Diana Frechilla

Phenylbutyrate ameliorates cognitive deficit and reduces tau pathology in an Alzheimer's disease mouse model.

Chromatin modification through histone acetylation is a molecular pathway involved in the regulation of transcription underlying memory storage. Sodium 4-phenylbutyrate (4-PBA) is a well-known histone deacetylase inhibitor, which increases gene transcription of a number of genes, and also exerts neuroprotective effects. In this study, we report that administration of 4-PBA reversed spatial learning and memory deficits in an established mouse model of Alzheimers disease (AD) without altering β-amyloid burden. We also observed that the phosphorylated form of tau was decreased in the AD mouse brain after 4-PBA treatment, an effect probably due to an increase in the inactive form of the glycogen synthase kinase 3β (GSK3β). Interestingly, we found a dramatic decrease in brain histone acetylation in the transgenic mice that may reflect an indirect transcriptional repression underlying memory impairment. The administration of 4-PBA restored brain histone acetylation levels and, as a most likely consequence, activated the transcription of synaptic plasticity markers such as the GluR1 subunit of the AMPA receptor, PSD95, and microtubule-associated protein-2. The results suggest that 4-PBA, a drug already approved for clinical use, may provide a novel approach for the treatment of AD.

Antibodies and Antisense Oligonucleotide for Probing the Distribution and Putative Functions of Central 5-HT6 Receptors

Among the recently cloned serotonin (5-hydroxytryptamine, 5-HT) receptors, the 5-HT6subtype is of special interest for at least two reasons: 1) it is abundant in limbic areas which participate in the control of mood and emotion; and 2) some antidepressants and antipsychotics are potent 5-HT6receptor antagonists. Studies using polyclonal anti-5-HT6receptor antibodies and an antisense oligonucleotide were performed in order to investigate further the function(s) of 5-HT6receptors in the rat brain. Immunocytochemistry at the light and electron microscope levels showed that 5-HT6receptors are mainly confined to the dendritic compartment, suggesting that they could mediate 5-HT actions on neuronal firing. In some limbic areas, 5-HT6receptor-like immunoreactivity is also associated with neuronal cilia with yet unknown functions. Continuous i.c.v. infusion with an antisense oligonucleotide for 3–4 days resulted in decreased 5-HT6receptor-like immunostaining of the nucleus accumbens and anxiogenic behaviours in the social interaction and elevated plus maze tests. Selective 5-HT6 receptor ligands are eagerly expected to investigate further the potential implication of these receptors in limbic-dependent behaviours.

Rosiglitazone rescues memory impairment in Alzheimer's transgenic mice: mechanisms involving a reduced amyloid and tau pathology.

Clinical studies suggest that agonists at peroxisome proliferator-activated receptor gamma (PPARγ) may exert beneficial effects in patients with mild-to-moderate Alzheimers disease (AD), but the mechanism for the potential therapeutic interest of this class of drugs has not yet been elucidated. Here, in mice overexpressing mutant human amyloid precursor protein, we found that chronic treatment with rosiglitazone, a high-affinity agonist at PPARγ, facilitated β-amyloid peptide (Aβ) clearance. Rosiglitazone not only reduced Aβ burden in the brain but, importantly, almost completely removed the abundant amyloid plaques observed in the hippocampus and entorhinal cortex of 13-month-old transgenic mice. In the hippocampus, neuropil threads containing phosphorylated tau, probably corresponding to dystrophic neurites, were also decreased by the drug. Rosiglitazone switched on the activated microglial phenotype, promoting its phagocytic ability, reducing the expression of proinflammatory markers and inducing factors for alternative differentiation. The decreased amyloid pathology may account for the reduction of p-tau-containing neuropil threads and for the rescue of impaired recognition and spatial memory in the transgenic mice. This study provides further insights into the mechanisms for the beneficial effect of rosiglitazone in AD patients.

Chronic antidepressant treatment increases the membrane expression of AMPA receptors in rat hippocampus

It has been proposed that potentiation of AMPA receptor (AMPAR) function may be useful in the treatment of depression. Here we studied the acute and chronic effect of the antidepressants desipramine and paroxetine, which differentially affect monoamine reuptake, on the expression of the AMPAR subunits GluR1 and GluR2/3, analyzed by Western blot, both in total and in membrane-enriched extracts from rat hippocampus. Acute antidepressant treatment did not produce any change in the expression of AMPAR subunits. In chronic treatments, rats were daily treated with the antidepressants (10 mg/kg/day) for 7, 14, or 21 days. In rats receiving either of the two antidepressant treatments for 21 consecutive days and killed 24 h after the last injection, an increase in GluR1 and GluR2/3 levels was found in the membrane fraction, with no significant change in the total extract, suggesting a trafficking of the AMPAR subunits from intracellular pools to synaptic sites in the hippocampus. This appeared to be a region-specific effect since no change in AMPAR subunit expression was found in the frontal cortex. Previously reported modifications in phosphorylating enzymes by chronic antidepressants could perhaps play a role in hippocampal membrane insertion of AMPAR subunits. When the survival time after the 21-day-treatment was longer - 72 instead of 24 h - the hippocampal membrane expression of GluR1, but not of GluR2/3 subunits, was still increased, as could be expected from the distinct mechanisms operating in synaptic delivery of GluR1 and GluR2/3 subunits. The antidepressant-induced increase in the number of GluR1- and GluR2/3-containing AMPARs at the synapses may indicate an enhanced AMPAR-mediated synaptic transmission which could help to counteract the alterations in neuronal connectivity which appear to underlie the pathophysiology of mood disorders.

Rosiglitazone reverses memory decline and hippocampal glucocorticoid receptor down-regulation in an Alzheimer's disease mouse model.

Clinical trials with rosiglitazone, a potent agonist at peroxisome proliferator-activated receptor gamma (PPARgamma) suggest an improvement of cognitive function in Alzheimers disease (AD) patients. The mechanisms mediating this potential beneficial effect remain to be fully elucidated. In mice overexpressing mutant human amyloid precursor protein (hAPP), a model of AD, we found that memory impairment in the object recognition test was prevented and also reversed by chronic rosiglitazone treatment. Given the possible involvement of glucocorticoid receptors (GR) in the actions of PPARgamma-ligands, we studied the effect of chronic rosiglitazone treatment on GR levels in the hippocampus of hAPP mice. An early down-regulation of GR, not related to elevated plasma corticosterone levels, was found in different hippocampal subfields of the transgenic mice and this decrease was prevented by rosiglitazone. In parallel with behavioural studies, rosiglitazone also normalized GR levels in older animals. This effect may contribute to explain the attenuation of memory decline by PPARgamma activation in an AD mouse model.

Serotonin 5-HT1A receptor expression is selectively enhanced in the striosomal compartment of chronic Parkinsonian monkeys

Cynomolgus monkeys (Macaca fascicularis) were chronically treated with the dopaminergic neurotoxin 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP) until stable parkinsonism was reached. Two months later, monkeys were sacrificed and monoamine content was measured in different brain regions of the lesioned monkeys and of age‐matched controls. 5‐HT1A serotonin receptor density was measured in coronal sections labeled with [3H]8‐OH‐DPAT. As expected, dopamine was virtually nonexistent in the caudate nucleus and putamen of MPTP‐treated monkeys. Serotonin levels were significantly reduced in different brain regions, particularly in the raphe nuclei. 5‐HT1A receptor density of control animals was high in the hippocampus, notably in the CA1 field and also in the raphe nuclei, and much lower in the striatum, where 5‐HT1A receptors showed a patchy distribution which corresponded to striosomes with poor calbindin immunostaining. 5‐HT1A receptor density was reduced in hippocampal fields and in the raphe nuclei of parkinsonian monkeys. Conversely, in the severely lesioned striatal nuclei 5‐HT1A receptor density was increased at caudal levels of the striatum, particularly in the putamen. The results tend to support the possibility of an increased synthesis of 5‐HT1A receptors in brain regions with higher neuronal cell death. Upregulation of this 5‐HT receptor subtype in the limbic compartment of the striatum may represent a compensatory event for the serotonergic dysfunction and associated mental disorders in neurodegenerative diseases such as Parkinson disease. Synapse 39:288–296, 2001.

Early Changes in Hippocampal Eph Receptors Precede the Onset of Memory Decline in Mouse Models of Alzheimer's Disease

Synapse loss occurs early in Alzheimers disease (AD) and is considered the best pathological correlate of cognitive decline. Ephrins and Eph receptors are involved in regulation of excitatory neurotransmission and play a role in cytoskeleton remodeling. We asked whether alterations in Eph receptors could underlie cognitive impairment in an AD mouse model overexpressing human amyloid-beta protein precursor (hA beta PP) with familial mutations (hA beta PP swe-ind mice). We found that EphA4 and EphB2 receptors were reduced in the hippocampus before the development of impaired object recognition and spatial memory. Similar results were obtained in another line of transgenic A beta PP mice, Tg2576. A reduction in Eph receptor levels was also found in postmortem hippocampal tissue from patients with incipient AD. At the time of onset of memory decline inhA beta PP swe-ind mice, no change in surface expression of AMPA or NMDA receptor subunits was apparent, but we found changes in Eph-receptor downstream signaling, in particular a decrease in membrane-associated phosho-cofilin levels that may cause cytoskeletal changes and disrupted synaptic activity. Consistent with this finding, Eph receptor activation in cell culture increased phosho-cofilin levels. The results suggest that alterations in Eph receptors may play a role in synaptic dysfunction in the hippocampus leading to cognitive impairment in a model of AD.

Sequential changes in BDNF mRNA expression and synaptic levels of AMPA receptor subunits in rat hippocampus after chronic antidepressant treatment.

Increased expression of brain-derived neurotrophic factor (BDNF) appears to be involved in the mechanism of action of antidepressant drugs. It has also been proposed that potentiation of the AMPA receptor (AMPAR) function may be useful in the treatment of depression. Here we looked for the time course of the effect of different doses of two antidepressants, desipramine (DMI) and paroxetine (PAR), which differentially affect monoamine reuptake, on BDNF mRNA expression in hippocampal subfields (CA1, CA3 and dentate gyrus) and levels of AMPAR subunits in total and membrane-enriched extracts from rat hippocampus. Acute antidepressant treatment changed neither BDNF mRNA expression nor AMPAR subunit levels. In chronic treatments, rats were treated daily with the antidepressants for 7-21 days. PAR produced a time- and dose-dependent increase of BDNF expression in the three hippocampal subfields examined. On the contrary, the effect of DMI on BDNF mRNA was neither dose- nor time-dependent. In rats receiving the same chronic antidepressant treatments, PAR produced a dose-dependent increase of GluR1 and GluR2/3 levels in the membrane fraction after a 3-week treatment, and not at earlier times. DMI increased the membrane levels of AMPAR subunits after a 3-week treatment with the lower dose tested. However, a higher dose, 15 mg/kg, did not produce any change in AMPAR subunits and reduced membrane levels of alpha-tubulin and PSD-95, possibly indicating a disorganization of membrane scaffolding proteins. The results suggest that paroxetine, but not desipramine, enhances synaptic plasticity in the hippocampus by increasing BDNF mRNA expression, which determines a later AMPAR subunit trafficking to synaptic membranes.

Overexpression of wild-type human APP in mice causes cognitive deficits and pathological features unrelated to Aβ levels

Transgenic mice expressing mutant human amyloid precursor protein (APP) develop an age-dependent amyloid pathology and memory deficits, but no overt neuronal loss. Here, in mice overexpressing wild-type human APP (hAPP(wt)) we found an early memory impairment, particularly in the water maze and to a lesser extent in the object recognition task, but beta-amyloid peptide (Abeta(42)) was barely detectable in the hippocampus. In these mice, hAPP processing was basically non-amyloidogenic, with high levels of APP carboxy-terminal fragments, C83 and APP intracellular domain. A tau pathology with an early increase in the levels of phosphorylated tau in the hippocampus, a likely consequence of enhanced ERK1/2 activation, was also observed. Furthermore, these mice presented a loss of synapse-associated proteins: PSD95, AMPA and NMDA receptor subunits and phosphorylated CaMKII. Importantly, signs of neurodegeneration were found in the hippocampal CA1 subfield and in the entorhinal cortex that were associated to a marked loss of MAP2 immunoreactivity. Conversely, in mice expressing mutant hAPP, high levels of Abeta(42) were found in the hippocampus, but no signs of neurodegeneration were apparent. The results support the notion of Abeta-independent pathogenic pathways in Alzheimers disease.

Effect of chronic antidepressant treatment on transcription factor binding activity in rat hippocampus and frontal cortex.

1. The effect of chronic antidepressant administration on CRE-, SP1- and GRE-binding activity was studied in rat hippocampus and frontal cortex. 2. Fluoxetine and desipramine (3 and 10 mg/kg/day respectively) were given to rats for 21 consecutive days. The animals were killed 3 hr after the last injection and nuclear extracts were prepared to perform the DNA-protein reaction with consensus CRE, SP1 and GRE oligonucleotides. 3. Gel-shift assays showed that CRE-binding activity was increased in both frontal cortex and hippocampus by chronic fluoxetine treatment. Desipramine, however, only enhanced this activity in the frontal cortex. 4. Chronic fluoxetine decreased SP1-binding activity in the two selected brain regions. Again, desipramine only produced a significant reduction in the frontal cortex. 5. GRE-binding in the hippocampus was only enhanced by desipramine. Since chronic desipramine, and not fluoxetine, is able to increase hippocampal glucocorticoid receptor (GR) expression, interactions of GR with CREB and SP1 may determine the lack of effect of desipramine on binding activity of the two latter transcription factors in this brain region. 6. Overall, the results show a differential and region-specific effect of chronic, and not acute, antidepressant treatment on the DNA-binding activities studied and are consonant with the possible role of changes in gene expression in the mechanism of antidepressant action.