Integrins regulate repulsion-mediated dendritic patterning of drosophila sensory neurons by restricting dendrites in a 2D space

Drosophila epidermal cells are intrinsically mechanosensitive and modulate nociceptive behavioral outputs

Somatosensory neurons (SSNs) that detect and transduce mechanical, thermal, and chemical stimuli densely innervate an animal's skin. However, although epidermal cells provide the first point of contact for sensory stimuli, our understanding of roles that epidermal cells play in SSN function, particularly nociception, remains limited. Here, we show that stimulating Drosophila epidermal cells elicits activation of SSNs including nociceptors and triggers a variety of behavior outputs, including avoidance and escape. Further, we find that epidermal cells are intrinsically mechanosensitive and that epidermal mechanically evoked calcium responses require the store-operated calcium channel Orai. Epidermal cell stimulation augments larval responses to acute nociceptive stimuli and promotes prolonged hypersensitivity to subsequent mechanical stimuli. Hence, epidermal cells are key determinants of nociceptive sensitivity and sensitization, acting as primary sensors of noxious stimuli that tune nociceptor output and drive protective behaviors.

Phagocytosis-driven neurodegeneration through opposing roles of an ABC transporter in neurons and phagocytes

Lipid homeostasis is critical to neuronal survival. ATP-binding cassette A (ABCA) proteins are lipid transporters associated with neurodegenerative diseases. How ABCA transporters regulate lipid homeostasis in neurodegeneration is an outstanding question. Here we report that the Drosophila ABCA protein engulfment ABC transporter in the ovary (Eato) regulates phagocytosis-dependent neurodegeneration by playing opposing roles in neurons and phagocytes: In neurons, Eato prevents dendrites and axons from being attacked by neighboring phagocytes; in phagocytes, Eato sensitizes the cell for detecting neurons as engulfment targets. Thus, Eato deficiency in neurons alone causes phagocytosis-dependent neurite degeneration, but additional Eato loss from phagocytes suppresses the neurite degeneration. Mechanistically, Eato functions by removing the eat-me signal phosphatidylserine from the cell surface in both neurons and phagocytes. Multiple human and worm ABCA homologs can rescue Eato loss in phagocytes but not in neurons, suggesting both conserved and cell type-specific activities of ABCA proteins. These results imply possible mechanisms of neuron-phagocyte interactions in neurodegenerative diseases.

Neurons exploit stochastic growth to rapidly and economically build dense radially oriented dendritic arbors

Dendrites grow by stochastic branching, elongation, and retraction. A key question is whether such a mechanism is sufficient to form highly branched dendritic morphologies. Alternatively, are signals from other cells or is the topological hierarchy of the growing network necessary for dendrite geometry? To answer these questions, we developed a mean-field model in which branch dynamics is isotropic and homogenous (i.e., no extrinsic instruction) and depends only on the average lengths and densities of branches. Branching is modeled as density-dependent nucleation so there are no tree structures and no network topology. Despite its simplicity, the model predicted several key morphological properties of class IV Drosophila sensory dendrites, including the exponential distribution of branch lengths, the parabolic scaling between dendrite number and length densities, the tight spacing of the dendritic meshwork (which required minimal total branch length), and the radial orientation of branches. Stochastic growth also accelerated the overall expansion rate of the arbor. Therefore, stochastic dynamics is an economical and rapid space-filling mechanism for building dendritic arbors without external guidance or hierarchical branching mechanisms. Our model provides a general theoretical framework for understanding how macroscopic branching patterns emerge from microscopic dynamics.

The inter-organelle cross-talk finely orchestrated in the amyloidogenic processing of amyloid precursor protein in dendritic arborization neurons of Drosophila

Background: Organelles in neuronal dendrites facilitate local metabolic processes and energy supply, crucial for dendrite development and neurodegenerative diseases. The distinct functions of dendritic organelles have been well studied, however, their crosstalk under physiological and pathological contexts remains elusive. We aimed to establish an in vivo model system of contacts between multi-organelles for investigating the modulation of inter-organelle crosstalk in Alzheimer's disease (AD). Methods: A dendrite model of organelle contacts was developed in Drosophila neurons using a set of proximity-driven probes and four-color Airyscan super-resolution imaging. The systematic modulations among multiple contact sites (CSs) between organelles were examined by manipulating CS tethers and vesicular transporters. Finally, perturbations of these CSs and the dendrite structure in the amyloidogenic processing of amyloid precursor protein (APP) were evaluated by introducing three stages of the processing in this model system. Results: A dynamic network, interconnected via CSs and organized with multi-organelle contacts, was presented among Golgi outposts, the endoplasmic reticulum, lysosomes, and mitochondria (GELM). The CS modulations were found to encompass both their density and motility. Notably, multi-CSs participated in complementary modulations spanning across different cellular pathways. Furthermore, the CS network was revealed to be progressively disturbed in APP amyloidogenic processing, with upregulations in density and motility extending from single- to multi-CSs. These CS perturbations, along with defects in dendrite structural plasticity, could be partially rescued by knocking down Miro. Conclusion: The elucidation of CS modulation modes in the GELM network model reveals a cascaded dysregulation of organelle crosstalk during APP amyloidogenic processing. It expands the mechanisms of inter-organelle communication and provides novel insights into neurodegeneration in AD pathology.

Study of Dendrite Differentiation Using Drosophila Dendritic Arborization Neurons

Neurons receive, process, and integrate inputs. These operations are organized by dendrite arbor morphology, and the dendritic arborization (da) neurons of the Drosophila peripheral sensory nervous system are an excellent experimental model for examining the differentiation processes that build and shape the dendrite arbor. Studies in da neurons are enabled by a wealth of fly genetic tools that allow targeted neuron manipulation and labeling of the neuron's cytoskeletal or organellar components. Moreover, as da neuron dendrite arbors cover the body wall, they are highly accessible for live imaging analysis of arbor patterning. Here, we outline the structure and function of different da neuron types and give examples of how they are used to elucidate central mechanisms of dendritic arbor formation.

Modeling neuropathic pain in a dish

The study of pain mechanisms has advanced significantly with the development of innovative in vitro models. This chapter explores those already used in or potentially useful for neuropathic pain research, emphasizing the complementary roles of animal and human cellular models to enhance translational success. Traditional animal models have provided foundational insights into the neurobiology of pain and remain invaluable for understanding complex pain pathways. However, integrating human cellular models addresses the need for better replication of human nociceptors. The chapter details methodologies for culturing rodent and human primary sensory neurons, including isolation and culture techniques, advantages, and limitations. It highlights the application of these models in neuropathic pain research, such as identifying pain-associated receptors and ion channels. Recent advancements in using induced pluripotent stem cell (iPSC)-derived sensory neurons are also discussed. Finally, the chapter explores advanced in vitro models, including 2D co-cultures and 3D organoids, and their implications for studying neuropathic pain. These models offer significant advantages for drug screening and ethical research practices, providing a more accurate representation of human pain pathways and paving the way for innovative therapeutic strategies. Despite challenges such as limited access to viable human tissue and variability between samples, these in vitro models, alongside traditional animal models, are indispensable for advancing our understanding of neuropathic pain and developing effective treatments.

C. elegans touch receptor neurons direct mechanosensory complex organization via repurposing conserved basal lamina proteins

The sense of touch is conferred by the conjoint function of somatosensory neurons and skin cells. These cells meet across a gap filled by a basal lamina, an ancient structure found in metazoans. Using Caenorhabditis elegans, we investigate the composition and ultrastructure of the extracellular matrix at the epidermis and touch receptor neuron (TRN) interface. We show that membrane-matrix complexes containing laminin, nidogen, and the MEC-4 mechano-electrical transduction channel reside at this interface and are central to proper touch sensation. Interestingly, the dimensions and spacing of these complexes correspond with the discontinuous beam-like extracellular matrix structures observed in serial-section transmission electron micrographs. These complexes fail to coalesce in touch-insensitive extracellular matrix mutants and in dissociated neurons. Loss of nidogen reduces the density of mechanoreceptor complexes and the amplitude of the touch-evoked currents they carry. Thus, neuron-epithelium cell interfaces are instrumental in mechanosensory complex assembly and function. Unlike the basal lamina ensheathing the pharynx and body wall muscle, nidogen recruitment to the puncta along TRNs is not dependent upon laminin binding. MEC-4, but not laminin or nidogen, is destabilized by point mutations in the C-terminal Kunitz domain of the extracellular matrix component, MEC-1. These findings imply that somatosensory neurons secrete proteins that actively repurpose the basal lamina to generate special-purpose mechanosensory complexes responsible for vibrotactile sensing.

De novo variants in FRYL are associated with developmental delay, intellectual disability, and dysmorphic features

FRY-like transcription coactivator (FRYL) belongs to a Furry protein family that is evolutionarily conserved from yeast to humans. The functions of FRYL in mammals are largely unknown, and variants in FRYL have not previously been associated with a Mendelian disease. Here, we report fourteen individuals with heterozygous variants in FRYL who present with developmental delay, intellectual disability, dysmorphic features, and other congenital anomalies in multiple systems. The variants are confirmed de novo in all individuals except one. Human genetic data suggest that FRYL is intolerant to loss of function (LoF). We find that the fly FRYL ortholog, furry (fry), is expressed in multiple tissues, including the central nervous system where it is present in neurons but not in glia. Homozygous fry LoF mutation is lethal at various developmental stages, and loss of fry in mutant clones causes defects in wings and compound eyes. We next modeled four out of the five missense variants found in affected individuals using fry knockin alleles. One variant behaves as a severe LoF variant, whereas two others behave as partial LoF variants. One variant does not cause any observable defect in flies, and the corresponding human variant is not confirmed to be de novo, suggesting that this is a variant of uncertain significance. In summary, our findings support that fry is required for proper development in flies and that the LoF variants in FRYL cause a dominant disorder with developmental and neurological symptoms due to haploinsufficiency.

Dendrite intercalation between epidermal cells tunes nociceptor sensitivity to mechanical stimuli in Drosophila larvae

An animal's skin provides a first point of contact with the sensory environment, including noxious cues that elicit protective behavioral responses. Nociceptive somatosensory neurons densely innervate and intimately interact with epidermal cells to receive these cues, however the mechanisms by which epidermal interactions shape processing of noxious inputs is still poorly understood. Here, we identify a role for dendrite intercalation between epidermal cells in tuning sensitivity of Drosophila larvae to noxious mechanical stimuli. In wild-type larvae, dendrites of nociceptive class IV da neurons intercalate between epidermal cells at apodemes, which function as body wall muscle attachment sites, but not at other sites in the epidermis. From a genetic screen we identified miR-14 as a regulator of dendrite positioning in the epidermis: miR-14 is expressed broadly in the epidermis but not in apodemes, and miR-14 inactivation leads to excessive apical dendrite intercalation between epidermal cells. We found that miR-14 regulates expression and distribution of the epidermal Innexins ogre and Inx2 and that these epidermal gap junction proteins restrict epidermal dendrite intercalation. Finally, we found that altering the extent of epidermal dendrite intercalation had corresponding effects on nociception: increasing epidermal intercalation sensitized larvae to noxious mechanical inputs and increased mechanically evoked calcium responses in nociceptive neurons, whereas reducing epidermal dendrite intercalation had the opposite effects. Altogether, these studies identify epidermal dendrite intercalation as a mechanism for mechanical coupling of nociceptive neurons to the epidermis, with nociceptive sensitivity tuned by the extent of intercalation.

A biologically inspired repair mechanism for neuronal reconstructions with a focus on human dendrites

Investigating and modelling the functionality of human neurons remains challenging due to the technical limitations, resulting in scarce and incomplete 3D anatomical reconstructions. Here we used a morphological modelling approach based on optimal wiring to repair the parts of a dendritic morphology that were lost due to incomplete tissue samples. In Drosophila, where dendritic regrowth has been studied experimentally using laser ablation, we found that modelling the regrowth reproduced a bimodal distribution between regeneration of cut branches and invasion by neighbouring branches. Interestingly, our repair model followed growth rules similar to those for the generation of a new dendritic tree. To generalise the repair algorithm from Drosophila to mammalian neurons, we artificially sectioned reconstructed dendrites from mouse and human hippocampal pyramidal cell morphologies, and showed that the regrown dendrites were morphologically similar to the original ones. Furthermore, we were able to restore their electrophysiological functionality, as evidenced by the recovery of their firing behaviour. Importantly, we show that such repairs also apply to other neuron types including hippocampal granule cells and cerebellar Purkinje cells. We then extrapolated the repair to incomplete human CA1 pyramidal neurons, where the anatomical boundaries of the particular brain areas innervated by the neurons in question were known. Interestingly, the repair of incomplete human dendrites helped to simulate the recently observed increased synaptic thresholds for dendritic NMDA spikes in human versus mouse dendrites. To make the repair tool available to the neuroscience community, we have developed an intuitive and simple graphical user interface (GUI), which is available in the TREES toolbox (www.treestoolbox.org).

Mechanisms of gas sensing by internal sensory neurons in Drosophila larvae

Internal sensory neurons monitor the chemical and physical state of the body, providing critical information to the central nervous system for maintaining homeostasis and survival. A population of larval Drosophila sensory neurons, tracheal dendrite (td) neurons, elaborate dendrites along respiratory organs and may serve as a model for elucidating the cellular and molecular basis of chemosensation by internal neurons. We find that td neurons respond to decreases in O2 levels and increases in CO2 levels. We assessed the roles of atypical soluble guanylyl cyclases (Gycs) and a gustatory receptor (Gr) in mediating these responses. We found that Gyc88E/Gyc89Db were necessary for responses to hypoxia, and that Gr28b was necessary for responses to CO2. Targeted expression of Gr28b isoform c in td neurons rescued responses to CO2 in mutant larvae and also induced ectopic sensitivity to CO2 in the td network. Gas-sensitive td neurons were activated when larvae burrowed for a prolonged duration, demonstrating a natural-like feeding condition in which td neurons are activated. Together, our work identifies two gaseous stimuli that are detected by partially overlapping subsets of internal sensory neurons, and establishes roles for Gyc88E/Gyc89Db in the detection of hypoxia, and Gr28b in the detection of CO2.

Interaction of human keratinocytes and nerve fiber terminals at the neuro-cutaneous unit

Traditionally, peripheral sensory neurons are assumed as the exclusive transducers of external stimuli. Current research moves epidermal keratinocytes into focus as sensors and transmitters of nociceptive and non-nociceptive sensations, tightly interacting with intraepidermal nerve fibers at the neuro-cutaneous unit. In animal models, epidermal cells establish close contacts and ensheath sensory neurites. However, ultrastructural morphological and mechanistic data examining the human keratinocyte-nerve fiber interface are sparse. We investigated this exact interface in human skin applying super-resolution array tomography, expansion microscopy, and structured illumination microscopy. We show keratinocyte ensheathment of afferents and adjacent connexin 43 contacts in native skin and have applied a pipeline based on expansion microscopy to quantify these parameter in skin sections of healthy participants versus patients with small fiber neuropathy. We further derived a fully human co-culture system, visualizing ensheathment and connexin 43 plaques in vitro. Unraveling human intraepidermal nerve fiber ensheathment and potential interaction sites advances research at the neuro-cutaneous unit. These findings are crucial on the way to decipher the mechanisms of cutaneous nociception.

Topology recapitulates morphogenesis of neuronal dendrites

Branching allows neurons to make synaptic contacts with large numbers of other neurons, facilitating the high connectivity of nervous systems. Neuronal arbors have geometric properties such as branch lengths and diameters that are optimal in that they maximize signaling speeds while minimizing construction costs. In this work, we asked whether neuronal arbors have topological properties that may also optimize their growth or function. We discovered that for a wide range of invertebrate and vertebrate neurons the distributions of their subtree sizes follow power laws, implying that they are scale invariant. The power-law exponent distinguishes different neuronal cell types. Postsynaptic spines and branchlets perturb scale invariance. Through simulations, we show that the subtree-size distribution depends on the symmetry of the branching rules governing arbor growth and that optimal morphologies are scale invariant. Thus, the subtree-size distribution is a topological property that recapitulates the functional morphology of dendrites.

Coordination of Pickpocket ion channel delivery and dendrite growth in Drosophila sensory neurons

Sensory neurons enable an organism to perceive external stimuli, which is essential for survival. The sensory capacity of a neuron depends on the elaboration of its dendritic arbor and the localization of sensory ion channels to the dendritic membrane. However, it is not well understood when and how ion channels localize to growing sensory dendrites and whether their delivery is coordinated with growth of the dendritic arbor. We investigated the localization of the DEG/ENaC/ASIC ion channel Pickpocket (Ppk) in the peripheral sensory neurons of developing fruit flies. We used CRISPR-Cas9 genome engineering approaches to tag endogenous Ppk1 and visualize it live, including monitoring Ppk1 membrane localization via a novel secreted split-GFP approach. Fluorescently tagged endogenous Ppk1 localizes to dendrites, as previously reported, and, unexpectedly, to axons and axon terminals. In dendrites, Ppk1 is present throughout actively growing dendrite branches and is stably integrated into the neuronal cell membrane during the expansive growth of the arbor. Although Ppk channels are dispensable for dendrite growth, we found that an over-active channel mutant severely reduces dendrite growth, likely by acting at an internal membrane and not the dendritic membrane. Our data reveal that the molecular motor dynein and recycling endosome GTPase Rab11 are needed for the proper trafficking of Ppk1 to dendrites. Based on our data, we propose that Ppk channel transport is coordinated with dendrite morphogenesis, which ensures proper ion channel density and distribution in sensory dendrites.

Dendrite intercalation between epidermal cells tunes nociceptor sensitivity to mechanical stimuli in Drosophila larvae

An animal's skin provides a first point of contact with the sensory environment, including noxious cues that elicit protective behavioral responses. Nociceptive somatosensory neurons densely innervate and intimately interact with epidermal cells to receive these cues, however the mechanisms by which epidermal interactions shape processing of noxious inputs is still poorly understood. Here, we identify a role for dendrite intercalation between epidermal cells in tuning sensitivity of Drosophila larvae to noxious mechanical stimuli. In wild-type larvae, dendrites of nociceptive class IV da neurons intercalate between epidermal cells at apodemes, which function as body wall muscle attachment sites, but not at other sites in the epidermis. From a genetic screen we identified miR-14 as a regulator of dendrite positioning in the epidermis: miR-14 is expressed broadly in the epidermis but not in apodemes, and miR-14 inactivation leads to excessive apical dendrite intercalation between epidermal cells. We found that miR-14 regulates expression and distribution of the epidermal Innexins ogre and Inx2 and that these epidermal gap junction proteins restrict epidermal dendrite intercalation. Finally, we found that altering the extent of epidermal dendrite intercalation had corresponding effects on nociception: increasing epidermal intercalation sensitized larvae to noxious mechanical inputs and increased mechanically evoked calcium responses in nociceptive neurons, whereas reducing epidermal dendrite intercalation had the opposite effects. Altogether, these studies identify epidermal dendrite intercalation as a mechanism for mechanical coupling of nociceptive neurons to the epidermis, with nociceptive sensitivity tuned by the extent of intercalation.

Magnetic chitosan hydrogel induces neuronal differentiation of neural stem cells by activating RAS-dependent signal cascade

Our aim was to modulate magnetic cues to influence the differentiation of neural stem cell (NSC) into neuron during nerve repair and to explore corresponding mechanisms. Here, a magnetic hydrogel composed of chitosan matrices and magnetic nanoparticles (MNPs) with different content was prepared as the magnetic-stimulation platform to apply intrinsically-present magnetic cue and externally-applied magnetic field to NSC grown on the hydrogel. The MNP content had regulatory effects on neuronal differentiation and the MNPs-50 samples exhibited the best neuronal potential and appropriate biocompatibility in vitro, as well as accelerated the subsequent neuronal regeneration in vivo. Remarkably, the use of proteomics analysis parsed the underlying mechanism of magnetic cue-mediated neuronal differentiation form the perspective of protein corona and intracellular signal transduction. The intrinsically-present magnetic cues in hydrogel contributed to the activation of intracellular RAS-dependent signal cascades, thus facilitating neuronal differentiation. Magnetic cue-dependent changes in NSCs benefited from the upregulation of adsorbed proteins related to "neuronal differentiation", "cell-cell interaction", "receptor", "protein activation cascade", and "protein kinase activity" in the protein corona. Additionally, magnetic hydrogel acted cooperatively with the exterior magnetic field, showing further improving neurogenesis. The findings clarified the mechanism for magnetic cue-mediated neuronal differentiation, coupling protein corona and intracellular signal transduction.

Direction Selectivity of TmY Neurites in Drosophila

The perception of motion is an important function of vision. Neural wiring diagrams for extracting directional information have been obtained by connectome reconstruction. Direction selectivity in Drosophila is thought to originate in T4/T5 neurons through integrating inputs with different temporal filtering properties. Through genetic screening based on synaptic distribution, we isolated a new type of TmY neuron, termed TmY-ds, that form reciprocal synaptic connections with T4/T5 neurons. Its neurites responded to grating motion along the four cardinal directions and showed a variety of direction selectivity. Intriguingly, its direction selectivity originated from temporal filtering neurons rather than T4/T5. Genetic silencing and activation experiments showed that TmY-ds neurons are functionally upstream of T4/T5. Our results suggest that direction selectivity is generated in a tripartite circuit formed among these three neurons-temporal filtering, TmY-ds, and T4/T5 neurons, in which TmY-ds plays a role in the enhancement of direction selectivity in T4/T5 neurons.

Gliotransmission and adenosine signaling promote axon regeneration

How glia control axon regeneration remains incompletely understood. Here, we investigate glial regulation of regenerative ability differences of closely related Drosophila larval sensory neuron subtypes. Axotomy elicits Ca2+ signals in ensheathing glia, which activates regenerative neurons through the gliotransmitter adenosine and mounts axon regenerative programs. However, non-regenerative neurons do not respond to glial stimulation or adenosine. Such neuronal subtype-specific responses result from specific expressions of adenosine receptors in regenerative neurons. Disrupting gliotransmission impedes axon regeneration of regenerative neurons, and ectopic adenosine receptor expression in non-regenerative neurons suffices to activate regenerative programs and induce axon regeneration. Furthermore, stimulating gliotransmission or activating the mammalian ortholog of Drosophila adenosine receptors in retinal ganglion cells (RGCs) promotes axon regrowth after optic nerve crush in adult mice. Altogether, our findings demonstrate that gliotransmission orchestrates neuronal subtype-specific axon regeneration in Drosophila and suggest that targeting gliotransmission or adenosine signaling is a strategy for mammalian central nervous system repair.

Loss of NDR1/2 kinases impairs endomembrane trafficking and autophagy leading to neurodegeneration

Autophagy is essential for neuronal development and its deregulation contributes to neurodegenerative diseases. NDR1 and NDR2 are highly conserved kinases, implicated in neuronal development, mitochondrial health and autophagy, but how they affect mammalian brain development in vivo is not known. Using single and double Ndr1/2 knockout mouse models, we show that only dual loss of Ndr1/2 in neurons causes neurodegeneration. This phenotype was present when NDR kinases were deleted both during embryonic development, as well as in adult mice. Proteomic and phosphoproteomic comparisons between Ndr1/2 knockout and control brains revealed novel kinase substrates and indicated that endocytosis is significantly affected in the absence of NDR1/2. We validated the endocytic protein Raph1/Lpd1, as a novel NDR1/2 substrate, and showed that both NDR1/2 and Raph1 are critical for endocytosis and membrane recycling. In NDR1/2 knockout brains, we observed prominent accumulation of transferrin receptor, p62 and ubiquitinated proteins, indicative of a major impairment of protein homeostasis. Furthermore, the levels of LC3-positive autophagosomes were reduced in knockout neurons, implying that reduced autophagy efficiency mediates p62 accumulation and neurotoxicity. Mechanistically, pronounced mislocalisation of the transmembrane autophagy protein ATG9A at the neuronal periphery, impaired axonal ATG9A trafficking and increased ATG9A surface levels further confirm defects in membrane trafficking, and could underlie the impairment in autophagy. We provide novel insight into the roles of NDR1/2 kinases in maintaining neuronal health.

Inter-organ Wingless/Ror/Akt signaling regulates nutrient-dependent hyperarborization of somatosensory neurons

Nutrition in early life has profound effects on an organism, altering processes such as organogenesis. However, little is known about how specific nutrients affect neuronal development. Dendrites of class IV dendritic arborization neurons in Drosophila larvae become more complex when the larvae are reared on a low-yeast diet compared to a high-yeast diet. Our systematic search for key nutrients revealed that the neurons increase their dendritic terminal densities in response to a combined deficiency in vitamins, metal ions, and cholesterol. The deficiency of these nutrients upregulates Wingless in a closely located tissue, body wall muscle. Muscle-derived Wingless activates Akt in the neurons through the receptor tyrosine kinase Ror, which promotes the dendrite branching. In larval muscles, the expression of wingless is regulated not only in this key nutrient-dependent manner, but also by the JAK/STAT signaling pathway. Additionally, the low-yeast diet blunts neuronal light responsiveness and light avoidance behavior, which may help larvae optimize their survival strategies under low-nutritional conditions. Together, our studies illustrate how the availability of specific nutrients affects neuronal development through inter-organ signaling.

Developmental pruning of sensory neurites by mechanical tearing in Drosophila

Mechanical forces actively shape cells during development, but little is known about their roles during neuronal morphogenesis. Developmental neurite pruning, a critical circuit specification mechanism, often involves neurite abscission at predetermined sites by unknown mechanisms. Pruning of Drosophila sensory neuron dendrites during metamorphosis is triggered by the hormone ecdysone, which induces local disassembly of the dendritic cytoskeleton. Subsequently, dendrites are severed at positions close to the soma by an unknown mechanism. We found that ecdysone signaling causes the dendrites to become mechanically fragile. Severing occurs during periods of increased pupal morphogenetic tissue movements, which exert mechanical forces on the destabilized dendrites. Tissue movements and dendrite severing peak during pupal ecdysis, a period of strong abdominal contractions, and abolishing ecdysis causes non-cell autonomous dendrite pruning defects. Thus, our data establish mechanical tearing as a novel mechanism during neurite pruning.

Sensory axons induce epithelial lipid microdomain remodeling and determine the distribution of junctions in the epidermis

Epithelial cell properties are determined by the polarized distribution of membrane lipids, the cytoskeleton, and adhesive junctions. Epithelia are often profusely innervated, but little work has addressed how neurites affect epithelial organization. We previously found that basal keratinocytes in the zebrafish epidermis enclose axons in ensheathment channels sealed by autotypic junctions. Here we characterized how axons remodel cell membranes, the cytoskeleton, and junctions in basal keratinocytes. At the apical surface of basal keratinocytes, axons organized lipid microdomains quantitatively enriched in reporters for PI(4,5)P2 and liquid-ordered (Lo) membranes. Lipid microdomains supported the formation of cadherin-enriched, F-actin protrusions, which wrapped around axons, likely initiating ensheathment. In the absence of axons, cadherin-enriched microdomains formed on basal cells but did not organize into contiguous domains. Instead, these isolated domains formed heterotypic junctions with periderm cells, a distinct epithelial cell type. Thus, axon endings dramatically remodel polarized epithelial components and regulate epidermal adhesion.

Shear stress activates nociceptors to drive Drosophila mechanical nociception

Mechanical nociception is essential for animal survival. However, the forces involved in nociceptor activation and the underlying mechanotransduction mechanisms remain elusive. Here, we address these problems by investigating nocifensive behavior in Drosophila larvae. We show that strong poking stimulates nociceptors with a mixture of forces including shear stress and stretch. Unexpectedly, nociceptors are selectively activated by shear stress, but not stretch. Both the shear stress responses of nociceptors and nocifensive behavior require transient receptor potential A1 (TrpA1), which is specifically expressed in nociceptors. We further demonstrate that expression of mammalian or Drosophila TrpA1 in heterologous cells confers responses to shear stress but not stretch. Finally, shear stress activates TrpA1 in a membrane-delimited manner, through modulation of membrane fluidity. Together, our study reveals TrpA1 as an evolutionarily conserved mechanosensitive channel specifically activated by shear stress and suggests a critical role of shear stress in activating nociceptors to drive mechanical nociception.

Cellular Pathogenesis of Chemotherapy-Induced Peripheral Neuropathy: Insights From Drosophila and Human-Engineered Skin Models

Chemotherapy-induced peripheral neuropathy (CIPN) is a highly prevalent and complex condition arising from chemotherapy cancer treatments. Currently, there are no treatment or prevention options in the clinic. CIPN accompanies pain-related sensory functions starting from the hands and feet. Studies focusing on neurons in vitro and in vivo models significantly advanced our understanding of CIPN pathological mechanisms. However, given the direct toxicity shown in both neurons and non-neuronal cells, effective in vivo or in vitro models that allow the investigation of neurons in their local environment are required. No single model can provide a complete solution for the required investigation, therefore, utilizing a multi-model approach would allow complementary advantages of different models and robustly validate findings before further translation. This review aims first to summarize approaches and insights from CIPN in vivo models utilizing small model organisms. We will focus on Drosophila melanogaster CIPN models that are genetically amenable and accessible to study neuronal interactions with the local environment in vivo. Second, we will discuss how these findings could be tested in physiologically relevant vertebrate models. We will focus on in vitro approaches using human cells and summarize the current understanding of engineering approaches that may allow the investigation of pathological changes in neurons and the skin environment.

Dynamic instability of dendrite tips generates the highly branched morphologies of sensory neurons

The highly ramified arbors of neuronal dendrites provide the substrate for the high connectivity and computational power of the brain. Altered dendritic morphology is associated with neuronal diseases. Many molecules have been shown to play crucial roles in shaping and maintaining dendrite morphology. However, the underlying principles by which molecular interactions generate branched morphologies are not understood. To elucidate these principles, we visualized the growth of dendrites throughout larval development of Drosophila sensory neurons and found that the tips of dendrites undergo dynamic instability, transitioning rapidly and stochastically between growing, shrinking, and paused states. By incorporating these measured dynamics into an agent-based computational model, we showed that the complex and highly variable dendritic morphologies of these cells are a consequence of the stochastic dynamics of their dendrite tips. These principles may generalize to branching of other neuronal cell types, as well as to branching at the subcellular and tissue levels.

The Drosophila fragile X mental retardation protein modulates the neuronal cytoskeleton to limit dendritic arborization

Dendritic arbor development is a complex, highly regulated process. Post-transcriptional regulation mediated by RNA-binding proteins plays an important role in neuronal dendrite morphogenesis by delivering on-site, on-demand protein synthesis. Here, we show how the Drosophila fragile X mental retardation protein (FMRP), a conserved RNA-binding protein, limits dendrite branching to ensure proper neuronal function during larval sensory neuron development. FMRP knockdown causes increased dendritic terminal branch growth and a resulting overelaboration defect due, in part, to altered microtubule stability and dynamics. FMRP also controls dendrite outgrowth by regulating the Drosophila profilin homolog chickadee (chic). FMRP colocalizes with chic mRNA in dendritic granules and regulates its dendritic localization and protein expression. Whereas RNA-binding domains KH1 and KH2 are both crucial for FMRP-mediated dendritic regulation, KH2 specifically is required for FMRP granule formation and chic mRNA association, suggesting a link between dendritic FMRP granules and FMRP function in dendrite elaboration. Our studies implicate FMRP-mediated modulation of both the neuronal microtubule and actin cytoskeletons in multidendritic neuronal architecture, and provide molecular insight into FMRP granule formation and its relevance to FMRP function in dendritic patterning.

Spatiotemporal Control of Neuronal Remodeling by Cell Adhesion Molecules: Insights From Drosophila

Developmental neuronal remodeling is required for shaping the precise connectivity of the mature nervous system. Remodeling involves pruning of exuberant neural connections, often followed by regrowth of adult-specific ones, as a strategy to refine neural circuits. Errors in remodeling are associated with neurodevelopmental disorders such as schizophrenia and autism. Despite its fundamental nature, our understanding of the mechanisms governing neuronal remodeling is far from complete. Specifically, how precise spatiotemporal control of remodeling and rewiring is achieved is largely unknown. In recent years, cell adhesion molecules (CAMs), and other cell surface and secreted proteins of various families, have been implicated in processes of neurite pruning and wiring specificity during circuit reassembly. Here, we review some of the known as well as speculated roles of CAMs in these processes, highlighting recent advances in uncovering spatiotemporal aspects of regulation. Our focus is on the fruit fly Drosophila, which is emerging as a powerful model in the field, due to the extensive, well-characterized and stereotypic remodeling events occurring throughout its nervous system during metamorphosis, combined with the wide and constantly growing toolkit to identify CAM binding and resulting cellular interactions in vivo. We believe that its many advantages pose Drosophila as a leading candidate for future breakthroughs in the field of neuronal remodeling in general, and spatiotemporal control by CAMs specifically.

Nociception and hypersensitivity involve distinct neurons and molecular transducers in Drosophila

SignificanceFunctional plasticity of the nociceptive circuit is a remarkable feature and is of clinical relevance. As an example, nociceptors lower their threshold upon tissue injury, a process known as allodynia that would facilitate healing by guarding the injured areas. However, long-lasting hypersensitivity could lead to chronic pain, a debilitating disease not effectively treated. Therefore, it is crucial to dissect the mechanisms underlying basal nociception and nociceptive hypersensitivity. In both vertebrate and invertebrate species, conserved transient receptor potential (Trp) channels are the primary transducers of noxious stimuli. Here, we provide a precedent that in Drosophila larvae, heat sensing in the nociception and hypersensitivity states is mediated by distinct heat-sensitive neurons and TrpA1 alternative isoforms.

Neuron-epidermal attachment protects hyper-fragile axons from mechanical strain

Axons experience significant strain caused by organismal development and movement. A combination of intrinsic mechanical resistance and external shielding by surrounding tissues prevents axonal damage, although the precise mechanisms are unknown. Here, we reveal a neuroprotective function of neuron-epidermal attachment in Caenorhabditis elegans. We show that a gain-of-function mutation in the epidermal hemidesmosome component LET-805/myotactin, in combination with a loss-of-function mutation in UNC-70/β-spectrin, disrupts the uniform attachment and subsequent embedment of sensory axons within the epidermis during development. This generates regions of high tension within axons, leading to spontaneous axonal breaks and degeneration. Completely preventing attachment, by disrupting HIM-4/hemicentin or MEC-5/collagen, eliminates tension and alleviates damage. Finally, we demonstrate that progressive neuron-epidermal attachment via LET-805/myotactin is induced by the axon during development, as well as during regeneration after injury. Together, these results reveal that establishment of uniform neuron-epidermal attachment is critical to protect axons from mechanical strain during development.

Molecular mechanisms regulating the spatial configuration of neurites

Precise neural networks, composed of axons and dendrites, are the structural basis for information processing in the brain. Therefore, the correct formation of neurites is critical for accurate neural function. In particular, the three-dimensional structures of dendrites vary greatly among neuron types, and the unique shape of each dendrite is tightly linked to specific synaptic connections with innervating axons and is correlated with its information processing. Although many systems are involved in neurite formation, the developmental mechanisms that control the orientation, size, and arborization pattern of neurites definitively defines their three-dimensional structure in tissues. In this review, we summarize these regulatory mechanisms that establish proper spatial configurations of neurites, especially dendrites, in invertebrates and vertebrates.

Finding Needles in a Haystack with Light: Resolving the Microcircuitry of the Brain with Fluorescence Microscopy

To understand the microcircuitry of the brain, the anatomical and functional connectivity among neurons must be resolved. One of the technical hurdles to achieving this goal is that the anatomical connections, or synapses, are often smaller than the diffraction limit of light and thus are difficult to resolve by conventional microscopy, while the microcircuitry of the brain is on the scale of 1 mm or larger. To date, the gold standard method for microcircuit reconstruction has been electron microscopy (EM). However, despite its rapid development, EM has clear shortcomings as a method for microcircuit reconstruction. The greatest weakness of this method is arguably its incompatibility with functional and molecular analysis. Fluorescence microscopy, on the other hand, is readily compatible with numerous physiological and molecular analyses. We believe that recent advances in various fluorescence microscopy techniques offer a new possibility for reliable synapse detection in large volumes of neural circuits. In this minireview, we summarize recent advances in fluorescence-based microcircuit reconstruction. In the same vein as these studies, we introduce our recent efforts to analyze the long-range connectivity among brain areas and the subcellular distribution of synapses of interest in relatively large volumes of cortical tissue with array tomography and superresolution microscopy.

The elegance of prickly sensations

Neurons sensing harmful mechanical forces in the larvae of fruit flies have a striking architecture of dendrites that are optimized to detect pointy objects.

The Immunoglobulin Superfamily Member Basigin Is Required for Complex Dendrite Formation in Drosophila

Coordination of dendrite growth with changes in the surrounding substrate occurs widely in the nervous system and is vital for establishing and maintaining neural circuits. However, the molecular basis of this important developmental process remains poorly understood. To identify potential mediators of neuron-substrate interactions important for dendrite morphogenesis, we undertook an expression pattern-based screen in Drosophila larvae, which revealed many proteins with expression in dendritic arborization (da) sensory neurons and in neurons and their epidermal substrate. We found that reporters for Basigin, a cell surface molecule of the immunoglobulin (Ig) superfamily previously implicated in cell-cell and cell-substrate interactions, are expressed in da sensory neurons and epidermis. Loss of Basigin in da neurons led to defects in morphogenesis of the complex dendrites of class IV da neurons. Classes of sensory neurons with simpler branching patterns were unaffected by loss of Basigin. Structure-function analyses showed that a juxtamembrane KRR motif is critical for this function. Furthermore, knock down of Basigin in the epidermis led to defects in dendrite elaboration of class IV neurons, suggesting a non-autonomous role. Together, our findings support a role for Basigin in complex dendrite morphogenesis and interactions between dendrites and the adjacent epidermis.

Tetris in the Nervous System: What Principles of Neuronal Tiling Can Tell Us About How Glia Play the Game

The precise organization and arrangement of neural cells is essential for nervous system functionality. Cellular tiling is an evolutionarily conserved phenomenon that organizes neural cells, ensuring non-redundant coverage of receptive fields in the nervous system. First recorded in the drawings of Ramon y Cajal more than a century ago, we now have extensive knowledge of the biochemical and molecular mechanisms that mediate tiling of neurons. The advent of live imaging techniques in both invertebrate and vertebrate model organisms has enhanced our understanding of these processes. Despite advancements in our understanding of neuronal tiling, we know relatively little about how glia, an essential non-neuronal component of the nervous system, tile and contribute to the overall spatial arrangement of the nervous system. Here, we discuss lessons learned from neurons and apply them to potential mechanisms that glial cells may use to tile, including cell diversity, contact-dependent repulsion, and chemical signaling. We also discuss open questions in the field of tiling and what new technologies need to be developed in order to better understand glial tiling.

The narrowing of dendrite branches across nodes follows a well-defined scaling law

The systematic variation of diameters in branched networks has tantalized biologists since the discovery of da Vinci's rule for trees. Da Vinci's rule can be formulated as a power law with exponent two: The square of the mother branch's diameter is equal to the sum of the squares of those of the daughters. Power laws, with different exponents, have been proposed for branching in circulatory systems (Murray's law with exponent 3) and in neurons (Rall's law with exponent 3/2). The laws have been derived theoretically, based on optimality arguments, but, for the most part, have not been tested rigorously. Using superresolution methods to measure the diameters of dendrites in highly branched Drosophila class IV sensory neurons, we have found that these types of power laws do not hold. In their place, we have discovered a different diameter-scaling law: The cross-sectional area is proportional to the number of dendrite tips supported by the branch plus a constant, corresponding to a minimum diameter of the terminal dendrites. The area proportionality accords with a requirement for microtubules to transport materials and nutrients for dendrite tip growth. The minimum diameter may be set by the force, on the order of a few piconewtons, required to bend membrane into the highly curved surfaces of terminal dendrites. Because the observed scaling differs from Rall's law, we propose that cell biological constraints, such as intracellular transport and protrusive forces generated by the cytoskeleton, are important in determining the branched morphology of these cells.

Transparent Touch: Insights From Model Systems on Epidermal Control of Somatosensory Innervation

Somatosensory neurons (SSNs) densely innervate our largest organ, the skin, and shape our experience of the world, mediating responses to sensory stimuli including touch, pressure, and temperature. Historically, epidermal contributions to somatosensation, including roles in shaping innervation patterns and responses to sensory stimuli, have been understudied. However, recent work demonstrates that epidermal signals dictate patterns of SSN skin innervation through a variety of mechanisms including targeting afferents to the epidermis, providing instructive cues for branching morphogenesis, growth control and structural stability of neurites, and facilitating neurite-neurite interactions. Here, we focus onstudies conducted in worms (Caenorhabditis elegans), fruit flies (Drosophila melanogaster), and zebrafish (Danio rerio): prominent model systems in which anatomical and genetic analyses have defined fundamental principles by which epidermal cells govern SSN development.

Brain-specific lipoprotein receptors interact with astrocyte derived apolipoprotein and mediate neuron-glia lipid shuttling

Lipid shuttling between neurons and glia contributes to the development, function, and stress responses of the nervous system. To understand how a neuron acquires its lipid supply from specific lipoproteins and their receptors, we perform combined genetic, transcriptome, and biochemical analyses in the developing Drosophila larval brain. Here we report, the astrocyte-derived secreted lipocalin Glial Lazarillo (GLaz), a homolog of human Apolipoprotein D (APOD), and its neuronal receptor, the brain-specific short isoforms of Drosophila lipophorin receptor 1 (LpR1-short), cooperatively mediate neuron-glia lipid shuttling and support dendrite morphogenesis. The isoform specificity of LpR1 defines its distribution, binding partners, and ability to support proper dendrite growth and synaptic connectivity. By demonstrating physical and functional interactions between GLaz/APOD and LpR1, we elucidate molecular pathways mediating lipid trafficking in the fly brain, and provide in vivo evidence indicating isoform-specific expression of lipoprotein receptors as a key mechanism for regulating cell-type specific lipid recruitment.

Upgraded CRISPR/Cas9 tools for tissue-specific mutagenesis in Drosophila

CRISPR/Cas9 has emerged as a powerful technology for tissue-specific mutagenesis. However, tissue-specific CRISPR/Cas9 tools currently available in Drosophila remain deficient in three significant ways. First, many existing gRNAs are inefficient, such that further improvements of gRNA expression constructs are needed for more efficient and predictable mutagenesis in both somatic and germline tissues. Second, it has been difficult to label mutant cells in target tissues with current methods. Lastly, application of tissue-specific mutagenesis at present often relies on Gal4-driven Cas9, which hampers the flexibility and effectiveness of the system. Here, we tackle these deficiencies by building upon our previous CRISPR-mediated tissue-restricted mutagenesis (CRISPR-TRiM) tools. First, we significantly improved gRNA efficiency in somatic tissues by optimizing multiplexed gRNA design. Similarly, we also designed efficient dual-gRNA vectors for the germline. Second, we developed methods to positively and negatively label mutant cells in tissue-specific mutagenesis by incorporating co-CRISPR reporters into gRNA expression vectors. Lastly, we generated genetic reagents for convenient conversion of existing Gal4 drivers into tissue-specific Cas9 lines based on homology-assisted CRISPR knock-in. In this way, we expand the choices of Cas9 for CRISPR-TRiM analysis to broader tissues and developmental stages. Overall, our upgraded CRISPR/Cas9 tools make tissue-specific mutagenesis more versatile, reliable, and effective in Drosophila These improvements may be also applied to other model systems.

Integrins protect sensory neurons in models of paclitaxel-induced peripheral sensory neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a major side effect from cancer treatment with no known method for prevention or cure in clinics. CIPN often affects unmyelinated nociceptive sensory terminals. Despite the high prevalence, molecular and cellular mechanisms that lead to CIPN are still poorly understood. Here, we used a genetically tractable Drosophila model and primary sensory neurons isolated from adult mouse to examine the mechanisms underlying CIPN and identify protective pathways. We found that chronic treatment of Drosophila larvae with paclitaxel caused degeneration and altered the branching pattern of nociceptive neurons, and reduced thermal nociceptive responses. We further found that nociceptive neuron-specific overexpression of integrins, which are known to support neuronal maintenance in several systems, conferred protection from paclitaxel-induced cellular and behavioral phenotypes. Live imaging and superresolution approaches provide evidence that paclitaxel treatment causes cellular changes that are consistent with alterations in endosome-mediated trafficking of integrins. Paclitaxel-induced changes in recycling endosomes precede morphological degeneration of nociceptive neuron arbors, which could be prevented by integrin overexpression. We used primary dorsal root ganglia (DRG) neuron cultures to test conservation of integrin-mediated protection. We show that transduction of a human integrin β-subunit 1 also prevented degeneration following paclitaxel treatment. Furthermore, endogenous levels of surface integrins were decreased in paclitaxel-treated mouse DRG neurons, suggesting that paclitaxel disrupts recycling in vertebrate sensory neurons. Altogether, our study supports conserved mechanisms of paclitaxel-induced perturbation of integrin trafficking and a therapeutic potential of restoring neuronal interactions with the extracellular environment to antagonize paclitaxel-induced toxicity in sensory neurons.

Furry is required for cell movements during gastrulation and functionally interacts with NDR1

Gastrulation is a key event in animal embryogenesis during which germ layer precursors are rearranged and the embryonic axes are established. Cell polarization is essential during gastrulation, driving asymmetric cell division, cell movements, and cell shape changes. The furry (fry) gene encodes an evolutionarily conserved protein with a wide variety of cellular functions, including cell polarization and morphogenesis in invertebrates. However, little is known about its function in vertebrate development. Here, we show that in Xenopus, Fry plays a role in morphogenetic processes during gastrulation, in addition to its previously described function in the regulation of dorsal mesoderm gene expression. Using morpholino knock-down, we demonstrate a distinct role for Fry in blastopore closure and dorsal axis elongation. Loss of Fry function drastically affects the movement and morphological polarization of cells during gastrulation and disrupts dorsal mesoderm convergent extension, responsible for head-to-tail elongation. Finally, we evaluate a functional interaction between Fry and NDR1 kinase, providing evidence of an evolutionarily conserved complex required for morphogenesis.

Molecular mechanisms that mediate dendrite morphogenesis

Neurons develop dendritic morphologies that bear cell type-specific features in dendritic field size and geometry, branch placement and density, and the types and distributions of synaptic contacts. Dendritic patterns influence the types and numbers of inputs a neuron receives, and the ways in which neural information is processed and transmitted in the circuitry. Even subtle alterations in dendritic structures can have profound consequences on neuronal function and are implicated in neurodevelopmental disorders. In this chapter, I review how growing dendrites acquire their exquisite patterns by drawing examples from diverse neuronal cell types in vertebrate and invertebrate model systems. Dendrite morphogenesis is shaped by intrinsic and extrinsic factors such as transcriptional regulators, guidance and adhesion molecules, neighboring cells and synaptic partners. I discuss molecular mechanisms that regulate dendrite morphogenesis with a focus on five aspects of dendrite patterning: (1) Dendritic cytoskeleton and cellular machineries that build the arbor; (2) Gene regulatory mechanisms; (3) Afferent cues that regulate dendritic arbor growth; (4) Space-filling strategies that optimize dendritic coverage; and (5) Molecular cues that specify dendrite wiring. Cell type-specific implementation of these patterning mechanisms produces the diversity of dendrite morphologies that wire the nervous system.

Extrinsic Factors Regulating Dendritic Patterning

Stereotypic dendrite arborizations are key morphological features of neuronal identity, as the size, shape and location of dendritic trees determine the synaptic input fields and how information is integrated within developed neural circuits. In this review, we focus on the actions of extrinsic intercellular communication factors and their effects on intrinsic developmental processes that lead to dendrite patterning. Surrounding neurons or supporting cells express adhesion receptors and secreted proteins that respectively, act via direct contact or over short distances to shape, size, and localize dendrites during specific developmental stages. The different ligand-receptor interactions and downstream signaling events appear to direct dendrite morphogenesis by converging on two categorical mechanisms: local cytoskeletal and adhesion modulation and global transcriptional regulation of key dendritic growth components, such as lipid synthesis enzymes. Recent work has begun to uncover how the coordinated signaling of multiple extrinsic factors promotes complexity in dendritic trees and ensures robust dendritic patterning.

Versatile CRISPR/Cas9-mediated mosaic analysis by gRNA-induced crossing-over for unmodified genomes

Mosaic animals have provided the platform for many fundamental discoveries in developmental biology, cell biology, and other fields. Techniques to produce mosaic animals by mitotic recombination have been extensively developed in Drosophila melanogaster but are less common for other laboratory organisms. Here, we report mosaic analysis by gRNA-induced crossing-over (MAGIC), a new technique for generating mosaic animals based on DNA double-strand breaks produced by CRISPR/Cas9. MAGIC efficiently produces mosaic clones in both somatic tissues and the germline of Drosophila. Further, by developing a MAGIC toolkit for 1 chromosome arm, we demonstrate the method's application in characterizing gene function in neural development and in generating fluorescently marked clones in wild-derived Drosophila strains. Eliminating the need to introduce recombinase-recognition sites into the genome, this simple and versatile system simplifies mosaic analysis in Drosophila and can in principle be applied in any organism that is compatible with CRISPR/Cas9.

The ELAV/Hu protein Found in neurons regulates cytoskeletal and ECM adhesion inputs for space-filling dendrite growth

Dendritic arbor morphology influences how neurons receive and integrate extracellular signals. We show that the ELAV/Hu family RNA-binding protein Found in neurons (Fne) is required for space-filling dendrite growth to generate highly branched arbors of Drosophila larval class IV dendritic arborization neurons. Dendrites of fne mutant neurons are shorter and more dynamic than in wild-type, leading to decreased arbor coverage. These defects result from both a decrease in stable microtubules and loss of dendrite-substrate interactions within the arbor. Identification of transcripts encoding cytoskeletal regulators and cell-cell and cell-ECM interacting proteins as Fne targets using TRIBE further supports these results. Analysis of one target, encoding the cell adhesion protein Basigin, indicates that the cytoskeletal defects contributing to branch instability in fne mutant neurons are due in part to decreased Basigin expression. The ability of Fne to coordinately regulate the cytoskeleton and dendrite-substrate interactions in neurons may shed light on the behavior of cancer cells ectopically expressing ELAV/Hu proteins.

Different types of neurons have different sizes and shapes that are tailored to their particular functions. In the fruit fly larva, a set of sensory neurons called class IV da neurons have highly branched trees of dendrites that cover the epidermis to sense potentially harmful stimuli. Neurons whose dendrites completely fill the territory they sample are also found in zebrafish, worms, mice and humans. We show that an RNA-binding protein called Fne plays an important role in coordinating different contributions to dendrite branching in class IV da neurons by impacting the organization of the cytoskeleton within the neuron and the ability of the dendrite to contact the substratum outside of it. The identification of mRNAs that code for cytoskeleton regulators and adhesive proteins as targets of Fne using a genome-wide approach further supports these results. While the ability of Fne to exert control over such proteins is crucial to generating the space-filling growth of neurons, it can be deleterious if not properly employed, such as when the homologs of Fne are expressed in cancer cells.

βIII spectrin controls the planarity of Purkinje cell dendrites by modulating perpendicular axon-dendrite interactions

The mechanism underlying the geometrical patterning of axon and dendrite wiring remains elusive, despite its crucial importance in the formation of functional neural circuits. The cerebellar Purkinje cell (PC) arborizes a typical planar dendrite, which forms an orthogonal network with granule cell (GC) axons. By using electrospun nanofiber substrates, we reproduce the perpendicular contacts between PC dendrites and GC axons in culture. In the model system, PC dendrites show a preference to grow perpendicularly to aligned GC axons, which presumably contribute to the planar dendrite arborization in vivo We show that βIII spectrin, a causal protein for spinocerebellar ataxia type 5, is required for the biased growth of dendrites. βIII spectrin deficiency causes actin mislocalization and excessive microtubule invasion in dendritic protrusions, resulting in abnormally oriented branch formation. Furthermore, disease-associated mutations affect the ability of βIII spectrin to control dendrite orientation. These data indicate that βIII spectrin organizes the mouse dendritic cytoskeleton and thereby regulates the oriented growth of dendrites with respect to the afferent axons.

Loss of Pseudouridine Synthases in the RluA Family Causes Hypersensitive Nociception in Drosophila

Nociceptive neurons of Drosophila melanogaster larvae are characterized by highly branched dendritic processes whose proper morphogenesis relies on a large number of RNA-binding proteins. Post-transcriptional regulation of RNA in these dendrites has been found to play an important role in their function. Here, we investigate the neuronal functions of two putative RNA modification genes, RluA-1 and RluA-2, which are predicted to encode pseudouridine synthases. RluA-1 is specifically expressed in larval sensory neurons while RluA-2 expression is ubiquitous. Nociceptor-specific RNAi knockdown of RluA-1 caused hypersensitive nociception phenotypes, which were recapitulated with genetic null alleles. These were rescued with genomic duplication and nociceptor-specific expression of UAS- RluA-1 -cDNA As with RluA-1, RluA-2 loss of function mutants also displayed hyperalgesia. Interestingly, nociceptor neuron dendrites showed a hyperbranched morphology in the RluA-1 mutants. The latter may be a cause or a consequence of heightened sensitivity in mutant nociception behaviors.

Achieving functional neuronal dendrite structure through sequential stochastic growth and retraction

Class I ventral posterior dendritic arborisation (c1vpda) proprioceptive sensory neurons respond to contractions in the Drosophila larval body wall during crawling. Their dendritic branches run along the direction of contraction, possibly a functional requirement to maximise membrane curvature during crawling contractions. Although the molecular machinery of dendritic patterning in c1vpda has been extensively studied, the process leading to the precise elaboration of their comb-like shapes remains elusive. Here, to link dendrite shape with its proprioceptive role, we performed long-term, non-invasive, in vivo time-lapse imaging of c1vpda embryonic and larval morphogenesis to reveal a sequence of differentiation stages. We combined computer models and dendritic branch dynamics tracking to propose that distinct sequential phases of stochastic growth and retraction achieve efficient dendritic trees both in terms of wire and function. Our study shows how dendrite growth balances structure-function requirements, shedding new light on general principles of self-organisation in functionally specialised dendrites.

Deterministic and Stochastic Rules of Branching Govern Dendrite Morphogenesis of Sensory Neurons

Dendrite morphology is necessary for the correct integration of inputs that neurons receive. The branching mechanisms allowing neurons to acquire their type-specific morphology remain unclear. Classically, axon and dendrite patterns were shown to be guided by molecules, providing deterministic cues. However, the extent to which deterministic and stochastic mechanisms, based upon purely statistical bias, contribute to the emergence of dendrite shape is largely unknown. We address this issue using the Drosophila class I vpda multi-dendritic neurons. Detailed quantitative analysis of vpda dendrite morphogenesis indicates that the primary branch grows very robustly in a fixed direction, though secondary branch numbers and lengths showed fluctuations characteristic of stochastic systems. Live-tracking dendrites and computational modeling revealed how neuron shape emerges from few local statistical parameters of branch dynamics. We report key opposing aspects of how tree architecture feedbacks on the local probability of branch shrinkage. Child branches promote stabilization of parent branches, although self-repulsion promotes shrinkage. Finally, we show that self-repulsion, mediated by the adhesion molecule Dscam1, indirectly patterns the growth of secondary branches by spatially restricting their direction of stable growth perpendicular to the primary branch. Thus, the stochastic nature of secondary branch dynamics and the existence of geometric feedback emphasize the importance of self-organization in neuronal dendrite morphogenesis.

An Efficient Screen for Cell-Intrinsic Factors Identifies the Chaperonin CCT and Multiple Conserved Mechanisms as Mediating Dendrite Morphogenesis

Dendritic morphology is inextricably linked to neuronal function. Systematic large-scale screens combined with genetic mapping have uncovered several mechanisms underlying dendrite morphogenesis. However, a comprehensive overview of participating molecular mechanisms is still lacking. Here, we conducted an efficient clonal screen using a collection of mapped P-element insertions that were previously shown to cause lethality and eye defects in Drosophila melanogaster. Of 280 mutants, 52 exhibited dendritic defects. Further database analyses, complementation tests, and RNA interference validations verified 40 P-element insertion genes as being responsible for the dendritic defects. Twenty-eight mutants presented severe arbor reduction, and the remainder displayed other abnormalities. The intrinsic regulators encoded by the identified genes participate in multiple conserved mechanisms and pathways, including the protein folding machinery and the chaperonin-containing TCP-1 (CCT) complex that facilitates tubulin folding. Mutant neurons in which expression of CCT4 or CCT5 was depleted exhibited severely retarded dendrite growth. We show that CCT localizes in dendrites and is required for dendritic microtubule organization and tubulin stability, suggesting that CCT-mediated tubulin folding occurs locally within dendrites. Our study also reveals novel mechanisms underlying dendrite morphogenesis. For example, we show that Drosophila Nogo signaling is required for dendrite development and that Mummy and Wech also regulate dendrite morphogenesis, potentially via Dpp- and integrin-independent pathways. Our methodology represents an efficient strategy for identifying intrinsic dendrite regulators, and provides insights into the plethora of molecular mechanisms underlying dendrite morphogenesis.