Activity-induced targeting of profilin and stabilization of dendritic spine morphology

Morphological changes in dendritic spines have been implicated in connective plasticity in brain circuitry, but the underlying pathway leading from synaptic transmission to structural change is unknown. Using primary neurons expressing GFP-tagged proteins, we found that profilin, a regulator of actin polymerization, is targeted to spine heads when postsynaptic NMDA receptors are activated and that actin-based changes in spine shape are concomitantly blocked. Profilin targeting was triggered by electrical stimulation patterns known to induce the long-term changes in synaptic responsiveness associated with memory formation. These results suggest that, in addition to electrophysiological changes, NMDA receptor activation initiates changes in the actin cytoskeleton of dendritic spines that stabilize synaptic structure.     

Cortical regulation of dopamine depletion-induced dendritic spine loss in striatal medium spiny neurons

The proximate cause of Parkinson's disease is striatal dopamine depletion. Although no overt toxicity to striatal neurons has been reported in Parkinson's disease, one of the consequences of striatal dopamine loss is a decrease in the number of dendritic spines on striatal medium spiny neurons (MSNs). Dendrites of these neurons receive cortical glutamatergic inputs onto the dendritic spine head and dopaminergic inputs from the substantia nigra onto the spine neck. This synaptic arrangement suggests that dopamine gates corticostriatal glutamatergic drive onto spines. Using triple organotypic slice cultures composed of ventral mesencephalon, striatum, and cortex of the neonatal rat, we examined the role of the cortex in dopamine depletion-induced dendritic spine loss in MSNs. The striatal dopamine innervation was lesioned by treatment of the cultures with the dopaminergic neurotoxin 1-methyl-4-phenylpyridinium (MPP+) or by removing the mesencephalon. Both MPP+ and mesencephalic ablation decreased MSN dendritic spine density. Analysis of spine morphology revealed that thin spines were preferentially lost after dopamine depletion. Removal of the cortex completely prevented dopamine depletion-induced spine loss. These data indicate that the dendritic remodeling of MSNs seen in parkinsonism occurs secondary to increases in corticostriatal glutamatergic drive, and suggest that modulation of cortical activity may be a useful therapeutic strategy in Parkinson's disease.     

Double dissociation between long-term depression and dendritic spine morphology in cerebellar Purkinje cells

Experiments in hippocampal area CA1 suggest that long-term potentiation could be associated with spine addition and enlargement, and long-term depression (LTD) with spine shrinkage and loss. Is this a general principle of synaptic plasticity? We used two-photon microscopy to measure dendritic spines in rat cerebellar Purkinje cells. Neither local synaptic induction of LTD nor global chemical induction of LTD changed spine number or size. Conversely, a manipulation that evoked persistent dendritic spine retraction did not alter parallel fiber-evoked excitatory postsynaptic currents.     

Changes in dendritic complexity and spine morphology in transgenic mice expressing human wild-type tau

Neurofibrillary tangles (NFTs) are composed of insoluble, hyperphosphorylated aggregates of the microtubule-associated protein tau and are present in various neurodegenerative diseases, including Alzheimer’s disease (AD). To investigate how tau affects neuronal function during NFT formation and subsequent neurodegeneration, we examined the morphology, spine density, spine type, and spine volume of layer III pyramidal neurons from the prefrontal cortex of mice expressing wild-type human tau (htau) over time. There were no significant alterations in apical dendritic arbor length in 3-, 6-, and 12-month-old htau mice; however, 12-month-old mice exhibited more complex arborization patterns. In addition, we observed a shift in spine morphology with fewer mushroom and more thin spines in both apical and basal dendrites as a function of htau accumulation. Interestingly, there was an overall decrease in volume of spines from 3 to 12 months. However, the volume of mushroom spines decreased from 3 to 6 months and increased from 6 to 12 months. This increase in complexity and branching in 12-month-old mice and the increase of volume of mushroom spines may represent compensatory mechanisms in the remaining intact neurons. As such, the accumulation of phosphorylated tau over time may contribute to the cognitive decline observed in AD by affecting neuronal structure and synaptic properties. Such alterations in dendrites and spines may result in the deterioration of neuronal function observed in AD, and provide a morphologic substrate for the relationship between synaptic integrity and cognitive decline.     

Impaired spatial memory and altered dendritic spine morphology in angiotensin II type 2 receptor-deficient mice

Mental retardation is the most frequent cause of serious handicap in children and young adults. Mutations in the human angiotensin II type 2 receptor (AT2) have been implicated in X-linked forms of mental retardation. We here demonstrate that mice lacking the AT2 receptor gene are significantly impaired in their performance in a spatial memory task and in a one-way active avoidance task. As no difference was observed between the genotypes in fear conditioning, the detected deficit in spatial memory may not relate to fear. Notably, receptor knockout mice showed increased motility in an activity meter and elevated plus maze. Importantly, these mice are characterized by abnormal dendritic spine morphology and length, both features also found to be associated with some cases of mental retardation. These findings suggest a crucial role of AT2 in normal brain function and that dysfunction of the receptor has impact on brain development and ultrastructural morphology with distinct consequences on learning and memory.

Control of hippocampal dendritic spine morphology through ephrin-A3/EphA4 signaling

Communication between glial cells and neurons is emerging as a critical parameter of synaptic function. However, the molecular mechanisms underlying the ability of glial cells to modify synaptic structure and physiology are poorly understood. Here we describe a repulsive interaction that regulates postsynaptic morphology through the EphA4 receptor tyrosine kinase and its ligand ephrin-A3. EphA4 is enriched on dendritic spines of pyramidal neurons in the adult mouse hippocampus, and ephrin-A3 is localized on astrocytic processes that envelop spines. Activation of EphA4 by ephrin-A3 was found to induce spine retraction, whereas inhibiting ephrin/EphA4 interactions distorted spine shape and organization in hippocampal slices. Furthermore, spine irregularities in pyramidal neurons from EphA4 knockout mice and in slices transfected with kinase-inactive EphA4 indicated that ephrin/EphA4 signaling is critical for spine morphology. Thus, our data support a model in which transient interactions between the ephrin-A3 ligand and the EphA4 receptor regulate the structure of excitatory synaptic connections through neuroglial cross-talk.     

Apolipoprotein E isoform-specific regulation of dendritic spine morphology in apolipoprotein E transgenic mice and Alzheimer's disease patients

Dendritic spines are postsynaptic sites of excitatory input in the mammalian nervous system. Apolipoprotein (apo) E participates in the transport of plasma lipids and in the redistribution of lipids among cells. A role for apoE is implicated in regeneration of synaptic circuitry after neural injury. The apoE4 allele is a major risk factor for late-onset familial and sporadic Alzheimer's disease (AD) and is associated with a poor outcome after brain injury. ApoE isoforms are suggested to have differential effects on neuronal repair mechanisms. In vitro studies have demonstrated the neurotrophic properties of apoE3 on neurite outgrowth. We have investigated the influence of apoE genotype on neuronal cell dendritic spine density in mice and in human postmortem tissue. In order to compare the morphology of neurons developing under different apoE conditions, gene gun labeling studies of dendritic spines of dentate gyrus (DG) granule cells of the hippocampus were carried out in wild-type (WT), human apoE3, human apoE4 expressing transgenic mice and apoE knockout (KO) mice; the same dendritic spine parameters were also assessed in human postmortem DG from individuals with and without the apoE4 gene. Quantitative analysis of dendritic spine length, morphology, and number was carried out on these mice at 3 weeks, 1 and 2 years of age. Human apoE3 and WT mice had a higher density of dendritic spines than human E4 and apoE KO mice in the 1 and 2 year age groups (P<0.0001), while at 3 weeks there were no differences between the groups. These age dependent differences in the effects of apoE isoforms on neuronal integrity may relate to the increased risk of dementia in aged individuals with the apoE4 allele. Significantly in human brain, apoE4 dose correlated inversely with dendritic spine density of DG neurons cell in the hippocampus of both AD (P=0.0008) and aged normal controls (P=0.0015). Our findings provide one potential explanation for the increased cognitive decline seen in aged and AD patients expressing apoE4.     

Chronic stress effects on dendritic morphology in medial prefrontal cortex: sex differences and estrogen dependence

A growing body of work has documented sex differences in many behavioral, neurochemical, and morphological responses to stress. Chronic stress alters morphology of dendrites in medial prefrontal cortex in male rats. However, potential sex differences in stress-induced morphological changes in medial prefrontal cortex have not been examined. Thus, in Experiment 1 we assessed dendritic morphology in medial prefrontal cortex in male and female rats after chronic stress. Male and female rats underwent either 3 hours of restraint daily for 1 week or were left unhandled except for weighing. On the final day of restraint, all rats were euthanized and brains were stained using a Golgi–Cox procedure. Pyramidal neurons in layer II–III of medial prefrontal cortex were drawn in three dimensions, and morphology of apical and basilar arbors was quantified. In males, stress decreased apical dendritic branch number and length, whereas in females, stress increased apical dendritic length. In Experiment 2, we assessed whether estradiol mediates this stress-induced dendritic hypertrophy in females by assessing the effects of restraint stress on female rats that had received either ovariectomy with or without 17-β-estradiol replacement or sham ovariectomy. Brains were processed and neurons reconstructed as described in Experiment 1. Both sham-operated and ovariectomized rats with estradiol implants showed stress-induced increases in apical dendritic material, whereas ovariectomy without estradiol replacement prevented the stress-induced increase. Thus, the stress-induced increase in apical dendritic material in females is estradiol-dependent.     

慢性脑缺血大鼠海马CA1区锥体细胞树突形态及树突棘密度的变化

目的: 研究慢性脑缺血大鼠海马CA1区锥体细胞树突形态及树突棘密度的变化。 方法: 对大鼠进行双侧颈总动脉永久性结扎(2VO)制备慢性脑缺血模型,分别于2周、4周、8周通过Morris水迷宫对各组大鼠进行行为学评价,筛选造模成功大鼠,进行Golgi染色,光镜下观察海马CA1区锥体细胞树突的分支、长度及树突棘密度的变化。结果: 与对照组相比,4周、8周模型组树突的分支及长度显著减少(P<0.01),各周模型组树突棘的密度均有显著减少(P<0.01);模型组内随着缺血时间延长,树突的分支及长度、树突棘密度均显著减少(P<0.05)。结论: 慢性脑缺血可导致海马CA1区锥体细胞树突及树突棘损伤性变化,从而构成进展性认知功能障碍的病理生理学基础。     

Dendrite and dendritic spine alterations in Alzheimer models

Synaptic damage and loss are factors that affect the degree of dementia experienced in Alzheimer disease (AD) patients. Multicolor DiOlistic labeling of the hippocampus has been undertaken which allows the full dendritic arbor of targeted neurons to be imaged. Using this labeling technique the neuronal morphology of two transgenic mouse lines (J20 and APP/PS1) expressing mutant forms of the Amyloid Precursor Protein (APP), at various ages, have been visualized and compared to Wild Type (WT) littermate controls. Swollen bulbous dystrophic neurites with loss of spines were apparent in the transgenic animals. Upon quantification, statistically significant reductions in the number of spines and total dendrite area was observed in both transgenic mouse lines at 11 months of age. Similar morphological abnormalities were seen in human AD hippocampal tissue both qualitatively and quantitatively. Immunohistochemistry and DiOlistic labeling was combined so that Abeta plaques were imaged in relation to the dendritic trees. No preferential localization of these abnormal dystrophic neurites was seen in regions with plaques. DiI labeled reative astrocytes were often apparent in close proximity to A beta plaques.     

Sexual dimorphism in locus coeruleus dendritic morphology: a structural basis for sex differences in emotional arousal

Stress-related psychiatric disorders, such as depression and anxiety, affect a disproportionate number of women. We previously demonstrated that the major brain norepinephrine (NE)-containing nucleus, locus coeruleus (LC) is more sensitive to stressors and to the stress-related neuropeptide, corticotropin-releasing factor (CRF) in female compared to male rats. Because the LC-NE system is a stress-responsive system that is thought to be dysregulated in affective disorders, sex differences in LC structure or function could play a role in female vulnerability to these diseases. The present study used different approaches to compare LC dendritic characteristics between male and female rats. Immunofluorescence labeling of tyrosine hydroxylase, the norepinephrine synthetic enzyme, revealed that LC dendrites of female rats extend further into the peri-LC region, covering a significantly greater area than those of males. Optical density measurements of dendrites in the peri-LC revealed increased dendritic density in females compared to their male counterparts. Additionally, immunoreactivity for synaptophysin, a synaptic vesicle protein, was significantly greater in the LC in female rats, suggesting an increased number of synaptic contacts onto LC processes. Individual LC neurons were juxtacellularly labeled with neurobiotin in vivo for morphological analysis. LC dendritic trees of females were longer and had more branch points and ends. Consistent with this, Sholl analysis determined that, compared to males, LC dendrites of females had a more complex pattern of branching. The greater dendritic extension and complexity seen in females predicts a higher probability of communication with diverse afferents that terminate in the peri-LC. This may be a structural basis for heightened arousal in females, an effect which may, in part, account for the sex bias in incidence of stress-related psychiatric disorders.     

Cortical dendritic pathology in human immunodeficiency virus encephalitis.

Previous neuropathological and morphometric studies of the cerebral cortex of patients with human immunodeficiency virus encephalitis (HIVE) have shown a decrease in the population of large neurons, moderate loss in synaptophysin immunoreactivity, and pathological changes in dendrites. To further characterize and quantify alterations in the dendritic tree of neocortical pyramidal neurons, we performed a modified Golgi impregnation technique on Formalin fixed blocks from the frontal cortex of 5 HIVE cases, 5 human immunodeficiency virus seropositive control cases without encephalitis, and 5 human immunodeficiency virus seronegative controls. Apical dendrites of HIVE cases were dilated, vacuolated, and tortuous with decreased length and branching. Basal and oblique dendrites also showed these alterations, but to a lesser extent. Some dendrites presented lacunae and filopodia consistent with remodeling. Computer aided quantification of HIVE cases showed a 40-60% decrease in spine density throughout the entire length of dendrites. Laser confocal imaging of Golgi impregnated sections displayed aberrant spines in regions of abnormal second order dendritic branches. These observations support the role of primary dendritic damage in HIVE in contrast to other neurodegenerative disorders where the primary pathology is presynaptic.     

Effects of chronic crowding stress on midbrain development: Changes in dendritic spine density and morphology in jewel fish optic tectum

This study investigates the effects of a biologically relevant stressor, crowding, on the development of neurons in the major teleostean brain area, the optic tectum. Adult jewel fish were reared for approximately 4 years under conditions of moderate density (3.3 liter/fish), or under uncrowded control conditions (25 liter/fish). Quantitative morphometric measures of Golgi-stained tissue were used to test whether long-term crowding at moderate density produced developmental deficits lasting beyond the juvenile period. Chronic crowding did not affect body size or gross tectal growth. However, crowding significantly decreased the density of dendritic spine formation on apical dendrites of pyriform interneurons in the basal region of the tectum (stratum album centrale). Additionally, the shapes of spines on this segment of the apical dendrite were altered by crowding: relative frequencies of overall spine length and spine stem length changed significantly. These results are interpreted in the context of cytoarchitectural changes produced by more extreme developmental stressors in previous studies.     

Molecular mechanisms of dendritic spine development and remodeling

Dendritic spines are small protrusions that cover the surface of dendrites and bear the postsynaptic component of excitatory synapses. Having an enlarged head connected to the dendrite by a narrow neck, dendritic spines provide a postsynaptic biochemical compartment that separates the synaptic space from the dendritic shaft and allows each spine to function as a partially independent unit. Spines develop around the time of synaptogenesis and are dynamic structures that continue to undergo remodeling over time. Changes in spine morphology and density influence the properties of neural circuits. Our knowledge of the structure and function of dendritic spines has progressed significantly since their discovery over a century ago, but many uncertainties still remain. For example, several different models have been put forth outlining the sequence of events that lead to the genesis of a spine. Although spines are small and apparently simple organelles with a cytoskeleton mainly composed of actin filaments, regulation of their morphology and physiology appears to be quite sophisticated. A multitude of molecules have been implicated in dendritic spine development and remodeling, suggesting that intricate networks of interconnected signaling pathways converge to regulate actin dynamics in spines. This complexity is not surprising, given the likely importance of dendritic spines in higher brain functions. In this review, we discuss the molecules that are currently known to mediate the exquisite sensitivity of spines to perturbations in their environment and we outline how these molecules interface with each other to mediate cascades of signals flowing from the spine surface to the actin cytoskeleton.     

Molecular mechanisms of dendritic spine development and maintenance

The large majority of excitatory synapses are located on dendritic spines which are discrete membrane protrusions present on neuronal dendrites. Interestingly the highly heterogeneous morphology of dendritic spines is thought to be the morphological basis for synaptic plasticity associated to learning and memory formation. Indeed dendritic spines structure is regulated by molecular mechanisms that are fine tuned and adjusted according to level and direction of synaptic activity, development, specific brain region, and different experimental behavioral conditions. This supports the idea that reciprocal changes between the structure and function of spines impact both local and global integration of signals within dendrites. An increasing number of proteins have been found to be morphogens for dendritic spines and provided new insights into the molecular mechanisms regulating spine formation and morphology. Thus determining the mechanisms that regulate spine formation and morphology is essential for understanding the cellular changes that underlie learning and memory in normal and pathological conditions.     

Postsynaptic membrane and spine apparatus: proximity in dendritic spines

We have reported previously that at dendritic spine synapses, the spine apparatus is associated with the synaptic spinule. In this report this association is shown, in serial thin sections, to involve intimate physical proximity between the postsynaptic density and membranous structures that are part of, or extend from, the spine apparatus itself. Because of the variegated shapes of synaptic spinules, the spine apparatus-postsynaptic density relation suggests participation in membrane distribution or synaptic remodeling phenomena.     

Granule cell dendritic spine density in the rat hippocampus varies with spine shape and location

The number of granule cell dendritic spines per micrometer of dendritic length in the dorsal and ventral leaves of the dentate gyrus was quantified using light microscopic-Golgi preparations of normal adult rats. Spines were counted in terms of 3 categories of spine form for the 3 afferent termination zones of the molecular layer and corrected for shading errors. Total spine density averaged 1.6 spines/micron of dendritic length in the dorsal leaf and 1.3 spines/micron of dendritic length in the ventral leaf. Statistically significant differences in spine density existed among the 3 shape categories. Variations in spine density occurred by shape category among the afferent termination zones.