Peripheral inflammatory biomarkers of Alzheimer's disease: The role of platelets

Alzheimer's disease is an age-dependent neurodegenerative disorder characterized by loss of neurons, synaptic degeneration, senile plaques and neurofibrillary tangles. Besides these hallmarks, increased accumulation of activated microglia, astrocytes and leukocytes adhering to postcapillary venules are observed in the affected brain areas, suggesting the presence of an ongoing inflammatory process. As neuroinflammation triggers the activation of peripheral immune system, many studies have analyzed circulating inflammatory biomarkers, including basal or stimulated levels of cytokines and related molecules in blood of Alzheimer's patients, but with conflicting results. Platelets are an important source of amyloid- (A) in the circulatory system and play an important pro-inflammatory role. Upon activation, they adhere to leukocytes and endothelial cells by means of adhesive proteins like P-selectin, platelet endothelial cell adhesion molecule-1 (PECAM) and intercellular adhesion molecule-1 and -2 (ICAM-1 and -2) and secrete inflammatory mediators (chemokines, interleukins). In addition, platelets contain important enzymes involved in inflammatory intermediary synthesis like phospholipase A₂ (PLA₂) and cyclooxygenase-2 (COX-2), and recent reports demonstrated significant changes in platelet levels and activities in Alzheimer's disease. Thus, as platelets represent an important link between A deposition and inflammatory reactions especially at endothelial level, they can be considered a valuable cellular model to evaluate potential peripheral inflammatory biomarkers in Alzheimer's disease.