Myogenic Differentiation of Mouse Embryonic Stem Cells That Lack a Functional Pax7 Gene

PAX7 Balances the Cell Cycle Progression via Regulating Expression of Dnmt3b and Apobec2 in Differentiating PSCs

PAX7 transcription factor plays a crucial role in embryonic myogenesis and in adult muscles in which it secures proper function of satellite cells, including regulation of their self renewal. PAX7 downregulation is necessary for the myogenic differentiation of satellite cells induced after muscle damage, what is prerequisite step for regeneration. Using differentiating pluripotent stem cells we documented that the absence of functional PAX7 facilitates proliferation. Such action is executed by the modulation of the expression of two proteins involved in the DNA methylation, i.e., Dnmt3b and Apobec2. Increase in Dnmt3b expression led to the downregulation of the CDK inhibitors and facilitated cell cycle progression. Changes in Apobec2 expression, on the other hand, differently impacted proliferation/differentiation balance, depending on the experimental model used.

Pax7 as molecular switch regulating early and advanced stages of myogenic mouse ESC differentiation in teratomas

Background

Pluripotent stem cells present the ability to self-renew and undergo differentiation into any cell type building an organism. Importantly, a lot of evidence on embryonic stem cell (ESC) differentiation comes from in vitro studies. However, ESCs cultured in vitro do not necessarily behave as cells differentiating in vivo. For this reason, we used teratomas to study early and advanced stages of in vivo ESC myogenic differentiation and the role of Pax7 in this process. Pax7 transcription factor plays a crucial role in the formation and differentiation of skeletal muscle precursor cells during embryonic development. It controls the expression of other myogenic regulators and also acts as an anti-apoptotic factor. It is also involved in the formation and maintenance of satellite cell population.

Methods

In vivo approach we used involved generation and analysis of pluripotent stem cell-derived teratomas. Such model allows to analyze early and also terminal stages of tissue differentiation, for example, terminal stages of myogenesis, including the formation of innervated and vascularized mature myofibers.

Results

We determined how the lack of Pax7 function affects the generation of different myofiber types. In Pax7−/− teratomas, the skeletal muscle tissue occupied significantly smaller area, as compared to Pax7+/+ ones. The proportion of myofibers expressing Myh3 and Myh2b did not differ between Pax7+/+ and Pax7−/− teratomas. However, the area of Myh7 and Myh2a myofibers was significantly lower in Pax7−/− ones. Molecular characteristic of skeletal muscles revealed that the levels of mRNAs coding Myh isoforms were significantly lower in Pax7−/− teratomas. The level of mRNAs encoding Pax3 was significantly higher, while the expression of Nfix, Eno3, Mck, Mef2a, and Itga7 was significantly lower in Pax7−/− teratomas, as compared to Pax7+/+ ones. We proved that the number of satellite cells in Pax7−/− teratomas was significantly reduced. Finally, analysis of neuromuscular junction localization in samples prepared with the iDISCO method confirmed that the organization of neuromuscular junctions in Pax7−/− teratomas was impaired.

Conclusions

Pax7−/− ESCs differentiate in vivo to embryonic myoblasts more readily than Pax7+/+ cells. In the absence of functional Pax7, initiation of myogenic differentiation is facilitated, and as a result, the expression of mesoderm embryonic myoblast markers is upregulated. However, in the absence of functional Pax7 neuromuscular junctions, formation is abnormal, what results in lower differentiation potential of Pax7−/− ESCs during advanced stages of myogenesis.

Screening identifies small molecules that enhance the maturation of human pluripotent stem cell-derived myotubes

Targeted differentiation of pluripotent stem (PS) cells into myotubes enables in vitro disease modeling of skeletal muscle diseases. Although various protocols achieve myogenic differentiation in vitro, resulting myotubes typically display an embryonic identity. This is a major hurdle for accurately recapitulating disease phenotypes in vitro, as disease commonly manifests at later stages of development. To address this problem, we identified four factors from a small molecule screen whose combinatorial treatment resulted in myotubes with enhanced maturation, as shown by the expression profile of myosin heavy chain isoforms, as well as the upregulation of genes related with muscle contractile function. These molecular changes were confirmed by global chromatin accessibility and transcriptome studies. Importantly, we also observed this maturation in three-dimensional muscle constructs, which displayed improved in vitro contractile force generation in response to electrical stimulus. Thus, we established a model for in vitro muscle maturation from PS cells.

The role of CXC receptors signaling in early stages of mouse embryonic stem cell differentiation

Interplay between CXCR7 and other CXC receptors, namely CXCR4 or CXCR3, binding such ligands as SDF-1 or ITAC, was shown to regulate multiple cellular processes. The developmental role of signaling pathways mediated by these receptors was proven by the phenotypes of mice lacking either functional CXCR4, or CXCR7, or SDF-1, showing that formation of certain lineages relies on these factors. In this study, using in vitro differentiating mouse embryonic stem cells that lacked the function of CXCR7, we asked the question about the role of CXCR mediated signaling during early steps of differentiation. Our analysis showed that interaction of SDF-1 or ITAC with CXC receptors is necessary for the regulation of crucial developmental regulators expression and that CXCR7 is involved in the control of ESC pluripotency and differentiation into mesodermal lineages.

Skeletal muscle-derived cell implantation for the treatment of sphincter-related faecal incontinence

Background

In an earlier pilot study with 10 women, we investigated a new approach for therapy of faecal incontinence (FI) due to obstetric trauma, involving ultrasound-guided injection of autologous skeletal muscle-derived cells (SMDC) into the external anal sphincter (EAS), and observed significant improvement. In the current study, we tested this therapeutic approach in an extended patient group: male and female patients suffering from FI due to EAS damage and/or atrophy. Furthermore, feasibility of lower cell counts and cryo-preserved SMDC was assessed.

Methods

In this single-centre, explorative, baseline-controlled clinical trial, each patient (n = 39; mean age 60.6 ± 13.81 years) received 79.4 ± 22.5 × 10⁶ cryo-preserved autologous SMDC. Changes in FI parameters, Fecal Incontinence Quality of Life (FIQL), anorectal manometry and safety from baseline to 1, 6 and 12 months post implantation were evaluated.

Results

SMDC used in this trial contained a high percentage of myogenic-expressing (CD56⁺) and muscle stem cell marker-expressing (Pax7⁺, Myf5⁺) cells. Intervention was well tolerated without any serious adverse events. After 12 months, the number of weekly incontinence episodes (WIE, primary variable), FIQL and patient condition had improved significantly. In 80.6% of males and 78.4% of females, the WIE frequency decreased by at least 50%; Wexner scores and severity of FI complaints decreased significantly, independent of gender and cause of FI.

Conclusions

Injection of SMDCs into the EAS effectively improved sphincter-related FI due to EAS damage and/or atrophy in males and females. When confirmed in a larger, placebo-controlled trial, this minimal invasive procedure has the potential to become first-line therapy for FI.

Trial registration

EU Clinical Trials Register, EudraCT 2010-023826-19 (Date of registration: 08.11.2010).

Transient MicroRNA Expression Enhances Myogenic Potential of Mouse Embryonic Stem Cells

MicroRNAs (miRNAs) are known regulators of various cellular processes, including pluripotency and differentiation of embryonic stem cells (ESCs). We analyzed differentiation of two ESC lines-D3 and B8, and observed significant differences in the expression of miRNAs and genes involved in pluripotency and differentiation. We also examined if transient miRNA overexpression could serve as a sufficient impulse modulating differentiation of mouse ESCs. ESCs were transfected with miRNA Mimics and differentiated in embryoid bodies and embryoid body outgrowths. miRNAs involved in differentiation of mesodermal lineages, such as miR145 and miR181, as well as miRNAs regulating myogenesis (MyomiRs)-miR1, miR133a, miR133b, and miR206 were tested. Using such approach, we proved that transient overexpression of molecules selected by us modulated differentiation of mouse ESCs. Increase in miR145 levels upregulated Pax3, Pax7, Myod1, Myog, and MyHC2, while miR181 triggered the expression of such crucial myogenic factors as Myf5 and MyHC2. As a result, the ability of ESCs to initiate myogenic differentiation and form myotubes was enhanced. Premature expression of MyomiRs had, however, an adverse effect on myogenic differentiation of ESCs. Stem Cells 2018;36:655-670.

Myogenic potential of mouse embryonic stem cells lacking functional Pax7 tested in vitro by 5-azacitidine treatment and in vivo in regenerating skeletal muscle

Regeneration of skeletal muscle relies on the presence of satellite cells. Satellite cells deficiency accompanying some degenerative diseases is the reason for the search for the "replacement cells" that can be used in the muscle therapies. Due to their unique properties embryonic stem cells (ESCs), as well as myogenic cells derived from them, are considered as a promising source of therapeutic cells. Among the factors crucial for the specification of myogenic precursor cells is Pax7 that sustains proper function of satellite cells. In our previous studies we showed that ESCs lacking functional Pax7 are able to form myoblasts in vitro when differentiated within embryoid bodies and their outgrowths. In the current study we showed that ESCs lacking functional Pax7, cultured in vitro in monolayer in the medium supplemented with horse serum and 5azaC, expressed higher levels of factors associated with myogenesis, such as Pdgfra, Pax3, Myf5, and MyoD. Importantly, skeletal myosin immunolocalization confirmed that myogenic differentiation of ESCs was more effective in case of cells lacking Pax7. Our in vivo studies showed that ESCs transplanted into regenerating skeletal muscles were detectable at day 7 of regeneration and the number of Pax7-/- ESCs detected was significantly higher than of control cells. Our results support the concept that lack of functional Pax7 promotes proliferation of differentiating ESCs and for this reason more of them can turn into myogenic lineage.

Cell cycle regulation of embryonic stem cells and mouse embryonic fibroblasts lacking functional Pax7

The transcription factor Pax7 plays a key role during embryonic myogenesis and in adult organisms in that it sustains the proper function of satellite cells, which serve as adult skeletal muscle stem cells. Recently we have shown that lack of Pax7 does not prevent the myogenic differentiation of pluripotent stem cells. In the current work we show that the absence of functional Pax7 in differentiating embryonic stem cells modulates cell cycle facilitating their proliferation. Surprisingly, deregulation of Pax7 function also positively impacts at the proliferation of mouse embryonic fibroblasts. Such phenotypes seem to be executed by modulating the expression of positive cell cycle regulators, such as cyclin E.