The solution structure of EMILIN1 globular C1q domain reveals a disordered insertion necessary for interaction with the alpha4beta1 integrin

The extracellular matrix protein EMILIN-1 impacts on the microenvironment by hampering gastric cancer development and progression

BACKGROUND

The contribution of the tumor microenvironment and extracellular matrix to the aggressive biology of Gastric Cancer (GC) has been recently characterized; however, the role of EMILIN-1 in this context is unknown. EMILIN-1 is an essential structural element for the maintenance of lymphatic vessel (LV) integrity and displays anti-proliferative properties as demonstrated in skin and colon cancer. Given the key role of LVs in GC progression, the aim of this study was to investigate the role of EMILIN-1 in GC mouse models.

METHODS

We used the syngeneic YTN16 cells which were injected subcutaneously and intraperitoneally in genetically modified EMILIN-1 mice. In alternative, carcinogenesis was induced using N-Methyl-N-nitrosourea (MNU). Mouse-derived samples and human biopsies were analyzed by IHC and IF to the possible correlation between EMILIN-1 expression and LV pattern.

RESULTS

Transgenic mice developed tumors earlier compared to WT animals. 20 days post-injection tumors developed in EMILIN-1 mutant mice were larger and displayed a significant increase of lymphangiogenesis. Treatment of transgenic mice with MNU associated with an increased number of tumors, exacerbated aggressive lesions and higher levels of LV abnormalities. A significant correlation between the levels of EMILIN-1 and podoplanin was detected also in human samples, confirming the results obtained with the pre-clinical models.

CONCLUSIONS

This study demonstrates for the first time that loss of EMILIN-1 in GC leads to lymphatic dysfunction and proliferative advantages that sustain tumorigenesis, and assess the use of our animal model as a valuable tool to verify the fate of GC upon loss of EMILIN-1.

Assessing nanobody interaction with SARS-CoV-2 Nsp9

The interaction between SARS-CoV-2 non-structural protein Nsp9 and the nanobody 2NSP90 was investigated by NMR spectroscopy using the paramagnetic perturbation methodology PENELOP (Paramagnetic Equilibrium vs Nonequilibrium magnetization Enhancement or LOss Perturbation). The Nsp9 monomer is an essential component of the replication and transcription complex (RTC) that reproduces the viral gRNA for subsequent propagation. Therefore preventing Nsp9 recruitment in RTC would represent an efficient antiviral strategy that could be applied to different coronaviruses, given the Nsp9 relative invariance. The NMR results were consistent with a previous characterization suggesting a 4:4 Nsp9-to-nanobody stoichiometry with the occurrence of two epitope pairs on each of the Nsp9 units that establish the inter-dimer contacts of Nsp9 tetramer. The oligomerization state of Nsp9 was also analyzed by molecular dynamics simulations and both dimers and tetramers resulted plausible. A different distribution of the mapped epitopes on the tetramer surface with respect to the former 4:4 complex could also be possible, as well as different stoichiometries of the Nsp9-nanobody assemblies such as the 2:2 stoichiometry suggested by the recent crystal structure of the Nsp9 complex with 2NSP23 (PDB ID: 8dqu), a nanobody exhibiting essentially the same affinity as 2NSP90. The experimental NMR evidence, however, ruled out the occurrence in liquid state of the relevant Nsp9 conformational change observed in the same crystal structure.

Multimerin-1 and cancer: a review

Multimerin-1 (MMRN1) is a platelet protein with a role in haemostasis and coagulation. It is also present in endothelial cells (ECs) and the extracellular matrix (ECM), where it may be involved in cell adhesion, but its molecular functions and protein–protein interactions in these cellular locations have not been studied in detail yet. In recent years, MMRN1 has been identified as a differentially expressed gene (DEG) in various cancers and it has been proposed as a possible cancer biomarker. Some evidence suggest that MMRN1 expression is regulated by methylation, protein interactions, and non-coding RNAs (ncRNAs) in different cancers. This raises the questions if a functional role of MMRN1 is being targeted during cancer development, and if MMRN1’s differential expression pattern correlates with cancer progression. As a result, it is timely to review the current state of what is known about MMRN1 to help inform future research into MMRN1’s molecular mechanisms in cancer.

Elastin MIcrofibriL INterfacer1 (EMILIN-1) is an alternative prosurvival VLA-4 ligand in chronic lymphocytic leukemia

CD49d, the α4 chain of the VLA-4 integrin, is a negative prognosticator in chronic lymphocytic leukemia (CLL) with a key role in CLL cell-microenvironment interactions mainly occurring via its ligands VCAM-1 and fibronectin. In the present study, we focused on EMILIN-1 (Elastin-MIcrofibriL-INterfacer-1), an alternative VLA-4 ligand whose role has been so far reported only in non-hematological settings, by investigating: i) the distribution of EMILIN-1 in CLL-involved tissues; ii) the capability of EMILIN-1 to operate, via its globular C1q (gC1q) domain, as additional adhesion ligand in CLL; iii) the functional meaning of EMILIN-1 gC1q/VLA-4 interactions in CLL. EMILIN-1 is widely present in the CLL-involved areas of bone marrow biopsies (BMBs) without difference between CD49d negative and positive cases, displaying at least three different expression patterns: "fibrillar", "dot-like" and "mixed". The lack in CLL-BMB of neutrophil elastase, whose proteolytic activity degrades EMILIN-1 and impairs EMILIN-1 function, suggests full functional EMILIN-1 in CLL independently of its expression pattern. Functionally, EMILIN-1 gC1q domain promotes adhesion of CLL cells through specific interaction with VLA-4, and releases pro-survival signals for CLL cells, as demonstrated by enhanced ERK and AKT phosphorylation and impairment of in-vitro-induced apoptosis. EMILIN-1/VLA-4 interaction can efficiently contribute to the maintenance of the neoplastic clone in CLL.

Extracellular Matrix Analysis of Human Renal Arteries in Both Quiescent and Active Vascular State

In vascular tissue engineering strategies, the addition of vascular-specific extracellular matrix (ECM) components may better mimic the in vivo microenvironment and potentially enhance cell-matrix interactions and subsequent tissue growth. For this purpose, the exact composition of the human vascular ECM first needs to be fully characterized. Most research has focused on characterizing ECM components in mature vascular tissue; however, the developing fetal ECM matches the active environment required in vascular tissue engineering more closely. Consequently, we characterized the ECM protein composition of active (fetal) and quiescent (mature) renal arteries using a proteome analysis of decellularized tissue. The obtained human fetal renal artery ECM proteome dataset contains higher levels of 15 ECM proteins versus the mature renal artery ECM proteome, whereas 16 ECM proteins showed higher levels in the mature tissue compared to fetal. Elastic ECM proteins EMILIN1 and FBN1 are significantly enriched in fetal renal arteries and are mainly produced by cells of mesenchymal origin. We functionally tested the role of EMILIN1 and FBN1 by anchoring the ECM secreted by vascular smooth muscle cells (SMCs) to glass coverslips. This ECM layer was depleted from either EMILIN1 or FBN1 by using siRNA targeting of the SMCs. Cultured endothelial cells (ECs) on this modified ECM layer showed alterations on the transcriptome level of multiple pathways, especially the Rho GTPase controlled pathways. However, no significant alterations in adhesion, migration or proliferation were observed when ECs were cultured on EMILIN1- or FNB1-deficient ECM. To conclude, the proteome analysis identified unique ECM proteins involved in the embryonic development of renal arteries. Alterations in transcriptome levels of ECs cultured on EMILIN1- or FBN1-deficient ECM showed that these candidate proteins could affect the endothelial (regenerative) response.

VLA-4 Expression and Activation in B Cell Malignancies: Functional and Clinical Aspects

Lineage commitment and differentiation of hematopoietic cells takes place in well-defined microenvironmental surroundings. Communication with other cell types is a vital prerequisite for the normal functions of the immune system, while disturbances in this communication support the development and progression of neoplastic disease. Integrins such as the integrin very late antigen-4 (VLA-4; CD49d/CD29) control the localization of healthy as well as malignant B cells within the tissue, and thus determine the patterns of organ infiltration. Malignant B cells retain some key characteristics of their normal counterparts, with B cell receptor (BCR) signaling and integrin-mediated adhesion being essential mediators of tumor cell homing, survival and proliferation. It is thus not surprising that targeting the BCR pathway using small molecule inhibitors has proved highly effective in the treatment of B cell malignancies. Attenuation of BCR-dependent lymphoma–microenvironment interactions was, in this regard, described as a main mechanism critically contributing to the efficacy of these agents. Here, we review the contribution of VLA-4 to normal B cell differentiation on the one hand, and to the pathophysiology of B cell malignancies on the other hand. We describe its impact as a prognostic marker, its interplay with BCR signaling and its predictive role for novel BCR-targeting therapies, in chronic lymphocytic leukemia and beyond.

Gene Expression Profiling of the Extracellular Matrix Signature in Macrophages of Different Activation Status: Relevance for Skin Wound Healing

The extracellular matrix (ECM) provides structural support for tissue architecture and is a major effector of cell behavior during skin repair and inflammation. Macrophages are involved in all stages of skin repair but only limited knowledge exists about macrophage-specific expression and regulation of ECM components. In this study, we used transcriptome profiling and bioinformatic analysis to define the unique expression of ECM-associated genes in cultured macrophages. Characterization of the matrisome revealed that most genes were constitutively expressed and that several genes were uniquely regulated upon interferon gamma (IFNγ) and dexamethasone stimulation. Among those core matrisome and matrisome-associated components transforming growth factor beta (TGFβ)-induced, matrix metalloproteinase 9 (MMP9), elastin microfibril interfacer (EMILIN)-1, netrin-1 and gliomedin were also present within the wound bed at time points that are characterized by profound macrophage infiltration. Hence, macrophages are a source of ECM components in vitro as well as during skin wound healing, and identification of these matrisome components is a first step to understand the role and therapeutic value of ECM components in macrophages and during wound healing.

A proteome comparison between human fetal and mature renal extracellular matrix identifies EMILIN1 as a regulator of renal epithelial cell adhesion

Cell-based approaches using tissue engineering and regenerative medicine to replace damaged renal tissue with 3D constructs is a promising emerging therapy for kidney disease. Besides living cells, a template provided by a scaffold based on biomaterials and bioactive factors is needed for successful kidney engineering. Nature's own template for a scaffolding system is the extracellular matrix (ECM). Research has focused on mapping the mature renal ECM; however, the developing fetal ECM matches more the active environment required in 3D renal constructs. Here, we characterized the differences between the human fetal and mature renal ECM using spectrometry-based proteomics of decellularized tissue. We identified 99 different renal ECM proteins of which the majority forms an overlapping core, but also includes proteins enriched in either the fetal or mature ECM. Relative protein quantification showed a significant dominance of EMILIN1 in the fetal ECM. We functionally tested the role of EMILIN1 in the ECM using a novel methodology that permits the reliable anchorage of native cell-secreted ECM to glass coverslips. Depletion of EMILIN1 from the ECM layer using siRNA mediated knock-down technologies does not affect renal epithelial cell growth, but does promote migration. Lack of EMILIN1 in the ECM layer reduces the adhesion strength of renal epithelial cells, shown by a decrease in focal adhesion points and associated stress fibers. We showed in this study the importance of a human renal fetal and mature ECM catalogue for identifying promising ECM components that have high implementation potential in scaffolds for 3D renal constructs.

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Proteomics revealed the differences between the renal fetal and mature extracellular matrix.

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EMILIN1 has a significant dominance in the fetal extracellular matrix.

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EMILIN1 depletion from the extracellular matrix reduces the adhesion strength and promotes migration of renal epithelial cells.

Integrin binding site within the gC1q domain orchestrates EMILIN-1-induced lymphangiogenesis

Lymphatic vessels (LVs) play a pivotal role in the control of tissue homeostasis and also have emerged as important regulators of immunity, inflammation and tumor metastasis. EMILIN-1 is the first ECM protein identified as a structural modulator of the growth and maintenance of LV; accordingly, Emilin1^-/- mice display lymphatic morphological alterations leading to functional defects as mild lymphedema, leakage and compromised lymph drainage. Many EMILIN-1 functions are exerted by the binding of its gC1q domain with the E933 residue of α4 and α9β1 integrins. To investigate the specific regulatory role of this domain on lymphangiogenesis, we generated a transgenic mouse model expressing an E933A-mutated EMILIN-1 (E1-E933A), unable to interact with α4 or α9 integrin. The mutant resulted in abnormal LV architecture with dense, tortuous and irregular networks; moreover, the number of anchoring filaments was reduced and collector valves had aberrant narrowed structures. E933A mutation also affected lymphatic function in lymphangiography assays and made the transgenic mice more prone to lymph node metastases. The finding that the gC1q/integrin interaction is crucial for a correct lymphangiogenesis response was confirmed and reinforced by functional in vitro tubulogenesis assays. In addition, ex vivo thoracic-duct ring assays revealed that E1-E933A-derived lymphatic endothelial cells had a severe reduction in sprouting capacity and were unable to organize into capillary-like structures. All these data provide evidence that the novel "regulatory structural" role of EMILIN-1 in the lymphangiogenic process is played by the integrin binding site within its gC1q domain.

The α4β1/EMILIN1 interaction discloses a novel and unique integrin-ligand type of engagement

EMILIN1, a homo-trimeric adhesive ECM glycoprotein, interacts with the α4β1 integrin through its gC1q domain. Uniquely among the C1q family members, the EMILIN1 gC1q presents only nine-stranded β-sandwich fold and the missing strand is substituted by a disordered 19-residue long segment spanning from Y927 to G945 at the apex of the gC1q domain. This unstructured loop exposes to the solvent the acidic residue E933, which plays a key role in the α4β1 integrin mediated interaction. Here, we experimentally determined that the three E933 residues (one from each monomer) are all required for ligand binding. By docking the NMR structure of the gC1q to a virtual α4β1 crystal structure based on the known structures of α4β7 and α5β1 integrins we built a model of α4β1-gC1q complex where three E933 residues are smoothly forced to coordinate the Mg²⁺ ion at the βI MIDAS site of the integrin. By bringing the three E933 close in space, the trimeric supramolecular organization of gC1q allows the formation of a proper 3D geometry and suggests a quaternary-structure-dependent mode of interaction. Furthermore, we experimentally identified R904 as a synergistic residue for cell adhesion. Accordingly, the model showed that this residue is able to form potential stabilizing intra-chain salt bridges with residues E928 and E930. This mode of interaction likely accounts for a more stable and durable α4β1-gC1q interaction in comparison with the prototypic CS1 ligand. To our knowledge, this is the first report describing the simultaneous involvement of all the three acidic residues of a trimeric ligand in the formation of a dimeric complex with the integrin βI domain.

The role of α9β1 integrin and its ligands in the development of autoimmune diseases

Adhesion of cells to extracellular matrix proteins through integrins expressed on the cell surface is important for cell adhesion/motility, survival, and differentiation. Recently, α9β1 integrin was reported to be important for the development of autoimmune diseases including rheumatoid arthritis, multiple sclerosis, and their murine models. In addition, ligands for α9β1 integrin, such as osteopontin and tenascin-C, are well established as key regulators of autoimmune diseases. Therefore, this review focused on the role of interactions between α9β1 integrin and its ligands in the development of autoimmune diseases.

Neutrophil elastase cleavage of the gC1q domain impairs the EMILIN1-α4β1 integrin interaction, cell adhesion and anti-proliferative activity

The extracellular matrix glycoprotein EMILIN1 exerts a wide range of functions mainly associated with its gC1q domain. Besides providing functional significance for adhesion and migration, the direct interaction between α4β1 integrin and EMILIN1-gC1q regulates cell proliferation, transducing net anti-proliferative effects. We have previously demonstrated that EMILIN1 degradation by neutrophil elastase (NE) is a specific mechanism leading to the loss of functions disabling its regulatory properties. In this study we further analysed the proteolytic activity of NE, MMP-3, MMP-9, and MT1-MMP on EMILIN1 and found that MMP-3 and MT1-MMP partially cleaved EMILIN1 but without affecting the functional properties associated with the gC1q domain, whereas NE was able to fully impair the interaction of gC1q with the α4β1 integrin by cleaving this domain outside of the E933 integrin binding site. By a site direct mutagenesis approach we mapped the bond between S913 and R914 residues and selected the NE-resistant R914W mutant still able to interact with the α4β1 integrin after NE treatment. Functional studies showed that NE impaired the EMILIN1-α4β1 integrin interaction by cleaving the gC1q domain in a region crucial for its proper structural conformation, paving the way to better understand NE effects on EMILIN1-cell interaction in pathological context.

Distinct methylation profiles characterize fusion-positive and fusion-negative rhabdomyosarcoma

Rhabdomyosarcoma comprises two major subtypes, fusion positive (PAX3-FOXO1 or PAX7-FOXO1) and fusion negative. To investigate the significance of DNA methylation in these subtypes, we analyzed methylation profiles of 37 rhabdomyosarcoma tumors and 10 rhabdomyosarcoma cell lines, as well as 8 normal tissues. Unsupervised clustering of DNA methylation clearly distinguished the fusion-positive and fusion-negative subsets. The fusion-positive tumors showed substantially lower overall levels of methylation compared with fusion-negative tumors. Comparison with the methylation pattern of normal skeletal muscle and bone marrow indicates that fusion-negative rhabdomyosarcoma is more similar to these normal tissues compared with fusion-positive rhabdomyosarcoma, and suggests that many of the methylation differences between these subtypes arise from 'aberrant' hyper- and hypomethylation events in fusion-positive rhabdomyosarcoma. Integrative methylation and gene expression analysis revealed that methylation differences between fusion-positive and fusion-negative tumors could either be positively or negatively associated with mRNA expression. There was no significant difference in the distribution of PAX3-FOXO1-binding sites between genes with and without differential methylation. However, the finding that PAX3-FOXO1-binding sites were enriched among genes that were both differentially methylated and differentially expressed suggests that the fusion protein interacts with DNA methylation to regulate target gene expression. An 11-gene DNA methylation signature, classifying the rhabdomyosarcoma tumors into fusion-positive and fusion-negative subsets, was established and validated by pyrosequencing assays. Notably, EMILIN1 (part of the 11-gene signature) showed higher methylation and lower mRNA expression in fusion-positive compared with fusion-negative tumors, and demonstrated demethylation and re-expression in multiple fusion-positive cell lines after treatment with 5-aza-2'-deoxycytidine. In conclusion, our study demonstrates that fusion-positive and fusion-negative rhabdomyosarcoma tumors possess characteristic methylation profiles that contribute to the expression differences between these fusion subtypes. These findings indicate an important relationship between fusion status and epigenetic changes in rhabdomyosarcoma, present a novel approach for ascertaining fusion status, and may identify new therapeutic targets in rhabdomyosarcoma.

EMILIN2 regulates platelet activation, thrombus formation, and clot retraction

Thrombosis, like other cardiovascular diseases, has a strong genetic component, with largely unknown determinants. EMILIN2, Elastin Microfibril Interface Located Protein2, was identified as a candidate gene for thrombosis in mouse and human quantitative trait loci studies. EMILIN2 is expressed during cardiovascular development, on cardiac stem cells, and in heart tissue in animal models of heart disease. In humans, the EMILIN2 gene is located on the short arm of Chromosome 18, and patients with partial and complete deletion of this chromosome region have cardiac malformations. To understand the basis for the thrombotic risk associated with EMILIN2, EMILIN2 deficient mice were generated. The findings of this study indicate that EMILIN2 influences platelet aggregation induced by adenosine diphosphate, collagen, and thrombin with both EMILIN2-deficient platelets and EMILIN2-deficient plasma contributing to the impaired aggregation response. Purified EMILIN2 added to platelets accelerated platelet aggregation and reduced clotting time when added to EMILIN2-deficient mouse and human plasma. Carotid occlusion time was 2-fold longer in mice with platelet-specific EMILIN2 deficiency, but stability of the clot was reduced in mice with both global EMILIN2 deficiency and with platelet-specific EMILIN2 deficiency. In vitro clot retraction was markedly decreased in EMILIN2 deficient mice, indicating that platelet outside-in signaling was dependent on EMILIN2. EMILIN1 deficient mice and EMILIN2:EMILIN1 double deficient mice had suppressed platelet aggregation and delayed clot retraction similar to EMILIN2 mice, but EMILIN2 and EMILIN1 had opposing affects on clot retraction, suggesting that EMILIN1 may attenuate the effects of EMILIN2 on platelet aggregation and thrombosis. In conclusion, these studies identify multiple influences of EMILIN2 in pathophysiology and suggest that its role as a prothrombotic risk factor may arise from its effects on platelet aggregation and platelet mediated clot retraction.

Neutrophil elastase-dependent cleavage compromises the tumor suppressor role of EMILIN1

Proteolysis of the extracellular matrix (ECM) is a key event in tumor growth and progression. The breakdown of ECM can lead to the generation of bioactive fragments that promote cell growth and spread. EMILIN1, a multidomain glycoprotein expressed in several tissues, exerts a crucial regulatory function through the engagement of α4/α9 integrins. Unlike the majority of ECM molecules that elicit a proliferative program, the signals emitting from EMILIN1 engaged by α4/α9β1 integrins are antiproliferative. In this study, aimed to demonstrate if the suppressor role of EMILIN1 was related to its structural integrity, we tested the possibility that EMILIN1 could be specifically cleaved. Among the proteolytic enzymes released in the tumor microenvironment we showed that neutrophil elastase cleaved EMILIN1 in three/four major fragments. The consequence of this proteolytic process was the impairment of its anti-proliferative role. Accordingly, EMILIN1 was digested in sarcomas and ovarian cancers. Sarcoma specimens were infiltrated by neutrophils (PMNs) and stained positively for elastase. The present findings highlight the peculiar activity of PMN elastase in disabling EMILIN1 suppressor function.

EMILIN1/α9β1 integrin interaction is crucial in lymphatic valve formation and maintenance

Lymphatic vasculature plays a crucial role in the maintenance of tissue interstitial fluid balance. The role of functional collecting lymphatic vessels in lymph transport has been recently highlighted in pathologies leading to lymphedema, for which treatments are currently unavailable. Intraluminal valves are of paramount importance in this process. However, valve formation and maturation have not been entirely elucidated yet, in particular, the role played by the extracellular matrix (ECM). We hypothesized that EMILIN1, an ECM multidomain glycoprotein, regulates lymphatic valve formation and maintenance. Using a mouse knockout model, we show that in the absence of EMILIN1, mice exhibit defects in lymphatic valve structure and in lymph flow. By applying morphometric in vitro and in vivo functional assays, we conclude that this impaired phenotype depends on the lack of α9β1 integrin engagement, the specific lymphatic endothelial cell receptor for EMILIN1, and the ensuing derangement of cell proliferation and migration. Our data demonstrate a fundamental role for EMILIN1-integrin α9 interaction in lymphatic vasculature, especially in lymphatic valve formation and maintenance, and underline the importance of this ECM component in displaying a regulatory function in proliferation and acting as a "guiding" molecule in migration of lymphatic endothelial cells.

An EMILIN1-negative microenvironment promotes tumor cell proliferation and lymph node invasion

The evidence that EMILIN1 (Elastic Microfibril Interface Located proteIN) deficiency in Emilin1(-/-) mice caused dermal and epidermal hyperproliferation and an abnormal lymphatic phenotype prompted us to hypothesize the involvement of this extracellular matrix component in tumor development and in lymphatic metastasis. Using the 12-dimethylbenz(α)anthracene/12-O-tetradecanoylphorbol-13-acetate (DMBA/TPA) two-stage model of skin carcinogenesis, we found that Emilin1(-/-) mice presented an accelerated formation, a higher incidence, and the development of a larger number of tumors compared with their wild-type littermates. EMILIN1-negative tumors showed more Ki67-positive proliferating cells and higher levels of pErk1/2. In these tumors, PTEN expression was lower. Emilin1(-/-) mice displayed enhanced lymphangiogenesis both in the tumor and in the sentinel lymph nodes. Accordingly, tumor growth and lymph node metastasis of transplanted syngenic tumors were also increased in Emilin1(-/-) mice. In vitro transmigration assays through lymphatic endothelial cells showed that EMILIN1 deficiency greatly facilitated tumor cell trafficking. Overall, these data established that EMILIN1 exerts a protective role in tumor growth, in tumor lymphatic vessel formation, as well as in metastatic spread to lymph nodes and reinforced the importance of its presence in the microenvironment to determine the tumor phenotype.

Polydom/SVEP1 is a ligand for integrin α9β1

A variety of proteins, including tenascin-C and osteopontin, have been identified as ligands for integrin α9β1. However, their affinities for integrin α9β1 are apparently much lower than those of other integrins (e.g. α3β1, α5β1, and α8β1) for their specific ligands, leaving the possibility that physiological ligands for integrin α9β1 still remain unidentified. In this study, we found that polydom (also named SVEP1) mediates cell adhesion in an integrin α9β1-dependent manner and binds directly to recombinant integrin α9β1 with an affinity that far exceeds those of the known ligands. Using a series of recombinant polydom proteins with N-terminal deletions, we mapped the integrin-binding site to the 21st complement control protein domain. Alanine-scanning mutagenesis revealed that the EDDMMEVPY sequence (amino acids 2636-2644) in the 21st complement control protein domain was involved in the binding to integrin α9β1 and that Glu(2641) was the critical acidic residue for the integrin binding. The importance of this sequence was further confirmed by integrin binding inhibition assays using synthetic peptides. Immunohistochemical analyses of mouse embryonic tissues showed that polydom colocalized with integrin α9 in the stomach, intestine, and other organs. Furthermore, in situ integrin α9β1 binding assays using frozen mouse tissues showed that polydom accounts for most, but not all, of the integrin α9β1 ligands in tissues. Taken together, the present findings indicate that polydom is a hitherto unknown ligand for integrin α9β1 that functions as a physiological ligand in vivo.

Integrins and their extracellular matrix ligands in lymphangiogenesis and lymph node metastasis

In the 1970s, the late Judah Folkman postulated that tumors grow proportionately to their blood supply and that tumor angiogenesis removed this limitation promoting growth and metastasis. Work over the past 40 years, varying from molecular examination to clinical trials, verified this hypothesis and identified a host of therapeutic targets to limit tumor angiogenesis, including the integrin family of extracellular matrix receptors. However, the propensity for some tumors to spread through lymphatics suggests that lymphangiogenesis plays a similarly important role. Lymphangiogenesis inhibitors reduce lymph node metastasis, the leading indicator of poor prognosis, whereas inducing lymphangiogenesis promotes lymph node metastasis even in cancers not prone to lymphatic dissemination. Recent works highlight a role for integrins in lymphangiogenesis and suggest that integrin inhibitors may serve as therapeutic targets to limit lymphangiogenesis and lymph node metastasis. This review discusses the current literature on integrin-matrix interactions in lymphatic vessel development and lymphangiogenesis and highlights our current knowledge on how specific integrins regulate tumor lymphangiogenesis.

The CD49d/CD29 complex is physically and functionally associated with CD38 in B-cell chronic lymphocytic leukemia cells

CD49d and CD38 are independent negative prognostic markers in chronic lymphocytic leukemia (CLL). Their associated expression marks a disease subset with a highly aggressive clinical course. Here, we demonstrate a constitutive physical association between the CD49d/CD29 integrin complex and CD38 in primary CLL cells and B-cell lines by (i) cocapping, (ii) coimmunoprecipitation and (iii) cell adhesion experiments using CD49d-specific substrates (vascular-cell adhesion molecule-1 or CS-1/H89 fibronectin fragments). The role of CD38 in CD49d-mediated cell adhesion was studied in CD49d(+)CD38(+) and CD49d(+)CD38(-) primary CLL cells, and confirmed using CD38 transfectants of the originally CD49d(+)CD38(-) CLL-derived cell line Mec-1. Results indicate that CD49d(+)CD38(+) cells adhered more efficiently onto CD49d-specific substrates than CD49d(+)CD38(-) cells (P < 0.001). Upon adhesion, CD49d(+)CD38(+) cells underwent distinctive changes in cell shape and morphology, with higher levels of phosphorylated Vav-1 than CD49d(+)CD38(-) cells (P = 0.0006) and a more complex distribution of F-actin to the adhesion sites. Lastly, adherent CD49d(+)CD38(+) cells were more resistant to serum-deprivation-induced (P < 0.001) and spontaneous (P = 0.03) apoptosis than the CD49d(+)CD38(-) counterpart. Altogether, our results point to a direct role for CD38 in enhancing CD49d-mediated adhesion processes in CLL, thus providing an explanation for the negative clinical impact exerted by these molecules when coexpressed in neoplastic cells.

The EMILIN/Multimerin family

Elastin microfibrillar interface proteins (EMILINs) and Multimerins (EMILIN1, EMILIN2, Multimerin1, and Multimerin2) constitute a four member family that in addition to the shared C-terminus gC1q domain typical of the gC1q/TNF superfamily members contain a N-terminus unique cysteine-rich EMI domain. These glycoproteins are homotrimeric and assemble into high molecular weight multimers. They are predominantly expressed in the extracellular matrix and contribute to several cellular functions in part associated with the gC1q domain and in part not yet assigned nor linked to other specific regions of the sequence. Among the latter is the control of arterial blood pressure, the inhibition of Bacillus anthracis cell cytotoxicity, the promotion of cell death, the proangiogenic function, and a role in platelet hemostasis. The focus of this review is to highlight the multiplicity of functions and domains of the EMILIN/Multimerin family with a particular emphasis on the regulatory role played by the ligand-receptor interactions of the gC1q domain. EMILIN1 is the most extensively studied member both from the structural and functional point of view. The structure of the gC1q of EMILIN1 solved by NMR highlights unique characteristics compared to other gC1q domains: it shows a marked decrease of the contact surface of the trimeric assembly and while conserving the jelly-roll topology with two β-sheets of antiparallel strands it presents a nine-stranded β-sandwich fold instead of the usual 10-stranded fold. This is likely due to the insertion of nine residues that disrupt the ordered strand organization and forma a highly dynamic protruding loop. In this loop the residue E933 is the site of interaction between gC1q and the α4β1 and α9β1 integrins, and contrary to integrin occupancy that usually upregulates cell growth, when gC1q is ligated by the integrin the cells reduce their proliferative activity.

Intrinsic disorder of the extracellular matrix

The extracellular matrix is very well organized at the supramolecular and tissue levels and little is known on the potential role of intrinsic disorder in promoting its organization. We predicted the amount of disorder and identified disordered regions in the human extracellular proteome with established computational tools. The extracellular proteome is significantly enriched in proteins comprising more than 50% of disorder compared to the complete human proteome. The enrichment is mostly due to long disordered regions containing at least 100 consecutive disordered residues. The amount of intrinsic disorder is heterogeneous in the extracellular protein families, with the most disordered being collagens and the small integrin-binding ligand N-linked glycoproteins. Although most domains found in extracellular proteins are structured, the fibronectin III domains contain a variable amount of disordered residues (up to 92%). Binding sites for heparin and integrins are found in disordered sequences of extracellular proteins. Intrinsic disorder is evenly distributed in hubs and ends in the interaction network of extracellular proteins with their extracellular partners. In contrast, extracellular hubs are significantly enriched in disorder in the network of extracellular proteins with their extracellular, membrane and intracellular partners. Disorder could thus provide the structural plasticity required for the hubs to interact with membrane and intracellular proteins. Organization and assembly of the extracellular matrix, development of mineralized tissues and cell-matrix adhesion are the biological processes overrepresented in the most disordered extracellular proteins. Extracellular disorder is associated with binding to growth factors, glycosaminoglycans and integrins at the molecular level.

EMILIN1-α4/α9 integrin interaction inhibits dermal fibroblast and keratinocyte proliferation

The α4/α9 integrins directly engage the ECM glycoprotein EMILIN1 to inhibit skin cell proliferation upstream of TGF-β signaling.

EMILIN1 promotes α4β1 integrin–dependent cell adhesion and migration and reduces pro–transforming growth factor–β processing. A knockout mouse model was used to unravel EMILIN1 functions in skin where the protein was abundantly expressed in the dermal stroma and where EMILIN1-positive fibrils reached the basal keratinocyte layer. Loss of EMILIN1 caused dermal and epidermal hyperproliferation and accelerated wound closure. We identified the direct engagement of EMILIN1 to α4β1 and α9β1 integrins as the mechanism underlying the homeostatic role exerted by EMILIN1. The lack of EMILIN1–α4/α9 integrin interaction was accompanied by activation of PI3K/Akt and Erk1/2 pathways as a result of the reduction of PTEN. The down-regulation of PTEN empowered Erk1/2 phosphorylation that in turn inhibited Smad2 signaling by phosphorylation of residues Ser245/250/255. These results highlight the important regulatory role of an extracellular matrix component in skin proliferation. In addition, EMILIN1 is identified as a novel ligand for keratinocyte α9β1 integrin, suggesting prospective roles for this receptor–ligand pair in skin homeostasis.

The extracellular matrix glycoprotein elastin microfibril interface located protein 2: a dual role in the tumor microenvironment

We have recently reported that elastin microfibril interface located protein 2 (EMILIN2), an extracellular matrix (ECM) glycoprotein, triggers cell death through a direct binding to death receptors. EMILIN2 thus influences cell viability through a mechanism that is unique for an ECM molecule. In the present work, we report an additional function for this molecule. First, we identify the region responsible for the proapoptotic effects, a 90-amino acid residue-long coiled-coil fragment toward the N-terminus of the molecule. The fragment recapitulates EMILIN2 proapoptotic mechanisms. In addition, using either the full molecule or the active fragment, for the first time, we demonstrate a significant antitumoral effect in vivo, likely due to a decrease in tumor cell viability. Unexpectedly, tumors treated with EMILIN2 or the deletion mutant display a significant increase of tumor angiogenesis. In view of this novel finding, the cotreatment of the growing tumors with an antiangiogenic drug led, in most cases, to a complete regression of tumor growth. These results grant further support to recent findings that pinpoint the microenvironment as an important regulator of cell fate under both physiological and pathological conditions and disclose the possibility of using EMILIN2 fragments as potent antineoplastic tools for cancer treatment.

NMR-based homology model for the solution structure of the C-terminal globular domain of EMILIN1

EMILIN1 is a glycoprotein of elastic tissues that has been recently linked to the pathogenesis of hypertension. The protein is formed by different independently folded structural domains whose role has been partially elucidated. In this paper the solution structure, inferred from NMR-based homology modelling of the C-terminal trimeric globular C1q domain (gC1q) of EMILIN1, is reported. The high molecular weight and the homotrimeric structure of the protein required the combined use of highly deuterated (15)N, (13)C-labelled samples and TROSY experiments. Starting from a homology model, the protein structure was refined using heteronuclear residual dipolar couplings, chemical shift patterns, NOEs and H-exchange data. Analysis of the gC1q domain structure of EMILIN1 shows that each protomer of the trimer adopts a nine-stranded beta sandwich folding topology which is related to the conformation observed for other proteins of the family. Distinguishing features, however, include a missing edge-strand and an unstructured 19-residue loop. Although the current data do not allow this loop to be precisely defined, the available evidence is consistent with a flexible segment that protrudes from each subunit of the globular trimeric assembly and plays a key role in inter-molecular interactions between the EMILIN1 gC1q homotrimer and its integrin receptor alpha4beta1.