In 1999, studies by Hirsch et al. [12] and Banati et al. [13] revealed the complexity in the pathogenesis of PD. These studies, and others, are based upon the examination of post mortem analyses of patient brains. Similarly, Banati et al. [13] found the typical features of apoptotic neurons in the substantia nigra of three PD patients using electron microscopy. On the other hand, other groups have criticized these data with respect to their limitations about sensitivity of the techniques used and the limited number of participating patients [12,13]. Another study using post­mortem brain tissues of PD patients shows the increase of caspase-1 and -3 activities and the expression of TNFR1 compared to controls [14].

      Studies based on the PD mouse model created by us­ing 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), indicates that a pro-apoptotic protein, Bax, is highly ex­pressed in nigral dopaminergic neurons, and that ablation of Bax renders mice more resistant to the dopaminergic neurotoxicity of MPTP [15]. Another mouse model cre­ated by using MPTP again shows that the blockade of APAF-1 mitigates the death of dopaminergic neurons [16].

      The 6-hydroxydopamine (6-OHDA) treatment of PC12 cells induced apoptosis and release of cytochrome c and Smac/Diablo from mitochondria, caspase-3 activation, cleavage of poly-ADP-ribose polymerase (PARP) and nuclear condensation. The 6-OHDA also induced the heat shock response, with increased levels of Hsp25 and Hsp27. In this study, prior heat shock or overexpression of Hsp27 delayed cytochrome c release, caspase activation and reduced the level of apoptosis caused by 6-OHDA [17]. Another study using PC12 cells revealed that 6-OHDA induces Puma (mRNA and protein) and Bim, both pro-apoptotic proteins. Using specific siRNAs, this study demonstrated that Puma is required for death evoked by 6-OHDA [18].

      An animal model using mice treated with MPTP re­vealed mitochondrial complex I deficiency, and this defect was associated with a recruitment of the mitochondria-dependent apoptotic pathway in vivo. On the other hand, isolated mitochondria from these mice showed that com­plex I dysfunction failed to directly activate apoptosis [19].

      Wild-type DJ-1 inhibits transcriptional silencing activ­ity of the PSF (pyrimidine tract-binding protein-associated splicing factor). In addition, PSF induces neuronal apop­tosis that can be reversed by wild-type DJ-1, and to a les­ser extent, by PD-associated DJ-1 mutants. Furthermore, these mutants exhibit decreased nuclear distribution and increased mitochondrial localization, resulting in dimin­ished co-localization with a co-activator p54nrb, and it is known that both DJ-1 and p54nrb block oxidative stress and mutant a-synuclein-induced cell death [20].

      The SH-SY5Y cells, which express wild-type PINK1 under basal and staurosporine-induced conditions, show that the number of TUNEL (terminal deoxynucleotidyl-transferase-mediated dUTP nick end labeling)-positive cells decreases compared with cells expressing PD-associ­ated mutant PINK1. Overexpression of wild-type PINK1 reduces both basal and staurosporine-induced caspase-3 activity and it also decreases the levels of cleaved caspase-9, -3, -7 and activated PARP [21].

A whole genome association study of PD using single nucleotide polymorphisms (SNPs) exhibited that an SNP within the semaphorin 5A (SEMA5A) gene has a statisti­cally significant association with PD, and it is already known that the protein encoded by this gene played an important role in neurogenesis and in neuronal apoptosis [22].