Protein arginine methylation by PRMT2 – a putative versatile posttranslational regulatory mechanism for controlling actin nucleation underlying proper neuromorphogenesis

大脑包含了一个复杂的神经细胞网络，其信号发送（轴突）和接收（树突）室的形态极其复杂。新生肌动蛋白丝的形成是神经元轴的发育和可塑性的主要驱动力之一。因此，肌动蛋白丝的组装需要严格的时间和空间控制。这种控制在调节肌动蛋白成核促进因子水平上是最有效的，因为这些因子是丝形成的关键成分。我们最近的研究结果表明，Cobl是一种肌动蛋白成核体，对神经元细胞树突的形成至关重要，它受精氨酸甲基化控制。这是出乎意料的，因为精氨酸甲基化是一种翻译后修饰，以前已知主要控制核过程。蛋白质精氨酸甲基转移酶PRMT2被确定与Cobl相关并调节其细胞骨架特性。重要的是，我们发现PRMT2也对树突树突化至关重要，其关键作用需要其酶活性和这种蛋白质精氨酸甲基转移酶特有的蛋白质相互作用结构域。在我们的质谱筛选中，鉴定出额外的高脑富集的肌动蛋白成核促进因子作为PRMT2的进一步相互作用伙伴，现在导致了令人兴奋的工作假设，即精氨酸甲基化可能是控制神经网络形成和正常大脑功能的更通用和通用的手段。因此，我们在此提出研究PRMT2是否也负责翻译后修饰的另一个肌动蛋白成核促进因子涉及神经形态发生在我们的质谱筛选中发现。随后的研究旨在揭示prmt2介导的精氨酸甲基化对细胞骨架特性及其调节蛋白网络相互作用的分子机制。神经元的功能研究将揭示prmt2介导的精氨酸甲基化在肌动蛋白成核促进因子介导的树突树突化中的普遍作用。神经网络形成的另一个重要方面是轴突的发育以及在神经突诱导和分支之后突触前和突触后特化的建立。由于神经元这些特殊形态区室的诱导和可塑性已被证明高度依赖于肌动蛋白的组织和动力学，我们提出揭示PRMT2在神经元形态控制中的功能是否以及在多大程度上超出了树突树突的控制。总之，这项工作将为针对肌动蛋白核促进因子的调控翻译后机制提供重要的见解，这些因子对神经形态发生至关重要，从而揭示了脊椎动物大脑中神经元网络形成的关键过程。

The brain encompasses a sophisticated network of neuronal cells with extremely elaborated morphologies of their signal-sending (axonal) and –receiving (dendritic) compartments. De novo actin filament formation is one of the major driving and steering forces for the development and plasticity of the neuronal arbor. Actin filament assembly thus requires tight temporal and spatial control. Such control is most effective at the level of regulating actin nucleation promoting factors, as these are key components for filament formation. Our recent results unveiled that Cobl, an actin nucleator critical for shaping the dendritic arbor of neuronal cells, is controlled by arginine methylation. This was unexpected, as arginine methylation is a posttranslational modification previously known to predominantly control nuclear processes. The protein arginine methyltransferase PRMT2 was identified to associate with Cobl and regulate its cytoskeletal properties. Importantly, we found that PRMT2 also was critical for dendritic arborization and that its crucial role required its enzymatic activity and a protein interaction domain unique for this protein arginine methyltransferase. The identification of additional highly brain-enriched actin nucleation promoting factors as further interaction partners of PRMT2 in our mass spectrometry screen now led to the exciting working hypothesis that arginine methylation may be a more general and versatile mean for controlling neuronal network formation and proper brain function. We therefore here propose to investigate whether PRMT2 is also responsible for posttranslational modification of an additional actin nucleation promoting factor implicated in neuromorphogenesis found in our mass spectrometry screen. Subsequent studies aim at unveiling the molecular mechanistic consequences of PRMT2-mediated arginine methylation of this important actin cytoskeletal component on its cytoskeletal properties and its regulatory protein network interactions. Functional studies in neurons will be conducted in order to uncover a putative generalized function of PRMT2-mediated arginine methylation in dendritic arborization mediated by actin nucleation promoting factors.Further important aspects in neuronal network formation are axonal development and the establishment of pre- and postsynaptic specializations subsequent to neurite induction and branching. Since also the induction and plasticity of these specialized morphological compartments of neurons have been demonstrated to be highly dependent on actin organization and dynamics, we propose to unveil whether and to what extent the function of PRMT2 in neuronal morphology control reaches beyond dendritic arborization.Together, this work will provide important insights into regulatory posttranslational mechanisms targeting actin nucleation-promoting factors crucial for neuromorphogenesis and thereby shed light on a key process for the formation of neuronal networks in vertebrate brains.