Microscopy-Based Automated Live Cell Screening for Small Molecules That Affect Ciliation

Interactome Analysis Reveals a Link of the Novel ALMS1-CEP70 Complex to Centrosomal Clusters

Alström syndrome (ALMS) is a very rare autosomal-recessive disorder, causing a broad range of clinical defects most notably retinal degeneration, type 2 diabetes, and truncal obesity. The ALMS1 gene encodes a complex and huge ∼0.5 MDa protein, which has hampered analysis in the past. The ALMS1 protein is localized to the centrioles and the basal body of cilia and is involved in signaling processes, for example, TGF-β signaling. However, the exact molecular function of ALMS1 at the basal body remains elusive and controversial. We recently demonstrated that protein complex analysis utilizing endogenously tagged cells provides an excellent tool to investigate protein interactions of ciliary proteins. Here, CRISPR/Cas9-mediated endogenously tagged ALMS1 cells were used for affinity-based protein complex analysis. Centrosomal and microtubule-associated proteins were identified, which are potential regulators of ALMS1 function, such as the centrosomal protein 70 kDa (CEP70). Candidate proteins were further investigated in ALMS1-deficient hTERT-RPE1 cells. Loss of ALMS1 led to shortened cilia with no change in structural protein localization, for example, acetylated and ɣ-tubulin, Centrin-3, or the novel interactor CEP70. Conversely, reduction of CEP70 resulted in decreased ALMS1 at the ciliary basal body. Complex analysis of CEP70 revealed domain-specific ALMS1 interaction involving the TPR-containing C-terminal (TRP-CT) fragment of CEP70. In addition to ALMS1, several ciliary proteins, including CEP135, were found to specifically bind to the TPR-CT domain. Data are available via ProteomeXchange with the identifier PXD046401. Protein interactors identified in this study provide candidate lists that help to understand ALMS1 and CEP70 function in cilia-related protein modification, cell death, and disease-related mechanisms.

Patterns of Ciliation and Ciliary Signaling in Cancer

Among the factors that have been strongly implicated in regulating cancerous transformation, the primary monocilium (cilium) has gained increasing attention. The cilium is a small organelle extending from the plasma membrane, which provides a localized hub for concentration of transmembrane receptors. These receptors transmit signals from soluble factors (including Sonic hedgehog (SHH), platelet-derived growth factor (PDGF-AA), WNT, TGFβ, NOTCH, and others) that regulate cell growth, as well as mechanosensory cues provided by flow or extracellular matrix. Ciliation is regulated by cell cycle, with most cells that are in G0 (quiescent) or early G1 ciliation and cilia typically absent in G2/M cells. Notably, while most cells organized in solid tissues are ciliated, cancerous transformation induces significant changes in ciliation. Most cancer cells lose cilia; medulloblastomas and basal cell carcinomas, dependent on an active SHH pathway, rely on ciliary maintenance. Changes in cancer cell ciliation are driven by core oncogenic pathways (EGFR, KRAS, AURKA, PI3K), and importantly ciliation status regulates functionality of those pathways. Ciliation is both influenced by targeted cancer therapies and linked to therapeutic resistance; recent studies suggest ciliation may also influence cancer cell metabolism and stem cell identity. We review recent studies defining the relationship between cilia and cancer.

Multi-color live-cell fluorescence imaging of primary ciliary membrane assembly and dynamics

The ciliary membrane is continuous with the plasma membrane but has distinct lipid and protein composition, which is key to defining the function of the primary cilium. Ciliary membranes dynamically assemble and disassemble in association with the cell cycle and directly transmit signals and molecules through budding membranes. Various imaging approaches have greatly advanced the understanding of the ciliary membrane function. In particular, fluorescence live-cell imaging has revealed important insights into the dynamics of ciliary membrane assembly by monitoring the changes of fluorescent-tagged ciliary proteins. Protein dynamics can be tracked simultaneously using multi-color live cell imaging by coupling ciliary-associated factors with different colored fluorescent tags. Ciliary membrane and membrane associated-proteins such as Smoothened, 5-HTr6, SSTR3, Rab8a, and Arl13b have been used to track ciliary membranes and centriole proteins like Centrin1/2, CEP164, and CEP83 are often used to mark the ciliary basal body. Here, we describe a method for studying ciliogenesis membrane dynamics using spinning disk confocal live-cell imaging.

Impaired Ciliary Beat Frequency and Ciliogenesis Alteration during Airway Epithelial Cell Differentiation in COPD

Chronic obstructive pulmonary disease (COPD) is a frequent respiratory disease. However, its pathophysiology remains partially elucidated. Epithelial remodeling including alteration of the cilium is a major hallmark of COPD, but specific assessments of the cilium have been rarely investigated as a diagnostic tool in COPD. Here we explore the dysregulation of the ciliary function (ciliary beat frequency (CBF)) and differentiation (multiciliated cells formation in air-liquid interface cultures) of bronchial epithelial cells from COPD (n = 17) and non-COPD patients (n = 15). CBF was decreased by 30% in COPD (11.15 +/- 3.37 Hz vs. 7.89 +/- 3.39 Hz, p = 0.037). Ciliary differentiation was altered during airway epithelial cell differentiation from COPD patients. While the number of multiciliated cells decreased (p < 0.005), the number of primary ciliated cells increased (p < 0.05) and primary cilia were shorter (p < 0.05). Altogether, we demonstrate that COPD can be considered as a ciliopathy through both primary non-motile cilia modifications (related to airway epithelial cell repair and remodeling) and motile cilia function impairment (associated with decrease sputum clearance and clinical respiratory symptoms). These observations encourage considering cilia-associated features in the complex COPD physiopathology and highlight the potential of cilia-derived biomarkers for diagnosis.

Renal Ciliopathies: Sorting Out Therapeutic Approaches for Nephronophthisis

Nephronophthisis (NPH) is an autosomal recessive ciliopathy and a major cause of end-stage renal disease in children. The main forms, juvenile and adult NPH, are characterized by tubulointerstitial fibrosis whereas the infantile form is more severe and characterized by cysts. NPH is caused by mutations in over 20 different genes, most of which encode components of the primary cilium, an organelle in which important cellular signaling pathways converge. Ciliary signal transduction plays a critical role in kidney development and tissue homeostasis, and disruption of ciliary signaling has been associated with cyst formation, epithelial cell dedifferentiation and kidney function decline. Drugs have been identified that target specific signaling pathways (for example cAMP/PKA, Hedgehog, and mTOR pathways) and rescue NPH phenotypes in in vitro and/or in vivo models. Despite identification of numerous candidate drugs in rodent models, there has been a lack of clinical trials and there is currently no therapy that halts disease progression in NPH patients. This review covers the most important findings of therapeutic approaches in NPH model systems to date, including hypothesis-driven therapies and untargeted drug screens, approached from the pathophysiology of NPH. Importantly, most animal models used in these studies represent the cystic infantile form of NPH, which is less prevalent than the juvenile form. It appears therefore important to develop new models relevant for juvenile/adult NPH. Alternative non-orthologous animal models and developments in patient-based in vitro model systems are discussed, as well as future directions in personalized therapy for NPH.

Hedgehog Signal and Genetic Disorders

The hedgehog (Hh) family comprises sonic hedgehog (Shh), Indian hedgehog (Ihh), and desert hedgehog (Dhh), which are versatile signaling molecules involved in a wide spectrum of biological events including cell differentiation, proliferation, and survival; establishment of the vertebrate body plan; and aging. These molecules play critical roles from embryogenesis to adult stages; therefore, alterations such as abnormal expression or mutations of the genes involved and their downstream factors cause a variety of genetic disorders at different stages. The Hh family involves many signaling mediators and functions through complex mechanisms, and achieving a comprehensive understanding of the entire signaling system is challenging. This review discusses the signaling mediators of the Hh pathway and their functions at the cellular and organismal levels. We first focus on the roles of Hh signaling mediators in signal transduction at the cellular level and the networks formed by these factors. Then, we analyze the spatiotemporal pattern of expression of Hh pathway molecules in tissues and organs, and describe the phenotypes of mutant mice. Finally, we discuss the genetic disorders caused by malfunction of Hh signaling-related molecules in humans.

Unexpected Activities in Regulating Ciliation Contribute to Off-target Effects of Targeted Drugs

PURPOSE

For many tumors, signaling exchanges between cancer cells and other cells in their microenvironment influence overall tumor signaling. Some of these exchanges depend on expression of the primary cilium on nontransformed cell populations, as extracellular ligands including Sonic Hedgehog (SHH), PDGFRα, and others function through receptors spatially localized to cilia. Cell ciliation is regulated by proteins that are themselves therapeutic targets. We investigated whether kinase inhibitors of clinical interest influence ciliation and signaling by proteins with ciliary receptors in cancer and other cilia-relevant disorders, such as polycystic kidney disease (PKD).

EXPERIMENTAL DESIGN

We screened a library of clinical and preclinical kinase inhibitors, identifying drugs that either prevented or induced ciliary disassembly. Specific bioactive protein targets of the drugs were identified by mRNA depletion. Mechanism of action was defined, and activity of select compounds investigated.

RESULTS

We identified multiple kinase inhibitors not previously linked to control of ciliation, including sunitinib, erlotinib, and an inhibitor of the innate immune pathway kinase, IRAK4. For all compounds, activity was mediated through regulation of Aurora-A (AURKA) activity. Drugs targeting cilia influenced proximal cellular responses to SHH and PDGFRα. In vivo, sunitinib durably limited ciliation and cilia-related biological activities in renal cells, renal carcinoma cells, and PKD cysts. Extended analysis of IRAK4 defined a subset of innate immune signaling effectors potently affecting ciliation.

CONCLUSIONS

These results suggest a paradigm by which targeted drugs may have unexpected off-target effects in heterogeneous cell populations in vivo via control of a physical platform for receipt of extracellular ligands.