Endoplasmic reticulum remains continuous and undergoes sheet-to-tubule transformation during cell division in mammalian cells

TLDR

The endoplasmic reticulum is a multifunctional organelle whose cell‑cycle–dependent morphological changes remain poorly understood. The study proposes that mitotic ER structural changes are linked to ribosomal activity on ER membranes. To mimic mitotic reorganization, puromycin was used to strip ribosomes from interphase ER, reproducing the ribosome loss observed during mitosis. Quantitative confocal and EM analyses, supported by 3D electron tomography, show that during mitosis the ER loses sheet structures and reorganizes into a continuous, branched tubular network, a change confirmed by redistribution of nuclear envelope markers.

Abstract

The endoplasmic reticulum (ER) is a multifaceted cellular organelle both structurally and functionally, and its cell cycle–dependent morphological changes are poorly understood. Our quantitative confocal and EM analyses show that the ER undergoes dramatic reorganization during cell division in cultured mammalian cells as mitotic ER profiles become shorter and more branched. 3D modeling by electron tomography reveals that the abundant interphase structures, sheets, are lost and subsequently transform into a branched tubular network that remains continuous. This is confirmed by observing the most prominent ER subdomain, the nuclear envelope (NE). A NE marker protein spreads to the mitotic ER tubules, although it does not show a homogenous distribution within the network. We mimicked the mitotic ER reorganization using puromycin to strip the membrane-bound ribosomes from the interphase ER corresponding to the observed loss of ribosomes normally occurring during mitosis. We propose that the structural changes in mitotic ER are linked to ribosomal action on the ER membranes.