Myotonic muscular dystrophy: abnormalities in fibroblast culture

Establishment of quantitative and consistent in vitro skeletal muscle pathological models of myotonic dystrophy type 1 using patient-derived iPSCs

Myotonic dystrophy type 1 (DM1) is caused by expanded CTG repeats (CTGexp) in the dystrophia myotonica protein kinase (DMPK) gene, and the transcription products, expanded CUG repeats, sequester muscleblind like splicing regulator 1 (MBNL1), resulting in the nuclear MBNL1 aggregation in the DM1 cells. Loss of MBNL1 function is the pivotal mechanism underlying the pathogenesis of DM1. To develop therapeutics for DM1, proper human in vitro models based on the pathologic mechanism of DM1 are required. In this study, we established robust in vitro skeletal muscle cell models of DM1 with patient-derived induced pluripotent stem cells (iPSCs) using the MyoD1-induced system and iPSCs-derived muscle stem cell (iMuSC) differentiation system. Our newly established DM1 models enable simple quantitative evaluation of nuclear MBNL1 aggregation and the downstream splicing defects. Quantitative analyses using the MyoD1-induced myotubes showed that CTGexp-deleted DM1 skeletal myotubes exhibited a reversal of MBNL1-related pathologies, and antisense oligonucleotide treatment recovered these disease phenotypes in the DM1-iPSCs-derived myotubes. Furthermore, iMuSC-derived myotubes exhibited higher maturity than the MyoD1-induced myotubes, which enabled us to recapitulate the SERCA1 splicing defect in the DM1-iMuSC-derived myotubes. Our quantitative and reproducible in vitro models for DM1 established using human iPSCs are promising for drug discovery against DM1.

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.

Myotubularin phosphoinositide phosphatases, protein phosphatases, and kinases: their roles in junction dynamics and spermatogenesis

Spermatogenesis in the seminiferous epithelium of the mammalian testis is a dynamic cellular event. It involves extensive restructuring at the Sertoli-germ cell interface, permitting germ cells to traverse the epithelium from basal to adluminal compartment. As such, Sertoli-germ cell actin-based adherens junctions (AJ), such as ectoplasmic specializations (ES), must disassemble and reassemble to facilitate this event. Recent studies have shown that AJ dynamics are regulated by intricate interactions between AJ integral membrane proteins (e.g., cadherins, alpha6beta1 integrins and nectins), phosphatases, kinases, adaptors, and the underlying cytoskeleton network. For instance, the myotubularin (MTM) phosphoinositide (PI) phosphatases, such as MTM related protein 2 (MTMR2), can form a functional complex with c-Src (a non-receptor protein tyrosine kinase). In turn, this phosphatase/kinase complex associates with beta-catenin, a constituent of the N-cadherin/beta-catenin functional unit at the AJ site. This MTMR2-c-Src-beta-catenin complex apparently regulates the phosphorylation status of beta-catenin, which determines cell adhesive function conferred by the cadherin-catenin protein complex in the seminiferous epithelium. In this review, we discuss the current status of research on selected phosphatases and kinases, and how these proteins potentially interact with adaptors at AJ in the seminiferous epithelium to regulate cell adhesion in the testis. Specific research areas that are open for further investigation are also highlighted.

Fibroblasts from patients with myotonic muscular dystrophy: cholesterol requirement for proliferation and sensitivity to polyene antibiotics

The genetic defect in myotonic muscular dystrophy (MMD) remains obscure. From the evidence that drugs blocking cholesterol biosynthesis induce myotonia and increased serum concentrations of deoxycholic acids are common among patients with MMD, evidence of the abnormal sterol metabolism in MMD fibroblasts was sought by comparing them with fibroblasts from control individuals and patients with Duchenne muscular dystrophy (DMD). Although early-onset type MMD and DMD fibroblasts have lower maximal cell densities than fibroblasts from age-matched control individuals do in medium containing 10% fetal bovine serum, we could not reveal any abnormalities in exogeneous cholesterol requirements for proliferation of MMD fibroblasts. This suggests that the sterol biosynthetic pathway in MMD fibroblasts is grossly intact. Furthermore, no difference were observed in sensitivities to polyene antibiotics, which bind to membrane sterols and presumably damage the cell membrane.

Connective tissue metabolism in normal and atrophic skeletal muscle

The first two enzymes of the pentose-phosphate pathway (G6PD and 6PGD) were studied in rabbit skeletal muscle. Their activities closely paralleled the connective tissue levels in each individual muscle, and they increased in atrophying muscles at the same rate as connective tissue. Skeletal muscle tendons and subcutaneously implanted polyvinyl sponges, both of which consist of relatively pure connective tissue, displayed much higher activity of these enzymes than the muscle homogenates. These results suggest that pentose synthesis in muscle is mostly confined to its connective tissue elements, raising the possibility of a role for connective tissue in the biosynthetic processes of skeletal muscle. The contribution of connective tissue to the results of muscle quantitative studies may be significant, and even overshadow other values in atrophic muscles, where the muscle cell population may be greatly reduced. The interpretation of quantitative biochemical studies on pathological muscle samples should take this factor into consideration.

Simulation of genetic mucopolysaccharidoses in normal human fibroblasts by alteration of pH of the medium

Catabolism of sulfated mucopolysaccharide by normal human fibroblasts in culture is progressively inhibited as the pH of the growth medium is raised from 6.8 to 8.0. The final cell density increases with the change in pH. The capacity to degrade mucopolysaccharide is rapidly restored by lowering the pH, and this reactivation does not require protein synthesis. Such pH dependence is not observed in cells from patients with genetic impairment of mucopolysaccharide degradation, such as the Hurler or Hunter syndromes. These results may have relevance not only to studies of mucopolysaccharide metabolism in cell culture, but also to the use of metachromasia as a genetic marker and to the observation that normal fibroblasts are released from contact inhibition of growth as the pH of the growth medium is raised.