Elevated expression of complement C4 in the mouse prefrontal cortex causes schizophrenia-associated phenotypes.

Accumulating evidence supports immune involvement in the pathogenesis of schizophrenia, a severe psychiatric disorder. In particular, high expression variants of C4, a gene of the innate immune complement system, were shown to confer susceptibility to schizophrenia. However, how elevated C4 expression may impact brain circuits remains largely unknown. We used in utero electroporation to overexpress C4 in the mouse prefrontal cortex. We found reduced glutamatergic input to pyramidal cells of juvenile and adult, but not of newborn C4-overexpressing (C4-OE) mice, together with decreased spine density, which mirrors spine loss observed in the schizophrenic cortex. Using time-lapse two-photon imaging in vivo, we observed that these deficits were associated with decreased dendritic spine gain and elimination in juvenile C4-OE mice, which may reflect poor formation and/or stabilization of immature spines. In juvenile and adult C4-OE mice, we found evidence for NMDA receptor hypofunction, another schizophrenia-associated phenotype, and synaptic accumulation of calcium-permeable AMPA receptors. Alterations in cortical GABAergic networks have been repeatedly associated with schizophrenia. We found that functional GABAergic transmission was reduced in C4-OE mice, in line with diminished GABA release probability from parvalbumin interneurons, lower GAD67 expression, and decreased intrinsic excitability in parvalbumin interneurons. These cellular abnormalities were associated with working memory impairment. Our results substantiate the causal relationship between an immunogenetic risk factor and several distinct cortical endophenotypes of schizophrenia and shed light on the underlying cellular mechanisms.

Early defects in translation elongation factor 1α levels at excitatory synapses in α-synucleinopathy.

Cognitive decline and dementia in neurodegenerative diseases are associated with synapse dysfunction and loss, which may precede neuron loss by several years. While misfolded and aggregated α-synuclein is recognized in the disease progression of synucleinopathies, the nature of glutamatergic synapse dysfunction and loss remains incompletely understood. Using fluorescence-activated synaptosome sorting (FASS), we enriched excitatory glutamatergic synaptosomes from mice overexpressing human alpha-synuclein (h-αS) and wild-type littermates to unprecedented purity. Subsequent label-free proteomic quantification revealed a set of proteins differentially expressed upon human alpha-synuclein overexpression. These include overrepresented proteins involved in the synaptic vesicle cycle, ER-Golgi trafficking, metabolism and cytoskeleton. Unexpectedly, we found and validated a steep reduction of eukaryotic translation elongation factor 1 alpha (eEF1A1) levels in excitatory synapses at early stages of h-αS mouse model pathology. While eEF1A1 reduction correlated with the loss of postsynapses, its immunoreactivity was found on both sides of excitatory synapses. Moreover, we observed a reduction in eEF1A1 immunoreactivity in the cingulate gyrus neuropil of patients with Lewy body disease along with a reduction in PSD95 levels. Altogether, our results suggest a link between structural impairments underlying cognitive decline in neurodegenerative disorders and local synaptic defects. eEF1A1 may therefore represent a limiting factor to synapse maintenance.

In vivo Ca2+ imaging of astrocytic microdomains reveals a critical role of the amyloid precursor protein for mitochondria.

The investigation of amyloid precursor protein (APP) has been mainly confined to its neuronal functions, whereas very little is known about its physiological role in astrocytes. Astrocytes exhibit a particular morphology with slender extensions protruding from somata and primary branches. Along these fine extensions, spontaneous calcium transients occur in spatially restricted microdomains. Within these microdomains mitochondria are responsible for local energy supply and Ca2+ buffering. Using two-photon in vivo Ca2+ imaging, we report a significant decrease in the density of active microdomains, frequency of spontaneous Ca2+ transients and slower Ca2+ kinetics in mice lacking APP. Mechanistically, these changes could be potentially linked to mitochondrial malfunction as our in vivo and in vitro data revealed severe, APP-dependent structural mitochondrial fragmentation in astrocytes. Functionally, such mitochondria exhibited prolonged kinetics and morphology dependent signal size of ATP-induced Ca2+ transients. Our results highlight a prominent role of APP in the modulation of Ca2+ activity in astrocytic microdomains whose precise functioning is crucial for the reinforcement and modulation of synaptic function. This study provides novel insights in APP physiological functions which are important for the understanding of the effects of drugs validated in Alzheimer's disease treatment that affect the function of APP.

TrpV1 receptor activation rescues neuronal function and network gamma oscillations from Aß-induced impairment in mouse hippocampus in vitro.

Amyloid-ß peptide (Aß) forms plaques in Alzheimer's disease (AD) and is responsible for early cognitive deficits in AD patients. Advancing cognitive decline is accompanied by progressive impairment of cognition-relevant EEG patterns such as gamma oscillations. The endocannabinoid anandamide, a TrpV1-receptor agonist, reverses hippocampal damage and memory impairment in rodents and protects neurons from Aß-induced cytotoxic effects. Here, we investigate a restorative role of TrpV1-receptor activation against Aß-induced degradation of hippocampal neuron function and gamma oscillations. We found that the TrpV1-receptor agonist capsaicin rescues Aß-induced degradation of hippocampal gamma oscillations by reversing both the desynchronization of AP firing in CA3 pyramidal cells and the shift in excitatory/inhibitory current balance. This rescue effect is TrpV1-receptor-dependent since it was absent in TrpV1 knockout mice or in the presence of the TrpV1-receptor antagonist capsazepine. Our findings provide novel insight into the network mechanisms underlying cognitive decline in AD and suggest TrpV1 activation as a novel therapeutic target.

Tcf4 regulates dendritic spine density and morphology in the adult brain.

Tcf4 is a transcription factor which regulates neurogenesis and neuronal migration in the brain. In humans, loss of function of Tcf4 leads to the rare neurodevelopmental disorder Pitt-Hopkins syndrome, which is characterized by intellectual disability, developmental delay and autistic behavior. We analyzed the consequences of functional loss of Tcf4 on dendritic spines in mature principal neurons. To this end, we crossed mice in which the DNA-binding domain of the Tcf4 gene is flanked by LoxP sites to mice expressing tamoxifen-inducible cre recombinase in a sparse subset of fluorescently labelled neurons (SlickV line). This resulted in a mouse model with an inducible functional knockout of Tcf4 in a subset of cortical and hippocampal neurons, in which we analyzed dendritic spines, which are the morphological correlate of excitatory postsynapses. Heterozygous as well as homozygous loss of Tcf4 led to a reduction in the number of dendritic spines in the cortex as well as in the hippocampus. This was accompanied by morphological changes of dendritic spines. These results suggest that Tcf4 is involved in synaptic plasticity in mature neurons, and functional loss of Tcf4 may contribute to the neurological symptoms in Pitt-Hopkins syndrome.

Beta-Site Amyloid Precursor Protein Cleaving Enzyme 1 Inhibition Impairs Synaptic Plasticity via Seizure Protein 6.

BACKGROUND: Beta-site amyloid precursor protein cleaving enzyme 1 (BACE1) is a promising drug target for the treatment of Alzheimer's disease. Prolonged BACE1 inhibition interferes with structural and functional synaptic plasticity in mice, most likely by altering the metabolism of BACE1 substrates. Seizure protein 6 (SEZ6) is predominantly cleaved by BACE1, and Sez6 knockout mice share some phenotypes with BACE1 inhibitor-treated mice. We investigated whether SEZ6 is involved in BACE1 inhibition-induced structural and functional synaptic alterations. METHODS: The function of NB-360, a novel blood-brain barrier penetrant and orally available BACE1 inhibitor, was verified by immunoblotting. In vivo microscopy was applied to monitor the impact of long-term pharmacological BACE1 inhibition on dendritic spines in the cerebral cortex of constitutive and conditional Sez6 knockout mice. Finally, synaptic functions were characterized using electrophysiological field recordings in hippocampal slices. RESULTS: BACE1 enzymatic activity was strongly suppressed by NB-360. Prolonged NB-360 treatment caused a reversible spine density reduction in wild-type mice, but it did not affect Sez6-/- mice. Knocking out Sez6 in a small subset of mature neurons also prevented the structural postsynaptic changes induced by BACE1 inhibition. Hippocampal long-term potentiation was decreased in both chronic BACE1 inhibitor-treated wild-type mice and vehicle-treated Sez6-/- mice. However, chronic NB-360 treatment did not alter long-term potentiation in CA1 neurons of Sez6-/- mice. CONCLUSIONS: Our results suggest that SEZ6 plays an important role in maintaining normal dendritic spine dynamics. Furthermore, SEZ6 is involved in BACE1 inhibition-induced structural and functional synaptic alterations.

Dementia-related Bri2 BRICHOS is a versatile molecular chaperone that efficiently inhibits Aß42 toxicity in Drosophila.

Formation of fibrils of the amyloid-ß peptide (Aß) is suggested to play a central role in neurodegeneration in Alzheimer's disease (AD), for which no effective treatment exists. The BRICHOS domain is a part of several disease-related proproteins, the most studied ones being Bri2 associated with familial dementia and prosurfactant protein C (proSP-C) associated with lung amyloid. BRICHOS from proSP-C has been found to be an efficient inhibitor of Aß aggregation and toxicity, but its lung-specific expression makes it unsuited to target in AD. Bri2 is expressed in the brain, affects processing of Aß precursor protein, and increased levels of Bri2 are found in AD brain, but the specific role of its BRICHOS domain has not been studied in vivo Here, we find that transgenic expression of the Bri2 BRICHOS domain in the Drosophila central nervous system (CNS) or eyes efficiently inhibits Aß42 toxicity. In the presence of Bri2 BRICHOS, Aß42 is diffusely distributed throughout the mushroom bodies, a brain region involved in learning and memory, whereas Aß42 expressed alone or together with proSP-C BRICHOS forms punctuate deposits outside the mushroom bodies. Recombinant Bri2 BRICHOS domain efficiently prevents Aß42-induced reduction in γ-oscillations in hippocampal slices. Finally, Bri2 BRICHOS inhibits several steps in the Aß42 fibrillation pathway and prevents aggregation of heat-denatured proteins, indicating that it is a more versatile chaperone than proSP-C BRICHOS. These findings suggest that Bri2 BRICHOS can be a physiologically relevant chaperone for Aß in the CNS and needs to be further investigated for its potential in AD treatment.

Amyloid precursor protein maintains constitutive and adaptive plasticity of dendritic spines in adult brain by regulating D-serine homeostasis.

Dynamic synapses facilitate activity-dependent remodeling of neural circuits, thereby providing the structural substrate for adaptive behaviors. However, the mechanisms governing dynamic synapses in adult brain are still largely unknown. Here, we demonstrate that in the cortex of adult amyloid precursor protein knockout (APP-KO) mice, spine formation and elimination were both reduced while overall spine density remained unaltered. When housed under environmental enrichment, APP-KO mice failed to respond with an increase in spine density. Spine morphology was also altered in the absence of APP The underlying mechanism of these spine abnormalities in APP-KO mice was ascribed to an impairment in D-serine homeostasis. Extracellular D-serine concentration was significantly reduced in APP-KO mice, coupled with an increase of total D-serine. Strikingly, chronic treatment with exogenous D-serine normalized D-serine homeostasis and restored the deficits of spine dynamics, adaptive plasticity, and morphology in APP-KO mice. The cognitive deficit observed in APP-KO mice was also rescued by D-serine treatment. These data suggest that APP regulates homeostasis of D-serine, thereby maintaining the constitutive and adaptive plasticity of dendritic spines in adult brain.