The Long Acidic Tail of High Mobility Group Box 1 (HMGB1) Protein Forms an Extended and Flexible Structure That Interacts with Specific Residues within and between the HMG Boxes

The acidic intrinsically disordered region of the inflammatory mediator HMGB1 mediates fuzzy interactions with CXCL12

Chemokine heterodimers activate or dampen their cognate receptors during inflammation. The CXCL12 chemokine forms with the fully reduced (fr) alarmin HMGB1 a physiologically relevant heterocomplex (frHMGB1•CXCL12) that synergically promotes the inflammatory response elicited by the G-protein coupled receptor CXCR4. The molecular details of complex formation were still elusive. Here we show by an integrated structural approach that frHMGB1•CXCL12 is a fuzzy heterocomplex. Unlike previous assumptions, frHMGB1 and CXCL12 form a dynamic equimolar assembly, with structured and unstructured frHMGB1 regions recognizing the CXCL12 dimerization surface. We uncover an unexpected role of the acidic intrinsically disordered region (IDR) of HMGB1 in heterocomplex formation and its binding to CXCR4 on the cell surface. Our work shows that the interaction of frHMGB1 with CXCL12 diverges from the classical rigid heterophilic chemokines dimerization. Simultaneous interference with multiple interactions within frHMGB1•CXCL12 might offer pharmacological strategies against inflammatory conditions.

Adenovirus protein VII binds the A-box of HMGB1 to repress interferon responses

Viruses hijack host proteins to promote infection and dampen host defenses. Adenovirus encodes the multifunctional protein VII that serves both to compact viral genomes inside the virion and disrupt host chromatin. Protein VII binds the abundant nuclear protein high mobility group box 1 (HMGB1) and sequesters HMGB1 in chromatin. HMGB1 is an abundant host nuclear protein that can also be released from infected cells as an alarmin to amplify inflammatory responses. By sequestering HMGB1, protein VII prevents its release, thus inhibiting downstream inflammatory signaling. However, the consequences of this chromatin sequestration on host transcription are unknown. Here, we employ bacterial two-hybrid interaction assays and human cell culture to interrogate the mechanism of the protein VII-HMGB1 interaction. HMGB1 contains two DNA binding domains, the A- and B-boxes, that bend DNA to promote transcription factor binding while the C-terminal tail regulates this interaction. We demonstrate that protein VII interacts directly with the A-box of HMGB1, an interaction that is inhibited by the HMGB1 C-terminal tail. By cellular fractionation, we show that protein VII renders A-box containing constructs insoluble, thereby acting to prevent their release from cells. This sequestration is not dependent on HMGB1's ability to bind DNA but does require post-translational modifications on protein VII. Importantly, we demonstrate that protein VII inhibits expression of interferon β, in an HMGB1-dependent manner, but does not affect transcription of downstream interferon-stimulated genes. Together, our results demonstrate that protein VII specifically harnesses HMGB1 through its A-box domain to depress the innate immune response and promote infection.

Adenovirus protein VII binds the A-box of HMGB1 to repress interferon responses

Viruses hijack host proteins to promote infection and dampen host defenses. Adenovirus encodes the multifunctional protein VII that serves both to compact viral genomes inside the virion and disrupt host chromatin. Protein VII binds the abundant nuclear protein high mobility group box 1 (HMGB1) and sequesters HMGB1 in chromatin. HMGB1 is an abundant host nuclear protein that can also be released from infected cells as an alarmin to amplify inflammatory responses. By sequestering HMGB1, protein VII prevents its release, thus inhibiting downstream inflammatory signaling. However, the consequences of this chromatin sequestration on host transcription are unknown. Here, we employ bacterial two-hybrid interaction assays and human cell biological systems to interrogate the mechanism of the protein VII-HMGB1 interaction. HMGB1 contains two DNA binding domains, the A- and B-boxes, that bend DNA to promote transcription factor binding while the C-terminal tail regulates this interaction. We demonstrate that protein VII interacts directly with the A-box of HMGB1, an interaction that is inhibited by the HMGB1 C-terminal tail. By cellular fractionation, we show that protein VII renders A-box containing constructs insoluble, thereby acting to prevent their release from cells. This sequestration is not dependent on HMGB1's ability to bind DNA but does require post-translational modifications on protein VII. Importantly, we demonstrate that protein VII inhibits expression of interferon β, in an HMGB1- dependent manner, but does not affect transcription of downstream interferon- stimulated genes. Together, our results demonstrate that protein VII specifically harnesses HMGB1 through its A-box domain to depress the innate immune response and promote infection.

High Mobility Group Box 1 (HMGB1): Potential Target in Sepsis-Associated Encephalopathy

Sepsis-associated encephalopathy (SAE) remains a challenge for intensivists that is exacerbated by lack of an effective diagnostic tool and an unambiguous definition to properly identify SAE patients. Risk factors for SAE development include age, genetic factors as well as pre-existing neuropsychiatric conditions. Sepsis due to certain infection sites/origins might be more prone to encephalopathy development than other cases. Currently, ICU management of SAE is mainly based on non-pharmacological support. Pre-clinical studies have described the role of the alarmin high mobility group box 1 (HMGB1) in the complex pathogenesis of SAE. Although there are limited data available about the role of HMGB1 in neuroinflammation following sepsis, it has been implicated in other neurologic disorders, where its translocation from the nucleus to the extracellular space has been found to trigger neuroinflammatory reactions and disrupt the blood–brain barrier. Negating the inflammatory cascade, by targeting HMGB1, may be a strategy to complement non-pharmacologic interventions directed against encephalopathy. This review describes inflammatory cascades implicating HMGB1 and strategies for its use to mitigate sepsis-induced encephalopathy.

New Short RNA Motifs Potentially Relevant in the SARS-CoV-2 Genome

Background

The coronavirus disease has led to an exhaustive exploration of the SARS-CoV-2 genome. Despite the amount of information accumulated, the prediction of short RNA motifs encoding peptides mediating protein-protein or protein-drug interactions has received limited attention.

Objective

The study aims to predict short RNA motifs that are interspersed in the SARS-CoV-2 genome.

Methods

A method in which 14 trinucleotide families, each characterized by being composed of triplets with identical nucleotides in all possible configurations, was used to find short peptides with biological relevance. The novelty of the approach lies in using these families to search how they are distributed across genomes of different CoV genera and then to compare the distributions of these families with each other.

Results

We identified distributions of trinucleotide families in different CoV genera and also how they are related, using a selection criterion that identified short RNA motifs. The motifs were reported to be conserved in SARS-CoVs; in the remaining CoV genomes analysed, motifs contained, exclusively, different configurations of the trinucleotides A, T, G and A, C, G. Eighty-eight short RNA motifs, ranging in length from 12 to 49 nucleotides, were found: 50 motifs in the 1a polyprotein-encoding orf, 27 in the 1b polyprotein-encoding orf, 5 in the spike-encoding orf, and 6 in the nucleocapsid-encoding orf. Although some motifs (~27%) were found to be intercalated or attached to functional peptides, most of them have not yet been associated with any known functions.

Conclusion

Some of the trinucleotide family distributions in different CoV genera are not random; they are present in short peptides that, in many cases, are intercalated or attached to functional sites of the proteome.

Structural Characteristics of High-Mobility Group Proteins HMGB1 and HMGB2 and Their Interaction with DNA

Non-histone nuclear proteins HMGB1 and HMGB2 (High Mobility Group) are involved in many biological processes, such as replication, transcription, and repair. The HMGB1 and HMGB2 proteins consist of a short N-terminal region, two DNA-binding domains, A and B, and a C-terminal sequence of glutamic and aspartic acids. In this work, the structural organization of calf thymus HMGB1 and HMGB2 proteins and their complexes with DNA were studied using UV circular dichroism (CD) spectroscopy. Post-translational modifications (PTM) of HMGB1 and HMGB2 proteins were determined with MALDI mass spectrometry. We have shown that despite the similar primary structures of the HMGB1 and HMGB2 proteins, their post-translational modifications (PTMs) demonstrate quite different patterns. The HMGB1 PTMs are located predominantly in the DNA-binding A-domain and linker region connecting the A and B domains. On the contrary, HMGB2 PTMs are found mostly in the B-domain and within the linker region. It was also shown that, despite the high degree of homology between HMGB1 and HMGB2, the secondary structure of these proteins is also slightly different. We believe that the revealed structural properties might determine the difference in the functioning of the HMGB1 and HMGB2 as well as their protein partners.

Dorsal switch protein 1 as a damage signal in insect gut immunity to activate dual oxidase via an eicosanoid, PGE2

Various microbiota including beneficial symbionts reside in the insect gut. Infections of pathogens cause dysregulation of the microflora and threaten insect survival. Reactive oxygen species (ROS) have been used in the gut immune responses, in which its production is tightly regulated by controlling dual oxidase (Duox) activity via Ca2+ signal to protect beneficial microflora and gut epithelium due to its high cytotoxicity. However, it was not clear how the insects discriminate the pathogens from the various microbes in the gut lumen to trigger ROS production. An entomopathogenic nematode (Steinernema feltiae) infection elevated ROS level in the gut lumen of a lepidopteran insect, Spodoptera exigua. Dorsal switch protein 1 (DSP1) localized in the nucleus in the midgut epithelium was released into plasma upon the nematode infection and activated phospholipase A2 (PLA2). The activated PLA2 led to an increase of PGE2 level in the midgut epithelium, in which rising Ca2+ signal up-regulated ROS production. Inhibiting DSP1 release by its specific RNA interference (RNAi) or specific inhibitor, 3-ethoxy-4-methoxyphenol, treatment failed to increase the intracellular Ca2+ level and subsequently prevented ROS production upon the nematode infection. A specific PLA2 inhibitor treatment also prevented the up-regulation of Ca2+ and subsequent ROS production upon the nematode infection. However, the addition of PGE2 to the inhibitor treatment rescued the gut immunity. DSP1 release was not observed at infection with non-pathogenic pathogens but detected in plasma with pathogenic infections that would lead to damage to the gut epithelium. These results indicate that DSP1 acts as a damage-associated molecular pattern in gut immunity through DSP1/PLA2/Ca2+/Duox.

The C-Terminal Acidic Tail Modulates the Anticancer Properties of HMGB1

Energy metabolism reprogramming was recently listed as a hallmark of cancer. In this process, the switch from pyruvate kinase isoenzyme type M1 to pyruvate kinase isoenzyme type M2 (PKM2) is believed to play a crucial role. Interestingly, the activity of the active form of PKM2 can efficiently be inhibited by the high-mobility group box 1 (HMGB1) protein, leading to a rapid blockage of glucose-dependent aerobic respiration and cancer cell death. HMGB1 is a member of the HMG protein family. It contains two DNA-binding HMG-box domains and an acidic C-terminal tail capable of positively or negatively modulating its biological properties. In this work, we report that the deletion of the C-terminal tail of HMGB1 increases its activity towards a large panel of cancer cells without affecting the viability of normal immortalized fibroblasts. Moreover, in silico analysis suggests that the truncated form of HMGB1 retains the capacity of the full-length protein to interact with PKM2. However, based on the capacity of the cells to circumvent oxidative phosphorylation inhibition, we were able to identify either a cytotoxic or cytostatic effect of the proteins. Together, our study provides new insights in the characterization of the anticancer activity of HMGB1.

O-GlcNAcylation of High Mobility Group Box 1 (HMGB1) Alters Its DNA Binding and DNA Damage Processing Activities

Protein O-GlcNAcylation is an essential and dynamic regulator of myriad cellular processes, including DNA replication and repair. Proteomic studies have identified the multifunctional nuclear protein HMGB1 as O-GlcNAcylated, providing a potential link between this modification and DNA damage responses. Here, we verify the protein's endogenous modification at S100 and S107 and found that the major modification site is S100, a residue that can potentially influence HMGB1-DNA interactions. Using synthetic protein chemistry, we generated site-specifically O-GlcNAc-modified HMGB1 at S100 and characterized biochemically the effect of the sugar modification on its DNA binding activity. We found that O-GlcNAc alters HMGB1 binding to linear, nucleosomal, supercoiled, cruciform, and interstrand cross-linked damaged DNA, generally resulting in enhanced oligomerization on these DNA structures. Using cell-free extracts, we also found that O-GlcNAc reduces the ability of HMGB1 to facilitate DNA repair, resulting in error-prone processing of damaged DNA. Our results expand our understanding of the molecular consequences of O-GlcNAc and how it affects protein-DNA interfaces. Importantly, our work may also support a link between upregulated O-GlcNAc levels and increased rates of mutations in certain cancer states.

Interactions of HMGB Proteins with the Genome and the Impact on Disease

High Mobility Group Box (HMGB) proteins are small architectural DNA binding proteins that regulate multiple genomic processes such as DNA damage repair, nucleosome sliding, telomere homeostasis, and transcription. In doing so they control both normal cellular functions and impact a myriad of disease states, including cancers and autoimmune diseases. HMGB proteins bind to DNA and nucleosomes to modulate the local chromatin environment, which facilitates the binding of regulatory protein factors to the genome and modulates higher order chromosomal organization. Numerous studies over the years have characterized the structure and function of interactions between HMGB proteins and DNA, both biochemically and inside cells, providing valuable mechanistic insight as well as evidence these interactions influence pathological processes. This review highlights recent studies supporting the roles of HMGB1 and HMGB2 in global organization of the genome, as well as roles in transcriptional regulation and telomere maintenance via interactions with G-quadruplex structures. Moreover, emerging models for how HMGB proteins function as RNA binding proteins are presented. Nuclear HMGB proteins have broad regulatory potential to impact numerous aspects of cellular metabolism in normal and disease states.

High-Mobility Group Box-1 and Its Potential Role in Perioperative Neurocognitive Disorders

Aseptic surgical trauma provokes the release of HMGB1, which engages the innate immune response after binding to pattern-recognition receptors on circulating bone marrow-derived monocytes (BM-DM). The initial systemic inflammation, together with HMGB1, disrupts the blood-brain barrier allowing penetration of CCR2-expressing BM-DMs into the hippocampus, attracted by the chemokine MCP-1 that is upregulated by HMGB1. Within the brain parenchyma quiescent microglia are activated and, together with the translocated BM-DMs, release proinflammatory cytokines that disrupt synaptic plasticity and hence memory formation and retention, resulting in postoperative cognitive decline (PCD). Neutralizing antibodies to HMGB1 prevents the inflammatory response to trauma and PCD.

Targeting Inflammation Driven by HMGB1 in Bacterial Keratitis-A Review

Pseudomonas (P.) aeruginosa is a Gram-negative bacteria that causes human infectionsinfections. It can cause keratitis, a severe eye infection, that develops quickly and is a major cause of ulceration of the cornea and ocular complications globally. Contact lens wear is the greatest causative reason in developed countries, but in other countries, trauma and predominates. Use of non-human models of the disease are critical and may provide promising alternative argets for therapy to bolster a lack of new antibiotics and increasing antibiotic resistance. In this regard, we have shown promising data after inhibiting high mobility group box 1 (HMGB1), using small interfering RNA (siRNA). Success has also been obtained after other means to inhinit HMGB1 and include: use of HMGB1 Box A (one of three HMGB1 domains), anti-HMGB1 antibody blockage of HMGB1 and/or its receptors, Toll like receptor (TLR) 4, treatment with thrombomodulin (TM) or vasoactive intestinal peptide (VIP) and glycyrrhizin (GLY, a triterpenoid saponin) that directly binds to HMGB1. ReducingHMGB1 levels in P. aeruginosa keratitis appears a viable treatment alternative.

Protein intrinsic disorder on a dynamic nucleosomal landscape

The complex nucleoprotein landscape of the eukaryotic cell nucleus is rich in dynamic proteins that lack a stable three-dimensional structure. Many of these intrinsically disordered proteins operate directly on the first fundamental level of genome compaction: the nucleosome. Here we give an overview of how disordered interactions with and within nucleosomes shape the dynamics, architecture, and epigenetic regulation of the genetic material, controlling cellular transcription patterns. We highlight experimental and computational challenges in the study of protein disorder and illustrate how integrative approaches are increasingly unveiling the fine details of nuclear interaction networks. We finally dissect sequence properties encoded in disordered regions and assess common features of disordered nucleosome-binding proteins. As drivers of many critical biological processes, disordered proteins are integral to a comprehensive molecular view of the dynamic nuclear milieu.

Dynamic Autoinhibition of the HMGB1 Protein via Electrostatic Fuzzy Interactions of Intrinsically Disordered Regions

Highly negatively charged segments containing only aspartate or glutamate residues ("D/E repeats") are found in many eukaryotic proteins. For example, the C-terminal 30 residues of the HMGB1 protein are entirely D/E repeats. Using nuclear magnetic resonance (NMR), fluorescence, and computational approaches, we investigated how the D/E repeats causes the autoinhibition of HMGB1 against its specific binding to cisplatin-modified DNA. By varying ionic strength in a wide range (40-900 mM), we were able to shift the conformational equilibrium between the autoinhibited and uninhibited states toward either of them to the full extent. This allowed us to determine the macroscopic and microscopic equilibrium constants for the HMGB1 autoinhibition at various ionic strengths. At a macroscopic level, a model involving the autoinhibited and uninhibited states can explain the salt concentration-dependent binding affinity data. Our data at a microscopic level show that the D/E repeats and other parts of HMGB1 undergo electrostatic fuzzy interactions, each of which is weaker than expected from the macroscopic autoinhibitory effect. This discrepancy suggests that the multivalent nature of the fuzzy interactions enables strong autoinhibition at a macroscopic level despite the relatively weak intramolecular interaction at each site. Both experimental and computational data suggest that the D/E repeats interact preferentially with other intrinsically disordered regions (IDRs) of HMGB1. We also found that mutations mimicking post-translational modifications relevant to nuclear export of HMGB1 can moderately modulate DNA-binding affinity, possibly by impacting the autoinhibition. This study illuminates a functional role of the fuzzy interactions of D/E repeats.

Cleavage of HMGB1 by Proteolytic Enzymes Associated with Inflammatory Conditions

Extracellular HMGB1 acts as an alarmin in multiple autoimmune diseases. While its release and functions have been extensively studied, there is a substantial lack of knowledge regarding HMGB1 regulation at the site of inflammation. Herein we show that enzymes present in arthritis-affected joints process HMGB1 into smaller peptides in vitro. Gel electrophoresis, Western blotting and mass spectrometry analyses indicate cleavage sites for human neutrophil elastase, cathepsin G, and matrix metalloproteinase 3 within the HMGB1 structure. While human neutrophil elastase and matrix metalloproteinase 3 might alter the affinity of HMGB1 to its receptors by cleaving the acidic C-terminal tail, cathepsin G rapidly and completely degraded the alarmin. Contrary to a previous report we demonstrate that HMGB1 is not a substrate for dipeptidyl peptidase IV. We also provide novel information regarding the presence of these proteases in synovial fluid of juvenile idiopathic arthritis patients. Correlation analysis of protease levels and HMGB1 levels in synovial fluid samples did not, however, reveal any direct relationship between the recorded levels. This study provides knowledge of proteolytic processing of HMGB1 relevant for the regulation of HMGB1 during inflammatory disease.

Discovery of 5,5'-Methylenedi-2,3-Cresotic Acid as a Potent Inhibitor of the Chemotactic Activity of the HMGB1·CXCL12 Heterocomplex Using Virtual Screening and NMR Validation

HMGB1 is a key molecule that both triggers and sustains inflammation following infection or injury, and is involved in a large number of pathologies, including cancer. HMGB1 participates in the recruitment of inflammatory cells, forming a heterocomplex with the chemokine CXCL12 (HMGB1·CXCL12), thereby activating the G-protein coupled receptor CXCR4. Thus, identification of molecules that disrupt this heterocomplex can offer novel pharmacological opportunities to treat inflammation-related diseases. To identify new HMGB1·CXCL12 inhibitors we have performed a study on the ligandability of the single HMG boxes of HMGB1 followed by a virtual screening campaign on both HMG boxes using Zbc Drugs and three different docking programs (Glide, AutoDock Vina, and AutoDock 4.2.6). The best poses in terms of scoring functions, visual inspection, and predicted ADME properties were further filtered according to a pharmacophore model based on known HMGB1 binders and clustered according to their structures. Eight compounds representative of the clusters were tested for HMGB1 binding by NMR. We identified 5,5'-methylenedi-2,3-cresotic acid (2a) as a binder of both HMGB1 and CXCL12; 2a also targets the HMGB1·CXCL12 heterocomplex. In cell migration assays 2a inhibited the chemotactic activity of HMGB1·CXCL12 with IC50 in the subnanomolar range, the best documented up to now. These results pave the way for future structure activity relationship studies to optimize the pharmacological targeting of HMGB1·CXCL12 for anti-inflammatory purposes.

Functional Diversity of Non-Histone Chromosomal Protein HmgB1

The functioning of DNA in the cell nucleus is ensured by a multitude of proteins, whose interactions with DNA as well as with other proteins lead to the formation of a complicated, organized, and quite dynamic system known as chromatin. This review is devoted to the description of properties and structure of the progenitors of the most abundant non-histone protein of the HMGB family-the HmgB1 protein. The proteins of the HMGB family are also known as "architectural factors" of chromatin, which play an important role in gene expression, transcription, DNA replication, and repair. However, as soon as HmgB1 goes outside the nucleus, it acquires completely different functions, post-translational modifications, and change of its redox state. Despite a lot of evidence of the functional activity of HmgB1, there are still many issues to be solved related to the mechanisms of the influence of HmgB1 on the development and treatment of different diseases-from oncological and cardiovascular diseases to pathologies during pregnancy and childbirth. Here, we describe molecular structure of the HmgB1 protein and discuss general mechanisms of its interactions with other proteins and DNA in cell.

An overview of high-mobility group box 1, a potent pro-inflammatory cytokine in asthma

High-mobility group box 1 (HMGB1), also called amphoterin, HMG1 and p30, is a highly conserved protein between different species that has various functions in nucleus such as stabilization of nucleosome formation, facilitation of deoxyribonucleic acid (DNA) bending and increasing the DNA transcription, replication and repair. It has also been indicated that HMGB1 acts as a potent pro-inflammatory cytokine with increasing concentrations in acute and chronic inflammatory diseases. Asthma is a common chronic respiratory disease associated with high morbidity and mortality rates. One central characteristic in its pathogenesis is airway inflammation. Considering the inflammatory role of HMGB1 and importance of inflammation in asthma pathogenesis, a better understanding of this protein is vital. This review describes the structure, cell surface receptors, signaling pathways and intracellular and extracellular functions of HMGB1, but also focuses on its inflammatory role in asthma. Moreover, this manuscript reviews experimental and clinical studies that investigated the pathologic role of HMGB1.

The acidic tail of HMGB1 regulates its secondary structure and conformational flexibility: A circular dichroism and molecular dynamics simulation study

High mobility group box 1 (HMGB1) is a damage-associated molecular pattern (DAMP) molecule that triggers the progression of several pro-inflammatory diseases such as diabetes, Alzheimer's disease and cancer, by inducing signals upon interaction with the receptors such as the receptor for advanced glycation end-products (RAGE) and toll-like receptors (TLRs). The acidic C-terminal tail of HMGB1 is an intrinsically disordered region of the protein which is known to determine the interaction of HMGB1 to DNA and histones. This study characterizes its structural properties using a combination of circular dichroism (CD) and molecular dynamics (MD) simulations. The full-length and tail-less forms of HMGB1 were compared to rationalise the role of the acidic tail in maintaining the stability of the entire structure of HMGB1 in atomistic detail. Consistent with experimental data, the acidic tail was predicted to adopt an extended conformation that allows it to make a range of hydrogen-bonding and electrostatic interactions with the box-like domains that stabilize the overall structure of HMGB1. Absence of the acidic tail was predicted to increase structural fluctuations of all amino acids, leading to changes in secondary structure from α-helical to more hydrophilic turns along with increased exposure of multiple amino acids to the surrounding solvent. These structural changes reveal the intrinsic conformational dynamics of HMGB1 that are likely to affect the accessibility of its receptors.

High-mobility group box 1 links sensing of reactive oxygen species by huntingtin to its nuclear entry

Huntington's disease (HD) is a neurodegenerative, age-onset disorder caused by a CAG DNA expansion in exon 1 of the HTT gene, resulting in a polyglutamine expansion in the huntingtin protein. Nuclear accumulation of mutant huntingtin is a hallmark of HD, resulting in elevated mutant huntingtin levels in cell nuclei. Huntingtin is normally retained at the endoplasmic reticulum via its N17 amphipathic α-helix domain but is released by oxidation of Met-8 during reactive oxygen species (ROS) stress. Huntingtin enters the nucleus via an importin β1- and 2-dependent proline-tyrosine nuclear localization signal (PY-NLS), which has a unique intervening sequence in huntingtin. Here, we have identified the high-mobility group box 1 (HMGB1) protein as an interactor of the intervening sequence within the PY-NLS. Nuclear levels of HMGB1 positively correlated with varying levels of nuclear huntingtin in both HD and normal human fibroblasts. We also found that HMGB1 interacts with the huntingtin N17 region and that this interaction is enhanced by the presence of ROS and phosphorylation of critical serine residues in the N17 region. We conclude that HMGB1 is a huntingtin N17/PY-NLS ROS-dependent interactor, and this protein bridging is essential for relaying ROS sensing by huntingtin to its nuclear entry during ROS stress. ROS may therefore be a critical age-onset stress that triggers nuclear accumulation of mutant huntington in Huntington's disease.

Redox-Mediated Mechanisms Fuel Monocyte Responses to CXCL12/HMGB1 in Active Rheumatoid Arthritis

Chemokine synergy-inducing molecules are emerging as regulating factors in cell migration. The alarmin HMGB1, in its reduced form, can complex with CXCL12 enhancing its activity on monocytes via the chemokine receptor CXCR4, while the form containing a disulfide bond, by binding to TLR2 or TLR4, initiates a cascade of events leading to production of cytokines and chemokines. So far, the possibility that the CXCL12/HMGB1 heterocomplex could be maintained in chronic inflammation was debated, due to the release of reactive oxygen species. Therefore, we have assessed if the heterocomplex could remain active in Rheumatoid Arthritis (RA) and its relevance in the disease assessment. Monocytes from RA patients with active disease require a low concentration of HMGB1 to enhance CXCL12-induced migration, in comparison to monocytes from patients in clinical remission or healthy donors. The activity of the heterocomplex depends on disease activity, on the COX2 and JAK/STAT pathways, and is determined by the redox potential of the microenvironment. In RA, the presence of an active thioredoxin system correlates with the enhanced cell migration, and with the presence of the heterocomplex in the synovial fluid. The present study highlights how, in an unbalanced microenvironment, the activity of the thioredoxin system plays a crucial role in sustaining inflammation. Prostaglandin E2 stimulation of monocytes from healthy donors is sufficient to recapitulate the response observed in patients with active RA. The activation of mechanisms counteracting the oxidative stress in the extracellular compartment preserves HMGB1 in its reduced form, and contributes to fuel the influx of inflammatory cells. Targeting the heterocomplex formation and its activity could thus be an additional tool for dampening the inflammation sustained by cell recruitment, for those patients with chronic inflammatory conditions who poorly respond to current therapies.

Ligation of free HMGB1 to TLR2 in the absence of ligand is negatively regulated by the C-terminal tail domain

Background

High mobility group box 1 (HMGB1) protein is a central endogenous inflammatory mediator contributing to the pathogenesis of several inflammatory disorders. HMGB1 interacts with toll-like receptors (TLRs) but contradictory evidence regarding its identity as a TLR2 ligand persists. The aim of this study was to investigate if highly purified HMGB1 interacts with TLR2 and if so, to determine the functional outcome.

Methods

Full length or C-terminal truncated (Δ30) HMGB1 was purified from E.coli. Binding to TLR2-Fc was investigated by direct-ELISA. For the functional studies, proteins alone or in complex with peptidoglycan (PGN) were added to human embryonic kidney (HEK) cells transfected with functional TLR2, TLR 1/2 or TLR 2/6 dimers, macrophages, whole blood or peripheral blood mononuclear cells (PBMCs). Cytokine levels were determined by ELISA.

Results

In vitro binding experiments revealed that Δ30 HMGB1, lacking the acidic tail domain, but not full length HMGB1 binds dose dependently to TLR2. Control experiments confirmed that the interaction was specific to TLR2 and could be inhibited by enzymatic digestion. Δ30 HMGB1 alone was unable to induce cytokine production via TLR2. However, full length HMGB1 and Δ30 HMGB1 formed complexes with PGN, a known TLR2 ligand, and synergistically potentiated the inflammatory response in PBMCs.

Conclusions

We have demonstrated that TLR2 is a receptor for HMGB1 and this binding is negatively regulated by the C-terminal tail. HMGB1 did not induce functional activation of TLR2 while both full length HMGB1 and Δ30 HMGB1 potentiated the inflammatory activities of the TLR2 ligand PGN. We hypothesize that Δ30 HMGB1 generated in vivo by enzymatic cleavage could act as an enhancer of TLR2-mediated inflammatory activities.

Electronic supplementary material

The online version of this article (10.1186/s10020-018-0021-x) contains supplementary material, which is available to authorized users.

Molecular signaling involving intrinsically disordered proteins in prostate cancer

Investigations on cellular protein interaction networks (PINs) reveal that proteins that constitute hubs in a PIN are notably enriched in Intrinsically Disordered Proteins (IDPs) compared to proteins that constitute edges, highlighting the role of IDPs in signaling pathways. Most IDPs rapidly undergo disorder-to-order transitions upon binding to their biological targets to perform their function. Conformational dynamics enables IDPs to be versatile and to interact with a broad range of interactors under normal physiological conditions where their expression is tightly modulated. IDPs are involved in many cellular processes such as cellular signaling, transcriptional regulation, and splicing; thus, their high-specificity/low-affinity interactions play crucial roles in many human diseases including cancer. Prostate cancer (PCa) is one of the leading causes of cancer-related mortality in men worldwide. Therefore, identifying molecular mechanisms of the oncogenic signaling pathways that are involved in prostate carcinogenesis is crucial. In this review, we focus on the aspects of cellular pathways leading to PCa in which IDPs exert a primary role.

Yeast HMO1: Linker Histone Reinvented

Eukaryotic genomes are packaged in chromatin. The higher-order organization of nucleosome core particles is controlled by the association of the intervening linker DNA with either the linker histone H1 or high mobility group box (HMGB) proteins. While H1 is thought to stabilize the nucleosome by preventing DNA unwrapping, the DNA bending imposed by HMGB may propagate to the nucleosome to destabilize chromatin. For metazoan H1, chromatin compaction requires its lysine-rich C-terminal domain, a domain that is buried between globular domains in the previously characterized yeast Saccharomyces cerevisiae linker histone Hho1p. Here, we discuss the functions of S. cerevisiae HMO1, an HMGB family protein unique in containing a terminal lysine-rich domain and in stabilizing genomic DNA. On ribosomal DNA (rDNA) and genes encoding ribosomal proteins, HMO1 appears to exert its role primarily by stabilizing nucleosome-free regions or "fragile" nucleosomes. During replication, HMO1 likewise appears to ensure low nucleosome density at DNA junctions associated with the DNA damage response or the need for topoisomerases to resolve catenanes. Notably, HMO1 shares with the mammalian linker histone H1 the ability to stabilize chromatin, as evidenced by the absence of HMO1 creating a more dynamic chromatin environment that is more sensitive to nuclease digestion and in which chromatin-remodeling events associated with DNA double-strand break repair occur faster; such chromatin stabilization requires the lysine-rich extension of HMO1. Thus, HMO1 appears to have evolved a unique linker histone-like function involving the ability to stabilize both conventional nucleosome arrays as well as DNA regions characterized by low nucleosome density or the presence of noncanonical nucleosomes.

Non-histone protein HMGB1 inhibits the repair of damaged DNA by cisplatin in NIH-3T3 murine fibroblasts

The nuclear non-histone protein high mobility group box (HMGB) 1 is known to having an inhibitory effect on the repair of DNA damaged by the antitumor drug cisplatin in vitro. To investigate the role of HMGB1 in living cells, we studied the DNA repair of cisplatin damages in mouse fibroblast cell line, NIH-3T3. We evaluated the effect of the post-synthetic acetylation and C-terminal domain of the protein by overexpression of the parental and mutant GFP fused forms of HMGB1. The results revealed that HMGB1 had also an inhibitory effect on the repair of cisplatin damaged DNA in vivo. The silencing of HMGB1 in NIH-3T3 cells increased the cellular DNA repair potential. The increased levels of repair synthesis could be "rescued" and returned to less than normal levels if the knockdown cells were transfected with plasmids encoding HMGB1 and HMGB1 K2A. In this case, the truncated form of HMGB1 also exhibited a slight inhibitory effect. [BMB Reports 2016; 49(2): 99-104].

The HMGB1 C-Terminal Tail Regulates DNA Bending

High mobility group box protein 1 (HMGB1) is an architectural protein that facilitates the formation of protein-DNA assemblies involved in transcription, recombination, DNA repair, and chromatin remodeling. Important to its function is the ability of HMGB1 to bend DNA non-sequence specifically. HMGB1 contains two HMG boxes that bind and bend DNA (the A box and the B box) and a C-terminal acidic tail. We investigated how these domains contribute to DNA bending by HMGB1 using single-molecule fluorescence resonance energy transfer (FRET), which enabled us to resolve heterogeneous populations of bent and unbent DNA. We found that full-length (FL) HMGB1 bent DNA more than the individual A and B boxes. Removing the C-terminal tail resulted in a protein that bent DNA to a greater extent than the FL protein. These data suggest that the A and B boxes simultaneously bind DNA in the absence of the C-terminal tail, but the tail modulates DNA binding and bending by one of the HMG boxes in the FL protein. Indeed, a construct composed of the B box and the C-terminal tail only bent DNA at higher protein concentrations. Moreover, in the context of the FL protein, mutating the A box such that it could not bend DNA resulted in a protein that bent DNA similar to a single HMG box and only at higher protein concentrations. We propose a model in which the HMGB1 C-terminal tail serves as an intramolecular damper that modulates the interaction of the B box with DNA.

High Mobility Group B Proteins, Their Partners, and Other Redox Sensors in Ovarian and Prostate Cancer

Cancer cells try to avoid the overproduction of reactive oxygen species by metabolic rearrangements. These cells also develop specific strategies to increase ROS resistance and to express the enzymatic activities necessary for ROS detoxification. Oxidative stress produces DNA damage and also induces responses, which could help the cell to restore the initial equilibrium. But if this is not possible, oxidative stress finally activates signals that will lead to cell death. High mobility group B (HMGB) proteins have been previously related to the onset and progressions of cancers of different origins. The protein HMGB1 behaves as a redox sensor and its structural changes, which are conditioned by the oxidative environment, are associated with different functions of the protein. This review describes recent advances in the role of human HMGB proteins and other proteins interacting with them, in cancerous processes related to oxidative stress, with special reference to ovarian and prostate cancer. Their participation in the molecular mechanisms of resistance to cisplatin, a drug commonly used in chemotherapy, is also revised.

Structure-specific nucleic acid recognition by L-motifs and their diverse roles in expression and regulation of the genome

The high-mobility group (HMG) domain containing proteins regulate transcription, DNA replication and recombination. They adopt L-shaped folds and are structure-specific DNA binding motifs. Here, I define the L-motif super-family that consists of DNA-binding HMG-box proteins and the L-motif of the histone mRNA binding domain of stem-loop binding protein (SLBP). The SLBP L-motif and HMG-box domains adopt similar L-shaped folds with three α-helices and two or three small hydrophobic cores that stabilize the overall fold, but have very different and distinct modes of nucleic acid recognition. A comparison of the structure, dynamics, protein-protein and nucleic acid interactions, and regulation by PTMs of the SLBP and the HMG-box L-motifs reveals the versatile and diverse modes by which L-motifs utilize their surfaces for structure-specific recognition of nucleic acids to regulate gene expression.

HMGB1 bound to cisplatin-DNA adducts undergoes extensive acetylation and phosphorylation in vivo

Cisplatin, one of the most effective anticancer drugs, is a DNA-damaging agent that induces cell death primarily by apoptosis. For many years, HMGB1 has been known to be a recognition protein for cisplatin-DNA lesions. Here, an application of a biomolecular probe based on a peptide-oligonucleotide conjugate is presented as a novel method for investigating this recognition process in vivo. Proteins known to be involved in the recognition of cisplatin-damaged DNA were pulled down and identified, including members of the HMGB family and a number of other proteins. Interestingly, at least 4 subforms of HMGB1 bind to cisplatin-DNA adducts. These proteins were further identified as post-translationally acetylated or phosphorylated forms of HMGB1. These results provide a rich pool of protein candidates whose roles in the mechanism of action of platinum drugs should be explored. These newly discovered molecular components of the DNA damage signalling cascade could serve as novel links between the initial cell responses to DNA damage and the downstream apoptotic or DNA repair pathways.

High mobility group box 1 protein is methylated and transported to cytoplasm in clear cell renal cell carcinoma

BACKGROUND

The high mobility group box 1 (HMGB1) protein is a widespread nuclear protein present in most cell types. It typically locates in the nucleus and functions as a nuclear cofactor in transcription regulation. However, HMGB1 can also localize in the cytoplasm and be released into extracellular matrix, where it plays critical roles in carcinogenesis and inflammation. However, it remains elusive whether HMGB1 is relocated to cytoplasm in clear cell renal cell carcinoma (ccRCC).

METHODS

Nuclear and cytoplasmic proteins were extracted by different protocols from 20 ccRCC samples and corresponding adjacent renal tissues. Western blotting and immunohistochemistry were used to identify the expression of HMGB1 in ccRCC. To elucidate the potential mechanism of HMGB1 cytoplasmic translocation, HMGB1 proteins were enriched by immunoprecipitation and analyzed by mass spectrometry (MS).

RESULTS

The HMGB1 protein was overexpressed and partially localized in cytoplasm in ccRCC samples (12/20, 60%, p<0.05). Immunohistochemistry results indicated that ccRCC of high nuclear grade possess more HMGB1 relocation than those with low grade (p<0.05). Methylation of HMGB1 at lysine 112 in ccRCC was detected by MS. Bioinformatics analysis showed that post-translational modification might affect the binding ability to DNA and mediate its translocation.

CONCLUSION

Relocation of HMGB1 to cytoplasm was confirmed in ccRCC. Methylation of HMGB1 at lysine 112 might the redistribution of this cofactor protein.

A nuclear factor of high mobility group box protein in Toxoplasma gondii

High mobility group box 1 (HMGB1) is a nuclear factor that usually binds DNA and modulates gene expression in multicellular organisms. Three HMGB1 orthologs were predicted in the genome of Toxoplasma gondii, an obligate intracellular protozoan pathogen, termed TgHMGB1a, b and c. Phylogenetic and bioinformatic analyses indicated that these proteins all contain a single HMG box and which shared in three genotypes. We cloned TgHMGB1a, a 33.9 kDa protein that can stimulates macrophages to release TNF-α, and, we demonstrated that the TgHMGB1a binds distorted DNA structures such as cruciform DNA in electrophoretic mobility shift assays (EMSA). Immunofluorescence assay indicated TgHMGB1a concentrated in the nucleus of intracellular tachyzoites but translocated into the cytoplasm while the parasites release to extracellular. There were no significant phenotypic changes when the TgHMGB1a B box was deleted, while transgenic parasites that overexpressed TgHMGB1a showed slower intracellular growth and caused delayed death in mouse, further quantitative RT-PCR analyses showed that the expression levels of many important genes, including virulence factors, increased when TgHMGB1a was overexpressed, but no significant changes were observed in TgHMGB1a B box-deficient parasites. Our findings demonstrated that TgHMGB1a is indeed a nuclear protein that maintains HMG box architectural functions and is a potential proinflammatory factor during the T.gondii infection. Further studies that clarify the functions of TgHMGB1s will increase our knowledge of transcriptional regulation and parasite virulence, and might provide new insight into host-parasite interactions for T. gondii infection.

The role of high mobility group box 1 in innate immunity

With growing accounts of inflammatory diseases such as sepsis, greater understanding the immune system and the mechanisms of cellular immunity have become primary objectives in immunology studies. High mobility group box 1 (HMGB1) is a ubiquitous nuclear protein that is implicated in various aspects of the innate immune system as a damage-associated molecular pattern molecule and a late mediator of inflammation, as well as in principal cellular processes, such as autophagy and apoptosis. HMGB1 functions in the nucleus as a DNA chaperone; however, it exhibits cytokine-like activity when secreted by injurious or infectious stimuli. Extracellular HMGB1 acts through specific receptors to promote activation of the NF-κB signaling pathway, leading to production of cytokines and chemokines. These findings further implicate HMGB1 in lethal inflammatory diseases as a crucial regulator of inflammatory, injurious, and infectious responses. In this paper, we summarize the role of HMGB1 in inflammatory and non-inflammatory states and assess potential therapeutic approaches targeting HMGB1 in inflammatory diseases.

Antibacterial and antiviral properties of tongue sole (Cynoglossus semilaevis) high mobility group B2 protein are largely independent on the acidic C-terminal domain

High mobility group box (HMGB) proteins are known to be involved in diverse functions in mammalian cells. In teleost, very limited studies on HMGB proteins have been documented. In this study, we reported identification of a HMGB homologue (named CsHMGB2) from tongue sole (Cynoglossus semilaevis) and examined its biological property. CsHMGB2 is 245 residues in length and contains two basic HMG boxes and an acidic C-terminal tail composed of 23 Asp/Glu residues. Quantitative real time RT-PCR (qRT-PCR) analysis showed that CsHMGB2 expression occurred in multiple tissues and was upregulated by bacterial and viral infection in a time-dependent manner. In vitro studies showed that when tongue sole peripheral blood leukocytes were treated with recombinant CsHMGB2 (rCsHMGB2) and the mutant rCsHMGB2M, which bears a deletion of the C-terminal acidic region, significant and comparable increases in cellular resistance against bacterial infection were observed. qRT-PCR detected enhanced expression of proinflammatory cytokines and chemokines in rCsHMGB2-treated cells. In vivo studies showed that when tongues sole were administered with rCsHMGB2 or rCsHMGB2M before being subjected to bacterial and viral infection, the pathogen loads in the spleen and kidney of the fish were significantly reduced. Taken together, these results suggest that CsHMGB2 possesses immunoregulatory properties that promote resistance against bacterial and viral infection in a manner that is largely independent on the highly conserved C-terminal acidic domain.

Role of the acidic tail of high mobility group protein B1 (HMGB1) in protein stability and DNA bending

High mobility group box (HMGB) proteins are abundant nonhistone proteins found in all eukaryotic nuclei and are capable of binding/bending DNA. The human HMGB1 is composed of two binding motifs, known as Boxes A and B, are L-shaped alpha-helix structures, followed by a random-coil acidic tail that consists of 30 Asp and Glu residues. This work aimed at evaluating the role of the acidic tail of human HMGB1 in protein stability and DNA interactions. For this purpose, we cloned, expressed and purified HMGB1 and its tailless form, HMGB1ΔC, in E. coli strain. Tryptophan fluorescence spectroscopy and circular dichroism (CD) experiments clearly showed an increase in protein stability promoted by the acidic tail under different conditions, such as the presence of the chemical denaturant guanidine hydrochloride (Gdn.HCl), high temperature and low pH. Folding intermediates found at low pH for both proteins were denatured only in the presence of chemical denaturant, thus showing a relatively high stability. The acidic tail did not alter the DNA-binding properties of the protein, although it enhanced the DNA bending capability from 76° (HMGB1ΔC) to 91° (HMGB1), as measured using the fluorescence resonance energy transfer technique. A model of DNA bending in vivo was proposed, which might help to explain the interaction of HMGB1 with DNA and other proteins, i.e., histones, and the role of that protein in chromatin remodeling.

Characterization of the interaction between HMGB1 and H3-a possible means of positioning HMGB1 in chromatin

High mobility group protein B1 (HMGB1) binds to the internucleosomal linker DNA in chromatin and abuts the nucleosome. Bending and untwisting of the linker DNA results in transmission of strain to the nucleosome core, disrupting histone/DNA contacts. An interaction between H3 and HMGB1 has been reported. Here we confirm and characterize the interaction of HMGB1 with H3, which lies close to the DNA entry/exit points around the nucleosome dyad, and may be responsible for positioning of HMGB1 on the linker DNA. We show that the interaction is between the N-terminal unstructured tail of H3 and the C-terminal unstructured acidic tail of HMGB1, which are presumably displaced from DNA and the HMG boxes, respectively, in the HMGB1-nucleosome complex. We have characterized the interaction by nuclear magnetic resonance spectroscopy and show that it is extensive for both peptides, and appears not to result in the acquisition of significant secondary structure by either partner.

Characterizations of two grass carp Ctenopharyngodon idella HMGB2 genes and potential roles in innate immunity

High-mobility group box 2 (HMGB2) protein is a chromatin-associated nonhistone protein, involved in transcriptional regulation and nucleic-acid-mediated innate immune responses in mammalian. However, the function of piscine HMGB2 in innate immune responses is still unknown. In the present study, two HMGB2 homologue genes (CiHMGB2a, CiHMGB2b) were identified and characterized in grass carp (Ctenopharyngodon idella). Both CiHMGB2a and CiHMGB2b genes encode proteins with 213 amino acids, sharing 71.4% identities and containing two basic HMG boxes and an acidic tail. The deduced protein sequences showed the most identities to HMGB2a (93%) and HMGB2b (86.4%) of zebrafish (Danio rerio), respectively. Quantitative real-time RT-PCR (qRT-PCR) analysis showed that CiHMGB2a and CiHMGB2b were constitutively expressed in all the 15 tested tissues. Post grass carp reovirus (GCRV) infection, mRNA levels of CiHMGB2a and CiHMGB2b were strongly up-regulated in spleen and head kidney and mildly modulated in C. idella kidney (CIK) cells. Meanwhile, mRNA expressions of CiHMGB2a and CiHMGB2b were significantly regulated by viral pathogen associated molecular patterns (PAMPs) polyinosinic-polycytidylic potassium salt (poly(I:C)) and bacterial PAMPs lipopolysaccharide (LPS), peptidoglycan (PGN) challenge in CIK cells. In CiHMGB2a and CiHMGB2b over-expression cells, expressions of CiHMGB2a and CiHMGB2b facilitated each other; transcription levels of CiTRIF, CiMyD88, CiIPS-1 and CiMx1 were remarkably enhanced, whereas CiIFN-I was inhibited, compared with those in cells transfected with pCMV (control plasmid); after GCRV challenge, all those tested genes were up-regulated with divergent expression profiles. Antiviral activities of CiHMGB2a and CiHMGB2b were manifested by the delayed appearance of cytopathic effect (CPE) and inhibition of GCRV yield. All those results demonstrate that CiHMGB2a and CiHMGB2b not only mediate antiviral immune responses but also involve in responding to viral/bacterial PAMPs challenge, which provides novel insights into the essential role of HMGB2 in innate immunity.

Generation and characterization of a polyclonal antibody against human high mobility group box 4

A human high mobility group box 4 (hHMGB4) expression construct (pET‑28a/hHMGB4) was generated by cloning the hHMGB4 full‑length cDNA in the expression vector pET‑28a(+). The hHMGB4 fusion protein with His6‑Tag was prepared using E.coli BL21 (DE3) transformed with pET‑28a/hHMGB4 and purified via preparative SDS‑PAGE plus electroelution. Immunization of rabbits with the purified hHMGB4 generated polyclonal antibodies. The titer of the antiserum was determined to be 1:102,400 by ELISA analysis. Western blotting analysis showed that the antibody specifically recognized the recombinant hHMGB4 protein and also the endogenous hHMGB4 protein in prostate cancer cells. In addition, immunohistochemical staining analysis using the prepared antibody revealed marked hHMGB4 staining in the nuclei of the human prostate tissue. These data demonstrate that the anti‑hHMGB4 polyclonal antibody may be a useful reagent for the functional study of hHMGB4.

Antimicrobial activity of high-mobility-group box 2: a new function to a well-known protein

The human intestinal tract is highly colonized by a vast number of microorganisms. Despite this permanent challenge, infections remain rare, due to a very effective barrier defense system. Essential effectors of this system are antimicrobial peptides and proteins (AMPs), which are secreted by intestinal epithelial and lymphoid cells, balance the gut microbial community, and prevent the translocation of microorganisms. Several antimicrobial proteins have already been identified in the gut. Nonetheless, we hypothesized that additional AMPs are yet to be discovered in this setting. Using biological screening based on antimicrobial function, here we identified competent antibacterial activity of high-mobility-group box 2 (HMGB2) against Escherichia coli. By recombinant expression, we confirmed this biologically new antimicrobial activity against different commensal and pathogenic bacteria. In addition, we demonstrated that the two DNA-binding domains (HMG boxes A and B) are crucial for the antibiotic function. We detected HMGB2 in several gastrointestinal tissues by mRNA analysis and immunohistochemical staining. In addition to the nuclei, we also observed HMGB2 in the cytoplasm of intestinal epithelial cells. Furthermore, HMGB2 was detectable in vitro in the supernatants of two different cell types, supporting an extracellular function. HMGB2 expression was not changed in inflammatory bowel disease but was detected in certain stool samples of patients, whereas it was absent from control individuals. Taken together, we characterized HMGB2 as an antimicrobial protein in intestinal tissue, complementing the diverse repertoire of gut mucosal defense molecules.

The multifunctional alarmin HMGB1 with roles in the pathophysiology of sepsis and cancer

Although originally described as a highly conserved nuclear protein involved in DNA replication, transcription and repair, high-mobility group box-1 protein (HMGB1) has emerged as a key mediator in the regulation of immune responses to infection and sterile injury by exhibiting all the properties of a prototypic 'alarmin'. These include rapid passive release in response to pathogenic infection and/or traumatic injury, active secretion providing for chemotactic and cytokine-like function and an ability to resolve inflammation, including tissue repair and remodelling. In this review, we will give an overview of the post-translational modifications necessary for such diversity in biological activity, concentrating particularly on how differences in oxidation of highly conserved redox-sensitive cysteine residues can potentiate inflammatory responses and dictate cellular fate. We will also review the most recent literature on HMGB1 and its involvement in the pathophysiology of sepsis and cancer, as well as cancer therapy-induced mucositis.

Localization and functional analysis of HmgB3p, a novel protein containing high-mobility-group-box domain from Tetrahymena thermophila

The high-mobility-group (HMG)-box domain represents a very versatile protein domain that mediates the DNA-binding of non-sequence-specific and sequence-specific proteins. HMG-box proteins are involved in various nuclear functions, including modulating chromatin structure and genomic stability. In this study, we identified the gene HMGB3 in Tetrahymena thermophila. The predicted HmgB3p contained a single HMG-box, an SK-rich-repeat domain and a neutral phosphorylated C-terminal. HMGB3 was expressed in the growth and starvation stages. Furthermore, HMGB3 showed a higher expression levels during the conjugation stage. HMGB3 knockout strains showed no obvious cytological defects, although initiation of HMGB3 knockout strain mating was delayed and maximum mating was decreased. HA-HmgB3p localized on the micronucleus (MIC) during the vegetative growth and starvation stages. Furthermore, HA-HmgB3p specially decorated the meiotic and mitotic functional MIC during the conjugation stage. Truncated HMGB3 lacking the HMG box domain disappeared from MICs and diffused in the cytoplasm. Overexpressed HmgB3p was abnormally maintained in newly developing macronuclei and affected the viability of progeny. Taken together, these results show that HmgB3p is a germline micronuclear-specific marker protein. It may bind to micronucleus-specific DNA sequences or structures and is likely to have some function specific to micronuclei of T. thermophila.

Real-time kinetics of high-mobility group box 1 (HMGB1) oxidation in extracellular fluids studied by in situ protein NMR spectroscopy

Some extracellular proteins are initially secreted in reduced forms via a non-canonical pathway bypassing the endoplasmic reticulum and become oxidized in the extracellular space. One such protein is HMGB1 (high-mobility group box 1). Extracellular HMGB1 has different redox states that play distinct roles in inflammation. Using a unique NMR-based approach, we have investigated the kinetics of HMGB1 oxidation and the half-lives of all-thiol and disulfide HMGB1 species in serum, saliva, and cell culture medium. In this approach, salt-free lyophilized (15)N-labeled all-thiol HMGB1 was dissolved in actual extracellular fluids, and the oxidation and clearance kinetics were monitored in situ by recording a series of heteronuclear (1)H-(15)N correlation spectra. We found that the half-life depends significantly on the extracellular environment. For example, the half-life of all-thiol HMGB1 ranged from ~17 min (in human serum and saliva) to 3 h (in prostate cancer cell culture medium). Furthermore, the binding of ligands (glycyrrhizin and heparin) to HMGB1 significantly modulated the oxidation kinetics. Thus, the balance between the roles of all-thiol and disulfide HMGB1 proteins depends significantly on the extracellular environment and can also be artificially modulated by ligands. This is important because extracellular HMGB1 has been suggested as a therapeutic target for inflammatory diseases and cancer. Our work demonstrates that the in situ protein NMR approach is powerful for investigating the behavior of proteins in actual extracellular fluids containing an enormous number of different molecules.

HMGB1-facilitated p53 DNA binding occurs via HMG-Box/p53 transactivation domain interaction, regulated by the acidic tail

Facilitated binding of p53 to DNA by high mobility group B1 (HMGB1) may involve interaction between the N-terminal region of p53 and the high mobility group (HMG) boxes, as well as HMG-induced bending of the DNA. Intramolecular shielding of the boxes by the HMGB1 acidic tail results in an unstable complex with p53 until the tail is truncated to half its length, at which point the A box, proposed to be the preferred binding site for p53(1-93), is exposed, leaving the B box to bind and bend DNA. The A box interacts with residues 38-61 (TAD2) of the p53 transactivation domain. Residues 19-26 (TAD1) bind weakly, but only in the context of p53(1-93) and not as a free TAD1 peptide. We have solved the structure of the A-box/p53(1-93) complex by nuclear magnetic resonance spectroscopy. The incipient amphipathic helix in TAD2 recognizes the concave DNA-binding face of the A box and may be acting as a single-stranded DNA mimic.

Mutually exclusive redox forms of HMGB1 promote cell recruitment or proinflammatory cytokine release

HMGB1 orchestrates leukocyte recruitment and their induction to secrete inflammatory cytokines by switching between mutually exclusive redox states.

Tissue damage causes inflammation, by recruiting leukocytes and activating them to release proinflammatory mediators. We show that high-mobility group box 1 protein (HMGB1) orchestrates both processes by switching among mutually exclusive redox states. Reduced cysteines make HMGB1 a chemoattractant, whereas a disulfide bond makes it a proinflammatory cytokine and further cysteine oxidation to sulfonates by reactive oxygen species abrogates both activities. We show that leukocyte recruitment and activation can be separated. A nonoxidizable HMGB1 mutant in which serines replace all cysteines (3S-HMGB1) does not promote cytokine production, but is more effective than wild-type HMGB1 in recruiting leukocytes in vivo. BoxA, a HMGB1 inhibitor, interferes with leukocyte recruitment but not with activation. We detected the different redox forms of HMGB1 ex vivo within injured muscle. HMGB1 is completely reduced at first and disulfide-bonded later. Thus, HMGB1 orchestrates both key events in sterile inflammation, leukocyte recruitment and their induction to secrete inflammatory cytokines, by adopting mutually exclusive redox states.

New developments in Toll-like receptor targeted therapeutics

Toll-like receptors (TLRs) play a crucial role in host defence and inflammation. Given that a significant amount of evidence implicates TLRs in the pathogenesis of immune diseases and cancer, and their activation occurs early in the inflammatory cascade, they are attractive targets for novel therapeutic agents. Potential therapeutics include TLR-targeted antibodies, small molecules and nucleic acid based drugs. Agonists are being tested in vaccines against hepatitis C and influenza as well as in allergic rhinitis and certain cancers. Antagonists are being tested in ischemia/reperfusion injury, systemic lupus erythematosus and psoriasis. The prospect of targeting TLRs in multiple pathologies continues to hold much promise.

H1 and HMGB1: modulators of chromatin structure

Histone H1 and HMGB1 (high-mobility group protein B1) are the most abundant chromosomal proteins apart from the core histones (on average, one copy per nucleosome and per ten nucleosomes respectively). They are both highly mobile in the cell nucleus, with high on/off rates for binding. In vivo and in vitro evidence shows that both are able to organize chromatin structure, with H1 binding resulting in a more stable structure and HMGB1 binding in a less stable structure. The binding sites for H1 and HMGB1 in chromatin are partially overlapping, and replacement of H1 by HMGB1 through the highly dynamic nature of their binding, possibly facilitated by interaction between them, could result in switching of chromatin states. Binding of HMGB1 to DNA or chromatin is regulated by its long and highly acidic tail, which is also involved in H1 binding. The present article focuses mainly on HMGB1 and its interaction with chromatin and H1, as well as its chaperone role in the binding of certain transcription factors (e.g. p53) to their cognate DNA.

HMGB1 promotes recruitment of inflammatory cells to damaged tissues by forming a complex with CXCL12 and signaling via CXCR4

CXCL12 forms a complex with HMGB1 that binds to the chemokine receptor CXCR4 and increases inflammatory cell migration.

After tissue damage, inflammatory cells infiltrate the tissue and release proinflammatory cytokines. HMGB1 (high mobility group box 1), a nuclear protein released by necrotic and severely stressed cells, promotes cytokine release via its interaction with the TLR4 (Toll-like receptor 4) receptor and cell migration via an unknown mechanism. We show that HMGB1-induced recruitment of inflammatory cells depends on CXCL12. HMGB1 and CXCL12 form a heterocomplex, which we characterized by nuclear magnetic resonance and surface plasmon resonance, that acts exclusively through CXCR4 and not through other HMGB1 receptors. Fluorescence resonance energy transfer data show that the HMGB1–CXCL12 heterocomplex promotes different conformational rearrangements of CXCR4 from that of CXCL12 alone. Mononuclear cell recruitment in vivo into air pouches and injured muscles depends on the heterocomplex and is inhibited by AMD3100 and glycyrrhizin. Thus, inflammatory cell recruitment and activation both depend on HMGB1 via different mechanisms.

Intrinsic disorder in cell signaling and gene transcription

Structural disorder, which enables unique modes of action often associated with molecular recognition and folding induced by a partner, is widespread in eukaryotic proteomes. Due to the ensuing advantages, such as specificity without strong binding, adaptability to multiple partners and subtle regulation by post-translational modification, structural disorder is prevalent in proteins of signaling and regulatory functions, such as membrane receptors, scaffold proteins, cytoskeletal proteins, transcription factors and nuclear hormone receptors. In this review we survey the most important aspects of structural disorder, with major focus on features and advantages pertinent to signal transduction. Our major goal is to elucidate how the functional requirements of these protein classes concur with specific functional modes disorder enables.