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MT-RNR2 | |||||||||||||||||||||||||||||||||||||||||||||||||||
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Identifiers | |||||||||||||||||||||||||||||||||||||||||||||||||||
Aliases | RNR2 , mitochondrially encoded 16S RNA, MTRNR2 | ||||||||||||||||||||||||||||||||||||||||||||||||||
External IDs | OMIM: 561010; MGI: 102492; GeneCards: RNR2; OMA:RNR2 - orthologs | ||||||||||||||||||||||||||||||||||||||||||||||||||
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Humanin is a micropeptide encoded in the mitochondrial genome by the 16S ribosomal RNA gene, MT-RNR2. Its structure contains a three-turn α-helix, and no symmetry.
In in vitro and animal models, it appears to have cytoprotective effects. [3] [4] [5] [6]
Humanin is encoded in the mitochondrial genome by the 16S ribosomal RNA gene, MT-RNR2. [7] Multiple paralogs are found in the nuclear genome (due to nuclear mitochondrial DNA segments) and are named MTRNR2L followed by a number. It is not entirely sure whether these paralogous isoforms are completely unexpressed. [8]
The expressed peptide [9] contains a three-turn α-helix, and has no symmetry. [9]
The length of the peptide depends on where it is produced. If it is produced inside the mitochondria it will be 21 amino acids long. [10] If it is produced outside the mitochondria, in the cytosol, it will be 24 amino acids long. [10] Both peptides have been shown to have biological activity. [10] [11]
Humanin is the most well-conserved of the mitochondria-derived peptides, found in such diverse species as humans, naked mole rats, and nematodes. [5] Overexpression of humanin in Caenorhabditis elegans has been shown to extend the lifespan of that nematode by increasing autophagy. [5]
The rat, Rattus norvegicus, has a gene, rattin ( C0HLU6 , "Humanin-like protein"), that encodes a 38 amino acid peptide homologous to humanin. [12] The two genes produce cDNAs that show 88% sequence identity. [12] The peptides are 81% identical, with the carboxyl terminal sequence in rattin being 14 amino acids longer than in humanin. [12] Of the 24 amino acids in the rest of the rat sequence, 20 are identical to the amino acids in the human sequence. [12]
The mouse MT-RNR2 humanin ortholog is a pseudogene, so no humanin is produced from the mtDNA. However, the nuclear genome harbors (like in humans) many copies of mitochondrial genomes, and one copy of the humanin homolog, Gm20594 ( J3QJY3 ), is actively expressed. [13]
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Humanin has several cytoprotective effects. [14]
Extracellular interaction with a tripartite receptor composed of gp130, WSX1, and CNTFR, as well as interaction with the formyl peptide receptor 2 (formylpeptide-like-1 receptor) have been published. [15] [16]
Intracellular interaction with BAX, tBID, IGFBP3, and TRIM11 may also be required for the effects of humanin. [11] [17] [18] [19]
Humanin was the first mitochondria-derived peptide to be discovered. [5] Humanin was independently found by three different labs looking at different parameters. The first to publish, in 2001, was the Nishimoto lab, which found humanin while looking for possible proteins that could protect cells from amyloid beta, a major component of Alzheimer's disease. [7] The Reed lab found humanin when screening for proteins that could interact with Bcl-2-associated X protein (Bax), a major protein involved in apoptosis. [11] The Pinchas Cohen lab independently discovered humanin when screening for proteins that interact with IGFBP3. [17]
Experiments using cultured cells have demonstrated that humanin has both neuroprotective as well as cytoprotective effects and experiments in rodents have found that it has protective effects in Alzheimer's disease models, Huntington's disease models and stroke models. [20]
Humanin is proposed to have myriad neuroprotective and cytoprotective effects. Both studies in cells and rodents have both found that administration of humanin or humanin derivatives increases survival and/or physiological parameters in Alzheimer's disease models. [21] [22] In addition to Alzheimer's disease, humanin has other neuroprotective effects against models of Huntington's disease, prion disease, and stroke. [23] [24] [25]
Beyond the possible neuroprotective effects, humanin protects against oxidative stress, atherosclerotic plaque formation, and heart attack. [26] [27] [28] [29] Humanin activates chaperone-mediated autophagy in a dose-dependent manner. [3] Humanin decreases production of inflammatory cytokines, which is part of its anti-apoptotic effect. [4] Metabolic effects have also been demonstrated and humanin helps improve survival of pancreatic beta-cells, which may help with type 1 diabetes, [30] and increases insulin sensitivity, which may help with type 2 diabetes. [31] [6] In rats, the humanin analog appears to normalize glucose levels and reduce diabetes symptoms. [32]
Rattin shows the same ability as humanin to defend neurons from the toxicity of beta-amyloid, associated with the degeneration in Alzheimer's disease. [12]
Small humanin-like peptides are a group of peptides found in the mitochondrial 16S rRNA, and also possess retrograde signaling functions.
Apoptosis is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemical events lead to characteristic cell changes (morphology) and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, DNA fragmentation, and mRNA decay. The average adult human loses 50 to 70 billion cells each day due to apoptosis. For the average human child between 8 and 14 years old, each day the approximate loss is 20 to 30 billion cells.
The cytochrome complex, or cyt c, is a small hemeprotein found loosely associated with the inner membrane of the mitochondrion where it plays a critical role in cellular respiration. It transfers electrons between Complexes III and IV. Cytochrome c is highly water-soluble, unlike other cytochromes. It is capable of undergoing oxidation and reduction as its iron atom converts between the ferrous and ferric forms, but does not bind oxygen. It also plays a major role in cell apoptosis. In humans, cytochrome c is encoded by the CYCS gene.
In excitotoxicity, nerve cells suffer damage or death when the levels of otherwise necessary and safe neurotransmitters such as glutamate become pathologically high, resulting in excessive stimulation of receptors. For example, when glutamate receptors such as the NMDA receptor or AMPA receptor encounter excessive levels of the excitatory neurotransmitter, glutamate, significant neuronal damage might ensue. Excess glutamate allows high levels of calcium ions (Ca2+) to enter the cell. Ca2+ influx into cells activates a number of enzymes, including phospholipases, endonucleases, and proteases such as calpain. These enzymes go on to damage cell structures such as components of the cytoskeleton, membrane, and DNA. In evolved, complex adaptive systems such as biological life it must be understood that mechanisms are rarely, if ever, simplistically direct. For example, NMDA in subtoxic amounts induces neuronal survival of otherwise toxic levels of glutamate.
Human mitochondrial genetics is the study of the genetics of human mitochondrial DNA. The human mitochondrial genome is the entirety of hereditary information contained in human mitochondria. Mitochondria are small structures in cells that generate energy for the cell to use, and are hence referred to as the "powerhouses" of the cell.
Neuroprotection refers to the relative preservation of neuronal structure and/or function. In the case of an ongoing insult the relative preservation of neuronal integrity implies a reduction in the rate of neuronal loss over time, which can be expressed as a differential equation.
Glucagon-like peptide-1 (GLP-1) is a 30- or 31-amino-acid-long peptide hormone deriving from the tissue-specific posttranslational processing of the proglucagon peptide. It is produced and secreted by intestinal enteroendocrine L-cells and certain neurons within the nucleus of the solitary tract in the brainstem upon food consumption. The initial product GLP-1 (1–37) is susceptible to amidation and proteolytic cleavage, which gives rise to the two truncated and equipotent biologically active forms, GLP-1 (7–36) amide and GLP-1 (7–37). Active GLP-1 protein secondary structure includes two α-helices from amino acid position 13–20 and 24–35 separated by a linker region.
Translocator protein (TSPO) is an 18 kDa protein mainly found on the outer mitochondrial membrane. It was first described as peripheral benzodiazepine receptor (PBR), a secondary binding site for diazepam, but subsequent research has found the receptor to be expressed throughout the body and brain. In humans, the translocator protein is encoded by the TSPO gene. It belongs to a family of tryptophan-rich sensory proteins. Regarding intramitochondrial cholesterol transport, TSPO has been proposed to interact with StAR to transport cholesterol into mitochondria, though evidence is mixed.
Mitofusin-2 is a protein that in humans is encoded by the MFN2 gene. Mitofusins are GTPases embedded in the outer membrane of the mitochondria. In mammals MFN1 and MFN2 are essential for mitochondrial fusion. In addition to the mitofusins, OPA1 regulates inner mitochondrial membrane fusion, and DRP1 is responsible for mitochondrial fission.
Diablo homolog (DIABLO) is a mitochondrial protein that in humans is encoded by the DIABLO gene on chromosome 12. DIABLO is also referred to as second mitochondria-derived activator of caspases or SMAC. This protein binds inhibitor of apoptosis proteins (IAPs), thus freeing caspases to activate apoptosis. Due to its proapoptotic function, SMAC is implicated in a broad spectrum of tumors, and small molecule SMAC mimetics have been developed to enhance current cancer treatments.
Serine protease HTRA2, mitochondrial is an enzyme that in humans is encoded by the HTRA2 gene. This protein is involved in caspase-dependent apoptosis and in Parkinson's disease.
17-β-Hydroxysteroid dehydrogenase X (HSD10) also known as 3-hydroxyacyl-CoA dehydrogenase type-2 is a mitochondrial enzyme that in humans is encoded by the HSD17B10 gene. Several alternatively spliced transcript variants have been identified, but the full-length nature of only two transcript variants has been determined. Human HSD10 cDNA was cloned from the brain (NM_004493), and the resulting protein, a homotetramer, was first characterized as a short chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD). Active sites of this enzyme can accommodate different substrates; 17β-HSD10 is involved in the oxidation of isoleucine, branched-chain fatty acids, and xenobiotics as well as the metabolism of sex hormones and neuroactive steroids.
Dynamin-1-like protein is a GTPase that regulates mitochondrial fission. In humans, dynamin-1-like protein, which is typically referred to as dynamin-related protein 1 (Drp1), is encoded by the DNM1L gene and is part of the dynamin superfamily (DSP) family of proteins.
Voltage-dependent anion-selective channel 1 (VDAC-1) is a beta barrel protein that in humans is encoded by the VDAC1 gene located on chromosome 5. It forms an ion channel in the outer mitochondrial membrane (OMM) and also the outer cell membrane. In the OMM, it allows ATP to diffuse out of the mitochondria into the cytoplasm. In the cell membrane, it is involved in volume regulation. Within all eukaryotic cells, mitochondria are responsible for synthesis of ATP among other metabolite needed for cell survival. VDAC1 therefore allows for communication between the mitochondrion and the cell mediating the balance between cell metabolism and cell death. Besides metabolic permeation, VDAC1 also acts as a scaffold for proteins such as hexokinase that can in turn regulate metabolism.
Mitochondrially encoded 16S RNA is the mitochondrial large subunit ribosomal RNA that in humans is encoded by the MT-RNR2 gene. The MT-RNR2 gene also encodes the Humanin polypeptide that has been the target of Alzheimer's disease research.
Tumstatin is a protein fragment cleaved from collagen that serves as both an antiangiogenic and proapoptotic agent. It has similar function to canstatin, endostatin, restin, and arresten, which also affect angiogenesis. Angiogenesis is the growth of new blood vessels from pre-existing blood vessels, and is important in tumor growth and metastasis. Angiogenesis is stimulated by many growth factors, the most prevalent of which is vascular endothelial growth factor (VEGF).
Pinchas Cohen is the dean of the USC Leonard Davis School of Gerontology, holds the William and Sylvia Kugel Dean's Chair in Gerontology and serves as the executive director of the Ethel Percy Andrus Gerontology Center.
Necroptosis is a programmed form of necrosis, or inflammatory cell death. Conventionally, necrosis is associated with unprogrammed cell death resulting from cellular damage or infiltration by pathogens, in contrast to orderly, programmed cell death via apoptosis. The discovery of necroptosis showed that cells can execute necrosis in a programmed fashion and that apoptosis is not always the preferred form of cell death. Furthermore, the immunogenic nature of necroptosis favors its participation in certain circumstances, such as aiding in defence against pathogens by the immune system. Necroptosis is well defined as a viral defense mechanism, allowing the cell to undergo "cellular suicide" in a caspase-independent fashion in the presence of viral caspase inhibitors to restrict virus replication. In addition to being a response to disease, necroptosis has also been characterized as a component of inflammatory diseases such as Crohn's disease, pancreatitis, and myocardial infarction.
Blarcamesine is an experimental drug which is under development for the treatment of Alzheimer's disease and a variety of other indications.
The ion channel hypothesis of Alzheimer's disease (AD), also known as the channel hypothesis or the amyloid beta ion channel hypothesis, is a more recent variant of the amyloid hypothesis of AD, which identifies amyloid beta (Aβ) as the underlying cause of neurotoxicity seen in AD. While the traditional formulation of the amyloid hypothesis pinpoints insoluble, fibrillar aggregates of Aβ as the basis of disruption of calcium ion homeostasis and subsequent apoptosis in AD, the ion channel hypothesis in 1993 introduced the possibility of an ion-channel-forming oligomer of soluble, non-fibrillar Aβ as the cytotoxic species allowing unregulated calcium influx into neurons in AD.
Small humanin-like peptides (SHLPs) are a group of peptides encoded in the 16S ribosomal RNA region of mitochondrial genome. Six peptides of this group (SHLP1–6) have been identified so far, each peptide is of 20-38 amino-acid long. They are derived from the mitochondria and act as important retrograde signaling molecules in the cell. Their names were given because the SHLPs share some similar biological effects with the mitochondrial peptide Humanin.