Cellular signalling by primary cilia in development, organ function and disease

Non-canonical CDK6 activity promotes cilia disassembly by suppressing axoneme polyglutamylation

Tubulin polyglutamylation is a posttranslational modification that occurs primarily along the axoneme of cilia. Defective axoneme polyglutamylation impairs cilia function and has been correlated with ciliopathies, including Joubert Syndrome (JBTS). However, the precise mechanisms regulating proper axoneme polyglutamylation remain vague. Here, we show that cyclin-dependent kinase 6 (CDK6), but not its paralog CDK4, localizes to the cilia base and suppresses axoneme polyglutamylation by phosphorylating RAB11 family interacting protein 5 (FIP5) at site S641, a critical regulator of cilia import of glutamylases. S641 phosphorylation disrupts the ciliary recruitment of FIP5 and its association with RAB11, thereby reducing the ciliary import of glutamylases. Encouragingly, the FDA-approved CDK4/6 inhibitor Abemaciclib can effectively restore cilia function in JBTS cells with defective glutamylation. In summary, our study elucidates the regulatory mechanisms governing axoneme polyglutamylation and suggests that developing CDK6-specific inhibitors could be a promising therapeutic strategy to enhance cilia function in ciliopathy patients.

TXNDC15, an ER-localized thioredoxin-like transmembrane protein, contributes to ciliary transition zone integrity

Primary cilia have specific proteins on their membrane to fulfill their sensory functions. Preservation of the specific protein composition of cilia relies on the barrier function of the transition zone (TZ) located at the ciliary base. Defects in cilia and the TZ cause ciliopathies, which have diverse clinical manifestations, including Meckel syndrome (MKS). Many of the proteins mutated in individuals with MKS are known to constitute the MKS module of the TZ. Although TXNDC15 (also known as MKS14) is a thioredoxin-related transmembrane protein that is localized mainly in the endoplasmic reticulum (ER) and is mutated in individuals with MKS, its role at the TZ or within cilia has not been characterized. Here, we show that TXNDC15-knockout cells have defects in MKS module assembly and in ciliary membrane protein localization. These defects in TXNDC15-knockout cells were not rescued by exogenous expression of any of the TXNDC15 constructs with MKS variations in the thioredoxin domain. Furthermore, TXNDC15 with mutations of two cysteine residues within the thioredoxin domain failed to rescue defects in TXNDC15-knockout cells, suggesting that TXNDC15 controls the TZ integrity from outside the TZ via its thioredoxin domain.

Regulation of the Cilia as a Potential Treatment for Senescence and Tumors: A Review

Millions of people worldwide die from malignant tumors every year, and the current clinical treatment is still based on radiotherapy and chemotherapy. Immunotherapy-adjuvant chemotherapy is widely applied, yet resistance to various factors persists in the management of advanced malignancies. Recently researchers have gradually discovered that the integrity of primary cilia is closely related to many diseases. The phenotypic changes in primary cilia are found in some cases of progeria, tumorigenesis, and drug resistance. Primary cilia seem to mediate signaling during these diseases. Hedgehog inhibitors have emerged in recent years to treat tumors by controlling signaling proteins on primary cilia. There is evidence for the use of anti-tumor drugs to treat senescence-related disease. Considering the close relationship between aging and obesity, as well as the obesity is the phenotype of many ciliopathies. Therefore, we speculate that some anti-tumor or anti-aging drugs can treat ciliopathies. Additionally, there is evidence suggesting that anti-aging drugs for tumor treatment, in which the process may be mediated by cilia. This review elucidates for the first time that cilia may be involved in the regulation of senescence, metabolic, tumorigenesis, and tumor resistance and hypothesizes that cilia can be regulated to treat these diseases in the future.

Impaired Glucose Metabolism, Primary Cilium Defects, and Kidney Cystogenesis in Glycogen Storage Disease Type Ia

Key Points

Metabolism adaptations due to glucose-6 phosphate accumulation in glycogen storage disease type Ia kidneys, toward a Warburg-like metabolism, promoted cell proliferation. Metabolic perturbations directly affected primary cilium structure and cystogenesis in glycogen storage disease type Ia kidneys.

Background

Glycogen storage disease type Ia (GSDIa) is a rare metabolic disorder caused by mutations in the catalytic subunit of glucose-6 phosphatase (G6PC1). This leads to severe hypoglycemia, and most young patients with GSDIa develop CKD. The kidney pathology is characterized by the development of cysts, which typically occur at an advanced stage of CKD.

Methods

To elucidate the molecular mechanisms responsible for cyst formation, we characterized renal metabolism, molecular pathways involved in cell proliferation, and primary cilium integrity using mice in which G6pc1 was specifically deleted in the kidney from an in utero stage.

Results

GSDIa mice exhibited kidney fibrosis, high inflammation, and cyst formation, leading to kidney dysfunction. In addition, the loss of G6PC1 led to the ectopic accumulation of glycogen and lipids in the kidneys and a metabolic shift toward a Warburg-like metabolism. This metabolic adaptation was due to an excess of glucose-6 phosphate, which supports cell proliferation, driven by the mitogen-activated protein kinase/extracellular signal–regulated kinases and protein kinase B/mammalian target of rapamycin pathways. Treatment of GSDIa mice with rapamycin, a target of the mammalian target of rapamycin pathway, reduced cell proliferation and kidney damage. Our results also identified lipocalin 2 as a contributor to renal inflammation and an early biomarker of CKD progression in GSDIa mice. Its inactivation partially prevented kidney lesions in GSDIa. Importantly, primary cilium defects were observed in the kidneys of GSDIa mice.

Conclusions

Metabolic adaptations because of glucose-6 phosphate accumulation in GSDIa renal tubules, toward a Warburg-like metabolism, promoted cell proliferation and cyst formation in a similar manner to that observed in various cystic kidney diseases. This was associated with downregulation of primary cilium gene expression and, consequently, altered cilium morphology.

Immunomodulatory potential of primary cilia in the skin

Primary cilia (PC) are essential signaling hubs for proper epithelial formation and the maintenance of skin homeostasis. Found on most cells in the human body, including skin cells, PC facilitate signal transduction that allows ciliated cells to interact with the immune system via multiple pathways, helping to maintain immune system homeostasis. PC can be altered by various microenvironmental stimuli to develop corresponding regulatory functions. Both PC and ciliary signaling pathways have been shown to be involved in the immune processes of various skin lesions. However, the mechanisms by which PC regulate cellular functions and maintain immune homeostasis in tissues are highly complex, and our understanding of them in the skin remains limited. In this paper, we discuss key ciliary signaling pathways and ciliated cells in the skin, with a focus on their immunomodulatory functions. We have compiled evidence from various cells, tissues and disease models to help explore the potential immunomodulatory effects of PC in the skin and their molecular mechanisms.

Influence of Hypoxia on the Airway Epithelium

The necessity of oxygen for metabolic processes means that hypoxia can lead to serious cell and tissue damage. On the other hand, in some situations, hypoxia occurs under physiological conditions and serves as an important regulation factor. The airway epithelium is specific in that it gains oxygen not only from the blood supply but also directly from the luminal air. Many respiratory diseases are associated with airway obstruction or excessive mucus production thus leading to luminal hypoxia. The main goal of this review is to point out how the airway epithelium reacts to hypoxic conditions. Cells detect low oxygen levels using molecular mechanisms involving hypoxia-inducible factors (HIFs). In addition, the cells of the airway epithelium appear to overexpress HIFs in hypoxic conditions. HIFs then regulate many aspects of epithelial cell functions. The effects of hypoxia include secretory cell stimulation and hyperplasia, epithelial barrier changes, and ciliogenesis impairment. All the changes can impair mucociliary clearance, exacerbate infection, and promote inflammation leading to damage of airway epithelium and subsequent airway wall remodeling. The modulation of hypoxia regulatory mechanisms may be one of the strategies for the treatment of obstructive respiratory diseases or diseases with mucus hyperproduction. Keywords: Secretory cells, Motile cilia, Epithelial barrier, Oxygenation, Obstructive respiratory diseases.

Motile Cilia in Female and Male Reproductive Tracts and Fertility

Motile cilia are evolutionarily conserved organelles. In humans, multiciliated cells (MCCs), assembling several hundred motile cilia on their apical surface, are components of the monolayer epithelia lining lower and upper airways, brain ventricles, and parts of the reproductive tracts, the fallopian tube and uterus in females, and efferent ductules in males. The coordinated beating of cilia generates a force that enables a shift of the tubular fluid, particles, or cells along the surface of the ciliated epithelia. Uncoordinated or altered cilia motion or cilia immotility may result in subfertility or even infertility. Here, we summarize the current knowledge regarding the localization and function of MCCs in the human reproductive tracts, discuss how cilia and cilia beating-generated fluid flow directly and indirectly contribute to the processes in these organs, and how lack or improper functioning of cilia influence human fertility.

C. elegans PPEF-type phosphatase (Retinal degeneration C ortholog) functions in diverse classes of cilia to regulate nematode behaviors

Primary (non-motile) cilia represent structurally and functionally diverse organelles whose roles as specialized cellular antenna are central to animal cell signaling pathways, sensory physiology and development. An ever-growing number of ciliary proteins, including those found in vertebrate photoreceptors, have been uncovered and linked to human disorders termed ciliopathies. Here, we demonstrate that an evolutionarily-conserved PPEF-family serine-threonine phosphatase, not functionally linked to cilia in any organism but associated with rhabdomeric (non-ciliary) photoreceptor degeneration in the Drosophila rdgC (retinal degeneration C) mutant, is a bona fide ciliary protein in C. elegans. The nematode protein, PEF-1, depends on transition zone proteins, which make up a 'ciliary gate' in the proximal-most region of the cilium, for its compartmentalization within cilia. Animals lacking PEF-1 protein function display structural defects to several types of cilia, including potential degeneration of microtubules. They also exhibit anomalies to cilium-dependent behaviors, including impaired responses to chemical, temperature, light, and noxious CO2 stimuli. Lastly, we demonstrate that PEF-1 function depends on conserved myristoylation and palmitoylation signals. Collectively, our findings broaden the role of PPEF proteins to include cilia, and suggest that the poorly-characterized mammalian PPEF1 and PPEF2 orthologs may also have ciliary functions and thus represent ciliopathy candidates.

Loss of Sc5d results in micrognathia due to a failure in osteoblast differentiation

INTRODUCTION

Mutations in genes related to cholesterol metabolism, or maternal diet and health status, affect craniofacial bone formation. However, the precise role of intracellular cholesterol metabolism in craniofacial bone development remains unclear.

OBJECTIVE

The aim of this study is to determine how cholesterol metabolism aberrations affect craniofacial bone development.

METHODS

Mice with a deficiency in Sc5d, which encodes an enzyme involved in cholesterol synthesis, were analyzed with histology, micro computed tomography (microCT), and cellular and molecular biological methods.

RESULTS

Sc5d null mice exhibited mandible hypoplasia resulting from defects in osteoblast differentiation. The activation of the hedgehog and WNT/β-catenin signaling pathways, which induce expression of osteogenic genes Col1a1 and Spp1, was compromised in the mandible of Sc5d null mice due to a failure in the formation of the primary cilium, a cell surface structure that senses extracellular cues. Treatments with an inducer of hedgehog or WNT/β-catenin signaling or with simvastatin, a drug that restores abnormal cholesterol production, partially rescued the defects in osteoblast differentiation seen in Sc5d mutant cells.

CONCLUSION

Our results indicate that loss of Sc5d results in mandibular hypoplasia through defective primary cilia-mediated hedgehog and WNT/β-catenin signaling pathways.

Large-scale genomic investigation of pediatric cholestasis reveals a novel hepatorenal ciliopathy caused by PSKH1 mutations

PURPOSE

Pediatric cholestasis is the phenotypic expression of clinically and genetically heterogeneous disorders of bile acid synthesis and flow. Although a growing number of monogenic causes of pediatric cholestasis have been identified, the majority of cases remain undiagnosed molecularly.

METHODS

In a cohort of 299 pediatric participants (279 families) with intrahepatic cholestasis, we performed exome sequencing as a first-tier diagnostic test.

RESULTS

A likely causal variant was identified in 135 families (48.56%). These comprise 135 families that harbor variants spanning 37 genes with established or tentative links to cholestasis. In addition, we propose a novel candidate gene (PSKH1) (HGNC:9529) in 4 families. PSKH1 was particularly compelling because of strong linkage in 3 consanguineous families who shared a novel hepatorenal ciliopathy phenotype. Two of the 4 families shared a founder homozygous variant, whereas the third and fourth had different homozygous variants in PSKH1. PSKH1 encodes a putative protein serine kinase of unknown function. Patient fibroblasts displayed abnormal cilia that are long and show abnormal transport. A homozygous Pskh1 mutant mouse faithfully recapitulated the human phenotype and displayed abnormally long cilia. The phenotype could be rationalized by the loss of catalytic activity observed for each recombinant PSKH1 variant using in vitro kinase assays.

CONCLUSION

Our results support the use of genomics in the workup of pediatric cholestasis and reveal PSKH1-related hepatorenal ciliopathy as a novel candidate monogenic form.

Rare homozygous cilia gene variants identified in consanguineous congenital heart disease patients

Congenital heart defects (CHD) appear in almost one percent of live births. Asian countries have the highest birth prevalence of CHD in the world. Recessive genotypes may represent a CHD risk factor in Asian populations with a high degree of consanguineous marriages. Genetic analysis of consanguineous families may represent a relatively unexplored source for investigating CHD etiology. To obtain insight into the contribution of recessive genotypes in CHD we analysed a cohort of forty-nine Pakistani CHD probands, originating from consanguineous unions. The majority (82%) of patient's malformations were septal defects. We identified protein altering, rare homozygous variants (RHVs) in the patient's coding genome by whole exome sequencing. The patients had a median of seven damaging RHVs each, and our analysis revealed a total of 758 RHVs in 693 different genes. By prioritizing these genes based on variant severity, loss-of-function intolerance and specific expression in the developing heart, we identified a set of 23 candidate disease genes. These candidate genes were significantly enriched for genes known to cause heart defects in recessive mouse models (P < 2.4e-06). In addition, we found a significant enrichment of cilia genes in both the initial set of 693 genes (P < 5.4e-04) and the 23 candidate disease genes (P < 5.2e-04). Functional investigation of ADCY6 in cell- and zebrafish-models verified its role in heart development. Our results confirm a significant role for cilia genes in recessive forms of CHD and suggest important functions of cilia genes in cardiac septation.

Structure, interaction and nervous connectivity of beta cell primary cilia

Primary cilia are sensory organelles present in many cell types, partaking in various signaling processes. Primary cilia of pancreatic beta cells play pivotal roles in paracrine signaling and their dysfunction is linked to diabetes. Yet, the structural basis for their functions is unclear. We present three-dimensional reconstructions of beta cell primary cilia by electron and expansion microscopy. These cilia are spatially confined within deep ciliary pockets or narrow spaces between cells, lack motility components and display an unstructured axoneme organization. Furthermore, we observe a plethora of beta cell cilia-cilia and cilia-cell interactions with other islet and non-islet cells. Most remarkably, we have identified and characterized axo-ciliary synapses between beta cell cilia and the cholinergic islet innervation. These findings highlight the beta cell cilia's role in islet connectivity, pointing at their function in integrating islet intrinsic and extrinsic signals and contribute to understanding their significance in health and diabetes.

Primary cilia shape postnatal astrocyte development through Sonic Hedgehog signaling

Primary cilia function as specialized signaling centers that regulate many cellular processes including neuron and glia development. Astrocytes possess cilia, but the function of cilia in astrocyte development remains largely unexplored. Critically, dysfunction of either astrocytes or cilia contributes to molecular changes observed in neurodevelopmental disorders. Here, we show that a sub-population of developing astrocytes in the prefrontal cortex are ciliated. This population corresponds to proliferating astrocytes and largely expresses the ciliary protein ARL13B. Genetic ablation of astrocyte cilia in vivo at two distinct stages of astrocyte development results in changes to Sonic Hedgehog (Shh) transcriptional targets. We show that Shh activity is decreased in immature and mature astrocytes upon loss of cilia. Furthermore, loss of cilia in immature astrocytes results in decreased astrocyte proliferation and loss of cilia in mature astrocytes causes enlarged astrocyte morphology. Together, these results indicate that astrocytes require cilia for Shh signaling throughout development and uncover functions for astrocyte cilia in regulating astrocyte proliferation and maturation. This expands our fundamental knowledge of astrocyte development and cilia function to advance our understanding of neurodevelopmental disorders.

ARL13B controls male reproductive tract physiology through primary and Motile Cilia

ARL13B is a small regulatory GTPase that controls ciliary membrane composition in both motile cilia and non-motile primary cilia. In this study, we investigated the role of ARL13B in the efferent ductules, tubules of the male reproductive tract essential to male fertility in which primary and motile cilia co-exist. We used a genetically engineered mouse model to delete Arl13b in efferent ductule epithelial cells, resulting in compromised primary and motile cilia architecture and functions. This deletion led to disturbances in reabsorptive/secretory processes and triggered an inflammatory response. The observed male reproductive phenotype showed significant variability linked to partial infertility, highlighting the importance of ARL13B in maintaining a proper physiological balance in these small ducts. These results emphasize the dual role of both motile and primary cilia functions in regulating efferent duct homeostasis, offering deeper insights into how cilia related diseases affect the male reproductive system.

Repulsive guidance molecules b (RGMb): molecular mechanism, function and role in diseases

Repulsive guidance molecule b (RGMb), a glycosylphosphatidylinositol-anchored member of the RGM family, is initially identified as a co-receptor of bone morphogenetic protein (BMP) in the nervous system. The expression of RGMb is transcriptionally regulated by dorsal root ganglion 11 (DRG11), which is a transcription factor expressed in embryonic DRG and dorsal horn neurons and plays an important role in the development of sensory circuits. RGMb is involved in important physiological processes such as embryonic development, immune response, intercellular adhesion and tumorigenesis. Furthermore, RGMb is mainly involved in the regulation of RGMb-neogenin-Rho and BMP signalling pathways. The recent discovery of programmed death-ligand 2 (PD-L2)-RGMb binding reveals that the cell signalling network and functional regulation centred on RGMb are extremely complex. The latest report suggests that down-regulation of the PD-L2-RGMb pathway in the gut microbiota promotes an anti-tumour immune response, which defines a potentially effective immune strategy. However, the biological function of RGMb in a variety of human diseases has not been fully determined, and will remain an active research field. This article reviews the properties and functions of RGMb, focusing on its role under various physiological and pathological conditions.

Permanent deconstruction of intracellular primary cilia in differentiating granule cell neurons

Primary cilia on granule cell neuron progenitors in the developing cerebellum detect sonic hedgehog to facilitate proliferation. Following differentiation, cerebellar granule cells become the most abundant neuronal cell type in the brain. While granule cell cilia are essential during early developmental stages, they become infrequent upon maturation. Here, we provide nanoscopic resolution of cilia in situ using large-scale electron microscopy volumes and immunostaining of mouse cerebella. In many granule cells, we found intracellular cilia, concealed from the external environment. Cilia were disassembled in differentiating granule cell neurons-in a process we call cilia deconstruction-distinct from premitotic cilia resorption in proliferating progenitors. In differentiating granule cells, cilia deconstruction involved unique disassembly intermediates, and, as maturation progressed, mother centriolar docking at the plasma membrane. Unlike ciliated neurons in other brain regions, our results show the deconstruction of concealed cilia in differentiating granule cells, which might prevent mitogenic hedgehog responsiveness. Ciliary deconstruction could be paradigmatic of cilia removal during differentiation in other tissues.

A review of CDKL: An underestimated protein kinase family

Cyclin-dependent kinase-like (CDKL) family proteins are serine/threonine protein kinases and is a specific branch of CMGC (including CDK, MAPK, GSK). Its name is due to the sequence similarity with CDK and it consists of 5 members. Their function in protein phosphorylation underpins their important role in cellular activities, including cell cycle, apoptosis, autophagy and microtubule dynamics. CDKL proteins have been demonstrated to regulate the length of primary cilium, which is a dynamic and diverse signaling hub and closely associated with multiple diseases. Furthermore, CDKL proteins have been shown to be involved in the development and progression of several diseases, including cancer, neurodegenerative diseases and kidney disease. In this review, we summarize the structural characteristics and discovered functions of CDKL proteins and their role in diseases, which might be helpful for the development of innovative therapeutic strategies for disease.

Increased ER stress by depletion of PDIA6 impairs primary ciliogenesis and enhances sensitivity to ferroptosis in kidney cells

Primary cilia are crucial for cellular balance, serving as sensors for external conditions. Nephronophthisis and related ciliopathies, which are hereditary and degenerative, stem from genetic mutations in cilia-related genes. However, the precise mechanisms of these conditions are still not fully understood. Our research demonstrates that downregulating PDIA6, leading to cilia removal, makes cells more sensitive to ferroptotic death caused by endoplasmic reticulum (ER) stress. The reduction of PDIA6 intensifies the ER stress response, while also impairing the regulation of primary cilia in various cell types. PDIA6 loss worsens ER stress, hastening ferroptotic death in proximal tubule epithelial cells, HK2 cells. Counteracting this ER stress can mitigate PDIA6 depletion effects, restoring both the number and length of cilia. Moreover, preventing ferroptosis corrects the disrupted primary ciliogenesis due to PDIA6 depletion in HK2 cells. Our findings emphasize the role of PDIA6 in primary ciliogenesis, and suggest its absence enhances ER stress and ferroptosis. These insights offer new therapeutic avenues for treating nephronophthisis and similar ciliopathies. [BMB Reports 2024; 57(10): 453-458].

Collaborative role of two distinct cilium-specific cytoskeletal systems in driving Hedgehog-responsive transcription factor trafficking

Calibrated transcriptional outputs in cellular signaling require fine regulation of transcription factor activity. In vertebrate Hedgehog (Hh) signaling, the precise output of the final effectors, the GLI (Glioma-associated-oncogene) transcription factors, depends on the primary cilium. In particular, the formation of the activator form of GLI upon pathway initiation requires its concentration at the distal cilium tip. However, the mechanisms underlying this critical step in Hh signaling are unclear. We developed a real-time imaging assay to visualize GLI2, the primary GLI activator isoform, at single particle resolution within the cilium. We observed that GLI2 is a cargo of Intraflagellar Transport (IFT) machinery and is transported with anterograde bias during a restricted time window following pathway activation. Our findings position IFT as a crucial mediator of transcription factor transport within the cilium for vertebrate Hh signaling, in addition to IFT's well-established role in ciliogenesis. Surprisingly, a conserved Hh pathway regulator, the atypical non-motile kinesin KIF7, is critical for the temporal control of GLI2 transport by IFT and the spatial control of GLI2 localization at the cilium tip. This discovery underscores the collaborative role of a motile and a non-motile cilium-specific cytoskeletal system in determining the transcriptional output during Hh signaling.

A prioritization tool for cilia-associated genes and their in vivo resources unveils new avenues for ciliopathy research

Defects in ciliary signaling or mutations in proteins that localize to primary cilia lead to a class of human diseases known as ciliopathies. Approximately 10% of mammalian genes encode cilia-associated proteins, and a major gap in the cilia research field is knowing which genes to prioritize to study and finding the in vivo vertebrate mutant alleles and reagents available for their study. Here, we present a unified resource listing the cilia-associated human genes cross referenced to available mouse and zebrafish mutant alleles, and their associated phenotypes, as well as expression data in the kidney and functional data for vertebrate Hedgehog signaling. This resource empowers researchers to easily sort and filter genes based on their own expertise and priorities, cross reference with newly generated -omics datasets, and quickly find in vivo resources and phenotypes associated with a gene of interest.

Amyloid precursor protein as a fibrosis marker in infants with biliary atresia

BACKGROUND

Biliary atresia (BA) is a rare condition of unknown origin in newborns with jaundice. In BA bile ducts are non-functional, causing neonatal cholestasis and following liver fibrosis and failure.

METHODS

This retrospective study included liver biopsies of 14 infants with BA aged [mean ± SD] 63 ± 23 days. Patients were grouped according to the clinical course (jaundice-free vs recurrent jaundice vs required liver transplantation or liver fibrosis (Ishak fibrosis score)) and followed for 1.61-5.64 years (mean 4.03). Transcriptome profiles were assessed using a panel of 768 fibrosis-specific genes, reanalyzed via qRT-PCR, and confirmed via immunostaining. Plasma from an additional 30 BA infants and 10 age-matched controls were used for amyloid precursor protein (APP) quantification by ELISA.

RESULTS

Different clinical outcome groups showed a homogeneous mRNA expression. Altered amyloid-metabolism-related gene expression was found between cases with Ishak fibrosis score greater than 4. Immunostaining confirmed a distinct presence of APP in the livers of all BA subjects. APP plasma levels were higher in BA than in age-matched controls and correlated with the histological fibrosis grade.

CONCLUSIONS

These results suggest that amyloidosis may contribute to BA and liver fibrosis, indicating that APP could serve as a potential liquid biomarker for these conditions.

IMPACT

Biliary atresia patients with higher fibrosis scores according to Ishak have higher hepatic expression of amyloid-related genes while amyloid precursor protein accumulates in the liver and increases in the circulation. After a recent study revealed beta-amyloid deposition as a mechanism potentially involved in biliary atresia, we were able to correlate amyloid-metabolism-related transcript levels as well as amyloid precursor protein tissue and plasma levels with the degree of hepatic fibrosis. These findings suggest that amyloid precursor protein is a fibrosis marker in infants with biliary atresia, reinforcing the role of amyloid metabolism in the pathogenesis of this serious disease.

Constructing condylar cartilage organoid to explore primary cilia functions

An organoid culture system better recapitulates the cellular structure, function, and interaction between cells and the extracellular matrix (ECM) than a two-dimensional (2D) culture system. We here constructed a condylar cartilage organoid to explore the regulatory role of primary cilia. Similar to the natural condylar cartilage, the condylar cartilage organoid exhibited abundant ECM and comprised superficial, proliferative, and hypertrophic zones. Primary cilia in the condylar cartilage organoid were shorter on average than those in the 2D culture chondrocytes, but their average length was equivalent to those in the natural condylar cartilage. Notably, primary cilia in each zone of the condylar cartilage organoid had an average length similar to that of primary cilia in the natural condylar cartilage. According to transcriptomic and biochemical analyses, the expression of cilia-related genes and cilia-related Hedgehog (HH) signaling differed between the condylar cartilage organoid and 2D culture systems. IFT88 knockdown promoted the protein levels of COL-Ⅹ, TRPV4, and HH signaling molecules in the condylar cartilage organoid, but decreased them in the 2D culture system. Notably, the protein levels of COL-Ⅹ, TRPV4, and HH signaling molecules increased in the superficial zone of the si IFT88 condylar cartilage organoid compared with the condylar cartilage organoid. However, the protein levels of aforementioned molecules were not significantly different in proliferative and hypertrophic zones. Collectively, we successfully constructed the condylar cartilage organoid with a better tissue structure and abundant ECM. Moreover, the condylar cartilage organoid is more suitable for exploring primary cilia functions.

Two functional forms of the Meckel-Gruber syndrome protein TMEM67 generated by proteolytic cleavage by ADAMTS9 mediate Wnt signaling and ciliogenesis

TMEM67 mutations are the major cause of Meckel-Gruber syndrome. TMEM67 is involved in both ciliary transition zone assembly, and non-canonical Wnt signaling mediated by its extracellular domain. How TMEM67 performs these two separate functions is not known. We identify a novel cleavage motif in the extracellular domain of TMEM67 cleaved by the extracellular matrix metalloproteinase ADAMTS9. This cleavage regulates the abundance of two functional forms: A C-terminal portion which localizes to the ciliary transition zone regulating ciliogenesis, and a non-cleaved form which regulates Wnt signaling. By characterizing three TMEM67 ciliopathy patient variants within the cleavage motif utilizing mammalian cell culture and C. elegans, we show the cleavage motif is essential for cilia structure and function, highlighting its clinical significance. We generated a novel non-cleavable TMEM67 mouse model which develop severe ciliopathies phenocopying Tmem67 -/- mice, but in contrast, undergo normal Wnt signaling, substantiating the existence of two functional forms of TMEM67.

Sonic Hedgehog activates prostaglandin signaling to stabilize primary cilium length

Sonic Hedgehog (SHH) is a driver of embryonic patterning that, when corrupted, triggers developmental disorders and cancers. SHH effector responses are organized through primary cilia (PC) that grow and retract with the cell cycle and in response to extracellular cues. Disruption of PC homeostasis corrupts SHH regulation, placing significant pressure on the pathway to maintain ciliary fitness. Mechanisms by which ciliary robustness is ensured in SHH-stimulated cells are not yet known. Herein, we reveal a crosstalk circuit induced by SHH activation of Phospholipase A2α that drives ciliary E-type prostanoid receptor 4 (EP4) signaling to ensure PC function and stabilize ciliary length. We demonstrate that blockade of SHH-EP4 crosstalk destabilizes PC cyclic AMP (cAMP) equilibrium, slows ciliary transport, reduces ciliary length, and attenuates SHH pathway induction. Accordingly, Ep4-/- mice display shortened neuroepithelial PC and altered SHH-dependent neuronal cell fate specification. Thus, SHH initiates coordination between distinct ciliary receptors to maintain PC function and length homeostasis for robust downstream signaling.

KIF2C/MCAK a prognostic biomarker and its oncogenic potential in malignant progression, and prognosis of cancer patients: a systematic review and meta-analysis as biomarker

KIF2C/MCAK (KIF2C) is the most well-characterized member of the kinesin-13 family, which is critical in the regulation of microtubule (MT) dynamics during mitosis, as well as interphase. This systematic review briefly describes the important structural elements of KIF2C, its regulation by multiple molecular mechanisms, and its broad cellular functions. Furthermore, it systematically summarizes its oncogenic potential in malignant progression and performs a meta-analysis of its prognostic value in cancer patients. KIF2C was shown to be involved in multiple crucial cellular processes including cell migration and invasion, DNA repair, senescence induction and immune modulation, which are all known to be critical during the development of malignant tumors. Indeed, an increasing number of publications indicate that KIF2C is aberrantly expressed in multiple cancer entities. Consequently, we have highlighted its involvement in at least five hallmarks of cancer, namely: genome instability, resisting cell death, activating invasion and metastasis, avoiding immune destruction and cellular senescence. This was followed by a systematic search of KIF2C/MCAK's expression in various malignant tumor entities and its correlation with clinicopathologic features. Available data were pooled into multiple weighted meta-analyses for the correlation between KIF2Chigh protein or gene expression and the overall survival in breast cancer, non-small cell lung cancer and hepatocellular carcinoma patients. Furthermore, high expression of KIF2C was correlated to disease-free survival of hepatocellular carcinoma. All meta-analyses showed poor prognosis for cancer patients with KIF2Chigh expression, associated with a decreased overall survival and reduced disease-free survival, indicating KIF2C's oncogenic potential in malignant progression and as a prognostic marker. This work delineated the promising research perspective of KIF2C with modern in vivo and in vitro technologies to further decipher the function of KIF2C in malignant tumor development and progression. This might help to establish KIF2C as a biomarker for the diagnosis or evaluation of at least three cancer entities.

Pleiotropy in FOXC1-attributable phenotypes involves altered ciliation and cilia-dependent signaling

Alterations to cilia are responsible for a wide range of severe disease; however, understanding of the transcriptional control of ciliogenesis remains incomplete. In this study we investigated whether altered cilia-mediated signaling contributes to the pleiotropic phenotypes caused by the Forkhead transcription factor FOXC1. Here, we show that patients with FOXC1-attributable Axenfeld-Rieger Syndrome (ARS) have a prevalence of ciliopathy-associated phenotypes comparable to syndromic ciliopathies. We demonstrate that altering the level of Foxc1 protein, via shRNA mediated inhibition, CRISPR/Cas9 mutagenesis and overexpression, modifies cilia length in vitro. These structural changes were associated with substantially perturbed cilia-dependent signaling [Hedgehog (Hh) and PDGFRα], and altered ciliary compartmentalization of the Hh pathway transcription factor, Gli2. Consistent with these data, in primary cultures of murine embryonic meninges, cilia length was significantly reduced in heterozygous and homozygous Foxc1 mutants compared to controls. Meningeal expression of the core Hh signaling components Gli1, Gli3 and Sufu was dysregulated, with comparable dysregulation of Pdgfrα signaling evident from significantly altered Pdgfrα and phosphorylated Pdgfrα expression. On the basis of these clinical and experimental findings, we propose a model that altered cilia-mediated signaling contributes to some FOXC1-induced phenotypes.

Cholangiocyte ciliary defects induce sustained epidermal growth factor receptor signaling

BACKGROUND AND AIMS

The primary cilium, an organelle that protrudes from cell surfaces, is essential for sensing extracellular signals. With disturbed cellular communication and chronic liver pathologies, this organelle's dysfunctions have been linked to disorders, including polycystic liver disease and cholangiocarcinoma. The goal of this study was to elucidate the relationship between primary cilia and the crucial regulator of cellular proliferation, the epidermal growth factor receptor (EGFR) signaling pathway, which has been associated with various clinical conditions.

APPROACH AND RESULTS

The study identified aberrant EGFR signaling pathways in cholangiocytes lacking functional primary cilia using liver-specific intraflagellar transport 88 knockout mice, a Pkhd1 mutant rat model, and human cell lines that did not have functional cilia. Cilia-deficient cholangiocytes showed persistent EGFR activation because of impaired receptor degradation, in contrast to their normal counterparts, where EGFR localization to the cilia promotes appropriate signaling. Using histone deacetylase 6 inhibitors to restore primary cilia accelerates EGFR degradation, thereby reducing maladaptive signaling. Importantly, experimental intervention with the histone deacetylase 6 inhibitor tubastatin A in an orthotopic rat model moved EGFR to cilia and reduced ERK phosphorylation. Concurrent administration of EGFR and histone deacetylase 6 inhibitors in cholangiocarcinoma and polycystic liver disease cells demonstrated synergistic antiproliferative effects, which were associated with the restoration of functioning primary cilia.

CONCLUSIONS

This study's findings shed light on ciliary function and robust EGFR signaling with slower receptor turnover. We could use therapies that restore the function of primary cilia to treat EGFR-driven diseases in polycystic liver disease and cholangiocarcinoma.

A class I PI3K signalling network regulates primary cilia disassembly in normal physiology and disease

Primary cilia are antenna-like organelles which sense extracellular cues and act as signalling hubs. Cilia dysfunction causes a heterogeneous group of disorders known as ciliopathy syndromes affecting most organs. Cilia disassembly, the process by which cells lose their cilium, is poorly understood but frequently observed in disease and upon cell transformation. Here, we uncover a role for the PI3Kα signalling enzyme in cilia disassembly. Genetic PI3Kα-hyperactivation, as observed in PIK3CA-related overgrowth spectrum (PROS) and cancer, induced a ciliopathy-like phenotype during mouse development. Mechanistically, PI3Kα and PI3Kβ produce the PIP3 lipid at the cilia transition zone upon disassembly stimulation. PI3Kα activation initiates cilia disassembly through a kinase signalling axis via the PDK1/PKCι kinases, the CEP170 centrosomal protein and the KIF2A microtubule-depolymerising kinesin. Our data suggest diseases caused by PI3Kα-activation may be considered 'Disorders with Ciliary Contributions', a recently-defined subset of ciliopathies in which some, but not all, of the clinical manifestations result from cilia dysfunction.

Primary cilia-associated signalling in squamous cell carcinoma of head and neck region

Squamous cell carcinoma (SCC) of the head and neck originates from the mucosal lining of the upper aerodigestive tract, including the lip, tongue, nasopharynx, oropharynx, larynx and hypopharynx. In this review, we summarise what is currently known about the potential function of primary cilia in the pathogenesis of this disease. As primary cilia represent a key cellular structure for signal transduction and are related to cell proliferation, an understanding of their role in carcinogenesis is necessary for the design of new treatment approaches. Here, we introduce cilia-related signalling in head and neck squamous cell carcinoma (HNSCC) and its possible association with HNSCC tumorigenesis. From this point of view, PDGF, EGF, Wnt and Hh signalling are discussed as all these pathways were found to be dysregulated in HNSCC. Moreover, we review the clinical potential of small molecules affecting primary cilia signalling to target squamous cell carcinoma of the head and neck area.

BET Inhibition Rescues Acinar-Ductal-Metaplasia and Ciliogenesis and Ameliorates Chronic Pancreatitis-Driven Changes in Mice With Loss of the Polarity Protein Par3

BACKGROUND & AIMS

The apical-basal polarity of pancreatic acinar cells is essential for maintaining tissue architecture. However, the mechanisms by which polarity proteins regulate acinar pancreas injury and regeneration are poorly understood.

METHODS

Cerulein-induced pancreatitis was induced in mice with conditional deletion of the polarity protein Par3 in the pancreas. The impact of Par3 loss on pancreas injury and regeneration was assessed by histologic analyses and transcriptional profiling by RNA sequencing. Mice were pretreated with the bromodomain and extraterminal domain (BET) inhibitor JQ1 before cotreatment with cerulein to determine the effect of BET inhibition on pancreas injury and regeneration.

RESULTS

Initially, we show that Par3 is increased in acinar-ductal metaplasia (ADM) lesions present in human and mouse chronic pancreatitis specimens. Although Par3 loss disrupts tight junctions, Par3 is dispensable for pancreatogenesis. However, with aging, Par3 loss results in low-grade inflammation, acinar degeneration, and pancreatic lipomatosis. Par3 loss exacerbates acute pancreatitis-induced injury and chronic pancreatitis-induced acinar cell loss, promotes pancreatic lipomatosis, and prevents regeneration. Par3 loss also results in suppression of chronic pancreatitis-induced ADM and primary ciliogenesis. Notably, targeting BET proteins attenuates chronic pancreatitis-induced loss of primary cilia and promotes ADM in mice lacking pancreatic Par3. Targeting BET proteins also attenuates cerulein-induced acinar cell loss and enhances recovery of acinar cell mass and body weight of mice lacking pancreatic Par3.

CONCLUSIONS

Combined, this study demonstrates how Par3 restrains chronic pancreatitis-induced changes in the pancreas and identifies a potential role for BET inhibitors to attenuate pancreas injury and facilitate regeneration.

Defects in diffusion barrier function of ciliary transition zone caused by ciliopathy variations of TMEM218

Primary cilia are antenna-like structures protruding from the surface of various eukaryotic cells, and have distinct protein compositions in their membranes. This distinct protein composition is maintained by the presence of the transition zone (TZ) at the ciliary base, which acts as a diffusion barrier between the ciliary and plasma membranes. Defects in cilia and the TZ are known to cause a group of disorders collectively called the ciliopathies, which demonstrate a broad spectrum of clinical features, such as perinatally lethal Meckel syndrome (MKS), relatively mild Joubert syndrome (JBTS), and nonsyndromic nephronophthisis (NPHP). Proteins constituting the TZ can be grouped into the MKS and NPHP modules. The MKS module is composed of several transmembrane proteins and three soluble proteins. TMEM218 was recently reported to be mutated in individuals diagnosed as MKS and JBTS. However, little is known about how TMEM218 mutations found in MKS and JBTS affect the functions of cilia. In this study, we found that ciliary membrane proteins were not localized to cilia in TMEM218-knockout cells, indicating impaired barrier function of the TZ. Furthermore, the exogenous expression of JBTS-associated TMEM218 variants but not MKS-associated variants in TMEM218-knockout cells restored the localization of ciliary membrane proteins. In particular, when expressed in TMEM218-knockout cells, the TMEM218(R115H) variant found in JBTS was able to restore the barrier function of cells, whereas the MKS variant TMEM218(R115C) could not. Thus, the severity of symptoms of MKS and JBTS individuals appears to correlate with the degree of their ciliary defects at the cellular level.

From Blur to Brilliance: The Ascendance of Advanced Microscopy in Neuronal Cell Biology

The intricate network of the brain's neurons and synapses poses unparalleled challenges for research, distinct from other biological studies. This is particularly true when dissecting how neurons and their functional units work at a cell biological level. While traditional microscopy has been foundational, it was unable to reveal the deeper complexities of neural interactions. However, an imaging renaissance has transformed our capabilities. Advancements in light and electron microscopy, combined with correlative imaging, now achieve unprecedented resolutions, uncovering the most nuanced neural structures. Maximizing these tools requires more than just technical proficiency. It is crucial to align research aims, allocate resources wisely, and analyze data effectively. At the heart of this evolution is interdisciplinary collaboration, where various experts come together to translate detailed imagery into significant biological insights. This review navigates the latest developments in microscopy, underscoring both the promise of and prerequisites for bending this powerful tool set to understanding neuronal cell biology.

Primary cilia abnormalities participate in the occurrence of spontaneous abortion through TGF-β/SMAD2/3 signaling pathway

Spontaneous abortion is the most common complication in early pregnancy, the exact etiology of most cases cannot be determined. Emerging studies suggest that mutations in ciliary genes may be associated with progression of pregnancy loss. However, the involvement of primary cilia on spontaneous abortion and the underlying molecular mechanisms remains poorly understood. We observed the number and length of primary cilia were significantly decreased in decidua of spontaneous abortion in human and lipopolysaccharide (LPS)-induced abortion mice model, accompanied with increased expression of proinflammatory cytokines interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α. The length of primary cilia in human endometrial stromal cell (hESC) was significantly shortened after TNF-α treatment. Knocking down intraflagellar transport 88 (IFT88), involved in cilia formation and maintenance, promoted the expression of TNF-α. There was a reverse regulatory relationship between cilia shortening and TNF-α expression. Further research found that shortened cilia impair decidualization in hESC through transforming growth factor (TGF)-β/SMAD2/3 signaling. Primary cilia were impaired in decidua tissue of spontaneous abortion, which might be mainly caused by inflammatory injury. Primary cilia abnormalities resulted in dysregulation of TGF-β/SMAD2/3 signaling transduction and decidualization impairment, which led to spontaneous abortion.

PKD2 regulates autophagy and forms a protein complex with BECN1 at the primary cilium of hypothalamic neuronal cells

The primary cilium, hereafter cilium, is an antenna-like organelle that modulates intracellular responses, including autophagy, a lysosomal degradation process essential for cell homeostasis. Dysfunction of the cilium is associated with impairment of autophagy and diseases known as "ciliopathies". The discovery of autophagy-related proteins at the base of the cilium suggests its potential role in coordinating autophagy initiation in response to physiopathological stimuli. One of these proteins, beclin-1 (BECN1), it which is necessary for autophagosome biogenesis. Additionally, polycystin-2 (PKD2), a calcium channel enriched at the cilium, is required and sufficient to induce autophagy in renal and cancer cells. We previously demonstrated that PKD2 and BECN1 form a protein complex at the endoplasmic reticulum in non-ciliated cells, where it initiates autophagy, but whether this protein complex is present at the cilium remains unknown. Anorexigenic pro-opiomelanocortin (POMC) neurons are ciliated cells that require autophagy to maintain intracellular homeostasis. POMC neurons are sensitive to metabolic changes, modulating signaling pathways crucial for controlling food intake. Exposure to the saturated fatty acid palmitic acid (PA) reduces ciliogenesis and inhibits autophagy in these cells. Here, we show that PKD2 and BECN1 form a protein complex in N43/5 cells, an in vitro model of POMC neurons, and that both PKD2 and BECN1 locate at the cilium. In addition, our data show that the cilium is required for PKD2-BECN1 protein complex formation and that PA disrupts the PKD2-BECN1 complex, suppressing autophagy. Our findings provide new insights into the mechanisms by which the cilium controls autophagy in hypothalamic neuronal cells.

STIL facilitates the development and malignant progression of triple-negative breast cancer through activation of Fanconi anemia pathway via interacting with KLF16

BACKGROUND

STIL is an important cell cycle-regulating protein specifically recruited to the mitotic centrosome to promote the replication of centrioles in dividing cells. However, the potential role of STIL in the regulation of the biological functions of triple-negative breast cancer remains still unclear.

METHODS

We screened for differentially expressed STIL in the Cancer Genome Atlas database. The expression of STIL protein in 10 pairs of breast cancer tissues and adjacent normal tissues was further assessed by western blotting. Functionally, the knockdown and overexpression of STIL have been used to explore the effects of STIL on breast cancer cell proliferation, migration, and invasion. Mechanistically, RNA-seq, dual-luciferase reporter assay, chromatin immunoprecipitation assay, mass spectrometry, immunoprecipitation assay, and DNA pull-down assay were performed.

RESULTS

Breast cancer tissues and cells have higher STIL expression than normal tissues and cells. STIL knockdown impairs breast cancer cell growth, migration, and invasion, whereas STIL overexpression accelerates these processes. STIL promotes breast cancer progression by regulating FANCD2 expression, and exploration of its molecular mechanism demonstrated that STIL interacts with KLF16 to regulate the expression of FANCD2.

CONCLUSIONS

Collectively, our findings identified STIL as a critical promoter of breast cancer progression that interacts with KLF16 to regulate Fanconi anemia pathway protein FANCD2. In summary, STIL is a potential novel biomarker and therapeutic target for breast cancer.

Both intrinsic and microenvironmental factors contribute to the regulation of stem cell quiescence

Precise regulation of stem cell quiescence is essential for tissue development and homeostasis. Therefore, its aberrant regulation is intimately correlated with various human diseases. However, the detailed mechanisms of stem cell quiescence and its specific role in the pathogenesis of various diseases remain to be determined. Recent studies have revealed that the intrinsic and microenvironmental factors are the potential candidates responsible for the orderly switch between the dormant and activated states of stem cells. In addition, defects in signaling pathways related to internal and external factors of stem cells might contribute to the initiation and development of diseases by altering the dormancy of stem cells. In this review, we focus on the mechanisms underlying stem cell quiescence, especially the involvement of intrinsic and microenvironmental factors. In addition, we discuss the relationship between the anomalies of stem cell quiescence and related diseases, hopefully providing therapeutic insights for developing novel treatments.

Ciliary biology intersects autism and congenital heart disease

Autism spectrum disorder (ASD) commonly co-occurs with congenital heart disease (CHD), but the molecular mechanisms underlying this comorbidity remain unknown. Given that children with CHD come to clinical attention by the newborn period, understanding which CHD variants carry ASD risk could provide an opportunity to identify and treat individuals at high risk for developing ASD far before the typical age of diagnosis. Therefore, it is critical to delineate the subset of CHD genes most likely to increase the risk of ASD. However, to date there is relatively limited overlap between high confidence ASD and CHD genes, suggesting that alternative strategies for prioritizing CHD genes are necessary. Recent studies have shown that ASD gene perturbations commonly dysregulate neural progenitor cell (NPC) biology. Thus, we hypothesized that CHD genes that disrupt neurogenesis are more likely to carry risk for ASD. Hence, we performed an in vitro pooled CRISPR interference (CRISPRi) screen to identify CHD genes that disrupt NPC biology similarly to ASD genes. Overall, we identified 45 CHD genes that strongly impact proliferation and/or survival of NPCs. Moreover, we observed that a cluster of physically interacting ASD and CHD genes are enriched for ciliary biology. Studying seven of these genes with evidence of shared risk (CEP290, CHD4, KMT2E, NSD1, OFD1, RFX3, TAOK1), we observe that perturbation significantly impacts primary cilia formation in vitro. While in vivo investigation of TAOK1 reveals a previously unappreciated role for the gene in motile cilia formation and heart development, supporting its prediction as a CHD risk gene. Together, our findings highlight a set of CHD risk genes that may carry risk for ASD and underscore the role of cilia in shared ASD and CHD biology.

Cilia-enriched oxysterol 7β,27-DHC is required for polycystin ion channel activation

Polycystin-1 (PC-1) and PC-2 form a heteromeric ion channel complex that is abundantly expressed in primary cilia of renal epithelial cells. This complex functions as a non-selective cation channel, and mutations within the polycystin complex cause autosomal dominant polycystic kidney disease (ADPKD). The spatial and temporal regulation of the polycystin complex within the ciliary membrane remains poorly understood. Using both whole-cell and ciliary patch-clamp recordings, we identify a cilia-enriched oxysterol, 7β,27-dihydroxycholesterol (DHC), that serves as a necessary activator of the polycystin complex. We further identify an oxysterol-binding pocket within PC-2 and showed that mutations within this binding pocket disrupt 7β,27-DHC-dependent polycystin activation. Pharmacologic and genetic inhibition of oxysterol synthesis reduces channel activity in primary cilia. In summary, our findings reveal a regulator of the polycystin complex. This oxysterol-binding pocket in PC-2 may provide a specific target for potential ADPKD therapeutics.

Ontogeny of Skin Stem Cells and Molecular Underpinnings

Skin stem cells (SCs) play a pivotal role in supporting tissue homeostasis. Several types of SCs are responsible for maintaining and regenerating skin tissue. These include bulge SCs and others residing in the interfollicular epidermis, infundibulum, isthmus, sebaceous glands, and sweat glands. The emergence of skin SCs commences during embryogenesis, where multipotent SCs arise from various precursor populations. These early events set the foundation for the diverse pool of SCs that will reside in the adult skin, ready to respond to tissue repair and regeneration demands. A network of molecular cues regulates skin SC behavior, balancing quiescence, self-renewal, and differentiation. The disruption of this delicate equilibrium can lead to SC exhaustion, impaired wound healing, and pathological conditions such as skin cancer. The present review explores the intricate mechanisms governing the development, activation, and differentiation of skin SCs, shedding light on the molecular signaling pathways that drive their fate decisions and skin homeostasis. Unraveling the complexities of these molecular drivers not only enhances our fundamental knowledge of skin biology but also holds promise for developing novel strategies to modulate skin SC fate for regenerative medicine applications, ultimately benefiting patients with skin disorders and injuries.

Retinal primary cilia and their dysfunction in retinal neurodegenerative diseases: beyond ciliopathies

Primary cilia are sensory organelles that extend from the cellular membrane and are found in a wide range of cell types. Cilia possess a plethora of vital components that enable the detection and transmission of several signaling pathways, including Wnt and Shh. In turn, the regulation of ciliogenesis and cilium length is influenced by various factors, including autophagy, organization of the actin cytoskeleton, and signaling inside the cilium. Irregularities in the development, maintenance, and function of this cellular component lead to a range of clinical manifestations known as ciliopathies. The majority of people with ciliopathies have a high prevalence of retinal degeneration. The most common theory is that retinal degeneration is primarily caused by functional and developmental problems within retinal photoreceptors. The contribution of other ciliated retinal cell types to retinal degeneration has not been explored to date. In this review, we examine the occurrence of primary cilia in various retinal cell types and their significance in pathology. Additionally, we explore potential therapeutic approaches targeting ciliopathies. By engaging in this endeavor, we present new ideas that elucidate innovative concepts for the future investigation and treatment of retinal ciliopathies.

Primary cilia mediate skeletogenic BMP and Hedgehog signaling in heterotopic ossification

Heterotopic ossification (HO), defined as the formation of extraskeletal bone in muscle and soft tissues, is a diverse pathological process caused by either genetic mutations or inciting trauma. Fibrodysplasia ossificans progressiva (FOP) is a genetic form of HO caused by mutations in the bone morphogenetic protein (BMP) type I receptor gene activin A receptor type 1 (ACVR1). These mutations make ACVR1 hypersensitive to BMP and responsive to activin A. Hedgehog (Hh) signaling also contributes to HO development. However, the exact pathophysiology of how skeletogenic cells contribute to endochondral ossification in FOP remains unknown. Here, we showed that the wild-type or FOP-mutant ACVR1 localized in the cilia of stem cells from human exfoliated deciduous teeth with key FOP signaling components, including activin A receptor type 2A/2B, SMAD family member 1/5, and FK506-binding protein 12kD. Cilia suppression by deletion of intraflagellar transport 88 or ADP ribosylation factor like GTPase 3 effectively inhibited pathological BMP and Hh signaling, subdued aberrant chondro-osteogenic differentiation in primary mouse or human FOP cells, and diminished in vivo extraskeletal ossification in Acvr1Q207D, Sox2-Cre; Acvr1R206H/+ FOP mice and in burn tenotomy-treated wild-type mice. Our results provide a rationale for early and localized suppression of cilia in affected tissues after injury as a therapeutic strategy against either genetic or acquired HO.

Basis of gene-specific transcription regulation by the Integrator complex

The Integrator complex attenuates gene expression via the premature termination of RNA polymerase II (RNAP2) at promoter-proximal pausing sites. It is required for stimulus response, cell differentiation, and neurodevelopment, but how gene-specific and adaptive regulation by Integrator is achieved remains unclear. Here, we identify two sites on human Integrator subunits 13/14 that serve as binding hubs for sequence-specific transcription factors (TFs) and other transcription effector complexes. When Integrator is attached to paused RNAP2, these hubs are positioned upstream of the transcription bubble, consistent with simultaneous TF-promoter tethering. The TFs co-localize with Integrator genome-wide, increase Integrator abundance on target genes, and co-regulate responsive transcriptional programs. For instance, sensory cilia formation induced by glucose starvation depends on Integrator-TF contacts. Our data suggest TF-mediated promoter recruitment of Integrator as a widespread mechanism for targeted transcription regulation.

Biliary atresia

Biliary atresia (BA) is a progressive inflammatory fibrosclerosing disease of the biliary system and a major cause of neonatal cholestasis. It affects 1:5,000-20,000 live births, with the highest incidence in Asia. The pathogenesis is still unknown, but emerging research suggests a role for ciliary dysfunction, redox stress and hypoxia. The study of the underlying mechanisms can be conceptualized along the likely prenatal timing of an initial insult and the distinction between the injury and prenatal and postnatal responses to injury. Although still speculative, these emerging concepts, new diagnostic tools and early diagnosis might enable neoadjuvant therapy (possibly aimed at oxidative stress) before a Kasai portoenterostomy (KPE). This is particularly important, as timely KPE restores bile flow in only 50-75% of patients of whom many subsequently develop cholangitis, portal hypertension and progressive fibrosis; 60-75% of patients require liver transplantation by the age of 18 years. Early diagnosis, multidisciplinary management, centralization of surgery and optimized interventions for complications after KPE lead to better survival. Postoperative corticosteroid use has shown benefits, whereas the role of other adjuvant therapies remains to be evaluated. Continued research to better understand disease mechanisms is necessary to develop innovative treatments, including adjuvant therapies targeting the immune response, regenerative medicine approaches and new clinical tests to improve patient outcomes.

A Short Sequence Targets Transmembrane Proteins to Primary Cilia

Primary cilia are finger-like sensory organelles that extend from the bodies of most cell types and have a distinct lipid and protein composition from the plasma membrane. This partitioning is maintained by a diffusion barrier that restricts the entry of non-ciliary proteins, and allows the selective entry of proteins harboring a ciliary targeting sequence (CTS). However, CTSs are not stereotyped and previously reported sequences are insufficient to drive efficient ciliary localisation across diverse cell types. Here, we describe a short peptide sequence that efficiently targets transmembrane proteins to primary cilia in all tested cell types, including human neurons. We generate human-induced pluripotent stem cell (hiPSC) lines stably expressing a transmembrane construct bearing an extracellular HaloTag and intracellular fluorescent protein, which enables the bright, specific labeling of primary cilia in neurons and other cell types to facilitate studies of cilia in health and disease. We demonstrate the utility of this resource by developing an image analysis pipeline for the automated measurement of primary cilia to detect changes in their length associated with altered signaling or disease state.

The cellular protrusions for inter-cellular material transfer: similarities between filopodia, cytonemes, tunneling nanotubes, viruses, and extracellular vesicles

Extracellular vesicles (EVs) are crucial for transferring bioactive materials between cells and play vital roles in both health and diseases. Cellular protrusions, including filopodia and microvilli, are generated by the bending of the plasma membrane and are considered to be rigid structures facilitating various cellular functions, such as cell migration, adhesion, and environment sensing. Compelling evidence suggests that these protrusions are dynamic and flexible structures that can serve as sources of a new class of EVs, highlighting the unique role they play in intercellular material transfer. Cytonemes are specialized filopodia protrusions that make direct contact with neighboring cells, mediating the transfer of bioactive materials between cells through their tips. In some cases, these tips fuse with the plasma membrane of neighboring cells, creating tunneling nanotubes that directly connect the cytosols of the adjacent cells. Additionally, virus particles can be released from infected cells through small bud-like of plasma membrane protrusions. These different types of protrusions, which can transfer bioactive materials, share common protein components, including I-BAR domain-containing proteins, actin cytoskeleton, and their regulatory proteins. The dynamic and flexible nature of these protrusions highlights their importance in cellular communication and material transfer within the body, including development, cancer progression, and other diseases.

The ciliary protein C2cd3 is required for mandibular musculoskeletal tissue patterning

The mandible is composed of several musculoskeletal tissues including bone, cartilage, and tendon that require precise patterning to ensure structural and functional integrity. Interestingly, most of these tissues are derived from one multipotent cell population called cranial neural crest cells (CNCCs). How CNCCs are properly instructed to differentiate into various tissue types remains nebulous. To better understand the mechanisms necessary for the patterning of mandibular musculoskeletal tissues we utilized the avian mutant talpid2 (ta2) which presents with several malformations of the facial skeleton including dysplastic tendons, mispatterned musculature, and bilateral ectopic cartilaginous processes extending off Meckel's cartilage. We found an ectopic epithelial BMP signaling domain in the ta2 mandibular prominence (MNP) that correlated with the subsequent expansion of SOX9+ cartilage precursors. These findings were validated with conditional murine models suggesting an evolutionarily conserved mechanism for CNCC-derived musculoskeletal patterning. Collectively, these data support a model in which cilia are required to define epithelial signal centers essential for proper musculoskeletal patterning of CNCC-derived mesenchyme.

Neurons dispose of hyperactive kinesin into glial cells for clearance

Microtubule-based kinesin motor proteins are crucial for intracellular transport, but their hyperactivation can be detrimental for cellular functions. This study investigated the impact of a constitutively active ciliary kinesin mutant, OSM-3CA, on sensory cilia in C. elegans. Surprisingly, we found that OSM-3CA was absent from cilia but underwent disposal through membrane abscission at the tips of aberrant neurites. Neighboring glial cells engulf and eliminate the released OSM-3CA, a process that depends on the engulfment receptor CED-1. Through genetic suppressor screens, we identified intragenic mutations in the OSM-3CA motor domain and mutations inhibiting the ciliary kinase DYF-5, both of which restored normal cilia in OSM-3CA-expressing animals. We showed that conformational changes in OSM-3CA prevent its entry into cilia, and OSM-3CA disposal requires its hyperactivity. Finally, we provide evidence that neurons also dispose of hyperactive kinesin-1 resulting from a clinic variant associated with amyotrophic lateral sclerosis, suggesting a widespread mechanism for regulating hyperactive kinesins.

Pkd1l1-deficiency drives biliary atresia through ciliary dysfunction in biliary epithelial cells

BACKGROUND & AIMS

Syndromic biliary atresia is a cholangiopathy characterized by fibro-obliterative changes in the extrahepatic bile duct (EHBD) and congenital malformations including laterality defects. The etiology remains elusive and faithful animal models are lacking. Genetic syndromes provide important clues regarding the pathogenic mechanisms underlying the disease. We investigated the role of the gene Pkd1l1 in the pathophysiology of syndromic biliary atresia.

METHODS

Constitutive and conditional Pkd1l1 knockout mice were generated to explore genetic pathology as a cause of syndromic biliary atresia. We investigated congenital malformations, EHBD and liver pathology, EHBD gene expression, and biliary epithelial cell turnover. Biliary drainage was functionally assessed with cholangiography. Histology and serum chemistries were assessed after DDC (3,5-diethoxycarbony l-1,4-dihydrocollidine) diet treatment and inhibition of the ciliary signaling effector GLI1.

RESULTS

Pkd1l1-deficient mice exhibited congenital anomalies including malrotation and heterotaxy. Pkd1l1-deficient EHBDs were hypertrophic and fibrotic. Pkd1l1-deficient EHBDs were patent but displayed delayed biliary drainage. Pkd1l1-deficient livers exhibited ductular reaction and periportal fibrosis. After DDC treatment, Pkd1l1-deficient mice exhibited EHBD obstruction and advanced liver fibrosis. Pkd1l1-deficient mice had increased expression of fibrosis and extracellular matrix remodeling genes (Tgfα, Cdkn1a, Hb-egf, Fgfr3, Pdgfc, Mmp12, and Mmp15) and decreased expression of genes mediating ciliary signaling (Gli1, Gli2, Ptch1, and Ptch2). Primary cilia were reduced on biliary epithelial cells and altered expression of ciliogenesis genes occurred in Pkd1l1-deficient mice. Small molecule inhibition of the ciliary signaling effector GLI1 with Gant61 recapitulated Pkd1l1-deficiency.

CONCLUSIONS

Pkd1l1 loss causes both laterality defects and fibro-proliferative EHBD transformation through disrupted ciliary signaling, phenocopying syndromic biliary atresia. Pkd1l1-deficient mice function as an authentic genetic model for study of the pathogenesis of biliary atresia.

IMPACT AND IMPLICATIONS

The syndromic form of biliary atresia is characterized by fibro-obliteration of extrahepatic bile ducts and is often accompanied by laterality defects. The etiology is unknown, but Pkd1l1 was identified as a potential genetic candidate for syndromic biliary atresia. We found that loss of the ciliary gene Pkd1l1 contributes to hepatobiliary pathology in biliary atresia, exhibited by bile duct hypertrophy, reduced biliary drainage, and liver fibrosis in Pkd1l1-deficient mice. Pkd1l1-deficient mice serve as a genetic model of biliary atresia and reveal ciliopathy as an etiology of biliary atresia. This model will help scientists uncover new therapeutic approaches for patients with biliary atresia, while pediatric hepatologists should validate the diagnostic utility of PKD1L1 variants.

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.

Inhibition of a cyclic nucleotide-gated channel on neuronal cilia activates unfolded protein response in intestinal cells to promote longevity

Ciliary defects are linked to ciliopathies, but impairments in the sensory cilia of Caenorhabditis elegans neurons extend lifespan, a phenomenon with previously unclear mechanisms. Our study reveals that neuronal cilia defects trigger the unfolded protein response of the endoplasmic reticulum (UPRER) within intestinal cells, a process dependent on the insulin/insulin-like growth factor 1 (IGF-1) signaling transcription factor and the release of neuronal signaling molecules. While inhibiting UPRER doesn't alter the lifespan of wild-type worms, it normalizes the extended lifespan of ciliary mutants. Notably, deactivating the cyclic nucleotide-gated (CNG) channel TAX-4 on the ciliary membrane promotes lifespan extension through a UPRER-dependent mechanism. Conversely, constitutive activation of TAX-4 attenuates intestinal UPRER in ciliary mutants. Administering a CNG channel blocker to worm larvae activates intestinal UPRER and increases adult longevity. These findings suggest that ciliary dysfunction in sensory neurons triggers intestinal UPRER, contributing to lifespan extension and implying that transiently inhibiting ciliary channel activity may effectively prolong lifespan.