Mechanisms of Methylmercury Toxicity in Neuromuscular Development

 说明(申请人提供)：甲基汞(MeHg)是一种持久性环境毒物，长期以来一直被认为是针对发育中的神经系统的。要了解甲基汞的风险与食用鱼类所带来的基本营养素的益处，需要更好地了解甲基汞在发育过程中导致神经缺陷的机制。胎儿接触甲基汞会导致婴儿和幼儿的运动和认知障碍。然而，运动缺陷源于肌肉发育改变的可能性在很大程度上被忽视了。在这项建议中，我们将描述胚胎肌肉作为甲基汞靶标的明显作用，从而将甲基汞神经毒性的基本领域扩展到发育中的运动单位。这项研究是以我们在果蝇中的早期发现为指导的，即甲基汞可以通过Notch受体途径激活发育信号，特别是上调裂解mDelta的Notch靶基因增强子(E(Spl)mδ)。我们还发现，甲基汞有破坏胚胎肌肉发育的倾向，与其对发育中的运动神经元的影响平行。我们现在发现，E(Spl)mδ在胚胎中胚层和肌源性谱系中固有地表达，并且，类似于甲基汞暴露，E(Spl)mδ在胚胎中的表达增强导致体肌表型和运动神经元突起的相应缺陷。通过全基因组关联分析(GWA)，我们发现了许多与甲基汞耐受相关的基本肌肉发育基因，特别是Kirre细胞黏附蛋白，它是肌原纤维形成中成肌细胞融合的核心成分。因此，我们的总体假设是，甲基汞对肌肉生成的干扰有助于神经肌肉发育的异常，从而导致运动障碍。这一假设提出了几个基本问题，我们将通过四个具体目标来解决这些问题。在目标1中，我们将确定肌肉发生在胚胎神经肌肉发育中作为甲基汞靶标的基本作用。在目标2中，我们进一步表征了Notch信号作为甲基汞毒性在神经肌肉发育中的不利结果途径的作用。对于目标3，我们将询问成肌细胞融合蛋白Kirre作为甲基汞对神经肌肉发育中成肌细胞融合的调节因子的功能。在目标4中，我们将我们的发现翻译到小鼠身上，并研究哺乳动物在培养的成肌细胞和产前暴露于甲基汞的小鼠中肌肉固有缺陷的推论。总而言之，这项研究将极大地促进目前对早期接触甲基汞可能导致运动障碍的理解，从而使实验毒理学家和临床医生了解甲基汞神经毒性的肌病成分的可能性。

 DESCRIPTION (provided by applicant): Methylmercury (MeHg) is a persistent environmental toxicant that has long been understood to target the developing nervous system. Understanding the risks of MeHg versus the benefits of essential nutrients that come with eating fish requires a greater understanding of the mechanisms by which MeHg induces neurological deficits during development. Fetal MeHg exposure can elicit both motor and cognitive deficits in infants and young children. Yet, the possibility that motor deficits stem from altered muscle development has been largely overlooked. In this proposal we will characterize an apparent role of embryonic muscle as a MeHg target, thereby extending the fundamental domain of MeHg neurotoxicity to the developing motor unit. This study is guided by our earlier discoveries in Drosophila that MeHg can activate developmental signaling through the Notch receptor pathway, specifically up-regulating the Notch target gene enhancer of split mDelta (E(spl)mδ). We also find that MeHg has a propensity to disrupt muscle development in the embryo in parallel with its effects on developing motor neurons. We now show that E(spl)mδ is intrinsically expressed in the embryonic mesoderm and myogenic lineages and that, akin to MeHg exposure, enhanced E(spl)mδ expression in the embryo induces a somatic muscle phenotype and corresponding defects in motor neuron outgrowth. Through a genome wide association analysis (GWA) we have uncovered numerous fundamental muscle development genes that associate with tolerance to MeHg, notably, the Kirre cell adhesion protein, a core component of myoblast fusion in myofibril formation. Thus, our overall hypothesis is that MeHg perturbation of myogenesis contributes to abnormal neuromuscular development leading to motor deficits. This hypothesis poses several fundamental questions that we will address through four Specific Aims. In Aim 1 we will determine the fundamental role of myogenesis as a MeHg target in neuromuscular development in the embryo. In Aim 2 we characterize further the role of Notch signaling as an adverse outcome pathway of MeHg toxicity in neuromuscular development. For Aim 3 we will interrogate the function of the myoblast fusion protein Kirre as moderator of MeHg effects on myoblast fusion in neuromuscular development. In Aim 4 we translate our findings to the mouse and investigate mammalian corollaries for muscle-intrinsic defects in cultured myoblasts and prenatally MeHg-exposed mice. In sum, this study will significantly advance the current understanding of how motor deficits are likely to arise from early life MeHg exposure and thereby inform both the experimental toxicologist and the clinician on the potential for a myopathic component of MeHg neurotoxicity.