Pathophysiology of Alzheimer's disease: advances in drug development and therapeutic innovations

BackgroundRecently, several studies suggested potential involvements of α‐synuclein in Alzheimer's disease (AD) pathophysiology. Higher concentrations of α‐synuclein were reported in cerebrospinal fluid (CSF) of AD patients with a positive correlation towards CSF tau, indicating its possible role in AD. We analyzed the CSF biomarkers to verify whether α‐synuclein could be an additional supported biomarker in AD diagnosis.MethodIn this cross‐sectional study, CSF samples of 71 early‐onset AD, 34 late‐onset AD, 11 mild cognitive impairment, 17 subjective cognitive decline, 45 Parkinson’s disease, and 32 healthy control (HC) were collected. CSF amyloid‐β1‐42 (A), total tau (N), and phosphorylated tau181 (T) were measured by commercial ELISA kits, and in‐house ELISA kit was developed to quantify α‐synuclein. The cognitive assessments and amyloid‐PET imaging were also performed.ResultCSF α‐synuclein manifested a tendency to increase in AD and to decreased in Parkinson’s disease compared to HC. The equilibrium states of total tau and α‐synuclein concentrations were changed significantly in AD, and the ratio of total tau/α‐synuclein (N/αS) was dramatically increased in AD than HC. Remarkably, N/αS revealed a strong positive correlation with tau phosphorylation rate. Also, the combination of N/αS with amyloid‐β1‐42/phosphorylated tau181 had the best diagnosis performance (AUC = 0.956, sensitivity = 96%, specificity = 87%). In concordance analysis, N/αS showed the higher diagnostic agreement with amyloid‐β1‐42 and amyloid‐PET. Analysis of biomarker profiling with N/αS had distinctive characteristics and clustering of each group. Especially, among the group of suspected non‐Alzheimer's disease pathophysiology, all A‐T+N+ patients with N/αS+ were reintegrated into AD.ConclusionThe high correlation of α‐synuclein with tau and the elevated N/αS in AD supported the involvement of α‐synuclein in AD pathophysiology. Importantly, N/αS improved the diagnostic performance, confirming the needs of incorporating α‐synuclein as a biomarker for neurodegenerative disorders. The incorporation of a biomarker group [N/αS] could contribute to provide better understanding and diagnosis of neurodegenerative disorders.

This manuscript outlines a model of Alzheimer's Disease (AD) pathophysiology in progressive layers, from its genesis to the development of biomarkers and then to symptom expression. Genetic predispositions are the major factor that leads to mitochondrial dysfunction and subsequent amyloid and tau protein accumulation, which have been identified as hallmarks of AD. Extending beyond these accumulations, we explore a broader spectrum of pathophysiological aspects, including the blood-brain barrier, blood flow, vascular health, gut-brain microbiodata, glymphatic flow, metabolic syndrome, energy deficit, oxidative stress, calcium overload, inflammation, neuronal and synaptic loss, brain matter atrophy, and reduced growth factors. Photobiomodulation (PBM), which delivers near-infrared light to selected brain regions using portable devices, is introduced as a therapeutic approach. PBM has the potential to address each of these pathophysiological aspects, with data provided by various studies. They provide mechanistic support for largely small published clinical studies that demonstrate improvements in memory and cognition. They inform of PBM's potential to treat AD pending validation by large randomized controlled studies. The presentation of brain network and waveform changes on electroencephalography (EEG) provide the opportunity to use these data as a guide for the application of various PBM parameters to improve outcomes. These parameters include wavelength, power density, treatment duration, LED positioning, and pulse frequency. Pulsing at specific frequencies has been found to influence the expression of waveforms and modifications of brain networks. The expression stems from the modulation of cellular and protein structures as revealed in recent studies. These findings provide an EEG-based guide for the use of artificial intelligence to personalize AD treatment through EEG data feedback.

The causes of late onset Alzheimer disease (AD) are poorly understood. Although beta-amyloid (Abeta) is thought to play a critical role in the pathophysiology of AD, no genetic evidence directly ties Abeta to late onset AD. This suggests that the accumulation of Abeta and neurodegeneration associated with AD might result from an abnormality that indirectly affects Abeta production or accumulation. Increasing evidence suggests that abnormalities in the metabolism of cholesterol and related molecules, such as cholseterol esters and 24(S) hydroxycholesterol might contribute to the pathophysiology of late onset AD by increasing production of Abeta. 24(S) Hydroxycholesterol is a member of a family of oxidized cholesterol catabolites, termed oxysterols, which function to regulate export of cholesterol from the cell and transcription of genes related to cholesterol metabolism. Cholesterol esters are cholesterol derivatives used for cholesterol storage. Levels of 24(S) hydroxycholesterol increase with AD. Polymorphisms in several different genes important for cholesterol physiology are associated with an increased load or level of Abeta in AD. These genes include apolipoprotein E, cholesterol 24 hydroxylase (Cyp46), acyl-CoA:cholesterol acetyltransferase (ACAT), and the cholesterol transporter ABCA1. Other studies show that levels of cholesterol, or its precursors, are elevated in subjects early in the course of AD. Finally, studies of the processing of amyloid precursor protein show that cholesterol and its catabolites modulate amyloid precursor protein processing and Abeta production. These lines of evidence raise the possibility that genetic abnormalities in cholesterol metabolism might contribute to the pathophysiology of AD.

Harnessing the potential of long non-coding RNAs in the pathophysiology of Alzheimer's disease.

The cdc2-like kinases (CLKs) are an evolutionarily conserved group of dual specificity kinases belonging to the CMGC (cyclin-dependent kinases (CDKs), mitogen-activated protein kinases (MAP kinases), glycogen synthase kinases (GSK) and CDK-like kinases). The CLK family consists of four isoforms namely CLK1, CLK2, CLK3 and CLK4. The human CLK1 encoded protein comprises 454 amino acids and the catalytic domain of CLK1 exhibits the typical protein kinase fold. CLK1 has been shown to autophosphorylate on serine, threonine and tyrosine residues and phosphorylate exogenous substrates on serine and threonine residues. CLK1 plays an important role in the regulation of RNA splicing through phosphorylation of members of the serine and arginine-rich (SR) family of splicing factors. CLK1 is involved in the pathophysiology of Alzheimer's disease by phosphorylating the serine residue in SR proteins. Nuclear speckles of the nucleoplasm contain the stored form of SR proteins and are moderately responsible for the choice of splicing sites during pre-mRNA splicing. Hence, the inhibition of CLK1 can be used as a therapeutic strategy for Alzheimer's disease. Many natural and synthetic molecules are reported to possess CLK1 inhibitory activity. Some specific examples are Marine alkaloid Leucettamine B and KH-CB19. Leucettamine B is a potent inhibitor of CLK1 (15 nM), Dyrk1A (40 nM), and Dyrk2 (35 nM) and a moderate inhibitor of CLK3 (4.5 µM) whereas KH-CB19 is a highly specific and potent inhibitor of the CLK1/CLK4. X-ray crystallographic studies have revealed the binding mode of marine sponge metabolite hymenialdisine and a dichloroindolyl enamino nitrile (KH-CB19) to CLK1. This review focuses on the role of CLKs in the pathophysiology of Alzheimer's disease and therapeutic potential of targeting CLK1 in Alzheimer's disease drug discovery and development. In addition, the recent developments in drug discovery efforts targeting human CLK1 are also highlighted.

Evidence indicates a critical role for cerebrovascular dysfunction in Alzheimer's disease (AD) pathophysiology. We have shown that fibrin(ogen), the principal blood-clotting protein, is deposited in the AD neurovasculature and interacts with beta-amyloid (Aβ), resulting in increased formation of blood clots. As apolipoprotein E (ApoE), a lipid-transporting protein with three human isoforms (E2, E3, and E4), also binds to Aβ, we hypothesized that ApoE and fibrin(ogen) may have a combined effect on the vascular pathophysiology in AD. We assessed whether APOE genotype differentially influences vascular fibrin(ogen) deposition in postmortem brain tissue using immunohistochemistry. An increased deposition of fibrin(ogen) was observed in AD cases compared with non-demented controls, and there was a strong correlation between cerebral amyloid angiopathy (CAA) severity and fibrin(ogen) deposition. Moreover, brains from AD cases homozygous for APOE ɛ4 showed increased deposition of fibrin(ogen), specifically in CAA- and oligomeric Aβ-positive vessels compared with AD APOE ɛ2 and ɛ3 allele carriers, an effect that was not directly linked to CAA severity and cerebrovascular atherosclerosis. These data further support a role for fibrin(ogen) in AD pathophysiology and link the APOE ɛ4/ɛ4 genotype with increased thrombosis and/or impaired fibrinolysis in the human AD brain.

Animal studies increasingly indicate that the gut microbiota composition and function can be involved in the pathophysiology and progression of Alzheimer's disease (AD) at multiple levels. However, few studies have investigated this putative gut-brain axis in human beings, and none of them considered diet as a determinant of intestinal microbiota composition. Epidemiological studies highlight that a high intake of fruit and vegetables, such as that typical of the Mediterranean diet, can modulate AD progression. Thus, nutritional interventions are being increasingly studied as a possible non-pharmacological strategy to slow down the progression of AD. In particular, polyphenols and fibers represent the nutritional compounds with the higher potential of counterbalancing the pathophysiological mechanisms of dementia due to their antioxidant, anti-inflammatory, and anti-apoptotic properties. These actions are mediated by the gut microbiota, that can transform polyphenols and fibers into biologically active compounds including, among others, phenyl-γ-valerolactones, urolithins, butyrate, and other short-chain fatty acids. In this review, the complex mechanisms linking nutrition, gut microbiota composition, and pathophysiology of cognitive decline in AD are discussed, with a particular focus on the role of polyphenols and fibers. The gaps between pre-clinical and clinical studies are particularly emphasized, as well as the urgent need for studies comprehensively evaluating the link between nutrition, microbiome, and clinical aspects of AD.

Chapter 11 - Stable Isotope Labeling Kinetics in CNS Translational Medicine: Introduction to SILK Technology

The role of white matter damage in the progression of Alzheimer's disease and the associated cognitive symptoms is becoming increasingly clearer. This is partly because of the advent of diffusion tensor imaging, which, in combination with other quantitative MRI techniques, offers unique insights into the patholophysiology of Alzheimer's disease in vivo. The purpose of this review is to integrate the most recent imaging findings, with respect to understanding Alzheimer's disease pathophysiology, and identifying potential biomarkers with diagnostic and prognostic value. Consistent with patterns of gray matter atrophy, white matter damage in Alzheimer's disease is localized within white matter tracts connecting the temporal lobe with the rest of the brain, including the cingulum, the uncinate fasciculus and the fornix. These abnormalities are often correlated with adjacent gray matter tissue loss, and with cognitive performance. The relationship between these findings and loss of functional connectivity supports the hypothesis of disconnection as a mechanism for the spread of Alzheimer's disease. White matter abnormalities occur early in Alzheimer's disease, and might actively contribute to the progression of the disease. Functional and structural gray matter abnormalities parallel the white matter changes, and successful biomarkers are likely to be multiparametric.

BackgroundMultiple studies have linked high cortisol levels, a frequently used biomarker of stress, with the Alzheimer's disease (AD) pathophysiology. However, the relationship between cerebrospinal fluid (CSF) cortisol levels and AD‐related brain atrophy is not fully understood. This study sought to investigate the cross‐sectional and longitudinal association between CSF cortisol levels and brain cortical thickness in patients across the biological and clinical continuum of AD.MethodsWe evaluated 310 individuals from the ADNI cohort with available baseline CSF cortisol concentrations, structural MRI and CSF Elecsys biomarkers (Aβ1‐42 and p‐tau181). Cross‐sectional analysis was conducted in Freesurfer (v7.1.1) through vertex‐wise analysis using general linear models (GLMs) corrected for multiple comparisons through cluster formation (p<0.01) and permutation (Monte Carlo simulation of 10,000 iterations). Longitudinal measures of an AD signature meta‐ROI were estimated from the surface area‐weighted average of the mean cortical thickness of the following ROIs: entorhinal, inferior temporal, middle temporal, and fusiform. Linear mixed‐effects (LME) models corrected for confounders were performed to evaluate cortical thickness longitudinal trajectories. The following models were used to assess the association between cortical thickness and cortisol levels: (1) cortisol as an independent variable; (2) cortisol, clinical diagnoses, and interaction; (3) cortisol, AD biomarkers positivity, and interaction.ResultsDemographics are depicted in Table 1. Cross‐sectional analysis revealed that higher cortisol levels were associated with higher brain atrophy in several brain regions, as depicted by cortical brain thickness measures (Figure 1). These analyses also showed that cortisol does not interact with clinical diagnoses or AD biomarkers to decrease cortical thickness. In the longitudinal analyses, the association between cortisol and cortical thickness and the interaction with clinical diagnoses were not significant. However, we found that cortisol significantly interacts with AD biomarkers positivity to reduce cortical thickness values over time (β=0.002, p=0.03; Figure 2).ConclusionsAltogether, our results support that cortisol levels potentiate AD pathophysiology effects on AD‐related brain atrophy. These findings suggest that CSF cortisol levels may affect brain vulnerability to AD pathophysiology in the long term through neurodegeneration. Thus, our results have potential implications for understating AD‐related brain atrophy and developing innovative therapeutic strategies.

Chapter 19 - Circadian changes in Alzheimer's disease: Neurobiology, clinical problems, and therapeutic opportunities

This increasing body of literature indicates that menopause hormonal replacement therapy (MHT) may substantially mitigate the risk of developing late-life cognitive decline due to progressive Alzheimer's disease (AD) pathophysiology. For the first time, we investigated the question whether MHT impacts AD biomarker-informed pathophysiological dynamics in de-novo diagnosed menopausal women. We analyzed baseline and longitudinal differences between MHT-taking and -not women in terms of concentrations of core pathophysiological AD plasma biomarkers, validated in symptomatic and cognitively healthy individuals, including biomarkers of (1) the amyloid-β (Aβ) pathway, (2) tau pathophysiology, (3) neuronal loss, and (4) axonal damage and neurodegeneration. We report a prominent and significant treatment response at the Aβ pathway biomarker level. Women at genetic risk for AD (APOE e4 allele carriers) have particularly shown favorable results from treatment. To our knowledge, we present first prospective clinical evidence on effects of MHT on AD pathophysiology during menopause.

BackgroundMultiple studies have linked high cortisol levels, a frequently used biomarker of stress, with the Alzheimer's disease (AD) pathophysiology. However, the relationship between cerebrospinal fluid (CSF) cortisol levels and AD‐related brain atrophy is not fully understood. This study sought to investigate the cross‐sectional and longitudinal association between CSF cortisol levels and brain cortical thickness in patients across the biological and clinical continuum of AD.MethodsWe evaluated 310 individuals from the ADNI cohort with available baseline CSF cortisol concentrations, structural MRI and CSF Elecsys biomarkers (Aß1‐42 and p‐tau181). Cross‐sectional analysis was conducted in Freesurfer (v7.1.1) through vertex‐wise analysis using general linear models (GLMs) corrected for multiple comparisons through cluster formation (p<0.01) and permutation (Monte Carlo simulation of 10,000 iterations). Longitudinal measures of an AD signature meta‐ROI were estimated from the surface area‐weighted average of the mean cortical thickness of the following ROIs: entorhinal, inferior temporal, middle temporal, and fusiform. Linear mixed‐effects (LME) models corrected for confounders were performed to evaluate cortical thickness longitudinal trajectories. The following models were used to assess the association between cortical thickness and cortisol levels: (1) cortisol as an independent variable; (2) cortisol, clinical diagnoses, and interaction; (3) cortisol, AD biomarkers positivity, and interaction.ResultsDemographics are depicted in Table 1. Cross‐sectional analysis revealed that higher cortisol levels were associated with higher brain atrophy in several brain regions, as depicted by cortical brain thickness measures (Figure 1). These analyses also showed that cortisol does not interact with clinical diagnoses or AD biomarkers to decrease cortical thickness. In the longitudinal analyses, the association between cortisol and cortical thickness and the interaction with clinical diagnoses were not significant. However, we found that cortisol significantly interacts with AD biomarkers positivity to reduce cortical thickness values over time (ß=0.002, p=0.03; Figure 2).ConclusionsAltogether, our results support that cortisol levels potentiate AD pathophysiology effects on AD‐related brain atrophy. These findings suggest that CSF cortisol levels may affect brain vulnerability to AD pathophysiology in the long term through neurodegeneration. Thus, our results have potential implications for understating AD‐related brain atrophy and developing innovative therapeutic strategies.

BackgroundVascular risk factors (VRFs) have an important role in the etiology and progression of Alzheimer´s Disease (AD). We recently described that VRF burden interacts with AD pathophysiology increasing plasma neurofilament light (NfL) levels, a biomarker of neuroaxonal damage. However, whether individual VRFs interact with AD pathophysiology to promote longitudinal neurodegeneration remains to be elucidated. Here, we aimed to assess the impact of individual VRFs in the longitudinal trajectory of plasma NfL in cognitively unimpaired (CU) individuals.MethodWe assessed 269 CU individuals from the ADNI cohort with available baseline medical data and cerebrospinal fluid (CSF) Elecsys biomarkers (Aβ1‐42 and p‐tau181), as well as longitudinal measures of plasma NfL. Individuals with both Aβ1‐42 and p‐tau181 positivity were defined as having preclinical AD (A+T+). The VRFs assessed in our analysis were history of cardiovascular disease (CAD), hypertension (HTN), diabetes mellitus (DM), hyperlipidemia (HLP), stroke or transient ischemic attack, smoking, atrial fibrillation, and left ventricular hypertrophy. Only those VRFs with at least 5% prevalence in the studied population were included in our analysis.ResultThe following VRFs were included in the final analysis based on prevalence: HTN, CAD, DM, and HLP. Linear mixed‐effects (LME) models revealed that no individual VRF significantly interacted with AD pathophysiology to increase longitudinal values of plasma NfL (HTN X AD pathophysiology X time, β =0.97 p=0.63; CAD X AD pathophysiology X time, β = 2.68, p= 0.36; HLP X AD pathophysiology X time, β= ‐2.1, p=0.3). DM was not present among individuals positive for AD pathophysiology. On the other hand, VRF burden significatively interacted with AD to increase NfL levels (VRF burden x AD pathology x time; β = 5.08, P = 0.016).ConclusionWe observed that no individual VRF interacted with AD pathophysiology to promote longitudinal neurodegeneration in CU individuals. Our findings suggest that the cumulative number of VRFs, rather than any VRF individually, impacts on neurodegeneration in the context of AD.

Alzheimer’s disease, a progressive and irreversible brain disorder predominantly affecting the elderly, is influenced by age, smoking, and head trauma. It disrupts memory, cognition, motor skills, speech, and more. Alzheimer’s disease pathophysiology is caused by two main processes: the formation of misfolded amyloid-beta plaques and misfolded tau tangles. While tau is a naturally occurring, microtubule-associated protein, amyloid-beta peptides are cleaved fragments of the transmembrane amyloid precursor protein. Accumulation of these plaques and tangles result in various negative mechanisms. Regarding the relationship between the two proteins, evidence suggests that amyloid beta induces the conversion of tau from a normal to toxic state, but they ultimately work together to contribute to Alzheimer’s disease pathogenesis. As of currently, there is still no cure for the disease, and patients rely on treatment methods that solely alleviate symptoms or benefit early stages to halt the disease’s progression. The main medications for Alzheimer’s disease are cholinesterase inhibitors such as Donepezil and Galantamine, but novel pharmacological and non-pharmacological treatments are being utilized as well, such as β-secretase inhibitors and deep brain stimulation respectively. This review investigates peer-reviewed publications on pathophysiology and treatment of Alzheimer’s disease, with a focus on novel approaches for treatment and intervention.