Mapping of the laminin-binding site of the N-terminal agrin domain (NtA)

Secreted Signaling Molecules at the Neuromuscular Junction in Physiology and Pathology

Signal transduction at the neuromuscular junction (NMJ) is affected in many human diseases, including congenital myasthenic syndromes (CMS), myasthenia gravis, Lambert–Eaton myasthenic syndrome, Isaacs’ syndrome, Schwartz–Jampel syndrome, Fukuyama-type congenital muscular dystrophy, amyotrophic lateral sclerosis, and sarcopenia. The NMJ is a prototypic cholinergic synapse between the motor neuron and the skeletal muscle. Synaptogenesis of the NMJ has been extensively studied, which has also been extrapolated to further understand synapse formation in the central nervous system. Studies of genetically engineered mice have disclosed crucial roles of secreted molecules in the development and maintenance of the NMJ. In this review, we focus on the secreted signaling molecules which regulate the clustering of acetylcholine receptors (AChRs) at the NMJ. We first discuss the signaling pathway comprised of neural agrin and its receptors, low-density lipoprotein receptor-related protein 4 (Lrp4) and muscle-specific receptor tyrosine kinase (MuSK). This pathway drives the clustering of acetylcholine receptors (AChRs) to ensure efficient signal transduction at the NMJ. We also discuss three secreted molecules (Rspo2, Fgf18, and connective tissue growth factor (Ctgf)) that we recently identified in the Wnt/β-catenin and fibroblast growth factors (FGF) signaling pathways. The three secreted molecules facilitate the clustering of AChRs by enhancing the agrin-Lrp4-MuSK signaling pathway.

Novel NtA and LG1 Mutations in Agrin in a Single Patient Causes Congenital Myasthenic Syndrome

Congenital myasthenic syndrome (CMS) is a group of genetic disorders of neuromuscular transmission that is characterized by muscle weakness. A mutation in the gene encoding agrin (AGRN) is a rare cause of CMS, and only a few families or isolated cases have been reported. We reported a pediatric proband exhibiting muscle weakness in the trunk and limbs with skeletal malformation and intellectual disability and performed whole-exome sequencing (WES) of the proband parent-offspring trio. Results revealed a new compound heterozygous mutation in AGRN: c.125A>C (p.Glu42Ala) in the N-terminal agrin domain (NtA) and c.4516G>A (p.Ala1506Thr) in the laminin G1 domain (LG1). Bioinformatic analysis predicted the mutation as possibly pathogenic. The new compound heterozygous mutation in AGRN may disrupt agrin's known function of bridging laminin and α-dystroglycan and undermine the formation and maintenance of the neuromuscular junction (NMJ) via both muscular and neural agrin pathways. It may also induce secondary peripheral neuropathy and skeletal malformation.

Engineering hydrogels with affinity-bound laminin as 3D neural stem cell culture systems

Laminin incorporation into biological or synthetic hydrogels has been explored to recapitulate the dynamic nature and biological complexity of neural stem cell (NSC) niches. However, the strategies currently explored for laminin immobilization within three-dimensional (3D) matrices do not address a critical aspect influencing cell-matrix interactions, which is the control over laminin conformation and orientation upon immobilization. This is a key feature for the preservation of the protein bioactivity. In this work, we explored an affinity-based approach to mediate the site-selective immobilization of laminin into a degradable synthetic hydrogel. Specifically, a four-arm maleimide terminated poly(ethylene glycol) (PEG-4MAL) macromer was functionalized with a mono-PEGylated recombinant human N-terminal agrin (NtA) domain, to promote high affinity binding of laminin. Different NtA concentrations (10, 50 and 100 μM) were used to investigate the impact of NtA density on laminin incorporation, hydrogel biophysical properties, and biological outcome. Laminin was efficiently incorporated for all the conditions tested (laminin incorporation >95%), and the developed hydrogels revealed mechanical properties (average storage modulus (G') ranging from 187 to 256 Pa) within the values preferred for NSC proliferation and neurite branching and extension. Affinity-bound laminin PEG-4MAL hydrogels better preserve laminin bioactivity, compared to unmodified hydrogels and hydrogels containing physically entrapped laminin, this effect being dependent on NtA concentration. This was evidenced by the 10 μM NtA-functionalized PEG-4MAL gels incorporating laminin that support enhanced human NSC proliferation and neurite extension, compared to the latter. Overall, this work highlights the potential of the proposed engineered matrices to be used as defined 3D platforms for the establishment of artificial NSC niches and as extracellular matrix-mimetic microenvironments to support human NSC transplantation.

Laminin-Inspired Cell-Instructive Microenvironments for Neural Stem Cells

Laminin is a heterotrimeric glycoprotein with a key role in the formation and maintenance of the basement membrane architecture and properties, as well as on the modulation of several biological functions, including cell adhesion, migration, differentiation and matrix-mediated signaling. In the central nervous system (CNS), laminin is differentially expressed during development and homeostasis, with an impact on the modulation of cell function and fate. Within neurogenic niches, laminin is one of the most important and well described extracellular matrix (ECM) proteins. Specifically, efforts have been made to understand laminin assembly, domain architecture, and interaction of its different bioactive domains with cell surface receptors, soluble signaling molecules, and ECM proteins, to gain insight into the role of this ECM protein and its receptors on the modulation of neurogenesis, both in homeostasis and during repair. This is also expected to provide a rational basis for the design of biomaterial-based matrices mirroring the biological properties of the basement membrane of neural stem cell niches, for application in neural tissue repair and cell transplantation. This review provides a general overview of laminin structure and domain architecture, as well as the main biological functions mediated by this heterotrimeric glycoprotein. The expression and distribution of laminin in the CNS and, more specifically, its role within adult neural stem cell niches is summarized. Additionally, a detailed overview on the use of full-length laminin and laminin derived peptide/recombinant laminin fragments for the development of hydrogels for mimicking the neurogenic niche microenvironment is given. Finally, the main challenges associated with the development of laminin-inspired hydrogels and the hurdles to overcome for these to progress from bench to bedside are discussed.

Alternative Splicing and the Intracellular Domain Mediate TM-agrin's Ability to Differentially Regulate the Density of Excitatory and Inhibitory Synapse-like Specializations in Developing CNS Neurons

Agrin is a multi-domain protein best known for its essential function during formation of the neuromuscular junction. Alternative mRNA splicing at sites named y and z in the C-terminal part of agrin regulates its interaction with a receptor complex consisting of the agrin-binding low-density lipoprotein receptor-related protein 4 (Lrp4) and the muscle-specific kinase (MuSK). Isoforms with inserts at both splice sites bind to Lrp4, activate MuSK and are synaptogenic at the neuromuscular junction. Agrin is also expressed as a transmembrane protein in the central nervous system (CNS) but its function during interneuronal synapse formation is unclear. Recently we demonstrated that transfection of a full-length cDNA coding for transmembrane agrin (TM-agrin) in cultured embryonic cortical neurons induced an Lrp4-dependent but MuSK-independent increase in dendritic glutamatergic synapses and an Lrp4- and MuSK-independent reduction of inhibitory synapses. Here we show that presynaptic specializations were similarly affected by TM-agrin overexpression. In addition, we mapped the regions within TM-agrin responsible for TM-agrin's effects on dendritic aggregates of synapse-associated proteins. We show that the presence of a four amino acid insert at splice site y is essential for the increase in the density of puncta containing the postsynaptic density protein 95 kDa. This effect was independent of splice site z. The reduction of the gephyrin puncta density was independent of the entire extracellular part of TM-agrin but required a highly conserved serine residue in the intracellular domain of TM-agrin. These results provide further evidence for a function of TM-agrin during CNS synaptogenesis and demonstrate that different domains and alternative splicing of TM-agrin differentially affect excitatory and inhibitory synapse formation in cultured embryonic CNS neurons.

Interaction studies of a protein and carbohydrate system using an integrated approach: a case study of the miniagrin-heparin system

The major challenges in biophysical characterization of human protein-carbohydrate interactions are obtaining monodispersed preparations of human proteins that are often post-translationally modified and lack of detection of carbohydrates by traditional detection systems. Light scattering (dynamic and static) techniques offer detection of biomolecules and their complexes based on their size and shape, and do not rely on chromophore groups (such as aromatic amino acid sidechains). In this study, we utilized dynamic light scattering, analytical ultracentrifugation and small-angle X-ray scattering techniques to investigate the solution properties of a complex resulting from the interaction between a 15 kDa heparin preparation and miniagrin, a miniaturized version of agrin. Results from dynamic light scattering, sedimentation equilibrium, and sedimentation velocity experiments signify the formation of a monodisperse complex with 1:1 stoichiometry, and low-resolution structures derived from the small-angle X-ray scattering measurements implicate an extended conformation for a side-by-side miniagrin‒heparin complex.

Brain-derived and glial cell line-derived neurotrophic factor fusion protein immobilization to laminin

Damage to the recurrent laryngeal nerve often causes hoarseness, dyspnea, dysphagia, and sometimes asphyxia due to vocal cord paralysis which result in a reduction of quality of life. Brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) play critical roles in peripheral nerve regeneration. However, methods for efficiently delivering these molecules are lacking, which limits their use in clinical applications. The present study reports an effective strategy for targeting BDNF and GDNF to laminin by fusing the N-terminal domains of these molecules with agrin (NtA). More specifically, laminin-binding efficacy was assessed and sustained release assays of the delivery of BDNF or GDNF fused with NtA (LBD-BDNF or LBD-GDNF) to laminin were conducted in vitro. In addition, the bioactivity of LBD-BDNF and LBD-GDNF on laminin in vitro was investigated. LBD-BDNF and LBD-GDNF were each able to specifically bind to laminin and maintain their activity in vitro. Moreover, neurotrophic factors with NtA retained higher concentrations and bioactivity levels compared with those without NtA. The ratio of LBD-BDNF and LBD-GDNF that produced optimal effects was 4:6. BDNF and GDNF fused with NtA were effective in specifically binding to laminin. As laminin is a major component of the extracellular matrix, LBD-BDNF and LBD-GDNF may prove useful in the repair of peripheral nerve injuries.

Cross-linking reveals laminin coiled-coil architecture

Laminin, an ∼800-kDa heterotrimeric protein, is a major functional component of the extracellular matrix, contributing to tissue development and maintenance. The unique architecture of laminin is not currently amenable to determination at high resolution, as its flexible and narrow segments complicate both crystallization and single-particle reconstruction by electron microscopy. Therefore, we used cross-linking and MS, evaluated using computational methods, to address key questions regarding laminin quaternary structure. This approach was particularly well suited to the ∼750-Å coiled coil that mediates trimer assembly, and our results support revision of the subunit order typically presented in laminin schematics. Furthermore, information on the subunit register in the coiled coil and cross-links to downstream domains provide insights into the self-assembly required for interaction with other extracellular matrix and cell surface proteins.

Dynamic light scattering: a practical guide and applications in biomedical sciences

Dynamic light scattering (DLS), also known as photon correlation spectroscopy (PCS), is a very powerful tool for studying the diffusion behaviour of macromolecules in solution. The diffusion coefficient, and hence the hydrodynamic radii calculated from it, depends on the size and shape of macromolecules. In this review, we provide evidence of the usefulness of DLS to study the homogeneity of proteins, nucleic acids, and complexes of protein-protein or protein-nucleic acid preparations, as well as to study protein-small molecule interactions. Further, we provide examples of DLS's application both as a complementary method to analytical ultracentrifugation studies and as a screening tool to validate solution scattering models using determined hydrodynamic radii.

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.

ECM receptors in neuronal structure, synaptic plasticity, and behavior

During central nervous system development, extracellular matrix (ECM) receptors and their ligands play key roles as guidance molecules, informing neurons where and when to send axonal and dendritic projections, establish connections, and form synapses between pre- and postsynaptic cells. Once stable synapses are formed, many ECM receptors transition in function to control the maintenance of stable connections between neurons and regulate synaptic plasticity. These receptors bind to and are activated by ECM ligands. In turn, ECM receptor activation modulates downstream signaling cascades that control cytoskeletal dynamics and synaptic activity to regulate neuronal structure and function and thereby impact animal behavior. The activities of cell adhesion receptors that mediate interactions between pre- and postsynaptic partners are also strongly influenced by ECM composition. This chapter highlights a number of ECM receptors, their roles in the control of synapse structure and function, and the impact of these receptors on synaptic plasticity and animal behavior.

Targeted delivery of neurogenin-2 protein in the treatment for cerebral ischemia-reperfusion injury

Neurogenin-2 (Ngn2), as a proneural gene that promotes the survival and differentiation of neural precursor cells, is an attractive candidate for therapy against cerebral ischemia-reperfusion injury. However, the delivery approach limits its clinical application. To deliver Ngn2 protein into the cerebral ischemic region and exert a therapeutic effect on injured neurons after ischemia, we here reported that the fusion protein TAT-LBD-Ngn2 was constructed by fusing a transactivator of transcription (TAT) domain and a laminin-binding domain (LBD) to Ngn2. TAT-LBD-Ngn2 promoted the outgrowth of neuronal neurite, increased the survival rate and alleviated apoptosis of hippocampal neurons exposed to oxygen glucose deprivation in vitro. Furthermore, a focal cerebral ischemia model in C57BL/6 mice showed that TAT-LBD-Ngn2 efficiently crossed the blood brain barrier, aggregated in the ischemic zone and was consistently incorporated into neurons. Moreover, TAT-LBD-Ngn2 transduced into brains attenuated neuronal degeneration and apoptosis in the ischemic zone. TAT-LBD-Ngn2 treatment resulted in a reduction of infarct volume that was associated with a parallel improvement in neurological functional outcomes after reperfusion. In conclusion, the targeted delivery of TAT-LBD-Ngn2 into the ischemic zone attenuated cerebral ischemia-reperfusion injury through the inhibition of neuronal degeneration and apoptosis, suggesting that TAT-LBD-Ngn2 is a promising target candidate for the treatment of ischemic stroke.

Structural mechanisms of the agrin-LRP4-MuSK signaling pathway in neuromuscular junction differentiation

The neuromuscular junction (NMJ) is the most extensively studied model of neuronal synaptogenesis. Acetylcholine receptor (AChR) clustering on the postsynaptic membrane is a cardinal event in the differentiation of NMJs. AChR clustering and postsynaptic differentiation is orchestrated by sophisticated interactions among three proteins: the neuron-secreted proteoglycan agrin, the co-receptor LRP4, and the muscle-specific receptor tyrosine kinase MuSK. LRP4 and MuSK act as scaffolds for multiple binding partners, resulting in a complex and dynamic network of interacting proteins that is required for AChR clustering. In this review, we discuss the structural basis for NMJ postsynaptic differentiation mediated by the agrin-LRP4-MuSK signaling pathway.

Site specific cleavage mediated by MMPs regulates function of agrin

BACKGROUND

Agrin is the key inducer of postsynaptic differentiations at the neuromuscular junction. The multidomain heparan sulfate proteoglycan is mediating via its N-terminal segment the interaction with laminin, whereas the C-terminal portion is responsible for Dystroglycan binding and clustering of the Acetylcholine receptor. Matrix metalloproteinases (MMP) are known to play essential roles in matrix remodeling, degradation and regulation of extracellular signaling networks.

PRINCIPAL FINDINGS

Site-specific processing of Agrin provides key insight into regulatory effects of Matrix metalloproteinases (MMPs). Here, we present a detailed study of agrin processing by different MMPs together with a molecular understanding of binding and cleavage at both terminal fragments. The data suggest for a regulatory effect of MMP cleavage at particularly important functional sites of agrin. Cleave of agrin abolishes the agrin-laminin complex formation and the Acetylcholine receptor clustering at the neuromuscular junction.

CONCLUSION/SIGNIFICANCE

Agrin is a target of specific MMP processing resulting in agrin subfragments with different regulatory activities. MMP processing is a powerful tool to regulate extracellular signaling networks.

Evidence for self-association of a miniaturized version of agrin from hydrodynamic and small-angle X-ray scattering measurements

Hydrodynamic studies of miniagrin indicate a molar mass that is 20% larger than the value calculated from the sequence of this genetically engineered protein. Consistent with this finding is the negative sign and also the magnitude of the second virial coefficient obtained from small-angle X-ray scattering measurements. The inference that miniagrin reversibly self-associates is confirmed by a sedimentation equilibrium study that yields an equilibrium constant of 0.24 L/g for a putative monomer-dimer interaction. Finally, Guinier analysis of the small-angle X-ray scattering (SAXS) results yields concentration-dependent values for the radius of gyration that may be described by the monomer-dimer model and respective R(g) values of 40 and 105 Å for the monomeric and dimeric miniagrin species. Although intermolecular protein interactions are endemic in the events leading to acetylcholine receptor aggregation by agrin, the matrix proteoglycan of which miniagrin is a miniaturized model, this investigation raises the possibility that agrin may itself self-associate.

T-shaped arrangement of the recombinant agrin G3-IgG Fc protein

Agrin is a large heparin sulphate proteoglycan with multiple domains, which is located in the extracellular matrix. The C-terminal G3 domain of agrin is functionally one of the most important domains. It harbors an α-dystroglycan binding site and carries out acetylcholine receptor clustering activities. In the present study, we have fused the G3 domain of agrin to an IgG Fc domain to produce a G3-Fc fusion protein that we intend to use as a tool to investigate new binding partners of agrin. As a first step of the study, we have characterized the recombinant fusion protein using a multidisciplinary approach using dynamic light scattering, analytical ultracentrifugation and small angle X-ray scattering (SAXS). Interestingly, our SAXS analysis using the high-resolution structures of G3 and Fc domain as models indicates that the G3-Fc protein forms a T-shaped molecule with the G3 domains extruding perpendicularly from the Fc scaffold. To validate our models, we have used the program HYDROPRO to calculate the hydrodynamic properties of the solution models. The calculated values are in excellent agreement with those determined experimentally.

An interdomain disulfide bridge links the NtA and first FS domain in agrin

Agrin is a multidomain heparan sulfate proteoglycan involved in postsynaptic differentiation at the neuromuscular junction. Binding of agrin to synaptic basal lamina is mediated by the N-terminal agrin (NtA) domain. The NtA domain of agrin is followed by a tandem of nine follistatin-like (FS) domains forming a rod-like spacer to the laminin G-like domains of the molecule. Here we report that the most C-terminal cysteine residue of NtA (Cys123) forms an interdomain disulfide bond with the FOLN subdomain of the FS module. Remarkably, this single cysteine is flanked by Leu117 and Val124, which are two essential beta-branched amino acids forming the heterocomplex of NtA with the gamma 1 chain of laminin. Moreover, we show that this covalent linkage compensates for the seven amino acid residue splice insert at the very C-terminal helix H3 and causes a rigid interface between NtA and FS independent of the alternative mRNA splice event. These results suggest that the interdomain disulfide bond between the NtA and the first FS domain might be important for the proper folding of agrin.

Laminin deposition in the extracellular matrix: a complex picture emerges

Laminins are structural components of basement membranes. In addition, they are key extracellular-matrix regulators of cell adhesion, migration, differentiation and proliferation. This Commentary focuses on a relatively understudied aspect of laminin biology: how is laminin deposited into the extracellular matrix? This topic has fascinated researchers for some time, particularly considering the diversity of patterns of laminin that can be visualized in the matrix of cultured cells. We discuss current ideas of how laminin matrices are assembled, the role of matrix receptors in this process and how laminin-associated proteins modulate matrix deposition. We speculate on the role of signaling pathways that are involved in laminin-matrix deposition and on how laminin patterns might play an important role in specifying cell behaviors, especially directed migration. We conclude with a description of new developments in the way that laminin deposition is being studied, including the use of tagged laminin subunits that should allow the visualization of laminin-matrix deposition and assembly by living cells.

Improvement of sciatic nerve regeneration using laminin-binding human NGF-beta

Background

Sciatic nerve injuries often cause partial or total loss of motor, sensory and autonomic functions due to the axon discontinuity, degeneration, and eventual death which finally result in substantial functional loss and decreased quality of life. Nerve growth factor (NGF) plays a critical role in peripheral nerve regeneration. However, the lack of efficient NGF delivery approach limits its clinical applications. We reported here by fusing with the N-terminal domain of agrin (NtA), NGF-β could target to nerve cells and improve nerve regeneration.

Methods

Laminin-binding assay and sustained release assay of NGF-β fused with NtA (LBD-NGF) from laminin in vitro were carried out. The bioactivity of LBD-NGF on laminin in vitro was also measured. Using the rat sciatic nerve crush injury model, the nerve repair and functional restoration by utilizing LBD-NGF were tested.

Findings

LBD-NGF could specifically bind to laminin and maintain NGF activity both in vitro and in vivo. In the rat sciatic nerve crush injury model, we found that LBD-NGF could be retained and concentrated at the nerve injury sites to promote nerve repair and enhance functional restoration following nerve damages.

Conclusion

Fused with NtA, NGF-β could bind to laminin specifically. Since laminin is the major component of nerve extracellular matrix, laminin binding NGF could target to nerve cells and improve the repair of peripheral nerve injuries.

Linker molecules between laminins and dystroglycan ameliorate laminin-alpha2-deficient muscular dystrophy at all disease stages

Mutations in laminin-α2 cause a severe congenital muscular dystrophy, called MDC1A. The two main receptors that interact with laminin-α2 are dystroglycan and α7β1 integrin. We have previously shown in mouse models for MDC1A that muscle-specific overexpression of a miniaturized form of agrin (mini-agrin), which binds to dystroglycan but not to α7β1 integrin, substantially ameliorates the disease (Moll, J., P. Barzaghi, S. Lin, G. Bezakova, H. Lochmuller, E. Engvall, U. Muller, and M.A. Ruegg. 2001. Nature. 413:302–307; Bentzinger, C.F., P. Barzaghi, S. Lin, and M.A. Ruegg. 2005. Matrix Biol. 24:326–332.). Now we show that late-onset expression of mini-agrin still prolongs life span and improves overall health, although not to the same extent as early expression. Furthermore, a chimeric protein containing the dystroglycan-binding domain of perlecan has the same activities as mini-agrin in ameliorating the disease. Finally, expression of full-length agrin also slows down the disease. These experiments are conceptual proof that linking the basement membrane to dystroglycan by specifically designed molecules or by endogenous ligands, could be a means to counteract MDC1A at a progressed stage of the disease, and thus opens new possibilities for the development of treatment options for this muscular dystrophy.

Activation of Muscle-specific Receptor Tyrosine Kinase and Binding to Dystroglycan Are Regulated by Alternative mRNA Splicing of Agrin

Agrin induces the aggregation of postsynaptic proteins at the neuromuscular junction (NMJ). This activity requires the receptor-tyrosine kinase MuSK. Agrin isoforms differ in short amino acid stretches at two sites, called A and B, that are localized in the two most C-terminal laminin G (LG) domains. Importantly, agrin isoforms greatly differ in their activities of inducing MuSK phosphorylation and of binding to alpha-dystroglycan. By using site-directed mutagenesis, we characterized the amino acids important for these activities of agrin. We find that the conserved tripeptide asparagineglutamate-isoleucine in the eight-amino acid long insert at the B-site is necessary and sufficient for full MuSK phosphorylation activity. However, even if all eight amino acids were replaced by alanines, this agrin mutant still has significantly higher MuSK phosphorylation activity than the splice version lacking any insert. We also show that binding to alpha-dystroglycan requires at least two LG domains and that amino acid inserts at the A and the B splice sites negatively affect binding.

Conjugation of LG Domains of Agrins and Perlecan to Polymerizing Laminin-2 Promotes Acetylcholine Receptor Clustering

Neuromuscular junction (NMJ) assembly is characterized by the clustering and neuronal alignment of acetylcholine receptors (AChRs). In this study we have addressed post-synaptic contributions to assembly that may arise from the NMJ basement membrane with cultured myotubes. We show that the cell surface-binding LG domains of non-neural (muscle) agrin and perlecan promote AChR clustering in the presence of laminin-2. This type of AChR clustering occurs with a several hour lag, requires muscle-specific kinase (MuSK), and is accompanied by tyrosine phosphorylation of MuSK and betaAChR. It also requires conjugation of the agrin or perlecan to laminin together with laminin polymerization. Furthermore, AChR clustering can be mimicked with antibody binding to non-neural agrin, supporting a mechanism of ligand aggregation. Neural agrin, in addition to its unique ability to cluster AChRs through its B/z sequence insert, also exhibits laminin-dependent AChR clustering, the latter enhancing and stabilizing its activity. Finally, we show that type IV collagen, which lacks clustering activity on its own, stabilizes laminin-dependent AChR clusters. These findings provide evidence for cooperative and partially redundant MuSK-dependent functions of basement membrane in AChR assembly that can enhance neural agrin activity yet operate in its absence. Such interactions may contribute to the assembly of aneural AChR clusters that precede neural agrin release as well as affect later NMJ development.

Structural and functional diversity generated by alternative mRNA splicing

Alternative mRNA splicing is becoming increasingly recognized as an important mechanism for the generation of structural and functional diversity in proteins. Recent estimations predict that approximately 50% of all eukaryotic proteins can be alternatively spliced. Several lines of evidence suggest that alternative mRNA splicing results in small changes in protein structure and is likely to fine-tune the function and specificity of the affected protein. However, knowledge of how alternative splicing regulates cellular processes on the molecular level is still limited. It is only recently that structures of alternatively spliced proteins have been solved. These studies have shown that alternative splicing affects the structure not only in the vicinity of the splice site but also at long distance.

Dystroglycan and Kir4.1 coclustering in retinal Müller glia is regulated by laminin-1 and requires the PDZ-ligand domain of Kir4.1

Inwardly rectifying potassium (Kir) channels in Müller glia play a critical role in the spatial buffering of potassium ions that accumulate during retinal activity. To this end, Kir channels show a polarized subcellular distribution with the predominant channel subunit in Müller glia, Kir4.1, clustered in the endfeet of these cells at the inner limiting membrane. However, the molecular mechanisms underlying their distribution have yet to be identified. Here, we show that laminin, agrin and alpha-dystroglycan (DG) codistribute with Kir4.1 at the inner limiting membrane in the retina and that laminin-1 induces the clustering of alpha-DG, syntrophin and Kir4.1 in Müller cell cultures. In addition, we found that alpha-DG clusters were enriched for agrin and sought to investigate the role of agrin in their formation using recombinant C-agrins. Both C-agrin 4,8 and C-agrin 0,0 failed to induce alpha-DG clustering and neither of them potentiated the alpha-DG clustering induced by laminin-1. Finally, our data reveal that deletion of the PDZ-ligand domain of Kir4.1 prevents their laminin-induced clustering. These findings indicate that both laminin-1 and alpha-DG are involved in the distribution of Kir4.1 to specific Müller cell membrane domains and that this process occurs via a PDZ-domain-mediated interaction. Thus, in the basal lamina laminin is an essential regulator involved in clearing excess potassium released during neuronal activity, thereby contributing to the maintenance of normal synaptic transmission in the retina.

Origin of acetylcholinesterase in the neuromuscular junction formed in the in vitro innervated human muscle

Synaptic basal lamina is interposed between the pre- and postsynaptic membrane of the neuromuscular junction (NMJ). This position permits deposition of basal lamina-bound NMJ components of both neuronal and muscle fibre origin. One such molecule is acetylcholinesterase (AChE). The origin of NMJ AChE has been investigated previously as the answer would elucidate the relative contributions of muscle fibers and motor neurons to NMJ formation. However, in the experimental models used in prior investigations either the neuronal or muscular components of the NMJs were removed, or the NMJs were poorly differentiated. Therefore, the question of AChE origin in the intact and functional NMJ remains open. Here, we have approached this question using an in vitro model in which motor neurons, growing from embryonic rat spinal cord explants, form well differentiated NMJs with cultured human myotubes. By immunocytochemical staining with species-specific anti-AChE antibodies, we are able to differentiate between human (muscular) and rat (neuronal) AChE at the NMJ. We observed strong signal at the NMJ after staining with human AChE antibodies, which suggests a significant muscular AChE contribution. However, a weaker, but still clearly recognizable signal is observed after staining with rat AChE antibodies, suggesting a smaller fraction of AChE was derived from motor neurons. This is the first report demonstrating that both motor neuron and myotube contribute synaptic AChE under conditions where they interact with each other in the formation of an intact and functional NMJ.

The Glycoprotein NOWA and Minicollagens Are Part of a Disulfidelinked Polymer That Forms the Cnidarian Nematocyst Wall

The nematocyst is a unique extrusive organelle involved in the defense and capture of prey in cnidarians. Minicollagens and the glycoprotein NOWA are major components of the nematocyst capsule wall, which resists osmotic pressure of 15 MPa. Here we present the recombinant expression of NOWA, which spontaneously assembles to globular macromolecular particles that are sensitive to reduction as the native wall structure. Ultra-structural analysis showed that the Hydra nematocyst wall is composed of several layers of globular particles, which are interconnected via radiating rodlike protrusions. Evidence is presented that native wall particles contain NOWA and minicollagen, supposed to be linked via disulfide bonds between their homologous cysteine-rich domains. Our data suggest a continuous suprastructure of the nematocyst wall, assembled from wall proteins that share a common oligomerization motif.

Role of the extracellular matrix in morphogenesis

The extracellular matrix is a complex, dynamic and critical component of all tissues. It functions as a scaffold for tissue morphogenesis, provides cues for cell proliferation and differentiation, promotes the maintenance of differentiated tissues and enhances the repair response after injury. Various amounts and types of collagens, adhesion molecules, proteoglycans, growth factors and cytokines or chemokines are present in the tissue- and temporal-specific extracellular matrices. Tissue morphogenesis is mediated by multiple extracellular matrix components and by multiple active sites on some of these components. Biologically active extracellular matrix components may have use in tissue repair, regeneration and engineering, and in programming stem cells for tissue replacement.