Data-driven modeling of mitochondrial dysfunction in Alzheimer's disease

Abstract

Intracellular accumulation of oligomeric forms of β amyloid (Aβ) are now believed to play a key role in the earliest phase of Alzheimer's disease (AD) as their rise correlates well with the early symptoms of the disease. Extensive evidence points to impaired neuronal Ca²⁺ homeostasis as a direct consequence of the intracellular Aβ oligomers. However, little is known about the downstream effects of the resulting Ca²⁺ rise on the many intracellular Ca²⁺-dependent pathways. Here we use multiscale modeling in conjunction with patch-clamp electrophysiology of single inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) and fluorescence imaging of whole-cell Ca²⁺ response, induced by exogenously applied intracellular Aβ₄₂ oligomers to show that Aβ₄₂ inflicts cytotoxicity by impairing mitochondrial function. Driven by patch-clamp experiments, we first model the kinetics of IP₃R, which is then extended to build a model for the whole-cell Ca²⁺ signals. The whole-cell model is then fitted to fluorescence signals to quantify the overall Ca²⁺ release from the endoplasmic reticulum by intracellular Aβ₄₂ oligomers through G-protein-mediated stimulation of IP₃ production. The estimated IP₃ concentration as a function of intracellular Aβ₄₂ content together with the whole-cell model allows us to show that Aβ₄₂ oligomers impair mitochondrial function through pathological Ca²⁺ uptake and the resulting reduced mitochondrial inner membrane potential, leading to an overall lower ATP and increased production of reactive oxygen species and H₂O₂. We further show that mitochondrial function can be restored by the addition of Ca²⁺ buffer EGTA, in accordance with the observed abrogation of Aβ₄₂ cytotoxicity by EGTA in our live cells experiments.

Keywords: Alzheimer's disease; Ca(2+) dyshomeostasis; Intracellular β amyloid; Mitochondrial dysfunction.

Fig. 1:

Equilibrium P_o of the single IP₃R channel in Xenopus laevis oocytes as a function of Ca²⁺ and IP₃ concentrations. P_o at [IP₃] = 10μM (black), 33nM (blue), 20nM (red), and 10nM (green) as we vary [Ca²⁺]_i. The symbols and lines represent experimental results [40, 41] and model fits respectively.

Fig. 2:

Schematic of the four-state model for single IP₃R Ca²⁺ channel. K_XY represents the transition rate from state X to Y where X,Y =, A, O, I , and R respectively.

Fig. 3:

Intracellular injection of Aβ₄₂ oligomers leads to IP₃ generation and consequently Ca²⁺ release from the ER into the cytoplasm. Time-traces of ΔF/F_o indicating [Ca²⁺]_i rise from experiment (circles) and model fits (solid lines) due to injection of 30μg/ml (blue), 10μg/ml (black), 3μg/ml (red), and 1μg/ml (green) [Aβ₄₂] oligomers (A) and corresponding IP₃ production (B). The experimental traces represent the mean time course of Ca²⁺dependent fluorescence recorded from 5 (30 μg/ml), 4 (10 μg/ml), 4 (3 μg/ml), and 6 (1 μg/ml) oocytes respectively [32].

Fig. 4:

High [Ca²⁺]_i in response to intracellular injection of Aβ₄₂ oligomers leads to impaired mitochondrial function. Time-traces of [Ca²⁺]_i (A), [Ca²⁺]_m (B), [Ca²⁺]_ER (C), △Ψ_m (D), [NADH] (E), [ATP] (F), ${\left[{\text{O}}_2^{-}\right]}_{\text{m}}$(G), and [H₂O₂] (H) responses due to injection of 30 μg/ml (blue), 10 μg/ml (black), 3 μg/ml (red), and 1 μg/ml (green) [Aβ₄₂] oligomers. No EGTA is added in these simulations.

Fig. 5:

EGTA can reverse the affect of Aβ₄₂ oligomers on Ca²⁺ signaling and mitochondrial function. Time-traces of [Ca²⁺]_i (A), [Ca²⁺]_m (B), [Ca²⁺]_ER (C), 4Ψ_m (D), [NADH] (E), [ATP] (F), ${\left[{\text{O}}_2^{-}\right]}_{\text{m}}$(G), and [H₂O₂] (H) response due to injection of 30 μg/ml (blue), 10 μg/ml (black), 3 μg/ml (red), and 1 μg/ml (green) [Aβ₄₂] oligomers with 1000 μM [EGTA] added to the cell.

Fig. 6:

Ca²⁺ buffer EGTA restores cell viability. No oocytes injected with 1 μg/ml of Aβ₄₂ oligomers remained viable after 37 hours of injection (black). Injecting 3mM of EGTA, restored the viability of oocytes to 70% [32]. Oocytes were bathed in a solution containing no added Ca²⁺ in order to eliminate influx from extracellular solution.

Fig. 7:

EGTA rescues mitochondrial function. Surface plots showing the dependance of Aβ₄₂ and [EGTA] on change (Δ) in values from steady state of [Ca²⁺]_i (A), [Ca²⁺]_m (B), ${\left[{\text{O}}_2^{-}\right]}_{\text{m}}$(C), △Ψ_m (D), [H₂O₂] (E), and [ATP] (F). Note that the change in panels (D) and (F) indicates a decrease with respect to resting values, while all other panels indicate an increase with respect to the resting levels.