Agrin in the developing CNS: new roles for a synapse organizer

Biomimetic strategies for the deputization of proteoglycan functions

Proteoglycans (PGs), which have glycosaminoglycan chains attached to their protein cores, are essential for maintaining the morphology and function of healthy body tissues. Extracellular PGs perform various functions, classified into the following four categories: i) the modulation of tissue mechanical properties; ii) the regulation and protection of the extracellular matrix; iii) protein sequestration; and iv) the regulation of cell signaling. The depletion of PGs may significantly impair tissue function, encompassing compromised mechanical characteristics and unregulated inflammatory responses. Since PGs play critical roles in the function of healthy tissues and their synthesis is complex, the development of PG mimetic molecules that recapitulate PG functions for tissue engineering and therapeutic applications has attracted the interest of researchers for more than 20 years. These approaches have ranged from semisynthetic graft copolymers to recombinant PG domains produced by cells that have undergone genetic modifications. This review discusses some essential extracellular PG functions and approaches to mimicking these functions.

Extracellular Matrix of Echinoderms

This review considers available data on the composition of the extracellular matrix (ECM) in echinoderms. The connective tissue in these animals has a rather complex organization. It includes a wide range of structural ECM proteins, as well as various proteases and their inhibitors. Members of almost all major groups of collagens, various glycoproteins, and proteoglycans have been found in echinoderms. There are enzymes for the synthesis of structural proteins and their modification by polysaccharides. However, the ECM of echinoderms substantially differs from that of vertebrates by the lack of elastin, fibronectins, tenascins, and some other glycoproteins and proteoglycans. Echinoderms have a wide variety of proteinases, with serine, cysteine, aspartic, and metal peptidases identified among them. Their active centers have a typical structure and can break down various ECM molecules. Echinoderms are also distinguished by a wide range of proteinase inhibitors. The complex ECM structure and the variety of intermolecular interactions evidently explain the complexity of the mechanisms responsible for variations in the mechanical properties of connective tissue in echinoderms. These mechanisms probably depend not only on the number of cross-links between the molecules, but also on the composition of ECM and the properties of its proteins.

The Role of Extracellular Matrix Components in the Spreading of Pathological Protein Aggregates

Several neurodegenerative diseases are characterized by the accumulation of aggregated misfolded proteins. These pathological agents have been suggested to propagate in the brain via mechanisms similar to that observed for the prion protein, where a misfolded variant is transferred from an affected brain region to a healthy one, thereby inducing the misfolding and/or aggregation of correctly folded copies. This process has been characterized for several proteins, such as α-synuclein, tau, amyloid beta (Aβ) and less extensively for huntingtin and TDP-43. α-synuclein, tau, TDP-43 and huntingtin are intracellular proteins, and their aggregates are located in the cytosol or nucleus of neurons. They have been shown to spread between cells and this event occurs, at least partially, via secretion of these protein aggregates in the extracellular space followed by re-uptake. Conversely, Aβ aggregates are found mainly extracellularly, and their spreading occurs in the extracellular space between brain regions. Due to the inherent nature of their spreading modalities, these proteins are exposed to components of the extracellular matrix (ECM), including glycans, proteases and core matrix proteins. These ECM components can interact with or process pathological misfolded proteins, potentially changing their properties and thus regulating their spreading capabilities. Here, we present an overview of the documented roles of ECM components in the spreading of pathological protein aggregates in neurodegenerative diseases with the objective of identifying the current gaps in knowledge and stimulating further research in the field. This could potentially lead to the identification of druggable targets to slow down the spreading and/or progression of these pathologies.

The vascular basement membrane in the healthy and pathological brain

The vascular basement membrane contributes to the integrity of the blood-brain barrier (BBB), which is formed by brain capillary endothelial cells (BCECs). The BCECs receive support from pericytes embedded in the vascular basement membrane and from astrocyte endfeet. The vascular basement membrane forms a three-dimensional protein network predominantly composed of laminin, collagen IV, nidogen, and heparan sulfate proteoglycans that mutually support interactions between BCECs, pericytes, and astrocytes. Major changes in the molecular composition of the vascular basement membrane are observed in acute and chronic neuropathological settings. In the present review, we cover the significance of the vascular basement membrane in the healthy and pathological brain. In stroke, loss of BBB integrity is accompanied by upregulation of proteolytic enzymes and degradation of vascular basement membrane proteins. There is yet no causal relationship between expression or activity of matrix proteases and the degradation of vascular matrix proteins in vivo. In Alzheimer's disease, changes in the vascular basement membrane include accumulation of Aβ, composite changes, and thickening. The physical properties of the vascular basement membrane carry the potential of obstructing drug delivery to the brain, e.g. thickening of the basement membrane can affect drug delivery to the brain, especially the delivery of nanoparticles.

A Novel Egr-1-Agrin Pathway and Potential Implications for Regulation of Synaptic Physiology and Homeostasis at the Neuromuscular Junction

Synaptic transmission requires intricate coordination of the components involved in processing of incoming signals, formation and stabilization of synaptic machinery, neurotransmission and in all related signaling pathways. Changes to any of these components cause synaptic imbalance and disruption of neuronal circuitry. Extensive studies at the neuromuscular junction (NMJ) have greatly aided in the current understanding of synapses and served to elucidate the underlying physiology as well as associated adaptive and homeostatic processes. The heparan sulfate proteoglycan agrin is a vital component of the NMJ, mediating synaptic formation and maintenance in both brain and muscle, but very little is known about direct control of its expression. Here, we investigated the relationship between agrin and transcription factor early growth response-1 (Egr-1), as Egr-1 regulates the expression of many genes involved in synaptic homeostasis and plasticity. Using chromatin immunoprecipitation (ChIP), cell culture with cell lines derived from brain and muscle, and animal models, we show that Egr-1 binds to the AGRN gene locus and suppresses its expression. When compared with wild type (WT), mice deficient in Egr-1 (Egr-1−/−) display a marked increase in AGRN mRNA and agrin full-length and cleavage fragment protein levels, including the 22 kDa, C-terminal fragment in brain and muscle tissue homogenate. Because agrin is a crucial component of the NMJ, we explored possible physiological implications of the Egr-1-agrin relationship. In the diaphragm, Egr-1−/− mice display increased NMJ motor endplate density, individual area and area of innervation. In addition to increased density, soleus NMJs also display an increase in fragmented and faint endplates in Egr-1−/− vs. WT mice. Moreover, the soleus NMJ electrophysiology of Egr-1−/− mice revealed increased quantal content and motor testing showed decreased movement and limb muscle strength compared with WT. This study provides evidence for the potential involvement of a novel Egr-1-agrin pathway in synaptic homeostatic and compensatory mechanisms at the NMJ. Synaptic homeostasis is greatly affected by the process of aging. These and other data suggest that changes in Egr-1 expression may directly or indirectly promote age-related pathologies.

Can agrin cerebrospinal fluid concentration be used as an early biomarker for Alzheimer's disease?

The need for effective treatments halting Alzheimer's disease (AD) urges the discovery of the earliest possible biomarkers. Agrin is increased in the early stages of AD and is involved in amyloid-β (Aβ) fibrillation and synaptogenesis. We investigated the potential of agrin as an early AD cerebrospinal fluid (CSF) biomarker. We analyzed the agrin CSF concentration in nondemented controls (n = 20) and those with mild (n = 20) and severe (n = 20) AD. The levels of agrin CSF were not significantly divergent among the different patient groups and did not correlate with the concentration of Aβ42, total tau, phosphorylated tau, or the Mini Mental State Examination scores. However, agrin strongly correlated with age in those with dementia. The results indicate that agrin cannot be used as an early AD CSF biomarker using the current immunoassay. However, our population was relatively young; thus, the correlation between agrin and age suggests that stronger differences in agrin concentrations might be found in older groups with more heterogeneous AD pathologic features.

The heparan sulfate proteoglycan agrin contributes to barrier properties of mouse brain endothelial cells by stabilizing adherens junctions

Barrier characteristics of brain endothelial cells forming the blood–brain barrier (BBB) are tightly regulated by cellular and acellular components of the neurovascular unit. During embryogenesis, the accumulation of the heparan sulfate proteoglycan agrin in the basement membranes ensheathing brain vessels correlates with BBB maturation. In contrast, loss of agrin deposition in the vasculature of brain tumors is accompanied by the loss of endothelial junctional proteins. We therefore wondered whether agrin had a direct effect on the barrier characteristics of brain endothelial cells. Agrin increased junctional localization of vascular endothelial (VE)-cadherin, β-catenin, and zonula occludens-1 (ZO-1) but not of claudin-5 and occludin in the brain endothelioma cell line bEnd5 without affecting the expression levels of these proteins. This was accompanied by an agrin-induced reduction of the paracellular permeability of bEnd5 monolayers. In vivo, the lack of agrin also led to reduced junctional localization of VE-cadherin in brain microvascular endothelial cells. Taken together, our data support the notion that agrin contributes to barrier characteristics of brain endothelium by stabilizing the adherens junction proteins VE-cadherin and β-catenin and the junctional protein ZO-1 to brain endothelial junctions.

Discovery of YB-1 as a new immunological target in neuroblastoma by vaccination in the context of regulatory T cell blockade

Neuroblastoma is one of the most common solid tumors in infancy and early childhood. Using the A/J mouse and a syngeneic neuroblastoma cell line AGN2a, we induced a strong anti-neuroblastoma cellular immune response when AGN2a transfected to express costimulatory molecules (CD80/CD86/CD54/CD137L) was used as a vaccine in the context of regulatory T cell blockade. Strong humoral immunity was induced by AGN2a-4p immunization in the context with regulatory T cell blockade. Serum from treated mice was used to screen an AGN2a cDNA expression library that was constructed with lambda ZAP express vector in order to identify tumor-associated antigens by SEREX. Twenty one clones were identified by sequencing and comparative analysis of gene pools. Most transcripts play some roles in the neuronal differentiation, cell metabolism, or have previously been identified as transcripts that are over-expressed in other malignancies. The most commonly identified tumor-associated antigen, using serum from AGN2a-4p immunization with Treg blockade mice, was YB-1 protein that also induced a T cell response. These results indicated that potential neuroblastoma-associated antigens were found by the sera from mice immunized with tumor cells expressing costimulatory molecules with regulatory T cell function blockade. The identification of YB-1 as tumor-associated antigens capable of eliciting a T cell response validates our experimental approach and argues for the antigens we have identified here to be evaluated as targets of effector immunity and as vaccine candidates.

The process-inducing activity of transmembrane agrin requires follistatin-like domains

Clustering or overexpression of the transmembrane form of the extracellular matrix proteoglycan agrin in neurons results in the formation of numerous highly motile filopodia-like processes extending from axons and dendrites. Here we show that similar processes can be induced by overexpression of transmembrane-agrin in several non-neuronal cell lines. Mapping of the process-inducing activity in neurons and non-neuronal cells demonstrates that the cytoplasmic part of transmembrane agrin is dispensable and that the extracellular region is necessary for process formation. Site-directed mutagenesis reveals an essential role for the loop between beta-sheets 3 and 4 within the Kazal subdomain of the seventh follistatin-like domain of TM-agrin. An aspartic acid residue within this loop is critical for process formation. The seventh follistatin-like domain could be functionally replaced by the first and sixth but not by the eighth follistatin-like domain, demonstrating a functional redundancy among some follistatin-like domains of agrin. Moreover, a critical distance of the seventh follistatin-like domain to the plasma membrane appears to be required for process formation. These results demonstrate that different regions within the agrin protein are responsible for synapse formation at the neuromuscular junction and for process formation in central nervous system neurons and suggest a role for agrin's follistatin-like domains in the developing central nervous system.

Transmembrane form agrin-induced process formation requires lipid rafts and the activation of Fyn and MAPK

Overexpression or clustering of the transmembrane form of the extracellular matrix heparan sulfate proteoglycan agrin (TM-agrin) induces the formation of highly dynamic filopodia-like processes on axons and dendrites from central and peripheral nervous system-derived neurons. Here we show that the formation of these processes is paralleled by a partitioning of TM-agrin into lipid rafts, that lipid rafts and transmembrane-agrin colocalize on the processes, that extraction of lipid rafts with methyl-beta-cyclodextrin leads to a dose-dependent reduction of process formation, that inhibition of lipid raft synthesis prevents process formation, and that the continuous presence of lipid rafts is required for the maintenance of the processes. Association of TM-agrin with lipid rafts results in the phosphorylation and activation of the Src family kinase Fyn and subsequently in the phosphorylation and activation of MAPK. Inhibition of Fyn or MAPK activation inhibits process formation. These results demonstrate that the formation of filopodia-like processes by TM-agrin is the result of the activation of a complex intracellular signaling cascade, supporting the hypothesis that TM-agrin is a receptor or coreceptor on neurons.

Agrin expression during synaptogenesis induced by traumatic brain injury

Interaction between extracellular matrix proteins and regulatory proteinases can mediate synaptic integrity. Previously, we documented that matrix metalloproteinase 3 (MMP-3) expression and activity increase following traumatic brain injury (TBI). We now report protein and mRNA analysis of agrin, a MMP-3 substrate, over the time course of trauma-induced synaptogenesis. Agrin expression during the successful synaptic reorganization of unilateral entorhinal cortical lesion (UEC) was compared with expression when normal synaptogenesis fails (combined fluid percussion TBI and bilateral entorhinal lesion [BEC]). We observed that agrin protein was increased in both models at 2 and 7 days postinjury, and immuohistochemical (IHC) co-localization suggested reactive astrocytes contribute to that increase. Agrin formed defined boundaries for sprouting axons along deafferented dendrites in the UEC, but failed to do so after combined insult. Similarly, Western blot analysis revealed greater increase in UEC agrin protein relative to the combined TBI+BEC model. Both models showed increased agrin transcription at 7 days postinjury and mRNA normalization by 15 days. Attenuation of synaptic pathology with the NMDA antagonist MK-801 reduced 7-day UEC agrin transcript to a level not different from unlesioned controls. By contrast, MK-801 in the combined insult failed to significantly change 7-day agrin transcript, mRNA levels remaining elevated over uninjured sham cases. Together, these results suggest that agrin plays an important role in the sprouting phase of reactive synaptogenesis, and that both its expression and distribution are correlated with extent of successful recovery after TBI. Further, when pathogenic conditions which induce synaptic plasticity are reduced, increase in agrin mRNA is attenuated.

Thermodynamic and structural studies of carbohydrate binding by the agrin-G3 domain

Agrin is a key heparan sulfate proteoglycan involved in the development and maintenance of synaptic junctions between nerves and muscles. Agrin's important functions include clustering acetylcholine receptors on the postsynaptic membranes of muscles and binding to the muscle protein alpha-dystroglycan through its glycan chains. ITC and NMR were used to study the interactions of the C-terminal domain, agrin-G3, with carbohydrates implicated in agrin's functions. Sialic acid caps the glycan chains of alpha-dystroglycan and occurs as a posttranslational modification on the muscle-specific kinase component of the agrin receptor. We found that agrin-G3 binds sialic acid in a Ca2+-dependent manner. ITC data indicate that binding is exothermic and occurs with a 1:1 stoichiometry. NMR chemical shift changes map the sialic acid binding site to the loops that control the domain's acetylcholine receptor clustering activity. By contrast, the glycosaminoglycans heparin and heparan sulfate bind independently of Ca2+. Binding is endothermic, and the binding site spans about 12 saccharide units. The binding site for heparin occupies a similar location but is distinct from that for sialic acid. NMR translational diffusion experiments show that agrin-G3 binds heparin with a 2:1 stoichiometry. Comparisons between the muscle (B0) and neuronal (B8) isoforms of the agrin domain showed very similar Ca2+ and carbohydrate binding properties. Our work identifies agrin-G3 as a functional analogue of the concanavalin A-type lectins, highlights functional similarities between agrin and laminin G domains, and provides mechanistic clues about the roles of carbohydrates in agrin's functions.

Agrin regulates growth cone turning of Xenopus spinal motoneurons

The pivotal role of agrin in inducing postsynaptic specializations at neuromuscular junctions has been well characterized. Increasing evidence suggests that agrin is also involved in neuronal development. In this study,we found that agrin inhibited neurite extension and, more importantly, a gradient of agrin induced repulsive growth-cone turning in cultured Xenopus spinal neurons. Incubation with a neutralizing antibody to agrin or expression of the extracellular domain of muscle-specific kinase, a component of the agrin receptor complex, abolished these effects of agrin. Agrin-induced repulsive growth-cone turning requires the activity of PI3-kinase and Ca2+ signaling. In addition, the expression of dominant-negative Rac1 inhibited neurite extension and blocked agrin-mediated growth-cone turning. Taken together, our findings suggest that agrin regulates neurite extension and provide evidence for an unanticipated role of agrin in growth-cone steering in developing neurons.