The TRA-1A sex determination protein of C. elegans regulates sexually dimorphic cell deaths by repressing the egl-1 cell death activator gene

Non-apoptotic role of EGL-1 in exopher production and neuronal health in Caenorhabditis elegans

While traditionally studied for their pro-apoptotic functions, recent research suggests BH3-only proteins also have non-apoptotic roles. Here, we find that EGL-1, the BH3-only protein in Caenorhabditis elegans , promotes the cell-autonomous production of exophers in adult neurons. Exophers are large, micron-scale vesicles that are ejected from the cell and contain cellular components such as mitochondria. EGL-1 facilitates exopher production potentially through regulation of mitochondrial dynamics. Moreover, an endogenous, low level of EGL-1 expression appears to benefit dendritic health. Our findings provide insights into the mechanistic role of BH3-only protein in mitochondrial dynamics, downstream exopher production, and ultimately neuronal health.

Significance statement

BH3-only proteins were known for their function in inducing cell death. Their presence in healthy adult neurons, however, suggests additional roles. Our study focused on the BH3-only protein EGL-1 in the nematode Caenorhabditis elegans , where its apoptotic role was discovered. We reveal a new role in cell-autonomously promoting exopher production - a process where neurons extrude large vesicles containing potentially harmful cell contents. EGL-1 appears to promote this by regulating mitochondrial dynamics. We also report that low levels of EGL-1 benefit neuronal health and function. These findings expand our understanding of BH3-only proteins, mitochondrial dynamics, and exopher production in neurons and provide insights for neurodegenerative diseases.

Let's talk about sex: Mechanisms of neural sexual differentiation in Bilateria

In multicellular organisms, sexed gonads have evolved that facilitate release of sperm versus eggs, and bilaterian animals purposefully combine their gametes via mating behaviors. Distinct neural circuits have evolved that control these physically different mating events for animals producing eggs from ovaries versus sperm from testis. In this review, we will describe the developmental mechanisms that sexually differentiate neural circuits across three major clades of bilaterian animals-Ecdysozoa, Deuterosomia, and Lophotrochozoa. While many of the mechanisms inducing somatic and neuronal sex differentiation across these diverse organisms are clade-specific rather than evolutionarily conserved, we develop a common framework for considering the developmental logic of these events and the types of neuronal differences that produce sex-differentiated behaviors. This article is categorized under: Congenital Diseases > Stem Cells and Development Neurological Diseases > Stem Cells and Development.

A genetic screen identifies C. elegans eif-3.H and hrpr-1 as pro-apoptotic genes and potential activators of egl-1 expression

During C. elegans development, 1090 somatic cells are generated of which 131 reproducibly die, many through apoptosis. The C. elegans BH3-only gene egl-1 is the key activator of apoptosis in somatic tissues, and it is predominantly expressed in 'cell death' lineages i.e. lineages in which apoptotic cell death occurs. egl-1 expression is regulated at the transcriptional and post-transcriptional level. For example, we previously showed that the miR-35 and miR-58 families of miRNAs repress egl-1 expression in mothers of 'unwanted' cells by binding to the 3' UTR of egl-1 mRNA, thereby increasing egl-1 mRNA turnover. In a screen for RNA-binding proteins with a role in the post-transcriptional control of egl-1 expression, we identified EIF-3.H (ortholog of human eIF3H) and HRPR-1 (ortholog human hnRNP R/Q) as potential activators of egl-1 expression. In addition, we demonstrate that the knockdown of the eif-3.H or hrpr-1 gene by RNA-mediated interference (RNAi) results in the inappropriate survival of unwanted cells during C. elegans development. Our study provides novel insight into how egl-1 expression is controlled to cause the reproducible pattern of cell death observed during C. elegans development.

A single neuron in C. elegans orchestrates multiple motor outputs through parallel modes of transmission

Animals generate a wide range of highly coordinated motor outputs, which allows them to execute purposeful behaviors. Individual neurons in the circuits that generate behaviors have a remarkable capacity for flexibility as they exhibit multiple axonal projections, transmitter systems, and modes of neural activity. How these multi-functional properties of neurons enable the generation of adaptive behaviors remains unknown. Here, we show that the HSN neuron in C. elegans evokes multiple motor programs over different timescales to enable a suite of behavioral changes during egg laying. Using HSN activity perturbations and in vivo calcium imaging, we show that HSN acutely increases egg laying and locomotion while also biasing the animals toward low-speed dwelling behavior over minutes. The acute effects of HSN on egg laying and high-speed locomotion are mediated by separate sets of HSN transmitters and different HSN axonal compartments. The long-lasting effects on dwelling are mediated in part by HSN release of serotonin, which is taken up and re-released by NSM, another serotonergic neuron class that directly evokes dwelling. Our results show how the multi-functional properties of a single neuron allow it to induce a coordinated suite of behaviors and also reveal that neurons can borrow serotonin from one another to control behavior.

PUF-8, a C. elegans ortholog of the RNA-binding proteins PUM1 and PUM2, is required for robustness of the cell death fate

During C. elegans development, 1090 somatic cells are generated, of which 959 survive and 131 die, many through apoptosis. We present evidence that PUF-8, a C. elegans ortholog of the mammalian RNA-binding proteins PUM1 and PUM2, is required for the robustness of this 'survival and death' pattern. We found that PUF-8 prevents the inappropriate death of cells that normally survive, and we present evidence that this anti-apoptotic activity of PUF-8 is dependent on the ability of PUF-8 to interact with ced-3 (a C. elegans ortholog of caspase) mRNA, thereby repressing the activity of the pro-apoptotic ced-3 gene. PUF-8 also promotes the death of cells that are programmed to die, and we propose that this pro-apoptotic activity of PUF-8 may depend on the ability of PUF-8 to repress the expression of the anti-apoptotic ced-9 gene (a C. elegans ortholog of Bcl2). Our results suggest that stochastic differences in the expression of genes within the apoptosis pathway can disrupt the highly reproducible and robust survival and death pattern during C. elegans development, and that PUF-8 acts at the post-transcriptional level to level out these differences, thereby ensuring proper cell number homeostasis.

A single neuron in C. elegans orchestrates multiple motor outputs through parallel modes of transmission

Animals generate a wide range of highly coordinated motor outputs, which allows them to execute purposeful behaviors. Individual neuron classes in the circuits that generate behavior have a remarkable capacity for flexibility, as they exhibit multiple axonal projections, transmitter systems, and modes of neural activity. How these multi-functional properties of neurons enable the generation of highly coordinated behaviors remains unknown. Here we show that the HSN neuron in C. elegans evokes multiple motor programs over different timescales to enable a suite of behavioral changes during egg-laying. Using HSN activity perturbations and in vivo calcium imaging, we show that HSN acutely increases egg-laying and locomotion while also biasing the animals towards low-speed dwelling behavior over longer timescales. The acute effects of HSN on egg-laying and high-speed locomotion are mediated by separate sets of HSN transmitters and different HSN axonal projections. The long-lasting effects on dwelling are mediated by HSN release of serotonin that is taken up and re-released by NSM, another serotonergic neuron class that directly evokes dwelling. Our results show how the multi-functional properties of a single neuron allow it to induce a coordinated suite of behaviors and also reveal for the first time that neurons can borrow serotonin from one another to control behavior.

Sexual dimorphism in Caenorhabditis elegans stress resistance

Physiological responses to the environment, disease, and aging vary by sex in many animals, but mechanisms of dimorphism have only recently begun to receive careful attention. The genetic model nematode Caenorhabditis elegans has well-defined mechanisms of stress response, aging, and sexual differentiation. C. elegans has males, but the vast majority of research only uses hermaphrodites. We found that males of the standard N2 laboratory strain were more resistant to hyperosmolarity, heat, and a natural pro-oxidant than hermaphrodites when in mixed-sex groups. Resistance to heat and pro-oxidant were also male-biased in three genetically and geographically diverse C. elegans strains consistent with a species-wide dimorphism that is not specific to domestication. N2 males were also more resistant to heat and pro-oxidant when keep individually indicating that differences in resistance do not require interactions between worms. We found that males induce canonical stress response genes by similar degrees and in similar tissues as hermaphrodites suggesting the importance of other mechanisms. We find that resistance to heat and pro-oxidant are influenced by the sex differentiation transcription factor TRA-1 suggesting that downstream organ differentiation pathways establish differences in stress resistance. Environmental stress influences survival in natural environments, degenerative disease, and aging. Understanding mechanisms of stress response dimorphism can therefore provide insights into sex-specific population dynamics, disease, and longevity.

DMRT Transcription Factors in the Control of Nervous System Sexual Differentiation

Sexual phenotypic differences in the nervous system are one of the most prevalent features across the animal kingdom. The molecular mechanisms responsible for sexual dimorphism throughout metazoan nervous systems are extremely diverse, ranging from intrinsic cell autonomous mechanisms to gonad-dependent endocrine control of sexual traits, or even extrinsic environmental cues. In recent years, the DMRT ancient family of transcription factors has emerged as being central in the development of sex-specific differentiation in all animals in which they have been studied. In this review, we provide an overview of the function of Dmrt genes in nervous system sexual regulation from an evolutionary perspective.

Deep conservation and co-option of programmed cell death facilitates evolution of alternative phenotypes at multiple biological levels

Alternative phenotypes, such as polyphenisms and sexual dimorphisms, are widespread in nature and appear at all levels of biological organization, from genes and cells to morphology and behavior. Yet, our understanding of the mechanisms through which alternative phenotypes develop and how they evolve remains understudied. In this review, we explore the association between alternative phenotypes and programmed cell death, a mechanism responsible for the elimination of superfluous cells during development. We discuss the ancient origins and deep conservation of programmed cell death (its function, forms and underlying core regulatory gene networks), and propose that it was co-opted repeatedly to generate alternative phenotypes at the level of cells, tissues, organs, external morphology, and even individuals. We review several examples from across the tree of life to explore the conditions under which programmed cell death is likely to facilitate the evolution of alternative phenotypes.

Identifying the Caenorhabditis elegans vulval transcriptome

Development of the Caenorhabditis elegans vulva is a classic model of organogenesis. This system, which starts with 6 equipotent cells, encompasses diverse types of developmental event, including developmental competence, multiple signaling events to control precise and faithful patterning of three cell fates, execution and proliferation of specific cell lineages, and a series of sophisticated morphogenetic events. Early events have been subjected to extensive mutational and genetic investigations and later events to cell biological analyses. We infer the existence of dramatically changing profiles of gene expression that accompanies the observed changes in development. Yet, except from serendipitous discovery of several transcription factors expressed in dynamic patterns in vulval lineages, our knowledge of the transcriptomic landscape during vulval development is minimal. This study describes the composition of a vulva-specific transcriptome. We used tissue-specific harvesting of mRNAs via immunoprecipitation of epitope-tagged poly(A) binding protein, PAB-1, heterologously expressed by a promoter known to express GFP in vulval cells throughout their development. The identified transcriptome was small but tightly interconnected. From this data set, we identified several genes with identified functions in development of the vulva and validated more with promoter-GFP reporters of expression. For one target, lag-1, promoter-GFP expression was limited but a fluorescent tag of the endogenous protein revealed extensive expression. Thus, we have identified a transcriptome of C. elegans vulval lineages as a launching pad for exploration of functions of these genes in organogenesis.

Molecular Mechanisms of Sexually Dimorphic Nervous System Patterning in Flies and Worms

Male and female brains display anatomical and functional differences. Such differences are observed in species across the animal kingdom, including humans, but have been particularly well-studied in two classic animal model systems, the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans. Here we summarize recent advances in understanding how the worm and fly brain acquire sexually dimorphic features during development. We highlight the advantages of each system, illustrating how the precise anatomical delineation of sexual dimorphisms in worms has enabled recent analysis into how these dimorphisms become specified during development, and how focusing on sexually dimorphic neurons in the fly has enabled an increasingly detailed understanding of sex-specific behaviors.

Sex Determination: TRA-1 Is a Non-binary Regulator of Sexual Identity

In nematodes, TRA-1 represses the transcription of genes involved in male differentiation, allowing XX animals to undergo normal hermaphrodite development. New reports show that this transcription factor also acts in XO males, to control the differentiation of many neurons.

One template, two outcomes: How does the sex-shared nervous system generate sex-specific behaviors?

Sex-specific behaviors are common in nature and are crucial for reproductive fitness and species survival. A key question in the field of sex/gender neurobiology is whether and to what degree the sex-shared nervous system differs between the sexes in the anatomy, connectivity and molecular identity of its components. An equally intriguing issue is how does the same sex-shared neuronal template diverge to mediate distinct behavioral outputs in females and males. This chapter aims to present the most up-to-date understanding of how this task is achieved in C. elegans. The vast majority of neurons in C. elegans are shared among the two sexes in terms of their lineage history, anatomical position and neuronal identity. Yet a substantial amount of evidence points to the hermaphrodite-male counterparts of some neurons expressing different genes and forming different synaptic connections. This, in turn, enables the same cells and circuits to transmit discrete signals in the two sexes and ultimately execute different functions. We review the various sex-shared behavioral paradigms that have been shown to be sexually dimorphic in recent years, discuss the mechanisms that underlie these examples, refer to the developmental regulation of neuronal dimorphism and suggest evolutionary concepts that emerge from the data.

PIG-1 MELK-dependent phosphorylation of nonmuscle myosin II promotes apoptosis through CES-1 Snail partitioning

The mechanism(s) through which mammalian kinase MELK promotes tumorigenesis is not understood. We find that the C. elegans orthologue of MELK, PIG-1, promotes apoptosis by partitioning an anti-apoptotic factor. The C. elegans NSM neuroblast divides to produce a larger cell that differentiates into a neuron and a smaller cell that dies. We find that in this context, PIG-1 MELK is required for partitioning of CES-1 Snail, a transcriptional repressor of the pro-apoptotic gene egl-1 BH3-only. pig-1 MELK is controlled by both a ces-1 Snail- and par-4 LKB1-dependent pathway, and may act through phosphorylation and cortical enrichment of nonmuscle myosin II prior to neuroblast division. We propose that pig-1 MELK-induced local contractility of the actomyosin network plays a conserved role in the acquisition of the apoptotic fate. Our work also uncovers an auto-regulatory loop through which ces-1 Snail controls its own activity through the formation of a gradient of CES-1 Snail protein.

Apoptosis is critical for the elimination of ‘unwanted’ cells. What distinguishes wanted from unwanted cells in developing animals is poorly understood. We report that in the C. elegans NSM neuroblast lineage, the level of CES-1, a Snail-family member and transcriptional repressor of the pro-apoptotic gene egl-1, contributes to this process. In addition, we demonstrate that C. elegans PIG-1, the orthologue of mammalian proto-oncoprotein MELK, plays a critical role in controlling CES-1^Snail levels. Specifically, during NSM neuroblast division, PIG-1^MELK controls partitioning of CES-1^Snail into one but not the other daughter cell thereby promoting the making of one wanted and one unwanted cell. Furthermore, we present evidence that PIG-1^MELK acts prior to NSM neuroblast division by locally activating the actomyosin network.

Cell death in animal development

Cell death is an important facet of animal development. In some developing tissues, death is the ultimate fate of over 80% of generated cells. Although recent studies have delineated a bewildering number of cell death mechanisms, most have only been observed in pathological contexts, and only a small number drive normal development. This Primer outlines the important roles, different types and molecular players regulating developmental cell death, and discusses recent findings with which the field currently grapples. We also clarify terminology, to distinguish between developmental cell death mechanisms, for which there is evidence for evolutionary selection, and cell death that follows genetic, chemical or physical injury. Finally, we suggest how advances in understanding developmental cell death may provide insights into the molecular basis of developmental abnormalities and pathological cell death in disease.

Signatures of sex: Sex differences in gene expression in the vertebrate brain

Women and men differ in disease prevalence, symptoms, and progression rates for many psychiatric and neurological disorders. As more preclinical studies include both sexes in experimental design, an increasing number of sex differences in physiology and behavior have been reported. In the brain, sex-typical behaviors are thought to result from sex-specific patterns of neural activity in response to the same sensory stimulus or context. These differential firing patterns likely arise as a consequence of underlying anatomic or molecular sex differences. Accordingly, gene expression in the brains of females and males has been extensively investigated, with the goal of identifying biological pathways that specify or modulate sex differences in brain function. However, there is surprisingly little consensus on sex-biased genes across studies and only a handful of robust candidates have been pursued in the follow-up experiments. Furthermore, it is not known how or when sex-biased gene expression originates, as few studies have been performed in the developing brain. Here we integrate molecular genetic and neural circuit perspectives to provide a conceptual framework of how sex differences in gene expression can arise in the brain. We detail mechanisms of gene regulation by steroid hormones, highlight landmark studies in rodents and humans, identify emerging themes, and offer recommendations for future research. This article is categorized under: Nervous System Development > Vertebrates: General Principles Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Gene Expression and Transcriptional Hierarchies > Sex Determination.

Sex-specific regulation of neuronal functions in Caenorhabditis elegans: the sex-determining protein TRA-1 represses goa-1/Gα(i/o)

Females and males differ substantially in various neuronal functions in divergent, sexually dimorphic animal species, including humans. Despite its developmental, physiological and medical significance, understanding the molecular mechanisms by which sex-specific differences in the anatomy and operation of the nervous system are established remains a fundamental problem in biology. Here, we show that in Caenorhabditis elegans (nematodes), the global sex-determining factor TRA-1 regulates food leaving (mate searching), male mating and adaptation to odorants in a sex-specific manner by repressing the expression of goa-1 gene, which encodes the Gα(i/o) subunit of heterotrimeric G (guanine–nucleotide binding) proteins triggering physiological responses elicited by diverse neurotransmitters and sensory stimuli. Mutations in tra-1 and goa-1 decouple behavioural patterns from the number of X chromosomes. TRA-1 binds to a conserved binding site located in the goa-1 coding region, and downregulates goa-1 expression in hermaphrodites, particularly during embryogenesis when neuronal development largely occurs. These data suggest that the sex-determination machinery is an important modulator of heterotrimeric G protein-mediated signalling and thereby various neuronal functions in this organism and perhaps in other animal phyla.

Dynamic Regulation of Adult-Specific Functions of the Nervous System by Signaling from the Reproductive System

Unlike juveniles, adult animals engage in suites of behaviors related to the search for and selection of potential mates and mating, including appropriate responses to sex pheromones. As in other species [1], male sex pheromones modulate several behaviors and physiological processes in C. elegans hermaphrodites [2-5]. In particular, one of these small-molecule signals, an ascaroside ascr#10, causes reduced exploration, more avid mating, and improved reproductive performance (see the accompanying paper by Aprison and Ruvinsky in this issue of Current Biology) [6]. Here, we investigated the mechanism that restricts pheromone response to adult hermaphrodites. Unexpectedly, we found that attainment of developmental adulthood was not alone sufficient for the behavioral response to the pheromone. To modify exploratory behavior in response to male pheromone, adult hermaphrodites also require functional germline and egg-laying apparatus. We show that this dependence of behavior on the reproductive system is due to feedback from the vulva muscles that reports ongoing reproduction to the nervous system. Our results reveal an activity-dependent conduit by which the reproductive system continuously licenses adult behaviors, including appropriate responses to the pheromones of the opposite sex. More broadly, our results suggest that signals from peripheral organs may serve as an important component of assuring age-appropriate functions of the nervous system.

Coordinated Behavioral and Physiological Responses to a Social Signal Are Regulated by a Shared Neuronal Circuit

Successful reproduction in animals requires orchestration of behavior and physiological processes. Pheromones can induce both "releaser" (behavioral) and "priming" (physiological) effects [1] in vertebrates [2, 3] and invertebrates [4, 5]. Therefore, understanding the mechanisms underlying pheromone responses could reveal how reproduction-related behaviors and physiology are coordinated. Here, we describe a neuronal circuit that couples the reproductive system and behavior in adult Caenorhabditis elegans hermaphrodites. We found that the response of the oogenic germline to the male pheromone requires serotonin signal from NSM and HSN neurons that acts via the mod-1 receptor in AIY and RIF interneurons and is antagonized by pigment-dispersing factor (PDF). Surprisingly, the same neurons and pathways have been previously implicated in regulation of exploratory behavior in the absence of male-produced signals [6]. We demonstrate that male pheromone acts via this circuit in hermaphrodites to reduce exploration and decrease mating latency, thereby tuning multiple fitness-proximal processes. Our results demonstrate how a single circuit could coordinate behavioral and physiological responses to the environment, even those that unfold on different timescales. Our findings suggest the existence of a centralized regulatory mechanism that balances organismal resources between reproductive investment and somatic maintenance.

Activity-Dependent Regulation of the Proapoptotic BH3-Only Gene egl-1 in a Living Neuron Pair in Caenorhabditis elegans

The BH3-only family of proteins is key for initiating apoptosis in a variety of contexts, and may also contribute to non-apoptotic cellular processes. Historically, the nematode Caenorhabditis elegans has provided a powerful system for studying and identifying conserved regulators of BH3-only proteins. In C. elegans, the BH3-only protein egl-1 is expressed during development to cell-autonomously trigger most developmental cell deaths. Here we provide evidence that egl-1 is also transcribed after development in the sensory neuron pair URX without inducing apoptosis. We used genetic screening and epistasis analysis to determine that its transcription is regulated in URX by neuronal activity and/or in parallel by orthologs of Protein Kinase G and the Salt-Inducible Kinase family. Because several BH3-only family proteins are also expressed in the adult nervous system of mammals, we suggest that studying egl-1 expression in URX may shed light on mechanisms that regulate conserved family members in higher organisms.

The Hox gene Abdominal-B uses DoublesexF as a cofactor to promote neuroblast apoptosis in the Drosophila central nervous system

Highly conserved DM domain-containing transcription factors (Doublesex/MAB-3/DMRT1) are responsible for generating sexually dimorphic features. In the Drosophila central nervous system, a set of Doublesex (Dsx)-expressing neuroblasts undergo apoptosis in females whereas their male counterparts proliferate and give rise to serotonergic neurons crucial for adult mating behaviour. Our study demonstrates that the female-specific isoform of Dsx collaborates with Hox gene Abdominal-B (Abd-B) to bring about this apoptosis. Biochemical results suggest that proteins AbdB and Dsx interact through their highly conserved homeodomain and DM domain, respectively. This interaction is translated into a cooperative binding of the two proteins on the apoptotic enhancer in the case of females but not in the case of males, resulting in female-specific activation of apoptotic genes. The capacity of AbdB to use the sex-specific isoform of Dsx as a cofactor underlines the possibility that these two classes of protein are capable of cooperating in selection and regulation of target genes in a tissue- and sex-specific manner. We propose that this interaction could be a common theme in generating sexual dimorphism in different tissues across different species.

Novel Technological Advances in Functional Connectomics in C. elegans

The complete structure and connectivity of the Caenorhabditis elegans nervous system ("mind of a worm") was first published in 1986, representing a critical milestone in the field of connectomics. The reconstruction of the nervous system (connectome) at the level of synapses provided a unique perspective of understanding how behavior can be coded within the nervous system. The following decades have seen the development of technologies that help understand how neural activity patterns are connected to behavior and modulated by sensory input. Investigations on the developmental origins of the connectome highlight the importance of role of neuronal cell lineages in the final connectivity matrix of the nervous system. Computational modeling of neuronal dynamics not only helps reconstruct the biophysical properties of individual neurons but also allows for subsequent reconstruction of whole-organism neuronal network models. Hence, combining experimental datasets with theoretical modeling of neurons generates a better understanding of organismal behavior. This review discusses some recent technological advances used to analyze and perturb whole-organism neuronal function along with developments in computational modeling, which allows for interrogation of both local and global neural circuits, leading to different behaviors. Combining these approaches will shed light into how neural networks process sensory information to generate the appropriate behavioral output, providing a complete understanding of the worm nervous system.

Molecular and cellular modulators for multisensory integration in C. elegans

In the natural environment, animals often encounter multiple sensory cues that are simultaneously present. The nervous system integrates the relevant sensory information to generate behavioral responses that have adaptive values. However, the neuronal basis and the modulators that regulate integrated behavioral response to multiple sensory cues are not well defined. Here, we address this question using a behavioral decision in C. elegans when the animal is presented with an attractive food source together with a repulsive odorant. We identify specific sensory neurons, interneurons and neuromodulators that orchestrate the decision-making process, suggesting that various states and contexts may modulate the multisensory integration. Among these modulators, we characterize a new function of a conserved TGF-β pathway that regulates the integrated decision by inhibiting the signaling from a set of central neurons. Interestingly, we find that a common set of modulators, including the TGF-β pathway, regulate the integrated response to the pairing of different foods and repellents. Together, our results provide mechanistic insights into the modulatory signals regulating multisensory integration.

The present study characterizes the modulation of a behavioral decision in C. elegans when the worm is presented with a food lawn that is paired with a repulsive smell. We show that multiple specific sensory neurons and interneurons play roles in making the decision. We also identify several modulatory molecules that are essential for the integrated decision when the animal faces a choice between the cues of opposing valence. We further show that many of these factors, which often represent different states and contexts, are common for behavioral decisions that integrate sensory information from different types of foods and repellents. Overall, our results reveal the molecular and cellular basis for integration of simultaneously present attractive and repulsive cues to fine-tune decision-making.

Sex-specific regulation of aging in Caenorhabditis elegans

A fascinating aspect of sexual dimorphism in various animal species is that the two sexes differ substantially in lifespan. In humans, for example, women's life expectancy exceeds that of men by 3-7 years. Whether this trait can be attributed to dissimilar lifestyles or genetic (regulatory) factors remains to be elucidated. Herein, we demonstrate that in the nematode Caenorhabditis elegans, the significantly longer lifespan of hermaphrodites-which are essentially females capable of sperm production-over males is established by TRA-1, the terminal effector of the sex-determination pathway. This transcription factor directly controls the expression of daf-16/FOXO, which functions as a major target of insulin/IGF-1 signaling (IIS) and key modulator of aging across diverse animal phyla. TRA-1 extends hermaphrodite lifespan through promoting daf-16 activity. Furthermore, TRA-1 also influences reproductive growth in a DAF-16-dependent manner. Thus, the sex-determination machinery is an important regulator of IIS in this organism. These findings provide a mechanistic insight into how longevity and development are specified unequally in the two genders. As TRA-1 is orthologous to mammalian GLI (glioma-associated) proteins, a similar sex-specific mechanism may also operate in humans to determine lifespan.

Neuronal Cell Death

Neuronal cell death occurs extensively during development and pathology, where it is especially important because of the limited capacity of adult neurons to proliferate or be replaced. The concept of cell death used to be simple as there were just two or three types, so we just had to work out which type was involved in our particular pathology and then block it. However, we now know that there are at least a dozen ways for neurons to die, that blocking a particular mechanism of cell death may not prevent the cell from dying, and that non-neuronal cells also contribute to neuronal death. We review here the mechanisms of neuronal death by intrinsic and extrinsic apoptosis, oncosis, necroptosis, parthanatos, ferroptosis, sarmoptosis, autophagic cell death, autosis, autolysis, paraptosis, pyroptosis, phagoptosis, and mitochondrial permeability transition. We next explore the mechanisms of neuronal death during development, and those induced by axotomy, aberrant cell-cycle reentry, glutamate (excitoxicity and oxytosis), loss of connected neurons, aggregated proteins and the unfolded protein response, oxidants, inflammation, and microglia. We then reassess which forms of cell death occur in stroke and Alzheimer's disease, two of the most important pathologies involving neuronal cell death. We also discuss why it has been so difficult to pinpoint the type of neuronal death involved, if and why the mechanism of neuronal death matters, the molecular overlap and interplay between death subroutines, and the therapeutic implications of these multiple overlapping forms of neuronal death.

Sexual Dimorphism and Sex Differences in Caenorhabditis elegans Neuronal Development and Behavior

As fundamental features of nearly all animal species, sexual dimorphisms and sex differences have particular relevance for the development and function of the nervous system. The unique advantages of the nematode Caenorhabditis elegans have allowed the neurobiology of sex to be studied at unprecedented scale, linking ultrastructure, molecular genetics, cell biology, development, neural circuit function, and behavior. Sex differences in the C. elegans nervous system encompass prominent anatomical dimorphisms as well as differences in physiology and connectivity. The influence of sex on behavior is just as diverse, with biological sex programming innate sex-specific behaviors and modifying many other aspects of neural circuit function. The study of these differences has provided important insights into mechanisms of neurogenesis, cell fate specification, and differentiation; synaptogenesis and connectivity; principles of circuit function, plasticity, and behavior; social communication; and many other areas of modern neurobiology.

Sexual modulation of sex-shared neurons and circuits in Caenorhabditis elegans

Studies using the nematode C. elegans have provided unique insights into the development and function of sex differences in the nervous system. Enabled by the relative simplicity of this species, comprehensive studies have solved the complete cellular neuroanatomy of both sexes as well as the complete neural connectomes of the entire adult hermaphrodite and the adult male tail. This work, together with detailed behavioral studies, has revealed three aspects of sex differences in the nervous system: sex-specific neurons and circuits; circuits with sexually dimorphic synaptic connectivity; and sex differences in the physiology and functions of shared neurons and circuits. At all of these levels, biological sex influences neural development and function through the activity of a well-defined genetic hierarchy that acts throughout the body to translate chromosomal sex into the state of a master autosomal regulator of sexual differentiation, the transcription factor TRA-1A. This Review focuses on the role of genetic sex in implementing sex differences in shared neurons and circuits, with an emphasis on linking the sexual modulation of specific neural properties to the specification and optimization of sexually divergent and dimorphic behaviors. An important and unexpected finding from these studies is that chemosensory neurons are a primary focus of sexual modulation, with genetic sex adaptively shaping chemosensory repertoire to guide behavioral choice. Importantly, hormone-independent functions of genetic sex are the principal drivers of all of these sex differences, making nematodes an excellent model for understanding similar but poorly understood mechanisms that likely act throughout the animal kingdom. © 2016 Wiley Periodicals, Inc.

Programmed Cell Death During Caenorhabditis elegans Development

Programmed cell death is an integral component of Caenorhabditis elegans development. Genetic and reverse genetic studies in C. elegans have led to the identification of many genes and conserved cell death pathways that are important for the specification of which cells should live or die, the activation of the suicide program, and the dismantling and removal of dying cells. Molecular, cell biological, and biochemical studies have revealed the underlying mechanisms that control these three phases of programmed cell death. In particular, the interplay of transcriptional regulatory cascades and networks involving multiple transcriptional regulators is crucial in activating the expression of the key death-inducing gene egl-1 and, in some cases, the ced-3 gene in cells destined to die. A protein interaction cascade involving EGL-1, CED-9, CED-4, and CED-3 results in the activation of the key cell death protease CED-3, which is tightly controlled by multiple positive and negative regulators. The activation of the CED-3 caspase then initiates the cell disassembly process by cleaving and activating or inactivating crucial CED-3 substrates; leading to activation of multiple cell death execution events, including nuclear DNA fragmentation, mitochondrial elimination, phosphatidylserine externalization, inactivation of survival signals, and clearance of apoptotic cells. Further studies of programmed cell death in C. elegans will continue to advance our understanding of how programmed cell death is regulated, activated, and executed in general.

miRNAs cooperate in apoptosis regulation during C. elegans development

Sherrard et al. demonstrate that, during embryogenesis, miR-35 and miR-58 bantam family miRNAs cooperate to prevent the precocious death of mothers of cells programmed to die by repressing the gene egl-1, which encodes a proapoptotic BH3-only protein.

Programmed cell death occurs in a highly reproducible manner during Caenorhabditis elegans development. We demonstrate that, during embryogenesis, miR-35 and miR-58 bantam family microRNAs (miRNAs) cooperate to prevent the precocious death of mothers of cells programmed to die by repressing the gene egl-1, which encodes a proapoptotic BH3-only protein. In addition, we present evidence that repression of egl-1 is dependent on binding sites for miR-35 and miR-58 family miRNAs within the egl-1 3′ untranslated region (UTR), which affect both mRNA copy number and translation. Furthermore, using single-molecule RNA fluorescent in situ hybridization (smRNA FISH), we show that egl-1 is transcribed in the mother of a cell programmed to die and that miR-35 and miR-58 family miRNAs prevent this mother from dying by keeping the copy number of egl-1 mRNA below a critical threshold. Finally, miR-35 and miR-58 family miRNAs can also dampen the transcriptional boost of egl-1 that occurs specifically in a daughter cell that is programmed to die. We propose that miRNAs compensate for lineage-specific differences in egl-1 transcriptional activation, thus ensuring that EGL-1 activity reaches the threshold necessary to trigger death only in daughter cells that are programmed to die.

"The persistence of memory"-Hermaphroditism in nematodes

Self-fertility has evolved many times in nematodes. This transition often produces an androdioecious species, with XX hermaphrodites and XO males. Although these hermaphrodites resemble females in most respects, early germ cells differentiate as sperm, and late ones as oocytes. The sperm then receive an activation signal, populate the spermathecae, and are stored for later use in self-fertilization. These traits are controlled by complex modifications to the sex-determination and sperm activation pathways, which have arisen independently during the evolution of each hermaphroditic species. This transformation in reproductive strategy then promotes other major changes in the development, evolution, and population structure of these animals. Mol. Reprod. Dev. 84: 144-157, 2017. © 2016 Wiley Periodicals, Inc.

Programmed cell death and clearance of cell corpses in Caenorhabditis elegans

Programmed cell death is critical to the development of diverse animal species from C. elegans to humans. In C. elegans, the cell death program has three genetically distinguishable phases. During the cell suicide phase, the core cell death machinery is activated through a protein interaction cascade. This activates the caspase CED-3, which promotes numerous pro-apoptotic activities including DNA degradation and exposure of the phosphatidylserine "eat me" signal on the cell corpse surface. Specification of the cell death fate involves transcriptional activation of the cell death initiator EGL-1 or the caspase CED-3 by coordinated actions of specific transcription factors in distinct cell types. In the cell corpse clearance stage, recognition of cell corpses by phagocytes triggers several signaling pathways to induce phagocytosis of apoptotic cell corpses. Cell corpse-enclosing phagosomes ultimately fuse with lysosomes for digestion of phagosomal contents. This article summarizes our current knowledge about programmed cell death and clearance of cell corpses in C. elegans.

Both the apoptotic suicide pathway and phagocytosis are required for a programmed cell death in Caenorhabditis elegans

Background

Programmed cell deaths in the nematode Caenorhabditis elegans are generally considered suicides. Dying cells are engulfed by neighboring cells in a process of phagocytosis. To better understand the interaction between the engulfment and death processes, we analyzed B.al/rapaav cell death, which has been previously described as engulfment-dependent and hence as a possible murder.

Results

We found that B.al/rapaav is resistant to caspase-pathway activation: the caspase-mediated suicide pathway initiates the cell-death process but is insufficient to cause B.al/rapaav death without the subsequent assistance of engulfment. When the engulfing cell P12.pa is absent, other typically non-phagocytic cells can display cryptic engulfment potential and facilitate this death.

Conclusions

We term this death an “assisted suicide” and propose that assisted suicides likely occur in other organisms. The study of assisted suicides might provide insight into non-cell autonomous influences on cell death. Understanding the mechanism that causes B.al/rapaav to be resistant to activation of the caspase pathway might reveal the basis of differences in the sensitivity to apoptotic stimuli of tumor and normal cells, a key issue in the field of cancer therapeutics.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0262-5) contains supplementary material, which is available to authorized users.

Neural circuits for sexually dimorphic and sexually divergent behaviors in Caenorhabditis elegans

Increasing interest in sex differences in Caenorhabditis elegans neurobiology is resulting from several advances, including the completion of the male tail connectome and the surprising discovery of two 'new' neurons in the male head. In this species, sex-specific circuits in the hermaphrodite and male control reproductive behaviors such as egg-laying and copulation, respectively. Studies of these systems are revealing interesting similarities and contrasts, particularly in the mechanisms by which nutritional status influences reproductive behaviors. Other studies have highlighted the importance of sexual modulation of shared neurons and circuits in optimizing behavioral strategies. Together, these findings indicate that C. elegans uses intertwined, distributed sex differences in circuit structure and function to implement sex-specific as well as sexually divergent, shared behaviors.

Cell-Specific mRNA Profiling of the Caenorhabditis elegans Somatic Gonadal Precursor Cells Identifies Suites of Sex-Biased and Gonad-Enriched Transcripts

The Caenorhabditis elegans somatic gonad differs greatly between the two sexes in its pattern of cell divisions, migration, and differentiation. Despite decades of study, the genetic pathways directing early gonadal development and establishing sexual dimorphism in the gonad remain largely unknown. To help define the genetic networks that regulate gonadal development, we employed cell-specific RNA-seq. We identified transcripts present in the somatic gonadal precursor cells and their daughter cells of each sex at the onset of sexual differentiation. We identified several hundred gonad-enriched transcripts, including the majority of known regulators of early gonadal development, and transgenic reporter analysis confirmed the effectiveness of this approach. Before the division of the somatic gonad precursors, few sex-biased gonadal transcripts were detectable; less than 6 hr later, after their division, we identified more than 250 sex-biased transcripts, of which about a third were enriched in the somatic gonad compared to the whole animal. This indicates that a robust sex-biased developmental program, some of it gonad-specific, initiates in the somatic gonadal precursor cells around the time of their first division. About 10% of male-biased transcripts had orthologs with male-biased expression in the early mouse gonad, suggesting possible conservation of gonad sex differentiation. Cell-specific analysis also identified approximately 70 previously unannotated mRNA isoforms that are enriched in the somatic gonad. Our data illustrate the power of cell-specific transcriptome analysis and suggest that early sex differentiation in the gonad is controlled by a relatively small suite of differentially expressed genes, even after dimorphism has become apparent.

Cell Death in C. elegans Development

Cell death is a common and important feature of animal development, and cell death defects underlie many human disease states. The nematode Caenorhabditis elegans has proven fertile ground for uncovering molecular and cellular processes controlling programmed cell death. A core pathway consisting of the conserved proteins EGL-1/BH3-only, CED-9/BCL2, CED-4/APAF1, and CED-3/caspase promotes most cell death in the nematode, and a conserved set of proteins ensures the engulfment and degradation of dying cells. Multiple regulatory pathways control cell death onset in C. elegans, and many reveal similarities with tumor formation pathways in mammals, supporting the idea that cell death plays key roles in malignant progression. Nonetheless, a number of observations suggest that our understanding of developmental cell death in C. elegans is incomplete. The interaction between dying and engulfing cells seems to be more complex than originally appreciated, and it appears that key aspects of cell death initiation are not fully understood. It has also become apparent that the conserved apoptotic pathway is dispensable for the demise of the C. elegans linker cell, leading to the discovery of a previously unexplored gene program promoting cell death. Here, we review studies that formed the foundation of cell death research in C. elegans and describe new observations that expand, and in some cases remodel, this edifice. We raise the possibility that, in some cells, more than one death program may be needed to ensure cell death fidelity.

Identification of vacuoles containing extraintestinal differentiated forms of Legionella pneumophila in colonized Caenorhabditis elegans soil nematodes

Legionella pneumophila, a causative agent of Legionnaires’ disease, is a facultative intracellular parasite of freshwater protozoa. Legionella pneumophila features a unique developmental network that involves several developmental forms including the infectious cyst forms. Reservoirs of L. pneumophila include natural and man-made freshwater systems; however, recent studies have shown that isolates of L. pneumophila can also be obtained directly from garden potting soil suggesting the presence of an additional reservoir. A previous study employing the metazoan Caenorhabditis elegans, a member of the Rhabditidae family of free-living soil nematodes, demonstrated that the intestinal lumen can be colonized with L. pneumophila. While both replicative forms and differentiated forms were observed in C. elegans, these morphologically distinct forms were initially observed to be restricted to the intestinal lumen. Using live DIC imaging coupled with focused transmission electron microscopy analyses, we report here that L. pneumophila is able to invade and establish Legionella-containing vacuoles (LCVs) in the intestinal cells. In addition, LCVs containing replicative and differentiated cyst forms were observed in the pseudocoelomic cavity and gonadal tissue of nematodes colonized with L. pneumophila. Furthermore, establishment of LCVs in the gonadal tissue was Dot/Icm dependent and required the presence of the endocytic factor RME-1 to gain access to maturing oocytes. Our findings are novel as this is the first report, to our knowledge, of extraintestinal LCVs containing L. pneumophila cyst forms in C. elegans tissues, highlighting the potential of soil-dwelling nematodes as an alternate environmental reservoir for L. pneumophila.

The C. elegans COE transcription factor UNC-3 activates lineage-specific apoptosis and affects neurite growth in the RID lineage

Mechanisms that regulate apoptosis in a temporal and lineage-specific manner remain poorly understood. The COE (Collier/Olf/EBF) transcription factors have been implicated in the development of many cell types, including neurons. Here, we show that the sole Caenorhabditis elegans COE protein, UNC-3, together with a histone acetyltransferase, CBP-1/P300, specifies lineage-specific apoptosis and certain aspects of neurite trajectory. During embryogenesis, the RID progenitor cell gives rise to the RID neuron and RID sister cell; the latter undergoes apoptosis shortly after cell division upon expression of the pro-apoptotic gene egl-1. We observe UNC-3 expression in the RID progenitor, and the absence of UNC-3 results in the failure of the RID lineage to express a Pegl-1::GFP reporter and in the survival of the RID sister cell. Lastly, UNC-3 interacts with CBP-1, and cbp-1 mutants exhibit a similar RID phenotype to unc-3. Thus, in addition to playing a role in neuronal terminal differentiation, UNC-3 is a cell lineage-specific regulator of apoptosis.

Developmental alterations of the C. elegans male anal depressor morphology and function require sex-specific cell autonomous and cell non-autonomous interactions

We studied the Caenorhabditis elegans anal depressor development in larval males and hermaphrodites to address how a differentiated cell sex-specifically changes its morphology prior to adulthood. In both sexes, the larval anal depressor muscle is used for defecation behavior. However in the adult males, the muscle's sarcomere is reorganized to facilitate copulation. To address when the changes occur in the anal depressor, we used YFP:actin to monitor, and mutant analysis, laser-ablation and transgenic feminization to perturb the cell's morphological dynamics. In L1 and L2 stage larva, the muscle of both sexes has similar sarcomere morphology, but the hermaphrodite sex-determination system promotes more growth. The male anal depressor begins to change in the L3 stage, first by retracting its muscle arm from the neurons of the defecation circuit. Then the muscle's ventral region develops a slit that demarcates an anterior and posterior domain. This demarcation is not dependent on the anal depressor's intrinsic genetic sex, but is influenced by extrinsic interactions with the developing male sex muscles. However, subsequent changes are dependent on the cell's sex. In the L4 stage, the anterior domain first disassembles the dorsal-ventral sarcomere region and develops filopodia that elongates anteriorly towards the spicule muscles. Later, the posterior domain dissembles the remnants of its sarcomere, but still retains a vestigial attachment to the ventral body wall. Finally, the anterior domain attaches to the sex muscles, and then reassembles an anterior-posteriorly oriented sarcomere. Our work identifies key steps in the dimorphic re-sculpting of the anal depressor that are regulated by genetic sex and by cell-cell signaling.

Patterning of sexually dimorphic neurogenesis in the caenorhabditis elegans ventral cord by Hox and TALE homeodomain transcription factors

BACKGROUND

Reproduction in animals requires development of distinct neurons in each sex. In C. elegans, most ventral cord neurons (VCNs) are present in both sexes, with the exception of six hermaphrodite-specific neurons (VCs) and nine pairs of male-specific neurons (CAs and CPs) that arise from analogous precursor cells. How are the activities of sexual regulators and mediators of neuronal survival, division, and fate coordinated to generate sex-specificity in VCNs?

RESULTS

To address this, we have developed a toolkit of VCN markers that allows us to examine sex-specific neurogenesis, asymmetric fates of daughters of a neuroblast division, and regional specification on the anteroposterior axis. Here, we describe the roles of the Hox transcription factors LIN-39 and MAB-5 in promoting survival, differentiation, and regionalization of VCNs. We also find that the TALE class homeodomain proteins CEH-20 and UNC-62 contribute to specification of neurotransmitter fate in males. Furthermore, we identify that VCN sex is determined during the L1 larval stage.

CONCLUSIONS

These findings, combined with future analyses made possible by the suite of VCN markers described here, will elucidate how Hox-mediated cell fate decisions and sex determination intersect to influence development of neuronal sex differences.

Differential regulation of germline apoptosis in response to meiotic checkpoint activation

In Caenorhabditis elegans, germline apoptosis is promoted by egl-1 and ced-13 in response to meiotic checkpoint activation. We report that the requirement for these two factors depends on which checkpoints are active. We also identify a regulatory region of egl-1 required to inhibit germline apoptosis in response to DNA damage incurred during meiotic recombination.

LIN-3/EGF promotes the programmed cell death of specific cells in Caenorhabditis elegans by transcriptional activation of the pro-apoptotic gene egl-1

Programmed cell death (PCD) is the physiological death of a cell mediated by an intracellular suicide program. Although key components of the PCD execution pathway have been identified, how PCD is regulated during development is poorly understood. Here, we report that the epidermal growth factor (EGF)-like ligand LIN-3 acts as an extrinsic signal to promote the death of specific cells in Caenorhabditis elegans. The loss of LIN-3 or its receptor, LET-23, reduced the death of these cells, while excess LIN-3 or LET-23 signaling resulted in an increase in cell deaths. Our molecular and genetic data support the model that the LIN-3 signal is transduced through LET-23 to activate the LET-60/RAS-MPK-1/ERK MAPK pathway and the downstream ETS domain-containing transcription factor LIN-1. LIN-1 binds to, and activates transcription of, the key pro-apoptotic gene egl-1, which leads to the death of specific cells. Our results provide the first evidence that EGF induces PCD at the whole organism level and reveal the molecular basis for the death-promoting function of LIN-3/EGF. In addition, the level of LIN-3/EGF signaling is important for the precise fine-tuning of the life-versus-death fate. Our data and the previous cell culture studies that say EGF triggers apoptosis in some cell lines suggest that the EGF-mediated modulation of PCD is likely conserved in C. elegans and humans.

Programmed cell death (PCD) is an evolutionarily conserved cellular process that is important for metazoan development and homeostasis. The epidermal growth factor (EGF) promotes cell proliferation, differentiation and survival during animal development. Surprisingly, we found that the EGF-like ligand LIN-3 also promotes the death of specific cells in Caenorhabditis elegans. We found that the LIN-3/EGF signal can be secreted from a cell to facilitate the demise of cells at a distance by activating the transcription of the PCD-promoting gene egl-1 in the doomed cells through the transcription factor LIN-1. LIN-1 binds to the egl-1 promoter in vitro and is positively regulated by the LIN-3/EGF, LET-23/EGF receptor, and the downstream MAPK signaling pathway. To our knowledge, LIN-3/EGF is the first extrinsic signal that has been shown to regulate the intrinsic PCD machinery during C. elegans development. In addition, the transcription factor LIN-31, which binds to LIN-1 and acts downstream of LIN-3/EGF, LET-23/EGF receptor, and the MAPK signaling pathway during vulval development, is dispensable for PCD. Thus, LIN-3/EGF promotes cell proliferation, differentiation, and PCD through common downstream signaling molecules but acts via distinct sets of transcription factors for different target gene expression.

The evolutionary origins and consequences of self-fertility in nematodes

Self-fertile hermaphrodites have evolved from male/female ancestors in many nematode species, and this transition occurred on three independent occasions in the genus Caenorhabditis. Genetic analyses in Caenorhabditis show that the origin of hermaphrodites required two types of changes: alterations to the sex-determination pathway that allowed otherwise female animals to make sperm during larval development, and the production of signals from the gonad that caused these sperm to activate and fertilize oocytes. Comparisons of C. elegans and C. briggsae hermaphrodites show that the ancestral sex-determination pathway has been altered in multiple unique ways. Some of these changes must have precipitated the production of sperm in XX animals, and others were modifying mutations that increased the efficiency of hermaphroditic reproduction. Reverse genetic experiments show that XX animals acquired the ability to activate sperm by co-opting one of the two redundant pathways that normally work in males. Finally, the adoption of a hermaphroditic lifestyle had profound effects on ecological and sexual interactions and genomic organization. Thus, nematode mating systems are ideal for elucidating the origin of novel traits, and studying the influence of developmental processes on evolutionary change.

The development of sexual dimorphism: studies of the Caenorhabditis elegans male

Studies of the development of the Caenorhabditis elegans male have been carried out with the aim of understanding the basis of sexual dimorphism. Postembryonic development of the two C. elegans sexes differs extensively. Development along either the hermaphrodite or male pathway is specified initially by the X to autosome ratio. The regulatory events initiated by this ratio include a male-determining paracrine intercellular signal. Expression of this signal leads to different consequences in three regions of the body: the nongonadal soma, the somatic parts of the gonad, and the germ line. In the nongonadal soma, activity of the key Zn-finger transcription factor TRA-1 determines hermaphrodite development; in its absence, the male pathway is followed. Only a few genes directly regulated by TRA-1 are currently known, including members of the evolutionarily conserved, male-determining DM domain Zn-finger transcription factors. In the somatic parts of the gonad and germ line, absence of TRA-1 activity is not sufficient for full expression of the male pathway. Several additional transcription factors involved have been identified. In the germ line, regulatory genes for sperm development that act at the level of RNA in the cytoplasm play a prominent role.

Sexual modulation of neural circuits and behavior in Caenorhabditis elegans

Sex differences in behavior-both sex-specific and shared behaviors-are fundamental to nearly all animal species. One often overlooked mechanism by which these behavioral differences can be generated is through sex-specific modulation of shared circuitry (i.e., circuits present in both sexes). In vertebrates this modulation is likely regulated by hormone-dependent mechanisms as well as by somatic sex itself; invertebrate models have particular promise for understanding the latter of these. Here we review molecular and behavioral evidence of sexual modulation of shared circuitry in the nematode Caenorhabditis elegans. Multiple behaviors in this species, both copulatory and not, are modulated by the genetic sex of shared neurons and circuit. These studies are close to uncovering the molecular mechanisms by which somatic sex modulates neural function in the worm, mechanisms which may be well conserved in more complex organisms. Improving our understanding of the modulation of neural circuit development and function by somatic sex may lend important insight into sex differences in the mammalian nervous system which, in turn, may have important implications for sex biases in disease.

A role for WDR5 in TRA-1/Gli mediated transcriptional control of the sperm/oocyte switch in C. elegans

The hermaphrodite germline of Caenorhabditis elegans initially engages in spermatogenesis and then switches to oogenesis during late stages of larval development. TRA-1, a member of the Ci/Gli family of transcriptional repressors, plays an essential role in this switch by repressing genes that promote spermatogenesis. WDR5 proteins are conserved components of histone methyltransferase complexes normally associated with gene activation. However, two C. elegans WDR5 homologs, wdr-5.1 and wdr-5.2 are redundantly required for normal TRA-1 dependent repression, and this function is independent of their roles in histone methylation. Animals lacking wdr-5.1/wdr-5.2 function fail to switch to oogenesis at 25°C, resulting in a masculinization of germline (Mog) phenotype. The Mog phenotype is caused by ectopic expression of fog-3, a direct target of TRA-1 repression. WDR-5.1 associates with the fog-3 promoter and is required for TRA-1 to bind to fog-3 promoter. Other direct targets of TRA-1 are similarly derepressed in the double mutant. These results show that WDR5 plays a novel and important role in stabilizing transcriptional repression during C. elegans sex determination, and provide evidence that this important protein may operate independently of its established role in histone methyltransferase complexes.

PDR-1/hParkin negatively regulates the phagocytosis of apoptotic cell corpses in Caenorhabditis elegans

Apoptotic cell death is an integral part of cell turnover in many tissues, and proper corpse clearance is vital to maintaining tissue homeostasis in all multicellular organisms. Even in tissues with high cellular turnover, apoptotic cells are rarely seen because of efficient clearance mechanisms in healthy individuals. In Caenorhabditis elegans, two parallel and partly redundant conserved pathways act in cell corpse engulfment. The pathway for cytoskeletal rearrangement requires the small GTPase CED-10 Rac1 acting for an efficient surround of the dead cell. The CED-10 Rac pathway is also required for the proper migration of the distal tip cells (DTCs) during the development of the C. elegans gonad. Parkin, the mammalian homolog of the C. elegans PDR-1, interacts with Rac1 in aged human brain and it is also implicated with actin dynamics and cytoskeletal rearrangements in Parkinsons's disease, suggesting that it might act on engulfment. Our genetic and biochemical studies indicate that PDR-1 inhibits apoptotic cell engulfment and DTC migration by ubiquitylating CED-10 for degradation.

Evolutionary change within a bipotential switch shaped the sperm/oocyte decision in hermaphroditic nematodes

A subset of transcription factors like Gli2 and Oct1 are bipotential--they can activate or repress the same target, in response to changing signals from upstream genes. Some previous studies implied that the sex-determination protein TRA-1 might also be bipotential; here we confirm this hypothesis by identifying a co-factor, and use it to explore how the structure of a bipotential switch changes during evolution. First, null mutants reveal that C. briggsae TRR-1 is required for spermatogenesis, RNA interference implies that it works as part of the Tip60 Histone Acetyl Transferase complex, and RT-PCR data show that it promotes the expression of Cbr-fog-3, a gene needed for spermatogenesis. Second, epistasis tests reveal that TRR-1 works through TRA-1, both to activate Cbr-fog-3 and to control the sperm/oocyte decision. Since previous studies showed that TRA-1 can repress fog-3 as well, these observations demonstrate that it is bipotential. Third, TRR-1 also regulates the development of the male tail. Since Cbr-tra-2 Cbr-trr-1 double mutants resemble Cbr-tra-1 null mutants, these two regulatory branches control all tra-1 activity. Fourth, striking differences in the relationship between these two branches of the switch have arisen during recent evolution. C. briggsae trr-1 null mutants prevent hermaphrodite spermatogenesis, but not Cbr-fem null mutants, which disrupt the other half of the switch. On the other hand, C. elegans fem null mutants prevent spermatogenesis, but not Cel-trr-1 mutants. However, synthetic interactions confirm that both halves of the switch exist in each species. Thus, the relationship between the two halves of a bipotential switch can shift rapidly during evolution, so that the same phenotype is produce by alternative, complementary mechanisms.

Neuronal repair: Apoptotic proteins make good

The potential of the central nervous system (CNS) to regenerate is regulated by a complex interaction of neuronal intrinsic and extrinsic factors that remain poorly understood. Significant research has been dedicated to identifying these factors to facilitate design of therapies that will treat the functional impairment associated with CNS injuries. Over the last decade, the development of in vivo laser severing of single axons in C. elegans has established an invaluable model for the genetic identification of novel regeneration factors. In a recent study we report the unexpected identification of the core apoptotic proteins CED-4/Apaf-1 and the executioner caspase CED-3 as important factors that promote early events in regeneration in C. elegans. Other upstream regulators of apoptosis do not influence regeneration, indicating the existence of a novel mechanism for activation of CED-4 and CED-3 in neuronal repair. CED-4 and CED-3 function downstream of injury-induced calcium transients and appear to act through the conserved DLK-1 pathway to promote regeneration. We propose a working model for calcium-dependent localized activation of CED-4 and CED-3 caspase and discuss questions raised including mechanisms for spatially regulating activated CED-3 and the possible substrates that it might cleave to initiate regeneration.

TRA-1 ChIP-seq reveals regulators of sexual differentiation and multilevel feedback in nematode sex determination

How sexual regulators translate global sexual fate into appropriate local sexual differentiation events is perhaps the least understood aspect of sexual development. Here we have used ChIP followed by deep sequencing (ChIP-seq) to identify direct targets of the nematode global sexual regulator Transformer 1 (TRA-1), a transcription factor acting at the interface between organism-wide and cell-specific sexual regulation to control all sex-specific somatic differentiation events. We identified 184 TRA-1-binding sites in Caenorhabditis elegans, many with temporal- and/or tissue-specific TRA-1 association. We also identified 78 TRA-1-binding sites in the related nematode Caenorhabditis briggsae, 19 of which are conserved between the two species. Some DNA segments containing TRA-1-binding sites drive male-specific expression patterns, and RNAi depletion of some genes adjacent to TRA-1-binding sites results in defects in male sexual development. TRA-1 binds to sites adjacent to a number of heterochronic regulatory genes, some of which drive male-specific expression, suggesting that TRA-1 imposes sex specificity on developmental timing. We also found evidence for TRA-1 feedback regulation of the global sex-determination pathway: TRA-1 binds its own locus and those of multiple upstream masculinizing genes, and most of these associations are conserved in C. briggsae. Thus, TRA-1 coordinates sexual development by reinforcing the sex-determination decision and directing downstream sexual differentiation events.