The concept of the stem cell niche is central both to the fundamental understanding of how stem cells are regulated by their microenvironment, and to clinical translation that targets the microenvironment for improving therapeutic outcome. The bone marrow (BM), where hematopoietic stem cells (HSCs) reside, is a crowded space packed with a diversity of cell types derived from both hematopoietic and nonhematopoietic precursors. A major challenge in studying the HSC niche has been the difficulty in identifying the rare HSCs and their neighboring cells in the native BM microenvironment. Elegant cell type-specific deletion of molecules critical for HSC maintenance has led to the identification of vascular endothelial cells (ECs) and CXCL12-abundant reticular (CAR) cells as two major cell types of the HSC niche. However, deletion of such factors impacts all ECs and CAR cells that are present throughout the BM, and are therefore not specific in terms of their local impact in the HSC niche. Direct imaging has the potential to uncover which cell types are in close contact with the HSCs, provided that specific markers are available for all cell types involved. As markers for HSCs are now just beginning to emerge, and visualization of minor cell types remains a challenge, the direct imaging approach has not progressed beyond resolving whether HSCs are in proximity to ECs, CAR cells, or bone-lining osteoblasts. Imaging on its own also does not provide the molecular information essential for understanding how the signals from the niche are communicated to the HSCs. We propose that two things are needed for the field to move forward. First, development of an HSC-specific reporter mouse will enable the identification of endogenous stem cells in their native microenvironment without transplantation. Second, development of a method to selectively isolate the cells in close proximity to the HSCs will enable unbiased profiling of cell types and their molecular signatures (for example, by single-cell RNA sequencing) involved in HSC maintenance. We have now taken steps to address both of these needs. First, we have developed (Camargo Lab) a dual genetic strategy in mice that restricts reporter labeling near exclusively to the most quiescent long-term subset of the HSC compartment (LT-HSCs). This reporter line is fully compatible with current intravital imaging approaches in the calvarial BM and enables live animal tracking of native HSCs (Lin Lab) based on the expression of the green fluorescent protein (GFP) alone, without the need for additional markers and without transplantation. In addition, we have developed a technique for micropipette aspiration of single cells and cell clusters directly from the BM under two-photon image guidance, enabling single cell analysis with high spatial definition. Here, we propose to bring the two teams together to work on an integrated approach for marking, isolating and profiling the native HSCs together with their neighboring ?niche cells?, whose cell types will be identified retrospectively from the transcriptome profiles.
Hematopoietic stem cells (HSCs) reside in specialized niches in the bone marrow where blood cells are produced. The identification of the niche cellular constituents and the elucidation of their unique molecular profiles are long-sought goals in the field, which have not been properly addressed because of historical technical limitations. Our project brings together experts in stem cell biology and live imaging to overcome these limitations. Successful execution of the aims in this proposal will usher in a new way to study the native HSC niche and how biological processes such as bone remodeling can impact the hematopoietic microenvironment.
