APC functions at the centrosome to stimulate microtubule growth

Nucleation of the destruction complex on the centrosome accelerates degradation of β-catenin and regulates Wnt signal transmission

Liquid–liquid phase separation (LLPS) governs a variety of mesoscale cellular processes. However, less is known about how cells utilize LLPS to drive cellular function. Here, we examined the destruction complex (DC), an organelle which controls Wnt signaling and whose components phase separate. Through a combination of advanced microscopy, CRISPR, computational modeling, and optogenetics, we find that the DC is nucleated by the centrosome and that this nucleation drives efficient signal transduction. Our work not only uncovers a biological function for LLPS but also highlights nucleation as a general method for controlling the function of intracellular condensates. Finally, our findings suggest a thermodynamic coupling between Wnt signal transduction and the cell cycle which could lead to insights into Wnt-driven cancers.

Wnt signal transduction is controlled by the destruction complex (DC), a condensate comprising scaffold proteins and kinases that regulate β-catenin stability. Overexpressed DC scaffolds undergo liquid–liquid phase separation (LLPS), but DC mesoscale organization at endogenous expression levels and its role in β-catenin processing were previously unknown. Here, we find that DC LLPS is nucleated by the centrosome. Through a combination of CRISPR-engineered custom fluorescent tags, finite element simulations, and optogenetic tools that allow for manipulation of DC concentration and multivalency, we find that centrosomal nucleation drives processing of β-catenin by colocalizing DC components to a single reaction crucible. Enriching GSK3β partitioning on the centrosome controls β-catenin processing and prevents Wnt-driven embryonic stem cell differentiation to mesoderm. Our findings demonstrate the role of nucleators in controlling biomolecular condensates and suggest tight integration between Wnt signal transduction and the cell cycle.

Cytoskeletal Control and Wnt Signaling-APC's Dual Contributions in Stem Cell Division and Colorectal Cancer

Intestinal epithelium architecture is sustained by stem cell division. In principle, stem cells can divide symmetrically to generate two identical copies of themselves or asymmetrically to sustain tissue renewal in a balanced manner. The choice between the two helps preserve stem cell and progeny pools and is crucial for tissue homeostasis. Control of spindle orientation is a prime contributor to the specification of symmetric versus asymmetric cell division. Competition for space within the niche may be another factor limiting the stem cell pool. An integrative view of the multiple links between intracellular and extracellular signals and molecular determinants at play remains a challenge. One outstanding question is the precise molecular roles of the tumour suppressor Adenomatous polyposis coli (APC) for sustaining gut homeostasis through its respective functions as a cytoskeletal hub and a down regulator in Wnt signalling. Here, we review our current understanding of APC inherent activities and partners in order to explore novel avenues by which APC may act as a gatekeeper in colorectal cancer and as a therapeutic target.

Spindle Assembly Disruption and Cancer Cell Apoptosis with a CLTC-Binding Compound

AK3 compounds are mitotic arrest agents that induce high levels of γH2AX during mitosis and apoptosis following release from arrest. We synthesized a potent AK3 derivative, AK306, that induced arrest and apoptosis of the HCT116 colon cancer cell line with an EC₅₀ of approximately 50 nmol/L. AK306 was active on a broad spectrum of cancer cell lines with total growth inhibition values ranging from approximately 25 nmol/L to 25 μmol/L. Using biotin and BODIPY-linked derivatives of AK306, binding to clathrin heavy chain (CLTC/CHC) was observed, a protein with roles in endocytosis and mitosis. AK306 inhibited mitosis and endocytosis, while disrupting CHC cellular localization. Cells arrested in mitosis by AK306 showed the formation of multiple microtubule-organizing centers consisting of pericentrin, γ-tubulin, and Aurora A foci, without apparent centrosome amplification. Cells released from AK306 arrest were unable to form bipolar spindles, unlike nocodazole-released cells that reformed spindles and completed division. Like AK306, CHC siRNA knockdown disrupted spindle formation and activated p53. A short-term (3-day) treatment of tumor-bearing APC-mutant mice with AK306 increased apoptosis in tumors, but not normal mucosa. These findings indicate that targeting the mitotic CHC complex can selectively induce apoptosis and may have therapeutic value.Implication: Disruption of clathrin with a small-molecule inhibitor, AK306, selectively induces apoptosis in cancer cells by disrupting bipolar spindle formation. Mol Cancer Res; 16(9); 1361-72. ©2018 AACR.

A look into centrosome abnormalities in colon cancer cells, how they arise and how they might be targeted therapeutically

Cancer cells have long been noted for alterations in centrosome structure, number, and function. Colorectal cancers are interesting in this regard since two frequently mutated genes, APC and CTNNB1 (β-catenin), encode proteins that directly interact with the centrosome and affect its ability to direct microtubule growth and establish cell polarity. Colorectal cancers also frequently display centrosome over-duplication and clustering. Efforts have been directed toward understanding how supernumerary centrosomes cluster and whether disrupting this clustering may be a way to induce aberrant/lethal mitoses of cancer cells. Given the important role of the centrosome in establishing spindle polarity and regulating some apoptotic signaling pathways, other approaches to centrosome targeting may be fruitful as well. Basic information on the nature and extent of centrosome defects in colorectal cancer, including why they over-duplicate and whether this over-duplication compensates for their functional defects, could provide a framework for the development of novel approaches for the therapeutic targeting of colorectal cancer.

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.

Insulin signaling regulates a functional interaction between adenomatous polyposis coli and cytoplasmic dynein

Diabetes is linked to an increased risk for colorectal cancer, but the mechanistic underpinnings of this clinically important effect are unclear. Here we describe an interaction between the microtubule motor cytoplasmic dynein, the adenomatous polyposis coli tumor suppressor protein (APC), and glycogen synthase kinase-3β (GSK-3β), which could shed light on this issue. GSK-3β is perhaps best known for glycogen regulation, being inhibited downstream in an insulin-signaling pathway. However, the kinase is also important in many other processes. Mutations in APC that disrupt the regulation of β-catenin by GSK-3β cause colorectal cancer in humans. Of interest, both APC and GSK-3β interact with microtubules and cellular membranes. We recently demonstrated that dynein is a GSK-3β substrate and that inhibition of GSK-3β promotes dynein-dependent transport. We now report that dynein stimulation in intestinal cells in response to acute insulin exposure (or GSK-3β inhibition) is blocked by tumor-promoting isoforms of APC that reduce an interaction between wild-type APC and dynein. We propose that under normal conditions, insulin decreases dynein binding to APC to stimulate minus end-directed transport, which could modulate endocytic and secretory systems in intestinal cells. Mutations in APC likely impair the ability to respond appropriately to insulin signaling. This is exciting because it has the potential to be a contributing factor in the development of colorectal cancer in patients with diabetes.

Characterization of Adenomatous Polyposis Coli Protein Dynamics and Localization at the Centrosome

The adenomatous polyposis coli (APC) tumor suppressor is a multifunctional regulator of Wnt signaling and acts as a mobile scaffold at different cellular sites. APC was recently found to stimulate microtubule (MT) growth at the interphase centrosome; however, little is known about its dynamics and localization at this site. To address this, we analysed APC dynamics in fixed and live cells by fluorescence microscopy. In detergent-extracted cells, we discovered that APC was only weakly retained at the centrosome during interphase suggesting a rapid rate of exchange. This was confirmed in living cells by fluorescence recovery after photobleaching (FRAP), which identified two pools of green fluorescent protein (GFP)-APC: a major rapidly exchanging pool (~86%) and minor retained pool (~14%). The dynamic exchange rate of APC was unaffected by C-terminal truncations implicating a targeting role for the N-terminus. Indeed, we mapped centrosome localization to N-terminal armadillo repeat (ARM) domain amino acids 334–625. Interestingly, the rate of APC movement to the centrosome was stimulated by intact MTs, and APC dynamics slowed when MTs were disrupted by nocodazole treatment or knockdown of γ-tubulin. Thus, the rate of APC recycling at the centrosome is enhanced by MT growth, suggesting a positive feedback to stimulate its role in MT growth.