Early embryonic programming of neuronal left/right asymmetry in C. elegans

Neurogenesis in Caenorhabditis elegans

Animals rely on their nervous systems to process sensory inputs, integrate these with internal signals, and produce behavioral outputs. This is enabled by the highly specialized morphologies and functions of neurons. Neuronal cells share multiple structural and physiological features, but they also come in a large diversity of types or classes that give the nervous system its broad range of functions and plasticity. This diversity, first recognized over a century ago, spurred classification efforts based on morphology, function, and molecular criteria. Caenorhabditis elegans, with its precisely mapped nervous system at the anatomical level, an extensive molecular description of most of its neurons, and its genetic amenability, has been a prime model for understanding how neurons develop and diversify at a mechanistic level. Here, we review the gene regulatory mechanisms driving neurogenesis and the diversification of neuron classes and subclasses in C. elegans. We discuss our current understanding of the specification of neuronal progenitors and their differentiation in terms of the transcription factors involved and ensuing changes in gene expression and chromatin landscape. The central theme that has emerged is that the identity of a neuron is defined by modules of gene batteries that are under control of parallel yet interconnected regulatory mechanisms. We focus on how, to achieve these terminal identities, cells integrate information along their developmental lineages. Moreover, we discuss how neurons are diversified postembryonically in a time-, genetic sex-, and activity-dependent manner. Finally, we discuss how the understanding of neuronal development can provide insights into the evolution of neuronal diversity.

Brain bilateral asymmetry - insights from nematodes, zebrafish, and Drosophila

Chirality is a fundamental trait of living organisms, encompassing the homochirality of biological molecules and the left-right (LR) asymmetry of visceral organs and the brain. The nervous system in bilaterian organisms displays a lateralized organization characterized by the presence of asymmetrical neuronal circuits and brain functions that are predominantly localized within one hemisphere. Although body asymmetry is relatively well understood, and exhibits robust phenotypic expression and regulation via conserved molecular mechanisms across phyla, current findings indicate that the asymmetry of the nervous system displays greater phenotypic, genetic, and evolutionary variability. In this review we explore the use of nematode, zebrafish, and Drosophila genetic models to investigate neuronal circuit asymmetry. We discuss recent discoveries in the context of body-brain concordance and highlight the distinct characteristics of nervous system asymmetry and its cognitive correlates.

Epigenome Defines Aberrant Brain Laterality in Major Mental Illnesses

Brain-hemisphere asymmetry/laterality is a well-conserved biological feature of normal brain development. Several lines of evidence, confirmed by the meta-analysis of different studies, support the disruption of brain laterality in mental illnesses such as schizophrenia (SCZ), bipolar disorder (BD), attention-deficit/hyperactivity disorder (ADHD), obsessive compulsive disorder (OCD), and autism. Furthermore, as abnormal brain lateralization in the planum temporale (a critical structure in auditory language processing) has been reported in patients with SCZ, it has been considered a major cause for the onset of auditory verbal hallucinations. Interestingly, the peripheral counterparts of abnormal brain laterality in mental illness, particularly in SCZ, have also been shown in several structures of the human body. For instance, the fingerprints of patients with SCZ exhibit aberrant asymmetry, and while their hair whorl rotation is random, 95% of the general population exhibit a clockwise rotation. In this work, we present a comprehensive literature review of brain laterality disturbances in mental illnesses such as SCZ, BD, ADHD, and OCD, followed by a systematic review of the epigenetic factors that may be involved in the disruption of brain lateralization in mental health disorders. We will conclude with a discussion on whether existing non-pharmacological therapies such as rTMS and ECT may be used to influence the altered functional asymmetry of the right and left hemispheres of the brain, along with their epigenetic and corresponding gene-expression patterns.

Identification of Differentially Expressed MicroRNAs in the Rat Hippocampus during Adolescence through an Epigenome-Wide Analysis

INTRODUCTION

Epigenetic mechanisms involving microRNAs (miRNAs) play a fundamental role in many biological processes, particularly during prenatal and early postnatal development. Their role in adolescent brain development, however, has been poorly described. The present study aimed to explore miRNA expression in the hippocampus during adolescence compared to adulthood in rats.

METHOD

The brains of female and male Wistar rats were extracted, and the hippocampus was freshly dissected at postnatal day 41 (adolescence) and postnatal day 98 (adulthood). An epigenome-wide analysis was conducted to identify the miRNAs significantly expressed in adolescence compared to adulthood. Additionally, target genes of such miRNAs were considered to perform an exploratory Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis.

RESULTS

We identified 16 differentially expressed miRNAs in adolescent male rats compared with adult male rats and 4 differentially expressed miRNAs in adolescent females compared with adult females. Enrichment analysis reinforced that the target genes found are related to neurodevelopmental processes such as cell proliferation, cell migration, and nervous system development.

CONCLUSION

Our findings suggest a complex pattern of miRNA expression during adolescence, which differs from that in adulthood. The differential expression of miRNA in the hippocampus during adolescence may be associated with the late developmental changes occurring in this brain region. Furthermore, the observed sex differences in miRNA expression patterns indicate potential sexual differentiation in hippocampal development. Further comprehensive investigations are needed to elucidate the roles of miRNA in normal brain development.

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. However, the limited sample size of these studies has impeded analyses into how variation of connectivity across populations may influence circuit architecture and behavior. Moreover, little is known about how experiences induce changes in circuit architecture. Here, we show that an asymmetric salt-sensing circuit in the nematode Caenorhabditis elegans exhibits variation that predicts the animals' salt preferences and undergoes restructuring during salt associative learning. Naive worms memorize and prefer the salt concentration they experience in the presence of food through a left-biased neural network architecture. However, animals conditioned at elevated salt concentrations change this left-biased network to a right-biased network. This change in circuit architecture occurs through the addition of new synapses in response to asymmetric, paracrine insulin signaling. Therefore, experience-dependent changes in an animal's neural connectome are induced by insulin signaling and are fundamental to learning and behavior.

Transcript accumulation rates in the early Caenorhabditis elegans embryo

Dynamic transcriptional changes are widespread in rapidly dividing developing embryos when cell fate decisions are made quickly. The Caenorhabditis elegans embryo overcomes these constraints partly through the rapid production of high levels of transcription factor mRNAs. Transcript accumulation rates for some developmental genes are known at single-cell resolution, but genome-scale measurements are lacking. We estimate zygotic mRNA accumulation rates from single-cell RNA sequencing data calibrated with single-molecule transcript imaging. Rapid transcription is common in the early C. elegans embryo with rates highest soon after zygotic transcription begins. High-rate genes are enriched for recently duplicated cell-fate regulators and share common genomic features. We identify core promoter elements associated with high rate and measure their contributions for two early endomesodermal genes, ceh-51 and sdz-31. Individual motifs modestly affect accumulation rates, suggesting multifactorial control. These results are a step toward estimating absolute transcription kinetics and understanding how transcript dosage drives developmental decisions.

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 Caenorhabditis elegans restructures an asymmetric salt sensing neural circuit during associative learning. Worms memorize and prefer the salt concentration at which they were raised in the presence of food through a left-biased network architecture. When conditioned at elevated salt concentrations, animals change the left-biased to a right-biased network, which explains the changed salt-seeking behavior. The changes in circuit architecture require new synapse formation induced through asymmetric, paracrine insulin-signaling. Therefore, experience-dependent changes in asymmetric network architecture rely on paracrine insulin signaling and are fundamental to learning and behavior.

Epigenetics in fetal alcohol spectrum disorder

During pregnancy, alcohol abuse and its detrimental effects on developing offspring are major public health, economic and social challenges. The prominent characteristic attributes of alcohol (ethanol) abuse during pregnancy in humans are neurobehavioral impairments in offspring due to damage to the central nervous system (CNS), causing structural and behavioral impairments that are together named fetal alcohol spectrum disorder (FASD). Development-specific alcohol exposure paradigms were established to recapitulate the human FASD phenotypes and establish the underlying mechanisms. These animal studies have offered some critical molecular and cellular underpinnings likely to account for the neurobehavioral impairments associated with prenatal ethanol exposure. Although the pathogenesis of FASD remains unclear, emerging literature proposes that the various genomic and epigenetic components that cause the imbalance in gene expression can significantly contribute to the development of this disease. These studies acknowledged numerous immediate and enduring epigenetic modifications, such as methylation of DNA, post-translational modifications (PTMs) of histone proteins, and regulatory networks related to RNA, using many molecular approaches. Methylated DNA profiles, PTMs of histone proteins, and RNA-regulated expression of genes are essential for synaptic and cognitive behavior. Thus, offering a solution to many neuronal and behavioral impairments reported in FASD. In the current chapter, we review the recent advances in different epigenetic modifications that cause the pathogenesis of FASD. The information discussed can help better explain the pathogenesis of FASD and thereby might provide a basis for finding novel therapeutic targets and innovative treatment strategies.

Molecular Insights into Epigenetics and Cannabinoid Receptors

The actions of cannabis are mediated by G protein-coupled receptors that are part of an endogenous cannabinoid system (ECS). ECS consists of the naturally occurring ligands N-arachidonylethanolamine (anandamide) and 2-arachidonoylglycerol (2-AG), their biosynthetic and degradative enzymes, and the CB1 and CB2 cannabinoid receptors. Epigenetics are heritable changes that affect gene expression without changing the DNA sequence, transducing external stimuli in stable alterations of the DNA or chromatin structure. Cannabinoid receptors are crucial candidates for exploring their functions through epigenetic approaches due to their significant roles in health and diseases. Epigenetic changes usually promote alterations in the expression of genes and proteins that can be evaluated by various transcriptomic and proteomic analyses. Despite the exponential growth of new evidence on the critical functions of cannabinoid receptors, much is still unknown regarding the contribution of various genetic and epigenetic factors that regulate cannabinoid receptor gene expression. Recent studies have identified several immediate and long-lasting epigenetic changes, such as DNA methylation, DNA-associated histone proteins, and RNA regulatory networks, in cannabinoid receptor function. Thus, they can offer solutions to many cellular, molecular, and behavioral impairments found after modulation of cannabinoid receptor activities. In this review, we discuss the significant research advances in different epigenetic factors contributing to the regulation of cannabinoid receptors and their functions under both physiological and pathological conditions. Increasing our understanding of the epigenetics of cannabinoid receptors will significantly advance our knowledge and could lead to the identification of novel therapeutic targets and innovative treatment strategies for diseases associated with altered cannabinoid receptor functions.

pop-1/TCF, ref-2/ZIC and T-box factors regulate the development of anterior cells in the C. elegans embryo

Patterning of the anterior-posterior axis is fundamental to animal development. The Wnt pathway plays a major role in this process by activating the expression of posterior genes in animals from worms to humans. This observation raises the question of whether the Wnt pathway or other regulators control the expression of the many anterior-expressed genes. We found that the expression of five anterior-specific genes in Caenorhabditis elegans embryos depends on the Wnt pathway effectors pop-1/TCF and sys-1/β-catenin. We focused further on one of these anterior genes, ref-2/ZIC, a conserved transcription factor expressed in multiple anterior lineages. Live imaging of ref-2 mutant embryos identified defects in cell division timing and position in anterior lineages. Cis-regulatory dissection identified three ref-2 transcriptional enhancers, one of which is necessary and sufficient for anterior-specific expression. This enhancer is activated by the T-box transcription factors TBX-37 and TBX-38, and surprisingly, concatemerized TBX-37/38 binding sites are sufficient to drive anterior-biased expression alone, despite the broad expression of TBX-37 and TBX-38. Taken together, our results highlight the diverse mechanisms used to regulate anterior expression patterns in the embryo.

Combinatorial Action of Temporally Segregated Transcription Factors

Combinatorial action of transcription factors (TFs) with partially overlapping expression is a widespread strategy to generate novel gene-expression patterns and, thus, cellular diversity. Known mechanisms underlying combinatorial activity require co-expression of TFs within the same cell. Here, we describe the mechanism by which two TFs that are never co-expressed generate a new, intersectional expression pattern in C. elegans embryos: lineage-specific priming of a gene by a transiently expressed TF generates a unique intersection with a second TF acting on the same gene four cell divisions later; the second TF is expressed in multiple cells but only activates transcription in those where priming occurred. Early induction of active transcription is necessary and sufficient to establish a competent state, maintained by broadly expressed regulators in the absence of the initial trigger. We uncover additional cells diversified through this mechanism. Our findings define a mechanism for combinatorial TF activity with important implications for generation of cell-type diversity.

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Lineage-specific priming enables asymmetric gene expression in L/R neuron pairs

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Transient, lineage-specific TFs prime a locus for later activation by a bilateral TF

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An early active transcriptional state is necessary and sufficient for priming

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Maintenance of asymmetric primed state occurs in a symmetric regulatory environment

Single cell RNA sequencing identifies early diversity of sensory neurons forming via bi-potential intermediates

Somatic sensation is defined by the existence of a diversity of primary sensory neurons with unique biological features and response profiles to external and internal stimuli. However, there is no coherent picture about how this diversity of cell states is transcriptionally generated. Here, we use deep single cell analysis to resolve fate splits and molecular biasing processes during sensory neurogenesis in mice. Our results identify a complex series of successive and specific transcriptional changes in post-mitotic neurons that delineate hierarchical regulatory states leading to the generation of the main sensory neuron classes. In addition, our analysis identifies previously undetected early gene modules expressed long before fate determination although being clearly associated with defined sensory subtypes. Overall, the early diversity of sensory neurons is generated through successive bi-potential intermediates in which synchronization of relevant gene modules and concurrent repression of competing fate programs precede cell fate stabilization and final commitment.

Neuronal specification in C. elegans: combining lineage inheritance with intercellular signaling

The nervous system is composed of a high diversity of neuronal types. How this diversity is generated during development is a key question in neurobiology. Addressing this question is one of the reasons that led Sydney Brenner to develop the nematode C. elegans as a model organism. While there was initially a debate on whether the neuronal specification follows a 'European' model (determined by ancestry) or an 'American' model (determined by intercellular communication), several decades of research have established that the truth lies somewhere in between. Neurons are specified by the combination of transcription factors inherited from the ancestor cells and signaling between neighboring cells (especially Wnt and Notch signaling). This converges to the activation in newly generated postmitotic neurons of a specific set of terminal selector transcription factors that initiate and maintain the differentiation of the neuron. In this review, we also discuss the evolution of these specification mechanisms in other nematodes and beyond.

Buffering and Amplifying Transcriptional Noise During Cell Fate Specification

The molecular processes that drive gene transcription are inherently noisy. This noise often manifests in the form of transcriptional bursts, producing fluctuations in gene activity over time. During cell fate specification, this noise is often buffered to ensure reproducible developmental outcomes. However, sometimes noise is utilized as a “bet-hedging” mechanism to diversify functional roles across a population of cells. Studies of bacteria, yeast, and cultured cells have provided insights into the nature and roles of noise in transcription, yet we are only beginning to understand the mechanisms by which noise influences the development of multicellular organisms. Here we discuss the sources of transcriptional noise and the mechanisms that either buffer noise to drive reproducible fate choices or amplify noise to randomly specify fates.

Subtle left-right asymmetry of gene expression profiles in embryonic and foetal human brains

Left-right laterality is an important aspect of human -and in fact all vertebrate- brain organization for which the genetic basis is poorly understood. Using RNA sequencing data we contrasted gene expression in left- and right-sided samples from several structures of the anterior central nervous systems of post mortem human embryos and foetuses. While few individual genes stood out as significantly lateralized, most structures showed evidence of laterality of their overall transcriptomic profiles. These left-right differences showed overlap with age-dependent changes in expression, indicating lateralized maturation rates, but not consistently in left-right orientation over all structures. Brain asymmetry may therefore originate in multiple locations, or if there is a single origin, it is earlier than 5 weeks post conception, with structure-specific lateralized processes already underway by this age. This pattern is broadly consistent with the weak correlations reported between various aspects of adult brain laterality, such as language dominance and handedness.

Cis-acting single nucleotide polymorphisms alter MicroRNA-mediated regulation of human brain-expressed transcripts

Substantial variability exists in the presentation of complex neurological disorders, and the study of single nucleotide polymorphisms (SNPs) has shed light on disease mechanisms and pathophysiological variability in some cases. However, the vast majority of disease-linked SNPs have unidentified pathophysiological relevance. Here, we tested the hypothesis that SNPs within the miRNA recognition element (MRE; the region of the target transcript to which the miRNA binds) can impart changes in the expression of those genes, either by enhancing or reducing transcript and protein levels. To test this, we cross-referenced 7,153 miRNA-MRE brain interactions with the SNP database (dbSNP) to identify candidates, and functionally assessed 24 SNPs located in the 3’UTR or the coding sequence (CDS) of targets. For over half of the candidates tested, SNPs either enhanced (4 genes) or disrupted (10 genes) miRNA binding and target regulation. Additionally, SNPs causing a shift from a common to rare codon within the CDS facilitated miRNA binding downstream of the SNP, dramatically repressing target gene expression. The biological activity of the SNPs on miRNA regulation was also confirmed in induced pluripotent stem cell (iPSC) lines. These studies strongly support the notion that SNPs in the 3’UTR or the coding sequence of disease-relevant genes may be important in disease pathogenesis and should be reconsidered as candidate modifiers.

Asymmetric development of the nervous system

The human nervous system consists of seemingly symmetric left and right halves. However, closer observation of the brain reveals anatomical and functional lateralization. Defects in brain asymmetry correlate with several neurological disorders, yet our understanding of the mechanisms used to establish lateralization in the human central nervous system is extremely limited. Here, we review left-right asymmetries within the nervous system of humans and several model organisms, including rodents, Zebrafish, chickens, Xenopus, Drosophila, and the nematode Caenorhabditis elegans. Comparing and contrasting mechanisms used to develop left-right asymmetry in the nervous system can provide insight into how the human brain is lateralized. Developmental Dynamics 247:124-137, 2018. © 2017 Wiley Periodicals, Inc.

Heterotaxy in Caenorhabditis: widespread natural variation in left-right arrangement of the major organs

Although the arrangement of internal organs in most metazoans is profoundly left-right (L/R) asymmetric with a predominant handedness, rare individuals show full (mirror-symmetric) or partial (heterotaxy) reversals. While the nematode Caenorhabditis elegans is known for its highly determinate development, including stereotyped L/R organ handedness, we found that L/R asymmetry of the major organs, the gut and gonad, varies among natural isolates of the species in both males and hermaphrodites. In hermaphrodites, heterotaxy can involve one or both bilaterally asymmetric gonad arms. Male heterotaxy is probably not attributable to relaxed selection in this hermaphroditic species, as it is also seen in gonochoristic Caenorhabditis species. Heterotaxy increases in many isolates at elevated temperature, with one showing a pregastrulation temperature-sensitive period, suggesting a very early embryonic or germline effect on this much later developmental outcome. A genome-wide association study of 100 isolates showed that male heterotaxy is associated with three genomic regions. Analysis of recombinant inbred lines suggests that a small number of loci are responsible for the observed variation. These findings reveal that heterotaxy is a widely varying quantitative trait in an animal with an otherwise highly stereotyped anatomy, demonstrating unexpected plasticity in an L/R arrangement of the major organs even in a simple animal.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.

The origin and evolution of cell types

Cell types are the basic building blocks of multicellular organisms and are extensively diversified in animals. Despite recent advances in characterizing cell types, classification schemes remain ambiguous. We propose an evolutionary definition of a cell type that allows cell types to be delineated and compared within and between species. Key to cell type identity are evolutionary changes in the 'core regulatory complex' (CoRC) of transcription factors, that make emergent sister cell types distinct, enable their independent evolution and regulate cell type-specific traits termed apomeres. We discuss the distinction between developmental and evolutionary lineages, and present a roadmap for future research.

Epigenetic Mechanisms in Developmental Alcohol-Induced Neurobehavioral Deficits

Alcohol consumption during pregnancy and its damaging consequences on the developing infant brain are significant public health, social, and economic issues. The major distinctive features of prenatal alcohol exposure in humans are cognitive and behavioral dysfunction due to damage to the central nervous system (CNS), which results in a continuum of disarray that is collectively called fetal alcohol spectrum disorder (FASD). Many rodent models have been developed to understand the mechanisms of and to reproduce the human FASD phenotypes. These animal FASD studies have provided several molecular pathways that are likely responsible for the neurobehavioral abnormalities that are associated with prenatal alcohol exposure of the developing CNS. Recently, many laboratories have identified several immediate, as well as long-lasting, epigenetic modifications of DNA methylation, DNA-associated histone proteins and microRNA (miRNA) biogenesis by using a variety of epigenetic approaches in rodent FASD models. Because DNA methylation patterns, DNA-associated histone protein modifications and miRNA-regulated gene expression are crucial for synaptic plasticity and learning and memory, they can therefore offer an answer to many of the neurobehavioral abnormalities that are found in FASD. In this review, we briefly discuss the current literature of DNA methylation, DNA-associated histone proteins modification and miRNA and review recent developments concerning epigenetic changes in FASD.

Disruption of Epithalamic Left-Right Asymmetry Increases Anxiety in Zebrafish

Differences between the left and right sides of the brain are found throughout the animal kingdom, but the consequences of altered neural asymmetry are not well understood. In the zebrafish epithalamus, the parapineal is located on the left side of the brain where it influences development of the adjacent dorsal habenular (dHb) nucleus, causing the left and right dHb to differ in their organization, gene expression, and connectivity. Left-right (L-R) reversal of parapineal position and dHb asymmetry occurs spontaneously in a small percentage of the population, whereas the dHb develop symmetrically following experimental ablation of the parapineal. The habenular region was previously implicated in modulating fear in both mice and zebrafish, but the relevance of its L-R asymmetry is unclear. We now demonstrate that disrupting directionality of the zebrafish epithalamus causes reduced exploratory behavior and increased cortisol levels, indicative of enhanced anxiety. Accordingly, exposure to buspirone, an anxiolytic agent, significantly suppresses atypical behavior. Axonal projections from the parapineal to the dHb are more variable when it is located on the right side of the brain, revealing that L-R reversals do not necessarily represent a neuroanatomical mirror image. The results highlight the importance of directional asymmetry of the epithalamus in the regulation of stress responses in zebrafish.

Diversity and convergence in the mechanisms establishing L/R asymmetry in metazoa

Differentiating left and right hand sides during embryogenesis represents a major event in body patterning. Left-Right (L/R) asymmetry in bilateria is essential for handed positioning, morphogenesis and ultimately the function of organs (including the brain), with defective L/R asymmetry leading to severe pathologies in human. How and when symmetry is initially broken during embryogenesis remains debated and is a major focus in the field. Work done over the past 20 years, in both vertebrate and invertebrate models, has revealed a number of distinct pathways and mechanisms important for establishing L/R asymmetry and for spreading it to tissues and organs. In this review, we summarize our current knowledge and discuss the diversity of L/R patterning from cells to organs during evolution.

Functional and structural comparison of visual lateralization in birds - similar but still different

Vertebrate brains display physiological and anatomical left-right differences, which are related to hemispheric dominances for specific functions. Functional lateralizations likely rely on structural left-right differences in intra- and interhemispheric connectivity patterns that develop in tight gene-environment interactions. The visual systems of chickens and pigeons show that asymmetrical light stimulation during ontogeny induces a dominance of the left hemisphere for visuomotor control that is paralleled by projection asymmetries within the ascending visual pathways. But structural asymmetries vary essentially between both species concerning the affected pathway (thalamo- vs. tectofugal system), constancy of effects (transient vs. permanent), and the hemisphere receiving stronger bilateral input (right vs. left). These discrepancies suggest that at least two aspects of visual processes are influenced by asymmetric light stimulation: (1) visuomotor dominance develops within the ontogenetically stronger stimulated hemisphere but not necessarily in the one receiving stronger bottom-up input. As a secondary consequence of asymmetrical light experience, lateralized top-down mechanisms play a critical role in the emergence of hemispheric dominance. (2) Ontogenetic light experiences may affect the dominant use of left- and right-hemispheric strategies. Evidences from social and spatial cognition tasks indicate that chickens rely more on a right-hemispheric global strategy whereas pigeons display a dominance of the left hemisphere. Thus, behavioral asymmetries are linked to a stronger bilateral input to the right hemisphere in chickens but to the left one in pigeons. The degree of bilateral visual input may determine the dominant visual processing strategy when redundant encoding is possible. This analysis supports that environmental stimulation affects the balance between hemispheric-specific processing by lateralized interactions of bottom-up and top-down systems.

Natural reversal of left-right gut/gonad asymmetry in C. elegans males is independent of embryonic chirality

Anatomical left-right (L/R) asymmetry in C. elegans is established in the four-cell embryo as a result of anteroposterior skewing of transverse mitotic spindles with a defined handedness. This event creates a chiral embryo and ultimately an adult body plan with fixed L/R positioning of internal organs and components of the nervous system. While this "dextral" configuration is invariant in hermaphrodites, it can be reversed by physical manipulation of the early embryo or by mutations that interfere with mitotic spindle orientation, which leads to viable, mirror-reversed (sinistral) animals. During normal development of the C. elegans male, the gonad develops on the right of the midline, with the gut bilaterally apposed on the left. However, we found that in males of the laboratory N2 strain and Hawaiian ("Hw") wild isolate, the gut/gonad asymmetry is frequently reversed in a temperature-dependent manner, independent of normal embryonic chirality. We also observed sporadic errors in gonad migration occurring naturally during early larval stages of these and other wild strains; however, the incidence of such errors does not correlate with the frequency of L/R gut/gonad reversals in these strains. Analysis of N2/Hw hybrids and recombinant inbred advanced intercross lines (RIAILs) indicate that the L/R organ reversals are likely to result from recessively acting variations in multiple genes. Thus, unlike the highly reproducible L/R asymmetries of most structures in hermaphrodites, the L/R asymmetry of the male C. elegans body plan is less rigidly determined and subject to natural variation that is influenced by a multiplicity of genes.

Two distinct types of neuronal asymmetries are controlled by the Caenorhabditis elegans zinc finger transcription factor die-1

Left/right asymmetric features of the body are either randomly distributed on either the left or right side within a population (antisymmetries) or found on one particular side (directional asymmetries). Nervous systems display both types of asymmetries, but it is unknown whether shared regulators establish them. Here, Cochella et al. show that the transcription factor die-1 controls both antisymmetry and directional asymmetry in distinct sensory systems in C. elegans. This study uncovers the first molecular link between two different kinds of body plan asymmetries.

Left/right asymmetric features of animals are either randomly distributed on either the left or right side within a population (“antisymmetries”) or found stereotypically on one particular side of an animal (“directional asymmetries”). Both types of asymmetries can be found in nervous systems, but whether the regulatory programs that establish these asymmetries share any mechanistic features is not known. We describe here an unprecedented molecular link between these two types of asymmetries in Caenorhabditis elegans. The zinc finger transcription factor die-1 is expressed in a directionally asymmetric manner in the gustatory neuron pair ASE left (ASEL) and ASE right (ASER), while it is expressed in an antisymmetric manner in the olfactory neuron pair AWC left (AWCL) and AWC right (AWCR). Asymmetric die-1 expression is controlled in a fundamentally distinct manner in these two neuron pairs. Importantly, asymmetric die-1 expression controls the directionally asymmetric expression of gustatory receptor proteins in the ASE neurons and the antisymmetric expression of olfactory receptor proteins in the AWC neurons. These asymmetries serve to increase the ability of the animal to discriminate distinct chemosensory inputs.

microRNA function in left-right neuronal asymmetry: perspectives from C. elegans

Left-right asymmetry in anatomical structures and functions of the nervous system is present throughout the animal kingdom. For example, language centers are localized in the left side of the human brain, while spatial recognition functions are found in the right hemisphere in the majority of the population. Disruption of asymmetry in the nervous system is correlated with neurological disorders. Although anatomical and functional asymmetries are observed in mammalian nervous systems, it has been a challenge to identify the molecular basis of these asymmetries. C. elegans has emerged as a prime model organism to investigate molecular asymmetries in the nervous system, as it has been shown to display functional asymmetries clearly correlated to asymmetric distribution and regulation of biologically relevant molecules. Small non-coding RNAs have been recently implicated in various aspects of neural development. Here, we review cases in which microRNAs are crucial for establishing left-right asymmetries in the C. elegans nervous system. These studies may provide insight into how molecular and functional asymmetries are established in the human brain.

Roles of microRNAs in the Caenorhabditis elegans nervous system

The first microRNA was discovered in Caenorhabditis elegans in 1993, and since then, thousands of microRNAs have been identified from almost all eukaryotic organisms examined. MicroRNAs function in many biological events such as cell fate determination, metabolism, apoptosis, and carcinogenesis. So far, more than 250 microRNAs have been identified in C. elegans; however, functions for most of these microRNAs are still unknown. A small number of C. elegans microRNAs are associated with known physiological roles such as developmental timing, cell differentiation, stress response, and longevity. In this review, we summarize known roles of microRNAs in neuronal differentiation and function of C. elegans, and discuss interesting perspectives for future studies.

Neurally Derived Tissues in Xenopus laevis Embryos Exhibit a Consistent Bioelectrical Left-Right Asymmetry

Consistent left-right asymmetry in organ morphogenesis is a fascinating aspect of bilaterian development. Although embryonic patterning of asymmetric viscera, heart, and brain is beginning to be understood, less is known about possible subtle asymmetries present in anatomically identical paired structures. We investigated two important developmental events: physiological controls of eye development and specification of neural crest derivatives, in Xenopus laevis embryos. We found that the striking hyperpolarization of transmembrane potential (V_mem) demarcating eye induction usually occurs in the right eye field first. This asymmetry is randomized by perturbing visceral left-right patterning, suggesting that eye asymmetry is linked to mechanisms establishing primary laterality. Bilateral misexpression of a depolarizing channel mRNA affects primarily the right eye, revealing an additional functional asymmetry in the control of eye patterning by V_mem. The ATP-sensitive K⁺ channel subunit transcript, SUR1, is asymmetrically expressed in the eye primordia, thus being a good candidate for the observed physiological asymmetries. Such subtle asymmetries are not only seen in the eye: consistent asymmetry was also observed in the migration of differentiated melanocytes on the left and right sides. These data suggest that even anatomically symmetrical structures may possess subtle but consistent laterality and interact with other developmental left-right patterning pathways.

Handedness of a motor program in C. elegans is independent of left-right body asymmetry

Complex animals display bilaterally asymmetric motor behavior, or "motor handedness," often revealed by preferential use of limbs on one side. For example, use of right limbs is dominant in a strong majority of humans. While the mechanisms that establish bilateral asymmetry in motor function are unknown in humans, they appear to be distinct from those for other handedness asymmetries, including bilateral visceral organ asymmetry, brain laterality, and ocular dominance. We report here that a simple, genetically homogeneous animal comprised of only ~1000 somatic cells, the nematode C. elegans, also shows a distinct motor handedness preference: on a population basis, males show a pronounced right-hand turning bias during mating. The handedness bias persists through much of adult lifespan, suggesting that, as in more complex animals, it is an intrinsic trait of each individual, which can differ from the population mean. Our observations imply that the laterality of motor handedness preference in C. elegans is driven by epigenetic factors rather than by genetic variation. The preference for right-hand turns is also seen in animals with mirror-reversed anatomical handedness and is not attributable to stochastic asymmetric loss of male sensory rays that occurs by programmed cell death. As with C. elegans, we also observed a substantial handedness bias, though not necessarily the same preference in direction, in several gonochoristic Caenorhabditis species. These findings indicate that the independence of bilaterally asymmetric motor dominance from overall anatomical asymmetry, and a population-level tendency away from ambidexterity, occur even in simple invertebrates, suggesting that these may be common features of bilaterian metazoans.

Embryonic priming of a miRNA locus predetermines postmitotic neuronal left/right asymmetry in C. elegans

The mechanisms by which functional left/right asymmetry arises in morphologically symmetric nervous systems are poorly understood. Here, we provide a mechanistic framework for how functional asymmetry in a postmitotic neuron pair is specified in C. elegans. A key feature of this mechanism is a temporally separated, two-step activation of the lsy-6 miRNA locus. The lsy-6 locus is first "primed" by chromatin decompaction in the precursor for the left neuron, but not the right neuron, several divisions before the neurons are born. lsy-6 expression is then "boosted" to functionally relevant levels several divisions later in the mother of the left neuron, through the activity of a bilaterally expressed transcription factor that can only activate lsy-6 in the primed neuron. This study shows how cells can become committed during early developmental stages to execute a specific fate much later in development and provides a conceptual framework for understanding the generation of neuronal diversity.

The microRNA mir-71 inhibits calcium signaling by targeting the TIR-1/Sarm1 adaptor protein to control stochastic L/R neuronal asymmetry in C. elegans

The Caenorhabditis elegans left and right AWC olfactory neurons communicate to establish stochastic asymmetric identities, AWC(ON) and AWC(OFF), by inhibiting a calcium-mediated signaling pathway in the future AWC(ON) cell. NSY-4/claudin-like protein and NSY-5/innexin gap junction protein are the two parallel signals that antagonize the calcium signaling pathway to induce the AWC(ON) fate. However, it is not known how the calcium signaling pathway is downregulated by nsy-4 and nsy-5 in the AWC(ON) cell. Here we identify a microRNA, mir-71, that represses the TIR-1/Sarm1 adaptor protein in the calcium signaling pathway to promote the AWC(ON) identity. Similar to tir-1 loss-of-function mutants, overexpression of mir-71 generates two AWC(ON) neurons. tir-1 expression is downregulated through its 3' UTR in AWC(ON), in which mir-71 is expressed at a higher level than in AWC(OFF). In addition, mir-71 is sufficient to inhibit tir-1 expression in AWC through the mir-71 complementary site in the tir-1 3' UTR. Our genetic studies suggest that mir-71 acts downstream of nsy-4 and nsy-5 to promote the AWC(ON) identity in a cell autonomous manner. Furthermore, the stability of mature mir-71 is dependent on nsy-4 and nsy-5. Together, these results provide insight into the mechanism by which nsy-4 and nsy-5 inhibit calcium signaling to establish stochastic asymmetric AWC differentiation.

Cerebral correlates of visual lateralization in Sepia

The common cuttlefish, Sepia officinalis (cephalopod mollusc) has recently become a relevant model for studying the setting-up of brain asymmetry among invertebrates. As the animals age from 3 to 30 days post hatching, they progressively develop a left-turning bias resulting from an eye-use preference. The aim of this study is to investigate whether anatomical (vertical, peduncle, inferior buccal, and optic lobes) or neurochemical (monoamines in optic lobes) brain asymmetries are present in the cuttlefish brain at 3 or at 30 post hatching days; and whether these correlate with side-turning preferences. We here find brain and behavioral asymmetry only at 30 post hatching days. Cuttlefish displayed a significant population bias towards a larger right peduncle lobe, and higher monoamine concentration in the left optic lobe (i.e. serotonin, dopamine and noradrenaline). None of these brain asymmetries were correlated to the studied side-turning bias. However, we found individual variation in the magnitude of the vertical and optic lobes asymmetry. A striking correlation was found with the behavioral results: the larger the right optic lobe and the right part of the vertical lobe, the stronger the bias to turn leftwards. To our knowledge, this is the first study to demonstrate a relationship at the individual level between brain and behavioral asymmetries in invertebrates.

Multidimensional regulation of gene expression in the C. elegans embryo

How cells adopt different expression patterns is a fundamental question of developmental biology. We quantitatively measured reporter expression of 127 genes, primarily transcription factors, in every cell and with high temporal resolution in C. elegans embryos. Embryonic cells are highly distinct in their gene expression; expression of the 127 genes studied here can distinguish nearly all pairs of cells, even between cells of the same tissue type. We observed recurrent lineage-regulated expression patterns for many genes in diverse contexts. These patterns are regulated in part by the TCF-LEF transcription factor POP-1. Other genes' reporters exhibited patterns correlated with tissue, position, and left–right asymmetry. Sequential patterns both within tissues and series of sublineages suggest regulatory pathways. Expression patterns often differ between embryonic and larval stages for the same genes, emphasizing the importance of profiling expression in different stages. This work greatly expands the number of genes in each of these categories and provides the first large-scale, digitally based, cellular resolution compendium of gene expression dynamics in live animals. The resulting data sets will be a useful resource for future research.

Cell-type-based analysis of microRNA profiles in the mouse brain

MicroRNAs (miRNA) are implicated in brain development and function but the underlying mechanisms have been difficult to study in part due to the cellular heterogeneity in neural circuits. To systematically analyze miRNA expression in neurons, we have established a miRNA tagging and affinity-purification (miRAP) method that is targeted to cell types through the Cre-loxP binary system in mice. Our studies of the neocortex and cerebellum reveal the expression of a large fraction of known miRNAs with distinct profiles in glutamatergic and GABAergic neurons and subtypes of GABAergic neurons. We further detected putative novel miRNAs, tissue or cell type-specific strand selection of miRNAs, and miRNA editing. Our method thus will facilitate a systematic analysis of miRNA expression and regulation in specific neuron types in the context of neuronal development, physiology, plasticity, pathology, and disease models, and is generally applicable to other cell types and tissues.

Notch-dependent induction of left/right asymmetry in C. elegans interneurons and motoneurons

Although nervous systems are largely bilaterally symmetric on a structural level, they display striking degrees of functional left/right (L/R) asymmetry. In Caenorhabditis elegans, two structurally symmetric pairs of sensory neurons, ASE and AWC, display two distinctly controlled types of functional L/R asymmetries (stereotyped versus stochastic asymmetry). Beyond these two cases, the extent of neuronal asymmetry in the C. elegans nervous system was unclear. Here, we report that the Beta3/Olig-type bHLH transcription factor hlh-16 is L/R asymmetrically expressed in several distinct, otherwise bilaterally symmetric interneuron and motoneuron pairs that are part of a known navigation circuit. We find that hlh-16 asymmetry is generated during gastrulation by an asymmetric LAG-2/Delta signal originating from the mesoderm that promotes hlh-16 expression in neurons on the left side through direct binding of the Notch effector LAG-1/Su(H)/CBF to a cis-regulatory element in the hlh-16 locus. Removal of hlh-16 reveals an unanticipated asymmetry in the ability of the axons of the AIY interneurons to extend into the nerve ring, with the left AIY axon requiring elevated hlh-16 expression for correct extension. Our study suggests that the extent of molecular L/R asymmetry in the C. elegans nervous system is broader than previously anticipated, establishes a novel signaling mechanism that crosses germ layers to diversify bilaterally symmetric neuronal lineages, and reveals L/R asymmetric control of axonal outgrowth of bilaterally symmetric neurons.

Microtubule-based localization of a synaptic calcium-signaling complex is required for left-right neuronal asymmetry in C. elegans

The axons of C. elegans left and right AWC olfactory neurons communicate at synapses through a calcium-signaling complex to regulate stochastic asymmetric cell identities called AWC(ON) and AWC(OFF). However, it is not known how the calcium-signaling complex, which consists of UNC-43/CaMKII, TIR-1/SARM adaptor protein and NSY-1/ASK1 MAPKKK, is localized to postsynaptic sites in the AWC axons for this lateral interaction. Here, we show that microtubule-based localization of the TIR-1 signaling complex to the synapses regulates AWC asymmetry. Similar to unc-43, tir-1 and nsy-1 loss-of-function mutants, specific disruption of microtubules in AWC by nocodazole generates two AWC(ON) neurons. Reduced localization of UNC-43, TIR-1 and NSY-1 proteins in the AWC axons strongly correlates with the 2AWC(ON) phenotype in nocodazole-treated animals. We identified kinesin motor unc-104/kif1a mutants for enhancement of the 2AWC(ON) phenotype of a hypomorphic tir-1 mutant. Mutations in unc-104, like microtubule depolymerization, lead to a reduced level of UNC-43, TIR-1 and NSY-1 proteins in the AWC axons. In addition, dynamic transport of TIR-1 in the AWC axons is dependent on unc-104, the primary motor required for the transport of presynaptic vesicles. Furthermore, unc-104 acts non-cell autonomously in the AWC(ON) neuron to regulate the AWC(OFF) identity. Together, these results suggest a model in which UNC-104 may transport some unknown presynaptic factor(s) in the future AWC(ON) cell that non-cell autonomously control the trafficking of the TIR-1 signaling complex to postsynaptic regions of the AWC axons to regulate the AWC(OFF) identity.

Left-right patterning in the C. elegans embryo: Unique mechanisms and common principles

The development of bilateral symmetry during the evolution of species probably 600 million years ago brought about several important innovations: It fostered efficient locomotion, streamlining and favored the development of a central nervous system through cephalization. However, to increase their functional capacities, many organisms exhibit chirality by breaking their superficial left-right (l-r) symmetry, which manifests in the lateralization of the nervous system or the l-r asymmetry of internal organs. In most bilateria, the mechanisms that maintain consistent l-r asymmetry throughout development are poorly understood. This review highlights insights into mechanisms that couple early embryonic l-r symmetry breaking to subsequent l-r patterning in the roundworm Caenorhabditis elegans. A recently identified strategy for l-r patterning in the early C. elegans embryo is discussed, the spatial separation of midline and anteroposterior axis, which relies on a rotational cellular rearrangement and non-canonical Wnt signaling. Evidence for a general relevance of rotational/torsional rearrangements during organismal l-r patterning and for non-canonical Wnt signaling/planar cell polarity as a common signaling mechanism to maintain l-r asymmetry is presented.

Establishment of medial fates along the proximodistal axis of the Drosophila leg through direct activation of dachshund by Distalless

The proximodistal (PD) axis of the Drosophila leg is thought to be established by the combined gradients of two secreted morphogens, Wingless (Wg) and Decapentaplegic (Dpp). According to this model, high [Wg+Dpp] activates Distalless (Dll) and represses dachshund (dac) in the distal cells of the leg disc, while intermediate [Wg+Dpp] activates dac in medial tissue. To test this model we identified and characterized a dac cis-regulatory element (dac RE) that recapitulates dac's medial expression domain during leg development. Counter to the gradient model, we find that Wg and Dpp do not act in a graded manner to activate RE. Instead, dac RE is activated directly by Dll and repressed distally by a combination of factors, including the homeodomain protein Bar. Thus, medial leg fates are established via a regulatory cascade in which Wg+Dpp activate Dll and then Dll directly activates dac, with Wg+Dpp as less critical, permissive inputs.

A Genome-Wide RNAi Screen for Factors Involved in Neuronal Specification in Caenorhabditis elegans

One of the central goals of developmental neurobiology is to describe and understand the multi-tiered molecular events that control the progression of a fertilized egg to a terminally differentiated neuron. In the nematode Caenorhabditis elegans, the progression from egg to terminally differentiated neuron has been visually traced by lineage analysis. For example, the two gustatory neurons ASEL and ASER, a bilaterally symmetric neuron pair that is functionally lateralized, are generated from a fertilized egg through an invariant sequence of 11 cellular cleavages that occur stereotypically along specific cleavage planes. Molecular events that occur along this developmental pathway are only superficially understood. We take here an unbiased, genome-wide approach to identify genes that may act at any stage to ensure the correct differentiation of ASEL. Screening a genome-wide RNAi library that knocks-down 18,179 genes (94% of the genome), we identified 245 genes that affect the development of the ASEL neuron, such that the neuron is either not generated, its fate is converted to that of another cell, or cells from other lineage branches now adopt ASEL fate. We analyze in detail two factors that we identify from this screen: (1) the proneural gene hlh-14, which we find to be bilaterally expressed in the ASEL/R lineages despite their asymmetric lineage origins and which we find is required to generate neurons from several lineage branches including the ASE neurons, and (2) the COMPASS histone methyltransferase complex, which we find to be a critical embryonic inducer of ASEL/R asymmetry, acting upstream of the previously identified miRNA lsy-6. Our study represents the first comprehensive, genome-wide analysis of a single neuronal cell fate decision. The results of this analysis provide a starting point for future studies that will eventually lead to a more complete understanding of how individual neuronal cell types are generated from a single-cell embryo.

Stochastic mechanisms of cell fate specification that yield random or robust outcomes

Although cell fate specification is tightly controlled to yield highly reproducible results and avoid extreme variation, developmental programs often incorporate stochastic mechanisms to diversify cell types. Stochastic specification phenomena are observed in a wide range of species and an assorted set of developmental contexts. In bacteria, stochastic mechanisms are utilized to generate transient subpopulations capable of surviving adverse environmental conditions. In vertebrate, insect, and worm nervous systems, stochastic fate choices are used to increase the repertoire of sensory and motor neuron subtypes. Random fate choices are also integrated into developmental programs controlling organogenesis. Although stochastic decisions can be maintained to produce a mosaic of fates within a population of cells, they can also be compensated for or directed to yield robust and reproducible outcomes.

Binary fate decisions in differentiating neurons

Neural cell fate programs must generate an enormous number of neurons with distinct adult functions. The decision to choose one neuronal subtype from two alternatives--a binary fate decision--is one way to diversify neuronal subtypes during nervous system development. Recent progress has been made in describing the genetic programs that define late-stage neuronal identity. Here, we review mechanisms that control how such fate decisions generate two different postmitotic, terminally differentiated neuronal subtypes. We survey examples from Caenorhabditis elegans and Drosophila that demonstrate different modes of binary neuronal fate specification that depend on cell division, lineage, stochastic gene expression, or extracellular signals. Comparison of these strategies reveals that, although organisms use diverse approaches to generate neural diversity, some common themes do exist.

Making a difference together: reciprocal interactions in C. elegans and zebrafish asymmetric neural development

Brain asymmetries are thought to increase neural processing capacity and to prevent interhemispheric conflict. In order to develop asymmetrically, neurons must be specified along the left-right axis, assigned left-side versus right-side identities and differentiate appropriately. In C. elegans and zebrafish, the cellular and molecular mechanisms that lead to neural asymmetries have recently come to light. Here, we consider recent insights into the mechanisms involved in asymmetrical neural development in these two species. Although the molecular details are divergent, both organisms use iterative cell-cell communication to establish left-right neuronal identity.

The C. elegans Tailless/TLX transcription factor nhr-67 controls neuronal identity and left/right asymmetric fate diversification

An understanding of the molecular mechanisms of cell fate determination in the nervous system requires the elucidation of transcriptional regulatory programs that ultimately control neuron-type-specific gene expression profiles. We show here that the C. elegans Tailless/TLX-type, orphan nuclear receptor NHR-67 acts at several distinct steps to determine the identity and subsequent left/right (L/R) asymmetric subtype diversification of a class of gustatory neurons, the ASE neurons. nhr-67 controls several broad aspects of sensory neuron development and, in addition, triggers the expression of a sensory neuron-type-specific selector gene, che-1, which encodes a zinc-finger transcription factor. Subsequent to its induction of overall ASE fate, nhr-67 diversifies the fate of the two ASE neurons ASEL and ASER across the L/R axis by promoting ASER and inhibiting ASEL fate. This function is achieved through direct expression activation by nhr-67 of the Nkx6-type homeobox gene cog-1, an inducer of ASER fate, that is inhibited in ASEL through the miRNA lsy-6. Besides controlling bilateral and asymmetric aspects of ASE development, nhr-67 is also required for many other neurons of diverse lineage history and function to appropriately differentiate, illustrating the broad and diverse use of this type of transcription factor in neuronal development.

Support for the selective chromatid segregation hypothesis advanced for the mechanism of left-right body axis development in mice

The somatic DNA strand-specific imprinting to effect gene regulation and selective chromatid segregation model was previously proposed to produce developmentally nonequivalent sister cells in mitosis. Such a mechanism might explain generation of stem-cell pattern of cell division in eukaryotes. The developmentally controlled process involves a pair of homologous chromosomes at a specific cell division to establish embryonic left-right body axis asymmetry. As a result, visceral organs in the two sides of vertebrate's body develop asymmetrically. The model was specifically proposed to explain the well-known axis randomization phenotype of the left-right dynein mutant mice where one-half of animals develop with standard visceral organ's positioning and the balance develops with the inverted arrangement. The model postulated that the specific dynein, a microtubule-based molecular motor protein, promotes the selective chromatid segregation process in mitosis. Thus, random segregation involving sister chromatids of a pair of specific chromosomes leads to axis randomization of the mutant. Moreover, the model uniquely predicts that 50 percent mutant embryos should produce symmetrical cell divisions because of random segregation; consequently, their either visceral side would develop as mirror image of the other side resulting in embryonic lethality. In view of this prediction, validity of prominent body axis-determination models is scrutinized here. Results supporting the cell-type regulated chromosome 6 and chromosome 7 selective chromatids segregation phenomenon existing in mouse cells are reviewed. Published results with the mutant mice are consistent with the chromosome segregation model for axis determination.