A novel, dynein-independent mechanism focuses the endoplasmic reticulum around spindle poles in dividingDrosophilaspermatocytes

Abstract

In dividing animal cells the endoplasmic reticulum (ER) concentrates around the poles of the spindle apparatus by associating with astral microtubules (MTs), and this association is essential for proper ER partitioning to progeny cells. The mechanisms that associate the ER with astral MTs are unknown. Because astral MT minus-ends are anchored by centrosomes at spindle poles, we tested the hypothesis that the MT minus-end motor dynein mediates ER concentration around spindle poles. Live in vivo imaging of Drosophila spermatocytes undergoing the first meiotic division revealed that dynein is required for ER concentration around centrosomes during interphase. In marked contrast, however, dynein suppression had no effect on ER association with astral MTs and concentration around spindle poles in early M-phase. Importantly though, there was a sudden onset of ER-astral MT association in Dhc64C RNAi cells, revealing activation of an M-phase specific mechanism. ER redistribution to spindle poles also did not require non-claret disjunctional (ncd), the other known Drosophila MT minus-end motor, nor Klp61F, a MT plus-end motor that generates spindle poleward forces. Collectively, our results suggest that a novel, M-phase specific mechanism of ER-MT association that is independent of MT minus-end motors is required for proper ER partitioning in dividing cells.

The Caenorhabditis elegans centrosome is surrounded by a membrane reticulum, the centriculum, that affects centrosome size and function

“…The existence of ER-derived membranes around centrosomes has been known for decades through studies using light microscopy (Bergman et al, 2015; Diaz et al, 2019; Harris, 1975; Karabasheva and Smyth, 2019; Poteryaev et al, 2005; Waterman-Storer et al, 1993), but their configuration and functional importance were unknown. Using vEM methodologies, we show that centrosome-associated membranes in C. elegans form a reticulum, leading us to name it the centriculum.…”

“…What might be the function(s) of the centriculum? It was previously suggested that the accumulation of ER around centrosomes in Drosophila serves to ensure that daughter cells receive adequate amounts of ER (Karabasheva and Smyth, 2019). We think that this is unlikely to be the case in C. elegans given that during mitosis, the ER is distributed throughout the cell and the fraction of the ER around centrosomes is relatively small (Fig.…”

“…1A; ER is detected by the signal peptidase, SP12, fused to mCherry). Centrosome-associated membranes were also observed in other systems, such as Drosophila, sea urchin, PtK2 cells and LLC-PK2 cells (Bergman et al, 2015;Diaz et al, 2019;Harris, 1975;Karabasheva and Smyth, 2019;Waterman-Storer et al, 1993). The configuration of these membrane and their function are unknown.…”