**10. Apoptosis**

transgenic mutant SOD1 mice. The expression of mutant SOD1 restricted to motor neurons *in vivo* was not enough or caused a mild neurodegeneration (Jaarsma et. al.; 2008). Indeed, when mutant SOD1 expression was reduced in microglia and macrophages there was a reduction in motor neuron degeneration (Boillee 2006, Wang 2009). In addition, mutant SOD1 expression in astrocytes is required to cause neurodegeneration by release of toxic factors (Gong et. al., 2000; Nagai et al. 2007). Co-cultures of healthy motor neurons with astrocytes expressing mutant SOD1 resulted in more than 50% motor neuron death (Marchetto et al, 2008), while astrocytes obtained from postmortem tissue from patients with fALS and sALS were both toxic to motor neurons (Haidet-Phillips et al., 2011). In agreement, mutant SOD1 knockdown in astrocytes attenuated toxicity towards motor neurons, suggesting that the mutant enzyme plays a role in both fALS and sALS (Phillips et al., 2011). SOD1G93A glial-restricted precursor cells transpanted into the cervical spinal cord of wild type rats survived and differentiated efficiently into astrocytes. These graft-derived SOD1G93A astrocytes induced host ubiquitina‐ tion and death of motor neurons, reactive astrocytosis, and reduction of the glial glutamate transporter GLT-1 expression that was associated with animal limb weakness and respiratory dysfunction (Papadeas et al., 2011). The SOD1G93A astrocyte-induced motor neuron death may

Abnormalities in the immune system have also been observed in ALS patients. Blood samples

However, early in the disease when motor features are still mild there is a reduction in CD4+

reduction in circulating T-reg cells could be due to the relocation of the cells into the central nervous system. Upon relocation, the T-reg cells would activate the innate immune cells like microglia, leading to the release of anti-inflammatory cytokines such as interleukin-10 and transforming growth factor-β to protect the affected area (Kipnis et al., 2004 and Mantonavi, et al., 2009). Indeed, immunostaining for the astrocytic marker glial fibrillary acid protein (GFAP) showed a significantly increased presence of astrocytes in the precentral gyrus of patients with both fALS and sALS. In addition, staining for activated microglia and macro‐ phages markers such as leukocyte common antigen (LCA), lymphocytes function associate molecule (LFA-1), complementary receptors CR3 (CD11b), and CR4 (CD11c) was also in‐ creased in motor cortex, brainstem, and corticospinal tract (Kawamata, et al., 1992; Papidimi‐ triou et al., 2010). Samples from brain and spinal cord from animal models and patient with ALS also showed a significant increase in activated or reactive astrocytes, an indication of

Astrocytes and microglia play an essential role in immune surveillance and response in the central nervous system. Reactive astrocytes recruited to the injured area reestablish the bloodbrain-barrier (BBB), release neurotrophins and growth factors (IGF-1), clear debri, and isolate the injured region through the formation of a glial scar (Papadimitriou et al 2010; Dong and Benviste 2001). Microglia are also activated in the presence of antigens exposed during neurodegeneration leading to the phagocytosis of cellular debri and the secretion of several neurotrophic factors, neurotrophins, and cytokines. However, a poor regulation of these factors could be harmful to motor neurons. Microglia seems to protect motor neurons from

cells and reduced levels of CD8+

monocytes. These observations suggest that the

T lymphocytes.

/

be madiated by host microglial activation (Papadeas et al., 2011).

of ALS patients have increased levels of CD4+

12 Current Advances in Amyotrophic Lateral Sclerosis

neuroinflammation (Sta et al., 2011).

T-regulatory cells (T-reg) and CD14+

CD25+

Apoptosis is a programmed cell death cascade involved in several physiological processes during development and aging. Cell death by apoptosis sustains the homeostasis of cell population in tissues including cell turnover, hormone dependent- and chemical induced-cell death, and immune system development. The programmed cell death also functions as a defense when cells are damaged by disease or noxious stimuli (Elmore, S; 2007). Thereby, inappropriate apoptosis is a potential mechanism implicated in the pathogenesis of several neurodegenerative disorders, including ALS (Elmore, S; 2007). There are two main apoptotic pathways, the extrinsic or death receptor pathway and the intrinsic or mitochondrial pathway (Igney and Krammer, 2002). The extrinsic pathway involves transmembrane receptormediated interactions between ligands and death receptors resulting in transmission of death signals from cell surface to the intracellular signaling pathways (Locksley et al., 2001). The most studied ligand and death receptor association are Fas ligand and Fas receptor (FasL/FasR) and tumor necrosis factor (TNF) and its receptor (TNFL/TNFR) (Hsu et al 1995; Wajant, 2002). The intrinsic pathway consists of non-receptor-mediated stimuli that cause changes in the inner mitochondrial membrane. These changes include the opening of mitochondrial mem‐ brane pores leading to loss of transmembrane potential and released of pro-apoptotic proteins such as cytochrome c, Smac/DIABLO, HtrA2/Omi, and others ending with the activation of caspases (Sealens, et al., 2004; Du et al., 2000; Van Loo et al., 2002; Garrido at al., 2005). The 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 al., 2012).

activation of the caspases, attenuated neuron degeneration and delayed disease onset and

Pathophysiology of Amyotrophic Lateral Sclerosis

http://dx.doi.org/10.5772/56562

15

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.

2 Burnett School of Biomedical Sciences, College of Medicine, University of Central Florida,

[1] Al-chalabi, A, Andersen, P. M, Nilsson, P, Chioza, B, Andersson, J. L, Russ, C, Shaw, C. E, Powell, J. F, & Leigh, P. N. Deletions of the heavy neurofilament subunit tail in

[2] Al-chalabi, A, & Leigh, P. N. (2000). Recent advances in amyotrophic lateral sclerosis.

[3] Alexianu, M. E, Ho, B. K, & Mohamed, A. H. La Bella V, Apple SH. The role of calci‐ um-binding protein in selective motor neuron vulnerability in Amyotrophic Lateral

[4] Arisato, T, Okubo, R, Arata, H, Abe, K, Fukada, K, Sakoda, S, Shimizu, A, Qin, X. H, Izumo, S, Osame, M, & Nakagawa, M. Clinical and pathological studies of familial

mortality (Vukosavic et al., 1999, Kostic et al., 1997).

Fabian H. Rossi1,2, Maria Clara Franco1,2 and Alvaro G. Estevez1,2

amyotrophic lateral sclerosis. Hum Mol Genet (1999).

Current Opinion in Neurology; , 13(4), 397-405.

Sclerosis. Annals of Neurology (1994).

1 Orlando VA Healthcare System, Orlando, USA

**11. Conclusion**

**Author details**

Orlando, USA

**References**

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 activation of the caspases, attenuated neuron degeneration and delayed disease onset and mortality (Vukosavic et al., 1999, Kostic et al., 1997).
