Roles for IFT172 and Primary Cilia in Cell Migration, Cell Division, and Neocortex Development

The role of cilia in the development, survival, and regeneration of hair cells

Mutations impacting cilia genes lead to a class of human diseases known as ciliopathies. This is due to the role of cilia in the development, survival, and regeneration of many cell types. We investigated the extent to which disrupting cilia impacted these processes in lateral line hair cells of zebrafish. We found that mutations in two intraflagellar transport (IFT) genes, ift88 and dync2h1, which lead to the loss of kinocilia, caused increased hair cell apoptosis. IFT gene mutants also have a decreased mitochondrial membrane potential, and blocking the mitochondrial uniporter causes a loss of hair cells in wild-type zebrafish but not mutants, suggesting mitochondria dysfunction may contribute to the apoptosis seen in these mutants. These mutants also showed decreased proliferation during hair cell regeneration but did not show consistent changes in support cell number or proliferation during hair cell development. These results show that the loss of hair cells seen following disruption of cilia through either mutations in anterograde or retrograde IFT genes appears to be due to impacts on hair cell survival but not necessarily development in the zebrafish lateral line.

Haploinsufficiency of intraflagellar transport protein 172 causes autism-like behavioral phenotypes in mice through BDNF

INTRODUCTION

Primary cilia are hair-like solitary organelles growing on most mammalian cells that play fundamental roles in embryonic patterning and organogenesis. Defective cilia often cause a suite of inherited diseases called ciliopathies with multifaceted manifestations. Intraflagellar transport (IFT), a bidirectional protein trafficking along the cilium, actively facilitates the formation and absorption of primary cilia. IFT172 is the largest component of the IFT-B complex, and its roles in Bardet-Biedl Syndrome (BBS) have been appreciated with unclear mechanisms.

OBJECTIVES

We performed a battery of behavioral tests with Ift172 haploinsufficiency (Ift172+/-) and WT littermates. We use RNA sequencing to identify the genes and signaling pathways that are differentially expressed and enriched in the hippocampus of Ift172+/- mice. Using AAV-mediated sparse labeling, electron microscopic examination, patch clamp and local field potential recording, western blot, luciferase reporter assay, chromatin immunoprecipitation, and neuropharmacological approach, we investigated the underlying mechanisms for the aberrant phenotypes presented by Ift172+/- mice.

RESULTS

Ift172+/- mice displayed excessive self-grooming, elevated anxiety, and impaired cognition. RNA sequencing revealed enrichment of differentially expressed genes in pathways relevant to axonogenesis and synaptic plasticity, which were further confirmed by less spine density and synaptic number. Ift172+/- mice demonstrated fewer parvalbumin-expressing neurons, decreased inhibitory synaptic transmission, augmented theta oscillation, and sharp-wave ripples in the CA1 region. Moreover, Ift172 haploinsufficiency caused less BDNF production and less activated BDNF-TrkB signaling pathway through transcription factor Gli3. Application of 7,8-Dihydroxyflavone, a potent small molecular TrkB agonist, fully restored BDNF-TrkB signaling activity and abnormal behavioral phenotypes presented by Ift172+/- mice. With luciferase and chip assays, we provided further evidence that Gli3 may physically interact with BDNF promoter I and regulate BDNF expression.

CONCLUSIONS

Our data suggest that Ift172 per se drives neurotrophic effects and, when defective, could cause neurodevelopmental disorders reminiscent of autism-like disorders.

[Improved Care and Treatment Options for Patients with Hyperphagia-Associated Obesity in Bardet-Biedl Syndrome]

Bardet-Biedl syndrome (BBS) is a rare, autosomal recessive multisystem disease. The pathophysiological origin is a dysfunction of the primary cilium. Clinical symptoms are heterogeneous and variable: retinal dystrophy, obesity, polydactyly, kidney abnormalities, hypogenitalism and developmental delays are the most common features. By the approval of the melanocortin 4 receptor agonist setmelanotide, a drug therapy for BBS-associated hyperphagia and obesity can be offered for the first time. Hyperphagia and severe obesity represent a considerable burden and are associated with comorbidity and increased mortality risk. Due to the limited experience with setmelanotide in BBS, a viable comprehensive therapy concept is to be presented. Therapy decision and management should be conducted in expert centers. For best therapeutic effects with setmelanotide adequate information of the patient about the modalities of the therapy (daily subcutaneous injection) and possible adverse drug events are necessary. Furthermore, the involvement of psychologists, nutritionists and nursing services (support for the application) should be considered together with the patient. The assessment of therapy response should be carried out with suitable outcome measurements and centrally reported to an adequate register.

Ventricular Netrin-1 deficiency leads to defective pyramidal decussation and mirror movement in mice

The corticospinal tract (CST) is the principal neural pathway responsible for conducting voluntary movement in the vertebrate nervous system. Netrin-1 is a well-known guidance molecule for midline crossing of commissural axons during embryonic development. Families with inherited Netrin-1 mutations display congenital mirror movements (CMM), which are associated with malformations of pyramidal decussation in most cases. Here, we investigated the role of Netrin-1 in CST formation by generating conditional knockout (CKO) mice using a Gfap-driven Cre line. A large proportion of CST axons spread laterally in the ventral medulla oblongata, failed to decussate and descended in the ipsilateral spinal white matter of Ntn1Gfap CKO mice. Netrin-1 mRNA was expressed in the ventral ventricular zone (VZ) and midline, while Netrin-1 protein was transported by radial glial cells to the ventral medulla, through which CST axons pass. The level of transported Netrin-1 protein was significantly reduced in Ntn1Gfap CKO mice. In addition, Ntn1Gfap CKO mice displayed increased symmetric movements. Our findings indicate that VZ-derived Netrin-1 deletion leads to an abnormal trajectory of the CST in the spinal cord due to the failure of CST midline crossing and provides novel evidence supporting the idea that the Netrin-1 signalling pathway is involved in the pathogenesis of CMM.

Morphological Observation and Transcriptome Analysis of Ciliogenesis in Urechis unicinctus (Annelida, Echiura)

During the early development of marine invertebrates, planktic larvae usually occur, and their body surfaces often form specific types of cilia that are involved in locomotion and feeding. The echiuran worm Urechis unicinctus sequentially undergoes the formation and disappearance of different types of body surface cilia during embryonic and larval development. The morphological characteristics and molecular mechanisms involved in the process remain unclear. In this study, we found that body surface cilia in U. unicinctus embryos and larvae can be distinguished into four types: body surface short cilia, apical tufts, circumoral cilia and telotrochs. Further, distribution and genesis of the body surface cilia were characterized using light microscope and electron microscope. To better understand the molecular mechanism during ciliogenesis, we revealed the embryonic and larval transcriptome profile of the key stages of ciliogenesis in U. unicinctus using RNA-Seq technology. A total of 29,158 differentially expressed genes (DEGs) were obtained from 24 cDNA libraries by RNA-Seq. KEGG pathway enrichment results showed that Notch, Wnt and Ca²⁺ signaling pathways were significantly enriched during the occurrence of apical tufts and circumoral cilia. Furthermore, all DEGs were classified according to their expression pattern, and DEGs with similar expression pattern were grouped into a module. All DEG co-expression modules were correlated with traits (body surface short cilia, apical tufts, circumoral cilia and telotrochs) by WGCNA, the results showed DEGs were divided into 13 modules by gene expression patterns and that the genes in No. 7, No. 8 and No. 10 modules were to be highly correlated with the occurrence of apical tufts, circumoral cilia and telotrochs. The top 10 hub genes in the above three modules were identified to be highly correlated with ciliogenesis, including the reported cilium-related gene Cnbd2 and unreported cilium-related candidate genes FAM181B, Capsl, Chst3, TMIE and Innexin. Notably, Innexin was included in the top10 hub genes of the two modules (No. 7 and No. 8), suggesting that Innexin may play an important role in U. unicinctus apical tufts, circumoral cilia and telotrochs genesis. This study revealed the characteristics of ciliogenesis on the body surface of U. unicinctus embryos and larvae, providing basic data for exploring the molecular mechanism of ciliogenesis on the body surface.

Technologies, strategies, and cautions when deconvoluting genome-wide association signals: FTO in focus

Genome-wide association studies have revealed a plethora of genetic variants that correlate with polygenic conditions. However, causal molecular mechanisms have proven challenging to fully define. Without such information, the associations are not physiologically useful or clinically actionable. By reviewing studies of the FTO locus in the genetic etiology of obesity, we wish to highlight advances in the field fueled by the evolution of technical and analytic strategies in assessing the molecular bases for genetic associations. Particular attention is drawn to extrapolating experimental findings from animal models and cell types to humans, as well as technical aspects used to identify long-range DNA interactions and their biological relevance with regard to the associated trait. A unifying model is proposed by which independent obesogenic pathways regulated by multiple FTO variants and genes are integrated at the primary cilium, a cellular antenna where signaling molecules that control energy balance convene.

Bardet-Biedl Syndrome: Current Perspectives and Clinical Outlook

The Bardet Biedl syndrome (BBS) is a rare inherited disorder considered a model of non-motile ciliopathy. It is in fact caused by mutations of genes encoding for proteins mainly localized to the base of the cilium. Clinical features of BBS patients are widely shared with patients suffering from other ciliopathies, especially autosomal recessive syndromic disorders; moreover, mutations in cilia-related genes can cause different clinical ciliopathy entities. Besides the best-known clinical features, as retinal degeneration, learning disabilities, polydactyly, obesity and renal defects, several additional clinical signs have been reported in BBS, expanding our understanding of the complexity of its clinical spectrum. The present review aims to describe the current knowledge of BBS i) pathophysiology, ii) clinical manifestations, highlighting both the most common and the less described features, iii) current and future perspective for treatment.

Primary cilia-associated protein IFT172 in ciliopathies

Cilium is a highly conserved antenna-like structure protruding from the surface of the cell membrane, which is widely distributed on most mammalian cells. Two types of cilia have been described so far which include motile cilia and immotile cilia and the latter are also known as primary cilia. Dysfunctional primary cilia are commonly associated with a variety of congenital diseases called ciliopathies with multifaceted presentations such as retinopathy, congenital kidney disease, intellectual disability, cancer, polycystic kidney, obesity, Bardet Biedl syndrome (BBS), etc. Intraflagellar transport (IFT) is a bi-directional transportation process that helps maintain a balanced flow of proteins or signaling molecules essential for the communication between cilia and cytoplasm. Disrupted IFT contributes to the abnormal structure or function of cilia and frequently promotes the occurrence of ciliopathies. Intraflagellar transport 172 (IFT172) is a newly identified member of IFT proteins closely involved in some rare ciliopathies such as Mainzer-Saldino syndrome (MZSDS) and BBS, though the underpinning causal mechanisms remain largely elusive. In this review, we summarize the key findings on the genetic and protein characteristic of IFT172, as well as its function in intraflagellar transport, to provide comprehensive insights to understand IFT172-related ciliopathies.

Brain heterotopia formation by ciliopathic breakdown of neuroepithelial and blood-cerebrospinal fluid barriers

The developmental functions of primary cilia and the downstream signaling pathways have been widely studied; however, the roles of primary cilia in the developing neurovascular system are not clearly understood. In this study, we found that ablation of genes encoding ciliary transport proteins such as intraflagellar transport homolog 88 (Ift88) and kinesin family member 3a (Kif3a) in cortical radial progenitors led to periventricular heterotopia during late mouse embryogenesis. Conditional mutation of primary cilia unexpectedly caused breakdown of both the neuroepithelial lining and the blood-choroid plexus barrier. Choroidal leakage was partially caused by enlargement of the choroid plexus in the cilia mutants. We found that the choroid plexus expressed platelet-derived growth factor A (Pdgf-A) and that Pdgf-A expression was ectopically increased in cilia-mutant embryos. Cortices obtained from embryos in utero electroporated with Pdgfa mimicked periventricular heterotopic nodules of the cilia mutant. These results suggest that defective ciliogenesis in both cortical progenitors and the choroid plexus leads to breakdown of cortical and choroidal barriers causing forebrain neuronal dysplasia, which may be related to developmental cortical malformation.

The Primary Cilium and Neuronal Migration

The primary cilium (PC) is a microtubule-based tiny sensory organelle emanating from the centrosome and protruding from the surface of most eukaryotic cells, including neurons. The extremely severe phenotypes of ciliopathies have suggested their paramount importance for multiple developmental events, including brain formation. Neuronal migration is an essential step of neural development, with all neurons traveling from their site of birth to their site of integration. Neurons perform a unique type of cellular migration called cyclic saltatory migration, where their soma periodically jumps along with the stereotyped movement of their centrosome. We will review here how the role of the PC on cell motility was first described in non-neuronal cells as a guide pointing to the direction of migration. We will see then how these findings are extended to neuronal migration. In neurons, the PC appears to regulate the rhythm of cyclic saltatory neuronal migration in multiple systems. Finally, we will review recent findings starting to elucidate how extracellular cues sensed by the PC could be intracellularly transduced to regulate the machinery of neuronal migration. The PC of migrating neurons was unexpectedly discovered to display a rhythmic extracellular emergence during each cycle of migration, with this transient exposure to the external environment associated with periodic transduction of cyclic adenosine monophosphate (cAMP) signaling at the centrosome. The PC in migrating neurons thus uniquely appears as a beat maker, regulating the tempo of cyclic saltatory migration.

Down-Regulation of Inpp5e Associated With Abnormal Ciliogenesis During Embryonic Neurodevelopment Under Inositol Deficiency

The inositol polyphosphate-5-phosphatase E (Inpp5e) gene is located on chromosome 9q34.3. The enzyme it encodes mainly hydrolyzes the 5-phosphate groups of phosphatidylinositol (3,4,5)-trisphosphate (PtdIns (3,4,5) P3) and phosphatidylinositol (4,5)-bisphosphate (PtdIns (4,5)P2), which are closely related to ciliogenesis and embryonic neurodevelopment, through mechanisms that are largely unknown. Here we studied the role of Inpp5e gene in ciliogenesis during embryonic neurodevelopment using inositol-deficiency neural tube defects (NTDs) mouse and cell models. Confocal microscopy and scanning electron microscope were used to examine the number and the length of primary cilia. The dynamic changes of Inpp5e expression in embryonic murine brain tissues were observed during Embryonic Day 10.5-13.5 (E 10.5-13.5). Immunohistochemistry, western blot, polymerase chain reaction (PCR) arrays were applied to detect the expression of Inpp5e and cilia-related genes of the embryonic brain tissues in inositol deficiency NTDs mouse. Real-time quantitative PCR (RT-qPCR) was used to validate the candidate genes in cell models. The levels of inositol and PtdIns(3,4) P2 were measured using gas chromatography-mass spectrometry (GC-MS) and enzyme linked immunosorbent assay (ELISA), respectively. Our results showed that the expression levels of Inpp5e gradually decreased in the forebrain tissues of the control embryos, but no stable trend was observed in the inositol deficiency NTDs embryos. Inpp5e expression in inositol deficiency NTDs embryos was significantly decreased compared with the control tissues. The expression levels of Inpp5e gene and the PtdIns (3,4) P2 levels were also significantly decreased in the inositol deficient cell model. A reduced number and length of primary cilia were observed in NIH3T3 cells when inositol deficient. Three important cilia-related genes (Ift80, Mkks, Smo) were down-regulated significantly in the inositol-deficient NTDs mouse and cell models, and Smo was highly involved in NTDs. In summary, these findings suggested that down-regulation of Inpp5e might be associated with abnormal ciliogenesis during embryonic neurodevelopment, under conditions of inositol deficiency.

Molecular genetics of renal ciliopathies

Renal ciliopathies are a heterogenous group of inherited disorders leading to an array of phenotypes that include cystic kidney disease and renal interstitial fibrosis leading to progressive chronic kidney disease and end-stage kidney disease. The renal tubules are lined with epithelial cells that possess primary cilia that project into the lumen and act as sensory and signalling organelles. Mutations in genes encoding ciliary proteins involved in the structure and function of primary cilia cause ciliopathy syndromes and affect many organ systems including the kidney. Recognised disease phenotypes associated with primary ciliopathies that have a strong renal component include autosomal dominant and recessive polycystic kidney disease and their various mimics, including atypical polycystic kidney disease and nephronophthisis. The molecular investigation of inherited renal ciliopathies often allows a precise diagnosis to be reached where renal histology and other investigations have been unhelpful and can help in determining kidney prognosis. With increasing molecular insights, it is now apparent that renal ciliopathies form a continuum of clinical phenotypes with disease entities that have been classically described as dominant or recessive at both extremes of the spectrum. Gene-dosage effects, hypomorphic alleles, modifier genes and digenic inheritance further contribute to the genetic complexity of these disorders. This review will focus on recent molecular genetic advances in the renal ciliopathy field with a focus on cystic kidney disease phenotypes and the genotypes that lead to them. We discuss recent novel insights into underlying disease mechanisms of renal ciliopathies that might be amenable to therapeutic intervention.

A cilium-independent role for intraflagellar transport 88 in regulating angiogenesis

Endothelial cilia are microtubule-based hair-like protrusions in the lumen ofblood vessels that function as fluid mechanosensors to regulate vascular hemodynamics.However, the functions of endothelial cilia in vascular development remain controversial. In this study, depletion of several key proteins responsible for ciliogenesis allows us to identify a cilium-independent role for intraflagellartransport88 (IFT88) in mammalian angiogenesis. Disruption of primary cilia by heat shock does not affect the angiogenic process. However, depletion of IFT88 significantly inhibits angiogenesis both in vitro and in vivo. IFT88 mediates angiogenesis by regulating the migration, polarization, proliferation, and oriented division of vascular endothelial cells. Further mechanistic studies demonstrate that IFT88 interacts with γ-tubulin and microtubule plus-end tracking proteins and promotes microtubule stability. Our findings indicate that IFT88 regulates angiogenesis through its actions in microtubule-based cellular processes, independent of its role in ciliogenesis.

Impaired Learning and Memory Ability Induced by a Bilaterally Hippocampal Injection of Streptozotocin in Mice: Involved With the Adaptive Changes of Synaptic Plasticity

Background: Alzheimer's disease (AD) is a neurodegenerative disease characterized by progressive cognitive decline, psychiatric symptoms and behavioral disorders, resulting in disability, and loss of self-sufficiency. Objective: To establish an AD-like mice model, investigate the behavioral performance, and explore the potential mechanism. Methods: Streptozotocin (STZ, 3 mg/kg) was microinjected bilaterally into the dorsal hippocampus of C57BL/6 mice, and the behavioral performance was observed. The serum concentrations of insulin and nesfatin-1 were measured by ELISA, and the activation of hippocampal microglia and astrocytes was assessed by immunohistochemistry. The protein expression of several molecular associated with the regulation of synaptic plasticity in the hippocampus and the pre-frontal cortex (PFC) was detected via western blotting. Results: The STZ-microinjected model mice showed a slower bodyweight gain and higher serum concentration of insulin and nesfatin-1. Although there was no significant difference between groups with regard to the ability of balance and motor coordination, the model mice presented a decline of spontaneous movement and exploratory behavior, together with an impairment of learning and memory ability. Increased activated microglia was aggregated in the hippocampal dentate gyrus of model mice, together with an increase abundance of Aβ_1-42 and Tau in the hippocampus and PFC. Moreover, the protein expression of NMDAR2A, NMDAR2B, SynGAP, PSD95, BDNF, and p-β-catenin/β-catenin were remarkably decreased in the hippocampus and the PFC of model mice, and the expression of p-GSK-3β (ser9)/GSK-3β were reduced in the hippocampus. Conclusion: A bilateral hippocampal microinjection of STZ could induce not only AD-like behavioral performance in mice, but also adaptive changes of synaptic plasticity against neuroinflammatory and endocrinal injuries. The underlying mechanisms might be associated with the imbalanced expression of the key proteins of Wnt signaling pathway in the hippocampus and the PFC.

The Multifaceted Roles of Primary Cilia in the Development of the Cerebral Cortex

The primary cilium, a microtubule based organelle protruding from the cell surface and acting as an antenna in multiple signaling pathways, takes center stage in the formation of the cerebral cortex, the part of the brain that performs highly complex neural tasks and confers humans with their unique cognitive capabilities. These activities require dozens of different types of neurons that are interconnected in complex ways. Due to this complexity, corticogenesis has been regarded as one of the most complex developmental processes and cortical malformations underlie a number of neurodevelopmental disorders such as intellectual disability, autism spectrum disorders, and epilepsy. Cortical development involves several steps controlled by cell–cell signaling. In fact, recent findings have implicated cilia in diverse processes such as neurogenesis, neuronal migration, axon pathfinding, and circuit formation in the developing cortex. Here, we will review recent advances on the multiple roles of cilia during cortex formation and will discuss the implications for a better understanding of the disease mechanisms underlying neurodevelopmental disorders.

Mediator Med23 Regulates Adult Hippocampal Neurogenesis

Mammalian Mediator (Med) is a key regulator of gene expression by linking transcription factors to RNA polymerase II (Pol II) transcription machineries. The Mediator subunit 23 (Med23) is a member of the conserved Med protein complex and plays essential roles in diverse biological processes including adipogenesis, carcinogenesis, osteoblast differentiation, and T-cell activation. However, its potential functions in the nervous system remain unknown. We report here that Med23 is required for adult hippocampal neurogenesis in mouse. Deletion of Med23 in adult hippocampal neural stem cells (NSCs) was achieved in Nestin-Cre^ER:Med23^flox/flox mice by oral administration of tamoxifen. We found an increased number of proliferating NSCs shown by pulse BrdU-labeling and immunostaining of MCM2 and Ki67, which is possibly due to a reduction in cell cycle length, with unchanged GFAP⁺/Sox2⁺ NSCs and Tbr2⁺ progenitors. On the other hand, neuroblasts and immature neurons indicated by NeuroD and DCX were decreased in number in the dentate gyrus (DG) of Med23-deficient mice. In addition, these mice also displayed defective dendritic morphogenesis, as well as a deficiency in spatial and contextual fear memory. Gene ontology (GO) analysis of hippocampal NSCs revealed an enrichment in genes involved in cell proliferation, Pol II-associated transcription, Notch signaling pathway and apoptosis. These results demonstrate that Med23 plays roles in regulating adult brain neurogenesis and functions.