Impaired Extraocular Muscle Innervation Is Present Before Eye Opening in a Mouse Model of Infantile Nystagmus Syndrome

Abstract

Purpose To determine if extraocular muscles (EOMs) from mice with nystagmus show abnormalities in myofiber composition and innervation, as seen in EOMs from human nystagmus patients, and to determine when in development those changes occur. Methods Balb/c albino mice were crossed to pigmented mice to generate heterozygous mice, which were mated to create experimental litters containing albinos and wild-type controls. Orbits were harvested from adult animals (12 weeks old); on postnatal day (P)0, P10, P14, and P21; and from 6-week-old animals. EOM sections were collected from the intraorbital portion of the muscles. Sections were immunostained for slow and fast myosin and for neuromuscular junctions (NMJs). The proportion of each myofiber subtype and the density and size of NMJs were quantified. Initial innervation patterns were assessed using whole-mount immunostaining of embryonic day (E)13.5 embryos expressing Isl MN : GFP . Results Adult albino EOMs display an increased proportion of slow myofibers, larger slow myofibers, and a decreased density of NMJs—similar to human nystagmus patients. The percentage of NMJs on slow myofibers is also lower in albino animals. The initial innervation pattern of the incoming ocular motor neurons is normal in E13.5 albino embryos. Differences in the proportion of slow and fast myofiber subtypes are present as early as P14, and a lower percentage of NMJs on slow myofibers is present by P21. There is a lower density of NMJs on albino EOMs as early as P10, prior to eye opening. Conclusions Changes in NMJ development observed before eye opening indicate that nystagmus is not solely secondary to poor vision.

Neuromuscular Junction Development Differs Between Extraocular and Skeletal Muscles and Between Different Extraocular Muscles

“…We previously showed that EOMs in albino animals with nystagmus have decreased innervation and an increased proportion of slow myofibers. 11 Changes in NMJ density are seen as early as P10, and increased slow myofiber percentage as early as P14. In that study, we examined only LR and MR, not vertical muscles.…”