Nematode Worms (Caenorhabditis Elegans)

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    Caenorhabditis elegans is a free-living transparent nematode worm about 1 mm in length[1] that lives in temperate soil environments. It is the type species of its genus.[2] The name is a blend of the Greek caeno- (recent), rhabditis (rod-like)[3] and Latin elegans (elegant). In 1900, Maupas initially named it Rhabditides elegans. Osche placed it in the subgenus Caenorhabditis in 1952, and in 1955, Dougherty raised Caenorhabditis to the status of genus.[4]

    C. elegans is an unsegmented pseudocoelomate and lacks respiratory or circulatory systems.[5] Most of these nematodes are hermaphrodites and a few are males.[6] Males have specialized tails for mating that include spicules.

    In 1963, Sydney Brenner proposed research into C. elegans, primarily in the area of neuronal development. In 1974, he began research into the molecular and developmental biology of C. elegans, which has since been extensively used as a model organism.[7] It was the first multicellular organism to have its whole genome sequenced, and in 2019 it was the first organism to have its connectome (neuronal "wiring diagram") completed.[8][9][10]

    Use as a Model Organism[edit | edit source]

    Movement of wild-type

    In 1963, Sydney Brenner proposed using C. elegans as a model organism for the investigation primarily of neural development in animals. It is one of the simplest organisms with a nervous system. The neurons do not fire action potentials, and do not express any voltage-gated sodium channels.[11] In the hermaphrodite, this system comprises 302 neurons[12] the pattern of which has been comprehensively mapped,[13] in what is known as a connectome,[14] and shown to be a small-world network.[15]

    Research has explored the neural and molecular mechanisms that control several behaviors of C. elegans, including chemotaxis, thermotaxis, mechanotransduction, learning, memory, and mating behaviour.[16] In 2019 the connectome of the male was published using a technique distinct from that used for the hermaphrodite. The same paper used the new technique to redo the hermaphrodite connectome, finding 1,500 new synapses.[13]

    It has been used as a model organism to study molecular mechanisms in metabolic diseases.[17] Brenner also chose it as it is easy to grow in bulk populations, and convenient for genetic analysis. It is a multicellular eukaryotic organism, yet simple enough to be studied in great detail. The transparency of C. elegans facilitates the study of cellular differentiation and other developmental processes in the intact organism. The spicules in the male clearly distinguish males from females. Strains are cheap to breed and can be frozen. When subsequently thawed, they remain viable, allowing long-term storage.[7] Maintenance is easy when compared to other multicellular model organisms. A few hundred nematodes can be kept on a single agar plate and suitable growth medium. Brenner described the use of a mutant of E. coli – OP50. OP50 is a uracil-requiring organism and its deficiency in the plate prevents the overgrowth of bacteria which would obscure the worms.[7] The use of OP50 does not demand any major laboratory safety measures, since it is non-pathogenic and easily grown in Luria-Bertani (LB) media overnight.[18]

    Ageing[edit | edit source]

    C. elegans has been a model organism for research into ageing; for example, the inhibition of an insulin-like growth factor signaling pathway has been shown to increase adult lifespan threefold;[19][20] while glucose feeding promotes oxidative stress and reduce adult lifespan by a half.[17] Similarly, induced degradation of an insulin/IGF-1 receptor late in life extended life expectancy of worms dramatically.[21] Long-lived mutants of C. elegans were demonstrated to be resistant to oxidative stress and UV light.[22] These long-lived mutants had a higher DNA repair capability than wild-type C. elegans.[22] Knockdown of the nucleotide excision repair gene Xpa-1 increased sensitivity to UV and reduced the life span of the long-lived mutants. These findings indicate that DNA repair capability underlies longevity.

    The capacity to repair DNA damage by the process of nucleotide excision repair declines with age.[23]

    C. elegans exposed to 5mM lithium chloride (LiCl) showed lengthened life spans.[24] When exposed to 10μM LiCl, reduced mortality was observed, but not with 1μM.[25]

    C. elegans has been instrumental in the identification of the functions of genes implicated in Alzheimer's disease, such as presenilin.[26] Moreover, extensive research on C. elegans has identified RNA-binding proteins as essential factors during germline and early embryonic development.[27]

    Telomeres, the length of which have been shown to correlate with increased lifespan and delayed onset of senescence in a multitude of organisms, from C. elegans[28][29] to humans,[30] show an interesting behaviour in C. elegans. While C. elegans maintains its telomeres in a canonical way similar to other eukaryotes, in contrast Drosophila melanogaster is noteworthy in its use of retrotransposons to maintain its telomeres,[31] during knock-out of the catalytic subunit of the telomerase (trt-1) C. elegans can gain the ability of alternative telomere lengthening (ALT). C. elegans was the first eukaryote to gain ALT functionality after knock-out of the canonical telomerase pathway.[32] ALT is also observed in about 10-15% of all clinical cancers.[33] Thus C. elegans is a prime candidate for ALT research.[34][35][36] Bayat et al. showed the paradoxical shortening of telomeres during trt-1 over-expression which lead to near sterility while the worms even exhibited a slight increase in lifespan, despite shortened telomeres.[37]

    See Also[edit | edit source]

    References[edit | edit source]

    1. Wood; "The Nematode Caenorhabditis elegans'" , pp. 1 , ISBN: 978-0-87969-433-3
    2. Sudhaus W, Kiontke K; "Phylogeny of Rhabditis subgenus Caenorhabditis (Rhabditidae, Nematoda)" , https://doi.org/10.1111/j.1439-0469.1996.tb00827.x
    3. καινός (caenos) = new, recent; ῥάβδος (rhabdos) = rod, wand.
    4. Caenorhabditis elegans, http://plpnemweb.ucdavis.edu/nemaplex/Taxadata/G900S2.htm
    5. Wallace RL, Ricci C, Melone G; "A cladistic analysis of pseudocoelomate (aschelminth) morphology." , https://doi.org/10.2307/3227041
    6. Introduction to sex determination, http://www.wormbook.org/chapters/www_introsexdetermination/introsexdetermination.html
    7. 7.0 7.1 7.2 Brenner S: The genetics of Caenorhabditis elegans. Genetics 1974. (PMID 4366476) [PubMed] [DOI] [Full text] Methods are described for the isolation, complementation and mapping of mutants of Caenorhabditis elegans, a small free-living nematode worm. About 300 EMS-induced mutants affecting behavior and morphology have been characterized and about one hundred genes have been defined. Mutations in 77 of these alter the movement of the animal. Estimates of the induced mutation frequency of both the visible mutants and X chromosome lethals suggests that, just as in Drosophila, the genetic units in C. elegans are large.
    8. White JG et al.: The structure of the nervous system of the nematode Caenorhabditis elegans. Philos Trans R Soc Lond B Biol Sci 1986. (PMID 22462104) [PubMed] [DOI] The structure and connectivity of the nervous system of the nematode Caenorhabditis elegans has been deduced from reconstructions of electron micrographs of serial sections. The hermaphrodite nervous system has a total complement of 302 neurons, which are arranged in an essentially invariant structure. Neurons with similar morphologies and connectivities have been grouped together into classes; there are 118 such classes. Neurons have simple morphologies with few, if any, branches. Processes from neurons run in defined positions within bundles of parallel processes, synaptic connections being made en passant. Process bundles are arranged longitudinally and circumferentially and are often adjacent to ridges of hypodermis. Neurons are generally highly locally connected, making synaptic connections with many of their neighbours. Muscle cells have arms that run out to process bundles containing motoneuron axons. Here they receive their synaptic input in defined regions along the surface of the bundles, where motoneuron axons reside. Most of the morphologically identifiable synaptic connections in a typical animal are described. These consist of about 5000 chemical synapses, 2000 neuromuscular junctions and 600 gap junctions.
    9. White JG: Getting into the mind of a worm--a personal view. WormBook 2013. (PMID 23801597) [PubMed] [DOI] [Full text]
    10. Template:cite magazine
    11. Clare JJ et al.: Voltage-gated sodium channels as therapeutic targets. Drug Discov Today 2000. (PMID 11084387) [PubMed] [DOI] Voltage-gated sodium channels (VGSCs) play a central role in the generation and propagation of action potentials in neurons and other cells. VGSC modulators have their origins in empirical pharmacology and are being used as local anaesthetics, antiarrhythmics, analgesics and antiepileptics, and for other disorders. However, the identification of a multigene family of VGSCs, along with tools to study the different subtypes in pathophysiology, is now providing a rational basis for selective intervention. Recent advances have addressed the technical challenges of expressing and assaying these complex proteins, enabling the correlation of empirical pharmacology to subtypes and the screening of individual subtypes for novel inhibitors with increased potency and selectivity.
    12. Adewumi O et al.: Characterization of human embryonic stem cell lines by the International Stem Cell Initiative. Nat Biotechnol 2007. (PMID 17572666) [PubMed] [DOI] The International Stem Cell Initiative characterized 59 human embryonic stem cell lines from 17 laboratories worldwide. Despite diverse genotypes and different techniques used for derivation and maintenance, all lines exhibited similar expression patterns for several markers of human embryonic stem cells. They expressed the glycolipid antigens SSEA3 and SSEA4, the keratan sulfate antigens TRA-1-60, TRA-1-81, GCTM2 and GCT343, and the protein antigens CD9, Thy1 (also known as CD90), tissue-nonspecific alkaline phosphatase and class 1 HLA, as well as the strongly developmentally regulated genes NANOG, POU5F1 (formerly known as OCT4), TDGF1, DNMT3B, GABRB3 and GDF3. Nevertheless, the lines were not identical: differences in expression of several lineage markers were evident, and several imprinted genes showed generally similar allele-specific expression patterns, but some gene-dependent variation was observed. Also, some female lines expressed readily detectable levels of XIST whereas others did not. No significant contamination of the lines with mycoplasma, bacteria or cytopathic viruses was detected.
    13. 13.0 13.1 Cook SJ et al.: Whole-animal connectomes of both Caenorhabditis elegans sexes. Nature 2019. (PMID 31270481) [PubMed] [DOI] [Full text] Knowledge of connectivity in the nervous system is essential to understanding its function. Here we describe connectomes for both adult sexes of the nematode Caenorhabditis elegans, an important model organism for neuroscience research. We present quantitative connectivity matrices that encompass all connections from sensory input to end-organ output across the entire animal, information that is necessary to model behaviour. Serial electron microscopy reconstructions that are based on the analysis of both new and previously published electron micrographs update previous results and include data on the male head. The nervous system differs between sexes at multiple levels. Several sex-shared neurons that function in circuits for sexual behaviour are sexually dimorphic in structure and connectivity. Inputs from sex-specific circuitry to central circuitry reveal points at which sexual and non-sexual pathways converge. In sex-shared central pathways, a substantial number of connections differ in strength between the sexes. Quantitative connectomes that include all connections serve as the basis for understanding how complex, adaptive behavior is generated.
    14. Brouillette et al.; "Mapping the brain to understand the mind" , https://knowablemagazine.org/article/mind/2022/mapping-brain-understand-mind , https://doi.org/10.1146/knowable-042122-1
    15. Watts DJ & Strogatz SH: Collective dynamics of 'small-world' networks. Nature 1998. (PMID 9623998) [PubMed] [DOI] Networks of coupled dynamical systems have been used to model biological oscillators, Josephson junction arrays, excitable media, neural networks, spatial games, genetic control networks and many other self-organizing systems. Ordinarily, the connection topology is assumed to be either completely regular or completely random. But many biological, technological and social networks lie somewhere between these two extremes. Here we explore simple models of networks that can be tuned through this middle ground: regular networks 'rewired' to introduce increasing amounts of disorder. We find that these systems can be highly clustered, like regular lattices, yet have small characteristic path lengths, like random graphs. We call them 'small-world' networks, by analogy with the small-world phenomenon (popularly known as six degrees of separation. The neural network of the worm Caenorhabditis elegans, the power grid of the western United States, and the collaboration graph of film actors are shown to be small-world networks. Models of dynamical systems with small-world coupling display enhanced signal-propagation speed, computational power, and synchronizability. In particular, infectious diseases spread more easily in small-world networks than in regular lattices.
    16. Schafer WR: Deciphering the neural and molecular mechanisms of C. elegans behavior. Curr Biol 2005. (PMID 16139205) [PubMed] [DOI] Because of its small and well-characterized nervous system and amenability to genetic manipulation, the nematode Caenorhabditis elegans offers the promise of understanding the mechanisms underlying a whole animal's behavior at the molecular and cellular levels. In fact, this goal was a primary motivation behind the development of C. elegans as an experimental organism 40 years ago. Yet it has proven surprisingly difficult to obtain a mechanistic understanding of how the C. elegans nervous system generates behavior, despite the existence of a 'wiring diagram' that contains a degree of information about neural connectivity unparalleled in any organism. This review describes three types of information--molecular data on cellular neurochemistry, temporal information about neural activity patterns, and behavioral data on the consequences of neural ablation and manipulation--that, along with genetic analysis, may ultimately lead to a complete functional map of the C. elegans nervous system.
    17. 17.0 17.1 Alcántar-Fernández J et al.: Caenorhabditis elegans respond to high-glucose diets through a network of stress-responsive transcription factors. PLoS One 2018. (PMID 29990370) [PubMed] [DOI] [Full text] High-glycemic-index diets, as well as a sedentary lifestyle are considered as determinant factors for the development of obesity, type 2 diabetes, and cardiovascular diseases in humans. These diets have been shown to shorten the life span of C. elegans in a manner that is dependent on insulin signaling, but the participation of other signaling pathways have not been addressed. In this study, we have determined that worms fed with high-glucose diets show alterations in glucose content and uptake, triglyceride content, body size, number of eggs laid, egg-laying defects, and signs of oxidative stress and accelerated aging. Additionally, we analyzed the participation of different key regulators of carbohydrate and lipid metabolism, oxidative stress and longevity such as SKN-1/NRF2, HIF-1/HIF1α, SBP-1/SREBP, CRH-1/CREB, CEP-1/p53, and DAF-16/FOXO, in the reduction of lifespan in glucose-fed worms.
    18. Behavior, http://www.wormbook.org/chapters/www_behavior/behavior.html#sec1
    19. Wolkow CA et al.: Regulation of C. elegans life-span by insulinlike signaling in the nervous system. Science 2000. (PMID 11021802) [PubMed] [DOI] An insulinlike signaling pathway controls Caenorhabditis elegans aging, metabolism, and development. Mutations in the daf-2 insulin receptor-like gene or the downstream age-1 phosphoinositide 3-kinase gene extend adult life-span by two- to threefold. To identify tissues where this pathway regulates aging and metabolism, we restored daf-2 pathway signaling to only neurons, muscle, or intestine. Insulinlike signaling in neurons alone was sufficient to specify wild-type life-span, but muscle or intestinal signaling was not. However, restoring daf-2 pathway signaling to muscle rescued metabolic defects, thus decoupling regulation of life-span and metabolism. These findings point to the nervous system as a central regulator of animal longevity.
    20. Ewald CY et al.: Dauer-independent insulin/IGF-1-signalling implicates collagen remodelling in longevity. Nature 2015. (PMID 25517099) [PubMed] [DOI] [Full text] Interventions that delay ageing mobilize mechanisms that protect and repair cellular components, but it is unknown how these interventions might slow the functional decline of extracellular matrices, which are also damaged during ageing. Reduced insulin/IGF-1 signalling (rIIS) extends lifespan across the evolutionary spectrum, and in juvenile Caenorhabditis elegans also allows the transcription factor DAF-16/FOXO to induce development into dauer, a diapause that withstands harsh conditions. It has been suggested that rIIS delays C. elegans ageing through activation of dauer-related processes during adulthood, but some rIIS conditions confer robust lifespan extension unaccompanied by any dauer-like traits. Here we show that rIIS can promote C. elegans longevity through a program that is genetically distinct from the dauer pathway, and requires the Nrf (NF-E2-related factor) orthologue SKN-1 acting in parallel to DAF-16. SKN-1 is inhibited by IIS and has been broadly implicated in longevity, but is rendered dispensable for rIIS lifespan extension by even mild activity of dauer-related processes. When IIS is decreased under conditions that do not induce dauer traits, SKN-1 most prominently increases expression of collagens and other extracellular matrix genes. Diverse genetic, nutritional, and pharmacological pro-longevity interventions delay an age-related decline in collagen expression. These collagens mediate adulthood extracellular matrix remodelling, and are needed for ageing to be delayed by interventions that do not involve dauer traits. By genetically delineating a dauer-independent rIIS ageing pathway, our results show that IIS controls a broad set of protective mechanisms during C. elegans adulthood, and may facilitate elucidation of processes of general importance for longevity. The importance of collagen production in diverse anti-ageing interventions implies that extracellular matrix remodelling is a generally essential signature of longevity assurance, and that agents promoting extracellular matrix youthfulness may have systemic benefit.
    21. Venz R et al.: End-of-life targeted degradation of DAF-2 insulin/IGF-1 receptor promotes longevity free from growth-related pathologies. Elife 2021. (PMID 34505574) [PubMed] [DOI] [Full text] Preferably, lifespan-extending therapies should work when applied late in life without causing undesired pathologies. Reducing insulin/insulin-like growth factor (IGF)-1 signaling (IIS) increases lifespan across species, but the effects of reduced IIS interventions in extreme geriatric ages remains unknown. Using the nematode Caenorhabditis elegans, we engineered the conditional depletion of the DAF-2/insulin/IGF-1 transmembrane receptor using an auxin-inducible degradation (AID) system. This allowed for the temporal and spatial reduction in DAF-2 protein levels at time points after which interventions such as RNAi become ineffective. Using this system, we found that AID-mediated depletion of DAF-2 protein surpasses the longevity of daf-2 mutants. Depletion of DAF-2 during early adulthood resulted in multiple adverse phenotypes, including growth retardation, germline shrinkage, egg retention, and reduced brood size. By contrast, AID-mediated depletion of DAF-2 post-reproduction, or specifically in the intestine in early adulthood, resulted in an extension of lifespan without these deleterious effects. Strikingly, at geriatric ages, when 75% of the population had died, AID-mediated depletion of DAF-2 protein resulted in a doubling in lifespan. Thus, we provide a proof-of-concept that even close to the end of an individual's lifespan, it is possible to slow aging and promote longevity. The goal of geroscience, or research into old age, is to promote health during old age, and thus, to increase lifespan. In the body, the groups of biochemical reactions, or ‘pathways’, that allow an organism to sense nutrients, and regulate growth and stress, play major roles in ensuring healthy aging. Indeed, organisms that do not produce a working version of the insulin/IGF-1 receptor, a protein involved in one such pathway, show increased lifespan. In the worm Caenorhabditis elegans, mutations in the insulin/IGF-1 receptor can even double their lifespan. However, it is unclear whether this increase can be achieved once the organism has reached old age. To answer this question, Venz et al. genetically engineered the nematode worm C. elegans so that they could trigger the rapid degradation of the insulin/IGF-1 receptor either in the entire organism or in a specific tissue. Venz et al. started by aging several C. elegans worms for three weeks, until about 75% had died. At this point, they triggered the degradation of the insulin/IGF-1 receptor in some of the remaining worms, keeping the rest untreated as a control for the experiment. The results showed that the untreated worms died within a few days, while worms in which the insulin/IGF-1 receptor had been degraded lived for almost one more month. This demonstrates that it is possible to double the lifespan of an organism at the very end of life. Venz et al.’s findings suggest that it is possible to make interventions to extend an organism’s lifespan near the end of life that are as effective as if they were performed when the organism was younger. This sparks new questions regarding the quality of this lifespan extension: do the worms become younger with the intervention, or is aging simply slowed down?
    22. 22.0 22.1 Hyun M et al.: Longevity and resistance to stress correlate with DNA repair capacity in Caenorhabditis elegans. Nucleic Acids Res 2008. (PMID 18203746) [PubMed] [DOI] [Full text] DNA repair is an important mechanism by which cells maintain genomic integrity. Decline in DNA repair capacity or defects in repair factors are thought to contribute to premature aging in mammals. The nematode Caenorhabditis elegans is a good model for studying longevity and DNA repair because of key advances in understanding the genetics of aging in this organism. Long-lived C. elegans mutants have been identified and shown to be resistant to oxidizing agents and UV irradiation, suggesting a genetically determined correlation between DNA repair capacity and life span. In this report, gene-specific DNA repair is compared in wild-type C. elegans and stress-resistant C. elegans mutants for the first time. DNA repair capacity is higher in long-lived C. elegans mutants than in wild-type animals. In addition, RNAi knockdown of the nucleotide excision repair gene xpa-1 increased sensitivity to UV and reduced the life span of long-lived C. elegans mutants. These findings support that DNA repair capacity correlates with longevity in C. elegans.
    23. Meyer JN et al.: Decline of nucleotide excision repair capacity in aging Caenorhabditis elegans. Genome Biol 2007. (PMID 17472752) [PubMed] [DOI] [Full text] BACKGROUND: Caenorhabditis elegans is an important model for the study of DNA damage and repair related processes such as aging, neurodegeneration, and carcinogenesis. However, DNA repair is poorly characterized in this organism. We adapted a quantitative polymerase chain reaction assay to characterize repair of DNA damage induced by ultraviolet type C (UVC) radiation in C. elegans, and then tested whether DNA repair rates were affected by age in adults. RESULTS: UVC radiation induced lesions in young adult C. elegans, with a slope of 0.4 to 0.5 lesions per 10 kilobases of DNA per 100 J/m2, in both nuclear and mitochondrial targets. L1 and dauer larvae were more than fivefold more sensitive to lesion formation than were young adults. Nuclear repair kinetics in a well expressed nuclear gene were biphasic in nongravid adult nematodes: a faster, first order (half-life about 16 hours) phase lasting approximately 24 hours and resulting in removal of about 60% of the photoproducts was followed by a much slower phase. Repair in ten nuclear DNA regions was 15% and 50% higher in more actively transcribed regions in young and aging adults, respectively. Finally, repair was reduced by 30% to 50% in each of the ten nuclear regions in aging adults. However, this decrease in repair could not be explained by a reduction in expression of nucleotide excision repair genes, and we present a plausible mechanism, based on gene expression data, to account for this decrease. CONCLUSION: Repair of UVC-induced DNA damage in C. elegans is similar kinetically and genetically to repair in humans. Furthermore, this important repair process slows significantly in aging C. elegans, the first whole organism in which this question has been addressed.
    24. McColl G et al.: Pharmacogenetic analysis of lithium-induced delayed aging in Caenorhabditis elegans. J Biol Chem 2008. (PMID 17959600) [PubMed] [DOI] [Full text] Lithium (Li(+)) has been used to treat mood affect disorders, including bipolar, for decades. This drug is neuroprotective and has several identified molecular targets. However, it has a narrow therapeutic range and the one or more underlying mechanisms of its therapeutic action are not understood. Here we describe a pharmacogenetic study of Li(+) in the nematode Caenorhabditis elegans. Exposure to Li(+) at clinically relevant concentrations throughout adulthood increases survival during normal aging (up to 46% median increase). Longevity is extended via a novel mechanism with altered expression of genes encoding nucleosome-associated functions. Li(+) treatment results in reduced expression of the worm ortholog of LSD-1 (T08D10.2), a histone demethylase; knockdown by RNA interference of T08D10.2 is sufficient to extend longevity ( approximately 25% median increase), suggesting Li(+) regulates survival by modulating histone methylation and chromatin structure.
    25. Zarse K et al.: Low-dose lithium uptake promotes longevity in humans and metazoans. Eur J Nutr 2011. (PMID 21301855) [PubMed] [DOI] [Full text] PURPOSE: Lithium is a nutritionally essential trace element predominantly contained in vegetables, plant-derived foods, and drinking water. Environmental lithium exposure and concurrent nutritional intake vary considerably in different regions. We here have analyzed the possibility that low-dose lithium exposure may affect mortality in both metazoans and mammals. METHODS: Based on a large Japanese observational cohort, we have used weighted regression analysis to identify putative effects of tap water-derived lithium uptake on overall mortality. Independently, we have exposed Caenorhabditis elegans, a small roundworm commonly used for anti-aging studies, to comparable concentrations of lithium, and have quantified mortality during this intervention. RESULTS: In humans, we find here an inverse correlation between drinking water lithium concentrations and all-cause mortality in 18 neighboring Japanese municipalities with a total of 1,206,174 individuals (β = -0.661, p = 0.003). Consistently, we find that exposure to a comparably low concentration of lithium chloride extends life span of C. elegans (p = 0.047). CONCLUSIONS: Taken together, these findings indicate that long-term low-dose exposure to lithium may exert anti-aging capabilities and unambiguously decreases mortality in evolutionary distinct species.
    26. Ewald CY & Li C: Understanding the molecular basis of Alzheimer's disease using a Caenorhabditis elegans model system. Brain Struct Funct 2010. (PMID 20012092) [PubMed] [DOI] [Full text] Alzheimer's disease (AD) is the major cause of dementia in the United States. At the cellular level, the brains of AD patients are characterized by extracellular dense plaques and intracellular neurofibrillary tangles whose major components are the beta-amyloid peptide and tau, respectively. The beta-amyloid peptide is a cleavage product of the amyloid precursor protein (APP); mutations in APP have been correlated with a small number of cases of familial Alzheimer's disease. APP is the canonical member of the APP family, whose functions remain unclear. The nematode Caenorhabditis elegans, one of the premier genetic workhorses, is being used in a variety of ways to address the functions of APP and determine how the beta-amyloid peptide and tau can induce toxicity. First, the function of the C. elegans APP-related gene, apl-1, is being examined. Although different organisms may use APP and related proteins, such as APL-1, in different functional contexts, the pathways in which they function and the molecules with which they interact are usually conserved. Second, components of the gamma-secretase complex and their respective functions are being revealed through genetic analyses in C. elegans. Third, to address questions of toxicity, onset of degeneration, and protective mechanisms, different human beta-amyloid peptide and tau variants are being introduced into C. elegans and the resultant transgenic lines examined. Here, we summarize how a simple system such as C. elegans can be used as a model to understand APP function and suppression of beta-amyloid peptide and tau toxicity in higher organisms.
    27. Hanazawa M et al.: PGL proteins self associate and bind RNPs to mediate germ granule assembly in C. elegans. J Cell Biol 2011. (PMID 21402787) [PubMed] [DOI] [Full text] Germ granules are germ lineage-specific ribonucleoprotein (RNP) complexes, but how they are assembled and specifically segregated to germ lineage cells remains unclear. Here, we show that the PGL proteins PGL-1 and PGL-3 serve as the scaffold for germ granule formation in Caenorhabditis elegans. Using cultured mammalian cells, we found that PGL proteins have the ability to self-associate and recruit RNPs. Depletion of PGL proteins from early C. elegans embryos caused dispersal of other germ granule components in the cytoplasm, suggesting that PGL proteins are essential for the architecture of germ granules. Using a structure-function analysis in vivo, we found that two functional domains of PGL proteins contribute to germ granule assembly: an RGG box for recruiting RNA and RNA-binding proteins and a self-association domain for formation of globular granules. We propose that self-association of scaffold proteins that can bind to RNPs is a general mechanism by which large RNP granules are formed.
    28. Coutts F et al.: The polygenic nature of telomere length and the anti-ageing properties of lithium. Neuropsychopharmacology 2019. (PMID 30559463) [PubMed] [DOI] [Full text] Telomere length is a promising biomarker for age-related disease and a potential anti-ageing drug target. Here, we study the genetic architecture of telomere length and the repositioning potential of lithium as an anti-ageing medication. LD score regression applied to the largest telomere length genome-wide association study to-date, revealed SNP-chip heritability estimates of 7.29%, with polygenic risk scoring capturing 4.4% of the variance in telomere length in an independent cohort (p = 6.17 × 10-5). Gene-enrichment analysis identified 13 genes associated with telomere length, with the most significant being the leucine rich repeat gene, LRRC34 (p = 3.69 × 10-18). In the context of lithium, we confirm that chronic use in a sample of 384 bipolar disorder patients is associated with longer telomeres (p = 0.03). As complementary evidence, we studied three orthologs of telomere length regulators in a Caenorhabditis elegans model of lithium-induced extended longevity and found all transcripts to be affected post-treatment (p < 0.05). Lithium may therefore confer its anti-ageing effects by moderating the expression of genes responsible for normal telomere length regulation. This is supported by our bipolar disorder sample, which shows that polygenic risk scores explain a higher proportion of the variance in telomere length amongst chronic lifetime lithium users (variance explained = 8.9%, p = 0.01), compared to non-users (p > 0.05). Consequently, this suggests that lithium may be catalysing the activity of endogenous mechanisms that promote telomere lengthening, whereby its efficacy eventually becomes limited by each individual's inherent telomere maintenance capabilities. Our work indicates a potential use of polygenic risk scoring for the prediction of adult telomere length and consequently lithium's anti-ageing efficacy.
    29. Raices M et al.: Uncoupling of longevity and telomere length in C. elegans. PLoS Genet 2005. (PMID 16151516) [PubMed] [DOI] [Full text] The nematode Caenorhabditis elegans, after completing its developmental stages and a brief reproductive period, spends the remainder of its adult life as an organism consisting exclusively of post-mitotic cells. Here we show that telomere length varies considerably in clonal populations of wild-type worms, and that these length differences are conserved over at least ten generations, suggesting a length regulation mechanism in cis. This observation is strengthened by the finding that the bulk telomere length in different worm strains varies considerably. Despite the close correlation of telomere length and clonal cellular senescence in mammalian cells, nematodes with long telomeres were neither long lived, nor did worm populations with comparably short telomeres exhibit a shorter life span. Conversely, long-lived daf-2 and short-lived daf-16 mutant animals can have either long or short telomeres. Telomere length of post-mitotic cells did not change during the aging process, and the response of animals to stress was found independent of telomere length. Collectively, our data indicate that telomere length and life span can be uncoupled in a post-mitotic setting, suggesting separate pathways for replication-dependent and -independent aging.
    30. Lulkiewicz M et al.: Telomere length: how the length makes a difference. Mol Biol Rep 2020. (PMID 32876842) [PubMed] [DOI] [Full text] Telomerase is perceived as an immortality enzyme that might provide longevity to cells and whole organisms. Importantly, it is generally inactive in most somatic cells of healthy, adult men. Consequently, its substrates, i.e. telomeres, get shorter in most human cells with time. Noteworthy, cell life limitation due to telomere attrition during cell divisions, may not be as bad as it looks since longer cell life means longer exposition to harmful factors. Consequently, telomere length (attrition rate) becomes a factor that is responsible for inducing the signaling that leads to the elimination of cells that lived long enough to acquire severe damage. It seems that telomere length that depends on many different factors (including telomerase activity but also genetic factors, a hormonal profile that reflects sex, etc.) might become a useful marker of aging and exposition to stress. Thus in the current paper, we review the factors that affect telomere length in human cells focusing on sex that all together with different environmental and hormonal regulations as well as parental aspect affect telomere attrition rate. We also raise some limitations in the assessment of telomere length that hinders a trustworthy meta-analysis that might lead to acknowledgment of the real value of this parameter.
    31. Pardue ML & DeBaryshe PG: Retrotransposons that maintain chromosome ends. Proc Natl Acad Sci U S A 2011. (PMID 21821789) [PubMed] [DOI] [Full text] Reverse transcriptases have shaped genomes in many ways. A remarkable example of this shaping is found on telomeres of the genus Drosophila, where retrotransposons have a vital role in chromosome structure. Drosophila lacks telomerase; instead, three telomere-specific retrotransposons maintain chromosome ends. Repeated transpositions to chromosome ends produce long head to tail arrays of these elements. In both form and function, these arrays are analogous to the arrays of repeats added by telomerase to chromosomes in other organisms. Distantly related Drosophila exhibit this variant mechanism of telomere maintenance, which was established before the separation of extant Drosophila species. Nevertheless, the telomere-specific elements still have the hallmarks that characterize non-long terminal repeat (non-LTR) retrotransposons; they have also acquired characteristics associated with their roles at telomeres. These telomeric retrotransposons have shaped the Drosophila genome, but they have also been shaped by the genome. Here, we discuss ways in which these three telomere-specific retrotransposons have been modified for their roles in Drosophila chromosomes.
    32. Meier B et al.: trt-1 is the Caenorhabditis elegans catalytic subunit of telomerase. PLoS Genet 2006. (PMID 16477310) [PubMed] [DOI] [Full text] Mutants of trt-1, the Caenorhabditis elegans telomerase reverse transcriptase, reproduce normally for several generations but eventually become sterile as a consequence of telomere erosion and end-to-end chromosome fusions. Telomere erosion and uncapping do not cause an increase in apoptosis in the germlines of trt-1 mutants. Instead, late-generation trt-1 mutants display chromosome segregation defects that are likely to be the direct cause of sterility. trt-1 functions in the same telomere replication pathway as mrt-2, a component of the Rad9/Rad1/Hus1 (9-1-1) proliferating cell nuclear antigen-like sliding clamp. Thus, the 9-1-1 complex may be required for telomerase to act at chromosome ends in C. elegans. Although telomere erosion limits replicative life span in human somatic cells, neither trt-1 nor telomere shortening affects postmitotic aging in C. elegans. These findings illustrate effects of telomere dysfunction in C. elegans mutants lacking the catalytic subunit of telomerase, trt-1.
    33. Cesare AJ & Reddel RR: Alternative lengthening of telomeres: models, mechanisms and implications. Nat Rev Genet 2010. (PMID 20351727) [PubMed] [DOI] Unlimited cellular proliferation depends on counteracting the telomere attrition that accompanies DNA replication. In human cancers this usually occurs through upregulation of telomerase activity, but in 10-15% of cancers - including some with particularly poor outcome - it is achieved through a mechanism known as alternative lengthening of telomeres (ALT). ALT, which is dependent on homologous recombination, is therefore an important target for cancer therapy. Although dissection of the mechanism or mechanisms of ALT has been challenging, recent advances have led to the identification of several genes that are required for ALT and the elucidation of the biological significance of some phenotypic markers of ALT. This has enabled development of a rapid assay of ALT activity levels and the construction of molecular models of ALT.
    34. Ijomone OM et al.: Null allele mutants of trt-1, the catalytic subunit of telomerase in Caenorhabditis elegans, are less sensitive to Mn-induced toxicity and DAergic degeneration. Neurotoxicology 2016. (PMID 27593554) [PubMed] [DOI] Exposure to manganese (Mn) represents an environmental risk factor for Parkinson's disease (PD). Recent evidence suggests that telomerase reverse transcriptase (TERT), the catalytic subunit of mammalian telomerase participates in non-telomeric functions and may play a role in cellular protection from oxidative stress and DNA damage. trt-1 is the catalytic subunit of telomerase in Caenorhabditis elegans (C. elegans). The present study investigated the relationship between trt-1 mutation and Mn-induced neurotoxicity. Wild-type (wt) and trt-1 worms were subjected to an acute Mn treatment of 1h at the first larval (L1) stage. Survival assay and behavior (Basal slowing response, chemotaxis) were assessed. Dopaminergic (DAergic) neurodegeneration was evaluated in successful crosses of trt-1 worms expressing green fluorescent protein (GFP) (dat-1:GFP worms). trt-1 worms were less sensitive to Mn-induced lethality compared to wt worms. Mn induced DAergic degeneration in wt worms, but not in trt-1 worms. Basal slowing was altered in both wt and trt-1 worms; however trt-1 worms were significantly less affected in their basal slowing behavior compared to wt worms. Mn treatment did not affect chemotaxis by NaCl in either wt or trt-1 mutants worms. Combined, the results establish that null mutation in trt-1 improves survival and attenuates damage to the DAergic system.
    35. Shtessel L et al.: Caenorhabditis elegans POT-1 and POT-2 repress telomere maintenance pathways. G3 (Bethesda) 2013. (PMID 23390606) [PubMed] [DOI] [Full text] Telomeres are composed of simple tandem DNA repeats that protect the ends of linear chromosomes from replicative erosion or inappropriate DNA damage response mechanisms. The mammalian Protection Of Telomeres (POT1) protein interacts with single-stranded telomeric DNA and can exert positive and negative effects on telomere length. Of four distinct POT1 homologs in the roundworm Caenorhabditis elegans, deficiency for POT-1 or POT-2 resulted in progressive telomere elongation that occurred because both proteins negatively regulate telomerase. We created a POT-1::mCherry fusion protein that forms discrete foci at C. elegans telomeres, independent of POT-2, allowing for live analysis of telomere dynamics. Transgenic pot-1::mCherry repressed telomerase in pot-1 mutants. Animals deficient for pot-1, but not pot-2, displayed mildly enhanced telomere erosion rates in the absence of the telomerase reverse transcriptase, trt-1. However, trt-1; pot-1 double mutants exhibited delayed senescence in comparison to trt-1 animals, and senescence was further delayed in trt-1; pot-2; pot-1 triple mutants, some of which survived robustly in the absence of telomerase. Our results indicate that POT-1 and POT-2 play independent roles in suppressing a telomerase-independent telomere maintenance pathway but may function together to repress telomerase.
    36. Kwon MS et al.: Paradoxical delay of senescence upon depletion of BRCA2 in telomerase-deficient worms. FEBS Open Bio 2016. (PMID 27761361) [PubMed] [DOI] [Full text] BRCA2 is a multifunctional tumor suppressor involved in homologous recombination (HR), mitotic checkpoint regulation, and telomere homeostasis. Absence of Brca2 in mice results in progressive shortening of telomeres and senescence, yet cells are prone to neoplastic transformation with elongated telomeres, suggesting that BRCA2 has positive and negative effects on telomere length regulation along the path to tumorigenesis. Using Caenorhabditis elegans as a model, we show here that depletion of BRC-2, an ortholog of BRCA2, paradoxically delays senescence in telomerase-deficient mutant worms. Telomerase-deficient worms (trt-1) exhibit early replication senescence due to short telomeres. It should be noted that worms mutated in brc-2 are not viable as well due to massive genotoxic insults. However, when BRC-2 is depleted by RNA interference in trt-1 mutant worms, the number of generations is unexpectedly increased with telomere length maintained, compared to telomerase mutants. Interestingly, depletion of other HR genes such as rad-51 and rad-54 exhibited similar effects. In worms doubly deficient of telomerase and brc-2, rad-51, or rad-54, extra telomeric C-circles were generated, suggesting that abrogation of HR induces an alteration in telomere environment favorable to illegitimate telomere maintenance when telomerase is absent. Collectively, absence of BRC-2 in telomerase-deficient background first leads to telomere shortening, followed by an induction of an as-yet-unknown telomere maintenance pathway, resulting in delay of senescence. The results have implications in the understanding of dysfunctional BRCA2-associated tumorigenesis.
    37. Bayat M et al.: Effects of telomerase overexpression in the model organism Caenorhabditis elegans. Gene 2020. (PMID 31954861) [PubMed] [DOI]