Pre-assembled Nuclear Pores Insert into the Nuclear Envelope during Early Development

Abstract

SummaryNuclear pore complexes (NPCs) span the nuclear envelope (NE) and mediate nucleocytoplasmic transport. In metazoan oocytes and early embryos, NPCs reside not only within the NE, but also at some endoplasmic reticulum (ER) membrane sheets, termed annulate lamellae (AL). Although a role for AL as NPC storage pools has been discussed, it remains controversial whether and how they contribute to the NPC density at the NE. Here, we show that AL insert into the NE as the ER feeds rapid nuclear expansion in Drosophila blastoderm embryos. We demonstrate that NPCs within AL resemble pore scaffolds that mature only upon insertion into the NE. We delineate a topological model in which NE openings are critical for AL uptake that nevertheless occurs without compromising the permeability barrier of the NE. We finally show that this unanticipated mode of pore insertion is developmentally regulated and operates prior to gastrulation.

Perforating the nuclear boundary – how nuclear pore complexes assemble

“…Importantly, despite a significant rearrangement of the nuclear envelope during annulate lamellae insertion, the permeability barrier between the cytoplasm and the nucleoplasm remains intact, as fluorescently labeled 155 kDa dextran injected into the cytoplasm is excluded from the nucleus (Hampoelz et al, 2016). It is possible that gaps in the expanding nuclear envelope are rapidly sealed, for example, by the ESCRT III complex as described above.…”

“…As annulate lamellae are tightly packed with ALPCs, they have been speculated to serve as storage compartments for NPC components (Spindler and Hemleben, 1982;Cordes et al, 1995); ALPCs might disassemble and their nucleoporins be used for NPC assembly. Interestingly, a recent study on annulate lamellae in Drosophila blastoderm embryos suggests an alternative scenario (Hampoelz et al, 2016). In these embryos, cell cycle progression is very rapid and limits the length of interphase to ∼10 min.…”

“…It is currently unclear how the opening in the nuclear envelope, which will be replaced by the annulate lamellae structure, originates. Any remaining gaps in the nuclear envelope, not sealed in telophase, might be starting points for this process (Hampoelz et al, 2016). Alternatively, mechanical nuclear envelope rupture or disassembly of a NPC might account for this.…”

“…Recent work proposes that in Drosophila blastoderm embryos, cytoplasmic double-membrane structures that are already filled with NPCs, so-called annulate lamellae, insert into the nuclear envelope to keep NPC density constant when these nuclei expand (Hampoelz et al, 2016). Annulate lamellae are cytoplasmic membrane cisternae that are continuous with the ER.…”

“…This raises the question of how the asymmetry of NPCs in the nuclear envelope, required for its transport function, is guaranteed if these complexes are already preassembled in cytoplasmic annulate lamellae? ALPCs, at least in Drosophila blastoderm embryos, are composed of only the symmetric part of NPCs (Hampoelz et al, 2016): the outer-and inner-ring comprising structural nucleoporins of the Y-complex and the Nup93 complex, as well as the barrierforming FG-nucleoporin Nup98. Two exceptions are the cytoplasmic nucleoporin Nup358 and MEL28, which are also found in ALPCs.…”