Asymmetric nuclear division of neural stem cells contributes to the formation of sibling nuclei with different identities

Background

Asymmetric divisions are a central feature in cellular differentiation, enabling the generation of a diversity of cell types from an initially homogenous cell population (Knoblich, 2010). This plays a key role during early embryonic development, whereby a specific signal (for example, the sperm entry site in C. elegans embryos) can initiate polarity establishment, leading to the unequal distribution of polarized components between daughter cells in subsequent cell divisions (Gönczy, 2008). Indeed, this symmetry breaking event has been shown to differentially trigger cell fate specification pathways in various organisms (Ajduk and Zernicka-Goetz, 2016).

In the stem cell niche, asymmetric divisions also generate daughter cells with distinct identities – differentiated cells and self-renewing stem cells (Knoblich, 2010). This is exemplified in Drosophila neuroblasts, whereby asymmetric division produces a larger neuroblast on the apical side and a smaller basal ganglion mother cell (GMC). While this classical example of asymmetric cell division has been intensely studied, most have focused on the asymmetric inheritance of polarized transcription factors that activate distinct transcriptional programs directing cell fate. In this new preprint, Roubinet et al. reveal an additional function of nuclear asymmetries, and dissect the interplay between cell and nuclear divisions during the asymmetric divisions driving fate specification in Drosophila neuroblasts.

Key findings

To characterize nuclear division dynamics during the asymmetric cell divisions of Drosophila neuroblasts, the authors performed live cell imaging of nuclear membrane and spindle markers during mitosis (Fig. 1). Unlike divisions occurring in mammalian cells where the nuclear envelope is disassembled and subsequently reformed at the end of mitosis (“open mitosis”), or mitotic events in other eukaryotes where an intact nuclear envelope is retained throughout division (“closed mitosis”), Drosophila neuroblasts follow a pattern of “semi-closed mitosis” – the nuclear compartment is retained throughout mitosis, but becomes permeable to cytoplasmic proteins with multiple perforations throughout the nuclear membranes. By the end of mitosis, the resultant neuroblasts and GMCs exhibit not only differences in cell volumes, but also distinct asymmetries in nuclear size and composition.

Figure 1. Nuclear membrane markers (green) and spindle marker (red) within two neuroblasts undergoing mitosis. Adapted from Figure 1B from Roubinet et al. 2020.

The retention of a nuclear envelope during “semi-closed” mitosis in this system necessitates a dynamic remodelling of the nuclear membrane during cell division to form two separate daughter nuclei of different sizes. Indeed, live cell imaging revealed formation of membrane extensions in the mitotic nuclear envelope, which ultimately enclosed individual daughter nuclei. Importantly, the authors find that the mitotic spindle defines the location of nuclear envelope resealing: the longer distance between the apical edge of the mitotic spindle and apical centrosome generates larger daughter nuclei in the apical neuroblasts, whereas the shorter distance between the basal mitotic spindle and basal centrosome produces smaller nuclei in basal GMCs. Strikingly, manipulations of spindle density and length to alter spindle asymmetries were able to induce corresponding changes in nuclear asymmetries. These results lend strong support to the role of mitotic spindle asymmetries in generating asymmetric nuclear envelope resealing and ultimately different daughter nuclei sizes.

Following nuclear envelope resealing, size asymmetry between daughter nuclei further increases. While nuclear size of GMCs rapidly stabilizes, neuroblast nuclei continue to grow before reaching their final nuclear size. The authors found that this nuclear growth differential could be attributed to the availability of ER membrane needed for nuclear envelope expansion to accommodate the nuclear size increase. Thus, a combination of asymmetric nuclear envelope resealing and differential nuclear growth regulates the nuclear size asymmetries between neuroblasts and GMCs.

What are the implications of asymmetric nuclear divisions? Firstly, differences in nuclear size can impose different physical constraints on packing and organization of chromatin, and the accumulation of epigenetic marks associated with stemness or neural differentiation. Secondly, the timing of asymmetric nuclear divisions appears to play a critical role – nuclear divisions are completed prior to the release of cortical proteins/fate determinants and their nuclear import, as well as the completion of cytokinesis. Crucially, cytokinetic failure led to cortical proteins being released into a common cytoplasmic pool, resulting in a loss of differential nuclear accumulation of fate determinants in daughter nuclei that is required to specify distinct daughter cell identities. Therefore, three factors are required in combination for the asymmetric division and generation of neuroblasts and GMCs: 1) asymmetric nuclear division, 2) cortical release of fate determinants, and 3) completion of cytokinesis.

What I like about this preprint

The authors use beautiful live cell imaging to uncover a novel role for asymmetric nuclear divisions in the specification of distinct cellular fates, during the asymmetric divisions producing neuroblasts and GMCs. Although important differences between the “semi-closed” form of mitosis in Drosophila neuroblasts and the “open” and “closed” models in other systems remain, this study presents a new perspective for understanding asymmetric divisions in general, by highlighting how nuclear size could impact chromatin organization and epigenetic differences driving differential fate specification in daughter cells. Indeed, others have identified asymmetries in histone inheritance during asymmetric cell divisions in Drosophila germline stem cells (Tran et al., 2012), but whether it is linked to nuclear size and/or composition remains to be investigated.

Questions for authors

1. The authors describe differences in both nuclear size and composition between neuroblasts and GMCs – for example, nuclear membranes of neuroblasts display greater accumulation of nuclear pore complexes as compared to those in GMCs. While the nuclear size differences can be attributed to spindle asymmetries and availability of ER membrane, how do the asymmetries in nuclear membrane composition arise?
2. Does altering nuclear asymmetry (via the spindle manipulations used in this study, or otherwise) induce differences in chromatin organization or acquisition of epigenetic markers?

References

1. Ajduk, A., and Zernicka-Goetz, M. (2016). Polarity and cell division orientation in the cleavage embryo: from worm to human. Mol. Hum. Reprod. 22, 691–703.
2. Gönczy, P. (2008). Mechanisms of asymmetric cell division: flies and worms pave the way. Nature Reviews Molecular Cell Biology 9, 355–366.
3. Knoblich, J.A. (2010). Asymmetric cell division: recent developments and their implications for tumour biology. Nature Reviews Molecular Cell Biology 11, 849–860.
4. Tran, V., Lim, C., Xie, J., and Chen, X. (2012). Asymmetric Division of Drosophila Male Germline Stem Cell Shows Asymmetric Histone Distribution. Science 338, 679–682.

 

doi: https://doi.org/10.1242/prelights.24394

Read preprint (2 votes)