Bahmanyar, S. | Search bioRxiv

bioRxiv2017

Nano-scale size holes in ER sheets provide an alternative to tubules for highly-curved membranes

The endoplasmic reticulum (ER) is composed of interconnected membrane sheets and tubules. Super-resolution microscopy recently revealed densely packed, rapidly moving ER tubules, highlighting the importance of revisiting classical views of ER structure with high spatial resolution in living cells. Using live-cell Stimulated Emission Depletion (STED) microscopy, we show highly dynamic, subdiffraction-sized holes in ER sheets. Holes coexist with uniform sheet regions and are distinct from tubular ER structures. The curvature-stabilizing reticulon protein Rtn4 localizes to these holes and the ER luminal tether Climp63 controls their diameter and mobility. Analytical modeling demonstrates that holes in ER sheets can serve as reservoirs for curvature-stabilizing proteins to support ER tubule extension and retraction, thus providing an explanation for how the ER locally alters its morphology on fast time-scales.\n\nOne Sentence SummaryDynamic nano-scale sized holes are prominent features of ER sheets that serve as reservoirs for curvature-stabilizing proteins to support ER tubule extension and retraction.

cell biology↗

bioRxiv2017

Dynein Pulling Forces On Ruptured Nuclei Counteract Lamin-Mediated Nuclear Envelope Repair Mechanisms In Vivo

Recent work done exclusively in tissue culture cells revealed that the nuclear envelope (NE) undergoes ruptures leading to transient mixing of nuclear and cytoplasmic components. The duration of transient NE ruptures depends on lamins, however the underlying mechanisms and the relevance to in vivo events is not known. Here, we use C. elegans embryos to show that dynein forces that position nuclei increase the severity of lamin-induced NE ruptures in vivo. In the absence of dynein forces, lamin prevents nuclear-cytoplasmic mixing caused by NE ruptures. By monitoring the dynamics of NE rupture events, we demonstrate that lamin is required for a distinct step in NE recovery that restricts nucleocytoplasmic mixing prior to the full restoration of NE rupture sites. We show that laser-induced puncture of the NE recapitulates phenotypes associated with NE recovery in wild type cells. Surprisingly, we find that embryonic lethality does not correlate with the incidence of NE rupture events suggesting that embryos survive transient losses of NE compartmentalization during early embryogenesis. In addition to presenting the first mechanistic analysis of transient NE ruptures in vivo, this work demonstrates that lamin controls the duration of NE ruptures by opposing dynein forces on ruptured nuclei to allow reestablishment of the NE permeability barrier and subsequent restoration of NE rupture sites.

developmental biology↗