Increased spatiotemporal resolution reveals highly dynamic dense tubular matrices in the peripheral ER

A dynamic view of the endoplasmic reticulum The endoplasmic reticulum (ER) is a complex membranous structure that extends from the nuclear envelope to the cell periphery. It has important roles in many cellular processes, and numerous proteins are involved in maintaining its structure. Nixon-Abell et al. used superresolution approaches to look at the ER at the periphery of the cell, where the ER contacts many other cellular organelles (see the Perspective by Terasaki). This peripheral ER has been thought to comprise tubules and sheets; however, the higher-resolution view revealed that most of the “sheets” consist of a dense clustering of tubules. This dynamic meshwork may allow the ER to change its conformation rapidly in response to cellular needs. Science, this issue p. 433; see also p. 415 Superresolution imaging shows that presumed sheets of endoplasmic reticulum comprise tightly packed, constantly changing tubules. INTRODUCTION The endoplasmic reticulum (ER) is a continuous, membrane-bound organelle, spanning from the nuclear envelope to the outer cell periphery, that contacts and influences nearly every other cellular organelle. In the peripheral ER, prevailing models propose a system of interconnected tubules and flattened sheets maintained by distinct proteins. Because mutations in these proteins and resultant ER irregularities coincide with various neurologic disorders, characterizing ER morphology is critical in understanding its roles in the basic biology of cells in both health and disease. Given limitations in imaging technologies, determining the dynamic rearrangements and fine ultrastructure of the peripheral ER has proven challenging. RATIONALE Previous work characterizing peripheral ER structure has relied extensively on diffraction-limited optical microscopy to describe gross morphology and dynamics, and electron microscopy (EM) for ultrastructural details. Regrettably, the respective spatial and temporal limitations of these techniques can obscure underlying cell processes where intricate morphology and/or rapid dynamism are important. Additionally, efforts to characterize protein distribution in the peripheral ER have presented confounding evidence regarding the localization of tubular junction-forming atlastin guanosine triphosphatases to sheets, and concerning the induction of sheet proliferation after atlastin overexpression. We exploited a variety of emerging superresolution (SR) microscopy techniques to collectively provide unprecedented spatiotemporal resolution that challenges prevailing models regarding peripheral ER morphology, dynamics, and protein distribution. RESULTS We used a combination of five SR technologies, with complementary strengths and weaknesses in the spatial and temporal domains, to image the peripheral ER in live and fixed cells. Using novel analytical approaches to study both protein and lipid components, we found that many structures previously proposed to be flat membrane sheets are instead densely packed tubular arrays—a previously undescribed structure we term an ER matrix. These matrices can become astoundingly compact, with spaces between the tubules far beneath the resolvable power of even most SR technologies. We observed dynamic oscillations of ER tubules and junctions, with matrices rapidly interconverting from tight to loose arrays, giving rise to different apparent morphologies dependent upon how closely their three-way junctions are clustered. We demonstrate how these ER matrices have been misinterpreted as a result of the spatiotemporal limitations of earlier imaging technologies. Finally, we account for the distribution of atlastin and other ER-shaping proteins within these structures. CONCLUSION The application of cutting-edge SR technologies to the peripheral ER has established a precedent for studying its dynamics and structural properties in living cells. The specific finding of dense tubular matrices in areas previously thought of as flat sheets provides a new model for maintaining and generating ER structure. Reorganization from tight to loose tubular network arrays may allow the ER to rapidly reach outward to the cell periphery during migration or other cell shape changes. Moreover, tight clusters of junctions may function as sites for sequestering excess membrane proteins and lipids or for contacting other organelles. Improved spatiotemporal resolution of ER structure and dynamics, as shown here, should help to address these and other key issues regarding ER function in healthy cells and during disease pathogenesis. Dense tubular matrices in the peripheral ER. New superresolution imaging modalities reveal that peripheral ER sheets are actually densely clustered tubules and interconnecting junctions. Shown is the distribution of an ER protein marker (3D-SIM) (upper right), internal cellular lipids (LLS-PAINT) (lower left), and an EM reconstruction (FIB-SEM) (upper left) demonstrating tubular matrices in the peripheral ER at high resolution. The endoplasmic reticulum (ER) is an expansive, membrane-enclosed organelle that plays crucial roles in numerous cellular functions. We used emerging superresolution imaging technologies to clarify the morphology and dynamics of the peripheral ER, which contacts and modulates most other intracellular organelles. Peripheral components of the ER have classically been described as comprising both tubules and flat sheets. We show that this system consists almost exclusively of tubules at varying densities, including structures that we term ER matrices. Conventional optical imaging technologies had led to misidentification of these structures as sheets because of the dense clustering of tubular junctions and a previously uncharacterized rapid form of ER motion. The existence of ER matrices explains previous confounding evidence that had indicated the occurrence of ER “sheet” proliferation after overexpression of tubular junction–forming proteins.