Laminin and s-laminin are produced and released by astrocytes, Schwann cells, and schwannomas in culture

Astrocytes: form, functions, and roles in disease

Astrocytes, once relegated to a mere supportive role in the central nervous system, are now recognized as a heterogeneous class of cells with many important and diverse functions. Major astrocyte functions can be grouped into three categories: guidance and support of neuronal migration during development, maintenance of the neural microenvironment, and modulation of immune reactions by serving as antigen-presenting cells. The concept of astrocytic heterogeneity is critical to understanding the functions and reactions of these cells in disease. Astrocytes from different regions of the brain have diverse biochemical characteristics and may respond in different ways to a variety of injuries. Astrocytic swelling and hypertrophy-hyperplasia are two common reactions to injury. This review covers the morphologic and pathophysiologic findings, time course, and determinants of these two responses. In addition to these common reactions, astrocytes may play a primary role in certain diseases, including epilepsy, neurological dysfunction in liver disease, neurodegenerative disorders such as Parkinson's and Huntington's diseases, and demyelination. Evidence supporting primary involvement of astrocytes in these diseases will be considered.

Evidence for the binding of Ng-CAM to laminin

Ng-CAM is a cell adhesion molecule mediating neuron-glia and neuron-neuron adhesion via different binding mechanisms. While its binding can be homophilic as demonstrated by the self-aggregation of Ng-CAM coated beads (Covaspheres), Ng-CAM has also been shown to bind to glia by a heterophilic mechanism. In the present study, we found that the extent of Ng-CAM Covasphere aggregation was strongly diminished in the presence of the extracellular matrix glycoprotein laminin. When proteolytic fragments of laminin were tested, the P1' fragment (obtained from the short arms by pepsin treatment) was found to inhibit aggregation of Ng-CAM-Covaspheres while the elastase fragments E3 and E8 (from the long arm) were ineffective. To provide other means of analyzing interactions between laminin and Ng-CAM, the two proteins were covalently linked to differently fluorescing Covaspheres and tested for coaggregation. Laminin-Covaspheres coaggregated with Ng-CAM-Covaspheres, and this binding was inhibited both by anti-Ng-CAM and by anti-laminin antibodies. Covaspheres coated with other proteins including BSA and fibronectin did not coaggregate with Ng-CAM-Covaspheres. Moreover, using a solid phase binding assay, we found that 125I-labeled Ng-CAM bound to laminin and to Ng-CAM but not to fibronectin. The results suggest that regions in the short arms of laminin can bind to Ng-CAM. To test whether Ng-CAM present on neurons could be involved in binding to laminin, adhesion of neurons to substrates coated with various proteins was tested in the presence of specific antibodies. Anti-Ng-CAM Fab' fragments inhibited neuronal binding to laminin but not binding to fibronectin. The combined results open the possibility that Ng-CAM on the surface of neurons may mediate binding to laminin in vivo, and that interactions with laminin can modulate homophilic Ng-CAM binding.

Effects of transforming growth factor-beta 1 on the extracellular matrix and cytoskeleton of cultured astrocytes

The present study was performed on primary cultures and subcultures of cerebellar astrocytes in order to investigate the effects of transforming growth factor-beta 1 (TGF beta 1) on proliferation, extracellular matrix (ECM) components, and cytoskeletal structures in relation to morphological changes. The expression and cellular distribution of the ECM components laminin and fibronectin and the cytoskeletal proteins glial fibrillary acidic protein (GFAP) and actin were investigated by immunoblotting, immunocytochemistry, and phalloidin staining. The proliferation of primary cultures was strongly inhibited by TGF beta 1. Treated cells became enlarged and spread onto the substratum. TGF beta 1 promoted the appearance of actin stress fibers and increased the cell actin content. It elicited a slight increase in GFAP expression and induced dispersion of thin filaments of GFAP. TGF beta 1 also stimulated the production of laminin and fibronectin and their incorporation into the ECM of primary cultures grown in medium with or without serum. Astrocytes grown in serum-containing medium for 1 day after subculturing responded strongly to TGF beta 1. Changes promoted by TGF beta 1 in cell shape, cytoskeleton, and ECM production of cultured astrocytes may have relevance for understanding the mechanisms of action of TGF beta 1 during brain development.

The laminins: a family of basement membrane glycoproteins important in cell differentiation and tumor metastases

Laminins are a family of basement membrane-derived glycoproteins that are very biologically active with a number of diverse cell types. The response of the cells is dependent on the cell type and various cell-specific intracellular events are activated. Multiple active sites on laminin and cellular receptors have been described. Both laminin and the synthetic peptides that define the active sites may have important clinical uses. For example, the neurite-promoting peptides may be useful in vivo in regeneration studies because of their potent activity with neural cells and their lack of antigenicity. Also, peptides, such as YIGSR, that inhibit angiogenesis are potentially useful for treating the vascularization of the eye that occurs in conditions such as diabetes mellitus. Likewise, the angiogenic peptide SIKVAV, because of its role in endothelial cell block vessel formation, may be useful for treating ischemia. The recent progress that has been made in characterizing basic mechanisms of action of laminin has laid the groundwork for more direct studies of its clinical relevance.

Laminin B1 and collagen type IV gene expression in transected peripheral nerve: reinnervation compared to denervation

The expression of B1 laminin and type IV collagen was followed in the microsurgically isolated endoneurium of transected rat sciatic nerves from 3 days until 8 weeks. Northern hybridizations revealed that after nerve transection the proximal stumps of denervated, as well as freely regenerating, nerves showed a markedly increased expression of laminin and type IV collagen which lasted from 3 days up to 8 weeks. In the distal stumps, close to the site of transection (2-7 mm), the expression of laminin, and to a certain extent that of type IV collagen, seemed to be enhanced if free axonal reinnervation was allowed. Further distally (10-15 mm), the patterns of B1 laminin and type IV collagen expression were similar in both experimental groups, so that an increased expression was noticed during the first 2 weeks. The present results suggest that laminin and type IV collagen gene expression is markedly different in different parts of transected rat sciatic nerve. During peripheral nerve regeneration, there is a long-lasting basement membrane gene expression in the proximal stump. In the distal part of the transected nerve, the axonal reinnervation possibly up-regulates, but is not essential for, the expression of B1 laminin and type IV collagen.

Ultrastructural development of the medial superior olive (MSO) in the ferret

When ferrets are born, four weeks before the onset of hearing, few synapses are evident in the medial superior olive (MSO). The synapses present are immature and almost exclusively found in the neuropil. The MSO somata are virtually devoid of synaptic contacts but are contacted by fine glial processes that increasingly ensheathe the somata during the first postnatal week. By P12, somatic synaptogenesis in the MSO is evident. Initially the terminals contain vesicles of irregular shape, size, and distribution. The glial lamellae appear to withdraw as the synaptic contacts form but continue to cover the asynaptic portions of the cell surface. The lamellae frequently extend from ensheathing the soma to encapsulate the immature terminals. During the next two weeks, synaptic density and terminal encapsulation proceed until the somata is surrounded by encapsulated synaptic terminals as in the adult ferret MSO. While most immature terminals contain round vesicles, during the first postnatal week some terminals with nonround vesicles can be distinguished. The first distinction between types of nonround vesicle-containing terminals, i.e., pleiomorphic and ovoid, is in the second postnatal week. This distinction becomes increasingly clear and by the end of the first postnatal month, terminal types can be reliably categorized. These observations indicate that: (1) synapses are present in the MSO neuropil one month prior to the onset of hearing, (2) the major period of synaptogenesis begins approximately two weeks prior to the onset of hearing, and (3) glial lamellae ensheathe MSO somata prior to the onset of somatic synaptogenesis, withdraw as synapses form, and subsequently re-extend to encapsulate newly formed synapses.

The subcellular distribution of chromosome 6-encoded dystrophin-related protein in the brain

Chromosome 6-encoded dystrophin-related-protein (DRP) shows significant structural similarities to dystrophin at the carboxyl terminus, though the two proteins are encoded on different chromosomes. Both DRP and dystrophin are expressed in muscle and brain and show some similarity in their subcellular localization. For example, in skeletal muscle both are expressed at neuromuscular and myotendinous junctions. However, while dystrophin is absent or severely reduced in Duchenne/Becker muscular dystrophy, DRP continues to be expressed. Within the brain, dystrophin is enriched at the postsynaptic regions of specific subsets of neurons, while the distribution of DRP is yet to be described. In this study we demonstrate a distinct though highly specific pattern of distribution of DRP in the brain. DRP is enriched in the choroid plexus, pia mater, intracerebral vasculature, and ependymal lining. Within the parenchyma proper, DRP is located at the inner plasma face of astrocytic foot processes at the abluminal aspect of the blood-brain barrier. The distribution of DRP is conserved across a large evolutionary distance, from mammals to elasmobranchs, suggesting that DRP may play a role in the maintenance of regional specializations in the brain.

Effects of TGF beta 1 on the proliferation and differentiation of an immortalized astrocyte cell line: relationship with extracellular matrix

The astrocyte cell line (C.LT.T.1.1.), which is immortalized and has retained a normal density-dependent regulation of growth, is a suitable model for studying the relationships between proliferation, differentiation, and the production of extracellular matrix. The growth factor TGF beta 1 was used to modulate these processes. When added to proliferative cells, it inhibited growth and caused morphological changes. It also suppressed the growth arrest at confluence, so that the cells formed multilayers of parallel spindle-shaped cells. Whereas untreated control cells expressed progressively the glial fibrillary acidic protein (GFAP) after arrest of multiplication, the addition of TGF beta 1 to proliferative cells prevented GFAP expression and accumulation of its mRNA. Concomitantly, it increased the amounts of laminin, fibronectin, and collagens synthesized during the growth phase and greatly altered the composition and the structure of the matrix deposited at confluence. In contrast, when added after cell differentiation had begun, TGF beta 1 did not alter the appearance of the matrix whereas it still stimulated, but to a lesser extent, extracellular matrix components production. The results show that TGF beta 1 prevents the transition from the proliferating to the differentiating state and correlatively alters the composition and structure of the extracellular matrix.

Expression of s-laminin and laminin in the developing rat central nervous system

The extracellular matrix component, s-laminin, is a homologue of the B1 subunit of laminin. S-laminin is concentrated in the synaptic cleft at the neuromuscular junction and contains a site that is adhesive for motor neurons, suggesting that it may influence neuromuscular development. To ascertain whether s-laminin may also play roles in the genesis of the central nervous system, we have examined its expression in the brain and spinal cord of embryonic and postnatal rats. S-laminin was not detectable in synapse-rich areas of adults. However, s-laminin was present in discrete subsets of three laminin-containing structures: (1) In the developing cerebral cortex, laminin and s-laminin were expressed in the subplate, a transient layer through which neuroblasts migrate and cortical afferents grow. Both laminin and s-laminin disappeared as embryogenesis proceeded; however, laminin was more widely distributed and present longer than s-laminin. (2) In the developing spinal cord, laminin was present throughout the pia. In contrast, s-laminin was concentrated in the pia that overlies the floor plate, a region in which extracellular cues have been postulated to guide growing axons. (3) In central capillaries, s-laminin appeared perinatally, an interval during which the blood-brain barrier matures. In contrast, laminin was present in capillary walls of both embryos and adults. To extend our immunohistochemical results, we used biochemical methods to characterize s-laminin in brain. We found that authentic s-laminin mRNA is present in the embryonic brain, but that brain-derived s-laminin differs (perhaps by a posttranslational modification) from that derived from nonneural tissues. We also used tissue culture methods to show that glia are capable of synthesizing "brain-like" s-laminin, and of assembling it into an extracellular matrix. Thus, glia may be one cellular source of s-laminin in brain. Together, these results demonstrate that s-laminin is present in the developing central nervous system, and raise the possibility that this molecule may influence developmental processes.

Laminin--developmental expression and role in axonal outgrowth in the peripheral nervous system of the chick

The possible role of laminin on axon outgrowth and guidance in vivo was examined by: (1) determining its developmental expression, and relationship to outgrowth of sensory, motor and sympathetic axons in the chick embryo; and (2) evaluating the changes in the pattern of sympathetic preganglionic projections subsequent to injections of laminin, antilaminin and other laminin function blockers (JG22, INO) into their pathways during axon outgrowth. Double immunofluorescent staining for laminin and neurofilaments in peripheral nerves prior to and during initial outgrowth showed no obvious relationship between laminin and potential nerve pathways. Even though weak laminin immunostaining is apparent throughout the mesenchyme through which axons grow, the most prominent laminin immunostaining is on basement membranes of the neural tube, notochord and dermamyotome. However, as peripheral nerves mature, laminin becomes localized to nerve fascicles throughout the peripheral nervous system, beginning with the dorsal and ventral roots, and progressing later to more distal spinal nerves. Microinjections of antilaminin, JG22 (a monoclonal antibody against laminin/fibronectin receptors) and INO (a monoclonal antibody against a laminin-heparan sulfate proteoglycan complex) into the pathway of sympathetic preganglionic axons prior to and during outgrowth had no effect on the spatio-temporal patterns of sympathetic preganglionic projections. An alternate laminin-rich pathway produced by injecting laminin into the region of the sympathetic trunk immediately adjacent but caudal to the T1 spinal level also did not alter the projection of T1 preganglionic axons. These results suggest that laminin may not be crucial to the initial of peripheral axons. The localization of laminin in nerve fascicles in later stages of development suggests instead that laminin may be important in the maintenance of these structures.

Purification and lectin-binding properties of s-laminin, a synaptic isoform of the laminin B1 chain

The extracellular matrix (ECM) at the vertebrate neuromuscular junction is a repository of functionally important molecules, some of which can regulate the formation of synapses during regeneration. One candidate molecule is s-laminin, a 185-kDa homologue of the laminin B1 chain. Whereas several members of the laminin family are present throughout the ECM ensheathing muscle fibers, immunoreactivity for s-laminin is found selectively at synaptic sites in adult and embryonic rats, and is detectable at a time when synaptogenesis is taking place during development. We have reported previously that a rat schwannoma cell line, D6P2T, produces and releases large amounts of s-laminin in culture. We have now purified s-laminin from medium conditioned by these cells by using a simple three-step procedure. Serum-free, conditioned medium is separated by ion-exchange chromatography on DEAE-Sephacel, followed by size-exclusion chromatography on 500 HR-Sephacryl. Finally, s-laminin is dissociated from other ECM components by agarose gel electrophoresis under reducing conditions and recovered in solution by extracting slices of agarose gel. The purified preparation displays one silver-stained band that is recognized by three monoclonal antibodies known to bind to different epitopes on s-laminin. Lectin-binding studies demonstrate that s-laminin is a glycoprotein and bears many of the carbohydrate moieties present on the B1 and B2 chains of laminin. Thus, the three 185-220-kDa members of the laminin family are related in both their protein and carbohydrate domains.

Expression of fibronectin and laminin by different types of mouse glial cells cultured in a serum-free medium

The expression of fibronectin and laminin by cultured glial cells was studied. The glial culture from neonatal mouse cerebra maintained in a chemically defined, serum-free medium consisted of type-1 astrocytes, oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells, oligodendrocytes and type-2 astrocytes. Double-labelling immunofluorescent experiments performed using the mixed glial culture indicated that fibronectin and laminin are expressed in different patterns among the glial subtypes. The staining intensities with anti-fibronectin or anti-laminin antibodies decreased in the order: type-1 astrocytes, O-2A progenitor cells and type-2 astrocytes. Both molecules were deposited in a fibrillar matrix underneath type-1 astrocytes, whereas only intracytoplasmic localization of these molecules was observed with O-2A progenitor cells and type-2 astrocytes. Western blot analysis showed that glial fibronectin has a slightly higher molecular weight than mouse plasma fibronectin (230 kDa) and that glial laminin is a variant with a 220 kDa B chain present and the 400 kDa A chain missing. Using enzyme-linked immunosorbent assays (ELISA), these molecules were detected in the glial extracellular matrix at the concentration of 4 ng/10(6) cells. A large amount of fibronectin (82 ng/10(6) cells) was secreted into the culture medium, while secretion of laminin was not detected.