Microglia modulation through external vagus nerve stimulation in a murine model of Alzheimer's disease

Chronically activated microglia contribute to the development of neurodegenerative diseases such as Alzheimer's disease (AD ) by the release of pro‐inflammatory mediators that compromise neuronal function and structure. Modulating microglia functions could be instrumental to interfere with disease pathogenesis. Previous studies have shown anti‐inflammatory effects of acetylcholine (AC h) or norepinephrine (NE ), which mainly activates the β‐receptors on microglial cells. Non‐invasive vagus nerve stimulation (nVNS ) is used in treatment of drug‐resistant depression, which is a risk factor for developing AD . The vagus nerve projects to the brainstem's locus coeruleus from which noradrenergic fibers reach to the Nucleus Basalis of Meynert (NBM ) and widely throughout the brain. Pilot studies showed first signs of cognitive‐enhancing effects of nVNS in AD patients. In this study, the effects of nVNS on mouse microglia cell morphology were analyzed over a period of 280 min by 2‐photon laser scanning in vivo microscopy. Total branch length, average branch order and number of branches, which are commonly used indicators for the microglial activation state were determined and compared between young and old wild‐type and amyloid precursor protein/presenilin‐1 (APP/PS1) transgenic mice. Overall, these experiments show strong morphological changes in microglia, from a neurodestructive to a neuroprotective phenotype, following a brief nVNS in aged animals, especially in APP/PS 1 animals, whereas microglia from young animals were morphologically unaffected.

     

Cholesterol as a causative factor in Alzheimer's disease: a debatable hypothesis

High serum/plasma cholesterol levels have been suggested as a risk factor for Alzheimer's disease (AD). Some reports, mostly retrospective epidemiological studies, have observed a decreased prevalence of AD in patients taking the cholesterol lowering drugs, statins. The strongest evidence causally linking cholesterol to AD is provided by experimental studies showing that adding/reducing cholesterol alters amyloid precursor protein (APP) and amyloid beta‐protein (Aβ) levels. However, there are problems with the cholesterol‐AD hypothesis. Cholesterol levels in serum/plasma and brain of AD patients do not support cholesterol as a causative factor in AD. Prospective studies on statins and AD have largely failed to show efficacy. Even the experimental data are open to interpretation given that it is well‐established that modification of cholesterol levels has effects on multiple proteins, not only amyloid precursor protein and Aβ. The purpose of this review, therefore, was to examine the above‐mentioned issues, discuss the pros and cons of the cholesterol‐AD hypothesis, involvement of other lipids in the mevalonate pathway, and consider that AD may impact cholesterol homeostasis.

     

A distinct subfraction of Aβ is responsible for the high‐affinity Pittsburgh compound B‐binding site in Alzheimer's disease brain

The positron emission tomography (PET) ligand ¹¹C‐labeled Pittsburgh compound B (PIB) is used to image β‐amyloid (Aβ) deposits in the brains of living subjects with the intent of detecting early stages of Alzheimer's disease (AD). However, deposits of human‐sequence Aβ in amyloid precursor protein transgenic mice and non‐human primates bind very little PIB. The high stoichiometry of PIB:Aβ binding in human AD suggests that the PIB‐binding site may represent a particularly pathogenic entity and/or report local pathologic conditions. In this study, ³H‐PIB was employed to track purification of the PIB‐binding site in > 90% yield from frontal cortical tissue of autopsy‐diagnosed AD subjects. The purified PIB‐binding site comprises a distinct, highly insoluble subfraction of the Aβ in AD brain with low buoyant density because of the sodium dodecyl sulfate‐resistant association with a limited subset of brain proteins and lipids with physical properties similar to lipid rafts and to a ganglioside:Aβ complex in AD and Down syndrome brain. Both the protein and lipid components are required for PIB binding. Elucidation of human‐specific biological components and pathways will be important in guiding improvement of the animal models for AD and in identifying new potential therapeutic avenues.

     

Nucleic acid oxidation: an early feature of Alzheimer's disease

Studies of oxidative damage during the progression of Alzheimer's disease (AD) suggest its central role in disease pathogenesis. To investigate levels of nucleic acid oxidation in both early and late stages of AD, levels of multiple base adducts were quantified in nuclear and mitochondrial DNA from the superior and middle temporal gyri (SMTG), inferior parietal lobule (IPL), and cerebellum (CER) of age‐matched normal control subjects, subjects with mild cognitive impairment, preclinical AD, late‐stage AD, and non‐AD neurological disorders (diseased control; DC) using gas chromatography/mass spectrometry. Median levels of multiple DNA adducts in nuclear and mitochondrial DNA were significantly (p ≤ 0.05) elevated in the SMTG, IPL, and CER in multiple stages of AD and in DC subjects. Elevated levels of fapyguanine and fapyadenine in mitochondrial DNA suggest a hypoxic environment early in the progression of AD and in DC subjects. Overall, these data suggest that oxidative damage is an early event not only in the pathogenesis of AD but is also present in neurodegenerative diseases in general.

     

Wnt signaling loss accelerates the appearance of neuropathological hallmarks of Alzheimer's disease in J20‐APP transgenic and wild‐type mice

Alzheimer's disease (AD ) is a neurodegenerative pathology characterized by aggregates of amyloid‐β (Aβ) and phosphorylated tau protein, synaptic dysfunction, and spatial memory impairment. The Wnt signaling pathway has several key functions in the adult brain and has been associated with AD , mainly as a neuroprotective factor against Aβ toxicity and tau phosphorylation. However, dysfunction of Wnt/β‐catenin signaling might also play a role in the onset and development of the disease. J20 APP swInd transgenic (Tg) mouse model of AD was treated i.p. with various Wnt signaling inhibitors for 10 weeks during pre‐symptomatic stages. Then, cognitive, biochemical and histochemical analyses were performed. Wnt signaling inhibitors induced severe changes in the hippocampus, including alterations in Wnt pathway components and loss of Wnt signaling function, severe cognitive deficits, increased tau phosphorylation and Aβ_1–42 peptide levels, decreased Aβ42/Aβ40 ratio and Aβ_1–42 concentration in the cerebral spinal fluid, and high levels of soluble Aβ species and synaptotoxic oligomers in the hippocampus, together with changes in the amount and size of senile plaques. More important, we also observed severe alterations in treated wild‐type (WT ) mice, including behavioral impairment, tau phosphorylation, increased Aβ_1–42 in the hippocampus, decreased Aβ_1–42 in the cerebral spinal fluid, and hippocampal dysfunction. Wnt inhibition accelerated the development of the pathology in a Tg AD mouse model and contributed to the development of Alzheimer's‐like changes in WT mice. These results indicate that Wnt signaling plays important roles in the structure and function of the adult hippocampus and suggest that inhibition of the Wnt signaling pathway is an important factor in the pathogenesis of AD .

Read the Editorial Highlight for this article on page 356 .

     

Hexokinase 2‐dependent hyperglycolysis driving microglial activation contributes to ischemic brain injury

The cytoplasmic trafficking of docosahexaenoic acid (DHA ), a cognitively beneficial fatty acid, across the blood–brain barrier (BBB ) is governed by fatty acid‐binding protein 5 (FABP 5). Lower levels of brain DHA have been observed in Alzheimer's disease (AD ), which is associated with diminished BBB expression of FABP 5. Therefore, up‐regulating FABP 5 expression at the BBB may be a novel approach for enhancing BBB transport of DHA in AD . DHA supplementation has been shown to be beneficial in various mouse models of AD , and therefore, the aim of this study was to determine whether DHA has the potential to up‐regulate the BBB expression of FABP 5, thereby enhancing its own uptake into the brain. Treating human brain microvascular brain endothelial (hCMEC /D3) cells with the maximum tolerable concentration of DHA (12.5 μM) for 72 h resulted in a 1.4‐fold increase in FABP 5 protein expression. Associated with this was increased expression of fatty acid transport proteins 1 and 4. To study the impact of dietary DHA supplementation, 6‐ to 8‐week‐old C57BL /6 mice were fed with a control diet or a DHA ‐enriched diet for 21 days. Brain microvascular FABP 5 protein expression was up‐regulated 1.7‐fold in mice fed the DHA ‐enriched diet, and this was associated with increased brain DHA levels (1.3‐fold). Despite an increase in brain DHA levels, reduced BBB transport of ¹⁴C‐DHA was observed over a 1 min perfusion, possibly as a result of competitive binding to FABP 5 between dietary DHA and ¹⁴C‐DHA . This study has demonstrated that DHA can increase BBB expression of FABP 5, as well as fatty acid transporters, overall increasing brain DHA levels.

     

Reduced blood‐brain barrier expression of fatty acid‐binding protein 5 is associated with increased vulnerability of APP/PS1 mice to cognitive deficits from low omega‐3 fatty acid diets

Lower levels of the cognitively beneficial docosahexaenoic acid (DHA) are often observed in Alzheimer's disease (AD) brains. Brain DHA levels are regulated by the blood‐brain barrier (BBB) transport of plasma‐derived DHA, a process facilitated by fatty acid‐binding protein 5 (FABP5). This study reports a 42.1 ± 12.6% decrease in the BBB transport of ¹⁴C‐DHA in 8‐month‐old AD transgenic mice (APPswe,PSEN1?E9) relative to wild‐type mice, associated with a 34.5 ± 6.7% reduction in FABP5 expression in isolated brain capillaries of AD mice. Furthermore, short‐term spatial and recognition memory deficits were observed in AD mice on a 6‐month n‐3 fatty acid‐depleted diet, but not in AD mice on control diet. This intervention led to a dramatic reduction (41.5 ± 11.9%) of brain DHA levels in AD mice. This study demonstrates FABP5 deficiency and impaired DHA transport at the BBB are associated with increased vulnerability to cognitive deficits in mice fed an n‐3 fatty acid‐depleted diet, in line with our previous studies demonstrating a crucial role of FABP5 in BBB transport of DHA and cognitive function.

     

A tale of three GTPases and a RIN in endothelial cell adhesion

Endothelial cell adhesion to the extracellular matrix regulates migration and outgrowth of blood vessels during angiogenesis. Cell adhesion is mediated by integrins, which transduce signals from the extracellular environment into the cell and, in turn, are regulated by intracellular signaling molecules. In a paper recently published in Cell Research, Sandri et al. show that RIN2 connects three GTPases, R-Ras, Rab5 and Rac1, to promote endothelial cell adhesion through the regulation of integrin internalization and Rac1 activation.The formation of the vascular tree during development requires the orderly growth of blood vessels to irrigate all organs and tissues. This process of blood vessel remodeling, termed angiogenesis, requires endothelial cell proliferation, adhesion, migration and tube formation¹. Pathological angiogenesis takes place during tumor growth as hypoxia within the tumor induces the release of pro-angiogenic mediators such as vascular endothelial growth factor (VEGF).Small GTPases are critical for the regulation of cell behavior and thus also play a central role in angiogenesis. Small GTPases are 20-25 kDa signaling proteins that cycle between an active GTP-bound and an inactive GDP-bound state. When active, GTPases associate with and activate diverse effector molecules that subsequently relay the signal to other molecules, ultimately leading to a specific cell response. Two classes of proteins facilitate GTPase cycling. Guanine exchange factors (GEFs) catalyze GDP unloading thereby promoting GTP binding and GTPase activation. Conversely, GTPase activating proteins (GAPs) enhance the intrinsic GTP hydrolysis activity of the GTPase leading to its inactivation. Small GTPases form a large superfamily with over 100 members in mammals. Based on structural and functional criteria, the GTPase superfamily is subdivided in Ras, Rab, Rho, Arf and Ran subfamilies, each of them generally, but not exclusively, specialized in the regulation of specific cellular events. For example, Rho GTPases primarily regulate cytoskeletal dynamics; Rab GTPases regulate intracellular membrane trafficking; and Ras GTPases function in the regulation of cell proliferation and survival. However, complex processes such as angiogenesis require the coordinated action of several GTPases. This is evidenced by the work of Sandri et al.² recently published in Cell Research. In their paper, Sandri et al. propose a mechanism for the regulation of endothelial cell adhesion and migration involving three GTPases belonging to different GTPase branches, R-Ras, Rab5 and Rac1. The protein RIN2 (Ras and Rab adaptor 2) brings together R-Ras and Rab5 to form a signaling module that orchestrates integrin trafficking and Rac1 activation, processes that are essential for cell adhesion and migration.Integrins are heterodimeric transmembrane extracellular matrix (ECM) receptors composed of one α and one β chain. In a process known as ''outside-in'' signaling, integrins transmit signals from the extracellular environment to intracellular adaptor and signaling molecules that regulate cell migration, survival and growth. Conversely, during ''inside-out'' signaling, integrins can be switched from an inactive to an active conformation by cytoplasmic signaling molecules leading to increased integrin affinity for the ECM.During 2D migration of adherent cells, nascent, highly dynamic focal contacts are formed at the leading edge lamellipodia where integrins mediate adhesion to the ECM. Some of these focal contacts disassemble and some mature into larger focal adhesions with a longer half-life. Failure in maintaining a dynamic assembly and disassembly of focal contacts will result in the inhibition of cell migration.Integrin-mediated adhesion can be regulated at different levels: (1) by changing integrin conformation and thus affinity for their ligand; (2) by modulating integrin avidity, i.e., by promoting integrin clustering on the plasma membrane; and (3) by changing the kinetics of integrin endocytosis and/or recycling³.The Ras GTPase R-Ras is primarily expressed in the vascular system (endothelial cells and vascular smooth muscle cells)⁴. Zhang et al.⁵ were the first to show that R-Ras is a potent regulator of cell adhesion when they reported that expression of active R-Ras was enough to induce ECM adhesion of suspension cells, whereas dominant negative R-Ras reduced adhesion of the adherent cell line CHO. Although R-Ras was shown to enhance integrin affinity⁵, this effect was not consistently observed^6,7. These contradictory findings could be explained by the fact that R-Ras may activate integrins indirectly through antagonizing H-Ras-mediated integrin inhibition⁶.Recent findings suggest that R-Ras stimulates adhesion through the regulation of integrin internalization into Rab11-positive endosomes⁸. Now, the data of Sandri et al.² support this model. The authors addressed the question on how R-Ras regulates cell adhesion of endothelial cells by performing a yeast-two-hybrid screen using constitutively-active R-Ras as bait. The screen revealed that RIN2 is a major R-Ras-interacting protein. RIN proteins (RIN1, 2 and 3) are downstream effectors of Ras GTPases that function as GEFs for Rab5⁹, a GTPase that regulates endocytosis. RIN1 was shown to mediate the stimulation of EGF receptor-mediated endocytosis by H-Ras through the activation of Rab5¹⁰. Surprisingly, Sandri et al. found that R-Ras dramatically impaired the Rab5 exchange activity of RIN2, while H-Ras had no effect. However, RIN2 was still able to specifically bind active Rab5. These data suggest that active R-Ras, RIN2 and active Rab5 form a signaling complex. Accordingly, Sandri et al. show that endogenous R-Ras, RIN2 and Rab5 are indeed found in a complex in endothelial cells. While active R-Ras and RIN2 colocalize at nascent focal contacts and on intracellular vesicles, colocalization with Rab5 takes place on endosomes. The deletion of either the Ras- or the Rab5-binding domains of RIN2 prevented the colocalization of the trio. Thus, RIN2 appears to facilitate the transport of active R-Ras to Rab5-positive endosomes. What is the functional relevance of these interactions? Sandri et al. show that silencing of endogenous RIN2 impaired the increase in adhesion induced by active R-Ras and by Rab5. A similar effect was obtained upon expression of RIN2 deletion mutants lacking Ras- or Rab5-binding domains. These data strongly suggest that the adaptor function of RIN2 in connecting R-Ras and Rab5 regulates endothelial cell adhesion to the ECM. But what is the mechanism? Previous work has shown that the pro-adhesive activity of active R-Ras is linked to its ability to regulate β1 integrin endocytosis⁸. Sandri et al. confirm these data by showing that silencing of R-Ras or RIN2 decreases the rate of endocytosis of active ECM-engaged β1 integrins. In addition, the authors set a step further as they show that the signaling complex R-Ras/RIN2/Rab5 mediates basal Rac1 GTPase activation. Rac1 regulates actin dynamics and ruffle formation at the leading edge of migrating cells and its activity is essential for cell adhesion and migration. TIAM-1-mediated activation of Rac1 on endosomes and subsequent polarized transport to the plasma membrane has been proposed as a way to restrict Rac activity to sites of membrane protrusion^11,12. In line with this model, Sandri et al. show that active R-Ras and RIN2 colocalize with Rac1 on endosomes and that the endosomal Rac GEF TIAM-1 is necessary for R-Ras- and RIN2-induced cell adhesion.Altogether, the data of Sandri et al. support a model in which, integrin-activated R-Ras recruits RIN2 to focal adhesions and induces RIN2 conversion from a Rab5 GEF to a Rab5-docking protein. Subsequently, the complex promotes the endocytosis of ECM-engaged integrins and moves to early endosomes where R-Ras activates the TIAM-1/Rac1 pathway¹³. Active Rac1 translocates to the plasma membrane where it promotes actin polymerization and formation of new focal contacts (Figure 1).Open in a separate windowFigure 1Model proposed by Sandri et al.² for the regulation of focal adhesion dynamics by R-Ras. (1) R-Ras is activated by ECM-engaged integrins, recruits RIN2 and converts it from a Rab5 GEF to a Rab5 adaptor; (2) RIN2 binding to active Rab5 mediates the endocytosis of integrins and the transport of active R-Ras to endosomes; (3) R-Ras contributes to the activation of the Rac1 GEF TIAM-1, which then activates Rac1; (4) Active Rac1 translocates to the plasma membrane and promotes actin polymerization and formation of new focal contacts.By bridging active R-Ras and Rab5, RIN2 combines two processes essential for cell adhesion: (1) focal contact dynamics through the internalization of ECM-engaged integrins; and (2) local Rac1 activation to ensure actin polymerization at lamellipodia. Similarly, RIN2 also connects H-Ras and Rab5 in the internalization of the epithelial cell-cell adhesion molecule E-cadherin¹⁴. Thus, RIN2 appears to be a universal effector of Ras-induced endocytosis of membrane receptors.Interestingly, the phenotype of a family with a homozygous mutation in RIN2 was recently described¹⁵. The affected individuals showed diverse abnormalities related to a defective connective tissue. Indeed, ultrastructural analysis of the skin showed an abnormal morphology of collagen fibrils. Collagen is a ligand for β1 integrins. Through simultaneous binding to collagen and to the intracellular cytoskeleton, integrins contribute to the assembly of the ECM by transmitting contraction forces from the cell to the ECM. It is tempting to speculate that the phenotype of the patients lacking RIN2 is due to a deficient β1 integrin function as found by Sandri et al. in their in vitro analysis. In addition, these patients bruise easily and present prolonged bleeding, which could be caused by deficient wound healing of blood vessels as a consequence of impaired R-Ras signaling.It should be noted, however, that R-Ras knockout mice have no major defects in vascular development but respond with increased angiogenesis to stress conditions such as tumor implantation⁴. On the contrary, the in vitro study by Sandri et al. suggests that R-Ras deficiency results in decreased endothelial cell migration. Further research is needed to clarify the role of R-Ras in angiogenesis. Likewise, it will be interesting to study vascular responses in RIN2-deficient mice in comparison to R-Ras knockout mice.     

p75 neurotrophin receptor interacts with and promotes BACE1 localization in endosomes aggravating amyloidogenesis

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by a progressive deposition of amyloid beta (Aβ) and dysregulation of neurotrophic signaling, causing synaptic dysfunction, loss of memory, and cell death. The expression of p75 neurotrophin receptor is elevated in the brain of AD patients, suggesting its involvement in this disease. However, the exact mechanism of its action is not yet clear. Here, we show that p75 interacts with beta‐site amyloid precursor protein cleaving enzyme‐1 (BACE1), and this interaction is enhanced in the presence of Aβ. Our results suggest that the colocalization of BACE1 and amyloid precursor protein (APP) is increased in the presence of both Aβ and p75 in cortical neurons. In addition, the localization of APP and BACE1 in early endosomes is increased in the presence of Aβ and p75. An increased phosphorylation of APP‐Thr668 and BACE1‐Ser498 by c‐Jun N‐terminal kinase (JNK) in the presence of Aβ and p75 could be responsible for this localization. In conclusion, our study proposes a potential involvement in amyloidogenesis for p75, which may represent a future therapeutic target for AD.

Cover Image for this Issue: doi. 10.1111/jnc.14163 .

     

Amyloid beta1–42 (Aβ42) up‐regulates the expression of sortilin via the p75NTR/RhoA signaling pathway

Sortilin, a Golgi sorting protein and a member of the VPS10P family, is the co‐receptor for proneurotrophins, regulates protein trafficking, targets proteins to lysosomes, and regulates low density lipoprotein metabolism. The aim of this study was to investigate the expression and regulation of sortilin in Alzheimer's disease (AD). A significantly increased level of sortilin was found in human AD brain and in the brains of 6‐month‐old swedish‐amyloid precursor protein/PS1dE9 transgenic mice. Aβ₄₂ enhanced the protein and mRNA expression levels of sortilin in a dose‐ and time‐dependent manner in SH‐SY5Y cells, but had no effect on sorLA. In addition, proBDNF also significantly increased the protein and mRNA expression of sortilin in these cells. The recombinant extracellular domain of p75^NTR (P75ECD‐FC), or the antibody against the extracellular domain of p75^NTR, blocked the up‐regulation of sortilin induced by Amyloid‐β protein (Aβ), suggesting that Aβ₄₂ increased the expression level of sortilin and mRNA in SH‐SY5Y via the p75^NTR receptor. Inhibition of ROCK, but not Jun N‐terminal kinase, suppressed constitutive and Aβ₄₂‐induced expression of sortilin. In conclusion, this study shows that sortilin expression is increased in the AD brain in human and mice and that Aβ₄₂ oligomer increases sortilin gene and protein expression through p75^NTR and RhoA signaling pathways, suggesting a potential physiological interaction of Aβ₄₂ and sortilin in Alzheimer's disease.

     

Amyloid β‐abrogated TrkA ubiquitination in PC12 cells analogous to Alzheimer's disease

Amyloid beta (Aβ) protein is the primary proteinaceous deposit found in the brains of patients with Alzheimer's disease (AD). Evidence suggests that Aβ plays a central role in the development of AD pathology. Here, we show in PC12 cells, Aβ impairs tropomyosin receptor kinase A (TrkA) ubiquitination, phosphorylation, and its association with p75^NTR, p62, and TRAF6 induced by nerve growth factor. The ubiquitination and tyrosine phosphorylation of TrkA was also found to be impaired in postmortem human AD hippocampus compared to control. Interestingly, the nitrotyrosylation of TrkA was increased in AD hippocampus and this explains why the phosphotyrosylation and ubiquitination of TrkA was impaired. In AD brain, the production of matrix metalloproteinase‐7 (MMP‐7), which cleaves proNGF, was reduced, thereby leading to the accumulation of pro‐NGF and a decrease in the level of active NGF. TrkA signaling events, including Ras/MAPK and phosphatidylinositol 3‐kinase (PI3K)/Akt pathways, are deactivated with Aβ and in the human AD hippocampus. Findings show that Aβ blocks the TrkA ubiquitination and downstream signaling similar to AD hippocampus.

     

Regulation of RAGE splicing by hnRNP A1 and Tra2β‐1 and its potential role in AD pathogenesis

The receptor for advanced glycation end products (RAGE) gene expresses two major alternative splicing isoforms, full‐length membrane‐bound RAGE (mRAGE) and secretory RAGE (esRAGE). Both isoforms play important roles in Alzheimer's disease (AD) pathogenesis, either via interaction of mRAGE with β‐amyloid peptide (Aβ) or inhibition of the mRAGE‐activated signaling pathway. In the present study, we showed that heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) and Transformer2β‐1 (Tra2β‐1) were involved in the alternative splicing of mRAGE and esRAGE. Functionally, two factors had an antagonistic effect on the regulation. Glucose deprivation induced an increased ratio of mRAGE/esRAGE via up‐regulation of hnRNP A1 and down‐regulation of Tra2β‐1. Moreover, the ratios of mRAGE/esRAGE and hnRNP A1/Tra2β‐1 were increased in peripheral blood mononuclear cells from AD patients. The results provide a molecular basis for altered splicing of mRAGE and esRAGE in AD pathogenesis.

     

Effects of ketone bodies in Alzheimer's disease in relation to neural hypometabolism, β‐amyloid toxicity,and astrocyte function

Diet supplementation with ketone bodies (acetoacetate and β‐hydroxybuturate) or medium‐length fatty acids generating ketone bodies has consistently been found to cause modest improvement of mental function in Alzheimer's patients. It was suggested that the therapeutic effect might be more pronounced if treatment was begun at a pre‐clinical stage of the disease instead of well after its manifestation. The pre‐clinical stage is characterized by decade‐long glucose hypometabolism in brain, but ketone body metabolism is intact even initially after disease manifestation. One reason for the impaired glucose metabolism may be early destruction of the noradrenergic brain stem nucleus, locus coeruleus, which stimulates glucose metabolism, at least in astrocytes. These glial cells are essential in Alzheimer pathogenesis. The β‐amyloid peptide Aβ interferes with their cholinergic innervation, which impairs synaptic function because of diminished astrocytic glutamate release. Aβ also reduces glucose metabolism and causes hyperexcitability. Ketone bodies are similarly used against seizures, but the effectively used concentrations are so high that they must interfere with glucose metabolism and de novo synthesis of neurotransmitter glutamate, reducing neuronal glutamatergic signaling. The lower ketone body concentrations used in Alzheimer's disease may owe their effect to support of energy metabolism, but might also inhibit release of gliotransmitter glutamate.

     

Pre‐synaptic C‐terminal truncated tau is released from cortical synapses in Alzheimer's disease

The microtubule‐associated protein tau has primarily been associated with axonal location and function; however, recent work shows tau release from neurons and suggests an important role for tau in synaptic plasticity. In our study, we measured synaptic levels of total tau using synaptosomes prepared from cryopreserved human postmortem Alzheimer's disease (AD) and control samples. Flow cytometry data show that a majority of synaptic terminals are highly immunolabeled with the total tau antibody (HT7) in both AD and control samples. Immunoblots of synaptosomal fractions reveal increases in a 20 kDa tau fragment and in tau dimers in AD synapses, and terminal‐specific antibodies show that in many synaptosome samples tau lacks a C‐terminus. Flow cytometry experiments to quantify the extent of C‐terminal truncation reveal that only 15–25% of synaptosomes are positive for intact C‐terminal tau. Potassium‐induced depolarization demonstrates release of tau and tau fragments from pre‐synaptic terminals, with increased release from AD compared to control samples. This study indicates that tau is normally highly localized to synaptic terminals in cortex where it is well‐positioned to affect synaptic plasticity. Tau cleavage may facilitate tau aggregation as well as tau secretion and propagation of tau pathology from the pre‐synaptic compartment in AD.