Isoform specificity of monoclonal hybridoma antibodies to quail skeletal muscle myosin subunits

Normal expression of myosin fast alkali light chain 3 in the hindlimb muscle of chick embryos paralyzed with curare

We have used a monoclonal antibody (Mab) raised against the fast alkali light chains of quail pectoral muscle myosin to study the expression of MLC1f and MLC3f in the hindlimb muscle of a staged series of control chick embryos and 16-day embryos that had been paralyzed with curare. The Mab (QBM-2) is highly specific for the fast myosin alkali light chains of chick, hamster, and human muscle myosin. On Western blots, MLC1f is detected in hindlimb actomyosin at all of the stages examined, whereas MLC3f cannot be detected until Embryonic Day 14 (E14). Most of the E16 embryos that had been paralyzed with curare since E4 express detectable levels of both MLC1f and MLC3f in their hindlimb muscles, even though embryos incubated under these conditions do not exhibit spontaneous limb movements. Contrary to other reports, our results demonstrate that muscle contraction is not required for the accumulation of MLC3f. In light of our previous finding that innervation is essential for MLC3f accumulation in limb buds grafted onto the chorioallantoic membrane of chick hosts, these results suggest that some neural influence other than contraction, possibly a trophic factor, may play a role in the developmentally regulated expression of MLC3f in avian limb muscle.

Temporal and tissue-specific expression of myosin heavy chain isoforms in developing and adult avian muscle

We have raised monoclonal antibodies (Mabs) to myosin heavy chain isoforms (MHCs) that have specific patterns of temporal expression during the development of quail pectoral muscle and that are expressed in very restricted, tissue-specific patterns in adult birds. We find that an early embryonic, a perinatal, and an adult-specific, fast myosin heavy chain are co-expressed at different levels in the pectoral muscle of 8-12 day quail embryos. The early embryonic MHC disappears from the pectoral muscle at approximately 14 days in ovo, whereas the perinatal MHC persists until 26 days post-hatching. The adult-specific MHC accumulates preferentially and eventually completely replaces the other isoforms. These Mabs cross-react with the homologous isoforms of the chick and detect a similar pattern of MHC expression in the pectoral muscle of developing chicks. Although the early embryonic and perinatal MHC isoforms recognized by our Mabs are expressed in the pectoral muscle only during distinct developmental stages, our Mabs also recognize MHC isoforms present in the heart and extraocular muscle of adult quail. Immunofingerprinting using Staphylococcus aureus protease V8 suggests that the early embryonic and perinatal MHC isoforms that we see are strongly homologous with the adult ventricular and extraocular muscle isoforms, respectively. These observations suggest that at least three distinct MHC isoforms, which are normally expressed in adult muscles, are co-expressed during the early development of the pectoral muscle in birds. In this respect, the pattern of expression of the MHCs recognized by our Mabs in developing, fast muscle is very similar to the patterns described for other muscle contractile proteins.

Growth factors, signaling pathways, and the regulation of proliferation and differentiation in BC3H1 muscle cells. I. A pertussis toxin-sensitive pathway is involved

Cells of the nonfusing muscle cell line BC3H1 stop proliferating and express a family of muscle-specific proteins when the FBS concentration is reduced from 20 to 0.5% (Munson, R., K.L. Caldwell, and L. Glaser. 1982. J. Cell Biol. 92:350-356). Several growth factors have been shown to block differentiation in this cell line. To begin to investigate the potential role of G proteins in signal transducing pathways from these receptors, we have examined the effects of cholera toxin (CT) and pertussis toxin (PT) on proliferation and differentiation in BC3H1 cells. PT specifically ADP ribosylates a protein with an apparent molecular mass of 40 kD in BC3H1 cell membranes, whereas CT specifically ADP ribosylates three proteins of 35-43 kD. When added to exponentially growing cells in 20% FBS, CT and PT inhibited [3H]thymidine incorporation by up to 75% in a dose-dependent fashion. We found the synthesis of creatine kinase (CK) and skeletal muscle myosin light chain was reversibly induced in cells in 20% FBS treated with PT, but no increased synthesis was seen in cells treated with CT or in control cells; Northern analysis indicated this induction was at the level of mRNA. In cells shifted to 0.5% FBS, CT inhibited the normally induced synthesis of CK whereas PT potentiated it by approximately 50%. Forskolin also inhibited growth in 20% FBS and differentiation in 0.5% FBS medium in a dose-dependent fashion. both forskolin and CT elevated cAMP levels compared with control or PT-treated cells, suggesting that CT is blocking proliferation and differentiation by elevating cAMP levels. These results establish that a PT-sensitive pathway is involved in regulating proliferation and differentiation in BC3H1 cells, and we postulate that PT functions by ADP ribosylating a G protein that transduces signals from growth factor receptors in these cells.

Myogenic lineage determination and differentiation: evidence for a regulatory gene pathway

Stable myogenic cell lines have been derived at a high frequency by transfection of a cloned multipotential mouse embryo cell line, C3H 10T1/2, with cloned human DNA linked to a selectable neomycin resistance gene. The myogenic phenotype remains linked to neomycin resistance during secondary transfections. Although proliferative in growth conditions, these cell lines maintain the ability to differentiate and express muscle-specific proteins. We conclude that there is a simple genetic basis for myogenic determination and that a single gene, myd, converts 10T1/2 cells to a myoblast lineage. Southern blot analysis demonstrates nonidentity of myd and the MyoD1 gene. Northern blot analysis shows that myd-transfected myogenic lineages express MyoD1 mRNA while parental 10T1/2 cells do not. These results suggest that a dependent regulatory gene pathway mediates myogenic determination and differentiation.

Reflective densitometry of Western blots to quantitate the developmentally regulated accumulation of myosin light chain 3

We have employed a monoclonal antibody to fast myosin alkali light chains to study the accumulation of myosin light chain 3 (MLC3f) in the breast and limb musculature of developing quail embryos using quantitative densitometry of Western blots. Our analyses reveal that MLC3f is first detected in the breast muscle of 11 day embryos and accumulates at a constant rate until hatching at day 16. This data suggests, by extrapolation, that MLC3f accumulation is initiated at day 10 in embryonic breast muscle. MLC3f is also first detected in the limb muscle of 11 day embryos, but does not accumulate rapidly until after day 13. These results demonstrate the effective use of reflective densitometry in the study of developmental problems and in the quantitation of Western blots in general.