Dopamine receptor type 5 in the primary cilia has dual chemo- and mechano-sensory roles

Structural Rearrangement of the AT1 Receptor Modulated by Membrane Thickness and Tension

Membrane-embedded mechanosensitive (MS) proteins, including ion channels and G-protein coupled receptors (GPCRs), are essential for the transduction of external mechanical stimuli into biological signals. The angiotensin II type 1 (AT1) receptor plays many important roles in cardiovascular regulation and is associated with diseases such as hypertension and congestive heart failure. The membrane-mediated activation of the AT1 receptor is not well understood, despite this being one of the most widely studied GPCRs within the context of biased agonism. Here, we use extensive molecular dynamics (MD) simulations to characterize the effect of the local membrane environment on the activation of the AT1 receptor. We show that membrane thickness plays an important role in the stability of active and inactive states of the receptor, as well as the dynamic interchange between states. Furthermore, our simulation results show that membrane tension is effective in driving large-scale structural changes in the inactive state such as the outward movement of transmembrane helix 6 to stabilize intermediate active-like conformations. We conclude by comparing our simulation observations with AlphaFold 2 predictions, as a proxy to experimental structures, to provide a framework for how membrane mediated stimuli can facilitate activation of the AT1 receptor through the β-arrestin signaling pathway.

Circadian cilia transcriptome in mouse brain across physiological and pathological states

Primary cilia are dynamic sensory organelles that continuously undergo structural modifications in response to environmental and cellular signals, many of which exhibit rhythmic patterns. Building on our previous findings of rhythmic cilia-related gene expression in diurnal primates (baboon), this study extends the investigation to the nocturnal mouse brain to identify circadian patterns of cilia gene expression across brain regions. We used computational techniques and transcriptomic data from four publicly available databases, to examine the circadian expression of cilia-associated genes within six brain areas: brainstem, cerebellum, hippocampus, hypothalamus, striatum, and suprachiasmatic nucleus. Our analysis reveals that a substantial proportion of cilia transcripts exhibit circadian rhythmicity across the examined regions, with notable overrepresentation in the striatum, hippocampus, and cerebellum. We also demonstrate region-specific variations in the abundance and timing of circadian cilia genes' peaks, indicating an adaptation to the distinct physiological roles of each brain region. Additionally, we show that the rhythmic patterns of cilia transcripts are shifted under various physiological and pathological conditions, including modulation of the dopamine system, high-fat diet, and epileptic conditions, indicating the adaptable nature of cilia transcripts' oscillation. While limited to a few mouse brain regions, our study provides initial insights into the distinct circadian patterns of cilia transcripts and highlights the need for future research to expand the mapping across wider brain areas to fully understand the role of cilia's spatiotemporal dynamics in brain functions.

Chronic endothelial dopamine receptor stimulation improves endothelial function and hemodynamics in autosomal dominant polycystic kidney disease

Altered polycystin-mediated endothelial flow mechanosensitivity contributes to the development of hypertension and cardiovascular complications in patients with autosomal dominant polycystic kidney disease (ADPKD). Stimulation of endothelial type 5 dopamine receptors (DR5) can acutely compensate for the endothelial consequences of polycystin deficiency, but the chronic impact of this approach must be evaluated in ADPKD. Nineteen patients with ADPKD on standard of care therapy were randomized to receive a 2-month treatment with the DR agonist rotigotine using transdermal patches, nine at 2 mg/24hours and ten at 4 mg/24hours or while ten were untreated. Rotigotine at the dose of 4 mg/24hours significantly increased nitric oxide release (nitrite levels from 10±30 to 46±34 nmol/L) and radial artery endothelium-dependent flow-mediated dilatation (from 16.4±6.3 to 22.5±7.3%) in response to hand skin heating. Systemic hemodynamics were not significantly modified but aplanation tonometry showed that rotigotine at 4 mg/24hours reduced aortic augmentation index and pulse pressure without affecting carotid-to femoral pulse wave velocity. Plasma creatinine and urea, urinary cyclic AMP, which contributes to cyst growth in ADPKD and copeptin, a surrogate marker of vasopressin, were not affected by rotigotine. In mice with a specific deletion of polycystin-1 in endothelial cells, chronic infusion of the peripheral DR5 agonist fenoldopam also improved mesenteric artery flow-mediated dilatation and reduced blood pressure. Thus, our study demonstrates that in patients with ADPKD, chronic administration of rotigotine improves conduit artery endothelial function through the restoration of flow-induced nitric oxide release as well as hemodynamics suggesting that endothelial DR5 activation may represent a promising pharmacological approach to prevent cardiovascular complications of ADPKD.

Magnetogenetics as a promising tool for controlling cellular signaling pathways

Magnetogenetics emerges as a transformative approach for modulating cellular signaling pathways through the strategic application of magnetic fields and nanoparticles. This technique leverages the unique properties of magnetic nanoparticles (MNPs) to induce mechanical or thermal stimuli within cells, facilitating the activation of mechano- and thermosensitive proteins without the need for traditional ligand-receptor interactions. Unlike traditional modalities that often require invasive interventions and lack precision in targeting specific cellular functions, magnetogenetics offers a non-invasive alternative with the capacity for deep tissue penetration and the potential for targeting a broad spectrum of cellular processes. This review underscores magnetogenetics' broad applicability, from steering stem cell differentiation to manipulating neuronal activity and immune responses, highlighting its potential in regenerative medicine, neuroscience, and cancer therapy. Furthermore, the review explores the challenges and future directions of magnetogenetics, including the development of genetically programmed magnetic nanoparticles and the integration of magnetic field-sensitive cells for in vivo applications. Magnetogenetics stands at the forefront of cellular manipulation technologies, offering novel insights into cellular signaling and opening new avenues for therapeutic interventions.

Regulatory role of primary cilia in oral and maxillofacial development and disease

Primary cilia have versatile functions, such as receiving signals from the extracellular microenvironment, mediating signaling transduction, and transporting ciliary substances, in tissue and organ development and clinical disease pathogenesis. During early development (embryos within 10 weeks) in the oral and maxillofacial region, defects in the structure and function of primary cilia can result in severe craniofacial malformations. For example, mice with mutations in the cilia-related genes Kif3a and IFT88 exhibit midline expansion and cleft lip/palate, which occur due to abnormalities in the fusion of the single frontonasal prominence and maxillary prominences. In the subsequent development of the oral and maxillofacial region, we discussed the regulatory role of primary cilia in the development of the maxilla, mandible, Meckel cartilage, condylar cartilage, lip, tongue, and tooth, among others. Moreover, primary cilia are promising regulators in some oral and maxillofacial diseases, such as tumors and malocclusion. We also summarize the regulatory mechanisms of primary cilia in oral and maxillofacial development and related diseases, including their role in various signaling transduction pathways. For example, aplasia of submandibular glands in the Kif3a mutant mice is associated with a decrease in SHH signaling within the glands. This review summarizes the similarities and specificities of the role of primary cilia in tissue and organ development and disease progression in the oral and maxillofacial region, which is expected to contribute several ideas for the treatment of primary cilia-related diseases.

Cilia Provide a Platform for the Generation, Regulated Secretion, and Reception of Peptidergic Signals

Cilia are microtubule-based cellular projections that act as motile, sensory, and secretory organelles. These structures receive information from the environment and transmit downstream signals to the cell body. Cilia also release vesicular ectosomes that bud from the ciliary membrane and carry an array of bioactive enzymes and peptide products. Peptidergic signals represent an ancient mode of intercellular communication, and in metazoans are involved in the maintenance of cellular homeostasis and various other physiological processes and responses. Numerous peptide receptors, subtilisin-like proteases, the peptide-amidating enzyme, and bioactive amidated peptide products have been localized to these organelles. In this review, we detail how cilia serve as specialized signaling organelles and act as a platform for the regulated processing and secretion of peptidergic signals. We especially focus on the processing and trafficking pathways by which a peptide precursor from the green alga Chlamydomonas reinhardtii is converted into an amidated bioactive product-a chemotactic modulator-and released from cilia in ectosomes. Biochemical dissection of this complex ciliary secretory pathway provides a paradigm for understanding cilia-based peptidergic signaling in mammals and other eukaryotes.

Mechanosensitive GPCRs and ion channels in shear stress sensing

As a universal mechanical cue, shear stress plays essential roles in many physiological processes, ranging from vascular morphogenesis and remodeling to renal transport and airway barrier function. Disrupted shear stress is commonly regarded as a major contributor to various human diseases such as atherosclerosis, hypertension, and chronic kidney disease. Despite the importance of shear stress in physiology and pathophysiology, our current understanding of mechanosensors that sense shear stress is far from complete. An increasing number of candidate mechanosensors have been proposed to mediate shear stress sensing in distinct cell types, including G protein-coupled receptors (GPCRs), G proteins, receptor tyrosine kinases, ion channels, glycocalyx proteins, and junctional proteins. Although multiple types of mechanosensors might be able to convert shear stress into downstream biochemical signaling events, in this review, we will focus on discussing the mechanosensitive GPCRs (angiotensin II type 1 receptor, GPR68, histamine H1 receptor, adhesion GPCRs) and ion channels (Piezo, TRP) that have been reported to be directly activated by shear stress.

Cardiovascular Manifestations and Management in ADPKD

Cardiovascular disease (CVD) is the major cause of mortality in autosomal dominant polycystic kidney disease (ADPKD) and contributes to significant burden of disease. The manifestations are varied, including left ventricular hypertrophy (LVH), intracranial aneurysms (ICAs), valvular heart disease, and cardiomyopathies; however, the most common presentation and a major modifiable risk factor is hypertension. The aim of this review is to detail the complex pathogenesis of hypertension and other extrarenal cardiac and vascular conditions in ADPKD drawing on preclinical, clinical, and epidemiological evidence. The main drivers of disease are the renin-angiotensin-aldosterone system (RAAS) and polycystin-related endothelial cell dysfunction, with the sympathetic nervous system (SNS), nitric oxide (NO), endothelin-1 (ET-1), and asymmetric dimethylarginine (ADMA) likely playing key roles in different disease stages. The reported rates of some manifestations, such as LVH, have decreased likely due to the use of antihypertensive therapies; and others, such as ischemic cardiomyopathy, have been reported with increased prevalence likely due to longer survival and higher rates of chronic disease. ADPKD-specific screening and management guidelines exist for hypertension, LVH, and ICAs; and these are described in this review.

Membrane mediated mechanical stimuli produces distinct active-like states in the AT1 receptor

The Angiotensin II Type 1 (AT1) receptor is one of the most widely studied GPCRs within the context of biased signaling. While the AT1 receptor is activated by agonists such as the peptide AngII, it can also be activated by mechanical stimuli such as membrane stretch or shear in the absence of a ligand. Despite the importance of mechanical activation of the AT1 receptor in biological processes such as vasoconstriction, little is known about the structural changes induced by external physical stimuli mediated by the surrounding lipid membrane. Here, we present a systematic simulation study that characterizes the activation of the AT1 receptor under various membrane environments and mechanical stimuli. We show that stability of the active state is highly sensitive to membrane thickness and tension. Structural comparison of membrane-mediated vs. agonist-induced activation shows that the AT1 receptor has distinct active conformations. This is supported by multi-microsecond free energy calculations that show unique landscapes for the inactive and various active states. Our modeling results provide structural insights into the mechanical activation of the AT1 receptor and how it may produce different functional outcomes within the framework of biased agonism.

Spatiotemporal Mapping of Brain Cilia Reveals Region-Specific Oscillation of Length and Orientation

In this study, we conducted high-throughput spatiotemporal analysis of primary cilia length and orientation across 22 mouse brain regions. We developed automated image analysis algorithms, which enabled us to examine over 10 million individual cilia, generating the largest spatiotemporal atlas of cilia. We found that cilia length and orientation display substantial variations across different brain regions and exhibit fluctuations over a 24-hour period, with region-specific peaks during light-dark phases. Our analysis revealed unique orientation patterns of cilia at 45 degree intervals, suggesting that cilia orientation within the brain is not random but follows specific patterns. Using BioCycle, we identified circadian rhythms of cilia length in five brain regions: nucleus accumbens core, somatosensory cortex, and three hypothalamic nuclei. Our findings present novel insights into the complex relationship between cilia dynamics, circadian rhythms, and brain function, highlighting cilia crucial role in the brain's response to environmental changes and regulation of time-dependent physiological processes.

Endothelium-Specific Deficiency of Polycystin-1 Promotes Hypertension and Cardiovascular Disorders

Background:

Autosomal dominant polycystic kidney disease is the most frequent hereditary kidney disease and is generally due to mutations in PKD1 and PKD2 , encoding polycystins 1 and 2. In autosomal dominant polycystic kidney disease, hypertension and cardiovascular disorders are highly prevalent, but their mechanisms are partially understood.

Methods:

Since endothelial cells express the polycystin complex, where it plays a central role in the mechanotransduction of blood flow, we generated a murine model with inducible deletion of Pkd1 in endothelial cells ( Cdh5-Cre ERT2 ; Pkd1 fl/fl ) to specifically determine the role of endothelial polycystin-1 in autosomal dominant polycystic kidney disease.

Results:

Endothelial deletion of Pkd1 induced endothelial dysfunction, as demonstrated by impaired flow-mediated dilatation of resistance arteries and impaired relaxation to acetylcholine, increased blood pressure and prevented the normal development of arteriovenous fistula. In experimental chronic kidney disease induced by subtotal nephrectomy, endothelial deletion of Pkd1 further aggravated endothelial dysfunction, vascular remodeling, and heart hypertrophy.

Conclusions:

Altogether, this study provides the first in vivo demonstration that specific deletion of Pkd1 in endothelial cells promotes endothelial dysfunction and hypertension, impairs arteriovenous fistula development, and potentiates the cardiovascular alterations associated with chronic kidney disease.

Primary cilia and SHH signaling impairments in human and mouse models of Parkinson's disease

Parkinson's disease (PD) as a progressive neurodegenerative disorder arises from multiple genetic and environmental factors. However, underlying pathological mechanisms remain poorly understood. Using multiplexed single-cell transcriptomics, we analyze human neural precursor cells (hNPCs) from sporadic PD (sPD) patients. Alterations in gene expression appear in pathways related to primary cilia (PC). Accordingly, in these hiPSC-derived hNPCs and neurons, we observe a shortening of PC. Additionally, we detect a shortening of PC in PINK1-deficient human cellular and mouse models of familial PD. Furthermore, in sPD models, the shortening of PC is accompanied by increased Sonic Hedgehog (SHH) signal transduction. Inhibition of this pathway rescues the alterations in PC morphology and mitochondrial dysfunction. Thus, increased SHH activity due to ciliary dysfunction may be required for the development of pathoetiological phenotypes observed in sPD like mitochondrial dysfunction. Inhibiting overactive SHH signaling may be a potential neuroprotective therapy for sPD.

Subtype-Selective Positive Modulation of KCa2.3 Channels Increases Cilia Length

Small-conductance Ca²⁺-activated potassium (K_Ca2.x) channels are gated exclusively by intracellular Ca²⁺. The activation of K_Ca2.3 channels induces hyperpolarization, which augments Ca²⁺ signaling in endothelial cells. Cilia are specialized Ca²⁺ signaling compartments. Here, we identified compound 4 that potentiates human K_Ca2.3 channels selectively. The subtype selectivity of compound 4 for human K_Ca2.3 over rat K_Ca2.2a channels relies on an isoleucine residue in the HA/HB helices. Positive modulation of K_Ca2.3 channels by compound 4 increased flow-induced Ca²⁺ signaling and cilia length, while negative modulation by AP14145 reduced flow-induced Ca²⁺ signaling and cilia length. These findings were corroborated by the increased cilia length due to the expression of Ca²⁺-hypersensitive K_Ca2.3_G351D mutant channels and the reduced cilia length resulting from the expression of Ca²⁺-hyposensitive K_Ca2.3_I438N channels. Collectively, we were able to associate functions of K_Ca2.3 channels and cilia, two crucial components in the flow-induced Ca²⁺ signaling of endothelial cells, with potential implications in vasodilation and ciliopathic hypertension.

Primary Cilia and Their Role in Acquired Heart Disease

Primary cilia are non-motile plasma membrane extrusions that display a variety of receptors and mechanosensors. Loss of function results in ciliopathies, which have been strongly linked with congenital heart disease, as well as abnormal development and function of most organ systems. Adults with congenital heart disease have high rates of acquired heart failure, and usually die from a cardiac cause. Here we explore primary cilia’s role in acquired heart disease. Intraflagellar Transport 88 knockout results in reduced primary cilia, and knockout from cardiac endothelium produces myxomatous degeneration similar to mitral valve prolapse seen in adult humans. Induced primary cilia inactivation by other mechanisms also produces excess myocardial hypertrophy and altered scar architecture after ischemic injury, as well as hypertension due to a lack of vascular endothelial nitric oxide synthase activation and the resultant left ventricular dysfunction. Finally, primary cilia have cell-to-cell transmission capacity which, when blocked, leads to progressive left ventricular hypertrophy and heart failure, though this mechanism has not been fully established. Further research is still needed to understand primary cilia’s role in adult cardiac pathology, especially heart failure.

Mechanical and chemical activation of GPR68 probed with a genetically encoded fluorescent reporter

G-protein-coupled receptor (GPCR) 68 (GPR68, or OGR1) couples extracellular acidifications and mechanical stimuli to G-protein signaling and plays important roles in vascular physiology, neuroplasticity and cancer progression. Inspired by previous GPCR-based reporters, here, we inserted a cyclic permuted fluorescent protein into the third intracellular loop of GPR68 to create a genetically encoded fluorescent reporter of GPR68 activation we call 'iGlow'. iGlow responds to known physiological GPR68 activators such as fluid shear stress and extracellular acidifications. In addition, iGlow responds to Ogerin, a synthetic GPR68-selective agonist, but not to a non-active Ogerin analog, showing the specificity of iGlow-mediated fluorescence signals. Flow-induced iGlow activation is not eliminated by pharmacological modulation of downstream G-protein signaling, disruption of actin filaments or application of GsMTx4, an inhibitor of certain mechanosensitive ion channels activated by membrane stretch. Deletion of the conserved helix 8, proposed to mediate mechanosensitivity in certain GPCRs, does not eliminate flow-induced iGlow activation. iGlow could be useful to investigate the contribution of GPR68-dependent signaling in health and disease.

Intraflagellar Transport Proteins as Regulators of Primary Cilia Length

Primary cilia are small, antenna-like organelles that detect and transduce chemical and mechanical cues in the extracellular environment, regulating cell behavior and, in turn, tissue development and homeostasis. Primary cilia are assembled via intraflagellar transport (IFT), which traffics protein cargo bidirectionally along a microtubular axoneme. Ranging from 1 to 10 μm long, these organelles typically reach a characteristic length dependent on cell type, likely for optimum fulfillment of their specific roles. The importance of an optimal cilia length is underscored by the findings that perturbation of cilia length can be observed in a number of cilia-related diseases. Thus, elucidating mechanisms of cilia length regulation is important for understanding the pathobiology of ciliary diseases. Since cilia assembly/disassembly regulate cilia length, we review the roles of IFT in processes that affect cilia assembly/disassembly, including ciliary transport of structural and membrane proteins, ectocytosis, and tubulin posttranslational modification. Additionally, since the environment of a cell influences cilia length, we also review the various stimuli encountered by renal epithelia in healthy and diseased states that alter cilia length and IFT.

Signal transduction in primary cilia - analyzing and manipulating GPCR and second messenger signaling

The primary cilium projects from the surface of most vertebrate cells, where it senses extracellular signals to regulate diverse cellular processes during tissue development and homeostasis. Dysfunction of primary cilia underlies the pathogenesis of severe diseases, commonly referred to as ciliopathies. Primary cilia contain a unique protein repertoire that is distinct from the cell body and the plasma membrane, enabling the spatially controlled transduction of extracellular cues. G-protein coupled receptors (GPCRs) are key in sensing environmental stimuli that are transmitted via second messenger signaling into a cellular response. Here, we will give an overview of the role of GPCR signaling in primary cilia, and how ciliary GPCR signaling can be targeted by pharmacology, chemogenetics, and optogenetics.

Primary Cilia and Atherosclerosis

In artery tree, endothelial function correlates with the distribution of shear stress, a dragging force generated by flowing blood. In laminar shear stress areas, endothelial cells (ECs) are available to prevent atherosclerosis, however, ECs in disturbed shear stress sites are featured with proinflammation and atherogenesis. Basic studies in the shear stress field that focused on the mechanosensors of ECs have attracted the interest of researchers. Among all the known mechanosensors, the primary cilium is distinctive because it is enriched in disturbed shear stress regions and sparse in laminar shear stress areas. The primary cilium, a rod liked micro-organelle, can transmit extracellular mechanical and chemical stimuli into intracellular space. In the cardiovascular system, primary cilia are enriched in disturbed shear stress regions, where blood flow is slow and oscillatory, such as the atrium, downstream of the aortic valve, branches, bifurcations, and inner curves of the artery. However, in the atrioventricular canal and straight vessels, blood flow is laminar, and primary cilia can barely be detected. Primary cilia in the heart cavity prevent ECs from mesenchymal transition and calcification by suppressing transforming growth factor (TGF) signaling. Besides, primary cilia in the vascular endothelium protected ECs against disturbed shear stress-induced cellular damage by triggering Ca²⁺ influx as well as nitric oxide (NO) release. Moreover, primary cilia inhibit the process of atherosclerosis. In the current review, we discussed ciliogenesis, ciliary structure, as well as ciliary distribution, function and the coordinate signal transduction with shear stress in the cardiovascular system.

[Cardiovascular disorders in autosomal dominant polycystic kidney disease]

Autosomal dominant polycystic kidney disease is the most frequent genetic kidney disease. Cardiovascular disorders associated with autosomal dominant polycystic kidney disease are multiple and may occur early in life. In autosomal dominant polycystic kidney disease cardiovascular morbidity and mortality are related both to the nonspecific consequences of chronic kidney disease and to the particular phenotype of autosomal dominant polycystic kidney disease. Compared to the general population, patients with autosomal dominant polycystic kidney disease present an increased prevalence of hypertension, left ventricular hypertrophy, atrial fibrillation, valvular diseases, aneurisms and arterial dissections. This review article provides an update on cardiovascular disorders associated with autosomal dominant polycystic kidney disease and recent pathophysiological developments.

Roles of CCN2 as a mechano-sensing regulator of chondrocyte differentiation

Cellular communication network factor 2 (CCN2) is a cysteine-rich secreted matricellular protein that regulates various cellular functions including cell differentiation. CCN2 is highly expressed under several types of mechanical stress, such as stretch, compression, and shear stress, in mesenchymal cells including chondrocytes, osteoblasts, and fibroblasts. In particular, CCN2 not only promotes cell proliferation and differentiation of various cells but also regulates the stability of mRNA of TRPV4, a mechanosensitive ion channel in chondrocytes. Of note, CCN2 behaves like a biomarker to sense suitable mechanical stress, because CCN2 expression is down-regulated when chondrocytes are subjected to excessive mechanical stress. These findings suggest that CCN2 is a mechano-sensing regulator. CCN2 expression is regulated by the activation of various mechano-sensing signaling pathways, e.g., mechanosensitive ion channels, integrin-focal adhesion-actin dynamics, Rho GTPase family members, Hippo-YAP signaling, and G protein-coupled receptors. This review summarizes the characterization of mechanoreceptors involved in CCN2 gene regulation and discusses the role of CCN2 as a mechano-sensing regulator of mesenchymal cell differentiation, with particular focus on chondrocytes.

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.

Regulation of blood flow in small arteries: mechanosensory events underlying myogenic vasoconstriction

As blood flow is proportional to the fourth power of the vascular radius small changes in the diameter of resistance arteries/arterioles following an increase in intraluminal pressure would be expected to substantially increase blood flow. However, arteriolar myocytes display an intrinsic ability to locally regulate blood flow according to metabolic demands by tuning the diameter of small arteries in response to local changes in he-modynamics. Critical to this, observations were made more than 100 years ago that mechanosensitive small arteries exhibit the "myogenic response" or pressure-induced vasoconstriction or vasodilation in re-sponse to increased or decreased intravascular pressure, respectively. Although cellular mechanisms underlying the myogenic response have now been studied extensively, the precise cellular mechanisms under-lying this intriguing phenomenon still remain uncertain. In particular, the biological machinery that senses changes in intravascular pressure in vascular smooth muscle cells have not been unquestionably identified and remain a significant issue in vascular biology to be fully elucidated. As such, this brief review focuses on putative mechanosensors that have been proposed to contribute to myogenic vasoreactivity. Specific attention is paid to the roles of integrins, G protein-coupled receptors, and cadherins.

Modulation of polycystic kidney disease by G-protein coupled receptors and cyclic AMP signaling

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a systemic disorder associated with polycystic liver disease (PLD) and other extrarenal manifestations, the most common monogenic cause of end-stage kidney disease, and a major burden for public health. Many studies have shown that alterations in G-protein and cAMP signaling play a central role in its pathogenesis. As for many other diseases (35% of all approved drugs target G-protein coupled receptors (GPCRs) or proteins functioning upstream or downstream from GPCRs), treatments targeting GPCR have shown effectiveness in slowing the rate of progression of ADPKD. Tolvaptan, a vasopressin V2 receptor antagonist is the first drug approved by regulatory agencies to treat rapidly progressive ADPKD. Long-acting somatostatin analogs have also been effective in slowing the rates of growth of polycystic kidneys and liver. Although no treatment has so far been able to prevent the development or stop the progression of the disease, these encouraging advances point to G-protein and cAMP signaling as a promising avenue of investigation that may lead to more effective and safe treatments. This will require a better understanding of the relevant GPCRs, G-proteins, cAMP effectors, and of the enzymes and A-kinase anchoring proteins controlling the compartmentalization of cAMP signaling. The purpose of this review is to provide an overview of general GPCR signaling; the function of polycystin-1 (PC1) as a putative atypical adhesion GPCR (aGPCR); the roles of PC1, polycystin-2 (PC2) and the PC1-PC2 complex in the regulation of calcium and cAMP signaling; the cross-talk of calcium and cAMP signaling in PKD; and GPCRs, adenylyl cyclases, cyclic nucleotide phosphodiesterases, and protein kinase A as therapeutic targets in ADPKD.

Helix 8 is the essential structural motif of mechanosensitive GPCRs

G-protein coupled receptors (GPCRs) are versatile cellular sensors for chemical stimuli, but also serve as mechanosensors involved in various (patho)physiological settings like vascular regulation, cardiac hypertrophy and preeclampsia. However, the molecular mechanisms underlying mechanically induced GPCR activation have remained elusive. Here we show that mechanosensitive histamine H₁ receptors (H₁Rs) are endothelial sensors of fluid shear stress and contribute to flow-induced vasodilation. At the molecular level, we observe that H₁Rs undergo stimulus-specific patterns of conformational changes suggesting that mechanical forces and agonists induce distinct active receptor conformations. GPCRs lacking C-terminal helix 8 (H8) are not mechanosensitive, and transfer of H8 to non-responsive GPCRs confers, while removal of H8 precludes, mechanosensitivity. Moreover, disrupting H8 structural integrity by amino acid exchanges impairs mechanosensitivity. Altogether, H8 is the essential structural motif endowing GPCRs with mechanosensitivity. These findings provide a mechanistic basis for a better understanding of the roles of mechanosensitive GPCRs in (patho)physiology.

GPCRs are versatile cellular sensors for chemical stimuli but the molecular mechanisms underlying mechanically induced GPCR activation have remained elusive. Here authors identify the C-terminal helix 8 (H8) as the essential structural motif endowing H₁R and other GPCRs with mechanosensitivity.

Cellular signalling by primary cilia in development, organ function and disease

Primary cilia project in a single copy from the surface of most vertebrate cell types; they detect and transmit extracellular cues to regulate diverse cellular processes during development and to maintain tissue homeostasis. The sensory capacity of primary cilia relies on the coordinated trafficking and temporal localization of specific receptors and associated signal transduction modules in the cilium. The canonical Hedgehog (HH) pathway, for example, is a bona fide ciliary signalling system that regulates cell fate and self-renewal in development and tissue homeostasis. Specific receptors and associated signal transduction proteins can also localize to primary cilia in a cell type-dependent manner; available evidence suggests that the ciliary constellation of these proteins can temporally change to allow the cell to adapt to specific developmental and homeostatic cues. Consistent with important roles for primary cilia in signalling, mutations that lead to their dysfunction underlie a pleiotropic group of diseases and syndromic disorders termed ciliopathies, which affect many different tissues and organs of the body. In this Review, we highlight central mechanisms by which primary cilia coordinate HH, G protein-coupled receptor, WNT, receptor tyrosine kinase and transforming growth factor-β (TGFβ)/bone morphogenetic protein (BMP) signalling and illustrate how defects in the balanced output of ciliary signalling events are coupled to developmental disorders and disease progression.

The Role of the Primary Cilium in Sensing Extracellular pH

Biosensors on the membrane of the vascular endothelium are responsible for sensing mechanical and chemical signals in the blood. Transduction of these stimuli into intracellular signaling cascades regulate cellular processes including ion transport, gene expression, cell proliferation, and/or cell death. The primary cilium is a well-known biosensor of shear stress but its role in sensing extracellular pH change has never been examined. As a cellular extension into the immediate microenvironment, the cilium could be a prospective sensor for changes in pH and regulator of acid response in cells. We aim to test our hypothesis that the primary cilium plays the role of an acid sensor in cells using vascular endothelial and embryonic fibroblast cells as in vitro models. We measure changes in cellular pH using pH-sensitive 2',7'-biscarboxyethy1-5,6-carboxyfluorescein acetoxy-methylester (BCECF) fluorescence and mitogen-activated protein kinase (MAPK) activity to quantify responses to both extracellular pH (pH_o) and intracellular pH (pH_i) changes. Our studies show that changes in pH_o affect pH_i in both wild-type and cilia-less Tg737 cells and that the kinetics of the pH_i response are similar in both cells. Acidic pH_o or pH_i was observed to change the length of primary cilia in wild-type cells while the cilia in Tg737 remained absent. Vascular endothelial cells respond to acidic pH through activation of ERK1/2 and p38-mediated signaling pathways. The cilia-less Tg737 cells exhibit delayed responsiveness to pH_o dependent and independent pH_i acidification as depicted in the phosphorylation profile of ERK1/2 and p38. Otherwise, intracellular pH homeostatic response to acidic pH_o is similar between wild-type and Tg737 cells, indicating that the primary cilia may not be the sole sensor for physiological pH changes. These endothelial cells respond to pH changes with a predominantly K⁺-dependent pH_i recovery mechanism, regardless of ciliary presence or absence.

Novel functions of the primary cilium in bone disease and cancer

The primary cilium, a sensory organelle that emanates from the cell surface of most mammalian cell types during growth arrest, has attracted the attention of many researchers over the past decade. Recently, a large number of new findings have assigned novel functions and roles to the primary cilium in signal transduction and related diseases, which has greatly augmented the importance of the cilium in human health and development. Here, we review emerging evidence supporting the primary cilium as a sensory organelle in signal transduction in microgravity, electromagnetic field sensing, chemosensation and tumorigenesis. We also present an overview of signal transduction crosstalk associated with the primary cilium in bone disease and cancer, including primary cilium-related Ca²⁺ signaling, parathyroid hormone signaling, cAMP signaling, BMP/Smad1/5/8 signaling and Wnt signaling. We anticipate that emerging discoveries about the function of the primary cilium will provide novel insight into the molecular mechanisms of stimulus sensation, signal transduction and pathogenesis.

Dopamine receptor antagonists as potential therapeutic agents for ADPKD

Autosomal dominant polycystic kidney disease (ADPKD) is caused mostly by mutations in polycystin-1 or polycystin-2. Fluid flow leads to polycystin-dependent calcium influx and nuclear export of histone deacetylase 5 (HDAC5), which facilitates the maintenance of renal epithelial architecture by de-repression of MEF2C target genes. Here, we screened a small-molecule library to find drugs that promotes nuclear export of HDAC5. We found that dopamine receptor antagonists, domperidone and loxapine succinate, stimulate export of HDAC5, even in Pkd1^–/–cells. Domperidone targets Drd3 receptor to modulate the phosphorylation of HDAC5. Domperidone treatment increases HDAC5 phosphorylation likely by reducing protein phosphatase 2A (PP2A) activity, thus shifting the equilibrium towards HDAC5-P and export from the nucleus. Treating Pkd1^–/–mice with domperidone showed significantly reduced cystic growth and cell proliferation. Further, treated mice displayed a reduction in glomerular cyst and increased body weight and activity. These results suggest that HDAC5 nucleocytoplasmic shuttling may be modulated to impede disease progression in ADPKD and uncovers an unexpected role for a class of dopamine receptors in renal epithelial morphogenesis.

Sensory primary cilium is a responsive cAMP microdomain in renal epithelia

Primary cilia are hair-like cellular extensions that sense microenvironmental signals surrounding cells. The role of adenylyl cyclases in ciliary function has been of interest because the product of adenylyl cyclase activity, cAMP, is relevant to cilia-related diseases. In the present study, we show that vasopressin receptor type-2 (V2R) is localized to cilia in kidney epithelial cells. Pharmacologic inhibition of V2R with tolvaptan increases ciliary length and mechanosensory function. Genetic knockdown of V2R, however, does not have any effect on ciliary length, although the effect of tolvaptan on ciliary length is dampened. Our study reveals that tolvaptan may have a cilia-specific effect independent of V2R or verapamil-sensitive calcium channels. Live-imaging of single cilia shows that V2R activation increases cilioplasmic and cytoplasmic cAMP levels, whereas tolvaptan mediates cAMP changes only in a cilia-specific manner. Furthermore, fluid-shear stress decreases cilioplasmic, but not cytoplasmic cAMP levels. Our data indicate that cilioplasmic and cytoplasmic cAMP levels are differentially modulated. We propose that the cilium is a critical sensor acting as a responsive cAMP microcompartment during physiologically relevant stimuli.

Ciliotherapy: Remote Control of Primary Cilia Movement and Function by Magnetic Nanoparticles

Patients with polycystic kidney disease (PKD) are characterized with uncontrolled hypertension. Hypertension in PKD is a ciliopathy, an abnormal function and/or structure of primary cilia. Primary cilia are cellular organelles with chemo and mechanosensory roles. In the present studies, we designed a cilia-targeted (CT) delivery system to deliver fenoldopam specifically to the primary cilia. We devised the iron oxide nanoparticle (NP)-based technology for ciliotherapy. Live imaging confirmed that the CT-Fe₂O₃-NPs specifically targeted primary cilia in cultured cells in vitro and vascular endothelia in vivo. Importantly, the CT-Fe₂O₃-NPs enabled the remote control of the movement and function of a cilium with an external magnetic field, making the nonmotile cilium exhibit passive movement. The ciliopathic hearts displayed hypertrophy with compromised functions in left ventricle pressure, stroke volume, ejection fraction, and overall cardiac output because of prolonged hypertension. The CT-Fe₂O₃-NPs significantly improved cardiac function in the ciliopathic hypertensive models, in which the hearts also exhibited arrhythmia, which was corrected with the CT-Fe₂O₃-NPs. Intraciliary and cytosolic Ca²⁺ were increased when cilia were induced with fluid flow or magnetic field, and this served as a cilia-dependent mechanism of the CT-Fe₂O₃-NPs. Fenoldopam-alone caused an immediate decrease in blood pressure, followed by reflex tachycardia. Pharmacological delivery profiles confirmed that the CT-Fe₂O₃-NPs were a superior delivery system for targeting cilia more specifically, efficiently, and effectively than fenoldopam-alone. The CT-Fe₂O₃-NPs altered the mechanical properties of nonmotile cilia, and these nano-biomaterials had enormous clinical potential for ciliotherapy. Our studies further indicated that ciliotherapy provides a possibility toward personalized medicine in ciliopathy patients.

[Current situation of researches on a sensor organelle, primary cilium, to understand the pathogenesis of ciliopathy]

Primary cilium is a membrane-protruding immotile sensory organelle. It had been supposed that the cilium was a static organelle for long periods. However, recent studies have uncovered that the cilium is dynamically organized organelle in a cell cycle-dependent manner; it is formed during G₀/G₁ phase and resorbed when the cells enter cell division cycle. Despite the primary cilium is very short and its surface area is extremely small, the cilium possesses a few kinds of G protein-coupled receptors, growth factor receptors and ion channels. Therefore, it can function as a signaling receptor for selective bioactive ligands and mechanical stresses. Dysregulation of the ciliary dynamics is linked with hereditary disorders, so called "ciliopathy", with clinical manifestations of microcephaly, polycystic kidney, situs inversus, polydactyly, and so on. No effective medical treatment for the ciliopathies has been available. Increasing evidences about the molecular mechanisms of ciliary dynamics and ciliary functions have revealed that enormous number of molecules regulate a cycle of ciliogenesis, cilium-derived signaling, ciliary resorption and elimination. However, it is a fact that research progress is far inferior to the full disclosure of the molecular mechanisms. Further studies are required to clarify the pathogenesis of the cilipathies. Moreover, efficient medical treatments are expected to be developed by pharmacological approaches.

Primary Cilia-An Underexplored Topic in Major Mental Illness

Though much progress has been made in recent years towards understanding the function and physiology of primary cilia, they remain a somewhat elusive organelle. Some studies have explored the role of primary cilia in the developing nervous system, and their dysfunction has been linked with several neurosensory deficits. Yet, very little has been written on their potential role in psychiatric disorders. This article provides an overview of some of the functions of primary cilia in signalling pathways, and demonstrates that they are a worthy candidate in psychiatric research. The links between primary cilia and major mental illness have been demonstrated to exist at several levels, spanning genetics, signalling pathways, and pharmacology as well as cell division and migration. The primary focus of this review is on the sensory role of the primary cilium and the neurodevelopmental hypothesis of psychiatric disease. As such, the primary cilium is demonstrated to be a key link between the cellular environment and cell behaviour, and hence of key importance in the considerations of the nature and nurture debate in psychiatric research.

Personalized Nanotherapy by Specifically Targeting Cell Organelles To Improve Vascular Hypertension

Ciliopathies caused by abnormal function of primary cilia include expanding spectrum of kidney, liver, and cardiovascular disorders. There is currently no treatment available for patients with cilia dysfunction. Therefore, we generated and compared two different (metal and polymer) cilia-targeted nanoparticle drug delivery systems (CTNDDS), CT-DAu-NPs and CT-PLGA-NPs, for the first time. These CTNDDS loaded with fenoldopam were further compared to fenoldopam-alone. Live-imaging of single-cell-single-cilium analysis confirmed that CTNDDS specifically targeted to primary cilia. While CTNDDS did not show any advantages over fenoldopam-alone in cultured cells in vitro, CTNDDS delivered fenoldopam more superior than fenoldopam-alone by eliminating the side effect of reflex tachycardia in murine models. Although slow infusion was required for fenoldopam-alone in mice, bolus injection was possible for CTNDDS. Though there were no significant therapeutic differences between CT-DAu-NPs and CT-PLGA-NPs, CT-PLGA-NPs tended to correct ciliopathy parameters closer to normal physiological levels, indicating CT-PLGA-NPs were better cargos than CT-DAu-NPs. Both CTNDDS showed no systemic adverse effect. In summary, our studies provided scientific evidence that existing pharmacological agent could be personalized with advanced nanomaterials to treat ciliopathy by targeting cilia without the need of generating new drugs.

The Roles of Primary Cilia in Cardiovascular Diseases

Primary cilia are microtubule-based organelles found in most mammalian cell types. Cilia act as sensory organelles that transmit extracellular clues into intracellular signals for molecular and cellular responses. Biochemical and molecular defects in primary cilia are associated with a wide range of diseases, termed ciliopathies, with phenotypes ranging from polycystic kidney disease, liver disorders, mental retardation, and obesity to cardiovascular diseases. Primary cilia in vascular endothelia protrude into the lumen of blood vessels and function as molecular switches for calcium (Ca²⁺) and nitric oxide (NO) signaling. As mechanosensory organelles, endothelial cilia are involved in blood flow sensing. Dysfunction in endothelial cilia contributes to aberrant fluid-sensing and thus results in vascular disorders, including hypertension, aneurysm, and atherosclerosis. This review focuses on the most recent findings on the roles of endothelial primary cilia within vascular biology and alludes to the possibility of primary cilium as a therapeutic target for cardiovascular disorders.

Signaling interplay between primary cilia and nitric oxide: A mini review

New discoveries into the functional role of primary cilia are on the rise. In little more than 20 years, research has shown the once vestigial organelle is a signaling powerhouse involved in a vast number of essential cellular processes. In the same decade that interest in primary cilia was burgeoning, nitric oxide won molecule of the year and a Nobel prize for its role as a near ubiquitous signaling molecule. Although primary cilia and nitric oxide are both involved in signaling, a direct relationship has not been investigated; however, after a quick review of the literature, parallels between their functions can be drawn. This review aims to suggest a possible interplay between primary cilia and nitric oxide signaling especially in the areas of vascular tissue homeostasis and cellular proliferation.

Role of endothelial primary cilia as fluid mechanosensors on vascular health

Primary cilia are microtubule-based organelles that protrude from the cell surface of many mammalian cell types, including endothelial and epithelial cells, osteoblasts, and neurons. These antennal-like projections enable cells to detect extracellular stimuli and elicit responses via intracellular signaling mechanisms. Primary cilia on endothelial cells lining blood vessels function as calcium-dependent mechanosensors that sense blood flow. In doing so, they facilitate the regulation of hemodynamic parameters within the vascular system. Defects in endothelial primary cilia result in inappropriate blood flow-induced responses and contribute to the development of vascular dysfunctions, including atherosclerosis, hypertension, and aneurysms. This review examines the current understanding of vascular endothelial cilia structure and function and their role in the vascular system. Future directions for primary cilia research and treatments for ciliary-based pathologies are discussed.

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.

Calcium-axonemal microtubuli interactions underlie mechanism(s) of primary cilia morphological changes

We have used cell culture of astrocytes aligned within microchannels to investigate calcium effects on primary cilia morphology. In the absence of calcium and in the presence of flow of media (10 μL.s^-1) the majority (90%) of primary cilia showed reversible bending with an average curvature of 2.1 ± 0.9 × 10^-4 nm^-1. When 1.0 mM calcium was present, 90% of cilia underwent bending. Forty percent of these cilia demonstrated strong irreversible bending, resulting in a final average curvature of 3.9 ± 1 × 10^-4 nm^-1, while 50% of cilia underwent bending similar to that observed during calcium-free flow. The average length of cilia was shifted toward shorter values (3.67 ± 0.34 μm) when exposed to excess calcium (1.0 mM), compared to media devoid of calcium (3.96 ± 0.26 μm). The number of primary cilia that became curved after calcium application was reduced when the cell culture was pre-incubated with 15 μM of the microtubule stabilizer, taxol, for 60 min prior to calcium application. Calcium caused single microtubules to curve at a concentration ≈1.0 mM in vitro, but at higher concentration (≈1.5 mM) multiple microtubule curving occurred. Additionally, calcium causes microtubule-associated protein-2 conformational changes and its dislocation from the microtubule wall at the location of microtubule curvature. A very small amount of calcium, that is 1.45 × 10¹¹ times lower than the maximal capacity of TRPPs calcium channels, may cause gross morphological changes (curving) of primary cilia, while global cytosol calcium levels are expected to remain unchanged. These findings reflect the non-linear manner in which primary cilia may respond to calcium signaling, which in turn may influence the course of development of ciliopathies and cancer.

Primary Cilium-Dependent Signaling Mechanisms

Primary cilia are hair-like organelles and play crucial roles in vertebrate development, organogenesis, health, and many genetic disorders. A primary cilium is a mechano-sensory organelle that responds to mechanical stimuli in the micro-environment. A cilium is also a chemosensor that senses chemical signals surrounding a cell. The overall function of a cilium is therefore to act as a communication hub to transfer extracellular signals into intracellular responses. Although intracellular calcium has been one of the most studied signaling messengers that transmit extracellular signals into the cells, calcium signaling by various ion channels remains a topic of interest in the field. This may be due to a broad spectrum of cilia functions that are dependent on or independent of utilizing calcium as a second messenger. We therefore revisit and discuss the calcium-dependent and calcium-independent ciliary signaling pathways of Hedgehog, Wnt, PDGFR, Notch, TGF-β, mTOR, OFD1 autophagy, and other GPCR-associated signaling. All of these signaling pathways play crucial roles in various cellular processes, such as in organ and embryonic development, cardiac functioning, planar cell polarity, transactivation, differentiation, the cell cycle, apoptosis, tissue homeostasis, and the immune response.

G-Protein-Coupled Receptor Signaling in Cilia

G-protein-coupled receptors (GPCRs) are the largest and most versatile family of signaling receptors in humans. They respond to diverse external signals, such as photons, proteins, peptides, chemicals, hormones, lipids, and sugars, and mediate a myriad of functions in the human body. Signaling through GPCRs can be optimized by enriching receptors and downstream effectors in discrete cellular domains. Many GPCRs have been found to be selectively targeted to cilia on numerous mammalian cell types. Moreover, investigations into the pathophysiology of human ciliopathies have implicated GPCR ciliary signaling in a number of developmental and cellular pathways. Thus, cilia are now appreciated as an increasingly important nexus for GPCR signaling. Yet, we are just beginning to understand the precise signaling pathways mediated by most ciliary GPCRs and how they impact cellular function and mammalian physiology.

Adhesion GPCRs as a Putative Class of Metabotropic Mechanosensors

Adhesion GPCRs as mechanosensors. Different aGPCR homologs and their cognate ligands have been described in settings, which suggest that they function in a mechanosensory capacity. For details, see text G protein-coupled receptors (GPCRs) constitute the most versatile superfamily of biosensors. This group of receptors is formed by hundreds of GPCRs, each of which is tuned to the perception of a specific set of stimuli a cell may encounter emanating from the outside world or from internal sources. Most GPCRs are receptive for chemical compounds such as peptides, proteins, lipids, nucleotides, sugars, and other organic compounds, and this capacity is utilized in several sensory organs to initiate visual, olfactory, gustatory, or endocrine signals. In contrast, GPCRs have only anecdotally been implicated in the perception of mechanical stimuli. Recent studies, however, show that the family of adhesion GPCRs (aGPCRs), which represents a large panel of over 30 homologs within the GPCR superfamily, displays molecular design and expression patterns that are compatible with receptivity toward mechanical cues (Fig. 1). Here, we review physiological and molecular principles of established mechanosensors, discuss their relevance for current research of the mechanosensory function of aGPCRs, and survey the current state of knowledge on aGPCRs as mechanosensing molecules.

Primary Cilia in Cystic Kidney Disease

Primary cilia are small, antenna-like structures that detect mechanical and chemical cues and transduce extracellular signals. While mammalian primary cilia were first reported in the late 1800s, scientific interest in these sensory organelles has burgeoned since the beginning of the twenty-first century with recognition that primary cilia are essential to human health. Among the most common clinical manifestations of ciliary dysfunction are renal cysts. The molecular mechanisms underlying renal cystogenesis are complex, involving multiple aberrant cellular processes and signaling pathways, while initiating molecular events remain undefined. Autosomal Dominant Polycystic Kidney Disease is the most common renal cystic disease, caused by disruption of polycystin-1 and polycystin-2 transmembrane proteins, which evidence suggests must localize to primary cilia for proper function. To understand how the absence of these proteins in primary cilia may be remediated, we review intracellular trafficking of polycystins to the primary cilium. We also examine the controversial mechanisms by which primary cilia transduce flow-mediated mechanical stress into intracellular calcium. Further, to better understand ciliary function in the kidney, we highlight the LKB1/AMPK, Wnt, and Hedgehog developmental signaling pathways mediated by primary cilia and misregulated in renal cystic disease.

RAPAMYCIN INCREASES LENGTH AND MECHANOSENSORY FUNCTION OF PRIMARY CILIA IN RENAL EPITHELIAL AND VASCULAR ENDOTHELIAL CELLS

Primary cilia arebiophysically-sensitive organelles responsible for sensing fluid-flow and transducing this stimulus into intracellular responses. Previous studies have shown that the primary cilia mediate flow-induced calcium influx, and sensitivity of cilia function to flow is correlated to cilia length. Cells with abnormal cilia length or function can lead to a host of diseases that are collectively termed as ciliopathies. Rapamycin, a potent inhibitor of mTOR (mammalian target of rapamycin), has been demonstrated to be a potential pharmacological agent against the aberrant mTOR signaling seen in ciliopathies such as polycystic kidney disease (PKD) and tuberous sclerosis complex (TSC). Here we look at the effects of rapamycin on ciliary length and function for the first time. Compared to controls, primary cilia in rapamycin-treated porcine renal epithelial and mouse vascular endothelial cells showed a significant increase in length. Graded increases in fluid-shear stress further indicates that rapamycin enhances cilia sensitivity to fluid flow. Treatment with rapamycin led to G0 arrest in porcine epithelial cells while no significant change in cell cycle were observed in rapamycin-treated mouse epithelial or endothelial cells, indicating a species-specific effect of rapamycin. Given the previousin vitro and in vivo studies establishing rapamycin as a potential therapeutic agent for ciliopathies, such as PKD and TSC, our studies show that rapamycin enhances ciliary function and sensitivity to fluid flow. The results of our studies suggest a potential ciliotherapeutic effect of rapamycin.

Vasopressin-related copeptin is a novel predictor of early endothelial dysfunction in patients with adult polycystic kidney disease

Background

In this study, we examined the relative usefulness of serum copeptin levels as a surrogate marker of vasopressin (AVP) in adult polycystic kidney disease (ADPKD) by correlating it with baseline and longitudinal changes in markers of both renal function and common CVD manifestations (hypertensive vascular disease, atherosclerosis and endothelial dysfunction) that accompany the progression of this disease.

Methods

We studied a cohort of young and otherwise healthy ADPKD patients (n = 235) and measured cardiovascular function using flow-mediation dilatation (FMD), carotid intima media thickness (cIMT) and pulse wave velocity (PWV), as well as serum copeptin (commercial ELISA, a stable marker of AVP activity). The same analyses were carried out at baseline and after 3 years of follow-up.

Results

At baseline, median eGFR was 69 mL/min./1.73 m², mean FMD 6.9 ± 0.9%, cIMT 0.7 ± 0.1 mm, and PWV 8.1 ± 1.2 m/s. At follow-up, equivalent values were 65 (44–75) mL/min./1.73 m², 5.8 ± 0.9%, 0.8 ± 0.1 mm. and 8.2 ± 1.3 m/s. with all changes statistically significant. Plasma copeptin also rose from 0.62 ± 0.12 to 0.94 ± 0.19 ng/mL and this change correlated with ΔeGFR (-0.33, p < 0.001), ΔFMD (0.599, p < 0.001), ΔcIMT (0.562, p < 0.001) and ΔPWV (0.27, p < 0.001) also after linear regression modeling to correct for confounders. Finally, ROC analysis was done for a high baseline copeptin with ΔeGFR [cut-off:≤59], ΔFMD [cut-off: ≤7.08], ΔcIMT [cut-off:>0.76], and ΔPWV [cut-off:≤7.80].

Conclusions

Vascular dysfunction as reflected by FMD and cIMT, but not PWV or an altered cardiac geometry, precede most other signs of disease in ADPKD but is predicted by elevated levels of the circulating AVP-marker copeptin.

The IN/OUT assay: a new tool to study ciliogenesis

Background

Nearly all cells have a primary cilia on their surface, which functions as a cellular antennae. Primary cilia assembly begins intracellularly and eventually emerges extracellularly. However, current ciliogenesis assays, which detect cilia length and number, do not monitor ciliary stages.

Methods

We developed a new assay that detects antibody access to a fluorescently tagged ciliary transmembrane protein, which revealed three ciliary states: classified as ‘inside,’ ‘outside,’ or ‘partial’ cilia.

Results

Strikingly, most cilia in RPE cells only partially emerged and many others were long and intracellular, which would be indistinguishable by conventional assays. Importantly, these states switch with starvation-induced ciliogenesis and the cilia can emerge both on the dorsal and ventral surface of the cell. Our assay further allows new molecular and functional studies of the ‘ciliary pocket,’ a deep plasma membrane invagination whose function is unclear. Molecularly, we show colocalization of EHD1, Septin 9 and glutamylated tubulin with the ciliary pocket.

Conclusions

Together, the IN/OUT assay is not only a new tool for easy and quantifiable visualization of different ciliary stages, but also allows molecular characterization of intermediate ciliary states.

Electronic supplementary material

The online version of this article (doi:10.1186/s13630-016-0044-2) contains supplementary material, which is available to authorized users.

Dopaminergic signaling within the primary cilia in the renovascular system

Activation of dopamine receptor type-5 (DR5) has been known to reduce systemic blood pressure, most likely by increasing renal vasodilation and enhancing natriuresis in the kidney. However, the mechanism of DR5 in natriuresis and vasodilation was not clearly known. We have previously shown that DR5 is localized to primary cilia of proximal renal epithelial and vascular endothelial cells. We here show that selective activation of DR5 specifically induces calcium influx only in the primary cilia, whereas non-selective activation of dopamine receptor induces calcium fluxes in both cilioplasm and cytoplasm. Cilia-independent signaling induced by thrombin only shows calcium signaling within cytoplasm. Furthermore, calcium activation in the cilioplasm by DR5 increases length and mechanosensory function of primary cilia, leading to a greater response to fluid-shear stress. We therefore propose a new mechanism by which DR5 induces vasodilation via chemical and mechanical properties that are specific to primary cilia.

Does dopamine connect the dots in ADPKD?

Healthy autosomal dominant polycystic kidney disease (ADPKD) patients with normal kidney function demonstrate reduced endothelial-dependent vasodilation that improves with increasing local dopamine levels. Dopamine regulates renal sodium excretion, and dopamine receptors are located on primary cilia in both vascular and renal tubular epithelial cells. The study by Lorthioir and colleagues links endothelial function and dopamine availability in ADPKD patients.

Sensing a sensor: identifying the mechanosensory function of primary cilia

Over the past decade, primary cilia have emerged as the premier means by which cells sense and transduce mechanical stimuli. Primary cilia are sensory organelles that have been shown to be vitally involved in the mechanosensation of urine in the renal nephron, bile in the hepatic biliary system, digestive fluid in the pancreatic duct, dentin in dental pulp, lacunocanalicular fluid in bone and cartilage, and blood in vasculature. The prevalence of primary cilia among mammalian cell types is matched by the tremendously varied disease states caused by both structural and functional defects in cilia. In the process of delineating the mechanisms behind these disease states, calcium fluorimetry has been widely utilized as a means of quantifying ciliary function to both fluid flow and pharmacological agents. In this review, we will discuss the approaches used in associating calcium levels to cilia function.

Polycystin deficiency induces dopamine-reversible alterations in flow-mediated dilatation and vascular nitric oxide release in humans

Autosomal dominant polycystic kidney disease (ADPKD) is a renal hereditary disorder associated with increased cardiovascular mortality, due to mutations in polycystin-1 and polycystin-2 genes. Endothelial polycystin-deficient cells have an altered mechanosensitivity to fluid shear stress and subsequent deficit in calcium-induced nitric oxide release, prevented by dopamine receptor stimulation. However, the impact of polycystin deficiency on endothelial function in ADPKD patients is still largely unknown. Here we assessed endothelium-dependent flow-mediated dilatation in 21 normotensive ADPKD patients and 21 healthy control subjects, during sustained (hand skin heating) and transient (postischemic hyperemia) flow stimulation. Flow-mediated dilatation was less marked in ADPKD patients than in controls during heating, but it was similar during postischemic hyperemia. There was no difference in endothelium-independent dilatation in response to glyceryl trinitrate. Local plasma nitrite, an indicator of nitric oxide availability, increased during heating in controls but not in patients. Brachial infusion of dopamine in a subset of ADPKD patients stimulated plasma nitrite increase during heating and improved flow-mediated dilatation. Thus, ADPKD patients display a loss of nitric oxide release and an associated reduction in endothelium-dependent dilatation of conduit arteries during sustained blood flow increase. The correction of these anomalies by dopamine suggests future therapeutic strategies that could reduce the occurrence of cardiovascular events in ADPKD.