Replicating myoblasts express a muscle-specific phenotype.

Abstract

During the terminal stage of skeletal myogenesis, myoblasts stop replicating, fuse to form multinucleate fibers, and express the genes that encode the proteins that convey contractile capacity. Because of this dramatic shift in proliferative state, morphology, and gene expression, it has been possible to readily identify and quantitate terminally differentiating myoblasts. In contrast, it is not clear whether the proliferating cells that give rise to postmitotic myoblasts are equally distinct in their phenotype and in fact whether distinct stages in skeletal myogenesis precede the onset of terminal differentiation. To address these questions, monoclonal antibodies and immunofluorescence microscopy were used to determine that replicating myoblasts from newborn rats do express a muscle-specific phenotype. To identify replicating cells, incorporation of 5-bromo-2'-deoxyuridine (BrdUrd) into DNA was assayed by using anti-BrdUrd antibody. The In what has come to be known as the terminal stage of skeletal muscle differentiation, myoblasts cease replicating and express a panoply of phenotypic characteristics intrinsic to this stage of development. During this same transition in myogenic development, skeletal myoblasts fuse to form multinucleate fibers in which assembly of myofibrillar proteins takes place. Once assembled in these fibers, these proteins provide the structural basis and enzymatic capacities that render the fibers functional contractile units, capable of responding to neuronal excitation. Expression of the genes that encode these proteins is largely regulated at the level of transcription (1-6); alternative splicing provides additional diversity and is often involved in the switching of isoforms (6, 7). Translation of muscle-specific transcripts may also be regulated (8-11).The earliest expression of the terminally differentiated phenotype takes place in myoblasts that normally do not replicate further (12, 13). However, there appears to be a period in which these postmitotic, terminally differentiating myoblasts can be manipulated to replicate again (14, 15): when replication is reinitiated, expression of the differentiated phenotype stops. Analysis of developmental mutants and the use of tumor viruses and growth factors likewise have led to the conclusion that continued proliferation is incompatible with terminal myogenic development (16-23). Studies using heterokaryons suggest that soluble trans-acting factors may mediate or initiate terminal differentiation (24,25). Transfection studies support this concept and indicate that the product of a single gene may mediate the onset of this stage of skeletal muscle development (26,27).By using antibodies and nucleic acid probes, and by measuring isozyme activities and myoblast fusion, it has been possible to identify and quantitate terminally differentiating myoblasts. In contrast, it is not clear whether the replicating cells that give rise to these postmitotic myoblasts have an equally distinct phenotype that distinguishes this particular stage of...

Characterization of proliferating human skeletal muscle‐derived cells in vitro: Differential modulation of myoblast markers by TGF‐β2

“…We have utilized co-expression of desmin and CD56 as a means to identify myoblasts (Kaufman and Foster, 1988;Belles-Isles et al, 1993). Dual fluorescent immunolabeling for detection of the myoblast markers CD56 and desmin combined with flow cytometric analysis revealed two populations of cells within a representative HuSkMC culture (strain A) after propagation through 3rd passage as described in Materials and Methods (Fig.…”

“…Activation of satellite cells and their propagation as myoblasts in vitro is achieved by enzymatic dissociation of cells in skeletal muscle and cultivation in mitogen-rich culture medium (Allen et al, 1997). The intermediate filament protein desmin is expressed in proliferating skeletal myoblasts (Kaufman and Foster, 1988;Lawson-Smith and McGeachie, 1998) and is prevalent in mature myocytes of skeletal muscle (Lazarides and Hubbard, 1976). Its up-regulation is a signal of myoblast differentiation.…”

Increased apoptosis, huntingtin inclusions and altered differentiation in muscle cell cultures from Huntington's disease subjects

“…Cultures were stained by immunocytochemistry using antibodies to desmin (Chemicon, Temecula, CA; Calbiochem/Oncogene Research Products, Cambridge, MA, USA), a protein expressed only in myogenic cells. 36 Myogenic purity, calculated as the proportion of cells expressing desmin, exceeded 98% in all cultures (Supplementary Figure 1). All experiments involved cell cultures at identical passages and data were obtained from experiments repeated in cultures at the second, third, and fourth passage.…”

Transformation-defective v-ski induces MyoD and myogenin expression but not myotube formation.

“…Therefore, we looked for musclespecific gene expression in tdM5i-QECs by staining cells with antibodies to muscle-specific proteins. Desmin expression, an early marker of the myogenic lineage (15,17), was completely absent in QECs but easily detectable in most cells infected by either SRA-SKV or tdM5i ( Fig. 2g to i).…”