Ca²⁺‐dependent endoplasmic reticulum stress correlates with astrogliosis in oligomeric amyloid β‐treated astrocytes and in a model of Alzheimer's disease

Abstract

SummaryNeurotoxic effects of amyloid b peptides are mediated through deregulation of intracellular Ca 2+ homeostasis and signaling, but relatively little is known about amyloid b modulation of Ca 2+ homeostasis and its pathological influence on glia. Here, we found that amyloid b oligomers caused a cytoplasmic Ca 2+ increase in cultured astrocytes, which was reduced by inhibitors of PLC and ER Ca 2+ release. Furthermore, amyloid b peptides triggered increased expression of glial fibrillary acidic protein (GFAP), as well as oxidative and ER stress, as indicated by eIF2a phosphorylation and overexpression of chaperone GRP78. These effects were decreased by ryanodine and 2APB, inhibitors of ryanodine receptors and InsP 3 receptors, respectively, in both primary cultured astrocytes and organotypic cultures of hippocampus and entorhinal cortex. Importantly, intracerebroventricular injection of amyloid b oligomers triggered overexpression of GFAP and GRP78 in astrocytes of the hippocampal dentate gyrus. These data were validated in a triple-transgenic mouse model of Alzheimer's disease (AD). Overexpression of GFAP and GRP78 in the hippocampal astrocytes correlated with the amyloid b oligomer load in 12-month-old mice, suggesting that this parameter drives astrocytic ER stress and astrogliosis in vivo. Together, these results provide evidence that amyloid b oligomers disrupt ER Ca 2+ homeostasis, which induces ER stress that leads to astrogliosis; this mechanism may be relevant to AD pathophysiology.

Astroglial calcium signalling in Alzheimer's disease

“…The data on acute effects of β-amyloid on astroglial cells in vitro and in slices are controversial. While several laboratories reported that β-amyloid (at 100 nM -5 µM) concentrations triggered transient [Ca 2+ ] i increases or [Ca 2+ ] i oscillations [98,[115][116][117][118][119], others have not found acute [Ca 2+ ] i responses to β-amyloid [112][113][114]. This seemingly incongruent findings could possibly be attributed to variability of β-amyloid species used.…”

“…Treatment of rodent astrocytes in vitro, in dissociated cell cultures and in situ in organotypic slices with β-amyloid in concentrations ranging between 100 nM and > 5 µM trigger astrogliotic response [97,98]. In the brains of transgenic AD mouse models, reactive astrocytes are generally associated with β-amyloid depositions and β-amyloid plaques in the hippocampus [96].…”

“…As dicussed earlier, in 3xTG-AD mice astrogliotic response is present in the hippocampus but not in the entorhinal and prefrontal cortices. The role for astroglial Ca 2+ signalling in triggering astrogliosis in response to β-amyloid was demonstrated in cultured astrocytes and in organotypic slices [98]. Exposure of these cells to β-amyloid triggered [Ca 2+ ] i oscillations originating from InsP 3 -mediated Ca 2+ release from the ER; pharmacological inhibition of these oscillations suppressed both [Ca 2+ ] i dynamics and astroglial reactivity [98].…”

“…The role for astroglial Ca 2+ signalling in triggering astrogliosis in response to β-amyloid was demonstrated in cultured astrocytes and in organotypic slices [98]. Exposure of these cells to β-amyloid triggered [Ca 2+ ] i oscillations originating from InsP 3 -mediated Ca 2+ release from the ER; pharmacological inhibition of these oscillations suppressed both [Ca 2+ ] i dynamics and astroglial reactivity [98]. When hippocampal astrocytes were compared to the astrocytes from the entorhinal cortex, it turned out that β-amyloid significantly up-regulated the expression of mGluR/InsP 3 Ca 2+ signalling pathway in the former but did not affect this cascade in the latter.…”

Destination Brain: the Past, Present, and Future of Therapeutic Gene Delivery

“…Similar to microglia, astrocytes secrete neuromodulatory molecules, which can be either protective or damaging depending on the stage of injury, making astrogliosis a ‘double-edged’ sword (Buffo et al, 2010). Other astrocyte intracellular processes that contribute to NDD pathologies include oxidative and mitochondrial stress (Manfredi and Xu, 2005), reduced excitatory amino acid transporter (EAAT)2 levels that lead to excitotoxicity (Ambrosi et al, 2014), calcium dysregulation (Alberdi et al, 2013) and attenuated neurotrophin secretion (Giralt et al, 2010). …”