Involvement of Wnt4 signaling during myogenic proliferation and differentiation of skeletal muscle

Delta-like noncanonical notch ligand 2 regulates the proliferation and differentiation of sheep myoblasts through the Wnt/β-catenin signaling pathway

This study delved into the role of delta-like noncanonical notch ligand 2 (DLK2) in the cell cycle, proliferation, apoptosis, and differentiation of myoblasts, as well as its interaction with the classical Wnt/β-catenin signaling pathway in regulating myoblast function. The research revealed that upregulation of DLK2 in myoblasts during the proliferation phase enhanced myoblast proliferation, facilitated cell cycle progression, and reduced apoptosis. Conversely, downregulation of DLK2 expression using siRNA during the differentiation phase promoted myoblast hypertrophy and fusion, suppressed the expression of muscle fiber degradation factors, and expedited the differentiation process. DLK2 regulates myoblasts function by influencing the expression of various factors associated with the Wnt/β-catenin signaling pathway, including CTNNB1, FZD1, FZD6, RSPO1, RSPO4, WNT4, WNT5A, and adenomatous polyposis coli. In essence, DLK2, with the involvement of the Wnt/β-catenin signaling pathway, plays a crucial regulatory role in the cell cycle, proliferation, apoptosis, and differentiation of myoblasts.

Wnt/β-catenin signaling pathway as an important mediator in muscle and bone crosstalk: A systematic review

Background

The interaction between muscle and bone is shown to be clinically important but the underlying mechanisms are largely unknown. The canonical Wnt/β-catenin signaling pathway is reported to be involved in muscle-bone crosstalk, but its detailed function remains unclear. This systematic review aims to investigate and elucidate the role of the Wnt/β-catenin signaling pathways in muscle-bone crosstalk.

Methods

We conducted a literature search on the Web of Science, PubMed, EBSCO and Embase with keywords "Wnt*", "bone*" and "muscle*". A systematic review was completed according to the guideline of preferred reporting items of systematic reviews and meta-analyses (PRISMA). Data synthesis included species (human, animal or cell type used), treatments involved, outcome measures and key findings with respect to Wnts.

Results

Seventeen papers were published from 2007 to 2021 and were extracted from a total of 1529 search results in the databases of Web of Science (468 papers), PubMed (457 papers), EBSCO (371) and Embase (233). 12 Wnt family members were investigated in the papers, including Wnt1, Wnt2, Wnt2b, Wnt3a, Wnt4, Wnt5a, Wnt8a, Wnt8b, Wnt9a, Wnt10a, Wnt10b and Wnt16. Many studies showed that muscles were able to increase or decrease osteogenesis of bone, while bone increased myogenesis of muscle through Wnt/β-catenin signaling pathways. Wnt3a, Wnt4 and Wnt10b were shown to play important roles in the crosstalk between muscle and bone.

Conclusions

Wnt3a, Wnt4 and Wnt10b are found to play important mediatory roles in muscle-bone crosstalk. The role of Wnt4 was mostly found to regulate muscle from the bone side. Whilst the role of Wnt10b during muscle ageing was proposed, current evidence is insufficient to clarify the specific role of Wnt/β-catenin signaling in the interplay between sarcopenia and osteoporosis. More future studies are required to investigate the exact regulatory roles of Wnts in muscle-bone crosstalk in musculoskeletal disease models such as sarcopenia and osteoporosis.

Translational potential of this article

The systematic review provides an extensive overview to reveal the roles of Wnt/β-catenin signaling pathways in muscle-bone crosstalk. These results provide novel research directions to further understand the underlying mechanism of sarcopenia, osteoporosis, and their crosstalk, finally helping the future development of new therapeutic interventions.

The Importance of on Sclerostin as Bone-Muscle Mediator Crosstalk

Loss of bone and muscle mass is a frequent aging condition and has become a growing public health problem. The term “osteosarcopenia” denotes close links between bone and muscle. Mechanical exercise was once thought to be the only mechanism of crosstalk between muscle and bone. Sclerostin is an important player in the process of unloading-induced bone loss and plays an important role in mechanotransduction in the bone. Furthermore, bones and muscles are categorized as endocrine organs because they produce hormone-like substances, resulting in “bone-muscle crosstalk.” Sclerostin, an inhibitor of bone development, has recently been shown to play a role in myogenesis. This review discusses the importance of sclerostin in bone-muscle crosstalk.

Facioscapulohumeral dystrophy weakened sarcomeric contractility is mimicked in induced pluripotent stem cells-derived innervated muscle fibres

BACKGROUND

Facioscapulohumeral dystrophy (FSHD) is a late-onset autosomal dominant form of muscular dystrophy involving specific groups of muscles with variable weakness that precedes inflammatory response, fat infiltration, and muscle atrophy. As there is currently no cure for this disease, understanding and modelling the typical muscle weakness in FSHD remains a major milestone towards deciphering the disease pathogenesis as it will pave the way to therapeutic strategies aimed at correcting the functional muscular defect in patients.

METHODS

To gain further insights into the specificity of the muscle alteration in this disease, we derived induced pluripotent stem cells from patients affected with Types 1 and 2 FSHD but also from patients affected with Bosma arhinia and microphthalmia. We differentiated these cells into contractile innervated muscle fibres and analysed their transcriptome by RNA Seq in comparison with cells derived from healthy donors. To uncover biological pathways altered in the disease, we applied MOGAMUN, a multi-objective genetic algorithm that integrates multiplex complex networks of biological interactions (protein-protein interactions, co-expression, and biological pathways) and RNA Seq expression data to identify active modules.

RESULTS

We identified 132 differentially expressed genes that are specific to FSHD cells (false discovery rate < 0.05). In FSHD, the vast majority of active modules retrieved with MOGAMUN converges towards a decreased expression of genes encoding proteins involved in sarcomere organization (P value 2.63e^-12 ), actin cytoskeleton (P value 9.4e^-5 ), myofibril (P value 2.19e^-12 ), actin-myosin sliding, and calcium handling (with P values ranging from 7.9e^-35 to 7.9e^-21 ). Combined with in vivo validations and functional investigations, our data emphasize a reduction in fibre contraction (P value < 0.0001) indicating that the muscle weakness that is typical of FSHD clinical spectrum might be associated with dysfunction of calcium release (P value < 0.0001), actin-myosin interactions, motor activity, mechano-transduction, and dysfunctional sarcomere contractility.

CONCLUSIONS

Identification of biomarkers of FSHD muscle remain critical for understanding the process leading to the pathology but also for the definition of readouts to be used for drug design, outcome measures, and monitoring of therapies. The different pathways identified through a system biology approach have been largely overlooked in the disease. Overall, our work opens new perspectives in the definition of biomarkers able to define the muscle alteration but also in the development of novel strategies to improve muscle function as it provides functional parameters for active molecule screening.

Integrated genomic and proteomic analyses identify stimulus-dependent molecular changes associated with distinct modes of skeletal muscle atrophy

Skeletal muscle atrophy is a debilitating condition that occurs with aging and disease, but the underlying mechanisms are incompletely understood. Previous work determined that common transcriptional changes occur in muscle during atrophy induced by different stimuli. However, whether this holds true at the proteome level remains largely unexplored. Here, we find that, contrary to this earlier model, distinct atrophic stimuli (corticosteroids, cancer cachexia, and aging) induce largely different mRNA and protein changes during muscle atrophy in mice. Moreover, there is widespread transcriptome-proteome disconnect. Consequently, atrophy markers (atrogenes) identified in earlier microarray-based studies do not emerge from proteomics as generally induced by atrophy. Rather, we identify proteins that are distinctly modulated by different types of atrophy (herein defined as “atroproteins”) such as the myokine CCN1/Cyr61, which regulates myofiber type switching during sarcopenia. Altogether, these integrated analyses indicate that different catabolic stimuli induce muscle atrophy via largely distinct mechanisms.

Skeletal muscle wasting is caused by many catabolic stimuli, which were thought to act via shared mechanisms. Hunt et al. now show that distinct catabolic stimuli induce muscle wasting via largely different molecular changes. The authors identify atrophy-associated proteins (“atroproteins”) that may represent diagnostic biomarkers and/or therapeutic targets.

Dact1 is expressed during chicken and mouse skeletal myogenesis and modulated in human muscle diseases

Vertebrate skeletal muscle development and repair relies on the precise control of Wnt signaling. Dact1 (Dapper/Frodo) is an important modulator of Wnt signaling, interacting with key components of the various Wnt transduction pathways. Here, we characterized Dact1 mRNA and protein expression in chicken and mouse fetal muscles in vivo and during the differentiation of chick primary and mouse C2C12 myoblasts in vitro. We also performed in silico analysis to investigate Dact1 gene expression in human myopathies, and evaluated the Dact1 protein structure to seek an explanation for the accumulation of Dact1 protein aggregates in the nuclei of myogenic cells. Our results show for the first time that in both chicken and mouse, Dact1 is expressed during myogenesis, with a strong upregulation as cells engage in terminal differentiation, cell cycle withdrawal and cell fusion. In humans, Dact1 expression was found to be altered in specific muscle pathologies, including muscular dystrophies. Our bioinformatic analyses of Dact1 proteins revealed long intrinsically disordered regions, which may underpin the ability of Dact1 to interact with its many partners in the various Wnt pathways. In addition, we found that Dact1 has strong propensity for liquid-liquid phase separation, a feature that explains its ability to form nuclear aggregates and points to a possible role as a molecular 'on'-'off' switch. Taken together, our data suggest Dact1 as a candidate, multi-faceted regulator of amniote myogenesis with a possible pathophysiological role in human muscular diseases.

Transcriptome Analysis Reveals the Profile of Long Non-coding RNAs During Chicken Muscle Development

The developmental complexity of muscle arises from elaborate gene regulation. Long non-coding RNAs (lncRNAs) play critical roles in muscle development through the regulation of transcription and post-transcriptional gene expression. In chickens, previous studies have focused on the lncRNA profile during the embryonic periods, but there are no studies that explore the profile from the embryonic to post-hatching period. Here, we reconstructed 14,793 lncRNA transcripts and identified 2,858 differentially expressed lncRNA transcripts and 4,282 mRNAs from 12-day embryos (E12), 17-day embryos (E17), 1-day post-hatch chicks (D1), 14-day post-hatch chicks (D14), 56-day post-hatch chicks (D56), and 98-day post-hatch chicks (D98), based on our published RNA-seq datasets. We performed co-expression analysis for the differentially expressed lncRNAs and mRNAs, using STEM, and identified two profiles with opposite expression trends: profile 4 with a downregulated pattern and profile 21 with an upregulated pattern. The cis- and trans-regulatory interactions between the lncRNAs and mRNAs were predicted within each profile. Functional analysis of the lncRNA targets showed that lncRNAs in profile 4 contributed to the cell proliferation process, while lncRNAs in profile 21 were mainly involved in metabolism. Our work highlights the lncRNA profiles involved in the development of chicken breast muscle and provides a foundation for further experiments on the role of lncRNAs in the regulation of muscle development.

Function of the Porcine TRPC1 Gene in Myogenesis and Muscle Growth

In animals, muscle growth is a quantitative trait controlled by multiple genes. Previously, we showed that the transient receptor potential channel 1 (TRPC1) gene was differentially expressed in muscle tissues between pig breeds with divergent growth traits base on RNA-seq. Here, we characterized TRPC1 expression profiles in different tissues and pig breeds and showed that TRPC1 was highly expressed in the muscle. We found two single nucleotide polymorphisms (SNPs) (C-1763T and C-1604T) in TRPC1 that could affect the promoter region activity and regulate pig growth rate. Functionally, we used RNAi and overexpression to illustrate that TRPC1 promotes myoblast proliferation, migration, differentiation, fusion, and muscle hypertrophy while inhibiting muscle degradation. These processes may be mediated by the activation of Wnt signaling pathways. Altogether, our results revealed that TRPC1 might promote muscle growth and development and plays a key role in Wnt-mediated myogenesis.

FGF signaling promotes myoblast proliferation through activation of wingless signaling

Indirect flight muscles (IFMs) are the largest muscles in Drosophila and are made up of hundreds of myonuclei. The generation of these giant muscles requires a large pool of wing disc associated adult muscle precursors (AMPs), however the factors that control proliferation to form this myoblast pool are incompletely known. Here, we examine the role of fibroblast growth factor (FGF) signaling in the proliferation of wing disc associated myoblasts. We find that the components of FGF signaling are expressed in myoblasts and surrounding epithelial cells of the wing disc. Next, we show that attenuation of FGF signaling results in a diminished myoblast pool. This reduction in the pool size is due to decreased myoblast proliferation. By contrast, activating the FGF signaling pathway increases the myoblast pool size and restores the proliferative capacity of FGF knockdown flies. Finally, our results demonstrate that the FGF receptor Heartless acts through up-regulating β-catenin/Armadillo signaling to promote myoblast proliferation. Our studies identify a novel role for FGF signaling during IFM formation and uncover the mechanism through which FGF coordinates with Wingless signaling to promote myoblast proliferation.

Expression of apoptosis and myogenesis related genes during prenatal life in two divergent breeds of pigs

We aimed to evaluate apoptosis and myogenesis related genes expression in embryos and fetuses from two divergent genetic groups of pigs: Piau breed and a commercial line. Thirty females (15 Piau and 15 commercial line) were selected at 120 days of age. Estrous cycle was observed and on the third estrus females were considered sexually mature. Gilts were inseminated with semen from males of the respective breed. Three females from each breed were slaughtered at five different gestational ages: 15, 30, 45, 60 and 90 days. Whole embryos (15 and 30 d) and samples of longissimus dorsi muscle from fetuses (45, 60 and 90 d) were collected for RNA extraction. Expression of apoptosis and myogenesis related genes (BAX, BCL2, FGF4, IHH, HHIP, SHH, SOX2, WNT1 and WNT4) were evaluated by quantitative real time PCR. There was significant effect of interaction between breeds and gestational ages for all genes evaluated (P < 0.05). The BCL2 gene expression differed throughout pregnancy in Piau group with lower expression on day 15. The IHH gene expression throughout pregnancy was lower on days 15 and 60 in Piau group and lower on day 90 in commercial line group; and the SHH gene expression throughout pregnancy was higher on day 30 and lower on day 60 in Piau group and lower on day 45 in commercial group. The WNT1 gene expression along pregnancy was lower on day 15 and higher on days 30 and 45 in Piau group, and it was higher on day 30 than days 15 and 60 in commercial group. WNT4 gene expression throughout pregnancy was lower on day 15 in Piau group, and it was lower on day 30 in commercial pigs. Piau group presented higher expression of the FGF4 gene on days 45, 60 and 90, and commercial group showed higher expression on day 15 and 90. SOX2 gene expression was lower on day 15 in Piau pigs and it was constant throughout pregnancy in commercial group. Overall, the expression of IHH, SHH, WNT1, WNT4 and FGF4 genes were higher in commercial than Piau pigs on day 15, besides expression of BCL2 gene in Piau embryos was lower on day 15; these results might indicate that the muscle precursor cells are allowed to proliferate for a longer time in commercial than in Piau embryos by the balance of proliferative and apoptotic genes. Therefore, the expression differential between breeds can stimulate proliferation and differentiation of cells in different ways, explaining the postnatal differences in the muscularity between pigs from Piau breed and a commercial line.

Myostatin gene promoter: structure, conservation and importance as a target for muscle modulation

Myostatin (MSTN) is one of the key factors regulating myogenesis. Because of its role as a negative regulator of muscle mass deposition, much interest has been given to its protein and, in recent years, several studies have analysed MSTN gene regulation. This review discusses the MSTN gene promoter, focusing on its structure in several animal species, both vertebrate and invertebrate. We report the important binding sites considering their degree of phylogenetic conservation and roles they play in the promoter activity. Finally, we discuss recent studies focusing on MSTN gene regulation via promoter manipulation and the potential applications they have both in medicine and agriculture.

Findings as a starting point to unravel the underlying mechanisms of in vivo interactions involving Wnt10a in bone, fat and muscle

Wnt10a is a member of the WNT family. Although deficiency of this gene causes symptoms related to teeth, hair, nails, and skin, we recently demonstrated a new phenotype of Wnt10a knockout (KO) mice involving bone and fat. The in vivo effect of the Wnt10a gene on bone and fat is unclear, and the relationship between bone/fat and muscle in Wnt10a signaling is also interesting. We aimed to evaluate the tissue changes in Wnt10a KO mice compared to wild-type mice and show the findings as a starting point to unravel the underlying mechanisms of in vivo interactions involving Wnt10a in bone, fat and muscle. Trabecular bone loss in the lower limbs of Wnt10a mice and decreased bone mineralization were observed. The adipose tissue in bone marrow was also decreased, and adipocyte differentiation was reduced. The body fat mass in Wnt10a KO mice was decreased, and white adipocytes in subcutaneous fat were converted to beige adipocytes. The muscle weight of the lower limbs was not decreased despite trabecular bone loss, but Gdf8/myostatin expression was reduced in the subcutaneous fat and gastrocnemius muscles of Wnt10a KO mice. Thus, in vivo deletion of Wnt10a inhibited osteogenic activity, promoted beige adipogenesis of white adipocytes and maintained muscle mass. These results suggest that regulation of Gdf8 by Wnt10a may help maintain the muscle mass of Wnt10a KO mice. This study was the first to histologically evaluate the bone, fat and muscle phenotypes of Wnt10a KO mice. The results of this study, which were obtained by investigating the three tissues together, could influence the understanding of in vivo interactions involving the Wnt10a gene.

Location, Location, Location: Signals in Muscle Specification

Muscles control body movement and locomotion, posture and body position and soft tissue support. Mesoderm derived cells gives rise to 700 unique muscles in humans as a result of well-orchestrated signaling and transcriptional networks in specific time and space. Although the anatomical structure of skeletal muscles is similar, their functions and locations are specialized. This is the result of specific signaling as the embryo grows and cells migrate to form different structures and organs. As cells progress to their next state, they suppress current sequence specific transcription factors (SSTF) and construct new networks to establish new myogenic features. In this review, we provide an overview of signaling pathways and gene regulatory networks during formation of the craniofacial, cardiac, vascular, trunk, and limb skeletal muscles.

Myostatin-deficiency in mice increases global gene expression at the Dlk1-Dio3 locus in the skeletal muscle

Myostatin, a member of the transforming growth factor-beta superfamily, is a negative regulator of skeletal muscle growth and development. Myostatin inhibition leads to increased skeletal muscle mass in mammals; hence, myostatin is considered a potential therapeutic target for skeletal muscle wasting. However, downstream molecules of myostatin in the skeletal muscle have not been fully elucidated. Here, we identified the Dlk1-Dio3 locus at the mouse chromosome 12qF1, also called as the callipyge locus in sheep, as a novel downstream target of myostatin. In skeletal muscle of myostatin knockout mice, the expression of mature miRNAs at the Dlk1-Dio3 locus was significantly increased. The increased miRNA levels are caused by the transcriptional activation of the Dlk1-Dio3 locus, because a significant increase in the primary miRNA transcript was observed in myostatin knockout mice. In addition, we found increased expression of coding and non-coding genes (Dlk1, Gtl2, Rtl1/Rtl1as, and Rian) at the Dlk1-Dio3 locus in myostatin-deficient skeletal muscle. Moreover, epigenetic changes, associated with the regulation of the Dlk1-Dio3 locus, were observed in myostatin knockout mice. Taken together, this is the first report demonstrating the role of myostatin in regulating the Dlk1-Dio3 (the callipyge) locus in the skeletal muscle.

Common and Distinctive Functions of the Hippo Effectors Taz and Yap in Skeletal Muscle Stem Cell Function

Hippo pathway downstream effectors Yap and Taz play key roles in cell proliferation and regeneration, regulating gene expression especially via Tead transcription factors. To investigate their role in skeletal muscle stem cells, we analyzed Taz in vivo and ex vivo in comparison with Yap. Small interfering RNA knockdown or retroviral‐mediated expression of wild‐type human or constitutively active TAZ mutants in satellite cells showed that TAZ promoted proliferation, a function shared with YAP. However, at later stages of myogenesis, TAZ also enhanced myogenic differentiation of myoblasts, whereas YAP inhibits such differentiation. Functionally, while muscle growth was mildly affected in Taz (gene Wwtr1^–/–) knockout mice, there were no overt effects on regeneration. Conversely, conditional knockout of Yap in satellite cells of Pax7^Cre‐ERT2/+: Yap^fl°^x/fl°^x:Rosa26^Lacz mice produced a regeneration deficit. To identify potential mechanisms, microarray analysis showed many common TAZ/YAP target genes, but TAZ also regulates some genes independently of YAP, including myogenic genes such as Pax7, Myf5, and Myod1 (ArrayExpress–E‐MTAB‐5395). Proteomic analysis revealed many novel binding partners of TAZ/YAP in myogenic cells, but TAZ also interacts with proteins distinct from YAP that are often involved in myogenesis and aspects of cytoskeleton organization (ProteomeXchange–PXD005751). Neither TAZ nor YAP bind members of the Wnt destruction complex but both regulated expression of Wnt and Wnt‐cross talking genes with known roles in myogenesis. Finally, TAZ operates through Tead4 to enhance myogenic differentiation. In summary, Taz and Yap have overlapping functions in promoting myoblast proliferation but Taz then switches to enhance myogenic differentiation. Stem Cells2017;35:1958–1972

Spatiotemporal expression of Wnt3a during striated muscle complex development in rat embryos with ethylenethiourea-induced anorectal malformations

Numerous patients with anorectal malformations (ARMs) continue to experience fecal incontinence and constipation following surgical procedures. One of the most important factors that influences defecation is the striated muscle complex (SMC). Wnt signaling regulates the expression of myogenic regulatory factors, which serve an important role in muscle development. Therefore, the present study aimed to investigate the expression pattern of Wnt3a protein (by immunohistochemistry and western blot analysis) and mRNA [by reverse transcription-quantitative polymerase chain reaction (RT-qPCR)] in the SMC of ARM model rats that were exposed to ethylenethiourea. Immunostaining revealed that the expression of Wnt3a exhibits space- and time-dependent changes in the developing SMC of ARM model rat embryos. Immunohistochemistry demonstrated that on embryonic day 17 (E17), Wnt3a-positive cells were observed in the SMC in normal embryos, and expression levels gradually increased as the rat embryos developed. Similar changes in Wnt3a protein expression were detected in ARM model rat embryos; however, the expression of Wnt3a was significantly reduced compared with the normal rat embryos. Western blotting and RT-qPCR also revealed lower expression levels of Wnt3a protein and mRNA, respectively, in the SMC of ARMs model rat embryos compared with normal rat embryos. These data revealed that the expression of Wnt3a in ARM embryos was notably reduced, indicating a potential role for Wnt3a in the maldevelopment of the SMC in patients with ARMs.

Wnt signaling in bone and muscle

Wnt signaling plays key roles in many aspects of development. In this review, we will briefly describe the components of signaling pathways induced by Wnt ligands and then describe the current state of research as this applies to aspects of development and disease as it relates to skeletal muscle and bone. We will conclude with a discussion of the parallels and differences in Wnt signaling in these two contexts and how these pathways are being (or could potentially be) targeted for therapeutic treatment of musculoskeletal diseases. This article is part of a Special Issue entitled "Muscle Bone Interactions".

miR-487b, miR-3963 and miR-6412 delay myogenic differentiation in mouse myoblast-derived C2C12 cells

Background

Skeletal muscle differentiation is a multistep, complex pathway in which several important signaling molecules are involved. Recently, microRNAs (miRNAs), endogenous non-coding small RNAs that regulate mRNAs, have been proposed to be involved in skeletal muscle differentiation. In this study, we identified skeletal muscle differentiation-associated miRNAs by comparing miRNA expression profiles between C2C12 cells and Wnt4 over-expressing C2C12 cells (W4-08), which can spontaneously differentiate into myotubes.

Results

We identified miR-206, miR-133a, and miR-133b as up-regulated miRNAs and miR-487b, miR-3963 and miR-6412 as down-regulated miRNAs in differentiating cells. We focused on the down-regulated miRNAs because their functions were largely unknown. Transfection of mimics of these miRNAs into C2C12 cells resulted in significantly reduced expression of myogenic differentiation markers, including troponin T and myosin heavy chain fast type and slow type, but did not affect the expression of the myogenic transcription factors, MyoD and myogenin.

Conclusions

These miRNAs were characterized as new myogenic differentiation-associated miRNAs which may delay late myogenic differentiation or maturation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12860-015-0061-9) contains supplementary material, which is available to authorized users.

Barx2 and Pax7 have antagonistic functions in regulation of wnt signaling and satellite cell differentiation

The canonical Wnt signaling pathway is critical for myogenesis and can induce muscle progenitors to switch from proliferation to differentiation; how Wnt signals integrate with muscle-specific regulatory factors in this process is poorly understood. We previously demonstrated that the Barx2 homeobox protein promotes differentiation in cooperation with the muscle regulatory factor (MRF) MyoD. Pax7, another important muscle homeobox factor, represses differentiation. We now identify Barx2, MyoD, and Pax7 as novel components of the Wnt effector complex, providing a new molecular pathway for regulation of muscle progenitor differentiation. Canonical Wnt signaling induces Barx2 expression in muscle progenitors and perturbation of Barx2 leads to misregulation of Wnt target genes. Barx2 activates two endogenous Wnt target promoters as well as the Wnt reporter gene TOPflash, the latter synergistically with MyoD. Moreover, Barx2 interacts with the core Wnt effectors β-catenin and T cell-factor 4 (TCF4), is recruited to TCF/lymphoid enhancer factor sites, and promotes recruitment of β-catenin. In contrast, Pax7 represses the Wnt reporter gene and antagonizes the activating effect of Barx2. Pax7 also binds β-catenin suggesting that Barx2 and Pax7 may compete for interaction with the core Wnt effector complex. Overall, the data show for the first time that Barx2, Pax7, and MRFs can act as direct transcriptional effectors of Wnt signals in myoblasts and that Barx2 and Wnt signaling participate in a regulatory loop. We propose that antagonism between Barx2 and Pax7 in regulation of Wnt signaling may help mediate the switch from myoblast proliferation to differentiation.

The role of Six1 in the genesis of muscle cell and skeletal muscle development

The sine oculis homeobox 1 (Six1) gene encodes an evolutionarily conserved transcription factor. In the past two decades, much research has indicated that Six1 is a powerful regulator participating in skeletal muscle development. In this review, we summarized the discovery and structural characteristics of Six1 gene, and discussed the functional roles and molecular mechanisms of Six1 in myogenesis and in the formation of skeletal muscle fibers. Finally, we proposed areas of future interest for understanding Six1 gene function.

Zebrafish Noxa promotes mitosis in early embryonic development and regulates apoptosis in subsequent embryogenesis

Noxa functions in apoptosis and immune system of vertebrates, but its activities in embryo development remain unclear. In this study, we have studied the role of zebrafish Noxa (zNoxa) by using zNoxa-specifc morpholino knockdown and overexpression approaches in developing zebrafish embryos. Expression pattern analysis indicates that zNoxa transcript is of maternal origin, which displays a uniform distribution in early embryonic development until shield stage, and the zygote zNoxa transcription is initiated from this stage and mainly localized in YSL of the embryos. The zNoxa expression alterations result in strong embryonic development defects, demonstrating that zNoxa regulates apoptosis from 75% epiboly stage of development onward, in which zNoxa firstly induces the expression of zBik, and then cooperates with zBik to regulate apoptosis. Moreover, zNoxa knockdown also causes a reduction in number of mitotic cells before 8 h.p.f., suggesting that zNoxa also promotes mitosis before 75% epiboly stage. The effect of zNoxa on mitosis is mediated by zWnt4b in early embryos, whereas zMcl1a and zMcl1b suppress the ability of zNoxa to regulate mitosis and apoptosis at different developmental stages. In addition, mammalian mouse Noxa (mNoxa) mRNA was demonstrated to rescue the arrest of mitosis when zNoxa was knocked down, suggesting that mouse and zebrafish Noxa might have similar dual functions. Therefore, the current findings indicate that Noxa is a novel regulator of early mitosis before 75% epiboly stage when it translates into a key mediator of apoptosis in subsequent embryogenesis.

Interaction of Wnt Signaling with BMP/Smad Signaling during the Transition from Cell Proliferation to Myogenic Differentiation in Mouse Myoblast-Derived Cells

Background. Wnt signaling is involved in muscle formation through β-catenin-dependent or -independent pathways, but interactions with other signaling pathways including transforming growth factor β/Smad have not been precisely elucidated. Results. As Wnt4 stimulates myogenic differentiation by antagonizing myostatin (GDF8) activity, we examined the role of Wnt4 signaling during muscle differentiation in the C2C12 myoblast cell line. Among several extrinsic signaling molecules examined in a microarray analysis of C2C12 cells during the transition from cell proliferation to differentiation after mitogen deprivation, bone morphogenetic protein 4 (BMP4) expression was prominently increased. Wnt4 overexpression had similar effects on BMP4 expression. BMP4 was able to inhibit muscle differentiation when added to the culture medium. BMP4 and noggin had no effects on the cellular localization of β-catenin induced by Wnt3a; however, the BMP4-induced phosphorylation of Smad1/5/8 was enhanced by Wnt4, but not by Wnt3a. The BMP antagonist noggin effectively stimulated muscle differentiation through binding to endogenous BMPs, and the effect of noggin was enhanced by the presence of Wnt3a and Wnt4. Conclusion. These results suggest that BMP/Smad pathways are modified through Wnt signaling during the transition from progenitor cell proliferation to myogenic differentiation, although Wnt/β-catenin signaling is not modified with BMP/Smad signaling.

Canonical Wnt signaling induces BMP-4 to specify slow myofibrogenesis of fetal myoblasts

Background

The Wnts are secreted proteins that play important roles in skeletal myogenesis, muscle fiber type diversification, neuromuscular junction formation and muscle stem cell function. How Wnt proteins orchestrate such diverse activities remains poorly understood. Canonical Wnt signaling stabilizes β-catenin, which subsequently translocate to the nucleus to activate the transcription of TCF/LEF family genes.

Methods

We employed TCF-reporter mice and performed analysis of embryos and of muscle groups. We further isolated fetal myoblasts and performed cell and molecular analyses.

Results

We found that canonical Wnt signaling is strongly activated during fetal myogenesis and weakly activated in adult muscles limited to the slow myofibers. Muscle-specific transgenic expression of a stabilized β-catenin protein led to increased oxidative myofibers and reduced muscle mass, suggesting that canonical Wnt signaling promotes slow fiber types and inhibits myogenesis. By TCF-luciferase reporter assay, we identified Wnt-1 and Wnt-3a as potent activators of canonical Wnt signaling in myogenic progenitors. Consistent with in vivo data, constitutive overexpression of Wnt-1 or Wnt-3a inhibited the proliferation of both C2C12 and primary myoblasts. Surprisingly, Wnt-1 and Wnt-3a overexpression up-regulated BMP-4, and inhibition of BMP-4 by shRNA or recombinant Noggin protein rescued the myogenic inhibitory effect of Wnt-1 and Wnt-3a. Importantly, Wnt-3a or BMP-4 recombinant proteins promoted slow myosin heavy chain expression during myogenic differentiation of fetal myoblasts.

Conclusions

These results demonstrate a novel interaction between canonical Wnt and BMP signaling that induces myogenic differentiation towards slow muscle phenotype.

Muscle development and obesity: Is there a relationship?

The formation of skeletal muscle from the epithelial somites involves a series of events triggered by temporally and spatially discrete signals resulting in the generation of muscle fibers which vary in their contractile and metabolic nature. The fiber type composition of muscles varies between individuals and it has now been found that there are differences in fiber type proportions between lean and obese animals and humans. Amongst the possible causes of obesity, it has been suggested that inappropriate prenatal environments may 'program' the fetus and may lead to increased risks for disease in adult life. The characteristics of muscle are both heritable and plastic, giving the tissue some ability to adapt to signals and stimuli both pre and postnatally. Given that muscle is a site of fatty acid oxidation and carbohydrate metabolism and that its development can be changed by prenatal events, it is interesting to examine the possible relationship between muscle development and the risk of obesity.

Marking the tempo for myogenesis: Pax7 and the regulation of muscle stem cell fate decisions

Post-natal growth and regeneration of skeletal muscle is highly dependent on a population of resident myogenic precursors known as satellite cells. Transcription factors from the Pax gene family, Pax3 and Pax7, are critical for satellite cell biogenesis, survival and potentially self-renewal; however, the underlying molecular mechanisms remain unsolved. This is particularly true in the case of Pax7, which appears to regulate myogenesis at multiple levels. Accordingly, recent data have highlighted the importance of a functional relationship between Pax7 and the MyoD family of muscle regulatory transcription factors during normal muscle formation and disease. Here we will critically review key findings suggesting that Pax7 may play a dual role by promoting resident muscle progenitors to commit to the skeletal muscle lineage while preventing terminal differentiation, thus keeping muscle progenitors poised to differentiate upon environmental cues. In addition, potential regulatory mechanisms for the control of Pax7 activity will be proposed.

Wnt signaling in myogenesis

The formation of skeletal muscle is a tightly regulated process that is critically modulated by Wnt signaling. Myogenesis is dependent on the precise and dynamic integration of multiple Wnt signals allowing self-renewal and progression of muscle precursors in the myogenic lineage. Dysregulation of Wnt signaling can lead to severe developmental defects and perturbation of muscle homeostasis. Recent work has revealed novel roles for the non-canonical planar cell polarity (PCP) and AKT/mTOR pathways in mediating the effects of Wnt on skeletal muscle. In this review, we discuss the role of Wnt signaling in myogenesis and in regulating the homeostasis of adult muscle.

Inhibition of myostatin reverses muscle fibrosis through apoptosis

Skeletal muscle fibrosis is a defining feature of the muscular dystrophies in which contractile myofibers are replaced by fibroblasts, adipocytes and extracellular matrix. This maladaptive response of muscle to repetitive injury is progressive, self-perpetuating and thus far, has been considered irreversible. We have previously shown that myostatin, a known endogenous modulator of muscle growth, stimulates normal muscle fibroblasts to proliferate. Here, we demonstrate that myostatin also regulates the proliferation of dystrophic muscle fibroblasts, and increases resistance of fibroblasts to apoptosis through Smad and MAPK signaling. Inhibition of myostatin signaling pathways with a soluble activin IIB receptor (ActRIIB.Fc) reduces resistance of muscle fibroblasts to apoptosis in vitro. Systemic administration of ActRIIB.Fc in senescent mdx mice, a model of muscular dystrophy, significantly increases the number of muscle fibroblasts undergoing apoptosis. This leads to the reversal of pre-existing muscle fibrosis as determined by histological, biochemical and radiographical criteria. These results demonstrate that skeletal muscle fibrosis can be pharmacologically reversed through induction of fibroblast apoptosis.

Fhl1 as a downstream target of Wnt signaling to promote myogenesis of C2C12 cells

Previous studies have shown that Wnt signaling is involved in postnatal mammalian myogenesis; however, the downstream mechanism of Wnt signaling is not fully understood. This study reports that the murine four-and-a-half LIM domain 1 (Fhl1) could be stimulated by β-catenin or LiCl treatment to induce myogenesis. In contrast, knockdown of the Fhl1 gene expression in C2C12 cells led to reduced myotube formation. We also adopted reporter assays to demonstrate that either β-catenin or LiCl significantly activated the Fhl1 promoter, which contains four putative consensus TCF/LEF binding sites. Mutations of two of these sites caused a significant decrease in promoter activity by luciferase reporter assay. Thus, we suggest that Wnt signaling induces muscle cell differentiation, at least partly, through Fhl1 activation.

Wnt4 participates in the formation of vertebrate neuromuscular junction

Neuromuscular junction (NMJ) formation requires the highly coordinated communication of several reciprocal signaling processes between motoneurons and their muscle targets. Identification of the early, spatially restricted cues in target recognition at the NMJ is still poorly documented, especially in mammals. Wnt signaling is one of the key pathways regulating synaptic connectivity. Here, we report that Wnt4 contributes to the formation of vertebrate NMJ in vivo. Results from a microarray screen and quantitative RT-PCR demonstrate that Wnt4 expression is regulated during muscle cell differentiation in vitro and muscle development in vivo, being highly expressed when the first synaptic contacts are formed and subsequently downregulated. Analysis of the mouse Wnt4−/− NMJ phenotype reveals profound innervation defects including motor axons overgrowing and bypassing AChR aggregates with 30% of AChR clusters being unapposed by nerve terminals. In addition, loss of Wnt4 function results in a 35% decrease of the number of prepatterned AChR clusters while Wnt4 overexpression in cultured myotubes increases the number of AChR clusters demonstrating that Wnt4 directly affects postsynaptic differentiation. In contrast, muscle structure and the localization of several synaptic proteins including acetylcholinesterase, MuSK and rapsyn are not perturbed in the Wnt4 mutant. Finally, we identify MuSK as a Wnt4 receptor. Wnt4 not only interacts with MuSK ectodomain but also mediates MuSK activation. Taken together our data reveal a new role for Wnt4 in mammalian NMJ formation that could be mediated by MuSK, a key receptor in synaptogenesis.

Canonical Wnt signaling is involved in switching from cell proliferation to myogenic differentiation of mouse myoblast cells

Background

Wnt/β-catenin signaling is involved in various aspects of skeletal muscle development and regeneration. In addition, Wnt3a and β-catenin are required for muscle-specific gene transcription in embryonic carcinoma cells and satellite-cell proliferation during adult skeletal muscle regeneration. Downstream targets of canonical Wnt signaling are cyclin D1 and c-myc. However both target genes are suppressed during differentiation of mouse myoblast cells, C2C12. Underlying molecular mechanisms of β-catenin signaling during myogenic differentiation remain unknown.

Results

Using C2C12 cells, we examined intracellular signaling and gene transcription during myoblast proliferation and differentiation. We confirmed that several Wnt signaling components, including Wnt9a, Sfrp2 and porcupine, were consistently upregulated in differentiating C2C12 cells. Troponin T-positive myotubes were decreased by Wnt3a overexpression, but not Wnt4. TOP/FOP reporter assays revealed that co-expression with Wnt4 reduced Wnt3a-induced luciferase activity, suggesting that Wnt4 signaling counteracted Wnt3a signaling in myoblasts. FH535, a small-molecule inhibitor of β-catenin/Tcf complex formation, reduced basal β-catenin in the cytoplasm and decreased myoblast proliferation. K252a, a protein kinase inhibitor, increased both cytosolic and membrane-bound β-catenin and enhanced myoblast fusion. Treatments with K252a or Wnt4 resulted in increased cytoplasmic vesicles containing phosphorylated β-catenin (Tyr654) during myogenic differentiation.

Conclusions

These results suggest that various Wnt ligands control subcellular β-catenin localization, which regulate myoblast proliferation and myotube formation. Wnt signaling via β-catenin likely acts as a molecular switch that regulates the transition from cell proliferation to myogenic differentiation.

Wnt4 activates the canonical β-catenin pathway and regulates negatively myostatin: functional implication in myogenesis

Expression of Wnt proteins is known to be important for developmental processes such as embryonic pattern formation and determination of cell fate. Previous studies have shown that Wn4 was involved in the myogenic fate of somites, in the myogenic proliferation, and differentiation of skeletal muscle. However, the function of this factor in adult muscle homeostasis remains not well understood. Here, we focus on the roles of Wnt4 during C2C12 myoblasts and satellite cells differentiation. We analyzed its myogenic activity, its mechanism of action, and its interaction with the anti-myogenic factor myostatin during differentiation. Established expression profiles indicate clearly that both types of cells express a few Wnts, and among these, only Wnt4 was not or barely detected during proliferation and was strongly induced during differentiation. As attested by myogenic factors expression pattern analysis and fusion index determination, overexpression of Wnt4 protein caused a strong increase in satellite cells and C2C12 myoblast differentiation leading to hypertrophic myotubes. By contrast, exposure of satellite and C2C12 cells to small interfering RNA against Wnt4 strongly diminished this process, confirming the myogenic activity of Wnt4. Moreover, we reported that Wnt4, which is usually described as a noncanonical Wnt, activates the canonical β-catenin pathway during myogenic differentiation in both cell types and that this factor regulates negatively the expression of myostatin and the regulating pathways associated with myostatin. Interestingly, we found that recombinant myostatin was sufficient to antagonize the differentiation-promoting activities of Wnt4. Reciprocally, we also found that the genetic deletion of myostatin renders the satellite cells refractory to the hypertrophic effect of Wnt4. These results suggest that the Wnt4-induced decrease of myostatin plays a functional role during hypertrophy. We propose that Wnt4 protein may be a key factor that regulates the extent of differentiation in satellite and C2C12 cells.

WNT pathways and upper limb anomalies

The various Wnt pathways that are related to upper limb anomalies are reviewed. Abnormalities in the Wnt7a pathway (located in the dorsal ectoderm) produce several clinically relevant conditions such as the palmar duplication syndrome, nail patella syndrome, ulnar ray deficiency, limb hypoplasia, polysyndactyly and the palmar nail syndrome. Abnormalities of the Wnt3/3a pathway (located in the apical ectodermal ridge) include tetra-amelia and loss of the distal phalanges/nails. Abnormalities of the Wnt5/5a pathway (located in the apical ectodermal ridge as well as in the mesoderm) will affect chondrogenesis of the developing limb and experimental Wnt5a(-/-) limbs have terminal adactyly. Chondrogenesis and limb muscle differentiation are both affected by several Wnt pathways and these will be reviewed in details. Abnormalities in LRP 5/6 (a co-receptor for Wnts) lead to congenital bone disease and Wnt4 is specifically involved in joint development. Finally, the relationship between the Wnt pathway and SALL4 (mutations of which cause Okihiro/Duane-radial ray deficiency in humans) are discussed.

Wnt4/β-catenin signaling induces VSMC proliferation and is associated with intimal thickening

RATIONALE

Vascular smooth muscle cell (VSMC) proliferation causes intimal thickening in atherosclerosis and restenosis. Previously, we demonstrated that Wnt/β-catenin signaling upregulates VSMC proliferation in vitro.

OBJECTIVE

We examined this pathway in vivo and investigated the involvement of specific Wnt proteins in VSMC proliferation.

METHODS AND RESULTS

Left carotid arteries of TOPgal (β-catenin signaling reporter) transgenic mice were ligated to induce intimal thickening. β-Catenin signaling was induced in the media and intima at 3 and 28 days after ligation, respectively, and was associated with VSMC proliferation and cyclin D1 expression. In vitro, a Wnt agonist promoted mouse VSMC proliferation, whereas Wnt inhibitory factor (WIF)-1 retarded platelet-derived growth factor-BB (PDGF-BB)-induced VSMC proliferation. Microarray analysis and quantitative PCR detected a significant induction of Wnt2 and Wnt4 mRNA in PDGF-BB-treated (proliferating) VSMCs compared to quiescent VSMCs. Western blotting revealed this increase was only translated into protein for Wnt4. Specific silencing RNA knockdown of Wnt4, but not Wnt2, significantly reduced VSMC proliferation. Recombinant Wnt4, but not Wnt2, significantly increased VSMC proliferation by ≈2-fold and silencing RNA knockdown revealed this is via Frizzled 1. Immunohistochemistry showed that increased Wnt4 protein correlated with VSMC proliferation and cyclin D1 expression (P<0.05 and P<0.001, respectively) during intimal thickening after rat carotid artery injury. Importantly, we also showed that intimal thickening and VSMC proliferation after carotid artery ligation was significantly retarded in Wnt4(+/-) compared to Wnt4(+/+) mice.

CONCLUSIONS

This study demonstrates that Wnt/β-catenin signaling occurs in proliferating VSMCs during intimal thickening and indicates that this is a result of Wnt4 upregulation.

Segregation of myoblast fusion and muscle-specific gene expression by distinct ligand-dependent inactivation of GSK-3β

Myogenic differentiation involves myoblast fusion and induction of muscle-specific gene expression, which are both stimulated by pharmacological (LiCl), genetic, or IGF-I-mediated GSK-3β inactivation. To assess whether stimulation of myogenic differentiation is common to ligand-mediated GSK-3β inactivation, myoblast fusion and muscle-specific gene expression were investigated in response to Wnt-3a. Moreover, crosstalk between IGF-I/GSK-3β/NFATc3 and Wnt/GSK-3β/β-catenin signaling was assessed. While both Wnt-3a and LiCl promoted myoblast fusion, muscle-specific gene expression was increased by LiCl, but not by Wnt-3a or β-catenin over-expression. Furthermore, LiCl and IGF-I, but not Wnt-3a, increased NFATc3 transcriptional activity. In contrast, β-catenin-dependent transcriptional activity was increased by Wnt-3a and LiCl, but not IGF-I. These results for the first time reveal a segregated regulation of myoblast fusion and muscle-specific gene expression following stimulation of myogenic differentiation in response to distinct ligand-specific signaling routes of GSK-3β inactivation.

Hypoxia preconditioned mesenchymal stem cells improve vascular and skeletal muscle fiber regeneration after ischemia through a Wnt4-dependent pathway

Mesenchymal stem cells (MSC) are multipotent postnatal stem cells, involved in the treatment of ischemic vascular diseases. We investigate the ability of MSC, exposed to short-term hypoxic conditions, to participate in vascular and tissue regeneration in an in vivo model of hindlimb ischemia. Transplantation of hypoxic preconditioned murine MSC (HypMSC) enhanced skeletal muscle regeneration at day 7, improved blood flow and vascular formation compared to injected nonpreconditioned MSC (NormMSC). These observed effects were correlated with an increase in HypMSC engraftment and a putative role in necrotic skeletal muscle fiber clearance. Moreover, HypMSC transplantation resulted in a large increase in Wnt4 (wingless-related MMTV integration site 4) expression and we demonstrate its functional significance on MSC proliferation and migration, endothelial cell (EC) migration, as well as myoblast differentiation. Furthermore, suppression of Wnt4 expression in HypMSC, abrogated the hypoxia-induced vascular regenerative properties of these cells in the mouse hindlimb ischemia model. Our data suggest that hypoxic preconditioning plays a critical role in the functional capabilities of MSC, shifting MSC location in situ to enhance ischemic tissue recovery, facilitating vascular cell mobilization, and skeletal muscle fiber regeneration via a paracrine Wnt-dependent mechanism.

Activation of Wnt/beta-catenin signaling increases insulin sensitivity through a reciprocal regulation of Wnt10b and SREBP-1c in skeletal muscle cells

Background

Intramyocellular lipid accumulation is strongly related to insulin resistance in humans, and we have shown that high glucose concentration induced de novo lipogenesis and insulin resistance in murin muscle cells. Alterations in Wnt signaling impact the balance between myogenic and adipogenic programs in myoblasts, partly due to the decrease of Wnt10b protein. As recent studies point towards a role for Wnt signaling in the pathogenesis of type 2 diabetes, we hypothesized that activation of Wnt signaling could play a crucial role in muscle insulin sensitivity.

Methodology/Principal Findings

Here we demonstrate that SREBP-1c and Wnt10b display inverse expression patterns during muscle ontogenesis and regeneration, as well as during satellite cells differentiation. The Wnt/β-catenin pathway was reactivated in contracting myotubes using siRNA mediated SREBP-1 knockdown, Wnt10b over-expression or inhibition of GSK-3β, whereas Wnt signaling was inhibited in myoblasts through silencing of Wnt10b. SREBP-1 knockdown was sufficient to induce Wnt10b protein expression in contracting myotubes and to activate the Wnt/β-catenin pathway. Conversely, silencing Wnt10b in myoblasts induced SREBP-1c protein expression, suggesting a reciprocal regulation. Stimulation of the Wnt/β-catenin pathway i) drastically decreased SREBP-1c protein and intramyocellular lipid deposition in myotubes; ii) increased basal glucose transport in both insulin-sensitive and insulin-resistant myotubes through a differential activation of Akt and AMPK pathways; iii) restored insulin sensitivity in insulin-resistant myotubes.

Conclusions/Significance

We conclude that activation of Wnt/β-catenin signaling in skeletal muscle cells improved insulin sensitivity by i) decreasing intramyocellular lipid deposition through downregulation of SREBP-1c; ii) increasing insulin effects through a differential activation of the Akt/PKB and AMPK pathways; iii) inhibiting the MAPK pathway. A crosstalk between these pathways and Wnt/β-catenin signaling in skeletal muscle opens the exciting possibility that organ-selective modulation of Wnt signaling might become an attractive therapeutic target in regenerative medicine and to treat obese and diabetic populations.

Comparative analysis of the mammalian WNT4 promoter

Background

WNT4 is a critical signalling molecule in embryogenesis and homeostasis, but the elements that control its transcriptional regulation are largely unknown. This study uses comparative cross species sequence and functional analyses between humans and a marsupial (the tammar wallaby,Macropus eugenii) to refine the mammalian Wnt4 promoter.

Results

We have defined a highly conserved 89 bp minimal promoter region in human WNT4 by comparative analysis with the tammar wallaby. There are many conserved transcription factor binding sites in the proximal promoter region, including SP1, MyoD, NFκB and AP2, as well as highly conserved CpG islands within the human, mouse and marsupial promoters, suggesting that DNA methylation may play an important role in WNT4 transcriptional regulation.

Conclusion

Using a marsupial model, we have been able to provide new information on the transcriptional regulators in the promoter of this essential mammalian developmental gene, WNT4. These transcription factor binding sites and CpG islands are highly conserved in two disparate mammals, and are likely key controlling elements in the regulation of this essential developmental gene.

Embryonic and fetal limb myogenic cells are derived from developmentally distinct progenitors and have different requirements for beta-catenin

Vertebrate muscle arises sequentially from embryonic, fetal, and adult myoblasts. Although functionally distinct, it is unclear whether these myoblast classes develop from common or different progenitors. Pax3 and Pax7 are expressed by somitic myogenic progenitors and are critical myogenic determinants. To test the developmental origin of embryonic and fetal myogenic cells in the limb, we genetically labeled and ablated Pax3(+) and Pax7(+) cells. Pax3(+)Pax7(-) cells contribute to muscle and endothelium, establish and are required for embryonic myogenesis, and give rise to Pax7(+) cells. Subsequently, Pax7(+) cells give rise to and are required for fetal myogenesis. Thus, Pax3(+) and Pax7(+) cells contribute differentially to embryonic and fetal limb myogenesis. To investigate whether embryonic and fetal limb myogenic cells have different genetic requirements we conditionally inactivated or activated beta-catenin, an important regulator of myogenesis, in Pax3- or Pax7-derived cells. beta-Catenin is necessary within the somite for dermomyotome and myotome formation and delamination of limb myogenic progenitors. In the limb, beta-catenin is not required for embryonic myoblast specification or myofiber differentiation but is critical for determining fetal progenitor number and myofiber number and type. Together, these studies demonstrate that limb embryonic and fetal myogenic cells develop from distinct, but related progenitors and have different cell-autonomous requirements for beta-catenin.

Differentiation-dependent PTPIP51 expression in human skeletal muscle cell culture

Protein tyrosine phosphatase-interacting protein 51 (PTPIP51) expression was analyzed in proliferating and differentiating human myogenic cells cultured in vitro. Satellite cell cultures derived from four different individuals were used in this study. To analyze the expression of PTPIP51, myoblasts were cultured under conditions promoting either proliferation or differentiation. In addition, further differentiation of already-differentiated myobtubes was inhibited by resubmitting the cells to conditions promoting proliferation. PTPIP51 protein and mRNA were investigated in samples taken at defined time intervals by immunostaining, immunoblotting, in situ hybridization, and PCR. Image analyses of fluorescence immunostainings were used to quantify PTPIP51 in cultured myoblasts and myotubes. Myoblasts grown in the presence of epidermal and fibroblast growth factors (EGF and FGF), both promoting proliferation, expressed PTPIP51 on a basic level. Differentiation to multinuclear myotubes displayed a linear increase in PTPIP51 expression. The rise in PTPIP51 protein was paralleled by an augmented expression of muscle-specific proteins, namely, sarcoplasmic reticulum Ca(2+) ATPase and myosin heavy-chain protein, both linked to a progressive state of myotubal differentiation. This differentiation-induced increase in PTPIP51 was partly reversible by resubmission of differentiated myotubes to conditions boosting proliferation. The results clearly point toward a strong association between PTPIP51 expression and differentiation in human muscle cells.