While our understanding of ALS and its genetic background has been growing rapidly in recent years, we are still far away from understanding the selective neuronal death in ALS. The presentations in the first basic science session of this year’s symposium on ALS/MND provided new ideas on this fascinating subject.
The session was opened by an interesting talk by Pico Caroni from the Friedrich Miescher Institute in Basel, Switzerland. Using the SOD1 mutant mouse model, he and his team showed that fast fatiguable (FF) motor neurons were preferentially affected and that misfolded SOD1 appeared within the first week of life in these motor neurons specifically. Little is known about these neurons, other than that they are less excitable than other motor neurons.
Interestingly, enhancing excitability of these neurons via stimulation at AMPA receptors was shown to decrease misfolded SOD1 accumulation, whereas decreasing excitability had unwanted effects. Such findings are counterintuitive given the established glutamate toxicity hypothesis of ALS.
A key role appears to be played by c-boutons, which are cholinergic synapses that increase excitability. Inhibition of these synapses resulted in accumulation of SOD1. C-boutons were shown to activate the mTOR pathway, the strongest known inhibitor of autophagy. Direct inhibition of the mTOR pathway by rapamycin was also detrimental to FF motor neuron survival, in a similar way to modulation of excitability. All these findings are somewhat unexpected and uncover new questions about the roles of both glutamate toxicity and of autophagy in ALS.
The second talk was by Fei Song from Wayne State University in Detroit. She showed histologic data from human spinal cords from ALS patients with and without upper motor neuron signs. In patients with UMN signs there was a loss of both myelin and axon at all spinal cord levels in both large and small diameter axons. In small diameter axons there appeared to be a slightly more severe loss caudally, evoking the idea of length dependent degeneration in this subgroup of neurons only. Patients without UMN signs only had mild large diameter axon loss. Finally Fei Song also showed some data on increased neuregulin activation (increased protein levels but not mRNA) that co-localised with increased microglial activation in spinal cords of patients with UMN symptoms.
In the third talk Anna King from the University of Tasmania, Australia, who used a compartmented microfluidic chamber to study the effects of kainic acid on both soma and axons of cortical neurons derived from mouse embryos. She and her co-workers showed that kainic acid induced excitotoxicity was associated with axonal caspase-3 expression but not MAP2 expression. Pre-treatment with taxol to stabilise microtubules was protective and reduced caspase-3 levels, particularly when applied to the axonal compartment, suggesting that microtubule stabilisation may be protective. Interestingly, expression of Wlds, the Slow Wallerian Degeneration Gene was also protective, implying parallels with mechanical axonal injury.
In the fourth talk by Tracey Dickson, also from the University of Tasmania, specifically looked at the susceptibility of interneurons in ALS. As increased excitability of motor neurons occurs early in ALS, reduced inhibition by interneurons is a potential explanation for this phenomenon. Tracey Dickson and her team looked at this question using interneuron specific markers in the SOD1 mouse and showed that subpopulations of interneurons that labelled positively with calretinin and vasoactive intestinal protein (VIP) were significantly decreased compared to wild type interneurons, while others, specifically those labelling with neuropeptide Y were significantly increased. Whether these changes in interneurons have a direct implication on subgroups of motor neuron populations remains to be shown.
The final talk was by Nilo Riva, from the Ospedale San Rafaelle in Milan. Together with others he characterised the peripheral nervous system damage in the hSOD1-G93A rat model using both magnetic resonance imaging of the sciatic nerve, neurophysiological examination and histology. The studies showed that the increased volume of the sciatic nerve and the neurophysiological findings correlated with the progressive axonal degeneration seen under the microscope: this data could prove very useful for in vivo studies in these animals.