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[
International Worm Meeting,
2015]
A connectome is a comprehensive map of all neural connections in an organism's nervous system. The first connectome was published almost 30 years ago by White et al. (1986) and described the structure of the nervous system of the nematode C. elegans adult hermaphrodite. Subsequent network analyses of this data have focused only on the synaptic connectivity of the nervous system, while neglecting much of the spatial information in the data. Initial spatial analyses of the C. elegans connectome reported in (White et al., 1983; Durbin, 1987) used only a sparse sampling of physical neuron contacts. Using the original electron micrographs from (White et al., 1986), we have extended this analysis by performing a 3D reconstruction of every neuron in the C. elegans nerve ring in both the L4 and adult. This represents the first complete volumetric reconstruction of the main neuropil of any animal from multiple developmental stages. With this enriched data set, we have been able to do a comparative analysis of synaptic connectivity, characterize the spatial distribution of synapses for each neuron and analyse the relationship between neuron contact and synapse formation in the C. elegans nerve ring. Similar to (White et al., 1983), we found that ~40% of all possible physical contacts result in a synapse or gap junction. We also found a positive correlation between the frequency of synapse formation and the amount of physical contact between neurons. Specifically, the frequency of synapse formation between two neurons approaches ~0.7 as the amount of physical contact approaches 10% of a neuron's total measured surface area. However, like (Durbin, 1987), we find that synapse probability and synapse number between any pair neurons does not depend strongly on the amount of shared physical contact. Furthermore, synapse volumes appear to be conserved between the L4 and adult, while the number of synapses between any two neurons appear to be, on average, greater in the adult. This could suggest that during late nervous system development, synaptic partnerships are reinforced by creating additional small synapses between neurons rather than enlarging the volume of current synapses. .
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Moyle, Mark, Bao, Zhirong, Fan, Li, Brittin, Christopher, Christensen, Ryan, Barnes, Kristopher, Santella, Anthony, Mohler, William, Colon-Ramos, Daniel, Kolotuev, Irina, Shroff, Hari
[
International Worm Meeting,
2021]
Neuropil formation requires structural organization in order to support information processing. While the organization of the C. elegans main neuropil (nerve ring) has been thoroughly mapped in the larva and adult, how these organizational features emerge from early (pre-neurite) collective cell behaviors is poorly understood. Prior to neurite outgrowth, we find that head neurons and surrounding cells collectively form a ring of multicellular rosettes that organize the topographic neighborhoods of the nerve ring. Neurite bundles that correspond directly to future topographic neighborhoods grow from rosette centers, travel along the ring on "bridge" cells that are simultaneously engaged in adjacent rosettes, and assemble into a scaffold that physically links adjacent topographic neighborhoods. Laser ablation of rosette cells leads to delays in nerve ring closure, while laser ablation of bridge cells leads to ectopic neurite growth between rosettes. Furthermore, we find that SAX-3/Robo is required for proper rosette formation and neurite outgrowth from rosette centers. Our results reveal how pre-neurite collective cell behaviors support complex neuropil patterning.
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[
International Worm Meeting,
2015]
Previous studies of real-world networks have suggested that networks motifs, defined as connectivity patterns that are significantly over-represented when compared to random networks with the same degree distribution, may arise due to evolutionary design principles and serve as computational units. For neural networks and other spatially embedded systems, where network connections form between physically proximate nodes, this approach carries a risk of overstating the statistical significance of connectivity patterns. Previous studies have attributed a number of network motifs to the C. elegans neuronal network and suggested that the worm's nervous system may be constructed from these computational modules (Milo et al., 2002; Reigl et al. 2004). However, other groups have conjectured that the high frequency of observed connectivity patterns may simply be a consequence of the organization and localized connectivity of the neuropile (White et al., 1983; Artzy-Randrup et al. 2004). To test these two hypotheses, we measured the spatial aggregation of neurons in the nematode C. elegans and used the data to construct a statistical model with a spatially constrained null-hypothesis. We found that a number of motifs, including the 3-node feed forward loop, are no longer over-represented in our spatially constrained model. Thus, the observed network structure in the C. elegans nervous system may simply be the consequence of how the neuropile is organized.
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[
International Worm Meeting,
2021]
Animal nervous system organization relies on features at every scale, from nano-level localization of synapses, through neuronal morphologies, to the high-level stereotyped connections between different regions of the brain. Previously, we developed methodologies for determining and characterizing multi-scale brain features from reconstructed serial sectioned electron micrographs (EM) of the L4 and adult nerve rings1. To obtain a development timeline of the nerve ring, we now integrate our analysis with results that we obtain from new EM datasets for the L1, L2, L3 and adult2. We find that membrane contacts between neurites are well described by a conserved core, embedded in a sea of variable contacts. We present a parsimonious model that consistently predicts that about 28-33% of conserved membrane contacts are actively targeted for synapse formation with high precision (≈93%), while the significant variability across datasets is accounted for by a non-negligible basal synaptic contact rate (≈20-30%), across all these developmental stages. Thus, while the numbers of membrane and synaptic contacts increases with age, our model predicts that the tendency to make synaptic contacts remains relatively constant. By clustering membrane contacts on each of the animals, we found a nerve ring organization of five spatial neighborhoods that supports a similarly modular information processing synaptic circuit. Extending this analysis over development, we find that the structural and synaptic modularity of the nerve ring is robust across all developmental stages, indicating that the macro-structure of the nerve ring is rooted in embryonic development (see accompanying abstract for a postulated mapping between this structure of the nerve ring and collective cell behaviors in the embryo3). We further present a developing brain map (a complete single cell resolution synaptic map of the C. elegans nerve ring) and use it to highlight key features of the post-embryonic development of the nerve ring. Our network analysis of the brain map points to a combination of individuality and robustness of brain organization that likely scale to larger nervous systems. 1. Brittin et al., (2021) Nature. https://doi.org/10.1038/s41586-021-03284-x 2. Witvliet et al. (2020), biorxiv. https://doi.org/10.1101/2020.04.30.066209 3. Brittin et al. (2021). Multicellular rosettes organize neuropil formation. [C. elegans conference abstract]
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[
Worm Breeder's Gazette,
1994]
THE MATERNAL GENE SKN-4 AND THE SPECIFICATION OF VENTRAL BLASTOMERE FATES IN THE EARLY C. ELEGANS EMBRYO Bruce Bowerman, Paula R. Martin, Christopher J. Thorpe, and Christopher A. Shelton. The Institute of Molecular Biology, University of Oregon, Eugene, OR 97403.
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[
Worm Breeder's Gazette,
2012]
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[
International Worm Meeting,
2005]
California Institute of Technology and University of California, San Diego have each developed practical, automated movement analysis systems to quantify aspects of worm behavior and morphology (see An imaging system for standardized quantitative analysis of C. elegans behavior, Zhaoyang Feng, et al, BMC Bioinformatics 2004, 5:115, An automated system for measuring parameters of nematode sinusoidal movement, Christopher J Cronin, et al, BMC Genetics 2005, 6:5, and previous meeting abstracts). We have consolidated our development efforts and merged our independent system designs to create a single, unified behavior analysis system. The combined system provides researchers with all of the quantification tools and database functionality of the individual UCSD and Caltech systems, but also establishes a common, open software and hardware foundation for continued development by Caltech, UCSD and the community. Current efforts include expanding system capabilities for the automated analysis of mating behavior with the challenge of tracking multiple moving individuals, both in contact with and separated from each other.
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Nguyen, Kenneth, Bloniarz, Adam E., Brittin, Christopher A., Cook, Steven J., Hall, David H., Emmons, Scott W., Xu, Meng, Jarrell, Travis A., Wang, Yi
[
International Worm Meeting,
2013]
The innate behavioral repertoires of the two sexes of a species are guided by differing reproductive priorities. C. elegans male copulation is controlled by a neural network in the tail in which a majority of the neurons and muscles are specific to the male. But known differences in olfactory preferences and exploratory tendencies emanate from behaviors controlled by circuits in the head, where the complement of neurons is nearly identical in the two sexes. We determined connectivity in the anterior nervous system of the adult male from a 1,500 section-long thin section EM series extending from near the tip of the nose, through the nerve ring, and part way into the retrovesicular ganglion. This region contains the bulk of the synapses, excluding ventral cord nmj's. To make a comparison to the hermaphrodite, we re-reconstructed legacy Cambridge micrographs using our software, which allows us to score synaptic weights (see abstract by Cook et al). While our analysis is at an early stage, we can already see the essential result: in the adjacency matrices that display the connectivity, it is difficult to spot differences that appear greater than would be expected given the inherent variability of neuronal wiring. Known circuits in navigation and other responses are conserved. Thus behavioral differences likely emerge from differing circuit properties rather than differing connectivity. There are two possible exceptions: AIM synapses onto AIB and RIA synapses onto RIB in the male only. One set of male-specific synapses expected involves the male-specific head CEM sensory neurons, and the tail EF interneurons, which receive extensive input from the copulatory circuits in the tail and extend processes through the ventral nerve cord into the nerve ring. Both of these neuron classes have as their strongest targets the AVB command interneurons for forward locomotion. This suggests one of their functions may be to inhibit forward locomotion when a hermaphrodite is sensed or during copulation. They make additional connections to be further explored.
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[
International Worm Meeting,
2013]
Suppressors of TDP-1 toxicity in Caenorhadbitis elegans Ilana Pena-Gonzalez1 and Christopher D Link1, 2 1 Integrative Physiology, University of Colorado, Boulder, CO 80309, USA 2 Institute for Behavioral Genetics, University of Colorado, Boulder, CO 80309, USA RNA binding protein TDP-43 forms damaging aggregates in multiple neurodegenerative diseases. We have found that the deletion of
tdp-1, the C. elegans ortholog of TDP-43, leads to increased accumulation of double stranded RNA. TDP-1 is not believed to bind to dsRNA itself; however the marked increase in dsRNA accumulation in worms deleted for
tdp-1 implies TDP-1 normally participates in limiting the stability and structure of dsRNA. Although worms deleted for TDP-1 do not have a severe phenotype, overexpressed nuclear TDP-1 is toxic in worms. One possible explanation for this is that too much TDP-1 leads to excessive disruption of structured RNAs needed for normal RNA metabolism. Making use of this overexpression model we have established a heat shock induction system to help identify components associated with TDP-1's prevention of dsRNA accumulation. Identifying mutations that suppress the neurotoxic effects of overexpressed TDP-1 could help identify others factors contributing to the complex mechanism of disassembling dsRNA. A mutagenesis screen has identified four mutant strains that partially suppress the toxicity of overexpressed TDP-1. We have also tested suppressors of other toxic proteins. Further identifying the pieces of the mechanism regulating dsRNA breakdown could help better explain the pathogenic pathway of TDP-1 and consequently TDP-43 in disease.
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Bulow, Hannes E., Brittin, Christopher A., Nguyen, Ken C.Q., Hall, David H., Tang, Leo T.-H., Jarrell, Travis A., Cook, Steven J., Yakovlev, Maksim, Emmons, Scott W., Wang, Yi, Hobert, Oliver, Bayer, Emily A.
[
International Worm Meeting,
2017]
We present the first whole-animal maps of synaptic connectivity, including anatomical connection strength, of both adult sexes of a species. Our results are based on analyses of legacy and new serial section electron micrographs (EMs) of the C. elegans nervous system and the tissues it innervates. In a graph representation of connectivity, the pathways of information flow can be arranged hierarchically, revealing a largely feed forward structure of shallow (1-5 synapses) depth. Our reconstruction has revealed that muscles and other end-organs are more extensively cross-connected than previously reported. Sensory information converges and diverges widely throughout the fully-connected neural network. The sexes differ not only by the addition of sex-specific neurons and muscles, but also at numerous points in the connectivity of shared neurons. Differences between the hermaphrodite and male reconstructions could be either inter-individual differences or differences due to genetic sex. To distinguish between these possibilities, we examined a subset of 7 synaptic connections that were respectively stronger in the male reconstruction, 4 that were stronger in the hermaphrodite reconstruction, and 4 that were similar, using in vivo trans-synaptic labeling. In each instance, the difference seen in the reconstructions was confirmed in multiple animals. Extrapolating these results to the number of statistically significant differences in the reconstructions, we conclude that there is an unexpectedly large number of sexually dimorphic connections. These connections were mainly located in the nerve ring, and embedded within the connectome at least one synapse away from any sex-specific neuron. Our results showed that AVA receives sex-specific input from ADL, ASH, and AVF in the hermaphrodite, while RIB receives sex-specific input from IL1, IL2, and RIA in the male. AIM, which has been reported to change its neurotransmitter from glutamate in the hermaphrodite to acetylcholine in the male, makes a strong male-specific connection to AIB. These hubs of sex-specific connectivity also maintained the majority of their sex-shared output. Our results suggest that the genetic sex of the nervous system allows for diverse synaptic patterns in a relatively small nervous system.