Abstract
Purpose :
Extraocular muscles (EOM) maintain a large population of activated satellite cells that provide for continuous remodeling throughout life. While limb skeletal muscles have some myonuclear turnover later in life and during injury, it is much more limited than seen in the EOM. MYOD is a myogenic regulatory factor that activates genes required for muscle differentiation. We have shown that knocking out MyoD causes nystagmic waveforms in mice and a reduction in myofiber size and number. We hypothesize that over time the EOM muscle fibers of MyoD knockout mice (MyoD KO) would reduce their ability to continuously remodel throughout life.
Methods :
We used a MyoD KO mouse to determine the effects of MYOD on EOM function at time points between 3-22 months of age. Eye tracking using optokinetic nystagmus (OKN) of MyoD KO and their wild type (WT) litter mate controls was performed on an ISCAN device and analyzed using R. Limb muscle functional tests were performed using a Force Gauge and a RotaRod. Morphometric analysis was performed using H&E staining for fiber size and number, and immunostaining for PITX2 and PAX7.
Results :
OKN waveforms showed normal eye movements in WT mice up to 20months, and nystagmic waveforms in MyoD KO mice up to 22months. Grip strength and average walking time were not significantly different for MyoD KO mice. Mean cross-sectional area and total fiber number of EOM in MyoD KO were reduced at older time points, but there was no significant reduction in the tibialis anterior (TA) of the same mice. In older mice there was an increase in PAX7 positive myogenic precursor cells in MyoD KO EOM, not in TA. The number of PITX2 precursor cells inside of the sarcolemma reduced significantly in MyoD KO EOM.
Conclusions :
In the absence of MYOD expression over time, normal control of OKN was significantly impaired. Lack of MYOD in aging mice caused a significant loss in EOM myofiber cross-sectional area and number, PITX2 positive cells inside of the sarcolemma of the EOM fibers, and an increase in PAX7 positive precursor cells. There was no change from WT to MyoD KO in limb skeletal muscles. Thus, nystagmus can develop in mice with a muscle-specific gene knockout, and nystagmus was maintained in aging despite muscle fiber loss.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.