The Rilp-like proteins Rilpl1 and Rilpl2 regulate ciliary membrane content

RILPL2 as a potential biomarker for predicting enhanced T cell infiltration in non-small cell lung cancer

Our previous bioinformatics analysis has revealed that Rab-interacting lysosomal protein-like 2 (RILPL2) is associated with tumor immune microenvironment in non-small cell lung cancer (NSCLC). In our study, we collected 140 patients with primary NSCLC to verify the RILPL2 expression and its prognostic value, the relationship between RILPL2 expression and CD4+, CD8+T cell infiltration. A total of 140 patients who had been diagnosed with primary NSCLC (including 66 lung adenocarcinomas and 74 lung squamous cell carcinomas) were enrolled in our study. Immunohistochemical (IHC) staining was performed to analyze the expression of RILPL2, CD4, and CD8 in these patients. Compared with peri-cancer tissues, the RILPL2 expression in NSCLC tissues was significantly lower (P < 0.0001). RILPL2 expression was significantly related to clinical stage (P = 0.019), and low RILPL2 expression indicated higher stage. Low RILPL2 expression predicted worse overall survival (OS) in NSCLC patients (P = 0.017). Correlational analyses revealed that RILPL2 expression was significantly positively correlated with CD4+T cell infiltration in NSCLC (R = 0.294, P < 0.001), LUAD subgroup (R = 0.256, P = 0.038), and LUSC subgroup (R = 0.333, P = 0.004); RILPL2 expression was also significantly positively correlated with CD8+ T cell infiltration in NSCLC (R = 0.263, P = 0.002), LUAD subgroup (R = 0.280, P = 0.023), and LUSC subgroup (R = 0.250, P = 0.031). In conclusion, RILPL2 expression was downregulated in NSCLC; low RILPL2 expression was significantly related to higher stage and worse prognosis; RILPL2 expression was significantly positively correlated with CD4+, CD8+T cell infiltration.

Rab10-CAV1 mediated intraluminal vesicle transport to migrasomes

Migrasomes, vesicular organelles generated on the retraction fibers of migrating cells, play a crucial role in migracytosis, mediating intercellular communication. The cargoes determine the functional specificity of migrasomes. Migrasomes harbor numerous intraluminal vesicles, a pivotal component of their cargoes. The mechanism underlying the transportation of these intraluminal vesicles to the migrasomes remains enigmatic. In this study, we identified that Rab10 and Caveolin-1 (CAV1) mark the intraluminal vesicles in migrasomes. Transport of Rab10-CAV1 vesicles to migrasomes required the motor protein Myosin Va and adaptor proteins RILPL2. Notably, the phosphorylation of Rab10 by the kinase LRRK2 regulated this process. Moreover, CSF-1 can be transported to migrasomes through this mechanism, subsequently fostering monocyte-macrophage differentiation in skin wound healing, which served as a proof of the physiological importance of this transporting mechanism.

RAB12-LRRK2 Complex Suppresses Primary Ciliogenesis and Regulates Centrosome Homeostasis in Astrocytes

Leucine-rich repeat kinase 2 (LRRK2) phosphorylates a subset of RAB GTPases, and the phosphorylation levels are elevated by Parkinson's disease (PD)-linked mutations of LRRK2. However, the precise function of the specific RAB GTPase targeted by LRRK2 signaling in the brain remains to be elucidated. Here, we identify RAB12 as a robust LRRK2 substrate in the mouse brains through phosphoproteomics profiling and solve the structure of RAB12-LRRK2 protein complex through Cryo-EM analysis. Mechanistically, RAB12 cooperates with LRRK2 to inhibit primary ciliogenesis and regulate centrosome homeostasis in astrocytes through enhancing the phosphorylation of RAB10 and recruiting Rab interacting lysosomal protein like 1 (RILPL1), while the functions of RAB12 require a direct interaction with LRRK2 and LRRK2 kinase activity. Furthermore, the ciliary deficits and centrosome alteration caused by the PD-linked LRRK2-G2019S mutation are prevented by the deletion of Rab12 in astrocytes. Thus, our study reveals a physiological function of the RAB12-LRRK2 complex in regulating ciliogenesis and centrosome homeostasis. The RAB12-LRRK2 structure offers a guidance in the therapeutic development of PD by targeting the RAB12-LRRK2 interaction.

Current advance on distal myopathy genetics

PURPOSE OF REVIEW

Distal myopathies are a clinically heterogenous group of rare, genetic muscle diseases, that present with weakness in hands and/or feet at onset. Some of these diseases remain accentuated in the distal muscles whereas others may later progress to the proximal muscles. In this review, the latest findings related to genetic and clinical features of distal myopathies are summarized.

RECENT FINDINGS

Variants in SMPX, DNAJB2, and HSPB6 have been identified as a novel cause of late-onset distal myopathy and neuromyopathy. In oculopharyngodistal myopathies, repeat expansions were identified in two novel disease-causing genes, RILPL1 and ABCD3. In multisystem proteinopathies, variants in HNRNPA1 and TARDBP, genes previously associated with amyotrophic lateral sclerosis, have been shown to cause late-onset distal myopathy without ALS. In ACTN2-related distal myopathy, the first recessive forms of the disease have been described, adding it to the growing list of genes were both dominant and recessive forms of myopathy are present.

SUMMARY

The identification of novel distal myopathy genes and pathogenic variants contribute to our ability to provide a final molecular diagnosis to a larger number of patients and increase our overall understanding of distal myopathy genetics and pathology.

Regulation of angiogenesis by endocytic trafficking mediated by cytoplasmic dynein 1 light intermediate chain 1

Dynein cytoplasmic 1 light intermediate chain 1 (LIC1, DYNC1LI1) is a core subunit of the dynein motor complex. The LIC1 subunit also interacts with various cargo adaptors to regulate Rab-mediated endosomal recycling and lysosomal degradation. Defects in this gene are predicted to alter dynein motor function, Rab binding capabilities, and cytoplasmic cargo trafficking. Here, we have identified a dync1li1 zebrafish mutant, harboring a premature stop codon at the exon 12/13 splice acceptor site, that displays increased angiogenesis. In vitro, LIC1-deficient human endothelial cells display increases in cell surface levels of the pro-angiogenic receptor VEGFR2, SRC phosphorylation, and Rab11-mediated endosomal recycling. In vivo, endothelial-specific expression of constitutively active Rab11a leads to excessive angiogenesis, similar to the dync1li1 mutants. Increased angiogenesis is also evident in zebrafish harboring mutations in rilpl1/2, the adaptor proteins that promote Rab docking to Lic1 to mediate lysosomal targeting. These findings suggest that LIC1 and the Rab-adaptor proteins RILPL1 and 2 restrict angiogenesis by promoting degradation of VEGFR2-containing recycling endosomes. Disruption of LIC1- and RILPL1/2-mediated lysosomal targeting increases Rab11-mediated recycling endosome activity, promoting excessive SRC signaling and angiogenesis.

LRRK2 and Parkinson's disease: from genetics to targeted therapy

LRRK2 variants are implicated in both familial and sporadic PD. LRRK2-PD has a generally benign clinical presentation and variable pathology, with inconsistent presence of Lewy bodies and marked Alzheimer's disease pathology. The mechanisms underlying LRRK2-PD are still unclear, but inflammation, vesicle trafficking, lysosomal homeostasis, and ciliogenesis have been suggested, among others. As novel therapies targeting LRRK2 are under development, understanding the role and function of LRRK2 in PD is becoming increasingly important. Here, we outline the epidemiological, pathophysiological, and clinical features of LRRK2-PD, and discuss the arising therapeutic approaches targeting LRRK2 and possible future directions for research.

Prognostic value of RILPL2 and its correlation with tumor immune microenvironment and glycolysis in non-small cell lung cancer

Rab-interacting lysosomal protein - like 2 (RILPL2) has been reported to be associated with prognosis and tumor biological functions in breast cancer and endometrial carcinoma. However, its expression and functional role in non-small cell lung cancer (NSCLC) remain unclear. The expression and clinical data of lung adenocarcinoma (LUAD) and lung squamous carcinoma (LUSC) were downloaded from the TCGA database. The expression of RILPL2 in NSCLC cell lines was verified by the Western blot. We used online databases and bioinformatics analysis tools to explore its prognostic value, potential biological functions, and correlations with tumor immune microenvironment.The expression of RILPL2 was significantly lower in NSCLC compared with adjacent normal tissues. Low RILPL2 expression was associated with worse overall survival (OS) in NSCLC. The GO analysis showed RILPL2 was comprehensively involved in immune activity. RILPL2 expression was significantly positively correlated with the infiltration levels of B cells, CD8+T cells, CD4+T cells, macrophages, neutrophils, dendritic cells (P < 0.001), and it was also significantly positively correlated with programmed cell death ligand 1 (PD-L1/CD274) (P < 0.001). High RILPL2 expression could predict better immunotherapy response and prognosis in the immunotherapy cohort. The GSEA analysis showed low RILPL2 expression was associated with glycolysis process in LUAD, which was verified in vitro.These results showed RILPL2 expression was correlated with prognosis, tumor microenvironment, and immunotherapy response in NSCLC. Besides, RILPL2 may regulate glycolysis in LUAD.

Insights into the cellular consequences of LRRK2-mediated Rab protein phosphorylation

Point mutations in leucine-rich repeat kinase 2 (LRRK2) which cause Parkinson's disease increase its kinase activity, and a subset of Rab GTPases have been identified as endogenous LRRK2 kinase substrates. Their phosphorylation correlates with a loss-of-function for the membrane trafficking steps they are normally involved in, but it also allows them to bind to a novel set of effector proteins with dominant cellular consequences. In this brief review, we will summarize novel findings related to the LRRK2-mediated phosphorylation of Rab GTPases and its various cellular consequences in vitro and in the intact brain, and we will highlight major outstanding questions in the field.

Ciliogenesis membrane dynamics and organization

Ciliogenesis is a complex multistep process used to describe assembly of cilia and flagella. These organelles play essential roles in motility and signaling on the surface of cells. Cilia are built at the distal ends of centrioles through the formation of an axoneme that is surrounded by the ciliary membrane. As is the case in the biogenesis of other cellular organelles, regulators of membrane trafficking play essential roles in ciliogenesis, albeit with a unique feature that membranes are organized around microtubule-based structures. Membrane association with the distal end of the centriole is a critical initiating step for ciliogenesis. Studies of this process in different cell types suggests that a singular mechanism may not be utilized to initiate cilium assembly. In this review, we focus on recent insights into cilium biogenesis and the roles membrane trafficking regulators play in described ciliogenesis mechanisms with relevance to human disease.

Appearing and disappearing acts of cilia

The past few decades have seen a rise in research on vertebrate cilia and ciliopathy, with interesting collaborations between basic and clinical scientists. This work includes studies on ciliary architecture, composition, evolution, and organelle generation and its biological role. The human body has cells that harbour any of the following four types of cilia: 9+0 motile, 9+0 immotile, 9+2 motile, and 9+2 immotile. Depending on the type, cilia play an important role in cell/fluid movement, mating, sensory perception, and development. Defects in cilia are associated with a wide range of human diseases afflicting the brain, heart, kidneys, respiratory tract, and reproductive system. These are commonly known as ciliopathies and affect millions of people worldwide. Due to their complex genetic etiology, diagnosis and therapy have remained elusive. Although model organisms like Chlamydomonas reinhardtii have been a useful source for ciliary research, reports of a fascinating and rewarding translation of this research into mammalian systems, especially humans, are seen. The current review peeks into one of the complex features of this organelle, namely its birth, the common denominators across the formation of both 9+0 and 9+2 ciliary types, the molecules involved in ciliogenesis, and the steps that go towards regulating their assembly and disassembly.

RILPL2 is associated with the progression and prognosis of cervical squamous cell carcinoma and endocervical adenocarcinoma

Cervical squamous cell carcinoma and endocervical adenocarcinoma (CESC) is one of the most common tumors among females worldwide. RILPL2 was recently reported to be a promising biomarker for the treatment of breast cancer. This study aimed to investigate the potential role of RILPL2 in CESC. Totally 302 CESC patients' data were downloaded from The Cancer Genome Atlas database. All patients were divided into high or low RILPL2 groups according to the median expression of RILPL2. Subsequently, survival analysis, multivariate Cox regression, and experimental validation were performed on all CESC patient data. The Ualcan database was used to analyze the expression level and prognostic value of RILPL2 in pan-cancer. The Gene Set Cancer Analysis database was used for drug sensitivity analysis. Functional KEGG pathways were analyzed using gene set enrichment analysis. RILPL2 was generally down-regulated in a variety of tumors, and a high level of RILPL2 was associated with a better prognosis in CESC patients. Immunohistochemistry, western blotting, and qRT-PCR results showed that RILPL2 was significantly down-regulated in CESC cells and tissues. Besides, along with the increase of TNM Stage, the RILPL2 expression tended to decrease gradually. Patients with high RILPL2 expression showed lower resistance to small molecule drugs used in CESC progressions, such as Methotrexate, AZD7762, and Vinblastine, and a higher response rate to immunotherapy. Additionally, we identified 267 co-expressing genes of RILPL2, all of which jointly affected CESC progression through 15 complex pathways. Low RILPL2 expression was closely associated with the onset, progression, and poor prognosis of CESC. RILPL2 might be a promising optional biomarker for CESC patients' diagnosis and prognosis.

Exome-wide association study to identify rare variants influencing COVID-19 outcomes: Results from the Host Genetics Initiative

Host genetics is a key determinant of COVID-19 outcomes. Previously, the COVID-19 Host Genetics Initiative genome-wide association study used common variants to identify multiple loci associated with COVID-19 outcomes. However, variants with the largest impact on COVID-19 outcomes are expected to be rare in the population. Hence, studying rare variants may provide additional insights into disease susceptibility and pathogenesis, thereby informing therapeutics development. Here, we combined whole-exome and whole-genome sequencing from 21 cohorts across 12 countries and performed rare variant exome-wide burden analyses for COVID-19 outcomes. In an analysis of 5,085 severe disease cases and 571,737 controls, we observed that carrying a rare deleterious variant in the SARS-CoV-2 sensor toll-like receptor TLR7 (on chromosome X) was associated with a 5.3-fold increase in severe disease (95% CI: 2.75-10.05, p = 5.41x10-7). This association was consistent across sexes. These results further support TLR7 as a genetic determinant of severe disease and suggest that larger studies on rare variants influencing COVID-19 outcomes could provide additional insights.

LRRK2 phosphorylation of Rab GTPases in Parkinson’s disease

Rab GTPases comprise a large family of conserved GTPases that are critical regulators of the secretory and endocytic pathways. The human genome encodes ~ 65 Rabs that localize to discrete membrane compartments and, when in their GTP-bound state, bind to effector proteins to carry out diverse functions. Activating mutations in LRRK2 kinase cause Parkinson's disease, and subsets of Rab GTPases are important LRRK2 substrates. LRRK2 phosphorylates a conserved threonine residue that is essential for Rab interaction with guanine nucleotide exchange factors, effectors, and GDI that recycles Rabs between membrane compartments. This brief review will highlight new findings related to LRRK2-mediated phosphorylation of Rab GTPases and its consequences. Remarkably, Rab phosphorylation flips a switch on Rab effector selection with dominant consequences for cell pathophysiology.

JIP3 interacts with dynein and kinesin-1 to regulate bidirectional organelle transport

The MAP kinase and motor scaffold JIP3 prevents excess lysosome accumulation in axons of vertebrates and invertebrates. How JIP3's interaction with dynein and kinesin-1 contributes to organelle clearance is unclear. We show that human dynein light intermediate chain (DLIC) binds the N-terminal RH1 domain of JIP3, its paralog JIP4, and the lysosomal adaptor RILP. A point mutation in RH1 abrogates DLIC binding without perturbing the interaction between JIP3's RH1 domain and kinesin heavy chain. Characterization of this separation-of-function mutation in Caenorhabditis elegans shows that JIP3-bound dynein is required for organelle clearance in the anterior process of touch receptor neurons. Unlike JIP3 null mutants, JIP3 that cannot bind DLIC causes prominent accumulation of endo-lysosomal organelles at the neurite tip, which is rescued by a disease-associated point mutation in JIP3's leucine zipper that abrogates kinesin light chain binding. These results highlight that RH1 domains are interaction hubs for cytoskeletal motors and suggest that JIP3-bound dynein and kinesin-1 participate in bidirectional organelle transport.

The LRRK2 signaling network converges on a centriolar phospho-Rab10/RILPL1 complex to cause deficits in centrosome cohesion and cell polarization

The Parkinson's-disease-associated LRRK2 kinase phosphorylates multiple Rab GTPases including Rab8 and Rab10, which enhances their binding to RILPL1 and RILPL2. The nascent interaction between phospho-Rab10 and RILPL1 blocks ciliogenesis in vitro and in the intact brain, and interferes with the cohesion of duplicated centrosomes in dividing cells. We show here that regulators of the LRRK2 signaling pathway including vps35 and PPM1H converge upon causing centrosomal deficits. The cohesion alterations do not require the presence of other LRRK2 kinase substrates including Rab12, Rab35 and Rab43 or the presence of RILPL2. Rather, they depend on the RILPL1-mediated centrosomal accumulation of phosphorylated Rab10. RILPL1 localizes to the subdistal appendage of the mother centriole, followed by recruitment of the LRRK2-phosphorylated Rab proteins to cause the centrosomal defects. The centrosomal alterations impair cell polarization as monitored by scratch wound assays which is reverted by LRRK2 kinase inhibition. These data reveal a common molecular pathway by which enhanced LRRK2 kinase activity impacts upon centrosome-related events to alter the normal biology of a cell.

Summary: The Parkinson's disease LRRK2 signaling pathway converges upon the formation of a complex at the subdistal appendage of the mother centriole which causes centrosomal deficits and impairs appropriate cell polarization.

ENKD1 is a centrosomal and ciliary microtubule-associated protein important for primary cilium content regulation

Centrioles and cilia are conserved, microtubule-based structures critical for cell function and development. Their dysfunction causes cancer and developmental disorders. How microtubules are organized into ordered structures by microtubule-associated proteins (MAPs) and tubulin modifications is best understood during mitosis but is largely unexplored for the centrioles and the ciliary axoneme, which are composed of stable microtubules that maintain their length at a steady-state. In particular, we know little about the identity of the centriolar and ciliary MAPs and how they work together during the assembly and maintenance of the cilium and centriole. Here, we identified the Enkurin domain containing 1 (ENKD1) as a component of the centriole wall and the axoneme in mammalian cells and showed that it has extensive proximity interactions with these compartments and MAPs. Using in vitro and cellular assays, we found that ENKD1 is a new MAP that regulates microtubule organization and stability. Consistently, we observed an increase in tubulin polymerization and microtubule stability, as well as disrupted microtubule organization in ENKD1 overexpression. Cells depleted for ENKD1 were defective in ciliary length and content regulation and failed to respond to Hedgehog pathway activation. Together, our results advance our understanding of the functional and regulatory relationship between MAPs and the primary cilium.

The CGG repeat expansion in RILPL1 is associated with oculopharyngodistal myopathy type 4

Recent studies indicate that CGG repeat expansions in LRP12, GIPC1, and NOTCH2NLC are associated with oculopharyngodistal myopathy (OPDM) types 1, 2, and 3, respectively. However, some clinicopathologically confirmed OPDM cases continue to have unknown genetic causes. Here, through a combination of long-read whole-genome sequencing (LRS), repeat-primed polymerase chain reaction (RP-PCR), and fluorescence amplicon length analysis PCR (AL-PCR), we found that a CGG repeat expansion in the 5' UTR of RILPL1 is associated with familial and simplex OPDM type 4 (OPDM4). The number of repeats ranged from 139 to 197. Methylation analysis indicates that the methylation levels in RILPL1 were unaltered in OPDM4 individuals. Analyses of muscle biopsies suggested that the expanded CGG repeat might be translated into a toxic poly-glycine protein that co-localizes with p62 in intranuclear inclusions. Moreover, analyses suggest that the toxic RNA gain-of-function effects also contributed to the pathogenesis of this disease. Intriguingly, all four types of OPDM have been found to be associated with the CGG repeat expansions located in 5' UTRs. This finding suggests that a common pathogenic mechanism, driven by the CGG repeat expansion, might underlie all cases of OPDM.

Rab GTPases in Parkinson's disease: a primer

Parkinson's disease is a prominent and debilitating movement disorder characterized by the death of vulnerable neurons which share a set of structural and physiological properties. Over the recent years, increasing evidence indicates that Rab GTPases can directly as well as indirectly contribute to the cellular alterations leading to PD. Rab GTPases are master regulators of intracellular membrane trafficking events, and alterations in certain membrane trafficking steps can be particularly disruptive to vulnerable neurons. Here, we describe current knowledge on the direct links between altered Rab protein function and PD pathomechanisms.

Prenatal alcohol exposure disrupts Sonic hedgehog pathway and primary cilia genes in the mouse neural tube

Neurulation-stage alcohol exposure (NAE; embryonic day [E] 8-10) is associated with midline craniofacial and CNS defects that likely arise from disruption of morphogen pathways, such as Sonic hedgehog (Shh). Notably, midline anomalies are also a hallmark of genetic ciliopathies such as Joubert syndrome. We tested whether NAE alters Shh pathway signaling and the number and function of primary cilia, organelles critical for Shh pathway transduction. Female C57BL/6 J mice were administered two doses of alcohol (2.9 g/kg/dose) or vehicle on E9. Embryos were collected 6, 12, or 24 h later, and changes to Shh, cell cycle genes, and primary cilia were measured in the rostroventral neural tube (RVNT). Within the first 24 h post-NAE, reductions in Shh pathway and cell cycle gene expression and the ratio of Gli3 forms in the full-length activator state were observed. RVNT volume and cell layer width were reduced at 12 h. In addition, altered expression of multiple cilia-related genes was observed at 6 h post-NAE. As a further test of cilia gene-ethanol interaction, mice heterozygous for Kif3a exhibited perturbed behavior during adolescence following NAE compared to vehicle-treated mice, and Kif3a heterozygosity exacerbated the hyperactive effects of NAE on exploratory activity. These data demonstrate that NAE downregulates the Shh pathway in a region of the neural tube that gives rise to alcohol-sensitive brain structures and identifies disruption of primary cilia function, or a "transient ciliopathy", as a possible cellular mechanism of prenatal alcohol pathogenesis.

The Regulation of Rab GTPases by Phosphorylation

Rab proteins are small GTPases that act as molecular switches for intracellular vesicle trafficking. Although their function is mainly regulated by regulatory proteins such as GTPase-activating proteins and guanine nucleotide exchange factors, recent studies have shown that some Rab proteins are physiologically phosphorylated in the switch II region by Rab kinases. As the switch II region of Rab proteins undergoes a conformational change depending on the bound nucleotide, it plays an essential role in their function as a ‘switch’. Initially, the phosphorylation of Rab proteins in the switch II region was shown to inhibit the association with regulatory proteins. However, recent studies suggest that it also regulates the binding of Rab proteins to effector proteins, determining which pathways to regulate. These findings suggest that the regulation of the Rab function may be more dynamically regulated by phosphorylation than just through the association with regulatory proteins. In this review, we summarize the recent findings and discuss the physiological and pathological roles of Rab phosphorylation.

Rab34 is necessary for early stages of intracellular ciliogenesis

Primary cilia are sensory organelles present on most vertebrate cells and are critical for development and health. Ciliary dysfunction is associated with a large class of human pathologies collectively known as ciliopathies. These include cystic kidneys, blindness, obesity, skeletal malformations, and other organ anomalies. Using a proximity biotinylation with Ift27 as bait, we identified the small guanosine triphosphatase (GTPase) Rab34 as a ciliary protein. Rab34 localizes to the centrosomes near the mother centriole, the axoneme of developed cilia, and highly dynamic tubule structures in the centrosomal region. Rab34 is required for cilia formation in fibroblasts, where we find that Rab34 loss blocks ciliogenesis at an early step of ciliary vesicle formation. In inner medullary collecting duct (IMCD3) epithelial cells, the requirement is more complex, with Rab34 needed in cells grown at low density but becoming less important as cell density increases. Ciliogenesis can proceed by an internal pathway where cilia form in the cytoplasm before being displayed on the ciliary surface or cilia can assemble by an external pathway where the centriole docks on the plasma membrane before ciliary assembly. Fibroblasts are thought to use the internal pathway, although IMCD3 cells are thought to use the external pathway. However, we find that IMCD3 cells can use the internal assembly pathway and significant numbers of internally assembling cilia are observed in low-density cells. Together, our work indicates that Rab34 is required for internal assembly of cilia, but not for cilia built on the cell surface.

LRRK2-phosphorylated Rab10 sequesters Myosin Va with RILPL2 during ciliogenesis blockade

Pathogenic LRRK2 phosphorylation of Rab10 GTPase dramatically redistributes Myosin Va and RILPL2 proteins to the mother centriole and sequesters Myosin Va at that location in a manner that likely interferes with its role in ciliogenesis.

Activating mutations in LRRK2 kinase causes Parkinson’s disease. Pathogenic LRRK2 phosphorylates a subset of Rab GTPases and blocks ciliogenesis. Thus, defining novel phospho-Rab interacting partners is critical to our understanding of the molecular basis of LRRK2 pathogenesis. RILPL2 binds with strong preference to LRRK2-phosphorylated Rab8A and Rab10. RILPL2 is a binding partner of the motor protein and Rab effector, Myosin Va. We show here that the globular tail domain of Myosin Va also contains a high affinity binding site for LRRK2-phosphorylated Rab10. In the presence of pathogenic LRRK2, RILPL2 and MyoVa relocalize to the peri-centriolar region in a phosphoRab10-dependent manner. PhosphoRab10 retains Myosin Va over pericentriolar membranes as determined by fluorescence loss in photobleaching microscopy. Without pathogenic LRRK2, RILPL2 is not essential for ciliogenesis but RILPL2 over-expression blocks ciliogenesis in RPE cells independent of tau tubulin kinase recruitment to the mother centriole. These experiments show that LRRK2 generated-phosphoRab10 dramatically redistributes a significant fraction of Myosin Va and RILPL2 to the mother centriole in a manner that likely interferes with Myosin Va’s role in ciliogenesis.

Pathogenic LRRK2 regulates ciliation probability upstream of tau tubulin kinase 2 via Rab10 and RILPL1 proteins

Mutations that activate LRRK2 protein kinase cause Parkinson's disease. We showed previously that Rab10 phosphorylation by LRRK2 enhances its binding to RILPL1, and together, these proteins block cilia formation in a variety of cell types, including patient derived iPS cells. We have used live-cell fluorescence microscopy to identify, more precisely, the effect of LRRK2 kinase activity on both the formation of cilia triggered by serum starvation and the loss of cilia seen upon serum readdition. LRRK2 activity decreases the overall probability of ciliation without changing the rates of cilia formation in R1441C LRRK2 MEF cells. Cilia loss in these cells is accompanied by ciliary decapitation, and kinase activity does not change the timing or frequency of decapitation or the rate of cilia loss but increases the percent of cilia that are lost upon serum addition. LRRK2 activity, or overexpression of RILPL1 protein, blocks release of CP110 from the mother centriole, a step normally required for early ciliogenesis; LRRK2 blockade of CP110 uncapping requires Rab10 and RILPL1 proteins and is due to failure to recruit TTBK2, a kinase needed for CP110 release. In contrast, deciliation probability does not change in cells lacking Rab10 or RILPL1 and relies on a distinct LRRK2 pathway. These experiments provide critical detail to our understanding of the cellular consequences of pathogenic LRRK2 mutation and indicate that LRRK2 blocks ciliogenesis upstream of TTBK2 and enhances the deciliation process in response to serum addition.

A comprehensive analysis of Rab GTPases reveals a role for Rab34 in serum starvation-induced primary ciliogenesis

Primary cilia are sensors of chemical and mechanical signals in the extracellular environment. The formation of primary cilia (i.e. ciliogenesis) requires dynamic membrane trafficking events, and several Rab small GTPases, key regulators of membrane trafficking, have recently been reported to participate in ciliogenesis. However, the precise mechanisms of Rab-mediated membrane trafficking during ciliogenesis remain largely unknown. In the present study, we used a collection of siRNAs against 62 human Rabs to perform a comprehensive knockdown screening for Rabs that regulate serum starvation-induced ciliogenesis in human telomerase reverse transcriptase retinal pigment epithelium 1 (hTERT-RPE1) cells and succeeded in identifying Rab34 as an essential Rab. Knockout (KO) of Rab34, but not of Rabs previously reported to regulate ciliogenesis (e.g. Rab8 and Rab10) in hTERT-RPE1 cells, drastically impaired serum starvation-induced ciliogenesis. Rab34 was also required for serum starvation-induced ciliogenesis in NIH/3T3 cells and MCF10A cells but not for ciliogenesis in Madin-Darby canine kidney (MDCK)-II cysts. We then attempted to identify a specific region(s) of Rab34 that is essential for ciliogenesis by performing deletion and mutation analyses of Rab34. Unexpectedly, instead of a specific sequence in the switch II region, which is generally important for recognizing effector proteins (e.g. Rab interacting lysosomal protein [RILP]), a unique long N-terminal region of Rab34 before the conserved GTPase domain was found to be essential. These findings suggest that Rab34 is an atypical Rab that regulates serum starvation-induced ciliogenesis through its unique N-terminal region.

X Caps the Phosphate for Phospho-Rab GTPase Recognition in Ciliogenesis and Parkinson's Disease

Some Rab GTPases, after activation by GDP to GTP exchange, are phosphorylated by the LRRK2 kinase implicated in Parkinson's disease. In the current issue of Structure, Waschbüsch et al. (2020) investigate the structural basis for recognition of active phospho-Rab GTPases by the RH2 domain of the effector protein RILPL2.

Transcriptome-Wide Regulation of Key Developmental Pathways in the Mouse Neural Tube by Prenatal Alcohol Exposure

BACKGROUND

Early gestational alcohol exposure is associated with severe craniofacial and CNS dysmorphologies and behavioral abnormalities during adolescence and adulthood. Alcohol exposure during the formation of the neural tube (gestational day [GD] 8 to 10 in mice; equivalent to4th week of human pregnancy) disrupts development of ventral midline brain structures such as the pituitary, septum, and ventricles. This study identifies transcriptomic changes in the rostroventral neural tube (RVNT), the region of the neural tube that gives rise to the midline structures sensitive to alcohol exposure during neurulation.

METHODS

Female C57BL/6J mice were administered 2 doses of alcohol (2.9 g/kg) or vehicle 4 hours apart on GD 9.0. The RVNTs of embryos were collected 6 or 24 hours after the first dose and processed for RNA-seq.

RESULTS

Six hours following GD 9.0 alcohol exposure (GD 9.25), over 2,300 genes in the RVNT were determined to be differentially regulated by alcohol. Enrichment analysis determined that PAE affected pathways related to cell proliferation, p53 signaling, ribosome biogenesis, and immune activation. In addition, over 100 genes involved in primary cilia formation and function and regulation of morphogenic pathways were altered 6 hours after alcohol exposure. The changes to gene expression were largely transient, as only 91 genes identified as differentially regulated by prenatal alcohol at GD 10 (24 hours postexposure). Functionally, the differentially regulated genes at GD 10 were related to organogenesis and cell migration.

CONCLUSIONS

These data give a comprehensive view of the changing landscape of the embryonic transcriptome networks in regions of the neural tube that give rise to brain structures impacted by a neurulation-stage alcohol exposure. Identification of gene networks dysregulated by alcohol will help elucidate the pathogenic mechanisms of alcohol's actions.

Structural Basis for Rab8a Recruitment of RILPL2 via LRRK2 Phosphorylation of Switch 2

Rab8a is associated with the dynamic regulation of membrane protrusions in polarized cells. Rab8a is one of several Rab GTPases that are substrates of leucine-rich repeat kinase 2 (LRRK2), a serine/threonine kinase that is linked to Parkinson's disease. Rab8a is phosphorylated at T72 (pT72) in its switch 2 helix and recruits the phospho-specific effector RILPL2, which subsequently regulates ciliogenesis. Here, we report the crystal structure of phospho-Rab8a (pRab8a) in complex with the RH2 (RILP homology) domain of RILPL2. The complex is a heterotetramer with RILPL2 forming a central α-helical dimer that bridges two pRab8a molecules. The N termini of the α helices cross over, forming an X-shaped cap (X-cap) that orients Arg residues from RILPL2 toward pT72. X-cap residues critical for pRab8a binding are conserved in JIP3 and JIP4, which also interact with LRRK2-phosphorylated Rab10. We propose a general mode of recognition for phosphorylated Rab GTPases by this family of phospho-specific effectors.

Advances in elucidating the function of leucine-rich repeat protein kinase-2 in normal cells and Parkinson's disease

Autosomal dominant missense mutations that hyperactivate the leucine-rich repeat protein kinase-2 (LRRK2) are a common cause of inherited Parkinson's disease and therapeutic efficacy of LRRK2 inhibitors is being tested in clinical trials. In this review, we discuss the nuts and bolts of our current understanding of how the LRRK2 is misregulated by mutations and how pathway activity is affected by LRRK2 binding to membrane, microtubule filaments, and 14-3-3, as well as by upstream components such as Rab29 and VPS35. We discuss recent work that points toward a subset of Rab proteins comprising key physiological substrates that bind new sets of effectors, such as RILPL1/2, JIP3 and JIP4 after phosphorylation by LRRK2. We explore what is known about how LRRK2 regulates ciliogenesis, the endosomal-lysosomal system, immune responses and interplay with alpha-synuclein and tau and how this might be linked to Parkinson's' disease.

Transport of Ciliary Membrane Proteins

Cilia and flagella are highly conserved organelles in eukaryotic cells that drive cell movement and act as cell antennae that receive and transmit signals. In addition to receiving and transducing external signals that activate signal cascades, cilia also secrete ciliary ectosomes that send signals to recipient cells, and thereby mediate cell–cell communication. Abnormal ciliary function leads to various ciliopathies, and the precise transport and localization of ciliary membrane proteins are essential for cilium function. This review summarizes current knowledge about the transport processes of ciliary membrane proteins after their synthesis at the endoplasmic reticulum: modification and sorting in the Golgi apparatus, transport through vesicles to the ciliary base, entrance into cilia through the diffusion barrier, and turnover by ectosome secretion. The molecular mechanisms and regulation involved in each step are also discussed. Transport of ciliary membrane proteins is a complex, precise cellular process coordinated among multiple organelles. By systematically analyzing the existing research, we identify topics that should be further investigated to promote progress in this field of research.

Novel fibrillar structure in the inversin compartment of primary cilia revealed by 3D single-molecule superresolution microscopy

Primary cilia in many cell types contain a periaxonemal subcompartment called the inversin compartment. Four proteins have been found to assemble within the inversin compartment: INVS, ANKS6, NEK8, and NPHP3. The function of the inversin compartment is unknown, but it appears to be critical for normal development, including left–right asymmetry and renal tissue homeostasis. Here we combine superresolution imaging of human RPE1 cells, a classic model for studying primary cilia in vitro, with a genetic dissection of the protein–protein binding relationships that organize compartment assembly to develop a new structural model. We observe that INVS is the core structural determinant of a compartment composed of novel fibril-like substructures, which we identify here by three-dimensional single-molecule superresolution imaging. We find that NEK8 and ANKS6 depend on INVS for localization to these fibrillar assemblies and that ANKS6-NEK8 density within the compartment is regulated by NEK8. Together, NEK8 and ANKS6 are required downstream of INVS to localize and concentrate NPHP3 within the compartment. In the absence of these upstream components, NPHP3 is redistributed within cilia. These results provide a more detailed structure for the inversin compartment and introduce a new example of a membraneless compartment organized by protein–protein interactions.

RAB8, RAB10 and RILPL1 contribute to both LRRK2 kinase-mediated centrosomal cohesion and ciliogenesis deficits

Mutations in the LRRK2 kinase are the most common cause of familial Parkinson's disease, and variants increase risk for the sporadic form of the disease. LRRK2 phosphorylates multiple RAB GTPases including RAB8A and RAB10. Phosphorylated RAB10 is recruited to centrosome-localized RILPL1, which may interfere with ciliogenesis in a disease-relevant context. Our previous studies indicate that the centrosomal accumulation of phosphorylated RAB8A causes centrosomal cohesion deficits in dividing cells, including in peripheral patient-derived cells. Here, we show that both RAB8 and RAB10 contribute to the centrosomal cohesion deficits. Pathogenic LRRK2 causes the centrosomal accumulation not only of phosho-RAB8 but also of phospho-RAB10, and the effects on centrosomal cohesion are dependent on RAB8, RAB10 and RILPL1. Conversely, the pathogenic LRRK2-mediated ciliogenesis defects correlate with the centrosomal accumulation of both phospho-RAB8 and phospho-RAB10. LRRK2-mediated centrosomal cohesion and ciliogenesis alterations are observed in patient-derived peripheral cells, as well as in primary astrocytes from mutant LRRK2 mice, and are reverted upon LRRK2 kinase inhibition. These data suggest that the LRRK2-mediated centrosomal cohesion and ciliogenesis defects are distinct cellular readouts of the same underlying phospho-RAB8/RAB10/RILPL1 nexus and highlight the possibility that either centrosomal cohesion and/or ciliogenesis alterations may serve as cellular biomarkers for LRRK2-related PD.

Physiological and pathological functions of LRRK2: implications from substrate proteins

Leucine-rich repeat kinase 2 (LRRK2) encodes a 2527-amino acid (aa) protein composed of multiple functional domains, including a Ras of complex proteins (ROC)-type GTP-binding domain, a carboxyl terminal of ROC (COR) domain, a serine/threonine protein kinase domain, and several repeat domains. LRRK2 is genetically involved in the pathogenesis of both sporadic and familial Parkinson's disease (FPD). Parkinson's disease (PD) is the second most common neurodegenerative disorder, manifesting progressive motor dysfunction. PD is pathologically characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta, and the presence of intracellular inclusion bodies called Lewy bodies (LB) in the remaining neurons. As the most frequent PD-causing mutation in LRRK2, G2019S, increases the kinase activity of LRRK2, an abnormal increase in LRRK2 kinase activity is believed to contribute to PD pathology; however, the precise biological functions of LRRK2 involved in PD pathogenesis remain unknown. Although biochemical studies have discovered several substrate proteins of LRRK2 including Rab GTPases and tau, little is known about whether excess phosphorylation of these substrates is the cause of the neurodegeneration in PD. In this review, we summarize latest findings regarding the physiological and pathological functions of LRRK2, and discuss the possible molecular mechanisms of neurodegeneration caused by LRRK2 and its substrates.

RAB7L1-Mediated Relocalization of LRRK2 to the Golgi Complex Causes Centrosomal Deficits via RAB8A

Mutations in the LRRK2 gene cause autosomal-dominant Parkinson’s disease (PD), and both LRRK2 as well as RAB7L1 have been implicated in increased susceptibility to idiopathic PD. RAB7L1 has been shown to increase membrane-association and kinase activity of LRRK2, and both seem to be mechanistically implicated in the same pathway. Another RAB protein, RAB8A, has been identified as a prominent LRRK2 kinase substrate, and our recent work demonstrates that aberrant LRRK2-mediated phosphorylation of RAB8A leads to centrosomal alterations. Here, we show that RAB7L1 recruits LRRK2 to the Golgi complex, which causes accumulation of phosphorylated RAB8A in a pericentrosomal/centrosomal location as well as centrosomal deficits identical to those observed with pathogenic LRRK2. The centrosomal alterations induced by wildtype LRRK2 in the presence of RAB7L1 depend on Golgi integrity. This is in contrast to pathogenic LRRK2 mutants, which cause centrosomal deficits independent of Golgi integrity or largely independent on RAB7L1 expression. Furthermore, centrosomal alterations in the presence of wildtype LRRK2 and RAB7L1 are at least in part mediated by aberrant LRRK2-mediated RAB8A phosphorylation, as abolished by kinase inhibitors and reduced upon knockdown of RAB8A. These results indicate that pathogenic LRRK2, as well as increased levels of RAB7L1, cause centrosomal deficits in a manner dependent on aberrant RAB8A phosphorylation and centrosomal/pericentrosomal accumulation, suggesting that centrosomal cohesion deficits may comprise a useful cellular readout for a broader spectrum of the disease.

ALMS1 and Alström syndrome: a recessive form of metabolic, neurosensory and cardiac deficits

Alström syndrome (AS) is characterised by metabolic deficits, retinal dystrophy, sensorineural hearing loss, dilated cardiomyopathy and multi-organ fibrosis. Elucidating the function of the mutated gene, ALMS1, is critical for the development of specific treatments and may uncover pathways relevant to a range of other disorders including common forms of obesity and type 2 diabetes. Interest in ALMS1 is heightened by the recent discovery of its involvement in neonatal cardiomyocyte cell cycle arrest, a process with potential relevance to regenerative medicine. ALMS1 encodes a ~ 0.5 megadalton protein that localises to the base of centrioles. Some studies have suggested a role for this protein in maintaining centriole-nucleated sensory organelles termed primary cilia, and AS is now considered to belong to the growing class of human genetic disorders linked to ciliary dysfunction (ciliopathies). However, mechanistic details are lacking, and recent studies have implicated ALMS1 in several processes including endosomal trafficking, actin organisation, maintenance of centrosome cohesion and transcription. In line with a more complex picture, multiple isoforms of the protein likely exist and non-centrosomal sites of localisation have been reported. This review outlines the evidence for both ciliary and extra-ciliary functions of ALMS1.

A pathway for Parkinson's Disease LRRK2 kinase to block primary cilia and Sonic hedgehog signaling in the brain

Parkinson’s disease-associated LRRK2 kinase phosphorylates multiple Rab GTPases, including Rab8A and Rab10. We show here that LRRK2 kinase interferes with primary cilia formation in cultured cells, human LRRK2 G2019S iPS cells and in the cortex of LRRK2 R1441C mice. Rab10 phosphorylation strengthens its intrinsic ability to block ciliogenesis by enhancing binding to RILPL1. Importantly, the ability of LRRK2 to interfere with ciliogenesis requires both Rab10 and RILPL1 proteins. Pathogenic LRRK2 influences the ability of cells to respond to cilia-dependent, Hedgehog signaling as monitored by Gli1 transcriptional activation. Moreover, cholinergic neurons in the striatum of LRRK2 R1441C mice show decreased ciliation, which will decrease their ability to sense Sonic hedgehog in a neuro-protective circuit that supports dopaminergic neurons. These data reveal a molecular pathway for regulating cilia function that likely contributes to Parkinson’s disease-specific pathology.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

The small G protein Arl8 contributes to lysosomal function and long-range axonal transport in Drosophila

The small GTPase Arl8 has emerged as a major regulatory GTPase on lysosomes. Studies in mammalian cells have shown that it regulates both fusion with late endosomes and also lysosomal motility. In its active GTP-bound state, it recruits to lysosomes the HOPS (homotypic fusion and protein sorting) endosomal tethering complex and also proteins that link lysosomes to microtubule motors such as the kinesin adaptor PLEKHM2. To gain further insights into Arl8 biology, we examined the single Drosophila ortholog. Drosophila Arl8 is essential for viability, and mitotic clones of mutant cells are able to continue to divide but show perturbation of the late endocytic pathway. Progeny-lacking Arl8 die as late larvae with movement-paralysis characteristic of defects in neuronal function. This phenotype was rescued by expression of Arl8 in motor neurons. Examination of these neurons in the mutant larvae revealed smaller synapses and axons with elevated levels of carriers containing synaptic components. Affinity chromatography revealed binding of Drosophila Arl8 to the HOPS complex, and to the Drosophila ortholog of RILP, a protein that, in mammals, recruits dynein to late endosomes, with dynein being known to be required for neuronal transport. Thus Drosophila Arl8 controls late endocytic function and transport via at least two distinct effectors.

This article has an associated First Person interview with the first author of the paper.

Summary:Drosophila Arl8 is essential for viability and is required for normal functioning of the late endocytic pathway and for long-range transport in axons.

Regulation of ciliary membrane protein trafficking and signalling by kinesin motor proteins

Primary cilia are antenna-like sensory organelles that regulate a substantial number of cellular signalling pathways in vertebrates, both during embryonic development as well as in adulthood, and mutations in genes coding for ciliary proteins are causative of an expanding group of pleiotropic diseases known as ciliopathies. Cilia consist of a microtubule-based axoneme core, which is subtended by a basal body and covered by a bilayer lipid membrane of unique protein and lipid composition. Cilia are dynamic organelles, and the ability of cells to regulate ciliary protein and lipid content in response to specific cellular and environmental cues is crucial for balancing ciliary signalling output. Here we discuss mechanisms involved in regulation of ciliary membrane protein trafficking and signalling, with main focus on kinesin-2 and kinesin-3 family members.

The Parkinson's disease VPS35[D620N] mutation enhances LRRK2-mediated Rab protein phosphorylation in mouse and human

Missense mutations in the LRRK2 (Leucine-rich repeat protein kinase-2) and VPS35 genes result in autosomal dominant Parkinson's disease. The VPS35 gene encodes for the cargo-binding component of the retromer complex, while LRRK2 modulates vesicular trafficking by phosphorylating a subgroup of Rab proteins. Pathogenic mutations in LRRK2 increase its kinase activity. It is not known how the only thus far described pathogenic VPS35 mutation, [p.D620N] exerts its effects. We reveal that the VPS35[D620N] knock-in mutation strikingly elevates LRRK2-mediated phosphorylation of Rab8A, Rab10, and Rab12 in mouse embryonic fibroblasts. The VPS35[D620N] mutation also increases Rab10 phosphorylation in mouse tissues (the lung, kidney, spleen, and brain). Furthermore, LRRK2-mediated Rab10 phosphorylation is increased in neutrophils as well as monocytes isolated from three Parkinson's patients with a heterozygous VPS35[D620N] mutation compared with healthy donors and idiopathic Parkinson's patients. LRRK2-mediated Rab10 phosphorylation is significantly suppressed by knock-out or knock-down of VPS35 in wild-type, LRRK2[R1441C], or VPS35[D620N] cells. Finally, VPS35[D620N] mutation promotes Rab10 phosphorylation more potently than LRRK2 pathogenic mutations. Available data suggest that Parkinson's patients with VPS35[D620N] develop the disease at a younger age than those with LRRK2 mutations. Our observations indicate that VPS35 controls LRRK2 activity and that the VPS35[D620N] mutation results in a gain of function, potentially causing PD through hyperactivation of the LRRK2 kinase. Our findings suggest that it may be possible to elaborate compounds that target the retromer complex to suppress LRRK2 activity. Moreover, patients with VPS35[D620N] associated Parkinson's might benefit from LRRK2 inhibitor treatment that have entered clinical trials in humans.

Systematic proteomic analysis of LRRK2-mediated Rab GTPase phosphorylation establishes a connection to ciliogenesis

We previously reported that Parkinson’s disease (PD) kinase LRRK2 phosphorylates a subset of Rab GTPases on a conserved residue in their switch-II domains (Steger et al., 2016) (PMID: 26824392). Here, we systematically analyzed the Rab protein family and found 14 of them (Rab3A/B/C/D, Rab5A/B/C, Rab8A/B, Rab10, Rab12, Rab29, Rab35 and Rab43) to be specifically phosphorylated by LRRK2, with evidence for endogenous phosphorylation for ten of them (Rab3A/B/C/D, Rab8A/B, Rab10, Rab12, Rab35 and Rab43). Affinity enrichment mass spectrometry revealed that the primary ciliogenesis regulator, RILPL1 specifically interacts with the LRRK2-phosphorylated forms of Rab8A and Rab10, whereas RILPL2 binds to phosphorylated Rab8A, Rab10, and Rab12. Induction of primary cilia formation by serum starvation led to a two-fold reduction in ciliogenesis in fibroblasts derived from pathogenic LRRK2-R1441G knock-in mice. These results implicate LRRK2 in primary ciliogenesis and suggest that Rab-mediated protein transport and/or signaling defects at cilia may contribute to LRRK2-dependent pathologies.

The molecular motor Myosin Va interacts with the cilia-centrosomal protein RPGRIP1L

Myosin Va (MyoVa) is an actin-based molecular motor abundantly found at the centrosome. However, the role of MyoVa at this organelle has been elusive due to the lack of evidence on interacting partners or functional data. Herein, we combined yeast two-hybrid screen, biochemical studies and cellular assays to demonstrate that MyoVa interacts with RPGRIP1L, a cilia-centrosomal protein that controls ciliary signaling and positioning. MyoVa binds to the C2 domains of RPGRIP1L via residues located near or in the Rab11a-binding site, a conserved site in the globular tail domain (GTD) from class V myosins. According to proximity ligation assays, MyoVa and RPGRIP1L can interact near the cilium base in ciliated RPE cells. Furthermore, we showed that RPE cells expressing dominant-negative constructs of MyoVa are mostly unciliated, providing the first experimental evidence about a possible link between this molecular motor and cilia-related processes.

Cby1 promotes Ahi1 recruitment to a ring-shaped domain at the centriole-cilium interface and facilitates proper cilium formation and function

Cby1 localizes to centrioles and antagonizes canonical Wnt signaling. Cby1^−/− mice have cystic kidneys, and Cby1 facilitates primary cilium formation and ciliary recruitment of Arl13b. Cby1 localizes to a distal centriolar domain with Ofd1 and Ahi1, and the amount of Ahi1 at the transition zone is reduced in Cby1^−/− cells.

Defects in centrosome and cilium function are associated with phenotypically related syndromes called ciliopathies. Cby1, the mammalian orthologue of the Drosophila Chibby protein, localizes to mature centrioles, is important for ciliogenesis in multiciliated airway epithelia in mice, and antagonizes canonical Wnt signaling via direct regulation of β-catenin. We report that deletion of the mouse Cby1 gene results in cystic kidneys, a phenotype common to ciliopathies, and that Cby1 facilitates the formation of primary cilia and ciliary recruitment of the Joubert syndrome protein Arl13b. Localization of Cby1 to the distal end of mature centrioles depends on the centriole protein Ofd1. Superresolution microscopy using both three-dimensional SIM and STED reveals that Cby1 localizes to an ∼250-nm ring at the distal end of the mature centriole, in close proximity to Ofd1 and Ahi1, a component of the transition zone between centriole and cilium. The amount of centriole-localized Ahi1, but not Ofd1, is reduced in Cby1^−/− cells. This suggests that Cby1 is required for efficient recruitment of Ahi1, providing a possible molecular mechanism for the ciliogenesis defect in Cby1^−/− cells.

Structural insights into functional overlapping and differentiation among myosin V motors

Myosin V (MyoV) motors have been implicated in the intracellular transport of diverse cargoes including vesicles, organelles, RNA-protein complexes, and regulatory proteins. Here, we have solved the cargo-binding domain (CBD) structures of the three human MyoV paralogs (Va, Vb, and Vc), revealing subtle structural changes that drive functional differentiation and a novel redox mechanism controlling the CBD dimerization process, which is unique for the MyoVc subclass. Moreover, the cargo- and motor-binding sites were structurally assigned, indicating the conservation of residues involved in the recognition of adaptors for peroxisome transport and providing high resolution insights into motor domain inhibition by CBD. These results contribute to understanding the structural requirements for cargo transport, autoinhibition, and regulatory mechanisms in myosin V motors.