Stress-induced protein disaggregation in the endoplasmic reticulum and the organelle`s morpho-regulatory control of calcium transport revealed by high-sensitivity optical biosensing

Perturbations to the performance of the endoplasmic reticulum (ER) underly pathological conditions which lead to diseases such as neurodegeneration. We seek understanding of how this compartment`s structural and biochemical properties and their malperformance define cell functioning and health (brain cells in particular). Implementing in-house-developed light microscopy-based biosensing of protein folding handling and the performance of ER network`s transport system, we find i. a morphogens’-dependent ER tubules dilation/narrowing modulates calcium supply across the cell and ii—an inducible proteostasis-defending mechanism to antagonise protein aggregates.

Thus, the latter describes an innate system for avoidance of toxicity originating in failed protein production/handling. Protein synthesis is supported by cellular machineries that ensure polypeptides fold to their native conformation whilst eliminating misfolded, aggregation-prone species. Protein aggregation underlies pathologies, including neurodegeneration. Aggregates` formation is antagonised by molecular chaperones, with cytoplasmic machinery resolving insoluble protein aggregates. However, it is unknown whether an analogous disaggregation system exists in the ER, where ~30% of the proteome is synthesised. We showed that the ER of a variety of mammalian cell types, including neurons, is endowed with the capability to resolve protein aggregates under stress. Utilising a purpose-developed protein aggregation probing system with a sub-organellar resolution, we observe steady-state aggregate accumulation in the ER. Pharmacological induction of ER stress does not augment aggregates but rather stimulates their clearance within hours. We show that this disaggregation activity is catalysed by the stress-responsive ER molecular chaperone – BiP. This work reveals a hitherto unknown, non-redundant strand of the proteostasis-restorative ER stress response.