Binding and calcium-induced aggregation of laminin onto lipid bilayers

Glycosaminoglycans Influence Extracellular Matrix of Human Trabecular Meshwork Cells Cultured on 3D Scaffolds

Glaucoma is a multifactorial progressive optic neuropathy characterized by the loss of retinal ganglion cells leading to irreversible blindness. It is the leading cause of global irreversible blindness and is currently affecting over 70 million people. Elevated intraocular pressure (IOP) is considered the only modifiable risk factor and is a target of numerous treatment modalities. Researchers have assigned this elevation of IOP to accumulation of extracellular matrix (ECM) components in the aqueous humor (AH) outflow pathway. The major drainage structure for AH outflow is the trabecular meshwork (TM). The ECM of the TM is important in regulating IOP in both normal and glaucomatous eyes. In this work, we have studied the role of exogeneous glycosaminoglycans (GAGs), glucocorticoids, and culture conditions on the expression of the ECM gene and proteins by human TM (hTM) cells cultured on biomaterial scaffolds. Gene and protein expression levels of elastin, laminin, and matrix metalloproteinase-2 (MMP-2) were evaluated using quantitative PCR and immunohistochemistry. Pressure gradient changes in cell-laden scaffolds in perfusion cultures were also monitored. Our findings show that GAGs and dexamethasone play an influencing role in hTM ECM turnover at both transcriptional and translational levels by altering expression levels of elastin, laminin, and MMP-2. Understanding the role of exogeneous factors on hTM cell behavior is helpful in gaining insights on glaucoma pathogenesis and ultimately pivotal in development of novel therapeutics against the disease.

Defective perlecan-associated basement membrane regeneration and altered modulation of transforming growth factor beta in corneal fibrosis

In the cornea, the epithelial basement membrane (EBM) and corneal endothelial Descemet's basement membrane (DBM) critically regulate the localization, availability and, therefore, the functions of transforming growth factor (TGF)β1, TGFβ2, and platelet-derived growth factors (PDGF) that modulate myofibroblast development. Defective regeneration of the EBM, and notably diminished perlecan incorporation, occurs via several mechanisms and results in excessive and prolonged penetration of pro-fibrotic growth factors into the stroma. These growth factors drive mature myofibroblast development from both corneal fibroblasts and bone marrow-derived fibrocytes, and then the persistence of these myofibroblasts and the disordered collagens and other matrix materials they produce to generate stromal scarring fibrosis. Corneal stromal fibrosis often resolves completely if the inciting factor is removed and the BM regenerates. Similar defects in BM regeneration are likely associated with the development of fibrosis in other organs where perlecan has a critical role in the modulation of signaling by TGFβ1 and TGFβ2. Other BM components, such as collagen type IV and collagen type XIII, are also critical regulators of TGF beta (and other growth factors) in the cornea and other organs. After injury, BM components are dynamically secreted and assembled through the cooperation of neighboring cells-for example, the epithelial cells and keratocytes for the corneal EBM and corneal endothelial cells and keratocytes for the corneal DBM. One of the most critical functions of these reassembled BMs in all organs is to modulate the pro-fibrotic effects of TGFβs, PDGFs and other growth factors between tissues that comprise the organ.

Laminin Polymerization and Inherited Disease: Lessons From Genetics

The laminins (LM) are a family of basement membranes glycoproteins with essential structural roles in supporting epithelia, endothelia, nerves and muscle adhesion, and signaling roles in regulating cell migration, proliferation, stem cell maintenance and differentiation. Laminins are obligate heterotrimers comprised of α, β and γ chains that assemble intracellularly. However, extracellularly these heterotrimers then assemble into higher-order networks via interaction between their laminin N-terminal (LN) domains. In vitro protein studies have identified assembly kinetics and the structural motifs involved in binding of adjacent LN domains. The physiological importance of these interactions has been identified through the study of pathogenic point mutations in LN domains that lead to syndromic disorders presenting with phenotypes dependent on which laminin gene is mutated. Genotype-phenotype comparison between knockout and LN domain missense mutations of the same laminin allows inferences to be drawn about the roles of laminin network assembly in terms of tissue function. In this review, we will discuss these comparisons in terms of laminin disorders, and the therapeutic options that understanding these processes have allowed. We will also discuss recent findings of non-laminin mediators of laminin network assembly and their implications in terms of basement membrane structure and function.

Conformable hierarchically engineered polymeric micromeshes enabling combinatorial therapies in brain tumours

The poor transport of molecular and nanoscale agents through the blood-brain barrier together with tumour heterogeneity contribute to the dismal prognosis in patients with glioblastoma multiforme. Here, a biodegradable implant (μMESH) is engineered in the form of a micrometre-sized poly(lactic-co-glycolic acid) mesh laid over a water-soluble poly(vinyl alcohol) layer. Upon poly(vinyl alcohol) dissolution, the flexible poly(lactic-co-glycolic acid) mesh conforms to the resected tumour cavity as docetaxel-loaded nanomedicines and diclofenac molecules are continuously and directly released into the adjacent tumour bed. In orthotopic brain cancer models, generated with a conventional, reference cell line and patient-derived cells, a single μMESH application, carrying 0.75 mg kg^-1 of docetaxel and diclofenac, abrogates disease recurrence up to eight months after tumour resection, with no appreciable adverse effects. Without tumour resection, the μMESH increases the median overall survival (∼30 d) as compared with the one-time intracranial deposition of docetaxel-loaded nanomedicines (15 d) or 10 cycles of systemically administered temozolomide (12 d). The μMESH modular structure, for the independent coloading of different molecules and nanomedicines, together with its mechanical flexibility, can be exploited to treat a variety of cancers, realizing patient-specific dosing and interventions.

Retinal capillary basement membrane thickening: Role in the pathogenesis of diabetic retinopathy

Vascular basement membrane (BM) thickening has been hailed over half a century as the most prominent histological lesion in diabetic microangiopathy, and represents an early ultrastructural change in diabetic retinopathy (DR). Although vascular complications of DR have been clinically well established, specific cellular and molecular mechanisms underlying dysfunction of small vessels are not well understood. In DR, small vessels develop insidiously as BM thickening occurs. Studies examining high resolution imaging data have established BM thickening as one of the foremost structural abnormalities of retinal capillaries. This fundamental structural change develops, at least in part, from excess accumulation of BM components. Although BM thickening is closely associated with the development of DR, its contributory role in the pathogenesis of DR is coming to light recently. DR develops over several years before clinical manifestations appear, and it is during this clinically silent period that hyperglycemia induces excess synthesis of BM components, contributes to vascular BM thickening, and promotes structural and functional lesions including cell death and vascular leakage in the diabetic retina. Studies using animal models show promising results in preventing BM thickening with subsequent beneficial effects. Several gene regulatory approaches are being developed to prevent excess synthesis of vascular BM components in an effort to reduce BM thickening. This review highlights current understanding of capillary BM thickening development, role of BM thickening in retinal vascular lesions, and strategies for preventing vascular BM thickening as a potential therapeutic strategy in alleviating characteristic lesions associated with DR.

Change the Laminin, Change the Cardiomyocyte: Improve Untreatable Heart Failure

No effective medical treatment exists for heart failure with preserved ejection fraction (HFpEF), accounting for approximately half of all heart failure cases. The elevated passive myocardial stiffness in HFpEF is attributed to a combination of alterations in the extracellular matrix (ECM) collagen content and modifications in the sarcomeric protein titin. Here, we propose polylaminin, a biomimetic polymer of laminin, as a promising approach for manipulating the titin isoform shift and phosphorylation in cardiomyocytes. Exploring the pleiotropic effects of polylaminin may be a novel strategy for alleviating symptoms in HFpEF's multifactorial pathophysiology.

Calcium in Cell-Extracellular Matrix Interactions

In multicellular organisms, the cells are surrounded by persistent, dynamic extracellular matrix (ECM), the largest calcium reservoir in animals. ECM regulates several aspects of cell behavior including cell migration and adhesion, survival, gene expression and differentiation, thus playing a significant role in health and disease. Calcium is reported to be important in the assembly of ECM, where it binds to many ECM proteins. While serving as a calcium reservoir, ECM macromolecules can directly interact with cell surface receptors resulting in calcium transport across the membrane. This chapter mainly focusses on the role of cell-ECM interactions in cellular calcium regulation and how calcium itself mediates these interactions.

Extracellular matrix scaffolding in angiogenesis and capillary homeostasis

The extracellular matrix (ECM) of blood vessels, which is composed of both the vascular basement membrane (BM) and the interstitial ECM is identified as a crucial component of the vasculature. We here focus on the unique molecular composition and scaffolding of the capillary ECM, which provides structural support to blood vessels and regulates properties of endothelial cells and pericytes. The major components of the BM are collagen IV, laminins, heparan sulfate proteoglycans and nidogen and also associated proteins such as collagen XVIII and fibronectin. Their organization and scaffolding in the BM is required for proper capillary morphogenesis and maintenance of vascular homeostasis. The BM also regulates vascular mechanosensing. A better understanding of the mechanical and structural properties of the vascular BM and interstitial ECM therefore opens new perspectives to control physiological and pathological angiogenesis and vascular homeostasis. The overall aim of this review is to explain how ECM scaffolding influences angiogenesis and capillary integrity.

Assembly, heterogeneity, and breaching of the basement membranes

Basement membranes are thin sheets of self-assembled extracellular matrices that are essential for embryonic development and for the homeostasis of adult tissues. They play a role in structuring, protecting, polarizing, and compartmentalizing cells, as well as in supplying them with growth factors. All basement membranes are built from laminin and collagen IV networks stabilized by nidogen/perlecan bridges. The precise composition of basement membranes, however, varies between different tissues. Even though basement membranes represent physical barriers that delimit different tissues, they are breached in many physiological or pathological processes, including development, the immune response, and tumor invasion. Here, we provide a brief overview of the molecular composition of basement membranes and the process of their assembly. We will then illustrate the heterogeneity of basement membranes using two examples, the epithelial basement membrane in the gut and the vascular basement membrane. Finally, we examine the different strategies cells use to breach the basement membrane.

Integrating Activities of Laminins that Drive Basement Membrane Assembly and Function

Studies on extracellular matrix proteins, cells, and genetically modified animals have converged to reveal mechanisms of basement membrane self-assembly as mediated by γ1 subunit-containing laminins, the focus of this chapter. The basic model is as follows: A member of the laminin family adheres to a competent cell surface and typically polymerizes followed by laminin binding to the extracellular adaptor proteins nidogen, perlecan, and agrin. Assembly is completed by the linking of nidogen and heparan sulfates to type IV collagen, allowing it to form a second stabilizing network polymer. The assembled matrix provides structural support, anchoring the extracellular matrix to the cytoskeleton, and acts as a signaling platform. Heterogeneity of function is created in part by the isoforms of laminin that vary in their ability to polymerize and to interact with integrins, dystroglycan, and other receptors. Mutations in laminin subunits, affecting expression or LN domain-specific functions, are a cause of human diseases that include those of muscle, nerve, brain, and kidney.

Polylaminin recognition by retinal cells

Polylaminin (polyLM) is a flat biomimetic polymer of laminin capable of promoting axonal growth both in vitro and in vivo. It is assembled in a cell-free system when laminin 111 is incubated in acidic pH, whereas incubation in neutral buffer leads to the formation of bulky and irregular polymers (LM). In the present work, we compared the behaviors of cells isolated from the P1 rat retina on polyLM and LM. PolyLM induced cellular spreading and the outgrowth of neurites in contact with the substrate, whereas LM led to the formation of large clusters of cells, with neurites growing only inward. After 24 hr in culture, the number of cells on polyLM increased threefold, and this increase was inhibited by 60% in the presence of the PKA inhibitor H89 and by 41% in the presence of the PKC inhibitor chelerythrine chloride, whereas both inhibitors abolished neuritogenesis. Neither the cell number nor the outgrowth of neurites was affected by the ERK1/2 inhibitor PD98059 on polyLM. On the other hand, PD98059 was able to reduce the cell number on LM, whereas the other inhibitors were not. Immunostaining of P1 retina with an antilaminin antibody revealed that the protein was expressed not only at its inner surface but also within the neuroblast layer in close contact with individual cells. Our results indicate that, when provided in its active polymerized form, laminin can influence both neuritogenesis and proliferation of retinal cells.

Laminins in basement membrane assembly

The heterotrimeric laminins are a defining component of all basement membranes and self-assemble into a cell-associated network. The three short arms of the cross-shaped laminin molecule form the network nodes, with a strict requirement for one α, one β and one γ arm. The globular domain at the end of the long arm binds to cellular receptors, including integrins, α-dystroglycan, heparan sulfates and sulfated glycolipids. Collateral anchorage of the laminin network is provided by the proteoglycans perlecan and agrin. A second network is then formed by type IV collagen, which interacts with the laminin network through the heparan sulfate chains of perlecan and agrin and additional linkage by nidogen. This maturation of basement membranes becomes essential at later stages of embryo development.

Scaffold-forming and Adhesive Contributions of Synthetic Laminin-binding Proteins to Basement Membrane Assembly

Laminins that possess three short arms contribute to basement membrane assembly by anchoring to cell surfaces, polymerizing, and binding to nidogen and collagen IV. Although laminins containing the alpha4 and alpha5 subunits are expressed in alpha2-deficient congenital muscular dystrophy, they may be ineffective substitutes because they bind weakly to cell surfaces and/or because they lack the third arm needed for polymerization. We asked whether linker proteins engineered to bind to deficient laminins that provide such missing activities would promote basement membrane assembly in a Schwann cell model. A chimeric fusion protein (alphaLNNd) that adds a short arm terminus to laminin through the nidogen binding locus was generated and compared with the dystrophy-ameliorating protein miniagrin (mAgrin) that binds to the laminin coiled-coil dystroglycan and sulfatides. alphaLNNd was found to mediate laminin binding to collagen IV, to bind to galactosyl sulfatide, and to selectively convert alpha-short arm deletion-mutant laminins LmDeltaalphaLN and LmDeltaalphaLN-L4b into polymerizing laminins. This protein enabled polymerization-deficient laminin but not an adhesion-deficient laminin lacking LG domains (LmDeltaLG) to assemble an extracellular matrix on Schwann cell surfaces. mAgrin, on the other hand, enabled LmDeltaLG to form an extracellular matrix on cell surfaces without increasing accumulation of non-polymerizing laminins. These gain-of-function studies reveal distinct polymerization and anchorage contributions to basement membrane assembly in which the three different LN domains mediate the former, and the LG domains provide primary anchorage with secondary contributions from the alphaLN domain. These findings may be relevant for an understanding of the pathogenesis and treatment of laminin deficiency states.

Basement membranes in skin are differently affected by lack of nidogen 1 and 2

Nidogens have been proposed to play a key role in basement membrane (BM) formation. However, recent findings using genetic approaches and organotypic coculture models demonstrated distinct tissue requirements thus changing the classical view of BM assembly. Toward this end, we have analyzed the dermo-epidermal junction and the microvasculature in skin of nidogen-deficient mice for their BM composition and structural assembly. Histology of nidogen double-null embryos at embryonic day (E)18.5 revealed overall normal skin morphology with a regularly differentiated epidermis. However, in the dermis, numerous erythrocytes had extravasated out of the microvasculature. Residual composition and ultrastructure of the dermo-epidermal BM are not altered in the absence of nidogens, demonstrating that the deposition of laminin, collagen IV, and perlecan occurs and allows cutaneous BM formation. In contrast, in capillaries, BM formation is severely impaired in the absence of nidogens, showing an irregular, patchy distribution and a dramatically reduced deposition of collagen IV, perlecan, and particularly laminin-411. Ultrastructure revealed thin fragile walls in the small blood vessels next to the epidermis, completely lacking a distinct endothelial BM. In summary, our results indicate that in skin the laminin composition of the various BMs determines whether nidogens are required for their assembly and stabilization.

Biological function of laminin-5 and pathogenic impact of its deficiency

The basement membrane glycoprotein laminin-5 is a key component of the anchoring complex connecting keratinocytes to the underlying dermis. It is secreted by keratinocytes as a cross-shaped heterotrimer of alpha3, beta3 and gamma2 chains and serves as a ligand of various transmembrane receptors, thereby regulating keratinocyte adhesion, motility and proliferation. In intact skin, laminin-5 provides essential links to both the hemidesmosomal alpha6beta4 integrin and the collagen type VII molecules which form the anchoring fibrils inserting into the dermis. If the basement membrane is injured, laminin-5 production increases rapidly. It then serves as a scaffold for cell migration, initiates the formation of hemidesmosomes and accelerates basement membrane restoration at the dermal-epidermal junction. Mutations of the laminin-5 genes or auto-antibodies against one of the subunits of laminin-5 may lead to a significant lack of this molecule in the epidermal basement membrane zone. The major contributions of laminin-5 to the resistance of the epidermis against frictional stress but also for basement membrane regeneration and repair of damaged skin are reflected by the phenotype of Herlitz junctional epidermolysis bullosa, which is caused by an inherited absence of functional laminin-5. This lethal disease becomes manifest in widespread blistering of skin and mucous membranes, impaired wound healing and chronic erosions containing exuberant granulation tissue. Here, we discuss current understanding of the biological functions of laminin-5, the pathogenic impact of its deficiency and implications on molecular approaches towards a therapy of junctional epidermolysis bullosa.

Role of Laminin Terminal Globular Domains in Basement Membrane Assembly

Laminins contribute to basement membrane assembly through interactions of their N- and C-terminal globular domains. To further analyze this process, recombinant laminin-111 heterotrimers with deletions and point mutations were generated by recombinant expression and evaluated for their ability to self-assemble, interact with nidogen-1 and type IV collagen, and form extracellular matrices on cultured Schwann cells by immunofluorescence and electron microscopy. Wild-type laminin and laminin without LG domains polymerized in contrast to laminins with deleted alpha1-, beta1-, or gamma1-LN domains or with duplicated beta1- or alpha1-LN domains. Laminins with a full complement of LN and LG domains accumulated on cell surfaces substantially above those lacking either LN or LG domains and formed a lamina densa. Accumulation of type IV collagen onto the cell surface was found to require laminin with separate contributions arising from the presence of laminin LN domains, nidogen-1, and the nidogen-binding site in laminin. Collectively, the data support the hypothesis that basement membrane assembly depends on laminin self-assembly through formation of alpha-, beta-, and gamma-LN domain complexes and LG-mediated cell surface anchorage. Furthermore, type IV collagen recruitment into the laminin extracellular matrices appears to be mediated through a nidogen bridge with a lesser contribution arising from a direct interaction with laminin.

Laminin-sulfatide binding initiates basement membrane assembly and enables receptor signaling in Schwann cells and fibroblasts

Endoneurial laminins (Lms), β1-integrins, and dystroglycan (DG) are important for Schwann cell (SC) ensheathment and myelination of axons. We now show that SC expression of galactosyl-sulfatide, a Lm-binding glycolipid, precedes that of Lms in developing nerves. This glycolipid anchors Lm-1 and -2 to SC surfaces by binding to their LG domains and enables basement membrane (BM) assembly. Revealingly, non–BM-forming fibroblasts become competent for BM assembly when sulfatides are intercalated into their cell surfaces. Assembly is characterized by coalescence of sulfatide, DG, and c-Src into a Lm-associated complex; by DG-dependent recruitment of utrophin and Src activation; and by integrin-dependent focal adhesion kinase phosphorylation. Collectively, our findings suggest that sulfated glycolipids are key Lm anchors that determine which cell surfaces can assemble Lms to initiate BM assembly and DG- and integrin-mediated signaling.

Laminin functions in tissue morphogenesis

Significant advances have been made in the application of genetics to probe the functions of basement membrane laminins. These studies have shown that different laminin subunits profoundly affect tissue morphogenesis, starting around the time of embryonic implantation and extending through organogenesis and into the postnatal period. Collectively they have revealed common functions that include the induction and maintenance of cell polarity, the establishment of barriers between tissue compartments, the organization of cells into tissues, and the protection of adherent cells from detachment-induced cell death, anoikis. Interpreted in light of what is known about laminin structure and self-assembly and binding activities, these advances have begun to provide insights into mechanisms of action. In this review we focus on the contributions of the laminins in invertebrate and vertebrate tissue morphogenesis.

Laminins: structure and genetic regulation

The laminins form a large family of modular proteins found in basement membranes, but also elsewhere. They function as structural components and are essential for morphogenesis, but in addition interact with cell surface receptors such as integrins and alpha-dystroglycan. By virtue of their receptor interactions, they initiate intracellular signalling events that regulate cellular organization and differentiation. The many interactions of laminins are mediated by binding sites, often contributed by single domains, which may differ between different forms of laminin. In the present article, we describe how the diversity of laminins and the genetic regulation of the expression of different laminin forms lead to the formation of extracellular matrices with variable laminin composition and thereby different biological properties.

Structure and function of laminin LG modules

Laminin G domain-like (LG) modules of approximately 180-200 residues are found in a number of extracellular and receptor proteins and often are present in tandem arrays. LG modules are implicated in interactions with cellular receptors (integrins, alpha-dystroglycan), sulfated carbohydrates and other extracellular ligands. The recently determined crystal structures of LG modules of the laminin alpha2 chain reveal a compact beta sandwich fold and identify a novel calcium binding site. Binding epitopes for heparin, sulfatides and alpha-dystroglycan have been mapped by site-directed mutagenesis and show considerable overlap. The epitopes are located in surface loops around the calcium site, which in other proteins (agrin, neurexins) are modified by alternative splicing. Efficient ligand binding often requires LG modules to be present in tandem. The close proximity of the N- and C-termini in the LG module, as well as a unique link region between laminin LG3 and LG4, impose certain constraints on the arrangement of LG tandems. Further modifications may be introduced by proteolytic processing of laminin G domains, which is known to occur in the alpha2, alpha3 and alpha4 chains.

Form and function: the laminin family of heterotrimers

The laminins are a family of glycoproteins that provide an integral part of the structural scaffolding of basement membranes in almost every animal tissue. Each laminin is a heterotrimer assembled from alpha, beta, and gamma chain subunits, secreted and incorporated into cell-associated extracellular matrices. The laminins can self-assemble, bind to other matrix macromolecules, and have unique and shared cell interactions mediated by integrins, dystroglycan, and other receptors. Through these interactions, laminins critically contribute to cell differentiation, cell shape and movement, maintenance of tissue phenotypes, and promotion of tissue survival. Recent advances in the characterization of genetic disruptions in humans, mice, nematodes and flies have revealed developmental roles for the different laminin subunits in diverse cell types, affecting differentiation from blastocyst formation to the post-natal period. These genetic defects have challenged some of the previous concepts about basement membranes and have shed new light on the diversity and complexity of laminin functions as well as established the molecular basis of several human diseases.

Self-assembly of laminin induced by acidic pH

The supramolecular architecture of the basement membrane is provided by two enmeshed networks of collagen IV and laminin. The laminin network is maintained exclusively by interactions among individual laminin molecules and does not depend on the presence of other extracellular matrix components. Laminin polymers can be obtained in vitro either in solution or in association with the surface of bilayers containing acidic lipids. In this work, we have tested the hypothesis that the negative charges present on acidic lipids establish an acid microenvironment that is directly responsible for inducing laminin aggregation. Using light-scattering measurements, we show that laminin does not aggregate on vesicles of neutral lipids, whereas instantaneous aggregation occurs to progressively greater extents as the proportion of acidic phospholipids in the vesicles is increased. Aggregation of laminin induced by vesicles containing acidic phospholipids occurs very rapidly, so that maximal aggregation for each condition is reached within 1 min after laminin dilution. Aggregation depends on the presence of Ca(2+) ions, is reversed by increasing ionic strength, and can be detected at laminin concentrations as low as 6 nM. In addition, we show that, in the absence of vesicles, acidification of the bulk solution can also induce laminin self-polymerization through a process that exhibits the same properties as lipid-induced polymerization. The fact that there is a correspondence between the processes of self-polymerization of laminin in acidic medium and in neutral medium but in the presence of vesicles containing negatively charged lipids leads us to propose that the microenvironment of an acidic surface may trigger the assembly of laminin networks. In vivo, such an acidic microenvironment would be provided by negatively charged sialic acid and sulfate groups present in the glycocalyx surrounding the cells.

Laminin polymerization induces a receptor-cytoskeleton network

The transition of laminin from a monomeric to a polymerized state is thought to be a crucial step in the development of basement membranes and in the case of skeletal muscle, mutations in laminin can result in severe muscular dystrophies with basement membrane defects. We have evaluated laminin polymer and receptor interactions to determine the requirements for laminin assembly on a cell surface and investigated what cellular responses might be mediated by this transition. We found that on muscle cell surfaces, laminins preferentially polymerize while bound to receptors that included dystroglycan and alpha7beta1 integrin. These receptor interactions are mediated through laminin COOH-terminal domains that are spatially and functionally distinct from NH2-terminal polymer binding sites. This receptor-facilitated self-assembly drives rearrangement of laminin into a cell-associated polygonal network, a process that also requires actin reorganization and tyrosine phosphorylation. As a result, dystroglycan and integrin redistribute into a reciprocal network as do cortical cytoskeleton components vinculin and dystrophin. Cytoskeletal and receptor reorganization is dependent on laminin polymerization and fails in response to receptor occupancy alone (nonpolymerizing laminin). Preferential polymerization of laminin on cell surfaces, and the resulting induction of cortical architecture, is a cooperative process requiring laminin- receptor ligation, receptor-facilitated self-assembly, actin reorganization, and signaling events.

Initiation of skin basement membrane formation at the epidermo-dermal interface involves assembly of laminins through binding to cell membrane receptors

To study the mechanism of basement membrane formation, we determined by immunochemistry temporal and spatial expression of laminin-5 (Ln-5), laminin-1 (Ln-1) and their integrin receptors during early skin morphogenesis. A 3-dimensional skin culture was used that allows the study of the sequential molecular events of basement membrane formation at the epidermodermal interface. During early anchorage of keratinocytes to the extracellular matrix there is expression of Ln-5, BP-230 antigen and alpha3, beta1 integrin subunits. During epidermal stratification and prior to the formation of the lamina densa there is assembly of Ln-5, Ln-1, collagen IV and nidogen accompanied by keratinocyte basal clustering of alpha2, alpha3, alpha6, beta1, and beta4+ integrin subunits. The assembly pattern of Ln-1 and Ln-5 can be disturbed with functional antibodies against the beta1 (AIIB2) and alpha6 (GoH3) integrin subunits. Ln-1 assembly can also be disturbed with antibodies against its E8 domain and by competitive inhibition with a synthetic peptide (AG-73) derived from its G-4 domain. Quantitative RT-PCR showed that the dermis contributes about 80% of the laminin gamma)1 chain mRNA while 20% is produced by the epidermis which emphasizes its dual tissue origin and the major contribution of the mesenchyma in laminin production. The laminin gamma2 chain mRNA, present in Ln-5, was mostly of epidermal origin. This study presents evidence that during the initiation of basement membrane formation, laminins bind to keratinocyte plasma membrane receptors and thus may serve as nucleation sites for further polymerization of these compounds by a self-assembly process.

Laminin-induced clustering of dystroglycan on embryonic muscle cells: comparison with agrin-induced clustering

The effect of laminin on the distribution of dystroglycan (DG) and other surface proteins was examined by fluorescent staining in cultures of muscle cells derived from Xenopus embryos. Western blotting confirmed that previously characterized antibodies are reactive in Xenopus. In control cultures, alphaDG, betaDG, and laminin binding sites were distributed as microclusters (<1 microm2 in area) over the entire dorsal surface of the muscle cells. Treatment with laminin induced the formation of macroclusters (1-20 microm2), accompanied by a corresponding decline in the density of the microclusters. With 6 nM laminin, clustering was apparent within 150 min and near maximal within 1 d. Laminin was effective at 30 pM, the lowest concentration tested. The laminin fragment E3, which competes with laminin for binding to alphaDG, inhibited laminin-induced clustering but did not itself cluster DG, thereby indicating that other portions of the laminin molecule in addition to its alphaDG binding domain are required for its clustering activity. Laminin-induced clusters also contained dystrophin, but unlike agrin-induced clusters, they did not contain acetylcholine receptors, utrophin, or phosphotyrosine, and their formation was not inhibited by a tyrosine kinase inhibitor. The results reinforce the notion that unclustered DG is mobile on the surface of embryonic muscle cells and suggest that this mobile DG can be trapped by at least two different sets of molecular interactions. Laminin self binding may be the basis for the laminin-induced clustering.

Laminin interactions important for basement membrane assembly are promoted by zinc and implicate laminin zinc finger-like sequences

Laminin is an abundant basement membrane (BM) glycoprotein which regulates specific cellular functions and participates in the assembly and maintenance of the BM superstructure. The assembly of BM is believed to involve the independent polymerization of collagen type IV and laminin, as well as high affinity interactions between laminin, entactin/nidogen, perlecan, and collagen type IV. We report here that Zn2+ can influence laminin binding activity, in vitro. Laminin contains 42 cysteine-rich repeats of which 12 contained nested zinc finger consensus sequences. Recently, the entactin binding site was mapped to one of these zinc finger-containing repeats on the laminin gamma chain (Mayer, U., Nischt, R., Poschl, E., Mann, K., Fukuda, K., Gerl, M., Yamada, Y., and Timpl, R. (1993) EMBO J. 12, 1879-1885). Based on these observations, the effect of a series of essential ions (Ca2+, Cd2+, Cu2+, Mg2+, Mn2+, and Zn2+) on laminin binding activity was evaluated. Zn2+ was found to be the most effective at enhancing laminin-entactin and laminin-collagen type IV binding. Laminin-bound Zn2+ was detected by flame atomic absorption spectroscopy at a maximum of 8 mol/mol of laminin. Furthermore, Ca2+-dependent laminin polymerization was unaffected by Zn2+, an observation consistent with the lack of zinc finger-containing repeats in the terminal globular domains required for polymerization. We conclude that Zn2+-laminin complexes may generate high affinity binding sites which contribute to BM cross-linking important for its assembly and homeostasis. Zinc is likely a cofactor for 2 kinds of cross-linking interactions; one involving direct binding between laminin and collagen type IV and the other a ternary complex of laminin-entactin-collagen type IV.

A novel role for alpha 3 beta 1 integrins in extracellular matrix assembly

To study the biological role of alpha 3 beta 1 integrins in cell adhesion, migration, and in the deposition of extracellular matrix, we stably expressed the human alpha 3 integrin subunit in the alpha 4, alpha 5 integrin deficient CHO cell line B2. The expression of alpha 3 beta 1 integrins enhanced cell adhesion on entactin (also known as nidogen), but not on fibronectin. Using recombinant GST-fusion proteins that span the entire length of the entactin molecule, we located cell adhesive activity to the G2 domain of entactin. These results suggest that the alpha 3 beta 1 integrin functions as an adhesion receptor interacting with the G2 domain of entactin. On the other hand, the expression of alpha 3 beta 1 integrins did not confer the ability to migrate on entactin. Strikingly, the expression of alpha 3 beta 1 dramatically increased the deposition of entactin and fibronectin into the pericellular matrix. This was accompanied by increased binding activity of the 29 kDa amino-terminal domain of fibronectin. Thus, similar to alpha 5 beta 1 integrins, alpha 3 beta 1 integrins can play an important role in modulating the assembly of pericellular matrices. However, unlike fibronectin deposition supported by alpha 5 beta 1, alpha 3 beta 1 supported fibronectin deposition into pericellular matrix was not inhibited by antibodies binding to the RGD containing cell adhesion domain of fibronectin, demonstrating that the two processes are mechanistically distinct. The role of alpha 3 beta 1 in pericellular matrix assembly potentially implicates this receptor in the assembly and/or recognition of entactin-containing pericellular matrices, an observation consistent with its apparent role in the renal glomerulus of the mammalian kidney.

Biosynthesis and secretion of laminin and S-laminin by human prostate carcinoma cell lines

Laminin has been previously shown to participate in tumor invasion and metastasis. Tumorigenesis by human prostate cell lines can be influenced by exogenous laminin. We have found that the human prostate tumor cell lines DU145, LNCaP, and PC-3 produce a variant form of the B1 chain of laminin, known as S-laminin. DU145 cells produce laminin containing a ratio of A:B1:B2:S of 1.8:1.0:2.5:1.0, respectively, compared to LNCaP cells, which have a ratio of A:B1:B2:S of 1.0:0:10.0:2.5. Kinetic analysis of laminin production showed that LNCaP cells retained most of their laminin production and secreted laminin forms characterized by decreased levels of n-acetyl glucosamine and sialic acid. In contrast, DU145 cells secreted most of their laminin in mature forms. LNCaP cells bound very little laminin to their surface. These features of LNCaP cells could contribute to their documented low adhesivity in vitro and its nontumorigenic phenotype.

Experimental methods for investigating protein adsorption kinetics at surfaces

The adsorption of proteins at the solid-liquid interface is a process of fundamental importance in nature. Extensive reviews (MacRitchie, 1978; Andrade & Hlady, 1986; Norde, 1986) testify to the strong interest which has been shown in the problem during the past few decades. Norde & Lyklema (1978) have rightly pointed out that protein adsorption is scientifically intriguing; the phenomenology is complicated and includes many presently apparently irreconcilable observations.

Cranin interacts specifically with the sulfatide-binding domain of laminin

Cranin is a 120 kDa integral membrane glycoprotein which binds laminin under conditions of physiologic ionic strength in a calcium-dependent manner. Here, binding of cranin to laminin has been characterized using both ligand-blotting assays and laminin affinity bead assays. Binding was specifically inhibited by anti-laminin antibodies against the A chain terminal domain G, but not by several other region-specific antibodies. Dextran sulfate, fucoidin, and sulfatide were potent inhibitors of binding (50% inhibition at 0.03, 0.5, and 1.7 micrograms/ml, respectively); heparin was a weaker inhibitor (50% inhibition approximately 5 micrograms/ml), and mannan and chondroitin sulfate were not inhibitory at 100 micrograms/ml. Binding was not inhibited by lactose or the A chain peptide PA22-2. The mobility of the broad, fuzzy cranin band was shifted after digestion with neuraminidase, N-glycanase, and O-glycanase, though none of these treatments decreased band heterogeneity nor destroyed the ability to bind laminin. Cranin bound to Jacalin lectin, which recognizes the Gal beta 1-3GalNAc linkage expressed in certain classes of mucins. These findings indicate that cranin binds at or near the high affinity sulfatide-binding site previously mapped to the E3 domain of laminin, which is known to exhibit bioactivity for neural cells. In view of the extremely low abundance of cranin in brain membranes (approximately 0.005%), its avid laminin-binding activity is remarkable, and strongly suggests that cranin may play a physiologic role in regulating specific neural cell interactions.

Total internal reflection fluorescence microscopy: application to substrate-supported planar membranes

The use of total internal reflection illumination in fluorescence microscopy (TIRFM) is reviewed with emphasis on application to fluorescent macromolecules that specifically and reversibly bind to planar model membranes supported on glass or quartz substrates. Several methods for characterizing macromolecular motion and organization are discussed: the measurement of equilibrium binding curves to obtain values for equilibrium binding constants; the measurement of fluorescence photobleaching recovery curves to obtain values of kinetic rate constants and surface diffusion coefficients; and the measurement of fluorescence intensities as a function of the evanescent field polarization to characterize orientational order. Applications to cell-substrate contact regions are summarized and future directions of TIRFM are outlined.

Calcium-dependence of laminin binding to phospholipid membranes

Laminin is the most abundant noncollagenous protein in basement membranes. Its self-assembly has been studied in solution, and it has been established that calcium ions induce partially reversible aggregation. The behavior at a lipid membrane surface is of greater biological significance, but it is difficult to study quantitatively binding kinetics at a surface. The present work uses a powerful new integrated optics technique to measure the absorption and aggregation of the laminin-nidogen at a bilayer lipid membrane surface. It is found that the binding of a single layer of laminin at the lipid membrane is independent of the presence of calcium, but that the building up of multilayer laminin membranes requires calcium, and that these may not be destroyed by a calcium-complexing agent.

Interaction of phospholipids with proteins and peptides. New advances III

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins and peptides. 2. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecules is also discussed.

The L2/HNK-1 carbohydrate mediates adhesion of neural cells to laminin

The L2/HNK-1 carbohydrate epitope shared by several neural adhesion molecules has been implicated in cell-to-cell and cell-to-laminin adhesion (Keilhauer et al., Nature, 316, 728-730, 1985; Künemund et al., J. Cell Biol., 106, 213-223, 1988). As demonstrated previously for chicken retinal ganglion cells (Cole et al., Neurosci. Lett., 93, 170-175, 1988), cerebral cortex astrocytes or cerebellar neurons could not be shown to adhere to the substrate-bound L2/HNK-1 carbohydrate. The cell-bound L2/HNK-1 carbohydrate, however, was a potent mediator of astrocytic and neuronal cell adhesion to laminin, which was strongly reduced in the presence of the L2/HNK-1 carbohydrate-carrying glycolipids or Fab fragments of a monoclonal antibody against it. Inhibition of adhesion could not be observed in the presence of the negatively charged gangliosides or sulphatide, but in the presence of heparin. To investigate whether the L2/HNK-1 carbohydrate and heparin use the same or different binding sites on laminin, adhesion of cells to laminin was determined in the presence of heparin and Fab fragments of a monoclonal L2 antibody, which gave an additive value of inhibition as compared to the inhibition caused by the single compounds. This result, as well as studies of the binding of the L2/HNK-1 glycolipids to laminin in the presence of heparin, indicates that the L2/HNK-1 carbohydrate and heparin are implicated in different aspects of neural cell adhesion to laminin.

Laminins and other strange proteins

Laminins are large multidomain proteins of the extracellular matrix (ECM) with important functions in the development and maintenance of cellular organization and supramolecular structure, in particular in basement membranes. Each molecule is composed of three polypeptide chains, A (300-400 kDa) and B1 and B2 (180-200 kDa), which together form the characteristic cross-shaped laminin structure with three short arms and one long arm. Many different domains have been identified in laminin by sequence analysis, structural investigations, and functional studies. Each short arm is formed by homologous N-terminal portions of one of the three chains. Structurally, each short arm contains two or three globular domains which are connected by rows of manyfold-repeated Cys-rich "EGF-like" domains. In all three chains this region is followed by a long heptad repeat region similar to those found in many alpha-helical coiled-coil proteins. These parts of the three laminin chains constitute a triple-stranded coiled-coil domain, which forms the extended rodlike structure of the long arm. This is the only domain in the protein which is made up of more than one chain and consequently serves the function of chain assembly. The two B chains are terminated by the coiled-coil domain, but the A chain contains an additional C-terminal segment which accounts for five globular domains located at the tip of the long arm. Several important functions of laminin have been assigned to individual domains in either the short arms or terminal regions of the long arm.(ABSTRACT TRUNCATED AT 250 WORDS)

Assembly of extracellular matrix

A great challenge in understanding how different extracellular matrices assemble is to sort through the vast number of possible interactions between and among matrix molecules. The most profound insights are likely to come from patients with defined defects of matrix molecules and the use of transgenic mice or other experimental technologies that mimic the complexity of the human system.

Formation of supported planar bilayers by fusion of vesicles to supported phospholipid monolayers

A technique for the production of supported phospholipid bilayers by adsorption and fusion of small unilamellar vesicles to supported phospholipid monolayers on quartz is described. The physical properties of these supported bilayers are compared with those of supported bilayers which are prepared by Langmuir-Blodgett deposition or by direct vesicle fusion to plain quartz slides. The time courses of vesicle adsorption, fusion and desorption are followed by total internal reflection fluorescence microscopy and the lateral diffusion of the lipids in the adsorbed layers by fluorescence recovery after photobleaching. Complete supported bilayers can be formed with phosphatidylcholine vesicles at concentrations as low as 35 microM. However, the adsorption, fusion and desorption kinetics strongly depend on the used lipid, NaCl and Ca2+ concentrations. Asymmetric negatively charged supported bilayers can be produced by incubating a phosphatidylcholine monolayer with vesicles composed of 80% phosphatidylcholine and 20% phosphatidylglycerol. Adsorbed vesicles can be removed by washing with buffer. The measured fluorescence intensities after washing are consistent with single supported bilayers. The lateral diffusion experiments confirm that continuous extended bilayers are formed by the monolayer-fusion technique. The measured lateral diffusion coefficient of NBD-labeled phosphatidylethanolamine is (3.6 +/- 0.5) x 10(-8) cm2/s in supported phosphatidylcholine bilayers, independent of the method by which the bilayers were prepared.