Project 2.  Characterization of the highly myogenic minority population of muscle stem cells. 

Terence Partridge, PhD.  Children’s National Medical Center, Washington DC

The loss of effective repair and growth and growth in muscle with age and in conditions of persistent or severe muscle damage reflect a derangement of the molecular and cellular control mechanisms that underlie these processes.  By careful analysis of the differences in patterns of gene expression that accompany these disruptive processes, and correlation with the observable changes in behaviour of cells responsible for the process of skeletal muscle maintenance and regeneration, we shall be able to determine rational approaches to ameliorating these problems.

With this in mind, our specific aims within a 3 year program, are to gain an understanding of the mechanisms that control satellite cell based myogenesis in skeletal muscle, so as to generate mechanisms of influencing the process to therapeutic advantage.  Our approach, is based on the study the behaviour of satellite cells present on isolated muscle fibres, both in vivo and in vitro.

Our most recent work in vitro (Zammit et al 2004) and in vivo (Collins et al. submitted) demonstrates that the satellite cell behaves as a self-replacing self-limiting stem cell when maintained in contact with its cognate muscle fibre, and that these properties are lost or severely compromised when separated from the fibre. In addition, in close parallel to haematopoietic stem cells, the stem cell like function of satellite cells when transplanted on their fibres -  a several thousand fold higher myogenic efficiency than separated cells –  is not seen unless the recipient environment is depleted of myogenic activity by prior irradiation.  This requirement for an empty niche re-inforces the parallels with the hematopoietic stem cell.  Likewise, in culture, satellite cells that are forced to retain their contacts with the muscle fibre by keeping them suspended, show strong parallels to their behaviour in an acute regenerative response in vivo by spontaneously dropping out of cell cycle after 3-4 days to either differentiate into skeletal muscle or to return to a quiescent satellite cell like condition. Comparable behaviour is only elicited in standard culture conditions by withdrawal of growth factors or by continuing growth to confluence: this spontaneous cessation of proliferation and return of a proportion of the cells to a quiescent non-differentiated state closely resembles the sequence of events observed in acute degeneration and regeneration of muscle in vivo.

We aim therefore to determine what factors of the satellite cell niche and what factors associated with prior irradiation of the graft site are implicated in generating a vigorous but self limiting myogenic response from satellite cells.  This knowledge would have to main areas of application: a) stimulation of the endogenous myogenic mechanisms to combat various sarcopaenic conditions such as bed-rest, aging, and cancer, and b) rebuilding of muscle lost asi a result of muscular dystrophies or to acute trauma.