Neuron's little helper: The role of primary cilia in neurogenesis

Novel CDKL5 targets identified in human iPSC-derived neurons

CDKL5 Deficiency Disorder (CDD) is a debilitating epileptic encephalopathy disorder affecting young children with no effective treatments. CDD is caused by pathogenic variants in Cyclin-Dependent Kinase-Like 5 (CDKL5), a protein kinase that regulates key phosphorylation events in neurons. For therapeutic intervention, it is essential to understand molecular pathways and phosphorylation targets of CDKL5. Using an unbiased phosphoproteomic approach we identified novel targets of CDKL5, including GTF2I, PPP1R35, GATAD2A and ZNF219 in human iPSC-derived neuronal cells. The phosphoserine residue in the target proteins lies in the CDKL5 consensus motif. We validated direct phosphorylation of GTF2I and PPP1R35 by CDKL5 using complementary approaches. GTF2I controls axon guidance, cell cycle and neurodevelopment by regulating expression of neuronal genes. PPP1R35 is critical for centriole elongation and cilia morphology, processes that are impaired in CDD. PPP1R35 interacts with CEP131, a known CDKL5 phospho-target. GATAD2A and ZNF219 belong to the Nucleosome Remodelling Deacetylase (NuRD) complex, which regulates neuronal activity-dependent genes and synaptic connectivity. In-depth knowledge of molecular pathways regulated by CDKL5 will allow a better understanding of druggable disease pathways to fast-track therapeutic development.

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

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

Neural stem cell homeostasis is affected in cortical organoids carrying a mutation in Angiogenin

Mutations in Angiogenin (ANG) and TARDBP encoding the 43 kDa transactive response DNA binding protein (TDP-43) are associated with amyotrophic lateral sclerosis and frontotemporal dementia (ALS-FTD). ANG is neuroprotective and plays a role in stem cell dynamics in the haematopoietic system. We obtained skin fibroblasts from members of an ALS-FTD family, one with mutation in ANG, one with mutation in both TARDBP and ANG, and one with neither mutation. We reprogrammed these fibroblasts to induced pluripotent stem cells (iPSCs) and generated cortical organoids as well as induced stage-wise differentiation of the iPSCs to neurons. Using these two approaches we investigated the effects of FTD-associated mutations in ANG and TARDBP on neural precursor cells, neural differentiation, and response to stress. We observed striking neurodevelopmental defects such as abnormal and persistent rosettes in the organoids accompanied by increased self-renewal of neural precursor cells. There was also a propensity for differentiation to later-born neurons. In addition, cortical neurons showed increased susceptibility to stress, which is exacerbated in neurons carrying mutations in both ANG and TARDBP. The cortical organoids and neurons generated from patient-derived iPSCs carrying ANG and TARDBP gene variants recapitulate dysfunctions characteristic of frontotemporal lobar degeneration observed in FTD patients. These dysfunctions were ameliorated upon treatment with wild type ANG. In addition to its well-established role during the stress response of mature neurons, ANG also appears to play a role in neural progenitor dynamics. This has implications for neurogenesis and may indicate that subtle developmental defects play a role in disease susceptibility or onset. © 2024 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Illumination of understudied ciliary kinases

Cilia are cellular signaling hubs. Given that human kinases are central regulators of signaling, it is not surprising that kinases are key players in cilia biology. In fact, many kinases modulate ciliogenesis, which is the generation of cilia, and distinct ciliary pathways. Several of these kinases are understudied with few publications dedicated to the interrogation of their function. Recent efforts to develop chemical probes for members of the cyclin-dependent kinase like (CDKL), never in mitosis gene A (NIMA) related kinase (NEK), and tau tubulin kinase (TTBK) families either have delivered or are working toward delivery of high-quality chemical tools to characterize the roles that specific kinases play in ciliary processes. A better understanding of ciliary kinases may shed light on whether modulation of these targets will slow or halt disease onset or progression. For example, both understudied human kinases and some that are more well-studied play important ciliary roles in neurons and have been implicated in neurodevelopmental, neurodegenerative, and other neurological diseases. Similarly, subsets of human ciliary kinases are associated with cancer and oncological pathways. Finally, a group of genetic disorders characterized by defects in cilia called ciliopathies have associated gene mutations that impact kinase activity and function. This review highlights both progress related to the understanding of ciliary kinases as well as in chemical inhibitor development for a subset of these kinases. We emphasize known roles of ciliary kinases in diseases of the brain and malignancies and focus on a subset of poorly characterized kinases that regulate ciliary biology.

Primary cilia promote the differentiation of human neurons through the WNT signaling pathway

BACKGROUND

Primary cilia emanate from most human cell types, including neurons. Cilia are important for communicating with the cell's immediate environment: signal reception and transduction to/from the ciliated cell. Deregulation of ciliary signaling can lead to ciliopathies and certain neurodevelopmental disorders. In the developing brain cilia play well-documented roles for the expansion of the neural progenitor cell pool, while information about the roles of cilia during post-mitotic neuron differentiation and maturation is scarce.

RESULTS

We employed ciliated Lund Human Mesencephalic (LUHMES) cells in time course experiments to assess the impact of ciliary signaling on neuron differentiation. By comparing ciliated and non-ciliated neuronal precursor cells and neurons in wild type and in RFX2 -/- mutant neurons with altered cilia, we discovered an early-differentiation "ciliary time window" during which transient cilia promote axon outgrowth, branching and arborization. Experiments in neurons with IFT88 and IFT172 ciliary gene knockdowns, leading to shorter cilia, confirm these results. Cilia promote neuron differentiation by tipping WNT signaling toward the non-canonical pathway, in turn activating WNT pathway output genes implicated in cyto-architectural changes.

CONCLUSIONS

We provide a mechanistic entry point into when and how ciliary signaling coordinates, promotes and translates into anatomical changes. We hypothesize that ciliary alterations causing neuron differentiation defects may result in "mild" impairments of brain development, possibly underpinning certain aspects of neurodevelopmental disorders.

Pioneer statoacoustic neurons guide neuroblast behaviour during otic ganglion assembly

Cranial ganglia are aggregates of sensory neurons that mediate distinct types of sensation. The statoacoustic ganglion (SAG) develops into several lobes that are spatially arranged to connect appropriately with hair cells of the inner ear. To investigate the cellular behaviours involved in the 3D organization of the SAG, we use high-resolution confocal imaging of single-cell, labelled zebrafish neuroblasts (NBs), photoconversion, photoablation, and genetic perturbations. We show that otic NBs delaminate out of the otic epithelium in an epithelial-mesenchymal transition-like manner, rearranging apical polarity and primary cilia proteins. We also show that, once delaminated, NBs require RhoGTPases in order to perform active migration. Furthermore, tracking of recently delaminated NBs revealed their directed migration and coalescence around a small population of pioneer SAG neurons. These pioneer SAG neurons, not from otic placode origin, populate the coalescence region before otic neurogenesis begins and their ablation disrupts delaminated NB migratory pathways, consequentially affecting SAG shape. Altogether, this work shows for the first time the role of pioneer SAG neurons in orchestrating SAG development.

Human LUHMES and NES cells as models for studying primary cilia in neurons

Primary cilia are antenna-like organelles emanating from the cell surface. They are involved in cell-to-cell communication and bidirectional signal transduction to/from the extracellular environment. During brain formation, cilia critically aid in neurogenesis and maturation of neuronal structures such as axons, dendrites and synapses. Aberrations in cilia function can induce neuron differentiation defects and pathological consequences of varying severity, resulting in ciliopathies and likely a number of neurodevelopmental disorders. Despite the documented relevance of cilia for proper brain development, human neuronal models to recognize and study cilia biology are still scarce. We have established two types of cell models, Lund Human Mesencephalic (LUHMES) cells and neuroepithelial stem (NES) cells derived from induced pluripotent stem cells (iPSC), to investigate cilia biology in both proliferating neuronal progenitors/precursors and during the entire neuron differentiation and maturation process. We employ improved immunocytochemistry assays able to specifically detect cilia by confocal and super-resolution microscopy. We provide straightforward and robust methods to easily maintain cells in culture, for immunostaining and characterization of cilia orientation, anatomy and shape in human neurons across all stages of differentiation.

Generation and characterization of Ccdc28b mutant mice links the Bardet-Biedl associated gene with mild social behavioral phenotypes

CCDC28B (coiled-coil domain-containing protein 28B) was identified as a modifier in the ciliopathy Bardet-Biedl syndrome (BBS). Our previous work in cells and zebrafish showed that CCDC28B plays a role regulating cilia length in a mechanism that is not completely understood. Here we report the generation of a Ccdc28b mutant mouse using CRISPR/Cas9 (Ccdc28b mut). Depletion of CCDC28B resulted in a mild phenotype. Ccdc28b mut animals i) do not present clear structural cilia affectation, although we did observe mild defects in cilia density and cilia length in some tissues, ii) reproduce normally, and iii) do not develop retinal degeneration or obesity, two hallmark features of reported BBS murine models. In contrast, Ccdc28b mut mice did show clear social interaction defects as well as stereotypical behaviors. This finding is indeed relevant regarding CCDC28B as a modifier of BBS since behavioral phenotypes have been documented in BBS. Overall, this work reports a novel mouse model that will be key to continue evaluating genetic interactions in BBS, deciphering the contribution of CCDC28B to modulate the presentation of BBS phenotypes. In addition, our data underscores a novel link between CCDC28B and behavioral defects, providing a novel opportunity to further our understanding of the genetic, cellular, and molecular basis of these complex phenotypes.

BBS is caused by mutations in any one of 22 genes known to date. In some families, BBS can be inherited as an oligogenic trait whereby mutations in more than one BBS gene collaborate in the presentation of the syndrome. In addition, CCDC28B was originally identified as a modifier of BBS, whereby a reduction in CCDC28B levels was associated with a more severe presentation of the syndrome. Different mechanisms, all relying on functional redundancy, have been proposed to explain these genetic interactions. The characterization of BBS proteins supported this functional redundancy hypothesis: BBS proteins play a role in cilia maintenance/function and subsets of BBS proteins can even interact directly in multiprotein complexes.

We have previously shown that CCDC28B also participates in cilia biology regulating the length of the organelle: knockdown of CCDC28B in cells results in cilia shortening and targeting ccdc28b in zebrafish also results in early embryonic phenotypes characteristic of other cilia mutants. In this work, we generated a Ccdc28b mutant mouse to determine whether abrogating Ccdc28b function would be sufficient to cause a ciliopathy phenotype in mammals, and to generate a tool to continue dissecting its modifying role in the context of BBS. Overall, Ccdc28b mutant mice presented a mild phenotype, a finding fully compatible with its role as a modifier, rather than a causal BBS gene. In addition, we found that Ccdc28b mutants showed behavioral phenotypes, similar to the deficits observed in rodent autism spectrum disorder (ASD) models. Thus, our results underscore a novel causal link between CCDC28B and behavioral phenotypes in mice.

Ciliary signaling in stem cells in health and disease: Hedgehog pathway and beyond

The primary cilium is a hair-like sensory compartment that protrudes from the cellular surface. The primary cilium is enriched in a variety of signaling molecules that regulate cellular activities. Stem cells have primary cilia. They reside in a specialized environment, called the stem cell niche. This niche contains a variety of secreted factors, and some of their receptors are localized in the primary cilia of stem cells. Here, we summarize the current understanding of the function of cilia in compartmentalized signaling in stem cells. We describe how ciliary signaling regulates stem cells and progenitor cells during development, tissue homeostasis and tumorigenesis. We summarize our understanding of cilia regulated signaling -primary involving the hedgehog pathway- in stem cells in diverse settings that include neuroepithelial cells, radial glia, cerebellar granule neuron precursors, hematopoietic stem cells, hair follicle stem cells, bone marrow mesenchymal stem cells and mammary gland stem cells. Overall, our review highlights a variety of roles that ciliary signaling plays in regulating stem cells throughout life.

STRADA-mutant human cortical organoids model megalencephaly and exhibit delayed neuronal differentiation

Genetic diseases involving overactivation of the mechanistic target of rapamycin (mTOR) pathway, so-called "mTORopathies," often manifest with malformations of cortical development (MCDs), epilepsy, and cognitive impairment. How mTOR pathway hyperactivation results in abnormal human cortical development is poorly understood. To study the effect of mTOR hyperactivity on early stages of cortical development, we focused on Pretzel Syndrome (polyhydramnios, megalencephaly, symptomatic epilepsy; PMSE syndrome), a rare mTORopathy caused by homozygous germline mutations in the STRADA gene. We developed a human cortical organoid (hCO) model of PMSE and examined morphology and size for the first 2 weeks of organoid growth, and cell type composition at weeks 2, 8, and 12 of differentiation. In the second week, PMSE hCOs enlarged more rapidly than controls and displayed an abnormal Wnt pathway-dependent increase in neural rosette structures. PMSE hCOs also exhibited delayed neurogenesis, decreased subventricular zone progenitors, increased proliferation and cell death, and an abnormal architecture of primary cilia. At week 8, PMSE hCOs had fewer deep layer neurons. By week 12, neurogenesis recovered in PMSE organoids, but they displayed increased outer radial glia, a cell type thought to contribute to the expansion of the human cerebral cortex. Together, these findings suggest that megalencephaly in PMSE arises from the expansion of neural stem cells in early corticogenesis and potentially also from increased outer radial glial at later gestational stages. The delayed neuronal differentiation in PMSE organoids demonstrates the important role the mTOR pathway plays in the maintenance and expansion of the stem cell pool.

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.

Hydrogen Sulfide Modulates Adult and Reparative Neurogenesis in the Cerebellum of Juvenile Masu Salmon, Oncorhynchus masou

Fish are a convenient model for the study of reparative and post-traumatic processes of central nervous system (CNS) recovery, because the formation of new cells in their CNS continues throughout life. After a traumatic injury to the cerebellum of juvenile masu salmon, Oncorhynchus masou, the cell composition of the neurogenic zones containing neural stem cells (NSCs)/neural progenitor cells (NPCs) in the acute period (two days post-injury) changes. The presence of neuroepithelial (NE) and radial glial (RG) neuronal precursors located in the dorsal, lateral, and basal zones of the cerebellar body was shown by the immunohistochemical (IHC) labeling of glutamine synthetase (GS). Progenitors of both types are sources of neurons in the cerebellum of juvenile O. masou during constitutive growth, thus, playing an important role in CNS homeostasis and neuronal plasticity during ontogenesis. Precursors with the RG phenotype were found in the same regions of the molecular layer as part of heterogeneous constitutive neurogenic niches. The presence of neuroepithelial and radial glia GS+ cells indicates a certain proportion of embryonic and adult progenitors and, obviously, different contributions of these cells to constitutive and reparative neurogenesis in the acute post-traumatic period. Expression of nestin and vimentin was revealed in neuroepithelial cerebellar progenitors of juvenile O. masou. Patterns of granular expression of these markers were found in neurogenic niches and adjacent areas, which probably indicates the neurotrophic and proneurogenic effects of vimentin and nestin in constitutive and post-traumatic neurogenesis and a high level of constructive metabolism. No expression of vimentin and nestin was detected in the cerebellar RG of juvenile O. masou. Thus, the molecular markers of NSCs/NPCs in the cerebellum of juvenile O. masou are as follows: vimentin, nestin, and glutamine synthetase label NE cells in intact animals and in the post-traumatic period, while GS expression is present in the RG of intact animals and decreases in the acute post-traumatic period. A study of distribution of cystathionine β-synthase (CBS) in the cerebellum of intact young O. masou showed the expression of the marker mainly in type 1 cells, corresponding to NSCs/NCPs for other molecular markers. In the post-traumatic period, the number of CBS+ cells sharply increased, which indicates the involvement of H₂S in the post-traumatic response. Induction of CBS in type 3 cells indicates the involvement of H₂S in the metabolism of extracellular glutamate in the cerebellum, a decrease in the production of reactive oxygen species, and also arrest of the oxidative stress development, a weakening of the toxic effects of glutamate, and a reduction in excitotoxicity. The obtained results allow us to consider H₂S as a biologically active substance, the numerous known effects of which can be supplemented by participation in the processes of constitutive neurogenesis and neuronal regeneration.

Dyslexia Candidate Gene and Ciliary Gene Expression Dynamics During Human Neuronal Differentiation

Developmental dyslexia (DD) is a neurodevelopmental condition with complex genetic mechanisms. A number of candidate genes have been identified, some of which are linked to neuronal development and migration and to ciliary functions. However, expression and regulation of these genes in human brain development and neuronal differentiation remain uncharted. Here, we used human long-term self-renewing neuroepithelial stem (lt-NES, here termed NES) cells derived from human induced pluripotent stem cells to study neuronal differentiation in vitro. We characterized gene expression changes during differentiation by using RNA sequencing and validated dynamics for selected genes by qRT-PCR. Interestingly, we found that genes related to cilia were significantly enriched among upregulated genes during differentiation, including genes linked to ciliopathies with neurodevelopmental phenotypes. We confirmed the presence of primary cilia throughout neuronal differentiation. Focusing on dyslexia candidate genes, 33 out of 50 DD candidate genes were detected in NES cells by RNA sequencing, and seven candidate genes were upregulated during differentiation to neurons, including DYX1C1 (DNAAF4), a highly replicated DD candidate gene. Our results suggest a role of ciliary genes in differentiating neuronal cells and show that NES cells provide a relevant human neuronal model to study ciliary and DD candidate genes.

Experience Recruits MSK1 to Expand the Dynamic Range of Synapses and Enhance Cognition

Experience powerfully influences neuronal function and cognitive performance, but the cellular and molecular events underlying the experience-dependent enhancement of mental ability have remained elusive. In particular, the mechanisms that couple the external environment to the genomic changes underpinning this improvement are unknown. To address this, we have used male mice harboring an inactivating mutation of mitogen- and stress-activated protein kinase 1 (MSK1), a brain-derived neurotrophic factor (BDNF)-activated enzyme downstream of the mitogen-activated protein kinase (MAPK) pathway. We show that MSK1 is required for the full extent of experience-induced improvement of spatial memory, for the expansion of the dynamic range of synapses, exemplified by the enhancement of hippocampal long-term potentiation (LTP) and long-term depression (LTD), and for the regulation of the majority of genes influenced by enrichment. In addition, and unexpectedly, we show that experience is associated with an MSK1-dependent downregulation of key MAPK and plasticity-related genes, notably of EGR1/Zif268 and Arc/Arg3.1, suggesting the establishment of a novel genomic landscape adapted to experience. By coupling experience to homeostatic changes in gene expression MSK1, represents a prime mechanism through which the external environment has an enduring influence on gene expression, synaptic function, and cognition.

SIGNIFICANCE STATEMENT Our everyday experiences strongly influence the structure and function of the brain. Positive experiences encourage the growth and development of the brain and support enhanced learning and memory and resistance to mood disorders such as anxiety. While this has been known for many years, how this occurs is not clear. Here, we show that many of the positive aspects of experience depend on an enzyme called mitogen- and stress-activated protein kinase 1 (MSK1). Using male mice with a mutation in MSK1, we show that MSK1 is necessary for the majority of gene expression changes associated with experience, extending the range over which the communication between neurons occurs, and for both the persistence of memory and the ability to learn new task rules.

Primary cilium remodeling mediates a cell signaling switch in differentiating neurons

Cellular differentiation leads to the formation of specialized cell types and complex morphological variations. Often, differentiating cells transition between states by switching how they respond to the signaling environment. However, the mechanisms regulating these transitions are poorly understood. Differentiating neurons delaminate from the neuroepithelium through the regulated process of apical abscission, which mediates an acute loss of polarity and primary cilium disassembly. Using high-resolution live-cell imaging in chick neural tube, we show that these cells retain an Arl13b+ particle, which elongates and initiates intraflagellar trafficking as it transits toward the cell body, indicating primary cilium remodeling. Notably, disrupting cilia during and after remodeling inhibits axon extension and leads to axon collapse, respectively. Furthermore, cilium remodeling corresponds to a switch from a canonical to noncanonical cellular response to Shh. This work transforms our understanding of how cells can rapidly reinterpret signals to produce qualitatively different responses within the same tissue context.

The DNA methylome in panic disorder: a case-control and longitudinal psychotherapy-epigenetic study

In panic disorder (PD), epigenetic mechanisms such as DNA methylation of candidate genes have been suggested to play a key role at the intersection of genetic and environmental factors. On an epigenome-wide level, however, only two studies in PD patients have been published so far, while to date no study has intra-individually analyzed dynamic epigenetic correlates of treatment-response in PD on a DNA methylome level. Here, an epigenome-wide association study (EWAS) was performed in a sample of 57 PD patients and matched healthy controls using the Illumina MethylationEPIC BeadChip, along with a longitudinal approach assessing changes on the DNA methylome level corresponding to clinical effects of a manualized six-week cognitive-behavioral therapy (CBT) in PD. While no epigenome-wide significant hits could be discerned, top suggestive evidence was observed for decreased methylation in PD at cg19917903 in the Cilia and Flagella Associated Protein 46 (CFAP46) gene, and for an increase in methylation after CBT at cg06943668 in the Interleukin 1 Receptor Type 1 (IL1R1) gene in treatment responders to CBT. Additional exploratory analyses based on biological validity and a combined statistical/biological ranking point to further new potential PD risk genes such as the CCL4L1 or GMNN genes, and suggest dynamic methylation of, e.g., the ZFP622 and the SLC43A2 genes along with response to CBT. These EWAS and first longitudinal epigenome-wide pilot data in PD add to the emerging candidate gene-based body of evidence for epigenetic mechanisms to be involved in PD pathogenesis and to possibly constitute dynamic biological correlates of therapeutic interventions.

The 5-HT6 receptor interactome: New insight in receptor signaling and its impact on brain physiology and pathologies

The serotonin (5-HT)₆ receptor is a Gs-coupled receptor exclusively expressed in the central nervous system. Highest receptor densities are found in brain regions implicated in mnemonic functions where the receptor is primarily but not exclusively located in the primary cilium of neurons. The 5-HT₆ receptor continues to raise particular interest for neuropharmacologists, given the pro-cognitive effects of antagonists in a wide range of cognitive impairment paradigms in rodents and human. The 5-HT₆ receptor also finely controls key neuro-developmental processes including neuron migration and differentiation. However, its influence upon neurodevelopment and cognition is not solely mediated by its coupling to the Gs-adenylyl cyclase pathway, suggesting alternative signal transduction mechanisms. This prompted studies aimed at characterizing the receptor interactome that identified 125 candidate receptor partners, making the 5-HT₆ receptor one of the G protein-coupled receptors with the most extensively characterized interactome. These studies showed that the receptor localization at the plasma membrane and, consequently, its signal transduction, are finely modulated by several receptor partners. They demonstrated that prefrontal 5-HT₆ receptors engage the mTOR pathway to compromise cognition in neurodevelopmental models of schizophrenia, and a role of the 5-HT₆-mTOR pathway in temporal epilepsy. Finally, they revealed that the receptor activates Cdk5 signaling in an agonist-independent manner through a mechanism involving receptor phosphorylation by the associated Cdk5 and highlighted its key role in the migration of neurons and neurite growth. These new receptor-operated signaling mechanisms should be considered in the future development of drugs acting on 5-HT₆ receptors. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

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.

Ciliary gene RPGRIP1L is required for hypothalamic arcuate neuron development

Intronic polymorphisms in the α-ketoglutarate-dependent dioxygenase gene (FTO) that are highly associated with increased body weight have been implicated in the transcriptional control of a nearby ciliary gene, retinitis pigmentosa GTPase regulator-interacting protein-1 like (RPGRIP1L). Previous studies have shown that congenital Rpgrip1l hypomorphism in murine proopiomelanocortin (Pomc) neurons causes obesity by increasing food intake. Here, we show by congenital and adult-onset Rpgrip1l deletion in Pomc-expressing neurons that the hyperphagia and obesity are likely due to neurodevelopmental effects that are characterized by a reduction in the Pomc/Neuropeptide Y (Npy) neuronal number ratio and marked increases in arcuate hypothalamic-paraventricular hypothalamic (ARH-PVH) axonal projections. Biallelic RPGRIP1L mutations result in fewer cilia-positive human induced pluripotent stem cell-derived (iPSC-derived) neurons and blunted responses to Sonic Hedgehog (SHH). Isogenic human ARH-like embryonic stem cell-derived (ESc-derived) neurons homozygous for the obesity-risk alleles at rs8050136 or rs1421085 have decreased RPGRIP1L expression and have lower numbers of POMC neurons. RPGRIP1L overexpression increases POMC cell number. These findings suggest that apparently functional intronic polymorphisms affect hypothalamic RPGRIP1L expression and impact development of POMC neurons and their derivatives, leading to hyperphagia and increased adiposity.

Neuronal and astrocytic primary cilia in the mature brain

Primary cilia are tiny microtubule-based signaling devices that regulate a variety of physiological functions, including metabolism and cell division. Defects in primary cilia lead to a myriad of diseases in humans such as obesity and cancers. In the mature brain, both neurons and astrocytes contain a single primary cilium. Although neuronal primary cilia are not directly involved in synaptic communication, their pathophysiological impacts on obesity and mental disorders are well recognized. In contrast, research on astrocytic primary cilia lags far behind. Currently, little is known about their functions and molecular pathways in the mature brain. Unlike neurons, postnatal astrocytes retain the capacity of cell division and can become reactive and proliferate in response to various brain insults such as epilepsy, ischemia, traumatic brain injury, and neurodegenerative β-amyloid plaques. Since primary cilia derive from the mother centrioles, astrocyte proliferation must occur in coordination with the dismantling and ciliogenesis of astrocyte cilia. In this regard, the functions, signal pathways, and structural dynamics of neuronal and astrocytic primary cilia are fundamentally different. Here we discuss and compare the current understanding of neuronal and astrocytic primary cilia.

Gli2 is required for the induction and migration of Xenopus laevis neural crest

The neural crest (NC) is a multipotent migratory embryonic population that is formed during late gastrulation and gives rise to a wide array of derivatives, including cells from the peripheral nervous system (PNS), the craniofacial bones and cartilages, peripheral glial cells, and melanocyte cells, among others. In this work we analyzed the role of the Hedgehog signaling pathway effector gli2 in Xenopus NC. We provide evidence that the gli2 gene is expressed in the prospective, premigratory and migratory NC. The use of a specific morpholino against gli2 and the pharmacological specific inhibitor GANT61 in different experimental approaches allowed us to determine that gli2 is required for the induction and specification of NC cells as a transcriptional activator. Moreover, gli2 also acts by reducing apoptosis in the NC without affecting its cell proliferation status. We also demonstrated that gli2 is required cell-autonomously for NC migration, and for the formation of NC derivatives such as the craniofacial cartilages, melanocytes and the cranial ganglia. Altogether, our results showed that gli2 is a key transcriptional activator to accomplish the proper specification and development of Xenopus NC cells.

The primary cilium - once a "rudimentary" organelle that is now a ubiquitous sensory cellular structure involved in many pathological disorders

This article looks mostly at the steps that have led to the primary cilium finding its place in our understanding of cell biology, developmental biology, and medical syndromes due to its aberrations. It is a personal account that stresses, if nothing else, the value of the adage "stick to your guns". My obsession with this organelle, following on from fascination with the centriole, has led to a whole career devoted to determining the nature and role of primary cilia in basic cell biology, which has proved much more important than had been appreciated for almost a century. They are heavily involved in very many aspects of cell physiology that have much wider implications with regard to human biology and probably throughout the animal kingdom. That aberrations, to the surprise of many researchers in their structure or functioning has led to their being implicated or perhaps deeply involved in an extraordinary range of medical conditions. This invitation allows me to raise crucial questions that need answers regarding the regulation of their genesis, their cache of both intracellular and extracellular signal, and their association with a multitude of development processes from embryo to adult status.

Coupling progenitor and neuronal diversity in the developing neocortex

The adult neocortex is composed of several types of glutamatergic neurons, which are sequentially born from progenitors during development. The extent and nature of progenitor diversity, and how it relates to neuronal diversity, is still poorly understood. In this review, we discuss key features of neocortical progenitors across several species, including their morphological properties, cell cycling behaviour and molecular signatures, and how these features relate to the competence of these cells to generate distinct types of progenies.