Mechanisms of a conserved niche endothelial program directing hematopoietic stem cell migration

About This Grant

PROJECT SUMMARY Hematopoietic stem cell transplantation (HSCT) is a potentially lifesaving treatment for patients with hematological and immune disorders but poses significant risk due to prolonged immunocompromised states before donor stem cell engraftment. Successful engraftment, where hematopoietic stem and progenitor cells (HSPCs) establish themselves in the recipient's bone marrow, requires that circulating donor HSPCs transmigrate the bone marrow endothelium to reach their supportive stem cell niche. Enhancing the transendothelial migration of HSPCs offers a promising strategy to speed engraftment and boost post- transplant immune system reconstitution. The full complement of receptors and intracellular machinery within the bone marrow niche endothelial cells (ECs) that promote HSPC migration remain incompletely understood. In mammals, the transendothelial migration of HSPCs occurs deep inside opaque bones, which are largely inaccessible to imaging and experimentation in live animals. The transparent zebrafish embryo, by contrast, which has a blood and vascular system highly similar to that of humans, enables direct visualization of HSPC migration in vivo. We previously identified a conserved gene expression signature unique to the hematopoietic niche ECs that regulate HSPC transmigration. This expression program includes genes with cell adhesion and endocytosis/vesicle trafficking functions. In my preliminary studies I disrupted candidate factors from the niche EC signature, which blocked endocytosis in the niche ECs and disrupted the niche migration of HSPCs. My data suggest that a combination of specific cell adhesion and endocytosis/vesicle trafficking machinery supports the endothelial transmigration of HSPCs, a hypothesis I will test with my proposed aims. In Aim 1, I use high-resolution in vivo imaging in the zebrafish paired with human cell culture systems to determine how a candidate adhesion molecule promotes HSPC migration. In Aim 2, I will use a rapid F0 zebrafish CRISPR knock-out platform to test whether different niche EC-expressed vesicle trafficking factors are required for HSPC migration. In parallel, I will use a transcription factor reprogramming approach to assess whether induction of the niche EC signature in different human cell types can trigger the transendothelial migration of HSPCs. Together, these proposed experiments will provide key insight into how niche EC-expressed factors promote the migration of HSPCs. Results from my proposed studies could inform methods to enhance bone marrow engraftment or to direct donor HSPCs to new and/or additional niches to improve HSCT outcomes.