Characterization of novel glycoprotein components of synaptic membranes and postsynaptic densities, gp65 and gp55, with a monoclonal antibody

Neuroplastin splice variants Np55 and Np65: Who is doing the job in macrophages?

Neuroplastin, a paralog of CD147/Basigin, is known as a neuronal cell adhesion molecule and as an auxiliary subunit of plasma membrane calcium ATPases in both neurons and adaptive immune cells. Recently, an interesting study by Ren et al. (2022) provided evidence for an important role of neuroplastin in macrophages during bacterial infection. Here, we critically discuss one aspect of this study, the assignment of this role to Np65 as one of two prominent splice variants of neuroplastin.

Disfunction of dorsal raphe nucleus-hippocampus serotonergic-HTR3 transmission results in anxiety phenotype of Neuroplastin 65-deficient mice

Anxiety disorders are the most common psychiatric condition, but the etiology of anxiety disorders remains largely unclear. Our previous studies have shown that neuroplastin 65 deficiency (NP65-/-) mice exhibit abnormal social and mental behaviors and decreased expression of tryptophan hydroxylase 2 (TPH2) protein. However, whether a causal relationship between TPH2 reduction and anxiety disorders exists needs to be determined. In present study, we found that replenishment of TPH2 in dorsal raphe nucleus (DRN) enhanced 5-HT level in the hippocampus and alleviated anxiety-like behaviors. In addition, injection of AAV-NP65 in DRN significantly increased TPH2 expression in DRN and hippocampus, and reduced anxiety-like behaviors. Acute administration of exogenous 5-HT or HTR3 agonist SR57227A in hippocampus mitigated anxiety-like behaviors in NP65-/- mice. Moreover, replenishment of TPH2 in DRN partly repaired the impairment of long-term potentiation (LTP) maintenance in hippocampus of NP65-/- mice. Finally, we found that loss of NP65 lowered transcription factors Lmx1b expression in postnatal stage and replenishment of NP65 in DRN reversed the decrease in Lmx1b expression of NP65-/- mice. Together, our findings reveal that NP65 deficiency induces anxiety phenotype by downregulating DRN-hippocampus serotonergic-HTR3 transmission. These studies provide a novel and insightful view about NP65 function, suggesting an attractive potential target for treatment of anxiety disorders.

Gender-related variation expressions of neuroplastin TRAF6, GluA1, GABA(A) receptor, and PMCA in cortex, hippocampus, and brainstem in an experimental epilepsy model

Epileptic seizures are seen as a result of changing excitability balance depending on the deterioration in synaptic plasticity in the brain. Neuroplastin, and its related molecules which are known to play a role in synaptic plasticity, neurotransmitter activities that provide balance of excitability and, different neurological diseases, have not been studied before in epilepsy. In this study, a total of 34 Sprague-Dawley male and female rats, 2 months old, weighing 250-300 g were used. The epilepsy model in rats was made via pentylenetetrazole (PTZ). After the completion of the experimental procedure, the brain tissue of the rats were taken and the histopathological changes in the hippocampus and cortex parts and the brain stem were investigated, as well as the immunoreactivity of the proteins related to the immunohistochemical methods. As a result of the histopathological evaluation, it was determined that neuron degeneration and the number of dilated blood vessels in the hippocampus, frontal cortex, and brain stem were higher in the PTZ status epilepticus (SE) groups than in the control groups. It was observed that neuroplastin and related proteins TNF receptor-associated factor 6 (TRAF6), Gamma amino butyric acid type A receptors [(GABA(A)], and plasma membrane Ca2+ ATPase (PMCA) protein immunoreactivity levels increased especially in the male hippocampus, and only AMPA receptor subunit type 1 (GluA1) immunoreactivity decreased, unlike other proteins. We believe this may be caused by a problem in the mechanisms regulating the interaction of neuroplastin and GluA1 and may cause problems in synaptic plasticity in the experimental epilepsy model. It may be useful to elucidate this mechanism and target GluA1 when determining treatment strategies.

Embigin deficiency leads to delayed embryonic lung development and high neonatal mortality in mice

Embigin (Gp70), a receptor for fibronectin and an ancillary protein for monocarboxylate transporters, is known to regulate stem cell niches in sebaceous gland and bone marrow. Here, we show that embigin expression is at high level during early mouse embryogenesis and that embigin is essential for lung development. Markedly increased neonatal mortality of Emb-/- mice can be explained by the compromised lung maturation: in Emb-/- mice (E17.5) the number and the size of the small airways and distal airspace are significantly smaller, there are fewer ATI and ATII cells, and the alkaline phosphatase activity in amniotic fluid is lower. Emb-/- lungs show less peripheral branching already at E12.5, and embigin is highly expressed in lung primordium. Thus, embigin function is essential at early pseudoglandular stage or even earlier. Furthermore, our RNA-seq analysis and Ki67 staining results support the idea that the development of Emb-/- lungs is rather delayed than defected.

Neuroplastin 65 deficiency reduces amyloid plaque formation and cognitive deficits in an Alzheimer's disease mouse model

Introduction

Alzheimer's disease (AD) is characterized by increasing cognitive dysfunction, progressive cerebral amyloid beta (Aβ) deposition, and neurofibrillary tangle aggregation. However, the molecular mechanisms of AD pathologies have not been completely understood. As synaptic glycoprotein neuroplastin 65 (NP65) is related with synaptic plasticity and complex molecular events underlying learning and memory, we hypothesized that NP65 would be involved in cognitive dysfunction and Aβ plaque formation of AD. For this purpose, we examined the role of NP65 in the transgenic amyloid precursor protein (APP)/presenilin 1 (PS1) mouse model of AD.

Methods

Neuroplastin 65-knockout (NP65^-/-) mice crossed with APP/PS1 mice to get the NP65-deficient APP/PS1 mice. In the present study, a separate cohort of NP65-deficient APP/PS1 mice were used. First, the cognitive behaviors of NP65-deficient APP/PS1 mice were assessed. Then, Aβ plaque burden and Aβ levels in NP65-deficient APP/PS1 mice were measured by immunostaining and western blot as well as ELISA. Thirdly, immunostaining and western blot were used to evaluate the glial response and neuroinflammation. Finally, protein levels of 5-hydroxytryptamin (serotonin) receptor 3A and synaptic proteins and neurons were measured.

Results

We found that loss of NP65 alleviated the cognitive deficits of APP/PS1 mice. In addition, Aβ plaque burden and Aβ levels were significantly reduced in NP65-deficient APP/PS1 mice compared with control animals. NP65-loss in APP/PS1 mice resulted in a decrease in glial activation and the levels of pro- and anti-inflammatory cytokines (IL-1β, TNF-α, and IL-4) as well as protective matrix YM-1 and Arg-1, but had no effect on microglial phenotype. Moreover, NP65 deficiency significantly reversed the increase in 5-hydroxytryptamine (serotonin) receptor 3A (Htr3A) expression levels in the hippocampus of APP/PS1 mice.

Discussion

These findings identify a previously unrecognized role of NP65 in cognitive deficits and Aβ formation of APP/PS1 mice, and suggest that NP65 may serve as a potential therapeutic target for AD.

Ca2+ signaling in postsynaptic neurons: Neuroplastin-65 regulates the interplay between plasma membrane Ca2+ ATPases and ionotropic glutamate receptors

Upon postsynaptic glutamate receptor activation, the cytosolic Ca²⁺ concentration rises and initiates signaling and plasticity in spines. The plasma membrane Ca²⁺ ATPase (PMCA) is a major player to limit the duration of cytosolic Ca²⁺ signals. It forms complexes with the glycoprotein neuroplastin (Np) isoforms Np55 and Np65 and functionally interplays with N-methyl-D-aspartate (NMDA)-type ionotropic glutamate receptors (iGluNRs). Moreover, binding of the Np65-specific extracellular domain to Ca²⁺-permeable GluA1-containing α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type ionotropic glutamate receptors (iGluA1Rs) was found to be required for long-term potentiation (LTP). However, the link between PMCA and iGluRs function to regulate cytosolic Ca²⁺ signals remained unclear. Here, we report that Np65 coordinates PMCA and iGluRs' functions to modulate the duration and amplitude of cytosolic Ca²⁺ transients in dendrites and spines of hippocampal neurons. Using live-cell Ca²⁺ imaging, acute pharmacological treatments, and GCaMP5G-expressing hippocampal neurons, we discovered that endogenous or Np65-promoted PMCA activity contributes to the restoration of basal Ca²⁺ levels and that this effect is dependent on iGluR activation. Super-resolution STED and confocal microscopy revealed that electrical stimulation increases the abundance of synaptic neuroplastin-PMCA complexes depending on iGluR activation and that low-rate overexpression of Np65 doubled PMCA levels and decreased cell surface levels of GluN2A and GluA1 in dendrites and Shank2-positive glutamatergic synapses. In neuroplastin-deficient hippocampi, we observed reduced PMCA and unchanged GluN2B levels, while GluN2A and GluA1 levels were imbalanced. Our electrophysiological data from hippocampal slices argues for an essential interplay of PMCA with GluN2A- but not with GluN2B-containing receptors upon induction of synaptic plasticity. Accordingly, we conclude that Np65 may interconnect PMCA with core players of glutamatergic neurotransmission to fine-tune the Ca²⁺ signal regulation in basal synaptic function and plasticity.

Retrograde Amnesia - A Question of Disturbed Calcium Levels?

Retrograde amnesia is the inability to remember events or information. The successful acquisition and memory of information is required before retrograde amnesia may occur. Often, the trigger for retrograde amnesia is a traumatic event. Loss of memories may be caused in two ways: either by loss/erasure of the memory itself or by the inability to access the memory, which is still present. In general, memories and learning are associated with a positive connotation although the extinction of unpleasant experiences and memories of traumatic events may be highly welcome. In contrast to the many experimental models addressing learning deficits caused by anterograde amnesia, the incapability to acquire new information, retrograde amnesia could so far only be investigated sporadically in human patients and in a limited number of model systems. Apart from models and diseases in which neurodegeneration or dementia like Alzheimer’s disease result in loss of memory, retrograde amnesia can be elicited by various drugs of which alcohol is the most prominent one and exemplifies the non-specific effects and the variable duration. External or internal impacts like traumatic brain injury, stroke, or electroconvulsive treatments may similarly result in variable degrees of retrograde amnesia. In this review, I will discuss a new genetic approach to induce retrograde amnesia in a mouse model and raise the hypothesis that retrograde amnesia is caused by altered intracellular calcium homeostasis. Recently, we observed that neuronal loss of neuroplastin resulted in retrograde amnesia specifically for associative memories. Neuroplastin is tightly linked to the expression of the main Ca²⁺ extruding pumps, the plasma membrane calcium ATPases (PMCAs). Therefore, neuronal loss of neuroplastin may block the retrieval and storage of associative memories by interference with Ca²⁺ signaling cascades. The possibility to elicit retrograde amnesia in a controlled manner allows to investigate the underlying mechanisms and may provide a deeper understanding of the molecular and circuit processes of memory.

Neuroplastin in Neuropsychiatric Diseases

Molecular mechanisms underlying neuropsychiatric and neurodegenerative diseases are insufficiently elucidated. A detailed understanding of these mechanisms may help to further improve medical intervention. Recently, intellectual abilities, creativity, and amnesia have been associated with neuroplastin, a cell recognition glycoprotein of the immunoglobulin superfamily that participates in synapse formation and function and calcium signaling. Data from animal models suggest a role for neuroplastin in pathways affected in neuropsychiatric and neurodegenerative diseases. Neuroplastin loss or disruption of molecular pathways related to neuronal processes has been linked to various neurological diseases, including dementia, schizophrenia, and Alzheimer's disease. Here, we review the molecular features of the cell recognition molecule neuroplastin, and its binding partners, which are related to neurological processes and involved in learning and memory. The emerging functions of neuroplastin may have implications for the treatment of diseases, particularly those of the nervous system.

Neuroplastin in human cognition: review of literature and future perspectives

Synaptic glycoprotein neuroplastin is involved in synaptic plasticity and complex molecular events underlying learning and memory. Studies in mice and rats suggest that neuroplastin is essential for cognition, as it is needed for long-term potentiation and associative memory formation. Recently, it was found that some of the effects of neuroplastin are related to regulation of calcium homeostasis through interactions with plasma membrane calcium ATPases. Neuroplastin is increasingly seen as a key factor in complex brain functions, but studies in humans remain scarce. Here we summarize present knowledge about neuroplastin in human tissues and argue its genetic association with cortical thickness, intelligence, schizophrenia, and autism; specific immunolocalization depicting hippocampal trisynaptic pathway; potential role in tissue compensatory response in neurodegeneration; and high, almost housekeeping, level of spatio-temporal gene expression in the human brain. We also propose that neuroplastin acts as a housekeeper of neuroplasticity, and that it may be considered as an important novel cognition-related molecule in humans. Several promising directions for future investigations are suggested, which may complete our understanding of neuroplastin actions in molecular basis of human cognition.

Heteromeric Solute Carriers: Function, Structure, Pathology and Pharmacology

Solute carriers form one of three major superfamilies of membrane transporters in humans, and include uniporters, exchangers and symporters. Following several decades of molecular characterisation, multiple solute carriers that form obligatory heteromers with unrelated subunits are emerging as a distinctive principle of membrane transporter assembly. Here we comprehensively review experimentally established heteromeric solute carriers: SLC3-SLC7 amino acid exchangers, SLC16 monocarboxylate/H⁺ symporters and basigin/embigin, SLC4A1 (AE1) and glycophorin A exchanger, SLC51 heteromer Ost α-Ost β uniporter, and SLC6 heteromeric symporters. The review covers the history of the heteromer discovery, transporter physiology, structure, disease associations and pharmacology - all with a focus on the heteromeric assembly. The cellular locations, requirements for complex formation, and the functional role of dimerization are extensively detailed, including analysis of the first complete heteromer structures, the SLC7-SLC3 family transporters LAT1-4F2hc, b^0,+AT-rBAT and the SLC6 family heteromer B⁰AT1-ACE2. We present a systematic analysis of the structural and functional aspects of heteromeric solute carriers and conclude with common principles of their functional roles and structural architecture.

Neuroplastin 65 modulates anxiety- and depression-like behavior likely through adult hippocampal neurogenesis and central 5-HT activity

Neuroplastin 65 (Np65) is a brain-specific cell adhesion molecule that is highly expressed in the hippocampus, amygdala, and cortex, regions of the brain that are associated with memory and emotions. However, the role of Np65 in regulation of emotional behavior is still unclear. In the present study, we show that Np65 knock-out (Np65 KO) mice display enhanced anxiety-like behavior, a reduction in some aspects of depressive-like behaviors, and increased sociability and memory. Biochemical investigations revealed that Np65 KO mice show increased adult-born neurons and proliferation in the hippocampus. In addition, the level of 5-hydroxytryptamine (5-HT) in the hippocampus was reduced. The expression of tryptophan hydroxylase 2 in the brainstem and the expression of the 5-HT_3A receptor were also decreased. Electrophysiological recordings confirmed an impaired maintenance of long-term potentiation in the hippocampus of Np65 KO mice. Together, our findings uncover a role for Np65 in regulating anxiety- and depressive-like behaviors and suggest that Np65 may be essential for the maintenance of emotional stability, indicating that it might be an attractive potential target for treatment of psychiatric disorders.

Increased Susceptibility to Ischemic Brain Injury in Neuroplastin 65-Deficient Mice Likely via Glutamate Excitotoxicity

Cell adhesion molecules (CAMs) are involved in synaptic plasticity and neuronal survival in the adult brain. Neuroplastin 65 (Np65), one member of the immunoglobulin superfamily of CAMs, is brain-specific and highly expressed in rodent forebrain. The roles of Np65 in synaptic plasticity have been confirmed, however, whether Np65 affects neuronal survival remains unknown. To address this gap, we generated, to our knowledge, the first Np65 knockout (KO) mice. By occluding middle cerebral artery to perform ischemic stroke model, we showed that Np65 KO mice exhibited more severe neurological deficits and larger infarction volume measured by TTC staining and more apoptotic cells confirmed by TUNEL staining compared to wild type (WT) mice. Besides, western blot analysis showed that the vesicular glutamate transporter-1(VGluT1), and N-Methyl D-Aspartate receptors, including NR1, NR2A, and NR2B were significantly increased in Np65 KO mice compared with WT mice. In contrast, vesicular gamma amino butyric acid transporter (VGAT) levels were unchanged in two genotypes after stroke. Additionally, phosphorylated-extracellular signal-regulated kinase 1/2 levels were significantly increased in Np65 KO mice compared with WT mice after stroke. Together, these results suggest that Np65 KO mice may be more susceptible to ischemic events in the brain.

Neuroplastin 65 mediates cognitive functions via excitatory/inhibitory synapse imbalance and ERK signal pathway

Neuroplastin 65 (NP65) is a brain-specific glycoprotein component of synaptic membrane, which is predominantly located in the forebrain such as the cortex, amygdala and striatum and hippocampus. Previous studies have shown that NP65 is implicated in synaptic plasticity, so it was hypothesized to play roles in cognitive functions. To test this hypothesis, we generated NP65 knock-out (KO) mice and found that the null mice exhibited enhanced hippocampus-dependent learning and memory as manifested by Morris water maze test and step-through passive avoidance test, but showed anxiety-like behaviors as manifested by open field test and light/dark exploration test. In addition, molecular and cellular studies revealed several alterations including: (1) the enhanced ratio of excitatory to inhibitory synapses; (2) increased expression of NMDA receptors NR2A; (3) enhanced activation of ERK signaling; (4) lowered number of the mushroom- and bifurcate-shaped dendritic spines in NP65 KO mice. Together, our findings suggest that NP65 may mediate cognitive functions.

Single nucleotide polymorphism in the neuroplastin locus associates with cortical thickness and intellectual ability in adolescents

Despite the recognition that cortical thickness is heritable and correlates with intellectual ability in children and adolescents, the genes contributing to individual differences in these traits remain unknown. We conducted a large-scale association study in 1583 adolescents to identify genes affecting cortical thickness. Single-nucleotide polymorphisms (SNPs; n=54,837) within genes whose expression changed between stages of growth and differentiation of a human neural stem cell line were selected for association analyses with average cortical thickness. We identified a variant, rs7171755, associating with thinner cortex in the left hemisphere (P=1.12 × 10(-)(7)), particularly in the frontal and temporal lobes. Localized effects of this SNP on cortical thickness differently affected verbal and nonverbal intellectual abilities. The rs7171755 polymorphism acted in cis to affect expression in the human brain of the synaptic cell adhesion glycoprotein-encoding gene NPTN. We also found that cortical thickness and NPTN expression were on average higher in the right hemisphere, suggesting that asymmetric NPTN expression may render the left hemisphere more sensitive to the effects of NPTN mutations, accounting for the lateralized effect of rs7171755 found in our study. Altogether, our findings support a potential role for regional synaptic dysfunctions in forms of intellectual deficits.

The Neuroplastin adhesion molecules: key regulators of neuronal plasticity and synaptic function

The Neuroplastins Np65 and Np55 are neuronal and synapse-enriched immunoglobulin superfamily molecules that play important roles in a number of key neuronal and synaptic functions including, for Np65, cell adhesion. In this review we focus on the physiological roles of the Neuroplastins in promoting neurite outgrowth, regulating the structure and function of both inhibitory and excitatory synapses in brain, and in neuronal and synaptic plasticity. We discuss the underlying molecular and cellular mechanisms by which the Neuroplastins exert their physiological effects and how these are dependent upon the structural features of Np65 and Np55, which enable them to bind to a diverse range of protein partners. In turn this enables the Neuroplastins to interact with a number of key neuronal signalling cascades. These include: binding to and activation of the fibroblast growth factor receptor; Np65 trans-homophilic binding leading to activation of p38 MAPK and internalization of glutamate (GluR1) receptor subunits; acting as accessory proteins for monocarboxylate transporters, thus affecting neuronal energy supply, and binding to GABAA α1, 2 and 5 subunits, thus regulating the composition and localization of GABAA receptors. An emerging theme is the role of the Neuroplastins in regulating the trafficking and subcellular localization of specific binding partners. We also discuss the involvement of Neuroplastins in a number of pathophysiological conditions, including ischaemia, schizophrenia and breast cancer and the role of a single nucleotide polymorphism in the human Neuroplastin (NPTN) gene locus in impairment of cortical development and cognitive functions. Neuroplastins are neuronal cell adhesion molecules, which induce neurite outgrowth and play important roles in synaptic maturation and plasticity. This review summarizes the functional implications of Neuroplastins for correct synaptic membrane protein localization, neuronal energy supply, expression of LTP and LTD, animal and human behaviour, and pathophysiology and disease. It focuses particularly on Neuroplastin binding partners and signalling mechanisms, and proposes perspectives for future research on these important immunoglobulin superfamily members.

The neuroplastins: multifunctional neuronal adhesion molecules--involvement in behaviour and disease

The neuroplastins np65 and np55 are neuronal and synapse-enriched immunoglobulin (Ig) superfamily cell adhesion molecules that contain 3 and 2 Ig domains, respectively. Np65 is neuron specific whereas np55 is expressed in many tissues. They are multifunctional proteins whose physiological roles are defined by the partner proteins they bind to and the signalling pathways they activate. The neuroplastins are implicated in activity-dependent long-term synaptic plasticity. Thus neuroplastin-specific antibodies and a recombinant peptide inhibit long-term potentiation in hippocampal neurones. This is mediated by activation of the p38MAP kinase signalling pathway, resulting in the downregulation of the surface expression of GluR1 receptors. Np65, but not np55, exhibits trans-homophilic binding. Both np65 and np55 induce neurite outgrowth and both activate the FGF receptor and associated downstream signalling pathways. Np65 binds to and colocalises with GABA(A) receptor subtypes and may play a role in anchoring them to specific synaptic and extrasynaptic sites. Most recently the neuroplastins have been shown to chaperone and support the monocarboxylate transporter MCT2 in transporting lactate across the neuronal plasma membrane. Thus the neuroplastins are multifunctional adhesion molecules which support neurite outgrowth, modulate long-term activity-dependent synaptic plasticity, regulate surface expression of GluR1 receptors, modulate GABA(A) receptor localisation, and play a key role in delivery of monocarboxylate energy substrates both to the synapse and to extrasynaptic sites. The diverse functions and range of signalling pathways activated by the neuroplastins suggest that they are important in modulating behaviour and in relation to human disease.

The neuroplastin adhesion molecules are accessory proteins that chaperone the monocarboxylate transporter MCT2 to the neuronal cell surface

BACKGROUND

The neuroplastins np65 and np55 are two synapse-enriched immunoglobulin (Ig) superfamily adhesion molecules that contain 3 and 2 Ig domains respectively. Np65 is implicated in long term, activity dependent synaptic plasticity, including LTP. Np65 regulates the surface expression of GluR1 receptor subunits and the localisation of GABA(A) receptor subtypes in hippocampal neurones. The brain is dependent not only on glucose but on monocarboxylates as sources of energy. The. monocarboxylate transporters (MCTs) 1-4 are responsible for the rapid proton-linked translocation of monocarboxylates including pyruvate and lactate across the plasma membrane and require association with either embigin or basigin, proteins closely related to neuroplastin, for plasma membrane expression and activity. MCT2 plays a key role in providing lactate as an energy source to neurons.

METHODOLOGY/FINDINGS

Here we use co-transfection of neuroplastins and monocarboxylate transporters into COS-7 cells to demonstrate that neuroplastins can act as ancillary proteins for MCT2. We also show that Xenopus laevis oocytes contain endogenous neuroplastin and its knockdown with antisense RNA reduces the surface expression of MCT2 and associated lactate transport. Immunocytochemical studies show that MCT2 and the neuroplastins are co-localised in rat cerebellum. Strikingly neuroplastin and MCT2 are enriched in the same parasagittal zebrin II-negative stripes.

CONCLUSIONS

These data strongly suggest that neuroplastins act as key ancillary proteins for MCT2 cell surface localisation and activity in some neuronal populations, thus playing an important role in facilitating the uptake of lactate for use as a respiratory fuel.

Neuroplastin expression in the hippocampus of mice lacking complex gangliosides

We report changes in neuroplastin gene and protein expression in the hippocampus of B4galnt1 null mice, which lacks complex ganglioside structures, compared with that of wild-type mice. Neuroplastin mRNA expression was significantly higher in the hippocampi of B4galnt1 null mice than in wild-type mice. Moreover, Western blot analysis shows increased neuroplastin protein levels of neuroplastin-55 isoform in B4galnt1 null hippocampal homogenates. Immunohistochemistry revealed a substantially different distribution of neuroplastin immunoreactivity in sagittal sections of the hippocampi derived from B4galnt1 null in comparison with those from wild-type mice. Most strikingly, B4galnt1 null mice had relatively little neuroplastin immunoreactivity in the pyramidal layer of CA1 and CA3, whereas wild-type mice had strong neuroplastin staining of pyramidal cells. Results of this study support the hypothesis that alterations of brain ganglioside expression influence the expression of neuroplastin. As both neuroplastin and gangliosides have important roles in synaptic transmission, synaptic plasticity, and neurite outgrowth, it will be of particular interest to unravel the molecular mechanisms underlying the relationship between ganglioside composition and neuroplastin transcript and protein expression in the mammalian nervous system.

The cell adhesion molecule neuroplastin-65 is a novel interaction partner of γ-aminobutyric acid type A receptors

γ-Aminobutyric acid type A (GABA(A)) receptors are pentameric ligand-gated ion channels that mediate fast inhibition in the central nervous system. Depending on their subunit composition, these receptors exhibit distinct pharmacological properties and differ in their ability to interact with proteins involved in receptor anchoring at synaptic or extra-synaptic sites. Whereas GABA(A) receptors containing α1, α2, or α3 subunits are mainly located synaptically where they interact with the submembranous scaffolding protein gephyrin, receptors containing α5 subunits are predominantly found extra-synaptically and seem to interact with radixin for anchorage. Neuroplastin is a cell adhesion molecule of the immunoglobulin superfamily that is involved in hippocampal synaptic plasticity. Our results reveal that neuroplastin and GABA(A) receptors can be co-purified from rat brain and exhibit a direct physical interaction as demonstrated by co-precipitation and Förster resonance energy transfer (FRET) analysis in a heterologous expression system. The brain-specific isoform neuroplastin-65 co-localizes with GABA(A) receptors as shown in brain sections as well as in neuronal cultures, and such complexes can either contain gephyrin or be devoid of gephyrin. Neuroplastin-65 specifically co-localizes with α1 or α2 but not with α3 subunits at GABAergic synapses. In addition, neuroplastin-65 also co-localizes with GABA(A) receptor α5 subunits at extra-synaptic sites. Down-regulation of neuroplastin-65 by shRNA causes a loss of GABA(A) receptor α2 subunits at GABAergic synapses. These results suggest that neuroplastin-65 can co-localize with a subset of GABA(A) receptor subtypes and might contribute to anchoring and/or confining GABA(A) receptors to particular synaptic or extra-synaptic sites, thus affecting receptor mobility and synaptic strength.

The immunolocalization of the synaptic glycoprotein neuroplastin differs substantially between the human and the rodent brain

Neuroplastin is a cell adhesion molecule of the immunoglobulin superfamily that exists in two splice isoforms, np65/np55, and that was reported to play a prominent role in synaptic plasticity processes. The splice isoform np65 associates with synapses in an activity-dependent manner and has been shown to play a role for the induction of hippocampal long-term potentiation in rodents. We have therefore analyzed the distribution of neuroplastins in human brain. Neuroplastin is present in many neuronal cell types of the forebrain and cerebellum and immunoreactive label covers the cell soma, neurites and also puncta in the neuropil were visible. Interestingly, we found some remarkable species differences in the expression patterns of neuroplastins between the human and the rodent brain. In human brain np65 is prominently present in cerebellum while np55 is the predominant isoform in mouse and rat cerebellum. Moreover, the parasagittal stripe-type of staining seen with np55 in mouse cerebellum is not found in human brain. In addition we found no segregation of np65 immunolabel in hippocampal subregions like it was reported previously for the rat. These results might indicate different cellular functions of the molecule in different species.

The cell adhesion molecule neuroplastin-65 inhibits hippocampal long-term potentiation via a mitogen-activated protein kinase p38-dependent reduction in surface expression of GluR1-containing glutamate receptors

Neuroplastin-65 is a brain-specific, synapse-enriched member of the immunoglobulin (Ig) superfamily of cell adhesion molecules. Previous studies highlighted the importance of neuroplastin-65 for long-term potentiation (LTP), but the mechanism was unclear. Here, we show how neuroplastin-65 activation of mitogen-activated protein kinase p38 (p38MAPK) modified synapse strength by altering surface glutamate receptor expression. Organotypic hippocampal slice cultures treated with the complete extracellular fragment of neuroplastin-65 (FcIg1-3) sustained an increase in the phosphorylation of p38MAPK and an inability to induce LTP at hippocampal synapses. The LTP block was reversed by application of the p38MAPK inhibitor SB202190, suggesting that p38MAPK activation occurred downstream of neuroplastin-65 binding and upstream of the loss of LTP. Further investigation revealed that the mechanism underlying neuroplastin-65-dependent prevention of LTP was a p38MAPK-dependent acceleration of the loss of surface-exposed glutamate receptor subunits that was reversed by pretreatment with the p38MAPK inhibitor SB202190. Our results indicate that neuroplastin-65 binding and associated stimulation of p38MAPK activity are upstream of a mechanism to control surface glutamate receptor expression and thereby influence plasticity at excitatory hippocampal synapses.

Comparison of neuroplastin and synaptic marker protein expression in acute and cultured organotypic hippocampal slices from rat

Organotypic hippocampal slice cultures can be used to study hippocampal biochemistry and physiology over a chronic period on the days to weeks timescale. In order to validate the organotypic hippocampal slice culture for our ongoing studies of synaptic function, we have compared, using Western blotting, the levels of a number of synaptic proteins from in vitro organotypic hippocampal slice cultures with those from in vivo hippocampal slices prepared from age-matched controls. We chose to follow the developmental expression of the neuroplastin (np) family of immunoglobulin related cell adhesion molecules (CAMs), np65, a brain specific isoform highly expressed in hippocampal neurones and np55 a more widely expressed isoform and two synaptic marker proteins, synaptophysin, a pre-synaptic marker and post-synaptic density protein-95, PSD95, a post-synaptic marker. All showed increasing expression over the developmental time period, both in vivo and in vitro. The level of both neuroplastins was also consistent between the in vivo and in vitro preparations, whereas the level of PSD95 was markedly increased in the organotypic hippocampal slice cultures while the level of synaptophysin was slightly decreased. Whilst these findings may indicate some differences in the composition and organisation of synapses, the developmental expression profiles of these synaptic proteins within organotypic hippocampal slice cultures suggests they are a valid model for the study of synapse function and development in vitro.

Expression of the immunoglobulin superfamily neuroplastin adhesion molecules in adult and developing mouse cerebellum and their localisation to parasagittal stripes

Neuroplastin (np) 55 and 65 are immunoglobulin superfamily members that arise by alternative splicing of the same gene and have been implicated in long-term activity-dependent synaptic plasticity. Both biochemical and immunocytochemical data suggest that np55 is the predominant isoform (>95% of total neuroplastin) in cerebellum. Neuroplastin immunoreactivity is concentrated in the molecular layer and synaptic glomeruli in the granule cell layer. Expression in the molecular layer appears to be postsynaptic. First, neuroplastin is associated with Purkinje cell dendrites in two mouse granuloprival cerebellar mutants, disabled and cerebellar deficient folia. Second, in an acid sphingomyelinase knockout mouse with widespread protein trafficking defects, neuroplastin accumulates in the Purkinje cell somata. Finally, primary cerebellar cultures show neuroplastin expression in Purkinje cell dendrites and somata lacking normal histotypic organization and synaptic connections, and high-magnification views indicate a preferential association with dendritic spines. In the molecular layer, differences in neuroplastin expression levels present as a parasagittal array of stripes that alternates with that revealed by the expression of another compartmentation antigen, zebrin II/aldolase c. Neuroplastin immunoreactivity is first detected weakly at postnatal day 3 (P3) in the anterior lobe vermis. By P5, parasagittal stripes are already apparent in the immature molecular layer. At this stage, punctate deposits are also localised at the perimeter of the Purkinje cell perikarya; these are no longer detected by P15. The data suggest a role for neuroplastins in the development and maintenance of normal synaptic connections in the cerebellum.

Brevican isoforms associate with neural membranes

Brevican is a neural-specific proteoglycan of the brain extracellular matrix, which is particularly abundant in the terminally differentiated CNS. It is expressed by neuronal and glial cells, and as a component of the perineuronal nets it decorates the surface of large neuronal somata and primary dendrites. One brevican isoform harbors a glycosylphosphatidylinositol anchor attachment site and, as shown by ethanolamine incorporation studies, is indeed glypiated in stably transfected HEK293 cells as well as in oligodendrocyte precursor Oli-neu cells. The major isoform is secreted into the extracellular space, although a significant amount appears to be tightly attached to the cell membrane, as it floats up in sucrose gradients. Flotation is sensitive to detergent treatment. Brevican is most prominent in the microsomal, light membrane and synaptosomal fractions of rat brain membrane preparations. The association with the particulate fraction is in part sensitive to chondroitinase ABC and phosphatidylinositol-specific phospholipase C treatment. Furthermore, brevican staining on the surface of hippocampal neurons in culture is diminished after hyaluronidase or chondroitinase ABC treatment. Taken together, this could provide a mechanism by which perineuronal nets are anchored on neuronal surfaces.

CD147 is tightly associated with lactate transporters MCT1 and MCT4 and facilitates their cell surface expression

CD147 is a broadly expressed plasma membrane glycoprotein containing two immunoglobulin-like domains and a single charge-containing transmembrane domain. Here we use co-immunoprecipitation and chemical cross-linking to demonstrate that CD147 specifically interacts with MCT1 and MCT4, two members of the proton-linked monocarboxylate (lactate) transporter family that play a fundamental role in metabolism, but not with MCT2. Studies with a CD2-CD147 chimera implicate the transmembrane and cytoplasmic domains of CD147 in this interaction. In heart cells, CD147 and MCT1 co-localize, concentrating at the t-tubular and intercalated disk regions. In mammalian cell lines, expression is uniform but cross-linking with anti-CD147 antibodies caused MCT1, MCT4 and CD147, but not GLUT1 or MCT2, to redistribute together into 'caps'. In MCT-transfected cells, expressed protein accumulated in a perinuclear compartment, whereas co-transfection with CD147 enabled expression of active MCT1 or MCT4, but not MCT2, in the plasma membrane. We conclude that CD147 facilitates proper expression of MCT1 and MCT4 at the cell surface, where they remain tightly bound to each other. This association may also be important in determining their activity and location.

The synaptic glycoprotein neuroplastin is involved in long-term potentiation at hippocampal CA1 synapses

Neuroplastin-65 and -55 (previously known as gp65 and gp55) are glycoproteins of the Ig superfamily that are enriched in rat forebrain synaptic membrane preparations. Whereas the two-Ig domain isoform neuroplastin-55 is expressed in many tissues, the three-Ig domain isoform neuroplastin-65 is brain-specific and enriched in postsynaptic density (PSD) protein preparations. Here, we have assessed the function of neuroplastin in long-term synaptic plasticity. Immunocytochemical studies with neuroplastin-65-specific antibodies differentially stain distinct synaptic neuropil regions of the rat hippocampus with most prominent immunoreactivity in the CA1 region and the proximal molecular layer of the dentate gyrus. Kainate-induced seizures cause a significant enhancement of neuroplastin-65 association with PSDs. Similarly, long-term potentiation (LTP) of CA1 synapses in hippocampal slices enhanced the association of neuroplastin-65 with a detergent-insoluble PSD-enriched protein fraction. Several antibodies against the neuroplastins, including one specific for neuroplastin-65, inhibited the maintenance of LTP. A similar effect was observed when recombinant fusion protein containing the three extracellular Ig domains of neuroplastin-65 was applied to hippocampal slices before LTP induction. Microsphere binding experiments using neuroplastin-F(c) chimeric proteins show that constructs containing Ig1-3 or Ig1 domains, but not Ig2-3 domains mediate homophilic adhesion. These data suggest that neuroplastin plays an essential role in implementing long-term changes in synaptic activity, possibly by means of a homophilic adhesion mechanism.

Immunoglobulin superfamily members gp65 and gp55: tissue distribution of glycoforms

Gp65 and gp55 are immunoglobulin superfamily members produced by alternative splicing of the same gene transcript, and originally identified as components of synaptic membranes. A monoclonal antibody specific for gp65 and gp55 has been used to detect immunoreactive species in a wide range of tissues. All immunoreactive species bind to concanavalin A and deglycosylation studies show that in all tissues tested other than brain the immunoreactive species are derived from gp55. HEK cells transfected with gp65 or gp55 express different glycoforms from brain showing that the pattern of glycosylation of these molecules is dependent upon the cell type in which they are expressed.

Synaptic membrane glycoproteins gp65 and gp55 are new members of the immunoglobulin superfamily

Glycoproteins gp65 and gp55 are major components of synaptic membranes prepared from rat forebrain. Both are recognized by the monoclonal antibody SMgp65. We have used SMgp65 to screen a rat brain cDNA expression library. Two sets of overlapping cDNAs that contain open reading frames of 397 and 281 amino acids were isolated. The deduced proteins are members of the immunoglobulin (Ig) superfamily containing three and two Ig domains, respectively. The common part has approximately 40% sequence identity with neurothelin/basigin. The identity of the proteins as gp65 and gp55 was confirmed by production of new antisera against a common recombinant protein fragment. These antisera immunoprecipitate gp65 and gp55. Furthermore, expression of gp65 and gp55 cDNAs in human 293 cells treated with tunicamycin results in the production of unglycosylated core proteins of identical size to deglycosylated gp65 and gp55. Northern analysis revealed that gp65 transcripts are brain-specific, whereas gp55 is expressed in most tissues and cell lines examined. The tissue distribution was confirmed at the protein level though the pattern of glycosylation of gp55 varies between tissues. In situ hybridization experiments with a common and a gp65-specific probe suggest differential expression of gp65 and gp55 transcripts in the rat brain.

Expression of PAC 1, an epitope associated with two synapse-enriched glycoproteins and a neuronal cytoskeleton-associated polypeptide in developing forebrain neurons

The monoclonal antibody PAC 1 (postsynaptic density and cytoskeleton enriched) recognizes an epitope present on two postsynaptic density-enriched glycoproteins of 130,000 (postsynaptic density-enriched glycoprotein 130) and 117,000 mol. wt (postsynaptic density-enriched glycoprotein 117), and a cytoskeleton-enriched polypeptide of 155,000 mol. wt (cp155). The PAC 1 antibody has been used to study the development of the PAC 1 antigens in the developing rat forebrain in vivo and in tissue culture. cp155 is detected by embryonic day 14 and its level continues to rise until the sixth postnatal week. Postsynaptic density-enriched glycoproteins 130 and 117 are also expressed in embryonic brain although the level of postsynaptic density-enriched glycoprotein 130 initially increases more rapidly than that of postsynaptic density-enriched glycoprotein 117. Peak values are observed at postnatal days 4 (postsynaptic density-enriched glycoprotein 117) and 9 (postsynaptic density-enriched glycoprotein 130). The level of post synaptic density-enriched glycoprotein 117 subsequently decreases to some 50% of the peak value by postnatal day 42. Immunocytochemical studies show that PAC 1 immunoreactivity in developing cerebral cortex, detectable by postnatal day 0, is primarily associated with the perikarya and dendrites of pyramidal cells. The immunoreactivity develops as patches of PAC 1-positive neurons, uniform staining of the cortex only being fully established after postnatal day 9. Double-immunofluorescence labelling studies of forebrain cultures prepared from embryonic day 18 animals shows that many, but not all, growth-associated protein 43-positive neurons exhibit PAC 1 immunoreactivity. Some non-neuronal cells also stain with the PAC 1 monoclonal antibody. The growth cones of cultured neurons exhibit PAC 1 immunoreactivity and the PAC 1 antigens are detected on immunodeveloped western blots of isolated growth cones. The PAC 1 epitope is intracellular, but immunoreactivity does not co-localize with F-actin as detected by rhod-amine-phalloidin or with tubulin immunoreactivity. Postsynaptic density-enriched glycoprotein 130 is readily detected on PAC 1 immunodeveloped western blots of forebrain cultures maintained for up to 14 days in vitro. Postsynaptic density-enriched glycoprotein 117 is only poorly expressed by these cultures. The PAC 1 glycoproteins are present in forebrain synaptic membranes and postsynaptic densities at an early stage of development. The synaptic membrane level of postsynaptic density-enriched glycoprotein 130 and postsynaptic density-enriched glycoprotein 117 increases markedly between postnatal days 3 and 8. The level of both glycoproteins detected in postsynaptic densities remain virtually constant from postnatal days 9-90. These results are consistent with functional roles for these molecules in neuronal and synapse development.

Molecular cloning of a novel mRNA using an antibody directed against synaptic glycoproteins

It has been suggested by a number of investigators that glycoproteins may play a role in the development and/or maintenance of synapses in the mammalian CNS. For many synaptic glycoproteins, however, little precise structural or functional information is available. In an effort to isolate probes specific to individual glycoproteins, we have screened a rat brain cDNA expression library with a mixed polyclonal antibody directed against concanavalin A-binding synaptic junctional glycoproteins. Using this approach, we have previously reported the cloning of SC1, a putative extracellular matrix glycoprotein found in adult brain (Johnston et al., Neuron 2:165-176, 1990). We now report the cloning and characterization of a second novel cDNA, which has been designated SC2. Northern blots show that this cDNA recognizes a 1.2-kb mRNA that is present throughout postnatal development in the rat. It is expressed at high levels in brain and is also found at lower levels in several other tissues. In situ hybridization suggests that the SC2 mRNA is strongly expressed by many types of neurons. Sequence data reveals a single open reading frame in the cDNA, encoding a putative hydrophobic protein with a calculated molecular weight of 36.1 kDa. Sequence analysis reveals some similarity between SC2 and 5 alpha-reductase, a microsomal membrane protein important in testosterone metabolism.

Molecular characterisation and structural relationship of the synapse-enriched glycoproteins gp65 and gp55

gp65 and gp55 are glycoprotein components of CNS synapses that are recognised by a single monoclonal antibody, SMgp65. This antibody has now been used to investigate the molecular properties of these two glycoproteins and the structural relationship between them. Both gp65 and gp55 occur in most brain regions as doublets of apparent molecular masses of 63 and 67 kDa, and 52 and 57 kDa, respectively. Striatal samples, however, are enriched in a novel gp65 isoform of 69 kDa. Removal of oligosaccharide residues from gp65 and gp55 with trifluoromethanesulphonic acid shows that gp65 and gp55 are composed of single polypeptide chains of 40 and 28 kDa, respectively. Removal of sialic acid residues with neuraminidase lowers the apparent molecular mass of both glycoproteins by 5-6 kDa. Triton X-114 phase partitioning and alkaline extraction of synaptic membranes indicate that both gp65 and gp55 are integral membrane glycoproteins. Treatment of synaptic membranes with phosphatidylinositol-specific phospholipase C does not solubilise either glycoprotein. One-dimensional peptide and epitope maps obtained by digestion of gp65 and gp55 with endoproteinase lys C or subtilisin are consistent with a close structural relationship between the two molecules. Tryptic digestion of samples enriched in gp65 and/or gp55 results in the formation of a novel immunoreactive 53-kDa species that is resistant to further trypsin degradation except in the presence of 0.1% (wt/vol) sodium dodecyl sulphate. Trypsin treatment of cultures of forebrain neurones in situ lowers the apparent molecular mass of gp65 to 53 kDa.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of a marrow stroma and thymus-associated antigen (ST3) in the rat brain: comparison with Thy-1

Cells of the immunohemopoietic and nervous systems express certain molecules that generally are not found in other tissues. One example is the 'ST3' antigen, which is present on the major population of fibroblastoid cells grown from rat bone marrow, but is not detected on adherent cells from most peripheral organs (e.g. lung). An immunohistological survey revealed ST3 also in the thymic cortex, the glomerular mesangial area, and the brain. Because this pattern of distribution is similar to that described for Thy-1, we compared the localization of the two antigens in the adult rat brain and found that there were areas where it was congruent and others where it was distinct. Staining for ST3 was absent from the white matter, but was especially notable in discrete layers of the frontal, orbital, parietal, and cingulate cortices, the substantia nigra, the inferior olivary nuclei, and the deep molecular layer of the cerebellum, as well as other scattered regions in the gray matter. This is in contrast to Thy-1, which stained more diffusely throughout the gray zones. In further experiments using primary brain cell cultures, ST3 was demonstrated on neurons, but not on oligodendrocytes or astrocytes. Similarly, it was found on the surface of cells of the PC12 neuronal line, but not on the C6 astrocytoma. This restricted distribution on a subpopulation of neurons raises the possibility that the ST3 epitope might be part of a cell interaction molecule of the marrow stroma, thymus, and brain.

PAC 1: an epitope associated with two novel glycoprotein components of isolated postsynaptic densities and a novel cytoskeleton-associated polypeptide

A monoclonal antibody has been raised which recognizes an epitope, PAC 1 (postsynaptic density and cytoskeleton enriched), which is specifically associated with two novel glycoprotein components of forebrain postsynaptic density preparations and a novel neuronal cytoskeletal-associated polypeptide. The monoclonal antibody has been used to study the cellular and subcellular localization of these molecules and for the partial characterization of all three PAC 1 antigens in the rat. The PAC 1 epitope is present on two concanavalin A binding glycoproteins of apparent molecular weights 130,000 (pgp130) and 117,000 (pgp117). Both species are enriched in preparations of rat forebrain postsynaptic densities and to a lesser extent in synaptic membranes. The epitope is also expressed by a polypeptide of 155,000 mol. wt, cp155. This molecule is highly enriched in cytoskeleton rather than membrane preparations. Enzymic removal of N-linked carbohydrate lowers the molecular weights of the PAC 1 glycoproteins pgp130 and pgp117 by 11,000 and 14,000 respectively, and suggests that cp155 is not glycosylated. Detergent, alkaline and salt extractions of postsynaptic densities and synaptic membranes indicate that pgp130 and pgp117 are integral membrane glycoproteins and are tightly bound components of postsynaptic density preparations. Immunocytochemical studies of adult rat forebrain show prominent staining of pyramidal cell dendrites and perikarya. There is no evidence of glial staining. Electron microscope studies show staining of microtubules together with punctate deposits of plasma membrane-associated reaction product. Several criteria have been used to show that pgp130 and pgp117 do not correspond to other known neuronal glycoproteins of similar molecular weight. We conclude that the PAC 1 epitope is expressed by two novel synaptic glycoproteins which are very probably integral components of the postsynaptic density and by a novel neuronal cytoskeleton-associated protein.

Molecular cloning of SC1: a putative brain extracellular matrix glycoprotein showing partial similarity to osteonectin/BM40/SPARC

We describe the cloning of SC1, a novel cDNA that was selected from a rat brain expression library using a mixed polyclonal antibody directed against synaptic junction glycoproteins. SC1 detects a 3.2 kb mRNA expressed throughout postnatal development of the brain and present at high levels in the adult. In situ hybridization reveals that the SC1 mRNA is expressed widely in the brain and is present in many types of neurons. DNA sequence data suggest that the SC1 product is a secreted, calcium binding glycoprotein. Strikingly, the carboxy-terminal region of the SC1 protein shows substantial similarity to the extracellular matrix glycoprotein osteonectin/BM40/SPARC. These data are consistent with the hypothesis that SC1 is an extracellular matrix glycoprotein in the brain.

Extracellular matrix of the superior olivary nuclei in the dog

The extracellular matrix around nerve cell bodies in canine lateral and medial superior olivary nuclei was examined by conventional electron microscopy, Golgi impregnation and histochemical techniques. Each neuron is surrounded by a region of myelin-free neuropil embedded amongst the myelinated fibres of the trapezoid body. In the myelin-free neuropil there are astrocytes, axons, synaptic boutons and extracellular matrix. The extracellular matrix fills the spaces between slender axons near the terminals, synaptic boutons and glial processes, but not the synaptic cleft. Golgi impregnation selectively stains the perineuronal nets which cover some of all of the nerve cell bodies and dendrites. The Golgi-EM method revealed that the impregnated profiles of the nets are restricted to the extracellular matrix. Synaptic boutons are situated in the holes of the perineuronal nets. Peanut (PNA) and soybean (SBA) agglutinins bound the extracellular matrix but not the synaptic boutons, glial processes, nerve cell bodies or basal lamina of blood capillaries. Light microscopic immunohistochemistry of the glial fibrillary acidic protein (GFAP) and S-100 protein did not stain a layer corresponding to the extracellular matrix and synapses but showed an intensely positive reaction immediately outside this layer. These data suggest the existence of a unique microenvironments associated with glycoconjugates around nerve cell bodies in canine superior olivary nuclei.