Primary Cilia Signaling Shapes the Development of Interneuronal Connectivity

Mechanisms of Weight Control by Primary Cilia

A primary cilium, a hair-like protrusion of the plasma membrane, is a pivotal organelle for sensing external environmental signals and transducing intracellular signaling. An interesting linkage between cilia and obesity has been revealed by studies of the human genetic ciliopathies Bardet-Biedl syndrome and Alström syndrome, in which obesity is a principal manifestation. Mouse models of cell type-specific cilia dysgenesis have subsequently demonstrated that ciliary defects restricted to specific hypothalamic neurons are sufficient to induce obesity and hyperphagia. A potential mechanism underlying hypothalamic neuron cilia-related obesity is impaired ciliary localization of G protein-coupled receptors involved in the regulation of appetite and energy metabolism. A well-studied example of this is melanocortin 4 receptor (MC4R), mutations in which are the most common cause of human monogenic obesity. In the paraventricular hypothalamus neurons, a blockade of ciliary trafficking of MC4R as well as its downstream ciliary signaling leads to hyperphagia and weight gain. Another potential mechanism is reduced leptin signaling in hypothalamic neurons with defective cilia. Leptin receptors traffic to the periciliary area upon leptin stimulation. Moreover, defects in cilia formation hamper leptin signaling and actions in both developing and differentiated hypothalamic neurons. The list of obesity-linked ciliary proteins is expending and this supports a tight association between cilia and obesity. This article provides a brief review on the mechanism of how ciliary defects in hypothalamic neurons facilitate obesity.

Impairment of ciliary dynamics in an APP knock-in mouse model of Alzheimer's disease

The primary cilium is a specialized microtubule-based sensory organelle that extends from the cell body of nearly all cell types. Neuronal primary cilia, which have their own unique signaling repertoire, are crucial for neuronal integrity and the maintenance of neuronal connectivity throughout adulthood. Dysfunction of cilia structure and ciliary signaling is associated with a variety of genetic syndromes, termed ciliopathies. One of the characteristic features of human ciliopathies is impairment of memory and cognition, which is also observed in Alzheimer's disease (AD). Amyloid β peptide (Aβ) is produced through the proteolytic processing of amyloid precursor protein (APP), and Aβ accumulation in the brain is proposed to be an early toxic event in the pathogenesis of AD. To evaluate the effect of increased Aβ level on primary cilia, we assessed ciliary dynamics in hippocampal neurons in an APP knock-in AD model (App^NL-G-F mice) compared to that in wild-type mice. Neuronal cilia length in the CA1, CA3, and dentate gyrus (DG) of wild-type mice increased significantly with age. In App^NL-G-F mice, such elongation was detected in the DG but not in the CA1 and CA3, where more Aβ accumulation was observed. We further demonstrated that Aβ1-42 treatment decreased cilia length both in hTERT-RPE1 cells and dissociated rat hippocampal neurons. There is growing evidence that reduced cilia length is associated with perturbations of synaptic connectivity and dendrite complexity. Thus, our observations raise the important possibility that structural alterations in neuronal cilia might have a role in AD development.

Multi-Omics Characterization of Early- and Adult-Onset Major Depressive Disorder

Age at depressive onset (AAO) corresponds to unique symptomatology and clinical outcomes. Integration of genome-wide association study (GWAS) results with additional “omic” measures to evaluate AAO has not been reported and may reveal novel markers of susceptibility and/or resistance to major depressive disorder (MDD). To address this gap, we integrated genomics with metabolomics using data-driven network analysis to characterize and differentiate MDD based on AAO. This study first performed two GWAS for AAO as a continuous trait in (a) 486 adults from the Pharmacogenomic Research Network-Antidepressant Medication Pharmacogenomic Study (PGRN-AMPS), and (b) 295 adults from the Combining Medications to Enhance Depression Outcomes (CO-MED) study. Variants from top signals were integrated with 153 p180-assayed metabolites to establish multi-omics network characterizations of early (<age 18) and adult-onset depression. The most significant variant (p = 8.77 × 10−8) localized to an intron of SAMD3. In silico functional annotation of top signals (p < 1 × 10−5) demonstrated gene expression enrichment in the brain and during embryonic development. Network analysis identified differential associations between four variants (in/near INTU, FAT1, CNTN6, and TM9SF2) and plasma metabolites (phosphatidylcholines, carnitines, biogenic amines, and amino acids) in early- compared with adult-onset MDD. Multi-omics integration identified differential biosignatures of early- and adult-onset MDD. These biosignatures call for future studies to follow participants from childhood through adulthood and collect repeated -omics and neuroimaging measures to validate and deeply characterize the biomarkers of susceptibility and/or resistance to MDD development.

Pathogenic LRRK2 control of primary cilia and Hedgehog signaling in neurons and astrocytes of mouse brain

Activating LRRK2 mutations cause Parkinson’s disease, and pathogenic LRRK2 kinase interferes with ciliogenesis. Previously, we showed that cholinergic interneurons of the dorsal striatum lose their cilia in R1441C LRRK2 mutant mice (Dhekne et al., 2018). Here, we show that cilia loss is seen as early as 10 weeks of age in these mice and also in two other mouse strains carrying the most common human G2019S LRRK2 mutation. Loss of the PPM1H phosphatase that is specific for LRRK2-phosphorylated Rab GTPases yields the same cilia loss phenotype seen in mice expressing pathogenic LRRK2 kinase, strongly supporting a connection between Rab GTPase phosphorylation and cilia loss. Moreover, astrocytes throughout the striatum show a ciliation defect in all LRRK2 and PPM1H mutant models examined. Hedgehog signaling requires cilia, and loss of cilia in LRRK2 mutant rodents correlates with dysregulation of Hedgehog signaling as monitored by in situ hybridization of Gli1 and Gdnf transcripts. Dopaminergic neurons of the substantia nigra secrete a Hedgehog signal that is sensed in the striatum to trigger neuroprotection; our data support a model in which LRRK2 and PPM1H mutant mice show altered responses to critical Hedgehog signals in the nigrostriatal pathway.

HMGB1 signaling phosphorylates Ku70 and impairs DNA damage repair in Alzheimer's disease pathology

DNA damage is increased in Alzheimer's disease (AD), while the underlying mechanisms are unknown. Here, we employ comprehensive phosphoproteome analysis, and identify abnormal phosphorylation of 70 kDa subunit of Ku antigen (Ku70) at Ser77/78, which prevents Ku70-DNA interaction, in human AD postmortem brains. The abnormal phosphorylation inhibits accumulation of Ku70 to the foci of DNA double strand break (DSB), impairs DNA damage repair and eventually causes transcriptional repression-induced atypical cell death (TRIAD). Cells under TRIAD necrosis reveal senescence phenotypes. Extracellular high mobility group box 1 (HMGB1) protein, which is released from necrotic or hyper-activated neurons in AD, binds to toll-like receptor 4 (TLR4) of neighboring neurons, and activates protein kinase C alpha (PKCα) that executes Ku70 phosphorylation at Ser77/78. Administration of human monoclonal anti-HMGB1 antibody to post-symptomatic AD model mice decreases neuronal DSBs, suppresses secondary TRIAD necrosis of neurons, prevents escalation of neurodegeneration, and ameliorates cognitive symptoms. TRIAD shares multiple features with senescence. These results discover the HMGB1-Ku70 axis that accounts for the increase of neuronal DNA damage and secondary enhancement of TRIAD, the cell death phenotype of senescence, in AD.

Large-scale analysis reveals spatiotemporal circadian patterns of cilia transcriptomes in the primate brain

Cilia are dynamic subcellular systems, with core structural and functional components operating in a highly coordinated manner. Since many environmental stimuli sensed by cilia are circadian in nature, it is reasonable to speculate that genes encoding cilia structural and functional components follow rhythmic circadian patterns of expression. Using computational methods and the largest spatiotemporal gene expression atlas of primates, we identified and analyzed the circadian rhythmic expression of cilia genes across 22 primate brain areas. We found that around 73% of cilia transcripts exhibited circadian rhythmicity across at least one of 22 brain regions. In 12 brain regions, cilia transcriptomes were significantly enriched with circadian oscillating transcripts, as compared to the rest of the transcriptome. The phase of the cilia circadian transcripts deviated from the phase of the majority of the background circadian transcripts, and transcripts coding for cilia basal body components accounted for the majority of cilia circadian transcripts. In addition, adjacent or functionally connected brain nuclei had large overlapping complements of circadian cilia genes. Most remarkably, cilia circadian transcripts shared across the basal ganglia nuclei and the prefrontal cortex peaked in these structures in sequential fashion that is similar to the sequential order of activation of the basal ganglia-cortical circuitry in connection with movement coordination, albeit on completely different timescales. These findings support a role for the circadian spatiotemporal orchestration of cilia gene expression in the normal physiology of the basal ganglia-cortical circuit and motor control. Studying orchestrated cilia rhythmicity in the basal ganglia-cortical circuits and other brain circuits may help develop better functional models, and shed light on the causal effects cilia functions have on these circuits and on the regulation of movement and other behaviors.

The localization of amyloid precursor protein to ependymal cilia in vertebrates and its role in ciliogenesis and brain development in zebrafish

Amyloid precursor protein (APP) is expressed in many tissues in human, mice and in zebrafish. In zebrafish, there are two orthologues, Appa and Appb. Interestingly, some cellular processes associated with APP overlap with cilia-mediated functions. Whereas the localization of APP to primary cilia of in vitro-cultured cells has been reported, we addressed the presence of APP in motile and in non-motile sensory cilia and its potential implication for ciliogenesis using zebrafish, mouse, and human samples. We report that Appa and Appb are expressed by ciliated cells and become localized at the membrane of cilia in the olfactory epithelium, otic vesicle and in the brain ventricles of zebrafish embryos. App in ependymal cilia persisted in adult zebrafish and was also detected in mouse and human brain. Finally, we found morphologically abnormal ependymal cilia and smaller brain ventricles in appa^−/−appb^−/− mutant zebrafish. Our findings demonstrate an evolutionary conserved localisation of APP to cilia and suggest a role of App in ciliogenesis and cilia-related functions.

Dynamic Changes of Brain Cilia Transcriptomes across the Human Lifespan

Almost all brain cells contain primary cilia, antennae-like microtubule sensory organelles, on their surface, which play critical roles in brain functions. During neurodevelopmental stages, cilia are essential for brain formation and maturation. In the adult brain, cilia play vital roles as signaling hubs that receive and transduce various signals and regulate cell-to-cell communications. These distinct roles suggest that cilia functions, and probably structures, change throughout the human lifespan. To further understand the age-dependent changes in cilia roles, we identified and analyzed age-dependent patterns of expression of cilia's structural and functional components across the human lifespan. We acquired cilia transcriptomic data for 16 brain regions from the BrainSpan Atlas and analyzed the age-dependent expression patterns using a linear regression model by calculating the regression coefficient. We found that 67% of cilia transcripts were differentially expressed genes with age (DEGAs) in at least one brain region. The age-dependent expression was region-specific, with the highest and lowest numbers of DEGAs expressed in the ventrolateral prefrontal cortex and hippocampus, respectively. The majority of cilia DEGAs displayed upregulation with age in most of the brain regions. The transcripts encoding cilia basal body components formed the majority of cilia DEGAs, and adjacent cerebral cortices exhibited large overlapping pairs of cilia DEGAs. Most remarkably, specific α/β-tubulin subunits (TUBA1A, TUBB2A, and TUBB2B) and SNAP-25 exhibited the highest rates of downregulation and upregulation, respectively, across age in almost all brain regions. α/β-tubulins and SNAP-25 expressions are known to be dysregulated in age-related neurodevelopmental and neurodegenerative disorders. Our results support a role for the high dynamics of cilia structural and functional components across the lifespan in the normal physiology of brain circuits. Furthermore, they suggest a crucial role for cilia signaling in the pathophysiological mechanisms of age-related psychiatric/neurological disorders.

Ventromedial hypothalamic primary cilia control energy and skeletal homeostasis

Dysfunction of primary cilia is related to dyshomeostasis, leading to a wide range of disorders. The ventromedial hypothalamus (VMH) is known to regulate several homeostatic processes, but those modulated specifically by VMH primary cilia are not yet known. In this study, we identify VMH primary cilia as an important organelle that maintains energy and skeletal homeostasis by modulating the autonomic nervous system. We established loss-of-function models of primary cilia in the VMH by either targeting IFT88 (IFT88-KOSF-1) using steroidogenic factor 1-Cre (SF-1-Cre) or injecting an adeno-associated virus Cre (AAV-Cre) directly into the VMH. Functional impairments of VMH primary cilia were linked to decreased sympathetic activation and central leptin resistance, which led to marked obesity and bone-density accrual. Obesity was caused by hyperphagia, decreased energy expenditure, and blunted brown fat function and was also associated with insulin and leptin resistance. The effect of bone-density accrual was independent of obesity, as it was caused by decreased sympathetic tone resulting in increased osteoblastic and decreased osteoclastic activities in the IFT88-KOSF-1 and VMH primary cilia knockdown mice. Overall, our current study identifies VMH primary cilia as a critical hypothalamic organelle that maintains energy and skeletal homeostasis.

Ciliary GPCR-based transcriptome as a key regulator of cilia length control

The primary cilium is a plasma membrane-protruding sensory organelle that efficiently conveys signaling cascades in a highly ordered microenvironment. Its signaling is mediated, in part, by a limited set of GPCRs preferentially enriched in the cilium membrane. This includes melanin-concentrating hormone (MCH) receptor 1 (MCHR1), which plays a role in feeding and mood. In addition to its receptor composition, the length of the cilium is a characteristic parameter that is implicated in its function. We previously found that MCH can dynamically shorten cilia length via the Gi/o and Akt pathways in both MCHR1-expressing hTERT-RPE1 cells (hRPE1 cells) and rat hippocampal neurons. However, the detailed mechanisms by which MCH regulates cilia length through ciliary MCHR1 remains unclear. In this study, we aimed to determine the transcriptome changes in MCHR1-expressing hRPE1 cells in response to MCH to identify the target molecules involved in cilia length regulation via MCHR1 activation. RNA sequencing analysis of ciliated cells subjected to MCH treatment showed upregulation of 424 genes and downregulation of 112 genes compared with static control cells. Validation by quantitative real-time PCR, knocking down, and CRISPR/Cas9-mediated knockout technology identified a molecule, PDZ and LIM domain-containing protein 5 (PDLIM5). Thus, it was considered as the most significant key factor for MCHR1-mediated shortening of cilia length. Additional analyses revealed that the actin-binding protein alpha-actinin 1/4 is a crucial downstream target of the PDLIM5 signaling pathway that exerts an effect on MCHR1-induced cilia shortening. In the endogenous MCHR1-expressing hippocampus, transcriptional upregulation of PDLIM5 and actinin 1/4, following the application of MCH, was detected when the MCHR1-positive cilia were shortened. Together, our transcriptome study based on ciliary MCHR1 function uncovered a novel and important regulatory step underlying cilia length control. These results will potentially serve as a basis for understanding the mechanism underlying the development of obesity and mood disorders.

NGS and phenotypic ontology-based approaches increase the diagnostic yield in syndromic retinal diseases

Syndromic retinal diseases (SRDs) are a group of complex inherited systemic disorders, with challenging molecular underpinnings and clinical management. Our main goal is to improve clinical and molecular SRDs diagnosis, by applying a structured phenotypic ontology and next-generation sequencing (NGS)-based pipelines. A prospective and retrospective cohort study was performed on 100 probands with an a priori diagnosis of non-Usher SRDs, using available clinical data, including Human Phenotype Ontology annotation, and further classification into seven clinical categories (ciliopathies, specific syndromes and five others). Retrospective molecular diagnosis was assessed using different molecular and bioinformatic methods depending on availability. Subsequently, uncharacterized probands were prospectively screened using other NGS approaches to extend the number of analyzed genes. After phenotypic classification, ciliopathies were the most common SRD (35%). A global characterization rate of 52% was obtained, with six cases incompletely characterized for a gene that partially explained the phenotype. An improved characterization rate was achieved addressing prospective cases (83%) and well-recognizable syndrome (62%) subgroups. The 27% of the fully characterized cases were reclassified into a different clinical category after identification of the disease-causing gene. Clinical-exome sequencing is the most appropriate first-tier approach for prospective cases, whereas whole-exome sequencing and bioinformatic reanalysis increases the diagnosis of uncharacterized retrospective cases to 45%, mostly those with unspecific symptoms. Our study describes a comprehensive approach to SRDs in daily clinical practice and the importance of thorough clinical assessment and selection of the most appropriate molecular test to be used to solve these complex cases and elucidate novel associations.

Mice Harboring a Non-Functional CILK1/ICK Allele Fail to Model the Epileptic Phenotype in Patients Carrying Variant CILK1/ICK

CILK1 (ciliogenesis associated kinase 1)/ICK (intestinal cell kinase) is a highly conserved protein kinase that regulates primary cilia structure and function. CILK1 mutations cause a wide spectrum of human diseases collectively called ciliopathies. While several CILK1 heterozygous variants have been recently linked to juvenile myoclonic epilepsy (JME), it remains unclear whether these mutations cause seizures. Herein, we investigated whether mice harboring either a heterozygous null Cilk1 (Cilk1^+/−) mutation or a heterozygous loss-of-function Cilk1 mutation (Cilk1^R272Q/+) have epilepsy. We first evaluated the spontaneous seizure phenotype of Cilk1^+/− and Cilk1^R272Q/+ mice relative to wildtype littermates. We observed no electrographic differences among the three mouse genotypes during prolonged recordings. We also evaluated electrographic and behavioral responses of mice recovering from isoflurane anesthesia, an approach recently used to measure seizure-like activity. Again, we observed no electrographic or behavioral differences in control versus Cilk1^+/− and Cilk1^R272Q/+ mice upon isoflurane recovery. These results indicate that mice bearing a non-functional copy of Cilk1 fail to produce electrographic patterns resembling those of JME patients with a variant CILK1 copy. Our findings argue against CILK1 haploinsufficiency being the mechanism that links CILK1 variants to JME.

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

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

Ciliary neuropeptidergic signaling dynamically regulates excitatory synapses in postnatal neocortical pyramidal neurons

Primary cilia are compartmentalized sensory organelles present on the majority of neurons in the mammalian brain throughout adulthood. Recent evidence suggests that cilia regulate multiple aspects of neuronal development, including the maintenance of neuronal connectivity. However, whether ciliary signals can dynamically modulate postnatal circuit excitability is unknown. Here we show that acute cell-autonomous knockdown of ciliary signaling rapidly strengthens glutamatergic inputs onto cultured rat neocortical pyramidal neurons and increases spontaneous firing. This increased excitability occurs without changes to passive neuronal properties or intrinsic excitability. Further, the neuropeptide receptor somatostatin receptor 3 (SSTR3) is localized nearly exclusively to excitatory neuron cilia both in vivo and in culture, and pharmacological manipulation of SSTR3 signaling bidirectionally modulates excitatory synaptic inputs onto these neurons. Our results indicate that ciliary neuropeptidergic signaling dynamically modulates excitatory synapses and suggest that defects in this regulation may underlie a subset of behavioral and cognitive disorders associated with ciliopathies.

HTR6 and SSTR3 targeting to primary cilia

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

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.

The essential role of primary cilia in cerebral cortical development and disorders

Primary cilium, first described in the 19th century in different cell types and organisms by Alexander Ecker, Albert Kolliker, Aleksandr Kowalevsky, Paul Langerhans, and Karl Zimmermann (Ecker, 1844; Kolliker, 1854; Kowalevsky, 1867; Langerhans, 1876; Zimmermann, 1898), play an essential modulatory role in diverse aspects of nervous system development and function. The primary cilium, sometimes referred to as the cell's 'antennae', can receive wide ranging inputs from cellular milieu, including morphogens, growth factors, neuromodulators, and neurotransmitters. Its unique structural and functional organization bequeaths it the capacity to hyper-concentrate signaling machinery in a restricted cellular domain approximately one-thousandth the volume of cell soma. Thus enabling it to act as a signaling hub that integrates diverse developmental and homestatic information from cellular milieu to regulate the development and function of neural cells. Dysfunction of primary cilia contributes to the pathophysiology of several brain malformations, intellectual disabilities, epilepsy, and psychiatric disorders. This review focuses on the most essential contributions of primary cilia to cerebral cortical development and function, in the context of neurodevelopmental disorders and malformations. It highlights the recent progress made in identifying the mechanisms underlying primary cilia's role in cortical progenitors, neurons and glia, in health and disease. A future challenge will be to translate these insights and advances into effective clinical treatments for ciliopathies.

Kif3a deletion prevents primary cilia assembly on oligodendrocyte progenitor cells, reduces oligodendrogenesis and impairs fine motor function

Primary cilia are small microtubule‐based organelles capable of transducing signals from growth factor receptors embedded in the cilia membrane. Developmentally, oligodendrocyte progenitor cells (OPCs) express genes associated with primary cilia assembly, disassembly, and signaling, however, the importance of primary cilia for adult myelination has not been explored. We show that OPCs are ciliated in vitro and in vivo, and that they disassemble their primary cilia as they progress through the cell cycle. OPC primary cilia are also disassembled as OPCs differentiate into oligodendrocytes. When kinesin family member 3a (Kif3a), a gene critical for primary cilium assembly, was conditionally deleted from adult OPCs in vivo (Pdgfrα‐CreER™:: Kif3a^fl/fl transgenic mice), OPCs failed to assemble primary cilia. Kif3a‐deletion was also associated with reduced OPC proliferation and oligodendrogenesis in the corpus callosum and motor cortex and a progressive impairment of fine motor coordination.

In vitro Time-lapse Imaging of Primary Cilium in Migrating Neuroblasts

Neuronal migration is a critical step for the development of neuronal circuits in the brain. Immature new neurons (neuroblasts) generated in the postnatal ventricular-subventricular zone (V-SVZ) show a remarkable potential to migrate for a long distance at a high speed in the postnatal mammalian brain, and are thus a powerful model to analyze the molecular and cellular mechanisms of neuronal migration. Here we describe a methodology for in vitro time-lapse imaging of the primary cilium and its related structures in migrating V-SVZ-derived neuroblasts using confocal or superresolution laser-scanning microscopy. The V-SVZ tissues are dissected from postnatal day 0-1 (P0-1) mouse brains and dissociated into single cells by trypsinization and gentle pipetting. These cells are then transduced with a plasmid(s) encoding a gene(s) of interest, aggregated by centrifugation, and cultured for 2 days in Matrigel. Time-lapse images of migratory behaviors of cultured neuroblasts and their ciliary structures, including the ciliary membrane and basal body, are acquired by confocal or superresolution laser-scanning microscopy. This method provides information about the spatiotemporal dynamics of neuroblasts' morphology and ciliary structures, and is widely applicable to various types of migrating neuronal and nonneuronal cells in various species.

Properties of primary cilia in melanin-concentrating hormone receptor 1-bearing hippocampal neurons in vivo and in vitro

The primary cilium is a solitary organelle that organizes a sensitive signaling hub in a highly ordered microenvironment. Cilia are plastic structures, changing their length in response to bioactive substances, and ciliary length may be regulated to ensure efficient signaling capacity. Mammalian brain neurons possess primary cilia that are enriched in a set of G protein-coupled receptors (GPCRs), including the feeding-related melanin-concentrating hormone (MCH) receptor 1 (MCHR1). We previously demonstrated a novel biological phenomenon, ciliary MCHR1-mediated cilia length shortening through Gi/o and Akt signaling, using a simple cell culture model of human retinal pigmented epithelial RPE1 cells exogenously expressing MCHR1. In the present study, we characterized the properties of endogenous MCHR1-expressing primary cilia in hippocampal neurons in rodents. Using cultured dissociated rat hippocampal neurons in vitro, we showed that MCH triggered cilia length reduction involved in MCHR1-Gi/o and -Akt signaling. In rat hippocampal slice cultures with preservation of the cytoarchitecture and cell populations, ciliary MCHR1 was abundantly located in the CA1 and CA3 regions, but not in the dentate gyrus. Notably, treatment of slice cultures with MCH induced Gi/o- and Akt-dependent cilia shortening in the CA1 region without influencing cilia length in the CA3 region. Regarding the in vivo mouse brain, we observed higher levels of ciliary MCHR1 in the CA1 and CA3 regions as well as in slice cultures. In the starved state mice, a marked increase in MCH mRNA expression was detected in the lateral hypothalamus. Furthermore, MCHR1-positive cilia length in the hippocampal CA1 region was significantly shortened in fasted mice compared with fed mice. The present findings focused on the hippocampus provide a potential approach to investigate how MCHR1-driven cilia shortening regulates neuronal activity and physiological function toward feeding and memory tasks.

Primary cilia mediate early life programming of adiposity through lysosomal regulation in the developing mouse hypothalamus

Hypothalamic neurons including proopiomelanocortin (POMC)-producing neurons regulate body weights. The non-motile primary cilium is a critical sensory organelle on the cell surface. An association between ciliary defects and obesity has been suggested, but the underlying mechanisms are not fully understood. Here we show that inhibition of ciliogenesis in POMC-expressing developing hypothalamic neurons, by depleting ciliogenic genes IFT88 and KIF3A, leads to adulthood obesity in mice. In contrast, adult-onset ciliary dysgenesis in POMC neurons causes no significant change in adiposity. In developing POMC neurons, abnormal cilia formation disrupts axonal projections through impaired lysosomal protein degradation. Notably, maternal nutrition and postnatal leptin surge have a profound impact on ciliogenesis in the hypothalamus of neonatal mice; through these effects they critically modulate the organization of hypothalamic feeding circuits. Our findings reveal a mechanism of early life programming of adult adiposity, which is mediated by primary cilia in developing hypothalamic neurons.

Ciliary defects and obesity has been associated, but the underlying mechanism is unclear. Here, the authors show that inhibition of ciliogenesis in POMC neurons during development results in lysosomal protein degradation-dependent axonal disruption and adult obesity in mice.

Neuron-specific cilia loss differentially alters locomotor responses to amphetamine in mice

The neural mechanisms that underlie responses to drugs of abuse are complex, and impacted by a number of neuromodulatory peptides. Within the past 10 years it has been discovered that several of the receptors for neuromodulators are enriched in the primary cilia of neurons. Primary cilia are microtubule-based organelles that project from the surface of nearly all mammalian cells, including neurons. Despite what we know about cilia, our understanding of how cilia regulate neuronal function and behavior is still limited. The primary objective of this study was to investigate the contributions of primary cilia on specific neuronal populations to behavioral responses to amphetamine. To test the consequences of cilia loss on amphetamine-induced locomotor activity we selectively ablated cilia from dopaminergic or GAD2-GABAergic neurons in mice. Cilia loss had no effect on baseline locomotion in either mouse strain. In mice lacking cilia on dopaminergic neurons, locomotor activity compared to wild- type mice was reduced in both sexes in response to acute administration of 3.0 mg/kg amphetamine. In contrast, changes in the locomotor response to amphetamine in mice lacking cilia on GAD2-GABAergic neurons were primarily driven by reductions in locomotor activity in males. Following repeated amphetamine administration (1.0 mg kg^-1  day^-1 over 5 days), mice lacking cilia on GAD2-GABAergic neurons exhibited enhanced sensitization of the locomotor stimulant response to the drug, whereas mice lacking cilia on dopaminergic neurons did not differ from wild-type controls. These results indicate that cilia play neuron-specific roles in both acute and neuroplastic responses to psychostimulant drugs of abuse.

Analysis of ciliary status via G-protein-coupled receptors localized on primary cilia

G-protein-coupled receptors (GPCRs) comprise the largest and most diverse cell surface receptor family, with more than 800 known GPCRs identified in the human genome. Binding of an extracellular cue to a GPCR results in intracellular G protein activation, after which a sequence of events, can be amplified and optimized by selective binding partners and downstream effectors in spatially discrete cellular environments. Because GPCRs are widely expressed in the body, they help to regulate an incredible range of physiological processes from sensation to growth to hormone responses. Indeed, it is estimated that ∼ 30% of all clinically approved drugs act by binding to GPCRs. The primary cilium is a sensory organelle composed of a microtubule axoneme that extends from the basal body. The ciliary membrane is highly enriched in specific signaling components, allowing the primary cilium to efficiently convey signaling cascades in a highly ordered microenvironment. Recent data demonstrated that a limited number of non-olfactory GPCRs, including somatostatin receptor 3 and melanin-concentrating hormone receptor 1 (MCHR1), are selectively localized to cilia on several mammalian cell types including neuronal cells. Utilizing cilia-specific cell biological and molecular biological approaches, evidence has accumulated to support the biological importance of ciliary GPCR signaling followed by cilia structural changes. Thus, cilia are now considered a unique sensory platform for integration of GPCR signaling toward juxtaposed cytoplasmic structures. Herein, we review ciliary GPCRs and focus on a novel role of MCHR1 in ciliary length control that will impact ciliary signaling capacity and neuronal function.

JNK Signaling Regulates Cellular Mechanics of Cortical Interneuron Migration

Aberrant migration of inhibitory interneurons can alter the formation of cortical circuitry and lead to severe neurologic disorders including epilepsy, autism, and schizophrenia. However, mechanisms involved in directing the migration of interneurons remain incompletely understood. Using a mouse model, we performed live-cell confocal microscopy to explore the mechanisms by which the c-Jun NH₂-terminal kinase (JNK) pathway coordinates leading process branching and nucleokinesis, two cell biological processes that are essential for the guided migration of cortical interneurons. Pharmacological inhibition of JNK signaling disrupts the kinetics of leading process branching, rate and amplitude of nucleokinesis, and leads to the rearward mislocalization of the centrosome and primary cilium to the trailing process. Genetic loss of Jnk from interneurons also impairs leading process branching and nucleokinesis, suggesting that important mechanics of interneuron migration depend on the intrinsic activity of JNK. These findings highlight key roles for JNK signaling in leading process branching, nucleokinesis, and the trafficking of centrosomes and cilia during interneuron migration, and further implicates JNK signaling as an important mediator of cortical development.

Lasting and Sex-Dependent Impact of Maternal Immune Activation on Molecular Pathways of the Amygdala

The prolonged and sex-dependent impact of maternal immune activation (MIA) during gestation on the molecular pathways of the amygdala, a brain region that influences social, emotional, and other behaviors, is only partially understood. To address this gap, we investigated the effects of viral-elicited MIA during gestation on the amygdala transcriptome of pigs, a species of high molecular and developmental homology to humans. Gene expression levels were measured using RNA-Seq on the amygdala for 3-week-old female and male offspring from MIA and control groups. Among the 403 genes that exhibited significant MIA effect, a prevalence of differentially expressed genes annotated to the neuroactive ligand-receptor pathway, glutamatergic functions, neuropeptide systems, and cilium morphogenesis were uncovered. Genes in these categories included corticotropin-releasing hormone receptor 2, glutamate metabotropic receptor 4, glycoprotein hormones, alpha polypeptide, parathyroid hormone 1 receptor, vasointestinal peptide receptor 2, neurotensin, proenkephalin, and gastrin-releasing peptide. These categories and genes have been associated with the MIA-related human neurodevelopmental disorders, including schizophrenia and autism spectrum disorders. Gene network reconstruction highlighted differential vulnerability to MIA effects between sexes. Our results advance the understanding necessary for the development of multifactorial therapies targeting immune modulation and neurochemical dysfunction that can ameliorate the effects of MIA on offspring behavior later in life.

A truncating Aspm allele leads to a complex cognitive phenotype and region-specific reductions in parvalbuminergic neurons

Neurodevelopmental disorders are heterogeneous and identifying shared genetic aetiologies and converging signalling pathways affected could improve disease diagnosis and treatment. Truncating mutations of the abnormal spindle-like microcephaly associated (ASPM) gene cause autosomal recessive primary microcephaly (MCPH) in humans. ASPM is a positive regulator of Wnt/β-Catenin signalling and controls symmetric to asymmetric cell division. This process balances neural progenitor proliferation with differentiation during embryogenesis, the malfunction of which could interfere with normal brain development. ASPM mutations may play a role also in other neurodevelopmental disorders, nevertheless, we lack the details of how or to what extent. We therefore assessed neurodevelopmental disease and circuit endophenotypes in mice with a truncating Aspm^1-7 mutation. Aspm^1-7 mice exhibited impaired short- and long-term object recognition memory and markedly enhanced place learning in the IntelliCage®. This behaviour pattern is reminiscent of a cognitive phenotype seen in mouse models and patients with a rare form of autism spectrum disorder (ASD) as well as in mouse models of altered Wnt signalling. These alterations were accompanied by ventriculomegaly, corpus callosum dysgenesis and decreased parvalbumin (PV)+ interneuron numbers in the hippocampal Cornu Ammonis (CA) region and thalamic reticular nucleus (TRN). PV+ cell number correlated to object recognition (CA and TRN) and place learning (TRN). This opens the possibility that, as well as causing MCPH, mutant ASPM potentially contributes to other neurodevelopmental disorders such as ASD through altered parvalbuminergic interneuron development affecting cognitive behaviour. These findings provide important information for understanding the genetic overlap and improved treatment of neurodevelopmental disorders associated with ASPM.

TTBK2 and primary cilia are essential for the connectivity and survival of cerebellar Purkinje neurons

Primary cilia are vital signaling organelles that extend from most types of cells, including neurons and glia. These structures are essential for development of many tissues and organs; however, their function in adult tissues, particularly neurons in the brain, remains largely unknown. Tau tubulin kinase 2 (TTBK2) is a critical regulator of ciliogenesis, and is also mutated in a hereditary neurodegenerative disorder, spinocerebellar ataxia type 11 (SCA11). Here, we show that conditional knockout of Ttbk2 in adult mice results in degenerative cerebellar phenotypes that recapitulate aspects of SCA11 including motor coordination deficits and defects to Purkinje cell (PC) integrity. We also find that the Ttbk2 conditional mutant mice quickly lose cilia throughout the brain. We show that conditional knockout of the key ciliary trafficking gene Ift88 in adult mice results in nearly identical cerebellar phenotypes to those of the Ttbk2 knockout, indicating that disruption of ciliary signaling is a key driver of these phenotypes. Our data suggest that primary cilia play an integral role in maintaining the function of PCs in the adult cerebellum and reveal novel insights into mechanisms involved in neurodegeneration.

Somatostatin receptors (SSTR1-5) on inhibitory interneurons in the barrel cortex

Inhibitory interneurons in the cerebral cortex contain specific proteins or peptides characteristic for a certain interneuron subtype. In mice, three biochemical markers constitute non-overlapping interneuron populations, which account for 80–90% of all inhibitory cells. These interneurons express parvalbumin (PV), somatostatin (SST), or vasoactive intestinal peptide (VIP). SST is not only a marker of a specific interneuron subtype, but also an important neuropeptide that participates in numerous biochemical and signalling pathways in the brain via somatostatin receptors (SSTR1-5). In the nervous system, SST acts as a neuromodulator and neurotransmitter affecting, among others, memory, learning, and mood. In the sensory cortex, the co-localisation of GABA and SST is found in approximately 30% of interneurons. Considering the importance of interactions between inhibitory interneurons in cortical plasticity and the possible GABA and SST co-release, it seems important to investigate the localisation of different SSTRs on cortical interneurons. Here, we examined the distribution of SSTR1-5 on barrel cortex interneurons containing PV, SST, or VIP. Immunofluorescent staining using specific antibodies was performed on brain sections from transgenic mice that expressed red fluorescence in one specific interneuron subtype (PV-Ai14, SST-Ai14, and VIP-Ai14 mice). SSTRs expression on PV, SST, and VIP interneurons varied among the cortical layers and we found two patterns of SSTRs distribution in L4 of barrel cortex. We also demonstrated that, in contrast to other interneurons, PV cells did not express SSTR2, but expressed other SSTRs. SST interneurons, which were not found to make chemical synapses among themselves, expressed all five SSTR subtypes.

Primary Cilia Signaling Promotes Axonal Tract Development and Is Disrupted in Joubert Syndrome-Related Disorders Models

Appropriate axonal growth and connectivity are essential for functional wiring of the brain. Joubert syndrome-related disorders (JSRD), a group of ciliopathies in which mutations disrupt primary cilia function, are characterized by axonal tract malformations. However, little is known about how cilia-driven signaling regulates axonal growth and connectivity. We demonstrate that the deletion of related JSRD genes, Arl13b and Inpp5e, in projection neurons leads to de-fasciculated and misoriented axonal tracts. Arl13b deletion disrupts the function of its downstream effector, Inpp5e, and deregulates ciliary-PI3K/AKT signaling. Chemogenetic activation of ciliary GPCR signaling and cilia-specific optogenetic modulation of downstream second messenger cascades (PI3K, AKT, and AC3) commonly regulated by ciliary signaling receptors induce rapid changes in axonal dynamics. Further, Arl13b deletion leads to changes in transcriptional landscape associated with dysregulated PI3K/AKT signaling. These data suggest that ciliary signaling acts to modulate axonal connectivity and that impaired primary cilia signaling underlies axonal tract defects in JSRD.

High-resolution chromosomal microarray analysis for copy-number variations in high-functioning autism reveals large aberration typical for intellectual disability

Copy-number variants (CNVs), in particular rare, small and large ones (< 1% frequency) and those encompassing brain-related genes, have been shown to be associated with neurodevelopmental disorders like autism spectrum disorders (ASDs), attention deficit hyperactivity disorder (ADHD), and intellectual disability (ID). However, the vast majority of CNV findings lack specificity with respect to autistic or developmental-delay phenotypes. Therefore, the aim of the study was to investigate the size and frequency of CNVs in high-functioning ASD (HFA) without ID compared with a random population sample and with published findings in ASD and ID. To investigate the role of CNVs for the "core symptoms" of high-functioning autism, we included in the present exploratory study only patients with HFA without ID. The aim was to test whether HFA have similar large rare (> 1 Mb) CNVs as reported in ASD and ID. We performed high-resolution chromosomal microarray analysis in 108 children and adolescents with HFA without ID. There was no significant difference in the overall number of rare CNVs compared to 124 random population samples. However, patients with HFA carried significantly more frequently CNVs containing brain-related genes. Surprisingly, six HFA patients carried very large CNVs known to be typically present in ID. Our findings provide new evidence that not only small, but also large CNVs affecting several key genes contribute to the genetic etiology/risk of HFA without affecting their intellectual ability.

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

Primary cilia project in a single copy from the surface of most vertebrate cell types; they detect and transmit extracellular cues to regulate diverse cellular processes during development and to maintain tissue homeostasis. The sensory capacity of primary cilia relies on the coordinated trafficking and temporal localization of specific receptors and associated signal transduction modules in the cilium. The canonical Hedgehog (HH) pathway, for example, is a bona fide ciliary signalling system that regulates cell fate and self-renewal in development and tissue homeostasis. Specific receptors and associated signal transduction proteins can also localize to primary cilia in a cell type-dependent manner; available evidence suggests that the ciliary constellation of these proteins can temporally change to allow the cell to adapt to specific developmental and homeostatic cues. Consistent with important roles for primary cilia in signalling, mutations that lead to their dysfunction underlie a pleiotropic group of diseases and syndromic disorders termed ciliopathies, which affect many different tissues and organs of the body. In this Review, we highlight central mechanisms by which primary cilia coordinate HH, G protein-coupled receptor, WNT, receptor tyrosine kinase and transforming growth factor-β (TGFβ)/bone morphogenetic protein (BMP) signalling and illustrate how defects in the balanced output of ciliary signalling events are coupled to developmental disorders and disease progression.

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

The cilium of a cell translates varied extracellular cues into intracellular signals that control embryonic development and organ function. The dynamic maintenance of ciliary structure and function requires balanced bidirectional cargo transport involving intraflagellar transport (IFT) complexes. IFT172 is a member of the IFT complex B, and IFT172 mutation is associated with pathologies including short rib thoracic dysplasia, retinitis pigmentosa and Bardet-Biedl syndrome, but how it underpins these conditions is not clear. We used the WIM cell line, derived from embryonic fibroblasts of Wimple mice (carrying homozygous Leu1564Pro mutation in Ift172), to probe roles of Ift172 and primary cilia in cell behavior. WIM cells had ablated cilia and deficiencies in directed migration (electrotaxis), cell proliferation and intracellular signaling. Additionally, WIM cells displayed altered cell cycle progression, with increased numbers of chromatids, highlighting dysfunctional centrosome status. Exposure to a physiological electric field promoted a higher percentage of primary cilia in wild-type cells. Interestingly, in situ hybridization revealed an extensive and dynamic expression profile of Ift172 in both developing and adult mouse cortex. In vivo manipulation of Ift172 expression in germinal regions of embryonic mouse brains perturbed neural progenitor proliferation and radial migration of post-mitotic neurons, revealing a regulatory role of Ift172 in cerebral morphogenesis. Our data suggest that Ift172 regulates a range of fundamental biological processes, highlighting the pivotal roles of the primary cilium in cell physiology and brain development.

Dynamic Changes in Ultrastructure of the Primary Cilium in Migrating Neuroblasts in the Postnatal Brain

New neurons, referred to as neuroblasts, are continuously generated in the ventricular-subventricular zone of the brain throughout an animal's life. These neuroblasts are characterized by their unique potential for proliferation, formation of chain-like cell aggregates, and long-distance and high-speed migration through the rostral migratory stream (RMS) toward the olfactory bulb (OB), where they decelerate and differentiate into mature interneurons. The dynamic changes of ultrastructural features in postnatal-born neuroblasts during migration are not yet fully understood. Here we report the presence of a primary cilium, and its ultrastructural morphology and spatiotemporal dynamics, in migrating neuroblasts in the postnatal RMS and OB. The primary cilium was observed in migrating neuroblasts in the postnatal RMS and OB in male and female mice and zebrafish, and a male rhesus monkey. Inhibition of intraflagellar transport molecules in migrating neuroblasts impaired their ciliogenesis and rostral migration toward the OB. Serial section transmission electron microscopy revealed that each migrating neuroblast possesses either a pair of centrioles or a basal body with an immature or mature primary cilium. Using immunohistochemistry, live imaging, and serial block-face scanning electron microscopy, we demonstrate that the localization and orientation of the primary cilium are altered depending on the mitotic state, saltatory migration, and deceleration of neuroblasts. Together, our results highlight a close mutual relationship between spatiotemporal regulation of the primary cilium and efficient chain migration of neuroblasts in the postnatal brain.SIGNIFICANCE STATEMENT Immature neurons (neuroblasts) generated in the postnatal brain have a mitotic potential and migrate in chain-like cell aggregates toward the olfactory bulb. Here we report that migrating neuroblasts possess a tiny cellular protrusion called a primary cilium. Immunohistochemical studies with zebrafish, mouse, and monkey brains suggest that the presence of the primary cilium in migrating neuroblasts is evolutionarily conserved. Ciliogenesis in migrating neuroblasts in the rostral migratory stream is suppressed during mitosis and promoted after cell cycle exit. Moreover, live imaging and 3D electron microscopy revealed that ciliary localization and orientation change during saltatory movement of neuroblasts. Our results reveal highly organized dynamics in maturation and positioning of the primary cilium during neuroblast migration that underlie saltatory movement of postnatal-born neuroblasts.

Ciliogenesis associated kinase 1: targets and functions in various organ systems

Ciliogenesis associated kinase 1 (CILK1) was previously known as intestinal cell kinase because it was cloned from that origin. However, CILK1 is now recognized as a widely expressed and highly conserved serine/threonine protein kinase. Mutations in the human CILK1 gene have been associated with ciliopathies, a group of human genetic disorders with defects in the primary cilium. In mice, both Cilk1 knock-out and Cilk1 knock-in mutations have recapitulated human ciliopathies. Thus, CILK1 has a fundamental role in the function of the cilium. Several candidate substrates have been proposed for CILK1 and the challenge is to relate these to the mutant phenotypes. In this review, we summarize what is known about CILK1 functions and targets, and discuss gaps in current knowledge that motivate further experimentation to fully understand the role of CILK1 in organ development in humans.

Characterization of Functional Primary Cilia in Human Induced Pluripotent Stem Cell-Derived Neurons

Recent advances in human induced pluripotent stem cells (hiPSCs) offer new possibilities for biomedical research and clinical applications. Neurons differentiated from hiPSCs may be promising tools to develop novel treatment methods for various neurological diseases. However, the detailed process underlying functional maturation of hiPSC-derived neurons remains poorly understood. Here, we analyze the developmental architecture of hiPSC-derived cortical neurons, iCell GlutaNeurons, focusing on the primary cilium, a single sensory organelle that protrudes from the surface of most growth-arrested vertebrate cells. To characterize the neuronal cilia, cells were cultured for various periods and evaluated immunohistochemically by co-staining with antibodies against ciliary markers Arl13b and MAP2. Primary cilia were detected in neurons within days, and their prevalence and length increased with increasing days in culture. Treatment with the mood stabilizer lithium led to primary cilia length elongation, while treatment with the orexigenic neuropeptide melanin-concentrating hormone caused cilia length shortening in iCell GlutaNeurons. The present findings suggest that iCell GlutaNeurons develop neuronal primary cilia together with the signaling machinery for regulation of cilia length. Our approach to the primary cilium as a cellular antenna can be useful for both assessment of neuronal maturation and validation of pharmaceutical agents in hiPSC-derived neurons.

Primary cilium and brain aging: role in neural stem cells, neurodegenerative diseases and glioblastoma

Brain aging is characterized by a progressive loss of tissue integrity and function as a consequence of impaired homeostasis and regeneration capacities. The primary cilium is a highly conserved organelle that projects from the cell surface in a single copy in virtually all mammalian cell types including neural stem/progenitors cells and neurons. Increasing evidence in the last decade points out that primary cilium could be a relevant mediator of neural stem cell activity, neurogenesis, neuronal maturation and maintenance, and brain tumorigenesis. In this review, we summarize the current knowledge about primary cilia roles in these processes. There is currently sufficient background to propose that defective primary cilia contribute to age-related cognitive decline and brain tumor development due to their critical roles in cell cycle control and signaling transduction. This might have potential applications on therapy against age-associated brain diseases.

Somatostatin Serves a Modulatory Role in the Mouse Olfactory Bulb: Neuroanatomical and Behavioral Evidence

Somatostatin (SOM) and somatostatin receptors (SSTR1-4) are present in all olfactory structures, including the olfactory bulb (OB), where SOM modulates physiological gamma rhythms and olfactory discrimination responses. In this work, histological, viral tracing and transgenic approaches were used to characterize SOM cellular targets in the murine OB. We demonstrate that SOM targets all levels of mitral dendritic processes in the OB with somatostatin receptor 2 (SSTR2) detected in the dendrites of previously uncharacterized mitral-like cells. We show that inhibitory interneurons of the glomerular layer (GL) express SSTR4 while SSTR3 is confined to the granule cell layer (GCL). Furthermore, SOM cells in the OB receive synaptic inputs from olfactory cortical afferents. Behavioral studies demonstrate that genetic deletion of SSTR4, SSTR2 or SOM differentially affects olfactory performance. SOM or SSTR4 deletion have no major effect on olfactory behavioral performances while SSTR2 deletion impacts olfactory detection and discrimination behaviors. Altogether, these results describe novel anatomical and behavioral contributions of SOM, SSTR2 and SSTR4 receptors in olfactory processing.

Primary Cilia-An Underexplored Topic in Major Mental Illness

Though much progress has been made in recent years towards understanding the function and physiology of primary cilia, they remain a somewhat elusive organelle. Some studies have explored the role of primary cilia in the developing nervous system, and their dysfunction has been linked with several neurosensory deficits. Yet, very little has been written on their potential role in psychiatric disorders. This article provides an overview of some of the functions of primary cilia in signalling pathways, and demonstrates that they are a worthy candidate in psychiatric research. The links between primary cilia and major mental illness have been demonstrated to exist at several levels, spanning genetics, signalling pathways, and pharmacology as well as cell division and migration. The primary focus of this review is on the sensory role of the primary cilium and the neurodevelopmental hypothesis of psychiatric disease. As such, the primary cilium is demonstrated to be a key link between the cellular environment and cell behaviour, and hence of key importance in the considerations of the nature and nurture debate in psychiatric research.

[Selective signaling pathway via feeding-related ciliary GPCR, melanin-concentrating hormone receptor 1]

G-protein-coupled receptors (GPCRs), which constitute a highly diverse family of seven transmembrane receptors, respond to external signals and regulate a variety of cellular and physiological processes. GPCRs are encoded by about 800 different genes in human and they represent the largest family of drug targets in clinical trials, which accounts for about 30% of approved drugs acting on 108 unique GPCRs. Signaling through GPCRs can be optimized by enriching receptors, selective binding partners, and downstream effectors in discrete cellular environment. The primary cilium is a ubiquitous organelle that functions as a sensory antenna for surrounding physical and chemical stimuli. Primary cilium's compartment is as little as 1/10,000th of the total cell volume. Therefore, the ciliary membrane is highly enriched for specific signaling molecules, allowing the primary cilium to organize signaling in a highly ordered microenvironment. Recently, a set of non-olfactory GPCRs such as somatostatin receptor 3 and melanin-concentrating hormone receptor 1 (MCHR1) have been found to be selectively targeted to cilia on several mammalian cell types including neuronal cells both in vitro and in vivo approaches. Moreover, investigations into the pathophysiology have implicated GPCR ciliary signaling in a number of developmental and cellular pathways. Thus, cilia are now considered as an increasingly important connection for GPCR signaling. This review summarizes our current understanding of the signaling pathways though ciliary GPCR, especially feeding- and mood-related GPCR MCHR1, along with specific biological phenomenon as cilia length shortening.

Microtubule asters anchored by FSD1 control axoneme assembly and ciliogenesis

Defective ciliogenesis causes human developmental diseases termed ciliopathies. Microtubule (MT) asters originating from centrosomes in mitosis ensure the fidelity of cell division by positioning the spindle apparatus. However, the function of microtubule asters in interphase remains largely unknown. Here, we reveal an essential role of MT asters in transition zone (TZ) assembly during ciliogenesis. We demonstrate that the centrosome protein FSD1, whose biological function is largely unknown, anchors MT asters to interphase centrosomes by binding to microtubules. FSD1 knockdown causes defective ciliogenesis and affects embryonic development in vertebrates. We further show that disruption of MT aster anchorage by depleting FSD1 or other known anchoring proteins delocalizes the TZ assembly factor Cep290 from centriolar satellites, and causes TZ assembly defects. Thus, our study establishes FSD1 as a MT aster anchorage protein and reveals an important function of MT asters anchored by FSD1 in TZ assembly during ciliogenesis.

Microtubule asters originate from centrosomes but their role during interphase remains largely unknown. Here, the authors find that microtubule asters anchored by previously-uncharacterized FSD1 play a role in ciliogenesis by maintaining the dynamic localization of centriolar satellites.

A rare human CEP290 variant disrupts the molecular integrity of the primary cilium and impairs Sonic Hedgehog machinery

The primary cilium is a microtubule-enriched cell-communication organelle that participates in mechanisms controlling tissue development and maintenance, including cerebellar architecture. Centrosomal protein of 290 kDa (CEP290) is a protein important for centrosomal function and ciliogenesis. Mutations in CEP290 have been linked to a group of multi-organ disorders - termed ciliopathies. The neurophysiological deficits observed in ciliopathies are sometimes associated with the progression of autistic traits. Here, the cellular function of two rare variants of CEP290 identified from recent exome sequencing of autistic individuals are investigated. Cells expressing Cep290 carrying the missense mutation R1747Q in mouse exhibited a defective Sonic hedgehog (Shh) signalling response, mislocalisation of the Shh receptor Smoothened (Smo), and dysregulation of ciliary protein mobility, which ultimately disrupted the proliferation of cerebellar granule progenitors (CGPs). This data was furthermore corroborated in an autism patient-derived iPSC line harbouring the R1746Q rare CEP290 variant. Evidence from this study suggests that the R1746Q mutation interferes with the function of CEP290 to maintain the ciliary diffusion barrier and disrupts the integrity of the molecular composition in the primary cilium, which may contribute to alterations in neuroarchitecture.

Signaling through the Primary Cilium

The presence of single, non-motile “primary” cilia on the surface of epithelial cells has been well described since the 1960s. However, for decades these organelles were believed to be vestigial, with no remaining function, having lost their motility. It wasn't until 2003, with the discovery that proteins responsible for transport along the primary cilium are essential for hedgehog signaling in mice, that the fundamental importance of primary cilia in signal transduction was realized. Little more than a decade later, it is now clear that the vast majority of signaling pathways in vertebrates function through the primary cilium. This has led to the adoption of the term “the cells's antenna” as a description for the primary cilium. Primary cilia are particularly important during development, playing fundamental roles in embryonic patterning and organogenesis, with a suite of inherited developmental disorders known as the “ciliopathies” resulting from mutations in genes encoding cilia proteins. This review summarizes our current understanding of the role of these fascinating organelles in a wide range of signaling pathways.

Neurodevelopmental disease mechanisms, primary cilia, and endosomes converge on the BLOC-1 and BORC complexes

The biogenesis of lysosome-related organelles complex-1 (BLOC-1) and the bloc-one-related complex (BORC) are the cytosolic protein complexes required for specialized membrane protein traffic along the endocytic route and the spatial distribution of endosome-derived compartments, respectively. BLOC-1 and BORC complex subunits and components of their interactomes have been associated with the risk and/or pathomechanisms of neurodevelopmental disorders. Thus, cellular processes requiring BLOC-1 and BORC interactomes have the potential to offer novel insight into mechanisms underlying behavioral defects. We focus on interactions between BLOC-1 or BORC subunits with the actin and microtubule cytoskeleton, membrane tethers, and SNAREs. These interactions highlight requirements for BLOC-1 and BORC in membrane movement by motors, control of actin polymerization, and targeting of membrane proteins to specialized cellular domains such as the nerve terminal and the primary cilium. We propose that the endosome-primary cilia pathway is an underappreciated hub in the genesis and mechanisms of neurodevelopmental disorders. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 311-330, 2018.

Cellular eyelashes help striatal neurons hook up

Primary cilia-based signaling provides critical infrastructure for formation of striatal neuronal networks.