The trafficking of circulating stem and progenitor cells to areas of tissue damage is poorly understood. The chemokine stromal cell-derived factor-1 (SDF-1 or CXCL12) mediates homing of stem cells to bone marrow by binding to CXCR4 on circulating cells. SDF-1 and CXCR4 are expressed in complementary patterns during embryonic organogenesis and guide primordial stem cells to sites of rapid vascular expansion. However, the regulation of SDF-1 and its physiological role in peripheral tissue repair remain incompletely understood. Here we show that SDF-1 gene expression is regulated by the transcription factor hypoxia-inducible factor-1 (HIF-1) in endothelial cells, resulting in selective in vivo expression of SDF-1 in ischemic tissue in direct proportion to reduced oxygen tension. HIF-1-induced SDF-1 expression increases the adhesion, migration and homing of circulating CXCR4-positive progenitor cells to ischemic tissue. Blockade of SDF-1 in ischemic tissue or CXCR4 on circulating cells prevents progenitor cell recruitment to sites of injury. Discrete regions of hypoxia in the bone marrow compartment also show increased SDF-1 expression and progenitor cell tropism. These data show that the recruitment of CXCR4-positive progenitor cells to regenerating tissues is mediated by hypoxic gradients via HIF-1-induced expression of SDF-1.

Diminished production of vascular endothelial growth factor (VEGF) and decreased angiogenesis are thought to contribute to impaired tissue repair in diabetic patients. We examined whether recombinant human VEGF 165 protein would reverse the impaired wound healing phenotype in genetically diabetic mice. Paired full-thickness skin wounds on the dorsum of db/db mice received 20 g of VEGF every other day for five doses to one wound and vehicle (phosphate-buffered saline) to the other. We demonstrate significantly accelerated repair in VEGF-treated wounds with an average time to resurfacing of 12 days versus 25 days in untreated mice. VEGF-treated wounds were characterized by an early leaky, malformed vasculature followed by abundant granulation tissue deposition. The VEGF-treated wounds demonstrated increased epithelialization, increased matrix deposition, and enhanced cellular proliferation, as assessed by uptake of 5-bromodeoxyuridine. Analysis of gene expression by real-time reverse transcriptase-polymerase chain reaction demonstrates a significant up-regulation of plateletderived growth factor-B and fibroblast growth factor-2 in VEGF-treated wounds, which corresponds with the increased granulation tissue in these wounds. These experiments also demonstrated an increase in the rate of repair of the contralateral phosphate-buffered salinetreated wound when compared to wounds in diabetic mice never exposed to VEGF (18 days versus 25 days), suggesting that topical VEGF had a systemic effect. We observed increased numbers of circulating VEGFR2 ؉

Ischemia is a known stimulus for vascular growth. Bone marrow (BM)-derived endothelial progenitor cells (EPCs) are believed to contribute to new blood vessel growth, but the mechanism for this contribution is unknown. To elucidate how BM cells are able to form new blood vessels, a novel murine model of soft tissue ischemia was developed in lethally irradiated mice with BM reconstituted from either tie2/lacZ or ROSA/green fluorescent protein (GFP) mice (n ‫؍‬ 24). BMderived EPCs were recruited to ischemic tissue within 72 hours, and the extent of recruitment was directly proportional to the degree of tissue ischemia. At 7 days, there were persistently elevated levels of vascular endothelial growth factor (VEGF) (2.5-fold) and circulating VEGF receptor-2/ CD11 ؊ (flk-1 ؉ /CD11 ؊ ) cells (18-fold) which correlated with increased numbers of BMderived EPCs within ischemic tissue. The cells were initially located extravascularly as proliferative clusters. By day 14, these clusters coalesced into vascular cords, which became functional vessels by day 21. In vitro examination of human EPCs from healthy volunteers (n ‫؍‬ 10) confirmed that EPC proliferation, adhesion, and chemotaxis were all significantly stimulated in hypoxic conditions. We conclude that BM-derived cells produce new blood vessels via localized recruitment, proliferation, and differentiation of circulating cells in a sequence of events markedly different from existing paradigms of angiogenesis.

Tissue ischemia promotes vasculogenesis through chemokine-induced recruitment of bone marrow-derived endothelial progenitor cells (EPCs). Diabetes significantly impairs this process. Because hyperglycemia increases reactive oxygen species in a number of cell types, and because many of the defects responsible for impaired vasculogenesis involve HIF1-regulated genes, we hypothesized that HIF1 function is impaired in diabetes because of reactive oxygen species-induced modification of HIF1␣ by the glyoxalase 1 (GLO1) substrate methylglyoxal. Decreasing superoxide in diabetic mice by either transgenic expression of manganese superoxide dismutase or by administration of an superoxide dismutase mimetic corrected post-ischemic defects in neovascularization, oxygen delivery, and chemokine expression, and normalized tissue survival. In hypoxic fibroblasts cultured in high glucose, overexpression of GLO1 prevented reduced expression of both the EPC mobilizing chemokine stromal cell-derived factor-1 (SDF-1) and of vascular epidermal growth factor, which modulates growth and differentiation of recruited EPCs. In hypoxic EPCs cultured in high glucose, overexpression of GLO1 prevented reduced expression of both the SDF-1 receptor CXCR4, and endothelial nitric-oxide synthase, an enzyme essential for EPC mobilization. HIF1␣ modification by methylglyoxal reduced heterodimer formation and HIF1␣ binding to all relevant promoters. These results provide a basis for the rational design of new therapeutics to normalize impaired ischemia-induced vasculogenesis in patients with diabetes.Studies in both experimental animals and humans have shown that diabetes impairs ischemia-driven neovascularization (1, 2). Diabetic animals have a decreased vascular density following hind limb ischemia (3, 4) and impaired wound healing (5). Human angiograms demonstrate fewer collateral vessels in diabetic patients compared with non-diabetic controls (1). Clinically, this contributes to increased rates of lower limb amputation, heart failure, and increased mortality after ischemic events.Ischemic tissue selectively recruits endothelial progenitor cells (EPCs) 3 from the bone marrow compartment by up-regulating the chemokine stromal cell-derived factor-1 (SDF-1). SDF-1 expression acts as a signal indicating the presence of tissue ischemia, and its expression is directly regulated by hypoxia-inducible factor-1 (5, 6). Ischemic tissue also up-regulates expression of VEGF, which modulates growth and differentiation of recruited EPCs (7). Endothelial cell mobilization from the bone marrow compartment is mediated by the SDF-1 receptor CXCR4, and endothelial nitric-oxide synthase plays an essential role in this mobilization (8). In diabetic mice, expression of SDF-1 in peripheral wound tissue is decreased, and in diabetic mice and humans, there is a significant decrease in circulating EPCs (5, 9), which exhibit impaired proliferation, adhesion, and incorporation into vascular structures (9). The mechanisms underlying defective ischemia-induced vasculogenesis ...

Pulsed electromagnetic fields (PEMF) have been shown to be clinically beneficial, but their mechanism of action remains unclear. The present study examined the impact of PEMF on angiogenesis, a process critical for successful healing of various tissues. PEMF increased the degree of endothelial cell tubulization (sevenfold) and proliferation (threefold) in vitro. Media from PEMF cultures had a similar stimulatory effect, but heat denaturation ablated this activity. In addition, conditioned media was able to induce proliferative and chemotactic changes in both human umbilical vein endothelial cells and fibroblasts, but had no effect on osteoblasts. Angiogenic protein screening demonstrated a fivefold increase in fibroblast growth factor beta-2 (FGF-2), as well as smaller increases in other angiogenic growth factors (angiopoietin-2, thrombopoietin, and epidermal growth factor). Northern blot analysis demonstrated an increase in FGF-2 transcription, and FGF-2 neutralizing antibody inhibited the effects of PEMF. In vivo, PEMF exposure increased angiogenesis more than twofold. We conclude that PEMF augments angiogenesis primarily by stimulating endothelial release of FGF-2, inducing paracrine and autocrine changes in the surrounding tissue. These findings suggest a potential role for PEMF in therapeutic angiogenesis.

Replacement of the donor graft vasculature by endothelial and endothelial progenitor cells from the recipient along preexisting channels is the predominant mechanism for skin graft revascularization. This mechanism is likely similar for all nonvascularized free grafts and suggests novel strategies for optimizing the vascularization of tissue constructs engineered in vitro.

This study demonstrates that pulsed electromagnetic fields are able to accelerate wound healing under diabetic and normal conditions by up-regulation of FGF-2-mediated angiogenesis. They also prevented tissue necrosis in response to a standardized ischemic insult, suggesting that noninvasive angiogenic stimulation by pulsed electromagnetic fields may be useful to prevent ulcer formation, necrosis, and amputation in diabetic patients.

Regenerative medicine focuses on new therapies to replace or restore lost, damaged, or aging cells in the human body to restore function. This goal is being realized by collaborative efforts in nonmammalian and human development, stem cell biology, genetics, materials science, bioengineering, and tissue engineering. At present, understanding existing reparative processes in humans and exploring the latent ability to regenerate tissue remains the focus in this field. This review covers recent work in limb regeneration, fetal wound healing, stem cell biology, somatic nuclear transfer, and tissue engineering as a foundation for developing new clinical therapies to augment and stimulate human regeneration.