Martine Ammassari-Teule

Caspase-3 triggers early synaptic dysfunction in a mouse model of Alzheimer's disease

Synaptic loss is the best pathological correlate of the cognitive decline in Alzheimers disease; however, the molecular mechanisms underlying synaptic failure are unknown. We found a non-apoptotic baseline caspase-3 activity in hippocampal dendritic spines and an enhancement of this activity at the onset of memory decline in the Tg2576-APPswe mouse model of Alzheimers disease. In spines, caspase-3 activated calcineurin, which in turn triggered dephosphorylation and removal of the GluR1 subunit of AMPA-type receptor from postsynaptic sites. These molecular modifications led to alterations of glutamatergic synaptic transmission and plasticity and correlated with spine degeneration and a deficit in hippocampal-dependent memory. Notably, pharmacological inhibition of caspase-3 activity in Tg2576 mice rescued the observed Alzheimer-like phenotypes. Our results identify a previously unknown caspase-3–dependent mechanism that drives synaptic failure and contributes to cognitive dysfunction in Alzheimers disease. These findings indicate that caspase-3 is a potential target for pharmacological therapy during early disease stages.

The Formation of Recent and Remote Memory Is Associated with Time-Dependent Formation of Dendritic Spines in the Hippocampus and Anterior Cingulate Cortex

Although hippocampal–cortical interactions are crucial for the formation of enduring declarative memories, synaptic events that govern long-term memory storage remain mostly unclear. We present evidence that neuronal structural changes, i.e., dendritic spine growth, develop sequentially in the hippocampus and anterior cingulate cortex (aCC) during the formation of recent and remote contextual fear memory. We found that mice placed in a conditioning chamber for one 7 min conditioning session and exposed to five footshocks (duration, 2 s; intensity, 0.7 mA; interstimulus interval, 60 s) delivered through the grid floor exhibited robust fear response when returned to the experimental context 24 h or 36 d after the conditioning. We then observed that their fear response at the recent, but not the remote, time point was associated with an increase in spine density on hippocampal neurons, whereas an inverse temporal pattern of spine density changes occurred on aCC neurons. At each time point, hippocampal or aCC structural alterations were achieved even in the absence of recent or remote memory tests, thus suggesting that they were not driven by retrieval processes. Furthermore, ibotenic lesions of the hippocampus impaired remote memory and prevented dendritic spine growth on aCC neurons when they were performed immediately after the conditioning, whereas they were ineffective when performed 24 d later. These findings reveal that gradual structural changes modifying connectivity in hippocampal–cortical networks underlie the formation and expression of remote memory, and that the hippocampus plays a crucial but time-limited role in driving structural plasticity in the cortex.

Spatial learning and memory, maze running strategies and cholinergic mechanisms in two inbred strains of mice

The acquisition process of the radial maze task was studied in two inbred strains of mice, C57BL/6 and DBA/2. A quantitative and qualitative evaluation of performance was performed and the pretest level of activity was measured. The results showed a significant correlation between activity and performance since the highly active C57BL/6 mice exhibited better performance of the radial maze task than the less active DBA/2 mice. Moreover, for correct trials, strain-dependent maze-running strategies were observed: while both strains displayed about the same percentage of clockwise and spatial strategies, it was observed that among the spatial strategies C57BL/6 used a larger number of different correct solutions. Subsequently, the effect of scopolamine administration on working memory processes was assessed in sequential and discrete trials. A different reactivity of each strain to anti-cholinergic treatment was found in discrete trials since only DBA/2 mice were impaired. The effect of scopolamine is discussed in relation to the different models of information processing involved in learning and memorizing the experimental rule.

Spine growth in the anterior cingulate cortex is necessary for the consolidation of contextual fear memory

Remodeling of cortical connectivity is thought to allow initially hippocampus-dependent memories to be expressed independently of the hippocampus at remote time points. Consistent with this, consolidation of a contextual fear memory is associated with dendritic spine growth in neurons of the anterior cingulate cortex (aCC). To directly test whether such cortical structural remodeling is necessary for memory consolidation, we disrupted spine growth in the aCC at different times following contextual fear conditioning in mice. We took advantage of previous studies showing that the transcription factor myocyte enhancer factor 2 (MEF2) negatively regulates spinogenesis both in vitro and in vivo. We found that increasing MEF2-dependent transcription in the aCC during a critical posttraining window (but not at later time points) blocked both the consolidation-associated dendritic spine growth and subsequent memory expression. Together, these data strengthen the causal link between cortical structural remodeling and memory consolidation and, further, identify MEF2 as a key regulator of these processes.

The differences shown by C57BL/6 and DBA/2 inbred mice in detecting spatial novelty are subserved by a different hippocampal and parietal cortex interplay

Inbred C57BL/6 (C57) and DBA/2 (DBA) mice with hippocampus, posterior parietal cortex or sham lesions were placed in an open-field containing five objects and their reactivity to the displacement (spatial novelty) or the substitution (object novelty) of some of these objects was examined. C57 mice reacted to spatial novelty by exploring more the displaced than the non-displaced objects while DBA mice did not show any consistent reaction. In the highly reactive C57 strain, the peak of exploratory responses directed towards the displaced objects was completely abolished by hippocampal and posterior parietal cortex lesions. In the non-reactive DBA strain, hippocampal lesions induced an aspecific decreased interest towards the two categories of objects while posterior parietal cortex lesions did not produce any behavioral modification. The high reactivity of C57 mice to spatial change appears to be subserved by the conjunctive participation of the hippocampus and the posterior parietal cortex. Conversely, the deficit shown by DBA mice in that situation seems to be related to: (i) a poorly functional hippocampus; and (ii) the non-involvement of the posterior parietal cortes. The present data suggest that the participation of the posterior parietal cortes to the detection of spatial novelty may depend on the degree of functionality of the hippocampus.

Learning in inbred mice: Strain-specific abilities across three radial maze problems

Mice belonging to the C57BL/6, DBA/2 (DBA), and C3H/He (C3H) strains were compared in three different eight0arm radial maze tasks requiring various degrees of spatial and nonspatial information processing. The results show that, on the standard radial maze task, C57 performed better than DBA, which, in turn, performed better than C3H. Fewer differences in the four-baited arm task and no difference in the cued version task were found between C57 and DBA, while C3H still performed more poorly. The high performance shown by C57 mice in all problems seems to be related to their ability to build up maze-running patterns based upon an optimal proportion of 45° angle turns, according to the demand of the situation. The cognitive and discriminative mechanisms involved in the solving of each task, the sensorial characteristics of the three strains, and the limits of an approach based upon neuroanatomical-behavioral correlations are discussed.

Plastic and behavioral abnormalities in experimental Huntington's disease: A crucial role for cholinergic interneurons

Huntingtons disease (HD) is a fatal hereditary neurodegenerative disease causing degeneration of striatal spiny neurons, whereas cholinergic interneurons are spared. This cell-type specific pathology produces an array of abnormalities including involuntary movements, cognitive impairments, and psychiatric disorders. Although the genetic mutation responsible for HD has been identified, little is known about the early synaptic changes occurring within the striatal circuitry at the onset of clinical symptoms. We therefore studied the synaptic plasticity of spiny neurons and cholinergic interneurons in two animal models of early HD. As a pathogenetic model, we used the chronic subcutaneous infusion of the mitochondrial toxin 3-nitropropionic acid (3-NP) in rats. This treatment caused striatal damage and impaired response flexibility in the cross-maze task as well as defective extinction of conditioned fear suggesting a perseverative behavior. In these animals, we observed a loss of depotentiation in striatal spiny neurons and a lack of long-term potentiation (LTP) in cholinergic interneurons. These abnormalities of striatal synaptic plasticity were also observed in R6/2 transgenic mice, a genetic model of HD, indicating that both genetic and phenotypic models of HD show cell-type specific alterations of LTP. We also found that in control rats, as well as in wild-type (WT) mice, depotentiation of spiny neurons was blocked by either scopolamine or hemicholinium, indicating that reversal of LTP requires activation of muscarinic receptors by endogenous acetylcholine. Our findings suggest that the defective plasticity of cholinergic interneurons could be the primary event mediating abnormal functioning of striatal circuits, and the loss of behavioral flexibility typical of early HD might largely depend on cell-type specific plastic abnormalities.

Viral-mediated expression of a constitutively active form of CREB in hippocampal neurons increases memory.

Synaptic activity‐dependent phosphorylation of the transcription factor cAMP response element binding protein (CREB) leads to CREB‐dependent gene transcription, a process thought to underlie long‐term hippocampal synaptic plasticity and memory formation. We previously reported that increasing CREB activity in glutamatergic neurons enhances synaptic plasticity and neuronal excitability. Whether these modifications are sufficient to promote hippocampal‐dependent memory formation was not determined. Here, we provide direct evidence that a brief increase in CREB‐dependent transcription in either CA1 or DG neurons, using in vivo viral vectors, is sufficient to boost memory for contextual representations, as tested in the contextual fear conditioning task, without affecting motor, pain, or anxiety behaviors.

N-Methyl-D-aspartate receptors in the nucleus accumbens⋅are involved in detection of spatial novelty in mice

Abstract The aim of this study was to investigate the role played by intra-accumbens N-methyl-D-aspartate (NMDA) receptors in spatial information encoding. For this purpose, the effect of local administration of both competitive (AP-5) and non-competitive (MK-801) NMDA antagonists was assessed in a task designed to estimate the ability of rodents to encode spatial relationships between discrete stimuli. The task consists of placing mice in an open field containing five objects and, after three sessions of habituation, examining their reactivity to object displacement (spatial novelty) and object substitution (object novelty). The results show that both doses of MK-801 (0.15 and 0.3 μg/side) induced a selective impairment in the capability of mice to detect spatial novelty. A similar effect was obtained by injecting the low dose of the competitive antagonist AP-5 (0.1 μg/side), whereas the high dose (0.15 μg/side) abolished detection of both spatial and object novelty. Taken together, these results show that intra-accumbens injections of low doses of competitive and non-competitive NMDA antagonists can produce selective deficits in processing spatial information resembling those observed after hippocampal damage. Moreover, the fact that pharmacological treatments spare memory processes involved in habituation suggests that NMDA antagonists may interfere with the formation of spatial representations rather than producing memory deficits per se.

Phosphodiesterase type IV inhibition prevents sequestration of CREB binding protein, protects striatal parvalbumin interneurons and rescues motor deficits in the R6/2 mouse model of Huntington's disease

The phosphodiesterase type IV inhibitor rolipram increases cAMP response element‐binding protein (CREB) phosphorylation and exerts neuroprotective effects in both the quinolinic acid rat model of Huntington’s disease ( DeMarch et al., 2007 ) and the R6/2 mouse including sparing of striatal neurons, prevention of neuronal intranuclear inclusion formation and attenuation of microglial reaction ( DeMarch et al., 2008 ). In this study, we sought to determine if rolipram has a beneficial role in the altered distribution of CREB binding protein in striatal spiny neurons and in the motor impairments shown by R6/2 mutants. Moreover, we investigated whether rolipram treatment altered the degeneration of parvalbuminergic interneurons typical of Huntington’s disease ( Fusco et al., 1999 ). Transgenic mice and their wild‐type controls from a stable colony maintained in our laboratory were treated with rolipram (1.5 mg/kg) or saline daily starting from 4 weeks of age. The cellular distribution of CREB binding protein in striatal spiny neurons was assessed by immunofluorescence, whereas parvalbuminergic neuron degeneration was evaluated by cell counts of immunohistochemically labeled tissue. Motor coordination and motor activity were also examined. We found that rolipram was effective in preventing CREB binding protein sequestration into striatal neuronal intranuclear inclusions, sparing parvalbuminergic interneurons of R6/2 mice, and rescuing their motor coordination and motor activity deficits. Our findings demonstrate the possibility of reversing pharmacologically the behavioral and neuropathological abnormalities of symptomatic R6/2 mice and underline the potential therapeutic value of phosphodiesterase type IV inhibitors in Huntington’s disease.