Macrophages arise from two distinct lineages of hematopoietic cells, Hematopoietic Stem Cell-derived progenitors and Erythromyeloid Progenitors. EMP derived macrophages are seeded in tissues early during embryogenesis and become specialized tissue resident macrophages such as microglia and Kupffer cells. HSC derived monocytes arise in the bone marrow and normally circulate via the bloodstream to reach tissues of the body and differentiate into macrophages which are cells that are constantly replenished by their progenitors. Monocytes are also recruited to inflamed tissues attracted by inflammatory signals, where they become macrophages and further contributed to the inflammatory process. These two macrophage populations may be implicated in different activities and have different functions, such as tissue repair or proinflammatory responses, respectively. Other cells that are unique in having both origins are osteoclasts, which during embryogenesis originate from EMP precursors and contribute to the formation of ossification centers of long bones and consequently the formation of bone marrow cavity. After birth, HSC derived monocytes contribute to their cell maintenance by direct cell fusion to pre-existing EMP-derived osteoclasts, eventually replacing them to become fully derived from HSC precursors, a mechanism that takes several month to complete in mice.
Cell lineage tracing is a powerful technique that allows for the labeling of specific cell populations in a specific time and space. The gold standard cell lineage tracing mouse model for the study of EMP derived cells including macrophages and osteoclasts in mice, Csf1rMeriCreMer;Rosa26LSL-YFP,1–3 uses a tamoxifen inducible Cre recombinase to induce expression of YFP and therefore permanently labels EMP progenitors and they progeny. Other mouse models to label macrophages and osteoclast have been used, which include Cx3cr1CreERT2;Rosa26LSL-tdTomato, however, recent evidence show spontaneous expression of tdTomato fluorescent protein in the absence of tamoxifen (unpulsed mice). Here we confirmed that Cre recombination resulted in tdTomato expression in the absence of tamoxifen in Cx3cr1CreERT2;Rosa26LSL-tdTomato mice. Therefore, we utilized a different fluorescent reporter gene instead, Rosa26LSL-YFP, and generated Cx3cr1CreERT2;Rosa26LSL-YFP mice for permanent labelling of EMP-derived cells, and provide evidence that no YFP expression is detected in unpulsed mice.
Further, we performed pulse labeling of the Cx3cr1CreERT2;Rosa26LSL-YFP model at E10.5 days of gestation. These experiments were notable for YFP expression in macrophages and the highest labeling efficiency reported in osteoclasts (~70%) at E18.5 embryonic days of development. Further, there was no YFP expression detected in HSCs or HSC-derived cells, confirming this model as a potential useful tool for precise and efficient labelling of EMP-derived macrophages and osteoclasts
| Identifer | oai:union.ndltd.org:bu.edu/oai:open.bu.edu:2144/48207 |
| Date | 26 February 2024 |
| Creators | Hacein-Bey, Camelia |
| Contributors | Divieti Pajevic, Paola, Jacome-Galarza, Christian |
| Source Sets | Boston University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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