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Clin Genet 2000: 57: 1625Printed in Ireland. All rights reser6ed

Developmental Biology: Frontiers for Clinical Genetics

Section Editor: Roderick R McInnes

e-mail: [email protected]

The molecular regulation of myogenesis

Sabourin LA, Rudnicki MA. The molecular regulation of myogenesis.Clin Genet 2000: 57: 1625. Munksgaard, 1999

Over the past years, several studies have unraveled important mecha-nisms by which the four myogenic regulatory factors (MRFs: MyoD,Myf-5, myogenin, and MRF4) control the specification and the differ-entiation of the muscle lineage. Early experiments led to the hypothesisthat these factors were redundant and could functionally replace oneanother. However, recent experiments using in 6i6o and in 6itro modelshave demonstrated that in fact different aspects of the myogenic pro-gram are controlled by different factors in 6i6o, suggesting that these

factors play distinct roles during myogenesis. The activity of the MRFsduring proliferation and differentiation of muscle precursor cells hasclearly been demonstrated to be dependent on specific cell-cycle controlmechanisms as well as distinct interactions with other regulatorymolecules, such as the ubiquitously expressed E proteins and severalother transcription factors. Furthermore, the observation that theMRFs can recruit chromatin remodeling proteins has shed some lighton the mechanisms by which the MRFs activate gene expression. Re-cently, a functional role for MyoD during satellite cell activation andmuscle repair has been identified in 6i6o, which cannot be substitutedfor by the other MRFs. This has put forward the hypothesis that thesefactors also play specific biological roles following muscle injury andrepair.

Luc A Sabourin and MichaelA Rudnicki

Institute for Molecular Biology and

Biotechnology, MOBIX, McMaster

University, Hamilton, Ontario, Canada

Corresponding author: Michael A Rudnicki,

McMaster University, 1280 Main St. West,

Life Sciences Rm 437, Hamilton, Ontario

L8S 4K1, Canada. Tel: +1 905 5259140

(ext: 27424); e-mail: [email protected]

Received 8 October 1999, revised and ac-

cepted for publication 14 October 1999

The myogenic regulatory factors (MRFs) are partof a superfamily of basic helix-loop-helix (bHLH)transcription factors including c-myc and acheate-scute (13). The MRF subfamily consists ofMyoD (Myf-3) (4), Myf-5 (5), myogenin (Myf-1)(6), and MRF4 (Myf-6/Herculin) (7 9). Twelveyears ago, the MyoD gene was first isolated fromsubtractive hybridization procedures using my-oblast-specific cDNA libraries (4). The MyoDcDNA was identified by virtue of its ability toconvert fibroblasts into myogenic cells. Subse-quently, low stringency library screens uncovered

three more MRFs all capable of inducing myo-genic conversion when overexpressed in a vastnumber of nonmuscle cell lines. The MRF proteinscontain a conserved basic DNA-binding domainessential for sequence-specific DNA binding and ahelix-loop-helix motif required for heterodimeriza-tion. Each of the MRFs has been shown to het-erodimerize in 6itro and in 6i6o with E proteins andto bind DNA in a sequence-specific manner at sitesknown as E-boxes (CANNTG). This DNA motif

is present in the promoters of many skeletal mus-cle-specific genes and mediates gene activation inan MRF-dependent manner (13, 1012).

Expression of the myogenic factors during

embryogenesis

The genes encoding the four MRFs have beenshown to be expressed in a temporally distinctpattern. As determined by in situ hybridization,activation of Myf-5 occurs first in the rostralsomites of the mouse around 8 days postcoitum

(dpc) and is down-regulated after day 14 (13). Theactivation of myogenin is observed at 8.5 dpcfollowed by MyoD at about 10.5 dpc along withmarkers of terminal differentiation (14). MRF4 isexpressed transiently between days 9 and 12 andrepressed until after birth (15). In the limb bud,Myf-5 is expressed transiently between days 10 and12, followed by co-expression of myogenin andMyoD after day 10.5 and MRF4 after day 16(1315). The specific promoter elements that gov-

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ern the temporo-spatial expression of the MRFshave yet to be fully defined. However, several studieshave identified specific enhancer elements for MyoDand myogenin (1623). Induction of the myogeningene has been shown to be dependent on an E-boxand a myocyte enhancer factor-2 element for properexpression in the somites and limbs of the develop-ing mouse, suggesting that its expression is MRF-dependent, in part (17, 23). The expression of the

MyoD gene during embryogenesis has been foundto be regulated by at least two distinct enhancers. Afirst element, located 5 kb upstream of the corepromoter region, directs MyoD expression duringthe terminal differentiation of myogenic precursorcells into myotubes and myofibers (18, 22). A secondenhancer, found 20 kb upstream of the MyoD startsite, has been demonstrated to direct the expressionof a LacZ reporter gene during embryogenesis inspecific somitic subdomains (19 21). The superposi-tion of the expression patterns generated by bothenhancers results in a pattern that is indistinguish-

able from the endogenous MyoD gene. The controlof MyoD expression through the 20-kb elementappears to be E-box-independent (24). In addition,demethylation of the MyoD locus has been demon-strated to play an important role during its activa-tion de no6o (25). Interestingly, Tajbakhsh andco-workers have shown that expression of MyoD isdependent on either Myf-5 or Pax-3 (a paired-boxdomain protein), demonstrating that MyoD actsdownstream of these genes during myogenesis (26)(and reviewed in (27)). In an independent study,ectopic expression of the Pax-3 gene in embryonictissues was shown to induce the expression of MyoD

and Myf-5 (28). In contrast, overexpression of Pax-3in cultured myoblasts inhibits terminal differentia-tion in 6itro and this phenomenon appears to bedependent on Pax-3 DNA-binding activity (29).Further dissection of the MyoD promoter will likelyuncover other enhancer elements important for theMyoD gene activation.

Targeted inactivation of the MRF genes

Gene targeting experiments have provided muchinsight into the functions of the MRFs in 6i6o. The

introduction of null mutations in the four MRFsinto the germline of mice has demonstrated theexistence of a hierarchical relationship among theMRFs. Inactivation of MyoD in mice results in anapparently normal muscle phenotype with a four-fold increase in Myf-5 expression (30). Similarly,Myf-5-deficient mice display normal skeletal mus-cles but die perinatally because of severe rib defects(31). Interestingly, introduction of the myogenincDNA into the Myf-5 locus is able to rescue the rib

defect and results in viable and fertile mice but isunable to fully compensate for the absence of Myf-5during myogenic determination (32, 33). However,mice deficient for both MyoD and Myf-5 die at birthowing to a complete absence of skeletal myoblastsand muscle (34). Mice lacking myogenin display anormal number of myoblasts but die at birth be-cause of an absence of myofibers (35, 36). In con-trast, inactivation of MRF4 results in viable mice

with apparently normal muscles but with a fourfoldincrease in myogenin expression (37 39). Takentogether, these experiments have then defined twogroups of MRFs. The primary MRFs, MyoD andMyf-5, appear to be required for myogenic determi-nation, whereas the secondary MRFs, myogeninand MRF4, are required downstream of MyoD andMyf-5 as differentiation factors (Fig. 1). In addition,these studies have demonstrated that some MRFscan substitute for one another without affectingoverall muscle development, suggesting the exis-tence of potential redundancy among the MRFs(40).

Recently, the use of MyoD-lacZ transgenic micebred into the MyoD- or Myf-5-deficient back-grounds has been used to address potential redun-dancy (41). MyoD-/- mice display normal epaxial(paraspinal and intercostal) muscle development,whereas hypaxial (limb and abdominal wall) devel-opment is delayed by 2.5 days (Fig. 2). In contrast,Myf-5-/- embryos exhibit normal muscle develop-ment in the limb buds and branchial arches, andmarkedly delayed development of epaxial muscles(41). Furthermore, normal migration of Pax-3-ex-pressing cells into the limb buds and subsequentinduction of Myf-5 in myogenic precursors areobserved, suggesting that Myf-5 expression in thelimb is insufficient for the normal progression ofmyogenic development (41). These observationssuggest that MyoD and Myf-5 play distinct rolesduring the formation of epaxial and hypaxial mus-cles and argue against the existence of redundancyamong the MRFs.

Fig. 1. Targeted inactivation of the MRFs has defined two

groups of factors. The primary MRFs, MyoD and Myf-5, are

required at the determination step for commitment of the

proliferating somitic cells to the myogenic lineage. The com-

mitted cells (myoblasts) can proliferate and further differenti-

ate into myocytes and mature into myofibers under the action

of the secondary MRFs, myogenin and MRF4.

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Sabourin and Rudnicki

Fig. 2. Myogenic cell lineages. The temporal expression pattern of a MyoD-LacZ transgene in different null background shows that

the 5-kb enhancer of MyoD, expressed in differentiated myocytes, is activated in the epaxial lineage (blue) in the absence of MyoD

and in the hypaxial domain (red) in the absence of Myf-5. MyoD-/- mice display normal epaxial muscle development, whereas

hypaxial development is delayed by 2.5 days, suggesting a specific role for Myf-5 in the establishment of the epaxial musculature.

In contrast, Myf-5-/- embryos exhibit normal muscle development in the limb buds and branchial arches, and markedly delayed

development of epaxial muscles, demonstrating a functional role for MyoD in the establishment of the hypaxial musculature.

Regulatory mechanisms of MRF activity

In 6itro differentiation systems have providedfurther into the regulation of MRFs activity andthe relationships between growth and differentia-tion. Several studies have demonstrated that theMRFs can efficiently heterodimerize with productsof the E2-2 (ITF2) and E2-5 genes (E12, E47, andITF1) (42 44) (see Fig. 3 and reviewed in (2)).These heterodimers activate muscle-specific tran-scription of E-box-containing muscle gene pro-moters (44). However, it is not clear whetherspecific heterodimers play distinct biological roles.

The MRFs have been shown to be negatively regu-lated by the HLH protein Id, which lacks a basicDNA-binding domain (45, 46). The Id factors,encoded by at least four different genes (Id1, Id2,Id3, and Id4), act in a dominant negative mannerby heterodimerizing with E proteins preventingtheir association with the MRFs and subsequentmuscle-specific gene activation (46) (Fig. 3). Simi-larly, MyoD activity has been shown to be inhib-ited in 6itro by the murine twist (mTwist) protein(47). The mTwist protein is also thought to se-quester E proteins, preventing MRF-E protein het-

erodimer formation. In addition to Id and mTwistproteins, MyoD has recently been shown to benegatively regulated by dimerization with Mist1, anovel bHLH factor that lacks a transactivationdomain (48). The resulting heterodimer does notbind E-box-containing promoters.

The activity of the MRFs is also tightly coupledto the cell cycle (reviewed in (49, 50)). For exam-ple, the hypophosphorylated form of theretinoblastoma protein (Rb) has been demon-

strated to associate with MyoD and to be requiredfor efficient transactivation of E-box-containingmuscle-specific promoters (51). The Rb protein isalso necessary to maintain the differentiated phe-notype of cultured myotubes. In addition, induc-tion of differentiation in cultured myoblasts resultsin up-regulation of the cell-cycle inhibitors p21(WAF-1, Cip1) and p16 (5254). The p21 gene hasbeen demonstrated to bear E-box in its upstreamregulatory regions and is activated by MyoD over-expression in transient assays (53). Supportingthese observations, high expression of p21 has been

correlated with the activation of the myogeningene during embryogenesis (55). The cell-cycleMRF connection is further substantiated by theobservation that overexpression of cyclin D1, aG1-S cyclin and a Cdk4 activator, inhibits MyoDactivity and subsequent transactivation of E-box-containing reporter genes (56, 57). Furthermore,MyoD activity has been demonstrated to be down-regulated by direct interaction with the C-terminaldomain of Cdk4 (58) (Fig. 3). This interactionappears to require the cyclin D1-dependent nucleartargeting of Cdk4. The activity of the MRFs is

further coupled to cellular proliferation by thenegative regulatory effect of the AP1 heterodimerFos/c-Jun. The proto-oncogene, c-Jun, directlybinds MyoD and inhibits its activity (59, 60). Fur-thermore, upon differentiation of cultured my-oblasts in 6itro, the c-Fos promoter isdown-regulated by MyoD through an E-boxwithin the c-Fos promoter (61). Various otheroncogenes such as c-myc, N-ras, and Ha-ras alsoinhibit muscle differentiation in 6itro, suggesting

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that growth-promoting factors negatively regulatethe MRFs (62 66). In addition, viral oncogenessuch as adenovirus E1a inhibit differentiation andMRF activity by direct physical interaction (67, 68).

MRF activity is also potentially negatively regu-lated through phosphorylation. For example, myo-genin is phosphorylated directly by protein kinase C(PKC) in 6itro and in 6i6o (69). Fibroblast growthfactor (FGF) treatment or PKC overexpression

results in threonine-87 phosphorylation in theDNA-binding domain and a loss of DNA-bindingactivity. Similarly, protein kinase A negatively reg-ulates myogenin through an indirect mechanism(70). Recently, the mitogen-activated protein kinase(MAPK) pathway has been shown to be activatedin differentiating muscle cells and to positivelyregulate the expression and activity of the MyoDprotein (71). However, in contrast, Bennet andco-workers have demonstrated that upon mitogenwithdrawal from C2C12 myoblasts, the MAPKp42Erk2 is inactivated concomitant with up-regula-

tion of muscle-specific genes (72). Supporting this,they showed that overexpression of MAPK phos-phatase-1 inhibited p42Erk2 activity and was suffi-cient to relieve the inhibitory effects of mitogens onmuscle-specific gene expression. Similarly, continu-ous activation of MEK, a MAP kinase kinase, isdetrimental to insulin-like growth factor-1- (IGF-1)or FGF-2-induced myogenesis (73). Recently, theactivity of stress-activated protein kinase 2(SAPK2/p38) has also been demonstrated to beimportant for the terminal differentiation of C2C12myoblasts (74). Whether different components ofthe MAPK pathway play distinct biological roles

during growth or differentiation remains to beelucidated.

Regulation of transcription through MRF/co-factor

interactions

In recent years, attempts at identifying the mecha-nisms underlying MRF functions have uncovered anumber of MRF-binding proteins and co-factors.

Interestingly, these co-activators are known to playimportant roles in chromatin remodeling, RNApolymerase II functions, or are transcription fac-tors themselves. Among the co-activators, MyoDhas been shown to interact directly with p300/CBP(75 78). This interaction was demonstrated bothin 6i6o and in 6itro and appeared to be required forthe terminal differentiation of cultured myoblasts.This interaction occurs through the carboxyl cys-teine/histidine-rich (C/H3) domain of p300 andincreases the ability of MyoD to transactivate anE-box-containing reporter construct (76, 78).

Active gene expression is associated with histoneacetylation and loss of histone/DNA interaction.Interestingly, in addition to p300/CBP, MyoD in-teracts with the histone acetyltransferase PCAF ina multiprotein complex also containing p300/CBP(78). Disruption of this complex by anti-PCAFantibody microinjection inhibits muscle differentia-tion, indicating that recruitment of histone acetyl-transferase activity of PCAF by MyoD, throughp300/CBP, is crucial for activation of the myogenicprogram. In a separate study, Gerber and co-work-ers have demonstrated that a cysteine-histidine-richregion of MyoD, upstream of the basic DNA-

Fig. 3. The activity of the MyoD family is coupled to cell-cycle control. In proliferating myoblasts, activated cyclin-dependent

kinases (Cdk4) inhibit MyoD activity through direct interaction. Expression of Id proteins precludes the formation of E

protein-MRFs heterodimers. Upon differentiation, withdrawal from the cell cycle is maintained by a positive feedback loop in

which high p21 and Rb expression prevents re-entry into the cell cycle and the MRF-E protein complex is activated. Green arrows

denote positive, whereas blunt red arrows denote negative regulatory relationships.

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binding domain, is necessary for chromatin remod-eling and gene activation by MyoD (79).

Although no known muscle diseases have beenassociated with genetic alterations in any of theMRFs (80), there is evidence that mutations inco-factors for the MRFs are contributing to thepathogenesis of rhabdomyosarcomas. Such geneticmodifications include the amplification of MDM2,for which overexpression has been shown to inhibit

myoblast differentiation (81). In addition, het-erokaryon formation studies have revealed thatrhabdomyosarcomas are deficient for an unknownfactor required for MyoD activity (82).

In addition to these factors, MyoD also interactswith components of the transcriptional machinery.Recently, the TATA-binding protein TFIID hasbeen identified as a novel MyoD-binding proteinand found to stabilize the binding of MyoD to itsconsensus binding site (83). Furthermore, MyoDhas been observed to facilitate the association ofTFIIB with the preinitiation complex subsequent

to DNA binding. Interaction of the MRF-E12dimers with muscle LIM protein also results inincreased DNA-binding activity and stimulation ofmyogenesis (84). Transcription factors such asMEF2-C, a member of the MEF2 family of tran-scription factors, has also been demonstrated tointeract directly and synergize with the MyoD-E12heterodimer but not with either protein alone (re-viewed in (8587)). Similarly, serum response fac-tor, a MEF2-related protein, has been shown tobind and enhance the activity of MRFs-E12 het-erodimers (88).

Muscle regeneration and satellite cell function: role

of MyoD

Satellite cells, the stem cells of adult skeletal mus-cles, reside beneath the basal lamina of adult skele-tal muscle closely juxtaposed against the musclefibers (89). Satellite cells arise around 17 dpc dur-ing mouse embryogenesis and are believed to rep-resent a unique myoblast lineage. Satellite cellsmediate the postnatal growth of muscle and con-tribute for the most part to the formation of theadult muscle mass (89). Satellite cells make up

2 7% of the nuclei associated with a particularmyofiber. This proportion varies with age and aparticular muscle group.

Satellite cells are normally mitotically quiescentbut are activated and re-enter the cell cycle inresponse to stress induced by weight-bearing exer-cise or trauma, including injury (89 91). Thedaughter cells of the activated satellite cells, calledmyogenic precursor cells (mpcs), undergo multiplerounds of division prior to fusion with the existing

or new myofibers. Satellite cells appear to form apopulation of stem cells that are biologically andbiochemically distinct from their descendant mpcs(89, 92). The total number of quiescent satellitecells in adult muscle remains relatively constantover multiple cycles of degeneration and regenera-tion, suggesting that self-renewal in the satellite cellcompartment maintains a population of quiescentcells (89). However, the numbers and proliferative

potential of satellite cells become progressively re-duced in muscle diseases presenting with muscularatrophy, such as Duchenne muscular dystrophy(DMD), which is likely due to high levels of ongo-ing regeneration (93, 94).

The essential role played by satellite cells inmuscle regeneration, muscle hypertrophy, andpostnatal muscle growth has been demonstratedextensively (89, 92, 95). However, the molecularmechanisms underlying the activation and functionof myogenic stem cells are still unclear. As deter-mined by polymerase chain reaction analysis, qui-

escent satellite cells display no detectable levels ofeither of the four MRFs (96). Upon injury andactivation, MyoD is rapidly up-regulated within 12h. This up-regulation of MyoD occurs prior to theexpression of proliferating cell nuclear antigen(PCNA), a marker for cell proliferation. The ex-pression of myogenin occurs last during the timeassociated with fusion and differentiation (97, 98).Analysis of gene expression by reverse transcrip-tion-polymerase chain reaction of individual satel-lite cells following their activation in intact musclefibers (96) substantiates that quiescent satellite cellsexpress no detectable MRFs but do express the

c-met receptor tyrosine kinase (the receptor forhepatocyte growth factor). Activated satellite cellsfirst express either Myf-5 or MyoD. Subsequently,both factors are co-expressed during the prolifera-tive phase. Following proliferation, myogenin andMRF4 are expressed in cells entering the terminaldifferentiation program. The absence of MRFmRNA in satellite cells prior to activation suggeststhat satellite cells represent a stem cell lineage thatis distinct from myoblasts. Furthermore, the deno6o induction of Myf-5 and MyoD transcriptionimplies that inductive signals are involved,

analogous to those that occur during embryogene-sis (27, 99).

The role of MyoD in satellite cell function hasbeen investigated by interbreeding MyoD-/- mice(30) with mdx mice. The mdx mouse carries a lossof function point mutation in the X-linked dys-trophin gene, and thus is an animal model forhuman Duchenne and Becker muscular dystrophy(93). The mdx mice display a high regenerativecapacity leading to muscle hypertrophy, making it

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an attractive model to investigate the role of theMRFs during muscle regeneration. Studies haveshown that the compound mutant mice(mdx:MyoD-/-) exhibit markedly increased pene-trance of the mdx phenotype characterized by mus-cle atrophy and increased myopathy leading topremature death (100). By 3 5 months of age,mdx:MyoD -/- mice develop a profound dorsal ventral curvature of the spine similar to the lordosis

and kyphosis of patients with DMD. Interestingly,unlike mdx mice, mdx:MyoD-/- animals also dis-play severe cardiomyopathy, a hallmark of DMDpatients (101).

Muscle regeneration is severely impaired inMyoD-/- mice and is characterized by an almostcomplete absence of proliferative myogenic precur-sor cells as determined by 3H-thymidine incorpora-tion or immunohistochemistry with antibodyreactive to PCNA (100). However, electron micro-scopic examination of MyoD-deficient muscle re-veals the presence of morphologically normal

satellite cells. However, cell counts show that theirrelative abundance is increased by 1.8-fold inMyoD-/- muscle and 13-fold in mdx:MyoD-/- mus-cle. These data suggest that up-regulation of MyoDis required for satellite cells to enter the mpc prolif-erative phase prior to terminal differentiation. Inthe absence of MyoD, myogenic stem cells undergoseveral rounds of division and return to a quiescentstate rather than progressing through the develop-mental program. Taken together, these experimentsstrongly support the hypothesis that satellite cellsform a stem cell compartment that is the source ofmyogenic precursor cells (100).

To gain insight into the regeneration deficit ofMyoD-/- muscle, satellite cell-derived primary cul-tures from adult MyoD-/- hind limb muscle weregenerated and analyzed for proliferative and differ-entiation potential. Low passage MyoD-/- myo-genic cells exhibit a fibroblast-like morphologydistinct from the bipolar morphology of wildtypemyoblasts (102). Myogenic cells lacking MyoDexpress c-met (96, 103), but do not express desmin,an intermediate filament protein typically expressedin myoblasts in 6itro and in 6i6o (104). Following theinduction of differentiation in 6itro, wildtype my-

oblasts undergo cell-cycle arrest and fuse into mult-inucleated myotubes, whereas MyoD-/- cellscontinue to proliferate and yield reduced numbersof predominantly mononuclear myocytes after sev-eral days in differentiation medium (102, 105). Inaddition, the expression of differentiation-specificmarkers is drastically reduced or absent in MyoD-/-cells. As for MyoD-/- muscle tissue, MyoD-/- my-oblast cultures display a fourfold increase in Myf-5mRNA expression, suggesting that overexpression

Fig. 4. Role of MyoD in satellite cell function. Upon activa-

tion, quiescent satellite cells, expressing c-met, first express

Myf-5 or MyoD before co-expressing both and progressing

through the developmental program. In the absence of MyoD,

satellite cells appear to exhibit a propensity for self-renewal

rather than progression through the differentiation program.

Expression of Myf-5 alone may allow self-renewal either before

returning to quiescence (yellow arrow) or up-regulating MyoD

and formation of proliferative mpcs (white arrows).

of Myf-5 cannot alleviate the differentiation defectimparted by the inactivation of MyoD in these cells.

Furthermore, culture mixing experiments usingLacZ-marked MyoD-/- cells has demonstrated thatthe MyoD-/- cellular phenotype is cell autonomous.Interestingly, expression of IGF-1 is markedly in-creased in MyoD-/- myogenic cells cultured underdifferentiation conditions, suggesting that MyoDnormally negatively regulates IGF-1 expression inprimary myogenic cells. One possibility is that IGF-1 promotes proliferation and inhibits differentiationof MyoD-/- myoblasts via an autocrine loop. Inaddition, the expression of M-cadherin is markedlyreduced in MyoD-/- myogenic cells and a require-

ment for M-cadherin has been reported for cell-cy-cle withdrawal and myoblast fusion (106, 107).Taken together, these results suggest that MyoD-/-myogenic cells represent an intermediate stage inthe satellite cell activation pathway downstream ofthe quiescent state but upstream of the mpc com-partment (102) (see Fig. 4). Since Myf-5-/- mice dieperinatally (31), a definitive role for Myf-5 in satel-lite cell activation has yet to be determined.

Future work

It is now well established that the MRFs constitutea group of four bHLH transcription factors thatplay a pivotal role during the specification anddifferentiation of muscle cells. To date, severalsignaling pathways that regulate MRF activityhave been identified. However, it will be of interestto identify additional regulatory components suchas kinases, phosphatases, and other transducersthat are directly controlling the activity of theMRFs under growth and differentiation condi-

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tions. Of great interest will be the identificationof the regulatory elements and the factors in-volved in the control of the de no6o MRF geneexpression during embryogenesis and satellite cellactivation. This is especially important for the in-duction of MyoD and Myf-5 as these determina-tion factors are likely to be subject to differentregulatory mechanisms, whether the cells are in acontext of embryogenesis or muscle regeneration.

The activity of the MyoD protein has beendemonstrated to be modulated by its interactionwith various nuclear co-factors and transactiva-tors. The identification of additional interactingco-factors or RNA polymerase II-associated com-ponents will provide further insights into our un-derstanding of the molecular mechanismsunderlying tissue-specific gene expression. In ad-dition, the identification of specific target genesfor which the expression is regulated by one ormore MRFs will be valuable for understandingthe distinct biological roles played by each of the

MRFs.Finally, the specific functions of each of theMRFs during satellite cell activation and muscleregeneration remain to be determined. The use ofMyoD-deficient and MRF4-/- mice will proveuseful in elucidating their role during satellite cellactivation but a major difficulty lies in the gener-ation of viable animals bearing mutations for theother two MRFs. The establishment of condi-tional mutant lines may circumvent the viabilityproblems encountered with the Myf-5-/- or myo-genin-null mice. Alternatively, the use of knock-ins, as demonstrated by Wang and co-workers

(33), may be helpful in this regard.

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