ATAC-Seq of single myofibers and muscle stem cells (MuSCs)

We report the application of single myofiber ATAC-Seq (smfATAC-Seq) to investigate the chromatin accessibility of a single myofiber without the presence of confounding muscle resident cell types. This method demonstrates that open chromatin regions of myonuclei can be tagmentated and high-quality sequencing ready libraries can be generated from these fragments. To perform comparative analysis as well as to demonstrate the applicability of the smfATAC-Sew to study changes in chromatin of myonuclei within different contexts, smfATAC-Seq was performed both on uninjured myofibers as well as injured myofibers seven days after induced injury. Furthermore, ATAC-Seq on 5000 muscle stem cells (MuSCs) was also performed according to the previously described OMNI ATAC-Seq protocol (Corces, M.R. et al. Nature Methods, 2017) in order to compare the sequencing quality of the smfATAC-Seq as well as to demonstrate the changes in open chromatin that occur from the stem cell state to the fully differentiated myofibers. smfATAC-seq resulted in comparable coverage and sequencing depth to the ATAC-seq performed on MuSCs and allowed for peak calling and differential peak analysis. These analysis revealed that the open chromatin state of uninjured and injured myofibers after seven days are mostly similar although some regions invovled in immune response remain in an open state in the injured myofibers compared to the uninjured myofibers and the regions involved in structural formation of the muscle are more accessible in the case of regeneration. Even though certain differences in the open chromatin are observed, smfATAC-Seq analysis suggest that overall, the open chromatin state of the myonuclei returns back to homeostasis after seven days of regeneration. Furthermore, smfATAC-Seq comparison with the ATAC-Seq from MuSCs show the differences in open chromatin regions between these conditions. Increased accessibility of genes involved in myogenesis and structural components can be observed in the myofibers compared to MuSCs while MuSCs show increased accessibility in regions involved in membrane permeability and signalling pathways. In addition, the regions that are accessible for both conditions include genes involved in mitochondrial transport, regulation of transcription and regulation of metabolites and energy. Overall, this study introduces smfATAC-Seq that succesfully assesses the genome-wide chromatin accessibility of a single myofiber with relatively high sequencing depth. The smfATAC-Seq can be used to perform comparative analysis between different conditions such as injury. However, this method can be readily applied to study differences between young and old myofibers or in the context of muscular dystrophy, cachexia, and exercise. smfATAC-Seq was performed on an uninjured single myofiber as well as injured myofiber isolated from Extensor Digitorum Longus (EDL). In addition, ATAC-seq on 5000 MuSCs is performed by following the OMNI ATAC-Seq protocol. The comparative analysis were performed between uninjured and injured myofibers and between uninjured myofibers and MuSCs.