Perlecan is a heparan sulfate proteoglycan that is expressed in all basement membranes (BMs), in cartilage, and several other mesenchymal tissues during development. Perlecan binds growth factors and interacts with various extracellular matrix proteins and cell adhesion molecules. Homozygous mice with a null mutation in the perlecan gene exhibit normal formation of BMs. However, BMs deteriorate in regions with increased mechanical stress such as the contracting myocardium and the expanding brain vesicles showing that perlecan is crucial for maintaining BM integrity. As a consequence, small clefts are formed in the cardiac muscle leading to blood leakage into the pericardial cavity and an arrest of heart function. The defects in the BM separating the brain from the adjacent mesenchyme caused invasion of brain tissue into the overlaying ectoderm leading to abnormal expansion of neuroepithelium, neuronal ectopias, and exencephaly. Finally, homozygotes developed a severe defect in cartilage, a tissue that lacks BMs. The chondrodysplasia is characterized by a reduction of the fibrillar collagen network, shortened collagen fibers, and elevated expression of cartilage extracellular matrix genes, suggesting that perlecan protects cartilage extracellular matrix from degradation.

Cellular replacement therapy has emerged as a novel strategy for the treatment of heart failure. The aim of our study was to determine the fate of injected mesenchymal stem cells (MSCs) and whole bone marrow (BM) cells in the infarcted heart. MSCs were purified from BM of transgenic mice and characterized using flow cytometry and in vitro differentiation assays. Myocardial infarctions were generated in mice and different cell populations including transgenic MSCs, unfractionated BM cells, or purified hematopoietic progenitors were injected. Encapsulated structures were found in the infarcted areas of a large fraction of hearts after injecting MSCs (22 of 43, 51.2%) and unfractionated BM cells (6 of 46, 13.0%). These formations contained calcifications and/or ossifications. In contrast, no pathological abnormalities were found after injection of purified hematopoietic progenitors ( IntroductionSevere heart failure is caused by an irreversible loss of cardiomyocytes and has a poor prognosis regardless of the underlying disease. 1 Since medical treatment is of only limited help, solid organ transplantation was considered until recently the only effective therapy. However, as organ availability decreases, there is an urgent need for alternative treatments. Studies in mice have suggested that myocardial infarctions can be repaired following transplantation of bone marrow (BM)-derived cells into the lesioned myocardium, either through generation of cardiomyocytes or angiogenesis. 2 An underlying assumption of this approach is that the environment will instruct as well as restrict the developmental fate of adult stem cells after their transplantation (for review see Laflamme and Murry 3 or Murry et al 4 ). However, the original findings in mice have recently been put into question, since we and others have demonstrated that BM-derived hematopoietic cells do not transdifferentiate into cardiomyocytes in the infarcted myocardium. [5][6][7] In this study, we focused on the potential of an enriched population of mesenchymal stem cells (MSCs) that are known to be present in the BM and are multipotent. 8 In contrast to hematopoietic progenitors, MSCs are easy to obtain and to expand in vitro and have therefore emerged as attractive candidates for cellular therapies in heart and other organs. 9,10 However, recent reports have questioned their "transdifferentiation" potential after injection into the myocardium and rather propose benefits via paracrine mechanisms. 11,12 Herein, we investigated and provide novel insights with regard to the fate of enriched populations of BM-derived MSCs as well as whole BM cells comprising both hematopoietic and mesenchymal progenitors after transplantation into the infarcted heart. Materials and methodsAll experiments were approved by the local ethics care committees at Bonn, Cologne, and Lund Universities. Cells for transplantation were isolated from transgenic C57Bl/6 mice expressing enhanced green fluorescent protein (EGFP) under control of the ␤-actin promoter. 13 Cell isolation and cultu...

SUMMARY Activation of the immune response during injury is a critical early event that determines whether the outcome of tissue restoration is regeneration or replacement of the damaged tissue with a scar. The mechanisms by which immune signals control these fundamentally different regenerative pathways are largely unknown. We have demonstrated that, during skin repair in mice, interleukin-4 receptor α (IL-4Rα)-dependent macrophage activation controlled collagen fibril assembly and that this process was important for effective repair while having adverse pro-fibrotic effects. We identified Relm-α as one important player in the pathway from IL-4Rα signaling in macrophages to the induction of lysyl hydroxylase 2 (LH2), an enzyme that directs persistent pro-fibrotic collagen cross-links, in fibroblasts. Notably, Relm-β induced LH2 in human fibroblasts, and expression of both factors was increased in lipodermatosclerosis, a condition of excessive human skin fibrosis. Collectively, our findings provide mechanistic insights into the link between type 2 immunity and initiation of pro-fibrotic pathways.

Excess exogenous retinoic acid (RA) has been well documented to have teratogenic effects in the limb and craniofacial skeleton. Malformations that have been observed in this context include craniosynostosis, a common developmental defect of the skull that occurs in 1 in 2500 individuals and results from premature fusion of the cranial sutures. Despite these observations, a physiological role for RA during suture formation has not been demonstrated. Here, we present evidence that genetically based alterations in RA signaling interfere with human development. We have identified human null and hypomorphic mutations in the gene encoding the RA-degrading enzyme CYP26B1 that lead to skeletal and craniofacial anomalies, including fusions of long bones, calvarial bone hypoplasia, and craniosynostosis. Analyses of murine embryos exposed to a chemical inhibitor of Cyp26 enzymes and zebrafish lines with mutations in cyp26b1 suggest that the endochondral bone fusions are due to unrestricted chondrogenesis at the presumptive sites of joint formation within cartilaginous templates, whereas craniosynostosis is induced by a defect in osteoblastic differentiation. Ultrastructural analysis, in situ expression studies, and in vitro quantitative RT-PCR experiments of cellular markers of osseous differentiation indicate that the most likely cause for these phenomena is aberrant osteoblast-osteocyte transitioning. This work reveals a physiological role for RA in partitioning skeletal elements and in the maintenance of cranial suture patency.