**2. Cellular senescence and mitochondria**

is a complex phenomenon with a large impact on society. Aging is recently an emerging topic since the life expectancy is rising and because aging itself is the basis for the development of age-related diseases such as cardiovascular disease (CVD), cancer, neurodegenerative diseases and degenerative metabolic diseases (e.g. *diabetes mellitus* type 2) [3]. Since age is the largest risk factor for CVD, the prevalence of these ailments increases dramatically with increasing age. Over 80% of all cases of coronary artery disease and more than 75% of those of congestive heart failure are observed in elderly patients [4]. In order to improve prevention and care for patients, it is important to analyze processes linked with cardiac aging. Prevalence of high blood pressure, obesity and metabolic syndrome correlate with age, and all these conditions facilitate the development of cardiomyopathies that are the major cause of chronic disability, morbidity and mortality in the elderly. Although the long-term exposure to cardiovascular risk factors plays a major role in the etiopathogenesis of CVD and neurodegeneration, intrinsic alterations in the heart and the vascular system occur during lifespan and render the cardiovascular system more vulnerable during senescence. The possible link between aging and senescence was first described in 1961 [5] and it based on the inability of telomeres to sustain their lengths. In addition to this, other events and signals were identified including non-telomeric/genotoxic stress generated by various signals, such as mitochondrial deterioration, oxidative stress, DNAreplication "stress" or activated oncogenes [6]. Several lines of evidence indicate mitochondrial dysfunction to be a major contributor to cardiovascular senescence. Damaged mitochondria are bioenergetically less efficient and they are producing excessive amounts of reactive oxygen species (ROS) with detrimental structural and functional consequences [7]. The ROS impair excitation-contraction coupling, cause arrhythmias, and contribute to cardiac remodeling by inducing cardiac hypertrophy, apoptosis, necrosis, and fibrosis [8]. However, antioxidant interventions in patients with CVD yielded only disappointing

The accumulation of abnormal/dysfunctional mitochondria is usually a consequence of impaired clearance of damaged organelles by autophagy and inadequate replenishment of the cellular mitochondrial pool by mitochondriogenesis [7]. Autophagic flux is generally decreased in aging hearts. Murine loss-of-function models for autophagy develop exacerbated cardiac dysfunction that is accompanied by accumulation of misfolded proteins and dysfunctional organelles. On the other hand, stimulation of autophagy in mouse models improves cardiac function and enables to study a protein aggregation by removing accumulated misfolded proteins, dysfunctional mitochondria, and damaged DNA, thereby alleviating aging-associated pathology in the heart. Multiple lines of evidence suggest that autophagy is required for many mechanisms that mediate lifespan extension, such as caloric restriction. These results are pointing out the possibility that autophagy may play an important role in combating the adverse effects of aging in the heart [10]. At the molecular level, the aging process is associated with accumulation of damaged proteins and organelles, partially due to defects in protein quality control systems. Since most cellular functions are performed by proteins, aging may be, in part, the consequence of a deregulation or malfunction of the cellular proteome [11]. Modern techniques enabled the investigation of the internal structure and morphology of mitochondria and revealed a highly complex compartmentalization [12]

results so far [9].

62 Mitochondrial Diseases

Cells undergo widespread changes and develop specific characteristics during senescence that are considered as senescence markers. However, no individual marker has been so far identified as entirely selective parameter for cellular senescence. Nevertheless, a combination of several markers might be evaluated and can help to define the current stage of senescence. Phenotypically, the increase in size and protein content was reported in senescent cells [16, 17] which is in agreement with our results. The data in **Table 1** show that the total protein concentration was elevated in senescent (27 months old) rat mitochondria by 40% (p < 0.01).


Values are expressed as Mean ± SEM of 5 individual experiments, \*\*p<0.01; significantly different in comparison to 6 months old rats.

**Table 1.** Protein concentration in homogenate and mitochondria during aging in heart (yet unpublished data).

However, the protein content in homogenates as well as in isolated mitochondrial fraction was maintained during overall aging process comparing the samples from old (14 months old) and adult (6 months old) rat hearts.

PTMs hamper electron flux along the ETC to provoke excessive O<sup>2</sup>

pathway [32].

rats.

by NADH dehydrogenase (complex I) and Cytochrome c reductase (complex III) causing functional uncoupling of the respiratory chain. The extent and way of individual ETC complexes inhibition is different. According to **Figure 1**, the decline in activity of complex I and cytochrome c oxidase (complex IV) was more obvious during aging when compared to the succinate dehydrogenase (complex II) and complex III activities. Literature data are inconsistent among the studies, mainly due to differences in the experimental age groups and animal models, isolation/purity of mitochondria or enzyme substrate/inhibitor used for study. Complex I is considered to be the most important player in the game of ROS production and/ or proton-motive cascade. Loss of its activity has been attributed to the mutations in mitochondrial DNA (mtDNA) in aging animals and recently was linked to the apoptotic cell death

Interplay Between Mitochondrial Proteins and Age-Associated Risk of Cardiovascular Diseases

Reduced complex I activity was seen in the rat heart [33], brain synaptic mitochondria [34] as well as continual decrease in the frontal cortex of Parkinson disease patients [35]. It is important to note that the activity and stability of this respiratory complex are determined by its abundance, PTMs and/or specific protein–protein interactions. In contrary, other studies have reported no age-related decrease in complex I activity [36]. Most of the inconsistencies are related to the complexes II, III and IV, where activities of respiratory complexes have been also shown to decline [37], remain unchanged or increased during aging [38]. Our data show age-dependent decrease in all ETC enzyme activities, although the extent of inhibition is different (**Figure 1**). Among them, complex IV was most affected throughout aging and reached 63.4% of adult respiring mitochondria. While complexes I, II and III maintained activities in 14 months old rat mitochondria, cytochrome c oxidase showed deprivation by

**Figure 1.** Activities of ETC complexes in heart mitochondria during aging (yet unpublished data). Values are expressed as Mean ± SEM of 5 individual experiments, \*\*p < 0.01, \*\*\*p < 0.001; significantly different in comparison to 6 months old

•− formation [31] mostly

65

http://dx.doi.org/10.5772/intechopen.71789

Further, senescent cells exhibit enlarged nuclei and lysosomes, which possess elevated senescence-associated β-galactosidase activity (SA-β-Gal), the most widely used marker [17]. They also enter a proliferative arrest state, detected by cell cycle inhibitor levels such as p53/p21, tumor suppressor p16INK4a [18, 19] and markers of proliferation like Ki-67 and 5-bromodeoxyuridine [20]. Other factors secreted during senescence are cytokines, chemokines, growth factors, proteases, fibronectin as well as ROS and reactive nitrogen species (RNS). Additionally, proteostatic changes during senescence accompanied by an increase in modified proteins, accumulation of protein aggregates and reduced functionality of the proteasomal and autophagy systems [3] will be discussed in following chapters. Only two parameters currently correlate with species longevity in the right sense: the mitochondrial rate of ROS production and the degree of fatty acid unsaturation of tissue membranes. Their basal level is in both cases low in long-lived animals. In addition, the best-known manipulation that extends longevity, dietary restriction, also decreases the rate of mitochondrial ROS production and oxidative damage to mtDNA [21]. The available information supports a mitochondrial free radical theory of aging focused on low generation of endogenous damage and low sensitivity of membranes to oxidation in long-lived animals.
