Improved cutaneous healing in diabetic mice exposed to healthy peripheral circulation

Wound Healing Impairment in Type 2 Diabetes Model of Leptin-Deficient Mice—A Mechanistic Systematic Review

Type II diabetes mellitus (T2DM) is one of the most prevalent diseases in the world, associated with diabetic foot ulcers and impaired wound healing. There is an ongoing need for interventions effective in treating these two problems. Pre-clinical studies in this field rely on adequate animal models. However, producing such a model is near-impossible given the complex and multifactorial pathogenesis of T2DM. A leptin-deficient murine model was developed in 1959 and relies on either dysfunctional leptin (ob/ob) or a leptin receptor (db/db). Though monogenic, this model has been used in hundreds of studies, including diabetic wound healing research. In this study, we systematically summarize data from over one hundred studies, which described the mechanisms underlying wound healing impairment in this model. We briefly review the wound healing dynamics, growth factors’ dysregulation, angiogenesis, inflammation, the function of leptin and insulin, the role of advanced glycation end-products, extracellular matrix abnormalities, stem cells’ dysregulation, and the role of non-coding RNAs. Some studies investigated novel chronic diabetes wound models, based on a leptin-deficient murine model, which was also described. We also discussed the interventions studied in vivo, which passed into human clinical trials. It is our hope that this review will help plan future research.

An agonistic monoclonal antibody targeting cMet attenuates inflammation and upregulates collagen synthesis and angiogenesis in Type 2 diabetic mouse wounds

BACKGROUND

Diabetic wounds account for 25%-50% of total diabetic healthcare costs annually, and present overall healing rates of less than 50%. Since delayed diabetic wound healing is associated with impaired fibroblast function, we hypothesize that tyrosine kinase Met (cMet) agonistic monoclonal antibody (mAb) will promote diabetic wound healing via stable activation of HGF/cMet signaling.

METHODS

Two 6 mm dorsal wounds were created in each mice (6-week-old, male BKS.Cg-Dock7m+/+Leprdb/J, n=5). After subcutaneous injections of agonist (20 mg/kg) at 0 and 72h, the wound sizes were measured at days 0, 1, 3, 6, and 10. Histological and immunohistochemical analyses were performed at day 10 (cMet, α-SMA, CD68, and TGF-β). In vitro cytotoxicity and migration tests with diabetic fibroblasts were performed with/without agonist treatment (1 or 10 nM). cMet pathway activation of fibroblasts was confirmed through p-p44/42MAPK, p-mTOR, p-cMet, and ROCK-1 expression.

RESULTS

cMet agonistic mAb-treated group showed 1.60-fold lower wound area (p=0.027), 1.54-fold higher collagen synthesis (p=0.001), and 1.79-fold lower inflammatory cell infiltration (p=0.032) than the saline-treated control. The agonist increased cMet (1.86-fold, p=0.029), α-SMA (1.20-fold, p=0.018), and VEGF (1.68-fold, p=0.029) expression but suppressed CD68 (1.25-fold, p=0.043), TFG-β (1.25-fold, p=0.022), and MMP-2 (2.59-fold, p=0.029) expression. In vitro agonist treatment (10 nM) of diabetic fibroblasts increased their migration by 8.98-fold (p=0.029) and activated HGF/cMet pathway.

CONCLUSIONS

cMet agonistic mAb treatment improved diabetic wound healing in mice and reduced wound-site inflammatory cell infiltration. These results need to be validated in large animals before piloting human trials.

Systematic review and meta-analysis of mouse models of diabetes-associated ulcers

Mouse models are frequently used to study diabetes-associated ulcers, however, whether these models accurately simulate impaired wound healing has not been thoroughly investigated. This systematic review aimed to determine whether wound healing is impaired in mouse models of diabetes and assess the quality of the past research. A systematic literature search was performed of publicly available databases to identify original articles examining wound healing in mouse models of diabetes. A meta-analysis was performed to examine the effect of diabetes on wound healing rate using random effect models. A meta-regression was performed to examine the effect of diabetes duration on wound healing impairment. The quality of the included studies was also assessed using two newly developed tools. 77 studies using eight different models of diabetes within 678 non-diabetic and 720 diabetic mice were included. Meta-analysis showed that wound healing was impaired in all eight models. Meta-regression suggested that longer duration of diabetes prior to wound induction was correlated with greater degree of wound healing impairment. Pairwise comparisons suggested that non-obese diabetic mice exhibited more severe wound healing impairment compared with db/db mice, streptozotocin-induced diabetic mice or high-fat fed mice at an intermediate stage of wound healing (p<0.01). Quality assessment suggested that the prior research frequently lacked incorporation of key clinically relevant characteristics. This systematic review suggested that impaired wound healing can be simulated in many different mouse models of diabetes but these require further refinement to become more clinically relevant.

Bone Marrow Adipocytes: The Enigmatic Components of the Hematopoietic Stem Cell Niche

Bone marrow adipocytes (BMA) exert pleiotropic roles beyond mere lipid storage and filling of bone marrow (BM) empty spaces, and we are only now beginning to understand their regulatory traits and versatility. BMA arise from the differentiation of BM mesenchymal stromal cells, but they seem to be a heterogeneous population with distinct metabolisms, lipid compositions, secretory properties and functional responses, depending on their location in the BM. BMA also show remarkable differences among species and between genders, they progressively replace the hematopoietic BM throughout aging, and play roles in a range of pathological conditions such as obesity, diabetes and anorexia. They are a crucial component of the BM microenvironment that regulates hematopoiesis, through mechanisms largely unknown. Previously considered as negative regulators of hematopoietic stem cell function, recent data demonstrate their positive support for hematopoietic stem cells depending on the experimental approach. Here, we further discuss current knowledge on the role of BMA in hematological malignancies. Early hints suggest that BMA may provide a suitable metabolic niche for the malignant growth of leukemic stem cells, and protect them from chemotherapy. Future in vivo functional work and improved isolation methods will enable determining the true essence of this elusive BM hematopoietic stem cell niche component, and confirm their roles in a range of diseases. This promising field may open new pathways for efficient therapeutic strategies to restore hematopoiesis, targeting BMA.

Migratory, metabolic and functional alterations of fibrocytes in type 2 diabetes

Fibrocytes are bloodborne mesenchymal progenitor cells that are recruited to injured tissue sites and contribute to the repair process by acquiring a myofibroblast-like phenotype and producing extracellular matrix components and growth factors. Treatment with normal fibrocytes or their exosomes restores the ability of genetically diabetic mice to heal skin wounds, suggesting the existence of dysfunctional alterations in diabetic fibrocytes. This study compared the migratory, metabolic and functional characteristics of fibrocytes from patients with type 2 diabetes (T2DPs) and healthy controls (HCs). It was found that the frequency of these cells was abnormally low in the peripheral blood of T2DPs. Diabetic fibrocytes showed reduced expression of the C-X-C motif and C-C motif chemokine receptors (CXCR)4, (CCR)5, and CCR7, and demonstrated reduced migration in response to their ligands (CXCL)12, (CCL)5, and CCL21. They exhibited increased expression of the receptor for advanced glycation end product, suppression of the alternative AGE receptor 1, increased intracellular concentrations of AGEs, decreased expression of sirtuin-1 and elevated oxidative stress. In short-term cultures, fibrocytes from T2DPs released larger amounts of proinflammatory cytokines than those from HCs. Unlike normal fibrocytes, diabetic fibrocytes did not exhibit increased expression of type I collagen and α-smooth muscle actin on stimulation with transforming growth factor (TGF)-β1 and this abnormal response was associated with downregulation of TGF-β1 type II receptor on the cell surface. Study findings uncover multiple migratory and functional alterations of diabetic fibrocytes that may contribute to explain why T2DPs experience impaired wound healing and chronic ulcers. © 2018 IUBMB Life, 70(11):1122-1132, 2018.

Pharmacological rescue of diabetic skeletal stem cell niches

Diabetes mellitus (DM) is a metabolic disease frequently associated with impaired bone healing. Despite its increasing prevalence worldwide, the molecular etiology of DM-linked skeletal complications remains poorly defined. Using advanced stem cell characterization techniques, we analyzed intrinsic and extrinsic determinants of mouse skeletal stem cell (mSSC) function to identify specific mSSC niche-related abnormalities that could impair skeletal repair in diabetic (Db) mice. We discovered that high serum concentrations of tumor necrosis factor-α directly repressed the expression of Indian hedgehog (Ihh) in mSSCs and in their downstream skeletogenic progenitors in Db mice. When hedgehog signaling was inhibited during fracture repair, injury-induced mSSC expansion was suppressed, resulting in impaired healing. We reversed this deficiency by precise delivery of purified Ihh to the fracture site via a specially formulated, slow-release hydrogel. In the presence of exogenous Ihh, the injury-induced expansion and osteogenic potential of mSSCs were restored, culminating in the rescue of Db bone healing. Our results present a feasible strategy for precise treatment of molecular aberrations in stem and progenitor cell populations to correct skeletal manifestations of systemic disease.

Human fibrocyte-derived exosomes accelerate wound healing in genetically diabetic mice

Diabetic ulcers represent a substantial societal and healthcare burden worldwide and scarcely respond to current treatment strategies. This study was addressed to evaluate the therapeutic potential of exosomes secreted by human circulating fibrocytes, a population of mesenchymal progenitors involved in normal wound healing via paracrine signaling. The exosomes released from cells sequentially stimulated with platelet-derived growth factor-BB and transforming growth factor-β1, in the presence of fibroblast growth factor 2, did not show potential immunogenicity. These exosomes exhibited in-vitro proangiogenic properties, activated diabetic dermal fibroblasts, induced the migration and proliferation of diabetic keratinocytes, and accelerated wound closure in diabetic mice in vivo. Important components of the exosomal cargo were heat shock protein-90α, total and activated signal transducer and activator of transcription 3, proangiogenic (miR-126, miR-130a, miR-132) and anti-inflammatory (miR124a, miR-125b) microRNAs, and a microRNA regulating collagen deposition (miR-21). This proof-of-concept study demonstrates the feasibility of the use of fibrocytes-derived exosomes for the treatment of diabetic ulcers.

Hyperglycemia Interacts with Ischemia in a Synergistic Way on Wound Repair and Myofibroblast Differentiation

Background:

Hyperglycemia is known to adversely affect the outcome of ischemic insults, but its interaction with ischemia has not been investigated in wound repair yet. In this study, we develop a new animal model allowing to investigate the interaction between hyperglycemia and ischemia during the wound repair process. We focus on myofibroblast differentiation, a key element of wound repair.

Methods:

Ischemia was inflicted in Wistar rats by resection of the femoral to popliteal arteries on the left side, whereas arteries were dissected without resection on the right side. Full-thickness skin wounds (1 cm²) were created on both feet. Hyperglycemia was induced by injection of streptozotocin. Normoglycemic animals served as control (n = 23/group). Blood flow, wound closure, and myofibroblast expression were measured.

Results:

Wound closure was significantly delayed in ischemic compared with nonischemic wounds in all rats. This delay was almost 5-fold exacerbated in hyperglycemic rats compared with normoglycemic rats, while hyperglycemia alone showed only a slight effect on wound repair. Delayed wound repair was associated with impaired wound contraction and myofibroblast differentiation.

Conclusions:

Our model allows to specifically quantify the effect of hyperglycemia and ischemia alone or in combination on wound repair. We show that hyperglycemia amplifies the inhibitory effect of ischemia on wound repair and myofibroblast expression. Our data reveal for the first time the synergic aspect of this interaction and therefore stress the importance of a strict glycemic control in the management of ischemic wounds.

Angiotensin II inhibitor facilitates epidermal wound regeneration in diabetic mice

Tissue regeneration and wound healing are severely impaired in diabetes and are associated with poor circulation and dysfunctional blood vessels. Angiotensin II inhibitors are anti-hypertensive drugs used in clinical practice to regulate blood pressure and could affect tissue remodeling. We hypothesize that blocking angiotensin II, using Losartan, could facilitate tissue regeneration in diabetic mice. To this end, we established an experimental model of wound healing in streptozotocin-induced diabetic mice. Our data demonstrated that Losartan accelerates wound repair and normalizes wound stromal responses, having a beneficial role in wounds of diabetic individuals. Our findings highlight a potential therapeutic use of Losartan in improving wound repair in diabetic conditions.

Adipose-derived stromal vascular fraction cells isolated from old animals exhibit reduced capacity to support the formation of microvascular networks

Adipose-derived regenerative and stem cells, defined collectively as the stromal vascular fraction (SVF), support the formation of neovascular networks at the site of implantation. The effect of advancing age on SVF cell population effectiveness towards stimulated neovascularization was evaluated.

METHODS

SVF was enzymatically isolated from adipose of young (ySVF, 4 months) or old (oSVF, 24 months) Fisher-344 rats, combined with type I collagen and polymerized. Encapsulated SVF was implanted subcutaneously into young Rag1 mice for two or four weeks. Angiogenic function of age-dependent SVF was also extensively evaluated in vitro using standard assays.

RESULTS

In vitro studies indicated no difference in angiogenic function between ySVF and oSVF (viability, proliferation, migration, and tube-formation). At two weeks post-implantation, there was no age-related difference in percent apoptosis in explanted constructs. By four weeks post-implantation, oSVF implants displayed 36% less total vessels/mm(2), 43% less perfused vessels/mm(2), and exhibited greater percent apoptosis compared to ySVF (n ≥ 12). Blocking thrombospondin-1 (Thbs-1), a protein found to be highly expressed in oSVF but not ySVF, increased the percent of perfused vascular volume and vessel diameters in oSVF constructs after two weeks compared to oSVF implants treated with control antibody.

CONCLUSIONS

Advancing donor age reduces the potential of adipose-derived SVF to derive a mature microcirculation, but does not hinder initial angiogenesis. However, modulation of Thbs-1 may improve this outcome. This data suggests that greater pruning, dysfunctional structural adaptation and/or poor maturation with initiation of blood flow may occur in oSVF.

Parabiotic model for differentiating local and systemic effects of continuous and intermittent hypoxia

Hypoxia can be damaging either because cells are directly sensitive to low oxygen pressure in their local microenvironment and/or because they are exposed to circulating factors systemically secreted in response to hypoxia. The conventional hypoxia model, breathing hypoxic air, does not allow one to distinguish between these local and systemic effects. Here we propose and validate a model for differentially applying local and systemic hypoxic challenges in an animal. We used parabiosis, two mice sharing circulation by surgical union through the skin, and tested the hypothesis that when one of the parabionts breathes room air and the other one is subjected to hypoxic air, both mice share systemic circulation but remain normoxic and hypoxic, respectively. We tested two common hypoxic paradigms in 10 parabiotic pairs: continuous hypoxia (10% O2) mimicking chronic lung diseases, and intermittent hypoxia (40 s, 21% O2; 20 s, 5% O2) simulating sleep apnea. Arterial oxygen saturation and oxygen partial pressure at muscle tissue were measured in both parabionts. Effective cross-circulation was assessed by intraperitoneally injecting a dye in one of the parabionts and measuring blood dye concentration in both animals after 2 h. The results confirmed the hypothesis that tissues of the parabiont under room air were perfused with normally oxygenated blood and, at the same time, were exposed to all of the systemic mediators secreted by the other parabiont actually subjected to hypoxia. In conclusion, combination of parabiosis and hypoxic/normoxic air breathing is a novel approach to investigate the effects of local and systemic hypoxia in respiratory diseases.

Diabetes irreversibly depletes bone marrow-derived mesenchymal progenitor cell subpopulations

Diabetic vascular pathology is largely attributable to impairments in tissue recovery from hypoxia. Circulating progenitor cells have been postulated to play a role in ischemic recovery, and deficiencies in these cells have been well described in diabetic patients. Here, we examine bone marrow-derived mesenchymal progenitor cells (BM-MPCs) that have previously been shown to be important for new blood vessel formation and demonstrate significant deficits in the context of diabetes. Further, we determine that this dysfunction is attributable to intrinsic defects in diabetic BM-MPCs that are not correctable by restoring glucose homeostasis. We identify two transcriptionally distinct subpopulations that are selectively depleted by both type 1 and type 2 diabetes, and these subpopulations have provasculogenic expression profiles, suggesting that they are vascular progenitor cells. These results suggest that the clinically observed deficits in progenitor cells may be attributable to selective and irreversible depletion of progenitor cell subsets in patients with diabetes.

Early kinetics of integration of collagen-glycosaminoglycan regenerative scaffolds in a diabetic mouse model

BACKGROUND

Collagen-glycosaminoglycan scaffolds, originally designed to treat severe burns, are now commonly used in patients with complex wounds associated with diabetes mellitus. In this study, the authors investigated how the thickness of the scaffold would affect cellular integration with the diabetic host and whether this can be accelerated using subatmospheric pressure wound therapy devices.

METHODS

Collagen-glycosaminoglycan scaffolds, 500 to 2000-μm thick, were applied to dorsal wounds in genetically diabetic mice. In addition, 1000-μm collagen-glycosaminoglycan scaffolds with and without silicone were treated with a subatmospheric pressure device (-125 mmHg). On days 5 and 10, cellular and vascular integration of tissues was studied by histology, immunohistochemistry, corrosion casting, and qRT-polymerase chain reaction.

RESULTS

Cells and vessels from the wound surface populated the scaffold to form layers with varying cellular density. Areas of high cell density and proliferation were noted at the bottom of the scaffold. Increasing the thickness of the scaffold did not affect the extent of cellular ingrowth, so that thicker scaffolds had a thicker residual acellular layer on the surface. The thickness of cellular ingrowth was stable between days 5 and 10, whereas vessels seen in the scaffolds on day 10 were not yet present on day 5. Subatmospheric pressure devices applied to silicone-covered collagen-glycosaminoglycan scaffolds minimized the granulation tissue formation beneath the scaffold, which enhanced vessel ingrowth.

CONCLUSIONS

The early kinetics of cellular integration into collagen-glycosaminoglycan scaffolds is independent of scaffold thickness in a diabetic wound model. Scaffold adherence to the wound and integration can be improved using a subatmospheric pressure device.

The acid-secreting parietal cell as an endocrine source of Sonic Hedgehog during gastric repair

Sonic Hedgehog (Shh) has been shown to regulate wound healing in various tissues. Despite its known function in tissue regeneration, the role of Shh secreted from the gastric epithelium during tissue repair in the stomach remains unknown. Here we tested the hypothesis that Shh secreted from the acid-secreting parietal cell is a fundamental circulating factor that drives gastric repair. A mouse model expressing a parietal cell-specific deletion of Shh (PC-ShhKO) was generated using animals bearing loxP sites flanking exon 2 of the Shh gene (Shh(flx/flx)) and mice expressing a Cre transgene under the control of the H(+),K(+)-ATPase β-subunit promoter. Shh(flx/flx), the H(+),K(+)-ATPase β-subunit promoter, and C57BL/6 mice served as controls. Ulcers were induced via acetic acid injury. At 1, 2, 3, 4, 5, and 7 days after the ulcer induction, gastric tissue and blood samples were collected. Parabiosis experiments were used to establish the effect of circulating Shh on ulcer repair. Control mice exhibited an increased expression of Shh in the gastric tissue and plasma that correlated with the repair of injury within 7 days after surgery. PC-ShhKO mice showed a loss of ulcer repair and reduced Shh tissue and plasma concentrations. In a parabiosis experiment whereby a control mouse was paired with a PC-ShhKO littermate and both animals subjected to gastric injury, a significant increase in the circulating Shh was measured in both parabionts. Elevated circulating Shh concentrations correlated with the repair of gastric ulcers in the PC-ShhKO parabionts. Therefore, the acid-secreting parietal cell within the stomach acts as an endocrine source of Shh during repair.

No Association between Glycemia and Wound Healing in an Experimental db/db Mouse Model

Impaired wound healing is a frequent problem in diabetes. Hyperglycemia may be an operative mechanism, but a link between glycemic control and wound healing has never been established. Wounds in db/db mice have been extensively studied. This study was undertaken to see if plasma glucose was a predictor of wound healing. An excisional wound was made (149 db/db mice). Wound closure was studied versus metabolic variables. The animals were 11.8 ± 0.2 weeks (mean ± standard error of the mean), obese (38.1 ± 0.5 g), and hyperglycemic (fasting plasma glucose 21.0 ± 0.7 mmol/L). Wound closure at day 13 was 30.1 ± 1.6%. In linear mixed model analyses neither fasting plasma glucose nor its change from start to end of experiment was a significant predictor of wound closure (β = 0.15, P = 0.07, 95% CI: −0.01 to 0.31 and β = 0.06, P = 0.5, 95% CI: −0.11 to 0.23, resp.). However, increase in body weight significantly and independently predicted wound closure (for weight change, β = 0.22, P = 0.008, 95% CI: 0.06 to 0.38). This study strongly suggests that wound healing in db/db mice is independent of prevailing glycemia but dependent on anabolic changes such as weight gain over time.

Growth differentiation factor 11 is a circulating factor that reverses age-related cardiac hypertrophy

The most common form of heart failure occurs with normal systolic function and often involves cardiac hypertrophy in the elderly. To clarify the biological mechanisms that drive cardiac hypertrophy in aging, we tested the influence of circulating factors using heterochronic parabiosis, a surgical technique in which joining of animals of different ages leads to a shared circulation. After 4 weeks of exposure to the circulation of young mice, cardiac hypertrophy in old mice dramatically regressed, accompanied by reduced cardiomyocyte size and molecular remodeling. Reversal of age-related hypertrophy was not attributable to hemodynamic or behavioral effects of parabiosis, implicating a blood-borne factor. Using modified aptamer-based proteomics, we identified the TGF-β superfamily member GDF11 as a circulating factor in young mice that declines with age. Treatment of old mice to restore GDF11 to youthful levels recapitulated the effects of parabiosis and reversed age-related hypertrophy, revealing a therapeutic opportunity for cardiac aging.

Increased saturated fatty acids in obesity alter resolution of inflammation in part by stimulating prostaglandin production

Extensive evidence indicates that nutrient excess associated with obesity and type 2 diabetes activates innate immune responses that lead to chronic, sterile low-grade inflammation, and obese and diabetic humans also have deficits in wound healing and increased susceptibility to infections. Nevertheless, the mechanisms that sustain unresolved inflammation during obesity remain unclear. In this study, we report that saturated free fatty acids that are elevated in obesity alter resolution of acute sterile inflammation by promoting neutrophil survival and decreasing macrophage phagocytosis. Using a targeted mass spectrometry-based lipidomics approach, we found that in db/db mice, PGE2/D2 levels were elevated in inflammatory exudates during the development of acute peritonitis. Moreover, in isolated macrophages, palmitic acid stimulated cyclooxygenase-2 induction and prostanoid production. Defects in macrophage phagocytosis induced by palmitic acid were mimicked by PGE2 and PGD2 and were reversed by cyclooxygenase inhibition or prostanoid receptor antagonism. Macrophages isolated from obese-diabetic mice expressed prostanoid receptors, EP2 and DP1, and contained significantly higher levels of downstream effector, cAMP, compared with wild-type mice. Therapeutic administration of EP2/DP1 dual receptor antagonist, AH6809, decreased neutrophil accumulation in the peritoneum of db/db mice, as well as the accumulation of apoptotic cells in the thymus. Taken together, these studies provide new insights into the mechanisms underlying altered innate immune responses in obesity and suggest that targeting specific prostanoid receptors may represent a novel strategy for resolving inflammation and restoring phagocyte defects in obese and diabetic individuals.

Liver-derived systemic factors drive β cell hyperplasia in insulin-resistant states

Integrative organ crosstalk regulates key aspects of energy homeostasis, and its dysregulation may underlie metabolic disorders such as obesity and diabetes. To test the hypothesis that crosstalk between the liver and pancreatic islets modulates β cell growth in response to insulin resistance, we used the liver-specific insulin receptor knockout (LIRKO) mouse, a unique model that exhibits dramatic islet hyperplasia. Using complementary in vivo parabiosis and transplantation assays, as well as in vitro islet culture approaches, we demonstrate that humoral, nonneural, non-cell-autonomous factor(s) induces β cell proliferation in LIRKO mice. Furthermore, we report that a hepatocyte-derived factor(s) stimulates mouse and human β cell proliferation in ex vivo assays, independent of ambient glucose and insulin levels. These data implicate the liver as a critical source of β cell growth factor(s) in insulin-resistant states.

Accelerated aging versus rejuvenation of the immune system in heterochronic parabiosis

The emergence of immune disorders in aging is explained by many factors, including thymus dysfunction, decrease in the proportion and function of naïve T cells, and so forth. There are several approaches to preventing these changes, such as thymus rejuvenation, stem cells recovery, modulation of hormone production, and others. Our investigations of heterochronic parabiosis have shown that benefits of a young immune system, e.g., actively working thymus and regular migration of young hematopoietic stem cells between parabiotic partners, appeared unable to restore the immune system of the old partner. At the same time, we have established a progressive immune impairment in the young heterochronic partners. The mechanism of age changes in the immune system in this model, which may lead to reduced life expectancy, has not been fully understood. The first age-related manifestation in the young partners observed 3 weeks after the surgery was a dramatic increase of CD8(+)44(+) cells population in the spleen. A detailed analysis of further changes revealed a progressive decline of most immunological functions observable for up to 3 months after the surgery. This article reviews possible mechanisms of induction of age-related changes in the immune system of young heterochronic partners. The data obtained suggest the existence of certain factors in the old organisms that trigger aging, thus preventing the rejuvenation process.

The stem cell niche in regenerative medicine

Stem cells are fundamental units for achieving regenerative therapies, which leads naturally to a theoretical and experimental focus on these cells for therapeutic screening and intervention. A growing body of data in many tissue systems indicates that stem cell function is critically influenced by extrinsic signals derived from the microenvironment, or "niche." In this vein, the stem cell niche represents a significant, and largely untapped, entry point for therapeutic modulation of stem cell behavior. This Perspective will discuss how the niche influences stem cells in homeostasis, in the progression of degenerative and malignant diseases, and in therapeutic strategies for tissue repair.

Fetal microchimerism in skin wound healing

Skin wound healing is a complex regenerative process involving various cell types. We recently investigated whether fetal microchimeric cells (FMCs) acquired during gestation contribute to maternal wound healing and used fetal microchimerism to investigate the recruitment of distant endothelial progenitor cells in skin wounds. Our study showed that fetal progenitor cells are recruited into maternal wounds and participate in inflammation and angiogenesis. These fetal cells might have beneficial effects in situations of maternal defective healing, and might also modify the adult maternal wound environment toward a scarless fetal-like wound healing.

Cross-circulation and cell distribution kinetics in parabiotic mice

Blood-borne nucleated cells participate not only in inflammation, but in tissue repair and regeneration. Because progenitor and stem cell populations have a low concentration in the blood, the circulation kinetics and tissue distribution of these cells is largely unknown. An important approach to tracking cell lineage is the use of fluorescent tracers and parabiotic models of cross-circulation. Here, we investigated the cross-circulation and cell distribution kinetics of C57/B6 GFP(+)/wild-type parabionts. Flow cytometry analysis of the peripheral blood after parabiosis demonstrated no evidence for a "parabiotic barrier" based on cell size or surface characterstics; all peripheral blood cell subpopulations in this study reached equilibrium within 14 days. Whole blood fluorescence analysis indicated that the mean exchange flow rate was 16 µl/h or 0.66% of the circulating blood volume per hour. Studies of peripheral lymphoid organs indicated differential cell distribution kinetics. Some subpopulations, such as CD8(+) and CD11c(+), equilibrated in both lymph nodes and spleen indicating a residence time <28 days; in contrast, other lymphocyte subpopulations, such as B220(+) and CD4(+) cells, had not yet reached equilibrium at 28 days. We conclude that parabiosis can provide important insights into defining tissue distribution, residence times, and recirculating pools using fluorochrome markers of cell lineage.

Impaired therapeutic capacity of autologous stem cells in a model of type 2 diabetes

Endogenous stem cells in the bone marrow respond to environmental cues and contribute to tissue maintenance and repair. In type 2 diabetes, a multifaceted metabolic disease characterized by insulin resistance and hyperglycemia, major complications are seen in multiple organ systems. To evaluate the effects of this disease on the endogenous stem cell population, we used a type 2 diabetic mouse model (db/db), which recapitulates these diabetic phenotypes. Bone marrow-derived mesenchymal stem cells (MSCs) from db/db mice were characterized in vitro using flow cytometric cell population analysis, differentiation, gene expression, and proliferation assays. Diabetic MSCs were evaluated for their therapeutic potential in vivo using an excisional splint wound model in both nondiabetic wild-type and diabetic mice. Diabetic animals possessed fewer MSCs, which were proliferation and survival impaired in vitro. Examination of the recruitment response of stem and progenitor cells after wounding revealed that significantly fewer endogenous MSCs homed to the site of injury in diabetic subjects. Although direct engraftment of healthy MSCs accelerated wound closure in both healthy and diabetic subjects, diabetic MSC engraftment produced limited improvement in the diabetic subjects and could not produce the same therapeutic outcomes as in their nondiabetic counterparts in vivo. Our data reveal stem cell impairment as a major complication of type 2 diabetes in mice and suggest that the disease may stably alter endogenous MSCs. These results have implications for the efficiency of autologous therapies in diabetic patients and identify endogenous MSCs as a potential therapeutic target.

Epoxyeicosanoids stimulate multiorgan metastasis and tumor dormancy escape in mice

Epoxyeicosatrienoic acids (EETs) are small molecules produced by cytochrome P450 epoxygenases. They are lipid mediators that act as autocrine or paracrine factors to regulate inflammation and vascular tone. As a result, drugs that raise EET levels are in clinical trials for the treatment of hypertension and many other diseases. However, despite their pleiotropic effects on cells, little is known about the role of these epoxyeicosanoids in cancer. Here, using genetic and pharmacological manipulation of endogenous EET levels, we demonstrate that EETs are critical for primary tumor growth and metastasis in a variety of mouse models of cancer. Remarkably, we found that EETs stimulated extensive multiorgan metastasis and escape from tumor dormancy in several tumor models. This systemic metastasis was not caused by excessive primary tumor growth but depended on endothelium-derived EETs at the site of metastasis. Administration of synthetic EETs recapitulated these results, while EET antagonists suppressed tumor growth and metastasis, demonstrating in vivo that pharmacological modulation of EETs can affect cancer growth. Furthermore, inhibitors of soluble epoxide hydrolase (sEH), the enzyme that metabolizes EETs, elevated endogenous EET levels and promoted primary tumor growth and metastasis. Thus, our data indicate a central role for EETs in tumorigenesis, offering a mechanistic link between lipid signaling and cancer and emphasizing the critical importance of considering possible effects of EET-modulating drugs on cancer.

Dynamic niches in the origination and differentiation of haematopoietic stem cells

Haematopoietic stem cells (HSCs) are multipotent, self-renewing progenitors that generate all mature blood cells. HSC function is tightly controlled to maintain haematopoietic homeostasis, and this regulation relies on specialized cells and factors that constitute the haematopoietic 'niche', or microenvironment. Recent discoveries, aided in part by technological advances in in vivo imaging, have engendered a new appreciation for the dynamic nature of the niche, identifying novel cellular and acellular niche components and uncovering fluctuations in the relative importance of these components over time. These new insights significantly improve our understanding of haematopoiesis and raise fundamental questions about what truly constitutes a stem cell niche.

Pulsed radiofrequency energy accelerates wound healing in diabetic mice

BACKGROUND

Impaired healing is a problematic and common complication of chronic wounds. Although pulsed radiofrequency energy has been used in the treatment of chronic wounds with promising efficacy, its mechanism is still poorly characterized. In this study, the authors used a diabetic mouse model to illustrate the action of pulsed radiofrequency energy on cutaneous wounds and set the stage to begin to understand its mechanism.

METHODS

Full-thickness cutaneous wounds were created in diabetic mice (n = 26). The experimental group (n = 13) was subject to pulsed radiofrequency energy treatment two times per day, whereas the sham group (n = 13) was subjected to sham devices. The rate of wound closure was evaluated by digital analysis of surface area of the wound bed, zone of reepithelialization, and rate of contraction. Mice were euthanized on days 7, 10, 22, and 42 and wounds were evaluated qualitatively and quantitatively by hematoxylin and eosin, Masson's trichrome, and Ki-67 assay for cell proliferation.

RESULTS

In the experimental group, the rate of wound closure was significantly accelerated, particularly beyond day 17. Contraction contributed to the wound healing process rather than reepithelialization. This was also associated with increased granulation tissue that was most prominent by day 22 and with enhanced dermal cell proliferation, with 25 percent and 45 percent Ki-67-positive nuclei on days 10 and 22, respectively, as compared with control animals.

CONCLUSION

These results indicate that pulsed radiofrequency energy accelerates impaired wound healing mainly through wound contraction by means of stimulating cell proliferation, granulation tissue formation, and collagen deposition.

Surgical approaches to create murine models of human wound healing

Wound repair is a complex biologic process which becomes abnormal in numerous disease states. Although in vitro models have been important in identifying critical repair pathways in specific cell populations, in vivo models are necessary to obtain a more comprehensive and pertinent understanding of human wound healing. The laboratory mouse has long been the most common animal research tool and numerous transgenic strains and models have been developed to help researchers study the molecular pathways involved in wound repair and regeneration. This paper aims to highlight common surgical mouse models of cutaneous disease and to provide investigators with a better understanding of the benefits and limitations of these models for translational applications.

Use of the parabiotic model in studies of cutaneous wound healing to define the participation of circulating cells

Previous experimental studies to assess the contribution of blood-borne circulating (BBC) cells to cutaneous wound healing have relied on discontinuous pulsing of labeled BBC elements or bone marrow transplant protocols. Such approaches do not allow the examination of stable BBC cells that have matured in a physiologically normal host. We have used a parabiotic murine model for cutaneous wound healing to evaluate the relative contribution of stable populations of peripheral blood cells expressing the green fluorescent protein (GFP) transgene in otherwise normal animals. Circulating cells (mature and immature) expressing the GFP transgene were easily detected and quantified in wounds of GFP- parabiotic twins during all evaluated stages of the healing response. Using multiple antibody probes, the relative contribution of various subsets of BBC cells could be comparatively assessed. In early wounds, some cells expressing mesenchymal epitopes were documented to be of hematopoietic origin, indicating the utility of this model in assessing cell plasticity in the context of tissue regeneration and repair. Application of this approach enables further investigation into the contribution of peripheral blood in normal and abnormal healing responses.

Parabiotic heterogenetic pairing of Abcc6-/-/Rag1-/- mice and their wild-type counterparts halts ectopic mineralization in a murine model of pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE), a pleiotropic heritable disorder, is characterized by ectopic mineralization of the connective tissues. This disease is caused by mutations in the ABCC6 gene, which is expressed primarily in the baso-lateral surface of hepatocytes, and Abcc6(-/-) mice develop progressive mineralization mimicking human PXE. To investigate the hypothesis that PXE is a metabolic disorder, potentially caused by the absence of antimineralization factor(s) in circulation, we used parabiotic pairing, ie, surgical joining of two mice, to create a shared circulation between various Abcc6 genotypic mice. To prevent immune reaction between the parabiotic animals, all mice were bred to be Rag1(-/-). Shared circulation between the parabiotic animals was confirmed by Evans blue dye injection and by quantitative PCR of blood cell genotypes. Pairing of Abcc6(-/-) mice with their wild-type counterparts halted the connective tissue mineralization in the knockout mice. Homogenetic wild-type and heterozygous pairings serving as controls were phenotypically unaffected by parabiosis. Consequently, the observations on the parabiotic mice support the notion that PXE is a metabolic disease, potentially due to absence of systemic antimineralization factor(s). These observations suggest that reintroduction of the critical antimineralization factors into circulation could provide a potential treatment for this, currently intractable, disease.