Hypoxia affects mesenchymal stromal cell osteogenic differentiation and angiogenic factor expression

Adipose stromal cells stimulate angiogenesis via promoting progenitor cell differentiation, secretion of angiogenic factors, and enhancing vessel maturation

Adipose-derived stromal cells (ASCs) are suggested to be potent candidates for cell therapy of ischemic conditions due to their ability to stimulate blood vessel growth. ASCs produce many angiogenic and anti-apoptotic growth factors, and their secretion is significantly enhanced by hypoxia. Utilizing a Matrigel implant model, we showed that hypoxia-treated ASCs stimulated angiogenesis as well as maturation of the newly formed blood vessels in vivo. To elucidate mechanisms of ASC angiogenic action, we used a co-culture model of ASCs with cells isolated from early postnatal hearts (cardiomyocyte fraction, CMF). CMF contained mature cardiomyocytes, endothelial cells, and progenitor cells. On the second day of culture CMF cells formed spontaneously beating colonies with CD31+ capillary-like structures outgrowing from those cell aggregates. However, these vessel-like structures were not stable, and disassembled within next 5 days. Co-culturing of CMF with ASCs resulted in the formation of stable and branched CD31+ vessel-like structures. Using immunomagnetic depletion of CMF from vascular cells as well as incubation of CMF with mitomycin C-treated ASCs, we showed that in co-culture ASCs enhance blood vessel growth not only by production of paracrine-acting factors but also by promoting the endothelial differentiation of cardiac progenitor cells. All these mechanisms of actions could be beneficial for the stimulation of angiogenesis in ischemic tissues by ASCs administration.

Differential effects of hypoxia on osteochondrogenic potential of human adipose-derived stem cells

Human adipose tissue-derived stem cells (hATSC) have been contemplated as reparative cells for cartilage engineering. Chondrogenic differentiation of hATSC can be induced by an enriched culture medium and a three-dimensional environment. Given that bone is vascularized and cartilage is not, oxygen tension has been suggested as a regulatory factor for osteochondrogenic differentiation. Our work aimed at determining whether hypoxia affects the osteochondrogenic potential of hATSC. hATSC were cultured in chondrogenic or osteogenic medium for 28 days, in pellets or monolayers, and under 5% or 20% oxygen tension. Cell differentiation was monitored by real-time PCR (COL2A1, aggrecan, Runx2, and osteocalcin). The chondrogenic differentiation was further evaluated by Alcian blue and immunohistological staining for glycosaminoglycans (GAGs) and type II collagen, respectively. Osteogenic differentiation was also assessed by the staining of mineralized matrix (Alizarin Red) and measurement of alkaline phosphatase (ALP) activity. The expression of chondrogenic markers was upregulated when hATSC were exposed to hypoxia in chondrogenic medium. Conversely, osteocalcin expression, mineralization, and ALP activity were severely reduced under hypoxic conditions even in the presence of osteogenic medium. Our data strongly suggest that hypoxia favors the chondrogenic differentiation of hATSC as evidenced by the expression of the chondrogenic markers, whereas it could alter their osteogenic potential. Our results highlight the differential regulatory role of hypoxia on the chondrogenic and osteogenic differentiation processes of hATSC. These data could help us exploit the potential of tissue engineering and stem cells to replace or restore the function of osteoarticular tissues.

Allogenic stem cell therapy improves right ventricular function by improving lung pathology in rats with pulmonary hypertension

Pulmonary arterial hypertension (PAH) is a chronic lung disease that leads to right ventricular (RV) hypertrophy (RVH), remodeling, and failure. We tested treatment with bone marrow-derived mesenchymal stem cells (MSCs) obtained from donor rats with monocrotaline (MCT)-induced PAH to recipient rats with MCT-induced PAH on pulmonary artery pressure, lung pathology, and RV function. This model was chosen to mimic autologous MSC therapy. On day 1, PAH was induced by MCT (60 mg/kg) in 20 female Wistar rats. On day 14, rats were treated with 106 MSCs intravenously (MCT + MSC) or with saline (MCT60). MSCs were obtained from donor rats with PAH at 28 days after MCT. A control group received saline on days 1 and 14. On day 28, the RV function of recipient rats was assessed, followed by isolation of the lungs and heart. RVH was quantified by the weight ratio of the RV/(left ventricle + interventricular septum). MCT induced an increase of RV peak pressure (from 27 ± 5 to 42 ± 17 mmHg) and RVH (from 0.25 ± 0.04 to 0.47 ± 0.12), depressed the RV ejection fraction (from 56 ± 11 to 43 ± 6%), and increased lung weight (from 0.96 ± 0.15 to 1.66 ± 0.32 g), including thickening of the arteriolar walls and alveolar septa. MSC treatment attenuated PAH (31 ± 4 mmHg) and RVH (0.32 ± 0.07), normalized the RV ejection fraction (52 ± 5%), reduced lung weight (1.16 ± 0.24 g), and inhibited the thickening of the arterioles and alveolar septa. We conclude that the application of MSCs from donor rats with PAH reduces RV pressure overload, RV dysfunction, and lung pathology in recipient rats with PAH. These results suggest that autologous MSC therapy may alleviate cardiac and pulmonary symptoms in PAH patients.

Enhanced chondrogenic differentiation of human bone marrow-derived mesenchymal stem cells in low oxygen environment micropellet cultures

Chondrogenesis of mesenchymal stem cells (MSCs) is typically induced when they are condensed into a single aggregate and exposed to transforming growth factor-beta (TGF-beta). Hypoxia, like aggregation and TGF-beta delivery, may be crucial for complete chondrogenesis. However, the pellet dimensions and associated self-induced oxygen gradients of current chondrogenic methods may limit the effectiveness of in vitro differentiation and subsequent therapeutic uses. Here we describe the use of embryoid body-forming technology to produce microscopic aggregates of human bone marrow MSCs (BM-MSCs) for chondrogenesis. The use of micropellets reduces the formation of gradients within the aggregates, resulting in a more homogeneous and controlled microenvironment. These micropellet cultures (approximately 170 cells/micropellet) as well as conventional pellet cultures (approximately 2 x 10(5) cells/pellet) were chondrogenically induced under 20% and 2% oxygen environments for 14 days. Compared to conventional pellets under both environments, micropellets differentiated under 2% O(2) showed significantly increased sulfated glycosaminoglycan (sGAG) production and more homogeneous distribution of proteoglycans and collagen II. Aggrecan and collagen II gene expressions were increased in pellet cultures differentiated under 2% O(2) relative to 20% O(2) pellets but 2% O(2) micropellets showed even greater increases in these genes, as well as increased SOX9. These results suggest a more advanced stage of chondrogenesis in the micropellets accompanied by more homogeneous differentiation. Thus, we present a new method for enhancing MSC chondrogenesis that reveals a unique relationship between oxygen tension and aggregate size. The inherent advantages of chondrogenic micropellets over a single macroscopic aggregate should allow for easy integration with a variety of cartilage engineering strategies.

Hypoxia-enhanced wound-healing function of adipose-derived stem cells: increase in stem cell proliferation and up-regulation of VEGF and bFGF

Adipose-derived stem cells (ADSCs) have been shown to induce wound-healing effects. Because inflammation near the wound area induces oxygen deficiency, it is interesting to elucidate the effect of hypoxia on the function of ADSCs. In this work, we asked: (1) does hypoxia alter the wound-healing function of ADSCs? and (2) what are the major factors responsible for the alteration in the wound-healing function? Effect of hypoxia on the proliferation of ADSCs was first examined that hypoxia (2% O(2)) enhanced the proliferation of ADSCs in either the presence of serum or in the absence of serum. The conditioned medium of ADSCs harvested under hypoxia (hypoCM) significantly promoted collagen synthesis and the migration of human dermal fibroblasts, compared with that in normoxia (norCM). In the animal studies, hypoCM significantly reduced the wound area compared with norCM. Furthermore, mRNA and protein measurements showed that hypoxia up-regulated growth factors such as vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF). Inhibition of VEGF and bFGF using neutralizing antibodies reversed the migration of the wounded human dermal fibroblasts and the healing of wounds in animal experiment. Collectively, these results suggest that hypoxia increases the proliferation of ADSCs and enhances the wound-healing function of ADSCs, at least partly, by up-regulating the secretion of VEGF and bFGF.

Toll-like receptor-mediated signaling in human adipose-derived stem cells: implications for immunogenicity and immunosuppressive potential

Human adipose-derived stem cells (hASCs) are mesenchymal stem cells with reduced immunogenicity and the capability to modulate immune responses. These properties make hASCs of special interest as therapeutic agents in the settings of chronic inflammatory and autoimmune diseases. Exogenous and endogenous toll-like receptor (TLR) ligands have been linked with the perpetuation of inflammation in a number of chronic inflammatory diseases such as inflammatory bowel disease and rheumatoid arthritis because of the permanent exposure of the immune system to TLR-specific stimuli. Therefore, hASCs employed in therapy are potentially exposed to TLR ligands, which may result in the modulation of hASC activity and therapeutic potency. In this study, we demonstrate that hASCs possess active TLR2, TLR3, and TLR4, because activation with specific ligands resulted in induction of nuclear factor kappa B-dependent genes, such as manganese superoxide dismutase and the release of interleukin (IL)-6 and IL-8. TLR3 and TLR4 ligands increased osteogenic differentiation, but no effect on adipogenic differentiation or proliferation was observed. Moreover, we show that TLR activation does not impair the immunogenic and immunosuppressive properties of hASCs. These results may have important implications with respect to the safety and efficacy of hASC-based cell therapies.

Phenotypic characterization, osteoblastic differentiation, and bone regeneration capacity of human embryonic stem cell-derived mesenchymal stem cells

To enhance the understanding of differentiation patterns and bone formation capacity of hESCs, we determined (1) the temporal pattern of osteoblastic differentiation of human embryonic stem cell-derived mesenchymal stem cells (hESC-MSCs), (2) the influence of a three-dimensional matrix on the osteogenic differentiation of hESC-MSCs in long-term culture, and (3) the bone-forming capacity of osteoblast-like cells derived from hESC-MSCs in calvarial defects. Incubation of hESC-MSCs in osteogenic medium induced osteoblastic differentiation of hESC-MSCs into mature osteoblasts in a similar chronological pattern to human bone marrow stromal cells and primary osteoblasts. Osteogenic differentiation was enhanced by culturing the cells on three-dimensional collagen scaffolds. Fluorescent-activated cell sorting of alkaline phosphatase expressing cells was used to obtain an enriched osteogenic cell population for in vivo transplantation. The identification of green fluorescence protein and expression of human-specific nuclear antigen in osteocytes in newly formed bone verified the role of transplanted human cells in the bone regeneration process. The current cell culture model and osteogenic cell enrichment method could provide large numbers of osteoprogenitor cells for analysis of differentiation patterns and cell transplantation to regenerate skeletal defects.

Impact of oxygen environment on mesenchymal stem cell expansion and chondrogenic differentiation

INTRODUCTION

In vitro expansion and differentiation of mesenchymal stem cells (MSC) rely on specific environmental conditions, and investigations have demonstrated that one crucial factor is oxygen environment.

OBJECTIVES

In order to understand the impact of oxygen tension on MSC culture and chondrogenic differentiation in vitro, we developed a mathematical model of these processes and applied it in predicting optimal assays.

METHODS AND RESULTS

We compared ovine MSCs under physiologically low and atmospheric oxygen tension. Low oxygen tension improved their in vitro population growth as demonstrated by monoclonal expansion and colony forming assays. Moreover, it accelerated induction of the chondrogenic phenotype in subsequent three-dimensional differentiation cultures. We introduced a hybrid stochastic multiscale model of MSC organization in vitro. The model assumes that cell adaptation to non-physiological high oxygen tension reversibly changes the structure of MSC populations with respect to differentiation. In simulation series, we demonstrated that these changes profoundly affect chondrogenic potential of the populations. Our mathematical model provides a consistent explanation of our experimental findings.

CONCLUSIONS

Our approach provides new insights into organization of MSC populations in vitro. The results suggest that MSC differentiation is largely reversible and that lineage plasticity is restricted to stem cells and early progenitors. The model predicts a significant impact of short-term low oxygen treatment on MSC differentiation and optimal chondrogenic differentiation at 10-11% pO(2).

IFATS collection: Adipose stromal cells adopt a proangiogenic phenotype under the influence of hypoxia

Evolving evidence suggests a possible role for adipose stromal cells (ASCs) in adult neovascularization, although the specific cues that stimulate their angiogenic behavior are poorly understood. We evaluated the effect of hypoxia, a central mediator of new blood vessel development within ischemic tissue, on proneovascular ASC functions. Murine ASCs were exposed to normoxia (21% oxygen) or hypoxia (5%, 1% oxygen) for varying lengths of time. Vascular endothelial growth factor (VEGF) secretion by ASCs increased as an inverse function of oxygen tension, with progressively higher VEGF expression at 21%, 5%, and 1% oxygen, respectively. Greater VEGF levels were also associated with longer periods in culture. ASCs were able to migrate towards stromal cell-derived factor (SDF)-1, a chemokine expressed by ischemic tissue, with hypoxia augmenting ASC expression of the SDF-1 receptor (CXCR4) and potentiating ASC migration. In vivo, ASCs demonstrated the capacity to proliferate in response to a hypoxic insult remote from their resident niche, and this was supported by in vitro studies showing increasing ASC proliferation with greater degrees of hypoxia. Hypoxia did not significantly alter the expression of endothelial surface markers by ASCs. However, these cells did assume an endothelial phenotype as evidenced by their ability to tubularize when seeded with differentiated endothelial cells on Matrigel. Taken together, these data suggest that ASCs upregulate their proneovascular activity in response to hypoxia, and may harbor the capacity to home to ischemic tissue and function cooperatively with existing vasculature to promote angiogenesis.

The use of a synthetic oxygen carrier-enriched hydrogel to enhance mesenchymal stem cell-based bone formation in vivo

A major hurdle to surmount in bone-tissue engineering is ensuring a sufficient oxygen supply to newly forming tissue to avoid cell death or delayed development of osteogenic features. We hypothesized that an oxygen-enriched hydrogel scaffold would enhance tissue-engineered bone formation in vivo. To test this, we used a well-characterized mesenchymal stem cell (MSC) line, Tet-off BMP2 MSC, whose cells were engineered to express recombinant human bone morphogenetic protein-2. Cells were suspended in hydrogel supplemented with perfluorotributylamine (PFTBA) and implanted subcutaneously in an ectopic site, a radial bone defect, or a lumbar paravertebral muscle (mouse model of spinal fusion) in C3H/HeN mice. For controls, we used cells suspended in the same gel without PFTBA. In the ectopic site, there were significant increases in bone formation (2.5-fold increase), cell survival, and osteocalcin activity in the PFTBA-supplemented groups. PFTBA supplementation significantly increased structural parameters of bone in radial bone defects and triggered a significant 1.4-fold increase in bone volume in the spinal fusion model. We conclude that synthetic oxygen carrier supplementation of tissue-engineered implants enhances ectopic bone formation and yields better bone quality and volume in bone-repair and spinal fusion models, probably due to increased cell survival.

Hypoxia and stem cell-based engineering of mesenchymal tissues

Stem cells have the ability for prolonged self-renewal and differentiation into mature cells of various lineages, which makes them important cell sources for tissue engineering applications. Their remarkable ability to replenish and differentiate in vivo is regulated by both intrinsic and extrinsic cellular mechanisms. The anatomical location where the stem cells reside, known as the "stem cell niche or microenvironment," provides signals conducive to the maintenance of definitive stem cell properties. Physiological condition including oxygen tension is an important component of the stem cell microenvironment and has been shown to play a role in regulating both embryonic and adult stem cells. This review focuses on oxygen as a signaling molecule and the way it regulates the stem cells' development into mesenchymal tissues in vitro. The physiological relevance of low oxygen tension as an environmental parameter that uniquely benefits stem cells' expansion and maintenance is described along with recent findings on the regulatory effects of oxygen on embryonic stem cells and adult mesenchymal stem cells. The relevance to tissue engineering is discussed in the context of the need to specifically regulate the oxygen content in the cellular microenvironment in order to optimize in vitro tissue development.

Fate of mesenchymal stem cells transplanted to osteonecrosis of femoral head

The purpose of this study was to investigate the survival and differentiation status of MSCs transplanted to ONFH. Traumatic ONFH was surgically produced in skeletally mature mongrel dogs. Osteonecrosis was treated with either saline (control) or autologous mesenchymal stem cells (MSCs) transplantation after decompression. Green fluorescent protein (GFP) was used to track the transplanted MSCs, the differentiation of MSCs were evaluated by fluorescent double-labeling with GFP between osteocalcin or von Willebrand factor (vWF) at 2nd, 8th, and 12th week after the transplantation. It was demonstrated that GFP-positive cells were present in the necrotic area up to 12 weeks after the transplantation, their number increased from 15% at 2nd week to 38% at 12th week (p < 0.05). Neither osteocalcin nor vWF was detected by immunocytochemistry in GFP-labeled MSCs in vitro, but osteocalcin was immunohistochemically positive in 90% of the GFP-labeled MSCs in vivo, while vWF was still negative. The vWF expression was of no significant difference between the control group and MSCs-transplanted group. The percentages of trabeculae bone volume were 9.36% and 8.42% at 2nd week (p > 0.05), 22.82% and 14.72% at 8th week, and 31.08% and 20.66% at 12th week (p < 0.05) in MSCs-transplanted group and control group, respectively; new trabeculae bone in MSCs-transplanted group was significantly increased as compared to that of control group at 8th and 12th week. The results demonstrated that the transplanted MSCs could survive, proliferate, and differentiate into osteoblasts directly, which contributed to the accelerated repair process. The possible mechanism is site-dependant differentiation.

Therapeutic effect of human umbilical cord multipotent mesenchymal stromal cells in a rat model of stroke

BACKGROUND

Human umbilical cord multipotent mesenchymal stromal cells (UC-MSC) have recently been identified as ideal candidate stem cells for cell-based therapy. The present study was designed to evaluate therapeutic potentials of intracerebral administration of UC-MSC in a rat model of stroke.

METHODS

Rats were subjected to 2-hr middle cerebral artery occlusion and received 2 10 UC-MSC or phosphate-buffered saline as a control. Neurologic function evaluation was conducted weekly after transplantation. Brain injury volume and in vivo differentiation of transplanted UC-MSC were detected 2 or 5 weeks after the UC-MSC treatment. In addition, vascular density, vascular endothelial growth factor, and basic fibroblast growth factor expression in ipsilateral hemisphere after treatment and in vitro angiogenic potential of UC-MSC were assessed.

RESULTS

The transplanted UC-MSC survived for at least 5 weeks in rat brain. Compared with the phosphate-buffered saline control, the UC-MSC treatment significantly reduced injury volume and neurologic functional deficits of rats after stroke. In ischemic brain, UC-MSC widely incorporated into cerebral vasculature and a subset of them was capable of differentiating into endothelial cells. Furthermore, the UC-MSC treatment substantially increased vascular density and vascular endothelial growth factor and basic fibroblast growth factor expression in ipsilateral hemisphere of stroke. In vitro induction and tube formation assay further confirmed their angiogenic properties.

CONCLUSIONS

UC-MSC transplantation could accelerate neurologic functional recovery of rats after stroke, which may be mediated by their ability to promote angiogenesis.

Subcutaneous-induced membranes have no osteoinductive effect on macroporous HA-TCP in vivo

Induced Membranes Technique was first described to enhance bone reconstruction of large osseous defects. Previous in vitro studies established their osteoinductive potential, due to the presence of opteoblasts precursors and to high amounts of growth factors contained within. The purpose of this study was to test in vivo the osteoinductive properties of induced membranes on a macroporous HA-TCP in a nonosseous subcutaneous site. Subcutaneous-induced membranes were obtained in 21 rabbits; 1 month later, the membranes were filled with a biphasic calcium phosphate material composed of 75% hydroxyapatite (HA) and 25% beta-tricalcium phosphate associated or not with autograft. Histological and immunohistochemical studies were performed on membrane biopsies. Undecalcified and decalcified sections were qualitatively and quantitatively analyzed. (45)Ca uptake was observed and quantified on the sections using microimager analysis. Dense vascularity was found in the induced membranes. New bone formation was detected in the HA-TCP + autograft samples and increased significantly from 3 to 6 months (p < 0.05). No bone was detected in the biomaterial graft alone in the induced membranes at any time. This study showed that induced membranes placed in a nonosseous site have no osteoinductive properties on a macroporous biphasic calcium phosphate biomaterial.

Recent developments in the biology of fracture repair

Fracture repair is dependent on local and systemic molecular and cellular processes. During fracture repair, mesenchymal stem cells are systemically recruited to the fracture site, and cytokines are released from the fracture site into the vascular system. In a significant minority of fractures, healing delays result from adverse clinical factors that interfere with these processes. Extrinsic factors, such as aging and smoking, adversely affect the molecular and cellular processes occurring locally in the fracture site. Fracture fixation affects healing through local changes in the biologic signaling within the fracture callus. Current biologic treatment of fractures includes the local application of osteoinductive bone morphogenetic proteins (ie, BMP-2, BMP-7) and cell-based therapies. Although clinical results with bone morphogenetic proteins have been satisfactory, they have not been as impressive as those reported in animal studies. Further understanding of the biology of fracture repair may lead to improved treatment modalities.

The effect of oxygen tension on the in vitro assay of human osteoblastic connective tissue progenitor cells

Connective tissue progenitors (CTPs) are defined as the heterogeneous set of stem and progenitor cells that reside in native tissues and are capable of proliferation and differentiation into one or more connective tissue phenotypes. CTPs play important roles in tissue formation, repair, and remodeling. Therefore, in vitro assays of CTP prevalence and biological potential have important scientific and clinical relevance. This study evaluated oxygen tension as an important variable in optimizing in vitro conditions for quantitative assays of human CTPs. Bone marrow aspirates were collected from 20 human subjects and cultured using established medium conditions at ambient oxygen tensions of 1, 5, 10, and 20%. Colony-forming efficiency (CFE), proliferation, and colony density were assessed. CFE and proliferation were greatest at 5% O(2). Traditional conditions using atmospheric oxygen tension (20% O(2)) reduced CFE by as much as 32%. CFE and proliferation at 1% O(2) were less than 5% O(2) but comparable to that seen at 20% O(2), suggesting that CTPs are relatively resilient under hypoxic conditions, a fact that may be relevant to their function in wound repair and their potential use in tissue engineering applications involving transplantation into settings of moderate to severe hypoxia. These data demonstrate that optimization of quantitative assays for CTPs will require control of oxygen tension.

5-Azacytidine-treated human mesenchymal stem/progenitor cells derived from umbilical cord, cord blood and bone marrow do not generate cardiomyocytes in vitro at high frequencies

BACKGROUND AND OBJECTIVES

Mesenchymal stem/progenitor cells (MSCs) are multipotent progenitors that differentiate into such lineages as bone, fat, cartilage and stromal cells that support haemopoiesis. Bone marrow MSCs can also contribute to cardiac repair, although the mechanism for this is unclear. Here, we examine the potential of MSCs from different sources to generate cardiomyocytes in vitro, as a means for predicting their therapeutic potential after myocardial infarction.

MATERIALS AND METHODS

Mesenchymal stem/progenitor cells were isolated from the perivascular tissue and Wharton's jelly of the umbilical cord and from cord blood. Their immunophenotype and differentiation potential to generate osteoblasts, chondrocytes, adipocytes and cardiomyoxcytes in vitro was compared with those of bone marrow MSCs.

RESULTS

Mesenchymal stem/progenitor cells isolated from umbilical cord and cord blood were phenotypically similar to bone marrow MSCs, the exception being in the expression of CD106, which was absent on umbilical cord MSCs, and CD146 that was highly expressed in cord blood MSCs. They have variable abilities to give rise to osteoblasts, chondrocytes and adipocytes, with bone marrow MSCs being the most robust. While a small proportion (approximately 0.07%) of bone marrow MSCs could generate cardiomyocyte-like cells in vitro, those from umbilical cord and cord blood did not express cardiac markers either spontaneously or after treatment with 5-azacytidine.

CONCLUSION

Although MSCs may be useful for such clinical applications as bone or cartilage repair, the results presented here indicate that such cells do not generate cardiomyocytes frequently enough for cardiac repair. Their efficacy in heart repair is likely to be due to paracrine mechanisms.

Hypoxic preconditioning results in increased motility and improved therapeutic potential of human mesenchymal stem cells

Mesenchymal stem cells (MSC) are adult multipotent cells found in bone marrow, adipose tissue, and other adult tissues. MSC have been shown to improve regeneration of injured tissues in vivo, but the mechanisms remain unclear. Typically, MSC are cultured under ambient, or normoxic, conditions (21% oxygen). However, the physiological niches for MSC in the bone marrow and other sites have much lower oxygen tension. When used as a therapeutic tool to repair tissue injuries, MSC cultured in standard conditions must adapt from 21% oxygen in culture to less than 1% oxygen in the ischemic tissue. We therefore examined the effects of preculturing human bone marrow-derived MSC in hypoxic conditions (1%-3% oxygen) to elucidate the best conditions that enhance their tissue regenerative potential. We demonstrated that MSC cultured in hypoxia activate the Akt signaling pathway while maintaining their viability and cell cycle rates. We also showed that MSC cultured in hypoxia induced expression of cMet, the major receptor for hepatocyte growth factor (HGF), and enhanced cMet signaling. MSC cultured in hypoxic conditions increased their migration rates. Since migration and HGF responsiveness are thought to be key mediators of MSC recruitment and/or activation in vivo, we next examined the tissue regenerative potential of MSC cultured under hypoxic conditions, using a murine hind limb ischemia model. We showed that local expression of HGF is increased in ischemic muscle in this model. Intra-arterial injection of MSC cultured in either normoxic or hypoxic conditions 24 hours after surgical induction of hind limb ischemia enhanced revascularization compared with saline controls. However, restoration of blood flow was observed significantly earlier in mice that had been injected with hypoxic preconditioned MSC. Collectively, these data suggest that preculturing MSC under hypoxic conditions prior to transplantation improves their tissue regenerative potential. Disclosure of potential conflicts of interest is found at the end of this article.

Relationships of maternal serum levels of vascular endothelial growth factor and tensile strength properties of the cervix in a rat model of chronic hypoxia

OBJECTIVE

It has been shown that hypoxia leads to alterations in maternal serum levels of vascular endothelial growth factor (VEGF). In this study, we sought to test the hypothesis that chronic hypoxia increases maternal serum levels of VEGF, which in turn cause measurable changes in the viscoelastic properties of the rat uterine cervix.

STUDY DESIGN

Timed-pregnant adult Sprague-Dawley rats were exposed to hypoxia beginning on day 17 of gestation (term = day 22). The following groups of animals were studied: (1) nonpregnant controls (NP, n = 6); (2) normoxia 21% fraction of inspired oxygen (FiO2) (NMX, n = 6); and (3) severe hypoxia 10% FiO2 (HPX; n = 5). A hypoxic chamber was used to assure consistent hypoxic environment. Animals were killed on day 21 of gestation (before labor). Maternal blood was collected immediately following anesthesia and prior to euthanasia. Free serum levels of VEGF were measured by highly specific immunoassays. Tensile strength properties of the cervix were assessed using a stretching regimen designed to mimic labor. Physical parameters measured were: indicators of viscoelasticity (slope; measure of stiffness), plasticity (yield point [YP]; moment the tissue changes its properties from elastic to plastic), strength (break point [BP]; moment of tissue disruption), and displacement at YP (marks the duration of the viscoelastic phase of the stretching) and BP (a measure of the strength of the material). Data were normalized to the dry weight of the cervix.

RESULTS

Hypoxia is associated with increased serum levels of VEGF compared to NP or NMX groups (P = .001). Cervical stiffness was lower in NMX, compared with NP animals (P = .004), and was not significantly influenced by hypoxia (P > .05). Overall there was a significant inverse correlation between slope and maternal serum levels of VEGF (r = -0.85, P < .001). The force required to reach YP was significantly higher for the NP, compared with NMX and HPX groups (P = .004). Hypoxia did not alter the force required to reach the YP (NMX vs HPX, P > .05). Conversely, hypoxia significantly decreased the displacement at YP, indicating a shortening of the elastic phase (NMX vs HPX, P = .021). There was a significant inverse correlation between maternal serum levels of VEGF and the displacement at YP (r = -0.68, P = .002). In vivo, hypoxia decreased the force required to reached the BP (NMX vs HPX, P = .025), but there was no correlation between the levels of maternal serum VEGF and this indicator (r = -0.35, P = .170).

CONCLUSION

Chronic hypoxia induces measurable changes in maternal serum levels of VEGF and tensile properties of the rat cervix, specifically a shortening of the elastic phase. Hypoxia decreases the cervical strength to stretch and predisposes to rupture, but this effect seems to be unrelated to maternal serum levels of VEGF.

Opportunities and challenges for mesenchymal stem cell-mediated heart repair

PURPOSE OF REVIEW

Mesenchymal stem cells (or multipotent stromal cells) are emerging as a potent cell type for cardiac cell therapy. This review describes the potential of cardiac mesenchymal stem cell therapy, but also highlights some recently discovered less favorable mesenchymal stem cell characteristics.

RECENT FINDINGS

Mesenchymal stem cells exert a beneficial effect on cardiac function upon administration to the ischemic myocardium. The mode of action does not seem to involve differentiation into cardiomyocytes and vascular cells. A robust effect on revascularization and remodeling is observed, however, most likely mediated by paracrine factors. Recently identified drawbacks associated with cardiac mesenchymal stem cell therapy include differentiation into unwanted mesenchymal cell types such as osteocytes and adipocytes, the occurrence of cytogenetic instability upon prolonged expansion, and immunization when used in an allogeneic setting.

SUMMARY

The application of mesenchymal stem cells is a novel strategy with therapeutic potential for cardiac repair. Strategies are needed, however, to optimize their therapeutic potential while minimizing their potential clinical risks.