Live Characterization Of Nascent Hematopoietic Stem And Progenitor Cells (HSPCs) Homing

Hematopoietic Stem Cells(HSCs)are able to self-renew and give rise to all blood lineages that support people's life.In mammals,HSCs emerge from embryonic AGM region,then migrate to fetal liver to expand,at last colonized the bone marrow to maintain the hematopoiesis through adulthood.During different stage of hematopoietic development,HSCs home to multiple hematopoietic tissue and are finely regulated by corresponding microenvironment.However,current understanding about the in vivo dynamic homing process and the molecular and cellular mechanism of homing is still very limited.The embryonic hematopoietic development of zebrafish is highly conserved with mammals.And as the transparent embryo of zebrafish develops in vitro,it is recognized as excellent model for research on dynamic hematopoietic process via live imaging.Therefore,we firstly combined photo-conversion and high-resolution live imaging technology to construct nascent hematopoietic stem and progenitor cells(HSPCs)live labelling and tracing system through the whole caudal hematopoietic tissue(CHT,equivalent to fetal liver of mammals).Compared to itga4 mutant with severe homing defect,we analyzed the retention time pattern of homing HSPC.When combined with track position,HSPCs retention hotspots appear along the upper side of the caudal vein plexus.To dissect the fine architecture of hotspots,using super high-resolution imaging and three-dimensional reconstruction,we revealed the unique venous capillary structure which constitutes the vasculature niche for HSPC homing.As there also exists normal venous capillary in itga4 mutants,we assumed that other niche component helps HSPCs homing in an ITGA4-dependent manner and did further research on VCAM-1,the only ligand of ITGA4 specifically expressed in the CHT.We surprisingly discovered that vasculature expressed VCAM-1 only to help HSPCs rolling on its bed,at the same time,a subpopulation of macrophages highly expressing VCAM-1 patrolled around the potential hotspot in a small scale and helped HSPCs homing to vascular niche through ITGA4-VCAM-1 coupling.We named the VCAM-1~+macrophages as“usher cell”.In conclusion,we constructed a brand-new system to lively label HSPC and its niche to explore the entire homing process in vivo and revealed the molecular and cellular mechanism of HSPCs homing.The study expands the knowledge of HSPCs homing and provides new research interest for clinical application of HSPCs.