**3.4. Turnover of mitochondrial proteins - role of mitophagy in cardiomyocytes**

Proper functioning of mitochondria is crucial for cardiac function. Damaged mitochondria produce less ATP, release greater amounts of ROS, and have a lower threshold for cytochrome c release resulting in apoptosis, undergo mitochondrial permeability transition pore opening resulting in necrosis or may release mitochondrial components into cytosol where are recognized by receptors for removal. Mitochondrial turnover is therefore an integral aspect of quality control in which dysfunctional mitochondria are selectively eliminated through autophagy or mitochondrial autophagy (mitophagy) and replaced through expansion of preexisting mitochondria (biogenesis). In the heart mitochondria turnover is with a half life of 14 days. Rat cardiomyocytes have roughly 1000 mitochondria per cell, suggesting that under basal resting conditions, one mitochondrion per cell is replaced every 40 minutes [73]. In order to facilitate and initiate mitophagy, mitochondrial fusion and fission play a critical role in mitochondrial turnover. Fission of mitochondria into smaller fragments is a crucial requirement for mitophagy to occur. The key regulator of this process is dynamin-related protein 1 (Drp1), which in concert with proteins fission 1 (Fis1), mitochondrial fission factor (Mff), mitochondrial dynamics proteins of 49 kDa (MiD49) and 51 kDa (MiD51) is responsible for mitochondrial fragmentation. The role of last three proteins appears essential. Mff assists in the assembly of Drp1. MiD49 and MiD51 may play a regulatory role by recruiting Drp1 and maintaining it in inactive state until fission is required [74]. Proteins that promote outer mitochondrial membrane (OMM) fusion such as Mitofusin 1 and 2 (Mfn1 and 2) are ubiquitinated and eliminated by the ubiquitin proteasome system. E3 ubiquitin ligase Parkin (also known as Park2) and PTEN-induced putative kinase 1 (PINK1) have been shown to play an important role in mitophagy. PINK1 targets to the mitochondria but is normally degraded by presenilin associated rhomboid-like protease (PARL). In response to loss of mitochondrial membrane potential, PARL is inactivated; PINK1 is stabilized and recruits Parkin. Parkin ubiquitinates several mitochondrial associated proteins and they are then recognized by p62 and bring mitochondria to the autophagosomes. Thus mitochondria with membrane potential loss can be selectively degraded. Parkin substrates include e.g. voltage-dependent anion-selective channel protein (VDAC), translocase of the outer membrane (TOM), mitochondrial fission 1 (FIS1), hexokinase, mitochondrial Rho-GTPase (MIRO) 1 and 2, although whether ubiquitination of these proteins is required or sufficient for mitophagy is unclear and highly dependent on the specific cellular context [75].

• Protein *S-Sulfenylation* leads to the production of sulfur-hydroxylation product (P-SOH), disulfide bond and sulfenyl-amide bond formation. It may be a precursor to the process of S-sulfinylation and S-sulfonylation. This type of modification is believed to be a fleeting

• *S-nitrosylation* occurs when NO is covalently incorporated into the Cysteine thiol group forming S-nitrosothiol (SNO). It plays important role in redox metabolism through GSH interaction in failing heart in rats [67] and human heart disease [69]. This PTM has been progressively implicated in virtually every NO-regulated process within the cardiovascular system. The current, widely-held paradigm is that S-nitrosylation plays an equivalent

• *S-glutathionylation* is covalent attachment of GSH to protein thiol groups. The function of protein S-glutathionylation reactions in metabolism is a rapid and reversible redox signaling mechanism that involves the conjugation and removal of glutathione from cysteine switches. Several observations have shown that unlike other redox modifications S-glutathionylation reactions fulfill the requisite criteria to serve as an effective PTM that controls protein function, links energy metabolism to redox signaling in mitochondria. Because of its role in modulation of ROS production in myocardial mitochondria, currently the usage of mitochondria penetrating antioxidants is discussed in context of the heart disease treat-

• During last decade *HNE modification* was under extensive research because of its dual role as pro- and anti-oxidant. This most abundant reactive aldehyde attacks predominantly nitrogen of histidine, lysine (less commonly arginine) or cysteine, and it is related to wide

Above mentioned PTMs occur the most frequently during oxidative stress-related aging process. However, there are virtually hundreds of additional PTMs that may occur in enzymes. To support results from protein profiling of mitochondria is important to focus on deep protein analyzes of individual selected proteins, their interactions within the individual compartments, between different organelles or the intercellular communication. For this purpose, proteomics is now integrated with molecular genetics, transcriptomics, and other areas lead-

Proper functioning of mitochondria is crucial for cardiac function. Damaged mitochondria produce less ATP, release greater amounts of ROS, and have a lower threshold for cytochrome c release resulting in apoptosis, undergo mitochondrial permeability transition pore opening resulting in necrosis or may release mitochondrial components into cytosol where are recognized by receptors for removal. Mitochondrial turnover is therefore an integral aspect of quality control in which dysfunctional mitochondria are selectively eliminated through autophagy or mitochondrial autophagy (mitophagy) and replaced through expansion of preexisting mitochondria (biogenesis). In the heart mitochondria turnover is with a half life of 14 days. Rat cardiomyocytes have roughly 1000 mitochondria per cell,

**3.4. Turnover of mitochondrial proteins - role of mitophagy in cardiomyocytes**

molecular switch that regulates non-enzymatic oxidative folding [68].

role as phosphorylation, providing a stable and controllable PTM [70].

ment [71].

74 Mitochondrial Diseases

range of metabolic diseases [72].

ing to systems biology strategies.

The importance of mitophagy for the preservation of cardiovascular homeostasis, the cardiomyocyte-specific deletion of Parkin and the expression of a mutant Mfn2 (mitofusin 2) at birth prevented the switch from fetal to adult mitochondria in the mice heart [76]. Another example, mice bearing a heart-specific deletion of Mfn2 prematurely succumbed to a progressive cardiomyopathy characterized by impaired contractile function [77]. Such a detrimental phenotype could be reversed, at least partially by Mfn2 to prevent the targeted mitochondria from rejoining the mitochondrial network through fusion. Under basal conditions Mfn2 functions in mitochondrial fusion events and links endoplasmic reticulum to mitochondria. Also acts as Parkin receptor during mitophagy following phosphorylation by PINK1 and recruiting Parkin to the mitochondria. So even in case of Parkin-dependent mitophagy, some outer mitochondrial membrane proteins are recycled through transfer to the endoplasmic reticulum. Most studies have relied on the systemic modulation of autophagy with nutritional or pharmacological interventions or the homozygous/heterozygous deletion of a relevant gene. Nutritional and pharmacological interventions commonly used to modulate autophagy in the cardiovascular system *in vivo*, including caloric restriction or caloric restriction mimetics, rapamycin, 3-methyladenine, and lysosomal inhibitors, are rather nonspecific. Genetic interventions offer increased specificity, but are not devoid of potential problems that should be kept under attentive consideration. Linking mitophagy with cardiomyocytes is the field of interest in many publications [78]. While mitophagy is responsible for bulk degradation of mitochondria, turnover of individual components may proceed at asynchronous rates through redistribution of components via fusion events, selective degradation of proteins by mitochondrial proteases, and proteasomal elimination of some outer mitochondrial membrane proteins. Some studies suggest that inner mitochondrial membrane proteins, especially oxidative phosphorylation constituents, may be primarily cleared via mitophagy. There is more to discuss, especially communication between intracellular organelles (mitochondria and endoplasmic reticulum) and oxidative phosphorylation events in mitochondria in relation to redox network, apoptosis/necrosis or mitophagy. These appear to be key players in cardiomyocytes survival during aging.

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

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

This work was supported by the project APVV-15-0107 and the project "Competence Center for Research and Development in the field of Diagnostics and Therapy of Oncological diseases", ITMS: 26220220153, co-financed from EU sources. We thank Mrs. Zdena Cetlova for

, Ivana Pilchova<sup>2</sup>

1 Department of Medical Biochemistry, Jessenius Faculty of Medicine in Martin, Comenius

2 Biomedical Center Martin, Jessenius Faculty of Medicine in Martin, Comenius University

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, Peter Racay1,2 and Peter Kaplan1,2

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

77

and age-related disorder(s).

**Acknowledgements**

technical assistance.

**Author details**

Zuzana Tatarkova<sup>1</sup>

**References**

\*, Martin Kolisek2

University in Bratislava, Martin, Slovakia

in Bratislava, Martin, Slovakia

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\*Address all correspondence to: tatarkova@jfmed.uniba.sk

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