Anti–VLA4/VCAM-1—Induced Mobilization Requires Cooperative Signaling Through the kit/mkit Ligand Pathway

Periosteal skeletal stem cells can migrate into the bone marrow and support hematopoiesis after injury

Skeletal stem cells have been isolated from various tissues, including periosteum and bone marrow, where they exhibit key functions in bone biology and hematopoiesis, respectively. The role of periosteal skeletal stem cells in bone regeneration and healing has been extensively studied, but their ability to contribute to the bone marrow stroma is still under debate. In the present study, we characterized a whole bone transplantation model that mimics the initial bone marrow necrosis and fatty infiltration seen after injury. Using this model and a lineage tracing approach, we observed the migration of periosteal skeletal stem cells into the bone marrow after transplantation. Once in the bone marrow, periosteal skeletal stem cells are phenotypically and functionally reprogrammed into bone marrow mesenchymal stem cells that express high levels of hematopoietic stem cell niche factors such as Cxcl12 and Kitl. In addition, using in-vitro and in-vivo approaches, we found that periosteal skeletal stem cells are more resistant to acute stress than bone marrow mesenchymal stem cells. These results highlight the plasticity of periosteal skeletal stem cells and their potential role in bone marrow regeneration after bone marrow injury.

Integrins, anchors and signal transducers of hematopoietic stem cells during development and in adulthood

Hematopoietic stem cells (HSCs), the apex of the hierarchically organized blood cell production system, are generated in the yolk sac, aorta-gonad-mesonephros region and placenta of the developing embryo. To maintain life-long hematopoiesis, HSCs emigrate from their site of origin and seed in distinct microenvironments, called niches, of fetal liver and bone marrow where they receive supportive signals for self-renewal, expansion and production of hematopoietic progenitor cells (HPCs), which in turn orchestrate the production of the hematopoietic effector cells. The interactions of hematopoietic stem and progenitor cells (HSPCs) with niche components are to a large part mediated by the integrin superfamily of adhesion molecules. Here, we summarize the current knowledge regarding the functional properties of integrins and their activators, Talin-1 and Kindlin-3, for HSPC generation, function and fate decisions during development and in adulthood. In addition, we discuss integrin-mediated mechanosensing for HSC-niche interactions, ex vivo protocols aimed at expanding HSCs for therapeutic use, and recent approaches targeting the integrin-mediated adhesion in leukemia-inducing HSCs in their protecting, malignant niches.

VCAM1 confers innate immune tolerance on haematopoietic and leukaemic stem cells

Haematopoietic stem cells (HSCs) home to the bone marrow via, in part, interactions with vascular cell adhesion molecule-1 (VCAM1)^1-3. Once in the bone marrow, HSCs are vetted by perivascular phagocytes to ensure their self-integrity. Here we show that VCAM1 is also expressed on healthy HSCs and upregulated on leukaemic stem cells (LSCs), where it serves as a quality-control checkpoint for entry into bone marrow by providing 'don't-eat-me' stamping in the context of major histocompatibility complex class-I (MHC-I) presentation. Although haplotype-mismatched HSCs can engraft, Vcam1 deletion, in the setting of haplotype mismatch, leads to impaired haematopoietic recovery due to HSC clearance by mononuclear phagocytes. Mechanistically, VCAM1 'don't-eat-me' activity is regulated by β2-microglobulin MHC presentation on HSCs and paired Ig-like receptor-B (PIR-B) on phagocytes. VCAM1 is also used by cancer cells to escape immune detection as its expression is upregulated in multiple cancers, including acute myeloid leukaemia (AML), where high expression associates with poor prognosis. In AML, VCAM1 promotes disease progression, whereas VCAM1 inhibition or deletion reduces leukaemia burden and extends survival. These results suggest that VCAM1 engagement regulates a critical immune-checkpoint gate in the bone marrow, and offers an alternative strategy to eliminate cancer cells via modulation of the innate immune tolerance.

Membrane-bound SCF and VCAM-1 synergistically regulate the morphology of hematopoietic stem cells

Membrane-bound factors expressed by niche stromal cells constitute a unique class of localized cues and regulate the long-term functions of adult stem cells, yet little is known about the underlying mechanisms. Here, we used a supported lipid bilayer (SLB) to recapitulate the membrane-bound interactions between hematopoietic stem cells (HSCs) and niche stromal cells. HSCs cluster membrane-bound stem cell factor (mSCF) at the HSC-SLB interface. They further form a polarized morphology with aggregated mSCF under a large protrusion through a synergy with VCAM-1 on the bilayer, which drastically enhances HSC adhesion. These features are unique to mSCF and HSCs among the factors and hematopoietic populations we examined. The mSCF-VCAM-1 synergy and the polarized HSC morphology require PI3K signaling and cytoskeletal reorganization. The synergy also enhances nuclear retention of FOXO3a, a crucial factor for HSC maintenance, and minimizes its loss induced by soluble SCF. Our work thus reveals a unique role and signaling mechanism of membrane-bound factors in regulating stem cell morphology and function.

Osteopontin is An Important Regulative Component of the Fetal Bone Marrow Hematopoietic Stem Cell Niche

Osteopontin (OPN) is an important component in both bone and blood regulation, functioning as a bridge between the two. Previously, thrombin-cleaved osteopontin (trOPN), the dominant form of OPN in adult bone marrow (BM), was demonstrated to be a critical negative regulator of adult hematopoietic stem cells (HSC) via interactions with α₄β₁ and α₉β₁ integrins. We now demonstrate OPN is also required for fetal hematopoiesis in maintaining the HSC and progenitor pool in fetal BM. Specifically, we showed that trOPN is highly expressed in fetal BM and its receptors, α₄β₁ and α₉β₁ integrins, are both highly expressed and endogenously activated on fetal BM HSC and progenitors. Notably, the endogenous activation of integrins expressed by HSC was attributed to high concentrations of three divalent metal cations, Ca²⁺, Mg²⁺ and Mn²⁺, which were highly prevalent in developing fetal BM. In contrast, minimal levels of OPN were detected in fetal liver, and α₄β₁ and α₉β₁ integrins expressed by fetal liver HSC were not in the activated state, thereby permitting the massive expansion of HSC and progenitors required during early fetal hematopoiesis. Consistent with these results, no differences in the number or composition of hematopoietic cells in the liver of fetal OPN^-/- mice were detected, but significant increases in the hematopoietic progenitor pool in fetal BM as well as an increase in the BM HSC pool following birth and into adulthood were observed. Together, the data demonstrates OPN is a necessary negative regulator of fetal and neonatal BM progenitors and HSC, and it exhibits preserved regulatory roles during early development, adulthood and ageing.

Phc2 controls hematopoietic stem and progenitor cell mobilization from bone marrow by repressing Vcam1 expression

The timely mobilization of hematopoietic stem and progenitor cells (HSPCs) is essential for maintaining hematopoietic and tissue leukocyte homeostasis. Understanding how HSPCs migrate between bone marrow (BM) and peripheral tissues is of great significance in the clinical setting, where therapeutic strategies for modulating their migration capacity determine the clinical outcome. Here, we identify an epigenetic regulator, Phc2, as a critical modulator of HSPC trafficking. The genetic ablation of Phc2 in mice causes a severe defect in HSPC mobilization through the derepression of Vcam1 in bone marrow stromal cells (BMSCs), ultimately leading to a systemic immunodeficiency. Moreover, the pharmacological inhibition of VCAM-1 in Phc2-deficient mice reverses the symptoms. We further determine that Phc2-dependent Vcam1 repression in BMSCs is mediated by the epigenetic regulation of H3K27me3 and H2AK119ub. Together, our data demonstrate a cell-extrinsic role for Phc2 in controlling the mobilization of HSPCs by finely tuning their bone marrow niche.

Mobilization of hematopoietic stem and progenitor cells (HSPCs) into the circulation is essential for maintaining homeostasis. Here, the authors show that Phc2 in bone marrow stromal cells represses the cell adhesion molecule Vcam1 and facilitates mobilization of HSPCs through regulation of epigenetic marks.

Ephrin ligands and Eph receptors contribution to hematopoiesis

Hematopoietic stem and progenitor cells reside predominantly in the bone marrow. They supply billions of mature blood cells every day during life through maturation into multilineage progenitors and self-renewal. Newly produced mature cells serve to replenish the pool of circulating blood cells at the end of their life-span. These mature blood cells and a few hematopoietic progenitors normally exit the bone marrow through the sinusoidal vessels, a specialized venous vascular system that spreads throughout the bone marrow. Many signals regulate the coordinated mobilization of hematopoietic cells from the bone marrow to the circulation. In this review, we present recent advances on hematopoiesis and hematopoietic cell mobilization with a focus on the role of Ephrin ligands and their Eph receptors. These constitute a large family of transmembrane ligands and receptors that play critical roles in development and postnatally. New insights point to distinct roles of ephrin and Eph in different aspects of hematopoiesis.

A tale of two niches: differential functions for VCAM-1 in satellite cells under basal and injured conditions

Cell-cell adhesion molecules play key roles in maintaining quiescence or promoting activation of various stem cells in their niche. Muscle stem cells called satellite cells (SC) are critical for skeletal muscle regeneration after injury, but little is known about the role of adhesion molecules in regulating the behavior of these stem cells. Vascular cell adhesion molecule-1 (VCAM-1) is a cell-cell adhesion protein expressed on quiescent and activated SC whose function is unknown in this context. We deleted Vcam1 from SC using an inducible Cre recombinase in young mice. In the injured niche, Vcam1^-/- SC underwent premature lineage progression to a more differentiated state as well as apoptosis leading to a transient delay in myofiber growth during regeneration. Apoptosis was also increased in Vcam1^-/- SC in vitro concomitant with decreased levels of phosphorylated Akt, a prosurvival signal activated by VCAM-1 signaling in other cell types. During muscle regeneration, we observed an influx of immune cells expressing α4 integrin, a component of the major, high-affinity VCAM-1 ligand, α4β1 integrin. Furthermore, α4 integrin mRNA and protein were induced in SC 2 days after injury. These results suggest that SC interact with other SC as well as immune cells through α4β1 integrin in the injured niche to promote expansion of SC. In the uninjured niche, multiple cell types also expressed α4 integrin. However, only basal fusion of Vcam1^-/- SC with myofibers was decreased, contributing to decreased myofiber growth. These studies define differential roles for VCAM-1 in SC depending on the state of their niche.

Complexity of bone marrow hematopoietic stem cell niche

Hematopoietic stem cells (HSCs) that produce a variety of hematopoietic lineage cells throughout the life reside in specialized microenvironment called "niche" in the bone marrow (BM) where they are tightly regulated. With the recent advances in experimental technologies enabling the selective deletion of molecules, various types of cells in the BM have been proposed to contribute to HSC niche activity. Among these are stromal cells closely associated with the vasculature. In this review, we provide an overview of recent advances in HSC niche research, and focus on the studies describing the functional roles of perivascular cells for HSC maintenance and mobilization. Not only for physiologic state, we also discuss the recent evidences suggesting the importance of microenvironment for emergence of malignant hematopoietic diseases.

LCN2 overexpression in bone enhances the hematopoietic compartment via modulation of the bone marrow microenvironment

Lipocalin-2 (LCN2) is a member of the lipocalin family whose expression is modulated in several conditions, including cell differentiation, innate immunity, stress, and cancer. Although it is known that it is expressed in bone, its function in this tissue remains poorly studied. To this end, we took advantage of transgenic mice lines that expressed LCN2 driven by a bone specific type I collagen (LCN2-Tg). In the bone marrow (BM) of LCN2-Tg mice we observed an increased number of phenotypically long-term hematopoietic stem cells (LT-HSC) that also displayed a higher proliferation rate compared to wild-type controls (Wt). Furthermore, hematopoietic progenitor cells, obtained from LCN2-Tg BM showed an increased clonogenic capacity compared to those obtained from LCN2-Tg spleen, a higher concentration of serum erythropoietin and a higher number of mature erythrocytes in the peripheral blood of old LCN2-Tg animals compared to aged-matched wt. The findings of a combined increase in the BM of the LCN2-Tg mice of SDF-1, SCF, and TIMP-1 levels along with the reduction of both MMP-9 activity and cathepsin K concentration may explain the observed effects on the HSC compartment. This study shows that LCN2 overexpression in bones modifies the BM microenvironment via modulation of the expression of key secreted factors and cytokines, which in turn regulate the HSC niche behavior enhancing both HSC homing in young mice and erythrocytes production in older mice.

The spleen microenvironment influences disease transformation in a mouse model of KITD816V-dependent myeloproliferative neoplasm

Activating mutations leading to ligand-independent signaling of the stem cell factor receptor KIT are associated with several hematopoietic malignancies. One of the most common alterations is the D816V mutation. In this study, we characterized mice, which conditionally express the humanized KIT^D816V receptor in the adult hematopoietic system to determine the pathological consequences of unrestrained KIT signaling during blood cell development. We found that KIT^D816V mutant animals acquired a myeloproliferative neoplasm similar to polycythemia vera, marked by a massive increase in red blood cells and severe splenomegaly caused by excessive extramedullary erythropoiesis. Moreover, we found mobilization of stem cells from bone marrow to the spleen. Splenectomy prior to KIT^D816V induction prevented expansion of red blood cells, but rapidly lead to a state of aplastic anemia and bone marrow fibrosis, reminiscent of post polycythemic myeloid metaplasia, the spent phase of polycythemia vera. Our results show that the extramedullary hematopoietic niche microenvironment significantly influences disease outcome in KIT^D816V mutant mice, turning this model a valuable tool for studying the interplay between functionally abnormal hematopoietic cells and their microenvironment during development of polycythemia vera-like disease and myelofibrosis.

Harnessing bone marrow resident regulatory T cells to improve allogeneic stem cell transplant outcomes

Regulatory T cells (Treg) are a suppressive T cell population which play a crucial role in the establishment of tolerance after stem cell transplantation (SCT) by controlling the effector T cell responses that drive acute and chronic GVHD. The BM compartment is enriched in a highly suppressive, activated/memory autophagy-dependent Treg population, which contributes to the HSC engraftment and the control of GVHD. G-CSF administration releases Treg from the BM through disruption of the CXCR4/SDF-1 axis and further improves Treg survival following SCT through the induction of autophagy. However, AMD3100 is more efficacious in mobilizing these Treg highlighting the potential for optimized mobilization regimes to produce more tolerogenic grafts. Notably, the disruption of adhesive interaction between integrins and their ligands contributes to HSC mobilization and may be relevant for BM Treg. Importantly, the Tregs in the BM niche contribute to maintenance of the HSC niche and appear required for optimal control of GVHD post-transplant. Although poorly studied, the BM Treg appear phenotypically and functionally unique to Treg in the periphery. Understanding the requirements for maintaining the enrichment, function and survival of BM Treg needs to be further investigated to improve therapeutic strategies and promote tolerance after SCT.

Enhanced Integrin α4β1-Mediated Adhesion Contributes to a Mobilization Defect of Endothelial Progenitor Cells in Diabetes

Diabetes is associated with a deficit of circulating endothelial progenitor cells (EPCs), which has been attributed to their defective mobilization from the bone marrow. The basis for this mobilization defect is not completely understood, and we sought to determine if hyperglycemic conditions enhanced EPC adhesion. We found that culturing EPCs in high glucose media increased adhesion to bone marrow stromal cells. This enhanced adhesion was associated with decreased expression of protein kinase A regulatory subunit 1β (PRKAR1β), activation of protein kinase A (PKA), and phosphorylation of α4-integrin on serine 988. This potentiated adhesion was reversed by treatment with a PKA inhibitor, overexpression of PRKAR1β, or expression of a phosphorylation-defective α4-integrin variant (α4[S988A]). Using a model of type 1 diabetes, we showed that α4(S988A)-expressing mice have more circulating EPCs than their wild-type counterparts. Moreover, diabetic α4(S988A) mice demonstrate enhanced revascularization after hind limb ischemia. Thus, we have identified a novel signaling mechanism activating PKA in diabetes (downregulation of an inhibitory regulatory subunit) that leads to deficits of circulating EPCs and impaired vascular repair, which could be reversed by α4-integrin mutation.

To condition or not to condition-That is the question: The evolution of nonmyeloablative conditions for transplantation

In 1985, Eugene Cronkite and his colleagues published, in Experimental Hematology, data indicating that five consecutive "transfusions" of large numbers of marrow cells significantly increase the number of donor-derived cells detected by day 10 of a spleen colony-forming assay, the most primitive hematopoietic cells detectable at that time, present in the host for as long as 2 months posttransfusion (Cronkite EP, Bullis JE, Brecher G. Marrow transfusions increase pluripotent stem cells in normal hosts. Exp Hematol 1985;13:802-805). These data provided the first evidence that donor hematopoietic stem cells (HSCs) may persist in vivo for some time in recipients when transfused and not transplanted, that is, not subjected to treatments that deplete their marrow niches of endogenous HSCs. The limited technology available at the time prevented Dr. Cronkite from pursuing this observation into the development of nonmyeloablated transplantation procedures, and his experiment, as well as the term bone marrow transfusion, has since been long forgotten. In recent years, the scientific need to clarify HSC functions in nonstressed hosts and the clinical need to develop transplantation procedures with levels of morbidity/mortality acceptable for curing inherited hematologic disorders have inspired the search for nonmyeloablative transplantation procedures, including methods that "outcompete" endogenous host HSCs such as those pioneered by Dr. Cronkite's experiments using high transfusion doses. This review describes the technical progress made since Dr. Cronkite's insightful work, which has finally found its path to the clinic.

Therapeutic targeting and rapid mobilization of endosteal HSC using a small molecule integrin antagonist

The inherent disadvantages of using granulocyte colony-stimulating factor (G-CSF) for hematopoietic stem cell (HSC) mobilization have driven efforts to identify alternate strategies based on single doses of small molecules. Here, we show targeting α₉β₁/α₄β₁ integrins with a single dose of a small molecule antagonist (BOP (N-(benzenesulfonyl)-L-prolyl-L-O-(1-pyrrolidinylcarbonyl)tyrosine)) rapidly mobilizes long-term multi-lineage reconstituting HSC. Synergistic engraftment augmentation is observed when BOP is co-administered with AMD3100. Impressively, HSC in equal volumes of peripheral blood (PB) mobilized with this combination effectively out-competes PB mobilized with G-CSF. The enhanced mobilization observed using BOP and AMD3100 is recapitulated in a humanized NODSCIDIL2Rγ^−/− model, demonstrated by a significant increase in PB CD34⁺ cells. Using a related fluorescent analogue of BOP (R-BC154), we show that this class of antagonists preferentially bind human and mouse HSC and progenitors via endogenously primed/activated α₉β₁/α₄β₁ within the endosteal niche. These results support using dual α₉β₁/α₄β₁ inhibitors as effective, rapid and transient mobilization agents with promising clinical applications.

Mobilizing haematopoietic stem cells to the peripheral blood has largely replaced bone marrow transplants as a strategy in the clinic. Here, Cao et al. report the use of an α₉β₁/α₄β₁ integrin antagonist to induce rapid mobilization of blood stem cells from the bone marrow in a humanized mouse model.

Regulation of long-term repopulating hematopoietic stem cells by EPCR/PAR1 signaling

The common developmental origin of endothelial and hematopoietic cells is manifested by coexpression of several cell surface receptors. Adult murine bone marrow (BM) long-term repopulating hematopoietic stem cells (LT-HSCs), endowed with the highest repopulation and self-renewal potential, express endothelial protein C receptor (EPCR), which is used as a marker to isolate them. EPCR/protease-activated receptor-1 (PAR1) signaling in endothelial cells has anticoagulant and anti-inflammatory roles, while thrombin/PAR1 signaling induces coagulation and inflammation. Recent studies define two new PAR1-mediated signaling cascades that regulate EPCR(+) LT-HSC BM retention and egress. EPCR/PAR1 signaling facilitates LT-HSC BM repopulation, retention, survival, and chemotherapy resistance by restricting nitric oxide (NO) production, maintaining NO(low) LT-HSC BM retention with increased VLA4 expression, affinity, and adhesion. Conversely, acute stress and clinical mobilization upregulate thrombin generation and activate different PAR1 signaling that overcomes BM EPCR(+) LT-HSC retention, inducing their recruitment to the bloodstream. Thrombin/PAR1 signaling induces NO generation, TACE-mediated EPCR shedding, and upregulation of CXCR4 and PAR1, leading to CXCL12-mediated stem and progenitor cell mobilization. This review discusses new roles for factors traditionally viewed as coagulation related, which independently act in the BM to regulate PAR1 signaling in bone- and blood-forming progenitor cells, navigating their fate by controlling NO production.

PAR1 signaling regulates the retention and recruitment of EPCR-expressing bone marrow hematopoietic stem cells

Retention of long-term repopulating hematopoietic stem cells (LT-HSCs) in the bone marrow is essential for hematopoiesis and for protection from myelotoxic injury. We report that signaling cascades that are traditionally viewed as coagulation related also control retention of endothelial protein C receptor-positive (EPCR(+)) LT-HSCs in the bone marrow and their recruitment to the blood via two pathways mediated by protease activated receptor 1 (PAR1). Thrombin-PAR1 signaling induces nitric oxide (NO) production, leading to EPCR shedding mediated by tumor necrosis factor-α-converting enzyme (TACE), enhanced CXCL12-CXCR4-induced motility and rapid stem and progenitor cell mobilization. Conversely, bone marrow blood vessels provide a microenvironment enriched with activated protein C (aPC) that retains EPCR(+) LT-HSCs by limiting NO generation, reducing Cdc42 activity and enhancing integrin VLA4 affinity and adhesion. Inhibition of NO production by aPC-EPCR-PAR1 signaling reduces progenitor cell egress from the bone marrow, increases retention of bone marrow NO(low) EPCR(+) LT-HSCs and protects mice from chemotherapy-induced hematological failure and death. Our study reveals new roles for PAR1 and EPCR in controlling NO production to balance maintenance and recruitment of bone marrow EPCR(+) LT-HSCs, with potential clinical relevance for stem cell transplantation.

c-kit(+) cells: the tell-tale heart of cardiac regeneration?

Cardiovascular disease is the leading cause of morbidity and mortality in the developed world. Although ongoing therapeutic strategies ameliorate symptoms and prolong life for patients with cardiovascular diseases, they do not solve the critical issue related to the loss of cardiac tissue. Accordingly, stem/progenitor cell therapy has emerged as a paramount approach for cardiac repair and regeneration. In this regard, c-kit(+) cells have animated much interest and controversy. These cells are self-renewing, clonogenic, and multipotent and display a noteworthy potential to differentiate into all cardiovascular lineages. However, their functional contribution to cardiomyocyte turnover is one of the centrally debated issues concerning their regenerative potential. Regardless, plentiful preclinical and clinical studies have been conducted which provide evidence for the capacity of c-kit(+) cells to improve cardiac function. The purpose of this review is to give a comprehensive, impartial, critical description and evaluation of the literature on c-kit(+) cells from bench to bedside in order to address their true potential, benefits and controversies.

Human adult stem cells maintain a constant phenotype profile irrespective of their origin, Basal media, and long term cultures

The study aims to identify the phenotypic marker expressions of different human adult stem cells derived from, namely, bone marrow, subcutaneous fat, and omentum fat, cultured in different media, namely, DMEM-Low Glucose, Alpha-MEM, DMEM-F12 and DMEM-KO and under long term culture conditions (>P20). We characterized immunophenotype by using various hematopoietic, mesenchymal, endothelial markers, and cell adhesion molecules in the long term cultures (Passages-P1, P3, P5, P9, P12, P15, and P20.) Interestingly, data revealed similar marker expression profiles irrespective of source, basal media, and extensive culturing. This demonstrates that all adult stem cell sources mentioned in this study share similar phenotypic marker and all media seem appropriate for culturing these sources. However, a disparity was observed in the markers such as CD49d, CD54, CD117, CD29, and CD106, thereby warranting further research on these markers. Besides the aforesaid objective, it is understood from the study that immunophenotyping acts as a valuable tool to identify inherent property of each cell, thereby leading to a valuable cell based therapy.

Concise review: Sowing the seeds of a fruitful harvest: hematopoietic stem cell mobilization

Hematopoietic stem cell transplantation is the only curative option for a number of malignant and nonmalignant diseases. As the use of hematopoietic transplant has expanded, so too has the source of stem and progenitor cells. The predominate source of stem and progenitors today, particularly in settings of autologous transplantation, is mobilized peripheral blood. This review will highlight the historical advances which led to the widespread use of peripheral blood stem cells for transplantation, with a look toward future enhancements to mobilization strategies.

New insights in the mobilization of hematopoietic stem cells in lymphoma and multiple myeloma patients

Following chemotherapy and/or the administration of growth factors, such as granulocyte-colony stimulated factor (G-CSF), hematopoietic stem cells (HSC) mobilize from bone marrow to peripheral blood. This review aims to systematically present the structure of the HSC "niche" and elucidate the mechanisms of their mobilization. However, this field is constantly evolving and new pathways and molecules have been shown to contribute to the mobilization process. Understanding the importance and the possible primary pathophysiologic role of each pathway is rather difficult, since they share various overlapping components. The primary initiating event for the mobilization of HSC is chemotherapy-induced endogenous G-CSF production or exogenous G-CSF administration. G-CSF induces proliferation and expansion of the myelomonocytic series, which leads to proteolytic enzyme activation. These enzymes result in disruption of various receptor-ligand bonds, which leads to the disanchorage of HSC from the bone marrow stroma. In everyday clinical practice, CXC chemokine receptor-4 (CXCR4) antagonists are now being used as mobilization agents in order to improve HSC collection. Furthermore, based on the proposed mechanisms of HSC mobilization, novel mobilizing agents have been developed and are currently evaluated in preclinical and clinical studies.

Niche anchorage and signaling through membrane-bound Kit-ligand/c-kit receptor are kinase independent and imatinib insensitive

Kit ligand (KitL) and its tyrosine kinase receptor c-kit are critical for germ cells, melanocytes, mastocytes, and hematopoietic stem cells. Alternative splicing of KitL generates membrane-bound KitL (mb-KitL) or soluble KitL, providing survival or cell migration, respectively. Here we analyzed whether c-kit can function both as an adhesion and signaling receptor to mb-KitL presented by the environmental niche. At contacts between fibroblasts and MC/9 mast cells, mb-KitL, and c-kit formed ligand/receptor clusters that formed stable complexes, which resisted dissociation by c-kit blocking mAbs and provided cell anchorage under physiological shear stresses. Clusters recruited tyrosine-phosphorylated proteins and induced spatially restricted F-actin polymerization. Mutational analysis of c-kit demonstrated kinase-independent mb-KitL/c-kit clustering, anchorage, F-actin polymerization, and Tyr567-dependent cluster phosphorylation. Kinase inhibition of c-kit by imatinib reduced cluster coalescence, but allowed cluster phosphorylation and F-actin polymerization, which required PI3K recruitment and a newly identified juxtamembrane residue. Synergies between integrin and c-kit-mediated spreading and adhesion of MC/9 cells were studied in vitro on immobilized-KitL/fibronectin surfaces. While c-kit blocking antibodies prevented spreading, imatinib blocked spreading induced by soluble- but not immobilized KitL. Thus, "mechanical" activation of c-kit provides signaling, niche-anchorage, and synergies with integrin-mediated adhesion, which is independent of kinase function and resistant to c-kit kinase inhibitors.-

Small molecules targeting in vivo tissue regeneration

The field of regenerative medicine has boomed in recent years thanks to milestone discoveries in stem cell biology and tissue engineering, which has been driving paradigm shifts in the pharmacotherapy of degenerative and ischemic diseases. Small molecule-mediated replenishment of lost and/or dysfunctional tissue in vivo, however, is still in its infancy due to a limited understanding of mechanisms that control such endogenous processes of tissue homeostasis or regeneration. Here, we discuss current progress using small molecules targeting in vivo aspects of regeneration, including adult stem cells, stem cell niches, and mechanisms of homing, mobilization, and engraftment as well as somatic cell proliferation. Many of these compounds derived from both knowledge-based design and screening campaigns, illustrating the feasibility of translating in vitro discovery to in vivo regeneration. These early examples of drug-mediated in vivo regeneration provide a glimpse of the future directions of in vivo regenerative medicine approaches.

Mechanobiology of bone marrow stem cells: from myosin-II forces to compliance of matrix and nucleus in cell forms and fates

Adult stem cells and progenitors are of great interest for their clinical application as well as their potential to reveal deep sensitivities to microenvironmental factors. The bone marrow is a niche for at least two types of stem cells, and the prototype is the hematopoietic stem cell/progenitors (HSC/Ps), which have saved many thousands of patients for several decades now. In bone marrow, HSC/Ps interact functionally with marrow stromal cells that are often referred to as mesenchymal stem cells (MSCs) or derivatives thereof. Myosin and matrix elasticity greatly affect MSC function, and these mechanobiological factors are now being explored with HSC/Ps both in vitro and in vivo. Also emerging is a role for the nucleus as a mechanically sensitive organelle that is semi-permeable to transcription factors which are modified for nuclear entry by cytoplasmic mechanobiological pathways. Since therapies envisioned with induced pluripotent stem cells and embryonic stem cells generally involve in vitro commitment to an adult stem cell or progenitor, a very deep understanding of stem cell mechanobiology is essential to progress with these multi-potent cells.

LNGFR(+)THY-1(+)VCAM-1(hi+) cells reveal functionally distinct subpopulations in mesenchymal stem cells

Human mesenchymal stem cells (hMSCs), which conventionally are isolated based on their adherence to plastic, are heterogeneous and have poor growth and differentiation, limiting our ability to investigate their intrinsic characteristics. We report an improved prospective clonal isolation technique and reveal that the combination of three cell-surface markers (LNGFR, THY-1, and VCAM-1) allows for the selection of highly enriched clonogenic cells (one out of three isolated cells). Clonal characterization of LNGFR⁺THY-1⁺ cells demonstrated cellular heterogeneity among the clones. Rapidly expanding clones (RECs) exhibited robust multilineage differentiation and self-renewal potency, whereas the other clones tended to acquire cellular senescence via P16INK4a and exhibited frequent genomic errors. Furthermore, RECs exhibited unique expression of VCAM-1 and higher cellular motility compared with the other clones. The combination marker LNGFR⁺THY-1⁺VCAM-1^hi+ (LTV) can be used selectively to isolate the most potent and genetically stable MSCs.

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The LNGFR⁺THY-1⁺ population is significantly enriched for CFU-Fs in human bone marrow

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Rapidly expanding clones (RECs) exhibited stem-like characteristics

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Expression of VCAM-1 correlated with proliferation and migration ability

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The combination marker LNGFR⁺THY-1⁺VCAM-1^hi+ is useful for isolating multipotent MSCs

The soluble form of LR11 protein is a regulator of hypoxia-induced, urokinase-type plasminogen activator receptor (uPAR)-mediated adhesion of immature hematological cells

A key property of hematopoietic stem and progenitor cells (HSPCs) regarding differentiation from the self-renewing quiescent to the proliferating stage is their adhesion to the bone marrow (BM) niche. An important molecule involved in proliferation and pool size of HSPCs in the BM is the hypoxia-induced urokinase-type plasminogen activator receptor (uPAR). Here, we show that the soluble form (sLR11) of LR11 (also called SorLA or SORL1) modulates the uPAR-mediated attachment of HSPCs under hypoxic conditions. Immunohistochemical and mRNA expression analyses revealed that hypoxia increased LR11 expression in hematological c-Kit(+) Lin(-) cells. In U937 cells, hypoxia induced a transient rise in LR11 transcription, production of cellular protein, and release of sLR11. Attachment to stromal cells of c-Kit(+) Lin(-) cells of lr11(-/-) mice was reduced by hypoxia much more than of lr11(+/+) animals. sLR11 induced the adhesion of U937 and c-Kit(+) Lin(-) cells to stromal cells. Cell attachment was increased by sLR11 and reduced in the presence of anti-uPAR antibodies. Furthermore, the fraction of uPAR co-immunoprecipitated with LR11 in membrane extracts of U937 cells was increased by hypoxia. CoCl2, a chemical inducer of HIF-1α, enhanced the levels of LR11 and sLR11 in U937 cells. The decrease in hypoxia-induced attachment of HIF-1α-knockdown cells was largely prevented by exogenously added sLR11. Finally, hypoxia induced HIF-1α binding to a consensus binding site in the LR11 promoter. Thus, we conclude that sLR11 regulates the hypoxia-enhanced adhesion of HSPCs via an uPAR-mediated pathway that stabilizes the hematological pool size by controlling cell attachment to the BM niche.

CD34⁺/CD38⁻ acute myelogenous leukemia cells aberrantly express CD82 which regulates adhesion and survival of leukemia stem cells

To identify molecular targets in leukemia stem cells (LSCs), this study compared the protein expression profile of freshly isolated CD34(+) /CD38(-) cells with that of CD34(+) /CD38(+) counterparts from individuals with acute myelogenous leukemia (n = 2, AML) using isobaric tags for relative and absolute quantitation (iTRAQ). A total of 98 proteins were overexpressed, while six proteins were underexpressed in CD34(+) /CD38(-) AML cells compared with their CD34(+) /CD38(+) counterparts. Proteins overexpressed in CD34(+) /CD38(-) AML cells included a number of proteins involved in DNA repair, cell cycle arrest, gland differentiation, antiapoptosis, adhesion, and drug resistance. Aberrant expression of CD82, a family of adhesion molecules, in CD34(+) /CD38(-) AML cells was noted in additional clinical samples (n = 12) by flow cytometry. Importantly, down-regulation of CD82 in CD34(+) /CD38(-) AML cells by a short hairpin RNA (shRNA) inhibited adhesion to fibronectin via up-regulation of matrix metalloproteinases 9 (MMP9) and colony forming ability of these cells as assessed by transwell assay, real-time RT-PCR, and colony forming assay, respectively. Moreover, we found that down-regulation of CD82 in CD34(+) /CD38(-) AML cells by an shRNA significantly impaired engraftment of these cells in severely immunocompromised mice. Taken together, aberrant expression of CD82 might play a role in adhesion of LSCs to bone marrow microenvironment and survival of LSCs. CD82 could be an attractive molecular target to eradicate LSCs.

Embryonic development of hematopoietic stem cells: implications for clinical use

Hematopoietic stem cell (HSC) transplantation is an important treatment modality for hematological malignancies or to correct congenital immunodeficiency disorders. Several stem cell sources are currently applied clinically, with a recent increased application of umbilical cord blood. The low number of HSCs available, particularly in umbilical cord blood, is a limiting factor, and different lines of research are ongoing to circumvent this issue. In this review, we will describe the research strategies developed to expand adult HSCs in vitro and to generate new HSCs from pluripotent stem cell lines. We will also discuss the importance of studying the embryonic microenvironment since it allows both generation and extensive expansion of HSCs. Understanding the mechanisms that underlie HSC production, self-renewal and differentiation is necessary for the establishment of optimal in vitro HSC cultures, where a limitless and manipulatable resource of HSCs would be available for both clinical and fundamental research.

Reconstruction of hematopoietic inductive microenvironment after transplantation of VCAM-1-modified human umbilical cord blood stromal cells

The hematopoietic inductive microenvironment (HIM) is where hematopoietic stem/progenitor cells grow and develop. Hematopoietic stromal cells were the key components of the HIM. In our previous study, we had successfully cultured and isolated human cord blood–derived stromal cells (HUCBSCs) and demonstrated that they could secret hemopoietic growth factors such as GM-CSF, TPO, and SCF. However, it is still controversial whether HUCBSCs can be used for reconstruction of HIM. In this study, we first established a co-culture system of HUCBSCs and cord blood CD34⁺ cells and then determined that using HUCBSCs as the adherent layer had significantly more newly formed colonies of each hematopoietic lineage than the control group, indicating that HUCBSCs had the ability to promote the proliferation of hematopoietic stem cells/progenitor cells. Furthermore, the number of colonies was significantly higher in vascular cell adhesion molecule-1 (VCAM-1)-modified HUCBSCs, suggesting that the ability of HUCBSCs in promoting the proliferation of hematopoietic stem cells/progenitor cells was further enhanced after having been modified with VCAM-1. Next, HUCBSCs were infused into a radiation-damaged animal model, in which the recovery of hematopoiesis was observed. The results demonstrate that the transplanted HUCBSCs were “homed in” to bone marrow and played roles in promoting the recovery of irradiation-induced hematopoietic damage and repairing HIM. Compared with the control group, the HUCBSC group had significantly superior effectiveness in terms of the recovery time for hemogram and myelogram, CFU-F, CFU-GM, BFU-E, and CFU-Meg. Such differences were even more significant in VCAM-1-modified HUCBSCs group. We suggest that HUCBSCs are able to restore the functions of HIM and promote the recovery of radiation-induced hematopoietic damage. VCAM-1 plays an important role in supporting the repair of HIM damage.

Progenitor cell mobilization and recruitment: SDF-1, CXCR4, α4-integrin, and c-kit

Progenitor cell retention and release are largely governed by the binding of stromal-cell-derived factor 1 (SDF-1) to CXC chemokine receptor 4 (CXCR4) and by α4-integrin signaling. Both of these pathways are dependent on c-kit activity: the mobilization of progenitor cells in response to either CXCR4 antagonism or α4-integrin blockade is impaired by the loss of c-kit kinase activity; and c-kit-kinase inactivation blocks the retention of CXCR4-positive progenitor cells in the bone marrow. SDF-1/CXCR4 and α4-integrin signaling are also crucial for the retention of progenitor cells in the ischemic region, which may explain, at least in part, why clinical trials of progenitor cell therapy have failed to display the efficacy observed in preclinical investigations. The lack of effectiveness is often attributed to poor retention of the transplanted cells and, to date, most of the trial protocols have mobilized cells with injections of granulocyte colony-stimulating factor (G-CSF), which activates extracellular proteases that irreversibly cleave cell-surface adhesion molecules, including α4-integrin and CXCR4. Thus, the retention of G-CSF-mobilized cells in the ischemic region may be impaired, and the mobilization of agents that reversibly disrupt SDF-1/CXCR4 binding, such as AMD3100, may improve patient response. Efforts to supplement SDF-1 levels in the ischemic region may also improve progenitor cell recruitment and the effectiveness of stem cell therapy.

Mobilization of hematopoietic stem/progenitor cells: general principles and molecular mechanisms

Hematopoietic stem/progenitor cell mobilization can be achieved by a variety of bone marrow niche modifications, although efficient mobilization requires simultaneous expansion of the stem/progenitor cell pool and niche modification. Many of the mechanisms involved in G-CSF-induced mobilization have been described. With regard to mobilization of hematopoietic stem/progenitor cells, challenges for the future include the analysis of genetic factors responsible for the great variability in mobilization responses, and the identification of predictors of mobilization efficiency, as well as the development of mobilizing schemes for poor mobilizers. Moreover, improved regimens for enhanced or even preferential mobilization of nonhematopoietic stem/progenitor cell types, and their therapeutic potential for endogenous tissue repair will be questions to be vigorously pursued in the near future.

Mobilization of hematopoietic stem and progenitor cells using inhibitors of CXCR4 and VLA-4

Successful hematopoietic stem cell transplant requires the infusion of a sufficient number of hematopoietic stem/progenitor cells (HSPCs) that are capable of homing to the bone marrow cavity and regenerating durable trilineage hematopoiesis in a timely manner. Stem cells harvested from peripheral blood are the most commonly used graft source in HSCT. Although granulocyte colony-stimulating factor (G-CSF) is the most frequently used agent for stem cell mobilization, the use of G-CSF alone results in suboptimal stem cell yields in a significant proportion of patients. Both the chemokine receptor CXCR4 and the integrin α(4)β(1) (very late antigen 4 (VLA-4)) have important roles in the homing and retention of HSPCs within the bone marrow microenvironment. Preclinical and/or clinical studies have shown that targeted disruption of the interaction of CXCR4 or VLA-4 with their ligands results in the rapid and reversible mobilization of hematopoietic stem cells into the peripheral circulation and is synergistic when combined with G-CSF. In this review, we discuss the development of small-molecule CXCR4 and VLA-4 inhibitors and how they may improve the utility and convenience of peripheral blood stem cell transplantation.

Hematopoietic stem cell mobilization with agents other than G-CSF

Hematopoietic stem and progenitor mobilization has revolutionized the field of hematopoietic transplantation. Currently, hematopoietic grafts acquired from the peripheral blood of patients or donors treated with granulocyte-colony stimulating factor (G-CSF) are the preferred source for transplantation. G-CSF mobilization regimens, however, are associated with known morbidities and a significant number of normal donors and patient populations fail to mobilize sufficient numbers of hematopoietic stem and progenitor cells for transplantation, necessitating the need for non-G-CSF mobilization strategies. Mechanistic studies evaluating hematopoietic bone marrow niche interactions have uncovered novel agents with the capacity for hematopoietic mobilization. This chapter provides a comprehensive overview of mobilizing agents, other than G-CSF, and experimental procedures and technical aspects important to evaluate and define their hematopoietic mobilizing activities alone and in combination.

Modulation of Tumor Cell Survival, Proliferation, and Differentiation by the Peptide Derived from Tenascin-C: Implication of β1-Integrin Activation

Cell adhesion to extracellular matrix (ECM) participates in various biological processes, such as cell survival, proliferation, differentiation, and migration. Since these processes are essential for keeping homeostasis, aberration of these processes leads to a variety of diseases including cancer. Previously, we found that a peptide derived from tenascin- (TN-) C, termed TNIIIA2, stimulates cell adhesion to ECM through activation of β1-integrin. It has been shown that TNIIIA2 can modulate cell proliferation and differentiation. Interestingly, TNIIIA2 could not only enhance cell proliferation but also induce apoptotic cell death, depending on cellular context. In this review, we show the function of the peptide TNIIIA2 in cell survival, proliferation, and differentiation and refer to the possibility of new strategy for tumor suppression by regulating cell adhesion status using the ECM-derived functional peptides.

Many mechanisms mediating mobilization: an alliterative review

PURPOSE OF REVIEW

Blood cell production is maintained by hematopoietic stem cells (HSCs) that reside in specialized niches within bone marrow. Treatment with granulocyte-colony stimulating factor (G-CSF) causes HSC egress from bone marrow niches and trafficking to the peripheral blood, a process termed 'mobilization'. Although the mobilization phenomenon has been known for some time and is utilized clinically to acquire HSC for transplant, the mechanisms mediating HSC release are not completely understood. We discuss recent advances and controversies in defining the mechanisms responsible for G-CSF-induced mobilization.

RECENT FINDINGS

New reports define a role for resident monocytes/macrophages in maintaining niche cells, which is diminished after G-CSF treatment, suggesting a new mechanism for mobilization. Although osteoblasts have been reported to be a primary component of the HSC niche, new results suggest a unique niche composed of innervated mesenchymal stem cells. Modulating bioactive lipid signaling also facilitates mobilization, and may define a future therapeutic strategy.

SUMMARY

Hematopoietic mobilization by G-CSF is primarily mediated by alterations to the bone marrow niche by both direct and indirect mechanisms, rather than directly altering HSC function. Further understanding of the processes mediating mobilization will advance our understanding on the cellular and molecular components of the HSC niche.

Mobilization of hematopoietic stem and leukemia cells

HSC mobilization is an essential homeostatic process during inflammation and for the maintenance of hematopoietic progenitors. It has been exploited for the therapeutic application of HSC transplantation. Recent evidence suggests that leukemic cells share surface molecules in common with stem cells and may be mobilized under similar conditions. This effect could be used for therapeutic interventions. In this review, we will provide evidence showing that leukemia cells and stem cells traffic similarly and may share a common niche. Studies are discussed comparing and contrasting the mechanism of normal stem cells and leukemic cell mobilization through the CXCR4/CXCL12 axis and other key intermediaries.

Diabetes impairs hematopoietic stem cell mobilization by altering niche function

Success with transplantation of autologous hematopoietic stem and progenitor cells (HSPCs) in patients depends on adequate collection of these cells after mobilization from the bone marrow niche by the cytokine granulocyte colony-stimulating factor (G-CSF). However, some patients fail to achieve sufficient HSPC mobilization. Retrospective analysis of bone marrow transplant patient records revealed that diabetes correlated with poor mobilization of CD34+ HSPCs. In mouse models of type 1 and type 2 diabetes (streptozotocin-induced and db/db mice, respectively), we found impaired egress of murine HSPCs from the bone marrow after G-CSF treatment. Furthermore, HSPCs were aberrantly localized in the marrow niche of the diabetic mice, and abnormalities in the number and function of sympathetic nerve termini were associated with this mislocalization. Aberrant responses to β-adrenergic stimulation of the bone marrow included an inability of marrow mesenchymal stem cells expressing the marker nestin to down-modulate the chemokine CXCL12 in response to G-CSF treatment (mesenchymal stem cells are reported to be critical for HSPC mobilization). The HSPC mobilization defect was rescued by direct pharmacological inhibition of the interaction of CXCL12 with its receptor CXCR4 using the drug AMD3100. These data suggest that there are diabetes-induced changes in bone marrow physiology and microanatomy and point to a potential intervention to overcome poor HSPC mobilization in diabetic patients.

Mobilization of hematopoietic stem cells from the bone marrow niche to the blood compartment

The vast majority of hematopoietic stem cells (HSCs) reside in specialized niches within the bone marrow during steady state, maintaining lifelong blood cell production. A small number of HSCs normally traffic throughout the body; however, exogenous stimuli can enhance their release from the niche and entry into the peripheral circulation. This process, termed mobilization, has become the primary means to acquire a stem cell graft for hematopoietic transplant at most transplant centers. Currently, the preferred method of HSC mobilization for subsequent transplantation is treatment of the donor with granulocyte colony-stimulating factor. The mobilizing effect of granulocyte colony-stimulating factor is not completely understood, but recent studies suggest that its capacity to mobilize HSCs, at least in part, is a consequence of alterations to the hematopoietic niche. The present article reviews some of the key mechanisms mediating HSC mobilization, highlighting recent advances and controversies in the field.

Mechanisms of G-CSF-mediated hematopoietic stem and progenitor mobilization

Under normal conditions, the great majority of hematopoietic stem/progenitors cells (HSPCs) reside in the bone marrow. The number of HSPCs in the circulation can be markedly increased in response to a number of stimuli, including hematopoietic growth factors, myeloablative agents and environmental stresses such as infection. The ability to 'mobilize' HSPCs from the bone marrow to the blood has been exploited clinically to obtain HSPCs for stem cell transplantation and, more recently, to stimulate therapeutic angiogenesis at sites of tissue ischemia. Moreover, there is recent interest in the use of mobilizing agents to sensitize leukemia and other hematopoietic malignancies to cytotoxic agents. Key to optimizing clinical mobilizing regimens is an understanding of the fundamental mechanisms of HSPC mobilization. In this review, we discuss recent advances in our understanding of the mechanisms by which granulocyte colony-stimulating factor (G-CSF), the prototypical mobilizing agent, induces HSPC mobilization.

CXCR4-mediated bone marrow progenitor cell maintenance and mobilization are modulated by c-kit activity

RATIONALE

The mobilization of bone marrow (BM) progenitor cells (PCs) is largely governed by interactions between stromal cell-derived factor (SDF)-1 and CXC chemokine receptor (CXCR)4. Ischemic injury disrupts the SDF-1-CXCR4 interaction and releases BM PCs into the peripheral circulation, where the mobilized cells are recruited to the injured tissue and contribute to vessel growth. BM PCs can also be mobilized by the pharmacological CXCR4 antagonist AMD3100, but the other components of the SDF-1-CXCR4 signaling pathway are largely unknown. c-kit, a membrane-bound tyrosine kinase and the receptor for stem cell factor, has also been shown to play a critical role in BM PC mobilization and ischemic tissue repair.

OBJECTIVE

To investigate the functional interaction between SDF-1-CXCR4 signaling and c-kit activity in BM PC mobilization.

METHODS AND RESULTS

AMD3100 administration failed to mobilize BM PCs in mice defective in c-kit kinase activity or in mice transplanted with BM cells that expressed a constitutively active c-kit mutant. Furthermore, BM levels of phosphorylated (phospho)-c-kit declined after AMD3100 administration and after CXCR4 deletion. In cells adhering to culture plates coated with vascular cell adhesion molecule 1, SDF-1 and stem cell factor increased phospho-c-kit levels, and AMD3100 treatment suppressed SDF-1-induced, but not SCF-induced, c-kit phosphorylation. SDF-1-induced c-kit phosphorylation also required the activation of Src nonreceptor tyrosine kinase: pretreatment of cells with a selective Src inhibitor blocked both c-kit phosphorylation and the interaction between c-kit and phospho-Src.

CONCLUSIONS

These findings indicate that the regulation of BM PC trafficking by SDF-1 and CXCR4 is dependent on Src-mediated c-kit phosphorylation.

Developmental changes of cell adhesion molecule expression in the fetal mouse liver

Developmental changes of cell adhesion molecule expression, especially in nonparenchymal cells, have hardly ever been analyzed in the murine liver. The present study was undertaken to immunohistochemically examine the expression of NCAM, ICAM, VCAM, and N-cadherin during mouse liver development and in fetal liver cell cultures. NCAM was transiently expressed in mesenchymal cells of the septum transversum and sinusoidal cells in liver development. In vitro studies demonstrated that desmin-positive stellate cells expressed this cell adhesion molecule. NCAM expression in periportal biliary epithelial cells and connective tissue cells also coincided well with bile duct remodeling processes in the perinatal periods. Expression of ICAM and VCAM was transiently restricted to hepatoblasts, hepatocytes and hemopoietic cells in fetal stages. N-cadherin was expressed not only in hepatoblasts and hepatocytes, but also in nonparenchymal cells such as endothelial cells, stellate cells and connective tissue cells, however the expression was weak. These results suggest that each cell adhesion molecule may play an important role during development in hepatic histogenesis, including hepatoblast/hepatocyte-stellate cell interactions, hemopoiesis, and bile duct morphogenesis.

The role of natalizumab in hematopoietic stem cell mobilization

The humanized monoclonal very late antigen 4 (VLA-4) antibody natalizumab is FDA approved for the treatment of patients with multiple sclerosis and Crohn's disease. In this review we focus on its role in the context of hematopoietic stem cell transplantation and stem cell diseases. The use of natalizumab alone or in combination with either cytotoxic drugs or other antibodies might be a new modality for stem cell mobilization and a therapeutic option for patients with hematologic malignancies.

On the adaptation of endosteal stem cell niche function in response to stress

Although the influence of microenvironmental "niche" on the function of a variety of stem cells is undisputed, the details of hematopoietic stem cell/niche interactions at the cellular and molecular level have sparked a continuous debate. We studied the microanatomic partitioning of transplanted normal and alpha4 integrin-deficient Lin-kit+ cells in trabecular and compact bone before and after irradiation and present robust quantitative data on both. We found that (1) the microanatomic distribution of normal highly enriched progenitor cells is random in nonirradiated recipients based on area distribution analyses, (2) in contrast, in irradiated hosts normal cells distribute preferentially near the endosteum, (3) the overall cell seeding efficiency was higher in trabecular versus compact bone both before and after irradiation, and (4) alpha4 integrin-deficient cells not only lodge with reduced overall efficiency confirming previous data, but fail to preferentially partition themselves into endosteal regions in irradiated hosts, as normal cells do. A similar phenotype was observed with cells rendered G(i)-protein signaling incompetent by pertussis toxin treatment, supporting an active stromal-derived factor 1 (SDF-1) gradient near endosteum after irradiation.