ALS-Linked SOD1 Mutants Enhance Neurite Outgrowth and Branching in Adult Motor Neurons

摘要

class="head no_bottom_margin" id="sec1title">IntroductionAmyotrophic lateral sclerosis (ALS) is a fatal, adult-onset neurodegenerative disorder in which there is selective loss of motor neurons in the cerebral cortex, brainstem, and spinal cord (). Approximately 90% of ALS cases are sporadic with unknown etiology; the remaining 10% are inherited and known as familial ALS (fALS), of which over 20% have mutations in the gene encoding Cu/Zn superoxide dismutase 1 (SOD1) (). To date, over 155 different mutations have been identified in SOD1 either in isolated cases of ALS or more commonly in patients from families showing autosomal dominant patterns of inheritance (, ). ALS-linked SOD1 mutations are thought to induce a toxic gain of function in the protein, which becomes prone to misfolding and subsequent aggregation (, ). However, expression of mutant SOD1 can affect a number of cellular processes, causing ER distress, mitochondrial dysfunction, excitotoxicity, defects in axonal transport, and inhibition of the proteasome (). Despite being the first gene identified with mutations that cause fALS () and providing the basis of the first ALS animal model (), there is still no consensus about how mutant SOD1 specifically alters motor neuron physiology.Although most studies have focused on the cellular mechanisms and genes that induce motor neuron death in ALS, less is known about the neurons that do survive, including their ability to resist stress-induced cell death and to compensate for dying motor neurons. Not all motor neurons are equally susceptible to cell death during ALS disease progression. ALS mostly targets motor neurons required for voluntary movement, whereas motor neurons of the autonomic system are less sensitive (). There is also a gradient of vulnerability among spinal motor neurons, whereby faster motor units become affected before slower muscle types (). Motor neurons that are less ALS susceptible can compensate for the cells that initially die by establishing new connections with the motor endplate, although many of these will eventually succumb to the disease (). This selective neuronal vulnerability is present in both sporadic ALS and familial ALS and is also recapitulated in rodent models, such as the SOD1^G93A mouse (, ).Most of our current knowledge about surviving spinal motor neurons in ALS mouse models has largely been generated by gene expression profiling of tissue and cells (, href="#bib6" rid="bib6" class=" bibr popnode">Brockington et al., 2013, href="#bib11" rid="bib11" class=" bibr popnode">de Oliveira et al., 2013, href="#bib12" rid="bib12" class=" bibr popnode">Ferraiuolo et al., 2007, href="#bib30" rid="bib30" class=" bibr popnode">Lobsiger et al., 2007, href="#bib47" rid="bib47" class=" bibr popnode">Saxena et al., 2009). However, these studies provide just a single snapshot of the motor neuron's biology and only allow for inferences to be made about how changes in gene expression alter motor neuron physiology, allow them to resist degeneration, or compensate for dying neurons by forming new motor endplate attachments. In the current study, we sought to functionally characterize ALS-resistant motor neurons by culturing them in vitro, where we would be able to directly assess dynamic cellular properties such as outgrowth, branching, and regulation of the cytoskeleton.