| htt |
HD |
Although largely cytosolic, htt is also present on outer mitochondrial membrane (OMM). htt expression correlated with elevated lactate levels, decreased mitochondrial membrane potential (MMP), decreased respiratory function through complex II and defects in mitochondrial calcium uptake, reduced mitochondrial mobility and mitochondrial ultrastructural changes. Rat cortical neurons treated with 3-nitropropionic acid have fragmentation and condensation of mitochondria that can be prevented by antioxidant treatment. In HeLa cells overexpressing mhtt with a 74-glutamine repeat, there is fragmentation of mitochondria, reduced mitochondrial fusion, reduced ATP, and increased cell death. Expression of either dominant-negative profission protein Drp1, or profusion protein Mfn2, not only prevents this change in mitochondrial morphology but also restores ATP levels and attenuates cell death. mhtt binds more tightly to Drp1 on mitochondria and triggers mitochondrial fragmentation. mhtt increases Drp1 enzymatic activity, which results in defective anterograde mitochondrial movement and synaptic deficiencies. |
| APP |
AD |
APP accumulates in the protein import channels of mitochondria from human AD brains where it was found to be stably associated with translocases of the outer and inner mitochondrial membrane (TOM40 and TIM23). Exposure of neuronal cells to conditioned medium from cells stably expressing mutant forms of APP leads to increased mitochondrial fission, loss of dendritic spines, and eventually cell death. This increase in mitochondrial fission was traced to elevated levels of S-nitrosylated Drp1. Increased levels of S-nitrosylated Drp1 were found in brain samples from patients with AD and AD mouse models. APP overexpression in M17 neuroblastoma cells resulted in predominantly fragmented mitochondria, decreased levels of Drp1 and OPA1, and a defect in neuronal differentiation. Overexpression of Drp1 or OPA1 could partially rescue different aspects of these defects. |
| Presenilins, PS1 and PS2 |
AD |
Presenilins form the γ-secretase complex that together with β-secretase cleaves APP to produce Aβ. PS1 and PS2 are enriched in mitochondria-associated membranes, which is a specialized subcompartment of the endoplasmic reticulum involved in lipid metabolism and calcium homeostasis. PS1 presence was also reported in mitochondria. Knockout of PS2 in contrast to PS1 results in reduced mitochondrial function in vivo. PS2 increases endoplasmic reticulum-mitochondria tethering, which could result in a chronic, toxic mitochondrial Ca+2 overload. |
| Aβ |
AD |
Mitochondrial accumulation of Aβ has been shown in patients with AD, APP transgenic mice, and cellular models of AD. Mitochondria localized Aβ was shown to inhibit Aβ binding alcohol dehydrogenase, disrupt mitochondrial permeability transition pore functions, and impair the respiratory chain complex III and IV. Intracellular accumulated Aβ induces age-dependent changes, including depletion of presynaptic mitochondria, slowdown of bidirectional transports of axonal mitochondria, decreased synaptic vesicles, increased large vacuoles, and elevated synaptic fatigue. Cells overproducing Aβ showed impairment of mitochondrial function such as reduced mitochondrial respiration, strongly altered morphology, and reduced intracellular mobility of mitochondria. Antioxidants reduced Aβ production and rescued mitochondrial function. Aβ increases expression of mitochondrial fission genes and reduces the expression of fusion genes in vitro. |
| Tau |
AD/PD |
Tau mediates the effects of Aβ on axonal transport and a reduction of tau protects against Aβ-induced axonal transport abnormalities. Hyperphosphorylated/overexpressed tau impairs axonal transport of mitochondria. N-terminal truncated tau localizes to mitochondrial membranes and was found to be highly enriched in mitochondria from cryopreserved synaptosomes of human AD brains. |
| α-Synuclein |
PD |
α-Synuclein is present predominantly on OMM in mouse brain and on inner mitochondrial membrane in human brain. Higher accumulation of α-synuclein was reported in mitochondria from striatum and substantia nigra of PD patients compared with normal subjects. Accumulation of α-synuclein in the mitochondria of human dopaminergic neurons reduced mitochondrial complex I activity and increased production of ROS. N-terminal 32 amino acids of α-synuclein were shown to function as a targeting sequence for the import of α-synuclein into mitochondria in an energy- and import channel-dependent manner. Mitochondrial association of α-synuclein in cells was linked to oxidation of mitochondrial proteins and increased levels of calcium and nitric oxide. Mitochondrial abnormalities were observed in transgenic mouse models overexpressing wild-type or mutant α-synuclein: selective oxidation of mitochondria-associated metabolic proteins; degenerating mitochondria containing α-synuclein; reduced complex IV activity; mitochondrial DNA damage; and increased mitochondrial pathology after treatment with MPTP. Studies in α-synuclein knockout mice suggest that α-synuclein controls synaptic vesicle dynamics and may regulate mitochondrial membrane lipid composition and complex I activity. |
| Parkin |
PD |
Parkin is a largely cytosolic E3 ubiquitin ligase, which, along with PINK1, is involved in mitochondrial quality control pathways, i.e. mitophagy. Although primarily cytosolic, Parkin is recruited to mitochondria under conditions of bioenergetic stress/loss of mitochondrial membrane potential such as in presence of chemical uncouplers and OXPHOS inhibitors; by relatively long exposure to oxidative stress (after treatment with paraquat); chronic loss of mitochondrial fusion, or loss of mtDNA integrity. Loss of Parkin in Drosophila melanogaster not only causes degeneration of flight muscles and dopaminergic neurons, but their mitochondria become dysmorphic and dysfunctional with less efficient OXPHOS and increased ROS production. Parkin ubiquitinates OMM proteins, voltage-dependent anion channel 1 in mammals and Marf (an ortholog of mammalian mitofusins) in D. melanogaster. Ubiquitination (therefore elimination/inhibition) of Marf by Parkin promotes mitochondrial fission. |
| PINK1 |
PD |
PINK1 gene encodes a mitochondrially localized serine/threonine kinase. PINK1 is normally kept at low levels on the OMM of healthy mitochondria. However, it is also imported into mitochondria and rapidly and selectively accumulates on the outer membrane of mitochondria that have lost membrane potential. Mutations in the PINK1 and Parkin genes result in enlarged or swollen mitochondria. D. melanogaster null for Pink1 have the same unusual phenotype as Parkin knockout flies. PINK1 was associated genetically with proteins involved in mitochondrial morphogenesis such as HtrA2, Drp1, and Opa1/Mfn2. PINK1 acts upstream of Parkin, and expression of PINK1 is necessary for the recruitment of Parkin to depolarized mitochondria. Parkin recruitment tags the impaired mitochondria for degradation (mitophagy). Mice null for Pink1 or Parkin exhibit synaptic dysfunction in neurons projecting to the striatum, and this synaptic dysfunction correlates with progressive loss of mitochondrial function and increased oxidative stress in the striatum with age. Loss-of-function mutations in PINK1 or Parkin have also been associated with mitochondrial dysfunction in cells from patients with familial forms of parkinsonism PINK1 phosphorylates Miro (which links mitochondria to microtubules via kinesin and Milton, for transport), resulting in Parkin-dependent proteasomal degradation of Miro. This prevents mitochondrial movement and probably quarantines damaged mitochondria before clearance by mitophagy. Phosphorylation of Drp1 and the mitochondrial intermembrane proteins, TRAP-1 and HtrA2/Omi, also depends on PINK1. HtrA2 phosphorylation increases its proteolytic activity and may serve to proteolytically degrade misfolded mitochondrial proteins to maintain mitochondrial integrity. HtrA2 phosphorylation is decreased in brains of patients with PD carrying mutations in PINK1. |
| DJ-1 |
PD |
DJ-1 protein is located mostly in the cytosol, and only a fraction is present in the nucleus and mitochondria, where it preferentially partitions to the matrix and intermembrane space of mitochondria and it impairs degradation. Upon oxidative stress, DJ-1 protein rapidly translocates to the mitochondria and, to a lesser extent, to the nucleus, acting as a neuroprotective intracellular redox sensor. DJ-1 and its mutants were found to be strongly associated with Hsp70, the cochaperone CHIP, and a mitochondria-resident Hsp70 complex in patients with PD. In vitro, association of wild-type DJ-1 with mitochondrial Hsp70 was further increased under oxidative stress, indicating that the translocation of DJ-1 to mitochondria may occur by binding to mitochondrial chaperones. DJ-1 protects dopaminergic neurons from oxidative stress by stabilizing Nrf2 and through up-regulation of glutathione synthesis and from the toxic consequences of A53T α-synuclein through increased expression of Hsp70. DJ-1 knockout in fly and mice produced decreased mtDNA levels, respiratory control ratio, and ATP levels. DJ-1-deficient mice are more sensitive to MPTP-induced loss of dopamine neurons. Loss of DJ-1 in flies and mice was associated with increased sensitivity of mitochondria to complex I inhibitors and oxidative stress. In vitro, reduction of DJ-1 was associated with lowered MMP, an increase in mitochondria fragmentation, autophagy and oxidative stress, and reduced mitochondrial fusion. DJ-1 was also found to directly bind to the mitochondrial complex I subunits; and loss of function of DJ-1 led to a decrease of mitochondrial complex I activity, but not complex III; whereas overexpression of DJ-1 conferred protection against the complex I inhibitor MPTP. DJ-1 inhibits the aggregation and toxicity of α-synuclein by increasing Nrf2, a direct interaction is not known; DJ-1 binds to PINK1 and increases PINK1 levels under conditions of PINK1 overexpression; DJ-1 and Parkin interact under conditions of oxidative stress. DJ-1 activates transcription of the Mn-SOD gene, which encodes an essential mitochondrial antioxidant enzyme. |
| LRRK2 |
PD |
LRRK2 immunoreactivity was shown to partially overlap with mitochondrial and lysosomal markers in the mammalian brain, and ultrastructural analysis revealed that LRRK2 is associated with intracellular membranes, including lysosomes, transport vesicles, and mitochondria. LRRK2 can bind to the OMM in mammalian brain. Approximately 10% of wild-type and mutant LRRK2 were present in the mitochondrial fraction in cells overexpressing the proteins. Patients with a G2019S mutation had a decrease in MMP and ATP. Overexpression of G2019S mutant LRRK2 in differentiated human neuroblastoma cells caused neurite retraction and shortening, which correlated with increased autophagy. LRRK2 mutations increase the kinase activity of LRRK2, which results in increased neurotoxicity. Overexpression of either wild-type or mutant (R1441C or G2019S) LRRK2 caused mitochondrial fragmentation, reduced mitochondrial fusion, and increased Drp1 recruitment to mitochondria by direct interaction with LRRK2 in vitro. |
| SOD1 |
ALS |
Wild-type SOD1 is predominantly cytosolic; however, a small fraction resides in mitochondria. SOD1 and its copper chaperone, CCS, enter mitochondria through the Erv1/Mia40 oxidative folding mechanism of import, which involves a formation of mixed disulfide bonds between CCS and Mia40, and is sensitive to oxygen concentrations. Wild-type SOD1 helps in the prevention of the oxidation of mitochondrial proteins and thus in the preservation of mitochondrial homeostasis. Mice lacking SOD1 show a distal motor neuropathy accompanied by decreased mitochondrial density and increased oxidative stress in mitochondria, which is rescued by SOD1 targeted to the mitochondrial intermembrane space. A proportion of SOD1 mutant protein is misfolded onto the cytoplasmic surface of mitochondria, and the axonal mitochondria of motor neurons are the primary in vivo targets for misfolded SOD1. A small fraction of the enzyme resides in various mitochondrial subcompartments, mutant SOD1 to a greater extent than wild type. Mutant SOD1 alters axonal mitochondrial morphology and distribution. Somal mitochondria are altered by mutant SOD1, with loss of the characteristic cylindrical, networked morphology and its replacement by a less elongated, more spherical shape. Mutant SOD1 motor neurons have impaired mitochondrial fusion in both the axons and cell bodies; there is selective impairment of retrograde axonal transport, smaller mitochondrial size, decreased mitochondrial density, and defective MMP. Expression of mutant SOD1 elicits a clear deficit in the electron transport chain, mishandling of mitochondrial calcium, increased production of ROS, and activation of the apoptotic pathway. Overexpression of mutant SOD1 induces the activation autophagy, as measured by the activation of LC3 (microtubule-associated protein1 light chain 3) and increases the association of PINK1 with mitochondria. |
