Mayorova Tatiana D, Koch Thomas Lund, Kachar Bechara, Jung Jae Hoon, Reese Thomas S, Smith Carolyn L
Laboratory of Neurobiology, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD, United States of America.
Department of Biochemistry, University of Utah, Salt Lake City, Utah, United States of America.
bioRxiv. 2025 Jan 16:2024.09.18.613768. doi: 10.1101/2024.09.18.613768.
Placozoa are millimeter-sized, flat, irregularly shaped ciliated animals that crawl on surfaces in warm oceans feeding on biofilms, which they digest externally. They stand out from other animals due to their simple body plans. They lack organs, body cavities, muscles and a nervous system and have only seven broadly defined morphological cell types, each with a unique distribution. Analyses of single cell transcriptomes of four species of placozoans revealed greater diversity of secretory cell types than evident from morphological studies, but the locations of many of these new cell types were unknown and it was unclear which morphological cell types they represent. Furthermore, there were contradictions between the conclusions of previous studies and the single cell RNAseq studies. To address these issues, we used mRNA probes for genes encoding secretory products expressed in different metacells in to localize cells in whole mounts and in dissociated cell cultures, where their morphological features could be visualized and identified. The nature and functions of their secretory granules were further investigated with electron microscopic techniques and by imaging secretion in live animals during feeding episodes. We found that two cell types participate in disintegrating prey, one resembling a lytic cell type in mammals and another combining features of zymogen gland cells and enterocytes. We identified secretory epithelial cells expressing glycoproteins or short peptides implicated in defense. We located seven peptidergic cell types and two types of mucocytes. Our findings reveal mechanisms that placozoans use to feed and protect themselves from pathogens and clues about neuropeptidergic signaling. We compare placozoan secretory cell types with cell types in other animal phyla to gain insight about general evolutionary trends in cell type diversification, as well as pathways leading to the emergence of synapomorphies.
扁盘动物是毫米级大小、扁平、形状不规则的纤毛动物，它们在温暖海洋的表面爬行，以生物膜为食，并在体外进行消化。它们因其简单的身体结构而与其他动物不同。它们没有器官、体腔、肌肉和神经系统，只有七种大致定义的形态学细胞类型，每种类型都有独特的分布。对四种扁盘动物的单细胞转录组分析显示，分泌细胞类型的多样性比形态学研究中明显的要大，但许多这些新细胞类型的位置尚不清楚，也不清楚它们代表哪些形态学细胞类型。此外，先前研究的结论与单细胞RNA测序研究之间存在矛盾。为了解决这些问题，我们使用了针对在不同亚细胞中表达的分泌产物编码基因的mRNA探针，来定位整装标本和离体细胞培养物中的细胞，在这些培养物中可以观察和识别它们的形态特征。我们用电子显微镜技术以及通过对进食期间活体动物的分泌进行成像，进一步研究了它们分泌颗粒的性质和功能。我们发现有两种细胞类型参与分解猎物，一种类似于哺乳动物中的溶细胞类型，另一种兼具酶原腺细胞和肠上皮细胞的特征。我们鉴定出了表达与防御有关的糖蛋白或短肽的分泌上皮细胞。我们定位了七种肽能细胞类型和两种黏液细胞类型。我们的研究结果揭示了扁盘动物用于进食和保护自己免受病原体侵害的机制，以及有关神经肽信号传导的线索。我们将扁盘动物的分泌细胞类型与其他动物门的细胞类型进行比较，以深入了解细胞类型多样化的一般进化趋势，以及导致共衍征出现的途径。
