Characterization of densin-180, a new brain-specific synaptic protein of the O-sialoglycoprotein family

Functions of CNKSR2 and Its Association with Neurodevelopmental Disorders

The Connector Enhancer of Kinase Suppressor of Ras-2 (CNKSR2), also known as CNK2 or MAGUIN, is a scaffolding molecule that contains functional protein binding domains: Sterile Alpha Motif (SAM) domain, Conserved Region in CNK (CRIC) domain, PSD-95/Dlg-A/ZO-1 (PDZ) domain, Pleckstrin Homology (PH) domain, and C-terminal PDZ binding motif. CNKSR2 interacts with different molecules, including RAF1, ARHGAP39, and CYTH2, and regulates the Mitogen-Activated Protein Kinase (MAPK) cascade and small GTPase signaling. CNKSR2 has been reported to control the development of dendrite and dendritic spines in primary neurons. CNKSR2 is encoded by the CNKSR2 gene located in the X chromosome. CNKSR2 is now considered as a causative gene of the Houge type of X-linked syndromic mental retardation (MRXHG), an X-linked Intellectual Disability (XLID) that exhibits delayed development, intellectual disability, early-onset seizures, language delay, attention deficit, and hyperactivity. In this review, we summarized molecular features, neuronal function, and neurodevelopmental disorder-related variations of CNKSR2.

Neurological involvement in monogenic podocytopathies

Genetic studies of hereditary nephrotic syndrome (NS) have identified more than 50 genes that, if mutated, are responsible for monogenic forms of steroid-resistant NS (SRNS), either isolated or syndromic. Most of these genes encode proteins expressed in the podocyte with various functions such as transcription factors, mitochondrial proteins, or enzymes, but mainly structural proteins of the slit diaphragm (SD) as well as cytoskeletal binding and regulator proteins. Syndromic NS is sometimes associated with neurological features. Over recent decades, various studies have established links between the physiology of podocytes and neurons, both morphologically (slit diaphragm and synapse) and functionally (signaling platforms). Variants in genes expressed in different compartments of the podocyte and neurons are responsible for phenotypes associating kidney lesions with proteinuria (mainly Focal and Segmental Glomerulosclerosis (FSGS) or Diffuse Mesangial Sclerosis (DMS)) and central and/or peripheral neurological disorders. The Galloway-Mowat syndrome (GAMOS, OMIM#251300) associates neurological defects, microcephaly, and proteinuria and is caused by variants in genes encoding proteins of various functions (microtubule cytoskeleton regulation (WDR73), regulation of protein synthesis via transfer RNAs (KEOPS and WDR4 complexes)). Pierson syndrome (OMIM#609049) associating congenital nephrotic syndrome and central neurological and ophthalmological anomalies is secondary to variants in LAMB2, involved in glomerular and ocular basement membranes. Finally, Charcot-Marie-Tooth-FSGS (OMIM#614455) combines peripheral sensory-motor neuropathy and proteinuria and arises from INF2 variants, resulting in cytoskeletal polymerization defects. This review focuses on genetic syndromes associating nephrotic range proteinuria and neurological involvement and provides the latest advances in the description of these neuro-renal disorders.

Role of densin-180 in mouse ventral hippocampal neurons in 24-hr retention of contextual fear conditioning

INTRODUCTION

Densin-180 interacts with postsynaptic molecules including calcium/calmodulin-dependent protein kinase IIα (CaMKIIα) but its function in learning and memory process has been unclear.

METHODS

To investigate a role of hippocampal densin-180 in contextual fear conditioning (CFC) learning and memory processes, knockdown (KD) of densin-180 in hippocampal subareas was applied.

RESULTS

First, ventral hippocampal (vHC) densin-180 KD impaired single-trial CFC (stCFC) memory one day later. stCFC caused freezing behaviors to reach the peak about one hour later in both control and KD mice, but then freezing was disappeared at 2 hr postshock in KD mice. Second, stCFC caused an immediate and transient reduction of vHC densin-180 in control mice, which was not observed in KD mice. Third, stCFC caused phosphorylated-T286 (p-T286) CaMKIIα to change similarly to densin-180, but p-T305 CaMKIIα was increased 1 hr later in control mice. In KD mice, these effects were gone. Moreover, both basal levels of p-T286 and p-T305 CaMKIIα were reduced without change in total CaMKIIα in KD mice. Fourth, we found double-trial CFC (dtCFC) memory acquisition and retrieval kinetics were different from those of stCFC in vHC KD mice. In addition, densin-180 in dorsal hippocampal area appeared to play its unique role during the very early retrieval period of both CFC memories.

CONCLUSION

This study shows that vHC densin-180 is necessary for stCFC memory formation and retrieval and suggests that both densin-180 and p-T305 CaMKIIα at 1 ~ 2 hr postshock are important for stCFC memory formation. We conclude that roles of hippocampal neuronal densin-180 in CFC are temporally dynamic and differential depending on the pattern of conditioning stimuli and its location along the dorsoventral axis of hippocampal formation.

Molecular architecture of postsynaptic Interactomes

The postsynaptic density (PSD) plays an essential role in the organization of the synaptic signaling machinery. It contains a set of core scaffolding proteins that provide the backbone to PSD protein-protein interaction networks (PINs). These core scaffolding proteins can be seen as three principal layers classified by protein family, with DLG proteins being at the top, SHANKs along the bottom, and DLGAPs connecting the two layers. Early studies utilizing yeast two hybrid enabled the identification of direct protein-protein interactions (PPIs) within the multiple layers of scaffolding proteins. More recently, mass-spectrometry has allowed the characterization of whole interactomes within the PSD. This expansion of knowledge has further solidified the centrality of core scaffolding family members within synaptic PINs and provided context for their role in neuronal development and synaptic function. Here, we discuss the scaffolding machinery of the PSD, their essential functions in the organization of synaptic PINs, along with their relationship to neuronal processes found to be impaired in complex brain disorders.

Interactions with PDZ proteins diversify voltage-gated calcium channel signaling

Voltage-gated Ca²⁺ (Ca_V ) channels are crucial for neuronal excitability and synaptic transmission upon depolarization. Their properties in vivo are modulated by their interaction with a variety of scaffolding proteins. Such interactions can influence the function and localization of Ca_V channels, as well as their coupling to intracellular second messengers and regulatory pathways, thus amplifying their signaling potential. Among these scaffolding proteins, a subset of PDZ (postsynaptic density-95, Drosophila discs-large, and zona occludens)-domain containing proteins play diverse roles in modulating Ca_V channel properties. At the presynaptic terminal, PDZ proteins enrich Ca_V channels in the active zone, enabling neurotransmitter release by maintaining a tight and vital link between channels and vesicles. In the postsynaptic density, these interactions are essential in regulating dendritic spine morphology and postsynaptic signaling cascades. In this review, we highlight the studies that demonstrate dynamic regulations of neuronal Ca_V channels by PDZ proteins. We discuss the role of PDZ proteins in controlling channel activity, regulating channel cell surface density, and influencing channel-mediated downstream signaling events. We highlight the importance of PDZ protein regulations of Ca_V channels and evaluate the link between this regulatory effect and human disease.

The Scribble family in cancer: twentieth anniversary

Among the more than 160 PDZ containing proteins described in humans, the cytoplasmic scaffold Scribble stands out because of its essential role in many steps of cancer development and dissemination. Its fame has somehow blurred the importance of homologous proteins, Erbin and Lano, all belonging to the LRR and PDZ (LAP) protein family first described twenty years ago. In this review, we will retrace the history of LAP family protein research and draw attention to their contribution in cancer by detailing the features of its members at the structural and functional levels, and highlighting their shared-but also different-implication in the tumoral process.

Lrrc7 mutant mice model developmental emotional dysregulation that can be alleviated by mGluR5 allosteric modulation

LRRC7 has been identified as a candidate gene for severe childhood emotional dysregulation. Direct experimental evidence for a role of LRRC7 in the disease is needed, as is a better understanding of its impact on neuronal structure and signaling, and hence potential treatment targets. Here, we generated and analyzed an Lrrc7 mutant mouse line. Consistent with a critical role of LRRC7 in emotional regulation, mutant mice had inappropriate juvenile aggressive behavior and significant anxiety-like behavior and social dysfunction in adulthood. The pivotal role of mGluR5 signaling was demonstrated by rescue of behavioral defects with augmentation of mGluR5 receptor activity by 3-Cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB). Intra-peritoneal injection of CDPPB alleviated abnormal juvenile behavior, as well as anxiety-like behavior and hypersociability at adulthood. Furthermore, mutant primary neurons had impaired neurite outgrowth which was rescued by CDPPB treatment. In conclusion, Lrrc7 mutant mice provide a valuable tool to model childhood emotional dysregulation and persistent mental health comorbidities. Moreover, our data highlight an important role of LRRC7 in mGluR5 signaling, which is a potential new treatment target for anxiety and social dysfunction.

Distribution of densin in neurons

Densin is a scaffold protein known to associate with key elements of neuronal signaling. The present study examines the distribution of densin at the ultrastructural level in order to reveal potential sites that can support specific interactions of densin. Immunogold electron microscopy on hippocampal cultures shows intense labeling for densin at postsynaptic densities (PSDs), but also some labeling at extrasynaptic plasma membranes of soma and dendrites and endoplasmic reticulum. At the PSD, the median distance of label from the postsynaptic membrane was ~27 nm, with the majority of label (90%) confined within 40 nm from the postsynaptic membrane, indicating predominant localization of densin at the PSD core. Depolarization (90 mM K⁺ for 2 min) promoted a slight shift of densin label within the PSD complex resulting in 77% of label remaining within 40 nm from the postsynaptic membrane. Densin molecules firmly embedded within the PSD may target a minor pool of CaMKII to substrates at the PSD core.

Regulation of cellular and PCP signalling by the Scribble polarity module

Since the first identification of the Scribble polarity module proteins as a new class of tumour suppressors that regulate both cell polarity and proliferation, an increasing amount of evidence has uncovered a broader role for Scribble, Dlg and Lgl in the control of fundamental cellular functions and their signalling pathways. Here, we review these findings as well as discuss more specifically the role of the Scribble module in PCP signalling.

Exome Pool-Seq in neurodevelopmental disorders

High throughput sequencing has greatly advanced disease gene identification, especially in heterogeneous entities. Despite falling costs this is still an expensive and laborious technique, particularly when studying large cohorts. To address this problem we applied Exome Pool-Seq as an economic and fast screening technology in neurodevelopmental disorders (NDDs). Sequencing of 96 individuals can be performed in eight pools of 12 samples on less than one Illumina sequencer lane. In a pilot study with 96 cases we identified 27 variants, likely or possibly affecting function. Twenty five of these were identified in 923 established NDD genes (based on SysID database, status November 2016) (ACTB, AHDC1, ANKRD11, ATP6V1B2, ATRX, CASK, CHD8, GNAS, IFIH1, KCNQ2, KMT2A, KRAS, MAOA, MED12, MED13L, RIT1, SETD5, SIN3A, TCF4, TRAPPC11, TUBA1A, WAC, ZBTB18, ZMYND11), two in 543 (SysID) candidate genes (ZNF292, BPTF), and additionally a de novo loss-of-function variant in LRRC7, not previously implicated in NDDs. Most of them were confirmed to be de novo, but we also identified X-linked or autosomal-dominantly or autosomal-recessively inherited variants. With a detection rate of 28%, Exome Pool-Seq achieves comparable results to individual exome analyses but reduces costs by >85%. Compared with other large scale approaches using Molecular Inversion Probes (MIP) or gene panels, it allows flexible re-analysis of data. Exome Pool-Seq is thus well suited for large-scale, cost-efficient and flexible screening in characterized but heterogeneous entities like NDDs.

LAP proteins are localized at the post-synaptic membrane of neuromuscular junctions and appear to modulate synaptic morphology and transmission

Erbin, Lano, Scribble, and Densin-180 belong to LAP (leucine-rich repeats and PDZ domain) adaptor proteins involved in cell signaling pathways. Previously, we identified Erbin, Lano, and Scribble, but not Densin-180, in muscle cells, where they are involved in regulating the aggregation of nicotinic acetylcholine receptors in vitro. Here, we analyzed their cellular localization at the neuromuscular junction (NMJ) in skeletal muscles of mice. Erbin, Lano, and Scribble were significantly accumulated at NMJs and localized in different synaptic cells. Moreover, we used mouse mutants to analyze the role of Erbin at the NMJ. We used two Erbin mutant mouse strains that either completely lack Erbin protein (Erbin^null/null ) or express a truncated Erbin mutant where the carboxy-terminal PDZ domain is replaced by β-galactosidase (Erbin^ΔC/ΔC ) thereby abolishing its interaction with ErbB receptor tyrosine kinases. Neither the lack of the PDZ domain of Erbin, nor its complete absence interfered with the general localization of LAP proteins at NMJs, but Lano and Scribble transcript levels were up-regulated in homozygous Erbin-null muscles. Furthermore, grip strength was reduced and neural transmission impaired in homozygous aged Erbin-null but not Erbin-ΔC mice. Erbin-null skeletal muscles did not reveal any conspicuous impairment of the muscle fiber. Localization of other NMJ marker proteins was not affected either. Quantitative 3D morphometry showed that NMJs of Erbin-null muscles were significantly smaller and fragmented in the soleus. We speculate that Erbin, Lano, and Scribble act at the post-synaptic membrane of NMJs in a concerted fashion to regulate nicotinic acetylcholine receptors cluster morphology and neural transmission. Cover Image for this issue: doi: 10.1111/jnc.13340.

Analysis of Gene Expression Profiles in the Human Brain Stem, Cerebellum and Cerebral Cortex

The human brain is one of the most mysterious tissues in the body. Our knowledge of the human brain is limited due to the complexity of its structure and the microscopic nature of connections between brain regions and other tissues in the body. In this study, we analyzed the gene expression profiles of three brain regions-the brain stem, cerebellum and cerebral cortex-to identify genes that are differentially expressed among these different brain regions in humans and to obtain a list of robust, region-specific, differentially expressed genes by comparing the expression signatures from different individuals. Feature selection methods, specifically minimum redundancy maximum relevance and incremental feature selection, were employed to analyze the gene expression profiles. Sequential minimal optimization, a machine-learning algorithm, was employed to examine the utility of selected genes. We also performed a literature search, and we discuss the experimental evidence for the important physiological functions of several highly ranked genes, including NR2E1, DAO, and LRRC7, and we give our analyses on a gene (TFAP2B) that have not been investigated or experimentally validated. As a whole, the results of our study will improve our ability to predict and understand genes related to brain regionalization and function.

Systematic family-wide analysis of sodium bicarbonate cotransporter NBCn1/SLC4A7 interactions with PDZ scaffold proteins

NBCn1 (SLC4A7) plays a role in transepithelial HCO3 (-) movement and intracellular pH maintenance in many tissues. In this study, we searched PDZ proteins capable of binding to NBCn1. We screened a protein array membrane, on which 96 different class I PDZ protein peptides were blotted, with the C-terminal domain of NBCn1 fused to GST. Thirteen proteins were identified in these screens: MAGI-3, NHERF-1, NHERF-2, PSD-95, chapsyn-110, ERBIN, MALS-1, densin-180, syntrophins α1, β2, γ2, MUPP1, and PDZK1. After determining these binding partners, we analyzed the database of known and predicted protein interactions to obtain an NBCn1 interaction network. The network shows NBCn1 being physically and functionally associated with a variety of membrane and cytosolic proteins via the binding partners. We then focused on syntrophin γ2 to examine the molecular and functional interaction between NBCn1 and one of the identified binding partners in the Xenopus oocyte expression system. GST/NBCn1 pulled down syntrophin γ2 and conversely GST/syntrophin γ2 pulled down NBCn1. Moreover, syntrophin γ2 increased intracellular pH recovery, from acidification, mediated by NBCn1's Na/HCO3 cotransport. Syntrophin γ2 also increased an ionic conductance produced by NBCn1 channel-like activity. Thus, syntrophin γ2 regulates NBCn1 activity. In conclusion, this study demonstrates that NBCn1 binds to many PDZ proteins, which in turn may allow the transporter to associate with other physiologically important proteins.

Brain-specific angiogenesis inhibitor-1 signaling, regulation, and enrichment in the postsynaptic density

Brain-specific angiogenesis inhibitor-1 (BAI1) is an adhesion G protein-coupled receptor that has been studied primarily for its anti-angiogenic and anti-tumorigenic properties. We found that overexpression of BAI1 results in activation of the Rho pathway via a Gα(12/13)-dependent mechanism, with truncation of the BAI1 N terminus resulting in a dramatic enhancement in receptor signaling. This constitutive activity of the truncated BAI1 mutant also resulted in enhanced downstream phosphorylation of ERK as well as increased receptor association with β-arrestin2 and increased ubiquitination of the receptor. To gain insights into the regulation of BAI1 signaling, we screened the C terminus of BAI1 against a proteomic array of PDZ domains to identify novel interacting partners. These screens revealed that the BAI1 C terminus interacts with a variety of PDZ domains from synaptic proteins, including MAGI-3. Removal of the BAI1 PDZ-binding motif resulted in attenuation of receptor signaling to Rho but had no effect on ERK activation. Conversely, co-expression with MAGI-3 was found to potentiate signaling to ERK by constitutively active BAI1 in a manner that was dependent on the PDZ-binding motif of the receptor. Biochemical fractionation studies revealed that BAI1 is highly enriched in post-synaptic density fractions, a finding consistent with our observations that BAI1 can interact with PDZ proteins known to be concentrated in the post-synaptic density. These findings demonstrate that BAI1 is a synaptic receptor that can activate both the Rho and ERK pathways, with the N-terminal and C-terminal regions of the receptor playing key roles in the regulation of BAI1 signaling activity.

Deficiency of Erbin induces resistance of cervical cancer cells to anoikis in a STAT3-dependent manner

Epithelial cell polarization and integration are essential to their function and loss of epithelial polarity and tissue architecture correlates with the development of aggressive tumors. Erbin is a basolateral membrane-associated protein. The roles of Erbin in establishing cell polarization and regulating cell adhesion have been suggested. Erbin is also a negative regulator in Ras-Raf-ERK (extracellular signal-regulated kinase) signaling pathway. However, the potential functions of Erbin in human cancer are basically unknown. In the present study, we show, for the first time, that loss of Erbin endows cervical cancer cells with resistance to anoikis both in vitro and in vivo and promotes the growth and metastasis of human cervical cancer xenografts in nude mice. We found that knockdown of Erbin induced the phosphorylation, nuclear translocation and transcriptional activities of signal transducer and activator of transcription factor 3 (STAT3) in cervical cancer cells. Overexpression of STAT3C or induction of endogenous STAT3 activation by interleukin (IL)-6 evidently inhibited anoikis of cervical cancer cells, whereas WP1066, a potent inhibitor of Janus-activated kinase 2 (Jak2)/STAT3, effectively blocked the effect of Erbin knockdown on cell survival under anchorage-independent conditions, indicating that loss of Erbin confers resistance of cervical cancer cells to anoikis in a STAT3-dependent manner. Interestingly, IL-6 induced STAT3 activation and Erbin expression simultaneously. Overexpression of STAT3C also significantly upregulated the level of Erbin, whereas the Jak2 inhibitor AG490 remarkably blocked not only STAT3 phosphorylation but also IL-6-induced Erbin expression. Knockdown of Erbin augmented the effects of IL-6 on STAT3 activation and anoikis resistance. In addition, by immunohistochemical analysis of Erbin expression, we demonstrate that the expression of Erbin is significantly decreased or even lost in cervical cancer tissues. These data reveal that Erbin is a novel negative regulator of STAT3, and the IL-6/STAT3/Erbin loop has a crucial role in cervical cancer progression and metastasis.

Polarity protein complex Scribble/Lgl/Dlg and epithelial cell barriers

The Scribble polarity complex or module is one of the three polarity modules that regulate cell polarity in multiple epithelia including blood-tissue barriers. This protein complex is composed of Scribble, Lethal giant larvae (Lgl) and Discs large (Dlg), which are well conserved across species from fruitflies and worms to mammals. Originally identified in Drosophila and C. elegans where the Scribble complex was found to work with the Par-based and Crumbs-based polarity modules to regulate apicobasal polarity and asymmetry in cells and tissues during embryogenesis, their mammalian homologs have all been identified in recent years. Components of the Scribble complex are known to regulate multiple cellular functions besides cell polarity, which include cell proliferation, assembly and maintenance of adherens junction (AJ) and tight junction (TJ), and they are also tumor suppressors. Herein, we provide an update on the Scribble polarity complex and how this protein complex modulates cell adhesion with some emphasis on its role in Sertoli cell blood-testis barrier (BTB) function. It should be noted that this is a rapidly developing field, in particular the role of this protein module in blood-tissue barriers, and this short chapter attempts to provide the information necessary for investigators studying reproductive biology and blood-tissue barriers to design future studies. We also include results of recent studies from flies and worms since this information will be helpful in planning experiments for future functional studies in the testis to understand how Scribble-based proteins regulate BTB dynamics and spermatogenesis.

Dynamic expression of the LAP family of genes during early development of Xenopus tropicalis

The leucine-rich repeats and PDZ (LAP) family of genes are crucial for the maintenance of cell polarity as well as for epithelial homeostasis and tumor suppression in both vertebrates and invertebrates. Four members of this gene family are known: densin, erbin, scribble and lano. Here, we identified the four members of the LAP gene family in Xenopus tropicalis and studied their expression patterns during embryonic development. The Xenopus LAP proteins show a conserved domain structure that is similar to their homologs in other vertebrates. In Xenopus embryos, these genes were detected in animal cap cells at the early gastrula stage. At later stages of development, they were widely expressed in epithelial tissues that are highly polar in nature, including the neural epithelia, optic and otic vesicles, and in the pronephros. These data suggest that the roles of the Xenopus LAP genes in the control of cell polarity and morphogenesis are conserved during early development. Erbin and lano show similar expression patterns in the developing head, suggesting potential functional interactions between the two molecules in vivo.

Genome-wide association study of the child behavior checklist dysregulation profile

OBJECTIVE

A potentially useful tool for understanding the distribution and determinants of emotional dysregulation in children is a Child Behavior Checklist profile, comprising the Attention Problems, Anxious/Depressed, and Aggressive Behavior clinical subscales (CBCL-DP). The CBCL-DP indexes a heritable trait that increases susceptibility for later psychopathology, including severe mood problems and aggressive behavior. We have conducted a genome-wide association study of the CBCL-DP in children with attention-deficit/hyperactivity disorder (ADHD).

METHOD

Families were ascertained at Massachusetts General Hospital and University of California, Los Angeles. Genotyping was conducted with the Illumina Human1M or Human1M-Duo BeadChip platforms. Genome-wide association analyses were conducted with the MQFAM multivariate extension of PLINK.

RESULTS

CBCL data were available for 341 ADHD offspring from 339 ADHD affected trio families from the UCLA (N = 128) and the MGH (N = 213) sites. We found no genome-wide statistically significant associations but identified several plausible candidate genes among findings at p < 5E-05: TMEM132D, LRRC7, SEMA3A, ALK, and STIP1.

CONCLUSIONS

We found suggestive evidence for developmentally expressed genes operant in hippocampal dependent memory and learning with the CBCL-DP.

Characterization of a central Ca2+/calmodulin-dependent protein kinase IIalpha/beta binding domain in densin that selectively modulates glutamate receptor subunit phosphorylation

The densin C-terminal domain can target Ca(2+)/calmodulin-dependent protein kinase IIα (CaMKIIα) in cells. Although the C-terminal domain selectively binds CaMKIIα in vitro, full-length densin associates with CaMKIIα or CaMKIIβ in brain extracts and in transfected HEK293 cells. This interaction requires a second central CaMKII binding site, the densin-IN domain, and an "open" activated CaMKII conformation caused by Ca(2+)/calmodulin binding, autophosphorylation at Thr-286/287, or mutation of Thr-286/287 to Asp. Mutations in the densin-IN domain (L815E) or in the CaMKIIα/β catalytic domain (I205/206K) disrupt the interaction. The amino acid sequence of the densin-IN domain is similar to the CaMKII inhibitor protein, CaMKIIN, and a CaMKIIN peptide competitively blocks CaMKII binding to densin. CaMKII is inhibited by both CaMKIIN and the densin-IN domain, but the inhibition by densin is substrate-selective. Phosphorylation of a model peptide substrate, syntide-2, or of Ser-831 in AMPA receptor GluA1 subunits is fully inhibited by densin. However, CaMKII phosphorylation of Ser-1303 in NMDA receptor GluN2B subunits is not effectively inhibited by densin in vitro or in intact cells. Thus, densin can target multiple CaMKII isoforms to differentially modulate phosphorylation of physiologically relevant downstream targets.

Trans-synaptic adhesions between netrin-G ligand-3 (NGL-3) and receptor tyrosine phosphatases LAR, protein-tyrosine phosphatase delta (PTPdelta), and PTPsigma via specific domains regulate excitatory synapse formation

Synaptic cell adhesion molecules regulate various steps of synapse formation. The trans-synaptic adhesion between postsynaptic NGL-3 (for netrin-G ligand-3) and presynaptic LAR (for leukocyte antigen-related) regulates excitatory synapse formation in a bidirectional manner. However, little is known about the molecular details of the NGL-3-LAR adhesion and whether two additional LAR family proteins, protein-tyrosine phosphatase delta (PTPdelta), and PTPsigma, also interact with NGL-3 and are involved in synapse formation. We report here that the leucine-rich repeat (LRR) domain of NGL-3, containing nine LRRs, interacts with the first two fibronectin III (FNIII) domains of LAR to induce bidirectional synapse formation. Moreover, Gln-96 in the first LRR motif of NGL-3 is critical for LAR binding and induction of presynaptic differentiation. PTPdelta and PTPsigma also interact with NGL-3 via their first two FNIII domains. These two interactions promote synapse formation in a different manner; the PTPsigma-NGL-3 interaction promotes synapse formation in a bidirectional manner, whereas the PTPdelta-NGL-3 interaction instructs only presynaptic differentiation in a unidirectional manner. mRNAs encoding LAR family proteins display overlapping and differential expression patterns in various brain regions. These results suggest that trans-synaptic adhesion between NGL-3 and the three LAR family proteins regulates excitatory synapse formation in shared and distinct neural circuits.

Identification and validation of novel spinophilin-associated proteins in rodent striatum using an enhanced ex vivo shotgun proteomics approach

Spinophilin regulates excitatory postsynaptic function and morphology during development by virtue of its interactions with filamentous actin, protein phosphatase 1, and a plethora of additional signaling proteins. To provide insight into the roles of spinophilin in mature brain, we characterized the spinophilin interactome in subcellular fractions solubilized from adult rodent striatum by using a shotgun proteomics approach to identify proteins in spinophilin immune complexes. Initial analyses of samples generated using a mouse spinophilin antibody detected 23 proteins that were not present in an IgG control sample; however, 12 of these proteins were detected in complexes isolated from spinophilin knock-out tissue. A second screen using two different spinophilin antibodies and either knock-out or IgG controls identified a total of 125 proteins. The probability of each protein being specifically associated with spinophilin in each sample was calculated, and proteins were ranked according to a chi(2) analysis of the probabilities from analyses of multiple samples. Spinophilin and the known associated proteins neurabin and multiple isoforms of protein phosphatase 1 were specifically detected. Multiple, novel, spinophilin-associated proteins (myosin Va, calcium/calmodulin-dependent protein kinase II, neurofilament light polypeptide, postsynaptic density 95, alpha-actinin, and densin) were then shown to interact with GST fusion proteins containing fragments of spinophilin. Additional biochemical and transfected cell imaging studies showed that alpha-actinin and densin directly interact with residues 151-300 and 446-817, respectively, of spinophilin. Taken together, we have developed a multi-antibody, shotgun proteomics approach to characterize protein interactomes in native tissues, delineating the importance of knock-out tissue controls and providing novel insights into the nature and function of the spinophilin interactome in mature striatum.

Tax1 enhances cancer cell proliferation via Ras-Raf-MEK-ERK signaling pathway

Erbin is an ErbB2 binding protein, which belongs to the LAP (leucine-rich repeat (LRR) and PDZ domain) protein family. We previously reported that Tax1, a protein of the human T-cell leukemia virus type I (HTLV-I), associated with Erbin by using Erbin PDZ domain as a bait to screen a human T lymphocyte cDNA library by a yeast two hybrid strategy. In the present study, we demonstrated that Tax1 enhances cancer cell proliferation via Ras-Raf-MEK-ERK signaling pathway by using molecular section strategy. The pull-down assay showed that the four amino acid domain, that is, Tax1 350-353, might specifically interact with Erbin, but not any other Tax1 deletion mutants. The coimmunoprecipitation assay confirmed that Tax1 350-353 domain bound with Erbin in vivo. Functional study demonstrated that overexpression of Tax1 in cancer cell lines of liver cancer SMMC-7721, colon cancer HCT-116, and breast cancer MCF-7 facilitated the cell proliferation. And the transfection of Tax1 353 in MCF-7 cells with endogenous Erbin expression markedly increased phosphorylation of Ras, Raf, MEK1/2, ERK1/2, PI3K, and IkappaBalpha, suggesting that Tax1-enhanced cell proliferation tracks Ras-Raf-MEK-ERK signaling pathway.

Densin-180: revised membrane topology, domain structure and phosphorylation status

Densin-180 is a core component of post-synaptic densities, the highly complex molecular assemblies that mediate signaling between neuronal cells. It is a multi-domain scaffold protein characterized by multiple leucine-rich repeat domains plus a single Psd95/Discs large/Zona occludens-1 domain. In its original topology model a single transmembrane segment was proposed with an extracellular N-terminus and an intracellular C-terminus. However, recently discovered in vivo phosphorylation sites are incompatible with this topology. Here, we discuss an all-intracellular and membrane-associated localization of Densin-180 that is consistent with and supported by all the latest experimental data. This revised topology which now includes also a phosphorylation-rich area will have deciding influence on future research involving Densin-180 and its signaling.

Developmentally regulated alternative splicing of densin modulates protein-protein interaction and subcellular localization

Densin is a member of the leucine-rich repeat (LRR) and PDZ domain (LAP) protein family that binds several signaling molecules via its C-terminal domains, including calcium/calmodulin-dependent protein kinase II (CaMKII). In this study, we identify several novel mRNA splice variants of densin that are differentially expressed during development. The novel variants share the LRR domain but are either prematurely truncated or contain internal deletions relative to mature variants of the protein (180 kDa), thus removing key protein-protein interaction domains. For example, CaMKIIalpha coimmunoprecipitates with densin splice variants containing an intact C-terminal domain from lysates of transfected HEK293 cells, but not with variants that only contain N-terminal domains. Immunoblot analyses using antibodies to peptide epitopes in the N- and C- terminal domains of densin are consistent with developmental regulation of splice variant expression in brain. Moreover, putative splice variants display different subcellular fractionation patterns in brain extracts. Expression of green fluorescent protein (GFP)-fused densin splice variants in HEK293 cells shows that the LRR domain can target densin to a plasma membrane-associated compartment, but that the splice variants are differentially localized and have potentially distinct effects on cell morphology. In combination, these data show that densin splice variants have distinct functional characteristics suggesting multiple roles during neuronal development.

Hepatocyte growth factor-induced enhancement of dendritic branching is blocked by inhibitors of N-methyl-D-aspartate receptors and calcium/calmodulin-dependent kinases

Hepatocyte growth factor (HGF) and its receptor, Met, are clustered at excitatory synapses and can enhance N-methyl-D-aspartate (NMDA) receptor current and promote formation of neurites and dendrites. In this study, we examine the effects of HGF on dendritic arborization in mature cultures of dissociated hippocampal neurons. Exogenous HGF treatment caused a dose-dependent increase in total dendritic branch tip number, total dendritic branch length, and dendritic complexity in these neurons. NMDA receptor activity has been linked to changes in dendritic structure, so we tested the effects of HGF on the dendritic arbor in the presence of DL-2-amino-5-phosphonopentanoic acid (APV), an NMDA receptor inhibitor. APV blocked the HGF-induced enhancement of the dendritic arbor in a dose-dependent manner. Similarly, pretreatment of neurons with KN62, an inhibitor of calcium-dependent kinases, suppressed changes in dendritic branching induced by HGF. These results suggest that HGF initiates Ca2+-dependent processes, so we examined the effect of HGF on intracellular calcium levels and autophosphorylation of the calcium/calmodulin-dependent protein kinase II (CaMKII). HGF caused a persistent increase in fluorescence in clusters along dendrites of neurons preloaded with the Ca2+ indicator Fluo-4. HGF treatment also enhanced autophosphorylation of CaMKII. The increases in Fluo-4 fluorescence and autophosphorylation of CaMKII were blocked by pretreatment of neurons with APV. These results indicate that HGF stimulates Ca2+ influx into dendrites through the NMDA receptor and that this effect is necessary for the changes in dendritic morphology induced by HGF.

The postsynaptic architecture of excitatory synapses: a more quantitative view

Excitatory (glutamatergic) synapses in the mammalian brain are usually situated on dendritic spines, a postsynaptic microcompartment that also harbors organelles involved in protein synthesis, membrane trafficking, and calcium metabolism. The postsynaptic membrane contains a high concentration of glutamate receptors, associated signaling proteins, and cytoskeletal elements, all assembled by a variety of scaffold proteins into an organized structure called the postsynaptic density (PSD). A complex machine made of hundreds of distinct proteins, the PSD dynamically changes its structure and composition during development and in response to synaptic activity. The molecular size of the PSD and the stoichiometry of many major constituents have been recently measured. The structures of some intact PSD proteins, as well as the spatial arrangement of several proteins within the PSD, have been determined at low resolution by electron microscopy. On the basis of such studies, a more quantitative and geometrically realistic view of PSD architecture is emerging.

More than just synaptic building blocks: scaffolding proteins of the post-synaptic density regulate dendritic patterning

The dendritic arbor is responsible for receiving and consolidating neuronal input. Outgrowth and morphogenesis of the arbor are complex stages of development that are poorly understood. However, recent findings have identified synaptic scaffolding proteins as novel regulators of these important events. Scaffolding proteins are enriched in the post-synaptic density where they bind and bring into close proximity neurotransmitter receptors, signaling molecules, and regulators of the actin cytoskeleton. This property is important for dendritic spine morphogenesis and maintenance in the mature neuron. Scaffolding proteins are now being described as key regulators of neurite outgrowth, dendritic development, and pattern formation in immature neurons. These proteins, which include post-synaptic-95, Shank and Densin-180, as well as many of their interacting partners, appear to regulate both the microtubule and actin cytoskeleton to influence dendrite morphology. Through a large array of protein-protein interaction domains, scaffolding proteins are able to form large macromolecular complexes that include cytoskeletal motor proteins as well as microtubule and actin regulatory molecules. Together, the new findings form a persuasive argument that scaffolding proteins deliver critical regulatory elements to sites of dendritic outgrowth and branching to modulate the formation and maintenance of the dendritic arbor.

Actin up: regulation of podocyte structure and function by components of the actin cytoskeleton

Podocytes of the renal glomerulus are unique cells with a complex cellular organization consisting of a cell body, major processes and foot processes. Podocyte foot processes form a characteristic interdigitating pattern with foot processes of neighboring podocytes, leaving in between the filtration slits that are bridged by the glomerular slit diaphragm. The highly dynamic foot processes contain an actin-based contractile apparatus comparable to that of smooth muscle cells or pericytes. Mutations affecting several podocyte proteins lead to rearrangement of the actin cytoskeleton, disruption of the filtration barrier and subsequent renal disease. The fact that the dynamic regulation of the podocyte cytoskeleton is vital to kidney function has led to podocytes emerging as an excellent model system for studying actin cytoskeleton dynamics in a physiological context.

Erbin inhibits transforming growth factor beta signaling through a novel Smad-interacting domain

Smad proteins are critical intracellular signaling mediators for the transforming growth factor beta (TGFbeta) superfamily. Here, we report that Erbin (for "ErbB2/Her2-interacting protein"), which contains leucine-rich repeats and a PDZ (PSD-95/DLG/ZO-1) domain, interacts specifically with Smad3 and, to a lesser extent, with Smad2 through a novel Smad-interacting domain (SID) adjacent to its PDZ domain. Increased expression of Erbin does not affect the level of TGFbeta-induced phosphorylation of Smad2/Smad3, but it physically sequesters Smad2/Smad3 from their association with Smad4 and hence negatively modulates TGFbeta-dependent transcriptional responses and cell growth inhibition. An isoform of Erbin encoded by an alternatively spliced transcript in human tissues lacks this SID and fails to inhibit TGFbeta responses. Consistently, knockdown of the endogenous Erbin gene with short hairpin RNA enhances TGFbeta-induced antiproliferative and transcriptional responses. In addition, Erbin suppresses activin/Smad2-dependent, but not BMP/Smad1-mediated, induction of endogenous gene expression in Xenopus embryos. Therefore, these results define Erbin as a novel negative modulator of Smad2/Smad3 functions and expand the physiological role of Erbin to the regulation of TGFbeta signaling.

Densin and beta-catenin form a complex and co-localize in cultured podocyte cell junctions

Densin is a member of LAP (leucine-rich repeat and PDZ domain) protein family that localizes in kidney to slit diaphragms, which are essential components of the glomerular filtration barrier. We have previously shown that densin interacts with a crucial slit diaphragm protein, nephrin. Here, we searched for novel binding partners of densin by yeast-two hybrid assay and identified beta-catenin. The interaction was confirmed by reciprocal co-immunoprecipitation assay and the binding site in densin was determined by GST-pull down assays. The GST-tagged densin was also able to pull down P-cadherin together with beta-catenin from human kidney glomerular lysates. Furthermore, densin co-localized with beta-catenin and F-actin in cell-cell contacts in cultured mouse podocytes. During cell-cell contact disruption and reformation densin and beta-catenin were dislocated from and relocated back to plasma membrane in a similar fashion. These and our previous findings suggest that densin may associate with the cadherin-catenin and nephrin complex(es), and may be involved in the formation of the cell-cell contacts including the slit diaphragm.

Nuclear relocation of the nephrin and CD2AP-binding protein dendrin promotes apoptosis of podocytes

Kidney podocytes and their slit diaphragms (SDs) form the final barrier to urinary protein loss. There is mounting evidence that SD proteins also participate in intracellular signaling pathways. The SD protein nephrin serves as a component of a signaling complex that directly links podocyte junctional integrity to actin cytoskeletal dynamics. Another SD protein, CD2-associated protein (CD2AP), is an adaptor molecule involved in podocyte homeostasis that can repress proapoptotic TGF-beta signaling in podocytes. Here we show that dendrin, a protein originally identified in telencephalic dendrites, is a constituent of the SD complex, where it directly binds to nephrin and CD2AP. In experimental glomerulonephritis, dendrin relocates from the SD to the nucleus of injured podocytes. High-dose, proapoptotic TGF-beta1 directly promotes the nuclear import of dendrin, and nuclear dendrin enhances both staurosporine- and TGF-beta1-mediated apoptosis. In summary, our results identify dendrin as an SD protein with proapoptotic signaling properties that accumulates in the podocyte nucleus in response to glomerular injury and provides a molecular target to tackle proteinuric kidney diseases. Nuclear relocation of dendrin may provide a mechanism whereby changes in SD integrity could translate into alterations of podocyte survival under pathological conditions.

Densin and filtrin in the pancreas and in the kidney, targets for humoral autoimmunity in patients with type 1 diabetes

BACKGROUND AND AIM

The development of autoantibodies against antigens of the pancreatic islet cells is a typical phenomenon in patients with type 1 diabetes. The expression of densin, recently shown to be present in kidney podocytes, was explored in the pancreas. Additionally, we studied whether densin and filtrin, another molecule shared between the kidney podocytes and pancreatic islet cells, can act as autoantigens and whether autoantibodies against these can be detected in patients with type 1 diabetes.

METHODS

Expression of pancreatic densin was studied with reverse transcription polymerase chain reaction (RT-PCR) and immunofluorescence. Children and adolescents (n = 66) with type 1 diabetes and control subjects were analysed for densin autoantibodies (DAA) and filtrin autoantibodies (FAA) using radioimmunoprecipitation assay. The serum samples were obtained at the time of diagnosis and after a duration of 2, 5 and 10 years.

RESULTS

Densin expression was observed in the pancreas, localising to the beta cells. DAA were detected in 33% of the patients and the positivity was typically seen already at diagnosis. FAA were observed in 11% of the patients. The proportion of islet cell antibody (ICA) positive, GADA positive and protein tyrosine phosphatase-related islet antigen 2 antibody (IA-2A)-positive patients decreased during the follow-up period, and a similar trend was seen for DAA but not for FAA. Among the 14 patients with signs of renal injury, four tested positive for DAA and two for FAA.

CONCLUSIONS

Densin is a novel molecule shared by the kidney glomerular podocytes and pancreatic islet cells. Densin and filtrin can act as autoantigens, and autoantibodies against these can be detected in patients with type 1 diabetes.

Densin is a novel cell membrane protein of sertoli cells in the testis

Cell-cell interactions between Sertoli cells and germ cells are crucial for the maturation of germ cells in spermatogenesis but the structural and functional aspects of the interactions remain to be fully elucidated. Densin is a junction protein suggested to play a role in establishment of specific cell-cell contacts in the post-synaptic densities of the brain and the slit diaphragm of the kidney podocyte. In the present study, densin was discovered to be expressed in the testis of the man and the mouse. Expression of densin at the gene and the protein level was studied by using RT-PCR and Western blotting analyses, and the localization of densin was explored with immunofluorescence staining. RT-PCR and Western blotting analyses showed that densin is expressed at the gene and the protein levels. Immunofluorescence staining localized the expression of densin to the cell membranes of Sertoli cells suggesting that densin may be an adherens junction protein between Sertoli cells and developing germ cells. Densin is a novel testicular protein expressed in the cell membranes of Sertoli cells. Its functional role remains to be assessed.

Leucine-rich repeat proteins of synapses

Leucine-rich repeats (LRRs) are 20-29-aa motifs that mediate protein-protein interactions and are present in a variety of membrane and cytoplasmic proteins. Many LRR proteins with neuronal functions have been reported. Here, we summarize an emerging group of synaptic LRR proteins, which includes densin-180, Erbin, NGL, SALM, and LGI1. These proteins have been implicated in the formation, differentiation, maintenance, and plasticity of neuronal synapses.

BRAG1, a Sec7 domain-containing protein, is a component of the postsynaptic density of excitatory synapses

The postsynaptic density (PSD) at excitatory synapses is a dynamic complex of glutamatergic receptors and associated proteins that governs synaptic structure and coordinates signal transduction. In this study, we report that BRAG1, a putative guanine nucleotide exchange factor for the Arf family of GTP-binding proteins, is a major component of the PSD. BRAG1 was identified in a 190 kDa band in the PSD fraction with the use of mass spectrometry coupled to searching of a protein sequence database. BRAG1 expression is abundant in the adult rat forebrain, and it is strongly enriched in the PSD fraction compared to forebrain homogenate and synaptosomes. Immunocytochemical localization of BRAG1 in dissociated hippocampal neurons shows that it forms discrete clusters that colocalize with the postsynaptic marker PSD-95 at sites along dendrites. BRAG1 contains a Sec7 domain, a domain that catalyzes exchange of GDP for GTP on the Arf family of small GTP-binding proteins. In their GTP-bound active state, Arfs regulate trafficking of vesicles and cytoskeletal structure. We demonstrate that the Sec7 domain of BRAG1 promotes binding of GTP to Arf in vitro. These data suggest that BRAG1 may modulate the functions of Arfs at synaptic sites.

Human scribble, a novel tumor suppressor identified as a target of high-risk HPV E6 for ubiquitin-mediated degradation, interacts with adenomatous polyposis coli

Recently, we have identified human scribble (hScrib), human homolog of the Drosophila tumor suppressor Scribble, as a substrate of human papillomavirus E6 oncoproteins for ubiquitin-mediated degradation dependent on ubiquitin-protein ligase E6AP. Human Scribble, classified as a LAP protein containing leucine-rich repeats and PDZ domains, interacts with E6 through its PDZ domains and C-terminal PDZ domain-binding motif of E6 protein. Interaction between human Discs Large (hDlg), which is a substrate of E6 for the ubiquitin-mediated degradation, and adenomatous polyposis coli (APC) has been shown. Here, we investigated whether hScrib and APC interact with each other in vitro and in vivo. Interaction between hScrib and APC is mediated by the PDZ domains 1 and 4 of hScrib and C-terminal PDZ domain-binding motif of APC. Human Scribble co-localized with APC at the synaptic sites of hippocampal neuron and at the tip of membrane protrusion in the epithelial cell line. Interference of the interaction between hScrib and APC caused disruption of adherens junction. Knockdown of hScrib expression by RNAi disrupts localization of APC at the adherens junction. These data suggest that hScrib may participate in the hDlg-APC complex through its PDZ domains and regulate cell cycle and neural function by associating with APC.

Convergent and Divergent Ligand Specificity among PDZ Domains of the LAP and Zonula Occludens (ZO) Families

We present a detailed comparative analysis of the PDZ domains of the human LAP proteins Erbin, Densin-180, and Scribble and the MAGUK ZO-1. Phage-displayed peptide libraries and in vitro affinity assays were used to define ligand binding profiles for each domain. The analysis reveals the importance of interactions with all four C-terminal residues of the ligand, which constitute a core recognition motif, and also the role of interactions with more upstream ligand residues that support and modulate the core binding interaction. In particular, the results highlight the importance of site(-1), which interacts with the penultimate residue of ligand C termini. Site(-1) was found to be monospecific in the Erbin PDZ domain (accepts tryptophan only), bispecific in the first PDZ domain of ZO-1 (accepts tryptophan or tyrosine), and promiscuous in the Scribble PDZ domains. Furthermore, it appears that the level of promiscuity within site(-1) greatly influences the range of potential biological partners and functions that can be associated with each protein. These findings show that subtle changes in binding specificity can significantly alter the range of biological partners for PDZ domains, and the insights enhance our understanding of this diverse family of peptide-binding modules.

The postsynaptic density

Glutamatergic synapses in the central nervous system are characterized by an electron-dense web underneath the postsynaptic membrane; this web is called the postsynaptic density (PSD). PSDs are composed of a dense network of several hundred proteins, creating a macromolecular complex that serves a wide range of functions. Prominent PSD proteins such as members of the MaGuk or ProSAP/Shank family build up a dense scaffold that creates an interface between clustered membrane-bound receptors, cell adhesion molecules and the actin-based cytoskeleton. Moreover, kinases, phosphatases and several proteins of different signalling pathways are specifically localized within the spine/PSD compartment. Small GTPases and regulating proteins are also enriched in PSDs being the molecular basis for regulated structural changes of cytoskeletal components within the synapse in response to external or internal stimuli, e.g. synaptic activation. This synaptic rearrangement (structural plasticity) is a rapid process and is believed to underlie learning and memory formation. The characterization of synapse/PSD proteins is especially important in the light of recent data suggesting that several mental disorders have their molecular defect at the synapse/PSD level.

Transient decrease in F-actin may be necessary for translocation of proteins into dendritic spines

It remains poorly understood as to how newly synthesized proteins that are required to act at specific synapses are translocated into only selected subsets of potentiated dendritic spines. Here, we report that F-actin, a major component of the skeletal structure of dendritic spines, may contribute to the regulation of synaptic specificity of protein translocation. We found that the stabilization of F-actin blocked the translocation of GFP-CaMKII and inhibited the diffusion of 3-kDa dextran into spines (in 2-3 weeks cultures). Neuronal activation in hippocampal slices and cultured neurons led to an increase in the activation (decrease in the phosphorylation) of the actin depolymerization factor, cofilin, and a decrease in F-actin. Furthermore, the induction of long-term potentiation by tetanic stimulation induced local transient depolymerization of F-actin both in vivo and in hippocampal slices (8-10 weeks), and this local F-actin depolymerization was blocked by APV, a N-methyl-D-aspartate (NMDA) receptor antagonist. These results suggest that F-actin may play a role in synaptic specificity by allowing protein translocation into only potentiated spines, gated through its depolymerization, which is probably triggered by the activation of NMDA receptors.

Molecular basis of the glomerular filtration: nephrin and the emerging protein complex at the podocyte slit diaphragm

For more than three decades, the molecular composition of the interpodocyte slit diaphragm of the glomerular filtration barrier has remained elusive. The first electron microscopic studies described the slit diaphragm as a porous, 'zipper-like' structure, but it was not until 1998 that the first transmembrane molecule of the slit diaphragm was identified: nephrin is a cell surface receptor of the immunoglobulin superfamily participating in cell-cell adhesion and signaling functions. Mutations in nephrin lead to the congenital nephrotic syndrome of the Finnish type, suggesting that nephrin is of pivotal importance for maintaining the filtration barrier. In recent years, the mapping of the genetic background of other inherited and acquired nephropathies and generation of transgenic animal models have led to a beginning of a new era in nephrology, possibly promising new targeted therapies and advanced diagnostics. This review article will briefly summarize the main findings that explain the molecular architecture of the glomerular filter itself and causes of some glomerular diseases that lead to proteinuria and, eventually, to renal failure.

Loss of Synaptic D1 Dopamine/N-Methyl-d-aspartate Glutamate Receptor Complexes in l-DOPA-Induced Dyskinesia in the Rat

Glutamate-mediated mechanisms are related to the motor complications of L-DOPA therapy in Parkinson's disease (PD). In striatal postsynaptic densities (PSD), the dopamine D1 receptor (D1R) is part of an oligomeric complex with the glutamate N-methyl-D-aspartate receptor (NMDAR), determining the strength of corticostriatal transmission. We studied D1R/NMDAR complex alterations induced by L-DOPA in the 6-hydroxydopamine-lesioned rat model of PD. L-DOPA-treated hemiparkinsonian rats were determined to be dyskinetic or nondyskinetic based on behavioral testing. D1R/NMDAR assemblies containing NR1-C2 and NR2B subunits were decreased in the PSD of lesioned striatum. Short-term L-DOPA administration improved akinesia and restored the synaptic abundance of D1R, NR1-C2 and NR2B. Prolonged L-DOPA treatment also normalized synaptic D1R/NMDAR complexes in nondyskinetic rats, but remarkably reduced them in the dyskinetic group without changing their interaction. This decrease involved NR1-C2, NR1-C2', NR2A, and NR2B subunits. The composition of residual synaptic D1R/NMDAR complexes in dyskinetic rats may thus be different from that observed in lesioned rats, suggesting that expression of different motor dysfunctions might be related to the receptor profile at corticostriatal synapses. The levels of D1R/NMDAR complexes were unchanged in total striatal membrane proteins, suggesting that the decrease of these species in the PSD is likely to reflect an altered receptor trafficking. In human embryonic kidney 293 cells expressing the D1R/NMDAR, complex costimulation of both D1R and NMDAR, but not individual receptor activation, promoted internalization, suggesting that development of dyskinesias might be related to agonist-mediated down-regulation of the D1R/NMDAR complex at corticostriatal synapses.

Dominant-negative effects of human P/Q-type Ca2+ channel mutations associated with episodic ataxia type 2

Episodic ataxia type 2 (EA2) is an inherited autosomal dominant disorder related to cerebellar dysfunction and is associated with mutations in the pore-forming alpha(1A)-subunits of human P/Q-type Ca(2+) channels (Cav2.1 channels). The majority of EA2 mutations result in significant loss-of-function phenotypes. Whether EA2 mutants may display dominant-negative effects in human, however, remains controversial. To address this issue, five EA2 mutants in the long isoform of human alpha(1A)-subunits were expressed in Xenopus oocytes to explore their potential dominant-negative effects. Upon coexpressing the cRNA of alpha(1A)-WT with each alpha(1A)-mutant in molar ratios ranging from 1:1 to 1:10, the amplitude of Ba(2+) currents through wild-type (WT)-Cav2.1 channels decreased significantly as the relative molar ratio of alpha(1A)-mutants increased, suggesting the presence of an alpha(1A)-mutant-specific suppression effect. When we coexpressed alpha(1A)-WT with proteins not known to interact with Cav2.1 channels, we observed no significant suppression effects. Furthermore, increasing the amount of auxiliary subunits resulted in partial reversal of the suppression effects in nonsense but not missense EA2 mutants. On the other hand, when we repeated the same coinjection experiments of alpha(1A)-WT and mutant using a splice variant of alpha(1A)-subunit that contained a considerably shorter COOH terminus (i.e., the short isoform), no significant dominant-negative effects were noted until we enhanced the relative molar ratio to 1:10. Altogether, these results indicate that for human WT-Cav2.1 channels comprising the long-alpha(1A)-subunit isoform, both missense and nonsense EA2 mutants indeed display prominent dominant-negative effects.