Parabiosis and transplantation models show no evidence of circulating dermal fibroblast progenitors in bleomycin-induced skin fibrosis

A Systematic Review of the Evidence of Hematopoietic Stem Cell Differentiation to Fibroblasts

Fibroblasts have an important role in the maintenance of the extracellular matrix of connective tissues by producing and remodelling extracellular matrix proteins. They are indispensable for physiological processes, and as such also associate with many pathological conditions. In recent years, a number of studies have identified donor-derived fibroblasts in various tissues of bone marrow transplant recipients, while others could not replicate these findings. In this systematic review, we provide an overview of the current literature regarding the differentiation of hematopoietic stem cells into fibroblasts in various tissues. PubMed, Embase, and Web of Science (Core Collection) were systematically searched for original articles concerning fibroblast origin after hematopoietic stem cell transplantation in collaboration with a medical information specialist. Our search found 5421 studies, of which 151 were analysed for full-text analysis by two authors independently, resulting in the inclusion of 104 studies. Only studies in animals and humans, in which at least one marker was used for fibroblast identification, were included. The results were described per organ of fibroblast engraftment. We show that nearly all mouse and human organs show evidence of fibroblasts of hematopoietic stem cell transfer origin. Despite significant heterogeneity in the included studies, most demonstrate a significant presence of fibroblasts of hematopoietic lineage in non-hematopoietic tissues. This presence appears to increase after the occurrence of tissue damage.

Analysis of the contribution of nonresident progenitor cells and hematopoietic cells to reparative dentinogenesis using parabiosis model in mice

INTRODUCTION

The aim of this study was to analyze the contribution of nonresident progenitor/stem cells and hematopoietic cells to reparative dentinogenesis.

METHODS

Parabiosis was established between C57BL/6-TgN(ACTbEGFP)10sb/J transgenic mice (GFP+) and C57BL/6 wild-type mice (GFP-) to ensure blood cross-circulation between animals. Reparative dentinogenesis was stimulated by pulp exposures and capping on the first maxillary molar in the GFP- mice. Histologic sections of injured molars from GFP- mice were analyzed by epifluorescence microscopy to examine the contributions of GFP+ cells (nonresident progenitor cells and hematopoietic cells originating from GFP+ mice) to reparative dentinogenesis.

RESULTS

GFP+ cells were detected in close association with reparative dentin formed at the site of pulp exposure in the maxillary first molars of the GFP- mice.

CONCLUSIONS

The present study suggests the participation of the nonresident progenitor cells and hematopoietic cells in reparative dentinogenesis.

Fibroblast abnormalities in the pathogenesis of systemic sclerosis

Systemic sclerosis (SSc) is a chronic systemic disease characterized by autoimmunity, vascular lesions and progressive fibrosis. The fibrotic component is dominant in SSc compared with other vascular or autoimmune diseases and determines its prognosis and therapeutic refractoriness. Fibroblasts are responsible for abnormal extracellular matrix accumulation. Studies in cultured SSc skin fibroblasts have facilitated the identification of potential pathways involved in their profibrotic phenotype. Profibrotic fibroblasts characterized by abnormal growth and extracellular matrix synthesis may differentiate or expand from normal resident fibroblasts. Recruitment of bone marrow-derived progenitors and transdifferentiation of different cell lineages might also be involved. Multiple factors and signaling pathways appear to be involved in the development or persistence of the SSc fibroblast phenotype. Although their relative relevance and interplay are unclear, aberrant TGF-β signaling seems pivotal and constitutes the best characterized therapeutic target.

Wnt-signaling pathways in progressive renal fibrosis

INTRODUCTION

The prevention and potential reversal of interstitial fibrosis is a central strategy for the treatment of progressive renal disease. This strategy requires a better understanding of the underlying pathophysiologic processes involved in progressive renal fibrosis.

AREAS COVERED

The developmental processes in which Wnt (combination of 'wingless' and 'INT')/frizzled signaling is involved is discussed in this review, including cell fate determination, cell polarity, tissue patterning and control of cell proliferation. These pathways are also active in the adult where they play key roles in the maintenance of tissue homeostasis, wound repair and chronic tissue damage.

EXPERT OPINION

Wnt biology helps to control cell polarity, moderates cell proliferation and underlies other processes linked to renal homeostasis. Reactivation and dysregulation of the Wnt pathways underlie chronic fibrosis and progressive renal failure. Wnt signaling is, however, context-dependent: the pathways are complex and undergo many levels of cross-talk with other regulatory systems and regulatory pathways. On one hand, this may help to explain the positive effects of Wnt-signaling blockades seen in some animal models of chronic renal damage and, on the other, this suggests that it may be difficult to predict how modifications of the Wnt pathway may influence a process.

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.

Differential contribution of dermal resident and bone marrow-derived cells to collagen production during wound healing and fibrogenesis in mice

Recent studies show that bone marrow (BM)-derived cells migrating into a dermal wound promote healing by producing collagen type I. However, their contribution to the repair process has not been fully verified yet. It is also unclear whether BM-derived cells participate in dermal fibrogenesis. We have addressed these issues using transgenic mice that harbor tissue-specific enhancer/promoter sequences of α2(I) collagen gene linked to either enhanced green fluorescent protein (COL/EGFP) or the luciferase (COL/LUC) reporter gene. Following dermal excision or subcutaneous bleomycin administration, a large number of EGFP-positive collagen-producing cells appeared in the dermis of COL/EGFP reporter mice. When wild-type mice were transplanted with BM cells from transgenic COL/EGFP animals and subjected to dermal excision, no EGFP-positive BM-derived collagen-producing cells were detected throughout the repair process. Luciferase assays of dermal tissues from COL/LUC recipient mice also excluded collagen production by BM-derived cells during dermal excision healing. In contrast, a limited but significant number of CD45-positive collagen-producing cells migrated from BM following bleomycin injection. These results indicate that resident cells in the skin are the major source of de novo collagen deposition in both physiological and pathological conditions, whereas BM-derived cells participate, in part, in collagen production during dermal fibrogenesis.

Pathogenetic mechanisms in radiation fibrosis

Deregulation of normal regenerative responses to physical, chemical and biological toxins in susceptible individuals leads to abnormal remodelling of extracellular matrix with pathological fibrosis. Processes deregulated after radiotherapy have much in common with processes associated with fibrotic diseases affecting the heart, skin, lungs, kidneys, gastro-intestinal tract and liver. Among the secreted factors driving fibrosis, transforming growth factor beta 1 (TGFβ1) produced by a wide range of inflammatory, mesenchymal and epithelial cells converts fibroblasts and other cell types into matrix-producing myofibroblasts. Even if required for the initiation of fibrosis, inflammation and the continued stimulus of TGFβ1 may not be needed to maintain it. After myofibroblast activation, collagen production can be perpetuated independently of TGFβ1 by autocrine induction of a cytokine called connective tissue growth factor. The role of inflammation, the origins and activation of myofibroblasts as biosynthetic cells and the downstream pathways of extracellular matrix synthesis in common fibrotic states are reviewed. Oxidative stress, hypoxia and microvascular damage are also considered, before examining the same processes in the context of radiotherapy. One of the main uncertainties is the relevance of very early events, including inflammatory responses in blood vessels, to fibrosis. Despite the power of animal models, including genetic systems, the potential contribution of research based on human tissue samples has never been greater. A closer interaction between scientists researching fibrosis and radiation oncologists holds enormous promise for therapeutic advances.

Parabiosis model does not show presence of circulating osteoprogenitor cells

The goal of this study was to determine the presence of osteoprogenitor cells in the peripheral blood. Experiments were conducted with a parabiosis model in which osteoblast specific transgenic mice (Col2.3GFP or hOC-GFP) were surgically joined with a transgenic mouse where herpes virus thymidine kinase gene is under the control of the collagen alpha1 promoter (Col2.3DeltaTK). This method permits conditional ablation of osteoblasts by ganciclovir (GCV) treatment. In parabionts treated with GCV for 15 days or 1.5-2 months, GFP (hOC-GFP or Col2.3GFP) expression was not detected in histological preparations or in marrow stromal cell cultures from the Col2.3DeltaTK parabiont. Finally, Col2.3GFP/Col2.3DeltaTK pairs were treated with GCV for 15 days and allowed to recover from GCV for 3 months. Again there was a failure to detect Col2.3GFP expressing cells in the Col2.3DeltaTK parabiont. These observations, at least within the limits of this model system, allow the conclusion that osteoprogenitor cells do not readily circulate.

Fibroblasts/myofibroblasts that participate in cutaneous wound healing are not derived from circulating progenitor cells

Dermal fibroblasts/myofibroblasts involved in the wound healing are thought to originate from the resident fibroblast progenitors. To test the hypothesis of an extra dermal origin of the dermal fibroblasts/myofibroblasts, bone marrow (BM) transplantation and parabiosis experiments were initiated utilizing a collagen promoter green fluorescent protein (GFP) reporter transgene as a visible marker for dermal fibroblasts/myofibroblasts. BM transplantation experiments using BM from Col3.6GFPsapph transgenic mice showed no evidence that BM derived progenitors differentiated into dermal fibroblasts/myofibroblasts at the wound site. Rather the GFP positive cells (GFP+) observed at the wound site were not dermal fibroblasts/myofibroblasts but immune cells. These GFP+ cells were also detected in the lung and spleen. Furthermore, GFP+ fibroblasts were not detected in primary dermal fibroblast cultures initiated from BM chimeras. Using the same transgenic mice, parabiotic pairs were generated. One partner in the parabiosis carried a GFP expressing transgene while the other partner was a non-transgenic C57BL/6 mouse. Similar to the BM transplantation experiments, GFP+ immune cells were detected in the wound of the non-transgenic parabiont, however, GFP expressing dermal fibroblasts/myofibroblasts were not observed. Collectively, these data suggest that dermal fibroblast/myofibroblast progenitors do not readily circulate. The expression of the Col3.6GFPsapph in the hematopoietic cells confirmed that our methods were sensitive enough to detect Col3.6GFP expressing dermal fibroblasts derived from the peripheral circulation if they had originated in the BM.

Adenosine A2A receptor blockade or deletion diminishes fibrocyte accumulation in the skin in a murine model of scleroderma, bleomycin-induced fibrosis

Peripheral blood fibrocytes are a newly identified circulating leukocyte subpopulation that migrates into injured tissue where it may display fibroblast-like properties and participate in wound healing and fibrosis of skin and other organs. Previous studies in our lab demonstrated that A(2A) receptor-deficient and A(2A) antagonist-treated mice were protected from developing bleomycin-induced dermal fibrosis, thus the aim of this study was to determine whether the adenosine A(2A) receptor regulates recruitment of fibrocytes to the dermis in this bleomycin-induced model of dermal fibrosis. Sections of skin from normal mice and bleomycin-treated wild type, A(2A) knockout and A(2A) antagonist-treated mice were stained for Procollagen alpha2 Type I and CD34 and the double stained cells, fibrocytes, were counted in the tissue sections. There were more fibrocytes in the dermis of bleomycin-treated mice than normal mice and the increase was abrogated by deletion or blockade of adenosine A(2A) receptors. Because fibrocytes play a central role in tissue fibrosis these results suggest that diminished adenosine A(2A) receptor-mediated recruitment of fibrocytes into tissue may play a role in the pathogenesis of fibrosing diseases of the skin. Moreover, these results provide further evidence that adenosine A(2A) receptors may represent a new target for the treatment of such fibrosing diseases as scleroderma or nephrogenic fibrosing dermopathy.

In perspective: murine models of scleroderma

An understanding of the complex pathogenesis of systemic sclerosis (SSc) has been slow to emerge, due in large part to the lack of an animal model recapitulating the three cardinal attributes of SSc: autoimmunity, vasculopathy, and fibrosis. Experimental manipulations in inbred murine strains can lead to conditions that mimic SSc fibrosis. Furthermore, genetic engineering has enabled the creation of novel murine strains that spontaneously develop fibrosis or are protected from fibrosis development. Studies of these mice shed light on the cell types, cell interactions, molecules, and pathways that contribute to SSc manifestations. High-throughput discovery technologies such as DNA microarrays in animal models can identify novel genes and regulatory networks that are important for disease manifestations and that may be targets for therapy. In this brief review, we highlight recent progress in the field and attempt to place the strengths and limitations of popular SSc murine models in perspective.