Differentiation of pluripotent stem cells to muscle fiber to model Duchenne muscular dystrophy

Ablation of the dystrophin Dp71f alternative C-terminal variant increases sarcoma tumour cell aggressiveness

Alterations in Dp71 expression, the most ubiquitous dystrophin isoform, have been associated with patient survival across tumours. Intriguingly, in certain malignancies, Dp71 acts as a tumour suppressor, while manifesting oncogenic properties in others. This diversity could be explained by the expression of two Dp71 splice variants encoding proteins with distinct C-termini, each with specific properties. Expression of these variants has impeded the exploration of their unique roles. Using CRISPR/Cas9, we ablated the Dp71f variant with the alternative C-terminus in a sarcoma cell line not expressing the canonical C-terminal variant, and conducted molecular (RNAseq) and functional characterisation of the knockout cells. Dp71f ablation induced major transcriptomic alterations, particularly affecting the expression of genes involved in calcium signalling and ECM-receptor interaction pathways. The genome-scale metabolic analysis identified significant downregulation of glucose transport via membrane vesicle reaction (GLCter) and downregulated glycolysis/gluconeogenesis pathway. Functionally, these molecular changes corresponded with, increased calcium responses, cell adhesion, proliferation, survival under serum starvation and chemotherapeutic resistance. Knockout cells showed reduced GLUT1 protein expression, survival without attachment and their migration and invasion in vitro and in vivo were unaltered, despite increased matrix metalloproteinases release. Our findings emphasise the importance of alternative splicing of dystrophin transcripts and underscore the role of the Dp71f variant, which appears to govern distinct cellular processes frequently dysregulated in tumour cells. The loss of this regulatory mechanism promotes sarcoma cell survival and treatment resistance. Thus, Dp71f is a target for future investigations exploring the intricate functions of specific DMD transcripts in physiology and across malignancies.

Coordination between endoderm progression and mouse gastruloid elongation controls endodermal morphotype choice

Embryonic development is highly robust. Morphogenetic variability between embryos (under ideal conditions) is largely quantitative. This robustness stands in contrast to in vitro embryo-like models, which, like most organoids, can display a high degree of tissue morphogenetic variability. The source of this difference is not fully understood. We use the mouse gastruloid model to study the morphogenetic progression of definitive endoderm (DE) and its divergence. We first catalog the different morphologies and characterize their statistics. We then learn predictive models for DE morphotype based on earlier expression and morphology measurements. Finally, we analyze these models to identify key drivers of morphotype variability and devise gastruloid-specific and global interventions that can lower this variability and steer morphotype choice. In the process, we identify two types of coordination lacking in the in vitro model but required for robust gut-tube formation. This approach can help improve the quality and usability of 3D embryo-like models.

Cellular interactions and microenvironment dynamics in skeletal muscle regeneration and disease

Skeletal muscle regeneration relies on the intricate interplay of various cell populations within the muscle niche-an environment crucial for regulating the behavior of muscle stem cells (MuSCs) and ensuring postnatal tissue maintenance and regeneration. This review delves into the dynamic interactions among key players of this process, including MuSCs, macrophages (MPs), fibro-adipogenic progenitors (FAPs), endothelial cells (ECs), and pericytes (PCs), each assuming pivotal roles in orchestrating homeostasis and regeneration. Dysfunctions in these interactions can lead not only to pathological conditions but also exacerbate muscular dystrophies. The exploration of cellular and molecular crosstalk among these populations in both physiological and dystrophic conditions provides insights into the multifaceted communication networks governing muscle regeneration. Furthermore, this review discusses emerging strategies to modulate the muscle-regenerating niche, presenting a comprehensive overview of current understanding and innovative approaches.

Generation of Skeletal Muscle Organoids from Human Pluripotent Stem Cells

Various protocols have been proven effective in the directed differentiation of mouse and human pluripotent stem cells into skeletal muscles and used to study myogenesis. Current 2D myogenic differentiation protocols can mimic muscle development and its alteration under pathological conditions such as muscular dystrophies. 3D skeletal muscle differentiation approaches can, in addition, model the interaction between the various cell types within the developing organoid. Our protocol ensures the differentiation of human embryonic/induced pluripotent stem cells (hESC/hiPSC) into skeletal muscle organoids (SMO) via cells with paraxial mesoderm and neuromesodermal progenitors' identity and further production of organized structures of the neural plate margin and the dermomyotome. Continuous culturing omits neural lineage differentiation and promotes fetal myogenesis, including the maturation of fibroadipogenic progenitors and PAX7-positive myogenic progenitors. The PAX7 progenitors resemble the late fetal stages of human development and, based on single-cell transcriptomic profiling, cluster close to adult satellite cells of primary muscles. To overcome the limited availability of muscle biopsies from patients with muscular dystrophy during disease progression, we propose to use the SMO system, which delivers a stable population of skeletal muscle progenitors from patient-specific iPSCs to investigate human myogenesis in healthy and diseased conditions. Key features • Development of skeletal muscle organoid differentiation from human pluripotent stem cells, which recapitulates myogenesis. • Analysis of early embryonic and fetal myogenesis. • Provision of skeletal muscle progenitors for in vitro and in vivo analysis for up to 14 weeks of organoid culture. • In vitro myogenesis from patient-specific iPSCs allows to overcome the bottleneck of muscle biopsies of patients with pathological conditions.

Heparan Sulfate Chain-Conjugated Laminin-E8 Fragments Advance Paraxial Mesodermal Differentiation Followed by High Myogenic Induction from hiPSCs

Human-induced pluripotent stem cells (hiPSCs) have great therapeutic potential. The cell source differentiated from hiPSCs requires xeno-free and robust methods for lineage-specific differentiation. Here, a system is described for differentiating hiPSCs on new generation laminin fragments (NGLFs), a recombinant form of a laminin E8 fragment conjugated to the heparan sulfate chains (HS) attachment domain of perlecan. Using NGLFs, hiPSCs are highly promoted to direct differentiation into a paraxial mesoderm state with high-efficiency muscle lineage generation. HS conjugation to the C-terminus of Laminin E8 fragments brings fibroblast growth factors (FGFs) bound to the HS close to the cell surface of hiPSCs, thereby facilitating stronger FGF signaling pathways stimulation and initiating HOX gene expression, which triggers the paraxial mesoderm differentiation of hiPSCs. This highly efficient differentiation system can provide a roadmap for paraxial mesoderm development and an infinite source of myocytes and muscle stem cells for disease modeling and regenerative medicine.

The adult environment promotes the transcriptional maturation of human iPSC-derived muscle grafts

Pluripotent stem cell (PSC)-based cell therapy is an attractive option for the treatment of multiple human disorders, including muscular dystrophies. While in vitro differentiating PSCs can generate large numbers of human lineage-specific tissue, multiple studies evidenced that these cell populations mostly display embryonic/fetal features. We previously demonstrated that transplantation of PSC-derived myogenic progenitors provides long-term engraftment and functional improvement in several dystrophic mouse models, but it remained unknown whether donor-derived myofibers mature to match adult tissue. Here, we transplanted iPAX7 myogenic progenitors into muscles of non-dystrophic and dystrophic mice and compared the transcriptional landscape of human grafts with respective in vitro-differentiated iPAX7 myotubes as well as human skeletal muscle biospecimens. Pairing bulk RNA sequencing with computational deconvolution of human reads, we were able to pinpoint key myogenic changes that occur during the in vitro-to-in vivo transition, confirm developmental maturity, and consequently evaluate their applicability for cell-based therapies.

Self-organization of vascularized skeletal muscle from bovine embryonic stem cells

Cultured beef holds promising potential as an alternative to traditional meat options. While adult stem cells are commonly used as the cell source for cultured beef, their proliferation and differentiation capacities are limited. To produce cultured beef steaks, current manufacturing plans often require the separate preparation of multiple cell types and intricate engineering for assembling them into structured tissues. In this study, we propose and report the co-induction of skeletal muscle, neuronal, and endothelial cells from bovine embryonic stem cells (ESCs) and the self-organization of tissue structures in 2- and 3-dimensional cultures. Bovine myocytes were induced in a stepwise manner through the induction of presomitic mesoderm (PSM) from bovine ESCs. Muscle fibers with sarcomeres appeared within 15 days, displaying calcium oscillations responsive to inputs from co-induced bovine spinal neurons. Bovine endothelial cells were also co-induced via PSM, forming uniform vessel networks inside tissues. Our serum-free, rapid co-induction protocols represent a milestone toward self-organizing beef steaks with integrated vasculature and innervation.

Functional specialisation and coordination of myonuclei

Myofibres serve as the functional unit for locomotion, with the sarcomere as fundamental subunit. Running the entire length of this structure are hundreds of myonuclei, located at the periphery of the myofibre, juxtaposed to the plasma membrane. Myonuclear specialisation and clustering at the centre and ends of the fibre are known to be essential for muscle contraction, yet the molecular basis of this regionalisation has remained unclear. While the 'myonuclear domain hypothesis' helped explain how myonuclei can independently govern large cytoplasmic territories, novel technologies have provided granularity on the diverse transcriptional programs running simultaneously within the syncytia and added a new perspective on how myonuclei communicate. Building upon this, we explore the critical cellular and molecular sources of transcriptional and functional heterogeneity within myofibres, discussing the impact of intrinsic and extrinsic factors on myonuclear programs. This knowledge provides new insights for understanding muscle development, repair, and disease, but also opens avenues for the development of novel and precise therapeutic approaches.

Motor neurons and endothelial cells additively promote development and fusion of human iPSC-derived skeletal myocytes

BACKGROUND

Neurovascular cells have wide-ranging implications on skeletal muscle biology regulating myogenesis, maturation, and regeneration. Although several in vitro studies have investigated how motor neurons and endothelial cells interact with skeletal myocytes independently, there is limited knowledge about the combined effect of neural and vascular cells on muscle maturation and development.

METHODS

Here, we report a triculture system comprising human-induced pluripotent stem cell (iPSC)-derived skeletal myocytes, human iPSC-derived motor neurons, and primary human endothelial cells maintained under controlled media conditions. Briefly, iPSCs were differentiated to generate skeletal muscle progenitor cells (SMPCs). These SMPCs were seeded at a density of 5 × 104 cells/well in 12-well plates and allowed to differentiate for 7 days before adding iPSC-derived motor neurons at a concentration of 0.5 × 104 cells/well. The neuromuscular coculture was maintained for another 7 days in coculture media before addition of primary human umbilical vein endothelial cells (HUVEC) also at 0.5 × 104 cells/well. The triculture was maintained for another 7 days in triculture media comprising equal portions of muscle differentiation media, coculture media, and vascular media. Extensive morphological, genetic, and molecular characterization was performed to understand the combined and individual effects of neural and vascular cells on skeletal muscle maturation.

RESULTS

We observed that motor neurons independently promoted myofiber fusion, upregulated neuromuscular junction genes, and maintained a molecular niche supportive of muscle maturation. Endothelial cells independently did not support myofiber fusion and downregulated expression of LRP4 but did promote expression of type II specific myosin isoforms. However, neurovascular cells in combination exhibited additive increases in myofiber fusion and length, enhanced production of Agrin, along with upregulation of several key genes like MUSK, RAPSYN, DOK-7, and SLC2A4. Interestingly, more divergent effects were observed in expression of genes like MYH8, MYH1, MYH2, MYH4, and LRP4 and secretion of key molecular factors like amphiregulin and IGFBP-4.

CONCLUSIONS

Neurovascular cells when cultured in combination with skeletal myocytes promoted myocyte fusion with concomitant increase in expression of various neuromuscular genes. This triculture system may be used to gain a deeper understanding of the effects of the neurovascular niche on skeletal muscle biology and pathophysiology.

Cellular and molecular control of vertebrate somitogenesis

Segmentation is a fundamental feature of the vertebrate body plan. This metameric organization is first implemented by somitogenesis in the early embryo, when paired epithelial blocks called somites are rhythmically formed to flank the neural tube. Recent advances in in vitro models have offered new opportunities to elucidate the mechanisms that underlie somitogenesis. Notably, models derived from human pluripotent stem cells introduced an efficient proxy for studying this process during human development. In this Review, we summarize the current understanding of somitogenesis gained from both in vivo studies and in vitro studies. We deconstruct the spatiotemporal dynamics of somitogenesis into four distinct modules: dynamic events in the presomitic mesoderm, segmental determination, somite anteroposterior polarity patterning, and epithelial morphogenesis. We first focus on the segmentation clock, as well as signalling and metabolic gradients along the tissue, before discussing the clock and wavefront and other models that account for segmental determination. We then detail the molecular and cellular mechanisms of anteroposterior polarity patterning and somite epithelialization.

Muscle stem cell dysfunction in rhabdomyosarcoma and muscular dystrophy

Muscle stem cells (MuSCs) are crucial to the repair and homeostasis of mature skeletal muscle. MuSC dysfunction and dysregulation of the myogenic program can contribute to the development of pathology ranging from cancers like rhabdomyosarcoma (RMS) or muscle degenerative diseases such as Duchenne muscular dystrophy (DMD). Both diseases exhibit dysregulation at nearly all steps of myogenesis. For instance, MuSC self-renewal processes are altered. In RMS, this leads to the creation of tumor propagating cells. In DMD, impaired asymmetric stem cell division creates a bias towards producing self-renewing stem cells instead of committing to differentiation. Hyperproliferation of these cells contribute to tumorigenesis in RMS and symmetric expansion of the self-renewing MuSC population in DMD. Both diseases also exhibit a repression of factors involved in terminal differentiation, halting RMS cells in the proliferative stage and thus driving tumor growth. Conversely, the MuSCs in DMD exhibit impaired differentiation and fuse prematurely, affecting myonuclei maturation and the integrity of the dystrophic muscle fiber. Finally, both disease states cause alterations to the MuSC niche. Various elements of the niche such as inflammatory and migratory signaling that impact MuSC behavior are dysregulated. Here we show how these seemingly distantly related diseases indeed have similarities in MuSC dysfunction, underlying the importance of considering MuSCs when studying the pathophysiology of muscle diseases.

Notch signalling influences cell fate decisions and HOX gene induction in axial progenitors

The generation of the post-cranial embryonic body relies on the coordinated production of spinal cord neurectoderm and presomitic mesoderm cells from neuromesodermal progenitors (NMPs). This process is orchestrated by pro-neural and pro-mesodermal transcription factors that are co-expressed in NMPs together with Hox genes, which are essential for axial allocation of NMP derivatives. NMPs reside in a posterior growth region, which is marked by the expression of Wnt, FGF and Notch signalling components. Although the importance of Wnt and FGF in influencing the induction and differentiation of NMPs is well established, the precise role of Notch remains unclear. Here, we show that the Wnt/FGF-driven induction of NMPs from human embryonic stem cells (hESCs) relies on Notch signalling. Using hESC-derived NMPs and chick embryo grafting, we demonstrate that Notch directs a pro-mesodermal character at the expense of neural fate. We show that Notch also contributes to activation of HOX gene expression in human NMPs, partly in a non-cell-autonomous manner. Finally, we provide evidence that Notch exerts its effects via the establishment of a negative-feedback loop with FGF signalling.

Generation of highly pure pluripotent stem cell-derived myogenic progenitor cells and myotubes

Driving efficient and pure skeletal muscle cell differentiation from pluripotent stem cells (PSCs) has been challenging. Here, we report an optimized protocol that generates skeletal muscle progenitor cells with high efficiency and purity in a short period of time. Human induced PSCs (hiPSCs) and murine embryonic stem cells (mESCs) were specified into the mesodermal myogenic fate using distinct and species-specific protocols. We used a specific maturation medium to promote the terminal differentiation of both human and mouse myoblast populations, and generated myotubes associated with a large pool of cell-cycle arrested PAX7+ cells. We also show that myotube maturation is modulated by dish-coating properties, cell density, and percentage of myogenic progenitor cells. Given the high efficiency in the generation of myogenic progenitors and differentiated myofibers, this protocol provides an attractive strategy for tissue engineering, modeling of muscle dystrophies, and evaluation of new therapeutic approaches in vitro.

Metabolic Changes during In Vivo Maturation of PSC-Derived Skeletal Myogenic Progenitors

In vitro-generated pluripotent stem cell (PSC)-derived Pax3-induced (iPax3) myogenic progenitors display an embryonic transcriptional signature, but upon engraftment, the profile of re-isolated iPax3 donor-derived satellite cells changes toward similarity with postnatal satellite cells, suggesting that engrafted PSC-derived myogenic cells remodel their transcriptional signature upon interaction within the adult muscle environment. Here, we show that engrafted myogenic progenitors also remodel their metabolic state. Assessment of oxygen consumption revealed that exposure to the adult muscle environment promotes overt changes in mitochondrial bioenergetics, as shown by the substantial suppression of energy requirements in re-isolated iPax3 donor-derived satellite cells compared to their in vitro-generated progenitors. Mass spectrometry-based metabolomic profiling further confirmed the relationship of engrafted iPax3 donor-derived cells to adult satellite cells. The fact that in vitro-generated myogenic progenitors remodel their bioenergetic signature upon in vivo exposure to the adult muscle environment may have important implications for therapeutic applications.

Efficient generation of a self-organizing neuromuscular junction model from human pluripotent stem cells

The complex neuromuscular network that controls body movements is the target of severe diseases that result in paralysis and death. Here, we report the development of a robust and efficient self-organizing neuromuscular junction (soNMJ) model from human pluripotent stem cells that can be maintained long-term in simple adherent conditions. The timely application of specific patterning signals instructs the simultaneous development and differentiation of position-specific brachial spinal neurons, skeletal muscles, and terminal Schwann cells. High-content imaging reveals self-organized bundles of aligned muscle fibers surrounded by innervating motor neurons that form functional neuromuscular junctions. Optogenetic activation and pharmacological interventions show that the spinal neurons actively instruct the synchronous skeletal muscle contraction. The generation of a soNMJ model from spinal muscular atrophy patient-specific iPSCs reveals that the number of NMJs and muscle contraction is severely affected, resembling the patient's pathology. In the future, the soNMJ model could be used for high-throughput studies in disease modeling and drug development. Thus, this model will allow us to address unmet needs in the neuromuscular disease field.

'Enhancing' skeletal muscle and stem cells in three-dimensions: genome regulation of skeletal muscle in development and disease

The noncoding genome imparts important regulatory control over gene expression. In particular, gene enhancers represent a critical layer of control that integrates developmental and differentiation signals outside the cell into transcriptional outputs inside the cell. Recently, there has been an explosion in genomic techniques to probe enhancer control, function, and regulation. How enhancers are regulated and integrate signals in stem cell development and differentiation is largely an open question. In this review, we focus on the role gene enhancers play in muscle stem cell specification, differentiation, and progression. We pay specific attention toward the identification of muscle-specific enhancers, the binding of transcription factors to these enhancers, and how enhancers communicate to their target genes via three-dimensional looping.

Regenerating human skeletal muscle forms an emerging niche in vivo to support PAX7 cells

Skeletal muscle stem and progenitor cells including those derived from human pluripotent stem cells (hPSCs) offer an avenue towards personalized therapies and readily fuse to form human-mouse myofibres in vivo. However, skeletal muscle progenitor cells (SMPCs) inefficiently colonize chimeric stem cell niches and instead associate with human myofibres resembling foetal niches. We hypothesized competition with mouse satellite cells (SCs) prevented SMPC engraftment into the SC niche and thus generated an SC ablation mouse compatible with human engraftment. Single-nucleus RNA sequencing of SC-ablated mice identified the absence of a transient myofibre subtype during regeneration expressing Actc1. Similarly, ACTC1+ human myofibres supporting PAX7+ SMPCs increased in SC-ablated mice, and after re-injury we found SMPCs could now repopulate into chimeric niches. To demonstrate ACTC1+ myofibres are essential to supporting PAX7 SMPCs, we generated caspase-inducible ACTC1 depletion human pluripotent stem cells, and upon SMPC engraftment we found a 90% reduction in ACTC1+ myofibres and a 100-fold decrease in PAX7 cell numbers compared with non-induced controls. We used spatial RNA sequencing to identify key factors driving emerging human niche formation between ACTC1+ myofibres and PAX7+ SMPCs in vivo. This revealed that transient regenerating human myofibres are essential for emerging niche formation in vivo to support PAX7 SMPCs.

The Clockwork Embryo: Mechanisms Regulating Developmental Rate

Organismal development requires the reproducible unfolding of an ordered sequence of discrete steps (cell fate determination, migration, tissue folding, etc.) in both time and space. Here, we review the mechanisms that grant temporal specificity to developmental steps, including molecular clocks and timers. Individual timing mechanisms must be coordinated with each other to maintain the overall developmental sequence. However, phenotypic novelties can also arise through the modification of temporal patterns over the course of evolution. Two main types of variation in temporal patterning characterize interspecies differences in developmental time: allochrony, where the overall developmental sequence is either accelerated or slowed down while maintaining the relative duration of individual steps, and heterochrony, where the duration of specific developmental steps is altered relative to the rest. New advances in in vitro modeling of mammalian development using stem cells have recently enabled the revival of mechanistic studies of allochrony and heterochrony. In both cases, differences in the rate of basic cellular functions such as splicing, translation, protein degradation, and metabolism seem to underlie differences in developmental time. In the coming years, these studies should identify the genetic differences that drive divergence in developmental time between species.

Human skeletal muscle organoids model fetal myogenesis and sustain uncommitted PAX7 myogenic progenitors

In vitro culture systems that structurally model human myogenesis and promote PAX7+ myogenic progenitor maturation have not been established. Here we report that human skeletal muscle organoids can be differentiated from induced pluripotent stem cell lines to contain paraxial mesoderm and neuromesodermal progenitors and develop into organized structures reassembling neural plate border and dermomyotome. Culture conditions instigate neural lineage arrest and promote fetal hypaxial myogenesis toward limb axial anatomical identity, with generation of sustainable uncommitted PAX7 myogenic progenitors and fibroadipogenic (PDGFRa+) progenitor populations equivalent to those from the second trimester of human gestation. Single-cell comparison to human fetal and adult myogenic progenitor /satellite cells reveals distinct molecular signatures for non-dividing myogenic progenitors in activated (CD44High/CD98+/MYOD1+) and dormant (PAX7High/FBN1High/SPRY1High) states. Our approach provides a robust 3D in vitro developmental system for investigating muscle tissue morphogenesis and homeostasis.

CRISPR/Cas9 Genome Editing in LGMD2A/R1 Patient-Derived Induced Pluripotent Stem and Skeletal Muscle Progenitor Cells

Large numbers of Calpain 3 (CAPN3) mutations cause recessive forms of limb-girdle muscular dystrophy (LGMD2A/LGMDR1) with selective atrophy of the proximal limb muscles. We have generated induced pluripotent stem cells (iPSC) from a patient with two mutations in exon 3 and exon 4 at the calpain 3 locus (W130C, 550delA). Two different strategies to rescue these mutations are devised: (i) on the level of LGMD2A-iPSC, we combined CRISPR/Cas9 genome targeting with a FACS and Tet transactivator-based biallelic selection strategy, which resulted in a new functional chimeric exon 3-4 without the two CAPN3 mutations. (ii) On the level of LGMD2A-iPSC-derived CD82+/Pax7+ myogenic progenitor cells, we demonstrate CRISPR/Cas9 mediated rescue of the highly prevalent exon 4 CAPN3 mutation. The first strategy specifically provides isogenic LGMD2A corrected iPSC for disease modelling, and the second strategy can be further elaborated for potential translational approaches.

Reconstructing human brown fat developmental trajectory in vitro

Brown adipocytes (BAs) represent a specialized cell type that is able to uncouple nutrient catabolism from ATP generation to dissipate energy as heat. In humans, the brown fat tissue is composed of discrete depots found throughout the neck and trunk region. BAs originate from a precursor common to skeletal muscle, but their developmental trajectory remains poorly understood. Here, we used single-cell RNA sequencing to characterize the development of interscapular brown fat in mice. Our analysis identified a transient stage of BA differentiation characterized by the expression of the transcription factor GATA6. We show that recapitulating the sequence of signaling cues identified in mice can lead to efficient differentiation of BAs in vitro from human pluripotent stem cells. These precursors can in turn be efficiently converted into functional BAs that can respond to signals mimicking adrenergic stimuli by increasing their metabolism, resulting in heat production.

Antioxidants restore store-operated Ca2+ entry in patient-iPSC-derived myotubes with tubular aggregate myopathy-associated Ile484ArgfsX21 STIM1 mutation via upregulation of binding immunoglobulin protein

Store-operated Ca2+ entry (SOCE) is indispensable for intracellular Ca2+ homeostasis in skeletal muscle, and constitutive activation of SOCE causes tubular aggregate myopathy (TAM). To understand the pathogenesis of TAM, we induced pluripotent stem cells (iPSCs) from a TAM patient with a rare mutation (c.1450_1451insGA; p. Ile484ArgfsX21) in the STIM1 gene. This frameshift mutation produces a truncated STIM1 with a disrupted C-terminal inhibitory domain (CTID) and was reported to diminish SOCE. Myotubes induced from the patient's-iPSCs (TAM myotubes) showed severely impaired SOCE, but antioxidants greatly restored SOCE partly via upregulation of an endoplasmic reticulum (ER) chaperone, BiP (GRP78), in the TAM myotubes. Our observation suggests that antioxidants are promising tools for treatment of TAM caused by reduced SOCE.

A transcriptional and regulatory map of mouse somite maturation

The mammalian body plan is shaped by rhythmic segmentation of mesoderm into somites, which are transient embryonic structures that form down each side of the neural tube. We have analyzed the genome-wide transcriptional and chromatin dynamics occurring within nascent somites, from early inception of somitogenesis to the latest stages of body plan establishment. We created matched gene expression and open chromatin maps for the three leading pairs of somites at six time points during mouse embryonic development. We show that the rate of somite differentiation accelerates as development progresses. We identified a conserved maturation program followed by all somites, but somites from more developed embryos concomitantly switch on differentiation programs from derivative cell lineages soon after segmentation. Integrated analysis of the somitic transcriptional and chromatin activities identified opposing regulatory modules controlling the onset of differentiation. Our results provide a powerful, high-resolution view of the molecular genetics underlying somitic development in mammals.

Bioprocessing Considerations towards the Manufacturing of Therapeutic Skeletal and Smooth Muscle Cells

Tissue engineering approaches within the muscle context represent a promising emerging field to address the current therapeutic challenges related with multiple pathological conditions affecting the muscle compartments, either skeletal muscle or smooth muscle, responsible for involuntary and voluntary contraction, respectively. In this review, several features and parameters involved in the bioprocessing of muscle cells are addressed. The cell isolation process is depicted, depending on the type of tissue (smooth or skeletal muscle), followed by the description of the challenges involving the use of adult donor tissue and the strategies to overcome the hurdles of reaching relevant cell numbers towards a clinical application. Specifically, the use of stem/progenitor cells is highlighted as a source for smooth and skeletal muscle cells towards the development of a cellular product able to maintain the target cell's identity and functionality. Moreover, taking into account the need for a robust and cost-effective bioprocess for cell manufacturing, the combination of muscle cells with biomaterials and the need for scale-up envisioning clinical applications are also approached.

Ex vivo adult stem cell characterization from multiple muscles in ambulatory children with cerebral palsy during early development of contractures

Cerebral palsy (CP) is one of the most common conditions leading to lifelong childhood physical disability. Literature reported previously altered muscle properties such as lower number of satellite cells (SCs), with altered fusion capacity. However, these observations highly vary among studies, possibly due to heterogeneity in patient population, lack of appropriate control data, methodology and different assessed muscle. In this study we aimed to strengthen previous observations and to understand the heterogeneity of CP muscle pathology. Myogenic differentiation of SCs from the Medial Gastrocnemius (MG) muscle of patients with CP (n = 16, 3-9 years old) showed higher fusion capacity compared to age-matched typically developing children (TD, n = 13). Furthermore, we uniquely assessed cells of two different lower limb muscles and showed a decreased myogenic potency in cells from the Semitendinosus (ST) compared to the MG (TD: n = 3, CP: n = 6). Longitudinal assessments, one year after the first botulinum toxin treatment, showed slightly reduced SC representations and lower fusion capacity (n = 4). Finally, we proved the robustness of our data, by assessing in parallel the myogenic capacity of two samples from the same TD muscle. In conclusion, these data confirmed previous findings of increased SC fusion capacity from MG muscle of young patients with CP compared to age-matched TD. Further elaboration is reported on potential factors contributing to heterogeneity, such as assessed muscle, CP progression and reliability of primary outcome parameters.

Population-level antagonism between FGF and BMP signaling steers mesoderm differentiation in embryonic stem cells

The mesodermal precursor populations for different internal organ systems are specified during gastrulation by the combined activity of extracellular signaling systems such as BMP, Wnt, Nodal and FGF. The BMP, Wnt and Nodal signaling requirements for the differentiation of specific mesoderm subtypes in mammals have been mapped in detail, but how FGF shapes mesodermal cell type diversity is not precisely known. It is also not clear how FGF signaling integrates with the activity of other signaling systems involved in mesoderm differentiation. Here, we address these questions by analyzing the effects of targeted signaling manipulations in differentiating stem cell populations at single-cell resolution. We identify opposing functions of BMP and FGF, and map FGF-dependent and -independent mesodermal lineages. Stimulation with exogenous FGF boosts the expression of endogenous Fgf genes while repressing Bmp ligand genes. This positive autoregulation of FGF signaling, coupled with the repression of BMP signaling, may contribute to the specification of reproducible and coherent cohorts of cells with the same identity via a community effect, both in the embryo and in synthetic embryo-like systems.

Chemokine/ITGA4 Interaction Directs iPSC-Derived Myogenic Progenitor Migration to Injury Sites in Aging Muscle for Regeneration

The failure of muscle to repair after injury during aging may be a major contributor to muscle mass loss. We recently generated muscle progenitor cells (MPCs) from human-induced pluripotent stem-cell (iPSC) cell lines using small molecules, CHIR99021 and Givinostat (Givi-MPCs) sequentially. Here, we test whether the chemokines overexpressed in injured endothelial cells direct MPC migration to the site by binding to their receptor, ITGA4. ITGA4 was heavily expressed in Givi-MPCs. To study the effects on the mobilization of Givi-MPCs, ITGA4 was knocked down by an ITGA4 shRNA lentiviral vector. With and without ITGA4 knocked down, cell migration in vitro and cell mobilization in vivo using aged NOD scid gamma (NSG) mice and mdx/scid mice were analyzed. The migration of shITGA4-Givi-MPCs was significantly impaired, as shown in a wound-healing assay. The knockdown of ITGA4 impaired the migration of Givi-MPCs towards human aortic endothelial cells (HAECs), in which CX3CL1 and VCAM-1 were up-regulated by the treatment of TNF-α compared with scramble ones using a transwell system. MPCs expressing ITGA4 sensed chemokines secreted by endothelial cells at the injury site as a chemoattracting signal to migrate to the injured muscle. The mobilization of Givi-MPCs was mediated by the ligand-receptor interaction, which facilitated their engraftment for repairing the sarcopenic muscle with injury.

Oct1 cooperates with the Smad family of transcription factors to promote mesodermal lineage specification

The transition between pluripotent and tissue-specific states is a key aspect of development. Understanding the pathways driving these transitions will facilitate the engineering of properly differentiated cells for experimental and therapeutic uses. Here, we showed that during mesoderm differentiation, the transcription factor Oct1 activated developmental lineage-appropriate genes that were silent in pluripotent cells. Using mouse embryonic stem cells (ESCs) with an inducible knockout of Oct1, we showed that Oct1 deficiency resulted in poor induction of mesoderm-specific genes, leading to impaired mesodermal and terminal muscle differentiation. Oct1-deficient cells exhibited poor temporal coordination of the induction of lineage-specific genes and showed inappropriate developmental lineage branching, resulting in poorly differentiated cell states retaining epithelial characteristics. In ESCs, Oct1 localized with the pluripotency factor Oct4 at mesoderm-associated genes and remained bound to those loci during differentiation after the dissociation of Oct4. Binding events for Oct1 overlapped with those for the histone lysine demethylase Utx, and an interaction between Oct1 and Utx suggested that these two proteins cooperate to activate gene expression. The specificity of the ubiquitous Oct1 for the induction of mesodermal genes could be partially explained by the frequent coexistence of Smad and Oct binding sites at mesoderm-specific genes and the cooperative stimulation of mesodermal gene transcription by Oct1 and Smad3. Together, these results identify Oct1 as a key mediator of mesoderm lineage-specific gene induction.

Protocols to generate and isolate mouse myogenic progenitors both in vitro and in vivo

Mouse embryonic stem cells (mESCs) can be directed to acquire cell-lineage-specific genetic programs and phenotypes by stepwise exposure to defined factors, allowing the development of in vitro models for studying disease and tissue generation. In this protocol, we describe the use of cultured mESCs to generate presomitic-like mesoderm cells undergoing further specification towards myogenic progenitors (MPs). Further, we describe here a procedure to obtain, dissect, and fluorescence-activated cell sorting (FACS)-isolate somitic cells from genetically labeled Pax7+/Cre; Rosa26YFP/+ embryos. For complete details on the use and execution of this protocol, please refer to Khateb et al.1.

Biological Significance of the Coupling Delay in Synchronized Oscillations

The significance of the coupling delay, which is the time required for interactions between coupled oscillators, in various oscillatory dynamics has been investigated mathematically for more than three decades, but its biological significance has been revealed only recently. In the segmentation clock, which regulates the periodic formation of somites in embryos, Hes7 expression oscillates synchronously between neighboring presomitic mesoderm (PSM) cells, and this synchronized oscillation is controlled by Notch signaling-mediated coupling between PSM cells. Recent studies have shown that inappropriate coupling delays dampen and desynchronize Hes7 oscillations, as simulated mathematically, leading to the severe fusion of somites and somite-derived tissues such as the vertebrae and ribs. These results indicate the biological significance of the coupling delay in synchronized Hes7 oscillations in the segmentation clock. The recent development of an in vitro PSM-like system will facilitate the detailed analysis of the coupling delay in synchronized oscillations.

Pluripotent Stem Cells in Disease Modeling and Drug Discovery for Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is a progressive multisystemic disease caused by the expansion of a CTG repeat tract within the 3' untranslated region (3' UTR) of the dystrophia myotonica protein kinase gene (DMPK). Although DM1 is considered to be the most frequent myopathy of genetic origin in adults, DM1 patients exhibit a vast diversity of symptoms, affecting many different organs. Up until now, different in vitro models from patients' derived cells have largely contributed to the current understanding of DM1. Most of those studies have focused on muscle physiopathology. However, regarding the multisystemic aspect of DM1, there is still a crucial need for relevant cellular models to cover the whole complexity of the disease and open up options for new therapeutic approaches. This review discusses how human pluripotent stem cell-based models significantly contributed to DM1 mechanism decoding, and how they provided new therapeutic strategies that led to actual phase III clinical trials.

Reconstruction and deconstruction of human somitogenesis in vitro

The vertebrate body displays a segmental organization that is most conspicuous in the periodic organization of the vertebral column and peripheral nerves. This metameric organization is first implemented when somites, which contain the precursors of skeletal muscles and vertebrae, are rhythmically generated from the presomitic mesoderm. Somites then become subdivided into anterior and posterior compartments that are essential for vertebral formation and segmental patterning of the peripheral nervous system^1-4. How this key somitic subdivision is established remains poorly understood. Here we introduce three-dimensional culture systems of human pluripotent stem cells called somitoids and segmentoids, which recapitulate the formation of somite-like structures with anteroposterior identity. We identify a key function of the segmentation clock in converting temporal rhythmicity into the spatial regularity of anterior and posterior somitic compartments. We show that an initial 'salt and pepper' expression of the segmentation gene MESP2 in the newly formed segment is transformed into compartments of anterior and posterior identity through an active cell-sorting mechanism. Our research demonstrates that the major patterning modules that are involved in somitogenesis, including the clock and wavefront, anteroposterior polarity patterning and somite epithelialization, can be dissociated and operate independently in our in vitro systems. Together, we define a framework for the symmetry-breaking process that initiates somite polarity patterning. Our work provides a platform for decoding general principles of somitogenesis and advancing knowledge of human development.

The emergence of the stem cell niche

Stem cell niches are composed of dynamic microenvironments that support stem cells over a lifetime. The emerging niche is distinct from the adult because its main role is to support the progenitors that build organ systems in development. Emerging niches mature through distinct stages to form the adult niche and enable proper stem cell support. As a model of emerging niches, this review highlights how differences in the skeletal muscle microenvironment influence emerging versus satellite cell (SC) niche formation in skeletal muscle, which is among the most regenerative tissue systems. We contrast how stem cell niches regulate intrinsic properties between progenitor and stem cells throughout development to adulthood. We describe new applications for generating emerging niches from human pluripotent stem cells (hPSCs) using developmental principles and highlight potential applications for regeneration and therapeutics.

Chicken genome editing for investigating poultry pathogens

Major advances in pathogen identification, treatment, vaccine development, and avian immunology have enabled the enormous expansion in global poultry production over the last 50 years. Looking forward, climate change, reduced feed, reduced water access, new avian pathogens and restrictions on the use of antimicrobials threaten to hamper further gains in poultry productivity and health. The development of novel in vitro cell culture systems, coupled with new genetic tools to investigate gene function, will aid in developing novel interventions for existing and newly emerging poultry pathogens. Our growing capacity to cryopreserve and generate genome-edited chicken lines will also be useful for developing improved chicken breeds for poultry farmers and conserving chicken genetic resources.

Efficient Muscle Regeneration by Human PSC-Derived CD82+ ERBB3+ NGFR+ Skeletal Myogenic Progenitors

Differentiation of pluripotent stem cells (PSCs) is a promising approach to obtaining large quantities of skeletal myogenic progenitors for disease modeling and cell-based therapy. However, generating skeletal myogenic cells with high regenerative potential is still challenging. We recently reported that skeletal myogenic progenitors generated from mouse PSC-derived teratomas possess robust regenerative potency. We have also found that teratomas derived from human PSCs contain a skeletal myogenic population. Here, we showed that these human PSC-derived skeletal myogenic progenitors had exceptional engraftability. A combination of cell surface markers, CD82, ERBB3, and NGFR enabled efficient purification of skeletal myogenic progenitors. These cells expressed PAX7 and were able to differentiate into MHC+ multinucleated myotubes. We further discovered that these cells are expandable in vitro. Upon transplantation, the expanded cells formed new dystrophin⁺ fibers that reconstituted almost ¾ of the total muscle volume, and repopulated the muscle stem cell pool. Our study, therefore, demonstrates the possibility of producing large quantities of engraftable skeletal myogenic cells from human PSCs.

In Vitro Maturation of Human Pluripotent Stem Cell-Derived Myotubes

Pluripotent stem cells have a multitude of potential applications in the areas of disease modeling, drug screening, and cell-based therapies for genetic diseases, including muscular dystrophies. The advent of induced pluripotent stem cell technology allows for the facile derivation of disease-specific pluripotent stem cells for any given patient. Targeted in vitro differentiation of pluripotent stem cells into the muscle lineage is a key step to enable all these applications. Transgene-based differentiation using conditional expression of the transcription factor PAX7 leads to the efficient derivation of an expandable and homogeneous population of myogenic progenitors suitable for both in vitro and in vivo applications. Here, we describe an optimized protocol for the derivation and expansion of myogenic progenitors from pluripotent stem cells using conditional expression of PAX7. Importantly, we further describe an optimized procedure for the terminal differentiation of myogenic progenitors into more mature myotubes, which are better suited for in vitro disease modeling and drug screening studies.

Latest developments in engineered skeletal muscle tissues for drug discovery and development

INTRODUCTION

With the advances in skeletal muscle tissue engineering, new platforms have arisen with important applications in biology studies, disease modeling, and drug testing. Current developments highlight the quest for engineering skeletal muscle tissues with higher complexity . These new human skeletal muscle tissue models will be powerful tools for drug discovery and development and disease modeling.

AREAS COVERED

The authors review the latest advances in in vitro models of engineered skeletal muscle tissues used for testing drugs with a focus on the use of four main cell culture techniques: Cell cultures in well plates, in microfluidics, in organoids, and in bioprinted constructs. Additional information is provided on the satellite cell niche.

EXPERT OPINION

In recent years, more sophisticated in vitro models of skeletal muscle tissues have been fabricated. Important developments have been made in stem cell research and in the engineering of human skeletal muscle tissue. Some platforms have already started to be used for drug testing, notably those based on the parameters of hypertrophy/atrophy and the contractibility of myotubes. More developments are expected through the use of multicellular types and multi-materials as matrices . The validation and use of these models in drug testing should now increase.

Differentiation of Human Fetal Muscle Stem Cells from Induced Pluripotent Stem Cells

Most muscular dystrophies are the result of genetic disorders. There is currently no effective treatment for these progressive diseases except palliative therapy. Muscle stem cells with potent self-renewal and regenerative potential are considered a target for treating muscular dystrophy. Human induced pluripotent stem cells have been expected as a source of MuSCs because of their infinite proliferation potential and less immunogenicity. However, the generation of engraftable MuSCs from hiPSCs is relatively difficult and encounters low efficiency and reproducibility. Here, we introduce a transgene-free protocol of hiPSCs differentiating into fetal MuSCs by identifying them as MYF5-positive cells. Flow cytometry analysis detected around 10% of MYF5-positive cells after 12 weeks of differentiation. Approximately 50 ~ 60% of MYF5-positive cells were positively identified using Pax7 immunostaining. This differentiation protocol is expected to be useful for not only the establishment of cell therapy but also the future drug discovery using patient-derived hiPSCs.

Producing Engraftable Skeletal Myogenic Progenitors from Pluripotent Stem Cells via Teratoma Formation

Generating engraftable skeletal muscle progenitor cells is a promising cell therapy approach to treating degenerating muscle diseases. Pluripotent stem cell (PSC) is an ideal cell source for cell therapy because of its unlimited proliferative capability and potential to differentiate into multiple lineages. Approaches such as ectopic overexpression of myogenic transcription factors and growth factors-directed monolayer differentiation, while able to differentiate PSCs into the skeletal myogenic lineage in vitro, are limited in producing muscle cells that reliably engraft upon transplantation. Here we present a novel method to differentiate mouse PSCs into skeletal myogenic progenitors without genetic modification or monolayer culture. We make use of forming a teratoma, in which skeletal myogenic progenitors can be routinely obtained. We first inject mouse PSCs into the limb muscle of an immuno-compromised mouse. Within 3-4 weeks, α7-integrin+ VCAM-1+ skeletal myogenic progenitors are purified by fluorescent-activated cell sorting. We further transplant these teratoma-derived skeletal myogenic progenitors into dystrophin-deficient mice to assess engraftment efficiency. This teratoma formation strategy is capable of generating skeletal myogenic progenitors with high regenerative potency from PSCs without genetic modifications or growth factors supplementation.

Stem Cell-Based Therapeutic Approaches in Genetic Diseases

Stem cells, which can self-renew and differentiate into different cell types, have become the keystone of regenerative medicine due to these properties. With the achievement of superior clinical results in the therapeutic approaches of different diseases, the applications of these cells in the treatment of genetic diseases have also come to the fore. Foremost, conventional approaches of stem cells to genetic diseases are the first approaches in this manner, and they have brought safety issues due to immune reactions caused by allogeneic transplantation. To eliminate these safety issues and phenotypic abnormalities caused by genetic defects, firstly, basic genetic engineering practices such as vectors or RNA modulators were combined with stem cell-based therapeutic approaches. However, due to challenges such as immune reactions and inability to target cells effectively in these applications, advanced molecular methods have been adopted in ZFN, TALEN, and CRISPR/Cas genome editing nucleases, which allow modular designs in stem cell-based genetic diseases' therapeutic approaches. Current studies in genetic diseases are in the direction of creating permanent treatment regimens by genomic manipulation of stem cells with differentiation potential through genome editing tools. In this chapter, the stem cell-based therapeutic approaches of various vital genetic diseases were addressed wide range from conventional applications to genome editing tools.

Metabolic regulation of species-specific developmental rates

Animals display substantial inter-species variation in the rate of embryonic development despite a broad conservation of the overall sequence of developmental events. Differences in biochemical reaction rates, including the rates of protein production and degradation, are thought to be responsible for species-specific rates of development^1-3. However, the cause of differential biochemical reaction rates between species remains unknown. Here, using pluripotent stem cells, we have established an in vitro system that recapitulates the twofold difference in developmental rate between mouse and human embryos. This system provides a quantitative measure of developmental speed as revealed by the period of the segmentation clock, a molecular oscillator associated with the rhythmic production of vertebral precursors. Using this system, we show that mass-specific metabolic rates scale with the developmental rate and are therefore higher in mouse cells than in human cells. Reducing these metabolic rates by inhibiting the electron transport chain slowed down the segmentation clock by impairing the cellular NAD⁺/NADH redox balance and, further downstream, lowering the global rate of protein synthesis. Conversely, increasing the NAD⁺/NADH ratio in human cells by overexpression of the Lactobacillus brevis NADH oxidase LbNOX increased the translation rate and accelerated the segmentation clock. These findings represent a starting point for the manipulation of developmental rate, with multiple translational applications including accelerating the differentiation of human pluripotent stem cells for disease modelling and cell-based therapies.

Induced Pluripotent Stem Cells for Modeling Physiological and Pathological Striated Muscle Complexity

Neuromuscular disorders (NMDs) are a large group of diseases associated with either alterations of skeletal muscle fibers, motor neurons or neuromuscular junctions. Most of these diseases is characterized with muscle weakness or wasting and greatly alter the life of patients. Animal models do not always recapitulate the phenotype of patients. The development of innovative and representative human preclinical models is thus strongly needed for modeling the wide diversity of NMDs, characterization of disease-associated variants, investigation of novel genes function, or the development of therapies. Over the last decade, the use of patient's derived induced pluripotent stem cells (hiPSC) has resulted in tremendous progress in biomedical research, including for NMDs. Skeletal muscle is a complex tissue with multinucleated muscle fibers supported by a dense extracellular matrix and multiple cell types including motor neurons required for the contractile activity. Major challenges need now to be tackled by the scientific community to increase maturation of muscle fibers in vitro, in particular for modeling adult-onset diseases affecting this tissue (neuromuscular disorders, cachexia, sarcopenia) and the evaluation of therapeutic strategies. In the near future, rapidly evolving bioengineering approaches applied to hiPSC will undoubtedly become highly instrumental for investigating muscle pathophysiology and the development of therapeutic strategies.

Evaluation of hiPSC-Derived Muscle Progenitor Cell Transplantation in a Mouse Duchenne Muscular Dystrophy Model

For cell therapy toward Duchenne muscle dystrophy (DMD), muscle progenitor cells derived from human-induced pluripotent stem cell (hiPSC-MuPCs) are recognized as a good candidate, and currently, cell transplantation of hiPSC-MuPCs is being tested with several DMD animal models. In this article, we describe an efficient method to dissociate, purify by cell sorting, transplant, and evaluate the transplantation efficacy of hiPSC-MuPCs.

LMNA Co-Regulated Gene Expression as a Suitable Readout after Precise Gene Correction

LMNA-related muscular dystrophy is an autosomal-dominant progressive disorder caused by mutations in LMNA. LMNA missense mutations are becoming correctable with CRISPR/Cas9-derived tools. Evaluating the functional recovery of LMNA after gene editing bears challenges as there is no reported direct loss of function of lamin A/C proteins in patient-derived cells. The proteins encoded by LMNA are lamins A/C, important ubiquitous nuclear envelope proteins but absent in pluripotent stem cells. We induced lamin A/C expression in induced pluripotent stem cells (iPSCs) of two patients with LMNA-related muscular dystrophy, NM_170707.4 (LMNA): c.1366A > G, p.(Asn456Asp) and c.1494G > T, p.(Trp498Cys), using a short three-day, serum-induced differentiation protocol and analyzed expression profiles of co-regulated genes, examples being COL1A2 and S100A6. We then performed precise gene editing of LMNA c.1366A > G using the near-PAMless (PAM: protospacer-adjacent motif) cytosine base editor. We show that the mutation can be repaired to 100% efficiency in individual iPSC clones. The fast differentiation protocol provided a functional readout and demonstrated increased lamin A/C expression as well as normalized expression of co-regulated genes. Collectively, our findings demonstrate the power of CRISPR/Cas9-mediated gene correction and effective outcome measures in a disease with, so far, little perspective on therapies.

Engineered skeletal muscle recapitulates human muscle development, regeneration and dystrophy

BACKGROUND

Human pluripotent stem cell-derived muscle models show great potential for translational research. Here, we describe developmentally inspired methods for the derivation of skeletal muscle cells and their utility in skeletal muscle tissue engineering with the aim to model skeletal muscle regeneration and dystrophy in vitro.

METHODS

Key steps include the directed differentiation of human pluripotent stem cells to embryonic muscle progenitors followed by primary and secondary foetal myogenesis into three-dimensional muscle. To simulate Duchenne muscular dystrophy (DMD), a patient-specific induced pluripotent stem cell line was compared to a CRISPR/Cas9-edited isogenic control line.

RESULTS

The established skeletal muscle differentiation protocol robustly and faithfully recapitulates critical steps of embryonic myogenesis in two-dimensional and three-dimensional cultures, resulting in functional human skeletal muscle organoids (SMOs) and engineered skeletal muscles (ESMs) with a regeneration-competent satellite-like cell pool. Tissue-engineered muscle exhibits organotypic maturation and function (up to 5.7 ± 0.5 mN tetanic twitch tension at 100 Hz in ESM). Contractile performance could be further enhanced by timed thyroid hormone treatment, increasing the speed of contraction (time to peak contraction) as well as relaxation (time to 50% relaxation) of single twitches from 107 ± 2 to 75 ± 4 ms (P < 0.05) and from 146 ± 6 to 100 ± 6 ms (P < 0.05), respectively. Satellite-like cells could be documented as largely quiescent PAX7⁺ cells (75 ± 6% Ki67^- ) located adjacent to muscle fibres confined under a laminin-containing basal membrane. Activation of the engineered satellite-like cell niche was documented in a cardiotoxin injury model with marked recovery of contractility to 57 ± 8% of the pre-injury force 21 days post-injury (P < 0.05 compared to Day 2 post-injury), which was completely blocked by preceding irradiation. Absence of dystrophin in DMD ESM caused a marked reduction of contractile force (-35 ± 7%, P < 0.05) and impaired expression of fast myosin isoforms resulting in prolonged contraction (175 ± 14 ms, P < 0.05 vs. gene-edited control) and relaxation (238 ± 22 ms, P < 0.05 vs. gene-edited control) times. Restoration of dystrophin levels by gene editing rescued the DMD phenotype in ESM.

CONCLUSIONS

We introduce human muscle models with canonical properties of bona fide skeletal muscle in vivo to study muscle development, maturation, disease and repair.

Generation of the First Human In Vitro Model for McArdle Disease Based on iPSC Technology

McArdle disease is a rare autosomal recessive disorder caused by mutations in the PYGM gene. This gene encodes for the skeletal muscle isoform of glycogen phosphorylase (myophosphorylase), the first enzyme in glycogenolysis. Patients with this disorder are unable to obtain energy from their glycogen stored in skeletal muscle, prompting an exercise intolerance. Currently, there is no treatment for this disease, and the lack of suitable in vitro human models has prevented the search for therapies against it. In this article, we have established the first human iPSC-based model for McArdle disease. For the generation of this model, induced pluripotent stem cells (iPSCs) from a patient with McArdle disease (harbouring the homozygous mutation c.148C>T; p.R50* in the PYGM gene) were differentiated into myogenic cells able to contract spontaneously in the presence of motor neurons and generate calcium transients, a proof of their maturity and functionality. Additionally, an isogenic skeletal muscle model of McArdle disease was created. As a proof-of-concept, we have tested in this model the rescue of PYGM expression by two different read-through compounds (PTC124 and RTC13). The developed model will be very useful as a platform for testing drugs or compounds with potential pharmacological activity.

Carnegie in 4D? Stem-cell-based models of human embryo development

How cells build embryos is still a major mystery. Many unresolved questions require the study of the processes that pattern and shape the embryo in live specimens, in toto, across spatial and temporal scales. In mammalian embryogenesis, this remains a major challenge as the embryo develops in utero, precluding easy accessibility. For human embryos, technical, ethical and legal limitations further hamper the in-depth investigation of embryogenesis, especially beyond gastrulation stages. This has resulted in an over-reliance on model organisms, particularly mice, to understand mammalian development. However, recent efforts show critical differences between rodent and primate embryos, including timing, architecture and transcriptional regulation. Thus, a human-centric understanding of embryogenesis is much needed. To empower this, novel in vitro approaches, which coax human pluripotent stem cells to form embryonic organoids that model embryo development, are pivotal. Here, we summarize these emergent technologies that recapitulate aspects of human development "in a dish". We show how these technologies can provide insights into the molecular, cellular and morphogenetic processes that fuel the formation of a fully formed fetus, and discuss the potential of these platforms to revolutionize our understanding of human development in health and disease. Despite their clear promise, we caution against over-interpreting the extent to which these in vitro platforms model the natural embryo. In particular, we discuss how fate, form and function - a tightly coupled trinity in vivo, can be disconnected in vitro. Finally, we propose how careful benchmarking of existing models, in combination with rational protocol design based on an increased understanding of in vivo developmental dynamics and insights from mouse in vitro models of embryo development, will help guide the establishment of better models of human embryo development.

Comparison of Muscle Regeneration after BMSC-Conditioned Medium, Syngeneic, or Allogeneic BMSC Injection

For many years optimal treatment for dysfunctional skeletal muscle characterized, for example, by impaired or limited regeneration, has been searched. Among the crucial factors enabling its development is finding the appropriate source of cells, which could participate in tissue reconstruction or serve as an immunomodulating agent (limiting immune response as well as fibrosis, that is, connective tissue formation), after transplantation to regenerating muscles. MSCs, including those derived from bone marrow, are considered for such applications in terms of their immunomodulatory properties, as their naive myogenic potential is rather limited. Injection of autologous (syngeneic) or allogeneic BMSCs has been or is currently being tested and compared in many potential clinical treatments. In the present study, we verified which approach, that is, the transplantation of either syngeneic or allogeneic BMSCs or the injection of BMSC-conditioned medium, would be the most beneficial for skeletal muscle regeneration. To properly assess the influence of the tested treatments on the inflammation, the experiments were carried out using immunocompetent mice, which allowed us to observe immune response. Combined analysis of muscle histology, immune cell infiltration, and levels of selected chemokines, cytokines, and growth factors important for muscle regeneration, showed that muscle injection with BMSC-conditioned medium is the most beneficial strategy, as it resulted in reduced inflammation and fibrosis development, together with enhanced new fiber formation, which may be related to, i.e., elevated level of IGF-1. In contrast, transplantation of allogeneic BMSCs to injured muscles resulted in a visible increase in the immune response, which hindered regeneration by promoting connective tissue formation. In comparison, syngeneic BMSC injection, although not detrimental to muscle regeneration, did not result in such significant improvement as CM injection.

TMEM132A ensures mouse caudal neural tube closure and regulates integrin-based mesodermal migration

Coordinated migration of the mesoderm is essential for accurate organization of the body plan during embryogenesis. However, little is known about how mesoderm migration influences posterior neural tube closure in mammals. Here, we show that spinal neural tube closure and lateral migration of the caudal paraxial mesoderm depend on transmembrane protein 132A (TMEM132A), a single-pass type I transmembrane protein, the function of which is not fully understood. Our study in Tmem132a-null mice and cell models demonstrates that TMEM132A regulates several integrins and downstream integrin pathway activation as well as cell migration behaviors. Our data also implicates mesoderm migration in elevation of the caudal neural folds and successful closure of the caudal neural tube. These results suggest a requirement for paraxial mesodermal cell migration during spinal neural tube closure, disruption of which may lead to spina bifida.

Transcriptomics, regulatory syntax, and enhancer identification in mesoderm-induced ESCs at single-cell resolution

Embryonic stem cells (ESCs) can adopt lineage-specific gene-expression programs by stepwise exposure to defined factors, resulting in the generation of functional cell types. Bulk and single-cell-based assays were employed to catalog gene expression, histone modifications, chromatin conformation, and accessibility transitions in ESC populations and individual cells acquiring a presomitic mesoderm fate and undergoing further specification toward myogenic and neurogenic lineages. These assays identified cis-regulatory regions and transcription factors presiding over gene-expression programs occurring at defined ESC transitions and revealed the presence of heterogeneous cell populations within discrete ESC developmental stages. The datasets were employed to identify previously unappreciated genomic elements directing the initial activation of Pax7 and myogenic and neurogenic gene-expression programs. This study provides a resource for the discovery of genomic and transcriptional features of pluripotent, mesoderm-induced ESCs and ESC-derived cell lineages.