Dendritic spine pathology in neuropsychiatric disorders

Actin isoforms in neuronal development and function

The actin cytoskeleton contributes directly or indirectly to nearly every aspect of neuronal development and function. This diversity of functions is often attributed to actin regulatory proteins, although how the composition of the actin cytoskeleton itself may influence its function is often overlooked. In neurons, the actin cytoskeleton is composed of two distinct isoforms, β- and γ-actin. Functions for β-actin have been investigated in axon guidance, synaptogenesis, and disease. Insight from loss-of-function in vivo studies has also revealed novel roles for β-actin in select brain structures and behaviors. Conversely, very little is known regarding functions of γ-actin in neurons. The dysregulation or mutation of both β- and γ-actin has been implicated in multiple human neurological disorders, however, demonstrating the critical importance of these still poorly understood proteins. This chapter highlights what is currently known regarding potential distinct functions for β- and γ-actin in neurons as well as the significant areas that remain unexplored.

Corticotropin-releasing factor and urocortin regulate spine and synapse formation: structural basis for stress-induced neuronal remodeling and pathology

Dendritic spines are important sites of excitatory neurotransmission in the brain with their function determined by their structure and molecular content. Alterations in spine number, morphology and receptor content are a hallmark of many psychiatric disorders, most notably those because of stress. We investigated the role of corticotropin-releasing factor (CRF) stress peptides on the plasticity of spines in the cerebellum, a structure implicated in a host of mental illnesses, particularly of a developmental origin. We used organotypic slice cultures of the cerebellum and restraint stress in behaving animals to determine whether CRF in vitro and stress in vivo affects Purkinje cell (PC) spine density. Application of CRF and urocortin (UCN) to cerebellar slice cultures increased the density of spines on PC signaling via CRF receptors (CRF-Rs) 1 and 2 and RhoA downregulation, although the structural phenotypes of the induced spines varied, suggesting that CRF-Rs differentially induce the outgrowth of functionally distinct populations of spines. Furthermore, CRF and UCN exert a trophic effect on the surface contact between synaptic elements by increasing active zones and postsynaptic densities and facilitating the alignment of pre- and post-synaptic membranes of synapses on PCs. In addition, 1 h of restraint stress significantly increased PC spine density compared with those animals that were only handled. This study provides unprecedented resolution of CRF pathways that regulate the structural machinery essential for synaptic transmission and provides a basis for understanding stress-induced mental illnesses.

Molecular mechanisms of dendrite morphogenesis

Dendrites are key integrators of synaptic information in neurons and play vital roles in neuronal plasticity. Hence, it is necessary that dendrite arborization is precisely controlled and coordinated with synaptic activity to ensure appropriate functional neural network integrity. In the past several years, it has become increasingly clear that several cell intrinsic and extrinsic mechanisms contribute to dendritic arborization. In this review, we will discuss some of the molecular mechanisms that regulate dendrite morphogenesis, particularly in cortical and hippocampal pyramidal neurons and some of the implications of aberrant dendritic morphology for human disease. Finally, we will discuss the current challenges and future directions in the field.

Dendritic spine alterations in the hippocampus and parietal cortex of alpha7 nicotinic acetylcholine receptor knockout mice

The α7 nicotinic acetylcholine receptor (nAChR) is involved in higher cognitive and memory functions, and is associated with the etiology of neurological diseases involving cognitive decline, including Alzheimer's disease (AD). We hypothesized that spine changes in the α7 knockout might help to explain the behavioral deficits observed in α7 knockout mice and prodromal hippocampal changes in AD. We quantified several measures of dendritic morphology in the CA1 region of the mouse hippocampus in Golgi-stained material from wildtype and α7 knockout mice at P24. The most significant difference was a 64% increase in thin (L-type) dendritic spines on the CA1 basilar tree in knockout mice (p<.05). There were small decreases in the number of in N-type (-15%), M-type (-14%) and D-type (-4%) spine densities. The CA1 basilar dendritic tree of knockout mice had significantly less branching in the regions near the soma in comparison with wildtype animals (p<.01), but not in the more distal branching. Changes in the configuration of CA1 basilar dendritic spines have been observed in a number of experimental paradigms, suggesting that basilar dendritic spines are highly plastic. One component of cognitive dysfunction may be through α7-modulated GABAergic interneurons synapsing on CA1 basal dendrites.

The novel synaptogenic protein Farp1 links postsynaptic cytoskeletal dynamics and transsynaptic organization

Synaptic adhesion organizes synapses, yet the signaling pathways that drive and integrate synapse development remain incompletely understood. We screened for regulators of these processes by proteomically analyzing synaptic membranes lacking the synaptogenic adhesion molecule SynCAM 1. This identified FERM, Rho/ArhGEF, and Pleckstrin domain protein 1 (Farp1) as strongly reduced in SynCAM 1 knockout mice. Farp1 regulates dendritic filopodial dynamics in immature neurons, indicating roles in synapse formation. Later in development, Farp1 is postsynaptic and its 4.1 protein/ezrin/radixin/moesin (FERM) domain binds SynCAM 1, assembling a synaptic complex. Farp1 increases synapse number and modulates spine morphology, and SynCAM 1 requires Farp1 for promoting spines. In turn, SynCAM 1 loss reduces the ability of Farp1 to elevate spine density. Mechanistically, Farp1 activates the GTPase Rac1 in spines downstream of SynCAM 1 clustering, and promotes F-actin assembly. Farp1 furthermore triggers a retrograde signal regulating active zone composition via SynCAM 1. These results reveal a postsynaptic signaling pathway that engages transsynaptic interactions to coordinate synapse development.

Amyloid plaque formation precedes dendritic spine loss

Amyloid-beta plaque deposition represents a major neuropathological hallmark of Alzheimer's disease. While numerous studies have described dendritic spine loss in proximity to plaques, much less is known about the kinetics of these processes. In particular, the question as to whether synapse loss precedes or follows plaque formation remains unanswered. To address this question, and to learn more about the underlying kinetics, we simultaneously imaged amyloid plaque deposition and dendritic spine loss by applying two-photon in vivo microscopy through a cranial window in double transgenic APPPS1 mice. As a result, we first observed that the rate of dendritic spine loss in proximity to plaques is the same in both young and aged animals. However, plaque size only increased significantly in the young cohort, indicating that spine loss persists even many months after initial plaque appearance. Tracking the fate of individual spines revealed that net spine loss is caused by increased spine elimination, with the rate of spine formation remaining constant. Imaging of dendritic spines before and during plaque formation demonstrated that spine loss around plaques commences at least 4 weeks after initial plaque formation. In conclusion, spine loss occurs, shortly but with a significant time delay, after the birth of new plaques, and persists in the vicinity of amyloid plaques over many months. These findings hence give further hope to the possibility that there is a therapeutic window between initial amyloid plaque deposition and the onset of structural damage at spines.

Electrophysiological changes precede morphological changes to frontal cortical pyramidal neurons in the rTg4510 mouse model of progressive tauopathy

Whole-cell patch-clamp recordings and high-resolution morphometry were used to assess functional and structural properties of layer 3 pyramidal neurons in early (<4 months) and advanced (>8 months) stages of tauopathy in frontal cortical slices prepared from rTg4510 tau mutant (P301L) mice. In early tauopathy, dendritic architecture is preserved. In advanced tauopathy, neurons can be categorized as either "atrophic" (58 %)-exhibiting marked atrophy of the apical tuft, or "intact" (42 %)-with normal apical tufts and, in some instances, proliferative sprouting of oblique branches of the apical trunk. Approximately equal numbers of atrophic and intact neurons contain neurofibrillary tangles (NFTs) or are tangle-free, lending further support to the idea that NFTs per se are not toxic. Spine density is decreased due to a specific reduction in mushroom spines, but filopodia are increased in both atrophic and intact neurons. By contrast to these morphological changes, which are robust only in the advanced stage, significant electrophysiological changes are present in the early stage and persist in the advanced stage in both atrophic and intact neurons. The most marked of these changes are: a depolarized resting membrane potential, an increased depolarizing sag potential and increased action potential firing rates-all indicative of hyperexcitability. Spontaneous excitatory postsynaptic currents are not reduced in frequency or amplitude in either stage. The difference in the time course of functionally important electrophysiological changes versus regressive morphological changes implies differences in pathogenic mechanisms underlying functional and structural changes to neurons during progressive tauopathy.

CELF4 regulates translation and local abundance of a vast set of mRNAs, including genes associated with regulation of synaptic function

RNA–binding proteins have emerged as causal agents of complex neurological diseases. Mice deficient for neuronal RNA–binding protein CELF4 have a complex neurological disorder with epilepsy as a prominent feature. Human CELF4 has recently been associated with clinical features similar to those seen in mutant mice. CELF4 is expressed primarily in excitatory neurons, including large pyramidal cells of the cerebral cortex and hippocampus, and it regulates excitatory but not inhibitory neurotransmission. We examined mechanisms underlying neuronal hyperexcitability in Celf4 mutants by identifying CELF4 target mRNAs and assessing their fate in the absence of CELF4 in view of their known functions. CELF4 binds to at least 15%–20% of the transcriptome, with striking specificity for the mRNA 3′ untranslated region. CELF4 mRNA targets encode a variety of proteins, many of which are well established in neuron development and function. While the overall abundance of these mRNA targets is often dysregulated in Celf4 deficient mice, the actual expression changes are modest at the steady-state level. In contrast, by examining the transcriptome of polysome fractions and the mRNA distribution along the neuronal cell body-neuropil axis, we found that CELF4 is critical for maintaining mRNA stability and availability for translation. Among biological processes associated with CELF4 targets that accumulate in neuropil of mutants, regulation of synaptic plasticity and transmission are the most prominent. Together with a related study of the impact of CELF4 loss on sodium channel Na_v1.6 function, we suggest that CELF4 deficiency leads to abnormal neuronal function by combining a specific effect on neuronal excitation with a general impairment of synaptic transmission. These results also expand our understanding of the vital roles RNA–binding proteins play in regulating and shaping the activity of neural circuits.

Epilepsy is a devastating brain disorder whereby a loss of regulation of electrochemical signals between neurons causes too much excitation and ultimately results in an “electrical storm” known as a seizure. Epilepsy can be heritable, but it is usually genetically complex, resulting from a collaboration of many genes. It is also a frequent feature of other common brain diseases, such as autism spectrum disorder and intellectual disability, likely because these diseases have a similar dysregulation of neuronal communication. To understand more about how the brain regulates electrical activity, we focused on an RNA–binding protein called CELF4, because a) mice that lack CELF4 have a complex form of epilepsy that includes features of other neurological diseases and b) this kind of protein has the potential to be a master regulator. We show that CELF4 binds to a vast array of mRNAs, and without CELF4 these mRNAs accumulate in the wrong places and can produce the wrong amount of protein. Moreover, many of these mRNAs encode key players in electrochemical signaling between neurons. Although the defects in individual mRNAs are modest, like a genetically complex disease, together these alterations collude to cause neurological symptoms including recurrent seizures.

Novel treatments in autism spectrum disorders: from synaptic dysfunction to experimental therapeutics

Recent discoveries and advances in genetics and neuroscience have provided deeper understanding of the complex neurobiology of ASD. The development of novel treatments is strictly dependent on these findings in order to design new strategies in the pharmacotherapy of ASD. At this time, therapeutics are limited to treating associated core, symptoms. Studies of single gene disorders, such as Phelan-McDermid syndrome, Fragile X and Tuberous Sclerosis, might be of significant help since the neurobiology of these disorders is clearer and clinical trials are already underway for these conditions. The pathogenesis paradigm shift of ASD towards synaptic abnormalities has led to current research of the pathways to disease, which involves multiple dynamic systems. Interest in oxytocin is growing as it has been recognized to be implicated in social development and affiliative behaviours. In the future, progress is expected in possible new options for therapeutics in ASD. Children and adolescents with ASD and their families can provide vital information about their experiences with new treatments, which should be a priority for future research.

Elimination of dendritic spines with long-term memory is specific to active circuits

Structural changes in brain circuits active during learning are thought to be important for long-term memory storage. If these changes support long-term information storage, they might be expected to be present at distant time points after learning, as well as to be specific to the circuit activated with learning, and sensitive to the contingencies of the behavioral paradigm. Here, we show such changes in the hippocampus as a result of contextual fear conditioning. There were significantly fewer spines specifically on active neurons of fear-conditioned mice. This spine loss did not occur in homecage mice or in mice exposed to the training context alone. Mice exposed to unpaired shocks showed a generalized reduction in spines. These learning-related changes in spine density could reflect a direct mechanism of encoding or alternately could reflect a compensatory adaptation to previously described enhancement in transmission due to glutamate receptor insertion.

Motifs in health and disease: the promise of circuit interrogation by optogenetics

Identifying the dominant dynamical motifs in cortical circuits and determining their functional relevance is of the utmost importance to understand the underlying mechanisms of psychiatric diseases and to develop effective therapies. Optogenetics can be used to interrogate cortical circuits to determine the dominant motif and thereby identify the relevant biophysical time scales that set the oscillation frequency. We review how computational models of cortical networks can help guide optogenetics experiments. We focus our attention on the pyramidal interneuron gamma motif, which is comprised of reciprocally connected excitatory and inhibitory neurons, and determine how the different biophysical time scales of the circuit components are reflected in the resonance of the power in the local field potential at the frequency of stimulation as a function of that frequency. Cardin et al. [J.A. Cardin et al. (2009)Nature, 459, 663-667] find that periodic stimulation of inhibitory cells leads to a resonance at gamma frequencies (30-80 Hz), but that stimulation of excitatory cells does not lead to a resonance. We can account for these results when the pyramidal cells are endowed with an intrinsic frequency preference due to a slow hyperpolarizing current. Furthermore, when fast α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-mediated excitatory currents are replaced by slow N-methyl-d-aspartate (NMDA)-mediated ones in inhibitory cells, the gamma frequency resonance is reduced; however, when the same replacement is made in excitatory cells, gamma oscillations are enhanced. The results are relevant to schizophrenia, because there is evidence that NMDA receptors on parvalbumin-positive cells are primarily affected and that the regulation of gamma oscillations is impaired.

Diverse types of genetic variation converge on functional gene networks involved in schizophrenia

Despite the successful identification of several relevant genomic loci, the underlying molecular mechanisms of schizophrenia remain largely unclear. We developed a computational approach (NETBAG+) that allows an integrated analysis of diverse disease-related genetic data using a unified statistical framework. The application of this approach to schizophrenia-associated genetic variations, obtained using unbiased whole-genome methods, allowed us to identify several cohesive gene networks related to axon guidance, neuronal cell mobility, synaptic function and chromosomal remodeling. The genes forming the networks are highly expressed in the brain, with higher brain expression during prenatal development. The identified networks are functionally related to genes previously implicated in schizophrenia, autism and intellectual disability. A comparative analysis of copy number variants associated with autism and schizophrenia suggests that although the molecular networks implicated in these distinct disorders may be related, the mutations associated with each disease are likely to lead, at least on average, to different functional consequences.

RNA-binding proteins involved in RNA localization and their implications in neuronal diseases

Very often, developmental abnormalities or subtle disturbances of neuronal function may yield brain diseases even if they become obvious only late in life. It is therefore our intention to highlight fundamental mechanisms of neuronal cell biology with a special emphasis on dendritic mRNA localization including local protein synthesis at the activated synapse. Furthermore, we would like to point out possible links to neuronal or synaptic dysfunction. In particular, we will focus on a series of well-known RNA-binding proteins that are involved in these processes and outline how their dysfunction might yield neurodevelopmental, neurodegenerative or neuropsychiatric disorders. We are convinced that increasing our understanding of RNA biology in general and the mechanisms underlying mRNA transport and subsequent translation at the synapse will ultimately generate important novel RNA-based tools in the near future that will allow us to hopefully treat some of these devastating diseases.

Kalrn plays key roles within and outside of the nervous system

Background

The human KALRN gene, which encodes a complex, multifunctional Rho GDP/GTP exchange factor, has been linked to cardiovascular disease, psychiatric disorders and neurodegeneration. Examination of existing Kalrn knockout mouse models has focused only on neuronal phenotypes. However, Kalirin was first identified through its interaction with an enzyme involved in the synthesis and secretion of multiple bioactive peptides, and studies in C.elegans revealed roles for its orthologue in neurosecretion.

Results

We used a broad array of tests to evaluate the effects of ablating a single exon in the spectrin repeat region of Kalrn (KalSR^KO/KO); transcripts encoding Kalrn isoforms containing only the second GEF domain can still be produced from the single remaining functional Kalrn promoter. As expected, KalSR^KO/KO mice showed a decrease in anxiety-like behavior and a passive avoidance deficit. No changes were observed in prepulse inhibition of acoustic startle or tests of depression-like behavior. Growth rate, parturition and pituitary secretion of growth hormone and prolactin were deficient in the KalSR^KO/KO mice. Based on the fact that a subset of Kalrn isoforms is expressed in mouse skeletal muscle and the observation that muscle function in C.elegans requires its Kalrn orthologue, KalSR^KO/KO mice were evaluated in the rotarod and wire hang tests. KalSR^KO/KO mice showed a profound decrease in neuromuscular function, with deficits apparent in KalSR^+/KO mice; these deficits were not as marked when loss of Kalrn expression was restricted to the nervous system. Pre- and postsynaptic deficits in the neuromuscular junction were observed, along with alterations in sarcomere length.

Conclusions

Many of the widespread and diverse deficits observed both within and outside of the nervous system when expression of Kalrn is eliminated may reflect its role in secretory granule function and its expression outside of the nervous system.

Long-term perturbation of spine plasticity results in distinct impairments of cognitive function

Dendritic spines serve as the post-synaptic structural component of synapses. The structure and function of dendritic spines are dynamically regulated by a number of signaling pathways and allow for normal neural processing, whereas aberrant spine changes are thought to contribute to cognitive impairment in neuropsychiatric and neurodegenerative disorders. However, spine changes within different brain regions and their contribution to specific cognitive functions, especially later in adulthood, is not well understood. In this study, we used late-adult KALRN-deficient mice as a tool to investigate the vulnerability of different cognitive functions to long-term perturbations in spine plasticity in different forebrain regions. We found that in these mice, loss of one or both copies of KALRN lead to genotype and brain region-dependent reductions in spine density. Surprisingly, heterozygote and knockout mice showed differential impairments in cognitive phenotypes, including working memory, social recognition, and social approach. Correlation analysis between the site and magnitude of spine loss and behavioral alterations suggests that the interplay between brain regions is critical for complex cognitive processing and underscores the importance of spine plasticity in normal cognitive function. Long-term perturbation of spine plasticity results in distinct impairments of cognitive function. Using genetically modified mice deficient in a central regulator of spine plasticity, we investigated the brain region-specific contribution of spine numbers to various cognitive functions. We found distinct cognitive functions display differential sensitivity to spine loss in the cortex and hippocampus. Our data support spines as neuronal structures important for cognition and suggest interplay between brain regions is critical for complex cognitive processing.

Effects of different amyloid β-protein analogues on synaptic function

Perisynaptic accumulations of amyloid β-protein (Aβ) play a critical role in the synaptic dysfunction underlying the cognitive impairment observed in Alzheimer's disease. The methionine residue at position 35 (Met35) in Aβ is highly subject to oxidation in Alzheimer's disease brains. In hippocampal brain slices we found that long-term potentiation at CA3-CA1 synapses was significantly inhibited by wild type Aβ42 in which Met35 is reduced, but not by Aβ42 harboring Met35 sulfoxide. Similar differences were observed when basal synaptic transmission was investigated in autaptic hippocampal neurons. The significant decreases in excitatory postsynaptic current amplitude, vesicle release probability and miniature excitatory postsynaptic current frequency caused by 20-minute exposure to wild type Aβ42 were not observed after exposure to Aβ42 harboring Met35 sulfoxide. With longer (24-hour) Aβ treatments, this early impairment of the presynaptic terminal function extended to involve the postsynaptic side as well. The Met35 oxidation also affected Aβ42 negative impact on dendritic spine density and expression of pre- and postsynaptic proteins (synaptophysin and postsynaptic density protein-95). Our findings suggest that oxidation of Met35 is critical for molecular, structural, and functional determinants of Aβ42 synaptotoxicity.

Mechanisms of psychostimulant-induced structural plasticity

Psychostimulants robustly induce alterations in neuronal structural plasticity throughout brain reward circuits. However, despite our extensive understanding of how these circuits modulate motivated behavior, it is still unclear whether structural plasticity within these regions drives pathological behavioral responses in addiction. Although these structural changes have been subjected to an exhaustive phenomenological characterization, we still have a limited understanding of the molecular mechanisms regulating their induction and the functional relevance of such changes in mediating addiction-like behavior. Here we have highlighted the known molecular pathways and intracellular signaling cascades that regulate psychostimulant-induced changes in neuronal morphology and synaptic restructuring, and we discuss them in the larger context of addiction behavior.

Arf4 determines dentate gyrus-mediated pattern separation by regulating dendritic spine development

The ability to distinguish between similar experiences is a critical feature of episodic memory and is primarily regulated by the dentate gyrus (DG) region of the hippocampus. However, the molecular mechanisms underlying such pattern separation tasks are poorly understood. We report a novel role for the small GTPase ADP ribosylation factor 4 (Arf4) in controlling pattern separation by regulating dendritic spine development. Arf4(+/-) mice at 4-5 months of age display severe impairments in a pattern separation task, as well as significant dendritic spine loss and smaller miniature excitatory post-synaptic currents (mEPSCs) in granule cells of the DG. Arf4 knockdown also decreases spine density in primary neurons, whereas Arf4 overexpression promotes spine development. A constitutively active form of Arf4, Arf4-Q71L, promotes spine density to an even greater extent than wildtype Arf4, whereas the inactive Arf4-T31N mutant does not increase spine density relative to controls. Arf4's effects on spine development are regulated by ASAP1, a GTPase-activating protein that modulates Arf4 GTPase activity. ASAP1 overexpression decreases spine density, and this effect is partially rescued by concomitant overexpression of wildtype Arf4 or Arf4-Q71L. In addition, Arf4 overexpression rescues spine loss in primary neurons from an Alzheimer's disease-related apolipoprotein (apo) E4 mouse model. Our findings suggest that Arf4 is a critical modulator of DG-mediated pattern separation by regulating dendritic spine development.

Scaffolding proteins of the post-synaptic density contribute to synaptic plasticity by regulating receptor localization and distribution: relevance for neuropsychiatric diseases

Synaptic plasticity represents the long lasting activity-related strengthening or weakening of synaptic transmission, whose well-characterized types are the long term potentiation and depression. Despite this classical definition, however, the molecular mechanisms by which synaptic plasticity may occur appear to be extremely complex and various. The post-synaptic density (PSD) of glutamatergic synapses is a major site for synaptic plasticity processes and alterations of PSD members have been recently implicated in neuropsychiatric diseases where an impairment of synaptic plasticity has also been reported. Among PSD members, scaffolding proteins have been demonstrated to bridge surface receptors with their intracellular effectors and to regulate receptors distribution and localization both at surface membranes and within the PSD. This review will focus on the molecular physiology and pathophysiology of synaptic plasticity processes, which are tuned by scaffolding PSD proteins and their close related partners, through the modulation of receptor localization and distribution at post-synaptic sites. We suggest that, by regulating both the compartmentalization of receptors along surface membrane and their degradation as well as by modulating receptor trafficking into the PSD, postsynaptic scaffolding proteins may contribute to form distinct signaling micro-domains, whose efficacy in transmitting synaptic signals depends on the dynamic stability of the scaffold, which in turn is provided by relative amounts and post-translational modifications of scaffolding members. The putative relevance for neuropsychiatric diseases and possible pathophysiological mechanisms are discussed in the last part of this work.

Silencing microRNA-134 produces neuroprotective and prolonged seizure-suppressive effects

Temporal lobe epilepsy is a common, chronic neurological disorder characterized by recurrent spontaneous seizures. MicroRNAs (miRNAs) are small, noncoding RNAs that regulate post-transcriptional expression of protein-coding mRNAs, which may have key roles in the pathogenesis of neurological disorders. In experimental models of prolonged, injurious seizures (status epilepticus) and in human epilepsy, we found upregulation of miR-134, a brain-specific, activity-regulated miRNA that has been implicated in the control of dendritic spine morphology. Silencing of miR-134 expression in vivo using antagomirs reduced hippocampal CA3 pyramidal neuron dendrite spine density by 21% and rendered mice refractory to seizures and hippocampal injury caused by status epilepticus. Depletion of miR-134 after status epilepticus in mice reduced the later occurrence of spontaneous seizures by over 90% and mitigated the attendant pathological features of temporal lobe epilepsy. Thus, silencing miR-134 exerts prolonged seizure-suppressant and neuroprotective actions; determining whether these are anticonvulsant effects or are truly antiepileptogenic effects requires additional experimentation.

Vezatin is essential for dendritic spine morphogenesis and functional synaptic maturation

Vezatin is an integral membrane protein associated with cell-cell adhesion complex and actin cytoskeleton. It is expressed in the developing and mature mammalian brain, but its neuronal function is unknown. Here, we show that Vezatin localizes in spines in mature mouse hippocampal neurons and codistributes with PSD95, a major scaffolding protein of the excitatory postsynaptic density. Forebrain-specific conditional ablation of Vezatin induced anxiety-like behavior and impaired cued fear-conditioning memory response. Vezatin knock-down in cultured hippocampal neurons and Vezatin conditional knock-out in mice led to a significantly increased proportion of stubby spines and a reduced proportion of mature dendritic spines. PSD95 remained tethered to presynaptic terminals in Vezatin-deficient hippocampal neurons, suggesting that the reduced expression of Vezatin does not compromise the maintenance of synaptic connections. Accordingly, neither the amplitude nor the frequency of miniature EPSCs was affected in Vezatin-deficient hippocampal neurons. However, the AMPA/NMDA ratio of evoked EPSCs was reduced, suggesting impaired functional maturation of excitatory synapses. These results suggest a role of Vezatin in dendritic spine morphogenesis and functional synaptic maturation.

S-nitrosylation of Cdk5: potential implications in amyloid-β-related neurotoxicity in Alzheimer disease

Aberrant activation of Cdk5 has been implicated in the process of neurodegenerative diseases such as Alzheimer's disease (AD). We recently reported that S-nitrosylation of Cdk5 (forming SNO-Cdk5) at specific cysteine residues results in excessive activation of Cdk5, contributing to mitochondrial dysfunction, synaptic damage, and neuronal cell death in models of AD. Furthermore, SNO-Cdk5 acts as a nascent S-nitrosylase, transnitrosylating the mitochondrial fission protein Drp1 and enhancing excessive mitochondrial fission in dendritic spines. However, a molecular mechanism that leads to the formation of SNO-Cdk5 in neuronal cells remained obscure. Here, we demonstrate that neuronal nitric oxide synthase (NOS1) interacts with Cdk5 and that the close proximity of the two proteins facilitates the formation of SNO-Cdk5. Interestingly, as a negative feedback mechanism, Cdk5 phosphorylates and suppresses NOS1 activity. Thus, together with our previous report, these findings delineate an S-nitrosylation pathway wherein Cdk5/NOS1 interaction enhances SNO-Cdk5 formation, mediating mitochondrial dysfunction and synaptic loss during the etiology of AD.

LTP induction translocates cortactin at distant synapses in wild-type but not Fmr1 knock-out mice

Stabilization of long-term potentiation (LTP) depends on reorganization of the dendritic spine actin cytoskeleton. The present study tested whether this involves activity-driven effects on the actin-regulatory protein cortactin, and whether such effects are disturbed in the Fmr1 knock-out (KO) model of fragile X syndrome, in which stabilization of both actin filaments and LTP is impaired. LTP induced by theta burst stimulation (TBS) in hippocampal slices from wild-type mice was associated with rapid, broadly distributed, and NMDA receptor-dependent decreases in synapse-associated cortactin. The reduction in cortactin content was blocked by blebbistatin, while basal levels were reduced by nocodazole, indicating that cortactin's movements into and away from synapses are regulated by microtubule and actomyosin motors, respectively. These results further suggest that synapse-specific LTP influences cytoskeletal elements at distant connections. The rapid effects of TBS on synaptic cortactin content were absent in Fmr1 KOs as was evidence for activity-driven phosphorylation of the protein or its upstream kinase, ERK1/2. Phosphorylation regulates cortactin's interactions with actin, and coprecipitation of the two proteins was reduced in the KOs. We propose that, in the KOs, excessive basal phosphorylation of ERK1/2 disrupts its interactions with cortactin, thereby blocking the latter protein's use of actomyosin transport systems. These impairments are predicted to compromise the response of the subsynaptic cytoskeleton to learning-related afferent activity, both locally and at distant sites.

Palmitoylation of A-kinase anchoring protein 79/150 regulates dendritic endosomal targeting and synaptic plasticity mechanisms

NMDA receptor-dependent long-term potentiation (LTP) and depression (LTD) are forms of synaptic plasticity underlying learning and memory that are expressed through increases and decreases, respectively, in dendritic spine size and AMPA receptor (AMPAR) phosphorylation and postsynaptic localization. The A-kinase anchoring protein 79/150 (AKAP79/150) signaling scaffold regulates AMPAR phosphorylation, channel activity, and endosomal trafficking associated with LTP and LTD. AKAP79/150 is targeted to dendritic spine plasma membranes by an N-terminal polybasic domain that binds phosphoinositide lipids, F-actin, and cadherin cell adhesion molecules. However, we do not understand how regulation of AKAP targeting controls AMPAR endosomal trafficking. Here, we report that palmitoylation of the AKAP N-terminal polybasic domain targets it to postsynaptic lipid rafts and dendritic recycling endosomes. AKAP palmitoylation was regulated by seizure activity in vivo and LTP/LTD plasticity-inducing stimuli in cultured rat hippocampal neurons. With chemical LTP induction, we observed AKAP79 dendritic spine recruitment that required palmityolation and Rab11-regulated endosome recycling coincident with spine enlargement and AMPAR surface delivery. Importantly, a palmitoylation-deficient AKAP79 mutant impaired regulation of spine size, endosome recycling, AMPAR trafficking, and synaptic potentiation. These findings emphasize the emerging importance of palmitoylation in controlling synaptic function and reveal novel roles for the AKAP79/150 signaling complex in dendritic endosomes.

S-SCAM/MAGI-2 is an essential synaptic scaffolding molecule for the GluA2-containing maintenance pool of AMPA receptors

Synaptic plasticity, the cellular basis of learning and memory, involves the dynamic trafficking of AMPA receptors (AMPARs) into and out of synapses. One of the remaining key unanswered aspects of AMPAR trafficking is the mechanism by which synaptic strength is preserved despite protein turnover. In particular, the identity of AMPAR scaffolding molecule(s) involved in the maintenance of GluA2-containing AMPARs is completely unknown. Here we report that the synaptic scaffolding molecule (S-SCAM; also called membrane-associated guanylate kinase inverted-2 and atrophin interacting protein-1) plays the critical role of maintaining synaptic strength. Increasing S-SCAM levels in rat hippocampal neurons led to specific increases in the surface AMPAR levels, enhanced AMPAR-mediated synaptic transmission, and enlargement of dendritic spines, without significantly effecting GluN levels or NMDA receptor (NMDAR) EPSC. Conversely, decreasing S-SCAM levels by RNA interference-mediated knockdown caused the loss of synaptic AMPARs, which was followed by a severe reduction in the dendritic spine density. Importantly, S-SCAM regulated synaptic AMPAR levels in a manner, dependent on GluA2 not GluA1, sensitive to N-ethylmaleimide-sensitive fusion protein interaction, and independent of activity. Further, S-SCAM increased surface AMPAR levels in the absence of PSD-95, while PSD-95 was dependent on S-SCAM to increase surface AMPAR levels. Finally, S-SCAM overexpression hampered NMDA-induced internalization of AMPARs and prevented the induction of long term-depression, while S-SCAM knockdown did not. Together, these results suggest that S-SCAM is an essential AMPAR scaffolding molecule for the GluA2-containing pool of AMPARs, which are involved in the constitutive pathway of maintaining synaptic strength.

GABA system dysfunction in autism and related disorders: from synapse to symptoms

Autism spectrum disorders (ASDs) are neurodevelopmental syndromes characterised by repetitive behaviours and restricted interests, impairments in social behaviour and relations, and in language and communication. These symptoms are also observed in a number of developmental disorders of known origin, including Fragile X Syndrome, Rett Syndrome, and Foetal Anticonvulsant Syndrome. While these conditions have diverse etiologies, and poorly understood pathologies, emerging evidence suggests that they may all be linked to dysfunction in particular aspects of GABAergic inhibitory signalling in the brain. We review evidence from genetics, molecular neurobiology and systems neuroscience relating to the role of GABA in these conditions. We conclude by discussing how these deficits may relate to the specific symptoms observed.

The polarity protein partitioning-defective 1 (PAR-1) regulates dendritic spine morphogenesis through phosphorylating postsynaptic density protein 95 (PSD-95)

The polarity protein PAR-1 plays an essential role in many cellular contexts, including embryogenesis, asymmetric cell division, directional migration, and epithelial morphogenesis. Despite its known importance in different cellular processes, the role of PAR-1 in neuronal morphogenesis is less well understood. In particular, its role in the morphogenesis of dendritic spines, which are sites of excitatory synaptic inputs, has been unclear. Here, we show that PAR-1 is required for normal spine morphogenesis in hippocampal neurons. We further show that PAR-1 functions through phosphorylating the synaptic scaffolding protein PSD-95 in this process. Phosphorylation at a conserved serine residue in the KXGS motif in PSD-95 regulates spine morphogenesis, and a phosphomimetic mutant of this site can rescue the defects of kinase-dead PAR-1. Together, our findings uncover a role of PAR-1 in spine morphogenesis in hippocampal neurons through phosphorylating PSD-95.

Astrocytes and developmental plasticity in fragile X

A growing body of research indicates a pivotal role for astrocytes at the developing synapse. In particular, astrocytes are dynamically involved in governing synapse structure, function, and plasticity. In the postnatal brain, their appearance at synapses coincides with periods of developmental plasticity when neural circuits are refined and established. Alterations in the partnership between astrocytes and neurons have now emerged as important mechanisms that underlie neuropathology. With overall synaptic function standing as a prominent link to the expression of the disease phenotype in a number of neurodevelopmental disorders and knowing that astrocytes influence synapse development and function, this paper highlights the current knowledge of astrocyte biology with a focus on their involvement in fragile X syndrome.

PCB-95 promotes dendritic growth via ryanodine receptor-dependent mechanisms

BACKGROUND

Aroclor 1254 (A1254) interferes with normal dendritic growth and plasticity in the developing rodent brain, but the mechanism(s) mediating this effect have yet to be established. Non-dioxin-like (NDL) polychlorinated biphenyls (PCBs) enhance the activity of ryanodine receptor (RyR) calcium ion (Ca(2+)) channels, which play a central role in regulating the spatiotemporal dynamics of intracellular Ca(2+) signaling. Ca(2+) signaling is a predominant factor in shaping dendritic arbors, but whether PCB potentiation of RyR activity influences dendritic growth is not known.

OBJECTIVE

We determined whether RyR activity is required for PCB effects on dendritic growth.

METHODS AND RESULTS

Golgi analysis of hippocampi from weanling rats confirmed that developmental exposure via the maternal diet to NDL PCB-95 (2,2',3,5'6-pentachlorobiphenyl), a potent RyR potentiator, phenocopies the dendrite-promoting effects of A1254. Dendritic growth in dissociated cultures of primary hippocampal neurons and in hippocampal slice cultures is similarly enhanced by PCB-95 but not by PCB-66 (2,3,4',4-tetrachlorobiphenyl), a congener with negligible effects on RyR activity. The dendrite-promoting effects of PCB-95 are evident at concentrations as low as 2 pM and are inhibited by either pharmacologic blockade or siRNA knockdown of RyRs.

CONCLUSIONS

Our findings demonstrate that environmentally relevant levels of NDL PCBs modulate neuronal connectivity via RyR-dependent effects on dendritic arborization. In addition, these findings identify RyR channel dysregulation as a novel mechanism contributing to dysmorphic dendritogenesis associated with heritable and environmentally triggered neurodevelopmental disorders.

PTEN signaling in autism spectrum disorders

PTEN germline mutations are found in a small subset of children diagnosed with autism spectrum disorder (ASD) and accompanying macrocephaly. In this review, we discuss recent advances that offer insight into the pathogenesis of this subgroup of autism patients. We provide an overview of how disrupting PTEN function influences neuronal cells, and describe efforts to decipher the cellular mechanisms associated with altered social behaviors. We discuss the PTEN downstream signaling pathways that likely mediate these cellular and behavioral effects. In addition, emerging data suggest that PTEN mutation can synergize with mutations in other autism susceptibility genes to contribute to the development of autistic behaviors. These studies extend our knowledge of PTEN and the PTEN signaling pathway, and offer molecular and cellular clues to better understand the etiology of ASDs.

Hippocampal dendritic spines modifications induced by perinatal asphyxia

Perinatal asphyxia (PA) affects the synaptic function and morphological organization. In previous works, we have shown neuronal and synaptic changes in rat neostriatum subjected to hypoxia leading to long-term ubi-protein accumulation. Since F-actin is highly concentrated in dendritic spines, modifications in its organization could be related with alterations induced by hypoxia in the central nervous system (CNS). In the present study, we investigate the effects of PA on the actin cytoskeleton of hippocampal postsynaptic densities (PSD) in 4-month-old rats. PSD showed an increment in their thickness and in the level of ubiquitination. Correlative fluorescence-electron microscopy photooxidation showed a decrease in the number of F-actin-stained spines in hippocampal excitatory synapses subjected to PA. Although Western Blot analysis also showed a slight decrease in β-actin in PSD in PA animals, the difference was not significant. Taken together, this data suggests that long-term actin cytoskeleton might have role in PSD alterations which would be a spread phenomenon induced by PA.

Optimization of cell morphology measurement via single-molecule tracking PALM

In neurons, the shape of dendritic spines relates to synapse function, which is rapidly altered during experience-dependent neural plasticity. The small size of spines makes detailed measurement of their morphology in living cells best suited to super-resolution imaging techniques. The distribution of molecular positions mapped via live-cell Photoactivated Localization Microscopy (PALM) is a powerful approach, but molecular motion complicates this analysis and can degrade overall resolution of the morphological reconstruction. Nevertheless, the motion is of additional interest because tracking single molecules provides diffusion coefficients, bound fraction, and other key functional parameters. We used Monte Carlo simulations to examine features of single-molecule tracking of practical utility for the simultaneous determination of cell morphology. We find that the accuracy of determining both distance and angle of motion depend heavily on the precision with which molecules are localized. Strikingly, diffusion within a bounded region resulted in an inward bias of localizations away from the edges, inaccurately reflecting the region structure. This inward bias additionally resulted in a counterintuitive reduction of measured diffusion coefficient for fast-moving molecules; this effect was accentuated by the long camera exposures typically used in single-molecule tracking. Thus, accurate determination of cell morphology from rapidly moving molecules requires the use of short integration times within each image to minimize artifacts caused by motion during image acquisition. Sequential imaging of neuronal processes using excitation pulses of either 2 ms or 10 ms within imaging frames confirmed this: processes appeared erroneously thinner when imaged using the longer excitation pulse. Using this pulsed excitation approach, we show that PALM can be used to image spine and spine neck morphology in living neurons. These results clarify a number of issues involved in interpretation of single-molecule data in living cells and provide a method to minimize artifacts in single-molecule experiments.

Neuroinflammation and synaptic loss

Neuroinflammation plays a critical role in the progression of many neurodegenerative, neuropsychiatric and viral diseases. In neuroinflammation, activated microglia and astrocytes release cytokines and chemokines as well as nitric oxide, which in turn activate many signal transduction pathways. The cytokines, interleukin-1 beta and tumor necrosis factor alpha, regulate transcription of a number of genes within the brain, which can lead to the formation of pro-inflammatory products of the arachidonic acid cascade. Formation of pro-inflammatory agents and associated cytotoxic products during neuroinflammation can be detrimental to neurons by altering synaptic proteins. Neuroinflammation as well as excitotoxic insults reduce synaptic markers such as synaptophysin and drebrin. Neurodegenerative, neuropsychiatric illnesses and viral infections are accompanied by loss of both pre- and post-synaptic proteins. These synaptic changes may contribute to the progressive cognitive decline and behavioral changes associated with these illnesses.

Spatially restricted actin-regulatory signaling contributes to synapse morphology

The actin cytoskeleton in dendritic spines is organized into microdomains, but how signaling molecules that regulate actin are spatially governed is incompletely understood. Here we examine how the localization of the RacGEF kalirin-7, a well-characterized regulator of actin in spines, varies as a function of post-synaptic density area and spine volume. Using serial section electron microscopy, we find that extrasynaptic, but not synaptic, expression of kalirin-7 varies directly with synapse size and spine volume. Moreover, we find that overall expression levels of kalirin-7 differ in spines bearing perforated and non-perforated synapses, due primarily to extrasynaptic pools of kalirin-7 expression in the former. Overall, our findings indicate that kalirin-7 is differentially compartmentalized in spines as a function of both synapse morphology and spine size.

Dendritic spine pathology in schizophrenia

Schizophrenia is a neurodevelopmental disorder whose clinical features include impairments in perception, cognition and motivation. These impairments reflect alterations in neuronal circuitry within and across multiple brain regions that are due, at least in part, to deficits in dendritic spines, the site of most excitatory synaptic connections. Dendritic spine alterations have been identified in multiple brain regions in schizophrenia, but are best characterized in layer 3 of the neocortex, where pyramidal cell spine density is lower. These spine deficits appear to arise during development, and thus are likely the result of disturbances in the molecular mechanisms that underlie spine formation, pruning, and/or maintenance. Each of these mechanisms may provide insight into novel therapeutic targets for preventing or repairing the alterations in neural circuitry that mediate the debilitating symptoms of schizophrenia.

The Emerging Role of microRNAs in Schizophrenia and Autism Spectrum Disorders

MicroRNAs (miRNAs) are small non-coding RNAs conserved throughout evolution whose perceived importance for brain development and maturation is increasingly being understood. Although a plethora of new discoveries have provided novel insights into miRNA-mediated molecular mechanisms that influence brain plasticity, their relevance for neuropsychiatric diseases with known deficits in synaptic plasticity, such as schizophrenia and autism, has not been adequately explored. In this review we discuss the intersection between current and old knowledge on the role of miRNAs in brain plasticity and function with a focus in the potential involvement of brain expressed miRNAs in the pathophysiology of neuropsychiatric disorders.

Arg kinase regulates prefrontal dendritic spine refinement and cocaine-induced plasticity

Adolescence is characterized by vulnerability to the development of neuropsychiatric disorders including drug addiction, as well as prefrontal cortical refinement that culminates in structural stability in adulthood. Neuronal refinement and stabilization are hypothesized to confer resilience to poor decision making and addictive-like behaviors, although intracellular mechanisms are largely unknown. We characterized layer V prefrontal dendritic spine development and refinement in adolescent wild-type mice and mice lacking the cytoskeletal regulatory protein Abl-related gene (Arg) kinase. Relative to hippocampal CA1 pyramidal neurons, which exhibited a nearly linear increase in spine density up to postnatal day 60 (P60), wild-type prefrontal spine density peaked at P31, and then declined by 18% by P56-P60. In contrast, dendritic spines in mice lacking Arg destabilized by P31, leading to a net loss in both structures. Destabilization corresponded temporally to the emergence of exaggerated psychomotor sensitivity to cocaine. Moreover, cocaine reduced dendritic spine density in wild-type orbitofrontal cortex and enlarged remaining spine heads, but arg(-/-) spines were unresponsive. Local application of Arg or actin polymerization inhibitors exaggerated cocaine sensitization, as did reduced gene dosage of the Arg substrate, p190RhoGAP. Genetic and pharmacological Arg inhibition also retarded instrumental reversal learning and potentiated responding for reward-related cues, providing evidence that Arg regulates both psychomotor sensitization and decision-making processes implicated in addiction. These findings also indicate that structural refinement in the adolescent orbitofrontal cortex mitigates psychostimulant sensitivity and support the emerging perspective that the structural response to cocaine may, at any age, have behaviorally protective consequences.

EphBs: an integral link between synaptic function and synaptopathies

The assembly and function of neuronal circuits rely on selective cell-cell interactions to control axon targeting, generate pre- and postsynaptic specialization and recruit neurotransmitter receptors. In neurons, EphB receptor tyrosine kinases mediate excitatory synaptogenesis early during development, and then later coordinate synaptic function by controlling synaptic glutamate receptor localization and function. EphBs direct synapse formation and function to regulate cellular morphology through downstream signaling mechanisms and by interacting with glutamate receptors. In humans, defective EphB-dependent regulation of NMDA receptor (NMDAR) localization and function is associated with neurological disorders, including neuropathic pain, anxiety disorders and Alzheimer's disease (AD). Here, we propose that EphBs act as a central organizer of excitatory synapse formation and function, and as a key regulator of diseases linked to NMDAR dysfunction.

Involvement of the γ-secretase-mediated EphA4 signaling pathway in synaptic pathogenesis of Alzheimer's disease

Loss of synapses is associated with cognitive impairment in Alzheimer's disease (AD). However, the molecular mechanism underlying this synaptic impairment is not well understood. EphA4 is a substrate of γ-secretase, and the γ-secretase-cleaved EphA4 intracellular domain (EICD) is known to enhance the formation of dendritic spines via activation of the Rac signaling pathway. Here, we show that the amount of Rac1 is significantly reduced, and correlated with the level of EICD in the frontal lobes of AD patients. Biochemical analyses revealed that the amount of membrane-associated EICD was decreased and strongly correlated with the level of membrane-associated Rac1, which is considered to be active Rac1. The synaptic scaffolding protein, postsynaptic density (PSD)-95, was specifically decreased in AD, and the amount of PSD-95 correlated with the level of Rac1. Moreover, the amounts of Rac1 and PSD-95 were negatively correlated with the extent of tau phosphorylation, which is crucial for neurofibrillary tangle formation. These results suggest that attenuation of the EICD-mediated Rac signaling pathway is involved in the synaptic pathogenesis of AD.

AAV2 mediated retrograde transduction of corticospinal motor neurons reveals initial and selective apical dendrite degeneration in ALS

Corticospinal motor neurons (CSMN) are the cortical component of motor neuron circuitry, which controls voluntary movement and degenerates in diseases such as amyotrophic lateral sclerosis, primary lateral sclerosis and hereditary spastic paraplegia. By using dual labeling combined with molecular marker analysis, we identified AAV2-2 mediated retrograde transduction as an effective approach to selectively target CSMN without affecting other neuron populations both in wild-type and hSOD1(G93A) transgenic ALS mice. This approach reveals very precise details of cytoarchitectural defects within vulnerable neurons in vivo. We report that CSMN vulnerability is marked by selective degeneration of apical dendrites especially in layer II/III of the hSOD1(G93A) mouse motor cortex, where cortical input to CSMN function is vastly modulated. While our findings confirm the presence of astrogliosis and microglia activation, they do not lend support to their direct role for the initiation of CSMN vulnerability. This study enables development of targeted gene replacement strategies to CSMN in the cerebral cortex, and reveals CSMN cortical modulation defects as a potential cause of neuronal vulnerability in ALS.

AMPA receptor activation causes silencing of AMPA receptor-mediated synaptic transmission in the developing hippocampus

Agonist-induced internalization of transmembrane receptors is a widespread biological phenomenon that also may serve as a mechanism for synaptic plasticity. Here we show that the agonist AMPA causes a depression of AMPA receptor (AMPAR) signaling at glutamate synapses in the CA1 region of the hippocampus in slices from developing, but not from mature, rats. This developmentally restricted agonist-induced synaptic depression is expressed as a total loss of AMPAR signaling, without affecting NMDA receptor (NMDAR) signaling, in a large proportion of the developing synapses, thus creating AMPAR silent synapses. The AMPA-induced AMPAR silencing is induced independently of activation of mGluRs and NMDARs, and it mimics and occludes stimulus-induced depression, suggesting that this latter form of synaptic plasticity is expressed as agonist-induced removal of AMPARs. Induction of long-term potentiation (LTP) rendered the developing synapses resistant to the AMPA-induced depression, indicating that LTP contributes to the maturation-related increased stability of these synapses. Our study shows that agonist binding to AMPARs is a sufficient triggering stimulus for the creation of AMPAR silent synapses at developing glutamate synapses.

Chemical genetic identification of NDR1/2 kinase substrates AAK1 and Rabin8 Uncovers their roles in dendrite arborization and spine development

Dendrite arborization and synapse formation are essential for wiring the neural circuitry. The evolutionarily conserved NDR1/2 kinase pathway, important for polarized growth from yeast to mammals, controls dendrite growth and morphology in the worm and fly. The function of NDR1/2 in mammalian neurons and their downstream effectors were not known. Here we show that the expression of dominant negative (kinase-dead) NDR1/2 mutants or siRNA increase dendrite length and proximal branching of mammalian pyramidal neurons in cultures and in vivo, whereas the expression of constitutively active NDR1/2 has the opposite effect. Moreover, NDR1/2 contributes to dendritic spine development and excitatory synaptic function. We further employed chemical genetics and identified NDR1/2 substrates in the brain, including two proteins involved in intracellular vesicle trafficking: AAK1 (AP-2 associated kinase) and Rabin8, a GDP/GTP exchange factor (GEF) of Rab8 GTPase. We finally show that AAK1 contributes to dendrite growth regulation, and Rabin8 regulates spine development.

β-Amyloid 42/40 ratio and kalirin expression in Alzheimer disease with psychosis

Psychosis in Alzheimer disease differentiates a subgroup with more rapid decline, is heritable, and aggregates within families, suggesting a distinct neurobiology. Evidence indicates that greater impairments of cerebral cortical synapses, particularly in dorsolateral prefrontal cortex, may contribute to the pathogenesis of psychosis in Alzheimer disease (AD) phenotype. Soluble β-amyloid induces loss of dendritic spine synapses through impairment of long-term potentiation. In contrast, the Rho guanine nucleotide exchange factor (GEF) kalirin is an essential mediator of spine maintenance and growth in cerebral cortex. We therefore hypothesized that psychosis in AD would be associated with increased soluble β-amyloid and reduced expression of kalirin in the cortex. We tested this hypothesis in postmortem cortical gray matter extracts from 52 AD subjects with and without psychosis. In subjects with psychosis, the β-amyloid(1-42)/β-amyloid(1-40) ratio was increased, due primarily to reduced soluble β-amyloid(1-40), and kalirin-7, -9, and -12 were reduced. These findings suggest that increased cortical β-amyloid(1-42)/β-amyloid(1-40) ratio and decreased kalirin expression may both contribute to the pathogenesis of psychosis in AD.

Deconstructing signal transduction pathways that regulate the actin cytoskeleton in dendritic spines

Dendritic spines are the sites of most excitatory synapses in the central nervous system. Recent studies have shown that spines function independently of each other, and they are currently the smallest known processing units in the brain. Spines exist in an array of morphologies, and spine structure helps dictate synaptic function. Dendritic spines are rich in actin, and actin rearrangements are critical regulators of spine morphology and density. In this review, we discuss the importance of actin in regulating dendritic spine morphogenesis, and discuss the upstream signal transduction pathways that either foster or inhibit actin polymerization. The understanding of actin regulatory pathways is best conceptualized as a hierarchical network in which molecules function in discrete levels defined by their molecular distance to actin. To this end, we focus on several classes of molecules, including guanine nucleotide exchange factors, small GTPases, small GTPase effectors, and actin binding proteins. We discuss how individual proteins in these molecular classes impact spine morphogenesis, and reveal the biochemical interactions in these networks that are responsible for shaping actin polymerization. Finally, we discuss the importance of these actin regulatory pathways in neuropsychiatric disorders.

Alteration of synaptic network dynamics by the intellectual disability protein PAK3

Several gene mutations linked to intellectual disability in humans code for synaptic molecules implicated in small GTPase signaling. This is the case of the Rac/Cdc42 effector p21-activated kinase 3 (PAK3). The mechanisms responsible for the intellectual defects and the consequences of the mutation on the development and wiring of brain networks remain unknown. Here we show that expression of PAK3 mutants, suppression of PAK3, or inhibition of PAK3 function in rat hippocampal slice cultures interfere with activity-mediated spine dynamics. Inhibition of PAK3 resulted in two main alterations: (1) an increased growth of new, unstable spines, occurring in clusters, and mediated by activity; and (2) an impairment of plasticity-mediated spine stabilization interfering with the formation of persistent spines. Additionally, we find that PAK3 is specifically recruited by activity from dendrites into spines, providing a new mechanism through which PAK3 could participate in the control of both spine stabilization and local spine growth. Together, these data identify a novel function of PAK3 in regulating activity-mediated rearrangement of synaptic connectivity associated with learning and suggest that defects in spine formation and refinement during development could account for intellectual disability.

Chronic calcium imaging in neuronal development and disease

Neuronal circuits develop, adjust to experience and degenerate in response to injury or disease in the course of weeks and months. Available recording techniques, however, typically sample physiological properties of identified neurons on the time scale of minutes and hours. Thus, in order to obtain a full understanding of a long term physiological process data need to be extrapolated from numerous experimental sessions and animals, often collected blindly and under variable conditions. The generation and ongoing engineering of genetically encoded calcium indicators creates an opportunity to repeatedly record activity from the same individual neurons in vivo over weeks, months and potentially the entire lifetime of a model organism. Chronic calcium imaging with genetically encoded indicators thus may allow to establish functional biographies of identified neuronal cell types in the brain and to reveal the physiological relevance of structural changes as they occur under natural and pathological conditions.