AMPA glutamate receptor subunits 1 and 2 regulate dendrite complexity and spine motility in neurons of the developing neocortex

Extracellular molecular signals shaping dendrite architecture during brain development

Proper growth and branching of dendrites are crucial for adequate central nervous system (CNS) functioning. The neuronal dendritic geometry determines the mode and quality of information processing. Any defects in dendrite development will disrupt neuronal circuit formation, affecting brain function. Besides cell-intrinsic programmes, extrinsic factors regulate various aspects of dendritic development. Among these extrinsic factors are extracellular molecular signals which can shape the dendrite architecture during early development. This review will focus on extrinsic factors regulating dendritic growth during early neuronal development, including neurotransmitters, neurotrophins, extracellular matrix proteins, contact-mediated ligands, and secreted and diffusible cues. How these extracellular molecular signals contribute to dendritic growth has been investigated in developing nervous systems using different species, different areas within the CNS, and different neuronal types. The response of the dendritic tree to these extracellular molecular signals can result in growth-promoting or growth-limiting effects, and it depends on the receptor subtype, receptor quantity, receptor efficiency, the animal model used, the developmental time windows, and finally, the targeted signal cascade. This article reviews our current understanding of the role of various extracellular signals in the establishment of the architecture of the dendrites.

Nrf2-mediated protective effect of alpha-lipoic acid on synaptic oxidative damage and inhibition of PKC/ERK/CREB pathway in bisphenol A-exposed HT-22 cells

The harmful effects of bisphenol A (BPA) on learning and memory may involve hippocampal oxidative damage; however, the underlying mechanism remains unclear. Antioxidants that antagonize BPA-induced neuronal oxidative damage lack research. This study aimed to develop an in vitro model using the HT-22 mouse hippocampal neuronal cell line to investigate the neurotoxic mechanism of BPA and the protective effect of alpha-lipoic acid (ALA) on nuclear factor erythroid 2-related factor 2 (Nrf2) inhibition. The results showed that ALA reduced BPA-induced reactive oxygen species and neuronal nitric oxide synthase (nNOS) levels; however, inhibiting Nrf2 weakened the protective effects of ALA. BPA reduced mitochondrial complex I/III activity and ATP levels, but ALA ameliorated this damage. ALA improved the BPA-induced downregulation of the kelch-like ECH-associated protein 1 (keap1)/Nrf2 system, synaptic-related proteins, and the protein kinase C (PKC)/extracellular signal-regulated kinase (ERK)/cAMP response element binding protein (CREB) pathway; however, the protective effects of ALA were weakened when Nrf2 was inhibited. Our results suggest that BPA causes oxidative damage to HT-22 cells by damaging mitochondrial function, nNOS, and the keap1/Nrf2 system, thereby impairing synaptic-related proteins and the PKC/ERK/CREB pathway. ALA counters BPA-induced damage via Nrf2, which may be a significant target for the protective action of ALA.

The Q/R editing site of AMPA receptor GluA2 subunit acts as an epigenetic switch regulating dendritic spines, neurodegeneration and cognitive deficits in Alzheimer's disease

BACKGROUND

RNA editing at the Q/R site of GluA2 occurs with ~99% efficiency in the healthy brain, so that the majority of AMPARs contain GluA2(R) instead of the exonically encoded GluA2(Q). Reduced Q/R site editing infcreases AMPA receptor calcium permeability and leads to dendritic spine loss, neurodegeneration, seizures and learning impairments. Furthermore, GluA2 Q/R site editing is impaired in Alzheimer's disease (AD), raising the possibility that unedited GluA2(Q)-containing AMPARs contribute to synapse loss and neurodegeneration in AD. If true, then inhibiting expression of unedited GluA2(Q), while maintaining expression of GluA2(R), may be a novel strategy of preventing synapse loss and neurodegeneration in AD.

METHODS

We engineered mice with the 'edited' arginine codon (CGG) in place of the unedited glutamine codon (CAG) at position 607 of the Gria2 gene. We crossbred this line with the J20 mouse model of AD and conducted anatomical, electrophysiological and behavioural assays to determine the impact of eliminating unedited GluA2(Q) expression on AD-related phenotypes.

RESULTS

Eliminating unedited GluA2(Q) expression in AD mice prevented dendritic spine loss and hippocampal CA1 neurodegeneration as well as improved working and reference memory in the radial arm maze. These phenotypes were improved independently of Aβ pathology and ongoing seizure susceptibility. Surprisingly, our data also revealed increased spine density in non-AD mice with exonically encoded GluA2(R) as compared to their wild-type littermates, suggesting an unexpected and previously unknown role for unedited GluA2(Q) in regulating dendritic spines.

CONCLUSION

The Q/R editing site of the AMPA receptor subunit GluA2 may act as an epigenetic switch that regulates dendritic spines, neurodegeneration and memory deficits in AD.

Contribution of the dihydropyrimidinase-like proteins family in synaptic physiology and in neurodevelopmental disorders

The dihydropyrimidinase-like (DPYSL) proteins, also designated as the collapsin response mediators (CRMP) proteins, constitute a family of five cytosolic phosphoproteins abundantly expressed in the developing nervous system but down-regulated in the adult mouse brain. The DPYSL proteins were initially identified as effectors of semaphorin 3A (Sema3A) signaling and consequently involved in regulation of growth cone collapse in young developing neurons. To date, it has been established that DPYSL proteins mediate signals for numerous intracellular/extracellular pathways and play major roles in variety of cellular process including cell migration, neurite extension, axonal guidance, dendritic spine development and synaptic plasticity through their phosphorylation status. The roles of DPYSL proteins at early stages of brain development have been described in the past years, particularly for DPYSL2 and DPYSL5 proteins. The recent characterization of pathogenic genetic variants in DPYSL2 and in DPYSL5 human genes associated with intellectual disability and brain malformations, such as agenesis of the corpus callosum and cerebellar dysplasia, highlighted the pivotal role of these actors in the fundamental processes of brain formation and organization. In this review, we sought to establish a detailed update on the knowledge regarding the functions of DPYSL genes and proteins in brain and to highlight their involvement in synaptic processing in later stages of neurodevelopment, as well as their particular contribution in human neurodevelopmental disorders (NDDs), such as autism spectrum disorders (ASD) and intellectual disability (ID).

Treating early postnatal circuit defect delays Huntington's disease onset and pathology in mice

Recent evidence has shown that even mild mutations in the Huntingtin gene that are associated with late-onset Huntington's disease (HD) disrupt various aspects of human neurodevelopment. To determine whether these seemingly subtle early defects affect adult neural function, we investigated neural circuit physiology in newborn HD mice. During the first postnatal week, HD mice have less cortical layer 2/3 excitatory synaptic activity than wild-type mice, express fewer glutamatergic receptors, and show sensorimotor deficits. The circuit self-normalizes in the second postnatal week but the mice nonetheless develop HD. Pharmacologically enhancing glutamatergic transmission during the neonatal period, however, rescues these deficits and preserves sensorimotor function, cognition, and spine and synapse density as well as brain region volume in HD adult mice.

Hyperexcitability and brain morphological differences in mice lacking the cystine/glutamate antiporter, system xc. J Neurosci Res 2021;99:3339-3353. [PMID: 34747522 DOI: 10.1002/jnr.24971]

System x_c ^- (Sx_c ^- ) is a heteromeric antiporter (L-cystine/L-glutamate exchanger) expressed predominately on astrocytes in the central nervous system. Its activity contributes importantly to the maintenance of the ambient extracellular glutamate levels, as well as, to cellular redox homeostasis. Since alterations in glutamate levels and redox modifications could cause structural changes, we analyzed gross regional morphology of thionin-stained brain sections and cellular and subcellular morphology of Golgi-Cox stained layer V pyramidal neurons in the primary motor cortex (PM1) of mice naturally null for SLC7A11 (SLC7A11^sut/sut )-the gene that encodes the substrate specific light chain (xCT) for Sx_c ^- . Intriguingly, in comparison to age- and sex-matched wild-type (SLC7A11^+/+ ) littermate controls, we found morphologic changes-including increased dendritic complexity and mushroom spine area in males and reduced corpus callosum and soma size in females-that have previously been described, in each case, as morphological correlates of excitability. Consistent with this, we found that both male and female SLC7A11^sut/sut mice had lower convulsive seizure thresholds and greater seizure severity than their sex-matched wild-type (SLC7A11^+/+ ) littermates after acute challenge with two pharmacologically distinct chemoconvulsants: the Glu receptor agonist, kainic acid (KA), or the GABA_A receptor antagonist, pentylenetetrazole (PTZ). These results suggest that the loss of Sx_c ^- signaling in males and females perturbs excitatory/inhibitory (E/I) balance in vivo, potentially through its regulation of cellular and subcellular morphology.

Protective effect of alpha-lipoic acid on bisphenol A-induced learning and memory impairment in developing mice: nNOS and keap1/Nrf2 pathway

The adverse effects of bisphenol A (BPA) on learning and memory may be related with oxidative stress, but the mechanisms are unclear. This study aimed to investigate the mechanism of damaged learning and memory caused by BPA through inducing oxidative stress, as well as to explore whether alpha-lipoic acid (ALA) show a protective action. Female mice were exposed to 0.1 μg/mL BPA, 0.2 μg/mL BPA, 0.6 mg/mL ALA, and 0.2 BPA + ALA through drinking water for 8 weeks. The results showed that ALA protected against the impairment of spatial, recognition, and avoidance memory caused by BPA. ALA replenished the reduce of hippocampus coefficient, serum estradiol (E2) level, and hippocampal neurotransmitters levels induced by BPA. ALA alleviated BPA-induced oxidative stress and hippocampal histological changes. BPA exposure reduced the levels of synaptic structural proteins and PKC/ERK/CREB pathway proteins, and ALA improved these reductions. ALA altered the protein levels of nNOS and keap1/Nrf2 pathway affected by BPA. Our results suggested that impairments of learning and memory caused by BPA was related to the damage of hippocampal synapses mediated by oxidative stress, and ALA protected learning and memory by reducing the oxidative stress induced by BPA through regulating the nNOS and keap1/Nrf2 pathway.

How Do Electric Fields Coordinate Neuronal Migration and Maturation in the Developing Cortex?

During development the vast majority of cells that will later compose the mature cerebral cortex undergo extensive migration to reach their final position. In addition to intrinsically distinct migratory behaviors, cells encounter and respond to vastly different microenvironments. These range from axonal tracts to cell-dense matrices, electrically active regions and extracellular matrix components, which may all change overtime. Furthermore, migrating neurons themselves not only adapt to their microenvironment but also modify the local niche through cell-cell contacts, secreted factors and ions. In the radial dimension, the developing cortex is roughly divided into dense progenitor and cortical plate territories, and a less crowded intermediate zone. The cortical plate is bordered by the subplate and the marginal zone, which are populated by neurons with high electrical activity and characterized by sophisticated neuritic ramifications. Neuronal migration is influenced by these boundaries resulting in dramatic changes in migratory behaviors as well as morphology and electrical activity. Modifications in the levels of any of these parameters can lead to alterations and even arrest of migration. Recent work indicates that morphology and electrical activity of migrating neuron are interconnected and the aim of this review is to explore the extent of this connection. We will discuss on one hand how the response of migrating neurons is altered upon modification of their intrinsic electrical properties and whether, on the other hand, the electrical properties of the cellular environment can modify the morphology and electrical activity of migrating cortical neurons.

Impairment of learning and memory induced by perinatal exposure to BPA is associated with ERα-mediated alterations of synaptic plasticity and PKC/ERK/CREB signaling pathway in offspring rats

The effect of bisphenol A (BPA) on learning and memory has attracted much attention recently, but its underlying mechanism remains unclear. We aimed to investigate whether the impairment of learning and memory induced by perinatal exposure to BPA was associated with the hippocampal estrogen receptor α (ERα)-mediated synaptic plasticity and PKC/ERK/CREB signaling pathway in different sex offspring rats. Pregnant Sprague-Dawley rats were treated with BPA (1 and 10 μg/mL) through drinking water from gestational day (GD) 6 to postnatal day (PND) 21. After weaning, offspring drank BPA-free water until PND 56. Morris water maze, placement and object recognition, and step-down passive avoidance task were performed. The serum estradiol (E2) levels, histopathology of hippocampus, and the expression of learning and memory related proteins were measured. The results showed that spatial and recognition memory were impaired in BPA-exposed female and male offspring, but the impaired passive avoidance memory presented only in males, not in females. The serum E2 levels were increased in BPA-exposed females and males. BPA altered the morphology and quantity of hippocampal neurons. The levels of ERα, NMDA receptor subunit 2B (NR2B), p-NR2B, AMPA receptor 1 (GluA1), p-GluA1, PSD-95, synapsin I, PKC, p-ERK and p-CREB protein expression were decreased in BPA exposed females and males, and there were interactions of sex × BPA exposure in ERα, p-NR2B and p-ERK levels. These findings suggested that perinatal exposure to BPA has sex-specific effects on learning and memory, which is associated with ERα-mediated impairment of synaptic plasticity and down-regulation of PKC/ERK/CREB signaling pathway.

Seizure-induced neuronal apoptosis is related to dysregulation of the RNA-edited GluR2 subunit in the developing mouse brain

Ca²⁺-permeable AMPA receptors (AMPAR) which crucially modify maturational programs of the developing brain are involved in seizure-induced glutamate excitotoxicity and apoptosis. Regulatory effects on AMPAR subunit composition and RNA-editing in the developing brain and their significance as therapeutic targets are not well understood. Here, we analyzed acute effects of recurrent pilocarpine-induced neonatal seizures on age- and region-specific expression of AMPAR subunits and adenosine deaminases (ADAR) in the developing mouse brain (P10). After recurrent seizure activity and regeneration periods of 6-72 h cerebral mRNA levels of GluR (glutamate receptor subunit) 1, GluR2, GluR3, and GluR4 were unaffected compared to controls. However, ratio of GluR2 and GluR4 to pooled GluR1-4 mRNA concentration significantly decreased in seizure-exposed brains in comparison to controls. After a regeneration period of 24-72 h ADAR1 and ADAR2 mRNA expression was significantly lower in seizure-exposed brains than in those of controls. This was confirmed at the protein level in the hippocampal CA3 region. We observed a regionally increased apoptosis (TUNEL+ and CC3+ cells) in the hippocampus, parietal cortex and subventricular zone of seizure-exposed brains in comparison to controls. Together, present in vivo data demonstrate the maturational age-specific, functional role of RNA-edited GluR2 in seizure-induced excitotoxicity in the developing mouse brain. In response to recurrent seizure activity, we observed reduced expression of GluR2 and the GluR2 mRNA-editing enzymes ADAR1 and ADAR2 accompanied by increased apoptosis in a region-specific manner. Thus, AMPA receptor subtype-specific mRNA editing is assessed as a promising target of novel neuroprotective treatment strategies in consideration of age-related developmental mechanisms.

The structural arrangement and dynamics of the heteromeric GluK2/GluK5 kainate receptor as determined by smFRET

Kainate receptors, which are glutamate activated excitatory neurotransmitter receptors, predominantly exist as heteromers of GluK2 and GluK5 subunits in the mammalian central nervous system. There are currently no structures of the full-length heteromeric kainate receptors. Here, we have used single molecule FRET to determine the specific arrangement of the GluK2 and GluK5 subunits within the dimer of dimers configuration in a full-length receptor. Additionally, we have also studied the dynamics and conformational heterogeneity of the amino-terminal and agonist-binding domain interfaces associated with the resting and desensitized states of the full-length heteromeric kainate receptor using FRET-based methods. The smFRET data are compared to similar experiments performed on the homomeric kainate receptor to provide insight into the differences in conformational dynamics that distinguish the two functionally. This article is part of a Special Issue entitled: Molecular biophysics of membranes and membrane proteins.

Ketamine increases the expression of GluR1 and GluR2 α-amino-3-hydroxy-5-methy-4-isoxazole propionate receptor subunits in human dopaminergic neurons differentiated from induced pluripotent stem cells

The mechanisms underlying the prolonged antidepressant effects after a single exposure to ketamine are only partially understood. Converging findings indicate a critical role of structural neuroplasticity, recently also proposed for dopaminergic (DA) neurons known to be involved in a depression core symptom, anhedonia. We recently showed that ketamine induces dendritic outgrowth in human DA neurons differentiated in-vitro from induced pluripotent stem cells of healthy donors, a phenomenon blocked by the α-amino-3-hydroxy-5-methy-4-isoxazole propionate receptor antagonist NBQX. As changes in the expression of AMPA receptor subunits GluR1 and GluR2 were observed in neuroplasticity of rodent DA neurons, we aimed to explore this phenomenon in human DA neurons. Using specific antibodies against GluR1 and GluR2 α-amino-3-hydroxy-5-methy-4-isoxazole propionate receptor subunits, we showed that GluR1 levels were significantly higher in soma than in dendrites, whereas for GluR2, levels were significantly higher in dendrites than in soma. One hour exposure to 1 µM ketamine increased the signal of both subunits in dendrites, but only of GluR2 in soma, at 24, 48, and 72 h. Nonlinear polynomial fitting of dendritic expression indicated that the two curves were significantly different, with stronger and more sustained effects on GluR2 expression. Overall, these data support a role for GluR1 and GluR2 dendritic upregulation in driving structural plasticity in human DA neurons depending on ketamine transient exposure, indicating translationally relevant downstream mechanism possibly involved in antidepressant effects.

A Method to Investigate Astrocyte and Microglial Morphological Changes in the Aging Brain of the Rhesus Macaque

With a rapidly aging population, studies of neuroinflammation and degeneration associated with eugeric aging are becoming critical. Using the unique archive at the Tulane National Primate Research Center as a resource, we have developed tools to quantify morphological changes in astrocytes and microglia across the life span of monkeys. This method can be used for morphometric studies of multiple parameters simultaneously in an unbiased manner.

The structural arrangement at intersubunit interfaces in homomeric kainate receptors

Kainate receptors are glutamate-gated cation-selective channels involved in excitatory synaptic signaling and are known to be modulated by ions. Prior functional and structural studies suggest that the dimer interface at the agonist-binding domain plays a key role in activation, desensitization, and ion modulation in kainate receptors. Here we have used fluorescence-based methods to investigate the changes and conformational heterogeneity at these interfaces associated with the resting, antagonist-bound, active, desensitized, and ion-modulated states of the receptor. These studies show that in the presence of Na+ ions the interfaces exist primarily in the coupled state in the apo, antagonist-bound and activated (open channel) states. Under desensitizing conditions, the largely decoupled dimer interface at the agonist-binding domain as seen in the cryo-EM structure is one of the states observed. However, in addition to this state there are several additional states with lower levels of decoupling. Replacing Na+ with Cs+ does not alter the FRET efficiencies of the states significantly, but shifts the population to the more decoupled states in both resting and desensitized states, which can be correlated with the lower activation seen in the presence of Cs+.

(2R,6R)-Hydroxynorketamine promotes dendrite outgrowth in human inducible pluripotent stem cell-derived neurons through AMPA receptor with timing and exposure compatible with ketamine infusion pharmacokinetics in humans

The mechanisms underlying the prolonged antidepressant effects after a single infusion of ketamine are only partially understood. Ketamine's half-life of ∼2 h cannot explain antidepressant effects that last for 1 week, suggesting the triggering of long-lasting neuroplasticity. Recent human pharmacokinetics (PK) data indicate that a ketamine metabolite, (2R,6R)-hydroxynorketamine (HNK), persists in the high submicromolar range for additional 6-12 h. As in rodents HNK can induce dendrite outgrowth through AMPA receptor-mediated mechanisms, in this work, we aimed to show that HNK produces similar effects in human neurons at concentrations and exposure time compatible with human PK after ketamine infusion. Human dopaminergic neurons were differentiated in-vitro from inducible pluripotent stem cells obtained from healthy donors. Exposure to submicromolar HNK for 6 h produced dendrite outgrowth when measured 3 days after exposure. This neuroplasticity was similar to that obtained with exposure to low micromolar concentrations of ketamine for 1 or 6 h. HNK and ketamine effects were blocked by pretreatment with the AMPA receptor antagonists NBQX and GYKI 52466, and by the mammalian target of rapamycin pathway blocker rapamycin. It is reasonable to conclude that the mechanistic similarity between ketamine and HNK and their diachronic brain exposure owing to the different plasma PK observed after single therapeutic ketamine infusion should contribute to the final sustained antidepressant action.

Neural stem cell differentiation into mature neurons: Mechanisms of regulation and biotechnological applications

The abilities of stem cells to self-renew and form different mature cells expand the possibilities of applications in cell-based therapies such as tissue recomposition in regenerative medicine, drug screening, and treatment of neurodegenerative diseases. In addition to stem cells found in the embryo, various adult organs and tissues have niches of stem cells in an undifferentiated state. In the central nervous system of adult mammals, neurogenesis occurs in two regions: the subventricular zone and the dentate gyrus in the hippocampus. The generation of the different neural lines originates in adult neural stem cells that can self-renew or differentiate into astrocytes, oligodendrocytes, or neurons in response to specific stimuli. The regulation of the fate of neural stem cells is a finely controlled process relying on a complex regulatory network that extends from the epigenetic to the translational level and involves extracellular matrix components. Thus, a better understanding of the mechanisms underlying how the process of neurogenesis is induced, regulated, and maintained will provide elues for development of novel for strategies for neurodegenerative therapies. In this review, we focus on describing the mechanisms underlying the regulation of the neuronal differentiation process by transcription factors, microRNAs, and extracellular matrix components.

Ketamine enhances structural plasticity in mouse mesencephalic and human iPSC-derived dopaminergic neurons via AMPAR-driven BDNF and mTOR signaling

Among neurobiological mechanisms underlying antidepressant properties of ketamine, structural remodeling of prefrontal and hippocampal neurons has been proposed as critical. The suggested mechanism involves downstream activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, which trigger mammalian target of rapamycin (mTOR)-dependent structural plasticity via brain-derived neurotrophic factor (BDNF) and protein neo-synthesis. We evaluated whether ketamine elicits similar molecular events in dopaminergic (DA) neurons, known to be affected in mood disorders, using a novel, translational strategy that involved mouse mesencephalic and human induced pluripotent stem cells-derived DA neurons. Sixty minutes exposure to ketamine elicited concentration-dependent increases of dendritic arborization and soma size in both mouse and human cultures as measured 72 hours after application. These structural effects were blocked by mTOR complex/signaling inhibitors like rapamycin. Direct evidence of mTOR activation by ketamine was revealed by its induction of p70S6 kinase. All effects of ketamine were abolished by AMPA receptor antagonists and mimicked by the AMPA-positive allosteric modulator CX614. Inhibition of BDNF signaling prevented induction of structural plasticity by ketamine or CX614. Furthermore, the actions of ketamine required functionally intact dopamine D3 receptors (D3R), as its effects were abolished by selective D3R antagonists and absent in D3R knockout preparations. Finally, the ketamine metabolite (2R,6R)-hydroxynorketamine mimicked ketamine effects at sub-micromolar concentrations. These data indicate that ketamine elicits structural plasticity by recruitment of AMPAR, mTOR and BDNF signaling in both mouse mesencephalic and human induced pluripotent stem cells-derived DA neurons. These observations are of likely relevance to the influence of ketamine upon mood and its other functional actions in vivo.

SPARC and GluA1-Containing AMPA Receptors Promote Neuronal Health Following CNS Injury

The proper formation and maintenance of functional synapses in the central nervous system (CNS) requires communication between neurons and astrocytes and the ability of astrocytes to release neuromodulatory molecules. Previously, we described a novel role for the astrocyte-secreted matricellular protein SPARC (Secreted Protein, Acidic and Rich in Cysteine) in regulating α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) and plasticity at developing synapses. SPARC is highly expressed by astrocytes and microglia during CNS development but its level is reduced in adulthood. Interestingly, SPARC has been shown to be upregulated in CNS injury and disease. However, the role of SPARC upregulation in these contexts is not fully understood. In this study, we investigated the effect of chronic SPARC administration on glutamate receptors on mature hippocampal neuron cultures and following CNS injury. We found that SPARC treatment increased the number of GluA1-containing AMPARs at synapses and enhanced synaptic function. Furthermore, we determined that the increase in synaptic strength induced by SPARC could be inhibited by Philanthotoxin-433, a blocker of homomeric GluA1-containing AMPARs. We then investigated the effect of SPARC treatment on neuronal health in an injury context where SPARC expression is upregulated. We found that SPARC levels are increased in astrocytes and microglia following middle cerebral artery occlusion (MCAO) in vivo and oxygen-glucose deprivation (OGD) in vitro. Remarkably, chronic pre-treatment with SPARC prevented OGD-induced loss of synaptic GluA1. Furthermore, SPARC treatment reduced neuronal death through Philanthotoxin-433 sensitive GluA1 receptors. Taken together, this study suggests a novel role for SPARC and GluA1 in promoting neuronal health and recovery following CNS damage.

In vivo and in vitro sex differences in the dendritic morphology of developing murine hippocampal and cortical neurons

Altered dendritic morphology is common in neurodevelopmental disorders (NDDs), many of which show sex biases in prevalence, onset and/or severity. However, whether dendritic morphology varies as a function of sex in juvenile mice or primary neuronal cell cultures is largely unknown even though both are widely used models for studying NDDs. To address this gap, we quantified dendritic morphology in CA1 pyramidal hippocampal and adjacent somatosensory pyramidal cortical neurons from male and female postnatal day (P)28 C57BL/6J mice. As determined by Sholl analysis of Golgi-stained brain sections, dendritic arbors of male hippocampal neurons are more complex than females. Conversely, dendritic morphology of female cortical neurons is more complex than males. In primary neuron-glia co-cultures from P0 mouse hippocampi, male neurons have more complex dendritic arbors than female neurons. Sex differences are less pronounced in cortical cultures. In vitro sex differences in dendritic morphology are driven in part by estrogen-dependent mechanisms, as evidenced by decreased dendritic complexity in male hippocampal neurons cultured in phenol red-free media or in the presence of an estrogen receptor antagonist. Evidence that sex influences dendritic morphogenesis in two models of neurodevelopment in a region-specific manner has significant mechanistic implications regarding sex biases in NDDs.

Low-density lipoprotein receptor-related protein 1 (LRP1) regulates the stability and function of GluA1 α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor in neurons

The low-density lipoprotein receptor-related protein 1 (LRP1) is a multifunctional endocytic receptor abundantly expressed in neurons. Increasing evidence demonstrates that LRP1 regulates synaptic integrity and function at the post synapses, at least partially by regulating glutamate receptors. The α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) are critical ionotropic glutamate receptors consisting of homotetramer or heterotetramer of GluA1-4 subunits and play an essential role in synaptic transmission and synaptic plasticity. Our previous work has shown that neuronal deletion of the Lrp1 gene in mice leads to decreased level of GluA1 and reduced long-term potentiation. To understand the underlying mechanism, we investigated the cellular and functional consequences of LRP1 deletion in primary neurons. Here, we show that LRP1 interacts with and regulates the cellular distribution and turnover of GluA1. LRP1 knockdown in mouse primary neurons led to accelerated turnover and decreased cell surface distribution of GluA1, which correspond to decreased phosphorylation of GluA1 at S845 and S831 sites. Decreased LRP1 expression also attenuated AMPA-evoked calcium influx and reduced GluA1-regulated neurite outgrowth and filopodia density. Our results reveal a novel mechanism by which LRP1 controls synaptic integrity and function, specifically by regulating GluA1 trafficking, phosphorylation and turnover. They further demonstrate that LRP1-GluA1 pathway may hold promises as a therapeutic target for restoring synaptic functions in neurodegenerative diseases.

Adolescent exposure to THC in female rats disrupts developmental changes in the prefrontal cortex

Current concepts suggest that exposure to THC during adolescence may act as a risk factor for the development of psychiatric disorders later in life. However, the molecular underpinnings of this vulnerability are still poorly understood. To analyze this, we investigated whether and how THC exposure in female rats interferes with different maturational events occurring in the prefrontal cortex during adolescence through biochemical, pharmacological and electrophysiological means. We found that the endocannabinoid system undergoes maturational processes during adolescence and that THC exposure disrupts them, leading to impairment of both endocannabinoid signaling and endocannabinoid-mediated LTD in the adult prefrontal cortex. THC also altered the maturational fluctuations of NMDA subunits, leading to larger amounts of gluN2B at adulthood. Adult animals exposed to THC during adolescence also showed increased AMPA gluA1 with no changes in gluA2 subunits. Finally, adolescent THC exposure altered cognition at adulthood. All these effects seem to be triggered by the disruption of the physiological role played by the endocannabinoid system during adolescence. Indeed, blockade of CB1 receptors from early to late adolescence seems to prevent the occurrence of pruning at glutamatergic synapses. These results suggest that vulnerability of adolescent female rats to long-lasting THC adverse effects might partly reside in disruption of the pivotal role played by the endocannabinoid system in the prefrontal cortex maturation.

A new tool for the quantitative analysis of dendritic filopodial motility

Dendritic filopodia are tiny and highly motile protrusions formed along the dendrites of neurons. During the search for future presynaptic partners, their shape and size change dynamically, with a direct impact on the formation, stabilization and maintenance of synaptic connections both in vivo and in vitro. In order to reveal molecular players regulating synapse formation, quantitative analysis of dendritic filopodia motility is needed. Defining the length or the tips of these protrusions manually, however, is time consuming, limiting the extent of studies as well as their statistical power. Additionally, area detection based on defining a single intensity threshold can lead to significant errors throughout the image series, as these small structures often have low contrast in fluorescent images. To overcome these problems, the open access Dendritic Filopodia Motility Analyzer, a semi-automated ImageJ/Fiji plugin was created. Our method calculates the displacement of the centre of mass (CoM) within a selected region based on the weighted intensity values of structure forming pixels, selected by upper and lower intensity thresholds. Using synthetic and real biological samples, we prove that the displacement of the weighted CoM reliably characterizes the motility of dendritic protrusions. Additionally, guidelines to define optimal parameters of live cell recordings from dendritic protrusions are provided. © 2014 International Society for Advancement of Cytometry.

Estradiol regulates markers of synaptic plasticity in the hypothalamic ventromedial nucleus and amygdala of female rats

Ovarian hormones act in multiple brain regions to modulate specific behaviors and emotional states. For example, ovarian hormones promote female sexual receptivity in the hypothalamic ventromedial nucleus (VMH) and modulate anxiety in the amygdala. Hormone-induced changes within the VMH include structural modifications, such as changes in dendritic spines, dendrite length and the number of synapses. In some situations, dendrite remodeling requires actin polymerization, which depends on phospho-deactivation of the enzyme cofilin, or the ionotropic AMPA-type glutamate receptors, especially the GluA1 and GluA2 subunits. The present experiments used immunohistochemistry to test the hypothesis that ovarian hormone-induced neural plasticity in the VMH and amygdala involves the regulation of phospho-cofilin, GluA1 and GluA2. These proteins were assessed acutely after estradiol administration (0.5, 1.0 and 4.0h), as well as three days after hormone treatment. Both brain regions displayed rapid (4.0h or less) and transient estradiol-induced increases in the level of phospho-cofilin. At the behaviorally relevant time point of three days, differential changes in AMPA receptor subunits were observed. Using Golgi impregnation, the effect of estradiol on amygdala dendrites was examined. Three days after estradiol treatment, an increase in the length of dendrites in the central nucleus of the amygdala was observed. Thus, estradiol initiates structural changes in dendrites in both the VMH and amygdala associated with an early phospho-deactivation of cofilin, followed by dynamic, brain region-specific changes in AMPA receptor composition.

MiR-223 regulates the differentiation of immature neurons

BACKGROUND

Small non-coding microRNA RNA molecules can regulate stem cell function. The role of microRNAs in neural stem/progenitor cells (NS/PCs) differentiation is not entirely clear.

METHODS

MiRNA profiling, loss and gain of function studies coupled with dendritic tree development morphometric analysis and calcium influx imaging were utilized to investigate the role of micoRNA-223 in differentiating NS/PCs.

RESULTS

MiRNA profiling in human NS/PCs before and after differentiation in vitro reveals modulation of miRNAs following differentiation of NS/PCs. MiR-223, a microRNA well characterized as a hematopoietic-specific miRNA was identified. Cell-autonomous inhibition of miR-223 in the adult mouse dentate gyrus NS/PCs led to a significant increase in immature neurons soma size, dendritic tree total length, branch number per neuron and complexity, while neuronal migration in the dentate gyrus remained unaffected. Overexpression of miR-223 decreased dendritic tree total length, branch number and complexity in neurons differentiated from human embryonic stem cells (hESCs). Inhibition of miR-223 enhanced N-methyl-D-aspartate (NMDA) induced calcium influx in human neurons differentiated from NS/PCs.

CONCLUSIONS

Taken together, these findings indicate that miR-223 regulates the differentiation of neurons derived from NS/PCs.

Postsynaptic GABAB receptor activity regulates excitatory neuronal architecture and spatial memory

Cognitive dysfunction is a common symptom in many neuropsychiatric disorders and directly correlates with poor patient outcomes. The majority of prolonged inhibitory signaling in the brain is mediated via GABAB receptors (GABABRs), but the molecular function of these receptors in cognition is ill defined. To explore the significance of GABABRs in neuronal activity and cognition, we created mice with enhanced postsynaptic GABABR signaling by mutating the serine 783 in receptor R2 subunit (S783A), which decreased GABABR degradation. Enhanced GABABR activity reduced the expression of immediate-early gene-encoded protein Arc/Arg3.1, effectors that are critical for long-lasting memory. Intriguingly, S783A mice exhibited increased numbers of excitatory synapses and surface AMPA receptors, effects that are consistent with decreased Arc/Arg3.1 expression. These deficits in Arc/Arg3.1 and neuronal morphology lead to a deficit in spatial memory consolidation. Collectively our results suggest a novel and unappreciated role for GABABR activity in determining excitatory neuronal architecture and spatial memory via their ability to regulate Arc/Arg3.1.

MiR-223 regulates the differentiation of immature neurons

Background

Small non-coding microRNA RNA molecules can regulate stem cell function. The role of microRNAs in neural stem/progenitor cells (NS/PCs) differentiation is not entirely clear.

Methods

MiRNA profiling, loss and gain of function studies coupled with dendritic tree development morphometric analysis and calcium influx imaging were utilized to investigate the role of micoRNA-223 in differentiating NS/PCs.

Results

MiRNA profiling in human NS/PCs before and after differentiation in vitro reveals modulation of miRNAs following differentiation of NS/PCs. MiR-223, a microRNA well characterized as a hematopoietic-specific miRNA was identified. Cell-autonomous inhibition of miR-223 in the adult mouse dentate gyrus NS/PCs led to a significant increase in immature neurons soma size, dendritic tree total length, branch number per neuron and complexity, while neuronal migration in the dentate gyrus remained unaffected. Overexpression of miR-223 decreased dendritic tree total length, branch number and complexity in neurons differentiated from human embryonic stem cells (hESCs). Inhibition of miR-223 enhanced N-methyl-D-aspartate (NMDA) induced calcium influx in human neurons differentiated from NS/PCs.

Conclusions

Taken together, these findings indicate that miR-223 regulates the differentiation of neurons derived from NS/PCs.

Electronic supplementary material

The online version of this article (doi:10.1186/2052-8426-2-18) contains supplementary material, which is available to authorized users.

Antihypertensive drug Valsartan promotes dendritic spine density by altering AMPA receptor trafficking

Recent studies demonstrated that the antihypertensive drug Valsartan improved spatial and episodic memory in mouse models of Alzheimer's Disease (AD) and human subjects with hypertension. However, the molecular mechanism by which Valsartan can regulate cognitive function is still unknown. Here, we investigated the effect of Valsartan on dendritic spine formation in primary hippocampal neurons, which is correlated with learning and memory. Interestingly, we found that Valsartan promotes spinogenesis in developing and mature neurons. In addition, we found that Valsartan increases the puncta number of PSD-95 and trends toward an increase in the puncta number of synaptophysin. Moreover, Valsartan increased the cell surface levels of AMPA receptors and selectively altered the levels of spinogenesis-related proteins, including CaMKIIα and phospho-CDK5. These data suggest that Valsartan may promote spinogenesis by enhancing AMPA receptor trafficking and synaptic plasticity signaling.

Perinatal exposure to bisphenol-A inhibits synaptogenesis and affects the synaptic morphological development in offspring male mice

Our previous study indicated that perinatal exposure to low-dose BPA, one of the most common environmental endocrine disrupters, alters behavioral development in offspring mice. Given that synaptic structure of the hippocampus is closely related to behaviors, in the present study, we examined the effects of perinatal exposure to BPA (0.04, 0.4, and 4.0 mg kg(-1) day(-1)) on the synaptic density and the synaptic structural modification of pyramidal cells in hippocampus region CA1 and the expressions of synaptic proteins such as synapsin I and PSD-95 and glutamate NMDA and AMPA receptors in male offspring mice on postnatal day (PND) 14, 21, and 56. The results of electron microscope measurement showed that BPA significantly reduced the numeric synaptic density and altered the structural modification of synaptic interface of pyramidal cells with the enlarged synaptic cleft, the shortened active zone, and the thinned postsynaptic density (PSD) on PND 14, 21, and 56 and the increased curvature of synaptic interface on PND 14 and 21. Further analyses of Western blot indicated that BPA markedly reduced the levels of synapsin I and PSD-95 on PND 14, 21, and 56 and down-regulated NMDA receptor subunit NR1 and AMPA receptor subunit GluR1 during development and young adulthood. These results suggest that perinatal exposure to low level of BPA inhibits synaptogenesis and affects synaptic structural modification after birth. The reduced expressions of synaptic proteins synapsin I and PSD-95 and glutamate NMDA and AMPA receptors may be involved in the negative changes in the synaptic plasticity.

Short-term, moderate exercise is capable of inducing structural, BDNF-independent hippocampal plasticity

Exercise is known to improve cognitive functions and to induce neuroprotection. In this study we used a short-term, moderate intensity treadmill exercise protocol to investigate the effects of exercise on usual markers of hippocampal synaptic and structural plasticity, such as synapsin I (SYN), synaptophysin (SYP), neurofilaments (NF), microtubule-associated protein 2 (MAP2), glutamate receptor subunits GluR1 and GluR2/3, brain-derived neurotrophic factor (BDNF) and glial fibrillary acidic protein (GFAP). Immunohistochemistry, Western blotting and real-time PCR were used. We also evaluated the number of cells positive for the proliferation marker 5-bromo-2-deoxyuridine (BrdU), the neurogenesis marker doublecortin (DCX) and the plasma corticosterone levels. Adult male Wistar rats were adapted to a treadmill and divided into 4 groups: sedentary (SED), 3-day exercise (EX3), 7-day exercise (EX7) and 15-day exercise (EX15). The protein changes detected were increased levels of NF68 and MAP2 at EX3, of SYN at EX7 and of GFAP at EX15, accompanied by a decreased level of GluR1 at EX3. Immunohistochemical findings revealed a similar pattern of changes. The real-time PCR analysis disclosed only an increase of MAP2 mRNA at EX7. We also observed an increased number of BrdU-positive cells and DCX-positive cells in the subgranular zone of the dentate gyrus at all time points and increased corticosterone levels at EX3 and EX7. These results reveal a positive effect of short-term, moderate treadmill exercise on hippocampal plasticity. This effect was in general independent of transcriptional processes and of BDNF upregulation, and occurred even in the presence of increased corticosterone levels.

Cell class-specific regulation of neocortical dendrite and spine growth by AMPA receptor splice and editing variants

Glutamatergic transmission converging on calcium signaling plays a key role in dendritic differentiation. In early development, AMPA receptor (AMPAR) transcripts are extensively spliced and edited to generate subunits that differ in their biophysical properties. Whether these subunits have specific roles in the context of structural differentiation is unclear. We have investigated the role of nine GluA variants and revealed a correlation between the expression of flip variants and the period of major dendritic growth. In interneurons, only GluA1(Q)-flip increased dendritic length and branching. In pyramidal cells, GluA2(Q)-flop, GluA2(Q)-flip, GluA3(Q)-flip and calcium-impermeable GluA2(R)-flip promoted dendritic growth, suggesting that flip variants with slower desensitization kinetics are more important than receptors with elevated calcium permeability. Imaging revealed significantly higher calcium signals in pyramidal cells transfected with GluA2(R)-flip as compared with GluA2(R)-flop, suggesting a contribution of voltage-activated calcium channels. Indeed, dendritic growth induced by GluA2(R)-flip in pyramidal cells was prevented by blocking NMDA receptors (NMDARs) or voltage-gated calcium channels (VGCCs), suggesting that they act downstream of AMPARs. Intriguingly, the action of GluA1(Q)-flip in interneurons was also dependent on NMDARs and VGCCs. Cell class-specific effects were not observed for spine formation, as GluA2(Q)-flip and GluA2(Q)-flop increased spine density in pyramidal cells as well as in interneurons. The results suggest that AMPAR variants expressed early in development are important determinants for activity-dependent dendritic growth in a cell type-specific and cell compartment-specific manner.

Calcium/calmodulin-dependent protein kinase IV mediates distinct features of basal and activity-dependent dendrite complexity

Intracellular signaling mechanisms translate extracellular signals, such as neuronal activity, into effects on dendrite complexity. Deciphering these mechanisms has considerable impact on understanding how the brain develops and what can go wrong in developmental disorders. How neurons regulate intracellular signaling to control their dendrite morphology remains poorly understood and is likely to be determined at the level of individual neuronal types. Calcium/calmodulin-dependent protein kinase IV (CaMKIV) is a signaling mechanism involved in the regulation of gene expression and dendrite growth. Expression of CaMKIV is developmentally regulated in the cerebral cortex, with highest expression occurring concomitant with the period of extensive dendrite growth and elaboration. Interestingly, cortical neurons heterogeneously expressed CaMKIV in postnatal rat cortices and cortical neurons in vitro. We tested if this differential CaMKIV expression mediated distinct arborization patterns in the dendrites of pyramidal and non-pyramidal neurons. In fact, CaMKIV mediated dendrite complexity via regulation of specific morphological features of the dendrite arbor: branching and elongation, but not primary dendrite formation. We found that small interfering RNA (siRNA) knockdown of CaMKIV decreased basal dendrite complexity indicating that endogenously expressed CaMKIV mediated dendrite complexity. CaMKIV was also required for activity-induced dendrite elaboration. Active CaMKIV expression in cortical neurons increased dendrite elaboration indicating that enzymatic activity was involved. These data indicated neuronal CaMKIV expression was required for basal and activity-induced dendrite complexity. Further, the data presented in this study indicate CaMKIV contributes to the diversity of dendrite arbors via restricted expression and regulation of distinct modes of dendrite elaboration.

Chronic fluoxetine treatment induces structural plasticity and selective changes in glutamate receptor subunits in the rat cerebral cortex

It has been postulated that chronic administration of antidepressant drugs induces delayed structural and molecular adaptations at glutamatergic forebrain synapses that might underlie mood improvement. To gain further insight into these changes in the cerebral cortex, rats were treated with fluoxetine (flx) for 4 weeks. These animals showed decreased anxiety and learned helplessness. N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunit levels (NR1, NR2A, NR2B, GluR1 and GluR2) were analysed in the forebrain by both western blot of homogenates and immunohistochemistry. Both methods demonstrated an upregulation of NR2A, GluR1 and GluR2 that was especially significant in the retrosplenial granular b cortex (RSGb). However, when analysing subunit content in postsynaptic densities and synaptic membranes, we found increases of NR2A and GluR2 but not GluR1. Instead, GluR1 was augmented in a microsomal fraction containing intracellular membranes. NR1 and GluR2 were co-immunoprecipitated from postsynaptic densities and synaptic membranes. In the immunoprecipitates, NR2A was increased while GluR1 was decreased supporting a change in receptor stoichiometry. The changes of subunit levels were associated with an upregulation of dendritic spine density and of large, mushroom-type spines. These molecular and structural adaptations might be involved in neuronal network stabilization following long-term flx treatment.

Cortical regulation of striatal medium spiny neuron dendritic remodeling in parkinsonism: modulation of glutamate release reverses dopamine depletion-induced dendritic spine loss

Striatal medium spiny neurons (MSNs) receive glutamatergic afferents from the cerebral cortex and dopaminergic inputs from the substantia nigra (SN). Striatal dopamine loss decreases the number of MSN dendritic spines. This loss of spines has been suggested to reflect the removal of tonic dopamine inhibitory control over corticostriatal glutamatergic drive, with increased glutamate release culminating in MSN spine loss. We tested this hypothesis in two ways. We first determined in vivo if decortication reverses or prevents dopamine depletion-induced spine loss by placing motor cortex lesions 4 weeks after, or at the time of, 6-hydroxydopamine lesions of the SN. Animals were sacrificed 4 weeks after cortical lesions. Motor cortex lesions significantly reversed the loss of MSN spines elicited by dopamine denervation; a similar effect was observed in the prevention experiment. We then determined if modulating glutamate release in organotypic cocultures prevented spine loss. Treatment of the cultures with the mGluR2/3 agonist LY379268 to suppress corticostriatal glutamate release completely blocked spine loss in dopamine-denervated cultures. These studies provide the first evidence to show that MSN spine loss associated with parkinsonism can be reversed and point to suppression of corticostriatal glutamate release as a means of slowing progression in Parkinson's disease.

Differential roles of NMDA Receptor Subtypes NR2A and NR2B in dendritic branch development and requirement of RasGRF1

N-methyl-D-aspartate receptors (NMDARs) are known to regulate axonal refinement and dendritic branching. However, because NMDARs are abundantly present as tri-heteromers (e.g., NR1/NR2A/NR2B) during development, the precise role of the individual subunits NR2A and NR2B in these processes has not been elucidated. Ventral spinal cord neurons (VSCNs) provide a unique opportunity to address this problem, because the expression of both NR2A and NR2B (but not NR1) is downregulated in culture. Exogenous NR2A or NR2B were introduced into these naturally NR2-null neurons at 4 DIV, and electrophysiological recordings at 11 DIV confirmed that synaptic NR1NR2A receptors and NR1NR2B receptors were formed, respectively. Analysis of the dendritic architecture showed that introduction of NR2B, but not NR2A, dramatically increased the number of secondary and tertiary dendritic branches of VSCNs. Whole cell patch-clamp recordings further indicated that the newly formed branches in NR2B-expressing neurons were able to establish functional synapses because the frequency of miniature AMPA-receptor synaptic currents was increased. Using previously described mutants, we also found that disruption of the interaction between NR2B and RasGRF1 dramatically impaired dendritic branch formation in VSCNs. The differential role of the NR2A and NR2B subunits and the requirement for RasGRF1 in regulating branch formation was corroborated in hippocampal cultures. We conclude that the association between NR1NR2B-receptors and RasGRF1 is needed for dendritic branch formation in VSCNs and hippocampal neurons in vitro. The dominated NR2A expression and the limited interactions of this subunit with the signaling protein RasGRF1 may contribute to the restricted dendritic arbor development in the adult CNS.