Synchronous Hyperactivity and Intercellular Calcium Waves in Astrocytes in Alzheimer Mice

Abstract

While senile plaques injure neurons focally, the functional response of astrocytes to Alzheimer's disease pathology is unknown. Using multiphoton fluorescence lifetime imaging microscopy in vivo, we quantitatively imaged astrocytic calcium homeostasis in a mouse model of Alzheimer's disease. Resting calcium was globally elevated in the astrocytic network, but was independent of proximity to individual plaques. Time lapse imaging revealed that calcium transients in astrocytes were more frequent, were synchronously coordinated across long distances, and were uncoupled from neuronal activity. Furthermore, rare intercellular calcium waves were observed, but only in mice with amyloid-β plaques, originating near plaques and spreading radially at least 200 µm. Thus, while neurotoxicity is observed near amyloid-β deposits, there exists a more general astrocyte-based network response to focal pathology.Growing evidence supports the hypothesis that in Alzheimer's disease (AD), synapses fail and dendritic spines are lost in the amyloid-β (Aβ) plaque micro-environment through a combination of changes to synaptic drive, calcium overload, and activation of calciumdependent degenerative processes (1-4). Neurons, however, make up only part of the brain's volume, with astrocytes making up the bulk of the remaining volume. Astrocytes form a structurally interconnected network that, in vitro, exhibit unique long-distance signaling properties that might be revealed in vivo only after pathological trauma. The idea that neural network dysfunction and degeneration fully mediates the memory loss in AD also does not reflect the growing in vivo evidence that astrocytes play an important role in cortical circuit function (5-7). In AD, pathological studies of human cases and mouse models have shown that astrocytes surround plaques and might play a critical role in Aβ deposition and clearance (8-10). Given the profound impact of Aβ deposition on nearby neuronal calcium homeostasis and synaptic function, it is reasonable to hypothesize that astrocyte networks would also be perturbed and might contribute to cortical dysfunction (11). We sought to test whether senile plaque deposition would similarly impact astrocyte calcium homeostasis or dynamic signaling in vivo in a mouse model of AD.To answer these questions, we used multiphoton fluorescent lifetime imaging microscopy (FLIM) to measure resting calcium levels in astrocytes of live mice with cortical plaques (12). We multiplexed the fluorescent properties of a small molecule calcium dye, OregonGreen BAPTA-1 AM (OGB), in the same experimental model and for the same group of cells (Fig. 1A); we used OGB both as a relative indicator of astrocytic activity (intensity) and as a quantitative measure of steady-state [Ca]i (lifetime). We used mice that express mutant human Aβ precursor protein (APP, swe) and mutant presenilin 1 (PS1, ΔE9) in neurons. These mutations lead to an increase in Aβ production and plaque deposition beginning at ~4.5 months

Calcium Signaling and Amyloid Toxicity in Alzheimer Disease

“…Alterations in Ca 2ϩ transients may exert global alterations in intracellular function and affect long-range coordination among cells mediated by intercellular Ca 2ϩ waves. This phenomenon is not limited to neurons, as increased Ca 2ϩ activity and synchronized Ca 2ϩ waves are observed across networks of astrocytes (84). Interestingly, astrocytic Ca 2ϩ signals differ from those in neurons in that they are independent of proximity to plaques.…”

“…However, recent evidence demonstrates functional associations between dense core plaques and Ca 2ϩ signaling alterations in AD mouse models. A series of in vivo imaging studies show that A␤ deposits result in intracellular Ca 2ϩ dysregulation in neurons and glia (83,84) and a structural breakdown of dendritic processes in later stages of AD pathology (85). Utilizing fluorescent live cell imaging techniques in plaque-bearing APP transgenic mice, increased resting Ca 2ϩ levels have been observed in neurites in close proximity (ϳ20 m) to dense core plaques (83), suggesting that plaques exert a direct pathogenic effect on steady-state Ca 2ϩ levels in dendrites and spines, regions critical for electrochemical signal transmission.…”

Astroglia as a cellular target for neuroprotection and treatment of neuro‐psychiatric disorders

“…Calcium homeostasis is also affected. Both resting calcium and intracellular calcium waves in astrocytes near plaques are increased, indicating that the astrocyte network contributes to AD pathology (Kuchibhotla, Lattarulo, Hyman, & Bacskai, 2009). Additionally, gap junctions between astrocytes are altered in AD (Nagy, Li, Hertzberg, & Marotta, 1996).…”

Astrocytes in Alzheimer's Disease

“…169 170,171 The abnormal, spontaneous Ca 2ϩ oscillations and Ca 2ϩ waves were also observed in vivo in astrocytes associated with neuritic plaques. 172 Furthermore, astrocytes in A␤ overexpressing transgenic mice demonstrated increased coupling in neocortical regions and had elevated expression of AMPA/kainate glutamate receptors and glutamate transporters. 173 In contrast, A␤ was reported to decrease expression and capacity of glutamate-aspartate transporter and glutamate transporter-1 mediated glutamate uptake in cultured astrocytes.…”