Calcineurin-mediated hypertrophy protects cardiomyocytes from apoptosis in vitro and in vivo: An apoptosis-independent model of dilated heart failure

A case study of a patient-centered reverse translational systems-based approach to understand adverse event profiles in drug development

Adverse drug reactions (ADRs) of targeted therapy drugs (TTDs) are frequently unexpected and long‐term toxicities detract from exceptional efficacy of new TTDs. In this proof‐of‐concept study, we explored how molecular causation involved in trastuzumab‐induced cardiotoxicity changes when trastuzumab was given in combination with doxorubicin, tamoxifen, paroxetine, or lapatinib. The data analytical platform Molecular Health Effect was utilized to map population ADR data from the US Food and Drug Administration (FDA) Adverse Event Reporting System to chemical and biological databases (such as UniProt and Reactome), for hypothesis generation regarding the underlying molecular mechanisms causing cardiotoxicity. Disproportionality analysis was used to assess the statistical relevance between adverse events of interest and molecular causation. Literature search was performed to compare the established hypotheses to published experimental findings. We found that the combination therapy of trastuzumab and doxorubicin may affect mitochondrial dysfunction in cardiomyocytes through different molecular pathways such as BCL‐X and PGC‐1α proteins, leading to a synergistic effect of cardiotoxicity. We found, on the other hand, that trastuzumab‐induced cardiotoxicity would be diminished by concomitant use of tamoxifen, paroxetine, and/or lapatinib. Tamoxifen and paroxetine may cause less cardiotoxicity through an increase in antioxidant activities, such as glutathione conjugation. Lapatinib may decrease the apoptotic effects in cardiomyocytes by altering the effects of trastuzumab on BCL‐X proteins. This patient‐centered systems‐based approach provides, based on the trastuzumab‐induced ADR cardiotoxicity, an example of how to apply reverse translation to investigate ADRs at the molecular pathway and target level to understand the causality and prevalence during drug development of novel therapeutics.

Catecholamine-induced cardiotoxicity: A critical element in the pathophysiology of stroke-induced heart injury

Cerebrovascular diseases such as ischemic stroke, brain hemorrhage, and subarachnoid hemorrhage provoke cardiac complications such as heart failure, neurogenic stress-related cardiomyopathy and Takotsubo cardiomyopathy. With regards to the pathophysiology of stroke-induced heart injury, several mechanisms have been postulated to contribute to this complex interaction between brain and heart, including damage from gut dysbiosis, immune and systematic inflammatory responses, microvesicle- and microRNA-mediated vascular injury and damage from a surge of catecholamines. All these cerebrovascular diseases may trigger pronounced catecholamine surges through diverse ways, including stimulation of hypothalamic-pituitary adrenal axis, dysregulation of autonomic system, and secretion of adrenocorticotropic hormone. Primary catecholamines involved in this pathophysiological response include norepinephrine (NE) and epinephrine. Both are important neurotransmitters that connect the nervous system with the heart, leading to cardiac damage via myocardial ischemia, calcium (Ca²⁺) overload, oxidative stress, and mitochondrial dysfunction. In this review, we will aim to summarize the molecular mechanisms behind catecholamine-induced cardiotoxicity including Ca²⁺ overload, oxidative stress, apoptosis, cardiac hypertrophy, interstitial fibrosis, and inflammation. In addition, we will focus on how synchronization among these pathways evokes cardiotoxicity.

RCAN1.4 attenuates renal fibrosis through inhibiting calcineurin-mediated nuclear translocation of NFAT2

Chronic kidney disease (CKD) is thus deemed to a global health problem. Renal fibrosis, characterized by accumulation of extracellular matrix (ECM) components in the kidney, is considered a common pathway leading to CKD. Regulator of calcineurin1 (RCAN1), identified as a competitive endogenous inhibitor of the phosphatase calcineurin, participates in ECM deposition in various organs. However, the role of RCAN1 in renal fibrosis remains unclear. Here, unilateral ureteral obstruction (UUO), a well-known model to induce renal fibrosis in vivo, was performed on mice for a week. To overexpress RCAN1.4 in vivo, recombinant adeno-associated virus 9-packed RCAN1.4 over-expression plasm was employed in mice kidney. Lentivirus-packed RCAN1.4 over-expression plasm was employed to transfer into HK-2 and NRK-49F cells in vitro. The results indicated that RCAN1.4 expression was impaired both in UUO-induced renal fibrosis in vivo and TGF-β1-induced renal fibrosis in vitro. However, knocking in of RCAN1.4 suppressed the production of extracellular matrix (ECM) both in vivo and in vitro. Furthermore, in vitro, the apoptosis-related proteins, including the ratio of Bax/Bcl-2 and cleaved-caspase3, were elevated in cells transfected with RCAN1.4 overexpression plasmid. In addition, we found that RCAN1.4 could rugulated NFAT2 nuclear distribution by inhibiting calcineurin pathway. So overexpression of RCAN1.4 could reverse renal fibrosis, attenuate ECM related protein accumulation, promote apoptosis of myofibroblast via inhibiting Calcineurin/NFAT2 signaling pathway. Taken together, our study demonstrated that targeting RCAN1.4 may be therapeutic efficacy in renal fibrosis.

A Review of Calcineurin Biophysics with Implications for Cardiac Physiology

Calcineurin, also known as protein phosphatase 2B, is a heterodimeric serine threonine phosphatase involved in numerous signaling pathways. During the past 50 years, calcineurin has been the subject of extensive investigation. Many of its cellular and physiological functions have been described, and the underlying biophysical mechanisms are the subject of active investigation. With the abundance of techniques and experimental designs utilized to study calcineurin and its numerous substrates, it is difficult to reconcile the available information. There have been a plethora of reports describing the role of calcineurin in cardiac disease. However, a physiological role of calcineurin in healthy cardiomyocyte function requires clarification. Here, we review the seminal biophysical and structural details that are responsible for the molecular function and inhibition of calcineurin. We then focus on literature describing the roles of calcineurin in cardiomyocyte physiology and disease.

Cardiac CaMKIIδ and Wenxin Keli Prevents Ang II-Induced Cardiomyocyte Hypertrophy by Modulating CnA-NFATc4 and Inflammatory Signaling Pathways in H9c2 Cells

Previous studies have demonstrated that calcium-/calmodulin-dependent protein kinase II (CaMKII) and calcineurin A-nuclear factor of activated T-cell (CnA-NFAT) signaling pathways play key roles in cardiac hypertrophy (CH). However, the interaction between CaMKII and CnA-NFAT signaling remains unclear. H9c2 cells were cultured and treated with angiotensin II (Ang II) with or without silenced CaMKIIδ (siCaMKII) and cyclosporine A (CsA, a calcineurin inhibitor) and subsequently treated with Wenxin Keli (WXKL). Patch clamp recording was conducted to assess L-type Ca²⁺ current (I_Ca-L), and the expression of proteins involved in signaling pathways was measured by western blotting. Myocardial cytoskeletal protein and nuclear translocation of target proteins were assessed by immunofluorescence. The results indicated that siCaMKII suppressed Ang II-induced CH, as evidenced by reduced cell surface area and I_Ca-L. Notably, siCaMKII inhibited Ang II-induced activation of CnA and NFATc4 nuclear transfer. Inflammatory signaling was inhibited by siCaMKII and WXKL. Interestingly, CsA inhibited CnA-NFAT pathway expression but activated CaMKII signaling. In conclusion, siCaMKII may improve CH, possibly by blocking CnA-NFAT and MyD88 signaling, and WXKL has a similar effect. These data suggest that inhibiting CaMKII, but not CnA, may be a promising approach to attenuate CH and arrhythmia progression.

Targeting the forkhead box protein P1 pathway as a novel therapeutic approach for cardiovascular diseases

Cardiovascular disease (CVD) is the leading cause of death worldwide and encompasses diverse diseases of the vasculature, myocardium, cardiac electrical circuit, and cardiac development. Forkhead box protein P1 (Foxp1) is a large multi-domain transcriptional regulator belonging to the Fox family with winged helix DNA-binding protein, which plays critical roles in cardiovascular homeostasis and disorders. The broad distribution of Foxp1 and alternative splicing isoforms implicate its distinct functions in diverse cardiac and vascular cells and tissue types. Foxp1 is essential for diverse biological processes and has been shown to regulate cellular proliferation, apoptosis, oxidative stress, fibrosis, angiogenesis, cardiovascular remodeling, and dysfunction. Notably, both loss-of-function and gain-of-function approaches have defined critical roles of Foxp1 in CVD. Genetic deletion of Foxp1 results in pathological cardiac remodeling, exacerbation of atherosclerotic lesion formation, prolonged occlusive thrombus formation, severe cardiac defects, and embryo death. In contrast, activation of Foxp1 performs a wide range of physiological effects, including cell growth, hypertrophy, differentiation, angiogenesis, and cardiac development. More importantly, Foxp1 exerts anti-inflammatory and anti-atherosclerotic effects in controlling coronary thrombus formation and myocardial infarction (MI). Thus, targeting for Foxp1 signaling has emerged as a pre-warning biomarker and a novel therapeutic approach against progression of CVD, and an increased understanding of cardiovascular actions of the Foxp1 signaling will help to develop effective interventions. In this review, we focus on the diverse actions and underlying mechanisms of Foxp1 highlighting its roles in CVD, including heart failure, MI, atherosclerosis, congenital heart defects, and atrial fibrillation.

Enalapril attenuates endoplasmic reticulum stress and mitochondrial injury induced by myocardial infarction via activation of the TAK1/NFAT pathway in mice

The present study investigated the effect of enalapril on myocardial infarction (MI) and its mechanism of action in mice. Treatment with enalapril significantly attenuated cellular apoptosis and death. In vivo, enalapril treatment alleviated MI injury, and decreased myocardial apoptosis and the size of the infarct area. This was paralleled by increased Bcl-2 expression, decreased Bax expression, a decreased caspase-3 level, decreased expression of endoplasmic reticulum stress-associated proteins, including activating transcription factor 6 and 78 kDa glucose-regulated protein, and fewer TUNEL-positive cells in the heart. Furthermore, enalapril-treatment increased transforming growth factor-activated kinase 1/nuclear factor of activated T cells 3 signaling, which protected the myocardium.

Reduced Compensatory β-Cell Proliferation in Nfatc3-Deficient Mice Fed on High-Fat Diet

BACKGROUND

High-fat-diet induces pancreatic β-cell compensatory proliferation, and impairments in pancreatic β-cell proliferation and function can lead to defects in insulin secretion and diabetes. NFATc3 is important for HFD-induced adipose tissue inflammation. But it is unknown whether NFATc3 is required for β cell compensatory growth in mice fed with HFD.

METHODS

NFATc3 mRNA and protein expression levels were quantified by RT-qPCR and Western blotting, respectively, in pancreatic islets of WT mice fed on HFD for 12-20 weeks. Adenoviral-mediated overexpression of NFATc3 were conducted in Min6 cells and cultured primary mouse islets. NFATc3-/- mice and WT control mice were fed with HFD and metabolic and functional parameters were measured.

RESULTS

We observed that the NFATc3 expression level was reduced in the islets of high-fat-diet (HFD)-fed mice. Adenovirus-mediated overexpression of NFATc3 enhanced glucose-stimulated insulin secretion and β-cell gene expression in cultured primary mouse islets. Nfatc3-/- mice initially developed similar glucose tolerance at 2-4 weeks after HFD feeding than HFD-fed WT mice, but Nfatc3-/- mice developed improved glucose tolerance and insulin sensitivity after 8 weeks of HFD feeding compared to Nfatc3+/+fed with HFD. Furthermore, Nfatc3-/- mice on HFD exhibited decreased β-cell mass and reduced expression of genes important for β-cell proliferation and function compared to Nfatc3+/+mice on HFD.

CONCLUSIONS

The findings suggested that NFATc3 plays a role in maintaining the pancreatic β-cell compensatory growth and gene expression in response to obesity.

Calreticulin Is Required for TGF-β-Induced Epithelial-to-Mesenchymal Transition during Cardiogenesis in Mouse Embryonic Stem Cells

Calreticulin, a multifunctional endoplasmic reticulum resident protein, is required for TGF-β-induced epithelial-to-mesenchymal transition (EMT) and subsequent cardiomyogenesis. Using embryoid bodies (EBs) derived from calreticulin-null and wild-type (WT) embryonic stem cells (ESCs), we show that expression of EMT and cardiac differentiation markers is induced during differentiation of WT EBs. This induction is inhibited in the absence of calreticulin and can be mimicked by inhibiting TGF-β signaling in WT cells. The presence of calreticulin in WT cells permits TGF-β-mediated signaling via AKT/GSK3β and promotes repression of E-cadherin by SNAIL2/SLUG. This is paralleled by induction of N-cadherin in a process known as the cadherin switch. We show that regulated Ca2+ signaling between calreticulin and calcineurin is critical for the unabated TGF-β signaling that is necessary for the exit from pluripotency and the cadherin switch during EMT. Calreticulin is thus a key mediator of TGF-β-induced commencement of cardiomyogenesis in mouse ESCs.

Cardiac Overexpression of S100A6 Attenuates Cardiomyocyte Apoptosis and Reduces Infarct Size After Myocardial Ischemia-Reperfusion

Background

Cardiomyocyte‐specific transgenic mice overexpressing S100A6, a member of the family of EF‐hand calcium‐binding proteins, develop less cardiac hypertrophy, interstitial fibrosis, and myocyte apoptosis after permanent coronary ligation, findings that support S100A6 as a potential therapeutic target after acute myocardial infarction. Our purpose was to investigate S100A6 gene therapy for acute myocardial ischemia‐reperfusion.

Methods and Results

We first performed in vitro studies to examine the effects of S100A6 overexpression and knockdown in rat neonatal cardiomyocytes. S100A6 overexpression improved calcium transients and protected against apoptosis induced by hypoxia‐reoxygenation via enhanced calcineurin activity, whereas knockdown of S100A6 had detrimental effects. For in vivo studies, human S100A6 plasmid or empty plasmid was delivered to the left ventricular myocardium by ultrasound‐targeted microbubble destruction in Fischer‐344 rats 2 days prior to a 30‐minute ligation of the left anterior descending coronary artery followed by reperfusion. Control animals received no therapy. Pretreatment with S100A6 gene therapy yielded a survival advantage compared to empty‐plasmid and nontreated controls. S100A6‐pretreated animals had reduced infarct size and improved left ventricular systolic function, with less myocyte apoptosis, attenuated cardiac hypertrophy, and less cardiac fibrosis.

Conclusions

S100A6 overexpression by ultrasound‐targeted microbubble destruction helps ameliorate myocardial ischemia‐reperfusion, resulting in lower mortality and improved left ventricular systolic function post–ischemia‐reperfusion via attenuation of apoptosis, reduction in cardiac hypertrophy, and reduced infarct size. Our results indicate that S100A6 is a potential therapeutic target for acute myocardial infarction.

Signaling Pathways in Cardiac Myocyte Apoptosis

Cardiovascular diseases, the number 1 cause of death worldwide, are frequently associated with apoptotic death of cardiac myocytes. Since cardiomyocyte apoptosis is a highly regulated process, pharmacological intervention of apoptosis pathways may represent a promising therapeutic strategy for a number of cardiovascular diseases and disorders including myocardial infarction, ischemia/reperfusion injury, chemotherapy cardiotoxicity, and end-stage heart failure. Despite rapid growth of our knowledge in apoptosis signaling pathways, a clinically applicable treatment targeting this cellular process is currently unavailable. To help identify potential innovative directions for future research, it is necessary to have a full understanding of the apoptotic pathways currently known to be functional in cardiac myocytes. Here, we summarize recent progress in the regulation of cardiomyocyte apoptosis by multiple signaling molecules and pathways, with a focus on the involvement of these pathways in the pathogenesis of heart disease. In addition, we provide an update regarding bench to bedside translation of this knowledge and discuss unanswered questions that need further investigation.

TRPC4α and TRPC4β Similarly Affect Neonatal Cardiomyocyte Survival during Chronic GPCR Stimulation

The Transient Receptor Potential Channel Subunit 4 (TRPC4) has been considered as a crucial Ca²⁺ component in cardiomyocytes promoting structural and functional remodeling in the course of pathological cardiac hypertrophy. TRPC4 assembles as homo or hetero-tetramer in the plasma membrane, allowing a non-selective Na⁺ and Ca²⁺ influx. Gαq protein-coupled receptor (GPCR) stimulation is known to increase TRPC4 channel activity and a TRPC4-mediated Ca²⁺ influx which has been regarded as ideal Ca²⁺ source for calcineurin and subsequent nuclear factor of activated T-cells (NFAT) activation. Functional properties of TRPC4 are also based on the expression of the TRPC4 splice variants TRPC4α and TRPC4β. Aim of the present study was to analyze cytosolic Ca²⁺ signals, signaling, hypertrophy and vitality of cardiomyocytes in dependence on the expression level of either TRPC4α or TRPC4β. The analysis of Ca²⁺ transients in neonatal rat cardiomyocytes (NRCs) showed that TRPC4α and TRPC4β affected Ca²⁺ cycling in beating cardiomyocytes with both splice variants inducing an elevation of the Ca²⁺ transient amplitude at baseline and TRPC4β increasing the Ca²⁺ peak during angiotensin II (Ang II) stimulation. NRCs infected with TRPC4β (Ad-C4β) also responded with a sustained Ca²⁺ influx when treated with Ang II under non-pacing conditions. Consistent with the Ca²⁺ data, NRCs infected with TRPC4α (Ad-C4α) showed an elevated calcineurin/NFAT activity and a baseline hypertrophic phenotype but did not further develop hypertrophy during chronic Ang II/phenylephrine stimulation. Down-regulation of endogenous TRPC4α reversed these effects, resulting in less hypertrophy of NRCs at baseline but a markedly increased hypertrophic enlargement after chronic agonist stimulation. Ad-C4β NRCs did not exhibit baseline calcineurin/NFAT activity or hypertrophy but responded with an increased calcineurin/NFAT activity after GPCR stimulation. However, this effect was not translated into an increased propensity towards hypertrophy but rather less hypertrophy during GPCR stimulation. Further analyses revealed that, although hypertrophy was preserved in Ad-C4α NRCs and even attenuated in Ad-C4β NRCs, cardiomyocytes had an increased apoptosis rate and thus were less viable after chronic GPCR stimulation. These findings suggest that TRPC4α and TRPC4β differentially affect Ca²⁺ signals, calcineurin/NFAT signaling and hypertrophy but similarly impair cardiomyocyte viability during GPCR stimulation.

Calcineurin proteolysis in astrocytes: Implications for impaired synaptic function

Mounting evidence suggests that astrocyte activation, found in most forms of neural injury and disease, is linked to the hyperactivation of the protein phosphatase calcineurin. In many tissues and cell types, calcineurin hyperactivity is the direct result of limited proteolysis. However, little is known about the proteolytic status of calcineurin in activated astrocytes. Here, we developed a polyclonal antibody to a high activity calcineurin proteolytic fragment in the 45-48kDa range (ΔCN) for use in immunohistochemical applications. When applied to postmortem human brain sections, the ΔCN antibody intensely labeled cell clusters in close juxtaposition to amyloid deposits and microinfarcts. Many of these cells exhibited clear activated astrocyte morphology. The expression of ΔCN in astrocytes near areas of pathology was further confirmed using confocal microscopy. Multiple NeuN-positive cells, particularly those within microinfarct core regions, also labeled positively for ΔCN. This observation suggests that calcineurin proteolysis can also occur within damaged or dying neurons, as reported in other studies. When a similar ΔCN fragment was selectively expressed in hippocampal astrocytes of intact rats (using adeno-associated virus), we observed a significant reduction in the strength of CA3-CA1 excitatory synapses, indicating that the hyperactivation of astrocytic calcineurin is sufficient for disrupting synaptic function. Together, these results suggest that proteolytic activation of calcineurin in activated astrocytes may be a central mechanism for driving and/or exacerbating neural dysfunction during neurodegenerative disease and injury.

Epithelial calcineurin controls microbiota-dependent intestinal tumor development

Inflammation-associated pathways are active in intestinal epithelial cells (IECs) and contribute to the pathogenesis of colorectal cancer (CRC). Calcineurin, a phosphatase required for the activation of the nuclear factor of activated T cells (NFAT) family of transcription factors, shows increased expression in CRC. We therefore investigated the role of calcineurin in intestinal tumor development. We demonstrate that calcineurin and NFAT factors are constitutively expressed by primary IECs and selectively activated in intestinal tumors as a result of impaired stratification of the tumor-associated microbiota and toll-like receptor signaling. Epithelial calcineurin supports the survival and proliferation of cancer stem cells in an NFAT-dependent manner and promotes the development of intestinal tumors in mice. Moreover, somatic mutations that have been identified in human CRC are associated with constitutive activation of calcineurin, whereas nuclear translocation of NFAT is associated with increased death from CRC. These findings highlight an epithelial cell-intrinsic pathway that integrates signals derived from the commensal microbiota to promote intestinal tumor development.

Stem cell death and survival in heart regeneration and repair

Cardiovascular diseases are major causes of mortality and morbidity. Cardiomyocyte apoptosis disrupts cardiac function and leads to cardiac decompensation and terminal heart failure. Delineating the regulatory signaling pathways that orchestrate cell survival in the heart has significant therapeutic implications. Cardiac tissue has limited capacity to regenerate and repair. Stem cell therapy is a successful approach for repairing and regenerating ischemic cardiac tissue; however, transplanted cells display very high death percentage, a problem that affects success of tissue regeneration. Stem cells display multipotency or pluripotency and undergo self-renewal, however these events are negatively influenced by upregulation of cell death machinery that induces the significant decrease in survival and differentiation signals upon cardiovascular injury. While efforts to identify cell types and molecular pathways that promote cardiac tissue regeneration have been productive, studies that focus on blocking the extensive cell death after transplantation are limited. The control of cell death includes multiple networks rather than one crucial pathway, which underlies the challenge of identifying the interaction between various cellular and biochemical components. This review is aimed at exploiting the molecular mechanisms by which stem cells resist death signals to develop into mature and healthy cardiac cells. Specifically, we focus on a number of factors that control death and survival of stem cells upon transplantation and ultimately affect cardiac regeneration. We also discuss potential survival enhancing strategies and how they could be meaningful in the design of targeted therapies that improve cardiac function.

TAK1 Regulates Myocardial Response to Pathological Stress via NFAT, NFκB, and Bnip3 Pathways

TAK1 (TGFβ-activated kinase-1) signaling is essential in regulating a number of important biological functions, including innate immunity, inflammatory response, cell growth and differentiation, and myocardial homeostasis. The precise role of TAK1 in the adult heart under pathological conditions remains largely unknown. Importantly, we observed that TAK1 is upregulated during compensatory hypertrophy but downregulated in end-stage heart failure. Here we generated transgenic mice with inducible expression of an active TAK1 mutant (TAK1ΔN) in the adult heart. TAK1ΔN transgenic mice developed greater cardiac hypertrophy compared with control mice after transverse aortic constriction (TAC), which was largely blocked by ablation of calcineurin Aβ. Expression of TAK1ΔN also promoted NFAT (nuclear factor of activated T-cells) transcriptional activity in luciferase reporter mice at baseline, which was further enhanced after TAC. Our results revealed that activation of TAK1 promoted adaptive cardiac hypertrophy through a cross-talk between calcineurin-NFAT and IKK-NFκB pathways. More significantly, adult-onset inducible expression of TAK1ΔN protected the myocardium from adverse remodeling and heart failure after myocardial infarction or long-term pressure overload, by preventing cardiac cell death and fibrosis. Mechanistically, TAK1 exerts its cardioprotective effect through activation of NFAT/NFκB, downregulation of Bnip3, and inhibition of cardiac cell death.

Ongoing controversies surrounding cardiac remodeling: is it black and white-or rather fifty shades of gray?

Cardiac remodeling describes the heart's multimodal response to a myriad of external or intrinsic stimuli and stressors most of which are probably only incompletely elucidated to date. Over many years the signaling molecules involved in these remodeling processes have been dichotomized according to a classic antagonistic view of black and white, i.e., attributed either a solely maladaptive or entirely beneficial character. By dissecting controversies, recent developments and shifts in perspective surrounding the three major cardiac signaling molecules calcineurin (Cn), protein kinase A (PKA) and calcium/calmodulin-dependent kinase II (CaMKII), this review challenges this dualistic view and advocates the nature and dignity of each of these key mediators of cardiac remodeling as a multilayered, highly context-sensitive and sophisticated continuum that can be markedly swayed and influenced by a multitude of environmental factors and crosstalk mechanisms. Furthermore this review delineates the importance and essential contributions of degradation and proteolysis to cardiac plasticity and homeostasis and finally aims to integrate the various aspects of protein synthesis and turnover into a comprehensive picture.

Na(+)/H (+) exchanger isoform 1 induced osteopontin expression in cardiomyocytes involves NFAT3/Gata4

Osteopontin (OPN), a multifunctional glycophosphoprotein, has been reported to contribute to the development and progression of cardiac remodeling and hypertrophy. Cardiac-specific OPN knockout mice were protected against hypertrophy and fibrosis mediated by Ang II. Recently, transgenic mice expressing the active form of the Na(+)/H(+) exchanger isoform 1 (NHE1) developed spontaneous hypertrophy in association with elevated levels of OPN. The mechanism by which active NHE1 induces OPN expression and contributes to the hypertrophic response remains unclear. To validate whether expression of the active form of NHE1 induces OPN, cardiomyocytes were stimulated with Ang II, a known inducer of both OPN and NHE1. Ang II induced hypertrophy and increased OPN protein expression (151.6 ± 28.19 %, P < 0.01) and NHE1 activity in H9c2 cardiomyoblasts. Ang II-induced hypertrophy and OPN protein expression were regressed in the presence of an NHE1 inhibitor, EMD 87580, or a calcineurin inhibitor, FK506. In addition, our results indicated that activation of NHE1-induced NFAT3 translocation into the nucleus and a significant activation of the transcription factor Gata4 (NHE1: 149 ± 28 % of control, P < 0.05). NHE1-induced activation of Gata4 was inhibited by FK506. In summary, our results suggest that activation of NHE1 induces hypertrophy through the activation of NFAT3/Gata4 and OPN expression.

High throughput phenotyping of left and right ventricular cardiomyopathy in calcineurin transgene mice

Consistent protocols for the assessment of diastolic and systolic cardiac function to assure the comparability of existing data on preclinical models are missing. Calcineurin transgene (CN) mice are a preclinical model for hypertrophic and failing hearts. We aimed at evaluating left and right ventricular structural and functional remodeling in CN hearts with an optimized phenotyping protocol. We developed a protocol using techniques and indices comparable to those from human diagnostics for comprehensive in vivo cardiac screening using high-frequency echocardiography, Doppler, electrocardiography and cardiac magnetic resonance (CMR) techniques. We measured left and right ventricular dimensions and function, pulmonary and mitral flow pattern and the hearts electrophysiology non-invasively in <1 h per mouse. We found severe biventricular dilation and a drastic decline in performance in accordance with a condition of heart failure (HF), diastolic dysfunction and defects in electrical conduction in 8-week-old calcineurin transgenic mice. Echocardiography of the left ventricle was performed with and without anesthesia. In all cases absolute values on echocardiography compared with CMR were smaller for LV dimension and wall thickness, resulting in higher fractional shorting and ejection fraction. The study protocol described here opens opportunities to assess the added value of combined echocardiography, Doppler, CMR and ECG recording techniques for the diagnosis of biventricular cardiac pathologies i.e. of HF and to study symptom occurrence and disease progression non-invasively in high-throughput. Phenotyping CN hearts revealed new symptom occurrence and allowed insights into the diverse phenotype of hypertrophic failing hearts.

Reduction of prohibitin expression contributes to left ventricular hypertrophy via enhancement of mitochondrial reactive oxygen species formation in spontaneous hypertensive rats

Left ventricular hypertrophy (LVH) in hypertension is characterized by thickening of myocardium and decrease in heart chamber volume in response to mechanical or pathological stress, but the underlying molecular mechanisms remain to be defined. In this work, we investigate whether mitochondrial prohibitin (PHB) was involved in the progression of LVH in spontaneous hypertensive rats (SHR). First, it was found that mitochondrial dysfunction occurred in left ventricles of SHR. Through analysis using quantitative reverse transcription polymerase chain reaction and Western blotting, it was found that PHB mRNA and mitochondrial PHB levels in left ventricles of SHR were significantly lower than that in Wistar-Kyoto rats. Furthermore, PHB mRNA levels were negatively correlated to left ventricles weight-to-body weight ratio in SHR. Knockdown of PHB led to increased formation of mitochondrial reactive oxygen species (ROS) and reduced activities of complex I, mitochondrial adenosine triphosphate generation and mitochondrial membrane potential in cultured cardiomyocytes. Knockdown of PHB contributed to the cardiomyocyte hypertrophy, which could be attenuated by treatment with the Tempol. Angiotensin II (AngII) was increased in plasma and left ventricles of SHR. Incubation with AngII reduced mitochondrial PHB expression in cardiomyocytes, which was reversed when pretreated with losartan. In conclusion, reduction of PHB expression in left ventricles in SHR contributed to LVH, at least in part, through promoting mitochondrial ROS formation.

Compensatory renal hypertrophy following uninephrectomy is calcineurin-independent

Calcineurin is a calcium-dependent phosphatase that is involved in many cellular processes including hypertrophy. Inhibition or genetic loss of calcineurin blocks pathological cardiac hypertrophy and diabetic renal hypertrophy. However, calcineurin does not appear to be involved in physiological cardiac hypertrophy induced by exercise. The role of calcineurin in a compensatory, non-pathological model of renal hypertrophy has not been tested. Therefore, in this study, we examined activation of calcineurin and the effect of calcineurin inhibition or knockout on compensatory hypertrophy following uninephrectomy (UNX). UNX induces ∼15% increase in the size of the remaining kidney; the data show no change in the generation of reactive oxygen species (ROS), Nox4 or transforming growth factor-β expression confirming the model as one of compensatory hypertrophy. Next, analyses of the remaining kidney reveal that total calcineurin activity is increased, and, to a lesser extent, transcriptional activity of the calcineurin substrate nuclear factor of activated T cell is up-regulated following UNX. However, inhibition of calcineurin with cyclosporine failed to prevent compensatory renal hypertrophy. Likewise, hypertrophy was comparable to WT in mice lacking either isoform of the catalytic subunit of calcineurin (CnAα−/− or CnAβ−/−). In conclusion, similar to its role in the heart, calcineurin is required for pathological but not compensatory renal hypertrophy. This separation of signalling pathways could therefore help further define key factors necessary for pathological hypertrophy including diabetic nephropathy.

Negative feedback regulation of calcineurin-dependent Prz1 transcription factor by the CaMKK-CaMK1 axis in fission yeast

Calcium signals trigger the translocation of the Prz1 transcription factor from the cytoplasm to the nucleus. The process is regulated by the calcium-activated phosphatase calcineurin, which activates Prz1 thereby maintaining active transcription during calcium signalling. When calcium signalling ceases, Prz1 is inactivated by phosphorylation and exported to the cytoplasm. In budding yeast and mammalian cells, different kinases have been reported to counter calcineurin activity and regulate nuclear export. Here, we show that the Ca(2+)/calmodulin-dependent kinase Cmk1 is first phosphorylated and activated by the newly identified kinase CaMKK2 homologue, Ckk2, in response to Ca(2+). Then, active Cmk1 binds, phosphorylates and inactivates Prz1 transcription activity whilst at the same time cmk1 expression is enhanced by Prz1 in response to Ca(2+). Furthermore, Cdc25 phosphatase is also phosphorylated by Cmk1, inducing cell cycle arrest in response to an increase in Ca(2+). Moreover, cmk1 deletion shows a high tolerance to chronic exposure to Ca(2+), due to the lack of cell cycle inhibition and elevated Prz1 activity. This work reveals that Cmk1 kinase activated by the newly identified Ckk2 counteracts calcineurin function by negatively regulating Prz1 activity which in turn is involved in activating cmk1 gene transcription. These results are the first insights into Cmk1 and Ckk2 function in Schizosaccharomyces pombe.

Transient receptor potential channels contribute to pathological structural and functional remodeling after myocardial infarction

RATIONALE

The cellular and molecular basis for post-myocardial infarction (MI) structural and functional remodeling is not well understood.

OBJECTIVE

Our aim was to determine if Ca2+ influx through transient receptor potential canonical (TRPC) channels contributes to post-MI structural and functional remodeling.

METHODS AND RESULTS

TRPC1/3/4/6 channel mRNA increased after MI in mice and was associated with TRPC-mediated Ca2+ entry. Cardiac myocyte-specific expression of a dominant-negative (loss-of-function) TRPC4 channel increased basal myocyte contractility and reduced hypertrophy and cardiac structural and functional remodeling after MI while increasing survival in mice. We used adenovirus-mediated expression of TRPC3/4/6 channels in cultured adult feline myocytes to define mechanistic aspects of these TRPC-related effects. TRPC3/4/6 overexpression in adult feline myocytes induced calcineurin (Cn)-nuclear factor of activated T-cells (NFAT)-mediated hypertrophic signaling, which was reliant on caveolae targeting of TRPCs. TRPC3/4/6 expression in adult feline myocytes increased rested state contractions and increased spontaneous sarcoplasmic reticulum Ca2+ sparks mediated by enhanced phosphorylation of the ryanodine receptor. TRPC3/4/6 expression was associated with reduced contractility and response to catecholamines during steady-state pacing, likely because of enhanced sarcoplasmic reticulum Ca2+ leak.

CONCLUSIONS

Ca2+ influx through TRPC channels expressed after MI activates pathological cardiac hypertrophy and reduces contractility reserve. Blocking post-MI TRPC activity improved post-MI cardiac structure and function.

The miR-19a/b family positively regulates cardiomyocyte hypertrophy by targeting atrogin-1 and MuRF-1

Progressive cardiac hypertrophy owing to pathological stimuli, such as pressure overload, is frequently associated with the development of heart failure, a major cause of morbidity and mortality worldwide. Growing evidence has shown that miRNAs are extensively involved in the pathogenesis of cardiac hypertrophy. In the present study, we examined the hypothesis that the miR-19a/b family acts as a key regulator of cardiac hypertrophy and apoptosis. Forced overexpression of miR-19a/b was sufficient to induce hypertrophy in rat neonatal cardiomyocytes. Luciferase assays revealed that miR-19a/b directly target the anti-hypertrophic genes atrogin-1 and MuRF-1 (muscle RING-finger protein-1). The endogenous expressions of the target genes were down-regulated by miR-19a/b. Pro-hypertrophic calcineurin/NFAT (nuclear factor of activated T-cells) signalling was elevated markedly in the presence of miR-19b, and the calcineurin inhibitor CsA (cyclosporin A) and the PKC (protein kinase C) inhibitor GF10923X significantly attenuated the miR-19b-mediated increase in cell size and expression of hypertrophic markers. Furthermore, miR-19b led to increased cell survival through up-regulation of the NFAT target gene encoding α-crystallin-B and repression of the pro-apoptotic gene Bim (Bcl-2-interacting mediator of cell death) under ER (endoplasmic reticulum) stress conditions. Taken together, the results of the present study demonstrate that the miR-19a/b family regulates phenotypes of cardiomyocytes via suppression of multiple direct target genes.

Chronic exercise downregulates myocardial myoglobin and attenuates nitrite reductase capacity during ischemia-reperfusion

The infarct sparing effects of exercise are evident following both long-term and short-term training regimens. Here we compared the infarct-lowering effects of nitrite therapy, voluntary exercise, and the combination of both following myocardial ischemia-reperfusion (MI/R) injury. We also compared the degree to which each strategy increased cardiac nitrite levels, as well as the effects of each strategy on the nitrite reductase activity of the heart. Mice subjected to voluntary wheel running (VE) for 4weeks displayed an 18% reduction in infarct size when compared to sedentary mice, whereas mice administered nitrite therapy (25mg/L in drinking water) showed a 53% decrease. However, the combination of VE and nitrite exhibited no further protection than VE alone. Although the VE and nitrite therapy mice showed similar nitrite levels in the heart, cardiac nitrite reductase activity was significantly reduced in the VE mice. Additionally, the cardiac protein expression of myoglobin, a known nitrite reductase, was also reduced after VE. Further studies revealed that cardiac NFAT activity was lower after VE due to a decrease in calcineurin activity and an increase in GSK3β activity. These data suggest that VE downregulates cardiac myoglobin levels by inhibiting calcineurin/NFAT signaling. Additionally, these results suggest that the modest infarct sparing effects of VE are the result of a decrease in the hearts ability to reduce nitrite to nitric oxide during MI/R.

Parsing good versus bad signaling pathways in the heart: role of calcineurin-nuclear factor of activated T-cells

Nine years ago, we published an article that suggested a specialized role for calcineurin–nuclear factor of activated T-cells (NFAT) signaling in regulating pathological cardiac hypertrophy preferentially over physiological growth and, in fact, the later response was associated with reduced calcineurin-NFAT activity. Since this time we and others have continued to uncover how this signaling effector pathway functions in the heart in regulating specific aspects of the growth response during disease and with exercise.

Physiologic functions of cyclophilin D and the mitochondrial permeability transition pore

This review focuses on the role of cyclophilin D (CypD) as a prominent mediator of the mitochondrial permeability transition pore (MPTP) and subsequent effects on cardiovascular physiology and pathology. Although a great number of reviews have been written on the MPTP and its effects on cell death, we focus on the biology surrounding CypD itself and the non-cell death physiologic functions of the MPTP. A greater understanding of the physiologic functions of the MPTP and its regulation by CypD will likely suggest novel therapeutic approaches for cardiovascular disease, both dependent and independent of programmed necrotic cell death mechanisms.  

Mechanisms involved in postconditioning protection of cardiomyocytes against acute reperfusion injury

Experimental and clinical studies demonstrated that postconditioning confers protection against myocardial ischemia/reperfusion injury. However the underlying cellular mechanisms responsible for the beneficial effect of postconditioning are still poorly understood. The aim of the present study was to examine the role of cytosolic and mitochondrial Ca(2+)-handling. For this purpose adult rat cardiomyocytes were subjected to simulated in vitro ischemia (glucose-free hypoxia at pH6.4) followed by simulated reperfusion with a normoxic buffer (pH7.4; 2.5 mmol/L glucose). Postconditioning, i.e., 2 repetitive cycles of normoxic (5s) and hypoxic (2.5 min) superfusion, was applied during the first 5 min of reoxygenation. Mitochondrial membrane potential (ΔΨm), cytosolic and mitochondrial Ca(2+) concentrations, cytosolic pH and necrosis were analysed applying JC-1, fura-2, fura-2/manganese, BCECF and propidium iodide, respectively. Mitochondrial permeability transition pore (MPTP) opening was detected by calcein release. Hypoxic treatment led to a reduction of ΔΨm, an increase in cytosolic and mitochondrial Ca(2+) concentration, and acidification of cardiomyocytes. During the first minutes of reoxygenation, ΔΨm transiently recovered, but irreversibly collapsed after 7 min of reoxygenation, which was accompanied by MPTP opening. Simultaneously, mitochondrial Ca(2+) increased during reperfusion and cardiomyocytes developed spontaneous cytosolic Ca(2+) oscillations and severe contracture followed by necrosis after 25 min of reoxygenation. In postconditioned cells, the collapse in ΔΨm as well as the leak of calcein, the increase in mitochondrial Ca(2+), cytosolic Ca(2+) oscillations, contracture and necrosis were significantly reduced. Furthermore postconditioning delayed cardiomyocyte pH recovery. Postconditioning by hypoxia/reoxygenation was as protective as treatment with cyclosporine A. Combining cyclosporine A and postconditioning had no additive effect. The data of the present study demonstrate that postconditioning by hypoxia/reoxygenation prevents reperfusion injury by limiting mitochondrial Ca(2+) load and thus opening of the MPTP in isolated cardiomyocytes. These effects seem to be supported by postconditioning-induced delay in pH recovery and suppression of Ca(2+) oscillations.

Nampt secreted from cardiomyocytes promotes development of cardiac hypertrophy and adverse ventricular remodeling

Nicotinamide phosphoribosyltransferase (Nampt) is an important coenzyme involved in cellular redox reactions. Inside the cell, Nampt (iNampt) functions as a rate-limiting enzyme in the NAD salvage pathway, and outside the cell (eNampt), it acts as a proinflammatory cytokine. High-circulating levels of Nampt are reported in different pathological conditions. This study was designed to examine the role of Nampt in the development of cardiac hypertrophy and ventricular remodeling. We studied the hypertrophic response in Nampt heterozygous (+/-) knockout and cardiac-specific overexpressing Nampt transgenic mice. Whereas Nampt(+/-) mice were protected against agonist (isoproterenol and angiotensin II)-induced hypertrophy, Nampt transgenic mice spontaneously developed cardiac hypertrophy at 6 mo of age. Experiments conducted to gain insight into the mechanism revealed that treatment of cardiomyocytes with recombinant (eNampt) or overexpression with Nampt-synthesizing adenovirus vector (Ad.Nampt) induced cardiomyocyte hypertrophy. The prohypertrophic effects of eNampt and Ad.Nampt were blocked by the addition of a Nampt-blocking antibody into cultures, thus suggesting that Nampt was in fact invoking hypertrophic response of cardiomyocytes by acting on the cell surface receptors. We also found increased Nampt levels in the supernatant of cardiomyocyte cultures subjected to stress by either serum starvation or H(2)O(2) treatment. Exploration of signaling pathways in Nampt-induced cardiac hypertrophy and fibrosis revealed increased activation of mitogen-activated protein kinases, namely, JNK1, p38, and ERK. This was also associated with increased calcineurin levels and nuclear factor of activated T-cell localization into the nucleus. From these studies we conclude that cardiomyocytes are capable of secreting Nampt during stress, and exogenous Nampt is a positive regulator of cardiac hypertrophy and adverse ventricular remodeling.

Anchoring proteins as regulators of signaling pathways

Spatial and temporal organization of signal transduction is coordinated through the segregation of signaling enzymes in selected cellular compartments. This highly evolved regulatory mechanism ensures the activation of selected enzymes only in the vicinity of their target proteins. In this context, cAMP-responsive triggering of protein kinase A is modulated by a family of scaffold proteins referred to as A-kinase anchoring proteins. A-kinase anchoring proteins form the core of multiprotein complexes and enable simultaneous but segregated cAMP signaling events to occur in defined cellular compartments. In this review we will focus on the description of A-kinase anchoring protein function in the regulation of cardiac physiopathology.

Regulation of RCAN1 translation and its role in oxidative stress-induced apoptosis

Abnormal expression of regulator of calcineurin 1 (RCAN1) has been implicated in Alzheimer's disease (AD) and Down's syndrome (DS). There are two major isoforms of RCAN1, isoforms 1 and 4. RCAN1 isoform 1 is predominantly expressed in the brain, particularly in neurons. In this report, we showed that there are two translation start codons in RCAN1 exon 1 serving as a functional translation initiation site to generate a longer 41-kDa isoform 1 (RCAN1.1L) and a shorter 31-kDa isoform 1 (RCAN1.1S). The first translation initiation site has higher translation efficiency than the downstream second one, and the translation initiation of two AUG sites is by a Cap-dependent mechanism. Short-term expression of RCAN1.1L protected SH-SY5Y cells from oxidative stress-induced apoptosis by inhibiting caspase-3 activation. However, long-term accumulation of RCAN1.1L in SH-SY5Y cells promoted oxidative stress-induced apoptosis via caspase-3 activation, and terminal deoxynucleotidyl transferase dUTP nick end labeling assay showed that the apoptosis ratio was increased to 499.03 ± 47.56% in SH-1.1L cells compared with 283.93 ± 28.66% in control cells. Furthermore, we found that RCAN1.1L is significantly elevated in the AD brains and patients with DS. RCAN1.1S is expressed at a low level in both human cells and brain tissues. Our results defined the regulatory mechanism underlying RCAN1 expression and the roles of RCAN1.1 in oxidative stress-induced neurodegeneration in AD and DS pathogenesis.

Calcineurin regulation of cytoskeleton organization: a new paradigm to analyse the effects of calcineurin inhibitors on the kidney

Calcineurin is a serine/threonine phosphatase originally involved in the immune response but is also known for its role as a central mediator in various non-immunological intracellular signals. The nuclear factor of activated T cell (NFAT) proteins are the most widely described substrates of calcineurin, but ongoing work has uncovered other substrates among which are the cytoskeleton organizing proteins (i.e. cofilin, synaptopodin, WAVE-1). Control over cytoskeletal proteins is of outmost interest because the phenotypic properties of cells are dependent on cytoskeleton architecture integrity, while rearrangements of the cytoskeleton are implicated in both physiological and pathological processes. Previous works investigating the role of calcineurin on the cytoskeleton have focused on neurite elongation, myocyte hypertrophic response and recently in kidney cells structure. Nuclear factor of activated T cell activation is expectedly identified in the signalling pathways for calcineurin-induced cytoskeleton organization, however new NFAT-independent pathways have also been uncovered. The aim of this review is to summarize the current knowledge on the effects of calcineurin on cytoskeletal proteins and related intracellular pathways. These newly described properties of calcineurin on cytoskeletal proteins may explain some of the beneficial or deleterious effects observed in kidney cells associated with the use of the calcineurin inhibitors, cyclosporine and tacrolimus.

Novel Ser/Thr protein phosphatases in cell death regulation

Cell death is regulated by a myriad of intracellular molecular pathways, with many involving protein phosphorylation and dephosphorylation. In this review, we will focus on Ser/Thr phosphatases-mediated regulation in cell apoptosis as well as on their potential roles in cell necrosis. The emerging functional importance of Ser/Thr protein phosphatases in cell death regulation adds new dimension to the signaling mechanisms of cellular function, physiology, and diseases.

O-GlcNAc signaling is essential for NFAT-mediated transcriptional reprogramming during cardiomyocyte hypertrophy

The regulation of cardiomyocyte hypertrophy is a complex interplay among many known and unknown processes. One specific pathway involves the phosphatase calcineurin, which regulates nuclear translocation of the essential cardiac hypertrophy transcription factor, nuclear factor of activated T-cells (NFAT). Although metabolic dysregulation is frequently described during cardiac hypertrophy, limited insights exist regarding various accessory pathways. One metabolically derived signal, beta-O-linked N-acetylglucosamine (O-GlcNAc), has emerged as a highly dynamic posttranslational modification of serine and threonine residues regulating physiological and stress processes. Given the metabolic dysregulation during hypertrophy, we hypothesized that NFAT activation is dependent on O-GlcNAc signaling. Pressure overload-induced hypertrophy (via transverse aortic constriction) in mice or treatment of neonatal rat cardiac myocytes with phenylephrine significantly enhanced global O-GlcNAc signaling. NFAT-luciferase reporter activity revealed O-GlcNAc-dependent NFAT activation during hypertrophy. Reversal of enhanced O-GlcNAc signaling blunted cardiomyocyte NFAT-induced changes during hypertrophy. Taken together, these results demonstrate a critical role of O-GlcNAc signaling in NFAT activation during hypertrophy and provide evidence that O-GlcNAc signaling is coordinated with the onset and progression of cardiac hypertrophy. This represents a potentially significant and novel mechanism of cardiac hypertrophy, which may be of particular interest in future in vivo studies of hypertrophy.

Polydatin, a natural polyphenol, protects arterial smooth muscle cells against mitochondrial dysfunction and lysosomal destabilization following hemorrhagic shock

The main objective of this study was to investigate the activity of polydatin on mitochondrial dysfunction and lysosomal stability of arteriolar smooth muscle cells (ASMCs) in severe shock. The experimental animals (rats) were divided into five groups: control, hemorrhagic shock, shock + CsA, shock + Res, and shock + PD (exposed to cyclosporin A, resveratrol, or polydatin following induction of hemorrhagic shock, respectively). The calcein-Co(2+) technique revealed opening of ASMC mitochondrial permeability transition pores (mPTP) after shock with resulting mitochondrial swelling, decreased mitochondrial membrane potential (ΔΨm), and reduced intracellular ATP levels. These alterations were all inhibited by exposure to PD, which was significantly more effective than CsA and Res. PD also preserved lysosomal stability, suppressed activation of K(ATP) channels, ASMC hyperpolarization, and reduced vasoresponsiveness to norepinephrine that normally follows severe shock. The results demonstrate that exposure to PD after initiation of severe shock effectively preserves ASMC mitochondrial integrity and has a significant therapeutic effect in severe shock. The effects may partially result from lysosomal stabilization against shock-induced oxidative stress and depressed relocation of hydrolytic enzymes and redox-active lysosomal iron that, in turn, may induce mPTP opening.

Isoproterenol cytotoxicity is dependent on the differentiation state of the cardiomyoblast H9c2 cell line

H9c2 cells are used as a surrogate for cardiac cells in several toxicological studies, which are usually performed with cells in their undifferentiated state, raising questions on the applicability of the results to adult cardiomyocytes. Since H9c2 myoblasts have the capacity to differentiate into skeletal and cardiac muscle cells under different conditions, the hypothesis of the present work was that cells in different differentiation states differ in their susceptibility to toxicants. In order to test the hypothesis, the effects of the cardiotoxicant isoproterenol (ISO) were investigated. The present work demonstrates that differentiated H9c2 cells are more susceptible to ISO toxicity. Cellular content of beta(1)-adrenergic receptors (AR), beta(3)-AR, and calcineurin is decreased as cells differentiate, as opposed to the content on the mitochondrial voltage-dependent anion channel (VDAC) and phosphorylated p38-MAPK, which increase. After ISO treatment, the pro-apoptotic protein Bax increases in all experimental groups, although only undifferentiated myoblasts up-regulate the anti-apoptotic Bcl-2. Calcineurin is decreased in differentiated H9c2 cells, which suggests an important role against ISO-induced cell death. The results indicate that the differentiation state of H9c2 myoblasts influence ISO toxicity, which may involve calcineurin, p38-MAPK, and Bax/Bcl-2 alterations. The data also provide new insights into cardiovascular toxicology during early development.

Cardiomyocyte calcineurin signaling in subcellular domains: from the sarcolemma to the nucleus and beyond

The serine-threonine phosphatase calcineurin is activated in cardiac myocytes in the diseased heart and induces pathological hypertrophy. Calcineurin activity is mainly triggered by calcium/calmodulin binding but also through calpain mediated cleavage. How controlled calcineurin activation is possible in cardiac myocytes, which typically show a 10-fold difference in cytosolic calcium concentration with every heartbeat, has remained enigmatic. It is now emerging that calcineurin activation and signaling occur in subcellular microdomains, in which it is brought together with target proteins and exceedingly high concentrations of calcium in order to induce downstream signaling. We review current evidence of subcellular calcineurin mainly at the sarcolemma and the nucleus, but also in association with the sarcoplasmic reticulum and mitochondria. We also suggest that knowledge about subcellular signaling could help to develop inhibitors of calcineurin in specific microdomains to avoid side-effects that may arise from complete calcineurin inhibition.

Alpha-1-adrenergic receptors: targets for agonist drugs to treat heart failure

Evidence from cell, animal, and human studies demonstrates that α1-adrenergic receptors mediate adaptive and protective effects in the heart. These effects may be particularly important in chronic heart failure, when catecholamine levels are elevated and β-adrenergic receptors are down-regulated and dysfunctional. This review summarizes these data and proposes that selectively activating α1-adrenergic receptors in the heart might represent a novel and effective way to treat heart failure. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

Mutation of calcineurin subunit B M118 influences the activities of NF-AT and p53, but not calcineurin expression level

Calcineurin (CN) is a Ca(2+)/calmodulin-dependent phosphatase, which consists of a catalytic A-subunit (CnA) and a regulatory B-subunit (CnB). Endogenous CnA and CnB have a strong corelationship in cancer cell lines. Through the introduction of CnB and its mutants in cells, we show that CnB does not increase the expression of CnA but protects it from degradation. CnB M118 is necessary for tight binding to CnA. Point mutations of CnB M118 also do not increase the expression of CnA but protect it from degradation. Furthermore, CnB M118K fails to enhance the activities of NF-AT and p53 induced by CnA in HeLa-s cells. Mutations in CnB M118 may prove to be a valuable marker in the diagnostics of some important illnesses such as Alzheimer's disease.

Myocardial AKT: the omnipresent nexus

One of the greatest examples of integrated signal transduction is revealed by examination of effects mediated by AKT kinase in myocardial biology. Positioned at the intersection of multiple afferent and efferent signals, AKT exemplifies a molecular sensing node that coordinates dynamic responses of the cell in literally every aspect of biological responses. The balanced and nuanced nature of homeostatic signaling is particularly essential within the myocardial context, where regulation of survival, energy production, contractility, and response to pathological stress all flow through the nexus of AKT activation or repression. Equally important, the loss of regulated AKT activity is primarily the cause or consequence of pathological conditions leading to remodeling of the heart and eventual decompensation. This review presents an overview compendium of the complex world of myocardial AKT biology gleaned from more than a decade of research. Summarization of the widespread influence that AKT exerts upon myocardial responses leaves no doubt that the participation of AKT in molecular signaling will need to be reckoned with as a seemingly omnipresent regulator of myocardial molecular biological responses.

Isoform-specific dephosphorylation of dynamin1 by calcineurin couples neurotrophin receptor endocytosis to axonal growth

Endocytic events are critical for neuronal survival in response to target-derived neurotrophic cues, but whether local axon growth is mediated by endocytosis-dependent signaling mechanisms remains unclear. Here, we report that Nerve Growth Factor (NGF) promotes endocytosis of its TrkA receptors and axon growth by calcineurin-mediated dephosphorylation of the endocytic GTPase dynamin1. Conditional deletion of calcineurin in sympathetic neurons disrupts NGF-dependent innervation of peripheral target tissues. Calcineurin signaling is required locally in sympathetic axons to support NGF-mediated growth in a manner independent of transcription. We show that calcineurin associates with dynamin1 via a PxIxIT interaction motif found only in specific dynamin1 splice variants. PxIxIT-containing dynamin1 isoforms colocalize with surface TrkA receptors, and their phosphoregulation is selectively required for NGF-dependent TrkA internalization and axon growth in sympathetic neurons. Thus, NGF-dependent phosphoregulation of dynamin1 is a critical event coordinating neurotrophin receptor endocytosis and axonal growth.

Calcineurin splicing variant calcineurin Aβ1 improves cardiac function after myocardial infarction without inducing hypertrophy

BACKGROUND

Calcineurin is a calcium-regulated phosphatase that plays a major role in cardiac hypertrophy. We previously described that alternative splicing of the calcineurin Aβ (CnAβ) gene generates the CnAβ1 isoform, with a unique C-terminal region that is different from the autoinhibitory domain present in all other CnA isoforms. In skeletal muscle, CnAβ1 is necessary for myoblast proliferation and stimulates regeneration, reducing fibrosis and accelerating the resolution of inflammation. Its role in the heart is currently unknown.

METHODS AND RESULTS

We generated transgenic mice overexpressing CnAβ1 in postnatal cardiomyocytes under the control of the α-myosin heavy chain promoter. In contrast to previous studies using an artificially truncated calcineurin, CnAβ1 overexpression did not induce cardiac hypertrophy. Moreover, transgenic mice showed improved cardiac function and reduced scar formation after myocardial infarction, with reduced neutrophil and macrophage infiltration and decreased expression of proinflammatory cytokines. Immunoprecipitation and Western blot analysis showed interaction of CnAβ1 with the mTOR complex 2 and activation of the Akt/SGK cardioprotective pathway in a PI3K-independent manner. In addition, gene expression profiling revealed that CnAβ1 activated the transcription factor ATF4 downstream of the Akt/mTOR pathway to promote the amino acid biosynthesis program, to reduce protein catabolism, and to induce the antifibrotic and antiinflammatory factor growth differentiation factor 15, which protects the heart through Akt activation.

CONCLUSIONS

Calcineurin Aβ1 shows a unique mode of action that improves cardiac function after myocardial infarction, activating different cardioprotective pathways without inducing maladaptive hypertrophy. These features make CnAβ1 an attractive candidate for the development of future therapeutic approaches.