Localization of Dlg at the mammalian neuromuscular junction

Requirement for Dlgh-1 in planar cell polarity and skeletogenesis during vertebrate development

The development of specialized organs is tightly linked to the regulation of cell growth, orientation, migration and adhesion during embryogenesis. In addition, the directed movements of cells and their orientation within the plane of a tissue, termed planar cell polarity (PCP), appear to be crucial for the proper formation of the body plan. In Drosophila embryogenesis, Discs large (dlg) plays a critical role in apical-basal cell polarity, cell adhesion and cell proliferation. Craniofacial defects in mice carrying an insertional mutation in Dlgh-1 suggest that Dlgh-1 is required for vertebrate development. To determine what roles Dlgh-1 plays in vertebrate development, we generated mice carrying a null mutation in Dlgh-1. We found that deletion of Dlgh-1 caused open eyelids, open neural tube, and misorientation of cochlear hair cell stereociliary bundles, indicative of defects in planar cell polarity (PCP). Deletion of Dlgh-1 also caused skeletal defects throughout the embryo. These findings identify novel roles for Dlgh-1 in vertebrates that differ from its well-characterized roles in invertebrates and suggest that the Dlgh-1 null mouse may be a useful animal model to study certain human congenital birth defects.

Evolution of the Discs large gene family provides new insights into the establishment of apical epithelial polarity and the etiology of mental retardation

Cell polarity is essential to the function of many cell types, such as epithelial cells and neurons. The Discs large (Dlg) scaffolding protein was identified in Drosophila as a major regulator of basolateral epithelial identity. Four Dlg orthologs (Dlg1 through 4) are found in vertebrates, and mutations in the human Dlg3 gene are associated with X-linked mental retardation. We recently found that Dlg3 controls apical epithelial polarity and tight junction formation and contributes to neural induction in mouse development.¹ During evolution, Dlg3 acquired specific PPxY motifs, which bind to the WW domains of the E3 ubiquitin ligases, Nedd4 and Nedd4-2. This interaction results in monoubiquitination of Dlg3, leading to directed microtubule-dependent protein trafficking, via the exocyst complex, in different polarized cell types. Directed trafficking of Dlg3 plays an important role, during both mammalian development and in adulthood, in the establishment and maintenance of specialized apical cell junctions, such as tight junctions in epithelial cells and synapses in neurons.

Proline-rich synapse-associated protein-1 and 2 (ProSAP1/Shank2 and ProSAP2/Shank3)-scaffolding proteins are also present in postsynaptic specializations of the peripheral nervous system

Proline-rich synapse-associated protein-1 and 2 (ProSAP1/Shank2 and ProSAP2/Shank3) were originally found as synapse-associated protein 90/postsynaptic density protein-95-associated protein (SAPAP)/guanylate-kinase-associated protein (GKAP) interaction partners and also isolated from synaptic junctional protein preparations of rat brain. They are essential components of the postsynaptic density (PSD) and are specifically targeted to excitatory asymmetric type 1 synapses. Functionally, the members of the ProSAP/Shank family are one of the postsynaptic key elements since they link and attach the postsynaptic signaling apparatus, for example N-methyl-d-aspartic acid (NMDA)-receptors via direct and indirect protein interactions to the actin-based cytoskeleton. The functional significance of ProSAP1/2 for synaptic transmission and the paucity of data with respect to the molecular composition of PSDs of the peripheral nervous system (PNS) stimulated us to investigate neuromuscular junctions (NMJs), synapses of the superior cervical ganglion (SCG), and synapses in myenteric ganglia as representative synaptic junctions of the PNS. Confocal imaging revealed ProSAP1/2-immunoreactivity (-iry) in NMJs of rat and mouse sternomastoid and tibialis anterior muscles. In contrast, ProSAP1/2-iry was only negligibly found in motor endplates of striated esophageal muscle probably caused by antigen masking or a different postsynaptic molecular anatomy at these synapses. ProSAP1/2-iry was furthermore detected in cell bodies and dendrites of superior cervical ganglion neurons and myenteric neurons in esophagus and stomach. Ultrastructural analysis of ProSAP1/2 expression in myenteric ganglia demonstrated that ProSAP1 and ProSAP2 antibodies specifically labelled PSDs of myenteric neurons. Thus, scaffolding proteins ProSAP1/2 were found within the postsynaptic specializations of synapses within the PNS, indicating a similar molecular assembly of central and peripheral postsynapses.

CASK localizes to nuclei in developing skeletal muscle and motor neuron culture models and is agrin-independent

Membrane-associated guanylate kinases (MAGUKs) are cytoplasmic multi-domain proteins that serve as scaffold proteins at cell junctions and synapses. Calmodulin-associated serine/threonine kinase (CASK) stabilizes the integrity of synapses in the brain. Additionally, CASK is capable of acting as a transcriptional co-activator and localizes to neuronal nuclei in the developing brain. We have recently described CASK localization to both the pre- and post-synaptic membranes of the neuromuscular junction (NMJ), where it forms a complex with discs large (Dlg). CASK also localizes to some, but not all nuclei in adult mouse skeletal muscle. To begin to dissect the roles of CASK in the cellular components of the NMJ, we investigated the localization of CASK during differentiation in cell culture models of skeletal muscle and motor neurons. We demonstrate that CASK localizes to the nucleus in undifferentiated myoblasts, but is pre-dominantly in the cytoplasm in differentiated myotubes of the C2C12 myogenic cell line. We also show nuclear localization of both CASK and Dlg in a motor neuron-neuroblastoma hybrid cell line, MN-1, suggesting a role for CASK and Dlg in nuclei of neurons in the peripheral nervous system. Finally, we demonstrate that CASK and Dlg do not co-cluster with acetylcholine receptors in C2C12 myotubes in response to agrin or laminin treatment, suggesting a novel mechanism of recruitment to the NMJ that is independent of acetylcholine receptor and utrophin complexes. These studies delineate important developmental characteristics of CASK and Dlg, and suggest dual roles for these proteins in both the skeletal muscle and motor neuron components of the NMJ.

Differential mRNA expression and protein localization of the SAP90/PSD-95-associated proteins (SAPAPs) in the nervous system of the mouse

The supramolecular anchoring/signaling complex at the postsynaptic density of glutamatergic synapses has been proposed to play a key role in regulating synaptic function and plasticity. One class of proteins present in the complex is the SAP90/PSD-95-associated protein family (SAPAPs). The SAPAPs, identified by their direct interaction with PSD-95 family proteins, were initially proposed to function in the anchoring/signaling complex as linker proteins between glutamate receptor binding proteins and the cytoskeleton. However, recent studies have indicated that the SAPAPs also bind to signaling molecules and may thus have multiple roles at synapses. Four homologous genes encoding SAPAP proteins have been previously identified. As a first step toward understanding the physiological function of the SAPAPs, we have investigated in detail, at both the mRNA and protein levels, the localization of the individual SAPAP genes in the adult murine nervous system. We find that the SAPAP mRNAs are highly, yet differentially, expressed in many regions of the brain, including the hippocampus and cerebellum. Furthermore, SAPAP3 mRNA is targeted to dendrites, whereas SAPAP1, -2, and -4 mRNAs are detected mainly in cell bodies. The SAPAP proteins are localized at synapses in a manner consistent with mRNA expression. Surprisingly, in addition to glutamatergic synapse localization, antibody staining also reveals that the SAPAP proteins are localized at cholinergic synapses, including neuronal cholinergic synapses and the neuromuscular junction. Together, these results indicate that the SAPAPs are general components of excitatory synapses and that each of these proteins may perform a distinct function.

PSD93 regulates synaptic stability at neuronal cholinergic synapses

Neuronal cholinergic synapses play important roles in both the PNS and CNS. However, the mechanisms that regulate the formation, maturation, and stability of neuronal cholinergic synapses are poorly understood. In this study, we use the readily accessible mouse superior cervical ganglion (SCG) and submandibular ganglion (SMG) to examine the assembly of the postsynaptic complex of neuronal cholinergic synapses. We find that novel splicing forms of PSD93 (postsynaptic density 93) are expressed in SCG. By immunostaining, we show that PSD93 proteins precisely colocalize with neuronal nicotinic acetylcholine receptors (nAChRs) at synapses of the SCG and SMG. Subcellular fractionation demonstrates that PSD93 is enriched in the PSD fraction of SCG, and coimmunoprecipitation shows that PSD93 and neuronal nAChRs form a complex in vivo. Furthermore, two additional components of the well characterized glutamatergic postsynaptic complex, GKAP/SAPAP (guanylate kinase domain-associated protein/synapse-associated protein-associated protein) and Shank/ProSAP family proteins, are also present at neuronal cholinergic synapses. To assess the function of this protein complex at neuronal cholinergic synapses in vivo, we examined ganglia in mice that lack PSD93. We find that neuronal cholinergic synapses form properly in PSD93 null mice. After denervation, however, synaptic clusters of nAChRs disassemble much faster in mice lacking PSD93 than those in wild-type mice. These results demonstrate that PSD93 is a key component of the postsynaptic scaffold at neuronal cholinergic synapses and plays an important role in synaptic stability. In addition, these results suggest that the mechanism of postsynaptic scaffolding is conserved between neuronal cholinergic and glutamatergic synapses.

CASK and Dlg form a PDZ protein complex at the mammalian neuromuscular junction

Membrane-associated guanylate kinases (MAGUKs) are modular adapter proteins that serve as scaffolding molecules and anchor channels and receptors via their PDZ (PSD-95, Dlg, Zo-1) domains. Calcium, calmodulin-associated serine/threonine kinase (CASK) is a MAGUK that is critical at synapses in the central nervous system and at cell-cell junctions because of its interactions with channels, receptors, and structural proteins. We show via confocal microscopy that CASK and another MAGUK, Discs Large (Dlg), are present at the mammalian neuromuscular junction in skeletal muscle. Immunoprecipitation data from mouse muscle show that CASK associates with Dlg, providing evidence of a MAGUK protein complex at this synapse. These data indicate that CASK and Dlg may act as a scaffold for organizing receptors and channels at the postsynaptic membrane of the neuromuscular junction.

Structure of the SH3-guanylate kinase module from PSD-95 suggests a mechanism for regulated assembly of MAGUK scaffolding proteins

Membrane-associated guanylate kinases (MAGUKs), such as PSD-95, are modular scaffolds that organize signaling complexes at synapses and other cell junctions. MAGUKs contain PDZ domains, which recruit signaling proteins, as well as a Src homology 3 (SH3) and a guanylate kinase-like (GK) domain, implicated in scaffold oligomerization. The crystal structure of the SH3-GK module from PSD-95 reveals that these domains form an integrated unit: the SH3 fold comprises noncontiguous sequence elements divided by a hinge region and the GK domain. These elements compose two subdomains that can assemble in either an intra- or intermolecular fashion to complete the SH3 fold. We propose a model for MAGUK oligomerization in which complementary SH3 subdomains associate by 3D domain swapping. This model provides a possible mechanism for ligand regulation of oligomerization.

The CASK/Lin-2 Drosophila homologue, Camguk, could play a role in epithelial patterning and in neuronal targeting

Drosophila Camguk (Cmg) is a member of the CAMGUK subfamily of the MAGUK family of proteins which are localized at cell junction and other plasma membrane specialized regions, from worms to mammals. The protein structure of Cmg, as the other CAMGUK proteins, is characterized by only one PDZ domain and an additional CaM kinase domain, similar to CaMKII. While the mammalian ortholog CASKs play an important role in synaptic protein targeting and in synaptic plasticity, the Drosophila Cmg role is unknown. To study its potential role, we reported a detailed analysis of mRNA distribution of the Drosophila cmg gene at cellular and developmental level, during embryonic, larval, pupal and adult stages. The transient cmg transcription in midgut and Malpighian tubules may suggest a potential function in cell junction formation and in epithelial tissue patterning. Interestingly, cmg transcription increases substantially during embryonic neuroblast proliferation, becoming predominant in the developing central nervous system (CNS) during embryonic and postembryonic development stages and in the mature brain. In addition, a high transcriptional level was detected in the eye imaginal discs and in the adult retina, demonstrating a specific and continuous expression of cmg in neuroblasts and photoreceptor neurons, from the onset of cytodifferentiation. Our findings suggest that Cmg could play a potential role in transmembrane protein targeting, particularly in synapses. These observations suggest the existence of a common highly conserved mechanism involved in forming and maintaining proper synaptic protein targeting, which are fundamental features of synaptic plasticity, learning and memory. Through its function, the CaM kinase domain-containing Cmg may be involved in signal transduction cascade. Its potential relation to Calmodulin and CaMKII is discussed.

Cloning and expression of three K+ channel cDNAs from Xenopus muscle

Embryonic Xenopus muscle cells grown in culture express voltage-gated K+ currents with inactivating and non-inactivating kinetics. Here we report the cloning of three K+ channel cDNAs, designated XKv1.2', XKv1.4 and XKv1.10, from muscle which may underlie these currents. XKv1.2' cDNA appears to be an allelic variant of the XKv1.2 previously cloned from Xenopus. The second cDNA encodes a homologue of Kv1.4 that has not been previously cloned from Xenopus. The predicted XKv1.4 protein shows 73% overall similarity to mouse and chick Kv1.4, but shows significant divergence in the region corresponding to the chain of the inactivating 'ball and chain' domain. The third K+ channel cDNA isolated from Xenopus muscle is a novel Kv1 isoform designated XKv1.10. The predicted protein shares about 70% similarity with other members of the Kv1 subfamily, and about 40% with members of the Kv2, Kv3 and Kv4 subfamilies. XKv1.4 mRNA appears as early as stage 10.5 in whole embryos and is prominent in muscle throughout development from stage 14 to adult. XKv1.2' mRNA is detected by stage 11.5 in whole embryos, but remains at low levels in embryonic skeletal muscle (stages 14 and 21), and is absent from adult muscle. XKv1.10 mRNA is first detected at stage 21 in whole embryos, and is present in muscle from this stage onwards. All three transcripts are present in spinal cord at stage 21. The results support the notion that channels encoded by XKv1.4 contribute to the inactivating K+ current observed in embryonic muscle cells in culture.

Erbin is a protein concentrated at postsynaptic membranes that interacts with PSD-95

Neuregulin is a factor essential for synapse-specific transcription of acetylcholine receptor genes at the neuromuscular junction. Its receptors, ErbB receptor tyrosine kinases, are localized at the postjunctional membrane presumably to ensure localized signaling. However, the molecular mechanisms underlying synaptic localization of ErbBs are unknown. Our recent studies indicate that ErbB4 interacts with postsynaptic density (PSD)-95 (SAP90), a PDZ domain-containing protein that does not interact with ErbB2 or ErbB3. Using as bait the ErbB2 C terminus, we identified Erbin, another PDZ domain-containing protein that interacts specifically with ErbB2. Erbin is concentrated in postsynaptic membranes at the neuromuscular junction and in the central nervous system, where ErbB2 is concentrated. Expression of Erbin increases the amount of ErbB2 labeled by biotin in transfected cells, suggesting that Erbin is able to increase ErbB2 surface expression. Furthermore, we provide evidence that Erbin interacts with PSD-95 in both transfected cells and synaptosomes. Thus ErbB proteins can interact with a network of PDZ domain-containing proteins. This interaction may play an important role in regulation of neuregulin signaling and/or subcellular localization of ErbB proteins.

Nitric oxide synthase (NOS-1) coclustered with agrin-induced AChR-specializations on cultured skeletal myotubes

Previously we reported that neuronal nitric oxide synthase type-1 (NOS-1) is expressed in skeletal myotubes in vitro. In the present paper we sought to determine whether agrin-induced membrane specializations known to include the nicotinic acetylcholine receptor (AChR) on cultured myotubes may also contain NOS-1 and related molecules. After treatment with various agrin constructs containing the full C-terminally AChR-clustering domain (fragments N2, N4), but not with fragment C2 (truncated), NOS-1 expressed in the cytosol of mouse C2C12 skeletal myotubes coclustered with AChR, 43K rapsyn, MuSK, and the dystrophin/utrophin glycoprotein-complex (DUGC). Agrin-induced specializations also included coaggregates of N-methyl-d-aspartic acid (NMDA)-receptor, alpha-sodium (NaCh), or Shaker-type K+ channel (KCh)/PSD-95 complexes, and NOS-1. We conclude that agrin is crucial for recruitment of preassembled multimolecular membrane clusters, including AChR, NMDAR, and ion channels linked to NOS-1. Coassembly of NOS-1 to postsynaptic molecules may reflect site-specific NO-signaling pathways in neuromuscular junction formation and functions.

Human homologue of the Drosophila discs large tumor suppressor protein forms an oligomer in solution. Identification of the self-association site

The human homologue of the Drosophila discs large tumor suppressor protein (hDlg), a member of the membrane-associated guanylate kinase (MAGUK) superfamily, interacts with K(+) channels, N-methyl-d-aspartate receptors, calcium ATPase, adenomatous polyposis coli, and PTEN tumor suppressor proteins, and several viral oncoproteins through its PDZ domains. MAGUKs play pivotal roles in the clustering and aggregation of receptors, ion channels, and cell adhesion molecules at the synapses. To investigate the physiological basis of hDlg interactions, we examined the self-association state of full-length hDlg as well as defined segments of hDlg expressed as recombinant proteins in bacteria and insect Sf9 cells. Gel permeation chromatography of full-length hDlg revealed that the purified protein migrates as a large particle of size >440 kDa. Similar measurements of defined domains of hDlg indicated that the anomalous mobility of hDlg originated from its amino-terminal domain. Ultrastructural analysis of hDlg by low angle rotary shadow electron microscopy revealed that the full-length hDlg protein as well as its amino-terminal domain exhibits a highly flexible irregular shape. Further evaluation of the self-association state of hDlg using sedimentation equilibrium centrifugation, matrix-assisted laser desorption/ionization mass spectrometry, and chemical cross-linking techniques confirmed that the oligomerization site of hDlg is contained within its amino-terminal domain. This unique amino-terminal domain mediates multimerization of hDlg into dimeric and tetrameric species in solution. Sedimentation velocity experiments demonstrated that the oligomerization domain exists as an elongated tetramer in solution. In vitro mutagenesis was used to demonstrate that a single cysteine residue present in the oligomerization domain of hDlg is not required for its self-association. Understanding the oligomerization status of hDlg may help to explicate the mechanism of hDlg association with multimeric K(+) channels and dimeric adenomatous polyposis coli tumor suppressor protein. Our findings, therefore, begin to rationalize the role of hDlg in the clustering of membrane channels and formation of multiprotein complexes necessary for signaling and cell proliferation pathways.

Brain dystrophin, neurogenetics and mental retardation

Duchenne muscular dystrophy (DMD) and the allelic disorder Becker muscular dystrophy (BMD) are common X-linked recessive neuromuscular disorders that are associated with a spectrum of genetically based developmental cognitive and behavioral disabilities. Seven promoters scattered throughout the huge DMD/BMD gene locus normally code for distinct isoforms of the gene product, dystrophin, that exhibit nervous system developmental, regional and cell-type specificity. Dystrophin is a complex plasmalemmal-cytoskeletal linker protein that possesses multiple functional domains, autosomal and X-linked homologs and associated binding proteins that form multiunit signaling complexes whose composition is unique to each cellular and developmental context. Through additional interactions with a variety of proteins of the extracellular matrix, plasma membrane, cytoskeleton and distinct intracellular compartments, brain dystrophin acquires the capability to participate in the modulatory actions of a large number of cellular signaling pathways. During neural development, dystrophin is expressed within the neural tube and selected areas of the embryonic and postnatal neuraxis, and may regulate distinct aspects of neurogenesis, neuronal migration and cellular differentiation. By contrast, in the mature brain, dystrophin is preferentially expressed by specific regional neuronal subpopulations within proximal somadendritic microdomains associated with synaptic terminal membranes. Increasing experimental evidence suggests that in adult life, dystrophin normally modulates synaptic terminal integrity, distinct forms of synaptic plasticity and regional cellular signal integration. At a systems level, dystrophin may regulate essential components of an integrated sensorimotor attentional network. Dystrophin deficiency in DMD/BMD patients and in the mdx mouse model appears to impair intracellular calcium homeostasis and to disrupt multiple protein-protein interactions that normally promote information transfer and signal integration from the extracellular environment to the nucleus within regulated microdomains. In DMD/BMD, the individual profiles of cognitive and behavioral deficits, mental retardation and other phenotypic variations appear to depend on complex profiles of transcriptional regulation associated with individual dystrophin mutations that result in the corresponding presence or absence of individual brain dystrophin isoforms that normally exhibit developmental, regional and cell-type-specific expression and functional regulation. This composite experimental model will allow fine-level mapping of cognitive-neurogenetic associations that encompass the interrelationships between molecular, cellular and systems levels of signal integration, and will further our understanding of complex gene-environmental interactions and the pathogenetic basis of developmental disorders associated with mental retardation.

Signaling pathways are focused at specialized regions of the plasma membrane by scaffolding proteins of the MAGUK family

The MAGUKs (membrane-associated guanylate kinase homologs) are a family of proteins that act as molecular scaffolds for signaling pathway components at the plasma membrane of animal cells. They are localized in and required for the formation of several types of cell junctions, including epithelial tight and septate junctions as well as synaptic and neuromuscular junctions. They are also localized at the plasma membrane of other cell types, including erythrocytes, where they contribute to cell shape maintenance. MAGUKs function mainly by binding directly to the cytoplasmic termini of transmembrane proteins as well as to other signal transduction proteins. They appear to hold together elements of individual signaling pathways, thereby contributing to the efficiency and specificity of signaling interactions while simultaneously maintaining the structural specializations of the plasma membrane. BioEssays 1999;21:912-921.