Luminal transport rates through intact endoplasmic reticulum limit the magnitude of localized Ca2+ signals

Abstract

The endoplasmic reticulum (ER) forms an interconnected network of tubules stretching throughout the cell. Understanding how ER functionality relies on its structural organization is crucial for elucidating cellular vulnerability to ER perturbations, which have been implicated in several neuronal pathologies. One of the key functions of the ER is enabling Ca2+ signalling by storing large quantities of this ion and releasing it into the cytoplasm in a spatiotemporally controlled manner. Through a combination of physical modeling and livecell imaging, we demonstrate that alterations in ER shape significantly impact its ability to support efficient local Ca2+ releases, due to hindered transport of luminal content within the ER. Our model reveals that rapid Ca2+ release necessitates mobile luminal buffer proteins with moderate binding strength, moving through a well-connected network of ER tubules. These findings provide insight into the functional advantages of normal ER architecture, emphasizing its importance as a kinetically efficient intracellular Ca2+ delivery system. Significance StatementThe peripheral endoplasmic reticulum forms a continuous network of tubules extending through the entire cell. One of the key functional roles of the ER is the release of Ca2+ ions into the cytosol to support a broad diversity of intracellular signaling processes. Such release events are enabled by the high Ca2+ storage capacity of the ER. This work demonstrates that mobile Ca2+binding buffer proteins and a well-connected lattice-like architecture of the ER network are optimal to supply local Ca2+ signals and that changes in ER structure can modulate Ca2+ release. By linking transport kinetics to Ca2+ release, we demonstrate a key functional role for the interconnected network architecture of the ER.