**1.4 Excitotoxicity in ALS**

378 Amyotrophic Lateral Sclerosis

sarcoma/translocated in liposarcoma (FUS/TLS), optineurin and valsolin containing protein (VCP) (Johnson et al.; Maruyama et al.; Rutherford et al., 2008; Van Deerlin et al., 2008; Del Bo et al., 2009; Kwiatkowski et al., 2009; Vance et al., 2009). Unfortunately, the discovery of these mutant genes has not yet progressed into useful ALS model organisms, so most of the work described below was conducted with mutant SOD1-

Multiple cell types contribute to the pathology making ALS a non-cell autonomous disease (Boillee et al., 2006a). By addition or deletion of mutant SOD1 in specific cell types, it is known that mutant SOD1 influences the disease depending on the cell type, including astrocytes (Yamanaka et al., 2008; Wang et al., 2011a), microglia (Boillee et al., 2006b), Schwann cells (Lobsiger et al., 2009) and motor neurons (Jaarsma et al., 2008). Additionally, ablation of T-cells (Beers et al., 2008; Chiu et al., 2008), B-cells (Naor et al., 2009), CD4+ and CD8+ cells (Beers et al., 2008) decrease survival of ALS mice, demonstrating the role of immune cells in disease progression. Although ALS is a non-cell autonomous disease, mutant SOD1 expressed solely in motor neurons is sufficient to initiate the disease, albeit with a slower disease progression (Jaarsma et al., 2008). Motor neurons in the motor cortex, brainstem and spinal cord undergo cell death selectively in patients. A number of hypotheses attempt to explain this cell type selectivity, including the long axons of the motor neurons (Fischer and Glass, 2007), their poor intracellular calcium buffering capacity (Grosskreutz et al., 2010) and motor neuron specific cell death pathways (Raoul et al., 2002;

The contribution of mutant SOD1 expressing astrocytes in the non-cell autonomous character of ALS has been studied by excising mutant SOD1 from astrocytes which increases survival in two different mutant SOD1 mouse models (Yamanaka et al., 2008; Wang et al., 2011a). These results denote the toxic character of mutant SOD1 in astrocytes that accelerate disease progression significantly by mechanisms such as, but not exclusively, the below described mechanisms of

Neuroinflammation occurs in a number of neurodegenative diseases, including ALS (reviewed in Papadimitriou et al., 2010 and Philips and Robberecht, 2011), and entails the reactive state of astrocytes (astrogliosis) and microglia (microgliosis) and the infiltration of lymphocytes. Initially perceived as a bystander effect, neuroinflammation is currently seen as beneficial at first, removing damaged cells and secreting supportive factors, and potentially detrimental thereafter by excessive release of cytokines (Beers et al., 2011a). Evidence of inflammation is detected in post mortem tissue (Schiffer et al., 1996; Anneser et al., 2004; Casula et al., 2011; Sta et al., 2011; Wang et al., 2011b), in cerebrospinal fluid (CSF) (Baron et al., 2005; Tateishi et al., 2010) and in blood samples of ALS patients (Poloni et al., 2000). In accordance, similar parameters of neuroinflammation are detected in ALS rodent models (among many others in Kiaei et al., 2006; Keller et al., 2009; Beers et al., 2011b). Inflammation is generally perceived as hazardous in ALS, as increasing inflammation in ALS models exacerbates disease progression and diminishes survival (Nguyen et al., 2004; Gowing et al., 2009). Fittingly, therapeutic strategies targeting inflammation are often

neuroinflammation and excitotoxicity. This is schematically presented in figure 1.

based ALS models.

**1.2 Non-cell autonomous ALS** 

Raoul et al., 2006; Genestine et al., 2011).

**1.3 Neuroinflammation observed in ALS** 

advantageous in ALS rodent models (see below).

An additional detrimental mechanism in ALS is excitotoxicity; an overstimulation of neurons causing neurodegeneration. Glutamate binds to the N-methyl D-aspartate (NMDA) or α-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) receptors, allowing extracellular sodium and calcium to enter motor neurons. Increased levels of intracellular calcium consequently cause neuronal cell death. The importance of excitotoxicity in ALS is demonstrated by the beneficial effects obtained by treating patients with riluzole. Although the precise mechanism of this drug is not yet known, it blocks NMDA receptors, enhances re-uptake of glutamate from the synaptic cleft and inhibits glutamate release by blocking voltage-gated sodium channels (Siniscalhi et al., 1999), thus preventing motor neuron cell death. Riluzole treatment increases predicted lifespan with a significant 12% in ALS mice (Bensimon et al., 1994; Lacomblez et al., 1996) and increases the probability of one year survival in patients by 9% (Miller et al., 2007). Unfortunately, it does not halt disease progression.

An overview of the current knowledge of the astrocytic contribution in ALS will be addressed in this chapter separately for the mechanisms inflammation and excitotoxicity.
