الفهرس 
Molecular Biology of MitochondrionOnly 14 pages are availabe for public view
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Abstract
Mitochondria play a central role in energy metabolism of cells. They usually provide most of the ATP by oxidative phosphorylation. Amajor consequence of the architecture of mitochondria is the impermeability of the inner membrane that facilitates the generation of a proton gradient, called the proton motive force.
The oxidative processes cells use to degrade fuel molecules yield NADH and FADH2 which are used as electron donors for the electron transport chain. The components of the chain are located in the inner mitochondrial membrane and include four complexes and some electron carriers.
With the current explosion of knowledge on the role of mitochondrial dysfunction in the genesis of various human disease states, there is an increased interest in targeting mitochondrial processes, pathways, and proteins for drug discovery efforts in cancer and cardiovascular, metabolic, and central nervous system diseases, the latter including neurodegenerative disorders.
A mitochondrial dysfunction and oxidative damage play role s in the pathogenesis of numerous disorders, e.g. PD, AD, HD, ALS, Wilson’s disease, Friedreich’s ataxia , multiple sclerosis and a number of inherited disorders of the mitochondrial genome, the mitochondrial encephalomyopathies (e.g., Leber’s disease with optic a trophy and dystonia, MELA S, MERR F, Leigh’ s disease , Kearns –Sayre syndrome). The list of mitochondria related diseases is growing rapidly: cancer, heart failure, diabetes, obesity, ischemia-reperfusion injury, atherosclerosis, certain liver diseases and asbestosis. They all share the common features of disturbances of the mitochondrial Ca2+, ATP or ROS metabolism
There is clear evidence that the deficiency of frataxin in Friedreich’s Ataxia results in increased mitochondrial iron, decreased respiratory chain function and elevated oxidative stress. However, current data suggests that the iron accumulation may be a relatively late event in the disease process, and a primary cause of the disease may well involve abnormal Fe-S synthesis. Frataxin has been suggested to act as a mitochondrial store of bio-available iron and/or delivery of iron for Fe-S centre synthesis. The resultant dysfunction of respiratory chain complexes I-III and aconitase will lead to a decrease in ATP synthesis and an increase in free radical production from the inhibited respiratory chain.
Mitochondrial dysfunction plays a critical role in mutant SOD1 mediated familial ALS. Various aspects of the underlying mechanisms as well as functional consequences of mitochondrial dysfunction include association of mutant SOD1 aggregates with mitochondria, abnormal mitochondrial morphology, impaired mitochondrial bioenergetics, loss of mitochondrial membrane potential, reduced mitochondrial calcium buffering capacity and disrupted calcium homestasis, impaired axonal transport of mitochondria, and potential imbalance of mitochondrial fission and fusion.
Numerous studies report the involvement of ROS in cell death after cerebral ischemia. ROS contribute not only to injury of macromolecules such as lipids, proteins, and DNA, but also to transduction of apoptotic signals.
Mitochondrial dysfunctions occur as a consequence of cerebral ischemia and promote ischemia induced neuronal cell death, especially the intrinsic pathway of apoptotic cell death. Conversely, endogenous protective pathways exist to counteract these detrimental effects induced by ischemia including mitochondria proteins UCP2 and SOD2, which are all regulated by PGC-1α. Therefore, mitochondria can be considered as a target for potential neuroprotective strategies in cerebral ischemia. Future studies of these cell death /survival mechanisms subsequent to ischemic attack may provide unique information regarding molecular targets for therapeutic strategies in clinical stroke. Protection of the mitochondria from bioenergetics failure and oxidative/nitrosative stress resulting in apoptosis in the ischemic tissue may open a new vista to the development of more effective neuroprotective strategies against ischemia-induced brain damage.
Oxidative stress is thought to promote tissue damage in MS. fumaric acid esters (FAE) enhance cellular resistance to free radicals via Nrf2, a transcriptional factor and could prove useful for MS treatment. Nrf2 plays an important role of anti-oxidative pathways for tissue protection. Furthermore, FAE are a new, orally available treatment option which targets the NRf2 pathway and which have already shown their efficacy in phase II/III MS trials.
The diagnosis of mitochondrial diseases is based first of all on a careful clinical evaluation. The application of strict clinical–pathological criteria is very diffcult because of the high clinical and genetic heterogeneity of this group of diseases.
Laboratory tests done in the usual evaluation of presenting symptoms may suggest the diagnosis: MRI for stroke showing infarct-like lesions crossing major arterial zones, lactic acid peaks by proton MR spectroscopy and lesions that remit and relapse are typical for MELAS. In cases with widespread brainstem dysfunction and ataxia, bilateral extensive T-2 hyperintensities in the brainstem and basal ganglia typify Leigh’s disease. Basal ganglia calcifications and subcortical luekoencephalopathy are seen in MNGIE and MELAS.
Specialised tests guided by the clinical picture provide supportive evidence but may not be pathognomonic and may be abnormal in other conditions.
Elevated serum lactic acid (>2.2 mmols/L) is a key finding. Blood sample is obtained after tourniquet is released to avoid spurious results and repeated if initially normal when clinical suspicion is strong. Serum pyruvate is helpful when lactate is high as the lactate/pyruvate ratio is typically in the range of 50:1 to 250:1 compared to normal of <25:1. CSF lactate is a good and stable marker for MELAS and other encephalo-myopahies. Evaluation of muscle biopsy is an excellent, standardised tool with or without clinical myopathy, amenable to histological, genetic and biochemical analysis. Abundant ragged red fibers, representing clusters of mitochondria, on modified Gomori stain, provide strong support for both mitochondrial and nuclear gene defects. Cytochrome c oxidsase (COX) stain reveals COX negative fibers seen in some mitochondrial gene lesions. Electron microscopy may detect morphologically distorted mitochondria. Activities of the respiratory chain enzymes in frozen samples are not routinely needed.
Genetic tests in sophisticated molecular laboratories can identify mutations in many syndromes of mitochondrial genes. But in only a few of the nuclear genes. Search can be focused for some syndromes: mitochondrial mutations of t-RNA for MELAS and MERRF, protein coding genes of NARP, LHON and KSS can be detected in lymphocytes. Muscle is tested for the single deletions of sporadic CPEO. A battery of tests is needed in cases without distinctive syndrome. Mutation of thymidine phosphorylase gene causing MNGIE is an example of nuclear gene defect. Early recognition of the common cardiac conduction disturbance and preventive pacemaking are important and life-saving.
Management is aimed primarily at minimizing disability, preventing complications, and providing prognostic information and genetic counseling based on current best practice.
A variety of pharmacological treatments are used but there is no consensus about the efficacy of these drugs. Mitochondria are an important source of free radicals, and increased production of free radicals has been proposed to be important in the pathogenesis of mitochondrial disorders which suggests that antioxidants may be beneficial for affected patients.
Oral natural supplements containing membrane phospholipids, CoQ10, microencapsulated NADH, L-carnitine, α-lipoic acid, and other nutrients can help restore mitochondrial function and reduce intractable fatigue in patients with chronic illnesses. The combination of these supplements can result in a safe and effective method to reduce fatigue and help restore quality of life.
Coenzyme Q has a dual function as an electron transporter in the respiratory chain and as a scavenger molecule and has been reported to have a beneficial effect on different symptoms in MELAS and Kearns Sayre syndrome.
Different compounds (e.g. riboflavin, tocopherol [vitamin E], succinate, ascorbate [vitamin C], menadione and nicotinamide) have been used to bypass blocks in the respiratory chain caused by deficiencies of specific respiratory chain enzyme complexes. Peptide nucleic acids (PNA) are synthetic polyamide nucleic acids and it is possible to design PNA molecules so that they are complementary to a short mtDNA sequence harbouring a point mutation or a deletion breakpoint.
Some patients with heteroplasmic tRNA mutations have high levels of mutated mtDNA in differentiated muscle and low levels in the surrounding satellite cells, which are responsible for muscle regeneration. This observation was recently used to develop a treatment strategy where a localized necrosis was induced in a small muscle region in a patient with mitochondrial myopathy.
Surprisingly, the regenerated muscle segment completely reversed the genotype and contained very low levels of mutated mtDNA with a resulting normal respiratory chain function. Activation of satellite cells may thus be an efficient treatment in particular cases of mitochondrial myopathy.
Gene therapy would be the ultimate treatment by replacing or repairing the defective gene, and this should lead to permanent health. However, it is currently impossible to introduce genes into mitochondria of in vitro cultured cells and this lack of a mitochondrial transformation system is a major obstacle for gene therapy of mtDNA disorders. Short pieces of nucleic acids can be attached to protein leader sequences and imported into isolated mitochondria, but the low frequency or absence of recombination in mammalian mitochondria may make this strategy unlikely to succeed.