Targeted inhibition of calcineurin prevents agonist-induced cardiomyocyte hypertrophy

SMURF1 controls the PPP3/calcineurin complex and TFEB at a regulatory node for lysosomal biogenesis

Macroautophagy/autophagy is a homeostatic process in response to multiple signaling, such as the lysosome-dependent recycling process of cellular components. Starvation-induced MTOR inactivation and PPP3/calcineurin activation were shown to promote the nuclear translocation of TFEB. However, the mechanisms via which signals from endomembrane damage are transmitted to activate PPP3/calcineurin and orchestrate autophagic responses remain unknown. This study aimed to show that autophagy regulator SMURF1 controlled TFEB nuclear import for transcriptional activation of the lysosomal biogenesis. We showed that blocking SMURF1 affected lysosomal biogenesis in response to lysosomal damage by preventing TFEB nuclear translocation. It revealed galectins recognized endolysosomal damage, and led to recruitment of SMURF1 and the PPP3/calcineurin apparatus on lysosomes. SMURF1 interacts with both LGALS3 and PPP3CB to form the LGALS3-SMURF1-PPP3/calcineurin complex. Importantly, this complex further stabilizes TFEB, thereby activating TFEB for lysosomal biogenesis. We determined that LLOMe-mediated TFEB nuclear import is dependent on SMURF1 under the condition of MTORC1 inhibition. In addition, SMURF1 is required for PPP3/calcineurin activity as a positive regulator of TFEB. SMURF1 controlled the phosphatase activity of the PPP3CB by promoting the dissociation of its autoinhibitory domain (AID) from its catalytic domain (CD). Overexpression of SMURF1 showed similar effects as the constitutive activation of PPP3CB. Thus, SMURF1, which bridges environmental stress with the core autophagosomal and autolysosomal machinery, interacted with endomembrane sensor LGALS3 and phosphatase PPP3CB to control TFEB activation.Abbreviations: ATG: autophagy-related; LLOMe: L-Leucyl-L-Leucine methyl ester; ML-SA1: mucolipin synthetic agonist 1; MTOR: mechanistic target of rapamycin kinase; PPP3CB: protein phosphatase 3 catalytic subunit beta; RPS6KB1/p70S6K: ribosomal protein S6 kinase B1; SMURF1: SMAD specific E3 ubiquitin protein ligase 1; TFEB: transcription factor EB.

Human disease-associated calmodulin mutations alter calcineurin function through multiple mechanisms

Calmodulin (CaM) is a ubiquitous, calcium-sensing protein that regulates a multitude of processes throughout the body. In response to changes in [Ca2+], CaM modifies, activates, and deactivates enzymes and ion channels, as well as many other cellular processes. The importance of CaM is highlighted by the conservation of an identical amino acid sequence in all mammals. Alterations to CaM amino acid sequence were once thought to be incompatible with life. During the last decade modifications to the CaM protein sequence have been observed in patients suffering from life-threatening heart disease (calmodulinopathy). Thus far, inadequate or untimely interaction between mutant CaM and several proteins (LTCC, RyR2, and CaMKII) have been identified as mechanisms underlying calmodulinopathy. Given the extensive number of CaM interactions in the body, there are likely many consequences for altering CaM protein sequence. Here, we demonstrate that disease-associated CaM mutations alter the sensitivity and activity of the Ca2+-CaM-enhanced serine/threonine phosphatase calcineurin (CaN). Biophysical characterization by circular dichroism, solution NMR spectroscopy, stopped-flow kinetic measurements, and MD simulations provide mechanistic insight into mutation dysfunction as well as highlight important aspects of CaM Ca2+ signal transduction. We find that individual CaM point mutations (N53I, F89L, D129G, and F141L) impair CaN function, however, the mechanisms are not the same. Specifically, individual point mutations can influence or modify the following properties: CaM binding, Ca2+ binding, and/or Ca2+kinetics. Moreover, structural aspects of the CaNCaM complex can be altered in manners that indicate changes to allosteric transmission of CaM binding to the enzyme active site. Given that loss of CaN function can be fatal, as well as evidence that CaN modifies ion channels already associated with calmodulinopathy, our results raise the possibility that altered CaN function contributes to calmodulinopathy.

Crim1 inhibits angiotensin II-induced hypertrophy and preserves Kv4.2 expression in cardiomyocytes

Objectives

Angiotensin II (Ang II) plays a key role in the regulation of myocardial hypertrophy via downstream cysteine-rich transmembrane bone morphogenetic protein regulator 1 (Crim1). However, it is still unclear whether Crim1 is involved in ionic channel remodeling. The study aimed to explore the effects of Crim1 on transient outward potassium current (Ito) and Kv4.2 (the main subunit of I_to channel) expression in hypertrophic ventricular cardiomyocytes.

Materials and Methods

The ventricular cardiomyocytes were isolated from the neonatal rats. Hypertrophy was induced by Ang II. Crim1 expression was modulated by using adenovirus transfection. The expression of myosin heavy chain beta (β-MHC), Crim1, and Kv4.2 was determined by RT-qPCR and western blot. The cellular surface area was assessed using Image J software. I_to was recorded by the whole-cell patch clamp technique.

Results

Ang II-induced hypertrophy in cardiomyocytes was identified by their larger cellular surface area and higher mRNA expression of β-MHC. Ang II significantly decreased the expression of Crim1 and Kv4.2 and reduced I_to current density. However, Crim1 overexpression abolished the Ang II-induced hypertrophy and preserved the expression of Kv4.2 and I_to current density.

Conclusion

Crim1 overexpression inhibits Ang II-induced hypertrophy and preserves I_to current density via up-regulating Kv4.2 in ventricular cardiomyocytes from neonatal rats. Crim1 could have a role in the development of ventricular arrhythmia in hypertrophic hearts.

Cardiovascular effects of immunosuppression agents

Immunosuppressive medications are widely used to treat patients with neoplasms, autoimmune conditions and solid organ transplants. Key drug classes, namely calcineurin inhibitors, mammalian target of rapamycin (mTOR) inhibitors, and purine synthesis inhibitors, have direct effects on the structure and function of the heart and vascular system. In the heart, immunosuppressive agents modulate cardiac hypertrophy, mitochondrial function, and arrhythmia risk, while in vasculature, they influence vessel remodeling, circulating lipids, and blood pressure. The aim of this review is to present the preclinical and clinical literature examining the cardiovascular effects of immunosuppressive agents, with a specific focus on cyclosporine, tacrolimus, sirolimus, everolimus, mycophenolate, and azathioprine.

When one says yes and the other says no; does calcineurin participate in physiologic cardiac hypertrophy?

Developing engaging activities that build skills for understanding and appreciating research is important for undergraduate and postgraduate science students. Comparing and contrasting opposing research studies does this, and more: it also appropriately for these cohorts challenges higher level cognitive processing. Here, we present and discuss one such scenario, that of calcineurin in the heart and its response to exercise training. This scenario is further accentuated by the existence of only two studies. The background is that regular aerobic endurance exercise training stimulates the heart to physiologically adapt to chronically increase its ability to produce a greater cardiac output to meet the increased demand for oxygenated blood in working muscles, and this happens by two main mechanisms: 1) increased cardiac contractile function and 2) physiologic hypertrophy. The major underlying mechanisms have been delineated over the last decades, but one aspect has not been resolved: the potential role of calcineurin in modulating physiologic hypertrophy. This is partly because the existing research has provided opposing and contrasting findings, one line showing that exercise training does activate cardiac calcineurin in conjunction with myocardial hypertrophy, but another line showing that exercise training does not activate cardiac calcineurin even if myocardial hypertrophy is blatantly occurring. Here, we review and present the current evidence in the field and discuss reasons for this controversy. We present real-life examples from physiology research and discuss how this may enhance student engagement and participation, widen the scope of learning, and thereby also further facilitate higher level cognitive processing.

Cardiac CIP protein regulates dystrophic cardiomyopathy

Heart failure is a leading cause of fatality in Duchenne muscular dystrophy (DMD) patients. Previously, we discovered that cardiac and skeletal-muscle-enriched CIP proteins play important roles in cardiac function. Here, we report that CIP, a striated muscle-specific protein, participates in the regulation of dystrophic cardiomyopathy. Using a mouse model of human DMD, we found that deletion of CIP leads to dilated cardiomyopathy and heart failure in young, non-syndromic mdx mice. Conversely, transgenic overexpression of CIP reduces pathological dystrophic cardiomyopathy in old, syndromic mdx mice. Genome-wide transcriptome analyses reveal that molecular pathways involving fibrogenesis and oxidative stress are affected in CIP-mediated dystrophic cardiomyopathy. Mechanistically, we found that CIP interacts with dystrophin and calcineurin (CnA) to suppress the CnA-Nuclear Factor of Activated T cells (NFAT) pathway, which results in decreased expression of Nox4, a key component of the oxidative stress pathway. Overexpression of Nox4 accelerates the development of dystrophic cardiomyopathy in mdx mice. Our study indicates CIP is a modifier of dystrophic cardiomyopathy and a potential therapeutic target for this devastating disease.

Calcineurin in the heart: New horizons for an old friend

Calcineurin, also known as PP2B or PPP3, is a member of the PPP family of protein phosphatases that also includes PP1 and PP2A. Together these three phosphatases carryout the majority of dephosphorylation events in the heart. Calcineurin is distinct in that it is activated by the binding of calcium/calmodulin (Ca2+/CaM) and therefore acts as a node for integrating Ca2+ signals with changes in phosphorylation, two fundamental intracellular signaling cascades. In the heart, calcineurin is primarily thought of in the context of pathological cardiac remodeling, acting through the Nuclear Factor of Activated T-cell (NFAT) family of transcription factors. However, calcineurin activity is also essential for normal heart development and homeostasis in the adult heart. Furthermore, it is clear that NFAT-driven changes in transcription are not the only relevant processes initiated by calcineurin in the setting of pathological remodeling. There is a growing appreciation for the diversity of calcineurin substrates that can impact cardiac function as well as the diversity of mechanisms for targeting calcineurin to specific sub-cellular domains in cardiomyocytes and other cardiac cell types. Here, we will review the basics of calcineurin structure, regulation, and function in the context of cardiac biology. Particular attention will be given to: the development of improved tools to identify and validate new calcineurin substrates; recent studies identifying new calcineurin isoforms with unique properties and targeting mechanisms; and the role of calcineurin in cardiac development and regeneration.

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.

Between Inflammation and Autophagy: The Role of Leptin-Adiponectin Axis in Cardiac Remodeling

Cardiac remodeling is the process by which the heart adapts to stressful stimuli, such as hypertension and ischemia/reperfusion; it ultimately leads to heart failure upon long-term exposure. Autophagy, a cellular catabolic process that was originally considered as a mechanism of cell death in response to detrimental stimuli, is thought to be one of the main mechanisms that controls cardiac remodeling and induces heart failure. Dysregulation of the adipokines leptin and adiponectin, which plays essential roles in lipid and glucose metabolism, and in the pathophysiology of the neuroendocrine and cardiovascular systems, has been shown to affect the autophagic response in the heart and to contribute to accelerate cardiac remodeling. The obesity-associated protein leptin is a pro-inflammatory, tumor-promoting adipocytokine whose elevated levels in obesity are associated with acute cardiovascular events, and obesity-related hypertension. Adiponectin exerts anti-inflammatory and anti-tumor effects, and its reduced levels in obesity correlate with the pathogenesis of obesity-associated cardiovascular diseases. Leptin- and adiponectin-induced changes in autophagic flux have been linked to cardiac remodeling and heart failure. In this review, we describe the different molecular mechanisms of hyperleptinemia- and hypoadiponectinemia-mediated pathogenesis of cardiac remodeling and the involvement of autophagy in this process. A better understanding of the roles of leptin, adiponectin, and autophagy in cardiac functions and remodeling, and the exact signal transduction pathways by which they contribute to cardiac diseases may well lead to discovery of new therapeutic agents for the treatment of cardiovascular remodeling.

Calcineurin Aβ gene knockdown inhibits transient outward potassium current ion channel remodeling in hypertrophic ventricular myocyte

It has been shown that the activation of calcineurin is involved in regulating ion channel remodeling in hypertrophic cardiomyocytes. But the precise role of calcineurin in the regulation of transient outward potassium current (I _to), an ion channel associated with fatal arrhythmia, remains controversial. This study aimed to examine the effects of calcineurin Aβ (CnAβ) gene knockdown on I _to channel remodeling and action potential duration (APD) in the hypertrophic ventricular myocytes of neonatal rats. Results showed that phenylephrine stimulation caused hypertrophy of ventricular myocytes, upregulation of CnAβ protein expression, downregulation of Kv4.2 mRNA and protein expression, a decrease in I _to current density, and prolongation of APD. CnAβ gene knockdown significantly inhibited the effects of phenylephrine stimulation. Our data indicate that CnAβ gene knockdown can inhibit I _to channel remodeling and APD prolongation in hypertrophic neonatal rat ventricular myocytes. This finding suggests that calcineurin may be a potential target for the prevention of malignant ventricular arrhythmia in a hypertrophic heart.

Role of calcineurin biosignaling in cell secretion and the possible regulatory mechanisms

Cyclic adenosine monophosphate (cAMP) and calcium ions (Ca²⁺) are two chemical molecules that play a central role in the stimulus-dependent secretion processes within cells. Ca²⁺ acts as the basal signaling molecule responsible to initiate cell secretion. cAMP primarily acts as an intracellular second messenger in a myriad of cellular processes by activating cAMP-dependent protein kinases through association with such kinases in order to mediate post-translational phosphorylation of those protein targets. Put succinctly, both Ca²⁺ and cAMP act by associating or activating other proteins to ensure successful secretion. Calcineurin is one such protein regulated by Ca²⁺; its action depends on the intracellular levels of Ca²⁺. Being a phosphatase, calcineurin dephosphorylate and other proteins, as is the case with most other phosphatases, such as protein phosphatase 2A (PP2A), PP2C, and protein phosphatase-1 (PP1), will likely be activated by phosphorylation. Via this process, calcineurin is able to affect different intracellular signaling with clinical importance, some of which has been the basis for development of different calcineurin inhibitors. In this review, the cAMP-dependent calcineurin bio-signaling, protein-protein interactions and their physiological implications as well as regulatory signaling within the context of cellular secretion are explored.

The endocrinological component and signaling pathways associated to cardiac hypertrophy

Although myocardial growth corresponds to an adaptive response to maintain cardiac contractile function, the cardiac hypertrophy is a condition that occurs in many cardiovascular diseases and typically precedes the onset of heart failure. Different endocrine factors such as thyroid hormones, insulin, insulin-like growth factor 1 (IGF-1), angiotensin II (Ang II), endothelin (ET-1), catecholamines, estrogen, among others represent important stimuli to cardiomyocyte hypertrophy. Thus, numerous endocrine disorders manifested as changes in the local environment or multiple organ systems are especially important in the context of progression from cardiac hypertrophy to heart failure. Based on that information, this review summarizes experimental findings regarding the influence of such hormones upon signalling pathways associated with cardiac hypertrophy. Understanding mechanisms through which hormones differentially regulate cardiac hypertrophy could open ways to obtain therapeutic approaches that contribute to prevent or delay the onset of heart failure related to endocrine diseases.

Multiplicity of Nitric Oxide and Natriuretic Peptide Signaling in Heart Failure

Heart failure (HF) is a common consequence of several cardiovascular diseases and is understood as a vicious cycle of cardiac and hemodynamic decline. The current inventory of treatments either alleviates the pathophysiological features (eg, cardiac dysfunction, neurohumoral activation, and ventricular remodeling) and/or targets any underlying pathologies (eg, hypertension and myocardial infarction). Yet, since these do not provide a cure, the morbidity and mortality associated with HF remains high. Therefore, the disease constitutes an unmet medical need, and novel therapies are desperately needed. Cyclic guanosine-3',5'-monophosphate (cGMP), synthesized by nitric oxide (NO)- and natriuretic peptide (NP)-responsive guanylyl cyclase (GC) enzymes, exerts numerous protective effects on cardiac contractility, hypertrophy, fibrosis, and apoptosis. Impaired cGMP signaling, which can occur after GC deactivation and the upregulation of cyclic nucleotide-hydrolyzing phosphodiesterases (PDEs), promotes cardiac dysfunction. In this study, we review the role that NO/cGMP and NP/cGMP signaling plays in HF. After considering disease etiology, the physiological effects of cGMP in the heart are discussed. We then assess the evidence from preclinical models and patients that compromised cGMP signaling contributes to the HF phenotype. Finally, the potential of pharmacologically harnessing cardioprotective cGMP to rectify the present paucity of effective HF treatments is examined.

New role of TRPM4 channel in the cardiac excitation-contraction coupling in response to physiological and pathological hypertrophy in mouse

The transient receptor potential Melastatin 4 (TRPM4) channel is a calcium-activated non-selective cation channel expressed widely. In the heart, using a knock-out mouse model, the TRPM4 channel has been shown to be involved in multiple processes, including β-adrenergic regulation, cardiac conduction, action potential duration and hypertrophic adaptations. This channel was recently shown to be involved in stress-induced cardiac arrhythmias in a mouse model overexpressing TRPM4 in ventricular cardiomyocytes. However, the link between TRPM4 channel expression in ventricular cardiomyocytes, the hypertrophic response to stress and/or cellular arrhythmias has yet to be elucidated. In this present study, we induced pathological hypertrophy in response to myocardial infarction using a mouse model of Trpm4 gene invalidation, and demonstrate that TRPM4 is essential for survival. We also demonstrate that the TRPM4 is required to activate both the Akt and Calcineurin pathways. Finally, using two hypertrophy models, either a physiological response to endurance training or a pathological response to myocardial infarction, we show that TRPM4 plays a role in regulating transient calcium amplitudes and leads to the development of cellular arrhythmias potentially in cooperation with the Sodium-calcium exchange (NCX). Here, we report two functions of the TRPM4 channel: first its role in adaptive hypertrophy, and second its association with NCX could mediate transient calcium amplitudes which trigger cellular arrhythmias.

Danshen (Salvia miltiorhiza) inhibits Leu27 IGF-II-induced hypertrophy in H9c2 cells

In this study, we used ICI 182 780 (ICI), an estrogen receptor (ER) antagonist, to investigate the estrogenic activity of Danshen, and to further explored whether Danshen extract can block Leu27IGF-II-induced hypertrophy in H9c2 cardiomyoblast cells. We first used an IGF-II analog Leu27IGF-II, which specifically activates IGF2R signaling cascades and induces H9c2 cardiomyoblast cell hypertrophy. However, Danshen extract completely inhibited Leu27IGF-II-induced cell size increase, ANP and BNP hypertrophic marker expression, and IGF2R induction. We also observed that Danshen extract inhibited calcineurin protein expression and NFAT3 nuclear translocation, leading to suppression of Leu27IGF-II-induced cardiac hypertrophy. Moreover, the anti-Leu27IGF-II-IGF2R signaling effect of Danshen was totally reversed by ICI, which suggest the cardio protective effect of Danshen is mediated through estrogen receptors. Our study suggests that, Danshen exerts estrogenic activity, and thus, it could be used as a selective ER modulator in IGFIIR induced hypertrophy model.

Cardiac Cyclic Nucleotide Phosphodiesterases: Roles and Therapeutic Potential in Heart Failure

The cyclic nucleotides cyclic adenosine-3′,5′-monophosphate (cAMP) and cyclic guanosine-3′,5′-monophosphate (cGMP) maintain physiological cardiac contractility and integrity. Cyclic nucleotide–hydrolysing phosphodiesterases (PDEs) are the prime regulators of cAMP and cGMP signalling in the heart. During heart failure (HF), the expression and activity of multiple PDEs are altered, which disrupt cyclic nucleotide levels and promote cardiac dysfunction. Given that the morbidity and mortality associated with HF are extremely high, novel therapies are urgently needed. Herein, the role of PDEs in HF pathophysiology and their therapeutic potential is reviewed. Attention is given to PDEs 1–5, and other PDEs are briefly considered. After assessing the role of each PDE in cardiac physiology, the evidence from pre-clinical models and patients that altered PDE signalling contributes to the HF phenotype is examined. The potential of pharmacologically harnessing PDEs for therapeutic gain is considered.

Cell-specific ablation of Hsp47 defines the collagen-producing cells in the injured heart

Collagen production in the adult heart is thought to be regulated by the fibroblast, although cardiomyocytes and endothelial cells also express multiple collagen mRNAs. Molecular chaperones are required for procollagen biosynthesis, including heat shock protein 47 (Hsp47). To determine the cell types critically involved in cardiac injury–induced fibrosis theHsp47 gene was deleted in cardiomyocytes, endothelial cells, or myofibroblasts. Deletion ofHsp47 from cardiomyocytes during embryonic development or adult stages, or deletion from adult endothelial cells, did not affect cardiac fibrosis after pressure overload injury. However, myofibroblast-specific ablation of Hsp47; blocked fibrosis and deposition of collagens type I, III, and V following pressure overload as well as significantly reduced cardiac hypertrophy. Fibroblast-specific Hsp47-deleted mice showed lethality after myocardial infarction injury, with ineffective scar formation and ventricular wall rupture. Similarly, only myofibroblast-specific deletion of Hsp47reduced fibrosis and disease in skeletal muscle in a mouse model of muscular dystrophy. Mechanistically, deletion of Hsp47 from myofibroblasts reduced mRNA expression of fibrillar collagens and attenuated their proliferation in the heart without affecting paracrine secretory activity of these cells. The results show that myofibroblasts are the primary mediators of tissue fibrosis and scar formation in the injured adult heart, which unexpectedly affects cardiomyocyte hypertrophy.

USP16-mediated deubiquitination of calcineurin A controls peripheral T cell maintenance

Calcineurin acts as a calcium-activated phosphatase that dephosphorylates various substrates, including members of the nuclear factor of activated T cells (NFAT) family, to trigger their nuclear translocation and transcriptional activity. However, the detailed mechanism regulating the recruitment of NFATs to calcineurin remains poorly understood. Here, we report that calcineurin A (CNA), encoded by PPP3CB or PPP3CC, is constitutively ubiquitinated on lysine 327, and this polyubiquitin chain is rapidly removed by ubiquitin carboxyl-terminal hydrolase 16 (USP16) in response to intracellular calcium stimulation. The K29-linked ubiquitination of CNA impairs NFAT recruitment and transcription of NFAT-targeted genes. USP16 deficiency prevents calcium-triggered deubiquitination of CNA in a manner consistent with defective maintenance and proliferation of peripheral T cells. T cell-specific USP16 knockout mice exhibit reduced severity of experimental autoimmune encephalitis and inflammatory bowel disease. Our data reveal the physiological function of CNA ubiquitination and its deubiquitinase USP16 in peripheral T cells. Notably, our results highlight a critical mechanism for the regulation of calcineurin activity and a novel immunosuppressive drug target for the treatment of autoimmune diseases.

Programmed Cell Death 5 Provides Negative Feedback on Cardiac Hypertrophy Through the Stabilization of Sarco/Endoplasmic Reticulum Ca2+-ATPase 2a Protein

PDCD5 (programmed cell death 5) is ubiquitously expressed in tissues, including the heart; however, the mechanism underlying the cardiac function of PDCD5 has not been understood. We investigated the mechanisms of PDCD5 in the pathogenesis of cardiac hypertrophy. Cardiac-specific PDCD5 knockout mice developed severe cardiac hypertrophy and impaired cardiac function, whereas PDCD5 protein was significantly increased in transverse aortic constriction mouse hearts and phenylephrine-stimulated cardiomyocytes. Overexpression of PDCD5 inhibited phenylephrine-induced cardiomyocyte hypertrophy, and knockdown of PDCD5 induced cardiomyocyte hypertrophy and aggravated phenylephrine-induced hypertrophy. The expression of PDCD5 protein was regulated by NFATc2 (nuclear factor of activated T cells c2) during hypertrophy. SERCA2a (sarco/endoplasmic reticulum Ca²⁺-ATPase 2a) expression was decreased in PDCD5-deficient mouse hearts because of increased ubiquitination. PDCD5-deficient cardiomyocytes displayed decreased calcium uptake rate, slowed decay of Ca²⁺ transients, decreased calcium stores, and diastolic dysfunction. Moreover, reintroduction of PDCD5 in PDCD5-deficient mouse hearts reserved SERCA2a protein, suppressed NFATc2 protein, and rescued the hypertrophy and cardiac dysfunction. Our results revealed that PDCD5 is a novel target of NFATc2 in the hypertrophic heart and provides negative feedback to protect the heart against excessive hypertrophy via the stabilization of SERCA2a protein.

Calcineurin-AKAP interactions: therapeutic targeting of a pleiotropic enzyme with a little help from its friends

The ubiquitous Ca²⁺ /calmodulin-dependent phosphatase calcineurin is a key regulator of pathological cardiac hypertrophy whose therapeutic targeting in heart disease has been elusive due to its role in other essential biological processes. Calcineurin is targeted to diverse intracellular compartments by association with scaffold proteins, including by multivalent A-kinase anchoring proteins (AKAPs) that bind protein kinase A and other important signalling enzymes determining cardiac myocyte function and phenotype. Calcineurin anchoring by AKAPs confers specificity to calcineurin function in the cardiac myocyte. Targeting of calcineurin 'signalosomes' may provide a rationale for inhibiting the phosphatase in disease.

miR-133: A Suppressor of Cardiac Remodeling?

Cardiac remodeling, which is characterized by mechanical and electrical remodeling, is a significant pathophysiological process involved in almost all forms of heart diseases. MicroRNAs (miRNAs) are a group of non-coding RNAs of 20–25 nucleotides in length that primarily regulate gene expression by promoting mRNA degradation or post-transcriptional repression in a sequence-specific manner. Three miR-133 genes have been identified in the human genome, miR-133a-1, miR-133a-2, and miR-133b, which are located on chromosomes 18, 20, and 6, respectively. These miRNAs are mainly expressed in muscle tissues and appear to repress the expression of non-muscle genes. Based on accumulating evidence, miR-133 participates in the proliferation, differentiation, survival, hypertrophic growth, and electrical conduction of cardiac cells, which are essential for cardiac fibrosis, cardiac hypertrophy, and arrhythmia. Nevertheless, the roles of miR-133 in cardiac remodeling are ambiguous, and the mechanisms are also sophisticated, involving many target genes and signaling pathways, such as RhoA, MAPK, TGFβ/Smad, and PI3K/Akt. Therefore, in this review, we summarize the critical roles of miR-133 and its potential mechanisms in cardiac remodeling.

Reduced expression of Na+/Ca2+ exchangers is associated with cognitive deficits seen in Alzheimer's disease model mice

Na⁺/Ca²⁺ exchangers (NCXs) are expressed primarily in the plasma membrane of most cell types, where they mediate electrogenic exchange of one Ca²⁺ for three Na⁺ ions, depending on Ca²⁺ and Na⁺ electrochemical gradients across the membrane. Three mammalian NCX isoforms (NCX1, NCX2, and NCX3) are each encoded by a distinct gene. Here, we report that NCX2 and NCX3 protein and mRNA levels are relatively reduced in hippocampal CA1 of APP23 and APP-KI mice. Likewise, NCX2^+/- or NCX3^+/- mice exhibited impaired hippocampal LTP and memory-related behaviors. Moreover, relative to controls, calcium/calmodulin-dependent protein kinase II (CaMKII) autophosphorylation significantly decreased in NCX2^+/- mouse hippocampus but increased in hippocampus of NCX3^+/- mice. NCX2 or NCX3 heterozygotes displayed impaired maintenance of hippocampal LTP, a phenotype that in NCX2^+/- mice was correlated with elevated calcineurin activity and rescued by treatment with the calcineurin (CaN) inhibitor FK506. Likewise, FK506 treatment significantly restored impaired hippocampal LTP in APP-KI mice. Moreover, Ca²⁺ clearance after depolarization following high frequency stimulation was slightly delayed in hippocampal CA1 regions of NCX2^+/- mice. Electron microscopy revealed relatively decreased synaptic density in CA1 of NCX2^+/- mice, while the number of spines with perforated synapses in CA1 significantly increased in NCX3^+/- mice. We conclude that memory impairment seen in NCX2^+/- and NCX3^+/- mice reflect dysregulated hippocampal CaMKII activity, which alters dendritic spine morphology, findings with implications for memory deficits seen in Alzheimer's disease model mice.

Reduced Cardiac Calcineurin Expression Mimics Long-Term Hypoxia-Induced Heart Defects in Drosophila

BACKGROUND

Hypoxia is often associated with cardiopulmonary diseases, which represent some of the leading causes of mortality worldwide. Long-term hypoxia exposures, whether from disease or environmental condition, can cause cardiomyopathy and lead to heart failure. Indeed, hypoxia-induced heart failure is a hallmark feature of chronic mountain sickness in maladapted populations living at high altitude. In a previously established Drosophila heart model for long-term hypoxia exposure, we found that hypoxia caused heart dysfunction. Calcineurin is known to be critical in cardiac hypertrophy under normoxia, but its role in the heart under hypoxia is poorly understood.

METHODS AND RESULTS

In the present study, we explore the function of calcineurin, a gene candidate we found downregulated in the Drosophila heart after lifetime and multigenerational hypoxia exposure. We examined the roles of 2 homologs of Calcineurin A, CanA14F, and Pp2B in the Drosophila cardiac response to long-term hypoxia. We found that knockdown of these calcineurin catalytic subunits caused cardiac restriction under normoxia that are further aggravated under hypoxia. Conversely, cardiac overexpression of Pp2B under hypoxia was lethal, suggesting that a hypertrophic signal in the presence of insufficient oxygen supply is deleterious.

CONCLUSIONS

Our results suggest a key role for calcineurin in cardiac remodeling during long-term hypoxia with implications for diseases of chronic hypoxia, and it likely contributes to mechanisms underlying these disease states.

Calcium Signaling and Transcriptional Regulation in Cardiomyocytes

Calcium (Ca 2+ ) is a universal regulator of various cellular functions. In cardiomyocytes, Ca 2+ is the central element of excitation–contraction coupling, but also impacts diverse signaling cascades and influences the regulation of gene expression, referred to as excitation–transcription coupling. Disturbances in cellular Ca 2+ -handling and alterations in Ca 2+ -dependent gene expression patterns are pivotal characteristics of failing cardiomyocytes, with several excitation–transcription coupling pathways shown to be critically involved in structural and functional remodeling processes. Thus, targeting Ca 2+ -dependent transcriptional pathways might offer broad therapeutic potential. In this article, we (1) review cytosolic and nuclear Ca 2+ dynamics in cardiomyocytes with respect to their impact on Ca 2+ -dependent signaling, (2) give an overview on Ca 2+ -dependent transcriptional pathways in cardiomyocytes, and (3) discuss implications of excitation–transcription coupling in the diseased heart.

Role of Mitochondrial Dysfunction in Hypertension and Obesity

Mitochondria are essential for the maintenance of normal physiological function of tissue cells. Mitochondria are subject to dynamic processes in order to establish a control system related to survival or cell death and adaptation to changes in the metabolic environment of cells. Mitochondrial dynamics includes fusion and fission processes, biogenesis, and mitophagy. Modifications of mitochondrial dynamics in organs involved in energy metabolism such as the pancreas, liver, skeletal muscle, and white adipose tissue could be of relevance for the development of insulin resistance, obesity, and type 2 diabetes. Mitochondrial dynamics and the factors involved in its regulation are also critical for neuronal development, survival, and function. Modifications in mitochondrial dynamics in either agouti-related peptide (AgRP) or pro-opiomelanocortin (POMC), circuits which regulates feeding behavior, are related to changes of food intake, energy balance, and obesity development. Activation of the sympathetic nervous system has been considered as a crucial point in the pathogenesis of hypertension among obese individuals and it also plays a key role in cardiac remodeling. Hypertension-related cardiac hypertrophy is associated with changes in metabolic substrate utilization, dysfunction of the electron transport chain, and ATP synthesis. Alterations in both mitochondrial dynamics and ROS production have been associated with endothelial dysfunction, development of hypertension, and cardiac hypertrophy. Finally, it might be postulated that alterations of mitochondrial dynamics in white adipose tissue could contribute to the development and maintenance of hypertension in obesity situations through leptin overproduction. Leptin, together with insulin, will induce activation of sympathetic nervous system with consequences at renal, vascular, and cardiac levels, driving to sodium retention, hypertension, and left ventricular hypertrophy. Moreover, both leptin and insulin will induce mitochondrial alterations into arcuate nucleus leading to signals driving to increased food intake and reduced energy expenditure. This, in turn would perpetuate white adipose tissue excess and its well-known metabolic and cardiovascular consequences.

Reduced CaM Kinase II and CaM Kinase IV Activities Underlie Cognitive Deficits in NCKX2 Heterozygous Mice

Among five members of the K⁺-dependent Na⁺/Ca²⁺ exchanger (NCKX) family (NCKX1-5), only NCKX2 is highly expressed in mouse brain. NCKX2 in plasma membranes mediates cytosolic calcium excretion through electrogenic exchange of 4 Na⁺ for 1 Ca²⁺ and 1 K⁺. Here, we observed significantly decreased levels of NCKX2 protein and mRNA in the CA1 region of APP23 mice, a model of Alzheimer's disease. We also found that, like APP23 mice, heterozygous NCKX2-mutant mice exhibit mildly impaired hippocampal LTP and memory acquisition, the latter based on novel object recognition and passive avoidance tasks. When we addressed underlying mechanisms, we found that both CaMKII autophosphorylation and CaMKIV phosphorylation significantly decreased in CA1 regions of NCKX2+/- relative to control mice. Likewise, phosphorylation of GluA1 (Ser-831) and CREB (Ser-133), respective downstream targets of CaMKII and CaMKIV, also significantly decreased in the CA1 region. BDNF protein and mRNA levels significantly decreased in CA1 of NCKX2+/- relative to control mice. Finally, CaN activity increased in CA1 of NCKX2+/- mice. Our findings suggest that like APP23 mice, NCKX2+/- mice may exhibit impaired learning and hippocampal LTP due to decreased CaM kinase II and CaM kinase IV activities.

Calcineurin signaling in the heart: The importance of time and place

The calcium-activated protein phosphatase, calcineurin, lies at the intersection of protein phosphorylation and calcium signaling cascades, where it provides an essential nodal point for coordination between these two fundamental modes of intracellular communication. In excitatory cells, such as neurons and cardiomyocytes, that experience rapid and frequent changes in cytoplasmic calcium, calcineurin protein levels are exceptionally high, suggesting that these cells require high levels of calcineurin activity. Yet, it is widely recognized that excessive activation of calcineurin in the heart contributes to pathological hypertrophic remodeling and the progression to failure. How does a calcium activated enzyme function in the calcium-rich environment of the continuously contracting heart without pathological consequences? This review will discuss the wide range of calcineurin substrates relevant to cardiovascular health and the mechanisms calcineurin uses to find and act on appropriate substrates in the appropriate location while potentially avoiding others. Fundamental differences in calcineurin signaling in neonatal verses adult cardiomyocytes will be addressed as well as the importance of maintaining heterogeneity in calcineurin activity across the myocardium. Finally, we will discuss how circadian oscillations in calcineurin activity may facilitate integration with other essential but conflicting processes, allowing a healthy heart to reap the benefits of calcineurin signaling while avoiding the detrimental consequences of sustained calcineurin activity that can culminate in heart failure.

Norisoboldine, an alkaloid from Radix linderae, inhibits NFAT activation and attenuates 2,4-dinitrofluorobenzene-induced dermatitis in mice

CONTEXT

The nuclear factor of activated T-cells (NFAT) is a family of transcription factors, essential for T-cell activation. Norisoboldine (NOR), an isoquinoline alkaloid from Radix linderae, has been demonstrated to possess anti-inflammatory activity.

OBJECTIVE

This study examines NOR's effect on NFAT activation and its therapeutic potential for atopic dermatitis (AD).

MATERIALS AND METHODS

The transcriptional activity of NFAT was examined with luciferase reporter assay, using K562-luc cells, stimulated with 20 ng/mL PMA plus 1 μM ionomycin. NFAT dephosphorylation was examined by immuno-blotting in K562-luc cells and Jurkat cells. Interleukin-2 (IL-2) expression in Jurkat cells was examined by real-time PCR. A mouse model of dermatitis, induced by 2,4-dinitrochlorobenzene (DNCB), was used to test NOR's therapeutic potential for AD.

RESULTS

NOR, dose-dependently, inhibited PMA and ionomycin-induced NFAT reporter gene expression in K562-luc cells in the range of 2-50 μM. NOR also inhibited PMA and ionomycin-induced NFAT dephosphorylation in K562-luc cells and Jurkat cells. Consequently, NOR suppressed PMA plus ionomycin-induced IL-2 expression in Jurkat cells. The administration of NOR (10 mg/kg, i.p.), alleviated DNCB-induced dermatitis in mice, by the reduction of ear swelling and attenuation of inflammatory infiltration into ear tissue. Moreover, mRNA levels of INF-γ, TNF-α, IL-4 and IL-6 in ears of NOR-treated mice were reduced by 78.4, 77.8, 72.3 and 73.9%, respectively, compared with untreated controls.

DISCUSSION AND CONCLUSION

This study demonstrates that NOR inhibits NFAT activation in T-cells and alleviates AD-like inflammatory reaction in a DNCB-induced dermatitis model, highlighting NOR as a potential therapeutic agent for AD.

Drosophila in the Heart of Understanding Cardiac Diseases: Modeling Channelopathies and Cardiomyopathies in the Fruitfly

Cardiovascular diseases and, among them, channelopathies and cardiomyopathies are a major cause of death worldwide. The molecular and genetic defects underlying these cardiac disorders are complex, leading to a large range of structural and functional heart phenotypes. Identification of molecular and functional mechanisms disrupted by mutations causing channelopathies and cardiomyopathies is essential to understanding the link between an altered gene and clinical phenotype. The development of animal models has been proven to be efficient for functional studies in channelopathies and cardiomyopathies. In particular, the Drosophila model has been largely applied for deciphering the molecular and cellular pathways affected in these inherited cardiac disorders and for identifying their genetic modifiers. Here we review the utility and the main contributions of the fruitfly models for the better understanding of channelopathies and cardiomyopathies. We also discuss the investigated pathological mechanisms and the discoveries of evolutionarily conserved pathways which reinforce the value of Drosophila in modeling human cardiac diseases.

MBNL1-mediated regulation of differentiation RNAs promotes myofibroblast transformation and the fibrotic response

The differentiation of fibroblasts into myofibroblasts mediates tissue wound healing and fibrotic remodelling, although the molecular programme underlying this process remains poorly understood. Here we perform a genome-wide screen for genes that control myofibroblast transformation, and identify the RNA-binding protein muscleblind-like1 (MBNL1). MBNL1 overexpression promotes transformation of fibroblasts into myofibroblasts, whereas loss of Mbnl1 abrogates transformation and impairs the fibrotic phase of wound healing in mouse models of myocardial infarction and dermal injury. Mechanistically, MBNL1 directly binds to and regulates a network of differentiation-specific and cytoskeletal/matrix-assembly transcripts to promote myofibroblast differentiation. One of these transcripts is the nodal transcriptional regulator serum response factor (SRF), whereas another is calcineurin Aβ. CRISPR-Cas9-mediated gene-editing of the MBNL1-binding site within the Srf 3′UTR impairs myofibroblast differentiation, whereas in vivo deletion of Srf in fibroblasts impairs wound healing and fibrosis. These data establish a new RNA-dependent paradigm for myofibroblast formation through MBNL1.

Fibroblast-to-myofibroblast differentiation is crucial for wound healing and regeneration. Davis et al. describe a new regulatory mechanism underlying myofibroblast differentiation via the RNA-binding protein MBNL1, which promotes the maturation of certain mRNA transcripts that are integral nodes in fibroblast differentiation.

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.

The Ca(2+)-activated cation channel TRPM4 is a negative regulator of angiotensin II-induced cardiac hypertrophy

Cardiac muscle adapts to hemodynamic stress by altering myocyte size and function, resulting in cardiac hypertrophy. Alteration in myocyte calcium homeostasis is known to be an initial signal in cardiac hypertrophy signaling. Transient receptor potential melastatin 4 protein (TRPM4) is a calcium-activated non-selective cation channel, which plays a role in regulating calcium influx and calcium-dependent cell functions in many cell types including cardiomyocytes. Selective deletion of TRPM4 from the heart muscle in mice resulted in an increased hypertrophic growth after chronic angiotensin (AngII) treatment, compared to WT mice. The enhanced hypertrophic response was also traceable by the increased expression of hypertrophy-related genes like Rcan1, ANP, and α-Actin. Intracellular calcium measurements on isolated ventricular myocytes showed significantly increased store-operated calcium entry upon AngII treatment in myocytes lacking the TRPM4 channel. Elevated intracellular calcium is a key factor in the development of pathological cardiac hypertrophy, leading to the activation of intracellular signaling pathways. In agreement with this, we observed significantly higher Rcan1 mRNA level, calcineurin enzyme activity and protein level in lysates from TRPM4-deficient mice heart compared to WT after AngII treatment. Collectively, these observations are consistent with a model in which TRPM4 is a regulator of calcium homeostasis in cardiomyocytes after AngII stimulation. TRPM4 contributes to the regulation of driving force for store-operated calcium entry and thereby the activation of the calcineurin–NFAT pathway and the development of pathological hypertrophy.

Cardiac CaM Kinase II genes δ and γ contribute to adverse remodeling but redundantly inhibit calcineurin-induced myocardial hypertrophy

BACKGROUND

Ca(2+)-dependent signaling through CaM Kinase II (CaMKII) and calcineurin was suggested to contribute to adverse cardiac remodeling. However, the relative importance of CaMKII versus calcineurin for adverse cardiac remodeling remained unclear.

METHODS AND RESULTS

We generated double-knockout mice (DKO) lacking the 2 cardiac CaMKII genes δ and γ specifically in cardiomyocytes. We show that both CaMKII isoforms contribute redundantly to phosphorylation not only of phospholamban, ryanodine receptor 2, and histone deacetylase 4, but also calcineurin. Under baseline conditions, DKO mice are viable and display neither abnormal Ca(2+) handling nor functional and structural changes. On pathological pressure overload and β-adrenergic stimulation, DKO mice are protected against cardiac dysfunction and interstitial fibrosis. But surprisingly and paradoxically, DKO mice develop cardiac hypertrophy driven by excessive activation of endogenous calcineurin, which is associated with a lack of phosphorylation at the auto-inhibitory calcineurin A site Ser411. Likewise, calcineurin inhibition prevents cardiac hypertrophy in DKO. On exercise performance, DKO mice show an exaggeration of cardiac hypertrophy with increased expression of the calcineurin target gene RCAN1-4 but no signs of adverse cardiac remodeling.

CONCLUSIONS

We established a mouse model in which CaMKII's activity is specifically and completely abolished. By the use of this model we show that CaMKII induces maladaptive cardiac remodeling while it inhibits calcineurin-dependent hypertrophy. These data suggest inhibition of CaMKII but not calcineurin as a promising approach to attenuate the progression of heart failure.

Galactokinase is a novel modifier of calcineurin-induced cardiomyopathy in Drosophila

Activated/uninhibited calcineurin is both necessary and sufficient to induce cardiac hypertrophy, a condition that often leads to dilated cardiomyopathy, heart failure, and sudden cardiac death. We expressed constitutively active calcineurin in the adult heart of Drosophila melanogaster and identified enlarged cardiac chamber dimensions and reduced cardiac contractility. In addition, expressing constitutively active calcineurin in the fly heart using the Gal4/UAS system induced an increase in heart wall thickness. We performed a targeted genetic screen for modifiers of calcineurin-induced cardiac enlargement based on previous calcineurin studies in the fly and identified galactokinase as a novel modifier of calcineurin-induced cardiomyopathy. Genomic deficiencies spanning the galactokinase locus, transposable elements that disrupt galactokinase, and cardiac-specific RNAi knockdown of galactokinase suppressed constitutively active calcineurin-induced cardiomyopathy. In addition, in flies expressing constitutively active calcineurin using the Gal4/UAS system, a transposable element in galactokinase suppressed the increase in heart wall thickness. Finally, genetic disruption of galactokinase suppressed calcineurin-induced wing vein abnormalities. Collectively, we generated a model for discovering novel modifiers of calcineurin-induced cardiac enlargement in the fly and identified galactokinase as a previously unknown regulator of calcineurin-induced cardiomyopathy in adult Drosophila.

Mitochondrial fission is required for cardiomyocyte hypertrophy mediated by a Ca2+-calcineurin signaling pathway

Cardiomyocyte hypertrophy has been associated with diminished mitochondrial metabolism. Mitochondria are crucial organelles for the production of ATP, and their morphology and function are regulated by the dynamic processes of fusion and fission. The relationship between mitochondrial dynamics and cardiomyocyte hypertrophy is still poorly understood. Here, we show that treatment of cultured neonatal rat cardiomyocytes with the hypertrophic agonist norepinephrine promotes mitochondrial fission (characterized by a decrease in mitochondrial mean volume and an increase in the relative number of mitochondria per cell) and a decrease in mitochondrial function. We demonstrate that norepinephrine acts through α1-adrenergic receptors to increase cytoplasmic Ca(2+), activating calcineurin and promoting migration of the fission protein Drp1 (encoded by Dnml1) to mitochondria. Dominant-negative Drp1 (K38A) not only prevented mitochondrial fission, it also blocked hypertrophic growth of cardiomyocytes in response to norepinephrine. Remarkably, an antisense adenovirus against the fusion protein Mfn2 (AsMfn2) was sufficient to increase mitochondrial fission and stimulate a hypertrophic response without agonist treatment. Collectively, these results demonstrate the importance of mitochondrial dynamics in the development of cardiomyocyte hypertrophy and metabolic remodeling.

CaMKII regulation of cardiac ryanodine receptors and inositol triphosphate receptors

Ryanodine receptors (RyRs) and inositol triphosphate receptors (InsP3Rs) are structurally related intracellular calcium release channels that participate in multiple primary or secondary amplified Ca(2+) signals, triggering muscle contraction and oscillatory Ca(2+) waves, or activating transcription factors. In the heart, RyRs play an indisputable role in the process of excitation-contraction coupling as the main pathway for Ca(2+) release from sarcoplasmic reticulum (SR), and a less prominent role in the process of excitation-transcription coupling. Conversely, InsP3Rs are believed to contribute in subtle ways, only, to contraction of the heart, and in more important ways to regulation of transcription factors. Because uncontrolled activity of either RyRs or InsP3Rs may elicit life-threatening arrhythmogenic and/or remodeling Ca(2+) signals, regulation of their activity is of paramount importance for normal cardiac function. Due to their structural similarity, many regulatory factors, accessory proteins, and post-translational processes are equivalent for RyRs and InsP3Rs. Here we discuss regulation of RyRs and InsP3Rs by CaMKII phosphorylation, but touch on other kinases whenever appropriate. CaMKII is emerging as a powerful modulator of RyR and InsP3R activity but interestingly, some of the complexities and controversies surrounding phosphorylation of RyRs also apply to InsP3Rs, and a clear-cut effect of CaMKII on either channel eludes investigators for now. Nevertheless, some effects of CaMKII on global cellular activity, such as SR Ca(2+) leak or force-frequency potentiation, appear clear now, and this constrains the limits of the controversies and permits a more tractable approach to elucidate the effects of phosphorylation at the single channel level.

Calcineurin regulates coordinated outgrowth of zebrafish regenerating fins

Vertebrates develop organs and appendages in a proportionally coordinated manner, and animals that regenerate them do so to the same dimensions as the original structures. Coordinated proportional growth involves controlled regulation between allometric and isometric growth programs, but it is unclear what executes this control. We show that calcineurin inhibition results in continued allometric outgrowth of regenerating fins beyond their original dimensions. Calcineurin inhibition also maintains allometric growth of juvenile fins and induces it in adult fins. Furthermore, calcineurin activity is low when the regeneration rate is highest, and its activity increases as the rate decreases. Growth measurements and morphometric analysis of proximodistal asymmetry indicate that calcineurin inhibition shifts fin regeneration from a distal growth program to a proximal program. This shift is associated with the promotion of retinoic acid signaling. Thus, we identified a calcineurin-mediated mechanism that operates as a molecular switch between position-associated isometric and allometric growth programs.

EBV-encoded LMP-1 sensitizes nasopharyngeal carcinoma cells to genotoxic drugs by down-regulating Cabin1 expression

The oncogenic latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) is involved in the pathogenesis of human nasopharyngeal carcinoma (NPC) and lymphoma. We and other authors have shown earlier that LMP1 induces apoptosis and inhibits xenograft tumor growth in mice, but the mechanism underlying these processes has not been investigated so far. In the present study, we show that knockdown of LMP1 renders the EBV-positive NPC cell line CG-1 resistant to various genotoxic drugs (cisplatin, etoposide, and adriamycin). LMP1 inhibits the expression of Cabin1, a Ca(2+) regulated protein shown earlier to inhibit calcineurin. Knockdown of calcineurin binding protein (Cabin1) with small hairpin RNA sensitizes CG-1 cells to genotoxic drugs. In contrast, LMP1 overexpression reduces Cabin1 level and renders both CG-1 cells and EBV-negative NPC cell lines sensitive to cisplatin. The c-Jun-N-terminal kinase (JNK) and ERK pathways are required for LMP1-induced suppression of Cabin1 at the transcriptional level. Chromatin immunoprecipitation assays further confirm that the JNK-activated transcription factor AP-1 mediates the LMP1-induced down-regulation of Cabin1 gene expression. LMP1 knockdown also increases the resistance of xenograph tumors to cisplatin in mice, therefore confirming the relevance of our findings in vivo. This study reveals the molecular mechanism underlying the pro-apoptotic activity of LMP1 during cisplatin-based NPC chemotherapy.

Mechanisms of altered Ca²⁺ handling in heart failure

Ca²⁺ plays a crucial role in connecting membrane excitability with contraction in myocardium. The hallmark features of heart failure are mechanical dysfunction and arrhythmias; defective intracellular Ca²⁺ homeostasis is a central cause of contractile dysfunction and arrhythmias in failing myocardium. Defective Ca²⁺ homeostasis in heart failure can result from pathological alteration in the expression and activity of an increasingly understood collection of Ca²⁺ homeostatic and structural proteins, ion channels, and enzymes. This review focuses on the molecular mechanisms of defective Ca²⁺ cycling in heart failure and considers how fundamental understanding of these pathways may translate into novel and innovative therapies.

Inducible re-expression of HEXIM1 causes physiological cardiac hypertrophy in the adult mouse

AIMS

The transcription factor hexamethylene-bis-acetamide-inducible protein 1 (HEXIM1) regulates myocardial vascularization and growth during cardiogenesis. Our aim was to determine whether HEXIM1 also has a beneficial role in modulating vascularization, myocardial growth, and function within the adult heart.

METHODS AND RESULTS

To achieve our objective, we created and investigated a mouse line wherein HEXIM1 was re-expressed in adult cardiomyocytes to levels found in the foetal heart. Our findings support a beneficial role for HEXIM1 through increased vascularization, myocardial growth, and increased ejection fraction within the adult heart. HEXIM1 re-expression induces angiogenesis, that is, essential for physiological hypertrophy and maintenance of cardiac function. The ability of HEXIM1 to co-ordinate processes associated with physiological hypertrophy may be attributed to HEXIM1 regulation of other transcription factors (HIF-1-α, c-Myc, GATA4, and PPAR-α) that, in turn, control many genes involved in myocardial vascularization, growth, and metabolism. Moreover, the mechanism for HEXIM1-induced physiological hypertrophy appears to be distinct from that involving the PI3K/AKT pathway.

CONCLUSION

HEXIM1 re-expression results in the induction of angiogenesis that allows for the co-ordination of tissue growth and angiogenesis during physiological hypertrophy.

Regulation of inflammatory responses and fibroblast-like synoviocyte apoptosis by calcineurin-binding protein 1 in mice with collagen-induced arthritis

OBJECTIVE

Calcineurin-binding protein 1 (CABIN-1) regulates calcineurin phosphatase activity as well as the activation, apoptosis, and inflammatory responses of fibroblast-like synoviocytes (FLS), which actively participate in the chronic inflammatory responses in rheumatoid arthritis (RA). However, the mechanism of action of CABIN-1 in FLS apoptosis is not clear. This study was undertaken to define the regulatory role of CABIN-1 in FLS from mice with collagen-induced arthritis (CIA).

METHODS

Transgenic mice overexpressing human CABIN-1 in joint tissue under the control of a type II collagen promoter were generated. Expression of human CABIN-1 (hCABIN-1) in joints and FLS was determined by reverse transcription-polymerase chain reaction (RT-PCR) and Western blot analysis. The expression of cytokines, matrix metalloproteinases (MMPs), and apoptosis-related genes in FLS was determined by enzyme-linked immunosorbent assay, gelatin zymography, and RT-PCR, respectively. Joints were stained with hematoxylin and eosin and with tartrate-resistant acid phosphatase for histologic analysis.

RESULTS

Human CABIN-1-transgenic mice with CIA had less severe arthritis than wild-type mice with CIA, as assessed according to hind paw thickness and histologic features. The milder arthritis was accompanied by significantly enhanced apoptosis in transgenic mice, evidenced by a significantly greater number of TUNEL-positive cells in synovial tissue. Expression of inflammatory cytokines and MMPs in the transgenic mice with CIA was reduced, and they exhibited decreased Akt activation and increased expression of p53, caspase 3, caspase 9, and Bax.

CONCLUSION

Our findings demonstrate that hCABIN-1 plays a critical role in promoting apoptosis of FLS and in attenuating inflammation and cartilage and bone destruction in RA. These results help elucidate the pathogenic mechanisms of RA and suggest that CABIN-1 is a potential target for treatment of this disease.

Promotion and inhibition of cardiac hypertrophy by A-kinase anchor proteins

Originally identified as mediators of cyclic adenosine monophosphate (cAMP) and protein kinase A signaling, A-kinase anchor proteins (AKAPs) are now recognized as a diverse family of molecular scaffolds capable of interacting with many other proteins. Members of the AKAP family within the heart can take on either pro- or anti-hypertrophic roles by interacting with a myriad of protein kinases and phosphatases in the process. AKAPs often form the core of large signaling complexes (or signalosomes) that allow multiple pathways to converge and functionally intertwine. Approximately 30% of AKAPs discovered to date are expressed in the heart, but the functions of many of these remain to be discovered. This review focuses on AKAPs that have been demonstrated to play roles in mediating cardiac hypertrophy.