The epithelial TJs formamorphological and functional gate, ultimately
restricting the paracellular permeability [21]. Chitosan is known to
adhere to the intestinal mucosa and affect TJ barrier properties, which
might bemediated by changes in cytosolic pH. This enhanced the permeability
of the paracellular route, which might not depend on changes of
just one member of the TJ proteins but rather on the whole TJ complex
[22]. In an intestinal epithelial cell model of Caco-2, Dorkoosh et al.
observed an accumulation of occludin at the cell borders [23], but a decrease
in the peripheral staining for occludin after treatment with chitosan
[24]. The TJ protein ZO-1 staining revealed a loss in the continuous
staining along cell–cell contact points after incubation with chitosan
[25]. A higher expression of claudin-1 within the TJ was observed after
treatment with trimethyl chitosan [23]. The mechanism of the activity
of reversible TJ opening induced by chitosan is known to involve in the
translocation of trans-membrane protein JAM-1 (a trans-membrane TJ
protein). However, the width of the opened TJs is limited,which restricts
the paracellular absorption of insoluble chitosan aggregates that form in themucus layer, while allowing the passage of soluble agents such as insulin
present in the vicinity of the opened TJsmaking chitosan a safe and
effective permeation enhancer [26]. Several studies demonstrated that
the effect of chitosan on the paracellular permeability is dependent on
its positive charge [27]. COS supplementation improved occludin protein
expression and had no effect on claudin-1 mRNA expression in piglets
challenged with ETEC in this study. The mechanism may be that its positive
charge binds with negative charge in nucleic acid within the bacterial
cells. This binding inhibits the generation of harmful bacteria such as
E. coli and bacterial toxins, which may enhance occludin protein expression,
and improve intestinal connectivity.