BACE1 inhibition induces a specific cerebrospinal fluid β-amyloid pattern that identifies drug effects in the central nervous system

Alzheimer's Disease Biomarker Analysis Using Targeted Mass Spectrometry

Alzheimer's disease (AD) is characterized by several neuropathological changes, mainly extracellular amyloid aggregates (plaques), intraneuronal inclusions of phosphorylated tau (tangles), as well as neuronal and synaptic degeneration, accompanied by tissue reactions to these processes (astrocytosis and microglial activation) that precede neuronal network disturbances in the symptomatic phase of the disease. A number of biomarkers for these brain tissue changes have been developed, mainly using immunoassays. In this review, we discuss how targeted mass spectrometry (TMS) can be used to validate and further characterize classes of biomarkers reflecting different AD pathologies, such as tau- and amyloid-beta pathologies, synaptic dysfunction, lysosomal dysregulation, and axonal damage, and the prospect of using TMS to measure these proteins in clinical research and diagnosis. TMS advantages and disadvantages in relation to immunoassays are discussed, and complementary aspects of the technologies are discussed.

Differential effects of familial Alzheimer's disease-causing mutations on amyloid precursor protein (APP) trafficking, proteolytic conversion, and synaptogenic activity

The amyloid precursor protein (APP) is a key player in Alzheimer`s disease (AD) and the precursor of the Aβ peptide, which is generated by consecutive cleavages of β- and γ-secretases. Familial Alzheimer's disease (FAD) describes a hereditary subgroup of AD that represents a low percentage of AD cases with an early onset of the disease. Different APP FAD mutations are thought to have qualitatively different effects on its proteolytic conversion. However, few studies have explored the pathogenic and putative physiological differences in more detail. Here, we compared different FAD mutations, located at the β- (Swedish), α- (Flemish, Arctic, Iowa) or γ-secretase (Iberian) cleavage sites. We examined heterologous expression of APP WT and FAD mutants in non-neuronal cells and their impact on presynaptic differentiation in contacting axons of co-cultured neurons. To decipher the underlying molecular mechanism, we tested the subcellular localization, the endocytosis rate and the proteolytic processing in detail by immunoprecipitation-mass spectrometry. Interestingly, we found that only the Iberian mutation showed altered synaptogenic function. Furthermore, the APP Iowa mutant shows significantly decreased α-secretase processing which is in line with our results that APP carrying the Iowa mutation was significantly increased in early endosomes. However, most interestingly, immunoprecipitation-mass spectrometry analysis revealed that the amino acid substitutions of APP FAD mutants have a decisive impact on their processing reflected in altered Aβ profiles. Importantly, N-terminally truncated Aβ peptides starting at position 5 were detected preferentially for APP Flemish, Arctic, and Iowa mutants containing amino acid substitutions around the α-secretase cleavage site. The strongest change in the ratio of Aβ40/Aβ42 was observed for the Iberian mutation while APP Swedish showed a substantial increase in Aβ1-17 peptides. Together, our data indicate that familial AD mutations located at the α-, β-, and γ-secretase cleavage sites show considerable differences in the underlying pathogenic mechanisms.

Mechanisms of amyloid-β34 generation indicate a pivotal role for BACE1 in amyloid homeostasis

The beta‑site amyloid precursor protein (APP) cleaving enzyme (BACE1) was discovered due to its "amyloidogenic" activity which contributes to the production of amyloid-beta (Aβ) peptides. However, BACE1 also possesses an "amyloidolytic" activity, whereby it degrades longer Aβ peptides into a non‑toxic Aβ34 intermediate. Here, we examine conditions that shift the equilibrium between BACE1 amyloidogenic and amyloidolytic activities by altering BACE1/APP ratios. In Alzheimer disease brain tissue, we found an association between elevated levels of BACE1 and Aβ34. In mice, the deletion of one BACE1 gene copy reduced BACE1 amyloidolytic activity by ~ 50%. In cells, a stepwise increase of BACE1 but not APP expression promoted amyloidolytic cleavage resulting in dose-dependently increased Aβ34 levels. At the cellular level, a mislocalization of surplus BACE1 caused a reduction in Aβ34 levels. To align the role of γ-secretase in this pathway, we silenced Presenilin (PS) expression and identified PS2-γ-secretase as the main γ-secretase that generates Aβ40 and Aβ42 peptides serving as substrates for BACE1's amyloidolytic cleavage to generate Aβ34.

BACE1 Overexpression Reduces SH-SY5Y Cell Viability Through a Mechanism Distinct from Amyloid-β Peptide Accumulation: Beta Prime-Mediated Competitive Depletion of sAβPPα

BACKGROUND

The Alzheimer's disease (AD)-associated amyloid-beta protein precursor (AβPP) can be cleaved by β-site AβPP cleaving enzyme 1 (BACE1) and the γ-secretase complex to yield neurotoxic amyloid-β (Aβ) peptides. However, AβPP can also be cleaved in a 'non-amyloidogenic' manner either by α-secretase to produce soluble AβPP alpha (sAβPPα) (a fragment with neuroprotective/neurogenic functions) or through alternative BACE1-mediated 'beta prime' activity yielding soluble AβPP beta prime (sAβPPβ').

OBJECTIVE

To determine whether sAβPPα depletion, as opposed to Aβ peptide accumulation, contributes to cytotoxicity in AD-relevant SH-SY5Y neuroblastoma cell models.

METHODS

AβPP proteolysis was characterized by immunoblotting in mock-, wild-type AβPP (wtAβPP)-, BACE1-, and Swedish mutant AβPP (SweAβPP)-transfected cells. AβPP beta prime cleavage was confirmed through secretase inhibitor studies and C-terminal fragment analysis. The roles of sAβPPα and sAβPPβ' in cell viability were confirmed by overexpression studies.

RESULTS

Despite producing enhanced Aβ peptide levels, wtAβPP- and SweAβPP-transfected cells did not exhibit reduced viability whereas BACE1-transfected cells did. sAβPPα generation in SH-SY5Y-BACE1 cells was virtually ablated in lieu of BACE1-mediated sAβPPβ' production. sAβPPα overexpression in SH-SY5Y-BACE1 cells restored viability whereas sAβPPβ' overexpression decreased viability further. The anti-AβPP 6E10 antibody was shown to cross-react with sAβPPβ'.

CONCLUSION

sAβPPα depletion and/or sAβPPβ' accumulation, but not elevated Aβ peptide levels, represent the cytotoxic mechanism following BACE1 overexpression in SH-SY5Y cells. These data support the novel concept that competitive sAβPPα depletion by BACE1 beta prime activity might contribute to AD. The cross-reactivity of 6E10 with AβPPβ'also questions whether previous studies assessing sAβPPα as a biomarker using this antibody should be revisited.

The Swedish dilemma - the almost exclusive use of APPswe-based mouse models impedes adequate evaluation of alternative β-secretases

Alzheimer's disease (AD) is the most common form of dementia, however incurable so far. It is widely accepted that aggregated amyloid β (Aβ) peptides play a crucial role for the pathogenesis of AD, as they cause neurotoxicity and deposit as so-called Aβ plaques in AD patient brains. Aβ peptides derive from the amyloid precursor protein (APP) upon consecutive cleavage at the β- and γ-secretase site. Hence, mutations in the APP gene are often associated with autosomal dominant inherited AD. Almost thirty years ago, two mutations at the β-secretase site were observed in two Swedish families (termed Swedish APP (APPswe) mutations), which led to early-onset AD. Consequently, APPswe was established in almost every common AD mouse model, as it contributes to early Aβ plaque formation and cognitive impairments. Analyzing these APPswe-based mouse models, the aspartyl protease BACE1 has been evolving as the prominent β-secretase responsible for Aβ release in AD and as the most important therapeutic target for AD treatment. However, with respect to β-secretase processing, the very rare occurring APPswe variant substantially differs from wild-type APP. BACE1 dominates APPswe processing resulting in the release of Aβ1-x, whereas N-terminally truncated Aβ forms are scarcely generated. However, these N-terminally truncated Aβ species such as Aβ2-x, Aβ3-x and Aβ4-x are elevated in AD patient brains and exhibit an increased potential to aggregate compared to Aβ1-x peptides. Proteases such as meprin β, cathepsin B and ADAMTS4 were identified as alternative β-secretases being capable of generating these N-terminally truncated Aβ species from wild-type APP. However, neither meprin β nor cathepsin B are capable of generating N-terminally truncated Aβ peptides from APPswe. Hence, the role of BACE1 for the Aβ formation during AD might be overrepresented through the excessive use of APPswe mouse models. In this review we critically discuss the consideration of BACE1 as the most promising therapeutic target. Shifting the focus of AD research towards alternative β secretases might unveil promising alternatives to BACE1 inhibitors constantly failing in clinical trials due to ineffectiveness and harmful side effects.

The Uppsala APP deletion causes early onset autosomal dominant Alzheimer's disease by altering APP processing and increasing amyloid β fibril formation

Point mutations in the amyloid precursor protein gene (APP) cause familial Alzheimer's disease (AD) by increasing generation or altering conformation of amyloid β (Aβ). Here, we describe the Uppsala APP mutation (Δ690-695), the first reported deletion causing autosomal dominant AD. Affected individuals have an age at symptom onset in their early forties and suffer from a rapidly progressing disease course. Symptoms and biomarkers are typical of AD, with the exception of normal cerebrospinal fluid (CSF) Aβ42 and only slightly pathological amyloid-positron emission tomography signals. Mass spectrometry and Western blot analyses of patient CSF and media from experimental cell cultures indicate that the Uppsala APP mutation alters APP processing by increasing β-secretase cleavage and affecting α-secretase cleavage. Furthermore, in vitro aggregation studies and analyses of patient brain tissue samples indicate that the longer form of mutated Aβ, AβUpp1-42_Δ19-24, accelerates the formation of fibrils with unique polymorphs and their deposition into amyloid plaques in the affected brain.

Matching Clinical Diagnosis and Amyloid Biomarkers in Alzheimer's Disease and Frontotemporal Dementia

BACKGROUND

The aims of this study were to compare the diagnostic accuracy, sensitivity, specificity, and positive and negative predictive values (PPV, NPV) of different cerebrospinal fluid (CSF) amyloid biomarkers and amyloid-Positron Emission Tomography (PET) in patients with a clinical diagnosis of Alzheimer's disease (AD) and Frontotemporal Dementia (FTD); to compare concordance between biomarkers; and to provide an indication of their use and interpretation.

METHODS

We included 148 patients (95 AD and 53 FTD), who underwent clinical evaluation, neuropsychological assessment, and at least one amyloid biomarker (CSF analysis or amyloid-PET). Thirty-six patients underwent both analyses. One-hundred-thirteen patients underwent Apolipoprotein E (ApoE) genotyping.

RESULTS

Amyloid-PET presented higher diagnostic accuracy, sensitivity, and NPV than CSF Aβ_1-42 but not Aβ_42/40 ratio. Concordance between CSF biomarkers and amyloid-PET was higher in FTD patients compared to AD cases. None of the AD patients presented both negative Aβ biomarkers.

CONCLUSIONS

CSF Aβ_42/40 ratio significantly increased the diagnostic accuracy of CSF biomarkers. On the basis of our current and previous data, we suggest a flowchart to guide the use of biomarkers according to clinical suspicion: due to the high PPV of both amyloid-PET and CSF analysis including Aβ_42/40, in cases of concordance between at least one biomarker and clinical diagnosis, performance of the other analysis could be avoided. A combination of both biomarkers should be performed to better characterize unclear cases. If the two amyloid biomarkers are both negative, an underlying AD pathology can most probably be excluded.

BACE1 Inhibitor, Neuroprotective, and Neuritogenic Activities of Melatonin Derivatives

Alzheimer’s disease (AD) is a common chronic neurodegenerative disorders. Melatonin (MLT) has been reported to be neuroprotective agent, and its modified structures exhibit potent antioxidant and anti-inflammation activities. Therefore, the activity of MLT and its derivatives against AD was investigated. Herein, the targeted enzymes, such as β-secretase (BACE1) and acetylcholinesterase (AChE), as well as the neuroprotective and neuritogenic effects on P19-derived neurons were evaluated. All the derivatives (1–5), including MLT, displayed potent inhibitory activity for BACE1, with inhibition values of more than 75% at 5 µM. A molecular docking study predicted that MLT, 5-MT, and 5 bound with BACE1 at catalytic amino acids Asp32 and the flap region, whereas 1–4 interacted with allosteric residue Thr232 and the flap region. The additional π-π interactions between 2, 3, and 5 with Tyr71 promoted ligand-enzyme binding. In addition, MLT, 1, 3, and 5 significantly protected neuron cells from oxidative stress by increasing the cell viability to 97.95, 74.29, 70.80, and 69.50% at 1 nM, respectively. Moreover, these derivatives significantly induced neurite outgrowth by increasing the neurite length and number. The derivatives 1, 3, and 5 should be thoroughly studied as potential AD treatment and neuroprotective agents.

NP03, a Microdose Lithium Formulation, Blunts Early Amyloid Post-Plaque Neuropathology in McGill-R-Thy1-APP Alzheimer-Like Transgenic Rats

Epidemiological, preclinical, and clinical studies have suggested a role for microdose lithium in reducing Alzheimer's disease (AD) risk by modulating key mechanisms associated with AD pathology. The novel microdose lithium formulation, NP03, has disease-modifying effects in the McGill-R-Thy1-APP transgenic rat model of AD-like amyloidosis at pre-plaque stages, before frank amyloid-β (Aβ) plaque deposition, during which Aβ is primarily intraneuronal. Here, we are interested in determining whether the positive effects of microdose lithium extend into early Aβ post-plaque stages. We administered NP03 (40μg Li/kg; 1 ml/kg body weight) to McGill-R-Thy1-APP transgenic rats for 12 weeks spanning the transition phase from plaque-free to plaque-bearing. The effect of NP03 on remote working memory was assessed using the novel object recognition task. Levels of human Aβ38, Aβ40, and Aβ42 as well as levels of pro-inflammatory mediators were measured in brain-extracts and plasma using electrochemiluminescent assays. Mature Aβ plaques were visualized with a thioflavin-S staining. Vesicular acetylcholine transporter (VAChT) bouton density and levels of chemokine (C-X-C motif) ligand 1 (CXCL1), interleukin-6 (IL-6), and 4-hydroxynonenal (4-HNE) were probed using quantitative immunohistochemistry. During the early Aβ post-plaque stage, we find that NP03 rescues functional deficits in object recognition, reduces loss of cholinergic boutons in the hippocampus, reduces levels of soluble and insoluble cortical Aβ42 and reduces hippocampal Aβ plaque number. In addition, NP03 reduces markers of neuroinflammation and cellular oxidative stress. Together these results indicate that microdose lithium NP03 is effective at later stages of amyloid pathology, after appearance of Aβ plaques.

CSF and Blood Biomarkers in Neuroinflammatory and Neurodegenerative Diseases: Implications for Treatment

Neuroinflammatory and neurodegenerative diseases are characterized by the interplay of a number of molecular pathways that can be assessed through biofluids, especially cerebrospinal fluid and blood. Accordingly, the definition and classification of these disorders will move from clinical and pathological to biological criteria. The consequences of this biomarker-based diagnostic and prognostic approach are highly relevant to the field of drug development. Indeed, in view of the availability of disease-modifying drugs, fluid biomarkers offer a unique opportunity for improving the quality and applicability of results from clinical trials. Herein, we discuss the benefits of using fluid biomarkers for patient stratification, target engagement, and outcome assessment, as well as the most recent developments in neuroinflammatory and neurodegenerative diseases.

Aβ5-x Peptides: N-Terminal Truncation Yields Tunable Cu(II) Complexes

The Aβ_5-x peptides (x = 38, 40, 42) are minor Aβ species in normal brains but elevated upon the application of inhibitors of Aβ processing enzymes. They are interesting from the point of view of coordination chemistry for the presence of an Arg-His metal binding sequence at their N-terminus capable of forming a 3-nitrogen (3N) three-coordinate chelate system. Similar sequences in other bioactive peptides were shown to bind Cu(II) ions in biological systems. Therefore, we investigated Cu(II) complex formation and reactivity of a series of truncated Aβ_5-x peptide models comprising the metal binding site: Aβ_5-9, Aβ_5-12, Aβ_5-12Y10F, and Aβ_5-16. Using CD and UV-vis spectroscopies and potentiometry, we found that all peptides coordinated the Cu(II) ion with substantial affinities higher than 3 × 10¹² M^-1 at pH 7.4 for Aβ_5-9 and Aβ_5-12. This affinity was elevated 3-fold in Aβ_5-16 by the formation of the internal macrochelate with the fourth coordination site occupied by the imidazole nitrogen of the His13 or His14 residue. A much higher boost of affinity could be achieved in Aβ_5-9 and Aβ_5-12 by adding appropriate amounts of the external imidazole ligand. The 3N Cu-Aβ_5-x complexes could be irreversibly reduced to Cu(I) at about -0.6 V vs Ag/AgCl and oxidized to Cu(III) at about 1.2 V vs Ag/AgCl. The internal or external imidazole coordination to the 3N core resulted in a slight destabilization of the Cu(I) state and stabilization of the Cu(III) state. Taken together these results indicate that Aβ_5-x peptides, which bind Cu(II) ions much more strongly than Aβ_1-x peptides and only slightly weaker than Aβ_4-x peptides could interfere with Cu(II) handling by these peptides, adding to copper dyshomeostasis in Alzheimer brains.

Partial reduction of amyloid β production by β-secretase inhibitors does not decrease synaptic transmission

BACKGROUND

Alzheimer's disease (AD) is the most common form of age-related neurodegenerative diseases. Cerebral deposition of Aβ peptides, especially Aβ42, is considered the major neuropathological hallmark of AD and the putative cause of AD-related neurotoxicity. Aβ peptides are produced by sequential proteolytic processing of APP, with β-secretase (BACE) being the initiating enzyme. Therefore, BACE has been considered an attractive therapeutic target in AD research and several BACE inhibitors have been tested in clinical trials, but so far, all have had negative outcomes or even led to worsening of cognitive function. AD can be triggered by Aβ years before the first symptoms appear and one reason for the failures could be that the clinical trials were initiated too late in the disease process. Another possible explanation could be that BACE inhibition alters physiological APP processing in a manner that impairs synaptic function, causing cognitive deterioration.

METHODS

The aim of this study was to investigate if partial BACE inhibition, mimicking the putative protective effect of the Icelandic mutation in the APP gene, could reduce Aβ generation without affecting synaptic transmission. To investigate this, we used an optical electrophysiology platform, in which effects of compounds on synaptic transmission in cultured neurons can be monitored. We employed this method on primary cortical rat neuronal cultures treated with three different BACE inhibitors (BACE inhibitor IV, LY2886721, and lanabecestat) and monitored Aβ secretion into the cell media.

RESULTS

We found that all three BACE inhibitors tested decreased synaptic transmission at concentrations leading to significantly reduced Aβ secretion. However, low-dose BACE inhibition, resulting in less than a 50% decrease in Aβ secretion, did not affect synaptic transmission for any of the inhibitors tested.

CONCLUSION

Our results indicate that Aβ production can be reduced by up to 50%, a level of reduction of relevance to the protective effect of the Icelandic mutation, without causing synaptic dysfunction. We therefore suggest that future clinical trials aimed at prevention of Aβ build-up in the brain should aim for a moderate CNS exposure of BACE inhibitors to avoid side effects on synaptic function.

Characterization of Cerebrospinal Fluid BACE1 Species

The β-site amyloid precursor protein cleaving enzyme 1 (BACE1) is the main brain β-secretase responsible for the amyloidogenic processing of the amyloid precursor protein (APP). Previous studies have suggested that cerebrospinal fluid (CSF) β-secretase activity may be a candidate diagnostic biomarker for Alzheimer's disease (AD), but biochemical characterization of BACE1 protein in CSF is needed. CSF samples from 19 AD patients and 19 age-matched non-AD controls (n = 19) were classified according to their Aβ42, total tau, and P-tau CSF biomarker levels. We found that β-secretase activity was higher in the CSF of AD subjects than in that of the controls. We found that the majority of the β-secretase activity in the CSF, measured using a peptide substrate homologous to the BACE1 cleavage site, was not inhibited by specific BACE1 inhibitors. We defined enzymatic activity attributable specifically to BACE1 as the activity that was blocked by the specific inhibitors, which is still higher in AD subjects. BACE1 protein levels were characterized by lectin binding, immunoprecipitation, blue native-PAGE, and western blotting using antibodies against specific protein domains. BACE1 was found to be present in human CSF as a mature form of ~ 70 kDa that probably comprised truncated and full-length species, and also as an immature form of ~ 50 kDa that retains the prodomain. CSF-BACE1 was found to assemble into hetero-complexes containing distinct species. Immunoblotting with an antibody against the C-terminus of BACE1 revealed significantly higher levels of the 70-kDa full-length BACE1, while the 50 kDa immature form remained unaltered. When the 70-kDa species was probed with an antibody against the N-terminus of BACE1 (which does not discriminate between truncated and full-length forms), no increase in immunoreactivity was observed, suggesting that truncated forms of BACE1 do not increase in AD. In conclusion, the complexity of BACE1 species in CSF has to be taken into consideration when determining BACE1 activity and protein levels in CSF as biomarkers of AD.

The BACE-1 inhibitor CNP520 for prevention trials in Alzheimer's disease

The beta‐site amyloid precursor protein cleaving enzyme‐1 (BACE‐1) initiates the generation of amyloid‐β (Aβ), and the amyloid cascade leading to amyloid plaque deposition, neurodegeneration, and dementia in Alzheimer's disease (AD). Clinical failures of anti‐Aβ therapies in dementia stages suggest that treatment has to start in the early, asymptomatic disease states. The BACE‐1 inhibitor CNP520 has a selectivity, pharmacodynamics, and distribution profile suitable for AD prevention studies. CNP520 reduced brain and cerebrospinal fluid (CSF) Aβ in rats and dogs, and Aβ plaque deposition in APP‐transgenic mice. Animal toxicology studies of CNP520 demonstrated sufficient safety margins, with no signs of hair depigmentation, retina degeneration, liver toxicity, or cardiovascular effects. In healthy adults ≥ 60 years old, treatment with CNP520 was safe and well tolerated and resulted in robust and dose‐dependent Aβ reduction in the cerebrospinal fluid. Thus, long‐term, pivotal studies with CNP520 have been initiated in the Generation Program.

Canine Cognitive Dysfunction and Alzheimer's Disease - Two Facets of the Same Disease?

Neurodegenerative diseases present a major and increasing burden in the societies worldwide. With aging populations, the prevalence of neurodegenerative diseases is increasing, yet there are no effective cures and very few treatment options are available. Alzheimer's disease is one of the most prevalent neurodegenerative conditions and although the pathology is well studied, the pathogenesis of this debilitating illness is still poorly understood. This is, among other reasons, also due to the lack of good animal models as laboratory rodents do not develop spontaneous neurodegenerative diseases and human Alzheimer's disease is only partially mimicked by transgenic rodent models. On the other hand, older dogs commonly develop canine cognitive dysfunction, a disease that is similar to Alzheimer's disease in many aspects. Dogs show cognitive deficits that could be paralleled to human symptoms such as disorientation, memory loss, changes in behavior, and in their brains, beta amyloid plaques are commonly detected both in extracellular space as senile plaques and around the blood vessels. Dogs could be therefore potentially a very good model for studying pathological process and novel treatment options for Alzheimer's disease. In the present article, we will review the current knowledge about the pathogenesis of canine cognitive dysfunction, its similarities and dissimilarities with Alzheimer's disease, and developments of novel treatments for these two diseases with a focus on canine cognitive dysfunction.

Aβ34 is a BACE1-derived degradation intermediate associated with amyloid clearance and Alzheimer's disease progression

The beta-site APP cleaving enzyme 1 (BACE1) is known primarily for its initial cleavage of the amyloid precursor protein (APP), which ultimately leads to the generation of Aβ peptides. Here, we provide evidence that altered BACE1 levels and activity impact the degradation of Aβ40 and Aβ42 into a common Aβ34 intermediate. Using human cerebrospinal fluid (CSF) samples from the Amsterdam Dementia Cohort, we show that Aβ34 is elevated in individuals with mild cognitive impairment who later progressed to dementia. Furthermore, Aβ34 levels correlate with the overall Aβ clearance rates in amyloid positive individuals. Using CSF samples from the PREVENT-AD cohort (cognitively normal individuals at risk for Alzheimer’s disease), we further demonstrate that the Aβ34/Aβ42 ratio, representing Aβ degradation and cortical deposition, associates with pre-clinical markers of neurodegeneration. We propose that Aβ34 represents a marker of amyloid clearance and may be helpful for the characterization of Aβ turnover in clinical samples.

Aβ34 is generated from degradation of Aβ40 and Aβ42 by β-secretase. Here, the authors show that Aβ34 is a marker for amyloid clearance and is elevated in the CSF of patients that go on to convert from mild cognitive impairment to Alzheimer’s disease, suggesting it may be a useful biomarker.

Increased Secondary Nucleation Underlies Accelerated Aggregation of the Four-Residue N-Terminally Truncated Aβ42 Species Aβ5-42

Aggregation of the amyloid-β (Aβ) peptide into plaques is believed to play a crucial role in Alzheimer's disease. Amyloid plaques consist of fibrils of full length Aβ peptides as well as N-terminally truncated species. β-Site amyloid precursor protein-cleaving enzyme (BACE1) cleaves amyloid precursor protein in the first step in Aβ peptide production and is an attractive therapeutic target to limit Aβ generation. Inhibition of BACE1, however, induces a unique pattern of Aβ peptides with increased levels of N-terminally truncated Aβ peptides starting at position 5 (Aβ5-X), indicating that these peptides are generated through a BACE1-independent pathway. Here we elucidate the aggregation mechanism of Aβ5-42 and its influence on full-length Aβ42. We find that, compared to Aβ42, Aβ5-42 is more aggregation prone and displays enhanced nucleation rates. Aβ5-42 oligomers cause nonspecific membrane disruption to similar extent as Aβ42 but appear at earlier time points in the aggregation reaction. Noteworthy, this implies similar toxicity of Aβ42 and Aβ5-42 and the toxic species are generated faster by Aβ5-42. The increased rate of secondary nucleation on the surface of existing fibrils originates from a higher affinity of Aβ5-42 monomers for fibrils, as compared to Aβ42: an effect that may be related to the reduced net charge of Aβ5-42. Moreover, Aβ5-42 and Aβ42 peptides coaggregate into heteromolecular fibrils and either species can elongate existing Aβ42 or Aβ5-42 fibrils but Aβ42 fibrils are more catalytic than Aβ5-42 fibrils. Our findings highlight the importance of the N-terminus for surface-catalyzed nucleation and thus the production of toxic oligomers.

Lack of β-amyloid cleaving enzyme-1 (BACE1) impairs long-term synaptic plasticity but enhances granule cell excitability and oscillatory activity in the dentate gyrus in vivo

BACE1 is a β-secretase involved in the cleavage of amyloid precursor protein and the pathogenesis of Alzheimer's disease (AD). The entorhinal cortex and the dentate gyrus are important for learning and memory, which are affected in the early stages of AD. Since BACE1 is a potential target for AD therapy, it is crucial to understand its physiological role in these brain regions. Here, we examined the function of BACE1 in the dentate gyrus. We show that loss of BACE1 in the dentate gyrus leads to increased granule cell excitability, indicated by enhanced efficiency of synaptic potentials to generate granule cell spikes. The increase in granule cell excitability was accompanied by prolonged paired-pulse inhibition, altered network gamma oscillations, and impaired synaptic plasticity at entorhinal-dentate synapses of the perforant path. In summary, this is the first detailed electrophysiological study of BACE1 deletion at the network level in vivo. The results suggest that BACE1 is important for normal dentate gyrus network function. This has implications for the use of BACE1 inhibitors as therapeutics for AD therapy, since BACE1 inhibition could similarly disrupt synaptic plasticity and excitability in the entorhinal-dentate circuitry.

Reduced Aβ secretion by human neurons under conditions of strongly increased BACE activity

The initial step in the amyloidogenic cascade of amyloid precursor protein (APP) processing is catalyzed by beta-site APP-cleaving enzyme (BACE), and this protease has increased activities in affected areas of Alzheimer's disease brains. We hypothesized that altered APP processing, because of augmented BACE activity, would affect the actions of direct and indirect BACE inhibitors. We therefore compared post-mitotic human neurons (LUHMES) with their BACE-overexpressing counterparts (BLUHMES). Although β-cleavage of APP was strongly increased in BLUHMES, they produced less full-length and truncated amyloid beta (Aβ) than LUHMES. Moreover, low concentrations of BACE inhibitors decreased cellular BACE activity as expected, but increased Aβ_1-40 levels. Several other approaches to modulate BACE activity led to a similar, apparently paradoxical, behavior. For instance, reduction in intracellular acidification by bepridil increased Aβ production in parallel with decreased BACE activity. In contrast to BLUHMES, the respective control cells (LUHMES or BLUHMES with catalytically inactive BACE) showed conventional pharmacological responses. Other non-canonical neurochemical responses (so-called 'rebound effects') are well-documented for the Aβ pathway, especially for γ-secretase: a partial block of its activity leads to an increased Aβ secretion by some cell types. We therefore compared LUHMES and BLUHMES regarding rebound effects of γ-secretase inhibitors and found an Aβ rise in LUHMES but not in BLUHMES. Thus, different cellular factors are responsible for the γ-secretase- versus BACE-related Aβ rebound. We conclude that increased BACE activity, possibly accompanied by an altered cellular localization pattern, can dramatically influence Aβ generation in human neurons and affect pharmacological responses to secretase inhibitors. OPEN PRACTICES: Open Science: This manuscript was awarded with the Open Materials Badge. For more information see: https://cos.io/our-services/open-science-badges/.

Are N- and C-terminally truncated Aβ species key pathological triggers in Alzheimer's disease?

The histopathology of Alzheimer's disease (AD) is characterized by neuronal loss, neurofibrillary tangles, and senile plaque formation. The latter results from an exacerbated production (familial AD cases) or altered degradation (sporadic cases) of 40/42-amino acid-long β-amyloid peptides (Aβ peptides) that are produced by sequential cleavages of Aβ precursor protein (βAPP) by β- and γ-secretases. The amyloid cascade hypothesis proposes a key role for the full-length Aβ42 and the Aβ40/42 ratio in AD etiology, in which soluble Aβ oligomers lead to neurotoxicity, tau hyperphosphorylation, aggregation, and, ultimately, cognitive defects. However, following this postulate, during the last decade, several clinical approaches aimed at decreasing full-length Aβ42 production or neutralizing it by immunotherapy have failed to reduce or even stabilize AD-related decline. Thus, the Aβ peptide (Aβ40/42)-centric hypothesis is probably a simplified view of a much more complex situation involving a multiplicity of APP fragments and Aβ catabolites. Indeed, biochemical analyses of AD brain deposits and fluids have unraveled an Aβ peptidome consisting of additional Aβ-related species. Such Aβ catabolites could be due to either primary enzymatic cleavages of βAPP or secondary processing of Aβ itself by exopeptidases. Here, we review the diversity of N- and C-terminally truncated Aβ peptides and their biosynthesis and outline their potential function/toxicity. We also highlight their potential as new pharmaceutical targets and biomarkers.

Cerebrospinal fluid and blood biomarkers for neurodegenerative dementias: An update of the Consensus of the Task Force on Biological Markers in Psychiatry of the World Federation of Societies of Biological Psychiatry

In the 12 years since the publication of the first Consensus Paper of the WFSBP on biomarkers of neurodegenerative dementias, enormous advancement has taken place in the field, and the Task Force takes now the opportunity to extend and update the original paper. New concepts of Alzheimer's disease (AD) and the conceptual interactions between AD and dementia due to AD were developed, resulting in two sets for diagnostic/research criteria. Procedures for pre-analytical sample handling, biobanking, analyses and post-analytical interpretation of the results were intensively studied and optimised. A global quality control project was introduced to evaluate and monitor the inter-centre variability in measurements with the goal of harmonisation of results. Contexts of use and how to approach candidate biomarkers in biological specimens other than cerebrospinal fluid (CSF), e.g. blood, were precisely defined. Important development was achieved in neuroimaging techniques, including studies comparing amyloid-β positron emission tomography results to fluid-based modalities. Similarly, development in research laboratory technologies, such as ultra-sensitive methods, raises our hopes to further improve analytical and diagnostic accuracy of classic and novel candidate biomarkers. Synergistically, advancement in clinical trials of anti-dementia therapies energises and motivates the efforts to find and optimise the most reliable early diagnostic modalities. Finally, the first studies were published addressing the potential of cost-effectiveness of the biomarkers-based diagnosis of neurodegenerative disorders.

The Metalloprotease Meprin β Is an Alternative β-Secretase of APP

The membrane bound metalloprotease meprin β is important for collagen fibril assembly in connective tissue formation and for the detachment of the intestinal mucus layer for proper barrier function. Recent proteomic studies revealed dozens of putative new substrates of meprin β, including the amyloid precursor protein (APP). It was shown that APP is cleaved by meprin β in distinct ways, either at the β-secretase site resulting in increased levels of Aβ peptides, or at the N-terminus releasing 11 kDa, and 20 kDa peptide fragments. The latter event was discussed to be rather neuroprotective, whereas the ectodomain shedding of APP by meprin β reminiscent to BACE-1 is in line with the amyloid hypothesis of Alzheimer's disease, promoting neurodegeneration. The N-terminal 11 kDa and 20 kDa peptide fragments represent physiological cleavage products, since they are found in human brains under different diseased or non-diseased states, whereas these fragments are completely missing in brains of meprin β knock-out animals. Meprin β is not only a sheddase of adhesion molecules, such as APP, but was additionally demonstrated to cleave within the prodomain of ADAM10. Activated ADAM10, the α-secretase of APP, is then able to shed meprin β from the cell surface thereby abolishing the β-secretase activity. All together meprin β seems to be a novel player in APP processing events, even influencing other enzymes involved in APP cleavage.

Neuritogenic activity of bi-functional bis-tryptoline triazole

Alzheimer's disease (AD) is a common neurodegenerative disorder, one of the hallmarks of which is the deposition of aggregated β-amyloid peptides (Aβ40,42) as plaques in the brain. Oligomers of these peptides have been reported to be toxic and to inhibit neurite outgrowth, as evidenced by neurite dystrophy and significant loss of synaptic connectivity of neurons in the AD brain resulting in cognitive decline. These peptides also react with biological metal in the brain to generate free radicals, thereby aggravating neuronal cell injury and death. Herein, multifunctional triazole-based compounds acting on multiple targets, namely β-secretase (BACE1), β-amyloid peptides (Aβ) as well as those possessing metal chelation and antioxidant properties, were developed and evaluated for neuritogenic activity in P19-derived neurons. At the non-cytotoxic concentration (1nM), all multifunctional compounds significantly enhanced neurite outgrowth. New bis-tryptoline triazole (BTT) increased the neurite length and neurite number, by 93.25% and 136.09% over the control, respectively. This finding demonstrates the ability of multifunctional compounds targeting Aβ to enhance neurite outgrowth in addition to their neuroprotective action.

Characterization of FRM-36143 as a new γ-secretase modulator for the potential treatment of familial Alzheimer's disease

Background

Familial Alzheimer’s disease (FAD) is caused by mutations in the amyloid precursor protein (APP) or presenilin (PS). Most PS mutations, which account for the majority of FAD cases, lead to an increased ratio of longer to shorter forms of the amyloid beta (Aβ) peptide. The therapeutic rationale of γ-secretase modulators (GSMs) for Alzheimer’s disease is based on this genetic evidence as well as on enzyme kinetics measurements showing changes in the processivity of the γ-secretase complex. This analysis suggests that GSMs could potentially offset some of the effects of PS mutations on APP processing, thereby addressing the root cause of early onset FAD. Unfortunately, the field has generated few, if any, molecules with good central nervous system (CNS) drug-like properties to enable proof-of-mechanism studies.

Method

We characterized the novel GSM FRM-36143 using multiple cellular assays to determine its in vitro potency and off-target activity as well as its potential to reverse the effect of PS mutations. We also tested its efficacy in vivo in wild-type mice and rats.

Results

FRM-36143 has much improved CNS drug-like properties compared to published GSMs. It has an in vitro EC₅₀ for Aβ₄₂ of 35 nM in H4 cells, can reduce Aβ₄₂ to 58 % of the baseline in rat cerebrospinal fluid, and also increases the non-amyloidogenic peptides Aβ₃₇ and Aβ₃₈. It does not inhibit Notch processing, nor does it inhibit 24-dehydrocholesterol reductase (DHCR24) activity. Most interestingly, it can reverse the effects of presenilin mutations on APP processing in vitro.

Conclusions

FRM-36143 possesses all the characteristics of a GSM in terms of Aβ modulation Because FRM-36143 was able to reverse the effect of PS mutations, we suggest that targeting patients with this genetic defect would be the best approach at testing the efficacy of a GSM in the clinic. While the amyloid hypothesis is still being tested with β-site APP-cleaving enzyme inhibitors and monoclonal antibodies in sporadic AD, we believe it is not a hypothesis for FAD. Since GSMs can correct the molecular defect caused by PS mutations, they have the promise to provide benefits to the patients when treated early enough in the course of the disease.

Profiling the dynamics of CSF and plasma Aβ reduction after treatment with JNJ-54861911, a potent oral BACE inhibitor

OBJECTIVES

Safety, tolerability, pharmacokinetics, and pharmacodynamics of a novel β-site amyloid precursor protein cleaving enzyme 1 (BACE1) inhibitor, JNJ-54861911, were assessed after single and multiple dosing in healthy participants.

METHODS

Two randomized, placebo-controlled, double-blind studies were performed using single and multiple ascending JNJ-54861911 doses (up to 14 days) in young and elderly healthy participants. Regular blood samples and frequent CSF samples, up to 36 hours after last dose, were collected to assess the pharmacokinetic and pharmacodynamic (Aβ, sAPPα,β,total levels) profiles of JNJ-54861911.

RESULTS

JNJ-54861911 was well-tolerated, adverse events were uncommon and unrelated to JNJ-54861911. JNJ-54861911 showed dose-proportional CSF and plasma pharmacokinetic profiles. Plasma- and CSF-Aβ and CSF-sAPPβ were reduced in a dose-dependent manner. Aβ reductions (up to 95%) outlasted exposure to JNJ-54861911. APOE ε4 carrier status and baseline Aβ levels did not influence Aβ/sAPPβ reductions.

CONCLUSION

JNJ-54861911, a potent brain-penetrant BACE1 inhibitor, achieved high and stable Aβ reductions after single and multiple dosing in healthy participants.

Solid-Phase Synthesis and Characterization of N-Terminally Elongated Aβ-3-x -Peptides

In addition to the prototypic amyloid-β (Aβ) peptides Aβ1-40 and Aβ1-42 , several Aβ variants differing in their amino and carboxy termini have been described. Synthetic availability of an Aβ variant is often the key to study its role under physiological or pathological conditions. Herein, we report a protocol for the efficient solid-phase peptide synthesis of the N-terminally elongated Aβ-peptides Aβ-3-38 , Aβ-3-40 , and Aβ-3-42 . Biophysical characterization by NMR spectroscopy, CD spectroscopy, an aggregation assay, and electron microscopy revealed that all three peptides were prone to aggregation into amyloid fibrils. Immunoprecipitation, followed by mass spectrometry, indicated that Aβ-3-38 and Aβ-3-40 are generated by transfected cells even in the presence of a tripartite β-site amyloid precursor protein cleaving enzyme 1 (BACE1) inhibitor. The elongated Aβ peptides starting at Val(-3) can be separated from N-terminally-truncated Aβ forms by high-resolution isoelectric-focusing techniques, despite virtually identical isoelectric points. The synthetic Aβ variants and the methods presented here are providing tools to advance our understanding of the potential roles of N-terminally elongated Aβ variants in Alzheimer's disease.

Characterization of a sensitive mouse Aβ40 PD biomarker assay for Alzheimer's disease drug development in wild-type mice

AIM

Transgenic mice that overexpress human amyloid precursor protein with Swedish or London (APPswe or APPlon) mutations have been widely used for preclinical Alzheimer's disease (AD) drug development. AD patients, however, rarely possess these mutations or overexpress APP.

RESULTS

We developed a sensitive ELISA that specifically and accurately measures low levels of endogenous Aβ40 in mouse plasma, brain and CSF. In wild-type mice treated with a bispecific anti-TfR/BACE1 antibody, significant Aβ reductions were observed in the periphery and the brain. APPlon transgenic mice showed a slightly less reduction, whereas APPswe mice did not have any decrease.

CONCLUSION

This sensitive and well-characterized mouse Aβ40 assay enables the use of wild-type mice for preclinical PK/PD and efficacy studies of potential AD therapeutics.

Harnessing Cerebrospinal Fluid Biomarkers in Clinical Trials for Treating Alzheimer's and Parkinson's Diseases: Potential and Challenges

No disease-modifying therapies (DMT) for neurodegenerative diseases (NDs) have been established, particularly for Alzheimer's disease (AD) and Parkinson's disease (PD). It is unclear why candidate drugs that successfully demonstrate therapeutic effects in animal models fail to show disease-modifying effects in clinical trials. To overcome this hurdle, patients with homogeneous pathologies should be detected as early as possible. The early detection of AD patients using sufficiently tested biomarkers could demonstrate the potential usefulness of combining biomarkers with clinical measures as a diagnostic tool. Cerebrospinal fluid (CSF) biomarkers for NDs are being incorporated in clinical trials designed with the aim of detecting patients earlier, evaluating target engagement, collecting homogeneous patients, facilitating prevention trials, and testing the potential of surrogate markers relative to clinical measures. In this review we summarize the latest information on CSF biomarkers in NDs, particularly AD and PD, and their use in clinical trials. The large number of issues related to CSF biomarker measurements and applications has resulted in relatively few clinical trials on CSF biomarkers being conducted. However, the available CSF biomarker data obtained in clinical trials support the advantages of incorporating CSF biomarkers in clinical trials, even though the data have mostly been obtained in AD trials. We describe the current issues with and ongoing efforts for the use of CSF biomarkers in clinical trials and the plans to harness CSF biomarkers for the development of DMT and clinical routines. This effort requires nationwide, global, and multidisciplinary efforts in academia, industry, and regulatory agencies to facilitate a new era.

High-abundant protein depletion strategies applied on dog cerebrospinal fluid and evaluated by high-resolution mass spectrometry

As the number of fully sequenced animal genomes and the performance of advanced mass spectrometry-based proteomics techniques are continuously improving, there is now a great opportunity to increase the knowledge of various animal proteomes. This research area is further stimulated by a growing interest from veterinary medicine and the pharmaceutical industry. Cerebrospinal fluid (CSF) is a good source for better understanding of diseases related to the central nervous system, both in humans and other animals.

In this study, four high-abundant protein depletion columns, developed for human or rat serum, were evaluated for dog CSF. For the analysis, a shotgun proteomics approach, based on nanoLC-LTQ Orbitrap MS/MS, was applied. All the selected approaches were shown to deplete dog CSF with different success. It was demonstrated that the columns significantly improved the coverage of the detected dog CSF proteome. An antibody-based column showed the best performance, in terms of efficiency, repeatability and the number of proteins detected in the sample. In total 983 proteins were detected. Of those, 801 proteins were stated as uncharacterized in the UniProt database. To the best of our knowledge, this is the so far largest number of proteins reported for dog CSF in one single study.

We evaluated four high-abundant protein depletion kits on dog CSF.

High abundant depletion kit developed for humans/rats can be used for dog CSF.

Protein depletion of dog CSF gives extended coverage of the CSF proteome.

In total, 983 dog proteins were identified in this study.

Independent information from cerebrospinal fluid amyloid-β and florbetapir imaging in Alzheimer's disease

Reduced cerebrospinal fluid amyloid-β42 and increased retention of florbetapir positron emission tomography are biomarkers reflecting cortical amyloid load in Alzheimer's disease. However, these measurements do not always agree and may represent partly different aspects of the underlying Alzheimer's disease pathology. The goal of this study was therefore to test if cerebrospinal fluid and positron emission tomography amyloid-β biomarkers are independently related to other Alzheimer's disease markers, and to examine individuals who are discordantly classified by these two biomarker modalities. Cerebrospinal fluid and positron emission tomography amyloid-β were measured at baseline in 769 persons [161 healthy controls, 68 subjective memory complaints, 419 mild cognitive impairment and 121 Alzheimer's disease dementia, mean age 72 years (standard deviation 7 years), 47% females] and used to predict diagnosis, APOE ε4 carriage status, cerebral blood flow, cerebrospinal fluid total-tau and phosphorylated-tau levels (cross-sectionally); and hippocampal volume, fluorodeoxyglucose positron emission tomography results and Alzheimer's Disease Assessment Scale-cognitive subscale scores (longitudinally). Cerebrospinal fluid and positron emission tomography amyloid-β were highly correlated, but adjusting one of these predictors for the other revealed that they both provided partially independent information when predicting diagnosis, APOE ε4, hippocampal volume, metabolism, cognition, total-tau and phosphorylated-tau (the 95% confidence intervals of the adjusted effects did not include zero). Cerebrospinal fluid amyloid-β was more strongly related to APOE ε4 whereas positron emission tomography amyloid-β was more strongly related to tau levels (P < 0.05). Discordance (mainly isolated cerebrospinal fluid amyloid-β positivity) differed by diagnostic group (P < 0.001) and was seen in 21% of cognitively healthy people but only 6% in dementia patients. The finding that cerebrospinal fluid and positron emission tomography amyloid-β provide partially independent information about a wide range of Alzheimer's measures supports the theory that these modalities represent partly different aspects of Alzheimer's pathology. The fact that mismatch, with positive cerebrospinal fluid amyloid-β but normal positron emission tomography amyloid-β, is relatively common in cognitively healthy people may be considered when using these biomarkers to identify early stage Alzheimer's disease. Reduced cerebrospinal fluid amyloid-β may be more strongly related to early stage Alzheimer's disease, whereas increased positron emission tomography amyloid-β may be more strongly related to disease progression.

Cerebrospinal fluid biomarkers in trials for Alzheimer and Parkinson diseases

Alzheimer disease (AD) and Parkinson disease (PD) are the most common neurodegenerative disorders. For both diseases, early intervention is thought to be essential to the success of disease-modifying treatments. Cerebrospinal fluid (CSF) can reflect some of the pathophysiological changes that occur in the brain, and the number of CSF biomarkers under investigation in neurodegenerative conditions has grown rapidly in the past 20 years. In AD, CSF biomarkers are increasingly being used in clinical practice, and have been incorporated into the majority of clinical trials to demonstrate target engagement, to enrich or stratify patient groups, and to find evidence of disease modification. In PD, CSF biomarkers have not yet reached the clinic, but are being studied in patients with parkinsonism, and are being used in clinical trials either to monitor progression or to demonstrate target engagement and downstream effects of drugs. CSF biomarkers might also serve as surrogate markers of clinical benefit after a specific therapeutic intervention, although additional data are required. It is anticipated that CSF biomarkers will have an important role in trials aimed at disease modification in the near future. In this Review, we provide an overview of CSF biomarkers in AD and PD, and discuss their role in clinical trials.

The Road Ahead to Cure Alzheimer's Disease: Development of Biological Markers and Neuroimaging Methods for Prevention Trials Across all Stages and Target Populations

Alzheimer's disease (AD) is a slowly progressing non-linear dynamic brain disease in which pathophysiological abnormalities, detectable in vivo by biological markers, precede overt clinical symptoms by many years to decades. Use of these biomarkers for the detection of early and preclinical AD has become of central importance following publication of two international expert working group's revised criteria for the diagnosis of AD dementia, mild cognitive impairment (MCI) due to AD, prodromal AD and preclinical AD. As a consequence of matured research evidence six AD biomarkers are sufficiently validated and partly qualified to be incorporated into operationalized clinical diagnostic criteria and use in primary and secondary prevention trials. These biomarkers fall into two molecular categories: biomarkers of amyloid-beta (Aβ) deposition and plaque formation as well as of tau-protein related hyperphosphorylation and neurodegeneration. Three of the six gold-standard ("core feasible) biomarkers are neuroimaging measures and three are cerebrospinal fluid (CSF) analytes. CSF Aβ1-42 (Aβ1-42), also expressed as Aβ1-42 : Aβ1-40 ratio, T-tau, and P-tau Thr181 & Thr231 proteins have proven diagnostic accuracy and risk enhancement in prodromal MCI and AD dementia. Conversely, having all three biomarkers in the normal range rules out AD. Intermediate conditions require further patient follow-up. Magnetic resonance imaging (MRI) at increasing field strength and resolution allows detecting the evolution of distinct types of structural and functional abnormality pattern throughout early to late AD stages. Anatomical or volumetric MRI is the most widely used technique and provides local and global measures of atrophy. The revised diagnostic criteria for "prodromal AD" and "mild cognitive impairment due to AD" include hippocampal atrophy (as the fourth validated biomarker), which is considered an indicator of regional neuronal injury. Advanced image analysis techniques generate automatic and reproducible measures both in regions of interest, such as the hippocampus and in an exploratory fashion, observer and hypothesis-indedendent, throughout the entire brain. Evolving modalities such as diffusion-tensor imaging (DTI) and advanced tractography as well as resting-state functional MRI provide useful additionally useful measures indicating the degree of fiber tract and neural network disintegration (structural, effective and functional connectivity) that may substantially contribute to early detection and the mapping of progression. These modalities require further standardization and validation. The use of molecular in vivo amyloid imaging agents (the fifth validated biomarker), such as the Pittsburgh Compound-B and markers of neurodegeneration, such as fluoro-2-deoxy-D-glucose (FDG) (as the sixth validated biomarker) support the detection of early AD pathological processes and associated neurodegeneration. How to use, interpret, and disclose biomarker results drives the need for optimized standardization. Multimodal AD biomarkers do not evolve in an identical manner but rather in a sequential but temporally overlapping fashion. Models of the temporal evolution of AD biomarkers can take the form of plots of biomarker severity (degree of abnormality) versus time. AD biomarkers can be combined to increase accuracy or risk. A list of genetic risk factors is increasingly included in secondary prevention trials to stratify and select individuals at genetic risk of AD. Although most of these biomarker candidates are not yet qualified and approved by regulatory authorities for their intended use in drug trials, they are nonetheless applied in ongoing clinical studies for the following functions: (i) inclusion/exclusion criteria, (ii) patient stratification, (iii) evaluation of treatment effect, (iv) drug target engagement, and (v) safety. Moreover, novel promising hypothesis-driven, as well as exploratory biochemical, genetic, electrophysiological, and neuroimaging markers for use in clinical trials are being developed. The current state-of-the-art and future perspectives on both biological and neuroimaging derived biomarker discovery and development as well as the intended application in prevention trials is outlined in the present publication.

β-site amyloid precursor protein-cleaving enzyme 1(BACE1) inhibitor treatment induces Aβ5-X peptides through alternative amyloid precursor protein cleavage

Introduction

The β-secretase enzyme, β-site amyloid precursor protein-cleaving enzyme 1 (BACE1), cleaves amyloid precursor protein (APP) in the first step in β-amyloid (Aβ) peptide production. Thus, BACE1 is a key target for candidate disease-modifying treatment of Alzheimer’s disease. In a previous exploratory Aβ biomarker study, we found that BACE1 inhibitor treatment resulted in decreased levels of Aβ1-34 together with increased Aβ5-40, suggesting that these Aβ species may be novel pharmacodynamic biomarkers in clinical trials. We have now examined whether the same holds true in humans.

Methods

In an investigator-blind, placebo-controlled and randomized study, healthy subjects (n =18) were randomly assigned to receive a single dose of 30 mg of LY2811376 (n =6), 90 mg of LY2811376 (n =6), or placebo (n =6). We used hybrid immunoaffinity-mass spectrometry (HI-MS) and enzyme-linked immunosorbent assays to monitor a variety of Aβ peptides.

Results

Here, we demonstrate dose-dependent changes in cerebrospinal fluid (CSF) Aβ1-34, Aβ5-40 and Aβ5-X after treatment with the BACE1-inhibitor LY2811376. Aβ5-40 and Aβ5-X increased dose-dependently, as reflected by two independent methods, while Aβ1-34 dose-dependently decreased.

Conclusion

Using HI-MS for the first time in a study where subjects have been treated with a BACE inhibitor, we confirm that CSF Aβ1-34 may be useful in clinical trials on BACE1 inhibitors to monitor target engagement. Since it is less hydrophobic than longer Aβ species, it is less susceptible to preanalytical confounding factors and may thus be a more stable marker. By independent measurement techniques, we also show that BACE1 inhibition in humans is associated with APP-processing into N-terminally truncated Aβ peptides via a BACE1-independent pathway.

Trial registration

ClinicalTrials.gov NCT00838084. Registered: First received: January 23, 2009, Last updated: July 14, 2009, Last verified: July 2009.

Biomarker modelling of early molecular changes in Alzheimer's disease

The preclinical phase of Alzheimer's disease (AD) occurs years, possibly decades, before the onset of clinical symptoms. Being able to detect the very earliest stages of AD is critical to improving understanding of AD biology, and identifying individuals at greatest risk of developing clinical symptoms with a view to treating AD pathophysiology before irreversible neurodegeneration occurs. Studies of dominantly inherited AD families and longitudinal studies of sporadic AD have contributed to knowledge of the earliest AD biomarkers. Here we appraise this evidence before reviewing novel, particularly fluid, biomarkers that may provide insights into AD pathogenesis and relate these to existing hypothetical disease models.

CSF in Alzheimer's disease

Alzheimer's disease (AD) is a progressive brain amyloidosis that injures brain regions involved in memory consolidation and other cognitive functions. Neuropathologically, the disease is characterized by accumulation of a 42-amino acid protein called amyloid beta, and N-terminally truncated fragments thereof, in extracellular senile plaques together with intraneuronal inclusions of hyperphosphorylated tau protein in neurofibrillary tangles, and neuronal and axonal degeneration and loss. Clinical chemistry tests for these pathologies have been developed for use on cerebrospinal fluid samples. Here, we review what these markers have taught us on the disease process in AD and how they can be implemented in routine clinical chemistry. We also provide an update on new marker development and ongoing analytical standardization effort.

N-truncation and pyroglutaminylation enhances the opsonizing capacity of Aβ-peptides and facilitates phagocytosis by macrophages and microglia

Abnormal accumulations of amyloid-β (Aβ)-peptides are one of the pathological hallmarks of Alzheimer's disease (AD). The precursor of the Aβ-peptides, the amyloid precursor protein (APP), is also found in peripheral blood cells, but its function in these cells remains elusive. We previously observed that mononuclear phagocytes release Aβ-peptides during activation and phagocytosis, suggesting a physiologic role in inflammatory processes. Here, we show that supplementing the media with soluble N-terminally truncated Aβ(2-40) and Aβ(2-42) as well as Aβ(1-42) induced the phagocytosis of polystyrene particles (PSPs) by primary human monocytes. If the PSPs were pre-incubated with Aβ-peptides, phagocytosis was induced by all tested Aβ-peptide species. N-terminally truncated Aβ(x-42) induced the phagocytosis of PSPs significantly more effectively than did Aβ(x-40). Similarly, the phagocytosis of Escherichia coli by GM-CSF- and M-CSF-elicited macrophages as well as microglia was particularly facilitated by pre-incubation with N-terminally truncated Aβ(x-42). The proinflammatory polarization of monocytes was indicated by the reduced MSRI expression and IL-10 secretion after phagocytosis of PSPs coated with Aβ(1-42), Aβ(2-42) and Aβ(3p-42). Polarization of the macrophages by GM-CSF reduced the phagocytic activity, but it did not affect the capabilities of Aβ-peptides to opsonize prey. Taken together, Aβ-peptides support phagocytosis as soluble factors and act as opsonins. Differential effects among the Aβ-peptide variants point to distinct mechanisms of interaction among monocytes/macrophages, prey and Aβ-peptides. A proinflammatory polarization induced by the phagocytosis of Aβ-peptide coated particles may provide a model for the chronic inflammatory reaction and sustained plaque deposition in AD.

CNS amyloid-β, soluble APP-α and -β kinetics during BACE inhibition

BACE, a β-secretase, is an attractive potential disease-modifying therapeutic strategy for Alzheimer's disease (AD) as it results directly in the decrease of amyloid precursor protein (APP) processing through the β-secretase pathway and a lowering of CNS amyloid-β (Aβ) levels. The interaction of the β-secretase and α-secretase pathway-mediated processing of APP in the rhesus monkey (nonhuman primate; NHP) CNS is not understood. We hypothesized that CNS inhibition of BACE would result in decreased newly generated Aβ and soluble APPβ (sAPPβ), with increased newly generated sAPPα. A stable isotope labeling kinetics experiment in NHPs was performed with a (13)C6-leucine infusion protocol to evaluate effects of BACE inhibition on CNS APP processing by measuring the kinetics of sAPPα, sAPPβ, and Aβ in CSF. Each NHP received a low, medium, or high dose of MBI-5 (BACE inhibitor) or vehicle in a four-way crossover design. CSF sAPPα, sAPPβ, and Aβ were measured by ELISA and newly incorporated label following immunoprecipitation and liquid chromatography-mass spectrometry. Concentrations, kinetics, and amount of newly generated APP fragments were calculated. sAPPβ and sAPPα kinetics were similar, but both significantly slower than Aβ. BACE inhibition resulted in decreased labeled sAPPβ and Aβ in CSF, without observable changes in labeled CSF sAPPα. ELISA concentrations of sAPPβ and Aβ both decreased and sAPPα increased. sAPPα increased by ELISA, with no difference by labeled sAPPα kinetics indicating increases in product may be due to APP shunting from the β-secretase to the α-secretase pathway. These results provide a quantitative understanding of pharmacodynamic effects of BACE inhibition on NHP CNS, which can inform about target development.

BACE1 as a therapeutic target in Alzheimer's disease: rationale and current status

Alzheimer's disease (AD) is a neurodegenerative disease of the central nervous system that causes dementia in a large percentage of the aged population and for which there are only symptomatic treatments. Disease-modifying therapies that are currently being pursued are based on the amyloid cascade theory. This states that accumulation of amyloid β (Aβ) in the brain triggers a cascade of cellular events leading to neurodegeneration. Aβ, which is the major constituent of amyloid plaques, is a peptidic fragment derived from proteolytic processing of the amyloid precursor protein (APP) by sequential cleavages that involve β-site APP-cleaving enzyme 1 (BACE1) and γ-secretase. Targeting BACE1 is a rational approach as its cleavage of APP is the rate-limiting step in Aβ production and this enzyme is elevated in the brain of patients with AD. Furthermore, knocking out the BACE1 gene in mice showed little apparent consequences. Ten years of intensive research has led to the design of efficacious BACE1 inhibitors with favorable pharmacological properties. Several drug candidates have shown promising results in animal models, as they reduce amyloid plaque pathology in the brain and rescue cognitive deficits. Phase I clinical trials indicate that these drugs are well tolerated, and the results from further trials in AD patients are now awaited eagerly. Yet, recent novel information on BACE1 biology, and the discovery that BACE1 cleaves a selection of substrates involved in myelination, retinal homeostasis, brain circuitry, and synaptic function, alert us to potential side effects of BACE1 inhibitors that will require further evaluation to provide a safe therapy for AD.