Tight Junctions

Tight junctions, TJs or zonula cccludens (ZO), are located at the border between the apical and lateral membrane cells.
TJs are present in all epithelial and endothelial cells. They are located apically from the other three junctions and form a belt-like structure surrounding the cell.

Transmission electron microscopy shows that the membranes of two neighboring cells form direct contact points at the TJs, which have been termed "kissing points". In these areas, the membranes of the neighboring cells appear to fuse and seal the intercellular space. This paracellular diffusion barrier is semipermeable: it is size- and charge-selective.

Tight junctions form bidirectional signaling platforms that receive signals from the cell interior that regulate their structure and function, and transmit the signals into the cell interior to control cell proliferation, migration, differentiation and survival.

It is known that cell-cell permeability correlates best with the properties of the proteins in the strands. The first of these proteins, occludin, was identified 20 years ago by the group of Shoichiro Tsukita.

In addition to occludin, the following proteins play a role in the function of TJs:

• the claudin family
• the Junctional Adhesion Molecules (JAMs 1-3)
• the zonula occludens proteins (ZOs 1, 2 and 3)
• the multiple PDZ domain protein 1 (MUPP1)
• the membrane-associated guanylate kinase isoforms (MAGIs) 1, 2 and 3
• the cell polarity proteins such as protein kinase C (PKC)
• the isotype-specific-interacting protein/protease-activated receptor-3 (ASIP/PAR-3) and PAR-6
• the non-PDZ-expressing proteins such as cingulin, symplekin, atypical protein kinase C (aPKC), protein phosphatase 2 (PP2A), the Ras-related protein Rab-3B (Rab3b) and Rab13

The assembly of the tight junctions is regulated via phosphorylation by various protein kinases. Thus, activation of the MAP kinase signaling pathways controls the interaction of ZO-1/occludin with the plasma membrane. The association of VASP with the tight junctions is regulated by PKA-dependent phosphorylation. In this way, tight junctions close the intercellular space, thereby controlling the flow of molecules across the epithelium.

Tight junctions have a so-called "fencefunction". They prevent the free movement of membrane components and thereby maintain cell polarity by dividing into apical and basal regions.

Tight junctions are an integral part of the evolutionarily conserved signalling mechanisms that control the polarization of epithelial cells and the formation of morphologically and functionally distinct apical domains.

The skin is often considered the largest organ of the human body. Given that the skin is the first line of defense against the hostile external environment, the integrity of TJ in the epidermis is critical due to its fencing and barrier functions and its role in homeostasis and immune response. The human epidermis expresses a variety of TJ molecules.

Of these, claudin-1, claudin-7, JAM-A and MUPP1 are found in all layers of the epidermis, whereas occludin and cingulin are restricted to the stratum granulosum. JAM-A is expressed in all layers of the human epidermis except the stratum corneum. Some TJ proteins, such as ZO-1, claudin-1 and claudin-4, are also localized in the hair follicles (Morita K et al.1998; Brandner JM et al. 2003).

Tight junctions are crucial for the function of the skin barrier. Changes in their expression have been found in various skin diseases such as atopic dermatitis. They also play a role in normal wound healing.

Claudin 1 knockout mice die shortly after birth due to skin barrier defects that lead to severe water loss. Knockout of claudin 1 and 4, occludin and ZO-1 in skin keratinocytes increases paracellular permeability to ions and larger molecules. In addition, the elimination of claudin-1 also reduces the water barrier function in the stratum corneum.

In atopic dermatitis (AD), ZO-1 and cla udin-1 are significantly reduced in lesional skin, suggesting that TJ barrier function is impaired in this skin disease (Yuki T. et al. 2016; Batista DI et al. 2015). In the non-lesional skin of AD, ZO-1 and claudin-4 are reduced. In a mouse model of atopic dermatitis, it was shown that the changes in the epithelial barrier function of the skin correlated with a reduced expression of claudin-1. Changes in paracellular flux and morphology of the stratum corneum indicated this (Tokumasu R et al. 2016).

In the early phase of psoriasis, occludin, ZO-1 and claudin-4 are expressed at higher levels than in normal skin. The expression of claudins 1 and 7 was reduced in the basal and uppermost layers (Kirschner N et al. 2009). Claudin expression was reduced in plaque-type psoriasis, which could explain the impaired barrier function of psoriatic skin.

In addition to their barrier function for water, ions and large and small molecules, TJs are also involved in cell proliferation and differentiation. In human skin keratinocytes, knockdown of claudin-1 by siRNA leads to disruption of TJ function, increased permeability to sodium fluorescein and increased proliferation.

In an AD mouse model, the absence of claudin-1 in the lower epidermal layers correlates with significantly increased proliferation of epithelial cells and changes in the expression of the differentiation markers keratin-10 and keratin-14 (Gruber R et al. 2015). Furthermore, knockdown of occludin in human skin keratinocytes leads to reduced adhesion between epithelial cells and cells, reduced susceptibility to apoptosis and altered expression of differentiation markers.

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