Differential effects of dopamine signalling on long-term memory formation and consolidation in rodent brain.

BACKGROUND: Using auditory discrimination learning in gerbils, we have previously shown that activation of auditory-cortical D1/D5 dopamine receptors facilitates mTOR-mediated, protein synthesis-dependent mechanisms of memory consolidation and anterograde memory formation. To understand molecular mechanisms of this facilitatory effect, we tested the impact of local pharmacological activation of different D1/D5 dopamine receptor signalling modes in the auditory cortex. To this end, protein patterns in soluble and synaptic protein-enriched fractions from cortical, hippocampal and striatal brain regions of ligand- and vehicle-treated gerbils were analysed by 2D gel electrophoresis and mass spectrometry 24 h after intervention. RESULTS: After auditory-cortical injection of SKF38393 - a D1/D5 dopamine receptor-selective agonist reported to activate the downstream effectors adenylyl cyclase and phospholipase C - prominent proteomic alterations compared to vehicle-treated controls appeared in the auditory cortex, striatum, and hippocampus, whereas only minor changes were detectable in the frontal cortex. In contrast, auditory-cortical injection of SKF83959 - a D1/D5 agonist reported to preferentially stimulate phospholipase C - induced pronounced changes in the frontal cortex. At the molecular level, we detected altered regulation of cytoskeletal and scaffolding proteins, changes in proteins with functions in energy metabolism, local protein synthesis, and synaptic signalling. Interestingly, abundance and/or subcellular localisation of the predominantly presynaptic protein α-synuclein displayed dopaminergic regulation. To assess the role of α-synuclein for dopaminergic mechanisms of memory modulation, we tested the impact of post-conditioning systemic pharmacological activation of different D1/D5 dopamine receptor signalling modes on auditory discrimination learning in α-synuclein-mutant mice. In C57BL/6JOlaHsd mice, bearing a spontaneous deletion of the α-synuclein-encoding gene, but not in the related substrains C57BL/6JCrl and C57BL/6JRccHsd, adenylyl cyclase-mediated signalling affected acquisition rates over future learning episodes, whereas phospholipase C-mediated signalling affected final memory performance. CONCLUSIONS: Dopamine signalling modes via D1/D5 receptors in the auditory cortex differentially impact protein profiles related to rearrangement of cytomatrices, energy metabolism, and synaptic neurotransmission in cortical, hippocampal, and basal brain structures. Altered dopamine neurotransmission in α-synuclein-deficient mice revealed that distinct D1/D5 receptor signalling modes may control different aspects of memory consolidation.

Dopamine modulates memory consolidation of discrimination learning in the auditory cortex.

In Mongolian gerbils, the auditory cortex is critical for discriminating rising vs. falling frequency-modulated tones. Based on our previous studies, we hypothesized that dopaminergic inputs to the auditory cortex during and shortly after acquisition of the discrimination strategy control long-term memory formation. To test this hypothesis, we studied frequency-modulated tone discrimination learning of gerbils in a shuttle box GO/NO-GO procedure following differential treatments. (i) Pre-exposure of gerbils to the frequency-modulated tones at 1 day before the first discrimination training session severely impaired the accuracy of the discrimination acquired in that session during the initial trials of a second training session, performed 1 day later. (ii) Local injection of the D1/D5 dopamine receptor antagonist SCH-23390 into the auditory cortex after task acquisition caused a discrimination deficit of similar extent and time course as with pre-exposure. This effect was dependent on the dose and time point of injection. (iii) Injection of the D1/D5 dopamine receptor agonist SKF-38393 into the auditory cortex after retraining caused a further discrimination improvement at the beginning of subsequent sessions. All three treatments, which supposedly interfered with dopamine signalling during conditioning and/or retraining, had a substantial impact on the dynamics of the discrimination performance particularly at the beginning of subsequent training sessions. These findings suggest that auditory-cortical dopamine activity after acquisition of a discrimination of complex sounds and after retrieval of weak frequency-modulated tone discrimination memory further improves memory consolidation, i.e. the correct association of two sounds with their respective GO/NO-GO meaning, in support of future memory recall.

Pregabalin improves axon regeneration and motor outcome in a rodent stroke model.

Ischaemic stroke remains a leading cause of death and disability worldwide. Surviving neurons in the peri-infarct area are able to establish novel axonal projections to juxtalesional regions, but this regeneration is curtailed by a growth-inhibitory environment induced by cells such as reactive astrocytes in the glial scar. Here, we found that the astroglial synaptogenic cue thrombospondin-1 is upregulated in the peri-infarct area, and hence tested the effects of the anticonvulsant pregabalin, a blocker of the neuronal thrombospondin-1 receptor Alpha2delta1/2, in a mouse model of cortical stroke. Studying axonal projections after cortical stroke in mice by three-dimensional imaging of cleared whole-brain preparations, we found that pregabalin, when administered systemically for 5 weeks after stroke, augments novel peri-infarct motor cortex projections and improves skilled forelimb motor function. Thus, the promotion of axon elongation across the glial scar by pregabalin represents a promising target beyond the acute phase after stroke to improve structural and functional recovery.

Slc1a3-2A-CreERT2 mice reveal unique features of Bergmann glia and augment a growing collection of Cre drivers and effectors in the 129S4 genetic background.

Genetic variation is a primary determinant of phenotypic diversity. In laboratory mice, genetic variation can be a serious experimental confounder, and thus minimized through inbreeding. However, generalizations of results obtained with inbred strains must be made with caution, especially when working with complex phenotypes and disease models. Here we compared behavioral characteristics of C57Bl/6-the strain most widely used in biomedical research-with those of 129S4. In contrast to 129S4, C57Bl/6 demonstrated high within-strain and intra-litter behavioral hyperactivity. Although high consistency would be advantageous, the majority of disease models and transgenic tools are in C57Bl/6. We recently established six Cre driver lines and two Cre effector lines in 129S4. To augment this collection, we genetically engineered a Cre line to study astrocytes in 129S4. It was validated with two Cre effector lines: calcium indicator gCaMP5g-tdTomato and RiboTag-a tool widely used to study cell type-specific translatomes. These reporters are in different genomic loci, and in both the Cre was functional and astrocyte-specific. We found that calcium signals lasted longer and had a higher amplitude in cortical compared to hippocampal astrocytes, genes linked to a single neurodegenerative disease have highly divergent expression patterns, and that ribosome proteins are non-uniformly expressed across brain regions and cell types.

Inhibition of Stat3-mediated astrogliosis ameliorates pathology in an Alzheimer's disease model.

Reactive astrogliosis is a hallmark of Alzheimer's disease (AD), but its role for disease initiation and progression has remained incompletely understood. We here show that the transcription factor Stat3 (signal transducer and activator of transcription 3), a canonical inducer of astrogliosis, is activated in an AD mouse model and human AD Therefore, using a conditional knockout approach, we deleted Stat3 specifically in astrocytes in the APP/PS1 model of AD We found that Stat3-deficient APP/PS1 mice show decreased ß-amyloid levels and plaque burden. Plaque-close microglia displayed a more complex morphology, internalized more ß-amyloid, and upregulated amyloid clearance pathways in Stat3-deficient mice. Moreover, astrocyte-specific Stat3-deficient APP/PS1 mice showed decreased pro-inflammatory cytokine activation and lower dystrophic neurite burden, and were largely protected from cerebral network imbalance. Finally, Stat3 deletion in astrocytes also strongly ameliorated spatial learning and memory decline in APP/PS1 mice. Importantly, these protective effects on network dysfunction and cognition were recapitulated in APP/PS1 mice systemically treated with a preclinical Stat3 inhibitor drug. In summary, our data implicate Stat3-mediated astrogliosis as an important therapeutic target in AD.

P2Y1 receptor blockade normalizes network dysfunction and cognition in an Alzheimer's disease model.

Astrocytic hyperactivity is an important contributor to neuronal-glial network dysfunction in Alzheimer's disease (AD). We have previously shown that astrocyte hyperactivity is mediated by signaling through the P2Y1 purinoreceptor (P2Y1R) pathway. Using the APPPS1 mouse model of AD, we here find that chronic intracerebroventricular infusion of P2Y1R inhibitors normalizes astroglial and neuronal network dysfunction, as measured by in vivo two-photon microscopy, augments structural synaptic integrity, and preserves hippocampal long-term potentiation. These effects occur independently from ß-amyloid metabolism or plaque burden but are associated with a higher morphological complexity of periplaque reactive astrocytes, as well as reduced dystrophic neurite burden and greater plaque compaction. Importantly, APPPS1 mice chronically treated with P2Y1R antagonists, as well as APPPS1 mice carrying an astrocyte-specific genetic deletion (Ip3r2-/-) of signaling pathways downstream of P2Y1R activation, are protected from the decline of spatial learning and memory. In summary, our study establishes the restoration of network homoeostasis by P2Y1R inhibition as a novel treatment target in AD.

α-Synuclein: the long distance runner.

Parkinson's disease is characterized by α-synuclein pathology in the form of Lewy bodies and Lewy neurites. Braak et al described the spatial and temporal spread of α-synuclein pathology in Parkinson's disease. Recent experimental studies have demonstrated that α-synuclein can transfer from cell to cell. In this review, we highlight the involvement of α-synuclein in Parkinson's disease and in Braak's staging of Parkinson's disease pathology. We discuss whether a prion-like mechanism of α-synuclein spread might contribute to Parkinson's disease pathology. We describe recent studies investigating cell-to-cell transfer of α-synuclein and focus our review on the long-distance axonal transport of α-synuclein along neurons.

Cordon-bleu is an actin nucleation factor and controls neuronal morphology.

Despite the wealth of different actin structures formed, only two actin nucleation factors are well established in vertebrates: the Arp2/3 complex and formins. Here, we describe a further nucleator, cordon-bleu (Cobl). Cobl is a brain-enriched protein using three Wiskott-Aldrich syndrome protein homology 2 (WH2) domains for actin binding. Cobl promotes nonbundled, unbranched filaments. Filament formation relies on barbed-end growth and requires all three Cobl WH2 domains and the extended linker L2. We suggest that the nucleation power of Cobl is based on the assembly of three actin monomers in cross-filament orientation. Cobl localizes to sites of high actin dynamics and modulates cell morphology. In neurons, induction of both neurites and neurite branching is dramatically increased by Cobl expression-effects that critically depend on Cobl's actin nucleation ability. Correspondingly, Cobl depletion results in decreased dendritic arborization. Thus, Cobl is an actin nucleator controlling neuronal morphology and development.