Mitophagy

In mammals

In mammalian cells, there are several mechanisms for the activation of mitophagy. The best studied pathway is PINK1 and parkin. This path begins by recognizing the differences between normal and damaged mitochondria. PTEN-induced kinase 1 (PINK1) is involved in determining the degree of mitochondrial damage. This protein contains an amino acid sequence that attracts it to mitochondria. In normal mitochondria, PINK1 is imported through the outer mitochondrial membrane through the complex and partially passes through the inner mitochondrial membrane through the TIM complex, so that it stops at the position penetrating the inner mitochondrial membrane. Import through the inner membrane is associated with cutting PINK1, so that its mass decreases from 64 kDa to 60 kDa. Then, with the participation of the protein, it is transformed into a protein of 52 kDa mass. The new form of PINK1 is degraded by mitochondrial proteases. Thus, the concentration of PINK1 in healthy mitochondria is controlled ^[5] .

In damaged mitochondria, the inner mitochondrial membrane becomes depolarized . The potential of this membrane is important for TIM-mediated protein imports. In the case of a depolarized membrane, PINK1 cannot pass through the inner mitochondrial membrane and is not cut by the PARL protein, so that the concentration of PINK1 in the outer membrane increases. PINK1 can then recruit a parkin. PINK1 is believed to phosphorylate ubiquitin attached to the remainder of serine 65 of parkin, and because of this, parkin is recruited to mitochondria ^{[6] [7]} . Parkin is a cytosolic E3 ubiquitin ligase ^[8] . When parkin enters the mitochondria, PINK1 phosphorylates parkin at the S65 residue (to the same site where ubiquitin was attached), and as a result of this phosphorylation, parkin dimers and activates. Parkin can now attach ubiquitin to other proteins ^[6] .

Since PINK1 attracts parkin to the surface of mitochondria, parkin can ubiquitinate the proteins of the outer mitochondrial membrane, including the proteins / and . Mitochondrial surface ubiquitinylation is one of the factors triggering mitophagy. Parkin launches the addition of ubiquitin chains to the lysine residues K63 and K48. K48 ubiquitinylation triggers protein degradation and can lead to passive destruction of mitochondria. K63 ubiquitinylation leads to the involvement of mitophagic adapter proteins MAP1LC3A / , which eventually trigger mitophagy. It is not known which proteins are necessary and sufficient for mitophagy, and how these proteins, when subjected to ubiquitinylation, trigger mitophagy.

Other ways of triggering mitophagy use mitophagy receptors on the side of the outer mitochondrial membrane facing the cytoplasm ^[9] . Mitophagy receptors include the NIX1, and proteins. All of these receptors contain a LIR that binds to MAP1LC3A / GABARAP, resulting in mitochondrial degradation. Under hypoxic conditions, BNIP3 expression is activated by HIF1α . Further, BNIP3 is phosphorylated at serine residues adjacent to the LIR sequence, and in phosphorylated form it promotes binding to MAP1LC3A. FUNDCI is also sensitive to hypoxia, although it is constantly present in the outer mitochondrial membrane under normal conditions ^[6] .

In the neurons, the mitochondria are unevenly distributed and tend to places requiring a large amount of energy, such as synapses and Ranvier intercepts . This distribution is supported by the transport of mitochondria along the axon , which is dependent on motor proteins ^[10] . In neurons, mitophagy occurs mainly in the cell body , although it can also occur in axon regions remote from the cell body; in both cases, mitophagy is activated along the paths of PINK1 and parkin ^[11] . In the nervous system, mitophagy can occur transcellularly, that is, damaged mitochondria of the ganglion cells of the retina can be transmitted to destroy nearby astrocytes . This process is known as transmitmitophagy ^[12] .

Yeast

The presence of mitophagy in yeast was suggested after the discovery of yeast mitochondrial escape genes ( English Yeast Mitochondrial Escape genes (yme) ), namely yme1. Like other proteins of the ume family, it activates the exit of mitochondrial DNA (mtDNA) from mitochondria, but only it increases mitochondrial degradation. Studying the proteins that mediate the release of mtDNA, the researchers discovered that the mitochondrial cycle is carried out by proteins ^[13] .

More information on the genetic control of mitophagy was obtained by studying UTH1. When searching for genes that regulate longevity, it was found that mitophagy was suppressed in ΔUTH1 strains , while no influence was exerted on the mechanisms of autophagy. The Uth1 protein has been shown to be necessary for the movement of mitochondria in vacuoles. This was evidence of the existence of a specialized mitophagy system. It has also been shown that phosphatase Aup1 marks mitochondria that need to be destroyed ^[13] .

Another yeast protein associated with mitophagy is Mdm38p / Mkh1p, a protein of the inner mitochondrial membrane. This protein is part of the complex, which provides the movement of K ⁺ / H ⁺ ions through the inner membrane. Deletion of this protein causes mitochondrial swelling, loss of membrane potential and mitochondrial fragmentation ^[13] .

It has recently been shown that the Atg32 protein plays a crucial role in mitophagy in yeast. It is localized in the mitochondria. Immediately after the start of mitophagy, Atg32 binds to Atg11, and mitochondria containing Atg32 protein are transferred to vacuole . Inhibition of Atg32 inhibits the involvement of autophagy machinery in mitochondria and mitochondrial degradation. Atg32 is not a protein necessary for other forms of autophagy ^{[14] [15]} .

Mitochondrial metabolism leads to the formation of by-products that lead to DNA damage and mutations . Thus, maintaining a population of normal mitochondria is necessary for cell activity. Previously, mitochondrial degradation was thought to be a random process, but mitophagy is now known to be a selective process ^[16] . The formation of ATP during oxidative phosphorylation leads to the appearance of a variety of reactive oxygen species (ROS) in mitochondria and submitochondrial particles. The formation of ROS as a by-product of mitochondrial activity ultimately leads to cell death. Due to the nature of their metabolism, mitochondria are very sensitive to ROS. When mitochondria are damaged, cytochrome c emerges , triggering caspases and, accordingly, apoptosis , and there is a lack of ATP. Damage to mitochondria occurs not only with oxidative stress or disease; normal mitochondria accumulate oxidative damage all the time, which is harmful not only for the mitochondria itself, but also for the cell. Due to the danger posed by damaged mitochondria to the cell, timely removal of damaged mitochondria is necessary to maintain cell integrity. A lack of mitochondria reduces the number of aging factors, and also leads to the fact that the cell must receive ATP using enhanced glycolysis ^[17] .

Cancer

As you know, the intensive growth of cancer cells is provided by a shift from oxidative phosphorylation to glycolysis due to hypoxia, and with the help of glycolysis the cancer cells produce the necessary ATP ( Warburg effect ). A decrease in oxidative phosphorylation leads to a decrease in the density of mitochondria. As a consequence of the Warburg effect, cancer cells accumulate a large amount of lactate . Excess lactate is released into the extracellular medium, which leads to a decrease in extracellular pH . Acidification of the medium leads to cellular stress, which, in turn, can lead to autophagy. Autophagy is triggered in response to various stimuli, such as lack of nutrients , hypoxia and activation of oncogenes . However, it appears that autophagy helps cancer cells survive under cellular stress, which leads to resistance to anticancer therapy, such as radiation therapy and chemotherapy . In addition, in the extracellular environment of cancer cells there is a lot of transcription factor induced by hypoxia 1-α (HIF1α), which activates the expression of BNIP3, an important mitophagic factor.

Parkinson's disease

Parkinson's disease is a neurodegenerative disease that is partially caused by the death of dopamine- producing cells in the substantia nigra . Parkinson's disease may be the result of certain mutations, including mutations that lead to loss of Parkin function ^[8] and PINK1 ^[18] . The loss of these proteins leads to the accumulation in cells of damaged mitochondria and protein clusters that cause cell death ^[19] .

It is believed that mitochondria are involved in the development of Parkinson's disease. In cases of spontaneous (that is, not related to genes ) Parkinson's disease, the disease is often caused by mitochondrial dysfunction, oxidative stress, changes in autophagy and the formation of protein clusters ^[20] . This can lead to swelling and depolarization of mitochondria. Disruptions in the release of energy in mitochondria cause destruction of cells, in particular, in the substantia nigra ^[21] .

• Starokadomsky, Peter. Autophagy, protophagy and the rest (neopr.) . // Website Biomolecula.ru (05.05.2003). Date of treatment March 21, 2018.
• Lebedev, Victor. 2016 Nobel Prize in Medicine and Physiology: for Samoyedism (neopr.) . // Website Biomolecula.ru (3.10.2016). Date of treatment March 21, 2018.