La Mitochondrie - Cours de Cytologie 2024/2025

General Characteristics of Mitochondria

Mitochondria (MTC) are specialized organelle structures found exclusively in eukaryotic cells. Often described as the "lung" of the cell, they play a fundamental role in cellular respiration and act as a central energy plant that provides the cell with the majority of its required energy. However, paradoxically, under certain conditions, mitochondria are involved in processes that lead to cellular illness, biological aging, or cell death. Within the human species, mitochondria are present in every cell type except for red blood cells (erythrocytes).

The inheritance of the mitochondrial pool is strictly maternal; the mother transmits her mitochondria via the ovule to the embryo, and paternal mitochondria are absent from the resulting embryo. Depending on the specific cell type and its physiological state, a single cell may contain between $1000$ and $3000$ mitochondria. Collectively, all the mitochondria in a cell form a dynamic, interconnected network known as the "CHONDRIOME." The movement of these organelles within the cell is primarily facilitated by interactions with the microtubule network of the cytoskeleton.

Mitochondria are unique among organelles because they possess their own genome in the form of circular DNA, which is remarkably similar to bacterial DNA. Although distributed throughout the hyaloplasm, they often cluster in preferred locations with high energy demands:

• Spermatozoa: Since flagellar beating consumes significant energy, a tight, spiral sleeve of mitochondria surrounds the base of the flagellum.
• Muscle Cells: Muscle contraction is energy-intensive; therefore, myofibrils are literally surrounded by mitochondria.
• Absorptive Epithelia: Permeative transport across the plasma membrane requires high energy. In these cells, mitochondria take on a filamentous appearance and insert themselves into long membrane invaginations perpendicular to the basal cell membrane.

Evolutionary Origin

The most widely accepted explanation for the origin of mitochondria is the endosymbiotic theory. This theory posits that mitochondria evolved from specific forms of aerobic prokaryotic bacteria that colonized a primitive anaerobic phagocytic eukaryotic cell. Instead of being degraded after ingestion, the bacteria formed a close, mutually beneficial relationship with the host—a process termed "symbiosis" occurring within ("endo") the host. Over evolutionary time, this integration gave rise to the modern eukaryotic cell.

Morphological Study of the Mitochondria

Light Microscopy (MO)

Under a light microscope, a mitochondrion typically measures between $2 imes 10^{-6}\,m$ and $10 imes 10^{-6}\,m$ ($2-10\,µm$) in length and between $0.5 imes 10^{-6}\,m$ and $1 imes 10^{-6}\,m$ ($0.5-1\,µm$) in width. This size makes them visible after the application of vital dyes. Their appearance is highly variable depending on the cell type and current level of cellular activity.

Electron Microscopy (ME)

Electron microscopy reveals that mitochondria are composed of four distinct compartments. From the outermost layer to the innermost, these are:

The Outer Membrane

This is a lipid bilayer with a composition similar to the plasma membrane. It is characterized by high permeability due to an abundance of porins—tunnel proteins that allow the passive passage of all molecules with a size smaller than $10\,KDa$. It contains complex proteins responsible for importing cytosol-synthesized proteins, known as the "TOM" (Translocation Outer Membrane) complex. Additionally, it contains ion channels and megachannels involved in apoptosis.

The Inter-membrane Space (Outer Chamber)

Located between the outer and inner membranes, this space has a thickness of $4\,nm$ to $7\,nm$. It contains metabolic substances diffusing from the outer membrane, ATP produced by the mitochondrion, and protons ($H^+$) pumped from the matrix during oxidative phosphorylation. It also houses Cytochrome c, which functions in both the respiratory chain and apoptosis. The enzyme adenyl kinase is present here to catalyze the reaction:
ATP + AMP ightleftharpoons 2ADP

The Inner Membrane

Unlike the outer membrane, the inner membrane is a lipid bilayer that is very protein-dense ($80\%$ proteins, $20\%$ lipids) and contains almost no cholesterol. It is rich in a specific phospholipid called cardiolipin (diphosphatidylglycerol), which the mitochondrion synthesizes itself. This lipid, first discovered in heart tissue, makes the membrane highly impermeable, specifically to protons, which is vital for energy production. The inner membrane forms invaginations called mitochondrial cristae, which increase the exchange surface area. The number of cristae correlates with metabolic activity. The membrane contains four main protein types:

1. Respiratory Chain Constituents: A series of complexes that transport electrons along an oxidation-reduction gradient.     * Complex I: NADH reductase.     * Complex II: SuccinateQ reductase.     * Complex III: Ubiquinol-Cytochrome C oxydo-reductase.     * Complex IV: Cytochrome C oxidase.     * Complex V: Reffered to as ATP synthase.     * Mobile Complexes: Ubiquinone (shuttles between I/III and II/III) and Cytochrome C (free in the inter-membrane space).
2. ATP Synthase (F0-F1 Complex): The machinery for ATP production.
3. Specific Transporters: Includes the ADP-ATP antiport (exports ATP to the cytosol), ion channels, and various symports for metabolite entry.
4. TIM Complexes: The Translocation Inner Membrane complex for protein importation.

The Mitochondrial Matrix (Inner Chamber)

The matrix contains various components essential for the organelle's semi-autonomous function:

• Mitoribosomes: Totaling $55S$, comprising a small subunit ($PSU\,28S$) and a large subunit ($GSU\,39S$). They contain more proteins than rRNA compared to standard ribosomes.
• mtDNA: Circular DNA without introns, containing $37$ genes that code for $13$ proteins, mostly central to the respiratory chain.
• mRNA and tRNA.
• Enzymatic Systems: Enzymes for the $\beta$-oxidation of short-chain fatty acids (less than $20$ carbons) via the Lynen Helix, pyruvate oxidation into acetyl-Coenzyme A, the Krebs cycle, and replication/transcription.
• Dense Granules: Inclusions primarily consisting of calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$).

Mitochondrial Renewal

Division

Mitochondria originate from the growth and division of pre-existing mitochondria. This occurs primarily through partition, where a crista extends to eventually split the organelle into two.

Lipid Importation

There are two prevailing theories on how mitochondria obtain lipids: either they form contact zones with the Endoplasmic Reticulum (ER) to take lipids directly, or carrier proteins shuttle lipids between the ER and the MTC.

Protein Importation

Most mitochondrial proteins are synthesized in the cytosol. They possess a targeting signal of approximately $30$ amino acids, rich in Lysine and Arginine at the N-terminal. Proteins enter via channels formed by the association of TOM and TIM complexes, which create docking zones between the two membranes.

Mitochondrial Functions

Cellular Respiration and ATP Synthesis

Cellular respiration involves degrading matter to produce energy while absorbing oxygen ($O_2$) and releasing Carbon Dioxide ($CO_2$). Human daily energy requirements are estimated at tens of kilograms of ATP, most of which is produced by the MTC.

Energetic Molecules and Aerobiosis

1. Pyruvate: Derived from glycolysis, it is oxidized into acetyl-CoA in the matrix to enter the Krebs cycle. In striated muscles during intense exercise with insufficient $O_2$, pyruvate undergoes lactic fermentation in the cytoplasm, turning the cell anaerobic.
2. Short-chain Fatty Acids ($C < 20$): These undergo $\beta$-oxidation cycles in the Lynen Helix.
3. Amino Acids: Their catabolism involves the urea cycle (occurring in both cytosol and matrix), feeding the Krebs cycle with arginino-succinate.

Steps of Respiration

1. The Krebs Cycle (Tricarboxylic Acid/Citric Acid Cycle): Occurs in the matrix. It produces high-energy reduced metabolites: $NADH/H^+$ and $FADH_2$.
2. Oxidative Phosphorylation: Occurs at the inner membrane involving the respiratory chain and ATP synthase.     * Respiratory Chain: Starts with the dehydrogenation of $NADH/H^+$, yielding two electrons to Complex I. This energy pumps protons into the inter-membrane space. Electrons flow: Ubiquinol → Complex III → Cytochrome C → Complex IV → Oxygen (final acceptor), forming $H_2O$. Complexes I, III, and IV act as proton pumps to create a gradient.     * FADH2: Donates electrons to Complex II, which does not pump protons. Consequently, one $NADH/H^+$ molecule generates $3\,ATP$, while one $FADH_2$ molecule generates only $2\,ATP$.     * ATP Synthesis: ATP synthase uses the proton flow back into the matrix to synthesize ATP from ADP. It requires $3\,H^+$ to produce one molecule of ATP. The ATP is then exported via ATP/ADP antiports.

Synthesis and Homeostasis

Mitochondria participate in synthesizing steroid hormones using cholesterol as a precursor, including Testosterone (testicles), Progesterone and Estradiol (ovaries), and Cortisol and Aldosterone (adrenal glands). They also work with the ER to regulate intracellular calcium concentration ($Ca^{2+}$ homeostasis).

Apoptosis

The intrinsic (mitochondrial) pathway of apoptosis is triggered by internal damage, such as DNA lesions. This involves the opening of megachannels in the outer membrane, releasing Cytochrome C into the cytoplasm. Cytochrome C then helps form the apoptosome, which triggers a cascade of caspase activation, leading to programmed cell death.

Medical Perspectives

Free Radicals

Mitochondria are the primary source of free radicals—unstable oxygen species with unpaired electrons. The main radical produced is the superoxide anion ($O_2^-$). The cell uses the enzyme superoxide dismutase to convert it into hydrogen peroxide ($H_2O_2$), which phagocytes use to kill bacteria. However, in excess, free radicals attack cellular components like DNA and MTC, leading to aging and chronic diseases such as cancer, diabetes, kidney failure, Alzheimer’s, and Parkinson’s.

Toxicology

Several toxins target the mitochondria:

1. Arsenic: Slow poisoning by blocking the citric acid cycle (Krebs), halting $NADH$ and $FADH_2$ production.
2. Cyanide: Inhibits Cytochrome c oxidase (Complex IV).
3. Rotenone and Antimycin: Used in pesticides; Rotenone blocks Complex I, while Antimycin blocks Complex III, leading to lethal mitochondrial dysfunction.