Page 72 - Haematologica April 2020

Hematopoiesis
Ferrata Storti Foundation
Haematologica 2020 Volume 105(4):914-924
Ephrin/Eph receptor interaction facilitates macrophage recognition of differentiating human erythroblasts
Lea A. Hampton-O’Neil,1,2,3 Charlotte E. Severn,1,2,3 Stephen J. Cross,1,4 Sonam Gurung,1 Catherine D. Nobes1 and Ashley M. Toye1,2,3
1School of Biochemistry, Biomedical Sciences Building, University of Bristol, University Walk, Bristol; 2Bristol Institute for Transfusion Sciences, NHS Blood and Transplant, Filton, Bristol; 3National Institute for Health Research (NIHR) Blood and Transplant Unit in Red Blood Cell Products, University of Bristol, Bristol and 4Wolfson Bioimaging Facility, Biomedical Sciences Building, University of Bristol, University Walk, Bristol, UK
ABSTRACT
Erythropoiesis is one of the most efficient cellular processes in the human body producing approximately 2.5 million red blood cells every second. This process occurs in a bone marrow niche comprised of a central resident macrophage surrounded by differentiating erythrob- lasts, termed an erythroblastic island. It is not known what initially attracts the macrophage to erythroblasts to form these islands. The ephrin/Eph receptor family are known to regulate heterophilic cell-cell adhesion. We find that human VCAM1+ and VCAM1– bone marrow macrophages and in vitro cultured macrophages are ephrin-B2 positive, whereas differentiating human erythroblasts express EPHB4, EPHB6 and EPHA4. Furthermore, we detect a rise in integrin activation on erythroblasts at the stage at which the cells bind which is independent of EPH receptor presence. Using a live cell imaging assay, we show that specific inhibitory peptides or shRNA deple- tion of EPHB4 cause a significant reduction in the ability of macrophages to interact with erythroblasts but do not affect integrin activation. This study demonstrates for the first time that EPHB4 expression is required on ery- throblasts to facilitate the initial recognition and subsequent interaction with macrophages, alongside the presence of active integrins.
Introduction
Erythropoiesis is the process whereby hematopoietic stem cells (HSC) develop to mature red blood cells by undergoing multiple stages of cell division and differen- tiation before enucleating to form nascent reticulocytes. In humans, this process occurs in the bone marrow (BM). HSC undergo asymmetric division and lineage restriction to form pro-erythroblasts in the HSC niche, where they bind a macrophage to form a specialized niche called an erythroblastic island. This niche is formed by a central resident macrophage which is surrounded by differentiating erythroblasts.1 The erythroblastic island is important for proliferation and terminal differentiation of erythroid cells, as macrophages are thought to supply nutrients to the surrounding erythroid cells, promote growth through survival signals, and phagocytose the pyrenocyte after enucleation.2-4
Multiple receptors are present on the surface of macrophages and erythroblasts which are involved in erythroblastic island interactions. These include intercellular adhesion molecule 4 (ICAM4), vascular cell adhesion molecule 1 (VCAM1), ery- throblast-macrophage protein (Emp), Fms related tyrosine kinase 3 (Flt3), proto- oncogene tyrosine-protein kinase MER (Mer-TK), dystroglycan (DG) receptor, inte- grins, and EPH receptors.4-10 It has already been established that ICAM4-/- mice formed significantly less erythroblastic islands than control mice6 and the loss of erythroblast-macrophage protein (Emp) in mice leads to apoptosis of erythroid pre- cursors and enucleation failure.5,11 Finally, integrin β3 knockout mice have a higher amount of early erythroblast release from erythroblastic islands.7 Overall, although we now know more about the importance of certain receptors for erythroblastic island integrity in mice, we do not know exactly which receptors are involved in
Correspondence:
ASHLEY TOYE
ash.m.toye@bristol.ac.uk
Received: December 20, 2018. Accepted: June 7, 2019. Pre-published: June 13, 2019.
doi:10.3324/haematol.2018.215160
Check the online version for the most updated information on this article, online supplements, and information on authorship & disclosures: www.haematologica.org/content/105/4/914
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