The Mitochondrion

The mitochondria is a network of sub-cellular, double membrane organelles, constantly fusing and dividing (Yaffe, 2003). The outer membrane is separated from the inner membrane by the intermembrane space. The inner membrane is folded into cristae, maximising its surface. The innermost compartment of the mitochondria is called the mitochondrial matrix, and contains the mitochondrial genome, ribosomes, tRNAs and various proteins and enzymes required for mitochondrial function. The main function of the mitochondria is to use oxygen to generate the cell’s major energy source, ATP.

The German pathologist Richard Altmann was the first to postulate that the mitochondria had metabolic and genetic autonomy (Altmann, 1890). The origin of the mitochondrion is believed to be an ancient bacterium, which infected a primordial cell and became symbiotic. The benefit of this partnership is thought to have been the removal of the toxic compound oxygen, and the production of ATP a welcome side effect. The free-living organism with a genome most similar to the mitochondrion is the a-protobacterium Rickettsia prowazekii (Andersson et al., 1998).

Mitochondrial DNA

The mitochondrion has its own genomic material, mitochondrial DNA, mtDNA. The mtDNA is a circular molecule residing within the mitochondrial matrix. Each isolated mitochondrion can contain up to ten copies of mtDNA, and a mammalian cell typically contains between 1,000 and 10,000 copies in total. The mature oocyte is the extreme with approximately 100,000 copies. MtDNA is maternally transmitted. The fertilized oocyte eliminates mtDNA carried by the sperm cell, leaving only maternal mtDNA to populate the growing embryo (Birky, 1995). Sometimes more than one species of mtDNA resides within a single cell. This state is called heteroplasmy. When a cell only contains one mtDNA species it is termed homoplasmy. The mitochondrion has dedicated replication, transcription and translation machineries, all of which resides within the mitochondrial matrix.

Structure

The size of the mtDNA molecule varies between species, ranging from almost 400 kb in Arabidopsis to less than 6kb in the Plasmodium families (Saccone et al., 2002). Mammalian mtDNAs are typically around 16kb. The human mtDNA is 16.6kb whereas the mouse mtDNA is 16.3kb. The mammalian mtDNA encodes 13 peptides, 2 ribosomal RNAs (rRNA) and 22 transfer RNAs (tRNA). It is highly compacted and contains no introns. The only longer non-coding part of the mtDNA is a 1.1kb regulatory region called the displacement loop, D-loop. The D-loop contains the heavy strand promoter (HSP), the light strand promoter (LSP), the origin of heavy strand replication (OH) as well as regulatory sequences necessary for mtDNA replication and transcription (Larsson and Clayton, 1995).

MtDNA Transcription and Replication

All proteins necessary for the replication and transcription of mtDNA are encoded by the nucleus, transcribed and translated in the cytoplasm, and imported into the mitochondrion. Initiation of mtDNA replication requires an RNA primer produced by mtDNA transcription. Therefore, defective mtDNA transcription will also affect mtDNA replication (Clayton, 2000). Table 1 lists the most studied nuclear-encoded proteins needed for mtDNA transcription and replication.

Mitochondrial oxidative phosphorylation

Most of the energy needed in mammalian cells comes from the process of oxidative phosphorylation, OXPHOS. In fact, more than 80% of the energy needed by the normal human adult is generated through OXPHOS. The remaining 20% is generated through glycolysis.

The respiratory chain

The respiratory chain (RC) is a set of five multi-subunit enzyme complexes (complex I to complex V) that resides embedded in the inner mitochondrial membrane. It carries out the process of oxidative phosphorylation, which uses oxygen and sugars to generate water and the cells’ main energy source, adenosine-tri-phosphate, ATP. The respiratory chain consists of over 100 different protein species, 13 of which are encoded by mtDNA (Larsson and Clayton, 1995). All complexes but complex II contain mtDNA-encoded subunits and are therefore dependent on mtDNA being present and decoded for their function. Complex I and II collect electrons from NADH and succinate, respectively, and pass them to ubiquinone (UQ). UQ is then oxidized by complex III. Cytochrome c (cyt c) transports electrons between complex III and IV (cytochrome c oxidase, COX). Complex I, III and IV couple the electron flow to proton pumping to make an electrochemical gradient across the inner mitochondrial membrane. Complex V uses this gradient to form ATP from ADP and phosphate in the form of Pi (Bindoff and Turnbull, 1990).

OXPHOS and reactive oxygen species

A potential harmful by-product of oxidative phosphorylation is reactive oxygen species, ROS. ROS are highly reactive compounds that can damage all molecules of the cell, such as proteins, DNA and lipid membranes (Davies, 1995). Under physiological conditions 1-2 % of all oxygen consumed by OXPHOS is converted to ROS (Boveris et al., 1972). Complex I, II and III generate superoxide, O2·-. Superoxide is converted into the less reactive molecule hydrogen peroxide, H2O2, by the mitochondrial superoxide dismutase, SOD2. The H2O2 is further reduced by glutathione peroxidase (GPX), which converts it into H2O (Davies, 2000). Normally, these defences are sufficient to protect the cell from ROS damage. However, if the ROS generation is elevated, defences might become inadequate and damage may occur.

Mitochondria and cell death

Typically, cell death is divided into two types; apoptosis and necrosis. Necrosis is accidental cell death, with uncontrolled swelling leading to leakage of the cell contents into the extracellular matrix, with inflammation as a consequence. Apoptosis, or programmed cell death, is suicidal. The membranes of an apoptotic cell keep their integrity, and the cell is finally engulfed by macrophages or neighbouring cells (Kerr et al., 1972). Apoptosis is a common feature in neurodegenerative disorders, and is often linked to mitochondrial dysfunction (Tatton and Olanow, 1999).