SATELLITE CELL

"Roles of Pitx, Dach and Meox genes in adult skeletal muscle stem cells"

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 Obiettivo del progetto (Objective)

'Stem cells are present in many adult tissues in a quiescent state. Their contribution to tissue repair depends on their activation, leading to proliferation and subsequent differentiation or return to quiescence. Skeletal muscle provides a model in which to study adult stem cell behavior. This tissue regenerates after injury and the muscle satellite cell is key to this process. These quiescent cells, located under the basal lamina of muscle fibres become activated upon injury, proliferate and differentiate into new muscle fibres or revert to quiescence. Examining the different states that a satellite cell can adopt under physiological conditions should lead to the identification of new regulators of myogenesis, and contribute to a better understanding of the mechanisms controlling quiescence and activation. Transcriptome analysis were performed on purified satellite cells isolated from adult muscles where these cells are quiescent and from growing or regenerating muscles, where they are activated. This analysis provided new insights into the genes differentially expressed in both states. My project will be to follow up on this work by focusing on three genes, Dach1, Meox2 and Pitx2/3. Those genes encode transcription factors whose profiles suggest they are involved in the function of muscle adult stem cells. Proliferation and differentiation of satellite cells will be studied in vivo in regenerating muscles of Pitx2/3 conditional mutant mice, or after inhibition of Dach1 and Meox2 expression by RNAi viral infection of injured muscle. Similarly, ex vivo proliferation and differentiation of satellite cells in single fibers and primary cultures will be analyzed after Dach1 or Meox2 knock-down by RNAi. Conversely, the consequences of the over-expression of these genes will be investigated in single fiber preparations and cultured satellite cells. These experiments are expected to give insights into the role of these genes in the physiology of satellite cells.'

Introduzione (Teaser)

Inherent tissue regeneration occurs through a small population of resident stem cells that differentiate to replenish the damaged cells. Investigation of the mechanisms that control quiescence and differentiation of tissue stem cells could open up novel avenues of exploitation in regenerative medicine.

Descrizione progetto (Article)

Endogenous muscle adult stem cells, which are known as satellite stem cells, get activated after skeletal muscle injury and differentiate to form new myofibers. The transcription factors Dach1, Meox2 and Pitx2/3 seem to be involved in the function of satellite cells.

The EU-funded 'Roles of Pitx, Dach and Meox genes in adult skeletal muscle stem cells' (SATELLITE CELLS) project set out to delineate the role of these transcription factors in adult skeletal muscle stem cells.

Pitx proteins are expressed during development and trigger the onset of myogenesis in the embryo. Expression analysis showed high Pitx2 levels in dormant satellite cells and decreasing levels upon differentiation. On the other hand, Pitx3 continued to be expressed in adult muscle fibres. Knock down of these genes in satellite cells demonstrated unique and overlapping functions in regulating cell behaviour. Pitx3 was essential for maintaining the stem cell pool while Pitx2 triggered the onset of differentiation.

In vivo studies in mice lacking Pitx2 showed delayed muscle regeneration upon injury compared to control animals. Pitx3-deficient animals presented a premature differentiation of stem cells while satellite cells lacking both Pitx proteins had a marked deficit in both proliferation and differentiation. This defect in satellite cell functionality hampered muscle regeneration in injured mice, suggesting their involvement in muscle tissue homeostasis.

Mutant cells exhibited a strong senescent phenotype due to excessive levels of reactive oxygen species and DNA damage. Treatment of mutant cells or animals with anti-oxidant compounds completely reversed the phenotype and increased their life expectancy.

Overall, the results of the SATELLITE CELLS study demonstrated an essential role of Pitx proteins in the maintenance and function of muscle satellite cells. The generated knowledge could find application in the treatment or symptom alleviation of muscular dystrophies.