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Small Molecules Boost Skeletal Muscle Differentiation

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Review of “Identification of Small Molecules Which Induce Skeletal Muscle Differentiation in Embryonic Stem Cells via Activation of the Wnt and Inhibition of Smad2/3 and Sonic Hedgehog Pathways” from Stem Cells by Stuart P. Atkinson

The production of muscle cells from human pluripotent stem cells (hPSCs) currently suffers from low differentiation efficiency [1, 2] or requires the expression of transgenes [3, 4]. Both these problems are barriers to the possible clinical application of muscle progenitors or fully differentiated myogenic cells. To get around these barriers, the laboratory of Matthias Mueller (Novartis Institute for Biomedical Research, Basel, Switzerland) designed a small molecule screen to identify compounds which act to induce skeletal muscle differentiation from embryonic stem cells (ESCs). Using Pax3 expression as a marker [3] they now describe the discovery of a new and potent inducer of skeletal muscle and its targets [5].

The study utilized an embryoid body (EB)-based differentiation screen to assess various compounds for the ability to upregulate Pax3 expression. From around a thousand tested compounds, the group identified two related compounds (denoted SMI1 and SMI2) with potent skeletal muscle promoting capabilities. Following treatment with these compounds, the authors saw a time-dependent increase in expression of the satellite cell marker Pax7, followed by the expression of markers of developed skeletal muscle (MyoD, Myf5, and Myogenin) which confirmed the myogenic induction capabilities of the SMI compounds.

So how do the SMIs boost skeletal muscle induction? The authors undertook gene expression analysis and signal pathway interrogation to show that the process requires Wnt pathway stimulation and Nodal pathway inhibition to start the process, followed by Shh signaling inhibition at a later time point. With this knowledge in hand, the authors attempted to recapitulate SMIs effects using specific inhibitors of the said pathways at distinct times. Encouragingly, the applied protocol (BIO/SB/Erismodegib (BSE) treatment) proved able to robustly induce skeletal muscle differentiation (See Myogenin and MHC expression in the adjoined figure) and, therefore, could represent an effective new strategy to generate this important tissue type.

High efficiency, low labor costs, high reliability, and no transgene expression - just what the skeletal muscle researcher ordered! Now this strategy needs to be recapitulated in human ESCs and induced pluripotent stem cells (iPSCs) and we also need to study the engraftment/functionality of the resultant cells. There is still work to do, but this a great first step.

References

  1. Kennedy KA, Porter T, Mehta V, et al. Retinoic acid enhances skeletal muscle progenitor formation and bypasses inhibition by bone morphogenetic protein 4 but not dominant negative beta-catenin. BMC Biol 2009;7:67.
  2. Ryan T, Liu J, Chu A, et al. Retinoic acid enhances skeletal myogenesis in human embryonic stem cells by expanding the premyogenic progenitor population. Stem Cell Rev 2012;8:482-493.
  3. Darabi R, Gehlbach K, Bachoo RM, et al. Functional skeletal muscle regeneration from differentiating embryonic stem cells. Nat Med 2008;14:134-143.
  4. Iacovino M, Bosnakovski D, Fey H, et al. Inducible cassette exchange: a rapid and efficient system enabling conditional gene expression in embryonic stem and primary cells. Stem Cells 2011;29:1580-1588.
  5. Lee H, Haller C, Manneville C, et al. Identification of Small Molecules Which Induce Skeletal Muscle Differentiation in Embryonic Stem Cells via Activation of the Wnt and Inhibition of Smad2/3 and Sonic Hedgehog Pathways. STEM CELLS 2016;34:299-310.