Structure modeling hints at a granular organization of the Golgi ribbon

Abstract

Background In vertebrate cells, the Golgi functional subunits, mini-stacks, are linked into a tri-dimensional network. How this “ribbon” architecture relates to Golgi functions remains unclear. Are all connections between mini-stacks equal? Is the local structure of the ribbon of functional importance? These are difficult questions to address, without a quantifiable readout of the output of ribbon-embedded mini-stacks. Endothelial cells produce secretory granules, the Weibel-Palade bodies (WPB), whose von Willebrand Factor (VWF) cargo is central to hemostasis. The Golgi apparatus controls WPB size at both mini-stack and ribbon levels. Mini-stack dimensions delimit the size of VWF "boluses” whilst the ribbon architecture allows their linear co-packaging, thereby generating WPBs of different lengths. This Golgi/WPB size relationship suits mathematical analysis. Results WPB lengths were quantized as multiples of the bolus size and mathematical modeling simulated the effects of different Golgi ribbon organizations on WPB size, to be compared with the ground truth of experimental data. An initial simple model, with the Golgi as a single long ribbon composed of linearly interlinked mini-stacks, was refined to a collection of mini-ribbons and then to a mixture of mini-stack dimers plus long ribbon segments. Complementing these models with cell culture experiments led to novel findings. Firstly, one-bolus sized WPBs are secreted faster than larger secretory granules. Secondly, microtubule depolymerization unlinks the Golgi into equal proportions of mini-stack monomers and dimers. Kinetics of binding/unbinding of mini-stack monomers underpinning the presence of stable dimers was then simulated. Assuming that stable mini-stack dimers and monomers persist within the ribbon resulted in a final model that predicts a “breathing” arrangement of the Golgi, where monomer and dimer mini-stacks within longer structures undergo continuous linking/unlinking, consistent with experimentally observed WPB size distributions. Conclusions Hypothetical Golgi organizations were validated against a quantifiable secretory output. The best-fitting Golgi model, accounting for stable mini-stack dimers, is consistent with a highly dynamic ribbon structure, capable of rapid rearrangement. Our modeling exercise therefore predicts that at the fine-grained level the Golgi ribbon is more complex than generally thought. Future experiments will confirm whether such a ribbon organization is endothelial-specific or a general feature of vertebrate cells.

Evolution of the ribbon-like organization of the Golgi apparatus in animal cells

“…We therefore analyzed sea urchin Golgi morphology at the ultrastructural level. As Golgi stack dimers have been observed in Drosophila melanogaster cells 21 , which notoriously display dispersed Golgi elements 22 , and even in mammalian cells after ribbon unlinking by microtubule depolymerization 23 , we defined Golgi centralization as "ribbon-like" only when three or more closely apposed stacks were observed in electron micrographs (Figure S1E). In sea urchin, at the ultrastructural level, the arrangement of Golgi elements recapitulated confocal microscopy observations.…”

RNA scaffolds the Golgi ribbon by forming condensates with GM130

“…In mammalian cells the Golgi is comprised of 100-200 stacks, but in contrast to non-vertebrate cells, the stacks are laterally fused together by tubular membranes into a twisted contiguous structure referred to as the Golgi ribbon (Nakamura et al, 2012). This higher-order organization into a joint compartment allows the Golgi to accommodate large proteins that are bigger than individual cisterna such as collagen or Weibel-Palade bodies (Lavieu et al, 2014; Page et al, 2022).…”