Investigating bone marrow megakaryocyte hetrogeneity in situ using spatial transcriptomics

Megakaryocytes are bone marrow (BM) resident cells that derive from hematopoietic stem cells. A pivotal function of megakaryocytes is the generation of platelets through the release of long protrusions called proplatelets into sinusoidal vessels. single-cell RNA-sequencing on murine BM megakaryocytes has previously revealed transcriptional heterogeneity with segmentation into four distinct categories. These studies postulated functions beyond platelet production with evidence for immunoregulatory and stem cell niche supporting subtypes, as well as a cycling population. The spatial context and transcriptional heterogeneity of megakaryocytes is of great interest as localization of for instance the vasculature is a necessity for platelet production. For single-cell RNA sequencing this spatial orientation is however lost due to the dissociation of tissues. Recent technological advances have enabled the interrogation of gene expression profiles of tissues in situ. This enables the integration of morphological, situational and transcriptional information to classify cells in the context of their microenvironment. In the following we present, for the first time, the application of this technology to BM megakaryocytes at a single cell level.巨核细胞（Megakaryocytes）是一类由造血干细胞（hematopoietic stem cells）分化而来、定居于骨髓（bone marrow, BM）的细胞。其核心功能之一是通过向血窦（sinusoidal vessels）释放被命名为前血小板（proplatelets）的长突起，进而生成血小板（platelets）。此前针对小鼠骨髓巨核细胞的单细胞RNA测序（single-cell RNA-sequencing）已揭示其存在转录异质性（transcriptional heterogeneity），且可被划分为四种不同的类别。这些研究推测巨核细胞还具备血小板生成以外的其他功能：有证据表明其存在免疫调节亚型、支持干细胞龛（stem cell niche）的亚型，以及一类增殖群体（cycling population）。巨核细胞的空间分布与转录异质性是当前研究的热点，因为诸如血管的定位是血小板生成的必要条件。然而，单细胞RNA测序会因组织解离而丢失细胞的空间定位信息。近年来的技术进步已实现了对组织原位（in situ）基因表达谱（gene expression profiles）的检测分析，该技术可整合细胞的形态学、定位信息与转录组学信息，从而在细胞微环境（microenvironment）的背景下对细胞进行分类。本文首次将该技术应用于单细胞层面的骨髓巨核细胞研究中。