in the T221A mutant. In summary, this study highlights that O-GlcNAcylation of TRPC6 is an important PTM needed to stabilize channel function, and its decline results in gain-of-function related diseases.

MTORC1-Regulated Metabolism Controlled by TSC2 Limits Cardiac Reperfusion Injury

RATIONALE

The mTORC1 (mechanistic target of rapamycin complex-1) controls metabolism and protein homeostasis and is activated following ischemia reperfusion (IR) injury and by ischemic preconditioning (IPC). However, studies vary as to whether this activation is beneficial or detrimental, and its influence on metabolism after IR is little reported. A limitation of prior investigations is their use of broad gain/loss of mTORC1 function, mostly applied before ischemic stress. This can be circumvented by regulating one serine (S1365) on TSC2 (tuberous sclerosis complex) to achieve bidirectional mTORC1 modulation but only with TCS2-regulated costimulation.

OBJECTIVE

We tested the hypothesis that reduced TSC2 S1365 phosphorylation protects the myocardium against IR and is required for IPC by amplifying mTORC1 activity to favor glycolytic metabolism.

METHODS AND RESULTS

Mice with either S1365A (TSC2^SA; phospho-null) or S1365E (TSC2^SE; phosphomimetic) knockin mutations were studied ex vivo and in vivo. In response to IR, hearts from TSC2^SA mice had amplified mTORC1 activation and improved heart function compared with wild-type and TSC2^SE hearts. The magnitude of protection matched IPC. IPC requited less S1365 phosphorylation, as TSC2^SE hearts gained no benefit and failed to activate mTORC1 with IPC. IR metabolism was altered in TSC2^SA, with increased mitochondrial oxygen consumption rate and glycolytic capacity (stressed/maximal extracellular acidification) after myocyte hypoxia-reperfusion. In whole heart, lactate increased and long-chain acylcarnitine levels declined during ischemia. The relative IR protection in TSC2^SA was lost by lowering glucose in the perfusate by 36%. Adding fatty acid (palmitate) compensated for reduced glucose in wild type and TSC2^SE but not TSC2^SA which had the worst post-IR function under these conditions.

CONCLUSIONS

TSC2-S1365 phosphorylation status regulates myocardial substrate utilization, and its decline activates mTORC1 biasing metabolism away from fatty acid oxidation to glycolysis to confer protection against IR. This pathway is also engaged and reduced TSC2 S1365 phosphorylation required for effective IPC. Graphic Abstract: A graphic abstract is available for this article.

Phosphorylation Modifications Regulating Cardiac Protein Quality Control Mechanisms

Many forms of cardiac disease, including heart failure, present with inadequate protein quality control (PQC). Pathological conditions often involve impaired removal of terminally misfolded proteins. This results in the formation of large protein aggregates, which further reduce cellular viability and cardiac function. Cardiomyocytes have an intricately collaborative PQC system to minimize cellular proteotoxicity. Increased expression of chaperones or enhanced clearance of misfolded proteins either by the proteasome or lysosome has been demonstrated to attenuate disease pathogenesis, whereas reduced PQC exacerbates pathogenesis. Recent studies have revealed that phosphorylation of key proteins has a potent regulatory role, both promoting and hindering the PQC machinery. This review highlights the recent advances in phosphorylations regulating PQC, the impact in cardiac pathology, and the therapeutic opportunities presented by harnessing these modifications.

CHIP phosphorylation by protein kinase G enhances protein quality control and attenuates cardiac ischemic injury

Proteotoxicity from insufficient clearance of misfolded/damaged proteins underlies many diseases. Carboxyl terminus of Hsc70-interacting protein (CHIP) is an important regulator of proteostasis in many cells, having E3-ligase and chaperone functions and often directing damaged proteins towards proteasome recycling. While enhancing CHIP functionality has broad therapeutic potential, prior efforts have all relied on genetic upregulation. Here we report that CHIP-mediated protein turnover is markedly post-translationally enhanced by direct protein kinase G (PKG) phosphorylation at S20 (mouse, S19 human). This increases CHIP binding affinity to Hsc70, CHIP protein half-life, and consequent clearance of stress-induced ubiquitinated-insoluble proteins. PKG-mediated CHIP-pS20 or expressing CHIP-S20E (phosphomimetic) reduces ischemic proteo- and cytotoxicity, whereas a phospho-silenced CHIP-S20A amplifies both. In vivo, depressing PKG activity lowers CHIP-S20 phosphorylation and protein, exacerbating proteotoxicity and heart dysfunction after ischemic injury. CHIP-S20E knock-in mice better clear ubiquitinated proteins and are cardio-protected. PKG activation provides post-translational enhancement of protein quality control via CHIP.

Targeting Protein Kinase G to Treat Cardiac Proteotoxicity

Impaired or insufficient protein kinase G (PKG) signaling and protein quality control (PQC) are hallmarks of most forms of cardiac disease, including heart failure. Their dysregulation has been shown to contribute to and exacerbate cardiac hypertrophy and remodeling, reduced cell survival and disease pathogenesis. Enhancement of PKG signaling and PQC are associated with improved cardiac function and survival in many pre-clinical models of heart disease. While many clinically used pharmacological approaches exist to stimulate PKG, there are no FDA-approved therapies to safely enhance cardiomyocyte PQC. The latter is predominantly due to our lack of knowledge and identification of proteins regulating cardiomyocyte PQC. Recently, multiple studies have demonstrated that PKG regulates PQC in the heart, both during physiological and pathological states. These studies tested already FDA-approved pharmacological therapies to activate PKG, which enhanced cardiomyocyte PQC and alleviated cardiac disease. This review examines the roles of PKG and PQC during disease pathogenesis and summarizes the experimental and clinical data supporting the utility of stimulating PKG to target cardiac proteotoxicity.

PKG1α Cysteine-42 Redox State Controls mTORC1 Activation in Pathological Cardiac Hypertrophy

RATIONALE

Stimulated PKG1α (protein kinase G-1α) phosphorylates TSC2 (tuberous sclerosis complex 2) at serine 1365, potently suppressing mTORC1 (mechanistic [mammalian] target of rapamycin complex 1) activation by neurohormonal and hemodynamic stress. This reduces pathological hypertrophy and dysfunction and increases autophagy. PKG1α oxidation at cysteine-42 is also induced by these stressors, which blunts its cardioprotective effects.

OBJECTIVE

We tested the dependence of mTORC1 activation on PKG1α C42 oxidation and its capacity to suppress such activation by soluble GC-1 (guanylyl cyclase 1) activation.

METHODS AND RESULTS

Cardiomyocytes expressing wild-type (WT) PKG1α (PKG1α^WT) or cysteine-42 to serine mutation redox-dead (PKG1α^CS/CS) were exposed to ET-1 (endothelin 1). Cells expressing PKG1α^WT exhibited substantial mTORC1 activation (p70 S6K [p70 S6 kinase], 4EBP1 [elF4E binding protein-1], and Ulk1 [Unc-51-like kinase 1] phosphorylation), reduced autophagy/autophagic flux, and abnormal protein aggregation; all were markedly reversed by PKG1α^CS/CS expression. Mice with global knock-in of PKG1α^CS/CS subjected to pressure overload (PO) also displayed markedly reduced mTORC1 activation, protein aggregation, hypertrophy, and ventricular dysfunction versus PO in PKG1α^WT mice. Cardioprotection against PO was equalized between groups by co-treatment with the mTORC1 inhibitor everolimus. TSC2-S1365 phosphorylation increased in PKG1α^CS/CS more than PKG1α^WT myocardium following PO. TSC2^{S1365A/S1365A} (TSC2 S1365 phospho-null, created by a serine to alanine mutation) knock-in mice lack TSC2 phosphorylation by PKG1α, and when genetically crossed with PKG1α^CS/CS mice, protection against PO-induced mTORC1 activation, cardiodepression, and mortality in PKG1α^CS/CS mice was lost. Direct stimulation of GC-1 (BAY-602770) offset disparate mTORC1 activation between PKG1α^WT and PKG1α^CS/CS after PO and blocked ET-1 stimulated mTORC1 in TSC2^S1365A-expressing myocytes.

CONCLUSIONS

Oxidation of PKG1α at C42 reduces its phosphorylation of TSC2, resulting in amplified PO-stimulated mTORC1 activity and associated hypertrophy, dysfunction, and depressed autophagy. This is ameliorated by direct GC-1 stimulation.

Abstract 346: Tuberin S1365 Phosphorylation Regulates Mechanistic Target of Rapamycin Complex 1 (mTORC1) Pathological Signaling While Sustaining Metabolic Sensor Function

Rationale: The Mechanistic Target of Rapamycin complex 1 (mTORC1) integrates signaling and sensory inputs to maintain cardiomyocyte homeostasis, and itself is negatively regulated by the signaling nexus tuberin (TSC2). We identified a novel TSC2 phosphorylation site S1365 (pS1365) targeted by protein kinase G (PKG), which suppressed hormonal growth factor (PE or ET1)-stimulated mTORC1 activity to attenuate pathological cardiomyocyte hypertrophy. This was recapitulated during growth factor stimulation with expression of a phospho-null (S1365A) or a phospho-mimetic (S1365E) TSC2 that exacerbated or blunted mTORC1 activation, respectively. The nature of TSC2 pS1365 as a potential metabolic sensor is unknown and will provide mechanistic insight into the TSC2 kinase input hierarchy that regulates the homeostatic function of mTORC1.

Objective: To determine how pS1365 affects the ability of TSC2 to integrate metabolic dependent signals to regulate mTORC1.

Methods/Results: TSC2 KO MEFs were infected with TSC2 WT or S1365A adenovirus, and then stimulated with ET1 (hormonal stress that also activates mTORC1), and 2-DG (AMPK stimulation). Both groups responded with similar decreases in mTORC1 activation regardless of pS1365. Phosphorylation of the AMPK site on TSC2 (S1387) was increased in all groups despite the presence of a phospho-null S1365. Both MEFs and neonatal rat cardiomyocytes (NRCMs) infected with TSC2 WT, S1365A, or S1365E adenovirus similarly increased mTORC1 activation with insulin (PI3K-Akt-TSC2 pathway) treatment. Serum withdrawal from NRCMs reduced mTORC1 activation in all groups regardless of whether a WT, S1365A, or S1365E TSC2 was expressed. In NRCMs subject to hypoxia (a combination of Erk, Akt, AMPK signaling), there was a similar observation with only nominal changes between WT, S1365A, and S1365E TSC2.

Conclusions: The energy and nutrient sensing role of the TSC2-mTORC1 pathway remains intact regardless of the phospho-status of TSC2 S1365. These findings provide important mechanistic insight into the function of TSC2 pS1365 as a potent suppressor of pathological mTORC1 activation while not affecting the ability of TSC2 to respond to the metabolic dependent signals – AMPK (energy), PI3K-Akt (insulin), and serum starvation.

Prevention of PKG-1α Oxidation Suppresses Antihypertrophic/Antifibrotic Effects From PDE5 Inhibition but not sGC Stimulation

BACKGROUND

Stimulation of sGC (soluble guanylate cyclase) or inhibition of PDE5 (phosphodiesterase type 5) activates PKG (protein kinase G)-1α to counteract cardiac hypertrophy and failure. PKG1α acts within localized intracellular domains; however, its oxidation at cysteine 42, linking homomonomers, alters this localization, impairing suppression of pathological cardiac stress. Because PDE5 and sGC reside in separate microdomains, we speculated that PKG1α oxidation might also differentially influence the effects from their pharmacological modulation.

METHODS AND RESULTS

Knock-in mice expressing a redox-dead PKG1α (PKG1α^C42S) or littermate controls (PKG1α^WT) were subjected to transaortic constriction to induce pressure overload and treated with a PDE5 inhibitor (sildenafil), sGC activator (BAY602770 [BAY]), or vehicle. In PKG1α^WT controls, sildenafil and BAY similarly enhanced PKG activity and reduced pathological hypertrophy/fibrosis and cardiac dysfunction after transaortic constriction. However, sildenafil failed to protect the heart in PKG1α^C42S, unlike BAY, which activated PKG and thereby facilitated protective effects. This corresponded with minimal PDE5 activation in PKG1α^C42S exposed to transaortic constriction versus higher activity in controls and little colocalization of PDE5 with PKG1α^C42S (versus colocalization with PKG1α^WT) in stressed myocytes.

CONCLUSIONS

In the stressed heart and myocytes, PKG1α C42-disulfide formation contributes to PDE5 activation. This augments the pathological role of PDE5 and so in turn enhances the therapeutic impact from its inhibition. PKG1α oxidation does not change the benefits from sGC activation. This finding favors the use of sGC activators regardless of PKG1α oxidation and may help guide precision therapy leveraging the cyclic GMP/PKG pathway to treat heart disease.

PKG1-modified TSC2 regulates mTORC1 activity to counter adverse cardiac stress

The mechanistic target of rapamycin complex-1 (mTORC1) coordinates regulation of growth, metabolism, protein synthesis, and autophagy¹. Its hyper-activation contributes to disease in many organs including the heart^1,2, though broad mTORC1 inhibition risks interference with its homeostatic roles. Tuberin (TSC2) is a GTPase-activating protein and prominent intrinsic regulator of mTORC1 by modulating Rheb (Ras homolog enriched in brain). TSC2 constitutively inhibits mTORC1, but this activity is modified by phosphorylation from multiple signaling kinases to in turn inhibit (AMPK, GSK3β) or stimulate (Akt, ERK, RSK-1) mTORC1 activity^3–9. Each kinase requires engagement of multiple serines, impeding analysis of their role in vivo. Here, we reveal phosphorylation or gain-or-loss of function mutations at either of two adjacent serines in TSC2 (S1365/1366 mouse; 1364/1365 human), with no prior known function, is sufficient to bi-directionally potently control growth-factor and hemodynamic-stress stimulated mTORC1 activity and consequent cell growth and autophagy. Basal mTORC1 activity, however, is unchanged. In heart, myocytes, and fibroblasts, phosphorylation occurs by protein kinase G (PKG), a primary effector of nitric oxide and natriuretic peptide signaling whose activation is protective against heart disease^10–13. PKG suppression of hypertrophy and stimulation of autophagy in myocytes requires TSC2 phosphorylation. Homozygous knock-in (KI) mice expressing a phospho-silenced TSC2 (S1365A) mutation develop far worse heart disease and mortality from sustained pressure-overload (PO) due to hyperactive mTORC1 that cannot be rescued by PKG stimulation. Heterozygote SA-KI are rescued, and KI-mice expressing a phospho-mimetic (S1365E) mutation are protected. Neither KI model alters resting mTORC1 activity. Thus, TSC2 phosphorylation is both required and sufficient for PKG-mediated cardiac protection against pressure-overload. These newly identified serines provide a genetic tool to bi-directionally regulate the amplitude of stress-stimulated mTORC1 activity.

Histone lysine dimethyl-demethylase KDM3A controls pathological cardiac hypertrophy and fibrosis

Left ventricular hypertrophy (LVH) is a major risk factor for cardiovascular morbidity and mortality. Pathological LVH engages transcriptional programs including reactivation of canonical fetal genes and those inducing fibrosis. Histone lysine demethylases (KDMs) are emerging regulators of transcriptional reprogramming in cancer, though their potential role in abnormal heart growth and fibrosis remains little understood. Here, we investigate gain and loss of function of an H3K9me2 specific demethylase, Kdm3a, and show it promotes LVH and fibrosis in response to pressure-overload. Cardiomyocyte KDM3A activates Timp1 transcription with pro-fibrotic activity. By contrast, a pan-KDM inhibitor, JIB-04, suppresses pressure overload-induced LVH and fibrosis. JIB-04 inhibits KDM3A and suppresses the transcription of fibrotic genes that overlap with genes downregulated in Kdm3a-KO mice versus WT controls. Our study provides genetic and biochemical evidence for a pro-hypertrophic function of KDM3A and proof-of principle for pharmacological targeting of KDMs as an effective strategy to counter LVH and pathological fibrosis.

Marked disparity of microRNA modulation by cGMP-selective PDE5 versus PDE9 inhibitors in heart disease

MicroRNAs (miRs) posttranscriptionally regulate mRNA and its translation into protein, and are considered master controllers of genes modulating normal physiology and disease. There is growing interest in how miRs change with drug treatment, and leveraging this for precision guided therapy. Here we contrast 2 closely related therapies, inhibitors of phosphodiesterase type 5 or type 9 (PDE5-I, PDE9-I), given to mice subjected to sustained cardiac pressure overload (PO). Both inhibitors augment cyclic guanosine monophosphate (cGMP) to activate protein kinase G, with PDE5-I regulating nitric oxide (NO) and PDE9-I natriuretic peptide-dependent signaling. While both produced strong phenotypic improvement of PO pathobiology, they surprisingly showed binary differences in miR profiles; PDE5-I broadly reduces more than 120 miRs, including nearly half those increased by PO, whereas PDE9-I has minimal impact on any miR (P < 0.0001). The disparity evolves after pre-miR processing and is organ specific. Lastly, even enhancing NO-coupled cGMP by different methods leads to altered miR regulation. Thus, seemingly similar therapeutic interventions can be barcoded by profound differences in miR signatures, and reversing disease-associated miR changes is not required for therapy success.

The role of heat shock proteins and co-chaperones in heart failure

The ongoing contractile and metabolic demands of the heart require a tight control over protein quality control, including the maintenance of protein folding, turnover and synthesis. In heart disease, increases in mechanical and oxidative stresses, post-translational modifications (e.g., phosphorylation), for example, decrease protein stability to favour misfolding in myocardial infarction, heart failure or ageing. These misfolded proteins are toxic to cardiomyocytes, directly contributing to the common accumulation found in human heart failure. One of the critical class of proteins involved in protecting the heart against these threats are molecular chaperones, including the heat shock protein70 (HSP70), HSP90 and co-chaperones CHIP (carboxy terminus of Hsp70-interacting protein, encoded by the Stub1 gene) and BAG-3 (BCL2-associated athanogene 3). Here, we review their emerging roles in the maintenance of cardiomyocytes in human and experimental models of heart failure, including their roles in facilitating the removal of misfolded and degraded proteins, inhibiting apoptosis and maintaining the structural integrity of the sarcomere and regulation of nuclear receptors. Furthermore, we discuss emerging evidence of increased expression of extracellular HSP70, HSP90 and BAG-3 in heart failure, with complementary independent roles from intracellular functions with important therapeutic and diagnostic considerations. While our understanding of these major HSPs in heart failure is incomplete, there is a clear potential role for therapeutic modulation of HSPs in heart failure with important contextual considerations to counteract the imbalance of protein damage and endogenous protein quality control systems.This article is part of the theme issue 'Heat shock proteins as modulators and therapeutic targets of chronic disease: an integrated perspective'.

Mast cells regulate myofilament calcium sensitization and heart function after myocardial infarction

Ngkelo et al. use a mast cell–deficient mouse model to reveal a protective role of mast cells in myocardial infarction, through regulation of the cardiac contractile machinery.

Acute myocardial infarction (MI) is a severe ischemic disease responsible for heart failure and sudden death. Inflammatory cells orchestrate postischemic cardiac remodeling after MI. Studies using mice with defective mast/stem cell growth factor receptor c-Kit have suggested key roles for mast cells (MCs) in postischemic cardiac remodeling. Because c-Kit mutations affect multiple cell types of both immune and nonimmune origin, we addressed the impact of MCs on cardiac function after MI, using the c-Kit–independent MC-deficient (Cpa3^Cre/+) mice. In response to MI, MC progenitors originated primarily from white adipose tissue, infiltrated the heart, and differentiated into mature MCs. MC deficiency led to reduced postischemic cardiac function and depressed cardiomyocyte contractility caused by myofilament Ca²⁺ desensitization. This effect correlated with increased protein kinase A (PKA) activity and hyperphosphorylation of its targets, troponin I and myosin-binding protein C. MC-specific tryptase was identified to regulate PKA activity in cardiomyocytes via protease-activated receptor 2 proteolysis. This work reveals a novel function for cardiac MCs modulating cardiomyocyte contractility via alteration of PKA-regulated force–Ca²⁺ interactions in response to MI. Identification of this MC-cardiomyocyte cross-talk provides new insights on the cellular and molecular mechanisms regulating the cardiac contractile machinery and a novel platform for therapeutically addressable regulators.

Abstract 358: Pacemaker Induced Transient Asynchrony (PITA) Restores Contractile Reserve in Synchronous Heart Failure

Heart failure (HF) with dyssynchrony treated with biventricular pacing (CRT) displays enhanced global and cellular function even compared to always synchronous HF. This suggests while HF is worsened by sustained dyssynchrony, it may paradoxically be improved by brief periods of Pacemaker Induced Transient Asynchrony (PITA). We tested this hypothesis in dogs tachypaced for 6 wks to induce HF. The HF group received atrial pacing and was compared to PITA (atrial pacing during day; right ventricular pacing, producing dyssynchrony, from 0000-0600). PITA blunted dilation (end diastolic and end systolic volumes reduced by 11 and 19%, respectively), reduced end-diastolic pressures from 22 to 13 mmHg, and improved the contractile response to dobutamine by 29%. Myocyte sarcomere shortening and calcium transient amplitude were depressed in HF and little improved by β adrenergic (βA) stimulation. PITA improved baseline function slightly, but virtually restored βA stimulated reserve. Membrane βA receptor density increased with PITA by 36% as well. Another contributor to the change in functional reserve was found in myofilament maximal calcium activated force (Fmax) normalized to cell cross sectional area (CSA). This declined ~40% in HF vs. Control, but was fully restored by PITA. However, as CSA was greater in HF and normalized by PITA, raw Fmax was similar despite hypertrophy in HF, suggesting HF myocytes had dysfunctional myofilaments, which PITA prevented. Electron microscopy confirmed normal myofilament structure in Control and PITA, whereas 40% of HF sarcomeres displayed deteriorated z-disks and loss of normal registration of the thick and thin filaments. In HF, 39% of HF isolated myofibrils produced virtually zero maximal force, whereas Control and PITA fibers functioned normally. Thus, there are two populations of myofibrils within HF hypertrophied cells, with ~40% structurally and functionally disrupted. PITA reverses this to restore force-calcium activation and with improved βA receptor signaling, restores functional reserve, suppressing chronic maladaptive remodeling. This surprising finding indicates PITA can ameliorate HF pathobiology and improve reserve function. Further studies are needed to test if such benefits translate to humans.

Genetically induced moderate inhibition of 20S proteasomes in cardiomyocytes facilitates heart failure in mice during systolic overload

The in vivo function status of the ubiquitin-proteasome system (UPS) in pressure overloaded hearts remains undefined. Cardiotoxicity was observed during proteasome inhibitor chemotherapy, especially in those with preexisting cardiovascular conditions; however, proteasome inhibition (PsmI) was also suggested by some experimental studies as a potential therapeutic strategy to curtail cardiac hypertrophy. Here we used genetic approaches to probe cardiac UPS performance and determine the impact of cardiomyocyte-restricted PsmI (CR-PsmI) on cardiac responses to systolic overload. Transgenic mice expressing an inverse reporter of the UPS (GFPdgn) were subject to transverse aortic constriction (TAC) to probe myocardial UPS performance during systolic overload. Mice with or without moderate CR-PsmI were subject to TAC and temporally characterized for cardiac responses to moderate and severe systolic overload. After moderate TAC (pressure gradient: ~40mmHg), cardiac UPS function was upregulated during the first two weeks but turned to functional insufficiency between 6 and 12weeks as evidenced by the dynamic changes in GFPdgn protein levels, proteasome peptidase activities, and total ubiquitin conjugates. Severe TAC (pressure gradients >60mmHg) led to UPS functional insufficiency within a week. Moderate TAC elicited comparable hypertrophic responses between mice with and without genetic CR-PsmI but caused cardiac malfunction in CR-PsmI mice significantly earlier than those without CR-PsmI. In mice subject to severe TAC, CR-PsmI inhibited cardiac hypertrophy but led to rapidly progressed heart failure and premature death, associated with a pronounced increase in cardiomyocyte death. It is concluded that cardiac UPS function is dynamically altered, with the initial brief upregulation of proteasome function being adaptive; and CR-PsmI facilitates cardiac malfunction during systolic overload.

Functional Amyloid Signaling via the Inflammasome, Necrosome, and Signalosome: New Therapeutic Targets in Heart Failure

As the most common cause of death and disability, globally, heart disease remains an incompletely understood enigma. A growing number of cardiac diseases are being characterized by the presence of misfolded proteins underlying their pathophysiology, including cardiac amyloidosis and dilated cardiomyopathy (DCM). At least nine precursor proteins have been implicated in the development of cardiac amyloidosis, most commonly caused by multiple myeloma light chain disease and disease-causing mutant or wildtype transthyretin (TTR). Similarly, aggregates with PSEN1 and COFILIN-2 have been identified in up to one-third of idiopathic DCM cases studied, indicating the potential predominance of misfolded proteins in heart failure. In this review, we present recent evidence linking misfolded proteins mechanistically with heart failure and present multiple lines of new therapeutic approaches that target the prevention of misfolded proteins in cardiac TTR amyloid disease. These include multiple small molecule pharmacological chaperones now in clinical trials designed specifically to support TTR folding by rational design, such as tafamidis, and chaperones previously developed for other purposes, such as doxycycline and tauroursodeoxycholic acid. Last, we present newly discovered non-pathological "functional" amyloid structures, such as the inflammasome and necrosome signaling complexes, which can be activated directly by amyloid. These may represent future targets to successfully attenuate amyloid-induced proteotoxicity in heart failure, as the inflammasome, for example, is being therapeutically inhibited experimentally in autoimmune disease. Together, these studies demonstrate multiple novel points in which new therapies may be used to primarily prevent misfolded proteins or to inhibit their downstream amyloid-mediated effectors, such as the inflammasome, to prevent proteotoxicity in heart failure.

Prevention of PKG1α oxidation augments cardioprotection in the stressed heart

The cGMP-dependent protein kinase-1α (PKG1α) transduces NO and natriuretic peptide signaling; therefore, PKG1α activation can benefit the failing heart. Disease modifiers such as oxidative stress may depress the efficacy of PKG1α pathway activation and underlie variable clinical results. PKG1α can also be directly oxidized, forming a disulfide bond between homodimer subunits at cysteine 42 to enhance oxidant-stimulated vasorelaxation; however, the impact of PKG1α oxidation on myocardial regulation is unknown. Here, we demonstrated that PKG1α is oxidized in both patients with heart disease and in rodent disease models. Moreover, this oxidation contributed to adverse heart remodeling following sustained pressure overload or Gq agonist stimulation. Compared with control hearts and myocytes, those expressing a redox-dead protein (PKG1α(C42S)) better adapted to cardiac stresses at functional, histological, and molecular levels. Redox-dependent changes in PKG1α altered intracellular translocation, with the activated, oxidized form solely located in the cytosol, whereas reduced PKG1α(C42S) translocated to and remained at the outer plasma membrane. This altered PKG1α localization enhanced suppression of transient receptor potential channel 6 (TRPC6), thereby potentiating antihypertrophic signaling. Together, these results demonstrate that myocardial PKG1α oxidation prevents a beneficial response to pathological stress, may explain variable responses to PKG1α pathway stimulation in heart disease, and indicate that maintaining PKG1α in its reduced form may optimize its intrinsic cardioprotective properties.

Phosphodiesterase 9A controls nitric-oxide-independent cGMP and hypertrophic heart disease

Cyclic guanosine monophosphate (cGMP) is a second messenger molecule that transduces nitric oxide (NO) and natriuretic peptide (NP) coupled signaling, stimulating phosphorylation changes by protein kinase G (PKG). Enhancing cGMP synthesis or blocking its degradation by phosphodiesterase type 5A (PDE5A) protects against cardiovascular disease^1,2. However, cGMP stimulation alone is limited by counter-adaptions including PDE upregulation³. Furthermore, though PDE5A regulates NO-generated cGMP^4,5, NO-signaling is often depressed by heart disease⁶. PDEs controlling NP-coupled cGMP remain uncertain. Here we show that cGMP-selective PDE9A^7,8 is expressed in mammalian heart including humans, and is upregulated by hypertrophy and cardiac failure. PDE9A regulates NP rather than NO-stimulated cGMP in heart myocytes and muscle, and its genetic or selective pharmacological inhibition protects against pathological responses to neuro-hormones, and sustained pressure-overload stress. PDE9A inhibition reverses pre-established heart disease independent of NO-synthase (NOS) activity, whereas PDE5A inhibition requires active NOS. Transcription factor activation and phospho-proteome analyses of myocytes with each PDE selectively inhibited reveals substantial differential targeting, with phosphorylation changes from PDE5A inhibition being more sensitive to NOS activation. Thus, unlike PDE5A, PDE9A can regulate cGMP signaling independent of the NO-pathway, and its role in stress-induced heart disease suggests potential as a therapeutic target.

Soluble guanylate cyclase is required for systemic vasodilation but not positive inotropy induced by nitroxyl in the mouse

Nitroxyl (HNO), the reduced and protonated form of nitric oxide (NO·), confers unique physiological effects including vasorelaxation and enhanced cardiac contractility. These features have spawned current pharmaceutical development of HNO donors as heart failure therapeutics. HNO interacts with selective redox sensitive cysteines to effect signaling but is also proposed to activate soluble guanylate cyclase (sGC) in vitro to induce vasodilation and potentially enhance contractility. Here, we tested whether sGC stimulation is required for these HNO effects in vivo and if HNO also modifies a redox-sensitive cysteine (C42) in protein kinase G-1α to control vasorelaxation. Intact mice and isolated arteries lacking the sGC-β subunit (sGCKO, results in full sGC deficiency) or expressing solely a redox-dead C42S mutant protein kinase G-1α were exposed to the pure HNO donor, CXL-1020. CXL-1020 induced dose-dependent systemic vasodilation while increasing contractility in controls; however, vasodilator effects were absent in sGCKO mice whereas contractility response remained. The CXL-1020 dose reversing 50% of preconstricted force in aortic rings was ≈400-fold greater in sGCKO than controls. Cyclic-GMP and cAMP levels were unaltered in myocardium exposed to CXL-1020, despite its inotropic-vasodilator activity. In protein kinase G-1α(C42S) mice, CXL-1020 induced identical vasorelaxation in vivo and in isolated aortic and mesenteric vessels as in littermate controls. In both groups, dilation was near fully blocked by pharmacologically inhibiting sGC. Thus, sGC and cGMP-dependent signaling are necessary and sufficient for HNO-induced vasodilation in vivo but are not required for positive inotropic action. Redox modulation of protein kinase G-1α is not a mechanism for HNO-mediated vasodilation.

Muscarinic 2 receptors modulate cardiac proteasome function in a protein kinase G-dependent manner

Proteasome function insufficiency and inadequate protein quality control are strongly implicated in a large subset of cardiovascular disease and may play an important role in their pathogenesis. Protein degradation by the ubiquitin proteasome system can be physiologically regulated. Cardiac muscarinic 2 (M2) receptors were pharmacologically interrogated in intact mice and cultured neonatal rat ventricular myocytes (NRVMs). Proteasome-mediated proteolysis was measured with a surrogate misfolded protein, proteasome peptidase assay, and by characterizing key proteasome subunits. Successful M2 receptor manipulation in cardiomyocytes was determined by measuring an endogenous protein substrate, and in mice, the cardiovascular physiological response. M2 receptor stimulation was associated with increased proteasome-mediated proteolysis and enhanced peptidase activities, while M2 receptor inhibition yielded opposing results. Additionally, M2 receptor manipulation did not alter abundance of the key proteasome subunits, Rpt6 and β5, but significantly shifted their isoelectric points. Inhibition of protein kinase G abrogated the stimulatory effects on proteasome-mediated proteolysis from M2 receptor activation. We conclude that M2 receptor stimulation enhances, whereas M2 receptor inhibition reduces, proteasome-mediated proteolysis likely through posttranslational modifications. Protein kinase G appears to be the mediator of the M2 receptors actions.

Abstract 300: Protein Kinase G Positively Regulates Proteasome-mediated Degradation of Misfolded Proteins

Objectives: Proteasome functional insufficiency is implicated in a large subset of cardiovascular diseases and may play an important role in their pathogenesis, evidenced by increased protein aggregates and ubiquitinated proteins. The regulation of proteasome function is poorly understood, hindering the development of effective strategies to improve proteasome function. We sought to establish the role of protein kinase G (PKG) in these debilitating conditions, for which there is currently no cure.

Methods and Results: PKG was manipulated genetically and pharmacologically in cultured cardiomyocytes. Activation of PKG increased proteasome peptidase activities, facilitated proteasome-mediated degradation of surrogate (GFPu) and bona fide misfolded proteins (CryABR120G), and attenuated CryABR120G overexpression-induced accumulation of ubiquitinated proteins and cellular injury. PKG inhibition elicited the opposite responses. Differences in the abundance of the key 26S proteasome subunits Rpt6 and β5 between PKG manipulated and the control groups were not statistically significant, but the isoelectric points of were shifted by PKG activation. In transgenic mice expressing a surrogate substrate (GFPdgn), PKG activation by sildenafil increased myocardial proteasome activities and significantly decreased myocardial GFPdgn protein levels. Sildenafil treatment significantly increased myocardial PKG activity and significantly reduced myocardial accumulation of CryABR120G, ubiquitin conjugates, and aberrant protein aggregates in mice with CryABR120G-based desmin-related cardiomyopathy. No discernible effect on bona fide native substrates of the ubiquitin-proteasome system was observed from PKG manipulation in vitro or in vivo.

Conclusions: PKG positively regulates proteasome activities and proteasome-mediated degradation of misfolded proteins likely through posttranslational modifications to proteasome subunits. Improved protein quality control is liekly a new mechanism underlying the benefit of PKG stimulation in treating cardiac diseases. Stimulation of PKG by measures such as sildenafil administration is potentially a novel therapeutic strategy to treat cardiac proteinopathies.

Protein kinase g positively regulates proteasome-mediated degradation of misfolded proteins

BACKGROUND

Proteasome functional insufficiency is implicated in a large subset of cardiovascular diseases and may play an important role in their pathogenesis. The regulation of proteasome function is poorly understood, hindering the development of effective strategies to improve proteasome function.

METHODS AND RESULTS

Protein kinase G (PKG) was manipulated genetically and pharmacologically in cultured cardiomyocytes. Activation of PKG increased proteasome peptidase activities, facilitated proteasome-mediated degradation of surrogate (enhanced green fluorescence protein modified by carboxyl fusion of degron CL1) and bona fide (CryAB(R120G)) misfolded proteins, and attenuated CryAB(R120G) overexpression-induced accumulation of ubiquitinated proteins and cellular injury. PKG inhibition elicited the opposite responses. Differences in the abundance of the key 26S proteasome subunits Rpt6 and β5 between the PKG-manipulated and control groups were not statistically significant, but the isoelectric points were shifted by PKG activation. In transgenic mice expressing a surrogate substrate (GFPdgn), PKG activation by sildenafil increased myocardial proteasome activities and significantly decreased myocardial GFPdgn protein levels. Sildenafil treatment significantly increased myocardial PKG activity and significantly reduced myocardial accumulation of CryAB(R120G), ubiquitin conjugates, and aberrant protein aggregates in mice with CryAB(R120G)-based desmin-related cardiomyopathy. No discernible effect on bona fide native substrates of the ubiquitin-proteasome system was observed from PKG manipulation in vitro or in vivo.

CONCLUSIONS

PKG positively regulates proteasome activities and proteasome-mediated degradation of misfolded proteins, likely through posttranslational modifications to proteasome subunits. This may be a new mechanism underlying the benefit of PKG stimulation in treating cardiac diseases. Stimulation of PKG by measures such as sildenafil administration is potentially a new therapeutic strategy to treat cardiac proteinopathies.

Abstract 320: Activation of Protein Kinase G Enhances Proteasome-Mediated Degradation of Misfolded Proteins

The Ubiquitin Proteasome System (UPS) plays a pivotal role in the intracellular protein homeostasis, thereby regulating a variety of cellular processes including protein quality control (PQC). Inadequate PQC and proteasome functional insufficiency are associated with and may play an important role in adverse cardiac remodeling and cardiac failure (CHF). This is best reflected by the increased presence of misfolded proteins and protein aggregates in failing human hearts. While the regulatory mechanisms of these processes remain poorly understood they carry significant clinical implications. Reduced protein kinase G (PKG) activity is well documented in CHF. Increasing the activity PKG, during various cardiovascular diseases improves patient prognosis. We hypothesize that PKG plays a role in regulating UPS function. Using both genetic and/or pharmacological manipulation of PKG activity in cultured cardiomyocytes and intact mice and taking advantage of a previously validated surrogate misfolded protein (GFPu or GFPdgn) as well as a bona fide misfolded protein (R120G-CryAB) that is known to cause proteinopathy in human and mice, our studies have revealed a significant role for PKG in UPS regulation. In cultured cardiomyocytes, PKG inhibition was achieved by PKG-specific siRNA transfection or administration of PKG pharmacological inhibitor while PKG activity was induced by adenoviral delivery of a constitutively active PKG or by the treatment of phosphodiesterase 5A inhibitor Sildenafil. PKG activation significantly stimulated UPS activity in cultured cardiomyocytes as indicated by a reduced protein level of GFPu and increased proteasome activities; while PKG inhibition reduced UPS proteolytic activity. Furthermore PKG activation resulted in enhanced removal of R120G-CryAB. These findings from cell culture studies were further corroborated by in vivo findings from manipulating the muscarinic (M) receptor in GFPdgn transgenic mice. The M receptor signals through PKG. Activating the M receptor deceased, while antagonizing the M receptor increased, the steady protein level of GFPdgn in the heart. Hence, we conclude that PKG positively regulates the UPS thereby supporting cardiomyocyte PQC.

Abstract P043: The COP9 Signalosome Regulates Autophagy

By indiscriminately degrading portions of cytoplasm for self-supply of nutrients, non-selective autophagy helps the cell to survive starvation. Selective autophagy, however, removes defective/surplus organelles and protein aggregates, thereby playing an important role in quality control in the cell. A utophagy is involved in the pathophysiology of a variety of disease, including common forms of heart disease. Mechanisms regulating autophagy, especially selective autophagy, remain poorly understood. The COP9 signalosome (CSN) is an evolutionarily conserved protein complex consisting of 8 subunits (CSN1 through CSN8). CSN was purported to regulate ubiquitin-proteasome system (UPS) mediated proteolysis. We recently reported UPS malfunction and the accumulation of ubiquitin positive aggregates in the cardiomyocytes of mice with perinatal cardiomyocyte-restricted Csn8 knockout (CR-Csn8KO), which displayed massive cardiomyocyte necrosis, congestive heart failure, and premature death. Here we report that Csn8/CSN is required for the removal of autophagosomes in cardiomyocytes, an exciting discovery that has not been reported in any types of cells. CR-Csn8KO mouse hearts show marked increases in LC3-II, indicative of increased autophagosomes. The increase in LC-II is accompanied by a significant increase in p62 protein levels, which is evident as early as 1 week of age, long before the accumulation of a surrogate UPS substrate becomes discernible. The increase in autophagosomes is confirmed by probing with a transgenic GFP-LC3 and by electron microscopy. Autophagic flux assessments reveal that the removal of autophagosomes in cardiomyocytes is impaired by Csn8 deficiency and the defective fusion between autophagosomes and lysosomes may be responsible. Rab7 transcript and protein levels in the heart are significantly decreased by Csn8 deficiency. Confocal microscopy reveals a striking reciprocal relationship between increases in GFP-LC3 puncta and the decreased Rab7 expression. Rab7 knockdown impairs the removal of autophagosomes in cultured cardiomyocytes. Hence, Csn8/CSN is a central regulator in not only the UPS but also autophagy. Csn8/CSN supports autophagosome-lysosome fusion likely by stimulating Rab7 expression.

Abstract P053: Impaired Autophagosome Removal in Cardiomyocytes Triggers Programmed Necrosis in Mouse Hearts

Programmed cell death includes apoptosis and programmed necrosis. The latter is also known as necroptosis. A well-recognized feature of necrosis is the loss of membrane integrity of the dying cell, resulting in inflammatory responses at the surrounding tissue. Cells undergoing apoptosis, however, maintain their membrane integrity very well and hence do not trigger inflammation. These features are effectively captivated to detect necrotic cells. By assessing Evans Blue Dye (EBD) uptake and CD45+ leukocytes, we previously detected massive cardiomyocyte (CM) necrosis, before increased apoptosis was evident, in mouse hearts deficient of COP9 signalosome (CSN) subunit 8 (Csn8) ( Circ Res 2011; 108:40–50 ). We show here that the detection of necrosis is confirmed by immuno-detection of endogenous IgG inside the CM. We found that virtually all EBD positive cells were also mouse IgG positive whereas some of the IgG positive CMs, as expected, did not show increased EBD uptake, suggesting that probing intracellular IgG offers a simpler and even more sensitive method for detecting necrotic CMs in vivo. Notably, we were able to demonstrate that both the EBD uptake assay and the immuno-detection of intracellular IgG clearly have a resolution at the single cell level. We then explored the mechanism by which Csn8/CSN deficiency causes necrosis. Autophagosomes are accumulated due to a defect in their fusion with lysosomes in mouse hearts with perinatal CM-restricted Csn8 knockout (CR-Csn8KO) but the accumulation is heterogeneous among CMs likely due to a temporal heterogeneity of Csn8 gene ablation and/or a spatial difference in the microenvironment and functionality of different CMs. A remarkably higher percentage of CMs with greater autophagosome accumulation were undergoing necrosis than those with less or without autophagosome accumulation. Importantly, chronic lysosomal inhibition caused CM necrosis in wild type mice. Moreover, the protein levels of RIP1, RIP3 kinases and the RIP1-bound RIP3 were all significantly increased in the CR-Csn8KO heart. Taken together, these novel findings strongly suggest that impaired autophagosome removal causes CM necrosis in Csn8/CSN deficient hearts through likely activating the RIP1-RIP3 mediated necroptotic pathway.

COP9 signalosome regulates autophagosome maturation

BACKGROUND

Autophagy is essential to intracellular homeostasis and is involved in the pathophysiology of a variety of diseases. Mechanisms regulating selective autophagy remain poorly understood. The COP9 signalosome (CSN) is a conserved protein complex consisting of 8 subunits (CSN1 through CSN8), and is known to regulate the ubiquitin-proteasome system. However, it is unknown whether CSN plays a role in autophagy.

METHODS AND RESULTS

Marked increases in the LC3-II and p62 proteins were observed on Csn8 depletion in the cardiomyocytes of mouse hearts with cardiomyocyte-restricted knockout of the gene encoding CSN subunit 8 (CR-Csn8KO). The increases in autophagosomes were confirmed by probing with green fluorescent protein-LC3 and electron microscopy. Autophagic flux assessments revealed that defective autophagosome removal was the cause of autophagosome accumulation and occurred before a global ubiquitin-proteasome system impairment in Csn8-deficient hearts. Analyzing the prevalence of different stages of autophagic vacuoles revealed defective autophagosome maturation. Downregulation of Rab7 was found to colocalize strikingly with the autophagosome accumulation at the individual cardiomyocyte level. A significantly higher percent of cardiomyocytes with autophagosome accumulation underwent necrosis in CR-Csn8KO hearts. Long-term lysosomal inhibition with chloroquine induced cardiomyocyte necrosis in mice. Rab7 knockdown impaired autophagosome maturation of nonselective and selective autophagy and exacerbated cell death induced by proteasome inhibition in cultured cardiomyocytes.

CONCLUSIONS

Csn8/CSN is a central regulator in not only the proteasomal proteolytic pathway, but also selective autophagy. Likely through regulating the expression of Rab7, Csn8/CSN plays a critical role in autophagosome maturation. Impaired autophagosome maturation causes cardiomyocytes to undergo necrosis.

Autophagy and p62 in cardiac proteinopathy

RATIONALE

Recent studies suggest an important role of autophagy in protection against αB-crystallin-based (CryAB(R120G)) desmin-related cardiomyopathies (DRC), but this has not been demonstrated in a different model of cardiac proteinopathy. Mechanisms underlying the response of cardiomyocytes to proteotoxic stress remain incompletely understood.

OBJECTIVE

Our first objective was to determine whether and how the autophagic activity is changed in a mouse model of desminopathy. We also investigated the role of p62 in the protein quality control of cardiomyocytes.

METHODS AND RESULTS

Using an autophagosome reporter and determining changes in LC3-II protein levels in response to lysosomal inhibition, we found significantly increased autophagic flux in mouse hearts with transgenic overexpression of a DRC-linked mutant desmin. Similarly, autophagic flux was increased in cultured neonatal rat ventricular myocytes (NRVMs) expressing a mutant desmin. Suppression of autophagy by 3-methyladenine increased, whereas enhancement of autophagy by rapamycin reduced the ability of a comparable level of mutant desmin overexpression to accumulate ubiquitinated proteins in NRVMs. Furthermore, p62 mRNA and protein expression was significantly up-regulated in cardiomyocytes by transgenic overexpression of the mutant desmin or CryAB(R120G) both in intact mice and in vitro. The p62 depletion impaired aggresome and autophagosome formation, exacerbated cell injury, and decreased cell viability in cultured NRVMs expressing the misfolded proteins.

CONCLUSIONS

Autophagic flux is increased in desminopathic hearts, and as previously suggested in CryAB(R120G)-based DRC, this increased autophagic flux serves as an adaptive response to overexpression of misfolded proteins. The p62 is up-regulated in mouse proteinopathic hearts. The p62 promotes aggresome formation and autophagy activation and protects cardiomyocytes against proteotoxic stress.

Activation of the ubiquitin-proteasome system in doxorubicin cardiomyopathy

Doxorubicin (Dox) is a very potent anticancer agent, but its use is limited by its dose-dependent, irreversible cardiotoxicity. Despite intensive research efforts, the mechanism of Dox cardiotoxicity remains poorly understood, so very limited means are available for its prevention or effective management. Recent studies have revealed that a therapeutic dose of Dox can activate proteolysis in cardiomyocytes that is mediated by the ubiquitin-proteasome system (UPS), and that the UPS-mediated degradation of a number of pivotal cardiac transcription factors and/or survival factors is enhanced by Dox treatment. These findings suggest that Dox-induced UPS activation may represent a new mechanism underlying Dox cardiotoxicity. Notably, recent experimental studies suggest that proteasome activation promotes cardiac remodeling during hypertension. This review surveys the current literature on the impact of Dox on the UPS and the potential mechanisms by which UPS activation may compromise the heart during Dox therapy.

Protein quality control and degradation in cardiomyocytes

The heart is constantly under stress and cardiomyocytes face enormous challenges to correctly fold nascent polypeptides and keep mature proteins from denaturing. To meet the challenge, cardiomyocytes have developed multi-layered protein quality control (PQC) mechanisms which are carried out primarily by chaperones and ubiquitin-proteasome system mediated proteolysis. Autophagy may also participate in PQC in cardiomyocytes, especially under pathological conditions. Cardiac PQC often becomes inadequate in heart disease, which may play an important role in the development of congestive heart failure.