Previous studies show that chronic stress induces synaptic structural alterations in brain regions involved in emotional processing such as the prefrontal cortex (PFC) and the basolateral amygdala (BLA). Yet, these studies are based mainly in animal exposure to unpredictable stressors or to restraint stress. On the other hand, studies using the chronic social defeat stress (CSDS), a relevant model of depression based on social conflict, are lacking. Here we aim to study the acute (24 h after CSDS) and long-term (one month after CSDS) effects of CSDS on dendritic and synaptic structures in the PFC and BLA of C57BL/6 mice. Specifically, BLA and PFC dendritic spine densities as well as BLA arborisation were analysed. Subsequently, we investigate in these regions the synaptic response to a friendly (interaction with a same strain mouse) or a fearful (interaction with a dominant strain mouse) social stimulus. Spine densities of the apical dendrites from the PFC pyramidal neurons were decreased by CSDS in the long-term (one month after CSDS). In addition, CSDS increased BLA stellate neurons spine density in the short-term (24 h after CSDS) and dendritic arborisation in the long-term. Moreover, long-term CSDS mice exposed to a fearful stimulus experienced a marked social avoidance and showed a significant increase in the expression of the immature form of the brain derived neurotrophic factor (proBDNF) in the amygdala. Taken together these results suggest the existence of persistent neuronal adaptations in the PFC and BLA in socially defeated mice. Specifically, spine density retraction in the PFC and increased BLA dendritic arborisation could represent an adaptive structural change allowing rapid expression of synaptic markers in response to fearful experiences.

Rictor associates with mTOR to form the mTORC2 complex, which activity regulates neuronal function and survival. Neurodegenerative diseases are characterized by the presence of neuronal dysfunction and cell death in specific brain regions such as for example Huntington's disease (HD), which is characterized by the loss of striatal projection neurons leading to motor dysfunction. Although HD is caused by the expression of mutant huntingtin, cell death occurs gradually suggesting that neurons have the capability to activate compensatory mechanisms to deal with neuronal dysfunction and later cell death. Here, we analyzed whether mTORC2 activity could be altered by the presence of mutant huntingtin. We observed that Rictor levels are specifically increased in the striatum of HD mouse models and in the putamen of HD patients. Rictor-mTOR interaction and the phosphorylation levels of Akt, one of the targets of the mTORC2 complex, were increased in the striatum of the R6/1 mouse model of HD suggesting increased mTORC2 signaling. Interestingly, acute downregulation of Rictor in striatal cells in vitro reduced mTORC2 activity, as shown by reduced levels of phospho-Akt, and increased mutant huntingtin-induced cell death. Accordingly, overexpression of Rictor increased mTORC2 activity counteracting cell death. Furthermore, normalization of endogenous Rictor levels in the striatum of R6/1 mouse worsened motor symptoms suggesting an induction of neuronal dysfunction. In conclusion, our results suggest that increased Rictor striatal levels could counteract neuronal dysfunction induced by mutant huntingtin.

Huntington's disease (HD) is a dominantly inherited neurodegenerative disease caused by expansion of a polyglutamine tract in the huntingtin protein. HD symptoms include severe motor, cognitive, and psychiatric impairments that result from dysfunction and later degeneration of medium-sized spiny neurons (MSNs) in the striatum. A key early pathogenic mechanism is dysregulated synaptic transmission due to enhanced surface expression of juvenile NMDA-type glutamate receptors containing G1uN3A subunits, which trigger the aberrant pruning of synapses formed by cortical afferents onto MSNs. Here, we tested the therapeutic potential of silencing G1uN3A expression in YAC128 mice, a well-established HD model. Recombinant adeno-associated viruses encoding a short-hairpin RNA against G1uN3A (rAAV-shGluN3A) were generated, and the ability of different serotypes to transduce MSNs was compared. A single injection of rAAV9-shGluN3A into the striatum of 1-month-old mice drove potent (>90%) and long-lasting reductions of G1uN3A expression in MSNs, prevented dendritic spine loss and improved motor performance in YAC128 mice. Later delivery, when spine pathology is already apparent, was also effective. Our data provide proof-of-concept for G1uN3A silencing as a beneficial strategy to prevent or reverse corticostriatal disconnectivity and motor impairment in HD and support the use of RNAi-based or small-molecule approaches for harnessing this therapeutic potential.

Excitotoxicity due to excessive activation of glutamate receptors is a primary mediator of cell death in acute and chronic neurological disorders, and NMDA-type glutamate receptors (NMDARs) are thought to be involved. NMDARs assemble from heteromeric combinations of GluNl, GluN2 and GluN3 subunits, yielding a variety of receptor subtypes that differ in biophysical properties, signaling, and synaptic targeting. Inclusion of inhibitory GluN3 subunits reduces Ca2+ influx via NMDAR channels and alters their synaptic targeting, thus modifying the two hallmarks of NMDARs that are critical for their roles on neuronal death and survival. Here we evaluated the neuroprotective potential of GluN3A subunits by analyzing the susceptibility to striatal excitotoxic damage of transgenic mice overexpressing GluN3A. We found that mild GluN3A overexpression protected susceptible striatal neurons from lesions induced by the neurotoxin 3-nitropropionic acid (3-NP), an inhibitor of mitochondrial complex 11/succinate dehydrogenase. GluN3A-mediated neuroprotection was dose-dependent and correlated with the levels of transgenic GluN3A expressed by two different mice strains. Neuroprotection was associated with a potent reduction of the activation of calpain, a Ca2+-dependent protease, which was measured as a decrease in 3-NP-induced fodrin and STEP cleavage in GluN3A transgenic mice relative to controls. We further show that transgenic GluN3A subunits incorporate into extrasynaptic compartments in mouse striatum, suggesting that reductions of toxic calpain activation might be linked to inhibition by GluN3A of pathological extrasynaptic NMDAR activity.

Alzheimer's disease (AD) and ageing are associated with impaired learning and memory, and recent findings point toward modulating chromatin remodeling through histone acetylation as a promising therapeutic strategy. Here we report that systemic administration of the HDAC inhibitor 4-phenylbutyrate (PBA) reinstated fear learning in the Tg2576 mouse model of AD. Tg2576 mice develop age-dependent amyloid pathology and cognitive decline that closely mimics disease progression in humans. Memory reinstatement by PBA was observed independently of the disease stage: both in 6-month-old Tg2576 mice, at the onset of the first symptoms, but also in aged, 12- to 16-month-old mice, when amyloid plaque deposition and major synaptic loss has occurred. Reversal of learning deficits was associated to a PBA-induced clearance of intraneuronal Aß accumulation, which was accompanied by mitigation of endoplasmic reticulum (ER) stress, and to restoration of dendritic spine densities of hippocampal CA1 pyramidal neurons to control levels. Furthermore, the expression of plasticity-related proteins such as the NMDA receptor subunit NR2B and the synaptic scaffold SAP102 was significantly increased by PBA. Our data suggest that the beneficial effects of PBA in memory are mediated both via its chemical chaperone-like activity and via the transcriptional activation of a cluster of proteins required for the induction of synaptic plasticity and structural remodeling.