Metabolic Signatures of Pregnancy-Induced Cardiac Growth

The goal of this study was to develop an atlas of the metabolic, transcriptional, and proteomic changes that occur with pregnancy in the maternal heart. Timed pregnancy studies in FVB/NJ mice revealed significant increases in heart size by day 8 of pregnancy (mid-pregnancy; MP), which was sustained throughout the rest of the term compared with non-pregnant controls. Cardiac hypertrophy and myocyte cross-sectional area were highest 7 d after birth (post-birth; PB) and were associated with significant increases in end-diastolic and end-systolic left ventricular volumes and cardiac output. Metabolomics analyses revealed that, by day 16 of pregnancy (late pregnancy; LP), metabolites associated with nitric oxide production as well as acylcholines, sphingomyelins, and fatty acid species were elevated, which coincided with a lower activation state of phosphofructokinase and higher levels of pyruvate dehydrogenase kinase 4 (Pdk4). In the postpartum period, urea cycle metabolites, polyamines, and phospholipid levels were markedly elevated in the maternal heart. Cardiac transcriptomics in LP revealed significant increases in not only Pdk4, but also genes that regulate glutamate and ketone body oxidation, which were preceded in MP by higher expression of transcripts controlling cell proliferation and angiogenesis. Proteomics analysis of the maternal heart in LP and PB revealed significant reductions in several contractile filaments and mitochondrial complex subunits. Collectively, these findings describe the coordinated molecular changes that occur in the maternal heart during and after pregnancy.

Considerations for using isolated cell systems to understand cardiac metabolism and biology

Changes in myocardial metabolic activity are fundamentally linked to cardiac health and remodeling. Primary cardiomyocytes, induced pluripotent stem cell-derived cardiomyocytes, and transformed cardiomyocyte cell lines are common models used to understand how (patho)physiological conditions or stimuli contribute to changes in cardiac metabolism. These cell models are helpful also for defining metabolic mechanisms of cardiac dysfunction and remodeling. Although technical advances have improved our capacity to measure cardiomyocyte metabolism, there is often heterogeneity in metabolic assay protocols and cell models, which could hinder data interpretation and discernment of the mechanisms of cardiac (patho)physiology. In this review, we discuss considerations for integrating cardiomyocyte cell models with techniques that have become relatively common in the field, such as respirometry and extracellular flux analysis. Furthermore, we provide overviews of metabolic assays that complement XF analyses and that provide information on not only catabolic pathway activity, but biosynthetic pathway activity and redox status as well. Cultivating a more widespread understanding of the advantages and limitations of metabolic measurements in cardiomyocyte cell models will continue to be essential for the development of coherent metabolic mechanisms of cardiac health and pathophysiology.

Abstract 526: Unraveling the Molecular Signature of Pregnancy Induced Hypertrophy

Cardiovascular disease is the leading cause of death in pregnant and postpartum women. During pregnancy, the maternal heart rapidly adapts to the increasing physiological and metabolic demands of the growing fetus. This adaptation often takes the form of a physiological hypertrophy in which the maternal heart grows to increase cardiac output; however, the molecular processes underlying pregnancy-induced hypertrophy (PIH) are poorly understood. The goal of this study was to examine the transcriptomic and metabolic signatures associated with the structural and functional adaptations of the heart to pregnancy. Therefore, we performed timed pregnancy studies in 12-week-old female FVB/NJ mice, which were distributed into the following groups: non-pregnant control (NP; n = 14), mid-pregnancy (MP, 6d pregnant; n = 11), late-pregnancy (LP, 16d pregnant; n = 13), and 1-wk post birth (PB; n = 8). Heart weight to tibia length were higher in MP (7.77±1.02 mg/mm; p <0.05), LP (7.84±0.87 mg/mm; p <0.05), and PB mice (9.86±1.14 mg/mm; p <0.05) compared with NP mice (6.54±0.74 mg/mm). The sustained increase in PB heart weight was associated with increased myocyte cross sectional area, consistent with cardiomyocyte hypertrophy. Compared with NP hearts, echocardiographic measurements suggest significant increases in both end diastolic (36.0±5.1 vs 61.2±5.9 μl; p <0.05) and systolic LV volume (9.4±3.8 vs 21.0±1.4 μl; p <0.05) in PB hearts. These changes in PB hearts were associated with a significant increase in LV mass and a decline in ejection fraction. In LP and PB hearts, we also found higher expression of markers of hypertrophy ( Nppa, Nppb, Myh7 ). Subsequent RNA-seq analyses revealed enrichment in genes involved in cell proliferation, cytokinesis, and transcription in MP hearts; in metabolism genes in LP hearts; and in fibrotic and extracellular matrix genes in PB hearts. Together, these findings reveal the key molecular signature underlying the structural and functional adaptation of the heart during pregnancy and parturition, and may shed light on the molecular processes underlying PIH.

Mitochondria-associated lactate dehydrogenase is not a biologically significant contributor to bioenergetic function in murine striated muscle

Previous studies indicate that mitochondria-localized lactate dehydrogenase (mLDH) might be a significant contributor to metabolism. In the heart, the presence of mLDH could provide cardiac mitochondria with a higher capacity to generate reducing equivalents directly available for respiration, especially during exercise when circulating lactate levels are high. The purpose of this study was to test the hypothesis that mLDH contributes to striated muscle bioenergetic function. Mitochondria isolated from murine cardiac and skeletal muscle lacked an appreciable ability to respire on lactate in the absence or presence of exogenous NAD⁺. Although three weeks of treadmill running promoted physiologic cardiac growth, mitochondria isolated from the hearts of acutely exercised or exercise-adapted mice showed no further increase in lactate oxidation capacity. In all conditions tested, cardiac mitochondria respired at >20-fold higher levels with provision of pyruvate compared with lactate. Similarly, skeletal muscle mitochondria showed little capacity to respire on lactate. Protease protection assays of isolated cardiac mitochondria confirmed that LDH is not localized within the mitochondrion. We conclude that mLDH does not contribute to cardiac bioenergetics in mice.

Corrigendum: FVB/NJ Mice Are a Useful Model for Examining Cardiac Adaptations to Treadmill Exercise

[This corrects the article on p. 636 in vol. 7, PMID: 28066267.].

Exercise-Induced Changes in Glucose Metabolism Promote Physiological Cardiac Growth

Supplemental Digital Content is available in the text.

Background:

Exercise promotes metabolic remodeling in the heart, which is associated with physiological cardiac growth; however, it is not known whether or how physical activity–induced changes in cardiac metabolism cause myocardial remodeling. In this study, we tested whether exercise-mediated changes in cardiomyocyte glucose metabolism are important for physiological cardiac growth.

Methods:

We used radiometric, immunologic, metabolomic, and biochemical assays to measure changes in myocardial glucose metabolism in mice subjected to acute and chronic treadmill exercise. To assess the relevance of changes in glycolytic activity, we determined how cardiac-specific expression of mutant forms of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase affect cardiac structure, function, metabolism, and gene programs relevant to cardiac remodeling. Metabolomic and transcriptomic screenings were used to identify metabolic pathways and gene sets regulated by glycolytic activity in the heart.

Results:

Exercise acutely decreased glucose utilization via glycolysis by modulating circulating substrates and reducing phosphofructokinase activity; however, in the recovered state following exercise adaptation, there was an increase in myocardial phosphofructokinase activity and glycolysis. In mice, cardiac-specific expression of a kinase-deficient 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase transgene (Glyco^Lo mice) lowered glycolytic rate and regulated the expression of genes known to promote cardiac growth. Hearts of Glyco^Lo mice had larger myocytes, enhanced cardiac function, and higher capillary-to-myocyte ratios. Expression of phosphatase-deficient 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase in the heart (Glyco^Hi mice) increased glucose utilization and promoted a more pathological form of hypertrophy devoid of transcriptional activation of the physiological cardiac growth program. Modulation of phosphofructokinase activity was sufficient to regulate the glucose–fatty acid cycle in the heart; however, metabolic inflexibility caused by invariantly low or high phosphofructokinase activity caused modest mitochondrial damage. Transcriptomic analyses showed that glycolysis regulates the expression of key genes involved in cardiac metabolism and remodeling.

Conclusions:

Exercise-induced decreases in glycolytic activity stimulate physiological cardiac remodeling, and metabolic flexibility is important for maintaining mitochondrial health in the heart.

FVB/NJ Mice Are a Useful Model for Examining Cardiac Adaptations to Treadmill Exercise

Mice are commonly used to examine the mechanisms by which exercise improves cardiometabolic health; however, exercise compliance and adaptations are often strain-dependent or are variable due to inconsistency in exercise training protocols. In this study, we examined nocturnal/diurnal behavior, treadmill exercise compliance, and systemic as well as cardiac-specific exercise adaptations in two commonly used mouse strains, C57BL/6J, and FVB/NJ mice. Metabolic cage analysis indicated a strong nocturnal nature of C57BL/6J mice, whereas FVB/NJ mice showed no circadian element to activity, food or water intake, VO₂, or VCO₂. Initial exercise capacity tests revealed that, compared with C57BL/6J mice, FVB/NJ mice are capable of achieving nearly 2-fold higher workloads prior to exhaustion. FVB/NJ mice tested during the day were capable of achieving significantly more work compared with their night-tested counterparts. Following 4 weeks of training, FVB/NJ mice showed significant increases in exercise capacity as well as physiologic cardiac growth characterized by enlarged myocytes and higher mitochondrial DNA content. C57BL/6J mice showed no increases in exercise capacity or cardiac growth regardless of whether they exercised during the day or the night. This lack of adaptation in C57BL/6J mice was attributable, at least in part, to their progressive loss of compliance to the treadmill training protocol. We conclude that the FVB/NJ strain is a useful and robust mouse model for examining cardiac adaptations to treadmill exercise and that treadmill training during daytime hours does not negatively affect exercise compliance or capacity.