Constitutive instability of muscle regulatory factor Myf5 is distinct from its mitosis-specific disappearance, which requires a D-box-like motif overlapping the basic domain

Nuclear translocation promotes proteasomal degradation of human Rad17 protein through the N-terminal destruction boxes

The ATR pathway is one of the major DNA damage checkpoints, and Rad17 is a DNA-binding protein that is phosphorylated upon DNA damage by ATR kinase. Rad17 recruits the 9-1-1 complex that mediates the checkpoint activation, and proteasomal degradation of Rad17 is important for recovery from the ATR pathway. Here, we identified several Rad17 mutants deficient in nuclear localization and resistant to proteasomal degradation. The nuclear localization signal was identified in the central basic domain of Rad17. Rad17 Δ230–270 and R240A/L243A mutants that were previously postulated to lack the destruction box, a sequence that is recognized by the ubiquitin ligase/anaphase-promoting complex that mediates degradation of Rad17, also showed cytoplasmic localization. Our data indicate that the nuclear translocation of Rad17 is functionally linked to the proteasomal degradation. The ATP-binding activity of Rad17, but not hydrolysis, is essential for the nuclear translocation, and the ATPase domain orchestrates the nuclear translocation, the proteasomal degradation, as well as the interaction with the 9-1-1 complex. The Rad17 mutant that lacked a nuclear localization signal was proficient in the interaction with the 9-1-1 complex, suggesting cytosolic association of Rad17 and the 9-1-1 complex. Finally, we identified two tandem canonical and noncanonical destruction boxes in the N-terminus of Rad17 as the bona fide destruction box, supporting the role of anaphase-promoting complex in the degradation of Rad17. We propose a model in which Rad17 is activated in the cytoplasm for translocation into the nucleus and continuously degraded in the nucleus even in the absence of exogenous DNA damage.

Whole-exome sequencing and genome-wide association studies identify novel sarcopenia risk genes in Han Chinese

Sarcopenia is a complex polygenic disease, and its molecular mechanism is still unclear. Whole lean body mass (WLBM) is a heritable trait predicting sarcopenia. To identify genomic loci underlying, we performed a whole-exome sequencing (WES) of WLBM variation with high sequencing depth (more than 40*) in 101 Chinese subjects. We then replicated in the major findings in the large-scale UK Biobank (UKB) cohort (N = 217,822) for WLBM. The results of four single-nucleotide polymorphisms (SNPs) were significant both in the discovery stage and replication stage: SNP rs740681 (discovery p = 1.66 × 10-6 , replication p = .05), rs2272303 (discovery p = 3.20 × 10-4 , replication p = 3.10 × 10-4 ), rs11170413 (discovery p = 3.99 × 10-4 , replication p = 2.90 × 10-4 ), and rs2272302 (discovery p = 9.13 × 10-4 , replication p = 3.10 × 10-4 ). We combined p values of the significant SNPs. Functional annotations highlighted two candidate genes, including FZR1 and SOAT2, that may exert pleiotropic effects to the development of body mass. Our findings provide useful insights that further enhance our understanding of genetic interplay in sarcopenia.

Protein 4.1R Influences Myogenin Protein Stability and Skeletal Muscle Differentiation

Protein 4.1R (4.1R) isoforms are expressed in both cardiac and skeletal muscle. 4.1R is a component of the contractile apparatus. It is also associated with dystrophin at the sarcolemma in skeletal myofibers. However, the expression and function of 4.1R during myogenesis have not been characterized. We now report that 4.1R expression increases during C2C12 myoblast differentiation into myotubes. Depletion of 4.1R impairs skeletal muscle differentiation and is accompanied by a decrease in the levels of myosin heavy and light chains and caveolin-3. Furthermore, the expression of myogenin at the protein, but not mRNA, level is drastically decreased in 4.1R knockdown myocytes. Similar results were obtained using MyoD-induced differentiation of 4.1R^-/- mouse embryonic fibroblast cells. von Hippel-Lindau (VHL) protein is known to destabilize myogenin via the ubiquitin-proteasome pathway. We show that 4.1R associates with VHL and, when overexpressed, reverses myogenin ubiquitination and stability. This suggests that 4.1R may influence myogenesis by preventing VHL-mediated myogenin degradation. Together, our results define a novel biological function for 4.1R in muscle differentiation and provide a molecular mechanism by which 4.1R promotes myogenic differentiation.

Trip12, a HECT domain E3 ubiquitin ligase, targets Sox6 for proteasomal degradation and affects fiber type-specific gene expression in muscle cells

Background

A sophisticated level of coordinated gene expression is necessary for skeletal muscle fibers to obtain their unique functional identities. We have previously shown that the transcription factor Sox6 plays an essential role in coordinating muscle fiber type differentiation by acting as a transcriptional suppressor of slow fiber-specific genes. Currently, mechanisms regulating the activity of Sox6 in skeletal muscle and how these mechanisms affect the fiber phenotype remain unknown.

Methods

Yeast two-hybrid screening was used to identify binding partners of Sox6 in muscle. Small interfering RNA (siRNA)-mediated knockdown of one of the Sox6 binding proteins, Trip12, was used to determine its effect on Sox6 activity in C2C12 myotubes using quantitative analysis of fiber type-specific gene expression.

Results

We found that the E3 ligase Trip12, a HECT domain E3 ubiquitin ligase, recognizes and polyubiquitinates Sox6. Inhibiting Trip12 or the 26S proteasome activity resulted in an increase in Sox6 protein levels in C2C12 myotubes. This control of Sox6 activity in muscle cells via Trip12 ubiquitination has significant phenotypic outcomes. Knockdown of Trip12 in C2C12 myotubes led to upregulation of Sox6 protein levels and concurrently to a decrease in slow fiber-specific Myh7 expression coupled with an increased expression in fast fiber-specific Myh4. Therefore, regulation of Sox6 cellular levels by the ubiquitin-proteasome system can induce identity-changing alterations in the expression of fiber type-specific genes in muscle cells.

Conclusions

Based on our data, we propose that in skeletal muscle, E3 ligases have a significant role in regulating fiber type-specific gene expression, expanding their importance in muscle beyond their well-established role in atrophy.

Ataxin-3 plays a role in mouse myogenic differentiation through regulation of integrin subunit levels

Background

During myogenesis several transcription factors and regulators of protein synthesis and assembly are rapidly degraded by the ubiquitin-proteasome system (UPS). Given the potential role of the deubiquitinating enzyme (DUB) ataxin-3 in the UPS, and the high expression of the murine ataxin-3 homolog in muscle during embryogenesis, we sought to define its role in muscle differentiation.

Methodology/Principal Findings

Using immunofluorescence analysis, we found murine ataxin-3 (mATX3) to be highly expressed in the differentiated myotome of E9.5 mouse embryos. C2C12 myoblasts depleted of mATX3 by RNA interference exhibited a round morphology, cell misalignment, and a delay in differentiation following myogenesis induction. Interestingly, these cells showed a down-regulation of α5 and α7 integrin subunit levels both by immunoblotting and immunofluorescence. Mouse ATX3 was found to interact with α5 integrin subunit and to stabilize this protein by repressing its degradation through the UPS. Proteomic analysis of mATX3-depleted C2C12 cells revealed alteration of the levels of several proteins related to integrin signaling.

Conclusions

Ataxin-3 is important for myogenesis through regulation of integrin subunit levels.

Characterisation and differential regulation of MAFbx/Atrogin-1 alpha and beta transcripts in skeletal muscle of Atlantic salmon (Salmo salar)

MAFbx is an E3 ubiquitin ligase which plays important roles in myogenesis and muscle atrophy. We characterised the Atlantic salmon MAFbx gene, identifying two alternatively spliced MAFbx isoforms. The mRNA sequence of Atlantic salmon MAFbx-alpha is 1698 nucleotides long including a 134 bp 5' UTR and 1065 bp coding sequence which encodes a 355 amino acid protein with a predicted mass of 41,657Da and pI 8.74. Two different 3' UTRs were identified of 495 and 314 bp in length. MAFbx-beta is produced by the removal of the 116 bp exon 2 from MAFbx-alpha, resulting in a frame shift mutation and introduction of a premature stop codon. In contrast to mammals, MAFbx-alpha and beta were ubiquitously expressed in all salmon tissues examined. In vivo, expression was 600-fold (MAFbx-alpha) and 200-fold (MAFbx-beta) higher in fasted individuals than following 21 days refeeding to satiation. In primary myogenic cell cultures, MAFbx-alpha mRNA was highest in differentiated myotubes while MAFbx-beta mRNA had peak expression in mono-nucleated cells. Starving cells of serum and amino acids resulted in a 6-fold increase in MAFbx-alpha, whereas MAFbx-beta remained similar to control levels. In starved cells, MAFbx-alpha mRNA levels declined in response to amino acid, IGF-I and IGF-II treatments whereas MAFbx-beta levels only decreased in response to IGF-I. Addition of amino acids and IGF or insulin to starved cells increased MAFbx-beta levels after 12 and 24h. These results indicate that regulation of MAFbx in Atlantic salmon occurs at both the transcriptional and post-transcriptional level through production of the alternatively spliced MAFbx-beta, which is a likely target for non-sense mediated decay.

The depletion of skeletal muscle satellite cells with age is concomitant with reduced capacity of single progenitors to produce reserve progeny

Satellite cells are myogenic progenitors that reside on the myofiber surface and support skeletal muscle repair. We used mice in which satellite cells were detected by GFP expression driven by nestin gene regulatory elements to define age-related changes in both numbers of satellite cells that occupy hindlimb myofibers and their individual performance. We demonstrate a reduction in satellite cells per myofiber with age that is more prominent in females compared to males. Satellite cell loss also persists with age in myostatin-null mice regardless of increased muscle mass. Immunofluorescent analysis of isolated myofibers from nestin-GFP/Myf5(nLacZ/+) mice reveals a decline with age in the number of satellite cells that express detectable levels of betagal. Nestin-GFP expression typically diminishes in primary cultures of satellite cells as myogenic progeny proliferate and differentiate, but GFP subsequently reappears in the Pax7(+) reserve population. Clonal analysis of sorted GFP(+) satellite cells from hindlimb muscles shows heterogeneity in the extent of cell density and myotube formation among colonies. Reserve cells emerge primarily within high-density colonies, and the number of clones that produce reserve cells is reduced with age. Thus, satellite cell depletion with age could be attributed to a reduced capacity to generate a reserve population.

A distinct profile of myogenic regulatory factor detection within Pax7+ cells at S phase supports a unique role of Myf5 during posthatch chicken myogenesis

Satellite cells are skeletal muscle stem cells that provide myogenic progeny for myofiber growth and repair. Temporal expression of muscle regulatory factors (MRFs) and the paired box transcription factor Pax7 defines characteristic phases of proliferation (Pax7(+)/MyoD(+)/myogenin(-)) and differentiation (Pax7(-)/MyoD(+)/myogenin(+)) during myogenesis of satellite cells. Here, using bromodeoxyuridine (BrdU) labeling and triple immunodetection, we analyzed expression patterns of Pax7 and the MRFs MyoD, Myf5, or myogenin within S phase myoblasts prepared from posthatch chicken muscle. Essentially, all BrdU incorporation was restricted to Pax7(+) cells, of which the majority also expressed MyoD. The presence of a minor BrdU(+)/Pax7(+)/myogenin(+) population in proliferation stage cultures suggests that myogenin up-regulation is alone insufficient for terminal differentiation. Myf5 was detected strictly within Pax7(+) cells and decreased during S phase while MyoD presence persisted in cycling cells. This study provides novel data in support of a unique role for Myf5 during posthatch myogenesis.

Ubiquitylation on canonical and non-canonical sites targets the transcription factor neurogenin for ubiquitin-mediated proteolysis

Polyubiquitylation targets multiple proteins for degradation by the proteasome. Typically, the first ubiquitin is linked to lysine residues in the substrate for degradation via an isopeptide bond, although rarely ubiquitin linkage to the N-terminal residue has also been observed. We have recently shown that Neurogenin (NGN), a basic helix-loop-helix transcription factor that plays a central role in regulating neuronal differentiation, is degraded by ubiquitin-mediated proteolysis. We have taken a biochemical and mutagenesis approach to investigate sites of ubiquitylation of NGN, initially using extracts of eggs from the frog Xenopus laevis as a source of ubiquitylation and degradation components. NGN can be targeted for destruction by ubiquitylation via lysines or the N terminus. However, we see that a modified NGN, where canonical lysine ubiquitylation and N-terminally linked ubiquitylation are prevented, is nevertheless ubiquitylated and degraded by the proteasome. We show that polyubiquitin chains covalently attach to non-canonical cysteine residues in NGN, and these non-canonical linkages alone are capable of targeting NGN protein for destruction. Importantly, canonical and non-canonical ubiquitylation occurs simultaneously in the native protein and may differ in importance for driving degradation in interphase and mitosis. We conclude that native NGN is ubiquitylated on multiple canonical and non-canonical sites by cellular ubiquitin ligases, and all types of linkage can contribute to protein turnover.

Pax3 regulation of FGF signaling affects the progression of embryonic progenitor cells into the myogenic program

Pax3/7-dependent stem cells play an essential role in skeletal muscle development. We now show that Fgfr4 lies genetically downstream from Pax3 and is a direct target. In chromatin immunoprecipitation (ChIP)-on-chip experiments, Pax3 binds to a sequence 3' of the Fgfr4 gene that directs Pax3-dependent expression at sites of myogenesis in transgenic mouse embryos. The activity of this regulatory element is also partially dependent on E-boxes, targets of the myogenic regulatory factors, which are expressed as progenitor cells enter the myogenic program. Other FGF signaling components, notably Sprouty1, are also regulated by Pax3. In vivo manipulation of Sprouty expression reveals that FGF signaling affects the balance between Pax-positive progenitor cells and committed myoblasts. These results provide new insight into the Pax-initiated regulatory network that modulates stem cell maintenance versus tissue differentiation.

Regulation of neurogenin stability by ubiquitin-mediated proteolysis

NGN (neurogenin), a proneural bHLH (basic helix-loop-helix) transcription factor, plays a central role in promoting neuronal specification and differentiation in many regions of the central nervous system. NGN activity has been shown extensively to be controlled at the transcriptional level. However, in addition, recent findings have indicated that the levels of NGN protein may also be regulated. In the present study, we have demonstrated that NGN protein stability was regulated in both Xenopus embryos and P19 embryonal carcinoma cells, a mammalian neuronal model system. In both systems, NGN was a highly unstable protein that was polyubiquitinated for destruction by the proteasome. NGN binds to DNA in complex with its heterodimeric E-protein partners E12 or E47. We observed that NGN was stabilized by the presence of E12/E47. Moreover, NGN was phosphorylated, and mutation of a single threonine residue substantially reduced E12-mediated stabilization of NGN. Thus E-protein partner binding and phosphorylation events act together to stabilize NGN, promoting its accumulation when it can be active.

The dual effects of Cdh1/APC in myogenesis

Ubiquitin-dependent proteolysis plays an important role in regulating fundamental biological functions, including cell division and cellular differentiation. Previous studies implicate the ubiquitin-proteasome system (UPS) in myogenic differentiation through regulating cell cycle progression and modulating myogenic factors such as MyoD and Myf5. Certain ubiquitin protein ligases, including the SCF complex and APC, have been suggested to govern terminal muscle differentiation. However, the underlying mechanism of regulation of both the cell cycle and myogenic factors by the UPS during this process remains unclear. We have dissected the role of the UPS in myogenic differentiation using an in vitro muscle differentiation system based on C2C12 cells. We demonstrate that Cdh1-APC regulates two critical proteins, Skp2 and Myf5, for proteolysis during muscle differentiation. The targeting of Skp2 by Cdh1-APC for destruction results in elevation of p21 and p27, which are crucial for coordinating cellular division and differentiation. Degradation of Myf5 by Cdh1-APC facilitates myogenic fusion. Knockdown of Cdh1 by siRNA significantly attenuates muscle differentiation. Taken together, Cdh1-APC is an important ubiquitin E3 ligase that modulates muscle differentiation through coordinating cell cycle progression and initiating the myogenic differentiation program.

MyoD undergoes a distinct G2/M-specific regulation in muscle cells

The transcription factors MyoD and Myf5 present distinct patterns of expression during cell cycle progression and development. In contrast to the mitosis-specific disappearance of Myf5, which requires a D-box-like motif overlapping the basic domain, here we describe a stable and inactive mitotic form of MyoD phosphorylated on its serine 5 and serine 200 residues by cyclin B-cdc2. In mitosis, these modifications are required for releasing MyoD from condensed chromosomes and inhibiting its DNA-binding and transcriptional activation ability. Then, nuclear MyoD regains instability in the beginning of G1 phase due to rapid dephosphorylation events. Moreover, a non-phosphorylable MyoD S5A/S200A is not excluded from condensed chromatin and alters mitotic progression with apparent abnormalities. Thus, the drop of MyoD below a threshold level and its displacement from the mitotic chromatin could present another window in the cell cycle for resetting the myogenic transcriptional program and to maintain the myogenic determination of the proliferating cells.

Pax3 and Pax7 have distinct and overlapping functions in adult muscle progenitor cells

The growth and repair of skeletal muscle after birth depends on satellite cells that are characterized by the expression of Pax7. We show that Pax3, the paralogue of Pax7, is also present in both quiescent and activated satellite cells in many skeletal muscles. Dominant-negative forms of both Pax3 and -7 repress MyoD, but do not interfere with the expression of the other myogenic determination factor, Myf5, which, together with Pax3/7, regulates the myogenic differentiation of these cells. In Pax7 mutants, satellite cells are progressively lost in both Pax3-expressing and -nonexpressing muscles. We show that this is caused by satellite cell death, with effects on the cell cycle. Manipulation of the dominant-negative forms of these factors in satellite cell cultures demonstrates that Pax3 cannot replace the antiapoptotic function of Pax7. These findings underline the importance of cell survival in controlling the stem cell populations of adult tissues and demonstrate a role for upstream factors in this context.

Multiple phosphorylation events control mitotic degradation of the muscle transcription factor Myf5

Background

The two myogenic regulatory factors Myf5 and MyoD are basic helix-loop-helix muscle transcription factors undergoing differential cell cycle dependent proteolysis in proliferating myoblasts. This regulated degradation results in the striking expression of these two factors at distinct phases of the cell cycle, and suggests that their precise and alternated disappearance is an important feature of myoblasts, maybe connected to the maintenance of the proliferative status and/or commitment to the myogenic lineage of these cells. One way to understand the biological function(s) of the cyclic expression of these proteins is to specifically alter their degradation, and to analyze the effects of their stabilization on cells. To this aim, we undertook the biochemical analysis of the mechanisms governing Myf5 mitotic degradation, using heterologous systems.

Results

We show here that mitotic degradation of Myf5 is conserved in non-myogenic cells, and is thus strictly under the control of the cell cycle apparatus. Using Xenopus egg extracts as an in vitro system to dissect the main steps of Myf5 mitotic proteolysis, we show that (1) Myf5 stability is regulated by a complex interplay of phosphorylation/dephosphorylation, probably involving various kinases and phosphatases, (2) Myf5 is ubiquitylated in mitotic extracts, and this is a prerequisite to its degradation by the proteasome and (3) at least in the Xenopus system, the E3 responsible for its mitotic degradation is not the APC/C (the major E3 during mitosis).

Conclusion

Altogether, our data strongly suggest that the mitotic degradation of Myf5 by the ubiquitin-proteasome system is precisely controlled by multiple phosphorylation of the protein, and that the APC/C is not involved in this process.

Mutant MyoD lacking Cdc2 phosphorylation sites delays M-phase entry

The transcription factors MyoD and Myf-5 control myoblast identity and differentiation. MyoD and Myf-5 manifest opposite cell cycle-specific expression patterns. Here, we provide evidence that MyoD plays a pivotal role at the G(2)/M transition by controlling the expression of p21(Waf1/Cip1) (p21), which is believed to regulate cyclin B-Cdc2 kinase activity in G(2). In growing myoblasts, MyoD reaccumulates during G(2) concomitantly with p21 before entry into mitosis; MyoD is phosphorylated on Ser5 and Ser200 by cyclin B-Cdc2, resulting in a decrease of its stability and down-regulation of both MyoD and p21. Inducible expression of a nonphosphorylable MyoD A5/A200 enhances the MyoD interaction with the coactivator P/CAF, thereby stimulating the transcriptional activation of a luciferase reporter gene placed under the control of the p21 promoter. MyoD A5/A200 causes sustained p21 expression, which inhibits cyclin B-Cdc2 kinase activity in G(2) and delays M-phase entry. This G(2) arrest is not observed in p21(-/-) cells. These results show that in cycling cells MyoD functions as a transcriptional activator of p21 and that MyoD phosphorylation is required for G(2)/M transition.

Ordered proteolysis in anaphase inactivates Plk1 to contribute to proper mitotic exit in human cells

We have found that key mitotic regulators show distinct patterns of degradation during exit from mitosis in human cells. Using a live-cell assay for proteolysis, we show that two of these regulators, polo-like kinase 1 (Plk1) and Aurora A, are degraded at different times after the anaphase-promoting complex/cyclosome (APC/C) switches from binding Cdc20 to Cdh1. Therefore, events in addition to the switch from Cdc20 to Cdh1 control the proteolysis of APC/C(Cdh1) substrates in vivo. We have identified a putative destruction box in Plk1 that is required for degradation of Plk1 in anaphase, and have examined the effect of nondegradable Plk1 on mitotic exit. Our results show that Plk1 proteolysis contributes to the inactivation of Plk1 in anaphase, and that this is required for the proper control of mitotic exit and cytokinesis. Our experiments reveal a role for APC/C-mediated proteolysis in exit from mitosis in human cells.

Active cyclin B1-Cdk1 first appears on centrosomes in prophase

Cyclin B1-Cdk1 is the key initiator of mitosis, but when and where activation occurs has not been precisely determined in mammalian cells. Activation may occur in the nucleus or cytoplasm, as just before nuclear envelope breakdown, Polo-like kinase1 (Plk1) is proposed to phosphorylate cyclin B1 in its nuclear export sequence (NES), to trigger rapid nuclear import. We raised phospho-specific antibodies against cyclin B1 that primarily recognise the active form of the complex. We show that cyclin B1 is initially phosphorylated on centrosomes in prophase and that Plk1 phosphorylates cyclin B1, but not in the NES. Furthermore, phosphorylation by Plk1 does not cause cyclin B1 to move into the nucleus. We conclude that cyclin B1-Cdk1 is first activated in the cytoplasm and that centrosomes may function as sites of integration for the proteins that trigger mitosis.

P311 induces a TGF-beta1-independent, nonfibrogenic myofibroblast phenotype

P311, also called PTZ17, was identified by suppressive subtraction hybridization as potentially involved in smooth muscle (SM) myogenesis. P311 is an 8-kDa protein with several PEST-like motifs found in neurons and muscle. P311 transfection into two fibroblast cell lines, NIH 3T3 and C3H10 T1/2, induced phenotypic changes consistent with myofibroblast transformation, including upregulation of SM alpha-actin and SM22, induction of FGF-2, VEGF, PDGF, and PDGF receptors, upregulation of integrins alpha3 and alpha5, and increased proliferation rate. The P311-mediated changes differed, however, from the well-characterized myofibroblast in that P311 inhibited TGF-beta1, TGF-beta receptor 2, and TGF-beta1-activating MMP-2 and MMP-9, with the resultant decrease in collagen 1 and 3 expression. The effect of P311 on collagen was overcome by exogenous TGF-beta1, indicating that the cells were responsive to TGF-beta1 paracrine stimulus. In support of a role for P311 in vivo, immunohistochemical examination of human wounds showed P311 only in myofibroblasts and their activated precursors. To our knowledge, these studies are the first to implicate P311 in myofibroblast transformation, to demonstrate that transformation may occur independently of TGF-beta1, and to suggest that P311 may prevent fibrosis.

The myogenic basic helix-loop-helix family of transcription factors shows similar requirements for SWI/SNF chromatin remodeling enzymes during muscle differentiation in culture

The myogenic basic helix-loop-helix family of transcription factors, MyoD, Myf5, myogenin, and MRF4, can each activate the muscle differentiation program when ectopically expressed in non-muscle cells. SWI/SNF complexes are ATP-dependent chromatin remodeling enzymes. We demonstrated previously that SWI/SNF enzymes promote MyoD-mediated muscle differentiation. To ascertain the requirement for SWI/SNF enzymes in muscle differentiation mediated by different MyoD family members, we examined MyoD, Myf5, MRF4, and myogenin-mediated induction of muscle differentiation in cells expressing dominant negative versions of BRG1 or BRM-based SWI/SNF enzymes. We demonstrated that expression of dominant negative BRG1 or BRM inhibited the induction of muscle-specific gene expression by Myf5 and MRF4; however, myogenin failed to induce measurable quantities of muscle-specific mRNAs, even in cells not expressing dominant negative SWI/SNF. In contrast, all four myogenic regulators induced expression of the cell cycle regulators p21, Rb, and cyclin D3 and promoted cell cycle arrest independently of the SWI/SNF enzymes. We proposed that SWI/SNF enzymes are required for the induction of all muscle-specific gene expression by MyoD, Myf5, and MRF4, whereas induction of the cell cycle regulators, p21, Rb, and cyclin D3 occurred independently of SWI/SNF function.

Cell density-dependent induction of endogenous myogenin (myf4) gene expression by Myf5

Transcription factors Myf5 and MyoD are critical for myoblast determination. Myogenin is a direct transcriptional target of these factors and its expression is associated with commitment to terminal differentiation. Here, we have used myogenic derivatives of human U20S cells expressing Myf5 or MyoD under control of a tetracycline-sensitive promoter to study expression of endogenous myogenin (myf4). We find that Myf5-mediated induction of myogenin shows striking dependence on cell density. At high cell density, Myf5 is a potent inducer of myogenin expression. At low cell density, Myf5 (unlike MyoD) is a poor inducer of myogenin expression, whilst retaining the capacity to direct expression of other muscle-specific genes. The permissive influence of high cell density on myogenin induction by Myf5 is not a consequence of serum depletion or cell cycle arrest, but is mimicked by a disruption adjacent to the basic region of Myf5 (Myf5/mt) which reduces its DNA binding affinity for E-boxes without compromising its ability to transactivate a reporter gene driven by the myogenin promoter. Coculture of cells expressing wild-type Myf5 and Myf5/mt leads to reduced myogenin induction in Myf5/mt cells. We propose that at low cell density Myf5 inhibits induction of myogenin.

Skeletal muscle formation in vertebrates

Research in the past year has added to our understanding of the signalling systems that specify myogenic identity in the embryo and of the regulation and roles of MyoD family members. New insights into the movement of muscle precursor cells include the demonstration that Lbx1 is essential for their migration from the somite to some but not all sites of muscle formation elsewhere. Later in development, ras as well as calcineurin signalling is now implicated in the definition of slow versus fast fibre types. The myogenic identity of precursor cells in the adult depends on Pax7, the orthologue of Pax3 which is required for early myogenesis; this finding is of major importance for muscle regeneration and the active field of stem cell research.