Mitophagy links oxidative stress conditions and neurodegenerative diseases

Abstract

Mitophagy is activated by a number of stimuli, including hypoxia, energy stress, and increased oxidative phosphorylation activity. Mitophagy is associated with oxidative stress conditions and central neurodegenerative diseases. Proper regulation of mitophagy is crucial for maintaining homeostasis; conversely, inadequate removal of mitochondria through mitophagy leads to the generation of oxidative species, including reactive oxygen species and reactive nitrogen species, resulting in various neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and amyotrophic lateral sclerosis. These diseases are most prevalent in older adults whose bodies fail to maintain proper mitophagic functions to combat oxidative species. As mitophagy is essential for normal body function, by targeting mitophagic pathways we can improve these disease conditions. The search for effective remedies to treat these disease conditions is an ongoing process, which is why more studies are needed. Additionally, more relevant studies could help establish therapeutic conditions, which are currently in high demand. In this review, we discuss how mitophagy plays a significant role in homeostasis and how its dysregulation causes neurodegeneration. We also discuss how combating oxidative species and targeting mitophagy can help treat these neurodegenerative diseases.

Introduction

Mitophagy is a term that was introduced by Le Masters in 2005 to describe the selective removal of mitochondria by autophagy (Lemasters, 2005). The degradation of mitochondria by mitophagy is especially important in cellular metabolism in which mitochondria play an essential role. The removal of dysfunctional and elderly mitochondria is essential for cell survival (Wallace, 2005). Additionally, neuronal cells are dependent on mitochondrial function, whereas its dysfunction is associated with neurodegenerative diseases. Disturbed mitochondrial function makes neurons especially sensitive to a wide variety of insults such as oxidative stress and bioenergetic defects. Thus, mitochondrial defects can greatly affect neuronal fate (Palomo and Manfredi, 2015).

Mitochondria are considered the main intracellular source of reactive oxygen species (ROS), which they produce during oxidative phosphorylation within all mammalian cells (Dai et al., 2014). ROS and reactive nitrogen species (RNS) play crucial roles in maintaining normal cellular behavior when regulated properly (Finkel and Holbrook, 2000). When ROS and RNS levels are excessive in terms of normal cellular requirements, it causes molecular damage and cellular debilitation. Higher levels of ROS may oxidize cellular constituents such as lipids, proteins and deoxyribonucleic acid (DNA), which interferes with cellular integrity (Finkel and Holbrook, 2000).

A previous study that used a mouse model of Purkinje cell degeneration demonstrated that altered mitophagy can cause excessive neuronal cell death, which was observed in the cerebellum. These results suggested that both uncontrollable mitophagy and inadequate mitophagy produce harmful effects (Kamat et al., 2014). Reduced autophagic function is believed to be responsible for many neurodegenerative diseases, such as Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), and amyotrophic lateral sclerosis (ALS). Therefore, mitochondria were recently considered a potential therapeutic drug target (Kamat et al., 2014).

In this review, we briefly discuss mitophagy and its involvement in the central nervous system (CNS) (i.e., AD, PD, HD, and ALS) and how these disease conditions occur when normal mitophagic function is compromised. By proper regulation of mitophagic pathways, the body can avoid harmful oxidative species, such as ROS and RNS, and harmful neurodegenerative diseases. Thus, by targeting these pathways, we can gain more knowledge about the therapeutic options to mitigate neurodegenerative disease conditions. Database search strategy is shown in Additional file 1.

Database search strategy- Mitophagy links oxidative stress conditions and neurodegenerative diseases

Serial No.	Article title	Eligibility criteria	Keywords/ Key terms	Publication date/Year	Database	Publishing language
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Mitophagy

Mitophagy is induced by oxidative stress. Direct production of mitochondrial ROS by using a mitochondrial-targeted photosensitizer can also induce mitophagy (Wang et al., 2012). The induction of autophagy results in the recruitment of autophagy-related genes (Atgs) to a particular subcellular location, termed the phagophore assembly site, and nucleation of an isolation membrane that forms a cup-shaped structure, termed a phagophore (Figure 1). Eventual elongation of the curved isolation membrane results in expansion of the phagophore into a sphere around a portion of the cytosol. The isolation membrane subsequently seals into a double-membraned vesicle termed the autophagosome (Figure 1), trapping the engulfed cytosolic material as autophagic cargo (Dikic and Elazar, 2018). Previous studies that utilized the autophagosome indicator green fluorescent-protein-light chain 3 (GFP-LC3) in vitro and in vivo demonstrated that autophagy is eminently maintained in neurons (Mizushima and Kuma, 2008). More recent studies have also revealed the distinctions of basal autophagy between non-neuronal and neuronal cells (Tsvetkov et al., 2009). For example, GFP-LC3-positive autophagosomes were rarely observed in normal neurons, as huge aggregations of autophagic vacuoles were observed under disease conditions (Lee, 2009).

Figure 1.

General process of autophagy.

At the begining of this process, cup-shaped phagophore is formed around the folded or aggregated proteins and with other cellular components, this is called nucleation. In the first step, the autophagic proteins (Atgs) such as Atg12, Atg 5, Atg 8, Beclin-1 (Bcl-1) and cargo materials are brought about through the ubiquitin-like conjugation systems Atg12-Atg5-Atg16L and Atg8 (LC3)-phosphatidylethanolamine (PE). In the second step, the expansion and maturation of the cup-shaped structure become rounded one and form autophagolysosomes which are double membraned vesicles with presence of LC3-I, LC3-II where the 3-methyladenine (3-MA) plays an inhibitatory role. In the third step, with an inhibitiory effect of bafilomycin A1 (Baf1), autophagosome is fused with lysosome and form single membraned autophagolysosome and this step is called fusion and autophagic vacuoles (AVs) and cytosolic proteins are seen. In the last step, the degradation of the autophagolysosome, with hyrolytic enzymes contributes to degradation of sequestered material, release of amino or fatty acids, and maintaineance of biogenesis.

After clearance of most Atgs and delivery along microtubules to the lysosome, the outer membrane of the autophagosome fuses with the lysosomal membrane to form an autophagolysosome (Figure 1). This fusion results in the release of a single-membrane autophagic body into the lysosomal lumen, followed by degradation of the autophagic body together with its cargo by the autolysosomal hydrolytic milieu (Abada and Elazar, 2014). Another study using mutant mice, in which Atg5 or Atg7 gene, specifically in the brain was deleted, showed the importance of basal autophagy in neurons (Komatsu et al., 2007). In the mutant mice, neurons lacked Atg5 or Atg7, and animals experienced continuous neurodegeneration (Koike et al., 2008). According to another experiment, rapamycin, which is involved in autophagy induction, conferred protection in animal models of neurodegenerative diseases (Sarkar et al., 2008).

Additionally, NIX, which is also known as beclin-2 (BCL2)/adenovirus E1B 19 kDa protein-interacting protein 3-like (BNIP3L), is transcriptionally upregulated in the period of reticulocyte maturation to erythrocytes. NIX/BNIP3L interacts directly with LC3B or Golgi-associated ATPase Enhancer of 16 kDa (GATE-16) via an LC3-interacting region (LIR), hence mediating the sequestration of NIX-expressing mitochondria via the growing phagophore (Novak et al., 2010). Interestingly, hypoxia also mediates the expression of NIX/BNIP3L and a related BH3 protein, BHIP3, demonstrating similar receptor-induced mitophagy mechanisms in injury-mediated mitophagy. Post-translational modifications also play a major role in mitophagy, permitting a more rapid response to hypoxic stress, as observed for the mitophagy receptor Fun14 domain-containing protein 1 (FUNDC1) (Lv et al., 2017).

Mitophagy is a selective form of autophagy that removes dysfunctional mitochondria and their harmful byproducts and oxidative species to help maintain homeostasis.

PINK1/PARKIN Pathway in the Regulation of Mitophagy

There are several pathways through which mitochondria are targeted for degeneration at the autophagosome; however, PTEN-induced putative kinase protein 1 (PINK1)/cytosolic E3 ubiquitin ligase PARKIN (PINK1/PARKIN-induced mitophagy is the most well-understood pathway regarding the maintenance of mitochondrial homeostasis in degenerative diseases (Kitagishi et al., 2017). PINK1 is a mitochondrial-targeted serine/threonine kinase that plays a protective role against mitochondrial dysfunction and apoptosis with mitochondrial quality control by activating PINK1/PARKIN-induced mitophagy (Figure 2) (Fivenson et al., 2017). The significance of PINK1 in the mitochondria is needed in cell-protective characteristics for combating oxidative stress (Eiyama and Okamoto, 2015). The role of PINK1 has been well-documented in neurodegenerative and aging-related diseases (Li and Hu, 2015).

Figure 2.

Protective roles of mitophagy.

Reactive oxygen species such as reactive oxygen species (ROS) and reactive nitrogen species (RNS) acts as a signaling mechanism to induce autophagy or mitophagy which has a role in bioenergetic pathway and protective roles in cell survival. On the other hand, an increase in mitochondrial fusion protects mitochondria through mitophagy. The PTEN-induced putative kinase protein 1 (PINK1) of mitochondria is destabilized by presenilin rhomboid like (PARL) and cytosolic E3 ubiquitin ligase PARKIN (PARKIN) ubiquitinates mitofusion (Mfn), voltage dependent protein channel 1 (VDAC1) etc. that results in elevated mitophagy. PINK1 with different proteins such as mitochondrial RhoGTase (MIRO), tumor necrosis factor receptor associated protein 1 (TRAP1), PARKIN combined with Mfn and voltage dependent anion channel 1 (VDAC1) has different inhibitory roles such as mitochondrial trafficking and mitochondrial remodeling. Clearance of damaged mitochondria by mitophagy enhances mitochondrial biogenesis and increases the rate of cell survival.

The cytosolic E3 ubiquitin ligase PARKIN and mitochondrial PINK1 have been implicated in the abnormal expression of genes associated with a recessive form of Parkinsonism (Schiavi and Ventura, 2014). However, the engagement of these proteins in the pathogenesis of PD remains unclear. Previous studies in Drosophila melanogaster have demonstrated that PINK1 and PARKIN function in the same genetic pathway to maintain mitochondrial network integrity (Greene et al., 2012). In healthy mitochondria, PINK1 is imported via the translocase complexes of the outer and inner mitochondrial membranes. PINK1 is then degraded by various proteases, such as mitochondria-processing protease (MPP⁺), the inner membrane presenilin-related rhomboid-like protease (Meissner et al., 2011). Following mitochondrial depolarization, PINK1 is translocated to the inner mitochondrial membrane, degraded, and sustained on the mitochondrial membrane (Lazarou et al., 2012). The aggregation of PINK on the mitochondrial surface induces mitophagy by volunteering PARKIN to degrade mitochondria via a mechanism that is not completely understood. Hence, PINK1 likely acts as a sensor for degraded mitochondria. Translocation of PARKIN to damaged mitochondria has been shown to weaken PINK1 function (Geisler et al., 2010).

As a consequence of its translocation, PARKIN ubiquitylates outer mitochondrial membrane proteins. Another adaptor molecule, such as p62, is then engaged to mitochondria to induce mitophagy. The mitochondrial fusion proteins mitofusion 1 (Mfn1) and mitofusion 2 (Mfn2) have been recognized as substrates of PARKIN (Rakovic et al., 2011), as illustrated in Figure 2. PARKIN hinders mitochondrial fusion via the degeneration of mitofusions, thus isolating damaged mitochondria from the healthy mitochondrial membrane proteins, such as the voltage-dependent anion channel (VADC), the mitochondrial RhoGTase (MIRO) 1 (Figure 2) and constituents of the mitochondrial translocase complex (TOM70, TOM40 and TOM20) (Yoshii et al., 2011). It is important to note that mitochondrial mobility is strongly maintained by the mitochondrial MIROs. MIRO1 and MIRO2 are both GTPases. MIRO function is essential for neuronal health: knockout of Miro1 in mice is lethal in the early postnatal period (Devine and Kittler, 2018).

Recessive mutations in PINK1 and PARKIN can cause PD and lead to a failure of mitophagy, causing mitochondrial damage (Kahle et al., 2009) and contributing to disease pathogenesis. Mitochondrial fission is also important for the function of neurons: dominant-negative dynamin-associated protein 1 (Drp1) mutation can cause a lethal infantile neurodegenerative phenotype. Drp1 knockout mice revealed embryonic lethality characterized by aberrant development of the brain and failure of synapse formation (Dagda et al., 2009).

Mitophagy and Neurodegeneration Diseases

The maintenance of mitochondrial physiology is essential for the nervous system because a disorder causes oxidative damage and many neurodegeneration diseases.

Alzheimer’s disease

AD is a chronic neurodegenerative disease characterized by the extracellular accumulation of β-amyloid (Aβ) (Wang et al., 2018). As the current therapies have limited effectiviness against AD, there is an urgent need for more research efforts concentrated at developing new agents for preventing the disease process (Aso and Ferrer, 2014).

ROS-mediated injury is observed in AD brains, and higher levels of malondialdehyde (MA) and 4-hydroxy nonenal (4-HNE) are observed in the brain (Figure 3) and cerebrospinal fluid of AD patients compared to controls (Butterfield and Lauderback, 2002). Transactive response DNA-binding protein of 43 kDa (TDP-43) pathology may be present as a comorbidity in approximately 20–50% of sporadic AD cases (Di et al., 2018). A recent study showed that resveratrol weakened Aβ_1–42-induced cell death and significantly increased mitophagy (i.e., increased the acidic vesicular organelle number, LC3-II/LC3-I ratio, Parkin and Beclin-1 (Bcl-1) expression and LC3 and TOMM20 co-localization in Aβ_1–42-treated PC12 cells) (Wang et al., 2018).

Figure 3.

Correlation of mitophagy and neurodegenerative diseases.

Upregulation or downregulation of autophagic and mitophagic function has a role in the development of many neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), Hungtington’s disease (HD), and amyotrophic lateral sclerosis (ALS). AD is caused by decreased mitophagic induction which causes higher levels of malondehyde (MA), 4-hydroxynonela (4-HNE), and Beclin-1 (Bcl-1), and increases amyloid beta (Aβ) aggregation and presenilin (PS1) mutations. PD is caused by loss of sequestration into autophagosome which causes loss of dopaminergic neurons in substantia nigra (SN) as well as knockdown of PINK1 expression or PARKIN mutations. ALS is caused by decreased lysosome or vesicle trafficking defects that result in formation of SOD1 formation and inpair axonal mitochondrial transport. HD is caused by mechanistic target of rapamycin (mTOR) sequestering into huntingtin protein (Htt) aggregates and oxidation of mitochondrial lipids which inhibits signaling that results in upregulation of mitophagy and polymorphism of autophagy related protein 7 (Atg7). It is shown that the dysfunction of mitochondria is responsible for generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) to cause malfunction in mitophagy which is vice versa.

Ultrastructural analysis revealed extensive dystrophy of virtually all neurites in the vicinity of and within β-amyloid deposits. There is also marked aggregation of vacuoles (mostly autophagic vacuoles [AVs] and smaller numbers of condensed mitochondria). The numbers of AVs in neuritic processes of AD brains have exceeded the incidence of AVs in cell bodies, although the AV numbers in neuronal Perikarya were also remarkably increased in AD (Nixon et al., 2005). On the other hand, AD may cause improper clearance of autophagosomes that contain both amyloid precursor protein (APP) and its processing enzymes, thus increasing the propensity to produce toxic Aβ peptides (Figure 3) (Butler et al., 2006). Aβ is transported to mitochondria where it interacts with mitochondrial proteins, causing an increase in ROS production, excess accumulation of mitochondrial Ca²⁺ and mitochondrial fragmentation, a decrease in the number of functionally active mitochondria and, ultimately, neuronal damage (Duchen, 2012). In an APP transgenic mouse model, the down- or up-regulation of Bcl-1 enhanced or decreased, respectively, extracellular Aβ aggregation and neurodegeneration, highlighting the importance of mitophagy in AD-associated pathology. Furthermore, a correlation between flavin adenine dinucleotide (FAD) and autophagy was currently noted, reporting that autophagy needs functional presenilin (PS-1) for lysosomal maturation, which is altered by Alzheimer-related presenilin 1 (PS-1) mutations (Lee et al., 2010). Hence, PS-1 alterations may indirectly affect mitochondrial function by impairing its recycling by mitophagy (García-Escudero et al., 2013).

An increasing number of studies have investigated the defensive aspect of mitophagy in various harmful situations, such as coenzyme Q10 (CoQ10) inadequacy, hypoxia (Zhang et al., 2008), and exposure to rotenone, thereby making mitophagy an appropriate target for therapeutic mediation. Similarly, injection of lentivirus-infected Bcl-1 in a mouse model of spinocerebellar ataxia type 3 (Machado-Joseph disease) elevated motor function and subsequently decreased protein accumulation (Hetz et al., 2013). Consistent with these results, haploinsufficiency of Bcl-1 promoted the advancement of experimental AD in vivo (Pickford et al., 2008). These phenomena were accompanied by the aggregation of p62, diminished levels of LC3-II and a modified equality between monomeric and oligomeric components of mutant superoxide dismutase 1 (SOD1) in the spinal cord (Nassif et al., 2014).

Using transgenic Drosophila expressing human tau, Iijima-Ando et al. (2012) demonstrated that RNAi-mediated Drosophila Miro (dMiro) knockdown enhanced human tau phosphorylation at the AD-related site Ser262 (phopgo-tau), resulting in enhanced levels of active PAR-1 and increased tau-mediated neurodegeneration. Moreover, knockdown of Miro generated late-onset neurodegeneration in the fly brain, an effect that was suppressed by knockdown of Drosophila tau or PAR-1 (Kay et al., 2018). Surprisingly, the heterozygous Miro mutation (miro[Sd32]) has been connected to mitochondrial mislocalization and the amyloid-β 42 (Aβ₄₂)-mediated onset of AD symptoms in an attenuated fly model (Kay et al., 2018).

AD is most prevalent in the elderly. It is defined by the accumulation of Aβ plaques and occurs when the normal mitophagic functions of the body are decreased; conversely, it produces ROS, which acts as an initiator of AD.

Parkinson’s disease

PD is the second most common progressive disorder of the CNS and is caused by a continuous loss of dopaminergic neurons (Tian et al., 2012). In dopaminergic neurons of the substantia nigra (SN), PD proteins such as Parkin, PINK1, DJ-1, and leucine-rich repeat serine/threonine-protein kinase 2 (LARRK2) as well as α-synucelin, play important roles in preventing cell death by maintaining normal mitochondrial function, protecting against oxidative stress, mediating mitophagy, and preventing apoptosis (Mukherjee et al., 2015). In addition to defective mitochondrial clearance, knockdown of PINK1 (Figure 3) also causes mitochondrial fragmentation followed by the activation of mitophagy (Dagda et al., 2009). A previous study also showed that oxidative stress is one of the most common causes of PD (Gaki and Papavassiliou, 2014).

Damaged mitochondria can also hinder movement via the PINK1-PARKIN-mediated degradation of MIRO1. MIRO1 turnover on degraded mitochondria is altered in fibroblasts from individuals with PD-related E3 ubiquitin protein ligase PARKIN (PARK2) mutations (Pickrell and Youle, 2015). The PD-related protein named Leucine-rich repeat kinase 2 (LRRK2) was recently shown to bind to MIRO1, inducing its degradation. A pathogenic mutation in LRRK2 impairs such binding, delaying the arrest and eventual removal of degraded mitochondria (Hsieh et al., 2016).

In a Drosophila PD model with loss of PINK1 function, weakened dMiro function improved the degenerative phenotype (as demonstrated in PINK1-mutant DA neurons). This result indicates a role for mitochondrial transport and Miro in PINK1-related PD pathogenesis (Pickrell and Youle, 2015), an idea further supported by the profound effects observed in altered PINK1 function or the transportation of axonal mitochondria in Drosophila larval motor neurons or mammalian hippocampal neurons (Kay et al., 2018).

Lee et al. (2018) reported that transgenic Drosophila melanogaster expressing fluorescent mitophagy affected PINK1/PARKIN mutations on basal mitophagy under physiological conditions. The author also showed that PINK1 and PARKIN are not essential for bulk basal mitophagy in Drosophila. More importantly, this is the first work to visualize mitophagy in fly models. The degree of/extent to which PINK1-triggered mitophagy is essential for mitochondrial quality control in the mammalian brain and the extent to which its deviated regulation is responsible for PD pathogenesis remain unclear (Chu, 2018). By contrast, a complementary study demonstrated the effect of PINK1 on the mito-QC reporter system in PINK1 knockout mice (McWilliams et al., 2018). The same study also showed that basal mitophagy is unaffected by the loss of PINK1 in most tissues (Lee et al., 2018).

Cardiolipin in mitochondria is redistributed to the surface of degraded mitochondria, where it engages LC3 to assist in the generation of autophagosomes centered on mitochondria termed mitosomes (Chu et al., 2013). In cardiolipin-mediated mitophagy, a cargo-targeting mechanism does not require PINK1 aggregation or PARKIN association with the mitochondria (Chu et al., 2013). Another study revealed that the Atg32 system in yeast cells, the association of LIR proteins such as BNIP3, BNIP3L/NIX, sequestosome 1 (SQSTM1), or FUNDC1, and the PINK1-PARKIN2/PARKIN pathway, which is defined by two proteins, are genetically linked to PD (Chu et al., 2014). However, according to another study, PINK1 along with PARKIN is not needed for receptor-induced mitophagy. A concurrent study reported that NIX compensated for the dysfunction of PINK1 or PARKIN in fibroblasts from PD patients (Koentjoro et al., 2017).

In general, defects in the formation of autophagosomes cause impaired mitophagy, which causes PARKIN mutations that further result in neurodegenerative disorders, such as PD (Figure 3). Moreover, AVs were recently observed in an experimental neurodegenerative model and in dying striatal neurons in PD; however, information on the extent to which autophagy is associated with neurodegeneration and its pathogenic significance is limited (Nixon et al., 2005).

In PD, the accumulation of α-synucelin in the SN, which results in excessive ROS, eventually impairs the normal mitophagic pathways to regulate the redox balance and homeostasis (Gaki and Papavassiliou, 2014).

Huntington’s disease

Motor deficits in HD patients are related to abnormal dopamine neurotransmission in the striatum (Vidoni et al., 2017). In HD, mitochondrial ROS production and oxidation of mitochondrial lipids play important roles in mitophagy (Johri et al., 2013).

In addition, it has been delineated that nitric oxide increases mitochondrial fission in neurons, initiating neuronal loss in a mouse model of stroke (Barsoum et al., 2006). In contrast, exhibition of Mfn or a dominant-negative Drp1 mutant in cultured neurons is defensive against oxidative insults. Apart from these, mechanistic target of rapamycin sequestration causes the aggregation of Huntington protein (Htt), which results in the upregulation of autophagy or polymorphisms in the autophagy-related gene Atg7 that further causes HD (Barsoum et al., 2006; Jahani-Asl et al., 2007) (Figure 3). Oxidative damage is found in the plasma of HD patients, HD postmortem brain tissue, lymphoblasts and cerebrospinal fluid (Khalil et al., 2015). In HD, degradation by autophagy is poorly understood, but the alterations in mitochondrial fission/fusion are likely to interfere with mitophagy, leading to the aggregation of degraded mitochondria in the cytoplasm. Martinez-Vicerte et al. (2010) showed that autophagosomes had a defect in cargo recognition that affects organelle sequestration by inducing autophagy, which may explain improper mitochondrial aggregation in HD cells. It was recently demonstrated that Htt is immensely associated with mitophagy by serving as a frame for both sequestosome 1 (SQSTM1/p62) and the autophagy-inducing kinase, UNC-51-like kinase-1 (ULK1), supporting the involvement of mutant Htt in these processes (Rui et al., 2015). In another study, dopamine-induced oxidative stress triggered apoptotic cell death in dopaminergic neuroblastoma SH-SY5Y cells that hyper-express mutant PolyQ Htt (PolyQ-Htt) protein. Dopamine toxicity was accompanied by impaired autophagy clearance of PolyQ-Htt aggregates. Dopamine also affected the stability and function of ATG4, a redox-sensitive cysteine protein associated with the process of LC3, a main step in autophagosome formation. Resveratrol, a dietary polyphenol with anti-oxidant and pro-autophagic characteristics, has demonstrated neuroprotective potential in HD (Vidoni et al., 2018).

Mitochondrial ROS plays an important role in the generation of HD, and abnormal ROS production imparts mitophagic dysregulation and fails to maintain the normal redox balance, resulting in impaired homeostasis.

Amyotrophic lateral sclerosis

ALS is a neurodegenerative disease affecting the spinal cord and brain motor neurons that ultimately leads to paralysis and early death (Mancuso and Navarro, 2017). Motor neuron death is caused by the dysfunction of mitochondria by directing them toward calcium-mediated excitotoxicity, by stimulating ROS generation and initiating the intrinsic apoptotic pathway (Julien, 2007). The particular mechanism of ALS is still under investigation because it is associated with cells other than neuronal cells. However, many lines of evidence suggest that huge amounts of autophagosomes and increased amounts of autophagic proteins and their activation are harmful for the survival of motor neurons. An increase in the LC3II macroautophagy marker protein and a decreased amount of phosphorylated mechanistic target of rapamycin-positive motor neurons revealed impaired mitophagy related to the loss of motor neurons in ALS (Okamoto et al., 2010). Various studies have reported dysfunctional Miro in ALS patients or animal models of the disease, including a report of significantly lower levels of Miro1 present in spinal cord samples of ALS patients (Zhang et al., 2015).

Mitochondrial fission and fusion hamper mitophagic clearance, which may also be affected by mutant SOD1 (Figure 3) (Albers and Beal, 2000). Glutathione (GSH) is a free radical scavenger tripeptide and acts as a main regulator of the intracellular redox state. GSH levels were lower in the motor cortex of ALS patients than those in the control volunteers (Weiduschat et al., 2014), and decreased levels of GSH result in neurological deficits and promoted the progression of mitochondrial pathology in the mutant SOD1 ALS mouse model (Vargas et al., 2011). Mutant SOD1 has been reported to impart Parkin-dependent degradation of MIRO1, which may explain the mitochondrial trafficking defect (Devine and Kittler, 2018). The same study also described Miro1-knockout mice, which exhibited upper motor neuron degeneration (Nguyen et al., 2014).

The expression of mutant TDP-43 in a motor neuron-like cell line induced oxidative species, mitochondrial disorder, and the accumulation of nuclear factor protein 2 (Nrf2), a modulator of oxidative species in a yeast model. TDP-43-expressing cells displayed increased markers of oxidative stress (Guareschi et al., 2012). Additionally, mitochondrial disorder was noticed, together with oxidative damage, as well as the induction of mitophagy in the mouse motor neuron-like cell line (NSC34) expressing wild-type or mutant TDP-43, representing a pathology resembling ALS. Moreover, motor neurons from these mice displayed cytoplasmic TDP-43-positive inclusions (Hong et al., 2012). In conclusion, lysosome or vesicle trafficking defects result in mutations in dynactin, which result in impaired mitophagy and ALS (Figure 3).

In a mouse model of motor neuron disease, full-length TDP-43 increased the involvement of mitochondria and blocked the TDP-43/mitochondria interaction, ameliorating mitochondrial TDP-43-interacting partners including VDAC1 and prohibitin 2 (PHB2), a vital mitophagy receptor (Davis et al., 2018). Mutant SOD1 impairs mitochondrial retrograde axonal transport (Magrané et al., 2013) along with mitochondrial network fragmentation, indicating the induction of mitophagy (Carrì et al., 2017).

Based on this review, we conclude that the loss of motor neurons and breakdown of the redox balance cause ALS in which ROS are an important component.

Conclusion and Future Perspectives

Mitophagy can prevent damaged mitochondrial aggregation and induce protective actions against cell demise. Clearing of degraded and aged mitochondria is an essential process for neuron survival. Focal mitophagy eradicates degraded mitochondria and decreases ROS-induced neuronal death (Kubli and Gustafsson, 2012). Li et al. (2014) demonstrated that rapamycin enhanced mitophagy, as evidenced by the increase in LC3-II and Bcl-1 expression in the mitochondria as well as p62 translocation to the mitochondria. Rapamycin decreased infarct volume, thus improving neurological outcomes, and decreased mitochondrial dysfunction compared with control animals. However, the mechanism by which rapamycin increases mitophagy should be further investigated (Li et al., 2014). In addition to 3-MA, other phosphoinositide 3-kinase (PI3K) inhibitors, such as bafilomycin and chloroquine, alter vascular and lysosomal pH and inhibit autophagosomal-lysosomal fusion, and E64 and pepstatin A prevent lysosomal protease activities. The prevention of autophagy usually leads to increased cell death; however, in some cases, autophagy leads to cytotoxicity. Investigating compounds that modulate autophagy and mitophagy will aid in the treatment of various diseases caused by oxidative protein modification aggregation within the cells (Zhang, 2013). It has been demonstrated in the abovementioned studies that mitophagy plays an important role in the course of neurodegenerative diseases by combating ROS in diseases such as AD, PD, HD, and ALS. We believe that by investigating different molecules that induce or inhibit mitophagy in vivo and in vitro, we can develop neuroprotective drugs.

Additional files:

Additional file 1: Database search strategy.

Additional file 2: Open peer review report 1 ^{(94.7KB, pdf)} .