Mechanisms of spindle positioning: cortical force generators in the limelight
Introduction
The mitotic spindle is a diamond-shaped microtubule-based structure that faithfully segregates sister chromatids during cell division. Several types of microtubules emanate from the spindle poles, including astral microtubules that reach out to the actin rich cortex located beneath the plasma membrane (Figure 1a). Pulling forces exerted on the plus-end of astral microtubules at the cell cortex are critical for accurately positioning the spindle with respect to cell-intrinsic or cell-extrinsic spatial cues. In turn, correct spindle positioning dictates placement of the cytokinesis furrow and is thus essential for determining the relative size and spatial disposition of the resulting daughter cells [2]. Accurate spindle positioning also ensures that cell fate determinants are appropriately segregated into daughter cells during development and in stem cell lineages [3].
What features of the cell cortex allow interactions with astral microtubules to orchestrate spindle positioning? Specialized cortical sites are key. Their importance has been suggested for instance by elegant experiments in Chaetopterus oocytes, in which the meiotic spindle pulled away from its cortical attachment site using a micro-needle returns to that location once released (Figure 1b). Such experiments illustrate the existence of a mechanical link between the spindle and specialized cortical regions [4, 5]. Laser microsurgery experiments in Fusarium solani or C. elegans suggested that this link corresponds to astral microtubules connecting the spindle poles with the cell cortex [6, 7, 8, 9, 10••]. Although not the focus of this review, there are instances where pulling forces are exerted along the length of astral microtubules instead of at their plus-end located at the cell cortex [11, 12, 13]. Work in several systems in recent years has increased our understanding of the basic principles governing spindle positioning and identified core molecular players and aspects of their mechanism of action. In this brief review, we will focus on cortically driven spindle positioning in one-cell C. elegans embryos and mammalian cells in culture. In doing so, we will discuss the nature of the ternary complex that anchors the motor protein dynein at the cell cortex and how dynein serves to generate pulling forces on astral microtubules. We will then review some of the mechanisms that regulate such cortical force generators and mention briefly the contribution of the actin cytoskeleton and of phosphatidylinositol lipids in spindle positioning. We will conclude by covering some of the exciting challenges that await the field.
Section snippets
The ternary complex: common players of an intricate game
Proteins governing spindle positioning in metazoan organisms have been identified notably through studies in the one-cell C. elegans embryo and in mammalian cells. In one-cell C. elegans embryos, the spindle assembles in the cell center, but is displaced under the influence of intrinsic anterior–posterior (A–P) polarity cues toward the posterior during metaphase and anaphase, resulting in unequal division (Figure 2a) [14]. Genetic and RNAi-based functional genomic screens have led to the
Cortical dynein: a force-generating motor
Dynein is a multisubunit motor protein complex critical for many basic cellular processes [30, 31]. In C. elegans embryos and human cells, co-immunoprecipitation experiments showed that LIN-5/NuMA associates with dynein [25, 32•, 33, 34]. The region mediating this association has been mapped to an N-terminal region of NuMA [25], but the molecular nature of the partner protein on the side of the dynein complex is not known. The presence of dynein at the cell cortex is compromised upon depletion
Balanced levels of cortical dynein are critical for proper spindle positioning
Alterations in ternary complex components have profound effects on cortical dynein levels and thus spindle positioning [25, 35]. As mentioned earlier, GDP-bound Gα is the relevant Gα species in the context of spindle positioning [42]. In line with this, work in C. elegans indicates that the Gα guanine nucleotide exchange factor (GEF) RIC-8 is important for generating pulling forces [43, 44]. Moreover, the Gα guanine nucleotide activating protein (GAP) RGS-7 also contributes to some extent,
Beyond the ternary complex
The role of the actin cytoskeleton and of associated motor proteins in spindle positioning has been extensively investigated. Here, we discuss only briefly the contribution of the actomyosin network in this process and refer readers to other recent reviews that offer a more extensive coverage of this aspect [55, 56]. In human cells, the importance of actin is illustrated for instance by the fact that actin depolymerizing drugs or RNAi-mediated depletion of the actin-associated protein Moesin
Future perspectives
Studies in a number of systems, including C. elegans embryos and human cells, have provided an initial understanding of the mechanisms governing spindle positioning. Exciting challenges lie ahead and novel insights are expected from several directions. Biophysical and structural analysis of proteins central to the force generating machinery is anticipated to be important. An illustration of such work is given by the recent atomic level characterization of the interaction between parts of LGN
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Note added in proof
Two recent publications shed further light on the mechanisms of spindle positioning in human cells. One study revealed the role of cortical dynein and asymmetric membrane elongation in positioning the anaphase spindle [73], whereas the other one focused on the interplay between CDK1 kinase and PPP2CA phosphatase in dictating levels of cortical dynein during mitosis [74].
Acknowledgements
We thank Marie Delattre, Virginie Hachet and Zoltan Spiro for useful comments on the manuscript. Work on spindle positioning in the laboratory of PG is supported by the Swiss National Science Foundation (3100A0-122500/1).
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