A CTDNEP1-lipin 1-mTOR regulatory network restricts ER membrane biogenesis to enable chromosome motions necessary for mitotic fidelity

Abstract

The endoplasmic reticulum (ER) dramatically restructures in open mitosis to become excluded from the mitotic spindle; however, the significance of ER reorganization to mitotic progression is not known. Here, we demonstrate that limiting ER membrane biogenesis enables mitotic chromosome movements necessary for chromosome biorientation and prevention of micronuclei formation. Aberrantly expanded ER membranes increase the effective viscosity of the mitotic cytoplasm to physically restrict chromosome dynamics - slowed chromosome motions impede correction of mitotic errors induced by transient spindle disassembly, leading to severe micronucleation. We define the mechanistic link between regulation of ER membrane biogenesis and mitotic fidelity by demonstrating that a CTDNEP1 lipin 1 mTOR regulatory network limits ER lipid synthesis to prevent chromosome missegregation. Together, this work shows that ER membranes reorganize in mitosis to enable chromosome movements necessary for mitotic error correction and reveal dysregulated lipid metabolism as a potential source of aneuploidy in cancer cells.

Endomembranes promote chromosome missegregation by ensheathing misaligned chromosomes

“…Our 3D EM images of ensheathed chromosomes show that microtubules face a difficult task to negotiate several layers of endomembranes in order to make the contact between kinetochore and spindle that is necessary for rescue and alignment. In cases where contact is made, endomembranes are also likely to impair the congression of the chromosome, as suggested by a recent study where excess ER was shown to slow chromosome motions (Merta et al, 2021). Since endomembranes are a risk factor for missegregation, their precise organization – for example the sheet-to-tubule ratio of the ER – may influence the likelihood for missegregation (Champion et al, 2017).…”

“…Our conclusion is that the endomembranes do not necessarily prevent contact with ensheathed chromosomes but they appear to make contact less likely or less productive. Even in cases where contact is made, endomembranes are likely to impair the congression of the chromosome, as suggested by a recent study where excess ER was shown to slow chromosome motions (Merta et al, 2021). Since endomembranes are a risk factor for missegregation, their precise organization -for example the sheet-to-tubule ratio of the ER -may influence the likelihood for missegregation (Champion et al, 2017).…”

“…This arrangement means that, although mitosis is open in mammalian cells, the spindle operates within a partially closed system. Several lines of evidence suggest that endomembranes must be cleared from the exclusion zone in order for the mitotic spindle to function normally (Vedrenne et al, 2005; Schlaitz et al, 2013; Champion et al, 2019; Kumar et al, 2019; Merta et al, 2021). In addition, it is thought that this arrangement is required to concentrate factors needed for spindle formation (Schweizer et al, 2015).…”

Ice2 promotes ER membrane biogenesis in yeast by inhibiting the conserved lipin phosphatase complex

“…Like in yeast, removal of lipins in protozoa, plants, worms, and flies causes ER expansion (Golden et al , 2009; Eastmond et al , 2010; Bahmanyar et al , 2014; Grillet et al , 2016; Pillai et al , 2017). Similarly, removal of the Nem1 ortholog CTDNEP1 causes ER expansion in human tissue culture cells (preprint: Merta et al , 2021). However, while both yeast and metazoa use the CDP‐DAG pathway to synthesize phosphatidylinositol, metazoa mainly generate phosphatidylcholine and phosphatidylethanolamine through the Kennedy pathway (Vance, 2015).…”

Differential reliance of CTD-nuclear envelope phosphatase 1 on its regulatory subunit in ER lipid synthesis and storage

“…Here, we focused on the protein phosphatase CTD-nuclear envelope phosphatase 1 (CTDNEP1, formerly known as Dullard; Nem1 in S. cerevisiae, CNEP-1 in C. elegans) because of its conserved role in controlling the production of ER and nuclear membranes (Siniossoglou et al, 1998;Santos-Rosa et al, 2005;O'Hara et al, 2006;Kim et al, 2007;Han et al, 2012;Bahmanyar et al, 2014;Jacquemyn et al, 2021;Merta et al, 2021). CTDNEP1 is enriched at the nuclear envelope and maintains a stable, dephosphorylated pool of the phosphatidic acid phosphatase lipin 1, the main enzyme that produces diacylglycerol (DAG) in the ER (Siniossoglou et al, 1998;Han et al, 2012;Merta et al, 2021;Lee et al, 2023). CTDNEP1 opposes stimulatory signals from nutrient sensing pathways that retain phosphorylated lipin 1 in the cytoplasm (Merta et al, 2021).…”