**6. Conclusions**

inclusions are observed in non-SOD1 ALS patients [134]. Several research groups have generated rodents either overexpressing mutated forms of these genes or carrying gene deletions, with the objective to model the ALS pathology. Overall, the current rodent models for TDP-43-ALS or FUS-ALS display relatively mild MN degeneration. Axonal degeneration has been seen in the transgenic TDP-43 rat model, which seems to selectively affect the largest MNs [135]. This observation suggests that selective vulnerability is also likely to occur in the

Recently, more emphasis has been placed on modeling C9ORF72-ALS. Mouse lines generated with a bacterial artificial chromosome carrying a pathogenic hexanucleotide expansion in the full human C9ORF72 gene did not develop any behavioral symptoms in two independent studies [136, 137]. By contrast, using AAV vectors to express the G4C2 repeat expansion in the mouse CNS led to both histological and behavioral defects similar to the pathology observed in C9ORF72-ALS/FTLD patients [138]. Although 20% of neuron death was measured in the cortex, motor cortex and cerebellum of these mice, the number of spinal cord MNs was not reported. With the continuous development of ALS models, it will hopefully be possible to more accurately design therapeutic approaches against pathogenic pathways that may be

The recent development of mouse and human ES and iPS cells could provide an alternative to *in vivo* models. Several hundred lines with various mutations have been generated and are publicly available. In the context of SOD1, these models have been used to explore the noncell autonomous pathogenic mechanisms. Survival of mES- or hES-derived MNs is decreased when the cells are grown on astrocytes differentiated from SOD1 or sporadic-ALS induced neural progenitors [76, 132]. MNs derived from patients carrying the C9ORF72 repeats display typical histopathological features, such as nuclear foci and the non-ATG translation of peptides encoded by the hexanucleotide repeats [139]. Remarkably, these neurons have an increased susceptibility to glutamate excitotoxicity. FUS, SOD1 and C9ORF72 ALS-derived MNs show electrophysiological abnormalities [140]. This technology presents the advantage to model a larger portion of ALS cases, allowing for comparative gene expression profiling experiments and testing of therapeutic compounds. However, it remains difficult to study the selective vulnerability of MN subtypes in such *in vitro* models, as they fail to replicate the differentiation

Lately, the transcriptome of the cerebellum and frontal cortex was analyzed *post mortem* in samples from C9ORF72-ALS and sALS patients [141]. The results show a prominent dysre‐ gulation of genes involved in the UPR and intracellular protein transport machinery in C9ORF72-ALS. In sALS, it is the genes involved in cytoskeleton organization and synaptic transmission, which are most affected. One should keep in mind that these molecular changes reflect only the late stage of the disease and in the future, it will be important to also access

More generally, it is tempting to speculate that the selective MN vulnerability observed in ALS may also apply to other MN disorders, such as spinal muscular atrophy (SMA) and Charcot-Marie-Tooth (CMT) diseases. In SMA patients, muscle biopsies show atrophy of the type II muscle fibers, with a compensatory hypertrophy of the type I fibers [142]. Moreover, the Onuf's

into cellular subtypes and the precise architecture of the CNS tissue.

spinal cord tissue to perform similar transcriptome analysis.

TDP-43 pathology.

180 Update on Amyotrophic Lateral Sclerosis

common to different forms of ALS.

The identification of ALS-causing mutations in the SOD1 gene in 1993 has raised hopes in the ALS scientific community [10]. One year later, Gurney *et al*. [8] generated the well-known SOD1G93A mouse model, which reproduced most of the ALS features, and opened the way for exploring the causes of the disease and developing therapies. However, the challenge turned out to be more difficult than expected. Indeed, more than 20 years later, the identification of therapeutic targets in the mSOD1 mouse model has failed to bring any effective treatment to ALS patients.

Nevertheless, the characterization of mouse models overexpressing mutant forms of SOD1 has dramatically improved our understanding of the cellular processes leading to ALS. In particular, the very reproducible course of the disease observed in these mice has been instrumental to study the different stages of the disease, highlighting the fact that not all MNs are equally affected, and that glial cells are important actors in the pathogenic process. Recently, several research groups have identified critical factors, which determine MN vulnerability, providing novel targets for therapeutic intervention. One obvious difficulty in designing treatments for ALS is that at a given stage of the disease, the various pools of MNs or glial cells may face different toxic mechanisms, depending on their intrinsic vulnerability. This may limit the therapeutic efficacy of a single drug.

It remains unknown whether the observed MN vulnerability pattern is specific to mSOD1, and if the identified molecular mechanisms can be applied to other forms of ALS. To address this question, it is critical to further develop animal and cellular models of ALS, based on the genes, which have recently been linked to the disease. By comparing these models, it will be possible to pinpoint common pathogenic pathways, and with the development of more specific biomarkers, apply therapies when and where they are the most likely to succeed.

#### **Acknowledgements**

This work was supported by the Swiss National Science Foundation (Grant 310030L\_156460) and by ERANET E-Rare FaSMALS (Grant 31ER30\_160673). NBM is supported by the Neuro‐ muscular Research Association Basel (NeRAB).
