Background Disruption of β-amyloid (Aβ) homeostasis is the initial culprit in Alzheimer’s disease (AD) pathogenesis. Astrocytes respond to emerging Aβ plaques by altering their phenotype and function, yet molecular mechanisms governing astrocytic response and their precise role in countering Aβ deposition remain ill-defined. Peroxiredoxin (PRDX) 6 is an enzymatic protein with independent glutathione peroxidase (Gpx) and phospholipase A2 (PLA2) activities involved in repair of oxidatively damaged cell membrane lipids and cellular signaling. In the CNS, PRDX6 is uniquely expressed by astrocytes and its exact function remains unexplored. Methods APPswe/PS1dE9 AD transgenic mice were once crossed to mice overexpressing wild-type Prdx6 allele or to Prdx6 knock out mice. Aβ pathology and associated neuritic degeneration were assessed in mice aged 10 months. Laser scanning confocal microscopy was used to characterize Aβ plaque morphology and activation of plaque-associated astrocytes and microglia. Effect of Prdx6 gene dose on plaque seeding was assessed in mice aged six months. Results We show that hemizygous knock in of the overexpressing Prdx6 transgene in APPswe/PS1dE9 AD transgenic mice promotes selective enticement of astrocytes to Aβ plaques and penetration of plaques by astrocytic processes along with increased number and phagocytic activation of periplaque microglia. This effects suppression of nascent plaque seeding and remodeling of mature plaques consequently curtailing brain Aβ load and Aβ-associated neuritic degeneration. Conversely, Prdx6 haplodeficiency attenuates astro- and microglia activation around Aβ plaques promoting Aβ deposition and neuritic degeneration. Conclusions We identify here PRDX6 as an important factor regulating response of astrocytes toward Aβ plaques. Demonstration that phagocytic activation of periplaque microglia vary directly with astrocytic PRDX6 expression level implies previously unappreciated astrocyte-guided microglia effect in Aβ proteostasis. Our showing that upregulation of PRDX6 attenuates Aβ pathology may be of therapeutic relevance for AD.

Prion diseases or prionoses are a group of rapidly progressing and invariably fatal neurodegenerative diseases. The pathogenesis of prionoses is associated with self-replication and connectomal spread of PrPSc, a disease specific conformer of the prion protein. Microglia undergo activation early in the course of prion pathogenesis and exert opposing roles in PrPSc mediated neurodegeneration. While clearance of PrPSc and apoptotic neurons have disease-limiting effect, microglia-driven neuroinflammation bears deleterious consequences to neuronal networks. Apolipoprotein (apo) E is a lipid transporting protein with pleiotropic functions, which include controlling of the phagocytic and inflammatory characteristics of activated microglia in neurodegenerative diseases. Despite the significance of microglia in prion pathogenesis, the role of apoE in prionoses has not been established. We showed here that infection of wild type mice with 22L mouse adapted scrapie strain is associated with significant increase in the total brain apoE protein and mRNA levels and also with a conspicuous cell-type shift in the apoE expression. There is reduced expression of apoE in activated astrocytes and marked upregulation of apoE expression by activated microglia. We also showed apoE ablation exaggerates PrPSc mediated neurodegeneration. Apoe−/− mice have shorter disease incubation period, increased load of spongiform lesion, pronounced neuronal loss, and exaggerated astro and microgliosis. Astrocytes of Apoe−/− mice display salient upregulation of transcriptomic markers defining A1 neurotoxic astrocytes while microglia show upregulation of transcriptomic markers characteristic for microglial neurodegenerative phenotype. There is impaired clearance of PrPSc and dying neurons by microglia in Apoe−/− mice along with increased level of proinflammatory cytokines. Our work indicates that apoE absence renders clearance of PrPSc and dying neurons by microglia inefficient, while the excess of neuronal debris promotes microglial neurodegenerative phenotype aggravating the vicious cycle of neuronal death and neuroinflammation.

The APOE gene is diversified by three alleles ε2, ε3, and ε4 encoding corresponding apolipoprotein (apo) E isoforms. Possession of the ε4 allele is signified by increased risks of age-related cognitive decline, Alzheimer’s disease (AD), and the rate of AD dementia progression. ApoE is secreted by astrocytes as high-density lipoprotein-like particles and these are internalized by neurons upon binding to neuron-expressed apoE receptors. ApoE isoforms differentially engage neuronal plasticity through poorly understood mechanisms. We examined here the effects of native apoE lipoproteins produced by immortalized astrocytes homozygous for ε2, ε3, and ε4 alleles on the maturation and the transcriptomic profile of primary hippocampal neurons. Control neurons were grown in the presence of conditioned media from Apoe–/– astrocytes. ApoE2 and apoE3 significantly increase the dendritic arbor branching, the combined neurite length, and the total arbor surface of the hippocampal neurons, while apoE4 fails to produce similar effects and even significantly reduces the combined neurite length compared to the control. ApoE lipoproteins show no systemic effect on dendritic spine density, yet apoE2 and apoE3 increase the mature spines fraction, while apoE4 increases the immature spine fraction. This is associated with opposing effects of apoE2 or apoE3 and apoE4 on the expression of NR1 NMDA receptor subunit and PSD95. There are 1,062 genes differentially expressed across neurons cultured in the presence of apoE lipoproteins compared to the control. KEGG enrichment and gene ontology analyses show apoE2 and apoE3 commonly activate expression of genes involved in neurite branching, and synaptic signaling. In contrast, apoE4 cultured neurons show upregulation of genes related to the glycolipid metabolism, which are involved in dendritic spine turnover, and those which are usually silent in neurons and are related to cell cycle and DNA repair. In conclusion, our work reveals that lipoprotein particles comprised of various apoE isoforms differentially regulate various neuronal arbor characteristics through interaction with neuronal transcriptome. ApoE4 produces a functionally distinct transcriptomic profile, which is associated with attenuated neuronal development. Differential regulation of neuronal transcriptome by apoE isoforms is a newly identified biological mechanism, which has both implication in the development and aging of the CNS.

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BackgroundBoth Alzheimer’s disease and prionoses are neurogenerative disorders, which involve misfolded protein deposition and transition of homeostatic microglia to their neurodegenerative phenotype. In prionoses, accumulation of toxic PrPSc protein constitutes the culprit of pathogenesis. Neurodegenerative phenotype microglia (MGnD) appear early in the course of disease and play opposing roles in PrPSc mediated neurodegeneration. Whilst clearance of PrPSc has a disease‐limiting effect, MGnD‐driven neuroinflammation is deleterious to neurons. Apolipoprotein (apo) E is a pleiotropic protein, which controls MGnD phagocytic and inflammatory characteristics. Despite prominent role of microglia in prionoses, involvement of apoE in their pathogenesis has not been established.MethodWild type (WT) and Apoe‐/‐ mice, both on the B6 background, were inoculated with 22L mouse adapted scrapie strain or normal brain homogenate. Neuropathological and biochemical analyses were performed 15 and 23 weeks post inoculation (WPI) on asymptomatic and overly symptomatic animals, respectively. NanoStringTM analysis of microglia and astrocytic transcriptomic markers was performed at 23WPI.ResultPrion infection of WT mice upregulates total brain apoE level and effects cell‐type shift in the apoE expression, downregulating astrocytic apoE while increasing microglial apoE 24‐folds. 22L Apoe‐/‐ mice have significantly shorter disease incubation period, higher load of spongiform changes, and increased total brain PrP and PrPSc levels compared to 22LWT mice. In 22L Apoe ‐/‐ mice astro and micro gliosis appears earlier and is more robust compared to 22LWT mice. Transcriptomic analysis shows significantly higher level of PAN and A1, but not A2 astrocytic markers and MGnD markers in 22L Apoe ‐/‐ mice compared to 22LWT animals. Microglia in 22L Apoe ‐/‐ mice feature impaired neuronal phagocytosis and produce increased level of pro‐inflammatory interleukins and cytokines. 22L Apoe ‐/‐ mice show higher neuronal loss and greater tempo of neuronal degeneration than 22LWT mice.ConclusionOur results indicate the net effect of apoE in prion pathogenesis is protective. ApoE facilitates clearance of PrPSc and dying neurons by microglia, which becomes ineffective in the absence of apoE. Remnants of disintegrating neurons promote MGnD phenotype transition and inflammatory cascade propagating the vicious cycle of neuronal death and neuroinflammation. Our data also indicate homeostatic microglia transit to the MGnD even if they do not express apoE.