Microglial motility is modulated by neuronal activity and correlates with dendritic spine plasticity in the hippocampus of awake mice.

Microglia, the resident immune cells of the brain, play a complex role in health and disease. They actively survey the brain parenchyma by physically interacting with other cells and structurally shaping the brain. Yet, the mechanisms underlying microglial motility and significance for synapse stability, especially in the hippocampus during adulthood, remain widely unresolved. Here, we investigated the effect of neuronal activity on microglial motility and the implications for the formation and survival of dendritic spines on hippocampal CA1 neurons in vivo. We used repetitive two-photon in vivo imaging in the hippocampus of awake and anesthetized mice to simultaneously study the motility of microglia and their interaction with dendritic spines. We found that CA3 to CA1 input is sufficient to modulate microglial process motility. Simultaneously, more dendritic spines emerged in mice after awake compared to anesthetized imaging. Interestingly, the rate of microglial contacts with individual dendritic spines and dendrites was associated with the stability, removal, and emergence of dendritic spines. These results suggest that microglia might sense neuronal activity via neurotransmitter release and actively participate in synaptic rewiring of the hippocampal neural network during adulthood. Further, this study has profound relevance for hippocampal learning and memory processes.

Hippocampal hyperactivity in a rat model of Alzheimer's disease.

Neuronal network dysfunction is a hallmark of Alzheimer's disease (AD). However, the underlying pathomechanisms remain unknown. We analyzed the hippocampal micronetwork in transgenic McGill-R-Thy1-APP rats (APPtg) at the beginning of extracellular amyloid beta (Aß) deposition. We established two-photon Ca2+ -imaging in vivo in the hippocampus of rats and found hyperactivity of CA1 neurons. Patch-clamp recordings in brain slices in vitro revealed increased neuronal input resistance and prolonged action potential width in CA1 pyramidal neurons. We did neither observe changes in synaptic inhibition, nor in excitation. Our data support the view that increased intrinsic excitability of CA1 neurons may precede inhibitory dysfunction at an early stage of Aß-deposition and disease progression.

Chronic 2P-STED imaging reveals high turnover of dendritic spines in the hippocampus in vivo.

Rewiring neural circuits by the formation and elimination of synapses is thought to be a key cellular mechanism of learning and memory in the mammalian brain. Dendritic spines are the postsynaptic structural component of excitatory synapses, and their experience-dependent plasticity has been extensively studied in mouse superficial cortex using two-photon microscopy in vivo. By contrast, very little is known about spine plasticity in the hippocampus, which is the archetypical memory center of the brain, mostly because it is difficult to visualize dendritic spines in this deeply embedded structure with sufficient spatial resolution. We developed chronic 2P-STED microscopy in mouse hippocampus, using a 'hippocampal window' based on resection of cortical tissue and a long working distance objective for optical access. We observed a two-fold higher spine density than previous studies and measured a spine turnover of ~40% within 4 days, which depended on spine size. We thus provide direct evidence for a high level of structural rewiring of synaptic circuits and new insights into the structure-dynamics relationship of hippocampal spines. Having established chronic super-resolution microscopy in the hippocampus in vivo, our study enables longitudinal and correlative analyses of nanoscale neuroanatomical structures with genetic, molecular and behavioral experiments.

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.

Fibrillar amyloid plaque formation precedes microglial activation.

In Alzheimer's disease (AD), hallmark ß-amyloid deposits are characterized by the presence of activated microglia around them. Despite an extensive characterization of the relation of amyloid plaques with microglia, little is known about the initiation of this interaction. In this study, the detailed investigation of very small plaques in brain slices in AD transgenic mice of the line APP-PS1(dE9) revealed different levels of microglia recruitment. Analysing plaques with a diameter of up to 10 µm we find that only the half are associated with clear morphologically activated microglia. Utilizing in vivo imaging of new appearing amyloid plaques in double-transgenic APP-PS1(dE9)xCX3CR1+/- mice further characterized the dynamic of morphological microglia activation. We observed no correlation of morphological microglia activation and plaque volume or plaque lifetime. Taken together, our results demonstrate a very prominent variation in size as well as in lifetime of new plaques relative to the state of microglia reaction. These observations might question the existing view that amyloid deposits by themselves are sufficient to attract and activate microglia in vivo.

Longitudinal assessment of cerebral ß-amyloid deposition in mice overexpressing Swedish mutant ß-amyloid precursor protein using 18F-florbetaben PET.

UNLABELLED: The progression of ß-amyloid deposition in the brains of mice overexpressing Swedish mutant ß-amyloid precursor protein (APP-Swe), a model of Alzheimer disease (AD), was investigated in a longitudinal PET study using the novel ß-amyloid tracer (18)F-florbetaben. METHODS: Groups of APP-Swe and age-matched wild-type (WT) mice (age range, 10-20 mo) were investigated. Dynamic emission recordings were acquired with a small-animal PET scanner during 90 min after the administration of (18)F-florbetaben (9 MBq, intravenously). After spatial normalization of individual PET recordings to common coordinates for mouse brain, binding potentials (BPND) and standardized uptake value ratios (SUVRs) were calculated relative to the cerebellum. Voxelwise analyses were performed using statistical parametric mapping (SPM). Histochemical analyses and ex vivo autoradiography were ultimately performed in a subset of animals as a gold standard assessment of ß-amyloid plaque load. RESULTS: SUVRs calculated from static recordings during the interval of 30-60 min after tracer injection correlated highly with estimates of BPND based on the entire dynamic emission recordings. (18)F-florbetaben binding did not significantly differ in APP-Swe mice and WT animals at 10 and 13 mo of age. At 16 mo of age, the APP-Swe mice had a significant 7.9% increase (P < 0.01) in cortical (18)F-florbetaben uptake above baseline and at 20 mo there was a 16.6% increase (P < 0.001), whereas WT mice did not show any temporal changes in tracer uptake during the interval of follow-up. Voxelwise SPM analyses revealed the first signs of increased cortical binding at 13 mo and confirmed progressive binding increases in both the frontal and the temporal cortices (P < 0.001 uncorrected) to 20 mo. The SUVR strongly correlated with percentage plaque load (R = 0.95, P < 0.001). CONCLUSION: In the first longitudinal PET study in an AD mouse model using the novel ß-amyloid tracer (18)F-florbetaben, the temporal and spatial progression of amyloidogenesis in the brain of APP-Swe mice were sensitively monitored. This method should afford the means for preclinical testing of novel therapeutic approaches to the treatment of AD.