Astrocytes form large gap junctional networks that contribute to ion and neurotransmitter homeostasis. Astrocytes concentrate in the lateral superior olive (LSO), a prominent auditory brainstem center. Compared to the LSO, astrocyte density is lower in the region dorsal to the LSO (dLSO) and in the internuclear space between the LSO, the superior paraolivary nucleus (SPN). We questioned whether astrocyte networks exhibit certain properties that reflect the precise neuronal arrangement. Employing whole-cell patch-clamp and concomitant injection of a gap junction-permeable tracer, we analyzed size and orientation of astrocyte networks in LSO, dLSO, and SPN-LSO in acute brainstem slices of mice at postnatal days 10-20. The majority of LSO networks exhibited an oval topography oriented orthogonally to the tonotopic axis, whereas dLSO networks showed no preferred orientation. This correlated with the overall astrocyte morphology in both regions, i.e. LSO astrocyte processes were oriented mainly orthogonally to the tonotopic axis. To assess the spread of small ions within LSO networks, we analyzed the diffusion of Na(+) signals between cells using Na(+) imaging. We found that Na(+) not only diffused between SR101(+) astrocytes, but also from astrocytes into SR101(-) cells. Using PLP-GFP mice for tracing, we could show that LSO networks contained astrocytes and oligodendrocytes. Together, our results demonstrate that LSO astrocytes and LSO oligodendrocytes form functional anisotropic panglial networks that are oriented predominantly orthogonally to the tonotopic axis. Thus, our results point toward an anisotropic ion and metabolite diffusion and a limited glial crosstalk between neighboring isofrequency bands in the LSO. GLIA 2016;64:1892-1911.

Increasing evidence suggests that synaptic functions of the amyloid precursor protein (APP), which is key to Alzheimer pathogenesis, may be carried out by its secreted ectodomain (APPs). The specific roles of APPsα and APPsβ fragments, generated by non-amyloidogenic or amyloidogenic APP processing, respectively, remain however unclear. Here, we expressed APPsα or APPsβ in the adult brain of conditional double knockout mice (cDKO) lacking APP and the related APLP2. APPsα efficiently rescued deficits in spine density, synaptic plasticity (LTP and PPF), and spatial reference memory of cDKO mice. In contrast, APPsβ failed to show any detectable effects on synaptic plasticity and spine density. The C-terminal 16 amino acids of APPsα (lacking in APPsβ) proved sufficient to facilitate LTP in a mechanism that depends on functional nicotinic α7-nAChRs. Further, APPsα showed high-affinity, allosteric potentiation of heterologously expressed α7-nAChRs in oocytes. Collectively, we identified α7-nAChRs as a crucial physiological receptor specific for APPsα and show distinct roles for APPsα versus APPsβ. This implies that reduced levels of APPsα that might occur during Alzheimer pathogenesis cannot be compensated by APPsβ.

The physiological role of the amyloid-precursor protein (APP) is insufficiently understood.Recent work has implicated APP in the regulation of synaptic plasticity. Substantial evidence exists for a role of APP and its secreted ectodomain APPsα in Hebbian plasticity. Here, we addressed the relevance of APP in homeostatic synaptic plasticity using organotypic tissue cultures prepared from APP -/mice of both sexes. In the absence of APP, dentate granule cells failed to strengthen their excitatory synapses homeostatically. Homeostatic plasticity is rescued by amyloid-(A and not by APPsα, and it is neither observed in APP +/+ tissue treated with -or -secretase inhibitors nor in synaptopodin-deficient cultures lacking the Ca 2+ -dependent molecular machinery of the spine apparatus. Together, these results suggest a role of APP processing via the amyloidogenic pathway in homeostatic synaptic plasticity, representing a function of relevance for brain physiology as well as for brain states associated with increased A levels. SIGNIFICANCE STATEMENTConsiderable effort has been directed to better understand the pathogenic role of the Amyloid Precursor Protein (APP) and its cleavage products in neurodegeneration -with a major focus on the accumulation and deposition of "synaptotoxic" Aβ peptides, which are produced by sequential cleavage of APP by β-and γ-secretases. Although the amyloidogenic APP processing pathway has recently been targeted in patients with Alzheimer's Diseases, the physiological role of APP/Aβ remains unclear, which limits our understanding how such interventions could influence brain functions in health and disease. Here, we report an essential role of Aβ (and not APPsα) in homeostatic synaptic plasticity, suggesting that this could be a major physiological function of Aβ in the healthy brain.

The physiological role of the amyloid-precursor protein (APP) is insufficiently understood. 29 Recent work has implicated APP in the regulation of synaptic plasticity. Substantial evidence 30 exists for a role of APP and its secreted ectodomain APPsα in Hebbian plasticity. Here, we 31 addressed the relevance of APP in homeostatic synaptic plasticity using organotypic tissue 32 cultures of APP -/mice. In the absence of APP, dentate granule cells failed to strengthen their 33 excitatory synapses homeostatically. Homeostatic plasticity is rescued by amyloid- (A and 34 not by APPsα, and it is neither observed in APP +/+ tissue treated with or -secretase 35 inhibitors nor in synaptopodin-deficient cultures lacking the Ca 2+ -dependent molecular 36 machinery of the spine apparatus. Together, these results suggest a role of APP processing via 37 the amyloidogenic pathway in homeostatic synaptic plasticity, representing a function of 38 relevance for brain physiology as well as for brain states associated with increased A levels. 39Hebbian plasticity, homeostatic plasticity 41 82 and report an essential role of Aβ in homeostatic plasticity of excitatory neurotransmission, 83 suggesting that this could be one of the major physiological functions of Aβ in the normal 84 brain. 85 5 RESULTSHomeostatic synaptic plasticity is not observed in dentate granule cells of APP-deficient 86 entorhinal-hippocampal tissue cultures 87 Considering the role of the hippocampal formation and specifically the dentate gyrus in 88 memory formation (Aimone et al., 2011; Friedman and Goldman-Rakic, 1988), 3-week-old 89 (18 days in vitro; div) organotypic tissue cultures containing the entorhinal cortex and the 90 hippocampus were prepared from APP +/+ and APP -/mice-including age-and time-matched 91 APP +/+ littermates obtained from APP +/intercrossing ( Figure 1A, B). Tissue cultures were 92 treated with tetrodotoxin (TTX; 2 µM; 2 days) to induce homeostatic synaptic plasticity, and 93 α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated 94 miniature excitatory postsynaptic currents (mEPSCs) were recorded from individual dentate 95 granule cells (Figure 1C) to assess compensatory (i.e., homeostatic) synaptic changes. 96 In line with previous work [e.g., (Echegoyen et al., 2007; Kim and Tsien, 2008; Strehl et 97 al., 2018; Turrigiano et al., 1998; Vlachos et al., 2013)], a homeostatic increase in excitatory 98 synaptic strength (i.e., a robust increase in mEPSC amplitudes) was observed in the wild-type 99 tissue cultures (Figure 1D, E). In APP -/preparations, no significant changes in mEPSC 100properties were observed in dentate granule cells ( Figure 1F, G). Specifically, mean mEPSC 101 amplitude was 11.5 ± 0.3 pA in vehicle-only-treated and 11.8 ± 0.4 pA TTX-treated APP -/-102 dentate granule cells (p = 0.4; Mann-Whitney-test). 103In an attempt to rescue the ability of granule cells to express homeostatic synaptic 104 plasticity, APP -/tissue cultures were transfected with a bicistronic adeno-associated ...

Alzheimer's disease (AD) is histopathologically characterized by Ab plaques and the accumulation of hyperphosphorylated Tau species, the latter also constituting key hallmarks of primary tauopathies. Whereas Ab is produced by amyloidogenic APP processing, APP processing along the competing nonamyloidogenic pathway results in the secretion of neurotrophic and synaptotrophic APPsa. Recently, we demonstrated that APPsa has therapeutic effects in transgenic AD model mice and rescues Ab-dependent impairments.Here, we examined the potential of APPsa to mitigate Tau-induced synaptic deficits in P301S mice (both sexes), a widely used mouse model of tauopathy. Analysis of synaptic plasticity revealed an aberrantly increased LTP in P301S mice that could be normalized by acute application of nanomolar amounts of APPsa to hippocampal slices, indicating a homeostatic function of APPsa on a rapid time scale. Further, AAV-mediated in vivo expression of APPsa restored normal spine density of CA1 neurons even at stages of advanced Tau pathology not only in P301S mice, but also in independent THY-Tau22 mice. Strikingly, when searching for the mechanism underlying aberrantly increased LTP in P301S mice, we identified an early and progressive loss of major GABAergic interneuron subtypes in the hippocampus of P301S mice, which may lead to reduced GABAergic inhibition of principal cells. Interneuron loss was paralleled by deficits in nest building, an innate behavior highly sensitive to hippocampal impairments. Together, our findings indicate that APPsa has therapeutic potential for Tau-mediated synaptic dysfunction and suggest that loss of interneurons leads to disturbed neuronal circuits that compromise synaptic plasticity as well as behavior.