The triage of damaged proteins: degradation by the ubiquitin-proteasome pathway or repair by molecular chaperones

The role of the co-chaperone HOP in plant homeostasis during development and stress

Proteins need to acquire their native structure in order to become fully functional. In specific cases, the active conformation is obtained spontaneously; nevertheless, many proteins need the assistance of chaperones and co-chaperones to be properly folded. These proteins help to maintain protein homeostasis under control conditions and under different stresses. HOP (HSP70-HSP90 organizing protein) is a highly conserved family of co-chaperones that assist HSP70 and HSP90 in the folding of specific proteins. In the last few years, findings in mammals and yeast have revealed novel functions of HOP and re-defined the role of HOP in protein folding. Here, we provide an overview of the most important aspects of HOP regulation and function in other eukaryotes and analyse whether these aspects are conserved in plants. In addition, we highlight the HOP clients described in plants and the role of HOP in plant development and stress response.

Trehalose Promotes Clearance of Proteotoxic Aggregation of Neurodegenerative Disease-Associated Aberrant Proteins

Accumulation of misfolded proteins compromises overall cellular health and fitness. The failure to remove misfolded proteins is a critical reason for their unwanted aggregation in dense cellular protein pools. The accumulation of various inclusions serves as a clinical feature for neurodegenerative diseases. Previous findings suggest that different cellular compartments can store these abnormal inclusions. Studies of transgenic mice and cellular models of neurodegenerative diseases indicate that depleted chaperone capacity contributes to the aggregation of damaged or aberrant proteins, which consequently disturb proteostasis and cell viability. However, improving these abnormal proteins' selective elimination is yet to be well understood. Still, molecular strategies that can promote the effective degradation of abnormal proteins without compromising cellular viability are unclear. Here, we reported that the trehalose treatment elevates endogenous proteasome levels and enhances the activities of the proteasome. Trehalose-mediated proteasomal activation elevates the removal of both bona fide misfolded and various neurodegenerative disease-associated proteins. Our current study suggests that trehalose may retain a proteasome activation potential, which seems helpful in the solubilization of different mutant misfolded proteins, improving cell viability. These results reveal a possible molecular approach to reduce the overload of intracellular misfolded proteins, and such cytoprotective functions may play a critical role against protein conformational diseases.

HSP70 positively regulates translation by interacting with the IRES and stabilizes the viral structural proteins VP1 and VP3 to facilitate duck hepatitis A virus type 1 replication

The maintenance of viral protein homeostasis depends on the interaction between host cell proteins and viral proteins. As a molecular chaperone, heat shock protein 70 (HSP70) has been shown to play an important role in viral infection. Our results showed that HSP70 can affect translation, replication, assembly, and release during the life cycle of duck hepatitis A virus type 1 (DHAV-1). We demonstrated that HSP70 can regulate viral translation by interacting with the DHAV-1 internal ribosome entry site (IRES). In addition, HSP70 interacts with the viral capsid proteins VP1 and VP3 and promotes their stability by inhibiting proteasomal degradation, thereby facilitating the assembly of DHAV-1 virions. This study demonstrates the specific role of HSP70 in regulating DHAV-1 replication, which are helpful for understanding the pathogenesis of DHAV-1 infection and provide additional information about the role of HSP70 in infection by different kinds of picornaviruses, as well as the interaction between picornaviruses and host cells.

CHIP: A Co-chaperone for Degradation by the Proteasome and Lysosome

Protein homeostasis relies on a balance between protein folding and protein degradation. Molecular chaperones like Hsp70 and Hsp90 fulfill well-defined roles in protein folding and conformational stability via ATP-dependent reaction cycles. These folding cycles are controlled by associations with a cohort of non-client protein co-chaperones, such as Hop, p23, and Aha1. Pro-folding co-chaperones facilitate the transit of the client protein through the chaperone-mediated folding process. However, chaperones are also involved in proteasomal and lysosomal degradation of client proteins. Like folding complexes, the ability of chaperones to mediate protein degradation is regulated by co-chaperones, such as the C-terminal Hsp70-binding protein (CHIP/STUB1). CHIP binds to Hsp70 and Hsp90 chaperones through its tetratricopeptide repeat (TPR) domain and functions as an E3 ubiquitin ligase using a modified RING finger domain (U-box). This unique combination of domains effectively allows CHIP to network chaperone complexes to the ubiquitin-proteasome and autophagosome-lysosome systems. This chapter reviews the current understanding of CHIP as a co-chaperone that switches Hsp70/Hsp90 chaperone complexes from protein folding to protein degradation.

Heat Shock Protein 60 Is Involved in Viral Replication Complex Formation and Facilitates Foot and Mouth Virus Replication by Stabilizing Viral Nonstructural Proteins 3A and 2C

The maintenance of viral protein homeostasis depends on the machinery of the infected host cells, giving us an insight into the interplay between host and virus. Accumulating evidence suggests that heat shock protein 60 (HSP60), as one molecular chaperone, is involved in regulating virus infection. However, the role of HSP60 during foot-and-mouth disease virus (FMDV) replication and its specific mechanisms have not been reported. We demonstrate that HSP60 modulates the FMDV life cycle. HSP60 plays a role at the postentry stage of the viral life cycle, including RNA replication and mRNA translation; however, HSP60 does not affect viral replication of Seneca Valley virus (SVA) or encephalomyocarditis virus (EMCV). We found that HSP60 is involved in FMDV replication complex (RC) formation. Furthermore, our results indicate that HSP60 interacts with FMDV nonstructural proteins 3A and 2C, key elements of the viral replication complex. We also show that HSP60 regulates the stability of 3A and 2C via caspase-dependent and autophagy-lysosome-dependent degradation, thereby promoting FMDV RNA synthesis and mRNA translation mediated by the RC. Additionally, we determined that the apical domain of HSP60 is responsible for interacting with 3A and 2C. The N terminus of 3A and ATPase domain of 2C are involved in binding to HSP60. Importantly, HSP60 depletion potently reduced FMDV pathogenicity in infected mice. Altogether, this study demonstrates a specific role of HSP60 in promoting FMDV replication. Furthermore, targeting host HSP60 will help us design the FMDV-specific antiviral drugs.

Cell Non-autonomous Proteostasis Regulation in Aging and Disease

Aging is a risk factor for a number of diseases, being the more notorious ones perhaps neurodegenerative diseases such as Alzheimer's and Parkinson's. These and other age-related pathologies are often associated with accumulation of proteotoxic material inside cells, as well as with the accumulation of protein deposits extracellularly. It is widely accepted that this accumulation of toxic proteins trails a progressive decline in the mechanisms that regulate protein homeostasis, or proteostasis, during aging. However, despite significant efforts, the progress in terms of novel or improved therapies targeting accumulation of proteotoxic material has been rather limited. For example, clinical trials for new drugs aimed at treating Alzheimer's disease, by preventing accumulation of toxic proteins, have notoriously failed. On the other hand, it is becoming increasingly apparent that regulation of proteostasis is not a cell autonomous process. In fact, cells rely on complex transcellular networks to maintain tissue and organ homeostasis involving endocrine and paracrine signaling pathways. In this review we will discuss the impact of cell non-autonomous proteostasis mechanisms and their impact in aging and disease. We will focus on how transcellular proteostasis networks can shed new light into stablished paradigms about the aging of organisms.

Potential Influence of Age and Diabetes Mellitus Type 1 on MSH2 (MutS homolog 2) Expression in a Rat Kidney Tissue

Background: Homeostasis of proliferating tissues is strongly dependent on intact DNA. Both neoplastic and non-neoplastic diseases have been associated with MSH2 (MutS homolog 2, a mismatch repair protein) deficiency. In this study, we examined how age and diabetes mellitus influence the expression of MSH2 in the kidney. Methods: To study the effect of age, three groups of healthy rats were formed: 2 months, 8 months, and 14 months old. Two groups of diabetic rats were formed: 8 months old and 14 months old. Expression of MSH2 in the kidney was studied by quantifying immunofluorescent staining. Results: Age was identified as the main factor that influences MSH2 expression in kidneys. The effect of age followed parabolic dynamics, with peak expression at 8 months of age and similar levels at 2 and 14 months. Diabetes had an age-dependent effect, which manifested as the increase of MSH2 expression in 14-month-old diabetic rats in comparison to healthy animals. Conclusions: Age influences MSH2 expression in the kidney more than diabetes mellitus. Since ageing is a risk factor for kidney neoplasia, downregulation of MSH2 in older rats might represent one of the pro-oncogenic mechanisms of ageing at a molecular level.

Endoplasmic reticulum stress induces degradation of glucose transporter proteins during hyperglycemic hepatotoxicity: Role of PERK-eIF2α-ATF4 axis

Hyperglycemia induced reactive oxygen species oxidize macromolecules including cellular proteins leading to their accumulation in Endoplasmic Reticulum (ER) lumen which in turn activates unfolded protein response (UPR) sensors including, PERK (Protein Kinase RNA-Like ER Kinase). Activated PERK induces ER associated degradation of misfolded proteins to lower the ER stress. In the present study, we hypothesized that ER stress leads to the degradation of glucose transporter proteins resulting in complex glucose metabolism. In vivo studies were carried out in the experimental model of hyperglycemia using streptozotocin/nicotinamide induced diabetic male Wistar rats. High glucose (30mM) treated HepG2 cells were used to perform the mechanistic study at different time points. PERK gene knockdown (siRNA transfection) and inhibition by ISRIB (Integrated Stress Response Inhibitor, a potent inhibitor of PERK signaling) confirmed the involvement of PERK axis in regulating the expression and translocation of hepatic glucose transporters. Co-immunoprecipitation and dual immunostaining studies further demonstrated increased degradation of GLUT proteins under high glucose conditions. Moreover, Morin (3,5,7,2',4' pentahydroxyflavone) treatment prevented PERK-eIF2α-ATF4 mediated degradation of glucose transporters and enhanced glucose uptake in both, HepG2 cells and diabetic rats. Targeting aberrant regulation of the expression and translocation of facilitative glucose transporter proteins (GLUT proteins) may provide novel therapeutic strategies for the better management of diabetes.

Measurement of erythrocyte membrane mannoses to assess splenic function

Red blood cells (RBCs) lose plasma membrane in the spleen as they age, but the cells and molecules involved are yet to be identified. Sickle cell disease and infection by Plasmodium falciparum cause oxidative stress that induces aggregates of cross‐linked proteins with N‐linked high‐mannose glycans (HMGs). These glycans can be recognised by mannose‐binding lectins, including the mannose receptor (CD206), expressed on macrophages and specialised phagocytic endothelial cells in the spleen to mediate the extravascular haemolysis characteristic of these diseases. We postulated this system might also mediate removal of molecules and membrane in healthy individuals. Surface expression of HMGs on RBCs from patients who had previously undergone splenectomy was therefore assessed: high levels were indeed observable as large membrane aggregates. Glycomic analysis by mass spectrometry identified a mixture of Man_5‐9GlcNAc₂ structures. HMG levels correlated well with manual pit counts (r = 0.75–0.85). To assess further whether HMGs might act as a splenic reticuloendothelial function test, we measured levels on RBCs from patients with potential functional hyposplenism, some of whom exhibited high levels that may indicate risk of complications.

Chaperone-assisted E3 ligase CHIP: A double agent in cancer

The carboxy-terminus of Hsp70-interacting protein (CHIP) is a ubiquitin ligase and co-chaperone belonging to Ubox family that plays a crucial role in the maintenance of cellular homeostasis by switching the equilibrium of the folding-refolding mechanism towards the proteasomal or lysosomal degradation pathway. It links molecular chaperones viz. HSC70, HSP70 and HSP90 with ubiquitin proteasome system (UPS), acting as a quality control system. CHIP contains charged domain in between N-terminal tetratricopeptide repeat (TPR) and C-terminal Ubox domain. TPR domain interacts with the aberrant client proteins via chaperones while Ubox domain facilitates the ubiquitin transfer to the client proteins for ubiquitination. Thus, CHIP is a classic molecule that executes ubiquitination for degradation of client proteins. Further, CHIP has been found to be indulged in cellular differentiation, proliferation, metastasis and tumorigenesis. Additionally, CHIP can play its dual role as a tumor suppressor as well as an oncogene in numerous malignancies, thus acting as a double agent. Here, in this review, we have reported almost all substrates of CHIP established till date and classified them according to the hallmarks of cancer. In addition, we discussed about its architectural alignment, tissue specific expression, sub-cellular localization, folding-refolding mechanisms of client proteins, E4 ligase activity, normal physiological roles, as well as involvement in various diseases and tumor biology. Further, we aim to discuss its importance in HSP90 inhibitors mediated cancer therapy. Thus, this report concludes that CHIP may be a promising and worthy drug target towards pharmaceutical industry for drug development.

CHIP-dependent regulation of the actin cytoskeleton is linked to neuronal cell membrane integrity

CHIP is an E3-ubiquitin ligase that contributes to healthy aging and has been characterized as neuroprotective. To elucidate dominant CHIP-dependent changes in protein steady-state levels in a patient-derived human neuronal model, CHIP function was ablated using gene-editing and an unbiased proteomic analysis conducted to compare knock-out and wild-type isogenic induced pluripotent stem cell (iPSC)-derived cortical neurons. Rather than a broad effect on protein homeostasis, loss of CHIP function impacted on a focused cohort of proteins from actin cytoskeleton signaling and membrane integrity networks. In support of the proteomics, CHIP knockout cells had enhanced sensitivity to induced membrane damage. We conclude that the major readout of CHIP function in cortical neurons derived from iPSC of a patient with elevate α-synuclein, Parkinson's disease and dementia, is the modulation of substrates involved in maintaining cellular "health". Thus, regulation of the actin cytoskeletal and membrane integrity likely contributes to the neuroprotective function(s) of CHIP.

Foot-and-Mouth Disease Virus Structural Protein VP1 Destroys the Stability of TPL2 Trimer by Degradation TPL2 to Evade Host Antiviral Immunity

Tumor progression locus 2 (TPL2) is a serine/threonine kinase that belongs to the mitogen-activated protein 3 kinase (MAP3K) family, and it plays an important role in pathogen infection. The trimer complex of TPL2, p105, and ABIN2 is essential for maintenance of TPL2 steady-state levels and host cell response to pathogens. Foot-and-mouth disease virus (FMDV) is a positive-strand RNA virus of the family Picornaviridae that encodes proteins capable of antagonizing host immune responses to achieve infection. The VP1 protein of FMDV is a multifunctional protein that can bind host cells and induce an immune response as well as cell apoptosis. However, the role and mechanisms of TPL2 in FMDV infection remain unknown. Here, we determined that FMDV infection could inhibit TPL2, p105, and ABIN2 at the transcription and protein levels, while VP1 could only inhibit TPL2, p105 and ABIN2 at protein level. TPL2 inhibited the replication of FMDV in vivo and in vitro, the 268 to 283 amino-acid region in the TPL2 kinase domain was essential for interaction with VP1. Moreover, VP1 promoted K48-linked polyubiquitination of TPL2 and degraded TPL2 by the proteasome pathway. However, VP1-induced degradation of p105 and ABIN2 was independent of proteasome, autophagy, lysosome, and caspase-dependent pathways. Further studies showed that VP1 destroyed the stability of the TPL2-p105-ABIN2 complex. Taken together, these results revealed that VP1 antagonized TPL2-meditated antivirus activity by degrading TPL2 and destroying its complex. These findings may contribute to understand FMDV-host interactions and improve development of a novel vaccine to prevent FMDV infection.Importance Virus-host interactions are critical for virus infection. This study was the first to demonstrate the antiviral effect of host TPL2 during FMDV replication by increasing production of interferons and antiviral cytokines. Both FMDV and VP1 protein can reduce host TPL2, ABIN2 and p105 to destroy TPL2-p105-ABIN2 trimer complex. VP1 interacted with TPL2 and degrade TPL2 via proteasome pathway to repress TPL2-mediated antivirus activity. This study provided new insights into FMDV immune evasion mechanisms, elucidating new informations regarding FMDV counteraction of host antivirus activity.

Lysine Acetylome Study of Human Hepatocellular Carcinoma Tissues for Biomarkers and Therapeutic Targets Discovery

Lysine acetylation is a vital post-translational modification (PTM) of proteins, which plays an important role in cancer development. In healthy human liver tissues, multiple non-histone proteins were identified with acetylation modification, however, the role of acetylated proteins in hepatocellular carcinoma (HCC) development remains largely unknown. Here we performed a quantitative acetylome study of tumor and normal liver tissues from HCC patients. Overall, 598 lysine acetylation sites in 325 proteins were quantified, and almost 59% of their acetylation levels were significantly changed. The differentially acetylated proteins mainly consisted of non-histone proteins located in mitochondria and cytoplasm, which accounted for 42% and 24%, respectively. Bioinformatics analysis showed that differentially acetylated proteins were enriched in metabolism, oxidative stress, and signal transduction processes. In tumor tissues, 278 lysine sites in 189 proteins showed decreased acetylation levels, which occupied 98% of differentially acetylated proteins. Moreover, we collected twenty pairs of tumor and normal liver tissues from HCC male patients, and found that expression levels of SIRT1 (p = 0.002), SIRT2 (p = 0.01), and SIRT4 (p = 0.045) were significantly up-regulated in tumor tissues. Over-expression of possibly accounted for the widespread deacetylation of non-histone proteins identified in HCC tumor tissues, which could serve as promising predictors of HCC. Taken together, our work illustrates abundant differentially acetylated proteins in HCC tumor tissues, and offered insights into the role of lysine acetylation in HCC development. It provided potential biomarker and drug target candidates for clinical HCC diagnosis and treatment.

Dietary supplementation with copper oxide nanoparticles ameliorates chronic heat stress in broiler chickens

Aims Heat stress (HS) is one of the most serious problems of poultry production. Copper (Cu) is an essential trace element that plays a crucial role in the organism defence against oxidative stress. Because of the low mineral availability of the commercial Cu salts, in a novel approach, copper oxide nanoparticles (CuO-NPs) were used to alleviate chronic heat stress-induced degenerative changes in two commercial broiler strains (Ross 308 and Cobb 500). Methods Birds of each broiler strain were divided into six groups, with three replicates each. The first group (N1) received 100% of the recommended Cu requirements as CuO and was housed under normal temperature (24 ± 2°C), the second and third groups (N2 and N3 respectively) received 100% and 50% of the recommended Cu requirements as CuO-NPs and were housed under normal temperature. The fourth, fifth and sixth groups (H1, H2 and H3 respectively) received the same level of Cu supplementation as did the first, second and third groups respectively, and they were housed under normal temperature until the age of 21 days, and then exposed to HS (33 ± 2°C/5 h per day for two successive weeks). Key results Dietary supplementation with CuO-NPs during HS altered the HS-induced responses of the birds, as confirmed by decreased liver malondehyde (MDA) concentration and enhanced superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx1) mRNA expression levels and enzyme activities (P < 0.001), with a distinct linear association between the gene expression level and enzyme activity. Copper oxide NPs also reduced HS-induced degenerative changes in the hepatic tissue. These nanoparticles modulated, although variably, liver HS protein 70 (HSP70), HS protein 90 (HSP90) and HS factor 3 (HSF3) mRNA transcript levels among Ross and Cobb chickens following HS (P < 0.001). Performance of both strains under HS was improved (as shown by a marked reduction in body temperature (P < 0.001) and a higher bodyweight (P < 0.01)) when CuO-NPs were supplemented in the diet, especially for the birds receiving 50% of the recommended Cu requirement, with different responses being noted in the two strains studied. Conclusion CuO-NPs could be used as a good alternative source of Cu in poultry nutrition during summer. Implications Dietary supplementation of CuO-NPs, especially at 50% of the birds’ recommended requirement, during heat stress could enhance bird performance, lower bird temperature and increase its resistance to negative consequences of elevated temperature.

Polyubiquitin Chains Linked by Lysine Residue 48 (K48) Selectively Target Oxidized Proteins In Vivo

Aims: Ubiquitin is a highly conserved protein modifier that heavily accumulates during the oxidative stress response. Here, we investigated the role of the ubiquitination system, particularly at the linkage level, in the degradation of oxidized proteins. The function of ubiquitin in the removal of oxidized proteins remains elusive because of the wide range of potential targets and different roles that polyubiquitin chains play. Therefore, we describe in detail the dynamics of the K48 ubiquitin response as the canonical signal for protein degradation. We identified ubiquitin targets and defined the relationship between protein ubiquitination and oxidation during the stress response. Results: Combining oxidized protein isolation, linkage-specific ubiquitination screens, and quantitative proteomics, we found that K48 ubiquitin accumulated at both the early and late phases of the stress response. We further showed that a fraction of oxidized proteins are conjugated with K48 ubiquitin. We identified ∼750 ubiquitinated proteins and ∼400 oxidized proteins that were modified during oxidative stress, and around half of which contain both modifications. These proteins were highly abundant and function in translation and energy metabolism. Innovation and Conclusion: Our work showed for the first time that K48 ubiquitin modifies a large fraction of oxidized proteins, demonstrating that oxidized proteins can be targeted by the ubiquitin/proteasome system. We suggest that oxidized proteins that rapidly accumulate during stress are subsequently ubiquitinated and degraded during the late phase of the response. This delay between oxidation and ubiquitination may be necessary for reprogramming protein dynamics, restoring proteostasis, and resuming cell growth.

Cellular DNAJA3, a Novel VP1-Interacting Protein, Inhibits Foot-and-Mouth Disease Virus Replication by Inducing Lysosomal Degradation of VP1 and Attenuating Its Antagonistic Role in the Beta Interferon Signaling Pathway

DnaJ heat shock protein family (Hsp40) member A3 (DNAJA3) plays an important role in viral infections. However, the role of DNAJA3 in replication of foot-and-mouth-disease virus (FMDV) remains unknown. In this study, DNAJA3, a novel binding partner of VP1, was identified using yeast two-hybrid screening. The DNAJA3-VP1 interaction was further confirmed by coimmunoprecipitation and colocalization in FMDV-infected cells. The J domain of DNAJA3 (amino acids 1 to 168) and the lysine at position 208 (K208) of VP1 were shown to be critical for the DNAJA3-VP1 interaction. Overexpression of DNAJA3 dramatically dampened FMDV replication, whereas loss of function of DNAJA3 elicited opposing effects against FMDV replication. Mechanistical study demonstrated that K208 of VP1 was critical for reducing virus titer caused by DNAJA3 using K208A mutant virus. DNAJA3 induced lysosomal degradation of VP1 by interacting with LC3 to enhance the activation of lysosomal pathway. Meanwhile, we discovered that VP1 suppressed the beta interferon (IFN-β) signaling pathway by inhibiting the phosphorylation, dimerization, and nuclear translocation of IRF3. This inhibitory effect was considerably boosted in DNAJA3-knockout cells. In contrast, overexpression of DNAJA3 markedly attenuated VP1-mediated suppression on the IFN-β signaling pathway. Poly(I⋅C)-induced phosphorylation of IRF3 was also decreased in DNAJA3-knockout cells compared to that in the DNAJA3-WT cells. In conclusion, our study described a novel role for DNAJA3 in the host's antiviral response by inducing the lysosomal degradation of VP1 and attenuating the VP1-induced suppressive effect on the IFN-β signaling pathway.IMPORTANCE This study pioneeringly determined the antiviral role of DNAJA3 in FMDV. DNAJA3 was found to interact with FMDV VP1 and trigger its degradation via the lysosomal pathway. In addition, this study is also the first to clarify the mechanism by which VP1 suppressed IFN-β signaling pathway by inhibiting the phosphorylation, dimerization, and nuclear translocation of IRF3. Moreover, DNAJA3 significantly abrogated VP1-induced inhibitive effect on the IFN-β signaling pathway. These data suggested that DNAJA3 plays an important antiviral role against FMDV by both degrading VP1 and restoring of IFN-β signaling pathway.

Short-term toxicity of dibutyl phthalate to mice intestinal tissue

The objective of this study was to investigate changes in intestinal histopathology and expression of heat-shock proteins (HSPs) in the small intestinal tissue of mouse after acute exposure to dibutyl phthalate (DBP). Forty-eight 60-day-old Institute of Cancer Research (ICR) mice were administered DBP by gavage once a day for 10 days. The mice were divided into three groups of 16 mice each: the high-dose group was administered 500 mg/kg body weight (BW) DBP; the low-dose group was administered 50 mg/kg BW; and the control group was not administered DBP. Significant increases in the uterine index, ovary index, and testicular index were observed in the DBP-exposed groups compared to those in the control group. Villus height and V/ C ratio significantly increased ( p < 0.05) in the duodenum and decreased ( p < 0.05) in the jejunum after the administration of DBP. The goblet cell number decreased in both the duodenum and the jejunum of mice exposed to DBP ( p < 0.05) compared to the number in the control group mice. Damage to the structure of the small intestine was accompanied by a marked increase in HSP27 expression and a decrease in the expression of HSP70 and HSP90 in both high-dose and low-dose groups. These results indicate that elevated HSP27 levels in the duodenum and jejunum may be important markers for acute DBP exposure and that HSP27 may act as a protective protein involved in intestinal mucosa repair.

Self-Repair of Structure and Bioactivity in a Supramolecular Nanostructure

Supramolecular nanostructures formed through self-assembly can have energy landscapes, which determine their structures and functions depending on the pathways selected for their synthesis and processing and on the conditions they are exposed to after their initial formation. We report here on the structural damage that occurs in supramolecular peptide amphiphile nanostructures, during freezing in aqueous media, and the self-repair pathways that restore their functions. We found that freezing converts long supramolecular nanofibers into shorter ones, compromising their ability to support cell adhesion, but a single heating and cooling cycle reverses the damage and rescues their bioactivity. Thermal energy in this cycle enables noncovalent interactions to reconfigure the nanostructures into the thermodynamically preferred long nanofibers, a repair process that is impeded by kinetic traps. In addition, we found that nanofibers disrupted during freeze-drying also exhibit the ability to undergo thermal self-repair and recovery of their bioactivity, despite the extra disruption caused by the dehydration step. Following both freezing and freeze-drying, which shorten the 1D nanostructures, their self-repair capacity through thermally driven elongation is inhibited by kinetically trapped states, which contain highly stable noncovalent interactions that are difficult to rearrange. These states decrease the extent of thermal nanostructure repair, an observation we hypothesize applies to supramolecular systems in general and is mechanistically linked to suppressed molecular exchange dynamics.

Hsp70 chaperone: a master player in protein homeostasis

Protein homeostasis (proteostasis) is an essential pillar for correct cellular function. Impairments in proteostasis are encountered both in aging and in several human disease conditions. Molecular chaperones are important players for proteostasis; in particular, heat shock protein 70 (Hsp70) has an essential role in protein folding, disaggregation, and degradation. We have recently proposed a model for Hsp70 functioning as a “multiple socket”. In the model, Hsp70 provides a physical platform for the binding of client proteins, other chaperones, and cochaperones. The final fate of the client protein is dictated by the set of Hsp70 interactions that occur in a given cellular context. Obtaining structural information of the different Hsp70-based protein complexes will provide valuable knowledge to understand the functional mechanisms behind the master role of Hsp70 in proteostasis. We additionally evaluate some of the challenges for attaining high-resolution structures of such complexes.

Stabilization of p27Kip1/CDKN1B by UBCH7/UBE2L3 catalyzed ubiquitinylation: a new paradigm in cell-cycle control

Ubiquitinylation drives many cellular processes by targeting proteins for proteasomal degradation. Ubiquitin conjugation enzymes promote ubiquitinylation and, thus, degradation of protein substrates. Ubiquitinylation is a well-known posttranslational modification controlling cell-cycle transitions and levels or/and activation levels of ubiquitin-conjugating enzymes change during development and cell cycle. Progression through the cell cycle is tightly controlled by CDK inhibitors such as p27^Kip1. Here we show that, in contrast to promoting its degradation, the ubiquitin-conjugating enzyme UBCH7/UBE2L3 specifically protects p27^Kip1 from degradation. Overexpression of UBCH7/UBE2L3 stabilizes p27^Kip1 and delays the G₁-to-S transition, while depletion of UBCH7/UBE2L3 increases turnover of p27^Kip1. Levels of p21^Cip1/Waf1, p57^Kip2, cyclin A and cyclin E, all of which are also involved in regulating the G₁/S transition are not affected by UBCH7/UBE2L3 depletion. The effect of UBCH7/UBE2L3 on p27^Kip1 is not due to alteration of the levels of any of the ubiquitin ligases known to ubiquitinylate p27^Kip1. Rather, UBCH7/UBE2L3 catalyzes the conjugation of heterotypic ubiquitin chains on p27^Kip1 that are proteolytically incompetent. These data reveal new controls and concepts about the ubiquitin proteasome system in which a ubiquitin-conjugating enzyme selectively inhibits and may even protect, rather than promote degradation of a crucial cell-cycle regulatory molecule.-Whitcomb, E. A., Tsai, Y. C., Basappa, J., Liu, K., Le Feuvre, A. K., Weissman, A. M., Taylor, A. Stabilization of p27^Kip1/CDKN1B by UBCH7/UBE2L3 catalyzed ubiquitinylation: a new paradigm in cell-cycle control.

Exercise, heat shock proteins and insulin resistance

Best known as chaperones, heat shock proteins (HSPs) also have roles in cell signalling and regulation of metabolism. Rodent studies demonstrate that heat treatment, transgenic overexpression and pharmacological induction of HSP72 prevent high-fat diet-induced glucose intolerance and skeletal muscle insulin resistance. Overexpression of skeletal muscle HSP72 in mice has been shown to increase endurance running capacity nearly twofold and increase mitochondrial content by 50%. A positive correlation between HSP72 mRNA expression and mitochondrial enzyme activity has been observed in human skeletal muscle, and HSP72 expression is markedly decreased in skeletal muscle of insulin resistant and type 2 diabetic patients. In addition, decreased levels of HSP72 correlate with insulin resistance and non-alcoholic fatty liver disease progression in livers from obese patients. These data suggest the targeted induction of HSPs could be a therapeutic approach for preventing metabolic disease by maintaining the body's natural stress response. Exercise elicits a number of metabolic adaptations and is a powerful tool in the prevention and treatment of insulin resistance. Exercise training is also a stimulus for increased HSP expression. Although the underlying mechanism(s) for exercise-induced HSP expression are currently unknown, the HSP response may be critical for the beneficial metabolic effects of exercise. Exercise-induced extracellular HSP release may also contribute to metabolic homeostasis by actively restoring HSP72 content in insulin resistant tissues containing low endogenous levels of HSPs.This article is part of the theme issue 'Heat shock proteins as modulators and therapeutic targets of chronic disease: an integrated perspective'.

The developmental-genetics of canalization

Canalization, or robustness to genetic or environmental perturbations, is fundamental to complex organisms. While there is strong evidence for canalization as an evolved property that varies among genotypes, the developmental and genetic mechanisms that produce this phenomenon are very poorly understood. For evolutionary biology, understanding how canalization arises is important because, by modulating the phenotypic variation that arises in response to genetic differences, canalization is a determinant of evolvability. For genetics of disease in humans and for economically important traits in agriculture, this subject is important because canalization is a potentially significant cause of missing heritability that confounds genomic prediction of phenotypes. We review the major lines of thought on the developmental-genetic basis for canalization. These fall into two groups. One proposes specific evolved molecular mechanisms while the other deals with robustness or canalization as a more general feature of development. These explanations for canalization are not mutually exclusive and they overlap in several ways. General explanations for canalization are more likely to involve emergent features of development than specific molecular mechanisms. Disentangling these explanations is also complicated by differences in perspectives between genetics and developmental biology. Understanding canalization at a mechanistic level will require conceptual and methodological approaches that integrate quantitative genetics and developmental biology.

Molecular Mechanisms Responsible for Increased Vulnerability of the Ageing Oocyte to Oxidative Damage

In their midthirties, women experience a decline in fertility, coupled to a pronounced increase in the risk of aneuploidy, miscarriage, and birth defects. Although the aetiology of such pathologies are complex, a causative relationship between the age-related decline in oocyte quality and oxidative stress (OS) is now well established. What remains less certain are the molecular mechanisms governing the increased vulnerability of the aged oocyte to oxidative damage. In this review, we explore the reduced capacity of the ageing oocyte to mitigate macromolecular damage arising from oxidative insults and highlight the dramatic consequences for oocyte quality and female fertility. Indeed, while oocytes are typically endowed with a comprehensive suite of molecular mechanisms to moderate oxidative damage and thus ensure the fidelity of the germline, there is increasing recognition that the efficacy of such protective mechanisms undergoes an age-related decline. For instance, impaired reactive oxygen species metabolism, decreased DNA repair, reduced sensitivity of the spindle assembly checkpoint, and decreased capacity for protein repair and degradation collectively render the aged oocyte acutely vulnerable to OS and limits their capacity to recover from exposure to such insults. We also highlight the inadequacies of our current armoury of assisted reproductive technologies to combat age-related female infertility, emphasising the need for further research into mechanisms underpinning the functional deterioration of the ageing oocyte.

Current Perspective in the Discovery of Anti-aging Agents from Natural Products

Aging is a process characterized by accumulating degenerative damages, resulting in the death of an organism ultimately. The main goal of aging research is to develop therapies that delay age-related diseases in human. Since signaling pathways in aging of Caenorhabditis elegans (C. elegans), fruit flies and mice are evolutionarily conserved, compounds extending lifespan of them by intervening pathways of aging may be useful in treating age-related diseases in human. Natural products have special resource advantage and with few side effect. Recently, many compounds or extracts from natural products slowing aging and extending lifespan have been reported. Here we summarized these compounds or extracts and their mechanisms in increasing longevity of C. elegans or other species, and the prospect in developing anti-aging medicine from natural products.

Hsp70 - a master regulator in protein degradation

Proteostasis, the controlled balance of protein synthesis, folding, assembly, trafficking and degradation, is a paramount necessity for cell homeostasis. Impaired proteostasis is a hallmark of ageing and of many human diseases. Molecular chaperones are essential for proteostasis in eukaryotic cells, and their function has traditionally been linked to protein folding, assembly and disaggregation. More recent findings suggest that chaperones also contribute to key steps in protein degradation. In particular, Hsp70 has an essential role in substrate degradation through the ubiquitin-proteasome system, as well as through different autophagy pathways. Accumulated knowledge suggests that the fate of an Hsp70 substrate is dictated by the combination of partners (cochaperones and other chaperones) that interact with Hsp70 in a given cell context.

Modulatory Effect of Monochromatic Blue Light on Heat Stress Response in Commercial Broilers

In a novel approach, monochromatic blue light was used to investigate its modulatory effect on heat stress biomarkers in two commercial broiler strains (Ross 308 and Cobb 500). At 21 days old, birds were divided into four groups including one group housed in white light, a second group exposed to blue light, a 3rd group exposed to white light + heat stress, and a 4th group exposed to blue light + heat stress. Heat treatment at 33°C lasted for five h for four successive days. Exposure to blue light during heat stress reduced MDA concentration and enhanced SOD and CAT enzyme activities as well as modulated their gene expression. Blue light also reduced the degenerative changes that occurred in the liver tissue as a result of heat stress. It regulated, though variably, liver HSP70, HSP90, HSF1, and HSF3 gene expression among Ross and Cobb chickens. Moreover, the Cobb strain showed better performance than Ross manifested by a significant reduction of rectal temperature in the case of H + B. Furthermore, a significant linear relationship was found between the lowered rectal temperature and the expression of all HSP genes. Generally, the performance of both strains by most assessed parameters under heat stress is improved when using blue light.

Multiple functions of the E3 ubiquitin ligase CHIP in immunity

The carboxyl terminal of Hsp70-interacting protein (CHIP) is an E3 ubiquitin ligase that plays a pivotal role in the protein quality control system by shifting the balance of the folding-refolding machinery toward the degradative pathway. However, the precise mechanisms by which nonnative proteins are selected for degradation by CHIP either directly or indirectly via chaperone Hsp70 or Hsp90 are still not clear. In this review, we aim to provide a comprehensive model of the mechanism by which CHIP degrades its substrate in a chaperone-dependent or direct manner. In addition, through tight regulation of the protein level of its substrates, CHIP plays important roles in many physiological and pathological conditions, including cancers, neurological disorders, cardiac diseases, bone metabolism, immunity, and so on. Nonetheless, the precise mechanisms underlying the regulation of the immune system by CHIP are still poorly understood despite accumulating developments in our understanding of the regulatory roles of CHIP in both innate and adaptive immune responses. In this review, we also aim to provide a view of CHIP-mediated regulation of immune responses and the signaling pathways involved in the model described. Finally, we discuss the roles of CHIP in immune-related diseases.

Proteomic analysis of physiological function response to hot summer in liver from lactating dairy cows

Lactation performance of dairy cattle is susceptible to heat stress. The liver is one of the most crucial organs affected by high temperature in dairy cows. However, the physiological adaption by the liver to hot summer conditions has not been well elucidated in lactating dairy cows. In the present study, proteomic analysis of the liver in dairy cows in spring and hot summer was performed using a label-free method. In total, 127 differentially expressed proteins were identified; most of the upregulated proteins were involved in protein metabolic processes and responses to stimuli, whereas most of the downregulated proteins were related to oxidation-reduction. Pathway analysis indicated that 3 upregulated heat stress proteins (HSP90α, HSP90β, and endoplasmin) were enriched in the NOD-like receptor signaling pathway, whereas several downregulated NADH dehydrogenase proteins were involved in the oxidative phosphorylation pathway. The protein-protein interaction network indicated that several upregulated HSPs (HSP90α, HSP90β, and GRP78) were involved in more interactions than other proteins and were thus considered as central hub nodes. Our findings provide novel insights into the physiological adaption of liver function in lactating dairy cows to natural high temperature.

Proteins that accumulate with age in human skeletal-muscle aggregates contribute to declines in muscle mass and function in Caenorhabditis elegans

Protein aggregation increases with age in normal tissues, and with pathology and age in Alzheimer's hippocampus and mouse cardiac muscle. We now ask whether human skeletal muscle accumulates aggregates with age. Detergent-insoluble protein aggregates were isolated from vastus lateralis biopsies from 5 young (23–27 years of age) and 5 older (64-80 years) adults. Aggregates, quantified after gel electrophoresis, contain 2.1-fold more protein (P<0.0001) when isolated from older subjects relative to young. Of 515 proteins identified by liquid chromatography coupled to tandem mass spectrometry, 56 (11%) were significantly more abundant in older muscle, while 21 (4%) were depleted with age (each P<0.05). Orthologs to seven of these proteins were then targeted in C. elegans by RNA interference. Six of the seven knockdown treatments decreased protein aggregation (range 6-45%, P<0.01 to <0.0001) and increased muscle mass (range 1.5- to 1.85-fold, P<0.01 to <0.0001) in aged nematodes, and rescued mobility (range 1.4 to 1.65-fold, P≤0.0005 each) in a nematode amyloidopathy model. We conclude that specific aggregate proteins, discovered as differentially abundant in aging human muscle, have orthologs that contribute functionally to aggregation and age-associated muscle loss in nematodes, and thus can be considered potential drug targets for sarcopenia in humans.

Utility of 17-(allylamino)-17-demethoxygeldanamycin treatment for skeletal muscle injury

Repeated eccentric contractions can injure skeletal muscle and result in functional deficits that take several weeks to fully recover. The 70-kDa heat shock protein (Hsp70) is a stress-inducible molecular chaperone that maintains protein quality and plays an integral role in the muscle's repair processes following injury. Here, we attempted to hasten this recovery by pharmacologically inducing Hsp70 expression in mouse skeletal muscle with 17-(allylamino)-17-demethoxygeldanamycin (17-AAG) (40 mg/kg) both prior to and throughout the first 7 days after an injurious bout of 150 maximal eccentric contractions. Hsp70 content in the injured skeletal muscle was strongly induced following the eccentric contractions and remained elevated over the next 7 days as the muscle underwent repair. Treatment with 17-AAG increased Hsp70 content ∼fivefold; however, this was significantly less than that induced by the injury. Moreover, 17-AAG treatment did not recover the decrements to in vivo isometric torque production following the bout of eccentric contractions. Together, these findings demonstrate that although Hsp70 content was induced in the uninjured skeletal muscle, treatment of 17-AAG (40 mg/kg) was not a preventive measure to either reduce the severity of skeletal muscle damage or enhance functional recovery following a bout of maximal eccentric contractions.

Melatonin enhances thermotolerance by promoting cellular protein protection in tomato plants

Melatonin is a pleiotropic signaling molecule that provides physiological protection against diverse environmental stresses in plants. Nonetheless, the mechanisms for melatonin-mediated thermotolerance remain largely unknown. Here, we report that endogenous melatonin levels increased with a rise in ambient temperature and that peaked at 40°C. Foliar pretreatment with an optimal dose of melatonin (10 μmol/L) or the overexpression of N-acetylserotonin methyltransferase (ASMT) gene effectively ameliorated heat-induced photoinhibition and electrolyte leakage in tomato plants. Both exogenous melatonin treatment and endogenous melatonin manipulation by overexpression of ASMT decreased the levels of insoluble and ubiquitinated proteins, but enhanced the expression of heat-shock proteins (HSPs) to refold denatured and unfolded proteins under heat stress. Meanwhile, melatonin also induced expression of several ATG genes and formation of autophagosomes to degrade aggregated proteins under the same stress. Proteomic profile analyses revealed that protein aggregates for a large number of biological processes accumulated in wild-type plants. However, exogenous melatonin treatment or overexpression of ASMT reduced the accumulation of aggregated proteins. Aggregation responsive proteins such as HSP70 and Rubisco activase were preferentially accumulated and ubiquitinated in wild-type plants under heat stress, while melatonin mitigated heat stress-induced accumulation and ubiquitination of aggregated proteins. These results suggest that melatonin promotes cellular protein protection through induction of HSPs and autophagy to refold or degrade denatured proteins under heat stress in tomato plants.

Age- and Hypertension-Associated Protein Aggregates in Mouse Heart Have Similar Proteomic Profiles

Neurodegenerative diseases are largely defined by protein aggregates in affected tissues. Aggregates contain some shared components as well as proteins thought to be specific for each disease. Aggregation has not previously been reported in the normal, aging heart or the hypertensive heart. Detergent-insoluble protein aggregates were isolated from mouse heart and characterized on 2-dimensional gels. Their levels increased markedly and significantly with aging and after sustained angiotensin II-induced hypertension. Of the aggregate components identified by high-resolution proteomics, half changed in abundance with age (392/787) or with sustained hypertension (459/824), whereas 30% (273/901) changed concordantly in both, each P<0.05. One fifth of these proteins were previously associated with age-progressive neurodegenerative or cardiovascular diseases, or both (eg, ApoE, ApoJ, ApoAIV, clusterin, complement C3, and others involved in stress-response and protein-homeostasis pathways). Because fibrosis is a characteristic of both aged and hypertensive hearts, we posited that aging of fibroblasts may contribute to the aggregates observed in cardiac tissue. Indeed, as cardiac myofibroblasts "senesced" (approached their replicative limit) in vitro, they accrued aggregates with many of the same constituent proteins observed in vivo during natural aging or sustained hypertension. In summary, we have shown for the first time that compact (detergent-insoluble) protein aggregates accumulate during natural aging, chronic hypertension, and in vitro myofibroblast senescence, sharing many common proteins. Thus, aggregates that arise from disparate causes (aging, hypertension, and replicative senescence) may have common underlying mechanisms of accrual.

Dietary glycemia as a determinant of health and longevity

The role of diet in extending lifespan and healthspan has been the subject of much research and debate. Our recent epidemiological and in vivo data suggest that carbohydrate quality can be a major determinant in prolonging eye health. Additionally, excessive carbohydrate intake can contribute to the exacerbation of many different diseases. The metabolic diversity of the tissues that are affected by excessive carbohydrate intake suggests that dietary carbohydrate quality may affect cellular homeostasis.

Biomarkers: Non-destructive Method for Predicting Meat Tenderization

Meat tenderness is the primary and most important quality attribute for the consumers worldwide. Tenderness is the process of breakdown of collagen tissue in meat to make it palatable. The earlier methods of tenderness evaluation like taste panels and shear force methods are destructive, time consuming and ill suited as they requires removing a piece of steak from the carcass for performing the test. Therefore, a non-destructive method for predicting the tenderness would be more desirable. The development of a meat quality grading and guarantee system through muscle profiling research can help to meet this demand. Biomarkers have the ability to identify if an exposure has occurred. Biomarkers of the meat quality are of prime importance for meat industry, which has ability to satisfy consumers' expectations. The biomarkers so far identified have been then sorted and grouped according to their common biological functions. All of them refer to a series of biological pathways including glycolytic and oxidative energy production, cell detoxification, protease inhibition and production of Heat Shock Proteins. On this basis, a detailed analysis of these metabolic pathways helps in identifying tenderization of meat having some domains of interest. It was, therefore, stressed forward that biomarkers can be used to determine meat tenderness. This review article summarizes the uses of several biomarkers for predicting the meat tenderness.

Isoform-selective Genetic Inhibition of Constitutive Cytosolic Hsp70 Activity Promotes Client Tau Degradation Using an Altered Co-chaperone Complement

The constitutively expressed heat shock protein 70 kDa (Hsc70) is a major chaperone protein responsible for maintaining proteostasis, yet how its structure translates into functional decisions regarding client fate is still unclear. We previously showed that Hsc70 preserved aberrant Tau, but it remained unknown if selective inhibition of the activity of this Hsp70 isoform could facilitate Tau clearance. Using single point mutations in the nucleotide binding domain, we assessed the effect of several mutations on the functions of human Hsc70. Biochemical characterization revealed that one mutation abolished both Hsc70 ATPase and refolding activities. This variant resembled the ADP-bound conformer at all times yet remained able to interact with cofactors, nucleotides, and substrates appropriately, resembling a dominant negative Hsc70 (DN-Hsc70). We then assessed the effects of this DN-Hsc70 on its client Tau. DN-Hsc70 potently facilitated Tau clearance via the proteasome in cells and brain tissue, in contrast to wild type Hsc70 that stabilized Tau. Thus, DN-Hsc70 mimics the action of small molecule pan Hsp70 inhibitors with regard to Tau metabolism. This shift in Hsc70 function by a single point mutation was the result of a change in the chaperome associated with Hsc70 such that DN-Hsc70 associated more with Hsp90 and DnaJ proteins, whereas wild type Hsc70 was more associated with other Hsp70 isoforms. Thus, isoform-selective targeting of Hsc70 could be a viable therapeutic strategy for tauopathies and possibly lead to new insights in chaperone complex biology.

CHIP: a co-chaperone for degradation by the proteasome

Protein homeostasis relies on a balance between protein folding and protein degradation. Molecular chaperones like Hsp70 and Hsp90 fulfil well-defined roles in protein folding and conformational stability via ATP dependent reaction cycles. These folding cycles are controlled by associations with a cohort of non-client protein co-chaperones, such as Hop, p23 and Aha1. Pro-folding co-chaperones facilitate the transit of the client protein through the chaperone mediated folding process. However, chaperones are also involved in ubiquitin-mediated proteasomal degradation of client proteins. Similar to folding complexes, the ability of chaperones to mediate protein degradation is regulated by co-chaperones, such as the C terminal Hsp70 binding protein (CHIP). CHIP binds to Hsp70 and Hsp90 chaperones through its tetratricopeptide repeat (TPR) domain and functions as an E3 ubiquitin ligase using a modified RING finger domain (U-box). This unique combination of domains effectively allows CHIP to network chaperone complexes to the ubiquitin-proteasome system. This chapter reviews the current understanding of CHIP as a co-chaperone that switches Hsp70/Hsp90 chaperone complexes from protein folding to protein degradation.

Proteasome activation delays aging in vitro and in vivo

Aging is a natural biological process that is characterized by a progressive accumulation of macromolecular damage. In the proteome, aging is accompanied by decreased protein homeostasis and function of the major cellular proteolytic systems, leading to the accumulation of unfolded, misfolded, or aggregated proteins. In particular, the proteasome is responsible for the removal of normal as well as damaged or misfolded proteins. Extensive work during the past several years has clearly demonstrated that proteasome activation by either genetic means or use of compounds significantly retards aging. Importantly, this represents a common feature across evolution, thereby suggesting proteasome activation to be an evolutionarily conserved mechanism of aging and longevity regulation. This review article reports on the means of function of these proteasome activators and how they regulate aging in various species.

Cellular senescence and protein degradation: breaking down cancer

Autophagy and the ubiquitin-proteasome pathway (UPP) are the major protein degradation systems in eukaryotic cells. Whereas the former mediate a bulk nonspecific degradation, the UPP allows a rapid degradation of specific proteins. Both systems have been shown to play a role in tumorigenesis, and the interest in developing therapeutic agents inhibiting protein degradation is steadily growing. However, emerging data point to a critical role for autophagy in cellular senescence, an established tumor suppressor mechanism. Recently, a selective protein degradation process mediated by the UPP was also shown to contribute to the senescence phenotype. This process is tightly regulated by E3 ubiquitin ligases, deubiquitinases, and several post-translational modifications of target proteins. Illustrating the complexity of UPP, more than 600 human genes have been shown to encode E3 ubiquitin ligases, a number which exceeds that of the protein kinases. Nevertheless, our knowledge of proteasome-dependent protein degradation as a regulated process in cellular contexts such as cancer and senescence remains very limited. Here we discuss the implications of protein degradation in senescence and attempt to relate this function to the protein degradation pattern observed in cancer cells.

Non-repair pathways for minimizing protein isoaspartyl damage in the yeast Saccharomyces cerevisiae

The spontaneous degradation of asparaginyl and aspartyl residues to isoaspartyl residues is a common type of protein damage in aging organisms. Although the protein-l-isoaspartyl (d-aspartyl) O-methyltransferase (EC 2.1.1.77) can initiate the repair of l-isoaspartyl residues to l-aspartyl residues in most organisms, no gene homolog or enzymatic activity is present in the budding yeast Saccharomyces cerevisiae. Therefore, we used biochemical approaches to elucidate how proteins containing isoaspartyl residues are metabolized in this organism. Surprisingly, the level of isoaspartyl residues in yeast proteins (50-300 pmol of isoaspartyl residues/mg of protein extract) is comparable with organisms with protein-l-isoaspartyl (d-aspartyl) O-methyltransferase, suggesting a novel regulatory pathway. Interfering with common protein quality control mechanisms by mutating and inhibiting the proteasomal and autophagic pathways in vivo did not increase isoaspartyl residue levels compared with wild type or uninhibited cells. However, the inhibition of metalloproteases in in vitro aging experiments by EDTA resulted in an ∼3-fold increase in the level of isoaspartyl-containing peptides. Characterization by mass spectrometry of these peptides identified several proteins involved in metabolism as targets of isoaspartyl damage. Further analysis of these peptides revealed that many have an N-terminal isoaspartyl site and originate from proteins with short half-lives. These results suggest that one or more metalloproteases participate in limiting isoaspartyl formation by robust proteolysis.

Epigallocatechin-3-gallate exerts protective effects against heat stress through modulating stress-responsive transcription factors in poultry

1. The aim of the study was to describe the effect of Epigallocatechin-3-gallate (EGCG), a polyphenol derived from green tea, on activator protein-1 (AP-1) components (phospho-c-Jun and c-Fos), cyclooxygenase-2 (COX-2) and heat shock proteins (HSPs) in the liver of heat-stressed quails. 2. A total of 180 5-week-old female Japanese quails were reared either at 22°C for 24 h/d (thermoneutral, TN) or 34°C for 8 h/d (heat stress, HS) for 12 weeks. Birds in both environments were randomly given 1 of 3 diets: basal diet and basal diet with 200 or 400 mg of EGCG added per kilogram of diet. 3. The hepatic c-Jun, c-Fos, COX-2 and HSPs gene expression for quails reared under the HS environment was greater than those reared under the TN environment. Supplemental EGCG decreased hepatic expression of these proteins at a greater extent under HS than TN. 4. In conclusion, suppression of AP-1 COX-2 and HSPs may partly account for the inhibitory effect of EGCG in heat-stressed quail.

Molecular chaperones and proteostasis regulation during redox imbalance

Free radicals originate from both exogenous environmental sources and as by-products of the respiratory chain and cellular oxygen metabolism. Sustained accumulation of free radicals, beyond a physiological level, induces oxidative stress that is harmful for the cellular homeodynamics as it promotes the oxidative damage and stochastic modification of all cellular biomolecules including proteins. In relation to proteome stability and maintenance, the increased concentration of oxidants disrupts the functionality of cellular protein machines resulting eventually in proteotoxic stress and the deregulation of the proteostasis (homeostasis of the proteome) network (PN). PN curates the proteome in the various cellular compartments and the extracellular milieu by modulating protein synthesis and protein machines assembly, protein recycling and stress responses, as well as refolding or degradation of damaged proteins. Molecular chaperones are key players of the PN since they facilitate folding of nascent polypeptides, as well as holding, folding, and/or degradation of unfolded, misfolded, or non-native proteins. Therefore, the expression and the activity of the molecular chaperones are tightly regulated at both the transcriptional and post-translational level at organismal states of increased oxidative and, consequently, proteotoxic stress, including ageing and various age-related diseases (e.g. degenerative diseases and cancer). In the current review we present a synopsis of the various classes of intra- and extracellular chaperones, the effects of oxidants on cellular homeodynamics and diseases and the redox regulation of chaperones.

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Free radicals originate from various sources and at physiological concentrations are essential for the modulation of cell signalling pathways.

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Abnormally high levels of free radicals induce oxidative stress and damage all cellular biomolecules, including proteins.

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Molecular chaperones facilitate folding of nascent polypeptides, as well as holding, folding, and/or degradation of damaged proteins.

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The expression and the activity of chaperones during oxidative stress are regulated at both the transcriptional and post-translational level.

E3 ubiquitin ligase CHIP and NBR1-mediated selective autophagy protect additively against proteotoxicity in plant stress responses

Plant stress responses require both protective measures that reduce or restore stress-inflicted damage to cellular structures and mechanisms that efficiently remove damaged and toxic macromolecules, such as misfolded and damaged proteins. We have recently reported that NBR1, the first identified plant autophagy adaptor with a ubiquitin-association domain, plays a critical role in plant stress tolerance by targeting stress-induced, ubiquitinated protein aggregates for degradation by autophagy. Here we report a comprehensive genetic analysis of CHIP, a chaperone-associated E3 ubiquitin ligase from Arabidopsis thaliana implicated in mediating degradation of nonnative proteins by 26S proteasomes. We isolated two chip knockout mutants and discovered that they had the same phenotypes as the nbr1 mutants with compromised tolerance to heat, oxidative and salt stresses and increased accumulation of insoluble proteins under heat stress. To determine their functional interactions, we generated chip nbr1 double mutants and found them to be further compromised in stress tolerance and in clearance of stress-induced protein aggregates, indicating additive roles of CHIP and NBR1. Furthermore, stress-induced protein aggregates were still ubiquitinated in the chip mutants. Through proteomic profiling, we systemically identified heat-induced protein aggregates in the chip and nbr1 single and double mutants. These experiments revealed that highly aggregate-prone proteins such as Rubisco activase and catalases preferentially accumulated in the nbr1 mutant while a number of light-harvesting complex proteins accumulated at high levels in the chip mutant after a relatively short period of heat stress. With extended heat stress, aggregates for a large number of intracellular proteins accumulated in both chip and nbr1 mutants and, to a greater extent, in the chip nbr1 double mutant. Based on these results, we propose that CHIP and NBR1 mediate two distinct but complementary anti-proteotoxic pathways and protein's propensity to aggregate under stress conditions is one of the critical factors for pathway selection of protein degradation.

Environmental stresses such as heat cause generation of misfolded and damaged proteins, which are highly toxic and must be efficiently removed. In plants, NBR1, the first isolated autophagy receptor with an ubiquitin-association domain, plays a critical role in plant stress tolerance by targeting ubiquitinated protein aggregates under stress conditions for degradation by autophagy. To study how stress-induced misfolded and damaged proteins are detected and ubiquitinated in plant cells, we analyzed the chaperone-associated E3 ubiquitin ligase CHIP from Arabidopsis thaliana for its role in protection against proteotoxicity in plant stress responses. Disruption of Arabidopsis CHIP caused increased sensitivity to a spectrum of abiotic stresses as found in the Arabidopsis nbr1 mutants. Disruption of both Arabidopsis CHIP and NBR1 further compromised plant stress tolerance, indicating that their roles are additive. Furthermore, in the chip nbr1 double mutant, compromised heat tolerance was associated with increased accumulation of insoluble proteins derived mostly from heat-sensitive but biologically important proteins such as Rubisco activase, catalases and proteins required for protein synthesis and folding. Importantly, stress-induced protein aggregates were still highly ubiquitinated in the chip mutants. These results strongly suggest that CHIP and NBR1 function in two distinct but complementary anti-proteotoxic pathways in plant stress responses.

Non-enzymatic post-translational protein modifications and proteostasis network deregulation in carcinogenesis

Organisms are constantly challenged by stressors and thus the maintenance of biomolecules functionality is essential for the assurance of cellular homeostasis. Proteins carry out the vast majority of cellular functions by mostly participating in multimeric protein assemblies that operate as protein machines. Cells have evolved a complex proteome quality control network for the rescue, when possible, or the degradation of damaged polypeptides. Nevertheless, despite these proteostasis ensuring mechanisms, new protein synthesis, and the replication-mediated dilution of proteome damage in mitotic cells, the gradual accumulation of stressors during aging (or due to lifestyle) results in increasingly damaged proteome. Non-enzymatic post-translational protein modifications mostly arise by unbalanced redox homeostasis and/or high glucose levels and may cause disruption of proteostasis as they can alter protein function. This outcome may then increase genomic instability due to reduced fidelity in processes like DNA replication or repair. Herein, we present a synopsis of the major non-enzymatic post-translation protein modifications and of the proteostasis network deregulation in carcinogenesis. We propose that activation of the proteostasis ensuring mechanisms in premalignant cells has tumor-preventive effects, whereas considering that over-activation of these mechanisms represents a hallmark of advanced tumors, their inhibition provides a strategy for the development of anti-tumor therapies. This article is part of a Special Issue entitled: Posttranslational Protein modifications in biology and Medicine.

Degradation of NF-κB, p53 and other regulatory redox-sensitive proteins by thiol-conjugating and -nitrosylating drugs in human tumor cells

The ionized cysteines present on the surfaces of many redox-sensitive proteins play functionally essential roles and are readily targeted by the reactive oxygen and reactive nitrogen species. Using disulfiram (DSF) and nitroaspirin (NCX4016) as the model compounds that mediate thiol-conjugating and nitrosylating reactions, respectively, we investigated the fate of p53, nuclear factor-kappaB (NF-κB) and other redox-responsive proteins following the exposure of human cancer cell lines to the drugs. Both drugs induced glutathionylation of bulk proteins in tumor cells and cell-free extracts. A prominent finding of this study was a time- and dose-dependent degradation of the redox-regulated proteins after brief treatments of tumor cells with DSF or NCX4016. DSF and copper-chelated DSF at concentrations of 50-200 µM induced the disappearance of wild-type p53, mutant p53, NF-κB subunit p50 and the ubiquitin-activating enzyme E1 (UBE1) in tumor cell lines. DSF also induced the glutathionylation of p53. The recombinant p53 protein modified by DSF was preferentially degraded by rabbit reticulocyte lysates. The proteasome inhibitor PS341 curtailed the DSF-induced degradation of p53 in HCT116 cells. Further, the NCX4016 induced a dose-dependent disappearance of the UBE1 and NF-κB p50 proteins in cell lines, besides a time-dependent degradation of aldehyde dehydrogenase in mouse liver after a single injection of 150 mg/kg. The loss of p53 and NF-kB proteins correlated with decreases in their specific binding to DNA. Our results demonstrate the hitherto unrecognized ability of the non-toxic thiolating and nitrosylating agents to degrade regulatory proteins and highlight the exploitable therapeutic benefits.

The ubiquitin-proteasome system in retinal health and disease

The ubiquitin-proteasome system (UPS) is the main intracellular pathway for modulated protein turnover, playing an important role in the maintenance of cellular homeostasis. It also exerts a protein quality control through degradation of oxidized, mutant, denatured, or misfolded proteins and is involved in many biological processes where protein level regulation is necessary. This system allows the cell to modulate its protein expression pattern in response to changing physiological conditions and provides a critical protective role in health and disease. Impairments of UPS function in the central nervous system (CNS) underlie an increasing number of genetic and idiopathic diseases, many of which affect the retina. Current knowledge on the UPS composition and function in this tissue, however, is scarce and dispersed. This review focuses on UPS elements reported in the retina, including ubiquitinating and deubiquitinating enzymes (DUBs), and alternative proteasome assemblies. Known and inferred roles of protein ubiquitination, and of the related, SUMO conjugation (SUMOylation) process, in normal retinal development and adult homeostasis are addressed, including modulation of the visual cycle and response to retinal stress and injury. Additionally, the relationship between UPS dysfunction and human neurodegenerative disorders affecting the retina, including Alzheimer's, Parkinson's, and Huntington's diseases, are dealt with, together with numerous instances of retina-specific illnesses with UPS involvement, such as retinitis pigmentosa, macular degenerations, glaucoma, diabetic retinopathy (DR), and aging-related impairments. This information, though still basic and limited, constitutes a suitable framework to be expanded in incoming years and should prove orientative toward future therapy design targeting sight-affecting diseases with a UPS underlying basis.