DWORF Extends Life Span in a PLN-R14del Cardiomyopathy Mouse Model by Reducing Abnormal Sarcoplasmic Reticulum Clusters

BACKGROUND

The p.Arg14del variant of the PLN (phospholamban) gene causes cardiomyopathy, leading to severe heart failure. Calcium handling defects and perinuclear PLN aggregation have both been suggested as pathological drivers of this disease. Dwarf open reading frame (DWORF) has been shown to counteract PLN regulatory calcium handling function in the sarco/endoplasmic reticulum (S/ER). Here, we investigated the potential disease-modulating action of DWORF in this cardiomyopathy and its effects on calcium handling and PLN aggregation.

METHODS

We studied a PLN-R14del mouse model, which develops cardiomyopathy with similar characteristics as human patients, and explored whether cardiac DWORF overexpression could delay cardiac deterioration. To this end, R14Δ/Δ (homozygous PLN-R14del) mice carrying the DWORF transgene (R14Δ/ΔDWORFTg [R14Δ/Δ mice carrying the DWORF transgene]) were used.

RESULTS

DWORF expression was suppressed in hearts of R14Δ/Δ mice with severe heart failure. Restoration of DWORF expression in R14Δ/Δ mice delayed cardiac fibrosis and heart failure and increased life span >2-fold (from 8 to 18 weeks). DWORF accelerated sarcoplasmic reticulum calcium reuptake and relaxation in isolated cardiomyocytes with wild-type PLN, but in R14Δ/Δ cardiomyocytes, sarcoplasmic reticulum calcium reuptake and relaxation were already enhanced, and no differences were detected between R14Δ/Δ and R14Δ/ΔDWORFTg. Rather, DWORF overexpression delayed the appearance and formation of large pathogenic perinuclear PLN clusters. Careful examination revealed colocalization of sarcoplasmic reticulum markers with these PLN clusters in both R14Δ/Δ mice and human p.Arg14del PLN heart tissue, and hence these previously termed aggregates are comprised of abnormal organized S/ER. This abnormal S/ER organization in PLN-R14del cardiomyopathy contributes to cardiomyocyte cell loss and replacement fibrosis, consequently resulting in cardiac dysfunction.

CONCLUSIONS

Disorganized S/ER is a major characteristic of PLN-R14del cardiomyopathy in humans and mice and results in cardiomyocyte death. DWORF overexpression delayed PLN-R14del cardiomyopathy progression and extended life span in R14Δ/Δ mice, by reducing abnormal S/ER clusters.

Mitochondrial microproteins: critical regulators of protein import, energy production, stress response pathways, and programmed cell death

Mitochondria rely upon the coordination of protein import, protein translation, and proper functioning of oxidative phosphorylation (OXPHOS) complexes I-V to sustain the activities of life for an organism. Each process is dependent upon the function of profoundly large protein complexes found in the mitochondria [translocase of the outer mitochondrial membrane (TOMM) complex, translocase of the inner mitochondrial membrane (TIMM) complex, OXPHOS complexes, mitoribosomes]. These massive protein complexes, in some instances more than one megadalton, are built up from numerous protein subunits of varying sizes, including many proteins that are ≤100-150 amino acids. However, these small proteins, termed microproteins, not only act as cogs in large molecular machines but also have important steps in inhibiting or promoting the intrinsic pathway of apoptosis, coordinate responses to cellular stress, and even act as hormones. This review focuses on microproteins that occupy the mitochondria and are critical for its function. Although the microprotein field is relatively new, researchers have long recognized the existence of these mitochondrial proteins as critical components of virtually all aspects of mitochondrial biology. Thus, recent studies estimating that hundreds of new microproteins of unknown function exist and are missing from current genome annotations suggests that the mitochondrial "microproteome" is a rich area for future biological investigation.

Microproteins: Overlooked regulators of physiology and disease

Ongoing efforts to generate a complete and accurate annotation of the genome have revealed a significant blind spot for small proteins (<100 amino acids) originating from short open reading frames (sORFs). The recent discovery of numerous sORF-encoded proteins, termed microproteins, that play diverse roles in critical cellular processes has ignited the field of microprotein biology. Large-scale efforts are currently underway to identify sORF-encoded microproteins in diverse cell-types and tissues and specialized methods and tools have been developed to aid in their discovery, validation, and functional characterization. Microproteins that have been identified thus far play important roles in fundamental processes including ion transport, oxidative phosphorylation, and stress signaling. In this review, we discuss the optimized tools available for microprotein discovery and validation, summarize the biological functions of numerous microproteins, outline the promise for developing microproteins as therapeutic targets, and look forward to the future of the field of microprotein biology.

Dwarf Open Reading Frame (DWORF) Gene Therapy Ameliorated Duchenne Muscular Dystrophy Cardiomyopathy in Aged mdx Mice

Background Cardiomyopathy is a leading health threat in Duchenne muscular dystrophy (DMD). Cytosolic calcium upregulation is implicated in DMD cardiomyopathy. Calcium is primarily removed from the cytosol by the sarcoendoplasmic reticulum calcium ATPase (SERCA). SERCA activity is reduced in DMD. Improving SERCA function may treat DMD cardiomyopathy. Dwarf open reading frame (DWORF) is a recently discovered positive regulator for SERCA, hence, a potential therapeutic target. Methods and Results To study DWORF's involvement in DMD cardiomyopathy, we quantified DWORF expression in the heart of wild-type mice and the mdx model of DMD. To test DWORF gene therapy, we engineered and characterized an adeno-associated virus serotype 9-DWORF vector. To determine if this vector can mitigate DMD cardiomyopathy, we delivered it to 6-week-old mdx mice (6×1012 vector genome particles/mouse) via the tail vein. Exercise capacity, heart histology, and cardiac function were examined at 18 months of age. We found DWORF expression was significantly reduced at the transcript and protein levels in mdx mice. Adeno-associated virus serotype 9-DWORF vector significantly enhanced SERCA activity. Systemic adeno-associated virus serotype 9-DWORF therapy reduced myocardial fibrosis and improved treadmill running, electrocardiography, and heart hemodynamics. Conclusions Our data suggest that DWORF deficiency contributes to SERCA dysfunction in mdx mice and that DWORF gene therapy holds promise to treat DMD cardiomyopathy.

Abstract P2009: MPTM1: A Cardiac-Enriched Microprotein That Is Upregulated During Heart Failure

Although they were originally misclassified as lncRNAs due to oversights in genome annotation methods, microproteins—proteins with open reading frames (ORFs) of 150 amino acids or less—execute critical roles in heart and skeletal muscle. We have identified a novel microprotein, MPTM1, that is enriched in cardiac and skeletal muscle tissues. The MPTM1 amino acid sequence is strongly conserved across multiple species, which indicates that it is a functional microprotein that has been maintained in response to evolutionary pressure. Genome-wide association studies (GWAS) of patients have demonstrated that MPTM1 is upregulated during heart failure. Additionally, MPTM1 expression strongly increases during skeletal muscle differentiation and maturation. In the present study, we assessed the hypothesis that MPTM1 promotes heart and skeletal muscle homeostasis using both in vitro and in vivo gain- and loss-of-function approaches. Furthermore, we generated C2C12 myoblast and NIH3T3 fibroblast cell lines null for MPTM1 using CRISPR/Cas9 gene editing methods. MPTM1 depletion in knockout cell lines resulted in striking membrane and nuclear abnormalities. Additionally, MPTM1 null C2C12 cells exhibited defects in myoblast differentiation characterized by premature myosin expression, altered cellular architecture, abnormal nuclear distribution, and the accumulation of intracellular vacuoles. We further evaluated MPTM1 cellular function using CRISPR/Cas9 homology-directed repair (HDR) to insert tandem affinity tags (3XHA-V5 or 3X FLAG) into the MPTM1 locus to assess binding partners in vitro using immunoprecipitation and mass spectrometry. Our results provide strong evidence that MPTM1 is a multi-localizing protein that is differentially recruited to the cytosol, nucleolus, and mitochondria, indicating it may act as a key intracellular signaling effector in response to stress. Additional studies in MPTM1 null mice are ongoing and will provide critical insight into MPTM1 function in the heart at under normal physiologic conditions and in response to pathological stress.

Abstract P2006: S100A1 And STRIT1 Redundantly Governs Responsiveness Of The Heart To Hemodynamic Stress Via Modulation Of SERCA2a Activity

Background & Objectives: SR Ca2+ load, which is sustained by SERCA2a pump activity, is a critical determinant for cardiac performance regulation & adaptation. Independent studies identified both S100A1 & STRIT1 as molecular enhancers of SERCA2a activity in the heart. S100A1 & STRIT1 decline in post-myocardial infarction hearts aggravated the transition to adverse cardiac remodelling & contractile failure. We therefore hypothesized that S100A1 & STRIT1 could act as independent but potentially redundant molecular switches for SERCA2a activity.

Methods & Results: S100a1 knock-out (SKO) mice display no overt cardiac contractile or structural abnormalities in the absence of stress. RNA-seq transcriptomic analysis of left ventricle (LV) of SKO & wild type (WT) identified Strit1 amongst the top 3 most upregulated genes in SKO LV. We validated STRIT1 upregulation by RT-PCR as well as by immunoblotting (IB) that yielded a 15-fold increase compared with WT. Age lapse-resolved RT-PCR analysis showed Strit1 response to S100a1 knockout begins at post-partum day 5 & reaches plateau in adulthood. Next, we generated Strit1-S100a1 double knock out mice (StSKO), which showed only a mild decline in cardiac contractile performance. Interestingly, LV tissue serin-16 phospholamban (PLN) phosphorylation levels & PLNs pentameric state were found to be enhanced. WT, SKO & StSKO mice were then subjected to transaortic constriction (TAC) & followed for 60 days, which fully unmasked the mutually compensatory functions of S100A1 & STRIT1. In TAC-StSKO hearts showed significantly higher decline in LV %EF, significantly increased LV end-systolic volume & LV end-systolic diameter, & significantly increased cardiac hypertrophic growth together with concordant molecular markers. TAC-SKO mice did not show any decline in STRIT1 protein upregulation, while TAC-WT hearts showed a putatively compensatory increase in the S100A1/STRIT1 protein ratio.

Conclusion: Our first results indicates that STRIT1 & S100A1 can act as compensatory molecular switches securing sufficient SERCA2a activity. As such, our study further sheds new light onto the novel concept of “molecular redundancy” to secure & protect cardiac key effector activities to cope with distinct hemodynamic stressors

The hidden world of membrane microproteins

Proteins are critical components of biological membranes and play key roles in many essential cellular processes. Membrane proteins are a structurally and functionally diverse family of proteins that have recently expanded to include a number of newly discovered tiny proteins called microproteins, or micropeptides. These microproteins are generated from small open reading frames, which produce protein products that are less than 100 amino acids in length. While not all microproteins are membrane proteins, this review will focus specifically on this subclass to highlight some of the important biological activities that have been ascribed to these molecules and to emphasize their promise as exciting new players in membrane biology.

MOXI Is a Mitochondrial Micropeptide That Enhances Fatty Acid β-Oxidation

Micropeptide regulator of β-oxidation (MOXI) is a conserved muscle-enriched protein encoded by an RNA transcript misannotated as non-coding. MOXI localizes to the inner mitochondrial membrane where it associates with the mitochondrial trifunctional protein, an enzyme complex that plays a critical role in fatty acid β-oxidation. Isolated heart and skeletal muscle mitochondria from MOXI knockout mice exhibit a diminished ability to metabolize fatty acids, while transgenic MOXI overexpression leads to enhanced β-oxidation. Additionally, hearts from MOXI knockout mice preferentially oxidize carbohydrates over fatty acids in an isolated perfused heart system compared to wild-type (WT) animals. MOXI knockout mice also exhibit a profound reduction in exercise capacity, highlighting the role of MOXI in metabolic control. The functional characterization of MOXI provides insight into the regulation of mitochondrial metabolism and energy homeostasis and underscores the regulatory potential of additional micropeptides that have yet to be identified.

Micropeptide regulator of b-oxidation (MOXI) is encoded by a muscle-enriched RNA transcript misannotated as noncoding. MOXI localizes to the inner mitochondrial membrane where it interacts with the trifunctional protein to modulate fatty acid b-oxidation and exercise capacity.

The DWORF micropeptide enhances contractility and prevents heart failure in a mouse model of dilated cardiomyopathy

Calcium (Ca2+) dysregulation is a hallmark of heart failure and is characterized by impaired Ca2+ sequestration into the sarcoplasmic reticulum (SR) by the SR-Ca2+-ATPase (SERCA). We recently discovered a micropeptide named DWORF (DWarf Open Reading Frame) that enhances SERCA activity by displacing phospholamban (PLN), a potent SERCA inhibitor. Here we show that DWORF has a higher apparent binding affinity for SERCA than PLN and that DWORF overexpression mitigates the contractile dysfunction associated with PLN overexpression, substantiating its role as a potent activator of SERCA. Additionally, using a well-characterized mouse model of dilated cardiomyopathy (DCM) due to genetic deletion of the muscle-specific LIM domain protein (MLP), we show that DWORF overexpression restores cardiac function and prevents the pathological remodeling and Ca2+ dysregulation classically exhibited by MLP knockout mice. Our results establish DWORF as a potent activator of SERCA within the heart and as an attractive candidate for a heart failure therapeutic.

Ten-year survivorship of primary total hip arthroplasty in patients 30 years of age or younger

Aims

For this retrospective cohort study, patients aged ≤ 30 years (very young) who underwent total hip arthroplasty (THA) were compared with patients aged ≥ 60 years (elderly) to evaluate the rate of revision arthroplasty, implant survival, the indications for revision, the complications, and the patient-reported outcomes.

Patients and Methods

We retrospectively reviewed all patients who underwent primary THA between January 2000 and May 2015 from our institutional database. A total of 145 very young and 1359 elderly patients were reviewed. The mean follow-up was 5.3 years (1 to 18). Logistic generalized estimating equations were used to compare characteristics and the revision rate. Survival was evaluated using Kaplan-Meier curves and hazard rates were created using Cox regression.

Results

The overall revision rate was 11% (16/145) in the very young and 3.83% (52/1359) in the elderly groups (odds ratio (OR) 2.58, 95% confidence interval (CI) 1.43 to 4.63). After adjusting for the American Society of Anesthesiologists (ASA) score, gender, and a history of previous surgery in a time-to-event model, the risk of revision remained greater in the very young (adjusted hazard ratio (HR) 2.48, 95% CI 1.34 to 4.58). Survival at ten years was 82% (95% CI, 71 to 89) in the very young and 96% (95% CI, 94 to 97) in the elderly group (p < 0.001). The very young had a higher rate of revision for complications related to metal-on-metal (MoM) bearing surfaces (p < 0.001). At last follow-up, the very young group had higher levels of physical function (p = 0.002), lower levels of mental health (p = 0.001), and similar levels of pain (p = 0.670) compared with their elderly counterparts.

Conclusion

The overall revision rate was greater in very young THA patients. This was largely explained by the use of MoM bearings. Young patients with non-MoM bearings had high survivorship with similar complication profiles to patients aged ≥ 60 years. Cite this article: Bone Joint J 2018;100-B:867-74.

Coenzyme A-mediated degradation of pyruvate dehydrogenase kinase 4 promotes cardiac metabolic flexibility after high-fat feeding in mice

Cardiac energy is produced primarily by oxidation of fatty acids and glucose, with the relative contributions of each nutrient being sensitive to changes in substrate availability and energetic demand. A major contributor to cardiac metabolic flexibility is pyruvate dehydrogenase (PDH), which converts glucose-derived pyruvate to acetyl-CoA within the mitochondria. PDH is inhibited by phosphorylation dependent on the competing activities of pyruvate dehydrogenase kinases (PDK1-4) and phosphatases (PDP1-2). A single high-fat meal increases cardiac PDK4 content and subsequently inhibits PDH activity, reducing pyruvate utilization when abundant fatty acids are available. In this study, we demonstrate that diet-induced increases in PDK4 are reversible and characterize a novel pathway that regulates PDK4 degradation in response to the cardiac metabolic environment. We found that PDK4 degradation is promoted by CoA (CoASH), the levels of which declined in mice fed a high-fat diet and normalized following transition to a control diet. We conclude that CoASH functions as a metabolic sensor linking the rate of PDK4 degradation to fatty acid availability in the heart. However, prolonged high-fat feeding followed by return to a low-fat diet resulted in persistent in vitro sensitivity of PDH to fatty acid-induced inhibition despite reductions in PDK4 content. Moreover, increases in the levels of proteins responsible for β-oxidation and rates of palmitate oxidation by isolated cardiac mitochondria following long-term consumption of high dietary fat persisted after transition to the control diet. We propose that these changes prime PDH for inhibition upon reintroduction of fatty acids.

Caveolae-localized L-type Ca2+ channels do not contribute to function or hypertrophic signalling in the mouse heart

Aims

L-type Ca2+ channels (LTCCs) in adult cardiomyocytes are localized to t-tubules where they initiate excitation-contraction coupling. Our recent work has shown that a subpopulation of LTCCs found at the surface sarcolemma in caveolae of adult feline cardiomyocytes can also generate a Ca2+ microdomain that activates nuclear factor of activated T-cells signaling and cardiac hypertrophy, although the relevance of this paradigm to hypertrophy regulation in vivo has not been examined.

Methods and results

Here we generated heart-specific transgenic mice with a putative caveolae-targeted LTCC activator protein that was ineffective in initiating or enhancing cardiac hypertrophy in vivo. We also generated transgenic mice with cardiac-specific overexpression of a putative caveolae-targeted inhibitor of LTCCs, and while this protein inhibited caveolae-localized LTCCs without effects on global Ca2+ handling, it similarly had no effect on cardiac hypertrophy in vivo. Cardiac hypertrophy was elicited by pressure overload for 2 or 12 weeks or with neurohumoral agonist infusion. Caveolae-specific LTCC activator or inhibitor transgenic mice showed no greater change in nuclear factor of activated T-cells activity after 2 weeks of pressure overload stimulation compared with control mice.

Conclusion

Our results indicate that LTCCs in the caveolae microdomain do not affect cardiac function and are not necessary for the regulation of hypertrophic signaling in the adult mouse heart.

Mining for Micropeptides

Advances in computational biology and large-scale transcriptome analyses have revealed that a much larger portion of the genome is transcribed than was previously recognized, resulting in the production of a diverse population of RNA molecules with both protein-coding and noncoding potential. Emerging evidence indicates that several RNA molecules have been mis-annotated as noncoding and in fact harbor short open reading frames (sORFs) that encode functional peptides and that have evaded detection until now due to their small size. sORF-encoded peptides (SEPs), or micropeptides, have been shown to have important roles in fundamental biological processes and in the maintenance of cellular homeostasis. These small proteins can act independently, for example as ligands or signaling molecules, or they can exert their biological functions by engaging with and modulating larger regulatory proteins. Given their small size, micropeptides may be uniquely suited to fine-tune complex biological systems.

MED12 regulates a transcriptional network of calcium-handling genes in the heart

The Mediator complex regulates gene transcription by linking basal transcriptional machinery with DNA-bound transcription factors. The activity of the Mediator complex is mainly controlled by a kinase submodule that is composed of 4 proteins, including MED12. Although ubiquitously expressed, Mediator subunits can differentially regulate gene expression in a tissue-specific manner. Here, we report that MED12 is required for normal cardiac function, such that mice with conditional cardiac-specific deletion of MED12 display progressive dilated cardiomyopathy. Loss of MED12 perturbs expression of calcium-handling genes in the heart, consequently altering calcium cycling in cardiomyocytes and disrupting cardiac electrical activity. We identified transcription factors that regulate expression of calcium-handling genes that are downregulated in the heart in the absence of MED12, and we found that MED12 localizes to transcription factor consensus sequences within calcium-handling genes. We showed that MED12 interacts with one such transcription factor, MEF2, in cardiomyocytes and that MED12 and MEF2 co-occupy promoters of calcium-handling genes. Furthermore, we demonstrated that MED12 enhances MEF2 transcriptional activity and that overexpression of both increases expression of calcium-handling genes in cardiomyocytes. Our data support a role for MED12 as a coordinator of transcription through MEF2 and other transcription factors. We conclude that MED12 is a regulator of a network of calcium-handling genes, consequently mediating contractility in the mammalian heart.

Widespread control of calcium signaling by a family of SERCA-inhibiting micropeptides

Micropeptides function as master regulators of calcium-dependent signaling in muscle. Sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA), the membrane pump that promotes muscle relaxation by taking up Ca²⁺ into the sarcoplasmic reticulum, is directly inhibited by three muscle-specific micropeptides: myoregulin (MLN), phospholamban (PLN), and sarcolipin (SLN). The widespread and essential function of SERCA across diverse cell types has raised questions as to how SERCA is regulated in cells that lack MLN, PLN, and SLN. We identified two transmembrane micropeptides, endoregulin (ELN) and another-regulin (ALN), that share key amino acids with their muscle-specific counterparts and function as direct inhibitors of SERCA pump activity. The distribution of transcripts encoding ELN and ALN mirrored that of SERCA isoform-encoding transcripts in nonmuscle cell types. Our findings identify additional members of the SERCA-inhibitory micropeptide family, revealing a conserved mechanism for the control of intracellular Ca²⁺ dynamics in both muscle and nonmuscle cell types.

Abstract 317: Med12 is a Transcriptional Mediator of Cardiac Contractile Function

The Mediator complex serves as a central governor of gene transcription by linking the basal transcriptional machinery with DNA-bound transcription factors. The activity of the Mediator complex is modulated by a kinase submodule comprised of four proteins: MED12, MED13, CDK8, and Cyclin C. The kinase submodule components are ubiquitously expressed, however recent studies support the hypothesis that these proteins differentially regulate gene expression in a tissue-specific manner. We previously demonstrated that MED13 acts in the heart to modulate systemic energy homeostasis through signaling to extracardiac tissues, but the role of the other kinase components in the heart has not yet been investigated. MED12 regulates development in a variety of cell types, and MED12 null mice are embryonically lethal. To investigate the cardiac functions of MED12, we generated mice with conditional cardiac-specific deletion of MED12. MED12 mutant mice displayed postnatal cardiomyopathy seven days after birth with a continual decline in cardiac function with age. RNAseq experiments identified specific dysregulation of genes involved in calcium homeostasis and contractility in the ventricles as early as postnatal day one. Defects in calcium handling were detected in cardiomyocytes isolated just after birth, suggesting that perturbed calcium transients lead to altered cardiac contractility and dysfunction. The consequences of cardiac deletion of MED12 and MED13 are clearly distinct, underscoring the unique functions of these Mediator components. These findings highlight the importance of the kinase submodule of the mediator complex in the control of cardiac function and reveal highly specific and distinct transcriptional actions of MED12 and MED13.

A peptide encoded by a transcript annotated as long noncoding RNA enhances SERCA activity in muscle

Muscle contraction depends on release of Ca(2+) from the sarcoplasmic reticulum (SR) and reuptake by the Ca(2+)adenosine triphosphatase SERCA. We discovered a putative muscle-specific long noncoding RNA that encodes a peptide of 34 amino acids and that we named dwarf open reading frame (DWORF). DWORF localizes to the SR membrane, where it enhances SERCA activity by displacing the SERCA inhibitors, phospholamban, sarcolipin, and myoregulin. In mice, overexpression of DWORF in cardiomyocytes increases peak Ca(2+) transient amplitude and SR Ca(2+) load while reducing the time constant of cytosolic Ca(2+) decay during each cycle of contraction-relaxation. Conversely, slow skeletal muscle lacking DWORF exhibits delayed Ca(2+) clearance and relaxation and reduced SERCA activity. DWORF is the only endogenous peptide known to activate the SERCA pump by physical interaction and provides a means for enhancing muscle contractility.

A mouse model for adult cardiac-specific gene deletion with CRISPR/Cas9

Clustered regularly interspaced short palindromic repeats (CRISPR)-associated (Cas)9 genomic editing has revolutionized the generation of mutant animals by simplifying the creation of null alleles in virtually any organism. However, most current approaches with this method require zygote injection, making it difficult to assess the adult, tissue-specific functions of genes that are widely expressed or which cause embryonic lethality when mutated. Here, we describe the generation of cardiac-specific Cas9 transgenic mice, which express high levels of Cas9 in the heart, but display no overt defects. In proof-of-concept experiments, we used Adeno-Associated Virus 9 (AAV9) to deliver single-guide RNA (sgRNA) that targets the Myh6 locus exclusively in cardiomyocytes. Intraperitoneal injection of postnatal cardiac-Cas9 transgenic mice with AAV9 encoding sgRNA against Myh6 resulted in robust editing of the Myh6 locus. These mice displayed severe cardiomyopathy and loss of cardiac function, with elevation of several markers of heart failure, confirming the effectiveness of this method of adult cardiac gene deletion. Mice with cardiac-specific expression of Cas9 provide a tool that will allow rapid and accurate deletion of genes following a single injection of AAV9-sgRNAs, thereby circumventing embryonic lethality. This method will be useful for disease modeling and provides a means of rapidly editing genes of interest in the heart.

Comparative effects of urocortins and stresscopin on cardiac myocyte contractility

RATIONALE

There is a current need for the development of new therapies for patients with heart failure.

OBJECTIVE

We test the effects of members of the corticotropin-releasing factor (CRF) family of peptides on myocyte contractility to validate them as potential heart failure therapeutics.

METHODS AND RESULTS

Adult feline left ventricular myocytes (AFMs) were isolated and contractility was assessed in the presence and absence of CRF peptides Urocortin 2 (UCN2), Urocortin 3 (UCN3), Stresscopin (SCP), and the β-adrenergic agonist isoproterenol (Iso). An increase in fractional shortening and peak Ca(2+) transient amplitude was seen in the presence of all CRF peptides. A decrease in Ca(2+) decay rate (Tau) was also observed at all concentrations tested. cAMP generation was measured by ELISA in isolated AFMs in response to the CRF peptides and Iso and significant production was seen at all concentrations and time points tested.

CONCLUSIONS

The CRF family of peptides effectively increases cardiac contractility and should be evaluated as potential novel therapeutics for heart failure patients.

Abstract 335: Regulation of Muscle Contractility by a Family of SERCA-Inhibitory Micropeptides

Functional micropeptides can be concealed within RNA transcripts that have been putatively annotated as non-coding. We recently discovered a muscle-specific micropeptide, named myoregulin (MLN), that inhibits the activity of SERCA, the membrane pump that controls muscle relaxation by regulating Ca2+ uptake into the sarcoplasmic reticulum (SR). Genetic deletion of MLN in mice enhances Ca2+ handling in skeletal muscle and improves exercise performance. MLN shares structural and functional similarity with phospholamban (PLN) and sarcolipin (SLN), two well-studied micropeptides that regulate cardiac contractility and disease. Here we identify an additional member of this micropeptide family, named endoregulin (ELN), that specifically overlaps with the expression of SERCA3, the dominant Ca2+ ATPase in endothelial cells that controls the contractility of vascular and visceral smooth muscles. ELN encodes a single transmembrane alpha helix that localizes to the endoplasmic reticulum (ER), where it forms a stable complex with SERCA3. In cell based assays, ELN inhibits SERCA-dependent Ca2+ uptake into the ER and controls ER calcium levels. Due to the essential role of SERCA3 in regulating vascular smooth muscle contractility, ELN represents a potential regulator of vascular tone and novel therapeutic target for the treatment of cardiovascular disease.

Abstract 339: DWORF: a Novel Cardiac Micropeptide That Enhances SERCA Activity and Cardiomyocyte Contractility

Background: Our lab has discovered a novel conserved micropeptide of 34 amino acids encoded by a muscle-specific RNA that was previously annotated as a putative long non-coding RNA (lncRNA). This micropeptide, which we named DWORF (DWarf Open Reading Frame), shares structural similarity with the known SERCA modulators phospholamban (PLN), sarcolipin (SLN) and myoregulin (MLN).

Objective: To define the functional role of DWORF in the heart and elucidate its mechanism of action.

Methods and Results: Immunofluorescence staining and imaging in isolated adult mouse cardiac myocytes indicates that DWORF specifically localizes to sarcoplasmic reticulum (SR) membranes where it co-localizes with SERCA. Co-IP experiments performed in COS or HEK cells co-transfected with DWORF and SERCA show that DWORF forms a stable complex with various SERCA isoforms. Further biochemical analysis indicates that DWORF binds to the same site on SERCA as PLN, and overexpression of DWORF is capable of competing with PLN for SERCA binding. Co-expression of GFP-tagged DWORF and PLN in the presence of SERCA followed by SERCA pull-down and GFP Western indicates that PLN and DWORF have similar affinities for SERCA binding. Transgenic mice with cardiac specific over-expression of DWORF show enhanced cellular contractility as evidenced by increased peak Ca2+ transient amplitude and faster cytosolic Ca2+ decay rates (reduced Tau values). Furthermore, Ca2+-dependent Ca2+-uptake assays performed in homogenates from DWORF transgenic and littermate control hearts show a statistically significant leftward shift of the Ca2+-dependence curve for SERCA indicating a higher affinity of SERCA for Ca2+. Intriguingly, DWORF expression is silenced in the heart in response to pathological calcineurin signaling, implicating this micropeptide in the cellular pathway for calcineurin action.

Conclusions: We have identified a novel muscle-specific micropeptide, DWORF, which is expressed in the heart at high levels and has the capacity to bind SERCA and modulate its function to increase contractility in transgenic mice. Our findings provide exciting data in support of DWORF as a previously unrecognized endogenous SERCA activator that plays a role in cardiac contractility.

Autologous c-Kit+ Mesenchymal Stem Cell Injections Provide Superior Therapeutic Benefit as Compared to c-Kit+ Cardiac-Derived Stem Cells in a Feline Model of Isoproterenol-Induced Cardiomyopathy

BACKGROUND

Cardiac- (CSC) and mesenchymal-derived (MSC) CD117+ isolated stem cells improve cardiac function after injury. However, no study has compared the therapeutic benefit of these cells when used autologously.

METHODS

MSCs and CSCs were isolated on day 0. Cardiomyopathy was induced (day 28) by infusion of L-isoproterenol (1,100 ug/kg/hour) from Alzet minipumps for 10 days. Bromodeoxyuridine (BrdU) was infused via minipumps (50 mg/mL) to identify proliferative cells during the injury phase. Following injury (day 38), autologous CSC (n = 7) and MSC (n = 4) were delivered by intracoronary injection. These animals were compared to those receiving sham injections by echocardiography, invasive hemodynamics, and immunohistochemistry.

RESULTS

Fractional shortening improved with CSC (26.9 ± 1.1% vs. 16.1 ± 0.2%, p = 0.01) and MSC (25.1 ± 0.2% vs. 12.1 ± 0.5%, p = 0.01) as compared to shams. MSC were superior to CSC in improving left ventricle end-diastolic (LVED) volume (37.7 ± 3.1% vs. 19.9 ± 9.4%, p = 0.03) and ejection fraction (27.7 ± 0.1% vs. 19.9 ± 0.4%, p = 0.02). LVED pressure was less in MSC (6.3 ± 1.3 mmHg) as compared to CSC (9.3 ± 0.7 mmHg) and sham (13.3 ± 0.7); p = 0.01. LV BrdU+ myocytes were higher in MSC (0.17 ± 0.03%) than CSC (0.09 ± 0.01%) and sham (0.06 ± 01%); p < 0.001.

CONCLUSIONS

Both CD117+ isolated CSC and MSC therapy improve cardiac function and attenuate pathological remodeling. However, MSC appear to confer additional benefit.

A micropeptide encoded by a putative long noncoding RNA regulates muscle performance

Functional micropeptides can be concealed within RNAs that appear to be noncoding. We discovered a conserved micropeptide, which we named myoregulin (MLN), encoded by a skeletal muscle-specific RNA annotated as a putative long noncoding RNA. MLN shares structural and functional similarity with phospholamban (PLN) and sarcolipin (SLN), which inhibit SERCA, the membrane pump that controls muscle relaxation by regulating Ca(2+) uptake into the sarcoplasmic reticulum (SR). MLN interacts directly with SERCA and impedes Ca(2+) uptake into the SR. In contrast to PLN and SLN, which are expressed in cardiac and slow skeletal muscle in mice, MLN is robustly expressed in all skeletal muscle. Genetic deletion of MLN in mice enhances Ca(2+) handling in skeletal muscle and improves exercise performance. These findings identify MLN as an important regulator of skeletal muscle physiology and highlight the possibility that additional micropeptides are encoded in the many RNAs currently annotated as noncoding.

Platelet endothelial cell adhesion molecule-1 mediates endothelial-cardiomyocyte communication and regulates cardiac function

Background

Dilated cardiomyopathy is characterized by impaired contractility of cardiomyocytes, ventricular chamber dilatation, and systolic dysfunction. Although mutations in genes expressed in the cardiomyocyte are the best described causes of reduced contractility, the importance of endothelial‐cardiomyocyte communication for proper cardiac function is increasingly appreciated. In the present study, we investigate the role of the endothelial adhesion molecule platelet endothelial cell adhesion molecule (PECAM‐1) in the regulation of cardiac function.

Methods and Results

Using cell culture and animal models, we show that PECAM‐1 expressed in endothelial cells (ECs) regulates cardiomyocyte contractility and cardiac function via the neuregulin‐ErbB signaling pathway. Conscious echocardiography revealed left ventricular (LV) chamber dilation and systolic dysfunction in PECAM‐1^−/− mice in the absence of histological abnormalities or defects in cardiac capillary density. Despite deficits in global cardiac function, cardiomyocytes isolated from PECAM‐1^−/− hearts displayed normal baseline and isoproterenol‐stimulated contractility. Mechanistically, absence of PECAM‐1 resulted in elevated NO/ROS signaling and NRG‐1 release from ECs, which resulted in augmented phosphorylation of its receptor ErbB2. Treatment of cardiomyocytes with conditioned media from PECAM‐1^−/− ECs resulted in enhanced ErbB2 activation, which was normalized by pre‐treatment with an NRG‐1 blocking antibody. To determine whether normalization of increased NRG‐1 levels could correct cardiac function, PECAM‐1^−/− mice were treated with the NRG‐1 blocking antibody. Echocardiography showed that treatment significantly improved cardiac function of PECAM‐1^−/− mice, as revealed by increased ejection fraction and fractional shortening.

Conclusions

We identify a novel role for PECAM‐1 in regulating cardiac function via a paracrine NRG1‐ErbB pathway. These data highlight the importance of tightly regulated cellular communication for proper cardiac function.

GRK5-mediated exacerbation of pathological cardiac hypertrophy involves facilitation of nuclear NFAT activity

RATIONALE

G protein-coupled receptor kinases (GRKs) acting in the cardiomyocyte regulate important signaling events that control cardiac function. Both GRK2 and GRK5, the predominant GRKs expressed in the heart, have been shown to be upregulated in failing human myocardium. Although the canonical role of GRKs is to desensitize G protein-coupled receptors via phosphorylation, it has been demonstrated that GRK5, unlike GRK2, can reside in the nucleus of myocytes and exert G protein-coupled receptor-independent effects that promote maladaptive cardiac hypertrophy and heart failure.

OBJECTIVE

To explore novel mechanisms by which GRK5 acting in the nucleus of cardiomyocytes participates in pathological cardiac hypertrophy.

METHODS AND RESULTS

In this study, we have found that GRK5-mediated pathological cardiac hypertrophy involves the activation of the nuclear factor of activated T cells (NFAT) because GRK5 causes enhancement of NFAT-mediated hypertrophic gene transcription. Transgenic mice with cardiomyocyte-specific GRK5 overexpression activate an NFAT-reporter in mice basally and after hypertrophic stimulation, including transverse aortic constriction and phenylephrine treatment. Complimentary to this, GRK5 null mice exhibit less NFAT transcriptional activity after transverse aortic constriction. Furthermore, the loss of NFATc3 expression in the heart protected GRK5 overexpressing transgenic mice from the exaggerated hypertrophy and early progression to heart failure seen after transverse aortic constriction. Molecular studies suggest that GRK5 acts in concert with NFAT to increase hypertrophic gene transcription in the nucleus via GRK5's ability to bind DNA directly without a phosphorylation event.

CONCLUSIONS

GRK5, acting in a kinase independent manner, is a facilitator of NFAT activity and part of a DNA-binding complex responsible for pathological hypertrophic gene transcription.

β-adrenergic receptor-mediated cardiac contractility is inhibited via vasopressin type 1A-receptor-dependent signaling

BACKGROUND

Enhanced arginine vasopressin levels are associated with increased mortality during end-stage human heart failure, and cardiac arginine vasopressin type 1A receptor (V1AR) expression becomes increased. Additionally, mice with cardiac-restricted V1AR overexpression develop cardiomyopathy and decreased β-adrenergic receptor (βAR) responsiveness. This led us to hypothesize that V1AR signaling regulates βAR responsiveness and in doing so contributes to development of heart failure.

METHODS AND RESULTS

Transaortic constriction resulted in decreased cardiac function and βAR density and increased cardiac V1AR expression, effects reversed by a V1AR-selective antagonist. Molecularly, V1AR stimulation led to decreased βAR ligand affinity, as well as βAR-induced Ca(2+) mobilization and cAMP generation in isolated adult cardiomyocytes, effects recapitulated via ex vivo Langendorff analysis. V1AR-mediated regulation of βAR responsiveness was demonstrated to occur in a previously unrecognized Gq protein-independent/G protein receptor kinase-dependent manner.

CONCLUSIONS

This newly discovered relationship between cardiac V1AR and βAR may be informative for the treatment of patients with acute decompensated heart failure and elevated arginine vasopressin.

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.

Imatinib activates pathological hypertrophy by altering myocyte calcium regulation

BACKGROUND

Imatinib mesylate is a selective tyrosine-kinase inhibitor used in the treatment of multiple cancers, most notably chronic myelogenous leukemia. There is evidence that imatinib can induce cardiotoxicity in cancer patients. Our hypothesis is that imatinib alters calcium regulatory mechanisms and can contribute to development of pathological cardiac hypertrophy.

METHODS AND RESULTS

Neonatal rat ventricular myocytes (NRVMs) were treated with clinical doses (low: 2 μM; high: 5 μM) of imatinib and assessed for molecular changes. Imatinib increased peak systolic Ca(2+) and Ca(2+) transient decay rates and Western analysis revealed significant increases in phosphorylation of phospholamban (Thr-17) and the ryanodine receptor (Ser-2814), signifying activation of calcium/calmodulin-dependent kinase II (CaMKII). Imatinib significantly increased NRVM volume as assessed by Coulter counter, myocyte surface area, and atrial natriuretic peptide abundance seen by Western. Imatinib induced cell death, but did not activate the classical apoptotic program as assessed by caspase-3 cleavage, indicating a necrotic mechanism of death in myocytes. We expressed AdNFATc3-green fluorescent protein in NRVMs and showed imatinib treatment significantly increased nuclear factor of activated T cells translocation that was inhibited by the calcineurin inhibitor FK506 or CaMKII inhibitors.

CONCLUSION

These data show that imatinib can activate pathological hypertrophic signaling pathways by altering intracellular Ca(2+) dynamics. This is likely a contributing mechanism for the adverse cardiac effects of imatinib.

The gut hormone ghrelin partially reverses energy substrate metabolic alterations in the failing heart

BACKGROUND

The gut-derived hormone ghrelin, especially its acylated form, plays a major role in the regulation of systemic metabolism and exerts also relevant cardioprotective effects; hence, it has been proposed for the treatment of heart failure (HF). We tested the hypothesis that ghrelin can directly modulate cardiac energy substrate metabolism.

METHODS AND RESULTS

We used chronically instrumented dogs, 8 with pacing-induced HF and 6 normal controls. Human des-acyl ghrelin [1.2 nmol/kg per hour] was infused intravenously for 15 minutes, followed by washout (rebaseline) and infusion of acyl ghrelin at the same dose. (3)H-oleate and (14)C-glucose were coinfused and arterial and coronary sinus blood sampled to measure cardiac free fatty acid and glucose oxidation and lactate uptake. As expected, cardiac substrate metabolism was profoundly altered in HF because baseline oxidation levels of free fatty acids and glucose were, respectively, >70% lower and >160% higher compared with control. Neither des-acyl ghrelin nor acyl ghrelin significantly affected function and metabolism in normal hearts. However, in HF, des-acyl and acyl ghrelin enhanced myocardial oxygen consumption by 10.2±3.5% and 9.9±3.7%, respectively (P<0.05), and cardiac mechanical efficiency was not significantly altered. This was associated, respectively, with a 41.3±6.7% and 32.5±10.9% increase in free fatty acid oxidation and a 31.3±9.2% and 41.4±8.9% decrease in glucose oxidation (all P<0.05).

CONCLUSIONS

Acute increases in des-acyl or acyl ghrelin do not interfere with cardiac metabolism in normal dogs, whereas they enhance free fatty acid oxidation and reduce glucose oxidation in HF dogs, thus partially correcting metabolic alterations in HF. This novel mechanism might contribute to the cardioprotective effects of ghrelin in HF.

Sorafenib cardiotoxicity increases mortality after myocardial infarction

RATIONALE

Sorafenib is an effective treatment for renal cell carcinoma, but recent clinical reports have documented its cardiotoxicity through an unknown mechanism.

OBJECTIVE

Determining the mechanism of sorafenib-mediated cardiotoxicity.

METHODS AND RESULTS

Mice treated with sorafenib or vehicle for 3 weeks underwent induced myocardial infarction (MI) after 1 week of treatment. Sorafenib markedly decreased 2-week survival relative to vehicle-treated controls, but echocardiography at 1 and 2 weeks post MI detected no differences in cardiac function. Sorafenib-treated hearts had significantly smaller diastolic and systolic volumes and reduced heart weights. High doses of sorafenib induced necrotic death of isolated myocytes in vitro, but lower doses did not induce myocyte death or affect inotropy. Histological analysis documented increased myocyte cross-sectional area despite smaller heart sizes after sorafenib treatment, further suggesting myocyte loss. Sorafenib caused apoptotic cell death of cardiac- and bone-derived c-kit+ stem cells in vitro and decreased the number of BrdU+ (5-bromo-2'-deoxyuridine+) myocytes detected at the infarct border zone in fixed tissues. Sorafenib had no effect on infarct size, fibrosis, or post-MI neovascularization. When sorafenib-treated animals received metoprolol treatment post MI, the sorafenib-induced increase in post-MI mortality was eliminated, cardiac function was improved, and myocyte loss was ameliorated.

CONCLUSIONS

Sorafenib cardiotoxicity results from myocyte necrosis rather than from any direct effect on myocyte function. Surviving myocytes undergo pathological hypertrophy. Inhibition of c-kit+ stem cell proliferation by inducing apoptosis exacerbates damage by decreasing endogenous cardiac repair. In the setting of MI, which also causes large-scale cell loss, sorafenib cardiotoxicity dramatically increases mortality.

Abstract 242: Bone-derived Stem Cells Repair The Heart After Myocardial Infarction Through Transdifferentiation And Paracrine Signaling Mechanisms

Rationale: Autologous bone marrow- or cardiac-derived stem cell therapy for heart disease has demonstrated safety and efficacy in clinical trials but has only offered limited functional improvements. Finding the optimal stem cell type best suited for cardiac regeneration remains a key goal toward improving clinical outcomes.

Objective: To determine the mechanism by which novel bone-derived stem cells support the injured heart.

Methods and Results: Cortical bone stem cells (CBSCs) were isolated from EGFP+ transgenic mice and were shown to express c-kit and Sca-1 as well as 8 paracrine factors involved in cardioprotection, angiogenesis and stem cell function. Wild-type C57BL/6 mice underwent sham operation (n=21) or myocardial infarction (MI) with injection of CBSCs (n=57) or saline (n=59). Cardiac function was monitored using echocardiography with strain analysis. EGFP+ stem cells in vivo were shown to express only 2/8 factors tested (basic fibroblast growth factor and vascular endothelial growth factor) and this expression was associated with increased neovascularization of the infarct border zone. CBSC therapy improved survival, cardiac function, attenuated adverse remodeling, and decreased infarct size relative to saline-treated MI controls. By 6 weeks post-MI, EGFP+ cardiomyocytes, vascular smooth muscle cells and endothelial cells could be identified on histology. Isolated EGFP+ myocytes were smaller, more frequently mononucleated, and demonstrated fractional shortening and calcium currents indistinguishable from EGFP- myocytes from the same hearts.

Conclusions: CBSCs improve survival, cardiac function, and attenuate remodeling by 1) secreting the proangiogenic factors bFGF and VEGF (stimulating endogenous neovascularization), and 2) differentiating into functional adult myocytes and vascular cells.

Abstract 164: Microdomain Specific Effects of Transient Receptor Potential Channels on Pathological Cardiac Hypertrophy and Myocyte Contractility

Hypothesis: Ca2+ influx through transient receptor potential canonical (TRPC) channels and L-type Ca2+ channels (LTCCs) within caveolin-3 (Cav3) stabilized signaling microdomains provide a unique source of Ca2+ to activate pathologic cardiac hypertrophy through calcineurin (Cn)-mediated nuclear factor of activated T-cells (NFAT) signaling. We suggest that a distinct and separate population of TRPC channels localized in excitation-contraction (EC) coupling microdomains may have potent effects on myocyte contractility independent of Cav3 signaling domains.

Methods and Results: Membrane localization studies and immunohistochemistry show that TRPC channels and LTCCs co-localize to Cav3 signaling microdomains. To explore a role for these caveolae based Ca2+ channels in the initiation of Cn-NFAT signaling we used an adenoviral NFAT-GFP reporter in cultured adult feline myocytes (AFMs). Infecting AFMs with ad-TRPC3 dramatically increased NFAT translocation, which was inhibited with dominant negative ad-dnTRPC6. Expression of a Cav3 targeted LTCC blocker (ad-Cav-Rem) reduced NFAT translocation while a targeted LTCC activator (ad-Cav-β2a) significantly increased NFAT activation. Neither LTCC modulator had significant effects on Ca2+ current or contractility in AFMs but we found that the expression of TRPC3 reduced myocyte contractility and induced spontaneous Ca2+ spark activity that was exacerbated by the DAG activator OAG. Moreover, dnTRPC6 blocked spontaneous Ca2+ sparks even in the presence of OAG. Immunohistochemistry analysis revealed the presence of TRPC channels in transverse tubules, consistent with the idea that they could have direct effects on EC coupling microdomains.

Conclusions: Our data show that TRPC channels and LTCCs co-localize to Cav3 signaling domains where they generate a unique Ca2+ microenvironment that directly regulates Cn-NFAT signaling. Our findings also suggest that a separate and distinct population of TRPC channels within EC coupling microdomains cause reduced myocyte contractility by inducing SR Ca2+ leak and Ca2+ spark activity.

Abstract 278: Exercise Training Ameliorates LV Dysfunction in Mice with a Calcium Influx Mediated Cardiomyopathy

Background— Ca 2+ influx through L-type Ca 2+ channels (LTCCs) induces Ca 2+ release from the SR to induce and regulate cardiac contraction. We have characterized a mouse model with cardiac-specific expression of the β2a subunit of the LTCC (β2a), leading to pathological hypertrophy due to excessive LTCC Ca 2+ entry. Chronic exercise causes physiological hypertrophy in normal hearts. We determined if swim training improved or further deranged the performance of β2a mice.

Methods and Results— β2a (n=12) and WT (n=20) mice were swim trained for 21 consecutive days. ECHO was performed at beginning and end of swim training. Ejection fraction (EF) and fractional shortening (FS) increased significantly with swim training in WT animals. (EF pre-swim: 57±2.02% vs. post-swim: 70±2.88%; FS pre-swim: 30±1.34% vs. post-swim: 39±2.42%). β2a mice were hypercontractile before swimming and had no significant change after training (EF pre-swim: 70±1.75% vs. post-swim: 74.06±2.72%; FS pre-training: 38±2.38% vs. post-training: 42±2.22%). Contractile performance was significantly greater in β2a mice versus WT before but not after training. There was a significant increase in HW/BW [mg/gm] ratio in trained WT (n=8) versus sedentary WT (n=8) (swim: 5.75±0.19 vs. sed: 4.63±0.2) but training did not cause additional hypertrophy in β2a (n=8) versus sedentary (n=7) (swim: 5.98±0.31 vs. sed: 5.96±0.25). Isolated WT and β2a hearts were placed on a Langendorff apparatus and a balloon was inserted into the LV to record isovolumic pressure. Each heart underwent 15 min of ischemia followed by 30 min of reperfusion. End diastolic pressure (EDP) increased in all hearts during ischemia. There were no protective effects of training on ischemia-induced increases in EDP in WT hearts. EDP increased more in sedentary β2a during ischemia but this effect was eliminated in trained β2a hearts. LV developed pressure (LVDP) fell with ischemia and partially recovered with reperfusion. LVDP recovery was significantly greater in trained versus sedentary WT hearts. Swim training also significantly improved LVDP recovery in β2a hearts.

Conclusions— Chronic exercise training reduces pathological hypertrophy and damaging effects of ischemia in a murine model of LTCC overexpression.

Bone-derived stem cells repair the heart after myocardial infarction through transdifferentiation and paracrine signaling mechanisms

RATIONALE

Autologous bone marrow-derived or cardiac-derived stem cell therapy for heart disease has demonstrated safety and efficacy in clinical trials, but functional improvements have been limited. Finding the optimal stem cell type best suited for cardiac regeneration is the key toward improving clinical outcomes.

OBJECTIVE

To determine the mechanism by which novel bone-derived stem cells support the injured heart.

METHODS AND RESULTS

Cortical bone-derived stem cells (CBSCs) and cardiac-derived stem cells were isolated from enhanced green fluorescent protein (EGFP+) transgenic mice and were shown to express c-kit and Sca-1 as well as 8 paracrine factors involved in cardioprotection, angiogenesis, and stem cell function. Wild-type C57BL/6 mice underwent sham operation (n=21) or myocardial infarction with injection of CBSCs (n=67), cardiac-derived stem cells (n=36), or saline (n=60). Cardiac function was monitored using echocardiography. Only 2/8 paracrine factors were detected in EGFP+ CBSCs in vivo (basic fibroblast growth factor and vascular endothelial growth factor), and this expression was associated with increased neovascularization of the infarct border zone. CBSC therapy improved survival, cardiac function, regional strain, attenuated remodeling, and decreased infarct size relative to cardiac-derived stem cells- or saline-treated myocardial infarction controls. By 6 weeks, EGFP+ cardiomyocytes, vascular smooth muscle, and endothelial cells could be identified in CBSC-treated, but not in cardiac-derived stem cells-treated, animals. EGFP+ CBSC-derived isolated myocytes were smaller and more frequently mononucleated, but were functionally indistinguishable from EGFP- myocytes.

CONCLUSIONS

CBSCs improve survival, cardiac function, and attenuate remodeling through the following 2 mechanisms: (1) secretion of proangiogenic factors that stimulate endogenous neovascularization, and (2) differentiation into functional adult myocytes and vascular cells.

Ca(2+) influx through L-type Ca(2+) channels and transient receptor potential channels activates pathological hypertrophy signaling

Common cardiovascular diseases such as hypertension and myocardial infarction require that myocytes develop greater than normal force to maintain cardiac pump function. This requires increases in [Ca(2+)]. These diseases induce cardiac hypertrophy and increases in [Ca(2+)] are known to be an essential proximal signal for activation of hypertrophic genes. However, the source of "hypertrophic" [Ca(2+)] is not known and is the topic of this study. The role of Ca(2+) influx through L-type Ca(2+) channels (LTCC), T-type Ca(2+) channels (TTCC) and transient receptor potential (TRP) channels on the activation of calcineurin (Cn)-nuclear factor of activated T cells (NFAT) signaling and myocyte hypertrophy was studied. Neonatal rat ventricular myocytes (NRVMs) and adult feline ventricular myocytes (AFVMs) were infected with an adenovirus containing NFAT-GFP, to determine factors that could induce NFAT nuclear translocation. Four millimolar Ca(2+) or pacing induced NFAT nuclear translocation. This effect was blocked by Cn inhibitors. In NRVMs Nifedipine (Nif, LTCC antagonist) blocked high Ca(2+)-induced NFAT nuclear translocation while SKF-96365 (TRP channel antagonist) and Nickel (Ni, TTCC antagonist) were less effective. The relative potency of these antagonists against Ca(2+) induced NFAT nuclear translocation (Nif>SKF-96365>Ni) was similar to their effects on Ca(2+) transients and the LTCC current. Infection of NRVM with viruses containing TRP channels also activated NFAT-GFP nuclear translocation and caused myocyte hypertrophy. TRP effects were reduced by SKF-96365, but were more effectively antagonized by Nif. These experiments suggest that Ca(2+) influx through LTCCs is the primary source of Ca(2+) to activate Cn-NFAT signaling in NRVMs and AFVMs. While TRP channels cause hypertrophy, they appear to do so through a mechanism involving Ca(2+) entry via LTCCs.

Abstract 111: High Activity Gating of Caveolae-Targeted L-Type Ca 2+ Channels Can Initiate Pathological Hypertrophy

Introduction: The source of Ca 2+ that activates pathological cardiac hypertrophy is not clearly defined. We hypothesize that high activity gating of L-type Ca 2+ channels (LTCCs) localized in caveolae signaling microdomains stabilized by caveolin-3 (Cav-3) provides the Ca 2+ influx that locally activates calcineurin (Cn)-mediated nuclear factor of activated T-cells (NFAT) to induce hypertrophic gene expression. We generated novel reagents that specifically activate or inhibit the gating of LTCCs in caveolae for analysis of the hypertrophic program, as well as any potential effect on ICa and contraction.

Methods and Results: We targeted the known LTCC inhibitory protein Rem or an LTCC subunit (β2a) that promotes high activity gating specifically to caveolae by fusing them to a caveolin-binding domain peptide (termed Cav-Rem and Cav-β2a). We infected adult feline left ventricular myocytes (AFLVMs) with adenoviruses containing Cav-Rem or Cav-β2a for membrane localization and functional studies. NFAT nuclear translocation was determined by co-infecting AFLVMs with ad-NFAT-GFP and either ad-Cav-Rem or ad-Cav-β2a and pacing cells to induce Ca2+ influx mediated nuclear NFAT-GFP translocation. Membrane fractionation experiments showed that Cav-3 membrane domains contain 26.2 +/- 12.7% of membrane targeted LTCCs and blocking these with Cav-Rem eliminated a small fraction of the LTCC current (<15%) and almost all Ca2+ influx induced NFAT nuclear translocation (>90%), but did not reduce myocyte contractility. Conversely, selective enhancement of LTCC activity within caveolae with Cav-β2a caused a significant increase in NFAT nuclear translocation (>70%) but had no significant effect on contractility.

Conclusions: We provide proof of concept that specific Cav-targeted reagents can be used to enhance or inhibit LTCC activity within caveolae microdomains to amplify or block the hypertrophic response. Our results suggest that high activity gating of LTCCs, which is known to be present in the hypertrophied failing human heart, can activate signaling pathways linked to cardiac hypertrophy. Selectively inhibiting these caveolae localized LTCCs could be a novel mechanism to block pathological hypertrophy without reducing cardiac contractility.

A characterization and targeting of the infarct border zone in a swine model of myocardial infarction

INTRODUCTION

Novel therapies for myocardial infarction (MI) involving stem cells, gene therapy, biomaterials, or revascularization strategies have shown promise in animal studies and clinical trials, but results have been limited partially due to the injection of therapeutics into ischemic myocardium that cannot support their mechanism of action. Accurate targeting of therapeutics precisely to the infarct border zone (BZ) may be essential for effective repair of the ischemic heart.

METHODS

Ischemia-reperfusion MI was induced in Yorkshire swine by inflation of an angioplasty balloon in the left anterior descending coronary artery. Fluorescent microspheres were injected into the BZ under NOGA catheter guidance, and this location was identified grossly then examined by immunohistochemistry and Western analysis.

RESULTS

Analysis of the infarct zone two hours post-MI revealed a frankly necrotic region devoid of contractile proteins with marked activation of caspase-3. The NOGA-defined BZ closely approximates the grossly-defined BZ and contains intact myocytes and vasculature. Western analysis detected Akt expression and levels of Ca(2+) handling proteins equivalent to that of viable tissues.

CONCLUSIONS

Histological and Western analysis revealed that NOGA mapping precisely identifies grossly and molecularly defined infarct BZ at a location where there are still viable cells and vessels capable of supporting novel therapeutic strategies.

Deletion of muscle GRP94 impairs both muscle and body growth by inhibiting local IGF production

Insulin-like growth factors (IGFs) are critical for development and growth of skeletal muscles, but because several tissues produce IGFs, it is not clear which source is necessary or sufficient for muscle growth. Because it is critical for production of both IGF-I and IGF-II, we ablated glucose-regulated protein 94 (GRP94) in murine striated muscle to test the necessity of local IGFs for normal muscle growth. These mice exhibited smaller skeletal muscles with diminished IGF contents but with normal contractile function and no apparent endoplasmic reticulum stress response. This result shows that muscles rely on GRP94 primarily to support local production of IGFs, a pool that is necessary for normal muscle growth. In addition, body weights were ∼30% smaller than those of littermate controls, and circulating IGF-I also decreased significantly, yet glucose homeostasis was maintained with little disruption to the growth hormone pathway. The growth defect was complemented on administration of recombinant IGF-I. Thus, unlike liver production of IGF-I, muscle IGF-I is necessary not only locally but also globally for whole-body growth.

Validation of transcatheter left ventricular electromechanical mapping for assessment of cardiac function and targeted transendocardial injection in a porcine ischemia-reperfusion model

Ischemic heart disease, despite advances in treatment, remains the major cause of mortality worldwide. NOGA 3D left ventricular electromechanical mapping allows accurate determination of cardiac function and precise identification of sites of injury. In a porcine model of ischemia-reperfusion injury, we validate the use of the NOGA mapping system for assessment of cardiac function along with the Myostar injection catheter for directed delivery of therapeutics to localized target sites in the setting of acute myocardial injury.

Hyperphosphorylation of the cardiac ryanodine receptor at serine 2808 is not involved in cardiac dysfunction after myocardial infarction

RATIONALE

Abnormal behavior of the cardiac ryanodine receptor (RyR2) has been linked to cardiac arrhythmias and heart failure (HF) after myocardial infarction (MI). It has been proposed that protein kinase A (PKA) hyperphosphorylation of the RyR2 at a single residue, Ser-2808, is a critical mediator of RyR dysfunction, depressed cardiac performance, and HF after MI.

OBJECTIVE

We used a mouse model (RyRS2808A) in which PKA hyperphosphorylation of the RyR2 at Ser-2808 is prevented to determine whether loss of PKA phosphorylation at this site averts post MI cardiac pump dysfunction.

METHODS AND RESULTS

MI was induced in wild-type (WT) and S2808A mice. Myocyte and cardiac function were compared in WT and S2808A animals before and after MI. The effects of the PKA activator Isoproterenol (Iso) on L-type Ca(2+) current (I(CaL)), contractions, and [Ca(2+)](I) transients were also measured. Both WT and S2808A mice had depressed pump function after MI, and there were no differences between groups. MI size was also identical in both groups. L type Ca(2+) current, contractions, Ca(2+) transients, and SR Ca(2+) load were also not significantly different in WT versus S2808A myocytes either before or after MI. Iso effects on Ca(2+) current, contraction, Ca(2+) transients, and SR Ca(2+) load were identical in WT and S2808A myocytes before and after MI at both low and high concentrations.

CONCLUSIONS

These results strongly support the idea that PKA phosphorylation of RyR-S2808 is irrelevant to the development of cardiac dysfunction after MI, at least in the mice used in this study.

A caveolae-targeted L-type Ca²+ channel antagonist inhibits hypertrophic signaling without reducing cardiac contractility

RATIONALE

The source of Ca(2+) to activate pathological cardiac hypertrophy is not clearly defined. Ca(2+) influx through the L-type Ca(2+) channels (LTCCs) determines "contractile" Ca(2+), which is not thought to be the source of "hypertrophic" Ca(2+). However, some LTCCs are housed in caveolin-3 (Cav-3)-enriched signaling microdomains and are not directly involved in contraction. The function of these LTCCs is unknown.

OBJECTIVE

To test the idea that LTCCs in Cav-3-containing signaling domains are a source of Ca(2+) to activate the calcineurin-nuclear factor of activated T-cell signaling cascade that promotes pathological hypertrophy.

METHODS AND RESULTS

We developed reagents that targeted Ca(2+) channel-blocking Rem proteins to Cav-3-containing membranes, which house a small fraction of cardiac LTCCs. Blocking LTCCs within this Cav-3 membrane domain eliminated a small fraction of the LTCC current and almost all of the Ca(2+) influx-induced NFAT nuclear translocation, but it did not reduce myocyte contractility.

CONCLUSIONS

We provide proof of concept that Ca(2+) influx through LTCCs within caveolae signaling domains can activate "hypertrophic" signaling, and this Ca(2+) influx can be selectively blocked without reducing cardiac contractility.

An essential role for ATP binding and hydrolysis in the chaperone activity of GRP94 in cells

Glucose-regulated protein 94 (GRP94) is an endoplasmic reticulum (ER) chaperone for which only few client proteins and no cofactors are known and whose mode of action is unclear. To decipher the mode of GRP94 action in vivo, we exploited our finding that GRP94 is necessary for the production of insulin-like growth factor (IGF)-II and developed a cell-based functional assay. Grp94(-/-) cells are hypersensitive to serum withdrawal and die. This phenotype can be complemented either with exogenous IGF-II or by expression of functional GRP94. Fusion proteins of GRP94 with monomeric GFP (mGFP) or mCherry also rescue the viability of transiently transfected, GRP94-deficient cells, demonstrating that the fusion proteins are functional. Because these constructs enable direct visualization of chaperone-expressing cells, we used this survival assay to assess the activities of GRP94 mutants that are defective in specific biochemical functions in vitro. Mutations that abolish binding of adenosine nucleotides cannot support growth in serum-free medium. Similarly, mutations of residues needed for ATP hydrolysis also render GRP94 partially or completely nonfunctional. In contrast, an N-terminal domain mutant that cannot bind peptides still supports cell survival. Thus the peptide binding activity in vitro can be uncoupled from the chaperone activity toward IGF in vivo. This mutational analysis suggests that the ATPase activity of GRP94 is essential for chaperone activity in vivo and that the essential protein-binding domain of GRP94 is distinct from the N-terminal domain.

Isolation and characterization of microsatellite markers from the acacia-ant Crematogaster mimosae

We describe 10 microsatellite loci developed from Crematogaster mimosae, an ant species that nests mutualistically in Acacia drepanolobium trees in east Africa. Polymorphism ranged from 4 to 16 alleles per locus (mean = 7.3). Observed and expected heterozygosities ranged from 0.485 to 0.813 (mean 0.626), and from 0.502 to 0.894 (mean 0.674), respectively. These markers will foster studies of the population structure, colony structure, and reproductive strategies of these ants.