The Janus face of HMGB1 in heart disease: a necessary update.

High mobility group box 1 (HMGB1) is a ubiquitous nuclear protein involved in transcription regulation, DNA replication and repair and nucleosome assembly. HMGB1 is passively released by necrotic tissues or actively secreted by stressed cells. Extracellular HMGB1 acts as a damage-associated molecular pattern (DAMPs) molecule and gives rise to several redox forms that by binding to different receptors and interactors promote a variety of cellular responses, including tissue inflammation or regeneration. Inhibition of extracellular HMGB1 in experimental models of myocardial ischemia/reperfusion injury, myocarditis, cardiomyopathies induced by mechanical stress, diabetes, bacterial infection or chemotherapeutic drugs reduces inflammation and is protective. In contrast, administration of HMGB1 after myocardial infarction induced by permanent coronary artery ligation ameliorates cardiac performance by promoting tissue regeneration. HMGB1 decreases contractility and induces hypertrophy and apoptosis in cardiomyocytes, stimulates cardiac fibroblast activities, and promotes cardiac stem cell proliferation and differentiation. Interestingly, maintenance of appropriate nuclear HMGB1 levels protects cardiomyocytes from apoptosis by preventing DNA oxidative stress, and mice with HMGB1cardiomyocyte-specific overexpression are partially protected from cardiac damage. Finally, higher levels of circulating HMGB1 are associated to human heart diseases. Hence, during cardiac injury, HMGB1 elicits both harmful and beneficial responses that may in part depend on the generation and stability of the diverse redox forms, whose specific functions in this context remain mostly unexplored. This review summarizes recent findings on HMGB1 biology and heart dysfunctions and discusses the therapeutic potential of modulating its expression, localization, and oxidative-dependent activities.

Non-oxidizable HMGB1 induces cardiac fibroblasts migration via CXCR4 in a CXCL12-independent manner and worsens tissue remodeling after myocardial infarction.

Myocardial infarction (MI) is a major health burden worldwide. Extracellular High mobility group box 1 (HMGB1) regulates tissue healing after injuries. The reduced form of HMGB1 (fr-HMGB1) exerts chemotactic activity by binding CXCL12 through CXCR4, while the disulfide form, (ds-HMGB1), induces cytokines expression by TLR4. Here, we assessed the role of HMGB1 redox forms and the non-oxidizable mutant (3S) on human cardiac fibroblast (hcFbs) functions and cardiac remodeling after infarction. Among HMGB1 receptors, hcFbs express CXCR4. Fr-HMGB1 and 3S, but not ds-HMGB1, promote hcFbs migration through Src activation, while none of HMGB1 redox forms induces proliferation or inflammatory mediators. 3S is more effective than fr-HMGB1 in stimulating hcFbs migration and Src phosphorylation being active at lower concentrations and in oxidizing conditions. Notably, chemotaxis toward both proteins is CXCR4-dependent but, in contrast to fr-HMGB1, 3S does not require CXCL12 since hcFbs migration persists in the presence of the CXCL12/CXCR4 inhibitor AMD3100 or an anti-CXCL12 antibody. Interestingly, 3S interacts with CXCR4 and induces a different receptor conformation than CXCL12. Mice undergoing MI and receiving 3S exhibit adverse LV remodeling owing to an excessive collagen deposition promoted by a higher number of myofibroblasts. On the contrary, fr-HMGB1 ameliorates cardiac performance enhancing neoangiogenesis and reducing the infarcted area and fibrosis. Altogether, our results demonstrate that non-oxidizable HMGB1 induce a sustained cardiac fibroblasts migration despite the redox state of the environment and by altering CXCL12/CXCR4 axis. This affects proper cardiac remodeling after an infarction.

The shedding-derived soluble receptor for advanced glycation endproducts sustains inflammation during acute Pseudomonas aeruginosa lung infection.

BACKGROUND: The membrane-bound isoform of the receptor for advanced glycation end products (FL-RAGE) is primarily expressed by alveolar epithelial cells and undergoes shedding by the protease ADAM10, giving rise to soluble cleaved RAGE (cRAGE). RAGE has been associated with the pathogenesis of several acute and chronic lung disorders. Whether the proteolysis of FL-RAGE is altered by a given inflammatory stimulus is unknown. Pseudomonas aeruginosa causes nosocomial infections in hospitalized patients and is the major pathogen associated with chronic lung diseases. METHODS: P. aeruginosa was injected in Rage-/- and wild-type mice and the impact on RAGE expression and shedding, levels of inflammation and bacterial growth was determined. RESULTS: Acute P. aeruginosa lung infection in mice induces a reduction of the active form of ADAM10, which determines an increase of FL-RAGE expression on alveolar cells and a concomitant decrease of pulmonary cRAGE levels. This was associated with massive recruitment of leukocytes and release of pro-inflammatory factors, tissue damage and relocation of cRAGE in the alveolar and bronchial cavities. The administration of sRAGE worsened bacterial burden and neutrophils infiltration. RAGE genetic deficiency reduced the susceptibility to P. aeruginosa infection, mitigating leukocyte recruitment, inflammatory molecules production, and bacterial growth. CONCLUSIONS: These data are the first to suggest that inhibition of FL-RAGE shedding, by affecting the FL-RAGE/cRAGE levels, is a novel mechanism for controlling inflammation to acute P. aeruginosa pneumonia. sRAGE in the alveolar space sustains inflammation in this setting. GENERAL SIGNIFICANCE: RAGE shedding may determine the progression of inflammatory lung diseases.

Abnormalities of the oculomotor function in type 1 diabetes and diabetic neuropathy.

AIMS: Abnormalities in the oculomotor system may represent an early sign of diabetic neuropathy and are currently poorly studied. We designed an eye-tracking-based test to evaluate oculomotor function in patients with type 1 diabetes. METHODS: We used the SRLab-Tobii TX300 Eye tracker®, an eye-tracking device, coupled with software that we developed to test abnormalities in the oculomotor system. The software consists of a series of eye-tracking tasks divided into 4 classes of parameters (Resistance, Wideness, Pursuit and Velocity) to evaluate both smooth and saccadic movement in different directions. We analyzed the oculomotor system in 34 healthy volunteers and in 34 patients with long-standing type 1 diabetes. RESULTS: Among the 474 parameters analyzed with the eye-tracking-based system, 11% were significantly altered in patients with type 1 diabetes (p < 0.05), with a higher proportion of abnormalities observed in the Wideness (24%) and Resistance (10%) parameters. Patients with type 1 diabetes without diabetic neuropathy showed more frequently anomalous measurements in the Resistance class (p = 0.02). The classes of Velocity and Pursuit were less frequently altered in patients with type 1 diabetes as compared to healthy subjects, with anomalous measurements mainly observed in patients with diabetic neuropathy. CONCLUSIONS: Abnormalities in oculomotor system function can be detected in patients with type 1 diabetes using a novel eye-tracking-based test. A larger cohort study may further determine thresholds of normality and validate whether eye-tracking can be used to non-invasively characterize early signs of diabetic neuropathy. TRIAL: NCT04608890.

PD-1 blockade counteracts post-COVID-19 immune abnormalities and stimulates the anti-SARS-CoV-2 immune response.

A substantial proportion of patients who have recovered from coronavirus disease-2019 (COVID-19) experience COVID-19-related symptoms even months after hospital discharge. We extensively immunologically characterized patients who recovered from COVID-19. In these patients, T cells were exhausted, with increased PD-1+ T cells, as compared with healthy controls. Plasma levels of IL-1ß, IL-1RA, and IL-8, among others, were also increased in patients who recovered from COVID-19. This altered immunophenotype was mirrored by a reduced ex vivo T cell response to both nonspecific and specific stimulation, revealing a dysfunctional status of T cells, including a poor response to SARS-CoV-2 antigens. Altered levels of plasma soluble PD-L1, as well as of PD1 promoter methylation and PD1-targeting miR-15-5p, in CD8+ T cells were also observed, suggesting abnormal function of the PD-1/PD-L1 immune checkpoint axis. Notably, ex vivo blockade of PD-1 nearly normalized the aforementioned immunophenotype and restored T cell function, reverting the observed post-COVID-19 immune abnormalities; indeed, we also noted an increased T cell-mediated response to SARS-CoV-2 peptides. Finally, in a neutralization assay, PD-1 blockade did not alter the ability of T cells to neutralize SARS-CoV-2 spike pseudotyped lentivirus infection. Immune checkpoint blockade ameliorates post-COVID-19 immune abnormalities and stimulates an anti-SARS-CoV-2 immune response.

High mobility group box 1 orchestrates tissue regeneration via CXCR4.

Inflammation and tissue regeneration follow tissue damage, but little is known about how these processes are coordinated. High Mobility Group Box 1 (HMGB1) is a nuclear protein that, when released on injury, triggers inflammation. We previously showed that HMGB1 with reduced cysteines is a chemoattractant, whereas a disulfide bond makes it a proinflammatory cytokine. Here we report that fully reduced HMGB1 orchestrates muscle and liver regeneration via CXCR4, whereas disulfide HMGB1 and its receptors TLR4/MD-2 and RAGE (receptor for advanced glycation end products) are not involved. Injection of HMGB1 accelerates tissue repair by acting on resident muscle stem cells, hepatocytes, and infiltrating cells. The nonoxidizable HMGB1 mutant 3S, in which serines replace cysteines, promotes muscle and liver regeneration more efficiently than the wild-type protein and without exacerbating inflammation by selectively interacting with CXCR4. Overall, our results show that the reduced form of HMGB1 coordinates tissue regeneration and suggest that 3S may be used to safely accelerate healing after injury in diverse clinical contexts.

The Mouse-Specific Splice Variant mRAGE_v4 Encodes a Membrane-Bound RAGE That Is Resistant to Shedding and Does Not Contribute to the Production of Soluble RAGE.

The receptor for advanced glycation end-products (RAGE) is involved in the onset and progression of several inflammatory diseases. The RAGE primary transcript undergoes numerous alternative splicing (AS) events, some of which are species-specific. Here, we characterize the mouse-specific mRAGE_v4 splice variant, which is conserved in rodents and absent in primates. mRAGE_v4 derives from exon 9 skipping and encodes a receptor (M-RAGE) that lacks 9 amino acids between the transmembrane and the immunoglobulin (Ig) domains. RNA-Seq data confirm that in mouse lung mRAGE_v4 is the most abundant RAGE mRNA isoform after mRAGE, which codes for full-length RAGE (FL-RAGE), while in heart all RAGE variants are almost undetectable. The proteins M-RAGE and FL-RAGE are roughly equally abundant in mouse lung. Contrary to FL-RAGE, M-RAGE is extremely resistant to shedding because it lacks the peptide motif recognized by both ADAM10 and MMP9, and does not contribute significantly to soluble cRAGE formation. Thus, a cassette exon in RAGE corresponds to a specific function of the RAGE protein-the ability to be shed. Given the differences in RAGE AS variants between rodents and humans, caution is due in the interpretation of results obtained in mouse models of RAGE-dependent human pathologies.