Biology subjects

Hobert, O.

Publications and source records attributed to Hobert, O..

46 records · Page 3Linked to original sources

bioRxiv2020

Temporal transitions in post-mitotic neurons throughout the C. elegans nervous system

In most animals, the majority of the nervous system is generated and assembled into neuronal circuits during embryonic development. However, during juvenile stages, nervous systems still undergo extensive anatomical and functional changes to eventually form a fully mature nervous system by the adult stage. The molecular changes in post-mitotic neurons across post-embryonic development and the genetic programs that control these temporal transitions are not well understood. Using the model organism C. elegans, we comprehensively characterized the distinct functional states (locomotor behavior) and corresponding distinct molecular states (transcriptome) of the post-mitotic nervous system across temporal transitions from early post-embryonic periods to adulthood. We observed pervasive changes in gene expression, many of which are controlled by the developmental upregulation of the conserved heterochronic miRNA lin-4/mir-125 and the subsequent promotion of a mature neuronal transcriptional program through the repression of its target, the transcription factor lin-14. The functional relevance of these molecular transitions are exemplified by a temporally regulated target gene of the lin-14 transcription factor, nlp-45, a neuropeptide-encoding gene. We found that nlp-45 is required for temporal transitions in exploratory activity across larval stages, across sexual maturation, and into a diapause arrest stage. Our studies provide new insights into regulatory strategies that control neuron-type specific gene batteries to modulate distinct behaviors states across temporal, sex and environmental dimensions of post-embryonic development, and also provides a rich atlas of post-embryonic molecular changes to uncover additional regulatory mechanisms.

neuroscience↗

bioRxiv2019

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 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. Contrasting the previous classification of pharyngeal neurons into distinct inter- and motorneuron classes, we provide evidence that most pharyngeal neurons are also likely sensory neurons and most, if not all, pharyngeal neurons also classify as motorneurons. Together with the extensive cross-connectivity among pharyngeal neurons, which is more widespread than previously realized, the sensory-motor characteristics of most neurons define a shallow network architecture of the pharyngeal connectome. Network analysis reveals that the patterns of neuronal connections are organized into putative computational modules that reflect the known functional domains of the pharynx. Compared to the somatic nervous system, pharyngeal neurons both physically associate with a larger fraction of their neighbors and create synapses with a greater proportion of their neighbors. We speculate that the overall architecture of the pharyngeal nervous system may be reminiscent of the architecture of ancestral, primitive nervous systems.

neuroscience↗

bioRxiv2019

Expression profiling of the mature C. elegans nervous system by single-cell RNA-Sequencing

A single neuron and its synapses define the fundamental structural motif of the brain but the underlying gene expression programs that specify individual neuron types are poorly understood. To address this question in a model organism, we have produced a gene expression profile of >90% of the individual neuron classes in the C. elegans nervous system, an ensemble of neurons for which both the anatomy and connectivity are uniquely defined at single cell resolution. We generated single cell transcriptomes for 52,412 neurons that resolve as clusters corresponding to 109 of the canonical 118 neuron classes in the mature hermaphrodite nervous system. Detailed analysis revealed molecular signatures that further subdivide identified classes into specific neuronal subtypes. Notably, neuropeptide-related genes are often differentially expressed between subtypes of the given neuron class which points to distinct functional characteristics. All of these data are publicly available at our website (http://www.cengen.org) and can be interrogated at the web application SCeNGEA (https://cengen.shinyapps.io/SCeNGEA). We expect that this gene expression catalog will spur the goal of delineating the underlying mechanisms that define the developmental lineage, detailed anatomy, synaptic connectivity and function of each type of C. elegans neuron.

neuroscience↗

bioRxiv2019

NeuroPAL: A Neuronal Polychromatic Atlas of Landmarks for Whole-Brain Imaging in C. elegans

Comprehensively resolving single neurons and their cellular identities from whole-brain fluorescent images is a major challenge. We achieve this in C. elegans through the engineering and use of a multicolor transgene called NeuroPAL (a Neuronal Polychromatic Atlas of Landmarks). NeuroPAL worms share a stereotypical multicolor fluorescence map for the entire hermaphrodite nervous system that allows comprehensive determination of neuronal identities. Neurons labeled with NeuroPAL do not exhibit fluorescence in the green, cyan, or yellow emission channels, allowing the transgene to be used with numerous reporters of gene expression or neuronal dynamics. Here we showcase three studies that leverage NeuroPAL for nervous-system-wide neuronal identification. First, we determine the brainwide expression patterns of all metabotropic receptors for acetylcholine, GABA, and glutamate, completing a map of this communication network. Second, we uncover novel changes in cell fate caused by transcription factor mutations. Third, we record brainwide activity in response to attractive and repulsive chemosensory cues, characterizing multimodal coding and novel neuronal asymmetries for these stimuli. We present a software package that enables semi-automated determination of all neuronal identities based on color and positional information. The NeuroPAL framework and software provide a means to design landmark atlases for other tissues and organisms. In conclusion, we expect NeuroPAL to serve as an invaluable tool for gene expression analysis, neuronal fate studies, and for mapping whole-brain activity patterns.

neuroscience↗

bioRxiv2019

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

The nematodes C. elegans and P. pacificus 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 P. pacificus. 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 P. pacificus and C. elegans. We found that neuronal number and soma position are highly conserved. However, the morphological elaborations of several amphid cilia are different between them, most notably in the absence of winged cilia morphology in P. pacificus. We established a synaptic wiring diagram of amphid sensory neurons and amphid interneurons in P. pacificus and found striking patterns of conservation and divergence in connectivity relative to C. elegans, but very little changes in relative neighborhood of neuronal processes.\n\nImpact StatementThe substrate for evolutionary divergence does not lie in changes in neuronal cell number or targeting, but rather in sensory perception and synaptic partner choice within invariant, prepatterned neuronal processes.

neuroscience↗

bioRxiv2018

Timing mechanism of sexually dimorphic nervous system differentiation

In all animals, sexual differentiation of somatic tissue is precisely timed, yet the molecular mechanisms that control the timing of sexual differentiation, particularly in the brain, are poorly understood. We have used sexually dimorphic molecular, anatomical and behavioral features of the C. elegans nervous system to decipher a regulatory pathway that controls the precise timing of sexual differentiation. We find that the sexually dimorphic differentiation of postmitotic neurons in the male nervous system is abrogated in animals that carry a mutation in the miRNA let-7 and prematurely executed in animals either lacking the let-7 inhibitor lin-28, or the direct let-7 target lin-41, an RNA-binding, posttranscriptional regulator. We show that an isoform of a phylogenetically conserved transcription factor, lin-29a, is a critical target of LIN-41 in controlling sexual maturation of sex-shared neurons. lin-29a is expressed in a male-specific manner in a subset of sex-shared neurons at the onset of sexual maturation. lin-29a acts cell-autonomously in these neurons to control the expression of sexually dimorphic neurotransmitter switches, sensory receptor expression, neurite anatomy and connectivity, and locomotor behavior. lin-29a is not only required but also sufficient to impose male-specific features at earlier stages of development and in the opposite sex. The temporal, sexual and spatial specificity of lin-29a expression is controlled intersectionally through the lin-28/let-7/lin-41 heterochronic pathway, sex chromosome configuration and neuron type-specific terminal selector transcription factors. Two Doublesex-like transcription factors represent additional neuron-type specific targets of LIN-41 and are regulated in a similar intersectional manner, indicating the existence of modular outputs downstream of the heterochronic pathway. In conclusion, we have provided insights into the molecular logic of the timing of sexual differentiation in the C. elegans nervous system. Remarkably, the lin28/let7 axis also controls the timing of sexual differentiation in mice and humans thereby hinting toward a striking universality of the control mechanisms of sexual differentiation.

neuroscience↗

bioRxiv2018

Plasticity of the electrical connectome of C. elegans

The patterns of electrical synapses of an animal nervous system (\"electrical connectome\"), as well as the functional properties and plasticity of electrical synapses, are defined by the neuron type-specific complement of electrical synapse constituents. We systematically examine here properties of the electrical connectome of the nematode C. elegans through a genome- and nervous system-wide analysis of the expression pattern of the central components of invertebrate electrical synapses, the innexins, revealing highly complex combinatorial patterns of innexin expression throughout the nervous system. We find that the complex expression patterns of 12 out of 14 neuronally expressed innexins change in a strikingly neuron type-specific manner throughout most of the nervous system, if animals encounter harsh environmental conditions and enter the dauer arrest stage. We systematically describe the plasticity of locomotory patterns of dauer stage animals and, by analyzing several individual electrical synapses, we demonstrate that dauer stage-specific electrical synapse remodeling is responsible for specific aspects of the altered locomotory patterns as well as altered chemosensory behavior of dauer stage animals. We describe an intersectional gene regulatory mechanism, involving terminal selector and FoxO transcription factors that are responsible for inducing innexin expression changes in a neuron type- and environment-specific manner. Taken together, our studies illustrate the remarkably dynamic nature of electrical synapses on a nervous system-wide level and describe regulatory strategies for how these alterations are achieved.

neuroscience↗

bioRxiv2017

An atlas of Caenorhabditis elegans chemoreceptor expression

One goal of modern day neuroscience is the establishment of molecular maps that assign unique features to individual neuron types. Such maps provide important starting points for neuron classification, for functional analysis and for developmental studies aimed at defining the molecular mechanisms of neuron identity acquisition and neuron identity diversification. In this resource paper, we describe a nervous system-wide map of the potential expression sites of 244 members of the largest gene family in the C. elegans genome, rhodopsin-like (class A) GPCR chemoreceptors, using classic gfp reporter gene technology. We cover representatives of all sequence families of chemoreceptors GPCRs, some of which were previously entirely uncharacterized. Most reporters are expressed in a very restricted number of cells, often just in single cells. We assign GPCR reporter expression to all but two of the 37 sensory neuron classes of the sex-shared, core nervous system. Some sensory neurons express a very small number of receptors, while others, particularly nociceptive neurons, co-express several dozen GPCR reporter genes. GPCR reporters are also expressed in a wide range of inter- and motorneurons, as well as nonneuronal cells, suggesting that GPCRs may constitute receptors not just for environmental signals, but also for internal cues. We observe only one notable, frequent association of coexpression patterns, namely in one nociceptive amphid (ASH) and two nociceptive phasmid sensory neurons (PHA, PHB). We identified GPCRs with sexually dimorphic expression and several GPCR reporters that are expressed in a left/right asymmetric manner. We identified a substantial degree of GPCR expression plasticity; particularly in the context of the environmentally-induced dauer diapause stage when one third of all tested GPCRs alter the cellular specificity of their expression within and outside the nervous system. Intriguingly, in a number of cases, the dauer-specific alterations of GPCR reporter expression in specific neuron classes are maintained during postdauer life and in some case new patterns are induced post-dauer, demonstrating that GPCR gene expression may serve as traits of life history. Taken together, our resource provides an entry point for functional studies and also offers a host of molecular markers for studying molecular patterning and plasticity of the nervous system.\n\nAUTHOR SUMMARYMaps of gene expression patterns in the nervous system provide an important resource for neuron classification, for functional analysis and for developmental studies that ask how different neurons acquire their unique identities. By analyzing transgenic gfp reporter strains, we describe here the expression pattern of 244 putative chemosensory receptor-encoding genes, which constitute the largest gene family in C.elegans. We show that, as expected, chemoreceptor expression is enriched in chemosensory neurons but it is also expressed in a wide range of interneurons, motorneurons, as well as non-neuronal cells, suggesting that putative chemosensory receptors may not just sense environmental signals but also internal cues. We find that each chemoreceptor is expressed in a few neuron types, often just one, but each neuron type can express a large number of chemoreceptors. Interestingly, we uncovered that chemoreceptor expression is remarkably plastic, particularly in the context of the environmentally-induced dauer diapause stage. Taken together, this molecular atlas of chemosensory receptors provides an entry point for functional studies and offers a host of markers for studying neuronal patterning and plasticity.

genetics↗

bioRxiv2017

pals-22, A Member Of An Expanded C. elegans Gene Family, Controls Silencing Of Repetitive DNA

Repetitive DNA sequences are subject to gene silencing in various animal species. Under specific circumstances repetitive DNA sequences can escape such silencing. For example, when exogenously added, extrachromosomal DNA sequences that are stably inherited in multicopy repetitive arrays in the nematode C. elegans are frequently silenced in the germline, whereas such silencing often does not occur in the soma. This indicates that somatic cells might utilize factors that prevent repetitive DNA silencing. Indeed, such \"anti-silencing\" factors have been revealed through genetic screens that identified mutant loci in which repetitive transgenic arrays are aberrantly silenced in the soma. We describe here a novel locus, pals-22 (for protein containing ALS2CR12 domain), required to prevent silencing of repetitive transgenes in neurons and other somatic tissue types. pals-22 deficiency also severely impacts animal vigor and confers phenotypes reminiscent of accelerated aging. We find that pals-22 is a member of a large family of divergent genes (39 members), defined by the presence of an ALS2CR12 domain. While gene family members are highly divergent, they show striking patterns of genomic clustering. The family expansion appears C. elegans-specific and has not occurred to the same extent in other nematode species. Previous transcriptome analysis has revealed that most of the pals genes are induced under stress conditions or upon infection by intracellular parasites. The transgene silencing phenotype observed upon loss of cytoplasmically localized PALS-22 protein depends on the biogenesis of small RNAs, since silencing is abolished in the RNAi defective mutant rde-4, suggesting that pals-22 might regulate RNAi dependent silencing in the cytoplasm of neurons and other tissues. We speculate that the pals gene family may be part of a species-specific cellular defense mechanism.

genetics↗

bioRxiv2016

Sexually dimorphic differentiation of a C. elegans hub neuron is cell-autonomously controlled by a conserved transcription factor

Functional and anatomical sexual dimorphisms in the brain are either the result of cells that are generated only in one sex, or a manifestation of sex-specific differentiation of neurons present in both sexes. The PHC neurons of the nematode C. elegans differentiate in a strikingly sex-specific manner. While in hermaphrodites the PHC neurons display a canonical pattern of synaptic connectivity similar to that of other sensory neurons, PHC differentiates into a densely connected hub sensory/interneuron in males, integrating a large number of male-specific synaptic inputs and conveying them to both male-specific and sex-shared circuitry. We describe that the differentiation into such a hub neuron involves the sex-specific scaling of several components of the synaptic vesicle machinery, including the vesicular glutamate transporter eat-4/VGLUT, induction of neuropeptide expression, changes in axonal projection morphology and a switch in neuronal function. We demonstrate that these molecular and anatomical remodeling events are controlled cell-autonomously by the phylogenetically conserved Doublesex homolog dmd-3, which is both required and sufficient for sex-specific PHC differentiation. Cellular specificity of dmd-3 action is ensured by its collaboration with non-sex specific terminal selector-type transcription factors whereas sex-specificity of dmd-3 action is ensured by the hermaphrodite-specific master regulator of hermaphroditic cell identity, the Gli-like transcription factor tra-1, which transcriptionally represses dmd-3 in hermaphrodite PHC. Taken together, our studies provide mechanistic insights into how neurons are specified in a sexually dimorphic manner.

neuroscience↗