Mitochondria

Image obtained by electronic microscopy of the lung tissue of a mammal, where two mitochondria are visualized.

The mitochondria are eukaryotic cell organelles responsible for supplying most of the energy necessary for cell activity through the process called cellular respiration. They act as power plants for the cell and synthesize ATP through expense of metabolic fuels (glucose, fatty acids and amino acids). Mitochondria have an outer membrane that is permeable to ions, metabolites, and many polypeptides. This is because it contains pore-forming proteins called porins or VDACs (voltage-gated anion channel), which allow molecules up to 5 kDa in mass to pass through. and an approximate diameter of 2 nm.

Discovery of mitochondria

The discovery of mitochondria was a collective event. The large number of terms that refer to this organelle is proof of this: Blepharoplast, chondriocont, chondriomites, chondroblasts, chondriosomes, chondriospheres, fila, fuchsinophilic granules, Korner, Fadenkörper, mitogel, parabasal bodies, vermicules, sarcosomes, interstitial bodies, plasmosomes, plastochondria, bioblasts. Cowdry tried in 1918, in a work later cited by Lehninger, to systematize and unify all the terms.

The first observations are probably due to the Swiss botanist Albert von Kölliker, who between 1880 and 1888 noted the presence of granules in muscle cells of insects which he called sarcosomes. He even concluded that they had a membrane. In 1882, the German Walther Flemming discovered a series of inclusions which he called a row. In 1884 they were also observed by Richard Altmann, who later in His work published in Leipzig Die Elementarorganismen describes a series of corpuscles that he observes using a special stain that includes fuchsin. He speculates that they are a kind of independent parasites, with their own metabolism and calls them bioblasts. The finding was rejected as an artifact of the preparation, and it was only later recognized as mitochondria by N. H. Cowdry (in the United States, in 1916). Also the "plastidules" of the Italian protozoologist Leopoldo Maggi could be early observations of mitochondria.

However, the name mitochondrion, which is the one that achieved the greatest success, is due to Carl Benda, who in 1889 gave this name to some granules that appeared with great brilliance in crystal violet stains and alizarin, and which had previously been called "cytomicrosomes" by Velette St. George. In 1904 F. Meves confirmed their presence in a plant, specifically in cells of the anther mat of Nymphaea, and in 1913 Otto Heinrich Warburg discovered the association with enzymes of the respiratory chain, although Kingsbury, in 1912, had already linked these organelles with cellular respiration. In 1934 they were isolated for the first time from liver homogenates and in 1948 Hogeboon, Schneider and Palade definitively established mitochondria as the place where cellular respiration occurs.

The presence of mitochondrial DNA was discovered by Margit M. K. Nass and Sylvan Nass in 1963.

Structure and composition

Structure of a mitochondria

The morphology of mitochondria is difficult to describe, as they are highly plastic structures that deform, divide, and fuse. They are usually depicted in an elongated shape. They range in size from 0.5 to 1 μm in diameter and up to 8 μm in length. Their number depends on the energy needs of the cell. The set of mitochondria in the cell is called the cellular chondrioma.

Mitochondria are surrounded by two membranes that are clearly different in their functions and enzymatic activities, which separate three spaces: the cytosol (or cytoplasmic matrix), the intermembranous space, and the mitochondrial matrix.

Outer membrane

It is an outer lipid bilayer permeable to ions, metabolites, and many polypeptides. That's because it contains pore-forming proteins called porins or VDACs (voltage-gated anion channel ), which allow the passage of large molecules up to 5,000 daltons and a diameter of about 20 Å. The outer membrane performs relatively few enzymatic or transport functions. It contains between 60 and 70% protein.

Internal membrane

The inner membrane contains more protein (80%), lacks pores, and is highly selective; it contains many enzyme complexes and transmembrane transport systems, which are involved in the translocation of molecules.

Mitochondrial Cristae

In most eukaryotes, the cristae form flattened septa perpendicular to the axis of the mitochondria, but in some protists they are tubular or discoidal in shape.

Recent studies have shown that the cristae membrane is not formed by invagination of the inner membrane, but instead forms a separate membranous system of the inner and outer membranes. They connect to the inner membrane at specific points that facilitate the transport of metabolites between the different compartments of the mitochondria.

Functions related to oxidative metabolism such as the respiratory chain or oxidative phosphorylation are carried out in the membrane of the cristae.

1. The electron transport chain, consisting of four fixed enzyme complexes and two mobile electron conveyors:
   1. Complex I or NADH dehydrogenase containing mononucleotide flavina (FMN).
   2. Complex II or dehydrogenase succinct; both yield electrons to coenzyme Q or ubiquinone.
   3. Complex III or cytochrome bc₁ that yields electrons to cytochrome c.
   4. Complex IV or cytochrome c oxidase that yields electrons to the O₂ to produce two water molecules.
2. An enzyme complex, the H channel⁺ ATP syntase that catalyzes ATP synthesis (oxidative phosphorylation).
3. Conveyor proteins that allow the passage of ions and molecules through them, such as fatty acids, piruvic acid, ADP, ATP, O₂ and water; they can be highlighted:
   1. Translocase adenine nucleotide. It is responsible for transporting to the mitochondrial matrix the cytolic ADP formed during the reactions that consume energy and, in parallel, translocates to the cytosol the newly synthesized ATP during oxidative phosphorylation.
   2. Translocation phosphate. Cytolic phosphate translocation along with a hydrogen to the matrix; phosphate is essential for phosphorylating the ADP during oxidative phosphorylation.

Intermembranous space

Between both membranes there is an intermembranous space that is made up of a liquid similar to hyaloplasm; it has a high concentration of protons as a result of the pumping of protons by the enzyme complexes of the respiratory chain. Various enzymes involved in the transfer of the high-energy bond of ATP are located in it, such as adenylate kinase or creatine kinase. Carnitine is also located, a molecule involved in the transport of fatty acids from the cytosol to the mitochondrial matrix, where they will be oxidized (beta-oxidation).

Mitochondrial Matrix

The mitochondrial matrix or mitosol contains fewer molecules than the cytosol, although it contains ions, metabolites to be oxidized, double-stranded circular DNA very similar to that of bacteria, ribosomes type 55S (70S in plants), called mitorribosomes, which carry out the synthesis of some mitochondrial proteins, and contains mitochondrial RNA; that is, they have the organelles that a free-living prokaryotic cell would have. Various key metabolic pathways for life take place in the mitochondrial matrix, such as the Krebs cycle and beta-oxidation of fatty acids; amino acids are also oxidized and some reactions of the synthesis of urea and heme groups are located.

Function

The main function of mitochondria is the oxidation of metabolites (Krebs cycle, beta-oxidation of fatty acids) and the obtaining of ATP through oxidative phosphorylation, which is dependent on the electron transport chain; the ATP produced in the mitochondria accounts for a very high percentage of the ATP synthesized by the cell. It also serves as a store for substances such as ions, water and some particles such as virus remains and proteins.

Protein uptake in mitochondria

Mitochondria have four compartments into which proteins can reach:

1. External mitochondrial membrane
2. Membrana mitochondrial interna
3. Intermembranous space
4. Matrix.

Most of the proteins destined for the inner mitochondrial membrane have internal leader sequences that mark their membership as part of the molecule. Before this protein can enter the mitochondria, it is thought to go through different phenomena, for example, it has to be in an unfolded or extended state. Chaperones such as Hsp70 and Hsp90 are involved in preparing polypeptides for uptake into mitochondria, including those that specifically target mitochondrial proteins to the cytosolic surface of the outer mitochondrial membrane.

The outer mitochondrial membrane contains a protein import complex called the TOM complex, which includes:

1. receptors that recognize and bind with mitochondrial proteins
2. channels covered by proteins through which the polypeptides deployed through the outer membrane pass.

Proteins destined for the inner mitochondrial membrane or matrix must pass through the intermembranous space and attach to a second protein import complex found in the inner mitochondrial membrane, the TIM complex.

The inner mitochondrial membrane contains two major TIM complexes: TIM22 and TIM23, TIM22 binds to integral proteins of the inner mitochondrial membrane that contain an internal leader sequence and inserts it into the lipid bilayer, while TIM23 binds to proteins that have amino-terminal presequence, including all matrix proteins, it recognizes and translates the proteins across the inner mitochondrial membrane and into the inner aqueous compartment. Translocation occurs at sites where the outer and inner mitochondrial membranes are in close proximity, so that the imported protein can cross both membranes at the same time.

Movement toward the matrix is driven by electrical potential, which, through the inner mitochondrial membrane, acts on the positively charged directing signal. Upon entering the matrix, a polypeptide interacts with mitochondrial chaperones that mediate entry into the aqueous compartment. Chaperones have also been proposed to act as force-generating motors that use energy derived from ATP hydrolysis to "pull" the unfolded polypeptide through the translocation pore, and are also proposed to aid polypeptide diffusion through the translocation pore. of the membrane.

Origin

The American scientist Lynn Margulis, together with other scientists, recovered an old hypothesis around 1980, reformulating it as an endosymbiotic theory. According to this updated version, 'about 2.309 million years ago,' a prokaryotic cell capable of obtaining energy from organic nutrients using molecular oxygen as an oxidant, fused at one point in evolution with another primitive prokaryotic or eukaryotic cell. by being engulfed without being immediately digested, a frequently observed phenomenon. In this way, a permanent symbiosis was produced between both types of beings: the phagocytosed prokaryote provided energy, especially in the form of ATP, and the host cell offered a stable environment rich in nutrients to the other. This mutual benefit caused the invading cell to become part of the larger organism, eventually becoming part of it: the mitochondria. Another factor that supports this theory is that bacteria and mitochondria have a lot in common, such as their size, structure, membrane components, and the way they produce energy, among other characteristics.

This hypothesis is based on the evidence that mitochondria have their own DNA, RNA, ribosomes and chromosomes and are covered by their own membrane. According to phylogenetic analyzes of DNA, ribosomal RNA, and proteome, mitochondria originated from a Rickettsia-like alphaproteobacterium, belonging to the order Rickettsiales. Other evidence supporting this hypothesis is that the genetic code of the Mitochondrial DNA is usually not the same as the genetic code of nuclear DNA. Throughout common history most mitochondrial genes have been transferred to the nucleus, such that mitochondria are not viable outside the host cell. and this is not usually without mitochondria.

Mitochondrial diseases

Human mitochondrial DNA contains genetic information for 13 mitochondrial proteins and some RNAs; however, most mitochondrial proteins are derived from genes located in the DNA of the cell nucleus and are synthesized by free ribosomes in the cytosol and then imported by the organelle. More than 150 mitochondrial diseases have been described, such as Luft's disease or Leber's hereditary optic neuropathy. Both mitochondrial DNA and nuclear DNA mutations give rise to mitochondrial genetic diseases that cause malfunctions of processes that take place in mitochondria, such as alterations in enzymes, RNA, components of the electron transport chain, and transport systems. the inner membrane; many of them affect skeletal muscle and the central nervous system.

Mitochondrial DNA can be damaged by free radicals formed in mitochondria, thus degenerative diseases associated with aging, such as Parkinson's disease, Alzheimer's disease, and heart disease, may be related to mitochondrial damage.

Loss of mitochondria by evolution

There are protists without mitochondria that lack them due to a secondary loss or their degeneration, to adapt to a parasitic, intracellular or anaerobic way of life.

The role of mitochondria in female fertility

Female fertility is affected by aging, having the peak of reproductive capacity at approximately 25 years of age, which declines with age, decreasing considerably after 37 years of age. Numerous studies suggest that a decrease in the quality of the oocyte is the cause of the decline in reproductive capacity related to age, and although the underlying mechanisms are still unknown, some scientists hypothesize that alterations in the mitochondria could be the key factors that mediate reproductive capacity (Benkhalifa et al., 2014; Ramalho-Santos et al., 2009; Schatten et al., 2014).

Mitochondria in the oocyte and embryo

As previously mentioned, the main function of mitochondria is to obtain energy via ATP, either through the Krebs Cycle or through the glycolytic pathway, and they are essential for cell function, since its deficiency can lead to apoptosis (death) of the cell. The processes that the oocyte goes through before ovulation and fertilization require ATP, as well as the first steps of embryonic development, the efficacy of which has been related to mitochondrial function and activity (Benkhalifa et al., 2014; Pang et al., 2013). The mitochondria of mature oocytes and pre-implanted embryos go through transformations in their structure and in their cytoplasmic distribution. In oocytes, the mitochondria are oval and have a dense cell matrix. Once past the morula stage, these structural changes result in an increase in ATP production and in its oxidative metabolism. It has been observed that in some embryos unable to develop there are mitochondria that have not undergone this morphological modification.

Mainly, the oocyte produces its energy almost entirely via oxidative phosphorylation, since glycolysis is limited by a low expression of phosphofructokinase. On the contrary, the cumulus oophorus cells have a great glycolytic capacity and provide the oocyte with pyruvate through the gap junctions, in order to produce ATP. The communication between both types of cells is bidirectional, meaning that alterations in the metabolic activity in the cumulus oophorus can modify the viability of the oocytes. Proof of this was a 2014 study by Hsu et al. where they demonstrated for the first time that women with endometriosis, when assisted reproductive techniques have been used, have a decrease in ATP by cumulus cells that causes the number of mature oocytes and implantation rates to be altered. Possibly this is because a mitochondrial dysfunction has occurred.

On the other hand, the use of hormones in ovarian stimulation could affect the female reproductive system, causing an increase in the number of cumulus cells in apoptosis, thus decreasing the pyruvate that reaches the oocytes, thus influencing their quality (Dell'Aquila et al., 2009; Eichenlaub-Ritter et al., 2011).

Effects of oxidative stress

Oxidative stress is caused by an instability between the production of reactive oxygen species (ROS) and the antioxidant molecules that neutralize it. Increased ROS production can lead to increased damage to mitochondrial enzymes. ROS production levels can be altered by factors intrinsic to the individual such as lifestyle —which causes it to accumulate over the years—, obesity, insulin resistance, endometriosis, polycystic ovary syndrome, etc. Also, there are external factors in assisted reproductive therapies, such as exposure to visible light, non-ideal pH, and temperature, which can increase ROS production levels. It is for all this that there is an increase in oxidative stress that leads to a lower probability of conception in these therapies. Although there are cells within ovarian tissue that could have an antioxidant effect, it is not enough to combat it. In addition, antioxidant capacity is lost over the years, so a 38-year-old woman will have increased oxidative stress compared to a 32-year-old woman.

It is believed that the negative effect of oxidative stress on conception and embryo development is due to the fact that it affects mitochondrial DNA, which can lead to a critical loss of function and, therefore, a decrease in ATP. There are studies that also link calcium levels with failures in mitochondrial function.

Mitochondrial therapies in assisted reproduction

Autologous mitochondria transplantation has been considered as a therapeutic option for women who have had several failed in vitro fertilization attempts. AUGMENT (germline mitochondrial energy transfer, Sinclair, 2013), for example, is a procedure that involves obtaining ovarian cell precursors from the ovarian cortex by laparoscopy. Subsequently, their mitochondria, which are in optimal conditions, would be isolated and transferred into the oocyte together with the sperm by intracytoplasmic injection. This technique has not been approved by the US Food and Drug Administration, and its effectiveness is unclear. However, the technique has worked for numerous women and has allowed them to conceive.