Lactate- and immunomagnetic-purified hiPSC-derived cardiomyocytes generate comparable engineered cardiac tissue constructs

Three-dimensional engineered cardiac tissue (ECT) using purified human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) has emerged as an appealing model system for the study of human cardiac biology and disease. A recent study reported widely used metabolic (lactate) purification of monolayer hiPSC-CM cultures results in an ischemic cardiomyopathy-like phenotype compared with magnetic antibody-based cell sorting (MACS) purification, complicating the interpretation of studies using lactate-purified hiPSC-CMs. Herein, our objective was to determine if use of lactate relative to MACS-purified hiPSC-CMs affects the properties of resulting hiPSC-ECTs. Therefore, hiPSC-CMs were differentiated and purified using either lactate-based media or MACS. Global proteomics revealed that lactate-purified hiPSC-CMs displayed a differential phenotype over MACS hiPSC-CMs. hiPSC-CMs were then integrated into 3D hiPSC-ECTs and cultured for 4 weeks. Structurally, there was no significant difference in sarcomere length between lactate and MACS hiPSC-ECTs. Assessment of isometric twitch force and Ca2+ transient measurements revealed similar functional performance between purification methods. High-resolution mass spectrometry-based quantitative proteomics showed no significant difference in protein pathway expression or myofilament proteoforms. Taken together, this study demonstrates that lactate- and MACS-purified hiPSC-CMs generate ECTs with comparable structural, functional, and proteomic features, and it suggests that lactate purification does not result in an irreversible change in a hiPSC-CM phenotype.

Neutral sphingomyelinase regulates mechanotransduction in human engineered cardiac tissues and mouse hearts

Cardiovascular disease is the leading cause of death in the USA and is known to be exacerbated by elevated mechanical stress from hypertension. Caveolae are plasma membrane structures that buffer mechanical stress but have been found to be reduced in pathological conditions associated with chronically stretched myocardium. To explore the physiological implications of the loss of caveolae, we used human engineered cardiac tissue (ECT) constructs, composed of human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes and hiPSC-derived cardiac fibroblasts, to develop a long-term cyclic stretch protocol that recapitulates the effects of hypertension on caveolae expression, membrane tension, and the β-adrenergic response. Leveraging this new stretch protocol, we identified neutral sphingomyelinases (nSMase) as mechanoregulated mediators of caveolae loss, ceramide production and the blunted β-adrenergic response in this human cardiac model. Specifically, in our ECT model, nSMase inhibition via GW4869 prevented stretch-induced loss of caveolae-like structures, mitigated nSMase-dependent ceramide production, and maintained the ECT contractile kinetic response to isoprenaline. These findings are correlated with a blood lipidomic analysis in middle-aged and older adults, which revealed an increase of the circulating levels of ceramides in adults with hypertension. Furthermore, we found that conduction slowing from increased pressure loading in mouse left ventricle was abolished in the context of nSMase inhibition. Collectively, these findings identify nSMase as a potent drug target for mitigating stretch-induced effects on cardiac function. KEY POINTS: We have developed a new stretch protocol for human engineered cardiac tissue that recapitulates changes in plasma membrane morphology observed in animal models of pressure/volume overload. Stretch of engineered cardiac tissue induces activation of neutral sphingomyelinase (nSMase), generation of ceramide, and disassembly of caveolae. Activation of nSMase blunts cardiac β-adrenergic contractile kinetics and mediates stretch-induced slowing of conduction and upstroke velocity. Circulating ceramides are increased in adults with hypertension, highlighting the clinical relevance of stretch-induced nSMase activity.

Top-down proteomics of myosin light chain isoforms define chamber-specific expression in the human heart

Myosin functions as the "molecular motor" of the sarcomere and generates the contractile force necessary for cardiac muscle contraction. Myosin light chains 1 and 2 (MLC-1 and -2) play important functional roles in regulating the structure of the hexameric myosin molecule. Each of these light chains has an 'atrial' and 'ventricular' isoform, so called because they are believed to exhibit chamber-restricted expression in the heart. However, recently the chamber-specific expression of MLC isoforms in the human heart has been questioned. Herein, we analyzed the expression of MLC-1 and -2 atrial and ventricular isoforms in each of the four cardiac chambers in adult non-failing donor hearts using top-down mass spectrometry (MS)-based proteomics. Strikingly, we detected an isoform thought to be ventricular, MLC-2v (gene: MYL2), in the atria and confirmed the protein sequence using tandem MS (MS/MS). For the first time, a putative deamidation post-translation modification (PTM) located on MLC-2v in atrial tissue was localized to amino acid N13. MLC-1v (MYL3) and MLC-2a (MYL7) were the only MLC isoforms exhibiting chamber-restricted expression patterns across all donor hearts. Importantly, our results unambiguously show that MLC-1v, not MLC-2v, is ventricle-specific in adult human hearts. Moreover, we found elevated MLC-2 phosphorylation in male hearts compared to female hearts across each cardiac chamber. Overall, top-down proteomics allowed an unbiased analysis of MLC isoform expression throughout the human heart, uncovering previously unexpected isoform expression patterns and PTMs.

Lactate and Immunomagnetic-purified iPSC-derived Cardiomyocytes Generate Comparable Engineered Cardiac Tissue Constructs

Three-dimensional engineered cardiac tissue (ECT) using purified human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) has emerged as an appealing model system for the study of human cardiac biology and disease. A recent study reported widely-used metabolic (lactate) purification of monolayer hiPSC-CM cultures results in an ischemic cardiomyopathy-like phenotype compared to magnetic antibody-based cell sorting (MACS) purification, complicating the interpretation of studies using lactate-purified hiPSC-CMs. Herein, our objective was to determine if use of lactate relative to MACs-purified hiPSC-CMs impacts the properties of resulting hiPSC-ECTs. Therefore, hiPSC-CMs were differentiated and purified using either lactate-based media or MACS. After purification, hiPSC-CMs were combined with hiPSC-cardiac fibroblasts to create 3D hiPSC-ECT constructs maintained in culture for four weeks. There were no structural differences observed, and there was no significant difference in sarcomere length between lactate and MACS hiPSC-ECTs. Assessment of isometric twitch force, Ca 2+ transients, and β-adrenergic response revealed similar functional performance between purification methods. High-resolution mass spectrometry (MS)-based quantitative proteomics showed no significant difference in any protein pathway expression or myofilament proteoforms. Taken together, this study demonstrates lactate- and MACS-purified hiPSC-CMs generate ECTs with comparable molecular and functional properties, and suggests lactate purification does not result in an irreversible change in hiPSC-CM phenotype.

cMyBP-C ablation in human engineered cardiac tissue causes progressive Ca2+-handling abnormalities

Truncation mutations in cardiac myosin binding protein C (cMyBP-C) are common causes of hypertrophic cardiomyopathy (HCM). Heterozygous carriers present with classical HCM, while homozygous carriers present with early onset HCM that rapidly progress to heart failure. We used CRISPR-Cas9 to introduce heterozygous (cMyBP-C+/-) and homozygous (cMyBP-C-/-) frame-shift mutations into MYBPC3 in human iPSCs. Cardiomyocytes derived from these isogenic lines were used to generate cardiac micropatterns and engineered cardiac tissue constructs (ECTs) that were characterized for contractile function, Ca2+-handling, and Ca2+-sensitivity. While heterozygous frame shifts did not alter cMyBP-C protein levels in 2-D cardiomyocytes, cMyBP-C+/- ECTs were haploinsufficient. cMyBP-C-/- cardiac micropatterns produced increased strain with normal Ca2+-handling. After 2 wk of culture in ECT, contractile function was similar between the three genotypes; however, Ca2+-release was slower in the setting of reduced or absent cMyBP-C. At 6 wk in ECT culture, the Ca2+-handling abnormalities became more pronounced in both cMyBP-C+/- and cMyBP-C-/- ECTs, and force production became severely depressed in cMyBP-C-/- ECTs. RNA-seq analysis revealed enrichment of differentially expressed hypertrophic, sarcomeric, Ca2+-handling, and metabolic genes in cMyBP-C+/- and cMyBP-C-/- ECTs. Our data suggest a progressive phenotype caused by cMyBP-C haploinsufficiency and ablation that initially is hypercontractile, but progresses to hypocontractility with impaired relaxation. The severity of the phenotype correlates with the amount of cMyBP-C present, with more severe earlier phenotypes observed in cMyBP-C-/- than cMyBP-C+/- ECTs. We propose that while the primary effect of cMyBP-C haploinsufficiency or ablation may relate to myosin crossbridge orientation, the observed contractile phenotype is Ca2+-mediated.

Identifying Features of Cardiac Disease Phenotypes Based on Mechanical Function in a Catecholaminergic Polymorphic Ventricular Tachycardia Model

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is characterized by an arrhythmogenic mechanism involving disruption of calcium handling. This genetic disease can lead to sudden death in children and young adults during physical or emotional stress. Prior CPVT studies have focused on calcium handling, but mechanical functionality has rarely been investigated in vitro. In this research we combine stem cell-derived cardiomyocytes from a CPVT patient (RyR2-H2464D mutation) and a healthy familial control with an engineered culture platform to evaluate mechanical function of cardiomyocytes. Substrates with Young's modulus ranging from 10 to 50 kPa were used in conjunction with microcontact printing of ECM proteins into defined patterns for subsequent attachment. Digital Image Correlation (DIC) was used to evaluate collections of contracting cells. The amplitude of contractile strain was utilized as a quantitative indicator of functionality and disease severity. We found statistically significant differences: the maximum contractile strain was consistently higher in patient samples compared to control samples on all substrate stiffnesses. Additionally, the patient cell line had a statistically significantly slower intrinsic contraction rate than the control, which agrees with prior literature. Differences in mechanical strain have not been previously reported, and hypercontractility is not a known characteristic of CPVT. However, functional changes can occur as the disease progresses, thus this observation may not represent behavior observed in adolescent and adult patients. These results add to the limited studies of mechanical function of CPVT CMs reported in literature and identify functional differences that should be further explored.

Stent interventions for pulmonary artery stenosis improve bi-ventricular flow efficiency in a swine model

BACKGROUND

Branch pulmonary artery (PA) stenosis (PAS) commonly occurs in patients with congenital heart disease (CHD). Prior studies have documented technical success and clinical outcomes of PA stent interventions for PAS but the impact of PA stent interventions on ventricular function is unknown. The objective of this study was to utilize 4D flow cardiovascular magnetic resonance (CMR) to better understand the impact of PAS and PA stenting on ventricular contraction and ventricular flow in a swine model of unilateral branch PA stenosis.

METHODS

18 swine (4 sham, 4 untreated left PAS, 10 PAS stent intervention) underwent right heart catheterization and CMR at 20 weeks age (55 kg). CMR included ventricular strain analysis and 4D flow CMR.

RESULTS

4D flow CMR measured inefficient right ventricular (RV) and left ventricular (LV) flow patterns in the PAS group (RV non-dimensional (n.d.) vorticity: sham 82 ± 47, PAS 120 ± 47; LV n.d. vorticity: sham 57 ± 5, PAS 78 ± 15 p < 0.01) despite the PAS group having normal heart rate, ejection fraction and end-diastolic volume. The intervention group demonstrated increased ejection fraction that resulted in more efficient ventricular flow compared to untreated PAS (RV n.d. vorticity: 59 ± 12 p < 0.01; LV n.d. vorticity: 41 ± 7 p < 0.001).

CONCLUSION

These results describe previously unknown consequences of PAS on ventricular function in an animal model of unilateral PA stenosis and show that PA stent interventions improve ventricular flow efficiency. This study also highlights the sensitivity of 4D flow CMR biomarkers to detect earlier ventricular dysfunction assisting in identification of patients who may benefit from PAS interventions.

Human iPSC-engineered cardiac tissue platform faithfully models important cardiac physiology

Cardiomyocytes derived from human induced pluripotent stem cells (hiPSC-CM) may provide an important bridge between animal models and the intact human myocardium. Fulfilling this potential is hampered by their relative immaturity, leading to poor physiological responsiveness. hiPSC-CMs grown in traditional two-dimensional (2D) culture lack a t-tubular system, have only rudimentary intracellular calcium-handling systems, express predominantly embryonic sarcomeric protein isoforms, and preferentially use glucose as an energy substrate. Culturing hiPSC-CM in a variety of three-dimensional (3D) environments and the addition of nutritional, pharmacological, and electromechanical stimuli have proven, to various degrees, to be beneficial for maturation. We present a detailed assessment of a novel model in which hiPSC-CMs and hiPSC-derived cardiac fibroblasts are cocultured in a 3D fibrin matrix to form engineered cardiac tissue constructs (hiPSC-ECTs). The hiPSC-ECTs are responsive to physiological stimuli, including stretch, frequency, and β-adrenergic stimulation, develop a t-tubular system, and demonstrate calcium-handling and contractile kinetics that compare favorably with ventricular human myocardium. Furthermore, transcript levels of various genes involved in calcium-handling and contraction are increased. These markers of maturation become more robust over a relatively short period of time in culture (6 wk vs. 2 wk in hiPSC-ECTs). A comparison of the hiPSC-ECT molecular and performance variables with those of human cardiac tissue and other available engineered tissue platforms is provided to aid selection of the most appropriate platform for the research question at hand. Important and noteworthy aspects of this human cardiac model system are its reliance on "off-the-shelf" equipment, ability to provide detailed physiological performance data, and the ability to achieve a relatively mature cardiac physiology without additional nutritional, pharmacological, and electromechanical stimuli that may elicit unintended effects on function.NEW & NOTEWORTHY This study seeks to provide an in-depth assessment of contractile performance of human iPSC-derived cardiomyocytes cultured together with fibroblasts in a 3-dimensional-engineered tissue and compares performance both over time as cells mature, and with corresponding measures found in the literature using alternative 3D culture configurations. The suitability of 3D-engineered human cardiac tissues to model cardiac function is emphasized, and data provided to assist in the selection of the most appropriate configuration based on the target application.

Functionally Integrated Top-Down Proteomics for Standardized Assessment of Human Induced Pluripotent Stem Cell-Derived Engineered Cardiac Tissues

Three-dimensional (3D) human induced pluripotent stem cell-derived engineered cardiac tissues (hiPSC-ECTs) have emerged as a promising alternative to two-dimensional hiPSC-cardiomyocyte monolayer systems because hiPSC-ECTs are a closer representation of endogenous cardiac tissues and more faithfully reflect the relevant cardiac pathophysiology. The ability to perform functional and molecular assessments using the same hiPSC-ECT construct would allow for more reliable correlation between observed functional performance and underlying molecular events, and thus is critically needed. Herein, for the first time, we have established an integrated method that permits sequential assessment of functional properties and top-down proteomics from the same single hiPSC-ECT construct. We quantitatively determined the differences in isometric twitch force and the sarcomeric proteoforms between two groups of hiPSC-ECTs that differed in the duration of time of 3D-ECT culture. Importantly, by using this integrated method we discovered a new and strong correlation between the measured contractile parameters and the phosphorylation levels of alpha-tropomyosin between the two groups of hiPSC-ECTs. The integration of functional assessments together with molecular characterization by top-down proteomics in the same hiPSC-ECT construct enables a holistic analysis of hiPSC-ECTs to accelerate their applications in disease modeling, cardiotoxicity, and drug discovery. Data are available via ProteomeXchange with identifier PXD022814.

Pulmonary artery and lung parenchymal growth following early versus delayed stent interventions in a swine pulmonary artery stenosis model

OBJECTIVES

Compare lung parenchymal and pulmonary artery (PA) growth and hemodynamics following early and delayed PA stent interventions for treatment of unilateral branch PA stenosis (PAS) in swine.

BACKGROUND

How the pulmonary circulation remodels in response to different durations of hypoperfusion and how much growth and function can be recovered with catheter directed interventions at differing time periods of lung development is not understood.

METHODS

A total of 18 swine were assigned to four groups: Sham (n = 4), untreated left PAS (LPAS) (n = 4), early intervention (EI) (n = 5), and delayed intervention (DI) (n = 5). EI had left pulmonary artery (LPA) stenting at 5 weeks (6 kg) with redilation at 10 weeks. DI had stenting at 10 weeks. All underwent right heart catheterization, computed tomography, magnetic resonance imaging, and histology at 20 weeks (55 kg).

RESULTS

EI decreased the extent of histologic changes in the left lung as DI had marked alveolar septal and bronchovascular abnormalities (p = .05 and p < .05 vs. sham) that were less prevalent in EI. EI also increased left lung volumes and alveolar counts compared to DI. EI and DI equally restored LPA pulsatility, R heart pressures, and distal LPA growth. EI and DI improved, but did not normalize LPA stenosis diameter (LPA/DAo ratio: Sham 1.27 ± 0.11 mm/mm, DI 0.88 ± 0.10 mm/mm, EI 1.01 ± 0.09 mm/mm) and pulmonary blood flow distributions (LPA-flow%: Sham 52 ± 5%, LPAS 7 ± 2%, DI 44 ± 3%, EI 40 ± 2%).

CONCLUSION

In this surgically created PAS model, EI was associated with improved lung parenchymal development compared to DI. Longer durations of L lung hypoperfusion did not detrimentally affect PA growth and R heart hemodynamics. Functional and anatomical discrepancies persist despite successful stent interventions that warrant additional investigation.

Technical Performance Score Predicts Perioperative Outcomes in Complex Congenital Heart Surgery Performed in a Small-to-Medium-Volume Program

As the quality of surgical outcomes depend on many factors, the development of validated tools to assess the different aspects of complex multidisciplinary teams' performance is crucial. The Technical Performance Score (TPS) has only been validated to correlate with outcomes in large-volume surgical programs. Here we assess the utility of TPS in correlation to perioperative outcomes for complex congenital heart surgeries (CHS) performed in a small-to-medium-volume program. 673 patients underwent CHS from 4/2012 to 12/2017 at our institution. Of those, 122 were STAT 4 and STAT 5. TPS was determined for each STAT 4 and STAT 5 operation using discharge echocardiogram: 1 = optimal, 2 = adequate, 3 = inadequate. Patient outcomes were compared including mortality, length of stay, ventilation times, and adverse events. 69 patients (57%) were neonates, 32 (26%) were infants, 17 (14%) were children, 4 (3%) were adults. TPS class 1 was assigned to 85 (70%) operations, TPS class 2 was assigned to 25 (20%) operations, and TPS class 3 was assigned to 12 (10%) operations. TPS was associated with re-intubation, ICU length of stay, postoperative length of stay, and mortality. TPS did not correlate with unplanned 30-day readmissions, need for reoperation, and inotropic score. Technical performance score was associated with perioperative outcomes and is a useful tool to assess the adequacy of repair for high complexity CHS in a small-to-medium-volume surgical program. TPS should be a part of program review in congenital heart programs of all sizes to identify strategies that may reduce postoperative morbidity and potentially improve long-term outcomes.

An Unbiased Proteomics Method to Assess the Maturation of Human Pluripotent Stem Cell-Derived Cardiomyocytes

RATIONALE

Human pluripotent stem cell (hPSC)-derived cardiomyocytes exhibit the properties of fetal cardiomyocytes, which limits their applications. Various methods have been used to promote maturation of hPSC-cardiomyocytes; however, there is a lack of an unbiased and comprehensive method for accurate assessment of the maturity of hPSC-cardiomyocytes.

OBJECTIVE

We aim to develop an unbiased proteomics strategy integrating high-throughput top-down targeted proteomics and bottom-up global proteomics for the accurate and comprehensive assessment of hPSC-cardiomyocyte maturation.

METHODS AND RESULTS

Utilizing hPSC-cardiomyocytes from early- and late-stage 2-dimensional monolayer culture and 3-dimensional engineered cardiac tissue, we demonstrated the high reproducibility and reliability of a top-down proteomics method, which enabled simultaneous quantification of contractile protein isoform expression and associated post-translational modifications. This method allowed for the detection of known maturation-associated contractile protein alterations and, for the first time, identified contractile protein post-translational modifications as promising new markers of hPSC-cardiomyocytes maturation. Most notably, decreased phosphorylation of α-tropomyosin was found to be associated with hPSC-cardiomyocyte maturation. By employing a bottom-up global proteomics strategy, we identified candidate maturation-associated markers important for sarcomere organization, cardiac excitability, and Ca²⁺ homeostasis. In particular, upregulation of myomesin 1 and transmembrane 65 was associated with hPSC-cardiomyocyte maturation and validated in cardiac development, making these promising markers for assessing maturity of hPSC-cardiomyocytes. We have further validated α-actinin isoforms, phospholamban, dystrophin, αB-crystallin, and calsequestrin 2 as novel maturation-associated markers, in the developing mouse cardiac ventricles.

CONCLUSIONS

We established an unbiased proteomics method that can provide accurate and specific assessment of the maturity of hPSC-cardiomyocytes and identified new markers of maturation. Furthermore, this integrated proteomics strategy laid a strong foundation for uncovering the molecular pathways involved in cardiac development and disease using hPSC-cardiomyocytes.

Abstract 405: Functional Maturation of Human iPSC-Derived Cardiomyocytes by Prolonged 3D Culture in Engineered Cardiac Tissue Constructs

The advent of human induced pluripotent stem cell (hiPSC) technology has revolutionized the way we study inherited diseases, as it allows us to study the effect of mutations in a human context. This is particularly true in the heart were species-specific differences has dramatic effects on heart rate, and expression of protein isoforms involved in excitation-contraction coupling. A major drawback of using hiPSCs is, however that cardiomyocytes derived from hiPSC (hiPSC-CM) are relatively immature, limiting their utility as a tool to study adult onset cardiac diseases.

We postulated that culturing hiPSC-CM in a 3D environment in engineered cardiac tissue constructs (ECT) would promote hiPSC-CM maturation.

ECTs were generated from lactate-selected day 30 hiPSCs that were mixed with isogenic hiPSC derived cardiac fibroblasts in a 10:1 ratio. Cells were mixed with fibrinogen and thrombin and seeded into molds under vacuum in FlexCell dishes to form ECT. ECTs were harvested 14-21 or 42-52 days later and subjected to functional testing, transcriptional analysis and top-down mass-spectrometry of sarcomeric proteins to assess maturation.

Functionally, we found that prolonged culture of hiPSCs in ECT resulted in increased calcium transient amplitude (0.856 vs. 0.462; P < 0.01) and an acceleration of calcium kinetics (calcium release time 67.7 ms vs. 109.3 ms; P < 0.01 and calcium decay time 148.3 ms vs. 188.0 ms; P < 0.05). This acceleration of calcium kinetics with time in culture was also true for twitch force kinetics (time to peak twitch force 178.6 ms vs. 208.0 ms; P < 0.05). Furthermore, beta adrenergic stimulation had a much greater effect on twitch kinetics in older ECTs (reduction in contraction time of 26.5% vs. 7.1%; P < 0.01). Transcriptional analysis revealed an increase in expression level of the beta 1 adrenergic receptor in older ECTs, while top-down mass spectrometry showed increased expression and phosphorylation of cardiac troponin I (cTnI) and mono-phosphorylated cTnI, as well as decreased phosphorylation of alpha-tropomyocin, all markers of myocardial maturation.

Taken together, these data supports our hypothesis that prolonged culture of hiPSC-CM in ECT promotes maturation of the calcium handling system and the contractile apparatus.

Novel Adult-Onset Systolic Cardiomyopathy Due to MYH7 E848G Mutation in Patient-Derived Induced Pluripotent Stem Cells

A novel myosin heavy chain 7 mutation (E848G) identified in a familial cardiomyopathy was studied in patient-specific induced pluripotent stem cell-derived cardiomyocytes. The cardiomyopathic human induced pluripotent stem cell-derived cardiomyocytes exhibited reduced contractile function as single cells and engineered heart tissues, and genome-edited isogenic cells confirmed the pathogenic nature of the E848G mutation. Reduced contractility may result from impaired interaction between myosin heavy chain 7 and cardiac myosin binding protein C.

Transcriptome Analysis of Cardiac Hypertrophic Growth in MYBPC3-Null Mice Suggests Early Responders in Hypertrophic Remodeling

Rationale: With a prevalence of 1 in 200 individuals, hypertrophic cardiomyopathy (HCM) is thought to be the most common genetic cardiac disease, with potential outcomes that include severe hypertrophy, heart failure, and sudden cardiac death (SCD). Though much research has furthered our understanding of how HCM-causing mutations in genes such as cardiac myosin-binding protein C (MYBPC3) impair contractile function, it remains unclear how such dysfunction leads to hypertrophy and/or arrhythmias, which comprise the HCM phenotype. Identification of early response mediators could provide rational therapeutic targets to reduce disease severity. Our goal was to differentiate physiologic and pathophysiologic hypertrophic growth responses and identify early genetic mediators in the development of cardiomegaly in the cardiac myosin-binding protein C-null (cMyBP-C^-/-) mouse model of HCM.

Methods and Results: We performed microarray analysis on left ventricles of wild-type (WT) and cMyBPC^-/- mice (n = 7 each) at postnatal day (PND) 1 and PND 9, before and after the appearance of an overt HCM phenotype. Applying the criteria of ≥2-fold change, we identified genes whose change was exclusive to pathophysiologic growth (n = 61), physiologic growth (n = 30), and genes whose expression changed ≥2-fold in both WT and cMyBP-C^-/- hearts (n = 130). Furthermore, we identified genes that were dysregulated in PND1 cMyBP-C^-/- hearts prior to hypertrophy, including genes in mechanosensing pathways and potassium channels linked to arrhythmias. One gene of interest, Xirp2, and its protein product, are regulated during growth but also show early, robust prehypertrophic upregulation in cMyBP-C^-/- hearts. Additionally, the transcription factor Zbtb16 also shows prehypertrophic upregulation at both gene and protein levels.

Conclusion: Our transcriptome analysis generated a comprehensive data set comparing physiologic vs. hypertrophic growth in mice lacking cMyBP-C. It highlights the importance of extracellular matrix pathways in hypertrophic growth and early dysregulation of potassium channels. Prehypertrophic upregulation of Xirp2 in cMyBP-C^-/- hearts supports a growing body of evidence suggesting Xirp2 has the capacity to elicit both hypertrophy and arrhythmias in HCM. Dysregulation of Xirp2, as well as Zbtb16, along with other genes associated with mechanosensing regions of the cardiomyocyte implicate stress-sensing in these regions as a potentially important early response in HCM.

The HCM-linked W792R mutation in cardiac myosin-binding protein C reduces C6 FnIII domain stability

Cardiac myosin-binding protein C (cMyBP-C) is a functional sarcomeric protein that regulates contractility in response to contractile demand, and many mutations in cMyBP-C lead to hypertrophic cardiomyopathy (HCM). To gain insight into the effects of disease-causing cMyBP-C missense mutations on contractile function, we expressed the pathogenic W792R mutation (substitution of a highly conserved tryptophan residue by an arginine residue at position 792) in mouse cardiomyocytes lacking endogenous cMyBP-C and studied the functional effects using three-dimensional engineered cardiac tissue constructs (mECTs). Based on complete conservation of tryptophan at this location in fibronectin type II (FnIII) domains, we hypothesized that the W792R mutation affects folding of the C6 FnIII domain, destabilizing the mutant protein. Adenoviral transduction of wild-type (WT) and W792R cDNA achieved equivalent mRNA transcript abundance, but not equivalent protein levels, with W792R compared with WT controls. mECTs expressing W792R demonstrated abnormal contractile kinetics compared with WT mECTs that were nearly identical to cMyBP-C-deficient mECTs. We studied whether common pathways of protein degradation were responsible for the rapid degradation of W792R cMyBP-C. Inhibition of both ubiquitin-proteasome and lysosomal degradation pathways failed to increase full-length mutant protein abundance to WT equivalence, suggesting rapid cytosolic degradation. Bacterial expression of WT and W792R protein fragments demonstrated decreased mutant stability with altered thermal denaturation and increased susceptibility to trypsin digestion. These data suggest that the W792R mutation destabilizes the C6 FnIII domain of cMyBP-C, resulting in decreased full-length protein expression. This study highlights the vulnerability of FnIII-like domains to mutations that alter domain stability and further indicates that missense mutations in cMyBP-C can cause disease through a mechanism of haploinsufficiency. NEW & NOTEWORTHY This study is one of the first to describe a disease mechanism for a missense mutation in cardiac myosin-binding protein C linked to hypertrophic cardiomyopathy. The mutation decreases stability of the fibronectin type III domain and results in substantially reduced mutant protein expression dissonant to transcript abundance.

Increased Postnatal Cardiac Hyperplasia Precedes Cardiomyocyte Hypertrophy in a Model of Hypertrophic Cardiomyopathy

Rationale: Hypertrophic cardiomyopathy (HCM) occurs in ~0.5% of the population and is a leading cause of sudden cardiac death (SCD) in young adults. Cardiomyocyte hypertrophy has been the accepted mechanism for cardiac enlargement in HCM, but the early signaling responsible for initiating hypertrophy is poorly understood. Mutations in cardiac myosin binding protein C (MYBPC3) are among the most common HCM-causing mutations. Ablation of Mybpc3 in an HCM mouse model (cMyBP-C^−/−) rapidly leads to cardiomegaly by postnatal day (PND) 9, though hearts are indistinguishable from wild-type (WT) at birth. This model provides a unique opportunity to explore early processes involved in the dramatic postnatal transition to hypertrophy.

Methods and Results: We performed microarray analysis, echocardiography, qPCR, immunohistochemistry (IHC), and isolated cardiomyocyte measurements to characterize the perinatal cMyBP-C^−/− phenotype before and after overt hypertrophy. cMyBP-C^−/− hearts showed elevated cell cycling at PND1 that transitioned to hypertrophy by PND9. An expanded time course revealed that increased cardiomyocyte cycling was associated with elevated heart weight to body weight ratios prior to cellular hypertrophy, suggesting that cell cycling resulted in cardiomyocyte proliferation. Animals heterozygous for the cMyBP-C deletion trended in the direction of the homozygous null, yet did not show increased heart size by PND9.

Conclusions: Results indicate that altered regulation of the cell cycling pathway and elevated proliferation precedes hypertrophy in the cMyBP-C^−/− HCM model, and suggests that increased cardiomyocyte number contributes to increased heart size in cMyBP-C^−/− mice. This pre-hypertrophic period may reflect a unique time during which the commitment to HCM is determined and disease severity is influenced.

Molecular Approaches in HFpEF: MicroRNAs and iPSC-Derived Cardiomyocytes

Heart failure with preserved left ventricular ejection fraction (HFpEF) has emerged as one of the largest unmet needs in cardiovascular medicine. HFpEF is increasing in prevalence and causes significant morbidity, mortality, and health care resource utilization. Patients have multiple co-morbidities which contribute to the disease complexity. To date, no effective treatment for HFpEF has been identified. The paucity of cardiac biopsies from this patient population and the absence of well-accepted animal models limit our understanding of the underlying molecular mechanisms of HFpEF. In this review, we discuss combining state-of-the-art technologies of microRNA profiling and human induced pluripotent cell-derived cardiomyocytes (iPSC-CMs) in order to uncover novel molecular pathways that may contribute to the development of HFpEF. Here, we focus the advantages and limitations of microRNA profiling and iPSC-CMs as a disease model system to discover molecular mechanisms in HFpEF.

Ontogeny of Vascular Growth Factors in Perinatal Sheep Myocardium

OBJECTIVE

To examine developmental changes in myocardial gene expression of previously identified regulators of vascular growth.

METHODS

Ovine left (LV) and right ventricle (RV) samples were obtained at four time points: 95 days' and 140 days' gestation (term = 145 days) and 7 days and 8 weeks postnatally. mRNA and protein levels of vascular endothelial growth factor (VEGF), its respective receptors (Flk-1 and Flt-1), basic fibroblast growth factor (bFGF), transforming growth factor-beta1 (TGF-beta1), and endothelial nitric oxide synthase (eNOS) were measured at these different time points.

RESULTS

RV but not LV VEGF mRNA levels decreased postnatally, although VEGF protein expression remained unchanged after birth. Flt-1 mRNA expression was divergent between ventricles, although the protein expression pattern was similar in RV and LV, decreasing with maturation. RV and LV Flk-1 mRNA decreased between 95 days and 140 days, remaining stable thereafter, while protein levels only decreased after birth. bFGF protein levels were highest in the LV at 140 days, and decreased after birth but remained unchanged in the RV throughout the period examined. TGF-beta1 and eNOS levels were highest early in gestation, decreasing with maturation in both ventricles.

CONCLUSION

Developmentally regulated ventricle-specific expression of VEGF, Flt-1, Flk-1, TGF-beta1, bFGF, and eNOS was demonstrated in the ovine myocardium. These findings suggest these proteins may participate in coronary vascular remodeling during the perinatal period and underscore the importance of studying the relationships among transcription factors, target genes, and anatomic/physiologic changes in the whole animal.

Diastolic Hypotension, Troponin Elevation, and Electrocardiographic Changes Associated With the Management of Moderate to Severe Asthma in Children

OBJECTIVE

The objective was to determine the occurrence of, and the factors associated with, diastolic hypotension and troponin elevation or electrocardiogram (ECG) ST-segment changes in a convenience sample of children with moderate to severe asthma receiving continuous albuterol nebulization.

METHODS

This was a prospective, descriptive study in a pediatric emergency department and an intensive care unit of a tertiary academic center. Fifty children with moderate to severe asthma (clinical asthma score > 8) who received 10 to 15 mg/hour continuous albuterol for >2 hours between June 5, 2007, and February 4, 2008, were approached. Hourly diastolic blood pressures were recorded. Cardiac troponin I (cTnI) and ECG tracings were obtained following the first 2 hours of albuterol and then subsequently every 12 hours while receiving continuous albuterol. Main outcome measures were: 1) incidence of diastolic hypotension, 2) incidence of troponin elevation, and 3) incidence of ECG ST-depression.

RESULTS

Fifty patients were enrolled. Thirty-three (66%) patients developed diastolic hypotension during the first 6 hours of continuous albuterol. Diastolic blood pressure declined from baseline at 1-6 hours (p < 0.01 vs. baseline). Twelve patients (24%) had elevated cTnI, 15 patients (30%) had ST-segment change, four patients (8%) had both, and 23 patients (46%, 95% confidence interval [CI] = 32 to 60) had either a cTnI elevation or an ECG ST-segment change. Troponin elevation and diastolic hypotension were not associated (RR = 1.2, 95% CI = 0.6 to 2.3).

CONCLUSIONS

In a subset of children with moderate to severe asthma, diastolic hypotension, troponin elevation, and ECG ST-segment change occur during administration of continuous albuterol. Future studies are necessary to determine the clinical significance of these findings.

Cardiac MyBP-C regulates the rate and force of contraction in mammalian myocardium

Cardiac myosin-binding protein-C (cMyBP-C) is a thick filament-associated protein that seems to contribute to the regulation of cardiac contraction through interactions with either myosin or actin or both. Several studies over the past several years have suggested that the interactions of cardiac myosin-binding protein-C with its binding partners vary with its phosphorylation state, binding predominantly to myosin when dephosphorylated and to actin when it is phosphorylated by protein kinase A or other kinases. Here, we summarize evidence suggesting that phosphorylation of cardiac myosin binding protein-C is a key regulator of the kinetics and amplitude of cardiac contraction during β-adrenergic stimulation and increased stimulus frequency. We propose a model for these effects via a phosphorylation-dependent regulation of the kinetics and extent of cooperative recruitment of cross bridges to the thin filament: phosphorylation of cardiac myosin binding protein-C accelerates cross bridge binding to actin, thereby accelerating recruitment and increasing the amplitude of the cardiac twitch. In contrast, enhanced lusitropy as a result of phosphorylation seems to be caused by a direct effect of phosphorylation to accelerate cross-bridge detachment rate. Depression or elimination of one or both of these processes in a disease, such as end-stage heart failure, seems to contribute to the systolic and diastolic dysfunction that characterizes the disease.

E258K HCM-causing mutation in cardiac MyBP-C reduces contractile force and accelerates twitch kinetics by disrupting the cMyBP-C and myosin S2 interaction

Mutations in cardiac myosin binding protein C (cMyBP-C) are prevalent causes of hypertrophic cardiomyopathy (HCM). Although HCM-causing truncation mutations in cMyBP-C are well studied, the growing number of disease-related cMyBP-C missense mutations remain poorly understood. Our objective was to define the primary contractile effect and molecular disease mechanisms of the prevalent cMyBP-C E258K HCM-causing mutation in nonremodeled murine engineered cardiac tissue (mECT). Wild-type and human E258K cMyBP-C were expressed in mECT lacking endogenous mouse cMyBP-C through adenoviral-mediated gene transfer. Expression of E258K cMyBP-C did not affect cardiac cell survival and was appropriately incorporated into the cardiac sarcomere. Functionally, expression of E258K cMyBP-C caused accelerated contractile kinetics and severely compromised twitch force amplitude in mECT. Yeast two-hybrid analysis revealed that E258K cMyBP-C abolished interaction between the N terminal of cMyBP-C and myosin heavy chain sub-fragment 2 (S2). Furthermore, this mutation increased the affinity between the N terminal of cMyBP-C and actin. Assessment of phosphorylation of three serine residues in cMyBP-C showed that aberrant phosphorylation of cMyBP-C is unlikely to be responsible for altering these interactions. We show that the E258K mutation in cMyBP-C abolishes interaction between N-terminal cMyBP-C and myosin S2 by directly disrupting the cMyBP-C-S2 interface, independent of cMyBP-C phosphorylation. Similar to cMyBP-C ablation or phosphorylation, abolition of this inhibitory interaction accelerates contractile kinetics. Additionally, the E258K mutation impaired force production of mECT, which suggests that in addition to the loss of physiological function, this mutation disrupts contractility possibly by tethering the thick and thin filament or acting as an internal load.

Ablation of cardiac myosin-binding protein-C accelerates contractile kinetics in engineered cardiac tissue

Hypertrophic cardiomyopathy (HCM) caused by mutations in cardiac myosin–binding protein-C (cMyBP-C) is a heterogenous disease in which the phenotypic presentation is influenced by genetic, environmental, and developmental factors. Though mouse models have been used extensively to study the contractile effects of cMyBP-C ablation, early postnatal hypertrophic and dilatory remodeling may overshadow primary contractile defects. The use of a murine engineered cardiac tissue (mECT) model of cMyBP-C ablation in the present study permits delineation of the primary contractile kinetic abnormalities in an intact tissue model under mechanical loading conditions in the absence of confounding remodeling events. We generated mechanically integrated mECT using isolated postnatal day 1 mouse cardiac cells from both wild-type (WT) and cMyBP-C–null hearts. After culturing for 1 wk to establish coordinated spontaneous contraction, we measured twitch force and Ca²⁺ transients at 37°C during pacing at 6 and 9 Hz, with and without dobutamine. Compared with WT, the cMyBP-C–null mECT demonstrated faster late contraction kinetics and significantly faster early relaxation kinetics with no difference in Ca²⁺ transient kinetics. Strikingly, the ability of cMyBP-C–null mECT to increase contractile kinetics in response to adrenergic stimulation and increased pacing frequency were severely impaired. We conclude that cMyBP-C ablation results in constitutively accelerated contractile kinetics with preserved peak force with minimal contractile kinetic reserve. These functional abnormalities precede the development of the hypertrophic phenotype and do not result from alterations in Ca²⁺ transient kinetics, suggesting that alterations in contractile velocity may serve as the primary functional trigger for the development of hypertrophy in this model of HCM. Our findings strongly support a mechanism in which cMyBP-C functions as a physiological brake on contraction by positioning myosin heads away from the thin filament, a constraint which is removed upon adrenergic stimulation or cMyBP-C ablation.

3D engineered cardiac tissue models of human heart disease: learning more from our mice

Mouse engineered cardiac tissue constructs (mECTs) are a new tool available to study human forms of genetic heart disease within the laboratory. The cultured strips of cardiac cells generate physiologic calcium transients and twitch force, and respond to electrical pacing and adrenergic stimulation. The mECT can be made using cells from existing mouse models of cardiac disease, providing a robust readout of contractile performance and allowing a rapid assessment of genotype-phenotype correlations and responses to therapies. mECT represents an efficient and economical extension to the existing tools for studying cardiac physiology. Human ECTs generated from iPSCMs represent the next logical step for this technology and offer significant promise of an integrated, fully human, cardiac tissue model.

Neonatal mouse-derived engineered cardiac tissue: a novel model system for studying genetic heart disease

RATIONALE

Cardiomyocytes cultured in a mechanically active 3-dimensional configuration can be used for studies that correlate contractile performance to cellular physiology. Current engineered cardiac tissue (ECT) models use cells derived from either rat or chick hearts. Development of a murine ECT would provide access to many existing models of cardiac disease and open the possibility of performing targeted genetic manipulation with the ability to directly assess contractile and molecular variables.

OBJECTIVE

To generate, characterize, and validate mouse ECT with a physiologically relevant model of hypertrophic cardiomyopathy.

METHODS AND RESULTS

We generated mechanically integrated ECT using isolated neonatal mouse cardiac cells derived from both wild-type and myosin-binding protein C (cMyBP-C)-null mouse hearts. The murine ECTs produced consistent contractile forces that followed the Frank-Starling law and accepted physiological pacing. cMyBP-C-null ECTs showed characteristic acceleration of contraction kinetics. Adenovirus-mediated expression of human cMyBP-C in murine cMyBP-C-null ECT restored contractile properties to levels indistinguishable from those of wild-type ECT. Importantly, the cardiomyocytes used to construct the cMyBP-C(-/-) ECT had yet to undergo the significant hypertrophic remodeling that occurs in vivo. Thus, this murine ECT model reveals a contractile phenotype that is specific to the genetic mutation rather than to secondary remodeling events.

CONCLUSIONS

Data presented here show mouse ECT to be an efficient and cost-effective platform to study the primary effects of genetic manipulation on cardiac contractile function. This model provides a previously unavailable tool to study specific sarcomeric protein mutations in an intact mammalian muscle system.

Ultrastructural, immunofluorescence, and RNA evidence support the hypothesis of a "new" virus associated with Kawasaki disease

BACKGROUND

Intracytoplasmic inclusion bodies (ICI) have been identified in ciliated bronchial epithelium of Kawasaki disease (KD) patients using a synthetic antibody derived from acute KD arterial IgA plasma cells; ICI may derive from the KD etiologic agent.

METHODS

Acute KD bronchial epithelium was subjected to immunofluorescence for ICI and cytokeratin, high-throughput sequencing, and transmission electron microscopy (TEM). Interferon pathway gene expression profiling was performed on KD lung.

RESULTS

An intermediate filament cytokeratin "cage" was not observed around KD ICI, making it unlikely that ICI are overproduced or misfolded human protein aggregates. Many interferon-stimulated genes were detected in the bronchial epithelium, and significant modulation of the interferon response pathway was observed in the lung tissue of KD patients. No known virus was identified by sequencing. Aggregates of virus-like particles (VLP) were detected by TEM in all 3 acute KD patients from whom nonembedded formalin-fixed lung tissue was available.

CONCLUSIONS

KD ICI are most likely virus induced; bronchial cells with ICI contain VLP that share morphologic features among several different RNA viral families. Expedited autopsies and tissue fixation from acute KD fatalities are urgently needed to more clearly ascertain the VLP. These findings are compatible with the hypothesis that the infectious etiologic agent of KD may be a "new" RNA virus.

Neonatal congestive heart failure due to a subclavian artery to subclavian vein fistula diagnosed by noninvasive procedures

Congestive heart failure in the neonate is usually due to intracardiac anomalies or cardiac dysfunction. Extracardiac causes are rare.

PATIENT

We report a newborn infant who presented with respiratory distress and cardiomegaly.

RESULT

Echocardiography identified a dilated right subclavian artery and vein and superior vena cava. Magnetic resonance imaging confirmed a subclavian artery to subclavian vein fistula that was treated with surgical ligation. The infant recovered fully. This case underscores the need for clinical suspicion of fistulous connection in unusual locations in the face of unexplained heart failure in the neonate.

CONCLUSION

Echocardiographic and magnetic resonance imaging are effective noninvasive modalities to confirm the diagnosis prior to surgical intervention.

Myocardial vascular and metabolic adaptations in chronically anemic fetal sheep

Little is known about the vascular and metabolic adaptations that take place in the fetal heart to maintain cardiac function in response to increased load. Chronic fetal anemia has previously been shown to result in increased ventricular mass, increased myocardial vascularization, and increased myocardial expression of hypoxia-inducible factor-1 (HIF-1) and vascular endothelial growth factor (VEGF). We therefore sought to determine whether chronic fetal anemia induces expression of HIF-1-regulated angiogenic factors and glycolytic enzymes in the fetal myocardium. Anemia was produced in chronically instrumented fetal sheep by daily isovolemic hemorrhage (80–100 ml) for either 3 ( n = 4) or 7 days ( n = 11) beginning at 134 days of gestation (term 145 days). Catheterized, nonbled twins served as controls. Isovolemic hemorrhage over 7 days resulted in decreased fetal hematocrit (37 ± 1 to 20 ± 1%) and arterial oxygen content (6.5 ± 0.4 to 2.8 ± 0.2 ml O2/dl). Myocardial blood flow and vascularization were significantly increased after 7 days of anemia. Myocardial HIF-1 protein expression and VEGF (left ventricular), VEGF receptor-1 (right ventricular), and VEGF receptor-2 (right ventricular, left ventricular) mRNA levels were elevated ( P < 0.05) in 7-day anemic compared with control animals. Myocardial expressions of the glycolytic enzymes aldolase, lactate dehydrogenase A, phosphofructokinase (liver), and phosphoglycerol kinase were also significantly elevated after 7 days of anemia. Despite the absence of a significant increase in myocardial HIF-1α protein in 3-day anemic fetuses, expressions of VEGF, VEGF receptor-1, and the glycolytic enzymes were greater in 3-day compared with 7-day anemic animals. These data suggest that HIF-1 likely participates in the fetal myocardial response to anemia by coordinating an increase in gene expressions that promote capillary growth and anaerobic metabolism. However, factors other than HIF-1 also appear important in the regulation of these genes. We speculate that the return of mRNA levels of angiogenic and glycolytic enzymes toward control levels in the 7-day anemic fetus is explained by a significantly increased resting myocardial blood flow, resulting from coronary vascular growth and increased coronary conductance, and a return to a state of adequate oxygen and nutrient delivery, obviating the need for enhanced transcription of genes encoding angiogenic and glycolytic enzymes.

Regulation of myocardial glucose transporters GLUT1 and GLUT4 in chronically anemic fetal lambs

Little is known about the chronic adaptations that take place in the fetal heart to allow for increased substrate delivery in response to chronic stress. Because glucose is an important fuel for the fetal cardiomyocytes, we hypothesized that myocardial glucose transporters 1 and 4 (GLUT1 and GLUT4, respectively) are up-regulated in the fetal sheep heart that is chronically stressed by anemia. Fetal sheep at 128 d gestation underwent daily isovolumic hemorrhage and determination of myocardial blood flow, oxygen consumption, and substrate utilization. At the end of 3 or 7 d of anemia, myocardial levels of GLUT1 and GLUT4 mRNA and protein were measured and subcellular localization was determined. Despite stable heart rate and blood pressure, anemia caused a nearly 4-fold increase in right and left ventricular (RV and LV) free wall blood flow. No significant change in myocardial glucose uptake was found and serum insulin levels remained stable. Both 3-d RV and LV and 7-d RV mRNA and protein levels of GLUT1 and GLUT4 were unchanged; 7-d LV GLUT1 and GLUT4 mRNA levels were also stable. However, LV GLUT1 protein levels declined significantly, whereas LV GLUT4 protein levels were increased. In the steady state, GLUT4 protein localized to the sarcolemma membrane. These findings suggest that the glucose transporters are post-transcriptionally regulated in myocardium of chronically anemic fetal sheep with changes that mimic normal postnatal development. Unlike the postnatal heart, localization of GLUT4 to the cell membrane suggests the importance of GLUT4 in basal glucose uptake in the stressed fetal heart.

Localization and function of the brain excitatory amino acid transporter type 1 in cardiac mitochondria

Glutamate is the only amino acid extracted by healthy myocardium in net amounts, with uptake further increased during hypoxic or ischemic conditions. Glutamate supplementation provides cardioprotection from hypoxic and reperfusion injury through several metabolic pathways that depend upon adequate transport of glutamate into the mitochondria. Glutamate transport across the inner mitochondrial membrane is a key component of the malate/aspartate shuttle. Glutamate transport in the brain has been well characterized since the discovery of the excitatory amino acid transporter (EAAT) family. We hypothesize that a protein similar to EAAT1 found in brain may function as a glutamate transporter in cardiac mitochondria. Rat heart total RNA was screened by reverse transcriptase-polymerase chain reaction with an array of primer pairs derived from the rat brain EAAT1 cDNA sequence, yielding a 3786-bp cDNA comprising a 1638-bp open reading frame identical to rat brain EAAT1 with flanking 5'- and 3'-untranslated regions. Northern blot analysis confirmed a 4-kb mRNA product in rat heart and brain, with greater abundance in brain. A protein of the predicted approximate 60-kD size was recognized in myocardial lysates by an anti-EAAT1 polyclonal antibody produced against an amino-terminal peptide from human EAAT1. The protein enriched in rat heart mitochondria by immunoblot, co-localized with the mitochondrial protein cytochrome c by immunohistochemistry, and further localized to the inner mitochondrial membrane upon digitonin fractionation of the mitochondria. In myocytes overexpressing EAAT1, activity of the malate/aspartate shuttle increased by 33% compared to non-transfected cells (P = 0.004). These data indicate that EAAT1 is expressed in myocardial mitochondria, and functions in the malate/aspartate shuttle, suggesting a role for EAAT1 in myocardial glutamate metabolism.

Correlation between myocardial malate/aspartate shuttle activity and EAAT1 protein expression in hyper- and hypothyroidism

In the heart, elevated thyroid hormone leads to upregulation of metabolic pathways associated with energy production and development of hypertrophy. The malate/aspartate shuttle, which transfers cytosolic-reducing equivalents into the cardiac mitochondria, is increased 33% in hyperthyroid rats. Within the shuttle, the aspartate-glutamate carrier is rate limiting. The excitatory amino acid transporter type 1 (EAAT1) functions as a glutamate carrier in the malate/aspartate shuttle. In this study, we hypothesize that EAAT1 is regulated by thyroid hormone. Adult rats were injected with triiodothyronine (T3) or saline over a period of 8-9 days or provided with propylthiouracil (PTU) in their drinking water for 2 mo. Steady-state mRNA levels of EAAT1 and aralar1 and citrin (both cardiac mitochondrial aspartate-glutamate transporters) were determined by Northern blot analysis and normalized to 18S rRNA. A spectrophotometric assay of maximal malate/aspartate shuttle activity was performed on isolated cardiac mitochondria from PTU-treated and control animals. Protein lysates from mitochondria were separated by SDS-PAGE and probed with a human anti-EAAT1 IgG. Compared with control, EAAT1 mRNA levels (arbitrary units) were increased nearly threefold in T3-treated (3.1 +/- 0.5 vs. 1.1 +/- 0.2; P < 0.05) and decreased in PTU-treated (2.0 +/- 0. 3 vs. 5.2 +/- 1; P < 0.05) rats. Aralar1 mRNA levels were unchanged in T3-treated and somewhat decreased in PTU-treated (7.1 +/- 1.0 vs. 9.3 +/- 0.1, P < 0.05) rats. Citrin mRNA levels were decreased in T3-treated and unchanged in PTU-treated rats. EAAT1 protein levels (arbitrary units) in T3-treated cardiac mitochondria were increased compared with controls (8.9 +/- 0.4 vs. 5.9 +/- 0.6; P < 0.005) and unchanged in PTU-treated mitochondria. No difference in malate/aspartate shuttle capacity was found between PTU-treated and control cardiac mitochondria. Hyperthyroidism in rats is related to an increase in cardiac expression of EAAT1 mRNA and protein. The 49% increase in EAAT1 mitochondrial protein level shows that malate/aspartate shuttle activity increased in hyperthyroid rat cardiac mitochondria. Although hypothyroidism resulted in a decrease in EAAT1 mRNA, neither the EAAT1 protein level nor shuttle activity was affected. EAAT1 regulation by thyroid hormone may facilitate increased metabolic demands of the cardiomyocyte during hyperthyroidism and impact cardiac function in hyperthyroidism.

Metabolic adaptation of the fetal and postnatal ovine heart: regulatory role of hypoxia-inducible factors and nuclear respiratory factor-1

Numerous metabolic adaptations occur in the heart after birth. Important transcription factors that regulate expression of the glycolytic and mitochondrial oxidative genes are hypoxia-inducible factors (HIF-1alpha and -2alpha) and nuclear respiratory factor-1 (NRF-1). The goal of this study was to examine expression of HIF-1alpha, HIF-2alpha, and NRF-1 and the genes they regulate in pre- and postnatal myocardium. Ovine right and left ventricular myocardium was obtained at four time points: 95 and 140 d gestation (term = 145 d) and 7 d and 8 wk postnatally. Steady-state mRNA and protein levels of HIF-1alpha and NRF-1 and protein levels of HIF-2alpha were measured along with mRNA of HIF-1alpha-regulated genes (aldolase A, alpha- and beta-enolase, lactate dehydrogenase A, liver and muscle phosphofructokinase) and NRF-1-regulated genes (cytochrome c, Va subunit of cytochrome oxidase, and carnitine palmitoyltransferase I ). HIF-1alpha protein was present in fetal myocardium but dropped below detectable levels at 7 d postnatally. HIF-2alpha protein levels were similar at the four time points. Steady-state mRNA levels of alpha-enolase, lactate dehydrogenase A, and liver phosphofructokinase declined significantly postnatally. Aldolase A, beta-enolase, and muscle phosphofructokinase mRNA levels increased postnatally. Steady-state mRNA and protein levels of NRF-1 decreased postnatally in contrast to the postnatal increases in cytochrome c, subunit Va of cytochrome oxidase, and carnitine palmitoyltransferase I mRNA levels. The in vivo postnatal regulation of enzymes encoding glycolytic and mitochondrial enzymes is complex. As transactivation response elements for the genes encoding metabolic enzymes continue to be characterized, studies using the fetal-to-postnatal metabolic transition of the heart will continue to help define the in vivo role of these transcription factors.