Erythrocyte membrane abnormalities in human myotonic dystrophy

Cell membrane integrity in myotonic dystrophy type 1: implications for therapy

Myotonic Dystrophy type 1 (DM1) is a multisystemic disease caused by toxic RNA from a DMPK gene carrying an expanded (CTG•CAG)n repeat. Promising strategies for treatment of DM1 patients are currently being tested. These include antisense oligonucleotides and drugs for elimination of expanded RNA or prevention of aberrant binding to RNP proteins. A significant hurdle for preclinical development along these lines is efficient systemic delivery of compounds across endothelial and target cell membranes. It has been reported that DM1 patients show elevated levels of markers of muscle damage or loss of sarcolemmal integrity in their serum and that splicing of dystrophin, an essential protein for muscle membrane structure, is abnormal. Therefore, we studied cell membrane integrity in DM1 mouse models commonly used for preclinical testing. We found that membranes in skeletal muscle, heart and brain were impermeable to Evans Blue Dye. Creatine kinase levels in serum were similar to those in wild type mice and expression of dystrophin protein was unaffected. Also in patient muscle biopsies cell surface expression of dystrophin was normal and calcium-positive fibers, indicating elevated intracellular calcium levels, were only rarely seen. Combined, our findings indicate that cells in DM1 tissues do not display compromised membrane integrity. Hence, the cell membrane is a barrier that must be overcome in future work towards effective drug delivery in DM1 therapy.

Abnormal insulin receptor binding in cultured monocytes in myotonic muscular dystrophy

Even though one of the characteristic features of myotonic dystrophy (MyD) is the high level of circulating insulin, 125I-insulin-binding data in MyD have been controversial. In the present study we utilized cultured monocytes to avoid problems of reproducibility and variability in age, and examined the affinity and binding characteristics of 125I-insulin binding in MyD patients and controls. The Bmax and mean affinity constant, Ka, were significantly lower, while the number of receptors per cell had increased in the patient group as compared to the controls. The data confirm our earlier findings that there is no primary defect in insulin receptors in MyD, and the disturbed insulin response may be due to an abnormality in the membrane environment. Since the insulin receptor is an integral membrane protein, abnormal plasma membrane lipid composition may lead to impaired lipid-protein interactions, and hence affect the binding characteristics in MyD.

Electrophysiological studies of the visual system in myotonic dystrophy

Ocular signs and electroretinal alterations frequently occur in Myotonic Dystrophy (MD). Surprisingly few reports describe VEP abnormalities for this syndrome. Since the evaluation of cortical visual responses is linked to an understanding of preceding retinal changes, we conducted a systematic study of the visual system including ophthalmological and electrophysiological (EOG, ERG, PERG, VEP) investigation in 14 confirmed myotonic patients. The various tests revealed consistent abnormalities, the most frequent of these being PERG and VEP changes. These alterations seemed to occur independently of one another, suggesting impaired function at different levels of visual pathway. A generalized defect of cell membrane has recently been proposed as etiopathogenesis of typical EMG and systemic features of the disease. Such membrane dysfunction might account for the early and marked abnormalities in electrophysiological tests, even in the absence of neuro-ophthalmological changes.

Transport enzymes in the cell membranes of cultured fibroblasts; alterations in dystrophic cells

In support of the widely held belief that membrane defects are present in the muscular dystrophies, alterations have been found in some transport-related enzymes of cells from affected donors. Cell membranes were isolated from cultured dermal fibroblasts of victims of myotonic muscular dystrophy, and of Duchenne's muscular dystrophy, and from cells of normal age- and sex-matched donors. Myotonic cells had an elevated Na+, K+ ATPase. gamma-Glutamyl transpeptidase was elevated in Duchenne cells. Among all cells' 5'nucleotide phosphatase exhibited a remarkably constant specific activity.

Analysis of erythrocyte membrane proteins from dystrophic hamsters

To search for potentially mutant proteins, we have investigated erythrocyte ghost proteins from normal and dystrophic hamster by two-dimensional gel electrophoresis. No significant differences are observed between dystrophic and normal erythrocytes in their peptide patterns on SDS-polyacrylamide gel electrophoresis while on two-dimensional gels a protein spot of approximate Mr 20 000 with an approximate isoelectric point of 4.5 is found in erythrocytes from dystrophic animals and is consistently absent in normal erythrocytes. A large population of erythrocyte (60%) from dystrophic hamsters shows distorted shape as visualized by scanning electron microscopy. The nature of this protein and its relevance in hamster muscular dystrophy are at present not known.

Erythrocyte flexibility, ATPase activities and Ca efflux in patients with Duchenne muscular dystrophy, myotonic muscular dystrophy and congenital myotonia

Erythrocyte flexibility measured by a polycarbonate membrane filtration method showed increased fragility (265 +/- 163 Hb mg/l vs. controls 86 +/- 72 Hb mg/l; mean +/- SD; P less than 0.0025) and increased rigidity (123 +/- 96 mm Hg vs. 79 +/- 19 mm Hg; P less than 0.05) in patients with congenital myotonia, while both parameters were normal in patients with Duchenne muscular dystrophy or with myotonic dystrophy. Erythrocyte ghosts obtained from patients with MyD displayed highly significant increases in both (Na+ + K+)-ATPase and (Ca2+ + Mg2+)-ATPase activities (P less than 0.005) and to a lesser extent in Mg2+-ATPase activity (P less than 0.05), while no difference was seen between patients with DMD and age-matched controls. The efflux of Ca2+ was increased from erythrocytes of patients with DMD as compared to age-matched controls (82 +/- 2% vs. 70 +/- 4%; P less than 0.005), while no difference was detected between patients with MyD and age-matched controls.

Erythrocyte membrane cation-stimulated ATPase activities in myotonic muscular dystrophy

The cation-stimulated ATPase activities of erythrocyte membranes from patients with myotonic muscular dystrophy (MyD) were compared with the activities in age- and sex-matched controls. The enzymes included ouabain-sensitive ATPase, Mg2+-ATPase and Ca2+ + Mg2+-ATPase. Sampling and processing of the materials from patients with MyD and controls were simultaneously done in each experiment. The enzyme activities were varied with or without EGTA in the reaction medium, or with different temperatures for membrane storage, but no significant differences between MyD and control were observed in any conditions. The present study indicates no specific abnormality of the cation-stimulated ATPase activities of erythrocyte membranes in MyD.

Quantitative measurement of [3H]ouabain binding to human skeletal muscle cryostat sections

The etiopathogenesis of myotonic muscular dystrophy is thought to involve a basic defect in muscle membrane. Biochemical investigations of human muscle membrane have been hampered by difficulty in obtaining large quantities of muscle at biopsy for the preparation of sarcolemma. We have determined [3H]ouabain binding to normal and myotonic dystrophy human skeletal muscle by using cryostat sections. The binding increased with increase in number of tissue sections (protein) and in concentrations of [3H]ouabain, ATP and Na+. The binding of [3H]ouabain in myotonic dystrophy patients was 2-3 fold higher than in normal and disease controls. Kinetic analysis revealed that the increased binding of ouabain to myotonic tissue sections was independent of low-affinity sites directed by ATP and Na+. These findings provide further evidence for the involvement of membrane abnormalities in myotonic muscular dystrophy.

Erythrocyte membrane in myotonic dystrophy. A study with acetylcholinesterase

Erythrocyte membrane in myotonic dystrophy (MyD) was studied with respect to acetylcholinesterase (AchE), an enzyme localized on the external surface of the membrane. The activity was determined over the temperature range of 7-41 degrees C (in hemolysates) and 25-41 degrees C (in ghosts). The activity, the transition temperature in the Arrhenius plots, and the activation energy either above or below the transition temperature did not differ between MyD and controls. The Hill coefficient for the inhibition by fluoride was approximately 1 in MyD and in controls at 13, 25 and 37 degrees C. The dose of fluoride for 50% inhibition of the enzyme activity differed between different temperatures but not between MyD and controls. There seems to be no gross abnormality in AchE or its environment in erythrocyte membrane in MyD.