Active Endoplasmic Reticulum luminal flow – mechanism of generation and reliant neuronal cell functionalities

The Endoplasmic Reticulum (ER), a network of membranous sheets and pipes, supports functions encompassing biogenesis of secretory proteins and delivery of functional solutes throughout the cell periphery. Molecular mobility through the ER network enables these functionalities. The diffusion-driven molecular motion (traditionally presumed by default), alone is not sufficient to explain the kinetics of luminal transport across supramicron distances. Understanding the ER structure-function relationship is critical to rationalizing how mutations in ER morphogenic proteins give rise to neurodegenerations. A purpose-developed super-resolution microscopy and data analysis approach enabled us to deconvolute the molecular motion inside the ER and observe structural dynamics in greater detail: Stochastic analysis of single particle trajectories of ER luminal proteins revealed that the topological organization of the ER correlates with distinct trafficking modes of its luminal content: with a dominant diffusive component in tubular junctions and a fast flow component in tubules (Holcman et al. 2018 Nature Cell Biology). Particle trajectory orientations resolved over time revealed an alternating current of the ER contents. Fast structured illumination microscopy identified energy-dependent tubule contraction events at specific points as a plausible mechanism for generating active ER luminal flow (a membrane-curving process resulting in a nanoperistalsis-like content propulsion). The discovery of active flow in the ER has implications for timely ER content distribution throughout the cell, particularly important for cells with expensive ER-containing projections e.g. neurons. Our modelling and experimental measurements of calcium dynamics in normal and pathological ER reveal the reveals how the ER morphoregulation and dynamics define calcium signalling.