Cellular Distribution Pattern of tjp1 (ZO-1) in Xenopus laevis Oocytes Heterologously Expressing Claudins

Cinnamic Acid and Caffeic Acid Effects on Gastric Tight Junction Proteins Analyzed in Xenopus laevis Oocytes

Analysis of secondary plant compounds for the development of novel therapies is a common focus of experimental biomedicine. Currently, multiple health-supporting properties of plant-derived molecules are known but still information on many mechanisms is scarce. Cinnamic acid and caffeic acid are two of the most abundant polyphenols in human dietary fruits and vegetables. In this study, we investigated cinnamic acid and caffeic acid effects on the gastric barrier, which is primarily provided by members of the transmembrane tight junction protein family of claudins. The Xenopus laevis oocyte has been established, in recent years, as a heterologous expression system for analysis of transmembrane tight junction protein interactions, by performing paired oocyte experiments to identify an effect on protein-protein interactions, in vitro. In our current study, human gastric claudin-4, -5, and -18.2. were expressed and detected in the oocyte plasma membrane by freeze fracture electron microscopy and immunoblotting. Oocytes were paired and incubated with 100 µM or 200 µM cinnamic acid or caffeic acid, or Ringer's solution, respectively. Caffeic acid showed no effect on the contact area strength of paired oocytes but led to an increased contact area size. In contrast, cinnamic acid-incubated paired oocytes revealed a reduced contact area and a strengthening effect on the contact area was identified. These results may indicate that caffeic acid and cinnamic acid both show an effect on gastric barrier integrity via direct effects on tight junction proteins.

Cannabidiol Strengthening of Gastric Tight Junction Complexes Analyzed in an Improved Xenopus Oocyte Assay

Cannabidiol (CBD), the non-psychoactive compound derived from the cannabis plant, has gained attention in recent years as a remedy against gastrointestinal disorders ranging from nausea and inflammation to abdominal pain. Recent advances demonstrated an effect on inflammatory pathways and barrier proteins. However, information on possible direct effects is scarce and needs to be addressed, as applications are currently increasing in popularity. To accomplish this, we have employed Xenopus laevis oocytes as a heterologous expression system for analysis of the direct effects on stomach-specific claudins and further developed tight junction (TJ) protein interaction assays. Human claudin-4, claudin-5, and claudin-18.2 were expressed in Xenopus oocytes, clustered in pairs to form contact areas, and analyzed in a two-cell model approach, including measurement of the contact area and contact strength. CLDN4/5/18 + CLDN4/5/18 oocyte pairs were incubated with 20 µM CBD or with 40 µM CBD and were compared to cells without CBD treatment (ctrl). For interaction analysis, the contact area was measured after 24 h and 48 h. Whereas CBD did not affect the size of the protein interaction area, Double Orbital Challenge experiments revealed an increased contact strength after 24 h incubation with CBD. In addition, the Xenopus oocyte experiments were accompanied by an analysis of claudin-4, -5, and -18 expression in gastric epithelium by immunoblotting and immunohistochemistry. Claudin-4, -5, and -18 were strongly expressed, indicating a major role for gastric epithelial barrier function. In summary, our study shows direct effects of 40 µM CBD on Xenopus oocytes heterologously expressing a stomach-specific claudin combination, indicating a supportive and beneficial effect of CBD on gastric TJ proteins.

Extracellular vesicles in the breast cancer brain metastasis: physiological functions and clinical applications

Breast cancer, which exhibits an increasing incidence and high mortality rate among cancers, is predominantly attributed to metastatic malignancies. Brain metastasis, in particular, significantly contributes to the elevated mortality in breast cancer patients. Extracellular vesicles (EVs) are small lipid bilayer vesicles secreted by various cells that contain biomolecules such as nucleic acids and proteins. They deliver these bioactive molecules to recipient cells, thereby regulating signal transduction and protein expression levels. The relationship between breast cancer metastasis and EVs has been extensively investigated. In this review, we focus on the molecular mechanisms by which EVs promote brain metastasis in breast cancer. Additionally, we discuss the potential of EV-associated molecules as therapeutic targets and their relevance as early diagnostic markers for breast cancer brain metastasis.

Small extracellular vesicles in breast cancer brain metastasis and the prospect of clinical application

Extracellular vesicles (EVs) are nanoscale extracellular particles that have received widespread scientific attention for carrying a variety of biomolecules such as nucleic acids and proteins and participating in the process of intercellular information exchange, making them become a research hotspot due to their potential diagnostic value. Breast cancer is the leading cause of cancer-related death in women, approximately 90% of patient deaths are due to metastasis complications. Brain metastasis is an important cause of mortality in breast cancer patients, about 10-15% of breast cancer patients will develop brain metastasis. Therefore, early prevention of brain metastasis and the development of new treatments are crucial. Small EVs have been discovered to be involved in the entire process of breast cancer brain metastasis (BCBM), playing an important role in driving organ-specific metastasis, forming pre-metastatic niches, disrupting the blood-brain barrier, and promoting metastatic tumor cell proliferation. We summarize the mechanisms of small EVs in the aforementioned pathological processes at the cellular and molecular levels, and anticipate their potential applications in the treatment of breast cancer brain metastasis, with the hope of providing new ideas for the precise treatment of breast cancer brain metastasis.

Functional Analysis of Gastric Tight Junction Proteins in Xenopus laevis Oocytes

The epithelial barrier is crucial for proper gastrointestinal function, preventing the unwanted passage of solutes and therefore representing a prerequisite for vectorial transport. Claudin-4 and claudin-18.2, two critical tight junction proteins of the gastric epithelium, seal neighboring cells in a physically and mechanically challenging environment. As the Xenopus laevis oocyte allows the functional and molecular analyses of claudin interaction, we have addressed the hypothesis that this interaction is not only dependent on mechanical force but also on pH. We expressed human claudin-4 and claudin-18 in Xenopus oocytes, and analyzed them in a two-cell model approach. Cells were clustered in pairs to form contact areas expressing CLDN18 + CLDN18, CLDN4/18 + CLDN4/18, and compared to controls, respectively. Contact areas in cells incubated in medium at pH 5.5 and 7.4 were quantified by employing transmitted light microscopy. After 24 h at pH 5.5, clustering of CLDN18 + CLDN18 and CLDN4/18 + CLDN4/18-expressing oocytes revealed a contact area reduced by 45% and 32%, compared with controls, respectively. A further approach, high-pressure impulse assay, revealed a stronger tight junction interaction at pH 5.5 in oocyte pairs expressing CLDN18 + CLDN18 or CLDN4/18 + CLDN4/18 indicating a protective role of claudin-18 for tight junction integrity during pH challenge. Thus, our current analysis of gastric tight junction proteins further establishes oocytes as an expression and two-cell screening model for tight junction integrity analysis of organ- and tissue-specific claudins by the characterization of homo- and heterophilic trans-interaction dependent on barrier effectors.