The cell surface hyaluronidase TMEM2 is essential for systemic hyaluronan catabolism and turnover

Hyaluronidase-induced matrix remodeling contributes to long-term synaptic changes

Extracellular brain space contains water, dissolved ions, and multiple other signaling molecules. The neural extracellular matrix (ECM) is also a significant component of the extracellular space. The ECM is synthesized by neurons, astrocytes, and other types of cells. Hyaluronan, a hyaluronic acid polymer, is a key component of the ECM. The functions of hyaluronan include barrier functions and signaling. In this article, we investigate physiological processes during the acute phase of enzymatic ECM removal. We found that hyaluronidase, an ECM removal agent, triggers simultaneous membrane depolarization and sharp calcium influx into neurons. Spontaneous action potential firing frequency increased rapidly after ECM destruction in interneurons, but not pyramidal neurons. Hyaluronidase-dependent calcium entry can be blocked by a selective antagonist of N-methyl-D-aspartate (NMDA) receptors, revealing these receptors as the main player in the observed phenomenon. Additionally, we demonstrate increased NMDA-dependent long-term potentiation at CA3-to-CA1 synapses during the acute phase of ECM removal. These findings suggest that hyaluronan is a significant synaptic player.

The roles of hyaluronan in kidney development, physiology and disease

The hyaluronan (HA) matrix in the tissue microenvironment is crucial for maintaining homeostasis by regulating inflammatory signalling, endothelial-mesenchymal transition and cell migration. During development, covalent modifications and osmotic swelling of HA create mechanical forces that initiate midgut rotation, vascular patterning and branching morphogenesis. Together with its main cell surface receptor, CD44, HA establishes a physicochemical scaffold at the cell surface that facilitates the interaction and clustering of growth factors and receptors that is required for normal physiology. High-molecular-weight HA, tumour necrosis factor-stimulated gene 6, pentraxin 3 and CD44 form a stable pericellular matrix that promotes tissue regeneration and reduces inflammation. By contrast, breakdown of high-molecular-weight HA into depolymerized fragments by hyaluronidases triggers inflammatory signalling, leukocyte migration and angiogenesis, contributing to tissue damage and fibrosis in kidney disease. Targeting HA metabolism is challenging owing to its dynamic regulation and tissue-specific functions. Nonetheless, modulating HA matrix functions by targeting its binding partners holds promise as a therapeutic strategy for restoring tissue homeostasis and mitigating pathological processes. Further research in this area is warranted to enable the development of novel therapeutic approaches for kidney and other diseases characterized by dysregulated HA metabolism.

Genet identity and season drive gene expression in outplanted Acropora palmata at different reef sites

Coral reefs are experiencing decreases in coral cover due to anthropogenic influences. Coral restoration is addressing this decline by outplanting large volumes of corals onto reef systems. Understanding how outplanted corals react at a transcriptomic level to different outplant locations over time is important, as it will highlight how habitat affects the coral host and influences physiological measures. In this study, the transcriptomic dynamics of four genets of outplanted Acropora palmata were assessed over a year at three reef sites in the Florida Keys. Genet identity was more important than time of sampling or outplant site, with differing levels of baseline immune and protein production the key drivers. Once accounting for genet, enriched growth processes were identified in the winter, and increased survival and immune expression were found in the summer. The effect of the reef site was small, with hypothesized differences in autotrophic versus heterotrophic dependent on outplant depth. We hypothesize that genotype identity is an important consideration for reef restoration, as differing baseline gene expression could play a role in survivorship and growth. Additionally, outplanting during cooler winter months may be beneficial due to higher expression of growth processes, allowing establishment of outplants on the reef system.

Genetic Deficiencies of Hyaluronan Degradation

Hyaluronan (HA) is a large polysaccharide that is broadly distributed and highly abundant in the soft connective tissues and embryos of vertebrates. The constitutive turnover of HA is very high, estimated at 5 g per day in an average (70 kg) adult human, but HA turnover must also be tightly regulated in some processes. Six genes encoding homologues to bee venom hyaluronidase (HYAL1, HYAL2, HYAL3, HYAL4, HYAL6P/HYALP1, SPAM1/PH20), as well as genes encoding two unrelated G8-domain-containing proteins demonstrated to be involved in HA degradation (CEMIP/KIAA1199, CEMIP2/TMEM2), have been identified in humans. Of these, only deficiencies in HYAL1, HYAL2, HYAL3 and CEMIP have been identified as the cause or putative cause of human genetic disorders. The phenotypes of these disorders have been vital in determining the biological roles of these enzymes but there is much that is still not understood. Deficiencies in these HA-degrading proteins have been created in mice and/or other model organisms where phenotypes could be analyzed and probed to expand our understanding of HA degradation and function. This review will describe what has been found in human and animal models of hyaluronidase deficiency and discuss how this has advanced our understanding of HA's role in health and disease.

Hyaluronan degradation by HYAL2 is essential for odontoblastic differentiation and migration of mouse dental papilla cells

The coordination between odontoblastic differentiation and directed cell migration of mesenchymal progenitors is necessary for regular dentin formation. The synthesis and degradation of hyaluronan (HA) in the extracellular matrix create a permissive niche that directly regulates cell behaviors. However, the role and mechanisms of HA degradation in dentin formation remain unknown. In this work, we present that HA digestion promotes odontoblastic differentiation and cell migration of mouse dental papilla cells (mDPCs). Hyaluronidase 2 (HYAL2) is responsible for promoting odontoblastic differentiation through degrading HA, while hyaluronidase 1 (HYAL1) exhibits negligible effect. Silencing Hyal2 generates an extracellular environment rich in HA, which attenuates F-actin and filopodium formation and in turn inhibits cell migration of mDPCs. In addition, activating PI3K/Akt signaling significantly rescues the effects of HA accumulation on cytodifferentiation. Taken together, the results confirm the contribution of HYAL2 to HA degradation in dentinogenesis and uncover the mechanism of the HYAL2-mediated HA degradation in regulating the odontoblastic differentiation and migration of mDPCs.

TMEM2 inhibits the development of Graves' orbitopathy through the JAK-STAT signaling pathway

A mouse model was used to investigate the role of the hyaluronidase, transmembrane protein 2 (TMEM2), on the progression of Graves' orbital (GO) disease. We established a GO mouse model through immunization with a plasmid expressing the thyroid stimulating hormone receptor. Orbital fibroblasts (OFs) were subsequently isolated from both GO and non-GO mice for comprehensive in vitro analyses. The expression of TMEM2 was assessed using qRT-PCR, Western blot and immunohistochemistry in vivo. Disease pathology was evaluated by H&E staining and Masson's trichrome staining in GO mouse tissues. Our investigation revealed a notable reduction in TMEM2 expression in GO mouse orbital tissues. Through overexpression and knockdown assays, we demonstrated that TMEM2 suppresses inflammatory cytokines and reactive oxygen species production. TMEM2 also inhibits the formation of lipid droplets in OFs and the expression of adipogenic factors. Further incorporating Gene Set Enrichment Analysis of relevant GEO datasets and subsequent in vitro cell experiments, robustly confirmed that TMEM2 overexpression was associated with a pronounced upregulation of the JAK/STAT signaling pathway. In vivo, TMEM2 overexpression reduced inflammatory cell infiltration, adipogenesis, and fibrosis in orbital tissues. These findings highlight the varied regulatory role of TMEM2 in GO pathogenesis. Our study reveals that TMEM2 plays a crucial role in mitigating inflammation, suppressing adipogenesis, and reducing fibrosis in GO. TMEM2 has potential as a therapeutic target and biomarker for treating or alleviating GO. These findings advance our understanding of GO pathophysiology and provide opportunities for targeted interventions to modulate TMEM2 for therapeutic purposes.

Hyaluronan Metabolism in Urologic Cancers

Hyaluronan (HA) is one of the major components of the extracellular matrix in tumor tissue. Recent reports have made it clear that the balance of HA synthesis and degradation is critical for tumor progression. HA is synthesized on the cytoplasmic surface of the plasma membrane by hyaluronan synthases (HAS) and extruded into the extracellular space. Excessive HA production in cancer is associated with enhanced HA degradation in the tumor microenvironment, leading to the accumulation of HA fragments with small molecular weight. These perturbations in both HA synthesis and degradation may play important roles in tumor progression. Recently, it has become increasingly clear that small HA fragments can induce a variety of biological events, such as angiogenesis, cancer-promoting inflammation, and tumor-associated immune suppression. Progression of urologic malignancies, particularly of prostate and bladder cancers, as well as of certain types of kidney cancer show markedly perturbed metabolism of tumor-associated HA. This review highlights the recent research findings regarding HA metabolism in tumor microenvironments with a special focus on urologic cancers. It also will discuss the potential implications of these findings for the development of novel therapeutic interventions for the treatment of prostate, bladder, and kidney cancers.

TMEM2 is a bona fide hyaluronidase possessing intrinsic catalytic activity

Transmembrane protein 2 (TMEM2) was originally identified as a membrane-anchored protein of unknown function. We previously demonstrated that TMEM2 can degrade hyaluronan (HA). Furthermore, we showed that induced global knockout of Tmem2 in adult mice results in rapid accumulation of incompletely degraded HA in bodily fluids and organs, supporting the identity of TMEM2 as a cell surface hyaluronidase. In spite of these advances, no direct evidence has been presented to demonstrate the intrinsic hyaluronidase activity of TMEM2. Here, we directly establish the catalytic activity of TMEM2. The ectodomain of TMEM2 (TMEM2ECD) was expressed as a His-tagged soluble protein and purified by affinity and size-exclusion chromatography. Both human and mouse TMEM2ECD robustly degrade fluorescein-labeled HA into 5 to 10 kDa fragments. TMEM2ECD exhibits this HA-degrading activity irrespective of the species of TMEM2 origin and the position of epitope tag insertion. The HA-degrading activity of TMEM2ECD is more potent than that of HYAL2, a hyaluronidase which, like TMEM2, has been implicated in cell surface HA degradation. Finally, we show that TMEM2ECD can degrade not only fluorescein-labeled HA but also native high-molecular weight HA. In addition to these core findings, our study reveals hitherto unrecognized confounding factors, such as the quality of reagents and the choice of assay systems, that could lead to erroneous conclusions regarding the catalytic activity of TMEM2. In conclusion, our results demonstrate that TMEM2 is a legitimate functional hyaluronidase. Our findings also raise cautions regarding the choice of reagents and methods for performing degradation assays for hyaluronidases.

Tmem2 Deficiency Leads to Enamel Hypoplasia and Soft Enamel in Mouse

Teeth consist of 3 mineralized tissues: enamel, dentin, and cementum. Tooth malformation, the most common craniofacial anomaly, arises from complex genetic and environmental factors affecting enamel structure, size, shape, and tooth eruption. Hyaluronic acid (HA), a primary extracellular matrix component, contributes to structural and physiological functions in periodontal tissue. Transmembrane protein 2 (TMEM2), a novel cell surface hyaluronidase, has been shown to play a critical role during embryogenesis. In this study, we demonstrate Tmem2 messenger RNA expression in inner enamel epithelium and presecretory, secretory, and mature ameloblasts. Tmem2 knock-in reporter mice reveal TMEM2 protein localization at the apical and basal ends of secretory ameloblasts. Micro-computed tomography analysis of epithelial-specific Tmem2 conditional knockout (Tmem2-CKO) mice shows a significant reduction in enamel layer thickness and severe enamel deficiency. Enamel matrix protein expression was remarkably downregulated in Tmem2-CKO mice. Scanning electron microscopy of enamel from Tmem2-CKO mice revealed an irregular enamel prism structure, while the microhardness and density of enamel were significantly reduced, indicating impaired ameloblast differentiation and enamel matrix mineralization. Histological evaluation indicated weak adhesion between cells and the basement membrane in Tmem2-CKO mice. The reduced and irregular expressions of vinculin and integrin β1 suggest that Tmem2 deficiency attenuated focal adhesion formation. In addition, abnormal HA accumulation in the ameloblast layer and weak claudin 1 immunoreactivity in Tmem2-CKO mice indicate impaired tight junction gate function. Irregular actin filament assembly was also observed at the apical and basal ends of secretory ameloblasts. Last, we demonstrated that Tmem2-deficient mHAT9d mouse ameloblasts exhibit defective adhesion to HA-containing substrates in vitro. Collectively, our data highlight the importance of TMEM2 in adhesion to HA-rich extracellular matrix, cell-to-cell adhesion, ameloblast differentiation, and enamel matrix mineralization.

Hyaluronic Acid in Rheumatology

Hyaluronic acid (HA), also known as hyaluronan, is an anionic glycosaminoglycan widely distributed throughout various tissues of the human body. It stands out from other glycosaminoglycans as it lacks sulfation and can attain considerable size: the average human synovial HA molecule weighs about 7 million Dalton (Da), equivalent to roughly 20,000 disaccharide monomers; although some sources report a lower range of 3-4 million Da. In recent years, HA has garnered significant attention in the field of rheumatology due to its involvement in joint lubrication, cartilage maintenance, and modulation of inflammatory and/or immune responses. This review aims to provide a comprehensive overview of HA's involvement in rheumatology, covering its physiology, pharmacology, therapeutic applications, and potential future directions for enhancing patient outcomes. Nevertheless, the use of HA therapy in rheumatology remains controversial with conflicting evidence regarding its efficacy and safety. In conclusion, HA represents a promising therapeutic option to improve joint function and alleviate inflammation and pain.

TMEM2 induces epithelial-mesenchymal transition and promotes resistance to temozolomide in GBM cells

Glioblastoma multiforme (GBM) is the most common intracranial malignant tumor and is notorious for its poor prognosis. An important element in the short overall survival of GBM patients is the lack of understanding the pathogenesis and progression of tumor and deficiency biomarkers that can be used for early diagnosis and therapeutic sensitivity monitoring. Studies have shown that transmembrane protein 2 (TMEM2) is participated in tumorigenesis of various human tumors, including rectal and breast cancers. Although Qiuyi Jiang et al. have reported that TMEM2 combined with IDH1/2 and 1p19q can predict the survival time of glioma patients based on bioinformatics, its expression and biological role of glioma remain unclear. In our study, we investigated the effect of TMEM2 expression level on glioma malignancy in public datasets and an independent internal dataset. We revealed TEMM2 expression was higher in GBM tissues than in non-tumor brain tissues (NBT). Moreover, the increase in TMEM2 expression level was closely related to tumor malignancy. The survival analysis showed that TMEM2 high expression reduces survival time in all glioma patients, including GBM and LGG patients. Subsequent experiments demonstrated that knockdown TMEM2 inhibited proliferation of GBM cells. In addition, we analyzed TMEM2 mRNA levels in different GBM subtypes, and demonstrated that TMEM2 expression was upregulated in mesenchymal subtype. Meanwhile, bioinformatics analysis and transwell assay indicated that knockdown TMEM2 suppressed epithelial-mesenchymal transition (EMT) in GBM. Importantly, Kaplan-Meier analysis demonstrated that TMEM2 high expression reduced the treatment response to TMZ in GBM patients. Knockdown of TMEM2 alone did not reduce apoptosis GBM cells, but significant apoptotic cells were observed in the group treated with additional TMZ. These studies may contribute to improving the accuracy of early diagnosis and evaluating the effectiveness of TMZ treatment in GBM patients.

Human TMEM2 is not a catalytic hyaluronidase, but a regulator of hyaluronan metabolism via HYBID (KIAA1199/CEMIP) and HAS2 expression

Cutaneous hyaluronan (HA) is depolymerized to intermediate sizes in the extracellular matrix, and further fragmented in the regional lymph nodes. Previously, we showed that the HA-binding protein involved in HA depolymerization (HYBID), also known as KIAA1199/CEMIP, is responsible for the first step of HA depolymerization. Recently, mouse transmembrane 2 (mTMEM2) with high structural similarity to HYBID was proposed to be a membrane-bound hyaluronidase. However, we showed that knockdown of human TMEM2 (hTMEM2) conversely promoted HA depolymerization in normal human dermal fibroblasts (NHDFs). Therefore, we examined the HA-degrading activity and function of hTMEM2 using HEK293T cells. We found that human HYBID and mTMEM2, but not hTMEM2, degraded extracellular HA, indicating that hTMEM2 does not function as a catalytic hyaluronidase. Analysis of the HA-degrading activity of chimeric TMEM2 in HEK293T cells suggested the importance of the mouse GG domain. Therefore, we focused on the amino acid residues that are conserved in active mouse and human HYBID and mTMEM2, but are substituted in hTMEM2. The HA-degrading activity of mTMEM2 was abolished when its His248 and Ala303 were simultaneously replaced by the corresponding residues of inactive hTMEM2 (Asn248 and Phe303). In NHDFs, enhancement of hTMEM2 expression by proinflammatory cytokines decreased HYBID expression and increased hyaluronan synthase 2 (HAS2)-dependent HA production. The effects of proinflammatory cytokines were abrogated by hTMEM2 knockdown. Moreover, a decreased HYBID expression by interleukin-1β and transforming growth factor-β was canceled by hTMEM2 knockdown. In conclusion, these results indicate that hTMEM2 is not a catalytic hyaluronidase, but a regulator of HA metabolism.

Structure of the transmembrane protein 2 (TMEM2) ectodomain and its apparent lack of hyaluronidase activity

Background: Hyaluronic acid (HA) is a major polysaccharide component of the extracellular matrix. HA has essential functions in tissue architecture and the regulation of cell behaviour. HA turnover needs to be finely balanced. Increased HA degradation is associated with cancer, inflammation, and other pathological situations. Transmembrane protein 2 (TMEM2) is a cell surface protein that has been reported to degrade HA into ~5 kDa fragments and play an essential role in systemic HA turnover. Methods: We produced the soluble TMEM2 ectodomain (residues 106-1383; sTMEM2) in human embryonic kidney cells (HEK293) and determined its structure using X-ray crystallography. We tested sTMEM2 hyaluronidase activity using fluorescently labelled HA and size fractionation of reaction products. We tested HA binding in solution and using a glycan microarray. Results: Our crystal structure of sTMEM2 confirms a remarkably accurate prediction by AlphaFold. sTMEM2 contains a parallel β-helix typical of other polysaccharide-degrading enzymes, but an active site cannot be assigned with confidence. A lectin-like domain is inserted into the β-helix and predicted to be functional in carbohydrate binding. A second lectin-like domain at the C-terminus is unlikely to bind carbohydrates. We did not observe HA binding in two assay formats, suggesting a modest affinity at best. Unexpectedly, we were unable to observe any HA degradation by sTMEM2. Our negative results set an upper limit for k _cat of approximately 10 ^-5 min ^-1. Conclusions: Although sTMEM2 contains domain types consistent with its suggested role in TMEM2 degradation, its hyaluronidase activity was undetectable. HA degradation by TMEM2 may require additional proteins and/or localisation at the cell surface.

Hyaluronan in liver fibrosis: basic mechanisms, clinical implications, and therapeutic targets

Hyaluronan (HA), also known as hyaluronic acid, is a glycosaminoglycan that is a critical component of the extracellular matrix (ECM). Production and deposition of ECM is a wound-healing response that occurs during chronic liver disease, such as cirrhosis. ECM production is a sign of the disease progression of fibrosis. Indeed, the accumulation of HA in the liver and elevated serum HA levels are used as biomarkers of cirrhosis. However, recent studies also suggest that the ECM, and HA in particular, as a functional signaling molecule, facilitates disease progression and regulation. The systemic and local levels of HA are regulated by de novo synthesis, cleavage, endocytosis, and degradation of HA, and the molecular mass of HA influences its pathophysiological effects. However, the regulatory mechanisms of HA synthesis and catabolism and the functional role of HA are still poorly understood in liver fibrosis. This review summarizes the role of HA in liver fibrosis at molecular levels as well as its clinical implications and discusses the potential therapeutic uses of targeting HA in liver fibrosis.

The role of hyaluronan in renal cell carcinoma

Renal cell carcinoma (RCC) is associated with high mortality rates worldwide and survival among RCC patients has not improved significantly in the past few years. A better understanding of the pathogenesis of RCC can enable the development of more effective therapeutic strategies against RCC. Hyaluronan (HA) is a glycosaminoglycan located in the extracellular matrix (ECM) that has several roles in biology, medicine, and physiological processes, such as tissue homeostasis and angiogenesis. Dysregulated HA and its receptors play important roles in fundamental cellular and molecular biology processes such as cell signaling, immune modulation, tumor progression and angiogenesis. There is emerging evidence that alterations in the production of HA regulate RCC development, thereby acting as important biomarkers as well as specific therapeutic targets. Therefore, targeting HA or combining it with other therapies are promising therapeutic strategies. In this Review, we summarize the available data on the role of abnormal regulation of HA and speculate on its potential as a therapeutic target against RCC.

Structure of the transmembrane protein 2 (TMEM2) ectodomain and its lack of hyaluronidase activity

Background: Hyaluronic acid (HA) is a major polysaccharide component of the extracellular matrix. HA has essential functions in tissue architecture and the regulation of cell behaviour. HA turnover needs to be finely balanced. Increased HA degradation is associated with cancer, inflammation, and other pathological situations. Transmembrane protein 2 (TMEM2) is a cell surface protein that has been reported to degrade HA into ~5 kDa fragments and play an essential role in systemic HA turnover. Methods: We produced the soluble TMEM2 ectodomain (residues 106-1383; sTMEM2) in human embryonic kidney cells (HEK293) and determined its structure using X-ray crystallography. We tested sTMEM2 hyaluronidase activity using fluorescently labelled HA and size fractionation of reaction products. Results: Our crystal structure of sTMEM2 confirms a remarkably accurate prediction by AlphaFold. sTMEM2 contains a parallel β-helix typical of other polysaccharide-degrading enzymes, but an active site cannot be assigned with confidence. A lectin-like domain is inserted into the β-helix and predicted to be functional in carbohydrate binding. A second lectin-like domain at the C-terminus is unlikely to bind carbohydrates. Unexpectedly, we were unable to observe any HA degradation by sTMEM2. Our negative results set an upper limit for    kcat of approximately 10-5 min-1. Conclusions: Although sTMEM2 contains domain types consistent with its suggested role in TMEM2 degradation, its hyaluronidase activity was undetectable. HA degradation by TMEM2 may require additional proteins and/or localisation at the cell surface.

Hyaluronan in Adipose Tissue, Metabolic Inflammation, and Diabetes: Innocent Bystander or Guilty Party?

Hyaluronic acid, or hyaluronan (HA), is a nonsulfated glucosaminoglycan that has long been recognized for its hydrophilic properties and is widely used as a dermal filler. Despite much attention given to the study of other extracellular matrix (ECM) components, in the field of ECM properties and their contribution to tissue fibroinflammation, little is known of HA's potential role in the extracellular milieu. However, recent studies suggest that it is involved in inflammatory response, diet-induced insulin resistance, adipogenesis, and autoimmunity in type 1 diabetes. Based on its unique physical property as a regulator of osmotic pressure, we emphasize underestimated implications in adipose tissue function, adipogenesis, and obesity-related dysfunction.

Aggrecan and Hyaluronan: The Infamous Cartilage Polyelectrolytes - Then and Now

Cartilages are unique in the family of connective tissues in that they contain a high concentration of the glycosaminoglycans, chondroitin sulfate and keratan sulfate attached to the core protein of the proteoglycan, aggrecan. Multiple aggrecan molecules are organized in the extracellular matrix via a domain-specific molecular interaction with hyaluronan and a link protein, and these high molecular weight aggregates are immobilized within the collagen and glycoprotein network. The high negative charge density of glycosaminoglycans provides hydrophilicity, high osmotic swelling pressure and conformational flexibility, which together function to absorb fluctuations in biomechanical stresses on cartilage during movement of an articular joint. We have summarized information on the history and current knowledge obtained by biochemical and genetic approaches, on cell-mediated regulation of aggrecan metabolism and its role in skeletal development, growth as well as during the development of joint disease. In addition, we describe the pathways for hyaluronan metabolism, with particular focus on the role as a "metabolic rheostat" during chondrocyte responses in cartilage remodeling in growth and disease.Future advances in effective therapeutic targeting of cartilage loss during osteoarthritic diseases of the joint as an organ as well as in cartilage tissue engineering would benefit from 'big data' approaches and bioinformatics, to uncover novel feed-forward and feed-back mechanisms for regulating transcription and translation of genes and their integration into cell-specific pathways.

Glucocorticoids Directly Affect Hyaluronan Production of Orbital Fibroblasts; A Potential Pleiotropic Effect in Graves' Orbitopathy

Orbital connective tissue expansion is a hallmark of Graves’ orbitopathy (GO). In moderate-to-severe active GO, glucocorticoids (GC) are the first line of treatment. Here we show that hydrocortisone (HC), prednisolone (P), methylprednisolone (MP), and dexamethasone (DEX) inhibit the hyaluronan (HA) production of orbital (OF) and dermal (DF) fibroblasts. HA production of GO OFs (n = 4), NON-GO OFs (n = 4) and DFs (n = 4) was measured by ELISA. mRNA expression of enzymes of HA metabolism and fibroblast proliferation was examined by RT-PCR and BrdU incorporation, respectively. After 24 h of GC treatment (1µM) HA production decreased by an average of 67.9 ± 3.11% (p < 0.0001) in all cell cultures. HAS2, HAS3 and HYAL1 expression in OFs also decreased (p = 0.009, p = 0.0005 and p = 0.015, respectively). Ten ng/mL PDGF-BB increased HA production and fibroblast proliferation in all cell lines (p < 0.0001); GC treatment remained effective and reduced HA production under PDGF-BB-stimulated conditions (p < 0.0001). MP and DEX reduced (p < 0.001, p = 0.002, respectively) PDGF-BB-induced HAS2 expression in OFs. MP and DEX treatment decreased PDGF-BB stimulated HAS3 expression (p = 0.035 and p = 0.029, respectively). None of the GCs tested reduced the PDGF-BB stimulated proliferation rate. Our results confirm that GCs directly reduce the HA production of OFs, which may contribute to the beneficial effect of GCs in GO.

Deregulated hyaluronan metabolism in the tumor microenvironment drives cancer inflammation and tumor-associated immune suppression

Hyaluronan (HA) is known to be a prominent component of the extracellular matrix in tumors, and many solid cancers are characterized by aberrant HA metabolism resulting in increased production in tumor tissue. HA has been implicated in regulating a variety of cellular functions in tumor cells and tumor-associated stromal cells, suggesting that altered HA metabolism can influence tumor growth and malignancy at multiple levels. Importantly, increased HA production in cancer is associated with enhanced HA degradation due to high levels of expression and activity of hyaluronidases (Hyal). Understanding the complex molecular and cellular mechanisms involved in abnormal HA metabolism and catabolism in solid cancers could have important implications for the design of future cancer therapeutic approaches. It appears that extensive crosstalk between immune cells and HA-enriched stroma contributes to tumor growth and progression in several ways. Specifically, the interaction of tumor-recruited Hyal2-expressing myeloid-derived suppressor cells (MDSCs) of bone marrow origin with HA-producing cancer-associated fibroblasts and epithelial tumor cells results in enhanced HA degradation and accumulation of small pro-inflammatory HA fragments, which further drives cancer-related inflammation. In addition, hyaluronan-enriched stroma supports the transition of tumor-recruited Hyal2⁺MDSCs to the PD-L1⁺ tumor-associated macrophages leading to the formation of an immunosuppressive and tolerogenic tumor microenvironment. In this review, we aim to discuss the contribution of tumor-associated HA to cancer inflammation, angiogenesis, and tumor-associated immune suppression. We also highlight the recent findings related to the enhanced HA degradation in the tumor microenvironment.

Endothelial Glycocalyx Degradation in Critical Illness and Injury

The endothelial glycocalyx is a gel-like layer on the luminal side of blood vessels that is composed of glycosaminoglycans and the proteins that tether them to the plasma membrane. Interest in its properties and function has grown, particularly in the last decade, as its importance to endothelial barrier function has come to light. Endothelial glycocalyx studies have revealed that many critical illnesses result in its degradation or removal, contributing to endothelial dysfunction and barrier break-down. Loss of the endothelial glycocalyx facilitates the direct access of immune cells and deleterious agents (e.g., proteases and reactive oxygen species) to the endothelium, that can then further endothelial cell injury and dysfunction leading to complications such as edema, and thrombosis. Here, we briefly describe the endothelial glycocalyx and the primary components thought to be directly responsible for its degradation. We review recent literature relevant to glycocalyx damage in several critical illnesses (sepsis, COVID-19, trauma and diabetes) that share inflammation as a common denominator with actions by several common agents (hyaluronidases, proteases, reactive oxygen species, etc.). Finally, we briefly cover strategies and therapies that show promise in protecting or helping to rebuild the endothelial glycocalyx such as steroids, protease inhibitors, anticoagulants and resuscitation strategies.

ROLE OF HYALURONAN IN PATHOPHYSIOLOGY OF VASCULAR1 ENDOTHELIAL AND SMOOTH MUSCLE CELLS

One of the main components of the extracellular matrix (ECM) of the blood vessel is hyaluronic acid or hyaluronan (HA). It is a ubiquitous polysaccharide belonging to the family of glycosaminoglycans, but, differently from other proteoglycan-associated glycosaminoglycans, it is synthesized on the plasma membrane by a family of three HA synthases (HAS). HA can be released as a free polymer in the extracellular space or remain associated with the membrane in the pericellular space via HAS or via binding proteins. In fact, several cell surface proteins can interact with HA working as HA receptors like CD44, RHAMM, and LYVE-1. In physiological conditions, HA is localized in the glycocalyx and in the adventitia and is responsible for the loose and hydrated vascular structure favoring flexibility and allowing the stretching of vessels in response to mechanical forces. During atherogenesis, ECM undergoes dramatic alterations which have a crucial role in lipoprotein retention and in triggering multiple signaling cascades that wake up cells from their quiescent status. HA becomes highly present in the media and neointima favoring smooth muscle cells dedifferentiation, migration, and proliferation that strongly contribute to vessel wall thickening. Further, HA is able to modulate immune cell recruitment both within the vessel wall and on the endothelial cell layer. This review is focused on the effects of HA on vascular cell behavior.

The Impact of Hyaluronan on Tumor Progression in Cutaneous Melanoma

The incidence of cutaneous melanoma is rapidly increasing worldwide. Cutaneous melanoma is an aggressive type of skin cancer, which originates from malignant transformation of pigment producing melanocytes. The main risk factor for melanoma is ultraviolet (UV) radiation, and thus it often arises from highly sun-exposed skin areas and is characterized by a high mutational burden. In addition to melanoma-associated mutations such as BRAF, NRAS, PTEN and cell cycle regulators, the expansion of melanoma is affected by the extracellular matrix surrounding the tumor together with immune cells. In the early phases of the disease, hyaluronan is the major matrix component in cutaneous melanoma microenvironment. It is a high-molecular weight polysaccharide involved in several physiological and pathological processes. Hyaluronan is involved in the inflammatory reactions associated with UV radiation but its role in melanomagenesis is still unclear. Although abundant hyaluronan surrounds epidermal and dermal cells in normal skin and benign nevi, its content is further elevated in dysplastic lesions and local tumors. At this stage hyaluronan matrix may act as a protective barrier against melanoma progression, or alternatively against immune cell attack. While in advanced melanoma, the content of hyaluronan decreases due to altered synthesis and degradation, and this correlates with poor prognosis. This review focuses on hyaluronan matrix in cutaneous melanoma and how the changes in hyaluronan metabolism affect the progression of melanoma.

Hyaluronan: A Neuroimmune Modulator in the Microbiota-Gut Axis

The commensal microbiota plays a fundamental role in maintaining host gut homeostasis by controlling several metabolic, neuronal and immune functions. Conversely, changes in the gut microenvironment may alter the saprophytic microbial community and function, hampering the positive relationship with the host. In this bidirectional interplay between the gut microbiota and the host, hyaluronan (HA), an unbranched glycosaminoglycan component of the extracellular matrix, has a multifaceted role. HA is fundamental for bacterial metabolism and influences bacterial adhesiveness to the mucosal layer and diffusion across the epithelial barrier. In the host, HA may be produced and distributed in different cellular components within the gut microenvironment, playing a role in the modulation of immune and neuronal responses. This review covers the more recent studies highlighting the relevance of HA as a putative modulator of the communication between luminal bacteria and the host gut neuro-immune axis both in health and disease conditions, such as inflammatory bowel disease and ischemia/reperfusion injury.

Endogenously-Produced Hyaluronan and Its Potential to Regulate the Development of Peritoneal Adhesions

Formation of peritoneal adhesions (PA) is one of the major complications following intra-abdominal surgery. It is primarily caused by activation of the mesothelial layer and underlying tissues in the peritoneal membrane resulting in the transition of mesothelial cells (MCs) and fibroblasts to a pro-fibrotic phenotype. Pro-fibrotic transition of MCs—mesothelial-to-mesenchymal transition (MMT), and fibroblasts activation to myofibroblasts are interconnected to changes in cellular metabolism and culminate in the deposition of extracellular matrix (ECM) in the form of fibrotic tissue between injured sides in the abdominal cavity. However, ECM is not only a mechanical scaffold of the newly synthetized tissue but reciprocally affects fibrosis development. Hyaluronan (HA), an important component of ECM, is a non-sulfated glycosaminoglycan consisting of N-acetyl-D-glucosamine (GlcNAc) and D-glucuronic acid (GlcUA) that can affect the majority of processes involved in PA formation. This review considers the role of endogenously produced HA in the context of different fibrosis-related pathologies and its overlap in the development of PA.