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

大脑包括一个复杂的神经元细胞网络，其信号发送（轴突）和接收（树突）室的形态极其复杂。肌动蛋白丝的重新形成是神经元发育和可塑性的主要驱动力和导向力之一。因此，肌动蛋白丝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的水平上是最有效的，因为这些是细丝形成的关键组分。我们最近的研究结果揭示，Cobl，肌动蛋白成核剂的关键塑造神经元细胞的树突状乔木，是由精氨酸甲基化控制。这是出乎意料的，因为精氨酸甲基化是先前已知主要控制核过程的翻译后修饰。蛋白质精氨酸甲基转移酶PRMT 2被鉴定为与Cobl相关并调节其细胞骨架性质。重要的是，我们发现PRMT 2对树突状分支也至关重要，并且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独特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度脑富集的肌动蛋白成核促进因子作为PRMT 2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元网络形成和适当脑功能的更普遍和通用的手段。因此，我们在这里建议调查是否PRMT 2也是负责翻译后修饰的一个额外的肌动蛋白成核促进因子牵连在我们的质谱屏幕中发现的神经形态发生。随后的研究旨在揭示PRMT 2介导的精氨酸甲基化这一重要的肌动蛋白细胞骨架组分对其细胞骨架特性及其调控蛋白网络相互作用的分子机制后果。在神经元中的功能研究将进行，以揭示一个假定的广义功能PRMT 2介导的精氨酸甲基化在树突状arborization介导的肌动蛋白成核promoting factors.Further重要方面在神经网络的形成是轴突的发展和建立突触前和突触后的专业化随后的轴突诱导和分支。由于神经元的这些特化形态区室的诱导和可塑性也已被证明高度依赖于肌动蛋白组织和动力学，我们提出揭示PRMT 2在神经元形态控制中的功能是否以及在多大程度上超越树突状树枝化。这项工作将提供重要的见解，以调节翻译后机制，针对肌动蛋白成核-促进神经形态发生的关键因素，从而揭示了脊椎动物大脑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.