Pitx2 Enhances the Fusion of Myogenic Cells in Extraocular Muscle

Objectives: Extraocular muscles (EOM) are spared from the degeneration that occurs in non-cranial muscles in muscular dystrophy. This sparing may be the result of developmental differences in cranial and non-cranial skeletal muscle. Pitx2, a bicoid-like homeobox transcription factor, is required for the development of EOM, but is not required for the development of limb muscle. We hypothesize that Pitx2 may be responsible for functional differences between EOM and limb cells that contribute to the sparing of EOM in muscular dystrophy.

Methods: Using flow cytometry, CD34⁺/Sca-1^-/CD45^-/CD31^- cells were isolated from extraocular and limb muscle. Pitx2 expression was knocked down in cultured cells using siRNA. Pitx2 transcript and protein levels were assessed with RT-qPCR and western blotting, respectively. The ability of myogenic cells to fuse into multinucleated myotubes was determined with a fusion index assay.

Results: Pitx2 transcript and protein levels are higher in cells from EOM compared to limb muscle. Loss of Pitx2 expression reduced the ability of myogenic cells from EOM and limb muscle to form multinucleated myotubes. 

Conclusions: Pitx2 enhances the fusion of myogenic cells in EOM. The higher expression of Pitx2 in EOM cells may contribute to their sparing in muscular dystrophy.