TRPV1 activation is required for hypertonicity-stimulated inflammatory cytokine release in human corneal epithelial cells

Calcium regulation by thermo- and osmosensing transient receptor potential vanilloid channels (TRPVs) in human conjunctival epithelial cells

Transient receptor potential vanilloid (TRPV) channels respond to polymodal stresses to induce pain, inflammation and tissue fibrosis. In this study, we probed for their functional expression in human conjunctival epithelial (HCjE) cells and ex vivo human conjunctivas. Notably, patients suffering from dry eye syndrome experience the same type of symptomology induced by TRPV channel activation in other ocular tissues. TRPV gene and protein expression were determined by RT-PCR and immunohistochemistry in HCjE cells and human conjunctivas (body donors). The planar patch-clamp technique was used to record nonselective cation channel currents. Ca(2+) transients were monitored in fura-2 loaded cells. Cultivated HCjE cells and human conjunctiva express TRPV1, TRPV2, and TRPV4 mRNA. TRPV1 and TRPV4 localization was identified in human conjunctiva. Whereas the TRPV1 agonist capsaicin (CAP) (5-20 μM) -induced Ca(2+) transients were blocked by capsazepine (CPZ) (10 μM), the TRPV4 activator 4α-PDD (10 μM) -induced Ca(2+) increases were reduced by ruthenium-red (RuR) (20 μM). Different heating (<40°C or >43°C) led to Ca(2+) increases, which were also reduced by RuR. Hypotonic challenges of either 25 or 50% induced Ca(2+) transients and nonselective cation channel currents. In conclusion, conjunctiva express TRPV1, TRPV2, and TRPV4 channels which may provide novel drug targets for dry eye therapeutics. Their usage may have fewer side effects than those currently encountered with less selective drugs.

Therapeutic efficacy of trehalose eye drops for treatment of murine dry eye induced by an intelligently controlled environmental system

PURPOSE

To determine whether eye drop instillation of the disaccharide trehalose (TT) alleviates ocular surface damage in a dry eye murine model.

METHODS

Dry eye was induced in mice using an intelligently controlled environmental system (ICES). After 21 days housed in the ICES without topical treatment, the mice were randomly divided into three groups: no eye drops (ICES) for three weeks, four times a day with PBS 0.01 M 10 µl/eye bilaterally (ICES+PBS), or with TT 87.6 mM 10 µl/eye bilaterally (ICES+TT). Another mice group that was not exposed to the ICES and received no treatment served as a control group (UT). The ocular surface integrity, in each group, was evaluated using Oregon Green dextran (OGD) and fluorescein staining. The expression and distribution of occludin, involucrin, and small proline-rich protein 2 were determined with immunohistology analysis on whole mounted corneas. Heat shock protein 70 (HSP70) and matrix metalloproteinase 9 (MMP-9) expression was estimated with immunohistology. Ocular surface inflammation associated with each treatment was estimated with real time-PCR of interleukin-1β (IL-1β), IL-2, IL-6, IL-17, and tumor necrosis factor-alpha in the conjunctiva.

RESULTS

OGD staining in the cornea epithelium was lower in the ICES+TT group than in the ICES and ICES+PBS groups. Corneal epithelial occludin staining was markedly more homogenous in the ICES+TT group than in ICES and ICES+PBS groups, and there were no desquamating apical epithelial cells. Involucrin and small proline-rich protein 2 labeling of whole mounted corneas revealed upregulation of their expression in the groups, which received no treatment or PBS instillation compared to the ICES+TT group. HSP70 and MMP-9 immunolabeling revealed a marked increase in corneal epithelial expression in response to the ICES. The group treated with trehalose showed a similar profile expression of HSP70 and MMP-9 as the control group (UT). Conjunctival IL-1β, IL-2, IL-6, IL-17, tumor necrosis factor-alpha (TNF-α), and MMP-9 mRNA expression was lower in the ICES+TT group than in the ICES or ICES+PBS group.

CONCLUSIONS

Trehalose application restored ocular surface integrity, suppressed inflammatory and proteolytic MMP-9 and HSP70 expression, and keratinization in mice with dry eye damaged by a desiccative model.

The corneal pain system. Part I: the missing piece of the dry eye puzzle

The traditional model of dry eye disease based on tear deficiency has presented us with many unanswered questions. Recent studies support the notion that dry eye-like symptoms represent non-specific corneal pain and provide new insights into the mechanisms that sustain the integrity of the optical tear layer. Thus, this enigmatic disease can be viewed with a new perspective, which involves the dysfunctional corneal pain system as a central pathogenetic feature of a series of disorders collectively known today as dry eye.

NF-κB feedback control of JNK1 activation modulates TRPV1-induced increases in IL-6 and IL-8 release by human corneal epithelial cells

PURPOSE

The corneal wound healing response to an alkali burn results in dysregulated inflammation and opacity. Transient receptor potential vanilloid type1 (TRPV1) ion channel activation by such a stress contributes to this unfavorable outcome. Accordingly, we sought to identify potential drug targets for mitigating this response, in human corneal epithelial cells (HCEC).

METHODS

SV40-immmortalized HCEC were transduced with lentiviral vectors to establish stable c-Jun N-terminal kinase1 (JNK1), nuclear factor-κB1 (NF-κB1), and dual specificity phsophatase1 (DUSP1) shRNAmir sublines. Immunoblotting evaluated the expression of NF-κB1, DUSP1, protein kinase Cδ (PKCδ), and the phosphorylation status of cell signaling mediators. Enzyme-linked immunosorbent assay (ELISA) evaluated interleukin-6 (IL-6) and interleukin-8 (IL-8) release.

RESULTS

Capsaicin (CAP; a selective TRPV1 agonist), induced time-dependent activation of transforming growth factor-activated kinase 1 (TAK1) and mitogen-activated protein kinase (MAPK) cascades temporally followed by increased nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor, alpha (IκBα) phosphorylation, rises in both PKCδ protein levels and IL-6 and IL-8 release. All of these responses were blocked by the TAK1 inhibitor 5z-7-oxozeaenol (5z-OX). In the JNK1 subline, CAP failed to increase IL-6/8 release, but still stimulated NF-κB by 50%. In the NF-κB1 subline, these IL-6/8 responses were absent, JNK1 activation was attenuated and there was a concomitant increase in DUSP1 expression compared to the control. In the DUSP1 subline, JNK1 phosphorylation was enhanced and prolonged and accompanied by larger increases in IL-6/8 release.

CONCLUSIONS

TRPV1 induced increases in IL-6/IL-8 release occur through TAK1 activation of JNK1-dependent and JNK1-independent signaling pathways. Their joint activation is required for NF-κB to elicit sufficient positive feedback control of JNK1/2 phosphorylation to elicit increases in IL-6/8 release. Such regulation depends on NF-κB modulation of DUSP1 expression levels and associated changes in PKCδ protein levels.

Transient receptor potential (TRP) gene superfamily encoding cation channels

Transient receptor potential (TRP) non-selective cation channels constitute a superfamily, which contains 28 different genes. In mammals, this superfamily is divided into six subfamilies based on differences in amino acid sequence homology between the different gene products. Proteins within a subfamily aggregate to form heteromeric or homomeric tetrameric configurations. These different groupings have very variable permeability ratios for calcium versus sodium ions. TRP expression is widely distributed in neuronal tissues, as well as a host of other tissues, including epithelial and endothelial cells. They are activated by environmental stresses that include tissue injury, changes in temperature, pH and osmolarity, as well as volatile chemicals, cytokines and plant compounds. Their activation induces, via intracellular calcium signalling, a host of responses, including stimulation of cell proliferation, migration, regulatory volume behaviour and the release of a host of cytokines. Their activation is greatly potentiated by phospholipase C (PLC) activation mediated by coupled GTP-binding proteins and tyrosine receptors. In addition to their importance in maintaining tissue homeostasis, some of these responses may involve various underlying diseases. Given the wealth of literature describing the multiple roles of TRP in physiology in a very wide range of different mammalian tissues, this review limits itself to the literature describing the multiple roles of TRP channels in different ocular tissues. Accordingly, their importance to the corneal, trabecular meshwork, lens, ciliary muscle, retinal, microglial and retinal pigment epithelial physiology and pathology is reviewed.