Abscission Is Regulated by the ESCRT-III Protein Shrub in Drosophila Germline Stem Cells

Abstract

Abscission is the final event of cytokinesis that leads to the physical separation of the two daughter cells. Recent technical advances have allowed a better understanding of the cellular and molecular events leading to abscission in isolated yeast or mammalian cells. However, how abscission is regulated in different cell types or in a developing organism remains poorly understood. Here, we characterized the function of the ESCRT-III protein Shrub during cytokinesis in germ cells undergoing a series of complete and incomplete divisions. We found that Shrub is required for complete abscission, and that levels of Shrub are critical for proper timing of abscission. Loss or gain of Shrub delays abscission in germline stem cells (GSCs), and leads to the formation of stem-cysts, where daughter cells share the same cytoplasm as the mother stem cell and cannot differentiate. In addition, our results indicate a negative regulation of Shrub by the Aurora B kinase during GSC abscission. Finally, we found that Lethal giant discs (lgd), known to be required for Shrub function in the endosomal pathway, also regulates the duration of abscission in GSCs.

Regulation of Ribosome Biogenesis and Protein Synthesis Controls Germline Stem Cell Differentiation

“…Using the frequency of 32-cell egg chambers as a readout for delay in GSC abscission (Figure 4B-C), we performed genetic interaction experiments combining mutations in ribosome assembly factors or eIF4E with those in Shrub. Some egg chambers containing 32 cells were always observed in heterozygous shrb alleles as previously described (Figure 4D; Matias et al, 2015), while 32-cell egg chambers were rarely observed in heterozygous mutants for eIF4E or ribosome assembly factors alone. In contrast, simultaneously lowering the levels of eIF4E and Shrub by combining mutant alleles in double heterozygous flies led to a significant increase in the frequency of 32-cell egg chamber when compared to their single heterozygous siblings.…”

“…Previous work has shown that the speed and outcome of GSC abscission, the last step of cytokinesis leading to physical separation of the two daughter cells, involves the activity of the Endosomal Sorting Complex Required for Transport-III (ESCRT-III) and is regulated by the Chromosomal Passenger Complex (CPC) and Cyclin B (Eikenes et al, 2015; Mathieu et al, 2013; Matias et al, 2015). For instance, increased activity of the CPC subunit Aurora B kinase as well as decreased levels of the ESCRT-III members such as Shrub (CHMP4) or ALIX delays abscission in GSCs, leading to the appearance of ‘stem cysts’ and the formation of abnormal egg chambers containing 32 cells.…”

“…How the activity of the translational apparatus affects the fate of the cytokinetic furrow in germ cells remains to be determined. Given the observed functional interaction with the CPC/ESCRT-III pathway (Eikenes et al, 2015; Mathieu et al, 2013; Matias et al, 2015), one exciting possibility is that abscission factors are exquisitely sensitive to the status of the translation machinery. Compelling evidence for targeted regulation was recently provided by the characterization of eIF4E haploinsufficient mice, which revealed that eIF4E levels are critical for the translation of a subset of transcripts during oncogenic transformation (Truitt et al, 2015).…”

“…In the light of our findings, we propose that the reported phenotype can be alternatively explained by the effect of ribosome assembly and translational machineries on abscission, independent of germ cell differentiation. Interestingly, the uncoupling of germline differentiation and incomplete cytokinesis also infers that mechanisms must exist to ensure proper abscission between GSC daughters in wild-type conditions (Eikenes et al, 2015; Mathieu et al, 2013; Matias et al, 2015). While it is known that the GSC stays attached to future CB until the G2 phase of the following cell cycle (de Cuevas and Spradling, 1998), what regulates this uncommon abscission process remains elusive.…”

Somatic Cell Encystment Promotes Abscission in Germline Stem Cells following a Regulated Block in Cytokinesis

“…Recent studies have found that shrub is negatively regulated by AurB in female GSCs (Matias et al, 2015). Though our results suggest that AurB activity should promote ESCRTIII function in the testis, it is compelling to speculate that AurB might control the Phase One-Phase Two transition through shrub.…”