Dysregulation of synaptic and developmental transcriptomic/proteomic profiles upon depletion of MUNC18-1

Absence of presynaptic protein MUNC18-1 (gene: Stxbp1) leads to neuronal cell death at an immature stage before synapse formation. Here, we performed transcriptomic and proteomic profiling of immature Stxbp1 knockout (KO) cells to discover which cellular processes depend on MUNC18-1. Hippocampi of Stxbp1 KO mice showed cell-type specific dysregulation of 2123 transcripts primarily related to synaptic transmission and immune response. To further investigate direct, neuron-specific effects of MUNC18-1 depletion, a proteomic screen was performed on murine neuronal cultures at two developmental timepoints prior to onset of neuron degeneration. 399 proteins were differentially expressed, which were primarily involved in synaptic function (especially synaptic vesicle exocytosis) and neuron development. We further show that many of the downregulated proteins upon loss of MUNC18-1 are normally upregulated during this developmental stage. Thus, absence of MUNC18-1 extensively dysregulates the transcriptome and proteome, primarily affecting synaptic and developmental profiles. Lack of synaptic activity is unlikely to underlie these effects, as the changes were observed in immature neurons without functional synapses, and minimal overlap was found to activity-dependent proteins. We hypothesize that presence of MUNC18-1 is essential to advance neuron development, serving as a 'checkpoint' for neurons to initiate cell death in its absence.Significance StatementPresynaptic protein MUNC18-1 is essential for neuronal functioning. Pathogenic variants in its gene, STXBP1, are among the most common found in patients with developmental delay and epilepsy. To discern the pathogenesis in these patients, a thorough understanding of MUNC18-1's function in neurons is required. Here, we show that loss of MUNC18-1 results in extensive dysregulation of synaptic and developmental proteins in immature neurons before synapse formation. Many of the downregulated proteins are normally upregulated during this developmental stage. This indicates that MUNC18-1 is a critical regulator of neuronal development, which could play an important role in the pathogenesis of STXBP1 variant carriers.

Stitching the synapse: Cross-linking mass spectrometry into resolving synaptic protein interactions

Synaptic transmission is the predominant form of communication in the brain. It requires functionally specialized molecular machineries constituted by thousands of interacting synaptic proteins. Here, we made use of recent advances in cross-linking mass spectrometry (XL-MS) in combination with biochemical and computational approaches to reveal the architecture and assembly of synaptic protein complexes from mouse brain hippocampus and cerebellum. We obtained 11,999 unique lysine-lysine cross-links, comprising connections within and between 2362 proteins. This extensive collection was the basis to identify novel protein partners, to model protein conformational dynamics, and to delineate within and between protein interactions of main synaptic constituents, such as Camk2, the AMPA-type glutamate receptor, and associated proteins. Using XL-MS, we generated a protein interaction resource that we made easily accessible via a web-based platform (http://xlink.cncr.nl) to provide new entries into exploration of all protein interactions identified.

A Potential Role for the STXBP5-AS1 Gene in Adult ADHD Symptoms

We aimed to detect Attention-deficit/hyperactivity (ADHD) risk-conferring genes in adults. In children, ADHD is characterized by age-inappropriate levels of inattention and/or hyperactivity-impulsivity and may persists into adulthood. Childhood and adulthood ADHD are heritable, and are thought to represent the clinical extreme of a continuous distribution of ADHD symptoms in the general population. We aimed to leverage the power of studies of quantitative ADHD symptoms in adults who were genotyped. Within the SAGA (Study of ADHD trait genetics in adults) consortium, we estimated the single nucleotide polymorphism (SNP)-based heritability of quantitative self-reported ADHD symptoms and carried out a genome-wide association meta-analysis in nine adult population-based and case-only cohorts of adults. A total of n = 14,689 individuals were included. In two of the SAGA cohorts we found a significant SNP-based heritability for self-rated ADHD symptom scores of respectively 15% (n = 3656) and 30% (n = 1841). The top hit of the genome-wide meta-analysis (SNP rs12661753; p-value = 3.02 × 10^-7) was present in the long non-coding RNA gene STXBP5-AS1. This association was also observed in a meta-analysis of childhood ADHD symptom scores in eight population-based pediatric cohorts from the Early Genetics and Lifecourse Epidemiology (EAGLE) ADHD consortium (n = 14,776). Genome-wide meta-analysis of the SAGA and EAGLE data (n = 29,465) increased the strength of the association with the SNP rs12661753. In human HEK293 cells, expression of STXBP5-AS1 enhanced the expression of a reporter construct of STXBP5, a gene known to be involved in "SNAP" (Soluble NSF attachment protein) Receptor" (SNARE) complex formation. In mouse strains featuring different levels of impulsivity, transcript levels in the prefrontal cortex of the mouse ortholog Gm28905 strongly correlated negatively with motor impulsivity as measured in the five choice serial reaction time task (r² = - 0.61; p = 0.004). Our results are consistent with an effect of the STXBP5-AS1 gene on ADHD symptom scores distribution and point to a possible biological mechanism, other than antisense RNA inhibition, involved in ADHD-related impulsivity levels.

Author Correction: Susceptible genes and disease mechanisms identified in frontotemporal dementia and frontotemporal dementia with Amyotrophic Lateral Sclerosis by DNA-methylation and GWAS

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Susceptible genes and disease mechanisms identified in frontotemporal dementia and frontotemporal dementia with Amyotrophic Lateral Sclerosis by DNA-methylation and GWAS

Frontotemporal dementia (FTD) is a neurodegenerative disorder predominantly affecting the frontal and temporal lobes. Genome-wide association studies (GWAS) on FTD identified only a few risk loci. One of the possible explanations is that FTD is clinically, pathologically, and genetically heterogeneous. An important open question is to what extent epigenetic factors contribute to FTD and whether these factors vary between FTD clinical subgroup. We compared the DNA-methylation levels of FTD cases (n = 128), and of FTD cases with Amyotrophic Lateral Sclerosis (FTD-ALS; n = 7) to those of unaffected controls (n = 193), which resulted in 14 and 224 candidate genes, respectively. Cluster analysis revealed significant class separation of FTD-ALS from controls. We could further specify genes with increased susceptibility for abnormal gene-transcript behavior by jointly analyzing DNA-methylation levels with the presence of mutations in a GWAS FTD-cohort. For FTD-ALS, this resulted in 9 potential candidate genes, whereas for FTD we detected 1 candidate gene (ELP2). Independent validation-sets confirmed the genes DLG1, METTL7A, KIAA1147, IGHMBP2, PCNX, UBTD2, WDR35, and ELP2/SLC39A6 among others. We could furthermore demonstrate that genes harboring mutations and/or displaying differential DNA-methylation, are involved in common pathways, and may therefore be critical for neurodegeneration in both FTD and FTD-ALS.

Cognitive flexibility deficits in a mouse model for the absence of full-length dystrophin

Duchenne muscular dystrophy (DMD) is a progressive muscle-wasting disorder, caused by mutations in the DMD gene and the resulting lack of dystrophin. The DMD gene has seven promoters, giving rise to multiple full-length and shorter isoforms. Besides the expression of dystrophin in muscles, the majority of dystrophin isoforms is expressed in brain and dystrophinopathy can lead to cognitive deficits, including intellectual impairments and deficits in executive function. In contrast to the muscle pathology, the impact of the lack of dystrophin on the brain is not very well studied. Here, we study the behavioral consequences of a lack of full-length dystrophin isoforms in mdx mice, particularly with regard to domains of executive functions and anxiety. We observed a deficit in cognitive flexibility in mdx mice in the absence of motor dysfunction or general learning impairments using two independent behavioral tests. In addition, increased anxiety was observed, but its expression depended on the context. Overall, these results suggest that the absence of full-length dystrophin in mice has specific behavioral effects that compare well to deficits observed in DMD patients.

Neurobiological changes by cytotoxic agents in mice

Cognitive deficit is a frequently reported side-effect of adjuvant chemotherapy. A large number of animal studies has been performed to examine the neurobiological mechanisms underlying this phenomenon, however, definite conclusions from these studies are restricted due to differences in experimental set-up. We systematically investigated the effects of 6 cytotoxic agents on various neurobiological parameters. C57Bl/6J mice were treated with cyclophosphamide, docetaxel, doxorubicin, 5-fluorouracil, methotrexate, or topotecan. The animals were sacrificed 3 or 15 weeks after treatment and the effect on neurogenesis, blood vessel density, and neuroinflammation was analyzed using immunohistochemistry. None of the cytostatic agents tested affected neurogenesis (cell survival or cell proliferation). Blood vessel density was increased in the hippocampus and prefrontal cortex 3 weeks after treatment with docetaxel and doxorubicin compared with control animals. A decrease in the number of microglial cells was observed in the prefrontal cortex after treatment with cyclophosphamide, docetaxel, 5-FU, and topotecan compared with control mice. The observed decrease in microglia cells is indicative of inflammation that occurred after treatment. Overall, the magnitude of the effects was relatively modest. Therefore, we conducted a similar study with topotecan in Abcg2;Abcb1a/b knock out and wildtype FVB mice. Animals were sacrificed 3 weeks after treatment and no notable effect was seen in hippocampal cell differentiation (DCX), microglia activation, or blood vessel density. Perhaps the FVB strain is more resistant to the neurotoxic effects of topotecan which makes this not the correct model to study the mechanism of chemotherapy-induced cognitive impairment.

HCN channels are a novel therapeutic target for cognitive dysfunction in Neurofibromatosis type 1

Cognitive impairments are a major clinical feature of the common neurogenetic disease neurofibromatosis type 1 (NF1). Previous studies have demonstrated that increased neuronal inhibition underlies the learning deficits in NF1, however, the molecular mechanism underlying this cell-type specificity has remained unknown. Here, we identify an interneuron-specific attenuation of hyperpolarization-activated cyclic nucleotide-gated (HCN) current as the cause for increased inhibition in Nf1 mutants. Mechanistically, we demonstrate that HCN1 is a novel NF1-interacting protein for which loss of NF1 results in a concomitant increase of interneuron excitability. Furthermore, the HCN channel agonist lamotrigine rescued the electrophysiological and cognitive deficits in two independent Nf1 mouse models, thereby establishing the importance of HCN channel dysfunction in NF1. Together, our results provide detailed mechanistic insights into the pathophysiology of NF1-associated cognitive defects, and identify a novel target for clinical drug development.

Progression and recovery of Parkinsonism in a chronic progressive MPTP-induction model in the marmoset without persistent molecular and cellular damage

Chronic exposure to low-dose 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in marmoset monkeys was used to model the prodromal stage of Parkinson's disease (PD), and to investigate mechanisms underlying disease progression and recovery. Marmosets were subcutaneously injected with MPTP for a period of 12weeks, 0.5mg/kg once per week, and clinical signs of Parkinsonism, motor- and non-motor behaviors were recorded before, during and after exposure. In addition, postmortem immunohistochemistry and proteomics analysis were performed. MPTP-induced parkinsonian clinical symptoms increased in severity during exposure, and recovered after MPTP administration was ended. Postmortem analyses, after the recovery period, revealed no alteration of the number and sizes of tyrosine hydroxylase (TH)-positive dopamine (DA) neurons in the substantia nigra. Also levels of TH in putamen and caudate nucleus were unaltered, no differences were observed in DA, serotonin or nor-adrenalin levels in the caudate nucleus, and proteomics analysis revealed no global changes in protein expression in these brain areas between treatment groups. Our findings indicate that parkinsonian symptoms can occur without detectable damage at the cellular or molecular level. Moreover, we show that parkinsonian symptoms may be reversible when diagnosed and treated early.

Functional characterization of the PCLO p.Ser4814Ala variant associated with major depressive disorder reveals cellular but not behavioral differences

Genome-wide association studies have suggested a role for a genetic variation in the presynaptic gene PCLO in major depressive disorder (MDD). As with many complex traits, the PCLO variant has a small contribution to the overall heritability and the association does not always replicate. One variant (rs2522833, p.Ser4814Ala) is of particular interest given that it is a common, nonsynonymous exon variant near a calcium-sensing part of PCLO. It has been suggested that the molecular effects of such variations penetrate to a variable extent in the population due to phenotypic and genotypic heterogeneity at the population level. More robust effects may be exposed by studying such variations in isolation, in a more homogeneous context. We tested this idea by modeling PCLO variation in a mouse knock-in model expressing the Pclo(SA)(/)(SA) variant. In the highly homogeneous background of inbred mice, two functional effects of the SA-variation were observed at the cellular level: increased synaptic Piccolo levels, and 30% increased excitatory synaptic transmission in cultured neurons. Other aspects of Piccolo function were unaltered: calcium-dependent phospholipid binding, synapse formation in vitro, and synaptic accumulation of synaptic vesicles. Moreover, anxiety, cognition and depressive-like behavior were normal in Pclo(SA)(/)(SA) mice. We conclude that the PCLO p.Ser4814Ala missense variant produces mild cellular phenotypes, which do not translate into behavioral phenotypes. We propose a model explaining how (subtle) cellular phenotypes do not penetrate to the mouse behavioral level but, due to genetic and phenotypic heterogeneity and non-linearity, can produce association signals in human population studies.

Cognitive impact of cytotoxic agents in mice

RATIONALE AND OBJECTIVES

Adjuvant chemotherapy is associated with changes in cognition in a subgroup of cancer patients. Chemotherapy is generally given as a combination of cytotoxic agents, which makes it hard to define the agent responsible for these observed changes. Literature on animal experiments has been difficult to interpret due to variance in experimental setup.

METHODS

We examined the effects of cytotoxic agents administered separately on various cognitive measures in a standardized animal model. Male C57Bl/6 mice received cyclophosphamide, docetaxel, doxorubicin, 5-fluorouracil, methotrexate, or topotecan. These agents represent different compound classes based on their working mechanism and are frequently prescribed in the clinic. A control group received saline. Behavioral testing started 2 or 15 weeks after treatment and included testing general measures of behavior and cognitive task performance: spontaneous behavior in an automated home cage, open field, novel location recognition (NLR), novel object recognition (NOR), Barnes maze, contextual fear conditioning, and a simple choice reaction time task (SCRTT).

RESULTS

Cyclophosphamide, docetaxel, and doxorubicin administration affected spontaneous activity in the automated home cage. All cytotoxic agents affected memory (NLR and/or NOR). Spatial memory measured in the Barnes maze was affected after administration with doxorubicin, 5-fluorouracil, and topotecan. Decreased inhibition in the SCRTT was observed after treatment with cyclophosphamide, docetaxel, and topotecan.

CONCLUSIONS

Our data show that, in mice, a single treatment with a cytotoxic agent causes cognitive impairment. Not all cytotoxic agents affected the same cognitive domains, which might be explained by differences in working mechanisms of the various agents.

Functional gene group analysis identifies synaptic gene groups as risk factor for schizophrenia

Schizophrenia is a highly heritable disorder with a polygenic pattern of inheritance and a population prevalence of ~1%. Previous studies have implicated synaptic dysfunction in schizophrenia. We tested the accumulated association of genetic variants in expert-curated synaptic gene groups with schizophrenia in 4673 cases and 4965 healthy controls, using functional gene group analysis. Identifying groups of genes with similar cellular function rather than genes in isolation may have clinical implications for finding additional drug targets. We found that a group of 1026 synaptic genes was significantly associated with the risk of schizophrenia (P=7.6 × 10(-11)) and more strongly associated than 100 randomly drawn, matched control groups of genetic variants (P<0.01). Subsequent analysis of synaptic subgroups suggested that the strongest association signals are derived from three synaptic gene groups: intracellular signal transduction (P=2.0 × 10(-4)), excitability (P=9.0 × 10(-4)) and cell adhesion and trans-synaptic signaling (P=2.4 × 10(-3)). These results are consistent with a role of synaptic dysfunction in schizophrenia and imply that impaired intracellular signal transduction in synapses, synaptic excitability and cell adhesion and trans-synaptic signaling play a role in the pathology of schizophrenia.

High-throughput phenotyping of avoidance learning in mice discriminates different genotypes and identifies a novel gene

Recognizing and avoiding aversive situations are central aspects of mammalian cognition. These abilities are essential for health and survival and are expected to have a prominent genetic basis. We modeled these abilities in eight common mouse inbred strains covering ∼75% of the species' natural variation and in gene-trap mice (>2000 mice), using an unsupervised, automated assay with an instrumented home cage (PhenoTyper) containing a shelter with two entrances. Mice visited this shelter for 20-1200 times/24 h and 71% of all mice developed a significant and often strong preference for one entrance. Subsequently, a mild aversive stimulus (shelter illumination) was automatically delivered when mice used their preferred entrance. Different genotypes developed different coping strategies. Firstly, the number of entries via the preferred entrance decreased in DBA/2J, C57BL/6J and 129S1/SvImJ, indicating that these genotypes associated one specific entrance with the aversive stimulus. Secondly, mice started sleeping outside (C57BL/6J, DBA/2J), indicating they associated the shelter, in general, with the aversive stimulus. Some mice showed no evidence for an association between the entrance and the aversive light, but did show markedly shorter shelter residence times in response to illumination, indicating they did perceive illumination as aversive. Finally, using this assay, we screened 43 different mutants, which yielded a novel gene, specc1/cytospinB. This mutant showed profound and specific delay in avoidance learning. Together, these data suggest that different genotypes express distinct learning and/or memory of associations between shelter entrance and aversive stimuli, and that specc1/cytospinB is involved in this aspect of cognition.

Reduction in hippocampal neurogenesis after social defeat is long-lasting and responsive to late antidepressant treatment

Major depressive disorder is a chronic disabling disease, often triggered and exacerbated by stressors of a social nature. Hippocampal volume reductions have been reported in depressed patients. In support of the neurogenesis theory of depression, in several stress-based animal models of depression, adult hippocampal neurogenesis was reduced and subsequently rescued by parallel antidepressant treatment. Here, we investigated whether repeated social defeat and subsequent individual housing for 3 months induces long-lasting changes in adult hippocampal neurogenesis in rats, and whether these can be normalized by late antidepressant treatment, as would match human depression. Neurogenesis was analysed by stereological quantification of the number of immature doublecortin (DCX)-immunopositive cells, in particular young (class I) and more mature (class II) DCX(+) cells, to distinguish differential effects of stress or drug treatment on these subpopulations. Using this social defeat paradigm, the total DCX(+) cell number was significantly reduced. This was most profound for older (class II) DCX(+) cells with long apical dendrites, whereas younger, class I cells remained unaffected. Treatment with the broad-acting tricyclic antidepressant imipramine, only during the last 3 weeks of the 3-month period after social defeat, completely restored the reduction in neurogenesis by increasing both class I and II DCX(+) cell populations. We conclude that despite the lack of elevated corticosterone plasma levels, neurogenesis is affected in a lasting manner by a decline in a distinct neuronal population of more mature newborn cells. Thus, the neurogenic deficit induced by this social defeat paradigm is long-lasting, but can still be normalized by late imipramine treatment.

Rotations, Vibrations and Structure in Solid C60: Investigations by Neutron Scattering

In a series of experiments on solid C60, the low-energy rotational dynamics of the molecules, the higher-energy vibrational spectra, and aspects of disorder in the static structure, have been studied. The nearly spherical shape of the C60 molecule, and the occurrence of an orientational phase transition at Tc=256 K, make the C60 solid an excellent system for investigations of orientational dynamics. Coherent quasi-elastic neutron scattering above Tc is described quantitatively by a rotational diffusion model. Below Tc, inelastic scattering froma soft librational mode is observed between 2 and 3 meV. Models of the strong, temperaturedependent diffuse scattering due to orientational disorder are discussed. Finally, measurements of the density of states for the intramolecular ( E ≥ 33 meV) vibrational modes are also presented.

Transcriptome analysis of the central nervous system of the mollusc Lymnaea stagnalis

Background

The freshwater snail Lymnaea stagnalis (L. stagnalis) has served as a successful model for studies in the field of Neuroscience. However, a serious drawback in the molecular analysis of the nervous system of L. stagnalis has been the lack of large-scale genomic or neuronal transcriptome information, thereby limiting the use of this unique model.

Results

In this study, we report 7,712 distinct EST sequences (median length: 847 nucleotides) of a normalized L. stagnalis central nervous system (CNS) cDNA library, resulting in the largest collection of L. stagnalis neuronal transcriptome data currently available. Approximately 42% of the cDNAs can be translated into more than 100 consecutive amino acids, indicating the high quality of the library. The annotated sequences contribute 12% of the predicted transcriptome size of 20,000. Surprisingly, approximately 37% of the L. stagnalis sequences only have a tBLASTx hit in the EST library of another snail species Aplysia californica (A. californica) even using a low stringency e-value cutoff at 0.01. Using the same cutoff, approximately 67% of the cDNAs have a BLAST hit in the NCBI non-redundant protein and nucleotide sequence databases (nr and nt), suggesting that one third of the sequences may be unique to L. stagnalis. Finally, using the same cutoff (0.01), more than half of the cDNA sequences (54%) do not have a hit in nematode, fruitfly or human genome data, suggesting that the L. stagnalis transcriptome is significantly different from these species as well. The cDNA sequences are enriched in the following gene ontology functional categories: protein binding, hydrolase, transferase, and catalytic enzymes.

Conclusion

This study provides novel molecular insights into the transcriptome of an important molluscan model organism. Our findings will contribute to functional analyses in neurobiology, and comparative evolutionary biology. The L. stagnalis CNS EST database is available at .

Proteomics analysis of immuno-precipitated synaptic protein complexes

Synapses are key neuronal elements of the brain. They are responsible for transmission, integration, and storage of information between nerve cells. A synapse is considered as the most complex cellular organelle consisting of approximately 1500 of proteins that are interacting in an activity dependent manner. We have initiated a series of immuno-precipitation experiments in conjunction with LC-MS/MS analysis in order to gain better insight into the organization of the synapse. In particular, we focused on proteins that have been implicated previously in the process of neuroplasticity, i.e., the glutamate receptor (GluR2), scaffolding proteins (PSD-95 and CASK), voltage gated potassium (KCNQ2 and Kv4.2) and calcium (CaV beta4) channel subunits, the signalling protein (GIT1) and synaptic vesicle protein (synaptophysin). This study confirms the previous reported protein-protein interactions and furthermore detects novel interactors. In conjunction with the literature reported protein-protein interaction a simple synaptic protein interactome was constructed. This model implicates the potential interaction of distinct protein complexes, and the engagement of single proteins, especially the scaffolding proteins, in multiple protein complexes.

Strain specificity and cholinergic modulation of visuospatial attention in three inbred mouse strains

The tremendous increase in the use of mouse inbred strains and mutant mice to study the molecular basis of psychiatric disorders urges for a better understanding of attentional performance in mice. To this aim, we investigated possible strain differences in performance and cholinergic modulation of visuospatial attention in three widely used mouse inbred strains (129S2/SvHsd, C57BL/6JOlaHsd and DBA/2OlaHsd) in the five-choice serial reaction time task. Results indicated that after extended training, performance of 129S2/SvHsd mice was superior to that of C57BL/6JOlaHsd and DBA/2OlaHsd mice in terms of attention, omission errors, inhibitory control and latencies to correct choice. Increasing the attentional load resulted in comparable decrements in attention in all strains and inhibitory control impairments that were most pronounced in DBA/2OlaHsd mice. Further pharmacological evaluation indicated that all strains showed attentional impairments after treatment with the muscarinic and nicotinic antagonists scopolamine and mecamylamine, respectively. 129S2/SvHsd mice were less sensitive, whereas DBA/2OlaHsd mice appeared more sensitive to the detrimental effects of mecamylamine. In addition, subchronic, but not acute, nicotine treatment slightly improved attentional performance in all strains to the same extent. In conclusion, our data indicate strain specificity with particularly good performance of 129S2/SvHsd mice and strong cholinergic involvement in visuospatial attention in mice.

Reduced expression of neuropeptide genes in a genome-wide screen of a secretion-deficient mouse

Activity-dependent changes in synapses rely on functional changes in resident proteins and on gene expression. We addressed the relationship between synapse activity and the expression of synaptic genes by comparing RNA levels in the neocortex of normal mice versus secretion-deficient and therefore synaptically silent munc18-1 (mammalian homologue of Caenorhabditis elegans uncoordinated locomotion-18) null mutants, using microarray expression analysis, real-time quantitative PCR and northern blotting. We hypothesized that genes under the control of synaptic activity would be differentially expressed between mutants and controls. We found that few synaptic genes were differentially expressed. However, most neuropeptide genes with detectable expression on the microarray were differentially expressed, being expressed 3-20-fold higher in control cortex. Several other secreted proteins were also differentially expressed, but genes encoding their receptors and many other synaptic components were not. Differential expression was confirmed by real-time quantitative PCR analysis. In situ hybridization indicated that the difference in neuropeptide expression was uniform and not due to the loss of specific cells in the mutant. In primary sensory neurons, which do not depend on synaptic activity for their input, the differential expression of neuropeptides was not observed. These data argue against a general relationship between the activity of synapses and the expression of their resident proteins, but suggest a link between secretion and the expression of genes encoding the secreted products.

Peptidomics Analysis of Neuropeptides Involved in Copulatory Behavior of the Mollusk Lymnaea stagnalis

Male copulation behavior in mollusks is controlled by an array of peptide messengers. In the present study, we have used a peptidomics approach employing liquid chromatography in conjunction with electrospray mass spectrometry to characterize peptides contained in the penial complex of the freshwater snail, Lymnaea stagnalis. In addition to the previously described peptides, we have identified a group of novel peptides that share the carboxyl termini of -FVRIamide. A cDNA cloning study revealed the organization of the precursor, which contains 20 peptide domains with the carboxyl termini of -F(X)RIamide which are flanked by many putative proteolytic sites including the KR and the less commonly occurring (G)K and (G)R sites. In addition, there are several monobasic R and dibasic RR and KK sites that may be used for processing. We then used MALDI-TOF/TOF-MS in a data-dependent mode, which selected all the molecular ion species with the predicted masses of the mature -F(X)RIamide peptides, and performed MS/MS analysis on these peptides. This approach allowed us to identify all the predicted -F(X)RIamide peptides. Immunocytochemistry showed the localization of -FVRIamide immunoreactive neurons in several central ganglia, and immunoreactive axons in the penial complex. Finally, application of synthetic -FVRIamide peptides to an in vitro posterior vas deferens preparation showed inhibitory effect on the spontaneous contraction/relaxation cycle of the vas deferens.

The molluscan RING-finger protein L-TRIM is essential for neuronal outgrowth

The tripartite motif proteins TRIM-2 and TRIM-3 have been put forward as putative organizers of neuronal outgrowth and structural plasticity. Here, we identified a molluscan orthologue of TRIM-2/3, named L-TRIM, which is up-regulated during in vitro neurite outgrowth of central neurons. In adult animals, L-Trim mRNA is ubiquitously expressed at low levels in the central nervous system and in peripheral tissues. Central nervous system expression of L-Trim mRNA is increased during postnatal brain development and during in vitro and in vivo neuronal regeneration. In vitro double-stranded RNA knock-down of L-Trim mRNA resulted in a >70% inhibition of neurite outgrowth. Together, our data establish a crucial role for L-TRIM in developmental neurite outgrowth and functional neuronal regeneration and indicate that TRIM-2/3 family members may have evolutionary conserved functions in neuronal differentiation.

Auto-inhibitory control of peptidergic molluscan neurons and reproductive senescence

We recently, characterized a novel peptide of the egg-laying controlling caudodorsal cells (CDC) of Lymnaea stagnalis. Here, we show that the novel peptide has autoinhibitory actions and its expression is significantly up-regulated in reproductively senescent animals. Intracellular recordings show that when bath-applied to the central nervous system in vitro, the peptide reduces the depolarizing after potential (DAP) in CDCs without affecting the action potential-threshold and -amplitude and the resting membrane potential. Moreover, peptide application can terminate an ongoing after discharge in the CDCs or, when electrical stimulation fails to induce an after discharge, can terminate the long-lasting depolarization. Semiquantitative peptide profiling by mass spectrometry demonstrated correct processing and targeting of peptides in the CDC somata and axon terminals of reproductively senescent animals. Interestingly, the level of the autoinhibitory peptide was selectively increased in the CDCs of reproductively senescent animals. Our results indicate that a shift in balance between excitatory and inhibitory auto-transmitter peptides in the CDC system of old non-egg-laying animals, plays a role in after discharge failure in CDCs of reproductively senescent Lymnaea.

LFRFamides: a novel family of parasitation-induced -RFamide neuropeptides that inhibit the activity of neuroendocrine cells in Lymnaea stagnalis

We report the characterization of a cDNA encoding a novel -RFamide neuropeptide precursor that is up-regulated during parasitation in the snail Lymnaea stagnalis. Processing of this precursor yields five structurally related neuropeptides, all but one ending with the C-terminal sequence -LFRFamide, as was confirmed by direct mass spectrometry of brain tissue. The LFRFamide gene is expressed in a small cluster of neurons in each buccal ganglion, three small clusters in each cerebral ganglion, and one cluster in each lateral lobe of the cerebral ganglia. Application of two of the LFRFamide peptides to neuroendocrine cells that control either growth and metabolism or reproduction induced similar hyperpolarizing K+-currents, and inhibited electrical activity. We conclude that up-regulation of inhibitory LFRFamide neuropeptides during parasitation probably reflects an evolutionary adaptation that allows endoparasites to suppress host metabolism and reproduction in order to fully exploit host energy recourses.

Spatio-temporal dynamics of the egg-laying-inducing peptides during an egg-laying cycle: a semiquantitative matrix-assisted laser desorption/ionization mass spectrometry approach

The activity-dependent release of peptides from the neuro-endocrine caudodorsal cell (CDC) system of the freshwater snail Lymnaea stagnalis regulates egg laying and related behaviors. In this study, we optimized a mass spectrometry-based approach to study the spatio-temporal dynamics of peptides that are largely derived from the CDC hormone precursor during an egg-laying cycle and a CDC discharge in vitro. Semi-quantitative peptide mass profiling using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) indicated a massive depletion of peptides from the neurohemal area in the cerebral commissure (COM) during egg laying and the existence of a reserve pool of peptides in the CDC somata that were transported to the COM to restore peptide levels. The depletion of CDC peptides from the COM was correlated to their release during an induced electrical discharge in vitro. Moreover, MALDI-MS of the releasate revealed extensive truncation of the carboxyl terminal peptide. Finally, two novel peptides of 1788 and 5895 Da, not encoded by the CDC hormone precursor, also exhibited temporal quantitative changes similar to those of CDC peptides. Sequencing of the peptide of 1788 Da by tandem mass spectrometry yielded the novel sequence HF(FH)FYGPYDVFQRDVamide. Together, this implicates a more complex set of CDC peptides for the regulation of egg laying than previously anticipated.

GABAA receptor maturation in relation to eye opening in the rat visual cortex

Changes in subunit composition of N-methyl-D-aspartate (NMDA) receptors have been reported to be affected by visual experience and may therefore form a major aspect of neuronal plasticity in the CNS during development. In contrast, putative alterations in the expression and functioning of the inhibitory GABAA receptor around eye opening have not been well defined yet. Here we describe the timing of changes in GABAA receptor subunit expression and the related synaptic functioning in the neonatal rat visual cortex and the influence of visual experience on this process. Quantitative analysis of all GABAA receptor subunit transcripts revealed a marked alpha3 to alpha1 subunit switch, in addition to a change in alpha4 and alpha5 expression. The changes were correlated with an acceleration of the decay of spontaneous inhibitory postsynaptic currents (sIPSCs). Both changes in receptor expression and synaptic functioning were initiated well before eye opening. Moreover, dark rearing could not prevent the robust upregulation of alpha1 or the change in sIPSC kinetics, indicating that this is not dependent of sensory (visual) input. Upon eye opening a positive correlation was observed between a faster decay of the sIPSCs and an increase in sIPSC frequency, which was absent in dark-reared animals. Thus, lack of extrinsic input to the cortex does not affect overall developmental regulation of synaptic functioning of GABAA receptors. However, we cannot exclude the possibility that visual experience is involved in proper shaping of the inhibitory network of the primary visual cortex.

Peptidergic modulation of male sexual behavior in Lymnaea stagnalis: structural and functional characterization of -FVamide neuropeptides

In the simultaneous hermaphrodite snail Lymnaea stagnalis, copulation as a male is controlled by neurons that send axons to the male copulatory organs via a single penis nerve. Using direct mass spectrometry of a penis nerve sample, we show that one of the molecular ions has a mass corresponding to GAPRFVamide, previously identified from the buccal ganglia, and named Lymnaea inhibitory peptide (LIP). The identity of this peptide is confirmed by partial peptide purification from the penis nerve, followed by post source decay mass spectrometry. We cloned the LIP-encoding cDNA, which predicts a prohormone that gives rise to five copies of LIP (now re-named LIP A), two other -FVamide peptides (LIPs B and C), and five structurally unrelated peptides. The LIP gene is expressed in neurons of the right cerebral ventral lobe that send their axons into the penis nerve. We show that the LIP A peptide is present in these neurons and in the penis nerve, and confirmed the presence of LIP B and C in the penis nerve by post source decay mass spectrometry. Finally, we demonstrate that LIP A, B and C inhibit the contractions of the penis retractor muscle, thereby implicating their role in male copulation behavior.

Perceptual ratings for pureed and molded peaches for individuals with and without impaired swallowing

Food molds have been created to alter the appearance of pureed foods by making the pureed consistencies look more like typically prepared foods. This study examined the perceptions of 15 adults with impaired swallowing and 15 adults with nonimpaired swallowing for three different types of pureed peach: a typically prepared pureed peach, a pureed peach with a thickening agent added, and a pureed peach with a thickening agent added and molded to look like peach slices. Results are presented for the attribute ratings of overall liking: liking of taste, texture, and appearance; and ease of chewing and swallowing. The results showed that the adults with impaired swallowing rated the attributes of chewing and swallowing significantly lower (i.e.. more difficult) for each type of puree. Significant differences within samples also were found for the attributes of overall liking and liking of taste. Overall, the pureed peach molds were not perceived favorably for any of the examined attributes. In addition, the molded peach was rated lower (more difficult) in its ease of chewing. The implications of these findings and ideas for further research are discussed.

Prostanoid receptor gene expression profile in human trabecular meshwork: a quantitative real-time PCR approach

PURPOSE

To assess the expression pattern of prostanoid receptor-encoding genes in trabecular meshwork (TM) of human donor eyes.

METHODS

Disposed human donor eyes (n = 10) were obtained from the Cornea Bank, Amsterdam. The TM was dissected from the scleral tissue and homogenized in lysis buffer, and total RNA was isolated. The RNA was converted into cDNA and used as a template for noncompetitive quantitative real-time polymerase chain reaction (PCR) using green fluorescent dye to quantify the accumulation of double-stranded PCR product. Specific primers for four housekeeping genes and DP, EP(1), EP(2), EP(3,) EP(4), FP, IP, and TP receptor-encoding transcripts were developed and tested for their efficiency.

RESULTS

The characterized expression profile was highly reproducible in all samples, with the EP(2) receptor-encoding transcript in the highest abundance, followed by FP, TP, IP, and EP(4) at levels that were approximately 10 to 15 times lower than that of the EP(2) subtype. DP and EP(3) were at the lowest levels, which were, on average, 45 times and 228 times lower than EP(2), respectively.

CONCLUSIONS

These data show that all prostanoid receptors are expressed at different levels in human TM tissue. Because the gene expression of the EP(2) receptor is, on average, 15 times more abundant than that of the EP(4) receptor, it may be expected that the increase in flow and cAMP levels in response to the activation of the EP receptors by application of prostaglandin E(1) (PGE(1)), is primarily mediated by the EP(2) receptor. These data should be considered when designing prostanoid receptor mimetics intended to enhance the aqueous humor outflow through the TM and Schlemm's canal.

The binding site of acetylcholine receptor as visualized in the X-Ray structure of a complex between alpha-bungarotoxin and a mimotope peptide

We have determined the crystal structure at 1.8 A resolution of a complex of alpha-bungarotoxin with a high affinity 13-residue peptide that is homologous to the binding region of the alpha subunit of acetylcholine receptor. The peptide fits snugly to the toxin and adopts a beta hairpin conformation. The structures of the bound peptide and the homologous loop of acetylcholine binding protein, a soluble analog of the extracellular domain of acetylcholine receptor, are remarkably similar. Their superposition indicates that the toxin wraps around the receptor binding site loop, and in addition, binds tightly at the interface of two of the receptor subunits where it inserts a finger into the ligand binding site, thus blocking access to the acetylcholine binding site and explaining its strong antagonistic activity.

Synapse formation between central neurons requires postsynaptic expression of the MEN1 tumor suppressor gene

Synapse formation is a crucial step in the development of neuronal circuits and requires precise coordination of presynaptic and postsynaptic activities. However, molecular mechanisms that control the formation of functionally mature synaptic contacts, in particular between central neurons, remain poorly understood. To identify genes that are involved in the formation of central synapses, we made use of molluscan neurons that in culture form synaptic contacts between their somata (soma-soma synapses) in the absence of neurite outgrowth. Using single-cell mRNA differential display, we have identified a molluscan homolog of the multiple endocrine neoplasia type 1 (MEN1) tumor suppressor gene encoding the transcription factor menin as a gene that is upregulated during synapse formation. In vitro antisense knock-down of MEN1 mRNA blocks the formation of mature synapses between different types of identified central neurons. Moreover, immunocytochemistry and cell-specific knock-down of MEN1 mRNA show that postsynaptic but not presynaptic expression is required for synapses to form. Together, our data demonstrate that menin is a synaptogenic factor that is critically involved in a general postsynaptic mechanism of synapse formation between central neurons.

Functional implications of neurotransmitter expression during axonal regeneration: serotonin, but not peptides, auto-regulate axon growth of an identified central neuron

We studied the regenerative properties of one of two electrically coupled molluscan neurons, the serotonergic cerebral giant cells (CGCs) of Lymnaea stagnalis, after axotomy. The CGCs play a crucial role in feeding behavior, and when both cells are disconnected from their target neurons, animals no longer feed. When one CGC was permanently disconnected from its targets and the other was reversibly damaged by a nerve crush, the latter one regenerated over a period of 2 weeks to reform functional synapses with specific target neurons. At the same time, recovery of the feeding behavior was observed. After the crush, neuropeptide gene expression in the CGC was downregulated to approximately 50%. Serotonin synthesis, on the other hand, remained unaffected, suggesting that serotonin might have an active role in regeneration. In primary neuron culture, CGCs failed to extend neurites in the presence of serotonin; in cells that extended neurites in the absence of serotonin, focally applied serotonin, but not neuropeptides, induced growth cone collapse. Using serotonin-sensitive sniffer cells, we show that CGC neurites and growth cones release serotonin in culture. Finally, both the spontaneous and stimulation-induced release of serotonin from CGCs in culture resulted in growth cone collapse responses that could be blocked by the serotonin receptor antagonist methysergide. Our data suggest that auto-released serotonin is inhibitory to CGC neurite outgrowth in vitro. During regeneration in vivo, serotonin release might fine-tune axon guidance and branching by inducing local collapse responses in extending neurites.

Crystal structure of an ACh-binding protein reveals the ligand-binding domain of nicotinic receptors

Pentameric ligand gated ion-channels, or Cys-loop receptors, mediate rapid chemical transmission of signals. This superfamily of allosteric transmembrane proteins includes the nicotinic acetylcholine (nAChR), serotonin 5-HT3, gamma-aminobutyric-acid (GABAA and GABAC) and glycine receptors. Biochemical and electrophysiological information on the prototypic nAChRs is abundant but structural data at atomic resolution have been missing. Here we present the crystal structure of molluscan acetylcholine-binding protein (AChBP), a structural and functional homologue of the amino-terminal ligand-binding domain of an nAChR alpha-subunit. In the AChBP homopentamer, the protomers have an immunoglobulin-like topology. Ligand-binding sites are located at each of five subunit interfaces and contain residues contributed by biochemically determined 'loops' A to F. The subunit interfaces are highly variable within the ion-channel family, whereas the conserved residues stabilize the protomer fold. This AChBP structure is relevant for the development of drugs against, for example, Alzheimer's disease and nicotine addiction.

A glia-derived acetylcholine-binding protein that modulates synaptic transmission

There is accumulating evidence that glial cells actively modulate neuronal synaptic transmission. We identified a glia-derived soluble acetylcholine-binding protein (AChBP), which is a naturally occurring analogue of the ligand-binding domains of the nicotinic acetylcholine receptors (nAChRs). Like the nAChRs, it assembles into a homopentamer with ligand-binding characteristics that are typical for a nicotinic receptor; unlike the nAChRs, however, it lacks the domains to form a transmembrane ion channel. Presynaptic release of acetylcholine induces the secretion of AChBP through the glial secretory pathway. We describe a molecular and cellular mechanism by which glial cells release AChBP in the synaptic cleft, and propose a model for how they actively regulate cholinergic transmission between neurons in the central nervous system.

Neurotrophic actions of a novel molluscan epidermal growth factor

The mammalian epidermal growth factor (EGF) is expressed in the developing and adult CNS, and it has been implicated in the control of cell proliferation, differentiation, and neurotrophic events. Despite extensive evolutionary conservation of the EGF motif in a range of different types of proteins, secreted EGF homologs with neurotrophic actions have not been reported in invertebrates. In this study, we present a novel member of the family of EGF-like growth factors, an EGF homolog from the mollusc Lymnaea stagnalis (L-EGF), and we demonstrate that this protein has neurotrophic activity. Purified L-EGF is a 43-residue peptide and retains the typical structural characteristics of the EGF motif. The L-EGF cDNA reveals a unique precursor organization. In contrast to the multidomain mammalian EGFs, it consists of only two domains, a signal peptide and a single EGF motif. Conspicuously, the L-EGF precursor lacks a transmembrane domain, setting it apart from all other members of the EGF-family. L-EGF mRNA is expressed throughout embryonic development, in the juvenile CNS, but not in the normal adult CNS. However, expression in the adult CNS is upregulated after injury, suggesting a role of L-EGF in repair functions. This notion is supported by the observation that L-EGF evokes neurite outgrowth in specific adult Lymnaea neurons in vitro, which could be inhibited by an EGF receptor tyrosine kinase inhibitor. In conclusion, our findings further substantiate the notion that the EGF family has an early phylogenetic origin, and our data support a neurotrophic role for L-EGF during development and injury repair.

Family of prohormone convertases in Lymnaea: characterization of two alternatively spliced furin-like transcripts and cell-specific regulation of their expression

The majority of neuropeptides in Lymnaea stagnalis are proteolytically processed from larger precursors at sites composed of single or multiple basic amino acid residues. Previous studies have identified several putative prohormone convertases in the brain of Lymnaea. To characterize the complete family, we undertook three independent approaches: reverse-transcribed polymerase chain reaction screening, and low-stringency cDNA and genomic library screenings. The central nervous system cDNA library screening yielded two cDNAs encoding Lfurin1 and its variant form, Lfurin1-X. Both proteins show the characteristic organization of (human) furin with a putative catalytic domain, a P domain, a Cys-rich domain, a transmembrane domain, and a cytoplasmic tail. Lfurin1 and Lfurin1-X are identical, apart from a putative alternatively spliced noncatalytic luminal protein domain, which is present exclusively in Lfurin1-X. In situ hybridization revealed that the Lfur1 gene is expressed throughout the Lymnaea brain, but that the level varies considerably from one neuron to another. Quantitative analysis of the expression level of the two alternatively spliced transcripts revealed that it is neuron type-specifically regulated. This probably indicates the functional importance of noncatalytic luminal protein domains in these enzymes. In addition, our findings suggest that apart from the identified convertases LPC2, Lfurin1/Lfurin1-X, and Lfurin2, additional prohormone convertase diversity is either not present or present only at low levels in the Lymnaea brain. Alternatively, additional prohormone convertases could exist with a lower degree of sequence conservation than the other Lymnaea prohormone convertase members. From our findings, it appears that the majority of prohormone processing in Lymnaea is carried out by the three thus far identified types of Kex2-related prohormone convertases despite the large number of neuropeptide precursors and diverse multiple basic cleavage sites hydrolyzed.

A molluscan peptide alpha-amidating enzyme precursor that generates five distinct enzymes

Mechanisms underlying the specificity and efficiency of enzymes, which modify peptide messengers, especially with the variable requirements of synthesis in the neuronal secretory pathway, are poorly understood. Here, we examine the process of peptide alpha-amidation in individually identifiable Lymnaea neurons that synthesize multiple proproteins, yielding complex mixtures of structurally diverse peptide substrates. The alpha-amidation of these peptide substrates is efficiently controlled by a multifunctional Lymnaea peptidyl glycine alpha-amidating monooxygenase (LPAM), which contains four different copies of the rate-limiting Lymnaea peptidyl glycine alpha-hydroxylating monooxygenase (LPHM) and a single Lymnaea peptidyl alpha-hydroxyglycine alpha-amidating lyase. Endogenously, this zymogen is converted to yield a mixture of monofunctional isoenzymes. In vitro, each LPHM displays a unique combination of substrate affinity and reaction velocity, depending on the penultimate residue of the substrate. This suggests that the different isoenzymes are generated in order to efficiently amidate the many peptide substrates that are present in molluscan neurons. The cellular expression of the LPAM gene is restricted to neurons that synthesize amidated peptides, which underscores the critical importance of regulation of peptide alpha-amidation.

The lymnaea cardioexcitatory peptide (LyCEP) receptor: a G-protein-coupled receptor for a novel member of the RFamide neuropeptide family

A novel G-protein-coupled receptor (GRL106) resembling neuropeptide Y and tachykinin receptors was cloned from the mollusc Lymnaea stagnalis. Application of a peptide extract from the Lymnaea brain to Xenopus oocytes expressing GRL106 activated a calcium-dependent chloride channel. Using this response as a bioassay, we purified the ligand for GRL106, Lymnaea cardioexcitatory peptide (LyCEP), an RFamide-type decapeptide (TPHWRPQGRF-NH2) displaying significant similarity to the Achatina cardioexcitatory peptide (ACEP-1) as well as to the recently identified family of mammalian prolactin-releasing peptides. In the Lymnaea brain, the cells that produce egg-laying hormone are the predominant site of GRL106 gene expression and appear to be innervated by LyCEP-containing fibers. Indeed, LyCEP application transiently hyperpolarizes isolated egg-laying hormone cells. In the Lymnaea pericardium, LyCEP-containing fibers end blindly at the pericardial lumen, and the heart is stimulated by LyCEP in vitro. These data confirm that LyCEP is an RFamide ligand for GRL106.

Early evolutionary origin of the neurotrophin receptor family

Neurotrophins and their Trk receptors play a crucial role in the development and maintenance of the vertebrate nervous system, but to date no component of this signalling system has been found in invertebrates. We describe a molluscan Trk receptor, designated Ltrk, from the snail Lymnaea stagnalis. The full-length sequence of Ltrk reveals most of the characteristics typical of Trk receptors, including highly conserved transmembrane and intracellular tyrosine kinase domains, and a typical extracellular domain of leucine-rich motifs flanked by cysteine clusters. In addition, Ltrk has a unique N-terminal extension and lacks immunoglobulin-like domains. Ltrk is expressed during development in a stage-specific manner, and also in the adult, where its expression is confined to the central nervous system and its associated endocrine tissues. Ltrk has the highest sequence identity with the TrkC mammalian receptor and, when exogenously expressed in fibroblasts or COS cells, binds human NT-3, but not NGF or BDNF, with an affinity of 2.5 nM. These findings support an early evolutionary origin of the Trk family as neuronal receptor tyrosine kinases and suggest that Trk signalling mechanisms may be highly conserved between vertebrates and invertebrates.

Direct mass spectrometric peptide profiling and sequencing of single neurons reveals differential peptide patterns in a small neuronal network

Mass spectrometry (MS) was employed to detect and structurally characterize peptides in two functionally related neurons, named VD1 and RPD2, which form a network involved in the modulation of heartbeat in Lymnaea. Matrix-assisted laser desorption/ionization MS, directly applied to single neurons VD1 and RPD2, showed overlapping yet distinct mass profiles, with a subset of putative peptides specifically present in neuron VD1. Direct tandem MS of a single VD1 neuron revealed the primary structures of the VD1-specific peptides, which were identified as members of the family of small cardioactive peptides. Based on the tandem MS data, a degenerate oligonucleotide was made for use in a polymerase chain reaction strategy to isolate the cDNA encoding the precursor to the small cardioactive peptides from a brain-specific cDNA library. The calculated masses of the mature, posttranslationally modified peptides, as predicted from the corresponding cDNA, agreed with the measured masses of the actual peptides, as detected in single-cell MS analysis. In situ hybridization studies showed that the transcript encoding the precursor is present in VD1, but not in RPD2, thus corroborating the single-cell MS analysis. Finally, the small cardioactive peptides were shown to enhance the contractions of the auricle in vitro.

Towards understanding the role of insulin in the brain: lessons from insulin-related signaling systems in the invertebrate brain

Insulin is a molecule that has played a key role in several of the most important landmarks in medical and biological research. It is one of the most extensively studied protein hormones, and its structure and function have been elucidated in many vertebrate species, ranging from man to hagfish and turkey. The structure, function as well as tissue of synthesis of vertebrate insulins are strictly conserved. The structural identification of insulin-related peptides from invertebrates has disrupted the picture of an evolutionary stable peptide hormone. Insulin-related peptides in molluscs and insects turned out to be a structurally diverse group encoded by large multi-gene families that are uniquely expressed in the brain and serve functions different from vertebrate insulin. In this review, we discuss invertebrate insulins in detail. We examine how these peptides relate to the model role that vertebrate insulin has played over the years; however, more importantly, we discuss several unique principles that can be learned from them. We show how diversity of these peptides is generated at the genetic level and how the structural diversity of the peptides is linked to the exclusive presence of a single type of neuronal insulin receptor-related receptor. We also discuss the fact that the invertebrate peptides, in addition to a hormonal role, may also act in a synaptic and/or nonsynaptic fashion as transmitters/neuromodulators on neurons in the brain. It can be expected that the use of well-defined neuronal preparations in invertebrates may lead to a further understanding of these novel functions and may act as guide preparations for a possible role of insulin and its relatives in the vertebrate brain.

Altered gene expression in the host brain caused by a trematode parasite: neuropeptide genes are preferentially affected during parasitosis

Schistosome parasites adjust the physiology and behavior of their intermediate molluscan hosts to their own benefit. Previous studies demonstrated effects of the avian-schistosome Trichobilharzia ocellata on peptidergic centers in the brain of the intermediate snail host Lymnaea stagnalis. In particular, electrophysiological properties and peptide release of growth- and reproduction-controlling neuroendocrine neurons were affected. We now have examined the possibility that the expression of genes that control physiology and behavior of the host might be altered during parasitosis. A cDNA library of the brain of parasitized Lymnaea was constructed and differentially screened by using mRNA from the brain of both parasitized and nonparasitized snails. This screening yielded a number of clones, including previously identified cDNAs as well as novel neuronal transcripts, which appear to be differentially regulated. The majority of these transcripts encode neuropeptides. Reverse Northern blot analysis confirmed that neuropeptide gene expression is indeed affected in parasitized animals. Moreover, the expression profiles of 10 transcripts tested showed a differential, parasitic stage-specific regulation. Changes in expression could in many cases already be observed between 1.5 and 5 hr postinfection, suggesting that changes in gene expression are a direct effect of parasitosis. We suggest that direct regulation of neuropeptide gene expression is a strategy of parasites to induce physiological and behavioral changes in the host.

Plasticity in fast synaptic inhibition of adult oxytocin neurons caused by switch in GABA(A) receptor subunit expression

We found that magnocellular oxytocin neurons in adult female rats exhibit an endogenous GABA(A) receptor subunit switch around parturition: a decrease in alpha1:alpha2 subunit mRNA ratio correlated with a decrease in allopregnanolone potentiation and increase in decay time constant of the GABA(A) receptor-mediated IPSCs in these cells. The causal relationship between changes in alpha1:alpha2 mRNA ratio and the ion channel kinetics was confirmed using in vitro antisense deletion. Further, GABA(A) receptors exhibited a tonic inhibitory influence upon oxytocin release in vivo, and allopregnanolone helped to restrain oxytocin neuron in vitro firing only before parturition, when the alpha1:alpha2 subunit mRNA ratio was still high. Such observations provide evidence for the physiological significance of GABA(A) receptor subunit heterogeneity and plasticity in the adult brain.

De novo protein synthesis in isolated axons of identified neurons

Neurons are highly polarized cells that contain a wealth of cytoplasmic and membrane proteins required for neurotransmission, synapse formation and various forms of neuronal plasticity. Typically, these proteins are differentially distributed over somatic, dendritic and axonal compartments. Until recently, it was believed that all proteins destined for various neuronal sites were synthesized exclusively in the somata and were subsequently targeted to appropriate extrasomal compartments. The discovery of various messenger RNA molecules in both dendrites and axons is suggestive of de novo protein synthesis in extrasomatic regions. The latter process has been demonstrated in few neuronal svrstems, but direct proof for the axonal transcription of a specific protein from a given messenger RNA is still lacking. This lack of fundamental knowledge in the field of cellular and molecular neurobiology is due primarily to both anatomical and experimental difficulties encountered in most animal preparations studied thus far. In this study we developed a neuronal experimental system comprising of individually identified neurons and their isolated axons from the mollusc Lymnaea stagnalis. We injected a foreign messenger RNA encoding a peptide precursor into the isolated axons of cultured neurons; and utilizing cellular, molecular and immunocytochemical techniques, we provide direct evidence for specific protein synthesis in isolated axons. The Lymnaea model provides us with an opportunity to examine the role and specificity of de novo protein synthesis in the extrasomal regions.

Deletion of two exons from the Lymnaea stagnalis beta1-->4-N-acetylglucosaminyltransferase gene elevates the kinetic efficiency of the encoded enzyme for both UDP-sugar donor and acceptor substrates

Lymnaea stagnalis UDP-GlcNAc:GlcNAcbeta-R beta1-->4-N-acetylglucosaminyltransferase (beta4-GlcNAcT) is an enzyme with structural similarity to mammalian UDP-Gal:GlcNAcbeta-R beta1-->4-galactosyltransferase (beta4-GalT). Here, we report that also the exon organization of the genes encoding these enzymes is very similar. The beta4-GlcNAcT gene (12.5 kilobase pairs, spanning 10 exons) contains four exons, encompassing sequences that are absent in the beta4-GalT gene. Two of these exons (exons 7 and 8) show a high sequence similarity to part of the preceding exon (exon 6), suggesting that they have originated by exon duplication. The exon in the beta4-GalT gene, corresponding to beta4-GlcNAcT exon 6, encodes a region that has been proposed to be involved in the binding of UDP-Gal. The question therefore arose, whether the repeating sequences encoded by exon 7 and 8 of the beta4-GlcNAcT gene would determine the specificity of the enzyme for UDP-GlcNAc, or for the less preferred UDP-GalNAc. It was found that deletion of only the sequence encoded by exon 8 resulted in a completely inactive enzyme. By contrast, deletion of the amino acid residues encoded by exons 7 and 8 resulted in an enzyme with an elevated kinetic efficiency for both UDP-sugar donors, as well as for its acceptor substrates. These results suggest that at least part of the donor and acceptor binding domains of the beta4-GlcNAcT are structurally linked and that the region encompassing the insertion contributes to acceptor recognition as well as to UDP-sugar binding and specificity.