New frontiers in the cGAS-STING intracellular DNA-sensing pathway.

The cGAS-STING intracellular DNA-sensing pathway has emerged as a key element of innate antiviral immunity and a promising therapeutic target. The existence of an innate immune sensor that can be activated by any double-stranded DNA (dsDNA) of any origin raises fundamental questions about how cGAS is regulated and how it responds to "foreign" DNA while maintaining tolerance to ubiquitous self-DNA. In this review, we summarize recent evidence implicating important roles for cGAS in the detection of foreign and self-DNA. We describe two recent and surprising insights into cGAS-STING biology: that cGAS is tightly tethered to the nucleosome and that the cGAMP product of cGAS is an immunotransmitter acting at a distance to control innate immunity. We consider how these advances influence our understanding of the emerging roles of cGAS in the DNA damage response (DDR), senescence, aging, and cancer biology. Finally, we describe emerging approaches to harness cGAS-STING biology for therapeutic benefit.

ABCC1 transporter exports the immunostimulatory cyclic dinucleotide cGAMP.

The DNA sensor cyclic GMP-AMP synthase (cGAS) is important for antiviral and anti-tumor immunity. cGAS generates cyclic GMP-AMP (cGAMP), a diffusible cyclic dinucleotide that activates the antiviral response through the adaptor protein stimulator of interferon genes (STING). cGAMP cannot passively cross cell membranes, but recent advances have established a role for extracellular cGAMP as an "immunotransmitter" that can be imported into cells. However, the mechanism by which cGAMP exits cells remains unknown. Here, we identifed ABCC1 as a direct, ATP-dependent cGAMP exporter in mouse and human cells. We show that ABCC1 overexpression enhanced cGAMP export and limited STING signaling and that loss of ABCC1 reduced cGAMP export and potentiated STING signaling. We demonstrate that ABCC1 deficiency exacerbated cGAS-dependent autoimmunity in the Trex1-/- mouse model of Aicardi-Goutières syndrome. Thus, ABCC1-mediated cGAMP export is a key regulatory mechanism that limits cell-intrinsic activation of STING and ameliorates STING-dependent autoimmune disease.

Transforming Growth Factor-ß and Interleukin-1ß Signaling Pathways Converge on the Chemokine CCL20 Promoter.

CCL20 is the only chemokine ligand for the chemokine receptor CCR6, which is expressed by the critical antigen presenting cells, dendritic cells. Increased expression of CCL20 is likely involved in the increased recruitment of dendritic cells observed in fibroinflammatory diseases such as chronic obstructive pulmonary disease (COPD). CCL20 expression is increased by the proinflammatory cytokine IL-1ß. We have determined that IL-1ß-dependent CCL20 expression is also dependent on the multifunctional cytokine TGF-ß. TGF-ß is expressed in a latent form that must be activated to function, and activation is achieved through binding to the integrin αvß8 (itgb8). Here we confirm correlative increases in αvß8 and IL-1ß with CCL20 protein in lung parenchymal lysates of a large cohort of COPD patients. How IL-1ß- and αvß8-mediated TGF-ß activation conspire to increase fibroblast CCL20 expression remains unknown, because these pathways have not been shown to directly interact. We evaluate the 5'-flanking region of CCL20 to determine that IL-1ß-driven CCL20 expression is dependent on αvß8-mediated activation of TGF-ß. We identify a TGF-ß-responsive element (i.e. SMAD) located on an upstream enhancer of the human CCL20 promoter required for efficient IL-1ß-dependent CCL20 expression. By chromatin immunoprecipitation, this upstream enhancer complexes with the p50 subunit of NF-κB on a NF-κB-binding element close to the transcriptional start site of CCL20. These interactions are confirmed by electromobility shift assays in nuclear extracts from human lung fibroblasts. These data define a mechanism by which αvß8-dependent activation of TGF-ß regulates IL-1ß-dependent CCL20 expression in COPD.

Computational design of constitutively active cGAS.

Cyclic GMP-AMP synthase (cGAS) is a pattern recognition receptor critical for the innate immune response to intracellular pathogens, DNA damage, tumorigenesis and senescence. Binding to double-stranded DNA (dsDNA) induces conformational changes in cGAS that activate the enzyme to produce 2'-3' cyclic GMP-AMP (cGAMP), a second messenger that initiates a potent interferon (IFN) response through its receptor, STING. Here, we combined two-state computational design with informatics-guided design to create constitutively active, dsDNA ligand-independent cGAS (CA-cGAS). We identified CA-cGAS mutants with IFN-stimulating activity approaching that of dsDNA-stimulated wild-type cGAS. DNA-independent adoption of the active conformation was directly confirmed by X-ray crystallography. In vivo expression of CA-cGAS in tumor cells resulted in STING-dependent tumor regression, demonstrating that the designed proteins have therapeutically relevant biological activity. Our work provides a general framework for stabilizing active conformations of enzymes and provides CA-cGAS variants that could be useful as genetically encoded adjuvants and tools for understanding inflammatory diseases.

Interleukin-1beta induces increased transcriptional activation of the transforming growth factor-beta-activating integrin subunit beta8 through altering chromatin architecture.

The integrin αvß8 is a cell surface receptor for the latent domain (LAP) of the multifunctional cytokine TGF-ß. Through its association with LAP, TGF-ß is maintained in a latent form that must be activated to function. Binding to the integrin αvß8 with subsequent metalloproteolytic cleavage of LAP represents a major mechanism of TGF-ß activation in vivo. Altered expression of the integrin ß8 subunit (ITGB8) is found in human chronic obstructive pulmonary disease, cancers, and brain vascular malformations. We have previously shown that the proinflammatory cytokine interleukin-1ß (IL-1ß) increases ITGB8 expression on lung fibroblasts, which increases αvß8-mediated TGF-ß activation in fibrosis and pathologic inflammation. Here we report the mechanism of increased ITGB8 expression by IL-1ß. Our data support a model where the chromatin architecture of the ITGB8 core promoter is altered by nucleosomal repositioning that enhances the interaction of an AP1 complex (containing c-Jun and ATF2). This repositioning is caused by the dissociation of HDAC2 with the ITGB8 core promoter, leading to increased histone H4 acetylation and a loosening of nucleosomal-DNA interactions allowing "opening" of the chromatin structure and increased association of c-Jun and ATF-2. These changes are mediated through NFκB- and p38-dependent pathways. Ultimately, these events culminate in increasing ITGB8 transcription, αvß8 surface expression, and αvß8-mediated TGFß activation.

Transcription of the transforming growth factor beta activating integrin beta8 subunit is regulated by SP3, AP-1, and the p38 pathway.

Integrin alphavbeta8 is a critical regulator of transforming growth factor beta activation in vasculogenesis during development, immune regulation, and endothelial/epithelial-mesenchymal homeostasis. Recent studies have suggested roles for integrin beta8 in the pathogenesis of chronic obstructive pulmonary disease, brain arteriovenous malformations, and select cancers (Araya, J., Cambier, S., Markovics, J. A., Wolters, P., Jablons, D., Hill, A., Finkbeiner, W., Jones, K., Broaddus, V. C., Sheppard, D., Barzcak, A., Xiao, Y., Erle, D. J., and Nishimura, S. L. (2007) J. Clin. Invest. 117, 3551-3562; Su, H., Kim, H., Pawlikowska, L., Kitamura, H., Shen, F., Cambier, S., Markovics, J., Lawton, M. T., Sidney, S., Bollen, A. W., Kwok, P. Y., Reichardt, L., Young, W. L., Yang, G. Y., and Nishimura, S. L. (2010) Am. J. Pathol. 176, 1018-1027; Culhane, A. C., and Quackenbush, J. (2009) Cancer Res. 69, 7480-7485; Cambier, S., Mu, D. Z., O'Connell, D., Boylen, K., Travis, W., Liu, W. H., Broaddus, V. C., and Nishimura, S. L. (2000) Cancer Res. 60, 7084-7093). Here we report the first identification and characterization of the promoter for ITGB8. We show that a SP binding site and a cyclic AMP response element (CRE) in the ITGB8 core promoter are required for its expression and that Sp1, Sp3, and several AP-1 transcription factors form a complex that binds to these sites in a p38-dependent manner. Furthermore, we demonstrate the requirement for Sp3, ATF-2, and p38 for the transcription and protein expression of integrin beta8. Additionally, reduction of SP3 or inhibition of p38 blocks alphavbeta8-mediated transforming growth factor beta activation. These results place integrin beta8 expression and activity under the control of ubiquitous transcription factors in a stress-activated and pro-inflammatory pathway.

Reduced expression of integrin alphavbeta8 is associated with brain arteriovenous malformation pathogenesis.

Brain arteriovenous malformations (BAVMs) are a rare but potentially devastating hemorrhagic disease. Transforming growth factor-beta signaling is required for proper vessel development, and defective transforming growth factor-beta superfamily signaling has been implicated in BAVM pathogenesis. We hypothesized that expression of the transforming growth factor-beta activating integrin, alphavbeta8, is reduced in BAVMs and that decreased beta8 expression leads to defective neoangiogenesis. We determined that beta8 protein expression in perivascular astrocytes was reduced in human BAVM lesional tissue compared with controls and that the angiogenic response to focal vascular endothelial growth factor stimulation in adult mouse brains with local Cre-mediated deletion of itgb8 and smad4 led to vascular dysplasia in newly formed blood vessels. In addition, common genetic variants in ITGB8 were associated with BAVM susceptibility, and ITGB8 genotypes associated with increased risk of BAVMs correlated with decreased beta8 immunostaining in BAVM tissue. These three lines of evidence from human studies and a mouse model suggest that reduced expression of integrin beta8 may be involved in the pathogenesis of sporadic BAVMs.

The side effects of phototherapy for neonatal jaundice: what do we know? What should we do?

Neonatal phototherapy (NNPT), a noninvasive, easily available therapy, has been widely used for the treatment of neonatal jaundice for more than half a century. Its efficiency in decreasing plasma bilirubin concentration is well documented, and NNPT leads to greatly reduced exchange transfusion rates for neonates with hyperbilirubinemia. It is generally accepted that the side effects of NNPT are not serious and seem to be well controlled. This review will focus on these possible side effects as well as the approaches to minimize them.

Squamous metaplasia amplifies pathologic epithelial-mesenchymal interactions in COPD patients.

Squamous metaplasia (SM) is common in smokers and is associated with airway obstruction in chronic obstructive pulmonary disease (COPD). A major mechanism of airway obstruction in COPD is thickening of the small airway walls. We asked whether SM actively contributes to airway wall thickening through alteration of epithelial-mesenchymal interactions in COPD. Using immunohistochemical staining, airway morphometry, and fibroblast culture of lung samples from COPD patients; genome-wide analysis of an in vitro model of SM; and in vitro modeling of human airway epithelial-mesenchymal interactions, we provide evidence that SM, through the increased secretion of IL-1beta, induces a fibrotic response in adjacent airway fibroblasts. We identify a pivotal role for integrin-mediated TGF-beta activation in amplifying SM and driving IL-1beta-dependent profibrotic mesenchymal responses. Finally, we show that SM correlates with increased severity of COPD and that fibroblast expression of the integrin alpha(v)beta(8), which is the major mediator of airway fibroblast TGF-beta activation, correlated with disease severity and small airway wall thickening in COPD. Our findings have identified TGF-beta as a potential therapeutic target for COPD.

Human DNA-PK activates a STING-independent DNA sensing pathway.

Detection of intracellular DNA by the cGAS-STING pathway activates a type I interferon-mediated innate immune response that protects from virus infection. Whether there are additional DNA sensing pathways, and how such pathways might function, remains controversial. We show here that humans-but not laboratory mice-have a second, potent, STING-independent DNA sensing pathway (SIDSP). We identify human DNA-dependent protein kinase (DNA-PK) as the sensor of this pathway and demonstrate that DNA-PK activity drives a robust and broad antiviral response. We show that the E1A oncoprotein of human adenovirus 5 and the ICP0 protein of herpes simplex virus 1 block this response. We found heat shock protein HSPA8/HSC70 as a target for inducible phosphorylation in the DNA-PK antiviral pathway. Last, we demonstrate that DNA damage and detection of foreign DNA trigger distinct modalities of DNA-PK activity. These findings reveal the existence, sensor, a specific downstream target, and viral antagonists of a SIDSP in human cells.

The integrin alpha(v)beta8 mediates epithelial homeostasis through MT1-MMP-dependent activation of TGF-beta1.

Integrins, matrix metalloproteases (MMPs), and the cytokine TGF-beta have each been implicated in homeostatic cell behaviors such as cell growth and matrix remodeling. TGF-beta exists mainly in a latent state, and a major point of homeostatic control is the activation of TGF-beta. Because the latent domain of TGF-beta1 possesses an integrin binding motif (RGD), integrins have the potential to sequester latent TGF-beta (SLC) to the cell surface where TGF-beta activation could be locally controlled. Here, we show that SLC binds to alpha(v)beta8, an integrin expressed by normal epithelial and neuronal cells in vivo. This binding results in the membrane type 1 (MT1)-MMP-dependent release of active TGF-beta, which leads to autocrine and paracrine effects on cell growth and matrix production. These data elucidate a novel mechanism of cellular homeostasis achieved through the coordination of the activities of members of three major gene families involved in cell-matrix interactions.

Tight nuclear tethering of cGAS is essential for preventing autoreactivity.

cGAS is an intracellular innate immune sensor that detects double-stranded DNA. The presence of billions of base pairs of genomic DNA in all nucleated cells raises the question of how cGAS is not constitutively activated. A widely accepted explanation for this is the sequestration of cGAS in the cytosol, which is thought to prevent cGAS from accessing nuclear DNA. Here, we demonstrate that endogenous cGAS is predominantly a nuclear protein, regardless of cell cycle phase or cGAS activation status. We show that nuclear cGAS is tethered tightly by a salt-resistant interaction. This tight tethering is independent of the domains required for cGAS activation, and it requires intact nuclear chromatin. We identify the evolutionarily conserved tethering surface on cGAS and we show that mutation of single amino acids within this surface renders cGAS massively and constitutively active against self-DNA. Thus, tight nuclear tethering maintains the resting state of cGAS and prevents autoreactivity.

Selective targeting of TGF-ß activation to treat fibroinflammatory airway disease.

Airway remodeling, caused by inflammation and fibrosis, is a major component of chronic obstructive pulmonary disease (COPD) and currently has no effective treatment. Transforming growth factor-ß (TGF-ß) has been widely implicated in the pathogenesis of airway remodeling in COPD. TGF-ß is expressed in a latent form that requires activation. The integrin αvß8 (encoded by the itgb8 gene) is a receptor for latent TGF-ß and is essential for its activation. Expression of integrin αvß8 is increased in airway fibroblasts in COPD and thus is an attractive therapeutic target for the treatment of airway remodeling in COPD. We demonstrate that an engineered optimized antibody to human αvß8 (B5) inhibited TGF-ß activation in transgenic mice expressing only human and not mouse ITGB8. The B5 engineered antibody blocked fibroinflammatory responses induced by tobacco smoke, cytokines, and allergens by inhibiting TGF-ß activation. To clarify the mechanism of action of B5, we used hydrodynamic, mutational, and electron microscopic methods to demonstrate that αvß8 predominantly adopts a constitutively active, extended-closed headpiece conformation. Epitope mapping and functional characterization of B5 revealed an allosteric mechanism of action due to locking-in of a low-affinity αvß8 conformation. Collectively, these data demonstrate a new model for integrin function and present a strategy to selectively target the TGF-ß pathway to treat fibroinflammatory airway diseases.

Mouse and human lung fibroblasts regulate dendritic cell trafficking, airway inflammation, and fibrosis through integrin αvß8-mediated activation of TGF-ß.

The airway is a primary portal of entry for noxious environmental stimuli that can trigger airway remodeling, which contributes significantly to airway obstruction in chronic obstructive pulmonary disease (COPD) and chronic asthma. Important pathologic components of airway remodeling include fibrosis and abnormal innate and adaptive immune responses. The positioning of fibroblasts in interstitial spaces suggests that they could participate in both fibrosis and chemokine regulation of the trafficking of immune cells such as dendritic cells, which are crucial antigen-presenting cells. However, physiological evidence for this dual role for fibroblasts is lacking. Here, in two physiologically relevant models - conditional deletion in mouse fibroblasts of the TGF-ß-activating integrin αvß8 and neutralization of αvß8 in human COPD fibroblasts - we have elucidated a mechanism whereby lung fibroblast chemokine secretion directs dendritic cell trafficking, in a manner that is critically dependent on αvß8-mediated activation of TGF-ß by fibroblasts. Our data therefore indicate that fibroblasts have a crucial role in regulating both fibrotic and immune responses in the lung.

Integrin-mediated transforming growth factor-beta activation regulates homeostasis of the pulmonary epithelial-mesenchymal trophic unit.

Trophic interactions between pulmonary epithelial and mesenchymal cell types, known as the epithelial-mesenchymal trophic unit (EMTU), are crucial in lung development and lung disease. Transforming growth factor (TGF)-beta is a key factor in mediating these interactions, but it is expressed in a latent form that requires activation to be functional. Using intact fetal tracheal tissue and primary cultures of fetal tracheal epithelial cells and fibroblasts, we demonstrate that a subset of integrins, alpha(v)beta(6) and alpha(v)beta(8), are responsible for almost all of the TGF-beta activation in the EMTU. Both alpha(v)beta(8) and alpha(v)beta(6) contribute to fetal tracheal epithelial activation of TGF-beta, whereas only alpha(v)beta(8) contributes to fetal tracheal fibroblast activation of TGF-beta. Interestingly, fetal tracheal epithelial alpha(v)beta(8)-mediated TGF-beta activation can be enhanced by phorbol esters, likely because of the increased activity of MT1-MMP, an essential co-factor in alpha(v)beta(8)-mediated activation of TGF-beta. Autocrine alpha(v)beta(8)-mediated TGF-beta activation by fetal tracheal fibroblasts results in suppression of both transcription and secretion of hepatocyte growth factor, which is sufficient to affect phosphorylation of the airway epithelial hepatocyte growth factor receptor, c-Met, as well as airway epithelial proliferation in a co-culture model of the EMTU. These findings elucidate the function and complex regulation of integrin-mediated activation of TGF-beta within the EMTU.

Integrin alpha(v)beta8-mediated activation of transforming growth factor-beta by perivascular astrocytes: an angiogenic control switch.

Brain hemorrhage is a severe complication of both neoplastic and nonneoplastic brain disease. Mice deficient in the alpha(v)beta8 integrin display defective brain vessel formation resulting in hemorrhage and perinatal death, but the mechanism of brain hemorrhage is unknown. Because the alpha(v)beta8 integrin is expressed by astrocytes and not expressed by endothelium, paracrine interactions between astrocytes and endothelial cells could contribute to the maintenance of brain vessel integrity. We have investigated the mechanisms underlying astrocytic-endothelial paracrine signaling and have found that integrin-mediated activation of transforming growth factor (TGF)-beta by astrocytes influences endothelial cell function. Thus, we identified the integrin alpha(v)beta8 in human perivascular glial cell processes surrounding developing blood vessels. Human astrocytic alpha(v)beta8 was a major cell surface receptor for latent TGF-beta, and alpha(v)beta8-dependent activation of TGF-beta was the major mechanism of TGF-beta activation in primary cultures of astrocytes or freshly dissociated fetal brain cells. This activation of TGF-beta was sufficient to inhibit endothelial migration in fibrin gels and to alter expression of genes affecting proteolytic and angiogenic pathways. Taken together, our data suggest that astrocytic alpha(v)beta8 acts as a central regulator of brain vessel homeostasis through regulation of TGF-beta activation and expression of TGF-beta-responsive genes that promote vessel differentiation and stabilization, most notably plasminogen activator inhibitor-1 and thrombospondin-1.

A 50-A separation of the integrin alpha v beta 3 extracellular domain C termini reveals an intermediate activation state.

The integrin alpha(v)beta(3) has been shown to exist in low and high affinity conformations. Activation to the high affinity state is thought to depend on the "switchblade-like" opening, from a low affinity bent conformation with a closed headpiece to an extended form of the integrin with an open headpiece. Activation has been shown to depend on separation of the cytoplasmic domains. How cytoplasmic domain separation is related to separation of the transmembrane domains is unknown, and the distance of separation of the transmembrane domains required for activation has not been defined. A constrained secreted form of alpha(v)beta(3) was engineered that introduced a 50-A separation of the integrin C-terminal tails of the extracellular domains of the alpha(v) and beta(3) subunits. Receptor binding and recognition by ligand-induced binding state (LIBS) monoclonal antibodies demonstrated that the mutant receptor was locked into a low affinity state that was likely in a partially extended conformation but with a closed headpiece. In the presence of RGD peptide, the constrained receptor was able to fully extend, as determined by full exposure of LIBS epitopes. In the presence of the appropriate LIBS antibody, high affinity ligand binding of the constrained receptor was achieved. The results support the existence of transient intermediate activation states of secreted alpha(v)beta(3). Furthermore, these results with the secreted alpha(v)beta(3) receptor support a model for the full-length membrane-bound form of alpha(v)beta(3), whereby a 50-A lateral separation of the integrin alpha(v) and beta(3) transmembrane domains would be sufficient to enforce the switchblade-like opening to the extended conformation but insufficient for full receptor activation.

Integrin alphavbeta8 functions as a receptor for foot-and-mouth disease virus: role of the beta-chain cytodomain in integrin-mediated infection.

Field isolates of foot-and-mouth disease virus (FMDV) have been shown to use three alphav integrins, alphavbeta1, alphavbeta3, and alphavbeta6, as cellular receptors. Binding to the integrin is mediated by a highly conserved RGD motif located on a surface-exposed loop of VP1. The RGD tripeptide is recognized by several other members of the integrin family, which therefore have the potential to act as receptors for FMDV. Here we show that SW480 cells are made susceptible to FMDV following transfection with human beta8 cDNA and expression of alphavbeta8 at the cell surface. The involvement of alphavbeta8 in infection was confirmed by showing that virus binding and infection of the transfected cells are inhibited by RGD-containing peptides and by function-blocking monoclonal antibodies specific for either the alphavbeta8 heterodimer or the alphav chain. Similar results were obtained with a chimeric alphavbeta8 including the beta6 cytodomain (alphavbeta8/6), showing that the beta6 cytodomain can substitute efficiently for the corresponding region of beta8. In contrast, virus binding to alphavbeta6 including the beta8 cytodomain (alphavbeta6/8) was lower than that of the wild-type integrin, and this binding did not lead to infection. Further, the alphavbeta6 chimera was recognized poorly by antibodies specific for the ectodomain of alphavbeta6 and displayed a relaxed sequence-binding specificity relative to that of wild-type integrin. These data suggest that the beta6 cytodomain is important for maintaining alphavbeta6 in a conformation required for productive infection by FMDV.

Integrin alphavbeta8-mediated activation of transforming growth factor-beta inhibits human airway epithelial proliferation in intact bronchial tissue.

Transforming growth factor (TGF)-beta is a potent multifunctional cytokine that is an essential regulator of epithelial proliferation. Because TGF-beta is expressed almost entirely in a latent state in vivo, a major source of regulation of TGF-beta function is its activation. A subset of integrins, alphavbeta8 and alphavbeta6, which are expressed in the human airway, has recently been shown to activate latent TGF-beta in vitro, suggesting a regulatory role for integrins in TGF-beta function in vivo. Here we have developed a novel, biologically relevant experimental model of human airway epithelium using intact human bronchial tissue. We have used this model to determine the function of integrin-mediated activation of TGF-beta in the airway. In human bronchial fragments cultured in vitro, authentic epithelial-stromal interactions were maintained and integrin and TGF-beta expression profiles correlated with profiles found in normal lung. In addition, in this model, we found that either the integrin alphavbeta8 or TGF-beta could inhibit airway epithelial cell proliferation. Furthermore, we found that one mechanism of integrin-alphavbeta8-dependent inhibition of cell proliferation was through activation of TGF-beta because anti-beta8 antibody blocked the majority (76%) of active TGF-beta released from bronchial fragments. These data provide compelling evidence for a functional role for integrin-mediated activation of TGF-beta in control of human airway epithelial proliferation in vivo.