2.3. Mitochondrial Dysfunction and Oxidative Stress

The dysfunction of mitochondria has long been regarded as a ma- jor mechanism contributing to aging and functional decline.[25] The accumulation of somatic mutations in genes (both nu- clear and mitochondrial) encoding proteins of mitochondrial respiratory chain or proteins involved in quality control mech- anisms leads to impaired oxidative phosphorylation, resulting in decreased adenosine triphosphate (ATP) synthesis and in- creased production of reactive oxygen species (ROS).[26] The ob- ≈ served decline in ATP production with age ( 8% per decade) causes energetic deficiency and likely contributes to lower phys- ical performance in elders.[26] Elevated intracellular levels of ROS together with an imbalance in antioxidant defenses provoke oxidative stress known to be involved in the development of many pathological conditions.[27] Recent studies have suggested a more complex relationship between oxidative stress and aging, as some low level of ROS may provoke signaling pathways associated with

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                                         se sae, Increased cellGenomic instability &                              : senescencemutation Quiscentcell 8-gall activityDysregulated Double strand break foctnutrient sensing Telomere ROS & oxidative byproductslength g Telomere lengthmTOR AMPK SASP factorsSIRTs IIS‘ DNA damage *.markers ) ° % ©Histone ™~, © srutationsmodificationSenescentcellEpigenetic modificationDNA methylation in CpG sitesDecreasedstemnessStem cell numberLoss of proteostasis   Misfolded proteins& aggregatesProteostasis networkmachinery                Mitochondrial dysfunction &oxidative stress                    Protein DNA/RNA Lipidoxidation oxidation —_peroxidationproducts products productsROS AGEs/ALES D  Altered intercellularcommunicationInflammatory factorsHormonesCirculating miRNAIntercellular matrix markers

Figure 1. Overview of molecular and cellular biomarkers of aging. Accumulation of molecular damage within a cell due to genomic instability and mutations, epigenetic modification, loss of proteostasis, mitochondrial dysfunction, and oxidative stress lead to loss of normal cell function and altered proliferation. As a result, cells can undergo a transition into an irreversible non-proliferative senescence state which—if the cells that became senescent с were stem cells—contributes to decreased stemness. Besides a senescence state, ells can go into a reversible non-proliferative quiescent state or die by various cell death mechanisms. Altered paracrine and endocrine communication, as well as intercellular matrix remodeling, also affect cell functioning.

formed due to direct oxidation of amino acid side chains by ROS or generated in chemical reactions with oxidative by-products.[30] Protein derivatives of side chains modifications, such as o- tyrosine, m-tyrosine, 3-chlorotyrosine, and 3-nitrotyrosine, also have been linked to various age-related pathologies.[30]

longevity[26,28]; moreover, most strategies to mitigate ROS have not led to lifespan extension in animal models.[28,29]

Oxidative stress can be measured directly by ROS or indirectly by specific chemical derivatives resulting from ROS-induced modification of biological macromolecules (nucleic acids, pro- teins, carbohydrates, and lipids), which are typically more stable than ROS molecules and presented in detectable quantities in biological fluids and tissues. The predominant products of ′ oxidative damage of nucleic acids are 8-oxo-2 -deoxyguanosine ′ (8-oxodG), 8-hydroxy-2 -deoxyguanosine (8-OHdG) and 8- hydroxyguanosine (8-OHG), associated with many age-related conditions, including arthritis, osteoporosis, atherosclero- sis, hypertension, cataracts, diabetes, and neurodegenerative diseases.[30] A reliable marker of lipid peroxidation is 8-iso prostaglandin F2𝛼 (F2-isoprostane), which is a peroxidation product of arachidonic acid or other polyunsaturated fatty acids presented in phospholipid membranes.[31] Similar to DNA damage markers, F2-isoprostane concentration increases with age and high levels are observed in a number of diseases com- monly encountered in elderly patients.[30] The most abundant markers of protein oxidative modification are protein carbonyls,

Proteins, lipids, or nucleic acids possessing free amino groups can non-enzymatically react with reducing sugars, and via a chain of intermediate reactions form cross-linked and non-degradable aggregates, termed advanced glycation end products (AGEs). Among the most representative and intensively studied AGEs are carboxymethyl-lysine, carboxymethyl-valine, and pentosidine.[32] AGEs accumulate slowly in human tissues during normal ag- ing but may accelerate when blood glucose levels are elevated, suggesting that AGEs contribute to the pathophysiology of dia- betic complications.[33,34] Some AGEs exhibit fluorescence, pro- viding opportunities for the noninvasive detection of AGEs depo- sition in human tissues.[32] Advanced lipoxidation end-products (ALEs) formed by a non-enzymatic reaction between macro- molecules and reactive aldehydic compounds, arising from lipid peroxidation, may also be used as an indicator of oxidative stress.[35]

proteins [IGFBPs]), extracellular proteinases and their inhibitors (matrix metalloproteinases [MMPs], tissue inhibitors of metallo- proteinases [TIMPs], plasminogen activator inhibitor-1 [PAI-1]) are among candidate biomarkers of aging.[37,47] Monitoring SASP factors can constitute a promising and feasible approach for the evaluation of general tissue or cell culture senescence status[48]; however, the profile of secreted proteins may vary significantly depending on cell type and stage of senescence,[37,49] complicat- ing the determination of optimal threshold values for all types of cells.