Super resolution imaging reveals details in hyperglycemic induced apoptosis in kidney cells

The role of the Bcl-family proteins in the mitochondrial apoptotic process is well described with biochemical and molecular methods in studies of isolated mitochondria and transfected cell lines. There is however little knowledge about the mechanisms for Bcl protein interaction leading to apoptosis in intact cells. In particular, the time sequence and location for Bcl protein interaction has so far only been described in hypothetical models.Here we have used Stimulated Emission Depletion (STED) microscopy and Single Molecule Localization Microscopy (SMLM) to study the apoptotic process in immune-stained rat renal epithelia cells exposed to 20 mM glucose (HG) and to study its rescue by ouabain. To assess distance between Bcl-2 proteins, we used the nearest-neighbor algorithm. The anti-apoptotic protein Bcl-xLl was predominantly expressed on mitochondria in control cells, and remained so throughout the process, although its abundance decreased. After 2h HG the apoptosis-inducing protein BAD had translocated from the cytoplasm to the mitochondria where it clustered with Bcl-xL. This occurred before an increase in reactive oxygen species and was dependent on activation of the PI3K –AKT pathway. According to current concepts, Bcl-xL interacts with the apoptotic protein Bax on the mitochondria under control conditions to translocate Bax back to the cytosol1. We found that Bax started to accumulate on the mitochondria after 4h HG and, surprisingly, that the interaction between Bcl-xL and Bax became more pronounced during the course of the apoptotic process. After 6h HG Bax also interacted with the non-specific ion transporter VDAC; an interaction described to lead to penetration of the inner mitochondrial membrane and mark the point of no return.
QC 20171003