Down-regulation of protein L-isoaspartyl methyltransferase in human epileptic hippocampus contributes to generation of damaged tubulin

Maize PIMT2 repairs damaged 3-METHYLCROTONYL COA CARBOXYLASE in mitochondria, affecting seed vigor

PROTEIN l-ISOASPARTYL O-METHYLTRANSFERASE (PIMT) affects seed vigor by repairing damaged proteins. While PIMT is capable of isoaspartyl (isoAsp) repair in all proteins, those proteins most susceptible to isoAsp formation have not been well characterized, and the mechanisms by which PIMT affects seed vigor remain largely unknown. Using co-immunoprecipitation and LC-MS/MS, we found that maize (Zea mays) PIMT2 (ZmPIMT2) interacted predominantly with both subunits of maize 3-METHYLCROTONYL COA CARBOXYLASE (ZmMCC). ZmPIMT2 is specifically expressed in the maize embryo. Both mRNA and protein levels of ZmPIMT2 increased during seed maturation and declined during imbibition. Maize seed vigor was decreased in the zmpimt2 mutant line, while overexpression of ZmPIMT2 in maize and Arabidopsis thaliana increased seed vigor upon artificial aging. ZmPIMT2 was localized in the mitochondria, as determined by subcellular localization assays using maize protoplasts. ZmPIMT2 binding to ZmMCCα was confirmed by luciferase complementation tests in both tobacco (Nicotiana benthamiana) leaves and maize protoplasts. Knockdown of ZmMCCα decreased maize seed aging tolerance. Furthermore, overexpression of ZmPIMT2 decreased the accumulation of isoAsp of ZmMCCα protein in seed embryos that underwent accelerated aging treatment. Taken together, our results demonstrate that ZmPIMT2 binds ZmMCCα in mitochondria, repairs isoAsp damage, and positively affects maize seed vigor.

Temporal Lobe Epilepsy: What do we understand about protein alterations?

During neuronal diseases, neuronal proteins get disturbed due to changes in the connections of neurons. As a result, neuronal proteins get disturbed and cause epilepsy. At the genetic level, many mutations may take place in proteins like axon guidance proteins, leucine-rich glioma inactivated 1 protein, microtubular protein, pore-forming, chromatin remodeling, and chemokine proteins which may lead toward temporal lobe epilepsy. These proteins can be targeted in the future for the treatment purpose of epilepsy. Novel avenues can be developed for therapeutic interventions by these new insights.

PROTEIN l-ISOASPARTYL METHYLTRANSFERASE (PIMT) in plants: regulations and functions

Proteins are essential molecules that carry out key functions in a cell. However, as a result of aging or stressful environments, the protein undergoes a range of spontaneous covalent modifications, including the formation of abnormal l-isoaspartyl residues from aspartyl or asparaginyl residues, which can disrupt the protein's inherent structure and function. PROTEIN l-ISOASPARTYL METHYLTRANSFERASE (PIMT: EC 2.1.1.77), an evolutionarily conserved ancient protein repairing enzyme (PRE), converts such abnormal l-isoaspartyl residues to normal l-aspartyl residues and re-establishes the protein's native structure and function. Although originally discovered in animals as a PRE, PIMT emerged as a key PRE in plants, particularly in seeds, in which PIMT plays a predominant role in preserving seed vigor and viability for prolonged periods of time. Interestingly, higher plants encode a second PIMT (PIMT2) protein which possesses a unique N-terminal extension, and exhibits several distinct features and far more complexity than non-plant PIMTs. Recent studies indicate that the role of PIMT is not restricted to preserving seed vigor and longevity but is also implicated in enhancing the growth and survivability of plants under stressful environments. Furthermore, expression studies indicate the tantalizing possibility that PIMT is involved in various physiological processes apart from its role in seed vigor, longevity and plant's survivability under abiotic stress. This review article particularly describes new insights and emerging interest in all facets of this enzyme in plants along with a concise comparative overview on isoAsp formation, and the role and regulation of PIMTs across evolutionary diverse species. Additionally, recent methods and their challenges in identifying isoaspartyl containing proteins (PIMT substrates) are highlighted.

Fluorogenic probes for disease-relevant enzymes

Traditional biochemical methods for enzyme detection are mainly based on antibody-based immunoassays, which lack the ability to monitor the spatiotemporal distribution and, in particular, the in situ activity of enzymes in live cells and in vivo. In this review, we comprehensively summarize recent progress that has been made in the development of small-molecule as well as material-based fluorogenic probes for sensitive detection of the activities of enzymes that are related to a number of human diseases. The principles utilized to design these probes as well as their applications are reviewed. Specific attention is given to fluorogenic probes that have been developed for analysis of the activities of enzymes including oxidases and reductases, those that act on biomacromolecules including DNAs, proteins/peptides/amino acids, carbohydrates and lipids, and those that are responsible for translational modifications. We envision that this review will serve as an ideal reference for practitioners as well as beginners in relevant research fields.

Emerging blood–brain-barrier-crossing nanotechnology for brain cancer theranostics

The advancements, perspectives, and challenges in blood–brain-barrier (BBB)-crossing nanotechnology for effective brain tumor delivery and highly efficient brain cancer theranostics.

Recent advances in mass spectrometric analysis of protein deamidation

Protein deamidation has been proposed to represent a "molecular clock" that progressively disrupts protein structure and function in human degenerative diseases and natural aging. Importantly, this spontaneous process can also modify therapeutic proteins by altering their purity, stability, bioactivity, and antigenicity during drug synthesis and storage. Deamidation occurs non-enzymatically in vivo, but can also take place spontaneously in vitro, hence artificial deamidation during proteomic sample preparation can hamper efforts to identify and quantify endogenous deamidation of complex proteomes. To overcome this, mass spectrometry (MS) can be used to conduct rigorous site-specific characterization of protein deamidation due to the high sensitivity, speed, and specificity offered by this technique. This article reviews recent progress in MS analysis of protein deamidation and discusses the strengths and limitations of common "top-down" and "bottom-up" approaches. Recent advances in sample preparation methods, chromatographic separation, MS technology, and data processing have for the first time enabled the accurate and reliable characterization of protein modifications in complex biological samples, yielding important new data on how deamidation occurs across the entire proteome of human cells and tissues. These technological advances will lead to a better understanding of how deamidation contributes to the pathology of biological aging and major degenerative diseases. © 2016 Wiley Periodicals, Inc. Mass Spec Rev 36:677-692, 2017.

Insight of brain degenerative protein modifications in the pathology of neurodegeneration and dementia by proteomic profiling

Dementia is a syndrome associated with a wide range of clinical features including progressive cognitive decline and patient inability to self-care. Due to rapidly increasing prevalence in aging society, dementia now confers a major economic, social, and healthcare burden throughout the world, and has therefore been identified as a public health priority by the World Health Organization. Previous studies have established dementia as a 'proteinopathy' caused by detrimental changes in brain protein structure and function that promote misfolding, aggregation, and deposition as insoluble amyloid plaques. Despite clear evidence that pathological cognitive decline is associated with degenerative protein modifications (DPMs) arising from spontaneous chemical modifications to amino acid side chains, the molecular mechanisms that promote brain DPMs formation remain poorly understood. However, the technical challenges associated with DPM analysis have recently become tractable due to powerful new proteomic techniques that facilitate detailed analysis of brain tissue damage over time. Recent studies have identified that neurodegenerative diseases are associated with the dysregulation of critical repair enzymes, as well as the misfolding, aggregation and accumulation of modified brain proteins. Future studies will further elucidate the mechanisms underlying dementia pathogenesis via the quantitative profiling of the human brain proteome and associated DPMs in distinct phases and subtypes of disease. This review summarizes recent developments in quantitative proteomic technologies, describes how these techniques have been applied to the study of dementia-linked changes in brain protein structure and function, and briefly outlines how these findings might be translated into novel clinical applications for dementia patients. In this review, only spontaneous protein modifications such as deamidation, oxidation, nitration glycation and carbamylation are reviewed and discussed.

Dementia-linked amyloidosis is associated with brain protein deamidation as revealed by proteomic profiling of human brain tissues

Background

Aggregation of malformed proteins is a key feature of many neurodegenerative diseases, but the mechanisms that drive proteinopathy in the brain are poorly understood. We aimed to characterize aggregated proteins in human brain tissues affected by dementia.

Results

To characterize amyloidal plaque purified from post-mortem brain tissue of dementia patient, we applied ultracentrifugation-electrostatic repulsion hydrophilic interaction chromatography (UC-ERLIC) coupled mass spectrometry-based proteomics technologies. Proteomics profiling of both soluble and aggregated amyloidal plaque demonstrated significant enrichment and deamidation of S100A9, ferritin, hemoglobin subunits, creatine kinase and collagen protein among the aggregated brain proteins. Amyloidal plaques were enriched in the deamidated variant of protein S100A9, and structural analysis indicated that both the low- and high-affinity calcium binding motifs of S100A9 were deamidated exclusively in the aggregated fraction, suggesting altered charge state and function of this protein in brain tissues affected by dementia. The multiple deamidated residues of S100A9 predicts introduction of negative charge that alter Ca⁺⁺ binding, suggesting increased capacity to form pathological aggregates in the brain.

Conclusion

UC-coupled proteomics revealed that brain amyloidal plaques are enriched in deamidated proteins, and suggested that altered charge state and calcium-binding capacity of S100A9 may enhance protein aggregation and promote neurodegeneration in the human brain.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-016-0200-z) contains supplementary material, which is available to authorized users.

Association of Microtubule Dynamics with Chronic Epilepsy

Approximately 30 % of epilepsy cases are refractory to current pharmacological treatments through unknown mechanisms. Much work has been done on the role of synaptic components in the pathogenesis of epilepsy, but relatively little attention has been given to the potential role of the microtubules. We investigated the level of microtubule dynamic in 30 human epileptic tissues and two different chronic epilepsy rat models. The administration of microtubule-modulating agent attenuated the progression of chronic epilepsy. By contrast, microtubule-depolymerizing agent aggravated the progression of chronic epilepsy. The electrophysiological index by whole-cell clamp was used to investigate the neuronal excitation and inhibitory synaptic transmission in brain slices after administration of microtubule-modulating agent and microtubule-depolymerizing agent. Interestingly, we found that microtubule-modulating agent significantly increased the frequency of action potential firing in interneurons, and significantly promoted the amplitudes and frequencies of miniature inhibitory postsynaptic currents. Microtubule-depolymerizing agent had an opposite effect. These findings suggest that modulating hyperdynamic microtubules may take an anti-epileptic effect via postsynaptic mechanisms in interneurons. It could represent a potential pharmacologic target in epilepsy treatment.

Temporal lobe proteins implicated in synaptic failure exhibit differential expression and deamidation in vascular dementia

Progressive synaptic failure precedes the loss of neurons and decline in cognitive function in neurodegenerative disorders, but the specific proteins and posttranslational modifications that promote synaptic failure in vascular dementia (VaD) remain largely unknown. We therefore used an isobaric tag for relative and absolute proteomic quantitation (iTRAQ) to profile the synapse-associated proteome of post-mortem human cortex from vascular dementia patients and age-matched controls. Brain tissue from VaD patients exhibited significant down-regulation of critical synaptic proteins including clathrin (0.29; p < 1.0⋅10(-3)) and GDI1 (0.51; p = 3.0⋅10(-3)), whereas SNAP25 (1.6; p = 5.5⋅10(-3)), bassoon (1.4; p = 1.3⋅10(-3)), excitatory amino acid transporter 2 (2.6; p = 9.2⋅10(-3)) and Ca(2+)/calmodulin dependent kinase II (1.6; p = 3.0⋅10(-2)) were substantially up-regulated. Our analyses further revealed divergent patterns of protein modification in the dementia patient samples, including a specific deamidation of synapsin1 predicted to compromise protein structure. Our results reveal potential molecular targets for intervention in synaptic failure and prevention of cognitive decline in VaD.

Brain proteomics supports the role of glutamate metabolism and suggests other metabolic alterations in protein l-isoaspartyl methyltransferase (PIMT)-knockout mice

Protein l-isoaspartyl methyltransferase (PIMT) repairs the isoaspartyl residues (isoAsp) that originate from asparagine deamidation and aspartic acid (Asp) isomerization to Asp residues. Deletion of the gene encoding PIMT in mice (Pcmt1) leads to isoAsp accumulation in all tissues measured, especially in the brain. These PIMT-knockout (PIMT-KO) mice have perturbed glutamate metabolism and die prematurely of epileptic seizures. To elucidate the role of PIMT further, brain proteomes of PIMT-KO mice and controls were analyzed. The isoAsp levels from two of the detected 67 isoAsp sites (residue 98 from calmodulin and 68 from glyceraldehyde-3-phosphate dehydrogenase) were quantified and found to be significantly increased in PIMT-KO mice (p < 0.01). Additionally, the abundance of at least 151 out of the 1017 quantified proteins was found to be altered in PIMT-KO mouse brains. Gene ontology analysis revealed that many down-regulated proteins are involved in cellular amino acid biosynthesis. For example, the serine synthesis pathway was suppressed, possibly leading to reduced serine production in PIMT-KO mice. Additionally, the abundances of enzymes in the glutamate-glutamine cycle were altered toward the accumulation of glutamate. These findings support the involvement of PIMT in glutamate metabolism and suggest that the absence of PIMT also affects other processes involving amino acid synthesis and metabolism.

Mitochondrial uncoupler carbonyl cyanide M-chlorophenylhydrazone induces the multimer assembly and activity of repair enzyme protein L-isoaspartyl methyltransferase

The protein L-isoaspartyl methyltransferase (PIMT) repairs damaged aspartyl residues in proteins. It is commonly described as a cytosolic protein highly expressed in brain tissues. Here, we report that PIMT is an active monomeric as well as a multimeric protein in mitochondria isolated from neuroblastoma cells. Upon treatments with mitochondrial uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP), PIMT monomers level decreased by half while that of PIMT multimers was higher. Gel electrophoresis under reducing conditions of CCCP-induced PIMT multimers led to PIMT monomers accumulation, indicating that multimers resulted from disulfide-linked PIMT monomers. The antioxidant ascorbic acid significantly lowered CCCP-induced formation of PIMT multimers, suggesting that reactive oxygen species contributed to PIMT multimerization. In addition, the elevation of PIMT multimers catalytic activity upon treatments with CCCP was severely inhibited by the reducing agent dithiothreitol. This indicated that PIMT monomers have lower enzymatic activity following CCCP treatments and that activation of PIMT multimers is essentially dependent on the formation of disulfide-linked monomers of PIMT. Furthermore, the perturbation of mitochondrial function by CCCP promoted the accumulation of damaged aspartyl residues in proteins with high molecular weights. Thus, this study demonstrates the formation of active PIMT multimers associated with mitochondria that could play a key role in repairing damaged proteins accumulating during mitochondrial dysfunction.

Protein l-isoaspartyl-O-methyltransferase of Vibrio cholerae: interaction with cofactors and effect of osmolytes on unfolding

Protein l-isoaspartyl-O-methyltransferase (PIMT) is an ubiquitous enzyme widely distributed in cells and plays a role in the repair of deamidated and isomerized proteins. In this study, we show that this enzyme is present in cytosolic extract of Vibrio cholerae, an enteric pathogenic Gram-negative bacterium and is enzymatically active. Additionally, we focus on the detailed biophysical characterization of the recombinant PIMT from V. cholerae to gain insight into its structure, stability and the cofactor binding. The equilibrium denaturation of PIMT has been studied using tryptophan fluorescence and CD spectroscopy. The far- and near-UV CD, as well as fluorescence experiments reveal the presence of a non-native intermediate in the folding pathway. Binding of the hydrophobic fluorescent probe, bis-ANS, to the intermediate occurs with high affinity because of the exposure of the hydrophobic clusters during the unfolding process. The existence of the probable intermediate has also been confirmed from limited tryptic digestion and DLS experiments. The protein shows higher binding affinity for AdoHcy, in comparison to AdoMet, and the binding increases the midpoint of thermal unfolding by 6 and 5 °C, respectively. Modeling and molecular dynamics simulations also support the higher stability of the protein in presence of AdoHcy.

Wortmannin reduces insulin signaling and death in seizure-prone Pcmt1-/- mice

L-isoaspartyl (D-aspartyl) O-methyltransferase deficient mice (Pcmt1^−/−) accumulate isomerized aspartyl residues in intracellular proteins until their death due to seizures at approximately 45 days. Previous studies have shown that these mice have constitutively activated insulin signaling in their brains, and that these brains are 20–30% larger than those from age-matched wild-type animals. To determine whether insulin pathway activation and brain enlargement is responsible for the fatal seizures, we administered wortmannin, an inhibitor of the phosphoinositide 3-kinase that catalyzes an early step in the insulin pathway. Oral wortmannin reduced the average brain size in the Pcmt1^−/− animals to within 6% of the wild-type DMSO administered controls, and nearly doubled the lifespan of Pcmt1^−/− at 60% survival of the original population. Immunoblotting revealed significant decreases in phosphorylation of Akt, PDK1, and mTOR in Pcmt1^−/− mice and Akt and PDK1 in wild-type animals upon treatment with wortmannin. These data suggest activation of the insulin pathway and its resulting brain enlargement contributes to the early death of Pcmt1−/− mice, but is not solely responsible for the early death observed in these animals.

Fibronectin: Functional character and role in alcoholic liver disease

Fibronectins are adhesive glycoproteins that can be found in tissue matrices and circulating in various fluids of the body. The variable composition of fibronectin molecules facilitates a diversity of interactions with cell surface receptors that suggest a role for these proteins beyond the structural considerations of the extracellular matrix. These interactions implicate fibronectin in the regulation of mechanisms that also determine cell behavior and activity. The two major forms, plasma fibronectin (pFn) and cellular fibronectin (cFn), exist as balanced amounts under normal physiological conditions. However, during injury and/or disease, tissue and circulating levels of cFn become disproportionately elevated. The accumulating cFn, in addition to being a consequence of prolonged tissue damage, may in fact stimulate cellular events that promote further damage. In this review, we summarize what is known regarding such interactions between fibronectin and cells that may influence the biological response to injury. We elaborate on the effects of cFn in the liver, specifically under a condition of chronic alcohol-induced injury. Studies have revealed that chronic alcohol consumption stimulates excess production of cFn by sinusoidal endothelial cells and hepatic stellate cells while impairing its clearance by other cell types resulting in the build up of this glycoprotein throughout the liver and its consequent increased availability to influence cellular activity that could promote the development of alcoholic liver disease. We describe recent findings by our laboratory that support a plausible role for cFn in the promotion of liver injury under a condition of chronic alcohol abuse and the implications of cFn stimulation on the pathogenesis of alcoholic liver disease. These findings suggest an effect of cFn in regulating cell behavior in the alcohol-injured liver that is worth further characterizing not only to gain a more comprehensive understanding of the role this reactive glycoprotein plays in the progression of injury but also for the insight further studies could provide towards the development of novel therapies for alcoholic liver disease.

The role of protein L-isoaspartyl/D-aspartyl O-methyltransferase (PIMT) in intracellular signal transduction

Under physiological conditions, L-aspartyl (L-Asp) and L-asparaginyl residues in proteins are spontaneously isomerized or racemized to D-aspartyl (D-Asp) or D,L-isoaspartyl (D,L-isoAsp) residue. These atypical Asp residues can interfere with protein activity and lead to disruption of cellular function. Protein L-isoaspartyl/D-aspartyl O-methyltransferase (PIMT) is a repair enzyme that initiates the conversion of L-isoAsp (or D-Asp) residues to L-Asp residues. PIMT-Deficient mice exhibit accumulation of L-isoAsp in several tissues and die from progressive epileptic seizures at a mean age of 42 days. However, the biological roles of PIMT are still largely unknown. To further our understanding of the function of this protein, we developed an assay to measure PIMT activity in cell lysates. Additionally, we generated PIMT-knockdown cells by stable transfection of HEK293 cells with PIMT small interfering (si) RNA. Northern blotting and immunoblot analysis revealed that PIMT mRNA and protein levels were significantly decreased in the knockdown cells. In addition, significant levels of proteins that contained isoAsp residues accumulated in these cells, and immunoblot analysis revealed that Raf-1, MEK, and ERK were hyperphosphorylated upon EGF stimulation compared to control cells. These results indicate that the ability to repair atypical Asp residues is important for normal MAP kinase signaling.

Substrates of the Arabidopsis thaliana protein isoaspartyl methyltransferase 1 identified using phage display and biopanning

The role of protein isoaspartyl methyltransferase (PIMT) in repairing a wide assortment of damaged proteins in a host of organisms has been inferred from the affinity of the enzyme for isoaspartyl residues in a plethora of amino acid contexts. The identification of PIMT target proteins in plant seeds, where the enzyme is highly active and proteome long-lived, has been hindered by large amounts of isoaspartate-containing storage proteins. Mature seed phage display libraries circumvented this problem. Inclusion of the PIMT co-substrate, S-adenosylmethionine (AdoMet), during panning permitted PIMT to retain aged phage in greater numbers than controls lacking co-substrate or when PIMT protein binding was poisoned with S-adenosyl homocysteine. After four rounds, phage titer plateaued in AdoMet-containing pans, whereas titer declined in both controls. This strategy identified 17 in-frame PIMT target proteins, including a cupin-family protein similar to those identified previously using on-blot methylation. All recovered phage had at least one susceptible Asp or Asn residue. Five targets were recovered independently. Two in-frame targets were produced in Escherichia coli as recombinant proteins and shown by on-blot methylation to acquire isoAsp, becoming a PIMT target. Both gained isoAsp rapidly in solution upon thermal insult. Mutant analysis of plants deficient in any of three in-frame PIMT targets resulted in demonstrable phenotypes. An over-representation of clones encoding proteins involved in protein production suggests that the translational apparatus comprises a subgroup for which PIMT-mediated repair is vital for orthodox seed longevity. Impaired PIMT activity would hinder protein function in these targets, possibly resulting in poor seed performance.

Mass spectrometric analysis of asparagine deamidation and aspartate isomerization in polypeptides

One of the most frequent modifications in proteins and peptides is the deamidation of asparagine, a spontaneous non-enzymatic reaction leading to a mixture of L,D-succinimidyl, L,D-aspartyl, and L,D-isoaspartyl forms, with L-isoaspartyl dominating. Spontaneous isomerization of L-Asp yields the same products. In vivo, these unusual forms of aspartate are repaired by the protein L-isoaspartyl O-methyltransferase enzyme, with the balance between isomerization and repair affecting the organism physiology. Mass spectrometric analysis of this balance involves isomer separation, iso-Asp/Asp quantification, and iso-Asp site identification. This review highlights the issues associated with these steps and discusses the prospects of high-throughput iso-Asp analysis.

The V119I polymorphism in protein L-isoaspartate O-methyltransferase alters the substrate-binding interface

Protein L-isoaspartate O-methyltransferase (PIMT) repairs isoaspartate residues in damaged proteins, and it contains a Val-Ile polymorphismin in alpha5, approximately 13 A from its active site. Val119 has lower activity and thermal stability but increased affinity for endogenous substrates. Studies suggest that heterozygosity for Val/Ile favors efficient isoaspartate repair. We have performed multiple molecular dynamics simulations of 119I and 119V PIMT. Both V119 and I119 interact with the same residues throughout all of the simulations. However, the larger Ile altered the orientations of alpha5 and beta5, both of which have co-substrate binding residues on their distal ends. I119 increases the flexibility of several residues, loosening up the S-adenosylmethionine (SAM)-binding site. These subtle changes are propagated towards the isoaspartate-docking site via residues common to both active sites. The increased mobility in 119I PIMT reorients alpha3, resulting in a salt-bridge network at the substrate-binding interface that disrupts several key side-chain interactions in the isoaspartate site. In contrast, 119V PIMT remains quite rigid with little change to the co-substrate binding site, which could hinder SAM's binding and release, accounting for the decreased activity. These results shed light on the molecular basis behind the decreased activity and increased specificity for endogenous substrates of 119V PIMT relative to the 119I variant. 119I PIMT catalyzes the methylation reaction but may have difficulties recognizing and orienting specific substrates due to its distorted substrate-binding site. Heterozygosity for both the Ile and Val alleles may provide the best of both worlds, allowing the fast and specific methylation of damaged proteins.

Disruption of normal cytoskeletal dynamics may play a key role in the pathogenesis of epilepsy

Epilepsy, a common disease affecting 1% to 2% of the population, is characterized by seizures, hyperexcitability at synapses, and aberrant extension of neurons following seizures. Much work has been done on the role of synaptic components in the pathogenesis of epilepsy, but relatively little attention has been given to the potential role of the cytoskeleton. The neuronal cytoskeleton consists of microtubules, actin filaments, intermediate filaments, and associated proteins. A number of mutations in both microtubule-associated proteins (MAPs) and actin-binding proteins, as well as altered expression levels of several cytoskeletal proteins, are known to be involved in epilepsy. These changes will affect the dynamics of the neuronal cytoskeleton and therefore are likely to contribute to the pathogenesis of epilepsy through mechanisms such as increased neurotrophic support to neurons and increased sprouting of mossy fibers. These changes may also contribute to hyperexcitability of neurons through an as yet unidentified mechanism.

Valproic acid enhances protein L-isoaspartyl methyltransferase expression by stimulating extracellular signal-regulated kinase signaling pathway

Proteins are susceptible to various non-enzymatic post-translational modifications occurring during aging and in certain pathological states. The protein L-isoaspartyl methyltransferase (PIMT) is an enzyme that recognizes and repairs the abnormal L-isoaspartyl residues in proteins. Recently, we reported that PIMT expression was stimulated by the anti-epileptic drug valproic acid and that this was mediated through the glycogen synthase kinase-3 (GSK-3)/beta-catenin pathway. In this study, to gain further insights into which of the signaling pathways activated by valproic acid regulate PIMT abundance, astrocytoma U-87 MG and neuroblastoma SH-SY5Y cells were treated with this drug to investigate the possible involvement of the extracellular-regulated kinase (ERK) pathway in PIMT induction. Valproic acid increased ERK1/2 phosphorylation on Thr202/Tyr204 and Thr185/Tyr187, respectively. Pharmacological inhibitors against the kinases Src, c-Raf, MEK1/2 and ERK1/2 abolished the ERK1/2 phosphorylation stimulated by valproic acid, thus preventing PIMT induction by the drug. Furthermore, MEK1/2 inhibition with U0126 blocked the higher phosphorylation of RSK-1 on Thr359/Ser363 and of GSK-3beta on Ser9 as well as the increased expression of RSK-1, beta-catenin and PIMT upon treatment with valproic acid. RSK-1 knockdown by interfering RNA abrogated the increased expression of RSK-1, beta-catenin and PIMT as well as the induced phosphorylation of RSK-1 and GSK-3beta due to valproic acid. Thus, our findings demonstrated that PIMT up-regulation by valproic acid required the activation of the ERK signaling pathway including RSK-1 the latter being responsible for inactivating GSK-3 and subsequently leading to beta-catenin stabilization.

Carnosine and its possible roles in nutrition and health

The dipeptide carnosine has been observed to exert antiaging activity at cellular and whole animal levels. This review discusses the possible mechanisms by which carnosine may exert antiaging action and considers whether the dipeptide could be beneficial to humans. Carnosine's possible biological activities include scavenger of reactive oxygen species (ROS) and reactive nitrogen species (RNS), chelator of zinc and copper ions, and antiglycating and anticross-linking activities. Carnosine's ability to react with deleterious aldehydes such as malondialdehyde, methylglyoxal, hydroxynonenal, and acetaldehyde may also contribute to its protective functions. Physiologically carnosine may help to suppress some secondary complications of diabetes, and the deleterious consequences of ischemic-reperfusion injury, most likely due to antioxidation and carbonyl-scavenging functions. Other, and much more speculative, possible functions of carnosine considered include transglutaminase inhibition, stimulation of proteolysis mediated via effects on proteasome activity or induction of protease and stress-protein gene expression, upregulation of corticosteroid synthesis, stimulation of protein repair, and effects on ADP-ribose metabolism associated with sirtuin and poly-ADP-ribose polymerase (PARP) activities. Evidence for carnosine's possible protective action against secondary diabetic complications, neurodegeneration, cancer, and other age-related pathologies is briefly discussed.

Up-regulation of protein L-isoaspartyl methyltransferase expression by lithium is mediated by glycogen synthase kinase-3 inactivation and beta-catenin stabilization

During cell aging, proteins accumulate damages, which affect their structure and activity. The protein l-isoaspartyl methyltransferase (PIMT) is involved in the repair of proteins containing abnormal L-isoaspartyl residues. Although its mechanism of action is well defined, little is known about the pathways involved in the regulation of PIMT expression. In this study, we demonstrated that glycogen synthase kinase-3 (GSK-3) and beta-catenin are involved in the regulation of PIMT expression. Treatment of astrocytoma cells (U-87) with direct pharmacological GSK-3 inhibitors such as lithium, SB-216763 and SB-415286 stimulated PIMT expression ( approximately twofold). As expected, GSK-3 inhibition led to an increase of phosphorylated GSK-3beta (Ser9) and to beta-catenin accumulation. PIMT induction by lithium was dependent on increased protein synthesis. In addition, RT-PCR analysis showed higher level of PIMT mRNA following GSK-3 inhibition, which was abolished by the transcriptional inhibitor actinomycin D. These results demonstrated regulation of PIMT expression by lithium at both the transcriptional and the translational levels. Additionally, inhibition by siRNA of GSK-3 and beta-catenin modulated the expression of the PIMT in accordance with GSK-3 pharmacological inhibition. Valproic acid, an antiepileptic drug with mood-stabilizing properties, up-regulated phospho-GSK-3beta (Ser9), beta-catenin and PIMT levels similarly to lithium. This study reports that PIMT expression is up-regulated by GSK-3 inhibition and beta-catenin stabilization upon treatments with lithium and valproic acid. These findings suggest a possible therapeutic role for PIMT in certain brain diseases including epilepsy.

Reactive oxygen species generated by thiol-modifying phenylarsine oxide stimulate the expression of protein L-isoaspartyl methyltransferase

Expression of the repair enzyme protein L-isoaspartyl methyltransferase (PIMT) has been reported to play important roles in brain. However, little is known about the regulation of PIMT expression following protein damage by oxidation in brain. Phenylarsine oxide (PAO) is an arsenical compound that alters proteins by forming disulfide bond with vicinal cysteinyl residues. Here we report that PIMT was rapidly up-regulated by PAO in U-87 human astroglioma cells. We also confirmed that PIMT up-regulation by PAO was mediated by the reaction with vicinal cysteines. Furthermore, we showed that PIMT induction by PAO was dependent on formation of reactive oxygen species (ROS). Crucially, both ROS formation and PIMT induction by PAO were inhibited by antioxidant N-acetyl-L-cysteine and NADPH oxidase inhibitor diphenyleneiodonium chloride. Importantly, down-regulation of PIMT by siRNA strikingly enhanced PAO-induced ROS. Together, these results highlight that PIMT expression is regulated by ROS and could primarily act as an antioxidant enzyme.

Role of transmethylation reactions in alcoholic liver disease

Alcoholic liver disease is a major health care problem worldwide. Findings from many laboratories, including ours, have demonstrated that ethanol feeding impairs several of the many steps involved in methionine metabolism. Ethanol consumption predominantly results in a decrease in the hepatocyte level of S-adenosylmethionine and the increases in two toxic metabolites, homocysteine and S-adenosylhomocysteine. These changes, in turn, result in serious functional consequences which include decreases in essential methylation reactions via inhibition of various methyltransferases. Of particular interest to our laboratory is the inhibition of three important enzymes, phosphatidylethanolamine methyltransferase, isoprenylcysteine carboxyl methyltransferase and protein L-isoaspartate methyltransferase. Decreased activity of these enzymes results in increased fat deposition, increased apoptosis and increased accumulation of damaged proteins-all of which are hallmark features of alcoholic liver injury. Of all the therapeutic modalities available, betaine has been shown to be the safest, least expensive and most effective in attenuating ethanol-induced liver injury. Betaine, by virtue of aiding in the remethylation of homocysteine, removes both toxic metabolites (homocysteine and S-adenosylhomocysteine), restores S-adenosylmethionine level, and reverses steatosis, apoptosis and damaged proteins accumulation. In conclusion, betaine appears to be a promising therapeutic agent in relieving the methylation and other defects associated with alcoholic abuse.

Accumulation of proteins bearing atypical isoaspartyl residues in livers of alcohol-fed rats is prevented by betaine administration: effects on protein-L-isoaspartyl methyltransferase activity

BACKGROUND/AIMS

Protein-L-isoaspartyl methyltransferase (PIMT) is a methyltransferase that plays a crucial role in the repair of damaged proteins. In this study, we investigated whether ethanol exposure causes an accumulation of modified proteins bearing atypical isoaspartyl residues that may be related to impaired PIMT activity. We further sought to determine whether betaine administration could prevent the accumulation of these types of damaged proteins.

METHODS

Livers of male Wistar rats, fed the Lieber DeCarli control, ethanol or 1% betaine-supplemented diets for 4 weeks, were processed for PIMT-related analyses.

RESULTS

We observed a significant increase in the accumulation of modified proteins bearing isoaspartyl residues, i.e. the substrates for PIMT, in homogenate samples and various subcellular fractions of livers from ethanol-fed rats. Betaine supplementation prevented this accumulation of damaged proteins. In contrast, ethanol exposure induced no changes in the PIMT enzyme activity levels as compared to controls. The accumulation of damaged proteins negatively correlated with hepatic S-adenosylmethionine (SAM) to S-adenosylhomocysteine (SAH) ratios.

CONCLUSIONS

Ethanol consumption results in the accumulation of modified proteins bearing atypical isoaspartyl residues via impaired in vivo PIMT activity. Betaine administration prevents the ethanol-induced accumulation of isoaspartyl-containing proteins by restoring the PIMT-catalyzed protein repair reaction through normalizing the hepatocellular SAM:SAH ratios.

Microtubule-associated protein 1B binds glyceraldehyde-3-phosphate dehydrogenase

Microtubule-associated protein 1B, MAP1B, is a major cytoskeletal protein during brain development and one of the largest brain MAPs associated with microtubules and microfilaments. Here, we identified several proteins that bind to MAP1B via immunoprecipitation with a MAP1B-specific antibody, by one and two-dimensional gel electrophoresis and subsequent mass spectrometry identification of precipitated proteins. In addition to tubulin and actin, a variety of proteins were identified. Among these proteins were glyceraldehyde-3-phosphate dehydrogenase (GAPDH), heat shock protein 8, dihydropyrimidinase related proteins 2 and 3, protein-L-isoaspartate O-methyltransferase, beta-spectrin, and clathrin protein MKIAA0034, linking either directly or indirectly to MAP1B. In particular, GAPDH, a key glycolytic enzyme, was bound in large quantity to the heavy chain of MAP1B in adult brain tissue. In vitro binding studies confirmed a direct binding of GAPDH to MAP1B. In PC12 cells, GAPDH was found in cytoplasm and nuclei and partially co-localized with MAP1B. It disappeared from the cytoplasm under oxidative stress or after a disruption of cytoskeletal elements after colcemid or cytochalasin exposure. GAPDH may be essential in the local energy provision of cytoskeletal structures and MAP1B may help to keep this key enzyme close to the cytoskeleton.

Regulation of protein L-isoaspartyl methyltransferase by cell-matrix interactions: involvement of integrin alphavbeta3, PI 3-kinase, and the proteasome

The enzyme L-isoaspartyl methyltransferase (PIMT) is known to repair damaged proteins that have accumulated abnormal aspartyl residues during cell aging. However, little is known about the mechanisms involved in the regulation of PIMT expression. Here we report that PIMT expression in bovine aortic endothelial cells is regulated by cell detachment and readhesion to a substratum. During cell detachment, the PIMT level was rapidly and strongly increased and correlated with a stimulation of protein synthesis. Aside from endothelial cells, PIMT levels were also regulated by cell adhesion in various cancer cell lines. The upregulation of PIMT expression could be prevented by an anti-alphavbeta3 antibody (LM609) or by a cyclic RGD peptide (XJ735) specific to integrin alphavbeta3, indicating that this integrin was likely involved in PIMT regulation. Moreover, we found that PIMT expression returned to the basal level when cells were replated on a substratum after detachment, though downregulation of PIMT expression could be partly prevented by the PI3K inhibitors LY294002 and wortmannin, as well as by the proteasome inhibitors MG-132, lactacystin, and beta-lactone. These findings support the assumption that the PIMT level was downregulated by proteasomal degradation, involving the PI3K pathway, during cell attachment. This study reports new insights on the molecular mechanisms responsible for the regulation of PIMT expression in cells. The regulation of PIMT level upon cell-substratum contact suggests a potential role for PIMT in biological processes such as wound healing, cell migration, and tumor metastasis dissemination.

Biochemistry of amino acid racemization and clinical application to musculoskeletal disease

During aging, proteins are subject to numerous forms of damage. Several types of non-enzymatic post-translational modifications have been described in aging proteins, including oxidation, nitration, glycation, and racemization. Racemization of amino acids is the spontaneous conversion of L-enantiomers to the D-form, which is dependent on temperature, pH, and time. Because of the time-dependent nature of racemization, it can be used to determine the relative age and turnover rates of long-lived proteins. There are many such long-lived proteins within the body; they are found in the brain, eye, and heart, but are particularly abundant in proteins found in musculoskeletal tissues such as bone and cartilage. During disease, musculoskeletal tissues have pathologically altered turnover rates. Because turnover rates can be estimated from levels of racemization, racemized musculoskeletal protein fragments may serve as useful biomarkers of disease. This review discusses the biochemistry of amino acid racemization in proteins and its clinical application to musculoskeletal disease.

Intracellular protein modification associated with altered T cell functions in autoimmunity

Posttranslational protein modifications influence a number of immunologic responses ranging from intracellular signaling to protein processing and presentation. One such modification, termed isoaspartyl (isoAsp), is the spontaneous nonenzymatic modification of aspartic acid residues occurring at physiologic pH and temperature. In this study, we have examined the intracellular levels of isoAsp residues in self-proteins from MRL(+/+), MRL/lpr, and NZB/W F(1) mouse strains compared with nonautoimmune B10.BR mice. In contrast to control B10.BR or NZB/W mice, the isoAsp content in MRL autoimmune mice increased and accumulated with age in erythrocytes, brain, kidney, and T lymphocytes. Moreover, T cells that hyperproliferate to antigenic stimulation in MRL mice also have elevated intracellular isoAsp protein content. Protein l-isoaspartate O-methyltransferase activity, a repair enzyme for isoAsp residues in vivo, remains stable with age in all strains of mice. These studies demonstrate a role for the accumulation of intracellular isoAsp proteins associated with T cell proliferative defects of MRL autoimmune mice.

Proteomic Identification of Novel Substrates of a Protein Isoaspartyl Methyltransferase Repair Enzyme

We report the use of a proteomic strategy to identify hitherto unknown substrates for mammalian protein l-isoaspartate O-methyltransferase. This methyltransferase initiates the repair of isoaspartyl residues in aged or stress-damaged proteins in vivo. Tissues from mice lacking the methyltransferase (Pcmt1(-/-)) accumulate more isoaspartyl residues than their wild-type littermates, with the most "damaged" residues arising in the brain. To identify the proteins containing these residues, brain homogenates from Pcmt1(-/-) mice were methylated by exogenous repair enzyme and the radiolabeled methyl donor S-adenosyl-[methyl-(3)H]methionine. Methylated proteins in the homogenates were resolved by both one-dimensional and two-dimensional electrophoresis, and methyltransferase substrates were identified by their increased radiolabeling when isolated from Pcmt1(-/-) animals compared with Pcmt1(+/+) littermates. Mass spectrometric analyses of these isolated brain proteins reveal for the first time that microtubule-associated protein-2, calreticulin, clathrin light chains a and b, ubiquitin carboxyl-terminal hydrolase L1, phosphatidylethanolamine-binding protein, stathmin, beta-synuclein, and alpha-synuclein, are all substrates for the l-isoaspartate methyltransferase in vivo. Our methodology for methyltransferase substrate identification was further supplemented by demonstrating that one of these methyltransferase targets, microtubule-associated protein-2, could be radiolabeled within Pcmt1(-/-) brain extracts using radioactive methyl donor and exogenous methyltransferase enzyme and then specifically immunoprecipitated with microtubule-associated protein-2 antibodies to recover co-localized protein with radioactivity. We comment on the functional significance of accumulation of relatively high levels of isoaspartate within these methyltransferase targets in the context of the histological and phenotypical changes associated with the methyltransferase knock-out mice.

Protein repair in the brain, proteomic analysis of endogenous substrates for protein L-isoaspartyl methyltransferase in mouse brain

Protein L-isoaspartyl methyltransferase (PIMT) catalyzes repair of L-isoaspartyl peptide bonds, a major source of protein damage under physiological conditions. PIMT knock-out (KO) mice exhibit brain enlargement and fatal epileptic seizures. All organs accumulate isoaspartyl proteins, but only the brain manifests an overt pathology. To further explore the role of PIMT in brain function, we undertook a global analysis of endogenous substrates for PIMT in mouse brain. Extracts from PIMT-KO mice were subjected to two-dimensional gel electrophoresis and blotted onto membranes. Isoaspartyl proteins were radiolabeled on-blot using [methyl-(3)H]S-adenosyl-L-methionine and recombinant PIMT. Fluorography of the blot revealed 30-35 (3)H-labeled proteins, 22 of which were identified by peptide mass fingerprinting. These isoaspartate-prone proteins represent a wide range of cellular functions, including neuronal development, synaptic transmission, cytoskeletal structure and dynamics, energy metabolism, nitrogen metabolism, pH homeostasis, and protein folding. The following five proteins, all of which are rich in neurons, accumulated exceptional levels of isoaspartate: collapsin response mediator protein 2 (CRMP2/ULIP2/DRP-2), dynamin 1, synapsin I, synapsin II, and tubulin. Several of the proteins identified here are prone to age-dependent oxidation in vivo, and many have been identified as autoimmune antigens, of particular interest because isoaspartate can greatly enhance the antigenicity of self-peptides. We propose that the PIMT-KO phenotype results from the cumulative effect of isoaspartate-related damage to a number of the neuron-rich proteins detected in this study. Further study of the isoaspartate-prone proteins identified here may help elucidate the molecular basis of one or more developmental and/or age-related neurological diseases.

Biological significance of isoaspartate and its repair system

Isomerization of L-aspartate and deamidation of L-asparagine in proteins or peptides dominantly give rise to L-isoaspartate by a non-enzymatic reaction via succinimide as a intermediate under physiological conditions. Isoaspartates have been identified in a variety of cellular proteins in vivo as well as pathologically deposited proteins in neurodegenerative brain tissue. We described here that the formation of isoaspartate is enhanced in amyloid-beta (Abeta) peptides in Alzheimer's disease (AD). Specific antibodies recognizing isoaspartate of Abeta revealed that isomerized Abeta peptides were deposited in senile plaques as well as amyloid-bearing vessels. Moreover, it was revealed that Abeta peptides, isomerized at position 7 or 23, were differentially deposited in senile plaques and vascular amyloids in AD brains. In vitro experiments showed that the modification at position 23 greatly enhanced the aggregation of Abeta. Furthermore, systematic proline substitution analyses revealed that the beta-turn structure at positions 22 and 23 of Abeta42 plays a crucial role in the aggregation and neurotoxicity of Abeta peptides. It is suggested that spontaneous isomerization at position 23 induces the conformational change to form a beta-turn at position 23, which plays a pathogenic role in the deposition of Abeta peptides in sporadic AD. Protein L-isoaspartyl methyltransferase (PIMT) is a putative protein repair enzyme, which converts L-isoaspartyl residues in damaged proteins to normal L-aspartyl residues. PIMT-deficient mice manifested neurodegenerative changes concomitant with the accumulation of L-isoaspartate in the brain. We discuss here the pathological implications of the formation of isoaspartate in damaged proteins during neurodegeneration in model mice and AD.

Mass spectrometry in aging research

This review covers the application of mass spectrometric techniques to aging research. Modern proteomic strategies will be discussed as well as the targeted analysis of specific proteins for the correlation of post-translational modifications with protein function. Selected examples will show both the power and also current limitations of the respective techniques. Experimental results and strategies are discussed in view of current theories of the aging process.

Expression and activity of l-isoaspartyl methyltransferase decrease in stage progression of human astrocytic tumors

Protein l-isoaspartyl methyltransferase (PIMT) functions as a repair enzyme that acts upon damaged proteins bearing abnormal aspartyl residues. We previously reported that PIMT expression and activity are reduced by half in human epileptic hippocampus. Here we investigated PIMT regulation in astrocytic tumors, which are the most common human brain tumors. PIMT expression and enzyme activity were significantly decreased in all grades of human astrocytic tumors. More precisely, PIMT levels were significantly lower by 76% in pilocytic astrocytomas (grade I), 46% in astrocytomas (grade II), 69% in anaplastic astrocytomas (grade III), and a marked 80% in glioblastomas (grade IV) as compared to normal brains. RT-PCR analysis showed that levels of type I PIMT mRNA were up-regulated while those of type II PIMT mRNA were down-regulated in glioblastomas. Furthermore, the reduced PIMT levels correlated closely with a decrease in the number of neuron cells in astrocytic tumors as assessed by measuring the neuron-specific enolase level. Many proteins with abnormal aspartyl residues accumulated in brain tumors and some were specific to individual grades of astrocytic tumors. Similar results were obtained, either by measuring the reduction in PIMT activity and expression or by measuring the formation of abnormal proteins, in an orthotopic rat brain tumor model implanted with invasive CNS-1 glioma cells. The novelty of these findings was to provide the first evidence for a marked reduction of PIMT expression and activity during stage progression of astrocytic tumors in humans.

Improved rotorod performance and hyperactivity in mice deficient in a protein repair methyltransferase

The protein L-isoaspartate (D-aspartate)-O-methyltransferase participates in the repair of age-induced protein damage by initiating the conversion of abnormal aspartyl residues within proteins to normal L-aspartyl residues. Previous studies have shown that mice deficient in the gene encoding this enzyme (Pcmt1-/-) accumulate damaged proteins, have altered levels of brain S-adenosylmethionine (AdoMet) and S-adenosylhomocysteine (AdoHcy), and suffer from epileptic seizures that result in death at an average age of about 42 days. In this study, we found that the behavior of Pcmt1-/- mice is abnormal in comparison to their wild-type (Pcmt1+/+) and heterozygous (Pcmt1+/-) littermates in two standard quantitative behavioral assays - the accelerating rotorod and the open-field test. On the accelerating rotorod, we found Pcmt1-/- mice actually perform significantly better than their heterozygous and wild-type littermates, a situation that has only been infrequently described in the literature and has not been described to date for epilepsy-prone mice. The Pcmt1-/- mice show, however, hyperactivity in the open-field test that becomes more pronounced with age, with a partial habituation with time in the chamber. Additionally, these mice demonstrate a strong thigmotaxic movement pattern. We present evidence that these phenotypes are not related to the alterations of the AdoMet/AdoHcy ratio in the brain and thus may be a function of the accumulation of damaged proteins. These results implicate a role for this enzyme in motor coordination and cerebellum development and suggest the importance of the function of the repair methyltransferase in hippocampal-dependent spatial learning.

Protein L-isoaspartyl methyltransferase repairs abnormal aspartyl residues accumulated in vivo in type-I collagen and restores cell migration

Abnormal aspartyl residue formation such as L-isoaspartates occurs frequently during aging in long-lived proteins, resulting in the alteration of their structures and biological functions. In this study, we investigated the alteration of aspartyl residues in extracellular matrix (ECM) proteins, type-I collagen and fibronectin, and in integrin- and ECM-binding motifs during aging, as well as the resulting effects on cell biological functions such as migration and attachment. Using protein L-isoaspartyl methyltransferase (PIMT) to monitor the presence of L-isoaspartyl residues, we showed their accumulation during in vivo aging in type-I collagen from rats. In vitro aging of fibronectin as well as of peptides containing an integrin- or ECM-binding motif such as RGDSR, KDGEA and KDDL also resulted in the formation of L-isoaspartyl residues. While aged fibronectin does not alter cell adhesion and migration, type-I collagen aged 20 months reduced by 65% cell motility, but not adhesion, when compared to 3-month-aged type-I collagen. Finally, by repairing 20-month-old type-I collagen with recombinant PIMT (rPIMT), cell migration was recovered by 72%. These results strongly suggest that L-isoaspartyl residue formation in ECM proteins such as type-I collagen could play an important role in reducing cell migration and that PIMT could be a therapeutic tool to restore normal cell migration in pathological conditions where cell motility is crucial.

Aging as war between chemical and biochemical processes: protein methylation and the recognition of age-damaged proteins for repair

Deamidated, isomerized, and racemized aspartyl and asparaginyl residues represent a significant part of the spontaneous damage to proteins that results from the aging process. The accumulation of these altered residues can lead to the loss of protein function and the consequent loss of cellular function. However, almost all cells in nature contain a methyltransferase that can recognize the major damaged form of the L-isoaspartyl residue, and some of these enzymes can also recognize the racemized D-aspartyl residue. The methyl esterification reaction can initiate the conversion of these altered residues to the normal L-aspartyl form, although there is no evidence yet that the L-asparaginyl form can be regenerated. This enzyme, the protein L-isoaspartate (D-aspartate) O-methyltransferase (EC 2.1.1.77), thus functions as a protein repair enzyme. The importance of this enzyme in attenuating age-related protein damage can be seen by the phenotypes of organisms where the gene encoding has been disrupted, or where its expression has been augmented.