Platelet immunoglobulin and amyloid precursor protein as potential peripheral biomarkers for Alzheimer's disease: findings from a pilot study

Neuroprotective astroglial response to neural damage and its relevance to affective disorders

Astrocytes not only support neuronal function with essential roles in synaptic neurotransmission, action potential propagation, metabolic support, or neuroplastic and developmental adaptations. They also respond to damage or dysfunction in surrounding neurons and oligodendrocytes by releasing neurotrophic factors and other molecules that increase the survival of the supported cells or contribute to mechanisms of structural and molecular restoration. The neuroprotective responsiveness of astrocytes is based on their ability to sense signals of degeneration, metabolic jeopardy and structural damage, and on their aptitude to locally deliver specific molecules to remedy threats to the molecular and structural features of their cellular partners. To the extent that neuronal and other glial cell disturbances are known to occur in affective disorders, astrocyte responsiveness to those disturbances may help to better understand the roles astrocytes play in affective disorders. The astrocytic sensing apparatus supporting those responses involves receptors for neurotransmitters, purines, cell adhesion molecules and growth factors. Astrocytes also share with the immune system the capacity of responding to cytokines released upon neuronal damage. In addition, in responses to specific signals astrocytes release unique factors such as clusterin or humanin that have been shown to exert potent neuroprotective effects. Astrocytes integrate the signals above to further deliver structural lipids, removing toxic metabolites, stabilizing the osmotic environment, normalizing neurotransmitters, providing anti-oxidant protection, facilitating synaptogenesis and acting as barriers to contain varied deleterious signals, some of which have been described in brain regions relevant to affective disorders and related animal models. Since various of the injurious signals that activate astrocytes have been implicated in different aspects of the etiopathology of affective disorders, particularly in relation to the diagnosis of depression, potentiating the corresponding astrocyte neuroprotective responses may provide additional opportunities to improve or complement available pharmacological and behavioral therapies for affective disorders.

Alpha-Synuclein in Peripheral Tissues as a Possible Marker for Neurological Diseases and Other Medical Conditions

The possible usefulness of alpha-synuclein (aSyn) determinations in peripheral tissues (blood cells, salivary gland biopsies, olfactory mucosa, digestive tract, skin) and in biological fluids, except for cerebrospinal fluid (serum, plasma, saliva, feces, urine), as a marker of several diseases, has been the subject of numerous publications. This narrative review summarizes data from studies trying to determine the role of total, oligomeric, and phosphorylated aSyn determinations as a marker of various diseases, especially PD and other alpha-synucleinopathies. In summary, the results of studies addressing the determinations of aSyn in its different forms in peripheral tissues (especially in platelets, skin, and digestive tract, but also salivary glands and olfactory mucosa), in combination with other potential biomarkers, could be a useful tool to discriminate PD from controls and from other causes of parkinsonisms, including synucleinopathies.

The potential of the gut microbiome for identifying Alzheimer's disease diagnostic biomarkers and future therapies

Being isolated from the peripheral system by the blood-brain barrier, the brain has long been considered a completely impervious tissue. However, recent findings show that the gut microbiome (GM) influences gastrointestinal and brain disorders such as Alzheimer's disease (AD). Despite several hypotheses, such as neuroinflammation, tau hyperphosphorylation, amyloid plaques, neurofibrillary tangles, and oxidative stress, being proposed to explain the origin and progression of AD, the pathogenesis remains incompletely understood. Epigenetic, molecular, and pathological studies suggest that GM influences AD development and have endeavored to find predictive, sensitive, non-invasive, and accurate biomarkers for early disease diagnosis and monitoring of progression. Given the growing interest in the involvement of GM in AD, current research endeavors to identify prospective gut biomarkers for both preclinical and clinical diagnoses, as well as targeted therapy techniques. Here, we discuss the most recent findings on gut changes in AD, microbiome-based biomarkers, prospective clinical diagnostic uses, and targeted therapy approaches. Furthermore, we addressed herbal components, which could provide a new venue for AD diagnostic and therapy research.

Platelet APP Processing: Is It a Tool to Explore the Pathophysiology of Alzheimer's Disease? A Systematic Review

The processing of the amyloid precursor protein (APP) is a critical event in the formation of amyloid plaques. Platelets contain most of the enzymatic machinery required for APP processing and correlates of intracerebral abnormalities have been demonstrated in platelets of patients with AD. The goal of the present paper was to analyze studies exploring platelet APP metabolism in Alzheimer's disease patients trying to assess potential reliable peripheral biomarkers, to offer new therapeutic solutions and to understand the pathophysiology of the AD. According to the PRISMA guidelines, we performed a systematic review through the PubMed database up to June 2020 with the search terms: "((((((APP) OR Amyloid Precursor Protein) OR AbetaPP) OR Beta Amyloid) OR Amyloid Beta) OR APP-processing) AND platelet". Thirty-two studies were included in this systematic review. The papers included are analytic observational studies, namely twenty-nine cross sectional studies and three longitudinal studies, specifically prospective cohort study. The studies converge in an almost unitary way in affirming that subjects with AD show changes in APP processing compared to healthy age-matched controls. However, the problem of the specificity and sensitivity of these biomarkers is still at issue and would deserve to be deepened in future studies.

Erythrocytes as Biomarkers for Dementia: Analysis of Protein Content and Alpha-Synuclein

BACKGROUND

Discovering biomarkers for dementia is a pivotal step toward successful early diagnosis and treatment. Although plasma biomarkers have been explored, no consensus has been reached. Alpha-synuclein (AS), a 14 kDa synaptic protein associated with several neurodegenerative diseases, exists natively within erythrocytes (ERC). This protein is characteristic of Lewy body diseases, in which it aggregates into toxic Lewy bodies. As ERC are implicated in dementia, they are a potential target for future biomarkers.

OBJECTIVE

The aims of this study were to assess AS levels within ERC and whether AS can be used as a peripheral biomarker to differentiate between dementia and aged matched healthy control subjects.

METHODS

A total of 114 samples (60 aging controls, 36 Alzheimer's disease, 12 vascular dementia (VaD) and 6 dementia with Lewy bodies (DLB) subjects) were analyzed. We used Bradford assay to measure protein concentration, indirect ELISA to detect levels of AS, and immunoblotting to identify AS composition. Data were analyzed with nonparametric tests.

RESULTS

AS oligomers were present in dementia blood samples, whereas in controls, AS was largely monomeric. There was a significant increase in AS levels in DLB whole blood (p = 0.005; Kruskal-Wallis test), with a sensitivity and specificity of 100.0% and 93.9%. Protein concentrations in ERC isolated at pH 5.7 were significantly increased in dementia patients compared to controls (17.58 versus 40.33μg/ml; p≤0.005; Mann-Whitney test). In the VaD group, the protein concentration in the pH5.7 ERC fraction had sensitivity and specificity of 91.7% and 62.1%.

CONCLUSIONS

ERC protein concentration and AS levels have a potential for development of a novel diagnostic dementia blood test.

Decrease in 130 kDa- amyloid protein precursor protein (APP) and APP protein ratio in schizophrenia platelets

Cognitive dysfunction is common among people with schizophrenia. The molecular substrates underlying this remain poorly understood. To address this, we analyzed changes in amyloid precursor protein (APP) in platelets of people with acute schizophrenia (n=24) and control subjects (n=20) by ECL-immunoblotting. APP bands corresponding to molecular masses of ∼130, ∼110 and ∼100 kDa, and the APP ratio (APPr: highest APP molecular mass vs lowest APP molecular mass bands) were quantified. The intensity of 130 kDa-APP and the APPr were significantly reduced in schizophrenia patients compared to control subjects. The age-associated decreases in the 130 kDa, ∼110 kDa proteins and APPr were present in patients, but not controls. Our results confirm peripheral APP metabolism is altered in people with schizophrenia. Further work is now warranted on a larger sample of diseased subjects with detailed cognitive assessment to determine the APP role in cognitive processing in schizophrenia, how it is related to severity and disease progression, as well as outcomes.

C-type lectin-like receptor 2 and zonulin are associated with mild cognitive impairment and Alzheimer's disease

OBJECTIVE

Increased permeability and changes in gut microbiota contributed to the pathogenesis of Alzheimer's disease (AD). Zonulin is a key modulator that regulates intestinal barrier function. Peripheral platelet alterations have been involved in AD pathology. C-type lectin-like receptor 2 (CLEC-2) is a receptor on the platelet surface for activation. The purpose of this study was to determine zonulin and CLEC-2 levels in mild cognitive impairment (MCI) and AD, and investigate the relationship between zonulin and CLEC-2.

METHODS

In this study, CLEC-2 and zonulin levels were measured using ELISA assay in 110 AD patients, 110 MCI patients, and 110 non-demented control subjects.

RESULTS

Increased CLEC-2 and zonulin levels were observed in MCI and AD patients. Furthermore, AD patients had higher CLEC-2 and zonulin levels compared with MCI patients. In addition, CLEC-2 levels were positively correlated with zonulin levels, after adjusting confounding factors (r = .592, P < .001). Multivariate analysis revealed that increased CLEC-2 and zonulin levels were significantly associated with reduced Mini-Mental State Examination (MMSE) score.

CONCLUSIONS

C-type lectin-like receptor 2 is correlated with zonulin after adjusting confounding covariates. Moreover, increased CLEC-2 and zonulin are the significant factors for reduced MMSE score in MCI and AD. Further studies are needed.

Platelets: Peripheral Biomarkers of Dementia?

Dementia continues to be the most burdening neurocognitive disorder, having a negative impact on the lives of millions. The search for biomarkers to improve the clinical diagnosis of dementia is ongoing, with the focus on effective use of readily accessible peripheral markers. In this review, we concentrate on platelets as biomarkers of dementia and analyze their potential as easily-accessible clinical biomarkers for various subtypes of dementia. Current platelet protein biomarkers that have been investigated for their clinical utility in the diagnosis of dementia, in particular Alzheimer's disease, include amyloid-β protein precursor (AβPP), the AβPP secretases (BACE1 and ADAM10), α-synuclein, tau protein, serotonin, cholesterol, phospholipases, clusterin, IgG, surface receptors, MAO-B, and coated platelets. Few of them, i.e., platelet tau, AβPP (particularly with regards to coated platelets) and secreted ADAM10 and BACE1 show the most promise to be taken forward into clinical setting to diagnose dementia. Aside from protein biomarkers, changes in factors such as mean platelet volume have the potential to play a very specific role in both the dementia diagnosis and prognosis. This review raises a number of research questions for consideration before application of the above biomarkers to routine clinical setting. It is without doubt that there is a need for more clarification on the effects of dementia on platelet morphology and protein content before these changes can be clinically applied as dementia biomarkers and explored further in differentiating distinct dementia subtypes.

Association between clusterin concentration and dementia: a systematic review and meta-analysis

Studies have showed that high clusterin (CLU) concentration was associated with increased risk of dementia. However, the results based on small samples remained controversial. The aim of our study was to determine the relationship between CLU concentration and the late-life cognitive outcomes including mild cognitive impairment (MCI), Alzheimer's disease (AD), vascular dementia (VAD), Parkinson's disease related dementia (PDD), Lewy body dementia (DLB) and frontotemporal dementia (FTD). A comprehensive search was conducted to screen the eligible studies in online database PubMed, Web of Science and Embase from 1950 to January 2017 according to the preferred reporting items for systematic reviews and meta-analyses (PRISMA) checklist. The CLU concentration data in brain tissue, cerebrospinal fluid (CSF), serum and plasma was collected to determine the strength of this association. The results were presented with standard difference of the mean (SDM) with 95% confidence intervals (CIs). A total of 28 studies were identified to calculate the association between CLU concentration and dementia. The results showed that the CLU concentration in the plasma (SDM = 0.73, 95% CI 0.26-1.19, P = 0.002) and brain tissue (SDM = 0.71, 95% CI 0.10-1.32, P = 0.022) was increased in dementia compared to normal control. Subgroup analysis showed that the plasma CLU concentration was significantly increased only in the AD group (SDM = 1.85, 95% CI 0.84-2.85, P < 0.001), but not in MCI or other dementias. No association was found between serum and CSF clusterin concentration and dementia. This meta-analysis indicates that high CLU concentration in the plasma and brain is associated with dementia, especially in AD.

Cognitive changes in prodromal Parkinson's disease: A review

Although other nonmotor phenomena representing possible prodromal symptoms of Parkinson's disease have been described in some detail, the occurrence and characteristics of cognitive decline in this early phase of the disease are less well understood. The aim of this review is to summarize the current state of research on cognitive changes in prodromal PD. Only a small number of longitudinal studies have been conducted that examined cognitive function in individuals with a subsequent PD diagnosis. However, when we consider data from at-risk groups, the evidence suggests that cognitive decline may occur in a substantial number of individuals who have the potential for developing PD. In terms of specific cognitive domains, executive function in particular and, less frequently, memory scores are reduced. Prospective longitudinal studies are thus needed to clarify whether cognitive, and specifically executive, decline might be added to the prodromal nonmotor symptom complex that may precede motor manifestations of PD by years and may help to update the risk scores used for early identification of PD. © 2017 International Parkinson and Movement Disorder Society.

Potential biomarkers and novel pharmacological targets in protein aggregation-related neurodegenerative diseases

The aggregation of specific proteins plays a pivotal role in the etiopathogenesis of several neurodegenerative diseases (NDs). β-Amyloid (Aβ) peptide-containing plaques and intraneuronal neurofibrillary tangles composed of hyperphosphorylated protein tau are the two main neuropathological lesions in Alzheimer's disease. Meanwhile, Parkinson's disease is defined by the presence of intraneuronal inclusions (Lewy bodies), in which α-synuclein (α-syn) has been identified as a major protein component. The current literature provides considerable insights into the mechanisms underlying oligomeric-related neurodegeneration, as well as the relationship between protein aggregation and ND, thus facilitating the development of novel putative biomarkers and/or pharmacological targets. Recently, α-syn, tau and Aβ have been shown to interact each other or with other "pathological proteins" to form toxic heteroaggregates. These latest findings are overcoming the concept that each neurodegenerative disease is related to the misfolding of a single specific protein. In this review, potential opportunities and pharmacological approaches targeting α-syn, tau and Aβ and their oligomeric forms are highlighted with examples from recent studies. Protein aggregation as a biomarker of NDs, in both the brain and peripheral fluids, is deeply explored. Finally, the relationship between biomarker establishment and assessment and their use as diagnostics or therapeutic targets are discussed.

Plasma and platelet clusterin ratio is altered in Alzheimer's disease patients with distinct neuropsychiatric symptoms: findings from a pilot study

BACKGROUND

Clusterin protein in plasma has been found to differentiate between people with and without cognitive changes. However, these findings are not conclusive, despite the clusterin gene variations repeatedly being linked to increased risk for dementia, in particular Alzheimer's disease (AD).

METHOD

We analysed the level of clusterin in platelet and plasma in 25 subjects with a clinical diagnosis of AD and 26 subjects with no cognitive impairment.

RESULTS

In the current study, we report that the levels of both plasma and platelet clusterin are similar between AD and cognitively intact individuals. Clusterin plasma and platelet levels, as well as the plasma/platelet clusterin ratio, were not affected by age, gender, cognitive impairment and/or overt behavioural symptomatology, including presence of hallucinations and delusions, as well as depression. However, the plasma/platelet clusterin ratio was positively associated in with the Neuropsychiatric Inventory measures of agitation, apathy, irritability and motor aberrant behaviour in AD subjects.

CONCLUSION

Previous inconsistencies in reported blood clusterin levels may be a result of underlying non-cognitive symptoms in people with AD. Our findings need now to be replicated in larger group of dementia subjects.

Platelets, a reliable source for peripheral Alzheimer's disease biomarkers?

Peripheral biomarkers play an indispensable role in quick and reliable diagnoses of any kind of disease. With the population ageing, the number of people suffering from age-related diseases is expected to rise dramatically over the coming decades. In particular, all types of cognitive deficits, such as Alzheimer's disease, will increase. Alzheimer's disease is characterised mainly by coexistence of amyloid plaques and neurofibrillary tangles in brain. Reliable identification of such molecular characteristics antemortem, however, is problematic due to restricted availability of appropriate sample material and definitive diagnosis is only possible postmortem. Currently, the best molecular biomarkers available for antemortem diagnosis originate from cerebrospinal fluid. Though, this is not convenient for routine diagnosis because of the required invasive lumbar puncture. As a consequence, there is a growing demand for additional peripheral biomarkers in a more readily accessible sample material. Blood platelets, due to shared biochemical properties with neurons, can constitute an attractive alternative as discussed here. This review summarises potential platelet Alzheimer's disease biomarkers, their role, implication, and alteration in the disease. For easy comparison of their performance, the Hedge effect size was calculated whenever data were available.

Intranasal delivery of plasma and platelet growth factors using PRGF-Endoret system enhances neurogenesis in a mouse model of Alzheimer's disease

Neurodegeneration together with a reduction in neurogenesis are cardinal features of Alzheimer’s disease (AD) induced by a combination of toxic amyloid-β peptide (Aβ) and a loss of trophic factor support. Amelioration of these was assessed with diverse neurotrophins in experimental therapeutic approaches. The aim of this study was to investigate whether intranasal delivery of plasma rich in growth factors (PRGF-Endoret), an autologous pool of morphogens and proteins, could enhance hippocampal neurogenesis and reduce neurodegeneration in an amyloid precursor protein/presenilin-1 (APP/PS1) mouse model. Neurotrophic and neuroprotective actions were firstly evident in primary neuronal cultures, where cell proliferation and survival were augmented by Endoret treatment. Translation of these effects in vivo was assessed in wild type and APP/PS1 mice, where neurogenesis was evaluated using 5-bromodeoxyuridine (BdrU), doublecortin (DCX), and NeuN immunostaining 5 weeks after Endoret administration. The number of BrdU, DCX, and NeuN positive cell was increased after chronic treatment. The number of degenerating neurons, detected with fluoro Jade-B staining was reduced in Endoret-treated APP/PS1 mice at 5 week after intranasal administration. In conclusion, Endoret was able to activate neuronal progenitor cells, enhancing hippocampal neurogenesis, and to reduce Aβ-induced neurodegeneration in a mouse model of AD.

Decreased mean platelet volume and platelet distribution width are associated with mild cognitive impairment and Alzheimer's disease

Neuroinflammation is a critical driving force underlying mild cognitive impairment (MCI) and Alzheimer's disease (AD) pathologies. Activated platelets play an important role in neuroinflammation and have been implicated in AD pathogenic mechanisms. Mean platelet volume (MPV), a marker of platelet activation, is involved in the pathophysiology of a variety of pro-inflammatory diseases. However, little research has been conducted to investigate the relationship between platelet indices and MCI and AD pathogenesis. In this cross-sectional study, we investigated the levels of platelet count, MPV and platelet distribution width (PDW) in 120 AD patients, 120 MCI patients, and 120 non-demented controls. Our study showed that MPV and PDW were significantly lower in patients with AD as compared with either MCI or controls. Moreover, MCI patients had lower MPV and PDW values compared with the controls (P < 0.001). In addition, there is a positive correlation between mini-mental state examination (MMSE) and MPV and PDW, after adjusting age, gender, and body mass index (r = 0.576, P < 0.001 for MPV; r = 0.465, P < 0.001 for PDW, respectively). Multivariate analysis showed that MPV and PDW were significantly associated with MMSE (β = 0.462; P < 0.001 for MPV; β = 0.245; P < 0.001 for PDW; respectively). In conclusion, MPV and PDW were decreased in MCI and AD patients. Further prospective research is warranted to determine the potential clinical application of MPV and PDW as biomarkers in the early diagnosis of AD.

Differential processing of amyloid precursor protein in brain and in peripheral blood leukocytes

Because amyloid precursor protein (APP) fragments exist in many tissues throughout the body, including the fluid compartments of blood, they have been the focus of numerous investigations into their potential as a biomarker of Alzheimer's disease. Using immunohistochemistry, immunoelectron microscopy, Western blot, and quantitative real-time-polymerase chain reaction (qRT-PCR) analysis we examined whether APP processing in leukocytes is analogous to APP processing in the brain. We show APP immunoreactivity at light and electron microscopic levels in the cytoplasm and nucleus of peripheral blood leukocytes (PBL) yet our Western blot analysis data demonstrated that brain and PBL contain different APP fragments and differentially expressed APP processing enzymes. A Disintegrin and Metalloproteinase domain 10 (ADAM10), nicastrin, and beta-secretase 2 (BACE2) were present in brain but were undetected in PBL. Presenilin 1 and beta-secretase 1 (BACE1) were detected in both tissues but showed different patterns in Western blots. Quantitative PCR results identified Neprilysin as the only processing enzyme we interrogated in which Western and quantitative PCR data coincided. Although our data on differential processing of APP in brain and PBL point to exercising caution when generalizing between blood and brain with regard to mechanisms, they have no implications regarding utility as biomarkers.