Perspective: Is therapeutic plasma exchange a viable option for treating Alzheimer's disease?

Association Between Plasma Amyloid-Beta 42 Ratio and Postoperative Delirium in Elderly Patients Undergoing Major Abdominal Surgery: Secondary Analysis of a Randomized Controlled Trial

INTRODUCTION

Cerebrospinal fluid Aβ42 has been proposed as a potential indicator for cerebral β-amyloidosis and may be involved in the pathophysiology of delirium. Whether perioperative plasma Aβ42 alternation is associated with postoperative delirium risk among elderly patients remains unknown.

METHODS

This was a secondary analysis of a randomized controlled trial evaluating the effects of acupuncture (intervention) compared to standard care (control) on the incidence of delirium in patients undergoing major abdominal surgery. Participants with blood samples collected were included in this cohort study. The exposure variable was the Aβ42 ratio, calculated with the plasma Aβ42 level immediately after surgery divided by the preoperative plasma Aβ42 level. The primary endpoint was the occurrence of delirium within the first 7 days following surgery or until hospital discharge, whichever happened first, evaluated using either the Confusion Assessment Method or the Confusion Assessment Method-intensive care unit for intubated patients. Delirium severity was a secondary outcome assessed by the Memorial Delirium Assessment Scale. The logistic regression models and a restricted cubic spline were performed to examine the association between the Aβ42 ratio and delirium incidence, with receiver operating characteristic curve (ROC) analysis for diagnostic power. The mediation effects of the matrix metalloproteinase-9 ratio were further explored by causal mediation analysis. The linear regression and generalized linear mixed models assessed the association between the Aβ42 ratio and delirium severity.

RESULTS

A total of 195 patients with blood samples collected were included in the final analysis. Among them, the mean age was 70.2 ± 4.2 years; 134 were female (68.7%), and 26 (13.3%) patients experienced postoperative delirium. The plasma Aβ42 ratio was positively correlated with an increased delirium risk (adjusted odds ratio 3.21, 95% confidence interval 1.71-6.05, p < 0.001) and delirium severity, as measured by the highest postoperative Memorial Delirium Assessment Scale score (adjusted β coefficient 3.04, 95% confidence interval 0.9-5.18, p = 0.006) in the fully adjusted multivariable analysis models. The restricted cubic spline indicated a linear relationship between the plasma Aβ42 ratio and delirium incidence (p = 0.202). The ROC showed that the area under the curve for the Aβ42 ratio to predict delirium risk was 0.698 (95% CI, 0.582-0.814), with the optimal cut-off point of 0.137. Mediation analyses showed that the Aβ42 ratio does not mediate postoperative delirium through the matrix metalloproteinase-9 ratio (proportion: 1.3%).

CONCLUSIONS

This cohort study showed that a higher Aβ42 ratio was associated with an increased delirium risk and severity, and the association was linear. The plasma Aβ42 ratio might be a mini-invasive biomarker to identify postoperative delirium.

One immune cell to bind them all: platelet contribution to neurodegenerative disease

Alzheimer's disease (AD) and related dementias (ADRD) collectively affect a significant portion of the aging population worldwide. The pathological progression of AD involves not only the classical hallmarks of amyloid beta (Aβ) plaque buildup and neurofibrillary tangle development but also the effects of vasculature and chronic inflammatory processes. Recently, platelets have emerged as central players in systemic and neuroinflammation. Studies have shown that patients with altered platelet receptor expression exhibit accelerated cognitive decline independent of traditional risk factors. Additionally, platelets from AD patients exhibit heightened unstimulated activation compared to control groups. Platelet granules contain crucial AD-related proteins like tau and amyloid precursor protein (APP). Dysregulation of platelet exocytosis contributes to disease phenotypes characterized by increased bleeding, stroke, and cognitive decline risk. Recent studies have indicated that these effects are not associated with the quantity of platelets present in circulation. This underscores the hypothesis that disruptions in platelet-mediated inflammation and healing processes may play a crucial role in the development of ADRD. A thorough look at platelets, encompassing their receptors, secreted molecules, and diverse roles in inflammatory interactions with other cells in the circulatory system in AD and ADRD, holds promising prospects for disease management and intervention. This review discusses the pivotal roles of platelets in ADRD.

Blood-based therapies to combat neurodegenerative diseases

Neurodegeneration, known as the progressive loss of neurons in terms of their structure and function, is the principal pathophysiological change found in the majority of brain-related disorders. Ageing has been considered the most well-established risk factor in most common neurodegenerative diseases, such as Parkinson's disease (PD) and Alzheimer's disease (AD). There is currently no effective treatment or cure for these diseases; the approved therapeutic options to date are only for palliative care. Ageing and neurodegenerative diseases are closely intertwined; reversing the aspects of brain ageing could theoretically mitigate age-related neurodegeneration. Ever since the regenerative properties of young blood on aged tissues came to light, substantial efforts have been focused on identifying and characterizing the circulating factors in the young and old systemic milieu that may attenuate or accentuate brain ageing and neurodegeneration. Later studies discovered the superiority of old plasma dilution in tissue rejuvenation, which is achieved through a molecular reset of the systemic proteome. These findings supported the use of therapeutic blood exchange for the treatment of degenerative diseases in older individuals. The first objective of this article is to explore the rejuvenating properties of blood-based therapies in the ageing brains and their therapeutic effects on AD. Then, we also look into the clinical applications, various limitations, and challenges associated with blood-based therapies for AD patients.

Plasma Exchange Reduces Aβ Levels in Plasma and Decreases Amyloid Plaques in the Brain in a Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is the most common type of dementia, characterized by the abnormal accumulation of protein aggregates in the brain, known as neurofibrillary tangles and amyloid-β (Aβ) plaques. It is believed that an imbalance between cerebral and peripheral pools of Aβ may play a relevant role in the deposition of Aβ aggregates. Therefore, in this study, we aimed to evaluate the effect of the removal of Aβ from blood plasma on the accumulation of amyloid plaques in the brain. We performed monthly plasma exchange with a 5% mouse albumin solution in the APP/PS1 mouse model from 3 to 7 months old. At the endpoint, total Aβ levels were measured in the plasma, and soluble and insoluble brain fractions were analyzed using ELISA. Brains were also analyzed histologically for amyloid plaque burden, plaque size distributions, and gliosis. Our results showed a reduction in the levels of Aβ in the plasma and insoluble brain fractions. Interestingly, histological analysis showed a reduction in thioflavin-S (ThS) and amyloid immunoreactivity in the cortex and hippocampus, accompanied by a change in the size distribution of amyloid plaques, and a reduction in Iba1-positive cells. Our results provide preclinical evidence supporting the relevance of targeting Aβ in the periphery and reinforcing the potential use of plasma exchange as an alternative non-pharmacological strategy for slowing down AD pathogenesis.

Toward Prevention and Reduction of Alzheimer's Disease

Different investigations lead to the urgent need to generate validated clinical protocols as a tool for medical doctors to orientate patients under risk for a preventive approach to control Alzheimer's disease. Moreover, there is consensus that the combined effects of risk factors for the disease can be modified according to lifestyle, thus controlling at least 40% of cases. The other fraction of cases are derived from candidate genes and epigenetic components as a relevant factor in AD pathogenesis. At this point, it appears to be of critical relevance the search for molecular biomarkers that may provide information on probable pathological events and alert about early detectable risks to prevent symptomatic events of the disease. These precocious detection markers will then allow early interventions of non-symptomatic subjects at risk. Here, we summarize the status and potential avenues of prevention and highlight the usefulness of biological and reliable markers for AD.

The Impact of Ageing on the CNS Immune Response in Alzheimer's Disease

Alzheimer's Disease (AD) is a progressive neurodegenerative disease strongly associated with increasing age. Neuroinflammation and the accumulation of amyloid protein are amongst the hallmarks of this disease and most translational research to date has focused on targeting these two processes. However, the exact etiology of AD remains to be fully elucidated. When compared alongside, the immune response in AD closely resembles the central nervous system (CNS) immune changes seen in elderly individuals. It is possible that AD is a pathological consequence of an aged immune system secondary to chronic stimulation by a previous or ongoing insult. Pathological changes like amyloid accumulation and neuronal cell death may reflect this process of immunosenescence as the CNS immune system fails to maintain homeostasis in the CNS. It is likely that future treatments designed to modulate the aged immune system may prove beneficial in altering the disease course. The development of new tests for appropriate biomarkers would also be essential in screening for patients most likely to benefit from such treatments.

Should drug discovery scientists still embrace the amyloid hypothesis for Alzheimer's disease or should they be looking elsewhere?

INTRODUCTION

Alzheimer's Disease (AD) represents a large and growing challenge to patients, carers and healthcare systems, yet extensive efforts to develop therapeutics to modify its course have been met with repeated failure in recent decades. Although the evident presence of accumulated β-amyloid (Aβ) in AD brains has singled it out as an obvious therapeutic target, the effective reduction of plaque load or soluble Aβ by numerous drug candidates has not produced commensurate clinical benefits - calling into question the Aβ cascade hypothesis of AD. A similar path is now unfolding in the pursuit of therapeutics targeting hyperphosphorylated tau-comprised neurofibrillary tangles.

AREAS COVERED

This perspective reviews the basis of the Aβ cascade hypothesis of AD and how clinical trials of anti-Aβ drugs have failed to support it, and reflects upon the early findings suggesting that a similar path is being followed with therapeutics targeting tau. Other potential approaches to identifying therapeutics for AD are explored herein.

EXPERT OPINION

The relevance of the Aβ cascade hypothesis to the development of therapeutics for AD appears disproven. Drugs targeting tau appear to be suffering the same fate but may yet produce better results. Alternative approaches are being pursued, some of them with initial small-scale, but promising, results.