Eric Van Leen

Victor

Pietro Florencia

Bellaïche Yohanns
email: yohanns.bellaiche@curie.fr

Oriented cell divisions in epithelia: from force generation to force anisotropy by tension, shape and vertices

Mitotic spindle orientation has been linked to asymmetric cell divisions, tissue morphogenesis and homeostasis. The canonical pathway to orient the mitotic spindle is composed of the cortical recruitment factor NuMA and the molecular motor dynein, which exerts pulling forces on astral microtubules to orient the spindle. Recent work has defined a novel role for NuMA as a direct contributor to force generation. In addition, the exploration of geometrical and physical cues combined with the study of classical polarity pathways has led to deeper insights into the upstream regulation of spindle orientation. Here, we focus on how cell shape, junctions and mechanical tension act to orient spindle pulling forces in epithelia, and discuss different roles for spindle orientation in epithelia.

Introduction

During animal development, complex epithelial structures are formed by regulating the spatiotemporal dynamics of cell rearrangements, cell shape changes, apoptosis and cell division [START_REF] Lecuit | Orchestrating size and shape during morphogenesis[END_REF][START_REF] Guirao | Unified quantitative characterization of epithelial tissue development[END_REF][START_REF] Pietro | Regulation of mitotic spindle orientation: an integrated view[END_REF][START_REF] Heisenberg | Forces in Tissue Morphogenesis and Patterning[END_REF]. Intensive work in the last 20 years highlighted that the orientation of cell division can play essential roles in asymmetric cell fate divisions and tissue architecture, and allowed the establishment of a paradigm for spindle orientation within the cell [START_REF] Pietro | Regulation of mitotic spindle orientation: an integrated view[END_REF][START_REF] Xiong | Interplay of Cell Shape and Division Orientation Promotes Robust Morphogenesis of Developing Epithelia[END_REF][START_REF] Morin | Mitotic Spindle Orientation in Asymmetric and Symmetric Cell Divisions during Animal Development[END_REF]. The axis of division is set up by cortical pulling forces acting on astral microtubules (MT) [START_REF] Bosveld | Epithelial tricellular junctions act as interphase cell shape sensors to orient mitosis[END_REF][START_REF] Grill | Polarity controls forces governing asymmetric spindle positioning in the Caenorhabditis elegans embryo[END_REF]. Therefore, the orientation of division can be ensured by locating pulling force generators at specific cortical locations within the cell. While force generation has been attributed to the MT minusend directed motor dynein, the recruitment task was assigned to NuMA (Mud in Drosophila, and Lin-5 in C. elegans), which in turn can be recruited to the cortex by LGN (Pins in Drosophila, and GPR-1/2 in C. elegans) and the heterotrimeric G protein Gαi [START_REF] Pietro | Regulation of mitotic spindle orientation: an integrated view[END_REF]. Recently, the relative roles of NuMA and dynein in force generation for spindle orientation have been further deciphered.

In symmetric cell divisions in epithelia, mitotic spindles are stereotypically oriented with respect to polarity axes within the tissue [START_REF] Gong | Planar cell polarity signalling controls cell division orientation during zebrafish gastrulation[END_REF][START_REF] Mao | Planar polarization of the atypical myosin Dachs orients cell divisions in Drosophila[END_REF]. Classical observations of dividing single cells have led to the proposition of Hertwig's rule, which states that cells divide according to their longest axis [START_REF] Hertwig | Das Problem der Befruchtung und der Isotropie des Eies, eine Theory der Vererbung[END_REF][START_REF] Minc | Influence of cell geometry on division-plane positioning[END_REF]. While true in many contexts, cell divisions that challenge this rule have been reported [START_REF] Gong | Planar cell polarity signalling controls cell division orientation during zebrafish gastrulation[END_REF][START_REF] Scarpa | Actomyosin-Driven Tension at Compartmental Boundaries Orients Cell Division Independently of Cell Geometry In Vivo[END_REF][START_REF] Finegan | Tissue tension and not interphase cell shape determines cell division orientation in the Drosophila follicular epithelium[END_REF]. Recent work has provided novel insights into whether and how epithelial cells adhere to Hertwig's rule, as well as how tension and polarity are integrated by cells to orient their spindles.

In this review, we first discuss how the cortical forces necessary for oriented cell divisions (OCD) are generated within the cell. Second, we describe recent data exploring how cell shape and local or global tissue tension influence spindle orientation in symmetric divisions in epithelia. Finally, we describe the function of spindle orientation in epithelial morphogenesis and homeostasis. For a review of mitotic spindle orientation in asymmetric cell divisions as well as a more complete oversight of the proteins involved in spindle orientation, we refer the readers to recent reviews [START_REF] Pietro | Regulation of mitotic spindle orientation: an integrated view[END_REF][START_REF] Morin | Mitotic Spindle Orientation in Asymmetric and Symmetric Cell Divisions during Animal Development[END_REF].

How are spindle orientation forces generated within the cell?

Mitotic spindle orientation commonly requires the exertion of cortical forces on the astral MT [START_REF] Bosveld | Epithelial tricellular junctions act as interphase cell shape sensors to orient mitosis[END_REF][START_REF] Grill | Polarity controls forces governing asymmetric spindle positioning in the Caenorhabditis elegans embryo[END_REF]. The current paradigm for force generation involves the recruitment of dynein and its cofactor dynactin to the cell cortex [START_REF] Bosveld | Epithelial tricellular junctions act as interphase cell shape sensors to orient mitosis[END_REF][START_REF] Grill | Polarity controls forces governing asymmetric spindle positioning in the Caenorhabditis elegans embryo[END_REF][START_REF] Kiyomitsu | Cortical Dynein and Asymmetric Membrane Elongation Coordinately Position the Spindle in Anaphase[END_REF][START_REF] Kotak | Cortical dynein is critical for proper spindle positioning in human cells[END_REF][START_REF] Okumura | proteins to the cortex in mammalian cells and found that NuMA but not dynein recruitment to the cortex is sufficient to orient the mitotic spindle[END_REF][START_REF] Nguyen-Ngoc | Coupling of cortical dynein and Gα proteins mediates spindle positioning in Caenorhabditis elegans[END_REF]. However, recent studies have indicated that cortical recruitment of dynein is insufficient for OCD [START_REF] Pirovano | Hexameric NuMA:LGN structures promote multivalent interactions required for planar epithelial divisions[END_REF][START_REF] Seldin | NuMA-microtubule interactions are critical for spindle orientation and the morphogenesis of diverse epidermal structures[END_REF]. Here we discuss recent findings on dynein activity and how its interactors assist dynein in force generation [START_REF] Okumura | proteins to the cortex in mammalian cells and found that NuMA but not dynein recruitment to the cortex is sufficient to orient the mitotic spindle[END_REF][START_REF] Pirovano | Hexameric NuMA:LGN structures promote multivalent interactions required for planar epithelial divisions[END_REF][START_REF] Seldin | NuMA-microtubule interactions are critical for spindle orientation and the morphogenesis of diverse epidermal structures[END_REF][START_REF] Gallini | NuMA Phosphorylation by Aurora-A Orchestrates Spindle Orientation[END_REF][START_REF] Fielmich | cortex in C. elegans and found that NuMA but not dynein recruitment to the cortex is sufficient to orient the mitotic spindle[END_REF].

Dynein force regulation and spindle orientation

Work in C. elegans and mammalian cells has led to the identification of different dynein regulators required for spindle orientation such as LIS-1, Nde1/NdeL1, and a subset of dynactin subunits including p150 [START_REF] Couwenbergs | Heterotrimeric G protein signaling functions with dynein to promote spindle positioning in C. elegans[END_REF][START_REF] Siller | Lis1/dynactin regulates metaphase spindle orientation in Drosophila neuroblasts[END_REF][START_REF] Yingling | Neuroepithelial stem cell proliferation requires LIS1 for precise spindle orientation and symmetric division[END_REF][START_REF] Moon | LIS1 controls mitosis and mitotic spindle organization via the LIS1-NDEL1-dynein complex[END_REF][START_REF] Di Pietro | An RNAi Screen in a Novel Model of Oriented Divisions Identifies the Actin-Capping Protein Z β as an Essential Regulator of Spindle Orientation[END_REF]. Recent structural analyses showed that dynein can form a ternary complex with dynactin and a cargo adaptor [START_REF] Urnavicius | dynein molecules. Complexes with two bound dyneins move at higher speed and generate more force[END_REF]. In vitro reconstitution indicates that this ternary complex is highly processive and capable of transporting cargo over large distances (Figure 1A) [START_REF] Mckenney | Activation of cytoplasmic dynein motility by dynactin-cargo adapter complexes[END_REF]. Two recent structures of dynein/dynactin complexed with the cargo adapters HOOK3 and BICD2 show that their interaction is facilitated by the coiled-coils of the adapter proteins which run along the length of dynactin [START_REF] Urnavicius | dynein molecules. Complexes with two bound dyneins move at higher speed and generate more force[END_REF]. As BICD2 is not essential for spindle orientation in cultured cells, this raises the question whether HOOK3 or another cargo adapter activates dynein during spindle orientation [START_REF] Di Pietro | An RNAi Screen in a Novel Model of Oriented Divisions Identifies the Actin-Capping Protein Z β as an Essential Regulator of Spindle Orientation[END_REF][START_REF] Olenick | Dynein activators and adaptors at a glance[END_REF]. Furthermore, depending on the cargo adapter, one or more dyneins can be part of the ternary complex. Complexes with two dyneins generate higher forces and walk faster on microtubules [START_REF] Urnavicius | dynein molecules. Complexes with two bound dyneins move at higher speed and generate more force[END_REF]. The binding of a second dynein is facilitated by the CAPZβ subunit of the dynactin barbed end (Figure 1A) [START_REF] Urnavicius | dynein molecules. Complexes with two bound dyneins move at higher speed and generate more force[END_REF]. Interestingly, recent experiments have revealed that CAPZβ knock-down leads to spindle misorientation in vivo, suggesting that the capacity of binding two dyneins contributes to division orientation regulation [START_REF] Di Pietro | An RNAi Screen in a Novel Model of Oriented Divisions Identifies the Actin-Capping Protein Z β as an Essential Regulator of Spindle Orientation[END_REF].

New roles for NuMA during spindle orientation

The respective roles of dynein and NuMA during spindle orientation have recently been investigated by using optogenetics to position components of the spindle orientation machinery at the cortex. In these studies, the authors independently found that optogenetic recruitment of NuMA to the cortex was sufficient to orient the mitotic spindle [START_REF] Okumura | proteins to the cortex in mammalian cells and found that NuMA but not dynein recruitment to the cortex is sufficient to orient the mitotic spindle[END_REF][START_REF] Fielmich | cortex in C. elegans and found that NuMA but not dynein recruitment to the cortex is sufficient to orient the mitotic spindle[END_REF]. By contrast and in agreement with other studies [START_REF] Pirovano | Hexameric NuMA:LGN structures promote multivalent interactions required for planar epithelial divisions[END_REF][START_REF] Seldin | NuMA-microtubule interactions are critical for spindle orientation and the morphogenesis of diverse epidermal structures[END_REF], recruitment of the dynein heavy chain/dynactin to the cortex was insufficient to generate pulling forces on the mitotic spindle [START_REF] Okumura | proteins to the cortex in mammalian cells and found that NuMA but not dynein recruitment to the cortex is sufficient to orient the mitotic spindle[END_REF][START_REF] Fielmich | cortex in C. elegans and found that NuMA but not dynein recruitment to the cortex is sufficient to orient the mitotic spindle[END_REF]. This suggests that NuMA is required not only for dynein recruitment, but also for force production without additional upstream components.

NuMA coiled-coil and MT binding domains are essential for spindle orientation

All NuMA homologs have a coiled-coil domain which has been shown to be required for spindle orientation in Drosophila and HeLa cells [START_REF] Bosveld | Epithelial tricellular junctions act as interphase cell shape sensors to orient mitosis[END_REF][START_REF] Okumura | proteins to the cortex in mammalian cells and found that NuMA but not dynein recruitment to the cortex is sufficient to orient the mitotic spindle[END_REF][START_REF] Kiyomitsu | The cortical force-generating machinery: how cortical spindlepulling forces are generated[END_REF]. Therefore, in the light of the recently established ternary dynein complex structure described above, NuMA is a potential candidate to work as a regulator of dynein activity (Figure 1B). Furthermore, Seldin and colleagues showed that NuMA localizes to MT tips in interphase and pro-metaphase cells, and that deletion of the domain responsible for this localization (AA:1811-1985 in human) impaired spindle orientation in cultured keratinocytes and in vivo [START_REF] Seldin | NuMA-microtubule interactions are critical for spindle orientation and the morphogenesis of diverse epidermal structures[END_REF]. In addition, Gallini and colleagues found a second NuMA MT binding domain, (AA: 2002-2115 in human) which is also essential for spindle orientation in cultured cells [START_REF] Pirovano | Hexameric NuMA:LGN structures promote multivalent interactions required for planar epithelial divisions[END_REF][START_REF] Gallini | NuMA Phosphorylation by Aurora-A Orchestrates Spindle Orientation[END_REF]. Accordingly, this second MT binding is necessary to orient the spindle upon optogenetic recruitment of NuMa to the cortex [START_REF] Okumura | proteins to the cortex in mammalian cells and found that NuMA but not dynein recruitment to the cortex is sufficient to orient the mitotic spindle[END_REF]. Exactly how NuMA and its MT binding domains assists in spindle orientation remains to be better defined. For instance, MT tip localized NuMA could regulate MT growth dynamics, cortical capture or cortical dwell time of astral MTs which have all been proposed to regulate spindle orientation [START_REF] Pietro | Regulation of mitotic spindle orientation: an integrated view[END_REF][START_REF] Kwon | Direct Microtubule-Binding by Myosin-10 Orients Centrosomes toward Retraction Fibers and Subcortical Actin Clouds[END_REF][START_REF] Bowman | The Drosophila NuMA Homolog Mud Regulates Spindle Orientation in Asymmetric Cell Division[END_REF].

In summary, these recent data illustrate that MT pulling force generation does not solely rely on dynein and its interaction with p150 and LIS1, but also requires NuMA. Direct analyses of astral MT dynamics and measurement of astral MT associated forces, combined with the depletion of the aforementioned NuMA domains and optogenetic recruitment will greatly contribute to a better understanding of dynein regulation in spindle orientation.

How do cell shape and tissue tension result in anisotropic spindle orientation forces?

The first exploration of the influence of cell geometry on division orientation was performed at the end of the 19 th century. In these experiments, rounded amphibian eggs were flattened between two glass slides resulting in the cleavage planes of the first two divisions being reproducibly positioned perpendicular to the long axis of the egg [START_REF] Hertwig | Das Problem der Befruchtung und der Isotropie des Eies, eine Theory der Vererbung[END_REF][START_REF] Pflüger | H: Entwicklungsmechanische Studien. I. Der Werth der beiden Furchungszellen der Echinodermenentwicklung[END_REF]35]. Predicting the orientation of division based on the cells' long axis later became known as Hertwig's rule. In depth study of the underlying mechanism was possible after advances in microfabrication enabled standardized regulation of cell shape and the application of defined mechanical stress. Combined with automated measurements of cell division orientation and physical modeling this led to important advances in linking geometry and division orientation [START_REF] Minc | Influence of cell geometry on division-plane positioning[END_REF][START_REF] Fink | External forces control mitotic spindle positioning[END_REF][START_REF] Théry | Experimental and theoretical study of mitotic spindle orientation[END_REF][START_REF] Théry | The extracellular matrix guides the orientation of the cell division axis[END_REF].

Théry and colleagues used fibronectin adhesive micropatterns to standardize the 2D shape of single adhesive cells to study the influence of 2D cell shape on spindle orientation. The authors found that it is not cell shape, but the specific distribution of actin rich retraction fibers connecting the mitotic cell to the substrate, which best predicts the angles of spindle orientation [START_REF] Théry | Experimental and theoretical study of mitotic spindle orientation[END_REF][START_REF] Théry | The extracellular matrix guides the orientation of the cell division axis[END_REF]. As the distribution of retraction fibers is controlled by the shape of the adhesion pattern, these fibers were proposed to provide a memory of the interphase adhesion pattern of the cell [START_REF] Fink | External forces control mitotic spindle positioning[END_REF][START_REF] Théry | Experimental and theoretical study of mitotic spindle orientation[END_REF][START_REF] Théry | The extracellular matrix guides the orientation of the cell division axis[END_REF]. Retraction fibers direct the distribution of cytoplasmic actin clouds that bias the forces exerted on astral MT. In particular, the unconventional MT bound Myosin-10 localizes to these actin clouds where it modulates astral MT dynamics [START_REF] Kwon | Direct Microtubule-Binding by Myosin-10 Orients Centrosomes toward Retraction Fibers and Subcortical Actin Clouds[END_REF][START_REF] Fink | External forces control mitotic spindle positioning[END_REF]. This mechanism was proposed to contribute to spindle orientation in parallel to the classical LGN/NuMA/dynein pathway [START_REF] Kwon | Direct Microtubule-Binding by Myosin-10 Orients Centrosomes toward Retraction Fibers and Subcortical Actin Clouds[END_REF][START_REF] Machicoane | SLK-dependent activation of ERMs controls LGN-NuMA localization and spindle orientation[END_REF]. Following Hertwig's seminal experiments, Minc and colleagues squeezed sea urchin eggs into microfabricated wells resulting in division orientation according to the long axis created by the applied external constraints [START_REF] Minc | Influence of cell geometry on division-plane positioning[END_REF]. By using quantitative modeling, they proposed that the applied rotational forces on centrosomes scale with MT length, linking cell geometry with division orientation for each specific cell shape in single cells [START_REF] Minc | Influence of cell geometry on division-plane positioning[END_REF].

In epithelia, several studies initially identified major roles for planar cell polarity (PCP) and apical-basal (AB) polarity in orienting cell divisions [START_REF] Gong | Planar cell polarity signalling controls cell division orientation during zebrafish gastrulation[END_REF][START_REF] Mao | Planar polarization of the atypical myosin Dachs orients cell divisions in Drosophila[END_REF]. These studies were instrumental in analyzing spindle orientation in multicellular contexts as well as the mechanisms of spindle orientation with respect to the apical-basal axis. Recent work explored whether and how geometry and tension play roles in regulating spindle orientation in epithelial tissues like in single cells, which we will discuss in the next sections.

Two axes of spindle orientation in epithelia: apical-basal and planar

In epithelia, mitotic spindles are typically oriented perpendicular to the apical-basal axis, which is critical to maintain both daughter cells in the plane of the epithelium. This orientation is generally dependent on the lateral distribution of pulling forces [START_REF] Chanet | Myosin 2-Induced Mitotic Rounding Enables Columnar Epithelial Cells to Interpret Cortical Spindle Positioning Cues[END_REF][START_REF] Nakajima | Epithelial junctions maintain tissue architecture by directing planar spindle orientation[END_REF][START_REF] Gloerich | Cell division orientation is coupled to cell-cell adhesion by the E-cadherin/LGN complex[END_REF]. Interestingly, when the actomyosin machinery necessary for cell rounding is impaired, the geometry can override the lateral pulling forces [START_REF] Chanet | Myosin 2-Induced Mitotic Rounding Enables Columnar Epithelial Cells to Interpret Cortical Spindle Positioning Cues[END_REF][START_REF] Nakajima | Epithelial junctions maintain tissue architecture by directing planar spindle orientation[END_REF][START_REF] Mckinley | Cellular aspect ratio and cell division mechanics underlie the patterning of cell progeny in diverse mammalian epithelia[END_REF]. Recently, Box and colleagues found that AB division orientation can respond to changes in cell geometry in mouse basal epidermal progenitors. Mutants for the PCP protein Vangl2 fail to close their neural tube, which in turn alters cell geometry, thereby indirectly perturbing division orientation relative to the AB axis [START_REF] Box | Epithelial geometry regulates spindle orientation and progenitor fate during formation of the mammalian epidermis[END_REF].

Spindle orientation is also regulated in the plane of the tissue (planar orientation, Figure 2 top center). In most tissues, planar cell divisions follow Hertwig's rule aligning the mitotic spindle with the cell interphase long axis (Figure 2A, 3A) [START_REF] Tamada | Square Cell Packing in the Drosophila Embryo through Spatiotemporally Regulated EGF Receptor Signaling[END_REF][START_REF] Mao | Differential proliferation rates generate patterns of mechanical tension that orient tissue growth[END_REF][START_REF] Da Silva | Oriented cell divisions in the extending germband of Drosophila[END_REF]. A large variety of cell anisotropies exist in tissues, ranging from highly anisotropic (elongated) cells to almost isotropic (round) cells [START_REF] Bosveld | Epithelial tricellular junctions act as interphase cell shape sensors to orient mitosis[END_REF][START_REF] Hart | E-cadherin and LGN align epithelial cell divisions with tissue tension independently of cell shape[END_REF][START_REF] Legoff | A global pattern of mechanical stress polarizes cell divisions and cell shape in the growing Drosophila wing disc[END_REF][START_REF] Nestor-Bergmann | Decoupling the Roles of Cell Shape and Mechanical Stress in Orienting and Cueing Epithelial Mitosis[END_REF]. Regardless of their interphase shape, cells round up during mitosis by increasing osmotic pressure and stiffening their actomyosin cortex [START_REF] Stewart | Hydrostatic pressure and the actomyosin cortex drive mitotic cell rounding[END_REF][START_REF] Sorce | Mitotic cells contract actomyosin cortex and generate pressure to round against or escape epithelial confinement[END_REF]. Rounding during mitosis poses the question of how the interphase long axis is sensed in cells that orient the spindle in the planar axis according to the long axis rule [START_REF] Chanet | Myosin 2-Induced Mitotic Rounding Enables Columnar Epithelial Cells to Interpret Cortical Spindle Positioning Cues[END_REF][START_REF] Stewart | Hydrostatic pressure and the actomyosin cortex drive mitotic cell rounding[END_REF][START_REF] Ramanathan | Cdk1dependent mitotic enrichment of cortical myosin II promotes cell rounding against confinement[END_REF]. Furthermore, epithelial cells are subjected to mechanical forces, which can directly affect planar spindle orientation and modulate cell shapes (Figure 3B) [START_REF] Heisenberg | Forces in Tissue Morphogenesis and Patterning[END_REF]. Central questions in the field are therefore whether tissue forces are able to orient the spindle, and whether this effect is dependent on cell shape changes.

In single adhesive cells, applying external forces without affecting cell shape was able to induce spindle rotation independently of cell shape [START_REF] Fink | External forces control mitotic spindle positioning[END_REF]. Initial studies in the context of Zebrafish epiboly, and in cultured epithelial monolayers stretched 30%, indicated that tension is able to orient cell divisions, while also orienting cell shape [START_REF] Campinho | Tensionoriented cell divisions limit anisotropic tissue tension in epithelial spreading during zebrafish epiboly[END_REF][START_REF] Wyatt | Emergence of homeostatic epithelial packing and stress dissipation through divisions oriented along the long cell axis[END_REF]. In the next section, we discuss several recent studies that investigated how epithelial cells sense their interphase cell shape, as well as how cell divisions are oriented by cell shape and/or tension [START_REF] Bosveld | Epithelial tricellular junctions act as interphase cell shape sensors to orient mitosis[END_REF][START_REF] Scarpa | Actomyosin-Driven Tension at Compartmental Boundaries Orients Cell Division Independently of Cell Geometry In Vivo[END_REF][START_REF] Finegan | Tissue tension and not interphase cell shape determines cell division orientation in the Drosophila follicular epithelium[END_REF][START_REF] Hart | E-cadherin and LGN align epithelial cell divisions with tissue tension independently of cell shape[END_REF][START_REF] Nestor-Bergmann | Decoupling the Roles of Cell Shape and Mechanical Stress in Orienting and Cueing Epithelial Mitosis[END_REF][START_REF] Campinho | Tensionoriented cell divisions limit anisotropic tissue tension in epithelial spreading during zebrafish epiboly[END_REF].

Sensing interphase cell shape in mitosis

Work in the Drosophila notum epithelium uncovered a mechanism that translates the interphase long axis into localized forces which can orient the mitotic spindle. This mechanism is based on the localization of Mud/NuMA to tricellular junctions (TCJ, the junctions where three cells come together). This localizes the forces exerted on astral MT to TCJ, leading to spindle orientation according to TCJ distribution, which generally aligns with the cell long axis (Figure 2B). In cases where the angle predicted by cell shape and the distribution of TCJ differ by more than 45 degrees, cells divide according to the TCJ distribution showing that the orientation of TCJ is a better predictor for spindle orientation than cell shape [START_REF] Bosveld | Epithelial tricellular junctions act as interphase cell shape sensors to orient mitosis[END_REF]. This new read-out of cell shape has later become known as the "TCJ rule".

Tension vs shape in spindle orientation

To analyze whether tension can orient the spindle independently of cell shape, a recent study subjected the Xenopus embryo animal cap to an uniaxial stretch (20%) with a PDMS based device [START_REF] Nestor-Bergmann | Decoupling the Roles of Cell Shape and Mechanical Stress in Orienting and Cueing Epithelial Mitosis[END_REF]. The externally applied stretch induced both cell shape elongation and cell division orientation along the axis of stretch. By comparing cell shape, cell area and the position of the TCJ vertices, the investigators found that TCJ align with the principal axis of local stress and are a better predictor of spindle orientation than global tissue stress [START_REF] Nestor-Bergmann | Decoupling the Roles of Cell Shape and Mechanical Stress in Orienting and Cueing Epithelial Mitosis[END_REF][START_REF] Nestor-Bergmann | Relating cell shape and mechanical stress in a spatially disordered epithelium using a vertex-based model[END_REF]. Indeed, in cells that do not elongate along the axis of stretch, the spindle is not oriented in the axis of stretch, but according to TCJ distribution (Figure 2B,3C). Mechanistically, TCJ are enriched in Cadherin and LGN, and this distribution is necessary to control planar spindle orientation in this context [START_REF] Nestor-Bergmann | Decoupling the Roles of Cell Shape and Mechanical Stress in Orienting and Cueing Epithelial Mitosis[END_REF]. In contrast to the findings in Xenopus epithelia, dividing cells within MDCK monolayers subjected to low uniaxial stretch (15%), oriented their spindle with respect to the stretch axis, independently of cell shape. The mechanism proposed here involves both Cadherin and the polarization of LGN towards the bicellular junctions (BCJ) that are perpendicular to the axis of stretch (Figure 3D) [START_REF] Hart | E-cadherin and LGN align epithelial cell divisions with tissue tension independently of cell shape[END_REF]. Therefore, the same molecules mediate the link between the orienting cue (geometry or tension) and force generation by localizing to TCJ or BCJ in Xenopus and MDCK cells respectively.

The link between tension and planar spindle orientation has been also studied in developing tissues which undergo tissue elongation or experience localized tensile stress. In the case of the Drosophila embryo, cells that are adjacent to high tensile supracellular actomyosin cables divide according to their long shape axis only if they are highly elongated. By contrast, slightly elongated cells divide according to the tension cue provided by the actomyosin cables, which run perpendicular to cell shape (Figure 2C) [START_REF] Scarpa | Actomyosin-Driven Tension at Compartmental Boundaries Orients Cell Division Independently of Cell Geometry In Vivo[END_REF]. Another tension dependent mechanism orienting cell divisions was identified in the elongating Drosophila follicular epithelium (FE). In these cells, the cell long axis is perpendicular to the axis of tissue elongation and thus to the stretch force [START_REF] Finegan | Tissue tension and not interphase cell shape determines cell division orientation in the Drosophila follicular epithelium[END_REF]. The embryo and FE constitute immature epithelia which do not harbor classical BCJ and TCJ septate junctions. No clear crescents or TCJ accumulation of LGN or NuMA could be observed and these proteins were not required for planar spindle orientation. In mature epithelia which do have septate junctions (larval wing disc, pupal wing and notum tissues), Mud is enriched at the level of the tricellular septate junction. Therefore, different mechanisms might orient cell divisions according to tension in immature as compared to mature epithelial tissues. In the embryo, the actomyosin cable is proposed to orient the spindle by capturing one centrosome in an actin network, while in the FE, anisotropic cortical stiffness has been proposed to explain the bias in spindle orientation [START_REF] Scarpa | Actomyosin-Driven Tension at Compartmental Boundaries Orients Cell Division Independently of Cell Geometry In Vivo[END_REF][START_REF] Finegan | Tissue tension and not interphase cell shape determines cell division orientation in the Drosophila follicular epithelium[END_REF]. What emerges from these experiments is that no clear one-size fits all rule can be applied. In highly elongated cells, the predominant cue seems to be cell shape, independently of the mechanism used by cells to interpret this cue [START_REF] Bosveld | Epithelial tricellular junctions act as interphase cell shape sensors to orient mitosis[END_REF][START_REF] Scarpa | Actomyosin-Driven Tension at Compartmental Boundaries Orients Cell Division Independently of Cell Geometry In Vivo[END_REF][START_REF] Nestor-Bergmann | Decoupling the Roles of Cell Shape and Mechanical Stress in Orienting and Cueing Epithelial Mitosis[END_REF]. In cells with moderate anisotropy, different scenarios are possible: i) in immature epithelial tissues or MDCK layers, where spindle orientation regulators do not localize to TCJ, spindle orientation responds to global tension independently of cell shape; ii) in mature epithelia, cells use TCJ bipolarity, which aligns with local tension, as the main cue for spindle orientation (Figure 2, 3) [START_REF] Bosveld | Epithelial tricellular junctions act as interphase cell shape sensors to orient mitosis[END_REF][START_REF] Scarpa | Actomyosin-Driven Tension at Compartmental Boundaries Orients Cell Division Independently of Cell Geometry In Vivo[END_REF][START_REF] Nestor-Bergmann | Decoupling the Roles of Cell Shape and Mechanical Stress in Orienting and Cueing Epithelial Mitosis[END_REF]. In this view, one could speculate that TCJ hot spots of LGN or NuMA are more efficient to orient the spindle than other mechanisms that depend on actomyosin forces. Actomyosin based spindle orientation mechanisms remain to be defined in epithelia: could actin clouds and Myosin-10 be involved in spindle orientation as observed in cultured cells?

Function of mitotic spindle orientation in epithelia

A central question in the field of mitotic spindle orientation is the function of OCD in epithelial tissues. Multiple roles have been proposed such as tissue layering, dissipation of tissue stress, changes in tissue morphology, and homeostasis (Figure 4) [START_REF] Heisenberg | Forces in Tissue Morphogenesis and Patterning[END_REF][START_REF] Nakajima | Epithelial junctions maintain tissue architecture by directing planar spindle orientation[END_REF][START_REF] Da Silva | Oriented cell divisions in the extending germband of Drosophila[END_REF][START_REF] Campinho | Tensionoriented cell divisions limit anisotropic tissue tension in epithelial spreading during zebrafish epiboly[END_REF][START_REF] Morin | Control of planar divisions by the G-protein regulator LGN maintains progenitors in the chick neuroepithelium[END_REF].

OCD were proposed to be required for axis elongation during Zebrafish gastrulation [START_REF] Gong | Planar cell polarity signalling controls cell division orientation during zebrafish gastrulation[END_REF]. The PCP pathway can regulate spindle orientation through Mud and via the Anthrax2 receptor [START_REF] Ségalen | The Fz-Dsh Planar Cell Polarity Pathway Induces Oriented Cell Division via Mud/NuMA in Drosophila and Zebrafish[END_REF][START_REF] Castanon | Anthrax toxin receptor 2a controls mitotic spindle positioning[END_REF]. Knock down of the PCP protein Dishevelled resulted in a shortened anterior-posterior axis [START_REF] Gong | Planar cell polarity signalling controls cell division orientation during zebrafish gastrulation[END_REF]. Yet, more directly perturbing spindle orientation by impairing dynein or NuMA function did not affect body axis elongation, challenging the idea that spindle orientation is involved in tissue elongation during Zebrafish gastrulation [START_REF] Ségalen | The Fz-Dsh Planar Cell Polarity Pathway Induces Oriented Cell Division via Mud/NuMA in Drosophila and Zebrafish[END_REF][START_REF] Quesada-Hernández | Stereotypical Cell Division Orientation Controls Neural Rod Midline Formation in Zebrafish[END_REF]. In Drosophila wing imaginal disc, cells in the central wing pouch divide preferentially along the proximal-distal axis, while cells in the periphery divide tangentially along the local tissue boundary [START_REF] Mao | Planar polarization of the atypical myosin Dachs orients cell divisions in Drosophila[END_REF][START_REF] Mao | Differential proliferation rates generate patterns of mechanical tension that orient tissue growth[END_REF][START_REF] Legoff | A global pattern of mechanical stress polarizes cell divisions and cell shape in the growing Drosophila wing disc[END_REF]. In this tissue, spindle orientation impairment due to loss of Mud/NuMA leads to the formation of smaller but normally shaped wings without gross phenotypic defects (Figure 4D-G) [START_REF] Zhou | Oriented Cell Divisions Are Not Required for Drosophila Wing Shape[END_REF]. Spindle misorientation can result in basally born daughter cells being lost from the tissue possibly explaining the observed size reduction [START_REF] Nakajima | Epithelial junctions maintain tissue architecture by directing planar spindle orientation[END_REF][START_REF] Zhou | Oriented Cell Divisions Are Not Required for Drosophila Wing Shape[END_REF]. As global tissue stress regulates OCD [START_REF] Mao | Planar polarization of the atypical myosin Dachs orients cell divisions in Drosophila[END_REF][START_REF] Mao | Differential proliferation rates generate patterns of mechanical tension that orient tissue growth[END_REF][START_REF] Legoff | A global pattern of mechanical stress polarizes cell divisions and cell shape in the growing Drosophila wing disc[END_REF], the authors proposed that loss of OCD contributing to tissue elongation can be buffered by an increase in tissue stress and cell rearrangements contributing to tissue elongation [START_REF] Zhou | Oriented Cell Divisions Are Not Required for Drosophila Wing Shape[END_REF]. Therefore, planar division within the wing disc epithelium might serve to aid planar tissue extension and reduce cell loss.

In the developing Zebrafish embryonic surface layer (pre-EVL), Xiong and colleagues proposed that OCD contribute to the layering of the tissue. In this model system, the cell shape predicts the orientation of division with squamous and cuboidal cells dividing in perpendicular directions (Figure 4A) [START_REF] Xiong | Interplay of Cell Shape and Division Orientation Promotes Robust Morphogenesis of Developing Epithelia[END_REF]. Cell divisions in cuboidal cells contributed to layering as one of the two daughter cells was displaced from the outer layer into a new layer below. Furthermore, cell division orientation was proposed to buffer changes in the global geometry of the embryo, permitting successful epiboly [START_REF] Heisenberg | Forces in Tissue Morphogenesis and Patterning[END_REF].

In the Zebrafish EVL, cell divisions tend to orient along the axis of tension during epiboly (Figure 4B) [START_REF] Campinho | Tensionoriented cell divisions limit anisotropic tissue tension in epithelial spreading during zebrafish epiboly[END_REF] and in stretched MDCK monolayers, cell divisions reorient according to the axis of stretch (Figure 3D, 4C) [START_REF] Gloerich | Cell division orientation is coupled to cell-cell adhesion by the E-cadherin/LGN complex[END_REF][START_REF] Wyatt | Emergence of homeostatic epithelial packing and stress dissipation through divisions oriented along the long cell axis[END_REF]. These orientations have been proposed to relieve tissue stress by increasing the number of cells along the stress axis [START_REF] Campinho | Tensionoriented cell divisions limit anisotropic tissue tension in epithelial spreading during zebrafish epiboly[END_REF][START_REF] Wyatt | Emergence of homeostatic epithelial packing and stress dissipation through divisions oriented along the long cell axis[END_REF].

Fine tuning of AB spindle orientation is essential to ensure faithful epithelial structure. During mouse embryonic epithelial development, a switch in spindle orientation, from parallel to perpendicular to the epithelial plane, is necessary for progenitor cells differentiation and skin stratification [START_REF] Kulukian | Spindle orientation and epidermal morphogenesis[END_REF]. Impaired AB spindle orientation can also lead to daughter cells being aberrantly displaced out of the epithelial layer [START_REF] Nakajima | Epithelial junctions maintain tissue architecture by directing planar spindle orientation[END_REF]. Therefore, OCD have also been proposed to prevent tumor initiation and/or tumor spreading. Nakajima and colleagues showed that by perturbing spindle orientation and inhibiting apoptosis, tumor like masses can form in fly wing disc epithelia (Figure 4G) [START_REF] Nakajima | Epithelial junctions maintain tissue architecture by directing planar spindle orientation[END_REF]. Alternatively, tumor spreading can also be prevented by reintegration of cells after misoriented divisions. In the Drosophila FE, the embryonic ectoderm, the neuroepithelium and the wing disc, misplaced daughter cells can reintegrate in the tissue (Figure 4G) [START_REF] Zhou | Oriented Cell Divisions Are Not Required for Drosophila Wing Shape[END_REF][START_REF] Bergstralh | Lateral adhesion drives reintegration of misplaced cells into epithelial monolayers[END_REF]. Further exploration of cell reintegration upon division misorientation will help to understand how epithelial tissues can prevent out of plane born daughter cells from becoming tumorigenic. Finally, how epithelial homeostasis and morphogenesis are affected by aberrant cell loss due to AB spindle misorientation remains to be further investigated.

Future directions

In summary, considerable progress has been made in understanding the mechanisms and roles of mitotic spindle orientation during symmetric epithelial cell division. Accordingly, complementary in vitro and in vivo studies have put forward novel hypotheses. For example, cortical clustering of NuMA is proposed to be critical for spindle orientation in cultured cells, while in epithelial tissues TCJ localization of spindle guidance cues seems to be a predominant mechanism to orient the spindle [START_REF] Okumura | proteins to the cortex in mammalian cells and found that NuMA but not dynein recruitment to the cortex is sufficient to orient the mitotic spindle[END_REF][START_REF] Pirovano | Hexameric NuMA:LGN structures promote multivalent interactions required for planar epithelial divisions[END_REF]. This suggests that NuMA clustering at TCJ, rather than homogenous cortical localization, could be more efficient to generate pulling forces in epithelia. Better characterization of the upstream regulators of spindle orientation and of the mechanisms of force generation will extend our understanding of the many roles of spindle orientation in multicellular contexts. Another exciting direction will be to explore how the interplay between tissue mechanics, vertex dynamics and apoptosis influences spindle orientation.

Figure 1 :

 1 Figure 1: Dynein can bind to dynactin and a cargo adapter such as HOOK3 to form a processive complex (A). Their interaction is facilitated by the coiled-coil in HOOK3. A second dynein can be part of the complex through interactions with the CAPZb subunit of dynactin. Knock-down of CAPZb leads to spindle misorientation. NuMA also possesses a long coiled-coil, which is required for spindle orientation. (B).

Figure 2 :

 2 Figure 2: Planar spindle orientation can be regulated through different mechanisms. Hertwig's rule states that cells divide according to their long axis (A). Cells with Mud orLGN at TCJ divide according to the position of TCJ (B). In the Drosophila embryo cells adjacent to the actomyosin cable divide according to cell shape if their cell shape is highly anisotropic, but spindle orientation is directed towards the cable in moderately anisotropic cells (C).

Figure 3 :

 3 Figure 3: In homeostasis, no apparent global direction of cell division can be observed (A). If a stretch is applied to the tissue, the cells become elongated and divide according to their long cell axis (B). Stretched Xenopus animal caps have enriched LGN at TCJ. The anisotropy of the cell as given by TCJ is the best predictor of local stress and cell division orientation (C). Cell divisions in MCDK layers do no follow cell shape, but divide along the direction of stretch by accumulation of LGN on junctions perpendicular to the axis of stretch (D).

Figure 4 :

 4 Figure 4: Multiple cellular processes have been linked to OCD. In Zebrafish, OCD contribute to the formation of multiple tissue layers in the pre-EVL stage (A). The spreading motion of epiboly generates tissue tension which can be mitigated by OCD along the axis of stretch (B). Similarly, OCD can relieve external stress imposed on MCDK layers (C). In Drosophila, spindle misorientation causes phenotypes in the plane (D) and along the AB axis of epithelia (E). Spindle misorientation in mud mutants leads to smaller wings (F). Furthermore, the loss of AB spindle orientation results in multiple cellular phenotypes. Firstly, a fraction of cells is able to reintegrate into the plane of the tissue. Alternatively, they can be lost through delamination after which the cell can be removed through apoptosis (G). When apoptosis is blocked, these cells can survive and form tumor like masses (G).

Acknowledgements

We thank Floris Bosveld for critical reading of the manuscript. Work in Y.B lab is supported by ERC Advanced (TiMorph, 340784), ARC (PGA1 RF20180206793), ANR Labex DEEP 11-LBX-0044, ANR-10-IDEX-0001-02 PSL grants for funding. E.v.L. is supported by the Marie Skłodowska-Curie Actions Innovative Training Network 675407 and F.d.P. by the Fondation pour la Recherche Medicale (FRM) postdoctoral grant SPF20170938661.