Dysfunction of epithelial permeability barrier induced
HMGB1 and Epithelial Permeability Barriers: Molecular Mechanisms in Airway and Intestinal Inflammation
Epithelial permeability barriers play a fundamental role in maintaining tissue homeostasis and protecting the body against environmental insults, pathogens, and inflammatory stimuli. In the respiratory and gastrointestinal systems, epithelial cells form specialized protective interfaces that regulate the movement of molecules while maintaining tissue integrity. Disruption of these barriers can contribute to chronic inflammation, infectious diseases, and various pathological conditions.
High mobility group box 1 (HMGB1) is a multifunctional protein involved in inflammatory signaling, immune responses, and cellular stress mechanisms. Although it primarily functions as a nuclear protein under physiological conditions, HMGB1 can be released into the extracellular environment following cellular damage or inflammatory stimulation. Once released, it acts as a damage-associated molecular pattern (DAMP), activating inflammatory pathways and contributing to epithelial barrier dysfunction.
Recent research has highlighted the involvement of HMGB1 in airway inflammation, inflammatory bowel disease, and epithelial hyperpermeability. Understanding the molecular mechanisms underlying these effects is essential for investigating new therapeutic approaches targeting inflammatory epithelial disorders.
Understanding Epithelial Permeability Barriers
Epithelial tissues form continuous cellular layers that separate internal organs from external environments. These barriers are particularly important in the respiratory tract and gastrointestinal system, where epithelial cells regulate the exchange of nutrients, ions, water, and other molecules.
The integrity of epithelial barriers depends on specialized intercellular junctions, including tight junctions, adherens junctions, and desmosomes. Among these structures, tight junctions are particularly important because they regulate paracellular permeability, controlling the passage of substances between adjacent epithelial cells.
In the respiratory system, epithelial barriers protect underlying tissues against airborne particles, microorganisms, and environmental pollutants. In the intestine, they regulate the absorption of nutrients while limiting the uncontrolled passage of potentially harmful substances.
Alterations in tight junction organization and epithelial permeability can lead to increased tissue exposure to inflammatory mediators and microbial components, contributing to the development and progression of inflammatory diseases.
High Mobility Group Box 1: Biological Functions and Inflammatory Activity
High mobility group box 1 is a highly conserved protein that belongs to the high mobility group family of chromatin-associated proteins. Under normal physiological conditions, HMGB1 is primarily located in the nucleus, where it participates in DNA organization, transcriptional regulation, and chromatin remodeling.
However, HMGB1 can also function as an extracellular signaling molecule when actively secreted by immune cells or passively released from damaged and necrotic cells. In the extracellular environment, it interacts with pattern recognition receptors, including the receptor for advanced glycation end products (RAGE) and Toll-like receptors such as TLR2 and TLR4.
These interactions activate multiple intracellular signaling pathways, including nuclear factor kappa B (NF-κB) and mitogen-activated protein kinase (MAPK) pathways. Consequently, HMGB1 can promote the production of pro-inflammatory cytokines, chemokines, and other inflammatory mediators.
Increased extracellular HMGB1 concentrations have been associated with several inflammatory conditions, making this protein an important research target in immunology and epithelial biology.
HMGB1-Induced Epithelial Hyperpermeability
Extracellular HMGB1 can interfere with epithelial barrier function by modifying the organization and regulation of intercellular junctions. Its inflammatory activity can stimulate signaling pathways that alter tight junction protein expression, distribution, and cellular localization.
These changes may weaken the epithelial permeability barrier and facilitate the uncontrolled movement of molecules through the paracellular pathway.
In airway and intestinal epithelial models, HMGB1-induced barrier dysfunction has been investigated using permeability assays that measure the passage of fluorescently labeled molecules across epithelial structures.
In particular, increased FITC-dextran permeability provides an experimental indication of alterations in epithelial barrier integrity. These findings help researchers investigate how inflammatory mediators influence molecular transport and intercellular junction organization.
The Role of Angulin-1/LSR in Tricellular Tight Junctions
Tricellular tight junctions are specialized structures located at the contact points where three epithelial cells meet. These regions represent important sites for regulating paracellular permeability because their structural organization differs from that of bicellular tight junctions.
Angulin-1, also known as lipolysis-stimulated lipoprotein receptor (LSR), is a transmembrane protein involved in the organization and maintenance of tricellular tight junctions.
LSR contributes to the formation of a functional epithelial barrier by regulating molecular passage at tricellular contacts. Alterations in its expression, localization, or activity may therefore contribute to increased epithelial permeability.
In the context of HMGB1-induced inflammation, investigating angulin-1/LSR provides important insight into the molecular mechanisms responsible for tricellular junction dysfunction.
Understanding the relationship between HMGB1 signaling and LSR-associated junctional organization may help identify new molecular targets for studying inflammatory disorders affecting airway and intestinal epithelial tissues.
Two-and-a-Half-Dimensional (2.5D) Cell Culture Models
Traditional two-dimensional cell culture systems provide a convenient experimental approach for studying cellular responses under controlled conditions. However, cells grown on flat surfaces may not reproduce the complex spatial organization and tissue-like architecture observed in vivo.
Two-and-a-half-dimensional (2.5D) culture models offer an alternative experimental approach by allowing epithelial cells to develop more complex three-dimensional arrangements while maintaining practical experimental accessibility.
These systems can support the formation of lumen-like structures and more organized cellular interfaces, providing useful models for investigating epithelial permeability and molecular transport.
In studies involving human lung, nasal, and intestinal epithelial cells, 2.5D culture assays have been used to investigate the effects of HMGB1 on epithelial barrier integrity.
Following HMGB1 treatment, increased penetration of FITC-dextran into epithelial lumens has been observed, providing evidence of altered permeability in these experimental models.
Such approaches contribute to a better understanding of inflammatory signaling and the mechanisms regulating epithelial barrier function.
HMGB1 Signaling Pathways in Airway and Intestinal Inflammation
The inflammatory effects of HMGB1 involve several interconnected molecular signaling pathways. Among the most extensively investigated are RAGE-dependent signaling, Toll-like receptor activation, NF-κB signaling, and MAPK pathway regulation.
Activation of these pathways can promote the transcription of inflammatory mediators and modify cellular responses associated with epithelial integrity.
In airway epithelial cells, HMGB1 signaling may contribute to inflammatory responses associated with respiratory infections and chronic inflammatory conditions. In intestinal epithelial tissues, increased HMGB1 activity has been linked to mechanisms involved in inflammatory bowel disease and impaired mucosal barrier function.
The interaction between extracellular HMGB1, inflammatory receptors, and tight junction-associated proteins represents an important area of investigation in epithelial pathophysiology.
HMGB1 and COVID-19: Implications for Respiratory Epithelial Barriers
The relationship between HMGB1 and severe respiratory inflammation has attracted particular research interest during the COVID-19 pandemic.
Studies have reported increased circulating HMGB1 concentrations in patients with severe COVID-19, suggesting a potential association between extracellular HMGB1 activity and systemic inflammatory responses.
Experimental findings have also indicated that exogenous HMGB1 can stimulate the expression of angiotensin-converting enzyme 2 (ACE2), a major cellular entry receptor for SARS-CoV-2, in alveolar epithelial cells through an advanced glycation end product receptor-dependent mechanism.
In human nasal epithelial cells (HNECs), HMGB1 treatment has been associated with increased expression of COVID-19-related genes, including cathepsin L (CTSL) and furin.
Cathepsin L is a cysteine protease involved in the processing of viral proteins and can contribute to SARS-CoV-2 entry into host cells. Furin is another protease implicated in the processing of the viral spike protein and its ability to facilitate host-cell infection.
These observations suggest that HMGB1-associated inflammatory signaling may influence several molecular processes involved in respiratory epithelial responses to SARS-CoV-2.
However, the precise contribution of HMGB1 to viral entry, epithelial barrier disruption, and disease severity requires further investigation.
Experimental Approaches for Studying Epithelial Barrier Dysfunction
Investigating epithelial permeability requires experimental techniques capable of evaluating both structural integrity and molecular transport across cellular barriers.
Several laboratory approaches are commonly used in this research area:
1. FITC-Dextran Permeability Assays
Fluorescein isothiocyanate-conjugated dextran is frequently used as a fluorescent tracer to evaluate epithelial permeability. Changes in its distribution or penetration can provide information about alterations in barrier function.
2. Immunofluorescence Microscopy
Immunofluorescence techniques allow researchers to visualize the localization and distribution of tight junction proteins, including claudins, occludin, and angulin-1/LSR.
3. Western Blot Analysis
Western blotting is used to investigate changes in the expression levels of proteins involved in inflammatory signaling and epithelial junction organization.
4. Quantitative Real-Time PCR
RT-qPCR enables the analysis of gene expression associated with inflammatory responses, epithelial barrier regulation, and cellular signaling pathways.
5. Cell Culture-Based Inflammatory Models
Human lung, nasal, and intestinal epithelial cell models provide experimental platforms for investigating the effects of HMGB1 and other inflammatory mediators on epithelial homeostasis.
Combining these techniques allows researchers to characterize molecular changes and functional alterations associated with epithelial barrier dysfunction.
Therapeutic Perspectives: Targeting HMGB1 in Inflammatory Diseases
Because HMGB1 participates in several inflammatory pathways, its extracellular activity has become an important subject of therapeutic research.
Strategies targeting HMGB1 signaling may involve inhibiting its release, interfering with receptor interactions, or modulating downstream inflammatory pathways.
Potential therapeutic approaches include HMGB1-neutralizing antibodies, receptor-targeting compounds, and inhibitors of associated intracellular signaling mechanisms.
In airway and intestinal inflammatory disorders, modulation of HMGB1-associated pathways could potentially help preserve epithelial barrier integrity and reduce excessive inflammatory responses.
Nevertheless, HMGB1 also performs essential intracellular functions, particularly in chromatin organization and cellular regulation. Therefore, therapeutic approaches must distinguish between its physiological nuclear activities and its extracellular inflammatory effects.
Further experimental and clinical investigations are necessary to determine the safety, specificity, and therapeutic relevance of HMGB1-targeted interventions.
Frequently Asked Questions About HMGB1 and Epithelial Permeability
1. What is HMGB1?
HMGB1 is a multifunctional protein involved in DNA organization, transcriptional regulation, and inflammatory signaling. When released extracellularly, it can act as a damage-associated molecular pattern and activate immune responses.
2. How does HMGB1 affect epithelial permeability?
Extracellular HMGB1 can activate inflammatory signaling pathways that modify tight junction organization and increase the passage of molecules across epithelial barriers.
3. What is angulin-1/LSR?
Angulin-1, also called lipolysis-stimulated lipoprotein receptor, is a transmembrane protein involved in the organization of tricellular tight junctions and the regulation of epithelial barrier integrity.
4. What is a 2.5D cell culture model?
A 2.5D culture model is an experimental cell culture approach that promotes more complex cellular organization than conventional flat two-dimensional cultures while retaining practical experimental accessibility.
5. Why is FITC-dextran used in permeability assays?
FITC-dextran is a fluorescent tracer used to evaluate the passage of molecules across epithelial barriers and investigate changes in paracellular permeability.
6. What is the relationship between HMGB1 and COVID-19?
Research has associated increased HMGB1 levels with severe COVID-19 inflammatory responses. Experimental studies have also investigated its effects on ACE2, cathepsin L, and furin expression in respiratory epithelial cells.
Conclusion
HMGB1 is an important mediator of inflammatory signaling and epithelial barrier dysfunction in respiratory and gastrointestinal tissues. Its extracellular activity can influence tight junction organization, increase epithelial permeability, and contribute to inflammatory processes associated with airway diseases and intestinal disorders.
The involvement of angulin-1/LSR in tricellular tight junction regulation, together with the use of advanced 2.5D cell culture models, provides valuable experimental opportunities for investigating the molecular mechanisms underlying epithelial hyperpermeability.
Furthermore, emerging research into HMGB1-associated signaling in COVID-19 highlights the potential importance of this protein in respiratory inflammation and epithelial responses to viral infection.
A better understanding of HMGB1-mediated mechanisms may contribute to the identification of new therapeutic targets and support the development of experimental strategies for preserving epithelial barrier integrity.