Noncanonical WNT5A controls the activation of latent TGF-ß to drive fibroblast activation and tissue fibrosis.

Transforming growth factor ß (TGF-ß) signaling is a core pathway of fibrosis, but the molecular regulation of the activation of latent TGF-ß remains incompletely understood. Here, we demonstrate a crucial role of WNT5A/JNK/ROCK signaling that rapidly coordinates the activation of latent TGF-ß in fibrotic diseases. WNT5A was identified as a predominant noncanonical WNT ligand in fibrotic diseases such as systemic sclerosis, sclerodermatous chronic graft-versus-host disease, and idiopathic pulmonary fibrosis, stimulating fibroblast-to-myofibroblast transition and tissue fibrosis by activation of latent TGF-ß. The activation of latent TGF-ß requires rapid JNK- and ROCK-dependent cytoskeletal rearrangements and integrin αV (ITGAV). Conditional ablation of WNT5A or its downstream targets prevented activation of latent TGF-ß, rebalanced TGF-ß signaling, and ameliorated experimental fibrosis. We thus uncovered what we believe to be a novel mechanism for the aberrant activation of latent TGF-ß in fibrotic diseases and provided evidence for targeting WNT5A/JNK/ROCK signaling in fibrotic diseases as a new therapeutic approach.

TANGO1 inhibitors reduce collagen secretion and limit tissue scarring.

Uncontrolled secretion of ECM proteins, such as collagen, can lead to excessive scarring and fibrosis and compromise tissue function. Despite the widespread occurrence of fibrotic diseases and scarring, effective therapies are lacking. A promising approach would be to limit the amount of collagen released from hyperactive fibroblasts. We have designed membrane permeant peptide inhibitors that specifically target the primary interface between TANGO1 and cTAGE5, an interaction that is required for collagen export from endoplasmic reticulum exit sites (ERES). Application of the peptide inhibitors leads to reduced TANGO1 and cTAGE5 protein levels and a corresponding inhibition in the secretion of several ECM components, including collagens. Peptide inhibitor treatment in zebrafish results in altered tissue architecture and reduced granulation tissue formation during cutaneous wound healing. The inhibitors reduce secretion of several ECM proteins, including collagens, fibrillin and fibronectin in human dermal fibroblasts and in cells obtained from patients with a generalized fibrotic disease (scleroderma). Taken together, targeted interference of the TANGO1-cTAGE5 binding interface could enable therapeutic modulation of ERES function in ECM hypersecretion, during wound healing and fibrotic processes.

Ablation of integrin-mediated cell-collagen communication alleviates fibrosis.

OBJECTIVES: Activation of fibroblasts is a hallmark of fibrotic processes. Besides cytokines and growth factors, fibroblasts are regulated by the extracellular matrix environment through receptors such as integrins, which transduce biochemical and mechanical signals enabling cells to mount appropriate responses according to biological demands. The aim of this work was to investigate the in vivo role of collagen-fibroblast interactions for regulating fibroblast functions and fibrosis. METHODS: Triple knockout (tKO) mice with a combined ablation of integrins α1ß1, α2ß1 and α11ß1 were created to address the significance of integrin-mediated cell-collagen communication. Properties of primary dermal fibroblasts lacking collagen-binding integrins were delineated in vitro. Response of the tKO mice skin to bleomycin induced fibrotic challenge was assessed. RESULTS: Triple integrin-deficient mice develop normally, are transiently smaller and reveal mild alterations in mechanoresilience of the skin. Fibroblasts from these mice in culture show defects in cytoskeletal architecture, traction stress generation, matrix production and organisation. Ablation of the three integrins leads to increased levels of discoidin domain receptor 2, an alternative receptor recognising collagens in vivo and in vitro. However, this overexpression fails to compensate adhesion and spreading defects on collagen substrates in vitro. Mice lacking collagen-binding integrins show a severely attenuated fibrotic response with impaired mechanotransduction, reduced collagen production and matrix organisation. CONCLUSIONS: The data provide evidence for a crucial role of collagen-binding integrins in fibroblast force generation and differentiation in vitro and for matrix deposition and tissue remodelling in vivo. Targeting fibroblast-collagen interactions might represent a promising therapeutic approach to regulate connective tissue deposition in fibrotic diseases.

TGFß promotes fibrosis by MYST1-dependent epigenetic regulation of autophagy.

Activation of fibroblasts is essential for physiological tissue repair. Uncontrolled activation of fibroblasts, however, may lead to tissue fibrosis with organ dysfunction. Although several pathways capable of promoting fibroblast activation and tissue repair have been identified, their interplay in the context of chronic fibrotic diseases remains incompletely understood. Here, we provide evidence that transforming growth factor-ß (TGFß) activates autophagy by an epigenetic mechanism to amplify its profibrotic effects. TGFß induces autophagy in fibrotic diseases by SMAD3-dependent downregulation of the H4K16 histone acetyltransferase MYST1, which regulates the expression of core components of the autophagy machinery such as ATG7 and BECLIN1. Activation of autophagy in fibroblasts promotes collagen release and is both, sufficient and required, to induce tissue fibrosis. Forced expression of MYST1 abrogates the stimulatory effects of TGFß on autophagy and re-establishes the epigenetic control of autophagy in fibrotic conditions. Interference with the aberrant activation of autophagy inhibits TGFß-induced fibroblast activation and ameliorates experimental dermal and pulmonary fibrosis. These findings link uncontrolled TGFß signaling to aberrant autophagy and deregulated epigenetics in fibrotic diseases and may contribute to the development of therapeutic interventions in fibrotic diseases.

An mTORC1-GRASP55 signaling axis controls unconventional secretion to reshape the extracellular proteome upon stress.

Cells communicate with their environment via surface proteins and secreted factors. Unconventional protein secretion (UPS) is an evolutionarily conserved process, via which distinct cargo proteins are secreted upon stress. Most UPS types depend upon the Golgi-associated GRASP55 protein. However, its regulation and biological role remain poorly understood. Here, we show that the mechanistic target of rapamycin complex 1 (mTORC1) directly phosphorylates GRASP55 to maintain its Golgi localization, thus revealing a physiological role for mTORC1 at this organelle. Stimuli that inhibit mTORC1 cause GRASP55 dephosphorylation and relocalization to UPS compartments. Through multiple, unbiased, proteomic analyses, we identify numerous cargoes that follow this unconventional secretory route to reshape the cellular secretome and surfactome. Using MMP2 secretion as a proxy for UPS, we provide important insights on its regulation and physiological role. Collectively, our findings reveal the mTORC1-GRASP55 signaling hub as the integration point in stress signaling upstream of UPS and as a key coordinator of the cellular adaptation to stress.

A story of fibers and stress: Matrix-embedded signals for fibroblast activation in the skin.

Our skin is continuously exposed to mechanical challenge, including shear, stretch, and compression. The extracellular matrix of the dermis is perfectly suited to resist these challenges and maintain integrity of normal skin even upon large strains. Fibroblasts are the key cells that interpret mechanical and chemical cues in their environment to turnover matrix and maintain homeostasis in the skin of healthy adults. Upon tissue injury, fibroblasts and an exclusive selection of other cells become activated into myofibroblasts with the task to restore skin integrity by forming structurally imperfect but mechanically stable scar tissue. Failure of myofibroblasts to terminate their actions after successful repair or upon chronic inflammation results in dysregulated myofibroblast activities which can lead to hypertrophic scarring and/or skin fibrosis. After providing an overview on the major fibrillar matrix components in normal skin, we will interrogate the various origins of fibroblasts and myofibroblasts in the skin. We then examine the role of the matrix as signaling hub and how fibroblasts respond to mechanical matrix cues to restore order in the confusing environment of a healing wound.

miR-127-3p Is an Epigenetic Activator of Myofibroblast Senescence Situated within the MicroRNA-Enriched Dlk1-Dio3‒Imprinted Domain on Mouse Chromosome 12.

During wound healing, fibroblasts differentiate into nonproliferative contractile myofibroblasts, contribute to skin repair, and eventually undergo apoptosis or become senescent. MicroRNAs are post-transcriptional regulators of gene expression networks that control cell fate and survival and may also regulate senescence. In this study, we determined the regulated microRNAs in myofibroblasts isolated from wounds and analyzed their role in senescent myofibroblast formation. Transcriptome profiling showed that a 200 kilobase pair region of the Dlk1-Dio3‒imprinted domain on mouse chromosome 12 encodes for most of the upregulated microRNAs in the entire genome of mouse myofibroblasts. Among those, miR-127-3p induced a myofibroblast-like phenotype associated with a block in proliferation. Molecular analysis revealed that miR-127-3p induced a prolonged cell cycle arrest with unique molecular features of senescence, including the activation of the senescence-associated ß-galactosidase, increase in p53 and p21 levels, inhibition of lamin B1, proliferation factors, and the production of senescence-associated inflammatory and extracellular matrix‒remodeling components. Hence, miR-127-3p emerges as an epigenetic activator regulating the transition from repair to remodeling during skin wound healing but may also induce age-related defects, pathological scarring, and fibrosis, all linked to myofibroblast senescence.

Role of collagen XII in skin homeostasis and repair.

Skin integrity and function depends to a large extent on the composition of the extracellular matrix, which regulates tissue organization. Collagen XII is a homotrimer with short collagenous domains that confer binding to the surface of collagen I-containing fibrils and extended flexible arms, which bind to non-collagenous matrix components. Thereby, collagen XII helps to maintain collagen suprastructure and to absorb stress. Mutant or absent collagen XII leads to reduced muscle and bone strength and lax skin, whereas increased collagen XII amounts are observed in tumor stroma, scarring and fibrosis. This study aimed at uncovering in vivo mechanisms by which collagen XII may achieve these contrasting outcomes. We analyzed skin as a model tissue that contains abundant fibrils, composed of collagen I, III and V with collagen XII decorating their surface, and which is subject to mechanical stress. The impact of different collagen XII levels was investigated in collagen XII-deficient (Col12-KO) mice and in mice with collagen XII overexpression in the dermis (Col12-OE). Unchallenged skin of these mice was histologically inconspicuous, but at the ultrastructural level revealed distinct aberrations in collagen network suprastructure. Repair of excisional wounds deviated from controls in both models by delayed healing kinetics, which was, however, caused by completely different mechanisms in the two mouse lines. The disorganized matrix in Col12-KO wounds failed to properly sequester TGFß, resulting in elevated numbers of myofibroblasts. These are, however, unable to contract and remodel the collagen XII-deficient matrix. Excess of collagen XII, in contrast, promotes persistence of M1-like macrophages in the wound bed, thereby stalling the wounds in an early inflammatory stage of the repair process and delaying healing. Taken together, we demonstrate that collagen XII is a key component that assists in orchestrating proper skin matrix structure, controls growth factor availability and regulates cellular composition and function. Together, these functions are pivotal for re-establishing homeostasis after injury.

Signaling pathways affected by mutations causing osteogenesis imperfecta.

Osteogenesis imperfecta (OI) is a clinically and genetically heterogeneous connective tissue disorder characterized by bone fragility and skeletal deformity. To maintain skeletal strength and integrity, bone undergoes constant remodeling of its extracellular matrix (ECM) tightly controlled by osteoclast-mediated bone resorption and osteoblast-mediated bone formation. There are at least 20 recognized OI-forms caused by mutations in the two collagen type I-encoding genes or genes implicated in collagen folding, posttranslational modifications or secretion of collagen, osteoblast differentiation and function, or bone mineralization. The underlying disease mechanisms of non-classical forms of OI that are not caused by collagen type I mutations are not yet completely understood, but an altered ECM structure as well as disturbed intracellular homeostasis seem to be the main defects. The ECM orchestrates local cell behavior in part by regulating bioavailability of signaling molecules through sequestration, release and activation during the constant bone remodeling process. Here, we provide an overview of signaling pathways that are associated with known OI-causing genes and discuss the impact of these genes on signal transduction. These pathways include WNT-, RANK/RANKL-, TGFß-, MAPK- and integrin-mediated signaling as well as the unfolded protein response.

Increased Collagen Turnover Impairs Tendon Microstructure and Stability in Integrin α2ß1-Deficient Mice.

Integrins are a family of transmembrane proteins, involved in substrate recognition and cell adhesion in cross-talk with the extra cellular matrix. In this study, we investigated the influence of integrin α2ß1 on tendons, another collagen type I-rich tissue of the musculoskeletal system. Morphological, as well as functional, parameters were analyzed in vivo and in vitro, comparing wild-type against integrin α2ß1 deficiency. Tenocytes lacking integrin α2ß1 produced more collagen in vitro, which is similar to the situation in osseous tissue. Fibril morphology and biomechanical strength proved to be altered, as integrin α2ß1 deficiency led to significantly smaller fibrils as well as changes in dynamic E-modulus in vivo. This discrepancy can be explained by a higher collagen turnover: integrin α2ß1-deficient cells produced more matrix, and tendons contained more residual C-terminal fragments of type I collagen, as well as an increased matrix metalloproteinase-2 activity. A greatly decreased percentage of non-collagenous proteins may be the cause of changes in fibril diameter regulation and increased the proteolytic degradation of collagen in the integrin-deficient tendons. The results reveal a significant impact of integrin α2ß1 on collagen modifications in tendons. Its role in tendon pathologies, like chronic degradation, will be the subject of future investigations.

Generation of a novel mouse strain with fibroblast-specific expression of Cre recombinase.

Cell-specific expression of genes offers the possibility to use their promoters to drive expression of Cre-recombinase, thereby allowing for detailed expression analysis using reporter gene systems, cell lineage tracing, conditional gene deletion, and cell ablation. In this context, current data suggest that the integrin α11 subunit has the potential to serve as a fibroblast biomarker in tissue regeneration and pathology, in particular in wound healing and in tissue- and tumor fibrosis. The mesenchyme-restricted expression pattern of integrin α11 thus prompted us to generate a novel ITGA11-driver Cre mouse strain using a ϕC31 integrase-mediated knock-in approach. In this transgenic mouse, the Cre recombinase is driven by regulatory promoter elements within the 3 kb segment of the human ITGA11 gene. ß-Galactosidase staining of embryonic tissues obtained from a transgenic ITGA11-Cre mouse line crossed with Rosa 26R reporter mice (ITGA11-Cre;R26R) revealed ITGA11-driven Cre expression and activity in mesenchymal cells in a variety of mesenchymal tissues in a pattern reminiscent of endogenous α11 protein expression in mouse embryos. Interestingly, X-gal staining of mouse embryonic fibroblasts (MEFs) isolated from the ITGA11-Cre;R26R mice indicated heterogeneity in the MEF population. ITGA11-driven Cre activity was shown in approximately 60% of the MEFs, suggesting that the expression of integrin α11 could be exploited for isolation of different fibroblast populations. ITGA11-driven Cre expression was found to be low in adult mouse tissues but was induced in granulation tissue of excisional wounds and in fibrotic hearts following aortic banding. We predict that the ITGA11-Cre transgenic mouse strain described in this report will be a useful tool in matrix research for the deletion of genes in subsets of fibroblasts in the developing mouse and for determining the function of subsets of pro-fibrotic fibroblasts in tissue fibrosis and in different subsets of cancer-associated fibroblasts in the tumor microenvironment.

Dual role of laminin­511 in regulating melanocyte migration and differentiation.

Laminins are the major basement membrane (BM) components and are heterotrimers composed of an α, a ß and a γ chain. In skin, laminins are present in basement membranes surrounding vascular structures, nerves, adipose tissue and in the specialized junctional BM between the epidermis and dermis. The main laminin isoforms in the dermo-epidermal BM are laminin­332, laminin­511 and laminin­211, the latter being restricted to hair follicles (HFs). The laminin γ1 chain is the most abundant γ chain; its global ablation in mice leads to early embryonic lethality at E5.5. To elucidate the cellular function of the γ1 chain in skin, we generated mice with keratinocyte-specific deletion of this chain (Lamc1EKO) by using the keratin (K)14-Cre/loxP system. These mice showed delayed coat pigmentation despite normal melanocyte counts in the skin. However, levels of differentiation-specific melanocyte enzymes TRP­1, TRP­2 and tyrosinase were reduced in Lamc1EKO mice, and melanocytes failed to migrate to their differentiation niche in HFs and accumulated in the IFE. These results suggested that the pigmentation defect results from impaired melanocyte migration. The impaired migratory capacity of melanocytes is due to the altered composition of laminins in the BM of Lamc1EKO mice: Loss of keratinocyte-derived pro-migratory laminin­511 is not compensated by ectopically deposited fibroblast-derived laminin­211. Furthermore, contact of melanocytes with recombinant laminin­511, but not with laminin­211, induces the expression of the chemokine receptor CXCR4 on melanocytes, needed for SDF­1 (stromal cell­derived factor­1)-mediated migration into HFs. We here demonstrate that laminin­511 controls the differentiation of melanocytes by regulating their migration from the epidermis into HFs and by activating CXCR4 expression on melanocytes required for their recruitment into HFs in an SDF­1-dependent manner.

Myeloid Cell-Restricted STAT3 Signaling Controls a Cell-Autonomous Antifibrotic Repair Program.

Myeloid cells can be beneficial as well as harmful in tissue regenerative responses. The molecular mechanisms by which myeloid cells control this critical decision of the immune system are not well understood. Using two different models of physiological acute or pathological chronic skin damage, in this study we identified myeloid cell-restricted STAT3 signaling as important and an injury context-dependent regulator of skin fibrosis. Targeted disruption of STAT3 signaling in myeloid cells significantly accelerated development of pathological skin fibrosis in a model of chronic bleomycin-induced tissue injury, whereas the impact on wound closure dynamics and quality of healing after acute excision skin injury was minor. Chronic bleomycin-mediated tissue damage in control mice provoked an antifibrotic gene signature in macrophages that was characterized by upregulated expression of IL-10, SOCS3, and decorin. In contrast, in STAT3-deficient macrophages this antifibrotic repair program was abolished whereas TGF-ß1 expression was increased. Notably, TGF-ß1 synthesis in cultured control bone marrow-derived macrophages (BMDMs) was suppressed after IL-10 exposure, and this suppressive effect was alleviated by STAT3 deficiency. Accordingly, coculture of IL-10-stimulated control BMDMs with fibroblasts suppressed expression of the TGF-ß1 downstream target connective tissue growth factor in fibroblasts, whereas this suppressive effect was lost by STAT3 deficiency in BMDMs. Our findings highlight a previously unrecognized protective role of myeloid cell-specific STAT3 signaling in immune cell-mediated skin fibrosis, and its regulatory pathway could be a potential target for therapy.

New developments on skin fibrosis - Essential signals emanating from the extracellular matrix for the control of myofibroblasts.

Many different diseases are associated with fibrosis of the skin. The clinical symptoms can vary considerably with a broad range from isolated small areas to the involvement of the entire integument. Fibrosis is triggered by a multitude of different stimuli leading to activation of the immune and vascular system that then initiate fibroblast activation and formation of matrix depositing and remodeling myofibroblasts. Ultimately, myofibroblasts deposit excessive amounts of extracellular matrix with a pathological architecture and alterations in growth factor binding and biomechanical properties, which culminates in skin hardening and loss of mobility. Treatment depends certainly on the specific type and cause of the disease, for the autoimmune driven localized and systemic scleroderma therapeutic options are still limited, but recent research has pointed out diverse molecular targets and mechanisms that can be exploited for the development of novel antifibrotic therapy.

TGFB1 is secreted through an unconventional pathway dependent on the autophagic machinery and cytoskeletal regulators.

TGFB1 (transforming growth factor beta 1) is a potent cytokine playing a driving role in development, fibrosis and cancer. It is synthesized as prodomain-growth factor complex that requires tethering to LTBP (latent transforming growth factor beta binding protein) for efficient secretion into the extracellular space. Upon release, this large latent complex is sequestered by anchorage to extracellular matrix (ECM) networks, from which the mature growth factor needs to be activated in order to reach its receptors and initiate signaling. Here, we uncovered a novel intracellular secretion pathway by which the latent TGFB1 complex reaches the plasma membrane and is released from fibroblasts, the key effector cells during tissue repair, fibrosis and in the tumor stroma. We show that secretion of latent TGFB1, but not of other selected cytokines or of bulk cargo, is regulated by fibroblast-ECM communication through ILK (integrin linked kinase) that restricts RHOA activity by interacting with ARHGAP26/GRAF1. Latent TGFB1 interacts with GORASP2/GRASP55 and is detected inside MAP1LC3-positive autophagosomal intermediates that are secreted by a RAB8A-dependent pathway. Interestingly, TGFB1 secretion is fully abrogated in human and murine fibroblasts and macrophages that lack key components of the autophagic machinery. Our data demonstrate an unconventional secretion mode of TGFB1 adding another level of control of its bioavailability and activity in order to effectively orchestrate cellular programs prone to dysregulation as seen in fibrosis and cancer.

Pathophysiological Mechanisms in Sclerosing Skin Diseases.

Sclerosing skin diseases represent a large number of distinct disease entities, which include systemic sclerosis, localized scleroderma, and scleredema adultorum. These pathologies have a common clinical appearance and share histological features. However, the specific interplay between cytokines and growth factors, which activate different mesenchymal cell populations and production of different extracellular matrix components, determines the biomechanical properties of the skin and the clinical features of each disease. A better understanding of the mechanisms underlying these events is prerequisite for developing novel targeted therapeutic approaches.

Cell-Populated Collagen Lattice Models.

Investigation of cell function is often hampered by the complexity of the tissue context. This problem is circumvented by isolating cells from tissues and analyzing their behavior in culture. Most cell types are cultured as monolayers on planar, rigid Petri dishes, an environment that does not reflect the spatial, three-dimensional cellular environment in vivo. Culture in three-dimensional collagen lattices has been devised to optimize in vitro culture conditions and to provide a more physiologic "in vivo-like" environment. Collagen lattices can easily be manipulated to suit diverse cell types and to provide variable mechanical forces. Cells can be imaged in such surroundings, and gene expression as well as protein production and activity can be monitored.

Deep Proteome Profiling Reveals Common Prevalence of MZB1-Positive Plasma B Cells in Human Lung and Skin Fibrosis.

RATIONALE: Analyzing the molecular heterogeneity of different forms of organ fibrosis may reveal common and specific factors and thus identify potential future therapeutic targets. OBJECTIVES: We sought to use proteome-wide profiling of human tissue fibrosis to (1) identify common and specific signatures across end-stage interstitial lung disease (ILD) cases, (2) characterize ILD subgroups in an unbiased fashion, and (3) identify common and specific features of lung and skin fibrosis. METHODS: We collected samples of ILD tissue (n = 45) and healthy donor control samples (n = 10), as well as fibrotic skin lesions from localized scleroderma and uninvolved skin (n = 6). Samples were profiled by quantitative label-free mass spectrometry, Western blotting, or confocal imaging. MEASUREMENTS AND MAIN RESULTS: We determined the abundance of more than 7,900 proteins and stratified these proteins according to their detergent solubility profiles. Common protein regulations across all ILD cases, as well as distinct ILD subsets, were observed. Proteomic comparison of lung and skin fibrosis identified a common upregulation of marginal zone B- and B1-cell-specific protein (MZB1), the expression of which identified MZB1+/CD38+/CD138+/CD27+/CD45-/CD20- plasma B cells in fibrotic lung and skin tissue. MZB1 levels correlated positively with tissue IgG and negatively with diffusing capacity of the lung for carbon monoxide. CONCLUSIONS: Despite the presumably high molecular and cellular heterogeneity of ILD, common protein regulations are observed, even across organ boundaries. The surprisingly high prevalence of MZB1-positive plasma B cells in tissue fibrosis warrants future investigations regarding the causative role of antibody-mediated autoimmunity in idiopathic cases of organ fibrosis, such as idiopathic pulmonary fibrosis.

Role of Integrins α1ß1 and α2ß1 in Wound and Tumor Angiogenesis in Mice.

Integrins are transmembrane receptors composed of one α subunit and one ß subunit and are involved in cellular growth, differentiation, and apoptosis. The collagen-binding integrins α1ß1 and α2ß1 have been shown to regulate wound and tumor vascularization by different mechanisms. In this study, we assessed wound and tumor vascularization in mice with genetic ablation of both integrin subunits α1 and α2, which resulted in loss of integrins α1ß1 and α2ß1. Wound angiogenesis was investigated in excisional wounds that were inflicted on the back skin of control and mice lacking integrin α1ß1 and α2ß1. Mutant mice displayed reduced wound angiogenesis, which correlated with decreased macrophage numbers at 3 and 7 days after injury, and showed significantly attenuated vascularization of sponge implants. Angiogenesis induced by tumors arising from intradermal injection of B16 F1 melanoma cells was also reduced in comparison to controls 7 days after injection. This reduction in angiogenesis correlated with increased levels and activity of circulating matrix metalloproteinase 9 and elevated angiostatin levels in plasma of mutant mice, which reduced endothelial cell proliferation. Ex vivo mutant aortic ring explants developed significantly fewer and thinner aortic sprouts with fewer branch points than controls because of impaired endothelial cell proliferation. In conclusion, the loss of integrins α1ß1 and α2ß1 in mice results in reduced wound and tumor angiogenesis by cell-autonomous and extrinsic mechanisms.