DEXELOPMEWCAL BIOLOGY 147,374-380 (1991) Transcription of the Embryonic Myosin Light Chain Gene Is Restricted to Type II Muscle Fibers in Human Adult Masseter NADIA SOUSSI-YANICOSTAS*,~ AND GILLIAN S. BUTLER-BRowNEt *h-tit& Pa&ear de Paris, Departewwnt de Biologic Mol.&.&ire, 25, rue du Dr. Ro-ux, 75015 Paris, France; and fFacult4 de Mkdeciw de Paris, .& rue des Saints-P&es, 75006 Paris, France Accepted July 3, 1991 We have previously demonstrated that the embryonic myosin light chain (MLClemh) isoform whose expression is restricted to the early fetal stages in most mammalian skeletal muscles, persists throughout development in human masseter muscle. In order to go further in this study, we have compared the developmental profile of MLClemb gene transcription in human masseter and quadriceps muscles using both Northern blotting and in situ hybridization tech- niques. Interestingly, whereas expression of this gene was observed in all fibers during fetal stages in both muscles, transcription in adult masseter was found to be restricted to type II fibers. Existence of a masseter-specific pathway of muscle gene regulation is discussed. o Isa Academic PWS, he. INTRODUCTION In vertebrates, myosin light chain (MLC)2 isoforms are encoded by a multigene family. Each isoform dis- plays a distinct pattern of stage- or fiber-specific ex- pression (Gauthier et c& 1982; Obinata et a& 1983). MLClf and MLC3f which are encoded by the same gene, are only expressed in fast skeletal muscle fibers, whereas the MLCls/MLClv is expressed in slow skele- tal and cardiac ventricular muscles (Barton and Buck- ingham, 1985a). Embryonic myosin light chain (MLClemb) shows a more complex profile of expression. It is expressed during the early stages of both cardiac and skeletal muscle development (Whalen et ah, 1978; Whalen and Sell, 1980; Cummins et at, 1980; Cummins, 1982; Strohman et al, 1983; Lyons et aZ., 1990) and disap- pears in most skeletal muscles and in the ventricle mus- cle at the end of fetal development. However, we have previously reported that the MLClemb persists throughout development in the human masseter muscle (Soussi-Yanicostas et uZ., 1990). Moreover, it has been demonstrated that the MLClA isoform expressed in atria1 muscle and the MLClemb are encoded by the same gene and display the same protein coding sequence (Barton et al, 1985; 1988; Arnold et uL, 1988; Kurabaya- shi et al, 1988). r To whom correspondence should be addressed c/o Dr. R. Whalen, 25, rue du Dr. Roux, 75015 Paris, France. ’ Abbreviations used: MLC, myosin light chain; MLClemb, embry- onic MLC, MLClA, atria1 MLC, MLClf and MLC3f; fast MLCs; MLCls/lv, slow/ventricular MLC; MHC, myosin heavy chain; MHCemb, embryonic MHC; MHCfet, fetal MHC. The human masseter muscle displays extensive dif- ferences compared to other skeletal muscles. In addition to type I, type IIA, and type IIB fibers found in all skele- tal muscles, fibers showing an intermediate ATPase staining reaction, IM fibers, were described in human masseter (Ringqvist, 1971; Serratrice et uL, 1976; Ringq- vist et a& 1982; Eriksson and Thornell, 1983). In addition to the unusual persistence of the MLClemb isoform during postnatal stages as mentioned above, the embry- onic and fetal MHC isoforms whose expression is re- stricted to early development in all other skeletal mus- cles (Whalen et uL, 1981; Fitzsimons and Hoh, 1981; Lyons et al., 1983; Lowey et uZ., 1983; Butler-Browne and Whalen, 1984; Butler-Browne et aL, 1990) are also ex- pressed in adult masseter (Butler-Browne et al, 1988; Soussi-Yanicostas et al, 1990). More recently, it has been shown that the human masseter muscle does ex- press the a-cardiac MHC (Bredman et al., 1991) whose expression has not been observed in other mammalian skeletal muscles (Hoh et uL, 1978; LomprC et uL, 1984; Mahdavi et al, 1984). Persistence of neonatal MHC isoform has also been reported in one chicken skeletal muscle (Crow and Stockdale, 1986). In this case, whereas neonatal MHC disappears in most avian skeletal muscles during the neonatal to adult transition, this isoform persists throughout development in the superficial biceps muscle. To obtain further insight into the expression and regu- lation of the MLClemb gene in masseter muscle, we have compared the spatial and temporal pattern of transcription of this gene in the human masseter and quadriceps muscles. 0012-1606/91 $3.00 374 Copyright 0 1991 by Academic Press. Inc. All rights of reproduction in any form reserved.