Mechanisms of spindle positioning: cortical force generators in the limelight

Abstract

Correct positioning of the spindle governs placement of the cytokinesis furrow and thus plays a crucial role in the partitioning of fate determinants and the disposition of daughter cells in a tissue. Converging evidence indicates that spindle positioning is often dictated by interactions between the plus-end of astral microtubules that emanate from the spindle poles and an evolutionary conserved cortical machinery that serves to pull on them. At the heart of this machinery lies a ternary complex (LIN-5/GPR-1/2/Ga in Caenorhabditis elegans and NuMA/LGN/Gai in Homo sapiens) that promotes the presence of the motor protein dynein at the cell cortex. In this review, we discuss how the above components contribute to spindle positioning and how the underlying mechanisms are precisely regulated to ensure the proper execution of this crucial process in metazoan organisms IntroductionThe mitotic spindle is a diamond-shaped microtubulebased 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 plusend 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 t...

Analysis and modeling of mitotic spindle orientations in three dimensions

“…Mitotic spindles are oriented by pulling forces acting on astral microtubules that connect the spindle poles to the cell cortex and attach to specific cortical sites (1,2,17). The genes PP4c and mInsc have been assigned distinct roles in this process.…”

“…The position of the mitotic spindle is regulated by pulling forces acting between the spindle poles and cortical microtubule attachment sites (1,2). The spindle position determines the cleavage plane and thereby influences the size and position of the newly forming daughter cells (3,4).…”

Small organelle, big responsibility: the role of centrosomes in development and disease

“…Only if the plane is vertical to the apical surface, do both daughter cells inherit a region of the apical membrane, important for maintenance of stem cell fate [68,69]. Mitotic spindle orientation is governed by centrosomenucleated astral microtubules that are captured by cortically tethered microtubule motor complexes [70]. For centrosomes to orient the mitotic spindle, they must nucleate astral microtubules and maintain stable association with the mitotic spindle poles, processes impaired in CDK5RAP2-and WDR62-deficient cells [24,37,71,72].…”

Mechanism of Nuclear Movements in a Multinucleated Cell

“…Lateral or side-on pulling occurs when cortex-anchored dynein walks on cMTs toward the minus end using its motor activity. This mode of action is independent of cMT shrinkage and may result in cMT sliding parallel to the cortex (Kotak and Gönczy, 2013;McNally, 2013;Akhmanova and van den Heuvel, 2016).A detailed mechanistic view for directing nuclei during the cell cycle is known in the budding yeast Saccharomyces cerevisiae, as outlined in Figure 1 and references therein. In contrast to most eukaryotic cells, the site of the cleavage plane in S. cerevisiae is selected early in the cell cycle by generating a ring structure (future bud neck) at the mother cell cortex at which the daughter cell (bud) will emerge.…”