“Caught in the net”: the extracellular matrix of the bone marrow in normal hematopoiesis and leukemia

•The ECM supports the HSC-supportive function of the bone marrow microenvironment.microenvironment•The ECM provides a scaffold and serves as a source of growth factors and proteases.•ECM proteins influence HSC function and the course of a leukemia.•Interactions between ECM proteins and HSCs or LSCs can be targeted therapeutically.•Novel technologies aid the understanding of the ECM in the BMM. The influence of the bone marrow microenvironment on normal hematopoiesis, but also leukemia, has largely been accepted. However, the focus has been predominantly on the role of various cell types or cytokines maintaining hematopoietic stem cells or protecting leukemia stem cells from different therapies. A frequently overlooked component of the bone marrow microenvironment is the extracellular matrix, which not only provides a mechanical scaffold, but also serves as a source of growth factors. We discuss here how extracellular matrix proteins directly or indirectly modulate hematopoietic stem cell physiology and influence leukemia progression. It is hoped that existing and future studies on this topic may propel forward the possibility of augmenting normal hematopoiesis and improving therapies for leukemia, for instance, by targeting of the extracellular matrix in the bone marrow. The influence of the bone marrow microenvironment on normal hematopoiesis, but also leukemia, has largely been accepted. However, the focus has been predominantly on the role of various cell types or cytokines maintaining hematopoietic stem cells or protecting leukemia stem cells from different therapies. A frequently overlooked component of the bone marrow microenvironment is the extracellular matrix, which not only provides a mechanical scaffold, but also serves as a source of growth factors. We discuss here how extracellular matrix proteins directly or indirectly modulate hematopoietic stem cell physiology and influence leukemia progression. It is hoped that existing and future studies on this topic may propel forward the possibility of augmenting normal hematopoiesis and improving therapies for leukemia, for instance, by targeting of the extracellular matrix in the bone marrow. After many decades of research, we can now postulate that hematopoiesis is a process in which hematopoietic stem cells (HSCs) are responsible for the production of mature differentiated blood cells via intermediate progenitor cells, but at the same time HSCs self-renew to maintain their own pool. 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Leukemias are blood cancers characterized by abnormalities of leukocytes. The concept of leukemia stem cells (LSCs) was introduced when it was found that CD34+CD38− cells from acute myeloid leukemia (AML) patients have leukemia-initiating capacity upon transplantation into immunosuppressed mice [4Bonnet D Dick JE. Human acute myeloid leukemia is organized as a hierarchy that originates from a primitive hematopoietic cell.Nat Med. 1997; 3: 730-737Crossref PubMed Scopus (4838) Google Scholar]. Therefore, a leukemia was proven to be composed of heterogeneous cells, with only a subset of them having self-renewal capacity similar to that of normal HSCs. Complexity has been added to this question by the discovery of the contributions of the bone marrow microenvironment (BMM) or niche to the development, progression, and therapy resistance of leukemia. The term stem cell niche, referring to a specific location and entity extrinsically orchestrating and controlling the self-renewal and differentiation capacity of HSCs, was coined in 1978 [5Schofield R. The relationship between the spleen colony-forming cell and the haemopoietic stem cell.Blood Cells. 1978; 4: 7-25PubMed Google Scholar]. The BMM has largely been recognized as a unique structure providing a complex cellular, chemical, and mechanical microenvironment [6Morrison SJ Scadden DT. The bone marrow niche for haematopoietic stem cells.Nature. 2014; 505: 327-334Crossref PubMed Scopus (1100) Google Scholar, 7Birbrair A Frenette PS. Niche heterogeneity in the bone marrow.Ann NY Acad Sci. 2016; 1370: 82-96Crossref PubMed Scopus (124) Google Scholar, 8Crane GM Jeffery E Morrison SJ Adult haematopoietic stem cell niches.Nat Rev Immunol. 2017; 17: 573-590Crossref PubMed Scopus (197) Google Scholar]. 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In general, it is understood that the BMM is a complex entity composed of several cellular and acellular elements such as mesenchymal stromal cells (MSCs), endothelial cells (ECs), osteoblasts, osteoclasts, osteocytes, adipocytes, C–X–C motif chemokine ligand (CXCL)12-abundant reticular (CAR) cells, fibroblasts, sympathetic neurons, megakaryocytes, and macrophages, as well as chemical and mechanical factors and the extracellular matrix (ECM) [34Mendez-Ferrer S Bonnet D Steensma DP et al.Bone marrow niches in haematological malignancies.Nat Rev Cancer. 2020; 20: 285-298Crossref PubMed Scopus (2) Google Scholar]. In addition, a specific pH, an oxygen gradient, cytokines, and mechanical factors, most of which are reviewed elsewhere [35Schepers K Campbell TB Passegue E Normal and leukemic stem cell niches: insights and therapeutic opportunities.Cell Stem Cell. 2015; 16: 254-267Abstract Full Text Full Text PDF PubMed Scopus (196) Google Scholar], contribute to the BMM's unique features (Figure 1). This review, however, is focused on the structure and biochemical properties of the proteins in the ECM of the BMM, as well as their functions in normal and malignant hematopoiesis. In general, the ECM is a three-dimensional, complex and highly dynamic noncellular network composed of (1) proteoglycans; (2) fibrous proteins such as collagens, fibronectins, elastins, tenascins, vitronectin, and laminins; and (3) glycosaminoglycans, such as hyaluronic acid, chondroitin sulfate, heparan sulfate, keratan sulfate, dermatan sulfate, and heparin; and (4) matricellular proteins such as osteocalcin and periostin, which assist in the linkage between other ECM proteins and cellular receptors [36Coutu DL Kokkaliaris KD Kunz L Schroeder T Three-dimensional map of nonhematopoietic bone and bone-marrow cells and molecules.Nat Biotechnol. 2017; 35: 1202-1210Crossref PubMed Scopus (28) Google Scholar,37Klamer S Voermans C. The role of novel and known extracellular matrix and adhesion molecules in the homeostatic and regenerative bone marrow microenvironment.Cell Adh Migr. 2014; 8: 563-577Crossref PubMed Scopus (38) Google Scholar]. 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The heparan sulfate proteoglycan perlecan was found in the bone matrix and arteriolar basement membrane [36Coutu DL Kokkaliaris KD Kunz L Schroeder T Three-dimensional map of nonhematopoietic bone and bone-marrow cells and molecules.Nat Biotechnol. 2017; 35: 1202-1210Crossref PubMed Scopus (28) Google Scholar]. Hyaluronan (HA) was highly expressed in the endothelium of blood vessels in the metaphysis, where it may be involved in the homing of hematopoietic stem and progenitor cells (HSPCs) [41Ellis SL Grassinger J Jones A et al.The relationship between bone, hemopoietic stem cells, and vasculature.Blood. 2011; 118: 1516-1524Crossref PubMed Scopus (103) Google Scholar]. 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In particular, mutations in the COL1A1 and COL1A2 genes, encoding the two α chains of type I collagen, cause autosomal dominant osteogenesis imperfecta. This results in abnormal synthesis and secretion of collagen type I, leading to bone fragility and deformities in patients [48Pollitt R McMahon R Nunn J et al.Mutation analysis of COL1A1 and COL1A2 in patients diagnosed with osteogenesis imperfecta type I–IV.Hum Mutat. 2006; 27: 716Crossref PubMed Google Scholar]. 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The extracellular matrix of the hematopoietic microenvironment.Experientia. 1995; 51: 914-926Crossref PubMed Scopus (0) Google Scholar] (Figure 2). The fact that ECM proteins can be extracted and purified from different animal sources and used widely for the coating of cell culture wells has formed the basis for studies of the interactions of HSCs with the ECM. In addition, novel technologies, as mentioned below, have provided insight into the biochemical-functional properties of the ECM. Changes in the cellular microenvironment, in general, are sensed by cell–ECM interactions and are mediated mainly by integrins [52Kechagia JZ Ivaska J Roca-Cusachs P Integrins as biomechanical sensors of the microenvironment.Nat Rev Mol Cell Biol. 2019; 20: 457-473Crossref PubMed Scopus (71) Google Scholar] as well as discoidin domain receptors (DDRs) [53Leitinger B Hohenester E. 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Murine HSPCs express integrin α4β1 (VLA4; ITGA4 and ITGB1), α5β1 (VLA5; ITGA5 and ITGB1), α6β1, αLβ2 (LFA1; ITGAL and ITGB2), and αMβ2 (MAC-1; ITGAM and ITGB2), while some HSPCs express integrin 3 (ITGB3) [37Klamer S Voermans C. The role of novel and known extracellular matrix and adhesion molecules in the homeostatic and regenerative bone marrow microenvironment.Cell Adh Migr. 2014; 8: 563-577Crossref PubMed Scopus (38) Google Scholar]. Binding of integrins to proteins of the ECM, also termed outside-in signaling "outside-in signaling", activates intracellular signaling pathways leading to modulation of the position of other membrane receptors, the actin cytoskeleton, and possibly the cell cycle. This regulates cell spreading and retraction, migration, survival, shape, and differentiation. “Inside-out signaling,” that is, cell-intrinsic processes, which may lead to clustering or conformational changes of integrins, in contrast, stimulates ligand binding by integrins [57Calderwood DA Integrin activation.J Cell Sci. 2004; 117: 657-666Crossref PubMed Scopus (348) Google Scholar] and regulates integrin adhesiveness. 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For example, deletion of Ext1, the gene encoding a glycosyltransferase essential for heparan sulfate production in BM stromal cells, led to mobilization of HSPCs from the BM [65Saez B Ferraro F Yusuf RZ et al.Inhibiting stromal cell heparan sulfate synthesis improves stem cell mobilization and enables engraftment without cytotoxic conditioning.Blood. 2014; 124: 2937-2947Crossref PubMed Scopus (17) Google Scholar]. Further, three-dimensional quantitative microscopy has revealed that sinusoidal endothelial cells and mesenchymal CXCL-12–abundant reticular cells closely associate with the ECM [66Gomariz A Helbling PM Isringhausen S et al.Quantitative spatial analysis of haematopoiesis-regulating stromal cells in the bone marrow microenvironment by 3D microscopy.Nat Commun. 2018; 9: 2532Crossref PubMed Scopus (28) Google Scholar]. 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