Transient production of alpha-smooth muscle actin by skeletal myoblasts during differentiation in culture and following intramuscular implantation

Nanoparticles systemically biodistribute to regenerating skeletal muscle in DMD

Skeletal muscle disease severity can often progress asymmetrically across muscle groups and heterogeneously within tissues. An example is Duchenne Muscular Dystrophy (DMD) in which lack of dystrophin results in devastating skeletal muscle wasting in some muscles whereas others are spared or undergo hypertrophy. An efficient, non-invasive approach to identify sites of asymmetry and degenerative lesions could enable better patient monitoring and therapeutic targeting of disease. In this study, we utilized a versatile intravenously injectable mesoporous silica nanoparticle (MSNP) based nanocarrier system to explore mechanisms of biodistribution in skeletal muscle of mdx mouse models of DMD including wildtype, dystrophic, and severely dystrophic mice. Moreover, MSNPs could be imaged in live mice and whole muscle tissues enabling investigation of how biodistribution is altered by different types of muscle pathology such as inflammation or fibrosis. We found MSNPs were tenfold more likely to aggregate within select mdx muscles relative to wild type, such as gastrocnemius and quadriceps. This was accompanied by decreased biodistribution in off-target organs. We found the greatest factor affecting preferential delivery was the regenerative state of the dystrophic skeletal muscle with the highest MSNP abundance coinciding with the regions showing the highest level of embryonic myosin staining and intramuscular macrophage uptake. To demonstrate, muscle regeneration regulated MSNP distribution, we experimentally induced regeneration using barium chloride which resulted in a threefold increase of intravenously injected MSNPs to sites of regeneration 7 days after injury. These discoveries provide the first evidence that nanoparticles have selective biodistribution to skeletal muscle in DMD to areas of active regeneration and that nanoparticles could enable diagnostic and selective drug delivery in DMD skeletal muscle.

Kinin receptors regulate skeletal muscle regeneration: differential effects for B1 and B2 receptors

OBJECTIVE AND DESIGN

After traumatic skeletal muscle injury, muscle healing is often incomplete and produces extensive fibrosis. Bradykinin (BK) reduces fibrosis in renal and cardiac damage models through the B2 receptor. The B1 receptor expression is induced by damage, and blocking of the kallikrein-kinin system seems to affect the progression of muscular dystrophy. We hypothesized that both kinin B1 and B2 receptors could play a differential role after traumatic muscle injury, and the lack of the B1 receptor could produce more cellular and molecular substrates for myogenesis and fewer substrates for fibrosis, leading to better muscle healing.

MATERIAL AND METHODS

To test this hypothesis, tibialis anterior muscles of kinin receptor knockout animals were subjected to traumatic injury. Myogenesis, angiogenesis, fibrosis, and muscle functioning were evaluated.

RESULTS

Injured B1KO mice showed a faster healing progression of the injured area with a larger amount of central nucleated fiber post-injury when compared to control mice. In addition, they exhibited higher neovasculogenic capacity, maintaining optimal tissue perfusion for the post-injury phase; had higher amounts of myogenic markers with less inflammatory infiltrate and tissue destruction. This was followed by higher amounts of SMAD7 and lower amounts of p-SMAD2/3, which resulted in less fibrosis. In contrast, B2KO and B1B2KO mice showed more severe tissue destruction and excessive fibrosis. B1KO animals had better results in post-injury functional tests compared to control animals.

CONCLUSIONS

We demonstrate that injured skeletal muscle tissues have a better repair capacity with less fibrosis in the presence of B2 receptor and absence of B1 receptor, including better performances in functional tests.

Evolution and function of calponin and transgelin

Calponin and transgelin (originally named SM22) are homologous cytoskeleton proteins that regulate actin-activated myosin motor functions in smooth muscle contraction and non-muscle cell motility during adhesion, migration, proliferation, phagocytosis, wound healing, and inflammatory responses. They are abundant cytoskeleton proteins present in multiple cell types whereas their physiological functions remain to be fully established. This focused review summarizes the evolution of genes encoding calponin and transgelin and their isoforms and discusses the structural similarity and divergence in vertebrate and invertebrate species in the context of functions in regulating cell motility. As the first literature review focusing on the evolution of the calponin-transgelin family of proteins in relevance to their structure-function relationship, the goal is to outline a foundation of current knowledge for continued investigations to understand the biological functions of calponin and transgelin in various cell types during physiological and pathological processes.

Proteomic and electron microscopy study of myogenic differentiation of alveolar mucosa multipotent mesenchymal stromal cells in three-dimensional culture

Myocyte differentiation is featured by adaptation processes, including mitochondria repopulation and cytoskeleton re-organization. The difference between monolayer and spheroid cultured cells at the proteomic level is uncertain. We cultivated alveolar mucosa multipotent mesenchymal stromal cells in spheroids in a myogenic way for the proper conditioning of ECM architecture and cell morphology, which induced spontaneous myogenic differentiation of cells within spheroids. Electron microscopy analysis was used for the morphometry of mitochondria biogenesis, and proteomic was used complementary to unveil events underlying differences between two-dimensional/three-dimensional myoblasts differentiation. The prevalence of elongated mitochondria with an average area of 0.097 μm² was attributed to monolayer cells 7 days after the passage. The population of small mitochondria with a round shape and area of 0.049 μm² (p < 0.05) was observed in spheroid cells cultured under three-dimensional conditions. Cells in spheroids were quantitatively enriched in proteins of mitochondria biogenesis (DNM1L, IDH2, SSBP1), respiratory chain (ACO2, ATP5I, COX5A), extracellular proteins (COL12A1, COL6A1, COL6A2), and cytoskeleton (MYL6, MYL12B, MYH10). Most of the Rab-related transducers were inhibited in spheroid culture. The proteomic assay demonstrated delicate mechanisms of mitochondria autophagy and repopulation, cytoskeleton assembling, and biogenesis. Differences in the ultrastructure of mitochondria indicate active biogenesis under three-dimensional conditions.

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.

"Inflammatory Leiomyosarcoma" and "Histiocyte-rich Rhabdomyoblastic Tumor": a clinicopathological, immunohistochemical and genetic study of 13 cases, with a proposal for reclassification as "Inflammatory Rhabdomyoblastic Tumor"

Inflammatory leiomyosarcoma (ILMS), defined as "a malignant neoplasm showing smooth muscle differentiation, a prominent inflammatory infiltrate, and near-haploidization", is a very rare soft tissue tumor with a generally favorable prognosis. The morphologic features of "histiocyte-rich rhabdomyoblastic tumor" (HRRMT) are similar to those of ILMS, although this lesion shows by definition a skeletal muscle phenotype. Recent gene expression profiling and immunohistochemical studies have also suggested that ILMS and HRRMT may be related. We studied the clinicopathologic, immunohistochemical and genetic features of four cases previously classified as ILMS and nine classified as HRRMT. Tumors from both groups tended to occur in the deep soft tissues of the extremities of young to middle-aged males and exhibited indolent behavior. Morphologically, all were well-circumscribed, often encapsulated, and showed a striking histiocyte-rich inflammatory infiltrate admixed with variably pleomorphic tumor cells showing spindled and epithelioid to rhabdoid morphology, eosinophilic cytoplasm, and prominent nucleoli, but few, if any, mitotic figures. Immunohistochemically, the tumor cells expressed desmin, alpha-smooth muscle actin, and the rhabdomyoblastic markers PAX7, MyoD1, and myogenin. H-caldesmon expression was absent in all cases, using the specific h-CD antibody. Karyotypic study (1 HRRMT) and genome-wide copy number analysis (7 HRRMT, OncoScan SNP assay), revealed near-haploidization in four cases, with subsequent genome doubling in one, an identical phenotype to that seen in ILMS. We propose reclassification of ILMS and HRRMT as "inflammatory rhabdomyoblastic tumor", a name which accurately describes the salient morphologic and immunohistochemical features of this distinctive tumor, as well as its intermediate (rarely metastasizing) clinical behavior.

Surface-directed engineering of tissue anisotropy in microphysiological models of musculoskeletal tissue

Here, we present an approach to model and adapt the mechanical regulation of morphogenesis that uses contractile cells as sculptors of engineered tissue anisotropy in vitro. Our method uses heterobifunctional cross-linkers to create mechanical boundary constraints that guide surface-directed sculpting of cell-laden extracellular matrix hydrogel constructs. Using this approach, we engineered linearly aligned tissues with structural and mechanical anisotropy. A multiscale in silico model of the sculpting process was developed to reveal that cell contractility increases as a function of principal stress polarization in anisotropic tissues. We also show that the anisotropic biophysical microenvironment of linearly aligned tissues potentiates soluble factor-mediated tenogenic and myogenic differentiation of mesenchymal stem cells. The application of our method is demonstrated by (i) skeletal muscle arrays to screen therapeutic modulators of acute oxidative injury and (ii) a 3D microphysiological model of lung cancer cachexia to study inflammatory and oxidative muscle injury induced by tumor-derived signals.

Identifying potential patient-specific predictors for anterior cruciate ligament reconstruction outcome - a diagnostic in vitro tissue remodeling platform

Purpose

Upon anterior cruciate ligament (ACL) rupture, reconstruction is often required, with the hamstring tendon autograft as most widely used treatment. Post-operative autograft remodeling enhances graft rupture risk, which occurs in up to 10% of the patient population, increasing up to 30% of patients aged under 20 years. Therefore, this research aimed to identify potential biological predictors for graft rupture, derived from patient-specific tissue remodeling-related cell properties in an in vitro micro-tissue platform.

Methods

Hamstring tendon-derived cells were obtained from remnant autograft tissue after ACL reconstructions (36 patients, aged 12–55 years), and seeded in collagen I gels on a micro-tissue platform. Micro-tissue compaction over time – induced by altering the boundary constraints – was monitored. Pro-collagen I expression was assessed using ELISA, and protein expression of tenomodulin and α-smooth muscle actin were measured using Western blot. Expression and activity of matrix metalloproteinase 2 were determined using gelatin zymography.

Results

Only micro-tissues corresponding to younger patients occasionally released themselves from the constraining posts. Pro-collagen I expression was significantly higher in younger patients. Differences in α-smooth muscle actin and tenomodulin expression between patients were found, but these were age-independent. Active matrix metalloproteinase 2 expression was slightly more abundant in younger patients.

Conclusions

The presented micro-tissue platform exposed patient-specific remodeling-related differences between tendon-derived cells, with the micro-tissues that released from constraining posts and pro-collagen I expression best reflecting the clinical age-dependency of graft rupture. These properties can be the starting point in the quest for potential predictors for identifying individual patients at risk for graft rupture.

αv integrins on mesenchymal cells regulate skeletal and cardiac muscle fibrosis

Mesenchymal cells expressing platelet-derived growth factor receptor beta (PDGFRβ) are known to be important in fibrosis of organs such as the liver and kidney. Here we show that PDGFRβ+ cells contribute to skeletal muscle and cardiac fibrosis via a mechanism that depends on αv integrins. Mice in which αv integrin is depleted in PDGFRβ+ cells are protected from cardiotoxin and laceration-induced skeletal muscle fibrosis and angiotensin II-induced cardiac fibrosis. In addition, a small-molecule inhibitor of αv integrins attenuates fibrosis, even when pre-established, in both skeletal and cardiac muscle, and improves skeletal muscle function. αv integrin blockade also reduces TGFβ activation in primary human skeletal muscle and cardiac PDGFRβ+ cells, suggesting that αv integrin inhibitors may be effective for the treatment and prevention of a broad range of muscle fibroses.

Differential characterization of myogenic satellite cells with linolenic and retinoic acid in the presence of thiazolidinediones from prepubertal Korean black goats

Objective

Myogenic satellite cells were isolated from semitendinosus muscle of prepubertal Korean black goat to observe the differential effect of linolenic and retinoic acid in thepresence of thiazolidinediones (TZD) and also to observe the production insulin sensitive preadipocyte.

Methods

Cells were characterized for their stemness with cluster of differentiation 34 (CD34), CD13, CD106, CD44, Vimentin surface markers using flow cytometry. Cells characterized themselves as possessing significant (p<0.05) levels of CD13, CD34, CD106, Vimentin revealing their stemness potential. Goat myogenic satellite cells also exhibited CD44, indicating that they possessed a % of stemness factors of adipose lineage apart from their inherent stemness of paxillin factors 3/7.

Results

Cells during proliferation stayed absolutely and firmly within the myogenic fate without any external cues and continued to show a significant (p<0.05) fusion index % to express myogenic differentiation, myosin heavy chain, and smooth muscle actin in 2% horse serum. However, confluent myogenic satellite cells were the ones easily turning into adipogenic lineage. Intriguingly, upregulation in adipose specific genetic markers such as peroxisome proliferation-activated receptor γ, adiponectin, lipoprotein lipase, and CCAAT/enhancer binding protein α were observed and confirmed in all given treatments. However, the amount of adipogenesis was found to be statistically significant (p<0.01) with linolenic acid as compared to retinoic acid in combination with TZD’s.

Conclusion

Retinoic acid was found to produce smaller preadipocytes which have been assumed to have insulin sensitization and hence retinoic acid could be used as a potential agent to sensitize tissues to insulin in combination with TZD’s to treat diabetic conditions in humans and animals in future.

The CSRP2BP histone acetyltransferase drives smooth muscle gene expression

The expression of nearly all smooth muscle genes are controlled by serum response factor binding sites in their promoter regions. However, SRF alone is not sufficient for regulating smooth muscle cell development. It associates with other cardiovascular specific cofactors to regulate smooth muscle gene expression. Previously, we showed that the transcription co-factor CRP2 was a regulator of smooth muscle gene expression. Here, we report that CSRP2BP, a coactivator for CRP2, is a histone acetyltransferase and a driver of smooth muscle gene expression. CSRP2BP directly interacted with SRF, CRP2 and myocardin. CSRP2BP synergistically activated smooth muscle gene promoters in an SRF-dependent manner. A combination of SRF, GATA6 and CRP2 required CSRP2BP for robust smooth muscle gene promoter activity. Knock-down of Csrp2bp in smooth muscle cells resulted in reduced smooth muscle gene expression. We conclude that the CSRP2BP histone acetyltransferase is a coactivator for CRP2 that works synergistically with SRF and myocardin to regulate smooth muscle gene expression.

PDGFRα signalling promotes fibrogenic responses in collagen-producing cells in Duchenne muscular dystrophy

Fibrosis is a characteristic of Duchenne muscular dystrophy (DMD), yet the cellular and molecular mechanisms responsible for DMD fibrosis are poorly understood. Utilizing the Collagen1a1-GFP transgene to identify cells producing Collagen-I matrix in wild-type mice exposed to toxic injury or those mutated at the dystrophin gene locus (mdx) as a model of DMD, we studied mechanisms of skeletal muscle injury/repair and fibrosis. PDGFRα is restricted to Sca1+, CD45- mesenchymal progenitors. Fate-mapping experiments using inducible CreER/LoxP somatic recombination indicate that these progenitors expand in injury or DMD to become PDGFRα+, Col1a1-GFP+ matrix-forming fibroblasts, whereas muscle fibres do not become fibroblasts but are an important source of the PDGFRα ligand, PDGF-AA. While in toxin injury/repair of muscle PDGFRα, signalling is transiently up-regulated during the regenerative phase in the DMD model and in human DMD it is chronically overactivated. Conditional expression of the constitutively active PDGFRα D842V mutation in Collagen-I+ fibroblasts, during injury/repair, hindered the repair phase and instead promoted fibrosis. In DMD, treatment of mdx mice with crenolanib, a highly selective PDGFRα/β tyrosine kinase inhibitor, reduced fibrosis, improved muscle strength, and was associated with decreased activity of Src, a downstream effector of PDGFRα signalling. These observations are consistent with a model in which PDGFRα activation of mesenchymal progenitors normally regulates repair of the injured muscle, but in DMD persistent and excessive activation of this pathway directly drives fibrosis and hinders repair. The PDGFRα pathway is a potential new target for treatment of progressive DMD. © 2016 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Mammalian Actins: Isoform-Specific Functions and Diseases

Actin is the central building block of the actin cytoskeleton, a highly regulated filamentous network enabling dynamic processes of cells and simultaneously providing structure. Mammals have six actin isoforms that are very conserved and thus share common functions. Tissue-specific expression in part underlies their differential roles, but actin isoforms also coexist in various cell types and tissues, suggesting specific functions and preferential interaction partners. Gene deletion models, antibody-based staining patterns, gene silencing effects, and the occurrence of isoform-specific mutations in certain diseases have provided clues for specificity on the subcellular level and its consequences on the organism level. Yet, the differential actin isoform functions are still far from understood in detail. Biochemical studies on the different isoforms in pure form are just emerging, and investigations in cells have to deal with a complex and regulated system, including compensatory actin isoform expression.

Conditions that promote primary human skeletal myoblast culture and muscle differentiation in vitro

Conditions under which skeletal myoblasts are cultured in vitro are critical to growth and differentiation of these cells into mature skeletal myofibers. We examined several culture conditions that promoted human skeletal myoblast (HSkM) culture and examined the effect of microRNAs and mechanical stimulation on differentiation. Culture conditions for HSkM are different from those that enable rapid C2C12 myoblast differentiation. Culture on a growth factor-reduced Matrigel (GFR-MG)-coated surface in 2% equine serum-supplemented differentiation medium to promote HSkM differentiation under static conditions was compared with culture conditions used for C2C12 cell differentiation. Such conditions led to a >20-fold increase in myogenic miR-1, miR-133a, and miR-206 expression, a >2-fold increase in myogenic transcription factor Mef-2C expression, and an increase in sarcomeric α-actinin protein. Imposing ±10% cyclic stretch at 0.5 Hz for 1 h followed by 5 h of rest over 2 wk produced a >20% increase in miR-1, miR-133a, and miR-206 expression in 8% equine serum and a >35% decrease in 2% equine serum relative to static conditions. HSkM differentiation was accelerated in vitro by inhibition of proliferation-promoting miR-133a: immunofluorescence for sarcomeric α-actinin exhibited accelerated development of striations compared with the corresponding negative control, and Western blotting showed 30% more α-actinin at day 6 postdifferentiation. This study showed that 100 μg/ml GFR-MG coating and 2% equine serum-supplemented differentiation medium enhanced HSkM differentiation and myogenic miR expression and that addition of antisense miR-133a alone can accelerate primary human skeletal muscle differentiation in vitro.

Distinct roles for Ste20-like kinase SLK in muscle function and regeneration

BACKGROUND

Cell growth and terminal differentiation are controlled by complex signaling systems that regulate the tissue-specific expression of genes controlling cell fate and morphogenesis. We have previously reported that the Ste20-like kinase SLK is expressed in muscle tissue and is required for cell motility. However, the specific function of SLK in muscle tissue is still poorly understood.

METHODS

To gain further insights into the role of SLK in differentiated muscles, we expressed a kinase-inactive SLK from the human skeletal muscle actin promoter. Transgenic muscles were surveyed for potential defects. Standard histological procedures and cardiotoxin-induced regeneration assays we used to investigate the role of SLK in myogenesis and muscle repair.

RESULTS

High levels of kinase-inactive SLK in muscle tissue produced an overall decrease in SLK activity in muscle tissue, resulting in altered muscle organization, reduced litter sizes, and reduced breeding capacity. The transgenic mice did not show any differences in fiber-type distribution but displayed enhanced regeneration capacity in vivo and more robust differentiation in vitro.

CONCLUSIONS

Our results show that SLK activity is required for optimal muscle development in the embryo and muscle physiology in the adult. However, reduced kinase activity during muscle repair enhances regeneration and differentiation. Together, these results suggest complex and distinct roles for SLK in muscle development and function.

Gingival rhabdomyosarcoma accompanied by an immature myogenic population immunoreactive for α-smooth muscle actin in a dog

A 3-year-old female shih tzu was presented with a white to dark red mass arising from the gingiva. Because of the rapid and invasive growth of the mass, the dog was humanely destroyed. Microscopically, round to polygonal anaplastic cells with strongly eosinophilic cytoplasm grew in an alveolar pattern separated by fibrous stroma. Mitotic figures were numerous. Multinucleated cells and 'strap cells' were observed, but cross striation and glycogen accumulation were absent. Immunohistochemically, the tumour cells were positive for vimentin, desmin, muscle-specific actin and MyoD1, and a small number of tumour cells were positive for α-smooth muscle actin (α-SMA). Based on the morphological and immunohistochemical features, the gingival mass was diagnosed as alveolar rhabdomyosarcoma accompanied by α-SMA-positive immature myogenic cells.

Muscle-derived cells for treatment of iatrogenic sphincter damage and urinary incontinence in men

Introduction. Aim of this study was to assess the safety and efficacy of injection of autologous muscle-derived cells into the urinary sphincter for treatment of postprostatectomy urinary incontinence in men and to characterize the injected cells prior to transplantation. Methods. 222 male patients with stress urinary incontinence and sphincter damage after uroloical procedures were treated with transurethral injection of autologous muscle-derived cells. The transplanted cells were investigated after cultivation and prior to application by immunocytochemistry using different markers of myogenic differentiation. Feasibility and functionality assessment was achieved with a follow-up of at least 12 months. Results. Follow-up was at least 12 months. Of the 222 treated patients, 120 responded to therapy of whom 26 patients (12%) were continent, and 94 patients (42%) showed improvement. In 102 (46%) patients, the therapy was ineffective. Clinical improvement was observed on average 4.7 months after transplantation and continued in all improved patients. The cells injected into the sphincter were at least ~50% of myogenic origin and representative for early stages of muscle cell differentiation. Conclusions. Transurethral injection of muscle-derived cells into the damaged urethral sphincter of male patients is a safe procedure. Transplanted cells represent different phases of myogenic differentiation.

Barx homeobox family in muscle development and regeneration

Homeobox transcription factors are key intrinsic regulators of myogenesis. In studies spanning several years, we have characterized the homeobox factor Barx2 as a novel marker for muscle progenitor cells and an important regulator of muscle growth and repair. In this review, we place the expression and function of Barx2 and its paralogue Barx1 in context with other muscle-expressed homeobox factors in both embryonic and adult myogenesis. We also describe the structure and regulation of Barx genes and possible gene/disease associations. The functional domains of Barx proteins, their molecular interactions, and cellular functions are presented with particular emphasis on control of genes and processes involved in myogenic differentiation. Finally, we describe the patterns of Barx gene expression in vivo and the phenotypes of various Barx gene perturbation models including null mice. We focus on the Barx2 null mouse model, which has demonstrated the critical roles of Barx2 in postnatal myogenesis including muscle maintenance during aging, and regeneration of acute and chronic muscle injury.

Engineering spatial control of multiple differentiation fates within a stem cell population

The capability to engineer microenvironmental cues to direct a stem cell population toward multiple fates, simultaneously, in spatially defined regions is important for understanding the maintenance and repair of multi-tissue units. We have previously developed an inkjet-based bioprinter to create patterns of solid-phase growth factors (GFs) immobilized to an extracellular matrix (ECM) substrate, and applied this approach to drive muscle-derived stem cells toward osteoblasts 'on-pattern' and myocytes 'off-pattern' simultaneously. Here this technology is extended to spatially control osteoblast, tenocyte and myocyte differentiation simultaneously. Utilizing immunofluorescence staining to identify tendon-promoting GFs, fibroblast growth factor-2 (FGF-2) was shown to upregulate the tendon marker Scleraxis (Scx) in C3H10T1/2 mesenchymal fibroblasts, C2C12 myoblasts and primary muscle-derived stem cells, while downregulating the myofibroblast marker α-smooth muscle actin (α-SMA). Quantitative PCR studies indicated that FGF-2 may direct stem cells toward a tendon fate via the Ets family members of transcription factors such as pea3 and erm. Neighboring patterns of FGF-2 and bone morphogenetic protein-2 (BMP-2) printed onto a single fibrin-coated coverslip upregulated Scx and the osteoblast marker ALP, respectively, while non-printed regions showed spontaneous myotube differentiation. This work illustrates spatial control of multi-phenotype differentiation and may have potential in the regeneration of multi-tissue units.

Skeletal muscle fibrosis: the effect of stromal-derived factor-1&#x03B1;-loaded collagen scaffolds

AIM

To develop a model for muscle fibrosis based on full-thickness muscle defects, and to evaluate the effects of implanted stromal-derived factor (SDF)-1α-loaded collagen scaffolds.

METHODS

Full-thickness defects 2 mm in diameter were made in the musculus soleus of 48 rats and either left alone or filled with SDF-1α-loaded collagen scaffolds. At 3, 10, 28 and 56 days postsurgery, muscles were analyzed for collagen deposition, satellite cells, myofibroblasts and macrophages.

RESULTS

A significant amount of collagen-rich fibrotic tissue was formed, which persisted over time. Increased numbers of satellite cells were present around, but not within, the wounds. Satellite cells were further upregulated in regenerating tissue when SDF-1α-loaded collagen scaffolds were implanted. The scaffolds also attracted macrophages, but collagen deposition and myofibroblast numbers were not affected.

CONCLUSION

Persistent muscle fibrosis is induced by full-thickness defects 2 mm in diameter. SDF-1α-loaded collagen scaffolds accelerated muscle regeneration around the wounds, but did not reduce muscle fibrosis.

The homeobox transcription factor Barx2 regulates plasticity of young primary myofibers

BACKGROUND

Adult mammalian muscle retains incredible plasticity. Muscle growth and repair involves the activation of undifferentiated myogenic precursors called satellite cells. In some circumstances, it has been proposed that existing myofibers may also cleave and produce a pool of proliferative cells that can re-differentiate into new fibers. Such myofiber dedifferentiation has been observed in the salamander blastema where it may occur in parallel with satellite cell activation. Moreover, ectopic expression of the homeodomain transcription factor Msx1 in differentiated C2C12 myotubes has been shown to induce their dedifferentiation. While it remains unclear whether dedifferentiation and redifferentiaton occurs endogenously in mammalian muscle, there is considerable interest in induced dedifferentiation as a possible regenerative tool.

METHODOLOGY/PRINCIPAL FINDINGS

We previously showed that the homeobox protein Barx2 promotes myoblast differentiation. Here we report that ectopic expression of Barx2 in young immature myotubes derived from cell lines and primary mouse myoblasts, caused cleavage of the syncytium and downregulation of differentiation markers. Microinjection of Barx2 cDNA into immature myotubes derived from primary cells led to cleavage and formation of mononucleated cells that were able to proliferate. However, injection of Barx2 cDNA into mature myotubes did not cause cleavage. Barx2 expression in C2C12 myotubes increased the expression of cyclin D1, which may promote cell cycle re-entry. We also observed differential muscle gene regulation by Barx2 at early and late stages of muscle differentiation which may be due to differential recruitment of transcriptional activator or repressor complexes to muscle specific genes by Barx2.

CONCLUSIONS/SIGNIFICANCE

We show that Barx2 regulates plasticity of immature myofibers and might act as a molecular switch controlling cell differentiation and proliferation.

Pax3 induces differentiation of juvenile skeletal muscle stem cells without transcriptional upregulation of canonical myogenic regulatory factors

Pax3 is an essential myogenic regulator of fetal and embryonic development, but its role in postnatal myogenesis remains a topic of debate. We show that constitutive expression of Pax3 in postnatal, juvenile mouse skeletal muscle stem cells, a subset of the heterogeneous satellite cell pool highly enriched for myogenic activity, potently induces differentiation. This differentiation-promoting activity stands in contrast to the differentiation-inhibiting effects of Pax3 in the commonly used mouse myoblast cell line C2C12. Pax3 mRNA levels in distinct muscles correlate with the rate of myogenic differentiation of their muscle stem cells. Although Pax3 controls embryonic myogenesis through regulation of the canonical myogenic regulatory factors (MRFs) Myf-5, MyoD, myogenin and Mrf4, we find that in postnatal muscle stem cells, ectopic Pax3 expression fails to induce expression of any of these factors. Unexpectedly, overexpression of neither Myf-5 nor myogenin is sufficient to induce differentiation of juvenile stem cells; and knockdown of Myf-5, rather than inhibiting differentiation, promotes it. Taken together, our results suggest that there are distinct myogenic regulatory pathways that control the embryonic development, juvenile myogenesis and adult regeneration of skeletal myofibers.

Absence of CD34 on murine skeletal muscle satellite cells marks a reversible state of activation during acute injury

BACKGROUND

Skeletal muscle satellite cells are myogenic progenitors that reside on myofiber surface beneath the basal lamina. In recent years satellite cells have been identified and isolated based on their expression of CD34, a sialomucin surface receptor traditionally used as a marker of hematopoietic stem cells. Interestingly, a minority of satellite cells lacking CD34 has been described.

METHODOLOGY/PRINCIPAL FINDINGS

In order to elucidate the relationship between CD34+ and CD34- satellite cells we utilized fluorescence-activated cell sorting (FACS) to isolate each population for molecular analysis, culture and transplantation studies. Here we show that unless used in combination with alpha7 integrin, CD34 alone is inadequate for purifying satellite cells. Furthermore, the absence of CD34 marks a reversible state of activation dependent on muscle injury.

CONCLUSIONS/SIGNIFICANCE

Following acute injury CD34- cells become the major myogenic population whereas the percentage of CD34+ cells remains constant. In turn activated CD34- cells can reverse their activation to maintain the pool of CD34+ reserve cells. Such activation switching and maintenance of reserve pool suggests the satellite cell compartment is tightly regulated during muscle regeneration.

Actin isoform expression patterns during mammalian development and in pathology: insights from mouse models

The dynamic actin cytoskeleton, consisting of six actin isoforms in mammals and a variety of actin binding proteins is essential for all developmental processes and for the viability of the adult organism. Actin isoform specific functions have been proposed for muscle contraction, cell migration, endo- and exocytosis and maintaining cell shape. However, these specific functions for each of the actin isoforms during development are not well understood. Based on transgenic mouse models, we will discuss the expression patterns of the six conventional actin isoforms in mammals during development and adult life. Ablation of actin genes usually leads to lethality and affects expression of other actin isoforms at the cell or tissue level. A good knowledge of their expression and functions will contribute to fully understand severe phenotypes or diseases caused by mutations in actin isoforms.

Barx2 controls myoblast fusion and promotes MyoD-mediated activation of the smooth muscle alpha-actin gene

Remodeling of the actin cytoskeleton is a critical early step in skeletal muscle differentiation. Smooth muscle alpha-actin (SMA) is one of the earliest markers of myoblast differentiation and is important for the migration and cell shape changes that precede fusion. We have found that satellite cell-derived primary myoblasts from mice lacking the Barx2 homeobox gene show altered patterns of actin remodeling, reduced cell migration, and delayed differentiation. Consistent with the role of SMA in these processes, Barx2(-)(/)(-) myoblasts also show reduced expression of SMA mRNA and protein. The proximal SMA promoter contains binding sites for muscle regulatory factors and serum response factor as well as a conserved homeodomain binding site (HBS). We found that Barx2 binds to the HBS element and potentiates up-regulation of SMA promoter activity by MyoD. We also show that Barx2, MyoD, and serum response factor simultaneously occupy the SMA promoter in cells and that Barx2 interacts with MyoD. Overall these data indicate that Barx2 cooperates with other muscle-expressed transcription factors to regulate the early cytoskeletal remodeling events that underlie efficient myoblast differentiation.

Isolation and characterization of stem cells from the human parathyroid gland

OBJECTIVES

Somatic stem cells can be obtained from a variety of adult human tissues. However, it was not clear whether human parathyroid glands, which secrete parathyroid hormones and are essential in maintaining homeostasis levels of calcium ions in the circulation, contained stem cells. We aimed to investigate the possibility of isolating such parathyroid-derived stem cells (PDSC).

MATERIALS AND METHODS

Surgically removed parathyroid glands were obtained with informed consent. Cell cytogenetics was used to observe chromosomal abnormalities. Surface phenotypes were characterized by flow cytometry. Telomerase repeat amplification protocol (TRAP) assay was performed to observe the telomerase activity. RT-PCR and real-time PCR was was used to detect gene expressions. Real-time calcium uptake imaging was performed for extent of calcium uptake and transmission electron microscopy and immunofluorecent staining for smooth muscle actin.

RESULTS

After enzymatic digestion and primary culture, plastic-adherent, fibroblast-like cells appeared in culture and a morphologically homogeneous population was derived from subsequent limiting dilution and clonal expansion. Karyotyping was normal and doubling time of clonal cell growth was estimated to be 70.7 +/- 14.5 h (mean +/- standard deviation). The surface phenotype of the cells was positive for CD73, CD166, CD29, CD49a, CD49b, CD49d, CD44, CD105, and MHC class I, and negative for CD34, CD133, CD117, CD114, CD31, CD62P, EGF-R, ICAM-3, CD26, CXCR4, CD106, CD90 and MHC class II, similar to mesenchymal stem cells (MSC). Detectable levels of telomerase activity along with pluripotency Sall4 gene expression were observed from the isolated PDSCs. Expression of calcium-sensing receptor gene along with alpha-smooth muscle actin was induced and cellular uptake of extracellular calcium ions was observed. Furthermore, PDSCs possessed osteogenic, chondrogenic and adipogenic differentiation potentials.

CONCLUSIONS

Our results reveal that PDSCs were similar phenotypically to MSCs and further studies are needed to formulate induction conditions to differentiate PDSCs into parathyroid hormone-secreting chief cells.

Murine Muscle Precursor Cells Survived and Integrated in a Cryoinjured Gastroesophageal Junction

BACKGROUND

Mini-invasive techniques for gastroesophageal reflux disease (GERD), such as endoscopic injections of inert materials, have been introduced in recent years. However, results are still preliminary. Cell injection has emerged as an alternative strategy in both vesicoureteral reflux and incontinence. Here we report, for the first time, the injection of muscle precursor cells (MPCs) in the gastroesophageal junction (GEJ).

MATERIALS AND METHODS

MPCs were derived from expanded satellite cells isolated from skeletal muscle fibers of green fluorescent protein (GFP) positive mice. Via laparotomy, GFP-negative mice were subjected to cryoinjury of GEJ followed by injection of MPCs (experimental animals), bone marrow derived cells, or saline (controls).

RESULTS

Immunofluorescence analyses of experimental GEJs demonstrated coexpression of GFP and desmin in grafted cells. GFP+ muscle neofibers were evident at 4 wk after injection. Coexpression of GFP and smooth muscle actin was also observed at 2 wk.

CONCLUSIONS

Satellite cells could be easily harvested, expanded in culture, and used as injectable substance in the GEJ. These results could be the background for the development of a new injection technique for GERD treatment, which might combine bulging and functional actions.

Localization of vascular response to VEGF is not dependent on heparin binding

The major vascular endothelial growth factor (VEGF) isoforms are splice variants from a single gene that differ in their extent of heparin affinity due to the absence of the heparin binding domain in the smallest isoform (mouse VEGF120, human VEGF121). A long-held assumption that has guided the use of VEGF isoforms clinically has been that their differences in heparin binding dictate their ability to diffuse through tissue, with VEGF121 moving most freely and that the distribution of recombinant VEGF would have therapeutically relevant consequences. To test this assumption, we delivered the genes encoding these isoforms by myoblast-mediated gene transfer, a means of delivering genes to highly localized sites within muscle. Surprisingly, all isoforms induced comparable extremely localized physiological effects. Significantly, irrespective of the isoform delivered, the vessels passing within several micrometers of muscle fibers expressing VEGF displayed sharply delineated changes in morphology. The induction of capillary wrapping around VEGF-producing fibers, and of vascular malformations in the muscle at high levels, did not differ among isoforms. These results indicate that heparin binding is not essential for the localization of VEGF in adult tissue and suggest that the preferential delivery of VEGF121 cDNA for clinical applications may not have a physiological basis.

Transcription Enhancer Factor-1-dependent Expression of the α-Tropomyosin Gene in the Three Muscle Cell Types

In vertebrates, the actin-binding proteins tropomyosins are encoded by four distinct genes that are expressed in a complex pattern during development and muscle differentiation. In this study, we have characterized the transcriptional machinery of the alpha-tropomyosin (alpha-Tm) gene in muscle cells. Promoter analysis revealed that a 284-bp proximal promoter region of the Xenopus laevis alpha-Tm gene is sufficient for maximal activity in the three muscle cell types. The transcriptional activity of this promoter in the three muscle cell types depends on both distinct and common cis-regulatory sequences. We have identified a 30-bp conserved sequence unique to all vertebrate alpha-Tm genes that contains an MCAT site that is critical for expression of the gene in all muscle cell types. This site can bind transcription enhancer factor-1 (TEF-1) present in muscle cells both in vitro and in vivo. In serum-deprived differentiated smooth muscle cells, TEF-1 was redistributed to the nucleus, and this correlated with increased activity of the alpha-Tm promoter. Overexpression of TEF-1 mRNA in Xenopus embryonic cells led to activation of both the endogenous alpha-Tm gene and the exogenous 284-bp promoter. Finally, we show that, in transgenic embryos and juveniles, an intact MCAT sequence is required for correct temporal and spatial expression of the 284-bp gene promoter. This study represents the first analysis of the transcriptional regulation of the alpha-Tm gene in vivo and highlights a common TEF-1-dependent regulatory mechanism necessary for expression of the gene in the three muscle lineages.

The cardiac isoform of α-actin in regenerating and atrophic skeletal muscle, myopathies and rhabdomyomatous tumors: an immunohistochemical study using monoclonal antibodies

The two sarcomeric isoforms of actins, cardiac and skeletal muscle alpha-actin, are highly homologous so that their immunohistochemical distinction is extremely difficult. Taking advantage of monoclonal antibodies distinguishing the two conservative amino acid exchanges near the aminoterminus, we have performed an extended immunohistochemical analysis of the cardiac alpha-actin (CAA) isoform in normal, regenerating, diseased and neoplastic human muscle tissues. Intense and uniform CAA staining is seen in fetal and adult myocardium and in fetal skeletal muscle while adult skeletal muscle is essentially negative, except for muscle spindle myocytes and a few scattered muscle fibres with overall reduced diameter. By contrast, CAA synthesis is markedly induced in regenerating skeletal muscle cells, in Duchenne muscular dystrophy and upon degenerative atrophy. CAA has also been detected in certain vascular and visceral smooth muscle cells. Among tumors, CAA has consistently been seen in rhabdomyosarcomas and rhabdomyomatous cells of nephroblastomas, whereas, smooth muscle tumors have shown only occasional staining. While the synthesis of this actin isoform is less restricted than previously thought, monoclonal antibodies against CAA provide a well-defined, reliable and sensitive diagnostic tool for the definition and detection of aberrant differentiation in diseased skeletal muscle and of striated muscle differentiation in rhabdomyosarcomas.

Mechanisms responsible for the promoter-specific effects of myocardin

Understanding the mechanism of smooth muscle cell (SMC) differentiation will provide the foundation for elucidating SMC-related diseases such as atherosclerosis, restenosis, and asthma. Recent studies have demonstrated that the interaction of SRF with the co-activator myocardin is a critical determinant of smooth muscle development. It has been proposed that the specific transcriptional activation of smooth muscle-restricted genes (as opposed to other SRF-dependent genes) by myocardin results from the presence of multiple CArG boxes in smooth muscle genes that facilitate myocardin homodimer formation. This proposal was further tested in the current study. Our results show that the SMC-specific telokin promoter, which contains only a single CArG box, is strongly activated by myocardin. Furthermore, myocardin and a dimerization defective mutant myocardin induce expression of endogenous telokin but not c-fos in 10T1/2 fibroblast cells. Knocking down myocardin by small interfering RNA decreased telokin promoter activity and expression in A10 SMCs. A series of telokin and c-fos promoter chimeric and mutant reporter genes was generated to determine the mechanisms responsible for the promoter-specific effects of myocardin. Data from these experiments demonstrated that the ets binding site in the c-fos promoter partially blocks the activation of this promoter by myocardin. However, the binding of ets factors alone was not sufficient to explain the promoter-specific effects of myocardin. Elements 3' of the CArG box in the c-fos promoter act in concert with the ets binding site to block the ability of myocardin to activate the promoter. Conversely, elements 5' and 3' of the CArG box in the telokin promoter act in concert with the CArG box to facilitate myocardin stimulation of the promoter. Together these data suggest that the promoter specificity of myocardin is dependent on complex combinatorial interactions of multiple cis elements and their trans binding factors.

Skeletal muscle satellite cells can spontaneously enter an alternative mesenchymal pathway

We show that muscle satellite cells, traditionally considered as committed myogenic precursors, are comprised of Pax7-expressing progenitors that preserve a mesenchymal repertoire extending beyond a mere myogenic potential. Mouse satellite cells from freshly isolated single myofibers, cultured individually in serum-rich growth medium, produced myogenic and non-myogenic clones. Only the myogenic clones expressed muscle-specific transcription factors and formed myotubes. Pax7 was initially expressed in all clones, but subsequently was associated only with the myogenic clones. Some cells in the non-myogenic clones expressed alpha-smooth muscle actin and nestin whereas others differentiated into mature adipocytes. This type of cell composition mirrors characteristics of mesenchymal stem cell progeny. Overall, individual myofibers persistently gave rise to both clonal phenotypes, but the ratio of myogenic to non-myogenic clones randomly varied among fibers. This randomness indicates that clonal dichotomy reflects satellite cell suppleness rather than pre-fated cell heterogeneity. We conclude that satellite cells possess mesenchymal plasticity, being able to commit either to myogenesis or to a mesenchymal alternative differentiation (MAD) program.

Mesoangioblasts--vascular progenitors for extravascular mesodermal tissues

Mesoangioblasts are multipotent progenitors of mesodermal tissues that express the key marker of angiopoietic progenitors, Flk1 (VEGF-receptor 2), and are physically associated with the embryonic dorsal aorta in avian and mammalian species. When transplanted in vivo, they give rise to multiple differentiated mesodermal phenotypes. Their ability to extensively self-renew in vitro, while retaining multipotency, qualifies mesoangioblasts as a novel class of stem cells. Mesoangioblasts disclose not only an unexpected source of progenitors for skeletal muscle and a variety of other mesoderm-derived tissues, but also establish a lineage kinship between progenitors of vascular and extravascular mesodermal tissues, with important basic and applicative implications.

Localized arteriole formation directly adjacent to the site of VEGF-induced angiogenesis in muscle

We have shown previously that implantation of myoblasts constitutively expressing the VEGF-A gene into nonischemic mouse skeletal muscle leads to overgrowth of capillary-like blood vessels and hemangioma formation. These aberrant effects occurred directly at the implantation site. We show here that these regions result from angiogenic capillary growth and involve a change in capillary growth pattern and that smooth muscle-coated vessels similar to arterioles form directly adjacent to the implantation site. Myoblasts genetically engineered to produce VEGF were implanted into mouse leg muscles. Implantation sites were surrounded by a zone of dense capillary-sized vessels, around which was a second zone of muscle containing larger, smooth-muscle-covered vessels but few capillaries, and an outer zone of muscle exhibiting normal capillary density. The lack of capillaries in the middle region suggests that the preexisting capillaries adjacent to the implantation site underwent enlargement and/or fusion and recruited a smooth muscle coat. Capillaries at the implantation site were frequently wrapped around VEGF-producing muscle fibers and were continuous with the circulation and were not observed to include bone-marrow-derived endothelial cells. In contrast with the distant arteriogenesis resulting from VEGF delivery described in previous studies, we report here that highly localized arterioles also form adjacent to the site of delivery.

The homeodomain protein Barx2 promotes myogenic differentiation and is regulated by myogenic regulatory factors

The homeobox protein Barx2 is expressed in both smooth and skeletal muscle and is up-regulated during differentiation of skeletal myotubes. Here we use antisense-oligonucleotide inhibition of Barx2 expression in limb bud cell culture to show that Barx2 is required for myotube formation. Moreover, overexpression of Barx2 accelerates the fusion of MyoD-positive limb bud cells and C2C12 myoblasts. However, overexpression of Barx2 does not induce ectopic MyoD expression in either limb bud cultures or in multipotent C3H10T1/2 mesenchymal cells, and does not induce fusion of C3H10T1/2 cells. These results suggest that Barx2 acts downstream of MyoD. To test this hypothesis, we isolated the Barx2 gene promoter and identified DNA regulatory elements that might control Barx2 expression during myogenesis. The proximal promoter of the Barx2 gene contained binding sites for several factors involved in myoblast differentiation including MyoD, myogenin, serum response factor, and myocyte enhancer factor 2. Co-transfection experiments showed that binding sites for both MyoD and serum response factor are necessary for activation of the promoter by MyoD and myogenin. Taken together, these studies indicate that Barx2 is a key regulator of myogenic differentiation that acts downstream of muscle regulatory factors.