Low aerobic capacity in McArdle disease: A role for mitochondrial network impairment?

BACKGROUND

McArdle disease is caused by myophosphorylase deficiency and results in complete inability for muscle glycogen breakdown. A hallmark of this condition is muscle oxidation impairment (e.g., low peak oxygen uptake (VO_2peak)), a phenomenon traditionally attributed to reduced glycolytic flux and Krebs cycle anaplerosis. Here we hypothesized an additional role for muscle mitochondrial network alterations associated with massive intracellular glycogen accumulation.

METHODS

We analyzed in depth mitochondrial characteristics-content, biogenesis, ultrastructure-and network integrity in skeletal-muscle from McArdle/control mice and two patients. We also determined VO_2peak in patients (both sexes, N = 145) and healthy controls (N = 133).

RESULTS

Besides corroborating very poor VO_2peak values in patients and impairment in muscle glycolytic flux, we found that, in McArdle muscle: (a) damaged fibers are likely those with a higher mitochondrial and glycogen content, which show major disruption of the three main cytoskeleton components-actin microfilaments, microtubules and intermediate filaments-thereby contributing to mitochondrial network disruption in skeletal muscle fibers; (b) there was an altered subcellular localization of mitochondrial fission/fusion proteins and of the sarcoplasmic reticulum protein calsequestrin-with subsequent alteration in mitochondrial dynamics/function; impairment in mitochondrial content/biogenesis; and (c) several OXPHOS-related complex proteins/activities were also affected.

CONCLUSIONS

In McArdle disease, severe muscle oxidative capacity impairment could also be explained by a disruption of the mitochondrial network, at least in those fibers with a higher capacity for glycogen accumulation. Our findings might pave the way for future research addressing the potential involvement of mitochondrial network alterations in the pathophysiology of other glycogenoses.

Coenzyme Q protects cells against serum withdrawal-induced apoptosis by inhibition of ceramide release and caspase-3 activation

Coenzyme Q10 (CoQ10) is a component of the antioxidant machinery that protects cell membranes from oxidative damage and decreases apoptosis in leukemic cells cultured in serum-depleted media. Serum deprivation induced apoptosis in CEM-C7H2 (CEM) and to a lesser extent in CEM-9F3, a subline overexpressing Bcl-2. Addition of CoQ10 to serum-free media decreased apoptosis in both cell lines. Serum withdrawal induced an early increase of neutral-sphingomyelinase activity, release of ceramide, and activation of caspase-3 in both cell lines, but this effect was more pronounced in CEM cells. CoQ10 prevented activation of this cascade of events. Lipids extracted from serum-depleted cultures activated caspase-3 independently of the presence of mitochondria in cell-free in vitro assays. Activation of caspase-3 by lipid extracts or ceramide was prevented by okadaic acid, indicating the implication of a phosphatase in this process. Our results support the hypothesis that plasma membrane CoQ10 regulate the initiation phase of serum withdrawal-induced apoptosis by preventing oxidative damage and thus avoiding activation of downstream effectors as neutral-sphingomyelinase and subsequent ceramide release and caspase activation pathways.

Antioxidant ascorbate is stabilized by NADH-coenzyme Q10 reductase in the plasma membrane

Plasma membranes isolated from K562 cells contain an NADH-ascorbate free radical reductase activity and intact cells show the capacity to reduce the rate of chemical oxidation of ascorbate leading to its stabilization at the extracellular space. Both activities are stimulated by CoQ10 and inhibited by capsaicin and dicumarol. A 34-kDa protein (p34) isolated from pig liver plasma membrane, displaying NADH-CoQ10 reductase activity and its internal sequence being identical to cytochrome b5 reductase, increases the NADH-ascorbate free radical reductase activity of K562 cells plasma membranes. Also, the incorporation of this protein into K562 cells by p34-reconstituted liposomes also increased the stabilization of ascorbate by these cells. TPA-induced differentiation of K562 cells increases ascorbate stabilization by whole cells and both NADH-ascorbate free radical reductase and CoQ10 content in isolated plasma membranes. We show here the role of CoQ10 and its NADH-dependent reductase in both plasma membrane NADH-ascorbate free radical reductase and ascorbate stabilization by K562 cells. These data support the idea that besides intracellular cytochrome b5-dependent ascorbate regeneration, the extracellular stabilization of ascorbate is mediated by CoQ10 and its NADH-dependent reductase.

Extracellular ascorbate stabilization: enzymatic or chemical process?

Ascorbate is stabilized in the presence of HL-60 cells. This stabilization has been questioned as a simple chemical effect. Further properties and controls about the enzymatic nature of this stabilization are described and discussed. Our results showed that cAMP derivatives and cAMP-increasing agents stimulated the ability of HL-60 cells to stabilize ascorbate. On the other hand, tunicamycin, a glycosylation-interfering agent, inhibited this ability. These data, together with hormonal regulation, support the hypothesis of an enzymatic redox system located at the plasma membrane as being responsible for the extracellular ascorbate stabilization by HL-60 cells.

Transplasma membrane redox system of HL-60 cells is controlled by cAMP

Transplasma membrane redox activity of HL-60 cells was determined by measuring the prevention of ascorbate chemical oxidation. The ascorbate free radical produced as the first step of ascorbate oxidation was reduced back by the transplasma membrane electron transport system, causing then the regeneration of extracellular ascorbate. Agents that increase intracellular cAMP, such as forskolin and dibutyryl cAMP (db-cAMP), increased the rate of ascorbate regeneration by HL-60 cells. Also, the phosphodiesterase-resistant cAMP analogue Sp-cAMP-S (agonist of the protein kinase A) increased the electron flow to the ascorbate free radical at the plasma membrane. Rp-cAMP-S, antagonist of the protein kinase A, partially inhibited the redox activity of cells and abolished the effect of Sp-cAMP-S. Inhibition obtained after preincubation of cells in Rp-cAMP-S was reversed by Sp-cAMP-S. Tunicamycin, a compound that inhibited the electron flow to the ascorbate free radical at the plasma membrane, also reduced the response of transplasma membrane redox system to Sp-cAMP-S. Lactate slightly affected the ascorbate regeneration in nonstimulated cells, but showed a significant effect on Sp-cAMP-S-stimulated plasma membrane electron flow. We show here a role for cAMP in the short-term modulation of transplasma membrane redox system measured as the regeneration of ascorbate at the cell surface of HL-60 cells, probably mediated by cAMP-dependent protein kinases.

A quantitative ultrastructural and cytochemical study of TPA-induced differentiation in HL-60 cells

The effects of the phorbol ester 12-O-tetradecanoyl phorbol-13-acetate on morphometric and stereological parameters have been studied using the HL-60 cell line as a differentiation model for the monocytic pathway. Evaluation of the differentiation was carried out by quantification of endoplasmic reticulum, Golgi apparatus, mitochondria and cytoplasmic granules. Changes in both nuclear and cytoplasmic volumes during TPA-induced differentiation led to a decrease of the nucleus-cytoplasmic ratio after 3 days of treatment. Plasma membrane glycoprotein pattern was also determined. The major change in cell surface was the presence of high amounts of glycoproteins containing N-acetyl glucosamine residues that make wheatgerm agglutinin lectin a valuable marker of the monocytic differentiation pathway in HL-60 cells.

NADH-ascorbate free radical and -ferricyanide reductase activities represent different levels of plasma membrane electron transport

Plasma membranes isolated from rat liver by two-phase partition exhibited dehydrogenase activities for ascorbate free radical (AFR) and ferricyanide reduction in a ratio of specific activities of 1:40. NADH-AFR reductase could not be solubilized by detergents from plasma membrane fractions. NADH-AFR reductase was inhibited in both clathrin-depleted membrane and membranes incubated with anti-clathrin antiserum. This activity was reconstituted in plasma membranes in proportion to the amount of clathrin-enriched supernatant added. NADH ferricyanide reductase was unaffected by both clathrin-depletion and antibody incubation and was fully solubilized by detergents. Also, wheat germ agglutinin only inhibited NADH-AFR reductase. The findings suggest that NADH-AFR reductase and NADH-ferricyanide reductase activities of plasma membrane represent different levels of the electron transport chain. The inability of the NADH-AFR reductase to survive detergent solubilization might indicate the involvement of more than one protein in the electron transport from NADH to the AFR but not to ferricyanide.

Ascorbate free radical stimulates the growth of a human promyelocytic leukemia cell line

Ascorbate free radical stimulates the growth of human promyelocytic leukemia cells (HL-60) in the presence of a limited amount of serum (1%) when added to the cells under conditions where it is impermeable. Maximum growth stimulation occurs at concentrations from 5 x 10(-9) to 2 x 10(-8) M. Ascorbate mimicks the stimulation effect of its free radical but stimulates at higher concentrations. Autoxidation of ascorbate by oxygen produces its free radical, which apparently causes growth stimulation. Ascorbate could be regenerated by intact cells in vitro, since prevention of autoxidation of ascorbate in the presence of cells is observed. Neither dehydroascorbate nor isoascorbate increases HL-60 cell growth. Short term incubation of cells in the presence of ascorbate free radical induced intracellular NADH oxidation. We propose that the stimulation of growth of HL-60 cells shown here could be caused by activation of the transplasma membrane electron transport system by the ascorbate free radical.