Not the usual suspects: the unexpected sources of tissue regeneration

Stem Cells in Cardiovascular Medicine: the Road to Regenerative Therapies

PURPOSE OF REVIEW

The purpose of this review is to provide a broad overview of current trends in stem cell research and its applications in cardiovascular medicine. Researches on different stem cell sources, their inherent characteristics, and the limitations they have in medical applications are discussed. Additionally, uses of stem cells for both modeling and treating cardiovascular disease are discussed, taking note of the obstacles these engineered interventions must overcome to be clinically viable.

RECENT FINDINGS

Tissue engineering aims to replace dysfunctional tissues with engineered constructs. Stem cell technologies have been a great enabling factor in working toward this goal. Many tissue-engineered products are in development that utilize stem cell technology. Although promising, some refinement must be made to these constructs with respect to safety and functionality. A deeper understanding of basic differentiation and tissue developmental mechanisms is required to allow these engineered tissues to be translated into the clinic.

Association of hepatocyte-derived growth factor receptor/caudal type homeobox 2 co-expression with mucosal regeneration in active ulcerative colitis

AIM: To characterize the regeneration-associated stem cell-related phenotype of hepatocyte-derived growth factor receptor (HGFR)-expressing cells in active ulcerative colitis (UC).

METHODS: On the whole 38 peripheral blood samples and 38 colonic biopsy samples from 18 patients with histologically proven active UC and 20 healthy control subjects were collected. After preparing tissue microarrays and blood smears HGFR, caudal type homeobox 2 (CDX2), prominin-1 (CD133) and Musashi-1 conventional and double fluorescent immunolabelings were performed. Immunostained samples were digitalized using high-resolution Mirax Desk instrument, and analyzed with the Mirax TMA Module software. For semiquantitative counting of immunopositive lamina propria (LP) cells 5 fields of view were counted at magnification × 200 in each sample core, then mean ± SD were determined. In case of peripheral blood smears, 30 fields of view with 100 μm diameter were evaluated in every sample and the number of immunopositive cells (mean ± SD) was determined. Using 337 nm UVA Laser MicroDissection system at least 5000 subepithelial cells from the lamina propria were collected. Gene expression analysis of HGFR, CDX2, CD133, leucine-rich repeat-containing G-protein coupled receptor 5 (Lgr5), Musashi-1 and cytokeratin 20 (CK20) were performed in both laser-microdisscted samples and blood samples by using real time reverse transcription polymerase chain reaction (RT-PCR).

RESULTS: By performing conventional and double fluorescent immunolabelings confirmed by RT-PCR, higher number of HGFR (blood: 6.7 ± 1.22 vs 38.5 ± 3.18; LP: 2.25 ± 0.85 vs 9.22 ± 0.65; P < 0.05), CDX2 (blood: 0 vs 0.94 ± 0.64; LP: 0.75 ± 0.55 vs 2.11 ± 0.75; P < 0.05), CD133 (blood: 1.1 ± 0.72 vs 8.3 ± 1.08; LP: 11.1 ± 0.85 vs 26.28 ± 1.71; P < 0.05) and Musashi-1 (blood and LP: 0 vs scattered) positive cells were detected in blood and lamina propria of UC samples as compared to controls. HGFR/CDX2 (blood: 0 vs 1 ± 0.59; LP: 0.8 ± 0.69 vs 2.06 ± 0.72, P < 0.05) and Musashi-1/CDX2 (blood and LP: 0 vs scattered) co-expressions were found in blood and lamina propria of UC samples. HGFR/CD133 and CD133/CDX2 co-expressions appeared only in UC lamina propria samples. CDX2, Lgr5 and Musashi-1 expressions in UC blood samples were not accompanied by CK20 mRNA expression.

CONCLUSION: In active UC, a portion of circulating HGFR-expressing cells are committed to the epithelial lineage, and may participate in mucosal regeneration by undergoing mesenchymal-to-epithelial transition.

The Role of Aerobic and Anaerobic Training Programs on CD(34+) Stem Cells and Chosen Physiological Variables

Exercise is one of the most powerful non-pharmacological strategies, which can affect nearly all cells and organs in the body. Changes in the behavior of adult stem cells have been shown to occur in response to exercise. Exercise may act on regenerative potential of tissues by altering the ability to generate new stem cells and differentiated cells that are able to carry out tissue specific functions. The purpose of this study was to reveal the role of aerobic and anaerobic training programs on CD34+ Stem Cells and chosen physiological variables. Twenty healthy male athletes aged 18-24 years were recruited for this study. Healthy low active males and BMI matched participants (n=10) aged 20-22 years were recruited as controls. Aerobic and anaerobic training programs for 12 weeks were conducted. VO2max pulse observation was carried out using the Astrand Rhyming protocol. RBCs, WBCs, HB and hematocrit were estimated using a coulter counter, lactate by the Accusport apparatus, CD34+ stem cells by flow cytometry. VO2max was increased significantly in case of the aerobic training program compared to anaerobic one (62±2.2 ml/kg/min vs. 54±2.1 ml/kg/min). Haemotological values increased significantly in the anaerobic program when compared to the aerobic one, RBCs (5.3±0.3 and 4.9±0.2 mln/ul), WBCs (6.6±0.5 and 6.1±0.4 thous/ul), HB (15.4±0.4 and 14.2±0.5 g/de), Hematocrit (4.6±1.2 and 4.4±1.1 %), CD34+ stem cells count increased significantly in case of the anaerobic program compared to the aerobic (251.6±21.64 and 130±14.61) and sedentary one (172±24.10). These findings suggest that anaerobic training programs provoke better adaptation to exercise and stem cell counts may differ between trained and sedentary subjects. Circulating immature cells are likely to be involved in angiogenesis and repair process, both mechanisms being associated with strenuous exercise. Knowledge of the physiological effects of training on stem cells might be of potential clinical use.

MEK, p38, and PI-3K mediate cross talk between EGFR and TNFR in enhancing hepatocyte growth factor production from human mesenchymal stem cells

Human bone marrow mesenchymal stem cells (MSCs) are a potent source of growth factors, which are partly responsible for their beneficial paracrine effects. We reported previously that transforming growth factor-alpha (TGF-alpha), a putative mediator of wound healing and the injury response, increases the release of vascular endothelial growth factor (VEGF), augments tumor necrosis factor-alpha (TNF-alpha)-stimulated VEGF production, and activates mitogen-activated protein kinases and phosphatidylinositol 3-kinase (PI-3K) pathway in human MSCs. The experiments described in this report indicate that TGF-alpha increases MSC-derived hepatocyte growth factor (HGF) production. TGF-alpha-stimulated HGF production was abolished by inhibition of MEK, p38, PI-3K, or by small interfering RNA (siRNA) targeting TNF receptor 2 (TNFR2), but was not attenuated by siRNA targeting TNF receptor 1 (TNFR1). Ablation of TNFR1 significantly increased basal and stimulated HGF. A potent synergy between TGF-alpha and TNF-alpha was noted in MSC HGF production. This synergistic effect was abolished by MEK, P38, PI-3K inhibition, or by ablation of both TNF receptors using siRNA. We conclude that 1) novel cross talk occurs between tumor necrosis factor receptor and TGF-alpha/epidermal growth factor receptor in stimulating MSC HGF production; 2) this cross talk is mediated, at least partially, via activation of MEK, p38, and PI-3K; 3) TGF-alpha stimulates MSCs to produce HGF by MEK, p38, PI-3K, and TNFR2-dependent mechanisms; and 4) TNFR1 acts to decrease basal TGF-alpha and TNF-alpha-stimulated HGF.

Role of tumor necrosis factor receptor 1 in sex differences of stem cell mediated cardioprotection

BACKGROUND

Mesenchymal stem cells (MSCs) hold great therapeutic potential for the repair and regeneration of ischemic tissue, possibly through the release of beneficial paracrine factors. Sex differences have been observed in the paracrine function of MSCs. Female stem cells produce lower proinflammatory cytokines and higher levels of growth factors compared with their male counterparts. Ablation of tumor necrosis factor receptor 1 (TNFR1) increases protective growth factor production by male, but not by female, MSCs. We therefore hypothesized the following: (1) that female MSCs would improve myocardial recovery compared with male MSCs after ischemia-reperfusion injury (I/R); and (2) that MSCs isolated from TNFR1 knock out male, but not female, mice, would improve postischemic myocardial recovery compared with their wild type (WT) counterparts.

METHODS

Male adult Sprague-Dawley rat hearts were subjected to I/R by Langendorff isolated heart preparation. The MSCs were harvested from adult mice and cultured under normal conditions. Immediately prior to ischemia, one million MSCs were infused into the coronary circulation. Cardiac functional parameters were recorded continuously.

RESULTS

Pretreatment with MSCs from either sex significantly increased postischemic myocardial recovery as evidenced by improved left ventricular developed pressure, contractility, and rate of relaxation. Infusion with female MSCs was associated with a greater degree of myocardial recovery after I/R compared with male MSCs. The TNFR1 deficiency increased the degree of myocardial recovery associated with male MSCs, but not with female MSCs. No additional cardioprotection was observed when TNFR1 was ablated in female MSCs.

CONCLUSION

Sex differences influence the cardioprotective effects of both WT and TNFR1 ablated MSCs.

Pleiotrophin induces nitric oxide dependent migration of endothelial progenitor cells

Pleiotrophin (PTN) is produced under ischemic conditions and has been shown to induce angiogenesis in vivo. We studied whether or not PTN exerts chemotaxis of pro-angiogenic early endothelial progenitor cells (EPCs), a population of circulating cells that have been reported to participate in and stimulate angiogenesis. Chemotaxis of EPCs, isolated from blood of healthy humans (n = 5), was measured in transwell assays. PTN at 10-500 ng/ml elicited dose-dependent chemotaxis of both EPCs and human umbilical vein endothelial cells (HUVECs), but not of human coronary artery smooth muscle cells (CASMCs) and T98G glioblastoma cells that lack PTN receptors. The degree of chemotaxis was comparable to that induced by the angiogenic factors VEGF and SDF-1alpha. Chemotaxis to PTN was blocked by the NOS inhibitors L-NNA and L-NMMA, the NO scavenger PTIO, the phosphoinositide-3 kinase inhibitor wortmannin, and the guanylyl cyclase inhibitor ODQ, suggesting dependence of EPC chemotaxis on these pathways. PTN induced NOS-dependent production of NO to a similar degree as did VEGF, as indicated by the NO indicator DAF-2. PTN increased proliferation in EPCs and HUVECs to a similar extent as VEGF, but did not induce proliferation of CASMCs. While L-NNA abolished PTN-induced migration in EPCs and HUVECs, it did not inhibit PTN- and VEGF-enhanced proliferation and also caused proliferation by itself. These data suggest that PTN may mediate its pro-angiogenic effects by increasing the local number of not only endothelial cells but also early EPCs at angiogenic sites.

Peripheral infusion of rat bone marrow derived endothelial progenitor cells leads to homing in acute lung injury

BACKGROUND

Bone marrow-derived progenitors for both epithelial and endothelial cells have been observed in the lung. Besides mature endothelial cells (EC) that compose the adult vasculature, endothelial progenitor cells (EPC) are supposed to be released from the bone marrow into the peripheral blood after stimulation by distinct inflammatory injuries. Homing of ex vivo generated bone marrow-derived EPC into the injured lung has not been investigated so far. We therefore tested the hypothesis whether homing of EPC in damaged lung tissue occurs after intravenous administration.

METHODS

Ex vivo generated, characterized and cultivated rat bone marrow-derived EPC were investigated for proliferation and vasculogenic properties in vitro. EPC were tested for their homing in a left-sided rat lung transplant model mimicking a severe acute lung injury. EPC were transplanted into the host animal by peripheral administration into the femoral vein (10(6) cells). Rats were sacrificed 1, 4 or 9 days after lung transplantation and homing of EPC was evaluated by fluorescence microscopy. EPC were tested further for their involvement in vasculogenesis processes occurring in subcutaneously applied Matrigel in transplanted animals.

RESULTS

We demonstrate the integration of intravenously injected EPC into the tissue of the transplanted left lung suffering from acute lung injury. EPC were localized in vessel walls as well as in destructed lung tissue. Virtually no cells were found in the right lung or in other organs. However, few EPC were found in subcutaneous Matrigel in transplanted rats.

CONCLUSION

Transplanted EPC may play an important role in reestablishing the endothelial integrity in vessels after severe injury or at inflammatory sites and might further contribute to vascular repair or wound healing processes in severely damaged tissue. Therapeutic applications of EPC transplantation may ensue.

Functional expression of HGF and HGF receptor/c-met in adult human mesenchymal stem cells suggests a role in cell mobilization, tissue repair, and wound healing

Human mesenchymal stem cells (hMSC) are adult stem cells with multipotent capacities. The ability of mesenchymal stem cells to differentiate into many cell types, as well as their high ex vivo expansion potential, makes these cells an attractive therapeutic tool for cell transplantation and tissue engineering. hMSC are thought to contribute to tissue regeneration, but the signals governing their mobilization, diapedesis into the bloodstream, and migration into the target tissue are largely unknown. Here we report that hepatocyte growth factor (HGF) and the cognate receptor HGFR/c-met are expressed in hMSC, on both the RNA and the protein levels. The expression of HGF was downregulated by transforming growth factor beta. HGF stimulated chemotactic migration but not proliferation of hMSC. Therefore the HGF/c-met signaling system may have an important role in hMSC recruitment sites of tissue regeneration. The controlled regulation of HGF/c-met expression may be beneficial in tissue engineering and cell therapy employing hMSC.

Bone marrow-derived stem cells initiate pancreatic regeneration

We show that transplantation of adult bone marrow-derived cells expressing c-kit reduces hyperglycemia in mice with streptozotocin-induced pancreatic damage. Although quantitative analysis of the pancreas revealed a low frequency of donor insulin-positive cells, these cells were not present at the onset of blood glucose reduction. Instead, the majority of transplanted cells were localized to ductal and islet structures, and their presence was accompanied by a proliferation of recipient pancreatic cells that resulted in insulin production. The capacity of transplanted bone marrow-derived stem cells to initiate endogenous pancreatic tissue regeneration represents a previously unrecognized means by which these cells can contribute to the restoration of organ function.

Circulating hematopoietic progenitor cells in runners

Because endurance exercise causes release of mediators and growth factors active on the bone marrow, we asked whether it might affect circulating hematopoietic progenitor cells (HPCs) in amateur runners [n = 16, age: 41.8 +/- 13.5 (SD) yr, training: 93.8 +/- 31.8 km/wk] compared with sedentary controls (n = 9, age: 39.4 +/- 10.2 yr). HPCs, plasma cortisol, interleukin (IL)-6, granulocyte colony-stimulating factor (G-CSF), and the growth factor fms-like tyrosine kinase-3 (flt3)-ligand were measured at rest and after a marathon (M; n = 8) or half-marathon (HM; n = 8). Circulating HPC counts (i.e., CD34(+) cells and their subpopulations) were three- to fourfold higher in runners than in controls at baseline. They were unaffected by HM or M acutely but decreased the morning postrace. Baseline cortisol, flt3-ligand, IL-6, and G-CSF levels were similar in runners and controls. IL-6 and G-CSF increased to higher levels after M compared with HM, whereas cortisol and flt3-ligand increased similarly postrace. Our data suggest that increased HPCs reflect an adaptation response to recurrent, exercise-associated release of neutrophils and stress and inflammatory mediators, indicating modulation of bone marrow activity by habitual running.