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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 hypothesized that the MT minus-end motor dynein mediates ER concentration around spindle poles. Live in vivo imaging of Drosophila spermatocytes revealed that dynein is required for ER concentration around centrosomes during late 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. In fact, there was a sudden onset of ER association with astral MTs in dynein RNAi cells, revealing activation of an M-phase specific mechanism of ER-MT association. 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.
Highlights
The endoplasmic reticulum (ER) cannot be formed by cells de novo and must be inherited during the process of cell division[1]
We present the surprising finding that dynein is required for peri-centrosomal focusing of the ER during late interphase, it does not mediate the astral MT-dependent spindle pole focusing of the ER around centrosomes during M-phase
Double labeling of ER and MTs by co-expression of ER-targeted RFP (RFP-KDEL) and GFP-tubulin, respectively, revealed that the ER was widely distributed throughout the cytoplasm during late interphase, and exhibited a distinct concentration around each cortical centrosome, with segments of the ER closely aligned along the centrosomal MTs (Fig. 1a)
Summary
The endoplasmic reticulum (ER) cannot be formed by cells de novo and must be inherited during the process of cell division[1]. Recent data suggest that proper partitioning of the ER during cell division, or M-phase, depends on specific association of the organelle with astral microtubules (MTs) of the mitotic spindle in both symmetrically and asymmetrically dividing cells[4]. There appears to be a shift from the balanced MT plus- and minus-end directed ER distribution during interphase to predominantly minus-end directed localization around spindle poles during M-phase Consistent with this conclusion, tracking of the ER with growing MT plus-ends is inhibited during cell division due to mitosis-specific phosphorylation of STIM112,13. The primary goal of this study was to test the hypothesis that dynein is required for astral MT association and spindle pole focusing of the ER in dividing cells We accomplished this by live timelapse imaging of Drosophila spermatocytes undergoing the first meiotic division of spermatogenesis in vivo. This report lays the groundwork for identification of this novel mechanism of ER-MT association that is essential for the process of ER inheritance
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