Lennart Dencker

Metabolism of arsenobetaine in mice, rats and rabbits☆

The distribution, retention and biotransformation of arsenobetaine, the most common organic arsenic compound in fish and crustacea, have been studied in mice, rats and rabbits by use of synthesized 73As-labelled arsenobetaine. Orally administered arsenobetaine was almost completely absorbed from the gastro-intestinal tract in mice. The urinary excretion for 3 days following intravenous injection was about 75% of the dose in the rabbits and more than 98% in the mice and rats. The rate of excretion in mice was independent of the dose level in the range 4 to 400 mg As/kg body weight. In both animal species the tissue distribution differed widely from that observed following exposure to inorganic arsenic. The clearance of arsenobetaine from plasma and most tissues was fast (somewhat faster in mice than in rabbits) and seemed to follow first-order kinetics. The clearance from cartilage, testes, epididymis, and in the rabbits also the muscles, was slower and consisted of more than one phase. 73As-arsenobetaine was the only labelled arsenic compound detected in urine and soluble extract of tissues, indicating that no biotransformation occurred.

Faced with inequality: chicken do not have a general dosage compensation of sex-linked genes

BackgroundThe contrasting dose of sex chromosomes in males and females potentially introduces a large-scale imbalance in levels of gene expression between sexes, and between sex chromosomes and autosomes. In many organisms, dosage compensation has thus evolved to equalize sex-linked gene expression in males and females. In mammals this is achieved by X chromosome inactivation and in flies and worms by up- or down-regulation of X-linked expression, respectively. While otherwise widespread in systems with heteromorphic sex chromosomes, the case of dosage compensation in birds (males ZZ, females ZW) remains an unsolved enigma.ResultsHere, we use a microarray approach to show that male chicken embryos generally express higher levels of Z-linked genes than female birds, both in soma and in gonads. The distribution of male-to-female fold-change values for Z chromosome genes is wide and has a mean of 1.4–1.6, which is consistent with absence of dosage compensation and sex-specific feedback regulation of gene expression at individual loci. Intriguingly, without global dosage compensation, the female chicken has significantly lower expression levels of Z-linked compared to autosomal genes, which is not the case in male birds.ConclusionThe pronounced sex difference in gene expression is likely to contribute to sexual dimorphism among birds, and potentially has implication to avian sex determination. Importantly, this report, together with a recent study of sex-biased expression in somatic tissue of chicken, demonstrates the first example of an organism with a lack of global dosage compensation, providing an unexpected case of a viable system with large-scale imbalance in gene expression between sexes.

Tissue distribution and retention of74As-dimethylarsinic acid in mice and rats

The metabolism of dimethylarsinic acid (DMA), a common pesticide and the primary metabolite of inorganic arsenic in mammals, has been studied in mice and rats. About 80% of an oral dose (0.4 mg As/kg body weight) was absorbed from the gastrointestinal tract. In the mice, more than 99% of the dose was eliminated within 3 days, as compared to about 50% in the rats, mainly due to accumulation in the blood. The tissue distribution in the mice was characterized by highest initial (0.5–6 hr) concentrations in kidneys, lungs, intestinal mucosa, stomach, and testes. Tissues with longest retention time were lungs, thyroid, intestinal walls and lens. No demethylation of the74As-DMA to inorganic arsenic was observed, but some of the74As-DMA in the tissues was apparently in a complexed form.

The cellular retinoic acid binding proteins

The two cellular retinoic acid binding proteins, CRABP I and CRABP II, belong to a family of small cytosolic lipid binding proteins and are highly conserved during evolution. Both proteins are expressed during embryogenesis, particularly in the developing nervous system, craniofacial region and limb bud. CRABP I is also expressed in several adult tissues, however, in contrast, CRABP II expression appears to be limited to the skin. It is likely that these proteins serve as regulators in the transport and metabolism of retinoic acid in the developing embryo and throughout adult life. It has been proposed that CRABP I sequesters retinoic acid in the cytoplasm and prevents nuclear uptake of retinoic acid. A role in catabolism of retinoic acid has also been proposed. Recent gene targeting experiments have shown that neither of the two CRABPs are essential for normal embryonic development or adult life. Examination of CRABP I expression at subcellular resolution reveals a differential cytoplasmic and/or nuclear localization of the protein. A regulated nuclear uptake of CRABP I implies a role for this protein in the intracellular transport of retinoic acid. A protein mediated mechanism which controls the nuclear uptake of retinoic acid may play an important role in the transactivation of the nuclear retinoic acid receptors.

Tissue distribution and subcellular binding of arsenic in Marmoset monkeys after injection of 74As-Arsenite

The distribution and retention of arsenic in Marmoset monkeys, given 74As-arsenite (0.4 mg As/kg body weight) i.p., were studied by means of whole-body autoradiography and determination of 74As-levels in tissues and excreta. Only about 30% of the dose was eliminated over 4 days, mainly via the kidneys. All of the arsenic in urine and tissues was found to be in inorganic form. Tissues with highest accumulation 4 days after dosing were liver (about 20% of the dose), squamous epithelium of oral cavity and esophagus, kidney cortex, skin, testes (mainly tubuli seminiferi) and intestinal wall. As a rule the major part of the arsenic in these tissues was found to be associated with cellular organelles. In the liver about 50% of the arsenic was strongly bound to the rough microsomal membranes. In the soluble extract of tissues, arsenic was mainly associated with macromolecular constituents. The long retention time and tight binding of arsenic could partly be explained by the fact that no biotransformation into methylated arsenic occurred, in contrast to all other species studied so far.

Prenatal coexposure to metallic mercury vapour and methylmercury produce interactive behavioural changes in adult rats

Pregnant rats were 1) administered methyl mercury (MeHg) by gavage, 2 mg/kg/day during days 6-9 of gestation, 2) exposed by inhalation to metallic mercury (Hg degrees) vapour (1.8 mg/m3 air for 1.5 h per day) during gestation days 14-19, 3) exposed to both MeHg by gavage and Hg degrees vapour by inhalation (MeHg + Hg degrees), or 4) were given combined vehicle administration for each of the two treatments (control). The inhalation regimen corresponded to an approximate dose of 0.1 mg Hg degrees/kg/day. Clinical observations and developmental markers up to weaning showed no differences between any of the groups. Testing of behavioural function was performed between 4 and 5 months of age and included spontaneous motor activity, spatial learning in a circular bath, and instrumental maze learning for food reward. Offspring of dams exposed to Hg degrees showed hyperactivity in the motor activity test chambers over all three parameters: locomotion, rearing and total activity; this effect was potentiated in the animals of the MeHg + Hg degrees group. In the swim maze test, the MeHg + Hg degrees and Hg degrees groups evidenced longer latencies to reach a submerged platform, which they had learned to mount the day before, compared to either the control or MeHg groups. In the modified, enclosed radial arm maze, both the MeHg + Hg degrees and Hg degrees groups showed more ambulations and rearings in the activity test prior to the learning test. During the learning trial, the same groups (i.e., MeHg + Hg degrees and Hg degrees) showed longer latencies and made more errors in acquiring all eight pellets. Generally, the results indicate that prenatal exposure to Hg degrees causes alterations to both spontaneous and learned behaviours, suggesting some deficit in adaptive functions. Coexposure to MeHg, which by itself did not alter these functions at the dose given in this study, served to significantly aggravate the changes.

Proteomic Evaluation of Neonatal Exposure to 2,2′,4,4′,5-Pentabromodiphenyl Ether

Exposure to the brominated flame retardant 2,2′,4,4′,5-pentabromodiphenyl ether (PBDE-99) during the brain growth spurt disrupts normal brain development in mice and results in disturbed spontaneous behavior in adulthood. The neurodevelopmental toxicity of PBDE-99 has been reported to affect the cholinergic and catecholaminergic systems. In this study we use a proteomics approach to study the early effect of PBDE-99 in two distinct regions of the neonatal mouse brain, the striatum and the hippocampus. A single oral dose of PBDE-99 (12 mg/kg body weight) or vehicle was administered to male NMRI mice on neonatal day 10, and the striatum and the hippocampus were isolated. Using two-dimensional fluorescence difference gel electrophoresis (2D-DIGE), we found 40 and 56 protein spots with significantly (p < 0.01) altered levels in the striatum and the hippocampus, respectively. We used matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI–ToF–MS) to determine the protein identity of 11 spots from the striatum and 10 from the hippocampus. We found that the levels of proteins involved in neurodegeneration and neuroplasticity (e.g., Gap-43/neuromodulin, stathmin) were typically altered in the striatum, and proteins involved in metabolism and energy production [e.g., α-enolase; γ-enolase; ATP synthase, H+ transporting, mitochondrial F1 complex, β subunit (Atp5b); and α-synuclein] were typically altered in the hippocampus. Interestingly, many of the identified proteins have been linked to protein kinase C signaling. In conclusion, we identify responses to early exposure to PBDE-99 that could contribute to persistent neurotoxic effects. This study also shows the usefulness of proteomics to identify potential biomarkers of developmental neurotoxicity of organohalogen compounds.

Sex-dependent gene expression in early brain development of chicken embryos

BackgroundDifferentiation of the brain during development leads to sexually dimorphic adult reproductive behavior and other neural sex dimorphisms. Genetic mechanisms independent of steroid hormones produced by the gonads have recently been suggested to partly explain these dimorphisms.ResultsUsing cDNA microarrays and real-time PCR we found gene expression differences between the male and female embryonic brain (or whole head) that may be independent of morphological differentiation of the gonads. Genes located on the sex chromosomes (ZZ in males and ZW in females) were common among the differentially expressed genes, several of which (WPKCI-8, HINT, MHM non-coding RNA) have previously been implicated in avian sex determination. A majority of the identified genes were more highly expressed in males. Three of these genes (CDK7, CCNH and BTF2-P44) encode subunits of the transcription factor IIH complex, indicating a role for this complex in neuronal differentiation.ConclusionIn conclusion, this study provides novel insights into sexually dimorphic gene expression in the embryonic chicken brain and its possible involvement in sex differentiation of the nervous system in birds.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) alters intrathymic T-cell development in mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) was administered to 2-4-week-old mice (5, 25, and 50 micrograms/kg body wt.) and to in vitro cultures (10(-9) M) of fetal thymi. By monitoring thymocyte populations with respect to the differentiation antigens CD4 and CD8, it was found that the cell number in all thymocyte populations except for CD8+ decreased significantly compared with controls. In vivo the most marked decrease occurred among double negative (DN) and double positive (DP) cells, whereas in vitro, the DP cells were most severely affected. The cell number had already decreased to some extent by day 1 after a dose of 50 micrograms/kg body wt. of TCDD, although a severe reduction did not become apparent until day 4. There was a clear dose/response relationship between 5 and 50 micrograms/kg body wt. Autoradiography and liquid scintillation counting studies showed that incorporation of [3H]thymidine in the thymus had already decreased 24 h after TCDD treatment, with the decrease being even more pronounced at 48 h. By 96 h, the rate of cell proliferation had returned to approximately normal values. The results show that TCDD has a long-lasting effect on thymocyte abundance together with a transient effect on cell proliferation. This indicates that in addition to the initial effects of TCDD on cell proliferation, it may also more permanently disturb the normal process of elimination by means of selection.

Accumulation of Toxic Metals in Male Reproduction Organs

Occupation exposure to metals has been related to impaired reproduction in males. This report summarizes autoradiographic studies on the distribution of cadmium (Cd), chromium (Cr) and arsenic (As) in the testis and epididymis of rodents. Cd and Cr strongly accumulated in the interstitial tissues, indicating an effect on hormone production (Leydig cells) and blood supply or both. Arsenic accumulated in the lumen of the duct of epididymis causing long-term exposure of the semen.