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CXCL12-Abundant Reticular (CAR) Cells Direct Megakaryocyte Protrusions across the Bone Marrow Sinusoid Wall.

ABSTRACT: Megakaryocytes (MKs) release platelets into the lumen of bone marrow (BM) sinusoids while remaining to reside within the BM. The morphogenetic events of this complex process are still not fully understood. We combined confocal laser scanning microscopy with transmission and serial block-face scanning electron microscopy followed by 3D-reconstruction on mouse BM tissue sections. These analyses revealed that MKs in close vicinity to BM sinusoid (BMS) wall first induce the lateral retraction of CXCL12-abundant reticular (CAR) cells (CAR), followed by basal lamina (BL) degradation enabling direct MK-sinusoidal endothelial cells (SECs) interaction. Subsequently, an endothelial engulfment starts that contains a large MK protrusion. Then, MK protrusions penetrate the SEC, transmigrate into the BMS lumen and form proplatelets that are in direct contact to the SEC surface. Furthermore, such processes are induced on several sites, as observed by 3D reconstructions. Our data demonstrate that MKs in interaction with CAR-cells actively induce BMS wall alterations, including CAR-cell retraction, BL degradation, and SEC engulfment containing a large MK protrusion. This results in SEC penetration enabling the migration of MK protrusion into the BMS lumen where proplatelets that are adherent to the luminal SEC surface are formed and contribute to platelet release into the blood circulation.

SUBMITTER: Wagner N

PROVIDER: S-EPMC8063926 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Decline in hematopoietic function in aged individuals is associated with expansion of phenotypic hematopoietic stem cells (HSCs) and a shift in their lineage potential toward production of myeloid cells. Both HSC-intrinsic changes, and extrinsic changes in the bone marrow (BM) microenvironment, have been identified in old mice and humans. However, to extend healthy and robust hematopoietic function from youth into older age, we need to understand and effectively target the processes that initiate functional hematopoietic decline. We recently identified decline in Insulin-Like Growth Factor 1 (IGF1) in the BM microenvironment as early as middle age to be an HSC-extrinsic initiating driver of HSC aging (Young et al., Cell Stem Cell 2021). As systemic IGF1 administration has significant undesirable side effects, we sought to comprehensively interrogate the cell population(s) in the BM microenvironment that are responsible for IGF1 decline, towards the goal of cell type-specific targeted therapy. We performed single cell RNA-seq to comprehensively profile hematopoietic and non-hematopoietic fractions of the BM in young (2-4mo; n = 5 biological replicates) and middle-aged (12-14mo; n = 10) mice. In young mice, we find Igf1 to be nearly entirely detected in the mesenchymal stromal cell populations Adipo-CAR and Osteo-CAR, and Igf1 is significantly reduced in expression in both populations in middle-aged mice. Using two independent mesenchymal stromal cell Cre mouse lines, Lepr-Cre and Prx1-CreERT2, we found that knockout of Igf1 resulted in myeloid-biased hematopoiesis that replicated aging phenotypes. This result was similar to our published work showing that knockout of Igf1 using Nestin-CreER causes myeloid-biased hematopoiesis. While these Cre models generally do not mark similar cell types, it has been shown that Lepr-Cre-expressing perisinusoidal stromal cells include cells that express certain Nestin transgenes. Using fluorescent reporters, we find that all three lines (Lepr-Cre, Prx1-CreERT2, and Nestin-CreER) overlap in expression in the CAR populations that abundantly express Igf1 in young mice. Taken together, our work identifies a new role for Cxcl12-abundant reticular cells in sustaining hematopoietic function through local IGF1 production and suggests that specifically targeting CAR cells to maintain or restore Igf1 expression during aging will have beneficial effects on lymphoid cell production and adaptive immunity.

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CXCL12-Abundant Reticular (CAR) Cells Direct Megakaryocyte Protrusions across the Bone Marrow Sinusoid Wall.

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