Alzheimer's disease: an update of the roles of receptors, astrocytes and primary cilia

The pathophysiological mechanisms underlying the onset and inexorable progression of the late-onset form of Alzheimer's disease (AD) are still the object of controversy. This review takes stock of some most recent advancements of this field concerning the complex roles played by the amyloid-β (Aβ)-binding p75 neurotrophin receptor (p75NTR) and calcium-sensing receptor (CaSR) and by the primary cilia in AD. Apart from their physiological roles, p75NTR is more intensely expressed in the hippocampus of human AD brains and Aβ-bound p75NTR triggers cell death, whereas Aβ-bound CaSR signalling induces the de novo synthesis and release of nitric oxide (NO), vascular endothelial growth factor (VEGF)-A and Aβ peptides (Aβs), particularly on the part of normal adult human astrocytes. The latter effect could significantly increase the pool of Aβ-and NO-producing nerve cells favouring the progressive spread of a self-sustaining and self-reinforcing 'infectious' mechanism of neural and vascular (i.e. blood-brain barrier) cell damage. Interestingly, primary cilia concentrate p75NTR receptors in their membranes and are abnormally structured/damaged in transgenic (Tg) AD-model mice, which could impact on the adult neurogenesis occurring in the dentate gyrus's subgranular zone (SGZ) that is necessary for new memory encoding, thereby favouring typical AD cognitive decline. Altogether, these findings may pave the way to novel therapeutic approaches to AD, particularly in its mild cognitive impairment (MCI) and pre-MCI stages of development.