Epidermal hyperplasia and papillomatosis in mice with a keratinocyte-restricted deletion of csk

The dipeptide prolyl-hydroxyproline promotes cellular homeostasis and lamellipodia-driven motility via active β1-integrin in adult tendon cells

Collagen-derived hydroxyproline (Hyp)-containing peptides have a variety of biological effects on cells. These bioactive collagen peptides are locally generated by the degradation of endogenous collagen in response to injury. However, no comprehensive study has yet explored the functional links between Hyp-containing peptides and cellular behavior. Here, we show that the dipeptide prolyl-4-hydroxyproline (Pro-Hyp) exhibits pronounced effects on mouse tendon cells. Pro-Hyp promotes differentiation/maturation of tendon cells with modulation of lineage-specific factors and induces significant chemotactic activity in vitro. In addition, Pro-Hyp has profound effects on cell proliferation, with significantly upregulated extracellular signal-regulated kinase phosphorylation and extracellular matrix production and increased type I collagen network organization. Using proteomics, we have predicted molecular transport, cellular assembly and organization, and cellular movement as potential linked-network pathways that could be altered in response to Pro-Hyp. Mechanistically, cells treated with Pro-Hyp demonstrate increased directional persistence and significantly increased directed motility and migration velocity. They are accompanied by elongated lamellipodial protrusions with increased levels of active β1-integrin-containing focal contacts, as well as reorganization of thicker peripheral F-actin fibrils. Pro-Hyp-mediated chemotactic activity is significantly reduced (p < 0.001) in cells treated with the mitogen-activated protein kinase kinase 1/2 inhibitor PD98059 or the α5β1-integrin antagonist ATN-161. Furthermore, ATN-161 significantly inhibits uptake of Pro-Hyp into adult tenocytes. Thus, our findings document the molecular basis of the functional benefits of the Pro-Hyp dipeptide in cellular behavior. These dynamic properties of collagen-derived Pro-Hyp dipeptide could lead the way to its application in translational medicine.

Transcription factor scleraxis vitally contributes to progenitor lineage direction in wound healing of adult tendon in mice

Tendon is a dense connective tissue that transmits high mechanical forces from skeletal muscle to bone. The transcription factor scleraxis (Scx) is a highly specific marker of both precursor and mature tendon cells (tenocytes). Mice lacking scx exhibit a specific and virtually complete loss of tendons during development. However, the functional contribution of Scx to wound healing in adult tendon has not yet been fully characterized. Here, using ScxGFP-tracking and loss-of-function systems, we show in an adult mouse model of Achilles tendon injury that paratenon cells, representing a stem cell antigen-1 (Sca-1)-positive and Scx-negative progenitor subpopulation, display Scx induction, migrate to the wound site, and produce extracellular matrix (ECM) to bridge the defect, whereas resident tenocytes exhibit a delayed response. Scx induction in the progenitors is initiated by transforming growth factor β (TGF-β) signaling. scx-deficient mice had migration of Sca-1-positive progenitor cell to the lesion site but impaired ECM assembly to bridge the defect. Mechanistically, scx-null progenitors displayed higher chondrogenic potential with up-regulation of SRY-box 9 (Sox9) coactivator PPAR-γ coactivator-1α (PGC-1α) in vitro, and knock-in analysis revealed that forced expression of full-length scx significantly inhibited Sox9 expression. Accordingly, scx-null wounds formed cartilage-like tissues that developed ectopic ossification. Our findings indicate a critical role of Scx in a progenitor-cell lineage in wound healing of adult mouse tendon. These progenitor cells could represent targets in strategies to facilitate tendon repair. We propose that this lineage-regulatory mechanism in tissue progenitors could apply to a broader set of tissues or biological systems in the body.

Systemic Loss of C-terminal Src Kinase Expression Elicits Spontaneous Suppurative Inflammation in Conditional Knockout Mice

C-terminal Src kinase (Csk) is one of the critical negative regulators of the Src family of kinases. The Src family of kinases are nonreceptor tyrosine kinases that regulate inflammation, cell proliferation, motility, and adhesion. To investigate potential histologic lesions associated with systemic loss of Csk gene activity in adult mice, conditional Csk-knockout mice were examined. Cre-mediated systemic excision of Csk induced by tamoxifen treatment resulted in multiorgan inflammation. Specifically, induction of Csk gene excision with three days of tamoxifen treatment resulted in greater than 90% gene excision. Strikingly, these mice developed enteritis that ranged from minimal and suppurative to severe, fibrinonecrosuppurative and hemorrhagic. Other inflammatory lesions included suppurative pneumonia, gastritis, and myocarditis, and increased numbers of inflammatory cells within the hepatic parenchyma. When tamoxifen treatment was reduced from three days to one day in an effort to lower the level of Csk gene excision and limit lesion development, the mice developed severe suppurative to pyogranulomatous pneumonia and minimal to mild suppurative enteritis. Lesions observed secondary to Csk gene excision suggest important roles for Csk in downregulating the proinflammatory activity of the Src family of kinases and limiting neutrophil-mediated inflammation.

Src Family Kinases Modulate the Loss of Endothelial Barrier Function in Response to TNF-α: Crosstalk with p38 Signaling

Activation of Src Family Kinase (SFK) signaling is required for the increase in endothelial permeability induced by a variety of cytokines and growth factors. However, we previously demonstrated that activation of endogenous SFKs by expression of dominant negative C-terminal Src Kinase (DN-Csk) is not sufficient to decrease endothelial adherens junction integrity. Basal SFK activity has been observed in normal venular endothelia and was not associated with increased basal permeability. The basal SFK activity however was found to contribute to increased sensitivity of the venular endothelium to inflammatory mediator-induced leakage. How SFK activation achieves this is still not well understood. Here, we show that SFK activation renders human dermal microvascular endothelial cells susceptible to low doses of TNF-α. Treatment of DN-Csk-expressing cells with 50 pg/ml TNF-α induced a loss of TEER as well as drastic changes in the actin cytoskeleton and focal adhesion proteins. This synergistic effect was independent of ROCK or NF-κB activity. TNF-α-induced p38 signaling was required for the synergistic effect on barrier function, and activation of the p38 MAPK alone was also able to induce changes in permeability only in monolayers with active SFKs. These results suggest that the activation of endogenous levels of SFK renders the endothelial barrier more susceptible to low, physiologic doses of TNF-α through activation of p38 which leads to a loss of endothelial tight junctions.

Molecular Mechanism Responsible for Fibronectin-controlled Alterations in Matrix Stiffness in Advanced Chronic Liver Fibrogenesis

Fibrosis is characterized by extracellular matrix (ECM) remodeling and stiffening. However, the functional contribution of tissue stiffening to noncancer pathogenesis remains largely unknown. Fibronectin (Fn) is an ECM glycoprotein substantially expressed during tissue repair. Here we show in advanced chronic liver fibrogenesis using a mouse model lacking Fn that, unexpectedly, Fn-null livers lead to more extensive liver cirrhosis, which is accompanied by increased liver matrix stiffness and deteriorated hepatic functions. Furthermore, Fn-null livers exhibit more myofibroblast phenotypes and accumulate highly disorganized/diffuse collagenous ECM networks composed of thinner and significantly increased number of collagen fibrils during advanced chronic liver damage. Mechanistically, mutant livers show elevated local TGF-β activity and lysyl oxidase expressions. A significant amount of active lysyl oxidase is released in Fn-null hepatic stellate cells in response to TGF-β1 through canonical and noncanonical Smad such as PI3 kinase-mediated pathways. TGF-β1-induced collagen fibril stiffness in Fn-null hepatic stellate cells is significantly higher compared with wild-type cells. Inhibition of lysyl oxidase significantly reduces collagen fibril stiffness, and treatment of Fn recovers collagen fibril stiffness to wild-type levels. Thus, our findings indicate an indispensable role for Fn in chronic liver fibrosis/cirrhosis in negatively regulating TGF-β bioavailability, which in turn modulates ECM remodeling and stiffening and consequently preserves adult organ functions. Furthermore, this regulatory mechanism by Fn could be translated for a potential therapeutic target in a broader variety of chronic fibrotic diseases.

Transforming growth factor-β-independent role of connective tissue growth factor in the development of liver fibrosis

We previously identified transforming growth factor (TGF)-β signaling as a fibronectin-independent mechanism of type I collagen fibrillogenesis following adult liver injury. To address the contribution of TGF-β signaling during the development of liver fibrosis, we generated adult mice lacking TGF-β type II receptor (TGF-βIIR) from the liver. TGF-βIIR knockout livers indeed showed a dominant effect in reducing fibrosis, but fibrosis still remained approximately 45% compared with control and fibronectin knockout livers. Unexpectedly, this was accompanied by significant up-regulation of connective tissue growth factor mRNA levels. Organized type I collagen networks in TGF-βIIR knockout livers colocalized well with fibronectin. We provide evidence that elimination of TGF-βIIR is not sufficient to completely prevent liver fibrosis. Our results indicate a TGF-β-independent mechanism of type I collagen production and suggest connective tissue growth factor as its potent mediator. We advocate combined elimination of TGF-β signaling and connective tissue growth factor as a potential therapeutic target by which to attenuate liver fibrosis.

Keratin 5-Cre-driven excision of nonmuscle myosin IIA in early embryo trophectoderm leads to placenta defects and embryonic lethality

In studies initially focused on roles of nonmuscle myosin IIA (NMIIA) in the developing mouse epidermis, we have discovered that a previously described cytokeratin 5 (K5)-Cre gene construct is expressed in early embryo development. Mice carrying floxed alleles of the nonmuscle myosin II heavy chain gene (NMHC IIA(flox/flox)) were crossed with the K5-Cre line. The progeny of newborn pups did not show a Mendelian genotype distribution, suggesting embryonic lethality. Analysis of post-implantation conceptuses from embryonic day (E)9.5 to E13.5 revealed poorly developed embryos and defective placentas, with significantly reduced labyrinth surface area and blood vessel vascularization. These results suggested the novel possibility that the bovine K5 promoter-driven Cre-recombinase was active early in trophoblast-lineage cells that give rise to the placenta. To test this possibility, K5-Cre transgenic mice were crossed with the mT/mG reporter mouse in which activation of GFP expression indicates Cre transgene expression. We observed activation of K5-Cre-driven GFP expression in the ectoplacental cone, in the extraembryonic ectoderm, and in trophoblast giant cells in the E6.5 embryo. In addition, we observed GFP expression at E11.5 to E13.5 in both the labyrinth of the placenta and the yolk sac. NMIIA expression was detected in these same cell types in normal embryos, as well as in E13.5 yolk sac and labyrinth. These findings taken together suggest that NMHC IIA may play critical roles in the early trophoblast-derived ectoplacental cone and extraembryonic ectoderm, as well as in the yolk sac and labyrinth tissues that form later. Our findings are consistent with phenotypes of constitutive NMIIA knockout mice made earlier, that displayed labyrinth and yolk sac-specific defects, but our findings extend those observations by suggesting possible NMIIA roles in trophoblast lineages as well. These results furthermore demonstrate that K5-Cre gene constructs, previously reported to be activated starting at approximately E12.5 in the forming epidermis, may be widely useful as drivers for activation of cre/lox based gene excision in early embryo extraembronic trophoblast tissues as well.

Critical role of factor XIII in the initial stages of carbon tetrachloride-induced adult liver remodeling

The transglutaminase-mediated, covalent cross-linking of proteins is an essential step in tissue remodeling after injury. This process provides tissues with extra rigidity and resistance against proteolytic degradation. Plasma coagulation factor XIII (FXIII) is a transglutaminase that promotes cross-linking of the extracellular matrix (ECM) components fibrin and fibronectin to form a provisional matrix in response to tissue damage. However, the functional requirement for this FXIII-mediated cross-linked provisional matrix in adult tissue remodeling remains to be defined. Although it has been proposed that the formation FXIII-mediated fibrin-fibronectin provisional matrix is a critical step for ECM remodeling, we show in an FXIII subunit A-deficient murine model of acute liver injury that the lack of FXIII subunit A did not interfere with collagen reconstruction and resolution after liver injury. Furthermore, FXIIIA deficiency caused significantly increased hepatocyte apoptosis and a delay in hepatocyte regeneration after injury, which were accompanied by a significantly high induction of p53 expression. These findings suggest novel functions of FXIII that the FXIII-mediated covalently cross-linked matrix could promote survival signals for hepatocytes in adult tissue remodeling.

A novel Src kinase inhibitor reduces tumour formation in a skin carcinogenesis model

The Src family tyrosine kinases are key modulators of cancer cell invasion and metastasis and a number of Src kinase inhibitors are currently in clinical development for the treatment of solid tumours. However, there is growing evidence that Src is also upregulated at very early stages of epithelial cancer development. We have investigated the role of Src in mouse skin, which is one of the most tractable models of epithelial homoeostasis and tumorigenesis. We found that Src protein expression and activity was regulated during the normal hair cycle and was increased specifically during the proliferative anagen phase and also in response to the tumour promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). AZD0530, a selective Src inhibitor, prevented the TPA-induced proliferation of basal keratinocytes both in vivo and in vitro. Moreover, treatment with AZD0530 reduced papilloma formation following the well-established 7,12-dimethylbenz(a)anthracene/TPA skin carcinogenesis protocol but did not inhibit the subsequent proliferation of the papillomas. Furthermore, AZD0530 did not alter the malignant conversion of papillomas to squamous cell carcinoma suggesting a role for Src in early tumour development in the skin carcinogenesis model, rather than at later stages of tumour progression. Src expression and activity were also seen in human actinic keratoses that are hyperproliferative pre-malignant skin lesions, indicating that Src may also play a role in the early stages of human skin tumour development. Thus, Src inhibitors such as AZD0530 may therefore have chemopreventative properties in patients with hyperproliferative epidermal disorders.

Differing Src signaling levels have distinct outcomes in Drosophila

High levels of Src activity are found in a broad spectrum of cancers. The roles of Src and its negative regulator Csk have been extensively studied, although results have often proved contradictory or the relevance to whole organisms is unclear. In Drosophila, overexpression of either Src orthologue resulted in apoptotic cell death, but paradoxically, reducing dCsk activity led to over-proliferation and tissue overgrowth. Here, we show that in Drosophila epithelia in situ, the levels of Src signaling determine the cellular outcome of Src activation. Apoptotic cell death was triggered specifically at high Src signaling levels; lower levels directed antiapoptotic signals while promoting proliferation. Furthermore, our data indicate that expression of kinase-dead Src isoforms do not necessarily act as dominant-negative factors, but can instead increase Src pathway activity, most likely by titrating Csk activity away from endogenous Src. The importance of Src activity levels was emphasized when we examined oncogenic cooperation between Src and Ras: malignant overgrowth was observed specifically when high Src signaling levels were achieved. We propose a model in which low levels of Src signaling promote survival and proliferation during early stages of tumorigenesis, whereas strong Src signaling, coupled with antiapoptotic signals, directs invasive migration and metastasis during advanced tumor stages.