A New Model of Hagfish Slime Mucous Vesicle Stabilization and Deployment

Abstract

Hagfishes thwart predators by releasing large volumes of gill-clogging slime, which consists of mucus and silklike fibers. The mucous fraction originates within gland mucous cells, which release numerous vesicles that swell and rupture when ejected into seawater. Several studies have examined the function of hagfish slime mucous vesicles in vitro, but a comprehensive model of their biophysics is lacking. Here, we tested the hypothesis that vesicles contain polyanionic glycoproteins stabilized by divalent cations and deploy in seawater via exchange of divalent for monovalent cations. We also tested the hypothesis that vesicle swelling and stabilization are governed by "Hofmeister effects". We found no evidence for either hypothesis. Our results show that hagfish mucous granules are only stabilized by multivalent anions, and pH titration experiments underscore these results. Our results lead us to the conclusion that the hagfish slime mucous gel is in fact polycationic in nature.

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“…285,286 Based on this finding, it was proposed that the glycoproteins in the mucin vesicles might be cationic in nature and dianionic species such as citrate or sulfate could stabilize the fluid phase. 287 Interestingly, there are parallels here to the stabilization of highly cationic mussel byssus adhesive proteins as a fluid condensate with sulfate dianions. 242…”

“… ,, While currently, very little is understood about the exact sequence and structure of these glycoproteins, compositional analysis of the mucin component from the Pacific hagfish species Eptatretus stoutii revealed it to consist primarily of protein (77%) and smaller amounts of carbohydrates (12%), sulfate (6%), and lipids (5%) by dry weight. , Based on amino acid analysis, the protein component of the mucin is enriched in threonine, valine, and similar to velvet worm slime, proline . It has been demonstrated in vitro that exposure of the mucin material to solutions of various salts results in the formation of slime, but that slime formation is inhibited by polyvalent anions including citrate. , Based on this finding, it was proposed that the glycoproteins in the mucin vesicles might be cationic in nature and dianionic species such as citrate or sulfate could stabilize the fluid phase . Interestingly, there are parallels here to the stabilization of highly cationic mussel byssus adhesive proteins as a fluid condensate with sulfate dianions …”

“…Slime formation occurs when the contents of the GTC skeins and GMC mucin vesicles are ejected by the animal and mixed turbulently in a seawater environment (Figure ). Once deployed into the saline and basic conditions of the marine milieu, the skeins of IF-like slime threads unravel to their full length ,, and the mucin vesicles rupture, ,− producing the native slime. Interestingly, the presence of specific ions (especially Ca 2+ and Na + ) results in a more controlled skein unravelling process, suggesting a key role of electrostatic interactions in the material assembly process. , Both components are critical for slime formation; the slime threads by themselves will not form a network, and the mucin material on its own lacks any mechanical consistency .…”