Rapid fluorescent detection of (anti)androgens with spiggin-gfp medaka.

Widespread environmental antiandrogen contamination has been associated with negative impacts on biodiversity and human health. In particular, many pesticides are antiandrogenic, creating a need for robust and sensitive environmental monitoring. Our aim was to develop a sensitive and specific transgenic medaka (Oryzias latipes) model bearing an androgen responsive fluorescent reporter construct for whole organism-based environmental screening of pro- and antiandrogens. We analyzed the 5' regions of the androgen responsive three-spined stickleback (Gasterosteus aculeatus) spiggin genes in silico, revealing conserved blocks of sequence harboring androgen response elements. Identified putative promoters were cloned upstream of GFP. Germinal transgenesis with spg1-gfp led to stable medaka lines. GFP induction was exclusive to the kidney, the site of spiggin protein production in sticklebacks. Significant GFP expression was induced by three or four-day androgen treatment of newly hatched fry, but not by estrogens, mineralocorticoids, glucocorticoids or progestogens. The model responded dose-dependently to androgens, with highest sensitivity to 17MT (1.5 µg/L). In addition to flutamide, the biocides fenitrothion, vinclozolin and linuron significantly inhibited 17MT-induced GFP induction, validating the model for detection of antiandrogens. The spg1-gfp medaka model provides a sensitive, specific, and physiologically pertinent biosensor system for analyzing environmental androgen activity.

Increased oligodendrogenesis and myelination in the subventricular zone of aged mice and gray mouse lemurs.

The adult rodent subventricular zone (SVZ) generates neural stem cells (NSCs) throughout life that migrate to the olfactory bulbs (OBs) and differentiate into olfactory interneurons. Few SVZ NSCs generate oligodendrocyte precursor cells (OPCs). We investigated how neurogliogenesis is regulated during aging in mice and in a non-human primate (NHP) model, the gray mouse lemur. In both species, neuronal commitment decreased with age, while OPC generation and myelin content unexpectedly increased. In the OBs, more tyrosine hydroxylase interneurons in old mice, but fewer in lemurs, marked a surprising interspecies difference that could relate to our observation of a continuous ventricle in lemurs. In the corpus callosum, aging promoted maturation of OPCs into mature oligodendrocytes in mice but blocked it in lemurs. The present study highlights similarities and dissimilarities between rodents and NHPs, revealing that NHPs are a more relevant model than mice to study the evolution of biomarkers of aging.

Developmental thyroid disruption permanently affects the neuroglial output in the murine subventricular zone.

Neural stem cells (NSCs) in the adult brain are a source of neural cells for brain injury repair. We investigated whether their capacity to generate new neurons and glia is determined by thyroid hormone (TH) during development because serum levels peak during postnatal reorganization of the main NSC niche, the subventricular zone (SVZ). Re-analysis of mouse transcriptome data revealed increased expression of TH transporters and deiodinases in postnatal SVZ NSCs, promoting local TH action, concomitant with a burst in neurogenesis. Inducing developmental hypothyroidism reduced NSC proliferation, disrupted expression of genes implicated in NSC determination and TH signaling, and altered the neuron/glia output in newborns. Three-month-old adult mice recovering from developmental hypothyroidism had fewer olfactory interneurons and underperformed on short-memory odor tests, dependent on SVZ neurogenesis. Our data provide readouts permitting comparison with adverse long-term events following thyroid disruptor exposure and ideas regarding the etiology of prevalent neurodegenerative diseases in industrialized countries.

Absence of Both Thyroid Hormone Transporters MCT8 and OATP1C1 Impairs Neural Stem Cell Fate in the Adult Mouse Subventricular Zone.

Adult neural stem cell (NSC) generation in vertebrate brains requires thyroid hormones (THs). How THs enter the NSC population is unknown, although TH availability determines proliferation and neuronal versus glial progenitor determination in murine subventricular zone (SVZ) NSCs. Mice display neurological signs of the severely disabling human disease, Allan-Herndon-Dudley syndrome, if they lack both MCT8 and OATP1C1 transporters, or MCT8 and deiodinase type 2. We analyzed the distribution of MCT8 and OATP1C1 in adult mouse SVZ. Both are strongly expressed in NSCs and at a lower level in neuronal cell precursors but not in oligodendrocyte progenitors. Next, we analyzed Mct8/Oatp1c1 double-knockout mice, where brain uptake of THs is strongly reduced. NSC proliferation and determination to neuronal fates were severely affected, but not SVZ-oligodendroglial progenitor generation. This work highlights how tight control of TH availability determines NSC function and glial-neuron cell-fate choice in adult brains.

The pyriproxyfen metabolite, 4'-OH-PPF, disrupts thyroid hormone signaling in neural stem cells, modifying neurodevelopmental genes affected by ZIKA virus infection.

North-Eastern Brazil saw intensive application of the insecticide pyriproxyfen (PPF) during the microcephaly outbreak caused by the Zika virus (ZIKV). ZIKV requires the neural RNA-binding protein Musashi-1 to replicate. Thyroid hormone (TH) represses MSI1. PPF is a suspected TH disruptor. We hypothesized that co-exposure to the main metabolite of PPF, 4'-OH-PPF, could exacerbate ZIKV effects through increased MSI1 expression. Exposing an in vivo reporter model, Xenopus laevis, to 4'-OH-PPF decreased TH signaling and increased msi1 mRNA and protein, confirming TH-antagonistic properties. Next, we investigated the metabolite's effects on mouse subventricular zone-derived neural stem cells (NSCs). Exposure to 4'-OH-PPF dose-dependently reduced neuroprogenitor proliferation and dysregulated genes implicated in neurogliogenesis. The highest dose induced Msi1 mRNA and protein, increasing cell apoptosis and the ratio of neurons to glial cells. Given these effects of the metabolite alone, we considered if combined infection with ZIKV worsened neurogenic events. Only at the fourth and last day of incubation did co-exposure of 4'-OH-PPF and ZIKV decrease viral replication, but viral RNA copies stayed within the same order of magnitude. Intracellular RNA content of NSCs was decreased in the combined presence of 4'-OH-PPF and ZIKV, suggesting a synergistic block of transcriptional machinery. Seven out of 12 tested key genes in TH signaling and neuroglial commitment were dysregulated by co-exposure, of which four were unaltered when exposed to 4'-OH-PPF alone. We conclude that 4'-OH-PPF is an active TH-antagonist, altering NSC processes known to underlie correct cortical development. A combination of the TH-disrupting metabolite and ZIKV could aggravate the microcephaly phenotype.

Transgenic Medaka Identify Embryonic Periods Sensitive to Disruption of Sex Determination.

Gonadal development in medaka (Oryzias latipes) is dependent on the synergy between estrogens and androgens. Disruption of steroid hormone levels can lead to ovo-testis. To determine the sensitive windows for hormonally induced sex reversal in medaka, we developed a novel 42sp50-GFP_ChgH-GFP transgenic medaka line, allowing the identification of female gonadal tissue by fluorescence present in developing oocytes. Germinal transgenesis resulted in a stable line exhibiting a strong green fluorescent protein signal constitutively in the ovaries and in the liver in response to estrogens. The sensitivity of this line to disruption of sex determination following 16-d chronic exposures was in the nanograms per liter range. To identify the developmental period sensitive to exogenous agents, fry were exposed to 24-h pulses of high concentrations of 17ß-estradiol (E2) or 5α-dihydrotestosterone (DHT) at various time points between days postfertilization (dpf) 0 and 12. Evaluation of phenotype followed by genotyping at 16 dpf revealed sensitivity to E2 between 1 and 8 dpf as well as 2 periods of susceptibility to DHT between 0 and 1 dpf and 4 and 8 dpf. No phenotypic sex reversal was detected after exposure to DHT or E2 on 11 or 12 dpf. The observed effects persisted to at least 24 dpf. The identified sensitive embryonic time periods for disruption of sex determination will aid future research on sex determination and the development of screening assays using early embryonic life stages. Environ Toxicol Chem 2020;39:842-851. © 2020 SETAC.

Gender-specific effects of transthyretin on neural stem cell fate in the subventricular zone of the adult mouse.

Choroid plexus epithelial cells produce and secrete transthyretin (TTR). TTR binds and distributes thyroid hormone (TH) to brain cells via the cerebrospinal fluid. The adult murine subventricular zone (SVZ) is in close proximity to the choroid plexus. In the SVZ, TH determines neural stem cell (NSC) fate towards a neuronal or a glial cell. We investigated whether the loss of TTR also disrupted NSC fate choice. Our results show a decreased neurogenic versus oligodendrogenic balance in the lateroventral SVZ of Ttr knockout mice. This balance was also decreased in the dorsal SVZ, but only in Ttr knockout male mice, concomitant with an increased oligodendrocyte precursor density in the corpus callosum. Quantitative RTqPCR analysis following FACS-dissected SVZs, or marked-coupled microbeads sorting of in vitro neurospheres, showed elevated Ttr mRNA levels in neuronal cells, as compared to uncommitted precursor and glial cells. However, TTR protein was undetectable in vivo using immunostaining, and this despite the presence of Ttr mRNA-expressing SVZ cells. Altogether, our data demonstrate that TTR is an important factor in SVZ neuro- and oligodendrogenesis. They also reveal important gender-specific differences and spatial heterogeneity, providing new avenues for stimulating endogenous repair in neurodegenerative diseases.

Using short-term bioassays to evaluate the endocrine disrupting capacity of the pesticides linuron and fenoxycarb.

Several short-term whole-organism bioassays based on transgenic aquatic models are now under validation by the OECD (Organization for Economic Co-operation and Development) to become standardized test guidelines for the evaluation of the endocrine activity of substances. Evaluation of the endocrine disrupting capacity of pesticides will be a domain of applicability of these future reference tests. The herbicide linuron and the insecticide fenoxycarb are two chemicals commonly used in agricultural practices. While numerous studies indicate that linuron is likely to be an endocrine disruptor, there is little information available on the effect of fenoxycarb on vertebrate endocrine systems. Using whole-organism bioassays based on transgenic Xenopus laevis tadpoles and medaka fry we assessed the potential of fenoxycarb and linuron to disrupt thyroid, androgen and estrogen signaling. In addition we used in silico approach to simulate the affinity of these two pesticides to human hormone receptors. Linuron elicited thyroid hormone-like activity in tadpoles at all concentrations tested and, showed an anti-estrogenic activity in medaka at concentrations 2.5mg/L and higher. Our experiments suggest that, in addition to its previously established anti-androgenic action, linuron exhibits thyroid hormone-like responses, as well as acting at the estrogen receptor level to inhibit estrogen signaling. Fenoxycarb on the other hand, did not cause any changes in thyroid, androgen or estrogen signaling at the concentrations tested.

Deiodinase activity is present in Xenopus laevis during early embryogenesis.

Thyroid hormones orchestrate amphibian metamorphosis. The type 2 and type 3 deiodinases make vital contributions to this process by controlling levels of the thyroid hormones T(4) and T(3) available to different tissues. Because the tadpole thyroid gland is not functional until stage NF44, it has been widely assumed that thyroid signaling is absent during amphibian early development, thyroid hormone only becoming a major regulator during premetamorphic stages. Similarly, in mammals, thyroid function is known to be essential to neuronal development, especially during the perinatal stages, but again little is known about early stages of development. Here we demonstrate that key elements of thyroid hormone signaling are present during early development of Xenopus. In particular, we find functional thyroid hormone-activating deiodinases and significant levels of their substrates, T(4) and T(3), during early embryogenesis. Furthermore, we have further characterized a recently identified deiodinase in amphibians, homologous to mammalian type 1 deiodinase (D1). This enzyme is expressed in marked, spatially defined patterns during embryogenesis. The patterns of expression of type 1 deiodinase are distinct from those of type 2 and type 3 deiodinases. Deiodinase expression is found in neurogenic areas from stage NF30 onward, both in the central and peripheral nervous systems. We conclude that both activating and inactivating deiodinases show dynamic patterns of expression during early embryogenesis in amphibians, particularly in neurogenic areas. These findings suggest that thyroid hormone signaling is a key component of early neuronal development in vertebrates.

Oestrogen reporter transgenic medaka for non-invasive evaluation of aromatase activity.

Vertebrate reproduction involves complex steroid hormone interplay and inter-conversion. A critical element in maintaining sex steroid levels is the enzyme aromatase (cytochrome P450 19A1) which converts androgens to oestrogens. In turn oestrogen signalling is targeted by numerous chemicals, from pharmaceuticals to agricultural chemicals, both frequent sources of contamination in waste waters and consequently rivers. Although many models are now available to address disruption of oestrogen signalling, there are currently no published protocols allowing discrimination between alterations in testosterone metabolism and in oestrogenic signalling. It was with this limitation in mind that we optimised this protocol. We show using a 48h protocol that pre-feeding fry of the choriogenin h-gfp (chgh-gfp) medaka line are sensitive to 0.05nM EE2 (15ng/L), within the range of the lowest published observable physiological effect concentrations for medaka. In addition, co-treatment with testosterone can reveal potential effects of test substances on aromatase enzymatic activity. As the measurements are visualised in real-time without affecting embryo viability, repeated measures are possible. We demonstrate the ability of this model to detect oestrogen receptor agonists, aromatisable androgens, P450 aromatase activity modulators and selective oestrogen response modulators. Importantly, the range of this assay is physiologically relevant.