Department of Environmental Medicine, University of Rochester School of Medicine and Dentistry, Rochester, NY, USA; ICF International, Durham, NC, USA.
Department of Environmental Medicine, University of Rochester School of Medicine and Dentistry, Rochester, NY, USA.
Neurotoxicol Teratol. 2021 Nov-Dec;88:107037. doi: 10.1016/j.ntt.2021.107037. Epub 2021 Oct 14.
The developmental toxicant, methylmercury (MeHg), can elicit motor deficits that last well into adulthood. Recent studies using Drosophila showed that the developing musculature is sensitive to high doses of MeHg, where a larval feeding paradigm resulted in compromised myotendinous junction (MTJ) formation during development, by a mechanism involving the NG2 homologue, kon-tiki (kon). Low-dose exposures to MeHg that do not produce muscle pathology during development, nevertheless result in impaired flight behavior later in adult life. The present study evaluated the potential for relatively low-dose exposure to produce latent adult muscle pathology and motor impairments, as assayed by climbing and flight, as well as to evaluate molecular mechanisms that may contribute to motor deficits. Wildtype larvae were fed 0, 2, 2.5, or 5 μM MeHg laden food until eclosion. The effect of 5 μM MeHg on MTJ-related gene expression during pupal development was assessed via quantitative RT-qPCR analysis. Upon eclosion, adults were transferred to standard food bottles for 4, 11, or 30 days prior to motor testing. Survivorship (%) was determined from a subset of 200 flies per treatment. Average climbing speed (cm/s) was quantified 4-days post-eclosion (PE). Flight ability was assayed 11- or 30-days PE by measuring landing height (cm) of flies dropped into an adhesive-lined vertical column. In parallel, total body mercury was measured to estimate the influence of residual MeHg at the time of motor testing. Muscle morphology was assessed using immuno-fluorescence microscopy. Exposure to 5uM MeHg significantly reduced climbing speed, and flight ability 4 and 11 - days PE, respectively. While age-related flight deficits were seen in each sex, flight deficits due to MeHg persisted to 30-day PE timepoints exclusively in males. Expression of kon was upregulated across the window of pupal development essential to establishing adult MTJ. However, experimentally restricting the induction of comparable levels of kon to muscle during the same periods did not recapitulate the flight deficits, indicating that muscle-specific induction of kon alone is not sufficient to contribute to latent flight impairments. Adult flight muscle morphology of 11-day PE flies treated with 5 μM MeHg was indistinct from controls, implying muscle structure is not grossly perturbed to impair flight. Collectively, the current data suggest that developmental exposure to 5 μM MeHg reduces flight ability in each sex at 11 day-PE and that latent deficits at 30-day PE are male-specific. It remains to be determined whether the developing MTJ of Drosophila is a sensitive target of MeHg, and whether or not kon acts in conjunction with additional MTJ factors to constitute a MeHg target.
发育性毒物甲基汞（MeHg）可引起运动功能障碍，这些障碍可持续到成年期。最近使用果蝇的研究表明，发育中的肌肉对高剂量的 MeHg 敏感，幼虫喂食模式导致发育过程中的肌腱连接（MTJ）形成受损，其机制涉及 NG2 同源物 kon-tiki（kon）。在发育过程中不会产生肌肉病理学的低剂量 MeHg 暴露，然而，在成年后会导致飞行行为受损。本研究评估了相对低剂量暴露产生潜在的成年肌肉病理学和运动障碍的可能性，如攀爬和飞行能力，以及评估可能导致运动障碍的分子机制。野生型幼虫在孵化前用 0、2、2.5 或 5μM 含 MeHg 的食物喂养，直至孵化。通过定量 RT-qPCR 分析评估 5μM MeHg 对蛹发育过程中与 MTJ 相关的基因表达的影响。孵化后，将成虫转移到标准食物瓶中，在进行运动测试前分别培养 4、11 或 30 天。从每个处理组的 200 只苍蝇中确定存活率（%）。平均攀爬速度（cm/s）在孵化后 4 天（PE）进行量化。飞行能力通过测量飞入粘性垂直柱的苍蝇的着陆高度（cm）在 11 或 30-PE 天进行测试。同时，测量总汞含量以估计运动测试时残留 MeHg 的影响。使用免疫荧光显微镜评估肌肉形态。暴露于 5μM MeHg 显著降低了 4 和 11-PE 天的攀爬速度和飞行能力。尽管每个性别都出现了与年龄相关的飞行缺陷，但仅在雄性中，由于 MeHg 导致的飞行缺陷持续到 30-PE 时间点。kon 的表达在蛹发育的关键时期被上调，这对于建立成年 MTJ 至关重要。然而，在相同时期将 kon 的诱导限制在肌肉中，实验并未重现飞行缺陷，这表明肌肉特异性诱导 kon 本身不足以导致潜在的飞行障碍。用 5μM MeHg 处理的 11-PE 天的成年飞行肌肉形态与对照无明显区别，这意味着肌肉结构没有受到严重干扰而导致飞行能力下降。总的来说，目前的数据表明，5μM MeHg 发育暴露会降低每个性别在 11-PE 天的飞行能力，而 30-PE 天的潜在缺陷是雄性特有的。尚不确定果蝇的发育性 MTJ 是否是 MeHg 的敏感靶点，以及 kon 是否与其他 MTJ 因子一起构成 MeHg 靶点。
