COP9 Signalosome Promotes Neointimal Hyperplasia via Deneddylation and CSN5-Mediated Nuclear Export

BACKGROUND

Neointimal hyperplasia (NH) is a common pathological response to vascular injury and mediated primarily by vascular smooth muscle cell (VSMC) migration and proliferation. The COP9 signalosome (CSN) is formed by 8 canonical subunits (CSN1 through CSN8) with its deneddylation activity residing in CSN5. Each or some of CSN subunits may have deneddylation-independent function. Despite strong evidence linking the CSN to cell cycle regulation in cancer cells, the role of the CSN in vascular biology remains obscure.

METHODS

Neointimal CSN5 expression in the lung tissue of pulmonary hypertension (PAH) patients was assessed with immunohistochemistry. Adult mice with smooth muscle cell-restricted CSN5 knockout (CSN5-SMKO) or CSN8 hypomorphism (CSN8-hypo) and cultured mouse VSMCs were studied to determine the role and governing mechanisms of the CSN in NH. NH was induced by ligation of the left common carotid artery (LCCA) and PDGF-BB stimulation was used to mimic the vascular injury in cell cultures.

RESULTS

Remarkably higher CSN5 levels were detected in the neointimal VSMCs of the pulmonary arteries of human PAH. LCCA ligation induced NH and significantly increased the mRNA and protein levels of CSN subunits in the LCCA wall of adult wild type mice. CSN5-SMKO impaired Cullin deneddylation and the nuclear export of p27 in vessel walls and markedly inhibited VSMC proliferation in mice. On the contrary, CSN8-hypo significantly exacerbated NH and VSMC proliferation in vivo and in cellulo . Cytoplasmic CSN5 mini-complexes and the nuclear export of p27 were significantly increased in CSN8-hypo mouse vessels and cultured CSN8-hypo VSMCs. Nuclear export inhibition with leptomycin attenuated the PDGF-BB-induced increases in VSMC proliferation in both CSN8-hypo and control VSMCs. Further, genetically disabling CSN5 nuclear export but not disabling CSN5 deneddylase activity suppressed the hyperproliferation and restored p27 nuclear localization in CSN8 hypomorphic VSMCs. Interestingly, CSN deneddylase inhibition by CSN5i-3 did not alter the hyperproliferation of cultured CSN8-hypo VSMCs but suppressed wild type VSMC proliferation in cellulo and in vivo and blocked neointimal formation in wild type mice.

CONCLUSION

The CSN promotes VSMC proliferation and NH in injured vessels through deneddylation activity and CSN5-mediated nuclear export.

JAB1 deletion in oligodendrocytes causes senescence-induced inflammation and neurodegeneration in mice

Oligodendrocytes are the primary target of demyelinating disorders and progressive neurodegenerative changes may evolve in the CNS. DNA damage and oxidative stress are considered key pathogenic events, but the underlying molecular mechanisms remain unclear. Moreover, animal models do not fully recapitulate human diseases, complicating the path to effective treatments. Here we report that mice with cell autonomous deletion of the nuclear COP9 signalosome component CSN5 (JAB1) in oligodendrocytes develop DNA damage and defective DNA repair in myelinating glial cells. Interestingly, oligodendrocytes lacking JAB1 expression underwent a senescence-like phenotype that fostered chronic inflammation and oxidative stress. These mutants developed progressive CNS demyelination, microglia inflammation and neurodegeneration, with severe motor deficits and premature death. Notably, blocking microglia inflammation did not prevent neurodegeneration, whereas the deletion of p21CIP1 but not p16INK4a pathway ameliorated the disease. We suggest that senescence is key to sustaining neurodegeneration in demyelinating disorders and may be considered a potential therapeutic target.

COP9 signalosome complex subunit 5, an IFT20 binding partner, is essential to maintain male germ cell survival and acrosome biogenesis†

Intraflagellar transport protein 20 (IFT20) is essential for spermatogenesis in mice. We discovered that COPS5 was a major binding partner of IFT20. COPS5 is the fifth component of the constitutive photomorphogenic-9 signalosome (COP9), which is involved in protein ubiquitination and degradation. COPS5 is highly abundant in mouse testis. Mice deficiency in COPS5 specifically in male germ cells showed dramatically reduced sperm numbers and were infertile. Testis weight was about one third compared to control adult mice, and germ cells underwent significant apoptosis at a premeiotic stage. Testicular poly (ADP-ribose) polymerase-1, a protein that helps cells to maintain viability, was dramatically decreased, and Caspase-3, a critical executioner of apoptosis, was increased in the mutant mice. Expression level of FANK1, a known COPS5 binding partner, and a key germ cell apoptosis regulator was also reduced. An acrosome marker, lectin PNA, was nearly absent in the few surviving spermatids, and expression level of sperm acrosome associated 1, another acrosomal component was significantly reduced. IFT20 expression level was significantly reduced in the Cops5 knockout mice, and it was no longer present in the acrosome, but remained in the Golgi apparatus of spermatocytes. In the conditional Ift20 mutant mice, COPS5 localization and testicular expression levels were not changed. COP9 has been shown to be involved in multiple signal pathways, particularly functioning as a co-factor for protein ubiquitination. COPS5 is believed to maintain normal spermatogenesis through multiple mechanisms, including maintaining male germ cell survival and acrosome biogenesis, possibly by modulating protein ubiquitination.

A Microfluidic Human Model of Blood-Brain Barrier Employing Primary Human Astrocytes

The neurovascular unit (NVU) is the most important biological barrier between vascular districts and central nervous system (CNS) parenchyma, which maintains brain homeostasis, protects the CNS from pathogens penetration, and mediates neuroimmune communication. T lymphocytes migration across the blood-brain barrier is heavily affected in different brain diseases, representing a major target for novel drug development. In vitro models of NVU could represent a primary tool to investigate the molecular events occurring at this interface. To move toward the establishment of personalized therapies, a patient-related NVU-model is set, incorporating human primary astrocytes integrated into a microfluidic platform. The model is morphologically and functionally characterized, proving to be an advantageous tool to investigate human T lymphocytes transmigration and thus the efficacy of potential novel drugs affecting this process.

Renal COP9 Signalosome Deficiency Alters CUL3-KLHL3-WNK Signaling Pathway

BACKGROUND

The familial hyperkalemic hypertension (FHHt) cullin 3 (CUL3) mutant does not degrade WNK kinases normally, thereby leading to thiazide-sensitive Na-Cl cotransporter (NCC) activation. CUL3 mutant (CUL3Δ9) does not bind normally to the COP9 signalosome (CSN), a deneddylase involved in regulating cullin-RING ligases. CUL3Δ9 also caused increased degradation of the CUL3-WNK substrate adaptor kelch-like 3 (KLHL3). Here, we sought to determine how defective CSN action contributes to the CUL3Δ9 phenotype.

METHODS

The Pax8/LC1 mouse system was used to generate mice in which the catalytically active CSN subunit, Jab1, was deleted only along the nephron, after full development (KS-Jab1 ^-/-).

RESULTS

Western blot analysis demonstrated that Jab1 deletion increased the abundance of neddylated CUL3. Moreover, total CUL3 expression was reduced, suggesting decreased CUL3 stability. KLHL3 was almost completely absent in KS-Jab1 ^-/- mice. Conversely, the protein abundances of WNK1, WNK4, and SPAK kinases were substantially higher. Activation of WNK4, SPAK, and OSR1 was indicated by higher phosphorylated protein levels and translocation of the proteins into puncta, as observed by immunofluorescence. The ratio of phosphorylated NCC to total NCC was also higher. Surprisingly, NCC protein abundance was low, likely contributing to hypokalemia and Na⁺ and K⁺ wasting. Additionally, long-term Jab1 deletion resulted in kidney damage.

CONCLUSIONS

Together, the results indicate that deficient CSN binding contributes importantly to the FHHt phenotype. Although defective CUL3Δ9-faciliated WNK4 degradation likely contributes, dominant effects on KLHL3 may be a second factor that is necessary for the phenotype.

T cell LFA-1-induced proinflammatory mRNA stabilization is mediated by the p38 pathway kinase MK2 in a process regulated by hnRNPs C, H1 and K

Activation of the β2 integrin lymphocyte function-associated antigen-1 (LFA-1) in T cells induces stabilization of proinflammatory AU-rich element (ARE)-bearing mRNAs, by triggering the nuclear-to-cytoplasmic translocation of the mRNA-binding and -stabilizing protein HuR. However, the mechanism by which LFA-1 engagement controls HuR localization is not known. Here, we identify and characterize four key regulators of LFA-1-induced changes in HuR activity: the p38 pathway kinase MK2 and the constitutive nuclear proteins hnRNPs C, H1 and K. LFA-1 engagement results in rapid, sequential activation of p38 and MK2. Post-LFA-1 activation, MK2 inducibly associates with both hnRNPC and HuR, resulting in the dissociation of HuR from hnRNPs C, H1 and K. Freed from the three hnRNPs, HuR translocates from the nucleus to the cytoplasm, and mediates the stabilization of labile cytokine transcripts. Our results suggest that the modulation of T cell cytokine mRNA half-life is an intricate process that is negatively regulated by hnRNPs C, H1 and K and requires MK2 as a critical activator.

Rac2 Modulates Atherosclerotic Calcification by Regulating Macrophage Interleukin-1β Production

OBJECTIVE

The calcium composition of atherosclerotic plaque is thought to be associated with increased risk for cardiovascular events, but whether plaque calcium itself is predictive of worsening clinical outcomes remains highly controversial. Inflammation is likely a key mediator of vascular calcification, but immune signaling mechanisms that promote this process are minimally understood.

APPROACH AND RESULTS

Here, we identify Rac2 as a major inflammatory regulator of signaling that directs plaque osteogenesis. In experimental atherogenesis, Rac2 prevented progressive calcification through its suppression of Rac1-dependent macrophage interleukin-1β (IL-1β) expression, which in turn is a key driver of vascular smooth muscle cell calcium deposition by its ability to promote osteogenic transcriptional programs. Calcified coronary arteries from patients revealed decreased Rac2 expression but increased IL-1β expression, and high coronary calcium burden in patients with coronary artery disease was associated with significantly increased serum IL-1β levels. Moreover, we found that elevated IL-1β was an independent predictor of cardiovascular death in those subjects with high coronary calcium burden.

CONCLUSIONS

Overall, these studies identify a novel Rac2-mediated regulation of macrophage IL-1β expression, which has the potential to serve as a powerful biomarker and therapeutic target for atherosclerosis.

Abstract 655: Rac2 is a Key Modulator of IL-1β -dependent Atherosclerotic Plaque Calcification

The calcium composition of atherosclerotic plaque has predictive value for increased risk of cardiovascular events. Inflammation is associated with atherosclerotic calcification, but the immune signaling that regulates calcium mineralization in plaque is minimally understood. The hematopoietic Rac family member, Rac2, modulates the activation of immune cells and has potential to influence plaque osteogenesis. Both aortic plaque from ApoE -/- mice fed a high fat diet and coronary plaque from patients revealed increased Rac1:Rac2 expression ratios, driven by dynamic Rac2 expression, to be associated with calcified plaque. On high fat diet, Rac2 -/- ApoE -/- mice demonstrated comparable serum cholesterol and plaque burden relative to ApoE -/- mice, but histology identified differences in plaque structure and cellularity. MicroCT and calcium-targeted imaging identified increased atherosclerotic calcification, which was associated with elevated expression of osteogenic transcription factors and was dependent on the hematopoietic compartment. Calcified plaque expressed higher IL-1β mRNA levels, and serum revealed increased IL-1β protein concentrations. Rac2 -/- ApoE -/- macrophages demonstrated increased activation of Rac1 and consequent Rac1-dependent IL-1β secretion. Downstream of Rac1, NF-κB and reactive oxygen species (ROS) signaling drove IL-1β production by increasing IL-1β mRNA expression and caspase1 activation. Cultured mouse aorta smooth muscle cells mineralized calcium in an IL-1β dose-dependent manner, and the enhanced atherosclerotic calcification in vivo was inhibited by IL-1 receptor antagonist, confirming a cause-and-effect relationship. In patients with stable coronary artery disease, high coronary calcium burden was associated with increased serum IL-1β, and patients with combined elevations in calcium and IL-1β had more events driven by higher mortality, reinforcing the relevance of this inflammatory calcification signaling axis to human disease. Therapeutic targeting of IL-1β expression through the balance of Rac activation has potential to impact patient care by modulating atherosclerotic calcification and consequent cardiovascular events.

A novel device to concurrently assess leukocyte extravasation and interstitial migration within a defined 3D environment

Leukocyte extravasation and interstitial migration are key events during inflammation. Traditional in vitro techniques address only specific steps of cell recruitment to tissues and fail to recapitulate the whole process in an appropriate three-dimensional (3D) microenvironment. Herein, we describe a device that enables us to qualitatively and quantitatively assess in 4D the interdependent steps underlying leukocyte trafficking in a close-to-physiology in vitro context. Real-time tracking of cells, from initial adhesion to the endothelium and subsequent diapedesis to interstitial migration towards the source of the chemoattractant within the 3D collagen matrix, is enabled by the use of optically transparent porous membranes laid over the matrix. Unique features of the device, such as the use of non-planar surfaces and the contribution of physiological flow to the establishment of a persistent chemoattractant gradient, were assessed by numerical simulations and validated by proof-of-concept, simultaneous testing of differentially treated primary mouse neutrophils. This microfluidic platform offers new and versatile tools to thoroughly investigate the stepwise process of circulating cell recruitment to target tissues in vitro and to test novel therapeutics targeting various steps of the process.

Loss of jab1 in osteochondral progenitor cells severely impairs embryonic limb development in mice

The transcriptional cofactor Jab1 controls cell proliferation, apoptosis, and differentiation in diverse developmental processes by regulating the activity of various transcription factors. To determine the role of Jab1 during early limb development, we developed a novel Jab1(flox/flox) ; Prx1-Cre conditional Knockout (cKO) mutant mouse model in which Jab1 was deleted in the osteochondral progenitor cells of the limb buds. Jab1 cKO mutant mice displayed drastically shortened limbs at birth. The short-limb defect became apparent in Jab1 cKO mutants at E15.5 and increasingly worsened thereafter. By E18.5, Jab1 cKO mutant mice exhibited significantly shorter limbs with: very few hypertrophic chondrocytes, disorganized chondrocyte columns, much smaller primary ossification centers, and significantly increased apoptosis. Real-time RT-PCR analysis showed decreased expression of Sox9, Col2a1, Ihh, and Col10a1 in Jab1 cKO mutant long bones, indicating impaired chondrogenesis. Furthermore, in a micromass culture model of early limb mesenchyme cells, alcian blue staining showed a significant decrease in chondrogenesis in Jab1 cKO limb bud cells. The expression of Sox9 and its downstream targets Col2a1 and Aggrecan, as well as BMP signaling downstream targets, Noggin, Id1, and Ihh, were significantly decreased in Jab1 cKO micromass cultures. Moreover, over-expression of SOX9 in Jab1 cKO micromass cultures partially restored Col2a1and Aggrecan expression. Jab1-deficient micromass cultures also exhibited decreased BMP signaling response and reduced BMP-specific reporter activity ex vivo. In summary, our study demonstrates that Jab1 is an essential regulator of early embryonic limb development in vivo, likely in part by co-activating Sox9 and BMP signaling.

Chemokine-coupled β2 integrin-induced macrophage Rac2-Myosin IIA interaction regulates VEGF-A mRNA stability and arteriogenesis

Myeloid cells are important contributors to arteriogenesis, but their key molecular triggers and cellular effectors are largely unknown. We report, in inflammatory monocytes, that the combination of chemokine receptor (CCR2) and adhesion receptor (β2 integrin) engagement leads to an interaction between activated Rac2 and Myosin 9 (Myh9), the heavy chain of Myosin IIA, resulting in augmented vascular endothelial growth factor A (VEGF-A) expression and induction of arteriogenesis. In human monocytes, CCL2 stimulation coupled to ICAM-1 adhesion led to rapid nuclear-to-cytosolic translocation of the RNA-binding protein HuR. This activation of HuR and its stabilization of VEGF-A mRNA were Rac2-dependent, and proteomic analysis for Rac2 interactors identified the 226 kD protein Myh9. The level of induced Rac2-Myh9 interaction strongly correlated with the degree of HuR translocation. CCL2-coupled ICAM-1 adhesion-driven HuR translocation and consequent VEGF-A mRNA stabilization were absent in Myh9(-/-) macrophages. Macrophage VEGF-A production, ischemic tissue VEGF-A levels, and flow recovery to hind limb ischemia were impaired in myeloid-specific Myh9(-/-) mice, despite preserved macrophage recruitment to the ischemic muscle. Micro-CT arteriography determined the impairment to be defective induced arteriogenesis, whereas developmental vasculogenesis was unaffected. These results place the macrophage at the center of ischemia-induced arteriogenesis, and they establish a novel role for Myosin IIA in signal transduction events modulating VEGF-A expression in tissue.

The PAR complex controls the spatiotemporal dynamics of F-actin and the MTOC in directionally migrating leukocytes

Inflammatory cells acquire a polarized phenotype to migrate towards sites of infection or injury. A conserved polarity complex comprising PAR-3, PAR-6 and atypical protein kinase C (aPKC) relays extracellular polarizing cues to control cytoskeletal and signaling networks affecting morphological and functional polarization. However, there is no evidence that myeloid cells use PAR signaling to migrate vectorially in three-dimensional (3D) environments in vivo. Using genetically encoded bioprobes and high-resolution live imaging, we reveal the existence of F-actin oscillations in the trailing edge and constant repositioning of the microtubule organizing center (MTOC) to direct leukocyte migration in wounded medaka fish larvae (Oryzias latipes). Genetic manipulation in live myeloid cells demonstrates that the catalytic activity of aPKC and the regulated interaction with PAR-3 and PAR-6 are required for consistent F-actin oscillations, MTOC perinuclear mobility, aPKC repositioning and wound-directed migration upstream of Rho kinase (also known as ROCK or ROK) activation. We propose that the PAR complex coordinately controls cytoskeletal changes affecting both the generation of traction force and the directionality of leukocyte migration to sites of injury.

The COP9 signalosome is a repressor of replicative stress responses and polyploidization in the regenerating liver

Aberrant DNA replication induced by deregulated or excessive proliferative stimuli evokes a "replicative stress response" leading to cell cycle restriction and/or apoptosis. This robust fail-safe mechanism is eventually bypassed by transformed cells, due to ill-defined epistatic interactions. The COP9 signalosome (CSN) is an evolutionarily conserved regulator of cullin ring ligases (CRLs), the largest family of ubiquitin ligases in metazoans. Conditional inactivation of the CSN in several tissues leads to activation of S- or G2-phase checkpoints resulting in irreversible cell cycle arrest and cell death. Herein we ablated COPS5, the CSNs catalytic subunit, in the liver, to investigate its role in cell cycle reentry by differentiated hepatocytes. Lack of COPS5 in regenerating livers causes substantial replicative stress, which triggers a CDKN2A-dependent genetic program leading to cell cycle arrest, polyploidy, and apoptosis. These outcomes are phenocopied by acute overexpression of c-Myc in COPS5 null hepatocytes of adult mice.

CONCLUSION

We propose that combined control of proto-oncogene product levels and proteins involved in DNA replication origin licensing may explain the deleterious consequences of CSN inactivation in regenerating livers and provide insight into the pathogenic role of the frequently observed overexpression of the CSN in hepatocellular carcinoma.

Jab1 regulates Schwann cell proliferation and axonal sorting through p27

Jab1 constitutes a regulatory molecule that integrates laminin211 signals in Schwann cells to govern cell cycle, cell number, and differentiation.

Axonal sorting is a crucial event in nerve formation and requires proper Schwann cell proliferation, differentiation, and contact with axons. Any defect in axonal sorting results in dysmyelinating peripheral neuropathies. Evidence from mouse models shows that axonal sorting is regulated by laminin211– and, possibly, neuregulin 1 (Nrg1)–derived signals. However, how these signals are integrated in Schwann cells is largely unknown. We now report that the nuclear Jun activation domain–binding protein 1 (Jab1) may transduce laminin211 signals to regulate Schwann cell number and differentiation during axonal sorting. Mice with inactivation of Jab1 in Schwann cells develop a dysmyelinating neuropathy with axonal sorting defects. Loss of Jab1 increases p27 levels in Schwann cells, which causes defective cell cycle progression and aberrant differentiation. Genetic down-regulation of p27 levels in Jab1-null mice restores Schwann cell number, differentiation, and axonal sorting and rescues the dysmyelinating neuropathy. Thus, Jab1 constitutes a regulatory molecule that integrates laminin211 signals in Schwann cells to govern cell cycle, cell number, and differentiation. Finally, Jab1 may constitute a key molecule in the pathogenesis of dysmyelinating neuropathies.

Microfluidics for in vitro biomimetic shear stress-dependent leukocyte adhesion assays

Recruitment of leukocytes from blood to tissues is a multi-step process playing a major role in the activation of inflammatory responses. Tethering and rolling of leukocytes along the vessel wall, followed by arrest and transmigration through the endothelium result from chemoattractant-dependent signals, inducing adhesive and migratory events. Shear forces exerted by the blood flow on leukocytes induce rolling via selectin-mediated interactions with endothelial cells and increase the probability of leukocytes to engage their chemokine receptors, facilitating integrin activation and consequent arrest. Flow-derived shear forces generate mechanical stimuli concurring with biochemical signals in the modulation of leukocyte-endothelial cell interactions. In the last few years, a host of in vitro studies have clarified the biochemical adhesion cascade and the role of shear stress in leukocyte extravasation. The limitation of the static environment in Boyden devices has been overcome both by the use of parallel-plate flow chambers and by custom models mimicking the in vivo conditions, along with widespread microfluidic approaches to in vitro modeling. These devices create an in vitro biomimetic environment where the multi-step transmigration process can be imaged and quantified under mechanical and biochemical controlled conditions, including fluid dynamic settings, channel design, materials and surface coatings. This paper reviews the technological solutions recently proposed to model, observe and quantify leukocyte adhesion behavior under shear flow, with a final survey of high-throughput solutions featuring multiple parallel assays as well as thorough and time-saving statistical interpretation of the experimental results.

The transcriptional co-regulator Jab1 is crucial for chondrocyte differentiation in vivo

The evolutionarily conserved transcriptional cofactor Jab1 plays critical roles in cell differentiation, proliferation, and apoptosis by modulating the activity of diverse factors and regulating the output of various signaling pathways. Although Jab1 can interact with the bone morphogenetic protein (BMP) downstream effector Smad5 to repress BMP signaling in vitro, the role of Jab1 in BMP-mediated skeletogenesis in vivo is still poorly understood. As a key regulator of skeletogenesis, BMP signaling regulates the critical Ihh-Pthrp feedback loop to promote chondrocyte hypertrophy. In this study, we utilized the loxP/Cre system to delineate the specific role of Jab1 in cartilage formation. Strikingly, Jab1 chondrocyte-specific knockout Jab1(flox/flox); Col2a1-Cre (cKO) mutants exhibited neonatal lethal chondrodysplasia with severe dwarfism. In the mutant embryos, all the skeletal elements developed via endochondral ossification were extremely small with severely disorganized chondrocyte columns. Jab1 cKO chondrocytes exhibited increased apoptosis, G2 phase cell cycle arrest, and increased expression of hypertrophic chondrocyte markers Col10a1 and Runx2. Jab1 can also inhibit the transcriptional activity of Runx2, a key regulator of chondrocyte hypertrophy. Notably, our study reveals that Jab1 is likely a novel inhibitor of BMP signaling in chondrocytes in vivo. In Jab1 cKO chondrocytes, there was heightened expression of BMP signaling components including Gdf10/Bmp3b and of BMP targets during chondrocyte hypertrophy such as Ihh. Furthermore, Jab1 cKO chondrocytes exhibited an enhanced response to exogenous BMP treatment. Together, our study demonstrates that Jab1 represses chondrocyte hypertrophy in vivo, likely in part by downregulating BMP signaling and Runx2 activity.

JAB1 is essential for B cell development and germinal center formation and inversely regulates Fas ligand and Bcl6 expression

Jun activation domain-binding protein 1 (JAB1) regulates ubiquitin-dependent protein degradation by deneddylation of cullin-based ubiquitin ligases and, therefore, plays a central role in regulating proliferation and apoptosis. Because these processes are decisive for B cell development, we investigated JAB1 functions in B cells by establishing a mouse strain with a B cell-specific JAB1 deletion. We show that JAB1 is essential for early B cell development, because the ablation of JAB1 expression blocks B cell development between the pro-B and pre-B cell stages. Furthermore, JAB1 deletion leads to aberrant expression of the apoptosis-triggering protein Fas ligand in pro-B cells. Concomitant B cell-specific overexpression of the antiapoptotic protein Bcl2 partially reverses the block in B cell development; rescued JAB1-deficient B cells reach the periphery and produce protective class-switched Abs after Borrelia burgdorferi infection. Interestingly, B cell-rescued mice exhibit no germinal centers but a striking extrafollicular plasma cell accumulation. In addition, JAB1 is essential for Bcl6 expression, a transcriptional repressor required for germinal center formation. These findings identify JAB1 as an important factor in checkpoint control during early B cell development, as well as in fate decisions in mature Ag-primed B cells.

Macrophage β2 integrin-mediated, HuR-dependent stabilization of angiogenic factor-encoding mRNAs in inflammatory angiogenesis

HuR is a member of the Drosophila Elav protein family that binds mRNA degradation sequences and prevents RNase-mediated degradation. Such HuR-mediated mRNA stabilization, which is stimulated by integrin engagement and is controlled at the level of HuR nuclear export, is critically involved in T-cell cytokine production. However, HuR's role in macrophage soluble factor production, in particular in response to angiogenic stimuli, has not yet been established. We show that the labile transcripts that encode vascular endothelial growth factor and matrix metalloproteinase-9 are stabilized when murine macrophages adhere to the β(2) integrin ligand intercellular adhesion molecule-1. This mRNA stabilization response was absent in bone marrow-derived macrophages obtained from conditional macrophage-specific HuR knockout mice. The microvascular angiogenic response to an inflammatory stimulus (ie, subcutaneous polyvinyl alcohol sponge implantation) was markedly diminished in these macrophage HuR knockout mice despite the equal levels of macrophage localization to those observed in littermate wild-type controls. Furthermore, blood flow recovery and ischemic muscle neovascularization after femoral artery ligation were impaired in the conditional macrophage-specific HuR knockout mice. These results demonstrate that dynamic effects on mRNA, mediated by the RNA-binding and RNA-stabilizing protein HuR, are required for macrophage production of angiogenic factors, which play critical roles in the neovascular responses to a variety of stimuli, including tissue ischemia.

T cell LFA-1 engagement induces HuR-dependent cytokine mRNA stabilization through a Vav-1, Rac1/2, p38MAPK and MKK3 signaling cascade

BACKGROUND

Engagement of the β2 integrin, lymphocyte function-associated antigen-1 (LFA-1), results in stabilization of T cell mRNA transcripts containing AU-rich elements (AREs) by inducing rapid nuclear-to-cytosolic translocation of the RNA-stabilizing protein, HuR. However, little is known regarding integrin-induced signaling cascades that affect mRNA catabolism. This study examines the role of the GTPases, Rac 1 and Rac 2, and their downstream effectors, in the LFA-1-induced effects on mRNA.

METHODOLOGY/PRINCIPAL FINDINGS

Engagement of LFA-1 to its ligand, ICAM-1, in human peripheral T cells resulted in rapid activation of Rac1 and Rac2. siRNA-mediated knockdown of either Rac1 or Rac2 prevented LFA-1-stimulated stabilization of the labile transcripts encoding IFN-γ and TNF-α, and integrin mediated IFN-γ mRNA stabilization was absent in T cells obtained from Rac2 gene-deleted mice. LFA-1 engagement-induced translocation of HuR and stabilization of TNF- α mRNA was lost in Jurkat cells deficient in the Rac guanine nucleotide exchange factor Vav-1 (J.Vav1). The transfection of J.Vav1 cells with constitutively active Rac1 or Rac2 stabilized a labile β-globin reporter mRNA, in a HuR-dependent manner. Furthermore, LFA-1-mediated mRNA stabilization and HuR translocation in mouse splenic T cells was dependent on the phosphorylation of the mitogen-activated protein kinase kinase, MKK3, and its target MAP kinase p38MAPK, and lost in T cells obtained from MKK3 gene-deleted mice.

CONCLUSIONS/SIGNIFICANCE

Collectively, these results demonstrate that LFA-1-induced stabilization of ARE-containing mRNAs in T cells is dependent on HuR, and occurs through the Vav-1, Rac1/2, MKK3 and p38MAPK signaling cascade. This pathway constitutes a molecular switch that enhances immune and pro-inflammatory gene expression in T cells undergoing adhesion at sites of activation and effector function.

Inhibitor of NF-kappa B kinases alpha and beta are both essential for high mobility group box 1-mediated chemotaxis [corrected]

Inhibitor of NF-kappaB kinases beta (IKKbeta) and alpha (IKKalpha) activate distinct NF-kappaB signaling modules. The IKKbeta/canonical NF-kappaB pathway rapidly responds to stress-like conditions, whereas the IKKalpha/noncanonical pathway controls adaptive immunity. Moreover, IKKalpha can attenuate IKKbeta-initiated inflammatory responses. High mobility group box 1 (HMGB1), a chromatin protein, is an extracellular signal of tissue damage-attracting cells in inflammation, tissue regeneration, and scar formation. We show that IKKalpha and IKKbeta are each critically important for HMGB1-elicited chemotaxis of fibroblasts, macrophages, and neutrophils in vitro and neutrophils in vivo. By time-lapse microscopy we dissected different parameters of the HMGB1 migration response and found that IKKalpha and IKKbeta are each essential to polarize cells toward HMGB1 and that each kinase also differentially affects cellular velocity in a time-dependent manner. In addition, HMGB1 modestly induces noncanonical IKKalpha-dependent p52 nuclear translocation and p52/RelB target gene expression. Akin to IKKalpha and IKKbeta, p52 and RelB are also required for HMGB1 chemotaxis, and p52 is essential for cellular orientation toward an HMGB1 gradient. RAGE, a ubiquitously expressed HMGB1 receptor, is required for HMGB1 chemotaxis. Moreover, IKKbeta, but not IKKalpha, is required for HMGB1 to induce RAGE mRNA, suggesting that RAGE is at least one IKKbeta target involved in HMGB1 migration responses, and in accord with these results enforced RAGE expression rescues the HMGB1 migration defect of IKKbeta, but not IKKalpha, null cells. Thus, proinflammatory HMGB1 chemotactic responses mechanistically require the differential collaboration of both IKK-dependent NF-kappaB signaling pathways.

Laser capture microdissection as a new tool to assess graft-infiltrating lymphocytes gene profile in islet transplantation

Innovative tolerogenic protocols in transplantation would take advantage of the development of new tools capable of evaluating the impact of these treatments on the immune system. These assays have potential for clinical application. Currently, many of these studies are based on the analysis of peripheral lymph nodes and blood-derived cells, where the percentage of alloantigen-specific cells can be low or even unpredictable. We combined a laser capture microdissection (LCM) technique with real-time PCR (RT-PCR) to evaluate gene profile of islet-infiltrating lymphocytes. Donor Lewis rats islets were transplanted under the kidney capsule in diabetic Brown Norway rats. Administration of anti-LFA1 mAb or anti-CD28 F(Ab)' was able to prolong islet survival, while the combined treatment resulted in indefinite survival. The analysis of gene expression profile for IL-2, IFN-gamma, and IL-10 production of graft-infiltrating cells revealed high IL-2, IFN-gamma, and IL-10 in untreated rats; on the contrary, the combined treatment selectively abrogated IL-2- and IFN-gamma-producing cells infiltrate. The comparison between cytokine profile in periphery (even during an allogenic extra stimulus) and in the graft revealed the dichotomy between graft and peripheral cytokine assessment. We thus propose that direct analysis of graft-infiltrating cells should be used whenever possible to evaluate the effects of a new immunomodulatory protocol.

JAM-A promotes neutrophil chemotaxis by controlling integrin internalization and recycling

The membrane-associated adhesion molecule JAM-A is required for neutrophil infiltration in inflammatory or ischemic tissues. JAM-A expressed in both endothelial cells and neutrophils has such a role, but the mechanism of action remains elusive. Here we show that JAM-A has a cell-autonomous role in neutrophil chemotaxis both in vivo and in vitro, which is independent of the interaction of neutrophils with endothelial cells. On activated neutrophils, JAM-A concentrates in a polarized fashion at the leading edge and uropod. Surprisingly, a significant amount of this protein is internalized in intracellular endosomal-like vesicles where it codistributes with integrin beta1. Clustering of beta1 integrin leads to JAM-A co-clustering, whereas clustering of JAM-A does not induce integrin association. Neutrophils derived from JAM-A-null mice are unable to correctly internalize beta1 integrins upon chemotactic stimuli and this causes impaired uropod retraction and cell motility. Consistently, inhibition of integrin internalization upon treatment with BAPTA-AM induces a comparable phenotype. These data indicate that JAM-A is required for the correct internalization and recycling of integrins during cell migration and might explain why, in its absence, the directional migration of neutrophils towards an inflammatory stimulus is markedly impaired.

Targeted inactivation of the COP9 signalosome impairs multiple stages of T cell development

Genetic programs promoting cell cycle progression, DNA repair, and survival are coordinately induced in developing T cells and require rapid turnover of effector molecules. As the COP9 signalosome (CSN) has been placed at the crossroads of these programs in lower organisms, we addressed its role by conditionally deleting CSN5/JAB1, its catalytic subunit, in developing thymocytes. CSN5/JAB1^del/del thymocytes show defective S phase progression and massive apoptosis at the double-negative (DN) 4–double-positive (DP) transition stage, which is paralleled by altered turnover of selected CSN-controlled substrates, including p53, IκB-α, and β-catenin. Combined dysregulation of the p53 and NF-κB pathways affects thymocyte survival by altering the mRNA and protein levels of selected Bcl-2 family members. Genetic complementation analysis performed on p53^−/−, Bcl-xL/Bcl-2A1, or T cell receptor transgenic backgrounds indicates that CSN5/JAB1 acts at distinct developmental stages to coordinate proliferation, survival, and positive selection of thymocytes by controlling the induction of defined genetic programs acting downstream of CSN-regulated transcription factors.

Pathophysiology of leukocyte-tissue interactions

Unlike most somatic cells, leukocytes are constitutively non-adherent. However, adhesive interactions are not only a required step in essentially all effector functions performed by leukocytes, but they also relay increasingly well-defined intracellular signals that affect the leukocyte as well as the surrounding tissues. Dissecting such signals in leukocytes has provided a wealth of information that contributes to our understanding of how adhesion controls higher-order biological responses, ranging from cell migration to proliferation, differentiation and survival.

LFA-1-dependent HuR nuclear export and cytokine mRNA stabilization in T cell activation

Lymphokine gene expression is a precisely regulated process in T cell-mediated immune responses. In this study we demonstrate that engagement of the beta(2) integrin LFA-1 in human peripheral T cells markedly extends the half-life of TNF-alpha, GM-CSF, and IL-3 mRNA, as well as a chimeric beta-globin mRNA reporter construct containing a strongly destabilizing class II AU-rich element from the GM-CSF mRNA 3'-untranslated region. This integrin-enhanced mRNA stability leads to augmented protein production, as determined by TNF-alpha ELISPOT assays. Furthermore, T cell stimulation by LFA-1 promotes rapid nuclear-to-cytoplasmic translocation of the mRNA-stabilizing protein HuR, which in turn is capable of binding an AU-rich element sequence in vitro. Abrogation of HuR function by use of inhibitory peptides, or marked reduction of HuR levels by RNA interference, prevents LFA-1 engagement-mediated stabilization of T cell TNF-alpha or IFN-gamma transcripts, respectively. Thus, HuR-mediated mRNA stabilization, stimulated by integrin engagement and controlled at the level of HuR nuclear export, is critically involved in T cell activation.

Dynamic partitioning into lipid rafts controls the endo-exocytic cycle of the alphaL/beta2 integrin, LFA-1, during leukocyte chemotaxis

Cell migration entails the dynamic redistribution of adhesion receptors from the cell rear toward the cell front, where they form new protrusions and adhesions. This process may involve regulated endo-exocytosis of integrins. Here we show that in primary neutrophils unengaged alphaL/beta2 integrin (LFA-1) is internalized and rapidly recycled upon chemoattractant stimulation via a clathrin-independent, cholesterol-sensitive pathway involving dynamic partitioning into detergent-resistant membranes (DRM). Persistent DRM association is required for recycling of the internalized receptor because 1) >90% of endocytosed LFA-1 is associated with DRM, and a large fraction of the internalized receptor colocalizes intracellularly with markers of DRM and the recycling endocytic compartment; 2) a recycling-defective mutant (alphaL/beta2Y735A) dissociates rapidly from DRM upon being endocytosed and is subsequently diverted into a late endosomal pathway; and 3) a dominant negative Rab11 mutant (Rab11S25N) induces intracellular accumulation of endocytosed alphaL/beta2 and prevents its enrichment in chemoattractant-induced lamellipodia. Notably, chemokine-induced migration of neutrophils over immobilized ICAM-1 is abrogated by cholesterol-sequestering agents. We propose that DRM-associated endocytosis allows efficient retrieval of integrins, as they detach from their ligands, followed by polarized recycling to areas of the plasma membrane, such as lamellipodia, where they establish new adhesive interactions and promote outside-in signaling events.

Synaptic clusters of MHC class II molecules induced on DCs by adhesion molecule-mediated initial T-cell scanning

Initial adhesive contacts between T lymphocytes and dendritic cells (DCs) facilitate recognition of peptide-MHC complexes by the TCR. In this report, we studied the dynamic behavior of adhesion and Ag receptors on DCs during initial contacts with T-cells. Adhesion molecules LFA-1- and ICAM-1,3-GFP as well as MHC class II-GFP molecules were very rapidly concentrated at the DC contact area. Binding of ICAM-3, and ICAM-1 to a lesser extent, to LFA-1 expressed by mature but not immature DC, induced MHC-II clustering into the immune synapse. Also, ICAM-3 binding to DC induced the activation of the Vav1-Rac1 axis, a regulatory pathway involved in actin cytoskeleton reorganization, which was essential for MHC-II clustering on DCs. Our results support a model in which ICAM-mediated MHC-II clustering on DC constitutes a priming mechanism to enhance antigen presentation to T-cells.

Expression pattern of the JAB1/CSN5 gene during murine embryogenesis: colocalization with NEDD8

The COP9 signalosome (CSN) is a conserved multiprotein complex, with an important developmental role in several organisms, ranging from plants to mammalians. The influence of the CSN on several signaling and developmental processes has been ascribed to its ability to regulate degradation of a number of signaling proteins by the ubiquitin-proteasome system. The CSN controls the function of the SCF ubiquitin-ligase complex through an enzymatic activity that removes the small ubiquitin-like molecule NEDD8 from the cullin component of the SCF and that requires subunit 5 of the CSN (JAB1/CSN5). Mutants of the CSN display early embryonic lethality, a feature that has hindered further characterization of the role of the CSN at later stages of mammalian development. Here we report the analysis of JAB1/CSN5 expression pattern in the mouse embryo. At early stages of development, JAB1/CSN5 transcripts were present with low expression levels in all tissues. Preferential expression in selected tissues was detected starting at E11.5, with higher levels in dorsal root ganglia; at later stages, prominent expression of JAB1/CSN5 transcripts was observed in cranial nerve, spinal and sympathetic ganglia, as well as in selected epithelia, such as the oral and the olfactory epithelium. In the adult brain, additional areas of JAB1/CSN5 expression were the hippocampus and the Purkinjie layer of the cerebellum. We also analyzed the temporal and spatial expression pattern of NEDD8, and found that it substantially overlapped JAB1/CSN5 expression at all stages analyzed, supporting the model of a functional interaction between the two proteins during developmental processes.

RanBPM is a phosphoprotein that associates with the plasma membrane and interacts with the integrin LFA-1

Integrin adhesion receptors can act as signaling receptors that transmit information from the extracellular environment to the interior of the cell, affecting many fundamental cellular processes, such as cell motility, proliferation, differentiation, and survival. Integrin signaling depends on the formation of organized sub-membrane complexes that comprise cytoskeletal, adapter, and signaling molecules. The identification of molecules that interact with the cytoplasmic domain of integrins has been the focus of research aimed to elucidating the mechanistic basis of integrin signal transduction. We have identified RanBPM as a novel interactor of the beta(2) integrin LFA-1 in a yeast-two-hybrid screen. In the same assay, RanBPM also interacted with the beta(1) integrin cytoplasmic domain. We demonstrate that RanBPM is a peripheral membrane protein and that integrins and RanBPM interact in vitro and in vivo and co-localize at the cell membrane. We find that RanBPM is phosphorylated on serine residues; phosphorylation of RanBPM is increased by stress stimuli and decreased by treatment with the p38 kinase inhibitor SB203580. Transfection of RanBPM synergizes with LFA-1-mediated adhesion in the transcriptional activation of an AP-1-dependent promoter, indicating that the two proteins interact functionally as well. We suggest that RanBPM may constitute a molecular scaffold that contributes to coupling LFA-1 and other integrins with intracellular signaling pathways.

T lymphocytes

T cells are major players in the adaptive immune response to pathogens. They express clonally distributed, highly polymorphic antigen receptors that enable them to recognize cell-associated antigen. Upon antigen recognition, T cells undergo clonal amplification and progressively acquire effector functions, ranging from the production of paracrine soluble factors that provide "help" to other immune cells to the ability to kill pathogen-infected cells with surgical precision. A pool of antigen-reactive T cells reverts to a state of quiescence and maintains a long-lasting memory of antigen encounter. T cells develop in the thymus through a rigorous selection process that recapitulates Darwinian phylogenesis: only the "fittest" survive, i.e. those that can efficiently recognize infectious non-self-antigens but ignore, or are silenced, by non-infectious self-antigens. Due to their ability to discriminate between self and non-self, T cells are the major effectors of allograft rejection. T cells are involved in the pathogenesis of several human disorders, resulting from their defective or dysregulated function. The former leads to a severe state of immunodeficiency, the latter to organ-specific or systemic autoimmunity.

The critical cytoplasmic regions of the alphaL/beta2 integrin in Rap1-induced adhesion and migration

Rap1 is a potent inside-out signal that increases LFA-1 adhesive activity. In this study, we have defined the cytoplasmic region of the alphaL and beta2 integrin that are required for Rap1-stimulated adhesion and subsequent migration on ICAM-1. Human LFA-1 bearing truncated and point-mutated alphaL and beta2 cytoplasmic regions were reconstituted in mouse IL-3-dependent proB cells, BAF/3. Truncation of the alphaL, but not beta2 subunit cytoplasmic region, abolished Rap1V12-dependent adhesion to ICAM-1. The alanine substitution of two lysine residues (K1097/K1099) in the alphaL subunit was found to be critical in adhesion induced by Rap1V12, but not PMA. This mutation suppressed Rap1V12-induced LFA-1 conformation changes and ligand-binding affinity. The K1097/K1099 mutation also impaired binding to ICAM-1 induced by TCR cross-linking or SDF-1. In contrast, the alanine substitution for tyrosine in the beta2 subunit endocytosis motif inhibited internalization of LFA-1, and severely impaired detachment at the cell rear, which resulted in long-elongated cell shapes. This result demonstrates that internalization of LFA-1 is a critical step in the deadhesion process. Our study revealed novel requirements of amino acid residues of the LFA-1 cytoplasmic region in the response to the inside-out signaling and the subsequent deadhesion process.

Characterization of human constitutive photomorphogenesis protein 1, a RING finger ubiquitin ligase that interacts with Jun transcription factors and modulates their transcriptional activity

RING finger proteins have been implicated in many fundamental cellular processes, including the control of gene expression. A key regulator of light-dependent development in Arabidopsis thaliana is the constitutive photomorphogenesis protein 1 (atCOP1), a RING finger protein that plays an essential role in translating light/dark signals into specific changes in gene transcription. atCOP1 binds the basic leucine zipper factor HY5 and suppresses its transcriptional activity through a yet undefined mechanism that results in HY5 degradation in response to darkness. Furthermore, the pleiotropic phenotype of atCOP1 mutants indicates that atCOP1 may be a central regulator of several transcriptional pathways. Here we report the cloning and characterization of the human orthologue of atCOP1. Human COP1 (huCOP1) distributes both to the cytoplasm and the nucleus of cells and shows a striking degree of sequence conservation with atCOP1, suggesting the possibility of a functional conservation as well. In co-immunoprecipitation assays huCOP1 specifically binds basic leucine zipper factors of the Jun family. As a functional consequence of this interaction, expression of huCOP1 in mammalian cells down-regulates c-Jun-dependent transcription and the expression of the AP-1 target genes, urokinase and matrix metalloproteinase 1. The RING domain of huCOP1 displays ubiquitin ligase activity in an autoubiquitination assay in vitro; however, suppression of AP-1-dependent transcription by huCOP1 occurs in the absence of changes in c-Jun protein levels, suggesting that this inhibitory effect is independent of c-Jun degradation. Our findings indicate that huCOP1 is a novel regulator of AP-1-dependent transcription sharing the important properties of Arabidopsis COP1 in the control of gene expression.

Macrophages exposed to Mycobacterium tuberculosis release chemokines able to recruit selected leucocyte subpopulations: focus on gammadelta cells

Granuloma is a typical feature of tuberculosis. We evaluated the chemotaxis of selected human leucocyte subsets induced by macrophages incubated with Mycobacterium tuberculosis (MT)-derived products in vitro. The release of monocyte chemotactic protein 1 (MCP-1) and interleukin-8 (IL-8) correlated with the specific induction of strong chemotaxis towards monocytes and polymorphonuclear leucocytes (PMNs). gammadelta and T helper type 1 (Th1) alphabeta lymphocytes were chemoattracted, while T-resting, IL-2-activated and Th2 lymphocytes were unaffected. Activation with mycobacterium-derived, phosphate-containing components, modulated the chemokine receptor profile of gammadelta T lymphocytes as well as their pattern of cyto-chemokine production, disclosing a potential for their active participation in granuloma formation. In particular, CXCR3 and IP-10, which we found to be released by MT-pulsed alveolar macrophages, seem to represent the receptor-counter-receptor pair implicated in the chemotaxis of gammadelta lymphocytes. Immunohistochemical analysis and in situ hybridization revealed the in vivo presence of IL-8, MCP-1 and IL-10 in lymph node and lung tuberculous granulomas. Our results underscore the role of MT extracts in the induction of macrophage-derived chemokines responsible for the orchestrated recruitment of PMNs, monocytes, and Th1 and gammadelta T cells, as well as in the regulation of gammadelta function.

Integrin-dependent regulation of gene expression in leukocytes

In addition to their role in strengthening intercellular adhesion, leukocyte integrins transduce signals which affect genetic programs, consequently defining cell phenotype and function. These signals can be independently sufficient, or can cooperate with other environmental stimuli to affect gene expression regulation. In the past several years, there has been an emergence of mechanistic data which contribute to our understanding of these critical integrin roles. In this review, we describe anchorage-dependent T lymphocyte proliferation and, in particular, how leukocyte integrin engagement overcomes the G1 to S cell cycle restriction point in antigen-activated T cells. The related role of alphaLbeta2 integrin (LFA-1) as a T cell co-stimulatory molecule is discussed. This includes defining mechanisms whereby LFA-1 engagement enhances transcriptional activation of numerous genes by regulating its association with transcription modulators such as JAB-1, and through interaction with other gene-activating signaling complexes such as JAK-STATs. Evidence is presented to support that leukocyte integrin engagement provides potent signals which stabilize otherwise labile activation mRNA transcripts, including those encoding cytokine and extracellular matrix degrading proteins. These integrin-dependent mechanisms, all described recently, play important roles in T cell differentiation and proliferation, immune surveillance and inflammatory responses.

Inhibition of interferon-gamma-mediated microvascular endothelial cell major histocompatibility complex class II gene activation by HMG-CoA reductase inhibitors

BACKGROUND

Graft vascular disease, a major cause of late graft failure in cardiac transplant patients, is associated with the presence of class II major histocompatibility complex molecules on the endothelium. 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase inhibitors, e.g., simvastatin, have been shown to reduce the incidence of graft vascular disease. We studied the effect of simvastatin on interferon (IFN)-gamma-induced human leukocyte antigen (HLA)-DR expression in human microvascular endothelial cells (MVECs).

METHODS AND RESULTS

Simvastatin pretreatment inhibited MVEC HILA-DR induction by IFN-gamma, as detected by flow cytometry. Simvastatin's inhibitory effect was reversed by the cholesterol synthesis pathway intermediates mevalonate and geranylgeranyl pyrophosphate but not squalene, indicating the involvement of protein prenylation in this process. Reverse transcription-polymerase chain reaction analysis demonstrated that induction of class II transactivator (CIITA), and consequently, HLA-DRalpha mRNA, is abrogated by simvastatin. Although signal transducer and activator of transcription (STAT)-1 is a critical CIITA gene transactivator, immunofluorescence studies, Western blotting, and electrophoretic mobility shift assays demonstrated that IFN-gamma-induced STAT-1 phosphorylation, nuclear translocation, and DNA binding are not affected by simvastatin. However, simvastatin inhibited IFN-gamma-induced transactivation of a CIITA promoter IV reporter construct, indicating the involvement of this promoter in the inhibitory effect of simvastatin.

CONCLUSIONS

Simvastatin pretreatment inhibits CIITA and consequent HLA-DR induction by IFN-gamma in MVECs through interference with protein prenylation. This inhibitory effect occurs at the level of transcription and is directed, at least in part, at the CIITA promoter IV. These results explain some of the beneficial effects of HMG-CoA reductase inhibitors in cardiac transplantation.

The localization of human cyclins B1 and B2 determines CDK1 substrate specificity and neither enzyme requires MEK to disassemble the Golgi apparatus

In this paper, we show that substrate specificity is primarily conferred on human mitotic cyclin-dependent kinases (CDKs) by their subcellular localization. The difference in localization of the B-type cyclin-CDKs underlies the ability of cyclin B1-CDK1 to cause chromosome condensation, reorganization of the microtubules, and disassembly of the nuclear lamina and of the Golgi apparatus, while it restricts cyclin B2-CDK1 to disassembly of the Golgi apparatus. We identify the region of cyclin B2 responsible for its localization and show that this will direct cyclin B1 to the Golgi apparatus and confer upon it the more limited properties of cyclin B2. Equally, directing cyclin B2 to the cytoplasm with the NH(2) terminus of cyclin B1 confers the broader properties of cyclin B1. Furthermore, we show that the disassembly of the Golgi apparatus initiated by either mitotic cyclin-CDK complex does not require mitogen-activated protein kinase kinase (MEK) activity.

Simvastatin modulates cytokine-mediated endothelial cell adhesion molecule induction: involvement of an inhibitory G protein

Endothelial cell adhesion molecules (CAMs) E-selectin, ICAM-1, and VCAM-1 play variably important roles in immune-mediated processes. They are induced by the proinflammatory cytokines IL-1 and TNF-alpha, and NF-kappaB is required for the regulated expression of all three genes. Regulators of this pathway could potentially be potent immune modulators. We studied the effect of a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, simvastatin, on cytokine-induced expression of CAMs in HUVEC. Unexpectedly, pretreatment with simvastatin potentiated the induction of all three endothelial CAMs by IL-1 and TNF, but not LPS or PMA, as detected by flow cytometry. Northern blot analysis demonstrated an increase in steady state IL-1-induced E-selectin mRNA levels in cells pretreated with simvastatin. This was associated with an increase in nuclear translocation of NF-kappaB, as detected by EMSA. The effect of simvastatin was reversed by mevalonate and geranylgeranyl pyrophosphate but not squalene, indicating that an inhibitory prenylated protein is involved in endothelial responses to proinflammatory cytokines. Pertussis toxin mimicked the effect of simvastatin, and the G protein activator NaF inhibited the cytokine-induced expression of endothelial CAMs, indicating that a Gialpha protein is involved. These results demonstrate that cytokine-mediated activation of the endothelium, and specifically CAM induction, can be modulated by a heterotrimeric G protein-coupled pathway. This may represent a "basal tone" of endothelial inactivation, which can either be disinhibited or amplified, depending on the stimulus.

Integrin LFA-1 interacts with the transcriptional co-activator JAB1 to modulate AP-1 activity

Integrin adhesion receptors transduce signals that control complex cell functions which require the regulation of gene expression, such as proliferation, differentiation and survival. Their intracellular domain has no catalytic function, indicating that interaction with other transducing molecules is crucial for integrin-mediated signalling. Here we have identified a protein that interacts with the cytoplasmic domain of the beta2 subunit of the alphaL/beta2 integrin LFA-1. This protein is JAB1 (Jun activation domain-binding protein 1), a coactivator of the c-Jun transcription factor. We found that JAB1 is present both in the nucleus and in the cytoplasm of cells and that a fraction of JAB1 colocalizes with LFA-1 at the cell membrane. LFA-1 engagement is followed by an increase of the nuclear pool of JAB1, paralleled by enhanced binding of c-Jun-containing AP-1 complexes to their DNA consensus site and increased transactivation of an AP-1-dependent promoter. We suggest that signalling through the LFA-1 integrin may affect c-Jun-driven transcription by regulating JAB1 nuclear localization. This represents a new pathway for integrin-dependent modulation of gene expression.

CD28 and LFA-1 contribute to cyclosporin A-resistant T cell growth by stabilizing the IL-2 mRNA through distinct signaling pathways

In clinical transplantation, the occurrence of cyclosporin A (CsA)-resistant production of IL-2 in vitro correlates with graft rejection in vivo. In this study we investigated the role of the costimulatory molecules CD28 and LFA-1 in this process in the setting of TCR-induced proliferation of primary T lymphocytes in vitro. Co-stimulation with ICAM-1 and B7.2 led to strong and CsA-resistant proliferation, which was found to be largely IL-2 dependent. All of the known calcineurin-dependent events, such as induction of NF-AT and NF-kappaB or stress-activated protein kinase activation, were markedly modulated by CsA independently of costimulation. In contrast, both ICAM-1 and B7.2 enhanced the half-life of the inducible IL-2 transcript in a CsA-resistant manner. LFA-1- but not CD28-induced IL-2 mRNA stabilization required the integrity of the actin-based cytoskeleton, suggesting that the two costimulatory molecules impact on qualitatively different signaling pathways. This is further suggested by the demonstration that LFA-1 and CD28 acted synergistically to confer CsA resistance in a model of co-stimulation using superantigen-pulsed dendritic cells. We propose that IL-2 transcript accumulation and subsequent T cell proliferation at the low transcriptional rate imposed by CsA are the result of co-stimulation-dependent stabilization of IL-2 mRNA.

Chemokines fail to up-regulate beta 1 integrin-dependent adhesion in human Th2 T lymphocytes

Th1 and Th2 cells are functionally distinct subsets of CD4+ T lymphocytes whose tissue-specific homing to sites of inflammation is regulated in part by the differential expression of P- and E-selectin ligands and selected chemokine receptors. Here we investigated the expression and function of beta 1 integrins in Th1 and Th2 cells polarized in vitro. Th1 lymphocytes adhere transiently to the extracellular matrix ligands laminin 1 and fibronectin in response to chemokines such as RANTES and stromal cell-derived factor-1, and this process is paralleled by the activation of the Rac1 GTPase and by a rapid burst of actin polymerization. Selective inhibitors of phosphoinositide-3 kinase prevent efficiently all of the above processes, whereas the protein kinase C inhibitor bisindolylmaleimide prevents chemokine-induced adhesion without affecting Rac1 activation and actin polymerization. Notably, chemokine-induced adhesion to beta 1 integrin ligands is markedly reduced in Th2 cells. Such a defect cannot be explained by a reduced sensitivity to chemokine stimulation in this T cell subset, nor by a defective activation of the signaling cascade involving phosphoinositide-3 kinase, Rac1, and actin turnover, as all these processes are activated at comparable levels by chemokines in the two subsets. We propose that reduced beta 1 integrin-mediated adhesion in Th2 cells may restrain their ability to invade and/or reside in sites of chronic inflammation, which are characterized by thickening of basement membranes and extensive fibrosis, requiring efficient interaction with organized extracellular matrices.

Circulating, Mycobacterium tuberculosis-specific lymphocytes from PPD skin test-negative patients with tuberculosis do not secrete interferon-gamma (IFN-gamma) and lack the cutaneous lymphocyte antigen skin-selective homing receptor

Individuals with a negative intradermal reaction to tuberculin PPD have long been described in the Mycobacterium tuberculosis exposed, immune-competent population. Here, we studied PPD-specific blood T lymphocytes from these subjects for phenotypic markers relevant to skin migration, including the expression of the skin-selective homing receptor, the cutaneous lymphocyte-associated antigen (CLA). Out of 82 patients with active tuberculosis we identified four subjects who were repeatedly PPD skin test-negative. CD4 T lymphocytes specific to mycobacterial antigens were derived from these individuals, which (i) proliferated in vitro to M. tuberculosis antigens comparably to those from PPD+ patients; (ii) secreted comparable amounts of IL-2 but lower amounts of IFN-gamma; (iii) were confined within the CLA-negative T cell subset. We conclude that the negative tuberculin reaction in a small subset of patients exposed to mycobacteria is associated with impaired production of IFN-gamma by circulating PPD-specific T cells that are lacking CLA expression. On this basis in vitro proliferation to PPD can discriminate bona fide non-responders from infected patients with a deficit in the margination of M. tuberculosis-specific T lymphocytes.

Upregulation of integrin alpha6/beta1 and chemokine receptor CCR1 by interleukin-12 promotes the migration of human type 1 helper T cells

CD4(+) T helper 1 (Th1) cells and Th2 cells are distinguished based on the pattern of cytokines they are able to produce. Selectin ligands and chemokine receptors are differentially expressed in Th1 and Th2 cells, providing a basis for tissue-specific recruitment of helper T-cell subsets. However, the modes and mechanisms regulating tissue-specific localization of Th1 and Th2 cells are still largely unknown. Here, we show the preferential expression on Th1 cells of the integrin alpha6/beta1, which is distinctly regulated by the Th1-inducing cytokines interleukin-12 (IL-12) and interferon-alfa (IFN-alpha). The pattern of integrin alpha6/beta1 regulation closely mirrors that of the chemokine receptor CCR1. Analysis of signal transducer and activator of transcription 4 (Stat4) activation by IL-12 and IFN-alpha shows distinct signaling kinetics by these cytokines, correlating with the pattern of CCR1 and integrin alpha6/beta1 expression. Unlike IFN-alpha, the ability of IL-12 to generate prolonged intracellular signals appears to be critical for inducing integrin alpha6/beta1 upregulation in Th1 cells. The expression and upregulation of CCR1 and alpha6/beta1 integrin promotes the migration of Th1 cells. These findings suggest that the exquisite regulation of integrin alpha6/beta1 and CCR1 may play an important role in tissue-specific localization of Th1 cells.

A tyrosine-based sorting signal in the beta2 integrin cytoplasmic domain mediates its recycling to the plasma membrane and is required for ligand-supported migration

Integrins play pivotal roles in supporting shear- and mechanical-stress-resistant cell adhesion and migration. These functions require the integrity of the short beta subunit cytoplasmic domains, which contain multiple, highly conserved tyrosine-based endocytic signals, typically found in receptors undergoing regulated, clathrin-dependent endocytosis. We hypothesized that these sequences may control surface integrin dynamics in statically adherent and/or locomoting cells via regulated internalization and polarized recycling of the receptors. By using site-directed mutagenesis and ectopic expression of the alphaL/beta2 integrin in Chinese hamster ovary cells, we found that Y735 in the membrane-proximal YRRF sequence is selectively required for recycling of spontaneously internalized receptors to the cell surface and to growth factor-induced membrane ruffles. Disruption of this motif by non-conservative substitutions has no effect on the receptor's adhesive function, but diverts internalized integrins from a recycling compartment into a degradative pathway. Conversely, the non-conservative F754A substitution in the membrane-proximal NPLF sequence abrogates ligand-dependent adhesion and spreading without affecting receptor recycling. Both of these mutants display a severe impairment in ligand-supported migration, suggesting the existence in integrin cytoplasmic domains of independent signals regulating apparently unrelated functions that are required to sustain cell migration over specific ligands.

Anchorage dependence of mitogen-induced G1 to S transition in primary T lymphocytes

Anchorage dependence defines the cellular requirement for integrin-mediated adhesion to substrate to initiate DNA replication in response to growth factors. In this study we investigated whether normal T cells, which spend extended periods in a nonadherent state, show similar requirements for cell cycle progression in response to TCR stimulation. Resting primary T lymphocytes were induced to enter the cell cycle by TCR triggering, and leukocyte integrins were either engaged using purified ICAM-1 or inhibited with function-blocking mAbs. Our data indicate that leukocyte integrins complement TCR-driven mitogenic signals not as a result of their direct clustering but, rather, via integrin-dependent organization of the actin cytoskeleton. Leukocyte integrin-dependent reorganization of the actin cytoskeleton cooperates with the TCR to effect mitogen-activated protein kinase activation, but also represents a required late (4-8 h poststimulation) component in the mitogenic response of normal T cells. Prolonged leukocyte integrin-dependent spreading, in the context of intercellular contact, is a requisite for the production of the mitogenic cytokine IL-2, which, in turn, is involved in the induction of D3 cyclin and is primarily responsible for the decrease in the cyclin-dependent kinase inhibitor p27kip, resulting in retinoblastoma protein inactivation and S phase entry. Thus, T lymphocytes represent a peculiar case of anchorage dependence, in which signals conveyed by integrins act sequentially with the activating stimulus to effect a sustained production of the essential mitogenic cytokine.

Anchorage Dependence of Mitogen-Induced G1 to S Transition in Primary T Lymphocytes

Anchorage dependence defines the cellular requirement for integrin-mediated adhesion to substrate to initiate DNA replication in response to growth factors. In this study we investigated whether normal T cells, which spend extended periods in a nonadherent state, show similar requirements for cell cycle progression in response to TCR stimulation. Resting primary T lymphocytes were induced to enter the cell cycle by TCR triggering, and leukocyte integrins were either engaged using purified ICAM-1 or inhibited with function-blocking mAbs. Our data indicate that leukocyte integrins complement TCR-driven mitogenic signals not as a result of their direct clustering but, rather, via integrin-dependent organization of the actin cytoskeleton. Leukocyte integrin-dependent reorganization of the actin cytoskeleton cooperates with the TCR to effect mitogen-activated protein kinase activation, but also represents a required late (4–8 h poststimulation) component in the mitogenic response of normal T cells. Prolonged leukocyte integrin-dependent spreading, in the context of intercellular contact, is a requisite for the production of the mitogenic cytokine IL-2, which, in turn, is involved in the induction of D3 cyclin and is primarily responsible for the decrease in the cyclin-dependent kinase inhibitor p27kip, resulting in retinoblastoma protein inactivation and S phase entry. Thus, T lymphocytes represent a peculiar case of anchorage dependence, in which signals conveyed by integrins act sequentially with the activating stimulus to effect a sustained production of the essential mitogenic cytokine.