Dendritic spine dynamics

Early-life adversity selectively interrupts the dendritic differentiation of dorsolateral striatal neurons in male mice

The effects of early-life adversity (ELA) on dendritic differentiation of striatal neurons were investigated in the dorsal striatum including the dorsomedial striatum and dorsolateral striatum (DMS and DLS, respectively). An animal model of ELA was created by changing the growth environment of newborn mouse pups by giving limited bedding and nesting materials from postnatal day 2 to day 9 (P2-P9). One week after the stress paradigm (P16), the dendritic branches and spines of striatal spiny neurons as well as the synapses represented by postsynaptic density protein-95 (PSD-95) in DMS and DLS were stereologically analyzed. Adverse experience in early life selectively affected the spiny neurons in DLS, leading to abundant proximal dendritic branches and an increased number of filopodia-like protrusions, but a reduced number of dendritic spines. The selective effects of stress on neurons in DLS were further identified by reduced expression of PSD-95, including a reduced optical density of PSD-95 immunoreactivity and fewer individual PSD-95 immunoreactive synapses in this region. Notably, stress in early life affected either D1 or D2 dopamine receptor-expressing DLS neurons. These findings suggest that adverse early-life experience delayed the maturation of dendritic spines on neurons in the dorsolateral striatum. Altered dendritic differentiation provoked by stress in early life may contribute critically to the formation of proper neuronal circuits in the dorsal striatum and, therefore, affect striatum-dependent habitual behavior and emotional function later in life.

Visualizing synaptic plasticity in vivo by large-scale imaging of endogenous AMPA receptors

Elucidating how synaptic molecules such as AMPA receptors mediate neuronal communication and tracking their dynamic expression during behavior is crucial to understand cognition and disease, but current technological barriers preclude large-scale exploration of molecular dynamics in vivo. We have developed a suite of innovative methodologies that break through these barriers: a new knockin mouse line with fluorescently tagged endogenous AMPA receptors, two-photon imaging of hundreds of thousands of labeled synapses in behaving mice, and computer vision-based automatic synapse detection. Using these tools, we can longitudinally track how the strength of populations of synapses changes during behavior. We used this approach to generate an unprecedentedly detailed spatiotemporal map of synapses undergoing changes in strength following sensory experience. More generally, these tools can be used as an optical probe capable of measuring functional synapse strength across entire brain areas during any behavioral paradigm, describing complex system-wide changes with molecular precision.

Learning-Dependent Dendritic Spine Plasticity Is Reduced in the Aged Mouse Cortex

Aging is accompanied by a progressive decrease in learning and memory function. Synaptic loss, one of the hallmarks of normal aging, likely plays an important role in age-related cognitive decline. But little is known about the impact of advanced age on synaptic plasticity and neuronal function in vivo. In this study, we examined the structural dynamics of postsynaptic dendritic spines as well as calcium activity of layer 5 pyramidal neurons in the cerebral cortex of young and old mice. Using transcranial two-photon microscopy, we found that in both sensory and motor cortices, the elimination rates of dendritic spines were comparable between young (3-5 months) and mature adults (8-10 months), but seemed higher in old mice (>20 months), contributing to a reduction of total spine number in the old brain. During the process of motor learning, old mice compared to young mice had fewer new spines formed in the primary motor cortex. Motor training-evoked somatic calcium activity in layer 5 pyramidal neurons of the motor cortex was also lower in old than young mice, which was associated with the decline of motor learning ability during aging. Together, these results demonstrate the effects of aging on learning-dependent synapse remodeling and neuronal activity in the living cortex and suggest that synaptic deficits may contribute to age-related learning impairment.

Pharmacological Targeting of CSF1R Inhibits Microglial Proliferation and Aggravates the Progression of Cerebral Ischemic Pathology

Ischemic stroke can induce rapid activation of the microglia. It has been reported that the microglia’s survival is dependent on colony-stimulating factor 1 receptor (CSF1R) signaling and that pharmacological inhibition of CSF1R leads to morphological changes in the microglia in the healthy brain. However, the impact of CSF1R inhibition on neuronal structures and motor ability after ischemia–reperfusion remains unclear. In this study, we investigated microglial de-ramification, proliferation, and activation after inhibition of CSF1R by a tyrosine kinase inhibitor (ki20227) in a mouse model of global cerebral ischemia induced by bilateral common carotid artery ligation (BCAL). In addition to microglial morphology, we evaluated the mRNA expression of cytokines, chemokines, and inflammatory receptors. Our results show that pharmacological inhibition of CSF1R in ischemic mice resulted in the blockade of microglial proliferation and a shift in microglial morphology reflected by excessive de-ramification and a more activated phenotype accompanied by an enhanced innate immune response. Furthermore, we show that pharmacological inhibition of CSF1R in ischemic mice resulted in the aggravation of neuronal degeneration and behavioral impairment. Intravital two-photon imaging revealed that although pharmacological inhibition of CSF1R did not affect the recovery of dendritic structures, it caused a significant increase in spine elimination during reperfusion in ischemic mice. These findings suggest that pharmacological inhibition of CSF1R induces a blockade of microglial proliferation and causes acute activation of the microglia accompanied by a severe inflammatory response. It aggravates neuronal degeneration, loss of dendritic spines, and behavioral deficits after transient global cerebral ischemia.

Structural plasticity on an accelerated analog neuromorphic hardware system

In computational neuroscience, as well as in machine learning, neuromorphic devices promise an accelerated and scalable alternative to neural network simulations. Their neural connectivity and synaptic capacity depend on their specific design choices, but is always intrinsically limited. Here, we present a strategy to achieve structural plasticity that optimizes resource allocation under these constraints by constantly rewiring the pre- and postsynaptic partners while keeping the neuronal fan-in constant and the connectome sparse. In particular, we implemented this algorithm on the analog neuromorphic system BrainScaleS-2. It was executed on a custom embedded digital processor located on chip, accompanying the mixed-signal substrate of spiking neurons and synapse circuits. We evaluated our implementation in a simple supervised learning scenario, showing its ability to optimize the network topology with respect to the nature of its training data, as well as its overall computational efficiency.

Cannabis points to the synaptic pathology of mental disorders: how aberrant synaptic components disrupt the highest psychological functions

Cannabis can elicit an acute psychotic reaction, and its long-term use is a risk factor for schizophrenia. The main active psychoactive ingredient ∆⁹-tetrahydrocannabinol (Δ⁹-THC) activates cannabinoid 1 (CB1) receptors, which are localized to the terminals of glutamate and GABA neurons in the brain. The endogenous cannabinoids are involved in information processing and plasticity at synapses in the hippocampus, basal ganglia, and cerebral cortex. Exogenously applied CB₁ receptor agonists disrupt neuronal dynamics and synaptic plasticity, resulting in cognitive deficits and impairment of the highest psychological functions. Various other pro-psychotic drugs, such as ketamine and methamphetamine, exert their effects in the same microdomain of synaptic spines as Δ⁹-THC. Additionally, many of the most robust findings in psychiatric genetics include components that localize to dendritic spines and have important roles in information processing and plasticity.


Reduced Dendritic Spines in the Visual Cortex Contralateral to the Optic Nerve Crush Eye in Adult Mice

Purpose

To determine alteration of dendritic spines and associated changes in the primary visual cortex (V1 region) related to unilateral optic nerve crush (ONC) in adult mice.

Methods

Adult unilateral ONC mice were established. Retinal nerve fiber layer (RNFL) thickness was measured by spectral-domain optical coherence tomography. Visual function was estimated by flash visual evoked potentials (FVEPs). Dendritic spines were observed in the V1 region contralateral to the ONC eye by two-photon imaging in vivo. The neurons, reactive astrocytes, oligodendrocytes, and activated microglia were assessed by NeuN, glial fibrillary acidic protein, CNPase, and CD68 in immunohistochemistry, respectively. Tropomyosin receptor kinase B (TrkB) and the markers in TrkB trafficking were estimated using western blotting and co-immunoprecipitation. Transmission electron microscopy and western blotting were used to evaluate autophagy.

Results

The amplitude and latency of FVEPs were decreased and delayed at 3 days, 1 week, 2 weeks, and 4 weeks after ONC, and RNFL thickness was decreased at 2 and 4 weeks after ONC. Dendritic spines were reduced in the V1 region contralateral to the ONC eye at 2, 3, and 4 weeks after ONC, with an unchanged number of neurons. Reactive astrocyte staining was increased at 2 and 4 weeks after ONC, but oligodendrocyte and activated microglia staining remained unchanged. TrkB was reduced with changes in the major trafficking proteins, and enhanced autophagy was observed in the V1 region contralateral to the ONC eye.

Conclusions

Dendritic spines were reduced in the V1 region contralateral to the ONC eye in adult mice. Reactive astrocytes and decreased TrkB may be associated with the reduced dendritic spines.

Structural dynamics and stability of corticocortical and thalamocortical axon terminals during motor learning

Synaptic plasticity is the cellular basis of learning and memory. When animals learn a novel motor skill, synaptic modifications are induced in the primary motor cortex (M1), and new postsynaptic dendritic spines relevant to motor memory are formed in the early stage of learning. However, it is poorly understood how presynaptic axonal boutons are formed, eliminated, and maintained during motor learning, and whether long-range corticocortical and thalamocortical axonal boutons show distinct structural changes during learning. In this study, we conducted two-photon imaging of presynaptic boutons of long-range axons in layer 1 (L1) of the mouse M1 during the 7-day learning of an accelerating rotarod task. The training-period-averaged rate of formation of boutons on axons projecting from the secondary motor cortical area increased, while the average rate of elimination of those from the motor thalamus (thalamic boutons) decreased. In particular, the elimination rate of thalamic boutons during days 4-7 was lower than that in untrained mice, and the fraction of pre-existing thalamic boutons that survived until day 7 was higher than that in untrained mice. Our results suggest that the late stabilization of thalamic boutons in M1 contributes to motor skill learning.

Characterizing spine issues: If offers novel therapeutics to Angelman syndrome

Angelman syndrome (AS) is a rare neurodevelopmental disorder characterized by severe mental retardation, microcephaly, speech impairment, frequent epilepsy, EEG abnormalities, ataxic movements, tongue protrusion, bursts of laughter, sleep abruptions, and hyperactivity. AS results from loss of function of the imprinted UBE3A (ubiquitin-protein ligase E3A) gene on chromosome 15q11-q13, including a mutation on the maternal allele of Ube3a, a large deletion of the maternally inherited chromosomal region 15q11-13, paternal uniparental disomy of chromosome 15q11-13, or an imprinting defect. The Ube3a maternal deleted mouse model recaptured the major phenotypes of AS patients include seizure, learning and memory impairments, sleep disturbance, and motor problems. Owing to the activity-dependent structural and functional plasticity, dendritic spines are believed as the basic subcellular compartment for learning and memory and the sites where LTP and LTD are induced. Defects of spine formation and dynamics are common among several neurodevelopmental disorders and neuropsychiatric disorders including AS and reflect the underlying synaptopathology, which drives clinically relevant behavioral deficits. This review will summarize the impaired spine density, morphology, and synaptic plasticity in AS and propose that future explorations on spine dynamics and synaptic plasticity may help develop novel interventions and therapy for neurodevelopmental disorders like AS.

AUTS2 Regulation of Synapses for Proper Synaptic Inputs and Social Communication

Impairments in synapse development are thought to cause numerous psychiatric disorders. Autism susceptibility candidate 2 (AUTS2) gene has been associated with various psychiatric disorders, such as autism and intellectual disabilities. Although roles for AUTS2 in neuronal migration and neuritogenesis have been reported, its involvement in synapse regulation remains unclear. In this study, we found that excitatory synapses were specifically increased in the Auts2-deficient primary cultured neurons as well as Auts2 mutant forebrains. Electrophysiological recordings and immunostaining showed increases in excitatory synaptic inputs as well as c-fos expression in Auts2 mutant brains, suggesting that an altered balance of excitatory and inhibitory inputs enhances brain excitability. Auts2 mutant mice exhibited autistic-like behaviors including impairments in social interaction and altered vocal communication. Together, these findings suggest that AUTS2 regulates excitatory synapse number to coordinate E/I balance in the brain, whose impairment may underlie the pathology of psychiatric disorders in individuals with AUTS2 mutations.

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AUTS2 regulates excitatory synapse number in forebrain pyramidal neurons

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Loss of Auts2 leads to increased spine formation in development and adulthood

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Loss of Auts2 alters the balance of excitatory and inhibitory synaptic inputs

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Auts2 mutant mice exhibit cognitive and sociobehavioral deficits

Functional Integration of Neuronal Precursors in the Adult Murine Piriform Cortex

The extent of functional maturation and integration of nonproliferative neuronal precursors, becoming neurons in the adult murine piriform cortex, is largely unexplored. We thus questioned whether precursors eventually become equivalent to neighboring principal neurons or whether they represent a novel functional network element. Adult brain neuronal precursors and immature neurons (complex cells) were labeled in transgenic mice (DCX-DsRed and DCX-CreERT2 /flox-EGFP), and their cell fate was characterized with patch clamp experiments and morphometric analysis of axon initial segments. Young (DCX+) complex cells in the piriform cortex of 2- to 4-month-old mice received sparse synaptic input and fired action potentials at low maximal frequency, resembling neonatal principal neurons. Following maturation, the synaptic input detected on older (DCX-) complex cells was larger, but predominantly GABAergic, despite evidence of glutamatergic synaptic contacts. Furthermore, the rheobase current of old complex cells was larger and the maximal firing frequency was lower than those measured in neighboring age-matched principal neurons. The striking differences between principal neurons and complex cells suggest that the latter are a novel type of neuron and new coding element in the adult brain rather than simple addition or replacement for preexisting network components.

Altered Neocortical Dynamics in a Mouse Model of Williams-Beuren Syndrome

Williams-Beuren syndrome (WBS) is a rare neurodevelopmental disorder characterized by moderate intellectual disability and learning difficulties alongside behavioral abnormalities such as hypersociability. Several structural and functional brain alterations are characteristic of this syndrome, as well as disturbed sleep and sleeping patterns. However, the detailed physiological mechanisms underlying WBS are mostly unknown. Here, we characterized the cortical dynamics in a mouse model of WBS previously reported to replicate most of the behavioral alterations described in humans. We recorded the laminar local field potential generated in the frontal cortex during deep anesthesia and characterized the properties of the emergent slow oscillation activity. Moreover, we performed micro-electrocorticogram recordings using multielectrode arrays covering the cortical surface of one hemisphere. We found significant differences between the cortical emergent activity and functional connectivity between wild-type mice and WBS model mice. Slow oscillations displayed Up states with diminished firing rate and lower high-frequency content in the gamma range. Lower firing rates were also recorded in the awake WBS animals while performing a marble burying task and could be associated with the decreased spine density and thus synaptic connectivity in this cortical area. We also found an overall increase in functional connectivity between brain areas, reflected in lower clustering and abnormally high integration, especially in the gamma range. These results expand previous findings in humans, suggesting that the cognitive deficits characterizing WBS might be associated with reduced excitability, plus an imbalance in the capacity to functionally integrate and segregate information.

Triiodothyronine modulates neuronal plasticity mechanisms to enhance functional outcome after stroke

The development of new therapeutic approaches for stroke patients requires a detailed understanding of the mechanisms that enhance recovery of lost neurological functions. The efficacy to enhance homeostatic mechanisms during the first weeks after stroke will influence functional outcome. Thyroid hormones (TH) are essential regulators of neuronal plasticity, however, their role in recovery related mechanisms of neuronal plasticity after stroke remains unknown. This study addresses important findings of 3,5,3′-triiodo-L-thyronine (T₃) in the regulation of homeostatic mechanisms that adjust excitability – inhibition ratio in the post-ischemic brain. This is valid during the first 2 weeks after experimental stroke induced by photothrombosis (PT) and in cultured neurons subjected to an in vitro model of acute cerebral ischemia. In the human post-stroke brain, we assessed the expression pattern of TH receptors (TR) protein levels, important for mediating T₃ actions.

Our results show that T₃ modulates several plasticity mechanisms that may operate on different temporal and spatial scales as compensatory mechanisms to assure appropriate synaptic neurotransmission. We have shown in vivo that long-term administration of T₃ after PT significantly (1) enhances lost sensorimotor function; (2) increases levels of synaptotagmin 1&2 and levels of the post-synaptic GluR2 subunit in AMPA receptors in the peri-infarct area; (3) increases dendritic spine density in the peri-infarct and contralateral region and (4) decreases tonic GABAergic signaling in the peri-infarct area by a reduced number of parvalbumin⁺ / c-fos⁺ neurons and glutamic acid decarboxylase 65/67 levels. In addition, we have shown that T₃ modulates in vitro neuron membrane properties with the balance of inward glutamate ligand-gated channels currents and decreases synaptotagmin levels in conditions of deprived oxygen and glucose. Interestingly, we found increased levels of TRβ1 in the infarct core of post-mortem human stroke patients, which mediate T₃ actions. Summarizing, our data identify T₃ as a potential key therapeutic agent to enhance recovery of lost neurological functions after ischemic stroke.

Low-dose bisphenol A exposure impairs learning and memory ability with alterations of neuromorphology and neurotransmitters in rats

To investigate the developmental neurotoxicity of environmental bisphenol A (BPA) exposure for infants and children, postnatal rats were used as the animal model and were divided into four groups. Then, they were treated with different concentrations of BPA (i.e., 0, 0.5, 50, or 5000 μg/kg·bw/day of BPA as the control, low-, medium- and high-exposed group) from postnatal days 7 to 21. Y-maze tests, Golgi-Cox assays and liquid chromatography-tandem mass spectrometry (LC/MS/MS) were performed to test the changes of learning and memory ability, hippocampal neuromorphology and neurotransmitter levels, respectively. The results showed that the BPA-exposed rats, especially the low- and high-exposed rats, needed more trials and longer times to qualify for the learned criterion than the control rats. Additionally, rats after low- or high-exposure to BPA exhibited decreased DG dendritic complexity and reduced CA1 and DG dendritic spine densities in the hippocampus. Low-dosage BPA treatment could significantly alter the neurotransmitter contents in the hippocampus. In male rats, the levels of glutamic acid (Glu) and acetylcholine increased, while the 5-hydroxytryptamine (5-HT) and γ-aminobutyric acid (GABA) levels decreased, which lead to an unbalanced Glu/GABA ratio. However, in female rats, only 5-HT levels decreased. In conclusion, postnatal exposure to BPA could sex- and dose-dependently disrupt dendritic development and neurotransmitter homeostasis in the rat hippocampus. The impaired spatial learning and memory ability of rats induced by low-dose BPA is associated with both disrupted dendritic development and neurotransmitter homeostasis in the hippocampus.

Helios modulates the maturation of a CA1 neuronal subpopulation required for spatial memory formation

Currently, molecular, electrophysiological and structural studies delineate several neural subtypes in the hippocampus. However, the precise developmental mechanisms that lead to this diversity are still unknown. Here we show that alterations in a concrete hippocampal neuronal subpopulation during development specifically affect hippocampal-dependent spatial memory. We observed that the genetic deletion of the transcription factor Helios in mice, which is specifically expressed in developing hippocampal calbindin-positive CA1 pyramidal neurons (CB-CA1-PNs), induces adult alterations affecting spatial memory. In the same mice, CA3-CA1 synaptic plasticity and spine density and morphology in adult CB-CA1-PNs were severely compromised. RNAseq experiments in developing hippocampus identified an aberrant increase on the Visinin-like protein 1 (VSNL1) expression in the hippocampi devoid of Helios. This aberrant increase on VSNL1 levels was localized in the CB-CA1-PNs. Normalization of VSNL1 levels in CB-CA1-PNs devoid of Helios rescued their spine loss in vitro. Our study identifies a novel and specific developmental molecular pathway involved in the maturation and function of a CA1 pyramidal neuronal subtype.

Bidirectional in vivo structural dendritic spine plasticity revealed by two-photon glutamate uncaging in the mouse neocortex

Most excitatory synapses in the brain form on dendritic spines. Two-photon uncaging of glutamate is widely utilized to characterize the structural plasticity of dendritic spines in brain slice preparations in vitro. In the present study, glutamate uncaging was used to investigate spine plasticity, for the first time, in vivo. A caged glutamate compound was applied to the surface of the mouse visual cortex in vivo, revealing the successful induction of spine enlargement by repetitive two-photon uncaging in a magnesium free solution. Notably, this induction occurred in a smaller fraction of spines in the neocortex in vivo (22%) than in hippocampal slices (95%). Once induced, the time course and mean long-term enlargement amplitudes were similar to those found in hippocampal slices. However, low-frequency (1–2 Hz) glutamate uncaging in the presence of magnesium caused spine shrinkage in a similar fraction (35%) of spines as in hippocampal slices, though spread to neighboring spines occurred less frequently than it did in hippocampal slices. Thus, the structural plasticity may occur similarly in the neocortex in vivo as in hippocampal slices, although it happened less frequently in our experimental conditions.

SYNGAP1 mutations: Clinical, genetic, and pathophysiological features

SYNGAP1 is a gene that encodes the cytosolic protein SYNGAP1 (SYNaptic GTPase Activating Protein), an essential component of the postsynaptic density at excitatory glutamatergic neurons. SYNGAP1 plays critical roles in synaptic development, structure, function, and plasticity. Mutations in SYNGAP1 result in a neurodevelopmental disorder termed Mental retardation-type 5 (MRD5, OMIM #612621) with a phenotype consisting of intellectual disability, motor impairments, and epilepsy, attesting to the importance of this protein for normal brain development. Here we review the clinical and pathophysiological aspects of SYNGAP1 mutations with a focus on their effect on synaptogenesis, neural circuit function, and cellular plasticity. We conclude by comparing the molecular pathogenesis of SYNGAP1 mutations with those of another neurodevelopmental disorder that affects dendritic function and cellular plasticity, fragile X syndrome. Insights into the molecular similarities and differences underlying these disorders could lead to rationale therapy development.

Close Homolog of L1 Regulates Dendritic Spine Density in the Mouse Cerebral Cortex Through Semaphorin 3B

Dendritic spines in the developing mammalian neocortex are initially overproduced and then eliminated during adolescence to achieve appropriate levels of excitation in mature networks. We show here that the L1 family cell adhesion molecule Close Homolog of L1 (CHL1) and secreted repellent ligand Semaphorin 3B (Sema3B) function together to induce dendritic spine pruning in developing cortical pyramidal neurons. Loss of CHL1 in null mutant mice in both genders resulted in increased spine density and a greater proportion of immature spines on apical dendrites in the prefrontal and visual cortex. Electron microscopy showed that excitatory spine synapses with postsynaptic densities were increased in the CHL1-null cortex, and electrophysiological recording in prefrontal slices from mutant mice revealed deficiencies in excitatory synaptic transmission. Mechanistically, Sema3B protein induced elimination of spines on apical dendrites of cortical neurons cultured from wild-type but not CHL1-null embryos. Sema3B was secreted by the cortical neuron cultures, and its levels increased when cells were treated with the GABA antagonist gabazine. In vivo CHL1 was coexpressed with Sema3B in pyramidal neuron subpopulations and formed a complex with Sema3B receptor subunits Neuropilin-2 and PlexinA4. CHL1 and NrCAM, a closely related L1 adhesion molecule, localized primarily to distinct spines and promoted spine elimination to Sema3B or Sema3F, respectively. These results support a new concept in which selective spine elimination is achieved through different secreted semaphorins and L1 family adhesion molecules to sculpt functional neural circuits during postnatal maturation.SIGNIFICANCE STATEMENT Dendritic spines in the mammalian neocortex are initially overproduced and then pruned in adolescent life through unclear mechanisms to sculpt maturing cortical circuits. Here, we show that spine and excitatory synapse density of pyramidal neurons in the developing neocortex is regulated by the L1 adhesion molecule, Close Homolog of L1 (CHL1). CHL1 mediated spine pruning in response to the secreted repellent ligand Semaphorin 3B and associated with receptor subunits Neuropilin-2 and PlexinA4. CHL1 and related L1 adhesion molecule NrCAM localized to distinct spines, and promoted spine elimination to Semaphorin 3B and -3F, respectively. These results support a new concept in which selective elimination of individual spines and nascent synapses can be achieved through the action of distinct secreted semaphorins and L1 adhesion molecules.

Distinct Impacts of Fullerene on Cognitive Functions of Dementia vs. Non-dementia Mice

Fullerene is a family of carbon materials widely applied in modern medicine and ecosystem de-contamination. Its wide application makes human bodies more and more constantly exposed to fullerene particles. Since fullerene particles are able to cross the blood-brain barrier (BBB) (Yamago et al. 1995), if and how fullerene would affect brain functions need to be investigated for human health consideration. For this purpose, we administered fullerene on subcortical ischemic vascular dementia (SIVD) model mice and sham mice, two types of mice with distinct penetration properties of BBB and hence possibly distinct vulnerabilities to fullerene. We studied the spatial learning and memory abilities of mice with Morris water maze (MWM) and the neuroplasticity properties of the hippocampus. Results showed that fullerene administration suppressed outcomes of MWM in sham mice, along with suppressed long-term potentiation (LTP) and dendritic spine densities. Oppositely, recoveries of MWM outcomes and neuroplasticity properties were observed in fullerene-treated SIVD mice. To further clarify the mechanism of the impact of fullerene on neuroplasticity, we measured the levels of postsynaptic density protein 95 (PSD-95), synaptophysin (SYP), brain-derived neurotrophic factor (BDNF), and tropomyosin receptor kinase B (TrkB) by western blot assay. Results suggest that the distinct impacts of fullerene on behavior test and neuroplasticity may be conducted through postsynaptic regulations that were mediated by BDNF.

Dendritic spines: Revisiting the physiological role

Dendritic spines are small, thin, specialized protrusions from neuronal dendrites, primarily localized in the excitatory synapses. Sophisticated imaging techniques revealed that dendritic spines are complex structures consisting of a dense network of cytoskeletal, transmembrane and scaffolding molecules, and numerous surface receptors. Molecular signaling pathways, mainly Rho and Ras family small GTPases pathways that converge on actin cytoskeleton, regulate the spine morphology and dynamics bi-directionally during synaptic activity. During synaptic plasticity the number and shapes of dendritic spines undergo radical reorganizations. Long-term potentiation (LTP) induction promote spine head enlargement and the formation and stabilization of new spines. Long-term depression (LTD) results in their shrinkage and retraction. Reports indicate increased spine density in the pyramidal neurons of autism and Fragile X syndrome patients and reduced density in the temporal gyrus loci of schizophrenic patients. Post-mortem reports of Alzheimer's brains showed reduced spine number in the hippocampus and cortex. This review highlights the spine morphogenesis process, the activity-dependent structural plasticity and mechanisms by which synaptic activity sculpts the dendritic spines, the structural and functional changes in spines during learning and memory using LTP and LTD processes. It also discusses on spine status in neurodegenerative diseases and the impact of nootropics and neuroprotective agents on the functional restoration of dendritic spines.

Dysregulation of Myosin Complex and Striated Muscle Contraction Pathway in the Brains of ALS-SOD1 Model Mice

Amyotrophic lateral sclerosis (ALS) is a progressive and fatal disease characterized by cortical and spinal motor neuron degeneration, some inherited cases of which are caused by mutations in the gene coding for copper-zinc superoxide dismutase-1 (SOD1). The SOD1^G93A mutant model mouse, which expresses large amounts of mutant SOD1, develops adult-onset neurodegeneration of spinal motor neurons and progressive motor deficits leading to paralysis. We used the Tandem Mass Tag technique to investigate the proteome profile of hippocampus, cerebral cortex, and medulla oblongata of the SOD1^G93A mutant model mice as compared with that of wild-type (WT) mice. Fifteen proteins were significantly increased or decreased (i.e., changed) in all three tissues. Gene ontology analysis revealed that the changed proteins were mainly enriched in negative regulation of reactive oxygen species, myosin complex and copper ion binding. In the Striated Muscle Contraction Pathway, most of the identified proteins were decreased in the SOD1^G93A mice compared with the WT mice. Myosin-1 (MYH1), fructose-2,6-bisphosphatase TIGAR (TIGAR), and sarcoplasmic/endoplasmic reticulum calcium ATPase 1 (ATP2A1) were significantly reduced in mutant vs WT mice, as confirmed by Western blot analysis. Since myosins and tropomyosins are specific for synapse function and drive actin dynamics in the maturation of dendritic spines, changes in these proteins may contribute to perturbations of brain neuronal circuitry in addition to spinal motor neuron disease.

Postnatal nectin-3 knockdown induces structural abnormalities of hippocampal principal neurons and memory deficits in adult mice

The early postnatal stage is a critical period of hippocampal neurodevelopment and also a period of high vulnerability to adverse life experiences. Recent evidence suggests that nectin-3, a cell adhesion molecule, mediates memory dysfunction and dendritic alterations in the adult hippocampus induced by postnatal stress. But it is unknown whether postnatal nectin-3 reduction alone is sufficient to alter hippocampal structure and function in adulthood. Here, we down regulated hippocampal expression of nectin-3 and its heterophilic adhesion partner nectin-1, respectively, from early postnatal stage by injecting adeno-associated virus (AAV) into the cerebral lateral ventricles of neonatal mice (postnatal day 2). We found that suppression of nectin-3, but not nectin-1, expression from the early postnatal stage impaired hippocampus-dependent novel object recognition and spatial object recognition in adult mice. Moreover, AAV-mediated nectin-3 knockdown significantly reduced dendritic complexity and spine density of pyramidal neurons throughout the hippocampus, whereas nectin-1 knockdown only induced the loss of stubby spines in CA3. Our data provide direct evidence that nectins, especially nectin-3, are necessary for postnatal hippocampal development of memory functions and structural integrity.

Density of small dendritic spines and microtubule-associated-protein-2 immunoreactivity in the primary auditory cortex of subjects with schizophrenia

Previously, we demonstrated that dendritic spine density (DSD) in deep layer 3 of the primary auditory cortex (A1) is lower, due to having fewer small spines, in subjects with schizophrenia (SZ) than non-psychiatric control (NPC) subjects. We also previously demonstrated that microtubule-associated-protein-2 immunoreactivity (MAP2-IR) in A1 deep layer 3 is lower, and positively correlated with DSD, in SZ subjects. Here, we first sought to confirm these findings in an independent cohort of 25 SZ-NPC subject pairs (cohort 1). We used immunohistochemistry and confocal microscopy to measure DSD and MAP2-IR in A1 deep layer 3. Consistent with previous studies, both DSD and MAP2-IR were lower in SZ subjects. We then tested the hypothesis that MAP2-IR mediates the effect of SZ on DSD in a cohort of 45 SZ-NPC subject pairs (combined cohort) that included all subjects from cohort 1 and two previously studied cohorts. Based on the distribution of MAP2-IR values in NPC subjects, we categorized each SZ subject as having either low MAP2-IR (SZ MAP2-IR(low)) or normal MAP2-IR (SZ MAP2-IR(normal)). Among SZ MAP-IR(low) subjects, mean DSD was significantly lower than in NPC subjects. However, mean DSD did not differ between SZ MAP2-IR(normal) and NPC subjects. Moreover, MAP2-IR statistically mediated small spine differences, with lower MAP2-IR values associated with fewer small spines. Our findings confirm that low density of small spines and low MAP2-IR are robust SZ phenotypes and suggest that MAP2-IR mediates the effect of SZ on DSD.

Chronic Stress Remodels Synapses in an Amygdala Circuit-Specific Manner

BACKGROUND

Chronic stress exposure increases the risk of developing various neuropsychiatric illnesses. The behavioral sequelae of stress correlate with dendritic hypertrophy and glutamate-related synaptic remodeling at basolateral amygdala projection neurons (BLA PNs). Yet, though BLA PNs are functionally heterogeneous with diverse corticolimbic targets, it remains unclear whether stress differentially impacts specific output circuits.

METHODS

Confocal imaging was used to reconstruct the morphology of mouse BLA PNs with the aid of retrograde tracing and biocytin staining. The synaptic activity in these neurons was measured with in vitro electrophysiology, and anxiety-like behavior of the mice was assessed with the elevated plus maze and open field test.

RESULTS

Chronic restraint stress (CRS) produced dendritic hypertrophy across mouse BLA PNs, regardless of whether they did (BLA→dorsomedial prefrontal cortex [dmPFC]) or did not (BLA↛dmPFC) target dmPFC. However, CRS increased the size of dendritic spine heads and the number of mature, mushroom-shaped spines only in BLA↛dmPFC PNs, sparing neighboring BLA→dmPFC PNs. Moreover, the excitatory glutamatergic transmission was also selectively increased in BLA↛dmPFC PNs, and this effect correlated with CRS-induced increases in anxiety-like behavior. Segregating BLA↛dmPFC PNs based on their targeting of ventral hippocampus (BLA→ventral hippocampus) or nucleus accumbens (BLA→nucleus accumbens) revealed that CRS increased spine density and glutamatergic signaling in BLA→ventral hippocampus PNs in a manner that correlated with anxiety-like behavior.

CONCLUSIONS

Chronic stress caused BLA PN neuronal remodeling with a previously unrecognized degree of circuit specificity, offering new insight into the pathophysiological basis of depression, anxiety disorders, and other stress-related conditions.

Primed to Sleep: The Dynamics of Synaptic Plasticity Across Brain States

It is commonly accepted that brain plasticity occurs in wakefulness and sleep. However, how these different brain states work in concert to create long-lasting changes in brain circuitry is unclear. Considering that wakefulness and sleep are profoundly different brain states on multiple levels (e.g., cellular, molecular and network activation), it is unlikely that they operate exactly the same way. Rather it is probable that they engage different, but coordinated, mechanisms. In this article we discuss how plasticity may be divided across the sleep-wake cycle, and how synaptic changes in each brain state are linked. Our working model proposes that waking experience triggers short-lived synaptic events that are necessary for transient plastic changes and mark (i.e., 'prime') circuits and synapses for further processing in sleep. During sleep, synaptic protein synthesis at primed synapses leads to structural changes necessary for long-term information storage.

Effect of physical activity on structural asymmetry of mouse hippocampus

The relevance of studies of adult neurogenesis is evident in connection with the potential use of these new neurons to replace neurons lost in the process of life. Despite considerable efforts, little is known about the fnal fate of these cells, the functional signifcance of their connections and the regulation of their development. It is known that physical activity signifcantly increases the number of fssile progenitors, the precursors of new neurons in the dentate gyrus of the hippocampus. The existing immunohistochemical methods for labeling new neurons do not allow tracing the temporal dynamics of changes in the volume of brain structures in the same animal, induced by external impacts, such as voluntary exercise. This makes it an urgent task to develop and improve methods for long­term control of changes that occur in the adult hippocampus due to the induction of neurogenesis. The main purpose of this work was to non­invasively track, by using magnetic resonance imaging (MRI), the temporal dynamics of changes in the volume of the hippocampus in the same animals that had voluntary physical activity. It was found that voluntary exercise did not change the total volume of the mouse hippocampus. However, the difference in the volume ratio between the right and left parts of the hippocampus was signifcantly lower compared with the control group. The reconstruction and analysis of protein­protein interactions that ensure the survival of a large number of new neurons and their integration into existing neural networks in the hippocampus have been carried out. The proposed approach allows the non­invasive registration of changes in the ratio of the volumes of these paired brain structures.

Is plasticity of synapses the mechanism of long-term memory storage?

It has been 70 years since Donald Hebb published his formalized theory of synaptic adaptation during learning. Hebb's seminal work foreshadowed some of the great neuroscientific discoveries of the following decades, including the discovery of long-term potentiation and other lasting forms of synaptic plasticity, and more recently the residence of memories in synaptically connected neuronal assemblies. Our understanding of the processes underlying learning and memory has been dominated by the view that synapses are the principal site of information storage in the brain. This view has received substantial support from research in several model systems, with the vast majority of studies on the topic corroborating a role for synapses in memory storage. Yet, despite the neuroscience community's best efforts, we are still without conclusive proof that memories reside at synapses. Furthermore, an increasing number of non-synaptic mechanisms have emerged that are also capable of acting as memory substrates. In this review, we address the key findings from the synaptic plasticity literature that make these phenomena such attractive memory mechanisms. We then turn our attention to evidence that questions the reliance of memory exclusively on changes at the synapse and attempt to integrate these opposing views.

Ketamine and selective activation of parvalbumin interneurons inhibit stress-induced dendritic spine elimination

Stress is a major risk factor for the onset of many psychiatric diseases. In rodent models, chronic stress induces depression and impairs excitatory neurotransmission. However, little is known about the effect of stress on synaptic circuitry during the development of behavioral symptoms. Using two-photon transcranial imaging, we studied the effect of repeated restraint stress on dendritic spine plasticity in the frontal cortex in vivo. We found that restraint stress induced dendritic spine loss by decreasing the rate of spine formation and increasing the rate of spine elimination. The N-methyl-D-aspartate receptor antagonist ketamine inhibited stress-induced spine loss mainly by protecting mushroom spines from elimination. Ketamine also induced re-formation of spines in close proximity to previously stress-eliminated spines. Electrophysiological and in vivo imaging experiments showed that ketamine enhanced activity of parvalbumin (PV) interneurons under stress and counterbalanced the stress-induced net loss of PV axonal boutons. In addition, selective chemogenetic excitation of PV interneurons mimicked the protective effects of ketamine on dendritic spines against stress. Collectively, our data provide new insights on the effects of ketamine on synaptic circuitry under stress and a possible mechanism to counteract stress-induced synaptic impairments through PV interneuron activation.

In Vivo Volume Dynamics of Dendritic Spines in the Neocortex of Wild-Type and Fmr1 KO Mice

Excitatory synapses are often formed at small protrusions of dendrite, called dendritic spines, in most projection neurons, and the spine-head volumes show strong correlations with synaptic connectivity. We examined the dynamics of spine volume in the adult mouse visual cortex using time-lapse in vivo two-photon imaging with a resonant Galvano scanner. Contrary to expectations, we found that the spines in the adult neocortex showed fluctuations to a similar degree as that observed in young hippocampal preparations, but there were systematic differences in how the dynamics were dependent on spine volumes, thus allowing for fewer fluctuations in small spines, which could account for the relatively low turnover rates of neocortical spines in vivo. We found that spine volumes fluctuated to a greater extent in a mouse model (Fmr1 knockout) of fragile X mental retardation than in wild-type mice, and the spine turnover rates were also higher in Fmr1 knock-out mice. Such features of spine dynamics in Fmr1 knock-out mice could be represented by a single slope factor in our model. Our data and model indicate a small but significant change in the average spine volume and more eminent differences in the statistical distribution in Fmr1 knock-out mice even in adulthood, which reflects the abnormal in vivo dynamics of spine volumes.

Memory corticalization triggered by REM sleep: mechanisms of cellular and systems consolidation

Once viewed as a passive physiological state, sleep is a heterogeneous and complex sequence of brain states with essential effects on synaptic plasticity and neuronal functioning. Rapid-eye-movement (REM) sleep has been shown to promote calcium-dependent plasticity in principal neurons of the cerebral cortex, both during memory consolidation in adults and during post-natal development. This article reviews the plasticity mechanisms triggered by REM sleep, with a focus on the emerging role of kinases and immediate-early genes for the progressive corticalization of hippocampus-dependent memories. The body of evidence suggests that memory corticalization triggered by REM sleep is a systemic phenomenon with cellular and molecular causes.

Inhibitory connectivity defines the realm of excitatory plasticity

Recent experiments demonstrate substantial volatility of excitatory connectivity in the absence of any learning. This challenges the hypothesis that stable synaptic connections are necessary for long-term maintenance of acquired information. Here we measure ongoing synaptic volatility and use theoretical modeling to study its consequences on cortical dynamics. We show that in the balanced cortex, patterns of neural activity are primarily determined by inhibitory connectivity, despite the fact that most synapses and neurons are excitatory. Similarly, we show that the inhibitory network is more effective in storing memory patterns than the excitatory one. As a result, network activity is robust to ongoing volatility of excitatory synapses, as long as this volatility does not disrupt the balance between excitation and inhibition. We thus hypothesize that inhibitory connectivity, rather than excitatory, controls the maintenance and loss of information over long periods of time in the volatile cortex.

The Human Interface of Biomedical Informatics

Biomedical informatics is the science of information, where information is defined as data with meaning. This definition identifies a fundamental challenge for informaticians: connecting with the healthcare team by enabling the acquisition, retrieval, and processing of information within the cognitive capabilities of the human brain. Informaticians can become aware of the constraints involved with cognitive processing and with workplace factors that impact how information is acquired and used to facilitate an improved user interface providing information to healthcare teams. Constraints affecting persons in the work environment include as follows: (1) cognitive processing of information; (2) cognitive load and memory capacity; (3) stress-affecting cognition; (4) cognitive distraction, attention, and multitasking; (5) cognitive bias and flexibility; (6) communication barriers; and (7) workplace environment. The human brain has a finite cognitive load capacity for processing new information. Short-term memory has limited throughput for processing of new informational items, while long-term memory supplies immediate simultaneous access to multiple informational items. Visual long-term memories can be extensive and detailed. Attention may be task dependent and highly variable among persons and requires maintaining control over distracting information. Multitasking reduces the effectiveness of working memory applied to each task. Transfer of information from person to person, or machine to person, is subject to cognitive bias and environmental stressors. High-stress levels increase emotional arousal to reduce memory formation and retrieval. The workplace environment can impact cognitive processes and stress, so maintaining civility augments cognitive abilities. Examples of human-computer interfaces employing principles of cognitive informatics inform design of systems to enhance the user interface.

Retrograde regulation of mossy fiber axon targeting and terminal maturation via postsynaptic Lnx1

Synapse formation relies on the coordination of dynamic pre- and postsynaptic structures during brain development. Liu et al. reveal that presynaptic terminal maturation of mossy fiber axons is retrogradely regulated by postsynaptic scaffold protein Lnx1 via stabilizing EphB receptor kinases.

Neuronal connections are initiated by axon targeting to form synapses. However, how the maturation of axon terminals is modulated through interacting with postsynaptic elements remains elusive. In this study, we find that ligand of Numb protein X 1 (Lnx1), a postsynaptic PDZ protein expressed in hippocampal CA3 pyramidal neurons, is essential for mossy fiber (MF) axon targeting during the postnatal period. Lnx1 deletion causes defective synaptic arrangement that leads to aberrant presynaptic terminals. We further identify EphB receptors as novel Lnx1-binding proteins to form a multiprotein complex that is stabilized on the CA3 neuron membrane through preventing proteasome activity. EphB1 and EphB2 are independently required to transduce distinct signals controlling MF pruning and targeting for precise DG-CA3 synapse formation. Furthermore, constitutively active EphB2 kinase rescues structure of the wired MF terminals in Lnx1 mutant mice. Our data thus define a retrograde trans-synaptic regulation required for integration of post- and presynaptic structure that participates in building hippocampal neural circuits during the adolescence period.

Development of the Adverse Outcome Pathway (AOP): Chronic binding of antagonist to N-methyl-d-aspartate receptors (NMDARs) during brain development induces impairment of learning and memory abilities of children

The Adverse Outcome Pathways (AOPs) are designed to provide mechanistic understanding of complex biological systems and pathways of toxicity that result in adverse outcomes (AOs) relevant to regulatory endpoints. AOP concept captures in a structured way the causal relationships resulting from initial chemical interaction with biological target(s) (molecular initiating event) to an AO manifested in individual organisms and/or populations through a sequential series of key events (KEs), which are cellular, anatomical and/or functional changes in biological processes. An AOP provides the mechanistic detail required to support chemical safety assessment, the development of alternative methods and the implementation of an integrated testing strategy. An example of the AOP relevant to developmental neurotoxicity (DNT) is described here following the requirements of information defined by the OECD Users' Handbook Supplement to the Guidance Document for developing and assessing AOPs. In this AOP, the binding of an antagonist to glutamate receptor N-methyl-d-aspartate (NMDAR) receptor is defined as MIE. This MIE triggers a cascade of cellular KEs including reduction of intracellular calcium levels, reduction of brain derived neurotrophic factor release, neuronal cell death, decreased glutamate presynaptic release and aberrant dendritic morphology. At organ level, the above mentioned KEs lead to decreased synaptogenesis and decreased neuronal network formation and function causing learning and memory deficit at organism level, which is defined as the AO. There are in vitro, in vivo and epidemiological data that support the described KEs and their causative relationships rendering this AOP relevant to DNT evaluation in the context of regulatory purposes.

Molecular Dissection of Neurobeachin Function at Excitatory Synapses

Spines are small protrusions from dendrites where most excitatory synapses reside. Changes in number, shape, and size of dendritic spines often reflect changes of neural activity in entire circuits or at individual synapses, making spines key structures of synaptic plasticity. Neurobeachin is a multidomain protein with roles in spine formation, postsynaptic neurotransmitter receptor targeting and actin distribution. However, the contributions of individual domains of Neurobeachin to these functions is poorly understood. Here, we used mostly live cell imaging and patch-clamp electrophysiology to monitor morphology and function of spinous synapses in primary hippocampal neurons. We demonstrate that a recombinant full-length Neurobeachin from humans can restore mushroom spine density and excitatory postsynaptic currents in neurons of Neurobeachin-deficient mice. We then probed the role of individual domains of Neurobeachin by comparing them to the full-length molecule in rescue experiments of knockout neurons. We show that the combined PH-BEACH domain complex is highly localized in spine heads, and that it is sufficient to restore normal spine density and surface targeting of postsynaptic AMPA receptors. In addition, we report that the Armadillo domain facilitates the formation of filopodia, long dendritic protrusions which often precede the development of mature spines, whereas the PKA-binding site appears as a negative regulator of filopodial extension. Thus, our results indicate that individual domains of Neurobeachin sustain important and specific roles in the regulation of spinous synapses. Since heterozygous mutations in Neurobeachin occur in autistic patients, the results will also improve our understanding of pathomechanism in neuropsychiatric disorders associated with impairments of spine function.

Increased BBB Permeability Enhances Activation of Microglia and Exacerbates Loss of Dendritic Spines After Transient Global Cerebral Ischemia

Ischemic stroke can induce rapid disruption of blood-brain barrier (BBB). It has been suggested that increased BBB permeability can affect the pathological progression of ischemic tissue. However, the impact of increased BBB permeability on microglial activation and synaptic structures following reperfusion after ischemia remains unclear. In this study, we investigated microglial activation, dendritic damage and plasticity of dendritic spines after increasing BBB permeability following transient global cerebral ischemia in the somatosensory cortices in mice. Bilateral common carotid artery ligation (BCAL) was used to induce transient global cerebral ischemia. Mannitol was used to increase the BBB permeability. Intravital two-photon imaging was performed to image the dendritic structures and BBB extravasation. Microglial morphology was quantitated using a skeletonization analysis method. To evaluate inflammation of cerebral cortex, the mRNA expression levels of integrin alpha M (CD11b), CD68, chemokine (C-X-C motif) ligand 10 (IP10) and tumor necrosis factor alpha (TNF-α) were measured by fluorescent quantitative PCR. Intravital two-photon imaging revealed that mannitol caused a drastic increase in BBB extravasation during reperfusion after transient global ischemia. Increased BBB permeability induced by mannitol had no significant effect on inflammation and dendritic spines in healthy mice but triggered a marked de-ramification of microglia; importantly, in ischemic animals, mannitol accelerated de-ramification of microglia and aggravated inflammation at 3 h but not at 3 days following reperfusion after ischemia. Although mannitol did not cause significant change in the percentage of blebbed dendrites and did not affect the reversible recovery of the dendritic structures, excessive extravasation was accompanied with significant decrease in spine formation and increase in spine elimination during reperfusion in ischemic mice. These findings suggest that increased BBB permeability induced by mannitol can lead to acute activation of microglia and cause excessive loss of dendritic spines after transient global cerebral ischemia.

Preferential Targeting of Lateral Entorhinal Inputs onto Newly Integrated Granule Cells

Mature dentate granule cells in the hippocampus receive input from the entorhinal cortex via the perforant path in precisely arranged lamina, with medial entorhinal axons innervating the middle molecular layer and lateral entorhinal cortex axons innervating the outer molecular layer. Although vastly outnumbered by mature granule cells, adult-generated newborn granule cells play a unique role in hippocampal function, which has largely been attributed to their enhanced excitability and plasticity (Schmidt-Hieber et al., 2004; Ge et al., 2007). Inputs from the medial and lateral entorhinal cortex carry different informational content. Thus, the distribution of inputs onto newly integrated granule cells will affect their function in the circuit. Using retroviral labeling in combination with selective optogenetic activation of medial or lateral entorhinal inputs, we examined the functional innervation and synaptic maturation of newly generated dentate granule cells in the mouse hippocampus. Our results indicate that lateral entorhinal inputs provide the majority of functional innervation of newly integrated granule cells at 21 d postmitosis. Despite preferential functional targeting, the dendritic spine density of immature granule cells was similar in the outer and middle molecular layers, which we speculate could reflect an unequal distribution of shaft synapses. However, chronic blockade of neurotransmitter release of medial entorhinal axons with tetanus toxin disrupted normal synapse development of both medial and lateral entorhinal inputs. Our results support a role for preferential lateral perforant path input onto newly generated neurons in mediating pattern separation, but also indicate that medial perforant path input is necessary for normal synaptic development.SIGNIFICANCE STATEMENT The formation of episodic memories involves the integration of contextual and spatial information. Newly integrated neurons in the dentate gyrus of the hippocampus play a critical role in this process, despite constituting only a minor fraction of the total number of granule cells. Here we demonstrate that these neurons preferentially receive information thought to convey the context of an experience. Each newly integrated granule cell plays this unique role for ∼1 month before reaching maturity.

O-GlcNAc cycling in the developing, adult and geriatric brain

Hundreds of proteins in the nervous system are modified by the monosaccharide O-GlcNAc. A single protein is often O-GlcNAcylated on several amino acids and the modification of a single site can play a crucial role for the function of the protein. Despite its complexity, only two enzymes add and remove O-GlcNAc from proteins, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). Global and local regulation of these enzymes make it possible for O-GlcNAc to coordinate multiple cellular functions at the same time as regulating specific pathways independently from each other. If O-GlcNAcylation is disrupted, metabolic disorder or intellectual disability may ensue, depending on what neurons are affected. O-GlcNAc's promise as a clinical target for developing drugs against neurodegenerative diseases has been recognized for many years. Recent literature puts O-GlcNAc in the forefront among mechanisms that can help us better understand how neuronal circuits integrate diverse incoming stimuli such as fluctuations in nutrient supply, metabolic hormones, neuronal activity and cellular stress. Here the functions of O-GlcNAc in the nervous system are reviewed.

Synapse Dysfunction of Layer V Pyramidal Neurons Precedes Neurodegeneration in a Mouse Model of TDP-43 Proteinopathies

TDP-43 is a major protein component of pathological neuronal inclusions that are present in frontotemporal dementia and amyotrophic lateral sclerosis. We report that TDP-43 plays an important role in dendritic spine formation in the cortex. The density of spines on YFP+ pyramidal neurons in both the motor and somatosensory cortex of Thy1-YFP mice, increased significantly from postnatal day 30 (P30), to peak at P60, before being pruned by P90. By comparison, dendritic spine density was significantly reduced in the motor cortex of Thy1-YFP::TDP-43A315T transgenic mice prior to symptom onset (P60), and in the motor and somatosensory cortex at symptom onset (P90). Morphological spine-type analysis revealed that there was a significant impairment in the development of basal mushroom spines in the motor cortex of Thy1-YFP::TDP-43A315T mice compared to Thy1-YFP control. Furthermore, reductions in spine density corresponded to mislocalisation of TDP-43 immunoreactivity and lowered efficacy of synaptic transmission as determined by electrophysiology at P60. We conclude that mutated TDP-43 has a significant pathological effect at the dendritic spine that is associated with attenuated neural transmission.

Palmitoylation of cdc42 Promotes Spine Stabilization and Rescues Spine Density Deficit in a Mouse Model of 22q11.2 Deletion Syndrome

22q11.2 deletion syndrome (22q11DS) is associated with learning and cognitive dysfunctions and a high risk of developing schizophrenia. It has become increasingly clear that dendritic spine plasticity is tightly linked to cognition. Thus, understanding how genes involved in cognitive disorders affect synaptic networks is a major challenge of modern biology. Several studies have pointed to a spine density deficit in 22q11DS transgenic mice models. Using the LgDel mouse model, we first quantified spine deficit at different stages using electron microscopy. Next we performed repetitive confocal imaging over several days on hippocampal organotypic cultures of LgDel mice. We show no imbalanced ratio between daily spine formation and spine elimination, but a decreased spine life expectancy. We corrected this impaired spine stabilization process by overexpressing ZDHHC8 palmitoyltransferase, whose gene belongs to the LgDel microdeletion. Overexpression of one of its substrates, the cdc42 brain-specific variant, under a constitutively active form (cdc42-palm-CA) led to the same result. Finally, we could rescue spine density in vivo, in adult LgDel mice, by injecting pups with a vector expressing cdc42-palm-CA. This study reveals a new role of ZDHHC8-cdc42-palm molecular pathway in postsynaptic structural plasticity and provides new evidence in favor of the dysconnectivity hypothesis for schizophrenia.

Dendritic Spine Elimination: Molecular Mechanisms and Implications

Dynamic modification of synaptic connectivity in response to sensory experience is a vital step in the refinement of brain circuits as they are established during development and modified during learning. In addition to the well-established role for new spine growth and stabilization in the experience-dependent plasticity of neural circuits, dendritic spine elimination has been linked to improvements in learning, and dysregulation of spine elimination has been associated with intellectual disability and behavioral impairment. Proper brain function requires a tightly regulated balance between spine formation and spine elimination. Although most studies have focused on the mechanisms of spine formation, considerable progress has been made recently in delineating the neural activity patterns and downstream molecular mechanisms that drive dendritic spine elimination. Here, we review the current state of knowledge concerning the signaling pathways that drive dendritic spine shrinkage and elimination in the cerebral cortex and we discuss their implication in neuropsychiatric and neurodegenerative disease.

A new therapy prevents intellectual disability in mouse with phenylketonuria

Untreated phenylketonuria (PKU) results in severe neurodevelopmental disorders, which can be partially prevented by an early and rigorous limitation of phenylalanine (Phe) intake. Enzyme substitution therapy with recombinant Anabaena variabilis Phe Ammonia Lyase (rAvPAL) proved to be effective in reducing blood Phe levels in preclinical and clinical studies of adults with PKU. Aims of present study were: a) to gather proofs of clinical efficacy of rAvPAL treatment in preventing neurological impairment in an early treated murine model of PKU; b) to test the advantages of an alternative delivering system for rAvPAL such as autologous erythrocytes. BTBR-Pah^enu2-/- mice were treated from 15 to 64 post-natal days with weekly infusions of erythrocytes loaded with rAvPAL. Behavioral, neurochemical, and brain histological markers denoting untreated PKU were examined in early treated adult mice in comparison with untreated and wild type animals. rAvPAL therapy normalized blood and brain Phe; prevented cognitive developmental failure, brain depletion of serotonin, dendritic spine abnormalities, and myelin basic protein reduction. No adverse events or inactivating immune reaction were observed. In conclusion present study testifies the clinical efficacy of rAvPAL treatment in a preclinical model of PKU and the advantages of erythrocytes as carrier of the enzyme in term of frequency of the administrations and prevention of immunological reactions.

The antipsychotic landscape: dopamine and beyond

Until recently, the actions of antipsychotic and pro-psychotic drugs have largely been evaluated in the framework of neuronal doctrine - namely, that neurons communicate by releasing transmitters, and that psychiatric disorders are caused by neurotransmitter imbalances. Moreover, the majority of studies have focused on single transmitter systems - neglecting the fact that in the nervous system, different transmitter systems work in concert and impact on not only their immediate receptors but also downstream pathways that shape structural plasticity. In this review, we discuss the history of understanding the antipsychotic and pro-psychotic actions of drugs, recent developments and future perspectives.

Visualization of Thalamocortical Axon Branching and Synapse Formation in Organotypic Cocultures

Axon branching and synapse formation are crucial processes for establishing precise neuronal circuits. During development, sensory thalamocortical (TC) axons form branches and synapses in specific layers of the cerebral cortex. Despite the obvious spatial correlation between axon branching and synapse formation, the causal relationship between them is poorly understood. To address this issue, we recently developed a method for simultaneous imaging of branching and synapse formation of individual TC axons in organotypic cocultures. This protocol describes a method which consists of a combination of an organotypic coculture and electroporation. Organotypic cocultures of the thalamus and cerebral cortex facilitate gene manipulation and observation of axonal processes, preserving characteristic structures such as laminar configuration. Two distinct plasmids encoding DsRed and EGFP-tagged synaptophysin (SYP-EGFP) were co-transfected into a small number of thalamic neurons by an electroporation technique. This method allowed us to visualize individual axonal morphologies of TC neurons and their presynaptic sites simultaneously. The method also enabled long-term observation which revealed the causal relationship between axon branching and synapse formation.

A novel therapeutic approach for treatment of catamenial epilepsy

Many women with epilepsy experience perimenstrual seizure exacerbation, referred to as catamenial epilepsy. There is no effective treatment for this condition, proposed to result from withdrawal of neurosteroid-mediated effects of progesterone. A double-blind, multicenter, phase III, clinical trial of catamenial epilepsy has failed to find a beneficial effect of progesterone. The neurosteroid-mediated effects of progesterone have been extensively studied in relation to catamenial epilepsy; however, the effects mediated by progesterone receptor activation have been overlooked. We determined whether progesterone increased excitatory transmission in the hippocampus via activation of progesterone receptors, which may play a role in regulating catamenial seizure exacerbation. In a double-blind study using a rat model of catamenial epilepsy, we found that treatment with RU-486, which blocks progesterone and glucocorticoid receptors, significantly attenuated neurosteroid withdrawal-induced seizures. Furthermore, progesterone treatment as well as endogenous rise in progesterone during estrous cycle increased the expression of GluA1 and GluA2 subunits of AMPA receptors in the hippocampi, and enhanced the AMPA receptor-mediated synaptic transmission of CA1 pyramidal neurons. The progesterone-induced plasticity of AMPA receptors was blocked by RU-486 treatment and progesterone also failed to increase AMPA receptor expression in progesterone receptor knockout mice. These studies demonstrate that progesterone receptor activation regulates AMPA receptor expression and may play a role in catamenial seizure exacerbation.

The Sensory Neocortex and Associative Memory

Most behaviors in mammals are directly or indirectly guided by prior experience and therefore depend on the ability of our brains to form memories. The ability to form an association between an initially possibly neutral sensory stimulus and its behavioral relevance is essential for our ability to navigate in a changing environment. The formation of a memory is a complex process involving many areas of the brain. In this chapter we review classic and recent work that has shed light on the specific contribution of sensory cortical areas to the formation of associative memories. We discuss synaptic and circuit mechanisms that mediate plastic adaptations of functional properties in individual neurons as well as larger neuronal populations forming topographically organized representations. Furthermore, we describe commonly used behavioral paradigms that are used to study the mechanisms of memory formation. We focus on the auditory modality that is receiving increasing attention for the study of associative memory in rodent model systems. We argue that sensory cortical areas may play an important role for the memory-dependent categorical recognition of previously encountered sensory stimuli.

The complement system: a gateway to gene-environment interactions in schizophrenia pathogenesis

The pathogenesis of schizophrenia is considered to be multi-factorial, with likely gene-environment interactions (GEI). Genetic and environmental risk factors are being identified with increasing frequency, yet their very number vastly increases the scope of possible GEI, making it difficult to identify them with certainty. Accumulating evidence suggests a dysregulated complement pathway among the pathogenic processes of schizophrenia. The complement pathway mediates innate and acquired immunity, and its activation drives the removal of damaged cells, autoantigens and environmentally derived antigens. Abnormalities in complement functions occur in many infectious and autoimmune disorders that have been linked to schizophrenia. Many older reports indicate altered serum complement activity in schizophrenia, though the data are inconclusive. Compellingly, recent genome-wide association studies suggest repeat polymorphisms incorporating the complement 4A (C4A) and 4B (C4B) genes as risk factors for schizophrenia. The C4A/C4B genetic associations have re-ignited interest not only in inflammation-related models for schizophrenia pathogenesis, but also in neurodevelopmental theories, because rodent models indicate a role for complement proteins in synaptic pruning and neurodevelopment. Thus, the complement system could be used as one of the 'staging posts' for a variety of focused studies of schizophrenia pathogenesis. They include GEI studies of the C4A/C4B repeat polymorphisms in relation to inflammation-related or infectious processes, animal model studies and tests of hypotheses linked to autoimmune diseases that can co-segregate with schizophrenia. If they can be replicated, such studies would vastly improve our understanding of pathogenic processes in schizophrenia through GEI analyses and open new avenues for therapy.