BBS7 is required for BBSome formation and its absence in mice results in Bardet-Biedl syndrome phenotypes and selective abnormalities in membrane protein trafficking

Forward genetic screen of the C. elegans million mutation library reveals essential, cell-autonomous contributions of BBSome proteins to dopamine signaling

The nematode Caenorhabditis elegans is well known for its ability to support forward genetic screens to identify molecules involved in neuronal viability and signaling. The proteins involved in C. elegans dopamine (DA) regulation are highly conserved across evolution, with prior work demonstrating that the model can serve as an efficient platform to identify novel genes involved in disease-associated processes. To identify novel players in DA signaling, we took advantage of a recently developed library of pre-sequenced mutant nematodes arising from the million mutation project (MMP) to identify strains that display the DA-dependent swimming-induced-paralysis phenotype (Swip). Our screen identified novel mutations in the dopamine transporter encoding gene dat-1, whose loss was previously used to identify the Swip phenotype, as well as multiple genes with previously unknown connections to DA signaling. Here, we present our isolation and characterization of one of these genes, bbs-1, previously linked to the function of primary cilia in worms and higher organisms, including humans, and where loss-of-function mutations result in a human disorder known as Bardet-Biedl syndrome. Our studies of C. elegans BBS-1 protein, as well as other proteins that are known to be assembled into a higher order complex (the BBSome) reveal that functional or structural disruption of this complex leads to exaggerated C. elegans DA signaling to produce Swip via a cell-autonomous mechanism. We provide evidence that not only does the proper function of cilia in C. elegans DA neurons support normal swimming behavior, but also that bbs-1 maintains normal levels of DAT-1 trafficking or function via a RHO-1 and SWIP-13/MAPK-15 dependent pathway where mutants may contribute to Swip independent of altered ciliary function. Together, these studies demonstrate novel contributors to DA neuron function in the worm and demonstrate the utility and efficiency of forward genetic screens using the MMP library.

Collaborative effort: managing Bardet-Biedl syndrome in pediatric patients. Case series and a literature review

Bardet-Biedl Syndrome (BBS) is an autosomal recessive non-motile ciliopathy, caused by mutations in more than twenty genes. Their expression leads to the production of BBSome-building proteins or chaperon-like proteins supporting its structure. The prevalence of the disease is estimated at 1: 140,000 - 160,000 of life births. Its main clinical features are retinal dystrophy, polydactyly, obesity, cognitive impairment, hypogonadism, genitourinary malformations, and kidney disease. BBS is characterized by heterogeneous clinical manifestation and the variable onset of signs and symptoms. We present a case series of eight pediatric patients with BBS (6 boys and 2 girls) observed in one clinical center including two pairs of siblings. The patients' age varies between 2 to 13 years (average age of diagnosis: 22 months). At presentation kidney disorders were observed in seven patients, polydactyly in six patients' obesity, and psychomotor development delay in two patients. In two patients with kidney disorders, the genetic tests were ordered at the age of 1 and 6 months due to the presence of symptoms suggesting BBS and having an older sibling with the diagnosis of the syndrome. The mutations in the following genes were confirmed: BBS10, MKKS, BBS7/BBS10, BBS7, BBS9. All described patients developed symptoms related to the urinary system and kidney-function impairment. Other most common symptoms are polydactyly and obesity. In one patient the obesity class 3 was diagnosed with multiple metabolic disorders. In six patients the developmental delay was diagnosed. The retinopathy was observed only in one, the oldest patient. Despite having the same mutations (siblings) or having mutations in the same gene, the phenotypes of the patients are different. We aimed to addresses gaps in understanding BBS by comparing our data and existing literature through a narrative review. This research includes longitudinal data and explores genotype-phenotype correlations of children with BBS. BBS exhibits diverse clinical features and genetic mutations, making diagnosis challenging despite defined criteria. Same mutations can result in different phenotypes. Children with constellations of polydactyly and/or kidney disorders and/or early-onset obesity should be managed towards BBS. Early diagnosis is crucial for effective monitoring and intervention to manage the multisystemic dysfunctions associated with BBS.

FBXO24 modulates mRNA alternative splicing and MIWI degradation and is required for normal sperm formation and male fertility

Spermiogenesis is a critical, post-meiotic phase of male gametogenesis, in which the proper gene expression is essential for sperm maturation. However, the underFlying molecular mechanism that controls mRNA expression in the round spermatids remains elusive. Here, we identify that FBXO24, an orphan F-box protein, is highly expressed in the testis of humans and mice and interacts with the splicing factors (SRSF2, SRSF3, and SRSF9) to modulate the gene alternative splicing in the round spermatids. Genetic mutation of FBXO24 in mice causes many abnormal splicing events in round spermatids, thus affecting a large number of critical genes related to sperm formation that were dysregulated. Further molecular and phenotypical analyses revealed that FBXO24 deficiency results in aberrant histone retention, incomplete axonemes, oversized chromatoid body, and abnormal mitochondrial coiling along sperm flagella, ultimately leading to male sterility. In addition, we discovered that FBXO24 interacts with MIWI and SCF subunits and mediates the degradation of MIWI via K48-linked polyubiquitination. Furthermore, we show that FBXO24 depletion could lead to aberrant piRNA production in testes, which suggests FBXO24 is required for normal piRNA counts. Collectively, these data demonstrate that FBXO24 is essential for sperm formation by regulating mRNA alternative splicing and MIWI degradation during spermiogenesis.

Primary cilia and actin regulatory pathways in renal ciliopathies

Ciliopathies are a group of rare genetic disorders caused by defects to the structure or function of the primary cilium. They often affect multiple organs, leading to brain malformations, congenital heart defects, and anomalies of the retina or skeletal system. Kidney abnormalities are among the most frequent ciliopathic phenotypes manifesting as smaller, dysplastic, and cystic kidneys that are often accompanied by renal fibrosis. Many renal ciliopathies cause chronic kidney disease and often progress to end-stage renal disease, necessitating replacing therapies. There are more than 35 known ciliopathies; each is a rare hereditary condition, yet collectively they account for a significant proportion of chronic kidney disease worldwide. The primary cilium is a tiny microtubule-based organelle at the apex of almost all vertebrate cells. It serves as a "cellular antenna" surveying environment outside the cell and transducing this information inside the cell to trigger multiple signaling responses crucial for tissue morphogenesis and homeostasis. Hundreds of proteins and unique cellular mechanisms are involved in cilia formation. Recent evidence suggests that actin remodeling and regulation at the base of the primary cilium strongly impacts ciliogenesis. In this review, we provide an overview of the structure and function of the primary cilium, focusing on the role of actin cytoskeleton and its regulators in ciliogenesis. We then describe the key clinical, genetic, and molecular aspects of renal ciliopathies. We highlight what is known about actin regulation in the pathogenesis of these diseases with the aim to consider these recent molecular findings as potential therapeutic targets for renal ciliopathies.

Formation and function of multiciliated cells

In vertebrates, multiciliated cells (MCCs) are terminally differentiated cells that line the airway tracts, brain ventricles, and reproductive ducts. Each MCC contains dozens to hundreds of motile cilia that beat in a synchronized manner to drive fluid flow across epithelia, the dysfunction of which is associated with a group of human diseases referred to as motile ciliopathies, such as primary cilia dyskinesia. Given the dynamic and complex process of multiciliogenesis, the biological events essential for forming multiple motile cilia are comparatively unelucidated. Thanks to advancements in genetic tools, omics technologies, and structural biology, significant progress has been achieved in the past decade in understanding the molecular mechanism underlying the regulation of multiple motile cilia formation. In this review, we discuss recent studies with ex vivo culture MCC and animal models, summarize current knowledge of multiciliogenesis, and particularly highlight recent advances and their implications.

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.

Neuronal primary cilia integrate peripheral signals with metabolic drives

Neuronal primary cilia have recently emerged as important contributors to the central regulation of energy homeostasis. As non-motile, microtubule-based organelles, primary cilia serve as signaling antennae for metabolic status. The impairment of ciliary structure or function can produce ciliopathies for which obesity is a hallmark phenotype and global ablation of cilia induces non-syndromic adiposity in mouse models. This organelle is not only a hub for metabolic signaling, but also for catecholamine neuromodulation that shapes neuronal circuitry in response to sensory input. The objective of this review is to highlight current research investigating the mechanisms of primary cilium-regulated metabolic drives for maintaining energy homeostasis.

Discovering genetic linkage between periodontitis and type 1 diabetes: A bioinformatics study

Background: Relationship between periodontitis (PD) and type 1 diabetes (T1D) has been reported, but the detailed pathogenesis requires further elucidation. This study aimed to reveal the genetic linkage between PD and T1D through bioinformatics analysis, thereby providing novel insights into scientific research and clinical treatment of the two diseases.

Methods: PD-related datasets (GSE10334, GSE16134, GSE23586) and T1D-related datasets(GSE162689)were downloaded from NCBI Gene Expression Omnibus (GEO). Following batch correction and merging of PD-related datasets as one cohort, differential expression analysis was performed (adjusted p-value <0.05 and ∣log2  fold change| > 0.5), and common differentially expressed genes (DEGs) between PD and T1D were extracted. Functional enrichment analysis was conducted via Metascape website. The protein-protein interaction (PPI) network of common DEGs was generated in The Search Tool for the Retrieval of Interacting Genes/Proteins (STRING) database. Hub genes were selected by Cytoscape software and validated by receiver operating characteristic (ROC) curve analysis.

Results: 59 common DEGs of PD and T1D were identified. Among these DEGs, 23 genes were commonly upregulated, and 36 genes were commonly downregulated in both PD- and T1D-related cohorts. Functional enrichment analysis indicated that common DEGs were mainly enriched in tube morphogenesis, supramolecular fiber organization, 9 + 0 non-motile cilium, plasma membrane bounded cell projection assembly, glomerulus development, enzyme-linked receptor protein signaling pathway, endochondral bone morphogenesis, positive regulation of kinase activity, cell projection membrane and regulation of lipid metabolic process. After PPI construction and modules selection, 6 hub genes (CD34, EGR1, BBS7, FMOD, IGF2, TXN) were screened out and expected to be critical in linking PD and T1D. ROC analysis showed that the AUC values of hub genes were all greater than 70% in PD-related cohort and greater than 60% in T1D-related datasets.

Conclusion: Shared molecular mechanisms between PD and T1D were revealed in this study, and 6 hub genes were identified as potential targets in treating PD and T1D.

Subretinal gene therapy delays vision loss in a Bardet-Biedl Syndrome type 10 mouse model

Blindness in Bardet-Biedl syndrome (BBS) is caused by dysfunction and loss of photoreceptor cells in the retina. BBS10, mutations of which account for approximately 21% of all BBS cases, encodes a chaperonin protein indispensable for the assembly of the BBSome, a cargo adaptor important for ciliary trafficking. The loss of BBSome function in the eye causes a reduced light sensitivity of photoreceptor cells, photoreceptor ciliary malformation, dysfunctional ciliary trafficking, and photoreceptor cell death. Cone photoreceptors lacking BBS10 have congenitally low electrical function in electroretinography. In this study, we performed gene augmentation therapy by injecting a viral construct subretinally to deliver the coding sequence of the mouse Bbs10 gene to treat retinal degeneration in a BBS10 mouse model. Long-term efficacy was assessed by measuring the electrical functions of the retina over time, imaging of the treated regions to visualize cell survival, conducting visually guided swim assays to measure functional vision, and performing retinal histology. We show that subretinal gene therapy slowed photoreceptor cell death and preserved retinal function in treated eyes. Notably, cone photoreceptors regained their electrical function after gene augmentation. Measurement of functional vision showed that subretinal gene therapy provided a significant benefit in delaying vision loss.

Ciliary signaling proteins are mislocalized in the brains of Bardet-Biedl syndrome 1-null mice

In the brain, primary cilia are found on most, if not all, central neurons. The importance of neuronal cilia is underscored by the fact that human diseases caused by primary cilia dysfunction, which are known as ciliopathies, are associated with neuropathologies, including neuropsychiatric disorders and learning and memory deficits. Neuronal cilia are enriched for certain G protein-coupled receptors and their downstream effectors, suggesting they sense and respond to neuromodulators in the extracellular milieu. GPCR ciliary localization is disrupted in neurons from mouse models of the ciliopathy Bardet-Biedl syndrome, with GPCRs failing to localize to cilia, indicating the Bardet-Biedl syndrome proteins are required for trafficking of G protein-coupled receptors into neuronal cilia. Yet, dopamine receptor 1 accumulates in cilia in the absence of Bardet-Biedl syndrome proteins, suggesting Bardet-Biedl syndrome proteins are required for normal ciliary import and export. To further explore the roles of the Bardet-Biedl syndrome proteins in neuronal cilia, we examined localization of ciliary signaling proteins in a new constitutive Bbs1 knockout mouse model. Interestingly, we find that two additional ciliary G protein-coupled receptors (Gpr161 and Gpr19) abnormally accumulate in cilia on Bardet-Biedl syndrome neurons. In addition, we find that the GPCR signaling protein β-arrestin accumulates in a subset of cilia in the brain, suggesting the presence of additional unidentified ciliary G protein-coupled receptors. These results confirm the importance of the Bardet-Biedl syndrome proteins in establishing ciliary GPCR pathways and indicate that loss of Bbs1 leads to complex changes in the localization of signaling proteins in the brain.

Retinal Degeneration Animal Models in Bardet-Biedl Syndrome and Related Ciliopathies

Retinal degeneration due to photoreceptor ciliary-related proteins dysfunction accounts for more than 25% of all inherited retinal dystrophies. The cilium, being an evolutionarily conserved and ubiquitous organelle implied in many cellular functions, can be investigated by way of many models from invertebrate models to nonhuman primates, all these models have massively contributed to the pathogenesis understanding of human ciliopathies. Taking the Bardet-Biedl syndrome (BBS) as an emblematic example as well as other related syndromic ciliopathies, the contribution of a wide range of models has enabled to characterize the role of the BBS proteins in the archetypical cilium but also at the level of the connecting cilium of the photoreceptors. There are more than 24 BBS genes encoding for proteins that form different complexes such as the BBSome and the chaperone proteins complex. But how they lead to retinal degeneration remains a matter of debate with the possible accumulation of proteins in the inner segment and/or accumulation of unwanted proteins in the outer segment that cannot return in the inner segment machinery. Many BBS proteins (but not the chaperonins for instance) can be modeled in primitive organisms such as Paramecium, Chlamydomonas reinardtii, Trypanosoma brucei, and Caenorhabditis elegans These models have enabled clarifying the role of a subset of BBS proteins in the primary cilium as well as their relations with other modules such as the intraflagellar transport (IFT) module, the nephronophthisis (NPHP) module, or the Meckel-Gruber syndrome (MKS)/Joubert syndrome (JBTS) module mostly involved with the transition zone of the primary cilia. Assessing the role of the primary cilia structure of the connecting cilium of the photoreceptor cells has been very much studied by way of zebrafish modeling (Danio rerio) as well as by a plethora of mouse models. More recently, large animal models have been described for three BBS genes and one nonhuman primate model in rhesus macaque for BBS7 In completion to animal models, human cell models can now be used notably thanks to gene editing and the use of induced pluripotent stem cells (iPSCs). All these models are not only important for pathogenesis understanding but also very useful for studying therapeutic avenues, their pros and cons, especially for gene replacement therapy as well as pharmacological triggers.

Rab-like small GTPases in the regulation of ciliary Bardet-Biedl syndrome (BBS) complex transport

Primary cilia, microtubule-based hair-like structures protruding from most cells, contain membranes enriched in signaling molecules and function as sensory and regulatory organelles critical for development and tissue homeostasis. Intraflagellar transport (IFT), cilia-specific bidirectional transport, is required for the assembly, maintenance, and function of cilia. BBSome, the coat complex, acts as the adaptor between the IFT complex and membrane proteins and is therefore essential for establishing the specific compartmentalization of signaling molecules in the cilia. Recent findings have revealed that three ciliary Rab-like small GTPases, IFT27, IFT22, and Rabl2, play critical regulatory roles in ciliary BBSome transport. In this review, we provide an overview of these three Rab-like small GTPases and their relationship with BBSome.

Temperature Incubation Influences Gonadal Gene Expression during Leopard Gecko Development

During development, sexual differentiation results in physiological, anatomical and metabolic differences that implicate not only the gonads but also other body structures. Sex in Leopard geckos is determined by egg incubation temperature. Based on the premise that the developmental decision of gender does not depend on a single gene, we performed an analysis on E. macularius to gain insights into the genes that may be involved in gonads' sexual differentiation during the thermosensitive period. All the genes were identified as differentially expressed at stage 30 during the labile phase of sex differentiation. In this way, the expression of genes known to be involved in gonadal sexual differentiation, such as WNT4, SOX9, DMRT1, Erα, Erβ, GnRH, P450 aromatase, PRLand PRL-R, was investigated. Other genes putatively involved in sex differentiation were sought by differential display. Our findings indicate that embryo exposure to a sex-determining temperature induces differential expression of several genes that are involved not only in gonadal differentiation, but also in several biological pathways (ALDOC, FREM1, BBIP1, CA5A, NADH5, L1 non-LTR retrotransposons, PKM). Our data perfectly fit within the new studies conducted in developmental biology, which indicate that in the developing embryo, in addition to gonadal differentiation, sex-specific tissue and metabolic polarization take place in all organisms.

A 5′ UTR Mutation Contributes to Down-Regulation of Bbs7 in the Berlin Fat Mouse

The Bardet–Biedl Syndrome 7 (Bbs7) gene was identified as the most likely candidate gene causing juvenile obesity in the Berlin Fat Mouse Inbred (BFMI) line. Bbs7 expression is significantly lower in the brain, adipose tissue, and liver of BFMI mice compared to lean C57BL/6NCrl (B6N) mice. A DNA sequence comparison between BFMI and B6N revealed 16 sequence variants in the Bbs7 promoter region. Here, we tested if these mutations contribute to the observed differential expression of Bbs7. In a cell-based dual-luciferase assay, we compared the effects of the BFMI and the B6N haplotypes of different regions of the Bbs7 promotor on the reporter gene expression. A single-nucleotide polymorphism (SNP) was identified causing a significant reduction in the reporter gene expression. This SNP (rs29947545) is located in the 5′ UTR of Bbs7 at Chr3:36.613.350. The SNP is not unique to BFMI mice but also occurs in several other mouse strains, where the BFMI allele is not associated with lower Bbs7 transcript amounts. Thus, we suggest a compensatory mutation in the other mouse strains that keeps Bbs7 expression at the normal level. This compensatory mechanism is missing in BFMI mice and the cell lines tested.

Progressive retinal degeneration of rods and cones in a Bardet-Biedl syndrome type 10 mouse model

Bardet-Biedl syndrome (BBS) is a multi-organ autosomal-recessive disorder caused by mutations in at least 22 different genes. A constant feature is early-onset retinal degeneration leading to blindness. Among the most common forms is BBS type 10 (BBS10), which is caused by mutations in a gene encoding a chaperonin-like protein. To aid in developing treatments, we phenotyped a Bbs10 knockout (Bbs10-/-) mouse model. Analysis by optical coherence tomography (OCT), electroretinography (ERG) and a visually guided swim assay (VGSA) revealed a progressive degeneration (from P19 to 8 months of age) of the outer nuclear layer that is visible by OCT and histology. Cone ERG was absent from at least P30, at which time rod ERG was reduced to 74.4% of control levels; at 8 months, rod ERG was 2.3% of that of controls. VGSA demonstrated loss of functional vision at 9 months. These phenotypes progressed more rapidly than retinal degeneration in the Bbs1M390R/M390R knock-in mouse. This study defines endpoints for preclinical trials that can be utilized to detect a treatment effect in the Bbs10-/- mouse and extrapolated to human clinical trials.

Retinal dystrophies: A look beyond the eyes

Purpose

To illustrate the importance of systemic evaluation in retinal dystrophies through examples of Alstrom syndrome, Bardet Biedl syndrome, and Refsum disease.

Observations

Detailed eye evaluations, including visual acuity, visual field, slit lamp examination, and indirect ophthalmoscopy were performed. Retinal imaging included fundus photography and spectral domain optical coherence tomography (SD-OCT). Functional testing of the retina was done using full field electroretinography (ffERG). In addition, molecular genetic testing was performed using a ciliopathy panel, a retinal dystrophy panel, and whole genome sequencing (WGS).

We report three individuals who presented with vision concerns first to ophthalmology, noted to have retinal dystrophy, and then referred to genomic medicine for genetic testing. Additional evaluation led to suspicion of specific groups of systemic disorders and guided appropriate genetic testing. The first individual presented with retinal dystrophy, obesity, and short stature with no reported neurocognitive deficits. Genetic testing included a ciliopathy panel that was negative followed by WGS that identified biallelic variants in ALMS: a novel frame-shift pathogenic variant c.6525dupT (p.Gln2176Serfs*17) and a rare nonsense pathogenic variant c.2035C > T (p.Arg679Ter) consistent with Alstrom syndrome. The second individual presented with retinal dystrophy, central obesity, and mild neurocognitive deficits. A ciliopathy genetic testing panel identified a homozygous pathogenic variant in BBS7: c.389_390del (p.Asn130Thrfs*4), confirming the diagnosis of Bardet Biedl syndrome. The third individual presented with progressive vision loss due to retinitis pigmentosa, anosmia, hearing loss, and shortened metatarsals and digits. Genetic testing identified two variants in PHYH: c.375_375del (p.Glu126Argfs*2) a pathogenic variant and c.536A > G (p.His179Arg), a variant of uncertain significance (VUS), suggestive of Refsum disease. Additional biochemical testing revealed markedly elevated phytanic acid with a low concentration of pristanic acid and normal concentrations of very long-chain fatty acids (C22:0, C24:0, C26:0), a pattern consistent with a diagnosis of Refsum disease.

Conclusions and importance

In individuals who present with retinal dystrophy to ophthalmologists, additional systemic manifestations such as sensorineural hearing loss, anosmia, or polydactyly, should be sought and a positive history or examination finding should prompt an immediate referral to a clinical geneticist for additional evaluation and appropriate genetic testing. This facilitates pre-test genetic counseling and allows for more accurate diagnosis, prognosis, and management of affected individuals along with better recurrence risk estimates for family members. Identification of an underlying etiology also enhances the understanding of the pathophysiology of disease and expands the genotypic and phenotypic spectrum. Ultimately, successful recognition of these diseases facilitates development of targeted therapies and surveillance of affected individuals.

Disruption of Dopamine Receptor 1 Localization to Primary Cilia Impairs Signaling in Striatal Neurons

A rod-shaped appendage called a primary cilium projects from the soma of most central neurons in the mammalian brain. The importance of cilia within the nervous system is highlighted by the fact that human syndromes linked to primary cilia dysfunction, collectively termed ciliopathies, are associated with numerous neuropathologies, including hyperphagia-induced obesity, neuropsychiatric disorders, and learning and memory deficits. Neuronal cilia are enriched with signaling molecules, including specific G-protein-coupled receptors (GPCRs) and their downstream effectors, suggesting that they act as sensory organelles that respond to neuromodulators in the extracellular space. We previously showed that GPCR ciliary localization is disrupted in neurons from mouse models of the ciliopathy Bardet-Biedl syndrome (BBS). Based on this finding, we hypothesized that mislocalization of ciliary GPCRs may impact receptor signaling and contribute to the BBS phenotypes. Here, we show that disrupting localization of the ciliary GPCR dopamine receptor 1 (D1) in male and female mice, either by loss of a BBS protein or loss of the cilium itself, specifically in D1-expressing neurons, results in obesity. Interestingly, the weight gain is associated with reduced locomotor activity, rather than increased food intake. Moreover, the loss of a BBS protein or cilia on D1-expressing neurons leads to a reduction in D1-mediated signaling. Together, these results indicate that cilia impact D1 activity in the nervous system and underscore the importance of neuronal cilia for proper GPCR signaling.SIGNIFICANCE STATEMENT Most mammalian neurons possess solitary appendages called primary cilia. These rod-shaped structures are enriched with signaling proteins, such as G-protein-coupled receptors (GPCRs), suggesting that they respond to neuromodulators. This study examines the consequences of disrupting ciliary localization of the GPCR dopamine receptor 1 (D1) in D1-expressing neurons. Remarkably, mice that have either an abnormal accumulation of D1 in cilia or a loss of D1 ciliary localization become obese. In both cases, the obesity is associated with lower locomotor activity rather than overeating. As D1 activation increases locomotor activity, these results are consistent with a reduction in D1 signaling. Indeed, we found that D1-mediated signaling is reduced in brain slices from both mouse models. Thus, cilia impact D1 signaling in the brain.

Cilia Action in Islets: Lessons From Mouse Models

Primary cilia as a signaling organelle have garnered recent attention as a regulator of pancreatic islet function. These rod-like sensors exist on all major islet endocrine cell types and transduce a variety of external cues, while dysregulation of cilia function contributes to the development of diabetes. The complex role of islet primary cilia has been examined using genetic deletion targeting various components of cilia. In this review, we summarize experimental models for the study of islet cilia and current understanding of mechanisms of cilia regulation of islet hormone secretion. Consensus from these studies shows that pancreatic cilia perturbation can cause both endocrine and exocrine defects that are relevant to human disease. We discuss future research directions that would further elucidate cilia action in distinct groups of islet cells, including paracrine and juxtacrine regulation, GPCR signaling, and endocrine-exocrine crosstalk.

Molecular basis of the morphogenesis of sperm head and tail in mice

Background

The spermatozoon has a complex molecular apparatus necessary for fertilization in its head and flagellum. Recently, numerous genes that are needed to construct the molecular apparatus of spermatozoa have been identified through the analysis of genetically modified mice.

Methods

Based on the literature information, the molecular basis of the morphogenesis of sperm heads and flagella in mice was summarized.

Main findings (Results)

The molecular mechanisms of vesicular trafficking and intraflagellar transport in acrosome and flagellum formation were listed. With the development of cryo‐electron tomography and mass spectrometry techniques, the details of the axonemal structure are becoming clearer. The fine structure and the proteins needed to form the central apparatus, outer and inner dynein arms, nexin‐dynein regulatory complex, and radial spokes were described. The important components of the formation of the mitochondrial sheath, fibrous sheath, outer dense fiber, and the annulus were also described. The similarities and differences between sperm flagella and Chlamydomonas flagella/somatic cell cilia were also discussed.

Conclusion

The molecular mechanism of formation of the sperm head and flagellum has been clarified using the mouse as a model. These studies will help to better understand the diversity of sperm morphology and the causes of male infertility.

A deletion containing a CTCF-element in intron 8 of the Bbs7 gene is partially responsible for juvenile obesity in the Berlin Fat Mouse

The Berlin Fat Mouse Inbred (BFMI) line is a model for juvenile obesity. Previous studies on crosses between BFMI and C57Bl/6N (B6N) have identified a recessive defect causing juvenile obesity on chromosome 3 (jObes1). Bbs7 was identified as the most likely candidate gene for the observed effect. Comparative sequence analysis showed a 1578 bp deletion in intron 8 of Bbs7 in BFMI mice. A CTCF-element is located inside this deletion. To investigate the functional effect of this deletion, it was introduced into B6N mice using CRISPR/Cas9. Two mice containing the target deletion were obtained (B6N Bbs7^emI8∆1 and Bbs7^emI8∆2) and were subsequently mated to BFMI and B6N to generate two families suitable for complementation. Inherited alleles were determined and body composition was measured by quantitative magnetic resonance. Evidence for a partial complementation (13.1-15.1%) of the jObes1 allele by the CRISPR/Cas9 modified B6N Bbs7^emI8∆1 and Bbs7^emI8∆2 alleles was found. Mice carrying the complementation alleles had a 23-27% higher fat-to-lean ratio compared to animals which have a B6N allele (P_{(Bbs7emI8∆1)} = 4.25 × 10^-7; P_{(Bbs7emI8∆2)} = 3.17 × 10^-5). Consistent with previous findings, the recessive effect of the BFMI allele was also seen for the B6N Bbs7^emI8∆1 and Bbs7^emI8∆2 alleles. However, the effect size of the B6N Bbs7^emI8∆1 and Bbs7^emI8∆2 alleles was smaller than the BFMI allele, and thus showed only a partial complementation. Findings suggest additional variants near Bbs7 in addition to or interacting with the deletion in intron 8.

Limited time window for retinal gene therapy in a preclinical model of ciliopathy

Retinal degeneration is a common clinical feature of ciliopathies, a group of genetic diseases linked to ciliary dysfunction, and gene therapy is an attractive treatment option to prevent vision loss. Although the efficacy of retinal gene therapy is well established by multiple proof-of-concept preclinical studies, its long-term effect, particularly when treatments are given at advanced disease stages, is controversial. Incomplete treatment and intrinsic variability of gene delivery methods may contribute to the variable outcomes. Here, we used a genetic rescue approach to 'optimally' treat retinal degeneration at various disease stages and examined the long-term efficacy of gene therapy in a mouse model of ciliopathy. We used a Bardet-Biedl syndrome type 17 (BBS17) mouse model, in which the gene-trap that suppresses Bbs17 (also known as Lztfl1) expression can be removed by tamoxifen administration, restoring normal gene expression systemically. Our data indicate that therapeutic effects of retinal gene therapy decrease gradually as treatments are given at later stages. These results suggest the presence of limited time window for successful gene therapy in certain retinal degenerations. Our study also implies that the long-term efficacy of retinal gene therapy may depend on not only the timing of treatment but also other factors such as the function of mutated genes and residual activities of mutant alleles.

Functional compartmentalization of photoreceptor neurons

Retinal photoreceptors are neurons that convert dynamically changing patterns of light into electrical signals that are processed by retinal interneurons and ultimately transmitted to vision centers in the brain. They represent the essential first step in seeing without which the remainder of the visual system is rendered moot. To support this role, the major functions of photoreceptors are segregated into three main specialized compartments-the outer segment, the inner segment, and the pre-synaptic terminal. This compartmentalization is crucial for photoreceptor function-disruption leads to devastating blinding diseases for which therapies remain elusive. In this review, we examine the current understanding of the molecular and physical mechanisms underlying photoreceptor functional compartmentalization and highlight areas where significant knowledge gaps remain.

Bardet-Biedl syndrome proteins regulate intracellular signaling and neuronal function in patient-specific iPSC-derived neurons

Bardet-Biedl syndrome (BBS) is a rare autosomal recessive disorder caused by mutations in genes encoding components of the primary cilium and is characterized by hyperphagic obesity. To investigate the molecular basis of obesity in human BBS, we developed a cellular model of BBS using induced pluripotent stem cell-derived (iPSC-derived) hypothalamic arcuate-like neurons. BBS mutations BBS1M390R and BBS10C91fsX95 did not affect neuronal differentiation efficiency but caused morphological defects, including impaired neurite outgrowth and longer primary cilia. Single-cell RNA sequencing of BBS1M390R hypothalamic neurons identified several downregulated pathways, including insulin and cAMP signaling and axon guidance. Additional studies demonstrated that BBS1M390R and BBS10C91fsX95 mutations impaired insulin signaling in both human fibroblasts and iPSC-derived neurons. Overexpression of intact BBS10 fully restored insulin signaling by restoring insulin receptor tyrosine phosphorylation in BBS10C91fsX95 neurons. Moreover, mutations in BBS1 and BBS10 impaired leptin-mediated p-STAT3 activation in iPSC-derived hypothalamic neurons. Correction of the BBS mutation by CRISPR rescued leptin signaling. POMC expression and neuropeptide production were decreased in BBS1M390R and BBS10C91fsX95 iPSC-derived hypothalamic neurons. In the aggregate, these data provide insights into the anatomic and functional mechanisms by which components of the BBSome in CNS primary cilia mediate effects on energy homeostasis.

The BBSome: a nexus controlling energy metabolism in the brain

Bardet-Biedl syndrome (BBS) is a syndromic ciliopathy that has obesity as a cardinal feature. BBS is caused by mutations in BBS genes. BBS proteins control primary cilia function, and BBS mutations therefore lead to dysfunctional primary cilia. Obesity in patients with BBS is mainly caused by hyperphagia due to dysregulated neuronal function in the brain, in particular in the hypothalamus. However, the mechanism by which mutations in BBS genes result in dysfunction in hypothalamic neurons is not well understood. In this issue of the JCI, Wang et al. used BBS and non-BBS patient-derived induced pluripotent stem cells to generate neurons and hypothalamic neurons. Using this human model system, the authors demonstrated that mutations in BBS genes affected primary cilia function, neuronal morphology, and signaling pathways regulating the function of hypothalamic neurons, which control energy homeostasis. This study provides important insights into the mechanisms of BBS-induced obesity.

A mouse model of Bardet-Biedl Syndrome has impaired fear memory, which is rescued by lithium treatment

Primary cilia are microtubule-based organelles present on most cells that regulate many physiological processes, ranging from maintaining energy homeostasis to renal function. However, the role of these structures in the regulation of behavior remains unknown. To study the role of cilia in behavior, we employ mouse models of the human ciliopathy, Bardet-Biedl Syndrome (BBS). Here, we demonstrate that BBS mice have significant impairments in context fear conditioning, a form of associative learning. Moreover, we show that postnatal deletion of BBS gene function, as well as congenital deletion, specifically in the forebrain, impairs context fear conditioning. Analyses indicated that these behavioral impairments are not the result of impaired hippocampal long-term potentiation. However, our results indicate that these behavioral impairments are the result of impaired hippocampal neurogenesis. Two-week treatment with lithium chloride partially restores the proliferation of hippocampal neurons which leads to a rescue of context fear conditioning. Overall, our results identify a novel role of cilia genes in hippocampal neurogenesis and long-term context fear conditioning.

Bardet-Biedl syndrome-7 (BBS7) shows treatment potential and a cone-rod dystrophy phenotype that recapitulates the non-human primate model

Purpose: To provide a detailed ophthalmic phenotype of two male patients with Bardet-Biedl Syndrome (BBS) due to mutations in the BBS7 geneMethods: Two brothers ages 26 (Patient 1, P1) and 23 (P2) underwent comprehensive ophthalmic evaluations over three years. Visual function was assessed with full-field electroretinograms (ffERGs), kinetic and chromatic perimetry, multimodal imaging with spectral domain optical coherence tomography (SD-OCT), fundus autofluorescence (FAF) with short- (SW) and near-infrared (NIR) excitation lights and adaptive optics scanning light ophthalmoscopy (AOSLO).Results: Both siblings had a history of obesity and postaxial polydactyly; P2 had diagnoses of type 1 Diabetes Mellitus, Addison's disease, high-functioning autism-spectrum disorder and -12D myopia. Visual acuities were better than 20/30. Kinetic fields were moderately constricted. Cone-mediated ffERGs were undetectable, rod ERGs were ~80% of normal mean. Static perimetry showed severe central cone and rod dysfunction. Foveal to parafoveal hypoautofluorescence, most obvious on NIR-FAF, co-localized with outer segment shortening/loss and outer nuclear layer thinning by SD-OCT, and with reduced photoreceptors densities by AOSLO. A structural-functional dissociation was confirmed for cone- and rod-mediated parameters. Worsening of the above abnormalities was documented by SD-OCT and FAF in P2 at 3 years. Gene screening identified compound heterozygous mutations in BBS7 (p.Val266Glu: c.797 T > A of maternal origin; c.1781_1783delCAT, paternal) in both patients.Conclusions: BBS7-associated retinal degeneration may present as a progressive cone-rod dystrophy pattern, reminiscent of both the murine and non-human primate models of the disease. Predominantly central retinal abnormalities in both cone and rod photoreceptors showed a structural-functional dissociation, an ideal scenario for gene augmentation treatments.

HTR6 and SSTR3 targeting to primary cilia

Primary cilia are hair-like projections of the cell membrane supported by an inner microtubule scaffold, the axoneme, which polymerizes out of a membrane-docked centriole at the ciliary base. By working as specialized signaling compartments, primary cilia provide an optimal environment for many G protein-coupled receptors (GPCRs) and their effectors to efficiently transmit their signals to the rest of the cell. For this to occur, however, all necessary receptors and signal transducers must first accumulate at the ciliary membrane. Serotonin receptor 6 (HTR6) and Somatostatin receptor 3 (SSTR3) are two GPCRs whose signaling in brain neuronal cilia affects cognition and is implicated in psychiatric, neurodegenerative, and oncologic diseases. Over a decade ago, the third intracellular loops (IC3s) of HTR6 and SSTR3 were shown to contain ciliary localization sequences (CLSs) that, when grafted onto non-ciliary GPCRs, could drive their ciliary accumulation. Nevertheless, these CLSs were dispensable for ciliary targeting of HTR6 and SSTR3, suggesting the presence of additional CLSs, which we have recently identified in their C-terminal tails. Herein, we review the discovery and mapping of these CLSs, as well as the state of the art regarding how these CLSs may orchestrate ciliary accumulation of these GPCRs by controlling when and where they interact with the ciliary entry and exit machinery via adaptors such as TULP3, RABL2 and the BBSome.

Xenopus to the rescue: A model to validate and characterize candidate ciliopathy genes

Cilia are present on most vertebrate cells and play a central role in development, growth, and homeostasis. Thus, cilia dysfunction can manifest into an array of diseases, collectively termed ciliopathies, affecting millions of lives worldwide. Yet, our understanding of the gene regulatory networks that control cilia assembly and functions remain incomplete. With the advances in next-generation sequencing technologies, we can now rapidly predict pathogenic variants from hundreds of ciliopathy patients. While the pace of candidate gene discovery is exciting, most of these genes have never been previously implicated in cilia assembly or function. This makes assigning the disease causality difficult. This review discusses how Xenopus, a genetically tractable and high-throughput vertebrate model, has played a central role in identifying, validating, and characterizing candidate ciliopathy genes. The review is focused on multiciliated cells (MCCs) and diseases associated with MCC dysfunction. MCCs harbor multiple motile cilia on their apical surface to generate extracellular fluid flow inside the airway, the brain ventricles, and the oviduct. In Xenopus, these cells are external and present on the embryonic epidermal epithelia, facilitating candidate genes analysis in MCC development in vivo. The ability to introduce patient variants to study their effects on disease progression makes Xenopus a powerful model to improve our understanding of the underlying disease mechanisms and explain the patient phenotype.

Compartmentalization of Photoreceptor Sensory Cilia

Functional compartmentalization of cells is a universal strategy for segregating processes that require specific components, undergo regulation by modulating concentrations of those components, or that would be detrimental to other processes. Primary cilia are hair-like organelles that project from the apical plasma membranes of epithelial cells where they serve as exclusive compartments for sensing physical and chemical signals in the environment. As such, molecules involved in signal transduction are enriched within cilia and regulating their ciliary concentrations allows adaptation to the environmental stimuli. The highly efficient organization of primary cilia has been co-opted by major sensory neurons, olfactory cells and the photoreceptor neurons that underlie vision. The mechanisms underlying compartmentalization of cilia are an area of intense current research. Recent findings have revealed similarities and differences in molecular mechanisms of ciliary protein enrichment and its regulation among primary cilia and sensory cilia. Here we discuss the physiological demands on photoreceptors that have driven their evolution into neurons that rely on a highly specialized cilium for signaling changes in light intensity. We explore what is known and what is not known about how that specialization appears to have driven unique mechanisms for photoreceptor protein and membrane compartmentalization.

Photoreceptor cilia, in contrast to primary cilia, grant entry to a partially assembled BBSome

The BBSome is a protein complex consisting of BBS1, BBS2, BBS4, BBS5, BBS7, BBS8, BBS9 and BBS18 that associates with intraflagellar transport complexes and specializes in ciliary trafficking. In primary cilia, ciliary entry requires the fully assembled BBSome as well as the small GTPase, ARL6 (BBS3). Retinal photoreceptors possess specialized cilia. In light of key structural and functional differences between primary and specialized cilia, we examined the principles of BBSome recruitment to photoreceptor cilia. We performed sucrose gradient fractionation using retinal lysates of Bbs2-/-, Bbs7-/-, Bbs8-/- and Bbs3-/- mice to determine the status of BBSome assembly, then determined localization of BBSome components using immunohistochemistry. Surprisingly, we found that a subcomplex of the BBSome containing at least BBS1, BBS5, BBS8 and BBS9 is recruited to cilia in the absence of BBS2 or BBS7. In contrast, a BBSome subcomplex consisting of BBS1, BBS2, BBS5, BBS7 and BBS9 is found in Bbs8-/- retinas and is denied ciliary entry in photoreceptor cells. In addition, the BBSome remains fully assembled in Bbs3-/- retinas and can be recruited to photoreceptor cilia in the absence of BBS3. We compared phenotypic severity of their retinal degeneration phenotypes. These findings demonstrate that unlike primary cilia, photoreceptor cilia admit a partially assembled BBSome meeting specific requirements. In addition, the recruitment of the BBSome to photoreceptor cilia does not require BBS3. These findings indicate that the ciliary entry of the BBSome is subjected to cell-specific regulation, particularly in cells with highly adapted forms of cilia such as photoreceptors.

Rabl2 GTP hydrolysis licenses BBSome-mediated export to fine-tune ciliary signaling

Cilia of higher animals sense various environmental stimuli. Proper ciliary signaling requires appropriate extent of BBSome-mediated export of membrane receptors across ciliary barrier transition zone (TZ) through retrograde intraflagellar transport (IFT) machinery. How the barrier passage is controlled, however, remains unknown. Here, we show that small GTPase Rabl2 functions as a molecular switch for the outward TZ passage. Rabl2-GTP enters cilia by binding to IFT-B complex. Its GTP hydrolysis enables the outward TZ passage of the BBSome and its cargos with retrograde IFT machinery, whereas its persistent association leads to their shedding from IFT-B during the passing process and consequently ciliary retention. Rabl2 deficiency or expression of a GTP-locked mutant impairs the ciliary hedgehog signaling without interfering with ciliation and respectively results in different spectrums of mouse developmental disorders. We propose that the switch role of Rabl2 ensures proper turnover of the BBSome and ciliary membrane receptors to fine-tune cilia-dependent signaling for normal embryonic development and organismic homeostasis.

Reproduction Function in Male Patients With Bardet Biedl Syndrome

PURPOSE

Bardet-Biedl syndrome (BBS) is a ciliopathy with a wide spectrum of symptoms due to primary cilia dysfunction, including genitourinary developmental anomalies as well as impaired reproduction, particularly in males. Primary cilia are known to be required at the following steps of reproduction function: (i) genitourinary organogenesis, (ii) in fetal firing of hypothalamo-pituitary axe, (iii) sperm flagellum structure, and (iv) first zygotic mitosis conducted by proximal sperm centriole. BBS phenotype is not fully understood.

METHODS

This study explored all steps of reproduction in 11 French male patients with identified BBS mutations.

RESULTS

BBS patients frequently presented with genitourinary malformations, such as cryptorchidism (5/11), short scrotum (5/8), and micropenis (5/8), but unexpectedly, with normal testis size (7/8). Ultrasonography highlighted epididymal cysts or agenesis of one seminal vesicle in some cases. Sexual hormones levels were normal in all patients except one. Sperm numeration was normal in 8 out of the 10 obtained samples. Five to 45% of sperm presented a progressive motility. Electron microscopy analysis of spermatozoa did not reveal any homogeneous abnormality. Moreover, a psychological approach pointed to a decreased self-confidence linked to blindness and obesity explaining why so few BBS patients express a child wish.

CONCLUSIONS

Primary cilia dysfunction in BBS impacts the embryology of the male genital tract, especially epididymis, penis, and scrotum through an insufficient fetal androgen production. However, in adults, sperm structure does not seem to be impacted. These results should be confirmed in a greater BBS patient cohort, focusing on fertility.

Cone Photoreceptor Degeneration and Neuroinflammation in the Zebrafish Bardet-Biedl Syndrome 2 (bbs2) Mutant Does Not Lead to Retinal Regeneration

Bardet-Biedl syndrome (BBS) is a heterogeneous and pleiotropic autosomal recessive disorder characterized by obesity, retinal degeneration, polydactyly, renal dysfunction, and mental retardation. BBS results from defects in primary and sensory cilia. Mutations in 21 genes have been linked to BBS and proteins encoded by 8 of these genes form a multiprotein complex termed the BBSome. Mutations in BBS2, a component of the BBSome, result in BBS as well as non-syndromic retinal degeneration in humans and rod degeneration in mice, but the role of BBS2 in cone photoreceptor survival is not clear. We used zebrafish bbs2^–/– mutants to better understand how loss of bbs2 leads to photoreceptor degeneration. Zebrafish bbs2^–/– mutants exhibited impaired visual function as larvae and adult zebrafish underwent progressive cone photoreceptor degeneration. Cone degeneration was accompanied by increased numbers of activated microglia, indicating an inflammatory response. Zebrafish exhibit a robust ability to regenerate lost photoreceptors following retinal damage, yet cone degeneration and inflammation was insufficient to trigger robust Müller cell proliferation. In contrast, high intensity light damage stimulated Müller cell proliferation and photoreceptor regeneration in both wild-type and bbs2^–/– mutants, although the bbs2^–/– mutants could only restore cones to pre-damaged densities. In summary, these findings suggest that cone degeneration leads to an inflammatory response in the retina and that BBS2 is necessary for cone survival. The zebrafish bbs2 mutant also represents an ideal model to identify mechanisms that will enhance retinal regeneration in degenerating diseases.

Dlec1 is required for spermatogenesis and male fertility in mice

Deleted in lung and esophageal cancer 1 (DLEC1) is a tumour suppressor gene that is downregulated in various cancers in humans; however, the physiological and molecular functions of DLEC1 are still unclear. This study investigated the critical role of Dlec1 in spermatogenesis and male fertility in mice. Dlec1 was significantly expressed in testes, with dominant expression in germ cells. We disrupted Dlec1 in mice and analysed its function in spermatogenesis and male fertility. Dlec1 deletion caused male infertility due to impaired spermatogenesis. Spermatogenesis progressed normally to step 8 spermatids in Dlec1^−/− mice, but in elongating spermatids, we observed head deformation, a shortened tail, and abnormal manchette organization. These phenotypes were similar to those of various intraflagellar transport (IFT)-associated gene-deficient sperm. In addition, DLEC1 interacted with tailless complex polypeptide 1 ring complex (TRiC) and Bardet–Biedl Syndrome (BBS) protein complex subunits, as well as α- and β-tubulin. DLEC1 expression also enhanced primary cilia formation and cilia length in A549 lung adenocarcinoma cells. These findings suggest that DLEC1 is a possible regulator of IFT and plays an essential role in sperm head and tail formation in mice.

Cilia interactome with predicted protein-protein interactions reveals connections to Alzheimer's disease, aging and other neuropsychiatric processes

Cilia are dynamic microtubule-based organelles present on the surface of many eukaryotic cell types and can be motile or non-motile primary cilia. Cilia defects underlie a growing list of human disorders, collectively called ciliopathies, with overlapping phenotypes such as developmental delays and cognitive and memory deficits. Consistent with this, cilia play an important role in brain development, particularly in neurogenesis and neuronal migration. These findings suggest that a deeper systems-level understanding of how ciliary proteins function together may provide new mechanistic insights into the molecular etiologies of nervous system defects. Towards this end, we performed a protein-protein interaction (PPI) network analysis of known intraflagellar transport, BBSome, transition zone, ciliary membrane and motile cilia proteins. Known PPIs of ciliary proteins were assembled from online databases. Novel PPIs were predicted for each ciliary protein using a computational method we developed, called High-precision PPI Prediction (HiPPIP) model. The resulting cilia "interactome" consists of 165 ciliary proteins, 1,011 known PPIs, and 765 novel PPIs. The cilia interactome revealed interconnections between ciliary proteins, and their relation to several pathways related to neuropsychiatric processes, and to drug targets. Approximately 184 genes in the cilia interactome are targeted by 548 currently approved drugs, of which 103 are used to treat various diseases of nervous system origin. Taken together, the cilia interactome presented here provides novel insights into the relationship between ciliary protein dysfunction and neuropsychiatric disorders, for e.g. interconnections of Alzheimer's disease, aging and cilia genes. These results provide the framework for the rational design of new therapeutic agents for treatment of ciliopathies and neuropsychiatric disorders.

Deletion in the Bardet-Biedl Syndrome Gene TTC8 Results in a Syndromic Retinal Degeneration in Dogs

In golden retriever dogs, a 1 bp deletion in the canine TTC8 gene has been shown to cause progressive retinal atrophy (PRA), the canine equivalent of retinitis pigmentosa. In humans, TTC8 is also implicated in Bardet–Biedl syndrome (BBS). To investigate if the affected dogs only exhibit a non-syndromic PRA or develop a syndromic ciliopathy similar to human BBS, we recruited 10 affected dogs to the study. The progression of PRA for two of the dogs was followed for 2 years, and a rigorous clinical characterization allowed a careful comparison with primary and secondary characteristics of human BBS. In addition to PRA, the dogs showed a spectrum of clinical and morphological signs similar to primary and secondary characteristics of human BBS patients, such as obesity, renal anomalies, sperm defects, and anosmia. We used Oxford Nanopore long-read cDNA sequencing to characterize retinal full-length TTC8 transcripts in affected and non-affected dogs, the results of which suggest that three isoforms are transcribed in the retina, and the 1 bp deletion is a loss-of-function mutation, resulting in a canine form of Bardet–Biedl syndrome with heterogeneous clinical signs.

The BBSome assembly is spatially controlled by BBS1 and BBS4 in human cells

Bardet-Biedl syndrome (BBS) is a pleiotropic ciliopathy caused by dysfunction of primary cilia. More than half of BBS patients carry mutations in one of eight genes encoding for subunits of a protein complex, the BBSome, which mediates trafficking of ciliary cargoes. In this study, we elucidated the mechanisms of the BBSome assembly in living cells and how this process is spatially regulated. We generated a large library of human cell lines deficient in a particular BBSome subunit and expressing another subunit tagged with a fluorescent protein. We analyzed these cell lines utilizing biochemical assays, conventional and expansion microscopy, and quantitative fluorescence microscopy techniques: fluorescence recovery after photobleaching and fluorescence correlation spectroscopy. Our data revealed that the BBSome formation is a sequential process. We show that the pre-BBSome is nucleated by BBS4 and assembled at pericentriolar satellites, followed by the translocation of the BBSome into the ciliary base mediated by BBS1. Our results provide a framework for elucidating how BBS-causative mutations interfere with the biogenesis of the BBSome.

Regulation of polycystin expression, maturation and trafficking

The major autosomal dominant polycystic kidney disease (ADPKD) genes, PKD1 and PKD2, are wildly expressed at the organ and tissue level. PKD1 encodes polycystin 1 (PC1), a large membrane associated receptor-like protein that can complex with the PKD2 product, PC2. Various cellular locations have been described for both PC1, including the plasma membrane and extracellular vesicles, and PC2, especially the endoplasmic reticulum (ER), but compelling evidence indicates that the primary cilium, a sensory organelle, is the key site for the polycystin complex to prevent PKD. As with other membrane proteins, the ER biogenesis pathway is key to appropriately folding, performing quality control, and exporting fully folded PC1 to the Golgi apparatus. There is a requirement for binding with PC2 and cleavage of PC1 at the GPS for this folding and export to occur. Six different monogenic defects in this pathway lead to cystic disease development, with PC1 apparently particularly sensitive to defects in this general protein processing pathway. Trafficking of membrane proteins, and the polycystins in particular, through the Golgi to the primary cilium have been analyzed in detail, but at this time, there is no clear consensus on a ciliary targeting sequence required to export proteins to the cilium. After transitioning though the trans-Golgi network, polycystin-bearing vesicles are likely sorted to early or recycling endosomes and then transported to the ciliary base, possibly via docking to transition fibers (TF). The membrane-bound polycystin complex then undergoes facilitated trafficking through the transition zone, the diffusion barrier at the base of the cilium, before entering the cilium. Intraflagellar transport (IFT) may be involved in moving the polycystins along the cilia, but data also indicates other mechanisms. The ciliary polycystin complex can be ubiquitinated and removed from cilia by internalization at the ciliary base and may be sent back to the plasma membrane for recycling or to lysosomes for degradation. Monogenic defects in processes regulating the protein composition of cilia are associated with syndromic disorders involving many organ systems, reflecting the pleotropic role of cilia during development and for tissue maintenance. Many of these ciliopathies have renal involvement, likely because of faulty polycystin signaling from cilia. Understanding the expression, maturation and trafficking of the polycystins helps understand PKD pathogenesis and suggests opportunities for therapeutic intervention.

Exploring Key Challenges of Understanding the Pathogenesis of Kidney Disease in Bardet-Biedl Syndrome

Bardet–Biedl syndrome (BBS) is a rare pleiotropic inherited disorder known as a ciliopathy. Kidney disease is a cardinal clinical feature; however, it is one of the less investigated traits. This study is a comprehensive analysis of the literature aiming to collect available information providing mechanistic insights into the pathogenesis of kidney disease by analyzing clinical and basic science studies focused on this issue. The analysis revealed that the syndrome is either clinically and genetically heterogenous, with 24 genes discovered to date, but with 3 genes (BBS1, BBS2, and BBS10) accounting for almost 50% of diagnoses; genotype–phenotype correlation studies showed that patients with BBS1 mutations have a less severe renal phenotype than the other 2 most common loci; in addition, truncating rather than missense mutations are more likely to cause kidney disease. However, significant intrafamilial clinical variability has been described, with no clear explanation to date. In mice kidneys, Bbs genes have relative low expression levels, in contrast with other common affected organs, like the retina; surprisingly, Bbs1 is the only locus with basal overexpression in the kidney. In vitro studies indicate that signalling pathways involved in embryonic kidney development and repair are affected in the context of BBS depletion; in mice, kidney disease does not have a full penetrance; when present, it resembles human phenotype and shows an age-dependent progression. Data on the exact contribution of local versus systemic consequences of Bbs dysfunction are scanty and further investigations are required to get firm conclusions.

Absence of SCAPER causes male infertility in humans and Drosophila by modulating microtubule dynamics during meiosis

BACKGROUND

Mutation in S-phase cyclin A-associated protein rin the endoplasmic reticulum (SCAPER) have been found across ethnicities and have been shown to cause variable penetrance of an array of pathological traits, including intellectual disability, retinitis pigmentosa and ciliopathies.

METHODS

Human clinical phenotyping, surgical testicular sperm extraction and testicular tissue staining. Generation and analysis of short spindle 3 (ssp3) (SCAPER orthologue) Drosophila CAS9-knockout lines. In vitro microtubule (MT) binding assayed by total internal reflection fluorescence microscopy.

RESULTS

We show that patients homozygous for a SCAPER mutation lack SCAPER expression in spermatogonia (SPG) and are azoospermic due to early defects in spermatogenesis, leading to the complete absence of meiotic cells. Interestingly, Drosophila null mutants for the ubiquitously expressed ssp3 gene are viable and female fertile but male sterile. We further show that male sterility in ssp3 null mutants is due to failure in both chromosome segregation and cytokinesis. In cells undergoing male meiosis, the MTs emanating from the centrosomes do not appear to interact properly with the chromosomes, which remain dispersed within dividing spermatocytes (SPCs). In addition, mutant SPCs are unable to assemble a normal central spindle and undergo cytokinesis. Consistent with these results, an in vitro assay demonstrated that both SCAPER and Ssp3 directly bind MTs.

CONCLUSIONS

Our results show that SCAPER null mutations block the entry into meiosis of SPG, causing azoospermia. Null mutations in ssp3 specifically disrupt MT dynamics during male meiosis, leading to sterility. Moreover, both SCAPER and Ssp3 bind MTs in vitro. These results raise the intriguing possibility of a common feature between human and Drosophila meiosis.

Cilia signaling and obesity

An emerging number of rare genetic disorders termed ciliopathies are associated with pediatric obesity. It is becoming clear that the mechanisms associated with cilia dysfunction and obesity in these syndromes are complex. In addition to ciliopathic syndromic forms of obesity, several cilia-associated signaling gene mutations also lead to morbid obesity. While cilia have critical and diverse functions in energy homeostasis including their roles in centrally mediated food intake as well as in peripheral tissues, many questions remain. Here, we briefly discuss the syndromic ciliopathies and monoallelic cilia signaling gene mutations associated with obesity. We also describe potential ways cilia may be involved in common obesity. We discuss how neuronal cilia impact food intake potentially through leptin signaling and changes in ciliary G protein-coupled receptor (GPCR) signaling. We highlight several recent studies that have implicated the potential for cilia in peripheral tissues such as adipose and the pancreas to contribute to metabolic dysfunction. Then we discuss the potential for cilia to impact energy homeostasis through their roles in both development and adult tissue homeostasis. The studies discussed in this review highlight how a comprehensive understanding of the requirement of cilia for the regulation of diverse biological functions will contribute to our understanding of common forms of obesity.

The absence of BBSome function decreases synaptogenesis and causes ectopic synapse formation in the retina

Photoreceptors possess ribbon synapses distinct from the conventional synapses in the brain. Little is known about the function of the BBSome, a complex integral in ciliary and intracellular trafficking, in ribbon synaptic formation. We performed immunohistochemistry using retinas from Bardet-Biedl Syndrome (BBS) mouse models and found that BBS mutant animals have significantly fewer ribbon synapses in the outer plexiform layer and increased ectopic synapses in the outer nuclear layer compared to controls. Many ectopic synapses in BBS mutant retinas are associated with horizontal cell axonal processes that aberrantly intrude into the outer nuclear layer. To determine whether this horizontal cell phenotype is a consequence of retinal degeneration, we examined this phenotype in mice with photoreceptor-specific inactivation of the BBSome induced by Cre recombinase driven by the rhodopsin promoter. At three months of age, despite retinal degeneration, Bbs8^{floxed/floxed}; Rho-Cre⁺ mice lack the aberrant intrusion of horizontal cell processes. At 6 months, some horizontal cell processes intrude into the outer nuclear layer in Bbs8^{floxed/floxed}; Rho-Cre⁺ mice, but the phenotype does not recapitulate the phenotypic severity observed in young congenital BBS mutant mice. Therefore, the lack of BBSome function negatively impacts retinal synaptogenesis, and causes horizontal cell defects in a potentially cell-autonomous fashion.

Structure and activation mechanism of the BBSome membrane protein trafficking complex

Bardet-Biedl syndrome (BBS) is a currently incurable ciliopathy caused by the failure to correctly establish or maintain cilia-dependent signaling pathways. Eight proteins associated with BBS assemble into the BBSome, a key regulator of the ciliary membrane proteome. We report the electron cryomicroscopy (cryo-EM) structures of the native bovine BBSome in inactive and active states at 3.1 and 3.5 Å resolution, respectively. In the active state, the BBSome is bound to an Arf-family GTPase (ARL6/BBS3) that recruits the BBSome to ciliary membranes. ARL6 recognizes a composite binding site formed by BBS1 and BBS7 that is occluded in the inactive state. Activation requires an unexpected swiveling of the β-propeller domain of BBS1, the subunit most frequently implicated in substrate recognition, which widens a central cavity of the BBSome. Structural mapping of disease-causing mutations suggests that pathogenesis results from folding defects and the disruption of autoinhibition and activation.

Cilia in cystic kidney and other diseases

Epithelial cells lining the ducts and tubules of the kidney nephron and collecting duct have a single non-motile cilium projecting from their surface into the lumen of the tubule. These organelles were long considered vestigial remnants left as a result of evolution from a ciliated ancestor, but we now recognize them as critical sensory antennae. In the kidney, the polycystins and fibrocystin, products of the major human polycystic kidney disease genes, localize to this organelle. The polycystins and fibrocystin, through an unknown mechanism, monitor the diameter of the kidney tubules and regulate the proliferation and differentiation of the cells lining the tubule. When the polycystins, fibrocystin or cilia themselves are defective, the cell perceives this as a pro-proliferative signal, which leads to tubule dilation and cystic disease. In addition to critical roles in preventing cyst formation in the kidney, cilia are also important in cystic and fibrotic diseases of the liver and pancreas, and ciliary defects lead to a variety of developmental abnormalities that cause structural birth defects in most organs.

Defining the layers of a sensory cilium with STORM and cryoelectron nanoscopy

Primary cilia carry out numerous signaling and sensory functions, and defects in them, "ciliopathies," cause a range of symptoms, including blindness. Understanding of their nanometer-scale ciliary substructures and their disruptions in ciliopathies has been hindered by limitations of conventional microscopic techniques. We have combined cryoelectron tomography, enhanced by subtomogram averaging, with superresolution stochastic optical reconstruction microscopy (STORM) to define subdomains within the light-sensing rod sensory cilium of mouse retinas and reveal previously unknown substructures formed by resident proteins. Domains are demarcated by structural features such as the axoneme and its connections to the ciliary membrane, and are correlated with molecular markers of subcompartments, including the lumen and walls of the axoneme, the membrane glycocalyx, and the intervening cytoplasm. Within this framework, we report spatial distributions of key proteins in wild-type (WT) mice and the effects on them of genetic deficiencies in 3 models of Bardet-Biedl syndrome.

Bardet-Biedl Syndrome in rhesus macaques: A nonhuman primate model of retinitis pigmentosa

The development of therapies for retinal disorders is hampered by a lack of appropriate animal models. Higher nonhuman primates are the only animals with retinal structure similar to humans, including the presence of a macula and fovea. However, few nonhuman primate models of genetic retinal disease are known. We identified a lineage of rhesus macaques with a frameshift mutation in exon 3 of the BBS7 gene c.160delG (p.Ala54fs) that is predicted to produce a non-functional protein. In humans, mutations in this and other BBS genes cause Bardet-Biedl syndrome, a ciliopathy and a syndromic form of retinitis pigmentosa generally occurring in conjunction with kidney dysfunction, polydactyly, obesity, and/or hypogonadism. Three full- or half-sibling monkeys homozygous for the BBS7 c.160delG variant, at ages 3.5, 4 and 6 years old, displayed a combination of severe photoreceptor degeneration and progressive kidney disease. In vivo retinal imaging revealed features of severe macular degeneration, including absence of photoreceptor layers, degeneration of the retinal pigment epithelium, and retinal vasculature atrophy. Electroretinography in the 3.5-year-old case demonstrated loss of scotopic and photopic a-waves and markedly reduced and delayed b-waves. Histological assessments in the 4- and 6-year-old cases confirmed profound loss of photoreceptors and inner retinal neurons across the posterior retina, with dramatic thinning and disorganization of all cell layers, abundant microglia, absent or displaced RPE cells, and significant gliosis in the subretinal space. Retinal structure, including presence of photoreceptors, was preserved only in the far periphery. Ultrasound imaging of the kidneys revealed deranged architecture, and renal histopathology identified distorted contours with depressed, fibrotic foci and firmly adhered renal capsules; renal failure occurred in the 6-year-old case. Magnetic resonance imaging obtained in one case revealed abnormally low total brain volume and unilateral ventricular enlargement. The one male had abnormally small testes at 4 years of age, but polydactyly and obesity were not observed. Thus, monkeys homozygous for the BBS7 c.160delG variant closely mirrored several key features of the human BBS syndrome. This finding represents the first identification of a naturally-occurring nonhuman primate model of BBS, and more broadly the first such model of retinitis pigmentosa and a ciliopathy with an associated genetic mutation. This important new preclinical model will provide the basis for better understanding of disease progression and for the testing of new therapeutic options, including gene and cell-based therapies, not only for BBS but also for multiple forms of photoreceptor degeneration.

Progressive Characterization of Visual Phenotype in Bardet-Biedl Syndrome Mutant Mice

Purpose

Bardet-Biedl syndrome (BBS) is an archetypical ciliopathy caused by defective ciliary trafficking and consequent function. Insights gained from BBS mouse models are applicable to other syndromic and nonsyndromic retinal diseases. This progressive characterization of the visual phenotype in three BBS mouse models sets a baseline for testing therapeutic interventions.

Methods

Longitudinal acquisition of electroretinograms, optical coherence tomography scans, and visual acuity using the optomotor reflex in Bbs6/Mkks, Bbs8/Ttc8, and Bbs5 knockout mice. Gene and protein expression analysis in vivo and in vitro.

Results

Complete loss of BBS5, BBS6, or BBS8 leads to different rates of retinal degeneration and visual function over time. BBS8-deficient mice showed the fastest rate of degeneration, and BBS8 seems to be required for cone photoreceptors to reach functional maturity. In contrast, the loss of BBS5 (a further BBSome component) showed very little degeneration. Loss of BBS8 versus BBS5 resulted in different physiologic responses both in vivo and in vitro. BBS6-deficient mice show a slower rate of degeneration with both rod and cone function reducing at a similar rate.

Conclusions

The mouse models analyzed show distinct and diverging courses of degeneration upon loss of BBS5, BBS6, or BBS8, which can be used as a benchmark to test therapeutic interventions. Close consideration of the different phenotypes reveal subtle but important differences relating to their function. Because we also see differences in terms of phenotype depending on the type of visual assessment used, our data highlight the importance of using a combinatorial approach for assessment of visual function.

Meta-analysis of genotype-phenotype associations in Bardet-Biedl syndrome uncovers differences among causative genes

Bardet-Biedl syndrome (BBS) is a recessive genetic disease causing multiple organ anomalies. Most patients carry mutations in genes encoding for the subunits of the BBSome, an octameric ciliary transport complex, or accessory proteins involved in the BBSome assembly or function. BBS proteins have been extensively studied using in vitro, cellular, and animal models. However, the molecular functions of particular BBS proteins and the etiology of the BBS symptoms are still largely elusive. In this study, we applied a meta-analysis approach to study the genotype-phenotype association in humans using our database of all reported BBS patients. The analysis revealed that the identity of the causative gene and the character of the mutation partially predict the clinical outcome of the disease. Besides their potential use for clinical prognosis, our analysis revealed functional differences of particular BBS genes in humans. Core BBSome subunits BBS2, BBS7, and BBS9 manifest as more critical for the function and development of kidneys than peripheral subunits BBS1, BBS4, and BBS8/TTC8, suggesting that incomplete BBSome retains residual function at least in the kidney.

The Molecular Architecture of Native BBSome Obtained by an Integrated Structural Approach

The unique membrane composition of cilia is maintained by a diffusion barrier at the transition zone that is breached when the BBSome escorts signaling receptors out of cilia. Understanding how the BBSome removes proteins from cilia has been hampered by a lack of structural information. Here, we present a nearly complete Cα model of BBSome purified from cow retina. The model is based on a single-particle cryo-electron microscopy density map at 4.9-Å resolution that was interpreted with the help of comprehensive Rosetta-based structural modeling constrained by crosslinking mass spectrometry data. We find that BBSome subunits have a very high degree of interconnectivity, explaining the obligate nature of the complex. Furthermore, like other coat adaptors, the BBSome exists in an autoinhibited state in solution and must thus undergo a conformational change upon recruitment to membranes by the small GTPase ARL6/BBS3. Our model provides the first detailed view of the machinery enabling ciliary exit.