Altered allosteric properties of cytoskeleton-bound phosphofructokinase in muscle from mice with X chromosome-linked muscular dystrophy (mdx)

Revisiting the dystrophin-ATP connection: How half a century of research still implicates mitochondrial dysfunction in Duchenne Muscular Dystrophy aetiology

Duchenne Muscular Dystrophy (DMD) is a fatal neuromuscular disease that is characterised by dystrophin-deficiency and chronic Ca(2+)-induced skeletal muscle wasting, which currently has no cure. DMD was once considered predominantly as a metabolic disease due to the myriad of metabolic insufficiencies evident in the musculature, however this aspect of the disease has been extensively ignored since the discovery of dystrophin. The collective historical and contemporary literature documenting these metabolic nuances has culminated in a series of studies that importantly demonstrate that metabolic dysfunction exists independent of dystrophin expression and a mild disease phenotype can be expressed even in the complete absence of dystrophin expression. Targeting and supporting metabolic pathways with anaplerotic and other energy-enhancing supplements has also shown therapeutic value. We explore the hypothesis that DMD is characterised by a systemic mitochondrial impairment that is central to disease aetiology rather than a secondary pathophysiological consequence of dystrophin-deficiency.

Combined in vivo and in silico investigations of activation of glycolysis in contracting skeletal muscle

The hypothesis was tested that the variation of in vivo glycolytic flux with contraction frequency in skeletal muscle can be qualitatively and quantitatively explained by calcium-calmodulin activation of phosphofructokinase (PFK-1). Ischemic rat tibialis anterior muscle was electrically stimulated at frequencies between 0 and 80 Hz to covary the ATP turnover rate and calcium concentration in the tissue. Estimates of in vivo glycolytic rates and cellular free energetic states were derived from dynamic changes in intramuscular pH and phosphocreatine content, respectively, determined by phosphorus magnetic resonance spectroscopy ((31)P-MRS). Computational modeling was applied to relate these empirical observations to understanding of the biochemistry of muscle glycolysis. Hereto, the kinetic model of PFK activity in a previously reported mathematical model of the glycolytic pathway (Vinnakota KC, Rusk J, Palmer L, Shankland E, Kushmerick MJ. J Physiol 588: 1961-1983, 2010) was adapted to contain a calcium-calmodulin binding sensitivity. The two main results were introduction of regulation of PFK-1 activity by binding of a calcium-calmodulin complex in combination with activation by increased concentrations of AMP and ADP was essential to qualitatively and quantitatively explain the experimental observations. Secondly, the model predicted that shutdown of glycolytic ATP production flux in muscle postexercise may lag behind deactivation of PFK-1 (timescales: 5-10 s vs. 100-200 ms, respectively) as a result of accumulation of glycolytic intermediates downstream of PFK during contractions.

Chronic AMPK stimulation attenuates adaptive signaling in dystrophic skeletal muscle

In the present study, we evaluated how a pharmacologically induced phenotype shift in dystrophic skeletal muscle would affect subsequent intracellular signaling in response to a complementary, adaptive physiological stimulus. mdx mice were treated with the AMP-activated protein kinase (AMPK) activator 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR; 500 mg·kg(-1)·day(-1)) for 30 days, and then one-half of the animals were subjected to a bout of treadmill running to induce acute AMPK and p38 MAPK signaling. The mRNA levels of phenotypic modifiers, including peroxisome proliferator-activated receptor-δ (PPARδ), PPARγ coactivator-1α (PGC-1α), receptor interacting protein 140 (RIP 140), and silent information regulator two ortholog 1 (SIRT1) were assessed in skeletal muscle, as well as the expression of the protein arginine methyltransferase genes PRMT1 and CARM1. We found unique AMPK and p38 phosphorylation and expression signatures between dystrophic and healthy muscle. In dystrophic skeletal muscle, treadmill running induced PPARδ, PGC-1α, and SIRT1 mRNAs, three molecules that promote the slow, oxidative myogenic program. In the mdx animals that received the chronic AICAR treatment, running-elicited AMPK and p38 phosphorylation was attenuated compared with vehicle-treated mice. Similarly, acute stress-evoked expression of PPARδ, PGC-1α, and SIRT1 was also blunted by chronic pharmacological AMPK stimulation. Skeletal muscle PRMT1 and CARM1 protein contents were higher in mdx mice compared with wild-type littermates. The acute running-evoked induction of PRMT1 and CARM1 mRNAs was also attenuated by the AICAR treatment. Our data demonstrate that prior pharmacological conditioning is a salient determinant in how dystrophic muscle adapts to subsequent complementary, acute physiological stress stimuli. These results provide insight into possible therapeutic applications of synthetic agonists in neuromuscular diseases, such as during chronic administration to Duchenne muscular dystrophy patients.

Differential expression of phosphofructokinase-1 isoforms correlates with the glycolytic efficiency of breast cancer cells

Cancer cells are characterized by increased aerobic glycolysis, which correlates with a negative prognosis. Although this correlation is well known, the mechanism of the elevated rate of glycolysis in cancer and the role of glycolytic enzymes have yet to be determined. The present work aims to evaluate the activity of the major enzymes that regulate glycolysis in breast cancer cell lines of varying aggressiveness. MCF10A, MCF-7 and MDA-mb-231 are human breast-derived cell lines with non-tumorigenic, tumorigenic and metastatic profiles, respectively. These cell lines have increasing degrees of glycolytic efficiency, i.e., lactate produced per glucose consumed, corresponding to their metastatic potential. Although, there are no differences in phosphofructokinase (PFK) or pyruvate kinase (PK) activities, the activity of hexokinase (HK) activity is higher in both tumorigenic cell lines compared to MCF10A cells. No difference in HK activity is observed between MCF-7 and MDA-mb-231 cells, suggesting that the difference in their glycolytic efficiency could not be attributed to this enzyme. However, we find that expression of the PFK-L isoform directly and strongly correlates with aggressiveness and glycolytic efficiency in these cell lines. Thus, we conclude that glycolytic efficiency, which is important for the survival of cancer cells, depends primarily on the preferential expression of PFK-L over the M and P isoforms.

Microtubular stability affects cardiomyocyte glycolysis by HIF-1alpha expression and endonuclear aggregation during early stages of hypoxia

Hypoxia-inducible factor (HIF)-1alpha is a key regulator of anaerobic energy metabolism. We asked the following question: Does the breakdown of microtubular structures influence glycolysis in hypoxic cardiomyocytes by regulating HIF-1alpha? Neonatal rat cardiomyocytes were cultured under hypoxic conditions, while microtubule-stabilizing (paclitaxel) and -depolymerizing (colchicine) agents were used to change microtubular structure. Models of high microtubule-associated protein 4 (MAP4) expression and RNA interference of microtubulin expression were established. Microtubular structural changes and intracellular HIF-1alpha protein distribution were observed with laser confocal scanning microscopy. Content of key glycolytic enzymes, viability, and energy content of cardiomyocytes were determined by colorimetry and high-performance liquid chromatography. HIF-1alpha protein content and mRNA expression were determined by Western blotting and real-time PCR, respectively. Low doses of microtubule-stabilizing agent (10 mumol/l paclitaxel) and enhanced expression of MAP4 stabilized the reticular microtubular structures in hypoxic cardiomyocytes, increased the content of key glycolytic enzymes, ameliorated energy supply and enhanced cell viability, and upregulated HIF-1alpha protein expression and endonuclear aggregation. In contrast, the microtubule-depolymerizing agent (10 mumol/l colchicine) or reduced microtubulin expression had adverse affects on the same parameters, in particular, HIF-1alpha protein content and endonuclear aggregation. We conclude that microtubular structural changes influence glycolysis in the early stages of hypoxia in cardiomyocytes by regulating HIF-1alpha content. Stabilizing microtubular structures increases endonuclear and total HIF-1alpha expression, content of key glycolytic enzymes, and energy supply. These findings provide potential therapeutic targets for ameliorating cell energy metabolism during early myocardial hypoxia.

Silencing of glycolysis in muscle: experimental observation and numerical analysis

The longstanding problem of rapid inactivation of the glycolytic pathway in skeletal muscle after contraction was investigated using (31)P NMR spectroscopy and computational modelling. Accumulation of phosphorylated glycolytic intermediates (hexose monophosphates) during cyclic contraction and subsequent turnover during metabolic recovery was measured in vivo in human quadriceps muscle using dynamic (31)P NMR spectroscopy. The concentration of hexose monophosphates in muscle peaked 40 s into metabolic recovery from maximal contractile work at 6.9 +/- 1.3 mm (mean +/- s.d.; n = 8) and subsequently declined at a rate of 0.009 +/- 0.001 mm s(1). It was next tested whether the current knowledge of the kinetic controls in the glycolytic pathway in muscle integrated in the Lambeth and Kushmerick computational model of skeletal muscle glycolysis explained the experimental data. It was found that the model underestimated the magnitude of deactivation of the glycolytic pathway in resting muscle, resulting in depletion of glycolytic intermediates and substrate for oxidative ATP synthesis. Numerical analysis of the model identified phosphofructokinase and pyruvate kinase as the kinetic control sites involved in deactivation of the glycolytic pathway. Ancillary 100-fold inhibition of both phosphofructokinase and pyruvate kinase was found necessary to predict glycolytic intermediate and ADP concentrations correctly in resting human muscle. Incorporation of this information into the model resulted in highly improved agreement between predicted and measured in vivo dynamics of hexose monophosphates in muscle following contraction. We concluded that silencing of the glycolytic pathway in muscle following contraction is most likely to be mediated by phosphofructokinase and pyruvate kinase inactivation on a time scale of seconds and minutes, respectively, and is necessary to prevent depletion of vital cellular substrates.

Association of neuronal nitric oxide synthase (nNOS) with alpha1-syntrophin at the sarcolemma

alpha1-syntrophin is a PDZ-containing dystrophin-associated protein, expressed predominantly in striated muscle and brain. alpha1-syntrophin null mice generated by gene targeting technique showed no overt muscular dystrophic phenotype. Though other dystrophin-associated proteins were localized at the sarcolemma, neuronal nitric oxide synthase (nNOS) was selectively lost from the membrane fraction but remained in the cytoplasm. Thus, the alpha1-syntrophin null mice are useful in the elucidation of the functional importance of nNOS targeting at the sarcolemma. In addition, the mice would facilitate identification of other signaling molecules, which are targeted to dystrophin complex via interaction with alpha1-syntrophin.

Hypoxia on hippocampal slices from mice deficient in dystrophin (mdx) and isoforms (mdx3cv)

Slices from control C57, mdx, and mdx3cv mice were made hypoxic until both field excitatory postsynaptic potential (fEPSP) and presynaptic afferent volley (AV) disappeared (H1). After reoxygenation and recovery of fEPSP, a second and longer hypoxic test (H2) lasted 3 minutes beyond the time required to block AV. When slices were kept in 10 mmol/L glucose, HI abolished AV 37 and 19% earlier in slices from mdr and mdx3cv mutants than in control slices (where HI = 12 +/- 4.6 minutes, mean +/- SD). During H2 or when slices were kept in 4 mmol/L glucose, AV vanished even more quickly, but the times to block did not differ significantly between slices from controls and mutants. After reoxygenation, AV fully recovered in most slices. Rates of blockade of fEPSPs were comparable in all slices, and most fEPSPs recovered fully after HI. But even in the presence of 10 mmol/L glucose, the second hypoxia suppressed fEPSPs irreversibly in some slices: 2 of 10 from control, 3 of 7 from mdx, and 1 of 6 from mdx3cv mice. Most slices in 4 mmol/L glucose showed no recovery at all: six of seven from control, three of five from mdx, and four of five from mdx3cv mice. Thus, slices from mdx mice were more susceptible than other slices to irreversible hypoxic failure when slices were kept in 10 mmol/L glucose, but they were less susceptible than other slices when kept in 4 mmol/L glucose. In conclusion, the lack of full-length dystrophin (427 kDa) predisposes to quicker loss of nerve conduction in slices from mdx and mdx3cv mutants and improved posthypoxic recovery of fEPSPs in 4 mmol/L glucose in slices from mdx but not mdx3cv mutants, perhaps because the 70-kDa and other C-terminal isoforms are still present in mdx mice.

Association of phosphofructokinase-M with caveolin-3 in differentiated skeletal myotubes. Dynamic regulation by extracellular glucose and intracellular metabolites

Caveolin-3 is a member of the caveolin family of proteins that is primarily expressed in striated muscle cell types (skeletal and cardiac). Here, we show that an approximately 80-kDa protein specifically co-immunoprecipitates with caveolin-3 expressed in differentiated skeletal C2C12 myotubes. Microsequence analysis of this approximately 80-kDa polypeptide revealed its identity as a key regulatory enzyme in the glycolytic pathway, namely phosphofructokinase-M (PFK-M). Pulse-chase experiments demonstrate that PFK-M associates with caveolin-3 with a significant time lag after the biosynthesis of PFK-M. In addition, we show that this interaction is (i) highly regulated by the extracellular concentration of glucose and (ii) can be stabilized by a number of relevant intracellular metabolites, such as fructose 1,6-bisphosphate and fructose 2,6-bisphosphate, which are known allosteric activators of PFK. While the bulk of these experiments were performed in C2C12 cells, identical results were obtained using mouse skeletal muscle extracts. Taken together, our results suggest that glucose-dependent plasma membrane recruitment of activated PFK-M by caveolin-3 could have important implications for understanding the mechanisms that regulate energy metabolism in skeletal muscle fibers.

Detachment of glycolytic enzymes from cytoskeleton of melanoma cells induced by calmodulin antagonists

Glycolysis, which is the primary energy source in cancer cells, is known to be controlled by allosteric regulators, as well as by reversible binding of glycolytic enzymes to cytoskeleton. We have previously found that different calmodulin antagonists decrease the levels of allosteric activators of glycolysis, and reduce ATP content and cell viability in B16 melanoma cells. Here we report of a novel, additional, mechanism of action of calmodulin antagonists in melanoma cells. We show that these drugs cause a detachment of the glycolytic enzymes, phosphofructokinase (ATP: D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) and aldolase (D-fructose-1,6-bisphosphate D-glyceraldehyde-3-phosphate-lyase, EC 4.1.2.13), from cytoskeleton of B16 melanoma cells. This effect was dose- and time-dependent, and preceded the decrease in cell viability. The detachment of glycolytic enzymes from cytoskeleton would reduce the provision of local ATP, in the vicinity of the cytoskeleton-membrane and would affect cytoskeleton structure. Since the cytoskeleton is being recognized as an important modulator of cell function, proliferation, differentiation and neoplasia, detachment of the glycolytic enzymes from cytoskeleton induced by calmodulin antagonists, as well as their reported inhibitory action on cell proliferation, make these drugs most promising agents in treatment of cancer.

Effects of carbamylcholine and pyridostigmine on cytoskeleton-bound and cytosolic phosphofructokinase and ATP levels in different rat tissues

1. The effects of carbamylcholine (CaCh) (acetylcholine agonist) and pyridostigmine (Pyr) (acetylcholinesterase inhibitor), on the activity of cytoskeleton-bound and cytosolic phosphofructokinase (PFK), the rate-limiting enzyme in glycolysis, and ATP levels, were studied in rat tibialis anterior (TA) muscle, heart, and brain. 2. In the TA muscle, a marked (about three-fold) increase in the allosteric activity of cytosolic (soluble) PFK was found, 3-5 min following the injection of CaCh or Pyr. The intracellular distribution of the enzyme was not affected by both drugs. Stimulation of glycolysis in this muscle was also expressed by a significant increase in the concentrations of glycolytic intermediates and lactate. Glucose 1,6-bisphosphate (Glc-1,6-P2) levels were unchanged, whereas fructose-2,6-bisphosphate (Fru-2,6-P2) was increased. Glycogenolysis was also stimulated, as deduced from the decrease in glycogen content. The stimulation of glycolysis, induced by both drugs, was accompanied by an increase in ATP level in the TA muscle. 3. In contrast to the stimulatory action of CaCh or Pyr on glycolysis in the TA muscle, both drugs had no effect on cytosolic and cytoskeletal PFK in heart and brain. However, ATP content in both heart and brain was markedly reduced by these drugs, most probably due to their reported harmful effects on mitochondrial function, leading to tissue damage. 4. Electron microscopic studies of TA muscle and heart from rats treated with CaCh or Pyr, revealed severe damage of heart but no harmful effects on TA muscle, which is a muscle with high glycolytic and low oxidative capacity. The present experiments suggest that the accelerated glycolysis in this muscle induced by both drugs, supplies ATP, thus preventing muscle damage.

Components of energy expenditure in the mdx mouse model of Duchenne muscular dystrophy

Previous observations showing that basal heat production rates and glucose metabolism were reduced in mdx mouse skeletal muscles incubated in vitro led us to study the components of total energy expenditure by open-circuit indirect calorimetry in the intact, free-moving mdx mouse. Our purpose was to verify if the mdx mouse exhibited whole-body alterations in energy metabolism. The results revealed that total and basal energy expenditure, as well as spontaneous activity, energetic cost of activity, and, therefore, energy expended in relation to activity were not significantly different in C57B1/10 (control) and in dystrophic (mdx) mice. In contrast, the thermic effect of food was 32% larger in mdx than in control mice and was accompanied by significant differences in post-prandial glucose and lipid oxidation. The present in vivo study could not show a direct demonstration that impaired glucose metabolism by skeletal muscles participated in this phenomenon. However, since post-prandial glucose metabolism by skeletal muscles contributes a significant part of the thermic effect of food, the present data are in line with previous studies in vitro that show that mdx mouse skeletal muscles probably suffer an impaired control of their energy metabolism.

Effects of low-frequency stimulation on soluble and structure-bound activities of hexokinase and phosphofructokinase in rat fast-twitch muscle

Several glycolytic enzymes exist in muscle as free and structure-bound forms. A fraction of hexokinase (HK) is associated with the outer mitochondrial membrane. Phosphofructokinase (PFK) and aldolase (ALD) bind to F-actin, and AMP deaminase (AMPase) interacts with myosin. Using low-frequency stimulation (10 Hz, 24 h/d), we studied in rat fast-twitch muscle effects of contractile activity on soluble and structure-bound forms of these enzymes. Phosphoglucose isomerase (PGI), a soluble enzyme, was also examined. Fractional extraction was applied to study the intracellular distribution of soluble and bound enzyme activities 5 min, 1 h, 3 h, 1 d, and 7 d after the onset of stimulation. Confirming previous findings, total HK activity increased 7-fold in 7-d-stimulated muscles, whereas PFK, ALD, and PGI were reduced, ranging between 55% and 80% of their normal activities. AMPase activity was unaltered. At the time points studied, no changes were found in the extraction behavior of PGI and AMPase. The fraction of bound ALD increased slightly (12%). However, the distribution of HK and PFK was markedly altered. Bound PFK increased from 50% in the control to 85% in 7-d-stimulated muscles. Bound HK rose from 52% to 83% during the same time period. The increase in PFK binding was steep and occurred mainly within the first minutes and hours. The increase in HK binding occurred with some delay, but was significant in muscles stimulated for more than 1 h. In view of the altered kinetic properties of F-actin-bound PFK (alleviated allosteric inhibition by ATP) and bound HK (elevated catalytic activity), these changes are interpreted as early responses to match the metabolic demands during maximal contractile activity imposed on a muscle not programmed for sustained activity: Enhanced binding of PFK serves to accelerate glycolytic flux immediately after the onset of stimulation, whereas mitochondrial binding of HK facilitates the phosphorylation of exogenous glucose when glycogen stores have been depleted.

Dystrophin-dependent efficiency of metabolic pathways in mouse skeletal muscles

Muscles from the mdx mouse (X-linked genetic disorder similar to Duchenne muscular dystrophy) lack dystrophin-associated transsarcolemmal proteins and show reduced maintenance metabolic rates. Here, microcalorimetric comparisons of metabolic stimulation by exogenous substrates in isolated muscles revealed substrate-selective limitation of chemical reaction rates through both glycolytic and TCA-cycle pathways, identical in slow- and fast-twitch mdx muscles. This systemic approach, as opposed to comparisons of single-enzyme activities, sheds new light on the function of dystrophin and associated proteins. The in vivo efficiency of metabolic pathways may depend on stabilization of enzyme complexes by dystrophin-associated elements of the cytoskeleton.

Decreased rates of Ca(2+)-dependent heat production in slow- and fast-twitch muscles from the dystrophic (mdx) mouse

Using a newly developed microcalorimetric approach to assess the rate of energy expenditure for intracellular [Ca2+] homeostasis in isolated muscles at rest, we found this was lower in mdx than in control mouse muscles, by 62% and 29% in soleus and extensor digitorum longus, respectively. Differences in total and Ca(2+)-dependent rates of specific heat production between mdx and control were enhanced during sustained, KCl-induced stimulation of energy dissipation. These results suggest that the low sarcoplasmic energy status of dystrophic muscles is not due to any excessive energy expenditure for intracellular [Ca2+] homeostasis.

Treatment of muscle damage, induced by high intracellular Ca2+, with calmodulin antagonists

1. Incubation of rat diaphragm muscles in the presence of Ca(2+)-ionophore A23187, which causes accumulation of free intracellular Ca2+, induced severe myofibrils damage. Electron microscopic studies have revealed that calmodulin (CaM) antagonists, trifluoperazine, thioridazine, pimozide and CGS 9343B, were most effective in preserving muscle structure. 2. The CaM antagonists raised the decreased glucose-1,6-bisphosphate levels, induced by high Ca2+, with a concomitant activation of the reduced cytosolic phosphofructokinase (the rate limiting enzyme of glycolysis) and thereby cytosolic glycolysis. 3. All four CaM inhibitors also prevented solubilization of cytoskeleton-bound glycolytic enzymes by high Ca2+. 4. The protective effect of these compounds on cytosolic and cytoskeletal glycolysis, was also expressed by their action in preserving muscle ATP levels. 5. The present experiments suggest that CaM antagonists may be effective drugs in treatment of muscle damage and various muscle diseases, which are characterized by a high pathological increase in intracellular Ca2+.