Amyloid β-Exposed Human Astrocytes Overproduce Phospho-Tau and Overrelease It within Exosomes, Effects Suppressed by Calcilytic NPS 2143—Further Implications for Alzheimer's Therapy

Abstract

The two main drivers of Alzheimer's disease (AD), amyloid-β (Aβ) and hyperphosphorylated Tau (p-Tau) oligomers, cooperatively accelerate AD progression, but a hot debate is still ongoing about which of the two appears first. Here we present preliminary evidence showing that Tau and p-Tau are expressed by untransformed cortical adult human astrocytes in culture and that exposure of such cells to an Aβ42 proxy, Aβ25−35, which binds the calcium-sensing receptor (CaSR) and activates its signaling, significantly increases intracellular p-Tau levels, an effect CaSR antagonist (calcilytic) NPS 2143 wholly hinders. The astrocytes also release both Tau and p-Tau by means of exosomes into the extracellular medium, an activity that could mediate p-Tau diffusion within the brain. Preliminary data also indicate that exosomal levels of p-Tau increase after Aβ25−35 exposure, but remain unchanged in cells pre-treated for 30-min with NPS 2143 before adding Aβ25−35. Thus, our previous and present findings raise the unifying prospect that Aβ•CaSR signaling plays a crucial role in AD development and progression by simultaneously activating (i) the amyloidogenic processing of amyloid precursor holoprotein, whose upshot is a surplus production and secretion of Aβ42 oligomers, and (ii) the GSK-3β-mediated increased production of p-Tau oligomers which are next released extracellularly inside exosomes. Therefore, as calcilytics suppress both effects on Aβ42 and p-Tau metabolic handling, these highly selective antagonists of pathological Aβ•CaSR signaling would effectively halt AD's progressive spread preserving patients' cognition and life quality.

Exosomal biomarkers in Down syndrome and Alzheimer's disease

“…As discussed in this review, toxic Aβ peptides and toxic species of P-Tau, neuroinflammation, and oxidative stress can all alter exosome content, secretion rate, and the cell-to-cell messages they carry. Several reports indicate that exosomes produced under pathological conditions can either confer protection or increased pathology to their target cells (Borland and Vilhardt 2017; Chiarini et al 2017; Goetzl et al 2016; Goetzl et al 2017; Kapogiannis et al 2015; Kouwaki et al 2016; Lee et al 2016; Li et al 2013; Papandreou and Tavernarakis 2017). Further study of these processes will provide new insights into how AD pathology spreads throughout the brain and the role of exosomes in this process.…”

“…In our recent study, which was a collaboration between several DS clinics both in the U.S. and abroad and with the Goetzl laboratory at the National Institute of Health (NIH), we hypothesized that neuron-derived exosomes might be more abundant in people with DS relative to an age-matched control population without DS (referred to herein as a ‘control’ population), which could serve to downregulate the increased levels of Aβ peptides and aberrant P-Tau proteins in neurons from people with DS. This assumption was based on previous work by others, showing increased production of exosomes from cells exposed to amyloid in vitro (Chiarini et al, 2017). To test our hypothesis, we isolated neuron-derived exosomes as previously described (Hamlett et al 2016), and quantified CD81 by enzyme-linked immunosorbent assay (ELISA) (Cusabio, Waltham, MA) against a standard curve of recombinant CD81 (Origene Technologies, Rockville, MD).…”

“…They displayed higher levels of both Aβ and p-Tau, as well as the β-site amyloid precursor protein cleaving enzyme 1 (BACE-1) than neuron-derived exosomes from the same samples. It has long been known that treatment of cells in culture with Aβ leads to P-Tau overexpression within the cells and recently it has also been shown that Aβ treatment of astrocytes leads to increased production of exosomes with P-Tau cargo (Chiarini et al 2017), thus providing an opportunity to study cell-cell interactions within the CNS with respect to AD pathology by examining both astrocyte- and neuron-derived exosomes and their cargo in blood.…”

Post-translational add-ons mark the path in exosomal protein sorting

“…The majority of modified (phosphoThr-181) Tau detected in human cerebrospinal fluid samples is secreted via exosomes, constituting a well-characterized biomarker for Alzheimer's disease (AD) [115]. In fact, phTau in combination with Aβ activate diverse signalling pathways, increasing the production of hyperphosphorylated Tau oligomers, which are then secreted by exosomes within the brain, accelerating neurodegeneration [110]. Moreover, some AD-related proteins such as ph-Tau, Aβ1-42, neurogranin (NRGN) and the repressor element 1-silencing transcription factor (REST) contained in neuronal-derived exosomes appear in plasma [117].…”

“…The localization of this protein depends on the phosphorylation of Tyr23, which prevents its endosomal degradation and allows its incorporation into exosomal membranes [109]. Other phosphorylated proteins found in exosomes include Tau, type 1 insulin receptor substrate (IRS-1), Crystalline alphaB (CryAB), Epidermal Growth Factor Receptor (EGFR), Na + -Cl − co-transporter (NCC), Aquaporin 2, FasL and proteins implicated in cellular processes such as apoptosis, survival and metabolism [79,101,108,[110][111][112][113][114][115][116][117] (Table 2 and Fig. 1).…”