In multicellular organisms, cells that are no longer needed are destroyed by a regulated process known as apoptosis [78,79,80]. Apoptosis is important for embryo development, tissue homeostasis, and regulation of the immune system as well as for the development of the nervous system [79,81]. Apoptosis may play a role in neurodegeneration and aging [80,82].

There are two apoptotic pathways in mammals; i.e., the extrinsic and the intrinsic pathways [83]. Both of these pathways involve the activation of caspases—proteolytic proteins that cleave their target polypeptides at specific locations without degradation of the target protein [84,85], thus creating gain-of-function or loss-of-function events that generate the apoptotic phenotype [85].

The intrinsic, or Bcl-2-regulated, mitochondrial pathway is triggered in response to several cellular stressors. Bcl-2 homology 3 (BH3)-only members either directly activate the pro-apoptotic Bcl-2 family members Bax and Bak, or antagonize anti-apoptotic Bcl-2 family members [86,87]. Bax and Bak are thought to homo-oligomerize and form pores in the outer mitochondrial membrane thereby increasing mitochondrial outer-membrane permeabilization (MOMP), considered as the ‘point of no return’ in apoptosis signaling. The MOMP allows the efflux of multiple pro-apoptotic proteins from the mitochondrial intermembrane space including cyt-c [85]; as a result, second mitochondrial activators of caspases (Smac or DIABLO) can pass from the intermembrane space into the cytoplasm. In the cytosol, cyt-c interacts with an adaptor protein, the apoptotic protease-activating factor-1 (Apaf-1), a crucial step of the intrinsic pathway [88]. The interaction between cyt-c and Apaf-1 induces Apaf-1 conformational changes driven by dATP hydrolysis [89]. The resulting complex recruits and binds pro-caspase-9 to the caspase recruitment domain (CARD) of Apaf-1 [84]. Caspase-9 is activated in a dATP/ATP-dependent process and the resulting complex cleaves, and activates caspases 3 and 7 [84,89,90,91]. These proteins mediate the molecular signals leading to cell death through the selective proteolysis of key protein substrates.

p53 is a key tumor-suppressor protein that abolishes genetically-unstable cells by inducing cell cycle arrest or apoptosis through transcriptional regulation or by direct interaction with apoptotic proteins [92]. p53 can be inactivated by post-translational modifications, mutations [93], or as a result of sequestration by binding proteins [94,95,96]. p53 has been implicated in transcriptional activation of several proteins (such as Ras, p21, Bax, BH3-only proteins Noxa and PUMA (p53-up-regulated modulator of apoptosis), PIG3, Killer/DR5, CD95 (Fas), p53AIP1 and Perp) or repression of genes involved in apoptosis [97].

Some studies have reported functional interactions between p53 and mortalin in the cytoplasm [23,52,53,92,98,99], leading to the inhibition of the transcriptional activation of p53 and control of centrosome duplication functions [92,99]. Specifically, p53 presents two binding sites for mortalin, one in the C-terminal domain and the other in the p53-tetramerization (TET) domain [23]; any of these domains is sufficient for a mortalin-p53 binding interaction. This interaction occurs through the PBD of mortalin [23,99] (Figure 1).

A recent study indicates that the mortalin-p53 interaction causes inactivation of p53-mediated apoptosis depending on the cellular stress levels [92]. Specifically, stress-associated induction of mortalin expression protects the cells against the initial insult, improves cell recovery, and improves resistance to subsequent stress signals. Unstressed or mildly stressed cells do not display mortalin-p53 interaction [92] (Figure 3). Mortalin may prevent the entry of p53 to the nucleus by physical entrapment that leads to proteasomal degradation [52]. During the late G1 phase, mortalin localizes in the centrosome and represses the p53-dependent suppression of centrosome duplication [98]. p53 can induce Bax activation, leading to changes in the mitochondrial membrane permeabilization; however, in the absence of mortalin, there is nuclear accumulation of p53, concomitant with increased levels of Bax, suggesting that the low levels (or absence) of mortalin cause activation of the p53-Bax apoptosis pathway [92]. Mortalin may also be associated with cell immortalization via binding to diphosphomevalonate decarboxylase (MVD1; previously known as mevalonate pyrophosphate decarboxylase or MPD), an inhibitory protein of p21 (Ras). Furthermore, co-expression of the human telomerase reverse transcriptase (hTERT) with mortalin can avoid cell death [33].

Another important protein in the mortalin/p53/OS-associated molecular mechanisms is the 66 kDa isoform of the SHC-transforming protein 1 or p66Shc, a protein that mediates OS-induced apoptotic mechanisms [53]. p66Shc is a downstream target of p53 that predominantly exists in the cytoplasm and is translocated into the mitochondria in a process mediated by mortalin and prolyl isomerase 1 (Pin1) [100]. In mitochondria, following pro-apoptotic stimulation, p66Shc oxidizes cyt-c, producing H₂O₂, which promotes the opening of the mitochondrial permeability transition pore triggering apoptosis [53].

The literature review presented here suggests that mortalin participates in apoptosis by regulating proteins that are implicated in cellular stress response mechanisms. Under low levels of stress, mortalin acts as an anti-apoptotic protein by inactivating p53 [32,92], and interfering with the ability of cyt-c and Apaf-1 to trigger the recruitment of procaspase-9; on the other hand, under stress conditions, mortalin alters mitochondrial functions while cytoplasmic p53 can induce apoptotic signals (Figure 3). These opposing functions point out to a mortalin protein that may represent a sensitive marker of stressed cells and apoptotic function associated with p53 activity.

Aging is a biological process characterized by a general and progressive deterioration in metabolic processes affecting tissues that exhibit a high rate of oxygen consumption, such as the brain [19]. Aging and neurodegeneration also affect the proteome. Oxidative protein damage results in protein aggregation, changes to secondary and tertiary structures, and loss of catalytic functions that may activate cell death-associated signal transduction pathways. Unfolded proteins have a strong tendency to form neurotoxic insoluble protein aggregates resulting in the impairment of the ubiquitin-proteasome degradation system, and suppression of the heat shock and OS response mechanisms [101]. The abnormal accumulation of unfolded and/or aggregated polypeptides usually leads to the loss of specific neuronal populations resulting in the onset and progression of several neurodegenerative diseases [34,77]. In general, the coupling of stress with impairment of the chaperone system can cause premature aging [19].

The level of oxidized proteins in a cell reflects the balance between the rates of protein oxidation (generation of ROS) and protein degradation (degradation of oxidatively-damaged proteins) [76,102]. Some studies have shown that there is an association between aging and oxidative damage of stress chaperones [21], like mortalin, in neurodegenerative diseases, including Alzheimer’s disease [11,21,102,103] and Parkinson’s disease [15,104]. Our studies have demonstrated that mortalin is oxidized in the brain tissues of an animal model of Alzheimer’s disease [21]. Another potential role of mortalin in neurodegeneration stems from the participation of mortalin in calcium channel regulation [58], a critical process for neuronal health.

Apolipoprotein E (ApoE) is important in the regulation of cholesterol and metabolism of triglycerides. There are three common ApoE isoforms: ε2, ε3 and ε4. The APOE4 allele is associated with an increased risk of Alzheimer’s disease [105,106,107,108]. Studies of ApoE4 transgenic and ApoE-deficient mice have confirmed an association between reduced ApoE activity, oxidative damage, and age-dependent neuronal alterations. Using proteomics, Osorio et al. performed a study of human ApoE4-Targeted Replacement mice (hApoE4-TR) compared to hApoE3-TR as control [20]. It was found that different mortalin isoforms are present in hApoE4-TR and hApoE3-TR mice brains, as well as between Alzheimer’s disease patients and age- and gender-matched controls [20]. In addition, using immunoprecipitation with ApoE- and with mortalin-antibodies, we have found that mortalin binds ApoE in hApoE-TR mice, as well as in human brains of Alzheimer’s disease patients (Figure 4). This binding, whose functional effect is under investigation, is different between diseased and non-diseased brains, and between APOE ε3/3 and APOE ε4/4 genotypes (Figure 4).

OS, and mitochondrial and proteosomal dysfunction have been implicated in the pathogenesis of Parkinson’s disease [15,109,110,111]. Parkinson’s disease is a progressive disorder characterized by dopaminergic neurodegeneration in the Substantia Nigra pars compacta (SNpc) and by the appearance of proteinaceous cytoplasmic inclusions (Lewy bodies) in the remaining nigral neurons [15,112]. A reduced expression level of mortalin has been observed in the affected brain regions of Parkinson’s disease patients [15,113] and in a cellular model of Parkinson’s disease [47]. Specifically, in dopaminergic neurons, manipulations of the level of mortalin resulted in changes to the sensitivity to Parkinson’s disease phenotypes via different pathways related to OS, mitochondrial and proteasomal function [47], correlating with reduced mitochondrial membrane potential and increased production of ROS [45].

Like in other neurodegenerative diseases, ROS is a key element in the pathophysiology of Parkinson’s disease [114]. Parkin, an E3 ubiquitin–protein ligase that mediates polyubiquitination of VDAC [115], is associated with mitochondrial dynamics [116], is involved in the regulation of mitochondria morphology, and is related with autosomal-recessive Parkinson’s disease. Parkin may also play a role in sporadic cases of Parkinson’s disease. There is evidence indicating that mortalin and Parkin provide a protective effect against oxidative damage, and that mortalin is involved in Parkinson’s disease-related abnormal mitochondrial morphology. Under OS, mortalin knockdown stimulates disintegration of mitochondrial connectivity and low-grade branching of mitochondria [15].

Dj-1 is an oncogene that protects cells against OS and cell death, and mutations in Dj-1 are associated with familial forms of Parkinson’s disease [117]. Dj-1 is associated with chaperones including Hsp70, CHIP and mortalin and undergoes OS-mediated translocation into mitochondria [118]. Mortalin has been identified as one of the five major proteins (mortalin, nucleolin, Grp94, calnexin and clathrin) that bind α-synuclein and Dj-1, two critical proteins in Parkinson’s disease pathogenesis [34,47].

Mortalin-null cells exposed to OS show disintegration of mitochondrial connectivity, suggesting that mortalin is implicated in the control of the mitochondrial dynamics and morphology [15]. It has also been reported that Tid-1, a chaperone protein involved in the regulation of cell survival, interacts with mortalin on an isoform-specific basis, and can mediate the reactivation of protein aggregates. It was suggested that mortalin can serve as a scavenger of toxic protein conformers in human mitochondria, making it an attractive target for therapies against protein conformational diseases [14].

Apoptosis allows the elimination of non-viable cells without affecting the neighboring cells. Unlike the rapid turnover of cells in proliferative tissues, neurons show only slight regeneration and normally stay alive for the entire life of the organism [119,120]. OS and metabolic stress are able to activate the chaperone system and can initiate neuronal apoptosis (Figure 5), and under OS there is inhibition of the electron transport chain and production of ROS in neurons [121]. Qu et al. demonstrated that overexpression of mortalin in neuroblastoma cells can reduce OS [30]. Mortalin increases the stress response capacity of the cells resulting in increased cell viability and extended longevity of an organism.

Mortalin responds to ROS accumulation under stress conditions while regulating other housekeeping functions, including control of cell proliferation, intracellular trafficking, or anti-apoptotic activity; this down-regulation of housekeeping functions may result in uncontrolled cell proliferation. For example, OS induces mortalin translocation into the mitochondria [104], leaving other proteins, including Apaf-1 and p53, unchecked, thus potentiating the disproportionate activation of apoptotic biochemical cascades.

From our observations that the mitochondrial proteome is affected by mortalin expression levels, we would expect that, under normal conditions, mortalin behaves as an anti-apoptotic protein that inactivates p53, resulting in cyt-c or Apaf-1 not being released. On the other hand, in the presence of oxidative stress, mortalin is responsible for mitochondrial homeostasis, allowing cytoplasmic p53 to induce apoptosis. These observations point to mortalin being a sensitive marker of stressed cells and the apoptotic function associated with p53 activity. Mortalin behaves as a regulatory protein that can alter cell function by associating with vital cellular proteins, including p53, Dj-1, FGF-1, and Hsp60. Regulating the functions of these proteins most likely affects signals involved in neurodegenerative diseases and apoptosis as discussed above. Bearing in mind the multiple functions of mortalin in cell control, is not surprising that over-expression of mortalin is able to promote cancer and may trigger features associated with neurodegenerative diseases, including Parkinson’s and Alzheimer’s diseases.

Cellular homeostasis is maintained by a strict regulation of the balance between ROS production, cell growth, and apoptosis. Many pathological states, including cancer and neurological diseases, are often associated with OS and disregulation of apoptotic pathways. Changes in mortalin expression are associated with cellular protection as they permit cells to survive lethal conditions modulating the cell’s lifespan [29,30,31]. Mortalin also has anti-apoptotic and pro-proliferative activities that influence the functions, dynamics, morphology and homeostasis of mitochondria.

Proteomic, molecular and biochemical data suggest that cell death, degeneration, and immortalization are not controlled by a single mechanism; they are regulated by a complex networks of proteins interconnected via multiple molecular pathways. The mitochondria and mitochondrial proteins play a fundamental role and are an indispensable part of this regulatory network. Several process that are related to the mitochondria, such as apoptosis, have been extensively studied for many years, but some of the processes, such as protein import, complex assembly, and the molecular mechanisms by which mortalin influences apoptosis have not yet been sufficiently elucidated. This review summarizes the present knowledge on mortalin and its relationship to apoptosis and neurodegenerative diseases, and mortalin’s role in OS and mitochondrial function. Although several cellular proteins are known to interact with mortalin, mortalin appears to be a regulatory protein that maintains the integrity of the cell via multiple molecular processes that are still under investigation. Further research on the function and dynamics of mortalin could provide valuable information about the complex balance between longevity, neurodegeneration, and apoptosis.