Compromised Astrocyte Swelling/Volume Regulation in the Hippocampus of the Triple Transgenic Mouse Model of Alzheimer’s Disease

Abstract

In this study, we aimed to disclose the impact of amyloid-β toxicity and tau pathology on astrocyte swelling, their volume recovery and extracellular space (ECS) diffusion parameters, namely volume fraction (α) and tortuosity (λ), in a triple transgenic mouse model of Alzheimer’s disease (3xTg-AD). Astrocyte volume changes, which reflect astrocyte ability to take up ions/neurotransmitters, were quantified during and after exposure to hypo-osmotic stress, or hyperkalemia in acute hippocampal slices, and were correlated with alterations in ECS diffusion parameters. Astrocyte volume and ECS diffusion parameters were monitored during physiological aging (controls) and during AD progression in 3-, 9-, 12- and 18-month-old mice. In the hippocampus of controls α gradually declined with age, while it remained unaffected in 3xTg-AD mice during the entire time course. Moreover, age-related increases in λ occurred much earlier in 3xTg-AD animals than in controls. In 3xTg-AD mice changes in α induced by hypo-osmotic stress or hyperkalemia were comparable to those observed in controls, however, AD progression affected α recovery following exposure to both. Compared to controls, a smaller astrocyte swelling was detected in 3xTg-AD mice only during hyperkalemia. Since we observed a large variance in astrocyte swelling/volume regulation, we divided them into high- (HRA) and low-responding astrocytes (LRA). In response to hyperkalemia, the incidence of LRA was higher in 3xTg-AD mice than in controls, which may also reflect compromised K+ and neurotransmitter uptake. Furthermore, we performed single-cell RT-qPCR to identify possible age-related alterations in astrocytic gene expression profiles. Already in 3-month-old 3xTg-AD mice, we detected a downregulation of genes affecting the ion/neurotransmitter uptake and cell volume regulation, namely genes of glutamate transporters, α2β2 subunit of Na+/K+-ATPase, connexin 30 or Kir4.1 channel. In conclusion, the aged hippocampus of 3xTg-AD mice displays an enlarged ECS volume fraction and an increased number of obstacles, which emerge earlier than in physiological aging. Both these changes may strongly affect intercellular communication and influence astrocyte ionic/neurotransmitter uptake, which becomes impaired during aging and this phenomenon is manifested earlier in 3xTg-AD mice. The increased incidence of astrocytes with limited ability to take up ions/neurotransmitters may further add to a cytotoxic environment.

Asymmetric dysregulation of glutamate dynamics across the synaptic cleft in a mouse model of Alzheimer’s disease

“…In either case, it is likely that the role of diffusion in clearing glutamate from synaptic sites has been underestimated, as further suggested by a recent paper that elegantly used iGluSnFR to quantify glutamate spread to neighboring synapses in the neuropil [ 68 ]. Importantly, at 6 months of age, the total volume of the extracellular space and its tortuosity are the same between WT and 3xTg mice [ 69 ], consistent with the conclusion that our observed effects are likely to be mediated by deficits in transporter-mediated uptake rather than altered diffusion. Using super-resolution imaging techniques, it is of interest for future studies to better understand the nanoscale spatial relationships between GLT-1 expression, perisynaptic astrocytic processes, and pre- and postsynaptic membranes in 3xTg mice.…”

“…Blocking Kir4.1 on astrocytes prolongs the depolarization of perisynaptic astrocytic processes, seemingly by increasing the time required to restore extracellular potassium levels [ 71 ]. Interestingly, Kir4.1 expression is reduced in 3xTg mice and 3xTg astrocytes exhibit reduced swelling in response to a high potassium challenge [ 69 ], suggesting that activity-dependent slowing of glutamate clearance may be exaggerated in 3xTg mice due to poor Kir4.1-mediated potassium buffering.…”

Local diffusion in the extracellular space of the brain

“…4B). This unusual relation between α and tortuosity was also observed in another Alzheimer's disease model, the 3xTg mice (Tureckova et al, 2022). This discrepancy has been hypothesized to be the result of an enlarged ECS due to amyloid deposition, which would also create an additional diffusion barrier that increases tortuosity.…”