Concise review: multipotent mesenchymal stromal cells in blood

Combination stem cell therapy for heart failure

Patients with congestive heart failure (CHF) that are not eligible for transplantation have limited therapeutic options. Stem cell therapy such as autologous bone marrow, mobilized peripheral blood, or purified cells thereof has been used clinically since 2001. To date over 1000 patients have received cellular therapy as part of randomized trials, with the general consensus being that a moderate but statistically significant benefit occurs. Therefore, one of the important next steps in the field is optimization. In this paper we discuss three ways to approach this issue: a) increasing stem cell migration to the heart; b) augmenting stem cell activity; and c) combining existing stem cell therapies to recapitulate a "therapeutic niche". We conclude by describing a case report of a heart failure patient treated with a combination stem cell protocol in an attempt to augment beneficial aspects of cord blood CD34 cells and mesenchymal-like stem cells.

Age-related molecular genetic changes of murine bone marrow mesenchymal stem cells

Background

Mesenchymal stem cells (MSC) are pluripotent cells, present in the bone marrow and other tissues that can differentiate into cells of all germ layers and may be involved in tissue maintenance and repair in adult organisms. Because of their plasticity and accessibility these cells are also prime candidates for regenerative medicine. The contribution of stem cell aging to organismal aging is under debate and one theory is that reparative processes deteriorate as a consequence of stem cell aging and/or decrease in number. Age has been linked with changes in osteogenic and adipogenic potential of MSCs.

Results

Here we report on changes in global gene expression of cultured MSCs isolated from the bone marrow of mice at ages 2, 8, and 26-months. Microarray analyses revealed significant changes in the expression of more than 8000 genes with stage-specific changes of multiple differentiation, cell cycle and growth factor genes. Key markers of adipogenesis including lipoprotein lipase, FABP4, and Itm2a displayed age-dependent declines. Expression of the master cell cycle regulators p53 and p21 and growth factors HGF and VEGF also declined significantly at 26 months. These changes were evident despite multiple cell divisions in vitro after bone marrow isolation.

Conclusions

The results suggest that MSCs are subject to molecular genetic changes during aging that are conserved during passage in culture. These changes may affect the physiological functions and the potential of autologous MSCs for stem cell therapy.

Emergence of fibrocytes showing morphological changes in the inflamed colonic mucosa

Fibrocytes contribute to wound healing and are uniquely defined by coexpression of hematopoietic and mesenchymal cell markers. In this study, trafficking of fibrocytes was determined in a murine model of colitis induced by administering 3% dextran sodium sulfate (DSS) for seven days. Colonic tissues were immunostained for CD45, collagen type I (Col I), and alpha-SMA. On day 0, there were no CD45(+)Col I(+) cells in colonic tissues. However, on day 7 when inflammatory cells showed remarkable accumulation, oval-shaped CD45(+)Col I(+) fibrocytes were obvious in the submucosal layer. On day 14 when colonic tissues were in the healing phase, numerous spindle-shaped CD45(+)Col I(+) fibrocytes were observed. Emergence of CD45(+)Col I(+) fibrocytes preceded the appearance of alpha-SMA(+) myofibroblasts. Oval-shaped fibrocytes recruited as early as the inflammatory phase of colitis are likely to differentiate into spindle-shaped fibrocytes in the healing phase, suggesting that fibrocytes may promote wound healing in inflamed colonic tissues.

Mesenchymal stem cells as anti-inflammatories: implications for treatment of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a lethal X-linked musculodegenerative condition consisting of an underlying genetic defect whose manifestation is augmented by inflammatory mechanisms. Previous treatment approaches using gene replacement, exon-skipping or allogeneic cell therapy have been relatively unsuccessful. The only intervention to mediate improvement in survival, albeit minor, is glucocorticoid treatment. Given this modality appears to function via suppression of underlying inflammation; we focus this review on the inflammatory response as a target for mesenchymal stem cell (MSC) therapy. In contrast to other cell based therapies attempted in DMD, MSC have the advantages of (a) ability to fuse with and genetically complement dystrophic muscle; (b) possess anti-inflammatory activities; and (c) produce trophic factors that may augment activity of endogenous repair cells. We conclude by describing one practical scenario of stem cell therapy for DMD.

Mesenchymal stem cells as a gene therapy carrier for treatment of fibrosarcoma

BACKGROUND AIMS

Cell-based gene therapy is an alternative to viral and non-viral gene therapy. Emerging evidence suggests that mesenchymal stem cells (MSC) are able to migrate to sites of tissue injury and have immunosuppressive properties that may be useful in targeted gene therapy for sustained specific tissue engraftment.

METHODS

In this study, we injected intravenously (i.v.) 1x10(6) MSC, isolated from green fluorescent protein (GFP) transgenic rats, into Rif-1 fibrosarcoma-bearing C3H/HeN mice. The MSC had been infected using a lentiviral vector to express stably the luciferase reporter gene (MSC-GFP-luci). An in vivo imaging system (IVIS 200) and Western blotting techniques were used to detect the distribution of MSC-GFP-luci in tumor-bearing animals.

RESULTS

We observed that xenogenic MSC selectively migrated to the tumor site, proliferated and expressed the exogenous gene in subcutaneous fibrosarcoma transplants. No MSC distribution was detected in other organs, such as the liver, spleen, colon and kidney. We further showed that the FGF2/FGFR pathways may play a role in the directional movement of MSC to the Rif-1 fibrosarcoma. We performed in vitro co-culture and in vivo tumor growth analysis, showing that MSC did not affect the proliferation of Rif-1 cells and fibrosarcoma growth compared with an untreated control group. Finally, we demonstrated that the xenogenic MSC stably expressing inducible nitric oxide synthase (iNOS) protein transferred by a lentivirus-based system had a significant inhibitory effect on the growth of Rif-1 tumors compared with MSC alone and the non-treatment control group.

CONCLUSIONS

iNOS delivered by genetically modified iNOS-MSC showed a significant anti-tumor effect both in vitro and in vivo. MSC may be used as a target gene delivery vehicle for the treatment of fibrosarcoma and other tumors.

A comprehensive review on mesenchymal stem cell growth and senescence

In recent years mesenchymal stem cells (MSCs) have generated a great deal of excitement as an attractive alternative to embryonic stem cells (ESCs) in cell-based regenerative medicine. In contrast to cells of embryonic origin, however, the clinical application of MSCs is heavily restricted by their finite ability of self-renewal, in which they resemble the rest of the somatic cells. Yet the mechanisms controlling MSC proliferation and senescence remain unclear. This review summarizes recent advances in our understanding of the factors affecting MSC expansion in vitro and discusses the pattern of their senescence with particular emphasis on the role of telomere shortening, activation of effectory pathways, and oxidative stress. The issues associated with MSC growth and senescence will be shown in the context of other somatic cells, and all of the parallels and disparities will be delineated precisely.

Optimization of primary culture condition for mesenchymal stem cells derived from umbilical cord blood with factorial design

Mesenchymal stem cells (MSCs) can not only support the expansion of hematopoietic stem cells in vitro, but also alleviate complications and accelerate recovery of hematopoiesis during hematopoietic stem cell transplantation. However, it proved challenging to culture MSCs from umbilical cord blood (UCB) with a success rate of 20-30%. Many cell culture parameters contribute to this outcome and hence optimization of culture conditions is critical to increase the probability of success. In this work, fractional factorial design was applied to study the effect of cell inoculated density, combination and dose of cytokines, and presence of serum and stromal cells. The cultured UCB-MSC-like cells were characterized by flow cytometry and their multilineage differentiation potentials were tested. The optimal protocol was identified achieving above 90% successful outcome: 2 x 10(6) cells/mL mononuclear cells inoculated in Iscove's modified Dulbecco's medium supplied with 10% FBS, 15 ng/mL IL-3, and 5 ng/mL Granulocyte-macrophage colony-stimulating factor (GM-CSF). Moreover, the UCB-MSC-like cells expressed MSC surface markers of CD13, CD29, CD105, CD166, and CD44 positively, and CD34, CD45, and human leukocyte antigens-DR (HLA-DR) negatively. Meanwhile, these cells could differentiate into osteoblasts, chondrocytes, and adipocytes similarly to MSCs derived from bone marrow. In conclusion, we have developed an efficient protocol for the primary culture of UCB-MSCs by adding suitable cytokines into the culture system.

Mesenchymal stem cell homing: the devil is in the details

The study of MSC trafficking is clinically relevant for minimally invasive cell therapy to promote regeneration of damaged tissue, to treat inflammation, and to promote angiogenesis. However, these studies are complicated by the diverse methods used to culture, characterize, and deliver MSCs and by the variety of methods used to assess homing events. This review provides a critical analysis of the methods used to track homing of exogenously infused MSCs and discusses strategies for enhancing their trafficking to particular tissues.

Non-expanded adipose stromal vascular fraction cell therapy for multiple sclerosis

The stromal vascular fraction (SVF) of adipose tissue is known to contain mesenchymal stem cells (MSC), T regulatory cells, endothelial precursor cells, preadipocytes, as well as anti-inflammatory M2 macrophages. Safety of autologous adipose tissue implantation is supported by extensive use of this procedure in cosmetic surgery, as well as by ongoing studies using in vitro expanded adipose derived MSC. Equine and canine studies demonstrating anti-inflammatory and regenerative effects of non-expanded SVF cells have yielded promising results. Although non-expanded SVF cells have been used successfully in accelerating healing of Crohn's fistulas, to our knowledge clinical use of these cells for systemic immune modulation has not been reported. In this communication we discuss the rationale for use of autologous SVF in treatment of multiple sclerosis and describe our experiences with three patients. Based on this rationale and initial experiences, we propose controlled trials of autologous SVF in various inflammatory conditions.

Multipotent progenitor cells are present in human peripheral blood

To determine whether the peripheral blood in humans contains a population of multipotent progenitor cells (MPCs), products of leukapheresis were obtained from healthy donor volunteers following the administration of granulocyte colony-stimulating factor. Small clusters of adherent proliferating cells were collected, and these cells continued to divide up to 40 population doublings without reaching replicative senescence and growth arrest. MPCs were positive for the transcription factors Nanog, Oct3/4, Sox2, c-Myc, and Klf4 and expressed several antigens characteristic of mesenchymal stem cells. However, they were negative for markers of hematopoietic stem/progenitor cells and bone marrow cell lineages. MPCs had a cloning efficiency of approximately 3%, and following their expansion, retained a highly immature phenotype. Under permissive culture conditions, MPCs differentiated into neurons, glial cells, hepatocytes, cardiomyocytes, endothelial cells, and osteoblasts. Moreover, the gene expression profile of MPCs partially overlapped with that of neural and embryonic stem cells, further demonstrating their primitive, uncommitted phenotype. Following subcutaneous transplantation in nonimmunosuppressed mice, MPCs migrated to distant organs and integrated structurally and functionally within the new tissue, acquiring the identity of resident parenchymal cells. In conclusion, undifferentiated cells with properties of embryonic stem cells can be isolated and expanded from human peripheral blood after granulocyte colony-stimulating factor administration. This cell pool may constitute a unique source of autologous cells with critical clinical import.

Increased number of mesenchymal stem cell-like cells in peripheral blood of patients with bone sarcomas

BACKGROUND AND AIMS

The number of peripheral blood mesenchymal stem cells (PBMSCs) may increase under pathological conditions. We sought to compare the number of MSC-like cells in the peripheral blood of patients with bone sarcomas with healthy controls and to analyze related cytokines in the peripheral blood plasma.

METHODS

Peripheral blood mononuclear cells (PBMNs) of patients with bone sarcomas and control subjects were isolated for culture and analyzed by flow cytometry for MSC phenotype. Cytokines in the plasma obtained after cell separation were analyzed using enzyme-linked immunosorbent assay (ELISA). Annexin-V and beta-galactosidase staining were used to investigate whether the cells died from apoptosis or senescence.

RESULTS

Flow cytometric analysis demonstrated an >9-fold increase in the number of cells with MSC-like phenotypes (CD34(-), CD45(-), CD105(+)) in patients with bone sarcomas compared with control subjects (p<0.05). ELISA results showed that concentrations of hepatocyte growth factor (HGF) and vascular endothelial growth factor (VEGF) in patients with bone sarcomas were statistically higher than those in the control subjects (p<0.05), whereas there was no significant difference in plasma concentrations of leptin and stromal cell-derived factor 1 between the two groups. A significant, positive correlation between the percentages of PBMSC-like cells and concentrations of HGF in all samples (R=0.618; p=0.011). Annexin-V staining of MSC-like cells was positive, whereas beta-galactosidase staining was negative.

CONCLUSIONS

Peripheral blood of patients with bone sarcomas has more cells with MSC phenotypes than blood of healthy persons. The increased number is accompanied by increased HGF and VEGF in the plasma.

Spartathlon, a 246 kilometer foot race: effects of acute inflammation induced by prolonged exercise on circulating progenitor reparative cells

Endothelial progenitor cells (EPCs) and the recently described circulating fibrocytes (CFs) are strongly associated with tissue repair. We investigated the kinetics of both "repair" progenitor cells in healthy athletes who participated in the "Spartahlon" ultradistance foot race (246 km continuous running exercise), which provides a unique model of inducing dramatic systemic inflammatory changes. Peripheral blood mononuclear cells (PBMCs) were isolated from 10 volunteer athletes, who completed successfully the race, before, at the end, and at 48 h post-race. EPCs and CFs were detected as endothelial colony-forming units (CFU-ECs) and as the number of adherent with a spindle-shaped morphology Collagen I(+) cells detected after 6-day culture of PBMCs, respectively. The marked increase of plasma levels of CRP, IL-6, SAA, MCP-1, IL-8, sVCAM-1, sICAM-1, thrombomodulin (sTM) and NT-pro-BNP at the end of race established acute inflammation and tissue injury. EPCs increased by nearly eleven-fold in peripheral blood at the end of the race from 44.5+/-2.5/ml to 494.6+/-27.9/ml and remained increased 428.5+/-31.5/ml at 48 h post-race (p<0.0001). The number of the fibrocytes cultured from PBMCs obtained before, at the end, and 48 h post-race did not reveal any significant difference. These findings indicate that bone marrow responses to acute inflammatory damage, induced by exhausting exercise, with a rapid release of EPCs but not CFs into circulation. Given the ability of EPCs to promote angiogenesis and vascular regeneration, we may suggest that this kind of cell mobilization may serve as a physiologic repair mechanism in acute inflammatory tissue injury.

Bone marrow-derived mesenchymal stem cells: isolation, expansion, characterization, viral transduction, and production of conditioned medium

Mesenchymal stem cells (MSCs) are defined as self-renewing and multipotent cells capable of differentiating into multiple cell types, including osteocytes, chondrocytes, adipocytes, hepatocytes, myocytes, neurons, and cardiomyocytes. MSCs were originally isolated from the bone marrow stroma but they have recently been identified also in other tissues, such as fat, epidermis, and cord blood. Several methods have been used for MSC isolation. The most common method is based on the ability of the MSCs to selectively adhere to plastic surfaces. Phenotypic characterization of MSCs is usually carried out using immunocytochemical detection or fluorescence-activated cell sorting (FACS) analysis of cell surface molecule expression. However, the lack of specific markers renders the characterization of MSCs difficult and sometimes ambiguous. MSCs posses remarkable expansion potential in culture and are highly amenable to genetic modification with various viral vectors rendering them optimal vehicles for cell-based gene therapy. Most importantly, MSC plasticity and the possibility to use them as autologous cells render MSCs suitable for cell therapy and tissue engineering. Furthermore, it is known that MSCs produce and secrete a great variety of cytokines and chemokines that play beneficial paracrine actions when MSCs are used for tissue repair. In this chapter, we describe methods for isolation, ex vivo expansion, phenotypic characterization, and viral infection of MSCs from mouse bone marrow. We also describe a method for preparation of conditioned and concentrated conditioned medium from MSCs. The conditioned medium can be easily tested both in vitro and in vivo when a particular paracrine effect (i.e., cytoprotection) is hypothesized to be an important mechanism of action of the MSCs and/or screened to identify a target paracrine/autocrine mediator.

Mesenchymal stem cells: the fibroblasts' new clothes?

Mesenchymal stem cells are adherent stromal cells, initially isolated from the bone marrow, characterized by their ability to differentiate into mesenchymal tissues such as bone, cartilage and fat. They have also been shown to suppress immune responses in vitro. Because of these properties, mesenchymal stem cells have recently received a very high profile. Despite the dramatic benefits reported in early phase clinical trials, their functions remain poorly understood. Particularly, several questions remain concerning the origin of mesenchymal stem cells and their relationship to other stromal cells such as fibroblasts. Whereas clear gene expression signatures are imprinted in stromal cells of different anatomical origins, the anti-proliferative effects of mesenchymal stem cells and fibroblasts and their potential to differentiate appear to be common features between these two cell types. In this review, we summarize recent studies in the context of historical and often neglected stromal cell literature, and present the evidence that mesenchymal stem cells and fibroblasts share much more in common than previously recognized.

Regeneration and adult stem cells in the human female reproductive tract

The human uterus is unique in that it exhibits a tremendous regenerative capacity that enables cyclical regeneration and remodeling throughout a woman’s reproductive life. This plasticity of the reproductive system has recently been highlighted. Regeneration and remodeling in the female reproductive tract alludes to the existence of endometrial and myometrial stem cell systems, which has been supported by increasing experimental evidence. Characterization of these stem cells, along with the study of the mechanisms controlling their regeneration, will improve the understanding of the physiology and pathophysiology of the female reproductive tract.

Gene expression analysis in osteoblastic differentiation from peripheral blood mesenchymal stem cells

MSCs are known to have an extensive proliferative potential and ability to differentiate in various cell types. Osteoblastic differentiation from mesenchymal progenitor cells is an important step of bone formation, though the pattern of gene expression during differentiation is not yet well understood. Here, to investigate the possibility to obtain a model for in vitro bone differentiation using mesenchymal stem cells (hMSCs) from human subjects non-invasively, we developed a method to obtain hMSCs-like cells from peripheral blood by a two step method that included an enrichment of mononuclear cells followed by depletion of unwanted cells. Using these cells, we analyzed the expression of transcription factor genes (runt-related transcription factor 2 (RUNX2) and osterix (SP7)) and bone related genes (osteopontin (SPP1), osteonectin (SPARC) and collagen, type I, alpha 1 (COLIA1)) during osteoblastic differentiation. Our results demonstrated that hMSCs can be obtained from peripheral blood and that they are able to generate CFU-F and to differentiate in osteoblast and adipocyte; in this study, we also identified a possible gene expression timing during osteoblastic differentiation that provided a powerful tool to study bone physiology.

Mesenchymal stem cells promote proliferation of endogenous neural stem cells and survival of newborn cells in a rat stroke model

Mesenchymal stem cells (MSCs) secrete bioactive factors that exert diverse responses in vivo. In the present study, we explored mechanism how MSCs may lead to higher functional recovery in the animal stroke model. Bone marrow-derived MSCs were transplanted into the brain parenchyma 3 days after induction of stroke by occluding middle cerebral artery for 2 h. Stoke induced proliferation of resident neural stem cells in subventricular zone. However, most of new born cells underwent cell death and had a limited impact on functional recovery after stroke. Transplantation of MSCs enhanced proliferation of endogenous neural stem cells while suppressing the cell death of newly generated cells. Thereby, newborn cells migrated toward ischemic territory and differentiated in ischemic boundaries into doublecortin+ neuroblasts at higher rates in animals with MSCs compared to control group. The present study indicates that therapeutic effects of MSCs are at least partly ascribed to dual functions of MSCs by enhancing endogenous neurogenesis and protecting newborn cells from deleterious environment. The results reinforce the prospects of clinical application using MSCs in the treatment of neurological disorders.

Evidence for cell fusion is absent in vascular lesions associated with pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a fatal disease associated with severe remodeling of the large and small pulmonary arteries. Increased accumulation of inflammatory cells and apoptosis-resistant cells are contributing factors. Proliferative apoptosis-resistant cells expressing CD133 are increased in the circulation of PAH patients. Circulating cells can contribute to tissue repair via cell fusion and heterokaryon formation. We therefore hypothesized that in the presence of increased leukocytes and CD133-positive (CD133(pos)) cells in PAH lung tissue, cell fusion and resulting genomic instability could account for abnormal cell proliferation and the genesis of vascular lesions. We performed analyses of CD45/CD133 localization, cell fusion, and proliferation during late-stage PAH in human lung tissue from control subjects and subjects with idiopathic (IPAH) and familial (FPAH) PAH. Localization, proliferation, and quantitation of cell populations in individual patients were performed by immunolocalization. The occurrence of cellular fusion in vascular lesions was analyzed in lung tissue by fluorescence in situ hybridization. We found the accumulation of CD45(pos) leukocytic cells in the tissue parenchyma and perivascular regions in PAH patients and less frequently observed myeloid cells (CD45/CD11b). CD133(pos) cells were detected in occlusive lesions and perivascular areas in those with PAH and were more numerous in those with IPAH lesions than in FPAH lesions. Cells coexpressing CD133 and smooth muscle alpha-actin were occasionally observed in occlusive lesions and perivascular areas. Proliferating cells were more prominent in IPAH lesions and colocalized with CD45 or CD133. We found no evidence of increased ploidy to suggest cell fusion. Taken together, these data suggest that abnormal lesion formation in PAH occurs in the absence of cell fusion.

PEDF from mouse mesenchymal stem cell secretome attracts fibroblasts

Conditioned medium (secretome) derived from an enriched stem cell culture stimulates chemotaxis of human fibroblasts. These cells are classified as multipotent murine mesenchymal stromal cells (mMSC) by immunochemical analysis of marker proteins. Proteomic analysis of mMSC secretome identifies nineteen secreted proteins, including extracellular matrix structural proteins, collagen processing enzymes, pigment epithelium-derived factor (PEDF) and cystatin C. Immunodepletion and reconstitution experiments show that PEDF is the predominant fibroblast chemoattractant in the conditioned medium, and immunofluorescence microscopy shows strong staining for PEDF in the cytoplasm, at the cell surface, and in intercellular space between mMSCs. This stimulatory effect of PEDF on fibroblast chemotaxis is in contrast to the PEDF-mediated inhibition of endothelial cell migration, reported previously. These differential functional effects of PEDF toward fibroblasts and endothelial cells may serve to program an ordered temporal sequence of scaffold building followed by angiogenesis during wound healing.

The isolation of novel mesenchymal stromal cell chemotactic factors from the conditioned medium of tumor cells

Bone marrow-derived mesenchymal stromal cells (MSCs) localize to solid tumors. Defining the signaling mechanisms that regulate this process is important in understanding the role of MSCs in tumor growth. Using a combination of chromatography and electrospray tandem mass spectrometry we have identified novel soluble signaling molecules that induce MSC chemotaxis present in conditioned medium of the breast carcinoma cell line MDA-MB231. Previous work has employed survey strategies using ELISA assay to identify known chemokines that promote MSC chemotaxis. While these studies provide valuable insights into the intercellular signals that impact MSC behavior, many less well-described, but potentially important soluble signaling molecules could be overlooked using these methods. Through the less directed method of column chromatography we have identified novel candidate MSC chemotactic peptides. Two proteins, cyclophilin B and hepatoma-derived growth factor were then further characterized and shown to promote MSC chemotaxis.

Immunosuppression by mesenchymal stromal cells: from culture to clinic

Extensive in vitro studies have shown that multipotent mesenchymal stromal cells (MSC) can exert profound immunosuppressive effects via modulation of both cellular and innate immune pathways. Their ability to be readily isolated from a number of tissues and expanded ex vivo makes them attractive candidates for systemic immunosuppressive therapy. In this article, we will review recent experimental data on the mechanisms by which MSC inhibit the alloproliferative response and the clinical relevance for their potential use in hematopoietic stem cell transplantation, solid organ transplantation, and treatment of autoimmune diseases. While in vitro data consistently demonstrate the immunosuppressive capability of MSC, current studies in animals and humans suggest that MSC are less effective in producing systemic immunosuppression. Further mechanistic studies and randomized controlled trials using standardized cell populations are needed to define the optimal conditions for the use of MSC as immunotherapy.

Banking on cord blood stem cells

Umbilical cord blood gifted to non-profit public cord blood banks is now routinely used as an alternative source of haematopoietic stem cells for allogeneic transplantation for children and adults with cancer, bone marrow failure syndromes, haemoglobinopathies and many genetic metabolic disorders. Because of the success and outcomes of public cord banking, many companies now provide private cord banking services. However, in the absence of any published transplant evidence to support autologous and non-directed family banking, commercial cord banks currently offer a superfluous service.

Endothelial colony forming cells and mesenchymal stem cells are enriched at different gestational ages in human umbilical cord blood

Endothelial progenitor cells (EPCs) are used for angiogenic therapies and as biomarkers of cardiovascular disease. Human umbilical cord blood (UCB) is a rich source of endothelial colony forming cells (ECFCs), which are EPCs with robust proliferative potential that may be useful for clinical vascular regeneration. Previous studies show that hematopoietic progenitor cells are increased in premature UCB compared with term controls. Based on this paradigm, we hypothesized that premature UCB would be an enriched source of ECFCs. Thirty-nine UCB samples were obtained from premature infants (24-37 wk gestational age (GA)) and term controls. ECFC colonies were enumerated, clonally isolated, and identified by expression of endothelial cell surface antigens and functional analysis. GA of 33-36 wk UCB yielded predominantly ECFC colonies at equivalent numbers to term infants. UCB from 24 to 28 wk GA infants had significantly fewer ECFCs. Surprisingly, 24-28 wk GA UCB yielded predominantly mesenchymal stem cell (MSC) colonies, capable of differentiating into adipocytes, chondrocytes, and osteocytes. MSCs were rarely identified in 37-40 wk GA UCB. These studies demonstrate that circulating MSCs and ECFCs appear at different GA in the human UCB, and that 24-28 wk GA UCB may be a novel source of MSCs for therapeutic use in human diseases.

IFATS collection: Combinatorial peptides identify alpha5beta1 integrin as a receptor for the matricellular protein SPARC on adipose stromal cells

The biological features of adipose stromal (stem) cells (ASC), which serve as progenitors for differentiated cells of white adipose tissue (WAT), are still largely undefined. In an initiative to identify functional ASC surface receptors, we screened a combinatorial library for peptide ligands binding to patient-derived ASC. We demonstrate that both primary and cultured human and mouse stromal cells express a conserved receptor targeted by peptides found to mimic SPARC, a matricellular protein that is required for normal WAT development. A signaling receptor for SPARC has not as yet been determined. By using the SPARC-mimicking peptides CMLAGWIPC (termed hPep) and CWLGEWLGC (termed mPep), isolated by panning on human and mouse cells, respectively, we identified the alpha5beta1 integrin complex as a candidate receptor for SPARC. On the basis of these results, we evaluated ASC responses to SPARC or SPARC-mimicking peptide exposure. Our results suggest that extracellular SPARC binds to alpha5beta1 integrin at sites of focal adhesions, an interaction disrupting firm attachment of ASC to extracellular matrix. We propose that SPARC-mediated mobilization of ASC through its effect on alpha5beta1 integrin complex provides a functional basis for the regulation of WAT body composition by SPARC. We also show that alpha5beta1 integrin is a potential target for ASC-selective intracellular delivery of bioactive peptides and gene therapy vectors directed by the SPARC-mimicking peptides. Disclosure of potential conflicts of interest is found at the end of this article.

Mesenchymal Progenitor Cells: Tissue Origin, Isolation and Culture

SUMMARY: Since the pioneering work of Alexander Friedenstein on multipotent mesenchymal stromal cells (MSCs), a tremendous amount of work has been done to isolate, characterize and culture such cells. Assay of colony forming unit-fibroblasts (CFU-Fs), the hallmark of MSCs, is used to estimate their frequency in tissue. MSCs are adherent cells, so they are easy to isolate, and they show contact inhibition. Thus, several parameters must be taken into account for culture: cell density, number of passages, culture medium, and growth factors used. The purity of the initial material is not a limiting parameter. Similar but not identical cell populations are found in almost all mammal or human tissues. MSCs seem to be very abundant in adipose tissue but at low frequency in blood from umbilical cord or in adult tissue. The culture conditions are very similar, whatever the source of cells. Because of their favorable properties, MSCs are very promising tools for regenerative medicine.

Mesenchymal stem cells in acute kidney injury

The potential role of mesenchymal stem cells (MSCs, also called mesenchymal stromal cells) in endogenous repair and cell-based therapies for acute kidney injury (AKI) is under intensive investigation. Preclinical studies indicate that administered MSCs both ameliorate renal injury and accelerate repair. These versatile cells home to sites of injury, where they modulate the repair process. The mechanisms responsible for their protective and regenerative effects are incompletely understood. Some have reported that MSCs are capable of direct engraftment into injured nephrons under certain circumstances. This is highly controversial, however, and even those who argue there is engraftment acknowledge that the primary means of repair by these cells most likely involves paracrine and endocrine effects, including mitogenic, antiapoptotic, anti-inflammatory, and angiogenic influences. There is a good deal of interest in MSC-based approaches for the treatment of human kidney injury, thanks to positive preclinical results, the strong clinical need for novel therapies to treat AKI, the ease of isolation and expansion of MSCs, and encouraging preliminary clinical trial results in other fields. This review summarizes current knowledge and identifies gaps in our understanding of MSC biology that will need to be filled in order to translate recent discoveries into therapies for AKI in humans.

Mesenchymal stem cells: a promising candidate in regenerative medicine

Mesenchymal stem cells were initially characterized as plastic adherent, fibroblastoid cells. In recent years, there has been an increasing focus on mesenchymal stem cells since they have great plasticity and are potential for therapeutic applications. Mesenchymal stem cells or mesenchymal stem cell-like cells have been shown to reside within the connective tissues of most organs. These cells can differentiate into osteogenic, adipogenic and chondrogenic lineages under appropriate conditions. A number of reports have also indicated that these cells possess the capacity to trans-differentiate into epithelial cells and lineages derived from the neuro-ectoderm, and in addition, mesenchymal stem cells can migrate to the sites of injury, inflammation, and to tumors. These properties of mesenchymal stem cells make them promising candidates for use in regenerative medicine and may also serve as efficient delivery vehicles in site-specific therapy.

Side population in human uterine myometrium displays phenotypic and functional characteristics of myometrial stem cells

Over the course of pregnancy, the human uterus undergoes a 500- to 1,000-fold increase in volume and a 24-fold increase in weight. The uterine smooth muscle layer or myometrium is remodeled, and both cell hypertrophy and hyperplasia are evident. The origin of the new smooth muscle cells, however, is unclear. They may arise from existing smooth muscle cells, or they may be the product of stem cell differentiation. This study describes a subset of myometrial cells isolated from nonpregnant human myometrium that represents the myometrial stem cell population. This was characterized as side population of myometrial cells (myoSP) by a distinct Hoechst dye efflux pattern. In contrast to the main population of myometrial cells (myoMP), myoSP resided in quiescence, underexpressed or lacked myometrial cell markers, and could proliferate and eventually differentiate into mature myometrial cells in vitro only under low oxygen concentration. Although myoMP displayed mature myometrial phenotypes before and after in vitro cultivation, only myoSP, not myoMP, generated functional human myometrial tissues efficiently when transplanted into the uteri of severely immunodeficient mice. Finally, myoSP were multipotent and made to differentiate into osteocytes and adipocytes in vitro under the appropriate differentiation-inducing conditions. Thus, myoSP exhibited phenotypic and functional characteristics of myometrial stem cells. Study of myoSP will improve the understanding of myometrial physiology and the pathogenesis of myometrium-derived diseases such as leiomyoma. myoSP may also represent a novel source of biological material that could be used in the reconstruction of not only the human uterus but also other organs as well.

The role of the fibrocyte, a bone marrow-derived mesenchymal progenitor, in reactive and reparative fibroses

Human fibrocytes are mesenchymal progenitors that exhibit mixed morphological and molecular characteristics of hematopoietic stem cells, monocytes and fibroblasts. They likely represent the obligate intermediate stage of differentiation into mature mesenchymal cells of a bone marrow-derived precursor of the monocyte lineage under permissive conditions. On in vitro stimulation with pro-fibrotic cytokines and growth factors, human fibrocytes produce large quantities of extracellular matrix components and further differentiate into cells identical to the contractile myofibroblasts that emerge at the tissue sites during repair processes and in some fibrotic lesions. Studies in various animal models of wound healing or fibrotic diseases have confirmed the ability of fibrocytes to differentiate into mature mesenchymal cells in vivo and have suggested a causal link between fibrocyte accumulation and ongoing tissue fibrogenesis or vascular remodeling in response to tissue damage or hypoxia. Fibrocytes synthesizing new collagen or acquiring myofibroblast markers have been detected in human hypertrophic scars, in the skin of patients affected by nephrogenic systemic fibrosis, in human atherosclerotic lesions, and in pulmonary diseases characterized by repeated cycles of inflammation and repair, like asthma. The presence of fibrocyte-like cells has been reported in human chronic pancreatitis and chronic cystitis. Similar cells also populate the stroma surrounding human benign tumors. The available data indicate that human fibrocytes serve as a source of mature mesenchymal cells during reparative processes and in fibrotic disorders or stromal reactions predominantly associated with a persistent inflammatory infiltrate or with the selective recruitment of monocytes induced by ischemic changes and tumor development. A deeper understanding of the mechanisms involved in fibrocyte differentiation in these pathological conditions may lead to the development of novel therapies for preventing detrimental tissue or vascular remodeling and metastatic progression of invasive tumors.

Rho inhibition induces migration of mesenchymal stromal cells

Although mesenchymal stromal cells (MSCs) are being increasingly used as cell therapeutics in clinical trials, the mechanisms that regulate their chemotactic migration behavior are incompletely understood. We aimed to better define the ability of the GTPase regulator of cytoskeletal activation, Rho, to modulate migration induction in MSCs in a transwell chemotaxis assay. We found that culture-expanded MSCs migrate poorly toward exogenous phospholipids lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) in transwell assays. Moreover, plasma-induced chemotactic migration of MSCs was even inhibited after pretreatment with LPA. LPA treatment activated intracellular Rho and increased actin stress fibers in resident MSCs. Very similar cytoskeletal changes were observed after microinjection of a cDNA encoding constitutively active RhoA (RhoAV14) in MSCs. In contrast, microinjection of cDNA encoding Rho inhibitor C3 transferase led to resolution of actin stress fibers, appearance of a looser actin meshwork, and increased numbers of cytoplasmic extensions in the MSCs. Surprisingly, in LPA-pretreated MSCs migrating toward plasma, simultaneous addition of Rho inhibitor C2I-C3 reversed LPA-induced migration suppression and led to improved migration. Moreover, addition of Rho inhibitor C2I-C3 resulted in an approximately 3- to 10-fold enhancement of chemotactic migration toward LPA, S1P, as well as platelet-derived growth factor or hepatocyte growth factor. Thus, inhibition of Rho induces rearrangement of actin cytoskeleton in MSCs and renders them susceptible to induction of migration by physiological stimuli. Disclosure of potential conflicts of interest is found at the end of this article.

Human mesenchymal stem cells in contact with their environment: surface characteristics and the integrin system

The identification of mesenchymal stem cells (MSCs) in adult human tissues and the disclosure of their self-renew-al and multi-lineage differentiation capabilities have provided exciting prospects for cell-based regeneration and tis-sue engineering. Although a considerable amount of data is available describing MSCs, there is still lack of information regarding the molecular mechanisms that govern their adhesion and migration. In this work, we will review the current state of knowledge on integrins and other adhesion molecules found to be expressed on MSCs. The discussed topics include the characteristics of MSCs and their clinical applications, integrins and their central role in cell-matrix attachment and migration, and comments on mobilization, differentiation and contribution to tumour development. Finally, by understanding the complex and fundamental pathways by which MSCs attach and migrate, it might be possible to fine-tune the strategies for effective and safe use of MSCs in regenerative therapies.