Freeman et al., Science 2020

Lipid-gated monovalent ion fluxes regulate endocytic traffic and support immune surveillance


>>Ion fluxes resolve organellar volume<<

EDITOR’s SUMMARY Animal cells continuously sample the surrounding medium, a feature accentuated in immune cells. Sampling is accomplished by trapping external medium into membrane-bound vesicles or vacuoles. These structures are promptly resolved, thus avoiding accumulation of endomembranes and volume expansion. In a variety of cultured cells, Freeman et al. found that this resolution entails conversion of spherical vacuoles into thin tubules, a process that involves marked changes in surface-to-volume ratio (see the Perspective by King and Smythe). Shrinkage of membrane-bound structures is driven by ion fluxes and subsequent osmotic transfer of water. Shriveled vacuoles attract curvature-sensing proteins that promote the extension of fine tubules. Ion channels thereby control membrane remodeling, enabling receptor recycling and proper routing of cellular cargo.

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ABSTRACT Despite ongoing (macro)pinocytosis of extracellular fluid, the volume of the endocytic pathway remains unchanged. To investigate the underlying mechanism, we used high-resolution video imaging to analyze the fate of macropinosomes formed by macrophages in vitro and in situ. Na+, the primary cationic osmolyte internalized, exited endocytic vacuoles via two-pore channels, accompanied by parallel efflux of Cl and osmotically coupled water. The resulting shrinkage caused crenation of the membrane, which fostered recruitment of curvature-sensing proteins. These proteins stabilized tubules and promoted their elongation, driving vacuolar remodeling, receptor recycling, and resolution of the organelles. Failure to resolve internalized fluid impairs the tissue surveillance activity of resident macrophages. Thus, osmotically driven increases in the surface-to-volume ratio of endomembranes promote traffic between compartments and help to ensure tissue homeostasis.

Failure of endosome resolution in PIKfyve-deficicent tissue macrophages (right) compared to wildtype cells (left).

The techniques & methods we used in this study:
+ Intravital 2P-microscopy
+ Multiplex confocal microscopy
+ In vivo cell biology (tissue biology)
+ Ex vivo cell biology

+ Dynamic spinning disk microscopy
+ Electron microscopy

Failure of endosome resolution (right) abrogates tissue surveillance and promotes neutrophil-driven inflammation (magenta) at stromal micro-lesions which are usually cloaked (left) by macrophages (green).