Pax7

Pax7 is a transcription factor of the paired box (PAX) family. It contains a paired DNA-binding domain and, in many species, a second DNA-binding motif, enabling regulation of target genes involved in myogenesis.

During vertebrate development, Pax7 is expressed in early muscle progenitors and in adult skeletal muscle satellite cells, which reside between the muscle fiber plasma membrane and basement membrane. Pax7 marks quiescent and activated satellite cells and is involved in maintaining their proliferative capacity and preventing premature differentiation. In embryogenesis, Pax7 function partly overlaps with Pax3: mice lacking Pax7 have reduced numbers of satellite cells and impaired muscle growth, but Pax3 can compensate in some lineages.

In postnatal muscle, Pax7 is essential for regeneration. Conditional deletion of Pax7 in adult satellite cells leads to depletion of the stem cell pool and defective muscle repair after injury, whereas enforced Pax7 expression can expand the pool of myogenic progenitors.

Pathologically, PAX7 can be involved in muscle tumors. In rhabdomyosarcoma, PAX family members (PAX3 or PAX7) can be rearranged with FOXO1 to form fusion oncoproteins that drive tumorigenic transcription programs. Immunohistochemical detection of PAX7 is used in the diagnosis of muscle-lineage tumors and to identify satellite-cell–derived components.

On the molecular level, Pax7 regulates a gene network with Myf5 and MyoD and coordinates with other myogenic regulatory factors to balance self-renewal and differentiation. Pax7 is widely used as a marker for skeletal muscle satellite cells in research and diagnostic contexts.