Tight junctions (TJ) are named according to their classical function to seal the cleft between epithelial and endothelial cells against unwanted passage of solutes and water. The main protein families of the TJ are claudins, TJ-associated MARVEL proteins (TAMP, including occludin and tricellulin), junctional adhesion molecules (JAM), and angulins, most of which are connected to the cytoskeleton via adapters such as zonula occludens (ZO) proteins. TJ proteins do not only form barriers but, in contrast, some constitute paracellular ion or water channels. The first molecular structures of claudins and models of TJ channel pores are published. This entry collection aims to provide further insight into the complex machinery of the development and control of tissue formation and cell differentiation.

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Topic Review
Expression, Regulation, and Physiological Relevance of Skin Aquaporins
The skin is the largest organ of the human body, serving as an effective mechanical barrier between the internal milieu and the external environment. The skin is widely considered the first-line defence of the body, with an essential function in rejecting pathogens and preventing mechanical, chemical, and physical damages. Keratinocytes are the predominant cells of the outer skin layer, the epidermis, which acts as a mechanical and water-permeability barrier. The epidermis is a permanently renewed tissue where undifferentiated keratinocytes located at the basal layer proliferate and migrate to the overlying layers. 
  • 94
  • 27 May 2022
Topic Review
Carbohydrate-Binding Modules of Potential Resources
Carbohydrate-binding modules (CBMs) are a class of multi-module enzyme proteins and their function is to respond to bind to the carbohydrate substrate. Cellulose-binding domains (CBDs) are the earliest-discovered CBMs which were used to be catergozied based on their sequence homology. However, with the in-depth study of carbohydrate hydrolases, more modules in carbohydrate-active enzymes were discovered that could bind, in addition to cellulose, to other types of carbohydrates such as chitin, glucan, xylan, or starch. 
  • 211
  • 07 May 2022
Topic Review
Astrocytes as Templates for Angiogenesis
 Angiogenesis is a key process in various physiological conditions in the nervous system and in the retina during postnatal life. Although an increasing number of studies have addressed the role of endothelial cells in this event, the astrocytes contribution in angiogenesis has received less attention.
  • 228
  • 08 Apr 2022
Topic Review
ydfD is a lytic gene from the Qin cryptic prophage that encodes a 63-amino-acid protein, the ectopic expression of which in Escherichia coli can cause nearly complete cell lysis rapidly. The bacterial 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway is responsible for synthesizing the isoprenoids uniquely required for sustaining bacterial growth.
  • 130
  • 21 Mar 2022
Topic Review
Interleukin-4 and Interleukin-13 Induced Atopic Dermatitis
Atopic dermatitis (AD) is a chronic inflammatory skin disease associated with widespread barrier dysfunction and T helper 2 (Th2) cytokines. AD is an imbalance in the Th2 immune response where there is an increase in the gene expression levels of major Th2 cytokines during the acute phase.
  • 240
  • 07 Mar 2022
Topic Review
MECP2-Related Disorders in Males
Methyl CpG binding protein 2 ( MECP2 ) is an unstructured protein that can adopt local secondary structures when binding to other molecules, which explains its involvement in multiple molecular interactions and thereby, functions. Thus, MECP2 is a multifunctional gene that acts as a transcriptional regulator (both activating and repressing) and a chromatin remodeler; it also interacts with the RNA splicing machinery and with microRNA processing machinery, among others. Post-translational modifications are also implicated in regulating its activity and interactions with other proteins.
  • 118
  • 08 Feb 2022
Topic Review
Claudins and Gastric Cancer
Despite recent improvements in diagnostic ability and treatment strategies, advanced gastric cancer (GC) has a high frequency of recurrence and metastasis, with poor prognosis. To improve the treatment results of GC, the search for new treatment targets from proteins related to epithelial–mesenchymal transition (EMT) and cell–cell adhesion is currently being conducted. EMT plays an important role in cancer metastasis and is initiated by the loss of cell–cell adhesion, such as tight junctions (TJs), adherens junctions, desmosomes, and gap junctions. Among these, claudins (CLDNs) are highly expressed in some cancers, including GC. Abnormal expression of CLDN1, CLDN2, CLDN3, CLDN4, CLDN6, CLDN7, CLDN10, CLDN11, CLDN14, CLDN17, CLDN18, and CLDN23 have been reported. Among these, CLDN18 is of particular interest. In The Cancer Genome Atlas, GC was classified into four new molecular subtypes, and CLDN18–ARHGAP fusion was observed in the genomically stable type. An anti-CLDN18.2 antibody drug was recently developed as a therapeutic drug for GC, and the results of clinical trials are highly predictable. Thus, CLDNs are highly expressed in GC as TJs and are expected targets for new antibody drugs. 
  • 225
  • 10 Jan 2022
Topic Review
Blood-Spinal Cord Barrier
The blood-spinal cord barrier (BSCB) has been long thought of as a functional equivalent to the blood-brain barrier (BBB), restricting blood flow into the spinal cord. The spinal cord is supported by various disc tissues that provide agility and has different local immune responses compared to the brain. Though physiologically, structural components of the BSCB and BBB share many similarities, the clinical landscape significantly differs. 
  • 215
  • 14 Jan 2022
Topic Review
Integration of Light and Brassinosteroid Signaling
Light and brassinosteroid (BR) are external stimuli and internal cue respectively, that both play critical roles in a wide range of developmental and physiological process. Seedlings grown in the light exhibit photomorphogenesis, while BR promotes seedling etiolation. Light and BR oppositely control the development switch from skotomorphogenesis in the dark to photomorphogenesis in the light. Recent progress report that substantial components have been identified as hubs to integrate light and BR signals. Photomorphogenic repressors including COP1, PIFs, and AGB1 have been reported to elevate BR response, while photomorphogenesis-promoting factors such as HY5, BZS1, and NF-YCs have been proven to repress BR signal. 
  • 121
  • 21 Dec 2021
Topic Review
Aquaporin-1 Facilitates Transmesothelial Water Permeability
Mesothelial cells in human peritoneum express the water channel aquaporin 1 (AQP1) at the plasma membrane, suggesting that, although in a non-physiological context, it may facilitate osmotic water exchange during peritoneal dialysis (PD).
  • 157
  • 08 Dec 2021
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