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 Ca homeostasis as a direct consequence of the intracellular Aβ oligomers. However, little is known about the downstream effects of the resulting Ca rise on the many intracellular Ca-dependent pathways. Here we use multiscale modeling in conjunction with patch-clamp electrophysiology of single inositol 1,4,5-trisphosphate (IP) receptor (IPR) and fluorescence imaging of whole-cell Ca response, induced by exogenously applied intracellular Aβ oligomers to show that Aβ inflicts cytotoxicity by impairing mitochondrial function. Driven by patch-clamp experiments, we first model the kinetics of IPR, which is then extended to build a model for the whole-cell Ca signals. The whole-cell model is then fitted to fluorescence signals to quantify the overall Ca release from the endoplasmic reticulum by intracellular Aβ oligomers through G-protein-mediated stimulation of IP production. The estimated IP concentration as a function of intracellular Aβ content together with the whole-cell model allows us to show that Aβ oligomers impair mitochondrial function through pathological Ca uptake and the resulting reduced mitochondrial inner membrane potential, leading to an overall lower ATP and increased production of reactive oxygen species and HO. We further show that mitochondrial function can be restored by the addition of Ca buffer EGTA, in accordance with the observed abrogation of Aβ cytotoxicity by EGTA in our live cells experiments.

The Function of Mitochondrial Calcium Uniporter at the Whole-Cell and Single Mitochondrion Levels in WT, MICU1 KO, and MICU2 KO Cells

“…This limiting step is taken into account in the model [ 36 ] and the flux due to MAS shuttle decreases with [Ca 2+ ] M and is almost at the minimum value in MICU1 KO cells ( Figure S7B ) where the resting [Ca 2+ ] M ~ 0.26 μM ( Figure S6A ). Finally, in the model, the oxidation of NADH in the Electron Transport Chain and the coupled extrusion of protons from the mitochondria are incorporated in a single equation ( Equation (S10) ) [ 36 , 38 , 59 , 60 , 61 , 62 , 63 ]. This flux declines for more hyperpolarized IMM, since it is difficult to pump protons against a large potential gradient and vice versa ( Figure S7C ).…”

“…Several models for mitochondrial Ca 2+ signaling have been developed over the past (for example, see [36,38,47,59,[61][62][63][67][68][69][70]). However, they are mostly based on Ca 2+ uptake in permeabilized cell cultures or isolated mitochondria suspended in aqueous solutions.…”

“…Finally, in the model, the oxidation of NADH in the Electron Transport Chain and the coupled extrusion of protons from the mitochondria are incorporated in a single equation (Eq. S10) [36,38,[59][60][61][62][63]. This flux declines for more hyperpolarized IMM, since it is difficult to pump protons against a large potential gradient and vice versa ( Figure S7C To investigate this counterintuitive result further, we look at the three fluxes that control [NADH] M in the model (Equation (S1)).…”

The Function of Mitochondrial Calcium Uniporter at the Whole-Cell and Single Mitochondrion Levels in WT, MICU1 KO, and MICU2 KO Cells

“…Several models for mitochondrial Ca 2+ signaling have been developed over the past (for example, see [35,37,43,55,[57][58][59][62][63][64][65]). However, they are mostly based on Ca 2+ uptake in permeabilized cell cultures or isolated mitochondria suspended in aqueous solutions.…”

“…Finally, in the model the oxidation of NADH in the Electron Transport Chain and the coupled extrusion of protons from the mitochondria are incorporated in a single equation (Eq. S10) [35,37,[55][56][57][58][59]. This flux declines for more hyperpolarized IMM, since it is difficult to pump protons against a large potential gradient and vice versa ( Figure S7C To test whether adding Ca 2+ buffer to the cytoplasm would restore mitochondrial function in MICU1 KO cells, we vary the buffer concentration from 0 to 1000 M.…”