Changes in cell characteristics due to retinoic acid; specifically, a decrease in the expression of claudin-1 and increase in claudin-4 within tight junctions in stratified oral keratinocytes

Claudin-4 Upregulation in Acantholytic and Autoimmune-Mediated Bullous Disorders

Claudin-4 is a key component of tight junctions, which play an important role in the formation of the epidermal barrier by forming a circumferential network in the granular layer that serves as a gatekeeper of the paracellular pathway. The aim of this study is to illustrate claudin-4 immunohistochemical staining patterns of different blistering disorders. We collected 35 cases, including two Hailey-Hailey disease, one Darier disease, three Grover disease, one acantholytic acanthoma, two warty dyskeratoma, 11 pemphigus vulgaris (PV) including six mucosal PV, and two pemphigus foliaceus. For comparison, we included five cases of normal skin, five eczema, and three bullous pemphigoid cases. Claudin-4 demonstrated weak-to-moderate expression in keratinocytes located in the stratum granulosum, keratinocytes surrounding hair follicles, and adnexal glands. Further, claudin-4 exhibited moderate-to-strong membranous staining in disrupted keratinocytes surrounding and within the acantholytic and bullous areas in 16/22 of the acantholytic cases (not seen in the six cases of mucosal PV) and all three bullous pemphigoids. This finding suggests that claudin-4 is upregulated in these conditions, which may be a compensatory response to the disrupted barrier function. This finding could shed light on the molecular mechanisms underlying disrupted barrier function in blistering disorders, independent of the specific underlying disease mechanism.

Micronutrient Improvement of Epithelial Barrier Function in Various Disease States: A Case for Adjuvant Therapy

The published literature makes a very strong case that a wide range of disease morbidity associates with and may in part be due to epithelial barrier leak. An equally large body of published literature substantiates that a diverse group of micronutrients can reduce barrier leak across a wide array of epithelial tissue types, stemming from both cell culture as well as animal and human tissue models. Conversely, micronutrient deficiencies can exacerbate both barrier leak and morbidity. Focusing on zinc, Vitamin A and Vitamin D, this review shows that at concentrations above RDA levels but well below toxicity limits, these micronutrients can induce cell- and tissue-specific molecular-level changes in tight junctional complexes (and by other mechanisms) that reduce barrier leak. An opportunity now exists in critical care—but also medical prophylactic and therapeutic care in general—to consider implementation of select micronutrients at elevated dosages as adjuvant therapeutics in a variety of disease management. This consideration is particularly pointed amidst the COVID-19 pandemic.

Inhibition of Retinoid X Receptor Improved the Morphology, Localization of Desmosomal Proteins, and Paracellular Permeability in Three-Dimensional Cultures of Mouse Keratinocytes

Retinoic acid (RA) plays an important role in epithelial homeostasis and influences the morphology, proliferation, differentiation and permeability of epithelial cells. Mouse keratinocytes, K38, reconstituted non-keratinized stratified epithelium in three-dimensional (3D) cultures with serum, which contains retinol (a source of RA), but the morphology was different from in vivo epithelium. The formed epithelium was thick, with loosened cell–cell contacts. Here, we investigated whether the inhibition of RA receptor (RAR)/retinoid X receptor (RXR)-mediated signaling by an RXR antagonist, HX 531, improved K38 3D cultures in terms of morphology and intercellular junctions. The epithelium formed by 0.5 μM HX531 was thin, and the intercellular space was narrowed because of the restoration of the layer-specific distribution of desmoglein (DSG)-1, DSG3 and plakoglobin (PG). Moreover, the levels of desmosomal proteins and tight junction proteins, including DSG1, DSG2, DSG3, PG, claudin (CLDN)-1 and CLDN4 increased, but the adherens junction protein, E-cadherin, did not show any change. Furthermore, CLDN1 was recruited to occludin-positive cell–cell contacts in the superficial cells and transepithelial electrical resistance was increased. Therefore, K38 3D cultures treated with 0.5 μM HX531 provides a useful in vitro model to study intercellular junctions in the non-keratinized epithelium.

Increased integrity of cell-cell junctions accompanied by increased expression of claudin 4 in keratinocytes stimulated with vitamin D3

The stratified squamous epithelium has a multilayer structure formed by the differentiation of the keratinized epithelium, which covers the skin and oral mucosa. The epithelium plays a central role in regulating the interactions between the immune system and pathogens. The tight junction (TJ) barrier, which is composed of adhesion molecules called claudins (CLDN), is critical for the homeostasis of the skin and oral mucosa. Furthermore, the crucial roles of vitamin D3 (VD3) in the pathogeneses of skin and oral mucosal disease have been suggested. The aim of this in vitro study was to observe the correlations between the integrity of the keratinocyte population and the expression levels of CLDN1 and CLDN4 in gingival epithelial cells, stimulated with VD3. CLDN 1 and 4 expression levels were down and upregulated, respectively, in the cells stimulated with VD3. Additionally, transepithelial electrical resistance (TEER) levels were increased in the stimulated cells when compared to the controls. These findings indicate that CLDN 4 may play a more important role in the TJ barrier than CLDN 1. Hence, the therapeutic effect of VD3 in skin and oral diseases may be regulated by the increase in the expression of CLDN 4.

Cutibacterium acnes regulates the epidermal barrier properties of HPV-KER human immortalized keratinocyte cultures

Our skin provides a physical barrier to separate the internal part of our body from the environment. Maintenance of complex barrier functions is achieved through anatomical structures in the skin, the stratified squamous epithelium specialized junctional organelles, called tight junctions (TJs). Several members of our microbial communities are known to affect the differentiation state and function of the colonized organ. Whether and how interactions between skin cells and cutaneous microbes, including Cutibacterium acnes (C. acnes), modify the structure and/or function of our skin is currently only partly understood. Thus, in our studies, we investigated whether C. acnes may affect the epidermal barrier using in vitro model systems. Real-time cellular analysis showed that depending on the keratinocyte differentiation state, the applied C. acnes strains and their dose, the measured impedance values change, together with the expression of selected TJ proteins. These may reflect barrier alterations, which can be partially restored upon antibiotic–antimycotic treatment. Our findings suggest that C. acnes can actively modify the barrier properties of cultured keratinocytes, possibly through alteration of tight cell-to-cell contacts. Similar events may play important roles in our skin, in the maintenance of cutaneous homeostasis.

IL-1β strengthens the physical barrier in gingival epithelial cells

Periodontitis is one of the most common oral diseases worldwide and is caused by a variety of interactions between oral bacteria and the host. Here, pathogens induce inflammatory host responses that cause the secretion of proinflammatory cytokines such as IL-1β, IL-6, and IL-8 by oral epithelial cells. In various systems, it has been shown that inflammation compromises physical barriers, which enables bacteria to invade the tissue. In this study, we investigated the barrier properties of the oral mucosa under physiological and inflamed conditions. For this purpose, we assessed the influence of IL-1β on the transepithelial electrical resistance and in particular on tight junctions in vitro in human stratified squamous epithelium models. Indirect immunofluorescence and western blot analyses were performed to investigate localization and expression of tight junction proteins in primary gingival cells, immortalized gingival cells and native gingiva. Furthermore, the TEER of gingival keratinocytes was assessed. The results showed that IL-1β led to strengthening of the gingival keratinocyte barrier. This was demonstrated by an increase in TEER, the upregulation of TJ proteins, and an increase in the formation of TJ strands. The IL-1β-mediated upregulation of occludin was prevented by the NF-κB inhibitor BAY 11-7085. These observations provide insights into host responses in the early stages of periodontal disease and offer information about TJ formation in human gingival epithelial cells under physiological and inflammatory conditions. Comprehensive knowledge of the physical barrier during inflammation may help in developing strategies to effectively target the inflammatory barrier to improve the bioavailability of drugs for the treatment of periodontitis.

The reduced susceptibility of mouse keratinocytes to retinoic acid may be involved in the keratinization of oral and esophageal mucosal epithelium

Keratinocytes take up serum-derived retinol (vitamin A) and metabolize it to all-trans-retinoic acid (atRA), which binds to the nuclear retinoic acid receptor (RAR). We previously reported that serum-affected keratinocyte differentiation and function; namely, it inhibited keratinization, decreased loricrin (LOR) and claudin (CLDN) 1 expression, increased keratin (K) 4 and CLDN4 levels, and reduced paracellular permeability in three-dimensional (3D) cultures of mouse keratinocytes (COCA). Contrarily, RAR inhibition reversed these changes. Here, we aimed to examine whether atRA exerted the same effects as serum, and whether it was involved in the differential oral mucosa keratinization among animal species. Porcine oral mucosal keratinocytes, which form non-keratinized epithelium in vivo, established keratinized epithelium in 3D cultures. Both mouse and porcine sera induced non-keratinized epithelium at 0.1% in COCA 3D cultures. Although atRA caused the same changes as serum, its effective concentration differed. atRA inhibited keratinization at 0.1 nM and 1 nM in porcine or human keratinocytes and COCA, respectively. Furthermore, atRA upregulated CLDN7 in the cytoplasm but not in cell-cell contacts. These atRA-induced changes were reverted by RAR inhibition. The results indicate that serum-induced changes are probably due to the effect of serum-derived atRA, and that mouse keratinocytes require higher atRA concentrations to suppress keratinization than porcine and human keratinocytes. We propose that the lower susceptibility of mouse keratinocytes to atRA, rather than a lower retinol concentration, is a possible reason for the keratinization of mouse oral mucosal epithelium.

All‑trans retinoic acid alters the expression of the tight junction proteins Claudin‑1 and ‑4 and epidermal barrier function‑associated genes in the epidermis

All-trans retinoic acid (ATRA) regulates skin cell proliferation and differentiation. ATRA is widely used in the treatment of skin diseases, but results in irritation, dryness and peeling, possibly due to an impaired skin barrier, although the exact mechanisms are unclear. The present study established an ATRA-associated dermatitis mouse model (n=32) in order to examine the molecular mechanisms of skin barrier impairment by ATRA. Changes in epidermal morphology and structure were observed using histological examination and transmission electron microscopy (TEM). Gene expression was analyzed by microarray chip assay. Histology and TEM demonstrated pronounced epidermal hyperproliferation and parakeratosis upon ATRA application. The stratum corneum layer displayed abnormal lipid droplets and cell-cell junctions, suggesting alterations in lipid metabolism and dysfunctional cell junctions. Gene expression profiling revealed that factors associated with epidermal barrier function were differentially expressed by ATRA, including those associated with tight junctions (TJs), cornified envelopes, lipids, proteases, protease inhibitors and transcription factors. In the mouse epidermis, Claudin-1 and -4 are proteins involved in TJs and have key roles in epidermal barrier function. ATRA reduced the expression and altered the localization of Claudin-1 in HaCaT immortalized keratinocytes and the mouse epidermis, which likely leads to the disruption of the epidermal barrier. By contrast, Claudin-4 was upregulated in HaCaT cells and the mouse epidermis following treatment with ATRA. In conclusion, ATRA exerts a dual effect on epidermal barrier genes: It downregulates the expression of Claudin-1 and upregulates the expression of Claudin-4. Claudin-4 upregulation may be a compensatory response for the disrupted barrier function caused by Claudin-1 downregulation.

Cell culture models of oral mucosal barriers: A review with a focus on applications, culture conditions and barrier properties

Understanding the function of oral mucosal epithelial barriers is essential for a plethora of research fields such as tumor biology, inflammation and infection diseases, microbiomics, pharmacology, drug delivery, dental and biomarker research. The barrier properties are comprised by a physical, a transport and a metabolic barrier, and all these barrier components play pivotal roles in the communication between saliva and blood. The sum of all epithelia of the oral cavity and salivary glands is defined as the blood-saliva barrier. The functionality of the barrier is regulated by its microenvironment and often altered during diseases. A huge array of cell culture models have been developed to mimic specific parts of the blood-saliva barrier, but no ultimate standard in vitro models have been established. This review provides a comprehensive overview about developed in vitro models of oral mucosal barriers, their applications, various cultivation protocols and corresponding barrier properties.

Retinoic Acid, Leaky Gut, and Autoimmune Diseases

A leaky gut has been observed in a number of autoimmune diseases including type 1 diabetes, multiple sclerosis, inflammatory bowel disease, and systemic lupus erythematosus. Previous studies from our laboratory have shown that lupus mice also bear a leaky gut and that the intestinal barrier function can be enhanced by gut colonization of probiotics such as Lactobacillus spp. Retinoic acid (RA) can increase the relative abundance of Lactobacillus spp. in the gut. Interestingly, RA has also been shown to strengthen the barrier function of epithelial cells in vitro and in the absence of probiotic bacteria. These reports bring up an interesting question of whether RA exerts protective effects on the intestinal barrier directly or through regulating the microbiota colonization. In this review, we will discuss the roles of RA in immunomodulation, recent literature on the involvement of a leaky gut in different autoimmune diseases, and how RA shapes the outcomes of these diseases.

Improvement of Human-Oral-Epithelial-Barrier Function and of Tight Junctions by Micronutrients

The oral epithelium represents a major interface between an organism and its external environment. Improving this barrier at the molecular level can provide an organism added protection from microbial-based diseases. Barrier function of the Gie-3B11-human-gingival-epithelial-cell-culture model is enhanced by the micronutrients zinc, quercetin, retinoic acid, and acetyl-11-keto-β-boswellic acid, as observed by a concentration-dependent increase in transepithelial electrical resistance and a decrease in transepithelial ¹⁴C-d-mannitol permeability. With this improvement of tight-junction (TJ)-barrier function (reduced leak) comes a pattern of micronutrient-induced changes in TJ claudin abundance that is specific to each individual micronutrient, along with changes in claudin subcellular localization. These micronutrients were effective not only when administered to both cell surfaces simultaneously but also when administered to the apical surface alone, the surface to which the micronutrients would be presented in routine clinical use. The biomedical implications of micronutrient enhancement of the oral-epithelial barrier are discussed.

Influence of retinoic acid on human gingival epithelial barriers

BACKGROUND AND OBJECTIVES

The gingival epithelium plays an important role in the protection of oral tissues from microbial challenge. Oral keratinocytes form a barrier and show various cellular contacts, including tight junctions (TJ). To analyse the barrier function in vitro the transepithelial electrical resistance (TER) is commonly used. Retinoic acid (RA) is an important signalling molecule in most tissues, including epithelial differentiation. RA signalling is mediated through three RA receptors. The aim of the study was to investigate the influence of RA on human gingival barriers in vitro.

MATERIAL AND METHODS

Immortalized human gingival keratinocytes were seeded on culture plate inserts. The effect of RA with and without infection with Porphyromonas gingivalis W83 on the barrier was analysed by TER measurements. The expression of TJ proteins was investigated by western blot.

RESULTS

During differentiation, mean TER increased from 16 (1 h), 43 (4 h) to 62 (6 h) Ohm × cm² . Addition of 15 μm RA increased TER by +19 after 1 h, +25 after 4 h and +16 Ohm × cm² after 6 h. The pan-RA receptor inhibitor BMS 493 resulted in TER values comparable to the control. The mean established TER of the control was approximately 110 Ohm × cm² . Addition of 15 μm RA elevated TER to 127 Ohm × cm² after 1 h, 150 Ohm × cm² after 4 h and 189 Ohm × cm² after 6 h (p ≤ 0.01). RA plus infection with P. gingivalis W83 further increased the TER increasing effect but could not prevent the destruction of TER induced by bacterial infection. The protein expression of the TJ proteins claudin 4 and occludin was enhanced while ZO-1 was downregulated after 1 h of RA incubation.

CONCLUSION

RA provides barrier-positive elements to the gingival epithelial cell model that is accompanied by altered expression of TJ proteins.

Alterations of intercellular junctions in peritoneal mesothelial cells from patients undergoing dialysis: effect of retinoic Acid

BACKGROUND

Dialysis patients are classified according to their peritoneal permeability as low transporter (LT, low solute permeability) or high transporter (HT, high solute permeability). Tight junction (TJ) proteins are critical to maintain ions, molecules and water paracellular transport through peritoneum. Exposure to peritoneal dialysis solutions causes damage to TJ in human peritoneal mesothelial cells (HPMCs). We analyzed the quantity, distribution and function of TJ proteins: claudin-1, -2 and -8, ZO-1 and occludin, in HPMC cultures from LT and HT patients. Since all-trans retinoic acid (ATRA) might modify the expression of TJ proteins, we studied its effect on HPMCs.

METHODS

Control HPMCs were isolated from human omentum, while HT or LT cells were obtained from dialysis effluents. Cells were cultured in presence of ATRA 0, 50 or 100 nM. Transepithelial electrical resistance (TER) measurement, immunostaining and Western blot analyses were performed.

RESULTS

HT exhibited lower TER than control and LT monolayers. Immunofluorescence for TJ was weak and discontinuous along the cell contour, in LT and HT. Furthermore, claudin-1, occludin and ZO-1 expressions were decreased. In all groups, claudin-2 was localized at nuclei. We observed that ATRA improved TJ distribution and increased TJ expression in HT. This retinoid did not modify claudin-2 and -8 expressions. All-trans retinoic acid decreased TER in HT, but had no effect in LT.

CONCLUSIONS

Tight junctions were altered in HPMCs from dialyzed patients. The HT monolayer has lower TER than LT, which might be associated with the peritoneal permeability in these patients. ATRA might be a therapeutic alternative to maintain mesothelial integrity, since it improved TJ localization and expression.

Retinoic acid promotes Sertoli cell differentiation and antagonises activin-induced proliferation

From puberty and throughout adult spermatogenesis, retinoid signalling is essential for germ cell differentiation and male fertility. The initiation of spermatogonial differentiation and germ cell meiosis occurs under the direction of local retinoid signalling in the testis, and corresponds with the final phase of somatic Sertoli cell differentiation at puberty. Here, we consider the cellular and molecular basis of retinoid actions upon Sertoli cell differentiation. Primary rat Sertoli cells were isolated during the pubertal proliferative and quiescent phases at postnatal days 10- and 20- respectively, and cultured with all-trans-retinoic acid. We show that retinoid signalling can potently suppress activin-induced proliferation by antagonising G1 phase progression and entry into the cell cycle. Retinoid signalling was also found to initiate tight junction formation in primary Sertoli cells, consistent with a pro-differentiative role. This study implicates retinoid signalling in the differentiation of both somatic and germ cells in the testis at puberty.

Distinct expression patterns and roles of aldehyde dehydrogenases in normal oral mucosa keratinocytes: differential inhibitory effects of a pharmacological inhibitor and RNAi-mediated knockdown on cellular phenotype and epithelial morphology

Aldehyde dehydrogenases (ALDHs), enzymes responsible for detoxification and retinoic acid biosynthesis, are considered a potent functional stem cell marker of normal and malignant cells in many tissues. To date, however, there are no available data on ALDH distributions and functions in oral mucosa. This study aims to clarify the levels and types of ALDH expression using immunohistochemistry with accompanying mRNA expression as well as an ALDEFLUOR assay, and to assess phenotypic and histological changes after manipulation of the ALDH activity of oral keratinocytes to increase the potency of a tissue-engineered oral mucosa by a specific ALDH inhibitor, diethylaminobenzaldehyde (DEAB), together with small interfering RNA of ALDH1A3 and ALDH3A1. Results showed the mRNA and cytoplasmic protein expression of ALDH1A3 and ALDH3A1 to be mostly localized in the upper suprabasal layer although no ALDH1A1 immunoreaction was detected throughout the epithelium. Oral keratinocytes with high ALDH activity exhibited a profile of differentiating cells. By pharmacological inhibition, the phenotypic analysis revealed the proliferating cell-population shifting to a more quiescent state compared with untreated cells. Furthermore, a well-structured epithelial layer showing a normal differentiation pattern and a decrease in Ki-67 immunopositive basal cells was developed by DEAB incubation, suggesting a slower turnover rate efficient to maintain undifferentiated cells. Histological findings of a regenerated oral epithelium by ALDH1A3 siRNA were similar to those when treated with DEAB while ALDH3A1 siRNA eradicated the epithelial regenerative capacity. These observations suggest the effects of phenotypic and morphological alterations by DEAB on oral keratinocytes are mainly consequent to the inhibition of ALDH1A3 activity.

Expression of claudins -2 and -4 and cingulin is coordinated with the start of stratification and differentiation in corneal epithelial cells: retinoic acid reversibly disrupts epithelial barrier

Although tight junctions (TJ) have been extensively studied in simple epithelial cells, it is still unknown whether their organization is coupled to cell differentiation in stratified epithelia. We studied the expression of TJ in RCE1(5T5) cells, an in vitro model which mimics the sequential steps of rabbit corneal epithelial differentiation. RCE1(5T5) cells expressed TJ components which were assembled once cells constituted differentiated epithelia, as suggested by the increase of transepithelial electrical resistance (TER) which followed a similar kinetic to the expression of the early differentiation marker Pax-6. TJ were functional as indicated by the establishment of an epithelial barrier nonpermeable to ruthenium red or a biotin tracer. In immunostaining experiments, TJ were located at the superficial cells from the suprabasal layers; Western blot and RT-PCR suggested that TJ were composed of claudins (cldn) -1, -2, -4, cingulin (cgn), occludin (ocln) and ZO-1. Semi-quantitative RT-PCR and TER measurements showed that TJ became organized when cells began to form a 3–5 layers stratified epithelium; TER increased once cells reached confluence, with a time course comparable to the raise in the expression of cgn, cldn-2 and -4. Nevertheless, cldn-1, -2, ZO-1 and ocln were present in the cells from the beginning of cultivation, suggesting that TER increases mainly depend on TJ assembly. While EGF increased epithelial barrier strength, retinoic acid disrupted it, increasing paracellular flux about 2-fold; this effect was concentration dependent and completely reversible. Our results suggest that TJ assembly is tightly linked to the expression of corneal epithelial terminal phenotype.

Expression of connexins and the effect of retinoic acid in oral keratinocytes

Differential expression of members of the connexin (Cx) gap junction multigene family permits formation of gap junctions with the varied physiological properties required by different tissues. The aim of this study was to characterize connexin expression and the influence of all-trans-retinoic acid (RA) in mouse gingival epithelial cells (GE1). The cells were treated with RA, and expression of Cxs was analyzed by immunofluorescence, reverse transcriptase-polymerase chain reaction (RT-PCR), and real-time PCR. RT-PCR revealed that GE1 cells expressed mRNA for Cx26, Cx30.3, Cx31.1, Cx32, and Cx43. In addition, real-time PCR revealed that RA significantly decreased expression of Cx31.1 as compared with control. These results indicate that GE1 cells are useful in analyzing the expression of connexin molecules in oral keratinocytes from oral mucosal lesions.