Insulin binding and internalization in cultured fibroblasts from myotonic muscular dystrophy

Cells of Matter-In Vitro Models for Myotonic Dystrophy

Myotonic dystrophy type 1 (DM1 also known as Steinert disease) is a multisystemic disorder mainly characterized by myotonia, progressive muscle weakness and wasting, cognitive impairments, and cardiac defects. This autosomal dominant disease is caused by the expression of nuclear retained RNAs containing pathologic expanded CUG repeats that alter the function of RNA-binding proteins in a tissue-specific manner, leading ultimately to neuromuscular dysfunction and clinical symptoms. Although considerable knowledge has been gathered on myotonic dystrophy since its first description, the development of novel relevant disease models remains of high importance to investigate pathophysiologic mechanisms and to assess new therapeutic approaches. In addition to animal models, in vitro cell cultures provide a unique resource for both fundamental and translational research. This review discusses how cellular models broke ground to decipher molecular basis of DM1 and describes currently available cell models, ranging from exogenous expression of the CTG tracts to variable patients' derived cells.

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.

Insulin resistance in multiple aspects of intermediary metabolism in myotonic dystrophy

The responses of circulating intermediary metabolites to a low-dose incremental insulin infusion (basal, 0.005, 0.01, and 0.05 U.kg-1.h-1) were examined in eight ambulant subjects with the multisystem disorder, myotonic dystrophy. Eight healthy subjects matched for age, gender, and body mass index served as controls. Oral glucose tolerance (75 g) was normal in all subjects. Basal (postabsorptive) hyperinsulinemia was observed in the subjects with myotonic dystrophy (8.4 +/- 2.0 v 2.3 +/- 0.2 mU/L, P less than .01) with increased basal C-peptide levels. Basal blood glycerol (0.09 +/- 0.02 v 0.05 +/- 0.01 mmol/L, P less than .05), lactate (1.14 +/- 0.12 v 0.77 +/- 0.07 mmol/L, P less than .02), and pyruvate (0.08 +/- 0.01 v 0.05 +/- 0.01 mmol/L, P less than .02) were also elevated in these subjects. During the incremental insulin infusion, circulating insulin (F = 8.2, P less than .02) and C-peptide (F = 5.1, P less than .05) levels were significantly higher in the myotonic subjects. Despite the hyperinsulinemia, circulating concentrations of lactate (F = 9.8, P less than .01), pyruvate (7.8, P less than .02), and glycerol (F = 7.5, P less than .02) were also higher in the subjects with myotonic dystrophy, providing prima facie evidence of insulin resistance in the regulation of these metabolites. During the highest insulin rate, isotopically determined metabolic clearance rate of glucose was significantly lower in the myotonic subjects (3.6 +/- 0.4 v 5.5 +/- 0.7 mL.kg-1.min-1, P less than .05), indicating impaired peripheral glucose metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)