Jason T. Ross

Identification of the haematopoietic stem cell niche and control of the niche size.

Haematopoietic stem cells (HSCs) are a subset of bone marrow cells that are capable of self-renewal and of forming all types of blood cells (multi-potential). However, the HSC ‘niche’—the in vivo regulatory microenvironment where HSCs reside—and the mechanisms involved in controlling the number of adult HSCs remain largely unknown. The bone morphogenetic protein (BMP) signal has an essential role in inducing haematopoietic tissue during embryogenesis. We investigated the roles of the BMP signalling pathway in regulating adult HSC development in vivo by analysing mutant mice with conditional inactivation of BMP receptor type IA (BMPRIA). Here we show that an increase in the number of spindle-shaped N-cadherin+CD45- osteoblastic (SNO) cells correlates with an increase in the number of HSCs. The long-term HSCs are found attached to SNO cells. Two adherens junction molecules, N-cadherin and β-catenin, are asymmetrically localized between the SNO cells and the long-term HSCs. We conclude that SNO cells lining the bone surface function as a key component of the niche to support HSCs, and that BMP signalling through BMPRIA controls the number of HSCs by regulating niche size.

BMP signaling inhibits intestinal stem cell self-renewal through suppression of Wnt-β-catenin signaling

In humans, mutations in BMPR1A, SMAD4 and PTEN are responsible for juvenile polyposis syndrome, juvenile intestinal polyposis and Cowden disease, respectively. The development of polyposis is a common feature of these diseases, suggesting that there is an association between BMP and PTEN pathways. The mechanistic link between BMP and PTEN pathways and the related etiology of juvenile polyposis is unresolved. Here we show that conditional inactivation of Bmpr1a in mice disturbs homeostasis of intestinal epithelial regeneration with an expansion of the stem and progenitor cell populations, eventually leading to intestinal polyposis resembling human juvenile polyposis syndrome. We show that BMP signaling suppresses Wnt signaling to ensure a balanced control of stem cell self-renewal. Mechanistically, PTEN, through phosphatidylinosital-3 kinase–Akt, mediates the convergence of the BMP and Wnt pathways on control of β-catenin. Thus, BMP signaling may control the duplication of intestinal stem cells, thereby preventing crypt fission and the subsequent increase in crypt number.

PTEN maintains haematopoietic stem cells and acts in lineage choice and leukaemia prevention

Haematopoietic stem cells (HSCs) must achieve a balance between quiescence and activation that fulfils immediate demands for haematopoiesis without compromising long-term stem cell maintenance, yet little is known about the molecular events governing this balance. Phosphatase and tensin homologue (PTEN) functions as a negative regulator of the phosphatidylinositol-3-OH kinase (PI(3)K)–Akt pathway, which has crucial roles in cell proliferation, survival, differentiation and migration. Here we show that inactivation of PTEN in bone marrow HSCs causes their short-term expansion, but long-term decline, primarily owing to an enhanced level of HSC activation. PTEN-deficient HSCs engraft normally in recipient mice, but have an impaired ability to sustain haematopoietic reconstitution, reflecting the dysregulation of their cell cycle and decreased retention in the bone marrow niche. Mice with PTEN-mutant bone marrow also have an increased representation of myeloid and T-lymphoid lineages and develop myeloproliferative disorder (MPD). Notably, the cell populations that expand in PTEN mutants match those that become dominant in the acute myeloid/lymphoid leukaemia that develops in the later stages of MPD. Thus, PTEN has essential roles in restricting the activation of HSCs, in lineage fate determination, and in the prevention of leukaemogenesis.

N-cadherin expression level distinguishes reserved versus primed states of hematopoietic stem cells.

Osteoblasts expressing the homophilic adhesion molecule N-cadherin form a hematopoietic stem cell (HSC) niche. Therefore, we examined how N-cadherin expression in HSCs relates to their function. We found that bone marrow (BM) cells highly expressing N-cadherin (N-cadherin(hi)) are not stem cells, being largely devoid of a Lineage(-)Sca1(+)cKit(+) population and unable to reconstitute hematopoietic lineages in irradiated recipient mice. Instead, long-term HSCs form distinct populations expressing N-cadherin at intermediate (N-cadherin(int)) or low (N-cadherin(lo)) levels. The minority N-cadherin(lo) population can robustly reconstitute the hematopoietic system, express genes that may prime them to mobilize, and predominate among HSCs mobilized from BM to spleen. The larger N-cadherin(int) population performs poorly in reconstitution assays when freshly isolated but improves in response to overnight in vitro culture. Their expression profile and lower cell-cycle entry rate suggest N-cadherin(int) cells are being held in reserve. Thus, differential N-cadherin expression reflects functional distinctions between two HSC subpopulations.

FGF signaling facilitates postinjury recovery of mouse hematopoietic system.

Previous studies have shown that fibroblast growth factor (FGF) signaling promotes hematopoietic stem and progenitor cell (HSPC) expansion in vitro. However, it is unknown whether FGF promotes HSPC expansion in vivo. Here we examined FGF receptor 1 (FGFR1) expression and investigated its in vivo function in HSPCs. Conditional knockout (CKO) of Fgfr1 did not affect phenotypical number of HSPCs and homeostatic hematopoiesis, but led to a reduced engraftment only in the secondary transplantation. When treated with 5-fluorouracil (5FU), the Fgfr1 CKO mice showed defects in both proliferation and subsequent mobilization of HSPCs. We identified megakaryocytes (Mks) as a major resource for FGF production, and further discovered a novel mechanism by which Mks underwent FGF-FGFR signaling dependent expansion to accelerate rapid FGF production under stress. Within HSPCs, we observed an up-regulation of nuclear factor κB and CXCR4, a receptor for the chemoattractant SDF-1, in response to bone marrow damage only in control but not in Fgfr1 CKO model, accounting for the corresponding defects in proliferation and migration of HSPCs. This study provides the first in vivo evidence that FGF signaling facilitates postinjury recovery of the mouse hematopoietic system by promoting proliferation and facilitating mobilization of HSPCs.

Recent advances in understanding extrinsic control of hematopoietic stem cell fate

Purpose of reviewHematopoietic stem cells are responsible for generating all types of blood cells. As such they are under a high degree of regulation, both internal and external. With the identification of the hematopoietic stem cell niche, there has been increased investigation into extrinsic regulation of hematopoietic stem cells with emphasis on developmental signaling pathways. The purpose of this review is to discuss recent advances and findings in how these different pathways interact to achieve a balanced control of these stem cells. Recent findingsStudies indicating the importance of pathways such as Wnt, Notch, bone morphogenic protein, Sonic hedgehog and fibroblast growth factor in controlling the fate of hematopoietic stem cells are the most significant recent findings. These pathways have been implicated to affect various aspects of hematopoietic stem cells, including self-renewal, proliferation and lineage determination. Equally important are studies showing, by inactivation of various pathway components, the complexity of signal integration at the stem cell level in vivo. Additionally, some recent reports have provided evidence for direct interaction or cross-talk between different signaling pathways in this regulation. SummaryWe review highlights of the recent advances made toward resolving the mechanisms of external regulation of hematopoietic stem cells. Understanding the interaction of different signaling pathways in the context of the hematopoietic stem cell niche is essential for increasing their therapeutic potential.

HSC mobilization: new incites and insights

In this issue of Blood, Ramirez and colleagues unveil a new small molecule antagonist of VCAM-1/VLA-4 signaling, BIO5192, that rapidly mobilizes HSPCs. Also in this issue, Christopher and colleagues show that suppression of CXCL12 (SDF-1) production by osteoblasts is essential for cytokine-mediated mobilization of HSPCs.

PTEN in Hematopoietic and Intestinal Stem Cells and Cancer

In this chapter, we discuss the roles of the tumor suppressor PTEN in regulating stem cells of the hematopoietic and intestinal systems as well as its contributions to carcinogenesis in these tissues. Stem cells in continually renewing tissues must balance the necessity to maintain their respective tissue with the requirement to preserve the stem cell pool throughout adult life. Hematopoietic stem cells (HSCs) are tasked with sustaining the various cell types of the blood while intestinal stem cells must continually regenerate the gut epithelium. PTEN, a dual-specificity phosphatase able to target proteins and lipids, is the sole antagonist of the PI3K/AKT signaling pathway. PI3K/AKT signaling is often activated by growth factors and typically results in the stimulation of cellular outcomes such as proliferation, inhibition of apoptosis, and migration. Functional studies of PTEN loss in animal models have indicated a role for PTEN as a protective agent for stem cells that promote quiescence, as PTEN-deficient animals exhibit overproliferative stem and progenitor cells and are prone to proliferative disorders and cancer development. However, from these and other studies including accumulated clinical evidence, it is not likely that PTEN acts alone to stimulate malignant transformation. Indeed, HSCs in PTEN-deficient animals become exhausted and unable to sustain a healthy hematopoietic system. Rather, PTEN appears to function as a restrictive factor that prevents unregulated proliferation, which leaves stem and progenitor cells susceptible to additional mutations/deregulation, such as in Wnt/β-catenin signaling, that results in overt cancer. Additional study into PTEN/PI3K/AKT signaling should provide further insight into self-renewal mechanisms of adult stem cells that may aid in distinguishing normal stem cells from their cancer-initiating counterparts.