kinase signaling cascade. The effect of the MAP kinase pathway on cell survival is mediated at least in part by the activation of downstream pp90 ribosomal S6 kinase (RSK) family members. Like Akt, RSK has been shown to phosphorylate and inhibit Bad. RSKs are also potent activators of CREB, which activates the transcription of the antiapoptotic gene Bcl-2 and a variety of immediateearly genes and delayed-response genes in regulating cell survival, axonal and dendritic growth, and neuronal differentiation. Thus PI3K-Akt and MAP kinase pathways converge on Bad and CREB to inhibit the apoptosis program.

1.3.2. Signals for death

The removal of NGF leads to a rapid inhibition of PI3K and MAP kinase activities, followed by a series of early metabolic changes, including increased production of reactive oxygen species, decreased glucose uptake and decreased RNA and protein synthesis, and increased c-Jun N-terminal kinase (JNK) activation or c-Jun phosphorylation.

The physiologic role of NGF in many systems is to promote neuronal survival by acting through the highaffinity TrkA receptor tyrosine kinase. NGF also acts through low-affinity receptor p75NTR during development to negatively regulate numbers and cholinergic phenotype of forebrain cholinergic neurons, because the number and the size of these neurons are increased in p75NTR–/– mice or in normal mice treated with a peptide that inhibits the binding of NGF to p75NTR. p75NTR is a member of the tumor necrosis factor (TNF) receptor superfamily. It binds mature neurotrophins with lower affinity, but binds pro-neurotrophins with higher affinity compared with that of Trk receptors. The neurotrophin-bound p75NTR can associate with several signaling partners, including Nogo receptor, sotilin, and LINGO-1, which are involved in regulating neurite outgrowth in response to myelin proteins such as Nogo, MAG, and OMgp. Formation of these different platforms may explain the multiple effects of p75NTR in different cell types and contexts.

The intracellular domain of p75NTR contains a region that bears similarity with the death domain of the TNF receptor family. In the condition of low or no Trk activity, highly activated p75NTR signals through ceramide, the JNK family, and NF-κB, similar to other members of TNF receptor family, to induce apoptosis. However, in the presence of high Trk activity, p75NTR-mediated apoptosis is suppressed. Instead, coactivation of p75NTR enhances Trk-mediated cell survival and differentiation. Concurrent stimulation by neurotrophins of p75NTR

and Trk receptors constitutes a dual growth control with antagonistic and synergistic elements aiming at optimal functional integration of cells and cell populations into their context.

2. PATHOLOGIC NEURONAL CELL DEATH

IN THE ADULT BRAIN

In the first part of this chapter, we discussed programmed neural cell death during development, which is required to achieve the accurate wiring of the nervous system. However, genetic or environmental factors can lead to nonprogrammed death of neurons during adult life due to neural insults caused by the onset of neurodegenerative disorders, stroke, or trauma. Pathological neuronal death can occur by apoptosis, necrosis, or a combination of both. The manner by which neuronal cell death is executed in a particular condition may depend on several factors, including the neuronal cell type involved, the nature and severity of the insult, and the energy content of the cells. For example, it has been shown that neurons at the core of an ischemic lesion undergo necrotic death and cannot be rescued by treatment with caspase inhibitors, whereas apoptotic neurons are more likely to be found at the penumbra, and cell death can be prevented in the presence of caspase inhibitors. In this part, we first discuss the evidence of apoptotic death in several neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), and amyotrophic lateral sclerosis (ALS). We also provide evidence for the involvement of caspase-independent cell death in neurodegenerative disorders, focusing on the proteolytic mechanism of calpains and cathepsins.

2.1. APOPTOSIS IN NEURODEGENERATIVE DISEASES

2.1.1. Alzheimer’s disease

AD is the most common form of dementia. A central role for amyloid-β (Aβ) protein in AD progression is supported by the effects of genetic mutations responsible for familial AD, which predisposes to amyloid plaque deposition in the brain. Aβ, a peptide that forms amyloid plaques, can directly induce apoptosis of cultured neurons. Fragmented nuclear DNA has been detected in brains of AD patients by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining. Biochemical evidence that supports the involvement of apoptosis in AD is provided by the detection of activated caspase-3, -8, and -9 in the hippocampal neurons of the brains affected by AD. Moreover, pharmacological

inhibition or genetic mutations of caspase family members, such as caspase-2, -3, -8, and -12, have been reported to offer partial or complete protection against Aβ-induced apoptotic cell death in vitro.

Aβ is a proteolytic cleavage product of γ-secretase presenilin-mediated processing of amyloid precursor protein (APP). Mutations in presenilin genes, responsible for a significant subset of early-onset familial AD, increase the production of a 42-residue form of Aβ, which is a major constituent of plaques in the AD patient brain, and neuronal sensitivity to apoptosis. In both cell culture and AD-affected brains, APP can also be cleaved by caspases, such as caspase-3, at the sites distinct from the presenilin-cleavage sites. Caspasemediated cleavage of APP not only releases Aβ, but also releases a carboxy-terminal peptide that is a potent inducer of apoptosis. Caspase-3 cleaved fragments of tau, a microtubule-associated protein that is the primary protein component of the filaments found in AD patient brains, have also been detected in postmortem samples. Despite all the evidence that supports a role of apoptosis in AD, questions have been raised regarding how apoptosis, a rapid form of cell death, might be compatible with the chronic progression of AD. In the AD brain, some neurons exhibit morphological features of apoptosis, but many degenerating neurons do not show evidence of apoptosis, suggesting that apoptosis might not be the only mechanism of degeneration in AD. Furthermore, evidence obtained from postmortem brain samples should be viewed with caution because increased caspase activation might have occurred in the context of a diseased brain postmortem.

Although the mechanism of Aβ neurotoxicity and its precise cellular locus of action are not settled, the evidence supporting the involvement of Aβ in AD is strong. Aβ has also been shown to induce oxidative stress and elevate intracellular Ca2+ levels, which activates several cell death signaling pathways. In addition, the increased presence of activated microglia, a prominent feature of AD patient brains, indicates the activation of inflammatory response. Microglial activation is associated with amyloid plaques and can be induced experimentally by Aβ. Microglial activation induced by Aβ leads to the secretion of TNF-α and other toxic factors that can induce neuronal apoptosis. Thus pathological neuronal death in AD might be a consequence of a complex interaction between neurons, microglia, and toxic factors.

2.1.2. Parkinson’s disease

PD is characterized by resting tremor, slowness of movement, rigidity, and postural instability. These symptoms

are attributed to the loss of dopamine (DA)-containing neurons in the substantia nigra pars compacta (SNPC). Biochemical assessment of apoptotic markers in PD patient brains revealed that both proapoptotic proteins, e.g. Bax, caspase-3, caspase-8, caspase-9, and antiapoptotic proteins (e.g., Bcl-xl) show increased expression or activation in DA neurons as compared with that of controls.

Early research using a 1-methyl-4-phenl-1,2,3,6- tetrahydropyridine (MPTP) induced mouse model of PD has provided evidence for the possible involvement of apoptosis in the pathogenesis of PD. MPTP, a byproduct during the chemical synthesis of a meperidine analogue, has potent heroin-like effects that can induce a disease syndrome in humans almost indistinguishable from PD. A metabolic product of MPTP, MPP+ , is concentrated in the mitochondria of DA neurons, where it inhibits the complex I of electron transport chain and results in an increased production of reactive oxygen species. Prolonged administration of a low dose of MPTP to mice induces downregulation of Bcl-2, upregulation of Bax, activation of caspase-9 and caspase-3, and morphologically defined apoptosis in DA neurons. Bax–/– mice and Bcl-2 transgenic mice are resistant to the toxicity of MPTP. The activation of p53 plays an important role in mediating the death of DA neurons after MPTP intoxication, as p53–/– mice are resistant to the MPTPinduced DA neuron death. In addition, pharmacological blockade of JNK activation results in a marked attenuation of MPTP-induced neurodegeneration. Blockage of the intrinsic apoptosis pathway (mitochondria pathway) by an intrastriatal injection of an adeno-associated viral vector containing a dominant-negative form of Apaf-1 also prevents the MPTP-induced activation of caspase-3 and SNPC neuronal death.

α-synuclein is an important component of the intracellular inclusions known as Lewy bodies, which are the neuropathological hallmark of PD. Dominantly inherited gain-of-function mutations in α-synuclein have been found in a subset of familial PD. Although the mechanism by which mutations in α-synuclein induce DA cell death has not been well established, deletion of α-synuclein in mice prevents MPTP-induced neurodegeneration, whereas α-synuclein transgenic mice show increased sensitivity to the toxin, and expression of mutant α-synuclein in cell cultures promotes apoptosis.

Recent progress in molecular genetics has identified several genes implicated in familiar forms of PD, including α-synuclein, leucine-rich repeat kinase 2 (LRRK2), Parkin, DJ-1, and phosphatase and tensin homolog (PTEN)–induced kinase 1 (PINK1), many of them coding for proteins found in Lewy bodies and/or implicated in