**11. Conclusion**

Bcl-2 family of proteins regulates the intrinsic apoptotic pathway (Cory and Adams 2002) and these proteins in turn are regulated by the tumor suppressor protein p53 (Schuler and Green, 2001). The Bcl-2 family includes pro-apoptotic and anti-apoptotic proteins. Some of the antiapoptotic proteins comprise Bcl2, Bcl-x, Bcl XL, Bcl-XS, Bcl-w, BAG, whereas the pro-apoptotic proteins include Bcl-10, BAX, Balk, Bid, Bad, Bim, Bik, and Blk. Both the intrinsic and extrinsic pathways require a specific stimuli to activate its own caspase initiator (caspase -2,-8,-9,-10). These two pathways, once activated, convey in the activation of a final execution pathway with cleavage of caspase-3, resulting in DNA fragmentation, cytoskeletal and nuclear protein cleavage, protein cross-linking, apoptotic bodies formation, expression of ligands for phago‐ cytic recognizion, and final uptake by phagocytic cells (Martinvalet, et al, 2005). There is compelling evidence in ALS, at least in mutant SOD1-ALS, that toxicity is mediated by apoptosis. In transgenic SOD1 mice there are numerous apoptotic findings such as DNA fragmentation, caspase activation, and altered expression of the anti-apoptotic protein Bcl-2 (Durham HD, et al. 1997; Spooren WP et al. 2000). Motor neuron degeneration in ALS struc‐ turally resembles apoptosis. The neuronal death progression is divided in 3 sequential stages: chromatolysis, somatodendritic attrition, and apoptosis. In ALS, in the spinal cord anterior horn and motor cortex there is DNA fragmentation and increased in caspase-3 activity. Vulnerable central nervous system regions affected by ALS show elevation of pro-apoptotic proteins Bax and Bak and reduction of the antiapoptotic protein Bcl-2 in mitochondrialenriched membrane compartment. Co-immunoprecipitation experiments show greater Bax-Bax interactions and lower Bax-Bcl-2 interactions in the mitochondrial-enriched membrane compartment of ALS motor cortex compared with controls, (Martin LJ, 1999). In mutant SOD1 mice apoptotic signals are activated in sequence, caspase 1, an inflammatory caspase, is activated at disease onset while activated caspase-3 is detected later in the curse of the disease (Pasinelli, P, et al. 2000). In SOD1 mice, intracerebroventricular injection of a broad caspaseinhibitor reduces caspase 1 and caspase 3 mRNA levels resulting in spear motor neurons at the spinal cord and delay in disease onset and progression compared with vehicle-infused mice (Li M, et al.; 2000). Overexpression of the antiapoptotic protein Bcl-2 and deletion of the pro-apoptotic protein Bax preserve motor function and prolong life in a SOD1G93A model. Genetic deletion of mitochondrial pro-apoptotic Bak and Bax proteins in a mouse model of ALS prevent neuronal loss and axonal degeneration, and delayed onset of disease (Reyes et

14 Current Advances in Amyotrophic Lateral Sclerosis

In SOD1G93A transgenic mice, cytosolic release of cytochrome c was observed (Pasinelli, et al., 2004; Kirkinezos et al., 2005; Takeuchi et al., 2002), and levels of pro-apoptotic proteins Bad and Bax were increased while those of anti-apoptotic proteins Bcl2, Bcl-xL and XIAP were decreased (Guegan et al., 2001; Vukosavic et al., 1999; Ishigaki et al., 2002). Caspase 1 and caspase 3 were also sequentially activated in motor neurons and astrocytes in SOD1G93A, SOD1G37R, and SOD1G85R mice (Li et al., 2000; Pasinelli et al., 1998; Pasinelli et al., 2000). Intraventricular administration of minocycline, which inhibits cytochrome c release from mitochondria, was shown to delay disease onset and extend survival (Zhu et al., 2002). However minocycline failed in human ALS patients (Gordon et al, 2007). Similar results were observed upon intraventricular administration of the broad-spectrum caspase inhibitor zVADfmk (Li et al., 2000). Additionally, over-expression of anti-apoptotic protein Bcl-2 delayed

al., 2012).

The research in the ALS field encounters many limitations, what is clearly reflected in the little progress accomplished in the therapy of this neurodegenerative disorder. Most of the studies describe the mechanisms involved in the pathogenesis of the familial form of ALS, which accounts for a minority of all the ALS cases. However, some of the hypothesis currently under investigation may also explain how the pathology develops in the sporadic forms of ALS. In the last two decades several experimental models *in vitro* and *in vivo* have shaded light into the pathogenesis of the disease. Several potential mechanisms have been implicated in ALS onset and progression including oxidative stress, excitotoxicity, mitochondrial dysfunction, glial activation, RNA-processing, and growth factor abnormalities. Whether these mechanisms intertwine, work in parallel or in sequence to cause neuronal death remains to be investigated.
