HSC Niche Biology and HSC Expansion Ex Vivo

Abstract

Hematopoietic stem cell (HSC) transplantation can restore a new functional hematopoietic system in recipients in cases where the system of the recipient is not functional or for example is leukemic. However, the number of available donor HSCs is often too low for successful transplantation. Expansion of HSCs and thus HSC self-renewal ex vivo would greatly improve transplantation therapy in the clinic. In vivo, HSCs expand significantly in the niche, but establishing protocols that result in HSC expansion ex vivo remains challenging. In this review we discuss current knowledge of niche biology, the intrinsic regulators of HSC self-renewal in vivo, and introduce novel niche-informed strategies of HSC expansion ex vivo.

Key Figure, Figure 1. Mammalian Bone Marrow HSC Niche

The diagram shows the cellular composition and cytokines/growth factors that can impact HSC self-renewal and function in the bone marrow (BM) niche. Recent research has identified the role of diverse BM niche cells and HSC progeny including osteoblasts, nestin⁺ mesenchymal stem cells, CAR cells, non-myelinating Schwann cells, BM endothelial cells and adipocytes, macrophages, megakaryocytes and neutrophils (PMN) in HSC self-renewal, differentiation and function. Niche cells also produce/release several cytokines/growth factors, such as SCF, TPO, TGF-β1, CXCL-4, CXCL-12, G-CSF, OPN, notch ligands, angiopoietin 1 and pleiotrophin to regulate HSC self-renewal, maintenance, survival, retention and function. The extra-cellular matrix (ECM) can also regulate HSC self-renewal and maintenance. SCF, stem cell factor; TPO, thrombopoietin; TGF-β1, transforming growth factor beta 1;CXCL-4, CXC chemokine ligand 4; CXCL-12, CXC chemokine ligand 12; G-CSF, Granulocyte-colony stimulating factor; OPN, osteopontin.

Figure 2. Metabolic Regulation of Mammalian HSCs

(A) Hematopoietic stem cells (HSCs) exhibit condense and immature mitochondria, low metabolic status and high glycolytic activity, as suggested by low ATP, low ROS and low membrane potential (ΔΨ) which uphold stemness of the HSCs, in contrast to progenitors and more differentiated cells that exhibit high mitochondrial activity and utilize oxidative phosphorylation (OXPHOS). Furthermore, stabilized hypoxia-inducible factor 1α (HIF1α) in HSCs can support self-renewal and stemness potential. (B) Mechanisms of metabolic regulation of HSC function. Under hypoxic conditions, cytokine TPO and MEIS1 can stabilize HIF1α to promote glycolysis by regulating glycolytic pathway enzymes including HK, hexokinase; LDHA, lactate dehydrogenase A; PFK, phosphofructokinase 2; PKM2, pyruvate kinase M2. HIF1α can also controls HOXB4 and pyruvate dehydrogenase kinase (PDK) activation that prevents pyruvate oxidation by suppressing the pyruvate dehydrogenase (PDH) complex, inhibiting oxidative phosphorylation and leading to maintenance of HSC quiescence and stemness. ROS, Reactive Oxygen Species: MEIS1, Meis Homeobox 1; TPO, thrombopoietin; HIF, hypoxia-inducible factor; HOXB4, Homeobox B4; PDK, pyruvate dehydrogenase kinase;

Figure 3. Metabolic Regulation of Mammalian HSC Self-Renewal

HSCs maintain low metabolic status and high glycolytic activity with low ROS and low membrane potential (ΔΨ) that helps to maintain stemness, during steady state or stress conditions. HSCs divide to produce stem and progenitor cells. Recent research has suggested that daughter cells fated to perform as progenitors through differentiation exhibit high mitochondrial activity with increased mitochondrial numbers, high membrane potential and utilize oxidative phosphorylation to produce more ROS, while daughter cells that receive low mitochondrial activity fate to perform as stem cells through self-renewal decisions. Furthermore, pharmacological modulation of mitochondrial activity using uncoupling agents or mitophagy can lead to increased stem cell self-renewal decisions.