Inactivation of a Gα(s)-PKA tumour suppressor pathway in skin stem cells initiates basal-cell carcinogenesis

The Hippo pathway in tissue homeostasis and regeneration

While several organs in mammals retain partial regenerative capability following tissue damage, the underlying mechanisms remain unclear. Recently, the Hippo signaling pathway, better known for its function in organ size control, has been shown to play a pivotal role in regulating tissue homeostasis and regeneration. Upon tissue injury, the activity of YAP, the major effector of the Hippo pathway, is transiently induced, which in turn promotes expansion of tissue-resident progenitors and facilitates tissue regeneration. In this review, with a general focus on the Hippo pathway, we will discuss its major components, functions in stem cell biology, involvement in tissue regeneration in different organs, and potential strategies for developing Hippo pathway-targeted regenerative medicines.

A reciprocal regulatory loop between TAZ/YAP and G-protein Gαs regulates Schwann cell proliferation and myelination

Schwann cell (SC) myelination in the peripheral nervous system is essential for motor function, and uncontrolled SC proliferation occurs in cancer. Here, we show that a dual role for Hippo effectors TAZ and YAP in SC proliferation and myelination through modulating G-protein expression and interacting with SOX10, respectively. Developmentally regulated mutagenesis indicates that TAZ/YAP are critical for SC proliferation and differentiation in a stage-dependent manner. Genome-wide occupancy mapping and transcriptome profiling reveal that nuclear TAZ/YAP promote SC proliferation by activating cell cycle regulators, while targeting critical differentiation regulators in cooperation with SOX10 for myelination. We further identify that TAZ targets and represses Gnas, encoding Gαs-protein, which opposes TAZ/YAP activities to decelerate proliferation. Gnas deletion expands SC precursor pools and blocks peripheral myelination. Thus, the Hippo/TAZ/YAP and Gαs-protein feedback circuit functions as a fulcrum balancing SC proliferation and differentiation, providing insights into molecular programming of SC lineage progression and homeostasis.

The Hippo pathway has recently been implicated in Schwann cell (SC) development and myelination. Here the authors reveal mechanistic insights into how TAZ and YAP regulate and interact with target genes; they further identify a negative feedback loop between TAZ/YAP and G protein Gαs that balances SC proliferation and differentiation.

Diversity of Precursor Lesions For Pancreatic Cancer: The Genetics and Biology of Intraductal Papillary Mucinous Neoplasm

Pancreatic ductal adenocarcinoma (PDA), one of the most lethal cancers worldwide, is associated with two main types of morphologically distinct precursors—pancreatic intraepithelial neoplasia (PanIN) and intraductal papillary mucinous neoplasm (IPMN). Although the progression of PanIN into invasive cancer has been well characterized, there remains an urgent need to understand the biology of IPMNs, which are larger radiographically detectable cystic tumors. IPMNs comprise a number of subtypes with heterogeneous histopathologic and clinical features. Although frequently remaining benign, a significant proportion exhibits malignant progression. Unfortunately, there are presently no accurate prognosticators for assessing cancer risk in individuals with IPMN. Moreover, the fundamental mechanisms differentiating PanIN and IPMN remain largely obscure, as do those that distinguish IPMN subtypes. Recent studies, however, have identified distinct genetic profiles between PanIN and IPMN, providing a framework to better understand the diversity of the precursors for PDA. Here, we review the clinical, biological, and genetic properties of IPMN and discuss various models for progression of these tumors to invasive PDA.

Epidermal loss of Gαq confers a migratory and differentiation defect in keratinocytes

G-protein coupled receptors (GPCRs), which activate heterotrimeric G proteins, are an essential class of transmembrane receptors that are responsible for a myriad of signaling events in normal and pathologic conditions. Two members of the G protein family, Gα_q and Gα₁₁, activate one of the main GPCR pathways and function as oncogenes by integrating mitogen-stimulated signaling cascades that are active under malignant conditions. Recently, it has been shown that targeted deletion of Gα₁₁ and Gα_q from endothelial cells impairs the Rho-mediated formation of focal adherens junctions, suggesting that Gα_11/q signaling may also play a significant role in cytoskeletal-mediated cellular responses in epithelial cells. Indeed, combined deletion of Gα₁₁ and Gα_q confers a significant migratory defect in keratinocytes that delays cutaneous wound healing in an in vivo setting. This delay can be attributed to a defect during the reepithelialization phase due to significantly attenuated migratory capacity of Gα_q-null keratinocytes under combined Gα₁₁ deficiency. In fact, cells lacking Gα_11/q demonstrate a severely reduced ability to respond to mitogenic and migratory signals in the microenvironment, leading to inappropriate and premature terminal differentiation. These results suggest that Gα_11/q signaling pathways may be critical for integrating mitogenic signals and cytoskeletal function to achieve normal physiological responses. Emergence of a malignant phenotype may therefore arise from both under- and overexpression of Gα_11/q signaling, implicating its upstream regulation as a potential therapeutic target in a host of pathologic conditions.

Hippo vs. Crab: tissue-specific functions of the mammalian Hippo pathway

The Hippo signaling pathway is a vital suppressor of tumorigenesis that is often inactivated in human cancers. In normal cells, the Hippo pathway is triggered by external forces such as cell crowding, or changes to the extracellular matrix or cell polarity. Once activated, Hippo signaling down-regulates transcription supported by the paralogous cofactors YAP1 and TAZ. The Hippo pathway's functions in normal and cancer biology have been dissected by studies of mutant mice with null or conditional tissue-specific mutations of Hippo signaling elements. In this review, we attempt to systematically summarize results that have been gleaned from detailed in vivo characterizations of these mutants. Our goal is to describe the physiological roles of Hippo signaling in several normal organ systems, as well as to emphasize how disruption of the Hippo pathway, and particularly hyperactivation of YAP1/TAZ, can be oncogenic.

Prostaglandin E2 Activates YAP and a Positive-Signaling Loop to Promote Colon Regeneration After Colitis but Also Carcinogenesis in Mice

BACKGROUND & AIMS

Prostaglandin E₂ (PGE₂) is mediator of inflammation that regulates tissue regeneration, but its continual activation has been associated with carcinogenesis. Little is known about factors in the PGE₂ signaling pathway that contribute to tumor formation. We investigated whether yes-associated protein 1 (YAP1), a transcriptional co-activator in the Hippo signaling pathway, mediates PGE₂ function.

METHODS

DLD-1 and SW480 colon cancer cell lines were transfected with vectors expressing transgenes or small hairpin RNAs and incubated with recombinant PGE₂, with or without pharmacologic inhibitors of signaling proteins, and analyzed by immunoblot, immunofluorescence, quantitative reverse-transcription polymerase chain reaction, transcriptional reporter, and proliferation assays. Dextran sodium sulfate (DSS) was given to induce colitis in C57/BL6 (control) mice, as well as in mice with disruption of the hydroxyprostaglandin dehydrogenase 15 gene (15-PGDH-knockout mice), Yap1 gene (YAP-knockout mice), and double-knockout mice. Some mice also were given indomethacin to block PGE₂ synthesis. 15-PGDH knockout mice were crossed with mice with intestine-specific disruption of the salvador family WW domain containing 1 gene (Sav1), which encodes an activator of Hippo signaling. We performed immunohistochemical analyses of colon biopsy samples from 26 patients with colitis-associated cancer and 51 age-and sex-matched patients with colorectal cancer (without colitis).

RESULTS

Incubation of colon cancer cell lines with PGE₂ led to phosphorylation of cyclic adenosine monophosphate-responsive element binding protein 1 and increased levels of YAP1 messenger RNA, protein, and YAP1 transcriptional activity. This led to increased transcription of the prostaglandin-endoperoxide synthase 2 gene (PTGS2 or cyclooxygenase 2) and prostaglandin E-receptor 4 gene (PTGER4 or EP4). Incubation with PGE₂ promoted proliferation of colon cancer cell lines, but not cells with knockdown of YAP1. Control mice developed colitis after administration of DSS, but injection of PGE₂ led to colon regeneration in these mice. However, YAP-knockout mice did not regenerate colon tissues and died soon after administration of DSS. 15-PGDH-knockout mice regenerated colon tissues more rapidly than control mice after withdrawal of DSS, and had faster recovery of body weight, colon length, and colitis histology scores. These effects were reversed by injection of indomethacin. SAV1-knockout or 15-PGDH-knockout mice did not develop spontaneous tumors after colitis induction, but SAV1/15-PGDH double-knockout mice developed polyps that eventually progressed to carcinoma in situ. Administration of indomethacin to these mice prevented spontaneous tumor formation. Levels of PGE₂ correlated with those of YAP levels in human sporadic colorectal tumors and colitis-associated tumors.

CONCLUSIONS

PGE₂ signaling increases the expression and transcriptional activities of YAP1, leading to increased expression of cyclooxygenase 2 and EP4 to activate a positive signaling loop. This pathway promotes proliferation of colon cancer cell lines and colon tissue regeneration in mice with colitis. Constitutive activation of this pathway led to formation of polyps and colon tumors in mice.

G Protein Alpha S Subunit Promotes Cell Proliferation of Renal Cell Carcinoma with Involvement of Protein Kinase A Signaling

Heterotrimeric G proteins, which are composed of Gα and Gβγ subunits, transduce signals sensed by the coupled surface receptors. Aberrant expressions of G proteins have been observed in many cancer types. This study aimed to determine the expression level of the stimulatory G protein alpha S subunit (Gαs, the main transcript encoded by the GNAS locus) and its biological function in renal cell carcinoma (RCC). Western blotting and quantitative reverse transcription-PCR results show that Gαs expression dramatically increased in RCC cell lines (ACHN, GRC-1, and 786-O) compared to normal renal epithelial cells HK-2. Knockdown of Gαs by small interfering RNA (siRNA) caused a significant inhibition on proliferation of ACHN cells as indicated by MTT assay and colony formation assay. Overexpression of Gαs in HK-2 cells promoted cell proliferation and led to a higher level of intracellular cyclic adenosine monophosphate (cAMP) in response to parathyroid hormone (PTH) compared to the cells transfected with empty vector. Notably, the growth of HK-2 cells overexpressing Gαs was efficiently inhibited in the presence of protein kinase A (PKA) inhibitor H89. Furthermore, in a xenograft model by subcutaneous injection of ACHN cells, tumor growth was also suppressed by H89. Taken together, these results suggest that Gαs plays a tumor-promoting role in RCC and possibly acts through a PKA-dependent pathway. Our findings may provide new clues for target therapy for RCC in the future.

Osteocrin, a peptide secreted from the heart and other tissues, contributes to cranial osteogenesis and chondrogenesis in zebrafish

The heart is an endocrine organ, as cardiomyocytes (CMs) secrete natriuretic peptide (NP) hormones. Since the discovery of NPs, no other peptide hormones that affect remote organs have been identified from the heart. We identified osteocrin (Ostn) as an osteogenesis/chondrogenesis regulatory hormone secreted from CMs in zebrafish. ostn mutant larvae exhibit impaired membranous and chondral bone formation. The impaired bones were recovered by CM-specific overexpression of OSTN. We analyzed the parasphenoid (ps) as a representative of membranous bones. In the shortened ps of ostn morphants, nuclear Yap1/Wwtr1-dependent transcription was increased, suggesting that Ostn might induce the nuclear export of Yap1/Wwtr1 in osteoblasts. Although OSTN is proposed to bind to NPR3 (clearance receptor for NPs) to enhance the binding of NPs to NPR1 or NPR2, OSTN enhanced C-type NP (CNP)-dependent nuclear export of YAP1/WWTR1 of cultured mouse osteoblasts stimulated with saturable CNP. OSTN might therefore activate unidentified receptors that augment protein kinase G signaling mediated by a CNP-NPR2 signaling axis. These data demonstrate that Ostn secreted from the heart contributes to bone formation as an endocrine hormone.

Cell lineage responses to photobiomodulation therapy

Photobiomodulation (PBM) therapy has been noted to promote cell proliferation and growth in many different cell types shown both in vitro and in vivo. Currently, treatment regimens are used in the clinic for a variety of ailments, including wound healing. However, most protocols treat an anatomical site without considering individual cell types constituting the target tissues. This study investigates the maximal dose threshold for oral keratinocyte and fibroblast cell types treated with near-infrared laser therapy. We observed keratinocytes have increased sensitivity to laser irradiances (>0.047 W/cm² , 300 sec, 14.2 J/cm² ) compared to the fibroblast cells (>0.057 W/cm² , 300 sec, 15.1 J/cm² ) (p < 0.0001). Laser treatments were noted to generate increased reactive oxygen species (ROS) levels in keratinocytes compared to fibroblasts that appeared to inversely correlate with higher basal catalase expression. To validate these observations, melatonin was used to treat keratinocytes to induce catalase activity (p < 0.0001). Increased melatonin-induced catalase levels were noted to significantly improve keratinocyte survival to phototoxic laser doses. These observations suggest that clinical laser dosing should account for differential effects of lasers on individual cell types to improve safety and clinical efficacy of PBM therapy.

Bimodal antagonism of PKA signalling by ARHGAP36

Protein kinase A is a key mediator of cAMP signalling downstream of G-protein-coupled receptors, a signalling pathway conserved in all eukaryotes. cAMP binding to the regulatory subunits (PKAR) relieves their inhibition of the catalytic subunits (PKAC). Here we report that ARHGAP36 combines two distinct inhibitory mechanisms to antagonise PKA signalling. First, it blocks PKAC activity via a pseudosubstrate motif, akin to the mechanism employed by the protein kinase inhibitor proteins. Second, it targets PKAC for rapid ubiquitin-mediated lysosomal degradation, a pathway usually reserved for transmembrane receptors. ARHGAP36 thus dampens the sensitivity of cells to cAMP. We show that PKA inhibition by ARHGAP36 promotes derepression of the Hedgehog signalling pathway, thereby providing a simple rationale for the upregulation of ARHGAP36 in medulloblastoma. Our work reveals a new layer of PKA regulation that may play an important role in development and disease.

Protein kinase A (PKA) is a key mediator of cyclic AMP signalling. Here, Eccles et al. show that ARHGAP36 antagonizes PKA by acting as a kinase inhibitor and targeting the catalytic subunit for endolysosomal degradation, thus reducing sensitivity of cells to cAMP and promoting Hedgehog signalling.

Functional characterization of AMP-activated protein kinase signaling in tumorigenesis

AMP-activated protein kinase (AMPK) is a ubiquitously expressed metabolic sensor among various species. Specifically, cellular AMPK is phosphorylated and activated under certain stressful conditions, such as energy deprivation, in turn to activate diversified downstream substrates to modulate the adaptive changes and maintain metabolic homeostasis. Recently, emerging evidences have implicated the potential roles of AMPK signaling in tumor initiation and progression. Nevertheless, a comprehensive description on such topic is still in scarcity, especially in combination of its biochemical features with mouse modeling results to elucidate the physiological role of AMPK signaling in tumorigenesis. Hence, we performed this thorough review by summarizing the tumorigenic role of each component along the AMPK signaling, comprising of both its upstream and downstream effectors. Moreover, their functional interplay with the AMPK heterotrimer and exclusive efficacies in carcinogenesis were chiefly explained among genetically altered mice models. Importantly, the pharmaceutical investigations of AMPK relevant medications have also been highlighted. In summary, in this review, we not only elucidate the potential functions of AMPK signaling pathway in governing tumorigenesis, but also potentiate the future targeted strategy aiming for better treatment of aberrant metabolism-associated diseases, including cancer.

PKA inhibits WNT signalling in adrenal cortex zonation and prevents malignant tumour development

Adrenal cortex physiology relies on functional zonation, essential for production of aldosterone by outer zona glomerulosa (ZG) and glucocorticoids by inner zona fasciculata (ZF). The cortex undergoes constant cell renewal, involving recruitment of subcapsular progenitors to ZG fate and subsequent lineage conversion to ZF identity. Here we show that WNT4 is an important driver of WNT pathway activation and subsequent ZG differentiation and demonstrate that PKA activation prevents ZG differentiation through WNT4 repression and WNT pathway inhibition. This suggests that PKA activation in ZF is a key driver of WNT inhibition and lineage conversion. Furthermore, we provide evidence that constitutive PKA activation inhibits, whereas partial inactivation of PKA catalytic activity stimulates β-catenin-induced tumorigenesis. Together, both lower PKA activity and higher WNT pathway activity lead to poorer prognosis in adrenocortical carcinoma (ACC) patients. These observations suggest that PKA acts as a tumour suppressor in the adrenal cortex, through repression of WNT signalling.

The adrenal cortex undergoes functional zonation to generate an outer zona glomerulosa (ZG) and inner zona fasciculata (ZF), but how this is regulated at a molecular level is unclear. Here, the authors show that ZG differentiation is stimulated by WNT signalling and that PKA blocks WNT signalling to allow ZF differentiation and also prevents WNT-induced cancer development.

mTOR inhibition prevents rapid-onset of carcinogen-induced malignancies in a novel inducible HPV-16 E6/E7 mouse model

The rising incidence of human papillomavirus (HPV)-associated malignancies, especially for oropharyngeal cancers, has highlighted the urgent need to understand how the interplay between high-risk HPV oncogenes and carcinogenic exposure results in squamous cell carcinoma (SCC) development. Here, we describe an inducible mouse model expressing high risk HPV-16 E6/E7 oncoproteins in adults, bypassing the impact of these viral genes during development. HPV-16 E6/E7 genes were targeted to the basal squamous epithelia in transgenic mice using a doxycycline inducible cytokeratin 5 promoter (cK5-rtTA) system. After doxycycline induction, both E6 and E7 were highly expressed, resulting in rapid epidermal hyperplasia with a remarkable expansion of the proliferative cell compartment to the suprabasal layers. Surprisingly, in spite of the massive growth of epithelial cells and their stem cell progenitors, HPV-E6/E7 expression was not sufficient to trigger mTOR activation, a key oncogenic driver in HPV-associated malignancies, and malignant progression to SCC. However, these mice develop SCC rapidly after a single exposure to a skin carcinogen, DMBA, which was increased by the prolonged exposure to a tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA). Thus, only few oncogenic hits may be sufficient to induce cancer in E6/E7 expressing cells. All HPV-E6/E7 expressing SCC lesions exhibited increased mTOR activation. Remarkably, rapamycin, an mTOR inhibitor, abolished tumor development when administered to HPV-E6/E7 mice prior to DMBA exposure. Our findings revealed that mTOR inhibition protects HPV-E6/E7 expressing tissues form SCC development upon carcinogen exposure, thus supporting the potential clinical use of mTOR inhibitors as a molecular targeted approach for prevention of HPV-associated malignancies.

The secretome of MUSE cells contains factors that may play a role in regulation of stemness, apoptosis and immunomodulation

Mesenchymal stromal cells (MSCs) are a heterogeneous population, which contain several cell phenotypes: mesenchymal stem cells, progenitor cells, fibroblasts and other type of cells. Previously, we identified unique stem cells that we named multilineage-differentiating stress enduring (Muse) cells as one to several percent of MSCs of the bone marrow, adipose tissue and dermis. Among different cell populations in MSCs, Muse cells, positive for pluripotent surface marker SSEA-3, may represent cells responsible for pluripotent-like property of MSCs, since they express pluripotency genes, able to differentiated into triploblastic cells from a single cells and are self-renewable. MSCs release biologically active factors that have profound effects on local cellular dynamics. A thorough examination of MSC secretome seems essential for understanding the physiological functions exerted by these cells in our organism and also for rational cellular therapy design. In this setting, studies on secretome of Muse cells may shed light on pathways that are associated with their specific features. Our findings evidenced that secretomes of MSCs and Muse cells contain factors that regulate extracellular matrix remodeling, ox-redox activities and immune system. Muse cells appear to secrete factors that may preserve their stem cell features, allow survival under stress conditions and may contribute to their immunomodulation capacity. In detail, the proteins belonging to protein kinase A signaling, FXR/RXR activation and LXR/RXR activation pathways may play a role in regulation of Muse stem cell features. These last 2 pathways together with proteins associated with antigen presentation pathway and coagulation system may play a role in immunomodulation.

cAMP-dependent protein kinase (PKA) complexes probed by complementary differential scanning fluorimetry and ion mobility-mass spectrometry

cAMP-dependent protein kinase (PKA) is an archetypal biological signaling module and a model for understanding the regulation of protein kinases. In the present study, we combine biochemistry with differential scanning fluorimetry (DSF) and ion mobility–mass spectrometry (IM–MS) to evaluate effects of phosphorylation and structure on the ligand binding, dynamics and stability of components of heteromeric PKA protein complexes in vitro. We uncover dynamic, conformationally distinct populations of the PKA catalytic subunit with distinct structural stability and susceptibility to the physiological protein inhibitor PKI. Native MS of reconstituted PKA R₂C₂ holoenzymes reveals variable subunit stoichiometry and holoenzyme ablation by PKI binding. Finally, we find that although a ‘kinase-dead’ PKA catalytic domain cannot bind to ATP in solution, it interacts with several prominent chemical kinase inhibitors. These data demonstrate the combined power of IM–MS and DSF to probe PKA dynamics and regulation, techniques that can be employed to evaluate other protein-ligand complexes, with broad implications for cellular signaling.

The Hippo pathway in intestinal regeneration and disease

The Hippo pathway is a signalling cascade conserved from Drosophila melanogaster to mammals. The mammalian core kinase components comprise MST1 and MST2, SAV1, LATS1 and LATS2 and MOB1A and MOB1B. The transcriptional co-activators YAP1 and TAZ are the downstream effectors of the Hippo pathway and regulate target gene expression. Hippo signalling has crucial roles in the control of organ size, tissue homeostasis and regeneration, and dysregulation of the Hippo pathway can lead to uncontrolled cell growth and malignant transformation. Mammalian intestine consists of a stem cell compartment as well as differentiated cells, and its ability to regenerate rapidly after injury makes it an excellent model system to study tissue homeostasis, regeneration and tumorigenesis. Several studies have established the important role of the Hippo pathway in these processes. In addition, crosstalk between Hippo and other signalling pathways provides tight, yet versatile, regulation of tissue homeostasis. In this Review, we summarize studies on the role of the Hippo pathway in the intestine on these physiological processes and the underlying mechanisms responsible, and discuss future research directions and potential therapeutic strategies targeting Hippo signalling in intestinal disease.

Activation of PKA leads to mesenchymal-to-epithelial transition and loss of tumor-initiating ability

The epithelial-to-mesenchymal transition enables carcinoma cells to acquire malignancy-associated traits and the properties of tumor-initiating cells (TICs). TICs have emerged in recent years as important targets for cancer therapy, owing to their ability to drive clinical relapse and enable metastasis. Here, we propose a strategy to eliminate mesenchymal TICs by inducing their conversion to more epithelial counterparts that have lost tumor-initiating ability. We report that increases in intracellular levels of the second messenger, adenosine 3',5'-monophosphate, and the subsequent activation of protein kinase A (PKA) induce a mesenchymal-to-epithelial transition (MET) in mesenchymal human mammary epithelial cells. PKA activation triggers epigenetic reprogramming of TICs by the histone demethylase PHF2, which promotes their differentiation and loss of tumor-initiating ability. This study provides proof-of-principle for inducing an MET as differentiation therapy for TICs and uncovers a role for PKA in enforcing and maintaining the epithelial state.

Hippo Pathway in Organ Size Control, Tissue Homeostasis, and Cancer

Two decades of studies in multiple model organisms have established the Hippo pathway as a key regulator of organ size and tissue homeostasis. By inhibiting YAP and TAZ transcription co-activators, the Hippo pathway regulates cell proliferation, apoptosis, and stemness in response to a wide range of extracellular and intracellular signals, including cell-cell contact, cell polarity, mechanical cues, ligands of G-protein-coupled receptors, and cellular energy status. Dysregulation of the Hippo pathway exerts a significant impact on cancer development. Further investigation of the functions and regulatory mechanisms of this pathway will help uncovering the mystery of organ size control and identify new targets for cancer treatment.

Proteomics of Primary Cilia by Proximity Labeling

While cilia are recognized as important signaling organelles, the extent of ciliary functions remains unknown because of difficulties in cataloguing proteins from mammalian primary cilia. We present a method that readily captures rapid snapshots of the ciliary proteome by selectively biotinylating ciliary proteins using a cilia-targeted proximity labeling enzyme (cilia-APEX). Besides identifying known ciliary proteins, cilia-APEX uncovered several ciliary signaling molecules. The kinases PKA, AMPK, and LKB1 were validated as bona fide ciliary proteins and PKA was found to regulate Hedgehog signaling in primary cilia. Furthermore, proteomics profiling of Ift27/Bbs19 mutant cilia correctly detected BBSome accumulation inside Ift27(-/-) cilia and revealed that β-arrestin 2 and the viral receptor CAR are candidate cargoes of the BBSome. This work demonstrates that proximity labeling can be applied to proteomics of non-membrane-enclosed organelles and suggests that proteomics profiling of cilia will enable a rapid and powerful characterization of ciliopathies.