An in vivo model to study and manipulate the hematopoietic stem cell niche

Bone Marrow Microenvironment as a Source of New Drug Targets for the Treatment of Acute Myeloid Leukaemia

Acute myeloid leukaemia (AML) is a heterogeneous disease with one of the worst survival rates of all cancers. The bone marrow microenvironment is increasingly being recognised as an important mediator of AML chemoresistance and relapse, supporting leukaemia stem cell survival through interactions among stromal, haematopoietic progenitor and leukaemic cells. Traditional therapies targeting leukaemic cells have failed to improve long term survival rates, and as such, the bone marrow niche has become a promising new source of potential therapeutic targets, particularly for relapsed and refractory AML. This review briefly discusses the role of the bone marrow microenvironment in AML development and progression, and as a source of novel therapeutic targets for AML. The main focus of this review is on drugs that modulate/target this bone marrow microenvironment and have been examined in in vivo models or clinically.

Mitochondrial calcium homeostasis in hematopoietic stem cell: Molecular regulation of quiescence, function, and differentiation

Hematopoiesis is based on the existence of hematopoietic stem cells (HSC) with the capacity to self-proliferate and self-renew or to differentiate into specialized cells. The hematopoietic niche is the essential microenvironment where stem cells reside and integrate various stimuli to determine their fate. Recent studies have identified niche containing high level of calcium (Ca²⁺) suggesting that HSCs are sensitive to Ca²⁺. This is a highly versatile and ubiquitous second messenger that regulates a wide variety of cellular functions. Advanced methods for measuring its concentrations, genetic experiments, cell fate tracing data, single-cell imaging, and transcriptomics studies provide information into its specific roles to integrate signaling into an array of mechanisms that determine HSC identity, lineage potential, maintenance, and self-renewal. Accumulating and contrasting evidence, are revealing Ca²⁺ as a previously unacknowledged feature of HSC, involved in functional maintenance, by regulating multiple actors including transcription and epigenetic factors, Ca²⁺-dependent kinases and mitochondrial physiology. Mitochondria are significant participants in HSC functions and their responsiveness to cellular demands is controlled to a significant extent via Ca²⁺ signals. Recent reports indicate that mitochondrial Ca²⁺ uptake also controls HSC fate. These observations reveal a physiological feature of hematopoietic stem cells that can be harnessed to improve HSC-related disease. In this review, we discuss the current knowledge Ca²⁺ in hematopoietic stem cell focusing on its potential involvement in proliferation, self-renewal and maintenance of HSC and discuss future research directions.

Ex Vivo Expansion of Hematopoietic Stem Cells for Therapeutic Purposes: Lessons from Development and the Niche

Expansion of hematopoietic stem cells (HSCs) for therapeutic purposes has been a “holy grail” in the field for many years. Ex vivo expansion of HSCs can help to overcome material shortage for transplantation purposes and genetic modification protocols. In this review, we summarize improved understanding in blood development, the effect of niche and conservative signaling pathways on HSCs in mice and humans, and also advances in ex vivo culturing protocols of human HSCs with cytokines or small molecule compounds. Different expansion protocols have been tested in clinical trials. However, an optimal condition for ex vivo expansion of human HSCs still has not been found yet. Translating and implementing new findings from basic research (for instance by using genetic modification of human HSCs) into clinical protocols is crucial to improve ex vivo expansion and eventually boost stem cell gene therapy.

An injectable bone marrow-like scaffold enhances T cell immunity after hematopoietic stem cell transplantation

The use of allogeneic hematopoietic stem cell transplantation (HSCT) to cure multiple disorders is limited by deficiency and dysregulation of T-cells. Here we report a biomaterial-based scaffold that mimics features of T-cell lymphopoiesis in the bone marrow. The bone marrow cryogel (BMC) releases bone morphogenetic protein-2 to recruit stromal cells, and presents the Notch ligand Delta-like ligand-4 to facilitate T-cell lineage specification of mouse and human hematopoietic progenitor cells. BMCs subcutaneously injected in mice at the time of HSCT enhanced T-cell progenitor seeding of the thymus, T-cell neogenesis and diversification of the T-cell receptor repertoire. Peripheral T-cell reconstitution increased ~6-fold in mouse HSCT and ~2-fold in human xenogeneic HSCT. Furthermore, BMCs promoted donor CD4⁺ regulatory T-cell generation and improved survival after allogeneic HSCT. Compared with adoptive transfer of T-cell progenitors, BMCs increased donor chimerism, T-cell generation and antigen-specific T-cell responses to vaccination. BMCs may provide an off-the-shelf approach for enhancing T-cell regeneration and mitigating graft-versus-host disease in HSCT.

Modeling the human bone marrow niche in mice: From host bone marrow engraftment to bioengineering approaches

Xenotransplantation of patient-derived samples in mouse models has been instrumental in depicting the role of hematopoietic stem and progenitor cells in the establishment as well as progression of hematological malignancies. The foundations for this field of research have been based on the development of immunodeficient mouse models, which provide normal and malignant human hematopoietic cells with a supportive microenvironment. Immunosuppressed and genetically modified mice expressing human growth factors were key milestones in patient-derived xenograft (PDX) models, highlighting the importance of developing humanized microenvironments. The latest major improvement has been the use of human bone marrow (BM) niche-forming cells to generate human-mouse chimeric BM tissues in PDXs, which can shed light on the interactions between human stroma and hematopoietic cells. Here, we summarize the methods used for human hematopoietic cell xenotransplantation and their milestones and review the latest approaches in generating humanized BM tissues in mice to study human normal and malignant hematopoiesis.

Generation and use of a humanized bone-marrow-ossicle niche for hematopoietic xenotransplantation into mice

Xenotransplantation is frequently used to study normal and malignant hematopoiesis of human cells. However, conventional mouse xenotransplantation models lack essential human-specific bone-marrow (BM)-microenvironment-derived survival, proliferation, and self-renewal signals for engraftment of normal and malignant blood cells. As a consequence, many human leukemias and other hematologic disorders do not robustly engraft in these conventional models. Here, we describe a complete workflow for the generation of humanized ossicles with an accessible BM microenvironment that faithfully recapitulates normal BM niche morphology and function. The ossicles, therefore, allow for accelerated and superior engraftment of primary patient-derived acute myeloid leukemia (AML) and other hematologic malignancies such as myelofibrosis (MF) in mice. The humanized ossicles are formed by in situ differentiation of BM-derived mesenchymal stromal cells (MSCs). Human hematopoietic cells can subsequently be transplanted directly into the ossicle marrow space or by intravenous injection. Using this method, a humanized engraftable BM microenvironment can be formed within 6-10 weeks. Engraftment of human hematopoietic cells can be evaluated by flow cytometry 8-16 weeks after transplantation. This protocol describes a robust and reproducible in vivo methodology for the study of normal and malignant human hematopoiesis in a more physiologic setting.

A PPARγ transcriptional cascade directs adipose progenitor cell-niche interaction and niche expansion

Adipose progenitor cells (APCs) reside in a vascular niche, located within the perivascular compartment of adipose tissue blood vessels. Yet, the signals and mechanisms that govern adipose vascular niche formation and APC niche interaction are unknown. Here we show that the assembly and maintenance of the adipose vascular niche is controlled by PPARγ acting within APCs. PPARγ triggers a molecular hierarchy that induces vascular sprouting, APC vessel niche affinity and APC vessel occupancy. Mechanistically, PPARγ transcriptionally activates PDGFRβ and VEGF. APC expression and activation of PDGFRβ promotes the recruitment and retention of APCs to the niche. Pharmacologically, targeting PDGFRβ disrupts APC niche contact thus blocking adipose tissue expansion. Moreover, enhanced APC expression of VEGF stimulates endothelial cell proliferation and expands the adipose niche. Consequently, APC niche communication and retention are boosted by VEGF thereby impairing adipogenesis. Our data indicate that APCs direct adipose tissue niche expansion via a PPARγ-initiated PDGFRβ and VEGF transcriptional axis.

Adipocyte progenitor cells (APCs) are found tethered to adipose tissue blood vessel walls and can differentiate into adipocytes. Here the authors show that PPARγ controls angiogenesis by stimulating APC–blood vessel interaction and retention via a transcriptional network that includes PDGFRβ and VEGF.

Extrinsic regulation of hematopoietic stem cells in development, homeostasis and diseases

Lifelong generation of blood and immune cells depends on hematopoietic stem cells (HSCs). Their function is precisely regulated by complex molecular networks that integrate and respond to ever changing physiological demands of the body. Over the past several years, significant advances have been made in understanding the extrinsic regulation of HSCs during development and in homeostasis. Propelled by technical advances in the field, the cellular and molecular components of the microenvironment that support HSCs in vivo are emerging. In addition, the interaction of HSCs with their niches is appreciated as a critical contributor to the pathogenesis of a number of hematologic disorders. Here, we review these advances in detail and highlight the extrinsic regulation of HSCs in the context of development, homeostasis, and diseases. WIREs Dev Biol 2017, 6:e279. doi: 10.1002/wdev.279 For further resources related to this article, please visit the WIREs website.

Extracellular matrix and α5β1 integrin signaling control the maintenance of bone formation capacity by human adipose-derived stromal cells

Stromal vascular fraction (SVF) cells of human adipose tissue have the capacity to generate osteogenic grafts with intrinsic vasculogenic properties. However, adipose-derived stromal/stem cells (ASC), even after minimal monolayer expansion, display poor osteogenic capacity in vivo. We investigated whether ASC bone-forming capacity may be maintained by culture within a self-produced extracellular matrix (ECM) that recapitulates the native environment. SVF cells expanded without passaging up to 28 days (Unpass-ASC) deposited a fibronectin-rich extracellular matrix and displayed greater clonogenicity and differentiation potential in vitro compared to ASC expanded only for 6 days (P0-ASC) or for 28 days with regular passaging (Pass-ASC). When implanted subcutaneously, Unpass-ASC produced bone tissue similarly to SVF cells, in contrast to P0- and Pass-ASC, which mainly formed fibrous tissue. Interestingly, clonogenic progenitors from native SVF and Unpass-ASC expressed low levels of the fibronectin receptor α₅ integrin (CD49e), which was instead upregulated in P0- and Pass-ASC. Mechanistically, induced activation of α₅β₁ integrin in Unpass-ASC led to a significant loss of bone formation in vivo. This study shows that ECM and regulation of α₅β₁-integrin signaling preserve ASC progenitor properties, including bone tissue-forming capacity, during in vitro expansion.

Engineering a humanized bone organ model in mice to study bone metastases

Current in vivo models for investigating human primary bone tumors and cancer metastasis to the bone rely on the injection of human cancer cells into the mouse skeleton. This approach does not mimic species-specific mechanisms occurring in human diseases and may preclude successful clinical translation. We have developed a protocol to engineer humanized bone within immunodeficient hosts, which can be adapted to study the interactions between human cancer cells and a humanized bone microenvironment in vivo. A researcher trained in the principles of tissue engineering will be able to execute the protocol and yield study results within 4-6 months. Additive biomanufactured scaffolds seeded and cultured with human bone-forming cells are implanted ectopically in combination with osteogenic factors into mice to generate a physiological bone 'organ', which is partially humanized. The model comprises human bone cells and secreted extracellular matrix (ECM); however, other components of the engineered tissue, such as the vasculature, are of murine origin. The model can be further humanized through the engraftment of human hematopoietic stem cells (HSCs) that can lead to human hematopoiesis within the murine host. The humanized organ bone model has been well characterized and validated and allows dissection of some of the mechanisms of the bone metastatic processes in prostate and breast cancer.

Versatile humanized niche model enables study of normal and malignant human hematopoiesis

The BM niche comprises a tightly controlled microenvironment formed by specific tissue and cells that regulates the behavior of hematopoietic stem cells (HSCs). Here, we have provided a 3D model that is tunable in different BM niche components and useful, both in vitro and in vivo, for studying the maintenance of normal and malignant hematopoiesis. Using scaffolds, we tested the capacity of different stromal cell types to support human HSCs. Scaffolds coated with human mesenchymal stromal cells (hMSCs) proved to be superior in terms of HSC engraftment and long-term maintenance when implanted in vivo. Moreover, we found that hMSC-coated scaffolds can be modulated to form humanized bone tissue, which was also able to support human HSC engraftment. Importantly, hMSC-coated humanized scaffolds were able to support the growth of leukemia patient cells in vivo, including the growth of samples that would not engraft the BM of immunodeficient mice. These results demonstrate that an s.c. implantation approach in a 3D carrier scaffold seeded with stromal cells is an effective in vivo niche model for studying human hematopoiesis. The various niche components of this model can be changed depending on the context to improve the engraftment of nonengrafting acute myeloid leukemia (AML) samples.

A humanized bone marrow ossicle xenotransplantation model enables improved engraftment of healthy and leukemic human hematopoietic cells

Xenotransplantation models represent powerful tools for the investigation of healthy and malignant human hematopoiesis. However, current models do not fully mimic the components of the human bone marrow (BM) microenvironment, and they enable only limited engraftment of samples from some human malignancies. Here we show that a xenotransplantation model bearing subcutaneous humanized ossicles with an accessible BM microenvironment, formed by in situ differentiation of human BM-derived mesenchymal stromal cells, enables the robust engraftment of healthy human hematopoietic stem and progenitor cells, as well as primary acute myeloid leukemia (AML) samples, at levels much greater than those in unmanipulated mice. Direct intraossicle transplantation accelerated engraftment and resulted in the detection of substantially higher leukemia-initiating cell (LIC) frequencies. We also observed robust engraftment of acute promyelocytic leukemia (APL) and myelofibrosis (MF) samples, and identified LICs in these malignancies. This humanized ossicle xenotransplantation approach provides a system for modeling a wide variety of human hematological diseases.

A T Cell View of the Bone Marrow

The majority of T cells present in the bone marrow (BM) represent an activated/memory phenotype and most of these, if not all, are circulating T cells. Their lodging in the BM keeps them activated, turning the BM microenvironment into a “memory reservoir.” This article will focus on how T cell activation in the BM results in both direct and indirect effects on the hematopoiesis. The hematopoietic stem cell niche will be presented, with its main components and organization, along with the role played by T lymphocytes in basal and pathologic conditions and their effect on the bone remodeling process. Also discussed herein will be how “normal” bone mass peak is achieved only in the presence of an intact adaptive immune system, with T and B cells playing critical roles in this process. Our main hypothesis is that the partnership between T cells and cells of the BM microenvironment orchestrates numerous processes regulating immunity, hematopoiesis, and bone remodeling.

Tissue engineered humanized bone supports human hematopoiesis in vivo

Advances in tissue-engineering have resulted in a versatile tool-box to specifically design a tailored microenvironment for hematopoietic stem cells (HSCs) in order to study diseases that develop within this setting. However, most current in vivo models fail to recapitulate the biological processes seen in humans. Here we describe a highly reproducible method to engineer humanized bone constructs that are able to recapitulate the morphological features and biological functions of the HSC niches. Ectopic implantation of biodegradable composite scaffolds cultured for 4 weeks with human mesenchymal progenitor cells and loaded with rhBMP-7 resulted in the development of a chimeric bone organ including a large number of human mesenchymal cells which were shown to be metabolically active and capable of establishing a humanized microenvironment supportive of the homing and maintenance of human HSCs. A syngeneic mouse-to-mouse transplantation assay was used to prove the functionality of the tissue-engineered ossicles. We predict that the ability to tissue engineer a morphologically intact and functional large-volume bone organ with a humanized bone marrow compartment will help to further elucidate physiological or pathological interactions between human HSCs and their native niches.

Engineered microenvironments provide new insights into ovarian and prostate cancer progression and drug responses

Tissue engineering technologies, which have originally been designed to reconstitute damaged tissue structure and function, can mimic not only tissue regeneration processes but also cancer development and progression. Bioengineered approaches allow cell biologists to develop sophisticated experimentally and physiologically relevant cancer models to recapitulate the complexity of the disease seen in patients. Tissue engineering tools enable three-dimensionality based on the design of biomaterials and scaffolds that re-create the geometry, chemistry, function and signalling milieu of the native tumour microenvironment. Three-dimensional (3D) microenvironments, including cell-derived matrices, biomaterial-based cell culture models and integrated co-cultures with engineered stromal components, are powerful tools to study dynamic processes like proteolytic functions associated with cancer progression, metastasis and resistance to therapeutics. In this review, we discuss how biomimetic strategies can reproduce a humanised niche for human cancer cells, such as peritoneal or bone-like microenvironments, addressing specific aspects of ovarian and prostate cancer progression and therapy response.

A tissue-engineered humanized xenograft model of human breast cancer metastasis to bone

The skeleton is a preferred homing site for breast cancer metastasis. To date, treatment options for patients with bone metastases are mostly palliative and the disease is still incurable. Indeed, key mechanisms involved in breast cancer osteotropism are still only partially understood due to the lack of suitable animal models to mimic metastasis of human tumor cells to a human bone microenvironment. In the presented study, we investigate the use of a human tissue-engineered bone construct to develop a humanized xenograft model of breast cancer-induced bone metastasis in a murine host. Primary human osteoblastic cell-seeded melt electrospun scaffolds in combination with recombinant human bone morphogenetic protein 7 were implanted subcutaneously in non-obese diabetic/severe combined immunodeficient mice. The tissue-engineered constructs led to the formation of a morphologically intact ‘organ’ bone incorporating a high amount of mineralized tissue, live osteocytes and bone marrow spaces. The newly formed bone was largely humanized, as indicated by the incorporation of human bone cells and human-derived matrix proteins. After intracardiac injection, the dissemination of luciferase-expressing human breast cancer cell lines to the humanized bone ossicles was detected by bioluminescent imaging. Histological analysis revealed the presence of metastases with clear osteolysis in the newly formed bone. Thus, human tissue-engineered bone constructs can be applied efficiently as a target tissue for human breast cancer cells injected into the blood circulation and replicate the osteolytic phenotype associated with breast cancer-induced bone lesions. In conclusion, we have developed an appropriate model for investigation of species-specific mechanisms of human breast cancer-related bone metastasis in vivo.

Epigenetic and in vivo comparison of diverse MSC sources reveals an endochondral signature for human hematopoietic niche formation

In the last decade there has been a rapid expansion in clinical trials using mesenchymal stromal cells (MSCs) from a variety of tissues. However, despite similarities in morphology, immunophenotype, and differentiation behavior in vitro, MSCs sourced from distinct tissues do not necessarily have equivalent biological properties. We performed a genome-wide methylation, transcription, and in vivo evaluation of MSCs from human bone marrow (BM), white adipose tissue, umbilical cord, and skin cultured in humanized media. Surprisingly, only BM-derived MSCs spontaneously formed a BM cavity through a vascularized cartilage intermediate in vivo that was progressively replaced by hematopoietic tissue and bone. Only BM-derived MSCs exhibited a chondrogenic transcriptional program with hypomethylation and increased expression of RUNX3, RUNX2, BGLAP, MMP13, and ITGA10 consistent with a latent and primed skeletal developmental potential. The humanized MSC-derived microenvironment permitted homing and maintenance of long-term murine SLAM(+) hematopoietic stem cells (HSCs), as well as human CD34(+)/CD38(-)/CD90(+)/CD45RA(+) HSCs after cord blood transplantation. These studies underscore the profound differences in developmental potential between MSC sources independent of donor age, with implications for their clinical use. We also demonstrate a tractable human niche model for studying homing and engraftment of human hematopoietic cells in normal and neoplastic states.

Hematopoietic stem cell niche maintenance during homeostasis and regeneration

The bone marrow niche has mystified scientists for many years, leading to widespread investigation to shed light into its molecular and cellular composition. Considerable efforts have been devoted toward uncovering the regulatory mechanisms of hematopoietic stem cell (HSC) niche maintenance. Recent advances in imaging and genetic manipulation of mouse models have allowed the identification of distinct vascular niches that have been shown to orchestrate the balance between quiescence, proliferation and regeneration of the bone marrow after injury. Here we highlight the recently discovered intrinsic mechanisms, microenvironmental interactions and communication with surrounding cells involved in HSC regulation, during homeostasis and in regeneration after injury and discuss their implications for regenerative therapy.

"Mesenchymal" stem cells

Two opposing descriptions of so-called mesenchymal stem cells (MSCs) exist at this time. One sees MSCs as the postnatal, self-renewing, and multipotent stem cells for the skeleton. This cell coincides with a specific type of bone marrow perivascular cell. In skeletal physiology, this skeletal stem cell is pivotal to the growth and lifelong turnover of bone and to its native regeneration capacity. In hematopoietic physiology, its role as a key player in maintaining hematopoietic stem cells in their niche and in regulating the hematopoietic microenvironment is emerging. In the alternative description, MSCs are ubiquitous in connective tissues and are defined by in vitro characteristics and by their use in therapy, which rests on their ability to modulate the function of host tissues rather than on stem cell properties. Here, I discuss how the two views developed, conceptually and experimentally, and attempt to clarify the confusion arising from their collision.

The Instructive Role of the Vasculature in Stem Cell Niches

An important hallmark of many adult stem cell niches is their proximity to the vasculature in vivo, a feature common to neural stem cells (NSCs), mesenchymal stem cells (MSCs) from bone marrow, adipose, and other tissues, hematopoietic stem cells (HSCs), and many tumor stem cells. This review summarizes key studies supporting the vasculature's instructive role in adult stem cell niches, and the putative underlying molecular mechanisms by which blood vessels in these niches exert control over progenitor cell fates. The importance of the perivascular niche for pathology, notably tumor metastasis and dormancy, is also highlighted. Finally, the implications of the perivascular regulation of stem and progenitor cells on biomaterial design and the impact on future research directions are discussed.

Controlling stem cell-mediated bone regeneration through tailored mechanical properties of collagen scaffolds

Mechanical properties of the extracellular matrix (ECM) play an essential role in cell fate determination. To study the role of mechanical properties of ECM in stem cell-mediated bone regeneration, we used a 3D in vivo ossicle model that recapitulates endochondral bone formation. Three-dimensional gelatin scaffolds with distinct stiffness were developed using 1-Ethyl-3-[3-dimethylaminopropyl] carbodiimide hydrochloride (EDC) mediated zero-length crosslinking. The mechanical strength of the scaffolds was significantly increased by EDC treatment, while the microstructure of the scaffold was preserved. Cell behavior on the scaffolds with different mechanical properties was evaluated in vitro and in vivo. EDC-treated scaffolds promoted early chondrogenic differentiation, while it promoted both chondrogenic and osteogenic differentiation at later time points. Both micro-computed tomography and histologic data demonstrated that EDC-treatment significantly increased trabecular bone formation by transplanted cells transduced with AdBMP. Moreover, significantly increased chondrogenesis was observed in the EDC-treated scaffolds. Based on both in vitro and in vivo data, we conclude that the high mechanical strength of 3D scaffolds promoted stem cell mediated bone regeneration by promoting endochondral ossification. These data suggest a new method for harnessing stem cells for bone regeneration in vivo by tailoring the mechanical properties of 3D scaffolds.

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.

Implantable microenvironments to attract hematopoietic stem/cancer cells

The environments that harbor hematopoietic stem and progenitor cells are critical to explore for a better understanding of hematopoiesis during health and disease. These compartments often are inaccessible for controlled and rapid experimentation, thus limiting studies to the evaluation of conventional cell culture and transgenic animal models. Here we describe the manufacture and image-guided monitoring of an engineered microenvironment with user-defined properties that recruits hematopoietic progenitors into the implant. Using intravital imaging and fluorescence molecular tomography, we show in real time that the cell homing and retention process is efficient and durable for short- and long-term engraftment studies. Our results indicate that bone marrow stromal cells, precoated on the implant, accelerate the formation of new sinusoidal blood vessels with vascular integrity at the microcapillary level that enhances the recruitment hematopoietic progenitor cells to the site. This implantable construct can serve as a tool enabling the study of hematopoiesis.

Prevalence of prostate cancer metastases after intravenous inoculation provides clues into the molecular basis of dormancy in the bone marrow microenvironment

Bone is the preferred metastasis site of advanced prostate cancer (PCa). Using an in vivo murine model of human PCa cell metastasis to bone, we noted that the majority of animals that develop skeletal metastasis have either spinal lesions or lesions in the bones of the hindlimb. Much less frequently, lesions develop in the bones of the forelimb. We therefore speculated whether the environment of the forelimb bones is not permissive for the growth of PCa. Consequently, data on tumor prevalence were normalized to account for the number of PCa cells arriving after intravascular injection, marrow cellularity, and number of hematopoietic stem cell niches. None of these factors were able to account for the observed differences in tumor prevalence. An analysis of differential gene and protein levels identified that growth arrest specific-6 (GAS6) levels were significantly greater in the forelimb versus hindlimb bone marrow. When murine RM1 cells were implanted into subcutaneous spaces in immune competent animals, tumor growth in the GAS6(-/-) animals was greater than in GAS6(+/+) wild-type animals. In an osseous environment, the human PC3 cell line grew significantly better in vertebral body transplants (vossicles) derived from GAS6(-/-) animals than in vossicles derived from GAS6(+/+) animals. Together, these data suggest that the differences in tumor prevalence after intravascular inoculation are a useful model to study the molecular basis of tumor dormancy. Importantly, these data suggest that therapeutic manipulation of GAS6 levels may prove useful as a therapy for metastatic disease.

Erythropoietin modulates the structure of bone morphogenetic protein 2-engineered cranial bone

The ideally engineered bone should have similar structural and functional properties to the native tissue. Although structural integrity is critical for functional bone regeneration, we know less about modulating the structural properties of the engineered bone elicited by bone morphogenetic protein (BMP) than efficacy and safety. Erythropoietin (Epo), a primary erythropoietic hormone, has been used to augment blood transfusion in orthopedic surgery. However, the effects of Epo on bone regeneration are not well known. Here, we determined the role of Epo in BMP2-induced bone regeneration using a cranial defect model. Epo administration improved the quality of BMP2-induced bone and more closely resembled natural cranial bone with a higher bone volume (BV) fraction and lower marrow fraction when compared with BMP2 treatment alone. Epo increased red blood cells (RBCs) in peripheral blood and also increased hematopoietic and mesenchymal stem cell (MSC) populations in bone marrow. Consistent with our previous work, Epo increased osteoclastogenesis both in vitro and in vivo. Results from a metatarsal organ culture assay suggested that Epo-promoted osteoclastogenesis contributed to angiogenesis because angiogenesis was blunted when osteoclastogenesis was blocked by alendronate (ALN) or osteoprotegerin (OPG). Earlier calcification of BMP2-induced temporary chondroid tissue was observed in the Epo+BMP group compared to BMP2 alone. We conclude that Epo significantly enhanced the outcomes of BMP2-induced cranial bone regeneration in part through its actions on osteoclastogenesis and angiogenesis.

Human very small embryonic-like cells generate skeletal structures, in vivo

Human very small embryonic-like (hVSEL) cells are a resident population of multipotent stem cells in the bone marrow involved in the turnover and regeneration of tissues. The levels of VSEL cells in blood are greatly increased in response to injury, and they have been shown to repair injured tissues. Adult hVSEL cells, SSEA-4(+)/CD133(+)/CXCR4(+)/Lin(-)/CD45(-), express the pluripotency markers (Oct-4 and Nanog) and may be able to differentiate into cells from all 3 germ lineages. hVSEL cells isolated from blood by apheresis following granulocyte-colony-stimulating factor mobilization were fractionated and enriched by elutriation and fluorescence activated cell sorting. Collagen sponge scaffolds containing 2,000-30,000 hVSEL cells were implanted into cranial defects generated in SCID mice. Analysis by microcomputed tomography showed that a cell population containing VSEL cells produced mineralized tissue within the cranial defects compared with controls at 3 months. Histologic studies showed significant bone formation and cellular organization within the defects compared with cellular or scaffold controls alone. Antibodies to human leukocyte antigens demonstrated that the newly generated tissues were of human origin. Moreover, human osteocalcin was identified circulating in the peripheral blood. There was evidence that some level of hVSEL cells migrated away from the defect site, using quantitative real-time polymerase chain reaction to detect for human-specific Alu sequences. This study demonstrates that hVSEL cells are able to generate human bone tissue in a mouse model of skeletal repair. These studies lay the foundation for future cell-based regenerative therapies for osseous and connective tissue disorders, including trauma and degenerative conditions, such as osteoporosis, fracture repair, and neoplastic repair.

Stem cells in the face: tooth regeneration and beyond

The face distinguishes one person from another. Postnatal orofacial tissues harbor rare cells that exhibit stem cell properties. Despite unmet clinical needs for reconstruction of tissues lost in congenital anomalies, infections, trauma, or tumor resection, how orofacial stem/progenitor cells contribute to tissue development, pathogenesis, and regeneration is largely obscure. This perspective article critically analyzes the current status of our understanding of orofacial stem/progenitor cells, identifies gaps in our knowledge, and highlights pathways for the development of regenerative therapies.

Human extramedullary bone marrow in mice: a novel in vivo model of genetically controlled hematopoietic microenvironment

The interactions between hematopoietic cells and the bone marrow (BM) microenvironment play a critical role in normal and malignant hematopoiesis and drug resistance. These interactions within the BM niche are unique and could be important for developing new therapies. Here, we describe the development of extramedullary bone and bone marrow using human mesenchymal stromal cells and endothelial colony-forming cells implanted subcutaneously into immunodeficient mice. We demonstrate the engraftment of human normal and leukemic cells engraft into the human extramedullary bone marrow. When normal hematopoietic cells are engrafted into the model, only discrete areas of the BM are hypoxic, whereas leukemia engraftment results in widespread severe hypoxia, just as recently reported by us in human leukemias. Importantly, the hematopoietic cell engraftment could be altered by genetical manipulation of the bone marrow microenvironment: Extramedullary bone marrow in which hypoxia-inducible factor 1α was knocked down in mesenchymal stromal cells by lentiviral transfer of short hairpin RNA showed significant reduction (50% ± 6%; P = .0006) in human leukemic cell engraftment. These results highlight the potential of a novel in vivo model of human BM microenvironment that can be genetically modified. The model could be useful for the study of leukemia biology and for the development of novel therapeutic modalities aimed at modifying the hematopoietic microenvironment.

Decreased soluble TGF-β1, Tie-2, and angiopoietins serum levels in bone marrow after treating healthy donors with granulocyte colony-stimulating factor

The goal of this study was to investigate the effect of granulocyte colony-stimulating factor (G-CSF) on Tie-2, angiopoietins, VEGF, and TGF-β1 in bone marrow (BM) of healthy donors. Soluble Tie-2, angiopoietins, VEGF, and TGF-β1 levels in the BM were determined via ELISA in 25 healthy donors before and after G-CSF treatment. The results showed that treating healthy donors with G-CSF significantly decrease serum levels of Tie-2, angiopoietin-1, angiopoietin-2, and TGF-β1. In contrast, median VEGF level in the G-CSF-primed BM was significantly higher than steady-state BM. Our results suggest that decreased soluble TGF-β1, Tie-2, and angiopoietins levels in the BM could be related to stem cell mobilization.

Experimental model of heterotopic ossification in Wistar rats

Heterotopic ossification (HO) is a metaplastic biological process in which there is newly formed bone in soft tissues adjacent to large joints, resulting in joint mobility deficit. In order to determine which treatment techniques are more appropriate for such condition, experimental models of induced heterotopic bone formation have been proposed using heterologous demineralized bone matrix implants and bone morphogenetic protein and other tissues. The objective of the present experimental study was to identify a reliable protocol to induce HO in Wistar rats, based on autologous bone marrow (BM) implantation, comparing 3 different BM volumes and based on literature evidence of this HO induction model in larger laboratory animals. Twelve male Wistar albino rats weighing 350/390 g were used. The animals were anesthetized for blood sampling before HO induction in order to quantify serum alkaline phosphatase (ALP). HO was induced by BM implantation in both quadriceps muscles of these animals, experimental group (EG). Thirty-five days after the induction, another blood sample was collected for ALP determination. The results showed a weight gain in the EG and no significant difference in ALP levels when comparing the periods before and after induction. Qualitative histological analysis confirmed the occurrence of heterotopic ossification in all 12 EG rats. In conclusion, the HO induction model was effective when 0.35 mL autologous BM was applied to the quadriceps of Wistar rats.

Structure and function of the solid tumor niche

Although the hematopoietic stem cell (HSC) niche has been an active area of study, the concept of the bone marrow microenvironment (BMM) harboring a niche for solid metastatic tumor cells has only recently been considered. The HSC niche and microenvironment that is thought to constitute the solid tumor niche share many of the same structural and functional components, suggesting the possibility that the HSC and tumor niche are one in the same. The osteoblast is a critical component for each of these niches, and is important for regulating cellular processes such homing and migration, growth and survival, and quiescence and dormancy. Current understanding of the HSC niche may provide more insight to better defining the solid tumor niche. As role of the niche in regulating these processes is better understood, new insights to the role of the BMM in metastatic disease may be gained, and provide more potential targets for therapy.

Human early fracture hematoma is characterized by inflammation and hypoxia

BACKGROUND

An effective immune system, especially during the inflammatory phase, putatively influences the quality and likelihood of bone healing. If and how this is reflected within the initial fracture hematoma is unclear.

QUESTIONS/PURPOSES

We therefore asked the following questions: (1) Does the local expression in fracture hematoma of genes involved in adaptation to hypoxia, migration, angiogenesis, and osteogenesis vary as compared to the peripheral blood? (2) Do these changes occur time dependently? (3) Is the gene expression during fracture hematoma formation altered by irradiation?

METHODS

Cells from fracture hematoma of 20 patients and hematomas formed in 40 patients after THA (20 without and 20 with preoperative radiation) were isolated and RNA was extracted to analyze the influence of oxygen deprivation during fracture healing on mRNA expression of genes (HIF1A, LDHA, and PGK1) involved in immunoregulation (IL6, IL8, CXCR4), angiogenesis (VEGF, IL8), and osteogenesis (SPP1, RUNX2) by quantitative PCR.

RESULTS

We observed locally increased LDHA gene expression in fracture hematoma cells (6-72 h post fracture) reflecting the adaptation to hypoxia. IL6, IL8, and VEGF upregulation indicated hypoxia-mediated inflammation and angiogenesis; increased CXCR4 expression reflected immigration of immune cells. Osteogenic differentiation was reflected in the increased expression of the SPP1 and RUNX2 genes. The increased expression of the LDHA, VEGF, IL8, SPP1 and RUNX2 genes occurred time dependently. Irradiation suppressed HIF1A, IL6, IL8, CXCR4, and RUNX2 gene expression.

CONCLUSIONS

Our data suggest cells in the fracture hematoma (1) adapt to hypoxia and (2) promote inflammation in fracture healing at the mRNA level, indicating early involvement of the immune system.

CLINICAL RELEVANCE

The initial fracture hematoma is important for the onset of angiogenesis, chemotaxis, and osteogenesis.

The stem cell niche should be a key issue for cell therapy in regenerative medicine

Recent advances in stem cell research have highlighted the role played by such cells and their environment (the stem cell niche) in tissue renewal and homeostasis. The control and regulation of stem cells and their niche are remaining challenges for cell therapy and regenerative medicine on several tissues and organs. These advances are important for both, the basic knowledge of stem cell regulation, and their practical translational applications into clinical medicine. This article is primarily concerned with the mesenchymal stem cells (MSCs) and it reviews the current aspects of their own niche. We discuss on the need for a deeper understanding of the identity of this cell type and its microenvironment in order to improve the effectiveness of any cell therapy for regenerative medicine. Ex vivo reproduction of the conditions of the natural stem cell niche, when necessary, would provide success to tissue engineering. The first challenge of regenerative medicine is to find cells able to replace and/or repair the lost function of tissues and organs by disease or aging and the trophic and immunomodulatory effects recently found for MSCs open up for new opportunities. If MSCs are pericytes, as it has been proposed, perhaps it may explain the ubiquity of these cells and their possible role in miscellaneous repairs throughout the body opening for new chances for extensive tissue repair.

Dynamic niches in the origination and differentiation of haematopoietic stem cells

Haematopoietic stem cells (HSCs) are multipotent, self-renewing progenitors that generate all mature blood cells. HSC function is tightly controlled to maintain haematopoietic homeostasis, and this regulation relies on specialized cells and factors that constitute the haematopoietic 'niche', or microenvironment. Recent discoveries, aided in part by technological advances in in vivo imaging, have engendered a new appreciation for the dynamic nature of the niche, identifying novel cellular and acellular niche components and uncovering fluctuations in the relative importance of these components over time. These new insights significantly improve our understanding of haematopoiesis and raise fundamental questions about what truly constitutes a stem cell niche.

Minireview: The stem cell next door: skeletal and hematopoietic stem cell "niches" in bone

Long known to be home to hematopoietic stem cells (HSC), the bone/bone marrow organ and its cellular components are directly implicated in regulating hematopoiesis and HSC function. Over the past few years, advances on the identity of HSC "niche" cells have brought into focus the role of cells of osteogenic lineage and of marrow microvessels. At the same time, the identity of self-renewing multipotent skeletal progenitors (skeletal stem cells, also known as mesenchymal stem cells) has also been more precisely defined, along with the recognition of their own microvascular niche. The two sets of evidence converge in delineating a picture in which two kinds of stem cells share an identical microanatomical location in the bone/bone marrow organ. This opens a new view on the manner in which the skeleton and hematopoiesis can cross-regulate via interacting stem cells but also a novel view of our general concept of stem cell niches.

Bone and the hematopoietic niche: a tale of two stem cells

The revived interest in (hematopoietic) stem cell (HSC) niches has highlighted the role of multiple cellular players found in the bone environment. Initially focused on the role of osteoblasts and sinusoid endothelial cells, the quest for HSC niche cells has recently focused on a unique role for osteoprogenitor cells (skeletal stem cells, mesenchymal stem cells). Strongly validated by observations of HSC dysregulation dictated by the dysregulation of osteoprogenitors, the role of osteoprogenitors in the HSC niche integrates data from different studies into a unified view. As preosteoblastic, periendothelial cells residing at the sinusoid wall, skeletal progenitors reconcile the notions of "osteoblastic" and "sinusoidal" niches with one another. In addition, they bring into focus the cross-regulation of skeletal and hematopoietic physiology as rooted into the interplay of two stem cells (hematopoietic and skeletal) sharing a single niche. As direct regulators of hematopoietic space formation, sinusoid development, and hematopoietic function(s), as well as direct progenitors of positive and negative regulators of HSCs such as osteoblasts and adipocytes, skeletal progenitors have emerged as pivotal organizers of a complex, highly plastic niche. This development seems to represents an evolutionary advance over the deterministic stem cell niches found in archetypal invertebrate systems.

Prospective identification and skeletal localization of cells capable of multilineage differentiation in vivo

A prospective in vivo assay was used to identify cells with potential for multiple lineage differentiation. With this assay, it was first determined that the 5-fluorouracil resistant cells capable of osseous tissue formation in vivo also migrated toward stromal derived factor-1 (SDF-1) in vitro. In parallel, an isolation method based on fluorescence-activated cell sorting was employed to identify a very small cell embryonic-like Lin-/Sca-1+CD45- cell that with as few as 500 cells was capable of forming bone-like structures in vivo. Differential marrow fractionation studies determined that the majority of the Lin-Sca-1+CD45- cells reside in the subendosteal regions of marrow. To determine whether these cells were capable of differentiating into multiple lineages, stromal cells harvested from Col2.3 Delta TK mice were implanted with a gelatin sponge into SCID mice to generate thymidine kinase sensitive ossicles. At 1.5 months, 2,000 green fluorescent protein (GFP)+ Lin-Sca-1+CD45- cells were injected into the ossicles. At harvest, colocalization of GFP-expressing cells with antibodies to the osteoblast-specific marker Runx-2 and the adipocyte marker PPAP gamma were observed. Based on the ability of the noncultured cells to differentiate into multiple mesenchymal lineages in vivo and the ability to generate osseous tissues at low density, we propose that this population fulfills many of the characteristics of mesenchymal stem cells.

The use of soluble signals to harness the power of the bone microenvironment for implant therapeutics

The use of soluble signals for modulation of bone formation has become a significant area of clinical research in recent years. Improvements in implant site preparation and osseointegration have already been achieved with the use of recombinant platelet-derived growth factor and bone morphogenetic proteins on osteogenic scaffolds. Other states of insufficient bone such as osteoporosis are frequently treated with inhibitors of osteoclast function or osteoblast anabolic agents. However, despite the existence of promising therapies targeting osteoblasts and osteoclasts directly, therapies utilizing indirect regulation through secondary cellular nodes of control (NOC) are just beginning to emerge. This article will review current strategies for regulation of bone formation by targeting two primary NOCs, the osteoblast and osteoclast, as well as four secondary NOCs, the vascular, hematopoietic, mesenchymal, and neural.

The endosteal 'osteoblastic' niche and its role in hematopoietic stem cell homing and mobilization

The concept of hematopoietic stem cell (HSC) niche was formulated in 1978, but HSC niches remained unidentified for the following two decades largely owing to technical limitations. Sophisticated live microscopy techniques and genetic manipulations have identified the endosteal region of the bone marrow (BM) as a preferential site of residence for the most potent HSC - able to reconstitute in serial transplants - with osteoblasts and their progenitors as critical cellular elements of these endosteal niches. This article reviews the path to the discovery of these endosteal niches (often called 'osteoblastic' niches) for HSC, what cell types contribute to these niches with their known physical and biochemical features. In the past decade, a first wave of research uncovered many mechanisms responsible for HSC homing to, and mobilization from, the whole BM tissue. However, the recent discovery of endosteal HSC niches has initiated a second wave of research focusing on the mechanisms by which most primitive HSC lodge into and migrate out of their endosteal niches. The second part of this article reviews the current knowledge of the mechanisms of HSC lodgment into, retention in and mobilization from osteoblastic niches.

Pluripotent stem cell-derived natural killer cells for cancer therapy

Human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) provide an accessible, genetically tractable, and homogenous starting cell population to efficiently study human blood cell development. These cell populations provide platforms to develop new cell-based therapies to treat both malignant and nonmalignant hematological diseases. Our group previously demonstrated the ability of hESC-derived hematopoietic precursors to produce functional natural killer (NK) cells as well as an explanation of the underlying mechanism responsible for the inefficient development of T and B cells from hESCs. hESCs and iPSCs, which can be engineered reliably in vitro, provide an important new model system to study human lymphocyte development and produce enhanced cell-based therapies with the potential to serve as a "universal" source of antitumor lymphocytes. This review will focus on the application of hESC-derived NK cells with currently used and novel therapeutics for clinical trials, barriers to translation, and future applications through genetic engineering approaches.