On November 1st, Michèle Sons has defended her thesis: "Roles of neuromuscular proteins at the neuromuscular junction". The majority of experimental work has been performed at the laboratory of co-promotor Dr. Jaap Plomp (Dept. of Molecular Cell Biology, Leiden University). Prof. dr. Jan Verschuuren (Neurology, LUMC) was promotor.

The study was aimed to elucidate the roles of several neuro-exocytotic proteins in neuromuscular synaptic transmission at the neuromuscular junction (NMJ). We made use of genetically engineered mice, lacking expression of either one or multiple neuro-exocytotic proteins. The role of proteins Munc18-1, Munc13, a-Neurexin, Rab3A and Rim1a in synaptic transmission was investigated using these knock-out mouse models. In addition, the roles of several of these proteins has been investigated under conditions mimicking Myasthenia Gravis. We made use of an established model for Myasthenia Gravis involving repeated injections of mice with the acetylcholine receptor (ACh) blocker a-Bungarotoxin. In myasthenic endplates, the reduction of postsynaptic sensitivity results in a compensatory increase in ACh release. This is regarded as an attempt of the synapse to reach sufficient depolarization of the postsynaptic membrane to elicit muscle action potentials, even with a reduced functional receptor number.

From the proteins investigated, all but RIM1a deletion showed a clear neuromuscular phenotype in the genetically modified mice. Often, this phenotype was less severe in the NMJ when compared to central synapses. For example, as described in Chapter 2 of the thesis, deletion of Munc13 completely inhibits neurotransmitter release in central synapses, while at the NMJ ACh release is still present albeit at a very low level. Munc18-1 and a-Neurexin were shown to be important for the up regulation of release in myasthenic endplates. Munc18-1 most likely is involved in regulating the number of release sites from where ACh is secreted. However, over expression of Munc18-1 protein did not improve compensatory mechanism for the reduced ACh sensitivity, making Munc-18-1 an unlikely drug target. a-Neurexin may play a role upstream from the release process participating in the signal transduction pathway. Alternatively the protein could be involved in calcium channel modulation as previously reported for central synapses.

This thesis contributes to our understanding of synaptic release mechanisms underscoring the redundancy but also pointing out specific roles of key proteins. The knowledge of the players of this release machinery is a prerequisite for identifying drug targets useful in treatment of muscular diseases with compromised neuromuscular transmission.

The online version of the thesis can be found here.