Red Blood Cell Contribution to Thrombosis in Polycythemia Vera and Essential Thrombocythemia.

Polycythemia vera (PV) and essential thrombocythemia (ET) are myeloproliferative neoplasms (MPN) characterized by clonal erythrocytosis and thrombocytosis, respectively. The main goal of therapy in PV and ET is to prevent thrombohemorrhagic complications. Despite a debated notion that red blood cells (RBCs) play a passive and minor role in thrombosis, there has been increasing evidence over the past decades that RBCs may play a biological and clinical role in PV and ET pathophysiology. This review summarizes the main mechanisms that suggest the involvement of PV and ET RBCs in thrombosis, including quantitative and qualitative RBC abnormalities reported in these pathologies. Among these abnormalities, we discuss increased RBC counts and hematocrit, that modulate blood rheology by increasing viscosity, as well as qualitative changes, such as deformability, aggregation, expression of adhesion proteins and phosphatidylserine and release of extracellular microvesicles. While the direct relationship between a high red cell count and thrombosis is well-known, the intrinsic defects of RBCs from PV and ET patients are new contributors that need to be investigated in depth in order to elucidate their role and pave the way for new therapeutical strategies.

Adhesion Molecules Involved in Stem Cell Niche Retention During Normal Haematopoiesis and in Acute Myeloid Leukaemia.

In the bone marrow (BM) of adult mammals, haematopoietic stem cells (HSCs) are retained in micro-anatomical structures by adhesion molecules that regulate HSC quiescence, proliferation and commitment. During decades, researchers have used engraftment to study the function of adhesion molecules in HSC's homeostasis regulation. Since the 90's, progress in genetically engineered mouse models has allowed a better understanding of adhesion molecules involved in HSCs regulation by BM niches and raised questions about the role of adhesion mechanisms in conferring drug resistance to cancer cells nested in the BM. This has been especially studied in acute myeloid leukaemia (AML) which was the first disease in which the concept of cancer stem cell (CSC) or leukemic stem cells (LSCs) was demonstrated. In AML, it has been proposed that LSCs propagate the disease and are able to replenish the leukemic bulk after complete remission suggesting that LSC may be endowed with drug resistance properties. However, whether such properties are due to extrinsic or intrinsic molecular mechanisms, fully or partially supported by molecular crosstalk between LSCs and surrounding BM micro-environment is still matter of debate. In this review, we focus on adhesion molecules that have been involved in HSCs or LSCs anchoring to BM niches and discuss if inhibition of such mechanism may represent new therapeutic avenues to eradicate LSCs.

Flow Cytometry Analysis of Mouse Hematopoietic Stem and Multipotent Progenitor Cells.

Flow cytometry has been widely used to detect a single event by means of multiparametric fluorescence measurements. Here we describe a method to analyze subsets of hematopoietic stem and progenitor cells isolated from long bones of mice. We further show that this method allows for comparing JAM-C protein expression between subsets of hematopoietic stem and progenitor cells.

GRASP55 Is Dispensable for Normal Hematopoiesis but Necessary for Myc-Dependent Leukemic Growth.

Grasp55 is a ubiquitous Golgi stacking protein involved in autophagy, protein trafficking, and glucose deprivation sensing. The function of Grasp55 in protein trafficking has been attributed to its PDZ-mediated interaction with the C-terminal PDZ-binding motifs of protein cargos. We have recently shown that such an interaction occurs between Grasp55 and the adhesion molecule Jam-C, which plays a central role in stemness maintenance of hematopoietic and spermatogenic cells. Accordingly, we have found that Grasp55-deficient mice suffer from spermatogenesis defects similar to Jam-C knockout mice. However, whether Grasp55 is involved in the maintenance of immunohematopoietic homeostasis through regulation of protein transport and Jam-C expression remains unknown. In this study, we show that Grasp55 deficiency does not affect hematopoietic stem cell differentiation, engraftment, or mobilization, which are known to depend on expression of Grasp55-dependent protein cargos. In contrast, using an Myc-dependent leukemic model addicted to autophagy, we show that knockdown of Grasp55 in leukemic cells reduces spleen and bone marrow tumor burden upon i.v. leukemic engraftment. This is not due to reduced homing of Grasp55-deficient cells to these organs but to increased spontaneous apoptosis of Grasp55-deficient leukemic cells correlated with increased sensitivity of the cells to glucose deprivation. These results show that Grasp55 plays a role in Myc-transformed hematopoietic cells but not in normal hematopoietic cells in vivo.