Spatiotemporal control of transgene expression using an infrared laser in the crustacean Daphnia magna.

The crustacean Daphnia magna is an emerging model for ecological and toxicological genomics. However, the lack of methods for spatial and temporal control of gene expression has impaired the elucidation of molecular mechanisms underlying responses to environments in vivo. Here we report local activation of the hsp70 promoter-driven gene cassette in D. magna by the infrared laser-evoked gene operator (IR-LEGO), a method for heating the target cells with infrared irradiation. We identified the heat-inducible promoter upstream of the D. magna hsp70-A gene. Using this promoter, we generated a transgenic Daphnia harboring the heat-shock responsive GFP reporter gene and confirmed that the GFP gene responds to heat treatment not only in juveniles and adults but also in embryos. We collected embryos from the reporter line and irradiated four different regions of interest in the embryos: a proximal region of the third thoracic segment, a part of the midline, a second maxilla, and a distal region of the endopodite of the second antenna, all of which increased GFP fluorescence with an infrared laser. Our results suggest that the IR-LEGO method is useful for spatial and temporal control of gene expression and would advance the functional genomics in D. magna.

Trehalose mediates salinity-stress tolerance in natural populations of a freshwater crustacean.

Salinization poses an increasing problem worldwide, threatening freshwater organisms and raising questions about their ability to adapt. We explored the mechanisms enabling a planktonic crustacean to tolerate elevated salinity. By gradually raising water salinity in clonal cultures from 185 Daphnia magna populations, we showed that salt tolerance strongly correlates with native habitat salinity, indicating local adaptation. A genome-wide association study (GWAS) further revealed a major effect of the Alpha,alpha-trehalose-phosphate synthase (TPS) gene, suggesting that trehalose production facilitates salinity tolerance. Salinity-tolerant animals showed a positive correlation between water salinity and trehalose concentrations, while intolerant animals failed to produce trehalose. Animals with a non-functional TPS gene, generated through CRISPR-Cas9, supported the trehalose role in salinity stress. Our study highlights how a keystone freshwater animal adapts to salinity stress using an evolutionary mechanism known in bacteria, plants, and arthropods.

Dragon fruit-derived oligosaccharides alter hemocyte-mediated immunity and expression of genes related to innate immunity and oxidative stress in Daphnia magna.

Dragon fruit oligosaccharide (DFO) is an indigestible prebiotic that enhances the growth and reproduction of Daphnia magna, increases the expression of genes involved in immunity, and reduces oxidative stress. This study investigated the effects of DFO on the expression of innate immunity- (Toll, Pelle, proPO, A2M, and CTL), oxidative stress- (Mn-SOD), and nitric oxide (NO) synthesis-related genes (NOS1, NOS2, and arginase) as well as NO localization and number of hemocytes in D. magna. For this ten-day-old D. magna were treated with 0 or 9 mg l-1 of DFO for 24 and 85 h. Gene expression levels, NO intensity and localization, and total hemocytes were evaluated. After 24 h, the expression of Toll and proPO increased significantly (p < 0.05), while that of C-type lectins (CTL) was reduced (p < 0.05). At 85 h, Mn-SOD and CTL expressions were markedly suppressed (p < 0.05). NO was mostly localized in the foregut, midgut, hindgut, and carapace. The expression of NOS1 was reduced after 24 h (p < 0.05). In addition, NO intensity at 24 h was insignificantly lower than the control (p > 0.05). At 85 h, the expression of NOS1, NOS2, and arginase was higher than control, but NO intensity did not differ significantly (p > 0.05). Furthermore, the total hemocyte count elevated remarkably at 85 h (p < 0.05). Our study suggested that 9 mg l-1 of DFO could alter the expression of the genes related to innate immunity, oxidative stress, and NO synthesis in D. magna and significantly stimulate hemocyte production.

Identification of gene isoforms and their switching events between male and female embryos of the parthenogenetic crustacean Daphnia magna.

The cladoceran crustacean Daphnia exhibits phenotypic plasticity, a phenomenon that leads to diverse phenotypes from one genome. Alternative usage of gene isoforms has been considered a key gene regulation mechanism for controlling different phenotypes. However, to understand the phenotypic plasticity of Daphnia, gene isoforms have not been comprehensively analyzed. Here we identified 25,654 transcripts derived from the 9710 genes expressed during environmental sex determination of Daphnia magna using the long-read RNA-Seq with PacBio Iso-Seq. We found that 14,924 transcripts were previously unidentified and 5713 genes produced two or more isoforms. By a combination of Illumina short-read RNA-Seq, we detected 824 genes that implemented switching of the highest expressed isoform between females and males. Among the 824 genes, we found isoform switching of an ortholog of CREB-regulated transcription coactivator, a major regulator of carbohydrate metabolism in animals, and a correlation of this switching event with the sexually dimorphic expression of carbohydrate metabolic genes. These results suggest that a comprehensive catalog of isoforms may lead to understanding the molecular basis for environmental sex determination of Daphnia. We also infer the applicability of the full-length isoform analyses to the elucidation of phenotypic plasticity in Daphnia.

Neonatal administration of synthetic estrogen, diethylstilbestrol to mice up-regulates inflammatory Cxclchemokines located in the 5qE1 region in the vaginal epithelium.

A synthetic estrogen, diethylstilbestrol (DES), is known to cause adult vaginal carcinoma by neonatal administration of DES to mice. However, the carcinogenic process remains unclear. By Cap Analysis of Gene Expression method, we found that neonatal DES exposure up-regulated inflammatory Cxcl chemokines 2, 3, 5, and 7 located in the 5qE1 region in the vaginal epithelium of mice 70 days after birth. When we examined the gene expressions of these genes much earlier stages, we found that neonatal DES exposure increased these Cxcl chemokine genes expression even after 17 days after birth. It implies the DES-mediated persistent activation of inflammatory genes. Intriguingly, we also detected DES-induced non-coding RNAs from a region approximately 100 kb far from the Cxcl5 gene. The non-coding RNA up-regulation by DES exposure was confirmed on the 17-day vagina and continued throughout life, which may responsible for the activation of Cxcl chemokines located in the same region, 5qE1. This study shows that neonatal administration of DES to mice causes long-lasting up-regulation of inflammatory Cxcl chemokines in the vaginal epithelium. DES-mediated inflammation may be associated with the carcinogenic process.

Lower Availability of Mitochondrial Complex I in Anterior Cingulate Cortex in Autism: A Positron Emission Tomography Study.

OBJECTIVE: Mitochondrial dysfunction has been implicated in the pathophysiology of autism spectrum disorder (ASD) in previous studies of postmortem brain or peripheral samples. The authors investigated whether and where mitochondrial dysfunction occurs in the living brains of individuals with ASD and to identify the clinical correlates of detected mitochondrial dysfunction. METHODS: This case-control study used positron emission tomography (PET) with 2-tert-butyl-4-chloro-5-{6-[2-(2-[18F]fluoroethoxy)-ethoxy]-pyridin-3-ylmethoxy}-2H-pyridazin-3-one ([18F]BCPP-EF), a radioligand that binds to the mitochondrial electron transport chain complex I, to examine the topographical distribution of mitochondrial dysfunction in living brains of individuals with ASD. Twenty-three adult males with high-functioning ASD, with no psychiatric comorbidities and free of psychotropic medication, and 24 typically developed males with no psychiatric diagnoses, matched with the ASD group on age, parental socioeconomic background, and IQ, underwent [18F]BCPP-EF PET measurements. Individuals with mitochondrial disease were excluded by clinical evaluation and blood tests for abnormalities in lactate and pyruvate levels. RESULTS: Among the brain regions in which mitochondrial dysfunction has been reported in postmortem studies of autistic brains, participants with ASD had significantly decreased [18F]BCPP-EF availability specifically in the anterior cingulate cortex compared with typically developed participants. The regional specificity was revealed by a significant interaction between diagnosis and brain regions. Moreover, the lower [18F]BCPP-EF availability in the anterior cingulate cortex was significantly correlated with the more severe ASD core symptom of social communication deficits. CONCLUSIONS: This study provides direct evidence to link in vivo brain mitochondrial dysfunction with ASD pathophysiology and its communicational deficits. The findings support the possibility that mitochondrial electron transport chain complex I is a novel therapeutic target for ASD core symptoms.

CELF1 represses Doublesex1 expression via its 5' UTR in the crustacean Daphnia magna.

In sex determination of the crustacean Daphnia magna, male-specific expression of DM-domain transcription factor Doublesex1 (Dsx1) orchestrates the male developmental program triggered by environmental stimuli. We previously identified the CELF1 ortholog as a candidate of proteins associated with the 5' UTR of the Dsx1α isoform. Here we report the CELF1-dependent suppression of Dsx1 expression in D. magna. During embryogenesis, CELF1 expression was not sexually dimorphic. Silencing of CELF1 led to the activation of Dsx1 expression both in female and male embryos. Overexpression of CELF1 in male embryos resulted in a reduction of Dsx1 expression. By these manipulations of CELF1 expression, the Dsx1 transcript level was not significantly changed. To investigate whether the CELF1 controls Dsx1 expression via its 5' UTR, we injected the GFP reporter mRNA having intact Dsx1α 5' UTR or mutated one lacking the GU-rich element (GRE) that is known as a binding site of the CELF1 ortholog. We found that deletion of the GRE significantly increased the reporter gene expression. These results indicate that CELF1 suppresses Dsx1 expression both in females and males, possibly at the post-transcriptional level. We speculate that CELF1 may avoid unintended Dsx1 expression and generation of sexual ambiguity by setting a threshold of Dsx1 expression.

Regulation of Doublesex1 Expression for Environmental Sex Determination in the Cladoceran Crustacean Daphnia.

The cladoceran crustacean Daphnia produces only females by parthenogenesis in a healthy population. However, in response to environmental declines such as crowding and lack of foods, it produces eggs destined to become males that are genetically identical to females. During the development of the sexually committed eggs, DM domain-containing transcription factor Doublesex1 (Dsx1) orchestrates male trait formation globally both in somatic and gonadal tissues. Recent studies have revealed that Dsx1 expression is tightly controlled at transcriptional, post-transcriptional, and epigenetic levels to avoid sexual ambiguity. In this review, together with basic information on Dsx1 structure and expression, we introduce the multi-layered Dsx1 regulation and discuss how each regulation is interconnected for controlling male development in environmental sex-determining Daphnia.

Cell-Free Synthesis of Human Endothelin Receptors and Its Application to Ribosome Display.

Engineering G-protein-coupled receptors (GPCRs) for improved stability or altered function is of great interest, as GPCRs consist of the largest protein family, are involved in many important signaling pathways, and thus, are one of the major drug targets. Here, we report the development of a high-throughput screening method for GPCRs using a reconstituted in vitro transcription-translation (IVTT) system. Human endothelin receptor type-B (ETBR), a class A GPCR that binds endothelin-1 (ET-1), a 21-residue peptide hormone, was synthesized in the presence of nanodisc (ND) composed of a phospholipid, 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG). The ET-1 binding of ETBR was significantly reduced or was undetectable when other phospholipids were used for ND preparation. However, when functional ETBR purified from Sf9 cells was reconstituted into NDs, ET-1 binding was observed with two different phospholipids tested, including POPG. These results suggest that POPG likely supports the folding of ETBR into its functional form in the IVTT system. Using the same conditions as ETBR, whose three-dimensional structure has been solved, human endothelin receptor type-A (ETAR), whose three-dimensional structure remains unsolved, was also synthesized in its functional form. By adding POPG-ND to the IVTT system, both ETAR and ETBR were successfully subjected to ribosome display, a method of in vitro directed evolution that facilitates the screening of up to 1012 mutants. Finally, using a mock library, we showed that ribosome display can be applied for gene screening of ETBR, suggesting that high-throughput screening and directed evolution of GPCRs is possible in vitro.

Development of transgenic Daphnia magna for visualizing homology-directed repair of DNA.

In the crustacean Daphnia magna, studying homology-directed repair (HDR) is important to understand genome maintenance during parthenogenesis, effects of environmental toxicants on the genome, and improvement of HDR-mediated genome editing. Here we developed a transgenic D. magna that expresses green fluorescence protein (GFP) upon HDR occurrence. We utilized the previously established reporter plasmid named DR-GFP that has a mutated eGFP gene (SceGFP) and the tandemly located donor GFP gene fragment (iGFP). Upon double-strand break (DSB) introduction on SceGFP, the iGFP gene fragment acts as the HDR template and restores functional eGFP expression. We customized this reporter plasmid to allow bicistronic expression of the mCherry gene under the control of the D. magna EF1α-1 promoter/enhancer. By CRISPR/Cas-mediated knock-in of this plasmid via non-homologous joining, we generated the transgenic D. magna that expresses mCherry ubiquitously, suggesting that the DR-GFP reporter gene is expressed in most cells. Introducing DSB on the SceGFP resulted in eGFP expression and this HDR event could be detected by fluorescence, genomic PCR, and quantitative reverse-transcription PCR, suggesting this line could be used for evaluating HDR. The established reporter line might expand our understanding of the HDR mechanism and also improve the HDR-based gene-editing system in this species.

Extrastriatal dopamine D2/3 receptor binding, functional connectivity, and autism socio-communicational deficits: a PET and fMRI study.

The social motivation hypothesis of autism proposes that social communication symptoms in autism-spectrum disorder (ASD) stem from atypical social attention and reward networks, where dopamine acts as a crucial mediator. However, despite evidence indicating that individuals with ASD show atypical activation in extrastriatal regions while processing reward and social stimuli, no previous studies have measured extrastriatal dopamine D2/3 receptor (D2/3R) availability in ASD. Here, we investigated extrastriatal D2/3R availability in individuals with ASD and its association with ASD social communication symptoms using positron emission tomography (PET). Moreover, we employed a whole-brain multivariate pattern analysis of resting-state functional magnetic resonance imaging (fMRI) to identify regions where functional connectivity atypically correlates with D2/3R availability depending on ASD diagnosis. Twenty-two psychotropic-free males with ASD and 24 age- and intelligence quotient-matched typically developing males underwent [11C]FLB457 PET, fMRI, and clinical symptom assessment. Participants with ASD showed lower D2/3R availability throughout the D2/3R-rich extrastriatal regions of the dopaminergic pathways. Among these, the posterior region of the thalamus, which primarily comprises the pulvinar, displayed the largest effect size for the lower D2/3R availability, which correlated with a higher score on the Social Affect domain of the Autism Diagnostic Observation Schedule-2 in participants with ASD. Moreover, lower D2/3R availability was correlated with lower functional connectivity of the thalamus-superior temporal sulcus and cerebellum-medial occipital cortex, specifically in individuals with ASD. The current findings provide novel molecular evidence for the social motivation theory of autism and offer a novel therapeutic target.

The freshwater water flea Daphnia magna NIES strain genome as a resource for CRISPR/Cas9 gene targeting: The glutathione S-transferase omega 2 gene.

The water flea Daphnia magna is a small freshwater planktonic animal in the Cladocera. In this study, we assembled the genome of the D. magna NIES strain, which is widely used for gene targeting but has no reported genome. We used the long-read sequenced data of the Oxford nanopore sequencing tool for assembly. Using 3,231 genetic markers, the draft genome of the D. magna NIES strain was built into ten linkage groups (LGs) with 483 unanchored contigs, comprising a genome size of 173.47 Mb. The N50 value of the genome was 12.54 Mb and the benchmarking universal single-copy ortholog value was 98.8%. Repeat elements in the D. magna NIES genome were 40.8%, which was larger than other Daphnia spp. In the D. magna NIES genome, 15,684 genes were functionally annotated. To assess the genome of the D. magna NIES strain for CRISPR/Cas9 gene targeting, we selected glutathione S-transferase omega 2 (GST-O2), which is an important gene for the biotransformation of arsenic in aquatic organisms, and targeted it with an efficient make-up (25.0%) of mutant lines. In addition, we measured reactive oxygen species and antioxidant enzymatic activity between wild type and a mutant of the GST-O2 targeted D. magna NIES strain in response to arsenic. In this study, we present the genome of the D. magna NIES strain using GST-O2 as an example of gene targeting, which will contribute to the construction of deletion mutants by CRISPR/Cas9 technology.

A Case of Nasal Mucosa Cautery With Reintubation Under Pharyngeal Suction for Massive Epistaxis After Extubation.

We describe a case of massive epistaxis that occurred after removal of a nasal endotracheal tube, prompting emergent reintubation. Mask ventilation could not be performed because the nasal cavity was packed with gauze and the airway was being evacuated with a suction catheter. Therefore, instead of inhalational anesthetics and muscle relaxants, boluses of midazolam and remifentanil were administered, and reintubation was promptly performed. Sedation was maintained with dexmedetomidine infusion and midazolam. Nasal cautery was performed near the left sphenopalatine foramen. The patient was extubated without agitation or additional hemorrhage. Immediate recognition of the potential for airway loss, sufficient control of active bleeding, and drug selection in accordance with the emergent circumstances enabled prompt resecuring of the airway without pulmonary aspiration of blood.

Sense-overlapping lncRNA as a decoy of translational repressor protein for dimorphic gene expression.

Long noncoding RNAs (lncRNAs) are vastly transcribed and extensively studied but lncRNAs overlapping with the sense orientation of mRNA have been poorly studied. We analyzed the lncRNA DAPALR overlapping with the 5´ UTR of the Doublesex1 (Dsx1), the male determining gene in Daphnia magna. By affinity purification, we identified an RNA binding protein, Shep as a DAPALR binding protein. Shep also binds to Dsx1 5´ UTR by recognizing the overlapping sequence and suppresses translation of the mRNA. In vitro and in vivo analyses indicated that DAPALR increased Dsx1 translation efficiency by sequestration of Shep. This regulation was impaired when the Shep binding site in DAPALR was deleted. These results suggest that Shep suppresses the unintentional translation of Dsx1 by setting a threshold; and when the sense lncRNA DAPALR is expressed, DAPALR cancels the suppression caused by Shep. This mechanism may be important to show dimorphic gene expressions such as sex determination and it may account for the binary expression in various developmental processes.

DNMT3.1 controls trade-offs between growth, reproduction, and life span under starved conditions in Daphnia magna.

The cladoceran crustacean Daphnia has long been a model of energy allocation studies due to its important position in the trophic cascade of freshwater ecosystems. However, the loci for controlling energy allocation between life history traits still remain unknown. Here, we report CRISPR/Cas-mediated target mutagenesis of DNA methyltransferase 3.1 (DNMT3.1) that is upregulated in response to caloric restriction in Daphnia magna. The resulting biallelic mutant is viable and did not show any change in growth rate, reproduction, and longevity under nutrient rich conditions. In contrast, under starved conditions, the growth rate of this DNMT3.1 mutant was increased but its reproduction was reciprocally reduced compared to the wild type when the growth and reproduction activities competed during a period from instar 4 to 8. The life span of this mutant was significantly shorter than that of the wild type. We also compared transcriptomes between DNMT3.1 mutant and wild type under nutrient-rich and starved conditions. Consistent with the DNMT3.1 mutant phenotypes, the starved condition led to changes in the transcriptomes of the mutant including differential expression of vitellogenin genes. In addition, we found upregulation of the I am not dead yet (INDY) ortholog, which has been known to shorten the life span in Drosophila, explaining the shorter life span of the DNMT3.1 mutant. These results establish DNMT3.1 as a key regulator for life span and energy allocation between growth and reproduction during caloric restriction. Our findings reveal how energy allocation is implemented by selective expression of a DNMT3 ortholog that is widely distributed among animals. We also infer a previously unidentified adaptation of Daphnia that invests more energy for reproduction than growth under starved conditions.

Oxytocin-induced increase in N,N-dimethylglycine and time course of changes in oxytocin efficacy for autism social core symptoms.

BACKGROUND: Oxytocin is expected as a novel therapeutic agent for autism spectrum disorder (ASD) core symptoms. However, previous results on the efficacy of repeated administrations of oxytocin are controversial. Recently, we reported time-course changes in the efficacy of the neuropeptide underlying the controversial effects of repeated administration; however, the underlying mechanisms remained unknown. METHODS: The current study explored metabolites representing the molecular mechanisms of oxytocin's efficacy using high-throughput metabolomics analysis on plasma collected before and after 6-week repeated intranasal administration of oxytocin (48 IU/day) or placebo in adult males with ASD (N = 106) who participated in a multi-center, parallel-group, double-blind, placebo-controlled, randomized controlled trial. RESULTS: Among the 35 metabolites measured, a significant increase in N,N-dimethylglycine was detected in the subjects administered oxytocin compared with those given placebo at a medium effect size (false discovery rate (FDR) corrected P = 0.043, d = 0.74, N = 83). Furthermore, subgroup analyses of the participants displaying a prominent time-course change in oxytocin efficacy revealed a significant effect of oxytocin on N,N-dimethylglycine levels with a large effect size (PFDR = 0.004, d = 1.13, N = 60). The increase in N,N-dimethylglycine was significantly correlated with oxytocin-induced clinical changes, assessed as changes in quantifiable characteristics of autistic facial expression, including both of improvements between baseline and 2 weeks (PFDR = 0.006, r = - 0.485, N = 43) and deteriorations between 2 and 4 weeks (PFDR = 0.032, r = 0.415, N = 37). LIMITATIONS: The metabolites changes caused by oxytocin administration were quantified using peripheral blood and therefore may not directly reflect central nervous system changes. CONCLUSION: Our findings demonstrate an association of N,N-dimethylglycine upregulation with the time-course change in the efficacy of oxytocin on autistic social deficits. Furthermore, the current findings support the involvement of the N-methyl-D-aspartate receptor and neural plasticity to the time-course change in oxytocin's efficacy. TRIAL REGISTRATION: A multi-center, parallel-group, placebo-controlled, double-blind, confirmatory trial of intranasal oxytocin in participants with autism spectrum disorders (the date registered: 30 October 2014; UMIN Clinical Trials Registry: https://upload.umin.ac.jp/cgi-open-bin/ctr_e/ctr_view.cgi?recptno=R000017703 ) (UMIN000015264).

Mutation of the Cytochrome P450 CYP360A8 Gene Increases Sensitivity to Paraquat in Daphnia magna.

The freshwater crustacean Daphnia magna has traditionally been a model for ecotoxicological studies owing to its sensitivity to many xenobiotics. Because it is used in many toxicity assessments, its detoxification mechanism for xenobiotics is important and requires further study. However, studies related to detoxification genes are limited to transcriptomic profiling, and there are no D. magna mutants for use in the understanding of xenobiotic metabolism in vivo. We report the generation of a D. magna CYP360A8 mutant-the gene is a cytochrome P450 (CYP) clan 3 gene. Based on RNA sequencing of adult D. magna, we found that CYP360A8 has the highest expression level among all CYP genes. At ovarian maturation, its expression level is up-regulated 6-fold compared to the juvenile stages and is maintained thereafter. Using the CRISPR/CRISPR-associated 9 (Cas9) system, we disrupted CYP360A8 by coinjecting CYP360A8-targeting guide RNA and Cas9 proteins into D. magna eggs and established one monoallelic CYP360A8 mutant line. This CYP360A8 mutant had a higher sensitivity to the herbicide paraquat compared to the wild type. We confirmed the up-regulation of CYP360A8 by paraquat. The results demonstrate the role of CYP360A8 in paraquat detoxification. The present study establishes a CYP mutant of D. magna, and this strategy can be a basic platform to document a range of CYP gene-xenobiotic relationships in this species. Environ Toxicol Chem 2021;40:1279-1288. © 2020 SETAC.

Reduction of histamine and enhanced spinning behavior of Daphnia magna caused by scarlet mutant.

The ABC transporter, Scarlet, and its binding partner, White are involved in pigment synthesis in the insect eye and mutations in these genes are used as genetic markers. Recent studies have suggested that these transporters also have additional functions in the neuronal system. In our previous study, we generated scarlet mutant in the small crustacean, Daphnia magna and showed that the mutant lacked the eye pigment in the mutant. Here, we show that the scarlet mutant exhibits spinning behavior. This phenotype is partly associated with the presence of light. Metabolomic analysis of a juvenile mutant revealed that the scarlet mutant has approximately one-tenth of the histamine content of the wild type. Application of histamine to the scarlet mutant rescued the spinning behavior in juveniles, suggesting that the spinning behavior of the mutant is caused by the reduction of histamine. However, the altered behavior was not rescued in the adult mutant by the addition of histamine, suggesting that Scarlet plays an irreversible role in the development of histaminergic neurons. These results suggest that Scarlet plays an important role in histaminergic signaling, which might be related to control the spinning behavior, in addition to its role in eye pigmentation.

Production of genome-edited Daphnia for heavy metal detection by fluorescence.

Aquatic heavy metal pollution is a growing concern. To facilitate heavy metal monitoring in water, we developed transgenic Daphnia that are highly sensitive to heavy metals and respond to them rapidly. Metallothionein A, which was a metal response gene, and its promoter region was obtained from Daphnia magna. A chimeric gene fusing the promoter region with a green fluorescent protein (GFP) gene was integrated into D. magna using the TALEN technique and transgenic Daphnia named D. magna MetalloG were produced. When D. magna MetalloG was exposed to heavy metal solutions for 1 h, GFP expression was induced only in their midgut and hepatopancreas. The lowest concentrations of heavy metals that activated GFP expression were 1.2 µM Zn2+, 130 nM Cu2+, and 70 nM Cd2+. Heavy metal exposure for 24 h could lower the thresholds even further. D. magna MetalloG facilitates aqueous heavy metal detection and might enhance water quality monitoring.

Caloric restriction upregulates the expression of DNMT3.1, lacking the conserved catalytic domain, in Daphnia magna.

DNA methylation plays an important role in many aspects of biology, including development, disease, and phenotypic plasticity. In the branchiopod crustacean, Daphnia, de novo DNA methylation has been detected in specific environmental contexts. However, fundamental information on de novo DNA methyltransferase DNMT3 orthologs, including domain organization, developmental expression, and response to environmental stimuli, is lacking. In this study, we examined two DNMT3 orthologs in Daphnia magna, DapmaDNMT3.1 and DapmaDNMT3.2. Amino acid sequence alignment revealed that DapmaDNMT3.1 and DapmaDNMT3.2 lack the conserved methyltransferase motifs of the catalytic domain and the PWWP domain, respectively. We profiled the expression of the two orthologs during embryogenesis and under various feeding levels. During embryogenesis, in contrast to the low DapmaDNMT3.1 expression, DapmaDNTM3.2 was highly expressed at specific stages, that is, in the one cell-stage and at 48 hr post ovulation. In nutrient-rich condition, both genes were lowly expressed, whereas DapmaDNMT3.1 was upregulated at the lower food levels, suggesting a potential role of DapmaDNMT3.1 in gene regulation in response to caloric restriction. These findings provide a basis for understanding the developmental stage- and stress-dependent function of DNMT3 orthologs in D. magna.