The role of cholesterol-sphingomyelin membrane nanodomains in the stability of intercellular membrane nanotubes

Maruša Lokar^1,*, Doron Kabaso^1,2,*, Nataša Resnik³, Kristina Sepcic⁵, Veronika Kralj-Iglic^4,6, Peter Veranic³, Robert Zorec², Aleš Iglic<sup>1,6

1</sup>Laboratory of Biophysics, Faculty of Electrical Engineering, ²Laboratory of Neuroendocrinology-Molecular Cell Physiology, Faculty of Medicine, ³Institute of Cell Biology, Faculty of Medicine, ⁴Faculty of Health Sciences, ⁵Department of Biology, Biotechnical Faculty, ⁶Laboratory of Clinical Biophysics, Department of Orthopedic Surgery, Faculty of Medicine, University of Ljubljana, Ljubljana, Slovenia

*These authors equally share first authorship

Abstract: Intercellular membrane nanotubes (ICNs) are highly curved tubular structures that connect neighboring cells. The stability of these structures depends on the inner cytoskeleton and the cell membrane composition. Yet, due to the difficulty in the extraction of ICNs, the cell membrane composition remains elusive. In the present study, a raft marker, ostreolysin, revealed the enrichment of cholesterol-sphingomyelin membrane nanodomains along ICNs in a T24 (malignant) urothelial cancer cell line. Cholesterol depletion, due to the addition of methyl-β-cyclodextrin, caused the dispersion of cholesterol-sphingomyelin membrane nanodomains and the retraction of ICNs. The depletion of cholesterol also led to cytoskeleton reorganization and to formation of actin stress fibers. Live cell imaging data revealed the possible functional coupling between the change from polygonal to spherical shape, cell separation, and the disconnection of ICNs. The ICN was modeled as an axisymmetric tubular structure, enabling us to investigate the effects of cholesterol content on the ICN curvature. The removal of cholesterol was predicted to reduce the positive spontaneous curvature of the remaining membrane components, increasing their curvature mismatch with the tube curvature. The mechanisms by which the increased curvature mismatch could contribute to the disconnection of ICNs are discussed.

Keywords: cyclodextrins, T24 urothelial cancer cell line, intercellular membrane nanotubes, cholesterol-sphingomyelin membrane nanodomains