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The endosomal–lysosomal apparatus is responsible for the intracellular digestion of externally and internally generated macromolecules. Endocrine glands are ductless glands that produce and release hormones to the blood through diffusion. Endocrine glands may be strictly endocrine, such as the pituitary, thyroid, parathyroid, adrenal, pineal and thymus; or they may be organs that have hormone production as one of many functions, such as the pancreas, gonads, hypothalamus, and others. Hormones are long-distance chemical signals that are secreted by the cells to the extracellular fluid and regulate the metabolic functions of other cells.
Most hormones are amino acid based, but gonadal and adrenocortical hormones are steroids, derived from cholesterol. Water-soluble hormones (all amino acid-based hormones except thyroid hormone) exert their effects through an intracellular second messenger that is activated when a hormone binds to a membrane receptor. Lipid-soluble hormones (steroids and thyroid hormone) diffuse into the cell, where they bind to intracellular receptors, migrate to the nucleus, and activate specific target sequences of DNA.
Target cell response depends on three factors: blood levels of the hormone, relative numbers of target cell receptors, and affinity of the receptor for the hormone.
The concentration of a hormone reflects its rate of release, and the rate of inactivation and removal from the body. The half-life of a hormone is the duration of time a hormone remains in the blood, and is shortest for water-soluble hormones. Permissiveness occurs when one hormone cannot exert its full effect without another hormone being present (reproductive hormones need thyroxine to properly stimulate development of reproductive organs). Synergism occurs when more than one hormone produces the same effects in a target cell, and their combined effects are amplified (glucagon + epinephrine together stimulate more glucose release from the liver than when each acts alone). Antagonism occurs when one hormone opposes the action of another hormone (glucagon antagonizes insulin). Nervous system modulation allows hormone secretion to be modified by the nervous stimulation in response to changing body needs. The pituitary gland is connected to the hypothalamus via a stalk, the infundibulum, and consists of two lobes: the anterior pituitary, or adenohypophysis, and the posterior pituitary, or neurohypophysis. Two neurohormones are synthesized by the hypothalamus and secreted by the posterior pituitary. Growth hormone (GH) indirectly (through insulin-like growth factors, IGFs) stimulates body cells to increase in size and divide. Direct effects are insulin-sparing: mobilization of fatty acids for fuel, inhibition of insulin activity, release of glucose from liver to blood, and stimulation of amino acid uptake by cells. The thyroid gland consists of hollow follicles with follicle cells that produce thyroglobulin, and parafollicular cells that produce calcitonin.
Thyroid hormone consists of two amine hormones: thyroxine (T4) and triiodothyronine (T3), that act on all body cells to increase basal metabolic rate and body heat production. The parathyroid glands contain chief cells that secrete parathyroid hormone, or parathormone. The adrenal glands, or suprarenal glands, consist of two regions: an inner adrenal medulla and an outer adrenal cortex. The adrenal cortex produces corticosteroids from three distinct regions: the zona glomerulosa, the zona fasciculata, and the zona reticularis.
The adrenal medulla contains chromaffin cells that synthesize epinephrine and norepinephrine (stimulus is acetylcholine released by preganglionic sympathetic fibers).
Insulin is an anabolic hormone and will stimulate not only glucose uptake but also storage in the form of glycogen (glycogenesis), fat (lipogenesis) and amino acid incorporation into proteins (inhibits amino acid breakdown by liver to form new glucose molecules - gluconeogenesis). Stimuli for insulin release are primarily high blood glucose levels but insulin release is also potentiated by rising blood levels of amino acids and fatty acids and release of acetylcholine by parasympathetic neurons (all of these things happen after a meal).
Glucagon is released by the pancreas in response to low blood glucose levels (primarily) and raises blood glucose levels back to within normal range by stimulating glycogenolysis, gluconeogenesis, and release of glucose to the blood by the liver. Indirectly receives input from the visual pathways in order to determine the timing of day and night. Adipose tissue produces leptin, which acts on the CNS to produce a feeling of satiety; secretion is proportional to fat stores.
Adipocytes also produce adiponectin, which enhances insulin activity, and resistin, an insulin antagonist. Osteoblasts in bone produce osteocalcin, which stimulates pancreatic beta cells to divide and secrete more insulin. Adiponectin levels are low in type II diabetes, suggesting higher levels may help reverse the insulin resistance characteristic of type II diabetes. Endocrine glands derived from mesoderm produce steroid hormones; those derived from ectoderm or endoderm produce amines, peptides, or protein hormones. Environmental pollutants have been demonstrated to have effects on sex hormones, thyroid hormone, and glucocorticoids. We will be provided with an authorization token (please note: passwords are not shared with us) and will sync your accounts for you. Pathological mutations in tRNA genes and tRNA processing enzymes are numerous and result in very complicated clinical phenotypes.
In this review, we survey the known and emerging connections between tRNA and human diseases, and the role of its key cellular partners in pathophysiology. Mitochondria perform the essential function of synthesizing ATP in eukaryotic cells, and this cellular energy resource powers the biosynthesis of key metabolites, mechanochemical and transport functions, and a vast array of other activities. It is well established that many different mt-tRNA mutations can cause a wide range of human diseases. The two classic well-characterized diseases associated with mt-tRNA mutations are mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) and myoclonic epilepsy with ragged red fibers (MERRF).
Other mt-tRNA mutations associated with MELAS and MERRF phenotypes have been isolated in the gene encoding mt-tRNAHis (Figure 1). Diseases related to mitochondrial tRNA mutations can manifest in very specific tissue types, leading to complex phenotypes.
In addition to the classic MELAS and MERRF cases, mutations in other mt-tRNAs are also linked to sensorineural defects. Like the auditory system, the visual system is similarly sensitive to mutations in mt-tRNA. In some systems, a tRNA-linked disorder that is primarily a sensory defect may exert negative effects on additional brain and motor neuron functions. While most cells are roughly spherical structures with diameters in the range of micrometers to tens of micrometers, neurons possess irregular structures with long dendrites and axons that extend out from the soma at distances from millimeters to a meter.
Neuronal synapses are rich in mitochondria, both at the presynaptic and postsynaptic nerve terminals, and recent work suggests that there may be differences between synaptic vs non-synaptic mitochondria (Gillingwater and Wishart, 2013). These issues bear on the investigation of the above described neurobiological phenotypes, where defects arising from mt-tRNA mutation or mt-ARS mutations cause losses of mitochondrial function that are concentrated at the synapse (Baloh, 2008).
Therapeutic progress toward cures for mitochondrial diseases has been slowed by the complex genetics associated with mitochondrial inheritance.
Other therapeutic options under active investigation for mitochondrial diseases include cytoplasmic transfer, gene therapy, or even eliminating dysfunctional mitochondria by altering mitochondrial dynamics to favor quality control mechanisms (Dimauro et al., 2013). Mitochondrial gene replacement offers one of the most dramatic routes to treatment of mitochondrial disease. Inhibition of host phospholipase D suppresses influenza replication in cell culture and in vivo.
Despite widespread vaccination programs, influenza and pneumonia together are the eighth leading cause of death in the United States, costing $40 billion dollars to the economy in 2005. The influenza virus enters a host cell following interaction between the viral hemagglutinin protein (HA) and a sialic acid-containing protein on the cell surface, which triggers endocytosis of the viral particle (Figure 1).
The two PLD isoforms, PLD1 and PLD2, catalyze the hydrolysis of phosphatidylcholine (PC) to phosphatidic acid (PA) (Figure 2A). The researchers began their investigation by looking at the effects of H1N1 influenza virus infection on PLD activity in A549 human adenocarcinomic alveolar basal epithelial cells. Inoculation of A549 cell cultures with influenza virus in the presence of n-butanol resulted in a reduced number of infected cells after 24 h, as compared to cultures inoculated in the presence of t-butanol, which cannot be utilized by PLD. To see if VU036739 could suppress influenza virus infection in vivo, the investigators treated mice every 8 h, starting 1 day before and continuing through 8 h after infection with an H1N1 strain of flu. Treatment of A549 cells with VU036739 resulted in an inhibition of endocytosis kinetics, as observed by a transferrin uptake assay. To evaluate the interaction between PLD2 inhibition and the immune response, the investigators used an siRNA screen of important viral immune mediators. Together, the results demonstrate that inhibition of PLD2 is a promising host-based target for therapy against influenza virus. Vanderbilt University is committed to principles of equal opportunity and affirmative action. As HIV-1 is an RNA virus, to integrate into the genome of its host cell it must first be converted into DNA by reverse transcription.
As shown in the accompanying figure, coated vesicles internalize most extracellular macromolecules by endocytosis to form early endosomes, which move from the plasma membrane towards the cell nucleus. RBX1 binds to the E2 ubiquitin-conjugating enzyme (UBC) that was previously charged with ubiquitin (Ub) by an E1 ubiquitin-activating enzyme (UBA). The stimulus for GHRH release is low blood levels of GH as well as hypoglycemia, low blood levels of fatty acids, and high blood levels of amino acids. Hypersecretion of GH in childhood results in gigantism; in adulthood hypersecretion of GH causes acromegaly (increase in size of flat bones after epiphyseal plates of long bones have sealed). Thryroid releasing hormone (TRH) from the hypothalamus stimulates TSH release; Thyroid hormone (Thyroxine) exerts negative feedback control of both TRH and TSH.

Excretion of ketoacids (with their negative charge) by the kidney is accompanied by loss of cations, particularly K+ and Na+. Secretion of resistin is proportional to fat stores; secretion of adiponectin is inversely proportional to fat stores. This means that you will not need to remember your user name and password in the future and you will be able to login with the account you choose to sync, with the click of a button. This page doesn't support Internet Explorer 6, 7 and 8.Please upgrade your browser or activate Google Chrome Frame to improve your experience. This report was followed by the demonstration that tRNALys is associated with MERRF (Shoffner et al., 1990). We begin in the mitochondria, as only mitochondrial tRNA mutations have been found, and briefly review how mitochondrial biology contributes to the effects of tRNA mutations. In addition to these functional roles, the mitochondria also generate and regulate the production of reactive oxygen species, as well as activate important cellular pathways such as apoptosis. High-energy consuming tissues such as muscular and nervous systems are particularly affected by mitochondrial defects. These are linked to mutations in mt-tRNALeu and mt-tRNALys respectively (Suzuki et al., 2011). Cybrid cells are created by fusion of cells that are free of mitochondria (rho 0 cells) with donor platelets, or by fusion of enucleated patient derived fibroblasts with osteosarcoma cells that lack mtDNA. For example, the heart is heavily reliant on mitochondrial ATP production for proper and synchronized muscle contraction, and is thus particularly sensitive to mt-tRNA mutations. A number of reports suggest that inner ear hair cells and their connecting neuronal circuitry may be specifically affected by mt-tRNA mutations.
For example, carriers of G12183A mutation in mt-tRNAHis often present with a pigmentary retinopathy that leads to photoreceptor degeneration, pigmentary deposits in the retina, and ultimately progressive loss of vision. While there was no evidence of maternal inheritance, patient blood samples were homoplasmic for the wild type mt-tRNAIle, and muscle tissue obtained by biopsy was heteroplasmic. One recent study reported that the pathogenic G14685A mutation in mt-tRNAGlu affects both visual and hearing systems (Lax et al., 2013). The synapse has the highest energy consumption of the neuron, and mitochondria are responsible for meeting this major energy demand.
In one model, specialized synaptic mitochondria may be synthesized at the soma and then trafficked to the synapse, where they adapt to the specific demands of this specialized environment. While past work underscores how the altered structure, expression, or lack of tRNA modifications resulting from mt-tRNA mutations may disrupt protein translation (Florentz et al., 2003), the apparent tissue specific phenotype of many of these pathophysiological mutations remains to be elucidated.
In yeast, overexpression of the corresponding mt-ARS provides a rescue mechanism for pathological mt-tRNA mutations (De Luca et al., 2006).
A recent noteworthy experiment featured the transfer of the nuclear genome from a primate oocyte to an enucleated oocyte of another primate containing only mitochondria (Tachibana et al., 2009). However, there are only 61 anticodons specified by the triplet code, so many of these identified tRNA genes share the same anticodon but differ in sequence elsewhere. Acidification of the endosome as it matures leads to a conformational change in the HA protein that results in fusion of the viral membrane with the endosomal membrane.
The virus binds to sialic acid-containing proteins on the cell surface through association with the viral hemagglutinin proteins (HA1, HA2). However, if available, the enzyme will preferentially use a small primary alcohol in the place of water, yielding a phosphatidylalcohol and choline as products. Using the phosphatidylbutanol assay, they discovered that influenza infection elicited an increase in PLD activity, and they found that PLD and the influenza nucleoprotein (NP) accumulate at the periphery of the cell and then move together to a perinuclear region.
They found reduced viral titres in the lungs of the inhibitor-treated mice compared to those of the vehicle-treated control mice. The inhibitor also suppressed the recruitment of the key membrane trafficking proteins, clathrin, Rab5, and CD63. They found that siRNA knockdown of IRF3, Rig-I, or MxA led to a failure of influenza virus suppression by VU036739.
It appears that PLD2 suppresses viral uptake and endosomal processing, giving the cell more time to mount an anti-viral defense. The viral DNA then forms part of the pre-integration complex with several proteins, including the integrase enzyme that splices it into the host DNA. Since then, multiple mutations in individual tRNA genes have been associated with multiple diseases, and individual diseases have been found to be caused by mutations in one of several tRNAs. Mutations in the canonical tRNA partner, aminoacyl tRNA-synthetases (ARSs), were first identified as causative agents of Charcot-Marie-Tooth in 2003 (Antonellis et al., 2003). Several tissue-specific diseases are reviewed, followed by a discussion of potential therapeutics. The 16.59 kb double stranded circular mitochondrial genome is located within the inner mitochondrial membrane, and encodes for a total of 37 genes. Unlike chromosomal DNA localized to the nucleus and subject to Mendelian inheritance, mitochondrial DNA is inherited solely from the mother and thousands of identical copies can exist per cell. Patients with MELAS present with seizures, recurrent headaches and vomiting, anorexia, exercise intolerance, and proximal limb weakness; these are often seen early in childhood. However, detailed immunohistochemistry and biochemical studies on the hearts of patients after transplant surgery indicated a large proportion of COX-deficient cardiomyocytes and defects in activity for respiratory chain complexes I, III, and IV (Giordano et al., 2013). In one report, a late onset hearing impairment in a Chinese family was linked to a T12201C mutation in the acceptor arm of mt-tRNAHis, with the severity of hearing impairment being linked to the degree of heteroplasmy (Figure 1) (Yan et al., 2011).
Specific muscles responsible for ocular movement are another target in the visual system susceptible to mt-tRNA mutations.
Analysis of the latter tissue indicated a link between the mutation and respiratory chain defects, leading to abnormal mitochondrial morphology. In this case, the 7 year old affected subject suffered from cataracts, peripheral retinal degeneration, and pigmentary retinopathy.
Among the important ATP dependent processes occurring at the synapse are vesicle exocytosis and endocytosis, maintenance of membrane potential by ion channels, and actin rearrangements of the cytoskeleton.
Alternatively, synapse specific mitochondria may be generated, selected for, and then trafficked to the synapse.
For diseases that affect basic neurobiologial functions, mtDNA tRNA mutations may affect processes beyond mitochondrial translation, including the mitochondrial inner membrane (MIM) lipid environment, dynamics, maintenance, and replication machinery (Dimauro et al., 2013). The oocytes generated contained the nuclear genome from two parents but mitochondria from the donor; when implanted in pseudopregnant mother they were able to successfully produce healthy rhesus macaque offspring. Remarkably, a human disease that is linked to a mutation in a cytoplasmic tRNA has not yet been reported, and this may be a direct result of the presence of multiple paralogs of the gene encoding each cytoplasmic tRNA molecule.
Drugs currently available for treatment of flu target proteins coded by the influenza virus. This allows the viral RNA and associated proteins to escape into the cell cytoplasm, which is followed by movement of the viral components to the nucleus. Frequently, investigators use this reaction to measure PLD activity or to assess the effect of preventing PA formation. Treating the cells with the PLD2-selective inhibitor VU0364739 (Figure 3) eliminated the increase in enzyme activity observed upon influenza virus infection.
Similarly, mice treated with VU036739 every 12 h starting one day before and continuing for 3 days after a lethal infection with H1N1 virus showed significantly improved survival and delayed mortality. These results suggest that the suppression of influenza virus replication observed in the presence of the PLD2 inhibitor is due, at least in part, to a suppression of viral endocytosis.
Thus, it is clear that inhibition of PLD2 works in concert with innate viral defense mechanisms to suppress influenza virus replication. VU036739 is remarkably nontoxic in cell culture and in vivo, further supporting the hypothesis that PLD2 is a viable therapeutic target. Jacque and Stevenson1 show that once the pre-integration complex has entered the nucleus (by an unknown mechanism), it targets a specific nuclear-membrane protein called emerin. This increasing acidity leads to the dissociation of lysosomal enzymes from mannose-6-phosphate receptors (see Fig. Disrupting this primary function affects protein synthesis and the expression, folding, and function of oxidative phosphorylation enzymes.
Next, we move into the cytoplasm to investigate tRNA interactions with other molecules—specifically modifiers and splicing endonucleases, followed by the role tRNA binding plays in regulating protein synthesis.
Thirteen of these encode for open reading frames that comprise most of the subunits of the respiratory chain complexes I (seven subunits), III (one subunit), IV (three subunits), and V (two subunits). Cells that carry a homogeneous population of the mitochondrial genome, wild type or mutated, are referred to as homoplasmic.
Recurring stroke-like episodes can progressively impair motor abilities, vision, hearing, and mentation.
Additionally, a homoplasmic A12146G mutation was identified in another MELAS patient (Calvaruso et al., 2011). Mutations discussed in this review that cause neurosensory disease (blue), cardiomyopathy (red), MELAS and MERRF (green), and other pathological reported mutations not discussed in this review (gray). Also, there appeared to be reduced expression of mt-tRNAIle in both left and right ventricle samples from two patients compared with control mt-tRNALeu. For this mutation, cellular and biochemical analysis point to reduction in the levels of mt-tRNAHis as being the source of reduced mitochondrial translation and subsequent respiratory chain defects. Chronic progressive external ophthalmoplegia (CPEO) is a neuromuscular disorder characterized by the loss of extraocular muscle mobility, which results in the inability to move the eyes.
The G4308A substitution leads to a misfolded conformation that is likely to be incompatible with 3′ end processing by tRNase Z. Accordingly, synaptic mitochondria may be docked or anchored in these specialized compartments in order to execute these essential functions (Court and Coleman, 2012; Sheng and Cai, 2012).

Mito-mouse models have been generated to recapitulate pathogenic mitochondrial DNA mutations (Inoue et al., 2000). Thus, selection pressure may lead to rapid acquisition of resistance through mutation of the genes for these proteins. There, translation and replication of the viral RNA produces new viral macromolecules, which are ultimately assembled into complete viral particles for release and infection of new cells. Acidification of the endosome causes a conformational change in the HA proteins that leads to fusion between the viral membrane and the endosomal membrane. However, complete ablation of PLD activity required siRNA-mediated knockdown of both PLD isoforms. Combination of PLD2 knockdown with exposure to VU036739 was not significantly more effective than either treatment alone, confirming that the mechanism of action in both cases is suppression of PLD2 activity.
Under these conditions, VU036739 produced relatively few cures, but its effects were remarkable considering the fact that the compound has not been optimized for in vivo administration.
Further work will be required to reach the full potential of this exciting novel approach to viral infection. Emerin forms part of the lamina beneath the nuclear membrane, where lamin proteins provide structural support for the nucleus. Mitochondrial tRNA mutations manifest in a wide panoply of diseases related to cellular energetics, including COX deficiency (cytochrome C oxidase), mitochondrial myopathy, MERRF (Myoclonic Epilepsy with Ragged Red Fibers), and MELAS (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes).
The primary role of tRNA is to deliver amino acids to the nascent polypeptide chain during protein translation, a seemingly irreplaceable function. Finally we review the canonical role of tRNA interactions with aminoacyl synthetases, with an emphasis on recent disease discoveries.
The remaining subunits of the respiratory chain complex are encoded by genes located in the nuclear genome. A condition in which more than one type of mitochondrial genome exists, either in a cell, tissue or in an individual, is referred to as heteroplasmy. MELAS is primarily caused by defects in respiratory chain complexes I and IV, which lead to impaired oxidative phosphorylation.
These wobble position modifications are both essential for proper decoding by the mitochondrial ribosome (Kirino et al., 2004). As in the previous example, an analysis of cybrid cells and patient muscle biopsy samples showed that deficiencies in various respiratory chain complexes enzymes were linked to the mt-tRNAHis mutation. Electron microscopy imaging carried out on patient cardiomyocytes showed a loss of sarcomere formation and an accumulation of mitochondria with aberrant morphology.
This suggests that the phenotype may arise from an increasing number of COX-deficient cardiomyocytes; however, the numbers of COX-deficient cells in cardiac muscle have been shown to increase with age.
Two related patients carrying a mt-tRNAHis inherited heteroplasmic G12183A mutation both developed sensorineural pathology, presenting with visual impairment around the average age of 9 (Crimi et al., 2003).
A G4302A mt-tRNAIle mutation located in the variable arm was also found in a small number of CPEO patients (Berardo et al., 2010). By the age of 40, the patient developed symptoms of spastic paraparesis, ataxia, slurred speech, and incontinence. Owing to their ability to be transported along filamentous structures in the cell, mitochondria are motile and can be readily localized at the synapses and other high demand areas.
In a zebrafish model, the loss-of-function mutation demonstrated that indeed transport of mitochondria along the axon is disrupted and may be the contributing factor in the CMT2A phenotype (Chapman et al., 2013).
Recently this group of investigators generated a MERRF mito-mouse model incorporating the mt-tRNALys G7731A mutation (Shimizu et al., 2014), arguably the first of many such models. This led Vanderbilt Institute of Chemical Biology members Alex Brown and Craig Lindsley to take a novel approach to antiviral therapeutics, namely targeting a host protein required for flu infection.
The involvement of PLD in endocytosis and membrane trafficking led the investigators to postulate that the enzyme may be required for successful virus infection. VU036739 suppressed replication of multiple influenza strains, including several that are highly virulent.
The host DNA may be associated with the lamina through the BAF protein, which binds to LEM-domain proteins such as emerin.
Late endosomes also fuse with primary lysosomes (which contain lysosomal hydrolases and bud from the Golgi) to form secondary lysosomes. Diseases caused by mt-tRNA mutations can also affect very specific tissue types, as in the case of neurosensory non-syndromic hearing loss and pigmentary retinopathy, diabetes mellitus, and hypertrophic cardiomyopathy.
Should a mutation spontaneously arise or be maternally inherited, mitochondria carrying the pathogenic mutated mtDNA can accumulate over the course of organismal development.
Additional details concerning the biochemical affects of these taurine defective mutations have been recently reviewed (Suzuki et al., 2011). Despite a predicted disruption of a conserved base pair in mt-tRNAIle, a muscle biopsy showed normal function of all of the respiratory chain complex proteins (Berardo et al., 2010).
Subsequent immunohistochemical (IHC) and biochemical analysis of muscle and brain tissues collected post mortem indicated respiratory chain complex I deficiencies. Additionally, mitochondria are dynamic and can undergo major structural changes such as fusion, fission, and fragmentation. The issue of how mitochondrial movement may be regulated, and the role of the other essential factors in that process is discussed in detail elsewhere (Schwarz, 2013). The animals generated carried varying levels of the mt-tRNALys G7731A mutation, and these levels correlated with the severity of the disease phenotype. They now report that a selective inhibitor of human phospholipase D (PLD) suppresses influenza virus replication in cells and in vivo (T.
Their discovery of potent and selective PLD inhibitors provided the necessary tools to test this hypothesis.
Importantly, mitochondrial heteroplasmy plays a role in disease severity and age of onset as well. All of the RNA molecules necessary for mitochondrial translation are provided for by the mitochondrial genome.
When the mutational load of a certain tissue type reaches a particular threshold, disease symptoms can become evident. This mt-tRNALeu A3243G mutation was initially confirmed in 5 patients with MELAS syndrome (Kirino et al., 2005). The older sibling had pigmentary retinopathy, neurosensory deafness, and some muscle atrophy and ataxia, while the younger sibling only experienced both visual and inner ear neurosensory deficits. Additional evidence suggested that this heteroplasmic mutation arose sporadically, with brain regions exhibiting the highest degree of heteroplasmy. Clearly, how specific mt-tRNA mutations affect biological functions of mitochondria is only beginning to be understood, and will remain a fertile area of research for the foreseeable future. While this three-parent in vitro fertilization shows tremendous promise as means to produce a child without mitochondrial DNA mutations that retains both parents' nuclear genome, the approach does not address curing existing patients with mitochondrial disease, or treating mtDNA mutations that arise spontaneously. Also shown is the entry mechanism for the human immunodeficiency virus (HIV-1), which is not discussed in this article. Not surprisingly, mutations in enzymes that modify cytoplasmic and mitochondrial tRNAs are also linked to a diverse range of clinical phenotypes.
Do tRNAs play additional roles in mitochondria or in the cell apart from translation, for example in organelle localization or replication, cell death, membrane environment, or DNA replication (Dimauro et al., 2013)?
However, many proteins necessary to perform protein synthesis, including mitochondrial aminoacyl tRNA synthetases (mt-ARS), ribosomal proteins, and tRNA processing enzymes are encoded in the nuclear genome.
Interestingly, the A8344G mutation associated with MERRF is also located in a wobble base position, in this case in mt-tRNALys.
Most reported CPEO cases involving MTTI mutations are heteroplasmic and restricted to skeletal muscle. This work highlights the promise of this technology in generating future mt-tRNA disease models. Secondary lysosomes might remain in the cell and become residual bodies, or be transported to the cell surface, where they fuse with the plasma membrane and exocytose their digested materials.
About 1700 mitochondrial proteins are coded in the nuclear genome, and it is unknown how mutated mt-tRNA might interact with these. MTTI mutations exist that are primarily homoplasmic and can be found in various tissue types.
To date there are 12 mutations in the MTTE gene encoding mt-tRNAGlu that cause mitochondrial pathologies with very different phenotypes, such as encephalomyopathy, retinopathy, MELAS, and Leber's hereditary optic neuropathy (LHON). Some of these pathological mutations in tRNAs and processing enzymes are likely to affect non-canonical tRNA functions, and contribute to the diseases without significantly impacting on translation. The molecular rationale for the tight linkage between mt-tRNAIle and CPEO provides a fascinating research question for future study. This chapter will review recent literature on the relation of mitochondrial and cytoplasmic tRNA, and enzymes that process tRNAs, to human disease. Mitochondrial tRNA structures can differ significantly from cytosolic tRNA (Florentz et al., 2003) and many require specific complex modifications in order to fold correctly.
We explore the mechanisms involved in the clinical presentation of these various diseases with an emphasis on neurological disease.
Here we will review some well-characterized and recently reported mt-tRNA mutations, particularly as they relate to neurobiology and specific human disease phenotypes.

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