Identification of a program of contractile protein gene expression initiated upon skeletal muscle differentiation.

The functional diversity of skeletal muscle is largely determined by the combinations of contractile protein isoforms that are expressed in different fibers. Just how the developmental expression of this large array of genes is regulated to give functional phenotypes is thus of great interest. In the present study, we performed a comprehensive analysis of contractile protein isoform mRNA profiles in skeletal muscle systems representing each generation of fiber formed: primary, secondary, and regenerating fibers. We find that in each system examined there is a common pattern of isoform gene expression during early differentiation for 5 of the 6 gene families we have investigated: myosin light chain (MLC)1, MLC2, tropomyosin, troponin (Tn)C, and TnI. We suggest that the common isoform patterns observed together represent a genetic program of skeletal muscle differentiation that is independent of the mature fiber phenotype and is found in all newly formed myotubes. Within each of these contractile protein gene families the program is independent of the isoforms of myosin heavy chain (MHC) expressed. The maintenance of such a program may reflect a specific requirement of the initial differentiation process.

Coordination of skeletal muscle gene expression occurs late in mammalian development.

The acquisition of specialized skeletal muscle fiber phenotypes during development is investigated by systematic measurement of the accumulation of 21 contractile protein mRNAs during hindlimb development in the rat and the human. During early myotube formation in both species there is no coordination of expression of either fast or slow contractile protein isoform genes, but rather some slow, some fast, and some cardiac isoforms are expressed. Some isoforms are not detected at all in early myotubes. From Embryonic Day 19 in the rat, and after 14 weeks in the human, a strong bias toward fast isoform expression is evident for all gene families examined. This results in the establishment of a coordinated fast isoform phenotype at birth in the rat, and by 24 weeks in the human fetus. Unexpectedly, during secondary myotube formation in the rat we observe sudden rises and falls in contractile protein gene output. We interpret these fluctuations in terms of periods of myoblast proliferation followed by synchronized fusion into myotubes. The data presented indicate that each contractile protein gene has its own determinants of mRNA accumulation and that the different myoblast populations which contribute to the developing limb are not intrinsically programmed to produce particular coordinated phenotypes with respect to the non-myosin heavy chain contractile proteins.

Cloning and characterization of a myoblast cell surface antigen defined by 24.1D5 monoclonal antibody.

Monoclonal antibody 24.1D5 reacts specifically with an epitope expressed on the cell surface of mononucleate myoblasts in primary cultures of human skeletal muscle cells, but not with either multinucleate myotubes or fibroblasts. Polypeptides of 60 and 100 X 10(3) Mr were identified by immunoblotting with the McAb. Human muscle cDNAs encoding the 24.1D5 epitope were used to study further the structure and expression of 24.1D5 during skeletal muscle development. Two mRNA species of 3.0 and 2.5 kb were identified in primary cultures of human skeletal muscle and in mouse muscle cell lines. The levels of both transcripts decreased during myotube formation in vitro and were similarly decreased during myogenesis in the mouse embryo. 24.1D5 mRNAs were expressed by multipotential cells and myoblast derivatives of the mouse embryonic cell line C3H10T1/2, suggesting that 24.1D5 is expressed at an early stage during skeletal muscle development.

Distinct dystrophin mRNA species are expressed in embryonic and adult mouse skeletal muscle.

We have examined dystrophin mRNA in embryonic, newborn and adult mouse skeletal muscle. A discrete nerve-independent increase in mRNA size was observed between embryonic and adult stages, indicating that a developmentally regulated mRNA isoform switch occurs in the expression of the Duchenne muscular dystrophy (DMD) gene in skeletal muscle. These distinct mRNAs are most likely generated via selection of alternative transcriptional start sites or RNA processing pathways. In addition, denervation of adult muscle was without effect on the expression pattern.

Alternative splicing generates a secreted form of N-CAM in muscle and brain.

A number of different membrane associated isoforms of the neural cell adhesion molecule (N-CAM) have previously been identified. Here the structure of a novel secreted isoform of N-CAM is established by analysis of a cDNA corresponding to an N-CAM mRNA from human skeletal muscle. The mRNA incorporates a novel sequence block into the extracellular domain, which introduces an in-frame stop codon and thus prematurely terminates the coding sequence, generating a truncated N-CAM polypeptide. Analysis of genomic clones indicates that the inserted sequence is present as a discrete exon within the human N-CAM gene, and Northern analysis shows it to be associated specifically with a 5.2 kb mRNA species from skeletal muscle and brain. Stable transfectants expressing the secreted isoform accumulate it in the cytoplasm and release it to the culture medium. In contrast, cells transfected with cDNA encoding lipid-tailed N-CAM express it predominantly at the cell surface. The existence of a secreted isoform may further expand the spectrum of N-CAM function beyond its known involvement in intercellular adhesion to extracellular matrix interactions.

Complete sequence and in vitro expression of a tissue-specific phosphatidylinositol-linked N-CAM isoform from skeletal muscle.

Neural cell adhesion molecules (N-CAMs) are a family of cell surface sialoglycoproteins encoded by a single copy gene. A full-length cDNA clone that encodes a nontransmembrane phosphatidylinositol (PI) linked N-CAM of Mr 125 x 10(3) has been isolated from a human skeletal muscle cDNA library. The deduced protein sequence encodes a polypeptide of 761 amino acids and is highly homologous to the N-CAM isoform in brain of Mr 120 x 10(3). The size difference between the 125 x 10(3). The size difference between the 125 x 10(3) Mr skeletal muscle form and the 120 x 10(3) Mr N-CAM form from brain is accounted for by the insertion of a block of 37 amino acids called MSD1, in the extracellular domain of the muscle form. Transient expression of the human cDNA in COS cells results in cell surface N-CAM expression via a putative covalent attachment to PI-containing phospholipid. Linked in vitro transcription and translation experiments followed by immunoprecipitation with anti-N-CAM antibodies demonstrate that the full-length clone of 761 amino acid coding potential produces a core polypeptide of Mr 110 x 10(3) which is processed by microsomal membranes to yield a 122 x 10(3) Mr species. Taken together, these results demonstrate that the cloned cDNA sequence encodes a lipid-linked, PI-specific phospholipase C releasable surface isoform of N-CAM with core glycopeptide molecular weight corresponding to the authentic muscle 125 x 10(3) Mr N-CAM isoform. This is the first direct correlation of cDNA and deduced protein sequence with a known PI-linked N-CAM isoform from skeletal muscle.

Human muscle neural cell adhesion molecule (N-CAM): identification of a muscle-specific sequence in the extracellular domain.

cDNA clones encoding neural cell adhesion molecule (N-CAM) mRNAs of 6.7, 5.2, and 4.3 kb from human skeletal muscle cells were isolated. A 6.7 kb mRNA encodes a transmembrane N-CAM isoform, present predominantly in mononucleate myoblasts, that shows sequence homology with chick brain N-CAM-140 and is down-regulated during myotube formation. In contrast, the 5.2 and 4.3 kb mRNAs encode nontransmembrane N-CAM isoforms that greatly increase during myoblast fusion. Furthermore, a discrete muscle-specific sequence domain (MSD1) was detected in the extracellular coding regions of the 5.2 and 4.3 kb mRNAs. This encodes a unique run of 37 amino acids and is not expressed in 7.2 and 6.7 kb mRNAs from human or chick brain or in the corresponding 6.7 kb muscle transcript. The MSD1 is also absent from chick and mouse brain mRNAs of 4.0 and 2.9 kb. These results show that diversity in N-CAM primary structure can be found in the extracellular domain in a tissue-specific manner.