Zonula cccludens proteins

Zonula occludens proteins are peripherally associated cytoplasmic membrane proteins belonging to the membrane-associated guanylate kinase homologs (MAGUK) family. Three different MAGUKs have been identified: ZO-1, ZO-2 and ZO-3, with molecular weights of 220, 160 and 130 kDa, respectively.

ZO-1, discovered in 1986, was the first identified member of the tight-junction family (Stevenson BR et al. (1986). The main features of the ZO protein family are its three structurally conserved PDZ domains, a Src homology 3 domain and a guanylate kinase domain. ZOs anchor transmembrane proteins such as claudins, occludin and JAMs (junctional adhesion molecules) to the actin cytoskeleton (Bauer H et al. 2010; Gonzalez-Mariscal L et al. 2000) and are expressed in important cell types involved, for example, in the wound healing process. ZO-1 and ZO-2 are expressed in both the epithelium and the endothelium, while ZO-3 is only expressed in the epithelium (Adachi M et al. 2006).

The absence of ZO-1 in mouse mammary gland epithelial cells leads to delayed recruitment of claudins and occludin to the TJs and delayed barrier formation. In MDCK cells, siRNA knockdown of ZO-2 leads to increased paracellular permeability, decreased fence function of TJs and defective recruitment of ZO-1, occludin and E-cadherin to newly formed junctions (Hernandez S et al. 2007). Knockouts of ZO-1 and ZO-2 have embryonic lethality. ZO-1 knockouts are associated with impaired yolk sac angiogenesis and apoptosis of embryonic cells. Embryos from mice with ZO-2 knockout show reduced proliferation, increased apoptosis and increased paracellular permeability, indicating impaired barrier function (Xu J et al. 2008). Embryos from ZO-3 knockout mice show no observed changes in phenotype, suggesting that ZO-3 may be a dispensable component of TJs ((Xu J et al. 2008).

Several research studies have also shown that, unlike other TJ component proteins, ZOs can move back and forth between the cytoplasm and the nucleus, particularly in response to injury or stressors. For example, ZO-2 accumulates in the nucleus of MDCK epithelial cells following heat shock or chemical injury.

In addition, a large body of work suggests that ZO-2 and other ZOs interact with a variety of nuclear proteins, including transcription factors, to modulate the growth and proliferation of epithelial and endothelial cells, both of which are important in the wound healing process [Bauer H et al. (2010).

ZO can be post-translationally modified with phosphate, and both PKA and PKC are responsible for increased phosphorylation of ZO-2. Hyperphosphorylation of ZO-2 correlates with decreased permeability and can be corrected by treatment with a PKC inhibitor (Avila-Flores A et al. 2001).

Similarly, ZO-1 phosphorylation is reduced in response to PKC inhibitors that induce the formation of TJs by increasing the calcium concentration in the cell culture medium (Stuart RO et al. (1995).

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10. Xu, J et al. (2009) Zona occludens-2 is critical for blood-testis barrier integrity and male fertility. Mol. Biol Cell 20: 4268-4277.