Developmental changes in heart and muscle phosphofructokinase isozymes

A Ratiometric Catalog of Protein Isoform Shifts in the Cardiac Fetal Gene Program

The fetal genetic program orchestrates cardiac development and the re-expression of fetal genes is thought to underlie cardiac disease and adaptation. Here, a proteomics ratio test using mass spectrometry is applied to find protein isoforms with statistically significant usage differences in the fetal vs. postnatal mouse heart. Changes in isoform usage ratios are pervasive at the protein level, with 104 significant events observed, including 88 paralog-derived isoform switching events and 16 splicing-derived isoform switching events between fetal and postnatal hearts. The ratiometric proteomic comparisons rediscovered hallmark fetal gene signatures including a postnatal switch from fetal β (MYH7) toward ɑ (MYH6) myosin heavy chains and from slow skeletal muscle (TNNI1) toward cardiac (TNNI3) troponin I. Altered usages in metabolic proteins are prominent, including a platelet to muscle phosphofructokinase (PFKP - PFKM), enolase 1 to 3 (ENO1 - ENO3), and alternative splicing of pyruvate kinase M2 toward M1 (PKM2 - PKM1) isoforms in glycolysis. The data also revealed a parallel change in mitochondrial proteins in cardiac development, suggesting the shift toward aerobic respiration involves also a remodeling of the mitochondrial protein isoform proportion. Finally, a number of glycolytic protein isoforms revert toward their fetal forms in adult hearts under pathological cardiac hypertrophy, suggesting their functional roles in adaptive or maladaptive response, but this reversal is partial. In summary, this work presents a catalog of ratiometric protein markers of the fetal genetic program of the mouse heart, including previously unreported splice isoform markers.

Mutant CHCHD10 causes an extensive metabolic rewiring that precedes OXPHOS dysfunction in a murine model of mitochondrial cardiomyopathy

Mitochondrial cardiomyopathies are fatal diseases, with no effective treatment. Alterations of heart mitochondrial function activate the mitochondrial integrated stress response (ISR^mt), a transcriptional program affecting cell metabolism, mitochondrial biogenesis, and proteostasis. In humans, mutations in CHCHD10, a mitochondrial protein with unknown function, were recently associated with dominant multi-system mitochondrial diseases, whose pathogenic mechanisms remain to be elucidated. Here, in CHCHD10 knockin mutant mice, we identify an extensive cardiac metabolic rewiring triggered by proteotoxic ISR^mt. The stress response arises early on, before the onset of bioenergetic impairments, triggering a switch from oxidative to glycolytic metabolism, enhancement of transsulfuration and one carbon (1C) metabolism, and widespread metabolic imbalance. In parallel, increased NADPH oxidases elicit antioxidant responses, leading to heme depletion. As the disease progresses, the adaptive metabolic stress response fails, resulting in fatal cardiomyopathy. Our findings suggest that early interventions to counteract metabolic imbalance could ameliorate mitochondrial cardiomyopathy associated with proteotoxic ISR^mt.

Sayles et al. report that mutant CHCHD10 proteotoxicity activates the mitochondrial integrated stress response (ISR^mt) in a mouse model of mitochondrial cardiomyopathy. Chronic ISR^mt causes profound metabolic imbalances, culminating in oxidative stress and iron dysregulation, ultimately resulting in mitochondrial dysfunction and contributing to disease pathogenesis.

Metabolic remodeling in early development and cardiomyocyte maturation

Aberrations in metabolism contribute to a large number of diseases, such as diabetes, obesity, cancer, and cardiovascular diseases, that have a substantial impact on the mortality rates and quality of life worldwide. However, the mechanisms leading to these changes in metabolic state--and whether they are conserved between diseases--is not well understood. Changes in metabolism similar to those seen in pathological conditions are observed during normal development in a number of different cell types. This provides hope that understanding the mechanism of these metabolic switches in normal development may provide useful insight in correcting them in pathological cases. Here, we focus on the metabolic remodeling observed both in early stage embryonic stem cells and during the maturation of cardiomyocytes.

Cardiac hypertrophy in the newborn delays the maturation of fatty acid β-oxidation and compromises postischemic functional recovery

During the neonatal period, cardiac energy metabolism progresses from a fetal glycolytic profile towards one more dependent on mitochondrial oxidative metabolism. In this study, we identified the effects of cardiac hypertrophy on neonatal cardiac metabolic maturation and its impact on neonatal postischemic functional recovery. Seven-day-old rabbits were subjected to either a sham or a surgical procedure to induce a left-to-right shunt via an aortocaval fistula to cause RV volume-overload. At 3 wk of age, hearts were isolated from both groups and perfused as isolated, biventricular preparations to assess cardiac energy metabolism. Volume-overload resulted in cardiac hypertrophy (16% increase in cardiac mass, P < 0.05) without evidence of cardiac dysfunction in vivo or in vitro. Fatty acid oxidation rates were 60% lower (P < 0.05) in hypertrophied hearts than controls, whereas glycolysis increased 246% (P < 0.05). In contrast, glucose and lactate oxidation rates were unchanged. Overall ATP production rates were significantly lower in hypertrophied hearts, resulting in increased AMP-to-ATP ratios in both aerobic hearts and ischemia-reperfused hearts. The lowered energy generation of hypertrophied hearts depressed functional recovery from ischemia. Decreased fatty acid oxidation rates were accompanied by increased malonyl-CoA levels due to decreased malonyl-CoA decarboxylase activity/expression. Increased glycolysis in hypertrophied hearts was accompanied by a significant increase in hypoxia-inducible factor-1α expression, a key transcriptional regulator of glycolysis. Cardiac hypertrophy in the neonatal heart results in a reemergence of the fetal metabolic profile, which compromises ATP production in the rapidly maturing heart and impairs recovery of function following ischemia.

Myocardial substrate metabolism in the normal and failing heart

The alterations in myocardial energy substrate metabolism that occur in heart failure, and the causes and consequences of these abnormalities, are poorly understood. There is evidence to suggest that impaired substrate metabolism contributes to contractile dysfunction and to the progressive left ventricular remodeling that are characteristic of the heart failure state. The general concept that has recently emerged is that myocardial substrate selection is relatively normal during the early stages of heart failure; however, in the advanced stages there is a downregulation in fatty acid oxidation, increased glycolysis and glucose oxidation, reduced respiratory chain activity, and an impaired reserve for mitochondrial oxidative flux. This review discusses 1) the metabolic changes that occur in chronic heart failure, with emphasis on the mechanisms that regulate the changes in the expression of metabolic genes and the function of metabolic pathways; 2) the consequences of these metabolic changes on cardiac function; 3) the role of changes in myocardial substrate metabolism on ventricular remodeling and disease progression; and 4) the therapeutic potential of acute and long-term manipulation of cardiac substrate metabolism in heart failure.

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.

Changes in phosphofructokinase isozymes during development of myoblasts to myotubes

The regulation of phosphofructokinase during development of C2C12 myoblasts to myotubes was investigated. Enzyme activity was markedly increased during myogenic development. The increase was observed when enzyme activity was measured under optimal conditions and was not due to changes in the allosteric kinetic properties of the enzyme. Immunoprecipitation of phosphofructokinase from [35S]methionine-labeled myogenic cells revealed that equal amounts of liver and muscle isozymes are present in myoblasts, while in myotubes there was a much higher level of the muscle isozyme. These results were confirmed using an immunoblotting technique. The increase in the level of muscle isozyme in myotubes is due to an increase in the rate of synthesis of the muscle isozyme and occurs in spite of a measurably small increase in its degradation rate. Northern blot analysis using a synthetic oligonucleotide probe showed a 25-fold increase in the level of muscle phosphofructokinase mRNA in myotubes. The conclusion is drawn that the increase in muscle isozyme in myotubes during myogenesis is due to an increase in its mRNA level.

Regulation of skeletal muscle 6-phosphofructo-1-kinase during aging and development

The purpose of this paper is to provide insight into the alterations of 6-phosphofructo-1-kinase (PFK) activity and isozyme types of rat skeletal muscle during development and aging. PFK isozymes are tetramers which may be comprised of one or any combination of the three subunit types, L, M, and C. The effects of fusion or terminal differentiation of cultured rat L6 myoblasts leading to formation of myotubles does not have a noticeable effect on total PFK activity. However, the amount of M-type subunit was increased; and the level of the C-type subunit decreased. These subunit changes caused shifts in the isozymic types. The ultimate effects of prenatal development of PFK were characterized in the near-term fetal muscle. This stage of development was accompanied by a significant loss of the C-type subunit and by two-fold increases in the L-type and M-type subunits which accounted for the 40% increase in total PFK activity. After birth, the M-type subunit increased dramatically as did the total PFK activity. Since the L-type and C-type subunits were gradually lost during the subsequent 3 weeks, the homotetramer of the M-type subunit (M4) was the only type which is present in mature muscle. M4 persisted as the only detectable form in skeletal muscle during the remainder of life, but the total PFK activity and amount of M4 decreased after 18 months of age. The decreased total PFK activity in aged skeletal muscle suggested that the expression of PFK genes may have reverted to an immature state when total PFK activity was lower. As shown by both the immunological analysis and direct quantification of subunit types, this clearly did not occur. That is, the loss of PFK activity in aged muscle is a consequence of decreased levels of the M-type subunit and not reappearance of other subunit types such as found in maturing muscle. Further, our examination of aged skeletal muscle indicates that no significant structural changes in M-type subunits had occurred and that inactive or partially active proteins which could crossreact with the M-type subunit were not detectable. It is suggested that the loss of M4 could cause a depression of the glycolytic rate leading to diminished ability of senile muscle to accommodate extreme energy demands.

On the ontogeny and interactions of phosphofructokinase in mouse tissues

The distribution and interactions of phosphofructokinase isozymes with cellular structure have been studied in the major tissues of the mouse during development. The ontogenic patterns of isozymes which were obtained were consistent with those observed for other species and are interpreted in terms of the presence of three genes and three homotetrameric forms of the enzyme (A4, B4 and C4) in the tissues of the mouse. In addition, the data provides a clear indication that interactions between the enzyme and cellular structure are appreciable in all major tissues and at all stages of development, with all isozyme types exhibiting such interactions. The significance of the study of subcellular interactions of these isozymes in contributing to a comprehensive physiological rationale for this mammalian enzyme and its multiple forms is discussed.

Properties of the phosphofructokinase regulatory factors

The work presented herein describes many of the physiological properties of the phosphofructokinase regulatory factors. Factor activity can be separated into two discrete fractions, which were designated factor A and factor B, based on their respective charges. A preparation containing both factor A and factor B did not protect the following key carbohydrate-metabolizing enzymes from thermal inactivation: glucokinase, glucose-6-phosphatase (solubilized or nonsolubilized forms), pyruvate kinase, glucose-6-P dehydrogenase, muscle-type phosphofructokinase, or the minor liver phosphofructokinase isozyme. Factor activity in this sample was found to be Pronase sensitive, irreversibly precipitated by trichloroacetic acid, reversibly precipitated by adjusting the sample to a pH of 3.0, and stable to heating at 98 degrees C for 20 min. Distribution studies indicated that factor activity was found only in the soluble cell fraction and not in the mitochondrial or nuclear fractions. Factor activity was retained by 12,000-14,000 molecular weight cut-off (MWCO) dialysis tubing, and not retained by 50,000 MWCO dialysis tubing. These studies indicate that fructose-2,6-P2, calmodulin, or insulin-generated mediator are not associated with factor activity. Although fructose-2,6-P2 did not, both factor preparations protect the major liver phosphofructokinase isozyme (liver PFK) from inactivation by lysosomal extracts. In the diabetic rat, the activities of both factors are greatly reduced but return to near normal levels after 48 h of insulin administration. These data suggest that factor B had little or no effect on the kinetic properties of liver PFK. However, factor A was a K-type activator with respect to fructose-6-P, increasing both the Km and Ki for ATP, and slightly increasing the Vm.

Isozyme composition and phosphorylation of brain phosphofructokinase

Rabbit brain phosphofructokinase was purified to homogeneity by a rapid procedure involving affinity chromatography and gel filtration. The enzyme consists of hybrids of the three phosphofructokinase subunit types C, A, and B. The molecular weights of these subunits are 86,000, 84,000, and 80,000, respectively; they are present in brain phosphofructokinase in a ratio of approximately 5:4:1.5. The enzyme as isolated from rabbit brain contains 0.16-0.18 mol phosphate per mole of subunit; another 0.4-0.5 mol phosphate per mole subunit can be incorporated in vitro in the presence of the catalytic subunit of cyclic AMP-dependent protein kinase. The initial rate of phosphorylation is increased by fructose 2,6-bisphosphate or AMP and decreased by citrate or high concentrations of ammonium sulfate. All three subunit types are phosphorylated in vitro, and the phosphorylation site on each subunit is sensitive to cleavage by trypsin at a terminal region of each subunit. However, these sites show different relative rates of phosphorylation in vitro in the presence of ammonium sulfate. In vitro phosphorylation of brain phosphofructokinase had no affect on specific activity, inhibition by ATP, or activation by fructose 2,6-bisphosphate.

Purification of homogeneous rat phosphofructokinase isozymes with high specific activities

The purification of rat muscle and liver phosphofructokinase (PFK) isozymes has been greatly facilitated by column chromatographic separation on immobilized Cibacron Blue F3GA. The homogeneous liver PFK isozyme exhibited a specific activity of greater than 200 units per mg of protein which is nearly two-fold greater than has been previously reported for this isozyme. The yields for this isozyme exceeded 40% of the original activity and the molecular weight of its subunit was about 85,000 as determined by SDS-polyacrylamide gel electrophoresis. The muscle PFK isozyme's specific activity was approximately 265 units/mg of protein which also is about twice the greatest specific activity previously reported. The overall yield for muscle PFK exceeded 50% of the original activity, and the molecular weight of its subunit was approximately 82,000. Using each homogeneous isozyme, antibodies were produced in rabbits; and the immunoglobin-G (IgG) fraction from the sera of these rabbits was highly specific for the PFK isozyme used as an antigen.