Mice lacking synapsin III show abnormalities in explicit memory and conditioned fear

Damaged mitochondria coincide with presynaptic vesicle loss and abnormalities in alzheimer's disease brain

Loss of synapses is the most robust pathological correlate of Alzheimer's disease (AD)-associated cognitive deficits, although the underlying mechanism remains incompletely understood. Synaptic terminals have abundant mitochondria which play an indispensable role in synaptic function through ATP provision and calcium buffering. Mitochondrial dysfunction is an early and prominent feature in AD which could contribute to synaptic deficits. Here, using electron microscopy, we examined synapses with a focus on mitochondrial deficits in presynaptic axonal terminals and dendritic spines in cortical biopsy samples from clinically diagnosed AD and age-matched non-AD control patients. Synaptic vesicle density within the presynaptic axon terminals was significantly decreased in AD cases which appeared largely due to significantly decreased reserve pool, but there were significantly more presynaptic axons containing enlarged synaptic vesicles or dense core vesicles in AD. Importantly, there was reduced number of mitochondria along with significantly increased damaged mitochondria in the presynapse of AD which correlated with changes in SV density. Mitochondria in the post-synaptic dendritic spines were also enlarged and damaged in the AD biopsy samples. This study provided evidence of presynaptic vesicle loss as synaptic deficits in AD and suggested that mitochondrial dysfunction in both pre- and post-synaptic compartments contribute to synaptic deficits in AD.

Synapsin III Regulates Dopaminergic Neuron Development in Vertebrates

Attention deficit and hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by alterations in the mesocorticolimbic and nigrostriatal dopaminergic pathways. Polymorphisms in the Synapsin III (Syn III) gene can associate with ADHD onset and even affect the therapeutic response to the gold standard ADHD medication, methylphenidate (MPH), a monoamine transporter inhibitor whose efficacy appears related with the stimulation of brain-derived neurotrophic factor (BDNF). Interestingly, we previously showed that MPH can bind Syn III, which can regulate neuronal development. These observations suggest that Syn III polymorphism may impinge on ADHD onset and response to therapy by affecting BDNF-dependent dopaminergic neuron development. Here, by studying zebrafish embryos exposed to Syn III gene knock-down (KD), Syn III knock-out (ko) mice and human induced pluripotent stem cells (iPSCs)-derived neurons subjected to Syn III RNA interference, we found that Syn III governs the earliest stages of dopaminergic neurons development and that this function is conserved in vertebrates. We also observed that in mammals Syn III exerts this function acting upstream of brain-derived neurotrophic factor (BDNF)- and cAMP-dependent protein kinase 5 (Cdk5)-stimulated dendrite development. Collectively, these findings own significant implications for deciphering the biological basis of ADHD.

Investigation of possible associations of the BDNF, SNAP-25 and SYN III genes with the neurocognitive measures: BDNF and SNAP-25 genes might be involved in attention domain, SYN III gene in executive function

OBJECTIVES

Attention-deficit/hyperactivity disorder (ADHD) is a heterogeneous disorder and Sluggish Cognitive Tempo (SCT) might be a second inattention disorder that might be even affected by different attention pathways. SCT is characterized by daydreaming, mental confusion, staring blankly and hypoactivity. In the present study, we evaluated 5 common variants (rs6265, rs3746544, rs1051312, rs133946 and rs133945) located in 3 candidate genes (BDNF, SNAP25 and SYN III) that are known to take part in synaptic plasticity and neurotransmitter transmission.

METHODS

We tested the effects of these variants on neuropsychological findings assessed by a computer-based neuropsychological test battery in children with inattention symptoms (SCT and/or ADHD).

RESULTS

BDNF (rs6265), SNAP25 (rs3746544 and rs1051312) and SYN III (rs133946 and rs133945) polymorphisms were associated with variable cognitive measures. BDNF gene (rs6265) polymorphism Met allele carriers and SNAP25 gene (rs3746544) T allele carriers had an association with the attention domain. SNAP25 gene (rs1051312) C allele carriers were only associated with reaction time scores. Cognitive flexibility, which is one of the key components of executive function evaluation and shifting attention test scores were associated with BDNF (rs6265) Met allele and SYN III (rs133946) gene G allele. SYN III (rs133945) gene C allele carriers had an association with verbal memory correct hit scores.

CONCLUSIONS

As a conclusion, BDNF, SNAP25 and SYN III genes were associated with specific neurocognitive outcomes in children with inattention symptoms. It is important to note that exploring genotyping effects on neurocognitive functions instead of a heterogeneous psychiatric diagnosis can improve our understanding of psychopathologies.

An updated reappraisal of synapsins: structure, function and role in neurological and psychiatric disorders

Synapsins (Syns) are phosphoproteins strongly involved in neuronal development and neurotransmitter release. Three distinct genes SYN1, SYN2 and SYN3, with elevated evolutionary conservation, have been described to encode for Synapsin I, Synapsin II and Synapsin III, respectively. Syns display a series of common features, but also exhibit distinctive localization, expression pattern, post-translational modifications (PTM). These characteristics enable their interaction with other synaptic proteins, membranes and cytoskeletal components, which is essential for the proper execution of their multiple functions in neuronal cells. These include the control of synapse formation and growth, neuron maturation and renewal, as well as synaptic vesicle mobilization, docking, fusion, recycling. Perturbations in the balanced expression of Syns, alterations of their PTM, mutations and polymorphisms of their encoding genes induce severe dysregulations in brain networks functions leading to the onset of psychiatric or neurological disorders. This review presents what we have learned since the discovery of Syn I in 1977, providing the state of the art on Syns structure, function, physiology and involvement in central nervous system disorders.

Dysfunction of the serotonergic system in the brain of synapsin triple knockout mice is associated with behavioral abnormalities resembling synapsin-related human pathologies

Synapsins (Syns) are a family of phosphoproteins associated with synaptic vesicles (SVs). Their main function is to regulate neurotransmitter release by maintaining a reserve pool of SVs at the presynaptic terminal. Previous studies reported that the deletion of one or more Syn genes in mice results in an epileptic phenotype and autism-related behavioral abnormalities. Here we aimed at characterizing the behavioral phenotype and neurobiological correlates of the deletion of Syns in a Syn triple knockout (TKO) mouse model. Wild type (WT) and TKO mice were tested in the open field, novelty suppressed feeding, light-dark box, forced swim, tail suspension and three-chamber sociability tests. Using in vivo electrophysiology, we recorded the spontaneous activity of dorsal raphe nucleus (DRN) serotonin (5-HT) and ventral tegmental area (VTA) dopamine (DA) neurons. Levels of 5-HT and DA in the frontal cortex and hippocampus of WT and TKO mice were also assessed using a High-Performance Liquid Chromatography. TKO mice displayed hyperactivity and impaired social and anxiety-like behavior. Behavioral dysfunctions were accompanied by reduced firing activity of DRN 5-HT, but not VTA DA, neurons. TKO mice also showed increased responsiveness of DRN 5-HT-1A autoreceptors, measured as a reduced dose of the 5-HT-1A agonist 8-OH-DPAT necessary to inhibit DRN 5-HT firing activity by 50%. Finally, hippocampal 5-HT levels were lower in TKO than in WT mice. Overall, Syns deletion in mice leads to a reduction in DRN 5-HT firing activity and hippocampal 5-HT levels along with behavioral alterations reminiscent of human neuropsychiatric conditions associated with Syn dysfunction.

Essential role for InSyn1 in dystroglycan complex integrity and cognitive behaviors in mice

Human mutations in the dystroglycan complex (DGC) result in not only muscular dystrophy but also cognitive impairments. However, the molecular architecture critical for the synaptic organization of the DGC in neurons remains elusive. Here, we report Inhibitory Synaptic protein 1 (InSyn1) is a critical component of the DGC whose loss alters the composition of the GABAergic synapses, excitatory/inhibitory balance in vitro and in vivo, and cognitive behavior. Association of InSyn1 with DGC subunits is required for InSyn1 synaptic localization. InSyn1 null neurons also show a significant reduction in DGC and GABA receptor distribution as well as abnormal neuronal network activity. Moreover, InSyn1 null mice exhibit elevated neuronal firing patterns in the hippocampus and deficits in fear conditioning memory. Our results support the dysregulation of the DGC at inhibitory synapses and altered neuronal network activity and specific cognitive tasks via loss of a novel component, InSyn1.

Synapsin I Controls Synaptic Maturation of Long-Range Projections in the Lateral Amygdala in a Targeted Selective Fashion

The amygdala, and more precisely its lateral nucleus, is thought to attribute emotional valence to external stimuli by generating long-term plasticity changes at long-range projections to principal cells. Aversive experience has also been shown to modify pre- and post-synaptic markers in the amygdala, suggesting their possible role in the structural organization of adult amygdala networks. Here, we focused on how the maturation of cortical and thalamic long-range projections occurs on principal neurons and interneurons in the lateral amygdala (LA). We performed dual electrophysiological recordings of identified cells in juvenile and adult GAD67-GFP mice after independent stimulation of cortical and thalamic afferent systems. The results demonstrate that synaptic strengthening occurs during development at synapses projecting to LA principal neurons, but not interneurons. As synaptic strengthening underlies fear conditioning which depends, in turn, on presence and increasing expression of synapsin I, we tested if synapsin I contributes to synaptic strengthening during development. Interestingly, the physiological synaptic strengthening of cortical and thalamic synapses projecting to LA principal neurons was virtually abolished in synapsin I knockout mice, but not differences were observed in the excitatory projections to interneurons. Immunohistochemistry analysis showed that the presence of synapsin I is restricted to excitatory contacts projecting to principal neurons in LA of adult mice. These results indicate that synapsin I is a key regulator of the maturation of synaptic connectivity in this brain region and that is expression is dependent on postsynaptic identity.

Postsynaptic Mechanisms Render Syn I/II/III Mice Highly Responsive to Psychostimulants

BACKGROUND

Synapsins are encoded by SYN I, SYN II, and SYN III, and they regulate neurotransmitter release by maintaining a reserve pool of synaptic vesicles.

METHODS

Presynaptic dopamine responses to cocaine were examined by microdialysis, and postsynaptic responses were evaluated to various dopamine receptor agonists in the open field with SynI/SynII/SynIII triple knockout mice.

RESULTS

Triple knockout mice showed enhanced spontaneous locomotion in a novel environment and were hyper-responsive to indirect and direct D1 and D2 dopamine agonists. Triple knockout animals appeared sensitized to cocaine upon first open field exposure; sensitization developed across days in wild-type controls. When mutants were preexposed to a novel environment before injection, cocaine-stimulated locomotion was reduced and behavioral sensitization retarded. Baseline dopamine turnover was enhanced in mutants and novel open field exposure increased their striatal dopamine synthesis rates. As KCl-depolarization stimulated comparable dopamine release in both genotypes, their readily releasable pools appeared indistinguishable. Similarly, cocaine-induced hyperlocomotion was indifferent to blockade of newly synthesized dopamine and depletion of releasable dopamine pools. Extracellular dopamine release was similar in wild-type and triple knockout mice preexposed to the open field and given cocaine or placed immediately into the arena following injection. Since motor effects to novelty and psychostimulants depend upon frontocortical-striatal inputs, we inhibited triple knockout medial frontal cortex with GABA agonists. Locomotion was transiently increased in cocaine-injected mutants, while their supersensitive cocaine response to novelty was lost.

CONCLUSIONS

These results reveal presynaptic dopamine release is not indicative of agonist-induced triple knockout hyperlocomotion. Instead, their novelty response occurs primarily through postsynaptic mechanisms and network effects.

GIT1 regulates synaptic structural plasticity underlying learning

The signaling scaffold protein GIT1 is expressed widely throughout the brain, but its function in vivo remains elusive. Mice lacking GIT1 have been proposed as a model for attention deficit-hyperactivity disorder, due to alterations in basal locomotor activity as well as paradoxical locomotor suppression by the psychostimulant amphetamine. Since we had previously shown that GIT1-knockout mice have normal locomotor activity, here we examined GIT1-deficient mice for ADHD-like behavior in more detail, and find neither hyperactivity nor amphetamine-induced locomotor suppression. Instead, GIT1-deficient mice exhibit profound learning and memory defects and reduced synaptic structural plasticity, consistent with an intellectual disability phenotype. We conclude that loss of GIT1 alone is insufficient to drive a robust ADHD phenotype in distinct strains of mice. In contrast, multiple learning and memory defects have been observed here and in other studies using distinct GIT1-knockout lines, consistent with a predominant intellectual disability phenotype related to altered synaptic structural plasticity.

Synapsin III is a key component of α-synuclein fibrils in Lewy bodies of PD brains

Lewy bodies (LB) and Lewy neurites (LN), which are primarily composed of α-synuclein (α-syn), are neuropathological hallmarks of Parkinson's disease (PD) and dementia with Lewy bodies (DLB). We recently found that the neuronal phosphoprotein synapsin III (syn III) controls dopamine release via cooperation with α-syn and modulates α-syn aggregation. Here, we observed that LB and LN, in the substantia nigra of PD patients and hippocampus of one subject with DLB, displayed a marked immunopositivity for syn III. The in situ proximity ligation assay revealed the accumulation of numerous proteinase K-resistant neuropathological inclusions that contained both α-syn and syn III in tight association in the brain of affected subjects. Most strikingly, syn III was identified as a component of α-syn-positive fibrils in LB-enriched protein extracts from PD brains. Finally, a positive correlation between syn III and α-syn levels was detected in the caudate putamen of PD subjects. Collectively, these findings indicate that syn III is a crucial α-syn interactant and a key component of LB fibrils in the brain of patients affected by PD.

Cellular diversity of the somatosensory cortical map plasticity

Sensory maps are representations of the sensory epithelia in the brain. Despite the intuitive explanatory power behind sensory maps as being neuronal precursors to sensory perception, and sensory cortical plasticity as a neural correlate of perceptual learning, molecular mechanisms that regulate map plasticity are not well understood. Here we perform a meta-analysis of transcriptional and translational changes during altered whisker use to nominate the major molecular correlates of experience-dependent map plasticity in the barrel cortex. We argue that brain plasticity is a systems level response, involving all cell classes, from neuron and glia to non-neuronal cells including endothelia. Using molecular pathway analysis, we further propose a gene regulatory network that could couple activity dependent changes in neurons to adaptive changes in neurovasculature, and finally we show that transcriptional regulations observed in major brain disorders target genes that are modulated by altered sensory experience. Thus, understanding the molecular mechanisms of experience-dependent plasticity of sensory maps might help to unravel the cellular events that shape brain plasticity in health and disease.

Parental origin impairment of synaptic functions and behaviors in cytoplasmic FMRP interacting protein 1 (Cyfip1) deficient mice

CYFIP1 maps to the interval between proximal breakpoint 1 (BP1) and breakpoint 2 (BP2) of chromosomal 15q11-q13 deletions that are implicated in the Angelman (AS) and Prader-Willi syndrome (PWS). There is only one breakpoint (BP3) at the distal end of deletion. CYFIP1 is deleted in AS patients with the larger class I deletion (BP1 to BP3) and the neurological presentations in these patients are more severe than that of patients with class II (BP2 to BP3) deletion. The haploinsufficiency of CYFIP1 is hypothesized to contribute to more severe clinical presentations in class I AS patients. The expression of CYFIP1 is suggested to be bi-allelic in literature but the possibility of parental origin of expression is not completely excluded. We generated and characterized Cyfip1 mutant mice. Homozygous Cyfip1 mice were early embryonic lethal. However, there was a parental origin specific effect between paternal Cyfip1 deficiency (m+/p-) and maternal deficiency (m-/p+) on both synaptic transmissions and behaviors in hippocampal CA1 synapses despite no evidence supporting the parental origin difference for the expression. Both m-/p+ and m+/p- showed the impaired input-output response and paired-pulse facilitation. While the long term-potentiation and group I mGluR mediated long term depression induced by DHPG was not different between Cyfip1 m-/p+ and m+/p- mice, the initial DHPG induced response was significantly enhanced in m-/p+ but not in m+/p- mice. m+/p- but not m-/p+ mice displayed increased freezing in cued fear conditioning and abnormal transitions in zero-maze test. The impaired synaptic transmission and behaviors in haploinsufficiency of Cyfip1 mice provide the evidence supporting the role of CYFIP1 modifying the clinical presentation of class I AS patients and in human neuropsychiatric disorders.

Autophosphorylation of CaMKK2 generates autonomous activity that is disrupted by a T85S mutation linked to anxiety and bipolar disorder

Mutations that reduce expression or give rise to a Thr85Ser (T85S) mutation of Ca(2+)-CaM-dependent protein kinase kinase-2 (CaMKK2) have been implicated in behavioural disorders such as anxiety, bipolar and schizophrenia in humans. Here we report that Thr85 is an autophosphorylation site that endows CaMKK2 with a molecular memory that enables sustained autonomous activation following an initial, transient Ca(2+) signal. Conversely, autophosphorylation of Ser85 in the T85S mutant fails to generate autonomous activity but instead causes a partial loss of CaMKK2 activity. The loss of autonomous activity in the mutant can be rescued by blocking glycogen synthase kinase-3 (GSK3) phosphorylation of CaMKK2 with the anti-mania drug lithium. Furthermore, CaMKK2 null mice representing a loss of function model the human behavioural phenotypes, displaying anxiety and manic-like behavioural disturbances. Our data provide a novel insight into CaMKK2 regulation and its perturbation by a mutation associated with behavioural disorders.

Phosphorylation by PKA and Cdk5 Mediates the Early Effects of Synapsin III in Neuronal Morphological Maturation

Synapsin III (SynIII) is a neuron-specific phosphoprotein that plays a unique role in neuronal development. SynIII is phosphorylated by cAMP-dependent protein kinase (PKA) at a highly conserved phosphorylation site and by cyclin-dependent kinase-5 (Cdk5) at a newly described site. Although SynIII is known to be involved in axon elongation in vitro, the role of its phosphorylation by PKA and Cdk5 in the modulation of this process is unknown. We expressed either wild-type (WT) or phosphorylation-site mutants of SynIII in primary SynIII knock-out (KO) mouse neurons at early stages of in vitro development. Whereas the neurite elongation phenotype of SynIII KO neurons was fully rescued by the expression of WT SynIII, the expression of nonphosphorylatable and pseudo-phosphorylated PKA mutants was ineffective. Also, the nonphosphorylatable Cdk5 mutant was unable to rescue the neurite elongation phenotype of SynIII KO neurons. By contrast, the pseudo-phosphorylated mutant rescued the delay in neuronal maturation and axonal elongation, revealing a Cdk5-dependent regulation of SynIII function. Interestingly, SynIII KO neurons also exhibited decreased survival that was fully rescued by the expression of WT SynIII, but not by its phosphorylation mutants, and was associated with increased activated caspase3 and altered tropomyosin receptor kinase B isoform expression. These results indicate that PKA and Cdk5 phosphorylation is required for the physiological action of SynIII on axon specification and neurite outgrowth and that the expression of a functional SynIII is crucial for cell survival. Significance statement: Synapsin III is an atypical member of the synapsin family of synaptic vesicle-associated phosphoproteins that is precociously expressed in neurons and is downregulated afterward. Although experimental evidence suggests a specific role for Synapsin III in neuronal development, the molecular mechanisms are still largely unknown. We found that Synapsin III plays a central role in early stages of neuronal development involving neuronal survival, polarization, and neuritic growth and that these effects are dependent on phosphorylation by cAMP-dependent protein kinase and cyclin-dependent protein kinase-5. These results explain the recently described neurodevelopmental defects in the migration and orientation of Synapsin III-depleted cortical neurons and support the potential association of Synapsin III with neurodevelopmental disorders such as schizophrenia.

Nitric oxide synthase 1 adaptor protein, a protein implicated in schizophrenia, controls radial migration of cortical neurons

BACKGROUND

Where a neuron is positioned in the brain during development determines neuronal circuitry and information processing needed for normal brain function. When aberrations in this process occur, cognitive disorders may result. Patients diagnosed with schizophrenia have been reported to show altered neuronal connectivity and heterotopias. To elucidate pathways by which this process occurs and become aberrant, we have chosen to study the long isoform of nitric oxide synthase 1 adaptor protein (NOS1AP), a protein encoded by a susceptibility gene for schizophrenia.

METHODS

To determine whether NOS1AP plays a role in cortical patterning, we knocked down or co-overexpressed NOS1AP and a green fluorescent protein or red fluorescent protein (TagRFP) reporter in neuronal progenitor cells of the embryonic rat neocortex using in utero electroporation. We analyzed sections of cortex (ventricular zone, intermediate zone, and cortical plate [CP]) containing green fluorescent protein or red fluorescent protein TagRFP positive cells and counted the percentage of positive cells that migrated to each region from at least three rats for each condition.

RESULTS

NOS1AP overexpression disrupts neuronal migration, resulting in increased cells in intermediate zone and less cells in CP, and decreases dendritogenesis. Knockdown results in increased migration, with more cells reaching the CP. The phosphotyrosine binding region, but not the PDZ-binding motif, is necessary for NOS1AP function. Amino acids 181 to 307, which are sufficient for NOS1AP-mediated decreases in dendrite number, have no effect on migration.

CONCLUSIONS

Our studies show for the first time a critical role for the schizophrenia-associated gene NOS1AP in cortical patterning, which may contribute to underlying pathophysiology seen in schizophrenia.

Synapsin III acts downstream of semaphorin 3A/CDK5 signaling to regulate radial migration and orientation of pyramidal neurons in vivo

Synapsin III (SynIII) is a phosphoprotein that is highly expressed at early stages of neuronal development. Whereas in vitro evidence suggests a role for SynIII in neuronal differentiation, in vivo evidence is lacking. Here, we demonstrate that in vivo downregulation of SynIII expression affects neuronal migration and orientation. By contrast, SynIII overexpression affects neuronal migration, but not orientation. We identify a cyclin-dependent kinase-5 (CDK5) phosphorylation site on SynIII and use phosphomutant rescue experiments to demonstrate its role in SynIII function. Finally, we show that SynIII phosphorylation at the CDK5 site is induced by activation of the semaphorin-3A (Sema3A) pathway, which is implicated in migration and orientation of cortical pyramidal neurons (PNs) and is known to activate CDK5. Thus, fine-tuning of SynIII expression and phosphorylation by CDK5 activation through Sema3A activity is essential for proper neuronal migration and orientation.

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Precise regulation of SynIII expression is essential during brain development

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SynIII regulates neuronal migration, orientation, and morphological maturation

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SynIII acts downstream of the Sema3A pathway, which involves NP1 and kinase CDK5

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Phosphorylation of SynIII by CDK5 on Ser⁴⁰⁴ is essential for SynIII function

Radioprotection of the brain white matter by Mn(III) n-Butoxyethylpyridylporphyrin-based superoxide dismutase mimic MnTnBuOE-2-PyP5+

Cranial irradiation is a standard therapy for primary and metastatic brain tumors. A major drawback of radiotherapy (RT), however, is long-term cognitive loss that affects quality of life. Radiation-induced oxidative stress in normal brain tissue is thought to contribute to cognitive decline. We evaluated the effectiveness of a novel mimic of superoxide dismutase enzyme (SOD), MnTnBuOE-2-PyP(5+)(Mn(III) meso-tetrakis(N-n-butoxyethylpyridinium-2-yl)porphyrin), to provide long-term neuroprotection following 8 Gy of whole brain irradiation. Long-term RT damage can only be assessed by brain imaging and neurocognitive studies. C57BL/6J mice were treated with MnTnBuOE-2-PyP(5+) before and after RT and evaluated three months later. At this time point, drug concentration in the brain was 25 nmol/L. Mice treated with MnTnBuOE-2-PyP(5+)/RT exhibited MRI evidence for myelin preservation in the corpus callosum compared with saline/RT treatment. Corpus callosum histology demonstrated a significant loss of axons in the saline/RT group that was rescued in the MnTnBuOE-2-PyP(5+)/RT group. In addition, the saline/RT groups exhibited deficits in motor proficiency as assessed by the rotorod test and running wheel tests. These deficits were ameliorated in groups treated with MnTnBuOE-2-PyP(5+)/RT. Our data demonstrate that MnTnBuOE-2-PyP(5+) is neuroprotective for oxidative stress damage caused by radiation exposure. In addition, glioblastoma cells were not protected by MnTnBuOE-2-PyP(5+) combination with radiation in vitro. Likewise, the combination of MnTnBuOE-2-PyP(5+) with radiation inhibited tumor growth more than RT alone in flank tumors. In summary, MnTnBuOE-2-PyP(5+) has dual activity as a neuroprotector and a tumor radiosensitizer. Thus, it is an attractive candidate for adjuvant therapy with RT in future studies with patients with brain cancer.

Loss of Cdc42 leads to defects in synaptic plasticity and remote memory recall

Cdc42 is a signaling protein important for reorganization of actin cytoskeleton and morphogenesis of cells. However, the functional role of Cdc42 in synaptic plasticity and in behaviors such as learning and memory are not well understood. Here we report that postnatal forebrain deletion of Cdc42 leads to deficits in synaptic plasticity and in remote memory recall using conditional knockout of Cdc42. We found that deletion of Cdc42 impaired LTP in the Schaffer collateral synapses and postsynaptic structural plasticity of dendritic spines in CA1 pyramidal neurons in the hippocampus. Additionally, loss of Cdc42 did not affect memory acquisition, but instead significantly impaired remote memory recall. Together these results indicate that the postnatal functions of Cdc42 may be crucial for the synaptic plasticity in hippocampal neurons, which contribute to the capacity for remote memory recall.

Neuron-specific Sumo1-3 knockdown in mice impairs episodic and fear memories

BACKGROUND

Growing evidence suggests that small ubiquitin-like modifier (SUMO) conjugation plays a key role in brain plasticity by modulating activity-dependent synaptic transmission. However, these observations are based largely on cell culture experiments. We hypothesized that episodic and fear memories would be affected by silencing SUMO1-3 expression.

METHODS

To investigate the role of SUMO conjugation in neuronal functioning in vivo, we generated a novel Sumo transgenic mouse model in which a Thy1 promoter drives expression of 3 distinct microRNAs to silence Sumo1-3 expression, specifically in neurons. Wild-type and Sumo1-3 knockdown mice were subjected to a battery of behavioural tests to elucidate whether Sumoylation is involved in episodic and emotional memory.

RESULTS

Expression of Sumo1-3 microRNAs and the corresponding silencing of Sumo expression were particularly pronounced in hippocampal, amygdala and layer V cerebral cortex neurons. The Sumo knockdown mice displayed anxiety-like responses and were impaired in episodic memory processes, contextual and cued fear conditioning and fear-potentiated startle.

LIMITATIONS

Since expression of Sumo1-3 was silenced in this mouse model, we need to verify in future studies which of the SUMO paralogues play the pivotal role in episodic and emotional memory.

CONCLUSION

Our results indicate that a functional SUMO conjugation pathway is essential for emotionality and cognition. This novel Sumo knockdown mouse model and the technology used in generating this mutant may help to reveal novel mechanisms that underlie a variety of neuropsychiatric conditions associated with anxiety and impairment of episodic and emotional memory.

In vivo and in vitro analyses of amygdalar function reveal a role for copper

Mice with a single copy of the peptide amidating monooxygenase (Pam) gene (PAM(+/-)) are impaired in contextual and cued fear conditioning. These abnormalities coincide with deficient long-term potentiation (LTP) at excitatory thalamic afferent synapses onto pyramidal neurons in the lateral amygdala. Slice recordings from PAM(+/-) mice identified an increase in GABAergic tone (Gaier ED, Rodriguiz RM, Ma XM, Sivaramakrishnan S, Bousquet-Moore D, Wetsel WC, Eipper BA, Mains RE. J Neurosci 30: 13656-13669, 2010). Biochemical data indicate a tissue-specific deficit in Cu content in the amygdala; amygdalar expression of Atox-1 and Atp7a, essential for transport of Cu into the secretory pathway, is reduced in PAM(+/-) mice. When PAM(+/-) mice were fed a diet supplemented with Cu, the impairments in fear conditioning were reversed, and LTP was normalized in amygdala slice recordings. A role for endogenous Cu in amygdalar LTP was established by the inhibitory effect of a brief incubation of wild-type slices with bathocuproine disulfonate, a highly selective, cell-impermeant Cu chelator. Interestingly, bath-applied CuSO₄ had no effect on excitatory currents but reversibly potentiated the disynaptic inhibitory current. Bath-applied CuSO₄ was sufficient to potentiate wild-type amygdala afferent synapses. The ability of dietary Cu to affect signaling in pathways that govern fear-based behaviors supports an essential physiological role for Cu in amygdalar function at both the synaptic and behavioral levels. This work is relevant to neurological and psychiatric disorders in which disturbed Cu homeostasis could contribute to altered synaptic transmission, including Wilson's, Menkes, Alzheimer's, and prion-related diseases.

Involvement of synaptic genes in the pathogenesis of autism spectrum disorders: the case of synapsins

Autism spectrum disorders (ASDs) are heterogeneous neurodevelopmental disorders characterized by deficits in social interaction and social communication, restricted interests, and repetitive behaviors. Many synaptic protein genes are linked to the pathogenesis of ASDs, making them prototypical synaptopathies. An array of mutations in the synapsin (Syn) genes in humans has been recently associated with ASD and epilepsy, diseases that display a frequent comorbidity. Syns are pre-synaptic proteins regulating synaptic vesicle traffic, neurotransmitter release, and short-term synaptic plasticity. In doing so, Syn isoforms control the tone of activity of neural circuits and the balance between excitation and inhibition. As ASD pathogenesis is believed to result from dysfunctions in the balance between excitatory and inhibitory transmissions in neocortical areas, Syns are novel ASD candidate genes. Accordingly, deletion of single Syn genes in mice, in addition to epilepsy, causes core symptoms of ASD by affecting social behavior, social communication, and repetitive behaviors. Thus, Syn knockout mice represent a good experimental model to define synaptic alterations involved in the pathogenesis of ASD and epilepsy.

Disruption of the expression of the proprotein convertase PC7 reduces BDNF production and affects learning and memory in mice

PC7 belongs to the proprotein convertase family, whose members are implicated in the cleavage of secretory precursors. The in vivo function of PC7 is unknown. Herein, we find that the precursor proBDNF is processed into mature BDNF in COS-1 cells coexpressing proBDNF with either PC7 or Furin. Conversely, the processing of proBDNF into BDNF is markedly reduced in the absence of either Furin or PC7 in mouse primary hepatocytes. In vivo we observe that BDNF and PC7 mRNAs are colocalized in mouse hippocampus and amygdala and that mature BDNF protein levels are reduced in these brain areas in PC7 KO mice but not in the hippocampus of PC1/3 KO mice. Various behavioral tests reveal that in PC7 KO mice spatial memory is intact and plasticity of responding is mildly abnormal. Episodic and emotional memories are severely impaired, but both are rescued with the tyrosine receptor kinase B agonist 7,8-dihydroxyflavone. Altogether, these results support an in vivo role for PC7 in the regulation of certain types of cognitive performance, in part via proBDNF processing. Because polymorphic variants of human PC7 are being characterized, it will be important in future studies to determine their effects on additional physiological and behavioral processes.

SYN2 is an autism predisposing gene: loss-of-function mutations alter synaptic vesicle cycling and axon outgrowth

An increasing number of genes predisposing to autism spectrum disorders (ASDs) has been identified, many of which are implicated in synaptic function. This 'synaptic autism pathway' notably includes disruption of SYN1 that is associated with epilepsy, autism and abnormal behavior in both human and mice models. Synapsins constitute a multigene family of neuron-specific phosphoproteins (SYN1-3) present in the majority of synapses where they are implicated in the regulation of neurotransmitter release and synaptogenesis. Synapsins I and II, the major Syn isoforms in the adult brain, display partially overlapping functions and defects in both isoforms are associated with epilepsy and autistic-like behavior in mice. In this study, we show that nonsense (A94fs199X) and missense (Y236S and G464R) mutations in SYN2 are associated with ASD in humans. The phenotype is apparent in males. Female carriers of SYN2 mutations are unaffected, suggesting that SYN2 is another example of autosomal sex-limited expression in ASD. When expressed in SYN2  knockout neurons, wild-type human Syn II fully rescues the SYN2 knockout phenotype, whereas the nonsense mutant is not expressed and the missense mutants are virtually unable to modify the SYN2 knockout phenotype. These results identify for the first time SYN2  as a novel predisposing gene for ASD and strengthen the hypothesis that a disturbance of synaptic homeostasis underlies ASD.

Autism-related behavioral abnormalities in synapsin knockout mice

Several synaptic genes predisposing to autism-spectrum disorder (ASD) have been identified. Nonsense and missense mutations in the SYN1 gene encoding for Synapsin I have been identified in families segregating for idiopathic epilepsy and ASD and genetic mapping analyses have identified variations in the SYN2 gene as significantly contributing to epilepsy predisposition. Synapsins (Syn I/II/III) are a multigene family of synaptic vesicle-associated phosphoproteins playing multiple roles in synaptic development, transmission and plasticity. Lack of SynI and/or SynII triggers a strong epileptic phenotype in mice associated with mild cognitive impairments that are also present in the non-epileptic SynIII(-/-) mice. SynII(-/-) and SynIII(-/-) mice also display schizophrenia-like traits, suggesting that Syns could be involved in the regulation of social behavior. Here, we studied social interaction and novelty, social recognition and social dominance, social transmission of food preference and social memory in groups of male SynI(-/-), SynII(-/-) and SynIII(-/-) mice before and after the appearance of the epileptic phenotype and compared their performances with control mice. We found that deletion of Syn isoforms widely impairs social behaviors and repetitive behaviors, resulting in ASD-related phenotypes. SynI or SynIII deletion altered social behavior, whereas SynII deletion extensively impaired various aspects of social behavior and memory, altered exploration of a novel environment and increased self-grooming. Social impairments of SynI(-/-) and SynII(-/-) mice were evident also before the onset of seizures. The results demonstrate an involvement of Syns in generation of the behavioral traits of ASD and identify Syn knockout mice as a useful experimental model of ASD and epilepsy.

Automated design of ligands to polypharmacological profiles

The clinical efficacy and safety of a drug is determined by its activity profile across many proteins in the proteome. However, designing drugs with a specific multi-target profile is both complex and difficult. Therefore methods to design drugs rationally a priori against profiles of several proteins would have immense value in drug discovery. Here we describe a new approach for the automated design of ligands against profiles of multiple drug targets. The method is demonstrated by the evolution of an approved acetylcholinesterase inhibitor drug into brain-penetrable ligands with either specific polypharmacology or exquisite selectivity profiles for G-protein-coupled receptors. Overall, 800 ligand-target predictions of prospectively designed ligands were tested experimentally, of which 75% were confirmed to be correct. We also demonstrate target engagement in vivo. The approach can be a useful source of drug leads when multi-target profiles are required to achieve either selectivity over other drug targets or a desired polypharmacology.

Pregnenolone rescues schizophrenia-like behavior in dopamine transporter knockout mice

Pregnenolone belongs to a class of endogenous neurosteroids in the central nervous system (CNS), which has been suggested to enhance cognitive functions through GABA(A) receptor signaling by its metabolites. It has been shown that the level of pregnenolone is altered in certain brain areas of schizophrenic patients, and clozapine enhances pregnenolone in the CNS in rats, suggesting that pregnenolone could be used to treat certain symptoms of schizophrenia. In addition, early phase proof-of-concept clinical trials have indicated that pregnenolone is effective in reducing the negative symptoms and cognitive deficits of schizophrenia patients. Here, we evaluate the actions of pregnenolone on a mouse model for schizophrenia, the dopamine transporter knockout mouse (DAT KO). DAT KO mice mirror certain symptoms evident in patients with schizophrenia, such as the psychomotor agitation, stereotypy, deficits of prepulse inhibition and cognitive impairments. Following acute treatment, pregnenolone was found to reduce the hyperlocomotion, stereotypic bouts and pre-pulse inhibition (PPI) deficits in DAT KO mice in a dose-dependent manner. At 60 mg/kg of pregnenolone, there were no significant differences in locomotor activities and stereotypy between wild-type and DAT KO mice. Similarly, acute treatment of 60 mg/kg of pregnenolone fully rescued PPI deficits of DAT KO mice. Following chronic treatment with pregnenolone at 60 mg/kg, the cognitive deficits of DAT KO mice were rescued in the paradigms of novel object recognition test and social transmission of food preference test. Pregnenolone thus holds promise as a therapeutic candidate in schizophrenia.

Rare structural variation of synapse and neurotransmission genes in autism

Autism spectrum disorders (ASDs) comprise a constellation of highly heritable neuropsychiatric disorders. Genome-wide studies of autistic individuals have implicated numerous minor risk alleles but few common variants, suggesting a complex genetic model with many contributing loci. To assess commonality of biological function among rare risk alleles, we compared functional knowledge of genes overlapping inherited structural variants in idiopathic ASD subjects relative to healthy controls. In this study we show that biological processes associated with synapse function and neurotransmission are significantly enriched, with replication, in ASD subjects versus controls. Analysis of phenotypes observed for mouse models of copy-variant genes established significant and replicated enrichment of observable phenotypes consistent with ASD behaviors. Most functional terms retained significance after excluding previously reported ASD loci. These results implicate several new variants that involve synaptic function and glutamatergic signaling processes as important contributors of ASD pathophysiology and suggest a sizable pool of additional potential ASD risk loci.

Too hungry to learn? Hungry broiler breeders fail to learn a Y-maze food quantity discrimination task

Choice tests may aid determining whether qualitative dietary restriction improves the welfare of feed-restricted broiler breeder chickens (Gallus gallus domesticus). However, hunger-stress may reduce competency to choose by impairing learning. The effect of chronic feed restriction on the ability of broiler breeders to learn a hunger-relevant discrimination task was investigated using a Y-maze paradigm. The task was to associate black and white arms with large and small quantities of feed. Birds were reared to three growth curves by means of severe (n = 12), moderate (n = 12) or very mild feed restriction (n = 12). Learning the task and selecting the larger food option allowed birds to increase their feed intake. Time taken to traverse the Y-maze was also measured. Birds from all treatment groups traversed the Y-maze more quickly over time, indicating that they had learnt that running down the Y-maze arms was associated with a rewarding outcome (food). However, feed restriction significantly reduced their ability to associate the black and white cues with differences in food quantity. Consequently, average pay-offs in terms of daily feed increments disproportionately accrued to the less feed-restricted treatment groups. It is concluded that feed restriction affected the performance of broiler breeders in this task, perhaps by narrowing their attention such that they ignore potentially hunger-relevant contextual cues. However, low overall group success rates demonstrate that this task was difficult to learn even for less severely feed-restricted birds. Therefore, Y-maze choice tests may not be the most appropriate method for determining hungry broiler breeder dietary preferences.

Synapsin III: role in neuronal plasticity and disease

Synapsin III was discovered in 1998, more than two decades after the first two synapsins (synapsins I and II) were identified. Although the biology of synapsin III is not as well understood as synapsins I and II, this gene is emerging as an important factor in the regulation of the early stages of neurodevelopment and dopaminergic neurotransmission, and in certain neuropsychiatric illnesses. Molecular genetic and clinical studies of synapsin III have determined that its neurodevelopmental effects are exerted at the levels of neurogenesis and axonogenesis. In vitro voltammetry studies have shown that synapsin III can control dopamine release in the striatum. Since dopaminergic dysfunction is implicated in many neuropsychiatric conditions, one may anticipate that polymorphisms in synapsin III can exert pervasive effects, especially since it is localized to extrasynaptic sites. Indeed, mutations in this gene have been identified in individuals diagnosed with schizophrenia, bipolar disorder and multiple sclerosis. These and other findings indicate that the roles of synapsin III differ significantly from those of synapsins I and II. Here, we focus on the unique roles of the newest synapsin, and where relevant, compare and contrast these with the actions of synapsins I and II.

SYN1 loss-of-function mutations in autism and partial epilepsy cause impaired synaptic function

Several genes predisposing to autism spectrum disorders (ASDs) with or without epilepsy have been identified, many of which are implicated in synaptic function. Here we report a Q555X mutation in synapsin 1 (SYN1), an X-linked gene encoding for a neuron-specific phosphoprotein implicated in the regulation of neurotransmitter release and synaptogenesis. This nonsense mutation was found in all affected individuals from a large French-Canadian family segregating epilepsy and ASDs. Additional mutations in SYN1 (A51G, A550T and T567A) were found in 1.0 and 3.5% of French-Canadian individuals with autism and epilepsy, respectively. The majority of these SYN1 mutations were clustered in the proline-rich D-domain which is substrate of multiple protein kinases. When expressed in synapsin I (SynI) knockout (KO) neurons, all the D-domain mutants failed in rescuing the impairment in the size and trafficking of synaptic vesicle pools, whereas the wild-type human SynI fully reverted the KO phenotype. Moreover, the nonsense Q555X mutation had a dramatic impact on phosphorylation by MAPK/Erk and neurite outgrowth, whereas the missense A550T and T567A mutants displayed impaired targeting to nerve terminals. These results demonstrate that SYN1 is a novel predisposing gene to ASDs, in addition to epilepsy, and strengthen the hypothesis that a disturbance of synaptic homeostasis underlies the pathogenesis of both diseases.

Haploinsufficiency in peptidylglycine alpha-amidating monooxygenase leads to altered synaptic transmission in the amygdala and impaired emotional responses

The mammalian amygdala expresses various neuropeptides whose signaling has been implicated in emotionality. Many neuropeptides require amidation for full activation by peptidylglycine α-amidating monooxygenase (PAM), a transmembrane vesicular cuproenzyme and regulator of the secretory pathway. Mice heterozygous for the Pam gene (PAM(+/-)) exhibit physiological and behavioral abnormalities related to specific peptidergic pathways. In the present study, we evaluated emotionality and examined molecular and cellular responses that characterize neurophysiological differences in the PAM(+/-) amygdala. PAM(+/-) mice presented with anxiety-like behaviors in the zero maze that were alleviated by diazepam. PAM(+/-) animals were deficient in short- and long-term contextual and cued fear conditioning and required higher shock intensities to establish fear-potentiated startle than their wild-type littermates. Immunohistochemical analysis of the amygdala revealed PAM expression in pyramidal neurons and local interneurons that synthesize GABA. We performed whole-cell recordings of pyramidal neurons in the PAM(+/-) amygdala to elucidate neurophysiological correlates of the fear behavioral phenotypes. Consistent with these observations, thalamic afferent synapses in the PAM(+/-) lateral nucleus were deficient in long-term potentiation. This deficit was apparent in the absence and presence of the GABA(A) receptor antagonist picrotoxin and was abolished when both GABA(A) and GABA(B) receptors were blocked. Both evoked and spontaneous excitatory signals were enhanced in the PAM(+/-) lateral nucleus. Phasic GABAergic signaling was also augmented in the PAM(+/-) amygdala, and this difference comprised activity-independent and -dependent components. These physiological findings represent perturbations in the PAM(+/-) amygdala that may underlie the aberrant emotional responses in the intact animal.

Toll-like receptor 3 inhibits memory retention and constrains adult hippocampal neurogenesis

Toll-like receptors (TLRs) are innate immune receptors that have recently emerged as regulators of neuronal survival and developmental neuroplasticity. Adult TLR3-deficient mice exhibited enhanced hippocampus-dependent working memory in the Morris water maze, novel object recognition, and contextual fear-conditioning tasks. In contrast, TLR3-deficient mice demonstrated impaired amygdala-related behavior and anxiety in the cued fear-conditioning, open field, and elevated plus maze tasks. Further, TLR3-deficient mice exhibited increased hippocampal CA1 and dentate gyrus volumes, increased hippocampal neurogenesis, and elevated levels of the AMPA receptor subunit GluR1 in the CA1 region of the hippocampus. In addition, levels of activated forms of the kinase ERK and the transcription factor CREB were elevated in the hippocampus of TLR3-deficient mice, suggesting that constitutive TLR3 signaling negatively regulates pathways known to play important roles in hippocampal plasticity. Direct activation of TLR3 by intracerebroventricular infusion of a TLR3 ligand impaired working memory, but not reference memory. Our findings reveal previously undescribed roles for TLR3 as a suppressor of hippocampal cellular plasticity and memory retention.

Synapsins differentially control dopamine and serotonin release

Synapsins are a family of synaptic vesicle proteins that are important for neurotransmitter release. Here we have used triple knock-out (TKO) mice lacking all three synapsin genes to determine the roles of synapsins in the release of two monoamine neurotransmitters, dopamine and serotonin. Serotonin release evoked by electrical stimulation was identical in substantia nigra pars reticulata slices prepared from TKO and wild-type mice. In contrast, release of dopamine in response to electrical stimulation was approximately doubled in striatum of TKO mice, both in vivo and in striatal slices, in comparison to wild-type controls. This was due to loss of synapsin III, because deletion of synapsin III alone was sufficient to increase dopamine release. Deletion of synapsins also increased the sensitivity of dopamine release to extracellular calcium ions. Although cocaine did not affect the release of serotonin from nigral tissue, this drug did enhance dopamine release. Cocaine-induced facilitation of dopamine release was a function of external calcium, an effect that was reduced in TKO mice. We conclude that synapsins play different roles in the control of release of dopamine and serotonin, with release of dopamine being negatively regulated by synapsins, specifically synapsin III, while serotonin release appears to be relatively independent of synapsins. These results provide further support for the concept that synapsin function in presynaptic terminals varies according to the neurotransmitter being released.

Suppressed phosphorylation of collapsin response mediator protein-2 in the hippocampus of HCNP precursor transgenic mice

We previously reported a novel peptide, Hippocampal Cholinergic Neurostimulating Peptide (HCNP), which induces acetylcholine synthesis by increasing the amount of choline acetyltransferase (ChAT) in medial septal nuclei. The HCNP precursor protein (HCNP-pp), composed of 186 amino acids, is an inhibitory factor of the c-Raf/MEK cascade and may be involved in fetal rat brain development via the inhibition of phosphorylation of Erk. To clarify the involvement of HCNP in hippocampal cholinergic circuitry, we previously generated HCNP-pp transgenic (HCNP-pp Tg) mice using the promoter of the α subunit of Ca(2+) calmodulin-dependent protein kinase II (CaMKIIα). These mice showed increased levels of ChAT in medial septal nuclei at 12 weeks of age, and the phenotype of depressive mood at 30 weeks of age. Here, through proteomic analysis we investigated the alteration of protein expression in the hippocampus of HCNP-pp Tg mice compared with wild-type littermate mice. We demonstrate that the activation of collapsin response mediator protein-2 (CRMP-2) is increased in the transgenic mice at 12 weeks of age when compared with wild-type littermate mice.