*Mitochondrial disorders are a heterogeneous group of multisystem diseases. They can result from mutations in hundreds of genes distributed across all of the chromosomes as well as the mitochondrial (mtDNA).
*Mitochondrial dysfunction is a common factor in a wide spectrum of metabolic and generative diseases, cancers, and aging.
*Progress is being made in developing genetic therapies for mitochondrial disease, although efforts to develop effective metabolic and pharmacological therapies have been surprisingly disappointing. A primary reason for difficulties in developing metabolic and pharmacological therapies is our lack of understanding of the complex bioenergetic and redox networks of the mitochondrion and the cell.
“Mitochondrial dysfunction has been linked to a wide range of modern age degenerative and metabolic diseases, cancer, and aging. All these clinical manifestations arise from the central role that energy sources have in our cells. Modern day therapies have been inadequate (to say the least) to solve disease. This failure results from the limited appreciation of the role of energetic sources in the cell as the interface between the environment and the cell. A systems approach, which, ironically, was first successfully applied over 80 years ago with the introduction of the ketogenic diet, is required. Analysis of the many ways that a shift from carbohydrate metabolism to a fat and ketone metabolism may modulate metabolism, signal transduction pathways, and the epigenome gives us an appreciation of the ketogenic diet and the potential for bioenergetic therapeutics.
“At present, our knowledge of energetic diseases stems from the past 20 years of studies on mitochondrial bioenergetics and diseases. Mitochondrial diseases are heterogeneous and often multisystemic. Because the mitochondrion provides much of the energy for the cell, mitochondrial disorders preferentially affect tissues with high-energy demands such as the brain, muscle, heart, and endocrine system, although any organ can be affected. Thus, energetic defects have been implicated in forms of blindness, deafness, movement disorders, dementias, cardiomyopathy, myopathy, renal dysfunction, and aging. Moreover, because the mitochondria lie at the interface between environmental calories and organ energetic demands, the mitochondrion is the likely mediator of the common metabolic disorders. As a consequence, mitochondrial dysfunction is probably central to diabetes, obesity, cardiovascular disease, and cancer.
“Although mitochondrial diseases had been assumed to be rare, epidemiological studies have concluded that the frequency of mtDNA diseases alone is on the order of 1 in 5000, and known pathogenic mtDNA mutations have been detected in the cord blood of 1 in 200 live births. Furthermore,nuclear DNA (nDNA) mutations affecting mitochondrial genes may be even more common than mitochondrial DNA (mtDNA) mutations. Thus, the genetic burden of energy dysfunction is likely to be enormous. Because the mitochondria are believed to generate the majority of our energy, understanding the pathophysiology of rare and common mitochondrial diseases and the development of energetic therapies must start with the mitochondrion.
“Each mammalian cell contains hundreds of mitochondria and thousands of mtDNA. When a mu-tation arises in a mtDNA, it creates a mixed population of normal and mutant mtDNAs [...] As the percentage of mutant mtDNAs increases, mitochondrial energetic function declines. When energy output is insufficient for normal tissue function, a threshold is crossed, symptoms appear, and apoptosis or necrosis [cell death] may be initiated.
“Current pharmacological or metabolic therapeutics have focused on increasing mitochondrial ATP [energy] production, reducing the mitochondrial reactive oxygen species (ROS) [inflammatory end products] produced, modifying Ca2+ uptake and toxicity, stabilizing the mitochondrial permeability transition pore (mtPTP), and inducing mitochondrial biogenesis.
“Ironically, one of the oldest therapeutic approaches—fasting and the ketogenic diet— remains the most promising treatment for mitochondrial defects. Hence, understanding the manifold potential actions of fasting and the ketogenic diet may point the way toward more comprehensive mitochondrial therapies.
“several of the affected pathways appear to be activated by the ketogenic diet. Therefore, in acquiring an understanding of the mechanisms of action of the ketogenic diet, we could obtain insights into how to design a more systematic approach to treating bioenergetic disease.
“Since the 1920s, fasting and the ketogenic diet have been known to be one of the most effective approaches for treatment of intractable seizures in children. Ketone bodies (beta-hydroxybutyrate and acetoacetate) are generated by the liver during fasting by beta oxidation of fatty acids [burning of fat]. Generally, the brain uses glucose as its main energy source, with a ratio of glucose to ketones of approximately 97:3. However, during short-term fasting this ratio can shift to ~70:30, and during starvation it can shift to ~30:70. Caloric restriction has also been shown to prolong life span in several organisms in association with reduced oxidative stress and increased antioxidant enzymes. This is the result of shifting metabolism from a more glucose-based glycolytic metabolism [carbohydrate metabolism] to a more fatty acid oxidation–based [fat burning metabolism] mitochondrial metabolism.
“In addition to preventing seizures in children, ketone treatment in adults has been found to (a) prevent symptoms of experimental hypoglycemia, including anxiety, fainting, hunger, irritability, and tremor, and (b) inhibit astrocytoma tumor [brain tumor] growth and vascularization. Beta-hydroxybutyrate also protects cultured mesencephalic neurons from MPP+ (1-methyl-4-phenylpyridinium) toxicity and hippocampal neurons from amyloid beta 42 toxicity.
“In addition to enhancing GABA production, the increased acetyl-CoA generated from betahy-droxybutyrate and acetoacetate [ketone bodies] also drives the tricarboxylic acid cycle [the Krebs cycle, to generate energy]. This increases mitochondrial reduced nicotinamide adenine nucleotide (NADH) [...] Because beta-hydroxybutyrate is more energy rich than pyruvate (the former has a 31% high caloric density per C2 unit), it produces more ATP. Moreover, in the heart, ketone metabolism increases the acetyl-CoA content 9- to 18-fold and citrate levels two- to fivefold; decreases succinate, oxaloacetate, and aspartate two- to threefold; increases the reduction status of NAD+ together with the oxidation status of CoQ; and increases heart hydraulic efficiency by 25% relative to pyruvate. [...] In the hippocampus, the ketogenic diet also results in the coordinated induction of energy metabolism gene transcripts, inducing 33 of the 34 energy metabolism genes (including 21 genes involved in oxidative phosphorylation) and 39 of the 42 genes encoding mitochondrial proteins. A ketogenic diet also results in a 46% increase in the mitochondrial density in the hippocampus of animals and increases the phosphocreatine-to-creatine ratio.
“Ketones have also been reported to reduce cellular and mitochondrial oxidative stress, to in-crease antioxidant scavenging molecules, and to stabilize the mitochondrial permeability transition pore (mtPTP). Studies on cultured rat neonatal cortical neurons revealed that beta-hydroxybutyrate and acetoacetate prevented neuronal cell death induced by glutamate. Ketones also significantly reduced mitochondrial production of reactive oxygen species (ROS) and associated cytotoxic effects in association with increased electron transport chain oxidation of NADH. Rats maintained on a ketogenic diet were found to have a twofold increase in hippocampal mitochondrial reduced glutathione levels[...] Mitochondria purified from the hippocampus of ketogenic diet–fed rats also showed a reduction in hydrogen peroxide production and were resistant to the hydrogen peroxide-induced mtDNA damage.
“Rat neocortical brain slices and neurons exposed to hydrogen peroxide showed protection against electrophysiological changes, loss of cell viability, and quenching of DCF (2′,7′-dichlorofluorescin) fluorescence when incubated with beta-hydroxybutyrate and acetoacetate, which were also protective against diamide thiol oxidation–induced electrophysiological changes and neuronal cell death. Ketones also directly increased the Ca2+ threshold of the mtPTP. [...] Therefore, ketone bodies reduce mitochondrial ROS production, increase antioxidant defenses, and stabilize the mtPTP.
“The reduction in mitochondrial ROS production and the stabilization of the mtPTP could also arise from increased mitochondrial oxidative phosphorylation [energetic output]. [...] The increased reduced glutathione/oxidized glutathione ratio could be the result of increased mitochondrial NADPH levels [...] Similarly, the antioxidant defenses in the cytosol [cell soup]could be enhanced by the export of the excess mitochondrial citrate into the cytosol, where it can be cleaved by ATP-citrate lyase into oxaloacetate and acetyl-CoA. The oxaloacetate could then be reduced by malate dehydrogenase, and the malate could be oxidized to pyruvate by malic enzyme, thereby reducing NADP+ to NADPH. The increased cytosolic NADPH could then reduce the cytosolic oxidized glutathione to reduced glutathione.
“The induction of mitochondrial bioenergetic genes could be explained by the increased generation of cytosolic acetyl-CoA produced from the mitochondrial citrate cleaved by ATP-citrate lyase. Elevated acetyl-CoA could then drive nuclear histone acetyltransferases to acetylate histones. This process would open the chromatin and increase the expression of the nDNA-encoded mitochondrial genes [...] [epigenetic changes]
“These speculations suggest that the ketogenic diet may act at multiple levels: It may decrease excitatory neuronal activity, increase the expression of bioenergetic genes, increase mitochon-drial biogenesis and oxidative energy production, and increase mitochondrial NADPH produc-tion, thus decreasing mitochondrial oxidative stress.
“Although it is becoming increasingly clear that mitochondrial dysfunction is a common factor in a wide range of complex diseases, effective therapeutic interventions are still not readily available. New approaches for genetic therapies for nDNA-encoded mitochondrial diseases as well as for mtDNA diseases are beginning to offer alternatives for individuals suffering from these devastating disorders. However, these approaches are not as likely to relieve the devastating symptoms suffered by individuals with bioenergetic diseases. Current metabolic interventions have been consistently disappointing, but new insights into the integrated energetic network of the cell are providing fresh insight into the mode of action of the older therapeutic approaches, such as fasting and the ketogenic diet, and suggesting more integrated approaches for restoring cellular energetics to homeostasis.
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