Article

Mitochondrial Repair in Dry Age-Related Macular Degeneration

Mitochondria-targeting drugs have potential for future interventions.

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Age-related macular degeneration (AMD) is considered the leading cause of blindness among elderly people in developed countries. Although several treatment regimens, such as anti-angiogenic agents, photodynamic therapy, and laser treatments, are available for wet AMD (which represents about 10% of the AMD population), to date there are no FDA-approved therapies for dry AMD. Because the retina is one of the most metabolically active tissues in the human body, there is strong evidence that disruption in major mitochondrial metabolic pathways contributes to AMD pathogenesis.1,2 This article will review some of the possible approaches for prevention and treatment of retinal pigment epithelial (RPE) dysfunction and cell loss that are key factors in the development of dry AMD.

MITOCHONDRIAL DNA AND ITS HAPLOGROUPS

The mitochondria in eukaryotes are believed to have originated from symbiotic relationships of specific bacteria with cells.3 Mitochondria have their own DNA that is circular and transferred through maternal lineage. Mitochondrial DNA (mtDNA) is made of 2 strands, which encode for 37 genes.4 Cells have a single nucleus but, depending upon their level of metabolic activity, can have hundreds of mitochondria, each containing 5-10 copies of mtDNA. Mitochondria have crucial roles in oxidative phosphorylation, ATP production, apoptosis, and retrograde signaling. Scientists previously believed that the nucleus is the “commander” and mitochondria’s role is reflected mainly in ATP formation. It is now accepted that retrograde signaling from mitochondria to the nucleus can modulate pathways involved in complement activation, inflammation, apoptosis, and angiogenesis, which are important for the development and progression of many diseases, including AMD.5

The mtDNA can be categorized into different haplogroups based on accumulations of specific single nucleotide polymorphisms (SNPs). These haplogroups have been formed over 150,000 years and are associated with the origin of distinct geographic populations. Some haplogroups have been associated with age-related diseases such as Alzheimer disease, Parkinson disease, and AMD.6 We have shown previously that in AMD, the mtDNA SNP variants linked with H (protective) versus J (associated with AMD) haplogroups can influence differences in reactive oxygen species (ROS); ATP and lactate formation; cell growth rates; and expression for genes involved in inflammation, oxidative stress, and apoptosis.7,8

MITOCHONDRIAL DAMAGE IN AMD

Molecular genetic studies have shown that AMD patients can have mtDNA SNP variants in their retinas, some of which cause amino acid changes in the proteins. These protein changes could weaken energy production, releasing additional ROS and consequently further damaging the mtDNA in AMD subjects.1

Photoreceptors and RPE cells are very metabolically active and possess large numbers of mitochondria to meet their energy requirements.9 Aging is accompanied by deterioration in mitochondrial structure and a decline in their numbers. In addition, high levels of mtDNA lesions and fragmentation are found in AMD RPE cells compared to age-matched controls.2

Interestingly, these abnormalities are in the mtDNA but not the nuclear DNA of RPE cells and correlate with the severity of AMD.13 It has been speculated that damaged mitochondria in AMD can lead to increased superoxide production, which further impairs the proteins, lipids, and DNA, creating a vicious cycle of injury.10 As mitochondrial membranes are altered,2 the toxic products accumulate within mitochondria, which can modulate changes in nuclear gene expression.11 However, this is a “chicken and egg” scenario, because oxidative damage can also occur as a response to AMD. Whichever is the case, RPE cell damage is considered the hallmark of dry AMD.12

CYBRID MODEL

To better understand the mitochondrial–nuclear interactions, we have used an experimental model of RPE cytoplasmic hybrids (cybrids), which have identical nuclei but possess mitochondria from different individuals. These ARPE-19 cybrid cell lines are formed by eliminating natural mtDNA from the ARPE-19 cells and then fusing platelets isolated from different individual AMD patients (Figure 1). This versatile model helps elucidate the important role AMD mitochondria play in the disease process. We have found that AMD mitochondria are dysfunctional and, when they are placed into the cybrid cells, they will cause cell death at a higher rate than the age-matched normal mitochondria. This is an excellent model for screening drugs that might protect and rescue the AMD mitochondria. Because cybrid cell lines can originate from AMD patients that have well defined responses to therapies, disease severity, and other clinical variables, the cell lines may help us predict responses to treatment and/or identify new therapeutic modalities that prevent mtDNA damage in the AMD patients.14

Figure 1. Schematic drawing for creation of transmitochondrial cybrids by fusion of platelets (originating from AMD or control subjects) with RPE cells devoid of mitochondrial DNA.

POTENTIAL TREATMENT TARGETS FOR MITOCHONDRIAL REPAIR

Currently, there is no established treatment for dry AMD. Finding potential effective modalities that decrease mitochondrial damage could lead to a treatment regimen for dry AMD. Below we describe the potential interventions in AMD based on the pathogenesis and molecular mechanisms related to mitochondria (Figure 2).

Figure 2. Schematic drawing of potentially effective modalities (green boxes) for mitochondrial repair.

Some researchers believe that mitochondria can self-repair to conserve mtDNA architecture. TOP1 (mitochondrial topoisomerase) is a protein known to break and fix the mtDNA when it is tangled. But free radicals may complicate the repair process by trapping TOP1 proteins within the mtDNA. Increasing TOP1 inside the cells may improve and boost the mtDNA repair mechanisms.15

Reduced levels of the DNA repair enzyme 8-oxoG DNA glycosylase1 (OGG1)16 were observed in RPE cells from aged donors or AMD patients, causing RPE loss and atrophy. This repair mechanism correlated with AMD severity and affected mainly the macular region. Vulnerability of mtDNA to damage may be related to lack of protective histones and nonhistone proteins leading to less protection against oxidative stress. Targeting mitochondria with human OGG1 could be a novel therapy that would boost mtDNA repair efficiency and protect against apoptosis that leads to RPE atrophy.17

MTP-131 (a novel mitochondrial peptide) is a topical drug that has shown promising results in cell culture and in a hydroquinone mouse model of AMD. MTP-131 has a high affinity for cardiolipin, and it prevented hydroquinone-induced mitochondrial dysfunction, activation of biochemical injury pathways, and cellular functions associated with sub-RPE deposits.18 Currently, topical MTP-131 is being used in an open-label dose-escalation study to evaluate its safety in diabetic macular edema and dry AMD patients.

Heat shock proteins inside the mitochondria (mtHsp70 and mtHsp60) act as molecular chaperones to prevent cellular damage from unfolded nuclear-encoded proteins. These mtHSP mediate mitochondrial membrane trafficking and protein folding, which protects against oxidative stresses. The mtHsp70 is decreased in RPE cells in early stages of AMD.19,20 Because RPE cells are postmitotic cells with limited regenerative capabilities, regulation of apoptosis is considered one of the most important factors for prevention of dry AMD.

Mitochondrial-derived peptides (MDPs) are coded from open-reading frames (ORFs) within the mtDNA. These MDPs have neuroprotection,21 cytoprotection,22 anti-oxidant,23 and anti-inflammatory properties.24 Humanin (HN) is the best studied of the several known MDPs. Like a Russian nesting doll, the HN gene is located within the 16S rRNA gene within the mitochondrial genome.25 Its beneficial effects against Alzheimer disease, oxidative stress, and ischemia reperfusion stresses have been reported.

In RPE cells, HN improves mitochondrial functions by increasing ATP levels, reserve capacity, oxygen consumption rate, and proton leak.26 Humanin G, a single amino acid variant of Humanin protects damaged AMD mitochondria in vitro.27 In AMD cybrids, humanin G protected AMD mitochondria, reduced proapoptosis RNA and protein levels, and increased the protection against amyloid-beta-induced damage. These properties make HN a potential candidate for prevention of dry AMD.27

The 16S region of the mtDNA codes for 6 MDPs identified as small humanin-like peptides (SHLP). Among them, SHLP2, a 26-amino-acid peptide, protects against mitochondrial damage by modulating cellular and mitochondrial functions,28 leading to higher production of ATP and oxygen consumption. We have shown that mtDNA-encoded MT-RNR2 gene, which harbors protective MDPs including HN and SHLPs, was downregulated significantly in AMD cybrids.27

Other damaged regions in AMD involve mtDNA genes for multiple subunits of complex I (NADH dehydrogenase S1, S4, S5, S6) and complex III (cytochrome b). Damage to these 2 complexes may lead to decreased subunit levels, mutations, and thus reduced ATP formation.16

Mitochondrial translation factor Tu (Tufm) saves translation defects, and its concentration affects the production of mtDNA-encoded proteins. Upregulation of mitochondrial translation factor Tu has been detected at the initial stages of AMD.20

Mitochondriotropics are compounds with affinity for mitochondrial membranes, which improve mitochondrial functions.29 Omega-3 fatty acids and carnitine or acetyl-l-carnitine affect mitochondrial membrane composition and improve mitochondrial lipid metabolism.30-32

Mitofilin can stabilize disorganized cristae that cause malfunctioning fission and fusion.2 RPE cells from AMD patients have impaired mtDNA fission and fusion with increased levels of mitofilin.20 Mitofilin levels could be targeted for a potential therapy.

The risk allele for complement factor H, regulator of the alternative complement pathway, increases the risk of mtDNA damage. This suggests a possible site of intervention for those patients harboring the allele.33

Antioxidant agents, such as alpha-lipoic acid, alpha-tocopherol, genistein, resveratrol, memantine, MitoQ, and Mito-CP, may preserve mitochondrial function by decelerating the progression to blindness in AMD patients.34-36

Idebenone, coenzyme Q10, creatine, EPI-743, and quinone analogues such as SkQ1 or SkQR1 regulate energy metabolism. These have shown promise for AMD treatment.37-39

Cyclosporine A prevents apoptosis by stabilizing mitochondrial membrane permeability, and the drug MDIV-1 inhibits the mitochondrial fission protein Drp1. Thus, these agents may have protective effects.40-44 Also, peroxisome proliferator-activated gamma coactivator 1-alpha (PGC-1α) and its analogues may be considered treatment targets for AMD by protecting RPE cells against ROS.45

“Epigenetics” describes changes in the genome that can be inherited but are also subject to environmental factors. Recently an association between levels of DNA methylation and acetylation and different mtDNA haplogroups has been shown. It seems that epigenetic changes in mtDNA could lead to long-term “metabolic memory,” which causes pathologic conditions associated with AMD.46

Mitochondria-targeting drugs present great potential for future interventions in patients with dry AMD. Mitochondria not only should be considered as the energy producing factory in highly active cells such as RPE, but also are of great importance to vital functions that promote cell survival and metabolism regulation. RP

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