The zinc-finger protein SEA-2 regulates larval developmental timing and adult lifespan in C. elegans

Zebrafish - The Neurobehavioural Model in Trend

Zebrafish (Danio rerio) is currently in vogue as a prevalently used experimental model for studies concerning neurobehavioural disorders and associated fields. Since the 1960s, this model has succeeded in breaking most barriers faced in the hunt for an experimental model. From its appearance to its high parity with human beings genetically, this model renders itself as an advantageous experimental lab animal. Neurobehavioural disorders have always posed an arduous task in terms of their detection as well as in determining their exact etiology. They are still, in most cases, diseases of interest for inventing or discovering novel pharmacological interventions. Thus, the need for a harbinger experimental model for studying neurobehaviours is escalating. Ensuring the same model is used for studying several neuro-studies conserves the results from inter-species variations. For this, we need a model that satisfies all the pre-requisite conditions to be made the final choice of model for neurobehavioural studies. This review recapitulates the progress of zebrafish as an experimental model with its most up-to-the-minute advances in the area. Various tests, assays, and responses employed using zebrafish in screening neuroactive drugs have been tabulated effectively. The tools, techniques, protocols, and apparatuses that bolster zebrafish studies are discussed. The probable research that can be done using zebrafish has also been briefly outlined. The various breeding and maintenance methods employed, along with the information on various strains available and most commonly used, are also elaborated upon, supplementing Zebrafish's use in neuroscience.

Avoidance behavior independent of innate-immune signaling seen in Caenorhabditis elegans challenged with Bacillus anthracis

Small organisms, like the nematode C. elegans, are emerging as insightful models in which to study host/pathogen interactions and the evolving interplay between host defenses and microbial offenses. In C. elegans the innate immune response has been shown to be connected to the DAF-2 insulin/insulin-like growth factor 1 (IGF-1) signal pathway, a critical transduction pathway that mediates stress response in the worms via the DAF-16 FOXO/forkhead transcription factor. Our studies of the C. elegans' phenotypes that are associated with behavioral innate immune response (avoidance behavior) and IGF-1 signaling perturbations (lifespan effects) led us to question the cause of the avoidance behavior observed when C. elegans are challenged with B. anthracis. While worms indeed avoid B. anthracis, and this behavior seems to be partly tied to IGF-1 signaling, the bacteria have neither nematocidal nor visible pathogenic effects on the worms. In fact, worms fed B. anthracis alone exhibit extended lifespans. We demonstrate that the extended lifespan phenotype seen in worms fed B. anthracis is likely the result of calorie restriction, and that worms do not eat B. anthracis even when avoidance behaviors have been suppressed. We further demonstrate a large time lag between the onset of avoidance behavior (which occurs upon contact with B. anthracis), and the induction of IGF-1 signaling (which occurs much later) in worms fed B. anthracis. Taken together, our data demonstrate behavioral avoidance that does not appear to be linked to a measurable immune response. We propose that, in some situations, avoidance behaviors categorized as immunological might be more accurately described as broad foraging behaviors induced in worms presented with a non-preferred food choice, or with a food choice that is either difficult or impossible for the worms to ingest.

The Makorin lep-2 and the lncRNA lep-5 regulate lin-28 to schedule sexual maturation of the C. elegans nervous system

Sexual maturation must occur on a controlled developmental schedule. In mammals, Makorin3 (MKRN3) and the miRNA regulators LIN28A/B are key regulators of this process, but how they act is unclear. In C. elegans, sexual maturation of the nervous system includes the functional remodeling of postmitotic neurons and the onset of adult-specific behaviors. Here, we find that the lin-28–let-7 axis (the ‘heterochronic pathway’) determines the timing of these events. Upstream of lin-28, the Makorin lep-2 and the lncRNA lep-5 regulate maturation cell-autonomously, indicating that distributed clocks, not a central timer, coordinate sexual differentiation of the C. elegans nervous system. Overexpression of human MKRN3 delays aspects of C. elegans sexual maturation, suggesting the conservation of Makorin function. These studies reveal roles for a Makorin and a lncRNA in timing of sexual differentiation; moreover, they demonstrate deep conservation of the lin-28–let-7 system in controlling the functional maturation of the nervous system.

Most animals develop from juveniles, which cannot reproduce, to sexually mature adults. The most obvious signs of this transition are changes in body shape and size. However, changes also take place in the brain that enable the animals to adapt their behavior to the demands of adulthood. For example, fully fed adult male roundworms will leave a food source to search for mates, whereas juvenile males will continue feeding.

The transition to sexual maturity needs to be carefully timed. Too early, and the animal risks compromising key stages of development. Too late, and the animal may be less competitive in the quest for reproductive success. Cues in the environment, such as the presence of food and mates, interact with timing mechanisms in the brain to trigger sexual maturity. But how these mechanisms work – in particular where and how an animal keeps track of its developmental stage – is not well understood.

In the roundworm species Caenorhabditis elegans, waves of gene activity, known collectively as the heterochronic pathway, determine patterns of cell growth as animals mature. Through further studies of these worms, Lawson et al. now show that these waves also control the time at which neural circuits mature. In addition, the waves of activity occur inside the nervous system itself, rather than in a tissue that sends signals to the nervous system. Moreover, they occur independently inside many different neurons. Each neuron thus has its own molecular clock for keeping track of development.

Several of the genes critical for developmental timekeeping in worms are also found in mammals, including two genes that help to control when puberty starts in humans. If one of these genes – called MKRN3 – does not work correctly, it can lead to a condition that causes individuals to go through puberty several years earlier than normal. Studying the mechanisms identified in roundworms may help us to better understand this disorder. More generally, future work that builds on the results presented by Lawson et al. will help to reveal how environmental cues and gene activity interact to control when we become adults.

Pheromones and Nutritional Signals Regulate the Developmental Reliance on let-7 Family MicroRNAs in C. elegans

Adverse environmental conditions can affect rates of animal developmental progression and lead to temporary developmental quiescence (diapause), exemplified by the dauer larva stage of the nematode Caenorhabditis elegans (C. elegans). Remarkably, patterns of cell division and temporal cell-fate progression in C. elegans larvae are not affected by changes in developmental trajectory. However, the underlying physiological and gene regulatory mechanisms that ensure robust developmental patterning despite substantial plasticity in developmental progression are largely unknown. Here, we report that diapause-inducing pheromones correct heterochronic developmental cell lineage defects caused by insufficient expression of let-7 family microRNAs in C. elegans. Moreover, two conserved endocrine signaling pathways, DAF-7/TGF-β and DAF-2/Insulin, that confer on the larva diapause and non-diapause alternative developmental trajectories interact with the nuclear hormone receptor, DAF-12, to initiate and regulate a rewiring of the genetic circuitry controlling temporal cell fates. This rewiring includes engagement of certain heterochronic genes, lin-46, lin-4, and nhl-2, that are previously associated with an altered genetic program in post-diapause animals, in combination with a novel ligand-independent DAF-12 activity, to downregulate the critical let-7 family target Hunchback-like-1 (HBL-1). Our results show how pheromone or endocrine signaling pathways can coordinately regulate both developmental progression and cell-fate transitions in C. elegans larvae under stress so that the developmental schedule of cell fates remains unaffected by changes in developmental trajectory.

The Long Non-Coding RNA lep-5 Promotes the Juvenile-to-Adult Transition by Destabilizing LIN-28

Biological roles for most long non-coding RNAs (lncRNAs) remain mysterious. Here, using forward genetics, we identify lep-5, a lncRNA acting in the C. elegans heterochronic (developmental timing) pathway. Loss of lep-5 delays hypodermal maturation and male tail tip morphogenesis (TTM), hallmarks of the juvenile-to-adult transition. We find that lep-5 is a ∼600 nt cytoplasmic RNA that is conserved across Caenorhabditis and possesses three essential secondary structure motifs but no essential open reading frames. lep-5 expression is temporally controlled, peaking prior to TTM onset. Like the Makorin LEP-2, lep-5 facilitates the degradation of LIN-28, a conserved miRNA regulator specifying the juvenile state. Both LIN-28 and LEP-2 associate with lep-5 in vivo, suggesting that lep-5 directly regulates LIN-28 stability and may function as an RNA scaffold. These studies identify a key biological role for a lncRNA: by regulating protein stability, it provides a temporal cue to facilitate the juvenile-to-adult transition.

LIN-28 balances longevity and germline stem cell number in Caenorhabditis elegans through let-7/AKT/DAF-16 axis

The RNA‐binding protein LIN‐28 was first found to control developmental timing in Caenorhabditis elegans. Later, it was found to play important roles in pluripotency, metabolism, and cancer in mammals. Here we report that a low dosage of lin‐28 enhanced stress tolerance and longevity, and reduced germline stem/progenitor cell number in C. elegans. The germline LIN‐28‐regulated microRNA let‐7 was required for these effects by targeting akt‐1/2 and decreasing their protein levels. AKT‐1/2 and the downstream DAF‐16 transcription factor were both required for the lifespan and germline stem cell effects of lin‐28. The pathway also mediated dietary restriction induced lifespan extension and reduction in germline stem cell number. Thus, the LIN‐28/let‐7/AKT/DAF‐16 axis we delineated here is a program that plays an important role in balancing reproduction and somatic maintenance and their response to the environmental energy level—a central dogma of the ‘evolutionary optimization’ of resource allocation that modulates aging.

Makorin ortholog LEP-2 regulates LIN-28 stability to promote the juvenile-to-adult transition in Caenorhabditis elegans

The heterochronic genes lin-28, let-7 and lin-41 regulate fundamental developmental transitions in animals, such as stemness versus differentiation and juvenile versus adult states. We identify a new heterochronic gene, lep-2, in Caenorhabditis elegans. Mutations in lep-2 cause a delay in the juvenile-to-adult transition, with adult males retaining pointed, juvenile tail tips, and displaying defective sexual behaviors. In both sexes, lep-2 mutants fail to cease molting or produce an adult cuticle. We find that LEP-2 post-translationally regulates LIN-28 by promoting LIN-28 protein degradation. lep-2 encodes the sole C. elegans ortholog of the Makorin (Mkrn) family of proteins. Like lin-28 and other heterochronic pathway members, vertebrate Mkrns are involved in developmental switches, including the timing of pubertal onset in humans. Based on shared roles, conservation and the interaction between lep-2 and lin-28 shown here, we propose that Mkrns, together with other heterochronic genes, constitute an evolutionarily ancient conserved module regulating switches in development.

A Stenotrophomonas maltophilia Strain Evades a Major Caenorhabditis elegans Defense Pathway

Stenotrophomonas maltophilia is a ubiquitous bacterium and an emerging nosocomial pathogen. This bacterium is resistant to many antibiotics, associated with a number of infections, and a significant health risk, especially for immunocompromised patients. Given that Caenorhabditis elegans shares many conserved genetic pathways and pathway components with higher organisms, the study of its interaction with bacterial pathogens has biomedical implications. S. maltophilia has been isolated in association with nematodes from grassland soils, and it is likely that C. elegans encounters this bacterium in nature. We found that a local S. maltophilia isolate, JCMS, is more virulent than the other S. maltophilia isolates (R551-3 and K279a) tested. JCMS virulence correlates with intestinal distension and bacterial accumulation and requires the bacteria to be alive. Many of the conserved innate immune pathways that serve to protect C. elegans from various pathogenic bacteria also play a role in combating S. maltophilia JCMS. However, S. maltophilia JCMS is virulent to normally pathogen-resistant DAF-2/16 insulin-like signaling pathway mutants. Furthermore, several insulin-like signaling effector genes were not significantly differentially expressed between S. maltophilia JCMS and avirulent bacteria (Escherichia coli OP50). Taken together, these findings suggest that S. maltophilia JCMS evades the pathogen resistance conferred by the loss of DAF-2/16 pathway components. In summary, we have discovered a novel host-pathogen interaction between C. elegans and S. maltophilia and established a new animal model with which to study the mode of action of this emerging nosocomial pathogen.

Molecular antagonism between X-chromosome and autosome signals determines nematode sex

Sex is determined in Caenorhabditis elegans by the ratio of X chromosomes to the sets of autosomes, the X:A signal. A set of genes called X signal elements (XSEs) communicates X-chromosome dose by repressing the masculinizing sex determination switch gene xol-1 (XO lethal) in a dose-dependent manner. xol-1 is active in 1X:2A embryos (males) but repressed in 2X:2A embryos (hermaphrodites). Here we showed that the autosome dose is communicated by a set of autosomal signal elements (ASEs) that act in a cumulative, dose-dependent manner to counter XSEs by stimulating xol-1 transcription. We identified new ASEs and explored the biochemical basis by which ASEs antagonize XSEs to determine sex. Multiple antagonistic molecular interactions carried out on a single promoter explain how different X:A values elicit different sexual fates. XSEs (nuclear receptors and homeodomain proteins) and ASEs (T-box and zinc finger proteins) bind directly to several sites on xol-1 to counteract each other's activities and thereby regulate xol-1 transcription. Disrupting ASE- and XSE-binding sites in vivo recapitulated the misregulation of xol-1 transcription caused by disrupting cognate signal element genes. XSE- and ASE-binding sites are distinct and nonoverlapping, suggesting that direct competition for xol-1 binding is not how XSEs counter ASEs. Instead, XSEs likely antagonize ASEs by recruiting cofactors with reciprocal activities that induce opposite transcriptional states. Most ASE- and XSE-binding sites overlap xol-1's -1 nucleosome, which carries activating chromatin marks only when xol-1 is turned on. Coactivators and corepressors tethered by proteins similar to ASEs and XSEs are known to deposit and remove such marks. The concept of a sex signal comprising competing XSEs and ASEs arose as a theory for fruit flies a century ago. Ironically, while the recent work of others showed that the fly sex signal does not fit this simple paradigm, our work shows that the worm signal does.

Autophagy modulates miRNA-mediated gene silencing and selectively degrades AIN-1/GW182 in C. elegans

MicroRNAs (miRNAs) post-transcriptionally repress gene expression via the miRNA-induced silencing complex (miRISC), which includes miRNA, Argonaute and a GW182 family member. Here we show that in Caenorhabditis elegans, miRNA-mediated gene silencing is modulated by macroautophagy, a lysosome-mediated degradation process. Loss of autophagy activity suppresses developmental defects caused by partially impaired silencing of miRNA targets including the let-7 family and lsy-6. The C. elegans GW182 homolog AIN-1 is itself selectively degraded by autophagy and colocalizes with the p62 homolog SQST-1 in autophagy mutants. Thus, autophagy activity modulates miRNA-mediated gene silencing and degrades a core miRISC component.

Curcumin is an early-acting stage-specific inducer of extended functional longevity in Drosophila

Larval feeding with curcumin induces an extended health span with significantly increased median and maximum longevities in the adult fly. This phenotype is diet insensitive and shows no additive effect on longevity when combined with an adult dietary restriction (DR) diet, suggesting that curcumin and DR operate via the same or overlapping pathways for this trait. This treatment significantly slows the aging rate so that it is comparable with that of genetically selected long lived animals. The larval treatment also enhances the adult animal's geotactic activity in an additive manner with DR, suggesting that curcumin and DR may use different pathways for different traits. Feeding the drug to adults during only the health span also results in a significantly extended health span with increased median and maximum life span. This extended longevity phenotype is induced only during these stage-specific periods. Feeding adults with the drug over their whole life results in a weakly negative effect on median longevity with no increase in maximum life span. There are no negative effects on reproduction, although larval curcumin feeding increases development time, and also apparently accelerates the normal late-life neuromuscular degeneration seen in the legs. Gene expression data from curcumin-fed larvae shows that the TOR pathway is inhibited in the larvae and the young to midlife adults, although several other genes involved in longevity extension are also affected. These data support the hypothesis that curcumin acts as if it is a DR mimetic nutraceutical. These data also suggest that the search for DR mimetics may be enhanced by the use of stage-specific screening of candidate molecules.

Differential function of the two Atg4 homologues in the aggrephagy pathway in Caenorhabditis elegans

The presence of multiple homologues of the same yeast Atg protein endows an additional layer of complexity on the autophagy pathway in higher eukaryotes. The physiological function of the individual genes, however, remains largely unknown. Here we investigated the role of the two Caenorhabditis elegans homologues of the cysteine protease Atg4 in the pathway responsible for degradation of protein aggregates. Loss of atg-4.1 activity causes defective degradation of a variety of protein aggregates, whereas atg-4.2 mutants remove these substrates normally. LGG-1 precursors accumulate in atg-4.1 mutants, but not atg-4.2 mutants. LGG-1 puncta, formation of which depends on lipidation of LGG-1, are present in atg-4.1 and atg-4.2 single mutants, but are completely absent in atg-4.1; atg-4.2 double mutants. In vitro enzymatic analysis revealed that ATG-4.1 processes LGG-1 precursors about 100-fold more efficiently than ATG-4.2. Expression of a mutant form LGG-1, which mimics the processed precursor, rescues the defective autophagic degradation of protein aggregates in atg-4.1 mutants and, to a lesser extent, in atg-4.1; atg-4.2 double mutants. Our study reveals that ATG-4.1 and ATG-4.2 are functionally redundant yet display differential LGG-1 processing and deconjugating activity in the aggrephagy pathway in C. elegans.