Inhibition of Mitochondrial Respiration Fragments ER Architecture and Remodels Organelle Contact Sites, as Revealed by FIB-SEM

The endoplasmic reticulum (ER) and mitochondria maintain a dynamic structural partnership essential for pancreatic β-cell homeostasis, yet the high-resolution 3D remodeling of these networks under stress conditions remains poorly defined.

We employed Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) to perform 3D reconstructions of INS1E cells subjected to mitochondrial respiratory chain inhibition, uncoupling, and exogenous oxidative stress. Quantitative analysis revealed that mitochondrial dysfunction induces profound ultrastructural transitions, characterized by significant luminal swelling of the ER, expansion of the perinuclear space, and mitochondrial diameter enlargement. 3D volume imaging identified a coordinated fragmentation of both ER and mitochondrial networks into discrete, spatially separated structures—a phenomenon distinct from the reticular morphology observed in control cells. The similarity between respiratory inhibition- and H ₂ O ₂ -induced phenotypes, together with preservation of ER structure following mitochondrial uncoupling, suggests a potential contribution of reactive oxygen species to the observed remodeling process.

Despite this extensive organelle breakdown, interorganelle membrane contact sites were not only preserved but expanded under stress conditions. We further provide a quantitative description of nuclear envelope–mitochondria contact sites (NAMs), demonstrating their selective remodeling during mitochondrial dysfunction. Our findings provide a high-resolution structural framework for organelle remodeling in β-cells, demonstrating that membrane contact sites are actively preserved and reorganized despite profound organelle fragmentation.