Dynamic constriction and fission of endoplasmic reticulum membranes by reticulon

Abstract

The endoplasmic reticulum (ER) is a continuous cell-wide membrane network. Network formation has been associated with proteins producing membrane curvature and fusion, such as reticulons and atlastin. Regulated network fragmentation, occurring in different physiological contexts, is less understood. Here we find that the ER has an embedded fragmentation mechanism based upon the ability of reticulon to produce fission of elongating network branches. In Drosophila, Rtnl1-facilitated fission is counterbalanced by atlastin-driven fusion, with the prevalence of Rtnl1 leading to ER fragmentation. Ectopic expression of Drosophila reticulon in COS-7 cells reveals individual fission events in dynamic ER tubules. Consistently, in vitro analyses show that reticulon produces velocity-dependent constriction of lipid nanotubes leading to stochastic fission via a hemifission mechanism. Fission occurs at elongation rates and pulling force ranges intrinsic to the ER, thus suggesting a principle whereby the dynamic balance between fusion and fission controlling organelle morphology depends on membrane motility.

In vivo Analysis of CRISPR/Cas9 Induced Atlastin Pathological Mutations in Drosophila

“…Thus, the severity of the CRISPR homozygous mutants suggests a graded loss of function effect, confirming the results obtained from the previous experiments. We note that average ER profile length in controls (w 1118 stock), while consistent with previous reports ( Orso et al, 2009 ; Espadas et al, 2019 ; Vajente et al, 2019 ), is shorter than in some mutants as if mutants had a greater baseline profile length ( Figures 4B,C ). However, because the graded effect of the mutations is solid and all lines are grown under the same environmental conditions, we hypothesize that this could be due to the potentially very different genetic background between w 1118 controls and mutants ( Chandler et al, 2013 ).…”

“…For this reason, we evaluated the length of ER profiles on thin EM sections of homozygous third instar larva mutant brains. ER profile length has been considered a parameter capable of reporting accurately on the functional state of atlastin since null atl 2 mutant neurons are characterized by a much shorter average ER profile length than w 1118 control neurons, a phenotype linked to increased ER fragmentation due to the prevalence of ER membrane fission in the absence of the fusion activity mediated by atlastin (Espadas et al, 2019). Our analysis reveals that the stronger the loss of function mutation, the shorter the neuronal ER profiles (Figures 4B,C).…”

Axonal Endoplasmic Reticulum Dynamics and Its Roles in Neurodegeneration

“…VAPA/B knockdown impairs tubular ER movements in axons (Lindhout et al, 2019), potentially by affecting ER MCSs, which may destabilize the ER network. Also, RTN overexpression reduces retraction speed and increases fission frequency of ER tubules (Espadas et al, 2019); since RTN is distributed along the tubular ER network, its role in ER dynamics might simply result from its effect on the physical properties of the ER tubules.…”

The entanglement between flaviviruses and ER-shaping proteins

“…It will be worth investigating if ER-shaping proteins are involved in ZIKV-induced cytoskeleton modifications [12] and the formation of vacuoles to understand if these proteins are related to ER stress, innate sensing of infection, and viral replication [30] [8] [29]. The interplay between different ER-shaping proteins [31] within the context of viral infection is also of potential interest. Finally, since mutations in several of the ER-shaping proteins are associated with inherited neurological complications, it would be interesting to determine if they are implicated in the pathology induced by neurotropic flaviviruses in relevant cell and animal models [32].…”