The development of neuronal morphology in insects

Comparative connectomics and escape behavior in larvae of closely related Drosophila species

Evolution has generated an enormous variety of morphological, physiological, and behavioral traits in animals. How do behaviors evolve in different directions in species equipped with similar neurons and molecular components? Here we adopted a comparative approach to investigate the similarities and differences of escape behaviors in response to noxious stimuli and their underlying neural circuits between closely related drosophilid species. Drosophilids show a wide range of escape behaviors in response to noxious cues, including escape crawling, stopping, head casting, and rolling. Here we find that D. santomea, compared with its close relative D. melanogaster, shows a higher probability of rolling in response to noxious stimulation. To assess whether this behavioral difference could be attributed to differences in neural circuitry, we generated focused ion beam-scanning electron microscope volumes of the ventral nerve cord of D. santomea to reconstruct the downstream partners of mdIV, a nociceptive sensory neuron in D. melanogaster. Along with partner interneurons of mdVI (including Basin-2, a multisensory integration neuron necessary for rolling) previously identified in D. melanogaster, we identified two additional partners of mdVI in D. santomea. Finally, we showed that joint activation of one of the partners (Basin-1) and a common partner (Basin-2) in D. melanogaster increased rolling probability, suggesting that the high rolling probability in D. santomea is mediated by the additional activation of Basin-1 by mdIV. These results provide a plausible mechanistic explanation for how closely related species exhibit quantitative differences in the likelihood of expressing the same behavior.

Insights into nanoparticles-induced neurotoxicity and cope up strategies

Nanoparticle applications are becoming increasingly popular in fields such as photonics, catalysis, magnetics, biotechnology, manufacturing of cosmetics, pharmaceuticals, and medicines. There is still a huge pile of undermining information about the potential toxicity of these products to humans, which can be encountered by neuroprotective antioxidants and anti-inflammatory compounds. Nanoparticles can be administered using a variety of methods, including oronasal, topical applications, and enteral and parenteral routes of administration. There are different properties of these nanomaterials that characterize different pathways. Crossing of the blood-brain barrier, a direct sensory nerve-to-brain pathway whose barriers are bypassed, these checks otherwise prevent the nanoparticles from entering the brain. This inflicts damage to sensory neurons and receptors by nanoparticles that lead to neurotoxicity of the central nervous system. A number of routes make nanoparticles able to penetrate through the skin. Exposure by various routes to these nanoparticles can result in oxidative stress, and immune suppression triggers inflammatory cascades and genome-level mutations after they are introduced into the body. To out-power, these complications, plant-based antioxidants, essential oils, and dietary supplements can be put into use. Direct nanoparticle transport pathways from sensory nerves to the brain via blood have been studied grossly. Recent findings regarding the direct pathways through which nanoparticles cross the blood-brain barriers, how nanoparticles elicit different responses on sensory receptors and nerves, how they cause central neurotoxicity and neurodegeneration through sensory nerve routes, and the possible mechanisms that outcast these effects are discussed.

EGFR-dependent suppression of synaptic autophagy is required for neuronal circuit development

The development of neuronal connectivity requires stabilization of dynamic axonal branches at sites of synapse formation. Models that explain how axonal branching is coupled to synaptogenesis postulate molecular regulators acting in a spatiotemporally restricted fashion to ensure branching toward future synaptic partners while also stabilizing the emerging synaptic contacts between such partners. We investigated this question using neuronal circuit development in the Drosophila brain as a model system. We report that epidermal growth factor receptor (EGFR) activity is required in presynaptic axonal branches during two distinct temporal intervals to regulate circuit wiring in the developing Drosophila visual system. EGFR is required early to regulate primary axonal branching. EGFR activity is then independently required at a later stage to prevent degradation of the synaptic active zone protein Bruchpilot (Brp). Inactivation of EGFR results in a local increase of autophagy in presynaptic branches and the translocation of active zone proteins into autophagic vesicles. The protection of synaptic material during this later interval of wiring ensures the stabilization of terminal branches, circuit connectivity, and appropriate visual behavior. Phenotypes of EGFR inactivation can be rescued by increasing Brp levels or downregulating autophagy. In summary, we identify a temporally restricted molecular mechanism required for coupling axonal branching and synaptic stabilization that contributes to the emergence of neuronal wiring specificity.

Binocular responsiveness of projection neurons of the praying mantis optic lobe in the frontal visual field

Praying mantids are the only insects proven to have stereoscopic vision (stereopsis): the ability to perceive depth from the slightly shifted images seen by the two eyes. Recently, the first neurons likely to be involved in mantis stereopsis were described and a speculative neuronal circuit suggested. Here we further investigate classes of neurons in the lobula complex of the praying mantis brain and their tuning to stereoscopically-defined depth. We used sharp electrode recordings with tracer injections to identify visual projection neurons with input in the optic lobe and output in the central brain. In order to measure binocular response fields of the cells the animals watched a vertical bar stimulus in a 3D insect cinema during recordings. We describe the binocular tuning of 19 neurons projecting from the lobula complex and the medulla to central brain areas. The majority of neurons (12/19) were binocular and had receptive fields for both eyes that overlapped in the frontal region. Thus, these neurons could be involved in mantis stereopsis. We also find that neurons preferring different contrast polarity (bright vs dark) tend to be segregated in the mantis lobula complex, reminiscent of the segregation for small targets and widefield motion in mantids and other insects.

A Segmentation Scheme for Complex Neuronal Arbors and Application to Vibration Sensitive Neurons in the Honeybee Brain

The morphology of a neuron is strongly related to its physiological properties, application of logical product and thus to information processing functions. Optical microscope images are widely used for extracting the structure of neurons. Although several approaches have been proposed to trace and extract complex neuronal structures from microscopy images, available methods remain prone to errors. In this study, we present a practical scheme for processing confocal microscope images and reconstructing neuronal structures. We evaluated this scheme using image data samples and associated “gold standard” reconstructions from the BigNeuron Project. In these samples, dendritic arbors belonging to multiple projection branches of the same neuron overlapped in space, making it difficult to automatically and accurately trace their structural connectivity. Our proposed scheme, which combines several software tools for image masking and filtering with an existing tool for dendritic segmentation and tracing, outperformed state-of-the-art automatic methods in reconstructing such neuron structures. For evaluating our scheme, we applied it to a honeybee local interneuron, DL-Int-1, which has complex arbors and is considered to be a critical neuron for encoding the distance information indicated in the waggle dance of the honeybee.

Post-molting development of wind-elicited escape behavior in the cricket

Arthropods including insects grow through several developmental stages by molting. The abrupt changes in their body size and morphology accompanying the molting are responsible for the developmental changes in behavior. While in holometabolous insects, larval behaviors are transformed into adult-specific behaviors with drastic changes in nervous system during the pupal stage, hemimetabolous insects preserve most innate behaviors whole life long, which allow us to trace the maturation process of preserved behaviors after the changes in body. Wind-elicited escape behavior is one of these behaviors and mediated by cercal system, which is a mechanosensory organ equipped by all stages of nymph in orthopteran insects like crickets. However, the maturation process of the escape behavior after the molt is unclear. In this study, we examined time-series of changes in the wind-elicited escape behavior just after the imaginal molt in the cricket. The locomotor activities are developed over the elapsed time, and matured 24h after the molt. In contrast, a stimulus-angle dependency of moving direction was unchanged over time, meaning that the cercal sensory system detecting airflow direction was workable immediately after the molt, independent from the behavioral maturation. The post-molting development of the wind-elicited behavior was considered to result not simply from maturation of the exoskeleton or musculature because the escape response to heat-shock stimulus did not change after the molt. No effect of a temporal immobilization after the imaginal molt on the maturation of the wind-elicited behavior also implies that the maturation may be innately programmed without experience of locomotion.

The Krüppel-Like Factor Dar1 Determines Multipolar Neuron Morphology

Neurons typically assume multipolar, bipolar, or unipolar morphologies. Little is known about the mechanisms underlying the development of these basic morphological types. Here, we show that the Krüppel-like transcription factor Dar1 determines the multipolar morphology of postmitotic neurons in Drosophila. Dar1 is specifically expressed in multipolar neurons and loss of dar1 gradually converts multipolar neurons into the bipolar or unipolar morphology without changing neuronal identity. Conversely, misexpression of Dar1 or its mammalian homolog in unipolar and bipolar neurons causes them to assume multipolar morphologies. Dar1 regulates the expression of several dynein genes and nuclear distribution protein C (nudC), which is an essential component of a specialized dynein complex that positions the nucleus in a cell. We further show that these genes are required for Dar1-induced multipolar neuron morphology. Dar1 likely functions as a terminal selector gene for the basic layout of neuron morphology by regulating both dendrite extension and the dendrite-nucleus coupling.

SIGNIFICANCE STATEMENT

The three basic morphological types of neurons--unipolar, bipolar, and multipolar--are important for information processing and wiring of neural circuits. Little progress has been made toward understanding the molecular and cellular programs that generate these types since their discovery over a century ago. It is generally assumed that basic morphological types of neurons are determined by the number of dendrites growing out from the cell body. Here, we show that this model alone is insufficient. We introduce the positioning of nucleus as a critical factor in this process and report that the transcription factor Dar1 determines multipolar neuron morphology in postmitotic neurons by regulating genes involved in nuclear positioning.

Context-aware modeling of neuronal morphologies

Neuronal morphologies are pivotal for brain functioning: physical overlap between dendrites and axons constrain the circuit topology, and the precise shape and composition of dendrites determine the integration of inputs to produce an output signal. At the same time, morphologies are highly diverse and variant. The variance, presumably, originates from neurons developing in a densely packed brain substrate where they interact (e.g., repulsion or attraction) with other actors in this substrate. However, when studying neurons their context is never part of the analysis and they are treated as if they existed in isolation. Here we argue that to fully understand neuronal morphology and its variance it is important to consider neurons in relation to each other and to other actors in the surrounding brain substrate, i.e., their context. We propose a context-aware computational framework, NeuroMaC, in which large numbers of neurons can be grown simultaneously according to growth rules expressed in terms of interactions between the developing neuron and the surrounding brain substrate. As a proof of principle, we demonstrate that by using NeuroMaC we can generate accurate virtual morphologies of distinct classes both in isolation and as part of neuronal forests. Accuracy is validated against population statistics of experimentally reconstructed morphologies. We show that context-aware generation of neurons can explain characteristics of variation. Indeed, plausible variation is an inherent property of the morphologies generated by context-aware rules. We speculate about the applicability of this framework to investigate morphologies and circuits, to classify healthy and pathological morphologies, and to generate large quantities of morphologies for large-scale modeling.

Cux1 and Cux2 selectively target basal and apical dendritic compartments of layer II-III cortical neurons

A number of recent reports implicate the differential regulation of apical and basal dendrites in autism disorders and in the higher functions of the human brain. They show that apical and basal dendrites are functionally specialized and that mechanisms regulating their development have important consequences for neuron function. The molecular identity of layer II-III neurons of the cerebral cortex is determined by the overlapping expression of Cux1 and Cux2. We previously showed that both Cux1 and Cux2 are necessary and nonredundant for normal dendrite development of layer II-III neurons. Loss of function of either gene reduced dendrite arbors, while overexpression increased dendritic complexity and suggested additive functions. We herein characterize the function of Cux1 and Cux2 in the development of apical and basal dendrites. By in vivo loss and gain of function analysis, we show that while the expression level of either Cux1 or Cux2 influences both apical and basal dendrites, they have distinct effects. Changes in Cux1 result in a marked effect on the development of the basal compartment whereas modulation of Cux2 has a stronger influence on the apical compartment. These distinct effects of Cux genes might account for the functional diversification of layer II-III neurons into different subpopulations, possibly with distinct connectivity patterns and modes of neuron response. Our data suggest that by their differential effects on basal and apical dendrites, Cux1 and Cux2 can promote the integration of layer II-III neurons in the intracortical networks in highly specific ways.

Dramatic expansion of the black widow toxin arsenal uncovered by multi-tissue transcriptomics and venom proteomics

Background

Animal venoms attract enormous interest given their potential for pharmacological discovery and understanding the evolution of natural chemistries. Next-generation transcriptomics and proteomics provide unparalleled, but underexploited, capabilities for venom characterization. We combined multi-tissue RNA-Seq with mass spectrometry and bioinformatic analyses to determine venom gland specific transcripts and venom proteins from the Western black widow spider (Latrodectus hesperus) and investigated their evolution.

Results

We estimated expression of 97,217 L. hesperus transcripts in venom glands relative to silk and cephalothorax tissues. We identified 695 venom gland specific transcripts (VSTs), many of which BLAST and GO term analyses indicate may function as toxins or their delivery agents. ~38% of VSTs had BLAST hits, including latrotoxins, inhibitor cystine knot toxins, CRISPs, hyaluronidases, chitinase, and proteases, and 59% of VSTs had predicted protein domains. Latrotoxins are venom toxins that cause massive neurotransmitter release from vertebrate or invertebrate neurons. We discovered ≥ 20 divergent latrotoxin paralogs expressed in L. hesperus venom glands, significantly increasing this biomedically important family. Mass spectrometry of L. hesperus venom identified 49 proteins from VSTs, 24 of which BLAST to toxins. Phylogenetic analyses showed venom gland specific gene family expansions and shifts in tissue expression.

Conclusions

Quantitative expression analyses comparing multiple tissues are necessary to identify venom gland specific transcripts. We present a black widow venom specific exome that uncovers a trove of diverse toxins and associated proteins, suggesting a dynamic evolutionary history. This justifies a reevaluation of the functional activities of black widow venom in light of its emerging complexity.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-366) contains supplementary material, which is available to authorized users.

BIOCAT: a pattern recognition platform for customizable biological image classification and annotation

Background

Pattern recognition algorithms are useful in bioimage informatics applications such as quantifying cellular and subcellular objects, annotating gene expressions, and classifying phenotypes. To provide effective and efficient image classification and annotation for the ever-increasing microscopic images, it is desirable to have tools that can combine and compare various algorithms, and build customizable solution for different biological problems. However, current tools often offer a limited solution in generating user-friendly and extensible tools for annotating higher dimensional images that correspond to multiple complicated categories.

Results

We develop the BIOimage Classification and Annotation Tool (BIOCAT). It is able to apply pattern recognition algorithms to two- and three-dimensional biological image sets as well as regions of interest (ROIs) in individual images for automatic classification and annotation. We also propose a 3D anisotropic wavelet feature extractor for extracting textural features from 3D images with xy-z resolution disparity. The extractor is one of the about 20 built-in algorithms of feature extractors, selectors and classifiers in BIOCAT. The algorithms are modularized so that they can be “chained” in a customizable way to form adaptive solution for various problems, and the plugin-based extensibility gives the tool an open architecture to incorporate future algorithms. We have applied BIOCAT to classification and annotation of images and ROIs of different properties with applications in cell biology and neuroscience.

Conclusions

BIOCAT provides a user-friendly, portable platform for pattern recognition based biological image classification of two- and three- dimensional images and ROIs. We show, via diverse case studies, that different algorithms and their combinations have different suitability for various problems. The customizability of BIOCAT is thus expected to be useful for providing effective and efficient solutions for a variety of biological problems involving image classification and annotation. We also demonstrate the effectiveness of 3D anisotropic wavelet in classifying both 3D image sets and ROIs.

An instruction language for self-construction in the context of neural networks

Biological systems are based on an entirely different concept of construction than human artifacts. They construct themselves by a process of self-organization that is a systematic spatio-temporal generation of, and interaction between, various specialized cell types. We propose a framework for designing gene-like codes for guiding the self-construction of neural networks. The description of neural development is formalized by defining a set of primitive actions taken locally by neural precursors during corticogenesis. These primitives can be combined into networks of instructions similar to biochemical pathways, capable of reproducing complex developmental sequences in a biologically plausible way. Moreover, the conditional activation and deactivation of these instruction networks can also be controlled by these primitives, allowing for the design of a "genetic code" containing both coding and regulating elements. We demonstrate in a simulation of physical cell development how this code can be incorporated into a single progenitor, which then by replication and differentiation, reproduces important aspects of corticogenesis.

Neuronal polarity in Drosophila: sorting out axons and dendrites

Drosophila neurons have identifiable axons and dendrites based on cell shape, but it is only just starting to become clear how Drosophila neurons are polarized at the molecular level. Dendrite-specific components including the Golgi complex, GABA receptors, neurotransmitter receptor scaffolding proteins, and cell adhesion molecules have been described. Proteins involved in constructing presynaptic specializations are concentrated in axons of some neurons. A very simple model for how these components are distributed to axons and dendrites can be constructed based on the opposite polarity of microtubules in axons and dendrites: dynein carries cargo into dendrites, and kinesins carry cargo into axons. The simple model works well for multipolar neurons, but will likely need refinement for unipolar neurons, which are common in Drosophila.

C. elegans bicd-1, homolog of the Drosophila dynein accessory factor Bicaudal D, regulates the branching of PVD sensory neuron dendrites

The establishment of cell type-specific dendritic arborization patterns is a key phase in the assembly of neuronal circuitry that facilitates the integration and processing of synaptic and sensory input. Although studies in Drosophila and vertebrate systems have identified a variety of factors that regulate dendrite branch formation, the molecular mechanisms that control this process remain poorly defined. Here, we introduce the use of the Caenorhabditis elegans PVD neurons, a pair of putative nociceptors that elaborate complex dendritic arbors, as a tractable model for conducting high-throughput RNAi screens aimed at identifying key regulators of dendritic branch formation. By carrying out two separate RNAi screens, a small-scale candidate-based screen and a large-scale screen of the ~3000 genes on chromosome IV, we retrieved 11 genes that either promote or suppress the formation of PVD-associated dendrites. We present a detailed functional characterization of one of the genes, bicd-1, which encodes a microtubule-associated protein previously shown to modulate the transport of mRNAs and organelles in a variety of organisms. Specifically, we describe a novel role for bicd-1 in regulating dendrite branch formation and show that bicd-1 is likely to be expressed, and primarily required, in PVD neurons to control dendritic branching. We also present evidence that bicd-1 operates in a conserved pathway with dhc-1 and unc-116, components of the dynein minus-end-directed and kinesin-1 plus-end-directed microtubule-based motor complexes, respectively, and interacts genetically with the repulsive guidance receptor unc-5.

Crossveinless-c, the Drosophila homolog of tumor suppressor DLC1, regulates directional elongation of dendritic branches via down-regulating Rho1 activity

Diverse neuronal subtypes develop distinctive morphologies of dendritic arbors that receive synaptic or sensory inputs. Dendritic arbors of many subtypes take on a polarized shape, and one underlying mechanism is unidirectionally biased elongation of dendritic branches. As reported herein, we found that Drosophila Crossveinless-c (Cv-c) was a key regulator for such directional growth. In the cv-c mutant, two subclass of multidendritic sensory neurons examined formed dorsally directed branches; however, dendritic branches had difficulty in growing along the anterior-posterior (A-P) body axis. Cv-c belongs to the family of Rho GTPase-activating proteins (RhoGAPs) and is the homolog of human tumor suppressor DLC1. The RhoGAP activity of Cv-c was required cell-autonomously for the A-P-oriented growth, and Cv-c elevated the GTPase activity of Rho1 and Cdc42 in a cell-free assay. Our analysis of genetic interactions suggested that Rho1 was the target of Cv-c in vivo. All of our results suggest that Cv-c contributes to sprouting and subsequent growth of the A-P-oriented branches through negative regulation of Rho1. We discuss a role of Cv-c in dendritic growth in response to environmental cues.

Establishment of axon-dendrite polarity in developing neurons

Neurons are among the most highly polarized cell types in the body, and the polarization of axon and dendrites underlies the ability of neurons to integrate and transmit information in the brain. Significant progress has been made in the identification of the cellular and molecular mechanisms underlying the establishment of neuronal polarity using primarily in vitro approaches such as dissociated culture of rodent hippocampal and cortical neurons. This model has led to the predominant view suggesting that neuronal polarization is specified largely by stochastic, asymmetric activation of intracellular signaling pathways. Recent evidence shows that extracellular cues can play an instructive role during neuronal polarization in vitro and in vivo. In this review, we synthesize the recent data supporting an integrative model whereby extracellular cues orchestrate the intracellular signaling underlying the initial break of neuronal symmetry leading to axon-dendrite polarization.

The microtubule network and neuronal morphogenesis: Dynamic and coordinated orchestration through multiple players

Nervous system function and plasticity rely on the complex architecture of neuronal networks elaborated during development, when neurons acquire their specific and complex shape. During neuronal morphogenesis, the formation and outgrowth of functionally and structurally distinct axons and dendrites require a coordinated and dynamic reorganization of the microtubule cytoskeleton involving numerous regulators. While most of these factors act directly on microtubules to stabilize them or promote their assembly, depolymerization or fragmentation, others are now emerging as essential regulators of neuronal differentiation by controlling tubulin availability and modulating microtubule dynamics. In this review, we recapitulate how the microtubule network is actively regulated during the successive phases of neuronal morphogenesis, and what are the specific roles of the various microtubule-regulating proteins in that process. We then describe the specific signaling pathways and inter-regulations that coordinate the different activities of these proteins to sustain neuronal development in response to environmental cues.

Identification of genes influencing dendrite morphogenesis in developing peripheral sensory and central motor neurons

Background

Developing neurons form dendritic trees with cell type-specific patterns of growth, branching and targeting. Dendrites of Drosophila peripheral sensory neurons have emerged as a premier genetic model, though the molecular mechanisms that underlie and regulate their morphogenesis remain incompletely understood. Still less is known about this process in central neurons and the extent to which central and peripheral dendrites share common organisational principles and molecular features. To address these issues, we have carried out two comparable gain-of-function screens for genes that influence dendrite morphologies in peripheral dendritic arborisation (da) neurons and central RP2 motor neurons.

Results

We found 35 unique loci that influenced da neuron dendrites, including five previously shown as required for da dendrite patterning. Several phenotypes were class-specific and many resembled those of known mutants, suggesting that genes identified in this study may converge with and extend known molecular pathways for dendrite development in da neurons. The second screen used a novel technique for cell-autonomous gene misexpression in RP2 motor neurons. We found 51 unique loci affecting RP2 dendrite morphology, 84% expressed in the central nervous system. The phenotypic classes from both screens demonstrate that gene misexpression can affect specific aspects of dendritic development, such as growth, branching and targeting. We demonstrate that these processes are genetically separable. Targeting phenotypes were specific to the RP2 screen, and we propose that dendrites in the central nervous system are targeted to territories defined by Cartesian co-ordinates along the antero-posterior and the medio-lateral axes of the central neuropile. Comparisons between the screens suggest that the dendrites of peripheral da and central RP2 neurons are shaped by regulatory programs that only partially overlap. We focused on one common candidate pathway controlled by the ecdysone receptor, and found that it promotes branching and growth of developing da neuron dendrites, but a role in RP2 dendrite development during embryonic and early larval stages was not apparent.

Conclusion

We identified commonalities (for example, growth and branching) and distinctions (for example, targeting and ecdysone response) in the molecular and organizational framework that underlies dendrite development of peripheral and central neurons.

Selective expression of Knot/Collier, a transcriptional regulator of the EBF/Olf-1 family, endows the Drosophila sensory system with neuronal class-specific elaborated dendritic patterns

Dendritic tree morphology is a hallmark of cellular diversity in the nervous system, and Drosophila dendritic arborization (da) neurons provide an excellent model system to study its molecular basis. The da neurons are classified into four classes I-IV in the order of increasing branching complexity. A transcriptional regulator of the early B-cell factor (EBF)/olfactory 1 (Olf-1) family, Knot (Kn)/Collier (Col) is expressed selectively in class IV neurons, which generate the most expansive and complicated dendritic trees in the four classes. Loss of kn function in class IV neurons greatly reduced the number of their dendritic branches. Conversely mis-expression of kn in classes I and II produced supernumerary higher-order branches, whereas class III-specific short and straight terminal branches was hardly formed by kn mis-expression. Neither kn loss of function nor mis-expression were associated with dramatic alterations in the expression patterns of two other transcriptional regulators, Abrupt (Ab) and Cut (Ct), which play important roles in shaping dendritic trees with distinct class specificity from Kn. In contrast, Kn was necessary and sufficient to drive expression of a gene that encodes a class IV-specific channel protein. Collectively, all of our results suggest that Kn exerts its cell-autonomous function to control the formation, and possibly the function, of class IV-like elaborated dendritic arbors.

Homophilic Dscam interactions control complex dendrite morphogenesis

Alternative splicing of the Drosophila gene Dscam results in up to 38,016 different receptor isoforms proposed to interact by isoform-specific homophilic binding. We report that Dscam controls cell-intrinsic aspects of dendrite guidance in all four classes of dendrite arborization (da) neurons. Loss of Dscam in single neurons causes a strong increase in self-crossing. Restriction of dendritic fields of neighboring class III neurons appeared intact in mutant neurons, suggesting that dendritic self-avoidance, but not heteroneuronal tiling, may depend on Dscam. Overexpression of the same Dscam isoforms in two da neurons with overlapping dendritic fields forced a spatial segregation of the two fields, supporting the model that dendritic branches of da neurons use isoform-specific homophilic interactions to ensure minimal overlap. Homophilic binding of the highly diverse extracellular domains of Dscam may therefore limit the use of the same "core" repulsion mechanism to cell-intrinsic interactions without interfering with heteroneuronal interactions.

Transcription factor Sp4 regulates dendritic patterning during cerebellar maturation

Integration of inputs by a neuron depends on dendritic arborization patterns. In mammals, the genetic programs that regulate dynamic remodeling of dendrites during development and in response to activity are incompletely understood. Here we report that knockdown of the transcription factor Sp4 led to an increased number of highly branched dendrites during maturation of cerebellar granule neurons in dissociated cultures and in cerebellar cortex. Time-course analysis revealed that depletion of Sp4 led to persistent generation of dendritic branches and a failure in resorption of transient dendrites. Depolarization induced a reduction in the number of dendrites, and knockdown of Sp4 blocked depolarization-induced remodeling. Furthermore, overexpression of Sp4 wild type, but not a mutant lacking the DNA-binding domain, was sufficient to promote dendritic pruning in nondepolarizing conditions. These findings indicate that the transcription factor Sp4 controls dendritic patterning during cerebellar development by limiting branch formation and promoting activity-dependent pruning.

Neuronal morphology and neuropil structure in the stomatogastric ganglion of the lobster, Homarus americanus

The stomatogastric nervous system (STNS) has long been used as a model system for the study of central pattern generation, neuromodulation, and network dynamics. Anatomical studies of the crustacean stomatogastric ganglion (STG) in different species have mostly been restricted to subsets of neurons and/or general structural features. For the first time, we describe the morphology of all STG neurons belonging to the two circuits that produce the well-described pyloric and gastric rhythms in the lobster, Homarus americanus. Somata sit on the dorsal and lateral surface of the STG and send a single primary neurite into the core of the neuropil, which is mostly made up of larger lower order branches. The perimeter of the neuropil consists mostly of finer higher order branches. Immunohistochemical labeling for synaptic proteins is associated with the small diameter branches. Somata positions are not constant but show preferred locations across individuals. The number of copies is constant for all neuron types except the PY and GM neurons (PY neuron number ranges from 3 to 7, and GM neuron number ranges from 6 to 9). Branch structure is largely nondichotomous, and branches can deviate substantially from cylindrical shape. Diameter changes at branch points can be as large as 20-fold. Clearly, the morphology of a specific neuron type can be quite variable from animal to animal.

Evolution and development of neural circuits in invertebrates

Developmental mechanisms can shed light on how evolutionary diversity has arisen. Invertebrate nervous systems offer a wealth of diverse structures and functions from which to relate development to evolution. Individual homologous neurons have been shown to have distinct roles in species with different behaviors. In addition, specific neurons have been lost or gained in some phylogenetic lineages. The ability to address the neural basis of behavior at the cellular level in invertebrates has facilitated discoveries showing that species-specific behavior can arise from differences in synaptic strength, in neuronal structure and in neuromodulation. The mechanisms involved in the development of neural circuits lead to these differences across species.

Got diversity? Wiring the fly brain with Dscam

The Drosophila gene Dscam, encoding Down syndrome cell-adhesion molecule, is required for the development of neural circuits. Alternative splicing of Dscam mRNA potentially generates 38016 isoforms of a cell-surface recognition protein of the immunoglobulin superfamily. These isoforms include 19008 different ectodomains joined to one of two alternative transmembrane segments. Each ectodomain comprises a unique combination of three variable immunoglobulin domains. Biochemical studies support a model in which each isoform preferentially binds to the same isoform on opposing cell surfaces. This homophilic binding requires matching at all three variable immunoglobulin domains. These findings raise the intriguing possibility that specificity of binding by the Dscam isoforms mediates cell-surface recognition events required for wiring the fly brain.

Development of Drosophila motoneurons: specification and morphology

In the Drosophila ventral nerve cord, segmentally repeated sets of approximately 80 motoneurons are generated during embryogenesis. Within each hemisegment, each motoneuron is characterised by its axonal projection and innervation of a particular target muscle as well as its dendritic tree in the central nervous system. Codes of transcriptional regulators appear to specify in a hierarchical fashion the cell type, motoneuron sub-types and eventually unique cellular identities. Recent studies show that patterns of connectivity in the periphery are mirrored by patterns of dendritic arborisation centrally thereby providing a neuronal correlate of connectivity to the anatomy of the motor system in the periphery. While the principal mechanisms that underlie the development of the peripheral neuromuscular system have been studied in some detail, much less is known about how the dendrites and their patterns of connections develop in the CNS.

Control of dendritic diversity

The dendritic trees of different neuronal types display an astonishing diversity in structure and function. How this diversity is generated remains incompletely understood. However, recent studies have revealed some of the underlying mechanisms by which intrinsic programs of cell-type specification and extrinsic factors exert their effects on the dendritic cytoskeleton to regulate patterns of growth and branching.