Cholangiocyte cilia express TRPV4 and detect changes in luminal tonicity inducing bicarbonate secretion

Biliary fibrosis is an important but neglected pathological feature in hepatobiliary disorders: from molecular mechanisms to clinical implications

Fibrosis resulting from pathological repair secondary to recurrent or persistent tissue damage often leads to organ failure and mortality. Biliary fibrosis is a crucial but easily neglected pathological feature in hepatobiliary disorders, which may promote the development and progression of benign and malignant biliary diseases through pathological healing mechanisms secondary to biliary tract injuries. Elucidating the etiology and pathogenesis of biliary fibrosis is beneficial to the prevention and treatment of biliary diseases. In this review, we emphasized the importance of biliary fibrosis in cholangiopathies and summarized the clinical manifestations, epidemiology, and aberrant cellular composition involving the biliary ductules, cholangiocytes, immune system, fibroblasts, and the microbiome. We also focused on pivotal signaling pathways and offered insights into ongoing clinical trials and proposing a strategic approach for managing biliary fibrosis-related cholangiopathies. This review will offer a comprehensive perspective on biliary fibrosis and provide an important reference for future mechanism research and innovative therapy to prevent or reverse fibrosis.

Primary cilia as a targetable node between biliary injury, senescence and regeneration in liver transplantation

BACKGROUND & AIMS

Biliary complications are a major cause of morbidity and mortality in liver transplantation. Up to 25% of patients that develop biliary complications require additional surgical procedures, re-transplantation or die in the absence of a suitable regraft. Here, we investigate the role of the primary cilium, a highly specialised sensory organelle, in biliary injury leading to post-transplant biliary complications.

METHODS

Human biopsies were used to study the structure and function of primary cilia in liver transplant recipients that develop biliary complications (n = 7) in comparison with recipients without biliary complications (n = 12). To study the biological effects of the primary cilia during transplantation, we generated murine models that recapitulate liver procurement and cold storage, and assessed the elimination of the primary cilia in biliary epithelial cells in the K19CreERTKif3afl/fl mouse model. To explore the molecular mechanisms responsible for the observed phenotypes we used in vitro models of ischemia, cellular senescence and primary cilia ablation. Finally, we used pharmacological and genetic approaches to target cellular senescence and the primary cilia, both in mouse models and discarded human donor livers.

RESULTS

Prolonged ischemic periods before transplantation result in ciliary shortening and cellular senescence, an irreversible cell cycle arrest that blocks regeneration. Our results indicate that primary cilia damage results in biliary injury and a loss of regenerative potential. Senescence negatively impacts primary cilia structure and triggers a negative feedback loop that further impairs regeneration. Finally, we explore how targeted interventions for cellular senescence and/or the stabilisation of the primary cilia improve biliary regeneration following ischemic injury.

CONCLUSIONS

Primary cilia play an essential role in biliary regeneration and we demonstrate that senolytics and cilia-stabilising treatments provide a potential therapeutic opportunity to reduce the rate of biliary complications and improve clinical outcomes in liver transplantation.

IMPACT AND IMPLICATIONS

Up to 25% of liver transplants result in biliary complications, leading to additional surgery, retransplants, or death. We found that the incidence of biliary complications is increased by damage to the primary cilium, an antenna that protrudes from the cell and is key to regeneration. Here, we show that treatments that preserve the primary cilia during the transplant process provide a potential solution to reduce the rates of biliary complications.

Store-operated calcium entry inhibits primary ciliogenesis via the activation of Aurora A

The primary cilium is an antenna-like organelle protruding from the cell surface that can detect physical and chemical stimuli in the extracellular space to activate specific signaling pathways and downstream gene expressions. Calcium ion (Ca2+ ) signaling regulates a wide spectrum of cellular processes, including fertilization, proliferation, differentiation, muscle contraction, migration, and death. This study investigated the effects of the regulation of cytosolic Ca2+ levels on ciliogenesis using chemical, genetic, and optogenetic approaches. We found that ionomycin-induced Ca2+ influx inhibited ciliogenesis and Ca2+ chelator BATPA-AM-induced Ca2+ depletion promoted ciliogenesis. In addition, store-operated Ca2+ entry and the endoplasmic reticulum Ca2+ sensor stromal interaction molecule 1 (STIM1) negatively regulated ciliogenesis. Moreover, an optogenetic platform was used to create different Ca2+ oscillation patterns by manipulating lighting parameters, including density, frequency, exposure time, and duration. Light-activated Ca2+ -translocating channelrhodopsin (CatCh) is activated by 470-nm blue light to induce Ca2+ influx. Our results show that high-frequency Ca2+ oscillations decrease ciliogenesis. Furthermore, the inhibition of cilia formation induced by Ca2+ may occur via the activation of Aurora kinase A. Cilia not only induce Ca2+ signaling but also regulate cilia formation by Ca2+ signaling.

Primary cilia as dynamic and diverse signalling hubs in development and disease

Primary cilia, antenna-like sensory organelles protruding from the surface of most vertebrate cell types, are essential for regulating signalling pathways during development and adult homeostasis. Mutations in genes affecting cilia cause an overlapping spectrum of >30 human diseases and syndromes, the ciliopathies. Given the immense structural and functional diversity of the mammalian cilia repertoire, there is a growing disconnect between patient genotype and associated phenotypes, with variable severity and expressivity characteristic of the ciliopathies as a group. Recent technological developments are rapidly advancing our understanding of the complex mechanisms that control biogenesis and function of primary cilia across a range of cell types and are starting to tackle this diversity. Here, we examine the structural and functional diversity of primary cilia, their dynamic regulation in different cellular and developmental contexts and their disruption in disease.

Proteomic Modulation in TGF-β-Treated Cholangiocytes Induced by Curcumin Nanoparticles

Curcumin is a natural polyphenol that exhibits a variety of beneficial effects on health, including anti-inflammatory, antioxidant, and hepato-protective properties. Due to its poor water solubility and membrane permeability, in the present study, we prepared and characterized a water-stable, freely dispersible nanoformulation of curcumin. Although the potential of curcumin nanoformulations in the hepatic field has been studied, there are no investigations on their effect in fibrotic pathological conditions involving cholangiocytes. Exploiting an in vitro model of transforming growth factor-β (TGF-β)-stimulated cholangiocytes, we applied the Sequential Window Acquisition of All Theoretical Mass Spectra (SWATH-MS)-based quantitative proteomic approaches to study the proteome modulation induced by curcumin nanoformulation. Our results confirmed the well-documented anti-inflammatory properties of this nutraceutic, highlighting the induction of programmed cell death as a mechanism to counteract the cellular damages induced by TGF-β. Moreover, curcumin nanoformulation positively influenced the expression of several proteins involved in TGF-β-mediated fibrosis. Given the crucial importance of deregulated cholangiocyte functions during cholangiopathies, our results provide the basis for a better understanding of the mechanisms associated with this pathology and could represent a rationale for the development of more targeted therapies.

Unconventional interactions of the TRPV4 ion channel with beta-adrenergic receptor ligands

The transient receptor potential vanilloid 4 (TRPV4) ion channel is present in different tissues including those of the airways. This channel is activated in response to stimuli such as changes in temperature, hypoosmotic conditions, mechanical stress, and chemicals from plants, lipids, and others. TRPV4's overactivity and/or dysfunction has been associated with several diseases, such as skeletal dysplasias, neuromuscular disorders, and lung pathologies such as asthma and cardiogenic lung edema and COVID-19-related respiratory malfunction. TRPV4 antagonists and blockers have been described; nonetheless, the mechanisms involved in achieving inhibition of the channel remain scarce, and the search for safe use of these molecules in humans continues. Here, we show that the widely used bronchodilator salbutamol and other ligands of β-adrenergic receptors inhibit TRPV4's activation. We also demonstrate that inhibition of TRPV4 by salbutamol is achieved through interaction with two residues located in the outer region of the pore and that salbutamol leads to channel closing, consistent with an allosteric mechanism. Our study provides molecular insights into the mechanisms that regulate the activity of this physiopathologically important ion channel.

TRPV4: Cell type-specific activation, regulation and function in the vertebrate eye

The architecture of the vertebrate eye is optimized for efficient delivery and transduction of photons and processing of signaling cascades downstream from phototransduction. The cornea, lens, retina, vasculature, ciliary body, ciliary muscle, iris and sclera have specialized functions in ocular protection, transparency, accommodation, fluid regulation, metabolism and inflammatory signaling, which are required to enable function of the retina-light sensitive tissue in the posterior eye that transmits visual signals to relay centers in the midbrain. This process can be profoundly impacted by non-visual stimuli such as mechanical (tension, compression, shear), thermal, nociceptive, immune and chemical stimuli, which target these eye regions to induce pain and precipitate vision loss in glaucoma, diabetic retinopathy, retinal dystrophies, retinal detachment, cataract, corneal dysfunction, ocular trauma and dry eye disease. TRPV4, a polymodal nonselective cation channel, integrate non-visual inputs with homeostatic and signaling functions of the eye. The TRPV4 gene is expressed in most if not all ocular tissues, which vary widely with respect to the mechanisms of TRPV4 channel activation, modulation, oligomerization, and participation in protein- and lipid interactions. Under- and overactivation of TRPV4 may affect intraocular pressure, maintenance of blood-retina barriers, lens accommodation, neuronal function and neuroinflammation. Because TRPV4 dysregulation precipitates many pathologies across the anterior and posterior eye, the channel could be targeted to mitigate vision loss.

Genetics, pathobiology and therapeutic opportunities of polycystic liver disease

Polycystic liver diseases (PLDs) are inherited genetic disorders characterized by progressive development of intrahepatic, fluid-filled biliary cysts (more than ten), which constitute the main cause of morbidity and markedly affect the quality of life. Liver cysts arise in patients with autosomal dominant PLD (ADPLD) or in co-occurrence with renal cysts in patients with autosomal dominant or autosomal recessive polycystic kidney disease (ADPKD and ARPKD, respectively). Hepatic cystogenesis is a heterogeneous process, with several risk factors increasing the odds of developing larger cysts. Depending on the causative gene, PLDs can arise exclusively in the liver or in parallel with renal cysts. Current therapeutic strategies, mainly based on surgical procedures and/or chronic administration of somatostatin analogues, show modest benefits, with liver transplantation as the only potentially curative option. Increasing research has shed light on the genetic landscape of PLDs and consequent cholangiocyte abnormalities, which can pave the way for discovering new targets for therapy and the design of novel potential treatments for patients. Herein, we provide a critical and comprehensive overview of the latest advances in the field of PLDs, mainly focusing on genetics, pathobiology, risk factors and next-generation therapeutic strategies, highlighting future directions in basic, translational and clinical research.

Polycystin-2 (TRPP2): Ion channel properties and regulation

Polycystin-2 (TRPP2, PKD2, PC2) is the product of the PKD2 gene, whose mutations cause Autosomal Dominant Polycystic Kidney Disease (ADPKD). PC2 belongs to the superfamily of TRP (Transient Receptor Potential) proteins that generally function as Ca²⁺-permeable nonselective cation channels implicated in Ca²⁺ signaling. PC2 localizes to various cell domains with distinct functions that likely depend on interactions with specific channel partners. Functions include receptor-operated, nonselective cation channel activity in the plasma membrane, intracellular Ca²⁺ release channel activity in the endoplasmic reticulum (ER), and mechanosensitive channel activity in the primary cilium of renal epithelial cells. Here we summarize our current understanding of the properties of PC2 and how other transmembrane and cytosolic proteins modulate this activity, providing functional diversity and selective regulatory mechanisms to its role in the control of cellular Ca²⁺ homeostasis.

Two Cases With Neonatal Cholestasis and Renal Disorders Due to DCDC2 Mutation

Ciliopathies are a heterogeneous group of diseases that are observed after deterioration of the ciliary structures on the cell surface that facilitate communication with the environment. Both liver and kidney involvement are frequently observed in this disease. Recently, a doublecortin domain containing protein 2 (DCDC2) mutation in a ciliopathy disease group was identified. Here, we present 2 patients with this mutation and with neonatal cholestasis and renal involvement.

Mitochondria regulate TRPV4-mediated release of ATP

BACKGROUND AND PURPOSE

Ca2+ influx via TRPV4 channels triggers Ca2+ release from the IP3 -sensitive internal store to generate repetitive oscillations. Although mitochondria are acknowledged regulators of IP3 -mediated Ca2+ release, how TRPV4-mediated Ca2+ signals are regulated by mitochondria is unknown. We show that depolarised mitochondria switch TRPV4 signalling from relying on Ca2+ -induced Ca2+ release at IP3 receptors to being independent of Ca2+ influx and instead mediated by ATP release via pannexins.

EXPERIMENTAL APPROACH

TRPV4-evoked Ca2+ signals were individually examined in hundreds of cells in the endothelium of rat mesenteric resistance arteries using the indicator Cal520.

KEY RESULTS

TRPV4 activation with GSK1016790A (GSK) generated repetitive Ca2+ oscillations that required Ca2+ influx. However, when the mitochondrial membrane potential was depolarised, by the uncoupler CCCP or complex I inhibitor rotenone, TRPV4 activation generated large propagating, multicellular, Ca2+ waves in the absence of external Ca2+ . The ATP synthase inhibitor oligomycin did not potentiate TRPV4-mediated Ca2+ signals. GSK-evoked Ca2+ waves, when mitochondria were depolarised, were blocked by the TRPV4 channel blocker HC067047, the SERCA inhibitor cyclopiazonic acid, the PLC blocker U73122 and the inositol trisphosphate receptor blocker caffeine. The Ca2+ waves were also inhibited by the extracellular ATP blockers suramin and apyrase and the pannexin blocker probenecid.

CONCLUSION AND IMPLICATIONS

These results highlight a previously unknown role of mitochondria in shaping TRPV4-mediated Ca2+ signalling by facilitating ATP release. When mitochondria are depolarised, TRPV4-mediated release of ATP via pannexin channels activates plasma membrane purinergic receptors to trigger IP3 -evoked Ca2+ release.

Distribution and Assembly of TRP Ion Channels

In the last several decades, a large family of ion channels have been identified and studied intensively as cellular sensors for diverse physical and/or chemical stimuli. Named transient receptor potential (TRP) channels, they play critical roles in various aspects of cellular physiology. A large number of human hereditary diseases are found to be linked to TRP channel mutations, and their dysregulations lead to acute or chronical health problems. As TRP channels are named and categorized mostly based on sequence homology rather than functional similarities, they exhibit substantial functional diversity. Rapid advances in TRP channel study have been made in recent years and reported in a vast body of literature; a summary of the latest advancements becomes necessary. This chapter offers an overview of current understandings of TRP channel distribution and subunit assembly.

Piezo1-Pannexin1 complex couples force detection to ATP secretion in cholangiocytes

Cholangiocytes actively contribute to the final composition of secreted bile. These cells are exposed to abnormal mechanical stimuli during obstructive cholestasis, which has a deep impact on their function. However, the effects of mechanical insults on cholangiocyte function are not understood. Combining gene silencing and pharmacological assays with live calcium imaging, we probed molecular candidates essential for coupling mechanical force to ATP secretion in mouse cholangiocytes. We show that Piezo1 and Pannexin1 are necessary for eliciting the downstream effects of mechanical stress. By mediating a rise in intracellular Ca²⁺, Piezo1 acts as a mechanosensor responsible for translating cell swelling into activation of Panx1, which triggers ATP release and subsequent signal amplification through P2X4R. Co-immunoprecipitation and pull-down assays indicated physical interaction between Piezo1 and Panx1, which leads to stable plasma membrane complexes. Piezo1–Panx1–P2X4R ATP release pathway could be reconstituted in HEK Piezo1 KO cells. Thus, our data suggest that Piezo1 and Panx1 can form a functional signaling complex that controls force-induced ATP secretion in cholangiocytes. These findings may foster the development of novel therapeutic strategies for biliary diseases.

Piezo1 helps bile on the pressure

JGP study shows that a mechanosensitive complex containing Piezo1 and Pannexin1 couples osmotic pressure to ATP secretion in bile duct cholangiocytes.

Application of single-cell RNA sequencing technology in liver diseases: a narrative review

Objective

This review aimed to summarize the application of single-cell transcriptome sequencing technology in liver diseases.

Background

The increasing application of single-cell ribonucleic acid (RNA) sequencing (scRNA-seq) in life science and biomedical research has greatly improved our understanding of cellular heterogeneity in immunology, oncology, and developmental biology. scRNA-seq has proven to be a powerful tool for identifying and classifying cell subsets, characterizing rare or small cell subsets and tracking cell differentiation along the dynamic cell stages. Globally, liver disease has high rates of morbidity and mortality, and its exact pathological mechanism remains unclear, current treatment options are limited to clearance of the underlying cause or liver transplantation, which cannot overwhelm and cure liver diseases. scRNA-seq provides many novel insights for healthy and diseased livers.

Methods

In this review, we searched for related articles in the PubMed database and summarized the advances of scRNA-seq in revealing the molecular mechanisms of liver development, regeneration, and disease. We also discussed the challenges and future application potential of scRNA-seq, which is expected to enhance the ability to explore the field of liver research and accelerate the clinical application of liver precision medicine.

Conclusions

With the continuous improvement of scRNA-seq technology, scRNA-seq is expected to unlock new avenues for liver biology exploration, liver disease diagnosis, and personalized treatment, which will pave the way for breakthrough innovation in personalized medicine.

Polycystic Liver Disease: Advances in Understanding and Treatment

Polycystic liver disease (PLD) is a group of genetic disorders characterized by progressive development of cholangiocyte-derived fluid-filled hepatic cysts. PLD is the most common manifestation of autosomal dominant and autosomal recessive polycystic kidney diseases and rarely occurs as autosomal dominant PLD. The mechanisms of PLD are a sequence of the primary (mutations in PLD-causative genes), secondary (initiation of cyst formation), and tertiary (progression of hepatic cystogenesis) interconnected molecular and cellular events in cholangiocytes. Nonsurgical, surgical, and limited pharmacological treatment options are currently available for clinical management of PLD. Substantial evidence suggests that pharmacological targeting of the signaling pathways and intracellular processes involved in the progression of hepatic cystogenesis is beneficial for PLD. Many of these targets have been evaluated in preclinical and clinical trials. In this review, we discuss the genetic, molecular, and cellular mechanisms of PLD and clinical and preclinical treatment strategies. Expected final online publication date for the Annual Review of Pathology: Mechanisms of Disease, Volume 17 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Exome sequencing identifies a disease variant of the mitochondrial ATP-Mg/Pi carrier SLC25A25 in two families with kidney stones

BACKGROUND

Calcium kidney stones are common and recurrences are often not preventable by available empiric remedies. Their etiology is multifactorial and polygenic, and an increasing number of genes are implicated. Their identification will enable improved management.

METHODS

DNA from three stone-formers in a Southampton family (UK) and two from an Italian family were analyzed independently by whole exome sequencing and selected variants were genotyped across all available members of both pedigrees. A disease variant of SLC25A25 (OMIM 608745), encoding the mitochondrial ATP-Mg/Pi carrier 3 (APC3) was identified, and analyzed structurally and functionally with respect to its calcium-regulated transport activity.

RESULTS

All five patients had a heterozygous dominant SLC25A25 variant (rs140777921; GRCh37.p13: chr 9 130868670 G>C; p.Gln349His; Reference Sequence NM_001006641.3). Non-stone formers also carried the variant indicating incomplete penetrance. Modeling suggests that the variant lacks a conserved polar interaction, which may cause structural instability. Calcium-regulated ATP transport was reduced to ~20% of the wild type, showing a large reduction in function.

CONCLUSION

The transporter is important in regulating mitochondrial ATP production. This rare variant may increase urine lithogenicity through impaired provision of ATP for solute transport processes in the kidney, and/or for purinergic signaling. Variants found in other genes may compound this abnormality.

Clinical and genetic heterogeneity of primary ciliopathies (Review)

Ciliopathies comprise a group of complex disorders, with involvement of the majority of organs and systems. In total, >180 causal genes have been identified and, in addition to Mendelian inheritance, oligogenicity, genetic modifications, epistatic interactions and retrotransposon insertions have all been described when defining the ciliopathic phenotype. It is remarkable how the structural and functional impairment of a single, minuscule organelle may lead to the pathogenesis of highly pleiotropic diseases. Thus, combined efforts have been made to identify the genetic substratum and to determine the pathophysiological mechanism underlying the clinical presentation, in order to diagnose and classify ciliopathies. Yet, predicting the phenotype, given the intricacy of the genetic cause and overlapping clinical characteristics, represents a major challenge. In the future, advances in proteomics, cell biology and model organisms may provide new insights that could remodel the field of ciliopathies.

TRPing to the Point of Clarity: Understanding the Function of the Complex TRPV4 Ion Channel

The transient receptor potential vanilloid 4 channel (TRPV4) belongs to the mammalian TRP superfamily of cation channels. TRPV4 is ubiquitously expressed, activated by a disparate array of stimuli, interacts with a multitude of proteins, and is modulated by a range of post-translational modifications, the majority of which we are only just beginning to understand. Not surprisingly, a great number of physiological roles have emerged for TRPV4, as have various disease states that are attributable to the absence, or abnormal functioning, of this ion channel. This review will highlight structural features of TRPV4, endogenous and exogenous activators of the channel, and discuss the reported roles of TRPV4 in health and disease.

Bile Acids and GPBAR-1: Dynamic Interaction Involving Genes, Environment and Gut Microbiome

Bile acids (BA) are amphiphilic molecules synthesized in the liver from cholesterol. BA undergo continuous enterohepatic recycling through intestinal biotransformation by gut microbiome and reabsorption into the portal tract for uptake by hepatocytes. BA are detergent molecules aiding the digestion and absorption of dietary fat and fat-soluble vitamins, but also act as important signaling molecules via the nuclear receptor, farnesoid X receptor (FXR), and the membrane-associated G protein-coupled bile acid receptor 1 (GPBAR-1) in the distal intestine, liver and extra hepatic tissues. The hydrophilic-hydrophobic balance of the BA pool is finely regulated to prevent BA overload and liver injury. By contrast, hydrophilic BA can be hepatoprotective. The ultimate effects of BA-mediated activation of GPBAR-1 is poorly understood, but this receptor may play a role in protecting the remnant liver and in maintaining biliary homeostasis. In addition, GPBAR-1 acts on pathways involved in inflammation, biliary epithelial barrier permeability, BA pool hydrophobicity, and sinusoidal blood flow. Recent evidence suggests that environmental factors influence GPBAR-1 gene expression. Thus, targeting GPBAR-1 might improve liver protection, facilitating beneficial metabolic effects through primary prevention measures. Here, we discuss the complex pathways linked to BA effects, signaling properties of the GPBAR-1, mechanisms of liver damage, gene-environment interactions, and therapeutic aspects.

Primary Cilia and Calcium Signaling Interactions

The calcium ion (Ca2+) is a diverse secondary messenger with a near-ubiquitous role in a vast array of cellular processes. Cilia are present on nearly every cell type in either a motile or non-motile form; motile cilia generate fluid flow needed for a variety of biological processes, such as left-right body patterning during development, while non-motile cilia serve as the signaling powerhouses of the cell, with vital singling receptors localized to their ciliary membranes. Much of the research currently available on Ca2+-dependent cellular actions and primary cilia are tissue-specific processes. However, basic stimuli-sensing pathways, such as mechanosensation, chemosensation, and electrical sensation (electrosensation), are complex processes entangled in many intersecting pathways; an overview of proposed functions involving cilia and Ca2+ interplay will be briefly summarized here. Next, we will focus on summarizing the evidence for their interactions in basic cellular activities, including the cell cycle, cell polarity and migration, neuronal pattering, glucose-mediated insulin secretion, biliary regulation, and bone formation. Literature investigating the role of cilia and Ca2+-dependent processes at a single-cellular level appears to be scarce, though overlapping signaling pathways imply that cilia and Ca2+ interact with each other on this level in widespread and varied ways on a perpetual basis. Vastly different cellular functions across many different cell types depend on context-specific Ca2+ and cilia interactions to trigger the correct physiological responses, and abnormalities in these interactions, whether at the tissue or the single-cell level, can result in diseases known as ciliopathies; due to their clinical relevance, pathological alterations of cilia function and Ca2+ signaling will also be briefly touched upon throughout this review.

Release of ATP by TRPV4 activation is dependent upon the expression of AQP2 in renal cells

Increasing evidence indicates that aquaporins (AQPs) exert an influence in cell signaling by the interplay with the transient receptor potential vanilloid 4 (TRPV4) channel. We previously found that TRPV4 physically and functionally interacts with AQP2 in cortical collecting ducts (CCD) cells, favoring cell volume regulation and cell migration. Because TRPV4 was implicated in ATP release in several tissues, we investigated the possibility that TRPV4/AQP2 interaction influences ATP release in CCD cells. Using two CCD cell lines expressing or not AQP2, we measured extracellular ATP (ATPe) under TRPV4 activation and intracellular Ca²⁺ under ATP addition. We found that AQP2 is critical for the release of ATP induced by TRPV4 activation. This ATP release occurs by an exocytic and a conductive route. ATPe, in turn, stimulates purinergic receptors leading to ATPe-induced ATP release by a Ca²⁺ -dependent mechanism. We propose that AQP2 by modulating Ca²⁺ and ATP differently could explain AQP2-increased cell migration.

The Enigmatic Role of Lipids in Cilia Signaling

Primary cilia are specialized cellular structures that project from the surface of most cell types in metazoans and mediate transduction of major signaling pathways. The ciliary membrane is contiguous with the plasma membrane, yet it exhibits distinct protein and lipid composition, which is essential for ciliary function. Diffusion barriers at the base of a cilium are responsible for establishing unique molecular composition of this organelle. Although considerable progress has been made in identifying mechanisms of ciliary protein trafficking in and out of cilia, it remains largely unknown how the distinct lipid identity of the ciliary membrane is achieved. In this mini review, I summarize recent developments in characterizing lipid composition and organization of the ciliary membrane and discuss the emerging roles of lipids in modulating activity of ciliary signaling components including ion channels and G protein-coupled receptors.

TRPV4 promotes acoustic wave-mediated BBB opening via Ca2+/PKC-δ pathway

Introduction

Numerous studies have shown the ability of low-energy acoustic waves such as focused ultrasound or shockwave to transiently open blood-brain barrier (BBB) and facilitate drug delivery to the brain. Preclinical and clinical evidences have well demonstrated the efficacy and safety in treating various brain disorders. However, the molecular mechanisms of acoustic waves on the BBB are still not fully understood.

Objectives

The present study aimed at exploring the possible molecular mechanisms of acoustic wave stimulation on brains.

Methods: Briefly describe the experimental design

The left hemisphere of the rat‘s brain was treated with pulsed ultrasound from a commercial focused shockwave or a planar ultrasound device, and the right hemisphere served as a control. One hour after the mechanical wave stimulation or overnight, the rats were sacrificed and the brains were harvested for protein or histological analysis. Agonists and antagonists related to the signal transduction pathways of tight junction proteins were used to investigate the possible intracellular mechanisms.

Results

Intracellular signal transduction analysis shows calcium influx through transient receptor potential vanilloid 4 (TRPV4) channels, and the activation of PKC-δ pathway to mediate dissociation of ZO-1 and occludin after acoustic wave stimulation. The activation of TRPV4 or PKC-δ signaling further increased the expression level of TRPV4, suggesting a feedback loop to regulate BBB permeability. Moreover, the tight junction proteins dissociation can be reversed by administration of PKC-δ inhibitor and TRPV4 antagonist.

Conclusion

The present study shows the crucial role of TRPV4 in acoustic wave-mediated BBB permeability, specifically its effect on compromising tight junction proteins, ZO-1 and occludin. Our findings provide a new molecular perspective to explain acoustic wave-mediated BBB opening. Moreover, activation of TRPV4 by agonists may reduce the threshold intensity level of acoustic waves for BBB opening, which may prevent undesirable mechanical damages while maintaining efficient BBB opening.

TRPV4: A Physio and Pathophysiologically Significant Ion Channel

Transient Receptor Potential (TRP) channels are a family of ion channels whose members are distributed among all kinds of animals, from invertebrates to vertebrates. The importance of these molecules is exemplified by the variety of physiological roles they play. Perhaps, the most extensively studied member of this family is the TRPV1 ion channel; nonetheless, the activity of TRPV4 has been associated to several physio and pathophysiological processes, and its dysfunction can lead to severe consequences. Several lines of evidence derived from animal models and even clinical trials in humans highlight TRPV4 as a therapeutic target and as a protein that will receive even more attention in the near future, as will be reviewed here.

Hepatoprotective impact of the bile acid receptor TGR5

During liver repair after injury, bile secretion has to be tightly modulated in order to preserve liver parenchyma from bile acid (BA)-induced injury. The mechanisms allowing the liver to maintain biliary homeostasis during repair after injury are not completely understood. Besides their historical role in lipid digestion, bile acids (BA) and their receptors constitute a signalling network with multiple impacts on liver repair, both stimulating regeneration and protecting the liver from BA overload. BA signal through nuclear (mainly Farnesoid X Receptor, FXR) and membrane (mainly G Protein-coupled BA Receptor 1, GPBAR-1 or TGR5) receptors to elicit a wide array of biological responses. While a great number of studies have been dedicated to the hepato-protective impact of FXR signalling, TGR5 is by far less explored in this context. Because the liver has to face massive and potentially harmful BA overload after partial ablation or destruction, BA-induced protective responses crucially contribute to spare liver repair capacities. Based on the available literature, the TGR5 BA receptor protects the remnant liver and maintains biliary homeostasis, mainly through the control of inflammation, biliary epithelial barrier permeability, BA pool hydrophobicity and sinusoidal blood flow. Mouse experimental models of liver injury reveal that in the lack of TGR5, excessive inflammation, leaky biliary epithelium and hydrophobic BA overload result in parenchymal insult and compromise optimal restoration of a functional liver mass. Translational perspectives are thus opened to target TGR5 with the aim of protecting the liver in the context of injury and BA overload.

A Bile Duct-on-a-Chip With Organ-Level Functions

BACKGROUND AND AIMS

Chronic cholestatic liver diseases, such as primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC), are frequently associated with damage to the barrier function of the biliary epithelium. Here, we report on a bile duct-on-a-chip that phenocopies not only the tubular architecture of the bile duct in three dimensions, but also its barrier functions.

APPROACH AND RESULTS

We showed that mouse cholangiocytes in the channel of the device became polarized and formed mature tight junctions, that the permeability of the cholangiocyte monolayer was comparable to ex vivo measurements, and that cholangiocytes in the device were mechanosensitive (as demonstrated by changes in calcium flux under applied luminal flow). Permeability decreased significantly when cells formed a compact monolayer with cell densities comparable to those observed in vivo. This device enabled independent access to the apical and basolateral surfaces of the cholangiocyte channel, allowing proof-of-concept toxicity studies with the biliary toxin, biliatresone, and the bile acid, glycochenodeoxycholic acid. The cholangiocyte basolateral side was more vulnerable than the apical side to treatment with either agent, suggesting a protective adaptation of the apical surface that is normally exposed to bile. Further studies revealed a protective role of the cholangiocyte apical glycocalyx, wherein disruption of the glycocalyx with neuraminidase increased the permeability of the cholangiocyte monolayer after treatment with glycochenodeoxycholic acid.

CONCLUSIONS

This bile duct-on-a-chip captured essential features of a simplified bile duct in structure and organ-level functions and represents an in vitro platform to study the pathophysiology of the bile duct using cholangiocytes from a variety of sources.

The primary cilium: Its role as a tumor suppressor organelle

The primary cilium is an organelle that nearly all cells within the body contain. Its function is to sense the extracellular environment through its abundance of receptors and linked signaling pathways, working as an antenna. Ciliary defects lead to different pathologies. In particular, many tumors lose primary cilia, and this is linked with negative implications for the cell such as an increase in malignancy. In this work we will go through the knowledge of the role of primary cilia in normal conditions, how it is involved in diverse signaling pathways, and in disease, particularly in cancer, highlighting its tumor suppressor properties.

Mechanosensor transient receptor potential vanilloid member 4 (TRPV4) regulates mouse cholangiocyte secretion and bile formation

Mechanosensitive signaling has emerged as a mechanism for the regulation of cholangiocyte transport and bile formation. The mechanical effect of fluid-flow, or shear, at the apical membrane of cholangiocytes regulates secretion through a process involving increases in [Ca²⁺]_i and activation of Ca²⁺-activated Cl^- channels. However, the initiating steps translating shear force to increases in intracellular calcium concentration ([Ca²⁺]_i) are unknown. Transient receptor potential vanilloid member 4 (TRPV4), a nonselective cation channel present in the apical membrane of cholangiocytes, has been proposed as a potential mechanosensor. The aim of the present studies was to determine the potential role of TRPV4 in initiating mechanosensitive signaling in response to fluid-flow in cholangiocytes. TRPV4 expression was confirmed in both small and large mouse cholangiocytes. Exposure of cells to either fluid flow or specific TRPV4 pharmacological agonists rapidly increased both [Ca²⁺]_i and membrane cation currents. Both flow- and agonist-stimulated currents displayed identical biophysical properties and were inhibited in the presence of TRPV4 antagonists or in cells after transfection with TRPV4 small interfering RNA. Transfection of mouse cholangiocytes with a TRPV4-enhanced green fluorescent protein construct increased the expression of TRPV4 and the magnitude of flow-stimulated currents. A specific TRPV4 agonist significantly increased the biliary concentration of ATP and bile flow in live mice when administered intravenously and increased ATP release from cholangiocyte monolayers when applied exogenously. The findings are consistent with a model in which activation of cholangiocyte TRPV4 translates shear force into an acute rise in membrane cation permeability, [Ca²⁺]_i, ATP release, and bile flow. Understanding the role of mechanosensitive transport pathways may provide novel insights to modulate bile flow for the treatment of cholestatic liver disorders.NEW & NOTEWORTHY These studies functionally characterize TRPV4 as a mechanosensitive channel in mouse cholangiocytes. By mediating a rapid rise in intracellular Ca²⁺, necessary for Ca²⁺-dependent secretion, TRPV4 represents a mechanosensor responsible for translating fluid flow into intracellular signaling and biliary secretion. Furthermore, intravenous infusion of a specific TRPV4 agonist increases bile flow in live mice. Understanding the role of TRPV4 in mechanosensitive transport pathways may provide novel insights to modulate bile flow during cholestasis.

Nucleus pulposus primary cilia alter their length in response to changes in extracellular osmolarity but do not control TonEBP-mediated osmoregulation

The nucleus pulposus (NP) cells adapt to their physiologically hyperosmotic microenvironment through Tonicity-responsive enhancer binding protein (TonEBP/nuclear factor of activated T-cell5 [NFAT5])-mediated osmoregulation. Primary cilia in different organs serve diverse roles including osmosensing, but its contribution to NP cell osmoadaptive response is unknown. A high percentage of cultured primary NP cells possessed primary cilia that changed length in response to osmotic stimuli. Stable silencing of Intraflagellar Transport 88 (Ift88) or Kinesin Family Member 3 A (Kif3a) to inhibit the formation of primary cilia did not affect hyperosmotic upregulation of TonEBP. While ShKif3a blocked hyperosmotic increase of TonEBP-Transactivation Domain (TAD) activity, overall the knockdown of either gene did not alter the hyperosmotic status of proximal promoter activities and transcription of key TonEBP targets. On the other hand, a small decrease in TonEBP level under hypoosmotic condition was attenuated by Ift88 or Kif3a knockdown. Noteworthy, none of the TonEBP target genes were responsive to hypoosmotic stimulus in control and Ift88 or Kif3a knockdown cells, suggesting the primary role of TonEBP in the hyperosmotic adaptation of NP cells. Similarly, in Kif3a null mouse embryonic fibroblasts (MEFs), the overall TonEBP-dependent hyperosmotic responses were preserved. Unlike NP cells, TonEBP targets were responsive to hypoosmolarity in wild-type MEFs, and these responses remained intact in Kif3a null MEFs. Together, these results suggest that primary cilia are dispensable for TonEBP-dependent osmoadaptive response.

Therapeutic perspectives for structural and functional abnormalities of cilia

Ciliopathies are a group of hereditary disorders that result from structural or functional abnormalities of cilia. Recent intense research efforts have uncovered the genetic bases of ciliopathies, and our understanding of the assembly and functions of cilia has been improved significantly. Although mechanism-specific therapies for ciliopathies have not yet received regulatory approval, the use of innovative therapeutic modalities such as oligonucleotide therapy, gene replacement therapy, and gene editing in addition to symptomatic treatments are expected to provide valid treatment options in the near future. Moreover, candidate chemical compounds for developing small molecule drugs to treat ciliopathies have been identified. This review introduces the key features of cilia and ciliopathies, and summarizes the advances as well as the challenges that remain with the development of therapies for treating ciliopathies.

The cilium as a force sensor-myth versus reality

Cells need to sense their mechanical environment during the growth of developing tissues and maintenance of adult tissues. The concept of force-sensing mechanisms that act through cell-cell and cell-matrix adhesions is now well established and accepted. Additionally, it is widely believed that force sensing can be mediated through cilia. Yet, this hypothesis is still debated. By using primary cilia sensing as a paradigm, we describe the physical requirements for cilium-mediated mechanical sensing and discuss the different hypotheses of how this could work. We review the different mechanosensitive channels within the cilium, their potential mode of action and their biological implications. In addition, we describe the biological contexts in which cilia are acting - in particular, the left-right organizer - and discuss the challenges to discriminate between cilium-mediated chemosensitivity and mechanosensitivity. Throughout, we provide perspectives on how quantitative analysis and physics-based arguments might help to better understand the biological mechanisms by which cells use cilia to probe their mechanical environment.

Cholangiocyte pathobiology

Cholangiocytes, the epithelial cells lining the intrahepatic and extrahepatic bile ducts, are highly specialized cells residing in a complex anatomic niche where they participate in bile production and homeostasis. Cholangiocytes are damaged in a variety of human diseases termed cholangiopathies, often causing advanced liver failure. The regulation of cholangiocyte transport properties is increasingly understood, as is their anatomical and functional heterogeneity along the biliary tract. Furthermore, cholangiocytes are pivotal in liver regeneration, especially when hepatocyte regeneration is compromised. The role of cholangiocytes in innate and adaptive immune responses, a critical subject relevant to immune-mediated cholangiopathies, is also emerging. Finally, reactive ductular cells are present in many cholestatic and other liver diseases. In chronic disease states, this repair response contributes to liver inflammation, fibrosis and carcinogenesis and is a subject of intense investigation. This Review highlights advances in cholangiocyte research, especially their role in development and liver regeneration, their functional and biochemical heterogeneity, their activation and involvement in inflammation and fibrosis and their engagement with the immune system. We aim to focus further attention on cholangiocyte pathobiology and the search for new disease-modifying therapies targeting the cholangiopathies.

[Bone mechanobiology, an emerging field: a review]

INTRODUCTION

Mechanobiology, at the interface between biology and biophysics, studies the impact of mechanical forces on tissues, cells and biomolecules. The application of orthodontic forces, followed by induced tooth displacement, is a striking example of its clinical application.

OBJECTIVE

The purpose of this article was to compile a review of the literature on the subject of mechanobiology; from its detection at bone level to the presentation of stimulated intracellular pathways.

MATERIALS AND METHODS

The literature search was conducted on the Pubmed database in April 2018, with associations of the terms "mechanobiology", "orthodontics", "cell culture", "physiopathology".

RESULTS

Three major areas of research were selected: highlighting of the phenomenon and its application in the field of bone biology; the cellular effectors of mechanobiology and its clinical applications. The use of mechanobiology in dentofacial orthopedics opens up a new field of reflection for clinicians regarding future advances in orthodontics.

Calcium Signaling in Cholangiocytes: Methods, Mechanisms, and Effects

Calcium (Ca²⁺) is a versatile second messenger that regulates a number of cellular processes in virtually every type of cell. The inositol 1,4,5-trisphosphate receptor (ITPR) is the only intracellular Ca²⁺ release channel in cholangiocytes, and is therefore responsible for Ca²⁺-mediated processes in these cells. This review will discuss the machinery responsible for Ca²⁺ signals in these cells, as well as experimental models used to investigate cholangiocyte Ca²⁺ signaling. We will also discuss the role of Ca²⁺ in the normal and abnormal regulation of secretion and apoptosis in cholangiocytes, two of the best characterized processes mediated by Ca²⁺ in this cell type.

Liver-Derived Exosomes and Their Implications in Liver Pathobiology

The liver has a wide range of physiological functions in the body, and its health is maintained by complex cross-talk among hepatic cells, including parenchymal hepatocytes and nonparenchymal cells. Exosomes, which are one method of cellular communication, are endosomal-derived small vesicles that are released by donor cells and delivered to the target cells at both short and long distances. Because exosomes carry a variety of cargoes, including proteins, mRNAs, microRNAs and other noncoding RNAs originating from donor cells, exosomes convey cellular information that enables them to potentially serve as biomarkers and therapeutics in liver diseases. Hepatocytes release exosomes to neighboring hepatocytes or nonparenchymal cells to regulate liver regeneration and repair. Nonparenchymal cells, including hepatic stellate cells, liver sinusoidal endothelial cells, and cholangiocytes, also secrete exosomes to regulate liver remodeling upon liver injury. Exosomes that are released from liver cancer cells create a favorable microenvironment for cancer growth and progression. In this review, we summarize and discuss the current findings and understanding of exosome-mediated intercellular communication in the liver, with a particular focus on the function of exosomes in both health and disease. Based on the current findings, we suggest the potential applications of exosomes as biomarkers and therapeutics for liver diseases.

Single cell RNA sequencing of human liver reveals distinct intrahepatic macrophage populations

The liver is the largest solid organ in the body and is critical for metabolic and immune functions. However, little is known about the cells that make up the human liver and its immune microenvironment. Here we report a map of the cellular landscape of the human liver using single-cell RNA sequencing. We provide the transcriptional profiles of 8444 parenchymal and non-parenchymal cells obtained from the fractionation of fresh hepatic tissue from five human livers. Using gene expression patterns, flow cytometry, and immunohistochemical examinations, we identify 20 discrete cell populations of hepatocytes, endothelial cells, cholangiocytes, hepatic stellate cells, B cells, conventional and non-conventional T cells, NK-like cells, and distinct intrahepatic monocyte/macrophage populations. Together, our study presents a comprehensive view of the human liver at single-cell resolution that outlines the characteristics of resident cells in the liver, and in particular provides a map of the human hepatic immune microenvironment.

The development of single cell RNA sequencing technologies has been instrumental in advancing our understanding of tissue biology. Here, MacParland et al. performed single cell RNA sequencing of human liver samples, and identify distinct populations of intrahepatic macrophages that may play specific roles in liver disease.

Role of Chemosensory TRP Channels in Lung Cancer

Transient receptor potential (TRP) channels represent a large family of cation channels and many members of the TRP family have been shown to act as polymodal receptor molecules for irritative or potentially harmful substances. These chemosensory TRP channels have been extensively characterized in primary sensory and neuronal cells. However, in recent years the functional expression of these proteins in non-neuronal cells, e.g., in the epithelial lining of the respiratory tract has been confirmed. Notably, these proteins have also been described in a number of cancer types. As sensor molecules for noxious compounds, chemosensory TRP channels are involved in cell defense mechanisms and influence cell survival following exposure to toxic substances via the modulation of apoptotic signaling. Of note, a number of cytostatic drugs or drug metabolites can activate these TRP channels, which could affect the therapeutic efficacy of these cytostatics. Moreover, toxic inhalational substances with potential involvement in lung carcinogenesis are well established TRP activators. In this review, we present a synopsis of data on the expression of chemosensory TRP channels in lung cancer cells and describe TRP agonists and TRP-dependent signaling pathways with potential relevance to tumor biology. Furthermore, we discuss a possible role of TRP channels in the non-genomic, tumor-promoting effects of inhalational carcinogens such as cigarette smoke.

MicroRNA (miR)-433 and miR-22 dysregulations induce histone-deacetylase-6 overexpression and ciliary loss in cholangiocarcinoma

Cholangiocytes normally express primary cilia, a multisensory organelle that detects signals from the cellular environment. Cilia are significantly reduced in cholangiocarcinoma (CCA) by a mechanism involving overexpression of histone deacetylase 6 (HDAC6). Despite HDAC6 overexpression in CCA, we found no differences in its mRNA level, suggesting a posttranscriptional regulation, possibly involving microRNAs (miRNAs). Here, we describe that at least two HDAC6-targeting miRNAs, miR-433 and miR-22, are down-regulated in CCA both in vitro and in vivo. Experimental restoration of these miRNAs in CCA cells reduced HDAC6 expression, induced ciliary restoration, and decreased the malignant phenotype. Furthermore, in contrast to the mature forms, levels of precursor forms of these miRNAs were higher in CCA compared to normal cholangiocytes and accumulated in the nuclei, suggesting a defective nuclear export. We assessed the expression of Exportin-5, the protein responsible for transporting miRNA precursors out of the nucleus, and found it to be reduced by 50% in CCA compared to normal cholangiocytes. Experimental overexpression of Exportin-5 in CCA cells restored precursor and mature forms of these miRNAs to normal levels, inducing a decrease in the expression of HDAC6 and a decrease in the malignant phenotype. Conversely, short hairpin RNA (shRNA) depletion of Exportin-5 in normal cholangiocytes resulted in increased nuclear retention of precursor miRNAs, decreased mature miRNAs, increased cell proliferation, and shorter cilia.

CONCLUSION

These data suggest that down-regulated Exportin-5 impairs the nuclear export of miR-433 and miR-22 precursor forms, causing a decrease in levels of mature miR-433 and miR-22 forms, and leading to overexpression of HDAC6 and ciliary loss in CCA. (Hepatology 2018).

Role of Bile Acids and the Biliary HCO3- Umbrella in the Pathogenesis of Primary Biliary Cholangitis

The biliary HCO₃^- umbrella hypothesis states that human cholangiocytes and hepatocytes create a protective apical alkaline barrier against millimolar concentrations of potentially toxic glycine-conjugated bile salts in bile by secreting HCO₃^- into the bile duct lumen. This alkaline barrier may retain biliary bile salts in their polar, deprotonated, and membrane-impermeant state to avoid uncontrolled invasion of apolar toxic bile acids, which initiate apoptosis, autophagy and senescence. In primary biliary cholangitis, defects of the biliary HCO₃^- umbrella, leading to impaired biliary HCO₃^- secretion have been identified. Current medical therapies stabilize the putatively defective biliary HCO₃^- umbrella and improve long-term prognosis.

Calcium Signaling in Liver Injury and Regeneration

The liver fulfills central roles in metabolic control and detoxification and, as such, is continuously exposed to a plethora of insults. Importantly, the liver has a unique ability to regenerate and can completely recoup from most acute, non-iterative insults. However, multiple conditions, including viral hepatitis, non-alcoholic fatty liver disease (NAFLD), long-term alcohol abuse and chronic use of certain medications, can cause persistent injury in which the regenerative capacity eventually becomes dysfunctional, resulting in hepatic scaring and cirrhosis. Calcium is a versatile secondary messenger that regulates multiple hepatic functions, including lipid and carbohydrate metabolism, as well as bile secretion and choleresis. Accordingly, dysregulation of calcium signaling is a hallmark of both acute and chronic liver diseases. In addition, recent research implicates calcium transients as essential components of liver regeneration. In this review, we provide a comprehensive overview of the role of calcium signaling in liver health and disease and discuss the importance of calcium in the orchestration of the ensuing regenerative response. Furthermore, we highlight similarities and differences in spatiotemporal calcium regulation between liver insults of different etiologies. Finally, we discuss intracellular calcium control as an emerging therapeutic target for liver injury and summarize recent clinical findings of calcium modulation for the treatment of ischemic-reperfusion injury, cholestasis and NAFLD.

Therapeutic Targets in Polycystic Liver Disease

Polycystic liver diseases (PLD) are a group of genetic disorders initiated by mutations in several PLD-related genes and characterized by the presence of multiple cholangiocyte-derived hepatic cysts that progressively replace liver tissue. PLD co-exists with Autosomal Dominant Polycystic Kidney Disease (ADPKD) and Autosomal Recessive PKD as well as occurs alone (i.e., Autosomal Dominant Polycystic Liver Disease [ADPLD]). PLD associated with ADPKD and ARPKD belong to a group of disorders known as cholangiociliopathies since many disease-causative and disease-related proteins are expressed in primary cilia of cholangiocytes. Aberrant expression of these proteins in primary cilia affects their structures and functions promoting cystogenesis. Current medical therapies for PLD include symptomatic management and surgical interventions. To date, the only available drug treatment for PLD patients that halt disease progression and improve quality of life are somatostatin analogs. However, the modest clinical benefits, need for long-term maintenance therapy, and the high cost of treatment justify the necessity for more effective treatment options. Substantial evidence suggests that experimental manipulations with components of the signaling pathways that influence cyst development (e.g., cAMP, intracellular calcium, receptor tyrosine kinase, transient receptor potential cation channel subfamily V member 4 (TRPV4) channel, mechanistic target of rapamycin (mTOR), histone deacetylase (HDAC6), Cdc25A phosphatase, miRNAs and metalloproteinases) attenuate growth of hepatic cysts. Many of these targets have been evaluated in pre-clinical trials suggesting their value as potential new therapies. This review outlines the current clinical and preclinical treatment strategies for PLD.

Primary Cilia in Tumor Biology: The Primary Cilium as a Therapeutic Target in Cholangiocarcinoma

Cilia are microtubule-based organelles, which are ubiquitously expressed in epithelial cells. Cholangiocytes, the epithelial cells lining the biliary tree, have primary cilia extending from their apical plasma membrane into the ductal lumen, where the cilia function as multisensory organelles transducing environmental cues into the cell interior. The decrease or loss of primary cilia has been described in several malignancies, including cholangiocarcinoma, suggesting that the loss of cilia is a common occurrence in neoplastic transformation. In this short review, we describe the expression of cilia in several cancers, explore the mechanisms and consequences of ciliary loss, and discuss the potential use of the primary cilia as therapeutic targets.

Pathobiology of biliary epithelia

Cholangiocytes are epithelial cells that line the intra- and extrahepatic biliary tree. They serve predominantly to mediate the content of luminal biliary fluid, which is controlled via numerous signaling pathways influenced by endogenous (e.g., bile acids, nucleotides, hormones, neurotransmitters) and exogenous (e.g., microbes/microbial products, drugs etc.) molecules. When injured, cholangiocytes undergo apoptosis/lysis, repair and proliferation. They also become senescent, a form of cell cycle arrest, which may prevent propagation of injury and/or malignant transformation. Senescent cholangiocytes can undergo further transformation to a senescence-associated secretory phenotype (SASP), where they begin secreting pro-inflammatory and pro-fibrotic signals that may contribute to disease initiation and progression. These and other concepts related to cholangiocyte pathobiology will be reviewed herein. This article is part of a Special Issue entitled: Cholangiocytes in Health and Disease edited by Jesus Banales, Marco Marzioni, Nicholas LaRusso and Peter Jansen.

The cholangiocyte primary cilium in health and disease

Cholangiocytes, like most cells, express primary cilia extending from their membranes. These organelles function as antennae which detect stimuli from bile and transmit the information into cells regulating several signaling pathways involved in secretion, proliferation and apoptosis. The ability of primary cilia to detect different signals is provided by ciliary associated proteins which are expressed in its membrane. Defects in the structure and/or function of these organelles lead to cholangiociliopathies that result in cholangiocyte hyperproliferation, altered fluid secretion and absorption. Since primary cilia dysfunction has been observed in several epithelial tumors, including cholangiocarcinoma (CCA), primary cilia have been proposed as tumor suppressor organelles. In addition, the loss of cilia is associated with dysregulation of several molecular pathways resulting in CCA development and progression. Thus, restoration of the primary cilia may be a potential therapeutic approach for several ciliopathies and CCA.

Calcium channels in primary cilia

PURPOSE OF REVIEW

Primary cilia have become important organelles implicated in embryonic development, organogenesis, health, and diseases. Although many studies in cell biology have focused on changes in ciliary length or ciliogenesis, the most common readout for evaluating ciliary function is intracellular calcium.

RECENT FINDINGS

Recent tools have allowed us to examine intracellular calcium in more precise locations, that is, the cilioplasm and cytoplasm. Advances in calcium imaging have also allowed us to identify which cilia respond to particular stimuli. Furthermore, direct electrophysiological measurement of ionic currents within a cilium has provided a wealth of information for understanding the sensory roles of primary cilia.

SUMMARY

Calcium imaging and direct measurement of calcium currents demonstrate that primary cilia are sensory organelles that house several types of functional calcium channels. Although intracellular calcium now allows a functional readout for primary cilia, discussions on the relative contributions of the several channel types have just begun. Perhaps, all of these calcium channels are required and necessary to differentiate stimuli in different microenvironments.

Primary cilia: Cell and molecular mechanosensors directing whole tissue function

Primary cilia are immotile, microtubule-based organelles extending from the surface of nearly every mammalian cell. Mechanical stimulation causes deflection of the primary cilium, initiating downstream signaling cascades to the rest of the cell. The cilium forms a unique subcellular microdomain, and defects in ciliary protein composition or physical structure have been associated with a myriad of human pathologies. In this review, we discuss the importance of ciliary mechanotransduction at the cell and tissue level, and how furthering our molecular understanding of primary cilia mechanobiology may lead to therapeutic strategies to treat human diseases.

Hyponatremia and the Brain

Hyponatremia is defined by low serum sodium concentration and is the most common electrolyte disorder encountered in clinical practice. Serum sodium is the main determinant of plasma osmolality, which, in turn, affects cell volume. In the presence of low extracellular osmolality, cells will swell if the adaptation mechanisms involved in the cell volume maintenance are inadequate. The most dramatic effects of hyponatremia on the brain are seen when serum sodium concentration decreases in a short period, allowing little or no adaptation. The brain is constrained inside a nonextensible envelope; thus, brain swelling carries a significant morbidity because of the compression of brain parenchyma over the rigid skull. Serum sodium concentration is an important determinant of several biological pathways in the nervous system, and recent studies have suggested that hyponatremia carries a significant risk of neurological impairment even in the absence of brain edema. The brain can also be affected by the treatment of hyponatremia, which, if not undertaken cautiously, could lead to osmotic demyelination syndrome, a rare demyelinating brain disorder that occurs after rapid correction of severe hyponatremia. This review summarizes the pathophysiology of brain complications of hyponatremia and its treatment.