A novel approach to induce cell cycle reentry in terminally differentiated muscle cells.

During terminal differentiation, skeletal muscle cells permanently retract from the cell cycle. We and others have shown previously that this cell cycle withdrawal is an actively maintained state that can be reversed by transient expression of the SV40 large T antigen. In an attempt to avoid the hazards of gene transfer and the difficulties of regulating transgene expression, we have now used this cellular system as a model to test whether direct protein delivery could constitute a feasible alternative or complementing strategy to gene therapy-based approaches. Taking advantage of the recently described intercellular trafficking properties of the herpes simplex virus I VP22 protein, we have constructed a chimeric VP22-SV40 large T antigen fusion protein and shown that it can spread into terminally differentiated myotubes where it accumulates in the nucleus. This fusion protein retains the ability to override the cell cycle arrest as shown for SV40 large T antigen alone. Our results clearly show that the transduced fusion protein remains capable of inducing S-phase and mitosis in these otherwise terminally differentiated cells and opens now the way to exploit this novel strategy for tissue regeneration.