Deciliation is associated with dramatic remodeling of epithelial cell junctions and surface domains

Endocardial primary cilia and blood flow are required for regulation of EndoMT during endocardial cushion development

Blood flow is critical for heart valve formation, and cellular mechanosensors are essential to translate flow into transcriptional regulation of development. Here, we identify a role for primary cilia in vivo in the spatial regulation of cushion formation, the first stage of valve development, by regionally controlling endothelial to mesenchymal transition (EndoMT) via modulation of Kruppel-like Factor 4 (Klf4) . We find that high shear stress intracardiac regions decrease endocardial ciliation over cushion development, correlating with KLF4 downregulation and EndoMT progression. Mouse embryos constitutively lacking cilia exhibit a blood-flow dependent accumulation of KLF4 in these regions, independent of upstream left-right abnormalities, resulting in impaired cushion cellularization. snRNA-seq revealed that cilia KO endocardium fails to progress to late-EndoMT, retains endothelial markers and has reduced EndoMT/mesenchymal genes that KLF4 antagonizes. Together, these data identify a mechanosensory role for endocardial primary cilia in cushion development through regional regulation of KLF4.

Genital stones: Radiological, histopathological, ultrastructural, and molecular analysis in rooster

Epididymal lithiasis, characterized by the formation of stones in the epididymis, has been associated with a decline in fertility in roosters. This study aimed to investigate the reproductive performance, ultrastructural characteristics, and expression of aromatase cytochrome P450 (CYP19) and aquaporin 9 (AQP9) in aged broiler breeder roosters affected by epididymal lithiasis. X-ray analysis confirmed the presence of genital stones in both the epididymis and testicular tissue regions. While there was a significant decrease in sperm concentration in the affected roosters compared to non-affected roosters, no significant differences were observed in total and progressive sperm motility between the two groups. Furthermore, the affected roosters exhibited significant abnormalities in semen parameters, except for sperm concentration and morphology. Transmission electron microscopy (TEM) revealed the depletion and deciliation of ciliated cells in the distal efferent ductules of the epididymis in affected roosters. Additionally, the expression of CYP19 and AQP9 was found to be increased in the epididymal region of affected roosters. Notably, we report the presence of testicular stones for the first time in this study, in addition to epididymal stones. Considering the male reproductive tract lesions observed, we propose the term "genital stones" to describe these conditions. Moreover, our findings suggest that the overexpression of AQP9, which is associated with a high copy number of the CYP19 gene in the epididymal region of affected aged roosters, may contribute to the formation of genital stones by promoting increased reabsorption of fluids in the epididymis. The condensation of epididymal duct contents and reduction in the population of ciliated cells further impairs semen movement and can lead to the blockage of extra-testicular ducts, resulting in the low fertility syndrome observed in aged roosters.

Changes in expression of mesothelial BBS genes in 2D and 3D after lithium chloride and ammonium sulphate induction of primary cilium disturbance: a pilot study

BACKGROUND

Malignant pleural mesothelioma (MPM), a rare and aggressive pleural tumor, has significant histological and molecular heterogeneity. Primary Cilium (PC), an organelle of emerging importance in malignancies, has been scarcely investigated in MPM. A critical molecular complex for the PC function is the BBSome and here we aimed at assessing its expression patterns in ordinary 2D and spheroid 3D cell cultures.

METHODS

A human benign mesothelial cell line (MeT-5A), MPM cell lines (M14K, epithelioid MPM; MSTO, biphasic MPM), and primary MPM cells (pMPM) were used. Primers specific for the human BBS1, 2, 4, 5, 7, 9, 18 transcripts were designed, and quantitative real-time PCR (qRT-PCR) was done with β-actin as the gene of reference. The relative gene expression across 2D and 3D cultures was analyzed by the expression factor (mean of 1/ΔCt values). With the 2-∆∆Ct method the gene expression fold changes were assessed from qRT-PCR data. Molecular changes using the PC-modulating drugs ammonium sulfate (AS) and lithium chloride (LC) were also determined.

RESULTS

PC was present in all cells used in the study at approximately 15% of the observed area. BBSome transcripts were differentially expressed in different dimensions of cell culture (2D vs. 3D) in all cell lines and pMPM. Treatment with AS and LC affected the expression of the ciliary BBS2 and BBS18 genes in the benign as well as in the MPM cells.

CONCLUSIONS

These data indicate distinct BBSome molecular profiles in human benign and MPM cells cultured in 2D and 3D dimensions and support the notion that PC genes should be investigated as potential MPM therapeutic targets.

Effects of pharmacological primary cilium disturbance in the context of in vitro 2D and 3D malignant pleura mesothelioma

INTRODUCTION

Primary cilium (PC) is a single non-motile antenna-like organelle composed of a microtubule core axon originating from the mother centriole of the centrosome. The PC is universal in all mammalian cells and protrudes to the extracellular environment receiving mechanochemical cues that it transmits in the cell.

AIM

To investigate the role of PC in mesothelial malignancy in the context of two-dimensional (2D) and three-dimensional (3D) phenotypes.

MATERIALS AND METHODS

The effect of pharmacological deciliation [using ammonium sulphate (AS) or chloral hydrate (CH)] and PC elongation [using lithium chloride (LC)] on cell viability, adhesion, and migration (2D cultures) as well as in mesothelial sphere formation, spheroid invasion and collagen gel contraction (3D cultures) was investigated in benign mesothelial MeT-5A cells and in malignant pleural mesothelioma (MPM) cell lines, M14K (epithelioid) and MSTO (biphasic), and primary malignant pleural mesothelioma cells (pMPM).

RESULTS

Pharmacological deciliation or elongation of the PC significantly affected cell viability, adhesion, migration, spheroid formation, spheroid invasion and collagen gel contraction in MeT-5A, M14K, MSTO cell lines and in pMPM cells compared to controls (no drug treatment).

CONCLUSIONS

Our findings indicate a pivotal role of the PC in functional phenotypes of benign mesothelial cells and MPM cells.

Brain heterotopia formation by ciliopathic breakdown of neuroepithelial and blood-cerebrospinal fluid barriers

The developmental functions of primary cilia and the downstream signaling pathways have been widely studied; however, the roles of primary cilia in the developing neurovascular system are not clearly understood. In this study, we found that ablation of genes encoding ciliary transport proteins such as intraflagellar transport homolog 88 (Ift88) and kinesin family member 3a (Kif3a) in cortical radial progenitors led to periventricular heterotopia during late mouse embryogenesis. Conditional mutation of primary cilia unexpectedly caused breakdown of both the neuroepithelial lining and the blood-choroid plexus barrier. Choroidal leakage was partially caused by enlargement of the choroid plexus in the cilia mutants. We found that the choroid plexus expressed platelet-derived growth factor A (Pdgf-A) and that Pdgf-A expression was ectopically increased in cilia-mutant embryos. Cortices obtained from embryos in utero electroporated with Pdgfa mimicked periventricular heterotopic nodules of the cilia mutant. These results suggest that defective ciliogenesis in both cortical progenitors and the choroid plexus leads to breakdown of cortical and choroidal barriers causing forebrain neuronal dysplasia, which may be related to developmental cortical malformation.

Oligodendroglial primary cilium heterogeneity during development and demyelination/remyelination

The primary cilium (PC) has emerged as an indispensable cellular antenna essential for signal transduction of important cell signaling pathways. The rapid acquisition of knowledge about PC biology has raised attention to PC as a therapeutic target in some neurological and psychiatric diseases. However, the role of PC in oligodendrocytes and its participation in myelination/remyelination remain poorly understood. Oligodendrocyte precursor cells (OPCs) give rise to oligodendrocytes during central nervous system (CNS) development. In adult, a small percentage of OPCs remains as undifferentiated cells located sparsely in the different regions of the CNS. These cells can regenerate oligodendrocytes and participate to certain extent in remyelination. This study aims characterize PC in oligodendrocyte lineage cells during post-natal development and in a mouse model of demyelination/remyelination. We show heterogeneity in the frequency of cilium presence on OPCs, depending on culture conditions in vitro and cerebral regions in vivo during development and demyelination/remyelination. In vitro, Lithium chloride (LiCl), Forskolin and Chloral Hydrate differentially affect cilium, depending on culture environment and PC length correlates with the cell differentiation state. Beside the role of PC as a keeper of cell proliferation, our results suggest its involvement in myelination/remyelination.

Exocyst complex mediates recycling of internal cilia

Primary cilia are slender, cellular antennae that sense extracellular stimuli, and their absence or dysfunction plays a role in numerous human diseases. Prior work has indicated a role of the exocyst tethering complex in cilia biogenesis and maintenance,^1-6 with the underlying paradigm that the exocyst targets vesicles to the ciliary base to deliver ciliary cargoes.^7-9 However, the role of the exocyst vis-à-vis to primary cilia in living cells and during stimulation is unknown. Herein, using advanced imaging and quantitative analysis reveals that serum stimulation increases the exocyst's localization to cilia by three-fold. This serum-stimulated localization is highly dynamic, and FRAP experiments show that exocysts at the cilia are highly mobile (60%-80%). Super resolution imaging reveals that the xocyst extends past the cilia base to the entire ciliary pocket. To visualize cilia exocytosis, we conducted live cell imaging with pH-sensitive cilia reporters in combination with extracellular pH switching. Strikingly, we observed that an exocyst-positive internal cilia fuses with the cell surface. These live cell results support a novel and dynamic role of the exocyst complex in the delivery of internalized cilia to the cell surface. Moreover, they suggest a novel pathway may be used to recycle primary cilia to the cell surface that engages the exocyst in response to stimuli. This new remarkable plasticity in cilia presence on the surface in response to extracellular stimuli suggest new means to potentially modulate cilia signaling.

Quinomycin A reduces cyst progression in polycystic kidney disease

Polycystic kidney disease (PKD) is a genetic disorder characterized by aberrant renal epithelial cell proliferation and formation and progressive growth of numerous fluid-filled cysts within the kidneys. Previously, we showed that there is elevated Notch signaling compared to normal renal epithelial cells and that Notch signaling contributes to the proliferation of cystic cells. Quinomycin A, a bis-intercalator peptide, has previously been shown to target the Notch signaling pathway and inhibit tumor growth in cancer. Here, we show that Quinomycin A decreased cell proliferation and cyst growth of human ADPKD cyst epithelial cells cultured within a 3D collagen gel. Treatment with Quinomycin A reduced kidney weight to body weight ratio and decreased renal cystic area and fibrosis in Pkd1^RC/RC ; Pkd2^+/- mice, an orthologous PKD mouse model. This was accompanied by reduced expression of Notch pathway proteins, RBPjk and HeyL and cell proliferation in kidneys of PKD mice. Quinomycin A treatments also normalized cilia length of cyst epithelial cells derived from the collecting ducts. This is the first study to demonstrate that Quinomycin A effectively inhibits PKD progression and suggests that Quinomycin A has potential therapeutic value for PKD patients.

Oxidative stress following acute kidney injury causes disruption of lung cell cilia and their release into the bronchoaveolar lavage fluid and lung injury, which are exacerbated by Idh2 deletion

Acute kidney injury (AKI) induces distant organ injury, which is a serious concern in patients with AKI. Recent studies have demonstrated that distant organ injury is associated with oxidative stress of organ and damage of cilium, an axoneme-based cellular organelle. However, the role of oxidative stress and cilia damage in AKI-induced lung injury remains to be defined. Here, we investigated whether AKI-induced lung injury is associated with mitochondrial oxidative stress and cilia disruption in lung cells. AKI was induced in isocitrate dehydrogenase 2 (Idh2, a mitochondrial antioxidant enzyme)-deleted (Idh2^−/−) and wild-type (Idh2^+/+) mice by kidney ischemia-reperfusion (IR). A group of mice were treated with Mito-TEMPO, a mitochondria-specific antioxidant. Kidney IR caused lung injuries, including alveolar septal thickening, alveolar damage, and neutrophil accumulation in the lung, and increased protein concentration and total cell number in bronchoalveolar lavage fluid (BALF). In addition, kidney IR caused fragmentation of lung epithelial cell cilia and the release of fragments into BALF. Kidney IR also increased the production of superoxide, lipid peroxidation, and mitochondrial and nuclei DNA oxidation in lungs and decreased IDH2 expression. Lung oxidative stress and injury relied on the degree of kidney injury. Idh2 deletion exacerbated kidney IR-induced lung injuries. Treatment with Mito-TEMPO attenuated kidney IR-induced lung injuries, with greater attenuation in Idh2^−/− than Idh2^+/+ mice. Our data demonstrate that AKI induces the disruption of cilia and damages cells via oxidative stress in lung epithelial cells, which leads to the release of disrupted ciliary fragments into BALF.

Centriole is the pivot coordinating dynamic signaling for cell proliferation and organization during early development in the vertebrates

Vertebrates have an elaborate and functionally segmented body. It evolves from a single cell by systematic cell proliferation but attains a complex body structure with exquisite precision. This development requires two cellular events: cell cycle and ciliogenesis. For these events, the dynamic molecular signaling is converged at the centriole. The cell cycle helps in cell proliferation and growth of the body and is a highly regulated and integrated process. Its errors cause malignancies and developmental disorders. The cells newly proliferated are organized during organogenesis. For a cellular organization, dedicated signaling hubs are developed in the cells, and most often cilia are utilized. The cilium is generated from one of the centrioles involved in cell proliferation. The developmental signaling pathways hosted in cilia are essential for the elaboration of the body plan. The cilium's compartmental seclusion is ideal for noise-free molecular signaling and is essential for the precision of the body layout. The dysfunctional centrioles and primary cilia distort the development of body layout that manifest as serious developmental disorders. Thus, centriole has a dual role in the growth and cellular organization. It organizes dynamically expressed molecules of cell cycle and ciliogenesis and plays a balancing act to generate new cells and organize them during development. A putative master molecule may regulate and coordinate the dynamic gene expression at the centrioles. The convergence of many critical signaling components at the centriole reiterates the idea that centriole is a major molecular workstation involved in elaborating the structural design and complexity in vertebrates. This article is protected by copyright. All rights reserved.

Primary ciliogenesis is a crucial step for multiciliated cell determinism in the respiratory epithelium

The alteration of the mucociliary clearance is a major hallmark of respiratory diseases related to structural and functional cilia abnormalities such as chronic obstructive pulmonary diseases (COPD), asthma and cystic fibrosis. Primary cilia and motile cilia are the two principal organelles involved in the control of cell fate in the airways. We tested the effect of primary cilia removal in the establishment of a fully differentiated respiratory epithelium. Epithelial barrier integrity was not altered while multiciliated cells were decreased and mucous-secreting cells were increased. Primary cilia homeostasis is therefore paramount for airway epithelial cell differentiation. Primary cilia-associated pathophysiologic implications require further investigations in the context of respiratory diseases.

MKS-NPHP module proteins control ciliary shedding at the transition zone

Ciliary shedding occurs from unicellular organisms to metazoans. Although required during the cell cycle and during neurogenesis, the process remains poorly understood. In all cellular models, this phenomenon occurs distal to the transition zone (TZ), suggesting conserved molecular mechanisms. The TZ module proteins (Meckel Gruber syndrome [MKS]/Nephronophtysis [NPHP]/Centrosomal protein of 290 kDa [CEP290]/Retinitis pigmentosa GTPase regulator-Interacting Protein 1-Like Protein [RPGRIP1L]) are known to cooperate to establish TZ formation and function. To determine whether they control deciliation, we studied the function of 5 of them (Transmembrane protein 107 [TMEM107], Transmembrane protein 216 [TMEM216], CEP290, RPGRIP1L, and NPHP4) in Paramecium. All proteins are recruited to the TZ of growing cilia and localize with 9-fold symmetry at the level of the most distal part of the TZ. We demonstrate that depletion of the MKS2/TMEM216 and TMEM107 proteins induces constant deciliation of some cilia, while depletion of either NPHP4, CEP290, or RPGRIP1L prevents Ca2+/EtOH deciliation. Our results constitute the first evidence for a role of conserved TZ proteins in deciliation and open new directions for understanding motile cilia physiology.

Functional analysis and subcellular localisation of the conserved transition zone proteins in the ciliate Paramecium tetraurelia demonstrates their involvement in the ciliary shedding process, opening new avenues fir understanding the molecular mechanism of deciliation.

Neutrophil gelatinase-associated lipocalin (NGAL) is localised to the primary cilium in renal tubular epithelial cells - A novel source of urinary biomarkers of renal injury

BACKGROUND

Primary cilia have been shown to play a central role in regulating epithelial cell differentiation during injury and repair. Growing evidence implicates structural and functional abnormalities of primary cilia in kidney epithelial cells in the onset and development of various kidney diseases including polycystic kidney disease (PKD). Neutrophil-gelatinase associated lipocalin (NGAL) has been identified as a reliable urinary biomarker of kidney injury. However, the mechanism by which this protein accumulates in patient urine samples has not been fully elucidated.

METHODS

Human renal tubular epithelial cells (RPTECs) were exposed to previously characterized deciliating agents to assess mechanisms of primary cilium loss. Confocal immunofluorescent imaging was employed to visualise the effects on cilia. Western blot analysis was utilised to quantify the ciliary protein Arl13b in both RPTEC whole cell lysates and supernatants. Co-immunoprecipitation was used to demonstrate co-localisation of Arl13b and NGAL in urinary samples from a clinical Chronic Allograft Nephropathy (CAN) cohort.

RESULTS

Immunofluorescent analysis revealed that NGAL was localised to the primary cilium in RPTECs, co-localizing with a ciliary specific protein, Arl13b. Deciliation experiments showed that loss of the cilia coincided with loss of NGAL from the cells.

CONCLUSION

The accumulation of NGAL in supernatants in vitro and in the urine of CAN patients was concurrent with loss of Arl13b, a specific ciliary protein. The findings of this study propose that increased NGAL urinary concentrations are directly linked to deciliation of the renal epithelial cells as a result of injury.

Primary cilium loss in mammalian cells occurs predominantly by whole-cilium shedding

The primary cilium is a central signaling hub in cell proliferation and differentiation and is built and disassembled every cell cycle in many animal cells. Disassembly is critically important, as misregulation or delay of cilia loss leads to cell cycle defects. The physical means by which cilia are lost are poorly understood but are thought to involve resorption of ciliary components into the cell body. To investigate cilium loss in mammalian cells, we used live-cell imaging to comprehensively characterize individual events. The predominant mode of cilium loss was rapid deciliation, in which the membrane and axoneme of the cilium was shed from the cell. Gradual resorption was also observed, as well as events in which a period of gradual resorption was followed by rapid deciliation. Deciliation resulted in intact shed cilia that could be recovered from culture medium and contained both membrane and axoneme proteins. We modulated levels of katanin and intracellular calcium, two putative regulators of deciliation, and found that excess katanin promotes cilia loss by deciliation, independently of calcium. Together, these results suggest that mammalian ciliary loss involves a tunable decision between deciliation and resorption.

Apical Shear Stress Enhanced Organic Cation Transport in Human OCT2/MATE1-Transfected Madin-Darby Canine Kidney Cells Involves Ciliary Sensing

Active transport by renal proximal tubules plays a significant role in drug disposition. During drug development, estimates of renal excretion are essential to dose determination. Kidney bioreactors that reproduce physiologic cues in the kidney, such as flow-induced shear stress, may better predict in vivo drug behavior than do current in vitro models. In this study, we investigated the role of shear stress on active transport of 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide (ASP+) by Madin-Darby canine kidney cells exogenously expressing the human organic cation transporters organic cation transporter 2 (OCT2) and multidrug and toxin extrusion protein 1 (MATE1). Cells cultured in a parallel plate under continuous media perfusion formed a tight monolayer with a high barrier to inulin. In response to increasing levels of shear stress (0.2-2 dynes/cm2), cells showed a corresponding increase in transport of ASP+, reaching a maximal 4.2-fold increase at 2 dynes/cm2 compared with cells cultured under static conditions. This transport was inhibited with imipramine, indicating active transport was present under shear stress conditions. Cells exposed to shear stress of 2 dynes/cm2 also showed an increase in RNA expression of both transfected human and endogenous OCT2 (3.7- and 2.0-fold, respectively). Removal of cilia by ammonium sulfate eliminated the effects of shear on ASP+ transport at 0.5 dynes/cm2 with no effect on ASP+ transport under static conditions. These results indicate that shear stress affects active transport of organic cations in renal tubular epithelial cells in a cilia-dependent manner.

A Flow Cytometry-Based Approach for the Isolation and Characterization of Neural Stem Cell Primary Cilia

In the adult mammalian brain, the apical surface of the subependymal zone (SEZ) is covered by many motile ependymal cilia and a few primary cilia originating from rare intermingled neural stem cells (NSCs). In NSCs the primary cilia are key for the transduction of essential extracellular signals such as Sonic hedgehog (SHH) and platelet-derived growth factor (PDGF). Despite their importance, the analysis of NSC primary cilia is greatly hampered by the fact that they are overwhelmingly outnumbered by the motile cilia. We here take advantage of flow cytometry to purify the two cilia types and allow their molecular characterization. Primary cilia were identified based on immunoreactivity to the marker adenylate cyclase type III (AC3) and differential levels of prominin-1 whereas motile cilia displayed immunoreactivity only to the latter. Consistent with the morphological differences between the two classes of cilia, enrichment of motile cilia positively correlated with size. Moreover, we observed age-dependent variations in the abundance of the two groups of ciliary organelles reflecting the changes associated with their development. The two cilia groups also differed with respect to the expression of signaling molecules, since PDGF receptor (PDGFR)α, smoothened (Smo) and CXC chemokine receptor (CXCR)4 were only detected in isolated primary but not motile cilia. Thus, our novel method of cilia isolation and characterization by flow cytometry has the potential to be extended to the study of cilia from different tissues and organs, providing a powerful tool for the investigation of primary cilia in physiological and pathological conditions.

Ciliary signalling in cancer

Although tumours initiate from oncogenic changes in a cancer cell, subsequent tumour progression and therapeutic response depend on interactions between the cancer cells and the tumour microenvironment (TME). The primary monocilium, or cilium, provides a spatially localized platform for signalling by Hedgehog, Notch, WNT and some receptor tyrosine kinase pathways and mechanosensation. Changes in ciliation of cancer cells and/or cells of the TME during tumour development enforce asymmetric intercellular signalling in the TME. Growing evidence indicates that some oncogenic signalling pathways as well as some targeted anticancer therapies induce ciliation, while others repress it. The links between the genomic profile of cancer cells, drug treatment and ciliary signalling in the TME likely affect tumour growth and therapeutic response.

Cilium structure, assembly, and disassembly regulated by the cytoskeleton

The cilium, once considered a vestigial structure, is a conserved, microtubule-based organelle critical for transducing extracellular chemical and mechanical signals that control cell polarity, differentiation, and proliferation. The cilium undergoes cycles of assembly and disassembly that are controlled by complex inter-relationships with the cytoskeleton. Microtubules form the core of the cilium, the axoneme, and are regulated by post-translational modifications, associated proteins, and microtubule dynamics. Although actin and septin cytoskeletons are not major components of the axoneme, they also regulate cilium organization and assembly state. Here, we discuss recent advances on how these different cytoskeletal systems­ affect cilium function, structure, and organization.

Can Tissue Cilia Lengths and Urine Cilia Proteins Be Markers of Kidney Diseases?

The primary cilium is an organelle which consists of a microtubule in the core and a surrounding cilia membrane, and has long been recognized as a “vestigial organelle”. However, new evidence demonstrates that the primary cilium has a notable effect on signal transduction in the cell and is associated with some genetic and non-genetic diseases. In the kidney, the primary cilium protrudes into the Bowman's space and the tubular lumen from the apical side of epithelial cells. The length of primary cilia is dynamically altered during the normal cell cycle, being shortened by retraction into the cell body at the entry of cell division and elongated at differentiation. Furthermore, the length of primary cilia is also dynamically changed in the cells, as a result and/or cause, during the progression of various kidney diseases including acute kidney injury and chronic kidney disease. Notably, recent data has demonstrated that the shortening of the primary cilium in the cell is associated with fragmentation, apart from retraction into the cell body, in the progression of diseases and that the fragmented primary cilia are released into the urine. This data reveals that the alteration of primary cilia length could be related to the progression of diseases. This review will consider if primary cilia length alteration is associated with the progression of kidney diseases and if the length of tissue primary cilia and the presence or increase of cilia proteins in the urine is indicative of kidney diseases.

NUP98 Sets the Size-Exclusion Diffusion Limit through the Ciliary Base

The primary cilium maintains a well-regulated complement of soluble and membrane proteins, allowing it to mediate a variety of signaling pathways that are essential for development and tissue homeostasis [1-3]. Entry into the cilium is regulated at the base, where a complex containing nucleoporins, referred to as the "ciliary pore complex" (CPC), has been proposed to set a size-exclusion limit for soluble molecule diffusion into the cilium [4-6]. Here, using a fluorescence-based diffusion trap system, we demonstrate that NUP98, a component of the phenylalanine-glycine (FG) hydrogel permeability barrier at the nuclear pore complex [7, 8], limits the diffusion of soluble molecules >70 kDa into the cilium in cultured mammalian cells. Small interfering RNA (siRNA)-mediated knockdown of NUP98 increases the rate of diffusion of molecules >100 kDa into the cilium. The tubulin heterodimer, the building block of the axoneme [9, 10], is approximately 100 kDa in size. After knockdown of NUP98, cilia become shorter, and their length is more sensitive to changes in cytoplasmic soluble tubulin levels. These data indicate a novel function of the ciliary pore complex, limiting diffusion of soluble tubulin between the ciliary matrix and the cytosol, allowing the cilium to regulate its length independently of cytosolic microtubule dynamics.

Tubulin glycylation controls primary cilia length

In motile cilia and flagella, tubulin glycylation is involved in axoneme stabilization. Using a newly developed antibody, Gadadhar et al. now show that glycylation also accumulates in primary cilia, where it controls ciliary length. This suggests an important role for this PTM in primary cilia homeostasis.

As essential components of the eukaryotic cytoskeleton, microtubules fulfill a variety of functions that can be temporally and spatially controlled by tubulin posttranslational modifications. Tubulin glycylation has so far been mostly found on motile cilia and flagella, where it is involved in the stabilization of the axoneme. In contrast, barely anything is known about the role of glycylation in primary cilia because of limitations in detecting this modification in these organelles. We thus developed novel glycylation-specific antibodies with which we detected glycylation in many primary cilia. Glycylation accumulates in primary cilia in a length-dependent manner, and depletion or overexpression of glycylating enzymes modulates the length of primary cilia in cultured cells. This strongly suggests that glycylation is essential for the homeostasis of primary cilia, which has important implications for human disorders related to primary cilia dysfunctions, such as ciliopathies and certain types of cancer.

Fluid shear stress-induced TGF-β/ALK5 signaling in renal epithelial cells is modulated by MEK1/2

Renal tubular epithelial cells are exposed to mechanical forces due to fluid flow shear stress within the lumen of the nephron. These cells respond by activation of mechano-sensors located at the plasma membrane or the primary cilium, having crucial roles in maintenance of cellular homeostasis and signaling. In this paper, we applied fluid shear stress to study TGF-β signaling in renal epithelial cells with and without expression of the Pkd1-gene, encoding a mechano-sensor mutated in polycystic kidney disease. TGF-β signaling modulates cell proliferation, differentiation, apoptosis, and fibrotic deposition, cellular programs that are altered in renal cystic epithelia. SMAD2/3-mediated signaling was activated by fluid flow, both in wild-type and Pkd1^−/− cells. This was characterized by phosphorylation and nuclear accumulation of p-SMAD2/3, as well as altered expression of downstream target genes and epithelial-to-mesenchymal transition markers. This response was still present after cilia ablation. An inhibitor of upstream type-I-receptors, ALK4/ALK5/ALK7, as well as TGF-β-neutralizing antibodies effectively blocked SMAD2/3 activity. In contrast, an activin-ligand trap was ineffective, indicating that increased autocrine TGF-β signaling is involved. To study potential involvement of MAPK/ERK signaling, cells were treated with a MEK1/2 inhibitor. Surprisingly, fluid flow-induced expression of most SMAD2/3 targets was further enhanced upon MEK inhibition. We conclude that fluid shear stress induces autocrine TGF-β/ALK5-induced target gene expression in renal epithelial cells, which is partially restrained by MEK1/2-mediated signaling.

The mechanosensitive BKα/β1 channel localizes to cilia of principal cells in rabbit cortical collecting duct (CCD)

Within the CCD of the distal nephron of the rabbit, the BK (maxi K) channel mediates Ca²⁺- and/or stretch-dependent flow-induced K⁺ secretion (FIKS) and contributes to K⁺ adaptation in response to dietary K⁺ loading. An unresolved question is whether BK channels in intercalated cells (ICs) and/or principal cells (PCs) in the CCD mediate these K⁺ secretory processes. In support of a role for ICs in FIKS is the higher density of immunoreactive apical BKα (pore-forming subunit) and functional BK channel activity than detected in PCs, and an increase in IC BKα expression in response to a high-K⁺ diet. PCs possess a single apical cilium which has been proposed to serve as a mechanosensor; direct manipulation of cilia leads to increases in cell Ca²⁺ concentration, albeit of nonciliary origin. Immunoperfusion of isolated and fixed CCDs isolated from control K⁺-fed rabbits with channel subunit-specific antibodies revealed colocalization of immunodetectable BKα- and β1-subunits in cilia as well as on the apical membrane of cilia-expressing PCs. Ciliary BK channels were more easily detected in rabbits fed a low-K⁺ vs. high-K⁺ diet. Single-channel recordings of cilia revealed K⁺ channels with conductance and kinetics typical of the BK channel. The observations that 1) FIKS was preserved but 2) the high-amplitude Ca²⁺ peak elicited by flow was reduced in microperfused CCDs subject to pharmacological deciliation suggest that cilia BK channels do not contribute to K⁺ secretion in this segment, but that cilia serve as modulators of cell signaling.

Probing Functional Changes in Exocyst Configuration with Monoclonal Antibodies

Spatial regulation of exocytosis relies on the exocyst, a hetero-octameric protein complex that tethers vesicles to fusion sites at the plasma membrane. Nevertheless, our understanding of mechanisms regulating exocyst assembly/disassembly, localization, and function are incomplete. Here, we have exploited a panel of anti-Sec6 monoclonal antibodies (mAbs) to probe possible configurational changes accompanying transitions in exocyst function in epithelial MDCK cells. Sec6 is quantitatively associated with Sec8 in high molecular weight complexes, as shown by gel filtration and co-immunoprecipitation studies. We mapped epitopes recognized by more than 20 distinct mAbs to one of six Sec6 segments. Surprisingly, mAbs that bound epitopes in each segment labeled distinct subcellular structures. In general, antibodies to epitopes in N-terminal domains labeled Sec6 in either cytosolic or nuclear pools, whereas those that bound epitopes in C-terminal domains labeled membrane-associated Sec6. In this latter group, we identified antibodies that labeled distinct Sec6 populations at the apical junctional complex, desmosomes, endoplasmic reticulum and vimentin-type intermediate filaments. That each antibody was specific was verified by both Sec6 RNAi and competition with fusion proteins containing each domain. Comparison of non-polarized and polarized cells revealed that many Sec6 epitopes either redistribute or become concealed during epithelial polarization. Transitions in exocyst configurations may be regulated in part by the actions of Ral GTPases, because the exposure of Sec6 C-terminal domain epitopes at the plasma membrane is significantly reduced upon RalA RNAi. To determine whether spatio-temporal changes in epitope accessibility was correlated with differential stability of interactions between Sec6 and other exocyst subunits, we quantified relative amounts of each subunit that co-immunoprecipitated with Sec6 when antibodies to N-terminal or C-terminal epitopes were used. Antibodies to Sec6NT co-precipitated substantially more Sec5, -10, -15, Exo70 and -84 than did those to Sec6CT. In contrast, antibodies to Sec6CT co-precipitated more Sec3 and Sec8 than did those to Sec6NT. These results are consistent with a model in which exocyst activation during periods of rapid membrane expansion is accompanied by molecular rearrangements within the holocomplex or association with accessory proteins, which expose the Sec6 C-terminal domain when the complex is membrane-bound and conceal it when the complex is cytoplasmic.

Discerning the role of mechanosensors in regulating proximal tubule function

All cells in the body experience external mechanical forces such as shear stress and stretch. These forces are sensed by specialized structures in the cell known as mechanosensors. Cells lining the proximal tubule (PT) of the kidney are continuously exposed to variations in flow rates of the glomerular ultrafiltrate, which manifest as changes in axial shear stress and radial stretch. Studies suggest that these cells respond acutely to variations in flow by modulating their ion transport and endocytic functions to maintain glomerulotubular balance. Conceptually, changes in the axial shear stress in the PT could be sensed by three known structures, namely, the microvilli, the glycocalyx, and primary cilia. The orthogonal component of the force produced by flow exhibits as radial stretch and can cause expansion of the tubule. Forces of stretch are transduced by integrins, by stretch-activated channels, and by cell-cell contacts. This review summarizes our current understanding of flow sensing in PT epithelia, discusses challenges in dissecting the role of individual flow sensors in the mechanosensitive responses, and identifies potential areas of opportunity for new study.

A Cilia Independent Role of Ift88/Polaris during Cell Migration

Ift88 is a central component of the intraflagellar transport (Ift) complex B, essential for the building of cilia and flagella from single cell organisms to mammals. Loss of Ift88 results in the absence of cilia and causes left-right asymmetry defects, disordered Hedgehog signaling, and polycystic kidney disease, all of which are explained by aberrant ciliary function. In addition, a number of extraciliary functions of Ift88 have been described that affect the cell-cycle, mitosis, and targeting of the T-cell receptor to the immunological synapse. Similarly, another essential ciliary molecule, the kinesin-2 subunit Kif3a, which transports Ift-B in the cilium, affects microtubule (MT) dynamics at the leading edge of migrating cells independently of cilia. We now show that loss of Ift88 impairs cell migration irrespective of cilia. Ift88 is required for the polarization of migrating MDCK cells, and Ift88 depleted cells have fewer MTs at the leading edge. Neither MT dynamics nor MT nucleation are dependent on Ift88. Our findings dissociate the function of Ift88 from Kif3a outside the cilium and suggest a novel extraciliary function for Ift88. Future studies need to address what unifying mechanism underlies the different extraciliary functions of Ift88.

Proliferation-Independent Initiation of Biliary Cysts in Polycystic Liver Diseases

Biliary cysts in adult patients affected by polycystic liver disease are lined by cholangiocytes that proliferate, suggesting that initiation of cyst formation depends on proliferation. Here, we challenge this view by analyzing cyst-lining cell proliferation and differentiation in Cpk mouse embryos and in livers from human fetuses affected by Autosomal Recessive Polycystic Kidney Disease (ARPKD), at early stages of cyst formation. Proliferation of fetal cholangiocyte precursors, measured by immunostaining in human and mouse livers, was low and did not differ between normal and ARPKD or Cpk livers, excluding excessive proliferation as an initiating cause of liver cysts. Instead, our analyses provide evidence that the polycystic livers exhibit increased and accelerated differentiation of hepatoblasts into cholangiocyte precursors, eventually coalescing into large biliary cysts. Lineage tracing experiments, performed in mouse embryos, indicated that the cholangiocyte precursors in Cpk mice generate cholangiocytes and periportal hepatocytes, like in wild-type animals. Therefore, contrary to current belief, cyst formation in polycystic liver disease does not necessarily depend on overproliferation. Combining our prenatal data with available data from adult livers, we propose that polycystic liver can be initiated by proliferation-independent mechanisms at a fetal stage, followed by postnatal proliferation-dependent cyst expansion.

Ciliary membrane proteins traffic through the Golgi via a Rabep1/GGA1/Arl3-dependent mechanism

Primary cilia contain specific receptors and channel proteins that sense the extracellular milieu. Defective ciliary function causes ciliopathies such as autosomal dominant polycystic kidney disease (ADPKD). However, little is known about how large ciliary transmembrane proteins traffic to the cilia. Polycystin-1 (PC1) and -2 (PC2), the two ADPKD gene products, are large transmembrane proteins that co-localize to cilia where they act to control proper tubular diameter. Here we describe that PC1 and PC2 must interact and form a complex to reach the trans-Golgi network (TGN) for subsequent ciliary targeting. PC1 must also be proteolytically cleaved at a GPS site for this to occur. Using yeast two-hybrid screening coupled with a candidate approach, we identify a Rabep1/GGA1/Arl3-dependent ciliary targeting mechanism, whereby Rabep1 couples the polycystin complex to a GGA1/Arl3-based ciliary trafficking module at the TGN. This study provides novel insights into the ciliary trafficking mechanism of membrane proteins.

The fate of the primary cilium during myofibroblast transition

Myofibroblasts, the culprit of organ fibrosis, can originate from mesenchymal and epithelial precursors through fibroblast-myofibroblast and epithelial-myofibroblast transition (EMyT). Because certain ciliopathies are associated with fibrogenesis, we sought to explore the fate and potential role of the primary cilium during myofibroblast formation. Here we show that myofibroblast transition from either precursor results in the loss of the primary cilium. During EMyT, initial cilium growth is followed by complete deciliation. Both EMyT and cilium loss require two-hit conditions: disassembly/absence of intercellular contacts and transforming growth factor-β1 (TGFβ) exposure. Loss of E-cadherin-dependent junctions induces cilium elongation, whereas both stimuli are needed for deciliation. Accordingly, in a scratch-wounded epithelium, TGFβ provokes cilium loss exclusively along the wound edge. Increased contractility, a key myofibroblast feature, is necessary and sufficient for deciliation, since constitutively active RhoA, Rac1, or myosin triggers, and down-regulation of myosin or myocardin-related transcription factor prevents, this process. Sustained myosin phosphorylation and consequent deciliation are mediated by a Smad3-, Rac1-, and reactive oxygen species-dependent process. Transitioned myofibroblasts exhibit impaired responsiveness to platelet-derived growth factor-AA and sonic hedgehog, two cilium-associated stimuli. Although the cilium is lost during EMyT, its initial presence contributes to the transition. Thus myofibroblasts represent a unique cilium-less entity with profoundly reprogrammed cilium-related signaling.

Peptide pills for brain diseases? Reality and future perspectives

The peptide therapeutic market is one of the fastest growth areas of the pharmaceutical industry. Although few orally administered peptides are marketed and many are in different phases of clinical development, there is no marketed oral peptide therapeutic used for CNS disorders. The major challenges involved in orally delivering peptides to the brain relate to their enzymatic instability and inability to permeate across physiological barriers. The paucity of therapies for the treatment of brain diseases and the presence of the blood-brain barrier excluding 98% of therapeutic molecules necessitates parenteral administration. Various approaches have been applied to enhance oral peptide bioavailability, but only nanoparticulate strategies were able to deliver orally therapeutic peptides to the brain. Although industry may be reluctant to invest in developing oral peptide nanomedicines, the increasingly unmet clinical need and economic burden associated with brain diseases will fuel the development of the first marketed oral-to-brain peptide therapy.

Reduction of oxidative stress during recovery accelerates normalization of primary cilia length that is altered after ischemic injury in murine kidneys

The primary cilium is a microtubule-based nonmotile organelle that extends from the surface of cells, including renal tubular cells. Here, we investigated the alteration of primary cilium length during epithelial cell injury and repair, following ischemia/reperfusion (I/R) insult, and the role of reactive oxygen species in this alteration. Thirty minutes of bilateral renal ischemia induced severe renal tubular cell damage and an increase of plasma creatinine (PCr) concentration. Between 8 and 16 days following the ischemia, the increased PCr returned to normal range, although without complete histological restoration. Compared with the primary cilium length in normal kidney tubule cells, the length was shortened 4 h and 1 day following ischemia, increased over normal 8 days after ischemia, and then returned to near normal 16 days following ischemia. In the urine of I/R-subjected mice, acetylated tubulin was detected. The cilium length of proliferating cells was shorter than that in nonproliferating cells. Mature cells had shorter cilia than differentiating cells. Treatment with Mn(III) tetrakis(1-methyl-4-pyridyl) porphyrin (MnTMPyP), an antioxidant, during the recovery of damaged kidneys accelerated normalization of cilia length concomitant with a decrease of oxidative stress and morphological recovery in the kidney. In the Madin-Darby canine kidney (MDCK) cells, H2O2 treatment caused released ciliary fragment into medium, and MnTMPyP inhibited the deciliation. The ERK inhibitor U0126 inhibited elongation of cilia in normal and MDCK cells recovering from H2O2 stress. Taken together, our results suggest that primary cilia length reflects cell proliferation and the length of primary cilium is regulated, at least, in part, by reactive oxygen species through ERK.

Regulation of cilium length and intraflagellar transport

Primary cilia are highly conserved sensory organelles that extend from the surface of almost all vertebrate cells. The importance of cilia is evident from their involvement in many diseases, called ciliopathies. Primary cilia contain a microtubular axoneme that is used as a railway for transport of both structural components and signaling proteins. This transport machinery is called intraflagellar transport (IFT). Cilia are dynamic organelles whose presence on the cell surface, morphology, length and function are highly regulated. It is clear that the IFT machinery plays an important role in this regulation. However, it is not clear how, for example environmental cues or cell fate decisions are relayed to modulate IFT and cilium morphology or function. This chapter presents an overview of molecules that have been shown to regulate cilium length and IFT. Several examples where signaling modulates IFT and cilium function are used to discuss the importance of these systems for the cell and for understanding of the etiology of ciliopathies.

Heat shock induces rapid resorption of primary cilia

Primary cilia are involved in important developmental and disease pathways, such as the regulation of neurogenesis and tumorigenesis. They function as sensory antennae and are essential in the regulation of key extracellular signalling systems. We have investigated the effects of cell stress on primary cilia. Exposure of mammalian cells in vitro, and zebrafish cells in vivo, to elevated temperature resulted in the rapid loss of cilia by resorption. In mammalian cells loss of cilia correlated with a reduction in hedgehog signalling. Heat-shock-dependent loss of cilia was decreased in cells where histone deacetylases (HDACs) were inhibited, suggesting resorption is mediated by the axoneme-localised tubulin deacetylase HDAC6. In thermotolerant cells the rate of ciliary resorption was reduced. This implies a role for molecular chaperones in the maintenance of primary cilia. The cytosolic chaperone Hsp90 localises to the ciliary axoneme and its inhibition resulted in cilia loss. In the cytoplasm of unstressed cells, Hsp90 is known to exist in a complex with HDAC6. Moreover, immediately after heat shock Hsp90 levels were reduced in the remaining cilia. We hypothesise that ciliary resorption serves to attenuate cilia-mediated signalling pathways in response to extracellular stress, and that this mechanism is regulated in part by HDAC6 and Hsp90.

Cholangiocyte cilia are abnormal in syndromic and non-syndromic biliary atresia

Biliary atresia (BA) is a neonatal disorder characterized by aggressive fibroinflammatory obliteration of the biliary tract. Approximately 20 percent of BA patients demonstrate left-right laterality defects (syndromic BA). Cilia participate in important physiological functions in cholangiocytes, and as some ciliopathies have been associated with both laterality defects and hepatic fibrosis, we hypothesized that patients with syndromic BA exhibit abnormalities of cholangiocyte cilia that disrupt cholangiocyte homeostasis. Nine BA specimens were studied, including pre-Kasai diagnostic biopsies (n=7) and liver explants (n=2). Five specimens were from patients with laterality defects. These were compared with normal pediatric livers, as well as livers affected by primary sclerosing cholangitis, Wilson's disease, and cardiac cirrhosis. Biopsy sections were stained with antibodies against keratin 19 (a cholangiocyte marker) and acetylated α-tubulin (a cilia marker) and were visualized by confocal microscopy. Computer-assisted relative quantification was used to compare staining of cilia within bile ducts among samples. Surprisingly, cilia in BA specimens were significantly shorter, abnormal in their orientation, and less abundant compared with normal liver and disease controls regardless of the presence of a laterality defect. There are significant abnormalities of cholangiocyte cilia in both syndromic and non-syndromic BA livers compared with normal livers and livers affected by other cholestatic diseases. Although this may result from severe cholestasis or inflammation, it may also reflect common mechanistic pathways in different forms of BA and may have important implications for understanding the progression of the disease.

Carcinogens induce loss of the primary cilium in human renal proximal tubular epithelial cells independently of effects on the cell cycle

The primary cilium is an immotile sensory and signaling organelle found on the majority of mammalian cell types. Of the multitude of roles that the primary cilium performs, perhaps some of the most important include maintenance of differentiation, quiescence, and cellular polarity. Given that the progression of cancer requires disruption of all of these processes, we have investigated the effects of several carcinogens on the primary cilium of the RPTEC/TERT1 human proximal tubular epithelial cell line. Using both scanning electron microscopy and immunofluorescent labeling of the ciliary markers acetylated tubulin and Arl13b, we confirmed that RPTEC/TERT1 cells express primary cilium upon reaching confluence. Treatment with the carcinogens ochratoxin A (OTA) and potassium bromate (KBrO(3)) caused a significant reduction in the number of ciliated cells, while exposure to nifedipine, a noncarcinogenic renal toxin, had no effect on primary cilium expression. Flow cytometric analysis of the effects of all three compounds on the cell cycle revealed that only KBrO(3) resulted in an increase in the proportion of cells entering the cell cycle. Microarray analysis revealed dysregulation of multiple pathways affecting ciliogenesis and ciliary maintenance following OTA and KBrO(3) exposure, which were unaffected by nifedipine exposure. The primary cilium represents a unique physical checkpoint with relevance to carcinogenesis. We have shown that the renal carcinogens OTA and KBrO(3) cause significant deciliation in a model of the proximal tubule. With KBrO(3), this was followed by reentry into the cell cycle; however, deciliation was not found to be associated with reentry into the cell cycle following OTA exposure. Transcriptomic analysis identified dysregulation of Wnt signaling and ciliary trafficking in response to OTA and KBrO(3) exposure.

A Rab8 guanine nucleotide exchange factor-effector interaction network regulates primary ciliogenesis

Primary cilia are microtubule-based solitary membrane projections on the cell surface that play important roles in signaling and development. Recent studies have demonstrated that polarized vesicular trafficking involving the small GTPase Rab8 and its guanine nucleotide exchange factor Rabin8 is essential for primary ciliogenesis. In this study, we show that a highly conserved region of Rabin8 is pivotal for its activation as a guanine nucleotide exchange factor for Rab8. In addition, in its activated conformation, Rabin8 interacts with Sec15, a subunit of the exocyst and downstream effector of Rab8. Expression of constitutively activated Rab8 promotes the association of Sec15 with Rabin8. Using immunofluorescence microscopy, we found that Sec15 co-localized with Rab8 along the primary cilium. Inhibition of Sec15 function in cells led to defects in primary ciliogenesis. The Rabin8-Rab8-Sec15 interaction may couple the activation of Rab8 to the recruitment of the Rab8 effector and is involved in the regulation of vesicular trafficking for primary cilium formation.

Emerging roles for renal primary cilia in epithelial repair

Primary cilia are microscopic sensory antennae that cells in many vertebrate tissues use to gather information about their environment. In the kidney, primary cilia sense urine flow and are essential for the maintenance of epithelial architecture. Defects of this organelle cause the cystic kidney disease characterized by epithelial abnormalities. These findings link primary cilia to the regulation of epithelial differentiation and proliferation, processes that must be precisely controlled during epithelial repair in the kidney. Here, we consider likely roles for primary cilium-based signaling during responses to renal injury and ensuing epithelial repair processes.

Shear stress reverses dome formation in confluent renal tubular cells

BACKGROUND/AIMS

It has been shown that MDCK cells, a cell line derived from canine renal tubules, develop cell domes due to fluid pumped under cell monolayer and focal detachment from the adhesion surface. In vitro studies have shown that primary cilia of kidney tubular epithelial cells act as mechanosensors, increasing intracellular calcium within seconds upon changes in fluid shear stress (SS) on cell membrane. We then studied the effect of prolonged SS exposure on cell dome formation in confluent MDCK cell monolayers.

METHODS

A parallel plate flow chamber was used to apply laminar SS at 2 dynes/cm(2) to confluent cell monolayers for 6 hours. Control MDCK cell monolayers were maintained in static condition. The effects of Ca(2+) blockade and cell deciliation on SS exposure were also investigated.

RESULTS

Seven days after reaching confluence, static cultures developed liquid filled domes, elevating from culture plate. Exposure to SS induced almost complete disappearance of cell domes (0.4±0.8 vs. 11.4±2.8 domes/mm(2), p < 0.01, n=14). SS induced dome disappearance took place within minutes to hours, as shown by time-lapse videomicroscopy. Exposure to SS importantly affected cell cytoskeleton altering actin stress fibers expression and organization, and the distribution of tight junction protein ZO-1. Dome disappearance induced by flow was completely prevented in the presence of EGTA or after cell deciliation.

CONCLUSIONS

These data indicate that kidney tubular cells are sensitive to apical flow and that these effects are mediated by primary cilia by regulation of Ca(2+) entry in to the cell. SS induced Ca(2+) entry provokes contraction of cortical actin ring that tenses cell-cell borders and decreases basal stress fibers. These processes may increase paracellular permeability and decrease basal adhesion making dome disappear. Elucidation of the effects of apical fluid flow on tubular cell function may open new insights on the pathophysiology of kidney diseases associated with cilia dysfunction.

Polycystin-2 and phosphodiesterase 4C are components of a ciliary A-kinase anchoring protein complex that is disrupted in cystic kidney diseases

Polycystic kidney disease (PKD) is a genetic disorder that is characterized by cyst formation in kidney tubules. PKD arises from abnormalities of the primary cilium, a sensory organelle located on the cell surface. Here, we show that the primary cilium of renal epithelial cells contains a protein complex comprising adenylyl cyclase 5/6 (AC5/6), A-kinase anchoring protein 150 (AKAP150), and protein kinase A. Loss of primary cilia caused by deletion of Kif3a results in activation of AC5 and increased cAMP levels. Polycystin-2 (PC2), a ciliary calcium channel that is mutated in human PKD, interacts with AC5/6 through its C terminus. Deletion of PC2 increases cAMP levels, which can be corrected by reexpression of wild-type PC2 but not by a mutant lacking calcium channel activity. Phosphodiesterase 4C (PDE4C), which catabolizes cAMP, is also located in renal primary cilia and interacts with the AKAP150 complex. Expression of PDE4C is regulated by the transcription factor hepatocyte nuclear factor-1β (HNF-1β), mutations of which produce kidney cysts. PDE4C is down-regulated and cAMP levels are increased in HNF-1β mutant kidney cells and mice. Collectively, these findings identify PC2 and PDE4C as unique components of an AKAP complex in primary cilia and reveal a common mechanism for dysregulation of cAMP signaling in cystic kidney diseases arising from different gene mutations.

Rabs and the exocyst in ciliogenesis, tubulogenesis and beyond

The exocyst, an octameric protein complex mediating vesicle tethering at the plasma membrane for exocytosis, is a downstream effector of the Rab proteins Rab8 and Rab11, which are key regulators of membrane trafficking from the trans-Golgi network and recycling endosome to the plasma membrane. Rab11 and Rab8 coordinate their actions via Rabin8, the guanine nucleotide exchange factor of Rab8. A cascade of protein-protein interactions involving the Rabs and the exocyst complex couples the generation of secretory vesicles at donor compartments to their docking and fusion at the plasma membrane. Here, we discuss recent work implicating Rab proteins and the exocyst in primary ciliogenesis and epithelial lumenogenesis. In addition, we discuss early work in the budding yeast Saccharomyces cerevisiae, which provided the initial insight into the molecular mechanisms of polarized exocytosis.

Aberrant activation of fatty acid synthesis suppresses primary cilium formation and distorts tissue development

Aberrant activation of fatty acid synthesis is a key feature of many advanced human cancers. Unlike in classical lipogenic tissues, this process has been implicated in membrane production required for rapid cell proliferation. Here, to gain further insight into the consequences of tumor-associated fatty acid synthesis, we have mimicked the lipogenic phenotype of cancer cells in Xenopus embryos by microinjection of RNA encoding the lipogenic transcription factor sterol regulatory element binding protein 1c (SREBP1c). Dramatic morphologic changes were observed that could be linked to alterations in Wnt and Hedgehog signaling, and ultimately to a distortion of the primary cilium. This is a sophisticated microtubular sensory organelle that is expressed on the surface of nearly every cell type and that is lost in many cancers. SREBP1c-induced loss of the primary cilium could be confirmed in mammalian Madin-Darby canine kidney (MDCK) cells and was mediated by changes in the supply of fatty acids. Conversely, inhibition of fatty acid synthesis in highly lipogenic human prostate cancer cells restored the formation of the primary cilium. Lipid-induced ciliary loss was associated with mislocalization of apical proteins, distortion of cell polarization, and aberrant epithelial tissue development as revealed in three-dimensional cultures of MDCK cells and in the developing mouse prostate. These data imply that tumor-associated lipogenesis, in addition to rendering cells more autonomous in terms of lipid supply, disturbs cilium formation and contributes to impaired environmental sensing, aberrant signaling, and distortion of polarized tissue architecture, which are all hallmarks of cancer.

Remodeling epithelial cell organization: transitions between front-rear and apical-basal polarity

Polarized epithelial cells have a distinctive apical-basal axis of polarity for vectorial transport of ions and solutes across the epithelium. In contrast, migratory mesenchymal cells have a front-rear axis of polarity. During development, mesenchymal cells convert to epithelia by coalescing into aggregates that undergo epithelial differentiation. Signaling networks and protein complexes comprising Rho family GTPases, polarity complexes (Crumbs, PAR, and Scribble), and their downstream effectors, including the cytoskeleton and the endocytic and exocytic vesicle trafficking pathways, together regulate the distributions of plasma membrane and cytoskeletal proteins between front-rear and apical-basal polarity. The challenge is to understand how these regulators and effectors are adapted to regulate symmetry breaking processes that generate cell polarities that are specialized for different cellular activities and functions.

Nucleofection disrupts tight junction fence function to alter membrane polarity of renal epithelial cells

Here, we compared the effects of nucleofection and lipid-based approaches to introduce siRNA duplexes on the subsequent development of membrane polarity in kidney cells. Nucleofection of Madin-Darby canine kidney (MDCK) cells, even with control siRNA duplexes, disrupted the initial surface polarity as well as the steady-state distribution of membrane proteins. Transfection using lipofectamine yielded slightly less efficient knockdown but did not disrupt membrane polarity. Polarized secretion was unaffected by nucleofection, suggesting a selective defect in the development of membrane polarity. Cilia frequency and length were not altered by nucleofection. However, the basolateral appearance of a fluorescent lipid tracer added to the apical surface of nucleofected cells was dramatically enhanced relative to untransfected controls or lipofectamine-treated cells. In contrast, [(3)H]inulin diffusion and transepithelial electrical resistance were not altered in nucleofected cells compared with untransfected ones. We conclude that nucleofection selectively hinders development of the tight junction fence function in MDCK cells.

Coordination of Rab8 and Rab11 in primary ciliogenesis

Primary cilia are microtubule-based membrane projections located at the surface of many cells. Defects in primary cilia formation have been implicated in a number of genetic disorders, such as Bardet-Biedl Syndrome and Polycystic Kidney Disease. Recent studies have demonstrated that polarized vesicular transport involving Rab8 and its guanine nucleotide-exchange factor Rabin8 is essential for primary ciliogenesis. Here we report that Rabin8 is a direct downstream effector of Rab11, which functions in membrane trafficking from the trans-Golgi network and recycling endosomes. Rab11, in its GTP-bound form, interacts with Rabin8 and kinetically stimulates the guanine nucleotide-exchange activity of Rabin8 toward Rab8. Rab11 is enriched at the base of the primary cilia and inhibition of Rab11 function by a dominant-negative mutant or RNA interference blocks primary ciliogenesis. Our results suggest that Rab GTPases coordinate with each other in the regulation of vesicular trafficking during primary ciliogenesis.

Primary cilia and the cell cycle

Cilia are microtubule-based structures that protrude from the cell surface and function as sensors for mechanical and chemical environmental cues that regulate cellular differentiation or division. In metazoans, ciliary signaling is important during organismal development and in the homeostasis controls of adult tissues, with receptors for the Hedgehog, platelet derived growth factor (PDGF), Wnt, and other signaling cascades arrayed and active along the ciliary membrane. In normal cells, cilia are dynamically regulated during cell cycle progression: present in G0 and G1 cells, and usually in S/G2 cells, but almost invariably resorbed before mitotic entry, to reappear post-cytokinesis. This periodic resorption and reassembly of cilia, specified by the intrinsic cell cycle the intrinsic cell cycle machinery, influences the susceptibility of cells to the influence of extrinsic signals with cilia-associated receptors. Pathogenic conditions of mammals associated with loss of or defects in ciliary integrity include a number of developmental disorders, cystic syndromes in adults, and some cancers. With the continuing expansion of the list of human diseases associated with ciliary abnormalities, the identification of the cellular mechanisms regulating ciliary growth and disassembly has become a topic of intense research interest. Although these mechanisms are far from being understood, a number of recent studies have begun to identify key regulatory factors that may begin to offer insight into disease pathogenesis and treatment. In this chapter we will discuss the current state of knowledge regarding cell cycle control of ciliary dynamics, and provide general methods that can be applied to investigate cell cycle-dependent ciliary growth and disassembly.