Expression and localization of agrin during sympathetic synapse formation in vitro

Alpha herpesvirus exocytosis from neuron cell bodies uses constitutive secretory mechanisms, and egress and spread from axons is independent of neuronal firing activity

Alpha herpesviruses naturally infect the peripheral nervous system, and can spread to the central nervous system, causing severe debilitating or deadly disease. Because alpha herpesviruses spread along synaptic circuits, and infected neurons exhibit altered electrophysiology and increased spontaneous activity, we hypothesized that alpha herpesviruses use activity-dependent synaptic vesicle-like regulated secretory mechanisms for egress and spread from neurons. Using live-cell fluorescence microscopy, we show that Pseudorabies Virus (PRV) particles use the constitutive Rab6 post-Golgi secretory pathway to exit from the cell body of primary neurons, independent of local calcium signaling. Some PRV particles colocalize with Rab6 in the proximal axon, but we did not detect colocalization/co-transport in the distal axon. Thus, the specific secretory mechanisms used for viral egress from axons remains unclear. To address the role of neuronal activity more generally, we used a compartmentalized neuron culture system to measure the egress and spread of PRV from axons, and pharmacological and optogenetics approaches to modulate neuronal activity. Using tetrodotoxin to silence neuronal activity, we observed no inhibition, and using potassium chloride or optogenetics to elevate neuronal activity, we also show no increase in virus spread from axons. We conclude that PRV egress from neurons uses constitutive secretory mechanisms: generally, activity-independent mechanisms in axons, and specifically, the constitutive Rab6 post-Golgi secretory pathway in cell bodies.

Satellite glial cells modulate cholinergic transmission between sympathetic neurons

Postganglionic sympathetic neurons and satellite glial cells are the two major cell types of the peripheral sympathetic ganglia. Sympathetic neurons project to and provide neural control of peripheral organs and have been implicated in human disorders ranging from cardiovascular disease to peripheral neuropathies. Here we show that satellite glia regulate synaptic activity of cultured postnatal sympathetic neurons, providing evidence for local ganglionic control of sympathetic drive. In addition to modulating neuron-to-neuron cholinergic neurotransmission, satellite glia promote synapse formation and contribute to neuronal survival. Examination of the cellular architecture of the rat sympathetic ganglia in vivo shows this regulation of neuronal properties takes place during a developmental period in which neuronal morphology and density are actively changing and satellite glia enwrap sympathetic neuronal somata. Cultured satellite glia make and release factors that promote neuronal activity and that can partially rescue the neurons from cell death following nerve growth factor deprivation. Thus, satellite glia play an early and ongoing role within the postnatal sympathetic ganglia, expanding our understanding of the contributions of local and target-derived factors in the regulation of sympathetic neuron function.

The role of agrin in synaptic development, plasticity and signaling in the central nervous system

Development of the neuromuscular junction (NMJ) requires secretion of specific isoforms of the proteoglycan agrin by motor neurons. Secreted agrin is widely expressed in the basal lamina of various tissues, whereas a transmembrane form is highly expressed in the brain. Expression in the brain is greatest during the period of synaptogenesis, but remains high in regions of the adult brain that show extensive synaptic plasticity. The well-established role of agrin in NMJ development and its presence in the brain elicited investigations of its possible role in synaptogenesis in the brain. Initial studies on the embryonic brain and neuronal cultures of agrin-null mice did not reveal any defects in synaptogenesis. However, subsequent studies in culture demonstrated inhibition of synaptogenesis by agrin antisense oligonucleotides or agrin siRNA. More recently, a substantial loss of excitatory synapses was found in the brains of transgenic adult mice that lacked agrin expression everywhere but in motor neurons. The mechanisms by which agrin influences synapse formation, maintenance and plasticity may include enhancement of excitatory synaptic signaling, activation of the "muscle-specific" receptor tyrosine kinase (MuSK) and positive regulation of dendritic filopodia. In this article I will review the evidence that agrin regulates synapse development, plasticity and signaling in the brain and discuss the evidence for the proposed mechanisms.

Nicotine-induced phosphorylation of phosphorylated cyclic AMP response element-binding protein (pCREB) in hippocampal neurons is potentiated by agrin

The scope of this study was to test whether increased levels of the extracellular matrix molecule (ECM) agrin might enhance nicotine effects on those molecular mechanisms that initiate neuroadaptative processes in the hippocampus, a key brain area for learning and memory. We studied the effects of repetitive applications of neuronal agrin to primary hippocampal cell culture on nicotine-induced phosphorylated cyclic AMP response element-binding protein (pCREB) expression, a marker of neuroadaptation, by using immunofluorescence-based assessment of pCREB-positive neurons. We also tested agrin effects on nicotine-induced expression of a marker of metabolic activation, the immediate early gene c-fos. Agrin was shown to significantly enhance nicotine-induced pCREB, but not c-fos, expression. By using Western blotting analysis, cumulative agrin has been shown to increase nicotine-induced pCREB phosphorylation. These analyses, however, showed that inhibition of the CaMKII pathway blocked general pCREB phosphorylation, whereas inhibition of the MAPK pathway potentiated the synergistic effect of cumulative agrin and nicotine. These findings suggest that increasing the concentration of an ECM molecule, i.e. agrin, may enhance nicotine effects on pCREB and that both MAPK and CaMKII signalling may play a regulatory role.

Agrin induced morphological and structural changes in growth cones of cultured hippocampal neurons

The role of agrin in synaptogenesis has been extensively studied. On the other hand, little is known about the function of this extracellular matrix protein during developmental processes that precede the formation of synapses. Recently, agrin was shown to regulate the rate of neurite elongation and the behavior of growth cones in hippocampal and spinal neurons, respectively. However, the molecular mechanisms underlying these effects have not been completely elucidated. In the present study, we analyzed the morphological and molecular changes induced by agrin in growth cones of hippocampal neurons that developed in culture. Morphometric analysis showed a significant enlargement of growth cones of hippocampal neurons cultured in the presence of agrin. These agrin-induced growth cone changes were accompanied by the formation of loops of microtubules highly enriched in acetylated tubulin and an increase in the content of the microtubule-associated protein (MAP)1B. Together, these data provide further insights into the potential molecular mechanisms underlying the effects of agrin on neurite outgrowth in rat central neurons.

Synaptic transmission is impaired at neuronal autonomic synapses in agrin-null mice

Neuronal synapse formation is a multistep process regulated by several pre- and postsynaptic adhesion and signaling proteins. Recently, we found that agrin acts as one such synaptogenic factor at neuronal synapses in the PNS by demonstrating that structural synapse formation is impaired in the superior cervical ganglia (SCG) of z+ agrin-deficient mice and in SCG cultures derived from those animals. Here, we tested whether synaptic function is defective in agrin-null (AGD-/-) ganglia and began to define agrin's mechanism of action. Our electrophysiological recordings of compound action potentials showed that presynaptic stimulation evoked action potentials in approximately 40% of AGD-/- ganglionic neurons compared to 90% of wild-type neurons; moreover, transmission could not be potentiated as in wild-type or z+ agrin-deficient ganglia. Intracellular recordings also showed that nerve-evoked excitatory postsynaptic potentials in AGD-/- neurons were only 1/3 the size of those in wild-type neurons and mostly subthreshold. Consistent with these defects in transmission, we found an approximately 40-50% decrease in synapse number in AGD-/- ganglia and cultures, and decreased levels of differentiation at the residual synapses in culture. Furthermore, surface levels of acetylcholine receptors (AChRs) were equivalent in cultured AGD-/- and wild-type neurons, and depolarization reduced the synaptic localization of AChRs in AGD-/- but not wild-type neurons. These findings provide the first direct demonstration that agrin is required for proper structural and functional development of an interneuronal synapse in vivo. Moreover, they suggest a novel role for agrin, in stabilizing the postsynaptic density of nAChR at nascent neuronal synapses.

Progesterone-induced agrin expression in astrocytes modulates glia-neuron interactions leading to synapse formation

Experimental evidence recently obtained suggests that synaptogenesis is a tripartite event in which not only pre- and post-synaptic neurons but also glial cells play a key role. However, the molecular mechanisms by which glia modulate the formation of synapses in the CNS remain poorly understood. In the present study, we analyzed the role of astrocytes in synapse formation in cultured hippocampal rat neurons. For these experiments, hippocampal neurons were cultured in the presence or absence of a monolayer of astrocytes. Our results indicated that hippocampal neurons cultured in the presence of astrocytes formed more synapses than the ones cultured in their absence only when kept in N2 serum-free medium. To get insights into the potential molecular mechanisms underlying this effect, we analyzed the expression of proteins known to induce synapse formation in hippocampal neurons. A significant increase in agrin expression was detected in astrocytes cultured in N2 serum-free medium when compared with the ones cultured in serum containing medium. Experiments performed using different components of the N2 mixture indicated that progesterone induced the expression of agrin in astrocytes. Taken collectively, these results provide evidence supporting a role for astrocytes in synapse formation in central neurons. Furthermore, they identified agrin as a potential mediator of this effect, and astrocytes as a bridge between the endocrine and nervous systems during synaptogenesis.

Differential targeting and function of alpha2A and alpha2C adrenergic receptor subtypes in cultured sympathetic neurons

Previous research suggested that alpha2A and alpha2C adrenergic receptor (AR) subtypes have overlapping but unique physiological roles in neuronal signaling; however, the basis for these dissimilarities is not completely known. To better understand the observed functional differences between these autoreceptors, we investigated targeting and signaling of endogenously expressed alpha2A and alpha2CARs in cultured sympathetic ganglion neurons (SGN). At Days 1 and 4, alpha2A and alpha2CARs could be readily detected in SGN from wild-type mice. By Day 8, alpha2A ARs were targeted to cell body, as well as axonal and dendritic sites, whereas alpha2C ARs were primarily localized to an intracellular vesicular pool within the cell body and proximal dendritic projections. Expression of synaptic vesicle marker protein SV2 did not differ at Day 8 nor co-localize with either subtype. By Day 16, however, alpha2C ARs had relocated to somatodendritic and axonal sites and, unlike alpha2A ARs, co-localized with SV2 at synaptic contact sites. Consistent with a functional role for alpha2 ARs, we also observed that dexmedetomidine stimulation of cultured SGN more efficiently inhibited depolarization-induced calcium entry into older, compared to younger, cultures. These results provide direct evidence of distinct developmental patterns of endogenous alpha2A and alpha2C AR targeting and function in a native cell system and that maturation of SGN in culture leads to alterations in neuronal properties required for proper targeting. More importantly, the co-localization at Day 16 of alpha2C ARs at sites of synaptic contact may partially explain the differential modulation of neurotransmitter release and responsiveness to action potential frequency observed between alpha2A and alpha2C ARs in SGN.

Identification of agrinSN isoform and muscle-specific receptor tyrosine kinase in sperm

We previously demonstrated several nicotinic acetylcholine receptor (nAChR) subunits and associated proteins in human sperm. Here, we identified in sperm for the first time two additional nAChR-associated molecules: (1) agrin(SN)Z(+) in human sperm localized in the posterior post-acrosomal, neck, and flagellar mid-piece regions; (2) a low-molecular weight isoform of muscle-specific receptor tyrosine kinase in human and mouse sperm localized in the flagellar mid-piece of human sperm.

Aberrant neuromuscular junctions and delayed terminal muscle fiber maturation in alpha-dystroglycanopathies

Recent studies have revealed an association between post-translational modification of alpha-dystroglycan (alpha-DG) and certain congenital muscular dystrophies known as secondary alpha-dystroglycanopathies (alpha-DGpathies). Fukuyama-type congenital muscular dystrophy (FCMD) is classified as a secondary alpha-DGpathy because the responsible gene, fukutin, is a putative glycosyltransferase for alpha-DG. To investigate the pathophysiology of secondary alpha-DGpathies, we profiled gene expression in skeletal muscle from FCMD patients. cDNA microarray analysis and quantitative real-time polymerase chain reaction showed that expression of developmentally regulated genes, including myosin heavy chain (MYH) and myogenic transcription factors (MRF4, myogenin and MyoD), in FCMD muscle fibers is inconsistent with dystrophy and active muscle regeneration, instead more of implicating maturational arrest. FCMD skeletal muscle contained mainly immature type 2C fibers positive for immature-type MYH. These characteristics are distinct from Duchenne muscular dystrophy, suggesting that another mechanism in addition to dystrophy accounts for the FCMD skeletal muscle lesion. Immunohistochemical analysis revealed morphologically aberrant neuromuscular junctions (NMJs) lacking MRF4 co-localization. Hypoglycosylated alpha-DG indicated a lack of aggregation, and acetylcholine receptor (AChR) clustering was compromised in FCMD and the myodystrophy mouse, another model of secondary alpha-DGpathy. Electron microscopy showed aberrant NMJs and neural terminals, as well as myotubes with maturational defects. Functional analysis of NMJs of alpha-DGpathy showed decreased miniature endplate potential and higher sensitivities to d-Tubocurarine, suggesting aberrant or collapsed formation of NMJs. Because alpha-DG aggregation and subsequent clustering of AChR are crucial for NMJ formation, hypoglycosylation of alpha-DG results in aberrant NMJ formation and delayed muscle terminal maturation in secondary alpha-DGpathies. Although severe necrotic degeneration or wasting of skeletal muscle fibers is the main cause of congenital muscular dystrophies, maturational delay of muscle fibers also underlies the etiology of secondary alpha-DGpathies.

Agrin mediates a rapid switch from electrical coupling to chemical neurotransmission during synaptogenesis

In contrast to its well-established actions as an organizer of synaptic differentiation at the neuromuscular junction, the proteoglycan agrin is still in search of a function in the nervous system. Here, we report an entirely unanticipated role for agrin in the dual modulation of electrical and chemical intercellular communication that occurs during the critical period of synapse formation. When applied at the developing splanchnic nerve–chromaffin cell cholinergic synapse in rat adrenal acute slices, agrin rapidly modified cell-to-cell communication mechanisms. Specifically, it led to decreased gap junction–mediated electrical coupling that preceded an increase in nicotinic synaptic transmission. This developmental switch from predominantly electrical to chemical communication was fully operational within one hour and depended on the activation of Src family–related tyrosine kinases. Hence, agrin may play a pivotal role in synaptogenesis in promoting a rapid switch between electrical coupling and synaptic neurotransmission.

Evidence that dystroglycan is associated with dynamin and regulates endocytosis

Disruption of the dystroglycan gene in humans and mice leads to muscular dystrophies and nervous system defects including malformation of the brain and defective synaptic transmission. To identify proteins that interact with dystroglycan in the brain we have used immunoaffinity purification followed by mass spectrometry (LC/MS-MS) and found that the GTPase dynamin 1 is a novel dystroglycan-associated protein. The beta-dystroglycan-dynamin 1 complex also included alpha-dystroglycan and Grb2. Overlay assays indicated that dynamin interacts directly with dystroglycan, and immunodepletion showed that only a pool of dynamin is associated with dystroglycan. Dystroglycan was associated and colocalized immunohistochemically with dynamin 1 in the central nervous system in the outer plexiform layer of retina where photoreceptor terminals are found. Endocytosis in neurons is both constitutive, as in non-neural cells, and regulated by neural activity. To assess the function of dystroglycan in the former, we have assayed transferrin uptake in fibroblastic cells differentiated from embryonic stem cells null for both dystroglycan alleles. In wild-type cells, dystroglycan formed a complex with dynamin and codistributed with cortactin at membrane ruffles, which are organelles implicated in endocytosis. Dystroglycan-null cells had a significantly greater transferrin uptake, a process well known to require dynamin. Expression of dystroglycan in null cells by infection with an adenovirus containing dystroglycan reduced transferrin uptake to levels seen in wild-type embryonic stem cells. These data suggest that dystroglycan regulates endocytosis possibly as a result of its interaction with dynamin.

Agrin becomes concentrated at neuroeffector junctions in developing rodent urinary bladder

The formation of somatic neuromuscular junctions in skeletal muscle is regulated by an extracellular matrix protein called agrin. Here, we have examined the expression and localization of agrin during development of the rodent urinary bladder, as a first step to examining its possible role at autonomic neuroeffector junctions in smooth muscle. We have found that agrin is expressed on the surface of developing smooth muscle cells and in the basement membrane underlying the urothelium. More importantly, agrin is progressively concentrated at parasympathetic varicosities during postnatal development and is present at virtually all junctions in mature muscle. Reverse transcription/polymerase chain reaction analysis has shown that pelvic ganglion neurons that innervate the bladder express LN/z8 agrin, whereas bladder smooth muscle expresses LN/z- agrin. Together, these results demonstrate that nerve and/or muscle agrin becomes localized at cholinergic parasympathetic varicosities in smooth muscle, where it could play a role in the maturation of the neuroeffector junction.

Targeting of recombinant agrin to axonal growth cones

Targeting of proteins to specific subcellular locations within pre- and postsynaptic neurons is essential for synapse formation. The heparan sulfate proteoglycan agrin orchestrates postsynaptic differentiation of the neuromuscular junction and may be involved in synaptic development and signaling in the central nervous system (CNS). Agrin is expressed as transmembrane and secretory isoforms with distinct N-termini. We examined the distribution of recombinant agrin in cultured motor and hippocampal neurons by transfection with agrin-GFP constructs. Immunostaining revealed a vesicular transport compartment within all neurites. Plasma membrane insertion and secretion of recombinant agrin were targeted to axonal growth cones of motor neurons; transmembrane agrin-GFP was targeted predominantly to axons and axonal growth cones in hippocampal neurons. We used agrin deletion mutants to show that axonal targeting of agrin depends on multiple domains that function in an additive fashion, including the very N-terminal portions and the C-terminal half of the molecule.

The COOH-terminal domain of agrin signals via a synaptic receptor in central nervous system neurons

Agrin is a motor neuron-derived factor that directs formation of the postsynaptic apparatus of the neuromuscular junction. Agrin is also expressed in the brain, raising the possibility that it might serve a related function at neuron-neuron synapses. Previously, we identified an agrin signaling pathway in central nervous system (CNS) neurons, establishing the existence of a neural receptor that mediates responses to agrin. As a step toward identifying this agrin receptor, we have characterized the minimal domains in agrin that bind and activate it. Structures required for agrin signaling in CNS neurons are contained within a 20-kD COOH-terminal fragment of the protein. Agrin signaling is independent of alternative splicing at the z site, but requires sequences that flank it because their deletion results in a 15-kD fragment that acts as an agrin antagonist. Thus, distinct regions within agrin are responsible for receptor binding and activation. Using the minimal agrin fragments as affinity probes, we also studied the expression of the agrin receptor on CNS neurons. Our results show that both agrin and its receptor are concentrated at neuron-neuron synapses. These data support the hypothesis that agrin plays a role in formation and/or function of CNS synapses.

PDZ-containing proteins provide a functional postsynaptic scaffold for nicotinic receptors in neurons

Protein scaffolds are essential for specific and efficient downstream signaling at synapses. Though nicotinic receptors are widely expressed in the nervous system and influence numerous cellular events due in part to their calcium permeability, no scaffolds have yet been identified for the receptors in neurons. Here we show that specific members of the PSD-95 family of PDZ-containing proteins are associated with specific nicotinic receptor subtypes. At postsynaptic sites, the PDZ scaffolds are essential for maturation of functional nicotinic synapses on neurons. They also help mediate downstream signaling as exemplified by activation of transcription factors. By tethering components to postsynaptic nicotinic receptors, PDZ scaffolds can organize synaptic structure and determine which calcium-dependent processes will be subject to nicotinic modulation.

New insights into the roles of agrin

The heparan sulphate proteoglycan agrin is expressed as several isoforms in various tissues. Agrin is best known as a crucial organizer of postsynaptic differentiation at the neuromuscular junction, but it has recently also been implicated in the formation of the immunological synapse, the organization of the cytoskeleton and the amelioration of function in diseased muscle. So the activities of agrin might be of broader significance than previously anticipated.

Nicotinic alpha 7 receptors: synaptic options and downstream signaling in neurons

Nicotinic receptors are cation-ion selective ligand-gated ion channels that are expressed throughout the nervous system. Most have significant calcium permeabilities, enabling them to regulate calcium-dependent events. One of the most abundant is a species composed of the alpha 7 gene product and having a relative calcium permeability equivalent to that of NMDA receptors. The alpha 7-containing receptors can be found presynaptically where they modulate transmitter release, and postsynaptically where they generate excitatory responses. They can also be found in perisynaptic locations where they modulate other inputs to the neuron and can activate a variety of downstream signaling pathways. The effects the receptors produce depend critically on the sites at which they are clustered. Instructive preparations for examining alpha 7-containing receptors are the rat hippocampus, where they are thought to play a modulatory role, and the chick ciliary ganglion, where they participate in throughput transmission as well as regulatory signaling. Relatively high levels of alpha 7-containing receptors are found in the two preparations, and the receptors display a variety of synaptic options and functions in the two cases. Progress is starting to be made in understanding the mechanisms responsible for localizing the receptors at specific sites and in identifying components tethered in the vicinity of the receptors that may facilitate signal transduction and downstream signaling.

Mapping sites responsible for interactions of agrin with neurons

The multidomain proteoglycan agrin is a critical organizer of postsynaptic differentiation at the skeletal neuromuscular junction. Agrin is also abundant in the brain, but its roles there are unknown. As a step toward understanding these roles, we mapped sites responsible for interactions of neurons with agrin. First, we used a series of recombinant agrin fragments to show that at least four sites on agrin interact with chick ciliary neurons. Use of blocking antibodies and peptides indicated that neurons adhere to a site in the second of three G domains by means of alphaVbeta1 integrin, and to a site in the last of four epidermal growth factor (EGF) repeats via a distinct beta1 integrin. A third, integrin-independent adhesion site is near to but distinct from the site that induces postsynaptic differentiation in muscles. These domains are insufficient, however, to account for neurite outgrowth-inhibiting properties of full-length agrin, which are mediated by the N-terminal half of the molecule. We then used a second set of agrin mutants to demonstrate and map a transmembrane domain in the amino-terminus of the SN-isoform of agrin. The extracellular matrix-bound form of agrin, called LN, bears an amino-terminus required for secretion and binding to laminin. The SN form, which is selectively expressed by neurons, bears a variant amino terminus that converts agrin from a secreted, matrix-associated protein to a type-II transmembrane protein, providing a mechanism for presenting agrin in central, as opposed to neuromuscular, synaptic clefts. The SN-amino terminus can mediate externalization and membrane anchoring of heterologous proteins, but is insufficient to target them to the synapse. Together, these studies define sites that contribute to the subcellular localization of and signaling by neuronal agrin.

Agrin plays an organizing role in the formation of sympathetic synapses

Agrin is a nerve-derived factor that directs neuromuscular synapse formation, however its role in regulating interneuronal synaptogenesis is less clear. Here, we examine agrin's role in synapse formation between cholinergic preganglionic axons and sympathetic neurons in the superior cervical ganglion (SCG) using agrin-deficient mice. In dissociated cultures of SCG neurons, we found a significant decrease in the number of synapses with aggregates of presynaptic synaptophysin and postsynaptic neuronal acetylcholine receptor among agrin-deficient neurons as compared to wild-type neurons. Moreover, the levels of pre- and postsynaptic markers at the residual synapses in agrin-deficient SCG cultures were also reduced, and these defects were rescued by adding recombinant neural agrin to the cultures. Similarly, we observed a decreased matching of pre- and postsynaptic markers in SCG of agrin-deficient embryos, reflecting a decrease in the number of differentiated synapses in vivo. Finally, in electrophysiological experiments, we found that paired-pulse depression was more pronounced and posttetanic potentiation was significantly greater in agrin-deficient ganglia, indicating that synaptic transmission is also defective. Together, these findings indicate that neural agrin plays an organizing role in the formation and/or differentiation of interneuronal, cholinergic synapses.

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.

Maturation of postsynaptic nicotinic structures on autonomic neurons requires innervation but not cholinergic transmission

Postsynaptic development at the neuromuscular junction depends on nicotinic transmission and secreted components from the presynaptic motor nerve terminal. Similarly, secreted components and synaptic activity are both thought to guide development of glutamatergic synapses in the CNS. Nicotinic synapses on chick ciliary neurons are structurally complex: a large presynaptic calyx engulfs the postsynaptic neuron and overlays a series of discrete mats of receptor-rich somatic spines tightly interwoven and folded against the soma. We used fluorescence imaging of alpha 7-containing nicotinic receptors and the spine constituent drebrin to monitor postsynaptic development. The results show that surgical disruption of the preganglionic input or removal of the ganglionic synaptic target tissue after synapses form in the ganglion does not disrupt the receptor-rich spine mats. Similarly, removal of the target tissue even prior to synapse formation in the ganglion does not prevent subsequent formation of the receptor clusters and associated spine constituents. Postsynaptic development is arrested, however, if normal innervation is prevented by ablating the preganglionic neurons prior to synapse formation. In this case the neurons express reduced levels of nicotinic receptors and cytoskeletal components and organize them only into early-stage clusters. Even low levels of residual innervation, however, can restore much of the normal postsynaptic receptor patterns. Chronic pharmacological blockade of cholinergic synaptic activity fails to replicate the effects of ablating the preganglionic nucleus. The results indicate that ciliary neurons are programmed to express postsynaptic components and can initiate clustering of alpha 7-containing receptors but need presynaptic guidance for maturation of the postsynaptic structure.

Agrin regulates neuronal responses to excitatory neurotransmitters in vitro and in vivo

Agrin mediates motor neuron-induced differentiation of the postsynaptic apparatus of the neuromuscular junction but its function in brain remains unknown. Here we report that expression of c-fos, induced by activation of nicotinic or glutamatergic receptors, was significantly lower in cortical neurons cultured from agrin-deficient mutant mouse embryos compared to wildtype. Agrin-deficient neurons also exhibited increased resistance to excitotoxic injury. Treatment with recombinant agrin restored glutamate-induced c-fos expression and excitotoxicity of the agrin-deficient neurons to near wild-type levels, confirming the agrin dependence of the phenotype. The observation that c-fos induction by activation of voltage-gated Ca2+ channels is also reduced in agrin-deficient neurons raises the possibility that agrin may play a wider role by regulating responses to Ca(2+)-mediated signals. Consistent with the decline in response of cultured mutant neurons to glutamate, decreases in kainic acid-induced seizure and mortality were observed in adult agrin heterozygous mice. Together, these data demonstrate that agrin plays an important role in defining neuronal responses to excitatory neurotransmitters both in vitro and in vivo.