Activation of transient receptor potential vanilloid subtype 1 increases expression and permeability of tight junction in normal and hyposecretory submandibular gland

Tight junction (TJ) is an important structure that regulates material transport through the paracellular pathway across the epithelium, but its significance in salivary physiology and pathogenesis of salivary dysfunctional diseases is not fully understood. We previously demonstrated that a functional transient receptor potential vanilloid subtype 1 (TRPV1) expresses in submandibular gland (SMG). However, association of TRPV1-induced saliva secretion with TJ remains unknown. Here we explored the effect of TRPV1 activation on expression and function of TJ of rabbit SMG in vitro and in vivo. RT-PCR and western blot analysis revealed that capsaicin upregulated expression of zonula occludin-1 (ZO-1), claudin (Cldn)-3, and -11, but not Cldn-1, -2, -4, -5, and -7 in cultured SMG cells. Capsaicin also increased the entering of 4 kDa FITC-dextran into the acinar lumen, induced redistribution of cytoskeleton F-actin under confocal microscope, and these effects were abolished by preincubation of capsazepine, a TRPV1 antagonist, indicating that activation of TRPV1 increases expression and permeability of TJ in SMG. Additionally, in a hyposecretory model induced by rabbit SMG transplantation, the expression of ZO-1, Cldn-3, and -11 was decreased, whereas other TJs remained unaltered. The structure of TJ was impaired and the width of apical TJs was reduced under transmission electron microscope, concomitant with diminished immunofluorescence of F-actin in peri-apicolateral region, indicating impaired TJ expression and decreased paracellular permeability in the transplanted SMG. Moreover, topical capsaicin cream increased secretion, decreased TJ structural injury, reversed TJ expression levels, and protected F-actin morphology from disarrangement in transplanted SMGs. These data provide the first evidence to demonstrate that TJ components, particularly ZO-1, Cldn-3, and -11 have important roles in secretion of SMG under both physiological and pathophysiological conditions. The injury in TJ integrity was involved in the hypofunctional SMGs, and TRPV1 might be a potential target to improve saliva secretion through modulating expression and function of TJs.