Myogenin, MyoD, and myosin heavy chain isoform expression following hindlimb suspension.
BACKGROUND
Myogenin expression is associated with a slow myofiber phenotype, and MyoD expression is associated with a fast myofiber phenotype. Hindlimb suspension (HS) will induce muscular atrophy, and a transition from a slow to fast myofiber phenotype in the rat soleus.


OBJECTIVE
Hindlimb suspension will induce myofiber atrophy, and a slow to fast myofiber type transition with corresponding changes in myogenin and MyoD expression.


METHODS
Myofiber phenotype was evaluated by electrophoretically separating the myosin heavy chain isoforms. Myogenin expression was evaluated by Northern analysis, while MyoD expression was evaluated by Northern analysis and semiquantitative RT-PCR.


RESULTS
After 28 d of hindlimb suspension, there was significantly (p < 0.05) less myosin heavy chain Type IIA, and more (p < 0.05) myosin heavy chain Type IIX in the soleus muscles of hindlimb suspended rats compared with soleus muscles from weight-bearing (WB) rats. Although there was a shift to a faster myosin heavy chain phenotype in soleus muscles from hindlimb suspended rats, there was no change in myogenin expression, and MyoD expression was undetectable by Northern analysis. Semi-quantitative RT-PCR revealed an up-regulation of MyoD expression following 14 d of hindlimb suspension.


CONCLUSIONS
Myogenin expression levels do not change during the slow to fast myofiber phenotypic transition that occurs during hindlimb suspension; MyoD expression appears to increase at the same time as the phenotypic transition. Thus, MyoD expression or the Myogenin: MyoD mRNA ratio may be important in the phenotypic transition. Neither myogenin nor MyoD appear to play a critical role in the muscular atrophy that occurs during weightlessness.
Mozdziak
P E
PE
Department of Anatomy, University of Wisconsin, Madison, USA.
Greaser
M L
ML
Schultz
E
E
D000818:Animals; D015152:Blotting, Northern; D004195:Disease Models, Animal; D005260:Female; D005786:Gene Expression Regulation / Q000235:genetics*; D019417:Hindlimb Suspension / Q000009:adverse effects* / Q000502:physiology*; D018656:Muscle Fibers, Fast-Twitch / Q000737:chemistry*; D018657:Muscle Fibers, Slow-Twitch / Q000737:chemistry*; D018482:Muscle, Skeletal / Q000737:chemistry*; D009133:Muscular Atrophy / Q000209:etiology* / Q000378:metabolism*; D017570:MyoD Protein / Q000032:analysis* / Q000235:genetics*; D018008:Myogenin / Q000032:analysis* / Q000235:genetics*; D018995:Myosin Heavy Chains / Q000032:analysis* / Q000235:genetics*; D010641:Phenotype; D051381:Rats; D011919:Rats, Inbred Strains; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013026:Space Flight; D013997:Time Factors; D016474:Weight-Bearing / Q000502:physiology
D017570:MyoD Protein; C503803:Myog protein, rat; D018008:Myogenin; D018995:Myosin Heavy Chains