MyoD- and nerve-dependent maintenance of MyoD expression in mature muscle fibres acts through the DRR/PRR element

In culture, MD6.0-lacZ expresses in myotubes but not myogenic cells, unlike endogenous MyoD. Reporter expression in vivo is in muscle fibre nuclei and is reduced in MyoD null mice. The MD6.0-lacZ reporter is down-regulated both in adult muscle fibres by denervation or muscle disuse and in cultured myotubes by inhibition of activity. Activity induces and represses MyoD through the DRR and PRR, respectively. During the postnatal period, accumulation of β-galactosidase correlates with maturation of innervation. Strikingly, endogenous MyoD expression is up-regulated in fibres by complete denervation, arguing for a separate activity-dependent suppression of MyoD requiring regulatory elements outside the DRR/PRR.The data show that MyoD regulation is more complex than previously supposed. Two factors, MyoD protein itself and fibre activity are required for essentially all expression of the 6 kb proximal enhancer/promoter (DRR/PRR) of MyoD in adult fibres. We propose that modulation of MyoD positive feedback by electrical activity determines the set point of MyoD expression in innervated fibres through the DRR/PRR element.Myogenic regulatory transcription factors (MRFs) are essential for skeletal myogenesis during embryonic development and for proper muscle regeneration [1-6]. Myf5 and MyoD are expressed in proliferating myoblasts, whereas myogenin and MRF4 are important in terminal differentiation [4,5,7,8]. In the absence of MyoD, muscle regeneration is impaired [9] possibly due to delayed differentiation of muscle precursor cells [5,7,10]. However, MyoD is also expressed in adult muscle fibres, albeit at low levels [11,12]. Conditions that damage muscle or change muscle phenotype often lead to changes of MyoD expression [13-15]. Nevertheless, when changes in MyoD expression occur, it is unclear how much is in fibres, myogenic cells or both [3,16]. Therefore, the activity and regulation of MyoD in normal muscle fibres is unknown.The role of MyoD within muscle fibres is un