Evidence for altered dendritic spine compartmentalization in Alzheimer's disease and functional effects in a mouse model.

Alzheimer's disease (AD) is associated with a progressive loss of synapses and neurons. Studies in animal models indicate that morphological alterations of dendritic spines precede synapse loss, increasing the proportion of large and short ("stubby") spines. Whether similar alterations occur in human patients, and what their functional consequences could be, is not known. We analyzed biopsies from AD patients and APP x presenilin 1 knock-in mice that were previously shown to present a loss of pyramidal neurons in the CA1 area of the hippocampus. We observed that the proportion of stubby spines and the width of spine necks are inversely correlated with synapse density in frontal cortical biopsies from non-AD and AD patients. In mice, the reduction in the density of synapses in the stratum radiatum was preceded by an alteration of spine morphology, with a reduction of their length and an enlargement of their neck. Serial sectioning examined with electron microscopy allowed us to precisely measure spine parameters. Mathematical modeling indicated that the shortening and widening of the necks should alter the electrical compartmentalization of the spines, leading to reduced postsynaptic potentials in spine heads, but not in soma. Accordingly, there was no alteration in basal synaptic transmission, but long-term potentiation and spatial memory were impaired. These results indicate that an alteration of spine morphology could be involved in the early cognitive deficits associated with AD.

Golgi localization of SARS-CoV-2 spike protein and interaction with furin in cerebral COVID-19 microangiopathy: a clue to the central nervous system involvement?

In a neuropathological series of 20 COVID-19 cases, we analyzed six cases (three biopsies and three autopsies) with multiple foci predominantly affecting the white matter as shown by MRI. The cases presented with microhemorrhages evocative of small artery diseases. This COVID-19 associated cerebral microangiopathy (CCM) was characterized by perivascular changes: arterioles were surrounded by vacuolized tissue, clustered macrophages, large axonal swellings and a crown arrangement of aquaporin-4 immunoreactivity. There was evidence of blood-brain-barrier leakage. Fibrinoid necrosis, vascular occlusion, perivascular cuffing and demyelination were absent. While no viral particle or viral RNA was found in the brain, the SARS-CoV-2 spike protein was detected in the Golgi apparatus of brain endothelial cells where it closely associated with furin, a host protease known to play a key role in virus replication. Endothelial cells in culture were not permissive to SARS-CoV-2 replication. The distribution of the spike protein in brain endothelial cells differed from that observed in pneumocytes. In the latter, the diffuse cytoplasmic labeling suggested a complete replication cycle with viral release, notably through the lysosomal pathway. In contrast, in cerebral endothelial cells the excretion cycle was blocked in the Golgi apparatus. Interruption of the excretion cycle could explain the difficulty of SARS-CoV-2 to infect endothelial cells in vitro and to produce viral RNA in the brain. Specific metabolism of the virus in brain endothelial cells could weaken the cell walls and eventually lead to the characteristic lesions of COVID-19 associated cerebral microangiopathy. Furin as a modulator of vascular permeability could provide some clues for the control of late effects of microangiopathy.

Alterations of Neuronal Lysosomes in Alzheimer's Disease and in APPxPS1-KI Mice.

BACKGROUND: The cellular and molecular alterations associated with synapse and neuron loss in Alzheimer's disease (AD) remain unclear. In transgenic mouse models that express mutations responsible for familial AD, neuronal and synaptic losses occur in populations that accumulate fibrillar amyloid-ß 42 (Aß42) intracellularly. OBJECTIVE: We aimed to study the subcellular localization of these fibrillar accumulations and whether such intraneuronal assemblies could be observed in the human pathology. METHODS: We used immunolabeling and various electron microscopy techniques on APP x presenilin1 - knock-in mice and on human cortical biopsies and postmortem samples. RESULTS: We found an accumulation of Aß fibrils in lipofuscin granule-like organelles in APP x presenilin1 - knock-in mice. Electron microscopy of human cortical biopsies also showed an accumulation of undigested material in enlarged lipofuscin granules in neurons from AD compared to age-matched non-AD patients. However, in those biopsies or in postmortem samples we could not detect intraneuronal accumulations of Aß fibrils, neither in the lipofuscin granules nor in other intraneuronal compartments. CONCLUSION: The intralysosomal accumulation of Aß fibrils in specific neuronal populations in APPxPS1-KI mice likely results from a high concentration of Aß42 in the endosome-lysosome system due to the high expression of the transgene in these neurons.

Zebrafish as a preclinical model for Extracellular Vesicle-based therapeutic development.

Extracellular Vesicles (EVs) are released during various pathophysiological processes and reflect the state of their cell of origin. Once released, they can propagate through biological fluids, target cells, deliver their content and elicit functional responses. These specific features would allow their harnessing as biomarkers, drug nano-vehicles and therapeutic intrinsic modulators. However, the further development of their potential therapeutic application is hampered by the lack of knowledge about how EVs behave in vivo. Recent advances in the field of imaging EVs in vivo now allow live-tracking of endogenous and exogenous EV in various model organisms at high spatiotemporal resolution to define their distribution, half-life and fate. This review highlights current imaging tools available to image EVs in vivo and how live imaging especially in the zebrafish embryo can bring further insights into the characterization of EVs dynamics, biodistribution and functions to potentiate their development for therapeutic applications.

Activity-induced MEMRI cannot detect functional brain anomalies in the APPxPS1-Ki mouse model of Alzheimer's disease.

Alzheimer's disease (AD) is the most common cause of dementia. Aside neuropathological lesions, abnormal neuronal activity and brain metabolism are part of the core symptoms of the disease. Activity-induced Manganese-Enhanced Magnetic Resonance Imaging (MEMRI) has been proposed as a powerful approach to visualize evoked brain activity in rodents. Here, we evaluated the relevance of MEMRI in measuring neuronal (dys-)function in the APPxPS1 knocked-in (KI) mouse model of AD. Brain anomalies were firstly demonstrated in APPxPS1-Ki mice using cognitive testing (memory impairment) and histological mapping of immediate early gene products (decreased density of fos-positive neurons). Paradoxically, MEMRI analyses were not able to confirm the occurrence of neuronal hypoactivities in vivo. We then performed a neuropathological analysis that highlighted an abnormal increased permeability of the blood-brain barrier (BBB) in APPxPS1-Ki mice. We hypothesized that diffuse weakening of the BBB results in an uncontrolled diffusion of the MR contrast agent and a lack of correlation between manganese accumulation and neuronal activity. These results bring to light a limitation of the activity-induced MEMRI approach when applied to the APPxPS1-Ki mouse model as well as other mouse models harboring a compromised BBB.