Cardiomyocyte-Specific Long Noncoding RNA Regulates Alternative Splicing of the Triadin Gene in the Heart.

BACKGROUND: Abnormalities in Ca2+ homeostasis are associated with cardiac arrhythmias and heart failure. Triadin plays an important role in Ca2+ homeostasis in cardiomyocytes. Alternative splicing of a single triadin gene produces multiple triadin isoforms. The cardiac-predominant isoform, mouse MT-1 or human Trisk32, is encoded by triadin exons 1 to 8. In humans, mutations in the triadin gene that lead to a reduction in Trisk32 levels in the heart can cause cardiac dysfunction and arrhythmias. Decreased levels of Trisk32 in the heart are also common in patients with heart failure. However, mechanisms that maintain triadin isoform composition in the heart remain elusive. METHODS: We analyzed triadin expression in heart explants from patients with heart failure and cardiac arrhythmias and in hearts from mice carrying a knockout allele for Trdn-as, a cardiomyocyte-specific long noncoding RNA encoded by the antisense strand of the triadin gene, between exons 9 and 11. Catecholamine challenge with isoproterenol was performed on Trdn-as knockout mice to assess the role of Trdn-as in cardiac arrhythmogenesis, as assessed by ECG. Ca2+ transients in adult mouse cardiomyocytes were measured with the IonOptix platform or the GCaMP system. Biochemistry assays, single-molecule fluorescence in situ hybridization, subcellular localization imaging, RNA sequencing, and molecular rescue assays were used to investigate the mechanisms by which Trdn-as regulates cardiac function and triadin levels in the heart. RESULTS: We report that Trdn-as maintains cardiac function, at least in part, by regulating alternative splicing of the triadin gene. Knockout of Trdn-as in mice downregulates cardiac triadin, impairs Ca2+ handling, and causes premature death. Trdn-as knockout mice are susceptible to cardiac arrhythmias in response to catecholamine challenge. Normalization of cardiac triadin levels in Trdn-as knockout cardiomyocytes is sufficient to restore Ca2+ handling. Last, Trdn-as colocalizes and interacts with serine/arginine splicing factors in cardiomyocyte nuclei and is essential for efficient recruitment of splicing factors to triadin precursor mRNA. CONCLUSIONS: These findings reveal regulation of alternative splicing as a novel mechanism by which a long noncoding RNA controls cardiac function. This study indicates potential therapeutics for heart disease by targeting the long noncoding RNA or pathways regulating alternative splicing.

LAMP-2B regulates human cardiomyocyte function by mediating autophagosome-lysosome fusion.

Mutations in lysosomal-associated membrane protein 2 (LAMP-2) gene are associated with Danon disease, which often leads to cardiomyopathy/heart failure through poorly defined mechanisms. Here, we identify the LAMP-2 isoform B (LAMP-2B) as required for autophagosome-lysosome fusion in human cardiomyocytes (CMs). Remarkably, LAMP-2B functions independently of syntaxin 17 (STX17), a protein that is essential for autophagosome-lysosome fusion in non-CMs. Instead, LAMP-2B interacts with autophagy related 14 (ATG14) and vesicle-associated membrane protein 8 (VAMP8) through its C-terminal coiled coil domain (CCD) to promote autophagic fusion. CMs derived from induced pluripotent stem cells (hiPSC-CMs) from Danon patients exhibit decreased colocalization between ATG14 and VAMP8, profound defects in autophagic fusion, as well as mitochondrial and contractile abnormalities. This phenotype was recapitulated by LAMP-2B knockout in non-Danon hiPSC-CMs. Finally, gene correction of LAMP-2 mutation rescues the Danon phenotype. These findings reveal a STX17-independent autophagic fusion mechanism in human CMs, providing an explanation for cardiomyopathy in Danon patients and a foundation for targeting defective LAMP-2B-mediated autophagy to treat this patient population.

Multiprotein Complex With TRPC (Transient Receptor Potential-Canonical) Channel, PDE1C (Phosphodiesterase 1C), and A2R (Adenosine A2 Receptor) Plays a Critical Role in Regulating Cardiomyocyte cAMP and Survival.

BACKGROUND: cAMP plays a critical role in regulating cardiomyocyte survival. Various cAMP signaling pathways behave distinctly or in opposition. We have previously reported that activation of cAMP hydrolysis by cyclic nucleotide phosphodiesterase 1C (PDE1C) promotes cardiomyocytes death/apoptosis, yet the underlying molecular mechanism remains unknown. In this study, we aimed to identify the specific cAMP signaling pathway modulated by PDE1C and determine the mechanism by which Ca2+/calmodulin-stimulated PDE1C is activated. METHODS: To study cardiomyocyte death/apoptosis, we used both isolated mouse adult cardiomyocytes in vitro and doxorubicin-induced cardiotoxicity in vivo. We used a variety of pharmacological activators and inhibitors as well as genetically engineered molecular tools to manipulate the expression and activity of proteins of interest. RESULTS: We found that the protective effect of PDE1C inhibition/deficiency on Ang II or doxorubicin-induced cardiomyocyte death/apoptosis is dependent on cAMP-generating adenosine A2 receptors (A2Rs), suggesting that PDE1C's cAMP-hydrolyzing activity selectively modulates A2R-cAMP signaling in cardiomyocytes. In addition, we found that the effects of PDE1C activation on Ang II-mediated cAMP reduction and cardiomyocyte death are dependent on transient receptor potential-canonical (TRPC) channels, in particular TRPC3. We also observed synergistic protective effects on cardiomyocyte survival from the combination of A2R stimulation together with PDE1 or TRPC inhibition. Coimmunostaining and coimmunoprecipitation studies showed that PDE1C is localized in proximity with A2R and TRPC3 in the plasma membrane and perhaps T tubules. It is important to note that we found that doxorubicin-induced cardiac toxicity and dysfunction in mice are attenuated by the PDE1 inhibitor IC86340 or in PDE1C knockout mice, and this protective effect is significantly diminished by A2R antagonism. CONCLUSIONS: We have characterized a novel multiprotein complex comprised of A2R, PDE1C, and TRPC3, in which PDE1C is activated by TRPC3-derived Ca2+, thereby antagonizing A2R-cAMP signaling and promoting cardiomyocyte death/apoptosis. Targeting these molecules individually or in combination may represent a compelling therapeutic strategy for potentiating cardiomyocyte survival.

PDE1C deficiency antagonizes pathological cardiac remodeling and dysfunction.

Cyclic nucleotide phosphodiesterase 1C (PDE1C) represents a major phosphodiesterase activity in human myocardium, but its function in the heart remains unknown. Using genetic and pharmacological approaches, we studied the expression, regulation, function, and underlying mechanisms of PDE1C in the pathogenesis of cardiac remodeling and dysfunction. PDE1C expression is up-regulated in mouse and human failing hearts and is highly expressed in cardiac myocytes but not in fibroblasts. In adult mouse cardiac myocytes, PDE1C deficiency or inhibition attenuated myocyte death and apoptosis, which was largely dependent on cyclic AMP/PKA and PI3K/AKT signaling. PDE1C deficiency also attenuated cardiac myocyte hypertrophy in a PKA-dependent manner. Conditioned medium taken from PDE1C-deficient cardiac myocytes attenuated TGF-ß-stimulated cardiac fibroblast activation through a mechanism involving the crosstalk between cardiac myocytes and fibroblasts. In vivo, cardiac remodeling and dysfunction induced by transverse aortic constriction, including myocardial hypertrophy, apoptosis, cardiac fibrosis, and loss of contractile function, were significantly attenuated in PDE1C-knockout mice relative to wild-type mice. These results indicate that PDE1C activation plays a causative role in pathological cardiac remodeling and dysfunction. Given the continued development of highly specific PDE1 inhibitors and the high expression level of PDE1C in the human heart, our findings could have considerable therapeutic significance.

Cyclic nucleotide phosphodiesterase 3A1 protects the heart against ischemia-reperfusion injury.

Phosphodiesterase 3A (PDE3A) is a major regulator of cAMP in cardiomyocytes. PDE3 inhibitors are used for acute treatment of congestive heart failure, but are associated with increased incidence of arrhythmias and sudden death with long-term use. We previously reported that chronic PDE3A downregulation or inhibition induced myocyte apoptosis in vitro. However, the cardiac protective effect of PDE3A has not been demonstrated in vivo in disease models. In this study, we examined the role of PDE3A in regulating myocardial function and survival in vivo using genetically engineered transgenic mice with myocardial overexpression of the PDE3A1 isozyme (TG). TG mice have reduced cardiac function characterized by reduced heart rate and ejection fraction (52.5±7.8% vs. 83.9±4.7%) as well as compensatory expansion of left ventricular diameter (4.19±0.19mm vs. 3.10±0.18mm). However, there was no maladaptive increase of fibrosis and apoptosis in TG hearts compared to wild type (WT) hearts, and the survival rates also remained the same. The diminution of cardiac contractile function is very likely attributed to a decrease in beta-adrenergic receptor (ß-AR) response in TG mice. Importantly, the myocardial infarct size (4.0±1.8% vs. 24.6±3.8%) and apoptotic cell number (1.3±1.0% vs. 5.6±1.5%) induced by ischemia/reperfusion (I/R) injury were significantly attenuated in TG mice. This was associated with decreased expression of inducible cAMP early repressor (ICER) and increased expression of anti-apoptotic protein BCL-2. To further verify the anti-apoptotic effects of PDE3A1, we performed in vitro apoptosis study in isolated adult TG and WT cardiomyocytes. We found that the apoptotic rates stimulated by hypoxia/reoxygenation or H2O2 were indeed significantly reduced in TG myocytes, and the differences between TG and WT myocytes were completely reversed in the presence of the PDE3 inhibitor milrinone. These together indicate that PDE3A1 negatively regulates ß-AR signaling and protects against I/R injury by inhibiting cardiomyocyte apoptosis.

Interferon hyperactivity impairs cardiogenesis in Down syndrome via downregulation of canonical Wnt signaling.

Congenital heart defects (CHDs) are frequent in children with Down syndrome (DS), caused by trisomy of chromosome 21. However, the underlying mechanisms are poorly understood. Here, using a human-induced pluripotent stem cell (iPSC)-based model and the Dp(16)1Yey/+ (Dp16) mouse model of DS, we identified downregulation of canonical Wnt signaling downstream of increased dosage of interferon (IFN) receptors (IFNRs) genes on chromosome 21 as a causative factor of cardiogenic dysregulation in DS. We differentiated human iPSCs derived from individuals with DS and CHDs, and healthy euploid controls into cardiac cells. We observed that T21 upregulates IFN signaling, downregulates the canonical WNT pathway, and impairs cardiac differentiation. Furthermore, genetic and pharmacological normalization of IFN signaling restored canonical WNT signaling and rescued defects in cardiogenesis in DS in vitro and in vivo. Our findings provide insights into mechanisms underlying abnormal cardiogenesis in DS, ultimately aiding the development of therapeutic strategies.

Transient hypercapnia reveals an underlying cerebrovascular pathology in a murine model for HIV-1 associated neuroinflammation: role of NO-cGMP signaling and normalization by inhibition of cyclic nucleotide phosphodiesterase-5.

BACKGROUND: Cerebral blood flow (CBF) is known to be dysregulated in persons with human immunodeficiency virus 1 (HIV-1), for uncertain reasons. This is an important issue because impaired vasoreactivity has been associated with increased risk of ischemic stroke, elevated overall cardiovascular risk and cognitive impairment. METHODS: To test whether dysregulation of CBF might be due to virally-induced neuroinflammation, we used a well-defined animal model (GFAP-driven, doxycycline-inducible HIV-1 Tat transgenic (Tat-tg) mice). We then exposed the mice to a brief hypercapnic stimulus, and assessed cerebrovascular reactivity by measuring 1) changes in cerebral blood flow, using laser Doppler flowmetry and 2) changes in vascular dilation, using in vivo two-photon imaging. RESULTS: Exposure to brief hypercapnia revealed an underlying cerebrovascular pathology in Tat-tg mice. In control animals, brief hypercapnia induced a brisk increase in cortical flow (20.8% above baseline) and vascular dilation, as measured by laser Doppler flowmetry and in vivo two-photon microscopy. These responses were significantly attenuated in Tat-tg mice (11.6% above baseline), but cortical microvascular morphology and capillary density were unaltered, suggesting that the functional pathology was not secondary to vascular remodeling. To examine the mechanistic basis for the diminished cerebrovascular response to brief hypercapnia, Tat-tg mice were treated with 1) gisadenafil, a phosphodiesterase 5 (PDE5) inhibitor and 2) tetrahydrobiopterin (BH4). Gisadenafil largely restored the normal increase in cortical flow following hypercapnia in Tat-tg mice (17.5% above baseline), whereas BH4 had little effect. Gisadenafil also restored the dilation of small (<25 µm) arterioles following hypercapnia (19.1% versus 20.6% diameter increase in control and Tat-tg plus gisadenafil, respectively), although it failed to restore full dilation of larger (>25 µm) vessels. CONCLUSIONS: Taken together, these data show that HIV-associated neuroinflammation can cause cerebrovascular pathology through effects on cyclic guanosine monophosphate (cGMP) metabolism and possibly on PDE5 metabolism.

Vinpocetine suppresses pathological vascular remodeling by inhibiting vascular smooth muscle cell proliferation and migration.

Abnormal vascular smooth muscle cell (SMC) activation is associated with various vascular disorders such as atherosclerosis, in-stent restenosis, vein graft disease, and transplantation-associated vasculopathy. Vinpocetine, a derivative of the alkaloid vincamine, has long been used as a cerebral blood flow enhancer for treating cognitive impairment. However, its role in pathological vascular remodeling remains unexplored. Herein, we show that systemic administration of vinpocetine significantly reduced neointimal formation in carotid arteries after ligation injury. Vinpocetine also markedly decreased spontaneous remodeling of human saphenous vein explants in ex vivo culture. In cultured SMCs, vinpocetine dose-dependently suppressed cell proliferation and caused G1-phase cell cycle arrest, which is associated with a decrease in cyclin D1 and an increase in p27Kip1 levels. In addition, vinpocetine dose-dependently inhibited platelet-derived growth factor (PDGF)-stimulated SMC migration as determined by the two-dimensional migration assays and three-dimensional aortic medial explant invasive assay. Moreover, vinpocetine significantly reduced PDGF-induced type I collagen and fibronectin expression. It is noteworthy that PDGF-stimulated phosphorylation of extracellular signal-regulated kinases 1/2 (ERK1/2), but not protein kinase B, was specifically inhibited by vinpocetine. Vinpocetine powerfully attenuated intracellular reactive oxidative species (ROS) production, which largely mediates the inhibitory effects of vinpocetine on ERK1/2 activation and SMC growth. Taken together, our results reveal a novel function of vinpocetine in attenuating neointimal hyperplasia and pathological vascular remodeling, at least partially through suppressing ROS production and ERK1/2 activation in SMCs. Given the safety profile of vinpocetine, this study provides insight into the therapeutic potential of vinpocetine in proliferative vascular disorders.

Dysfunctional sarcomeric relaxation in the heart.

Since cardiac relaxation is commonly impaired in heart failure caused by many different etiologies, identifying druggable targets is a common goal. While many factors contribute to cardiac relaxation, this review focuses on sarcomeric relaxation and dysfunction. Any alteration in how sarcomeric proteins interact can lead to significant shifts in sarcomeric relaxation that may contribute to diastolic dysfunction. Considering examples of sarcomeric dysfunction that have been reported in 3 different pathologies, hypertrophic cardiomyopathy, restrictive cardiomyopathy, and heart failure with preserved ejection fraction, will provide insights into the role sarcomeric dysfunction plays in impaired cardiac relaxation. This will ultimately improve our understanding of sarcomeric physiology and uncover new therapeutic targets.

Natural variation in Arabidopsis leads to the identification of REME1, a pentatricopeptide repeat-DYW protein controlling the editing of mitochondrial transcripts.

In vascular plants, organelle RNAs are edited by C-to-U base modification. Hundreds of mitochondrial C residues are targeted for editing in flowering plants. In this study, we exploited naturally occurring variation in editing extent to identify Required for Efficiency of Mitochondrial Editing1 (REME1), an Arabidopsis (Arabidopsis thaliana) pentatricopeptide repeat protein-encoding gene belonging to the DYW subclass that promotes editing of at least two C residues on different mitochondrial transcripts. Positional cloning identified REME1 unambiguously as the gene controlling editing of nad2-558. Virus-induced gene silencing of REME1 confirmed its role in editing of nad2-558 and allowed us to identify orfX-552 as a second C whose editing is positively controlled by REME1. An unexpected outcome of REME1 silencing was the finding of a number of mitochondrial C targets whose editing extent exhibits a significant and reproducible increase in silenced tissues. That increase was shown to be partly due to the virus inoculation and partly to REME1-specific silencing. Analysis of an insertional T-DNA mutant within the REME1 coding sequence confirmed the findings of the virus-induced gene silencing experiments: decrease in editing extent of nad2-558 and orfX-552 and increase in editing extent of two sites, matR-1771 and rpl5-92. Transgenic complementation of the low-edited accession (Landsberg erecta) restored the editing of nad2-558 and orfX-552 to high-edited accession (Columbia)-type levels or to even higher levels than Columbia. There was no effect of the transgene on editing extent of matR-1771 and rpl5-92. The strategy and tools used in this report can be applied to identify additional genes that affect editing extent in plant mitochondria.

A simple protocol to produce mature human-induced pluripotent stem cell-derived cardiomyocytes.

When cultured under typical conditions, human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are structurally and functionally immature. We have previously demonstrated that culture of hiPSC-CMs in maturation medium containing fatty acids, in combination with culture on micropatterned surfaces, produces cells that demonstrate a more mature phenotype compared to standard approaches. Here, we show in detail the steps needed to produce mature hiPSC-CMs. Compared with many approaches, our protocol is relatively simple and can be easily adapted to new laboratories. For complete details on the use and execution of this protocol, please refer to Knight et al. (2021).

Maturation of Pluripotent Stem Cell-Derived Cardiomyocytes Enables Modeling of Human Hypertrophic Cardiomyopathy.

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are a powerful platform for biomedical research. However, they are immature, which is a barrier to modeling adult-onset cardiovascular disease. Here, we sought to develop a simple method that could drive cultured hiPSC-CMs toward maturity across a number of phenotypes, with the aim of utilizing mature hiPSC-CMs to model human cardiovascular disease. hiPSC-CMs were cultured in fatty acid-based medium and plated on micropatterned surfaces. These cells display many characteristics of adult human cardiomyocytes, including elongated cell morphology, sarcomeric maturity, and increased myofibril contractile force. In addition, mature hiPSC-CMs develop pathological hypertrophy, with associated myofibril relaxation defects, in response to either a pro-hypertrophic agent or genetic mutations. The more mature hiPSC-CMs produced by these methods could serve as a useful in vitro platform for characterizing cardiovascular disease.

Ex vivo Methods for Measuring Cardiac Muscle Mechanical Properties.

Cardiovascular disease continues to be the leading cause of morbidity and mortality in the United States and thousands of manuscripts each year are aimed at elucidating mechanisms underlying cardiac disease. The methods for quantifying cardiac performance are quite varied, with each technique assessing unique features of cardiac muscle mechanical properties. Accordingly, in this review, we discuss current ex vivo methods for quantifying cardiac muscle performance, highlighting what can be learned from each method, and how each technique can be used in conjunction to complement others for a more comprehensive understanding of cardiac function. Importantly, cardiac function can be assessed at several different levels, from the whole organ down to individual protein-protein interactions. Here, we take a reductionist view of methods that are commonly used to measure the distinct aspects of cardiac mechanical function, beginning with whole heart preparations and finishing with the in vitro motility assay. While each of the techniques are individually well-documented in the literature, there is a significant need for a comparison of the techniques, delineating the mechanical parameters that can are best measured with each technique, as well as the strengths and weaknesses inherent to each method. Additionally, we will consider complementary techniques and how these methods can be used in combination to improve our understanding of cardiac mechanical function. By presenting each of these methods, with their strengths and limitations, in a single manuscript, this review will assist cardiovascular biologists in understanding the existing literature on cardiac mechanical function, as well as designing future experiments.

Roles of PDE1 in Pathological Cardiac Remodeling and Dysfunction.

Pathological cardiac hypertrophy and dysfunction is a response to various stress stimuli and can result in reduced cardiac output and heart failure. Cyclic nucleotide signaling regulates several cardiac functions including contractility, remodeling, and fibrosis. Cyclic nucleotide phosphodiesterases (PDEs), by catalyzing the hydrolysis of cyclic nucleotides, are critical in the homeostasis of intracellular cyclic nucleotide signaling and hold great therapeutic potential as drug targets. Recent studies have revealed that the inhibition of the PDE family member PDE1 plays a protective role in pathological cardiac remodeling and dysfunction by the modulation of distinct cyclic nucleotide signaling pathways. This review summarizes recent key findings regarding the roles of PDE1 in the cardiac system that can lead to a better understanding of its therapeutic potential.

Therapeutic potential of PDE modulation in treating heart disease.

Altered cyclic nucleotide-mediated signaling plays a critical role in the development of cardiovascular pathology. By degrading cAMP/cGMP, the action of cyclic nucleotide PDEs is essential for controlling cyclic nucleotide-mediated signaling intensity, duration, and specificity. Altered expression, localization and action of PDEs have all been implicated in causing changes in cyclic nucleotide signaling in cardiovascular disease. Accordingly, pharmacological inhibition of PDEs has gained interest as a treatment strategy and as an area of drug development. While targeting of certain PDEs has the potential to ameliorate cardiovascular disease, inhibition of others might actually worsen it. This review will highlight recent research on the physiopathological role of cyclic nucleotide signaling, especially with regard to PDEs. While the physiological roles and biochemical properties of cardiovascular PDEs will be summarized, the primary emphasis will be pathological. Research into the potential benefits and hazards of PDE inhibition will also be discussed.