EASTBIO: Compensatory motor units: What is the problem and how can we help

Deanery of Biomedical Sciences

Lower motor neurons conduct electrical signals (action potentials) from the spinal cord to muscles, to trigger contraction in tens to thousands of muscle fibres. The motor neuron and the set of muscle fibres it innervates thereby constitute the functional motor unit (MU). In aging, some motor neurones die, so MUs are lost. However, remaining lower motor neurons possess a remarkable capacity for regeneration. Indeed, surviving motor neurons are able to grow new branches to muscle fibres losing those connections. The formation of these new connections results in an increase in the territory of the surviving motor neuron, and an increase in MU size. The process is a critical process in retaining muscle strength as we age. However, motor neurons existing in this enlarged state for a prolonged period of time can have a deleterious effect on their survival. We aim to understand what pressures are put upon these over-extended cells and find ways to support them and enhance their longevity. To do this, we will first produce a model system, using partial denervation in mice, and then analyse the effect on the remaining enlarged motor neurons, with 3 main aims.

Aim 1, will determine degree of MU enlargement in the model. We will partially denervate with a L5 spinal nerve ligation in mice, and use electrophysiology and immunofluorescence to quantify the degree of motor unit enlargement. Aim 2 will evaluate the levels of oxidative stress induced by this compensatory enlargement of motor neurons. 6-12 months after partial denervation, the spinal cords will be analysed. A combination of immunofluorescence and western blot will be used to assess the levels of protein oxidation, DNA damage and proteins involved in the response to oxidative stress. Aim 3 will determine the level of transcriptional of changes in enlarged motor neurons following partial denervation. We will induce motor unit enlargement as in Aim 1. 12 months post injury, we will perform RNAseq on laser captured motor neurons from injured vs non-injured sides. Bioinformatics will be used to identify transcripts and pathways which are significantly altered, to understand which metabolic pathways might require support. 

MU enlargement is a critical compensatory process during aging. Understanding the pressures put upon motor neurons, to identify ways to support them throughout the lifespan of the individual, can help preserve motor function and slow motor decline in our ageing population.