Cerebrovascular Myosin Light Chain Phosphorylation in Fetus, Newborn, and Adult

描述(申请人提供)：心血管不稳定是早产儿临床管理中最困难的挑战之一，这是由于新生儿心血管反应的独特模式，特别是在脑循环中。与成年动脉相比，未成熟的脑动脉更多地依赖于钙内流而不是细胞内钙的释放来启动收缩，但也表现出明显增强的肌丝钙敏感性。这种增强的胎儿肌丝钙敏感性的一个可能的解释是，调节性肌球蛋白轻链(MLC20)的磷酸化，一个关键的收缩蛋白，在未成熟的大脑动脉中上调。以前研究MLC20原位磷酸化的尝试受到技术障碍的限制，包括无法测量完整动脉中MLC20磷酸化的快速时间过程。申请人实验室最近开发的方法克服了这些问题。一种定制设计的仪器现在可以在小到足以容纳胎儿大脑动脉的试管中，以毫秒级的分辨率同时测量完整动脉的胞浆钙瞬变和MLC20磷酸化。现在已经有了蛋白质表达系统，可以大量生产适合动力学测量的单一亚型MLC20。新的共聚焦共定位方法能够解决这些小动脉中多个不同收缩蛋白之间的相互作用。基于这些令人兴奋的新方法，这一建议旨在探索以下主要假设：与成熟的脑动脉相比，未成熟的脑动脉肌丝钙敏感性上调是由于MLC20磷酸化速度加快所致。一个得到强有力的初步数据支持的推论认为，MLC20在脑血管平滑肌中的超微结构组织是其作为磷酸化底物能力的重要决定因素。为了解决这些问题，我们设计了三个具体的目标：1)确定在有和没有磷酸酶抑制剂存在和不存在的情况下破碎细胞标本中MLC20磷酸化的底物-速度关系；(2)确定在有和没有磷酸酶抑制剂存在和不存在的情况下，完整细胞标本中MLC20磷酸化的速度，以响应自发的和标准的胞内钙瞬变；以及3)确定MLC20与肌球蛋白轻链激酶、肌球蛋白轻链磷酸酶、平滑肌肉1-肌动蛋白和肌球蛋白重链异构体相关的丰度、分布和共定位。这些实验将在足月胎羊、新生羔羊和未怀孕的成年绵羊的大脑中动脉中进行，以确定出生后年龄对这些调节机制的影响。对这些机制的进一步了解为完善临床策略提供了潜在的机会，以管理那些有脑血管损伤风险的婴儿，以及那些患有许多其他疾病的婴儿，如哮喘、败血症和结肠炎，在这些疾病中，MLC20磷酸化的改变是一个关键因素。

DESCRIPTION (provided by applicant): Cardiovascular instability is one of the most difficult challenges in clinical management of pre-term infants due to unique neonatal patterns of cardiovascular reactivity, particularly in the cerebral circulation. Compared to adult arteries, immature cerebral arteries exhibit greater reliance on Ca++ influx than on intracellular Ca++ release for initiation of contraction, but also exhibit markedly enhanced myofilament Ca++ sensitivity. One possible explanation for this enhanced fetal myofilament Ca++ sensitivity is that phosphorylation of regulatory myosin light chain (MLC20), a key contractile protein, is upregulated in immature cerebral arteries. Previous attempts to study phosphorylation of MLC20 in situ have been limited by technical barriers, including the inability to measure the rapid time course of MLC20 phosphorylation in intact arteries. Methods recently developed in the applicant's laboratory have surmounted these problems. A custom designed apparatus now can simultaneously measure cytosolic Ca++ transients and MLC20 phosphorylation with millisecond resolution in intact arteries in a cuvette small enough to accommodate fetal cerebral arteries. Protein expression systems are now available that can produce single isoforms of MLC20 in quantities suitable for kinetic measurements. New confocal colocalization methods are capable of resolving interactions among multiple different contractile proteins in these small arteries. Based on these exciting new methods, this proposal was designed to explore the main hypothesis that myofilament Ca++ sensitivity is upregulated in immature compared to mature cerebral arteries due to an increased velocity of MLC20 phosphorylation. A corollary hypothesis supported by strong preliminary data predicts that the ultra-structural organization of MLC20 within cerebrovascular smooth muscle is an important determinant of its ability to serve as a substrate for phosphorylation. To address these ideas, three specific aims have been designed: 1) determine substrate- velocity relations for MLC20 phosphorylation in broken cell preparations in the presence and absence of phosphatase inhibitors with age-specific sources of purified ovine MLC20 and synthesized single isoforms of Non-Muscle and Smooth-Muscle MLC20; 2) determine the velocities of MLC20 phosphorylation in intact cell preparations in the presence and absence of phosphatase inhibitors, in response to both spontaneous and standardized cytosolic Ca++ transients; and 3) determine the abundance, distribution, and co-localization of MLC20 in relation to myosin light chain kinase, myosin light chain phosphatase, smooth muscle 1-actin, and myosin heavy chain isoforms. These experiments will be conducted in middle cerebral arteries from term fetal lambs, newborn lambs, and non-pregnant adult sheep to define the effects of postnatal age on these regulatory mechanisms. Further understanding of these mechanisms offers potential to refine clinical strategies for management of infants at risk for cerebrovascular injury, as well as those afflicted by numerous other diseases such as asthma, sepsis, and colitis, in which altered rates of MLC20 phosphorylation are a key factor.