Novel Insights into the Role of Keratinocytes-Expressed TRPV3 in the Skin

TRPV3 is a non-selective cation channel that is highly expressed in keratinocytes in the skin. Traditionally, keratinocytes-expressed TRPV3 is involved in multiple physiological and pathological functions of the skin, such as itching, heat pain, and hair development. Although the underlying mechanisms by which TRPV3 functions in vivo remain obscure, recent research studies suggest that several cytokines and EGFR signaling pathways may be involved. However, there have also been other studies with opposite results that question the role of TRPV3 in heat pain. In addition, an increasing number of studies have suggested a novel role of TRPV3 in promoting skin regeneration, indicating that TRPV3 may become a new potential target for regulating skin regeneration. This paper not only reviews the role of keratinocytes-expressed TRPV3 in the physiological and pathological processes of itching, heat pain, hair development, and skin regeneration, but also reviews the relationship between TRPV3 gene mutations and skin diseases such as atopic dermatitis (AD) and Olmsted syndrome (OS). This review will lay a foundation for further developing our understanding of the mechanisms by which TRPV3 is involved in itching, heat pain, and hair development, as well as the treatments for TRPV3-related skin diseases.

Presence of TRPV3 in macrophage lysosomes helps in skin wound healing against bacterial infection

Transient Receptor Potential Vanilloid subtype 3 (TRPV3) is a non-selective cation channel that is known to be activated by physiological temperature and endogenous ligands. Involvement of TRPV3 in different skin functions has been reported. In this work, we demonstrate that activation of TRPV3 by FPP, an endogenous ligand enhances skin wound healing and bacterial clearance there. We report for the first time that TRPV3 is endogenously expressed in macrophages and activation of TRPV3 results in efficient bacterial clearance. At the subcellular level, TRPV3 is present in the lysosome and also in the nucleolus. We demonstrate that pharmacological modulation of TRPV3 protects lysosomal functions at hyperthermic shock conditions. The localization of TRPV3 at the nucleolus is specific, more in case of LPS-treatment and dynamic with respect to the cell signalling. We demonstrate that at certain conditions, the nucleolar localization of TRPV3 is correlated with the presence of TRPV3 at the lysosome and with the cellular stress in general. We propose that TRPV3 act as a lysosomal regulator and sensor for cellular stress. These findings may have broad implications in understanding the cellular stress and TRPV3-induced channelopathies and may have clinical relevance to skin infection treatment.

Stratum corneum as polymer sheet: concept and cornification processes

The skin barrier protects our body from external insults and water loss through a specialized layer, the stratum corneum. The stratum corneum, an accumulation of dead keratinocytes (corneocytes), comprises lipids and supporting cell bodies. We propose a framework of lipid-filled polymer sheet of corneocytes, a unique structure that achieves flexibility and robustness, updating the rigid image of the historical bricks-and-mortar model. The polymerization of polymer sheet (cornification) by cell death of keratinocytes (corneoptosis) is delicately and dynamically controlled by cytoplasmic calcium ion and pH. Understanding the structure and formation of the stratum corneum can lead to better treatments for skin diseases and a better understanding of the evolution of the stratum corneum.

Transient Receptor Potential Vanilloid 3 Expression Is Increased In Non-Lesional Skin Of Atopic Dermatitis Patients

TRPV3 (transient receptor potential vanilloid 3) is a pro-inflammatory ion channel mostly expressed by keratinocytes of the human skin. Previous studies have shown that the expression of TRPV3 is markedly upregulated in the lesional epidermis of atopic dermatitis (AD) patients suggesting a potential pathogenetic role of the ion channel in the disease. In the current study, we aimed at defining the molecular and functional expression of TRPV3 in non-lesional skin of AD patients as previous studies implicated that healthy-appearing skin in AD are markedly distinct from normal skin with respect to terminal differentiation and certain immune function abnormalities. By using multiple, complementary immunolabeling and RT-qPCR technologies on full-thickness and epidermal shave biopsy samples from AD patients (lesional, non-lesional) and healthy volunteers, we provide the first evidence that the expression of TRPV3 is markedly upregulated in non-lesional human AD epidermis, similar to lesional AD samples. Of further importance, by using the patch-clamp method on cultured healthy and non-lesional AD keratinocytes, we also show that this upregulation is functional as determined by the significantly augmented TRPV3-specific ion current (induced by agonists) on cultured non-lesional AD keratinocytes when compared to healthy ones.

Hair Loss Caused by Gain-of-Function Mutant TRPV3 Is Associated with Premature Differentiation of Follicular Keratinocytes

Gain-of-function mutations in the TRPV3 gene can cause Olmsted syndrome characterized by palmoplantar and periorificial keratoderma, itch, and hair loss. The mechanism underlying the hair loss remains unclear. In this study, we engineered an Olmsted syndrome mouse model by introducing the point mutation G568V to the corresponding Trpv3 locus in the mice. These mice developed fully penetrant hair loss. The hair loss was associated with premature differentiation of follicular keratinocytes characterized by precocious degeneration of trichohyalin and keratins, increased production of deiminated proteins, elevated apoptosis, and attenuation of transcription regulators (Foxn1, Msx2, Dlx3, and Gata3) known to regulate hair follicle differentiation. These abnormalities occurred in the medial‒proximal region of the inner root sheath and the hair shaft, where Trpv3 is highly expressed, and correlated with an impaired formation of the hair canal and the hair shaft. The mutant Trpv3 mice also exhibited increased proliferation in the outer root sheath, accelerated hair cycle, reduction of hair follicle stem cells, and miniaturization of regenerated hair follicles. Findings from this study suggest that precocious maturation of postmitotic follicular keratinocytes drives hair loss in patients with Olmsted syndrome.

Genotype‒Phenotype Correlation of TRPV3-Related Olmsted Syndrome

We have previously shown that gain-of-function variations in transient receptor potential vanilloid-3 (TRPV3) underlay Olmsted syndrome, a rare hyperkeratotic skin channelopathy. In this study, we attempt to establish a genotype‒phenotype correlation in Olmsted syndrome, which has been unclear owing to the rarity and heterogeneity of the condition. We identified five previously unreported TRPV3 variations (R416Q, R416W, L655P, W692S, and L694P) and three recurrent variations (G568D, G568V, and L673F) in nine unrelated patients. Seven variants were expressed in human embryonic kidney 293 cells, and channel behavior was characterized electrophysiologically, with results compared with the clinical severity. These variant TRPV3 channels, in either homomeric or heteromeric form, exhibited differentially elevated basal open probability, increased voltage sensitivity, and cytotoxicity. Functional changes were particularly pronounced in variants corresponding to severer Olmsted syndrome (e.g., L673F and W692S) but not in mild Olmsted syndrome variants (e.g., R416Q). Interestingly, the extent of functional rescue by wild-type TRPV3 in vitro was also consistent with the clinical severity of the variants. These findings, in combination with all reported cases, indicate a preliminary genotype‒phenotype correlation, that is, variations in the S4‒S5 linker and transient receptor potential domain of TRPV3 significantly enhance channel function, causing severe phenotype, whereas other variations appear to exert milder effects on channel function and disease phenotype.

Identification of Transient Receptor Potential Vanilloid 3 Antagonists from Achillea alpina L. and Separation by Liquid-Liquid-Refining Extraction and High-Speed Counter-Current Chromatography

Bioassay-guided fractionation of the ethanol extract of whole herbs of Achilleaalpina led to the isolation of isochlorogenic acids A and B as transient receptor potential vanilloid 3 (TRPV3) channel antagonists by using a calcium fluorescent assay. The structures were identified by spectroscopic analysis and the inhibitory activities of isochlorogenic acids A and B were confirmed by whole-cell patch clamp recordings of human embryonic kidney 293 (HEK293) cells expressing human TRPV3. Molecular docking results revealed that these two compounds reside in the same active pocket of human TRPV3 channel protein with lower binding energy than the agonist 2-aminoethoxydiphenyl borate (2-APB). High-speed counter-current chromatography (HSCCC) coupled with a liquid-liquid extraction approach was successfully established for the separation of isochlorogenic acids A and B from the whole herbs of A. alpina. Ethyl acetate and n-hexane-ethyl acetate-water (3:3:4 and 1:5:4, v/v/v) were selected as liquid-liquid extraction solvent systems to remove high- and low-polarity impurities in the mixture. Sixty g of ethanol extract was refined by solvent partition to yield 1.7 g of the enriched fraction, of which 480 mg in turn obtained 52.5 mg of isochlorogenic acid B (purity 98.3%) and 37.6 mg isochlorogenic acid A (purity 96.2%) after HSCCC with n-hexane-ethyl acetate-water containing 1% acetic acid (1:4:8, v/v/v).

Establishment of the experimental procedure for prediction of conjugation capacity in mutant UGT1A1

UDP-glucuronosyltransferase 1A1 (UGT1A1) is an enzyme that is found in the endoplasmic reticulum membrane and can reportedly have a large number of amino acid substitutions that result in the reduction of glucuronidation capacity. For example, adverse drug reactions when patients receive CPT-11 (irinotecan) such as in cancer chemotherapy are caused by amino acid substitutions in UGT1A1. We previously found that the extent of the docking when the hydroxyl residue of bilirubin was oriented toward UDP-glucuronic acid correlated with in vitro conjugation capacity. In this study, we analyzed the conformation of mutant UGT1A1s by means of structural optimization with water and lipid bilayers instead of the optimization in vacuo that we used in our previous study. We then derived a mathematical model that can predict the conjugation capacities of mutant UGT1A1s by using results of substrate docking in silico and results of in vitro analysis of glucuronidation of acetaminophen and 17β-estradiol by UGT1A1s. This experimental procedure showed that the in silico conjugation capacities of other mutant UGT1A1s with bilirubin or SN-38 were similar to reported in vitro conjugation capacities. Our results suggest that this experimental procedure described herein can correctly predict the conjugation capacities of mutant UGT1A1s and any substrate.

TRP Channels as Drug Targets to Relieve Itch

Although acute itch has a protective role by removing irritants to avoid further damage, chronic itch is debilitating, significantly impacting quality of life. Over the past two decades, a considerable amount of stimulating research has been carried out to delineate mechanisms of itch at the molecular, cellular, and circuit levels. There is growing evidence that transient receptor potential (TRP) channels play important roles in itch signaling. The purpose of this review is to summarize our current knowledge about the role of TRP channels in the generation of itch under both physiological and pathological conditions, thereby identifying them as potential drug targets for effective anti-itch therapies.

Olmsted Syndrome Caused by a Heterozygous p.Gly568Val Missense Mutation in TRPV3 Gene

Olmsted syndrome (OS) is a rare congenital skin disorder characterized by severe palmoplantar and periorificial keratoderma, alopecia, onychodystrophy, and severe pruritus. Recently, pathogenic 'gain-of-function' mutations of the transient receptor potential vanilloid 3 gene (TRPV3), which encodes a cation channel involved in keratinocyte differentiation and proliferation, hair growth, inflammation, pain and pruritus, have been identified to cause OS. Due to the rarity, the pattern of inheritance of OS is still unclear. We report a case of OS in a 3-year-old Korean girl and its underlying gene mutation. The patient presented with a disabling, bilateral palmoplantar keratoderma with onychodystrophy. She also exhibited pruritic eczematous skin lesions around her eyes, ears and gluteal fold. Genetic analysis identified a heterozygous p.Gly568Val missense mutation in the exon 13 of TRPV3. To our knowledge, this is the first case of OS in the Korean population showing a missense mutation p.Gly573Ser.

Co-precipitation molecules hemopexin and transferrin may be key molecules for fibrillogenesis in TTR V30M amyloidogenesis

The disease model of familial amyloidotic polyneuropathy—7.2-hMet30 mice—manifests amyloid deposition that consists of a human amyloidogenic mutant transthyretin (TTR) (TTR V30M). Our previous study found amyloid deposits in 14 of 27 7.2-hMet30 mice at 21–24 months of age. In addition, non-fibrillar TTR deposits were found in amyloid-negative 7.2hMet30 mice. These results suggested that TTR amyloidogenesis required not only mutant TTR but also an additional factor (or factors) as an etiologic molecule. To determine the differences in serum proteome in amyloid-positive and amyloid-negative mice in the 7.2-hMet30 model, we used proteomic analyses and studied serum samples obtained from these mice. Hemopexin (HPX) and transferrin (Tf) were detected in the serum samples from amyloid-positive mice and were also found in amyloid deposits via immunohistochemistry, but serum samples from amyloid-negative mice did not contain HPX and Tf. These two proteins were also not detected in non-fibrillar TTR deposits. In addition, in silico analyses suggested that HPX and Tf facilitate destabilization of TTR secondary structures and misfolding of TTR. These results suggest that HPX and Tf may be associated with TTR amyloidogenesis after fibrillogenesis in vivo.