A laminin substrate promotes myogenesis in rat skeletal muscle cultures: analysis of replication and development using antidesmin and anti-BrdUrd monoclonal antibodies

Myocyte Culture with Decellularized Skeletal Muscle Sheet with Observable Interaction with the Extracellular Matrix

In skeletal muscles, muscle fibers are highly organized and bundled within the basement membrane. Several microfabricated substrate models have failed to mimic the macrostructure of native muscle, including various extracellular matrix (ECM) proteins. Therefore, we developed and evaluated a system using decellularized muscle tissue and mouse myoblasts C2C12 to analyze the interaction between native ECM and myocytes. Chicken skeletal muscle was sliced into sheets and decellularized to prepare decellularized skeletal muscle sheets (DSMS). C2C12 was then seeded and differentiated on DSMS. Immunostaining for ECM molecules was performed to examine the relationship between myoblast adhesion status, myotube orientation, and collagen IV orientation. Myotube survival in long-term culture was confirmed by calcein staining. C2C12 myoblasts adhered to scaffolds in DSMS and developed adhesion plaques and filopodia. Furthermore, C2C12 myotubes showed orientation along the ECM orientation within DSMS. Compared to plastic dishes, detachment was less likely to occur on DSMS, and long-term incubation was possible. This culture technique reproduces a cell culture environment reflecting the properties of living skeletal muscle, thereby allowing studies on the interaction between the ECM and myocytes.

Myogenic Potential of Extracellular Matrix Derived from Decellularized Bovine Pericardium

Skeletal muscles represent 40% of body mass and its native regenerative capacity can be permanently lost after a traumatic injury, congenital diseases, or tumor ablation. The absence of physiological regeneration can hinder muscle repair preventing normal muscle tissue functions. To date, tissue engineering (TE) represents one promising option for treating muscle injuries and wasting. In particular, hydrogels derived from the decellularized extracellular matrix (dECM) are widely investigated in tissue engineering applications thanks to their essential role in guiding muscle regeneration. In this work, the myogenic potential of dECM substrate, obtained from decellularized bovine pericardium (Tissuegraft Srl), was evaluated in vitro using C2C12 murine muscle cells. To assess myotubes formation, the width, length, and fusion indexes were measured during the differentiation time course. Additionally, the ability of dECM to support myogenesis was assessed by measuring the expression of specific myogenic markers: α-smooth muscle actin (α-sma), myogenin, and myosin heavy chain (MHC). The results obtained suggest that the dECM niche was able to support and enhance the myogenic potential of C2C12 cells in comparison of those grown on a plastic standard surface. Thus, the use of extracellular matrix proteins, as biomaterial supports, could represent a promising therapeutic strategy for skeletal muscle tissue engineering.

Recapitulating pathophysiology of skeletal muscle diseases in vitro using primary mouse myoblasts on a nanofibrous platform

Tissue engineering approaches are used to mimic the microenvironment of the skeletal muscle in vitro. However, the validation of a bioengineered muscle as a model to study diseases is inadequate. Here, we present polycaprolactone nanofibers as a robust platform that mimics cellular organization and recapitulates critical functions of the myotubes observed in vivo. We isolated myoblasts from mice following a simplified protocol and cultured them on aligned nanofibers. Myotubes grown on aligned nanofibers maintained alignment for 14 days and exhibited a time-dependent increase in levels of p-AKT upon insulin stimulation. Treatment with matrix-assisted integrin inhibitor led to reduction in p-AKT levels, underscoring the critical role of environment on the biological processes. We demonstrate the suitability of myotubes grown on nanofibrous platform to study corticosteroid-induced muscle degeneration. This study, thus, demonstrates that aligned nanofibers retain myotubes in culture for longer duration and recapitulate the functions of skeletal muscle under pathophysiological conditions.

Effect of cell-extracellular matrix interaction on myogenic characteristics and artificial skeletal muscle tissue

Although various types of artificial skeletal muscle tissue have been reported, the contractile forces generated by tissue-engineered artificial skeletal muscles remain to be improved for biological model and clinical applications. In this study, we investigated the effects of extracellular matrix (ECM) and supplementation of a small molecule, which has been reported to enhance α7β1 integrin expression (SU9516), on cell migration speed, cell fusion rate, myoblast (mouse C2C12 cells) differentiation and contractile force generation of tissue-engineered artificial skeletal muscles. When cells were cultured on varying ECM coated-surfaces, we observed significant enhancement in the migration speed, while the myotube formation (differentiation ratio) decreased in all except for cells cultured on Matrigel coated-surfaces. In contrast, SU9516 supplementation resulted in an increase in both the myotube width and differentiation ratio. Following combined culture with a Matrigel-coated surface and SU9516 supplementation, myotube width was further increased. Additionally, contractile forces produced by the tissue-engineered artificial skeletal muscles was augmented following combined culture. These findings indicate that regulation of the cell-ECM interaction is a promising approach to improve the function of tissue-engineered artificial skeletal muscles.

Microcarriers for Upscaling Cultured Meat Production

Due to the considerable environmental impact and the controversial animal welfare associated with industrial meat production, combined with the ever-increasing global population and demand for meat products, sustainable production alternatives are indispensable. In 2013, the world's first laboratory grown hamburger made from cultured muscle cells was developed. However, coming at a price of $300.000, and being produced manually, substantial effort is still required to reach sustainable large-scale production. One of the main challenges is scalability. Microcarriers (MCs), offering a large surface/volume ratio, are the most promising candidates for upscaling muscle cell culture. However, although many MCs have been developed for cell lines and stem cells typically used in the medical field, none have been specifically developed for muscle stem cells and meat production. This paper aims to discuss the MCs' design criteria for skeletal muscle cell proliferation and subsequently for meat production based on three scenarios: (1) MCs are serving only as a temporary substrate for cell attachment and proliferation and therefore they need to be separated from the cells at some stage of the bioprocess, (2) MCs serve as a temporary substrate for cell proliferation but are degraded or dissolved during the bioprocess, and (3) MCs are embedded in the final product and therefore need to be edible. The particularities of each of these three bioprocesses will be discussed from the perspective of MCs as well as the feasibility of a one-step bioprocess. Each scenario presents advantages and drawbacks, which are discussed in detail, nevertheless the third scenario appears to be the most promising one for a production process. Indeed, using an edible material can limit or completely eliminate dissociation/degradation/separation steps and even promote organoleptic qualities when embedded in the final product. Edible microcarriers could also be used as a temporary substrate similarly to scenarios 1 and 2, which would limit the risk of non-edible residues.

Skeletal muscle as an experimental model of choice to study tissue aging and rejuvenation

Skeletal muscle is among the most age-sensitive tissues in mammal organisms. Significant changes in its resident stem cells (i.e., satellite cells, SCs), differentiated cells (i.e., myofibers), and extracellular matrix cause a decline in tissue homeostasis, function, and regenerative capacity. Based on the conservation of aging across tissues and taking advantage of the relatively well-characterization of the myofibers and associated SCs, skeletal muscle emerged as an experimental system to study the decline in function and maintenance of old tissues and to explore rejuvenation strategies. In this review, we summarize the approaches for understanding the aging process and for assaying the success of rejuvenation that use skeletal muscle as the experimental system of choice. We further discuss (and exemplify with studies of skeletal muscle) how conflicting results might be due to variations in the techniques of stem cell isolation, differences in the assays of functional rejuvenation, or deciding on the numbers of replicates and experimental cohorts.

Histological, transcriptomic and in vitro analysis reveal an intrinsic activated state of myogenic precursors in hyperplasic muscle of trout

BACKGROUND

The dramatic increase in myotomal muscle mass in post-hatching fish is related to their ability to lastingly produce new muscle fibres, a process termed hyperplasia. The molecular and cellular mechanisms underlying fish muscle hyperplasia largely remain unknown. In this study, we aimed to characterize intrinsic properties of myogenic cells originating from hyperplasic fish muscle. For this purpose, we compared in situ proliferation, in vitro cell behavior and transcriptomic profile of myogenic precursors originating from hyperplasic muscle of juvenile trout (JT) and from non-hyperplasic muscle of fasted juvenile trout (FJT) and adult trout (AT).

RESULTS

For the first time, we showed that myogenic precursors proliferate in hyperplasic muscle from JT as shown by in vivo BrdU labeling. This proliferative rate was very low in AT and FJT muscle. Transcriptiomic analysis revealed that myogenic cells from FJT and AT displayed close expression profiles with only 64 differentially expressed genes (BH corrected p-val < 0.001). In contrast, 2623 differentially expressed genes were found between myogenic cells from JT and from both FJT and AT. Functional categories related to translation, mitochondrial activity, cell cycle, and myogenic differentiation were inferred from genes up regulated in JT compared to AT and FJT myogenic cells. Conversely, Notch signaling pathway, that signs cell quiescence, was inferred from genes down regulated in JT compared to FJT and AT. In line with our transcriptomic data, in vitro JT myogenic precursors displayed higher proliferation and differentiation capacities than FJT and AT myogenic precursors.

CONCLUSIONS

The transcriptomic analysis and examination of cell behavior converge to support the view that myogenic cells extracted from hyperplastic muscle of juvenile trout are intrinsically more potent to form myofibres than myogenic cells extracted from non-hyperplasic muscle. The generation of gene expression profiles in myogenic cell extracted from muscle of juvenile trout may yield insights into the molecular and cellular mechanisms controlling hyperplasia and provides a useful list of potential molecular markers of hyperplasia.

Implication of SPARC in the modulation of the extracellular matrix and mitochondrial function in muscle cells

Secreted protein, acidic and rich in cysteine (SPARC) is differentially associated with cell proliferation and extracellular matrix (ECM) assembly. We show here the effect of exogenous SPARC inhibition/induction on ECM and mitochondrial proteins expression and on the differentiation of C2C12 cells. The cells were cultured in growth medium (GM) supplemented with different experimental conditions. The differentiation of myoblasts was studied for 5 days, the expressions of ECM and mitochondrial proteins were measured and the formation of the myotubes was quantified after exogenous induction/inhibition of SPARC. The results indicate that the addition of recombinant SPARC protein (rSPARC) in cell culture medium increased the differentiation of C2C12 myoblasts and myogenin expression during the myotube formation. However, the treatment with antibody specific for SPARC (anti-SPARC) prevented the differentiation and decreased myogenin expression. The induction of SPARC in the proliferating and differentiating C2C12 cells increased collagen 1a1 protein expression, whereas the inhibition decreased it. The effects on fibronectin protein expression were opposite. Furthermore, the addition of rSPARC in C2C12 myoblast increased the expression of mitochondrial proteins, ubiquinol-cytochrome c reductase core protein II (UQCRC2) and succinate dehydrogenase iron-sulfur subunit (SDHB), whereas the anti-SPARC decreased them. During the differentiation, only the anti-SPARC had the effects on mitochondrial proteins, NADH dehydrogenase ubiquinone 1 beta subcomplex subunit 8 (NADHB8), SDHB and cytochrome c oxidase 1 (MTCO1). Thus, SPARC plays a crucial role in the proliferation and differentiation of C2C12 and may be involved in the link between the ECM remodeling and mitochondrial function.

Differentiation and sarcomere formation in skeletal myocytes directly prepared from human induced pluripotent stem cells using a sphere-based culture

Human induced-pluripotent stem cells (iPSCs) are a promising resource for propagation of myogenic progenitors. Our group recently reported a unique protocol for the derivation of myogenic progenitors directly (without genetic modification) from human pluripotent cells using free-floating spherical culture. Here we expand our previous efforts and attempt to determine how differentiation duration, culture surface coatings, and nutrient supplements in the medium influence progenitor differentiation and formation of skeletal myotubes containing sarcomeric structures. A long differentiation period (over 6 weeks) promoted the differentiation of iPSC-derived myogenic progenitors and subsequent myotube formation. These iPSC-derived myotubes contained representative sarcomeric structures, consisting of organized myosin and actin filaments, and could spontaneously contract. We also found that a bioengineering approach using three-dimensional (3D) artificial muscle constructs could facilitate the formation of elongated myotubes. Lastly, we determined how culture surface coating matrices and different supplements would influence terminal differentiation. While both Matrigel and laminin coatings showed comparable effects on muscle differentiation, B27 serum-free supplement in the differentiation medium significantly enhanced myogenesis compared to horse serum. Our findings support the possibility to create an in vitro model of contractile sarcomeric myofibrils for disease modeling and drug screening to study neuromuscular diseases.

Intramuscular Migration of Myoblasts Transplanted after Muscle Pretreatment with Metalloproteinases

The effect of pretreatments of host muscles with metalloproteinases (MMPs) or with notexin on the migration of transplanted myoblasts was investigated. Transgenic TnILacZ mice in which the β-galactosidase gene is under the control of a quail fast skeletal troponin I gene promoter were used as donors. A polyethylene microtube with four perforations was inserted in the tibialis anterior (TA) of CD1 mice. Both pretreatment substances and cells were slowly injected through that microtube. Muscles were pretreated 2 days before myoblast injection either with a mixture of collagenase, matrilysin, and notexin or with only collagenase and matrilysin or only notexin. As control for our experiments, TnILacZ and C2C12 myoblasts were also injected in TA muscles not pretreated. Comparison of short and long-term myoblast radial migration was performed using a dye (PKH26) and X-gal staining, respectively. The recipient mice were immunosuppressed with FK506. Two days after myoblast transplantation, the cell movement in muscles pretreated with collagenase, matrilysin, and notexin was slightly greater than in muscles pretreated only with collagenase and matrilysin but was about twice that observed in muscles treated with notexin alone. Almost no radial migration of TnILacZ myoblasts was observed in untreated muscles. The C2C12 myoblasts showed a four-to fivefold higher migration capacity than TnILacZ myoblasts. At 15 days after TnILacZ myoblast transplantation, the farthest positive β-gal muscle fibers show a two- to threefold extension of the initial migration observed at 2 days, demonstrating the ability of myoblasts to continue the migration following all pretreatments and even in the untreated muscles. In addition, more muscle fibers expressed the β-gal reporter gene in muscles pretreated only with MMPs. Our results clearly demonstrate that muscle pretreatments with MMPs increase myoblast migration and fusion with host muscle fibers after transplantation and that the C2C12 cell line producing MMPs has a higher migratory capacity.

Knockdown of subunit 3 of the COP9 signalosome inhibits C2C12 myoblast differentiation via NF-KappaB signaling pathway

Background

The COP9 signalosome (CSN) is a conserved protein complex composed of 8 subunits designated CSN1-CSN8. CSN3 represents the third subunit of the CSN and maintains the integrity of the complex. CSN3 binds to the striated muscle-specific β1D integrin tail, and its subcellular localization is altered in differentiated skeletal muscle cells. However, the role of CSN3 in skeletal muscle differentiation is unknown. The main goal of this study was to identify whether CSN3 participates in myoblast differentiation and the signalling mechanisms involved using C2C12 cells as a skeletal muscle cell model.

Methods

Small-hairpin (shRNA) was used to knockdown CSN3 in C2C12 cells. Differentiation was evaluated by immunostaining and confocal microscopy. Markers of differentiation, NF-κB signaling and CSN subunits expression, were assessed by immunoblotting and/or immunostaining. Cell proliferation was analysed by cell counting, flow cytometry and a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Data were analyzed by one or two-way analysis of variance (ANOVA) followed by post-hoc testing.

Results

Transduction of C2C12 cells with two distinct CSN3 shRNAs led to the production of two cells lines expressing 7% of CSN3 protein (shCSN3-Low) and 43% of CSN3 protein (CSN3-Med) compared to controls. Knockdown of CSN3 was accompanied by destabilization of several CSN subunits and increased nuclear NF-κB localization. shCSN3-Med cells expressed less myogenin and formed shorter and thinner myotubes. In contrast, the shCSN3-Low cells expressed higher levels of myogenin prior and during the differentiation and remained mononucleated throughout the differentiation period. Both CSN3 knockdown cell lines failed to express sarcomeric myosin heavy chain (MHC) protein during differentiation. The fusion index was significantly higher in control cells than in shCSN3-Med cells, whereas shCSN3-Low cells showed no cell fusion. Interestingly, CSN3 knockdown cells exhibited a significantly slower growth rate relative to the control cells. Cell cycle analysis revealed that CSN3 knockdowns delayed in S phase and had increased levels of nuclear p21/Cip1 and p27/Kip1.

Conclusions

This study clarifies the first step toward unrevealing the CSN3/CSN-mediated pathways that controls C2C12 differentiation and proliferation. Further in vivo characterization of CSN/CSN3 may lead to the discovery of novel therapeutic target of skeletal muscle diseases such as muscular dystrophies.

The role of laminins in the organization and function of neuromuscular junctions

The synapse between motor neurons and skeletal muscle is known as the neuromuscular junction (NMJ). Proper alignment of presynaptic and post-synaptic structures of motor neurons and muscle fibers, respectively, is essential for efficient motor control of skeletal muscles. The synaptic cleft between these two cells is filled with basal lamina. Laminins are heterotrimer extracellular matrix molecules that are key members of the basal lamina. Laminin α4, α5, and β2 chains specifically localize to NMJs, and these laminin isoforms play a critical role in maintenance of NMJs and organization of synaptic vesicle release sites known as active zones. These individual laminin chains exert their role in organizing NMJs by binding to their receptors including integrins, dystroglycan, and voltage-gated calcium channels (VGCCs). Disruption of these laminins or the laminin-receptor interaction occurs in neuromuscular diseases including Pierson syndrome and Lambert-Eaton myasthenic syndrome (LEMS). Interventions to maintain proper level of laminins and their receptor interactions may be insightful in treating neuromuscular diseases and aging related degeneration of NMJs.

Influence of exercise and aging on extracellular matrix composition in the skeletal muscle stem cell niche

Skeletal muscle is endowed with a remarkable capacity for regeneration, primarily due to the reserve pool of muscle resident satellite cells. The satellite cell is the physiologically quiescent muscle stem cell that resides beneath the basal lamina and adjacent to the sarcolemma. The anatomic location of satellite cells is in close proximity to vasculature where they interact with other muscle resident stem/stromal cells (e.g., mesenchymal stem cells and pericytes) through paracrine mechanisms. This mini-review describes the components of the muscle stem cell niche, as well as the influence of exercise and aging on the muscle stem cell niche. Although exercise promotes ECM reorganization and stem cell accumulation, aging is associated with dense ECM deposition and loss of stem cell function resulting in reduced regenerative capacity and strength. An improved understanding of the niche elements will be valuable to inform the development of therapeutic interventions aimed at improving skeletal muscle regeneration and adaptation over the life span.

Impaired primary mouse myotube formation on crosslinked type I collagen films is enhanced by laminin and entactin

In skeletal muscle, the stem cell niche is important for controlling the quiescent, proliferation and differentiation states of satellite cells, which are key for skeletal muscle regeneration after wounding. It has been shown that type I collagen, often used as 3D-scaffolds for regenerative medicine purposes, impairs myoblast differentiation. This is most likely due to the absence of specific extracellular matrix proteins providing attachment sites for myoblasts and/or myotubes. In this study we investigated the differentiation capacity of primary murine myoblasts on type I collagen films either untreated or modified with elastin, laminin, type IV collagen, laminin/entactin complex, combinations thereof, and Matrigel as a positive control. Additionally, increased reactive oxygen species (ROS) and ROCK signaling might also be involved. To measure ROS levels with live-cell microscopy, fibronectin-coated glass coverslips were additionally coated with type I collagen and Matrigel onto which myoblasts were differentiated. On type I collagen-coated coverslips, myotube formation was impaired while ROS levels were increased. However, anti-oxidant treatment did not enhance myotube formation. ROCK inhibition, which generally improve cellular attachment to uncoated surfaces or type I collagen, enhanced myoblast attachment to type I collagen-coated coverslips and -films, but slightly enhanced myotube formation. Only modification of type I collagen films by Matrigel and a combination of laminin/entactin significantly improved myotube formation. Our results indicate that type I collagen scaffolds can be modified by satellite cell niche factors of which specifically laminin and entactin enhanced myotube formation. This offers a promising approach for regenerative medicine purposes to heal skeletal muscle wounds.

STATEMENT OF SIGNIFICANCE

In this manuscript we show for the first time that impaired myotube formation on type I collagen scaffolds can be completely restored by modification with laminin and entactin, two extracellular proteins from the satellite cell niche. This offers a promising approach for regenerative medicine approaches to heal skeletal muscle wounds.

Basal Lamina Mimetic Nanofibrous Peptide Networks for Skeletal Myogenesis

Extracellular matrix (ECM) is crucial for the coordination and regulation of cell adhesion, recruitment, differentiation and death. Therefore, equilibrium between cell-cell and cell-matrix interactions and matrix-associated signals are important for the normal functioning of cells, as well as for regeneration. In this work, we describe importance of adhesive signals for myoblast cells' growth and differentiation by generating a novel ECM mimetic peptide nanofiber scaffold system. We show that not only structure but also composition of bioactive signals are important for cell adhesion, growth and differentiation by mimicking the compositional and structural properties of native skeletal muscle basal lamina. We conjugated laminin-derived integrin binding peptide sequence, "IKVAV", and fibronectin-derived well known adhesive sequence, "RGD", into peptide nanostructures to provide adhesive and myogenic cues on a nanofibrous morphology. The myogenic and adhesive signals exhibited a synergistic effect on model myoblasts, C2C12 cells. Our results showed that self-assembled peptide nanofibers presenting laminin derived epitopes support adhesion, growth and proliferation of the cells and significantly promote the expression of skeletal muscle-specific marker genes. The functional peptide nanofibers used in this study present a biocompatible and biodegradable microenvironment, which is capable of supporting the growth and differentiation of C2C12 myoblasts into myotubes.

Biomechanical Origins of Muscle Stem Cell Signal Transduction

Skeletal muscle, the most abundant and widespread tissue in the human body, contracts upon receiving electrochemical signals from the nervous system to support essential functions such as thermoregulation, limb movement, blinking, swallowing and breathing. Reconstruction of adult muscle tissue relies on a pool of mononucleate, resident muscle stem cells, known as "satellite cells", expressing the paired-box transcription factor Pax7 necessary for their specification during embryonic development and long-term maintenance during adult life. Satellite cells are located around the myofibres in a niche at the interface of the basal lamina and the host fibre plasma membrane (i.e., sarcolemma), at a very low frequency. Upon damage to the myofibres, quiescent satellite cells are activated and give rise to a population of transient amplifying myogenic progenitor cells, which eventually exit the cell cycle permanently and fuse to form new myofibres and regenerate the tissue. A subpopulation of satellite cells self-renew and repopulate the niche, poised to respond to future demands. Harnessing the potential of satellite cells relies on a complete understanding of the molecular mechanisms guiding their regulation in vivo. Over the past several decades, studies revealed many signal transduction pathways responsible for satellite cell fate decisions, but the niche cues driving the activation and silencing of these pathways are less clear. Here we explore the scintillating possibility that considering the dynamic changes in the biophysical properties of the skeletal muscle, namely stiffness, and the stretch and shear forces to which a myofibre can be subjected to may provide missing information necessary to gain a full understanding of satellite cell niche regulation.

Regulation of the muscle fiber microenvironment by activated satellite cells during hypertrophy

Our aim in the current study was to determine the necessity of satellite cells for long-term muscle growth and maintenance. We utilized a transgenic Pax7-DTA mouse model, allowing for the conditional depletion of > 90% of satellite cells with tamoxifen treatment. Synergist ablation surgery, where removal of synergist muscles places functional overload on the plantaris, was used to stimulate robust hypertrophy. Following 8 wk of overload, satellite cell-depleted muscle demonstrated an accumulation of extracellular matrix (ECM) and fibroblast expansion that resulted in reduced specific force of the plantaris. Although the early growth response was normal, an attenuation of hypertrophy measured by both muscle wet weight and fiber cross-sectional area occurred in satellite cell-depleted muscle. Isolated primary myogenic progenitor cells (MPCs) negatively regulated fibroblast ECM mRNA expression in vitro, suggesting a novel role for activated satellite cells/MPCs in muscle adaptation. These results provide evidence that satellite cells regulate the muscle environment during growth.

The origin and fate of muscle satellite cells

Satellite cells represent the primary population of stem cells resident in skeletal muscle. These adult muscle stem cells facilitate the postnatal growth, remodeling, and regeneration of skeletal muscle. Given the remarkable regenerative potential of satellite cells, there is great promise for treatment of muscle pathologies such as the muscular dystrophies with this cell population. Various protocols have been developed which allow for isolation, enrichment, and expansion of satellite cell derived muscle stem cells. However, isolated satellite cells have yet to translate into effective modalities for therapeutic intervention. Broadening our understanding of satellite cells and their niche requirements should improve our in vivo and ex vivo manipulation of these cells to expedite their use for regeneration of diseased muscle. This review explores the fates of satellite cells as determined by their molecular signatures, ontogeny, and niche dependent programming.

Matrigel, but not collagen I, maintains the differentiation capacity of muscle derived cells in vitro

Satellite cells are key cells for post-natal muscle growth and regeneration and they play a central role in the search for therapies to treat muscle injuries. In this study the proliferation and differentiation capacity of muscle progenitor cells was studied in 2D and 3D cultures with collagen type I and Matrigel, which contain the niche factors laminin and collagen type IV. Muscle progenitor cells were cultured to induce proliferation and differentiation in collagen- or Matrigel-coated surfaces (2D) or in gels (3D). In the 2D cultures, muscle progenitor cells proliferated faster in Matrigel than in collagen. The numbers of Pax7(+) and MyoD(+) cells were also significantly higher in Matrigel than in collagen. During differentiation, muscle progenitor cells formed more and larger MyoD(+) and myogenin(+) myotubes in Matrigel. In the 3D cultures, muscle progenitor cells in Matrigel expressed higher mRNA levels of MyoD and myogenin, and formed elongated myotubes expressing myogenin and myosin. In collagen gels, the myotubes were short and rounded. In conclusion, muscle progenitor cells, both in 2D and 3D, lose their differentiation capacity in collagen but not in Matrigel. Although Matrigel contains growth factors, our results indicate that the kind of biomaterial steers the maintenance of the myogenic potential and their proper differentiation to achieve optimal skeletal muscle restoration.

Alignment of multi-layered muscle cells within three-dimensional hydrogel macrochannels

This work describes the development and testing of poly(ethylene glycol) (PEG) hydrogels with independently controlled dimensions of wide and deep macrochannels for their ability to promote alignment of skeletal myoblasts and myoblast differentiation. A UV-photopatterned thiol-ene mold was employed to produce long channels, which ranged from ∼40 to 200 μm in width and from ∼100 to 200 μm in depth, within a PEG-RGD hydrogel. Skeletal myoblasts (C2C12) were successfully cultured multiple cell layers deep within the channels. Decreasing channel width, increasing channel depth and, interestingly, increasing cell layer away from the channel base all contributed to a decreased interquartile range of cell angle relative to the long axis of the channel wall, indicating improved cell alignment. Differentiation of skeletal myoblasts into myotubes was confirmed by gene expression for myoD, myogenin and MCH IIb, and myotube formation for all channel geometries, but was not dependent on channel size. Qualitatively, myotubes were characteristically different, as myotubes were larger and had more nuclei in larger channels. Overall, our findings demonstrate that relatively large features, which do not readily facilitate cell alignment in two dimensions, promote cell alignment when presented in three dimensions, suggesting an important role for three-dimensional spatial cues.

Defects in the peripheral taste structure and function in the MRL/lpr mouse model of autoimmune disease

While our understanding of the molecular and cellular aspects of taste reception and signaling continues to improve, the aberrations in these processes that lead to taste dysfunction remain largely unexplored. Abnormalities in taste can develop in a variety of diseases, including infections and autoimmune disorders. In this study, we used a mouse model of autoimmune disease to investigate the underlying mechanisms of taste disorders. MRL/MpJ-Fas(lpr)/J (MRL/lpr) mice develop a systemic autoimmunity with phenotypic similarities to human systemic lupus erythematosus and Sjögren's syndrome. Our results show that the taste tissues of MRL/lpr mice exhibit characteristics of inflammation, including infiltration of T lymphocytes and elevated levels of some inflammatory cytokines. Histological studies reveal that the taste buds of MRL/lpr mice are smaller than those of wild-type congenic control (MRL/+/+) mice. 5-Bromo-2'-deoxyuridine (BrdU) pulse-chase experiments show that fewer BrdU-labeled cells enter the taste buds of MRL/lpr mice, suggesting an inhibition of taste cell renewal. Real-time RT-PCR analyses show that mRNA levels of several type II taste cell markers are lower in MRL/lpr mice. Immunohistochemical analyses confirm a significant reduction in the number of gustducin-positive taste receptor cells in the taste buds of MRL/lpr mice. Furthermore, MRL/lpr mice exhibit reduced gustatory nerve responses to the bitter compound quinine and the sweet compound saccharin and reduced behavioral responses to bitter, sweet, and umami taste substances compared with controls. In contrast, their responses to salty and sour compounds are comparable to those of control mice in both nerve recording and behavioral experiments. Together, our results suggest that type II taste receptor cells, which are essential for bitter, sweet, and umami taste reception and signaling, are selectively affected in MRL/lpr mice, a model for autoimmune disease with chronic inflammation.

At the Start of the Sarcomere: A Previously Unrecognized Role for Myosin Chaperones and Associated Proteins during Early Myofibrillogenesis

The development of striated muscle in vertebrates requires the assembly of contractile myofibrils, consisting of highly ordered bundles of protein filaments. Myofibril formation occurs by the stepwise addition of complex proteins, a process that is mediated by a variety of molecular chaperones and quality control factors. Most notably, myosin of the thick filament requires specialized chaperone activity during late myofibrillogenesis, including that of Hsp90 and its cofactor, Unc45b. Unc45b has been proposed to act exclusively as an adaptor molecule, stabilizing interactions between Hsp90 and myosin; however, recent discoveries in zebrafish and C. elegans suggest the possibility of an earlier role for Unc45b during myofibrillogenesis. This role may involve functional control of nonmuscle myosins during the earliest stages of myogenesis, when premyofibril scaffolds are first formed from dynamic cytoskeletal actin. This paper will outline several lines of evidence that converge to build a model for Unc45b activity during early myofibrillogenesis.

Extrinsic regulation of satellite cell specification

Cellular commitment during vertebrate embryogenesis is controlled by an interplay of intrinsic regulators and morphogenetic signals. These mechanisms recruit a subset of cells in the developing organism to become the ancestors of skeletal muscle. Signals that control progression through the myogenic lineage converge on a battery of hierarchically organized transcription factors which modulate the cells to either remain in a primitive state or allow their commitment and differentiation into skeletal muscle fibers. A small population of cells will retain a largely unspecified state throughout development. Such stem cells, in conjunction with more committed myogenic progenitors, form a heterogeneous population that colonizes adult skeletal muscle as satellite cells. The satellite cell pool is responsible for the remarkable regenerative capacity of skeletal muscle. Similar to their counterparts during embryonic development, satellite cells are capable of self-renewal and can give rise to myogenic progeny. Impaired satellite cell homeostasis has been associated with numerous muscular disorders. Due to intense research efforts in the past two decades, the complex biology of muscle stem cells has now revealed some of its secrets and new avenues for the development of therapeutic molecules have emerged. In the present review we focus on the extrinsic mechanisms that control self-renewal, specification and differentiation of satellite cells and their significance for the development of biologic drugs.

Lipopolysaccharide-induced inflammation attenuates taste progenitor cell proliferation and shortens the life span of taste bud cells

Background

The mammalian taste bud, a complex collection of taste sensory cells, supporting cells, and immature basal cells, is the structural unit for detecting taste stimuli in the oral cavity. Even though the cells of the taste bud undergo constant turnover, the structural homeostasis of the bud is maintained by balancing cell proliferation and cell death. Compared with nongustatory lingual epithelial cells, taste cells express higher levels of several inflammatory receptors and signalling proteins. Whether inflammation, an underlying condition in some diseases associated with taste disorders, interferes with taste cell renewal and turnover is unknown. Here we report the effects of lipopolysaccharide (LPS)-induced inflammation on taste progenitor cell proliferation and taste bud cell turnover in mouse taste tissues.

Results

Intraperitoneal injection of LPS rapidly induced expression of several inflammatory cytokines, including tumor necrosis factor (TNF)-α, interferon (IFN)-γ, and interleukin (IL)-6, in mouse circumvallate and foliate papillae. TNF-α and IFN-γ immunoreactivities were preferentially localized to subsets of cells in taste buds. LPS-induced inflammation significantly reduced the number of 5-bromo-2'-deoxyuridine (BrdU)-labeled newborn taste bud cells 1-3 days after LPS injection, suggesting an inhibition of taste bud cell renewal. BrdU pulse-chase experiments showed that BrdU-labeled taste cells had a shorter average life span in LPS-treated mice than in controls. To investigate whether LPS inhibits taste cell renewal by suppressing taste progenitor cell proliferation, we studied the expression of Ki67, a cell proliferation marker. Quantitative real-time RT-PCR revealed that LPS markedly reduced Ki67 mRNA levels in circumvallate and foliate epithelia. Immunofluorescent staining using anti-Ki67 antibodies showed that LPS decreased the number of Ki67-positive cells in the basal regions surrounding circumvallate taste buds, the niche for taste progenitor cells. PCR array experiments showed that the expression of cyclin B2 and E2F1, two key cell cycle regulators, was markedly downregulated by LPS in the circumvallate and foliate epithelia.

Conclusions

Our results show that LPS-induced inflammation inhibits taste progenitor cell proliferation and interferes with taste cell renewal. LPS accelerates cell turnover and modestly shortens the average life span of taste cells. These effects of inflammation may contribute to the development of taste disorders associated with infections.

A defined long-term in vitro tissue engineered model of neuromuscular junctions

Neuromuscular junction (NMJ) formation, occurring between motoneurons and skeletal muscle, is a complex multistep process involving a variety of signaling molecules and pathways. In vitro motoneuron-muscle co-cultures are powerful tools to study the role of different growth factors, hormones and cellular structures involved in NMJ formation. In this study, a serum-free culture system utilizing defined temporal growth factor application and a non-biological substrate resulted in the formation of robust NMJs. The system resulted in long-term survival of the co-culture and selective expression of neonatal myosin heavy chain, a marker of myotube maturation. NMJ formation was verified by colocalization of dense clusters of acetylcholine receptors visualized using alpha-bungarotoxin and synaptophysin containing vesicles present in motoneuron axonal terminals. This model will find applications in basic NMJ research and tissue engineering applications such as bio-hybrid device development for limb prosthesis and regenerative medicine as well as for high-throughput drug and toxin screening applications.

Automating the expansion process of human skeletal muscle myoblasts with suppression of myotube formation

An intelligent culture system accompanied by automated operations (liquid transfer and cell passage) was newly developed to perform serial cultures of human skeletal muscle myoblasts. To realize a desired performance, a laminin-coated surface was applied to myoblast expansion in a culture flask. It was found that the laminin coating enhanced the overall growth ability attributable not to shortening of the doubling time but to prevention of differentiation toward myotube formation, compared with that on a conventional plain surface. In addition, the effects of seeding density and confluence degree on the growth were investigated quantitatively in terms of cell attachment and division as well as proliferative cell population in the culture on the laminin-coated surface. With increasing in seeding density, the number of proliferative cells decreased at the end of culture accompanied by an increase in the confluence degree, which caused poor attachment of the passaged cells on the surface in the subsequent culture. The quantitative analyses of these cell behaviors helped us determine the appropriate seeding density and attainable confluence degree during one passage, which were 1.0 x 10(3) cells/cm(2) and 0.5 as the initial and boundary conditions, respectively. An automated culture system that could manage two serial cultures by monitoring the confluence degree was constructed. The automated operation with the intelligent determination of the time for passage was successfully performed without serious loss of growth activity, compared with manual operation using conventional flasks. These results indicated that the monitoring of confluence degree is effective to perform the culture passage of myoblasts, being contributable to automating the cell expansion process.

Skeletal muscle tissue engineering: a maturation model promoting long-term survival of myotubes, structural development of the excitation-contraction coupling apparatus and neonatal myosin heavy chain expression

The use of defined in vitro systems to study the developmental and physiological characteristics of a variety of cell types is increasing, due in large part to their ease of integration with tissue engineering, regenerative medicine, and high-throughput screening applications. In this study, myotubes derived from fetal rat hind limbs were induced to develop several aspects of mature muscle including: sarcomere assembly, development of the excitation-contraction coupling apparatus and myosin heavy chain (MHC) class switching. Utilizing immunocytochemical analysis, anisotropic and isotropic band formation (striations) within the myotubes was established, indicative of sarcomere formation. In addition, clusters of ryanodine receptors were colocalized with dihydropyridine complex proteins which signaled development of the excitation-contraction coupling apparatus and transverse tubule biogenesis. The myotubes also exhibited MHC class switching from embryonic to neonatal MHC. Lastly, the myotubes survived significantly longer in culture (70-90 days) than myotubes from our previously developed system (20-25 days). These results were achieved by modifying the culture timeline as well as the development of a new medium formulation. This defined model system for skeletal muscle maturation supports the goal of developing physiologically relevant muscle constructs for use in tissue engineering and regenerative medicine as well as for high-throughput screening applications.

A functional connection between pRB and transforming growth factor beta in growth inhibition and mammary gland development

Transforming growth factor beta (TGF-beta) is a crucial mediator of breast development, and loss of TGF-beta-induced growth arrest is a hallmark of breast cancer. TGF-beta has been shown to inhibit cyclin-dependent kinase (CDK) activity, which leads to the accumulation of hypophosphorylated pRB. However, unlike other components of TGF-beta cytostatic signaling, pRB is thought to be dispensable for mammary development. Using gene-targeted mice carrying subtle missense changes in pRB (Rb1(DeltaL) and Rb1(NF)), we have discovered that pRB plays a critical role in mammary gland development. In particular, Rb1 mutant female mice have hyperplastic mammary epithelium and defects in nursing due to insensitivity to TGF-beta growth inhibition. In contrast with previous studies that highlighted the inhibition of cyclin/CDK activity by TGF-beta signaling, our experiments revealed that active transcriptional repression of E2F target genes by pRB downstream of CDKs is also a key component of TGF-beta cytostatic signaling. Taken together, our work demonstrates a unique functional connection between pRB and TGF-beta in growth control and mammary gland development.

The influence of extracellular matrix derived from skeletal muscle tissue on the proliferation and differentiation of myogenic progenitor cells ex vivo

Skeletal muscle relies upon regeneration to maintain homeostasis and repair injury. This process involves the recruitment of the tissue's resident stem cell, the muscle progenitor cell, and a subsequent proliferative response by newly generated myoblasts, which must then align and fuse to generate new muscle fibers. During regeneration, cells rely on environmental input for direction. Extracellular matrix (ECM) represents a crucial component of a cell's microenvironment that aids in guiding muscle regeneration. We hypothesized that ECM extracted from skeletal muscle would provide muscle progenitor cells and myoblasts with an ideal substrate for growth and differentiation ex vivo. To test this hypothesis, we developed a method to extract ECM from the large thigh muscles of adult rats and present it to cells as a surface coating. Myogenic cells cultured on ECM extract experienced enhanced proliferation and differentiation relative to standard growth surfaces. As the methodology can be applied to any size muscle, these results demonstrate that bioactive ECM can be readily obtained from skeletal muscle and used to develop biomaterials that enhance muscle regeneration. Furthermore, the model system demonstrated here can be applied to the study of interactions between the ECM of a particular tissue and a cell population of interest.

Genetically determined proteolytic cleavage modulates alpha7beta1 integrin function

The dystrophin-glycoprotein complex and the alpha7beta1 integrin are trans-sarcolemmal linkage systems that connect and transduce contractile forces between muscle fibers and the extracellular matrix. alpha7beta1 is the major laminin binding integrin in skeletal muscle. Different functional variants of this integrin are generated by alternative splicing and post-translational modifications such as glycosylation and ADP-ribosylation. Here we report a species-specific difference in alpha7 chains that results from an intra-peptide proteolytic cleavage, by a serine protease, at the 603RRQ605 site. Site-directed mutagenesis of RRQ to GRQ prevents this cleavage. This RRQ sequence in the alpha7 integrin chain is highly conserved among vertebrates but it is absent in mice. Protein structure modeling indicates this cleavage site is located in an open region between the beta-propeller and thigh domains of the alpha7 chain. Compared with the non-cleavable alpha7 chain, the cleaved form enhances cell adhesion and spreading on laminin. Cleavage of the alpha7 chain is elevated upon myogenic differentiation, and this cleavage may be mediated by urokinase-type plasminogen activator. These results suggest proteolytic cleavage is a novel mechanism that regulates alpha7 integrin functions in skeletal muscle, and that the generation of such cleavage sites is another evolutionary mechanism for expanding and modifying protein functions.

Differentiation of human adipose-derived stem cells towards cardiomyocytes is facilitated by laminin

Adipose-derived stem cells (ASCs) are promising candidates for therapy in myocardial infarction (MI). However, the frequency of human ASCs that differentiate towards cardiomyocytes is low. We hypothesized that adherence to extracellular matrix molecules that are upregulated after MI might increase human stem cell differentiation towards cardiomyocytes. We analysed putative ASC differentiation on fibronectin-coated, laminin-coated and uncoated culture plates. Expression of cardiac markers in cells was analysed 1, 3 and 5 weeks after stimulation with 5-aza-2-deoxycytidine. After 1 week, mRNA expression of myosin light chain-2alpha (MLC-2alpha), an early marker in cardiomyocyte development, was increased significantly in treated cells, independent of coating. At 5 weeks, however, mRNA expression of the late cardiomyocyte development marker SERCA2alpha was only significantly increased in 5-aza-2-deoxycytidine-treated cells cultured on laminin. Significantly higher numbers of cells were immunopositive for MLC-2alpha in cultures of treated cells grown on laminin-coated wells, when compared with cultures of treated cells grown on uncoated wells, both at 1 week and at 5 weeks. Furthermore, after 3 weeks, significantly more alpha-actinin- and desmin-positive cells were detected after treatment with 5-aza-2-deoxycytidine, but only in uncoated wells. After 5 weeks, however, the number of desmin-positive cells was only significantly increased after treatment of cells with 5-aza-2-deoxycytidine and culture on laminin (61% positive cells). Thus, we have found that a high percentage of human ASCs can be differentiated towards cardiomyocytes; this effect can be improved by laminin, especially during late differentiation.

Stem cell review series: aging of the skeletal muscle stem cell niche

Declining stem cell function during aging contributes to impaired tissue function. Muscle-specific stem cells ('satellite cells') are responsible for generating new muscle in response to injury in the adult. However, aged muscle displays a significant reduction in regenerative abilities and an increased susceptibility to age-related pathologies. This review describes components of the satellite cell niche and addresses how age-related changes in these components impinge on satellite cell function. In particular, we review changes in the key niche elements, the myofiber and the basal lamina that are in intimate contact with satellite cells. We address how these elements are influenced by factors secreted by interstitial cells, cells of the immune system, and cells associated with the vasculature, all of which change with age. In addition, we consider more distant sources of influence on the satellite cell niche that change with age, such as neural-mediated trophic factors and electrical activity and systemic factors present in the circulation. A better understanding of the niche elements and their influence on the satellite cell will facilitate the development of therapeutic interventions aimed at improving satellite cell activity and ultimately tissue response to injury in aged individuals.

Cell culture-based analysis of postsynaptic membrane assembly in muscle cells

We report a method for studying postsynaptic membrane assembly utilizing the replating of aneural cultures of differentiated skeletal muscle cells onto laminin-coated surfaces. A significant limitation to the current cell culturebased approaches has been their inability to recapitulate the multistage surface acetylcholine receptor (AChR) redistribution events that produce complex AChR clusters found at the intact neuromuscular junction (NMJ). By taking advantage of the ability of substrate laminin to induce advanced maturation of AChR aggregates on the surface of myotubes, we have developed a secondary-plating method that allows more precise analysis of the signaling events connecting substrate laminin stimulation to complex AChR cluster formation. We validate the utility of this method for biochemical and microscopy studies by demonstrating the roles of RhoGTPases in substrate laminin-induced complex cluster assembly.

Increasing alpha 7 beta 1-integrin promotes muscle cell proliferation, adhesion, and resistance to apoptosis without changing gene expression

The dystrophin-glycoprotein complex maintains the integrity of skeletal muscle by associating laminin in the extracellular matrix with the actin cytoskeleton. Several human muscular dystrophies arise from defects in the components of this complex. The alpha(7)beta(1)-integrin also binds laminin and links the extracellular matrix with the cytoskeleton. Enhancement of alpha(7)-integrin levels alleviates pathology in mdx/utrn(-/-) mice, a model of Duchenne muscular dystrophy, and thus the integrin may functionally compensate for the absence of dystrophin. To test whether increasing alpha(7)-integrin levels affects transcription and cellular functions, we generated alpha(7)-integrin-inducible C2C12 cells and transgenic mice that overexpress the integrin in skeletal muscle. C2C12 myoblasts with elevated levels of integrin exhibited increased adhesion to laminin, faster proliferation when serum was limited, resistance to staurosporine-induced apoptosis, and normal differentiation. Transgenic expression of eightfold more integrin in skeletal muscle did not result in notable toxic effects in vivo. Moreover, high levels of alpha(7)-integrin in both myoblasts and in skeletal muscle did not disrupt global gene expression profiles. Thus increasing integrin levels can compensate for defects in the extracellular matrix and cytoskeleton linkage caused by compromises in the dystrophin-glycoprotein complex without triggering apparent overt negative side effects. These results support the use of integrin enhancement as a therapy for muscular dystrophy.

Modulation of satellite cell adhesion and motility following BMP2-induced differentiation to osteoblast lineage

Quiescent satellite cells represent pluripotent stem cells capable of differentiating into other lineages. To define the potential changes in adhesion and motility in these differentiating cells, we utilized an established model system of murine-derived satellite cells induced with BMP2 to undergo osteoblastic differentiation. When mouse myogenic satellite cells were treated with BMP2, myogenesis was inhibited, and interaction with extracellular matrix ligands was altered. alpha7 integrin expression was rapidly downregulated with attenuation of adhesion and migration on laminin substrates. BMP2 also induced alpha2 integrin expression with increased adhesion and motility on collagen substrates as the pluripotent myoblasts develop into the osteogenic lineage. We examined the effect of BMP2 on alpha7 promoter activity in myoblasts using a CAT reporter gene. BMP2 was found to suppress integrin expression through a transcriptional mechanism. The results identify a novel role for BMP2 in modulating satellite cell integrin expression and altering their interactions with the microenvironment during osteoblastic differentiation.

Multifunctional roles of MT1-MMP in myofiber formation and morphostatic maintenance of skeletal muscle

Sequential activation of muscle-specific transcription factors is the critical basis for myogenic differentiation. However, the complexity of this process does not exclude the possibility that other molecules and systems are regulatory as well. We observed that myogenic differentiation proceeded through three distinct stages of proliferation, elongation and fusion, which are distinguishable by their cellular morphologies and gene expression patterns of proliferation- and differentiation-specific markers. Treatment of the differentiating myoblasts with inhibitors of matrix metalloproteinases (MMPs) revealed that MMP activity at the elongation stage is a critical prerequisite to complete the successive myoblast cell fusion. The MMP regulated the myogenic differentiation independently from the genetic program that governs expression of the myogenic genes. Membrane-type 1 matrix metalloproteinase (MT1-MMP) was identified as a major contributor to this checkpoint for morphological differentiation and degraded fibronectin, a possible inhibitory factor for myogenic cell fusion. A MT1-MMP deficiency caused similar myogenic impediments forming smaller myofibers in situ. Additionally, the mutant mice demonstrated some central nucleation of the myofibers typically found in muscular dystrophy and MT1-MMP was found to cleave laminin-2/4 in the basement membrane. Thus, MT1-MMP is a new multilateral regulator for muscle differentiation and maintenance through processing of stage-specific distinct ECM substrates.

Lamin A/C and emerin are critical for skeletal muscle satellite cell differentiation

Mutations within LMNA, encoding A-type nuclear lamins, are associated with multiple tissue-specific diseases, including Emery-Dreifuss (EDMD2/3) and Limb-Girdle muscular dystrophy (LGMD1B). X-linked EDMD results from mutations in emerin, a lamin A-associated protein. The mechanisms through which these mutations cause muscular dystrophy are not understood. Here we show that most, but not all, cultured muscle cells from lamin A/C knockout mice exhibit impaired differentiation kinetics and reduced differentiation potential. Similarly, normal muscle cells that have been RNA interference (RNAi) down-regulated for either A-type lamins or emerin have impaired differentiation potentials. Replicative myoblasts lacking A-type lamins or emerin also have decreased levels of proteins important for muscle differentiation including pRB, MyoD, desmin, and M-cadherin; up-regulated Myf5; but no changes in Pax3, Pax7, MEF2C, MEF2D, c-met, and beta-catenin. To determine whether impaired myogenesis is linked to reduced MyoD or desmin levels, these proteins were individually expressed in Lmna(-/-) myoblasts that were then induced to undergo myogenesis. Expression of either MyoD or, more surprisingly, desmin in Lmna(-/-) myoblasts resulted in increased differentiation potential. These studies indicate roles for A-type lamins and emerin in myogenic differentiation and also suggest that these effects are at least in part due to decreased endogenous levels of other critical myoblast proteins. The delayed differentiation kinetics and decreased differentiation potential of lamin A/C-deficient and emerin-deficient myoblasts may in part underlie the dystrophic phenotypes observed in patients with EDMD.

Transgenic expression of {alpha}7{beta}1 integrin maintains muscle integrity, increases regenerative capacity, promotes hypertrophy, and reduces cardiomyopathy in dystrophic mice

We previously reported that enhanced expression of the alpha7beta1 integrin ameliorates the development of muscular dystrophy and extends longevity in alpha7BX2-mdx/utr(-/-) transgenic mice (Burkin DJ, Wallace GQ, Nicol KJ, Kaufman DJ, Kaufman SJ: Enhanced expression of the alpha7beta1 integrin reduces muscular dystrophy and restores viability in dystrophic mice. We now report on the mechanism by which these mice were rescued by the integrin. As a result of increased integrin in alpha7BX2-mdx/utr(-/-) mice the structural integrity of the myotendinous and neuromuscular junctions are maintained. A twofold increase in satellite cells in alpha7BX2-mdx/utr(-/-) skeletal muscle was detected by immunofluorescence using the satellite cell marker c-met. These cells enhanced the regenerative capacity of muscle in the transgenic animals as determined by fusion of BrdUrd-labeled cells into muscle fibers. Increased integrin also leads to hypertrophy. Finally, transgenic expression of alpha7BX2 integrin chain in skeletal muscle secondarily reduces the development of cardiomyopathy, the ultimate cause of death in these animals. We believe this multiplicity of responses to increased alpha7beta1 integrin collectively inhibits the development of muscle disease and increases longevity in these mice.

Integrin repertoire on myogenic cells changes during the course of primary myogenesis in the mouse

Cells interact with the extracellular matrix through receptors, most commonly of the integrin family. We (Cachaco et al. [2003] Development 130:1659-1671) and others (Schwander et al. [2003] Dev. Cell 4:673-685) have demonstrated a role for beta1 integrins in mouse primary myogenesis. However, it is unclear what alpha subunits pair with beta1 during this process in vivo. Here, we determined alpha subunit expression patterns at embryonic day (E) 11.5-E14.5. Differentiated myotomal myocytes express all alpha subunits studied. As the muscle masses form both in trunk (E12.5) and limbs (E11.5-E12.5), laminin receptors alpha6beta1 and alpha7beta1 are undetectable, and an assembled laminin matrix is absent. Instead alpha1beta1, alpha4beta1, alpha5beta1, and an alpha v-containing integrin are expressed and unassembled laminin and fibronectin are abundant around myogenic cells. At E13.5-E14.5, alpha6beta1 and alpha7beta1 are expressed, and a laminin matrix forms around individual myotubes. Thus, myogenic cells change their integrin expression pattern during the course of primary myogenesis in the mouse, suggesting different roles for fibronectin- and laminin-containing matrices in this process.

Role for the α7β1 integrin in vascular development and integrity

The alpha7beta1 integrin is a laminin receptor that has been implicated in muscle disease and the development of neuromuscular and myotendinous junctions. Studies have shown the alpha7beta1 integrin is also expressed in nonskeletal muscle tissues. To identify the expression pattern of the alpha7 integrin in these tissues during embryonic development, alpha7 integrin chain knockout mice were generated by a LacZ knockin strategy. In these mice, expression from the alpha7 promoter is reported by beta-galactosidase. From embryonic day (ED) 11.5 to ED14.5, beta-galactosidase was detected in the developing central and peripheral nervous systems and vasculature. The loss of the alpha7 integrin gene resulted in partial embryonic lethality. Several alpha7 null embryos were identified with cerebrovascular hemorrhages and showed reduced vascular smooth muscle cells and cerebral vascularization. The alpha7 null mice that survived to birth exhibited vascular smooth muscle defects, including hyperplasia and hypertrophy. In addition, altered expression of alpha5 and alpha6B integrin chains was detected in the cerebral arteries of alpha7 null mice, which may contribute to the vascular phenotype. Our results demonstrate for the first time that the alpha7beta1 integrin is important for the recruitment or survival of cerebral vascular smooth muscle cells and that this integrin plays an important role in vascular development and integrity.

Gender dimorphism influences extracellular matrix expression and regeneration of muscular tissue in mdx dystrophic mice

Mdx mouse, the animal model of Duchenne muscular dystrophy, lacks dystrophin and develops an X-linked recessive inflammatory myopathy characterized by degeneration of skeletal muscle fibers and connective tissue replacement. The present work aimed to assess whether gender dimorphism in mdx mice would influence skeletal muscle pathology at ages corresponding to main histological changes in the microenvironment of muscular tissue: myonecrosis, regeneration, and fibrosis. At the height of myonecrosis (6 weeks postnatal), skeletal muscles of male mdx mice showed increased sarcolemmal permeability, numerous inflammatory foci, and marked deposition of the extracellular matrix components (ECM) type I collagen and laminin. In contrast, age-matched mdx females showed mild ECM deposition, discrete myonecrosis, but increased numbers of regenerating fibers expressing the satellite cell marker NCAM. In contrast ovariectomized mdx females showed decreased numbers of regenerating fibers. Older (24 and 48 weeks postnatal) mdx females showed extensive fibrosis with increased sarcolemmal permeability and marked deposition of ECM components than corresponding males. These results suggest a role for female hormones in the control of myonecrosis probably by promoting regeneration of muscular tissue and mitigating inflammation especially at ages under the critical influence of sex hormones.

Role of extracellular matrix in adaptation of tendon and skeletal muscle to mechanical loading

The extracellular matrix (ECM), and especially the connective tissue with its collagen, links tissues of the body together and plays an important role in the force transmission and tissue structure maintenance especially in tendons, ligaments, bone, and muscle. The ECM turnover is influenced by physical activity, and both collagen synthesis and degrading metalloprotease enzymes increase with mechanical loading. Both transcription and posttranslational modifications, as well as local and systemic release of growth factors, are enhanced following exercise. For tendons, metabolic activity, circulatory responses, and collagen turnover are demonstrated to be more pronounced in humans than hitherto thought. Conversely, inactivity markedly decreases collagen turnover in both tendon and muscle. Chronic loading in the form of physical training leads both to increased collagen turnover as well as, dependent on the type of collagen in question, some degree of net collagen synthesis. These changes will modify the mechanical properties and the viscoelastic characteristics of the tissue, decrease its stress, and likely make it more load resistant. Cross-linking in connective tissue involves an intimate, enzymatical interplay between collagen synthesis and ECM proteoglycan components during growth and maturation and influences the collagen-derived functional properties of the tissue. With aging, glycation contributes to additional cross-linking which modifies tissue stiffness. Physiological signaling pathways from mechanical loading to changes in ECM most likely involve feedback signaling that results in rapid alterations in the mechanical properties of the ECM. In developing skeletal muscle, an important interplay between muscle cells and the ECM is present, and some evidence from adult human muscle suggests common signaling pathways to stimulate contractile and ECM components. Unaccostumed overloading responses suggest an important role of ECM in the adaptation of myofibrillar structures in adult muscle. Development of overuse injury in tendons involve morphological and biochemical changes including altered collagen typing and fibril size, hypervascularization zones, accumulation of nociceptive substances, and impaired collagen degradation activity. Counteracting these phenomena requires adjusted loading rather than absence of loading in the form of immobilization. Full understanding of these physiological processes will provide the physiological basis for understanding of tissue overloading and injury seen in both tendons and muscle with repetitive work and leisure time physical activity.

Nerve-independent formation of a topologically complex postsynaptic apparatus

As the mammalian neuromuscular junction matures, its acetylcholine receptor (AChR)–rich postsynaptic apparatus is transformed from an oval plaque into a pretzel-shaped array of branches that precisely mirrors the branching pattern of the motor nerve terminal. Although the nerve has been believed to direct postsynaptic maturation, we report here that myotubes cultured aneurally on matrix-coated substrates form elaborately branched AChR-rich domains remarkably similar to those seen in vivo. These domains share several characteristics with the mature postsynaptic apparatus, including colocalization of multiple postsynaptic markers, clustering of subjacent myonuclei, and dependence on the muscle-specific kinase and rapsyn for their formation. Time-lapse imaging showed that branched structures arise from plaques by formation and fusion of AChR-poor perforations through a series of steps mirroring that seen in vivo. Multiple fluorophore imaging showed that growth occurs by circumferential, asymmetric addition of AChRs. Analysis in vivo revealed similar patterns of AChR addition during normal development. These results reveal the sequence of steps by which a topologically complex domain forms on a cell and suggest an unexpected nerve-independent role for the postsynaptic cell in generating this topological complexity.

The muscle integrin binding protein (MIBP) interacts with alpha7beta1 integrin and regulates cell adhesion and laminin matrix deposition

Integrins are alphabeta transmembrane receptors that function in key cellular processes, including cell adhesion, differentiation, and extracellular matrix deposition through interactions with extracellular, membrane, and cytoplasmic proteins. We previously identified and cloned a muscle beta1 integrin cytoplasmic binding protein termed MIBP and found that the expression level of MIBP is critical in the decision-making process of terminal myogenic differentiation. We report here that MIBP interacts with the alpha7beta1 integrin but not the alpha5beta1 integrin in C2C12 myoblasts, suggesting an important role of integrin alpha chains in the regulation of the beta1-MIBP interaction. Furthermore, consistent with its selective binding activity toward the alpha7beta1 laminin receptor, we have found that overexpression of MIBP in C2C12 myoblasts resulted in a significant reduction of cell adhesion to laminin and inhibition of laminin matrix deposition. By contrast, neither cell adhesion to fibronectin nor fibronectin matrix deposition was significantly altered in cells overexpressing MIBP. Finally, we show that both the protein level and tyrosine phosphorylation of paxillin, a key signaling molecule involved in the cellular control of myogenic differentiation, are increased by MIBP. These results suggest that MIBP functions in the control of myogenic differentiation by regulating alpha7beta1 integrin-mediated cell interactions with laminin matrix and intracellular signaling through paxillin.