Unravelling human hematopoietic progenitor cell diversity through association with intrinsic regulatory factors.

Hematopoietic stem and progenitor cell (HSPC) transplantation is an essential therapy for hematological conditions, but finer definitions of human HSPC subsets with associated function could enable better tuning of grafts and more routine, lower-risk application. To deeply phenotype HSPCs, following a screen of 328 antigens, we quantified 41 surface proteins and functional regulators on millions of CD34+ and CD34- cells, spanning four primary human hematopoietic tissues: bone marrow, mobilized peripheral blood, cord blood, and fetal liver. We propose more granular definitions of HSPC subsets and provide new, detailed differentiation trajectories of erythroid and myeloid lineages. These aspects of our revised human hematopoietic model were validated with corresponding epigenetic analysis and in vitro clonal differentiation assays. Overall, we demonstrate the utility of using molecular regulators as surrogates for cellular identity and functional potential, providing a framework for description, prospective isolation, and cross-tissue comparison of HSPCs in humans.

Assay optimization for the objective quantification of human multilineage colony-forming units.

Colony-forming unit (CFU) assays are a powerful tool in hematopoietic research because they allow researchers to functionally test the lineage potential of individual stem and progenitor cells. Assaying for lineage potential is important for determining and validating the identity of progenitor populations isolated by methods such as fluorescence-activated cell sorting (FACS). However, current methods for CFU assays are limited in their ability to robustly assay multipotent progenitors with the ability to differentiate down the myeloid, erythroid, and megakaryocytic lineages because of the lack of specific growth factors necessary for certain lineage outputs. In addition, manual counting of colony types is subjective resulting in user to user variability in assessments of cell types based on colony and cell morphologies. We demonstrate that the addition of granulocyte colony-stimulating factor (G-CSF), macrophage (M)-CSF, and granulocyte-macrophage (GM)-CSF into a collagen-based MegaCult medium containing IL-3, IL-6, SCF, EPO, and TPO allows for the differentiation of common myeloid progenitors into expected proportions of colonies containing granulocytic (G), monocytic (M), erythroid (E), and megakaryocytic (Mk) cells. Additionally, we demonstrate an objective method using in situ immunofluorescence (IF) with anti-CD66b, anti-CD14, anti-CD235a, and anti-CD41 to detect G, M, E, and Mk cells, respectively. IF stained colonies can be analyzed individually at a microscope or using high-throughput microscopy. Thus, our improvements to the culture conditions and method for assay readout increase the accuracy, reproducibility, and throughput of the myeloid CFU assay.

Multiparameter analysis of timelapse imaging reveals kinetics of megakaryocytic erythroid progenitor clonal expansion and differentiation.

Single-cell assays have enriched our understanding of hematopoiesis and, more generally, stem and progenitor cell biology. However, these single-end-point approaches provide only a static snapshot of the state of a cell. To observe and measure dynamic changes that may instruct cell fate, we developed an approach for examining hematopoietic progenitor fate specification using long-term (> 7-day) single-cell time-lapse imaging for up to 13 generations with in situ fluorescence staining of primary human hematopoietic progenitors followed by algorithm-assisted lineage tracing. We analyzed progenitor cell dynamics, including the division rate, velocity, viability, and probability of lineage commitment at the single-cell level over time. We applied a Markov probabilistic model to predict progenitor division outcome over each generation in culture. We demonstrated the utility of this methodological pipeline by evaluating the effects of the cytokines thrombopoietin and erythropoietin on the dynamics of self-renewal and lineage specification in primary human bipotent megakaryocytic-erythroid progenitors (MEPs). Our data support the hypothesis that thrombopoietin and erythropoietin support the viability and self-renewal of MEPs, but do not affect fate specification. Thus, single-cell tracking of time-lapse imaged colony-forming unit assays provides a robust method for assessing the dynamics of progenitor self-renewal and lineage commitment.

Epithelial (E)-Cadherin is a Novel Mediator of Platelet Aggregation and Clot Stability.

Cadherins play a major role in mediating cell-cell adhesion, which shares many parallels with platelet-platelet interactions during aggregate formation and clot stabilization. Platelets express epithelial (E)-cadherin, but its contribution to platelet function and/or platelet production is currently unknown. To assess the role of E-cadherin in platelet production and function in vitro and in vivo, we utilized a megakaryocyte-specific E-cadherin knockout mouse model. Loss of E-cadherin in megakaryocytes does not affect megakaryocyte maturation, platelet number or size. However, platelet dysfunction in the absence of E-cadherin is revealed when conditional knockout mice are challenged with acute antibody-mediated platelet depletion. Unlike wild-type mice that recover fully, knockout mice die within 72 hours post-antibody administration, likely from haemorrhage. Furthermore, conditional knockout mice have prolonged tail bleeding times, unstable clot formation, reduced clot retraction and reduced fibrin deposition in in vivo injury models. Murine platelet aggregation in vitro in response to thrombin and thrombin receptor activating peptide is compromised in E-cadherin null platelets, while aggregation in response to adenosine diphosphate (ADP) is not significantly different. Consistent with this, in vitro aggregation of primary human platelets in response to thrombin is decreased by an inhibitory E-cadherin antibody. Integrin activation and granule secretion in response to ADP and thrombin are not affected in E-cadherin null platelets, but Akt and glycogen synthase kinase 3ß (GSK3ß) activation are attenuated, suggesting a that E-cadherin contributes to aggregation, clot stabilization and retraction that is mediated by phosphoinositide 3-kinase/Akt/GSK3ß signalling. In summary, E-cadherin plays a salient role in platelet aggregation and clot stability.