Primary cilium-dependent cAMP/PKA signaling at the centrosome regulates neuronal migration

Emerging mechanistic understanding of cilia function in cellular signalling

Primary cilia are solitary, immotile sensory organelles present on most cells in the body that participate broadly in human health, physiology and disease. Cilia generate a unique environment for signal transduction with tight control of protein, lipid and second messenger concentrations within a relatively small compartment, enabling reception, transmission and integration of biological information. In this Review, we discuss how cilia function as signalling hubs in cell-cell communication using three signalling pathways as examples: ciliary G-protein-coupled receptors (GPCRs), the Hedgehog (Hh) pathway and polycystin ion channels. We review how defects in these ciliary signalling pathways lead to a heterogeneous group of conditions known as 'ciliopathies', including metabolic syndromes, birth defects and polycystic kidney disease. Emerging understanding of these pathways' transduction mechanisms reveals common themes between these cilia-based signalling pathways that may apply to other pathways as well. These mechanistic insights reveal how cilia orchestrate normal and pathophysiological signalling outputs broadly throughout human biology.

Ciliogenesis defects after neurulation impact brain development and neuronal activity in larval zebrafish

Cilia are slender, hair-like structures extending from cell surfaces and playing essential roles in diverse physiological processes. Within the nervous system, primary cilia contribute to signaling and sensory perception, while motile cilia facilitate cerebrospinal fluid flow. Here, we investigated the impact of ciliary loss on neural circuit development using a zebrafish line displaying ciliogenesis defects. We found that cilia defects after neurulation affect neurogenesis and brain morphology, especially in the cerebellum, and lead to altered gene expression profiles. Using whole brain calcium imaging, we measured reduced light-evoked and spontaneous neuronal activity in all brain regions. By shedding light on the intricate role of cilia in neural circuit formation and function in the zebrafish, our work highlights their evolutionary conserved role in the brain and sets the stage for future analysis of ciliopathy models.

Loss of SIL1 Affects Actin Dynamics and Leads to Abnormal Neural Migration

SIL1 is a nucleotide exchange factor for the molecular chaperone protein Bip in the endoplasmic reticulum that plays a crucial role in protein folding. The Sil1 gene is currently the only known causative gene of Marinesco-Sjögren syndrome (MSS). Intellectual developmental disability is the main symptom of MSS, and its mechanism has not been fully elucidated. Studies have shown that mutations in the Sil1 gene can delay neuronal migration during cortical development, but the underlying molecular mechanisms remain unclear. To further identify potential molecules involved in the regulation of central nervous system development by SIL1, we established a cortical neuron model with SIL1 protein deficiency and used proteomic analysis to screen for differentially expressed proteins after Sil1 silencing, followed by GO functional enrichment and protein‒protein interaction (PPI) network analysis. We identified 68 upregulated and 137 downregulated proteins in total, and among them, 10 upregulated and 3 downregulated proteins were mainly related to actin cytoskeleton dynamics. We further validated the differential changes in actin-related molecules using qRT‒PCR and Western blotting of a Sil1 gene knockout (Sil1-/-) mouse model. The results showed that the protein levels of ACTN1 and VIM decreased, while their mRNA levels increased as a compensatory response to protein deficiency. The mRNA and protein levels of IQGAP1 both showed a secondary increase. In conclusion, we identified ACTN1 and VIM as the key molecules regulated by SIL1 that are involved in neuronal migration during cortical development.

FMRP regulates postnatal neuronal migration via MAP1B

The fragile X syndrome (FXS) represents the most prevalent form of inherited intellectual disability and is the first monogenic cause of autism spectrum disorder. FXS results from the absence of the RNA-binding protein FMRP (fragile X messenger ribonucleoprotein). Neuronal migration is an essential step of brain development allowing displacement of neurons from their germinal niches to their final integration site. The precise role of FMRP in neuronal migration remains largely unexplored. Using live imaging of postnatal rostral migratory stream (RMS) neurons in Fmr1-null mice, we observed that the absence of FMRP leads to delayed neuronal migration and altered trajectory, associated with defects of centrosomal movement. RNA-interference-induced knockdown of Fmr1 shows that these migratory defects are cell-autonomous. Notably, the primary Fmrp mRNA target implicated in these migratory defects is microtubule-associated protein 1B (MAP1B). Knocking down MAP1B expression effectively rescued most of the observed migratory defects. Finally, we elucidate the molecular mechanisms at play by demonstrating that the absence of FMRP induces defects in the cage of microtubules surrounding the nucleus of migrating neurons, which is rescued by MAP1B knockdown. Our findings reveal a novel neurodevelopmental role for FMRP in collaboration with MAP1B, jointly orchestrating neuronal migration by influencing the microtubular cytoskeleton.

Research Advances in Neuroblast Migration in Traumatic Brain Injury

Neuroblasts were first derived from the adult mammalian brains in the 1990s by Reynolds et al. Since then, persistent neurogenesis in the subgranular zone (SGZ) of the hippocampus and subventricular zone (SVZ) has gradually been recognized. To date, reviews on neuroblast migration have largely investigated glial cells and molecular signaling mechanisms, while the relationship between vasculature and cell migration remains a mystery. Thus, this paper underlines the partial biological features of neuroblast migration and unravels the significance and mechanisms of the vasculature in the process to further clarify theoretically the neural repair mechanism after brain injury. Neuroblast migration presents three modes according to the characteristics of cells that act as scaffolds during the migration process: gliophilic migration, neurophilic migration, and vasophilic migration. Many signaling molecules, including brain-derived neurotrophic factor (BDNF), stromal cell-derived factor 1 (SDF-1), vascular endothelial growth factor (VEGF), and angiopoietin-1 (Ang-1), affect vasophilic migration, synergistically regulating the migration of neuroblasts to target areas along blood vessels. However, the precise role of blood vessels in the migration of neuroblasts needs to be further explored. The in-depth study of neuroblast migration will most probably provide theoretical basis and breakthrough for the clinical treatment of brain injury diseases.

Identification of the growth cone as a probe and driver of neuronal migration in the injured brain

Axonal growth cones mediate axonal guidance and growth regulation. We show that migrating neurons in mice possess a growth cone at the tip of their leading process, similar to that of axons, in terms of the cytoskeletal dynamics and functional responsivity through protein tyrosine phosphatase receptor type sigma (PTPσ). Migrating-neuron growth cones respond to chondroitin sulfate (CS) through PTPσ and collapse, which leads to inhibition of neuronal migration. In the presence of CS, the growth cones can revert to their extended morphology when their leading filopodia interact with heparan sulfate (HS), thus re-enabling neuronal migration. Implantation of an HS-containing biomaterial in the CS-rich injured cortex promotes the extension of the growth cone and improve the migration and regeneration of neurons, thereby enabling functional recovery. Thus, the growth cone of migrating neurons is responsive to extracellular environments and acts as a primary regulator of neuronal migration.

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.

Role of lipids in the control of autophagy and primary cilium signaling in neurons

The brain is, after the adipose tissue, the organ with the greatest amount of lipids and diversity in their composition in the human body. In neurons, lipids are involved in signaling pathways controlling autophagy, a lysosome-dependent catabolic process essential for the maintenance of neuronal homeostasis and the function of the primary cilium, a cellular antenna that acts as a communication hub that transfers extracellular signals into intracellular responses required for neurogenesis and brain development. A crosstalk between primary cilia and autophagy has been established; however, its role in the control of neuronal activity and homeostasis is barely known. In this review, we briefly discuss the current knowledge regarding the role of autophagy and the primary cilium in neurons. Then we review the recent literature about specific lipid subclasses in the regulation of autophagy, in the control of primary cilium structure and its dependent cellular signaling in physiological and pathological conditions, specifically focusing on neurons, an area of research that could have major implications in neurodevelopment, energy homeostasis, and neurodegeneration.

FMRP regulates tangential neuronal migration via MAP1B

The Fragile X Syndrome (FXS) represents the most prevalent form of inherited intellectual disability and is the first monogenic cause of Autism Spectrum Disorder. FXS results from the absence of the RNA-binding protein FMRP (Fragile X Messenger Ribonucleoprotein). Neuronal migration is an essential step of brain development allowing displacement of neurons from their germinal niches to their final integration site. The precise role of FMRP in neuronal migration remains largely unexplored. Using live imaging of postnatal Rostral Migratory Stream (RMS) neurons in Fmr1-null mice, we observed that the absence of FMRP leads to delayed neuronal migration and altered trajectory, associated with defects of centrosomal movement. RNA-interference-induced knockdown of Fmr1 shows that these migratory defects are cell-autonomous. Notably, the primary FMRP mRNA target implicated in these migratory defects is MAP1B (Microtubule-Associated Protein 1B). Knocking-down MAP1B expression effectively rescued most of the observed migratory defects. Finally, we elucidate the molecular mechanisms at play by demonstrating that the absence of FMRP induces defects in the cage of microtubules surrounding the nucleus of migrating neurons, which is rescued by MAP1B knockdown. Our findings reveal a novel neurodevelopmental role for FMRP in collaboration with MAP1B, jointly orchestrating neuronal migration by influencing the microtubular cytoskeleton.

Circadian oscillation in primary cilium length by clock genes regulates fibroblast cell migration

Various mammalian cells have autonomous cellular clocks that are produced by the transcriptional cycle of clock genes. Cellular clocks provide circadian rhythms for cellular functions via transcriptional and cytoskeletal regulation. The vast majority of mammalian cells possess a primary cilium, an organelle protruding from the cell surface. Here, we investigated the little-known relationship between circadian rhythm and primary cilia. The length and number of primary cilia showed circadian dynamics both in vitro and in vivo. The circadian rhythm of primary cilium length was abolished by SR9011 and Bmal1 knockout. A centrosomal protein, pericentrin, transiently accumulates in centriolar satellites, the base of primary cilia at the shortest cilia phase, and induces elongation of primary cilia at the longest cilia phase in the circadian rhythm of primary cilia. In addition, rhythmic cell migration during wound healing depends on the length of primary cilia and affects the rate of wound healing. Our findings demonstrate that the circadian dynamics of primary cilium length by clock genes control fibroblast migration and could provide new insights into chronobiology.

CXCL12 targets the primary cilium cAMP/cGMP ratio to regulate cell polarity during migration

Directed cell migration requires sustained cell polarisation. In migrating cortical interneurons, nuclear movements are directed towards the centrosome that organises the primary cilium signalling hub. Primary cilium-elicited signalling, and how it affects migration, remain however ill characterised. Here, we show that altering cAMP/cGMP levels in the primary cilium by buffering cAMP, cGMP or by locally increasing cAMP, influences the polarity and directionality of migrating interneurons, whereas buffering cAMP or cGMP in the apposed centrosome compartment alters their motility. Remarkably, we identify CXCL12 as a trigger that targets the ciliary cAMP/cGMP ratio to promote sustained polarity and directed migration. We thereby uncover cAMP/cGMP levels in the primary cilium as a major target of extrinsic cues and as the steering wheel of neuronal migration.

Primary cilia-mediated regulation of microglial secretion in Alzheimer's disease

Alzheimer's disease (AD) is a brain disorder manifested by a gradual decline in cognitive function due to the accumulation of extracellular amyloid plaques, disruptions in neuronal substance transport, and the degeneration of neurons. In affected neurons, incomplete clearance of toxic proteins by neighboring microglia leads to irreversible brain inflammation, for which cellular signaling is poorly understood. Through single-cell transcriptomic analysis, we discovered distinct regional differences in the ability of microglia to clear damaged neurites. Specifically, microglia in the septal region of wild type mice exhibited a transcriptomic signature resembling disease-associated microglia (DAM). These lateral septum (LS)-enriched microglia were associated with dense axonal bundles originating from the hippocampus. Further transcriptomic and proteomic approaches revealed that primary cilia, small hair-like structures found on cells, played a role in the regulation of microglial secretory function. Notably, primary cilia were transiently observed in microglia, and their presence was significantly reduced in microglia from AD mice. We observed significant changes in the secretion and proteomic profiles of the secretome after inhibiting the primary cilia gene intraflagellar transport particle 88 (Ift88) in microglia. Intriguingly, inhibiting primary cilia in the septal microglia of AD mice resulted in the expansion of extracellular amyloid plaques and damage to adjacent neurites. These results indicate that DAM-like microglia are present in the LS, a critical target region for hippocampal nerve bundles, and that the primary ciliary signaling system regulates microglial secretion, affecting extracellular proteostasis. Age-related primary ciliopathy probably contributes to the selective sensitivity of microglia, thereby exacerbating AD. Targeting the primary ciliary signaling system could therefore be a viable strategy for modulating neuroimmune responses in AD treatments.

Phosphoregulation of the septin cytoskeleton in neuronal development and disease

Septins are highly conserved GTP-binding proteins that oligomerize and form higher order structures. The septin cytoskeleton plays an important role in cellular organization, intracellular transport, and cytokinesis. Kinase-mediated phosphorylation of septins regulates various aspects of their function, localization, and dynamics. Septins are enriched in the mammalian nervous system where they contribute to neurodevelopment and neuronal function. Emerging research has implicated aberrant changes in septin cytoskeleton in several human diseases. The mechanisms through which aberrant phosphorylation by kinases contributes to septin dysfunction in neurological disorders are poorly understood and represent an important question for future research with therapeutic implications. This review summarizes the current state of knowledge of the diversity of kinases that interact with and phosphorylate mammalian septins, delineates how phosphoregulation impacts septin dynamics, and describes how aberrant septin phosphorylation contributes to neurological disorders.

Role of purines in brain development, from neuronal proliferation to synaptic refinement

The purinergic system includes P1 and P2 receptors, which are activated by ATP and its metabolites. They are expressed in adult neuronal and glial cells and are crucial in brain function, including neuromodulation and neuronal signaling. As P1 and P2 receptors are expressed throughout embryogenesis and development, purinergic signaling also has an important role in the development of the peripheral and central nervous system. In this review, we present the expression pattern and activity of purinergic receptors and of their signaling pathways during embryonic and postnatal development of the nervous system. In particular, we review the involvement of the purinergic signaling in all the crucial steps of brain development i.e. in neural stem cell proliferation, neuronal differentiation and migration as well as in astrogliogenesis and oligodendrogenesis. Then, we review data showing a crucial role of the ATP and adenosine signaling pathways in the formation of the peripheral neuromuscular junction and of central GABAergic and glutamatergic synapses. Finally, we examine the consequences of deregulation of the purinergic system during development and discuss the therapeutic potential of targeting it at adult stage in diseases with reactivation of the ATP and adenosine pathway.

Compartmentalised cAMP signalling in the primary cilium

cAMP is a universal second messenger that relies on precise spatio-temporal regulation to control varied, and often opposing, cellular functions. This is achieved via selective activation of effectors embedded in multiprotein complexes, or signalosomes, that reside at distinct subcellular locations. cAMP is also one of many pathways known to operate within the primary cilium. Dysfunction of ciliary signaling leads to a class of diseases known as ciliopathies. In Autosomal Dominant Polycystic Kidney Disease (ADPKD), a ciliopathy characterized by the formation of fluid-filled kidney cysts, upregulation of cAMP signaling is known to drive cystogenesis. For decades it has been debated whether the primary cilium is an independent cAMP sub-compartment, or whether it shares a diffusible pool of cAMP with the cell body. Recent studies now suggest it is a specific pool of cAMP generated in the cilium that propels cyst formation in ADPKD, supporting the notion that this antenna-like organelle is a compartment within which cAMP signaling occurs independently from cAMP signaling in the bulk cytosol. Here we present examples of cAMP function in the cilium which suggest this mysterious organelle is home to more than one cAMP signalosome. We review evidence that ciliary membrane localization of G-Protein Coupled Receptors (GPCRs) determines their downstream function and discuss how optogenetic tools have contributed to establish that cAMP generated in the primary cilium can drive cystogenesis.

Temporal static and dynamic imaging of skeletal muscle in vivo

A major challenge in the study of living systems is understanding how tissues and organs are established, maintained during homeostasis, reconstituted following injury or deteriorated during disease. Most of the studies that interrogate in vivo cell biological properties of cell populations within tissues are obtained through static imaging approaches. However, in vertebrates, little is known about which, when, and how extracellular and intracellular signals are dynamically integrated to regulate cell behaviour and fates, due largely to technical challenges. Intravital imaging of cellular dynamics in mammalian models has exposed surprising properties that have been missed by conventional static imaging approaches. Here we highlight some selected examples of intravital imaging in mouse intestinal stem cells, hematopoietic stem cells, hair follicle stem cells, and neural stem cells in the brain, each of which have distinct features from an anatomical and niche-architecture perspective. Intravital imaging of mouse skeletal muscles is comparatively less advanced due to several technical constraints that will be discussed, yet this approach holds great promise as a complementary investigative method to validate findings obtained by static imaging, as well as a method for discovery.

Cilia at the Crossroads of Tumor Treating Fields and Chemotherapy

Glioblastoma (GBM), the most common and lethal primary brain tumor in adults, requires multi-treatment intervention which unfortunately barely shifts the needle in overall survival. The treatment options after diagnosis and surgical resection (if possible) include irradiation, temozolomide (TMZ) chemotherapy, and now tumor treating fields (TTFields). TTFields are electric fields delivered locoregionally to the head/tumor via a wearable medical device (Optune®). Overall, the concomitant treatment of TTFields and TMZ target tumor cells but spare normal cell types in the brain. Here, we examine whether primary cilia, microtubule-based "antennas" found on both normal brain cells and GBM cells, play specific roles in sensitizing tumor cells to treatment. We discuss evidence supporting GBM cilia being exploited by tumor cells to promote their growth and treatment resistance. We review how primary cilia on normal brain and GBM cells are affected by GBM treatments as monotherapy or concomitant modalities. We also focus on latest findings indicating a differential regulation of GBM ciliogenesis by TTFields and TMZ. Future studies await arrival of intracranial TTFields models to determine if GBM cilia carry a prognostic capacity.

The genetic background of hydrocephalus in a population-based cohort: implication of ciliary involvement

Hydrocephalus is one of the most common congenital disorders of the central nervous system and often displays psychiatric co-morbidities, in particular autism spectrum disorder. The disease mechanisms behind hydrocephalus are complex and not well understood, but some association with dysfunctional cilia in the brain ventricles and subarachnoid space has been indicated. A better understanding of the genetic aetiology of hydrocephalus, including the role of ciliopathies, may bring insights into a potentially shared genetic aetiology. In this population-based case-cohort study, we, for the first time, investigated variants of postulated hydrocephalus candidate genes. Using these data, we aimed to investigate potential involvement of the ciliome in hydrocephalus and describe genotype-phenotype associations with an autism spectrum disorder. One-hundred and twenty-one hydrocephalus candidate genes were screened in a whole-exome-sequenced sub-cohort of the Lundbeck Foundation Initiative for Integrative Psychiatric Research study, comprising 72 hydrocephalus patients and 4181 background population controls. Candidate genes containing high-impact variants of interest were systematically evaluated for their involvement in ciliary function and an autism spectrum disorder. The median age at diagnosis for the hydrocephalus patients was 0 years (range 0-27 years), the median age at analysis was 22 years (11-35 years), and 70.5% were males. The median age for controls was 18 years (range 11-26 years) and 53.3% were males. Fifty-two putative hydrocephalus-associated variants in 34 genes were identified in 42 patients (58.3%). In hydrocephalus cases, we found increased, but not significant, enrichment of high-impact protein altering variants (odds ratio 1.51, 95% confidence interval 0.92-2.51, P = 0.096), which was driven by a significant enrichment of rare protein truncating variants (odds ratio 2.71, 95% confidence interval 1.17-5.58, P = 0.011). Fourteen of the genes with high-impact variants are part of the ciliome, whereas another six genes affect cilia-dependent processes during neurogenesis. Furthermore, 15 of the 34 genes with high-impact variants and three of eight genes with protein truncating variants were associated with an autism spectrum disorder. Because symptoms of other diseases may be neglected or masked by the hydrocephalus-associated symptoms, we suggest that patients with congenital hydrocephalus undergo clinical genetic assessment with respect to ciliopathies and an autism spectrum disorder. Our results point to the significance of hydrocephalus as a ciliary disease in some cases. Future studies in brain ciliopathies may not only reveal new insights into hydrocephalus but also, brain disease in the broadest sense, given the essential role of cilia in neurodevelopment.

Disrupting the ciliary gradient of active Arl3 affects rod photoreceptor nuclear migration

The small GTPase Arl3 is important for the enrichment of lipidated proteins to primary cilia, including the outer segment of photoreceptors. Human mutations in the small GTPase Arl3 cause both autosomal recessive and dominant inherited retinal dystrophies. We discovered that dominant mutations result in increased active G-protein-Arl3-D67V has constitutive activity and Arl3-Y90C is fast cycling-and their expression in mouse rods resulted in a displaced nuclear phenotype due to an aberrant Arl3-GTP gradient. Using multiple strategies, we go on to show that removing or restoring the Arl3-GTP gradient within the cilium is sufficient to rescue the nuclear migration defect. Together, our results reveal that an Arl3 ciliary gradient is involved in proper positioning of photoreceptor nuclei during retinal development.

The Primary Cilium and Neuronal Migration

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

Insights Into the Properties, Biological Functions, and Regulation of USP21

Deubiquitylating enzymes (DUBs) antagonize ubiquitination by removing ubiquitin from their substrates. The role of DUBs in controlling various physiological and pathological processes has been extensively studied, and some members of DUBs have been identified as potential therapeutic targets in diseases ranging from tumors to neurodegeneration. Ubiquitin-specific protease 21 (USP21) is a member of the ubiquitin-specific protease family, the largest subfamily of DUBs. Although USP21 was discovered late and early research progress was slow, numerous studies in the last decade have gradually revealed the importance of USP21 in a wide variety of biological processes. In particular, the pro-carcinogenic effect of USP21 has been well elucidated in the last 2 years. In the present review, we provide a comprehensive overview of the current knowledge on USP21, including its properties, biological functions, pathophysiological roles, and cellular regulation. Limited pharmacological interventions for USP21 have also been introduced, highlighting the importance of developing novel and specific inhibitors targeting USP21.

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.

Ciliary Type III Adenylyl Cyclase in the VMH Is Crucial for High-Fat Diet-Induced Obesity Mediated by Autophagy

Neuronal primary cilia are crucial for body weight maintenance. Type III adenylyl cyclase (AC3) is abundantly enriched in neuronal cilia, and mice with global AC3 ablation are obese. However, whether AC3 regulates body weight through its ciliary expression and the mechanism underlying this potential regulation are not clear. In this study, humanized AC3 knock-in mice that are resistant to high-fat diet (HFD)-induced obesity are generated, and increases in the number and length of cilia in the ventromedial hypothalamus (VMH) are shown. It is demonstrated that mice with specifically knocked down ciliary AC3 expression in the VMH show pronounced HFD-induced obesity. In addition, in vitro and in vivo analyses of the VMH show that ciliary AC3 regulates autophagy by binding an autophagy-related gene, gamma-aminobutyric acid A receptor-associated protein (GABARAP). Mice with GABARAP knockdown in the VMH exhibit exacerbated HFD-induced obesity. Overall, the findings may reveal a potential mechanism by which ciliary AC3 expression regulates body weight in the mouse VMH.

Control of the Hedgehog pathway by compartmentalized PKA in the primary cilium

The Hedgehog (Hh) signaling is one of the essential signaling pathways during embryogenesis and in adults. Hh signal transduction relies on primary cilium, a specialized cell surface organelle viewed as the hub of cell signaling. Protein kinase A (PKA) has been recognized as a potent negative regulator of the Hh pathway, raising the question of how such a ubiquitous kinase specifically regulates one signaling pathway. We reviewed recent genetic, molecular and biochemical studies that have advanced our mechanistic understanding of PKA's role in Hh signaling in vertebrates, focusing on the compartmentalized PKA at the centrosome and in the primary cilium. We outlined the recently developed genetic and optical tools that can be harvested to study PKA activities during the course of Hh signal transduction.

Olfactory Optogenetics: Light Illuminates the Chemical Sensing Mechanisms of Biological Olfactory Systems

The mammalian olfactory system has an amazing ability to distinguish thousands of odorant molecules at the trace level. Scientists have made great achievements on revealing the olfactory sensing mechanisms in decades; even though many issues need addressing. Optogenetics provides a novel technical approach to solve this dilemma by utilizing light to illuminate specific part of the olfactory system; which can be used in all corners of the olfactory system for revealing the olfactory mechanism. This article reviews the most recent advances in olfactory optogenetics devoted to elucidate the mechanisms of chemical sensing. It thus attempts to introduce olfactory optogenetics according to the structure of the olfactory system. It mainly includes the following aspects: the sensory input from the olfactory epithelium to the olfactory bulb; the influences of the olfactory bulb (OB) neuron activity patterns on olfactory perception; the regulation between the olfactory cortex and the olfactory bulb; and the neuromodulation participating in odor coding by dominating the olfactory bulb. Finally; current challenges and future development trends of olfactory optogenetics are proposed and discussed.

Cellular context shapes cyclic nucleotide signaling in neurons through multiple levels of integration

Intracellular signaling with cyclic nucleotides are ubiquitous signaling pathways, yet the dynamics of these signals profoundly differ in different cell types. Biosensor imaging experiments, by providing direct measurements in intact cellular environment, reveal which receptors are activated by neuromodulators and how the coincidence of different neuromodulators is integrated at various levels in the signaling cascade. Phosphodiesterases appear as one important determinant of cross-talk between different signaling pathways. Finally, analysis of signal dynamics reveal that striatal medium-sized spiny neuron obey a different logic than other brain regions such as cortex, probably in relation with the function of this brain region which efficiently detects transient dopamine.

Kinase-anchoring proteins in ciliary signal transduction

Historically, the diffusion of chemical signals through the cell was thought to occur within a cytoplasmic soup bounded by the plasma membrane. This theory was predicated on the notion that all regulatory enzymes are soluble and moved with a Brownian motion. Although enzyme compartmentalization was initially rebuffed by biochemists as a 'last refuge of a scoundrel', signal relay through macromolecular complexes is now accepted as a fundamental tenet of the burgeoning field of spatial biology. A-Kinase anchoring proteins (AKAPs) are prototypic enzyme-organizing elements that position clusters of regulatory proteins at defined subcellular locations. In parallel, the primary cilium has gained recognition as a subcellular mechanosensory organelle that amplifies second messenger signals pertaining to metazoan development. This article highlights advances in our understanding of AKAP signaling within the primary cilium and how defective ciliary function contributes to an increasing number of diseases known as ciliopathies.

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.

Intrinsic Mechanisms Regulating Neuronal Migration in the Postnatal Brain

Neuronal migration is a fundamental brain development process that allows cells to move from their birthplaces to their sites of integration. Although neuronal migration largely ceases during embryonic and early postnatal development, neuroblasts continue to be produced and to migrate to a few regions of the adult brain such as the dentate gyrus and the subventricular zone (SVZ). In the SVZ, a large number of neuroblasts migrate into the olfactory bulb (OB) along the rostral migratory stream (RMS). Neuroblasts migrate in chains in a tightly organized micro-environment composed of astrocytes that ensheath the chains of neuroblasts and regulate their migration; the blood vessels that are used by neuroblasts as a physical scaffold and a source of molecular factors; and axons that modulate neuronal migration. In addition to diverse sets of extrinsic micro-environmental cues, long-distance neuronal migration involves a number of intrinsic mechanisms, including membrane and cytoskeleton remodeling, Ca²⁺ signaling, mitochondria dynamics, energy consumption, and autophagy. All these mechanisms are required to cope with the different micro-environment signals and maintain cellular homeostasis in order to sustain the proper dynamics of migrating neuroblasts and their faithful arrival in the target regions. Neuroblasts in the postnatal brain not only migrate into the OB but may also deviate from their normal path to migrate to a site of injury induced by a stroke or by certain neurodegenerative disorders. In this review, we will focus on the intrinsic mechanisms that regulate long-distance neuroblast migration in the adult brain and on how these pathways may be modulated to control the recruitment of neuroblasts to damaged/diseased brain areas.

Role of phosphodiesterases in the pathophysiology of neurodevelopmental disorders

Phosphodiesterases (PDEs) are enzymes involved in the homeostasis of both cAMP and cGMP. They are members of a family of proteins that includes 11 subfamilies with different substrate specificities. Their main function is to catalyze the hydrolysis of cAMP, cGMP, or both. cAMP and cGMP are two key second messengers that modulate a wide array of intracellular processes and neurobehavioral functions, including memory and cognition. Even if these enzymes are present in all tissues, we focused on those PDEs that are expressed in the brain. We took into consideration genetic variants in patients affected by neurodevelopmental disorders, phenotypes of animal models, and pharmacological effects of PDE inhibitors, a class of drugs in rapid evolution and increasing application to brain disorders. Collectively, these data indicate the potential of PDE modulators to treat neurodevelopmental diseases characterized by learning and memory impairment, alteration of behaviors associated with depression, and deficits in social interaction. Indeed, clinical trials are in progress to treat patients with Alzheimer's disease, schizophrenia, depression, and autism spectrum disorders. Among the most recent results, the application of some PDE inhibitors (PDE2A, PDE3, PDE4/4D, and PDE10A) to treat neurodevelopmental diseases, including autism spectrum disorders and intellectual disability, is a significant advance, since no specific therapies are available for these disorders that have a large prevalence. In addition, to highlight the role of several PDEs in normal and pathological neurodevelopment, we focused here on the deregulation of cAMP and/or cGMP in Down Syndrome, Fragile X Syndrome, Rett Syndrome, and intellectual disability associated with the CC2D1A gene.

Cilia, ciliopathies and hedgehog-related forebrain developmental disorders

Development of the forebrain critically depends on the Sonic Hedgehog (Shh) signaling pathway, as illustrated in humans by the frequent perturbation of this pathway in holoprosencephaly, a condition defined as a defect in the formation of midline structures of the forebrain and face. The Shh pathway requires functional primary cilia, microtubule-based organelles present on virtually every cell and acting as cellular antennae to receive and transduce diverse chemical, mechanical or light signals. The dysfunction of cilia in humans leads to inherited diseases called ciliopathies, which often affect many organs and show diverse manifestations including forebrain malformations for the most severe forms. The purpose of this review is to provide the reader with a framework to understand the developmental origin of the forebrain defects observed in severe ciliopathies with respect to perturbations of the Shh pathway. We propose that many of these defects can be interpreted as an imbalance in the ratio of activator to repressor forms of the Gli transcription factors, which are effectors of the Shh pathway. We also discuss the complexity of ciliopathies and their relationships with forebrain disorders such as holoprosencephaly or malformations of cortical development, and emphasize the need for a closer examination of forebrain defects in ciliopathies, not only through the lens of animal models but also taking advantage of the increasing potential of the research on human tissues and organoids.

Differentiation of ciliated human midbrain-derived LUHMES neurons

Many human cell types are ciliated, including neural progenitors and differentiated neurons. Ciliopathies are characterized by defective cilia and comprise various disease states, including brain phenotypes, where the underlying biological pathways are largely unknown. Our understanding of neuronal cilia is rudimentary, and an easy-to-maintain, ciliated human neuronal cell model is absent. The Lund human mesencephalic (LUHMES) cell line is a ciliated neuronal cell line derived from human fetal mesencephalon. LUHMES cells can easily be maintained and differentiated into mature, functional neurons within one week. They have a single primary cilium as proliferating progenitor cells and as postmitotic, differentiating neurons. These developmental stages are completely separable within one day of culture condition change. The sonic hedgehog (SHH) signaling pathway is active in differentiating LUHMES neurons. RNA-sequencing timecourse analyses reveal molecular pathways and gene-regulatory networks critical for ciliogenesis and axon outgrowth at the interface between progenitor cell proliferation, polarization and neuronal differentiation. Gene expression dynamics of cultured LUHMES neurons faithfully mimic the corresponding in vivo dynamics of human fetal midbrain. In LUHMES cells, neuronal cilia biology can be investigated from proliferation through differentiation to mature neurons.

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.