Early alterations in the MCH system link aberrant neuronal activity and sleep disturbances in a mouse model of Alzheimer's disease.

Early Alzheimer's disease (AD) is associated with hippocampal hyperactivity and decreased sleep quality. Here we show that homeostatic mechanisms transiently counteract the increased excitatory drive to CA1 neurons in AppNL-G-F mice, but that this mechanism fails in older mice. Spatial transcriptomics analysis identifies Pmch as part of the adaptive response in AppNL-G-F mice. Pmch encodes melanin-concentrating hormone (MCH), which is produced in sleep-active lateral hypothalamic neurons that project to CA1 and modulate memory. We show that MCH downregulates synaptic transmission, modulates firing rate homeostasis in hippocampal neurons and reverses the increased excitatory drive to CA1 neurons in AppNL-G-F mice. AppNL-G-F mice spend less time in rapid eye movement (REM) sleep. AppNL-G-F mice and individuals with AD show progressive changes in morphology of CA1-projecting MCH axons. Our findings identify the MCH system as vulnerable in early AD and suggest that impaired MCH-system function contributes to aberrant excitatory drive and sleep defects, which can compromise hippocampus-dependent functions.

Human iPSC-derived astrocytes transplanted into the mouse brain undergo morphological changes in response to amyloid-ß plaques.

BACKGROUND: Increasing evidence for a direct contribution of astrocytes to neuroinflammatory and neurodegenerative processes causing Alzheimer's disease comes from molecular and functional studies in rodent models. However, these models may not fully recapitulate human disease as human and rodent astrocytes differ considerably in morphology, functionality, and gene expression. RESULTS: To address these challenges, we established an approach to study human astrocytes within the mouse brain by transplanting human induced pluripotent stem cell (hiPSC)-derived astrocyte progenitors into neonatal brains. Xenografted hiPSC-derived astrocyte progenitors differentiated into astrocytes that integrated functionally within the mouse host brain and matured in a cell-autonomous way retaining human-specific morphologies, unique features, and physiological properties. In Alzheimer´s chimeric brains, transplanted hiPSC-derived astrocytes responded to the presence of amyloid plaques undergoing morphological changes that seemed independent of the APOE allelic background. CONCLUSIONS: In sum, we describe here a promising approach that consist of transplanting patient-derived and genetically modified astrocytes into the mouse brain to study human astrocyte pathophysiology in the context of Alzheimer´s disease.

Loss of Bin1 Promotes the Propagation of Tau Pathology.

Tau pathology propagates within synaptically connected neuronal circuits, but the underlying mechanisms are unclear. BIN1-amphiphysin2 is the second most prevalent genetic risk factor for late-onset Alzheimer's disease. In diseased brains, the BIN1-amphiphysin2 neuronal isoform is downregulated. Here, we show that lowering BIN1-amphiphysin2 levels in neurons promotes Tau pathology propagation whereas overexpression of neuronal BIN1-amphiphysin2 inhibits the process in two in vitro models. Increased Tau propagation is caused by increased endocytosis, given our finding that BIN1-amphiphysin2 negatively regulates endocytic flux. Furthermore, blocking endocytosis by inhibiting dynamin also reduces Tau pathology propagation. Using a galectin-3-binding assay, we show that internalized Tau aggregates damage the endosomal membrane, allowing internalized aggregates to leak into the cytoplasm to propagate pathology. Our work indicates that lower BIN1 levels promote the propagation of Tau pathology by efficiently increasing aggregate internalization by endocytosis and endosomal trafficking.

The Dynamics and Turnover of Tau Aggregates in Cultured Cells: INSIGHTS INTO THERAPIES FOR TAUOPATHIES.

Filamentous tau aggregates, the hallmark lesions of Alzheimer disease (AD), play key roles in neurodegeneration. Activation of protein degradation systems has been proposed to be a potential strategy for removing pathological tau, but it remains unclear how effectively tau aggregates can be degraded by these systems. By applying our previously established cellular model system of AD-like tau aggregate induction using preformed tau fibrils, we demonstrate that tau aggregates induced in cells with regulated expression of full-length mutant tau can be gradually cleared when soluble tau expression is suppressed. This clearance is at least partially mediated by the autophagy-lysosome pathway, although both the ubiquitin-proteasome system and the autophagy-lysosome pathway are deficient in handling large tau aggregates. Importantly, residual tau aggregates left after the clearance phase leads to a rapid reinstatement of robust tau pathology once soluble tau expression is turned on again. Moreover, we succeeded in generating monoclonal cells persistently carrying tau aggregates without obvious cytotoxicity. Live imaging of GFP-tagged tau aggregates showed that tau inclusions are dynamic structures constantly undergoing "fission" and "fusion," which facilitate stable propagation of tau pathology in dividing cells. These findings provide a greater understanding of cell-to-cell transmission of tau aggregates in dividing cells and possibly neurons.

Synaptic Contacts Enhance Cell-to-Cell Tau Pathology Propagation.

Accumulation of insoluble Tau protein aggregates and stereotypical propagation of Tau pathology through the brain are common hallmarks of tauopathies, including Alzheimer's disease (AD). Propagation of Tau pathology appears to occur along connected neurons, but whether synaptic contacts between neurons are facilitating propagation has not been demonstrated. Using quantitative in vitro models, we demonstrate that, in parallel to non-synaptic mechanisms, synapses, but not merely the close distance between the cells, enhance the propagation of Tau pathology between acceptor hippocampal neurons and Tau donor cells. Similarly, in an artificial neuronal network using microfluidic devices, synapses and synaptic activity are promoting neuronal Tau pathology propagation in parallel to the non-synaptic mechanisms. Our work indicates that the physical presence of synaptic contacts between neurons facilitate Tau pathology propagation. These findings can have implications for synaptic repair therapies, which may turn out to have adverse effects by promoting propagation of Tau pathology.

LRRK2 functions in synaptic vesicle endocytosis through a kinase-dependent mechanism.

Mutations in leucine-rich repeat kinase 2 (LRRK2) are associated with Parkinson's disease, but the precise physiological function of the protein remains ill-defined. Recently, our group proposed a model in which LRRK2 kinase activity is part of an EndoA phosphorylation cycle that facilitates efficient vesicle formation at synapses in the Drosophila melanogaster neuromuscular junctions.Flies harbor only one Lrrk gene, which might encompass the functions of both mammalian LRRK1 and LRRK2. We therefore studied the role of LRRK2 in mammalian synaptic function and provide evidence that knockout or pharmacological inhibition of LRRK2 results in defects in synaptic vesicle endocytosis, altered synaptic morphology and impairments in neurotransmission. In addition, our data indicate that mammalian endophilin A1 (EndoA1,also known as SH3GL2) is phosphorylated by LRRK2 in vitro at T73 and S75, two residues in the BAR domain. Hence, our results indicate that LRRK2 kinase activity has an important role in the regulation of clathrin-mediated endocytosis of synaptic vesicles and subsequent neurotransmission at the synapse.