The co-chaperone BAG3 regulates Herpes Simplex Virus replication

Critical Role of Viral Protein Hexon in Hypervirulent Fowl Adenovirus Serotype-4-Induced Autophagy by Interaction with BAG3 and Promotion of Viral Replication in LMH Cells

Hepatitis-pericardial syndrome (HHS) is an acute highly infectious avian disease caused by fowl adenovirus serotype 4 (FAdV-4), characterized by fulminant hepatitis and hydropericardium in broilers. Since 2015, a widespread epidemic has occurred in China due to the emergence of hypervirulent FAdV-4 (HPFAdV-4), causing huge losses to the stakeholders. However, the pathogenesis of HPFAdV-4 and the host responses to its infection remain elusive. Here, we show that infection of leghorn male hepatocellular (LMH) cells by HPFAdV-4 induced complete autophagy in cells and that the autophagy induced by recombinant HPFAdV-4-ON1 (rHPFAdV-4-ON1), a viral strain generated by replacing the hexon gene of wild-type HPFAdV-4 (HPFAdV-4-WT) with the one of nonpathogenic strain FAdV-4-ON1, was remarkably mitigated compared to that of the rHPFAdV-4-WT control, suggesting that HPFAdV-4 hexon is responsible for virus-induced autophagy. Importantly, we found that hexon interacted with a cellular protein, BAG3, a host protein that initiates autophagy, and that BAG3 expression increased in cells infected with HPFAdV-4. Furthermore, knockdown of BAG3 by RNA interference (RNAi) significantly inhibited HPFAdV-4- or hexon-induced autophagy and suppressed viral replication. On the contrary, expression of hexon markedly upregulated the expression of BAG3 via activating the P38 signaling pathway, triggering autophagy. Thus, these findings reveal that HPFAdV-4 hexon interacts with the host protein BAG3 and promotes BAG3 expression by activating P38 signaling pathway, thereby inducing autophagy and enhancing viral proliferation, which immensely furthers our understanding of the pathogenesis of HPFAdV-4 infection. IMPORTANCE HHS, mainly caused by HPFAdV-4, has caused large economic losses to the stakeholders in recent years. Infection of leghorn male hepatocellular (LMH) cells by HPFAdV-4 induced complete autophagy that is essential for HPFAdV-4 replication. By a screening strategy, the viral protein hexon was found responsible for virus-induced autophagy in cells. Importantly, hexon was identified as a factor promoting viral replication by interaction with BAG3, an initiator of host cell autophagy. These findings will help us to better understand the host response to HPFAdV-4 infection, providing a novel insight into the pathogenesis of HPFAdV-4 infection.

Mode of Action of Heat Shock Protein (HSP) Inhibitors against Viruses through Host HSP and Virus Interactions

Misfolded proteins after stress-induced denaturation can regain their functions through correct re-folding with the aid of molecular chaperones. As a molecular chaperone, heat shock proteins (HSPs) can help client proteins fold correctly. During viral infection, HSPs are involved with replication, movement, assembly, disassembly, subcellular localization, and transport of the virus via the formation of macromolecular protein complexes, such as the viral replicase complex. Recent studies have indicated that HSP inhibitors can inhibit viral replication by interfering with the interaction of the virus with the HSP. In this review, we describe the function and classification of HSPs, the transcriptional mechanism of HSPs promoted by heat shock factors (HSFs), discuss the interaction between HSPs and viruses, and the mode of action of HSP inhibitors at two aspects of inhibiting the expression of HSPs and targeting the HSPs, and elaborate their potential use as antiviral agents.

The chaperone-assisted selective autophagy complex dynamics and dysfunctions

Each protein must be synthesized with the correct amino acid sequence, folded into its native structure, and transported to a relevant subcellular location and protein complex. If any of these steps fail, the cell has the capacity to break down aberrant proteins to maintain protein homeostasis (also called proteostasis). All cells possess a set of well-characterized protein quality control systems to minimize protein misfolding and the damage it might cause. Autophagy, a conserved pathway for the degradation of long-lived proteins, aggregates, and damaged organelles, was initially characterized as a bulk degradation pathway. However, it is now clear that autophagy also contributes to intracellular homeostasis by selectively degrading cargo material. One of the pathways involved in the selective removal of damaged and misfolded proteins is chaperone-assisted selective autophagy (CASA). The CASA complex is composed of three main proteins (HSPA, HSPB8 and BAG3), essential to maintain protein homeostasis in muscle and neuronal cells. A failure in the CASA complex, caused by mutations in the respective coding genes, can lead to (cardio)myopathies and neurodegenerative diseases. Here, we summarize our current understanding of the CASA complex and its dynamics. We also briefly discuss how CASA complex proteins are involved in disease and may represent an interesting therapeutic target.Abbreviation ALP: autophagy lysosomal pathway; ALS: amyotrophic lateral sclerosis; AMOTL1: angiomotin like 1; ARP2/3: actin related protein 2/3; BAG: BAG cochaperone; BAG3: BAG cochaperone 3; CASA: chaperone-assisted selective autophagy; CMA: chaperone-mediated autophagy; DNAJ/HSP40: DnaJ heat shock protein family (Hsp40); DRiPs: defective ribosomal products; EIF2A/eIF2α: eukaryotic translation initiation factor 2A; EIF2AK1/HRI: eukaryotic translation initiation factor 2 alpha kinase 1; GABARAP: GABA type A receptor-associated protein; HDAC6: histone deacetylase 6; HSP: heat shock protein; HSPA/HSP70: heat shock protein family A (Hsp70); HSP90: heat shock protein 90; HSPB8: heat shock protein family B (small) member 8; IPV: isoleucine-proline-valine; ISR: integrated stress response; KEAP1: kelch like ECH associated protein 1; LAMP2A: lysosomal associated membrane protein 2A; LATS1: large tumor suppressor kinase 1; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MTOC: microtubule organizing center; MTOR: mechanistic target of rapamycin kinase; NFKB/NF-κB: nuclear factor kappa B; NFE2L2: NFE2 like bZIP transcription factor 2; PLCG/PLCγ: phospholipase C gamma; polyQ: polyglutamine; PQC: protein quality control; PxxP: proline-rich; RAN translation: repeat-associated non-AUG translation; SG: stress granule; SOD1: superoxide dismutase 1; SQSTM1/p62: sequestosome 1; STUB1/CHIP: STIP1 homology and U-box containing protein 1; STK: serine/threonine kinase; SYNPO: synaptopodin; TBP: TATA-box binding protein; TARDBP/TDP-43: TAR DNA binding protein; TFEB: transcription factor EB; TPR: tetratricopeptide repeats; TSC1: TSC complex subunit 1; UBA: ubiquitin associated; UPS: ubiquitin-proteasome system; WW: tryptophan-tryptophan; WWTR1: WW domain containing transcription regulator 1; YAP1: Yes1 associated transcriptional regulator.

Functional characterization of BAG3 in orange-spotted grouper (Epinephelus coioides) during viral infection

Bcl-2-associated athanogene 3 (BAG3) is a cochaperone protein that interacts with Bcl-2 and mediate cell death. However, little is known about the roles of fish BAG3 during viral infection. In this study, we characterized a BAG3 homolog from orange-spotted grouper (Epinephelus coioides) (EcBAG3) and investigated its roles during viral infection. The EcBAG3 protein encoded 579 amino acids with typical WW, PXXP and BAG domains, which shared high identities with reported fish BAG3. Quantitative real-time PCR (qRT-PCR) analysis revealed that EcBAG3 was highly expressed in brain and heart. And the expression of EcBAG3 was significantly up-regulated after red-spotted grouper nervous necrosis virus (RGNNV) stimulation in vitro. EcBAG3 overexpression could promoted the expression of viral genes (coat protein (CP) and RNA-dependent RNA polymerase (RdRp)), which was enhanced by co-transfection with Hsp70 and Hsp22. Also, EcBAG3 overexpression up-regulated the expression of LC3-Ⅱ and down-regulated the expression of Bax and BNIP3, the IFN- (IRF1, IRF3, IRF7, IFP35, Mx1) or inflammation-related (IL-1β and TNFα) factors, as well as decreased the activities of NF-κB, ISRE and IFN-3. While knockdown of EcBAG3 decreased the transcripts of RGNNV CP gene and RdRp gene. Further studies showed that EcBAG3 knockdown impaired the expression level of autophagy factor LC3-Ⅱ, and promoted the expression level of Bax and BNIP3, inflammatory factors and interferon factors. These data indicate that EcBAG3 can affect viral infection through modulating virus-induced cell death, regulating the expression of IFN- and inflammation-related factors, which will be helpful to further explore the immune response of fish during viral infection.

The role of BAG3 in health and disease: A "Magic BAG of Tricks"

The multi-domain structure of Bcl-2-associated athanogene 3 (BAG3) facilitates its interaction with many different proteins that participate in regulating a variety of biological pathways. After revisiting the BAG3 literature published over the past ten years with Citespace software, we classified the BAG3 research into several clusters, including cancer, cardiomyopathy, neurodegeneration, and viral propagation. We then highlighted recent key findings in each cluster. To gain greater insight into the roles of BAG3, we analyzed five different published mass spectrometry data sets of proteins that co-immunoprecipitate with BAG3. These data gave us insight into universal, as well as cell-type-specific BAG3 interactors in cancer cells, cardiomyocytes, and neurons. Finally, we mapped variable BAG3 SNPs and also mutation data from previous publications to further explore the link between the domains and function of BAG3. We believe this review will provide a better understanding of BAG3 and direct future studies towards understanding BAG3 function in physiological and pathological conditions.

Hsp90 is involved in pseudorabies virus virion assembly via stabilizing major capsid protein VP5

Many viruses utilize molecular chaperone heat shock protein 90 (Hsp90) for protein folding and stabilization, however, the role of Hsp90 in herpesvirus lifecycle is obscure. Here, we provide evidence that Hsp90 participates in pseudorabies virus (PRV) replication. Viral growth kinetics assays show that Hsp90 inhibitor geldanamycin (GA) abrogates PRV replication at the post-penetration step. Transmission electron microscopy demonstrates that dysfunction of Hsp90 diminishes the quantity of PRV nucleocapsids. Overexpression and knockdown of Hsp90 suggest that de novo Hsp90 is involved in PRV replication. Mechanismly, dysfunction of Hsp90 inhibits PRV major capsid protein VP5 expression. Co-immunoprecipitation and indirect immunofluorescence assays indicate that Hsp90 interacts with VP5. Interestingly, Hsp70, a collaborator of Hsp90, also interacts with VP5, but doesn't affect PRV growth. Finally, inhibition of Hsp90 results in PRV VP5 degradation in a proteasome-dependent manner. Collectively, our data suggest that Hsp90 contributes to PRV virion assembly and replication via stabilization of VP5.

Are Hsp90 inhibitors good candidates against Covid-19?

Drug reposition, or repurposing, has become a promising strategy in therapeutics due to its advantages in several aspects of drug therapy. General drug development is expensive and can take more than 10 years to go through the designing, development, and necessary approval steps. However, established drugs have already overcome these steps and thus a potential candidate may be already available decreasing the risks and costs involved. Viruses invade cells, usually provoking biochemical changes, leading to tissue damage, alteration of normal physiological condition in organisms and can even result in death. Inside the cell, the virus finds the machinery necessary for its multiplication, as for instance the protein quality control system, which involves chaperones and Hsps (heat shock proteins) that, in addition to physiological functions, help in the stabilization of viral proteins. Recently, many inhibitors of Hsp90 have been developed as therapeutic strategies against diseases such as the Hsp90 inhibitors used in anticancer therapy. Several shreds of evidence indicate that these inhibitors can also be used as therapeutic strategies against viruses. Therefore, since a drug treatment for COVID-19 is urgently needed, this review aims to discuss the potential use of Hsp90 inhibitors in the treatment of this globally threatening disease.

Host BAG3 Is Degraded by Pseudorabies Virus pUL56 C-Terminal 181L-185L and Plays a Negative Regulation Role during Viral Lytic Infection

Bcl2-associated athanogene (BAG) 3, which is a chaperone-mediated selective autophagy protein, plays a pivotal role in modulating the life cycle of a wide variety of viruses. Both positive and negative modulations of viruses by BAG3 were reported. However, the effects of BAG3 on pseudorabies virus (PRV) remain unknown. To investigate whether BAG3 could modulate the PRV life cycle during a lytic infection, we first identified PRV protein UL56 (pUL56) as a novel BAG3 interactor by co-immunoprecipitation and co-localization analyses. The overexpression of pUL56 induced a significant degradation of BAG3 at protein level via the lysosome pathway. The C-terminal mutations of ¹⁸¹L/A, ¹⁸⁵L/A, or ¹⁸¹L/A-¹⁸⁵L/A in pUL56 resulted in a deficiency in pUL56-induced BAG3 degradation. In addition, the pUL56 C-terminal mutants that lost Golgi retention abrogated pUL56-induced BAG3 degradation, which indicates a Golgi retention-dependent manner. Strikingly, BAG3 was not observed to be degraded in either wild-type or UL56-deleted PRV infected cells as compared to mock infected ones, whereas the additional two adjacent BAG3 cleaved products were found in the infected cells in a species-specific manner. Overexpression of BAG3 significantly suppressed PRV proliferation, while knockdown of BAG3 resulted in increased viral yields in HEK293T cells. Thus, these data indicated a negative regulation role of BAG3 during PRV lytic infection. Collectively, our findings revealed a novel molecular mechanism on host protein degradation induced by PRV pUL56. Moreover, we identified BAG3 as a host restricted protein during PRV lytic infection in cells.

Proteostasis in Viral Infection: Unfolding the Complex Virus-Chaperone Interplay

Viruses are obligate intracellular parasites that rely on their hosts for protein synthesis, genome replication, and viral particle production. As such, they have evolved mechanisms to divert host resources, including molecular chaperones, facilitate folding and assembly of viral proteins, stabilize complex structures under constant mutational pressure, and modulate signaling pathways to dampen antiviral responses and prevent premature host death. Biogenesis of viral proteins often presents unique challenges to the proteostasis network, as it requires the rapid and orchestrated production of high levels of a limited number of multifunctional, multidomain, and aggregation-prone proteins. To overcome such challenges, viruses interact with the folding machinery not only as clients but also as regulators of chaperone expression, function, and subcellular localization. In this review, we summarize the main types of interactions between viral proteins and chaperones during infection, examine evolutionary aspects of this relationship, and discuss the potential of using chaperone inhibitors as broad-spectrum antivirals.

The Role of the Multifunctional BAG3 Protein in Cellular Protein Quality Control and in Disease

In neurons, but also in all other cells the complex proteostasis network is monitored and tightly regulated by the cellular protein quality control (PQC) system. Beyond folding of newly synthesized polypeptides and their refolding upon misfolding the PQC also manages the disposal of aberrant proteins either by the ubiquitin-proteasome machinery or by the autophagic-lysosomal system. Aggregated proteins are primarily degraded by a process termed selective macroautophagy (or aggrephagy). One such recently discovered selective macroautophagy pathway is mediated by the multifunctional HSP70 co-chaperone BAG3 (BCL-2-associated athanogene 3). Under acute stress and during cellular aging, BAG3 in concert with the molecular chaperones HSP70 and HSPB8 as well as the ubiquitin receptor p62/SQSTM1 specifically targets aggregation-prone proteins to autophagic degradation. Thereby, BAG3-mediated selective macroautophagy represents a pivotal adaptive safeguarding and emergency system of the PQC which is activated under pathophysiological conditions to ensure cellular proteostasis. Interestingly, BAG3-mediated selective macroautophagy is also involved in the clearance of aggregated proteins associated with age-related neurodegenerative disorders, like Alzheimer’s disease (tau-protein), Huntington’s disease (mutated huntingtin/polyQ proteins), and amyotrophic lateral sclerosis (mutated SOD1). In addition, based on its initial description BAG3 is an anti-apoptotic protein that plays a decisive role in other widespread diseases, including cancer and myopathies. Therefore, in the search for novel therapeutic intervention avenues in neurodegeneration, myopathies and cancer BAG3 is a promising candidate.

Chaperone-Mediated Autophagy Protein BAG3 Negatively Regulates Ebola and Marburg VP40-Mediated Egress

Ebola (EBOV) and Marburg (MARV) viruses are members of the Filoviridae family which cause outbreaks of hemorrhagic fever. The filovirus VP40 matrix protein is essential for virus assembly and budding, and its PPxY L-domain motif interacts with WW-domains of specific host proteins, such as Nedd4 and ITCH, to facilitate the late stage of virus-cell separation. To identify additional WW-domain-bearing host proteins that interact with VP40, we used an EBOV PPxY-containing peptide to screen an array of 115 mammalian WW-domain-bearing proteins. Using this unbiased approach, we identified BCL2 Associated Athanogene 3 (BAG3), a member of the BAG family of molecular chaperone proteins, as a specific VP40 PPxY interactor. Here, we demonstrate that the WW-domain of BAG3 interacts with the PPxY motif of both EBOV and MARV VP40 and, unexpectedly, inhibits budding of both eVP40 and mVP40 virus-like particles (VLPs), as well as infectious VSV-EBOV recombinants. BAG3 is a stress induced protein that regulates cellular protein homeostasis and cell survival through chaperone-mediated autophagy (CMA). Interestingly, our results show that BAG3 alters the intracellular localization of VP40 by sequestering VP40 away from the plasma membrane. As BAG3 is the first WW-domain interactor identified that negatively regulates budding of VP40 VLPs and infectious virus, we propose that the chaperone-mediated autophagy function of BAG3 represents a specific host defense strategy to counteract the function of VP40 in promoting efficient egress and spread of virus particles.

BAG3 down-modulation sensitizes HPV18(+) HeLa cells to PEITC-induced apoptosis and restores p53

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BAG3 down-modulation by siRNA technology restored p53.

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Reduced BAG3 expression sensitized HeLa but not C33A (HPV-negative) cells to PEITC.

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Reduced BAG3 expression was associated with a decrease of E6 viral protein levels.

BAG3 is a multi-functional component of tumor cell pro-survival machinery, and its biological functions have been largely associated to proteasome system. Here, we show that BAG3 down-modulation resulted in reduced cell viability and enhanced PEITC-induced apoptosis largely more extensively in HeLa (HPV18⁺) rather than in C33A (HPV⁻) cervical carcinoma cell lines. Moreover, we demonstrate that BAG3 suppression led to a decrease of viral E6 oncoprotein and a concomitant recovery of p53 tumor suppressor, the best recognized target of E6 for proteasome degradation. E6 and p53 expression were modulated at protein level, since their respective mRNAs were unaffected. Taken together our findings reveal a novel role for BAG3 as host protein contributing to HPV18 E6-activated pro-survival strategies, and suggest a possible relevance of its expression levels in drug/radiotherapy-resistance of HPV18-bearing cervical carcinomas.

Bcl2-associated athanogene 3 interactome analysis reveals a new role in modulating proteasome activity

Bcl2-associated athanogene 3 (BAG3), a member of the BAG family of co-chaperones, plays a critical role in regulating apoptosis, development, cell motility, autophagy, and tumor metastasis and in mediating cell adaptive responses to stressful stimuli. BAG3 carries a BAG domain, a WW domain, and a proline-rich repeat (PXXP), all of which mediate binding to different partners. To elucidate BAG3's interaction network at the molecular level, we employed quantitative immunoprecipitation combined with knockdown and human proteome microarrays to comprehensively profile the BAG3 interactome in humans. We identified a total of 382 BAG3-interacting proteins with diverse functions, including transferase activity, nucleic acid binding, transcription factors, proteases, and chaperones, suggesting that BAG3 is a critical regulator of diverse cellular functions. In addition, we characterized interactions between BAG3 and some of its newly identified partners in greater detail. In particular, bioinformatic analysis revealed that the BAG3 interactome is strongly enriched in proteins functioning within the proteasome-ubiquitination process and that compose the proteasome complex itself, suggesting that a critical biological function of BAG3 is associated with the proteasome. Functional studies demonstrated that BAG3 indeed interacts with the proteasome and modulates its activity, sustaining cell survival and underlying resistance to therapy through the down-modulation of apoptosis. Taken as a whole, this study expands our knowledge of the BAG3 interactome, provides a valuable resource for understanding how BAG3 affects different cellular functions, and demonstrates that biologically relevant data can be harvested using this kind of integrated approach.

Chaperone-assisted proteostasis is essential for mechanotransduction in mammalian cells

Maintaining the dynamic proteome of a living cell in the face of an ever-changing environment depends on a fine-tuned balance of protein synthesis and protein degradation. Molecular chaperones exert key functions during protein homeostasis (proteostasis). They associate with nonnative client proteins following synthesis or damage and facilitate client sorting and folding. When client proteins are terminally misfolded, chaperones cooperate with protein degradation systems to dispose of such clients. This dual proteostasis activity of chaperones is essential for maintaining cell function under normal growth conditions and becomes even more important under stress conditions such as heat and oxidative stress. The recent identification of chaperone-assisted selective autophagy (CASA) as a tension-induced autophagy pathway highlights the critical role of molecular chaperones in mechanically strained cells and tissues. The CASA complex, assembled by the cochaperone BAG3, coordinates protein degradation and protein synthesis in response to mechanical force. Here we describe the composition and function of this chaperone complex in mammals and discuss its relevance for tissue homeostasis and the regulation of cell adhesion, migration and proliferation. We provide a unifying concept for the function of BAG3, which integrates its involvement in muscle maintenance, tumor formation and virus infection.

BAG3-dependent noncanonical autophagy induced by proteasome inhibition in HepG2 cells

Emerging lines of evidence have shown that blockade of ubiquitin-proteasome system (UPS) activates autophagy. The molecular players that regulate the relationship between them remain to be elucidated. Bcl-2 associated athanogene 3 (BAG3) is a member of the BAG co-chaperone family that regulates the ATPase activity of heat shock protein 70 (HSP70) chaperone family. Studies on BAG3 have demonstrated that it plays multiple roles in physiological and pathological processes, including antiapoptotic activity, signal transduction, regulatory role in virus infection, cell adhesion and migration. Recent studies have attracted much attention on its role in initiation of autophagy. The current study, for the first time, demonstrates that proteasome inhibitors elicit noncanonical autophagy, which was not suppressed by inhibitors of class III phosphatidylinositol 3-kinase (PtdIns3K) or shRNA against Beclin 1 (BECN1). In addition, we demonstrate that BAG3 is ascribed to activation of autophagy elicited by proteasome inhibitors and MAPK8/9/10 (also known as JNK1/2/3 respectively) activation is also implicated via upregulation of BAG3. Moreover, we found that noncanonical autophagy mediated by BAG3 suppresses responsiveness of HepG2 cells to proteasome inhibitors.

A novel, non-apoptotic role for Scythe/BAT3: a functional switch between the pro- and anti-proliferative roles of p21 during the cell cycle

Background

Scythe/BAT3 is a member of the BAG protein family whose role in apoptosis has been extensively studied. However, since the developmental defects observed in Bat3-null mouse embryos cannot be explained solely by defects in apoptosis, we investigated whether BAT3 is also involved in cell-cycle progression.

Methods/Principal Findings

Using a stable-inducible Bat3-knockdown cellular system, we demonstrated that reduced BAT3 protein level causes a delay in both G1/S transition and G2/M progression. Concurrent with these changes in cell-cycle progression, we observed a reduction in the turnover and phosphorylation of the CDK inhibitor p21, which is best known as an inhibitor of DNA replication; however, phosphorylated p21 has also been shown to promote G2/M progression. Our findings indicate that in Bat3-knockdown cells, p21 continues to be synthesized during cell-cycle phases that do not normally require p21, resulting in p21 protein accumulation and a subsequent delay in cell-cycle progression. Finally, we showed that BAT3 co-localizes with p21 during the cell cycle and is required for the translocation of p21 from the cytoplasm to the nucleus during the G1/S transition and G2/M progression.

Conclusion:

Our study reveals a novel, non-apoptotic role for BAT3 in cell-cycle regulation. By maintaining a low p21 protein level during the G1/S transition, BAT3 counteracts the inhibitory effect of p21 on DNA replication and thus enables the cells to progress from G1 to S phase. Conversely, during G2/M progression, BAT3 facilitates p21 phosphorylation by cyclin A/Cdk2, an event required for G2/M progression. BAT3 modulates these pro- and anti-proliferative roles of p21 at least in part by regulating cyclin A abundance, as well as p21 translocation between the cytoplasm and the nucleus to ensure that it functions in the appropriate intracellular compartment during each phase of the cell cycle.

Broad action of Hsp90 as a host chaperone required for viral replication

Viruses are intracellular pathogens responsible for a vast number of human diseases. Due to their small genome size, viruses rely primarily on the biosynthetic apparatus of the host for their replication. Recent work has shown that the molecular chaperone Hsp90 is nearly universally required for viral protein homeostasis. As observed for many endogenous cellular proteins, numerous different viral proteins have been shown to require Hsp90 for their folding, assembly, and maturation. Importantly, the unique characteristics of viral replication cause viruses to be hypersensitive to Hsp90 inhibition, thus providing a novel therapeutic avenue for the development of broad-spectrum antiviral drugs. The major developments in this emerging field are hereby discussed. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).

Involvement of JNK and NF-κB pathways in lipopolysaccharide (LPS)-induced BAG3 expression in human monocytic cells

Lipopolysaccharide (LPS) is an outer-membrane glycolipid component of Gram-negative bacteria known for its fervent ability to activate monocytic cells and for its potent proinflammatory capabilities. Bcl-2-associated athanogene 3 (BAG3) is a survival protein that has been shown to be stimulated during cell response to stressful conditions, such as exposure to high temperature, heavy metals, proteasome inhibition, and human immunodeficiency virus 1 (HIV-1) infection. In addition, BAG3 regulates replication of Varicella-Zoster Virus (VZV) and Herpes Simplex Virus (HSV) replication, suggesting that BAG3 could participate in the host response to infection. In the current study, we found that LPS increased the expression of BAG3 in a dose- and time-dependent manner. Actinomycin D completely blocked the LPS-induced BAG3 accumulation, as well as LPS activated the proximal promoter of BAG3 gene, supported that the induction by LPS occurred at the level of gene transcription. LPS-induced BAG3 expression was blocked by JNK or NF-κB inhibition, suggesting that JNK and NF-κB pathways participated in BAG3 induction by LPS. In addition, we also found that induction of BAG3 was implicated in monocytic cell adhesion to extracellular matrix induced by LPS. Overall, the data support that BAG3 is induced by LPS via JNK and NF-κB-dependent signals, and involved in monocytic cell-extracellular matrix interaction, suggesting that BAG3 may have a role in the host response to LPS stimulation.

BAG3: a multifaceted protein that regulates major cell pathways

Bcl2-associated athanogene 3 (BAG3) protein is a member of BAG family of co-chaperones that interacts with the ATPase domain of the heat shock protein (Hsp) 70 through BAG domain (110–124 amino acids). BAG3 is the only member of the family to be induced by stressful stimuli, mainly through the activity of heat shock factor 1 on bag3 gene promoter. In addition to the BAG domain, BAG3 contains also a WW domain and a proline-rich (PXXP) repeat, that mediate binding to partners different from Hsp70. These multifaceted interactions underlie BAG3 ability to modulate major biological processes, that is, apoptosis, development, cytoskeleton organization and autophagy, thereby mediating cell adaptive responses to stressful stimuli. In normal cells, BAG3 is constitutively present in a very few cell types, including cardiomyocytes and skeletal muscle cells, in which the protein appears to contribute to cell resistance to mechanical stress. A growing body of evidence indicate that BAG3 is instead expressed in several tumor types. In different tumor contexts, BAG3 protein was reported to sustain cell survival, resistance to therapy, and/or motility and metastatization. In some tumor types, down-modulation of BAG3 levels was shown, as a proof-of-principle, to inhibit neoplastic cell growth in animal models. This review attempts to outline the emerging mechanisms that can underlie some of the biological activities of the protein, focusing on implications in tumor progression.

Autoregulation of co-chaperone BAG3 gene transcription

The Bcl-2-associated athanogene, BAG, protein family through their BAG domain associates with the heat shock protein 70 (HSP-70) and modulates its chaperone activity. One member of this family, BAG3, appears to play an important role in protein homeostasis, as its expression promotes cell survival. Expression of BAG3 is enhanced by a variety of stress-inducing agents. Here we describe a role for BAG3 to modulate transcription of its own promoter through a positive feedback loop involving its 5'-UTR sequence. Activation of the BAG3 promoter is mediated by the BAG domain and is independent of BAG3 association with the UTR sequence. Autoactivation of the BAG3 gene is observed in several cultures of human glial cells including gliomas, but not in several other non-glial cell lines such as He La and others. Results from cell fractionation and immunocytochemistry showed BAG3 in the cytoplasm as well as the nuclei of glial cells. These observations suggest that BAG3 gene expression is controlled by its own product and that this may be critical for the biological activity of BAG3 in some cell types.

The RING finger domain of Varicella-Zoster virus ORF61p has E3 ubiquitin ligase activity that is essential for efficient autoubiquitination and dispersion of Sp100-containing nuclear bodies

Varicella zoster virus encodes an immediate-early (IE) protein termed ORF61p that is orthologous to the herpes simplex virus IE protein ICP0. Although these proteins share several functional properties, ORF61p does not fully substitute for ICP0. The greatest region of similarity between these proteins is a RING finger domain. We demonstrate that disruption of the ORF61p RING finger domain by amino acid substitution (Cys19Gly) alters ORF61p intranuclear distribution and abolishes ORF61p-mediated dispersion of Sp100-containing nuclear bodies. In addition, we demonstrate that an intact ORF61p RING finger domain is necessary for E3 ubiquitin ligase activity and is required for autoubiquitination and regulation of protein stability.

Quantitative proteomics analysis reveals BAG3 as a potential target to suppress severe acute respiratory syndrome coronavirus replication

The discovery of a novel coronavirus (CoV) as the causative agent of severe acute respiratory syndrome (SARS) has highlighted the need for a better understanding of CoV replication. The replication of SARS-CoV is highly dependent on host cell factors. However, relatively little is known about the cellular proteome changes that occur during SARS-CoV replication. Recently, we developed a cell line expressing a SARS-CoV subgenomic replicon and used it to screen inhibitors of SARS-CoV replication. To identify host proteins important for SARS-CoV RNA replication, the protein profiles of the SARS-CoV replicon cells and parental BHK21 cells were compared using a quantitative proteomic strategy termed "stable-isotope labeling by amino acids in cell culture-mass spectrometry" (SILAC-MS). Our results revealed that, among the 1,081 host proteins quantified in both forward and reverse SILAC measurements, 74 had significantly altered levels of expression. Of these, significantly upregulated BCL2-associated athanogene 3 (BAG3) was selected for further functional studies. BAG3 is involved in a wide variety of cellular processes, including cell survival, cellular stress response, proliferation, migration, and apoptosis. Our results show that inhibition of BAG3 expression by RNA interference led to significant suppression of SARS-CoV replication, suggesting the possibility that upregulation of BAG3 may be part of the machinery that SARS-CoV relies on for replication. By correlating the proteomic data with these functional studies, the findings of this study provide important information for understanding SARS-CoV replication.

AtBAG7, an Arabidopsis Bcl-2-associated athanogene, resides in the endoplasmic reticulum and is involved in the unfolded protein response

The Bcl-2-associated athanogene (BAG) family is an evolutionarily conserved, multifunctional group of cochaperones that perform diverse cellular functions ranging from proliferation to growth arrest and cell death in yeast, in mammals, and, as recently observed, in plants. The Arabidopsis genome contains seven homologs of the BAG family, including four with domain organization similar to animal BAGs. In the present study we show that an Arabidopsis BAG, AtBAG7, is a uniquely localized endoplasmic reticulum (ER) BAG that is necessary for the proper maintenance of the unfolded protein response (UPR). AtBAG7 was shown to interact directly in vivo with the molecular chaperone, AtBiP2, by bimolecular fluorescence complementation assays, and the interaction was confirmed by yeast two-hybrid assay. Treatment with an inducer of UPR, tunicamycin, resulted in accelerated cell death of AtBAG7-null mutants. Furthermore, AtBAG7 knockouts were sensitive to known ER stress stimuli, heat and cold. In these knockouts heat sensitivity was reverted successfully to the wild-type phenotype with the addition of the chemical chaperone, tauroursodexycholic acid (TUDCA). Real-time PCR of ER stress proteins indicated that the expression of the heat-shock protein, AtBiP3, is selectively up-regulated in AtBAG7-null mutants upon heat and cold stress. Our results reveal an unexpected diversity of the plant's BAG gene family and suggest that AtBAG7 is an essential component of the UPR during heat and cold tolerance, thus confirming the cytoprotective role of plant BAGs.

Chaperone-assisted selective autophagy is essential for muscle maintenance

How are biological structures maintained in a cellular environment that constantly threatens protein integrity? Here we elucidate proteostasis mechanisms affecting the Z disk, a protein assembly essential for actin anchoring in striated muscles, which is subjected to mechanical, thermal, and oxidative stress during contraction [1]. Based on the characterization of the Drosophila melanogaster cochaperone Starvin (Stv), we define a conserved chaperone machinery required for Z disk maintenance. Instead of keeping Z disk proteins in a folded conformation, this machinery facilitates the degradation of damaged components, such as filamin, through chaperone-assisted selective autophagy (CASA). Stv and its mammalian ortholog BAG-3 coordinate the activity of Hsc70 and the small heat shock protein HspB8 during disposal that is initiated by the chaperone-associated ubiquitin ligase CHIP and the autophagic ubiquitin adaptor p62. CASA is thus distinct from chaperone-mediated autophagy, previously shown to facilitate the ubiquitin-independent, direct translocation of a client across the lysosomal membrane [2]. Impaired CASA results in Z disk disintegration and progressive muscle weakness in flies, mice, and men. Our findings reveal the importance of chaperone-assisted degradation for the preservation of cellular structures and identify muscle as a tissue that highly relies on an intact proteostasis network, thereby shedding light on diverse myopathies and aging.

Herpes simplex virus type 1 immediate-early protein ICP22 is required for VICE domain formation during productive viral infection

During productive infection, herpes simplex virus type 1 (HSV-1) induces the formation of discrete nuclear foci containing cellular chaperone proteins, proteasomal components, and ubiquitinated proteins. These structures are known as VICE domains and are hypothesized to play an important role in protein turnover and nuclear remodeling in HSV-1-infected cells. Here we show that VICE domain formation in Vero and other cells requires the HSV-1 immediate-early protein ICP22. Since ICP22 null mutants replicate efficiently in Vero cells despite being unable to induce VICE domain formation, it can be concluded that VICE domain formation is not essential for HSV-1 productive infection. However, our findings do not exclude the possibility that VICE domain formation is required for viral replication in cells that are nonpermissive for ICP22 mutants. Our studies also show that ICP22 itself localizes to VICE domains, suggesting that it could play a role in forming these structures. Consistent with this, we found that ICP22 expression in transfected cells is sufficient to reorganize the VICE domain component Hsc70 into nuclear inclusion bodies that resemble VICE domains. An N-terminal segment of ICP22, corresponding to residues 1 to 146, is critical for VICE domain formation in infected cells and Hsc70 reorganization in transfected cells. We previously found that this portion of the protein is dispensable for ICP22's effects on RNA polymerase II phosphorylation. Thus, ICP22 mediates two distinct regulatory activities that both modify important components of the host cell nucleus.

Virus-Induced Chaperone-Enriched (VICE) domains function as nuclear protein quality control centers during HSV-1 infection

Virus-Induced Chaperone-Enriched (VICE) domains form adjacent to nuclear viral replication compartments (RC) during the early stages of HSV-1 infection. Between 2 and 3 hours post infection at a MOI of 10, host protein quality control machinery such as molecular chaperones (e.g. Hsc70), the 20S proteasome and ubiquitin are reorganized from a diffuse nuclear distribution pattern to sequestration in VICE domains. The observation that VICE domains contain putative misfolded proteins suggests that they may be similar to nuclear inclusion bodies that form under conditions in which the protein quality control machinery is overwhelmed by the presence of misfolded proteins. The detection of Hsc70 in VICE domains, but not in nuclear inclusion bodies, indicates that Hsc70 is specifically reorganized by HSV-1 infection. We hypothesize that HSV-1 infection induces the formation of nuclear protein quality control centers to remodel or degrade aberrant nuclear proteins that would otherwise interfere with productive infection. Detection of proteolytic activity in VICE domains suggests that substrates may be degraded by the 20S proteasome in VICE domains. FRAP analysis reveals that GFP-Hsc70 is dynamically associated with VICE domains, suggesting a role for Hsc70 in scanning the infected nucleus for misfolded proteins. During 42°C heat shock, Hsc70 is redistributed from VICE domains into RC perhaps to remodel viral replication and regulatory proteins that have become insoluble in these compartments. The experiments presented in this paper suggest that VICE domains are nuclear protein quality control centers that are modified by HSV-1 to promote productive infection.

Protein quality control is a protective cellular mechanism by which damaged proteins are refolded or degraded so that they cannot interfere with essential cellular processes. In the event that protein quality control machinery cannot refold or degrade damaged proteins, sequestration of misfolded protein is an alternative protective mechanism for reducing the toxic effects of misfolded protein. Several neurological diseases result from the accumulation of toxic misfolded proteins that cannot be efficiently refolded or degraded. In neurons from patients afflicted with Huntington's disease, misfolded huntingtin protein is sequestered in large aggregates in the nucleus called inclusion bodies. Inclusion bodies also contain protein quality control machinery including molecular chaperones, the proteasome and ubiquitin. Here we report that analogous structures called Virus-Induced Chaperone-Enriched (VICE) domains form in the nucleus of cells infected with Herpes Simplex Virus type 1 (HSV-1). VICE domains contain misfolded protein, chaperones and protein degradation activity. VICE domain formation is efficient in infected cells taxed with high levels of viral protein production. We hypothesize that misfolded proteins that arise in HSV-1-infected cells are sequestered in VICE domains to promote remodeling of misfolded proteins.

Complementation of a herpes simplex virus ICP0 null mutant by varicella-zoster virus ORF61p

The herpes simplex virus (HSV) ICP0 protein acts to overcome intrinsic cellular defenses that repress viral alpha gene expression. In that vein, viruses that have mutations in ICP0's RING finger or are deleted for the gene are sensitive to interferon, as they fail to direct degradation of promyelocytic leukemia protein (PML), a component of host nuclear domain 10s. While varicella-zoster virus is also insensitive to interferon, ORF61p, its ICP0 ortholog, failed to degrade PML. A recombinant virus with each coding region of the gene for ICP0 replaced with sequences encoding ORF61p was constructed. This virus was compared to an ICP0 deletion mutant and wild-type HSV. The recombinant degraded only Sp100 and not PML and grew to higher titers than its ICP0 null parental virus, but it was sensitive to interferon, like the virus from which it was derived. This analysis permitted us to compare the activities of ICP0 and ORF61p in identical backgrounds and revealed distinct biologic roles for these proteins.

Components of nuclear domain 10 bodies regulate varicella-zoster virus replication

PML, Sp100, and Daxx are proteins that normally reside within nuclear domains 10 (ND10s). They associate with DNA virus genomes and repress the very early stages of the DNA virus replication cycle. Virus-encoded proteins counteract this innate antiviral response. ICP0, a herpes simplex virus (HSV) immediate-early protein, is necessary and sufficient to dissociate ND10s and target their two major components, PML and Sp100, for proteasomal degradation. In this report, we show that ORF61p, the varicella-zoster virus (VZV) ortholog of ICP0, does not degrade PML and alters Sp100 levels only slightly. Furthermore, we demonstrate that other virus proteins cannot substitute for this lack of function during infection. By using short interfering RNAs, we depleted PML, Sp100, and Daxx and studied their roles in plaquing efficiency, virus protein accumulation, infectious-center titer, and virus spread. The results of these studies show that components of ND10s can accelerate VZV replication but do not ultimately control cell-associated virus titers. We conclude that while both ICP0 and ORF61p activate virus gene expression, they modulate host innate repression mechanisms in two different ways. As a result, HSV and VZV commandeer their host cells by distinct mechanisms to ensure their replication and spread.