Centrosome-Associated Degradation Limits β-Catenin Inheritance by Daughter Cells after Asymmetric Division

Modulating β-catenin homeostasis for cancer therapy

β-Catenin is a well-established driver of many cancers; however, there are challenges in developing agents targeting β-catenin for clinical use. Recent progress has indicated that most of the pathological changes in β-catenin may be commonly caused by loss of protein homeostasis. Modulation of β-catenin homeostasis, especially by hyperactivation of β-catenin, potentially leads to robust antitumor outcomes. Here, we comprehensively dissect the protein homeostasis of β-catenin in terms of time, compartmentalization, supramolecular assemblies, and dynamics, with emphasis on changes in β-catenin homeostasis upon oncogenic mutations. We propose that altered β-catenin homeostasis could be deleterious for β-catenin-dependent cancers and that modulation of β-catenin homeostasis offers a novel avenue for targeting β-catenin for cancer therapy.

SYS-1/beta-catenin inheritance and regulation by Wnt-signaling during asymmetric cell division

Asymmetric cell division (ACD) allows daughter cells of a polarized mother to acquire different developmental fates. In C. elegans , the Wnt/β-catenin Asymmetry (WβA) pathway oversees many embryonic and larval ACDs; here, a Wnt gradient induces an asymmetric distribution of Wnt signaling components within the dividing mother cell. One terminal nuclear effector of the WβA pathway is the transcriptional activator SYS-1/β-catenin. SYS-1 is sequentially negatively regulated during ACD; first by centrosomal regulation and subsequent proteasomal degradation and second by asymmetric activity of the β-catenin "destruction complex" in one of the two daughter cells, which decreases SYS-1 levels in the absence of WβA signaling. However, the extent to which mother cell SYS-1 influences cell fate decisions of the daughters is unknown. Here, we quantify inherited SYS-1 in the differentiating daughter cells and the role of SYS-1 inheritance in Wnt-directed ACD. Photobleaching experiments demonstrate the GFP::SYS-1 present in daughter cell nuclei is comprised of inherited and de novo translated SYS-1 pools. We used a photoconvertible DENDRA2::SYS-1, to directly observe the dynamics of inherited SYS-1. Photoconversion during mitosis reveals that SYS-1 clearance at the centrosome preferentially degrades older SYS-1, and this accumulation is regulated via dynein trafficking. Photoconversion of the EMS cell during Wnt-driven ACD shows daughter cell inheritance of mother cell SYS-1. Additionally, loss of centrosomal SYS-1 increased inherited SYS-1 and, surprisingly, loss of centrosomal SYS-1 also resulted in increased levels of de novo SYS-1 in both EMS daughter cells. Lastly, we show that daughter cell negative regulation of SYS-1 via the destruction complex member APR-1/APC is key to limit both the de novo and the inherited SYS-1 pools in both the E and the MS cells. We conclude that regulation of both inherited and newly translated SYS-1 via centrosomal processing in the mother cell and daughter cell regulation via Wnt signaling are critical to maintain sister SYS-1 asymmetry during ACD.

Wnt signalling in cell division: from mechanisms to tissue engineering

Wnt signalling is an essential player in tissue formation, notably in the regulation of stem cell function. Wnt signalling is best known for its roles in G1/S progression. However, a complex Wnt programme that also mediates mitotic progression and asymmetric cell division (ACD) is emerging. Recent developments in this area have provided mechanistic insights as well as tools to engineer or target Wnt signalling for translational and therapeutic purposes. Here, we discuss the bidirectional relationship between Wnt activity and mitosis. We emphasise how various Wnt-dependent mechanisms control spindle dynamics, chromosome segregation, and ACD. Finally, we illustrate how knowledge about these mechanisms has been successfully employed in tissue engineering for regenerative medicine applications.

pop-1/TCF, ref-2/ZIC and T-box factors regulate the development of anterior cells in the C. elegans embryo

Patterning of the anterior-posterior axis is fundamental to animal development. The Wnt pathway plays a major role in this process by activating the expression of posterior genes in animals from worms to humans. This observation raises the question of whether the Wnt pathway or other regulators control the expression of the many anterior-expressed genes. We found that the expression of five anterior-specific genes in Caenorhabditis elegans embryos depends on the Wnt pathway effectors pop-1/TCF and sys-1/β-catenin. We focused further on one of these anterior genes, ref-2/ZIC, a conserved transcription factor expressed in multiple anterior lineages. Live imaging of ref-2 mutant embryos identified defects in cell division timing and position in anterior lineages. Cis-regulatory dissection identified three ref-2 transcriptional enhancers, one of which is necessary and sufficient for anterior-specific expression. This enhancer is activated by the T-box transcription factors TBX-37 and TBX-38, and surprisingly, concatemerized TBX-37/38 binding sites are sufficient to drive anterior-biased expression alone, despite the broad expression of TBX-37 and TBX-38. Taken together, our results highlight the diverse mechanisms used to regulate anterior expression patterns in the embryo.

Centrosomal Enrichment and Proteasomal Degradation of SYS-1/β-catenin Requires the Microtubule Motor Dynein

The Caenorhabditis elegans Wnt/β-catenin asymmetry (WβA) pathway utilizes asymmetric regulation of SYS-1/β-catenin and POP-1/TCF coactivators. WβA differentially regulates gene expression during cell fate decisions, specifically by asymmetric localization of determinants in mother cells to produce daughters biased toward their appropriate cell fate. Despite the induction of asymmetry, β-catenin localizes symmetrically to mitotic centrosomes in both mammals and C. elegans. Owing to the mitosis-specific localization of SYS-1 to centrosomes and enrichment of SYS-1 at kinetochore microtubules when SYS-1 centrosomal loading is disrupted, we investigated active trafficking in SYS-1 centrosomal localization. Here, we demonstrate that trafficking by microtubule motor dynein is required to maintain SYS-1 centrosomal enrichment, by dynein RNA interference (RNAi)-mediated decreases in SYS-1 centrosomal enrichment and by temperature-sensitive allele of the dynein heavy chain. Conversely, we observe depletion of microtubules by nocodazole treatment or RNAi of dynein-proteasome adapter ECPS-1 exhibits increased centrosomal enrichment of SYS-1. Moreover, disruptions to SYS-1 or negative regulator microtubule trafficking are sufficient to significantly exacerbate SYS-1 dependent cell fate misspecifications. We propose a model whereby retrograde microtubule-mediated trafficking enables SYS-1 enrichment at centrosomes, enhancing its eventual proteasomal degradation. These studies support the link between centrosomal localization and enhancement of proteasomal degradation, particularly for proteins not generally considered “centrosomal.”

Centrosomes are required for proper β-catenin processing and Wnt response

The Wnt/β-catenin signaling pathway is central to metazoan development and routinely dysregulated in cancer. Wnt/β-catenin signaling initiates transcriptional reprogramming upon stabilization of the transcription factor β-catenin, which is otherwise posttranslationally processed by a destruction complex and degraded by the proteasome. Since various Wnt signaling components are enriched at centrosomes, we examined the functional contribution of centrosomes to Wnt signaling, β-catenin regulation, and posttranslational modifications. In HEK293 cells depleted of centrosomes we find that β-catenin synthesis and degradation rates are unaffected but that the normal accumulation of β-catenin in response to Wnt signaling is attenuated. This is due to accumulation of a novel high-molecular-weight form of phosphorylated β-catenin that is constitutively degraded in the absence of Wnt. Wnt signaling operates by inhibiting the destruction complex and thereby reducing destruction complex–phosphorylated β-catenin, but high-molecular-weight β-catenin is unexpectedly increased by Wnt signaling. Therefore these studies have identified a pool of β-catenin effectively shielded from regulation by Wnt. We present a model whereby centrosomes prevent inappropriate β-catenin modifications that antagonize normal stabilization by Wnt signals.

Re-enforcing hypoxia-induced polyploid cardiomyocytes enter cytokinesis through activation of β-catenin

Cardiomyocyte (CM) loss is a characteristic of various heart diseases, including ischaemic heart disease. Cardiac regeneration has been suggested as a promising strategy to address CM loss. Although many studies of regeneration have focused mainly on mononucleated or diploid CM, the limitations associated with the cytokinesis of polyploid and multinucleated CMs remain less well known. Here, we show that β-catenin, a key regulator in heart development, can increase cytokinesis in polyploid multinucleated CMs. The activation of β-catenin increases the expression of the cytokinesis-related factor epithelial cell transforming 2 (ECT2), which regulates the actomyosin ring and thus leads to the completion of cytokinesis in polyploid CMs. In addition, hypoxia can induce polyploid and multinucleated CMs by increasing factors related to the G1-S-anaphase of the cell cycle, but not those related to cytokinesis. Our study therefore reveals that the β-catenin can promote the cytokinesis of polyploid multinucleated CMs via upregulation of ECT2. These findings suggest a potential field of polyploid CM research that may be exploitable for cardiac regeneration therapy.

C. elegans Runx/CBFβ suppresses POP-1 TCF to convert asymmetric to proliferative division of stem cell-like seam cells

A correct balance between proliferative and asymmetric cell divisions underlies normal development, stem cell maintenance and tissue homeostasis. What determines whether cells undergo symmetric or asymmetric cell division is poorly understood. To gain insight into the mechanisms involved, we studied the stem cell-like seam cells in the Caenorhabditis elegans epidermis. Seam cells go through a reproducible pattern of asymmetric divisions, instructed by divergent canonical Wnt/β-catenin signaling, and symmetric divisions that increase the seam cell number. Using time-lapse fluorescence microscopy we observed that symmetric cell divisions maintain asymmetric localization of Wnt/β-catenin pathway components. Our observations, based on lineage-specific knockout and GFP-tagging of endogenous pop-1, support the model that POP-1^TCF induces differentiation at a high nuclear level, whereas low nuclear POP-1 promotes seam cell self-renewal. Before symmetric division, the transcriptional regulator RNT-1^Runx and cofactor BRO-1^CBFβ temporarily bypass Wnt/β-catenin asymmetry by downregulating pop-1 expression. Thereby, RNT-1/BRO-1 appears to render POP-1 below the level required for its repressor function, which converts differentiation into self-renewal. Thus, we found that conserved Runx/CBFβ-type stem cell regulators switch asymmetric to proliferative cell division by opposing TCF-related transcriptional repression.

Summary: To switch asymmetric to proliferative cell division, the C. elegans RNT-1/BRO-1 transcriptional repressor opposes POP-1 TCF expression in seam stem cells, which turns POP-1-induced differentiation into self-renewal.

A centrosomal view of CNS growth

Embryonic development of the central nervous system (CNS) requires the proliferation of neural progenitor cells to be tightly regulated, allowing the formation of an organ with the right size and shape. This includes regulation of both the spatial distribution of mitosis and the mode of cell division. The centrosome, which is the main microtubule-organizing centre of animal cells, contributes to both of these processes. Here, we discuss the impact that centrosome-mediated control of cell division has on the shape of the overall growing CNS. We also review the intrinsic properties of the centrosome, both in terms of its molecular composition and its signalling capabilities, and discuss the fascinating notion that intrinsic centrosomal asymmetries in dividing neural progenitor cells are instructive for neurogenesis. Finally, we discuss the genetic links between centrosome dysfunction during development and the aetiology of microcephaly.

Inactivation of PLK4-STIL Module Prevents Self-Renewal and Triggers p53-Dependent Differentiation in Human Pluripotent Stem Cells

Centrioles account for centrosomes and cilia formation. Recently, a link between centrosomal components and human developmental disorders has been established. However, the exact mechanisms how centrosome abnormalities influence embryogenesis and cell fate are not understood. PLK4-STIL module represents a key element of centrosome duplication cycle. We analyzed consequences of inactivation of the module for early events of embryogenesis in human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs). We demonstrate that blocking of PLK4 or STIL functions leads to centrosome loss followed by both p53-dependent and -independent defects, including prolonged cell divisions, upregulation of p53, chromosome instability, and, importantly, reduction of pluripotency markers and induction of differentiation. We show that the observed loss of key stem cells properties is connected to alterations in mitotic timing and protein turnover. In sum, our data define a link between centrosome, its regulators, and the control of pluripotency and differentiation in PSCs.

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Blocking of PLK4-STIL module in hESCs/hiPSCs leads to:

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Centrosome loss, prolonged and error-prone mitosis;

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p53-dependent differentiation;

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Reduction of pluripotency linked to altered protein turnover

The connections of Wnt pathway components with cell cycle and centrosome: side effects or a hidden logic?

Wnt signaling cascade has developed together with multicellularity to orchestrate the development and homeostasis of complex structures. Wnt pathway components - such as β-catenin, Dishevelled (DVL), Lrp6, and Axin-- are often dedicated proteins that emerged in evolution together with the Wnt signaling cascade and are believed to function primarily in the Wnt cascade. It is interesting to see that in recent literature many of these proteins are connected with cellular functions that are more ancient and not limited to multicellular organisms - such as cell cycle regulation, centrosome biology, or cell division. In this review, we summarize the recent literature describing this crosstalk. Specifically, we attempt to find the answers to the following questions: Is the response to Wnt ligands regulated by the cell cycle? Is the centrosome and/or cilium required to activate the Wnt pathway? How do Wnt pathway components regulate the centrosomal cycle and cilia formation and function? We critically review the evidence that describes how these connections are regulated and how they help to integrate cell-to-cell communication with the cell and the centrosomal cycle in order to achieve a fine-tuned, physiological response.

Wnt Signaling Polarizes C. elegans Asymmetric Cell Divisions During Development

Asymmetric cell division is a common mode of cell differentiation during the invariant lineage of the nematode, C. elegans. Beginning at the four-cell stage, and continuing throughout embryogenesis and larval development, mother cells are polarized by Wnt ligands, causing an asymmetric inheritance of key members of a Wnt/β-catenin signal transduction pathway termed the Wnt/β-catenin asymmetry pathway. The resulting daughter cells are distinct at birth with one daughter cell activating Wnt target gene expression via β-catenin activation of TCF, while the other daughter displays transcriptional repression of these target genes. Here, we seek to review the body of evidence underlying a unified model for Wnt-driven asymmetric cell division in C. elegans, identify global themes that occur during asymmetric cell division, as well as highlight tissue-specific variations. We also discuss outstanding questions that remain unanswered regarding this intriguing mode of asymmetric cell division.

Quantifying β-catenin subcellular dynamics and cyclin D1 mRNA transcription during Wnt signaling in single living cells

Signal propagation from the cell membrane to a promoter can induce gene expression. To examine signal transmission through sub-cellular compartments and its effect on transcription levels in individual cells within a population, we used the Wnt/β-catenin signaling pathway as a model system. Wnt signaling orchestrates a response through nuclear accumulation of β-catenin in the cell population. However, quantitative live-cell measurements in individual cells showed variability in nuclear β-catenin accumulation, which could occur in two waves, followed by slow clearance. Nuclear accumulation dynamics were initially rapid, cell cycle independent and differed substantially from LiCl stimulation, presumed to mimic Wnt signaling. β-catenin levels increased simultaneously at adherens junctions and the centrosome, and a membrane-centrosome transport system was revealed. Correlating β-catenin nuclear dynamics to cyclin D1 transcriptional activation showed that the nuclear accumulation rate of change of the signaling factor, and not actual protein levels, correlated with the transcriptional output of the pathway.

DOI:http://dx.doi.org/10.7554/eLife.16748.001

Cells in an animal’s body must communicate with one another to coordinate many processes that are essential to life. One way that cells do this is by releasing molecules that bind to receptors located on the surface of others cells; this binding then triggers a signaling pathway in the receiving cell that passes information from the surface of the cell to its interior. The last stage of these pathways typically involves specific genes being activated, which changes the cell’s overall activity.

Wnt is one protein that animal cells release to control how nearby cells grow and divide. One arm of the Wnt signaling pathway involves a protein called β-catenin. In the absence of a Wnt signal, there is little β-catenin in the cell. When Wnt binds to its receptor, β-catenin accumulates and enters the cell’s nucleus to activate its target genes. One of these genes, called cyclin D1, controls cell division. However it was not understood how β-catenin builds up in response to a Wnt signal and influences the activity of genes.

Using microscopy, Kafri et al. have now examined how the activities of β-catenin and the cyclin D1 gene change in living human cells. These analyses were initially performed in a population of cells, and confirmed that β-catenin rapidly accumulates after a Wnt signal and that the cyclin D1 gene becomes activated.

Individual cells in a population can respond differently to signaling events. To assess whether human cells differ in their responses to Wnt, Kafri et al. examined the dynamics of β-catenin in single cells in real time. In most cells, β-catenin accumulated after Wnt activation. However, the time taken to accumulate β-catenin, and this protein’s levels, varied between individual cells. Most cells showed the “average” response, with one major wave of accumulation that peaked about two hours after the Wnt signal. Notably, in some cells, β-catenin accumulated in the cell’s nucleus in two waves; in other words, the levels in this compartment of the cell increased, dropped slightly and then increased again.

So how does β-catenin in the nucleus activate target genes? Kafri et al. saw that the absolute number of β-catenin molecules in the nucleus did not affect the activity of cyclin D1. Instead, cells appeared to sense how quickly the amount of β-catenin in the nucleus changes over time, and this rate influences the activation of cyclin D1.

Importantly, problems with Wnt signaling have been linked to diseases in humans; and different components of the Wnt signaling pathway are mutated in many cancers. An important next challenge will be to uncover how the dynamics of this pathway change during disease. Furthermore, a better understanding of Wnt signaling may in future help efforts to develop new drugs that can target the altered pathway in cancer cells.

DOI:http://dx.doi.org/10.7554/eLife.16748.002

The benefits of local depletion: The centrosome as a scaffold for ubiquitin-proteasome-mediated degradation

The centrosome is the major microtubule-organizing center in animal cells but is dispensable for proper microtubule spindle formation in many biological contexts and is thus thought to fulfill additional functions. Recent observations suggest that the centrosome acts as a scaffold for proteasomal degradation in the cell to regulate a variety of biological processes including cell fate acquisition, cell cycle control, stress response, and cell morphogenesis. Here, we review the body of studies indicating a role for the centrosome in promoting proteasomal degradation of ubiquitin-proteasome substrates and explore the functional relevance of this system in different biological contexts. We discuss a potential role for the centrosome in coordinating local degradation of proteasomal substrates, allowing cells to achieve stringent spatiotemporal control over various signaling processes.

Unique and redundant β-catenin regulatory roles of two Dishevelled paralogs during C. elegans asymmetric cell division

The Wnt/β-catenin signaling pathway is utilized across metazoans. However, the mechanism of signal transduction, especially dissociation of the β-catenin destruction complex by Dishevelled proteins, remains controversial. Here, we describe the function of the Dishevelled paralogs DSH-2 and MIG-5 in the Wnt/β-catenin asymmetry (WβA) pathway in Caenorhabditis elegans, where WβA drives asymmetric cell divisions throughout development. We find that DSH-2 and MIG-5 redundantly regulate cell fate in hypodermal seam cells. Similarly, both DSH-2 and MIG-5 are required for positive regulation of SYS-1 (a C. elegans β-catenin), but MIG-5 has a stronger effect on the polarity of SYS-1 localization. We show that MIG-5 controls cortical APR-1 (the C. elegans APC) localization. DSH-2 and MIG-5 both regulate the localization of WRM-1 (another C. elegans β-catenin), acting together as negative regulators of WRM-1 nuclear localization. Finally, we demonstrate that overexpression of DSH-2 or MIG-5 in seam cells leads to stabilization of SYS-1 in the anterior seam daughter, solidifying the Dishevelled proteins as positive regulators of SYS-1. Overall, we have further defined the role of Dishevelled in the WβA signaling pathway, and demonstrated that DSH-2 and MIG-5 regulate cell fate, β-catenin nuclear levels and the polarity of β-catenin regulation.

Quantitative Differences in Nuclear β-catenin and TCF Pattern Embryonic Cells in C. elegans

The Wnt signaling pathway plays a conserved role during animal development in transcriptional regulation of distinct targets in different developmental contexts but it remains unclear whether quantitative differences in the nuclear localization of effector proteins TCF and β-catenin contribute to context-specific regulation. We investigated this question in Caenorhabditis elegans embryos by quantifying nuclear localization of fluorescently tagged SYS-1/β-catenin and POP-1/TCF and expression of Wnt ligands at cellular resolution by time-lapse microscopy and automated lineage tracing. We identified reproducible, quantitative differences that generate a subset of Wnt-signaled cells with a significantly higher nuclear concentration of the TCF/β-catenin activating complex. Specifically, β-catenin and TCF are preferentially enriched in nuclei of daughter cells whose parents also had high nuclear levels of that protein, a pattern that could influence developmental gene expression. Consistent with this, we found that expression of synthetic reporters of POP-1-dependent activation is biased towards cells that had high nuclear SYS-1 in consecutive divisions. We identified new genes whose embryonic expression patterns depend on pop-1. Most of these require POP-1 for either transcriptional activation or repression, and targets requiring POP-1 for activation are more likely to be expressed in the cells with high nuclear SYS-1 in consecutive divisions than those requiring POP-1 for repression. Taken together, these results indicate that SYS-1 and POP-1 levels are influenced by the parent cell’s SYS-1/POP-1 levels and this may provide an additional mechanism by which POP-1 regulates distinct targets in different developmental contexts.

The Wnt signaling pathway is active during the development of all multi-cellular animals and also improperly re-activated in many cancers. Here, we use time-lapse microscopy to quantify the nuclear localization of several proteins in response to Wnt signaling throughout early embryonic development in the nematode worm, C. elegans. We find that cells that received a Wnt signal in the previous division respond more strongly to a Wnt signal in the next division, in part by localizing more of the regulator β-catenin to the nucleus. This causes the relative enrichment of Wnt pathway proteins in the nuclei of repeatedly signaled cells, which we show likely impacts the activation of Wnt target genes. This represents a novel mechanism for the regulation of Wnt pathway targets in development and disease.