Martin E. Young

Although studied for decades, metabolic therapies that target cardiac lipid metabolism are underdeveloped, and most approaches have thus far failed as a heart failure treatment. In contrast, new therapies for diabetes and obesity are widely used for the prevention and treatment of heart failure. The heart depends heavily on lipid uptake and utilization for proper function. Too much or too little cardiac lipid catabolism becomes detrimental and causes heart failure, either due to lipotoxicity or energetic depletion. For this reason, cardiac lipid metabolism is carefully controlled and balanced. Moreover, cardiac fatty acid oxidation affects systemic energy metabolism, as evidenced by changes in circulating levels of fatty acids and lipoproteins. This review describes mechanisms of regulation of lipid uptake and metabolism by cardiomyocytes, how fatty acid and glucose use are coordinated, how mitochondrial fatty acid metabolism is regulated at the transcriptional and posttranslational levels, as well as how fed/fasting cycles and circadian clocks modulate heart metabolism during the day. We review studies that used cultured cells, animal models, human tissue, and nuclear tracing. Our objective is to present current knowledge on mechanisms that control cardiac lipid metabolism, thereby suggesting experimental directions that could lead to new metabolic therapies.

Background: Changes in circadian-related behaviors (e.g., the timing of food intake, sleep cycles) and the environment (e.g., light-dark cycles) increase the risk of numerous cardiometabolic diseases, including diabetes mellitus and cardiac disease. Recent studies indicate a close interrelationship between circadian clocks and the posttranslational modification, protein O-GlcNAcylation. The current study was designed to investigate whether a modest elevation of protein O-GlcNAcylation in the adult mouse heart, similar to levels observed during pathologic states, influenced circadian governance of the heart.Methods and results: Cardiomyocyte-specific expression of a dominant negative O-GlcNAcase (dnOGAh) for a 2-week period resulted in an approximate 1.5-fold increase in cardiac protein O-GlcNAcylation, impacting 70% of core circadian clock components in the heart at the mRNA level. Further interrogation of cardiac mRNA species in dnOGAh hearts at candidate (RT-PCR) and unbiased (RNAseq) levels revealed a 95% loss of circadian governance of the cardiac transcriptome. This was despite persistent/augmented 24 h oscillations of the core circadian clock proteins BMAL1, REVERBα, and PER2 in dnOGAh hearts. Direct comparison of dnOGAh hearts with cardiomyocyte-specific BMAL1 knockout (CBK) hearts underscored an apparent uncoupling of the core clock mechanism from clock control of downstream target genes in dnOGAh hearts, and highlighted that loss of circadian governance results in interstitial fibrosis.Conclusions: Sustained protein O-GlcNAcylation in the heart causes loss of circadian governance, likely downstream of the core circadian clock mechanism. Moreover, interstitial fibrosis appears to be a universal adverse outcome following impaired circadian governance.

Energetic demand and nutrient supply fluctuate as a function of time-of-day, in alignment with sleep-wake and fasting-feeding cycles. These daily rhythms are mirrored by 24-hour oscillations in numerous cardiovascular functional parameters, including blood pressure, heart rate, and myocardial contractility. It is, therefore, not surprising that metabolic processes also fluctuate over the course of the day, to ensure temporal needs for ATP, building blocks, and metabolism-based signaling molecules are met. What has become increasingly clear is that in addition to classic signal-response coupling (termed reactionary mechanisms), cardiovascular-relevant cells use autonomous circadian clocks to temporally orchestrate metabolic pathways in preparation for predicted stimuli/stresses (termed anticipatory mechanisms). Here, we review current knowledge regarding circadian regulation of metabolism, how metabolic rhythms are synchronized with cardiovascular function, and whether circadian misalignment/disruption of metabolic processes contribute toward the pathogenesis of cardiovascular disease.

Time-of-day significantly influences the severity and incidence of stroke. Evidence has emerged not only for circadian governance over stroke risk factors, but also for important determinants of clinical outcome. In this review, we provide a comprehensive overview of the interplay between chronobiology and cerebrovascular disease. We discuss circadian regulation of pathophysiological mechanisms underlying stroke onset or tolerance as well as in vascular dementia. This includes cell death mechanisms, metabolism, mitochondrial function, and inflammation/immunity. Furthermore, we present clinical evidence supporting the link between disrupted circadian rhythms and increased susceptibility to stroke and dementia. We propose that circadian regulation of biochemical and physiological pathways in the brain increase susceptibility to damage after stroke in sleep and attenuate treatment effectiveness during the active phase. This review underscores the importance of considering circadian biology for understanding the pathology and treatment choice for stroke and vascular dementia and speculates that considering a patient’s chronotype may be an important factor in developing precision treatment following stroke.

Mitochondria morphology and function, and their quality control by mitophagy, are essential for heart function. We investigated whether these are influenced by time of the day (TOD), sex, and fed or fasting status, using transmission electron microscopy (EM), mitochondrial electron transport chain (ETC) activity, and mito-QC reporter mice. We observed peak mitochondrial number at ZT8 in the fed state, which was dependent on the intrinsic cardiac circadian clock, as hearts from cardiomyocyte-specific BMAL1 knockout (CBK) mice exhibit different TOD responses. In contrast to mitochondrial number, mitochondrial ETC activities do not fluctuate across TOD, but decrease immediately and significantly in response to fasting. Concurrent with the loss of ETC activities, ETC proteins were decreased with fasting, simultaneous with significant increases of mitophagy, mitochondrial antioxidant protein SOD2, and the fission protein DRP1. Fasting-induced mitophagy was lost in CBK mice, indicating a direct role of BMAL1 in regulating mitophagy. This is the first of its kind report to demonstrate the interactions between sex, fasting, and TOD on cardiac mitochondrial structure, function and mitophagy. These studies provide a foundation for future investigations of mitochondrial functional perturbation in aging and heart diseases.

Circadian disruption increases cardiovascular disease (CVD) risk, through poorly understood mechanisms. Given that small RNA species are critical modulators of cardiac physiology/pathology, we sought to determine the extent to which cardiomyocyte circadian clock (CCC) disruption impacts cardiac small RNA species. Accordingly, we collected hearts from cardiomyocyte-specific Bmal1 knockout (CBK; a model of CCC disruption) and littermate control (CON) mice at multiple times of the day, followed by small RNA-seq. The data reveal 47 differentially expressed miRNAs species in CBK hearts. Subsequent bioinformatic analyses predict that differentially expressed miRNA species in CBK hearts influence processes such as circadian rhythmicity, cellular signaling, and metabolism. Of the induced miRNAs in CBK hearts, 7 are predicted to be targeted by the transcriptional repressors REV-ERBα/β (integral circadian clock components that are directly regulated by BMAL1). Similar to CBK hearts, cardiomyocyte-specific Rev-erbα/β double knockout (CM-RevDKO) mouse hearts exhibit increased let-7c-1-3p, miR-23b-5p, miR-139-3p, miR-5123, and miR-7068-3p levels. Importantly, 19 putative targets of these 5 miRNAs are commonly repressed in CBK and CM-RevDKO heart (of which 16 are targeted by let-7c-1-3p). These observations suggest that disruption of the circadian BMAL1–REV-ERBα/β regulatory network in the heart induces distinct miRNAs, whose mRNA targets impact critical cellular functions.

The cardiomyocyte circadian clock temporally governs fundamental cellular processes, leading to 24-h rhythms in cardiac properties (such as electrophysiology and contractility). The importance of this cell-autonomous clock is underscored by reports that the disruption of the mechanism leads to adverse cardiac remodeling and heart failure. In healthy non-stressed mice, the cardiomyocyte circadian clock modestly augments both cardiac protein synthesis (~14%) and mass (~11%) at the awake-to-sleep transition (relative to their lowest values in the middle of the awake period). However, the increased capacity for cardiac growth at the awake-to-sleep transition exacerbates the responsiveness of the heart to pro-hypertrophic stimuli/stresses (e.g., adrenergic stimulation, nutrients) at this time. The cardiomyocyte circadian clock orchestrates time-of-day-dependent rhythms in cardiac growth through numerous mechanisms. Both ribosomal RNA (e.g., 28S) and the PI3K/AKT/mTOR/S6 signaling axis are circadian regulated, peaking at the awake-to-sleep transition in the heart. Conversely, the negative regulators of translation (including PER2, AMPK, and the integrated stress response) are elevated in the middle of the awake period in a coordinated fashion. We speculate that persistent circadian governance of cardiac growth during non-dipping/nocturnal hypertension, sleep apnea, and/or shift work may exacerbate left ventricular hypertrophy and cardiac disease development, highlighting a need for the advancement of chronotherapeutic interventions.

Nitric oxide (NO) is a gaseous signaling molecule, which plays crucial roles in various biological processes, including inflammatory responses, metabolism, cardiovascular functions, and cognitive function. NO bioavailability is reduced with aging and cardiometabolic disorders in humans and rodents. NO stimulates metabolic rate by increasing mitochondrial biogenesis and brown fat activation. Therefore, we propose a novel technology of providing exogenous NO to improve metabolic rate and cognitive function by promoting the development of brown adipose tissue. In the present study, we demonstrate the effects of the peptide amphiphiles-NO-releasing nanomatrix gel (PANO gel) in high-fat diet (HFD)-induced obesity, insulin resistance, and cognitive functions. Eight-week-old male C57BL/6 mice were subcutaneously injected in the brown fat area with the PANO gel or vehicle (PA gel) every two weeks for 12 weeks. The PANO gel-injected mice gained less body weight, improved glucose tolerance, and decreased fasting serum insulin and leptin levels compared with the PA gel-injected mice. Insulin signaling in muscle, liver and epididymal white adipose tissue (eWAT) was improved by the PANO gel injection. The PANO gel reduced inflammation, increased lipolysis in the eWAT, decreased serum lipids, and liver triglycerides. Interestingly, the PANO gel stimulated uncoupled protein 1 gene expression in the brown and beige fat tissues. Furthermore, the PANO gel increased cerebral blood flow and improved learning and memory abilities. Our results suggest that using the PANO gel to supply exogenous NO is a novel technology to treat metabolic disorders and cognitive dysfunctions.