Crossing the Worm-Brain Barrier by Using Caenorhabditis elegans to Explore Fundamentals of Human Psychiatric Illness

SNX27: A trans-species cognitive modulator with implications for anxiety and stress susceptibility

Sorting Nexin 27 (SNX27) is a brain-enriched endosome-associated cargo adaptor that shapes excitatory control, being relevant for cognitive and reward processing, and for several neurological conditions. Despite this, SNX27's role in the nervous system remains poorly explored. To further understand SNX27 function, we performed an extensive behavioral characterization comprising motor, cognitive and emotional dimensions of SNX27+/- mice. Furthermore, attending on the recently described association between SNX27 function and cellular stress signaling mechanisms in vitro, we explored SNX27-stress interplay using a Caenorhabditis elegans Δsnx-27 mutant and wild-type (WT) rodents after stress exposure. SNX27+/- mice, as C. elegans Δsnx-27 mutants, present cognitive impairments, highlighting a conserved role for SNX27 in cognitive modulation across species. Interestingly, SNX27 downmodulation leads to anxiety-like behavior in mice evaluated in the Elevated Plus Maze (EPM). This anxious phenotype is associated with increased dendritic complexity of the bed nucleus of the stria terminalis (BNST) neurons, and increased complexity of the basolateral amygdala (BLA) pyramidal neurons. These findings highlight the still unknown role of SNX27 in anxiety regulation. Moreover, we uncovered a direct link between SNX27 dysfunction and stress susceptibility in C. elegans and found that stress-exposed rodents display decreased SNX27 levels in stress-susceptible brain regions. Altogether, we provided new insights on SNX27's relevance in anxiety-related behaviors and neuronal structure in stress-associated brain regions.

Evolutionary conservation of putative suicidality-related risk genes that produce diminished motivation corrected by clozapine, lithium and antidepressants

Background

Genome wide association studies (GWAS) and candidate gene analyses have identified genetic variants and genes that may increase the risk for suicidal thoughts and behaviors (STBs). Important unresolved issues surround these tentative risk variants such as the characteristics of the associated genes and how they might elicit STBs.

Methods

Putative suicidality-related risk genes (PSRGs) were identified by comprehensive literature search and were characterized with respect to evolutionary conservation, participation in gene interaction networks and associated phenotypes. Evolutionary conservation was established with database searches and BLASTP queries, whereas gene-gene interactions were ascertained with GeneMANIA. We then examined whether mutations in risk-gene counterparts in C. elegans produced a diminished motivation phenotype previously connected to suicide risk factors.

Results and conclusions

From the analysis, 105 risk-gene candidates were identified and found to be: 1) highly conserved during evolution, 2) enriched for essential genes, 3) involved in significant gene-gene interactions, and 4) associated with psychiatric disorders, metabolic disturbances and asthma/allergy. Evaluation of 17 mutant strains with loss-of-function/deletion mutations in PSRG orthologs revealed that 11 mutants showed significant evidence of diminished motivation that manifested as immobility in a foraging assay. Immobility was corrected in some or all of the mutants with clozapine, lithium and tricyclic antidepressant drugs. In addition, 5-HT2 receptor and muscarinic receptor antagonists restored goal-directed behavior in most or all of the mutants. These studies increase confidence in the validity of the PSRGs and provide initial clues about possible mechanisms that mediate STBs.

Pre-Pulse Inhibition of an escape response in adult fruit fly, Drosophila melanogaster

Pre-Pulse Inhibition (PPI) is a neural process where suppression of a startle response is elicited by preceding the startling stimulus (Pulse) with a weak, non-startling one (Pre-Pulse). Defective PPI is widely employed as a behavioural endophenotype in humans and mammalian disorder-relevant models for neuropsychiatric disorders. We have developed a user-friendly, semi-automated, high-throughput-compatible Drosophila light-off jump response PPI paradigm, with which we demonstrate that PPI, with similar parameters measured in mammals, exists in adults of this model organism. We report that Drosophila PPI is affected by reduced expression of Dysbindin and both reduced and increased expression of Nmdar1 (N-methyl-D-aspartate receptor 1), perturbations associated with schizophrenia. Studying the biology of PPI in an organism that offers a plethora of genetic tools and a complex and well characterized connectome will greatly facilitate our efforts to gain deeper insight into the aetiology of human mental disorders, while reducing the need for mammalian models.

Ketamine induces apical extracellular matrix modifications in Caenorhabditis elegans

Ketamine is a widely used anesthetic agent since 1960s and has recently been exploited for its rapid antidepressant action at subanesthetic doses. It has been demonstrated that ketamine induces alterations in extracellular matrix (ECM) in rodent models which in part plays a role in its anti-depressant action. The nematode Caenorhabditis elegans serves as a powerful tool for understanding mechanisms of drug action with its short life cycle, genetic amenability and conserved cellular processes. Further investigation is required particularly in in vivo systems to gain broader understanding of ketamine's actions. In this study, we aimed to decipher ketamine-mediated alterations using C. elegans as a model. We show that ketamine specifically induces apical extracellular matrix modifications (aECM) in the vulva and the cuticle. Ketamine treatment phenocopies neuronal migration and vulval invagination defects of chondroitin mutants despite wild-type like chondroitin staining pattern. Normal vulval expansion and defective vulval eversion phenotypes of ketamine-treated animals are suggestive of alterations in the network of aECM factors which do not impinge on chondroitin. Ketamine ameliorates impaired movement of a group of roller mutants characterised with collagen defects in the cuticle and RNA-seq identifies that 30% of the cuticular collagens are upregulated in response to ketamine. Ketamine alters aECM, neuronal migration and collagen expression in C. elegans. We propose C. elegans as a putative animal model to investigate ketamine-mediated ECM modifications.

Loss of the Schizophrenia-Linked Furin Protein from Drosophila Mushroom Body Neurons Results in Antipsychotic-Reversible Habituation Deficits

Habituation is a conserved adaptive process essential for incoming information assessment, which drives the behavioral response decrement to recurrent inconsequential stimuli and does not involve sensory adaptation or fatigue. Although the molecular mechanisms underlying the process are not well understood, habituation has been reported to be defective in a number of disorders including schizophrenia. We demonstrate that loss of furin1, the Drosophila homolog of a gene whose transcriptional downregulation has been linked to schizophrenia, results in defective habituation to recurrent footshocks in mixed sex populations. The deficit is reversible by transgenic expression of the Drosophila or human Furin in adult α'/β' mushroom body neurons and by acute oral delivery of the typical antipsychotic haloperidol and the atypical clozapine, which are commonly used to treat schizophrenic patients. The results validate the proposed contribution of Furin downregulation in schizophrenia and suggest that defective footshock habituation is a Drosophila protophenotype of the human disorder.SIGNIFICANCE STATEMENT Genome-wide association studies have revealed a number of loci linked to schizophrenia, but most have not been verified experimentally in a relevant behavioral task. Habituation deficits constitute a schizophrenia endophenotype. Drosophila with attenuated expression of the schizophrenia-linked highly conserved Furin gene present delayed habituation reversible with acute exposure to antipsychotics. This strongly suggests that footshock habituation defects constitute a schizophrenia protophenotype in Drosophila Furthermore, determination of the neurons whose regulated activity is required for footshock habituation provides a facile metazoan system to expediently validate putative schizophrenia genes and variants in a well understood simple brain.

Translational relevance of forward genetic screens in animal models for the study of psychiatric disease

Psychiatric disorders represent a significant burden in our societies. Despite the convincing evidence pointing at gene and gene-environment interaction contributions, the role of genetics in the etiology of psychiatric disease is still poorly understood. Forward genetic screens in animal models have helped elucidate causal links. Here we discuss the application of mutagenesis-based forward genetic approaches in common animal model species: two invertebrates, nematodes (Caenorhabditis elegans) and fruit flies (Drosophila sp.); and two vertebrates, zebrafish (Danio rerio) and mice (Mus musculus), in relation to psychiatric disease. We also discuss the use of large scale genomic studies in human populations. Despite the advances using data from human populations, animal models coupled with next-generation sequencing strategies are still needed. Although with its own limitations, zebrafish possess characteristics that make them especially well-suited to forward genetic studies exploring the etiology of psychiatric disorders.

Catalytic Reaction Model of Suicide

Suicide is a devastating outcome of unresolved issues that affect mental health, general wellbeing and socioeconomic stress. The biology of suicidal behavior is still poorly understood, although progress has been made. Suicidal behavior runs in families and genetic studies have provided initial glimpses into potential genes that contribute to suicide risk. Here, we attempt to unify the biology and behavioral dimensions into a model that can guide research in this area. The proposed model envisions suicidal behavior as a catalytic reaction that may result in suicide depending on the conditions, analogously to enzyme catalysis of chemical reactions. A wide array of substrates or reactants, such as hopelessness, depression, debilitating illnesses and diminished motivation can mobilize suicidal thoughts and behaviors (STBs), which can then catalyze the final step/act of suicide. Here, we focus on three biological substrates in particular: threat assessment, motivation to engage in life and impulsivity. Genetic risk factors can affect each of these processes and tilt the balance toward suicidal behavior when existential crises (real or perceived) emerge such as loss of a loved one, sudden changes in social status or serious health issues. Although suicide is a uniquely human behavior, many of the fundamental biological processes are evolutionarily conserved. Insights from animal models may help to shape our understanding of suicidal behavior in man. By examining counterparts of the major biological processes in other organisms, new ideas about the role of genetic risk factors may emerge along with possible therapeutic interventions or preventive measures.

Deconstructing and reconstructing behaviour relevant to mental health disorders: The benefits of a psychological approach, with a focus on addiction

RUTHERFORD, L.G. and Milton, A.L. Deconstructing and reconstructing behaviour relevant to mental health disorders: what can psychology offer? NEUROSCI BIOBEHAV REV XX(X)XXX-XXX, 2021. - Current treatments for mental health disorders are successful only for some patients, and there is an unmet clinical need for new treatment development. One challenge for treatment development has been how best to model complex human conditions in animals, where mechanism can be more readily studied with a range of neuroscientific techniques. We suggest that an approach to modelling based on associative animal learning theory provides a good framework for deconstructing complex mental health disorders such that they can be studied in animals. These individual simple models can subsequently be used in combination to 'reconstruct' a more complex model of the mental health disorder of interest. Using examples primarily from the field of drug addiction, we explore the 'psychological approach' and suggest that in addition to facilitating translation and backtranslation of tasks between animal models and patients, it is also highly concordant with the concept of triangulation.

Neuronal membrane glycoprotein (nmgp-1) gene deficiency affects chemosensation-related behaviors, dauer exit and egg-laying in Caenorhabditis elegans

The nervous system monitors the environment to maintain homeostasis, which can be affected by stressful conditions. Using mammalian models of chronic stress, we previously observed altered brain levels of GPM6A, a protein involved in neuronal morphology. However, GPM6A's role in systemic stress responses remains unresolved. The nematode Caenorhabditis elegans expresses a GPM6A ortholog, the neuronal membrane glycoprotein 1 (NMGP-1). Because of the shared features between nematode and mammalian nervous systems and the vast genetic tools available in C. elegans, we used the worm to elucidate the role of GPM6A in the stress response. We first identified nmgp-1 expression in different amphid and phasmid neurons. To understand the nmgp-1 role, we characterized the behavior of nmgp-1(RNAi) animals and two nmgp-1 mutant alleles. Compared to control animals, mutant and RNAi-treated worms exhibited increased recovery time from the stress-resistant dauer stage, altered SDS chemosensation and reduced egg-laying rate resulting in egg retention (bag-of-worms phenotype). Silencing of nmgp-1 expression induced morphological abnormalities in the ASJ sensory neurons, partly responsible for dauer exit. These results indicate that nmgp-1 is required for neuronal morphology and for behaviors associated with chemosensation. Finally, we propose nmgp-1 mutants as a tool to screen drugs for human nervous system pathologies.

Analysis of Major Depression Risk Genes Reveals Evolutionary Conservation, Shared Phenotypes, and Extensive Genetic Interactions

Major depressive disorder (MDD) affects around 15% of the population at some stage in their lifetime. It can be gravely disabling and it is associated with increased risk of suicide. Genetics play an important role; however, there are additional environmental contributions to the pathogenesis. A number of possible risk genes that increase liability for developing symptoms of MDD have been identified in genome-wide association studies (GWAS). The goal of this study was to characterize the MDD risk genes with respect to the degree of evolutionary conservation in simpler model organisms such as Caenorhabditis elegans and zebrafish, the phenotypes associated with variation in these genes and the extent of network connectivity. The MDD risk genes showed higher conservation in C. elegans and zebrafish than genome-to-genome comparisons. In addition, there were recurring themes among the phenotypes associated with variation of these risk genes in C. elegans. The phenotype analysis revealed enrichment for essential genes with pleiotropic effects. Moreover, the MDD risk genes participated in more interactions with each other than did randomly-selected genes from similar-sized gene sets. Syntenic blocks of risk genes with common functional activities were also identified. By characterizing evolutionarily-conserved counterparts to the MDD risk genes, we have gained new insights into pathogenetic processes relevant to the emergence of depressive symptoms in man.

Using touchscreen-delivered cognitive assessments to address the principles of the 3Rs in behavioral sciences

Despite considerable advances in both in silico and in vitro approaches, in vivo studies that involve animal model systems remain necessary in many research disciplines. Neuroscience is one such area, with studies often requiring access to a complete nervous system capable of dynamically selecting between and then executing a full range of cognitive and behavioral outputs in response to a given stimulus or other manipulation. The involvement of animals in research studies is an issue of active public debate and concern and is therefore carefully regulated. Such regulations are based on the principles of the 3Rs of Replacement, Reduction and Refinement. In the sub-specialty of behavioral neuroscience, Full/Absolute Replacement remains a major challenge, as the complete ex vivo recapitulation of a system as complex and dynamic as the nervous system has yet to be achieved. However, a number of very positive developments have occurred in this area with respect to Relative Replacement and to both Refinement and Reduction. In this review, we discuss the Refinement- and Reduction-related benefits yielded by the introduction of touchscreen-based behavioral assessment apparatus. We also discuss how data generated by a specific panel of behavioral tasks developed for this platform might substantially enhance monitoring of laboratory animal welfare and provide robust, quantitative comparisons of husbandry techniques to define and ensure maintenance of best practice.

Increased Stress Resistance and Lifespan in Chaenorhabditis elegans Wildtype and Knockout Mutants-Implications for Depression Treatment by Medicinal Herbs

Depression and anxiety disorders are widespread diseases, and they belong to the leading causes of disability and greatest burdens on healthcare systems worldwide. It is expected that the numbers will dramatically rise during the COVID-19 pandemic. Established medications are not sufficient to adequately treat depression and are not available for everyone. Plants from traditional medicine may be promising alternatives to treat depressive symptoms. The model organism Chaenorhabditis elegans was used to assess the stress reducing effects of methanol/dichlormethane extracts from plants used in traditional medicine. After initial screening for antioxidant activity, nine extracts were selected for in vivo testing in oxidative stress, heat stress, and osmotic stress assays. Additionally, anti-aging properties were evaluated in lifespan assay. The extracts from Acanthopanax senticosus, Campsis grandiflora, Centella asiatica, Corydalis yanhusuo, Dan Zhi, Houttuynia cordata, Psoralea corylifolia, Valeriana officinalis, and Withaniasomnifera showed antioxidant activity of more than 15 Trolox equivalents per mg extract. The extracts significantly lowered ROS in mutants, increased resistance to heat stress and osmotic stress, and the extended lifespan of the nematodes. The plant extracts tested showed promising results in increasing stress resistance in the nematode model. Further analyses are needed, in order to unravel underlying mechanisms and transfer results to humans.

Ursolic acid enhances stress resistance, reduces ROS accumulation and prolongs life span in C. elegans serotonin-deficient mutants

INTRODUCTION

Depression and anxiety disorders contribute to the global disease burden. Ursolic acid (UA), a natural compound present in many vegetables, fruits and medicinal plants, was tested in vivo for its effect on (1) enhancing resistance to stress and (2) its effect on life span.

METHODS

The compound was tested for its antioxidant activity in C. elegans. Stress resistance was tested in the heat and osmotic stress assay. Additionally, the influence on normal life span was examined. RT-PCR was used to assess possible serotonin targets.

RESULTS

UA prolonged the life span of C. elegans. Additionally, UA significantly lowered reactive oxygen species (ROS). Molecular docking studies, PCR analysis and microscale thermophoresis (MST) supported the results that UA acts through serotonin receptors to enhance stress resistance.

DISCUSSION

Considering the urgent need for new and safe medications in the treatment of depression and anxiety disorders, our results indicate that UA may be a promising new drug candidate.

Loss of egli-1, the Caenorhabditis elegans Orthologue of a Downstream Target of SMN, Leads to Abnormalities in Sensorimotor Integration

The connectome of Caenorhabditis elegans has been extensively studied and fully mapped, allowing researchers to more confidently conclude on the impact of any change in neuronal circuits based on behavioral data. One of the more complex sensorimotor circuits in nematodes is the one that regulates the integration of feeding status with the subsequent behavioral responses that allow animals to adapt to environmental conditions. Here, we have characterized a Caenorhabditis elegans knockout model of the egli-1 gene (previously known as tag-175). This is an orthologue of the stasimon/tmem41b gene, a downstream target of SMN, the depleted protein in spinal muscular atrophy (SMA), which partially recapitulates the SMA phenotype in fly and zebrafish models when mutated. Surprisingly, egli-1 mutants reveal no deficits in motor function. Instead, they show functional impairment of a specific neuronal circuit, leading to defects in the integration of sensorial information related to food abundance, with consequences at the level of locomotion adaptation, egg laying, and the response to aversive chemical stimuli. This work has demonstrated for the first time the relevance of egli-1 in the nervous system, as well as revealed a function for this gene, which had remained elusive so far.

Role of Astrocytes in Manganese Neurotoxicity Revisited

Manganese (Mn) overexposure is a public health concern due to its widespread industrial usage and the risk for environmental contamination. The clinical symptoms of Mn neurotoxicity, or manganism, share several pathological features of Parkinson's disease (PD). Biologically, Mn is an essential trace element, and Mn in the brain is preferentially localized in astrocytes. This review summarizes the role of astrocytes in Mn-induced neurotoxicity, specifically on the role of neurotransmitter recycling, neuroinflammation, and genetics. Mn overexposure can dysregulate astrocytic cycling of glutamine (Gln) and glutamate (Glu), which is the basis for Mn-induced excitotoxic neuronal injury. In addition, reactive astrocytes are important mediators of Mn-induced neuronal damage by potentiating neuroinflammation. Genetic studies, including those with Caenorhabditis elegans (C. elegans) have uncovered several genes associated with Mn neurotoxicity. Though we have yet to fully understand the role of astrocytes in the pathologic changes characteristic of manganism, significant strides have been made over the last two decades in deciphering the role of astrocytes in Mn-induced neurotoxicity and neurodegeneration.

Towards High-Throughput Chemobehavioural Phenomics in Neuropsychiatric Drug Discovery

Identifying novel marine-derived neuroactive chemicals with therapeutic potential is difficult due to inherent complexities of the central nervous system (CNS), our limited understanding of the molecular foundations of neuro-psychiatric conditions, as well as the limited applications of effective high-throughput screening models that recapitulate functionalities of the intact CNS. Furthermore, nearly all neuro-modulating chemicals exhibit poorly characterized pleiotropic activities often referred to as polypharmacology. The latter renders conventional target-based in vitro screening approaches very difficult to accomplish. In this context, chemobehavioural phenotyping using innovative small organism models such as planarians and zebrafish represent powerful and highly integrative approaches to study the impact of new chemicals on central and peripheral nervous systems. In contrast to in vitro bioassays aimed predominantly at identification of chemicals acting on single targets, phenotypic chemobehavioural analysis allows for complex multi-target interactions to occur in combination with studies of polypharmacological effects of chemicals in a context of functional and intact milieu of the whole organism. In this review, we will outline recent advances in high-throughput chemobehavioural phenotyping and provide a future outlook on how those innovative methods can be utilized for rapidly screening and characterizing marine-derived compounds with prospective applications in neuropharmacology and psychosomatic medicine.

Surprising conservation of schizophrenia risk genes in lower organisms reflects their essential function and the evolution of genetic liability

Schizophrenia is a devastating psychiatric illness that affects approximately 1% of the population. Genetic variation in multiple genes causes elevated risk for the disorder, but the molecular basis is inadequately understood and it is not clear how risk genes have evolved and persisted in the genome. To address these issues, we have identified orthologs/homologs of 344 schizophrenia risk genes (from the Psychiatric Genomics Consortium dataset) in lower organisms, including C. elegans, Drosophila and zebrafish, along with phenotypes produced by genetic disruption in C. elegans. Schizophrenia risk genes were evolutionarily conserved at significantly higher rates in C. elegans (81%) and zebrafish (88%) than genes in general for these two species (40-70%). The risk-gene equivalents were highly (~3-fold) enriched for essential genes consistent with polygenic mutation threshold models, which propose that genetic susceptibility results from the inevitable expression of harmful combinations of risk variants in the population. Most notably, numerous examples of cross-species synteny revealed how blocks of risk genes geared toward a shared biological purpose coalesced into proximity during evolution. We obtained initial evidence that schizophrenia risk genes affected different stages of development, potentially allowing differential modulation by the environment. Taken together, studies of the conservation of schizophrenia risk genes in simple model organisms provided novel insights into the molecular basis for genetic susceptibility to a complex human psychiatric disorder.