Mitogenic and adhesive effects of tenascin-C on human hematopoietic cells are mediated by various functional domains

Tenascin-C fibronectin D domain is involved in the fine-tuning of glial response to CNS injury in vitro

Understanding processes that occur after injuries to the central nervous system is essential in order to gain insight into how the restoration of function can be improved. Extracellular glycoprotein tenascin-C (TnC) has numerous functions in wound healing process depending on the expression time, location, isoform and binding partners which makes it interesting to study in this context. We used an in vitro injury model, the mixed culture of cortical astrocytes and microglia, and observed that without TnC microglial cells tend to populate gap area in greater numbers and proliferate more, whereas astrocytes build up in the border region to promote faster gap closure. Alternatively spliced domain of TnC, fibronectin type III-like repeat D (FnD) strongly affected physiological properties and morphology of both astrocytes and microglia in this injury model. The rate of microglial proliferation in the injury region decreased significantly with the addition of FnD. Additionally, density of microglia also decreased, in part due to reduced proliferation, and possibly due to reduced migration and increased contact inhibition between enlarged FnD-treated cells. Overall morphology of FnD-treated microglia resembled the activated pro-inflammatory cells, and elevated expression of iNOS was in accordance with this phenotype. The effect of FnD on astrocytes was different, as it did not affect their proliferation, but stimulated migration of reactivated astrocytes into the scratched area 48 h after the lesion. Elevated expression and secretion of TNF-α and IL-1β upon FnD treatment indicated the onset of inflammation. Furthermore, on Western blots we observed increased intensity of precursor bands of β1 integrin and appearance of monomeric bands of P2Y12R after FnD treatment which substantiates and clarifies its role in cellular shape and motility changes. Our results show versatile functions of TnC and in particular FnD after injury, mostly contributing to ongoing inflammation in the injury region. Based on our findings, FnD might be instrumental in limiting immune cell infiltration, and promoting astrocyte migration within the injury region, thus influencing spaciotemporal organization of the wound and surrounding area.

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.

The extracellular matrix of hematopoietic stem cell niches

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Comprehensive overview of different classes of ECM molecules in the HSC niche.

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Overview of current knowledge on role of biophysics of the HSC niche.

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Description of approaches to create artificial stem cell niches for several application.

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Importance of considering ECM in drug development and testing.

Hematopoietic stem cells (HSCs) are the life-long source of all types of blood cells. Their function is controlled by their direct microenvironment, the HSC niche in the bone marrow. Although the importance of the extracellular matrix (ECM) in the niche by orchestrating niche architecture and cellular function is widely acknowledged, it is still underexplored. In this review, we provide a comprehensive overview of the ECM in HSC niches. For this purpose, we first briefly outline HSC niche biology and then review the role of the different classes of ECM molecules in the niche one by one and how they are perceived by cells. Matrix remodeling and the emerging importance of biophysics in HSC niche function are discussed. Finally, the application of the current knowledge of ECM in the niche in form of artificial HSC niches for HSC expansion or targeted differentiation as well as drug testing is reviewed.

Chondroitin Sulfate as a Potential Modulator of the Stem Cell Niche in Cornea

Chondroitin sulfate (CS) is an important component of the extracellular matrix in multiple biological tissues. In cornea, the CS glycosaminoglycan (GAG) exists in hybrid form, whereby some of the repeating disaccharides are dermatan sulfate (DS). These CS/DS GAGs in cornea, through their presence on the proteoglycans, decorin and biglycan, help control collagen fibrillogenesis and organization. CS also acts as a regulatory ligand for a spectrum of signaling molecules, including morphogens, cytokines, chemokines, and enzymes during corneal growth and development. There is a growing body of evidence that precise expression of CS or CS/DS with specific sulfation motifs helps define the local extracellular compartment that contributes to maintenance of the stem cell phenotype. Indeed, recent evidence shows that CS sulfation motifs recognized by antibodies 4C3, 7D4, and 3B3 identify stem cell populations and their niches, along with activated progenitor cells and transitional areas of tissue development in the fetal human elbow. Various sulfation motifs identified by some CS antibodies are also specifically located in the limbal region at the edge of the mature cornea, which is widely accepted to represent the corneal epithelial stem cell niche. Emerging data also implicate developmental changes in the distribution of CS during corneal morphogenesis. This article will reflect upon the potential roles of CS and CS/DS in maintenance of the stem cell niche in cornea, and will contemplate the possible involvement of CS in the generation of eye-like tissues from human iPS (induced pluripotent stem) cells.

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

The influence of the bone marrow microenvironment on normal hematopoiesis, but also leukemia, has largely been accepted. However, the focus has been predominantly on the role of various cell types or cytokines maintaining hematopoietic stem cells or protecting leukemia stem cells from different therapies. A frequently overlooked component of the bone marrow microenvironment is the extracellular matrix, which not only provides a mechanical scaffold, but also serves as a source of growth factors. We discuss here how extracellular matrix proteins directly or indirectly modulate hematopoietic stem cell physiology and influence leukemia progression. It is hoped that existing and future studies on this topic may propel forward the possibility of augmenting normal hematopoiesis and improving therapies for leukemia, for instance, by targeting of the extracellular matrix in the bone marrow.

Tenascin-C deficiency impairs alveolarization and microvascular maturation during postnatal lung development

After the airways have been formed by branching morphogenesis the gas exchange area of the developing lung is enlarged by the formation of new alveolar septa (alveolarization). The septa themselves mature by a reduction of their double-layered capillary networks to single-layered ones (microvascular maturation). Alveolarization in mice is subdivided into a first phase (postnatal days 4-21, classical alveolarization), where new septa are lifted off from immature preexisting septa, and a second phase (day 14 to adulthood, continued alveolarization), where new septa are formed from mature septa. Tenascin-C (TNC) is a multidomain extracellular matrix protein contributing to organogenesis and tumorigenesis. It is highly expressed during classical alveolarization, but afterward its expression is markedly reduced. To study the effect of TNC deficiency on postnatal lung development, the formation and maturation of the alveolar septa were followed stereologically. Furthermore, the number of proliferating (Ki-67-positive) and TUNEL-positive cells was estimated. In TNC-deficient mice for both phases of alveolarization a delay and catch-up were observed. Cell proliferation was increased at days 4 and 6; at day 7, thick septa with an accumulation of capillaries and cells were observed; and the number of TUNEL-positive cells (dying cells or DNA repair) was increased at day 10. Whereas at days 15 and 21 premature microvascular maturation was detected, the microvasculature was less mature at day 60 compared with wild type. No differences were observed in adulthood. We conclude that TNC contributes to the formation of new septa, to microvascular maturation, and to cell proliferation and migration during postnatal lung development.NEW & NOTEWORTHY Previously, we showed that the extracellular matrix protein tenascin-C takes part in prenatal lung development by controlling branching morphogenesis. Now we report that tenascin-C is also important during postnatal lung development, because tenascin-C deficiency delays the formation and maturation of the alveolar septa during not only classical but also continued alveolarization. Adult lungs are indistinguishable from wild type because of a catch-up formation of new septa.

Matricellular proteins as regulators of cancer metastasis to bone

Metastasis is the major cause of death in cancer patients, and a frequent site of metastasis for many cancers is the bone marrow. Therefore, understanding the mechanisms underlying the metastatic process is necessary for future prevention and treatment. The tumor microenvironment is now known to play a role in the metastatic cascade, both at the primary tumor and in metastatic sites, and includes both cellular and non-cellular components. The extracellular matrix (ECM) provides structural support and signaling cues to cells. One particular group of molecules associated with the ECM, known as matricellular proteins, modulate multiple aspects of tumor biology, including growth, migration, invasion, angiogenesis and metastasis. These proteins are also important for normal function in the bone by regulating bone formation and bone resorption. Recent studies have described a link between some of these proteins and metastasis of various tumors to the bone. The aim of this review is to summarize what is currently known about matricellular protein influence on bone metastasis. Particular attention to the contribution of both tumor cells and non-malignant cells in the bone has been given.

Hematopoietic Stem and Progenitor Cell Expansion in Contact with Mesenchymal Stromal Cells in a Hanging Drop Model Uncovers Disadvantages of 3D Culture

Efficient ex vivo expansion of hematopoietic stem cells with a concomitant preservation of stemness and self-renewal potential is still an unresolved ambition. Increased numbers of methods approaching this issue using three-dimensional (3D) cultures were reported. Here, we describe a simplified 3D hanging drop model for the coculture of cord blood-derived CD34⁺ hematopoietic stem and progenitor cells (HSPCs) with bone marrow-derived mesenchymal stromal cells (MSCs). When seeded as a mixed cell suspension, MSCs segregated into tight spheroids. Despite the high expression of niche-specific extracellular matrix components by spheroid-forming MSCs, HSPCs did not migrate into the spheroids in the initial phase of coculture, indicating strong homotypic interactions of MSCs. After one week, however, HSPC attachment increased considerably, leading to spheroid collapse as demonstrated by electron microscopy and immunofluorescence staining. In terms of HSPC proliferation, the conventional 2D coculture system was superior to the hanging drop model. Furthermore, expansion of primitive hematopoietic progenitors was more favored in 2D than in 3D, as analyzed in colony-forming assays. Conclusively, our data demonstrate that MSCs, when arranged with a spread (monolayer) shape, exhibit better HSPC supportive qualities than spheroid-forming MSCs. Therefore, 3D systems are not necessarily superior to traditional 2D culture in this regard.

Tenascin C in metastasis: A view from the invasive front

The extracellular matrix protein tenascin C (TNC) is a large glycoprotein expressed in connective tissues and stem cell niches. TNC over-expression is repeatedly observed in cancer, often at the invasive tumor front, and is associated with poor clinical outcome in several malignancies. The link between TNC expression and poor survival in cancer patients suggests a role for TNC in metastatic progression, which is responsible for the majority of cancer related deaths. Indeed, functional studies using mouse models are revealing new roles of TNC in cancer progression and underscore its important contribution to the development of metastasis. TNC has a pleiotropic role in advancing metastasis by promoting migratory and invasive cell behavior, angiogenesis and cancer cell viability under stress. TNC is an essential component of the metastatic niche and modulates stem cell signaling within the niche. This may be crucial for the fitness of disseminated cancer cells confronted with a foreign environment in secondary organs, that can exert a strong selective pressure on invading cells. TNC is a compelling example of how an extracellular matrix protein can provide a molecular context that is imperative to cancer cell fitness in metastasis.

Matrix metalloproteinases in stem cell mobilization

Hematopoietic stem cells (HSCs) have the capability to migrate back and forth between their preferred microenvironment in bone marrow niches and the peripheral blood, but under steady-state conditions only a marginal number of stem cells can be found in the circulation. Different mobilizing agents, however, which create a highly proteolytic milieu in the bone marrow, can drastically increase the number of circulating HSCs. Among other proteases secreted and membrane-bound matrix metalloproteinases (MMPs) are known to be involved in the induced mobilization process and can digest niche-specific extracellular matrix components and cytokines responsible for stem cell retention to the niches. Iatrogenic stem cell mobilization and stem cell homing to their niches are clinically employed on a routine basis, although the exact mechanisms of both processes are still not fully understood. In this review we provide an overview on the various roles of MMPs in the induced release of HSCs from the bone marrow.

Tenascin C promotes hematoendothelial development and T lymphoid commitment from human pluripotent stem cells in chemically defined conditions

The recent identification of hemogenic endothelium (HE) in human pluripotent stem cell (hPSC) cultures presents opportunities to investigate signaling pathways that are essential for blood development from endothelium and provides an exploratory platform for de novo generation of hematopoietic stem cells (HSCs). However, the use of poorly defined human or animal components limits the utility of the current differentiation systems for studying specific growth factors required for HE induction and manufacturing clinical-grade therapeutic blood cells. Here, we identified chemically defined conditions required to produce HE from hPSCs growing in Essential 8 (E8) medium and showed that Tenascin C (TenC), an extracellular matrix protein associated with HSC niches, strongly promotes HE and definitive hematopoiesis in this system. hPSCs differentiated in chemically defined conditions undergo stages of development similar to those previously described in hPSCs cocultured on OP9 feeders, including the formation of VE-Cadherin⁺CD73⁻CD235a/CD43⁻ HE and hematopoietic progenitors with myeloid and T lymphoid potential.

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Hemogenic endothelium is generated in a completely defined xenogen-free system

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The system reproduces all stages of hematopoietic development

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Tenascin C enhances hematoendothelial development from pluripotent stem cells

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Tenesacin C uniquely supports T cell specification

The role of Tenascin C in the lymphoid progenitor cell niche

Hemopoietic stem cells (HSCs) are extrinsically controlled by the bone marrow (BM) microenvironment. Mice devoid of the extracellular matrix molecule Tenascin-C (TNC) were reported to develop normally. The current study explores the relationship between TNC and hemopoiesis, from HSCs within their niche to maturing progenitors in alternate niches. Although the absence of TNC did not alter the size of the BM stem cell pool, we report decreased thymic T cell progenitors with redistribution to other lymphoid organs, suggesting an anchoring role for TNC. TNC did not play an essential role in stem and progenitor cell homing to BM, but significantly altered lymphoid primed progenitor cell homing. These cells express the TNC receptor, integrin α9β1, with the same reduced homing evident in the absence of this integrin. The absence of TNC also resulted in an increased proportion and number of mature circulating T cells. In addition, the absence of TNC significantly impaired hemopoietic reconstitution after transplant and increased stem and progenitor cell mobilization. In summary, our analysis revealed unidentified roles for TNC in hemopoiesis: in lineage commitment of thymic T cell progenitors, peripheral T cell migration, and hemopoietic reconstitution.

A naturally derived cardiac extracellular matrix enhances cardiac progenitor cell behavior in vitro

Myocardial infarction (MI) produces a collagen scar, altering the local microenvironment and impeding cardiac function. Cell therapy is a promising therapeutic option to replace the billions of myocytes lost following MI. Despite early successes, chronic function remains impaired and is likely a result of poor cellular retention, proliferation, and differentiation/maturation. While some efforts to deliver cells with scaffolds have attempted to address these shortcomings, they lack the natural cues required for optimal cell function. The goal of this study was to determine whether a naturally derived cardiac extracellular matrix (cECM) could enhance cardiac progenitor cell (CPC) function in vitro. CPCs were isolated via magnetic sorting of c-kit(+) cells and were grown on plates coated with either cECM or collagen I (Col). Our results show an increase in early cardiomyocyte markers on cECM compared with Col, as well as corresponding protein expression at a later time. CPCs show stronger serum-induced proliferation on cECM compared with Col, as well as increased resistance to apoptosis following serum starvation. Finally, a microfluidic adhesion assay demonstrated stronger adhesion of CPCs to cECM compared with Col. These data suggest that cECM may be optimal for CPC therapeutic delivery, as well as providing potential mechanisms to overcome the shortcomings of naked cell therapy.

Endogenous activation of adaptive immunity: tenascin-C drives interleukin-17 synthesis in murine arthritic joint disease

OBJECTIVE

Rheumatoid arthritis is characterized by persistent synovial inflammation and progressive joint destruction, which are mediated by innate and adaptive immune responses. Cytokine blockade successfully treats some patient subsets; however, ∼50% do not respond to this approach. Targeting of pathogenic T lymphocytes is emerging as an effective alternative/complementary therapeutic strategy, yet the factors that control T cell activation in joint disease are not well understood. Tenascin-C is an arthritogenic extracellular matrix glycoprotein that is not expressed in healthy synovium but is elevated in the rheumatoid joint, where high levels are produced by myeloid cells. Among these cells, tenascin-C expression is most highly induced in activated dendritic cells (DCs). The aim of this study was to examine the role of tenascin-C in this cell type.

METHODS

We systematically compared the phenotype of DCs isolated from wild-type mice or mice with a targeted deletion of tenascin-C by assessing cell maturation, cytokine synthesis, and T cell polarization.

RESULTS

Dendritic cells derived from tenascin-C-null mice exhibited no defects in maturation; induction of the class II major histocompatibility complex and the costimulatory molecules CD40 and CD86 was unimpaired. Dendritic cells that did not express tenascin-C, however, produced lower levels of inflammatory cytokines than did cells from wild-type mice and exhibited specific defects in Th17 cell polarization. Moreover, tenascin-C-null mice displayed ablated levels of interleukin-17 in the joint during experimental arthritis.

CONCLUSION

These data demonstrate that tenascin-C is important in DC-mediated polarization of Th17 lymphocytes during inflammation and suggest a key role for this endogenous danger signal in driving adaptive immunity in erosive joint disease.

The role of tenascin-C in tissue injury and tumorigenesis

The extracellular matrix molecule tenascin-C is highly expressed during embryonic development, tissue repair and in pathological situations such as chronic inflammation and cancer. Tenascin-C interacts with several other extracellular matrix molecules and cell-surface receptors, thus affecting tissue architecture, tissue resilience and cell responses. Tenascin-C modulates cell migration, proliferation and cellular signaling through induction of pro-inflammatory cytokines and oncogenic signaling molecules amongst other mechanisms. Given the causal role of inflammation in cancer progression, common mechanisms might be controlled by tenascin-C during both events. Drugs targeting the expression or function of tenascin-C or the tenascin-C protein itself are currently being developed and some drugs have already reached advanced clinical trials. This generates hope that increased knowledge about tenascin-C will further improve management of diseases with high tenascin-C expression such as chronic inflammation, heart failure, artheriosclerosis and cancer.

The role of tenascin-C in tissue injury and tumorigenesis

The extracellular matrix molecule tenascin-C is highly expressed during embryonic development, tissue repair and in pathological situations such as chronic inflammation and cancer. Tenascin-C interacts with several other extracellular matrix molecules and cell-surface receptors, thus affecting tissue architecture, tissue resilience and cell responses. Tenascin-C modulates cell migration, proliferation and cellular signaling through induction of pro-inflammatory cytokines and oncogenic signaling molecules amongst other mechanisms. Given the causal role of inflammation in cancer progression, common mechanisms might be controlled by tenascin-C during both events. Drugs targeting the expression or function of tenascin-C or the tenascin-C protein itself are currently being developed and some drugs have already reached advanced clinical trials. This generates hope that increased knowledge about tenascin-C will further improve management of diseases with high tenascin-C expression such as chronic inflammation, heart failure, artheriosclerosis and cancer.

Characterization and functional analysis of osteoblast-derived fibulins in the human hematopoietic stem cell niche

OBJECTIVE

In the bone marrow stem cell niche, osteoblasts lining the endosteum are of major importance in supporting hematopoietic stem cell maintenance. Our objective was to analyze expression of the fibulins, highly conserved calcium-binding glycoproteins, which are components of the extracellular matrix of human osteoblasts, and to provide insights into their functional interactions with hematopoietic progenitor cells.

MATERIALS AND METHODS

Expression of the fibulins by human osteoblasts was determined by reverse transcription polymerase chain reaction analysis and by immunofluorescence staining and immunoblotting using fibulin-specific antisera. Recombinant fibulins were used in cell proliferation and differentiation assays with human CD34(+) hematopoietic progenitor cells. Adhesive interactions of CD34(+) cells with fibulins were investigated using cell-adhesion assays.

RESULTS

Human osteoblasts strongly express and secrete fibulin-1 and -2. Whereas fibulin-1 is secreted in its intact form, fibulin-2 synthesized by human osteoblasts undergoes rapid proteolytic degradation. The matrix metalloproteinase-2, which is constitutively expressed by the osteoblasts, seems to be responsible for fibulin-2 degradation. Fibulin-1 showed an inhibitory effect on short-term CD34(+) hematopoietic progenitor cell proliferation. Both fibulin-1 and fibulin-2 were able to diminish erythroid and myeloid colony formation. The CD34(+) cell line KG1a strongly attached to fibulin-2, whereas magnetic-activated cell sorted CD34(+) hematopoietic progenitors did not adhere to either fibulin-1 or fibulin-2. On the other hand, fibulin-1 can strongly interfere with CD34(+) cell adhesion to fibronectin.

CONCLUSION

Fibulins seem to be important components of the extracellular matrix of osteoblasts and are likely to negatively influence the proliferation rate of stem cells and the overall adhesive properties of the endosteal stem cell niche.

Role of Annexin-II in GI cancers: interaction with gastrins/progastrins

The role of the gastrin peptide hormones (G17, G34) and their precursors (progastrins, PG; gly-extended gastrin, G-gly), in gastrointestinal (GI) cancers has been extensively reviewed in recent years [W. Rengifo-Cam, P. Singh, Role of progastrins and gastrins and their receptors in GI and pancreatic cancers: targets for treatment, Curr. Pharm. Des. 10 (19) (2004) 2345-2358; M. Dufresne, C. Seva, D. Fourmy, Cholecystokinin and gastrin receptors, Physiol. Rev. 86 (3) (2006) 805-847; A. Ferrand, T.C. Wang, Gastrin and cancer: a review, Cancer Lett. 238 (1) (2006) 15-29]. A possible important role of progastrin peptides in colon carcinogenesis has become evident from experiments with transgenic mouse models [W. Rengifo-Cam, P. Singh, (2004); A. Ferrand, T.C. Wang, (2006)]. It is now known that growth stimulatory and co-carcinogenic effects of gastrin/PG peptides are mediated by both proliferative and anti-apoptotic effects of the peptides on target cells [H. Wu, G.N. Rao, B. Dai, P. Singh, Autocrine gastrins in colon cancer cells Up-regulate cytochrome c oxidase Vb and down-regulate efflux of cytochrome c and activation of caspase-3, J. Biol. Chem. 275 (42) (2000) 32491-32498; H. Wu, A. Owlia, P. Singh, Precursor peptide progastrin(1-80) reduces apoptosis of intestinal epithelial cells and upregulates cytochrome c oxidase Vb levels and synthesis of ATP, Am. J. Physiol. Gastrointest. Liver Physiol. 285 (6) (2003) G1097-G1110]. Several receptor subtypes have been described that mediate growth effects of gastrin peptides [W. Rengifo-Cam, P. Singh (2004); M. Dufresne, C. Seva, D. Fourmy, (2006)]. Recently, we identified Annexin II as a high affinity binding protein for gastrin/PG peptides [P. Singh, H. Wu, C. Clark, A. Owlia, Annexin II binds progastrin and gastrin-like peptides, and mediates growth factor effects of autocrine and exogenous gastrins on colon cancer and intestinal epithelial cells, Oncogene (2006), doi:10.1038/sj.onc.1209798]. Importantly, the expression of Annexin II was required for mediating growth stimulatory effects of gastrin and PG peptides on intestinal epithelial and colon cancer cells [P. Singh, H. Wu, C. Clark, A. Owlia, Annexin II binds progastrin and gastrin-like peptides, and mediates growth factor effects of autocrine and exogenous gastrins on colon cancer and intestinal epithelial cells, Oncogene (2006), doi:10.1038/sj.onc.1209798], suggesting that Annexin-II may represent the elusive novel receptor for gastrin/PG peptides. The importance of this finding in relation to the structure and function of Annexin-II, especially in GI cancers, is described below. Since this surprising finding represents a new front in our understanding of the mechanisms involved in mediating growth effects of gastrin/PG peptides in GI cancers, our current understanding of the role of Annexin-II in proliferation and metastasis of cancer cells is additionally reviewed.

The Integrin alpha9beta1 contributes to granulopoiesis by enhancing granulocyte colony-stimulating factor receptor signaling

The integrin alpha9beta1 is widely expressed on neutrophils, smooth muscle, hepatocytes, endothelia, and some epithelia. We now show that mice lacking this integrin have a dramatic defect in neutrophil development, with decreased numbers of granulocyte precursors in bone marrow and impaired differentiation of bone marrow cells into granulocytes. In response to granulocyte colony-stimulating factor (G-CSF), alpha9-deficient bone marrow cells or human bone marrow cells incubated with alpha9beta1-blocking antibody demonstrated decreased phosphorylation of signal transducer and activator of transcription 3 and extracellular signal-regulated protein kinase. These effects depended on the alpha9 subunit cytoplasmic domain, which was required for formation of a physical complex between alpha9beta1 and ligated G-CSF receptor. Integrin alpha9beta1 was required for granulopoiesis and played a permissive role in the G-CSF-signaling pathway, suggesting that this integrin could play an important role in disorders of granulocyte development and other conditions characterized by defective G-CSF signaling.

Tenascin-C induced signaling in cancer

Tenascin-C is an adhesion modulatory extracellular matrix molecule that is highly expressed in the microenvironment of most solid tumors. High tenascin-C expression reduces the prognosis of disease-free survival in patients with some cancers. The possible role of tenascin-C in tumor initiation and progression is addressed with emphasis on underlying signaling mechanisms. How tenascin-C affects malignant transformation, uncontrolled proliferation, angiogenesis, metastasis and escape from tumor immunosurveillance is summarized. Finally, we discuss how the phenotypes of tenascin-C knock-out mice may help define the roles of tenascin-C in tumorigenesis and how this knowledge could be applied to cancer therapy.

The Corneal Epithelial Stem Cell Niche

In recent years, it has become generally accepted that the corneal epithelial stem cells are localized in the basal cell layer of the limbal epithelium. However, a number of questions remain regarding the number, markers, generation, and maintenance of the corneal epithelial stem cells. One of the key questions concerns what makes up the microenvironment or niche that is responsible for allowing the stem cells to remain and function throughout the life of the tissue. This review will consider the unique aspects of the limbus and compare these to what is known about other stem cell niches.

Immunofluorescence and confocal laser scanning microscopy of chronic myeloproliferative disorders on archival formaldehyde-fixed bone marrow

Spatial analysis of the histoarchitecture and photographic documentation at high resolution are the principal advantages of confocal laser scanning microscopy (CLSM) over conventional fluorescence microscopy (CFM) if combined with appropriate software. Restrictions for the use of CFM and CLSM, on the other hand, include nonspecific background fluorescence, fading of photolabile fluorochromes, and both tissue-specific and fixation-induced autofluorescence. Most of those shortcomings can now be avoided. Autofluorescence, the most limiting factor of high-resolution CLSM, was recently controlled also for paraffin sections of archival formaldehyde-fixed tissues. This allowed the present study on cytoskeletal fibers and extracellular matrix proteins in both neoplastic cells of myeloproliferative disorders and in medullary stromal cells using CLSM under proper autofluorescence control. By multiple fluorescence labeling, we found that the intracellular smooth muscle alpha-actin (SMA) fibers and the two extracellular adhesive matrix proteins tenascin and fibronectin vary in their presence in stromal and/or myeloid cells according to the degree of bone marrow fibrosis in chronic myeloproliferative disorders (CMPDs). CLSM offers further insight in our attempts to understand a complex interplay between the two cellular compartments.

Involvement of large tenascin-C splice variants in breast cancer progression

Alternative splicing of fibronectin-like type III (FNIII) repeats of tenascin-C (Tn-C) generates a number of splice variants. The distribution of large variants, typical components of provisional extracellular matrices that are up-regulated during tumor stroma remodeling, was here studied by immunoblotting and immunohistochemistry using a monoclonal antibody against the FNIII B domain (named 4C8MS) in a series of human breast cancers. Large Tn-C variants were found at only low levels in normal breast tissues, but were highly expressed at invading sites of intraductal cancers and in the stroma of invasive ductal cancers, especially at invasion fronts. There was a positive correlation between the expression of large Tn-C variants and the cell proliferation rate determined by immunolabeling of the Ki-67 antigen. Of the Tn-C recombinant fragments (all FNIII repeats or mFNIII FL, the conserved FNIII domain only, the epidermal growth factor-like domain, and the fibrinogen-like domain) which were expressed by CHO-K1 cells transfected with mouse Tn-C cDNAs, only the mFNIII FL enhanced in vitro migration and mitotic activity of mammary cancer cells derived from a Tn-C-null mouse. Addition of 4C8MS blocked the function of mFNIII FL. These findings provide strong evidence that the FNIII alternatively spliced region has important roles in tumor progression of breast cancer.

The alternatively spliced domain TnFnIII A1A2 of the extracellular matrix protein tenascin-C suppresses activation-induced T lymphocyte proliferation and cytokine production

Several lines of evidences have suggested that T cell activation could be impaired in the tumor environment, a condition referred to as tumor-induced immunosuppression. We have previously shown that tenascin-C, an extracellular matrix protein highly expressed in the tumor stroma, inhibits T lymphocyte activation in vitro, raising the possibility that this molecule might contribute to tumor-induced immunosuppression in vivo. However, the region of the protein mediating this effect has remained elusive. Here we report the identification of the minimal region of tenascin-C that can inhibit T cell activation. Recombinant fragments corresponding to defined regions of the molecule were tested for their ability to inhibit in vitro activation of human peripheral blood T cells induced by anti-CD3 mAbs in combination with fibronectin or IL-2. A recombinant protein encompassing the alternatively spliced fibronectin type III domains of tenascin-C (TnFnIII A-D) vigorously inhibited both early and late lymphocyte activation events including activation-induced TCR/CD8 down-modulation, cytokine production, and DNA synthesis. In agreement with this, full length recombinant tenascin-C containing the alternatively spliced region suppressed T cell activation, whereas tenascin-C lacking this region did not. Using a series of smaller fragments and deletion mutants issued from this region, we have identified the TnFnIII A1A2 domain as the minimal region suppressing T cell activation. Single TnFnIII A1 or A2 domains were no longer inhibitory, while maximal inhibition required the presence of the TnFnIII A3 domain. Altogether, these data demonstrate that the TnFnIII A1A2 domain mediate the ability of tenascin-C to inhibit in vitro T cell activation and provide insights into the immunosuppressive activity of tenascin-C in vivo.

Knockout mice reveal a contribution of the extracellular matrix molecule tenascin-C to neural precursor proliferation and migration

The extracellular matrix glycoprotein tenascin-C is widely expressed in the vertebrate central nervous system (CNS) during development and repair. Despite multiple effects of tenascin-C on cell behaviour in culture, no structural abnormalities of the CNS and other organs have been found in adult tenascin-C-null mice, raising the question of whether this glycoprotein has a significant role in vivo. Using a transgenic approach, we have demonstrated that tenascin-C regulates both cell proliferation and migration in oligodendrocyte precursors during development. Knockout mice show increased rates of oligodendrocyte precursor migration along the optic nerve and reduced rates of oligodendrocyte precursor proliferation in different regions of the CNS. Levels of programmed cell death were reduced in areas of myelination at later developmental stages,providing a potential corrective mechanism for any reduction in cell numbers that resulted from the proliferation phenotype. The effects on cell proliferation are mediated via the αvβ3 integrin and an interaction with the platelet-derived growth factor-stimulated mitogenic pathway, emphasising the importance of both CNS extracellular matrix and integrin growth factor interactions in the regulation of neural precursor behaviour.

Involvement of E-cadherin in the Development of Erythroid Cells; Subject Heading

The cadherins represent a large family of structurally and functionally related cell adhesion molecules involved in morphogenesis of multicellular organisms and maintenance of solid tissues. In the hematopoietic system, however, almost nothing was known about the involvement of this family. PCR screening of RNA of human bone marrow mononuclear cells with specific primers for different classical cadherins revealed that members of this family are also expressed by bone marrow cells. Here we report that E-cadherin, which is mainly expressed by cells of epithelial origin, plays a critical role in the development of human erythrocytes. FACS analysis with human E-cadherin-specific antibodies and the use of immunoaffinity columns revealed that expression of E-cadherin is restricted to defined maturation stages of the erythropoietic cell lineage. Erythroblasts and normoblasts express E-cadherin, but mature erythrocytes do not. Lymphoid and all the other myeloid cell lineages do not express E-cadherin at any developmental stage. The differentiation of the erythroid lineage in vitro could be influenced by addition of anti-E-cadherin antibodies in a concentration dependent manner indicating a direct involvement of this cell adhesion molecule in the differentiation process. In line with these in vitro data is the finding that E-cadherin is down regulated during erythroleukemia on the developing erythroid cells. Our results suggest an unanticipated function of E-cadherin in the hematopoietic system.

Blockade of alpha 5 beta 1 integrins reverses the inhibitory effect of tenascin on chemotaxis of human monocytes and polymorphonuclear leukocytes through three-dimensional gels of extracellular matrix proteins

Tenascin is an extracellular matrix protein found in adults in T cell-dependent areas of lymphoid tissues, sites of inflammation, and tumors. We report here that it inhibited chemotaxis of chemoattractant-stimulated human monocytes and chemoattractant-stimulated polymorphonuclear leukocytes (PMN) through three-dimensional gels composed of collagen I or Matrigel, and chemotaxis of leukotriene B4-stimulated PMN through fibrin gels. The inhibitory effect of tenascin on monocyte or PMN chemotaxis through these matrices was reversed by Abs directed against alpha5beta1 integrins or by a peptide (GRGDSP) that binds to beta1 integrins. Tenascin did not affect leukotriene B4- or fMLP-stimulated expression of beta1 or beta2 integrins, but did exert a small inhibitory effect on PMN adhesion and closeness of apposition to fibrin(ogen)-containing surfaces. Thus, alpha5beta1 integrins mediate the inhibitory effect of tenascin on monocyte and PMN chemotaxis, without promoting close apposition between these leukocytes and surfaces coated with tenascin alone or with tenascin bound to other matrix proteins. This contrasts with the role played by alpha5beta1 integrins in promoting close apposition between fMLP-stimulated PMN and fibrin containing surfaces, thereby inhibiting chemotaxis of fMLP-stimulated PMN through fibrin gels. Thus, chemoattractants and matrix proteins regulate chemotaxis of phagocytic leukocytes by at least two different mechanisms: one in which specific chemoattractants promote very tight adhesion of leukocytes to specific matrix proteins and another in which specific matrix proteins signal cessation of migration without markedly affecting strength of leukocyte adhesion.

Identification and characterization of a conformational heparin-binding site involving two fibronectin type III modules of bovine tenascin-X

Tenascin-X is known as a heparin-binding molecule, but the localization of the heparin-binding site has not been investigated until now. We show here that, unlike tenascin-C, the recombinant fibrinogen-like domain of tenascin-X is not involved in heparin binding. On the other hand, the two contiguous fibronectin type III repeats b10 and b11 have a predicted positive charge at physiological pH, hence a set of recombinant proteins comprising these domains was tested for interaction with heparin. Using solid phase assays and affinity chromatography, we found that interaction with heparin was conformational and involved both domains 10 and 11. Construction of a three-dimensional model of domains 10 and 11 led us to predict exposed residues that were then submitted to site-directed mutagenesis. In this way, we identified the basic residues within each domain that are crucial for this interaction. Blocking experiments using antibodies against domain 10 were performed to test the efficiency of this site within intact tenascin-X. Binding was significantly reduced, arguing for the activity of a heparin-binding site involving domains 10 and 11 in the whole molecule. Finally, the biological significance of this site was tested by cell adhesion studies. Heparan sulfate cell surface receptors are able to interact with proteins bearing domains 10 and 11, suggesting that tenascin-X may activate different signals to regulate cell behavior.

Signal-regulatory protein alpha (SIRPalpha) but not SIRPbeta is involved in T-cell activation, binds to CD47 with high affinity, and is expressed on immature CD34(+)CD38(-) hematopoietic cells

Signal-regulatory proteins (SIRPs) represent a new family of inhibitory/activating receptor pairs. They consist of 3 highly homologous immunoglobulin (Ig)-like domains in their extracellular regions, but differ in their cytoplasmic regions by the presence (SIRPalpha) or absence (SIRPbeta) of immunoreceptor tyrosine-based inhibitory motifs (ITIMs). To analyze the differential expression on hematopoietic cells, function and ligand binding capacity of SIRPalpha and SIRPbeta molecules, soluble fusion proteins consisting of the extracellular domains of SIRPalpha1, SIRPalpha2, and SIRPbeta1, as well as SIRPalpha/beta-specific and SIRPbeta-specific monoclonal antibodies (MoAbs) were generated. In contrast to SIRPalpha1 and SIRPalpha2, no adhesion of SIRPbeta1 to CD47 could be detected by cell attachment assays and flow cytometry. Using deletion constructs of SIRPalpha1, the epitope responsible for SIRPalpha1 binding to CD47 could be confined to the N-terminal Ig-like loop. Flow cytometry analysis with SIRPalpha/beta- and SIRPbeta-specific MoAbs revealed that SIRPalpha but not SIRPbeta is expressed on CD34(+)CD38(-) hematopoietic cells. In addition, a strong SIRPalpha expression was also observed on primary myeloid dendritic cells (DCs) from peripheral blood as well as on in vitro generated DCs. Analysis of the T-cell stimulatory capacity of in vitro generated DCs in the presence of soluble SIRPalpha1 fusion proteins as well as SIRPalpha/beta-specific and CD47-specific MoAbs revealed a significant reduction of T-cell proliferation in mixed lymphocyte reaction and inhibition of induction of primary T-cell responses under these conditions. In contrast, soluble SIRPalpha or SIRPbeta-specific antibodies had no effect. The data suggest that the interaction of SIRPalpha with CD47 plays an important role during T-cell activation and induction of antigen-specific cytotoxic T-lymphocyte responses by DCs.

Binding of tenascin-X to decorin

Tenascin-X (TN-X) is an extracellular matrix protein whose absence results in an alteration of the mechanical properties of connective tissue. To understand the mechanisms of integration of TN-X in the extracellular matrix, overlay blot assays were performed on skin extracts. A 100 kDa molecule interacting with TN-X was identified by this method and this interaction was abolished when the extract was digested by chondroitinase. By solid-phase assays, we showed that dermatan sulfate chains of decorin bind to the heparin-binding site included within the fibronectin-type III domains 10 and 11 of TN-X. We thus postulate that the association of TN-X with collagen fibrils is mediated by decorin and contributes to the integrity of the extracellular network.

N-cadherin is developmentally regulated and functionally involved in early hematopoietic cell differentiation

The cadherins, an important family of cell adhesion molecules, are known to play major roles during embryonic development and in the maintenance of solid tissue architecture. In the hematopoietic system, however, little is known of the role of this cell adhesion family. By RT-PCR, western blot analysis and immunofluorescence staining we show that N-cadherin, a classical type I cadherin mainly expressed on neuronal, endothelial and muscle cells, is expressed on the cell surface of resident bone marrow stromal cells. FACS analysis of bone marrow mononuclear cells revealed that N-cadherin is also expressed on a subpopulation of early hematopoietic progenitor cells. Triple-color FACS analysis defined a new CD34(+) CD19(+) N-cadherin(+) progenitor cell population. During further differentiation, however, N-cadherin expression is lost. Treatment of CD34(+) progenitor cells with function-perturbing N-cadherin antibodies drastically diminished colony formation, indicating a direct involvement of N-cadherin in the differentiation program of early hematopoietic progenitors. N-cadherin can also mediate adhesive interactions within the bone marrow as demonstrated by inhibition of homotypic interactions of bone-marrow-derived cells with N-cadherin antibodies. Together, these data strongly suggest that N-cadherin is involved in the development and retention of early hematopoietic progenitors within the bone marrow microenvironment.

Characterization and functional analysis of laminin isoforms in human bone marrow

Laminins are a family of disulfide-linked heterotrimeric proteins consisting of 3 different subunits termed α, β, and γ chains. Combinations of 11 characterized laminin subunits (α1-α5, β1-β3, and γ1-γ3) generate at least 12 laminin isoforms, which can serve different functions. Although expression of laminin in the hematopoietic microenvironment has been known for many years, the nature of the laminin isoforms present in the human bone marrow is poorly characterized. The present study attempts to clarify this issue. Reverse transcriptase–polymerase chain reaction analysis of human bone marrow stromal cells suggested the expression of many laminin isoforms in the marrow. Northern blot and immunoblot analysis, however, showed that laminin-8/9 and laminin-10/11 are the most abundant laminin isoforms synthesized by human bone marrow stromal cells. Other isoforms, if present, certainly play a minor role in the hematopoietic microenvironment. Functionally, laminin-10/11 preparations showed strong adhesive interactions with human CD34+ cell lines. Antibodies against the β1 integrin subunit inhibited these interactions. Other laminin isoforms, especially laminin-1 and laminin-2/4, showed only weak or no adhesive interactions with the hematopoietic cell lines tested, explaining former negative results. In addition to its adhesion-mediating properties, laminin-10/11 preparations also showed a mitogenic activity for human hematopoietic progenitor cells. Taken together, these data suggest that laminin in the bone marrow plays a hitherto unexpected important function in the development of hematopoietic progenitor cells.

Characterization and functional analysis of laminin isoforms in human bone marrow

Laminins are a family of disulfide-linked heterotrimeric proteins consisting of 3 different subunits termed α, β, and γ chains. Combinations of 11 characterized laminin subunits (α1-α5, β1-β3, and γ1-γ3) generate at least 12 laminin isoforms, which can serve different functions. Although expression of laminin in the hematopoietic microenvironment has been known for many years, the nature of the laminin isoforms present in the human bone marrow is poorly characterized. The present study attempts to clarify this issue. Reverse transcriptase–polymerase chain reaction analysis of human bone marrow stromal cells suggested the expression of many laminin isoforms in the marrow. Northern blot and immunoblot analysis, however, showed that laminin-8/9 and laminin-10/11 are the most abundant laminin isoforms synthesized by human bone marrow stromal cells. Other isoforms, if present, certainly play a minor role in the hematopoietic microenvironment. Functionally, laminin-10/11 preparations showed strong adhesive interactions with human CD34+ cell lines. Antibodies against the β1 integrin subunit inhibited these interactions. Other laminin isoforms, especially laminin-1 and laminin-2/4, showed only weak or no adhesive interactions with the hematopoietic cell lines tested, explaining former negative results. In addition to its adhesion-mediating properties, laminin-10/11 preparations also showed a mitogenic activity for human hematopoietic progenitor cells. Taken together, these data suggest that laminin in the bone marrow plays a hitherto unexpected important function in the development of hematopoietic progenitor cells.

The tenascin family of ECM glycoproteins: structure, function, and regulation during embryonic development and tissue remodeling

The determination of animal form depends on the coordination of events that lead to the morphological patterning of cells. This epigenetic view of development suggests that embryonic structures arise as a consequence of environmental influences acting on the properties of cells, rather than an unfolding of a completely genetically specified and preexisting invisible pattern. Specialized cells of developing multicellular organisms are surrounded by a complex extracellular matrix (ECM), comprised largely of different collagens, proteoglycans, and glycoproteins. This ECM is a substrate for tissue morphogenesis, lends support and flexibility to mature tissues, and acts as an epigenetic informational entity in the sense that it transduces and integrates intracellular signals via distinct cell surface receptors. Consequently, ECM-receptor interactions have a profound influence on major cellular programs including growth, differentiation, migration, and survival. In contrast to many other ECM proteins, the tenascin (TN) family of glycoproteins (TN-C, TN-R, TN-W, TN-X, and TN-Y) display highly restricted and dynamic patterns of expression in the embryo, particularly during neural development, skeletogenesis, and vasculogenesis. These molecules are reexpressed in the adult during normal processes such as wound healing, nerve regeneration, and tissue involution, and in pathological states including vascular disease, tumorigenesis, and metastasis. In concert with a multitude of associated ECM proteins and cell surface receptors that include members of the integrin family, TN proteins impart contrary cellular functions, depending on their mode of presentation (i.e., soluble or substrate-bound) and the cell types and differentiation states of the target tissues. Expression of tenascins is regulated by a variety of growth factors, cytokines, vasoactive peptides, ECM proteins, and biomechanical factors. The signals generated by these factors converge on particular combinations of cis-regulatory elements within the recently identified TN gene promoters via specific transcriptional activators or repressors. Additional complexity in regulating TN gene expression is achieved through alternative splicing, resulting in variants of TN polypeptides that exhibit different combinations of functional protein domains. In this review, we discuss some of the recent advances in TN biology that provide insights into the complex way in which the ECM is regulated and how it functions to regulate tissue morphogenesis and gene expression.

Mechanical modulation of tenascin-C and collagen-XII expression during avian synovial joint formation

The objective of this study was to investigate how temporal and spatial patterns of characteristic extracellular matrix molecules are altered in the absence of normal functional skeletal muscle contractions during avian synovial joint development. By using in situ detection of protein and mRNA expression in developing avian feet and femorotibial joints from a wide range of developmental stages, we demonstrate that the morphological abnormalities that result from embryonic immobilization are associated with altered patterns of tenascin-C and collagen-XII expression within developing joint structures. As the joints fuse in immobilized embryos, the cells of the presumptive articular surface differentiate from flattened fibroblasts to more rounded chondrocytes and collagens XII and I are no longer detected at sites of complete joint fusion. Although the expression of collagen XII persists at normal levels elsewhere within the immobilized joint, tenascin-C expression is diminished within the chondroepiphysis, synovium, and tendons, as well as within the remains of the fibrous articular surface. This effect is most notable for the shortest tenascin variant (Tn190) within the chondroepiphysis and the largest variant (Tn230) within tendons, synovium, and the fibrous surface layer of the joint. This study thus provides in vivo support of previous in vitro work that suggests that tenascin expression is sensitive to external changes in mechanical loading environment. However, these data do not support a similar conclusion for collagen XII during early development.

Integrin expression during epithelial migration and restratification in the tenascin-C-deficient mouse cornea

In the unwounded cornea, tenascin-C localizes to a short stretch of the basement membrane zone at the corneoscleral junction or limbus. To determine whether the function of the limbus is affected by the absence of tenascin-C, mice possessing a deletion of tenascin-C and strain-matched wild-type mice are used in corneal debridement wounding experiments. The expression of integrins (alpha3, alpha9, and beta4) in the tenascin-C knockout corneas is evaluated by producing polyclonal cytoplasmic domain antipeptide sera and performing immunofluorescence microscopy. In addition, we evaluate the localization of several other proteins involved in wound healing, including fibronectin, laminin beta1, nidogen/entactin, and VCAM-1, in both the tenascin knockout and wild-type mice. There are no differences in healing rate, scarring, or neovascularization after corneal debridement wounds. alpha9 integrin is expressed at the limbal border of unwounded tenascin-C knockout animals and is upregulated during migration only after the larger wounds. At 8 weeks after larger wounds, the localization of alpha9 again becomes restricted to the limbal border. Results show that tenascin-C is not required for development or maintenance of the corneal limbus or for normal re-epithelialization of corneal epithelial cells after debridement wounding.

Human Signal-Regulatory Protein Is Expressed on Normal, But Not on Subsets of Leukemic Myeloid Cells and Mediates Cellular Adhesion Involving Its Counterreceptor CD47

Signal-regulatory proteins (SIRPs) comprise a novel transmembrane glycoprotein family involved in the negative regulation of receptor tyrosine kinase-coupled signaling pathways. To analyze the expression and function of SIRPs, we prepared soluble recombinant fusion proteins of the extracellular regions of SIRP1 and SIRP2, as well as a variety of monoclonal antibodies (MoAbs) against these domains. The antibodies reacted predominantly with monocytes, granulocytes, dendritic cells, and their precursors, as well as with bone marrow CD34+, AC133+, CD90+hematopoietic stem/progenitor cells. In contrast, SIRP expression was absent or significantly reduced on the majority of myeloid blasts from patients with acute myeloid leukemia (AML) or chronic myeloid leukemia (CML). Functional studies showed that the extracellular domains of SIRP1 and SIRP2 support adhesion of a number of primary hematopoietic cells and cell lines. This interaction could be blocked by 4 of 7 SIRP1-reactive MoAbs. In addition, SIRP1 and SIRP2 competed for the same cell binding site, suggesting a common widely expressed SIRP ligand. In an approach to identify this molecule, MoAbs were generated against the SIRP-binding cell line CCRF-CEM, and MoAb CC2C6 was selected because of its capacity to inhibit cell binding to SIRP1. Further analysis showed that this antibody recognized CD47, a ubiquitously expressed plasma membrane protein previously implicated in integrin function, host defense action, and neutrophil migration. In this study, we identify CD47 as the extracellular ligand for human SIRP and show that these two counterreceptors are involved in cellular adhesion.

Human Signal-Regulatory Protein Is Expressed on Normal, But Not on Subsets of Leukemic Myeloid Cells and Mediates Cellular Adhesion Involving Its Counterreceptor CD47

Signal-regulatory proteins (SIRPs) comprise a novel transmembrane glycoprotein family involved in the negative regulation of receptor tyrosine kinase-coupled signaling pathways. To analyze the expression and function of SIRPs, we prepared soluble recombinant fusion proteins of the extracellular regions of SIRP1 and SIRP2, as well as a variety of monoclonal antibodies (MoAbs) against these domains. The antibodies reacted predominantly with monocytes, granulocytes, dendritic cells, and their precursors, as well as with bone marrow CD34+, AC133+, CD90+hematopoietic stem/progenitor cells. In contrast, SIRP expression was absent or significantly reduced on the majority of myeloid blasts from patients with acute myeloid leukemia (AML) or chronic myeloid leukemia (CML). Functional studies showed that the extracellular domains of SIRP1 and SIRP2 support adhesion of a number of primary hematopoietic cells and cell lines. This interaction could be blocked by 4 of 7 SIRP1-reactive MoAbs. In addition, SIRP1 and SIRP2 competed for the same cell binding site, suggesting a common widely expressed SIRP ligand. In an approach to identify this molecule, MoAbs were generated against the SIRP-binding cell line CCRF-CEM, and MoAb CC2C6 was selected because of its capacity to inhibit cell binding to SIRP1. Further analysis showed that this antibody recognized CD47, a ubiquitously expressed plasma membrane protein previously implicated in integrin function, host defense action, and neutrophil migration. In this study, we identify CD47 as the extracellular ligand for human SIRP and show that these two counterreceptors are involved in cellular adhesion.

The tenascin-C knockout revisited

In the past seven years, two groups have independently produced tenascin-C-knockout mice. These mice are born alive and, originally, were described as showing no abnormalities. More recent studies, many involving pathological intervention, have shown that tenascin-C-knockout mice have several defects. The mice exhibit abnormal behaviour, as well as abnormalities in brain chemistry. They also show defects in structure and repair of neuromuscular junctions, in the ability to recover from snake-venom-induced glomerulonephritis and in chemically induced dermatitis. Healing of skin wounds is morphologically normal, but the mice exhibit defects in healing after suture injury of corneas. In both skin and corneal wounds, fibronectin expression is abnormally low in tenascin-C-knockout mice. Finally, in vitro studies indicate that haemopoietic activity is defective in bone marrow from these mice. When examined together, these studies provide evidence for precise functions for tenascin-C, as well as an explanation for why the sequence of tenascin-C is so highly phylogenetically conserved.

Detailed characterization of the human aorta-gonad-mesonephros region reveals morphological polarity resembling a hematopoietic stromal layer

The definitive long-term repopulating human hematopoietic stem cell, which seeds the adult blood system, was previously thought to derive from the extra-embryonic yolk sac. However, there is now considerable evidence that in both avian and murine systems, yolk sac hematopoietic cells are largely a transient, embryonic population and the definitive stem cell, in fact, derives from a distinct region within the embryonic mesoderm, the aorta-gonad-mesonephros region. In the human embryo, an analogous region has been found to contain a cluster of cells distinct from, but closely associated with, the ventral endothelium of the dorsal aorta, the appearance of which is restricted both spatially and temporally. We have used antibodies recognising hematopoietic regulatory factors to further characterise this region in the human embryo. These studies indicate that all factors examined, including vascular endothelial growth factor and its receptor FLK-1, Flt-3 ligand and its receptor STK-1, and stem cell leukemia transcription factor, are expressed by both hematopoietic cells in the cluster and endothelial cells. However, there is some discontinuity in cells directly underlying the cluster. Furthermore, we have identified a morphologically distinct region of densely-packed, rounded cells in the mesenchyme directly beneath the ventral wall of the dorsal aorta, and running along its entire length. In the preumbilical AGM region, directly underlying the hematopoietic cluster, but not at more rostral and caudal levels, this region of mesenchyme expresses tenascin-C, an extracellular matrix glycoprotein known to facilitate cell-cell interactions and migration. This region of cells may therefore provide the microenvironmental support for the intraembryonic development of definitive hematopoietic stem cells, a process in which tenascin-C may play a pivotal role.

Adhesive interactions of human multiple myeloma cell lines with different extracellular matrix molecules

Multiple myeloma represents a human B cell malignancy which is characterized by a predominant localization of the malignant cell clone within the bone marrow. With the exception of the terminal stage of the disease the myeloma tumor cells do not circulate in the peripheral blood. The bone marrow microenvironment is believed to play an important role in homing, proliferation and terminal differentiation of myeloma cells. Here we have studied the expression of several extracellular matrix (ECM) molecules in the bone marrow of multiple myeloma patients and analyzed their adhesive capacities with four different human myeloma-derived cell lines. All ECM molecules analyzed (tenascin, laminin, fibronectin, collagen types I, III, V and VI) could be detected in bone marrow cryostat sections of multiple myeloma patients. Adhesion assays showed that only laminin, the microfibrillar collagen type VI and fibronectin were strong adhesive components for the myeloma cell lines U266, IM-9, OPM-2 and NCI-H929. Tenascin and collagen type I were only weak adhesive substrates for these myeloma cells. Adhesion to laminin and fibronectin was beta 1-integrin-mediated since addition of anti-beta 1-integrin antibodies could inhibit the binding of the four different cell types to both matrix molecules. In contrast, integrins do not seem to be involved in binding of the myeloma cells to collagen type VI. Instead, inhibition of binding by heparin suggested that membrane-bound heparan sulfate proteoglycans are responsible ligands for binding to collagen type VI. Adhesion assays with several B-cell lines resembling earlier differentiation stages revealed only weak interactions with tenascin and no interactions with collagen type VI, laminin or fibronectin. In summary, the interactions of human myeloma cells with the extracellular matrix may explain the specific retention of the plasma cells within the bone marrow.