Closed mitosis is a type of cell division where, unlike in open mitosis, the nuclear envelope (NE) does not break down, but instead expands and chromosomes are segregated inside the nucleus. This NE expansion requires increased production of membrane glycerophospholipids (GPLs), most notably from phosphatidic acid (PA) and diacylglycerol (DG). However, how GPLs are synthesised to meet this demand and how they are spatiotemporally regulated is not known. In this study (Foo et al., 2023), Snezhana Oliferenko and colleagues use fluorescent lipid sensors to track changes in PA and DG levels and localisation during closed mitosis in fission yeast. Here, they find that DG is depleted from the inner nuclear membrane (INM), and this is attributed to conversion of DG into PA, catalysed by the diacylglycerol kinase Dgk1, to produce GPLs de novo. Interestingly, some dgk1 knockout cells are still able to divide normally and show a decrease in DG levels at the INM analogous to that in wild-type cells. The authors show that this is achieved by production of GPLs by an alternative mechanism, the Kennedy pathway, which synthesises GPLs directly from DG, catalysed by the enzyme Ept1. Together, these findings show that production of GPLs, through both de novo biosynthesis and the Kennedy pathway, are important in controlling NE expansion and thus mitotic fidelity during closed mitosis.
