Myoblast deactivation within engineered human skeletal muscle creates a transcriptionally heterogeneous population of quiescent satellite-like cells

Mini-MEndR: a miniaturized 96-well predictive assay to evaluate muscle stem cell-mediated repair

Background

Functional evaluation of molecules that are predicted to promote stem cell mediated endogenous repair often requires in vivo transplant studies that are low throughput and hinder the rate of discovery. To offer greater throughput for functional validation studies, we miniaturized, simplified and expanded the functionality of a previously developed muscle endogenous repair (MEndR) in vitro assay that was shown to capture significant events of in vivo muscle endogenous repair.

Methods

The mini-MEndR assay consists of miniaturized cellulose scaffolds designed to fit in 96-well plates, the pores of which are infiltrated with human myoblasts encapsulated in a fibrin-based hydrogel to form engineered skeletal muscle tissues. Pre-adsorbing thrombin to the cellulose scaffolds facilitates in situ tissue polymerization, a critical modification that enables new users to rapidly acquire assay expertise. Following the generation of the 3D myotube template, muscle stem cells (MuSCs), enriched from digested mouse skeletal muscle tissue using an improved magnetic-activated cell sorting protocol, are engrafted within the engineered template. Murine MuSCs are fluorescently labeled, enabling co-evaluation of human and mouse Pax7+ cell responses to drug treatments. A regenerative milieu is introduced by injuring the muscle tissue with a myotoxin to initiate endogenous repair "in a dish". Phenotypic data is collected at endpoints with a high-content imaging system and is analyzed using ImageJ-based image analysis pipelines.

Results

The miniaturized format and modified manufacturing protocol cuts reagent costs in half and hands-on seeding time ~ threefold, while the image analysis pipelines save 40 h of labour. By evaluating multiple commercially available human primary myoblast lines in 2D and 3D culture, we establish quality assurance metrics for cell line selection that standardizes myotube template quality. In vivo outcomes (enhanced muscle production and Pax7+ cell expansion) to a known modulator of MuSC mediated repair (p38/β MAPK inhibition) are recapitulated in the miniaturized culture assay, but only in the presence of stem cells and the regenerative milieu.

Discussion

The miniaturized predictive assay offers a simple, scaled platform to co-investigate human and mouse skeletal muscle endogenous repair molecular modulators, and thus is a promising strategy to accelerate the muscle endogenous repair discovery pipeline.

Supplementary Information

The online version contains supplementary material available at 10.1186/s44330-024-00005-4.

Complex or not too complex? One size does not fit all in next generation microphysiological systems

In the attempt to overcome the increasingly recognized shortcomings of existing in vitro and in vivo models, researchers have started to implement alternative models, including microphysiological systems. First examples were represented by 2.5D systems, such as microfluidic channels covered by cell monolayers as blood vessel replicates. In recent years, increasingly complex microphysiological systems have been developed, up to multi-organ on chip systems, connecting different 3D tissues in the same device. However, such an increase in model complexity raises several questions about their exploitation and implementation into industrial and clinical applications, ranging from how to improve their reproducibility, robustness, and reliability to how to meaningfully and efficiently analyze the huge amount of heterogeneous datasets emerging from these devices. Considering the multitude of envisaged applications for microphysiological systems, it appears now necessary to tailor their complexity on the intended purpose, being academic or industrial, and possibly combine results deriving from differently complex stages to increase their predictive power.

Methods to Isolate Muscle Stem Cells for Cell-Based Cultured Meat Production: A Review

Cultured meat production relies on various cell types, including muscle stem cells (MuSCs), embryonic stem cell lines, induced pluripotent cell lines, and naturally immortalized cell lines. MuSCs possess superior muscle differentiation capabilities compared to the other three cell lines, making them key for cultured meat development. Therefore, to produce cultured meat using MuSCs, they must first be effectively separated from muscles. At present, the methods used to isolate MuSCs from muscles include (1) the pre-plating method, using the ability of cells to adhere differently, which is a biological characteristic of MuSCs; (2) the density gradient centrifugation method, using the intrinsic density difference of cells, which is a physical characteristic of MuSCs; and (3) fluorescence- and magnetic-activated cell sorting methods, using the surface marker protein on the cell surface of MuSCs, which is a molecular characteristic of MuSCs. Further efficient and valuable methods for separating MuSCs are expected to be required as the cell-based cultured meat industry develops. Thus, we take a closer look at the four methods currently in use and discuss future development directions in this review.

MicroRNA control of the myogenic cell transcriptome and proteome: the role of miR-16

MicroRNAs (miRs) control stem cell biology and fate. Ubiquitously expressed and conserved miR-16 was the first miR implicated in tumorigenesis. miR-16 is low in muscle during developmental hypertrophy and regeneration. It is enriched in proliferating myogenic progenitor cells but is repressed during differentiation. The induction of miR-16 blocks myoblast differentiation and myotube formation, whereas knockdown enhances these processes. Despite a central role for miR-16 in myogenic cell biology, how it mediates its potent effects is incompletely defined. In this investigation, global transcriptomic and proteomic analyses after miR-16 knockdown in proliferating C2C12 myoblasts revealed how miR-16 influences myogenic cell fate. Eighteen hours after miR-16 inhibition, ribosomal protein gene expression levels were higher relative to control myoblasts and p53 pathway-related gene abundance was lower. At the protein level at this same time point, miR-16 knockdown globally upregulated tricarboxylic acid (TCA) cycle proteins while downregulating RNA metabolism-related proteins. miR-16 inhibition induced specific proteins associated with myogenic differentiation such as ACTA2, EEF1A2, and OPA1. We extend prior work in hypertrophic muscle tissue and show that miR-16 is lower in mechanically overloaded muscle in vivo. Our data collectively point to how miR-16 is implicated in aspects of myogenic cell differentiation. A deeper understanding of the role of miR-16 in myogenic cells has consequences for muscle developmental growth, exercise-induced hypertrophy, and regenerative repair after injury, all of which involve myogenic progenitors.

The mini-IDLE 3D biomimetic culture assay enables interrogation of mechanisms governing muscle stem cell quiescence and niche repopulation

Adult skeletal muscle harbours a population of muscle stem cells (MuSCs) that are required for repair after tissue injury. In youth, MuSCs return to a reversible state of cell-cycle arrest termed 'quiescence' after injury resolution. Conversely, some MuSCs in aged muscle remain semi-activated, causing a premature response to injuries that results in incomplete repair and eventual stem cell depletion. Regulating this balance between MuSC quiescence and activation may hold the key to restoring tissue homeostasis with age, but is incompletely understood. To fill this gap, we developed a simple and tractable in vitro method, to rapidly inactivate MuSCs freshly isolated from young murine skeletal muscle, and return them to a quiescent-like state for at least 1-week, which we name mini-IDLE (Inactivation and Dormancy LEveraged in vitro). This was achieved by introducing MuSCs into a 3D bioartificial niche comprised of a thin sheet of mouse myotubes, which we demonstrate provides the minimal cues necessary to induce quiescence. With different starting numbers of MuSCs, the assay revealed cellular heterogeneity and population-level adaptations that converged on a common niche repopulation density; behaviours previously observed only in vivo. Quiescence-associated hallmarks included a Pax7⁺CalcR⁺DDX6⁺MyoD^-c-FOS^- signature, quiescent-like morphologies, and polarized niche markers. Leveraging high-content bioimaging pipelines, we demonstrate a relationship between morphology and cell fate signatures for possible real-time morphology-based screening. When using MuSCs from aged muscle, they displayed aberrant proliferative activities and delayed inactivation kinetics, among other quiescence-associated defects that we show are partially rescued by wortmannin treatment. Thus, the assay offers an unprecedented opportunity to systematically investigate long-standing queries in areas such as regulation of pool size and functional heterogeneity within the MuSC population, and to uncover quiescence regulators in youth and age.