Monovalent cation transport in myotonic dystrophy. Na-K pump ratio in erythrocytes

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 M-wave responses during exercise in myotonic muscular dystrophy: a Na(+)--K+ pump defect?

Maximum M-waves (muscle compound action potentials) were studied in the abductor pollicis brevis (APB), extensor digitorum brevis (EDB), and tibialis anterior (TA) muscles of 12 patients with myotonic muscular dystrophy (MMD) and in the same number of control subjects, matched for age and sex. The peak-to-peak amplitudes and voltage-time areas of the responses were measured at rest, between 40 maximum voluntary contractions (each lasting 3 s) and also during a 2-min recovery period. In 34 of the 36 control muscles the M-waves potentiated during the period of intermittent voluntary contractions. In the MMD patients, however, the M waves exhibited initial declines in 25 of 30 muscles. In the APB and EDB muscles the normalized mean values for the smallest M-waves, recorded during the 350 s total observation periods, differed significantly between the 2 groups of subjects. It is suggested that the sarcolemmal Na(+)-K+ pump has a raised threshold for activation in MMD patients.

The sialic acid and galactose concentrations in erythrocyte membranes in patients with myotonic dystrophy, limb-girdle and facioscapulohumeral dystrophy

Sialic acid is an important constituent of membrane-bound glycoproteins and glycolipids. It occurs linked to galactose at the surface of the membrane and is involved in, e.g., cation exchange, receptor function, maintenance of membrane polarity and intercellular interactions. In myotonic dystrophy there is evidence of an as yet basically undefined plasma-membrane abnormality. Considering the importance of sialic acid in membrane function, sialic acid as well as galactose concentrations were measured in erythrocyte membranes from 17 patients with myotonic dystrophy and compared to 17 matched healthy controls. There was a highly significant (P less than 0.0005) reduction of the sialic acid concentration in the patients, while no significant difference in galactose concentration was found. In 16 patients with limb-girdle and facioscapulohumeral dystrophy, sialic acid and galactose concentrations did not differ from matched controls. The possible importance of a reduced concentration of membrane-bound sialic acid in myotonic dystrophy is discussed in relation to previously reported biochemical membrane abnormalities in this disease.

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.

Glucose transport and oxidation in adipose tissue of patients with myotonic dystrophy

The effect of insulin on the transport of 2-deoxyglucose and the oxidation of glucose in chopped adipose tissue was investigated in 14 myotonic dystrophy (MyD) patients and 28 age and size-matched control subjects. The transport of 0.55 mM 2-deoxyglucose was measured over 3 min at 37 degrees C both with and without 32 ng/ml of insulin. Oxidation was determined at 37 degrees C for 90 min by the measurement of 14CO2 released from a system containing 0.55 mM glucose with and without 50 ng/ml of insulin. Basal 2-deoxyglucose transport was not reduced in MyD subjects but insulin-stimulated 2-deoxyglucose transport in MyD was significantly less at 0.512 +/- 0.220 nmole compared to control subjects with 0.906 +/- 0.160 nmole/100 mg tissue/3 min (P less than 0.02). Both the basal and insulin-stimulated glucose oxidation were significantly less in the MyD group. Insulin-stimulated oxidation was 2.92 +/- 0.21 nmole in the control subjects compared to 2.20 +/- 0.27 nmole/100 mg tissue/90 min in the MyD cases (P less than 0.02). Similar findings were obtained when calculations were based on nmoles of 2-deoxyglucose transport and glucose oxidation/100 mg lipid. The findings indicate that both glucose transport and oxidation are impaired in MyD.

Erythrocyte membrane abnormalities in human myotonic dystrophy

Membrane-bound enzyme activities and cardiac glycoside binding were determined in red blood cell membrane preparations from patients with myotonic dystrophy and in age matched controls. Na+-K+-activated ATPase activity was significantly increased in myotonic patients. [3H]Ouabain binding to erythrocyte membranes was also significantly increased in myotonic dystrophy patients. The Mg2+-ATPase (ouabain-insensitive) was, however, unchanged. The K+-stimulated paranitrophenyl phosphatase (KPNPPase) activity was markedly enhanced in myotonic patients as compared to controls. The kinetic analysis showed a marked change in Vmax of Na+-K+ ATPase with respect to the activation by Na+, K+ and ATP. However, the Km values were the same in control as well as in myotonic groups. The increased erythrocyte membrane Na+-K+-ATPase activity, KPNPPase and [3H]ouabain binding in myotonic patients supports the hypothesis that generalized membrane abnormality may be involved in pathogenesis of the human myotonic dystrophy.

Red blood cell alterations in muscular dystrophy: the role of lipids

Biochemical, morphologic, and biophysical studies support the concept that the red blood cell (RBC) membrane is altered in both myotonic muscular dystrophy (MyD) and Duchenne muscular dystrophy (DMD). These studies have not identified a primary metabolic defect that would explain the various alterations of membrane properties. Since the lipid milieu of the membrane affects most membrane properties, it has been extensively investigated in MyD and DMD. Although some studies have suggested specific lipid abnormalities, no reproducible alterations have been reported in the major lipid constituents of the RBC membrane in these disorders. These findings suggest that major alterations of the predominant membrane lipids are not involved in these diseases. Furthermore, studies of the RBC membrane do not provide definitive statements as to the inborn error of metabolism, whether proteins or lipid constituents are primarily affected, or even whether the described alterations are intrinsic to the membrane or are secondary to some circulating factors. Nevertheless, RBCs have proved useful in demonstrating the involvement of the plasma membrane in muscle disorders and should be important in defining how such membrane perturbations affect transport mechanisms.