Analysis of proteoglycan expression in developing chicken brain: characterization of a heparan sulfate proteoglycan that interacts with the neural cell adhesion molecule

Deconstruction of Neurotrypsin Reveals a Multi-factorially Regulated Activity Affecting Myotube Formation and Neuronal Excitability

Neurotrypsin (NT) is a highly specific nervous system multi-domain serine protease best known for its selective processing of the potent synaptic organizer agrin. Its enzymatic activity is thought to influence processes of synaptic plasticity, with its deregulation causing accelerated neuromuscular junction (NMJ) degeneration or contributing to forms of mental retardation. These biological effects are likely to stem from NT-based regulation of agrin signaling. However, dissecting the exact biological implications of NT-agrin interplay is difficult, due to the scarce molecular detail regarding NT activity and NT-agrin interactions. We developed a strategy to reliably produce and purify a catalytically competent engineered variant of NT called "NT-mini" and a library of C-terminal agrin fragments, with which we performed a thorough biochemical and biophysical characterization of NT enzyme functionality. We studied the regulatory effects of calcium ions and heparin, identified NT's heparin-binding domain, and discovered how zinc ions induce modulation of enzymatic activity. Additionally, we investigated myotube differentiation and hippocampal neuron excitability, evidencing a dose-dependent increase in neuronal activity alongside a negative impact on myoblast fusion when using the active NT enzyme. Collectively, our results provide in vitro and cellular foundations to unravel the molecular underpinnings and biological significance of NT-agrin interactions.

A link between agrin signalling and Cav3.2 at the neuromuscular junction in spinal muscular atrophy

SMN protein deficiency causes motoneuron disease spinal muscular atrophy (SMA). SMN-based therapies improve patient motor symptoms to variable degrees. An early hallmark of SMA is the perturbation of the neuromuscular junction (NMJ), a synapse between a motoneuron and muscle cell. NMJ formation depends on acetylcholine receptor (AChR) clustering triggered by agrin and its co-receptors lipoprotein receptor-related protein 4 (LRP4) and transmembrane muscle-specific kinase (MuSK) signalling pathway. We have previously shown that flunarizine improves NMJs in SMA model mice, but the mechanisms remain elusive. We show here that flunarizine promotes AChR clustering in cell-autonomous, dose- and agrin-dependent manners in C2C12 myotubes. This is associated with an increase in protein levels of LRP4, integrin-beta-1 and alpha-dystroglycan, three agrin co-receptors. Furthermore, flunarizine enhances MuSK interaction with integrin-beta-1 and phosphotyrosines. Moreover, the drug acts on the expression and splicing of Agrn and Cacna1h genes in a muscle-specific manner. We reveal that the Cacna1h encoded protein Cav3.2 closely associates in vitro with the agrin co-receptor LRP4. In vivo, it is enriched nearby NMJs during neonatal development and the drug increases this immunolabelling in SMA muscles. Thus, flunarizine modulates key players of the NMJ and identifies Ca_v3.2 as a new protein involved in the NMJ biology.

Dissecting the Extracellular Complexity of Neuromuscular Junction Organizers

Synapse formation is a very elaborate process dependent upon accurate coordination of pre and post-synaptic specialization, requiring multiple steps and a variety of receptors and signaling molecules. Due to its relative structural simplicity and the ease in manipulation and observation, the neuromuscular synapse or neuromuscular junction (NMJ)-the connection between motor neurons and skeletal muscle-represents the archetype junction system for studying synapse formation and conservation. This junction is essential for survival, as it controls our ability to move and breath. NMJ formation requires coordinated interactions between motor neurons and muscle fibers, which ultimately result in the formation of a highly specialized post-synaptic architecture and a highly differentiated nerve terminal. Furthermore, to ensure a fast and reliable synaptic transmission following neurotransmitter release, ligand-gated channels (acetylcholine receptors, AChRs) are clustered on the post-synaptic muscle cell at high concentrations in sites opposite the presynaptic active zone, supporting a direct role for nerves in the organization of the post-synaptic membrane architecture. This organized clustering process, essential for NMJ formation and for life, relies on key signaling molecules and receptors and is regulated by soluble extracellular molecules localized within the synaptic cleft. Notably, several mutations as well as auto-antibodies against components of these signaling complexes have been related to neuromuscular disorders. The recent years have witnessed strong progress in the understanding of molecular identities, architectures, and functions of NMJ macromolecules. Among these, prominent roles have been proposed for neural variants of the proteoglycan agrin, its receptor at NMJs composed of the lipoprotein receptor-related protein 4 (LRP4) and the muscle-specific kinase (MuSK), as well as the regulatory soluble synapse-specific protease Neurotrypsin. In this review we summarize the current state of the art regarding molecular structures and (agrin-dependent) canonical, as well as (agrin-independent) non-canonical, MuSK signaling mechanisms that underscore the formation of neuromuscular junctions, with the aim of providing a broad perspective to further stimulate molecular, cellular and tissue biology investigations on this fundamental intercellular contact.

Cell Adhesion Molecules and Ubiquitination-Functions and Significance

Cell adhesion molecules of the immunoglobulin (Ig) superfamily represent the biggest group of cell adhesion molecules. They have been analyzed since approximately 40 years ago and most of them have been shown to play a role in tumor progression and in the nervous system. All members of the Ig superfamily are intensively posttranslationally modified. However, many aspects of their cellular functions are not yet known. Since a few years ago it is known that some of the Ig superfamily members are modified by ubiquitin. Ubiquitination has classically been described as a proteasomal degradation signal but during the last years it became obvious that it can regulate many other processes including internalization of cell surface molecules and lysosomal sorting. The purpose of this review is to summarize the current knowledge about the ubiquitination of cell adhesion molecules of the Ig superfamily and to discuss its potential physiological roles in tumorigenesis and in the nervous system.

New Insights into Hyperglycemia-induced Molecular Changes in Microvascular Cells

Hyperglycemia is the most prevalent characteristic of diabetes and plays a central role in mediating adverse effects on vascular cells during the progression of diabetic vascular complications. In diabetic microangiopathy, hyperglycemia induces biochemical and molecular changes in microvascular cells that ultimately progress to retinal, renal, and neural complications and extends to other complications, including advanced periodontal disease. In this review, we describe changes involving basement membrane thickening, tissue remodeling, gap junctions, inflammation, cytokines, and transcription factors, and their effects on the pathogenesis of diabetic microvascular complications. The majority of the changes described relate to retinal microangiopathy, since ultrastructural, structural, and biochemical alterations have been well-characterized in this tissue.

Specific detection of CD56 (NCAM) isoforms for the identification of aggressive malignant neoplasms with progressive development

Alternative splicing of transcripts from many cancer-associated genes is believed to play a major role in carcinogenesis as well as in tumor progression. Alternative splicing of one such gene, the neural cell adhesion molecule CD56 (NCAM), impacts the progression, inadequate therapeutic response, and reduced total survival of patients who suffer from numerous malignant neoplasms. Although previous investigations have determined that CD56 exists in three major isoforms (CD56(120kD), CD56(140kD), and CD56(180kD)) with individual structural and functional properties, neither the expression profiles nor the functional relevance of these isoforms in malignant tumors have been consistently investigated. Using new quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) strategies and novel CD56 isoform-specific antibodies, CD56(140kD) was shown to be exclusively expressed in a number of highly malignant CD56(+) neoplasms and was associated with the progression of CD56(+) precursor lesions of unclear malignant potential. Moreover, only CD56(140kD) induced antiapoptotic/proliferative pathways and specifically phosphorylated calcium-dependent kinases that are relevant for tumorigenesis. We conclude, therefore, that the specific detection of CD56 isoforms will help to elucidate their individual functions in the pathogenesis and progression of malignant neoplasms and may have a positive impact on the development of CD56-based immunotherapeutic strategies.

A novel role for embigin to promote sprouting of motor nerve terminals at the neuromuscular junction

Adult skeletal muscle accepts ectopic innervation by foreign motor axons only after section of its own nerve, suggesting that the formation of new neuromuscular junctions is promoted by muscle denervation. With the aim to identify new proteins involved in neuromuscular junction formation we performed an mRNA differential display on innervated versus denervated adult rat muscles. We identified transcripts encoding embigin, a transmembrane protein of the immunoglobulin superfamily (IgSF) class of cell adhesion molecules to be strongly regulated by the state of innervation. In innervated muscle it is preferentially localized to neuromuscular junctions. Forced overexpression in innervated muscle of a full-length embigin transgene, but not of an embigin fragment lacking the intracellular domain, promotes nerve terminal sprouting and the formation of additional acetylcholine receptor clusters at synaptic sites without affecting terminal Schwann cell number or morphology, and it delays the retraction of terminal sprouts following re-innervation of denervated endplates. Conversely, knockdown of embigin by RNA interference in wild-type muscle accelerates terminal sprout retraction, both by itself and synergistically with deletion of neural cell adhesion molecule. These findings indicate that embigin enhances neural cell adhesion molecule-dependent neuromuscular adhesion and thereby modulates neuromuscular junction formation and plasticity.

A polysaccharide isolated from the brown seaweed Sargassum stenophyllum exerts antivasculogenic effects evidenced by modified morphogenesis

A polysaccharide (Sarg) extracted from the brown marine alga Sargassum stenophyllum was studied for its antivasculogenic effects in both in vivo and in vitro assays, as well as for its capacity to modify embryonic morphogenetic processes endogenously regulated by bFGF, a well-known angiogenic stimulator. The antivasculogenic activity of Sarg (6-1500 microg/implant) was evaluated in a chick yolk sac membrane assay and the embryonic morphogenesis was measured as the percentage cephalic length. Sarg alone (96-1500 microg/implant) and co-administered with hydrocortisone (HC; 156 microg/implant) decreased the vitelline vessel number by 23-100% and 54-100% respectively. The polysaccharide potentiated the antivasculogenic effect of HC (42% inhibition). Basic fibroblast growth factor-stimulated vasculogenesis (141% of vessels as compared to control) was partially reversed by Sarg. The treatment with Sarg also decreased the percentage cephalic length of 3.5- and 4-day chick embryos (as cultured in vivo and in vitro, respectively), uncoupled from any impairment in the body shape or embryotoxic effect. Due to polyanionic characteristics of Sarg, which are similar to those seen in the heparin molecule, we suggest that this polysaccharide should modulate the activity of heparin-binding vascular growth factors (such as bFGF, which also acts as a morphogen) mimetically interfering with heparan sulfate proteoglycans during microvessel formation.

Alteration of serum and tumoral neural cell adhesion molecule (NCAM) isoforms in patients with brain tumors

The neural cell adhesion molecule (NCAM) is involved in the intercellular junctions of neurons and glial cells. We investigated its relevance as a biomarker in gliomas which main characteristic is their high invasiveness. We studied by Western blot the pattern of serum NCAM bands in patients with gliomas (n = 34), with brain metastasis of different primary cancers (n = 27) and with benign brain tumors (n = 22)] compared with healthy controls (n = 69). For densitometric analysis NCAM bands > or = 130 kDa (HMW) and <130 kDa (LMW) were clustered. We observed that glioma patients presented higher NCAM HMW and lower NCAM LMW levels than control subjects (P < 0.01). A similar pattern was found in patients with brain metastasis or brain benign tumors, suggesting that the pattern of serum NCAM bands would be useful to detect brain tumor pathology. On the other hand, serum NCAM expression was not associated with the main clinicopathological features of gliomas, including overall survival. Interestingly, we found that 9/12 patients with glioma showed a significant decrease in NCAM HMW/LMW ratio between 1-3 months after successful tumor removal. Thus, serum NCAM could be a useful marker for monitoring treatment.NCAM expression was also analyzed at tissular level in 59 glioma sections from paraffined tumors. We observed that NCAM immunostaining was inversely correlated with the histological grade of malignancy, remaining this association in a multivariate analysis. Besides, loss of NCAM staining was significantly associated with bad prognosis in an univariate analysis.

Blockade of natural killer cell-mediated lysis by NCAM140 expressed on tumor cells

Expression of the neural cell adhesion molecule (NCAM) on malignant cells of neuroendocrine, epithelial and hematopoeitic origin has been reported, but its role for tumor cell recognition by the immune system remained uncertain so far. We have studied the cytotoxicity of the natural killer (NK) cell line NK92 and polyclonal NK cells from different donors, against NCAM-deficient and NCAM-transfected tumors. While the pancreatic carcinoma PANC-1 and the glioblastoma T98G showed no enhanced susceptibility to NK lysis after NCAM transfection, de novo NCAM expression in HeLa cervical carcinoma, SHEP neuroblastoma and the multiple myeloma lines RPMI-8226 and LP-1 was associated with significantly decreased lysis by NK cells. Binding of an NCAM-specific monoclonal antibody to NCAM-positive target cells was able to reverse the reduced lysis susceptibility. Conjugate formation of NCAM-expressing tumor cells with NK cells was blocked and could be restored by anti-NCAM. NK cell-expressed NCAM molecules which might engage in homotypic cis- or trans-interactions had no apparent inhibitory function. The known cis-ligands of NCAM, heparan sulfate proteoglycan and L1-CAM, were also not directly involved in NK inhibition. ICAM-1 mRNA and cell surface expression was downmodulated in NCAM-transfected HeLa cells. ICAM-1 is involved in killer cell immune synapse formation. Its downmodulation may therefore contribute to the reduced lysis of NCAM-expressing target cells. We conclude that aberrant expression of NCAM on tumor cells of different histogenetic origin can lead to inhibition of target cell recognition and lysis by NK cells.

Agrin is required for posterior development and motor axon outgrowth and branching in embryonic zebrafish

Although recent studies have extended our understanding of agrin's function during development, its function in the central nervous system (CNS) is not clearly understood. To address this question, zebrafish agrin was identified and characterized. Zebrafish agrin is expressed in the developing CNS and in nonneural structures such as somites and notochord. In agrin morphant embryos, acetylcholine receptor (AChR) cluster number and size on muscle fibers at the choice point were unaffected, whereas AChR clusters on muscle fibers in the dorsal and ventral regions of the myotome were reduced or absent. Defects in the axon outgrowth by primary motor neurons, subpopulations of branchiomotor neurons, and Rohon-Beard sensory neurons were also observed, which included truncation of axons and increased branching of motor axons. Moreover, agrin morphants exhibit significantly inhibited tail development in a dose-dependent manner, as well as defects in the formation of the midbrain-hindbrain boundary and reduced size of eyes and otic vesicles. Together these results show that agrin plays an important role in both peripheral and CNS development and also modulates posterior development in zebrafish.

Antiangiogenic and antitumoral properties of a polysaccharide isolated from the seaweed Sargassum stenophyllum

The potential antiangiogenic and antitumoral properties of SargA, a polysaccharide extracted from the brown marine alga Sargassum stenophyllum, were studied in assays carried out in chick embryos and mice. Gelfoam plugs containing SargA (2-1500 microg/plug) implanted in vivo into fertilized 6-day-old chicken eggs induced dose-related antiangiogenic activity in the chorioallantoic membrane (CAM). By day 8, the highest dose of SargA alone decreased the vessel number in the CAM by 64%, but coadministered with hydrocortisone (156 microg/plug, which alone caused 30% inhibition) failed to potentiate its antiangiogenic effect. Combined with basic fibroblast growth factor (50 ng/plug), SargA (1500 microg/plug) abolished angiogenesis stimulated by this factor in both chick embryo CAM and in subcutaneous (s.c.) Gelfoam plugs implanted in the dorsal skin of Swiss mice (measured as plug hemoglobin content). Repeated s.c. injections of SargA (1.5 or 150 microg per animal per day for 3 days) close to B16F10 melanoma cell tumors in the dorsal skin of mice markedly decreased tumor growth in a dose-related fashion (by 40% and 80% at 2 weeks after the first injection, respectively), without evident signs of toxicity. SargA caused graded inhibitions of migration and viability of cultured B16F10 cells and also displayed antithrombotic activity in human plasma (5 mg/ml increased thrombin time 2.5-fold relative to saline). Thus, SargA exhibits pronounced antiangiogenic as well as antitumoral properties. Although the latter action of SargA might be related to the inhibition of angiogenesis, the polysaccharide also exerts cytotoxic effects on tumor cells. Because of its chemical characteristics and polyanionic constituents, we postulate that the polysaccharide SargA might modulate the activity of heparin-binding angiogenic growth factors.

Glycosaminoglycan-dependent and -independent inhibition of neurite outgrowth by agrin

Agrin is a proteoglycan that can inhibit neurite outgrowth from multiple neuronal types when present as a substrate. Agrin's neurite inhibitory activity is confined to the N-terminal segment of the protein (agrin N150), which contains heparan sulfate (HS) and chondroitin sulfate (CS) side chains. We have examined the activities of various purified recombinant agrin fragments and their glycosaminoglycan (GAG) side chains in neurite outgrowth inhibition. Inhibitory activity was tested using dissociated chick ciliary ganglion neurons or dorsal root ganglion explants growing on laminin or N-cadherin. Initial experiments demonstrated that agrin N150 lacking GAG chains inhibited neurite outgrowth. Both halves of N150, each containing HS and/or CS side chains, could also inhibit neurite growth. Experiments using agrin fragments in which the GAG acceptor residues were mutated, or using agrin fragments purified from cells deficient in GAG synthesis, demonstrated that inhibition by the N-terminal portion of N150 requires GAGs, but that inhibition from the C-terminal part of N150 does not. Thus, the core protein or other types of glycosylation are important for inhibition from the more C-terminal region. Our results suggest that there are two distinct mechanisms for neurite outgrowth inhibition by agrin, one that is GAG-dependent and one that is GAG-independent.

The heparan sulfate proteoglycan agrin modulates neurite outgrowth mediated by FGF-2

Although the role of agrin in the formation of the neuromuscular junction is well established, other functions for agrin have remained elusive. The present study was undertaken to assess the role of agrin in neurite outgrowth mediated by the heparin-binding growth factor basic fibroblast growth factor (FGF-2), which we have shown previously to bind to agrin with high affinity and that has been shown to mediate neurite outgrowth from a number of neuronal cell types. Using both an established neuronal cell line, PC12 cells, and primary chick retina neuronal cultures, we find that agrin potentiates the ability of FGF-2 to stimulate neurite outgrowth. In PC12 cells and retinal neurons agrin increases the efficacy of FGF-2 stimulation of neurite outgrowth mediated by the FGF receptor, as an inhibitor of the FGF receptor abolished neurite outgrowth in the presence of agrin and FGF-2. We also examined possible mechanisms by which agrin may modulate neurite outgrowth, analyzing ERK phosphorylation and c-fos phosphorylation. These studies indicate that agrin augments a transient early phosphorylation of ERK in the presence of FGF-2, and augments and sustains FGF-2 mediated increases in c-fos phosphorylation. These data are consistent with established mechanisms where heparan sulfate proteoglycans such as agrin may increase the affinity between FGF-2 and the FGF receptor. In summary, our studies suggest that neural agrin contributes to the establishment of axon pathways by modulating the function of neurite promoting molecules such as FGF-2.

Agrin in the CNS: a protein in search of a function?

The extracellular matrix molecule agrin mediates the motor neuron induced accumulation of acetylcholine receptors (AChR) at the neuromuscular junction. Agrin is also present in the CNS. However, while its spatiotemporal pattern of expression is consistent with a function in neuron-neuron synapse formation, it also suggests a role for agrin in other aspects of neural tissue morphogenesis. Here we review the data supporting these synaptic and non-synaptic functions of agrin in the CNS. The results of studies aimed at identifying a neuronal receptor for agrin (NRA) and its associated signal transduction pathways are examined. Possible roles for agrin in the etiology of diseases affecting the brain are also discussed.

A neuronal inhibitory domain in the N-terminal half of agrin

Agrin is required for appropriate pre- and postsynaptic differentiation of neuromuscular junctions. While agrin's ability to orchestrate postsynaptic differentiation is well documented, more recent experiments have suggested that agrin is also a "stop signal" for the presynaptic neuron, and that agrin has actions on neurons in the CNS. To elucidate the neuronal activities of agrin and to define the receptor(s) responsible for these functions, we have examined adhesions of neurons and their neurite-outgrowth responses to purified agrin in vitro. We find that both full-length agrin and the C-terminal 95 kDa of agrin (agrin c95), which is sufficient to induce postsynaptic differentiation, are adhesive for chick ciliary ganglion (CG) and forebrain neurons. Consistent with previous findings, our results show that N-CAM binds to full-length agrin, and suggest that alpha-dystroglycan is a neuronal receptor for agrin c95. In neurite outgrowth assays, full-length agrin inhibited both laminin- and N-cadherin-induced neurite growth from CG neurons. The N-terminal 150 kDa fragment of agrin, but not agrin c95, inhibited neurite outgrowth, indicating that domains in the N-terminal portion of agrin are sufficient for this function. Adhesion assays using protein-coated beads and agrin-expressing cells revealed differential interactions of agrin with members of the immunoglobulin superfamily of cell adhesion molecules. However, none of these, including N-CAM, appeared to be critical for neuronal adhesion. In summary, our results suggest that the N-terminal half of agrin is involved in agrin's ability to inhibit neurite outgrowth. Our results further suggest that neither alpha-dystroglycan nor N-CAM, two known binding proteins for agrin, mediate this effect.

NCAM regulates cell motility

Cell migration is required during development of the nervous system. The regulatory mechanisms for this process, however, are poorly elucidated. We show here that expression of or exposure to the neural cell adhesion molecule (NCAM) strongly affected the motile behaviour of glioma cells independently of homophilic NCAM interactions.

Expression of the transmembrane 140 kDa isoform of NCAM (NCAM-140) caused a significant reduction in cellular motility, probably through interference with factors regulating cellular attachment, as NCAM-140-expressing cells exhibited a decreased attachment to a fibronectin substratum compared with NCAM-negative cells. Ectopic expression of the cytoplasmic part of NCAM-140 also inhibited cell motility, presumably via the non-receptor tyrosine kinase p59fyn with which NCAM-140 interacts.

Furthermore, we showed that the extracellular part of NCAM acted as a paracrine inhibitor of NCAM-negative cell locomotion through a heterophilic interaction with a cell-surface receptor. As we showed that the two N-terminal immunoglobulin modules of NCAM, which are known to bind to heparin, were responsible for this inhibition, we presume that this receptor is a heparan sulfate proteoglycan. A model for the inhibitory effect of NCAM is proposed, which involves competition between NCAM and extracellular components for the binding to membrane-associated heparan sulfate proteoglycan.

Heparan sulfate proteoglycan expression in the optic chiasm of mouse embryos

Previous studies have demonstrated that heparan sulfate (HS) proteoglycans (PGs) regulate neurite outgrowth through binding to a variety of cell surface molecules, extracellular matrix proteins, and growth factors. The present study investigated the possible involvement of HS-PGs in retinal axon growth by examining its expression in the retinofugal pathway of mouse embryos by using a monoclonal antibody against the HS epitope. Immunoreactive HS was first detected in all regions of the retina at embryonic day (E) 11. The staining was gradually lost in the central regions and restricted to the retinal periphery at later developmental stages (E12--E16). Prominent staining for HS was consistently found in the retinal fiber layer and at the optic disk, indicating a possible supportive role of HS-PGs in axon growth in the retina. At the ventral diencephalon, immunostaining for HS was first detected at E12, before arrival of any retinal axons. The staining matched closely the neurons that are immunopositive for the stage-specific embryonic antigen 1 (SSEA-1). At E13 to E16, when axons are actively exploring their paths across the chiasm, immunoreactivity for HS was particularly intense at the midline. This characteristic expression pattern suggests a role for HS-PGs in defining the path of early axons in the chiasm and in regulating development of axon divergence at the midline. Furthermore, HS immunoreactivity is substantially reduced at regions flanking both sides of the midline, which coincides spatially to the position of actin-rich growth cones from subpial surface to the deep regions of the optic axon layer at the chiasm. Moreover, at the threshold of the optic tract, immunoreactive HS was localized to deep parts of the fiber layer. These findings indicate that changes in age-related fiber order in the optic chiasm and optic tract of mouse embryos are possibly regulated by a spatially restricted expression of HS-PGs.

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.

Structural and functional alterations of neuromuscular junctions in NCAM-deficient mice

The role of neural cell adhesion molecule (NCAM) in the development and maturation of the neuromuscular junction (NMJ) was explored by characterizing structurally and functionally NMJs from postnatal day 11 (P11) to P30 +/+, +/-, and -/- NCAM null mutant mice. Differences in NCAM levels resulted in alterations in the size and shape of NMJs, with -/- NMJs being smaller. Additionally both the withdrawal of polyneuronal innervation and the selective accumulation of synaptic vesicle protein in the presynaptic terminal were delayed. These observations suggest that the bidirectional signaling responsible for these events is impaired at -/- NMJs. Functionally, miniature end plate potential size, end plate potential size, and quantal content did not differ from that of wild type under either normal or low release conditions. However at normal release conditions, -/- NMJs, unlike +/+ NMJs, lacked paired-pulse facilitation. The most striking abnormality was the inability of NCAM null junctions to maintain transmitter output with repetitive stimuli. Combined electrophysiological and FM1-43-labeling studies suggest that NCAM null junctions are unable either to dock or to mobilize a sufficient number of vesicles at high but physiological rates of transmitter release. Taken together our observations show that many aspects of transmission are normal and, thus, that many presynaptic and postsynaptic molecules have assembled properly in the absence of NCAM. However, the fact that NCAM was required for specific aspects of transmission, including paired-pulse facilitation and reliable transmission with repetitive stimuli, suggests that NCAM either is directly involved in these processes or is required for the proper organization and/or function of other molecules underlying these processes.

Polysialic acid regulates chain formation by migrating olfactory interneuron precursors

Olfactory interneuron precursors in the rostral migration stream migrate in chains and through long distances to the olfactory bulb. The migration is inhibited when polysialic acid moiety of NCAM is removed. How polysialic acid regulates chain migration has remained unknown. Previous studies in other systems have indicated the polysialic acid as a negative regulator of cell-cell interactions. Thus, polysialic acid may prevent cells in chains from interacting too tightly. To test this hypothesis and examine how polysialic acid regulates chain migration, the effect of polysialic acid depletion was evaluated in vitro and in vivo. Surprisingly, removal of polysialic acid often resulted in the dispersion of chains into single cells in both subventricular zone cultures and in adult mice where chain migration was observed. These results indicate that polysialic acid plays an important role in the formation of chains by olfactory interneuron precursors.

Synaptic localization and axonal targeting of agrin secreted by ventral spinal cord neurons in culture

Agrin secreted by motor neurons is a critical signal for postsynaptic differentiation at the developing neuromuscular junction. We used cultures of chick ventral spinal cord neurons with rat myotubes and immunofluorescence with species-specific antibodies to determine the distribution of agrin secreted by neurons and compare it to the distribution of agrin secreted by myotubes. In addition, we determined the distribution of agrin secreted by isolated chick ventral spinal cord neurons and rat motor neurons grown on a substrate that binds agrin. In cocultures, neuronal agrin was concentrated along axons at sites of axon-induced acetylcholine receptor (AChR) aggregation and was found at every such synaptic site, consistent with its role in synaptogenesis. Smaller amounts of agrin were found on dendrites and cell bodies and rarely were associated with AChR aggregation. Muscle agrin, recognized by an antibody against rat agrin, was found at nonsynaptic sites of AChR aggregation but was not detected at synaptic sites, in contrast to neuronal agrin. In cultures of isolated chick neurons or rat motor neurons, agrin was deposited relatively uniformly around axons and dendrites during the first 2-3 days in culture. In older cultures, agrin immunoreactivity was markedly more intense around axons than dendrites, indicating that motor neurons possess an intrinsic, developmentally regulated program to target agrin secretion to axons.

Agrin binds to beta-amyloid (Abeta), accelerates abeta fibril formation, and is localized to Abeta deposits in Alzheimer's disease brain

Agrin is an extracellular matrix heparan sulfate proteoglycan (HSPG) well known for its role in modulation of the neuromuscular junction during development. Although agrin is one of the major HSPGs of the brain, its function there remains elusive. Here we provide evidence suggesting a possible function for agrin in Alzheimer's disease brain. Agrin protein binds the amyloidogenic peptide Abeta (1-40) in its fibrillar state via a mechanism that involves the heparan sulfate glycosaminoglycan chains of agrin. Furthermore, agrin is able to accelerate Abeta fibril formation and protect Abeta (1-40) from proteolysis, in vitro. Supporting a biological significance for these in vitro data, immunocytochemical studies demonstrate agrin's presence within senile plaques and cerebrovascular amyloid deposits, and agrin immunostained capillaries exhibit pathological alterations in AD brain. These data therefore suggest that agrin may be an important factor in the progression of Abeta peptide aggregation and/or its persistence in Alzheimer's disease brain.

Heparan sulfate in the inner limiting membrane of embryonic chicken retina binds basic fibroblast growth factor to promote axonal outgrowth

During neural development retinal ganglion cell axons migrate over the retinal basal lamina (inner limiting membrane, ILM) in directed growth toward the optic nerve. We found that both growth rate and distribution density of the ganglion cell axons on isolated cell-free ILM was greatly inhibited by pretreatment with heparitinase but not with chondroitinase ABC. The persistence of radioactively labeled proteoglycans added to the culture medium eliminated residual heparitinase as an explanation for the inhibition. A cell binding assay showed that heparitinase acted on the ILM to influence axonal behavior without apparent inhibition of cell adhesion. These results indicated that the neurite outgrowth promoting activity of the ILM depended on the heparan sulfate (HS) side chains of its proteoglycans. Basic fibroblast growth factor (bFGF) stimulated additional neuronal sprouting and neurite elongation on the ILM. This neurotropic activity of bFGF was inhibited by heparitinase pretreatment of the ILM, suggesting that bFGF bound to HS on the ILM. The activity of bFGF was enhanced by exogenous heparin added to the culture medium; although heparin alone failed to stimulate either neurite extension or neuronal cell sprouting. These results demonstrate that HS in the ILM possesses neurotropic activity for axons of the ganglion cells by binding bFGF for presentation to cell-surface receptors and may, therefore, play a significant role in stimulating axonal outgrowth during development.

Formation of the neuromuscular junction. Agrin and its unusual receptors

Synapses are essential relay stations for the transmission of information between neurones and other cells. An ordered and tightly regulated formation of these structures is crucial for the functioning of the nervous system. The induction of the intensively studied synapse between nerve and muscle is initiated by the binding of neurone-specific isoforms of the basal membrane protein agrin to receptors on the surface of myotubes. Agrin activates a receptor complex that includes the muscle-specific kinase and most likely additional, yet to be identified, components. Receptor activation leads to the aggregation of acetylcholine receptors (AChR) and other proteins of the postsynaptic apparatus. This activation process has unique features which distinguish it from other receptor tyrosine kinases. In particular, the autophosphorylation of the kinase domain, which usually induces the recruitment of adaptor and signalling molecules, is not sufficient for AChR aggregation. Apparently, interactions of the extracellular domain with unknown components are also required for this process. Agrin binds to a second protein complex on the muscle surface known as the dystrophin-associated glycoprotein complex. This binding forms one end of a molecular link between the extracellular matrix and the cytoskeleton. While many components of the machinery triggering postsynaptic differentiation have now been identified, our picture of the molecular pathway causing the redistribution of synaptic proteins is still incomplete.

I-type lectins in the nervous system

The number of animal lectins, basically defined upon their interaction with specific carbohydrate structures, is growing considerably during the last few years. Among these proteins the recently identified subfamily of I-type lectins consists of mainly transmembranous glycoproteins belonging to the immunoglobulin superfamily. Most of the I-type lectins participate in cell adhesion events, as are the different sialoadhesins recognizing sialylated glycan structures, which represent the best characterized subgroup. I-type lectins are abundant in the nervous system and have been implicated in a number of morphogenetic processes as fundamental as axon growth, myelin formation and growth factor signaling. In the present review, we summarize the structural and functional properties of I-type lectins expressed in neural tissues with a main focus on the sialoadhesin myelin-associated glycoprotein, the neural cell adhesion molecule and the fibroblast growth factor receptors.

Identification of extracellular matrix ligands for the heparan sulfate proteoglycan agrin

Agrin is a major brain heparan sulfate proteoglycan which is expressed in nearly all basal laminae and in early axonal pathways of the developing central nervous system. To further understand agrin's function during nervous system development, we have examined agrin's ability to interact with several heparin-binding extracellular matrix proteins. Our data show that agrin binds FGF-2 and thrombospondin by a heparan sulfate-dependent mechanism, merosin and laminin by both heparan sulfate-dependent and -independent mechanisms, and tenascin solely via agrin's protein core. Furthermore, agrin's heparan sulfate side chains encode a specificity in interactions with heparin-binding molecules since fibronectin and the cell adhesion molecule L1 do not bind agrin. Surface plasmon resonance studies (BIAcore) reveal a high affinity for agrin's interaction with FGF-2 and merosin (2.5 and 1.8 nM, respectively). Demonstrating a biological significance for these interactions, FGF-2, laminin, and tenascin copurify with immunopurified agrin and immunohistochemistry reveals a partial codistribution of agrin and its ECM ligands in the chick developing visual system. These studies and our previous studies, showing that merosin and NCAM also colocalize with agrin, provide evidence that agrin plays a crucial role in the function of the extracellular matrix and suggest a role for agrin in axon pathway development.

Alternatively spliced isoforms of nerve- and muscle-derived agrin: their roles at the neuromuscular junction

Agrin induces synaptic differentiation at the skeletal neuromuscular junction (NMJ); both pre- and postsynaptic differentiation are drastically impaired in its absence. Multiple alternatively spliced forms of agrin that differ in binding characteristics and bioactivity are synthesized by nerve and muscle cells. We used surgical chimeras, isoform-specific mutant mice, and nerve-muscle cocultures to determine the origins and nature of the agrin required for synaptogenesis. We show that agrin containing Z exons (Z+) is a critical nerve-derived inducer of postsynaptic differentiation, whereas neural isoforms containing a heparin binding site (Y+) and all muscle-derived isoforms are dispensable for major steps in synaptogenesis. Our results also suggest that the requirement of agrin for presynaptic differentiation is mediated indirectly by its ability to promote postsynaptic production or localization of appropriate retrograde signals.

A role for polysialic acid in neural cell adhesion molecule heterophilic binding to proteoglycans

The neural cell adhesion molecule (NCAM) is known to participate in both homophilic and heterophilic binding, the latter including mechanisms that involve interaction with proteoglycans. The polysialic acid (PSA) moiety of NCAM can serve as a negative regulator of homophilic binding, but indirect evidence has suggested that PSA can also be involved in heterophilic binding. We have examined this potential positive role for PSA in terms of the adhesion of PSA-expressing mouse F11 cells and chick embryonic brain cells to substrates composed of the purified heparan sulfate proteoglycans agrin and 6C4. This adhesion was specifically inhibited by polyclonal anti-NCAM Fab antibodies, monoclonal anti-PSA antibodies, PSA itself, and enzymatic removal of either PSA or heparan sulfate side chains. By contrast, the adhesion was not affected by chondroitinase, and cell binding to laminin was not inhibited by any of these treatments. A specific NCAM-heparan sulfate interaction in this adhesion was further indicated by its inhibition with monoclonal anti-NCAM Fab antibodies that recognize the known heparin-binding domain of NCAM and with the HBD-2 peptide derived from this region, but not with antibodies directed against other regions of the protein including the homophilic binding region. Together, the results suggest that PSA can act in vitro either as a receptor in NCAM heterophilic adhesion or as a promoter of binding between heparan sulfate proteoglycans and the NCAM heparin-binding domain.

Signals mediating ion channel clustering at the neuromuscular junction

High densities of acetylcholine receptors and sodium channels in the crests and troughs of the postsynaptic folds, respectively, ensure reliable neuromuscular signalling. Clustering of both ion channels is mediated by agrin. In the case of acetylcholine receptors, agrin activates the tyrosine kinase receptor muscle-specific kinase (MuSK), initiating a process requiring rapsyn and possibly also receptor phosphorylation. In many respects, the interactions between agrin and MuSK and their downstream effectors are atypical of conventional receptor tyrosine kinase signalling systems. A new understanding of the structural features of rapsyn involved in receptor clustering, as well as syntrophin's role in sodium channel targeting, has recently been revealed. Perhaps the most surprising result of the past year with regard to synaptogenesis is a negative one--mice lacking both dystrophin and utrophin have nearly normal neuromuscular junctions.

Agrin orchestrates synaptic differentiation at the vertebrate neuromuscular junction

The synapse is a key structure that is involved in perception, learning and memory. Understanding the sequence of steps that is involved in establishing synapses during development might also help to understand mechanisms that cause changes in synapses during learning and memory. For practical reasons, most of our current knowledge of synapse development is derived from studies of the vertebrate neuromuscular junction (NMJ). Several lines of evidence strongly suggest that motor axons release the molecule agrin to induce the formation of the postsynaptic apparatus in muscle fibers. Recent advances implicate proteins such as dystroglycan, MuSK, and rapsyn in the transduction of agrin signals. Recently, additional functions of agrin have been discovered, including the upregulation of gene transcription in myonuclei and the control of presynaptic differentiation. Agrin therefore appears to play a unique role in controlling synaptic differentiation on both sides of the NMJ.

Evidence that agrin directly influences presynaptic differentiation at neuromuscular junctions in vitro

The synaptic protein agrin is required for aspects of both pre- and postsynaptic differentiation at neuromuscular junctions. Although a direct effect of agrin on postsynaptic differentiation, presumably through the MuSK receptor, is established, it is not clear whether agrin directly affects the presynaptic nerve. To provide evidence on this point, we used anti-agrin IgG to disrupt agrin function in chick ciliary ganglion (CG) neuron/myotube cocultures. In cocultures grown in the presence of 200 microg/ml anti-agrin IgG, clustering of acetylcholine receptors (AChRs), extracellular matrix proteins, and the synaptic vesicle protein synaptotagmin (syt) at nerve-muscle contacts was inhibited. Syt clustering was still inhibited in the presence of 100 microg/ml blocking antibody, while the postsynaptic clustering of AChRs, heparan sulphate proteoglycan, and s-laminin was retained. Additionally, in CG neurons cultured with COS cells expressing agrin A0B0, which lacks the ability to signal postsynaptic differentiation, syt clustering was induced and this clustering was also blocked by anti-agrin IgG. Our results demonstrate that agrin function is acutely required for pre- and postsynaptic differentiation in vitro, and strongly suggest that agrin is directly involved in the induction of presynaptic differentiation.

In vitro differentiation of chick spinal cord neurons in the presence of Reissner's fibre, an ependymal brain secretion

The subcommissural organ (SCO), which belongs to the circumventricular organs, is a specialized ependymal structure of the brain that secretes glycoproteins into the cerebrospinal fluid (CSF) which condense to form a thread-like structure, the Reissner's fibre (RF). Regarding the presence of this ependymal brain secretion all along the central canal of the developing spinal cord, we analysed a putative developmental activity of RF on neuronal spinal cord cells. The effects of RF proper and soluble RF-material were examined in primary cultures of dissociated spinal cord cells from day 6 chicken embryos. In serum-containing mixed glial/neuronal cell cultures, both RF and soluble RF-material promoted neuronal survival. This effect was blocked by addition of specific antibodies raised against bovine RF into the culture medium. In serum-free neuron-enriched cultures, no neuronal survival activity was observed; however, under these conditions RF proper induced neuronal aggregation and neuritic outgrowth of spinal cord cells. Interestingly, neurites extending from the aggregates appeared mainly unfasciculated. Our results suggest a direct modulation of cell-cell interactions by SCO/RF glycoproteins and an indirect survival effect on neurons. These data strengthen the hypothesis of the involvement of SCO/RF complex in the development of the central nervous system (CNS) and are discussed regarding molecular features of SCO-spondin, a novel glycoprotein recently identified in this complex.

Intercellular communication that mediates formation of the neuromuscular junction

Reciprocal signals between the motor axon and myofiber induce structural and functional differentiation in the developing neuromuscular junction (NMJ). Elevation of presynaptic acetylcholine (ACh) release on nerve-muscle contact and the correlated increase in axonal-free calcium are triggered by unidentified membrane molecules. Restriction of axon growth to the developing NMJ and formation of active zones for ACh release in the presynaptic terminal may be induced by molecules in the synaptic basal lamina, such as S-laminin, heparin binding growth factors, and agrin. Acetylcholine receptor (AChR) synthesis by muscle cells may be increased by calcitonin gene-related peptide (CGRP), ascorbic acid, and AChR-inducing activity (ARIA)/heregulin, which is the best-established regulator. Heparin binding growth factors, proteases, adhesion molecules, and agrin all may be involved in the induction of AChR redistribution to form postsynaptic-like aggregates. However, the strongest case has been made for agrin's involvement. "Knockout" experiments have implicated agrin as a primary anterograde signal for postsynaptic differentiation and muscle-specific kinase (MuSK), as a putative agrin receptor. It is likely that both presynaptic and postsynaptic differentiation are induced by multiple molecular signals. Future research should reveal the physiological roles of different molecules, their interactions, and the identity of other molecular participants.

Genetic analysis of postsynaptic differentiation at the vertebrate neuromuscular junction

As neuromuscular junctions form in vertebrate skeletal muscle, nicotinic acetylcholine receptors (AChRs) become concentrated in the postsynaptic membrane. The nerve directs this redistribution, using multiple signals to regulate AChRs at both transcriptional and post-translational levels. Recent studies in vitro have led to the identification of candidate nerve-derived signaling molecules (such as agrin, ARIA/neuregulin, and calcitonin gene-related peptide) and components of their intramuscular signaling pathways (including dystroglycan, MuSK, erbB kinases, utrophin, and rapsyn). Studies of knock-out mice are now making it possible to test which signals and pathways are responsible for postsynaptic differentiation in vivo.

Localization of cranin (dystroglycan) at sites of cell-matrix and cell-cell contact: recruitment to focal adhesions is dependent upon extracellular ligands

We report that cranin (dystroglycan) can become recruited to focal adhesions of cultured rat REF 52 fibroblasts and human aortic smooth muscle cells. Within mature focal adhesions, cranin was present within the plaque region defined by beta 1 integrin, vinculin and phosphotyrosine staining, but occupied a larger domain corresponding to the terminal segments of stress fibers that was more precisely co-extensive with the cytoskeletal proteins alpha-actinin, utrophin and aciculin. When REF 52 fibroblasts were plated on different substrata in the absence of protein synthesis and secretion in serum-free medium, focal clusters of cranin readily formed within 2 hours on matrix proteins that bind cranin directly (laminin or agrin) which were maintained as the focal adhesions became mature. In contrast, cranin failed to become targeted to cell-substratum attachment sites, either at early or later times, when cells were plated on a variety of other substrata that elicit formation of focal adhesions but do not bind cranin directly (fibronectin, vitronectin, collagen type IV, or anti-beta 1 integrin antibody TS2/16). These data strongly suggest that targeting of cranin to focal adhesions was dependent upon the presence of an extracellular ligand capable of binding cranin directly. However, some cultured nonmuscle cell lines (e.g., human umbilical vein endothelial cells, NIH 3T3 and CHO cells) failed to localize cranin to focal adhesions, even when plated on laminin. Cranin was also enriched at cell-cell adherens-type junctions of human normal breast MCF-10 epithelial cells, and at growth cones of E17 rat hippocampal axons. That cranin can become targeted to sites of cell-cell and cell-substratum contact in diverse cell types supports the hypothesis that cranin may be involved in mediating or regulating cell adhesion. The absence of muscle-specific and synapse-specific proteins within fibroblasts and epithelial cells provides a different context for thinking about cranin (dystroglycan) that may aid in discerning general principles of its structure and function.

Soluble material from Reissner's fiber displays anti-aggregative activity in primary cultures of chick cortical neurons

The subcommissural organ (SCO), which belongs to the circumventricular organs, is a specialized ependymal structure of the brain that secretes glycoproteins into the cerebrospinal fluid (CSF) which condense to form a thread-like structure, Reissner's fiber (RF). The effects of soluble material released by RF were examined in primary cultures of dissociated cortical cells from embryonic (day 8) chick brain. Under serum-free conditions, the presence in the cultures of soluble RF material markedly impaired neuronal cell aggregation. This effect was completely blocked by addition into the culture medium of specific antibodies raised against bovine RF. The anti-aggregative effect of soluble RF material is observed on poly-L-lysine as well as on different extracellular matrix proteins including collagen and laminin, but was less effective on fibronectin. The continuous exposure of the cultures to soluble RF material for 7 days significantly decreased choline acetyltransferase activity. On the other hand, soluble RF material did not appear to have mitogenic activity on neuronal cultures. Modulation of cell-cell interactions by SCO/RF glycoproteins strengthens the hypothesis of the involvement of RF in developmental events of the central nervous system.

Defective neuromuscular synaptogenesis in agrin-deficient mutant mice

During neuromuscular synapse formation, motor axons induce clustering of acetylcholine receptors (AChRs) in the muscle fiber membrane. The protein agrin, originally isolated from the basal lamina of the synaptic cleft, is synthesized and secreted by motoneurons and triggers formation of AChR clusters on cultured myotubes. We show here postsynaptic AChR aggregates are markedly reduced in number, size, and density in muscles of agrin-deficient mutant mice. These results support the hypothesis that agrin is a critical organizer of postsynaptic differentiation does occur in the mutant, suggesting the existence of a second-nerve-derived synaptic organizing signal. In addition, we show that intramuscular nerve branching and presynaptic differentiation are abnormal in the mutant, phenotypes which may reflect either a distinct effect of agrin or impaired retrograde signaling from a defective postsynaptic apparatus.

NCAM-mediated adhesion of transfected cells to agrin

The vertebrate neural cell adhesion molecule NCAM mediates heterophilic adhesion to heparan sulfate proteoglycans in embryonic chick brain membranes. In this study, mouse L cells transfected with chicken NCAM were used to identify two of these ligands as agrin and the target of the 6C4 monoclonal antibody. A third heparan sulfate proteoglycan, perlecan, appeared not to support NCAM-mediated adhesion. Enzymatic degradation of chondroitin sulfates decreased adhesion in agrin-containing membrane fractions but increased adhesion if the agrin had previously been removed by immunoprecipitation, suggesting that interactions between heparan sulfate and chondroitin sulfate proteoglycans have important influences on adhesion. Our experiments support the view that NCAM can interact with multiple, but not with all, heparan sulfate and chondroitin sulfate proteoglycans in chick brain membranes in both positive and negative ways to influence cell adhesion.

Unilateral containment of retinal axons by tectal glia: a possible role for sulfated proteoglycans

(1) A distinct group of radial glia resides along the roofplate of the mesencephalon. Results of experiments, in which the neonatal tectum is manipulated surgically, point to the involvement of these glia in compartmentalizing retinotectal axons to one side of the midbrain. (2) Immunohistochemical studies document that the GAGs CS and KS are expressed along these midline glia during development: their expression occurs after the intertectal axons grow across the midline, but is coincident with the time of ingrowth of retinotectal axons, which fail to cross the midline. Together with results of in vitro experiments from other laboratories, these observations suggest that CS and KS are involved in the barrier function of the midline cells. (3) Preliminary data on biochemical characterization of PGs in developing tectum indicate that similar PG core proteins are found in the midline region as well as in the lateral tectum, whereas metabolic labeling shows a significantly higher uptake of radioactive sulfates along the midline. Thus differential glycosylation of proteins along the midline is likely, along with the possibility that it is the sugar chains which contribute to the barrier function of the raphe glia. Taken in the context of what we currently know about the biochemical heterogeneity of PGs, their developmental expression, and their functions in relation to the growth of axons from a variety of different neuronal cell types, it is clear that the analyses of interactions between PGs and growing axons must occur at several different levels, not the least of which involves a detailed understanding of the milieu in vivo within which these interactions take place.

An amino-terminal extension is required for the secretion of chick agrin and its binding to extracellular matrix

Agrin is an extracellular matrix (ECM) protein with a calculated relative molecular mass of more than 200 kD that induces the aggregation of acetylcholine receptors (AChRs) at the neuromuscular junction. This activity has been mapped to its COOH terminus. In an attempt to identify the functions of the NH2-terminal end, we have now characterized full-length chick agrin. We show that chick agrin encoded by a previously described cDNA is not secreted from transfected cells. Secretion is achieved with a construct that includes an additional 350 bp derived from the 5' end of chick agrin mRNA. Recombinant agrin is a heparan sulfate proteoglycan (HSPG) of more than 400 kD with glycosaminoglycan side chains attached only to the NH2-terminal half. Endogenous agrin in tissue homogenates also has an apparent molecular mass of > 400 kD. While the amino acid sequence encoded by the 350-bp extension has no homology to published rat agrin, it includes a stretch of 15 amino acids that is 80% identical to a previously identified bovine HSPG. The extension is required for binding of agrin to ECM. AChR aggregates induced by recombinant agrin that includes the extension are considerably smaller than those induced by agrin fragments, suggesting that binding of agrin to ECM modulates the size of receptor clusters. In addition, we found a site encoding seven amino acids at the NH2-terminal end of agrin that is alternatively spliced. While motor neurons express the splice variant with the seven amino acid long insert, muscle cells mainly synthesize isoforms that lack this insert. In conclusion, the cDNAs described here code for chick agrin that has all the characteristics previously allocated to endogenous agrin.

Agrin is a differentiation-inducing "stop signal" for motoneurons in vitro

Proteins of the synaptic basal lamina are important in directing the differentiation of motor nerve terminals. One synaptic basal lamina protein, agrin, which influences postsynaptic muscle differentiation, has been suggested to influence nerve terminals as well. To test this hypothesis, we cocultured chick ciliary ganglion neurons with agrin-expressing CHO cells. Ciliary ganglion neurons, but not sensory neurons, adhered five times as well to agrin-expressing cells as to untransfected cells. Further, ciliary ganglion neurites were growth inhibited upon contact with agrin-expressing cells. Finally, the synaptic vesicle protein synaptotagmin became concentrated at contacts between ciliary ganglion neurites and agrin-expressing cells. These activities were shared by neuronal and muscle-derived agrin isoforms, consistent with the hypothesis that muscle agrin may influence the presynaptic axon. Our results suggest that agrin influences the growth and differentiation of motoneurons in vivo.

Identification of a novel alternatively spliced agrin mRNA that is preferentially expressed in non-neuronal cells

A novel agrin isoform was identified based on the isolation of an agrin cDNA from E9 chick brain that lacked 21 base pairs (bp) in the NH2-terminal encoding region of the agrin mRNA. Reverse transcription-polymerase chain reaction (RT-PCR) of E9 chick brain mRNA confirmed the existence of this agrin isoform in brain, although the novel splice variant represents a minor fraction of agrin mRNA in brain. However, upon analysis of chick brain astrocyte mRNA, smooth muscle mRNA, and cardiac muscle mRNA by RT-PCR, we show that this novel agrin isoform is the predominant agrin isoform in these non-neuronal cell populations. We extended our analyses to examine the expression of this agrin mRNA isoform during chick development and show that the agrin mRNA lacking this 21-bp exon is up-regulated with brain development, consistent with the increase in glial number during brain development, while the agrin isoform that does not undergo splicing and thus contains the 21-bp exon is down-regulated in brain development. Because the 21-bp exon is inserted in the region of chick agrin which encodes the putative signal sequence of agrin, with the signal peptidase site immediately preceding the putative first amino acid of the mature protein being deleted as a result of splicing, these data raise the interesting possibility that the presence or absence of this alternatively spliced exon may differentially regulate processing of the agrin protein in neuronal and non-neuronal cells, respectively.