Ex Vivo Expansion and In Vivo Self-Renewal of Human Muscle Stem Cells

Abstract

Adult skeletal muscle stem cells, or satellite cells (SCs), regenerate functional muscle following transplantation into injured or diseased tissue. To gain insight into human SC (huSC) biology, we analyzed transcriptome dynamics by RNA sequencing of prospectively isolated quiescent and activated huSCs. This analysis indicated that huSCs differentiate and lose proliferative potential when maintained in high-mitogen conditions ex vivo. Further analysis of gene expression revealed that p38 MAPK acts in a transcriptional network underlying huSC self-renewal. Activation of p38 signaling correlated with huSC differentiation, while inhibition of p38 reversibly prevented differentiation, enabling expansion of huSCs. When transplanted, expanded huSCs differentiated to generate chimeric muscle and engrafted as SCs in the sublaminar niche with a greater frequency than freshly isolated cells or cells cultured without p38 inhibition. These studies indicate characteristics of the huSC transcriptome that promote expansion ex vivo to allow enhanced functional engraftment of a defined population of self-renewing huSCs.

Graphical abstract

Figure 1

Identification and Prospective Isolation of HuSCs (A) Cell-sorting scheme used to isolate huSCs by FACS for a representative specimen of latissimus dorsi muscle. Red gates indicate subpopulations containing huSCs. Numbers indicate percentage of total events falling within each gate; given error represents SD (n = 15). (B) Immunofluorescence (IF) analysis of PAX7 expression in purified huSCs at low (top) and high (bottom) magnification 3 days after isolation. Cells were stained with antibodies against PAX7 and with DAPI to identify nuclei. (C) qRT-PCR analysis of PAX7, PAX3, MYF5, MYOD, MYOG, and MEF2C mRNA levels in the huSC and huSC-depleted cell populations after a period of 7 days in culture is shown (n = 4). Error bars represent SD. ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001.

Figure 2

Transcriptome Profiles of Prospectively Isolated Quiescent and Activated HuSCs (A) Schematic shows RNA-seq analysis of prospectively isolated quiescent and ex vivo activated huSCs. (B) Representative RNA-seq analysis shows quiescent (blue) and activated (red) muscle SCs isolated from latissimus dorsi muscle. (C) RNA-seq analysis shows MYF5 expression in quiescent and activated huSCs. (D) RNA-seq analysis shows expression of differentiation-associated transcription factors in quiescent and activated huSCs. (E) Quantification of DUSP1 expression in quiescent and activated huSCs based on RNA-seq analyses is shown. (F) Transcription factor regulatory network of huSCs constructed from analysis of differential gene expression in quiescent and activated huSCs (see Experimental Procedures for details). Highlighted in red are transcription factors with known associations with the p38 MAPK signaling pathway; highlighted in green are myogenic regulatory factors. The strength of evidence for a given interaction is reflected by the hue of the edge connecting two nodes, with darker edges indicating greater confidence. For RNA-seq data, reads from two biological replicates per condition are shown mapped to the reference genome. The number of sequenced fragments per kilobase of exon per million fragments mapped (FPKM) in each condition is shown for individual genes (n = 2 biological replicates per condition). Error bars represent SD.

Figure 3

Regulation of p38 MAPK Signaling Controls HuSC Fate (A) IF analysis shows activated p-p38 in quiescent huSCs in vivo (top) and activated huSCs ex vivo (bottom). (B) RNA-seq analysis of markers of terminal differentiation in control untreated (red) and p38i-treated (green) activated huSCs. Sequencing reads from two biological replicates per condition are shown mapped to the reference genome. The FPKM in each condition is shown for individual genes (n = 2 biological replicates per condition). (C) Representative fluorescence microscopy analysis of EdU incorporation in control and p38i-treated huSCs cultured for 9 days. The percentage of huSCs in each condition incorporating EdU during a 1-hr pulse was quantified (n = 4). Error bars represent SD. ^∗∗p < 0.01.

Figure 4

Xenotransplantation Reveals Regenerative Potential of Expanded HuSCs (A) Four weeks after transplantation, engraftment of freshly isolated quiescent huSCs, control activated huSCs, and p38i-treated activated huSCs was determined by IF analysis of human-specific ITGB1 expression (magenta); 3 × 10⁴ cells were used for each transplant. Anti-laminin (green) detects laminin of mouse and human origin. (B) Representative low-magnification (left) and high-magnification (right) images of engrafted p38i-treated activated huSCs 4 weeks after transplantation into mouse muscle. The yellow arrowhead identifies a sublaminar PAX7-expressing cell of human origin; the yellow arrow identifies a nearby sublaminar Pax7-expressing cell of mouse origin. (C) Representative flow cytometric analysis of human-specific ITGB1 expression in the bulk population of mononuclear cells isolated from mouse muscle transplanted with 3 × 10⁴ huSCs per transplant. Quantification of total mononuclear human ITGB1-expressing cells per transplanted muscle revealed a 3.6-fold and 7.7-fold increase in engraftment of p38i-treated huSCs relative to quiescent huSCs and untreated activated huSCs, respectively. ^∗∗∗p < 0.001 comparing p38i-treated activated huSCs to control activated huSCs or quiescent huSCs (n = 4 per condition). Error bars represent SD.