2. Myogenic regulatory factors

Abstract

Members of the MyoD family of basic helix-loop-helix (bHLH) transcription factors have a central role in the determination and differentiation of vertebrate skeletal muscle. Collectively referred to as myogenic regulatory factors (MRFs), their mammalian counterparts include the unlinked genes MyoD and myogenin, and the closely linked genes myf-5 and MRF4 (myf-6). MRFs form heterodimers with E protein products such as E12 and E47 of the ubiquitously expressed E2A gene family and bind to the E-box DNA sequences (CANNTG) present in the regulatory regions of many muscle-specific genes. Another important regulator of skeletal muscle differentiation is the myocyte-specific enhancer factor 2 (MEF2) family of transcription factors which form homodimers and bind to an A/T-rich sequence present in many muscle-specific promotors and enhancers. MEF2 isoforms regulate myogenic bHLH genes and cooperate with MRFs in activating skeletal muscle-specific transcription. We have recently cloned cDNAs encoding MRFs of myogenin, MyoD, and myf-5, together with E12, from embryos and larvae of the common carp. MEF2 cDNAs were also isolated from an adult carp cDNA library. During the period of somite formation in carp, myf-5 was first expressed followed by MEF2C and MyoD, then myogenin and MEF2A, finally skeletal myosin heavy chain (MyHC) and a-actin. E12 was detected from embryos at various developmental stages as well as in juveniles. We also examined by Northern blot analysis the changes in the accumulated mRNA levels of MyHC isoforms in carp fast skeletal muscle during water temperature acclimation from 20 to 300C in relation to those of MyoD family and MEF2 family members. There was a dramatic decrease in the transcripts of the MyHC isoform predominantly expressed in cold-acclimated carp and a significant increase in the transcripts of the dominant MyHC isoform of warm-acclimated carp. Over such an acclimation period, the transcription levels of myogenin, MEF2A, and MEF2C were changed significantly, whereas MyoD transcripts were rather constant. MRFs and related transcription factors probably play important roles not only during development, but also during temperature acclimation of adult fish.