Binding properties and protease stability of recombinant human nidogen

Basement membrane dynamics and mechanics in tissue morphogenesis

The basement membrane (BM) is a thin, planar-organized extracellular matrix that underlies epithelia and surrounds most organs. During development, the BM is highly dynamic and simultaneously provides mechanical properties that stabilize tissue structure and shape organs. Moreover, it is important for cell polarity, cell migration, and cell signaling. Thereby BM diverges regarding molecular composition, structure, and modes of assembly. Different BM organization leads to various physical features. The mechanisms that regulate BM composition and structure and how this affects mechanical properties are not fully understood. Recent studies show that precise control of BM deposition or degradation can result in BMs with locally different protein densities, compositions, thicknesses, or polarization. Such heterogeneous matrices can induce temporospatial force anisotropy and enable tissue sculpting. In this Review, I address recent findings that provide new perspectives on the role of the BM in morphogenesis.

Dynamics, structure and assembly of the basement membrane in developing salivary glands revealed by an exogenous EGFP-tagged nidogen probe

Most epithelial tissues rapidly become complex during embryonic development while being surrounded by the basement membrane (BM). Thus, the BM shape is thought to change dramatically as the epithelium grows, but the underlying mechanism is not yet clear. Nidogen-1 is ubiquitous in the BM and binds to various other BM components, including laminin and type IV collagen. To elucidate the behavior of the BM during epithelial morphogenesis, we attempted to live-label the developing BM with recombinant human nidogen-1 fused to an enhanced green fluorescent protein (hNid1-EGFP). Submandibular glands of mouse embryos were cultured in glass-bottomed dishes and incubated in media containing hNid1-EGFP. Subsequent confocal microscopy clearly visualized the BMs surrounding the epithelial end buds. On three-dimensional reconstruction from Z-series confocal sections, the epithelial BM was observed as a thin sheet that expanded continuously around the entire epithelial basal surface. Because the explants continued to grow well in the presence of hNid1-EGFP, time-lapse confocal microscopy was performed to follow the dynamics of the BM. We found that the epithelial BM is an adaptive structure that deforms in accordance with the rapid shape changes of the developing epithelium. Furthermore, hNid1-EGFP was found to be incorporated differently into the epithelial BM compared with that reported for fibronectin or type IV collagen, suggesting that individual BM components assemble in different ways to form the BM.

Vertebrate extracellular matrix protein hemicentin-1 interacts physically and genetically with basement membrane protein nidogen-2

Hemicentins are large proteins of the extracellular matrix that belong to the fibulin family and play pivotal roles during development and homeostasis of a variety of invertebrate and vertebrate tissues. However, bona fide interaction partners of hemicentins have not been described as yet. Here, applying surface plasmon resonance spectroscopy and co-immunoprecipitation, we identify the basement membrane protein nidogen-2 (NID2) as a binding partner of mouse and zebrafish hemicentin-1 (HMCN1), in line with the formerly described essential role of mouse HMCN1 in basement membrane integrity. We show that HMCN1 binds to the same protein domain of NID2 (G2) as formerly shown for laminins, but with an approximately 3.5-fold lower affinity and in a competitive manner. Furthermore, immunofluorescence and immunogold labeling revealed that HMCN1/Hmcn1 is localized close to basement membranes and in partial overlap with NID2/Nid2a in different tissues of mouse and zebrafish. Genetic knockout and antisense-mediated knockdown studies in zebrafish further show that loss of Nid2a leads to similar defects in fin fold morphogenesis as the loss of Laminin-α5 (Lama5) or Hmcn1. Finally, combined partial loss-of-function studies indicated that nid2a genetically interacts with both hmcn1 and lama5. Together, these findings suggest that despite their mutually exclusive physical binding, hemicentins, nidogens, and laminins tightly cooperate and support each other during formation, maintenance, and function of basement membranes to confer tissue linkage.

Nidogen-1 expression is associated with overall survival and temozolomide sensitivity in low-grade glioma patients

We investigated the prognostic significance of nidogen-1 (NID1) in glioma. Oncomine, GEPIA, UALCAN, CCGA database analyses showed that NID1 transcript levels were significantly upregulated in multiple cancer types, including gliomas. Quantitative RT-PCR analyses confirmed that NID1 expression was significantly upregulated in glioma tissues compared to paired adjacent normal brain tissue samples (n=9). NID1 silencing enhanced in vitro apoptosis and the temozolomide sensitivity of U251 and U87-MG glioma cells. Protein-protein interaction network analysis using the STRING and GeneMANIA databases showed that NID1 interacts with several extracellular matrix proteins. TIMER database analysis showed that NID1 expression in low-grade gliomas was associated with tumor infiltration of B cells, CD4⁺ and CD8⁺ T cells, macrophages, neutrophils, and dendritic cells. Kaplan-Meier survival curve analysis showed that low-grade gliomas patients with high NID1 expression were associated with shorter overall survival. However, NID1 expression was not associated with overall survival in glioblastoma multiforme patients. These findings demonstrate that NID1 expression in glioma tissues is associated with overall survival of low-grade glioma patients and temozolomide sensitivity. NID1 is thus a potential prognostic biomarker and therapeutic target in low-grade glioma patients.

Temporal characterization of optic fissure basement membrane composition suggests nidogen may be an initial target of remodeling

Fusion of the optic fissure is necessary to complete retinal morphogenesis and ensure proper function of the optic stalk. Failure of this event leads to congenital coloboma, one of the leading causes of pediatric blindness. Mechanistically it is widely accepted that the basement membrane (BM) surrounding the maturing retina needs to be remodeled within the fissure in order to facilitate subsequent epithelial sheet fusion. However, the mechanism driving BM remodeling has yet to be elucidated. As a first step to understanding this critical molecular event we comprehensively characterized the core composition of optic fissure BMs in the zebrafish embryos. Zebrafish optic fissure BMs were found to express laminin a1, a4, b1a, c1 and c3, nidogen 1a, 1b and 2a, collagen IV a1 and a2 as well as perlecan. Furthermore, we observed that laminin, perlecan and collagen IV expression persists in the fissure during fusion, up to 56 hpf, while nidogen expression is downregulated upon initiation of fusion, at 36 hpf. Using immunohistochemistry we also show that nidogen is removed from the BM prior to that of laminin, indicating that remodeling of the BM is an ordered event. Lastly, we characterized retinal morphogenesis in the absence of nidogen function and documented retinal malformation similar to what is observed in laminin mutants. Taken together, we propose a model of BM remodeling where nidogen acts as a linchpin during initiation of optic fissure fusion.

Characterization of Drosophila Nidogen/entactin reveals roles in basement membrane stability, barrier function and nervous system patterning

Basement membranes (BMs) are specialized layers of extracellular matrix (ECM) mainly composed of Laminin, type IV Collagen, Perlecan and Nidogen/entactin (NDG). Recent in vivo studies challenged the initially proposed role of NDG as a major ECM linker molecule by revealing dispensability for viability and BM formation. Here, we report the characterization of the single Ndg gene in Drosophila. Embryonic Ndg expression was primarily observed in mesodermal tissues and the chordotonal organs, whereas NDG protein localized to all BMs. Although loss of Laminin strongly affected BM localization of NDG, Ndg-null mutants exhibited no overt changes in the distribution of BM components. Although Drosophila Ndg mutants were viable, loss of NDG led to ultrastructural BM defects that compromised barrier function and stability in vivo Moreover, loss of NDG impaired larval crawling behavior and reduced responses to vibrational stimuli. Further morphological analysis revealed accompanying defects in the larval peripheral nervous system, especially in the chordotonal organs and the neuromuscular junction (NMJ). Taken together, our analysis suggests that NDG is not essential for BM assembly but mediates BM stability and ECM-dependent neural plasticity during Drosophila development.

Role of meprin metalloproteinases in cytokine processing and inflammation

Meprin metalloendopeptidases, comprising α and β isoforms, are widely expressed in mammalian cells and organs including kidney, intestines, lungs, skin, and bladder, and in a variety of immune cells and cancer cells. Meprins proteolytically process many inflammatory mediators, including cytokines, chemokines, and other bioactive proteins and peptides that control the function of immune cells. The knowledge of meprin-mediated processing of inflammatory mediators and other target substrates provides a pathophysiologic link for the involvement of meprins in the pathogenesis of many inflammatory disorders. Meprins are now known to play important roles in inflammatory diseases including acute kidney injury, sepsis, urinary tract infections, bladder inflammation, and inflammatory bowel disease. The proteolysis of epithelial and endothelial barriers including cell junctional proteins by meprins promotes leukocyte influx into areas of tissue damage to result in inflammation. Meprins degrade extracellular matrix proteins; this ability of meprins is implicated in the cell migration of leukocytes and the invasion of tumor cells that express meprins. Proteolytic processing and maturation of procollagens provides evidence that meprins are involved in collagen maturation and deposition in the fibrotic processes involved in the formation of keloids and hypertrophic scars and lung fibrosis. This review highlights recent progress in understanding the role of meprins in inflammatory disorders in both human and mouse models.

Dissection of Nidogen function in Drosophila reveals tissue-specific mechanisms of basement membrane assembly

Basement membranes (BMs) are thin sheet-like specialized extracellular matrices found at the basal surface of epithelia and endothelial tissues. They have been conserved across evolution and are required for proper tissue growth, organization, differentiation and maintenance. The major constituents of BMs are two independent networks of Laminin and Type IV Collagen in addition to the proteoglycan Perlecan and the glycoprotein Nidogen/entactin (Ndg). The ability of Ndg to bind in vitro Collagen IV and Laminin, both with key functions during embryogenesis, anticipated an essential role for Ndg in morphogenesis linking the Laminin and Collagen IV networks. This was supported by results from cultured embryonic tissue experiments. However, the fact that elimination of Ndg in C. elegans and mice did not affect survival strongly questioned this proposed linking role. Here, we have isolated mutations in the only Ndg gene present in Drosophila. We find that while, similar to C.elegans and mice, Ndg is not essential for overall organogenesis or viability, it is required for appropriate fertility. We also find, alike in mice, tissue-specific requirements of Ndg for proper assembly and maintenance of certain BMs, namely those of the adipose tissue and flight muscles. In addition, we have performed a thorough functional analysis of the different Ndg domains in vivo. Our results support an essential requirement of the G3 domain for Ndg function and unravel a new key role for the Rod domain in regulating Ndg incorporation into BMs. Furthermore, uncoupling of the Laminin and Collagen IV networks is clearly observed in the larval adipose tissue in the absence of Ndg, indeed supporting a linking role. In light of our findings, we propose that BM assembly and/or maintenance is tissue-specific, which could explain the diverse requirements of a ubiquitous conserved BM component like Nidogen.

Basement membranes (BMs) are thin layers of specialized extracellular matrices present in every tissue of the human body. Its main constituents are two networks of laminin and Type IV Collagen linked by Nidogen (Ndg) and proteoglycans. They form an organized scaffold that regulates organ morphogenesis and function. Mutations affecting BM components are associated with organ dysfunction and several congenital diseases. Thus, a better comprehension of BM assembly and maintenance will not only help to learn more about organogenesis but also to a better understanding and, hopefully, treatment of these diseases. Here, we have used the fruit fly Drosophila to analyse the role of Ndg in BM formation in vivo. Elimination of Ndg in worms and mice does not affect survival, strongly questioning its proposed linking role, derived from in vitro experiments. Here, we show that in the fly, Ndg is dispensable for BM assembly and preservation in many tissues, but absolutely required in others. Furthermore, our functional study of the different Ndg domains challenges the significance of some interactions between BM components derived from in vitro experiments, while confirming others, and reveals a new key requirement for the Rod domain in Ndg function and incorporation into BMs.

The interplay of extracellular matrix and microbiome in urothelial bladder cancer

Many pathological changes in solid tumours are caused by the accumulation of genetic mutations and epigenetic molecular alterations. In addition, tumour progression is profoundly influenced by the environment surrounding the transformed cells. The interplay between tumour cells and their microenvironment has been recognized as one of the key determinants of cancer development and is being extensively investigated. Data suggest that both the extracellular matrix and the microbiota represent microenvironments that contribute to the onset and progression of tumours. Through the introduction of omics technologies and pyrosequencing analyses, a detailed investigation of these two microenvironments is now possible. In urological research, assessment of their dysregulation has become increasingly important to provide diagnostic, prognostic and predictive biomarkers for urothelial bladder cancer. Understanding the roles of the extracellular matrix and microbiota, two key components of the urothelial mucosa, in the sequelae of pathogenic events that occur in the development and progression of urothelial carcinomas will be important to overcome the shortcomings in current bladder cancer treatment strategies.

Basement membrane protein nidogen-1 is a target of meprin β in cisplatin nephrotoxicity

Meprins are oligomeric metalloproteinases that are abundantly expressed in the brush-border membranes of renal proximal tubules. During acute kidney injury (AKI) induced by cisplatin or ischemia-reperfusion, membrane-bound meprins are shed and their localization is altered from the apical membranes toward the basolateral surface of the proximal tubules. Meprins are capable of cleaving basement membrane proteins in vitro, however, it is not known whether meprins are able to degrade extracellular matrix proteins under pathophysiological conditions in vivo. The present study demonstrates that a basement membrane protein, nidogen-1, is cleaved and excreted in the urine of mice subjected to cisplatin-induced nephrotoxicity, a model of AKI. Cleaved nidogen-1 was not detected in the urine of untreated mice, but during the progression of cisplatin nephrotoxicity, the excretion of cleaved nidogen-1 increased in a time-dependent manner. The meprin inhibitor actinonin markedly prevented urinary excretion of the cleaved nidogen-1. In addition, meprin β-deficient mice, but not meprin α-deficient mice, subjected to cisplatin nephrotoxicity significantly suppressed excretion of cleaved nidogen-1, further suggesting that meprin β is involved in the cleavage of nidogen-1. These studies provide strong evidence for a pathophysiological link between meprin β and urinary excretion of cleaved nidogen-1 during cisplatin-induced AKI.

Skin basement membrane: the foundation of epidermal integrity--BM functions and diverse roles of bridging molecules nidogen and perlecan

The epidermis functions in skin as first defense line or barrier against environmental impacts, resting on extracellular matrix (ECM) of the dermis underneath. Both compartments are connected by the basement membrane (BM), composed of a set of distinct glycoproteins and proteoglycans. Herein we are reviewing molecular aspects of BM structure, composition, and function regarding not only (i) the dermoepidermal interface but also (ii) the resident microvasculature, primarily focusing on the per se nonscaffold forming components perlecan and nidogen-1 and nidogen-2. Depletion or functional deficiencies of any BM component are lethal at some stage of development or around birth, though BM defects vary between organs and tissues. Lethality problems were overcome by developmental stage- and skin-specific gene targeting or by cell grafting and organotypic (3D) cocultures of normal or defective cells, which allows recapitulating BM formation de novo. Thus, evidence is accumulating that BM assembly and turnover rely on mechanical properties and composition of the adjacent ECM and the dynamics of molecular assembly, including further "minor" local components, nidogens largely functioning as catalysts or molecular adaptors and perlecan as bridging stabilizer. Collectively, orchestration of BM assembly, remodeling, and the role of individual players herein are determined by the developmental, tissue-specific, or functional context.

Cleavage of nidogen-1 by cathepsin S impairs its binding to basement membrane partners

Cathepsin S (catS), which is expressed in normal human keratinocytes and localized close to the dermal-epidermal junction (DEJ) degrades some of major basement membrane (BM) constituents. Among them, catS readily hydrolyzed in a time and dose dependent manner human nidogen-1 (nid-1) and nidogen-2, which are key proteins in the BM structure. CatS preferentially cleaved nid-1 at both acid and neutral pH. Hydrolysis of nid-1 was hampered in murine ctss^−/− spleen lysates pretreated with inhibitors of other classes of proteases. Nid-1 was cleaved within its G2 and G3 globular domains that are both involved in interactions with other BM components. Binding assays with soluble and immobilized ligands indicated that catS altered the formation of complexes between nid-1 and other BM components. Assuming that the cleavage of nid-1 impairs its ability to crosslink with BM partners and perturbs the viscoelastic properties of BM matrix, these data indicate that catS may participate in BM proteolysis, in addition to already identified proteases.

The epidermal basement membrane is a composite of separate laminin- or collagen IV-containing networks connected by aggregated perlecan, but not by nidogens

The basement membrane between the epidermis and the dermis is indispensable for normal skin functions. It connects, and functionally separates, the epidermis and the dermis. To understand the suprastructural and functional basis of these connections, heterotypic supramolecular aggregates were isolated from the dermal-epidermal junction zone of human skin. Individual suprastructures were separated and purified by immunomagnetic beads, each recognizing a specific, molecular component of the aggregates. The molecular compositions of the suprastructures were determined by immunogold electron microscopy and immunoblotting. A composite of two networks was obtained from fibril-free suspensions by immunobeads recognizing either laminin 332 or collagen IV. After removal of perlecan-containing suprastructures or after enzyme digestion of heparan sulfate chains, a distinct network with a diffuse electron-optical appearance was isolated with magnetic beads coated with antibodies to collagen IV. The second network was more finely grained and comprised laminin 332 and laminins with α5-chains. The core protein of perlecan was an exclusive component of this network whereas its heparan sulfate chains were integrated into the collagen IV-containing network. Nidogens 1 and 2 occurred in both networks but did not form strong molecular cross-bridges. Their incorporation into one network appeared to be masked after their incorporation into the other one. We conclude that the epidermal basement membrane is a composite of two structurally independent networks that are tightly connected in a spot-welding-like manner by perlecan-containing aggregates.

Basement membrane components are key players in specialized extracellular matrices

More than three decades ago, basement membranes (BMs) were described as membrane-like structures capable of isolating a cell from and connecting a cell to its environment. Since this time, it has been revealed that BMs are specialized extracellular matrices (sECMs) with unique components that support important functions including differentiation, proliferation, migration, and chemotaxis of cells during development. The composition of these sECM is as unique as the tissues to which they are localized, opening the possibility that such matrices can fulfill distinct functions. Changes in BM composition play significant roles in facilitating the development of various diseases. Furthermore, tissues have to provide sECM for their stem cells during development and for their adult life. Here, we briefly review the latest research on these unique sECM and their components with a special emphasis on embryonic and adult stem cells and their niches.

Differential expression of entactin-1/nidogen-1 and entactin-2/nidogen-2 in myogenic differentiation

Here, we show that entactin-2 expression is strongly, but transiently, induced in myogenic differentiation. Treatment of C2C12 myoblasts with actinomycin D in parallel to the induction of differentiation could demonstrate that this is due to enhanced transcription of the entactin-2 gene. Furthermore, treatment with the translation inhibitor cycloheximide could show that entactin-2 is a primary response gene. As p38 MAP kinase is an important regulator of myogenic differentiation, we also analyzed the possibility that entactin-2 might be a target of this pathway. However, using various p38 MAPK inhibitors, we could not detect involvement of p38 in entactin-2 up-regulation. Most remarkably, expression of the entactin-2 homolog entactin-1 dramatically declined in myogenesis, suggesting different functions of the two entactins in this process. A similar effect was seen in primary myoblasts isolated from two different mouse strains. Expression of high levels of entactin-1 in myoblasts using a retroviral expression system led to a higher proliferation rate both in growth and in differentiation medium and to reduced expression of various myogenic differentiation markers after the induction of differentiation. Furthermore, decreased expression of the entactin-2 gene after treatment of the cells with ent-2-specific siRNA preparation led to reduced expression of the cell cycle inhibitor p21. These data suggest important and distinct functions of entactin-1 and -2 in myogenic differentiation.

Identification of substrates of the extracellular protease ADAMTS1 by DIGE proteomic analysis

Proteolytic modification of components of the extracellular milieu by metalloproteinases plays important roles in the regulation of multiple cellular and physiological processes and pathological conditions. ADAMTS1 is a secreted enzyme of the ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs) family of proteases, which is related to angiogenesis and inflammation processes. Here, we describe a proteomic screening for putative ADAMTS1 substrates by analyzing the protein profiles obtained from cultures of transfected cells overexpressing the protease as compared to parental cells. Conditioned medium proteins of cultures of the two cell lines have been quantitatively compared by DIGE. Proteins showing differential levels have been identified by MS techniques leading to the finding of five potential new substrates of ADAMTS1: the basement membrane proteins nidogen-1 and -2, the desmosomal protein desmocollin-3, and the extracellular glycoproteins dystroglycan 1 and Mac-2-binding protein. Nidogen-1 and -2 have been further validated as substrates by immunochemical analysis. Our results demonstrate the utility of the DIGE proteomic technique for the discovery of specific substrates of matrix proteases.

Basement membrane composition in the early mouse embryo day 7

Basement membranes (BM) are specialized structures of the extracellular matrix known to be involved in various early developmental processes. Despite numerous investigations on the localization of BM components, it remains unknown which molecules are expressed in early developmental stages and by which germ layers these proteins are produced. Therefore, we tested for all known laminin chains, nidogens, collagen type IV, and perlecan by means of light microscopic immunostaining and performed in situ reverse transcriptase-polymerase chain reaction to detect the mRNAs specific for laminin alpha1, laminin beta1, the alpha1 chain of collagen type IV, nidogen-2, and perlecan in the early mouse embryo, day 7, in vivo. Only the laminin chains alpha1, beta1, and gamma1 were detected immunohistochemically throughout the entire endodermal and ectodermal BM zones of the embryo proper. The mRNA of laminin alpha1, laminin beta1, collagen type IV, nidogen-2 and perlecan were expressed in the ectoderm-derived mesoderm, in the endoderm as well as in the ectoderm. In contrast, Reichert's membrane was positive for all laminin chains except for the alpha4, alpha5, beta3, and gamma3 chains. Moreover, maternal epithelial as well as mesenchymal cells expressed laminins, nidogen-1 and nidogen-2, collagen type IV, and perlecan. In conclusion, laminin-1 might be the only laminin isoform in the early mouse embryo that, together with the other main BM components, nidogens, collagen type IV, and perlecan, is synthesized by all three germ layers.

Laminin γ3 Chain Binds to Nidogen and Is Located in Murine Basement Membranes

Recently a novel laminin gamma3 chain was identified in mouse and human and shown to have the same modular structure as the laminin gamma1 chain. We expressed two fragments of the gamma3 chain in mammalian cells recombinantly. The first, domain VI/V, consisting of laminin N-terminal (domain VI) and four laminin-type epidermal growth factor-like (domain V) and laminin N-terminal modules, was shown to be essential for self-assembly of laminins. The other was domain III3-5, which consists of three laminin-type epidermal growth factor-like modules and is predicted to bind to nidogens. The gamma3 VI/V fragment was a poor inhibitor for laminin-1 polymerization as was the beta2 VI/V fragment. The gamma3 III3-5 fragment bound to nidogen-1 and nidogen-2 with lower affinity than the gamma1 III3-5 fragment. These data suggested that laminins containing the gamma3 chain may assemble networks independent of other laminins. Polyclonal antibodies raised against gamma3 VI/V and gamma3 III3-5 showed no cross-reaction with homologous fragments from the gamma1 and gamma2 chains of laminin and allowed the establishment of gamma chain-specific radioimmunoassays and light and electron microscopic immunostaining of tissues. This demonstrated a 20-100-fold lower content of the gamma3 chain compared with the gamma1 chain in various tissue extracts of adult mice. The expression of gamma3 chain was highly tissue-specific. In contrast to earlier assumptions, the antibodies against the gamma3 chain showed light microscopic staining exclusively in basement membrane zones of adult and embryonic tissues, such as the brain, kidney, skin, muscle, and testis. Ultrastructural immunogold staining localized the gamma3 chain to basement membranes of these tissues.

Scaffolds, Stem Cells, and Tissue Engineering: A Potent Combination!

Stem cells, either from embryonic or adult sources, have demonstrated the potential to differentiate into a wide range of tissues depending on culture conditions. This makes them prime candidates for use in tissue engineering applications. Current technology allows us to process biocompatible and biodegradable polymers into three-dimensional (3D) configurations, either as solid porous scaffolds or hydrogels, with controlled macro and/or micro spatial geometry and surface chemistry. Such control provides us with the ability to present highly controlled microenvironments to a chosen cell type. However, the precise microenvironments required for optimal expansion and/or differentiation of stem cells are only now being elucidated, and hence the controlled use of stem cells in tissue engineering remains a very young field. We present here a brief review of the current literature detailing interactions between stem cells and 3D scaffolds of varying morphology and chemical properties, concluding with remaining challenges for those interested in tissue engineering using tailored scaffolds and stem cells.

Evidence of nidogen-2 compensation for nidogen-1 deficiency in transgenic mice

Previous studies have shown that inhibition of nidogen-laminin binding interferes with basement membrane stabilization in various mouse organ cultures while no overt phenotype has been observed following inactivation of the nidogen-1 gene in mice. We have now used recombinant mouse nidogen-1 and nidogen-2 in order to evaluate a possible compensation between the two isoforms in the knock-out mice. Essentially, a comparable in vitro binding of nidogens-1 and -2 to the same laminin gamma1 chain structure and to several other basement membrane proteins has been revealed. Quantitative radioimmuno-assays have demonstrated high concentrations of nidogen-1 exceeding those of laminin gamma1 and nidogen-2 by factors of 5 and 20-50, respectively, in tissue extracts of wild-type mice. A three- to sevenfold increase in nidogen-2 was observed in heart and muscle of mice with nidogen-1 deficiency and confirmed by a similar increase in the intensity of immunogold staining of these tissues. However, a few of the tissues from mice with the gene knock-out still contained some nidogen-1-like immunoreactivity (1% of wild-type). Furthermore, both nidogen isoforms showed a similar distribution in various organs during embryonic development which, however, as shown previously, changed in some adult tissues. The data support the nidogen-2 compensation hypothesis to explain the limited phenotype observed following elimination of the nidogen-1 gene.

Binding of mouse nidogen-2 to basement membrane components and cells and its expression in embryonic and adult tissues suggest complementary functions of the two nidogens

Nidogen-1 binds several basement membrane components by well-defined, domain-specific interactions. Organ culture and gene targeting approaches suggest that a high-affinity nidogen-binding site of the laminin gamma1 chain (gamma1III4) is important for kidney development and for nerve guidance. Other proteins may also bind gamma1III4, although human nidogen-2 binds poorly to the mouse laminin gamma1 chain. We therefore characterized recombinant mouse nidogen-2 and its binding to basement membrane proteins and cells. Mouse nidogen-1 and -2 interacted at comparable levels with collagen IV, perlecan, and fibulin-2 and, most notably, also with laminin-1 fragments P1 and gamma1III3-5, which both contain the gamma1III4 module. In embryos, nidogen-2 mRNA was produced by mesenchyme at sites of epithelial-mesenchymal interactions, but the protein was deposited on epithelial basement membranes, as previously shown for nidogen-1. Hence, binding of both nidogens to the epithelial laminin gamma1 chain is dependent on epithelial-mesenchymal interactions. Epidermal growth factor stimulated expression of both nidogens in embryonic submandibular glands. Both nidogens were found in all studied embryonic and adult basement membranes. Nidogen-2 was more adhesive than nidogen-1 for some cell lines and was mainly mediated by alpha3beta1 and alpha6beta1 integrins as shown by antibody inhibition. These findings revealed extensive coregulation of nidogen-1 and -2 expression and much more complementary functions of the two nidogens than previously recognized.

Gene structure and functional analysis of the mouse nidogen-2 gene: nidogen-2 is not essential for basement membrane formation in mice

Nidogens are highly conserved proteins in vertebrates and invertebrates and are found in almost all basement membranes. According to the classical hypothesis of basement membrane organization, nidogens connect the laminin and collagen IV networks, so stabilizing the basement membrane, and integrate other proteins. In mammals two nidogen proteins, nidogen-1 and nidogen-2, have been discovered. Nidogen-2 is typically enriched in endothelial basement membranes, whereas nidogen-1 shows broader localization in most basement membranes. Surprisingly, analysis of nidogen-1 gene knockout mice presented evidence that nidogen-1 is not essential for basement membrane formation and may be compensated for by nidogen-2. In order to assess the structure and in vivo function of the nidogen-2 gene in mice, we cloned the gene and determined its structure and chromosomal location. Next we analyzed mice carrying an insertional mutation in the nidogen-2 gene that was generated by the secretory gene trap approach. Our molecular and biochemical characterization identified the mutation as a phenotypic null allele. Nidogen-2-deficient mice show no overt abnormalities and are fertile, and basement membranes appear normal by ultrastructural analysis and immunostaining. Nidogen-2 deficiency does not lead to hemorrhages in mice as one may have expected. Our results show that nidogen-2 is not essential for basement membrane formation or maintenance.

A new generation of Melan-A/MART-1 peptides that fulfill both increased immunogenicity and high resistance to biodegradation: implication for molecular anti-melanoma immunotherapy

Intense efforts of research are made for developing antitumor vaccines that stimulate T cell-mediated immunity. Tumor cells specifically express at their surfaces antigenic peptides presented by MHC class I and recognized by CTL. Tumor antigenic peptides hold promise for the development of novel cancer immunotherapies. However, peptide-based vaccines face two major limitations: the weak immunogenicity of tumor Ags and their low metabolic stability in biological fluids. These two hurdles, for which separate solutions exist, must, however, be solved simultaneously for developing improved vaccines. Unfortunately, attempts made to combine increased immunogenicity and stability of tumor Ags have failed until now. Here we report the successful design of synthetic derivatives of the human tumor Ag Melan-A/MART-1 that combine for the first time both higher immunogenicity and high peptidase resistance. A series of 36 nonnatural peptide derivatives was rationally designed on the basis of knowledge of the mechanism of degradation of Melan-A peptides in human serum and synthesized. Eight of them were efficiently protected against proteolysis and retained the antigenic properties of the parental peptide. Three of the eight analogs were twice as potent as the parental peptide in stimulating in vitro Melan-specific CTL responses in PBMC from normal donors. We isolated these CTL by tetramer-guided cell sorting and expanded them in vitro. The resulting CTL efficiently lysed tumor cells expressing Melan-A Ag. These Melan-A/MART-1 Ag derivatives should be considered as a new generation of potential immunogens in the development of molecular anti-melanoma vaccines.

Recombinant domains of mouse nidogen-1 and their binding to basement membrane proteins and monoclonal antibodies

The basement membrane protein, nidogen-1, was previously shown to consist of three globular domains, G1 to G3, and two connecting segments. Nidogen-1 is a major mediator in the formation of ternary complexes with laminins, collagen IV, perlecan and fibulins. In the present study, we have produced recombinant proteins of these predicted domains in mammalian cells and used these proteins for crystallographic and binding epitope analyses. These fragments included G1, G2, the rod domain and a slightly larger G3 structure; all were obtained in good yields and were shown to be properly folded using electron microscopy. Surface plasmon resonance assays demonstrated high affinity binding (Kd = 3-9 nM) of domain G2 for collagen IV, perlecan domain IV-1 and fibulin-2, and a more moderate Kd for fibulin-1C. Domain G3 contained high affinity binding sites for the laminin gamma1 chain and collagen IV (Kd = 1 nM) and weaker binding sites for fibulin-1C and fibulin-2. A moderate binding affinity was also observed between domain G1 and fibulin-2, while no activity could be detected for the nidogen rod domain. Together, these data indicate the potential of nidogen-1 for multiple interactions within basement membranes. A similar binding repertoire was also identified for seven rat monoclonal antibodies that bound with Kd = 2-30 nM to either G1, G1-G2, G2, the rod domain or G3. Three of the antibodies showed strongly reduced binding to G2 and G3 after complex formation with either a perlecan domain or laminin-1.

Mapping of binding sites for nidogens, fibulin-2, fibronectin and heparin to different IG modules of perlecan

Perlecan, a major basement membrane proteoglycan, has a complex modular structure designed for the binding of many cellular and extracellular ligands. Its domain IV, which consists of a tandem of immunoglobulin-like modules (IG2-IG15), is rich in such binding sites, which have been mapped to different modules obtained by recombinant production. Heparin/sulfatide binding was restricted to IG5 and shown to depend on four arginine residues that are close in space in beta strands B and E of the C-type IG fold. The nidogen-1 and nidogen-2 isoforms bind to IG3 with high affinity (K(d) approximately 10 nM). This interaction depends on the globular nidogen domain G2 and is crucial for the formation of ternary complexes with laminins. Two loops of IG3 located between beta strands B/C and F/G, which are spatially close, make a major contribution to binding. Fibronectin binding was localized to IG4-5 and fibulin-2 binds to IG2 and IG13-15 with different affinities. This implicates a complex cluster of heterotypic interaction sites apparently important for the supramolecular organization of perlecan in tissues.

Still more complexity in mammalian basement membranes

At the epithelial/mesenchymal interface of most tissues lies the basement membrane (BM). These thin sheets of highly specialized extracellular matrix vary in composition in a tissue-specific manner, and during development and repair. For about two decades it has been apparent that all BMs contain laminins, entactin-1/nidogen-1, Type IV collagen, and proteoglycans. However, within the past few years this complexity has increased as new components are described. The entactin/nidogen (E/N) family has expanded with the recent description of a new isoform, E/N-2/osteonidogen. Agrin and Type XVIII collagen have been reclassified as heparan sulfate proteoglycans (HSPGs), expanding the repertoire of HSPGs in the BM. The laminin family has become more diverse as new alpha-chains have been characterized, increasing the number of laminin isoforms. Interactions between BM components are now appreciated to be regulated through multiple, mostly domain-specific mechanisms. Understanding the functions of individual BM components and their assembly into macromolecular complexes is a considerable challenge that may increase as further BM and cell surface ligands are discovered for these proteins.

The absence of nidogen 1 does not affect murine basement membrane formation

Nidogen 1 is a highly conserved protein in mammals, Drosophila melanogaster, Caenorhabditis elegans, and ascidians and is found in all basement membranes. It has been proposed that nidogen 1 connects the laminin and collagen IV networks, so stabilizing the basement membrane, and integrates other proteins, including perlecan, into the basement membrane. To define the role of nidogen 1 in basement membranes in vivo, we produced a null mutation of the NID-1 gene in embryonic stem cells and used these to derive mouse lines. Homozygous animals produce neither nidogen 1 mRNA nor protein. Surprisingly, they show no overt abnormalities and are fertile, their basement membrane structures appearing normal. Nidogen 2 staining is increased in certain basement membranes, where it is normally only found in scant amounts. This occurs by either redistribution from other extracellular matrices or unmasking of nidogen 2 epitopes, as its production does not appear to be upregulated. The results show that nidogen 1 is not required for basement membrane formation or maintenance.

Effect of brown spider venom on basement membrane structures

Loxoscelism or necrotic arachnidism are terms used to describe lesions and reactions induced by bites (envenomation) from spiders of the genus Loxosceles. Envenomation has been reported to provoke dermonecrosis and haemorrhage at the bite site and haemolysis, disseminated intravascular coagulation and renal failure. The purpose of this work was to study the effect of the venom of the brown spider Loxosceles intermedia on basement membrane structures and on its major constituent molecules. Light microscopy observations showed that L. intermedia venom obtained through electric shock, which reproduces two major signals of Loxoscelism in the laboratory, exhibits activity toward basement membrane structures in mouse Engelbreth-Holm-Swarm (EHS) sarcoma. Basement degradation was seen by a reduced periodic acid-Schiff (PAS) and alcian blue staining as well as by a reduced immunostaining for laminin when compared to control experiments. Electron microscopy studies confirmed the above results, showing the action of the venom on EHS-basement membranes and demonstrating that these tissue structures are susceptible to the venom. Using purified components of the basement membrane, we determined through SDS-PAGE and agarose gel that the venom is not active toward laminin or type IV collagen, but is capable of cleaving entactin and endothelial heparan sulphate proteoglycan. In addition, when EHS tissue was incubated with venom we detected a release of laminin into the supernatant, corroborating the occurrence of some basement membrane disruption. The venom-degrading effect on entactin was blocked by 1, 10-phenanthroline, but not by other protease inhibitors such as PMSF, NEM or pepstatin-A. By using light microscopy associated with PAS staining we were able to identify that 1,10-phenanthroline also inhibits EHS-basement membrane disruption evoked by venom, corroborating that a metalloprotease of venom is involved in these effects. Degradation of these extracellular matrix molecules and the observed susceptibility of the basement membrane could lead to loss of vessel and glomerular integrity, resulting in haemorrhage and renal problems after envenomation.

Structural organization of distinct domains within the non-collagenous N-terminal region of collagen type XI

Collagen XI is a heterotrimeric molecule found predominantly in heterotypic cartilage fibrils, where it is involved in the regulation of fibrillogenesis. This function is thought to involve the complex N-terminal domain. The goal of this current study was to examine its structural organization to further elucidate the regulatory mechanism. The amino-propeptide (alpha1-Npp) alone or with isoforms of the variable region were recombinantly expressed and purified by affinity and molecular sieve chromatography. Cys-1-Cys-4 and Cys-2-Cys-3 disulfide bonds were detected by liquid chromatography-tandem mass spectrometry. This pattern is identical to the homologous alpha2-Npp, indicating that the recombinant proteins were folded correctly. Anomalous elution on molecular sieve chromatography suggested that the variable region was extended, which was confirmed using rotary shadowing; the alpha1-Npp formed a globular "head" and the variable region an extended "tail." Circular dichroism spectra analysis determined that the alpha1-Npp comprised 33% beta-sheet, whereas the variable region largely comprised non-periodic structure. Taken together, these results imply that the alpha1-Npp cannot be accommodated within the core of the fibril and that the variable region and/or minor helix facilitates its exclusion to the fibril surface. This provides further support for regulation of fibril diameter by steric hindrance or by interactions with other matrix components that affect fibrillogenesis.

Nidogen-1. Expression and ultrastructural localization during the onset of mesoderm formation in the early mouse embryo

Nidogen-1, a key component of basement membranes, is considered to function as a link between laminin and collagen Type IV networks and is expressed by mesenchymal cells during embryonic and fetal development. It is not clear which cells produce nidogen-1 in early developmental stages when no mesenchyme is present. We therefore localized nidogen-1 and its corresponding mRNA at the light and electron microscopic level in Day 7 mouse embryos during the onset of mesoderm formation by in situ hybridization, light microscopic immunostaining, and immunogold histochemistry. Nidogen-1 mRNA was found not only in the cells of the ectoderm-derived mesoderm but also in the cytoplasm of the endoderm and ectoderm, indicating that all three germ layers express it. Nidogen-1 was localized only in fully developed basement membranes of the ectoderm and was not seen in the developing endodermal basement membrane or in membranes disrupted during mesoderm formation. In contrast, laminin-1 and collagen Type IV were present in all basement membrane types at this developmental stage. The results indicate that, in the early embryo, nidogen-1 may be expressed by epithelial and mesenchymal cells, that both cell types contribute to embryonic basement membrane formation, and that nidogen-1 might serve to stabilize basement membranes in vivo. (J Histochem Cytochem 48:229-237, 2000)

Basement membranes in development

The aim of this review is to introduce the reader to the main ECM constituents and to some of their roles in development. The main functions of the ECM during embryogenesis are the production, promotion, and regulation of normal tissue structure. Among the ECM components, LMs have been the most extensively studied in relation to embryo-genesis. Skin and skeletal muscle disorders have been shown to be caused by LM alterations. Additional experiments, e.g., with knockout mice, will help enormously to elucidate the functional significance of many ECM constituents and their involvement in development and disease.

Recombinant domain IV of perlecan binds to nidogens, laminin-nidogen complex, fibronectin, fibulin-2 and heparin

Domain IV of mouse perlecan, which consists of 14 immunoglobulin superfamily (IG) modules, was prepared from recombinant human cell culture medium in the form of two fragments, IV-1 (IG2-9, 100 kDa) and IV-2 (IG10-15, 66 kDa). Both fragments bound to a heparin column, being eluted at ionic strengths either below (IV-2) or above (IV-1) physiological level, and could thus be readily purified. Electron microscopy demonstrated an elongated shape (20-25 nm), and folding into a native structure was indicated by immunological assay and CD spectroscopy. Solid-phase and surface plasmon resonance assays demonstrated strong binding of fragment IV-1 to fibronectin, nidogen-1, nidogen-2 and the laminin-1-nidogen-1 complex, with Kd values in the range 4-17 nM. The latter binding apparently occurs through nidogen-1, as shown by the formation of ternary complexes. Only moderate binding was observed for fibulin-2 and collagen IV and none for fibulin-1 and BM-40. Fragment IV-2 showed a more restricted pattern of binding, with only weaker binding to fibronectin and fibulin-2. None of these activities could be demonstrated for recombinant fragments corresponding to the N-terminal perlecan domains I to III. This indicates a special role for domain IV in the integration of perlecan into basement membranes and other extracellular structures via protein-protein interactions.

Nidogen-2: a new basement membrane protein with diverse binding properties

Human nidogen-2 was cloned and sequenced (1375 residues) and found to share 46% sequence identity and a similar domain arrangement with the previously characterized basement membrane protein nidogen-1. Recombinant nidogen-2 was purified as a 200 kDa protein from transfected mammalian cell medium, showed a high level of N and O-glycosylation, and could be clearly distinguished from nidogen-1 (150 kDa) by specific antibodies. Electron microscopy demonstrated that the two isoforms have a similar shape, consisting of three globular domains connected by two threads, but differ somewhat in length. Northern blots and immunological assays demonstrated co-expression of the nidogens in various tissues and cultured cells. Immunofluoresence revealed colocalization in vessel walls and other basement membrane zones but some differences in heart and skeletal muscle. Nidogen-2 interacted with collagens I and IV, and perlecan at a comparable level to nidogen-1 but failed to bind to fibulins. Nidogen-2 bound to laminin-1, but only moderately to the epitope on the laminin gamma1 chain, which promotes high-affinity binding of nidogen-1. Both nidogens were cell-adhesive for a restricted number of cell lines, with nidogen-2 having a higher activity. Together, these data suggest that nidogen-2 can compensate for some but not all functional activities ascribed to nidogen-1.

A correlation between epithelial proliferation rates, basement membrane component localization patterns, and morphogenetic potential in the embryonic mouse lung

Lung epithelial branching morphogenesis results from a repetitive series of cleft and bud formation, a process dependent upon a complex interaction with the surrounding mesenchyme. The present study describes these cleft- and bud-forming regions as autonomous morphogenetic compartments within the embryonic day 11.5 (E11.5) mouse lung and directly correlates their identity with differences in epithelial proliferation rates and the localization pattern of specific basement membrane components. Lung buds were cultured in vitro, in two-dimensional planes, and labeled with a series of 5-bromo-2'-deoxyuridine (BrdU) pulses. Collectively, epithelial cells within actively budding regions of the bronchiolar tree demonstrated an at least 2.5-fold greater proliferation rate than those situated in the adjacent cleft-forming regions. Epithelial proliferation rates showed an inverse relationship with the degree of immunoreactivity of nidogen, laminin-1, fibronectin, and collagen IV within the underlying basement membrane. Epithelial cells dissected free from mesenchyme demonstrated cell-cell contact-dependent proliferation, thus revealing a hierarchy between mesenchymal signaling and direct epithelial cell-cell communication during branch formation. Dissection of the E11.5 bronchiolar tree into specific distalbud and interbud regions and their in vitro culture demonstrated differences in their autonomous morphogenetic potential. Tissue dissected from the distal tips of the lung continued to branch, whereas tissue dissected from immediately adjacent cleft regions seldom branched. Isolated distalbud tissue also continued to correlate regional differences in epithelial proliferation rates and immunolocalization patterns of nidogen, laminin-1, fibronectin, and collagen IV with branch formation. These results support the basement membrane remodeling hypothesis, thus connecting nidogen, collagen type IV, fibronectin, and laminin-1 localization with the molecular processes directing epithelial proliferation and supporting bud outgrowth and cleft formation/stabilization during lung morphogenesis.

Drosophila laminin binds to mammalian nidogen and to heparan sulfate proteoglycan

A Drosophila laminin that has the chain composition alpha5 beta1 gamma1, relative to mammalian laminins, bound human and mouse nidogen almost as strongly as mouse laminin-1 (alpha1 beta1 gamma1) in solid-phase assays, and had only a fourfold lower affinity in a radioligand competition test. This is due to a short, highly conserved sequence that occurs in both laminin gamma1 chains and which binds nidogen. When the single conservative amino acid difference between the two sequences (Tyr-->His) was introduced into the mouse laminin binding module gamma1 III4 it failed to cause any change of binding. A high affinity between Drosophila laminin and mouse nidogen resulted in the formation of a stable complex in solution. Drosophila laminin also bound to the mouse heparan sulfate proteoglycan perlecan and the formation of this complex was inhibited by heparin, but not by chondroitin sulfate. In addition, a weaker connection between the core protein of mouse perlecan and Drosophila laminin can be mediated through nidogen. Elastase and other proteases degraded Drosophila laminin to a restricted number of larger fragments (40-300 kDa), almost all of which were bound to a heparin affinity column. Three fragments could be displaced at low salt concentration and were derived from the short arms of the Drosophila laminin, as shown by sequence analysis. A more strongly bound 50-kDa fragment apparently comprised the globular domains LG2 and LG3 derived from the C-terminal part of its alpha chain. Therefore, Drosophila laminin and mouse laminin-1 differ in certain aspects of protease stability and heparin-binding sites that, in part, can be attributed to their different alpha chains. The data suggest the existence of a nidogen analog and heparan sulfate proteoglycans in Drosophila, which remain to be identified.

Importance of nidogen binding to laminin gamma1 for branching epithelial morphogenesis of the submandibular gland

Epithelial-mesenchymal interactions are major driving forces for the development of most solid organs. The importance of these interactions was first shown for the embryonic submandibular gland more than 40 years ago. We here present evidence that interactions between two basement membrane components, nidogen (entactin) and laminin gamma1 chain, could be important for epithelial-mesenchymal interactions in this gland. Nidogen mRNA was detected by in situ hybridization in the mesenchyme, and yet the protein was detected in epithelial and endothelial basement membranes. The role of nidogen-laminin interactions for epithelial morphogenesis was studied by applying antibodies to submandibular gland organ cultures. Antibodies reacting strongly with the nidogen-binding site of laminin gamma1 chain drastically perturbed branching epithelial morphogenesis. Electron microscopy of the epithelial-mesenchymal interface showed that blocking antibodies disrupted the formation of the basement membrane. Epidermal growth factor was shown to increase the expression of nidogen in mesenchyme, and could counteract the effect of the blocking antibodies. We suggest that nidogen could be an important mesenchymal factor for submandibular gland development.

Alpha 3 beta 1 integrin has a crucial role in kidney and lung organogenesis

A mutation was targeted to the murine alpha3 integrin gene. Homozygous mutant mice survived to birth, but died during the neonatal period. The mutation caused abnormal kidney and lung development. Mutant kidneys displayed decreased branching of the medullary collecting ducts, although the number of nephrons was not altered. Proximal tubules exhibited two distinct subsets of abnormalities, with the epithelial cells either containing excess lysosomes or becoming microcystic. In addition, glomerular development was markedly affected. In mutant kidneys, the extent of branching of glomerular capillary loops was decreased, with capillary lumina being wider than normal. The glomerular basement membrane was disorganized and glomerular podocytes were unable to form mature foot processes. Branching of the bronchi in lungs of mutant mice was also decreased and the large bronchi extended to the periphery. These results indicate a role for integrin receptors in basement membrane organization and branching morphogenesis.

Macromolecular organization of basement membranes

A considerable variety of basement membrane components, including in particular more than ten laminin isoforms and their novel alpha chains (alpha3, alpha4 and alpha5), has been characterized in recent studies. The functional properties of these components are increasingly being analyzed by recombinant technologies and by structural studies at atomic resolution, techniques which led to the elucidation of the nidogen-binding epitope on the laminin gamma1 chain. Novel insights into functions of basement membrane components have been obtained from gene-targeting experiments and studies of mutated genes identified in inherited disorders.

Different susceptibilities of fibulin-1 and fibulin-2 to cleavage by matrix metalloproteinases and other tissue proteases

Fibulin-1 and fibulin-2 are two novel rod-like proteins which occur either in basement membranes or in interstitial fibrils in close association with fibronectin. They were examined for their sensitivity to proteolysis by matrix metalloproteinases (stromelysin, matrilysin), circulating proteases (thrombin, plasmin, kallikrein), leucocyte elastase and mast cell chymase. Fibulin-1 (95 kDa) was readily cleaved by leucocyte elastase, weakly by matrilysin and not by the other proteases. Cleavage occurred in a domain-connecting link region close to the N-terminus, giving rise to fragments of 70 kDa and 26 kDa. A much more extensive cleavage by all seven proteases was observed for fibulin-2 (195 kDa), giving rise to many fragments in the range 15-150 kDa. Vulnerable sites included two central link regions, the cysteine-free part of the large N-terminal globular domain but also several regions of epidermal-growth-factor(EGF)-like repeats which are a major part of the rod-like domain. The latter domain became much more sensitive to proteolysis in the presence of EDTA, demonstrating that calcium is required for stabilization. Edman degradation demonstrated cleavage of peptide bonds corresponding to the known specificities of these proteases. A similar proteolysis was also observed for fibulin-2 deposited by cultured fibroblasts into a dense fibrillar network. Since fibulin-2 is an abundant component of small and large blood vessels it could be a major target for proteolysis during vascular injuries.