Mitotic spindle misorientation in cancer--out of alignment and into the fire

LKB1 as a Tumor Suppressor in Uterine Cancer: Mouse Models and Translational Studies

The LKB1 tumor suppressor was identified in 1998 as the gene mutated in the Peutz-Jeghers Syndrome (PJS), a hereditary cancer predisposition characterized by gastrointestinal polyposis and a high incidence of cancers, particularly carcinomas, at a variety of anatomic sites including the gastrointestinal tract, lung, and female reproductive tract. Women with PJS have a high incidence of carcinomas of the uterine corpus (endometrium) and cervix. The LKB1 gene is also somatically mutated in human cancers arising at these sites. Work in mouse models has highlighted the potency of LKB1 as an endometrial tumor suppressor and its distinctive roles in driving invasive and metastatic growth. These in vivo models represent tractable experimental systems for the discovery of underlying biological principles and molecular processes regulated by LKB1 in the context of tumorigenesis and also serve as useful preclinical model systems for experimental therapeutics. Here we review LKB1's known roles in mTOR signaling, metabolism, and cell polarity, with an emphasis on human pathology and mouse models relevant to uterine carcinogenesis, including cancers of the uterine corpus and cervix.

Are Tumor Cell Lineages Solely Shaped by Mechanical Forces?

This paper investigates cell proliferation dynamics in small tumor cell aggregates using an individual-based model (IBM). The simulation model is designed to study the morphology of the cell population and of the cell lineages as well as the impact of the orientation of the division plane on this morphology. Our IBM model is based on the hypothesis that cells are incompressible objects that grow in size and divide once a threshold size is reached, and that newly born cell adhere to the existing cell cluster. We performed comparisons between the simulation model and experimental data by using several statistical indicators. The results suggest that the emergence of particular morphologies can be explained by simple mechanical interactions.

The tumor suppressor CYLD controls epithelial morphogenesis and homeostasis by regulating mitotic spindle behavior and adherens junction assembly

Epithelial morphogenesis and homeostasis are essential for animal development and tissue regeneration, and epithelial disorganization is associated with developmental disorders and tumorigenesis. However, the molecular mechanisms that contribute to the morphogenesis and homeostasis of the epithelium remain elusive. Herein, we report a novel role for the cylindromatosis (CYLD) tumor suppressor in these events. Our results show that CYLD depletion disrupts epithelial organization in both Drosophila egg chambers and mouse skin and intestinal epithelia. Microscopic analysis of proliferating cells in mouse epithelial tissues and cultured organoids reveals that loss of CYLD synergizes with tumor-promoting agents to cause the misorientation of the mitotic spindle. Mechanistic studies show that CYLD accumulates at the cell cortex in epithelial tissues and cultured cells, where it promotes the formation of epithelial adherens junctions through the modulation of microtubule dynamics. These data suggest that CYLD controls epithelial morphogenesis and homeostasis by modulating the assembly of adherens junctions and ensuring proper orientation of the mitotic spindle. Our findings thus provide novel insight into the role of CYLD in development, tissue homeostasis, and tumorigenesis.

E-cadherin and LGN align epithelial cell divisions with tissue tension independently of cell shape

Tissue morphogenesis requires the coordinated regulation of cellular behavior, which includes the orientation of cell division that defines the position of daughter cells in the tissue. Cell division orientation is instructed by biochemical and mechanical signals from the local tissue environment, but how those signals control mitotic spindle orientation is not fully understood. Here, we tested how mechanical tension across an epithelial monolayer is sensed to orient cell divisions. Tension across Madin-Darby canine kidney cell monolayers was increased by a low level of uniaxial stretch, which oriented cell divisions with the stretch axis irrespective of the orientation of the cell long axis. We demonstrate that stretch-induced division orientation required mechanotransduction through E-cadherin cell-cell adhesions. Increased tension on the E-cadherin complex promoted the junctional recruitment of the protein LGN, a core component of the spindle orientation machinery that binds the cytosolic tail of E-cadherin. Consequently, uniaxial stretch triggered a polarized cortical distribution of LGN. Selective disruption of trans engagement of E-cadherin in an otherwise cohesive cell monolayer, or loss of LGN expression, resulted in randomly oriented cell divisions in the presence of uniaxial stretch. Our findings indicate that E-cadherin plays a key role in sensing polarized tensile forces across the tissue and transducing this information to the spindle orientation machinery to align cell divisions.

Xenopus as a model for studies in mechanical stress and cell division

We exist in a physical world, and cells within biological tissues must respond appropriately to both environmental forces and forces generated within the tissue to ensure normal development and homeostasis. Cell division is required for normal tissue growth and maintenance, but both the direction and rate of cell division must be tightly controlled to avoid diseases of over-proliferation such as cancer. Recent studies have shown that mechanical cues can cause mitotic entry and orient the mitotic spindle, suggesting that physical force could play a role in patterning tissue growth. However, to fully understand how mechanics guides cells in vivo, it is necessary to assess the interaction of mechanical strain and cell division in a whole tissue context. In this mini-review we first summarise the body of work linking mechanics and cell division, before looking at the advantages that the Xenopus embryo can offer as a model organism for understanding: (1) the mechanical environment during embryogenesis, and (2) factors important for cell division. Finally, we introduce a novel method for applying a reproducible strain to Xenopus embryonic tissue and assessing subsequent cell divisions.

Cell reintegration: Stray epithelial cells make their way home

Ongoing work shows that misplaced epithelial cells have the capacity to reintegrate back into tissue layers. This movement appears to underlie tissue stability and may also control aspects of tissue structure. A recent study reveals that cell reintegration in at least one tissue, the Drosophila follicular epithelium, is based on adhesion molecules that line lateral cell surfaces. In this article we will review these observations, discuss their implications for epithelial tissue development and maintenance, and identify future directions for study.

27 T ultra-high static magnetic field changes orientation and morphology of mitotic spindles in human cells

Purified microtubules have been shown to align along the static magnetic field (SMF) in vitro because of their diamagnetic anisotropy. However, whether mitotic spindle in mammalian cells can be aligned by magnetic field has not been experimentally proved. In particular, the biological effects of SMF of above 20 T (Tesla) on mammalian cells have never been reported. Here we found that in both CNE-2Z and RPE1 human cells spindle orients in 27 T SMF. The direction of spindle alignment depended on the extent to which chromosomes were aligned to form a planar metaphase plate. Our results show that the magnetic torque acts on both microtubules and chromosomes, and the preferred direction of spindle alignment relative to the field depends more on chromosome alignment than microtubules. In addition, spindle morphology was also perturbed by 27 T SMF. This is the first reported study that investigated the mammalian cellular responses to ultra-high magnetic field of above 20 T. Our study not only found that ultra-high magnetic field can change the orientation and morphology of mitotic spindles, but also provided a tool to probe the role of spindle orientation and perturbation in developmental and cancer biology.

DOI:http://dx.doi.org/10.7554/eLife.22911.001

Nowadays, a number of methods can be used to ‘look’ inside the body to investigate potential health problems. One of these is a technique called magnetic resonance imaging (MRI) that uses magnetic fields that are several hundred times stronger than a fridge magnet (or over 10,000 times stronger than the Earth’s natural magnetic field) to generate images of the inside of the body. In general, stronger magnetic fields enable higher quality images to be obtained. However, the effects of exposing the body’s cells to these magnetic fields have not been fully determined.

Like most other biological materials, protein polymers called microtubules can respond to high magnetic fields – for example, by aligning with the field. Microtubules play a number of roles inside cells. This includes forming the mitotic spindle that separates copies of chromosomes – the structures in which the majority of a cell’s genetic material is stored – equally between dividing cells.

The orientation of the mitotic spindle determines the direction in which a cell will divide. This direction is important for generating different types of cells and tissues. Furthermore, many cancerous cells have incorrectly oriented spindles.

Zhang, Hou et al. have now exposed cancerous and normal human cells to magnetic fields of varying strengths. The maximum magnetic field strength tested (27 Tesla – or around 10 times the highest field strengths produced by standard hospital MRI scanners) did not kill the cells after four hours of exposure, but the orientation of the spindles inside the cells did change. In addition, the 27 Tesla magnetic field caused spindles that were perpendicular to the direction of the field to widen. At an intermediate field strength (9 Tesla – a magnetic field strength that has been used in some experimental MRI scanners), the orientation of the spindle only changed after three days of continuous exposure to the magnetic field. Lower field strengths (such as those currently used in hospital MRI scanners) did not alter the orientation of the spindle even after seven days of exposure.

Zhang, Hou et al. also observed that the magnetic field acts on both the microtubules and chromosomes. However, the alignment of the chromosomes in the cell was the greatest determinant of the direction in which the spindle would align itself in response to the magnetic field.

The next step is to analyze the consequences of magnetic field-induced spindle orientation changes – can these lead to cancer or reduce cancer growth, or change how animal tissues develop? Understanding how to control the position of the spindle could also ultimately make it possible to use ultra-high magnetic fields to engineer tissues or stimulate their regeneration.

DOI:http://dx.doi.org/10.7554/eLife.22911.002

Cell division orientation is coupled to cell-cell adhesion by the E-cadherin/LGN complex

Both cell–cell adhesion and oriented cell division play prominent roles in establishing tissue architecture, but it is unclear how they might be coordinated. Here, we demonstrate that the cell–cell adhesion protein E-cadherin functions as an instructive cue for cell division orientation. This is mediated by the evolutionarily conserved LGN/NuMA complex, which regulates cortical attachments of astral spindle microtubules. We show that LGN, which adopts a three-dimensional structure similar to cadherin-bound catenins, binds directly to the E-cadherin cytosolic tail and thereby localizes at cell–cell adhesions. On mitotic entry, NuMA is released from the nucleus and competes LGN from E-cadherin to locally form the LGN/NuMA complex. This mediates the stabilization of cortical associations of astral microtubules at cell–cell adhesions to orient the mitotic spindle. Our results show how E-cadherin instructs the assembly of the LGN/NuMA complex at cell–cell contacts, and define a mechanism that couples cell division orientation to intercellular adhesion.

Cell–cell adhesion and oriented cell division play key roles in tissue architecture, but how they are coordinated is not known. Here, the authors show that E-cadherin interacts with LGN, and thereby provides a cortical cue that serves to stabilize cortical attachment of astral microtubules at cell–cell adhesions, thus orienting the mitotic spindle.

A basal cell defect promotes budding of prostatic intraepithelial neoplasia

Basal cells in a simple secretory epithelium adhere to the extracellular matrix (ECM), providing contextual cues for ordered repopulation of the luminal cell layer. Early high-grade prostatic intraepithelial neoplasia (HG-PIN) tissue has enlarged nuclei and nucleoli, luminal layer expansion and genomic instability. Additional HG-PIN markers include loss of α6β4 integrin or its ligand laminin-332, and budding of tumor clusters into laminin-511-rich stroma. We modeled the invasive budding phenotype by reducing expression of α6β4 integrin in spheroids formed from two normal human stable isogenic prostate epithelial cell lines (RWPE-1 and PrEC 11220). These normal cells continuously spun in culture, forming multicellular spheroids containing an outer laminin-332 layer, basal cells (expressing α6β4 integrin, high-molecular-weight cytokeratin and p63, also known as TP63) and luminal cells that secrete PSA (also known as KLK3). Basal cells were optimally positioned relative to the laminin-332 layer as determined by spindle orientation. β4-integrin-defective spheroids contained a discontinuous laminin-332 layer corresponding to regions of abnormal budding. This 3D model can be readily used to study mechanisms that disrupt laminin-332 continuity, for example, defects in the essential adhesion receptor (β4 integrin), laminin-332 or abnormal luminal expansion during HG-PIN progression.

Symmetry Does not Come for Free: Cellular Mechanisms to Achieve a Symmetric Cell Division

During mitosis cells can divide symmetrically to proliferate or asymmetrically to generate tissue diversity. While the mechanisms that ensure asymmetric cell division have been extensively studied, it is often assumed that a symmetric cell division is the default outcome of mitosis. Recent studies, however, imply that the symmetric nature of cell division is actively controlled, as they reveal numerous mechanisms that ensure the formation of equal-sized daughter cells as cells progress through cell division. Here we review our current knowledge of these mechanisms and highlight possible key questions in the field.

Centrosomes in spindle organization and chromosome segregation: a mechanistic view

Centrosomes are complex structures, which are embedded into the opposite poles of the mitotic spindle of most animals, acting as microtubule organizing centres. Surprisingly, in several biological systems, such as flies, chicken, or human cells, centrosomes are not essential for cell division. Nonetheless, they ensure faithful chromosome segregation. Moreover, mis-functioning centrosomes can act in a dominant-negative manner, resulting in erroneous mitotic progression. Here, I review the mechanisms by which centrosomes contribute to proper spindle organization and faithful chromosome segregation under physiological conditions and discuss how errors in centrosome function impair transmission of the genomic material in a pathological setting.

PRL-3 disrupts epithelial architecture by altering the post-mitotic midbody position

Disruption of epithelial architecture is a fundamental event during epithelial tumorigenesis. We show that the expression of the cancer-promoting phosphatase PRL-3 (PTP4A3), which is overexpressed in several epithelial cancers, in polarized epithelial MDCK and Caco2 cells leads to invasion and the formation of multiple ectopic, fully polarized lumens in cysts. Both processes disrupt epithelial architecture and are hallmarks of cancer. The pathological relevance of these findings is supported by the knockdown of endogenous PRL-3 in MCF-7 breast cancer cells grown in three-dimensional branched structures, showing the rescue from multiple-lumen- to single-lumen-containing branch ends. Mechanistically, it has been previously shown that ectopic lumens can arise from midbodies that have been mislocalized through the loss of mitotic spindle orientation or through the loss of asymmetric abscission. Here, we show that PRL-3 triggers ectopic lumen formation through midbody mispositioning without altering the spindle orientation or asymmetric abscission, instead, PRL-3 accelerates cytokinesis, suggesting that this process is an alternative new mechanism for ectopic lumen formation in MDCK cysts. The disruption of epithelial architecture by PRL-3 revealed here is a newly recognized mechanism for PRL-3-promoted cancer progression.

Detection and tracking of overlapping cell nuclei for large scale mitosis analyses

Background

Cell culture on printed micropatterns slides combined with automated fluorescent microscopy allows for extraction of tens of thousands of videos of small isolated growing cell clusters. The analysis of such large dataset in space and time is of great interest to the community in order to identify factors involved in cell growth, cell division or tissue formation by testing multiples conditions. However, cells growing on a micropattern tend to be tightly packed and to overlap with each other. Consequently, image analysis of those large dynamic datasets with no possible human intervention has proven impossible using state of the art automated cell detection methods.

Results

Here, we propose a fully automated image analysis approach to estimate the number, the location and the shape of each cell nucleus, in clusters at high throughput. The method is based on a robust fit of Gaussian mixture models with two and three components on each frame followed by an analysis over time of the fitting residual and two other relevant features. We use it to identify with high precision the very first frame containing three cells. This allows in our case to measure a cell division angle on each video and to construct division angle distributions for each tested condition. We demonstrate the accuracy of our method by validating it against manual annotation on about 4000 videos of cell clusters.

Conclusions

The proposed approach enables the high throughput analysis of video sequences of isolated cell clusters obtained using micropatterns. It relies only on two parameters that can be set robustly as they reduce to the average cell size and intensity.

A ligand-independent integrin β1 mechanosensory complex guides spindle orientation

Control of spindle orientation is a fundamental process for embryonic development, morphogenesis and tissue homeostasis, while defects are associated with tumorigenesis and other diseases. Force sensing is one of the mechanisms through which division orientation is determined. Here we show that integrin β1 plays a critical role in this process, becoming activated at the lateral regions of the cell cortex in a ligand-independent manner. This activation is force dependent and polar, correlating with the spindle capture sites. Inhibition of integrin β1 activation on the cortex and disruption of its asymmetric distribution leads to spindle misorientation, even when cell adhesion is β1 independent. Examining downstream targets reveals that a cortical mechanosensory complex forms on active β1, and regulates spindle orientation irrespective of cell context. We propose that ligand-independent integrin β1 activation is a conserved mechanism that allows cell responses to external stimuli.

The dynein cortical anchor Num1 activates dynein motility by relieving Pac1/LIS1-mediated inhibition

Upon offloading to Num1 cortical receptor sites in budding yeast, cytoplasmic dynein motility is switched “on” by a mechanism that likely involves Num1-mediated dissociation of the Pac1 inhibitor, a homologue of human LIS1.

Cortically anchored dynein orients the spindle through interactions with astral microtubules. In budding yeast, dynein is offloaded to Num1 receptors from microtubule plus ends. Rather than walking toward minus ends, dynein remains associated with plus ends due in part to its association with Pac1/LIS1, an inhibitor of dynein motility. The mechanism by which dynein is switched from “off” at the plus ends to “on” at the cell cortex remains unknown. Here, we show that overexpression of the coiled-coil domain of Num1 specifically depletes dynein–dynactin–Pac1/LIS1 complexes from microtubule plus ends and reduces dynein-Pac1/LIS1 colocalization. Depletion of dynein from plus ends requires its microtubule-binding domain, suggesting that motility is required. An enhanced Pac1/LIS1 affinity mutant of dynein or overexpression of Pac1/LIS1 rescues dynein plus end depletion. Live-cell imaging reveals minus end–directed dynein–dynactin motility along microtubules upon overexpression of the coiled-coil domain of Num1, an event that is not observed in wild-type cells. Our findings indicate that dynein activity is directly switched “on” by Num1, which induces Pac1/LIS1 removal.

Connections between cadherin-catenin proteins, spindle misorientation, and cancer

Cadherin-catenin mediated adhesion is an important determinant of tissue architecture in multicellular organisms. Cancer progression and maintenance is frequently associated with loss of their expression or functional activity, which not only leads to decreased cell-cell adhesion, but also to enhanced tumor cell proliferation and loss of differentiated characteristics. This review is focused on the emerging implications of cadherin-catenin proteins in the regulation of polarized divisions through their connections with the centrosomes, cytoskeleton, tissue tension and signaling pathways; and illustrates how alterations in cadherin-catenin levels or functional activity may render cells susceptible to transformation through the loss of their proliferation-differentiation balance.

Lateral adhesion drives reintegration of misplaced cells into epithelial monolayers

Cells in simple epithelia orient their mitotic spindles in the plane of the epithelium so that both daughter cells are born within the epithelial sheet. This is assumed to be important to maintain epithelial integrity and prevent hyperplasia, because misaligned divisions give rise to cells outside the epithelium. Here we test this assumption in three types of Drosophila epithelium; the cuboidal follicle epithelium, the columnar early embryonic ectoderm, and the pseudostratified neuroepithelium. Ectopic expression of Inscuteable in these tissues reorients mitotic spindles, resulting in one daughter cell being born outside the epithelial layer. Live imaging reveals that these misplaced cells reintegrate into the tissue. Reducing the levels of the lateral homophilic adhesion molecules Neuroglian or Fasciclin 2 disrupts reintegration, giving rise to extra-epithelial cells, whereas disruption of adherens junctions has no effect. Thus, the reinsertion of misplaced cells seems to be driven by lateral adhesion, which pulls cells born outside the epithelial layer back into it. Our findings reveal a robust mechanism that protects epithelia against the consequences of misoriented divisions.

The hallmarks of cancer: relevance to the pathogenesis of polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a progressive inherited disorder in which renal tissue is gradually replaced with fluid-filled cysts, giving rise to chronic kidney disease (CKD) and progressive loss of renal function. ADPKD is also associated with liver ductal cysts, hypertension, chronic pain and extra-renal problems such as cerebral aneurysms. Intriguingly, improved understanding of the signalling and pathological derangements characteristic of ADPKD has revealed marked similarities to those of solid tumours, even though the gross presentation of tumours and the greater morbidity and mortality associated with tumour invasion and metastasis would initially suggest entirely different disease processes. The commonalities between ADPKD and cancer are provocative, particularly in the context of recent preclinical and clinical studies of ADPKD that have shown promise with drugs that were originally developed for cancer. The potential therapeutic benefit of such repurposing has led us to review in detail the pathological features of ADPKD through the lens of the defined, classic hallmarks of cancer. In addition, we have evaluated features typical of ADPKD, and determined whether evidence supports the presence of such features in cancer cells. This analysis, which places pathological processes in the context of defined signalling pathways and approved signalling inhibitors, highlights potential avenues for further research and therapeutic exploitation in both diseases.

Direct Microtubule-Binding by Myosin-10 Orients Centrosomes toward Retraction Fibers and Subcortical Actin Clouds

Positioning of centrosomes is vital for cell division and development. In metazoan cells, spindle positioning is controlled by a dynamic pool of subcortical actin that organizes in response to the position of retraction fibers. These actin "clouds" are proposed to generate pulling forces on centrosomes and mediate spindle orientation. However, the motors that pull astral microtubules toward these actin structures are not known. Here, we report that the unconventional myosin, Myo10, couples actin-dependent forces from retraction fibers and subcortical actin clouds to centrosomes. Myo10-mediated centrosome positioning requires its direct microtubule binding. Computational image analysis of large microtubule populations reveals a direct effect of Myo10 on microtubule dynamics and microtubule-cortex interactions. Myo10's role in centrosome positioning is distinct from, but overlaps with, that of dynein. Thus, Myo10 plays a key role in integrating the actin and microtubule cytoskeletons to position centrosomes and mitotic spindles.

Regulation of NDR1 activity by PLK1 ensures proper spindle orientation in mitosis

Accurate chromosome segregation during mitosis requires the physical separation of sister chromatids which depends on correct position of mitotic spindle relative to membrane cortex. Although recent work has identified the role of PLK1 in spindle orientation, the mechanisms underlying PLK1 signaling in spindle positioning and orientation have not been fully illustrated. Here, we identified a conserved signaling axis in which NDR1 kinase activity is regulated by PLK1 in mitosis. PLK1 phosphorylates NDR1 at three putative threonine residues (T7, T183 and T407) at mitotic entry, which elicits PLK1-dependent suppression of NDR1 activity and ensures correct spindle orientation in mitosis. Importantly, persistent expression of non-phosphorylatable NDR1 mutant perturbs spindle orientation. Mechanistically, PLK1-mediated phosphorylation protects the binding of Mob1 to NDR1 and subsequent NDR1 activation. These findings define a conserved signaling axis that integrates dynamic kinetochore-microtubule interaction and spindle orientation control to genomic stability maintenance.

Syndecan defines precise spindle orientation by modulating Wnt signaling in C. elegans

Wnt signals orient mitotic spindles in development, but it remains unclear how Wnt signaling is spatially controlled to achieve precise spindle orientation. Here, we show that C. elegans syndecan (SDN-1) is required for precise orientation of a mitotic spindle in response to a Wnt cue. We find that SDN-1 is the predominant heparan sulfate (HS) proteoglycan in the early C. elegans embryo, and that loss of HS biosynthesis or of the SDN-1 core protein results in misorientation of the spindle of the ABar blastomere. The ABar and EMS spindles both reorient in response to Wnt signals, but only ABar spindle reorientation is dependent on a new cell contact and on HS and SDN-1. SDN-1 transiently accumulates on the ABar surface as it contacts C, and is required for local concentration of Dishevelled (MIG-5) in the ABar cortex adjacent to C. These findings establish a new role for syndecan in Wnt-dependent spindle orientation.

Dlg1 controls planar spindle orientation in the neuroepithelium through direct interaction with LGN

Dlg1 recruits LGN to the cortex of cells in the chick neuroepithelium and may provide instructive cues that drive planar spindle orientation.

Oriented cell divisions are necessary for the development of epithelial structures. Mitotic spindle orientation requires the precise localization of force generators at the cell cortex via the evolutionarily conserved LGN complex. However, polarity cues acting upstream of this complex in vivo in the vertebrate epithelia remain unknown. In this paper, we show that Dlg1 is localized at the basolateral cell cortex during mitosis and is necessary for planar spindle orientation in the chick neuroepithelium. Live imaging revealed that Dlg1 is required for directed spindle movements during metaphase. Mechanistically, we show that direct interaction between Dlg1 and LGN promotes cortical localization of the LGN complex. Furthermore, in human cells dividing on adhesive micropatterns, homogenously localized Dlg1 recruited LGN to the mitotic cortex and was also necessary for proper spindle orientation. We propose that Dlg1 acts primarily to recruit LGN to the cortex and that Dlg1 localization may additionally provide instructive cues for spindle orientation.

PTEN phosphatase-independent maintenance of glandular morphology in a predictive colorectal cancer model system

Organotypic models may provide mechanistic insight into colorectal cancer (CRC) morphology. Three-dimensional (3D) colorectal gland formation is regulated by phosphatase and tensin homologue deleted on chromosome 10 (PTEN) coupling of cell division cycle 42 (cdc42) to atypical protein kinase C (aPKC). This study investigated PTEN phosphatase-dependent and phosphatase-independent morphogenic functions in 3D models and assessed translational relevance in human studies. Isogenic PTEN-expressing or PTEN-deficient 3D colorectal cultures were used. In translational studies, apical aPKC activity readout was assessed against apical membrane (AM) orientation and gland morphology in 3D models and human CRC. We found that catalytically active or inactive PTEN constructs containing an intact C2 domain enhanced cdc42 activity, whereas mutants of the C2 domain calcium binding region 3 membrane-binding loop (M-CBR3) were ineffective. The isolated PTEN C2 domain (C2) accumulated in membrane fractions, but C2 M-CBR3 remained in cytosol. Transfection of C2 but not C2 M-CBR3 rescued defective AM orientation and 3D morphogenesis of PTEN-deficient Caco-2 cultures. The signal intensity of apical phospho-aPKC correlated with that of Na(+)/H(+) exchanger regulatory factor-1 (NHERF-1) in the 3D model. Apical NHERF-1 intensity thus provided readout of apical aPKC activity and associated with glandular morphology in the model system and human colon. Low apical NHERF-1 intensity in CRC associated with disruption of glandular architecture, high cancer grade, and metastatic dissemination. We conclude that the membrane-binding function of the catalytically inert PTEN C2 domain influences cdc42/aPKC-dependent AM dynamics and gland formation in a highly relevant 3D CRC morphogenesis model system.

Force and the spindle: mechanical cues in mitotic spindle orientation

The mechanical environment of a cell has a profound effect on its behaviour, from dictating cell shape to driving the transcription of specific genes. Recent studies have demonstrated that mechanical forces play a key role in orienting the mitotic spindle, and therefore cell division, in both single cells and tissues. Whilst the molecular machinery that mediates the link between external force and the mitotic spindle remains largely unknown, it is becoming increasingly clear that this is a widely used mechanism which could prove vital for coordinating cell division orientation across tissues in a variety of contexts.

Accumulation of extracellular hyaluronan by hyaluronan synthase 3 promotes tumor growth and modulates the pancreatic cancer microenvironment

Extensive accumulation of the glycosaminoglycan hyaluronan is found in pancreatic cancer. The role of hyaluronan synthases 2 and 3 (HAS2, 3) was investigated in pancreatic cancer growth and the tumor microenvironment. Overexpression of HAS3 increased hyaluronan synthesis in BxPC-3 pancreatic cancer cells. In vivo, overexpression of HAS3 led to faster growing xenograft tumors with abundant extracellular hyaluronan accumulation. Treatment with pegylated human recombinant hyaluronidase (PEGPH20) removed extracellular hyaluronan and dramatically decreased the growth rate of BxPC-3 HAS3 tumors compared to parental tumors. PEGPH20 had a weaker effect on HAS2-overexpressing tumors which grew more slowly and contained both extracellular and intracellular hyaluronan. Accumulation of hyaluronan was associated with loss of plasma membrane E-cadherin and accumulation of cytoplasmic β-catenin, suggesting disruption of adherens junctions. PEGPH20 decreased the amount of nuclear hypoxia-related proteins and induced translocation of E-cadherin and β-catenin to the plasma membrane. Translocation of E-cadherin was also seen in tumors from a transgenic mouse model of pancreatic cancer and in a human non-small cell lung cancer sample from a patient treated with PEGPH20. In conclusion, hyaluronan accumulation by HAS3 favors pancreatic cancer growth, at least in part by decreasing epithelial cell adhesion, and PEGPH20 inhibits these changes and suppresses tumor growth.

Spindle orientation: what if it goes wrong?

The angle of cell division is critical in at least two contexts. It can determine cell fate, as it does in developing neural tissue. It can also dictate tissue architecture, as it does in many epithelia. One way to ensure the correct angle of cell division is through controlled orientation of the spindle at metaphase. What happens when that control is lost? Ongoing work suggests that the consequence of metaphase spindle misorientation may be significant, but multiple mechanisms exist to protect the cell and the tissue. We speculate that one such mechanism involves a recently identified anaphase activity for two of the key players at metaphase: NuMA (Mud, LIN-5) and dynein.

Scribble modulates the MAPK/Fra1 pathway to disrupt luminal and ductal integrity and suppress tumour formation in the mammary gland

Polarity coordinates cell movement, differentiation, proliferation and apoptosis to build and maintain complex epithelial tissues such as the mammary gland. Loss of polarity and the deregulation of these processes are critical events in malignant progression but precisely how and at which stage polarity loss impacts on mammary development and tumourigenesis is unclear. Scrib is a core polarity regulator and tumour suppressor gene however to date our understanding of Scrib function in the mammary gland has been limited to cell culture and transplantation studies of cell lines. Utilizing a conditional mouse model of Scrib loss we report for the first time that Scrib is essential for mammary duct morphogenesis, mammary progenitor cell fate and maintenance, and we demonstrate a critical and specific role for Scribble in the control of the early steps of breast cancer progression. In particular, Scrib-deficiency significantly induced Fra1 expression and basal progenitor clonogenicity, which resulted in fully penetrant ductal hyperplasia characterized by high cell turnover, MAPK hyperactivity, frank polarity loss with mixing of apical and basolateral membrane constituents and expansion of atypical luminal cells. We also show for the first time a role for Scribble in mammalian spindle orientation with the onset of mammary hyperplasia being associated with aberrant luminal cell spindle orientation and a failure to apoptose during the final stage of duct tubulogenesis. Restoring MAPK/Fra1 to baseline levels prevented Scrib-hyperplasia, whereas persistent Scrib deficiency induced alveolar hyperplasia and increased the incidence, onset and grade of mammary tumours. These findings, based on a definitive genetic mouse model provide fundamental insights into mammary duct maturation and homeostasis and reveal that Scrib loss activates a MAPK/Fra1 pathway that alters mammary progenitor activity to drive premalignancy and accelerate tumour progression.

The special case of hepatocytes: unique tissue architecture calls for a distinct mode of cell division

Columnar epithelia (e.g., kidney, intestine) and hepatocytes embody the two major organizational phenotypes of non-stratified epithelial cells. Columnar epithelia establish their apical and basal domains at opposing poles and organize in monolayered cysts and tubules, in which their apical surfaces form a single continuous lumen whereas hepatocytes establish their apical domains in the midst of their basolateral domains and organize a highly branched capillary luminal network, the bile canaliculi, in which a single hepatocyte can engage in lumen formation with multiple neighbors. To maintain their distinct tissue architectures, columnar epithelial cells bisect their luminal domains during symmetric cell divisions, while the cleavage furrow in dividing hepatocytes avoids bisecting the bile canalicular domains. We discuss recently discovered molecular mechanisms that underlie the different cell division phenotypes in columnar and hepatocytic model cell lines. The serine/threonine kinase Par1b determines both the epithelial lumen polarity and cell division phenotype via cell adhesion signaling that converges on the small GTPase RhoA.

Plk2 regulates mitotic spindle orientation and mammary gland development

Disruptions in polarity and mitotic spindle orientation contribute to the progression and evolution of tumorigenesis. However, little is known about the molecular mechanisms regulating these processes in vivo. Here, we demonstrate that Polo-like kinase 2 (Plk2) regulates mitotic spindle orientation in the mammary gland and that this might account for its suggested role as a tumor suppressor. Plk2 is highly expressed in the mammary gland and is required for proper mammary gland development. Loss of Plk2 leads to increased mammary epithelial cell proliferation and ductal hyperbranching. Additionally, a novel role for Plk2 in regulating the orientation of the mitotic spindle and maintaining proper cell polarity in the ductal epithelium was discovered. In support of a tumor suppressor function for Plk2, loss of Plk2 increased the formation of lesions in multiparous glands. Collectively, these results demonstrate a novel role for Plk2 in regulating mammary gland development.

CYLD regulates spindle orientation by stabilizing astral microtubules and promoting dishevelled-NuMA-dynein/dynactin complex formation

Oriented cell division is critical for cell fate specification, tissue organization, and tissue homeostasis, and relies on proper orientation of the mitotic spindle. The molecular mechanisms underlying the regulation of spindle orientation remain largely unknown. Herein, we identify a critical role for cylindromatosis (CYLD), a deubiquitinase and regulator of microtubule dynamics, in the control of spindle orientation. CYLD is highly expressed in mitosis and promotes spindle orientation by stabilizing astral microtubules and deubiquitinating the cortical polarity protein dishevelled. The deubiquitination of dishevelled enhances its interaction with nuclear mitotic apparatus, stimulating the cortical localization of nuclear mitotic apparatus and the dynein/dynactin motor complex, a requirement for generating pulling forces on astral microtubules. These findings uncover CYLD as an important player in the orientation of the mitotic spindle and cell division and have important implications in health and disease.

NETs and cell cycle regulation

There are many ways that the nuclear envelope can influence the cell cycle. In addition to roles of lamins in regulating the master cell cycle regulator pRb and nuclear envelope breakdown in mitosis, many other nuclear envelope proteins influence the cell cycle through regulatory or structural functions. Of particular note among these are the nuclear envelope transmembrane proteins (NETs) that appear to influence cell cycle regulation through multiple separate mechanisms. Some NETs and other nuclear envelope proteins accumulate on the mitotic spindle, suggesting functional or structural roles in the cell cycle. In interphase exogenous overexpression of some NETs promotes an increase in G1 populations, while others promote an increase in G2/M populations, sometimes associated with the induction of senescence. Intriguingly, most of the NETs linked to the cell cycle are highly restricted in their tissue expression; thus, their misregulation in cancer could contribute to the many tissue-specific types of cancer.

Par1b induces asymmetric inheritance of plasma membrane domains via LGN-dependent mitotic spindle orientation in proliferating hepatocytes

Proliferating hepatocytes in the liver show an atypical, asymmetric mode of cell division, which is coordinated by Par1b and LGN and may explain the unique tissue architecture of the liver.

The development and maintenance of polarized epithelial tissue requires a tightly controlled orientation of mitotic cell division relative to the apical polarity axis. Hepatocytes display a unique polarized architecture. We demonstrate that mitotic hepatocytes asymmetrically segregate their apical plasma membrane domain to the nascent daughter cells. The non-polarized nascent daughter cell can form a de novo apical domain with its new neighbor. This asymmetric segregation of apical domains is facilitated by a geometrically distinct “apicolateral” subdomain of the lateral surface present in hepatocytes. The polarity protein partitioning-defective 1/microtubule-affinity regulating kinase 2 (Par1b/MARK2) translates this positional landmark to cortical polarity by promoting the apicolateral accumulation of Leu-Gly-Asn repeat-enriched protein (LGN) and the capture of nuclear mitotic apparatus protein (NuMA)–positive astral microtubules to orientate the mitotic spindle. Proliferating hepatocytes thus display an asymmetric inheritance of their apical domains via a mechanism that involves Par1b and LGN, which we postulate serves the unique tissue architecture of the developing liver parenchyma.

The development and maintenance of the polarized epithelial architecture and function of organs that form tubular “lumen” structures is important for normal physiology and, when deregulated, gives rise to disease. Recent studies have highlighted the importance of a strict coordination of the orientation of mitotic divisions relative to an internal axis of asymmetry in proliferating epithelial cells during this process. Hepatocytes are the predominant epithelial cells of the liver. Hepatocytes display a unique lumen-forming architecture and cellular asymmetry, but the molecular basis for this special polarized architecture is not well understood. Our study now reveals an unexpected mode of plasma membrane domain inheritance that is coupled to a cellular axis of asymmetry in proliferating mammalian hepatocytes. We show that mitotic hepatocytes asymmetrically segregate their apical plasma membrane (the membrane facing the lumen structure) along with the lumen to their daughter cells. We demonstrate that the coordinated action of two proteins, Par1b and LGN, constitutes a fundamental part of the underlying molecular mechanism. This coupling of cell division and polarity in hepatocytes is distinct from that established in other epithelial cell types. These findings are important for understanding the unique polarized tissue architecture in the developing liver.

Nucleoporin Nup62 maintains centrosome homeostasis

Centrosomes are comprised of 2 orthogonally arranged centrioles surrounded by the pericentriolar material (PCM), which serves as the main microtubule organizing center of the animal cell. More importantly, centrosomes also control spindle polarity and orientation during mitosis. Recently, we and other investigators discovered that several nucleoporins play critical roles during cell division. Here, we show that nucleoporin Nup62 plays a novel role in centrosome integrity. Knockdown of Nup62 induced mitotic arrest in G₂/M phases and mitotic cell death. Depletion of Nup62 using RNA interference results in defective centrosome segregation and centriole maturation during the G₂ phase. Moreover, Nup62 depletion in human cells leads to the appearance of multinucleated cells and induces the formation of multipolar centrosomes, centriole synthesis defects, dramatic spindle orientation defects, and centrosome component rearrangements that impair cell bi-polarity. Our results also point to a potential role of Nup62 in targeting gamma-tubulin and SAS-6 to the centrioles.

Breaking the epithelial polarity barrier in cancer: the strange case of LKB1/PAR-4

The PAR clan of polarity regulating genes was initially discovered in a genetic screen searching for genes involved in asymmetric cell divisions in the Caenorhabditis elegans embryo. Today, investigations in worms, flies and mammals have established PAR proteins as conserved and fundamental regulators of animal cell polarization in a broad range of biological phenomena requiring cellular asymmetries. The human homologue of invertebrate PAR-4, a serine-threonine kinase LKB1/STK11, has caught attention as a gene behind Peutz-Jeghers polyposis syndrome and as a bona fide tumour suppressor gene commonly mutated in sporadic cancer. LKB1 functions as a master regulator of AMP-activated protein kinase (AMPK) and 12 other kinases referred to as the AMPK-related kinases, including four human homologues of PAR-1. The role of LKB1 as part of the energy sensing LKB1-AMPK module has been intensively studied, whereas the polarity function of LKB1, in the context of homoeostasis or cancer, has gained less attention. Here, we focus on the PAR-4 identity of LKB1, discussing the weight of evidence indicating a role for LKB1 in regulation of cell polarity and epithelial integrity across species and highlight recent investigations providing new insight into the old question: does the PAR-4 identity of LKB1 matter in cancer?

A cancer-associated mutation in atypical protein kinase Cι occurs in a substrate-specific recruitment motif

Atypical protein kinase Cι (PKCι) has roles in cell growth, cellular polarity, and migration, and its abundance is frequently increased in cancer. We identified a protein interaction surface containing a dibasic motif (RIPR) that bound a distinct subset of PKCι substrates including lethal giant larvae 2 (LLGL2) and myosin X, but not other substrates such as Par3. Further characterization demonstrated that Arg471 in this motif was important for binding to LLGL2, whereas Arg474 was critical for interaction with myosin X, indicating that multiple complexes could be formed through this motif. A somatic mutation of the dibasic motif (R471C) was the most frequent mutation of PKCι in human cancer, and the intact dibasic motif was required for normal polarized epithelial morphogenesis in three-dimensional cysts. Thus, the R471C substitution is a change-of-function mutation acting at this substrate-specific recruitment site to selectively disrupt the polarizing activity of PKCι.

Oriented divisions, fate decisions

During development, the establishment of proper tissue architecture depends upon the coordinated control of cell divisions not only in space and time, but also direction. Execution of an oriented cell division requires establishment of an axis of polarity and alignment of the mitotic spindle along this axis. Frequently, the cleavage plane also segregates fate determinants, either unequally or equally between daughter cells, the outcome of which is either an asymmetric or symmetric division, respectively. The last few years have witnessed tremendous growth in understanding both the extrinsic and intrinsic cues that position the mitotic spindle, the varied mechanisms in which the spindle orientation machinery is controlled in diverse organisms and organ systems, and the manner in which the division axis influences the signaling pathways that direct cell fate choices.

Epithelial junctions maintain tissue architecture by directing planar spindle orientation

During epithelial cell proliferation, planar alignment of the mitotic spindle coordinates the local process of symmetric cell cleavage with the global maintenance of polarized tissue architecture. Although the disruption of planar spindle alignment is proposed to cause epithelial to mesenchymal transition and cancer, the in vivo mechanisms regulating mitotic spindle orientation remain elusive. Here we demonstrate that the actomyosin cortex and the junction-localized neoplastic tumour suppressors Scribbled and Discs large 1 have essential roles in planar spindle alignment and thus the control of epithelial integrity in the Drosophila imaginal disc. We show that defective alignment of the mitotic spindle correlates with cell delamination and apoptotic death, and that blocking the death of misaligned cells is sufficient to drive the formation of basally localized tumour-like masses. These findings indicate a key role for junction-mediated spindle alignment in the maintenance of epithelial integrity, and also reveal a previously unknown cell-death-mediated tumour-suppressor function inherent in the polarized architecture of epithelia.

Formin-mediated actin polymerization cooperates with Mushroom body defect (Mud)-Dynein during Frizzled-Dishevelled spindle orientation

To position the mitotic spindle, cytoskeletal components must be coordinated to generate cortical forces on astral microtubules. Although the dynein motor is common to many spindle orientation systems, 'accessory pathways' are often also required. In this work, we identified an accessory spindle orientation pathway in Drosophila that functions with Dynein during planar cell polarity, downstream of the Frizzled (Fz) effector Dishevelled (Dsh). Dsh contains a PDZ ligand and a Dynein-recruiting DEP domain that are both required for spindle orientation. The Dsh PDZ ligand recruits Canoe/Afadin and ultimately leads to Rho GTPase signaling mediated through RhoGEF2. The formin Diaphanous (Dia) functions as the Rho effector in this pathway, inducing F-actin enrichment at sites of cortical Dsh. Chimeric protein experiments show that the Dia-actin accessory pathway can be replaced by an independent kinesin (Khc73) accessory pathway for Dsh-mediated spindle orientation. Our results define two 'modular' spindle orientation pathways and show an essential role for actin regulation in Dsh-mediated spindle orientation.

Sox9 induction, ectopic Paneth cells, and mitotic spindle axis defects in mouse colon adenomatous epithelium arising from conditional biallelic Apc inactivation

We generated transgenic mice in which human CDX2 gene elements control expression of a tamoxifen-regulated Cre protein (CDX2P-CreER(T2)) to allow for inducible gene targeting in intestinal epithelium. After tamoxifen dosing of CDX2P-CreER(T2) mice, Cre activity was detected in the distal ileal, cecal, colonic, and rectal epithelium, with selected crypt base, transit amplifying, and surface cells all capable of activating Cre function. Four weeks after tamoxifen dosing of CDX2P-CreER(T2) mice carrying a Cre-activated fluorescent reporter, single crypts were uniformly fluorescence positive or negative, reflecting Cre activation in crypt stem cells. Biallelic inactivation of the Apc tumor suppressor gene via the CDX2P-CreER(T2) transgene in colon epithelium led to acute alterations in cell proliferation, apoptosis, and morphology, along with mitotic spindle misorientation, β-catenin nuclear localization, and induction of the intestinal stem cell markers Lgr5 and Musashi-1 and the Sox9 transcription factor. Normal mouse colon epithelium lacks Paneth cells, a key small intestine niche cell type, and Paneth cell differentiation is dependent on Sox9 function. In Apc-deficient colon epithelium, ectopic Paneth-like cells were seen outside the crypt base, such as new crypt budding sites. Our data indicate Apc inactivation via CDX2P-CreER(T2) targeting in mouse colon epithelium is sufficient to induce adenomatous changes and the generation of Paneth-like cells from neoplastic progenitors, with potentially significant roles in colon adenoma development and progression.

Molecular pathways regulating mitotic spindle orientation in animal cells

Orientation of the cell division axis is essential for the correct development and maintenance of tissue morphology, both for symmetric cell divisions and for the asymmetric distribution of fate determinants during, for example, stem cell divisions. Oriented cell division depends on the positioning of the mitotic spindle relative to an axis of polarity. Recent studies have illuminated an expanding list of spindle orientation regulators, and a molecular model for how cells couple cortical polarity with spindle positioning has begun to emerge. Here, we review both the well-established spindle orientation pathways and recently identified regulators, focusing on how communication between the cell cortex and the spindle is achieved, to provide a contemporary view of how positioning of the mitotic spindle occurs.

Parkin deficiency contributes to pancreatic tumorigenesis by inducing spindle multipolarity and misorientation

Parkin, an E3 ubiquitin ligase well known for its role in the pathogenesis of juvenile Parkinson disease, has been considered as a candidate tumor suppressor in certain types of cancer. It remains unknown whether parkin is involved in the development of pancreatic cancer, the fourth leading cause of cancer-related deaths worldwide. Herein, we demonstrate the downregulation and copy number loss of the parkin gene in human pancreatic cancer specimens. The expression of parkin negatively correlates with clinicopathological parameters indicating the malignancy of pancreatic cancer. In addition, knockdown of parkin expression promotes the proliferation and tumorigenic properties of pancreatic cancer cells both in vitro and in mice. We further find that parkin deficiency increases the proportion of cells with spindle multipolarity and multinucleation. Parkin-depleted cells also show a significant increase in spindle misorientation. These findings indicate crucial involvement of parkin deficiency in the pathogenesis of pancreatic cancer.

Mitotic spindle (DIS)orientation and DISease: cause or consequence?

Correct alignment of the mitotic spindle during cell division is crucial for cell fate determination, tissue organization, and development. Mutations causing brain diseases and cancer in humans and mice have been associated with spindle orientation defects. These defects are thought to lead to an imbalance between symmetric and asymmetric divisions, causing reduced or excessive cell proliferation. However, most of these disease-linked genes encode proteins that carry out multiple cellular functions. Here, we discuss whether spindle orientation defects are the direct cause for these diseases, or just a correlative side effect.

Molecular mechanisms in spindle positioning: structures and new concepts

Coordination of cell cleavage with respect to cell geometry, cell polarity and neighboring tissues is critical for tissue maintenance, malignant transformation and metastasis. The position of the mitotic spindle within the cell determines where cell cleavage occurs. Spindle positioning is often mediated through capture of astral microtubules by motor proteins at the cell cortex. Recently, the core dynein anchor complex has been structurally resolved. Junctional complexes were shown to provide additional capture sites for astral microtubules in proliferating tissues. Finally, latest studies show that signals from centrosomes control spindle positioning and propose novel concepts for generation of centrosome identity.

An oblique view on the role of spindle orientation in vertebrate neurogenesis

Neurogenesis is a dynamic process that produces a diverse number of glial and neural cell types from a limited number of neural stem cells throughout development and into adulthood. After an initial period of amplification through symmetric division, neural stem cells rely on asymmetric modes of division to self-renew while producing more committed progeny. Understanding the molecular mechanisms regulating the choice between symmetric and asymmetric modes of division is essential to understand human brain development and pathologies, and to explain the increasing cortical complexity observed in evolution. A popular model states the existence of a causal relationship between the orientation of the axis of division of stem cells and the fate of their progeny in many different tissues, but the validity of the model in neural stem cells is not clear. In this review, we briefly present the diversity of neural stem cells and intermediate progenitors in the developing central nervous system. We then draw a historic overview of the assumed causal relationship between spindle orientation and fate determination. We show how this prompted a search for regulators of spindle orientation, and present the current state of knowledge on the mechanism. Finally, we review data on the effect of defective spindle orientation and try to integrate conflicting observations by presenting alternative mechanisms that may regulate the choice between symmetric and asymmetric outcomes.

The tumor suppressor Apc controls planar cell polarities central to gut homeostasis

Asymmetric stem cell divisions controlled by Apc in the intestinal crypt result in regulated, anisotropic movement of daughter cells away from the niche.

The stem cells (SCs) at the bottom of intestinal crypts tightly contact niche-supporting cells and fuel the extraordinary tissue renewal of intestinal epithelia. Their fate is regulated stochastically by populational asymmetry, yet whether asymmetrical fate as a mode of SC division is relevant and whether the SC niche contains committed progenitors of the specialized cell types are under debate. We demonstrate spindle alignments and planar cell polarities, which form a novel functional unit that, in SCs, can yield daughter cell anisotropic movement away from niche-supporting cells. We propose that this contributes to SC homeostasis. Importantly, we demonstrate that some SC divisions are asymmetric with respect to cell fate and provide data suggesting that, in some SCs, mNumb displays asymmetric segregation. Some of these processes were altered in apparently normal crypts and microadenomas of mice carrying germline Apc mutations, shedding new light on the first stages of progression toward colorectal cancer.

The LKB1 tumor suppressor controls spindle orientation and localization of activated AMPK in mitotic epithelial cells

Orientation of mitotic spindles plays an integral role in determining the relative positions of daughter cells in a tissue. LKB1 is a tumor suppressor that controls cell polarity, metabolism, and microtubule stability. Here, we show that germline LKB1 mutation in mice impairs spindle orientation in cells of the upper gastrointestinal tract and causes dramatic mislocalization of the LKB1 substrate AMPK in mitotic cells. RNAi of LKB1 causes spindle misorientation in three-dimensional MDCK cell cysts. Maintaining proper spindle orientation, possibly mediated by effects on the downstream kinase AMPK, could be an important tumor suppressor function of LKB1.