Adgrf5 contributes to patterning of the endothelial deep layer in retina

Neovascularization of the inner retinal space is a major cause of vision loss. In retinal angiomatous proliferation (RAP) syndrome, newly formed vessels originate from the retinal plexus and invade the inner retinal space. However, the molecular pathways preventing subretinal vascularization remain largely unknown. In most murine models of RAP, pathological neovascularization occurs concomitantly with the development of the retinal vasculature. Here, we demonstrate that disturbing the sequence of morphogenetic events that shape the three-layered retinal vascular network leads to subretinal vascularization. Sprouts emerging from the perivenous region after the first postnatal week extended toward the retinal space where they merged into the deep layer. The small GTPase Rac1 was required for the formation of these vascular extensions and the vascular inner plexus is formed coaxially to the overarching veins. The adhesion receptor Adgrf5 was highly expressed in the endothelium of the central nervous system, where it regulates blood–brain barrier formation. The vascular superficial plexus of Adgrf5 mutant mouse retinae exhibited an increased vascular density in the perivenous areas with increased projections toward the inner plexus where they subsequently created hyper-dense endothelial cells (EC) clusters. Disturbing the perivenous pool of EC thus significantly altered the inner plexus formation. These abnormalities culminated in transient vascular protrusions in the inner retinal space. Taken together, these results reveal a previously unobserved vascular morphogenetic defect in Adgrf5 knockout mice, implicating a role for ADGRF5 in the initiation of subretinal vascularization. Our findings also illustrate how vein-derived EC shape the inner retinal layer formation and could control the appearance of angiomatous malformations.

Loss of RhoA promotes skin tumor formation and invasion by upregulation of RhoB

Cellular movement is controlled by small GTPases, such as RhoA. Although migration is crucial for cancer cell invasion, the specific role of RhoA in tumor formation is unclear. Inducing skin tumors in mice with a keratinocyte-restricted loss of RhoA, we observed increased tumor frequency, growth and invasion. In vitro invasion assays revealed that in the absence of RhoA cell invasiveness is increased in a Rho-associated protein kinase (ROCK) activation and cell contraction-dependent manner. Surprisingly, loss of RhoA causes increased Rho signaling via overcompensation by RhoB because of reduced lysosomal degradation of RhoB in Gamma-aminobutyric acid receptor-associated protein (GABARAP)+ autophagosomes and endosomes. In the absence of RhoA, RhoB relocalized to the plasma membrane and functionally replaced RhoA with respect to invasion, clonogenic growth and survival. Our data demonstrate for the first time that RhoA is a tumor suppressor in 7,12-dimethylbenz[a]anthracene/12-O-tetradecanoylphorbol 13-acetate skin carcinogenesis and identify Rho signaling dependent on RhoA and RhoB as a potent driver of tumor progression.

Critical off-target effects of the widely used Rac1 inhibitors NSC23766 and EHT1864 in mouse platelets

BACKGROUND

Platelet aggregation at sites of vascular injury is essential for normal hemostasis, but may also cause pathologic vessel occlusion. Rho GTPases are molecular switches that regulate essential cellular processes, and they have pivotal functions in the cardiovascular system. Rac1 is an important regulator of platelet cytoskeletal reorganization, and contributes to platelet activation. Rac1 inhibitors are thought to be beneficial in a wide range of therapeutic settings, and have therefore been tested in vivo for a variety of disorders. Two small-molecule inhibitors, NSC23766 and EHT1864, have been characterized in different cell types, demonstrating high specificity for Rac1 and Rac, respectively.

OBJECTIVES

To analyze the specificity of NSC23766 and EHT1864.

METHODS

Platelet function was assessed in mouse wild-type and Rac1-deficient platelets by the use of flow cytometric analysis of cellular activation and aggregometry. Platelet spreading was analyzed with differential interference contrast microscopy, and activation of effector molecules was analyzed with biochemical approaches.

RESULTS

NSC23766 and EHT1864 showed strong and distinct Rac1-independent effects at 100 μm in platelet function tests. Both inhibitors induced Rac1-specific inhibition of platelet spreading, but also markedly impaired agonist-induced activation of Rac1(-/-) platelets. Furthermore, glycoprotein Ib-mediated signaling was dramatically inhibited by NSC23766 in both wild-type and Rac1-deficient platelets. Importantly, these inhibitors directly affected the activation of the Rac1 effectors p21-activated kinase (PAK)1 and PAK2.

CONCLUSIONS

Our results reveal critical off-target effects of NSC23766 and EHT1864 at 100 μm in mammalian cells, raising questions about their utility as specific Rac1/Rac inhibitors in biochemical studies at these concentrations and possibly as therapeutic agents.

Rac1 is crucial for Ras-dependent skin tumor formation by controlling Pak1-Mek-Erk hyperactivation and hyperproliferation in vivo

Rac1 has a role in proliferation and survival of tumor cells in vitro. The exact effects of Rac1 on growth, apoptosis and corresponding signaling pathways during tumorigenesis in vivo, however, have not been explored yet. Using mice with a keratinocyte-restricted deletion of the Rac1 gene, we found that Rac1 is essential for DMBA/TPA-induced skin tumor formation. This corresponded to a decreased keratinocyte hyperproliferation, although apoptosis was not detectably altered. Activated Rac1 promoted Erk-dependent hyperproliferation by Pak1-mediated Mek activation independent of Mek1 phosporylation at serine 298. Rac1 was furthermore required for Pak2-dependent hyperactivation of Akt, which under in vivo condition was restricted to the suprabasal cell layers corresponding to a suprabasal-specific expression of Pak2. It is surprising that none of these signaling pathways was altered in untreated Rac1-deficient skin, indicating a hyperproliferation-specific function of Rac1 in vivo. These data suggest that blocking of Rac1 function might allow tumor-specific growth repression, as Rac1 is not required for normal growth and growth signaling controlling pathways in skin in vivo.

beta1-integrin-mediated signaling essentially contributes to cell survival after radiation-induced genotoxic injury

Integrin-mediated adhesion to extracellular matrix proteins confers resistance to radiation- or drug-induced genotoxic injury. To analyse the underlying mechanisms specific for beta1-integrins, wild-type beta1A-integrin-expressing GD25beta1A cells were compared to GD25beta1B cells, which express signaling-incompetent beta1B variants. Cells grown on fibronectin, collagen-III, beta1-integrin-IgG or poly-l-lysine were exposed to 0-6 Gy X-rays in presence or depletion of growth factors and phosphatidylinositol-3 kinase (PI3K) inhibitors (LY294002, wortmannin). In order to test the relevance of these findings in tumor cells, human A-172 glioma cells were examined under the same conditions after siRNA-mediated silencing of beta1-integrins. We found that beta1A-integrin-mediated adhesion to fibronectin, collagen-III or beta1-IgG was essential for cell survival after radiation-induced genotoxic injury. Mediated by PI3K, pro-survival beta1A-integrin/Akt signaling was critically involved in this process. Additionally, the beta1-integrin downstream targets p130Cas and paxillin-impaired survival-regulating PI3K-dependent JNK. In A-172 glioma cells, beta1-integrin knockdown and PI3K inhibition confirmed the central role of beta1-integrins in Akt- and p130Cas/paxillin-mediated prosurvival signaling. These findings suggest beta1-integrins as critical regulators of cell survival after radiation-induced genotoxic injury. Elucidation of the molecular circuitry of prosurvival beta1-integrin-mediated signaling in tumor cells may promote the development of innovative molecular-targeted therapeutic antitumor strategies.

Leukocyte extravasation as a target for anti-inflammatory therapy - Which molecule to choose?

In view of the central pathogenic importance of leukocyte extravasation in inflammatory skin diseases, therapeutic interference with this - surprisingly complex - process is clearly a promising new approach for treating these dermatoses. Despite some disappointments during the clinical use of these agents and despite their crippling price tag, the recent incorporation of biologicals that target defined molecular controls of leukocyte extravasation into dermatological and rheumatological practise, consequently, has greatly enriched our therapeutic options for battling major, chronic, inflammatory dermatoses such as psoriasis. However, the - as yet unresolved and still rather controversially discussed - critical question is: Which of the multiple steps that control leukocyte extravasation in the human system really offer the most promising, most pragmatic, and safest molecular targets for therapeutic intervention for which disease entity? The current debate intends to stimulate public and rational debate of this crucial issue, beyond the evident commercial interests that are touched by whatever stand one takes.

Short arm region of laminin-5 gamma2 chain: structure, mechanism of processing and binding to heparin and proteins

Laminin-5 is a typical component of several epithelial tissues and contains a unique gamma2 chain which can be proteolytically processed by BMP-1. This occurs in the N-terminal half of the gamma2 chain (606 residues), which consists of two rod-like tandem arrays of LE modules, LE1-3 and LE4-6, that flank a globular L4m module containing the cleavage site. Recombinant analysis of L4m, which includes an additional imperfect LE module essential for proper folding, demonstrated an unusual pattern of disulfide bonding. These connectivities prevented the release of gamma2LE1-3L4 m after BMP-1 cleavage which required in addition disulfide reshuffling by isomerases. The liberated segment bound through its L4 m module to heparin, nidogen-1, fibulin-1 and fibulin-2. A further heparin/sulfatide-binding site could be attributed to some arginine residues in module LE1. The gamma2LE4-6 segment remaining in processed laminin-5 showed only a strong binding to fibulin-2. Immunological studies showed a similar partial processing in cell culture and tissues and the persistence of the released fragment in tissues. This indicated that both N-terminal regions of the gamma2 chain may have a function in vivo.

Mice lacking desmocollin 1 show epidermal fragility accompanied by barrier defects and abnormal differentiation

The desmosomal cadherin desmocollin (Dsc)1 is expressed in upper epidermis where strong adhesion is required. To investigate its role in vivo, we have genetically engineered mice with a targeted disruption in the Dsc1 gene. Soon after birth, null mice exhibit flaky skin and a striking punctate epidermal barrier defect. The epidermis is fragile, and acantholysis in the granular layer generates localized lesions, compromising skin barrier function. Neutrophils accumulate in the lesions and further degrade the tissue, causing sloughing (flaking) of lesional epidermis, but rapid wound healing prevents the formation of overt lesions. Null epidermis is hyperproliferative and overexpresses keratins 6 and 16, indicating abnormal differentiation. From 6 wk, null mice develop ulcerating lesions resembling chronic dermatitis. We speculate that ulceration occurs after acantholysis in the fragile epidermis because environmental insults are more stringent and wound healing is less rapid than in neonatal mice. This dermatitis is accompanied by localized hair loss associated with formation of utriculi and dermal cysts, denoting hair follicle degeneration. Possible resemblance of the lesions to human blistering diseases is discussed. These results show that Dsc1 is required for strong adhesion and barrier maintenance in epidermis and contributes to epidermal differentiation.

Neurocan is dispensable for brain development

Neurocan is a component of the extracellular matrix in brain. Due to its inhibition of neuronal adhesion and outgrowth in vitro and its expression pattern in vivo it was suggested to play an important role in axon guidance and neurite growth. To study the role of neurocan in brain development we generated neurocan-deficient mice by targeted disruption of the neurocan gene. These mice are viable and fertile and have no obvious deficits in reproduction and general performance. Brain anatomy, morphology, and ultrastructure are similar to those of wild-type mice. Perineuronal nets surrounding neurons appear largely normal. Mild deficits in synaptic plasticity may exist, as maintenance of late-phase hippocampal long-term potentiation is reduced. These data indicate that neurocan has either a redundant or a more subtle function in the development of the brain.

Functional consequences of integrin gene mutations in mice

Integrins are cell-surface receptors responsible for cell attachment to extracellular matrices and to other cells. The application of mouse genetics has significantly increased our understanding of integrin function in vivo. In this review, we summarize the phenotypes of mice carrying mutant integrin genes and compare them with phenotypes of mice lacking the integrin ligands.

Distinct roles for dystroglycan, (β)1 integrin and perlecan in cell surface laminin organization

Dystroglycan (DG) is a cell surface receptor for several extracellular matrix (ECM) molecules including laminins, agrin and perlecan. Recent data indicate that DG function is required for the formation of basement membranes in early development and the organization of laminin on the cell surface. Here we show that DG-mediated laminin clustering on mouse embryonic stem (ES) cells is a dynamic process in which clusters are consolidated over time into increasingly more complex structures. Utilizing various null-mutant ES cell lines, we define roles for other molecules in this process. In (β)1 integrin-deficient ES cells, laminin-1 binds to the cell surface, but fails to organize into more morphologically complex structures. This result indicates that (β)1 integrin function is required after DG function in the cell surface-mediated laminin assembly process. In perlecan-deficient ES cells, the formation of complex laminin-1 structures is defective, implicating perlecan in the laminin matrix assembly process. Moreover, laminin and perlecan reciprocally modulate the organization of the other on the cell surface. Taken together, the data support a model whereby DG serves as a receptor essential for the initial binding of laminin on the cell surface, whereas (β)1 integrins and perlecan are required for laminin matrix assembly processes after it binds to the cell.

Tie-1-directed expression of Cre recombinase in endothelial cells of embryoid bodies and transgenic mice

Tissue-specific gene inactivation using the Cre-loxP system has become an important tool to unravel functions of genes when the conventional null mutation is lethal. We report here the generation of a transgenic mouse line expressing Cre recombinase in endothelial cells. In order to avoid the production and screening of multiple transgenic lines we used embryonic stem cell and embryoid body technology to identify recombinant embryonic stem cell clones with high, endothelial-specific Cre activity. One embryonic stem cell clone that showed high Cre activity in endothelial cells was used to generate germline chimeras. The in vivo efficiency and specificity of the transgenic Cre was analysed by intercrossing the tie-1-Cre line with the ROSA26R reporter mice. At initial stages of vascular formation (E8-9), LacZ staining was detected in almost all cells of the forming vasculature. Between E10 and birth, LacZ activity was detected in most endothelial cells within the embryo and of extra-embryonic tissues such as yolk sac and chorioallantoic placenta. Ectopic expression of Cre was observed in approximately 12–20% of the adult erythroid, myeloid and lymphoid cells and in subregions of the adult brain. These results show that the tie-1-Cre transgenic strain can efficiently direct deletion of floxed genes in endothelial cells in vivo.

Rhodocytin (Aggretin) Activates Platelets Lacking α2β1 Integrin, Glycoprotein VI, and the Ligand-binding Domain of Glycoprotein Ibα

Although alpha(2)beta(1) integrin (glycoprotein Ia/IIa) has been established as a platelet collagen receptor, its role in collagen-induced platelet activation has been controversial. Recently, it has been demonstrated that rhodocytin (also termed aggretin), a snake venom toxin purified from the venom of Calloselasma rhodostoma, induces platelet activation that can be blocked by monoclonal antibodies against alpha(2)beta(1) integrin. This finding suggested that clustering of alpha(2)beta(1) integrin by rhodocytin is sufficient to induce platelet activation and led to the hypothesis that collagen may activate platelets by a similar mechanism. In contrast to these findings, we provided evidence that rhodocytin does not bind to alpha(2)beta(1) integrin. Here we show that the Cre/loxP-mediated loss of beta(1) integrin on mouse platelets has no effect on rhodocytin-induced platelet activation, excluding an essential role of alpha(2)beta(1) integrin in this process. Furthermore, proteolytic cleavage of the 45-kDa N-terminal domain of glycoprotein (GP) Ibalpha either on normal or on beta(1)-null platelets had no significant effect on rhodocytin-induced platelet activation. Moreover, mouse platelets lacking both alpha(2)beta(1) integrin and the activating collagen receptor GPVI responded normally to rhodocytin. Finally, even after additional proteolytic removal of the 45-kDa N-terminal domain of GPIbalpha rhodocytin induced aggregation of these platelets. These results demonstrate that rhodocytin induces platelet activation by mechanisms that are fundamentally different from those induced by collagen.

Skin and hair follicle integrity is crucially dependent on beta 1 integrin expression on keratinocytes

beta 1 integrins are ubiquitously expressed receptors that mediate cell-cell and cell-extracellular matrix interactions. To analyze the function of beta1 integrin in skin we generated mice with a keratinocyte-restricted deletion of the beta 1 integrin gene using the cre-loxP system. Mutant mice developed severe hair loss due to a reduced proliferation of hair matrix cells and severe hair follicle abnormalities. Eventually, the malformed hair follicles were removed by infiltrating macrophages. The epidermis of the back skin became hyperthickened, the basal keratinocytes showed reduced expression of alpha 6 beta 4 integrin, and the number of hemidesmosomes decreased. Basement membrane components were atypically deposited and, at least in the case of laminin-5, improperly processed, leading to disruption of the basement membrane and blister formation at the dermal-epidermal junction. In contrast, the integrity of the basement membrane surrounding the beta 1-deficient hair follicle was not affected. Finally, the dermis became fibrotic. These results demonstrate an important role of beta 1 integrins in hair follicle morphogenesis, in the processing of basement membrane components, in the maintenance of some, but not all basement membranes, in keratinocyte differentiation and proliferation, and in the formation and/or maintenance of hemidesmosomes.

Increased levels of apoptosis in the prefusion neural folds underlie the craniofacial disorder, Treacher Collins syndrome

Treacher Collins syndrome (TCS) is an autosomal dominant disorder of human craniofacial development that results from loss-of-function mutations in the gene TCOF1. Although this gene has been demonstrated to encode the nucleolar phosphoprotein treacle, the developmental mechanism underlying TCS remains elusive, particularly as expression studies have shown that the murine orthologue, Tcof1, is widely expressed. To investigate the molecular pathogenesis of TCS, we replaced exon 1 of Tcof1 with a neomycin-resistance cassette via homologous recombination in embryonic stem cells. Tcof1 heterozygous mice die perinatally as a result of severe craniofacial anomalies that include agenesis of the nasal passages, abnormal development of the maxilla, exencephaly and anophthalmia. These defects arise due to a massive increase in the levels of apoptosis in the prefusion neural folds, which are the site of the highest levels of Tcof1 expression. Our results demonstrate that TCS arises from haploinsufficiency of a protein that plays a crucial role in craniofacial development and indicate that correct dosage of treacle is essential for survival of cephalic neural crest cells.

Fetal and adult hematopoietic stem cells require beta1 integrin function for colonizing fetal liver, spleen, and bone marrow

Homing of hematopoietic stem cells (HSCs) into hematopoietic organs is a prerequisite for the establishment of hematopoiesis during embryogenesis and after bone marrow transplantation. We show that beta1 integrin-deficient HSCs from the para-aortic splanchnopleura and the fetal blood had hematolymphoid differentiation potential in vitro and in fetal organ cultures but were unable to seed fetal and adult hematopoietic tissues. Adult beta1 integrin null HSCs isolated from mice carrying loxP-tagged beta1 integrin alleles and ablated for beta1 integrin expression by retroviral cre transduction failed to engraft irradiated recipient mice. Moreover, absence of beta1 integrin resulted in sequestration of HSCs in the circulation and their reduced adhesion to endothelioma cells. These findings define beta1 integrin as an essential adhesion receptor for the homing of HSCs.

Discoidin domain receptor 1 is activated independently of beta(1) integrin

Various types of collagen have been identified as potential ligands for the two mammalian discoidin domain receptor (DDR) tyrosine kinases, DDR1 and DDR2. It is presently unclear whether collagen-induced DDR receptor activation, which occurs with very slow kinetics, involves additional proteins with kinase activity or membrane-anchored proteins serving as coreceptors. In particular, the role of the collagen-binding integrins alpha(1)beta(1) or alpha(2)beta(1) in the DDR activation process is undefined. Here, we provide three lines of evidence suggesting that DDR1 signaling is distinct from integrin activation. First we demonstrate that the enzymatic activity of DDR1 is essential for receptor tyrosine phosphorylation. Collagen-induced DDR receptor autophosphorylation can be blocked either by a dominant negative mutant or by a preparation of recombinant extracellular domain. Second, we show DDR1 signals independent of the epidermal growth factor (EGF) receptor. In cells that endogenously express both DDR1 and the EGF receptor, stimulation with EGF does not induce DDR activation. Third, we detected full DDR1 activation after collagen stimulation in cells that have been treated with blocking antibodies for alpha(2)beta(1) integrin or in cells with a targeted deletion of the beta(1) integrin gene. Finally, we show that overexpression of dominant negative DDR1 in the myoblast cell line C2C12 blocks cellular differentiation and the formation of myofibers.

Beta1 integrin promotes but is not essential for metastasis of ras-myc transformed fibroblasts

To investigate the role of beta1 integrin during tumor metastasis, we established a ras-myc transformed fibroblastoid cell line with a disrupted beta1 integrin gene on both alleles (GERM 11). Stable transfection of this cell line with an expression vector encoding beta1A integrin resulted in beta1A integrin-expressing sublines. Tumors were induced by subcutaneous injection of GERM 11 cells and 3 independent beta1 integrin expressing sublines (GERM 116, 1A10, 2F2) into syngeneic mice. After 10 days tumors were surgically removed. While average weights of GERM 11 and GERM 116 tumors were similar, tumors induced by the high expressing clones 1A10 and 2F2 were markedly smaller, suggesting an inverse correlation of tumor growth and beta1 integrin expression. The metastasis potential of all three beta1 integrin-expressing GERM 11 sublines tested was significantly higher than that of the beta1-deficient GERM 11 cells. GERM 116 tumors led in all animals to severe metastasis in lung and liver, while GERM 11 tumors induced only a few metastatic foci in the lung. Stroma of both tumors contained nidogen and high amounts of tenascin C, but only a few very low levels of fibronectin, laminin-1, and collagen type I. Beta1 integrin, therefore, increases but is not essential for metastasis of ras-myc transformed fibroblasts.

Isolation and functional characterization of the human 90K promoter

90K is a secreted protein thought to be involved in the body's defense against pathogens and cancer. To elucidate its transcriptional regulation, the promoter of human 90K (HGMW-approved symbol LGAL S3BP) was isolated and characterized. Analysis of the 3. 3-kb 5'-flanking region revealed that it is a TATA-less promoter, but neither GC-rich nor dependent on SP1 sites. RNase protection assays detected one major transcription start site (+1) and several minor transcription start sites upstream and downstream. Deletion studies defined a minimal promoter (-103 --> -49) and indirectly suggested positive synergism between different elements within it. Consistent with the proposed function of 90K, its promoter activity could be stimulated by poly(I). poly(C), mimicking viral infection. Two regions mediating induction by poly(I). poly(C) (-171 --> -112, -32 --> 46) were identified by deletion mutants. A small region around the minimal promoter (-99 --> -12) was highly homologous between human and mouse. While both human and mouse minimal promoters contained an interferon-responsive element (IRF-E), the human minimal promoter was not inducible by poly(I). poly(C) in contrast to that of the mouse. Point mutations 30 bp upstream of the IRF-E, however, conferred inducibility to the human minimal promoter, suggesting interaction between different promoter elements.

Mac-2 binding protein is a cell-adhesive protein of the extracellular matrix which self-assembles into ring-like structures and binds beta1 integrins, collagens and fibronectin

Human Mac-2 binding protein (M2BP) was prepared in recombinant form from the culture medium of 293 kidney cells and consisted of a 92 kDa subunit. The protein was obtained in a native state as indicated by CD spectroscopy, demonstrating alpha-helical and beta-type structure, and by protease resistance and immunological analysis. It was highly modified by N- and O-glycosylation but not by glycosaminoglycans. Ultracentrifugation showed non-covalent association into oligomers with molar masses of 1000-1500 kDa. Electron microscopy showed ring-like shapes with diameters of 30-40 nm. M2BP bound in solid-phase assays to collagens IV, V and VI, fibronectin and nidogen, but not to fibrillar collagens I and III or other basement membrane proteins. The protein also mediated adhesion of cell lines at comparable strength with laminin. Adhesion to M2BP was inhibited by antibodies to integrin beta1 subunits but not to alpha2 and alpha6 subunits, RGD peptide or lactose. This distinguishes cell adhesion of M2BP from that of laminin and excludes involvement of lactose-binding galectin-3. Immunological assays demonstrated variable secretion by cultured human cells of M2BP, which was detected in the extracellular matrix of several mouse tissues.

Genetic analysis of beta1 integrin function: confirmed, new and revised roles for a crucial family of cell adhesion molecules

Integrins are heterodimeric cell adhesion proteins connecting the extracellular matrix to the cytoskeleton and transmitting signals in both directions. These integrins are suggested to be involved in many different biological processes such as growth, differentiation, migration, and cell death. Of more than 20 known integrins, 10 contain the nearly ubiquitously expressed beta1 integrin subunit. Disruption of the beta1 integrin gene by homologous recombination allows us to assess the supposed functions of beta1 containing integrins in vivo in a new way. This review will present and discuss recent findings derived from such studies concerning the biological roles of beta1 integrins in early development, differentiation and migration, hematopoiesis, tumorigenesis, and supramolecular assembly of extracellular matrix proteins. While several former results were confirmed, others were contradicted and new functions found, significantly changing the previous view of beta1 integrin function in vivo.

Beta 1 integrin is essential for teratoma growth and angiogenesis

Teratomas are benign tumors that form after ectopic injection of embryonic stem (ES) cells into mice and contain derivatives of all primitive germ layers. To study the role of beta 1 integrin during teratoma formation, we compared teratomas induced by normal and beta1-null ES cells. Injection of normal ES cells gave rise to large teratomas. In contrast, beta 1-null ES cells either did not grow or formed small teratomas with an average weight of <5% of that of normal teratomas. Histological analysis of beta 1-null teratomas revealed the presence of various differentiated cells, however, a much lower number of host-derived stromal cells than in normal teratomas. Fibronectin, collagen I, and nidogen were expressed but, in contrast to normal teratomas, diffusely deposited in beta1-null teratomas. Basement membranes were present but with irregular shape and detached from the cell surface. Normal teratomas had large blood vessels with a smooth inner surface, containing both host- and ES cell-derived endothelial cells. In contrast, beta 1-null teratomas had small vessels that were loosely embedded into the connective tissue. Furthermore, endothelial cells were always of host-derived origin and formed blood vessels with an irregular inner surface. Although beta 1- deficient endothelial cells were absent in teratomas, beta 1-null ES cells could differentiate in vitro into endothelial cells. The formation of a complex vasculature, however, was significantly delayed and of poor quality in beta1-null embryoid bodies. Moreover, while vascular endothelial growth factor induced proliferation of endothelial cells as well as an extensive branching of blood vessels in normal embryoid bodies, it had no effect in beta 1-null embryoid bodies.

Sequence and chromosomal localization of the mouse brevican gene

Brevican is a brain-specific proteoglycan belonging to the aggrecan family. Phage clones containing the complete mouse brevican open reading frame of 2649 bp and the complete 3'-untranslated region of 341 bp were isolated from a mouse brain cDNA library, and cosmid clones containing the mouse brevican gene were isolated from a genomic library using a PCR-generated DNA fragment as probe. The obtained genomic sequence of 13,700 nucleotides revealed that the murine gene has a size of approximately 13 kb and contains the sequence of the mRNA for the secreted brevican isoform on 14 exons. The exon-intron structure reflected the structural organization of the multidomain protein brevican. No consensus TATA sequence was found upstream of the first exon, and RNase protection experiments revealed multiple transcriptional start sites for the brevican gene. The first part of the sequence of intron 8 corresponded to an alternative brevican cDNA, coding for a GPI-linked isoform. Single strand conformation polymorphism analysis mapped the brevican gene (Bcan) to chromosome 3 between the microsatellite markers D3Mit22 and D3Mit11.

Expression of the 90K immunostimulator gene is controlled by a promoter with unique features

90K is a secreted glycoprotein with tumor suppressive functions, which is up-regulated in various types of cancer and in AIDS. In order to understand the regulation of its expression, the mouse 90K gene was isolated and analyzed. The gene spans about 8.8-kilobase pairs and consists of 6 exons and was localized on chromosome 11, region E. RNase protection identified one major transcription start site (+1) and three minor ones (-3, +32, +34). The mouse 90K gene was found to have a TATA-less promoter of unusual structure. The 2. 3-kilobase pair 5'-flanking region exhibited strong promoter activity in NIH 3T3 cells; however, it contained neither a TATA-box nor a SP1 site and was not GC-rich. No known initiator motif was found around the transcription start site. 5'- and 3'-deletions defined a minimal promoter of 51 base pairs (-66 --> -16), not including the start site, essential and sufficient for promoter activity. This minimal promoter showed increased activity after stimulation with interferon-gamma or poly(I.C), a substance mimicking viral infection. Essential for both inductions was the integrity of an interferon regulatory factor element within this sequence, a potential binding site for the anti-oncogenic transcription factor interferon regulatory factor-1.

Functional discrepancies between tumor necrosis factor and lymphotoxin alpha explained by trimer stability and distinct receptor interactions

Tumor necrosis factor (TNF) and lymphotoxin alpha (LT alpha) are closely related cytokines which bind with nearly identical affinities to the same pair of cell surface receptors, p55 and p75TNFR. Therefore it is assumed that TNF and LT alpha are redundant cytokines. This study, however, demonstrates that TNF and LT alpha differ significantly with regard to their mitogenic and cytotoxic potentials. LT alpha's superior mitogenic effect could be explained by its formation of a more stable trimer. In contrast to the TNF trimer, which disintegrated under physiological conditions into biologically inactive monomers, the LT alpha trimer remained stable for several days. Accordingly, LT alpha more effectively induced fibroblast growth which demands long-term presence of the cytokine. TNF's superior cytotoxicity, which requires only short-term impact of the cytokine, could be attributed to a distinct interaction with the human p55TNFR. This was demonstrated in NIH 3T3 cells transfected with the human p55TNFR, where cytotoxicity is mediated exclusively by the transfected receptor. Although the p55ATNFR had virtually identical affinities for TNF and LT alpha, as defined by Scatchard analysis, it nevertheless discriminated between binding of each cytokine and showed a 200-fold enhanced cytotoxicity mediated by TNF.

Suppression of tumor growth in vivo by local and systemic 90K level increase

Expression levels of the immunostimulatory 90K antigen in mammary carcinoma, glioblastoma, and other tumor-derived cell lines inversely correlate with their tumorigenicity in athymic mice. Engineered enhancement of 90K expression results in significant (> 80%) tumor growth inhibition, not by direct action on the tumor cell, but by stimulation of the residual cell-mediated immune defense of the nude mouse. Enhanced 90K level effects are both localized and systemic and involve induction of ICAM-1 and VCAM-1 in the tumor endothelium. The findings presented suggest a role for 90K as a molecular alarm signal for the body's cellular defense against pathogens, which in a subset of tumors is suppressed to allow cancer progression.

Structural requirements for inducible shedding of the p55 tumor necrosis factor receptor

Induced shedding of the p55 tumor necrosis factor receptor (p55-R) was previously shown to be independent of the amino acid sequence properties of the intracellular domain of this receptor. We now find it also independent of the sequence properties of the transmembrane domain and of the cysteine-rich region that constitutes most of the extracellular domain of the receptor. The shedding is shown to depend solely on the sequence properties of a small region within the spacer that links the cysteine-rich region in the extracellular domain to the transmembrane domain. Detailed tests of effects of mutations in the spacer on the shedding indicate that the process is independent of the amino acid side-chain identity in this region except for a limited dependence on the identity of 1 residue (Val-173), located downstream to the putative major cleavage site of the receptor. It is strongly affected, however, by some mutations that seem to change the conformation of the spacer region. These findings suggest that a short amino acid sequence in the p55-R is essential and sufficient for its shedding and that the shedding is mediated either by a protease with limited sequence specificity or by several different proteases that recognize different amino acid sequences, yet it strictly depends on some conformational features of the cleavage region in the receptor.

Dual role of the p75 tumor necrosis factor (TNF) receptor in TNF cytotoxicity

Whereas there is ample evidence for involvement of the p55 tumor necrosis factor (TNF) receptor (p55-R) in the cytocidal effect of TNF, the role of the p75 TNF receptor (p75-R) in this effect is a matter of debate. In this study, we probed the function of p75-R in cells sensitive to the cytotoxicity of TNF using a wide panel of antibodies (Abs) against the receptor's extracellular domain. Two distinct Ab effects were observed. The Abs triggered signaling for cytotoxicity. This effect: (a) was correlated with the extent of p75-R expression by the cells; (b) was dependent on receptor cross-linking by the Abs; (c) occurred in HeLa cells, but not in A9 cells transfected with human p75-R or in HeLa cells expressing cytoplasmically truncated p75-R mutants, indicating that it involves cell-specific activities of the intracellular domain of the receptor; (d) was synergistic with the cytocidal effect of Abs against p55-R. Moreover, it seemed to reverse induced desensitization to the cytocidal effect of anti p55-R Abs, suggesting that it involves mechanisms different from those of the signaling by the p55 TNF-R. In addition, the Abs affected the response to TNF in a way that does not involve the signaling activity of p75-R. These effects: (a) could be observed also in cells in which only p55-R signaled for the cytocidal effect; (b) were not dependent on receptor cross-linking by the Abs; (c) varied according to the site at which the Abs bound to the receptor; and (d) were correlated inversely with the effects of the Abs on TNF binding to p75-R. That is, Abs binding to the membrane-distal part of the receptor's extracellular domain displaced TNF from the p75 receptor and enhanced cytocidal effect, whereas Abs that bind to the membrane-proximal part of the extracellular domain--a region at which a conformational change seems to take place upon TNF binding--decreased the dissociation of TNF from p75-R and inhibited its cytocidal effect. The above findings suggest that p75-R contributes to the cytocidal effect of TNF both by its own signaling and by regulating the access of TNF to p55-R.

Selective decrease in cell surface expression and mRNA level of the 55-kDa tumor necrosis factor receptor during differentiation of HL-60 cells into macrophage-like but not granulocyte-like cells

Expression of the two known receptors for TNF was studied in the promyelocytic leukemia cell line HL-60 before and after differentiation of the cells along the granulocyte lineage (induced by incubation with retinoic acid), or along the macrophage lineage (induced by incubation with the phorbol diester, PMA). The extent of inhibition of TNF binding by receptor-specific antisera, as well as the size of the complexes formed after cross-linking TNF to its receptors on intact cells, indicated that both receptor species were expressed on the surface of the undifferentiated HL60 cells. Differentiation into granulocyte-like cells resulted in some increase in TNF binding. The increase was apparently due to enhanced expression of the 75-kDa TNF-R, whereas the amounts of the 55-kDa TNF-R did not change significantly. In contrast, in HL-60 cells induced to differentiate into macrophage-like cells, expression of the 55-kDa TNF-R species was completely abolished. The pattern of TNF-R expression in the differentiated HL-60 cells was similar to that observed in leukocytes isolated from peripheral blood: on granulocytes, there were about equal amounts of both receptor species, whereas on monocytes the 75-kDa receptor was predominant. The loss of 55-kDa receptors during differentiation of HL-60 cells into macrophage-like cells was accompanied by a pronounced decrease in the level of the mRNA for that receptor, suggesting that at least part of the change in TNF-R expression is due to mechanisms that control the amounts of receptor mRNA. Although little is yet known regarding the functional differences between the two receptor species, marked changes in the pattern of their expression, as observed during HL-60 cell differentiation, are likely to alter the kind of response of the cells to TNF and may therefore play an important role in the coordination of TNF effects in the organism.

Selective decrease in cell surface expression and mRNA level of the 55-kDa tumor necrosis factor receptor during differentiation of HL-60 cells into macrophage-like but not granulocyte-like cells

Expression of the two known receptors for TNF was studied in the promyelocytic leukemia cell line HL-60 before and after differentiation of the cells along the granulocyte lineage (induced by incubation with retinoic acid), or along the macrophage lineage (induced by incubation with the phorbol diester, PMA). The extent of inhibition of TNF binding by receptor-specific antisera, as well as the size of the complexes formed after cross-linking TNF to its receptors on intact cells, indicated that both receptor species were expressed on the surface of the undifferentiated HL60 cells. Differentiation into granulocyte-like cells resulted in some increase in TNF binding. The increase was apparently due to enhanced expression of the 75-kDa TNF-R, whereas the amounts of the 55-kDa TNF-R did not change significantly. In contrast, in HL-60 cells induced to differentiate into macrophage-like cells, expression of the 55-kDa TNF-R species was completely abolished. The pattern of TNF-R expression in the differentiated HL-60 cells was similar to that observed in leukocytes isolated from peripheral blood: on granulocytes, there were about equal amounts of both receptor species, whereas on monocytes the 75-kDa receptor was predominant. The loss of 55-kDa receptors during differentiation of HL-60 cells into macrophage-like cells was accompanied by a pronounced decrease in the level of the mRNA for that receptor, suggesting that at least part of the change in TNF-R expression is due to mechanisms that control the amounts of receptor mRNA. Although little is yet known regarding the functional differences between the two receptor species, marked changes in the pattern of their expression, as observed during HL-60 cell differentiation, are likely to alter the kind of response of the cells to TNF and may therefore play an important role in the coordination of TNF effects in the organism.

Cytoplasmic truncation of the p55 tumour necrosis factor (TNF) receptor abolishes signalling, but not induced shedding of the receptor

The mechanistic relationship between the signalling for the TNF effects by the human p55 TNF receptor (hu-p55-TNF-R) and the formation of a soluble form of the receptor, which is inhibitory to these effects, was explored by examining the function of C-terminally truncated mutants of the receptor, expressed in rodent cells. The 'wild-type' receptor signalled for a cytocidal effect when cross-linked with specific antibodies and exhibited spontaneous shedding. Shedding of the receptor was not affected by TNF but was markedly enhanced by 4 beta-phorbol-12-myristate-13-acetate (PMA). Receptor mutants with 53%, 83% and 96% C-terminal deletions could not signal for the cytocidal effect. Furthermore, they were found to associate with the endogenous rodent receptors, interfering with their signalling. Yet even the deletion of 96% of the intracellular domain did not abolish shedding of the receptor in response to PMA. These findings suggest that signalling and shedding of the p55 TNF-R are mechanistically distinct.

Stabilization of the bioactivity of tumor necrosis factor by its soluble receptors

The receptors for tumor necrosis factor (TNF) exist in cell-associated as well as soluble forms, both binding specifically to TNF. Since the soluble forms of TNF receptors (sTNF-Rs) can compete with the cell-associated TNF receptors for TNF, it was suggested that they function as inhibitors of TNF activity; at high concentrations, the sTNF-Rs indeed inhibit TNF effects. However, we report here that in the presence of low concentrations of the sTNF-Rs, effects of TNF whose induction depend on prolonged treatment with this cytokine are augmented, reflecting an attenuation by the sTNF-Rs of spontaneous TNF activity decay. Evidence that this stabilization of TNF activity by the sTNF-Rs follows from stabilization of TNF structure within the complexes that TNF forms with the sTNF-Rs is presented here, suggesting that the sTNF-Rs can affect TNF activity not only by interfering with its binding to cells but also by stabilizing its structure and preserving its activity, thus augmenting some of its effects.

Soluble forms of tumor necrosis factor receptors (TNF-Rs). The cDNA for the type I TNF-R, cloned using amino acid sequence data of its soluble form, encodes both the cell surface and a soluble form of the receptor

Two proteins which specifically bind tumor necrosis factor (TNF) have recently been isolated from human urine in our laboratory. The two proteins cross-react immunologically with two species of cell surface TNF receptors (TNF-R). Antibodies against one of the two TNF binding proteins (TBPI) were found to have effects characteristic of TNF, including stimulating phosphorylation of specific cellular proteins. Oligonucleotide probes designed on the basis of the NH2-terminal amino acid sequence of TBPI were used to clone the cDNA for the structurally related cell surface type 1 TNF-R. It is notable that although this receptor can signal the phosphorylation of cellular proteins, it appears from its amino acid sequence to be devoid of intrinsic protein kinase activity. The extracellular domain of the receptor is composed of four internal cysteine-rich repeats, homologous to structures repeated four times in the extracellular domains of the nerve growth factor receptor and the B lymphocytes surface antigen CDw40. The amino acid composition and size of the extracellular domain of the type I TNF-R closely resemble those of TBPI. The COOH-terminal amino acid sequence of the four cysteine rich repeats within the extracellular domain of the type I TNF-R matches the COOH-terminal sequence of TBPI. Amino acid sequences in the extracellular domain also fully match other sequences found in TBPI. On the other hand, amino acid sequences in the soluble form of the type II TNF-R (TBPII), while indicating a marked homology of structure, did not suggest any identity between this protein and the extracellular domain of the type I TNF-R. CHO cells transfected with type I TNF-R cDNA produced both cell surface and soluble forms of the receptor. The receptor produced by CHO cells was recognized by several monoclonal antibodies against TBPI, reacting with several distinct epitopes in this molecule. These data suggest that the soluble forms of the TNF-Rs are structurally identical to the extracellular cytokine binding domains of these receptors and are consistent with the notion that the soluble forms are, at least partly, derived from the same transcripts that encode the cell surface receptors.

Antibodies to a soluble form of a tumor necrosis factor (TNF) receptor have TNF-like activity

Immunological cross-reactivity between tumor necrosis factor (TNF) binding proteins which are present in human urine (designated TBPI and TBPII) and two molecular species of the cell surface receptors for TNF is demonstrated. The two TNF receptors are shown to be immunologically distinct, to differ in molecular weight (58,000 and 73,000), and to be expressed differentially in different cells. It is further shown that polyclonal antibodies against one of the TNF binding proteins (TBPI) display, by virtue of their ability to bind the TNF receptor, activities which are very similar to those of TNF. These antibodies are cytotoxic to cells which are sensitive to TNF toxicity, induce resistance to TNF toxicity, enhance the incorporation of thymidine into normal fibroblasts, inhibit the growth of chlamydiae, and induce the synthesis of prostaglandin E2. Monovalent F(ab) fragments of the polyclonal antibodies lack TNF-like activities, but acquire them upon cross-linking with anti-F(ab)2 antibodies, suggesting that the ability of the anti-TBPI antibodies to mimic TNF correlates with their ability to cross-link the TNF receptors. This notion was further supported by data obtained in a comparative study of the TNF-like cytotoxicity of a panel of monoclonal antibodies against TBPI. The induction of TNF-like effects by antibodies to a TNF receptor suggests that TNF is not directly involved in intracellular signalling. Rather, it is the receptors to this cytokine which, when properly triggered in a process which appears to involve clustering of these receptors, transduce the signal for response to TNF into the cell's interior.