Purpose: Myoblast determination protein 1 (MYOD) is a critical myogenic regulatory factor in muscle development, differentiation, myofiber repair, and regeneration. As the extraocular muscles significantly remodel their myofibers throughout life compared with limb skeletal muscles, we hypothesized that the absence of MYOD would result in their abnormal structure and function. To assess structural and functional changes in the extraocular muscles in MyoD-/- mice, fiber size and number and optokinetic nystagmus reflex (OKN) responses were examined.
Methods: OKN was measured in MyoD-/- mice and littermate wild-type controls at 3, 6, and 12 months. The extraocular muscles were examined histologically for changes in mean myofiber cross-sectional area, total myofiber number, and nuclei immunostained for PAX7 and PITX2, markers of myogenic precursor cells.
Results: The MyoD-/- mice developed nystagmus, with both jerk and pendular waveforms, in the absence and in the presence of moving visual stimulation. At 12 months, there were significant losses in mean myofiber cross-sectional area and in total number of orbital layer fibers in all rectus muscles, as well as in global layer fibers in the superior and inferior rectus muscles. Haploinsufficient mice showed abnormal OKN responses. PITX2-positive cell entry into myofibers of the MyoD-/- mice was significantly reduced.
Conclusions: This study is the first demonstration of the development of nystagmus in the constitutive absence of expression of the muscle-specific transcription factor MYOD. We hypothesize that myofiber loss over time may alter anterograde and/or retrograde communication between the motor nerves and extraocular muscles that are critical for maintaining normalcy of extraocular muscle function.