Characterizing how the microenvironment, or niche, regulates stem cell activity is central to understanding stem cell biology and to developing strategies for therapeutic manipulation of stem cells1. Low oxygen tension (hypoxia) is commonly thought to be a shared niche characteristic in maintaining quiescence in multiple stem cell types2–4. However, support for the existence of a hypoxic niche has largely come from indirect evidence such as proteomic analysis5, expression of HIF-1 and related genes6, and staining with surrogate hypoxic markers (e.g. pimonidazole)6–8. Here we perform direct in vivo measurements of local oxygen tension (pO2) in the bone marrow (BM) of live mice. Using two-photon phosphorescence lifetime microscopy (2PLM), we determined the absolute pO2 of the BM to be quite low (<32 mmHg) despite very high vascular density. We further uncovered heterogeneities in local pO2, with the lowest pO2 (~9.9 mmHg, or 1.3%) found in deeper peri-sinusoidal regions. The endosteal region, by contrast, is less hypoxic as it is perfused with small arteries that are often positive for the marker nestin. These pO2 values change dramatically after radiation and chemotherapy, pointing to the role of stress in altering the stem cell metabolic microenvironment.

Ultraviolet radiation (UVR) has widespread effects on the biology and integrity of the skin barrier. Research on the mechanisms that drive these changes, as well as their effect on skin barrier function has been ongoing since the 1980s. However, no studies have examined the impact of UVR on nanoparticle skin penetration. Nanoparticles (NP) are commonly used in sunscreens and other cosmetics, and since consumer use of sunscreen is often applied to sun damaged skin, the effect of UVR on NP skin penetration is a concern due to potential toxicity. In this study we investigate nanoparticle skin penetration by employing an in vivo semiconductor quantum dot nanoparticle (QD) model system. This model system improves NP imaging capabilities and provides additional primary interest due to widespread and expanding use of QD in research applications and manufacturing. In our experiments, carboxylated QD were applied to the skin of SKH-1 mice in a glycerol vehicle with and without UVR exposure. The skin collection and penetration patterns were evaluated 8 and 24 hours after QD application using tissue histology, confocal microscopy, and transmission electron microscopy (TEM) and EDAX analysis. Low levels of penetration were seen in both the non-UVR exposed mice and the UVR exposed mice. Qualitatively higher levels of penetration were observable in the UVR exposed mice. These results are the first for in vivo QD skin penetration, and provide important insight into the ability of QD to penetrate intact and UVR compromised skin barrier. Our findings raise concern that NP of similar size and surface chemistry, such as metal oxide NP found in sunscreens, may also penetrate UV damaged skin.

Corneal epithelial homeostasis and regeneration are sustained by limbal stem cells (LSCs)1–3, and LSC deficiency is a major cause of blindness worldwide4. Transplantation is often the only therapeutic option available to patients with LSC deficiency. However, while transplant success depends foremost on LSC frequency within grafts5, a gene allowing for prospective LSC enrichment has not been identified so far5. Here we show that ATP-binding cassette, sub-family B, member 5 (ABCB5)6,7 marks LSCs and is required for LSC maintenance, corneal development and repair. Furthermore, we demonstrate that prospectively isolated human or murine ABCB5-positive LSCs possess the exclusive capacity to fully restore the cornea upon grafting to LSC-deficient mice in xenogeneic or syngeneic transplantation models. ABCB5 is preferentially expressed on label-retaining LSCs2 in mice and p63α-positive LSCs8 in humans. Consistent with these findings, ABCB5-positive LSC frequency is reduced in LSC-deficient patients. Abcb5 loss of function in Abcb5 knockout mice causes depletion of quiescent LSCs due to enhanced proliferation and apoptosis, and results in defective corneal differentiation and wound healing. Our results from gene knockout studies, LSC tracing and transplantation models, as well as phenotypic and functional analyses of human biopsy specimens, provide converging lines of evidence that ABCB5 identifies mammalian LSCs. Identification and prospective isolation of molecularly defined LSCs with essential functions in corneal development and repair has important implications for the treatment of corneal disease, particularly corneal blindness due to LSC deficiency.

One of the greatest challenges in cell therapy is to minimally invasively deliver a large quantity of viable cells to a tissue of interest with high engraftment efficiency. Low and inefficient homing of systemically delivered mesenchymal stem cells (MSCs), for example, is thought to be a major limitation of existing MSC-based therapeutic approaches, caused predominantly by inadequate expression of cell surface adhesion receptors. Using a platform approach that preserves the MSC phenotype and does not require genetic manipulation, we modified the surface of MSCs with a nanometer-scale polymer construct containing sialyl Lewisx (sLex) that is found on the surface of leukocytes and mediates cell rolling within inflamed tissue. The sLex engineered MSCs exhibited a robust rolling response on inflamed endothelium in vivo and homed to inflamed tissue with higher efficiency compared with native MSCs. The modular approach described herein offers a simple method to potentially target any cell type to specific tissues via the circulation.

Key Points mRNA transfection is an effective tool to simultaneously engineer MSCs for enhanced homing and improved secretome. MSCs can be systemically targeted to sites of inflammation to achieve therapeutically relevant concentrations of biological agents.

SummaryPost-natal skeletal stem cells expressing PRX1 (pnPRX1+) have been identified in the calvaria and in the axial skeleton. Here we characterize the location and functional capacity of the calvarial pnPRX1+ cells. We found that pnPRX1+ reside exclusively in the calvarial suture niche and decrease in number with age. They are distinct from preosteoblasts and osteoblasts of the sutures, respond to WNT signaling in vitro and in vivo by differentiating into osteoblasts, and, upon heterotopic transplantation, are able to regenerate bone. Diphtheria toxin A (DTA)-mediated lineage ablation of pnPRX1+ cells and suturectomy perturb regeneration of calvarial bone defects and confirm that pnPRX1+ cells of the sutures are required for bone regeneration. Orthotopic transplantation of sutures with traceable pnPRX1+ cells into wild-type animals shows that pnPRX1+ cells of the suture contribute to calvarial bone defect regeneration. DTA-mediated lineage ablation of pnPRX1+ does not, however, interfere with calvarial development.

BackgroundMyocarditis is characterized by inflammatory cell infiltration of the heart and subsequent deterioration of cardiac function. Monocytes are the most prominent population of accumulating leucocytes. We investigated whether in vivo administration of nanoparticle-encapsulated siRNA targeting chemokine (C-C motif) receptor 2 (CCR2)-a chemokine receptor crucial for leucocyte migration in humans and mice-reduces inflammation in autoimmune myocarditis. Methods and resultsIn myocardium of patients with myocarditis, CCL2 mRNA levels and CCR2 + cells increased (P , 0.05), motivating us to pursue CCR2 silencing. Flow cytometric analysis showed that siRNA silencing of CCR2 (siCCR2) reduced the number of Ly6C high monocytes in hearts of mice with acute autoimmune myocarditis by 69% (P , 0.05), corroborated by histological assessment. The nanoparticle-delivered siRNA was not only active in monocytes but also in bone marrow haematopoietic progenitor cells. Treatment with siCCR2 reduced the migration of bone marrow granulocyte macrophage progenitors into the blood. Cellular magnetic resonance imaging (MRI) after injection of macrophage-avid magnetic nanoparticles detected myocarditis and therapeutic effects of RNAi non-invasively. Mice with acute myocarditis showed enhanced macrophage MRI contrast, which was prevented by siCCR2 (P , 0.05). Follow-up MRI volumetry revealed that siCCR2 treatment improved ejection fraction (P , 0.05 vs. control siRNA-treated mice). ConclusionThis study highlights the importance of CCR2 in the pathogenesis of myocarditis. In addition, we show that siCCR2 affects leucocyte progenitor trafficking. The data also point to a novel therapeutic strategy for the treatment of myocarditis.

Volumetric muscle loss (VML) injury is characterized by a non-recoverable loss of muscle fibers due to ablative surgery or severe orthopaedic trauma, that results in chronic functional impairments of the soft tissue. Currently, the effects of VML on the oxidative capacity and adaptability of the remaining injured muscle are unclear. A better understanding of this pathophysiology could significantly shape how VML-injured patients and clinicians approach regenerative medicine and rehabilitation following injury. Herein, the data indicated that VML-injured muscle has diminished mitochondrial content and function (i.e., oxidative capacity), loss of mitochondrial network organization, and attenuated oxidative adaptations to exercise. However, forced PGC-1α over-expression rescued the deficits in oxidative capacity and muscle strength. This implicates physiological activation of PGC1-α as a limiting factor in VML-injured muscle’s adaptive capacity to exercise and provides a mechanistic target for regenerative rehabilitation approaches to address the skeletal muscle dysfunction.