Supplementary table 2: Vesicle diameters measured in a cell of Salpingoeca rosetta and Monosiga brevicollis. from Choanoflagellates and the ancestry of neurosecretory vesicles

Neurosecretory vesicles are highly specialized trafficking organelles that store neurotransmitters which are released at pre-synaptic nerve endings and are, therefore, important for animal cell–cell signalling. Despite considerable anatomical and functional diversity of neurons in animals, the protein composition of neurosecretory vesicles in bilaterians appears to be similar. This similarity points towards a common evolutionary origin. Moreover, many putative homologues of key neurosecretory vesicle proteins predate the origin of the first neurons, and some even the origin of the first animals. However, little is known about the molecular toolkit of these vesicles in non-bilaterian animals and their closest unicellular relatives, making inferences about the evolutionary origin of neurosecretory vesicles extremely difficult. By comparing 28 proteins of the core neurosecretory vesicle proteome in 13 different species, we demonstrate that most of the proteins are present in unicellular organisms. Surprisingly, we find that the vesicular membrane-associated soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein synaptobrevin is localized to the vesicle-rich apical and basal pole in the choanoflagellate Salpingoeca rosetta. Our 3D vesicle reconstructions reveal that the choanoflagellates S. rosetta and Monosiga brevicollis exhibit a polarized and diverse vesicular landscape reminiscent to the polarized organization of chemical synapses that secrete the content of neurosecretory vesicles into the synaptic cleft. This study sheds light on the ancestral molecular machinery of neurosecretory vesicles and provides a framework to understand the origin and evolution of secretory cells, synapses and neurons.This article is part of the theme issue ‘Basal cognition: multicellularity, neurons and the cognitive lens’.

神经分泌囊泡（neurosecretory vesicles）是一类高度特化的转运细胞器，负责储存神经递质，此类递质经突触前神经末梢释放，因此对动物细胞间信号传导至关重要。尽管动物体内神经元在解剖结构与功能上存在显著多样性，但两侧对称动物（bilaterians）的神经分泌囊泡蛋白质组成却呈现出高度相似性。这种相似性暗示了它们拥有共同的演化起源。此外，诸多关键神经分泌囊泡蛋白的推定同源物（putative homologues）的出现时间早于首个神经元的起源，部分同源物的起源甚至早于首只动物的出现。然而，学界对于非两侧对称动物及其最近缘单细胞类群体内这类囊泡的分子工具库所知甚少，这使得推断神经分泌囊泡的演化起源极具挑战性。本研究通过比对13个不同物种的核心神经分泌囊泡蛋白质组（core neurosecretory vesicle proteome）中的28种蛋白，证实其中多数蛋白均存在于单细胞生物体内。令人意外的是，研究发现囊泡膜相关可溶性N-乙基马来酰亚胺敏感因子附着蛋白受体（soluble N-ethylmaleimide-sensitive factor attachment protein receptor, SNARE）蛋白突触泡蛋白（synaptobrevin）在领鞭毛虫（choanoflagellate）玫瑰旋领虫（Salpingoeca rosetta）中，定位于富含囊泡的顶端极与基底极。我们的囊泡三维重构结果显示，领鞭毛虫玫瑰旋领虫（S. rosetta）与短领鞭毛虫（Monosiga brevicollis）呈现出极化且多样的囊泡分布模式，这与将神经分泌囊泡内容物释放至突触间隙的化学突触（chemical synapses）极化组织形式极为相似。本研究阐明了神经分泌囊泡的祖先分子机制，并为理解分泌细胞、突触与神经元的起源及演化提供了研究框架。本文隶属于专题专辑“基础认知：多细胞性、神经元与认知视角”（Basal cognition: multicellularity, neurons and the cognitive lens）。