Dynamic distribution and formation of a para-sarcomeric banding pattern ofprosomes during myogenic differentiation of satellite cells in vitro

Myogenesis proceeds by fusion of proliferating myoblasts into myotubes unde r the control of various transcription factors, In adult skeletal muscle, m yogenic stem cells are represented by the satellite cells which carl be cul tured and differentiate in vitro. This system was used to investigate the s ubcellular distribution of a particular type of prosomes at different steps of the myogenic process. Prosomes constitute the MCP core of the 26S prote asomes but were first observed as subcomplexes of the untranslated mRNPs; r ecently, their RNase activity was discovered. A monoclonal antibody raised against the p27K subunit showed that the p27K subunit-specific prosomes mov e transiently into the nucleus prior to the onset of myoblast fusion into m yotubes; this represents possibly one of the first signs of myoblast switch ing into the differentiation pathway. Prior to fusion, the prosomes contain ing the p27K subunit return to the cytoplasm, where they align with the gra dually formed lengthwise-running desmin-type intermediate filaments and the microfilaments, co-localizing finally with the actin bundles. The prosomes progressively form discontinuous punctate structures which eventually deve lop a pseudo-sarcomeric banding pattern. In myotubes just formed in vitro, the formation of this pattern seems to preceed that produced by the muscle- specific sarcomeric a-actin. Interestingly, this pattern of prosomes of myo tubes in terminal in vitro differentiation was very similar to that of pros omes observed in vivo in foetal and adult muscle. These observations are di scussed in relation to molecular myogenesis and prosome/proteasome function .