The myogenic basic Helix-Loop-Helix transcription factors, including Myf5, MyoD, myogenin (myg) and MRF4, play important roles in skeletal muscle development. The phenotypes of mutant mice deficient in either gene are different, suggesting that each gene may have a unique function in vivo. We previously showed that targeting myogenin into the Myf5 locus (Myf5(myg-ki)) rescued the rib cage truncation in the Myf5-null mutant, hence demonstrating functional redundancy between Myf5 and myogenin in skeletal morphogenesis. Here we present the results of crossing myogenin knock-in (myg-ki) mice with either MyoD-null or myogenin-null mutants. The Myf5(myg-ki) allele rescued early myogenesis, but Myf5(myg-ki/myg-ki);MyoD(−/−) mutant mice died immediately after birth owing to reduced muscle formation. Therefore, myogenin, expressed from the Myf5 locus, is not able to completely replace the function of Myf5 in muscle development although it is capable of determining and/or maintaining myogenic lineage. Myf5(myg-ki/myg-ki);myg(−/−) mutant mice displayed the same phenotype as myg(−/−) mutants. This indicates that the earlier expression of myogenin cannot promote myogenic terminal differentiation, which is normally initiated by the endogenous myogenin. Thus, our results are consistent with the notion that Myf5 and myogenin are functionally interchangeable in determining myogenic lineage and assuring normal rib formation. Our experiment revealed, however, that some aspects of myogenesis may be unique to a given myogenic factor and are due to either different regulatory sequences that control their temporal and spatial expression or different functional protein domains.

REFERENCES

Asakura
A.
,
Lyons
G. E.
,
Tapscott
S. J.
(
1995
)
The regulation of MyoD gene expression: conserved elements mediate expression in embryonic axial muscle.
Dev. Biol
171
,
386
98
Black
B. L.
,
Martin
J. F.
,
Olson
E. N.
(
1995
)
The mouse MRF4 promoter is trans-activated directly and indirectly by muscle-specific transcription factors.
J. Biol. Chem
270
,
2889
92
Bober
E.
,
Lyons
G. E.
,
Braun
T.
,
Cossu
G.
,
Buckingham
M.
,
Arnold
H. H.
(
1991
)
The muscle regulatory gene, Myf-6, has a biphasic pattern of expression during early mouse development.
J. Cell Biol
113
,
1255
65
Braun
T.
,
Rudnicki
M. A.
,
Arnold
H. H.
,
Jaenisch
R.
(
1992
)
Targeted inactivation of the muscle regulatory gene Myf-5 results in abnormal rib development and perinatal death.
Cell
71
,
369
82
Buckingham
M.
(
1994
)
Muscle differentiation. Which myogenic factors make muscle?.
Curr. Biol
4
,
61
3
Cheng
T. C.
,
Wallace
M. C.
,
Merlie
J. P.
,
Olson
E. N.
(
1993
)
Separable regulatory elements governing myogenin transcription in mouse embryogenesis.
Science
261
,
215
8
Chomczynski
P.
,
Sacchi
N.
(
1987
)
Single-step method of RNA isolation by acid guanidinium thiocyanate-phenol-chloroform extraction.
Analyt. Biochem
162
,
156
159
Christ
B.
,
Jacob
H.
,
Jacob
M.
(
1978
)
On the formation of the myotomes in avian embryos. An experimental and scanning electron microscope study.
Experientia
34
,
514
516
Cusella-De Angelis
M. G.
,
Lyons
G.
,
Sonnino
C.
,
De Angelis
L.
,
Vivarelli
E.
,
Farmer
K.
,
Wright
W. E.
,
Molinaro
M.
,
Bouche
M.
,
Buckingham
M.
, et al. 
(
1992
)
MyoD, myogenin independent differentiation of primordial myoblasts in mouse somites.
J. Cell Biol
116
,
1243
55
Gerber
A. N.
,
Klesert
A. R.
,
Bergstrom
D. A.
,
Tapscott
S. J.
(
1997
)
Two domains of MyoD mediate transcriptional activation of genes in repressive chromatin: a mechanism for lineage determination in myogenesis.
Genes Dev
11
,
436
50
Goldhamer
D. J.
,
Brunk
B. P.
,
Faerman
A.
,
King
A.
,
Shani
M.
,
Emerson
C. P.
Jr
(
1995
)
Embryonic activation of the myoD gene is regulated by a highly conserved distal control element.
Development
121
,
637
49
Hasty
P.
,
Bradley
A.
,
Morris
J. H.
,
Edmondson
D. G.
,
Venuti
J. M.
,
Olson
E. N.
,
Klein
W. H.
(
1993
)
Muscle deficiency and neonatal death in mice with a targeted mutation in the myogenin gene.
Nature
364
,
501
6
Hinterberger
T. J.
,
Sassoon
D. A.
,
Rhodes
S. J.
,
Konieczny
S. F.
(
1991
)
Expression of the muscle regulatory factor MRF4 during somite and skeletal myofiber development.
Dev. Biol
147
,
144
56
Lassar
A.
,
Munsterberg
A.
(
1994
)
Wiring diagrams: regulatory circuits and the control of skeletal myogenesis.
Curr. Opin.Cell Biol
6
,
432
42
Megeney
L. A.
,
Kablar
B.
,
Garrett
K.
,
Anderson
J. E.
,
Rudnicki
M. A.
(
1994
)
MyoD is required for myogenic stem cell function in adult skeletal muscle.
Genes Dev
10
,
1173
83
Nabeshima
Y.
,
Hanaoka
K.
,
Hayasaka
M.
,
Esumi
E.
,
Li
S.
,
Nonaka
I.
,
Nabeshima
Y.
(
1993
)
Myogenin gene disruption results in perinatal lethality because of severe muscle defect.
Nature
364
,
532
5
Naidu
P. S.
,
Ludolph
D. C.
,
To
R. Q.
,
Hinterberger
T. J.
,
Konieczny
S. F.
(
1995
)
Myogenin and MEF2 function synergistically to activate the MRF4 promoter during myogenesis.
Mol. Cell Biol
15
,
2707
18
Olson
E. N.
(
1990
)
MyoD family: a paradigm for development?.
Genes Dev
4
,
1454
61
Olson
E. N.
,
Klein
W. H.
(
1994
)
bHLH factors in muscle development: dead lines and commitments, what to leave in and what to leave out.
Genes Dev
8
,
1
8
Olson
E. N.
,
Arnold
H. H.
,
Rigby
P. W.
,
Wold
B. J.
(
1996
)
Know your neighbors: three phenotypes in null mutants of the myogenic bHLH gene MRF4.
Cell
85
,
11
4
Ordahl
C. P.
,
Le Douarin
N. M.
(
1992
)
Two myogenic lineages within the developing somite.
Development
114
,
339
353
Ott
M. O.
,
Bober
E.
,
Lyons
G.
,
Arnold
H.
,
Buckingham
M.
(
1991
)
Early expression of the myogenic regulatory gene, myf-5, in precursor cells of skeletal muscle in the mouse embryo.
Development
111
,
1097
107
Patapoutian
A.
,
Miner
J. H.
,
Lyons
G. E.
,
Wold
B.
(
1993
)
Isolated sequences from the linked Myf-5 and MRF4 genes drive distinct patterns of muscle-specific expression in transgenic mice.
Development
118
,
61
9
Patapoutian
A.
,
Yoon
J. K.
,
Miner
J. H.
,
Wang
S.
,
Stark
K.
,
Wold
B.
(
1995
)
Disruption of the mouse MRF4 gene identifies multiple waves of myogenesis in the myotome.
Development
121
,
3347
58
Rawls
A.
,
Morris
J. H.
,
Rudnicki
M.
,
Braun
T.
,
Arnold
H. H.
,
Klein
W. H.
,
Olson
E. N.
(
1995
)
Myogenin's functions do not overlap with those of MyoD or Myf-5 during mouse embryogenesis.
Dev. Biol
172
,
37
50
Rudnicki
M. A.
,
Braun
T.
,
Hinuma
S.
,
Jaenisch
R.
(
1992
)
Inactivation of MyoD in mice leads to up-regulation of the myogenic HLH gene Myf-5 and results in apparently normal muscle development.
Cell
71
,
383
90
Rudnicki
M. A.
,
Jaenisch
R.
(
1995
)
The MyoD family of transcription factors and skeletal myogenesis.
BioEssays
17
,
203
9
Rudnicki
M. A.
,
Schnegelsberg
P. N.
,
Stead
R. H.
,
Braun
T.
,
Arnold
H. H.
,
Jaenisch
R.
(
1993
)
MyoD or Myf-5 is required for the formation of skeletal muscle.
Cell
75
,
1351
9
Sassoon
D.
,
Lyons
G.
,
Wright
W. E.
,
Lin
V.
,
Lassar
A.
,
Weintraub
H.
,
Buckingham
M.
(
1989
)
Expression of two myogenic regulatory factors myogenin and MyoD1 during mouse embryogenesis.
Nature
341
,
303
7
Sassoon
D. A.
(
1993
)
Myogenic regulatory factors: dissecting their role and regulation during vertebrate embryogenesis.
Dev. Biol
156
,
11
23
Smith
T. H.
,
Kachinsky
A. M.
,
Miller
J. B.
(
1994
)
Somite subdomains, muscle cell origins, and the four muscle regulatory factor proteins.
J. Cell Biol
127
,
95
105
Wang
Y.
,
Schnegelsberg
P. N. J.
,
Dausman
J.
,
Jaenisch
R.
(
1996
)
Functional redundancy of the muscle-specific transcription factors Myf5 and myogenin.
Nature
379
,
823
5
Weintraub
H.
(
1993
)
The MyoD family and myogenesis: redundancy, networks, and thresholds.
Cell
75
,
1241
4
Weintraub
H.
,
Davis
R.
,
Tapscott
S.
,
Thayer
M.
,
Krause
M.
,
Benezra
R.
,
Blackwell
T. K.
,
Turner
D.
,
Rupp
R.
,
Hollenberg
S.
, et al. 
(
1991
)
The myoD gene family: nodal point during specification of the muscle cell lineage.
Science
251
,
761
6
Yee
S. P.
,
Rigby
P. W.
(
1993
)
The regulation of myogenin gene expression during the embryonic development of the mouse.
Genes Dev
7
,
1277
89
Zhang
W.
,
Behringer
R. R.
,
Olson
E. N.
(
1995
)
Inactivation of the myogenic bHLH gene MRF4 results in up-regulation of myogenin and rib anomalies.
Genes Dev
9
,
1388
99
This content is only available via PDF.