BACKGROUND: The defining feature of eukaryotic cells is the presence of membranebound organelles of diverse kinds, each with specialized functions. Most organelles have multiple copies in cells. In contrast, each cell contains only one endoplasmic reticulum (ER). However, the ER consists of an elaborated network of membrane cisternae and tubules that extends throughout the cell and occupies a large fraction of the cytoplasmic volume. While compartmentalization of biochemical reactions and processes in these organelles has obvious advantages, it also poses challenges for their coordinated activity, requiring mechanisms for regulated inter-organelle communication. However, these have remained elusive and the quintessential textbook cartoon still pictures organelles in isolation, floating in a cytoplasmic sea. The last decade radically changed this view and membrane contact sites (MCSs) between different organelles were brought to the center stage as prime, highly regulated routes for inter-organelle communication essential for cell homeostasis. ADVANCES: The presence of organelle contacts was recognized long ago. However, the significance of these structures remained unclear. Recent advances in the resolution of microscopy and the development of unique fluorophores have dramatically advanced our ability to study inter-organelle MCSs. The 3D structure of ER MCSs with other organelles and the plasma membrane can be visualized at nanometer resolution by electron microscopy (EM). Multi-spectral live-cell fluorescence microscopy displays the behavior of MCSs over time and in response to stimuli. Together these data have revealed the general features of MCSs. For example, EM has revealed that MCSs are closely opposed and tethered, but not fused membranes; MCSs are spaced at 10-30nm; and ribosomes are excluded from the ER surface at these sites. Fluorescence microscopy demonstrates that organelles can remain attached to ER tubules as they traffic along microtubules. The combinations of these tools with classical molecular biology and biochemical tools have identified molecules implicated in several MCSs and elucidated their functions, including lipid and ion transport between organelles and organelles positioning and division. OUTLOOK: MCSs are central to normal cell physiology. Moreover, several MCSs proteins are linked to various diseases: Seipin, Protrudin, and Spastin to hereditary spastic paraplegia; VAPA and VAPB to amyotrophic lateral sclerosis; Dnm2 and Mfn2 to charcot marie tooth; Stim1 and Orai1 to tubular aggregate myopathy; and ACBD5 to retinal dystrophy. Whether defects in MCSs functions cause these diseases directly or indirectly remain to be explored. Recent progress has begun to identify some of the molecular machineries that regulate MCSs formation. Dissecting roles of these factors will strengthen our understanding of the integrative nature of MCSs. The advancement of diverse microscopy techniques will allow us to track multiple factors at MCSs simultaneously in real time and in high resolution, and this may help us gain a more detailed view of MCSs biology and their related physiological processes. Our textbook image of organelles has changed. Instead of isolated cellular compartments, the picture now emerging shows organelles as largely interdependent structures that can communicate through membrane contact sites (MCSs). MCSs are sites where opposing organelles are tethered but do not fuse. MCSs provide a hybrid location where the toolkits of two different organelles can work together to perform vital cellular functions, such as lipid and ion transfer, signaling, and organelle division. Here we concentrate on MCSs involving the endoplasmic reticulum (ER), an organelle forming an extensive network of cisternae and tubules. We will highlight how the dynamic ER network regulates a plethora of cellular processes through MCSs with various organelles and with the plasma membrane (PM). Fig. 0. Endoplasmic reticulum (ER) membrane contacts sites (MCSs) with other organelles and the plasma membrane (PM). The ER forms MCSs with mitochondria, Golgi, endosomes, peroxisomes, lipid droplets and the PM. These MCSs are closely opposed but not fused membranes containing various molecular machineries. Factors localized to these MCSs mediate essential cellular processes including lipid and ion exchange, organelle positioning and biogenesis.
