Polycystins

Mechanosensing by Piezo1 and its implications in the kidney

Piezo1 is an essential mechanosensitive transduction ion channel in mammals. Its unique structure makes it capable of converting mechanical cues into electrical and biological signals, modulating biological and (patho)physiological processes in a wide variety of cells. There is increasing evidence demonstrating that the piezo1 channel plays a vital role in renal physiology and disease conditions. This review summarizes the current evidence on the structure and properties of Piezo1, gating modulation, and pharmacological characteristics, with special focus on the distribution and (patho)physiological significance of Piezo1 in the kidney, which may provide insights into potential treatment targets for renal diseases involving this ion channel.

Insights into the mechanisms of neuron generation and specification in the zebrafish ventral spinal cord

The vertebrate nervous system is composed of a wide range of neurons and complex synaptic connections, raising the intriguing question of how neuronal diversity is generated. The spinal cord provides an excellent model for exploring the mechanisms governing neuronal diversity due to its simple neural network and the conserved molecular processes involved in neuron formation and specification during evolution. This review specifically examines two distinct progenitor domains present in the zebrafish ventral spinal cord: the lateral floor plate (LFP) and the p2 progenitor domain. The LFP is responsible for the production of GABAergic Kolmer-Agduhr neurons (KA″), glutamatergic V3 neurons, and intraspinal serotonergic neurons, while the p2 domain generates V2 precursors that subsequently differentiate into three unique subpopulations of V2 neurons, namely glutamatergic V2a, GABAergic V2b, and glycinergic V2s. Based on recent findings, we will examine the fundamental signaling pathways and transcription factors that play a key role in the specification of these diverse neurons and neuronal subtypes derived from the LFP and p2 progenitor domains.

Convergence of Calcium Channel Regulation and Mechanotransduction in Skeletal Regenerative Biomaterial Design

Cells are known to perceive their microenvironment through extracellular and intracellular mechanical signals. Upon sensing mechanical stimuli, cells can initiate various downstream signaling pathways that are vital to regulating proliferation, growth, and homeostasis. One such physiologic activity modulated by mechanical stimuli is osteogenic differentiation. The process of osteogenic mechanotransduction is regulated by numerous calcium ion channels-including channels coupled to cilia, mechanosensitive and voltage-sensitive channels, and channels associated with the endoplasmic reticulum. Evidence suggests these channels are implicated in osteogenic pathways such as the YAP/TAZ and canonical Wnt pathways. This review aims to describe the involvement of calcium channels in regulating osteogenic differentiation in response to mechanical loading and characterize the fashion in which those channels directly or indirectly mediate this process. The mechanotransduction pathway is a promising target for the development of regenerative materials for clinical applications due to its independence from exogenous growth factor supplementation. As such, also described are examples of osteogenic biomaterial strategies that involve the discussed calcium ion channels, calcium-dependent cellular structures, or calcium ion-regulating cellular features. Understanding the distinct ways calcium channels and signaling regulate these processes may uncover potential targets for advancing biomaterials with regenerative osteogenic capabilities.

A Comparison of PKD2L1-Expressing Cerebrospinal Fluid Contacting Neurons in Spinal Cords of Rodents, Carnivores, and Primates

Cerebrospinal fluid contacting neurons (CSF-cNs) are a specific type of neurons located around the ventricles in the brain and the central canal in the spinal cord and have been demonstrated to be intrinsic sensory neurons in the central nervous system. One of the important channels responsible for the sensory function is the polycystic kidney disease 2-like 1 (PKD2L1) channel. Most of the studies concerning the distribution and function of the PKD2L1-expressing CSF-cNs in the spinal cord have previously been performed in non-mammalian vertebrates. In the present study immunohistochemistry was performed to determine the distribution of PKD2L1-immunoreactive (IR) CSF-cNs in the spinal cords of four mammalian species: mouse, rat, cat, and macaque monkey. Here, we found that PKD2L1-expressing CSF-cNs were present at all levels of the spinal cord in these animal species. Although the distribution pattern was similar across these species, differences existed. Mice and rats presented a clear PKD2L1-IR cell body labeling, whereas in cats and macaques the PKD2L1-IR cell bodies were more weakly labeled. Ectopic PKD2L1-IR neurons away from the ependymal layer were observed in all the animal species although the abundance and the detailed locations varied. The apical dendritic protrusions with ciliated fibers were clearly seen in the lumen of the central canal in all the animal species, but the sizes of protrusion bulbs were different among the species. PKD2L1-IR cell bodies/dendrites were co-expressed with doublecortin, MAP2 (microtubule-associated protein 2), and aromatic L-amino acid decarboxylase, but not with NeuN (neuronal nuclear protein), indicating their immature properties and ability to synthesize monoamine transmitters. In addition, in situ hybridization performed in rats revealed PKD2L1 mRNA expression in the cells around the central canal. Our results indicate that the intrinsic sensory neurons are conserved across non-mammalian and mammalian vertebrates. The similar morphology of the dendritic bulbs with ciliated fibers (probably representing stereocilia and kinocilia) protruding into the central canal across different animal species supports the notion that PKD2L1 is a chemo- and mechanical sensory channel that responds to mechanical stimulations and maintains homeostasis of the spinal cord. However, the differences of PKD2L1 distribution and expression between the species suggest that PKD2L1-expressing neurons may receive and process sensory signals differently in different animal species.

Cerebrospinal fluid-contacting neuron tracing reveals structural and functional connectivity for locomotion in the mouse spinal cord

Cerebrospinal fluid-contacting neurons (CSF-cNs) are enigmatic mechano- or chemosensory cells lying along the central canal of the spinal cord. Recent studies in zebrafish larvae and lampreys have shown that CSF-cNs control postures and movements via spinal connections. However, the structures, connectivity, and functions in mammals remain largely unknown. Here we developed a method to genetically target mouse CSF-cNs that highlighted structural connections and functions. We first found that intracerebroventricular injection of adeno-associated virus with a neuron-specific promoter and Pkd2l1-Cre mice specifically labeled CSF-cNs. Single-cell labeling of 71 CSF-cNs revealed rostral axon extensions of over 1800 μm in unmyelinated bundles in the ventral funiculus and terminated on CSF-cNs to form a recurrent circuitry, which was further determined by serial electron microscopy and electrophysiology. CSF-cNs were also found to connect with axial motor neurons and premotor interneurons around the central canal and within the axon bundles. Chemogenetic CSF-cNs inactivation reduced speed and step frequency during treadmill locomotion. Our data revealed the basic structures and connections of mouse CSF-cNs to control spinal motor circuits for proper locomotion. The versatile methods developed in this study will contribute to further understanding of CSF-cN functions in mammals.

Graded spikes differentially signal neurotransmitter input in cerebrospinal fluid contacting neurons of the mouse spinal cord

The action potential and its all-or-none nature is fundamental to neural communication. Canonically, the action potential is initiated once voltage-activated Na⁺ channels are activated, and their rapid kinetics of activation and inactivation give rise to the action potential's all-or-none nature. Here we demonstrate that cerebrospinal fluid contacting neurons (CSFcNs) surrounding the central canal of the mouse spinal cord employ a different strategy. Rather than using voltage-activated Na⁺ channels to generate binary spikes, CSFcNs use two different types of voltage-activated Ca²⁺ channel, enabling spikes of different amplitude. T-type Ca²⁺ channels generate small amplitude spikes, whereas larger amplitude spikes require high voltage-activated Cd²⁺-sensitive Ca²⁺ channels. We demonstrate that these different amplitude spikes can signal input from different transmitter systems; purinergic inputs evoke smaller T-type dependent spikes whereas cholinergic inputs evoke larger spikes that do not rely on T-type channels. Different synaptic inputs to CSFcNs can therefore be signaled by the spike amplitude.

Fetal nuchal edema and developmental anomalies caused by gene mutations in mice

Fetal nuchal edema, a subcutaneous accumulation of extracellular fluid in the fetal neck, is detected as increased nuchal translucency (NT) by ultrasonography in the first trimester of pregnancy. It has been demonstrated that increased NT is associated with chromosomal anomalies and genetic syndromes accompanied with fetal malformations such as defective lymphatic vascular development, cardiac anomalies, anemia, and a wide range of other fetal anomalies. However, in many clinical cases of increased NT, causative genes, pathogenesis and prognosis have not been elucidated in humans. On the other hand, a large number of gene mutations have been reported to induce fetal nuchal edema in mouse models. Here, we review the relationship between the gene mutants causing fetal nuchal edema with defective lymphatic vascular development, cardiac anomalies, anemia and blood vascular endothelial barrier anomalies in mice. Moreover, we discuss how studies using gene mutant mouse models will be useful in developing diagnostic method and predicting prognosis.

TRPP2 ion channels: The roles in various subcellular locations

TRPP2 (PC2, PKD2 or Polycytin-2), encoded by PKD2 gene, belongs to the nonselective cation channel TRP family. Recently, the three-dimensional structure of TRPP2 was constructed. TRPP2 mainly functions in three subcellular compartments: endoplasmic reticulum, plasma membrane and primary cilia. TRPP2 can act as a calcium-activated intracellular calcium release channel on the endoplasmic reticulum. TRPP2 also interacts with other Ca²⁺ release channels to regulate calcium release, like IP3R and RyR2. TRPP2 acts as an ion channel regulated by epidermal growth factor through activation of downstream factors in the plasma membrane. TRPP2 binding to TRPC1 in the plasma membrane or endoplasmic reticulum is associated with mechanosensitivity. In cilium, TRPP2 was found to combine with PKD1 and TRPV4 to form a complex related to mechanosensitivity. Because TRPP2 is involved in regulating intracellular ion concentration, TRPP2 mutations often lead to autosomal dominant polycystic kidney disease, which may also be associated with cardiovascular disease. In this paper, we review the molecular structure of TRPP2, the subcellular localization of TRPP2, the related functions and mechanisms of TRPP2 at different sites, and the diseases related to TRPP2.

Cilia-Localized Counterregulatory Signals as Drivers of Renal Cystogenesis

Primary cilia play counterregulatory roles in cystogenesis-they inhibit cyst formation in the normal renal tubule but promote cyst growth when the function of polycystins is impaired. Key upstream cilia-specific signals and components involved in driving cystogenesis have remained elusive. Recent studies of the tubby family protein, Tubby-like protein 3 (TULP3), have provided new insights into the cilia-localized mechanisms that determine cyst growth. TULP3 is a key adapter of the intraflagellar transport complex A (IFT-A) in the trafficking of multiple proteins specifically into the ciliary membrane. Loss of TULP3 results in the selective exclusion of its cargoes from cilia without affecting their extraciliary pools and without disrupting cilia or IFT-A complex integrity. Epistasis analyses have indicated that TULP3 inhibits cystogenesis independently of the polycystins during kidney development but promotes cystogenesis in adults when polycystins are lacking. In this review, we discuss the current model of the cilia-dependent cyst activation (CDCA) mechanism in autosomal dominant polycystic kidney disease (ADPKD) and consider the possible roles of ciliary and extraciliary polycystins in regulating CDCA. We then describe the limitations of this model in not fully accounting for how cilia single knockouts cause significant cystic changes either in the presence or absence of polycystins. Based on available data from TULP3/IFT-A-mediated differential regulation of cystogenesis in kidneys with deletion of polycystins either during development or in adulthood, we hypothesize the existence of cilia-localized components of CDCA (cCDCA) and cilia-localized cyst inhibition (CLCI) signals. We develop the criteria for cCDCA/CLCI signals and discuss potential TULP3 cargoes as possible cilia-localized components that determine cystogenesis in kidneys during development and in adult mice.

Cnidarian hair cell development illuminates an ancient role for the class IV POU transcription factor in defining mechanoreceptor identity

Although specialized mechanosensory cells are found across animal phylogeny, early evolutionary histories of mechanoreceptor development remain enigmatic. Cnidaria (e.g. sea anemones and jellyfishes) is the sister group to well-studied Bilateria (e.g. flies and vertebrates), and has two mechanosensory cell types – a lineage-specific sensory effector known as the cnidocyte, and a classical mechanosensory neuron referred to as the hair cell. While developmental genetics of cnidocytes is increasingly understood, genes essential for cnidarian hair cell development are unknown. Here, we show that the class IV POU homeodomain transcription factor (POU-IV) – an indispensable regulator of mechanosensory cell differentiation in Bilateria and cnidocyte differentiation in Cnidaria – controls hair cell development in the sea anemone cnidarian Nematostella vectensis. N. vectensis POU-IV is postmitotically expressed in tentacular hair cells, and is necessary for development of the apical mechanosensory apparatus, but not of neurites, in hair cells. Moreover, it binds to deeply conserved DNA recognition elements, and turns on a unique set of effector genes – including the transmembrane receptor-encoding gene polycystin 1 – specifically in hair cells. Our results suggest that POU-IV directs differentiation of cnidarian hair cells and cnidocytes via distinct gene regulatory mechanisms, and support an evolutionarily ancient role for POU-IV in defining the mature state of mechanosensory neurons.

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.

Transcriptional profiling of identified neurons in leech

Background

While leeches in the genus Hirudo have long been models for neurobiology, the molecular underpinnings of nervous system structure and function in this group remain largely unknown. To begin to bridge this gap, we performed RNASeq on pools of identified neurons of the central nervous system (CNS): sensory T (touch), P (pressure) and N (nociception) neurons; neurosecretory Retzius cells; and ganglia from which these four cell types had been removed.

Results

Bioinformatic analyses identified 3565 putative genes whose expression differed significantly among the samples. These genes clustered into 9 groups which could be associated with one or more of the identified cell types. We verified predicted expression patterns through in situ hybridization on whole CNS ganglia, and found that orthologous genes were for the most part similarly expressed in a divergent leech genus, suggesting evolutionarily conserved roles for these genes. Transcriptional profiling allowed us to identify candidate phenotype-defining genes from expanded gene families. Thus, we identified one of eight hyperpolarization-activated cyclic-nucleotide gated (HCN) channels as a candidate for mediating the prominent sag current in P neurons, and found that one of five inositol triphosphate receptors (IP3Rs), representing a sub-family of IP3Rs absent from vertebrate genomes, is expressed with high specificity in T cells. We also identified one of two piezo genes, two of ~ 65 deg/enac genes, and one of at least 16 transient receptor potential (trp) genes as prime candidates for involvement in sensory transduction in the three distinct classes of leech mechanosensory neurons.

Conclusions

Our study defines distinct transcriptional profiles for four different neuronal types within the leech CNS, in addition to providing a second ganglionic transcriptome for the species. From these data we identified five gene families that may facilitate the sensory capabilities of these neurons, thus laying the basis for future work leveraging the strengths of the leech system to investigate the molecular processes underlying and linking mechanosensation, cell type specification, and behavior.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07526-0.

Canonical Transient Receptor Potential (TRPC) Channels in Nociception and Pathological Pain

Chronic pathological pain is one of the most intractable clinical problems faced by clinicians and can be devastating for patients. Despite much progress we have made in understanding chronic pain in the last decades, its underlying mechanisms remain elusive. It is assumed that abnormal increase of calcium levels in the cells is a key determinant in the transition from acute to chronic pain. Exploring molecular players mediating Ca²⁺ entry into cells and molecular mechanisms underlying activity-dependent changes in Ca²⁺ signaling in the somatosensory pain pathway is therefore helpful towards understanding the development of chronic, pathological pain. Canonical transient receptor potential (TRPC) channels form a subfamily of nonselective cation channels, which permit the permeability of Ca²⁺ and Na+ into the cells. Initiation of Ca²⁺ entry pathways by these channels triggers the development of many physiological and pathological functions. In this review, we will focus on the functional implication of TRPC channels in nociception with the elucidation of their role in the detection of external stimuli and nociceptive hypersensitivity.

The primary cilia in diabetic kidney disease: A tubulocentric view?

Diabetic kidney disease is growing exponentially. This review aims to discuss alternate therapeutic approaches beyond the glomerulocentric view and to consider a novel tubulocentric approach with focus on the primary cilia. Renin-angiotensin-aldosterone system blockade to decrease glomerular capillary pressure and prevent albuminuria has been the mainstay of treatment for diabetic and non-diabetic proteinuric kidney disease. Landmark clinical trials have also shown cardiorenal benefit with sodium-glucose linked co-transporter 2 inhibitors and glucagon-like peptide 1 receptor analogues in patients with type 2 diabetes. Effective renoprotective drugs seem to have a common mechanistic mode of reducing glomerular hyperfiltration/hypertension. In the tubules, primary cilia act as "antennae" to detect mechanosensory changes such as glomerular hyperfiltration and trgger intracellular signalling pathways. They are also implicated in obesity and metabolic disorders linked to diabetes. To conclude, primary cilia of the kidney tubules offer a novel therapeutic target and may complement the current glomerulocentric approaches.

Structure and function of polycystins: insights into polycystic kidney disease

Mutations in the polycystins PC1 or PC2 cause autosomal dominant polycystic kidney disease (ADPKD), which is characterized by the formation of fluid-filled renal cysts that disrupt renal architecture and function, ultimately leading to kidney failure in the majority of patients. Although the genetic basis of ADPKD is now well established, the physiological function of polycystins remains obscure and a matter of intense debate. The structural determination of both the homomeric PC2 and heteromeric PC1-PC2 complexes, as well as the electrophysiological characterization of PC2 in the primary cilium of renal epithelial cells, provided new valuable insights into the mechanisms of ADPKD pathogenesis. Current findings indicate that PC2 can function independently of PC1 in the primary cilium of renal collecting duct epithelial cells to form a channel that is mainly permeant to monovalent cations and is activated by both membrane depolarization and an increase in intraciliary calcium. In addition, PC2 functions as a calcium-activated calcium release channel at the endoplasmic reticulum membrane. Structural studies indicate that the heteromeric PC1-PC2 complex comprises one PC1 and three PC2 channel subunits. Surprisingly, several positively charged residues from PC1 occlude the ionic pore of the PC1-PC2 complex, suggesting that pathogenic polycystin mutations might cause ADPKD independently of an effect on channel permeation. Emerging reports of novel structural and functional findings on polycystins will continue to elucidate the molecular basis of ADPKD.

The role of motile cilia in the development and physiology of the nervous system

Motile cilia are miniature, whip-like organelles whose beating generates a directional fluid flow. The flow generated by ciliated epithelia is a subject of great interest, as defective ciliary motility results in severe human diseases called motile ciliopathies. Despite the abundance of motile cilia in diverse organs including the nervous system, their role in organ development and homeostasis remains poorly understood. Recently, much progress has been made regarding the identity of motile ciliated cells and the role of motile-cilia-mediated flow in the development and physiology of the nervous system. In this review, we will discuss these recent advances from sensory organs, specifically the nose and the ear, to the spinal cord and brain ventricles. This article is part of the Theo Murphy meeting issue 'Unity and diversity of cilia in locomotion and transport'.

Expression and localization of transient receptor potential channels in the bovine uterus epithelium throughout the estrous cycle

Transient receptor potential (TRP) channels are expressed in the endometrium but it is unknown if they are modulated through the estrous cycle (EC). This study was undertaken to identify the modulation of the TRPC gene and protein isoforms in bovine uterine epithelium, as a model for human, throughout the EC. Changes in the expression of TRPC genes in bovine uterine epithelium throughout the EC were measured using Real-Time PCR, while immunohistochemistry and immunocytochemistry were used to determine the localization of these channels. Out of the 7 members of the TRPC family, TRPC1, 2, 3, 4 and 6 genes were expressed in bovine uterine epithelial tissue and TRPC 5 and 7 were not. Gene expression levels of all TRPC isoforms underwent cyclical changes throughout the EC. Moreover, cyclical changes were detected in the protein levels of TRPC1 and TRPC6 throughout the EC. These findings show that TRPC channels are modulated through the EC and therefore may have a role in reproductive events.

Pkd2l1 is required for mechanoception in cerebrospinal fluid-contacting neurons and maintenance of spine curvature

Defects in cerebrospinal fluid (CSF) flow may contribute to idiopathic scoliosis. However, the mechanisms underlying detection of CSF flow in the central canal of the spinal cord are unknown. Here we demonstrate that CSF flows bidirectionally along the antero-posterior axis in the central canal of zebrafish embryos. In the cfap298^tm304 mutant, reduction of cilia motility slows transport posteriorly down the central canal and abolishes spontaneous activity of CSF-contacting neurons (CSF-cNs). Loss of the sensory Pkd2l1 channel nearly abolishes CSF-cN calcium activity and single channel opening. Recording from isolated CSF-cNs in vitro, we show that CSF-cNs are mechanosensory and require Pkd2l1 to respond to pressure. Additionally, adult pkd2l1 mutant zebrafish develop an exaggerated spine curvature, reminiscent of kyphosis in humans. These results indicate that CSF-cNs are mechanosensory cells whose Pkd2l1-driven spontaneous activity reflects CSF flow in vivo. Furthermore, Pkd2l1 in CSF-cNs contributes to maintenance of natural curvature of the spine.

Alteration of cerebrospinal fluid (CSF) flow and cilia defects are clinically associated with idiopathic scoliosis. This study shows that transient receptor potential channel Pkd2l1 is required for mechanosensory function of neurons detecting CSF flow and normal spine curvature development in zebrafish.

A case of congenital left ventricular diverticulum in a patient with autosomal dominant polycystic kidney disease: possible mechanistic link between polycystin and ventricular diverticulum

A 40-year-old woman had been followed as an outpatient to manage chronic kidney disease secondary to autosomal dominant polycystic kidney disease (ADPKD). Atrial premature contraction was found incidentally on an electrocardiogram during her regular follow-up examination. Subsequent transthoracic echocardiography detected an abnormal structure located very close to the left ventricular outflow tract (23 mm long × 15 mm wide in diastole). The structure was finally diagnosed as congenital left ventricular diverticulum (CLVD) using transesophageal echocardiography, contrast-enhanced computed tomography, and magnetic resonance imaging. Although CLVD occasionally causes intraventricular coagulation, lethal arrhythmia, and congestive heart failure, the size and location of her diverticulum remained unchanged over time and a 24-h Holter electrocardiogram showed no lethal arrhythmias. Accordingly, neither anticoagulation therapy nor surgical resection of the diverticulum was performed. To the best of our knowledge, ours is the first case of CLVD in a patient with ADPKD. Because gene abnormalities in polycystin coding are mechanistically related to the development of colonic diverticulum and abnormal cyst formation in ADPKD patients, we suspected that CLVD and abnormal cyst formation were related to the same gene abnormality in ADPKD. More case reports, case series studies, and basic research are required to determine whether CLVD in ADPKD is mechanistically associated with abnormal polycystin or just a coincidence.

The Significant Role of c-Abl Kinase in Barrier Altering Agonists-mediated Cytoskeletal Biomechanics

Exploration of human pulmonary artery endothelial cell (EC) as a prototypical biomechanical system has important pathophysiologic relevance because this cell type plays a key role in the development of a wide variety of clinical conditions. The complex hierarchical organization ranging from the molecular scale up to the cellular level has an intimate and intricate relationship to the barrier function between lung tissue and blood. To understand the innate molecule-cell-tissue relationship across varied length-scales, the functional role of c-Abl kinase in the cytoskeletal nano-biomechanics of ECs in response to barrier-altering agonists was investigated using atomic force microscopy. Concurrently, the spatially specific arrangement of cytoskeleton structure and dynamic distribution of critical proteins were examined using scanning electron microscopy and immunofluorescence. Reduction in c-Abl expression by siRNA attenuates both thrombin- and sphingosine 1-phosphate (S1P)-mediated structural changes in ECs, specifically spatially-defined changes in elastic modulus and distribution of critical proteins. These results indicate that c-Abl kinase is an important determinant of cortical actin-based cytoskeletal rearrangement. Our findings directly bridge the gap between kinase activity, structural complexity, and functional connectivity across varied length-scales, and suggest that manipulation of c-Abl kinase activity may be a potential target for the treatment of pulmonary barrier disorders.

Molecular basis of dental sensitivity: The odontoblasts are multisensory cells and express multifunctional ion channels

Odontoblasts are the dental pulp cells responsible for the formation of dentin. In addition, accumulating data strongly suggest that they can also function as sensory cells that mediate the early steps of mechanical, thermic, and chemical dental sensitivity. This assumption is based on the expression of different families of ion channels involved in various modalities of sensitivity and the release of putative neurotransmitters in response to odontoblast stimulation which are able to act on pulp sensory nerve fibers. This review updates the current knowledge on the expression of transient-potential receptor ion channels and acid-sensing ion channels in odontoblasts, nerve fibers innervating them and trigeminal sensory neurons, as well as in pulp cells. Moreover, the innervation of the odontoblasts and the interrelationship been odontoblasts and nerve fibers mediated by neurotransmitters was also revisited. These data might provide the basis for novel therapeutic approaches for the treatment of dentin sensibility and/or dental pain.

Hyperosmotic stress stimulates autophagy via polycystin-2

Various intracellular mechanisms are activated in response to stress, leading to adaptation or death. Autophagy, an intracellular process that promotes lysosomal degradation of proteins, is an adaptive response to several types of stress. Osmotic stress occurs under both physiological and pathological conditions, provoking mechanical stress and activating various osmoadaptive mechanisms. Polycystin-2 (PC2), a membrane protein of the polycystin family, is a mechanical sensor capable of activating the cell signaling pathways required for cell adaptation and survival. Here we show that hyperosmotic stress provoked by treatment with hyperosmolar concentrations of sorbitol or mannitol induces autophagy in HeLa and HCT116 cell lines. In addition, we show that mTOR and AMPK, two stress sensor proteins involved modulating autophagy, are downregulated and upregulated, respectively, when cells are subjected to hyperosmotic stress. Finally, our findings show that PC2 is required to promote hyperosmotic stress-induced autophagy. Downregulation of PC2 prevents inhibition of hyperosmotic stress-induced mTOR pathway activation. In conclusion, our data provide new insight into the role of PC2 as a mechanosensor that modulates autophagy under hyperosmotic stress conditions.

The dual developmental origin of spinal cerebrospinal fluid-contacting neurons gives rise to distinct functional subtypes

Chemical and mechanical cues from the cerebrospinal fluid (CSF) can affect the development and function of the central nervous system (CNS). How such cues are detected and relayed to the CNS remains elusive. Cerebrospinal fluid-contacting neurons (CSF-cNs) situated at the interface between the CSF and the CNS are ideally located to convey such information to local networks. In the spinal cord, these GABAergic neurons expressing the PKD2L1 channel extend an apical extension into the CSF and an ascending axon in the spinal cord. In zebrafish and mouse spinal CSF-cNs originate from two distinct progenitor domains characterized by distinct cascades of transcription factors. Here we ask whether these neurons with different developmental origins differentiate into cells types with different functional properties. We show in zebrafish larva that the expression of specific markers, the morphology of the apical extension and axonal projections, as well as the neuronal targets contacted by CSF-cN axons, distinguish the two CSF-cN subtypes. Altogether our study demonstrates that the developmental origins of spinal CSF-cNs give rise to two distinct functional populations of sensory neurons. This work opens novel avenues to understand how these subtypes may carry distinct functions related to development of the spinal cord, locomotion and posture.

Mechano-sensitive regulation of gene expression during the embryonic development

Cell movements during embryogenesis produce mechanical tensions that shape the embryo and can also regulate gene expression, thereby affecting cell differentiation. Increasing evidence indicates that mechanosensitive regulation of gene expression plays important roles during embryogenesis by coupling the processes of morphogenesis and differentiation. However, the molecular mechanisms of this phenomenon remain poorly understood. This review focuses on the molecular mechanisms that "translate" mechanical stimuli into gene expression.

Identification and Expression Analysis of the Complete Family of Zebrafish pkd Genes

Polycystic kidney disease (PKD) proteins are trans-membrane proteins that have crucial roles in many aspects of vertebrate development and physiology, including the development of many organs as well as left–right patterning and taste. They can be divided into structurally-distinct PKD1-like and PKD2-like proteins and usually one PKD1-like protein forms a heteromeric polycystin complex with a PKD2-like protein. For example, PKD1 forms a complex with PKD2 and mutations in either of these proteins cause Autosomal Dominant Polycystic Kidney Disease (ADPKD), which is the most frequent potentially-lethal single-gene disorder in humans. Here, we identify the complete family of pkd genes in zebrafish and other teleosts. We describe the genomic locations and sequences of all seven genes: pkd1, pkd1b, pkd1l1, pkd1l2a, pkd1l2b, pkd2, and pkd2l1. pkd1l2a/pkd1l2b are likely to be ohnologs of pkd1l2, preserved from the whole genome duplication that occurred at the base of the teleosts. However, in contrast to mammals and cartilaginous and holostei fish, teleosts lack pkd2l2, and pkdrej genes, suggesting that these have been lost in the teleost lineage. In addition, teleost, and holostei fish have only a partial pkd1l3 sequence, suggesting that this gene may be in the process of being lost in the ray-finned fish lineage. We also provide the first comprehensive description of the expression of zebrafish pkd genes during development. In most structures we detect expression of one pkd1-like gene and one pkd2-like gene, consistent with these genes encoding a heteromeric protein complex. For example, we found that pkd2 and pkd1l1 are expressed in Kupffer's vesicle and pkd1 and pkd2 are expressed in the developing pronephros. In the spinal cord, we show that pkd1l2a and pkd2l1 are co-expressed in KA cells. We also identify potential co-expression of pkd1b and pkd2 in the floor-plate. Interestingly, and in contrast to mouse, we observe expression of all seven pkd genes in regions that may correspond to taste receptors. Taken together, these results provide a crucial catalog of pkd genes in an important model system for elucidating cell and developmental processes and modeling human diseases and the most comprehensive analysis of embryonic pkd gene expression in any vertebrate.

The functions of TRPP2 in the vascular system

TRPP2 (polycystin-2, PC2 or PKD2), encoded by the PKD2 gene, is a non-selective cation channel with a large single channel conductance and high Ca(2+) permeability. In cell membrane, TRPP2, along with polycystin-1, TRPV4 and TRPC1, functions as a mechanotransduction channel. In the endoplasmic reticulum, TRPP2 modulates intracellular Ca(2+) release associated with IP3 receptors and the ryanodine receptors. Noteworthily, TRPP2 is widely expressed in vascular endothelial and smooth muscle cells of all major vascular beds, and contributes to the regulation of vessel function. The mutation of the PKD2 gene is a major cause of autosomal dominant polycystic kidney disease (ADPKD), which is not only a common genetic disease of the kidney but also a systemic disorder associated with abnormalities in the vasculature; cardiovascular complications are the leading cause of mortality and morbidity in ADPKD patients. This review provides an overview of the current knowledge regarding the TRPP2 protein and its possible role in cardiovascular function and related diseases.

Phosphodiesterase Isoform Regulation of Cell Proliferation and Fluid Secretion in Autosomal Dominant Polycystic Kidney Disease

cAMP stimulates cell proliferation and Cl(-)-dependent fluid secretion, promoting the progressive enlargement of renal cysts in autosomal dominant polycystic kidney disease (ADPKD). Intracellular cAMP levels are determined by the balance of cAMP synthesis by adenylyl cyclases and degradation by phosphodiesterases (PDEs). Therefore, PDE isoform expression and activity strongly influence global and compartmentalized cAMP levels. We report here that PDE3 and PDE4 expression levels are lower in human ADPKD tissue and cells compared with those of normal human kidneys (NHKs), whereas PDE1 levels are not significantly different. Inhibition of PDE4 caused a greater increase in basal and vasopressin (AVP)-stimulated cAMP levels and Cl(-) secretion by ADPKD cells than inhibition of PDE1, and inhibition of PDE4 induced cyst-like dilations in cultured mouse Pkd1(-/-) embryonic kidneys. In contrast, inhibition of PDE1 caused greater stimulation of extracellular signal-regulated kinase (ERK) and proliferation of ADPKD cells than inhibition of PDE4, and inhibition of PDE1 enhanced AVP-induced ERK activation. Notably, inhibition of PDE1, the only family of Ca(2+)-regulated PDEs, also induced a mitogenic response to AVP in NHK cells, similar to the effect of restricting intracellular Ca(2+). PDE1 coimmunoprecipitated with B-Raf and A-kinase anchoring protein 79, and AVP increased this interaction in ADPKD but not NHK cells. These data suggest that whereas PDE4 is the major PDE isoform involved in the regulation of global intracellular cAMP and Cl(-) secretion, PDE1 specifically affects the cAMP signal to the B-Raf/MEK/ERK pathway and regulates AVP-induced proliferation of ADPKD cells.

The fundamental role of mechanical properties in the progression of cancer disease and inflammation

The role of mechanical properties in cancer disease and inflammation is still underinvestigated and even ignored in many oncological and immunological reviews. In particular, eight classical hallmarks of cancer have been proposed, but they still ignore the mechanics behind the processes that facilitate cancer progression. To define the malignant transformation of neoplasms and finally reveal the functional pathway that enables cancer cells to promote cancer progression, these classical hallmarks of cancer require the inclusion of specific mechanical properties of cancer cells and their microenvironment such as the extracellular matrix as well as embedded cells such as fibroblasts, macrophages or endothelial cells. Thus, this review will present current cancer research from a biophysical point of view and will therefore focus on novel physical aspects and biophysical methods to investigate the aggressiveness of cancer cells and the process of inflammation. As cancer or immune cells are embedded in a certain microenvironment such as the extracellular matrix, the mechanical properties of this microenvironment cannot be neglected, and alterations of the microenvironment may have an impact on the mechanical properties of the cancer or immune cells. Here, it is highlighted how biophysical approaches, both experimental and theoretical, have an impact on the classical hallmarks of cancer and inflammation. It is even pointed out how these biophysical approaches contribute to the understanding of the regulation of cancer disease and inflammatory responses after tissue injury through physical microenvironmental property sensing mechanisms. The recognized physical signals are transduced into biochemical signaling events that guide cellular responses, such as malignant tumor progression, after the transition of cancer cells from an epithelial to a mesenchymal phenotype or an inflammatory response due to tissue injury. Moreover, cell adaptation to mechanical alterations, in particular the understanding of mechano-coupling and mechano-regulating functions in cell invasion, appears as an important step in cancer progression and inflammatory response to injuries. This may lead to novel insights into cancer disease and inflammatory diseases and will overcome classical views on cancer and inflammation. In addition, this review will discuss how the physics of cancer and inflammation can help to reveal whether cancer cells will invade connective tissue and metastasize or how leukocytes extravasate and migrate through the tissue. In this review, the physical concepts of cancer progression, including the tissue basement membrane a cancer cell is crossing, its invasion and transendothelial migration as well as the basic physical concepts of inflammatory processes and the cellular responses to the mechanical stress of the microenvironment such as external forces and matrix stiffness, are presented and discussed. In conclusion, this review will finally show how physical measurements can improve classical approaches that investigate cancer and inflammatory diseases, and how these physical insights can be integrated into classical tumor biological approaches.

Investigation of spinal cerebrospinal fluid-contacting neurons expressing PKD2L1: evidence for a conserved system from fish to primates

Over 90 years ago, Kolmer and Agduhr identified spinal cerebrospinal fluid-contacting neurons (CSF-cNs) based on their morphology and location within the spinal cord. In more than 200 vertebrate species, they observed ciliated neurons around the central canal that extended a brush of microvilli into the cerebrospinal fluid (CSF). Although their morphology is suggestive of a primitive sensory cell, their function within the vertebrate spinal cord remains unknown. The identification of specific molecular markers for these neurons in vertebrates would benefit the investigation of their physiological roles. PKD2L1, a transient receptor potential channel that could play a role as a sensory receptor, has been found in cells contacting the central canal in mouse. In this study, we demonstrate that PKD2L1 is a specific marker for CSF-cNs in the spinal cord of mouse (Mus musculus), macaque (Macaca fascicularis) and zebrafish (Danio rerio). In these species, the somata of spinal PKD2L1⁺ CSF-cNs were located below or within the ependymal layer and extended an apical bulbous extension into the central canal. We found GABAergic PKD2L1-expressing CSF-cNs in all three species. We took advantage of the zebrafish embryo for its transparency and rapid development to identify the progenitor domains from which pkd2l1⁺ CSF-cNs originate. pkd2l1⁺ CSF-cNs were all GABAergic and organized in two rows—one ventral and one dorsal to the central canal. Their location and marker expression is consistent with previously described Kolmer–Agduhr cells. Accordingly, pkd2l1⁺ CSF-cNs were derived from the progenitor domains p3 and pMN defined by the expression of nkx2.2a and olig2 transcription factors, respectively. Altogether our results suggest that a system of CSF-cNs expressing the PKD2L1 channel is conserved in the spinal cord across bony vertebrate species.

The TRPP subfamily and polycystin-1 proteins

It has been exciting times since the identification of polycystic kidney disease 1 (PKD1) and PKD2 as the genes mutated in autosomal dominant polycystic kidney disease (ADPKD). Biological roles of the encoded proteins polycystin-1 and TRPP2 have been deduced from phenotypes in ADPKD patients, but recent insights from vertebrate and invertebrate model organisms have significantly expanded our understanding of the physiological functions of these proteins. The identification of additional TRPP (TRPP3 and TRPP5) and polycystin-1-like proteins (PKD1L1, PKD1L2, PKD1L3, and PKDREJ) has added yet another layer of complexity to these fascinating cellular signalling units. TRPP proteins assemble with polycystin-1 family members to form receptor-channel complexes. These protein modules have important biological roles ranging from tubular morphogenesis to determination of left-right asymmetry. The founding members of the polycystin family, TRPP2 and polycystin-1, are a prime example of how studying human disease genes can provide insights into fundamental biological mechanisms using a so-called "reverse translational" approach (from bedside to bench). Here, we discuss the current literature on TRPP ion channels and polycystin-1 family proteins including expression, structure, physical interactions, physiology, and lessons from animal model systems and human disease.

Lateral boundary mechanosensing by adherent cells in a collagen gel system

Cell adhesion responses to in-depth physical properties such as substrate roughness and topography are well described but little is known about the influence of lateral physical cues such as tissue boundaries on the function of adherent cells. Accordingly, we developed a model system to examine remote cell sensing of lateral boundaries. The model employs floating thin collagen gels supported by rigid grids of varying widths. The dynamics, lengths, and numbers of cell extensions were regulated by grid opening size, which in turn determined the distance of cells from rigid physical boundaries. In smaller grids (200 μm and 500 μm wide), cell-induced deformation fields extended to, and were resisted by, the grid boundaries. However, in larger grids (1700 μm wide), the deformation field did not extend to the grid boundaries, which strongly affected the mean length and number of cell extensions (∼60% reduction). The generation of cell extensions in collagen gels required expression of the β1 integrin, focal adhesion kinase and actomyosin activity. We conclude that the presence of physical boundaries interrupts the process of cell-mediated collagen compaction and fiber alignment in the collagen matrix and enhances the formation of cell extensions. This new cell culture platform provides a geometry that more closely approximates the native basement membrane and will help to elucidate the roles of cell extensions and lateral mechanosensing on extracellular matrix remodeling by invasion and degradation.

TRP channels

TRP channels constitute a large superfamily of cation channel forming proteins, all related to the gene product of the transient receptor potential (trp) locus in Drosophila. In mammals, 28 different TRP channel genes have been identified, which exhibit a large variety of functional properties and play diverse cellular and physiological roles. In this article, we provide a brief and systematic summary of expression, function, and (patho)physiological role of the mammalian TRP channels.

Piezo1-dependent stretch-activated channels are inhibited by Polycystin-2 in renal tubular epithelial cells

Mechanical forces associated with fluid flow and/or circumferential stretch are sensed by renal epithelial cells and contribute to both adaptive or disease states. Non-selective stretch-activated ion channels (SACs), characterized by a lack of inactivation and a remarkably slow deactivation, are active at the basolateral side of renal proximal convoluted tubules. Knockdown of Piezo1 strongly reduces SAC activity in proximal convoluted tubule epithelial cells. Similarly, overexpression of Polycystin-2 (PC2) or, to a greater extent its pathogenic mutant PC2-740X, impairs native SACs. Moreover, PC2 inhibits exogenous Piezo1 SAC activity. PC2 coimmunoprecipitates with Piezo1 and deletion of its N-terminal domain prevents both this interaction and inhibition of SAC activity. These findings indicate that renal SACs depend on Piezo1, but are critically conditioned by PC2.

TNF-α blockade is ineffective in animal models of established polycystic kidney disease

Background

Given the large medical burden of polycystic kidney disease (PKD) and recent clinical trial failures, there is a need for novel, safe and effective treatments for the disorder.

Methods

In PCK rat and PKD2^(ws25/w183) mouse models, entanercept was administered once every three days at 5 or 10 mg/kg, once daily. Mozavaptan was administered as a pilot control, provided continuously via milled chow at 0.1%. Animals were assessed for measures of pharmacodynamic response, and improvements in measures of polycystic kidney disease.

Results

Entanercept treatment modulated inflammatory markers, but provided limited therapeutic benefit in multiple animal models of established polycystic kidney disease. Kidney weight, cyst burden and renal function markers remained unchanged following administration of etanercept at various dose levels and multiple treatment durations.

Conclusions

While it remains possible that TNF-α inhibition may be effective in truly preventative settings, our observations suggest this pathway is less likely to exhibit therapeutic or disease-modifying efficacy following the standard clinical diagnosis of disease.

From mechanosensitivity to inflammatory responses: new players in the pathology of glaucoma

PURPOSE OF THE STUDY

Many blinding diseases of the inner retina are associated with degeneration and loss of retinal ganglion cells (RGCs). Recent evidence implicates several new signaling mechanisms as causal agents associated with RGC injury and remodeling of the optic nerve head. Ion channels such as Transient receptor potential vanilloid isoform 4 (TRPV4), pannexin-1 (Panx1) and P2X7 receptor are localized to RGCs and act as potential sensors and effectors of mechanical strain, ischemia and inflammatory responses. Under normal conditions, TRPV4 may function as an osmosensor and a polymodal molecular integrator of diverse mechanical and chemical stimuli, whereas P2X7R and Panx1 respond to stretch- and/or swelling-induced adenosine triphosphate release from neurons and glia. Ca(2+) influx, induced by stimulation of mechanosensitive ion channels in glaucoma, is proposed to influence dendritic and axonal remodeling that may lead to RGC death while (at least initially) sparing other classes of retinal neuron. The secondary phase of the retinal glaucoma response is associated with microglial activation and an inflammatory response involving Toll-like receptors (TLRs), cluster of differentiation 3 (CD3) immune recognition molecules associated with the T-cell antigen receptor, complement molecules and cell type-specific release of neuroactive cytokines such as tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β). The retinal response to mechanical stress thus involves a diversity of signaling pathways that sense and transduce mechanical strain and orchestrate both protective and destructive secondary responses.

CONCLUSIONS

Mechanistic understanding of the interaction between pressure-dependent and independent pathways is only beginning to emerge. This review focuses on the molecular basis of mechanical strain transduction as a primary mechanism that can damage RGCs. The damage occurs through Ca(2+)-dependent cellular remodeling and is associated with parallel activation of secondary ischemic and inflammatory signaling pathways. Molecules that mediate these mechanosensory and immune responses represent plausible targets for protecting ganglion cells in glaucoma, optic neuritis and retinal ischemia.

Novel role of ouabain as a cystogenic factor in autosomal dominant polycystic kidney disease

The classic role of the Na-K-ATPase is that of a primary active transporter that utilizes cell energy to establish and maintain transmembrane Na(+) and K(+) gradients to preserve cell osmotic stability, support cell excitability, and drive secondary active transport. Recent studies have revealed that Na-K-ATPase located within cholesterol-containing lipid rafts serves as a receptor for cardiotonic steroids, including ouabain. Traditionally, ouabain was viewed as a toxin produced only in plants, and it was used in relatively high concentrations to experimentally block the pumping action of the Na-K-ATPase. However, the new and unexpected role of the Na-K-ATPase as a signal transducer revealed a novel facet for ouabain in the regulation of a myriad of cell functions, including cell proliferation, hypertrophy, apoptosis, mobility, and metabolism. The seminal discovery that ouabain is endogenously produced in mammals and circulates in plasma has fueled the interest in this endogenous molecule as a potentially important hormone in normal physiology and disease. In this article, we review the role of the Na-K-ATPase as an ion transporter in the kidney, the experimental evidence for ouabain as a circulating hormone, the function of the Na-K-ATPase as a signal transducer that mediates ouabain's effects, and novel results for ouabain-induced Na-K-ATPase signaling in cystogenesis of autosomal dominant polycystic kidney disease.

Cell cycle progression by the repression of primary cilia formation in proliferating cells

In most cell types, primary cilia protrude from the cell surface and act as major hubs for cell signaling, cell differentiation, and cell polarity. With the exception of some cells ciliated during cell proliferation, most cells begin to disassemble their primary cilia at cell cycle re-entry. Although the role of primary cilia disassembly on cell cycle progression is still under debate, recent data have emerged to support the idea that primary cilia exert influence on cell cycle progression. In this review, we emphasize a non-mitotic role of Aurora-A not only in the ciliary resorption at cell cycle re-entry but also in continuous suppression of cilia regeneration during cell proliferation. We also summarize recent new findings indicating that forced induction/suppression of primary cilia can affect cell cycle progression, in particular the transition from G0/G1 to S phase. In addition, we speculate how (de)ciliation affects cell cycle progression.

Novel insights into TRPV4 function in the kidney

Kidneys are complex highly organized paired organs of nearly one million nephrons each. They rigorously process about 180 l of plasma daily to keep whole body homeostasis. To effectively perform such a titanic work, kidneys rely on mechanisms able to sense dynamic changes in composition and flow rates of protourine along the renal tubule. It is envisioned that Ca(2+)-permeable transient receptor potential (TRP) channels, and specifically mechanosensitive TRPV4, can serve to interpret these external mechanical cues in the form of elevated intracellular Ca(2+) concentration. This, in turn, initiates multiple cellular responses and adaptation mechanisms. The current review summarizes up-to-date knowledge about the sites of TRPV4 expression in renal tissue as well as discusses the functional role of the channel in cellular responses to hypotonicity and tubular flow. We will also provide insights as to how TRPV4 fits into classical polycystin mechanosensory complex in cilia and will speculate about previously underappreciated clinical implication of pharmacological TRPV4 targeting in treatment of polycystic kidney disease.

Endothelial mechanosensors of shear stress as regulators of atherogenesis

PURPOSE OF REVIEW

Differences in local blood flow patterns along the endothelium may trigger abnormal vascular responses which can have profound pathophysiological consequences. While endothelial cells exposed to laminar blood flow (high shear stress) are protected from atherosclerosis formation, turbulent or disturbed blood flow, which occurs at bends and bifurcations of blood vessels, facilitates atherosclerosis formation. Here, we will highlight the endothelial cell mechanisms involved in detecting shear stress and their translation into downstream biochemical signals.

RECENT FINDINGS

Prior evidence supports a role for integrins as mechanotransducers in the endothelium by promoting phosphorylation of different targets through the activation of focal adhesion kinase. Our recent findings show that integrins contact integrin-linked kinase and regulate vasomotor responses by an endothelial nitric oxide synthase-dependent mechanism, which stabilizes the production of vasoactive factor nitric oxide. In addition, different structures of endothelial cells, mainly primary cilia, are investigated, as they can explain the differential responses to laminar versus disturbed flow.

SUMMARY

The discovery of a connection between endothelial cell structures such as cilia, integrin, extracellular matrix, and signaling events opens today a new chapter in our understanding of the molecular mechanisms regulating vascular responses to the changes in flow.

Calmodulin-sensitive adenylyl cyclases mediate AVP-dependent cAMP production and Cl- secretion by human autosomal dominant polycystic kidney cells

In autosomal dominant polycystic kidney disease (ADPKD), binding of AVP to the V2 receptor (V2R) increases cAMP and accelerates cyst growth by stimulating cell proliferation and Cl(-)-dependent fluid secretion. Basal cAMP is elevated in human ADPKD cells compared with normal human kidney (NHK) cells. V2R mRNA levels are elevated in ADPKD cells; however, AVP caused a greater increase in global cAMP in NHK cells, suggesting an intrinsic difference in cAMP regulation. Expression, regulatory properties, and receptor coupling of specific adenylyl cyclases (ACs) provide temporal and spatial regulation of the cAMP signal. ADPKD and NHK cells express mRNAs for all nine ACs. Ca(2+)-inhibited ACs 5 and 6 are increased in ADPKD cells, while Ca(2+)/CaM-stimulated ACs 1 and 3 are downregulated. ACs 1, 3, 5, and 6 were detected in cyst cells in situ, and codistribution with aquaporin-2 suggests that these cysts were derived from collecting ducts. To determine the contribution of CaM-sensitive ACs to AVP signaling, cells were treated with W-7, a CaM inhibitor. W-7 decreased AVP-induced cAMP production and Cl(-) secretion by ADPKD cells. CaMKII inhibition increased AVP-induced cAMP, suggesting that cAMP synthesis is mediated by AC3. In contrast, CaM and CaMKII inhibition in NHK cells did not affect AVP-induced cAMP production. Restriction of intracellular Ca(2+) switched the response in NHK cells, such that CaM inhibition decreased AVP-induced cAMP production. We suggest that a compensatory response to decreased Ca(2+) in ADPKD cells switches V2R coupling from Ca(2+)-inhibited ACs 5/6 to Ca(2+)/CaM-stimulated AC3, to mitigate high cAMP levels in response to continuous AVP stimulation.

Keeping the balance between proliferation and differentiation: the primary cilium

Primary cilia are post-mitotic cellular organelles that are present in the vast majority of cell types in the human body. An extensive body of data gathered in recent years is demonstrating a crucial role for this organelle in a number of cellular processes that include mechano and chemo-sensation as well as the transduction of signaling cascades critical for the development and maintenance of different tissues and organs. Consequently, cilia are currently viewed as cellular antennae playing a critical role at the interphase between cells and their environment, integrating a range of stimuli to modulate cell fate decisions including cell proliferation, migration and differentiation. Importantly, this regulatory role is not just a consequence of their participation in signal transduction but is also the outcome of both the tight synchronization/regulation of ciliogenesis with the cell cycle and the role of individual ciliary proteins in cilia-dependent and independent processes. Here we review the role of primary cilia in the regulation of cell proliferation and differentiation and illustrate how this knowledge has provided insight to understand the phenotypic consequences of ciliary dysfunction.

Progesterone induced mesenchymal differentiation and rescued cystic dilation of renal tubules of Pkd1(-/-) mice

Autosomal dominant polycystic kidney disease (ADPKD), the most common hereditary disease affecting the kidneys, is caused in 85% of cases by mutations in the PKD1 gene. The protein encoded by this gene, polycystin-1, is a renal epithelial cell membrane mechanoreceptor, sensing morphogenetic cues in the extracellular environment, which regulate the tissue architecture and differentiation. However, how such mutations result in the formation of cysts is still unclear. We performed a precise characterization of mesenchymal differentiation using PAX2, WNT4 and WT1 as a marker, which revealed that impairment of the differentiation process preceded the development of cysts in Pkd1(-/-) mice. We performed an in vitro organ culture and found that progesterone and a derivative thereof facilitated mesenchymal differentiation, and partially prevented the formation of cysts in Pkd1(-/-) kidneys. An injection of progesterone or this derivative into the intraperitoneal space of pregnant females also improved the survival of Pkd1(-/-) embryos. Our findings suggest that compounds which enhance mesenchymal differentiation in the nephrogenesis might be useful for the therapeutic approach to prevent the formation of cysts in ADPKD patients.

The role of the cilium in normal and abnormal cell cycles: emphasis on renal cystic pathologies

The primary cilium protrudes from the cell surface and acts as a sensor for chemical and mechanical growth cues, with receptors for a number of growth factors (PDGFα, Hedgehog, Wnt, Notch) concentrated within the ciliary membrane. In normal tissues, the cilium assembles after cells exit mitosis and is resorbed as part of cell cycle re-entry. Although regulation of the cilium by cell cycle transitions has been appreciated for over 100 years, only recently have data emerged to indicate the cilium also exerts influence on the cell cycle. The resorption/protrusion cycle, regulated by proteins including Aurora-A, VHL, and GSK-3β, influences cell responsiveness to growth cues involving cilia-linked receptors; further, resorption liberates the ciliary basal body to differentiate into the centrosome, which performs discrete functions in S-, G2-, and M-phase. Besides these roles, the cilium provides a positional cue that regulates polarity of cell division, and thus directs cells towards fates of differentiation versus proliferation. In this review, we summarize the specific mechanisms mediating the cilia-cell cycle dialog. We then emphasize the examples of polycystic kidney disease (PKD), nephronopthisis (NPHP), and VHL-linked renal cysts as cases in which defects of ciliary function influence disease pathology, and may also condition response to treatment.

Mechanoprotection by polycystins against apoptosis is mediated through the opening of stretch-activated K(2P) channels

How renal epithelial cells respond to increased pressure and the link with kidney disease states remain poorly understood. Pkd1 knockout or expression of a PC2 pathogenic mutant, mimicking the autosomal dominant polycystic kidney disease, dramatically enhances mechanical stress-induced tubular apoptotic cell death. We show the presence of a stretch-activated K(+) channel dependent on the TREK-2 K(2P) subunit in proximal convoluted tubule epithelial cells. Our findings further demonstrate that polycystins protect renal epithelial cells against apoptosis in response to mechanical stress, and this function is mediated through the opening of stretch-activated K(2P) channels. Thus, to our knowledge, we establish for the first time, both in vitro and in vivo, a functional relationship between mechanotransduction and mechanoprotection. We propose that this mechanism is at play in other important pathologies associated with apoptosis and in which pressure or flow stimulation is altered, including heart failure or atherosclerosis.

Tolvaptan inhibits ERK-dependent cell proliferation, Cl⁻ secretion, and in vitro cyst growth of human ADPKD cells stimulated by vasopressin

In autosomal dominant polycystic kidney disease (ADPKD), arginine vasopressin (AVP) accelerates cyst growth by stimulating cAMP-dependent ERK activity and epithelial cell proliferation and by promoting Cl(-)-dependent fluid secretion. Tolvaptan, a V2 receptor antagonist, inhibits the renal effects of AVP and slows cyst growth in PKD animals. Here, we determined the effect of graded concentrations of tolvaptan on intracellular cAMP, ERK activity, cell proliferation, and transcellular Cl(-) secretion using human ADPKD cyst epithelial cells. Incubation of ADPKD cells with 10(-9) M AVP increased intracellular cAMP and stimulated ERK and cell proliferation. Tolvaptan caused a concentration-dependent inhibition of AVP-induced cAMP production with an apparent IC(50) of ∼10(-10) M. Correspondingly, tolvaptan inhibited AVP-induced ERK signaling and cell proliferation. Basolateral application of AVP to ADPKD cell monolayers grown on permeable supports caused a sustained increase in short-circuit current that was completely blocked by the Cl(-) channel blocker CFTR(inh-172), consistent with AVP-induced transepithelial Cl(-) secretion. Tolvaptan inhibited AVP-induced Cl(-) secretion and decreased in vitro cyst growth of ADPKD cells cultured within a three-dimensional collagen matrix. These data demonstrate that relatively low concentrations of tolvaptan inhibit AVP-stimulated cell proliferation and Cl(-)-dependent fluid secretion by human ADPKD cystic cells.

Orientation of primary cilia of articular chondrocytes in three-dimensional space

Primary cilia have functions as sensory organelles integral to signal transduction and establishment of cell polarity. In articular cartilage the primary cilium has been hypothesized to function as an antenna to sense the biomechanical environment, regulate the secretion of extracellular matrix components, and maintain cellular positional information, leading to high tissue anisotropy. We used analysis of electron microscopy serial sections to demonstrate positional attributes of the primary cilium of adult equine articular chondrocytes in situ. Data for ~500 axonemes, comparing superficial to radiate chondrocytes from both load-bearing and non-load-bearing regions, were graphed using spherical co-ordinates θ, φ. The data demonstrate the axoneme has a definable orientation in 3D space differing in superficial and radiate zone chondrocytes, cells that differ by 90° in the orientation of their major axes to the articular surface. Axonemal orientation is more definable in load-bearing than in non-load-bearing areas. The position of emergence of the axoneme from the cell also is variable. In load-bearing regions of the superficial zone, extension of the axoneme is from the cellular side facing the subchondral bone. In radiate zone cells, axonemes extend from either face of the chondrocyte, that is, both toward the articular surface or toward the subchondral bone. In non-load-bearing regions this consistency is lost. These observations relate to current hypotheses concerning establishment of tissue anisotropy in articular cartilage during development, involving both migration of cells from the joint periphery and a restricted zone of division within the tissue resulting in the columnar arrangement of radiate zone cells.