An arrhythmogenic metabolite in atrial fibrillation

BACKGROUND

Long-chain acyl-carnitines (ACs) are potential arrhythmogenic metabolites. Their role in atrial fibrillation (AF) remains incompletely understood. Using a systems medicine approach, we assessed the contribution of C18:1AC to AF by analysing its in vitro effects on cardiac electrophysiology and metabolism, and translated our findings into the human setting.

METHODS AND RESULTS

Human iPSC-derived engineered heart tissue was exposed to C18:1AC. A biphasic effect on contractile force was observed: short exposure enhanced contractile force, but elicited spontaneous contractions and impaired Ca2+ handling. Continuous exposure provoked an impairment of contractile force. In human atrial mitochondria from AF individuals, C18:1AC inhibited respiration. In a population-based cohort as well as a cohort of patients, high C18:1AC serum concentrations were associated with the incidence and prevalence of AF.

CONCLUSION

Our data provide evidence for an arrhythmogenic potential of the metabolite C18:1AC. The metabolite interferes with mitochondrial metabolism, thereby contributing to contractile dysfunction and shows predictive potential as novel circulating biomarker for risk of AF.

Hdac1-deficiency affects the cell cycle axis Cdc25-Cdk1 causing impaired G2/M phase progression and reduced cardiomyocyte proliferation in zebrafish

Zebrafish have the ability to fully regenerate their hearts after injury since cardiomyocytes subsequently dedifferentiate, re-enter cell cycle, and proliferate to replace damaged myocardial tissue. Recent research identified the reactivation of dormant developmental pathways during cardiac regeneration in adult zebrafish, suggesting pro-proliferative pathways important for developmental heart growth to be also critical for regenerative heart growth after injury. Histone deacetylase 1 (Hdac1) was recently shown to control both, embryonic as well as adult regenerative cardiomyocyte proliferation in the zebrafish model. Nevertheless, regulatory pathways controlled by Hdac1 are not defined yet. By analyzing RNA-seq-derived transcriptional profiles of the Hdac1-deficient zebrafish mutant baldrian, we here identified DNA damage response (DDR) pathways activated in baldrian mutant embryos. Surprisingly, although the DDR signaling pathway was transcriptionally activated, we found the complete loss of protein expression of the known DDR effector and cell cycle inhibitor p21. Consequently, we observed an upregulation of the p21-downstream target Cdk2, implying elevated G1/S phase transition in Hdac1-deficient zebrafish hearts. Remarkably, Cdk1, another p21-but also Cdc25-downstream target was downregulated. Here, we found the significant downregulation of Cdc25 protein expression, explaining reduced Cdk1 levels and suggesting impaired G2/M phase progression in Hdac1-deficient zebrafish embryos. To finally prove defective cell cycle progression due to Hdac1 loss, we conducted Cytometer-based cell cycle analyses in HDAC1-deficient murine HL-1 cardiomyocytes and indeed found impaired G2/M phase transition resulting in defective cardiomyocyte proliferation. In conclusion, our results suggest a critical role of Hdac1 in maintaining both, regular G1/S and G2/M phase transition in cardiomyocytes by controlling the expression of essential cell cycle regulators such as p21 and Cdc25.

Histone deacetylase 1 controls cardiomyocyte proliferation during embryonic heart development and cardiac regeneration in zebrafish

In contrast to mammals, the zebrafish maintains its cardiomyocyte proliferation capacity throughout adulthood. However, neither the molecular mechanisms that orchestrate the proliferation of cardiomyocytes during developmental heart growth nor in the context of regeneration in the adult are sufficiently defined yet. We identified in a forward genetic N-ethyl-N-nitrosourea (ENU) mutagenesis screen the recessive, embryonic-lethal zebrafish mutant baldrian (bal), which shows severely impaired developmental heart growth due to diminished cardiomyocyte proliferation. By positional cloning, we identified a missense mutation in the zebrafish histone deacetylase 1 (hdac1) gene leading to severe protein instability and the loss of Hdac1 function in vivo. Hdac1 inhibition significantly reduces cardiomyocyte proliferation, indicating a role of Hdac1 during developmental heart growth in zebrafish. To evaluate whether developmental and regenerative Hdac1-associated mechanisms of cardiomyocyte proliferation are conserved, we analyzed regenerative cardiomyocyte proliferation after Hdac1 inhibition at the wound border zone in cryoinjured adult zebrafish hearts and we found that Hdac1 is also essential to orchestrate regenerative cardiomyocyte proliferation in the adult vertebrate heart. In summary, our findings suggest an important and conserved role of Histone deacetylase 1 (Hdac1) in developmental and adult regenerative cardiomyocyte proliferation in the vertebrate heart.

Heart disease is one of the most common causes of death in all developed countries. While zebrafish cardiomyocytes are able to proliferate throughout adulthood, mammalian cardiomyocytes lose this ability during early development, and therefore are not capable to replace and renew cardiomyocytes after injury. The underlying mechanisms of cardiomyocyte proliferation are still not completely resolved. Understanding how zebrafish cardiomyocytes preserve their proliferating state, would be a valuable information to foster cardiac regeneration, e.g. after myocardial infarction in patients. Knowledge of the signaling pathways that need to be activated, or deactivated in order to induce cardiomyocyte proliferation after acute or chronic injury will pave the way for the development of genetic and/or pharmacological treatment options. In an ENU-mutagenesis screen, we identified the zebrafish mutant baldrian, which shows reduced embryonic cardiomyocyte proliferation. As genetic cause of the observed phenotype, we identified a missense mutation in the hdac1 gene. By treatment of heart-injured adult fish with the HDAC1 inhibitor Mocetinostat, we were able to show a reduced rate of cardiomyocyte proliferation also in the adult zebrafish heart in vivo, suggesting a role of Hdac1 in embryonic heart growth and adult regenerative cardiomyocyte proliferation in zebrafish.

Genetics of Cardiovascular Disease: Fishing for Causality

Cardiovascular disease (CVD) is still the leading cause of death in all western world countries and genetic predisposition in combination with traditional risk factors frequently mediates their manifestation. Genome-wide association (GWA) studies revealed numerous potentially disease modifying genetic loci often including several SNPs and associated genes. However, pure genetic association does not prove direct or indirect relevance of the modifier region on pathogenesis, nor does it define within the associated region the exact genetic driver of the disease. Therefore, the relevance of the identified genetic disease associations needs to be confirmed either in monogenic traits or in experimental in vivo model system by functional genomic studies. In this review, we focus on the use of functional genomic approaches such as gene knock-down or CRISPR/Cas9-mediated genome editing in the zebrafish model to validate disease-associated genomic loci and to identify novel cardiovascular disease genes. We summarize the benefits of the zebrafish for cardiovascular research and highlight examples demonstrating the successful combination of GWA studies and functional genomics in zebrafish to broaden our knowledge on the genetic and molecular underpinnings of cardiovascular diseases.

Conversion of twice-daily tacrolimus to once-daily tacrolimus formulation in stable pediatric kidney transplant recipients: pharmacokinetics and efficacy

The pharmacokinetics, efficacy and safety of once-daily tacrolimus formulation (Tac-OD) were assessed in 34 stable pediatric kidney transplant recipients. Enrolled patients received their dose of twice-daily tacrolimus formulation (Tac-BID) on study Days 0 through 7. On the morning of study Day 8, the total daily doses for patients were converted to Tac-OD on a 1:1 basis and maintained on a once-daily morning dosing regimen. Tacrolimus pharmacokinetic profiles were obtained on study Days 7, 14 and 28 (after dose adjustment). Although the mean C0 concentrations (4.10 ± 1.16-3.53 ± 1.10 ng/mL, p = 0.004), and AUC0-24 (151.8 ± 41.6-129.8 ± 39.3 ng h/mL, p < 0.001) were decreased significantly after a 1:1 based conversion, there was high interindividual variability. The dose of Tac-OD was decreased in 26.5% and increased in 44.1% of patients. The resultant tacrolimus dose and pharmacokinetic profiles on study Day 28 were comparable to those on Day 7. There were no serious adverse events. In conclusion, Tac-BID can be safely converted to Tac-OD in stable pediatric kidney transplant patients with the heightened therapeutic drug monitoring. Effects of drug conversion on the cardiovascular risk factors, neurological side effects and adherence should be further evaluated.

Spinal cord neurons are vulnerable to rapidly triggered kainate neurotoxicity in vitro

Initial studies found glutamate injury to murine spinal cultures (14-17 days in vitro) to reflect contributions of both NMDA and AMPA/kainate receptors. Subsequent experiments found the spinal cultures to be more sensitive than cortical cultures to injury from prolonged low level kainate exposures, and, unlike cortical cultures, to be significantly damaged by relatively brief (30-60 min) kainate exposures. This rapidly triggered kainate damage to spinal neurons is Ca(2+)-dependent. Also, more than 40% of spinal neurons (in comparison to about 15% of cortical neurons) are subject to kainate-activated Co2+ uptake (Co2+(+) neurons), a histochemical technique that labels neurons with Ca(2+)-permeable AMPA/kainate channels. These spinal Co2+(+) neurons are very sensitive to Ca(2+)-dependent kainate injury, and show greater kainate-induced elevations in intracellular Ca2+ concentrations ([Ca2+]i) than other spinal neurons during low level kainate exposures. Thus, the heightened vulnerability of spinal neurons to kainate toxicity may at least in part reflect the large proportion that possess Ca2+ permeable AMPA/kainate channels, permitting receptor activation to trigger rapid Ca2+ influx and overwhelm the cells Ca2+ homeostatic capabilities.

Infantile gastrointestinal hemangioma as a cause of chronic anemia

Gastrointestinal hemangiomas, though rare, can be an important source of GI bleeding in infants. In the past, diagnosis has been hampered by a lack of reliable methods. With the recent advent of selective mesenteric angiography, the suspected diagnosis can be readily confirmed. Surgical resection remains the definitive therapy; steroid therapy has produced conflicting results.