Tau Protein Mediates APP Intracellular Domain (AICD)-Induced Alzheimer's-Like Pathological Features in Mice.

Amyloid precursor protein (APP) is cleaved by gamma-secretase to simultaneously generate amyloid beta (Aß) and APP Intracellular Domain (AICD) peptides. Aß plays a pivotal role in Alzheimer's disease (AD) pathogenesis but recent studies suggest that amyloid-independent mechanisms also contribute to the disease. We previously showed that AICD transgenic mice (AICD-Tg) exhibit AD-like features such as tau pathology, aberrant neuronal activity, memory deficits and neurodegeneration in an age-dependent manner. Since AD is a tauopathy and tau has been shown to mediate Aß-induced toxicity, we examined the role of tau in AICD-induced pathological features. We report that ablating endogenous tau protects AICD-Tg mice from deficits in adult neurogenesis, seizure severity, short-term memory deficits and neurodegeneration. Deletion of tau restored abnormal phosphorylation of NMDA receptors, which is likely to underlie hyperexcitability and associated excitotoxicity in AICD-Tg mice. Conversely, overexpression of wild-type human tau aggravated receptor phosphorylation, impaired adult neurogenesis, memory deficits and neurodegeneration. Our findings show that tau is essential for mediating the deleterious effects of AICD. Since tau also mediates Aß-induced toxic effects, our findings suggest that tau is a common downstream factor in both amyloid-dependent and-independent pathogenic mechanisms and therefore could be a more effective drug target for therapeutic intervention in AD.

A randomized controlled study to evaluate the effect of bexarotene on amyloid-ß and apolipoprotein E metabolism in healthy subjects.

INTRODUCTION: We conducted a phase Ib proof of mechanism trial to determine whether bexarotene (Targretin) increases central nervous system (CNS) apolipoprotein E (apoE) levels and alters Aß metabolism in normal healthy individuals with the APOE ε3/ε3 genotype. METHODS: We used stable isotope labeling kinetics (SILK-ApoE and SILK-Aß) to measure the effect of bexarotene on the turnover rate of apoE and Aß peptides and stable isotope spike absolute quantitation (SISAQ) to quantitate their concentrations in the cerebrospinal fluid (CSF). Normal subjects were treated for 3 days with bexarotene (n = 3 women, 3 men, average 32 years old) or placebo (n = 6 women, average 30.2 years old) before administration of C13-leucine and collection of plasma and CSF over the next 48 hours. Bexarotene concentrations in plasma and CSF were also measured. RESULTS: Oral administration of bexarotene resulted in plasma levels of 1 to 2 µM; however, only low nM levels were found in CSF. Bexarotene increased CSF apoE by 25% but had no effect on metabolism of Aß peptides. DISCUSSION: Bexarotene has poor CNS penetration in normal human subjects. Drug treatment resulted in a modest increase in CSF apoE levels. The absence of an effect on Aß metabolism is likely reflective of the low CNS levels of bexarotene achieved. This study documents the utility of SILK-ApoE technology in measuring apoE kinetics in humans. TRIAL REGISTRATION: This trial is registered at clinicaltrials.gov (NCT02061878).

Omega-3 Fatty Acids Augment the Actions of Nuclear Receptor Agonists in a Mouse Model of Alzheimer's Disease.

Alzheimer's disease (AD) is a highly prevalent disorder for which there are no effective therapies. Accumulation of amyloid ß (Aß) peptides in the brain is associated with impaired cognition and memory, pronounced inflammatory dysregulation, and subsequent amyloid plaque deposition. Thus, drugs that promote the clearance of Aß peptides and resolution of inflammation may represent viable therapeutic approaches. Agonists of nuclear receptors LXR:RXR and PPAR:RXR act to ameliorate AD-related cognitive impairment and amyloid accumulation in murine models of AD. The use of an agonist to the nuclear receptor RXR, bexarotene, as monotherapy against AD, presents potential challenges due to the metabolic perturbations it induces in the periphery, most prominently hypertriglyceridemia. We report that the ω-3 fatty acid docosahexaenoic acid (DHA), in combination with bexarotene, enhances LXR:RXR target gene expression of Abca1 and ApoE, reduces soluble forms of Aß, and abrogates release of pro-inflammatory cytokines and mediators both in vitro and in a mouse model of AD. Moreover, DHA abrogates bexarotene-induced hypertriglyceridemia in vivo. Importantly, dual therapy promotes reductions in AD pathology and resultant amelioration of cognitive deficits. While monotherapy with either bexarotene or DHA resulted in modest effects in vitro and in vivo, combined treatment with both agents produced a significant additive benefit on associated AD-related phenotypes, suggesting that targeted combinatorial agents may be beneficial over single agents alone in treating AD.

The SHH/Gli pathway is reactivated in reactive glia and drives proliferation in response to neurodegeneration-induced lesions.

In response to neurodegeneration, the adult mammalian brain activates a cellular cascade that results in reactive astrogliosis and microgliosis. The mechanism through which astrocytes become reactive and the physiological consequences of their activation in response to neurodegeneration is complex. While the activation and proliferation of astrocytes has been shown to occur during massive neuronal cell death, the functional relationship between these two events has not been clearly elucidated. Here we show that in response to kainic acid- (KA) induced neurodegeneration, the mitogen sonic hedgehog (SHH) is upregulated in reactive astrocytes. SHH activity peaks at 7 days and is accompanied by increased Gli activity and elevated proliferation in several cell types. To determine the functional role of SHH-Gli signaling following KA lesions, we used a pharmacological approach to show that SHH secreted by astrocytes drives the activation and proliferation of astrocytes and microglia. The consequences of SHH-Gli signaling in KA-induced lesions appear to be independent of the severity of neurodegeneration.

The apolipoprotein-E-mimetic COG112 protects amyloid precursor protein intracellular domain-overexpressing animals from Alzheimer's disease-like pathological features.

BACKGROUND: Amyloid-ß (Aß) peptides derive from the amyloid precursor protein (APP) and play a pivotal role in Alzheimer's disease (AD) pathogenesis. Our previous work showed that the APP intracellular domain (AICD), which is produced simultaneously with Aß, also contributes to the development of AD-like features. Studies show that administration of apolipoprotein E (apoE) and apoE-derived small peptide mimetics protect AD mouse models against these AD-like features. However, the effects of apoE-mimetic treatment on AICD-mediated AD-like pathologies remain to be elucidated. OBJECTIVE: To study the effects of an apoE mimetic (COG112) on neuroinflammation, hyperphosphorylation of tau and defects in adult neurogenesis in AICD- overexpressing transgenic mice (FeCγ25 line). METHODS: Beginning at 1 month of age, animals were administered subcutaneous COG112 3 times per week for 3 months, followed by immunohistochemical analysis for neuroinflammation, neurogenesis and phosphorylated tau. RESULTS: Treatment with COG112 significantly reduced neuroinflammation in AICD mice and protected against impaired adult hippocampal neurogenesis. We also found that COG112 treatment reduced hyperphosphorylation and somatodendritic accumulation of tau in the hippocampus and cerebral cortex of AICD mice. CONCLUSIONS: Reduction of neuroinflammation by the apoE-mimetic COG112 protects against impaired neurogenesis and tau pathology in AICD transgenic mice. These data suggest that neuroinflammation plays an important role in AICD-induced AD-like pathologies.

Amyloid precursor protein: more than just neurodegeneration.

Amyloid precursor protein (APP) fascinates cell biologists because it is proteolytically processed to generate multiple peptides, including amyloid-ß, which is implicated in Alzheimer's disease. However, a large body of data also shows that the extracellular soluble fragment of APP produced by α-secretase (sAPPα) is neuroprotective and promotes neuronal outgrowth. A study by Demars and colleagues appearing in the previous issue provides data showing that sAPPα is a general growth factor for stem cells of multiple lineages. Thus, APP seems to play complex and disparate roles in neurodegeneration and neuroprotection.

Aging and excitotoxic stress exacerbate neural circuit reorganization in amyloid precursor protein intracellular domain transgenic mice.

The cleavage of amyloid precursor protein (APP) by presenilins simultaneously generates amyloid-ß (Aß) and APP intracellular Domain (AICD) peptides. Aß plays a pivotal role in Alzheimer's disease (AD) pathology and recently AICD was also shown to contribute to AD. Transgenic mice overexpressing AICD show age-dependent tau phosphorylation and aggregation, memory deficits, and neurodegeneration. Moreover, these mice show aberrant electrical activity and silent seizures beginning at 3-4 months of age. Here we show that AICD mice also displayed abnormal mossy fiber sprouting beginning about the same time and that this sprouting intensified as the animals aged. Expression of neuropeptide Y was increased in mossy fiber terminals in aged but not young AICD mice. Importantly, young AICD mice injected with kainic acid showed similar pathology to that observed in aged AICD mice. These data show that elevated levels of AICD render neurons hypersensitive to stress and induce hippocampal circuit reorganization, which can further exacerbate hyperexcitability. These results further demonstrate that AICD, in addition to Aß, can play a significant role in AD pathogenesis.

APP intracellular domain impairs adult neurogenesis in transgenic mice by inducing neuroinflammation.

BACKGROUND: A devastating aspect of Alzheimer's disease (AD) is the progressive deterioration of memory due to neuronal loss. Amyloid precursor protein (APP) occupies a central position in AD and APP-derived amyloid-beta (Abeta) peptides are thought to play a pivotal role in disease pathogenesis. Nonetheless, it is becoming clear that AD etiology is highly complex and that factors other than Abeta also contribute to AD pathogenesis. APP intracellular domain (AICD) is generated together with Abeta and we recently showed that AICD transgenic mice recapitulate pathological features of AD such as tau hyperphosphorylation, memory deficits and neurodegeneration without increasing the Abeta levels. Since impaired adult neurogenesis is shown to augment memory deficits in AD mouse models, here we examined the status of adult neurogenesis in AICD transgenic mice. METHODOLOGY/PRINCIPAL FINDING: We previously generated transgenic mice co-expressing 59-residue long AICD fragment and its binding partner Fe65. Hippocampal progenitor cell proliferation was determined by BrdU incorporation at 1.5, 3 and 12 months of age. Only male transgenic and their respective wilt type littermate control mice were used. We find age-dependent decrease in BrdU incorporation and doublecortin-positive cells in the dentate gyrus of AICD transgenic mice suggesting impaired adult neurogenesis. This deficit resulted from decreased proliferation and survival, whereas neuronal differentiation remained unaffected. Importantly, this impairment was independent of Abeta since APP-KO mice expressing AICD also exhibit reduced neurogenesis. The defects in adult neurogenesis are prevented by long-term treatment with the non-steroidal anti-inflammatory agents ibuprofen or naproxen suggesting that neuroinflammation is critically involved in impaired adult neurogenesis in AICD transgenic mice. CONCLUSION/SIGNIFICANCE: Since adult neurogenesis is crucial for spatial memory, which is particularly vulnerable in AD, these findings suggest that AICD can exacerbate memory defects in AD by impairing adult neurogenesis. Our findings further establish that AICD, in addition to Abeta, contributes to AD pathology and that neuroinflammation plays a much broader role in AD pathogenesis than previously thought.

Agonistic behavior and electrical stimulation of the antennae induces Fos-like protein expression in the male cricket brain.

Immediate early genes (IEG) such as c-Fos and Fos-related antigens (FRA) have been used as markers of neuronal activation. In this study, we determined whether the expression of c-Fos/FRAs is increased in the brains of adult male Acheta domesticus crickets following agonistic interactions. We looked for c-Fos/FRA proteins in the brain of un-fought, control male crickets and of dominant and subordinate male crickets sacrificed at different time periods following an agonistic interaction. Using immunoblot analysis, we found four different c-Fos/FRA-like proteins in the adult cricket brain. Continuous agonistic interaction increased c-Fos/FRA protein expression in the brains of subordinate males compared to control and dominant males. In addition, direct electrical stimulation of the male cricket antennae increased c-Fos/FRA-like protein in the brain. We identified the specific brain regions that exhibit c-Fos/FRA-like immunoreactivity in crickets. We detected c-Fos/FRA-like cellular immunoreactivity in different functional regions of the adult brain including the pars intercerebralis, protocerebrum, deutocerebrum, and the cortex of the mushroom bodies.

Alzheimer's disease-like pathological features in transgenic mice expressing the APP intracellular domain.

The hypothesis that amyloid-beta (Abeta) peptides are the primary cause of Alzheimer's disease (AD) remains the best supported theory of AD pathogenesis. Yet, many observations are inconsistent with the hypothesis. Abeta peptides are generated when amyloid precursor protein (APP) is cleaved by presenilins, a process that also produces APP intracellular domain (AICD). We previously generated AICD-overexpressing transgenic mice that showed abnormal activation of GSK-3beta, a pathological feature of AD. We now report that these mice exhibit additional AD-like characteristics, including hyperphosphorylation and aggregation of tau, neurodegeneration and working memory deficits that are prevented by treatment with lithium, a GSK-3beta inhibitor. Consistent with its potential role in AD pathogenesis, we find AICD levels to be elevated in brains from AD patients. The in vivo findings that AICD can contribute to AD pathology independently of Abeta have important therapeutic implications and may explain some observations that are discordant with the amyloid hypothesis.

Agonistic behavior enhances adult neurogenesis in male Acheta domesticus crickets.

We examined the effect of agonistic behavior on cell proliferation and neurogenesis in the central nervous system (CNS) of adult male Acheta domesticus crickets. We combined 5-bromo,2'deoxyuridine (BrdU)-labeling of dividing cells with immunocytochemical detection of the neuronal marker horseradish peroxidase to examine the proliferation of progenitor cells and the survival of newborn neurons. In crickets, the mushroom bodies of the brain contain clusters of proliferative cells that divide and generate new neurons in adulthood. Pairs of male crickets were allowed to fight and establish social rank and were then injected with BrdU. Proliferation of mushroom body neurogenic cluster cells was unaffected by agonistic interactions; 24 h after a fight, the number of BrdU positive cells in fought and un-fought males did not significantly differ. However, agonistic interactions did influence cell survival. Two weeks after an agonistic interaction, fought males had more newborn neurons than males that did not fight. There was also a rank-specific effect because dominant males had significantly more new neurons than subordinates. We also report for the first time that neurogenesis in adult crickets can occur in other regions of the brain and in other CNS ganglia, including the terminal abdominal ganglion (TAG). Agonistic interactions enhanced the proliferation of these distributed precursor cells but did not increase the survival of the newborn neurons generated by these cells.