**4. Conclusion**

108 Amyotrophic Lateral Sclerosis

major improvement in the development of *in vitro* models of the disease. The culture of NPCs in methylcellulose scaffold enriched with laminin has been shown to reduce apoptosis and enhance survival, differentiation into neurons, astrocytes or oligodendrocytes and neurite extension (Cullen, et al., 2007; Stabenfeldt, et al., 2010). Motor neurons axonal migration can also be analyzed when these cells are cultured on the top of a threedimensional reconstructed connective tissue made of a collagen sponge populated with fibroblasts. The addition of neurotrophic factors underneath the construct promotes neurite growth from the top to the bottom of the tissue. The main advantage of this model is to enable myelin sheath formation by Schwann cells around axons in the connective tissue and the possibility to add various glial cells (astrocytes, microglia) in the motor neuron layer (Fig 5) (Gingras, et al., 2008). In addition, it is possible to easily combine cells from different origin, like WT motor neurons with ALS glial cells, and vice-versa in order to further explore the non-cell autonomous effect observed in SOD1-linked ALS pathology. Such combinations could also be very informative in the study of sporadic ALS. Alternatively, a 3 dimensional reconstructed muscle model has been developed to study *in vitro* the effect of muscle stretching on mRNA expression of muscle cells. This model was prepared using primary muscle cultures from human control subjects and ALS patients. The cells were embedded in a collagen gel tethered to a Culture Force Monitor to analyze gel contraction

Finally, the combination of ALS patient NPC-derived neural cells in tissue-engineered reconstructed spinal cord models is a promising strategy to develop the next generation of

Fig. 5. Three-dimensional model of motor neuron axonal migration and myelin sheath

(Cheema, et al., 2003; Evans, et al., 2010).

*in vitro* models of ALS.

formation.

A very large body of knowledge has been built over the years with the development of various *in vivo* models to better understand ALS. Particularly, the generation of a large number of different transgenic mice has been extremely useful to study familial ALS, but has met with limited success so far to study SALS or to identify drugs alleviating the disease symptoms. This could be explained by the existence of redundant compensatory mechanisms present in higher vertebrates. The use of invertebrates together with higher animal models will hopefully allow the identification of novel pathogenic pathways and novel therapeutic avenues in order to cure this devastating disease.

In parallel, the development of *in vitro* models will give valuable information about the intracellular modulations induced by the disease. In addition, the promise of building sophisticated *in vitro* models using patient's cells will also be crucial to better understand the disease. However, the results obtained from *in vitro* models may not always be relevant because they are oversimplified compared to the *in vivo* situation. Therefore, the best strategy to reduce the limitations inherent to all these different models, and to benefit from their specific advantages is to combine both *in vivo* and *in vitro* models in order to better model the disease. Future work, focusing on different neuronal in vitro system and animal models, will certainly increases our knowledge in the biology of ALS and hopefully leads to more translational researches in order to find a cure or to slow down the course of this yet untreatable disease.
