Control of hippocampal synaptic plasticity by microglia-dendrite interactions depends on genetic context in mouse models of Alzheimer's disease.

INTRODUCTION: Human data suggest susceptibility and resilience to features of Alzheimer's disease (AD) such as microglia activation and synaptic dysfunction are under genetic control. However, causal relationships between these processes, and how genomic diversity modulates them remain systemically underexplored in mouse models. METHODS: AD-vulnerable hippocampal neurons were virally labeled in inbred (C57BL/6J) and wild-derived (PWK/PhJ) APP/PS1 and wild-type mice, and brain microglia depleted from 4 to 8 months of age. Dendrites were assessed for synapse plasticity changes by evaluating spine densities and morphologies. RESULTS: In C57BL/6J, microglia depletion blocked amyloid-induced synaptic density and morphology changes. At a finer scale, synaptic morphology on individual branches was dependent on microglia-dendrite physical interactions. Conversely, synapses from PWK/PhJ mice showed remarkable stability in response to amyloid, and no evidence of microglia contact-dependent changes on dendrites. DISCUSSION: These results demonstrate that microglia-dependent synaptic alterations in specific AD-vulnerable projection pathways are differentially controlled by genetic context.

Genetic context controls early microglia-synaptic interactions in mouse models of Alzheimer's disease.

Common features of Alzheimer's disease (AD) include amyloid pathology, microglia activation and synaptic dysfunction, however, the causal relationships amongst them remains unclear. Further, human data suggest susceptibility and resilience to AD neuropathology is controlled by genetic context, a factor underexplored in mouse models. To this end, we leveraged viral strategies to label an AD-vulnerable neuronal circuit in CA1 dendrites projecting to the frontal cortex in genetically diverse C57BL/6J (B6) and PWK/PhJ (PWK) APP/PS1 mouse strains and used PLX5622 to non-invasively deplete brain microglia. Reconstructions of labeled neurons revealed microglia-dependent changes in dendritic spine density and morphology in B6 wild-type (WT) and APP/PS1 yet a marked stability of spines across PWK mice. We further showed that synaptic changes depend on direct microglia-dendrite interactions in B6. APP/PS1 but not PWK. APP/PS1 mice. Collectively, these results demonstrate that microglia-dependent synaptic alterations in a specific AD-vulnerable projection pathway are differentially controlled by genetic context.

Hippocampal and thalamic afferents form distinct synaptic microcircuits in the mouse infralimbic frontal cortex.

The selection of goal-directed behaviors is supported by neural circuits located within the frontal cortex. Frontal cortical afferents arise from multiple brain areas, yet the cell-type-specific targeting of these inputs is unclear. Here, we use monosynaptic retrograde rabies mapping to examine the distribution of afferent neurons targeting distinct classes of local inhibitory interneurons and excitatory projection neurons in mouse infralimbic frontal cortex. Interneurons expressing parvalbumin, somatostatin, or vasoactive intestinal peptide receive a large proportion of inputs from the hippocampus, while interneurons expressing neuron-derived neurotrophic factor receive a large proportion of inputs from thalamic regions. A similar dichotomy is present among the four different excitatory projection neurons. These results show a prominent bias among long-range hippocampal and thalamic afferent systems in their targeting to specific sets of frontal cortical neurons. Moreover, they suggest the presence of two distinct local microcircuits that control how different inputs govern frontal cortical information processing.

Region-specific inhibition of 14-3-3 proteins induces psychomotor behaviors in mice.

The 14-3-3 family of proteins is genetically linked to several psychiatric disorders, including schizophrenia. Our 14-3-3 functional knockout (FKO) mice, as well as other 14-3-3 knockout models, have been shown to exhibit behavioral endophenotypes related to schizophrenia. While specific forebrain regions, such as the prefrontal cortex (PFC) and hippocampus (HP), have been implicated in schizophrenic pathophysiology, the role of these brain regions in the top-down control of specific schizophrenia-associated behaviors has not been examined. Here, we used an adeno-associated virus (AAV) delivered shRNA to knock down the expression of the 14-3-3-inhibitor transgene, thus selectively restoring the function of 14-3-3 in the forebrain of the 14-3-3 FKO mice, we found that injection of the AAV-shRNA into both the PFC and the HP is necessary to attenuate psychomotor activity of the 14-3-3 FKO mice. Furthermore, we found that acute inhibition of 14-3-3, through the delivery of an AAV expressing the 14-3-3 inhibitor to both the PFC and HP, can trigger psychomotor agitation. Interestingly, when assessing the two brain regions separately, we determined that AAV-mediated expression of the 14-3-3 inhibitor specifically within the HP alone is sufficient to induce several behavioral deficits including hyperactivity, impaired associative learning and memory, and reduced sensorimotor gating. In addition, we show that post-synaptic NMDA receptor levels are regulated by acute 14-3-3 manipulations. Taken together, findings from this study directly link 14-3-3 inhibition in specific forebrain regions to certain schizophrenia-associated endophenotypes.

Inhibition of 14-3-3 Proteins Leads to Schizophrenia-Related Behavioral Phenotypes and Synaptic Defects in Mice.

BACKGROUND: The 14-3-3 family of proteins is implicated in the regulation of several key neuronal processes. Previous human and animal studies suggested an association between 14-3-3 dysregulation and schizophrenia. METHODS: We characterized behavioral and functional changes in transgenic mice that express an isoform-independent 14-3-3 inhibitor peptide in the brain. RESULTS: We recently showed that 14-3-3 functional knockout mice (FKO) exhibit impairments in associative learning and memory. We report here that these 14-3-3 FKO mice display other behavioral deficits that correspond to the core symptoms of schizophrenia. These behavioral deficits may be attributed to alterations in multiple neurotransmission systems in the 14-3-3 FKO mice. In particular, inhibition of 14-3-3 proteins results in a reduction of dendritic complexity and spine density in forebrain excitatory neurons, which may underlie the altered synaptic connectivity in the prefrontal cortical synapse of the 14-3-3 FKO mice. At the molecular level, this dendritic spine defect may stem from dysregulated actin dynamics secondary to a disruption of the 14-3-3-dependent regulation of phosphorylated cofilin. CONCLUSIONS: Collectively, our data provide a link between 14-3-3 dysfunction, synaptic alterations, and schizophrenia-associated behavioral deficits.

14-3-3 proteins are required for hippocampal long-term potentiation and associative learning and memory.

14-3-3 is a family of regulatory proteins highly expressed in the brain. Previous invertebrate studies have demonstrated the importance of 14-3-3 in the regulation of synaptic functions and learning and memory. However, the in vivo role of 14-3-3 in these processes has not been determined using mammalian animal models. Here, we report the behavioral and electrophysiological characterization of a new animal model of 14-3-3 proteins. These transgenic mice, considered to be a 14-3-3 functional knock-out, express a known 14-3-3 inhibitor in various brain regions of different founder lines. We identify a founder-specific impairment in hippocampal-dependent learning and memory tasks, as well as a correlated suppression in long-term synaptic plasticity of the hippocampal synapses. Moreover, hippocampal synaptic NMDA receptor levels are selectively reduced in the transgenic founder line that exhibits both behavioral and synaptic plasticity deficits. Collectively, our findings provide evidence that 14-3-3 is a positive regulator of associative learning and memory at both the behavioral and cellular level.

14-3-3 protein targets misfolded chaperone-associated proteins to aggresomes.

The aggresome is a key cytoplasmic organelle for sequestration and clearance of toxic protein aggregates. Although loading misfolded proteins cargos to dynein motors has been recognized as an important step in the aggresome formation process, the molecular machinery that mediates the association of cargos with the dynein motor is poorly understood. Here, we report a new aggresome-targeting pathway that involves isoforms of 14-3-3, a family of conserved regulatory proteins. 14-3-3 interacts with both the dynein-intermediate chain (DIC) and an Hsp70 co-chaperone Bcl-2-associated athanogene 3 (BAG3), thereby recruiting chaperone-associated protein cargos to dynein motors for their transport to aggresomes. This molecular cascade entails functional dimerization of 14-3-3, which we show to be crucial for the formation of aggresomes in both yeast and mammalian cells. These results suggest that 14-3-3 functions as a molecular adaptor to promote aggresomal targeting of misfolded protein aggregates and may link such complexes to inclusion bodies observed in various neurodegenerative diseases.