BACE/APPV717F double-transgenic mice develop cerebral amyloidosis and inflammation

Critical analysis of the use of β-site amyloid precursor protein-cleaving enzyme 1 inhibitors in the treatment of Alzheimer's disease

Alzheimer’s disease (AD) is the major cause of dementia in the elderly and an unmet clinical challenge. A variety of therapies that are currently under development are directed to the amyloid cascade. Indeed, the accumulation and toxicity of amyloid-β (Aβ) is believed to play a central role in the etiology of the disease, and thus rational interventions are aimed at reducing the levels of Aβ in the brain. Targeting β-site amyloid precursor protein-cleaving enzyme (BACE)-1 represents an attractive strategy, as this enzyme catalyzes the initial and rate-limiting step in Aβ production. Observation of increased levels of BACE1 and enzymatic activity in the brain, cerebrospinal fluid, and platelets of patients with AD and mild cognitive impairment supports the potential benefits of BACE1 inhibition. Numerous potent inhibitors have been generated, and many of these have been proved to lower Aβ levels in the brain of animal models. Over 10 years of intensive research on BACE1 inhibitors has now culminated in advancing half a dozen of these drugs into human trials, yet translating the in vitro and cellular efficacy of BACE1 inhibitors into preclinical and clinical trials represents a challenge. This review addresses the promises and also the potential problems associated with BACE1 inhibitors for AD therapy, as the complex biological function of BACE1 in the brain is becoming unraveled.

Unfolded protein response signaling by transcription factor XBP-1 regulates ADAM10 and is affected in Alzheimer's disease

In Alzheimer's disease (AD), disturbed homeostasis of the proteases competing for amyloid precursor protein processing has been reported: a disintegrin and metalloproteinase 10 (ADAM10), the physiological α-secretase, is decreased in favor of the amyloid-β-generating enzyme BACE-1. To identify transcription factors that modulate the expression of either protease, we performed a screening approach: 48 transcription factors significantly interfered with ADAM10/BACE-1-promoter activity. One selective inducer of ADAM10 gene expression is the X-box binding protein-1 (XBP-1). This protein regulates the unfolded protein-response pathway. We demonstrate that particularly the spliced XBP-1 variant dose dependently regulates ADAM10 expression, which can be synergistically enhanced by 100 nM insulin. Analysis of 2 different transgenic mouse models (APP/PS1 and 5xFAD) revealed that at early time points in pathology XBP-1 metabolism is induced. This is accompanied by a 2-fold augmented ADAM10 amount as compared with nontransgenic littermates (P=0.011). Along with aging of the mice, the system is counterregulated, and XBP-1 together with ADAM10 expression level decreased to ∼50% as compared with control animals. Analyses of expression levels in human AD brains showed that ADAM10 mRNA correlated with active XBP-1 (r=0.3120), but expression did not reach levels of healthy age-matched controls, suggesting deregulation of XBP-1 signaling. Our results demonstrate that XBP-1 is a driver of ADAM10 gene expression and that disturbance of this pathway might contribute to development or progression of AD.

Preclinical non-human models to combat dementia

Dementia is characterized by a certain degree of memory loss with disabled intellectual functioning, which mostly presents as Alzheimer's disease. The underlying causes range from gene mutations, lifestyle factors, and other environmental influences to brain injuries and normal aging. Although there have been many rodent and non-human primate models created by various drugs, neurotoxins and genetic ablation but the current scenario does not exhibit a well characterized animal model to evaluate novel compounds and various treatment strategies for dementia. Therefore, a comprehensive model exhibiting the pathologies and neuro-behavioral parameters close to this syndrome is very much needed. This report discusses the various experimental strategies to create animal models of dementia.

PEGylation enhances the therapeutic potential for insulin-like growth factor I in central nervous system disorders

OBJECTIVE

Due to its potent neurotrophic activity, insulin-like growth factor I (IGF-I) has been proposed many times for therapeutic application in disorders of the central nervous system (CNS). However, insufficient brain delivery to yield beneficial central without peripheral side effects have prevented clinical development in most instances.

DESIGN

We recently reported the generation of a polyethylene-glycol modified IGF-I variant (PEG-IGF-I) with prolonged half-life and less acute side effects, but with fully maintained slow anabolic activity. Here we investigated if these beneficial properties result in improved brain availability of the drug, thereby reaching therapeutically relevant steady-state concentrations to elicit beneficial effects on neuronal function.

RESULTS

After a single subcutaneous injection, PEG-IGF-I reached much higher steady-state levels in brain tissue and cerebrospinal fluid compared with IGF-I. Two weeks treatment with PEG-IGF-I was sufficient to modulate brain plasticity processes, as judged by changes in synaptic proteins and related animal behavior. Furthermore, chronic treatment of a mouse model of brain amyloidosis with PEG-IGF-I reverted deficits in insulin/IGF-I signaling, synaptic proteins and cognitive performance.

CONCLUSIONS

Our data generate the therapeutic potential for PEG-IGF-I to treat CNS disorders by systemic drug application, and in addition scientifically support its application in disorders of synaptic function and neuronal development.

Phosphorylation of Tau at S422 is enhanced by Abeta in TauPS2APP triple transgenic mice

Amyloid beta peptides and microtubule-associated protein Tau are misfolded and form aggregates in brains of Alzheimer's disease patients. To examine their specific roles in the pathogenesis of Alzheimer's disease and their relevance in neurodegenerative processes, we have created TauPS2APP triple transgenic mice that express human mutated Amyloid Precursor Protein, presenilin 2 and Tau. We present a cross-sectional analysis of these mice at 4, 8, 12 and 16 months of age. By comparing with single transgenic Tau mice, we demonstrate that accumulation of Abeta in TauPS2APP triple transgenic mice impacts on Tau pathology by increasing the phosphorylation of Tau at serine 422, as determined by a novel immunodetection method that is able to reliably measure phospho-Tau species in transgenic mouse brains. The TauPS2APP triple transgenic mouse model will be very useful for studying the effect of new therapeutic paradigms on amyloid deposition and downstream neurofibrillary tangle development.

Phosphorylation of the translation initiation factor eIF2alpha increases BACE1 levels and promotes amyloidogenesis

beta-site APP cleaving enzyme-1 (BACE1), the rate-limiting enzyme for beta-amyloid (Abeta) production, is elevated in Alzheimer's disease (AD). Here, we show that energy deprivation induces phosphorylation of the translation initiation factor eIF2alpha (eIF2alpha-P), which increases the translation of BACE1. Salubrinal, an inhibitor of eIF2alpha-P phosphatase PP1c, directly increases BACE1 and elevates Abeta production in primary neurons. Preventing eIF2alpha phosphorylation by transfection with constitutively active PP1c regulatory subunit, dominant-negative eIF2alpha kinase PERK, or PERK inhibitor P58(IPK) blocks the energy-deprivation-induced BACE1 increase. Furthermore, chronic treatment of aged Tg2576 mice with energy inhibitors increases levels of eIF2alpha-P, BACE1, Abeta, and amyloid plaques. Importantly, eIF2alpha-P and BACE1 are elevated in aggressive plaque-forming 5XFAD transgenic mice, and BACE1, eIF2alpha-P, and amyloid load are correlated in humans with AD. These results strongly suggest that eIF2alpha phosphorylation increases BACE1 levels and causes Abeta overproduction, which could be an early, initiating molecular mechanism in sporadic AD.

Upregulation of BACE1 and beta-amyloid protein mediated by chronic cerebral hypoperfusion contributes to cognitive impairment and pathogenesis of Alzheimer's disease

Chronic cerebral hypoperfusion (CCH) increases the risk of Alzheimer disease (AD) through several biologically plausible pathways, but the relationship between CCH and the development of AD remains uncertain. To investigate expression of APP, BACE1 and A beta in the hippocampus of BCCAO rats and study pathophysiological mechanism of AD from CCH. CCH rat model was established by chronic bilateral common carotid artery occlusion (BCCAO). Behavior was evaluated after BCCAO with Morris water maze and open-field task. Expression of A beta was measured by enzyme linked immunosorbent assay (ELISA). beta-Amyloid precursor protein cleavage enzyme 1 (BACE1) and beta-amyloid precursor protein (APP) were tested by ELISA, Western blotting and RT-PCR. Cognitive impairment occurred with CCH by Morris water maze test and open-field task. The BACE1 and A beta level in BCCAO rats was more increased than sham-operation control rats (P < 0.01) but APP had no difference(P > 0.05). The expression of BACE1 and A beta has no inter-group difference in BCCAO rats (P > 0.05). The level of BACE1 and A beta had positive correlation with cognitive impairment (P < 0.01) while no correlation was observed between APP and cognitive impairment. Chronic cerebral ischemia contributes to cognitive impairment and vascular pathogenesis of Alzheimer's disease that chronic cerebral hypoperfusion increases BACE1 and A beta level in brain.

Generation of Abeta38 and Abeta42 is independently and differentially affected by familial Alzheimer disease-associated presenilin mutations and gamma-secretase modulation

Alzheimer disease amyloid beta-peptide (Abeta) is generated via proteolytic processing of the beta-amyloid precursor protein by beta- and gamma-secretase. Gamma-secretase can be blocked by selective inhibitors but can also be modulated by a subset of non-steroidal anti-inflammatory drugs, including sulindac sulfide. These drugs selectively reduce the generation of the aggregation-prone 42-amino acid Abeta(42) and concomitantly increase the levels of the rather benign Abeta(38). Here we show that Abeta(42) and Abeta(38) generation occur independently from each other. The amount of Abeta(42) produced by cells expressing 10 different familial Alzheimer disease (FAD)-associated mutations in presenilin (PS) 1, the catalytic subunit of gamma-secretase, appeared to correlate with the respective age of onset in patients. However, Abeta(38) levels did not show a negative correlation with the age of onset. Modulation of gamma-secretase activity by sulindac sulfide reduced Abeta(42) in the case of wild type PS1 and two FAD-associated PS1 mutations (M146L and A285V). The remaining eight PS1 FAD mutants showed either no reduction of Abeta(42) or only rather subtle effects. Strikingly, even the mutations that showed no effect on Abeta(42) levels allowed a robust increase of Abeta(38) upon treatment with sulindac sulfide. Similar observations were made for fenofibrate, a compound known to increase Abeta(42) and to decrease Abeta(38). For mutants that predominantly produce Abeta(42), the ability of fenofibrate to further increase Abeta(42) levels became diminished, whereas Abeta(38) levels were altered to varying extents for all mutants analyzed. Thus, we conclude that Abeta(38) and Abeta(42) production do not depend on each other. Using an independent non-steroidal anti-inflammatory drug derivative, we obtained similar results for PS1 as well as for PS2. These in vitro results were confirmed by in vivo experiments in transgenic mice expressing the PS2 N141I FAD mutant. Our findings therefore have strong implications on the selection of transgenic mouse models used for screening of the Abeta(42)-lowering capacity of gamma-secretase modulators. Furthermore, human patients with certain PS mutations may not respond to gamma-secretase modulators.

BACE1 and mutated presenilin-1 differently modulate Abeta40 and Abeta42 levels and cerebral amyloidosis in APPDutch transgenic mice

APPDutch transgenic (tg) mice develop cerebral amyloid angiopathy (CAA) that consists mainly of AbetaDutch40, with virtually no parenchymal amyloid plaques. To modulate cerebral amyloidosis, we crossbred APPDutch mice with either BACE1 tg mice to increase total AbetaDutch, or with G384A-mutated PS1 tg mice to elevate the ratio of AbetaDutch42 to AbetaDutch40. We analyzed all mice at 22 months of age. Compared to APPDutch mice, double-tg APPDutch/BACE1 mice revealed increased CAA mainly due to extensive vascular amyloid accumulation in the thalamus. In addition, they developed parenchymal amyloid in cortex and subiculum. In contrast, APPDutch/G384A-PS1 mice showed extensive, predominantly parenchymal amyloid throughout the entire brain, interestingly, even in the thalamus. The amyloid, composed largely of AbetaDutch42, was different compared to that in APPDutch/BACE1 mice which was composed mainly of AbetaDutch40. In summary, these mouse models reveal a broad variety and region-specificity of parenchymal versus vascular cerebral amyloid. This is partially explained by the absolute amount of neuronally produced AbetaDutch42 and AbetaDutch40 and ratio between the two. We conclude that the absolute levels of Abeta in combination with the ratio of Abeta42 to Abeta40 play a key role in determining the cerebral compartment and brain region in which Abeta is deposited.

Beta-site amyloid precursor protein cleaving enzyme 1 levels become elevated in neurons around amyloid plaques: implications for Alzheimer's disease pathogenesis

Beta-site amyloid precursor protein cleaving enzyme 1 (BACE1) (beta-secretase) initiates generation of beta-amyloid (Abeta), which plays an early role in Alzheimer's disease (AD). BACE1 levels are increased in postmortem AD brain, suggesting BACE1 elevation promotes Abeta production and AD. Alternatively, the BACE1 increase may be an epiphenomenon of late-stage AD. To distinguish between these possibilities, we analyzed BACE1 elevation using a highly specific BACE1 antibody, BACE-Cat1, made in BACE1-/- mice, which mount a robust anti-BACE1 immune response. Previous BACE1 immunohistochemical studies lack consistent results because typical BACE1 antibodies produce nonspecific background, but BACE-Cat1 immunolabels BACE1 only. BACE1 elevation was recapitulated in two amyloid precursor protein (APP) transgenic mouse lines. 5XFAD mice form amyloid plaques at young ages and exhibit neuron loss. In contrast, Tg2576 form plaques at a more advanced age and do not show cell death. These two mouse lines allow differentiation between early Abeta-induced events and late phenomena related to neuron death. BACE1 levels became elevated in parallel with amyloid burden in each APP transgenic, starting early in 5XFAD and late in Tg2576. The increase in BACE1 protein occurred without any change in BACE1 mRNA level, indicating a posttranscriptional mechanism. In APP transgenic and AD brains, high BACE1 levels were observed in an annulus around Abeta42-positive plaque cores and colocalized with neuronal proteins. These results demonstrate that amyloid plaques induce BACE1 in surrounding neurons at early stages of pathology before neuron death occurs. We conclude that BACE1 elevation is most likely triggered by the amyloid pathway and may drive a positive-feedback loop in AD.