The ubiquitin proteasome system as a regulator of neuronal function

Homeostatic balance between protein synthesis, turn over and degradation is vital for the stability and viability of all living cells. The primary function of the Ubiquitin Proteasome System is to degrade intracellular proteins, however perturbation of this system has been associated with the pathogenesis of neurodegenerative diseases. There is also mounting evidence for the role of the proteasome in the healthy functioning of neurons, however an investigation into the effect of proteasomal inhibition on the physiology, morphology and behaviour within a vertebrate species has yet to be carried out. In Chapters 3 and 4, whole-cell patch clamp technique has been used to determine how pharmacological inhibition of the proteasome influences the synaptic and intrinsic properties of motoneurons within the spinal cord of larval zebrafish. Proteasomal inhibition increased the frequency of glycinergic miniature inhibitory post-synaptic currents during chronic and short-term incubations. However, proteasome inhibition did not affect glutamatergic miniature excitatory post-synaptic currents or firing properties of motoneurons. In Chapter 5, changes in the synaptic properties of the neuromuscular junction and the morphology primary motoneurons following pharmacological proteasomal inhibition were investigated. Proteasomal inhibition increased the frequency of miniature endplate potentials, suggesting that inhibition of the proteasome selectively affects neurotransmitter release at specific synapses. Thereafter, using immunohistochemistry, the effect of proteasomal inhibition on the morphology of primary motoneurons was assessed. Inhibition of the proteasome does not affect the outgrowth of primary motoneurons or the formation of neuromuscular junctions. Finally, in Chapter 6, the effect of proteasomal inhibition on the free-swimming behaviour of larval zebrafish was assessed. Inhibition of the proteasome does not affect overall swimming distance or time spent swimming. However, proteasomal inhibition increased velocity of swimming, suggesting that changes in synaptic transmission due to inhibited proteasomes affects the locomotive behaviour of larval zebrafish.