Knowledge Bank

Skeletal muscle is composed of two muscle fiber-types: Slow Oxidative (Type 1) and Fast Glycolytic (Type 2). As each muscle’s functionality is dependent upon its fiber-type ratio; disruption of muscle fiber-type differentiation directly disrupts muscle physiology. Since fiber-type and overall muscle physiology are partially dependent upon innervation of a muscle fiber by a motor neuron, many muscular dystrophies and atrophies are associated with abnormalities of the neuromuscular junction (NMJ). Here, we characterize novel glutamate receptors at the NMJ, focusing primarily on the obligate Nmethyl- D-aspartate (NMDA) receptor subunit, NR1. First, we determine the timing of localization to the junction and begin to identify which NR1 isoforms are expressed in muscle. Then, we analyze electrophysiology experiments to detect changes in skeletal muscle membrane potential after treatment with glutamate receptor agonists/antagonists. Finally, we use Cre-loxP technology to create muscle-specific knockout mice for NR1. From these studies, we determine that NR1 localizes to the NMJ between 1 day and 1 week of age in mice. We further conclude that glutamate receptors at the NMJ are functional, based upon changes in quantal content and mini end-plate potential (mEPP) frequency after antagonist treatment. Our preliminary data suggests the presence of at least two alternatively spliced NR1 isoforms in muscle; however we were unable to definitively identify the sequences of these isoforms. Overall, these results challenge the conventional dogma of a “simple synapse” mechanism of neuromuscular transmission, providing the possibility of alternative methods of treatment and therapeutics for muscular dystrophies and other neuromuscular disorders. Advisor: Jill Rafael-Fortney