Initial Aβ seeds as therapeutic target for Alzheimer’s disease

Alzheimer’s disease (AD) is the leading form of dementia interfering with daily life due to progressive memory loss and cognitive disabilities. With more than 45 million people suffering from dementia worldwide, AD is one of the costliest health conditions to society. Because of an increasing proportion of elderly people the number of individuals living with dementia is expected to more than triple by 2050. Although there are symptomatic treatments available that temporarily slow the worsening of clinical symptoms, a disease-modifying cure is still missing. Therefore, dementia, and AD in particular, is becoming a public health priority evoking worldwide efforts to delay or even prevent the disease from ever developing. The amyloid cascade hypothesis explained in detail in chapter one of this thesis proposes that an impaired homeostasis of production and clearance of the amyloid-β (Aβ) peptide is the trigger initiating a sequence of pathogenic events causing AD. Aβ misfolding and aggregation leads to the accumulation of cerebral amyloid plaques, a typical hallmark of AD. As the pathology progresses, plaques continue to develop and grow, which is suggested to lead to the second disease characteristic, neurofibrillary tangles (NFTs), consisting of hyperphosphorylated tau proteins. Aβ as the driving force of this pathological cascade has been regarded as the most reasonable therapeutic target. Therefore the second chapter is dedicated to different therapeutic approaches for AD with particular focus on the β-site amyloid precursor protein (APP)-cleaving enzyme 1 (BACE1), which is responsible for regulating the production of Aβ. To this end APP transgenic (tg) mice that mimic β-amyloidosis-related features of AD were treated with a potent BACE inhibitor for six months. In response to this long-term therapy, brain Aβ as well as plaque deposition were reduced to levels comparable to six months younger animals. Surprisingly, BACE inhibition also exhibited downstream effects preventing the pathology-dependent increase of tau in the cerebrospinal fluid (CSF). Thus, BACE inhibitors are valuable therapeutic agents and their effectiveness can be predicted by CSF tau measurements in clinical trials. These findings have been published in Alzheimer’s & Dementia: The Journal of the Alzheimer’s Association (Schelle et al., 2017, Alzheimer’s & Dementia: The Journal of the Alzheimer’s Association, 13(6), pp.701-709). The third part of this thesis investigated the stability of small Aβ aggregates (Aβ seeds), which share pathogenic properties with the prion protein implicated in transmissible spongiform encephalopathies. Whereas in Aβ seeds inoculated APP tg mice the presence of host-derived Aβ together with the exogenously applied seeds induced their propagation, the injected material was undetectable in App null mice 30 days post inoculation. However, reinoculation with brain extracts from App null mice inoculated with Aβ seeds up to six months harboring Aβ levels below detection, still induced cerebral β-amyloidosis in APP tg hosts. In conclusion, Aβ seeds can persist even in the absence of host Aβ and regain their pathogenic activity as soon as sufficient Aβ becomes available. This discovery indicates on the one hand that lowering Aβ production inhibits the formation of new Aβ seeds and on the other hand that therapeutic intervention is most effective when applied at early stages. These results have been published in Nature Neuroscience (Ye, Fritschi, Schelle et al., 2015, Nature neuroscience, 18(11), pp.1559-61). Recent biomarker studies in familial AD subjects, which revealed that first pathological changes occur up to 25 years before clinical disease onset, supported this idea. Therefore, in the last part, pre-depositing APP tg mice were treated with a combinational therapy based on anti-Aβ immunization to remove Aβ seeds and a BACE inhibitor to block the production of soluble Aβ. Using two different APP tg mouse models results revealed that brain Aβ levels and plaque formation were reduced acutely after the treatment. Moreover, this short but early intervention delayed amyloid pathology after discontinuation of the treatment. Thus, targeting initial Aβ seeds by anti-Aβ combination therapy might be the most promising strategy to effectively prevent cerebral β-amyloidosis. These findings are now prepared for publication (Schelle et al., 2017). In summary, this doctoral thesis highlights the importance of early therapeutic intervention with anti-Aβ drugs in order to prevent AD. Previous studies have shown that treatment of AD patients who have already developed irreversible neurodegeneration might be inadequate to stop the progression of this devastating disorder and could be one reason to explain recent failures of anti-amyloid agents in numerous clinical trials. The results presented in this doctoral thesis indicate that the treatment focus should be shifted toward earlier stages of AD and even toward primary prevention before symptom onset targeting initial Aβ seeds

Increase in CSF Abeta during the very early phase of cerebral Abeta deposition in mouse models

Abnormalities in the brain of Alzheimer’s Disease (AD) patients are thought to start long before the first clinical symptoms emerge. The identification of affected individuals at this “preclinical AD” stage relies on biomarkers such as decreased levels of the β-amyloid peptide (Aβ) in the cerebrospinal fluid (CSF) and brain retention of amyloid-binding agents using positron emission tomography. However, these biomarkers are limited by the lack of longitudinal profiles and lack pathological conformation in brain. To this end we have studied CSF Aβ changes in three genetically-defined amyloid precursor protein (APP) transgenic mouse models focusing our analysis on the time of the initial Aβ deposition in brain, which differs significantly between the models studied. Remarkably, while we confirmed the CSF Aβ decrease during the course of brain amyloid deposition, a temporary 20-30% increase in CSF Aβ40 and 42 was found at the time of the appearance of the first individual Aβ plaques in all the three models. These results together with emerging indications of similar CSF Aβ increases at very early stages in familial and sporadic AD suggest that increased CSF Aβ levels may constitute the first detectable biomarker change in the AD pathological process. This important observation opens new perspectives in patient selection and stratification for preventive treatment strategies and is an incentive to the discovery of additional “preclinical AD” biomarkers

Neurofilament light chain in blood and CSF as marker of disease progression in mouse models of neurodegenerative diseases

Current disease-modifying therapeutic approaches for Alzheimer´s disease (AD) and Parkinson´s disease (PD) target the formation of the characteristic proteopathic lesions (α-synuclein, Tau, Aβ). To monitor such treatments, fluid biomarkers reflecting the underlying disease process are of crucial importance. Genetically-induced mouse models that develop α-synuclein, Tau, or Aβ lesions are ideally suited for the required translational studies comparing biomarker changes directly with the brain pathology as they avoid the diagnostic uncertainty and interfering comorbidities frequently present in humans. We now report very robust increases of neurofilament light chain (NfL) ranging from 10 to 1000- and 3 to 100-fold in CSF and blood plasma, respectively, in murine models of α-synucleinopathies, taupathy, and ß-amyloidosis. Blood and CSF NfL levels showed a strong correlation and NfL changes coincided with the onset and correlated in magnitude with the corresponding proteopathic lesions in brain. Experimental induction of α-synuclein lesions in α-synuclein transgenic mice increased both CSF and blood NfL levels, while BACE1 inhibition in Aβ precursor protein transgenic mice prevented Aβ lesions and the concomitant increase of NfL in CSF and blood. Our results suggest that CSF and in particular blood NfL can serve as reliable and easily accessible biomarker to monitor disease progression and treatment response in mouse models developing proteopathic lesions and potentially also in AD and PD patients

Prevention of tau increase in cerebrospinal fluid of APP transgenic mice suggests downstream effect of BACE1 inhibition

The inhibition of the beta-site amyloid precursor protein-cleaving enzyme 1 (BACE1) is a main therapeutic approach for the treatment of Alzheimer's disease (AD). The characterization of BACE1 inhibitors has largely focused on direct effects, i.e. the reduction of β-amyloid (Aβ) generation and deposition in the brain. We reported previously the age-related increase of tau protein in the cerebrospinal fluid (CSF) of Aβ precursor protein (APP) transgenic mice reminiscent of similar changes in AD CSF. Using a novel high-sensitivity tau sandwich immunoassay we now demonstrate that BACE1 inhibition prevents CSF tau increase in both, early-depositing APP tg mice and mice with moderate Aβ pathology. Our results demonstrate that BACE1 inhibition not only reduces Aβ generation but also downstream AD pathologies. The tight correlation between Aβ aggregation in brain and tau levels in CSF renders CSF tau a valuable marker to predict the effectiveness of BACE inhibitors in current clinical trials

Aβ reduction at early disease stages prevents progression of cerebral amyloid angiopathy in a mouse model of hereditary cerebral hemorrhage with amyloidosis - Dutch type

Introduction: Deposition of -amyloid peptide (A) within cerebral vessels (cerebral amyloid angiopathy, CAA) contributes to intracerebral bleedings and altered brain function. CAA occurs with aging and to various degree in Alzheimer’s disease. Methods: APPDutch mice model hereditary cerebral hemorrhage with amyloidosis – Dutch-type (HCHWA-D) and develop CAA in the vast absence of parenchymal amyloid. 3D-ultramicroscopy and immunoassays were used to assess CAA onset and progression. An inhibitor to the β-site amyloid precursor protein-cleaving enzyme 1 (BACE1) was used to lower A generation. Results: CAA was first detected in frontal leptomeningeal and superficial cortical vessels followed by vessels penetrating the cortical layers. Upon CAA onset, A in the CSF was decreased. BACE1-inhibition initiated at the onset of CAA, and continued for 4 months, largely prevented CAA progression and associated microgliosis. Discussion: Results provide a preclinical basis for A-reducing treatments in patients at risk of CAA and in presymptomatic HCHWA-D mutation carriers