Exposure to arsenic during embryogenesis impairs olfactory sensory neuron differentiation and function into adulthood.

Arsenic is a contaminant of food and drinking water. Epidemiological studies have reported correlations between arsenic exposure and neurodevelopmental abnormalities, such as reduced sensory functioning, while in vitro studies have shown that arsenic reduces neurogenesis and alters stem cell differentiation. The goal of this study was assess whether arsenic exposure during embryogenesis reduced olfactory stem cell function and/or numbers, and if so, whether those changes persist into adulthood. Killifish (Fundulus heteroclitus) embryos were exposed to 0, 10, 50 or 200 ppb arsenite (AsIII) until hatching, and juvenile fish were raised in clean water. At 0, 2, 4, 8, 16, 28 and 40 weeks of age, odorant response tests were performed to assess specific olfactory sensory neuron (OSN) function. Olfactory epithelia were then collected for immunohistochemical analysis of stem cell (Sox2) and proliferating cell numbers (PCNA), as well as the number and expression of ciliated (calretinin) and microvillus OSNs (Gαi3) at 0, 4, 16 and 28 weeks. Odorant tests indicated that arsenic exposure during embryogenesis increased the start time of killifish responding to pheromones, and this altered start time persisted to 40 weeks post-exposure. Response to the odorant taurocholic acid (TCA) was also reduced through week 28, while responses to amino acids were not consistently altered. Immunohistochemistry was used to determine whether changes in odorant responses were correlated to altered cell numbers in the olfactory epithelium, using markers of proliferating cells, progenitor cells, and specific OSNs. Comparisons between response to pheromones and PCNA + cells indicated that, at week 0, both parameters in exposed fish were significantly reduced from the control group. At week 28, all exposure are still significantly different than control fish, but now with higher PCNA expression coupled with reduced pheromone responses. A similar trend was seen in the comparisons between Sox2-expressing progenitor cells and response to pheromones, although Sox2 expression in the 28 week-old fish only recovers back to the level of control fish rather than being significantly higher. Comparisons between calretinin expression (ciliated OSNs) and response to TCA demonstrated that both parameters were reduced in the 200 ppb arsenic-exposed fish in at weeks 4, 16, and 28. Correlations between TCA response and the number of PCNA + cells revealed that, at 28 weeks of age, all arsenic exposure groups had reductions in response to TCA, but higher PCNA expression, similar to that seen with the pheromones. Few changes in Gαi3 (microvillus OSNs) were seen. Thus, it appears that embryonic-only exposure to arsenic has long-term reductions in proliferation and differentiation of olfactory sensory neurons, leading to persistent effects in their function.

Embryonic arsenic exposure reduces intestinal cell proliferation and alters hepatic IGF mRNA expression in killifish (Fundulus heteroclitus).

Arsenic (As) is a toxicant found in food and water throughout the world, and studies suggested that exposure early in life reduces growth. Thus, the goal of this study was to examine mechanisms by which As impacted organismal growth. Killifish (Fundulus heteroclitus) were exposed to 0, 10, 50, or 200 ppb As as embryos and, after hatching, were reared in clean water for up to 40 weeks. Metabolism studies revealed that killifish biotransform As such that monomethylated and dimethylated arsenicals account for 15-17% and 45-61%, respectively, of the total metal. Growth, as measured by condition factor (CF), was significantly and dose-dependently reduced at 8 weeks of age but was similar to controls by 40 weeks. To determine mechanisms underlying the observed initial decrease, intestinal proliferation and morphology were examined. Arsenic-exposed fish exhibited significant 1.3- to 1.5-fold reduction in intestinal villus height and 1.4- to 1.6-fold decrease in proliferating cell nuclear antigen (PCNA+) intestinal cells at all weeks examined. In addition, there were significant correlations between CF, PCNA+ cells, and intestinal villus height. Upon examining whether fish might compensate for the intestinal changes, it was found that hepatic mRNA expression of insulin-like growth factor 1 (IGF-1) and its binding protein (IGFBP-1) were dose-dependently increased. These results indicate that embryonic exposure initially diminished growth, and while intestinal cell proliferation remained reduced, fish appear to compensate by enhancing transcript levels of hepatic IGF-1 and IGFBP-1.

Embryonic-only arsenic exposure in killifish (Fundulus heteroclitus) reduces growth and alters muscle IGF levels one year later.

Arsenic is a contaminant of drinking water and crops in many parts of the world. Epidemiological studies have shown that arsenic exposure is linked to decreased birth weight, weight gain, and proper skeletal muscle function. The goal of this study was to use killifish (Fundulus heteroclitus) as a model to determine the long-term effects of embryonic-only arsenic exposure on muscle growth and the insulin-like growth factor (IGF) pathway. Killifish embryos were exposed to 0, 50, 200 or 800ppb AsIII from fertilization until hatching. Juvenile fish were reared in clean water and muscle samples were collected at 16, 28, 40 and 52 weeks of age. There were significant reductions in condition factors, ranging from 12 to 17%, in the fish exposed to arsenic at 16, 28 and 40 weeks of age. However, by 52 weeks, no significant changes in condition factors were seen. Alterations in IGF-1R and IGF-1 levels were assessed as a potential mechanism by which growth was reduced. While there no changes in hepatic IGF-1 transcripts, skeletal muscle cells can also produce their own IGF-1 and/or alter IGF-1 receptor levels to help enhance growth. After a 200 and 800ppb embryonic exposure, fish grown in clean water for 16 weeks had IGF-1R transcripts that were 2.8-fold and 2-fold greater, respectively, than unexposed fish. Through 40 weeks of age, IGF1-R remained elevated in the 200ppb and 800ppb embryonic exposure groups by 1.8-3.9-fold, while at 52 weeks of age, IGF-1R levels were still significantly increased in the 800ppb exposure group. Skeletal muscle IGF-1 transcripts were also significantly increased by 1.9-5.1 fold through the 52 weeks of grow-out in clean by water in the 800ppb embryonic exposure group. Based on these results, embryonic arsenic exposure has long-term effects in that it reduces growth and increases both IGF-1 and IGF-1R levels in skeletal muscle even 1year after the exposure has ended.