Paula Merino-Serrais

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.

Widespread Changes in Dendritic Spines in a Model of Alzheimer's Disease

The mechanism by which dementia occurs in patients with Alzheimers disease (AD) is not known. We assessed changes in hippocampal dendritic spines of APP/PS1 transgenic mice that accumulate amyloid beta throughout the brain. Three-dimensional analysis of 21,507 dendritic spines in the dentate gyrus, a region crucial for learning and memory, revealed a substantial decrease in the frequency of large spines in plaque-free regions of APP/PS1 mice. Plaque-related dendrites also show striking alterations in spine density and morphology. However, plaques occupy only 3.9% of the molecular layer volume. Because large spines are considered to be the physical traces of long-term memory, widespread decrease in the frequency of large spines likely contributes to the cognitive impairments observed in this AD model.

The influence of phospho-tau on dendritic spines of cortical pyramidal neurons in patients with Alzheimer’s disease

The dendritic spines on pyramidal cells represent the main postsynaptic elements of cortical excitatory synapses and they are fundamental structures in memory, learning and cognition. In the present study, we used intracellular injections of Lucifer yellow in fixed tissue to analyse over 19 500 dendritic spines that were completely reconstructed in three dimensions along the length of the basal dendrites of pyramidal neurons in the parahippocampal cortex and CA1 of patients with Alzheimer’s disease. Following intracellular injection, sections were immunostained for anti-Lucifer yellow and with tau monoclonal antibodies AT8 and PHF-1, which recognize tau phosphorylated at Ser202/Thr205 and at Ser396/404, respectively. We observed that the diffuse accumulation of phospho-tau in a putative pre-tangle state did not induce changes in the dendrites of pyramidal neurons, whereas the presence of tau aggregates forming intraneuronal neurofibrillary tangles was associated with progressive alteration of dendritic spines (loss of dendritic spines and changes in their morphology) and dendrite atrophy, depending on the degree of tangle development. Thus, the presence of phospho-tau in neurons does not necessarily mean that they suffer severe and irreversible effects as thought previously but rather, the characteristic cognitive impairment in Alzheimer’s disease is likely to depend on the relative number of neurons that have well developed tangles.

Antagomirs targeting microRNA-134 increase hippocampal pyramidal neuron spine volume in vivo and protect against pilocarpine-induced status epilepticus

Emerging data support roles for microRNA (miRNA) in the pathogenesis of various neurologic disorders including epilepsy. MicroRNA-134 (miR-134) is enriched in dendrites of hippocampal neurons, where it negatively regulates spine volume. Recent work identified upregulation of miR-134 in experimental and human epilepsy. Targeting miR-134 in vivo using antagomirs had potent anticonvulsant effects against kainic acid-induced seizures and was associated with a reduction in dendritic spine number. In the present study, we measured dendritic spine volume in mice injected with miR-134-targeting antagomirs and tested effects of the antagomirs on status epilepticus triggered by the cholinergic agonist pilocarpine. Morphometric analysis of over 6,400 dendritic spines in Lucifer yellow-injected CA3 pyramidal neurons revealed increased spine volume in mice given antagomirs compared to controls that received a scrambled sequence. Treatment of mice with miR-134 antagomirs did not alter performance in a behavioral test (novel object location). Status epilepticus induced by pilocarpine was associated with upregulation of miR-134 within the hippocampus of mice. Pretreatment of mice with miR-134 antagomirs reduced the proportion of animals that developed status epilepticus following pilocarpine and increased animal survival. In antagomir-treated mice that did develop status epilepticus, seizure onset was delayed and total seizure power was reduced. These studies provide in vivo evidence that miR-134 regulates spine volume in the hippocampus and validation of the seizure-suppressive effects of miR-134 antagomirs in a model with a different triggering mechanism, indicating broad conservation of anticonvulsant effects.

Morphological alterations to neurons of the amygdala and impaired fear conditioning in a transgenic mouse model of Alzheimer's disease

Patients with Alzheimers disease (AD) suffer from impaired memory and emotional disturbances, the pathogenesis of which is not entirely clear. In APP/PS1 transgenic mice, a model of AD in which amyloid β (Aβ) accumulates in the brain, we have examined neurons in the lateral nucleus of the amygdala (LA), a brain region crucial to establish cued fear conditioning. We found that although there was no neuronal loss in this region and Aβ plaques only occupy less than 1% of its volume, these mice froze for shorter times after auditory fear conditioning when compared to their non‐transgenic littermates. We performed a three‐dimensional analysis of projection neurons and of thousands of dendritic spines in the LA. We found changes in dendritic tree morphology and a substantial decrease in the frequency of large spines in plaque‐free neurons of APP/PS1 mice. We suggest that these morphological changes in the neurons of the LA may contribute to the impaired auditory fear conditioning seen in this AD model. Copyright

Facilitation of AMPA receptor synaptic delivery as a molecular mechanism for cognitive enhancement

A small peptide from a neuronal cell adhesion molecule enhances synaptic plasticity in the hippocampus and results in improved cognitive performance in rats.

Layer‐specific alterations to CA1 dendritic spines in a mouse model of Alzheimer's disease

Why memory is a particular target for the pathological changes in Alzheimers Disease (AD) has long been a fundamental question when considering the mechanisms underlying this disease. It has been established from numerous biochemical and morphological studies that AD is, at least initially, a consequence of synaptic malfunction provoked by Amyloid β (Aβ) peptide. APP/PS1 transgenic mice accumulate Aβ throughout the brain, and they have therefore been employed to investigate the effects of Aβ overproduction on brain circuitry and cognition. Previous studies show that Aβ overproduction affects spine morphology in the hippocampus and amygdala, both within and outside plaques (Knafo et al., (2009) Cereb Cortex 19:586‐592; Knafo et al., (in press) J Pathol). Hence, we conducted a detailed analysis of dendritic spines located in the stratum oriens and stratum radiatum of the CA1 hippocampal subfield of APP/PS1 mice. Three‐dimensional analysis of 18,313 individual dendritic spines revealed a substantial layer‐specific decrease in spine neck length and an increase in the frequency of spines with a small head volume. Since dendritic spines bear most of the excitatory synapses in the brain, changes in spine morphology may be one of the factors contributing to the cognitive impairments observed in this AD model.

Abnormal tau phosphorylation in the thorny excrescences of CA3 hippocampal neurons in patients with Alzheimer's disease.

A key symptom in the early stages of Alzheimers disease (AD) is the loss of declarative memory. The anatomical substrate that supports this kind of memory involves the neural circuits of the medial temporal lobe, and in particular, of the hippocampal formation and adjacent cortex. A main feature of AD is the abnormal phosphorylation of the tau protein and the presence of tangles. The sequence of cellular changes related to tau phosphorylation and tangle formation has been studied with an antibody that binds to diffuse phosphotau (AT8). Moreover, another tau antibody (PHF-1) has been used to follow the pathway of neurofibrillary (tau aggregation) degeneration in AD. We have used a variety of quantitative immunocytochemical techniques and confocal microscopy to visualize and characterize neurons labeled with AT8 and PHF-1 antibodies. We present here the rather unexpected discovery that in AD, there is conspicuous abnormal phosphorylation of the tau protein in a selective subset of dendritic spines. We identified these spines as the typical thorny excrescences of hippocampal CA3 neurons in a pre-tangle state. Since thorny excrescences represent a major synaptic target of granule cell axons (mossy fibers), such aberrant phosphorylation may play an essential role in the memory impairment typical of AD patients.

Synaptic Changes in the Dentate Gyrus of APP/PS1 Transgenic Mice Revealed by Electron Microscopy

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WIP Is a Negative Regulator of Neuronal Maturation and Synaptic Activity

Wiskott-Aldrich syndrome protein (WASP) -interacting protein (WIP) is an actin-binding protein involved in the regulation of actin polymerization in cells, such as fibroblasts and lymphocytes. Despite its recognized function in non-neuronal cells, the role of WIP in the central nervous system has not been examined previously. We used WIP-deficient mice to examine WIP function both in vivo and in vitro. We report here that WIP(-)(/-) hippocampal neurons exhibit enlargement of somas as well as overgrowth of neuritic and dendritic branches that are more evident in early developmental stages. Dendritic arborization and synaptogenesis, which includes generation of postsynaptic dendritic spines, are actin-dependent processes that occur in parallel at later stages. WIP deficiency also increases the amplitude and frequency of miniature excitatory postsynaptic currents, suggesting that WIP(-)(/-) neurons have more mature synapses than wild-type neurons. These findings reveal WIP as a previously unreported regulator of neuronal maturation and synaptic activity.