Myofibers cultured in viscoelastic hydrogels reveal the effects of integrin-binding and mechanosensing on muscle satellite cells.

Quiescent skeletal muscle satellite cells (SCs) located on myofibers activate in response to muscle injury to regenerate muscle; however, identifying the role of specific matrix signals on SC behavior in vivo is difficult. Therefore, we developed a viscoelastic hydrogel with tunable properties to encapsulate myofibers while maintaining stem cell niche polarity and SC-myofiber interactions to investigate how matrix signals, including viscoelasticity and the integrin-binding ligand arginyl-glycyl-aspartic acid (RGD), influence SC behavior during muscle regeneration. Viscoelastic hydrogels support myofiber culture while preserving SC stemness for up to 72 hours post-encapsulation, minimizing myofiber hypercontraction and SC hyperproliferation compared to Matrigel.

Aging disrupts gene expression timing during muscle regeneration.

Skeletal muscle function and regenerative capacity decline during aging, yet factors driving these changes are incompletely understood. Muscle regeneration requires temporally coordinated transcriptional programs to drive myogenic stem cells to activate, proliferate, fuse to form myofibers, and to mature as myonuclei, restoring muscle function after injury. We assessed global changes in myogenic transcription programs distinguishing muscle regeneration in aged mice from young mice by comparing pseudotime trajectories from single-nucleus RNA sequencing of myogenic nuclei.

Photo-expansion microscopy enables super-resolution imaging of cells embedded in 3D hydrogels.

Hydrogels are extensively used as tunable, biomimetic three-dimensional cell culture matrices, but optically deep, high-resolution images are often difficult to obtain, limiting nanoscale quantification of cell-matrix interactions and outside-in signalling. Here we present photopolymerized hydrogels for expansion microscopy that enable optical clearance and tunable ×4.6-6.

The regenerating skeletal muscle niche drives satellite cell return to quiescence.

Skeletal muscle stem cells, or satellite cells (SCs), are essential to regenerate and maintain muscle. Quiescent SCs reside in an asymmetric niche between the basal lamina and myofiber membrane. To repair muscle, SCs activate, proliferate, and differentiate, fusing to repair myofibers or reacquiring quiescence to replenish the SC niche.

Injury-mediated stiffening persistently activates muscle stem cells through YAP and TAZ mechanotransduction.

The skeletal muscle microenvironment transiently remodels and stiffens after exercise and injury, as muscle ages, and in myopathic muscle; however, how these changes in stiffness affect resident muscle stem cells (MuSCs) remains understudied. Following muscle injury, muscle stiffness remained elevated after morphological regeneration was complete, accompanied by activated and proliferative MuSCs. To isolate the role of stiffness on MuSC behavior and determine the underlying mechanotransduction pathways, we cultured MuSCs on strain-promoted azide-alkyne cycloaddition hydrogels capable of in situ stiffening by secondary photocrosslinking of excess cyclooctynes.

Cosmc is required for T cell persistence in the periphery.

T lymphocytes, a key arm of adaptive immunity, are known to dynamically regulate O-glycosylation during T cell maturation and when responding to stimuli; however, the direct role of O-glycans in T cell maturation remains largely unknown. Using a conditional knockout of the gene (C1GalT1C1 or Cosmc) encoding the specific chaperone Cosmc, we generated mice whose T cells lack extended O-glycans (T cell conditional Cosmc knock out or TCKO mice) and homogeneously express the truncated Tn antigen. Loss of Cosmc is highly deleterious to T cell persistence, with near-complete elimination of Cosmc-null T cells from spleen and lymph nodes.

Myo-granules Connect Physiology and Pathophysiology.

A hallmark of many neuromuscular diseases including Alzheimer disease, inclusion body myositis, amyotrophic lateral sclerosis, frontotemporal lobar dementia, and ocular pharyngeal muscular dystrophy is large cytoplasmic aggregates containing the RNA-binding protein, TDP-43. Despite acceptance that cytoplasmic TDP-43 aggregation is pathological, cytoplasmic TDP-43 assemblies form in healthy regenerating muscle. These recently discovered ribonucleoprotein assemblies, termed myo-granules, form in healthy muscle following injury and are readily cleared as the myofibers mature.

Muscling in on the Awesome Proliferative Power of the Terrible Teratoma.

In this issue of Cell Stem Cell, Chan et al. (2018) report that in vivo differentiation of pluripotent stem cells in induced teratomas produces functional embryonic-like muscle stem cells. These purified muscle stem cells engraft with high efficiency and regenerate serially injured muscle.

Biochemical Isolation of Myonuclei from Mouse Skeletal Muscle Tissue.

Skeletal muscle provides the contractile force necessary for movement, swallowing, and breathing and, consequently, is necessary for survival. Skeletal muscle cells are unique in that they are extremely large cells containing thousands of nuclei. These nuclei must all work in concert to maintain skeletal muscle function and thereby maintain life.

Non-equivalence of nuclear import among nuclei in multinucleated skeletal muscle cells.

Skeletal muscle is primarily composed of large myofibers containing thousands of post-mitotic nuclei distributed throughout a common cytoplasm. Protein production and localization in specialized myofiber regions is crucial for muscle function. Myonuclei differ in transcriptional activity and protein accumulation, but how these differences among nuclei sharing a cytoplasm are achieved is unknown.

The RNA-binding protein, ZC3H14, is required for proper poly(A) tail length control, expression of synaptic proteins, and brain function in mice.

A number of mutations in genes that encode ubiquitously expressed RNA-binding proteins cause tissue specific disease. Many of these diseases are neurological in nature revealing critical roles for this class of proteins in the brain. We recently identified mutations in a gene that encodes a ubiquitously expressed polyadenosine RNA-binding protein, ZC3H14 (Zinc finger CysCysCysHis domain-containing protein 14), that cause a nonsyndromic, autosomal recessive form of intellectual disability.

Biochemical isolation of myonuclei employed to define changes to the myonuclear proteome that occur with aging.

Skeletal muscle aging is accompanied by loss of muscle mass and strength. Examining changes in myonuclear proteins with age would provide insight into molecular processes which regulate these profound changes in muscle physiology. However, muscle tissue is highly adapted for contraction and thus comprised largely of contractile proteins making the nuclear proteins difficult to identify from whole muscle samples.