Tight junctions in inflammatory bowel diseases and inflammatory bowel disease associated colorectal cancer

Table 1 Tight junctions of interest in inflammatory bowel diseases

TJ protein	Function	Expression in CD	Expression in UC	Expressionin pouchitis
Claudin 1	Tightens the epithelium[17,23] able to initiate formation of TJ strands[14]	Active: ↑[46,65] ↔[50]	Active: ↑[65],	↓[40]
		Inactive: ↔[65]	↔[45]
			Inactive: ↔[65]
Claudin 2	Important pore-forming TJ protein[15,16,23], able to initiate formation of TJ strands[14], decreases barrier function of CLDN1 and CLDN4[13]	↑[44,50,65,101]	↑↑[44,50,65,101]	↑[40]
Claudin 4	Tightens the epithelium, decreases paracellular conductance through decrease in sodium permeability[19]	↓[44]	Active: ↓[44,45]	↔[40]
		Active inflammation: ↑[65]	Active Inflammation: ↑[65]
		↔[50]
Claudin 5	Tightens the epithelium[20]	↓[50]		↔[40]
Claudin 8	Tightens the epithelium[21]	↓[50]
Claudin 12	Tightens the epithelium	↑[48]
Claudin 18	Uncertain function		↑[47]
Occludin	Binds ZO-1, regulates paracellular permeability, function in cellular adhesion[102]	↓[46,50,52]	↓[52]	↔[40]
		↔[65]	↔[65]
ZO-1	Mediates protein-protein interactions, link to actin cytoskeleton: “anchoring” protein[4,103]	↓[68]	↔[64] (Mees et al: “in patients with a history of UC”)	↓[54] (chronic pouchitis)

TJ: Tight junction; ZO-1: Zona occludens-1; CD: Crohn’s disease; UC: Ulcerative colitis.