Soluble Abeta produces an increase in cytosolic and mitochondrial calcium in astrocytes in vivo, in part by direct effect in astroglial cells

Abstract

AbstractBackgroundBy using in vivo multiphoton microscopy, we have previously reported that astrocytic cytosolic resting calcium (Ca2+) is globally elevated in a transgenic mouse model of cerebral β‐amyloidosis (APPswe/PS1dE9) (PMID:19251629). However, this phenomenon is independent of the proximity of astrocytes to amyloid β (Aβ) plaques. In addition, soluble Aβ oligomers (Aβo) are thought to mediate neuronal toxicity by increasing neuronal calcium and causing synapse loss. For those reasons, we hypothesized that Aβo, rather than Aβ plaques, contribute to the Ca2+ insult in astroglial cytosol and mitochondria in vivo.MethodTo test this hypothesis, we topically applied naturally secreted soluble Aβo (conditioned medium from cultured transgenic primary neurons) onto the brain surface of 4‐6‐month old wild‐type mice. We investigated changes on astrocyte and mitochondrial Ca2+ levels relative to baseline via intravital multiphoton imaging, by using a ratiometric genetically‐encoded FRET‐based Ca2+ indicator (Yellow Cameleon 3.6) targeted to astroglial cytosol or mitochondria.ResultWe observed that Ca2+ levels dramatically increased upon topical soluble Aβo application, in all cytosolic astrocyte compartments (soma, branches and end feet), but also in astrocyte mitochondria. Aβ‐immunodepleted transgenic conditioned media and wild‐type conditioned media did not alter astrocyte cytosolic or mitochondrial Ca2+, supporting the specificity of the observed effects. Furthermore, we have also seen this effect in cultured astrocytes, suggesting that Aβo is directly interacting with astroglial cells, and not through an effect on other cell types, such as neurons or microglia. In addition, we previously reported that upon blocking the mitochondrial calcium uniporter (MCU) with Ru360, calcium levels in neuronal mitochondria were restored (PMID:32358564). However, there was still an increase of mitochondrial calcium in astrocytes in presence of soluble Aβo and Ru360 in vivo. Thus, soluble Aβo is disrupting calcium levels in astrocytes through a different mechanism.ConclusionUltimately, these results together support a detrimental role of Aβo which leads to astrocyte Ca2+ dyshomeostasis in vivo, implying that Aβo are involved in the astrocytic dysfunction observed in AD. Future studies will continue to explore the source of soluble Aβo‐induced Ca2+ increase, including intracellular Ca2+ stores and channels involved, which could have therapeutic value.