Comparative connectomics of two distantly related nematode species reveals patterns of nervous system evolution.

Understanding the evolution of the bilaterian brain requires a detailed exploration of the precise nature of cellular and subcellular differences between related species. We undertook an electron micrographic reconstruction of the brain of the predatory nematode and compared the results with the brain of , which diverged at least 100 million years ago. We revealed changes in neuronal cell death, neuronal cell position, axodendritic projection patterns, and synaptic connectivity of homologous neurons that display no obvious changes in overall neurite morphology and projection patterns.

Evolution of lateralized gustation in nematodes.

Animals with small nervous systems have a limited number of sensory neurons that must encode information from a changing environment. This problem is particularly exacerbated in nematodes that populate a wide variety of distinct ecological niches but only have a few sensory neurons available to encode multiple modalities. How does sensory diversity prevail within this constraint in neuron number? To identify the genetic basis for patterning different nervous systems, we demonstrate that sensory neurons in respond to various salt sensory cues in a manner that is partially distinct from that of the distantly related nematode .

Whole-brain chemosensory responses of both sexes.

Sexually-dimorphic neural circuits play a critical role in shaping sex-specific animal behaviors. Maps of the structural dimorphisms in these circuits have been explored by analyzing "synaptic connectomes", electron micrograph reconstructions of synaptic connectivity. Nevertheless, recent studies in the model organism have shown little to no correlation between the synaptic connectome and dynamic neural activity.

Evolution of lateralized gustation in nematodes.

Animals with small nervous systems have a limited number of sensory neurons that must encode information from a changing environment. This problem is particularly exacerbated in nematodes that populate a wide variety of distinct ecological niches but only have a few sensory neurons available to encode multiple modalities. How does sensory diversity prevail within this constraint in neuron number? To identify the genetic basis for patterning different nervous systems, we demonstrate that sensory neurons in respond to various salt sensory cues in a manner that is partially distinct from that of the distantly related nematode .

Anatomical restructuring of a lateralized neural circuit during associative learning by asymmetric insulin signaling.

Studies of neuronal connectivity in model organisms, i.e., of their connectomes, have been instrumental in dissecting the structure-function relationship of nervous systems.

Neuronal contact predicts connectivity in the C. elegans brain.

Axons must project to particular brain regions, contact adjacent neurons, and choose appropriate synaptic targets to form a nervous system. Multiple mechanisms have been proposed to explain synaptic partnership choice. In a "lock-and-key" mechanism, first proposed by Sperry's chemoaffinity model, a neuron selectively chooses a synaptic partner among several different, adjacent target cells, based on a specific molecular recognition code.

Restructuring of an asymmetric neural circuit during associative learning.

Asymmetric brain function is common across the animal kingdom and involved in language processing, and likely in learning and memory. What regulates asymmetric brain function remains elusive. Here, we show that the nematode restructures an asymmetric salt sensing neural circuit during associative learning.

Widespread employment of conserved C. elegans homeobox genes in neuronal identity specification.

Homeobox genes are prominent regulators of neuronal identity, but the extent to which their function has been probed in animal nervous systems remains limited. In the nematode Caenorhabditis elegans, each individual neuron class is defined by the expression of unique combinations of homeobox genes, prompting the question of whether each neuron class indeed requires a homeobox gene for its proper identity specification. We present here progress in addressing this question by extending previous mutant analysis of homeobox gene family members and describing multiple examples of homeobox gene function in different parts of the C.

The bHLH-PAS gene is expressed in the AVH, not AVJ interneurons.

Single neuron-specific drivers are important tools for visualizing neuron anatomy, manipulating neuron activity and gene rescue experiments. We report here that genomic regions upstream of the bHLH-PAS gene can be used to drive gene expression exclusively in the AVH interneuron pair and not, as previously reported, the AVJ interneuron pair.

Molecular topography of an entire nervous system.

We have produced gene expression profiles of all 302 neurons of the C. elegans nervous system that match the single-cell resolution of its anatomy and wiring diagram. Our results suggest that individual neuron classes can be solely identified by combinatorial expression of specific gene families.

The Prop1-like homeobox gene specifies the identity of synaptically connected neurons.

Many neuronal identity regulators are expressed in distinct populations of cells in the nervous system, but their function is often analyzed only in specific isolated cellular contexts, thereby potentially leaving overarching themes in gene function undiscovered. We show here that the Prop1-like homeobox gene is expressed in 15 distinct sensory, inter- and motor neuron classes throughout the entire nervous system. Strikingly, all 15 neuron classes expressing are synaptically interconnected, prompting us to investigate whether controls the functional properties of this circuit and perhaps also the assembly of these neurons into functional circuitry.

A multi-scale brain map derived from whole-brain volumetric reconstructions.

Animal nervous system organization is crucial for all body functions and its disruption can lead to severe cognitive and behavioural impairment. This organization relies on features across scales-from the localization of synapses at the nanoscale, through neurons, which possess intricate neuronal morphologies that underpin circuit organization, to stereotyped connections between different regions of the brain. The sheer complexity of this organ means that the feat of reconstructing and modelling the structure of a complete nervous system that is integrated across all of these scales has yet to be achieved.

Direct glia-to-neuron transdifferentiation gives rise to a pair of male-specific neurons that ensure nimble male mating.

Sexually dimorphic behaviours require underlying differences in the nervous system between males and females. The extent to which nervous systems are sexually dimorphic and the cellular and molecular mechanisms that regulate these differences are only beginning to be understood. We reveal here a novel mechanism by which male-specific neurons are generated in through the direct transdifferentiation of sex-shared glial cells.

The connectome of the Caenorhabditis elegans pharynx.

Detailed anatomical maps of individual organs and entire animals have served as invaluable entry points for ensuing dissection of their evolution, development, and function. The pharynx of the nematode Caenorhabditis elegans is a simple neuromuscular organ with a self-contained, autonomously acting nervous system, composed of 20 neurons that fall into 14 anatomically distinct types. Using serial electron micrograph (EM) reconstruction, we re-evaluate here the connectome of the pharyngeal nervous system, providing a novel and more detailed view of its structure and predicted function.

Evolution of neuronal anatomy and circuitry in two highly divergent nematode species.

The nematodes and populate diverse habitats and display distinct patterns of behavior. To understand how their nervous systems have diverged, we undertook a detailed examination of the neuroanatomy of the chemosensory system of . Using independent features such as cell body position, axon projections and lipophilic dye uptake, we have assigned homologies between the amphid neurons, their first-layer interneurons, and several internal receptor neurons of and We found that neuronal number and soma position are highly conserved.

Decreased function of survival motor neuron protein impairs endocytic pathways.

Spinal muscular atrophy (SMA) is caused by depletion of the ubiquitously expressed survival motor neuron (SMN) protein, with 1 in 40 Caucasians being heterozygous for a disease allele. SMN is critical for the assembly of numerous ribonucleoprotein complexes, yet it is still unclear how reduced SMN levels affect motor neuron function. Here, we examined the impact of SMN depletion in Caenorhabditis elegans and found that decreased function of the SMN ortholog SMN-1 perturbed endocytic pathways at motor neuron synapses and in other tissues.

Glia-derived neurons are required for sex-specific learning in C. elegans.

Sex differences in behaviour extend to cognitive-like processes such as learning, but the underlying dimorphisms in neural circuit development and organization that generate these behavioural differences are largely unknown. Here we define at the single-cell level-from development, through neural circuit connectivity, to function-the neural basis of a sex-specific learning in the nematode Caenorhabditis elegans. We show that sexual conditioning, a form of associative learning, requires a pair of male-specific interneurons whose progenitors are fully differentiated glia.

The Cell Death Pathway Regulates Synapse Elimination through Cleavage of Gelsolin in Caenorhabditis elegans Neurons.

Synapse elimination occurs in development, plasticity, and disease. Although the importance of synapse elimination has been documented in many studies, the molecular mechanisms underlying this process are unclear. Here, using the development of C.

Directional Trans-Synaptic Labeling of Specific Neuronal Connections in Live Animals.

Understanding animal behavior and development requires visualization and analysis of their synaptic connectivity, but existing methods are laborious or may not depend on trans-synaptic interactions. Here we describe a transgenic approach for in vivo labeling of specific connections in Caenorhabditis elegans, which we term iBLINC. The method is based on BLINC (Biotin Labeling of INtercellular Contacts) and involves trans-synaptic enzymatic transfer of biotin by the Escherichia coli biotin ligase BirA onto an acceptor peptide.

Computer assisted assembly of connectomes from electron micrographs: application to Caenorhabditis elegans.

A rate-limiting step in determining a connectome, the set of all synaptic connections in a nervous system, is extraction of the relevant information from serial electron micrographs. Here we introduce a software application, Elegance, that speeds acquisition of the minimal dataset necessary, allowing the discovery of new connectomes. We have used Elegance to obtain new connectivity data in the nematode worm Caenorhabditis elegans.

CGEF-1 and CHIN-1 regulate CDC-42 activity during asymmetric division in the Caenorhabditis elegans embryo.

The anterior-posterior axis of the Caenorhabditis elegans embryo is elaborated at the one-cell stage by the polarization of the partitioning (PAR) proteins at the cell cortex. Polarization is established under the control of the Rho GTPase RHO-1 and is maintained by the Rho GTPase CDC-42. To understand more clearly the role of the Rho family GTPases in polarization and division of the early embryo, we constructed a fluorescent biosensor to determine the localization of CDC-42 activity in the living embryo.

Insights to false positive total cyanide measurements in wastewater plant effluents.

Many publicly owned treatment works in North America are exceeding permitted limits for total cyanide in their wastewater treatment effluents. A recently introduced rapid, segmented, flow-injection analysis procedure using UV digestion and amperometric detection of the membrane-separated cyanide was used to investigate the various scenarios by which elevated cyanide levels might be present in wastewater treatment plant effluent. A number of significant interferences can produce false positive bias during sample analysis with the traditional acid distillation technique, but are minimized or absent with the new analytical method.

Segmented flow injection, UV digestion, and amperometric detection for the determination of total cyanide in wastewater treatment plant effluents.

The currently approved method for the analysis of total cyanide (TCN) in wastewaters has remained virtually unchanged in the 25 years since its initial use; this despite its subjection to a number of interferences, many of which provide a positive bias in cyanide measurements, including the formation of TCN during sample processing and some of which remain undocumented to this day. In particular, many municipal wastewater treatment plant chlorinated effluents throughout North America have often been cited for permit violations on the levels of total cyanide in their effluents measured using this methodology. A recently developed procedure for the analysis of TCN in various matrixes that utilizes segmented flow injection for sample transport and reaction, on-line acidic UV digestion for conversion of complexed cyanide to HCN, and amperometric detection achieved within 4 min of sample injection is demonstrated on chlorinated effluents discharged from municipal wastewater treatment plants.