Mechanisms and molecules of neuronal wiring: a primer

Single-cell transcriptome profiles of Drosophila fruitless-expressing neurons from both sexes

Drosophila melanogaster reproductive behaviors are orchestrated by fruitless neurons. We performed single-cell RNA-sequencing on pupal neurons that produce sex-specifically spliced fru transcripts, the fru P1-expressing neurons. Uniform Manifold Approximation and Projection (UMAP) with clustering generates an atlas containing 113 clusters. While the male and female neurons overlap in UMAP space, more than half the clusters have sex differences in neuron number, and nearly all clusters display sex-differential expression. Based on an examination of enriched marker genes, we annotate clusters as circadian clock neurons, mushroom body Kenyon cell neurons, neurotransmitter- and/or neuropeptide-producing, and those that express doublesex. Marker gene analyses also show that genes that encode members of the immunoglobulin superfamily of cell adhesion molecules, transcription factors, neuropeptides, neuropeptide receptors, and Wnts have unique patterns of enriched expression across the clusters. In vivo spatial gene expression links to the clusters are examined. A functional analysis of fru P1 circadian neurons shows they have dimorphic roles in activity and period length. Given that most clusters are comprised of male and female neurons indicates that the sexes have fru P1 neurons with common gene expression programs. Sex-specific expression is overlaid on this program, to build the potential for vastly different sex-specific behaviors.

The factors affecting neurogenesis after stroke and the role of acupuncture

Stroke induces a state of neuroplasticity in the central nervous system, which can lead to neurogenesis phenomena such as axonal growth and synapse formation, thus affecting stroke outcomes. The brain has a limited ability to repair ischemic damage and requires a favorable microenvironment. Acupuncture is considered a feasible and effective neural regulation strategy to improve functional recovery following stroke via the benign modulation of neuroplasticity. Therefore, we summarized the current research progress on the key factors and signaling pathways affecting neurogenesis, and we also briefly reviewed the research progress of acupuncture to improve functional recovery after stroke by promoting neurogenesis. This study aims to provide new therapeutic perspectives and strategies for the recovery of motor function after stroke based on neurogenesis.

Motor neurons use push-pull signals to direct vascular remodeling critical for their connectivity

The nervous system requires metabolites and oxygen supplied by the neurovascular network, but this necessitates close apposition of neurons and endothelial cells. We find motor neurons attract vessels with long-range VEGF signaling, but endothelial cells in the axonal pathway are an obstacle for establishing connections with muscles. It is unclear how this paradoxical interference from heterotypic neurovascular contacts is averted. Through a mouse mutagenesis screen, we show that Plexin-D1 receptor is required in endothelial cells for development of neuromuscular connectivity. Motor neurons release Sema3C to elicit short-range repulsion via Plexin-D1, thus displacing endothelial cells that obstruct axon growth. When this signaling pathway is disrupted, epaxial motor neurons are blocked from reaching their muscle targets and concomitantly vascular patterning in the spinal cord is altered. Thus, an integrative system of opposing push-pull cues ensures detrimental axon-endothelial encounters are avoided while enabling vascularization within the nervous system and along peripheral nerves.

In situ cell-type-specific cell-surface proteomic profiling in mice

Cell-surface proteins (CSPs) mediate intercellular communication throughout the lives of multicellular organisms. However, there are no generalizable methods for quantitative CSP profiling in specific cell types in vertebrate tissues. Here, we present in situ cell-surface proteome extraction by extracellular labeling (iPEEL), a proximity labeling method in mice that enables spatiotemporally precise labeling of cell-surface proteomes in a cell-type-specific environment in native tissues for discovery proteomics. Applying iPEEL to developing and mature cerebellar Purkinje cells revealed differential enrichment in CSPs with post-translational protein processing and synaptic functions in the developing and mature cell-surface proteomes, respectively. A proteome-instructed in vivo loss-of-function screen identified a critical, multifaceted role for Armh4 in Purkinje cell dendrite morphogenesis. Armh4 overexpression also disrupts dendrite morphogenesis; this effect requires its conserved cytoplasmic domain and is augmented by disrupting its endocytosis. Our results highlight the utility of CSP profiling in native mammalian tissues for identifying regulators of cell-surface signaling.

Molecular insights into the axon guidance molecules Sidestep and Beaten path

The transmembrane protein Sidestep (Side) functions as a substrate-bound attractant for motor axons in Drosophila. Outgrowing motor axons recognize Side via Beaten path Ia (Beat) and migrate along Side-expressing tissues. Here, we report a structure-function analysis of these guidance molecules using a variety of mutant lines and transgenic constructs. Investigation of Side mutants shows that the exchange of a single amino acid (L241H) in the second immunoglobulin domain disturbs Side function and subcellular localization. Overexpression of Side and Beat deletion constructs in S2 cells and muscles demonstrate that the first Ig domains of both proteins are necessary for their interaction. Furthermore, subcellular distributions of several Beat constructs identify functional domains and suggest a potential posttranslational processing step in ER compartments. In fact, fusing full-length Beat at both the N- and C-terminus with GFP and mCherry, respectively, shows that the N-terminal domain is transported to the plasma membrane and exposed on the cell surface, while the C-terminal domain accumulated in the nucleus. Taken together, these results give insights into the interaction of Side and Beat and imply that Beat might be subject to proteolytic cleavage during maturation.

Molecular neuroscience community shares perspectives

In May, an interdisciplinary group gathered in Crete for the Molecular Neurobiology Workshop. Scientists shared data acquired by vastly diverse techniques to understand how the nervous system, with only a limited number of components, is assembled to respond to infinite stimuli. Ideas of molecular cues, timing, switching, and context emerged.

Regulation of Off-track bidirectional signaling by Semaphorin-1a and Wnt signaling in the Drosophila motor axon guidance

Off-track receptor tyrosine kinase (OTK) has been shown to play an important role in the Drosophila motor axon pathfinding. The results of biochemical and genetic interactions previously suggested that OTK acts as a component of Semaphorin-1a/Plexin A (Sema-1a/PlexA) signaling during embryonic motor axon guidance and further showed that OTK binds to Wnt family members Wnt2 and Wnt4 and their common receptor Frizzled (Fz). However, the molecular mechanisms underlying the motor axon guidance function of OTK remain elusive. Here, we conclude that OTK mediates the forward and reverse signaling required for intersegmental nerve b (ISNb) motor axon pathfinding and we also demonstrate that the loss of two copies of Sema-1a synergistically enhances the bypass phenotype observed in otk mutants. Furthermore, the amorphic wnt2 mutation resulted in increased premature branching phenotypes, and the loss of fz function caused a frequent inability of ISNb motor axons to defasciculate at specific choice points. Consistent with a previous study, wnt4 mutant axons were often defective in recognizing target muscles. Interestingly, the bypass phenotype of otk mutants was robustly suppressed by loss of function mutations in wnt2, wnt4, or fz. In contrast, total ISNb defects of otk were increased by the loss-of-function alleles in wnt2 and wnt4, but not fz. These findings indicate that OTK may participate in the crosstalk between the Sema-1a/PlexA and Wnt signaling pathways, thereby contributing to ISNb motor axon pathfinding and target recognition.

Adhesion molecule Amigo2 is involved in the fasciculation process of the fasciculus retroflexus

BACKGROUND

The fasciculus retroflexus is the prominent efferent pathway from the habenular complex. Medial habenular axons form a core packet whereas lateral habenular axons course in a surrounding shell. Both groups of fibers share the same initial pathway but differ in the final segment of the tract, supposedly regulated by surface molecules. The gene Amigo2 codes for a membrane adhesion molecule with an immunoglobulin-like domain 2 and is selectively expressed in the medial habenula. We present it as a candidate for controlling the fasciculation behavior of medial habenula axons.

RESULTS

First, we studied the development of the habenular efferents in an Amigo2 lack of function mouse model. The fasciculus retroflexus showed a variable defasciculation phenotype. Gain of function experiments allowed us to generate a more condensed tract and rescued the Amigo2 knock-out phenotype. Changes in Amigo2 function did not alter the course of habenular fibers.

CONCLUSION

We have demonstrated that Amigo2 plays a subtle role in the fasciculation of the fasciculus retroflexus.

Semaphorin signaling restricts neuronal regeneration in C. elegans

Extracellular signaling proteins serve as neuronal growth cone guidance molecules during development and are well positioned to be involved in neuronal regeneration and recovery from injury. Semaphorins and their receptors, the plexins, are a family of conserved proteins involved in development that, in the nervous system, are axonal guidance cues mediating axon pathfinding and synapse formation. The Caenorhabditis elegans genome encodes for three semaphorins and two plexin receptors: the transmembrane semaphorins, SMP-1 and SMP-2, signal through their receptor, PLX-1, while the secreted semaphorin, MAB-20, signals through PLX-2. Here, we evaluate the locomotion behavior of knockout animals missing each of the semaphorins and plexins and the neuronal morphology of plexin knockout animals; we described the cellular expression pattern of the promoters of all plexins in the nervous system of C. elegans; and we evaluated their effect on the regrowth and reconnection of motoneuron neurites and the recovery of locomotion behavior following precise laser microsurgery. Regrowth and reconnection were more prevalent in the absence of each plexin, while recovery of locomotion surpassed regeneration in all genotypes.

Computational methods and diffusion theory in triangulation sensing to model neuronal navigation

Computational methods are now recognized as powerful and complementary approaches in various applied sciences such as biology. These computing methods are used to explore the gap between scales such as the one between molecular and cellular. Here we present recent progress in the development of computational approaches involving diffusion modeling, asymptotic analysis of the model partial differential equations, hybrid methods and simulations in the generic context of cell sensing and guidance via external gradients. Specifically, we highlight the reconstruction of the location of a point source in two and three dimensions from the steady-state diffusion fluxes arriving to narrow windows located on the cell. We discuss cases in which these windows are located on the boundary of a two-dimensional plane or three-dimensional half-space, on a disk in free space or inside a two-dimensional corridor, or a ball in three dimensions. The basis of this computational approach is explicit solutions of the Neumann-Green's function for the mentioned geometry. This analysis can be used to design hybrid simulations where Brownian paths are generated only in small regions in which the local spatial organization is relevant. Particle trajectories outside of this region are only implicitly treated by generating exit points at the boundary of this domain of interest. This greatly accelerates the simulation time by avoiding the explicit computation of Brownian paths in an infinite domain and serves to generate statistics, without following all trajectories at the same time, a process that can become numerically expensive quickly. Moreover, these computational approaches are used to reconstruct a point source and estimating the uncertainty in the source reconstruction due to an additive noise perturbation present in the fluxes. We also discuss the influence of various window configurations (cluster vs uniform distributions) on recovering the source position. Finally, the applications in developmental biology are formulated into computational principles that could underly neuronal navigation in the brain.

Multiomic Analysis of Neurons with Divergent Projection Patterns Identifies Novel Regulators of Axon Pathfinding

Axon pathfinding is a key step in neural circuits formation. However, the transcriptional mechanisms regulating its progression remain poorly understood. The binary decision of crossing or avoiding the midline taken by some neuronal axons during development represents a robust model to investigate the mechanisms that control the selection of axonal trajectories. Here, to identify novel regulators of axon guidance, this work compares the transcriptome and chromatin occupancy profiles of two neuronal subpopulations, ipsilateral (iRGC) and contralateral retinal ganglion cells (cRGC), with similar functions but divergent axon trajectories. These analyses retrieved a number of genes encoding for proteins not previously implicated in axon pathfinding. In vivo functional experiments confirm the implication of some of these candidates in axonal navigation. Among the candidate genes, γ-synuclein is identified as essential for inducing midline crossing. Footprint and luciferase assays demonstrate that this small-sized protein is regulated by the transcription factor (TF) Pou4f1 in cRGCs. It is also shown that Lhx2/9 are specifically expressed in iRGCs and control a program that partially overlaps with that regulated by Zic2, previously described as essential for iRGC specification. Overall, the analyses identify dozens of new molecules potentially involved in axon guidance and reveal the regulatory logic behind the selection of axonal trajectories.

Modulation of DRG neurons response to semaphorin 3A via substrate stiffness

Semaphorin 3A (Sema3a) is a chemotropic protein that acts as a neuronal guidance cue and plays a major role in dorsal root ganglion (DRG) sensory neurons projection during embryo development. The present study evaluated the impact of stiffness in the repulsive response of DRG neurons to Sema3a when cultured over substrates of variable stiffness. Stiffness modified DRG neurons morphology and regulated their response to Sema3a, reducing the collapse of growth cones when they were cultured on softer substrates. Sema3a receptors expression was also regulated by stiffness, neuropilin-1 was overexpressed and plexin A4 mRNA was downregulated in stiffer substrates. Cytoskeleton distribution was also modified by stiffness. In softer substrates, βIII-tubulin and actin co-localized up to the leading edge of the growth cones, and as the substrate became stiffer, βIII-tubulin was confined to the transition and peripheral domains of the growth cone. Moreover, a decrease in the α-actinin adaptor protein was also observed in softer substrates. Our results show that substrate stiffness plays an important role in regulating the collapse response to Sema3a and that the modulation of cytoskeleton distribution and Sema3a receptors expression are related to the differential collapse responses of the growth cones.

Neuronal paxillin and drebrin mediate BDNF-induced force transduction and growth cone turning in a soft-tissue-like environment

Soft tissue environments govern neuronal morphogenesis. However, the precise molecular mechanisms underlying chemotropism-directed axonal growth cone movement in extremely soft environments remain unclear. Here, we show that drebrin, a growth cone T-zone protein, modulates growth cone turning in response to brain-derived neurotrophic factor (BDNF) coated on a soft substrate. Structurally, axonal growth cones of rodent hippocampal neurons grown on 0.1 kPa hydrogels possess an expanded T zone in which drebrin is highly integrated with both F-actin and microtubules. Biochemically, we identify paxillin as interacting with drebrin in cells grown on 0.1 kPa hydrogels but not on glass coverslips. When grown on 0.1 kPa substrates, growth cones asymmetrically exposed to BDNF-bound stripes exhibit enhanced paxillin-drebrin interaction on the side facing the stripes, an activity that is PKA and AAK1 dependent but independent of Src kinase. Functionally, we show that BDNF-induced growth cone turning and force generation on soft substrates require drebrin phosphorylation and paxillin-drebrin association.

The impact of early exposure to general anesthesia on visual and neurocognitive development

Every year millions of children are exposed to general anesthesia while undergoing surgical and diagnostic procedures. In the field of ophthalmology, 44,000 children are exposed to general anesthesia annually for strabismus surgery alone. While it is clear that general anesthesia is necessary for sedation and pain minimization during surgical procedures, the possibility of neurotoxic impairments from its exposure is of concern. In animals there is strong evidence linking early anesthesia exposure to abnormal neural development. but in humans the effects of anesthesia are debated. In humans many aspects of vision develop within the first year of life, making the visual system vulnerable to early adverse experiences and potentially vulnerable to early exposure to general anesthesia. We attempt to address whether the visual system is affected by early postnatal exposure to general anesthesia. We first summarize key mechanisms that could account for the neurotoxic effects of general anesthesia on the developing brain and review existing literature on the effects of early anesthesia exposure on the visual system in both animals and humans and on neurocognitive development in humans. Finally, we conclude by proposing future directions for research that could address unanswered questions regarding the impact of general anesthesia on visual development.

Local mRNA translation and cytoskeletal reorganization: Mechanisms that tune neuronal responses

Neurons are highly polarized cells with significantly long axonal and dendritic extensions that can reach distances up to hundreds of centimeters away from the cell bodies in higher vertebrates. Their successful formation, maintenance, and proper function highly depend on the coordination of intricate molecular networks that allow axons and dendrites to quickly process information, and respond to a continuous and diverse cascade of environmental stimuli, often without enough time for communication with the soma. Two seemingly unrelated processes, essential for these rapid responses, and thus neuronal homeostasis and plasticity, are local mRNA translation and cytoskeletal reorganization. The axonal cytoskeleton is characterized by high stability and great plasticity; two contradictory attributes that emerge from the powerful cytoskeletal rearrangement dynamics. Cytoskeletal reorganization is crucial during nervous system development and in adulthood, ensuring the establishment of proper neuronal shape and polarity, as well as regulating intracellular transport and synaptic functions. Local mRNA translation is another mechanism with a well-established role in the developing and adult nervous system. It is pivotal for axonal guidance and arborization, synaptic formation, and function and seems to be a key player in processes activated after neuronal damage. Perturbations in the regulatory pathways of local translation and cytoskeletal reorganization contribute to various pathologies with diverse clinical manifestations, ranging from intellectual disabilities (ID) to autism spectrum disorders (ASD) and schizophrenia (SCZ). Despite the fact that both processes are essential for the orchestration of pathways critical for proper axonal and dendritic function, the interplay between them remains elusive. Here we review our current knowledge on the molecular mechanisms and specific interaction networks that regulate and potentially coordinate these interconnected processes.

Region-specific microRNA alterations in marmosets carrying SLC6A4 polymorphisms are associated with anxiety-like behavior

BACKGROUND

Psychiatric diseases such as depression and anxiety are multifactorial conditions, highly prevalent in western societies. Human studies have identified a number of high-risk genetic variants for these diseases. Among them, polymorphisms in the promoter region of the serotonin transporter gene (SLC6A4) have attracted much attention. However, due to the paucity of experimental models, molecular alterations induced by these genetic variants and how they correlate to behavioral deficits have not been examined. In this regard, marmosets have emerged as a powerful model in translational neuroscience to investigate molecular underpinnings of complex behaviors.

METHODS

Here, we took advantage of naturally occurring genetic polymorphisms in marmoset SLC6A4 gene that have been linked to anxiety-like behaviors. Using FACS-sorting, we profiled microRNA contents in different brain regions of genotyped and behaviorally-phenotyped marmosets.

FINDINGS

We revealed that marmosets bearing different SLC6A4 variants exhibit distinct microRNAs signatures in a region of the prefrontal cortex whose activity has been consistently altered in patients with depression/anxiety. We also identified Deleted in Colorectal Cancer (DCC), a gene previously linked to these diseases, as a downstream target of the differently expressed microRNAs. Significantly, we showed that levels of both microRNAs and DCC in this region were highly correlated to anxiety-like behaviors.

INTERPRETATION

Our findings establish links between genetic variants, molecular modifications in specific cortical regions and complex behavioral responses, providing new insights into gene-behavior relationships underlying human psychopathology.

FUNDING

This work was supported by France National Agency, NRJ Foundation, Celphedia and Fondation de France as well as the Wellcome Trust.

Cracking the combinatorial code of neuronal wiring

How transcription factors orchestrate the combinatorial expression of cell-surface proteins that, in turn, specify the wiring of the nervous system is an open question. In this issue of Neuron, Xie et al. reveal a new, unexpected layer of complexity.

The Nogo-66 Receptors NgR1 and NgR3 Are Required for Commissural Axon Pathfinding

Nogo-66 receptors (NgR1-3) are glycosylphosphatidyl inositol-linked proteins that belong to the leucine-rich repeat superfamily. Through binding to myelin-associated inhibitors, NgRs contribute to the inhibition of axonal regeneration after spinal cord injury. Their role in limiting synaptic plasticity and axonal outgrowth in the adult CNS has been described previously, but not much is known about their role during the development of the nervous system. Here, we show that NgR1 and NgR3 mRNAs are expressed during spinal cord development of the chicken embryo. In particular, they are expressed in the dI1 subpopulation of commissural neurons during the time when their axons navigate toward and across the floorplate, the ventral midline of the spinal cord. To assess a potential role of NgR1 and NgR3 in axon guidance, we downregulated them using in ovo RNAi and analyzed the trajectory of commissural axons by tracing them in open-book preparations of spinal cords. Our results show that loss of either NgR1 or NgR3 causes axons to stall in the midline area and to interfere with the rostral turn of postcrossing axons. In addition, we also show that NgR1, but not NgR3, requires neuronal PlexinA2 for the regulation of commissural axon guidance.SIGNIFICANCE STATEMENT Over the last decades, many studies have focused on the role of NgRs, particularly NgR1, in axonal regeneration in the injured adult CNS. Here, we show a physiological role of NgRs in guiding commissural axons during early development of the chicken spinal cord in vivo Both NgR1 and NgR3 are required for midline crossing and subsequent turning of postcrossing axons into the longitudinal axis of the spinal cord. NgR1, but not NgR3, forms a receptor complex with PlexinA2 during axon guidance. Overall, these findings provide a link between neural regenerative mechanisms and developmental processes.

Transcription factor Acj6 controls dendrite targeting via a combinatorial cell-surface code

Transcription factors specify the fate and connectivity of developing neurons. We investigate how a lineage-specific transcription factor, Acj6, controls the precise dendrite targeting of Drosophila olfactory projection neurons (PNs) by regulating the expression of cell-surface proteins. Quantitative cell-surface proteomic profiling of wild-type and acj6 mutant PNs in intact developing brains, and a proteome-informed genetic screen identified PN surface proteins that execute Acj6-regulated wiring decisions. These include canonical cell adhesion molecules and proteins previously not associated with wiring, such as Piezo, whose mechanosensitive ion channel activity is dispensable for its function in PN dendrite targeting. Comprehensive genetic analyses revealed that Acj6 employs unique sets of cell-surface proteins in different PN types for dendrite targeting. Combined expression of Acj6 wiring executors rescued acj6 mutant phenotypes with higher efficacy and breadth than expression of individual executors. Thus, Acj6 controls wiring specificity of different neuron types by specifying distinct combinatorial expression of cell-surface executors.

Scaffold Protein Lnx1 Stabilizes EphB Receptor Kinases for Synaptogenesis

Postsynaptic structure assembly and remodeling are crucial for functional synapse formation during the establishment of neural circuits. However, how the specific scaffold proteins regulate this process during the development of the postnatal period is poorly understood. In this study, we find that the deficiency of ligand of Numb protein X 1 (Lnx1) leads to abnormal development of dendritic spines to impair functional synaptic formation. We further demonstrate that loss of Lnx1 promotes the internalization of EphB receptors from the cell surface. Constitutively active EphB2 intracellular signaling rescues synaptogenesis in Lnx1 mutant mice. Our data thus reveal a molecular mechanism whereby the Lnx1-EphB complex controls postsynaptic structure for synapse maturation during the adolescent period.

Sema6D Regulates Zebrafish Vascular Patterning and Motor Neuronal Axon Growth in Spinal Cord

Vessels and nerves are closely associated in anatomy as well as functions. Accumulating evidences have demonstrated that axon-guiding signals may affect endothelial cells migration and path finding, which is crucial for the patterning of both the complex vascular network and neural system. However, studies regarding the functional overlap between vascular and neuronal orchestrating are still incomplete. Semaphorin6D (Sema6D) belongs to the Semaphorin family and has been identified as an important regulating factor in diverse biological processes. Its roles in vascular development are still unclear. Here, we confirmed that sema6D is enriched in neural system and blood vessels of zebrafish embryos by in situ hybridization. Then, the deficiency of sema6D caused by specific antisense morpholino-oligonucleotides (MO) led to dramatic path finding defects in both intersegmental vessels (ISVs) and primary motor neurons (PMNs) of spinal cord in zebrafish embryos. Furthermore, these defective phenotypes were confirmed in F0 generation of sema6D knockouts and rescue experiments by overexpression of sema6D mRNA in sema6D morphants. These data collectively indicate that sema6D regulates zebrafish vascular patterning and motor neuronal axon growth in the spinal cord, which might be of great therapeutical use to regulate vessel and nerve guidance in the relevant diseases that affect both systems.

Transcriptomic analysis of dorsal and ventral subiculum after induction of acute seizures by electric stimulation of the perforant pathway in rats

Preconditioning is a mechanism in which injuries induced by non-lethal hypoxia or seizures trigger cellular resistance to subsequent events. Norwood et al., in a 2010 study, showed that an 8-h-long period of electrical stimulation of the perforant pathway in rats is required for the induction of hippocampal sclerosis. However, in order to avoid generalized seizures, status epilepticus (SE), and death, a state of resistance to seizures must be induced in the hippocampus by a preconditioning paradigm consisting of two daily 30-min stimulation periods. Due to the importance of the subiculum in the hippocampal formation, this study aims to investigate differential gene expression patterns in the dorsal and ventral subiculum using RNA-sequencing, after induction of a preconditioning protocol by electrical stimulation of the perforant pathway. The dorsal (dSub) and ventral (vSub) subiculum regions were collected by laser-microdissection 24 h after preconditioning protocol induction in rats. RNA sequencing was performed in a Hiseq 4000 platform, reads were aligned using the STAR and DESEq2 statistics package was used to estimate gene expression. We identified 1176 differentially expressed genes comparing control to preconditioned subiculum regions, 204 genes were differentially expressed in dSub and 972 in vSub. The gene ontology enrichment analysis showed that the most significant common enrichment pathway considering up-regulated genes in dSub and vSub was steroid metabolism. In contrast, the most significant enrichment pathway considering down-regulated genes in vSub was axon guidance. Our results indicate that preconditioning induces changes in the expression of genes related to synaptic reorganization, increased cholesterol metabolism, and astrogliosis in both dSub and vSub. Both regions also presented a decrease in the expression of genes related to glutamatergic transmission and an increase in expression of genes related to complement system activation and GABAergic transmission. The down-regulation of proapoptotic and axon guidance genes in the ventral subiculum suggests that preconditioning may induce a neuroprotective environment in this region.

Intertissue mechanical interactions shape the olfactory circuit in zebrafish

While the chemical signals guiding neuronal migration and axon elongation have been extensively studied, the influence of mechanical cues on these processes remains poorly studied in vivo. Here, we investigate how mechanical forces exerted by surrounding tissues steer neuronal movements and axon extension during the morphogenesis of the olfactory placode in zebrafish. We mainly focus on the mechanical contribution of the adjacent eye tissue, which develops underneath the placode through extensive evagination and invagination movements. Using quantitative analysis of cell movements and biomechanical manipulations, we show that the developing eye exerts lateral traction forces on the olfactory placode through extracellular matrix, mediating proper morphogenetic movements and axon extension within the placode. Our data shed new light on the key participation of intertissue mechanical interactions in the sculpting of neuronal circuits.

Fzd3 Expression Within Inner Ear Afferent Neurons Is Necessary for Central Pathfinding

During development the afferent neurons of the inner ear make precise wiring decisions in the hindbrain reflective of their topographic distribution in the periphery. This is critical for the formation of sensory maps capable of faithfully processing both auditory and vestibular input. Disorganized central projections of inner ear afferents in Fzd3 null mice indicate Wnt/PCP signaling is involved in this process and ear transplantation in Xenopus indicates that Fzd3 is necessary in the ear but not the hindbrain for proper afferent navigation. However, it remains unclear in which cell type of the inner ear Fzd3 expression is influencing the guidance of inner ear afferents to their proper synaptic targets in the hindbrain. We utilized Atoh1-cre and Neurod1-cre mouse lines to conditionally knockout Fzd3 within the mechanosensory hair cells of the organ of Corti and within the inner ear afferents, respectively. Following conditional deletion of Fzd3 within the hair cells, the central topographic distribution of inner ear afferents was maintained with no gross morphological defects. In contrast, conditional deletion of Fzd3 within inner ear afferents leads to central pathfinding defects of both cochlear and vestibular afferents. Here, we show that Fzd3 is acting in a cell autonomous manner within inner ear afferents to regulate central pathfinding within the hindbrain.

Receptor-Ribosome Coupling: A Link Between Extrinsic Signals and mRNA Translation in Neuronal Compartments

Axons receive extracellular signals that help to guide growth and synapse formation during development and to maintain neuronal function and survival during maturity. These signals relay information via cell surface receptors that can initiate local intracellular signaling at the site of binding, including local messenger RNA (mRNA) translation. Direct coupling of translational machinery to receptors provides an attractive way to activate this local mRNA translation and change the local proteome with high spatiotemporal resolution. Here, we first discuss the increasing evidence that different external stimuli trigger translation of specific subsets of mRNAs in axons via receptors and thus play a prominent role in various processes in both developing and mature neurons. We then discuss the receptor-mediated molecular mechanisms that regulate local mRNA translational with a focus on direct receptor-ribosome coupling. We advance the idea that receptor-ribosome coupling provides several advantages over other translational regulation mechanisms and is a common mechanism in cell communication.

Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Performing Single-Cell Clonal Analysis in the Mouse Brain Using Mosaic Analysis with Double Markers (MADM)

A central question in neuroscience is how 100 billion neurons come together to build the human brain. The wiring, morphology, survival, and death of each neuron are controlled by genes that encode intrinsic and extrinsic factors. Determining the function of these genes at a high spatiotemporal resolution is a critical step toward understanding brain development and function. Moreover, an increasing number of somatic mutations are being discovered in many brain disorders. However, neurons are embedded in complex networks, making it difficult to distinguish cell-autonomous from non-cell-autonomous function of any given gene in the brain. Here, I describe MADM (mosaic analysis with double markers), a genetic method that allows for labeling and manipulating gene function at the single-cell level within the mouse brain. I present mouse breeding schemes to employ MADM analysis and important considerations for experimental design. This powerful system can be adapted to make fundamental neuroscience discoveries by targeting genetically defined cell types in the mouse brain with high spatiotemporal resolution.

The Potential Role of Exercise Training and Mechanical Loading on Bone-Associated Skeletal Nerves

The spatial distribution, innervation, and functional role of the bone-associated skeletal nerves have been previously reported in detail. However, studies examining exercise-induced associations between skeletal nerves and bone metabolism are limited. This review introduces a potential relationship between exercise and the skeletal nerves and discusses how it can contribute to exercise-induced bone anabolism. First, the background and current understanding of nerve fiber types and their functions in the skeleton are provided. Next, the influence of exercise and mechanical loading on the skeletal nervous system is elaborated. Effective synthesis of recent studies could serve as an established baseline for the novel discovery of the effects of exercise on skeletal nerve density and bone anabolic activity in the future. Therefore, this review overviews the existing evidence for the neural control of bone metabolism and the potential positive effects of exercise on the peripheral skeletal nervous system. The influence of exercise training models on the relationships of sensory nerve signals with osteoblast-mediated bone formation and the increased bone volume provides the first insight on the potential importance of exercise training in stimulating positive adaptations in the skeletal nerve-bone interaction and its downstream effect on bone metabolism, thereby highlighting its therapeutic potential in a variety of clinical populations.

Multiple Functions of Draxin/Netrin-1 Signaling in the Development of Neural Circuits in the Spinal Cord and the Brain

Axon guidance proteins play key roles in the formation of neural circuits during development. We previously identified an axon guidance cue, named draxin, that has no homology with other axon guidance proteins. Draxin is essential for the development of various neural circuits including the spinal cord commissure, corpus callosum, and thalamocortical projections. Draxin has been shown to not only control axon guidance through netrin-1 receptors, deleted in colorectal cancer (Dcc), and neogenin (Neo1) but also modulate netrin-1-mediated axon guidance and fasciculation. In this review, we summarize the multifaceted functions of draxin and netrin-1 signaling in neural circuit formation in the central nervous system. Furthermore, because recent studies suggest that the distributions and functions of axon guidance cues are highly regulated by glycoproteins such as Dystroglycan and Heparan sulfate proteoglycans, we discuss a possible function of glycoproteins in draxin/netrin-1-mediated axon guidance.

CRMP4-mediated fornix development involves Semaphorin-3E signaling pathway

Neurodevelopmental axonal pathfinding plays a central role in correct brain wiring and subsequent cognitive abilities. Within the growth cone, various intracellular effectors transduce axonal guidance signals by remodeling the cytoskeleton. Semaphorin-3E (Sema3E) is a guidance cue implicated in development of the fornix, a neuronal tract connecting the hippocampus to the hypothalamus. Microtubule-associated protein 6 (MAP6) has been shown to be involved in the Sema3E growth-promoting signaling pathway. In this study, we identified the collapsin response mediator protein 4 (CRMP4) as a MAP6 partner and a crucial effector in Sema3E growth-promoting activity. CRMP4-KO mice displayed abnormal fornix development reminiscent of that observed in Sema3E-KO mice. CRMP4 was shown to interact with the Sema3E tripartite receptor complex within detergent-resistant membrane (DRM) domains, and DRM domain integrity was required to transduce Sema3E signaling through the Akt/GSK3 pathway. Finally, we showed that the cytoskeleton-binding domain of CRMP4 is required for Sema3E's growth-promoting activity, suggesting that CRMP4 plays a role at the interface between Sema3E receptors, located in DRM domains, and the cytoskeleton network. As the fornix is affected in many psychiatric diseases, such as schizophrenia, our results provide new insights to better understand the neurodevelopmental components of these diseases.

With the Permission of Microtubules: An Updated Overview on Microtubule Function During Axon Pathfinding

During the establishment of neural circuitry axons often need to cover long distances to reach remote targets. The stereotyped navigation of these axons defines the connectivity between brain regions and cellular subtypes. This chemotrophic guidance process mostly relies on the spatio-temporal expression patterns of extracellular proteins and the selective expression of their receptors in projection neurons. Axon guidance is stimulated by guidance proteins and implemented by neuronal traction forces at the growth cones, which engage local cytoskeleton regulators and cell adhesion proteins. Different layers of guidance signaling regulation, such as the cleavage and processing of receptors, the expression of co-receptors and a wide variety of intracellular cascades downstream of receptors activation, have been progressively unveiled. Also, in the last decades, the regulation of microtubule (MT) assembly, stability and interactions with the submembranous actin network in the growth cone have emerged as crucial effector mechanisms in axon pathfinding. In this review, we will delve into the intracellular signaling cascades downstream of guidance receptors that converge on the MT cytoskeleton of the growing axon. In particular, we will focus on the microtubule-associated proteins (MAPs) network responsible of MT dynamics in the axon and growth cone. Complementarily, we will discuss new evidences that connect defects in MT scaffold proteins, MAPs or MT-based motors and axon misrouting during brain development.

Reduced hippocampal inhibition and enhanced autism-epilepsy comorbidity in mice lacking neuropilin 2

The neuropilin receptors and their secreted semaphorin ligands play key roles in brain circuit development by regulating numerous crucial neuronal processes, including the maturation of synapses and migration of GABAergic interneurons. Consistent with its developmental roles, the neuropilin 2 (Nrp2) locus contains polymorphisms in patients with autism spectrum disorder (ASD). Nrp2-deficient mice show autism-like behavioral deficits and propensity to develop seizures. In order to determine the pathophysiology in Nrp2 deficiency, we examined the hippocampal numbers of interneuron subtypes and inhibitory regulation of hippocampal CA1 pyramidal neurons in mice lacking one or both copies of Nrp2. Immunostaining for interneuron subtypes revealed that Nrp2^-/- mice have a reduced number of parvalbumin, somatostatin, and neuropeptide Y cells, mainly in CA1. Whole-cell recordings identified reduced firing and hyperpolarized shift in resting membrane potential in CA1 pyramidal neurons from Nrp2^+/- and Nrp2^-/- mice compared to age-matched wild-type controls indicating decrease in intrinsic excitability. Simultaneously, the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs) are reduced in Nrp2-deficient mice. A convulsive dose of kainic acid evoked electrographic and behavioral seizures with significantly shorter latency, longer duration, and higher severity in Nrp2^-/- compared to Nrp2^+/+ animals. Finally, Nrp2^+/- and Nrp2^-/- but not Nrp2^+/+, mice have impaired cognitive flexibility demonstrated by reward-based reversal learning, a task associated with hippocampal circuit function. Together these data demonstrate a broad reduction in interneuron subtypes and compromised inhibition in CA1 of Nrp2^-/- mice, which could contribute to the heightened seizure susceptibility and behavioral deficits consistent with an ASD/epilepsy phenotype.

Advancing models of neural development with biomaterials

Human pluripotent stem cells have emerged as a promising in vitro model system for studying the brain. Two-dimensional and three-dimensional cell culture paradigms have provided valuable insights into the pathogenesis of neuropsychiatric disorders, but they remain limited in their capacity to model certain features of human neural development. Specifically, current models do not efficiently incorporate extracellular matrix-derived biochemical and biophysical cues, facilitate multicellular spatio-temporal patterning, or achieve advanced functional maturation. Engineered biomaterials have the capacity to create increasingly biomimetic neural microenvironments, yet further refinement is needed before these approaches are widely implemented. This Review therefore highlights how continued progression and increased integration of engineered biomaterials may be well poised to address intractable challenges in recapitulating human neural development.

Cellular bases of olfactory circuit assembly revealed by systematic time-lapse imaging

Neural circuit assembly features simultaneous targeting of numerous neuronal processes from constituent neuron types, yet the dynamics is poorly understood. Here, we use the Drosophila olfactory circuit to investigate dynamic cellular processes by which olfactory receptor neurons (ORNs) target axons precisely to specific glomeruli in the ipsi- and contralateral antennal lobes. Time-lapse imaging of individual axons from 30 ORN types revealed a rich diversity in extension speed, innervation timing, and ipsilateral branch locations and identified that ipsilateral targeting occurs via stabilization of transient interstitial branches. Fast imaging using adaptive optics-corrected lattice light-sheet microscopy showed that upon approaching target, many ORN types exhibiting "exploring branches" consisted of parallel microtubule-based terminal branches emanating from an F-actin-rich hub. Antennal nerve ablations uncovered essential roles for bilateral axons in contralateral target selection and for ORN axons to facilitate dendritic refinement of postsynaptic partner neurons. Altogether, these observations provide cellular bases for wiring specificity establishment.

Mechanisms Underlying Target Selectivity for Cell Types and Subcellular Domains in Developing Neocortical Circuits

The cerebral cortex contains numerous neuronal cell types, distinguished by their molecular identity as well as their electrophysiological and morphological properties. Cortical function is reliant on stereotyped patterns of synaptic connectivity and synaptic function among these neuron types, but how these patterns are established during development remains poorly understood. Selective targeting not only of different cell types but also of distinct postsynaptic neuronal domains occurs in many brain circuits and is directed by multiple mechanisms. These mechanisms include the regulation of axonal and dendritic guidance and fine-scale morphogenesis of pre- and postsynaptic processes, lineage relationships, activity dependent mechanisms and intercellular molecular determinants such as transmembrane and secreted molecules, many of which have also been implicated in neurodevelopmental disorders. However, many studies of synaptic targeting have focused on circuits in which neuronal processes target different lamina, such that cell-type-biased connectivity may be confounded with mechanisms of laminar specificity. In the cerebral cortex, each cortical layer contains cell bodies and processes from intermingled neuronal cell types, an arrangement that presents a challenge for the development of target-selective synapse formation. Here, we address progress and future directions in the study of cell-type-biased synaptic targeting in the cerebral cortex. We highlight challenges to identifying developmental mechanisms generating stereotyped patterns of intracortical connectivity, recent developments in uncovering the determinants of synaptic target selection during cortical synapse formation, and current gaps in the understanding of cortical synapse specificity.

A Co-purification Method for Efficient Production and Src Kinase-mediated Phosphorylation of Aplysia Cortactin

Cortactin is an actin-binding protein that regulates processes like cell migration, endocytosis, and tumor cell metastasis. Although cortactin is associated with actin-cytoskeletal dynamics in non-neuronal cells and cell-free systems, the exact mechanisms underlying its fundamental roles in neuronal growth cones are not fully explored. Recent reports show that Aplysia Src2 tyrosine kinase induces phosphorylation of cortactin as a mechanism to control lamellipodia protrusion and filopodia formation in cultured Aplysia bag cell neurons ( He et al., 2015 ; Ren et al., 2019 ). In order to provide in vitro evidence for Src2-mediated phosphorylation of cortactin, we developed a robust and cost-effective method for the efficient expression and purification of Aplysia cortactin and Src2 kinase that can be used for biochemical studies including phosphorylation assays. By co-purifying cortactin and Src kinase with a phosphatase (YopH) from Yersinia enterocolitica, we eliminated the problem of non-specific phosphorylation of induced proteins by bacterial kinases and also reduced costs by bypassing the need for commercial enzymatic treatments. This protocol is reproducible and can be modified to produce homogenous non-phosphorylated proteins during recombinant protein expression in Escherichia coli.

Architectures of neuronal circuits

[Figure: see text].

Coming of Age for the Photoreceptor Synapse

Purpose

To discuss the potential contribution of rod and cone synapses to the loss of visual function in retinal injury and disease.

Methods

The published literature and the authors' own work were reviewed.

Results

Retinal detachment is used as a case study of rod spherule and cone pedicle plasticity after injury. Both rod and cone photoreceptors terminals are damaged after detachment although the structural changes observed are only partially overlapping. For second-order neurons, only those associated with rod spherules respond consistently to injury by remodeling. Examination of signaling pathways involved in plasticity of conventional synapses and in neural development has been and may continue to be productive in discovering novel therapeutic targets. Rho kinase (ROCK) inhibition is an example of therapy that may reduce synaptic damage by preserving normal synaptic structure of rod and cone cells.

Conclusions

We hypothesize that synaptic damage contributes to poor visual restoration after otherwise successful anatomical repair of retinal detachment. A similar situation may exist for patients with degenerative retinal disease. Thus, synaptic structure and function should be routinely studied, as this information may disclose therapeutic strategies to mitigate visual loss.

Reconstructing a point source from diffusion fluxes to narrow windows in three dimensions

We develop a computational approach to locate the source of a steady-state gradient of diffusing particles from the fluxes through narrow windows distributed either on the boundary of a three-dimensional half-space or on a sphere. This approach is based on solving the mixed boundary stationary diffusion equation with Neumann–Green’s function. The method of matched asymptotic expansions enables the computation of the probability fluxes. To explore the range of validity of this expansion, we develop a fast analytical-Brownian numerical scheme. This scheme accelerates the simulation time by avoiding the explicit computation of Brownian trajectories in the infinite domain. The results obtained from our derived analytical formulae and the fast numerical simulation scheme agree on a large range of parameters. Using the analytical representation of the particle fluxes, we show how to reconstruct the location of the point source. Furthermore, we investigate the uncertainty in the source reconstruction due to additive fluctuations present in the fluxes. We also study the influence of various window configurations: clustered versus uniform distributions on recovering the source position. Finally, we discuss possible applications for cell navigation in biology.

Molecular Mechanisms Underlying Motor Axon Guidance in Drosophila

Decoding the molecular mechanisms underlying axon guidance is key to precise understanding of how complex neural circuits form during neural development. Although substantial progress has been made over the last three decades in identifying numerous axon guidance molecules and their functional roles, little is known about how these guidance molecules collaborate to steer growth cones to their correct targets. Recent studies in Drosophila point to the importance of the combinatorial action of guidance molecules, and further show that selective fasciculation and defasciculation at specific choice points serve as a fundamental strategy for motor axon guidance. Here, I discuss how attractive and repulsive guidance cues cooperate to ensure the recognition of specific choice points that are inextricably linked to selective fasciculation and defasciculation, and correct pathfinding decision-making.

Mechanosensitive axon outgrowth mediated by L1-laminin clutch interface

Mechanical properties of the extracellular environment modulate axon outgrowth. Growth cones at the tip of extending axons generate traction force for axon outgrowth by transmitting the force of actin filament retrograde flow, produced by actomyosin contraction and F-actin polymerization, to adhesive substrates through clutch and cell adhesion molecules. A molecular clutch between the actin filament flow and substrate is proposed to contribute to cellular mechanosensing. However, the molecular identity of the clutch interface responsible for mechanosensitive growth cone advance is unknown. We previously reported that mechanical coupling between actin filament retrograde flow and adhesive substrates through the clutch molecule shootin1a and the cell adhesion molecule L1 generates traction force for axon outgrowth and guidance. Here, we show that cultured mouse hippocampal neurons extend longer axons on stiffer substrates under elastic conditions that correspond to the soft brain environments. We demonstrate that this stiffness-dependent axon outgrowth requires actin-adhesion coupling mediated by shootin1a, L1, and laminin on the substrate. Speckle imaging analyses showed that L1 at the growth cone membrane switches between two adhesive states: L1 that is immobilized and that undergoes retrograde movement on the substrate. The duration of the immobilized phase was longer on stiffer substrates; this was accompanied by increases in actin-adhesion coupling and in the traction force exerted on the substrate. These data suggest that the interaction between L1 and laminin is enhanced on stiffer substrates, thereby promoting force generation for axon outgrowth.

Genetic influences on hub connectivity of the human connectome

Brain network hubs are both highly connected and highly inter-connected, forming a critical communication backbone for coherent neural dynamics. The mechanisms driving this organization are poorly understood. Using diffusion-weighted magnetic resonance imaging in twins, we identify a major role for genes, showing that they preferentially influence connectivity strength between network hubs of the human connectome. Using transcriptomic atlas data, we show that connected hubs demonstrate tight coupling of transcriptional activity related to metabolic and cytoarchitectonic similarity. Finally, comparing over thirteen generative models of network growth, we show that purely stochastic processes cannot explain the precise wiring patterns of hubs, and that model performance can be improved by incorporating genetic constraints. Our findings indicate that genes play a strong and preferential role in shaping the functionally valuable, metabolically costly connections between connectome hubs.

Roles and Mechanisms of Axon-Guidance Molecules in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease that is characterized by progressive memory decline and cognitive dysfunctions. Although the causes of AD have not yet been established, many mechanisms have been proposed. Axon-guidance molecules play the roles in the occurrence and development of AD by participating in different mechanisms. Therefore, what roles do axon-guidance molecules play in AD? This study aimed at elucidating how axon-guidance molecules Netrins, Slits, Semaphorins, and Ephrins regulate the levels of Aβ, hyperphosphorylation of tau protein, Reelin, and other ways through different signaling pathways, in order to show the roles of axon-guidance molecules in the occurrence and development of AD. And it is hoped that this study can provide a theoretical basis and new perspectives in the search for new therapeutic targets for AD.

Netrin 1-Mediated Role of the Substantia Nigra Pars Compacta and Ventral Tegmental Area in the Guidance of the Medial Habenular Axons

The fasciculus retroflexus is an important fascicle that mediates reward-related behaviors and is associated with different psychiatric diseases. It is the main habenular efference and constitutes a link between forebrain regions, the midbrain, and the rostral hindbrain. The proper functional organization of habenular circuitry requires complex molecular programs to control the wiring of the habenula during development. However, the mechanisms guiding the habenular axons toward their targets remain mostly unknown. Here, we demonstrate the role of the mesodiencephalic dopaminergic neurons (substantia nigra pars compacta and ventral tegmental area) as an intermediate target for the correct medial habenular axons navigation along the anteroposterior axis. These neuronal populations are distributed along the anteroposterior trajectory of these axons in the mesodiencephalic basal plate. Using in vitro and in vivo experiments, we determined that this navigation is the result of netrin 1 attraction generated by the mesodiencephalic dopaminergic neurons. This attraction is mediated by the receptor deleted in colorectal cancer (DCC), which is strongly expressed in the medial habenular axons. The increment in our knowledge on the fasciculus retroflexus trajectory guidance mechanisms opens the possibility of analyzing if its alteration in mental health patients could account for some of their symptoms.

Reciprocal repulsions instruct the precise assembly of parallel hippocampal networks

Mammalian medial and lateral hippocampal networks preferentially process spatial- and object-related information, respectively. However, the mechanisms underlying the assembly of such parallel networks during development remain largely unknown. Our study shows that, in mice, complementary expression of cell surface molecules teneurin-3 (Ten3) and latrophilin-2 (Lphn2) in the medial and lateral hippocampal networks, respectively, guides the precise assembly of CA1-to-subiculum connections in both networks. In the medial network, Ten3-expressing (Ten3+) CA1 axons are repelled by target-derived Lphn2, revealing that Lphn2- and Ten3-mediated heterophilic repulsion and Ten3-mediated homophilic attraction cooperate to control precise target selection of CA1 axons. In the lateral network, Lphn2-expressing (Lphn2+) CA1 axons are confined to Lphn2+ targets via repulsion from Ten3+ targets. Our findings demonstrate that assembly of parallel hippocampal networks follows a "Ten3→Ten3, Lphn2→Lphn2" rule instructed by reciprocal repulsions.

Understanding, engineering, and modulating the growth of neural networks: An interdisciplinary approach

A deeper understanding of the brain and its function remains one of the most significant scientific challenges. It not only is required to find cures for a plethora of brain-related diseases and injuries but also opens up possibilities for achieving technological wonders, such as brain-machine interface and highly energy-efficient computing devices. Central to the brain's function is its basic functioning unit (i.e., the neuron). There has been a tremendous effort to understand the underlying mechanisms of neuronal growth on both biochemical and biophysical levels. In the past decade, this increased understanding has led to the possibility of controlling and modulating neuronal growth in vitro through external chemical and physical methods. We provide a detailed overview of the most fundamental aspects of neuronal growth and discuss how researchers are using interdisciplinary ideas to engineer neuronal networks in vitro. We first discuss the biochemical and biophysical mechanisms of neuronal growth as we stress the fact that the biochemical or biophysical processes during neuronal growth are not independent of each other but, rather, are complementary. Next, we discuss how utilizing these fundamental mechanisms can enable control over neuronal growth for advanced neuroengineering and biomedical applications. At the end of this review, we discuss some of the open questions and our perspectives on the challenges and possibilities related to controlling and engineering the growth of neuronal networks, specifically in relation to the materials, substrates, model systems, modulation techniques, data science, and artificial intelligence.

Glycosylation in Axonal Guidance

How millions of axons navigate accurately toward synaptic targets during development is a long-standing question. Over decades, multiple studies have enriched our understanding of axonal pathfinding with discoveries of guidance molecules and morphogens, their receptors, and downstream signalling mechanisms. Interestingly, classification of attractive and repulsive cues can be fluid, as single guidance cues can act as both. Similarly, guidance cues can be secreted, chemotactic cues or anchored, adhesive cues. How a limited set of guidance cues generate the diversity of axonal guidance responses is not completely understood. Differential expression and surface localization of receptors, as well as crosstalk and spatiotemporal patterning of guidance cues, are extensively studied mechanisms that diversify axon guidance pathways. Posttranslational modification is a common, yet understudied mechanism of diversifying protein functions. Many proteins in axonal guidance pathways are glycoproteins and how glycosylation modulates their function to regulate axonal motility and guidance is an emerging field. In this review, we discuss major classes of glycosylation and their functions in axonal pathfinding. The glycosylation of guidance cues and guidance receptors and their functional implications in axonal outgrowth and pathfinding are discussed. New insights into current challenges and future perspectives of glycosylation pathways in neuronal development are discussed.

A developmental framework linking neurogenesis and circuit formation in the Drosophila CNS

The mechanisms specifying neuronal diversity are well characterized, yet it remains unclear how or if these mechanisms regulate neural circuit assembly. To address this, we mapped the developmental origin of 160 interneurons from seven bilateral neural progenitors (neuroblasts) and identify them in a synapse-scale TEM reconstruction of the Drosophila larval central nervous system. We find that lineages concurrently build the sensory and motor neuropils by generating sensory and motor hemilineages in a Notch-dependent manner. Neurons in a hemilineage share common synaptic targeting within the neuropil, which is further refined based on neuronal temporal identity. Connectome analysis shows that hemilineage-temporal cohorts share common connectivity. Finally, we show that proximity alone cannot explain the observed connectivity structure, suggesting hemilineage/temporal identity confers an added layer of specificity. Thus, we demonstrate that the mechanisms specifying neuronal diversity also govern circuit formation and function, and that these principles are broadly applicable throughout the nervous system.

Autophagy in axonal and presynaptic development

The study of autophagy in the nervous system has predominantly centered on degeneration. Evidence is now cementing crucial roles for autophagy in neuronal development and growth, especially in axonal and presynaptic compartments. A picture is emerging that autophagy typically promotes the growth of axons and reduces presynaptic stability. Nonetheless, these are not rigid principles, and it remains unclear why autophagy does not always display these relationships during axonal and presynaptic development. Recent progress has identified mechanisms underlying spatiotemporal control of autophagy in neurons and begun to unravel how autophagy is integrated with other cellular processes, such as proteasomal degradation and axon guidance. Ultimately, understanding how autophagy is regulated and its role in the developing nervous system is key to comprehending how the nervous system assembles its stereotyped yet plastic configuration. It is also likely to inform how we think about neurodevelopmental disorders and neurodegenerative diseases.

FEZ1 Forms Complexes with CRMP1 and DCC to Regulate Axon and Dendrite Development

Elaboration of neuronal processes is an early step in neuronal development. Guidance cues must work closely with intracellular trafficking pathways to direct expanding axons and dendrites to their target neurons during the formation of neuronal networks. However, how such coordination is achieved remains incompletely understood. Here, we characterize an interaction between fasciculation and elongation protein zeta 1 (FEZ1), an adapter involved in synaptic protein transport, and collapsin response mediator protein (CRMP)1, a protein that functions in growth cone guidance, at neuronal growth cones. We show that similar to CRMP1 loss-of-function mutants, FEZ1 deficiency in rat hippocampal neurons causes growth cone collapse and impairs axonal development. Strikingly, FEZ1-deficient neurons also exhibited a reduction in dendritic complexity stronger than that observed in CRMP1-deficient neurons, suggesting that the former could partake in additional developmental signaling pathways. Supporting this, FEZ1 colocalizes with VAMP2 in developing hippocampal neurons and forms a separate complex with deleted in colorectal cancer (DCC) and Syntaxin-1 (Stx1), components of the Netrin-1 signaling pathway that are also involved in regulating axon and dendrite development. Significantly, developing axons and dendrites of FEZ1-deficient neurons fail to respond to Netrin-1 or Netrin-1 and Sema3A treatment, respectively. Taken together, these findings highlight the importance of FEZ1 as a common effector to integrate guidance signaling pathways with intracellular trafficking to mediate axo-dendrite development during neuronal network formation.