Striated Rootlet and Nonfilamentous Forms of Rootletin Maintain Ciliary Function

SummaryPrimary cilia are microtubule-based sensory organelles whose structures and functions must be actively maintained throughout animal lifespan to support signal transduction pathways essential for development and physiological processes such as vision and olfaction [1]. Remarkably, few cellular components aside from the intraflagellar transport (IFT) machinery are implicated in ciliary maintenance [2]. Rootletin, an evolutionarily conserved protein found as prominent striated rootlets or a nonfilamentous form, both of which are associated with cilium-anchoring basal bodies, represents a likely candidate given its well-known role in preventing ciliary photoreceptor degeneration in a mouse model [3, 4]. Whether rootletin is universally required for maintaining ciliary integrity, and if so, by what mechanism, remains unresolved. Here, we demonstrate that the gene disrupted in the previously isolated C. elegans chemosensory mutant che-10 encodes a rootletin ortholog that localizes proximally and distally to basal bodies of cilia harboring or lacking conspicuous rootlets. In vivo analyses reveal that CHE-10/rootletin maintains ciliary integrity partly by modulating the assembly, motility, and flux of IFT particles, which are critical for axoneme length control. Surprisingly, CHE-10/rootletin is also essential for stabilizing ciliary transition zones and basal bodies, roles not ascribed to IFT. Unifying these findings, we provide evidence that the underlying molecular defects in the che-10 mutant stem from disrupted organization/function of the periciliary membrane, affecting the efficient delivery of basal body-associated and ciliary components and resulting in cilium degeneration. Together, our cloning and functional analyses of C. elegans che-10 provide the first mechanistic insights into how filamentous and nonfilamentous forms of rootletin play essential roles in maintaining ciliary function in metazoans

CDKL Family Kinases Have Evolved Distinct Structural Features and Ciliary Function

Various kinases, including a cyclin-dependent kinase (CDK) family member, regulate the growth and functions of primary cilia, which perform essential roles in signaling and development. Neurological disorders linked to CDK-Like (CDKL) proteins suggest that these underexplored kinases may have similar functions. Here, we present the crystal structures of human CDKL1, CDKL2, CDKL3, and CDKL5, revealing their evolutionary divergence from CDK and mitogen-activated protein kinases (MAPKs), including an unusual ?J helix important for CDKL2 and CDKL3 activity. C. elegans CDKL-1, most closely related to CDKL1-4 and localized to neuronal cilia transition zones, modulates cilium length; this depends on its kinase activity and ?J helix-containing C terminus. Human CDKL5, linked to Rett syndrome, also localizes to cilia, and it impairs ciliogenesis when overexpressed. CDKL5 patient mutations modeled in CDKL-1 cause localization and/or cilium length defects. Together, our studies establish a disease model system suggesting cilium length defects as a pathomechanism for neurological disorders, including epilepsy

Mechanism of transport of IFT particles in C. elegans cilia by the concerted action of kinesin-II and OSM-3 motors

The assembly and function of cilia on Caenorhabditis elegans neurons depends on the action of two kinesin-2 motors, heterotrimeric kinesin-II and homodimeric OSM-3–kinesin, which cooperate to move the same intraflagellar transport (IFT) particles along microtubule (MT) doublets. Using competitive in vitro MT gliding assays, we show that purified kinesin-II and OSM-3 cooperate to generate movement similar to that seen along the cilium in the absence of any additional regulatory factors. Quantitative modeling suggests that this could reflect an alternating action mechanism, in which the motors take turns to move along MTs, or a mechanical competition, in which the motors function in a concerted fashion to move along MTs with the slow motor exerting drag on the fast motor and vice versa. In vivo transport assays performed in Bardet-Biedl syndrome (BBS) protein and IFT motor mutants favor a mechanical competition model for motor coordination in which the IFT motors exert a BBS protein–dependent tension on IFT particles, which controls the IFT pathway that builds the cilium foundation

The inner junction protein CFAP20 functions in motile and non-motile cilia and is critical for vision

Motile and non-motile cilia are associated with mutually-exclusive genetic disorders. Motile cilia propel sperm or extracellular fluids, and their dysfunction causes primary ciliary dyskinesia. Non-motile cilia serve as sensory/signalling antennae on most cell types, and their disruption causes single-organ ciliopathies such as retinopathies or multi-system syndromes. CFAP20 is a ciliopathy candidate known to modulate motile cilia in unicellular eukaryotes. We demonstrate that in zebrafish, cfap20 is required for motile cilia function, and in C. elegans, CFAP-20 maintains the structural integrity of non-motile cilia inner junctions, influencing sensory-dependent signalling and development. Human patients and zebrafish with CFAP20 mutations both exhibit retinal dystrophy. Hence, CFAP20 functions within a structural/functional hub centered on the inner junction that is shared between motile and non-motile cilia, and is distinct from other ciliopathy-associated domains or macromolecular complexes. Our findings suggest an uncharacterised pathomechanism for retinal dystrophy, and potentially for motile and non-motile ciliopathies in general

EFHC1, Implicated in Juvenile Myoclonic Epilepsy, Functions at the Cilium and Synapse to Modulate Dopamine Signaling

Neurons throughout the mammalian brain possess non-motile cilia, organelles with varied functions in sensory physiology and cellular signaling. Yet, the roles of cilia in these neurons are poorly understood. To shed light into their functions, we studied EFHC1, an evolutionarily conserved protein required for motile cilia function and linked to a common form of inherited epilepsy in humans, juvenile myoclonic epilepsy (JME). We demonstrate that C. elegans EFHC-1 functions within specialized non-motile mechanosensory cilia, where it regulates neuronal activation and dopamine signaling. EFHC-1 also localizes at the synapse, where it further modulates dopamine signaling in cooperation with the orthologue of an R-type voltage-gated calcium channel. Our findings unveil a previously undescribed dual-regulation of neuronal excitability at sites of neuronal sensory input (cilium) and neuronal output (synapse). Such a distributed regulatory mechanism may be essential for establishing neuronal activation thresholds under physiological conditions, and when impaired, may represent a novel pathomechanism for epilepsy

Damaged DNA Binding Protein 2 Plays a Role in Breast Cancer Cell Growth

The Damaged DNA binding protein 2 (DDB2), is involved in nucleotide excision repair as well as in other biological processes in normal cells, including transcription and cell cycle regulation. Loss of DDB2 function may be related to tumor susceptibility. However, hypothesis of this study was that DDB2 could play a role in breast cancer cell growth, resulting in its well known interaction with the proliferative marker E2F1 in breast neoplasia. DDB2 gene was overexpressed in estrogen receptor (ER)-positive (MCF-7 and T47D), but not in ER-negative breast cancer (MDA-MB231 and SKBR3) or normal mammary epithelial cell lines. In addition, DDB2 expression was significantly (3.0-fold) higher in ER-positive than in ER-negative tumor samples (P = 0.0208) from 16 patients with breast carcinoma. Knockdown of DDB2 by small interfering RNA in MCF-7 cells caused a decrease in cancer cell growth and colony formation. Inversely, introduction of the DDB2 gene into MDA-MB231 cells stimulated growth and colony formation. Cell cycle distribution and 5 Bromodeoxyuridine incorporation by flow cytometry analysis showed that the growth-inhibiting effect of DDB2 knockdown was the consequence of a delayed G1/S transition and a slowed progression through the S phase of MCF-7 cells. These results were supported by a strong decrease in the expression of S phase markers (Proliferating Cell Nuclear Antigen, cyclin E and dihydrofolate reductase). These findings demonstrate for the first time that DDB2 can play a role as oncogene and may become a promising candidate as a predictive marker in breast cancer

MKS and NPHP modules cooperate to establish basal body/transition zone membrane associations and ciliary gate function during ciliogenesis

Eight proteins, defects in which are associated with Meckel-Gruber syndrome and nephronophthisis ciliopathies, work together as two functional modules at the transition zone to establish basal body/transition zone connections with the membrane and barricade entry of non-ciliary components into this organelle

Mutations in a Guanylate Cyclase GCY-35/GCY-36 Modify Bardet-Biedl Syndrome–Associated Phenotypes in Caenorhabditis elegans

Ciliopathies are pleiotropic and genetically heterogeneous disorders caused by defective development and function of the primary cilium. Bardet-Biedl syndrome (BBS) proteins localize to the base of cilia and undergo intraflagellar transport, and the loss of their functions leads to a multisystemic ciliopathy. Here we report the identification of mutations in guanylate cyclases (GCYs) as modifiers of Caenorhabditis elegans bbs endophenotypes. The loss of GCY-35 or GCY-36 results in suppression of the small body size, developmental delay, and exploration defects exhibited by multiple bbs mutants. Moreover, an effector of cGMP signalling, a cGMP-dependent protein kinase, EGL-4, also modifies bbs mutant defects. We propose that a misregulation of cGMP signalling, which underlies developmental and some behavioural defects of C. elegans bbs mutants, may also contribute to some BBS features in other organisms

Localization of a Guanylyl Cyclase to Chemosensory Cilia Requires the Novel Ciliary MYND Domain Protein DAF-25

In harsh conditions, Caenorhabditis elegans arrests development to enter a non-aging, resistant diapause state called the dauer larva. Olfactory sensation modulates the TGF-β and insulin signaling pathways to control this developmental decision. Four mutant alleles of daf-25 (abnormal DAuer Formation) were isolated from screens for mutants exhibiting constitutive dauer formation and found to be defective in olfaction. The daf-25 dauer phenotype is suppressed by daf-10/IFT122 mutations (which disrupt ciliogenesis), but not by daf-6/PTCHD3 mutations (which prevent environmental exposure of sensory cilia), implying that DAF-25 functions in the cilia themselves. daf-25 encodes the C. elegans ortholog of mammalian Ankmy2, a MYND domain protein of unknown function. Disruption of DAF-25, which localizes to sensory cilia, produces no apparent cilia structure anomalies, as determined by light and electron microscopy. Hinting at its potential function, the dauer phenotype, epistatic order, and expression profile of daf-25 are similar to daf-11, which encodes a cilium-localized guanylyl cyclase. Indeed, we demonstrate that DAF-25 is required for proper DAF-11 ciliary localization. Furthermore, the functional interaction is evolutionarily conserved, as mouse Ankmy2 interacts with guanylyl cyclase GC1 from ciliary photoreceptors. The interaction may be specific because daf-25 mutants have normally-localized OSM-9/TRPV4, TAX-4/CNGA1, CHE-2/IFT80, CHE-11/IFT140, CHE-13/IFT57, BBS-8, OSM-5/IFT88, and XBX-1/D2LIC in the cilia. Intraflagellar transport (IFT) (required to build cilia) is not defective in daf-25 mutants, although the ciliary localization of DAF-25 itself is influenced in che-11 mutants, which are defective in retrograde IFT. In summary, we have discovered a novel ciliary protein that plays an important role in cGMP signaling by localizing a guanylyl cyclase to the sensory organelle