**4. Conclusions**

Maintenance of mitochondrial function and energy/redox homeostasis requires both generation of newly synthesized and elimination of dysfunctional mitochondria. Taken together, age-dependent decline of mitophagy inhibits removal of dysfunctional or redundant mitochondria as well as impairs mitochondrial biogenesis. It results to progressive mitochondrial accretion and consequently, deterioration of cardiomyocytes function. At present it is pointed out that the steady-state amounts of structural damage accumulated during life are relatively small. However, changes in the redox state reflected by the decline in redox potential, increases in production of H2 O2 and level of protein modifications are significant and ubiquitous. Activities of ETC complexes revealed that the most affected throughout aging was complex IV, in contrast to relatively small age-related changes in complex II. These non-uniform changes in ETC enzyme complexes may lead to altered electron transfer through the chain, leading to impairment of ATP synthesis and overall functionality of cardiomyocytes during aging. The ability of the main regulatory Krebs cycle enzymes to supplement NADH to ETC was altered in senescent mitochondria. ICDH and KGDH had only half activity in senescent mitochondria; however ICDH was not affected in group of 14 months old. This might support promotion of H2 O2 in senescent cardiac myocytes where peroxiredoxin 2 ability to trap H2 O2 was also significantly affected. Decrease in GSH levels indicates low GSH/GSSG ratio leading to more oxidized environment in senescent cardiomyocytes but 14-month old hearts were able to maintain basal levels of adult mitochondria. Despite the variations in enzyme activities, the overall proteomic analyses revealed only 12 significantly affected proteins during aging. All these were more or less deprived but question is what is responsible for such a changes. Are these proteins really relevant? Is change in protein amount important? These and other questions are waiting for us and other researchers to be answered.

Specific marker of senescence was yet not identified, but evaluating a series of markers can help to define the senescent state. Senescent cells have greater proportion of protein content, which might be modulated by several PTMs. If these protein modifications are connected to functional consequences and protein-protein interactions are revealed, link may lead to the solution. Unfortunately, the proteome is much more complicated than the genome. To fit all the proteins together with their different characteristics in various pathways and cellular compartments is almost infeasible. At present, various nutritional, pharmacological and genetic interventions have higher or lower specificity, but are not devoid of potential problems that should be kept under attentive consideration. Collectively, these studies suggest that dysfunctional proteostasis has a causative role in aging and that restoration of protein homeostasis is protective against age-related diseases. New window is opened and hopefully with help of bioinformatics collecting huge amounts of data from proteomics, genomics and other scientific approaches will lead to personalized therapeutic procedures for individual patient and age-related disorder(s).
