In this study, we tested several culture protocols to find an improved alternative protocol to the basic muscle differentiation method using MYOD1-overexpressed hiPSCs. In our improved method, the application of a specific combination of commercially available media at specific timings is critical. Our newly developed optimized protocol successfully improved skeletal muscle maturation with high reproducibility, which was confirmed by testing several hiPSC lines, and hence maximized the benefits of the MYOD1-mediated muscle differentiation approach. Myotubes cultured using the optimized protocol were multi nucleated and showed increased levels of muscle related proteins and gene expression of various MYH subtypes within 10 days of culture.
Since the discovery of iPSCs in 20061, remarkable progress has been reported in the generation of myogenic cells from hiPSCs. The culture conditions were modified and adapted using growth factors and various kinds of media in each report. In the field of skeletal muscle development using hiPSCs, transgenic and nontransgenic approaches for generating differentiated muscle cells from hiPSCs have been shown to have both advantages and disadvantages26,27. The forced expression of transcription factors can effectively generate muscle cells with high purity, although the myotubes are immature and show neither multi nucleation nor sarcomeric structures. In addition, myotube maturation can be promoted using bioengineering culture techniques, such as 3D bioprinting architecture and electric-field stimulation, although specific devices are necessary to use these techniques in the experimental setting4. Compared with methods using transgenic approaches, direct muscle differentiation protocols require longer times as the cells need to undergo several differentiation stages28; however, it is possible to generate mature myotubes, which are able to undergo spontaneous self-contractions5.
The clinical severity of LMNA-associated muscular dystrophies are evaluated by the levels of skeletal muscle weakness and atrophy, cardiac dysfunction, joint contractures, and spinal deformities29,30. Regarding the laminopathies, understanding their tissue-specific phenotypes is particularly important toward gaining a better understanding of the specific roles of lamin A/C in different tissues, although the genetic and pathogenic mechanisms of these diseases still remain unclear. Owing to the rarity of the skeletal muscle laminopathies, only a small amount of data from studies using patient-derived iPSCs is available. Recent studies established and analyzed 3 MYOD1-overexpressed LMNA-mutant hiPSCs (LMNA L35P, R249W, and K32del) by applying a 3D artificial skeletal muscle platform using fibrin hydrogels11,24. This tissue engineering technique enabled the recapitulation of the nuclear shape abnormalities in differentiated myotubes, in which nuclear elongation was the predominant abnormality, consistent with previous reports using muscle biopsies and primary myoblasts from laminopathy patients31,32. However, it is still challenging to apply these bioengineering culture techniques for high-throughput drug screening25.
Here, we established a novel laminopathy patient-derived iPSC line (E33del) and its isogenic control generated by CRISPR-Cas9 technology, using their direct differentiation into myogenic cells by MYOD1 overexpression. We clearly demonstrated that nuclear shape abnormalities, including nuclear elongation, were specific phenotypes in the LMNA hiPSCs, which were recovered in the Rescue hiPSCs cultured by the standard protocol. Moreover, promoting myotube maturation using the optimized protocol increased the percentage of nuclear abnormalities by up to 18% in the patient-derived iPSCs. In skeletal muscle sections from EDMD/LGMD1B patients, about 20% of myonuclei have irregular shapes33; therefore, applying the optimized protocol can recapitulate the abnormalities of skeletal muscle laminopathies, even at the monolayer culture level. These unique nuclear phenotypes are thought to result from the increased fragility, altered stiffness, and irregular localization of highly condensed heterochromatin, as demonstrated by studies using murine models of the laminopathies34–36; however, the underlying mechanisms and correlation with clinical phenotypes still remain unclear. One benefit of using LMNA patient-derived iPSCs to study the genotype-phenotype correlations is that nuclear shape abnormalities can be analyzed in differentiated muscle cells. In contrast to the impaired muscle cell differentiation that is observed in primary muscle stem cells from laminopathy patients (K32del, L380S, and R249W)37, myotube fusion and differentiation were well-maintained in the LMNA hiPSCs, which were similarly reported in previous studies11,24. We also detected the mislocalization of NE proteins in nuclei with morphological defects from differentiated LMNA-mutant myotubes. In our model, lamin A/C was localized evenly in myotube nuclei with normal, elongated, blebs, and string shapes, whereas lamin B1 was not observed within nuclear blebs. The uneven distribution of lamin A/C and lamin B1 was also reported in iPS-mesenchymal stem cells derived from patients with Hutchinson-Gilford progeria syndrome, which is caused by point mutations in the LMNA gene38, and in fibroblasts from patients with familial partial lipodystrophy of the Dunnigan type, which is caused by missense heterozygous mutations in the LMNA gene39. Other NE proteins, such as NUP153 located in the nuclear pore complex, and LAP2 located in the inner nuclear membrane were localized throughout the entire nuclei, including in the abnormal nuclear regions. The localization of emerin was the most prominent feature of NE proteins expressed in abnormal myonuclei from laminopathy patient-derived iPSCs. Emerin is a ubiquitously expressed nuclear inner membrane protein40, and mutations in the emerin gene cause EDMD41. Emerin directly interacts with lamin A/C42,43, and the depletion of these NE proteins affects chromatin organization in cancer cells in vitro44. Moreover, recent studies clearly demonstrated that emerin plays key roles in front-rear cell polarity in myoblasts45, and in nuclear migration to reseal injured muscle membranes46. In LMNA hiPSCs, the accumulation of emerin was particularly detected in the protruded regions, and on 1 side of differentiated myonuclei, which is similarly observed in cells seeded onto plastic-bottom, glass-bottom, and silicone-bottom culture materials. Although it is still unknown whether the accumulation of emerin contributes to the progression of nuclear abnormalities or to protection of the nuclear membrane against mechanical stress, the reproduction of abnormal myonuclear shapes in differentiated myotubes using LMNA hiPSCs could provide the foundations for elucidating the pathological mechanisms of laminopathy. Moreover, our simple optimized protocol for skeletal myogenic differentiation of hiPSCs can be applied for high-throughput screening to investigate potential therapeutic drugs in the future.
In conclusion, we established an optimized monolayer myogenic differentiation culture method to maximize the advantages of a previously established MYOD1 overexpression strategy. This new approach was effective for developing more mature myotubes using 5 different hiPSC lines, including a new skeletal muscle laminopathy model and its isogenic control; therefore, our approach can be applied to other MYOD1-overexpressed hiPSCs derived from patients with various muscle diseases. The limitations of this study include the utilization of the Dox-inducible MYOD1-overexpression system, and the existence of unknown components in the myotube fusion medium used in the optimized protocol. It is unclear whether the protocol can improve the muscle differentiation of hiPSCs generated by other transgenic approaches or direct myogenic induction systems without transgenes. Taken together, our monolayer culture approach is beneficial for improving myotube formation, and is easy to adapt for studying other muscular dystrophy-hiPSCs models.