Cerebrospinal Fluid Levels of Amyloid Beta 1-43 in Patients with Amnestic Mild Cognitive Impairment or Early Alzheimer's Disease: A 2-Year Follow-Up Study

Oligomer Formation by Physiologically Relevant C-Terminal Isoforms of Amyloid β-Protein

Alzheimer's disease (AD) is a neurological disorder associated with amyloid β-protein (Aβ) assembly into toxic oligomers. In addition to the two predominant alloforms, Aβ1-40 and Aβ1-42, other C-terminally truncated Aβ peptides, including Aβ1-38 and Aβ1-43, are produced in the brain. Here, we use discrete molecular dynamics (DMD) and a four-bead protein model with amino acid-specific hydropathic interactions, DMD4B-HYDRA, to examine oligomer formation of Aβ1-38, Aβ1-40, Aβ1-42, and Aβ1-43. Self-assembly of 32 unstructured monomer peptides into oligomers is examined using 32 replica DMD trajectories for each of the four peptides. In a quasi-steady state, Aβ1-38 and Aβ1-40 adopt similar unimodal oligomer size distributions with a maximum at trimers, whereas Aβ1-42 and Aβ1-43 oligomer size distributions are multimodal with the dominant maximum at trimers or tetramers, and additional maxima at hexamers and unidecamers (for Aβ1-42) or octamers and pentadecamers (for Aβ1-43). The free energy landscapes reveal isoform- and oligomer-order specific structural and morphological features of oligomer ensembles. Our results show that oligomers of each of the four isoforms have unique features, with Aβ1-42 alone resulting in oligomers with disordered and solvent-exposed N-termini. Our findings help unravel the structure-function paradigm governing oligomers formed by various Aβ isoforms.

Aβ43 levels determine the onset of pathological amyloid deposition

About 2% of Alzheimer's disease (AD) cases have early onset (FAD) and are caused by mutations in either Presenilins (PSEN1/2) or amyloid-β precursor protein (APP). PSEN1/2 catalyze production of Aβ peptides of different length from APP. Aβ peptides are the major components of amyloid plaques, a pathological lesion that characterizes AD. Analysis of mechanisms by which PSEN1/2 and APP mutations affect Aβ peptide compositions lead to the implication of the absolute or relative increase in Aβ42 in amyloid-β plaques formation. Here, to elucidate the formation of pathogenic Aβ cocktails leading to amyloid pathology, we utilized FAD rat knock-in models carrying the Swedish APP (Apps allele) and the PSEN1 L435F (Psen1LF allele) mutations. To accommodate the differences in the pathogenicity of rodent and human Aβ, these rat models are genetically engineered to express human Aβ species as both the Swedish mutant allele and the WT rat allele (called Apph) have been humanized in the Aβ-coding region. Analysis of the eight possible FAD mutant permutations indicates that the CNS levels of Aβ43, rather than absolute or relative increases in Aβ42, determine the onset of pathological amyloid deposition in FAD knock-in rats. Notably, Aβ43 was found in amyloid plaques in late onset AD and mild cognitive impairment cases, suggesting that the mechanisms initiating amyloid pathology in FAD knock-in rat reflect disease mechanisms driving amyloid pathology in late onset AD. This study helps clarifying the molecular determinants initiating amyloid pathology and supports therapeutic interventions targeting Aβ43 in AD.

Alzheimer’s Disease Biomarkers Revisited From the Amyloid Cascade Hypothesis Standpoint

Alzheimer’s disease (AD) is the most common neurodegenerative disease worldwide. Amyloid beta (Aβ) is one of the proteins which aggregate in AD, and its key role in the disease pathogenesis is highlighted in the amyloid cascade hypothesis, which states that the deposition of Aβ in the brain parenchyma is a crucial initiating step in the future development of AD. The sensitivity of instruments used to measure proteins in blood and cerebrospinal fluid has significantly improved, such that Aβ can now successfully be measured in plasma. However, due to the peripheral production of Aβ, there is significant overlap between diagnostic groups. The presence of pathological Aβ within the AD brain has several effects on the cells and surrounding tissue. Therefore, there is a possibility that using markers of tissue responses to Aβ may reveal more information about Aβ pathology and pathogenesis than looking at plasma Aβ alone. In this manuscript, using the amyloid cascade hypothesis as a starting point, we will delve into how the effect of Aβ on the surrounding tissue can be monitored using biomarkers. In particular, we will consider whether glial fibrillary acidic protein, triggering receptor expressed on myeloid cells 2, phosphorylated tau, and neurofilament light chain could be used to phenotype and quantify the tissue response against Aβ pathology in AD.

Transitioning from cerebrospinal fluid to blood tests to facilitate diagnosis and disease monitoring in Alzheimer's disease

Alzheimer's disease (AD) is increasingly prevalent worldwide, and disease-modifying treatments may soon be at hand; hence, now, more than ever, there is a need to develop techniques that allow earlier and more secure diagnosis. Current biomarker-based guidelines for AD diagnosis, which have replaced the historical symptom-based guidelines, rely heavily on neuroimaging and cerebrospinal fluid (CSF) sampling. While these have greatly improved the diagnostic accuracy of AD pathophysiology, they are less practical for application in primary care, population-based and epidemiological settings, or where resources are limited. In contrast, blood is a more accessible and cost-effective source of biomarkers in AD. In this review paper, using the recently proposed amyloid, tau and neurodegeneration [AT(N)] criteria as a framework towards a biological definition of AD, we discuss recent advances in biofluid-based biomarkers, with a particular emphasis on those with potential to be translated into blood-based biomarkers. We provide an overview of the research conducted both in CSF and in blood to draw conclusions on biomarkers that show promise. Given the evidence collated in this review, plasma neurofilament light chain (N) and phosphorylated tau (p-tau; T) show particular potential for translation into clinical practice. However, p-tau requires more comparisons to be conducted between its various epitopes before conclusions can be made as to which one most robustly differentiates AD from non-AD dementias. Plasma amyloid beta (A) would prove invaluable as an early screening modality, but it requires very precise tests and robust pre-analytical protocols.

The Amyloid, Tau, and Neurodegeneration (A/T/N) Classification Applied to a Clinical Research Cohort with Long-Term Follow-Up

Background:

The unbiased amyloid, tau, and neurodegeneration (A/T/N) classification is designed to characterize individuals in the Alzheimer continuum and is currently little explored in clinical cohorts.

Objective:

A retrospective comparison of the A/T/N classification system with the results of a two-year clinical study, with extended follow-up up to 10 years after inclusion.

Methods:

Patients (n = 102) clinically diagnosed as Alzheimer’s disease (AD) with dementia or amnestic mild cognitive impairment (MCI), and 61 cognitively healthy control individuals were included. Baseline cerebrospinal fluid core biomarkers for AD (Aβ₄₂, phosphorylated tau, and total tau) were applied to the A/T/N classification using the final clinical diagnosis at extended follow-up as the gold standard.

Results:

A + T + N+ was a strong predictor for AD dementia, even among cognitively healthy individuals. Amnestic MCI was heterogenous, considering both clinical outcome and distribution within A/T/N. Some individuals with amnestic MCI progressed to clinical AD dementia within all four major A/T/N groups. The highest proportion of progression was among triple positive cases, but progression was also common in individuals with suspected non-Alzheimer pathophysiology (A-T + N+), and those with triple negative status. A-T-N- individuals who were cognitively healthy overwhelmingly remained cognitively intact over time, but in amnestic MCI the clinical outcome was heterogenous, including AD dementia, other dementias, and recovery.

Conclusion:

The A/T/N framework accentuates biomarkers over clinical status. However, when selecting individuals for research, a combination of the two may be necessary since the prognostic value of the A/T/N framework depends on clinical status.

Quantitative Measurement of Cerebrospinal Fluid Amyloid-β Species by Mass Spectrometry

Background:

High sensitivity liquid chromatography mass spectrometry (LC-MS/MS) was recently introduced to measure amyloid-β (Aβ) species, allowing for a simultaneous assay that is superior to ELISA, which requires more assay steps with multiple antibodies.

Objective:

We validated the Aβ_1-38, Aβ_1-40, Aβ_1-42, and Aβ_1-43 assay by LC-MS/MS and compared it with ELISA using cerebrospinal fluid (CSF) samples to investigate its feasibility for clinical application.

Methods:

CSF samples from 120 subjects [8 Alzheimer’s disease (AD) with dementia (ADD), 2 mild cognitive dementia due to Alzheimer’s disease (ADMCI), 14 cognitively unimpaired (CU), and 96 neurological disease subjects] were analyzed. Aβ species were separated using the Shimadzu Nexera X2 system and quantitated using a Qtrap 5500 LC-MS/MS system. Aβ_1-40 and Aβ_1-42 levels were validated using ELISA.

Results:

CSF levels in CU were 666±249 pmol/L in Aβ_1-38, 2199±725 pmol/L in Aβ_1-40, 153.7±79.7 pmol/L in Aβ_1-42, and 9.78±4.58 pmol/L in Aβ_1-43. The ratio of the amounts of Aβ_1-38, Aβ_1-40, Aβ_1-42, and Aβ_1-43 was approximately 68:225:16:1. Linear regression analyses showed correlations among the respective Aβ species. Both Aβ_1-40 and Aβ_1-42 values were strongly correlated with ELISA measurements. No significant differences were observed in Aβ_1-38 or Aβ_1-40 levels between AD and CU. Aβ_1-42 and Aβ_1-43 levels were significantly lower, whereas the Aβ_1-38/1-42, Aβ_1-38/1-43, and Aβ_1-40/Aβ_1-43 ratios were significantly higher in AD than in CU. The basic assay profiles of the respective Aβ species were adequate for clinical usage.

Conclusion:

A quantitative LC-MS/MS assay of CSF Aβ species is as reliable as specific ELISA for clinical evaluation of CSF biomarkers for AD.

Plasma isoprostanoids assessment as Alzheimer's disease progression biomarkers

Alzheimer's Disease (AD) is the most common neurodegenerative disease worldwide. So, there is a need to identify AD early diagnosis and monitoring biomarkers in blood samples. The aim of this study was to analyse the utility of lipid peroxidation biomarkers in AD progression evaluation. Participants (n = 19) were diagnosed with AD at early stages (Time 0, T0), and they were re-evaluated 2 years later (Time 1, T1). Plasma biomarkers from AD patients were determined at both times. Some analytes, such as dihomo-isoprostanes (17-epi-17-F2t-dihomo-IsoP, 17-F2t-dihomo-IsoP, Ent-7(RS)-7-F2t-dihomo-IsoP), and neuroprostanes (10-epi-10-F4t-NeuroP) showed very high probability of showing an increasing trend over time. Baseline values allowed to develop an affordable preliminary regression model to predict long-term cognitive status. So, some lipid peroxidation biomarkers would deserve consideration as useful progression AD biomarkers. The developed prediction model would constitute an important minimally invasive approach in AD personalized prognosis and perhaps could have some interest also in experimental treatments evaluation.

Amyloid-β1-43 cerebrospinal fluid levels and the interpretation of APP, PSEN1 and PSEN2 mutations

Background

Alzheimer’s disease (AD) mutations in amyloid precursor protein (APP) and presenilins (PSENs) could potentially lead to the production of longer amyloidogenic Aβ peptides. Amongst these, Aβ_1–43 is more prone to aggregation and has higher toxic properties than the long-known Aβ_1–42. However, a direct effect on Aβ_1–43 in biomaterials of individuals carrying genetic mutations in the known AD genes is yet to be determined.

Methods

N = 1431 AD patients (n = 280 early-onset (EO) and n = 1151 late-onset (LO) AD) and 809 control individuals were genetically screened for APP and PSENs. For the first time, Aβ_1–43 levels were analysed in cerebrospinal fluid (CSF) of 38 individuals carrying pathogenic or unclear rare mutations or the common PSEN1 p.E318G variant and compared with Aβ_1–42 and Aβ_1–40 CSF levels.

The soluble sAPPα and sAPPβ species were also measured for the first time in mutation carriers.

Results

A known pathogenic mutation was identified in 5.7% of EOAD patients (4.6% PSEN1, 1.07% APP) and in 0.3% of LOAD patients. Furthermore, 12 known variants with unclear pathogenicity and 11 novel were identified. Pathogenic and unclear mutation carriers showed a significant reduction in CSF Aβ_1–43 levels compared to controls (p = 0.037; < 0.001). CSF Aβ_1–43 levels positively correlated with CSF Aβ_1–42 in both pathogenic and unclear carriers and controls (all p < 0.001). The p.E318G carriers showed reduced Aβ_1–43 levels (p < 0.001), though genetic association with AD was not detected. sAPPα and sAPPβ CSF levels were significantly reduced in the group of unclear (p = 0.006; 0.005) and p.E318G carriers (p = 0.004; 0.039), suggesting their possible involvement in AD. Finally, using Aβ_1–43 and Aβ_1–42 levels, we could re-classify as “likely pathogenic” 3 of the unclear mutations.

Conclusion

This is the first time that Aβ_1–43 levels were analysed in CSF of AD patients with genetic mutations in the AD causal genes. The observed reduction of Aβ_1–43 in APP and PSENs carriers highlights the pathogenic role of longer Aβ peptides in AD pathogenesis. Alterations in Aβ_1–43 could prove useful in understanding the pathogenicity of unclear APP and PSENs variants, a critical step towards a more efficient genetic counselling.

Cerebrospinal Fluid Aβ43 Is Reduced in Early-Onset Compared to Late-Onset Alzheimer's Disease, But Has Similar Diagnostic Accuracy to Aβ42

Background: Amyloid beta 1-43 (Aβ43) may be a useful additional biomarker for diagnosing Alzheimer's disease (AD). We have investigated cerebrospinal fluid (CSF) levels of Aβ43 in patients with early-onset AD in contrast to levels in late-onset AD. For comparison, in addition to the 'core' biomarkers, several other analytes were also determined [YKL-40, neurofilament light (NF-L), glial fibrillary acidic protein (GFAP), and progranulin]. Material and Methods: Cerebrospinal fluid samples were obtained from patients with early-onset AD (age ≤ 62, n = 66), late-onset AD (age ≥ 68, n = 25), and groups of cognitively intact individuals (age ≤ 62, n = 41, age ≥ 68, n = 39). Core CSF AD biomarkers [amyloid beta 1-42 (Aβ42), total tau, phosphorylated tau] were analyzed, as well as levels of Aβ43 and other analytes, using commercially available enzyme-linked immunosorbent assays. Results: Cerebrospinal fluid Aβ43 was significantly reduced in early-onset AD compared to late-onset AD (14.8 ± 7.3 vs. 21.8 ± 9.4 pg/ml, respectively), whereas the levels of Aβ42 in the two AD groups were not significantly different (474.9 ± 142.0 vs. 539.6 ± 159.9 pg/ml, respectively). Aβ43 and all core biomarkers were significantly altered in patients with AD compared to corresponding controls. NF-L was significantly increased in early-onset AD compared to younger controls, an effect not found between the older groups. Relationships between the Aβ peptides and tau proteins, YKL-40, NF-L, GFAP and progranulin were also investigated without finding marked associations. However, age-associated increases in levels of tau proteins, YKL-40, NF-L and GFAP were found with respect to age in healthy controls. Results for these other analytes were similar to previously published data. Aβ43 did not improve diagnostic accuracy in either AD group compared to Aβ42.

DISCUSSION

Cerebrospinal fluid Aβ43, but not Aβ42 levels, varied significantly with age in patients with AD. If CSF levels of Aβ peptides reflect amyloid deposition in brain, the possibility arises that there is a difference between Aβ43 and Aβ42 deposition in younger compared to older brain. However, the level of Aβ43 in CSF shows no improvement over Aβ42 regarding diagnostic accuracy.

Cerebrospinal Fluid Levels of Amyloid Beta 1-43 Mirror 1-42 in Relation to Imaging Biomarkers of Alzheimer's Disease

Introduction: Amyloid beta 1-43 (Aβ43), with its additional C-terminal threonine residue, is hypothesized to play a role in early Alzheimer’s disease pathology possibly different from that of amyloid beta 1-42 (Aβ42). Cerebrospinal fluid (CSF) Aβ43 has been suggested as a potential novel biomarker for predicting conversion from mild cognitive impairment (MCI) to dementia in Alzheimer’s disease. However, the relationship between CSF Aβ43 and established imaging biomarkers of Alzheimer’s disease has never been assessed.

Materials and Methods: In this observational study, CSF Aβ43 was measured with ELISA in 89 subjects; 34 with subjective cognitive decline (SCD), 51 with MCI, and four with resolution of previous cognitive complaints. All subjects underwent structural MRI; 40 subjects on a 3T and 50 on a 1.5T scanner. Forty subjects, including 24 with SCD and 12 with MCI, underwent ¹⁸F-Flutemetamol PET. Seventy-eight subjects were assessed with ¹⁸F-fluorodeoxyglucose PET (21 SCD/7 MCI and 11 SCD/39 MCI on two different scanners). Ten subjects with SCD and 39 with MCI also underwent diffusion tensor imaging.

Results: Cerebrospinal fluid Aβ43 was both alone and together with p-tau a significant predictor of the distinction between SCD and MCI. There was a marked difference in CSF Aβ43 between subjects with ¹⁸F-Flutemetamol PET scans visually interpreted as negative (37 pg/ml, n = 27) and positive (15 pg/ml, n = 9), p < 0.001. Both CSF Aβ43 and Aβ42 were negatively correlated with standardized uptake value ratios for all analyzed regions; CSF Aβ43 average rho -0.73, Aβ42 -0.74. Both CSF Aβ peptides correlated significantly with hippocampal volume, inferior parietal and frontal cortical thickness and axial diffusivity in the corticospinal tract. There was a trend toward CSF Aβ42 being better correlated with cortical glucose metabolism. None of the studied correlations between CSF Aβ43/42 and imaging biomarkers were significantly different for the two Aβ peptides when controlling for multiple testing.

Conclusion: Cerebrospinal fluid Aβ43 appears to be strongly correlated with cerebral amyloid deposits in the same way as Aβ42, even in non-demented patients with only subjective cognitive complaints. Regarding imaging biomarkers, there is no evidence from the present study that CSF Aβ43 performs better than the classical CSF biomarker Aβ42 for distinguishing SCD and MCI.