SummaryThere is no consensus in the stem cell field as to what constitutes the mature cardiac myocyte. Thus, helping formalize a molecular signature for cardiac myocyte maturation would advance the field. In the mammalian heart, inactivation of the “fetal” TNNI gene, TNNI1 (ssTnI), together in temporal concert with its stoichiometric replacement by the adult TNNI gene product, TNNI3 (cTnI), represents a quantifiable ratiometric maturation signature. We examined the TNNI isoform transition in human induced pluripotent stem cell (iPSC) cardiac myocytes (hiPSC-CMs) and found the fetal TNNI signature, even during long-term culture. Rodent stem cell-derived and primary myocytes, however, transitioned to the adult TnI profile. Acute genetic engineering of hiPSC-CMs enabled a rapid conversion toward the mature TnI profile. While there is no single marker to denote the mature cardiac myocyte, we propose that tracking the cTnI:ssTnI protein isoform ratio provides a valuable maturation signature to quantify myocyte maturation status across laboratories.

Summary Mesp1 is regarded as the master regulator of cardiovascular development, initiating the cardiac transcription factor cascade to direct the generation of cardiac mesoderm. To define the early embryonic cell population that responds to Mesp1, we performed pulse inductions of gene expression over tight temporal windows following embryonic stem cell differentiation. Remarkably, instead of promoting cardiac differentiation in the initial wave of mesoderm, Mesp1 binds to the Tal1 (Scl) +40k enhancer and generates Flk-1+ precursors expressing Etv2 (ER71) and Tal1 that undergo hematopoietic differentiation. The second wave of mesoderm responds to Mesp1 by differentiating into PDGFRα+ precursors that undergo cardiac differentiation. Furthermore, in the absence of serum-derived factors, Mesp1 promotes skeletal myogenic differentiation. Lineage tracing revealed that the majority of yolk sac and many adult hematopoietic cells derive from Mesp1+ precursors. Thus, Mesp1 is a context-dependent determination factor, integrating stage of differentiation and signaling environment to specify different lineage outcomes.

Derivation of functional skeletal muscle stem cells from pluripotent cells without genetic modification has proven elusive. Here we show that teratomas formed in adult skeletal muscle differentiate in vivo to produce large numbers of α7-Integrin+ VCAM-1+ myogenic progenitors. When FACS-purified and transplanted into diseased muscles, mouse teratoma-derived myogenic progenitors demonstrate very high engraftment potential. As few as 40,000 cells can reconstitute ∼80% of the tibialis anterior muscle volume. Newly generated fibers are innervated, express adult myosins, and ameliorate dystrophy-related force deficit and fatigability. Teratoma-derived myogenic progenitors also contribute quiescent PAX7+ muscle stem cells, enabling long-term maintenance of regenerated muscle and allowing muscle regeneration in response to subsequent injuries. Transcriptional profiling reveals that teratoma-derived myogenic progenitors undergo embryonic-to-adult maturation when they contribute to the stem cell compartment of regenerated muscle. Thus, teratomas are a rich and accessible source of potent transplantable skeletal muscle stem cells. VIDEO ABSTRACT.

Codevelopment of the lungs and heart underlies key evolutionary innovations in the transition to terrestrial life. Cardiac specializations that support pulmonary circulation, including the atrial septum, are generated by second heart field (SHF) cardiopulmonary progenitors (CPPs). It has been presumed that transcription factors required in the SHF for cardiac septation, e.g., Tbx5, directly drive a cardiac morphogenesis gene-regulatory network. Here, we report instead that TBX5 directly drives Wnt ligands to initiate a bidirectional signaling loop between cardiopulmonary mesoderm and the foregut endoderm for endodermal pulmonary specification and, subsequently, atrial septation. We show that Tbx5 is required for pulmonary specification in mice and amphibians but not for swim bladder development in zebrafish. TBX5 is non-cell-autonomously required for pulmonary endoderm specification by directly driving Wnt2 and Wnt2b expression in cardiopulmonary mesoderm. TBX5 ChIPsequencing identified cis-regulatory elements at Wnt2 sufficient for endogenous Wnt2 expression domains in vivo and required for Wnt2 expression in precardiac mesoderm in vitro. Tbx5 cooperated with Shh signaling to drive Wnt2b expression for lung morphogenesis. Tbx5 haploinsufficiency in mice, a model of Holt-Oram syndrome, caused a quantitative decrement of mesodermal-to-endodermal Wnt signaling and subsequent endodermal-to-mesodermal Shh signaling required for cardiac morphogenesis. Thus, Tbx5 initiates a mesodermendoderm-mesoderm signaling loop in lunged vertebrates that provides a molecular basis for the coevolution of pulmonary and cardiac structures required for terrestrial life. lung development | heart development | TBX5 | Wnt signaling | Hedgehog signaling

Facioscapulohumeral muscular dystrophy is a slowly progressive but devastating myopathy caused by loss of repression of the transcription factor DUX4; however, DUX4 expression is very low, and protein has not been detected directly in patient biopsies. Efforts to model DUX4 myopathy in mice have foundered either in being too severe, or in lacking muscle phenotypes. Here we show that the endogenous facioscapulohumeral muscular dystrophy-specific DUX4 polyadenylation signal is surprisingly inefficient, and use this finding to develop an facioscapulohumeral muscular dystrophy mouse model with muscle-specific doxycycline-regulated DUX4 expression. Very low expression levels, resulting in infrequent DUX4 + myonuclei, evoke a slow progressive degenerative myopathy. The degenerative process involves inflammation and a remarkable expansion in the fibroadipogenic progenitor compartment, leading to fibrosis. These animals also show high frequency hearing deficits and impaired skeletal muscle regeneration after injury. This mouse model will facilitate in vivo testing of therapeutics, and suggests the involvement of fibroadipogenic progenitors in facioscapulohumeral muscular dystrophy.

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SummaryRecent reports have documented the differentiation of human pluripotent stem cells toward the skeletal myogenic lineage using transgene- and cell purification-free approaches. Although these protocols generate myocytes, they have not demonstrated scalability, safety, and in vivo engraftment, which are key aspects for their future clinical application. Here we recapitulate one prominent protocol, and show that it gives rise to a heterogeneous cell population containing myocytes and other cell types. Upon transplantation, the majority of human donor cells could not contribute to myofiber formation. As a proof-of-principle, we incorporated the inducible PAX7 lentiviral system into this protocol, which then enabled scalable expansion of a homogeneous population of skeletal myogenic progenitors capable of forming myofibers in vivo. Our findings demonstrate the methods for scalable expansion of PAX7+ myogenic progenitors and their purification are critical for practical application to cell replacement treatment of muscle degenerative diseases.

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