The influence of the MuSK system and the L25 transgene on the Neuromuscular Junction

Abstract

The MuSK system is important in the development and maintenance of the neuromuscular junction. The neuromuscular junction is a chemical synapse that is important in muscle contraction. Research has shown that aged NMJ experience structural impairments, resulting in decreased efficiency of synaptic transmission and subsequent muscle contraction. Previous studies concerning reduced gene dosage of proteins of the MuSK system focused on homozygous knockouts. However, the prenatal death of the null mice prevented study of the role of the MuSK system in aged mice. Here, I studied the influence of the MuSK system in aged mice, by comparing mice with reduced gene dosage of agrin and rapsyn from the MuSK system. In addition, I studied the influence of elevated MuSK expression. I hypothesised that reduced gene dosage of agrin and rapsyn (major proteins of the MuSK system) may exacerbate and/or prematurely reveal age associated changes of the NMJ. On the other hand, I predicted that elevated MuSK expression would delay and/or prevent age associated changes at the NMJ. With the use of fluorescence confocal microscopy and immunohistochemical techniques I quantified and analysed pre-synaptic and post-synaptic structural integrity of the NMJ in wild type controls and affected groups. The experimental results do not seem to lend evidence to my hypotheses. They suggest that reduced rapsyn or agrin dosage have no effect on the structural integrity of the neuromuscular junction. Similar neuromuscular junction integrity was also noted in MuSK elevated samples, suggesting increased MuSK dosage has no effect. The L25 transgenic line is generated by the random insertion of the growth promoter sef gene. A subset of the offspring from L25+/- and L25+/- mating developed motor symptoms reminiscent of mouse models of motor neuron disease (most likely due to the disruption of an endogenous gene). The affected L25 mice were recently found to displayed hind limb spasticity, body tremor and paralysis (Eva Kitchkin, Honours Thesis 2014). Here, using the same techniques as in the study of the MuSK system, I investigate whether symptoms from the L25 mice line involve changes in motor endplate innervation, which would imply the involvement of lower motor neurons. The results from this study suggests that the nerve terminals, as well as lower motor neurons, are not involved in the affected L25 animals.