A quadripotential mesenchymal progenitor cell isolated from the marrow of an adult mouse

Treatment of neural injury with marrow stromal cells

We describe our preclinical studies on the use of bone-marrow stromal cells (MSC; an uncharacterised mixed population of plastic-adherent cells) in the treatment of neural injury. These cells obtained from donor rats or human beings have been directly transplanted into brain or administered intra-arterially or intravenously. MSC selectively target injured tissue and promote functional recovery. Signals that target inflammatory cells to injured tissue probably direct MSC to injury sites. Although some MSC express proteins typical of neural cells, the possibility that benefit is derived by replacement of infarcted tissue with differentiated MSC is highly unlikely. MSC activate endogenous restorative responses in injured brain, which include angiogenesis, neurogenesis, and synaptogenesis. Given the robust therapeutic benefit of these cells in the treatment of experimental neural injury, and the fact that MSC have been used in the treatment of other human disease, there is justification for further preclinical studies leading to clinical trials for the treatment of neural injury such as stroke.

Articular cartilage repair: basic science and clinical progress. A review of the current status and prospects

OBJECTIVE

To review the basic scientific status of repair in articular cartilage tissue and to assess the efficiency of current clinical therapies instigated for the treatment of structural lesions generated therein as a result of trauma or during the course of various diseases, notably osteoarthritis (OA). Current scientific trends and possible directions for the future will also be discussed.

DESIGN

A systematic and critical analysis is undertaken, beginning with a description of the spontaneous repair responses in different types of lesion. Surgical interventions aimed at inducing repair without the use of active biologics will then be considered, followed by those involving active biologics and those drawing on autogenic and allogeneic tissue transplantation principles. Cell transplantation approaches, in particular novel tissue engineering concepts, will be critically presented. These will include growth-factor-based biological treatments and gene transfection protocols. A number of technical problems associated with repair interventions, such as tissue integration, tissue retention and the role of mechanical factors, will also be analysed.

RESULTS

A critical analysis of the literature reveals the existence of many novel and very promising biologically-based approaches for the induction of articular cartilage repair, the vast majority of which are still at an experimental phase of development. But prospective, double-blinded clinical trials comparing currently practiced surgical treatments have, unfortunately, not been undertaken.

CONCLUSION

The existence of many new and encouraging biological approaches to cartilage repair justifies the future investment of time and money in this research area, particularly given the extremely high socio-economic importance of such therapeutic strategies in the prevention and treatment of these common joint diseases and traumas. Clinical epidemiological and prospective trials are, moreover, urgently needed for an objective, scientific appraisal of current therapies and future novel approaches.

Rat extramedullary adipose tissue as a source of osteochondrogenic progenitor cells

Human liposuction aspirates contain pluripotent adipose-derived mesodermal stem cells that have previously been shown to differentiate into various mesodermal cell types, including osteoblasts and chondrocytes. To develop an autologous research model of bone and cartilage tissue engineering, the authors sought to determine whether rat inguinal fat pads contain a similar population of osteochondrogenic precursor cells. It was hypothesized that the rat inguinal fat pad contains adipose-derived multipotential cells that resemble human adipose-derived mesodermal stem cells in their osteochondrogenic capacity. To test this, the authors assessed the ability of cells isolated from the rat inguinal fat pad to differentiate into osteoblasts and chondrocytes by a variety of lineage-specific histologic stains. Rat inguinal fat pads were isolated and processed from Sprague-Dawley rats into a fibroblast-like cell population. Cell cultures were placed in pro-osteogenic media containing dexamethasone, ascorbic acid, and beta-glycerol phosphate. Osteogenic differentiation was assessed at 2, 4, and 6 weeks. Alkaline phosphatase activity and von Kossa staining were performed to assess osteoblastic differentiation and the production of a calcified extracellular matrix. Cell cultures were also placed in prochondrogenic conditions and media supplemented with transforming growth factor-beta1, insulin, transferrin, and ascorbic acid. Chondrogenic differentiation was assessed at 2, 7, and 14 days by the presence of positive Alcian blue staining and type II collagen immunohistochemistry. Cells placed in osteogenic conditions changed in structure to a more cuboidal shape, formed bone nodules, stained positively for alkaline phosphatase activity, and secreted calcified extracellular matrix by 2 weeks. Cells placed in chondrogenic conditions formed cartilaginous nodules within 48 hours that stained positively for Alcian blue and type II collagen. The authors identified the rat inguinal fat pad as a source of osteochondrogenic precursors and developed a straightforward technique to isolate osteochondrogenic precursors from a small animal source. This relatively easily obtained source of osteochondrogenic cells from the rat may be useful for study of tissue engineering strategies and the basic science of stem cell biology.

Human marrow-derived mesenchymal progenitor cells: isolation, culture expansion, and analysis of differentiation

A number of adult mesenchymal tissues contain subpopulations of undifferentiated cells, which retain the capacity to differentiate along multiple lineages. These mesenchymal progenitor cells may be cultured in an undifferentiated state and, when given the appropriate signals, differentiate into an expanding list of several mesenchymal and a few ectodermal derived tissues. The maintenance and propagation of the multipotential nature of these progenitor cell populations are crucially dependent on the isolation protocol, the culture expansion conditions, particularly the properties of the fetal bovine serum supplement in the culture medium. This article describes a method for selection of the appropriate serum lot, and introduces a simplified isolation technique to optimize the yield of progenitor cells that maintain the capability of undergoing multilineage differentiation in response to appropriate cues. Cell populations isolated and culture expanded in this manner, by virtue of their multiple differentiation potential, should serve as ideal candidate cells for tissue engineering applications for the repair and regeneration of tissue damaged by disease and or trauma.

Exposure of KS483 cells to estrogen enhances osteogenesis and inhibits adipogenesis

Osteoblasts and adipocytes arise from a common progenitor cell in bone marrow. Whether estrogen directly regulates the progenitor cells differentiating into osteoblasts or adipocytes remains unknown. Using a mouse clonal cell line KS483 cultured in charcoal-stripped fetal bovine serum (FBS), we showed that 17beta-estradiol (E2) stimulates the differentiation of progenitor cells into osteoblasts and concurrently inhibits adipocyte formation in an estrogen receptor (ER)-dependent way. E2 increased alkaline phosphate (ALP) activity and nodule formation and stimulated messenger RNA (mRNA) expression of core-binding factor alpha-1 (Cbfa1), parathyroid hormone/parathyroid hormone-related protein receptors (PTH/PTHrP-Rs), and osteocalcin. In contrast, E2 decreased adipocyte numbers and down-regulated mRNA expression of peroxisome proliferator-activated receptor-gamma (PPARgamma)2, adipocyte protein 2 (aP2), and lipoprotein lipase (LPL). Furthermore, the reciprocal control of osteoblast and adipocyte differentiation by E2 was observed also in the presence of the adipogenic mixture of isobutylmethylxanthine, dexamethasone, and insulin. Immunohistochemical staining showed that ERalpha and ERbeta were present in osteoblasts and adipocytes. A new mouse splice variant ERbeta2 was identified, which differed in two amino acid residues from the rat isoform. E2 down-regulated mRNA expression of ERalpha, ERbeta1, and ERbeta2. The effects of E2 are not restricted to the KS483 cell line because similar results were obtained in mouse bone marrow cell cultures. Our results indicate that estrogen, in addition to stimulation of osteogenesis, inhibits adipogenesis, which might explain the clinical observations that estrogen-deficiency leads to an increase in adipocytes.

Human BMP-7/OP-1 induces the growth and differentiation of adipocytes and osteoblasts in bone marrow stromal cell cultures

We studied the effects of BMP-7/OP-1 on growth and differentiation of bone marrow stromal cells. BMS2, a mouse bone marrow stromal cell line capable of differentiating into adipocytes and osteoblasts, were treated in a serum-free medium containing differentiation agents that favor the expression of both lineages. BMP-7/OP-1 stimulated cell proliferation and differentiation concomitantly. These effects were dose- and growth phase-dependent. Cells were more sensitive to the treatment early in the culture (30-40% confluence) with a significant increase in cell proliferation and markers of differentiation at low concentrations. When treated later in the growth phase (90-100% confluence), no significant increase in cell proliferation was seen. The concentration requirement for cells later in the culture to reach an equivalent degree of differentiation was 3-10- fold higher than for cells treated early. In both cases, the effects on adipocyte differentiation were biphasic; low concentrations stimulated adipocyte differentiation which was inhibited at higher concentrations where stimulation of osteoblast markers were observed. We conclude that cell proliferation and cell differentiation into adipocyte/osteoblast can occur simultaneously under BMP-7/OP-1 treatment.

MicroSAGE analysis of 2,353 expressed genes in a single cell-derived colony of undifferentiated human mesenchymal stem cells reveals mRNAs of multiple cell lineages

Mesenchymal stem cells (MSCs) isolated from the bone marrow of adult organisms are capable of differentiating into adipocytes, chondrocytes, myoblasts, osteoblasts, and hematopoiesis-supporting stroma. We recently demonstrated that MSCs also adopt glial cell fates when transplanted into the developing central nervous system and hence can produce tissue elements derived from a separate embryonic layer. Despite these remarkable properties, it has been difficult to establish specific criteria to characterize MSCs. Using a modified protocol for micro-serial analysis of gene expression, we cataloged 2,353 unique genes expressed by a single cell-derived colony of undifferentiated human MSCs. This analysis revealed that the MSC colony simultaneously expressed transcripts characteristic of various mesenchymal cell lineages including chondrocytes, myoblasts, osteoblasts, and hematopoiesis-supporting stroma. Therefore, the profile of expressed transcripts reflects the developmental potential of the cells. Additionally, the MSC colony expressed mRNAs characteristic of endothelial, epithelial and neuronal cell lineages, a combination that provides a unique molecular signature for the cells. Other expressed transcripts included various products involved in wound repair as well as several neurotrophic factors. A total of 268 novel transcripts were also identified, one of which is the most abundantly expressed mRNA in MSCs. This study represents the first extensive gene expression analysis of MSCs and as such reveals new insight into the biology, ontogeny, and in vivo function of the cells.

Extracellular matrix mineralization and osteoblast gene expression by human adipose tissue-derived stromal cells

Human adipose tissue represents an abundant reservoir of stromal cells with potential utility for tissue engineering. The current study demonstrates the ability of human adipose tissue-derived stromal cells to display some of the hallmarks of osteoblast differentiation in vitro. Following treatment with ascorbate, beta-glycerophosphate, dexamethasone, and 1,25 dihydroxy vitamin D(3), adipose tissue-derived stromal cells mineralize their extracellular matrix based on detection of calcium phosphate deposits using Alizarin Red and von Kossa histochemical stains. Fourier transform infrared analysis demonstrates the apatitic nature of these crystals. Mineralization is accompanied by increased expression or activity of the osteoblast-associated proteins osteocalcin and alkaline phosphatase. These and other osteoblast-associated gene markers are detected based on polymerase chain reaction. In contrast, the adipocyte gene markers--leptin, lipoprotein lipase, and peroxisome proliferator activated receptor gamma2--are reduced under mineralization conditions, consistent with the reciprocal relationship postulated to exist between adipocytes and osteoblasts. The current work supports the presence of a multipotent stromal cell population within human extramedullary adipose tissue. These findings have potential implications for human bone tissue bioengineering.

High-dose chemotherapy shows a dose-dependent toxicity to bone marrow osteoprogenitors: a mechanism for post-bone marrow transplantation osteopenia

BACKGROUND

Osteoporosis is a sequela of hemopoietic cell transplantation with a complex multifactorial pathogenesis in which the relative role of chemotherapy and irradiation is not completely understood. Therefore, the authors investigated the toxicity of chemotherapy-only conditioning regimens on bone homeostasis and bone marrow osteoprogenitors, its dose dependency, and the mechanism of chemotherapy-induced osteopenia.

METHODS

Fifty-one patients with high-grade non-Hodgkin lymphoma or breast carcinoma who had been treated previously with high-dose + peripheral blood progenitor cell or conventional chemotherapy or who had not received any treatment (prechemotherapy) were enrolled. The authors measured the bone marrow colony-forming unit fibroblast (CFU-f) and long-term culture-initiating cell frequency, forearm bone mineral density, serum osteotropic hormones and metabolic markers of bone formation (plasma osteocalcin), and resorption (urinary collagen I C-crosslinks).

RESULTS

Both high-dose chemotherapy regimens caused a 50% reduction in CFU-f frequency, independently of gonadal function status, whereas conventional chemotherapy and prechemotherapy groups were unaffected. Bone mineral density was measured in 26 non-Hodgkin lymphoma patients and again only high-dose chemotherapy caused a 10% loss in cortical bone and 20% in trabecular bone. No endocrine abnormality was found except for the secondary amenorrhea uniformly induced in the high-dose chemotherapy group. In these patients, plasma osteocalcin unexpectedly failed to increase in response to the menopausal increase in bone resorption rate, showing a selective impairment of the osteoblast compartment to cope with increased functional demand.

CONCLUSIONS

Chemotherapy without irradiation shows a dose-dependent toxicity to bone marrow stromal osteoprogenitors and can cause osteopenia by direct damage of the osteoblastic compartment, as a mechanism distinct from and summable to hypogonadism.

Phenotypic differentiation of TGF-beta1-responsive pluripotent premesenchymal prehematopoietic progenitor (P4 stem) cells from murine bone marrow

On the horizon of modern molecular medicine is the requisite technology to capture multipotent human stem cells that are capable of self-renewal and to direct these stem cells along defined lineages for therapeutic purposes. In this article, we describe the hematopoietic and mesenchymal differentiation potential of a unique population of transforming growth factor-beta1 (TGF-beta1)-responsive stem cells derived from murine bone marrow. Stringent selection of the stem cells was accomplished under low serum conditions by virtue of an inherent survival response to a TGF-beta1-vWF fusion protein that was bound to collagen matrices. The TGF-beta1-responsive stem cells initially exhibited a non-adherent and uniformly blastoid morphology, underwent expansion into colonies upon serum reconstitution, and were capable of overt cytodifferentiation along fibrogenic, osteogenic, chondrogenic, or adipogenic lineages upon growth factor stimulation. Remarkably, these stem cells also underwent rapid expansion in the presence of either hematopoietic stem cell factor (SCF) or interleukin3 (IL-3), and differentiated into myeloid and lymphoid phenotypes upon exposure to the latter. Taken together, these results support the hypothesis that pluripotent premesenchymal prehematopoietic progenitor cells, designated P4 stem cells, are present postnatally in murine bone marrow and, thus, may be summarily isolated for various cell-based experimental therapies.

Bone marrow stromal damage after chemo/radiotherapy: occurrence, consequences and possibilities of treatment

High dose chemotherapy (CT) followed by bone marrow transplant (BMT) is increasingly used for the treatment of both hematological and solid neoplasms, but an understanding of its late consequences on the marrow microenvironment is still only at its beginning. It is in fact known that marrow stroma is damaged by high-dose cytotoxic therapy and by radiation exposure. However little is known on the extent of this damage and on the self-repair ability of the stroma. The damage of the stromal microenvironment affects the long-term stem cell engraftment and the maintenance of hemopoietic functions. Furthermore, marrow stroma also represents a progenitor compartment for endosteal osteoblasts, and therefore its damage implies alterations of bone metabolism. Indeed, osteoporosis has recently been recognized as a consequence, of BMT, but only a few studies have been performed to establish the functional status of the stromal compartment after treatment with cytotoxic drugs with or without total body irradiation (TBI) and its role in post-BMT sequelae.

Col1a1-driven transgenic markers of osteoblast lineage progression

The modular organization of the type I collagen promoter allows creation of promoter-reporter constructs with preferential activity in different type I collagen-producing tissues that might be useful to mark cells at different stages of osteoblastic differentiation. Primary marrow stromal cell (MSC) and mouse calvarial osteoblast (mCOB) cultures were established from transgenic mice harboring different Col1a1 promoter fragments driving chloramphenicol acetyltransferase (CAT). In these models, Col1a1 messenger RNA (mRNA) and alkaline phosphatase (ALP) are the first markers of differentiation appearing soon after the colonies develop. Bone sialoprotein (BSP) is detected 2-3 days later, followed by osteocalcin (OC) expression and nodule mineralization. A 3.6 Col1a1 fragment (ColCAT3.6) initiated activity concomitant with ALP staining and type I collagen mRNA expression. In contrast, a 2.3 Col1a1 fragment (ColCAT2.3) became active coincident with BSP expression. The pattern of transgene expression assessed by immunostaining was distinctly different. ColCAT3.6 was expressed within and at the periphery of developing nodules whereas the ColCAT2.3 expression was restricted to the differentiated nodules. The feasibility of using green fluorescent protein (GFP) as a marker of osteoblast differentiation was evaluated in ROS17/2.8 cells. A 2.3-kilobase (kb) Col1a1 promoter driving GFP (pOB4Col2.3GLP) was stably transfected into the cell line and positive clones were selected. Subcultures lost and then regained GFP expression that was localized in small clusters of cells throughout the culture. This suggests that expression from the 2.3-kb Col1A1 fragment is determined by the state of differentiation of the ROS17/2.8 cells. Col1a1 transgenes should be useful in appreciating the heterogeneity of a primary or immortalized culture undergoing osteoblastic differentiation.

Use of VSV-G pseudotyped retroviral vectors to target murine osteoprogenitor cells

Marrow stromal cells (MSC) and neonatal calvarial cells have the potential to differentiate and express markers of mature osteoblasts. Furthermore, MSCs can generate multiple differentiated connective tissue phenotypes. These properties and their ability to be expanded ex vivo make them good models for ex vivo gene therapy. In this study we examined the ability of vesicular stomatitis virus (VSV-G) pseudotyped retroviral vectors to transduce osteoprogenitor cells derived from bone marrow and from neonatal calvaria. Retrovectors encoding either beta-galactosidase or green fluorescent protein (eGFP) were used for transduction of primary murine marrow stromal and primary neonatal calvarial cell cultures. High infection efficiency was demonstrated by fluorescence-activated cell analysis when GFP was used as a marker or by estimating the number of beta-galactosidase-positive cells. Expression of markers of differentiated bone cells, including Col1a1, bone sialoprotein, and osteocalcin mRNA and alkaline phosphatase activity was not impaired by retroviral transduction. Our data suggest that VSV-G pseudotypes retroviral vectors are suitable for introducing genes into osteoprogenitor cells without affecting osteoprogenitor lineage progression.

Mesenchymal stem cells

Within the bone marrow stroma there exists a subset of nonhematopoietic cells referred to as mesenchymal stem or mesenchymal progenitor cells. These cells can be ex vivo expanded and induced, either in vitro or in vivo, to terminally differentiate into osteoblasts, chondrocytes, adipocytes, tenocytes, myotubes, neural cells, and hematopoietic-supporting stroma. The multipotential of these cells, their easy isolation and culture, as well as their high ex vivo expansive potential make these cells an attractive therapeutic tool. In this work we will review the information dealing with the biology of mesenchymal progenitors as it has been revealed mainly by ex vivo studies performed with bone marrow-derived cells. The discussed topics include, among others, characteristics of mesenchymal progenitors, evidence for the existence of a vast repertoire of uncommitted and committed progenitors both in the bone marrow and in mesenchymal tissues, a diagram for their proliferative hierarchy, and comments on mobilization, microenvironment, and clinical use of mesenchymal progenitors. Despite the enormous data available at molecular and cellular levels, it is evident that a number of fundamental questions still need to be resolved before mesenchymal progenitors can be used for safe and effective clinical applications in the context of both cell and gene therapies.

Mesenchymal stem cells: building blocks for molecular medicine in the 21st century

Mesenchymal stem sells (MSCs) are present in a variety of tissues during human development, and in adults they are prevalent in bone marrow. From that readily available source, MSCs can be isolated, expanded in culture, and stimulated to differentiate into bone, cartilage, muscle, marrow stroma, tendon, fat and a variety of other connective tissues. Because large numbers of MSCs can be generated in culture, tissue-engineered constructs principally composed of these cells could be re-introduced into the in vivo setting. This approach is now being explored to regenerate tissues that the body cannot naturally repair or regenerate when challenged. Moreover, MSCs can be transduced with retroviral and other vectors and are, thus, potential candidates to deliver somatic gene therapies for local or systemic pathologies. Untapped applications include both diagnostic and prognostic uses of MSCs and their descendents in healthcare management. Finally, by understanding the complex, multistep and multifactorial differentiation pathway from MSC to functional tissues, it might be possible to manipulate MSCs directly in vivo to cue the formation of elaborate, composite tissues in situ.

Reciprocal control of osteoblast/chondroblast and osteoblast/adipocyte differentiation of multipotential clonal human marrow stromal F/STRO-1(+) cells

The regulation of human bone marrow stromal precursor cell differentiation toward the chondrocyte, osteoblast or adipocyte lineages is not known. In this study, we assessed the lineage-specific differentiation and conversion of immortalized clonal F/STRO-1(+) A human fetal bone marrow stromal cells under the control of dexamethasone (Dex), indomethacin/insulin (Indo/Ins) and linoleic acid (LA). Under basal conditions, F/STRO-1(+) A cells expressed markers mRNAs or proteins of the osteoblast lineage [CBFA1, osteocalcin (OC), alkaline phosphatase (ALP), type 1 collagen], of the chondrocyte lineage (aggrecan, types 2, 9 and 10 collagen), and of the adipocyte lineage (PPARgamma2, C/EBPalpha, aP2, G3PDH, lipoprotein lipase, leptin). Treatment with Dex increased CBFA1, OC and ALP mRNA and protein levels. Exposure to LA enhanced expression of adipocytic genes and cytoplasmic triglycerides accumulation, and suppressed the Dex-induced stimulation of osteoblast marker genes. Indo/Ins stimulated the synthesis of aggrecan and type 2 collagen and increased types 9 and 10 collagen mRNA levels, and suppressed both basal and Dex-promoted expression of osteoblast markers. Conversely, stimulation of osteoblastogenesis by Dex suppressed both basal and Indo/Ins-stimulated chondrocyte genes. Thus, the clonal human fetal bone marrow stromal F/STRO-1(+) A cell line is a lineage-unrestricted common progenitor that expresses tripotential adipocyte, osteoblast or chondrocyte characteristics. Our data also show that differentiation towards one pathway in response to Dex, Indo/Ins and LA restricts expression of other lineage-specific genes, and provide evidence for a controlled reciprocal regulation of osteoblast/chondroblast and osteoblast/adipocyte differentiation of clonal F/STRO-1(+) human bone marrow stromal cells.

Multilineage cells from human adipose tissue: implications for cell-based therapies

Future cell-based therapies such as tissue engineering will benefit from a source of autologous pluripotent stem cells. For mesodermal tissue engineering, one such source of cells is the bone marrow stroma. The bone marrow compartment contains several cell populations, including mesenchymal stem cells (MSCs) that are capable of differentiating into adipogenic, osteogenic, chondrogenic, and myogenic cells. However, autologous bone marrow procurement has potential limitations. An alternate source of autologous adult stem cells that is obtainable in large quantities, under local anesthesia, with minimal discomfort would be advantageous. In this study, we determined if a population of stem cells could be isolated from human adipose tissue. Human adipose tissue, obtained by suction-assisted lipectomy (i.e., liposuction), was processed to obtain a fibroblast-like population of cells or a processed lipoaspirate (PLA). These PLA cells can be maintained in vitro for extended periods with stable population doubling and low levels of senescence. Immunofluorescence and flow cytometry show that the majority of PLA cells are of mesodermal or mesenchymal origin with low levels of contaminating pericytes, endothelial cells, and smooth muscle cells. Finally, PLA cells differentiate in vitro into adipogenic, chondrogenic, myogenic, and osteogenic cells in the presence of lineage-specific induction factors. In conclusion, the data support the hypothesis that a human lipoaspirate contains multipotent cells and may represent an alternative stem cell source to bone marrow-derived MSCs.

Basic science of articular cartilage repair

As the ability to understand the peculiarities of successful healing of articular cartilage defects moves forward, it becomes clear that this complex orthopaedic problem soon will be successfully addressed. A multidisciplinary approach, combining clinical experience, cogent biomaterial designs, new cell biologic processes, biomechanical assessment, and modern molecular biology, clearly is leading toward clinically acceptable, viable, and consistent articular cartilage regeneration.

Adult stem cell plasticity and methods of detection

The ability to selectively produce one or more differentiated cell types at will from totipotent stem cells would be of profound clinical importance, as it would enable the specific replacement of damaged/dysfunctional cell types within the body, potentially curing numerous diseases. Until recently, it was thought that the only cells that possessed sufficient immaturity to be capable of giving rise to more than one tissue type in vitro and in vivo were the embryonic stem cells. However, recent studies have now provided compelling evidence that the adult bone marrow, brain and skeletal muscle contain stem cells that possess the remarkable ability to trans-differentiate and give rise to progeny of alternate embryologic derivations. These recent findings have shattered the existing dogma that the stages of embryologic development are irreversible. In this review, we present a brief summary of the most significant findings in the field of stem cell plasticity, emphasizing studies involving the hematopoietic system, discussing the models used thus far, and finishing with our findings on human stem cell plasticity using the fetal sheep model.

Differentiation of osteoblasts and in vitro bone formation from murine embryonic stem cells

Pluripotent embryonic stem (ES) cells have the potential to differentiate to all fetal and adult cell types and might represent a useful cell source for tissue engineering and repair. Here we show that differentiation of ES cells toward the osteoblast lineage can be enhanced by supplementing serum-containing media with ascorbic acid, beta-glycerophosphate, and/or dexamethasone/retinoic acid or by co-culture with fetal murine osteoblasts. ES cell differentiation into osteoblasts was characterized by the formation of discrete mineralized bone nodules that consisted of 50-100 cells within an extracellular matrix of collagen-1 and osteocalcin. Dexamethasone in combination with ascorbic acid and beta-glycerophosphate induced the greatest number of bone nodules and was dependent on time of stimulation with a sevenfold increase when added to ES cultures after, but not before, 14 days. Co-culture with fetal osteoblasts also provided a potent stimulus for osteogenic differentiation inducing a fivefold increase in nodule number relative to ES cells cultured alone. These data demonstrate the application of a quantitative assay for the derivation of osteoblast lineage progenitors from pluripotent ES cells. This could be applied to obtain purified osteoblasts to analyze mechanisms of osteogenesis and for use of ES cells in skeletal tissue repair.

Extracellular calcium sensing and extracellular calcium signaling

The cloning of a G protein-coupled extracellular Ca(2+) (Ca(o)(2+))-sensing receptor (CaR) has elucidated the molecular basis for many of the previously recognized effects of Ca(o)(2+) on tissues that maintain systemic Ca(o)(2+) homeostasis, especially parathyroid chief cells and several cells in the kidney. The availability of the cloned CaR enabled the development of DNA and antibody probes for identifying the CaR's mRNA and protein, respectively, within these and other tissues. It also permitted the identification of human diseases resulting from inactivating or activating mutations of the CaR gene and the subsequent generation of mice with targeted disruption of the CaR gene. The characteristic alterations in parathyroid and renal function in these patients and in the mice with "knockout" of the CaR gene have provided valuable information on the CaR's physiological roles in these tissues participating in mineral ion homeostasis. Nevertheless, relatively little is known about how the CaR regulates other tissues involved in systemic Ca(o)(2+) homeostasis, particularly bone and intestine. Moreover, there is evidence that additional Ca(o)(2+) sensors may exist in bone cells that mediate some or even all of the known effects of Ca(o)(2+) on these cells. Even more remains to be learned about the CaR's function in the rapidly growing list of cells that express it but are uninvolved in systemic Ca(o)(2+) metabolism. Available data suggest that the receptor serves numerous roles outside of systemic mineral ion homeostasis, ranging from the regulation of hormonal secretion and the activities of various ion channels to the longer term control of gene expression, programmed cell death (apoptosis), and cellular proliferation. In some cases, the CaR on these "nonhomeostatic" cells responds to local changes in Ca(o)(2+) taking place within compartments of the extracellular fluid (ECF) that communicate with the outside environment (e.g., the gastrointestinal tract). In others, localized changes in Ca(o)(2+) within the ECF can originate from several mechanisms, including fluxes of calcium ions into or out of cellular or extracellular stores or across epithelium that absorb or secrete Ca(2+). In any event, the CaR and other receptors/sensors for Ca(o)(2+) and probably for other extracellular ions represent versatile regulators of numerous cellular functions and may serve as important therapeutic targets.

Dedifferentiation of mammalian myotubes induced by msx1

The process of cellular differentiation culminating in terminally differentiated mammalian cells is thought to be irreversible. Here, we present evidence that terminally differentiated murine myotubes can be induced to dedifferentiate. Ectopic expression of msx1 in C2C12 myotubes reduced the nuclear muscle proteins MyoD, myogenin, MRF4, and p21 to undetectable levels in 20%-50% of the myotubes. Approximately 9% of the myotubes cleave to produce either smaller multinucleated myotubes or proliferating, mononucleated cells. Finally, clonal populations of the myotube-derived mononucleated cells can be induced to redifferentiate into cells expressing chondrogenic, adipogenic, myogenic, and osteogenic markers. These results suggest that terminally differentiated mammalian myotubes can dedifferentiate when stimulated with the appropriate signals and that msx1 can contribute to the dedifferentiation process.

Ex vivo synthesis of articular cartilage

This review discusses modern methods used for the synthesis of articular cartilage ex vivo. The value of culturing articular chondrocytes as a monolayer and in three-dimensional lattices is discussed. Of particular interest are techniques involving seeding of chondrocytes onto synthetic, biodegradable, polymeric scaffolds, and natural materials, such as collagen and agarose. Also discussed is the use of bioreactors to modulate the fluid-flow-induced shear environment of cell-seeded scaffolds. Biodegradable scaffolds are central to the efforts to tissue engineer articular cartilage ex vivo. A review of salient efforts to design and use such scaffolds is presented, along with our thoughts on potential future improvements.

Analysis of the microenvironment necessary for engraftment: role of the vascular smooth muscle-like stromal cells

This is a review of recent data concerning the phenotype of human and murine stroma, as grown in long-term cultures. Using data on cytoskeletal and extracellular matrix protein expression, a sequential model of differentiation from mesenchymal (stem) cells to vascular-smooth muscle-like stromal cells is proposed. This model would apply, at least in the mouse, to stromal cells generated from different sites of hematopoiesis (bone marrow, fetal liver, spleen, and yolk sac). The in vivo counterparts of vascular-smooth muscle-like stromal cells in the different sites of definitive hematopoiesis are discussed.

Mesenchymal stem cells from osteoporotic patients produce a type I collagen-deficient extracellular matrix favoring adipogenic differentiation

Mesenchymal stem cells (MSCs), precursor cells resident in the bone marrow, have the capacity to differentiate into bone, cartilage, fat, and connective tissue. We have recently reported that MSCs from "healthy" donors differ from cells obtained from osteoporotic postmenopausal women in their proliferation rate, mitogenic response to osteogenic growth factors, and potential to mineralize. The purpose of this study was to examine the factors that explain the differential capacity of MSCs derived from "healthy" control and osteoporotic postmenopausal women to support mineralization. In addition, we examined the factors that regulate the differentiation of osteoporotic cells into adipocytes. For this purpose, we isolated MSCs from bone marrow of donors and analyzed the synthesis and deposition of type I collagen, the main component of bone extracellular matrix, the time course of gelatinolytic activity expression, the deposition of transforming growth factor beta (TGF-beta), and the ability of cells to differentiate into adipocytes. Our results indicate that cells derived from osteoporotic donors synthesized 50% less type I collagen than normal cells and maintained higher levels of gelatinolytic activity under differentiation conditions (70% versus 15% after 14 days in culture). MSCs derived from osteoporotic women produced 60-65% less TGF-beta and expressed higher adipogenic capacity. We conclude that the capacity of MSCs derived from osteoporotic postmenopausal women to generate and maintain type I collagen-rich extracellular matrix is decreased, favoring their adipogenic differentiation. These observations may explain the decreased mineralization previously observed in these types of cells.

Biology and clinical utilization of mesenchymal progenitor cells

Within the complex cellular arrangement found in the bone marrow stroma there exists a subset of nonhematopoietic cells referred to as mesenchymal progenitor cells (MPC). These cells can be expanded ex vivo and induced, either in vitro or in vivo, to terminally differentiate into at least seven types of cells: osteocytes, chondrocytes, adipocytes, tenocytes, myotubes, astrocytes and hematopoietic-supporting stroma. This broad multipotentiality, the feasibility to obtain MPC from bone marrow, cord and peripheral blood and their transplantability support the impact that the use of MPC will have in clinical settings. However, a number of fundamental questions about the cellular and molecular biology of MPC still need to be resolved before these cells can be used for safe and effective cell and gene therapies intended to replace, repair or enhance the physiological function of the mesenchymal and/or hematopoietic systems.

Novel approaches to fracture healing

The fractures that occur as a result of trauma frequently require multiple stage surgical procedures to achieve adequate union. Bone grafting with autogenous cancellous or cortico cancellous bone grafts is the traditional method used to repair bone defects. Most fractures will heal using this traditional procedure, however a number of fractures, up to 10% of the cases in United States alone, will result in delayed or impaired healing. Novel approaches are currently being investigated for the augmentation and acceleration of fracture healing. Some of these approaches include the use of biodegradable matrices; cell based approaches supplemented with osteogenic factors and genetic therapy. Cell based approaches for fracture healing have roused intense interest because of the great advance in the isolation and expansion of cells from the marrow that have the ability to differentiate into various types of cells including osteoblasts. In addition, the discovery and cloning of several proteins (bone morphogenetic proteins) that have the ability to induce bone formation, have contributed to the investigation of novel approaches to augment fracture healing. Use of genetic therapy for the augmentation of fracture healing has also recently gained strong interest. The attractive feature of gene therapy is that therapeutic proteins can be delivered locally to the fracture site in relatively high concentrations and in a sustained fashion. This review discusses these novel approaches and presents an assessment of their future clinical applicability.

Human bone marrow stromal cell: coexpression of markers specific for multiple mesenchymal cell lineages

The role of hematopoietic stem cells in blood cell development is reasonably understood, whereas the identity and the function of bone marrow stromal cells are much less clear. Using stromal cells in bone marrow cultures of the Dexter type, a favorite medium for the study of hematopoiesis, we show that stromal cells actually represent a unique cell type. Conventional wisdom has held that stromal cells in Dexter cultures comprise a mixture of macrophages, hematopoietic cells, adipocytes, osteoblasts, fibroblasts, muscle cells, and endothelial cells. Our findings demonstrate that Dexter cultures consist of three cell types: macrophages ( approximately 35%), hematopoietic cells ( approximately 5%), and nonhematopoietic cells ( approximately 60%). We have purified the nonhematopoietic cells free of macrophages and hematopoietic cells to produce compelling evidence that they in fact represent a single cell type (multidifferentiated mesenchymal progenitor cell, MPC) which coexpresses genes specific for various mesenchymal cell lineages including adipocytes, osteoblasts, fibroblasts, and muscle cells. We further show that these multi- or pluridifferentiated MPCs are capable of supporting hematopoiesis by demonstrating the expression of several hematopoietic growth factors and extracellular matrix receptors including G-CSF, SCF, VCAM-1, ICAM-1, and ALCAM. Since the MPCs can be easily purified to near homogeneity (95%), they can be of value in enhancing engraftment of hematopoietic stem cells. Also, this new understanding of bone marrow stromal cells as "one cell with many different faces" promises to advance our knowledge of regulatory cellular interactions within bone marrow.

Mesenchymal progenitor cells in human umbilical cord blood

Haemopoiesis is sustained by two main cellular components, the haematopoietic cells (HSCs) and the mesenchymal progenitor cells (MPCs). MPCs are multipotent and are the precursors for marrow stroma, bone, cartilage, muscle and connective tissues. Although the presence of HSCs in umbilical cord blood (UCB) is well known, that of MPCs has been not fully evaluated. In this study, we examined the ability of UCB harvests to generate in culture cells with characteristics of MPCs. Results showed that UCB-derived mononuclear cells, when set in culture, gave rise to adherent cells, which exhibited either an osteoclast- or a mesenchymal-like phenotype. Cells with the osteoclast phenotype were multinucleated, expressed TRAP activity and antigens CD45 and CD51/CD61. In turn, cells with the mesenchymal phenotype displayed a fibroblast-like morphology and expressed several MPC-related antigens (SH2, SH3, SH4, ASMA, MAB 1470, CD13, CD29 and CD49e). Our results suggest that preterm, as compared with term, cord blood is richer in mesenchymal progenitors, similar to haematopoietic progenitors.

Adenoviral-mediated gene transfer into ex vivo expanded human bone marrow mesenchymal progenitor cells

OBJECTIVE

Based on their differentiation properties and facilely of ex vivo expansion, human bone marrow mesenchymal progenitor cells (MPC), are considered as attractive targets to deliver foreign genes to the bone marrow or other mesenchymal tissues. In this study we investigated the feasibility of transduce MPC with adenoviral vectors (Adv).

METHODS

MPC were expanded ex vivo and transduced with replication-defective Adv-containing reporter genes (lacZ or GFP) under the control of CMV promoter. Transfection efficiency was assessed by microscopical scoring or by flow cytometry. Expression and involvement of Adv-attachment (CAR) and Adv-internalization (integrins alphav) receptors were evaluated by flow cytometric studies.

RESULTS

Transgene expression analysis showed that only 19%+/-3% of cells expressed the transgenes at high levels. MPC express the attachment and internalization receptors required for Adv infection. While integrins alphavbeta3 and alphavbeta5 are expressed by all MPC, CAR is solely expressed by a fraction of low size cells. Antibodies against CAR and alphavbeta5, but not against alphavbeta3, blocked Adv-mediated gene transfer into MPC, showing that CAR and alphavbeta5 are required for infection. Because alphavbeta5, as compared with CAR, is overexpressed in MPC, the results suggest that the efficiency of Adv-mediated gene transfer into MPC depends on the level of CAR expression.

CONCLUSION

These findings demonstrate that Adv may be useful to engineer a subpopulation of ex vivo expanded human mesenchymal progenitors, with a high level of transgene expression.

Clonal mesenchymal progenitors from human bone marrow differentiate in vitro according to a hierarchical model

Bone marrow stromal cells can give rise to several mesenchymal lineages. The existence of a common stem/progenitor cell, the mesenchymal stem cell, has been proposed, but which developmental stages follow this mesenchymal multipotent progenitor is not known. Based on experimental evidence, a model of mesenchymal stem cell differentiation has been proposed in which individual lineages branch directly from the same progenitor. We have verified this model by using clonal cultures of bone marrow derived stromal fibroblasts. We have analyzed the ability of 185 non-immortalized human bone marrow stromal cell clones to differentiate into the three main lineages: osteo-, chondro- and adipogenic. All clones but one differentiated into the osteogenic lineage. About one third of the clones differentiated into all three lineages analyzed. Most clones (60-80%) displayed an osteo-chondrogenic potential. We have never observed clones with a differentiation potential limited to the osteo-adipo- or to the chondro-adipogenic phenotype, nor pure chondrogenic and adipogenic clones. How long the differentiation potential of a number of clones was maintained was assessed throughout their life span. Clones progressively lost their adipogenic and chondrogenic differentiation potential at increasing cell doublings. Our data suggest a possible model of predetermined bone marrow stromal cells differentiation where the tripotent cells can be considered as early mesenchymal progenitors that display a sequential loss of lineage potentials, generating osteochondrogenic progenitors which, in turn, give rise to osteogenic precursors.

Review: tissue engineering for regeneration of articular cartilage

Joint pain due to cartilage degeneration is a serious problem, affecting people of all ages. Although many techniques, often surgical, are currently employed to treat this affliction, none have had complete success. Recent advances in biology and materials science have pushed tissue engineering to the forefront of new cartilage repair techniques. This review seeks to condense information for the biomaterialist interested in developing materials for this application. Articular cartilage anatomy, types of injury, and current repair methods are explained. The need for biomaterials, current commonly used materials for tissue-engineered cartilage, and considerations in scale-up of cell-biomaterial constructs are summarized.

Multipotency of a bone marrow stromal cell line, TBR31-2, established from ts-SV40 T antigen gene transgenic mice

Bone marrow is believed to contain multipotential stromal stem cells which can differentiate into osteoblasts, chondrocytes, adipocytes, and myoblasts (Prockop, D. J. Science 276, 71-74, 1997). Therefore, characterization and identification of the stem-like cell within the stromal cells are important to understand bone marrow function in relation to the hematopoietic microenvironment, and repair/regeneration of tissue defects. TBR31-2 cell, a bone marrow stromal cell line established from bone marrow of transgenic mice harboring temperature-sensitive (ts) simian virus (SV) 40T-antigen gene for immortality, is induced toward both adipocytic and osteogenic cells under conditions of the inactivation of T-antigen (Okuyama, R., Yanai, N., Obinata, M. Exp. Cell Res. 218, 424-429, 1995). In this work, using a semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analysis, mRNA expressions of tissue-specific differentiation markers for adipocyte (lipoprotein lipase), osteoblast (type I collagen and osteocalcin), chondrocyte (type II and X collagen), and muscle cell (desmin) were examined during a long-term culture of the cell. In addition, histochemical studies showed the appearance of adipocytic, osteoblastic, chondrocytic, and muscle cells during this long-term culture. Thus, TBR31-2, which has characteristics of an undifferentiated cell, has the potential to express the multipotential cell lineages. These results indicated that a multipotential progenitor cell including potential to differentiate into a muscle cell and which is situated in the mesenchymal cell lineage was first obtained.

The nuclear factor of activated T cells (NFAT) transcription factor NFATp (NFATc2) is a repressor of chondrogenesis

Nuclear factor of activated T cells (NFAT) transcription factors regulate gene expression in lymphocytes and control cardiac valve formation. Here, we report that NFATp regulates chondrogenesis in the adult animal. In mice lacking NFATp, resident cells in the extraarticular connective tissues spontaneously differentiate to cartilage. These cartilage cells progressively differentiate and the tissue undergoes endochondral ossification, recapitulating the development of endochondral bone. Proliferation of already existing articular cartilage cells also occurs in some older animals. At both sites, neoplastic changes in the cartilage cells occur. Consistent with these data, NFATp expression is regulated in mesenchymal stem cells induced to differentiate along a chondrogenic pathway. Lack of NFATp in articular cartilage cells results in increased expression of cartilage markers, whereas overexpression of NFATp in cartilage cell lines extinguishes the cartilage phenotype. Thus, NFATp is a repressor of cartilage cell growth and differentiation and also has the properties of a tumor suppressor.

Mesenchymal precursor cells in the blood of normal individuals

STATEMENT OF FINDINGS: Mesenchymal precursor cells found in the blood (BMPCs) of normal persons adhere to plastic and glass and proliferate logarithmically in DMEM-20% fetal calf serum (FCS) without growth factors. They form cells with fibroblast-like and stromal morphology, which is not affected by eliminating CD34, CD3, or CD14 cells. Osteogenic supplements (dexamethasone, ascorbic acid, and beta-glycerophosphate) added to the culture inhibited fibroblast formation, and BMPCs assumed the cuboidal shape of osteoblasts. After 5 days in supplemented medium, the elutriated cells displayed alkaline phosphatase (AP), and the addition of bone morphogenetic protein (BMP)2 (1 ng) doubled AP production (P < 0.04). Two weeks later, 30% of the cells were very large and reacted with anti-osteocalcin antibody. The same cultures also contained sudanophlic adipocytes and multinucleated giant cells that stained for tartrate-resistant acid phosphatase (TRAP) and vitronectin receptors. Cultured BMPCs immunostain with antibodies to vimentin, type I collagen, and BMP receptors, heterodimeric structures expressed on mesenchymal lineage cells. In addition, BMPCs stain with anti-CD105 (endoglin), a putative marker for bone-marrow mesenchymal stem cells (MSCs).