Expression and functional evaluation of transient receptor potential channel 4 in bovine corneal endothelial cells

Transient Receptor Potential Channels: Important Players in Ocular Pain and Dry Eye Disease

Dry eye disease (DED) is a multifactorial disorder in which the eyes respond to minor stimuli with abnormal sensations, such as dryness, blurring, foreign body sensation, discomfort, irritation, and pain. Corneal pain, as one of DED’s main symptoms, has gained recognition due to its increasing prevalence, morbidity, and the resulting social burden. The cornea is the most innervated tissue in the body, and the maintenance of corneal integrity relies on a rich density of nociceptors, such as polymodal nociceptor neurons, cold thermoreceptor neurons, and mechano-nociceptor neurons. Their sensory responses to different stimulating forces are linked to the specific expression of transient receptor potential (TRP) channels. TRP channels are a group of unique ion channels that play important roles as cellular sensors for various stimuli. These channels are nonselective cation channels with variable Ca²⁺ selectivity. TRP homologs are a superfamily of 28 different members that are subdivided into 7 different subfamilies based on differences in sequence homology. Many of these subtypes are expressed in the eye on both neuronal and non-neuronal cells, where they affect various stress-induced regulatory responses essential for normal vision maintenance. This article reviews the current knowledge about the expression, function, and regulation of TRPs in ocular surface tissues. We also describe their implication in DED and ocular pain. These findings contribute to evidence suggesting that drug-targeting TRP channels may be of therapeutic benefit in the clinical setting of ocular pain.

Temperature-sensitive transient receptor potential channels in corneal tissue layers and cells

We here provide a brief summary of the characteristics of transient receptor potential channels (TRPs) identified in corneal tissue layers and cells. In general, TRPs are nonselective cation channels which are Ca(2+) permeable. Most TRPs serve as thermosensitive molecular sensors (thermo-TRPs). Based on their functional importance, the possibilities are described for drug-targeting TRP activity in a clinical setting. TRPs are expressed in various tissues of the eye including both human corneal epithelial and endothelial layers as well as stromal fibroblasts and stromal nerve fibers. TRP vanilloid type 1 (TRPV1) heat receptor, also known as capsaicin receptor, along with TRP melastatin type 8 (TRPM8) cold receptor, which is also known as menthol receptor, are prototypes of the thermo-TRP family. The TRPV1 functional channel is the most investigated TRP channel in these tissues, owing to its contribution to maintaining tissue homeostasis as well as eliciting wound healing responses to injury. Other thermo-TRP family members identified in these tissues are TRPV2, 3 and 4. Finally, there is the TRP ankyrin type 1 (TRPA1) cold receptor. All of these thermo-TRPs can be activated within specific temperature ranges and transduce such inputs into chemical and electrical signals. Although several recent studies have begun to unravel complex roles for thermo-TRPs such as TRPV1 in corneal layers and resident cells, additional studies are needed to further elucidate their roles in health and disease.

TRPC4- and TRPC4-containing channels

TRPC4 proteins comprise six transmembrane domains, a putative pore-forming region, and an intracellularly located amino- and carboxy-terminus. Among eleven splice variants identified so far, TRPC4α and TRPC4β are the most abundantly expressed and functionally characterized. TRPC4 is expressed in various organs and cell types including the soma and dendrites of numerous types of neurons; the cardiovascular system including endothelial, smooth muscle, and cardiac cells; myometrial and skeletal muscle cells; kidney; and immune cells such as mast cells. Both recombinant and native TRPC4-containing channels differ tremendously in their permeability and other biophysical properties, pharmacological modulation, and mode of activation depending on the cellular environment. They vary from inwardly rectifying store-operated channels with a high Ca(2+) selectivity to non-store-operated channels predominantly carrying Na(+) and activated by Gαq- and/or Gαi-coupled receptors with a complex U-shaped current-voltage relationship. Thus, individual TRPC4-containing channels contribute to agonist-induced Ca(2+) entry directly or indirectly via depolarization and activation of voltage-gated Ca(2+) channels. The differences in channel properties may arise from variations in the composition of the channel complexes, in the specific regulatory pathways in the corresponding cell system, and/or in the expression pattern of interaction partners which comprise other TRPC proteins to form heteromultimeric channels. Additional interaction partners of TRPC4 that can mediate the activity of TRPC4-containing channels include (1) scaffolding proteins (e.g., NHERF) that may mediate interactions with signaling molecules in or in close vicinity to the plasma membrane such as Gα proteins or phospholipase C and with the cytoskeleton, (2) proteins in specific membrane microdomains (e.g., caveolin-1), or (3) proteins on cellular organelles (e.g., Stim1). The diversity of TRPC4-containing channels hampers the development of specific agonists or antagonists, but recently, ML204 was identified as a blocker of both recombinant and endogenous TRPC4-containing channels with an IC50 in the lower micromolar range that lacks activity on most voltage-gated channels and other TRPs except TRPC5 and TRPC3. Lanthanides are specific activators of heterologously expressed TRPC4- and TRPC5-containing channels but can block individual native TRPC4-containing channels. The biological relevance of TRPC4-containing channels was demonstrated by knockdown of TRPC4 expression in numerous native systems including gene expression, cell differentiation and proliferation, formation of myotubes, and axonal regeneration. Studies of TRPC4 single and TRPC compound knockout mice uncovered their role for the regulation of vascular tone, endothelial permeability, gastrointestinal contractility and motility, neurotransmitter release, and social exploratory behavior as well as for excitotoxicity and epileptogenesis. Recently, a single-nucleotide polymorphism (SNP) in the Trpc4 gene was associated with a reduced risk for experience of myocardial infarction.

TRP channels in vascular endothelial cells

Endothelial cells regulate multiple vascular functions, such as vascular tone, permeability, remodeling, and angiogenesis. It is known for long that cytosolic Ca(2+) level ([Ca(2+)](i)) and membrane potential of endothelial cells are crucial factors to initiate the signal transduction cascades, leading to diverse vascular functions. Among the various kinds of endothelial ion channels that regulate ion homeostasis, transient receptor potential (TRP) channels emerge as the prime mediators for a diverse range of vascular signaling. The characteristics of TRP channels, including subunit heteromultimerization, diverse ion selectivity, and multiple modes of activation, permit their versatile functional roles in vasculatures. Substantial amount of evidence demonstrates that many TRP channels in endothelial cells participate in physiological and pathophysiological processes of vascular system. In this article, we summarize the recent findings of TRP research in endothelial cells, aiming at providing up-to-date information to the researchers in this rapidly growing field.

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.

Role of carbonic anhydrase IV in corneal endothelial HCO3- transport

PURPOSE

Carbonic anhydrase activity has a central role in corneal endothelial function. The authors examined the role of carbonic anhydrase IV (CAIV) in facilitating CO(2) flux, HCO(3)(-) permeability, and HCO(3)(-) flux across the apical membrane.

METHODS

Primary cultures of bovine corneal endothelial cells were established on membrane-permeable filters. Apical CAIV was inhibited by benzolamide or siRNA knockdown of CAIV. Apical CO(2) fluxes and HCO(3)(-) permeability were determined by measuring pH(i) changes in response to altering the CO(2) or HCO(3)(-) gradient across the apical membrane. Basolateral to apical (B-to-A) HCO(3)(-) flux was determined by measuring the pH of a weakly buffered apical bath in the presence of basolateral bicarbonate-rich Ringer solution. In addition, the effects of benzolamide and CAIV knockdown on steady state DeltapH (apical-basolateral compartment pH) after 4-hour incubation in DMEM were measured.

RESULTS

CAIV expression was confirmed, and CAIV was localized exclusively to the apical membrane by confocal microscopy. Both 10 microM benzolamide and CAIV siRNA reduced apparent apical CO(2) flux by approximately 20%; however, they had no effect on HCO(3)(-) permeability or HCO(3)(-) flux. The steady state apical-basolateral pH gradient at 4 hours was reduced by 0.12 and 0.09 pH units in benzolamide- and siRNA-treated cells, respectively, inconsistent with a net cell-to-apical compartment CO(2) flux.

CONCLUSIONS

CAIV does not facilitate steady state cell-to-apical CO(2) flux, apical HCO(3)(-) permeability, or B-to-A HCO(3)(-) flux. Steady state pH changes, however, suggest that CAIV may have a role in buffering the apical surface.

Corticolimbic expression of TRPC4 and TRPC5 channels in the rodent brain

The canonical transient receptor potential (TRPC) channels are a family of non-selective cation channels that are activated by increases in intracellular Ca(2+) and G(q)/phospholipase C-coupled receptors. We used quantitative real-time PCR, in situ hybridization, immunoblots and patch-clamp recording from several brain regions to examine the expression of the predominant TRPC channels in the rodent brain. Quantitative real-time PCR of the seven TRPC channels in the rodent brain revealed that TRPC4 and TRPC5 channels were the predominant TRPC subtypes in the adult rat brain. In situ hybridization histochemistry and immunoblotting further resolved a dense corticolimbic expression of the TRPC4 and TRPC5 channels. Total protein expression of HIP TRPC4 and 5 proteins increased throughout development and peaked late in adulthood (6-9 weeks). In adults, TRPC4 expression was high throughout the frontal cortex, lateral septum (LS), pyramidal cell layer of the hippocampus (HIP), dentate gyrus (DG), and ventral subiculum (vSUB). TRPC5 was highly expressed in the frontal cortex, pyramidal cell layer of the HIP, DG, and hypothalamus. Detailed examination of frontal cortical layer mRNA expression indicated TRPC4 mRNA is distributed throughout layers 2-6 of the prefrontal cortex (PFC), motor cortex (MCx), and somatosensory cortex (SCx). TRPC5 mRNA expression was concentrated specifically in the deep layers 5/6 and superficial layers 2/3 of the PFC and anterior cingulate. Patch-clamp recording indicated a strong metabotropic glutamate-activated cation current-mediated depolarization that was dependent on intracellular Ca(2+)and inhibited by protein kinase C in brain regions associated with dense TRPC4 or 5 expression and absent in regions lacking TRPC4 and 5 expression. Overall, the dense corticolimbic expression pattern suggests that these Gq/PLC coupled nonselective cation channels may be involved in learning, memory, and goal-directed behaviors.

Molecular expression and functional involvement of the bovine calcium-activated chloride channel 1 (bCLCA1) in apical HCO3- permeability of bovine corneal endothelium

Corneal endothelium secretes HCO(3)(-) from basolateral (stroma) to apical (anterior chamber) compartments. Apical HCO(3)(-) permeability can be enhanced by increasing [Ca(2+)](i). We hypothesized that the bovine calcium-activated chloride channel 1 (bCLCA1), shown previously by PCR screening to be expressed in corneal endothelium, is involved in Ca(2+) activated apical HCO(3)(-) permeability. bCLCA1 expression in cultured bovine corneal endothelial cells (CBCEC) was examined by in situ hybridization analysis, immunoblotting, immunofluorescence and confocal microscopy. Rabbit polyclonal antibodies were generated using a 14 aa polypeptide (417-430) from the predicted sequence of bCLCA1. The small interference RNA (siRNA) knock down technique was used to evaluate the functional involvement of bCLCA1 in apical HCO(3)(-) permeability. In situ hybridization confirmed prominent bCLCA1-specific mRNA expression in CBCEC. bCLCA1 antiserum detected the heterologously expressed bCLCA1 in HEK293 cells and a 90kDa band in CBCEC, which was absent when using the pre-immune serum or antigen absorption of serum. Immunofluoresence staining with anti-bCLCA1 antibody and confocal microscopy indicates an apical membrane location in CBCEC. In CBCEC transfected with bCLCA1 specific siRNA, bCLCA1 expression was reduced by 80%, while transfection with siControl scrambled sequence had no effect. Increasing [Ca(i)(2+)] by application of ATPgammaS or cyclopiazonic acid (CPA) increased apical HCO(3)(-) permeability in siControl transfected CBCEC, while having no effect on apical HCO(3)(-) permeability in bCLCA1 specific siRNA transfected cells. Baseline HCO(3)(-) permeability, however, was not different between controls and siRNA treated cells. We conclude that the calcium-activated chloride channel (bCLCA1) is expressed in bovine corneal endothelial cells and can contribute to Ca(2+) dependent apical HCO(3)(-) permeability, but not resting permeability, across the corneal endothelium.