Cultured myoblasts from patients affected by myotonic dystrophy type 2 exhibit senescence-related features: ultrastructural evidence

Abstract

Myotonic dystrophy type 2 (DM2) is an autosomal dominant disorder caused by the expansion of the tetranucleotidic repeat (CCTG)n in the first intron of the Zinc Finger Protein-9 gene. In DM2 tissues, the expanded mutant transcripts accumulate in nuclear focal aggregates where splicing factors are sequestered, thus affecting mRNA processing. Interestingly, the ultrastructural alterations in the splicing machinery observed in the myonuclei of DM2 skeletal muscles are reminiscent of the nuclear changes occurring in age-related muscle atrophy. Here, we investigated in vitro structural and functional features of satellite cell-derived myoblasts from biceps brachii, in the attempt to investigate cell senescence indices in DM2 patients by ultrastructural cytochemistry. We observed that in satellite cell-derived DM2 myoblasts, cell-senescence alterations such as cytoplasmic vacuolization, reduction of the proteosynthetic apparatus, accumulation of heterochromatin and impairment of the pre-mRNA maturation pathways occur earlier than in myoblasts from healthy patients. These results, together with preliminary in vitro observations on the early onset of defective structural features in DM2 myoblast derived-myotubes, suggest that the regeneration capability of DM2 satellite cells may be impaired, thus contributing to the muscular dystrophy in DM2 patients.

Muscleblind-like1 undergoes ectopic relocation in the nuclei of skeletal muscles in myotonic dystrophy and sarcopenia

“…1,11 , 28-34 More generally, it could be hypothesised that the accumulation/delocalization of mRNA processing factors, including MBNL1, could hamper the functionality of the whole splicing machinery and slow down the intranuclear molecular trafficking, leading to a lower metabolic activity of myonuclei, consistent with the reduced protein synthesis observed in DM1 and DM2 myoblasts, 68,69 and the misregulated protein turnover resulting in a structural imbalance between protein synthesis and degradation in aged muscles 70 . It is worth noting that myoblasts from DM2 patients grown in culture show cell-senescence structural alterations and impairment of the pre-mRNA maturation pathways much earlier than the myoblasts from healthy subjects 71 . The skeletal muscle of DM patients seems therefore to share intriguing similarities with the muscle from aged mammals, with special reference to the alterations in the nuclear RNP-containing structures involved in pre-mRNA transcription and splicing.…”

Myotonic Dystrophy Type 2: An Update on Clinical Aspects, Genetic and Pathomolecular Mechanism

“…However, in vitro primary myoblasts obtained from muscle of adult DM patients do not show evident morphological abnormalities and are capable of normally differentiating [74, 102, 103]. Conversely, recent data demonstrated that DM2 myoblasts are characterized by senescence related features mainly consisting in the early appearance of cytological alterations and impairment of the pre-mRNA maturation pathways [104]. Moreover, Renna et al [105] reported that DM1 and DM2 myoblasts are characterized by a premature proliferative growth arrest compared to healthy myoblasts through a mechanism similar to senescence since both DM1 and DM2 cells expressed biomarkers usually observed in senescent cells.…”

In vitro culture of primary human myoblasts by using the dextran microcarriers Cytodex3®

“…Myoblast cell therapy can be also potentially offered to dystrophic patients. It is well documented that muscular dystrophies are frequently linked to premature cell aging and exhaustion of satellite cells pool [24] so dystrophic myoblasts obtained from relatively young individuals should be considered as the adult ones demonstrating of diminished proliferative potential.…”