The structure and global distribution of the endoplasmic reticulum network is actively regulated by lysosomes

Abstract

28The endoplasmic reticulum (ER) comprises morphologically and functionally distinct domains, 29 sheets and interconnected tubules. These domains undergo dynamic reshaping, in response to 30 changes in the cellular environment. However, the mechanisms behind this rapid remodeling 31 within minutes are largely unknown. Here, we report that ER remodeling is actively driven by 32 lysosomes, following lysosome repositioning in response to changes in nutritional status. The 33 anchorage of lysosomes to ER growth tips is critical for ER tubule elongation and connection. We 34 validate this causal link via the chemo-and optogenetically driven re-positioning of lysosomes, 35 which leads to both a redistribution of the ER tubules and its global morphology. Lysosomes sense 36 metabolic change in the cell and regulate ER tubule distribution accordingly. Dysfunction in this 37 mechanism during axonal extension may lead to axonal growth defects. Our results demonstrate a 38 critical role of lysosome-regulated ER dynamics and reshaping in nutrient responses and neuronal 39 development. 40 41 Main text 42 43The structure of the ER is constantly adapted for the particular needs of the cell (1): the dynamic 44 transitions between ER sheets and tubules allow it to rapidly respond to the changing cellular 45 environment. A group of ER-shaping proteins have been identified as maintaining ER morphology 46 (1), mutations in which are linked to diseases such as hereditary spastic paraplegias (HSPs) (2). 47 (3, 4). Previous work has shown that ER tubule elongation can be driven by three mechanisms: 1/ 50 force generation by motors moving along microtubules (5), which can be classified as sliding, 2/ 51 coupling to microtubule growth using a tip assembly complex (TAC), and 3/ hitchhiking by 52 connecting to other organelles. Whether such reshaping in local domains of the ER tubules could 53 lead to the global reorganization and redistribution of ER remains an open question, and, if this is 54 the case, how is this process regulated? The ER is known to contact other motile organelles, 55 including endosomes, lysosomes, mitochondria, peroxisomes et cetera (6). Among these, 56 lysosomes are particularly interesting, as they make a great number of contacts with the ER (7) 57 and their positioning is regulated by different nutritional status (8). Although ER has been reported 58 to regulate lysosome motions (9), it is not clear whether lysosomes can modulate ER reshaping 59 and distribution, for example via coupled motion (4). We hypothesized that a causal link exists 60 between lysosome motion and ER redistributing and asked whether this provides a mechanism for 61 ER morphological response to nutritional status, given that lysosomes are known to act as signaling 62 hubs for metabolic sensing (10). 63 We first investigated the correlation of motions between lysosomes and the ER network by rapid 64 live-cell imaging. We visualized ER with GFP-tagged vesicle-associated membrane protein-65 associated protein A (VAP...

Axonal Organelles as Molecular Platforms for Axon Growth and Regeneration after Injury

“…MCSs between endoplasmic reticulum and the endocytic pathways have emerged as regulators of endosomal function and localization, and their association is normally microtubule-associated and increases as endosomes mature [282,283]. Interestingly, ER tubule positioning along the axon and its morphology were recently shown to depend on motile lysosomes in response to chemical, light or metabolic stimuli [284]. Ablation of ERlysosome MCS protein, VAP-A, resulted in ER fragmentation and reduced axonal length in growing RGC axons [284].The molecular composition of MCSs between ER and lysosomes/late endosomes is still not very well studied.…”

“…Interestingly, ER tubule positioning along the axon and its morphology were recently shown to depend on motile lysosomes in response to chemical, light or metabolic stimuli [284]. Ablation of ERlysosome MCS protein, VAP-A, resulted in ER fragmentation and reduced axonal length in growing RGC axons [284].The molecular composition of MCSs between ER and lysosomes/late endosomes is still not very well studied. One protein shown to modulate the interaction between the ER and late endosomes (LE) is ER-resident protein Protrudin.…”

Complex Interactions Between Membrane-Bound Organelles, Biomolecular Condensates and the Cytoskeleton

“…In mammals, several examples of this type of organelle hitchhiking have also been recently discovered. For instance, it was found that the ER can tether itself onto late endosomes/lysosomes for hitchhiking in cell lines and neurons ( Guo et al, 2018 ; Lu et al, 2020 ). GI-SIM imaging also revealed that mitochondria can hitchhike on motile late endosomes/lysosomes for their translocation along MTs ( Guo et al, 2018 ).…”

“…Dual-color single particle tracking in COS7 cells revealed that 98% of lysosomes are moving simultaneously with the ER and the growing tips of the ER associated with lysosomes are elongated and merged with the existing ER network to form three-way junctions whilst the ER growth tips that are not associated with lysosomes cannot maintain elongation and are retracted ( Lu et al, 2020 ). In addition, manipulation of lysosome positioning by coupling lysosomes to dynein for retrograde movement to the perinuclear area or kinesins for anterograde movement to the cell periphery has been recently shown to cause a reduction in ER tubules or extension of ER tubules to the periphery of cells, respectively ( Lu et al, 2020 ). Moreover, live-cell imaging in cultured Xenopus laevis retinal ganglion cell (RGC) axons showed that the ER can rearrange its structure into a ring arrangement that tightly wraps around lysosomes.…”

“…These lysosomes then pull out the ER tubule for its extension. This suggests that ER tubule elongation is driven by lysosome positioning and is essential for ER tubule remodeling in axons to support axonal growth ( Lu et al, 2020 ).…”