Cargo adapters expand the transport range of intraflagellar transport

Cilia at the crossroad: convergence of regulatory mechanisms to govern cilia dynamics during cell signaling and the cell cycle

Cilia are versatile, microtubule-based organelles that facilitate cellular signaling, motility, and environmental sensing in eukaryotic cells. These dynamic structures act as hubs for key developmental signaling pathways, while their assembly and disassembly are intricately regulated along cell cycle transitions. Recent findings show that factors regulating ciliogenesis and cilia dynamics often integrate their roles across other cellular processes, including cell cycle regulation, cytoskeletal organization, and intracellular trafficking, ensuring multilevel crosstalk of mechanisms controlling organogenesis. Disruptions in these shared regulators lead to broad defects associated with both ciliopathies and cancer. This review explores the crosstalk of regulatory mechanisms governing cilia assembly, disassembly, and maintenance during ciliary signaling and the cell cycle, along with the broader implications for development, tissue homeostasis, and disease.

An amino acid-resolution interactome for motile cilia identifies the structure and function of ciliopathy protein complexes

Motile cilia are ancient, evolutionarily conserved organelles whose dysfunction underlies motile ciliopathies, a broad class of human diseases. Motile cilia contain a myriad of different proteins that assemble into an array of distinct machines, and understanding the interactions and functional hierarchies among them presents an important challenge. Here, we defined the protein interactome of motile axonemes using cross-linking mass spectrometry in Tetrahymena thermophila. From over 19,000 cross-links, we identified over 4,700 unique amino acid interactions among over 1,100 distinct proteins, providing both macromolecular and atomic-scale insights into diverse ciliary machines, including the intraflagellar transport system, axonemal dynein arms, radial spokes, the 96-nm ruler, and microtubule inner proteins. Guided by this dataset, we used vertebrate multiciliated cells to reveal functional interactions among several poorly defined human ciliopathy proteins. This dataset provides a resource for studying the biology of an ancient organelle and the molecular etiology of human genetic disease.

Molecular mechanisms of the obesity associated with Bardet-Biedl syndrome: An update

Obesity is a prominent feature of Bardet-Biedl syndrome (BBS), which represents a major and growing public health problem. More than half of BBS patients carry mutations in one of eight genes that encode subunits of a protein complex known as the BBSome, which has emerged as a key regulator of energy and glucose homeostasis. However, the mechanisms underlying obesity in BBS are complex. Numerous studies have identified a high prevalence of insulin resistance and metabolic syndrome among individuals with BBS. However, the exact mechanisms are not fully understood. This review summarized evidence from experiments using mouse and cell models, focusing on the energy imbalance that leads to obesity in patients with BBS. The studies discussed in this review contribute to understanding the functional role of the BBSome in the obesity associated with BBS, laying the foundation for developing new preventive or therapeutic strategies for obese patients.

Integrative in silico and biochemical analyses demonstrate direct Arl3-mediated ODA16 release from the intraflagellar transport machinery

Outer dynein arms (ODAs) are essential for ciliary motility and are preassembled in the cytoplasm before trafficking into cilia by intraflagellar transport (IFT). ODA16 is a key adaptor protein that links ODAs to the IFT machinery via direct interaction with the IFT46 protein. However, the molecular mechanisms regulating the assembly, transport, and release of ODAs remain poorly understood. Here, we employ AlphaPulldown, an in silico screening method, to identify direct interactors of ODA16, including the dynein adaptor IDA3 and the small GTPase Arl3. We use structural modeling, biochemical, and biophysical assays on Chlamydomonas and human proteins to elucidate the interactions and regulatory mechanisms governing the IFT of ODAs. We identify a conserved N-terminal motif in Chlamydomonas IFT46 that mediates its binding to one side of the ODA16 structure. Biochemical dissection reveals that IDA3 and Arl3 bind to the same surface of ODA16 (the C-terminal β-propeller face), which is opposite to the IFT46 binding site, enabling them to dissociate ODA16 from IFT46, likely through an allosteric mechanism. Our findings provide mechanistic insights into the concerted actions of IFT and adaptor proteins in ODA transport and regulation.

BBSome deficiency in Lotmaria passim reveals divergent functions in trypanosomatid parasites

BACKGROUND

The BBSome is an octameric protein complex crucial for ciliary transport, though it also participates in multiple other cellular processes. These diverse functions may result from the co-option of its ancestral roles. Studying the BBSome in flagellated protists can provide insights into these ancestral functions and their subsequent adaptations.

METHODS

We examined the functions of the BBSome (LpBBS1 and LpBBS2) in Lotmaria passim, a monoxenous trypanosomatid parasite infecting honey bees. The phenotypes resulting from depletion of LpBBS1 using the auxin-inducible degron system and disruption of LpBBS2 were characterized.

RESULTS

Parasites deficient in LpBBS2 are smaller and less motile compared with wild-type. Although intraflagellar transport of a marker membrane protein is only mildly impaired, its association with lipid rafts is significantly disrupted in the mutants. This suggests that the BBSome is essential for maintaining lipid raft integrity in L. passim. Transcriptomic comparisons between wild-type and LpBBS2-deficient parasites reveal that the BBSome may also influence processes related to metabolism, membrane localization of specific proteins, DNA repair, microtubules, and mitochondria.

CONCLUSIONS

In contrast to Leishmania mexicana, the BBSome in L. passim is crucial for efficient infection of the honey bee gut, demonstrating that its cellular functions vary between related trypanosomatid species. The BBSome is likely an adaptor that links multiple proteins in a species-specific manner under various cellular contexts.

ciBAR1 loss in mice causes laterality defects, pancreatic degeneration, and altered glucose tolerance

Bin/Amphiphysin/Rvs (BAR) domains are highly conserved domains found in all eukaryotes. BAR domain proteins form crescent-shaped dimers that sense and sculpt curved lipid membranes and play key roles in various cellular processes. However, their functions in mammalian development are poorly understood. We previously demonstrated that Chibby1-interacting BAR domain-containing 1 (ciBAR1, formerly known as FAM92A) localizes to the ciliary base and plays a critical role in ciliogenesis. Here, we report ciliopathy phenotypes of ciBAR1-KO mice. We found that ∼28% of ciBAR1-KO mice show embryonic lethality because of randomized left-right asymmetry; the rest survive into adulthood with no gross morphological abnormalities. Histological assessments of ciliated tissues revealed exocrine pancreatic lesions. Although overall endocrine islet morphology appeared to be normal, ciBAR1-KO mice showed impaired glucose tolerance. Examination of ductal and islet cilia revealed that cilia number and length were significantly reduced in ciBAR1-KO pancreata. ciBAR1-KO MEFs also exhibited ciliary defects. Our findings indicate that ciBAR1 plays a critical role in ciliogenesis depending on the tissue and cell type in mice.

Tonic ubiquitination of the central body weight regulator melanocortin receptor 4 (MC4R) promotes its constitutive exit from cilia

The G protein-coupled receptor (GPCR) melanocortin receptor 4 (MC4R) is an essential regulator of body weight homeostasis. MC4R is unusual among GPCRs in that its activity is regulated by 2 opposing physiological ligands, the agonist ⍺-MSH and the antagonist/inverse agonist AgRP. Paradoxically, while MC4R localizes and functions at the cilium of hypothalamic neurons, the ciliary levels of MC4R are very low under unrestricted feeding conditions. Here, we find that the constitutive activity of MC4R is responsible for the continuous depletion of MC4R from cilia and that inhibition of MC4R's activity via AgRP leads to a robust accumulation of MC4R in cilia. Ciliary targeting of MC4R is mediated by its partner MRAP2 and the constitutive exit of MC4R from cilia relies on the sensor of activation β-arrestin, on ubiquitination, and on the BBSome ciliary trafficking complex. Thus, while MC4R exits cilia via conventional mechanisms, it only accumulates in cilia when its activity is suppressed by AgRP.

The Role of Visual Electrophysiology in Systemic Hereditary Syndromes

Visual electrophysiology is a valuable tool for evaluating the visual system in various systemic syndromes. This review highlights its clinical application in a selection of syndromes associated with hearing loss, mitochondrial dysfunction, obesity, and other multisystem disorders. Techniques such as full-field electroretinography (ffERG), multifocal electroretinography (mfERG), pattern electroretinography (PERG), visual evoked potentials (VEP), and electrooculography (EOG) offer insights into retinal and optic nerve function, often detecting abnormalities before clinical symptoms manifest. In hearing loss syndromes like Refsum disease, Usher syndrome (USH), and Wolfram syndrome (WS), electrophysiology facilitates the detection of early retinal changes that precede the onset of visual symptoms. For mitochondrial disorders such as maternally-inherited diabetes and deafness (MIDD), Kearns-Sayre syndrome (KSS), and neuropathy, ataxia, and retinitis pigmentosa (NARP) syndrome, these tests can be useful in characterizing retinal degeneration and optic neuropathy. In obesity syndromes, including Bardet-Biedl syndrome (BBS), Alström syndrome, and Cohen syndrome, progressive retinal degeneration is a hallmark feature. Electrophysiological techniques aid in pinpointing retinal dysfunction and tracking disease progression. Other syndromes, such as Alagille syndrome (AGS), abetalipoproteinemia (ABL), Cockayne syndrome (CS), Joubert syndrome (JS), mucopolysaccharidosis (MPS), Neuronal ceroid lipofuscinoses (NCLs), and Senior-Løken syndrome (SLS), exhibit significant ocular involvement that can be evaluated using these methods. This review underscores the role of visual electrophysiology in diagnosing and monitoring visual system abnormalities across a range of syndromes, potentially offering valuable insights for early diagnosis, monitoring of progression, and management.

Melanocortin 1 receptor mediates melanin production by interacting with the BBSome in primary cilia

Production of melanin pigments is a protective mechanism of the skin against ultraviolet (UV)-induced damage and carcinogenesis. However, the molecular basis for melanogenesis is still poorly understood. Herein, we demonstrate a critical interplay between the primary cilium and the melanocortin 1 receptor (MC1R) signaling. Our data show that UV and α-melanocyte-stimulating hormone (α-MSH) trigger cilium formation in human melanocytes and melanoma cells. Deficiency of MC1R or the presence of its red hair color (RHC) variations significantly attenuates the UV/α-MSH-induced ciliogenesis. Further investigation reveals that MC1R enters the cilium upon UV/α-MSH stimulation, which is facilitated by the interaction of MC1R with the BBSome and the palmitoylation of MC1R. MC1R interacts with the BBSome through the second and third intercellular loops, which contain the common RHC variant alleles (R151C and R160W). These RHC variants of MC1R exhibit attenuated ciliary localization, and enforced ciliary localization of these variants elevates melanogenesis. Ciliary MC1R triggers a sustained cAMP signaling and selectively stimulates Sox9, which appears to up-regulate melanogenesis-related genes as the transcriptional cofactor for MITF. These findings reveal a previously unrecognized nexus between MC1R and cilia and suggest an important mechanism for RHC variant-related pigmentary defects.

The Role of HDAC6 in Glioblastoma Multiforme: A New Avenue to Therapeutic Interventions?

Despite the great advances in basic research results, glioblastoma multiforme (GBM) still remains an incurable tumour. To date, a GBM diagnosis is a death sentence within 15-18 months, due to the high recurrence rate and resistance to conventional radio- and chemotherapy approaches. The effort the scientific community is lavishing on the never-ending battle against GBM is reflected by the huge number of clinical trials launched, about 2003 on 10 September 2024. However, we are still far from both an in-depth comprehension of the biological and molecular processes leading to GBM onset and progression and, importantly, a cure. GBM is provided with high intratumoral heterogeneity, immunosuppressive capacity, and infiltrative ability due to neoangiogenesis. These features impact both tumour aggressiveness and therapeutic vulnerability, which is further limited by the presence in the tumour core of niches of glioblastoma stem cells (GSCs) that are responsible for the relapse of this brain neoplasm. Epigenetic alterations may both drive and develop along GBM progression and also rely on changes in the expression of the genes encoding histone-modifying enzymes, including histone deacetylases (HDACs). Among them, HDAC6-a cytoplasmic HDAC-has recently gained attention because of its role in modulating several biological aspects of GBM, including DNA repair ability, massive growth, radio- and chemoresistance, and de-differentiation through primary cilia disruption. In this review article, the available information related to HDAC6 function in GBM will be presented, with the aim of proposing its inhibition as a valuable therapeutic route for this deadly brain tumour.

Distribution and bulk flow analyses of the intraflagellar transport (IFT) motor kinesin-2 support an "on-demand" model for Chlamydomonas ciliary length control

Most cells tightly control the length of their cilia. The regulation likely involves intraflagellar transport (IFT), a bidirectional motility of multi-subunit particles organized into trains that deliver building blocks into the organelle. In Chlamydomonas, the anterograde IFT motor kinesin-2 consists of the motor subunits FLA8 and FLA10 and the nonmotor subunit KAP. KAP dissociates from IFT at the ciliary tip and diffuses back to the cell body. This observation led to the diffusion-as-a-ruler model of ciliary length control, which postulates that KAP is progressively sequestered into elongating cilia because its return to the cell body will require increasingly more time, limiting motor availability at the ciliary base, train assembly, building block supply, and ciliary growth. Here, we show that Chlamydomonas FLA8 also returns to the cell body by diffusion. However, more than 95% of KAP and FLA8 are present in the cell body and, at a given time, just ~1% of the motor participates in IFT. After repeated photobleaching of both cilia, IFT of fluorescent kinesin subunits continued indicating that kinesin-2 cycles from the large cell-body pool through the cilia and back. Furthermore, growing and full-length cilia contained similar amounts of kinesin-2 subunits and the size of the motor pool at the base changed only slightly with ciliary length. These observations are incompatible with the diffusion-as-a-ruler model, but rather support an "on-demand model," in which the cargo load of the trains is regulated to assemble cilia of the desired length.

ARL3 GTPases facilitate ODA16 unloading from IFT in motile cilia

Eukaryotic cilia and flagella are essential for cell motility and sensory functions. Their biogenesis and maintenance rely on the intraflagellar transport (IFT). Several cargo adapters have been identified to aid IFT cargo transport, but how ciliary cargos are discharged from the IFT remains largely unknown. During our explorations of small GTPases ARL13 and ARL3 in Trypanosoma brucei, we found that ODA16, a known IFT cargo adapter present exclusively in motile cilia, is a specific effector of ARL3. In the cilia, active ARL3 GTPases bind to ODA16 and dissociate ODA16 from the IFT complex. Depletion of ARL3 GTPases stabilizes ODA16 interaction with the IFT, leading to ODA16 accumulation in cilia and defects in axonemal assembly. The interactions between human ODA16 homolog HsDAW1 and ARL GTPases are conserved, and these interactions are altered in HsDAW1 disease variants. These findings revealed a conserved function of ARL GTPases in IFT transport of motile ciliary components, and a mechanism of cargo unloading from the IFT.

Contribution of intraflagellar transport to compartmentalization and maintenance of the photoreceptor cell

The first steps of vision take place in the ciliary outer segment compartment of photoreceptor cells. The protein composition of outer segments is uniquely suited to perform this function. The most abundant among these proteins is the visual pigment, rhodopsin, whose outer segment trafficking involves intraflagellar transport (IFT). Here, we report three major findings from the analysis of mice in which ciliary transport was acutely impaired by conditional knockouts of IFT-B subunits. First, we demonstrate the existence of a sorting mechanism whereby mislocalized rhodopsin is recruited to and concentrated in extracellular vesicles prior to their release, presumably to protect the cell from adverse effects of protein mislocalization. Second, reducing rhodopsin expression significantly delays photoreceptor degeneration caused by IFT disruption, suggesting that controlling rhodopsin levels may be an effective therapy for some cases of retinal degenerative disease. Last, the loss of IFT-B subunits does not recapitulate a phenotype observed in mutants of the BBSome (another ciliary transport protein complex relying on IFT) in which non-ciliary proteins accumulate in the outer segment. Whereas it is widely thought that the role of the BBSome is to primarily participate in ciliary transport, our data suggest that the BBSome has another major function independent of IFT and possibly related to maintaining the diffusion barrier of the ciliary transition zone.

MI-181 Modulates Cilia Length and Restores Cilia Length in Cells with Defective Shortened Cilia

Primary cilia are membrane-covered microtubule-based structures that protrude from the cell surface and are critical for cell signaling and homeostasis during human development and adulthood. Dysregulation of cilia formation, length, and function can lead to a spectrum of human diseases and syndromes known as ciliopathies. Although some genetic and chemical screens have been performed to define important factors that modulate cilia biogenesis and length control, there are currently no clinical treatments that restore cilia length in patients. We report that the microtubule-targeting agent MI-181(mitotic inhibitor-181) is a potent modulator of cilia length and biogenesis. Treatment of retinal pigment epithelial-1 cells with MI-181 induced an increase in the average size of cilia and in the percent ciliated cells under nonstarved conditions. Importantly, MI-181 was effective at rescuing cilia length and ciliation defects in cells that had been treated with the intraflagellar transport inhibitor Ciliobrevin D or the O-GlcNAc transferase inhibitor OSMI-1. Most importantly, MI-181 induced an increase in cilia length and restored ciliation in cells with compromised shortened cilia at low nanomolar concentrations and did not show an inhibitory response at high concentrations. Therefore, MI-181 represents a lead molecule for developing drugs targeting ciliopathies characterized by shortened cilia.

LRRC56 is an IFT cargo required for assembly of the distal dynein docking complex in Trypanosoma brucei

Outer dynein arms (ODAs) are responsible for ciliary beating in eukaryotes. They are assembled in the cytoplasm and shipped by intraflagellar transport (IFT) before attachment to microtubule doublets via the docking complex. The LRRC56 protein has been proposed to contribute to ODAs maturation. Mutations or deletion of the LRRC56 gene lead to reduced ciliary motility in all species investigated so far, but with variable impact on dynein arm presence. Here, we investigated the role of LRRC56 in the protist Trypanosoma brucei, where its absence results in distal loss of ODAs, mostly in growing flagella. We show that LRRC56 is a transient cargo of IFT trains during flagellum construction and surprisingly, is required for efficient attachment of a subset of docking complex proteins present in the distal portion of the organelle. This relation is interdependent since the knockdown of the distal docking complex prevents LRRC56's association with the flagellum. Intriguingly, lrrc56-/- cells display shorter flagella whose maturation is delayed. Inhibition of cell division compensates for the distal ODAs absence thanks to the redistribution of the proximal docking complex, restoring ODAs attachment but not the flagellum length phenotype. This work reveals an unexpected connection between LRRC56 and the docking complex.

Molecular and structural perspectives on protein trafficking to the primary cilium membrane

The primary cilium is a dynamic subcellular compartment templated from the mother centriole or basal body. Cilia are solitary and tiny, but remarkably consequential in cellular pathways regulating proliferation, differentiation, and maintenance. Multiple transmembrane proteins such as G-protein-coupled receptors, channels, enzymes, and membrane-associated lipidated proteins are enriched in the ciliary membrane. The precise regulation of ciliary membrane content is essential for effective signal transduction and maintenance of tissue homeostasis. Surprisingly, a few conserved molecular factors, intraflagellar transport complex A and the tubby family adapter protein TULP3, mediate the transport of most membrane cargoes into cilia. Recent advances in cryogenic electron microscopy provide fundamental insights into these molecular players. Here, we review the molecular players mediating cargo delivery into the ciliary membrane through the lens of structural biology. These mechanistic insights into ciliary transport provide a framework for understanding of disease variants in ciliopathies, enable precise manipulation of cilia-mediated pathways, and provide a platform for the development of targeted therapeutics.

Deciphering vesicle-assisted transport mechanisms in cytoplasm to cilium trafficking

The cilium, a pivotal organelle crucial for cell signaling and proper cell function, relies on meticulous macromolecular transport from the cytoplasm for its formation and maintenance. While the intraflagellar transport (IFT) pathway has traditionally been the focus of extensive study concerning ciliogenesis and ciliary maintenance, recent research highlights a complementary and alternative mechanism-vesicle-assisted transport (VAT) in cytoplasm to cilium trafficking. Despite its potential significance, the VAT pathway remains largely uncharacterized. This review explores recent studies providing evidence for the dynamics of vesicle-related diffusion and transport within the live primary cilium, employing high-speed super-resolution light microscopy. Additionally, we analyze the spatial distribution of vesicles in the cilium, mainly relying on electron microscopy data. By scrutinizing the VAT pathways that facilitate cargo transport into the cilium, with a specific emphasis on recent advancements and imaging data, our objective is to synthesize a comprehensive model of ciliary transport through the integration of IFT-VAT mechanisms.

1-L Transcription of SARS-CoV-2 Spike Protein S1 Subunit

The COVID-19 pandemic prompted rapid research on SARS-CoV-2 pathogenicity. Consequently, new data can be used to advance the molecular understanding of SARS-CoV-2 infection. The present bioinformatics study discusses the "spikeopathy" at the molecular level and focuses on the possible post-transcriptional regulation of the SARS-CoV-2 spike protein S1 subunit in the host cell/tissue. A theoretical protein-RNA recognition code was used to check the compatibility of the SARS-CoV-2 spike protein S1 subunit with mRNAs in the human transcriptome (1-L transcription). The principle for this method is elucidated on the defined RNA binding protein GEMIN5 (gem nuclear organelle-associated protein 5) and RNU2-1 (U2 spliceosomal RNA). Using the method described here, it was shown that 45% of the genes/proteins identified by 1-L transcription of the SARS-CoV-2 spike protein S1 subunit are directly linked to COVID-19, 39% are indirectly linked to COVID-19, and 16% cannot currently be associated with COVID-19. The identified genes/proteins are associated with stroke, diabetes, and cardiac injury.

Motor proteins, spermatogenesis and testis function

The role of motor proteins in supporting intracellular transports of vesicles and organelles in mammalian cells has been known for decades. On the other hand, the function of motor proteins that support spermatogenesis is also well established since the deletion of motor protein genes leads to subfertility and/or infertility. Furthermore, mutations and genetic variations of motor protein genes affect fertility in men, but also a wide range of developmental defects in humans including multiple organs besides the testis. In this review, we seek to provide a summary of microtubule and actin-dependent motor proteins based on earlier and recent findings in the field. Since these two cytoskeletons are polarized structures, different motor proteins are being used to transport cargoes to different ends of these cytoskeletons. However, their involvement in germ cell transport across the blood-testis barrier (BTB) and the epithelium of the seminiferous tubules remains relatively unknown. It is based on recent findings in the field, we have provided a hypothetical model by which motor proteins are being used to support germ cell transport across the BTB and the seminiferous epithelium during the epithelial cycle of spermatogenesis. In our discussion, we have highlighted the areas of research that deserve attention to bridge the gap of research in relating the function of motor proteins to spermatogenesis.

Protein subcellular relocalization and function of duplicated flagellar calcium binding protein genes in honey bee trypanosomatid parasite

The honey bee trypanosomatid parasite, Lotmaria passim, contains two genes that encode the flagellar calcium binding protein (FCaBP) through tandem duplication in its genome. FCaBPs localize in the flagellum and entire body membrane of L. passim through specific N-terminal sorting sequences. This finding suggests that this is an example of protein subcellular relocalization resulting from gene duplication, altering the intracellular localization of FCaBP. However, this phenomenon may not have occurred in Leishmania, as one or both of the duplicated genes have become pseudogenes. Multiple copies of the FCaBP gene are present in several Trypanosoma species and Leptomonas pyrrhocoris, indicating rapid evolution of this gene in trypanosomatid parasites. The N-terminal flagellar sorting sequence of L. passim FCaBP1 is in close proximity to the BBSome complex, while that of Trypanosoma brucei FCaBP does not direct GFP to the flagellum in L. passim. Deletion of the two FCaBP genes in L. passim affected growth and impaired flagellar morphogenesis and motility, but it did not impact host infection. Therefore, FCaBP represents a duplicated gene with a rapid evolutionary history that is essential for flagellar structure and function in a trypanosomatid parasite.

The Small Interactor of PKD2 protein promotes the assembly and ciliary entry of the Chlamydomonas PKD2-mastigoneme complexes

In Chlamydomonas, the channel polycystin 2 (PKD2) is primarily present in the distal region of cilia, where it is attached to the axoneme and mastigonemes, extracellular polymers of MST1. In a smaller proximal ciliary region that lacks mastigonemes, PKD2 is more mobile. We show that the PKD2 regions are established early during ciliogenesis and increase proportionally in length as cilia elongate. In chimeric zygotes, tagged PKD2 rapidly entered the proximal region of PKD2-deficient cilia, whereas the assembly of the distal region was hindered, suggesting that axonemal binding of PKD2 requires de novo assembly of cilia. We identified the protein Small Interactor of PKD2 (SIP), a PKD2-related, single-pass transmembrane protein, as part of the PKD2-mastigoneme complex. In sip mutants, stability and proteolytic processing of PKD2 in the cell body were reduced and PKD2-mastigoneme complexes were absent from the cilia. Like the pkd2 and mst1 mutants, sip mutant cells swam with reduced velocity. Cilia of the pkd2 mutant beat with an increased frequency but were less efficient in moving the cells, suggesting a structural role for the PKD2-SIP-mastigoneme complex in increasing the effective surface of Chlamydomonas cilia.

The retrograde IFT dynein is required for normal function of diverse mechanosensory cilia in Drosophila

Introduction

Cilia biogenesis relies on intraflagellar transport (IFT), a conserved transport mechanism which functions bi-directionally to bring protein complexes to the growing ciliary tip and recycle signaling and transport proteins between the cilium and cell body. In Drosophila, anterograde IFT is critical for assembly of sensory cilia in the neurons of both chordotonal (ch) organs, which have relatively long ciliary axonemes, and external sensory (es) organs, which have short axonemal segments with microtubules in distal sensory segments forming non-axonemal bundles. We previously isolated the beethoven (btv) mutant in a mutagenesis screen for auditory mutants. Although many btv mutant flies are deaf, some retain a small residual auditory function as determined both by behavior and by auditory electrophysiology.

Results

Here we molecularly characterize the btv gene and demonstrate that it encodes the IFT-associated dynein-2 heavy chain Dync2h1. We also describe morphological changes in Johnston's organ as flies age to 30 days, and we find that morphological and electrophysiological phenotypes in this ch organ of btv mutants become more severe with age. We show that NompB protein, encoding the conserved IFT88 protein, an IFT complex B component, fails to be cleared from chordotonal cilia in btv mutants, instead accumulating in the distorted cilia. In macrochaete bristles, a class of es organ, btv mutants show a 50% reduction in mechanoreceptor potentials.

Discussion

Thus, the btv-encoded Dync2h1 functions as the retrograde IFT motor in the assembly of long ciliary axonemes in ch organs and is also important for normal function of the short ciliary axonemes in es organs.

An amino acid-resolution interactome for motile cilia illuminates the structure and function of ciliopathy protein complexes

Motile cilia are ancient, evolutionarily conserved organelles whose dysfunction underlies motile ciliopathies, a broad class of human diseases. Motile cilia contain myriad different proteins that assemble into an array of distinct machines, so understanding the interactions and functional hierarchies among them presents an important challenge. Here, we defined the protein interactome of motile axonemes using cross-linking mass spectrometry (XL/MS) in Tetrahymena thermophila. From over 19,000 XLs, we identified 4,757 unique amino acid interactions among 1,143 distinct proteins, providing both macromolecular and atomic-scale insights into diverse ciliary machines, including the Intraflagellar Transport system, axonemal dynein arms, radial spokes, the 96 nm ruler, and microtubule inner proteins, among others. Guided by this dataset, we used vertebrate multiciliated cells to reveal novel functional interactions among several poorly-defined human ciliopathy proteins. The dataset therefore provides a powerful resource for studying the basic biology of an ancient organelle and the molecular etiology of human genetic disease.

The IFT81-IFT74 complex acts as an unconventional RabL2 GTPase-activating protein during intraflagellar transport

Cilia are important cellular organelles for signaling and motility and are constructed via intraflagellar transport (IFT). RabL2 is a small GTPase that localizes to the basal body of cilia via an interaction with the centriolar protein CEP19 before downstream association with the IFT machinery, which is followed by initiation of IFT. We reconstituted and purified RabL2 with CEP19 or IFT proteins to show that a reconstituted pentameric IFT complex containing IFT81/74 enhances the GTP hydrolysis rate of RabL2. The binding site on IFT81/74 that promotes GTP hydrolysis in RabL2 was mapped to a 70-amino-acid-long coiled-coil region of IFT81/74. We present structural models for RabL2-containing IFT complexes that we validate in vitro and in cellulo and demonstrate that Chlamydomonas IFT81/74 enhances GTP hydrolysis of human RabL2, suggesting an ancient evolutionarily conserved activity. Our results provide an architectural understanding of how RabL2 is incorporated into the IFT complex and a molecular rationale for why RabL2 dissociates from anterograde IFT trains soon after departure from the ciliary base.

Organization, functions, and mechanisms of the BBSome in development, ciliopathies, and beyond

The BBSome is an octameric protein complex that regulates ciliary transport and signaling. Mutations in BBSome subunits are closely associated with ciliary defects and lead to ciliopathies, notably Bardet-Biedl syndrome. Over the past few years, there has been significant progress in elucidating the molecular organization and functions of the BBSome complex. An improved understanding of BBSome-mediated biological events and molecular mechanisms is expected to help advance the development of diagnostic and therapeutic approaches for BBSome-related diseases. Here, we review the current literature on the structural assembly, transport regulation, and molecular functions of the BBSome, emphasizing its roles in cilium-related processes. We also provide perspectives on the pathological role of the BBSome in ciliopathies as well as how these can be exploited for therapeutic benefit.

Primary cilia as dynamic and diverse signalling hubs in development and disease

Primary cilia, antenna-like sensory organelles protruding from the surface of most vertebrate cell types, are essential for regulating signalling pathways during development and adult homeostasis. Mutations in genes affecting cilia cause an overlapping spectrum of >30 human diseases and syndromes, the ciliopathies. Given the immense structural and functional diversity of the mammalian cilia repertoire, there is a growing disconnect between patient genotype and associated phenotypes, with variable severity and expressivity characteristic of the ciliopathies as a group. Recent technological developments are rapidly advancing our understanding of the complex mechanisms that control biogenesis and function of primary cilia across a range of cell types and are starting to tackle this diversity. Here, we examine the structural and functional diversity of primary cilia, their dynamic regulation in different cellular and developmental contexts and their disruption in disease.