Adenomatous polyposis coli nucleates actin assembly to drive cell migration and microtubule-induced focal adhesion turnover

Direct targeting of host microtubule and actin cytoskeletons by a chlamydial pathogenic effector protein

To propagate within a eukaryotic cell, pathogenic bacteria hijack and remodulate host cell functions. The Gram-negative obligate intracellular Chlamydiaceae, which pose a serious threat to human and animal health, attach to host cells and inject effector proteins that reprogram host cell machineries. Members of the conserved chlamydial TarP family have been characterized as major early effectors that bind to and remodel the host actin cytoskeleton. We now describe a new function for the Chlamydia pneumoniae TarP member CPn0572, namely the ability to bind and alter the microtubule cytoskeleton. Thus, CPn0572 is unique in being the only prokaryotic protein that directly modulates both dynamic cytoskeletons of a eukaryotic cell. Ectopically expressed GFP-CPn0572 associates in a dose-independent manner with either cytoskeleton singly or simultaneously. In vitro, CPn0572 binds directly to microtubules. Expression of a microtubule-only CPn0572 variant resulted in the formation of an aberrantly thick, stabilized microtubule network. Intriguingly, during infection, secreted CPn0572 also colocalized with altered microtubules, suggesting that this protein also affects microtubule dynamics during infection. Our analysis points to a crosstalk between actin and microtubule cytoskeletons via chlamydial CPn0572.

Focal adhesions are controlled by microtubules through local contractility regulation

Microtubules regulate cell polarity and migration via local activation of focal adhesion turnover, but the mechanism of this process is insufficiently understood. Molecular complexes containing KANK family proteins connect microtubules with talin, the major component of focal adhesions. Here, local optogenetic activation of KANK1-mediated microtubule/talin linkage promoted microtubule targeting to an individual focal adhesion and subsequent withdrawal, resulting in focal adhesion centripetal sliding and rapid disassembly. This sliding is preceded by a local increase of traction force due to accumulation of myosin-II and actin in the proximity of the focal adhesion. Knockdown of the Rho activator GEF-H1 prevented development of traction force and abolished sliding and disassembly of focal adhesions upon KANK1 activation. Other players participating in microtubule-driven, KANK-dependent focal adhesion disassembly include kinases ROCK, PAK, and FAK, as well as microtubules/focal adhesion-associated proteins kinesin-1, APC, and αTAT. Based on these data, we develop a mathematical model for a microtubule-driven focal adhesion disruption involving local GEF-H1/RhoA/ROCK-dependent activation of contractility, which is consistent with experimental data.

Tumor invasiveness is regulated by the concerted function of APC, formins, and Arp2/3 complex

Tumor cell invasion is the initial step in metastasis, the leading cause of death from cancer. Invasion requires protrusive cellular structures that steer the migration of leader cells emanating from the tumor mass toward neighboring tissues. Actin is central to these processes and is therefore the prime target of drugs known as migrastatics. However, the broad effects of general actin inhibitors limit their therapeutic use. Here, we delineate the roles of specific actin nucleators in tuning actin-rich invasive protrusions and pinpoint potential pharmacological targets. We subject colorectal cancer spheroids embedded in collagen matrix-a preclinical model mirroring solid tumor invasiveness-to pharmacologic and/or genetic treatment of specific actin arrays to assess their roles in invasiveness. Our data reveal coordinated yet distinct involvement of actin networks nucleated by adenomatous polyposis coli, formins, and actin-related protein 2/3 complex in the biogenesis and maintenance of invasive protrusions. These findings may open avenues for better targeted therapies.

Actin Cytoskeleton Remodeling Accompanied by Redistribution of Adhesion Proteins Drives Migration of Cells in Different EMT States

Epithelial-mesenchymal transition (EMT) is a process during which epithelial cells lose epithelial characteristics and gain mesenchymal features. Here, we used several cell models to study migratory activity and redistribution of cell-cell adhesion proteins in cells in different EMT states: EGF-induced EMT of epithelial IAR-20 cells; IAR-6-1 cells with a hybrid epithelial-mesenchymal phenotype; and their more mesenchymal derivatives, IAR-6-1-DNE cells lacking adherens junctions. In migrating cells, the cell-cell adhesion protein α-catenin accumulated at the leading edges along with ArpC2/p34 and α-actinin. Suppression of α-catenin shifted cell morphology from fibroblast-like to discoid and attenuated cell migration. Expression of exogenous α-catenin in MDA-MB-468 cells devoid of α-catenin drastically increased their migratory capabilities. The Y654 phosphorylated form of β-catenin was detected at integrin adhesion complexes (IACs). Co-immunoprecipitation studies indicated that α-catenin and pY654-β-catenin were associated with IAC proteins: vinculin, zyxin, and α-actinin. Taken together, these data suggest that in cells undergoing EMT, catenins not participating in assembly of adherens junctions may affect cell migration.

CKAP5 enables formation of persistent actin bundles templated by dynamically instable microtubules

Cytoskeletal rearrangements and crosstalk between microtubules and actin filaments are vital for living organisms. Recently, an abundantly present microtubule polymerase, CKAP5 (XMAP215 homolog), has been reported to play a role in mediating crosstalk between microtubules and actin filaments in the neuronal growth cones. However, the molecular mechanism of this process is unknown. Here, we demonstrate, in a reconstituted system, that CKAP5 enables the formation of persistent actin bundles templated by dynamically instable microtubules. We explain the templating by the difference in CKAP5 binding to microtubules and actin filaments. Binding to the microtubule lattice with higher affinity, CKAP5 enables the formation of actin bundles exclusively on the microtubule lattice, at CKAP5 concentrations insufficient to support any actin bundling in the absence of microtubules. Strikingly, when the microtubules depolymerize, actin bundles prevail at the positions predetermined by the microtubules. We propose that the local abundance of available CKAP5-binding sites in actin bundles allows the retention of CKAP5, resulting in persisting actin bundles. In line with our observations, we found that reducing CKAP5 levels in vivo results in a decrease in actin-microtubule co-localization in growth cones and specifically decreases actin intensity at microtubule plus ends. This readily suggests a mechanism explaining how exploratory microtubules set the positions of actin bundles, for example, in cytoskeleton-rich neuronal growth cones.

Effects of different protein phosphorylation levels on the tenderness of different ultimate pH beef

This study investigated the relationship between tenderness and protein phosphorylation levels of normal ultimate pH (pHu, 5.4-5.8, NpHu), intermediate pHu (5.8-6.2, IpHu) and high pHu (≥6.2, HpHu) Longissimus lumborum from beef. During 21 d of ageing, the HpHu group had the lowest Warner-Bratzler shear force (WBSF) values, while the IpHu group showed the highest and even after 21 days of ageing still had high levels. In the late stage of the 24 h post-mortem period the faster degradation rate of troponin T and earlier activation of caspase 9 in the HpHu group were the key reasons for the lower WBSF compared with the NpHu and IpHu groups. The activity of caspase 3 cannot explain the tenderness differences between IpHu and HpHu groups, since their activities did not show any difference. At 24 h post-mortem, 17 common differential phosphorylated peptides were detected among pHu groups, of which nine were associated with pHu and WBSF. The higher phosphorylation level of glycogen synthase may have caused the delay of meat tenderization in the IpHu group.

The Mechanics of Tumor Cells Dictate Malignancy via Cytoskeleton-Mediated APC/Wnt/β-Catenin Signaling

Tumor cells progressively remodel cytoskeletal structures and reduce cellular stiffness during tumor progression, implicating the correlation between cell mechanics and malignancy. However, the roles of tumor cell cytoskeleton and the mechanics in tumor progression remain incompletely understood. We report that softening/stiffening tumor cells by targeting actomyosin promotes/suppresses self-renewal in vitro and tumorigenic potential in vivo. Weakening/strengthening actin cytoskeleton impairs/reinforces the interaction between adenomatous polyposis coli (APC) and β-catenin, which facilitates β-catenin nuclear/cytoplasmic localization. Nuclear β-catenin binds to the promoter of Oct4, which enhances its transcription that is crucial in sustaining self-renewal and malignancy. These results demonstrate that the mechanics of tumor cells dictate self-renewal through cytoskeleton-APC-Wnt/β-catenin-Oct4 signaling, which are correlated with tumor differentiation and patient survival. This study unveils an uncovered regulatory role of cell mechanics in self-renewal and malignancy, and identifies tumor cell mechanics as a hallmark not only for cancer diagnosis but also for mechanotargeting.

APC-driven actin nucleation powers collective cell dynamics in colorectal cancer cells

Cell remodeling relies on dynamic rearrangements of cell contacts powered by the actin cytoskeleton. The tumor suppressor adenomatous polyposis coli (APC) nucleate actin filaments (F-actin) and localizes at cell junctions. Whether APC-driven actin nucleation acts in cell junction remodeling remains unknown. By combining bioimaging and genetic tools with artificial intelligence algorithms applied to colorectal cancer cell, we found that the APC-dependent actin pool contributes to sustaining levels of F-actin, as well as E-cadherin and occludin protein levels at cell junctions. Moreover, this activity preserved cell junction length and angle, as well as vertex motion and integrity. Loss of this F-actin pool led to larger cells with slow and random cell movement within a sheet. Our findings suggest that APC-driven actin nucleation promotes cell junction integrity and dynamics to facilitate collective cell remodeling and motility. This offers a new perspective to explore the relevance of APC-driven cytoskeletal function in gut morphogenesis.

CLASP2 facilitates dynamic actin filament organization along the microtubule lattice

Coordination between the microtubule and actin networks is essential for cell motility, neuronal growth cone guidance, and wound healing. Members of the CLASP (cytoplasmic linker-associated protein) family of proteins have been implicated in the cytoskeletal cross-talk between microtubules and actin networks; however, the molecular mechanisms underlying the role of CLASP in cytoskeletal coordination are unclear. Here, we investigate CLASP2α's cross-linking function with microtubules and F-actin. Our results demonstrate that CLASP2α cross-links F-actin to the microtubule lattice in vitro. We find that the cross-linking ability is retained by L-TOG2-S, a minimal construct containing the TOG2 domain and serine-arginine-rich region of CLASP2α. Furthermore, CLASP2α promotes the accumulation of multiple actin filaments along the microtubule, supporting up to 11 F-actin landing events on a single microtubule lattice region. CLASP2α also facilitates the dynamic organization of polymerizing actin filaments templated by the microtubule network, with F-actin forming bridges between individual microtubules. Finally, we find that depletion of CLASPs in vascular smooth muscle cells results in disorganized actin fibers and reduced coalignment of actin fibers with microtubules, suggesting that CLASP and microtubules contribute to higher-order actin structures. Taken together, our results indicate that CLASP2α can directly cross-link F-actin to microtubules and that this microtubule-CLASP-actin interaction may influence overall cytoskeletal organization in cells.

The compass to follow: Focal adhesion turnover

How cells move is a fundamental biological question. The directionality of adherent migrating cells depends on the assembly and disassembly (turnover) of focal adhesions (FAs). FAs are micron-sized actin-based structures that link cells to the extracellular matrix. Traditionally, microtubules have been considered key to triggering FA turnover. Through the years, advancements in biochemistry, biophysics, and bioimaging tools have been invaluable for many research groups to unravel a variety of mechanisms and molecular players that contribute to FA turnover, beyond microtubules. Here, we discuss recent discoveries of key molecular players that affect the dynamics and organization of the actin cytoskeleton to enable timely FA turnover and consequently proper directed cell migration.

Confocal Laser Scanning Imaging of Cell Junctions in Human Colon Cancer Cells

The intestinal epithelium is formed by a single layer of cells. These cells originate from self-renewal stem cells that give rise to various lineages of cells: Paneth, transit-amplifying, and fully differentiated cells (as enteroendocrine, goblet cells, and enterocytes). Enterocytes, also known as absorptive epithelial cells, are the most abundant cell type in the gut. Enterocytes have the potential to polarize as well as form tight junctions with neighbor cells which altogether serve to ensure both the absorption of "good" substances into the body and the blockage of "bad" substances, among other functions. Culture cell models such as the Caco-2 cell line have been proved to be valuable tools to study the fascinating functions of the intestine. In this chapter we outline some experimental procedures to grow, differentiate, and stain intestinal Caco-2 cells, as well as image them using two modes of confocal laser scanning microscopy.

ERRα coordinates actin and focal adhesion dynamics

Cell migration depends on the dynamic organisation of the actin cytoskeleton and assembly and disassembly of focal adhesions (FAs). However, the precise mechanisms coordinating these processes remain poorly understood. We previously identified the oestrogen-related receptor α (ERRα) as a major regulator of cell migration. Here, we show that loss of ERRα leads to abnormal accumulation of actin filaments that is associated with an increased level of inactive form of the actin-depolymerising factor cofilin. We further show that ERRα depletion decreases cell adhesion and results in defective FA formation and turnover. Interestingly, specific inhibition of the RhoA-ROCK-LIMK-cofilin pathway rescues the actin polymerisation defects resulting from ERRα silencing, but not cell adhesion. Instead, we found that MAP4K4 is a direct target of ERRα and down-regulation of its activity rescues cell adhesion and FA formation in the ERRα-depleted cells. Altogether, our results highlight a crucial role of ERRα in coordinating the dynamic of actin network and FAs through the independent regulation of the RhoA and MAP4K4 pathways.

The CLIP-170 N-terminal domain binds directly to both F-actin and microtubules in a mutually exclusive manner

The cooperation between the actin and microtubule (MT) cytoskeletons is important for cellular processes such as cell migration and muscle cell development. However, a full understanding of how this cooperation occurs has yet to be sufficiently developed. The MT plus-end tracking protein CLIP-170 has been implicated in this actin-MT coordination by associating with the actin-binding signaling protein IQGAP1 and by promoting actin polymerization through binding with formins. Thus far, the interactions of CLIP-170 with actin were assumed to be indirect. Here, we demonstrate using high-speed cosedimentation assays that CLIP-170 can bind to filamentous actin (F-actin) directly. We found that the affinity of this binding is relatively weak but strong enough to be significant in the actin-rich cortex, where actin concentrations can be extremely high. Using CLIP-170 fragments and mutants, we show that the direct CLIP-170-F-actin interaction is independent of the FEED domain, the region that mediates formin-dependent actin polymerization, and that the CLIP-170 F-actin-binding region overlaps with the MT-binding region. Consistent with these observations, in vitro competition assays indicate that CLIP-170-F-actin and CLIP-170-MT interactions are mutually exclusive. Taken together, these observations lead us to speculate that direct CLIP-170-F-actin interactions may function to reduce the stability of MTs in actin-rich regions of the cell, as previously proposed for MT end-binding protein 1.

The tumor suppressor adenomatous polyposis coli regulates T lymphocyte migration

Adenomatous polyposis coli (APC) is a tumor suppressor whose mutations underlie familial adenomatous polyposis (FAP) and colorectal cancer. Although its role in intestinal epithelial cells is well characterized, APC importance in T cell biology is ill defined. APC regulates cytoskeleton organization, cell polarity, and migration in various cell types. Here, we address whether APC plays a role in T lymphocyte migration. Using a series of cell biology tools, we unveiled that T cells from FAP patients carrying APC mutations display impaired adhesion and motility in constrained environments. We further dissected the cellular mechanisms underpinning these defects in APC-depleted CEM T cell line that recapitulate the phenotype observed in FAP T cells. We found that APC affects T cell motility by modulating integrin-dependent adhesion and cytoskeleton reorganization. Hence, APC mutations in FAP patients not only drive intestinal neoplasms but also impair T cell migration, potentially contributing to inefficient antitumor immunity.

Wnt signaling polarizes cortical actin polymerization to increase daughter cell asymmetry

Asymmetric positioning of the mitotic spindle contributes to the generation of two daughter cells with distinct sizes and fates. Here, we investigated an asymmetric division in the Caenorhabditis elegans Q neuroblast lineage. In this division, beginning with an asymmetrically positioned spindle, the daughter-cell size differences continuously increased during cytokinesis, and the smaller daughter cell in the posterior eventually underwent apoptosis. We found that Arp2/3-dependent F-actin assembled in the anterior but not posterior cortex during division, suggesting that asymmetric expansion forces generated by actin polymerization may enlarge the anterior daughter cell. Consistent with this, inhibition of cortical actin polymerization or artificially equalizing actin assembly led to symmetric cell division. Furthermore, disruption of the Wnt gradient or its downstream components impaired asymmetric cortical actin assembly and caused symmetric division. Our results show that Wnt signaling establishes daughter cell asymmetry by polarizing cortical actin polymerization in a dividing cell.

An emergent Wnt5a/YAP/TAZ regulatory circuit and its possible role in cancer

Wnt5a is a ligand that plays several roles in development, homeostasis, and disease. A growing body of evidence indicates that Wnt5a is involved in cancer progression. Despite extensive research in this field, our knowledge about how Wnt5a is precisely involved in cancer is still incomplete. It is usually thought that certain combinations of Frizzled receptors and co-receptors might explain the observed effects of Wnt5a either as a tumor suppressor or by promoting migration and invasion. While accepting this 'receptor context' model, this review proposes that Wnt5a is integrated within a larger regulatory circuit involving β-catenin, YAP/TAZ, and LATS1/2. Remarkably, WNT5A and YAP1 are transcriptionally regulated by the Hippo and Wnt pathways, respectively, and might form a regulatory circuit acting through LATS kinases and secreted Wnt/β-catenin inhibitors, including Wnt5a itself. Therefore, understanding the precise role of Wnt5a and YAP in cancer requires a systems biology perspective.

Cyclical phosphorylation of LRAP35a and CLASP2 by GSK3β and CK1δ regulates EB1-dependent MT dynamics in cell migration

Mammalian cell cytoskeletal reorganization for efficient directional movement requires tight coordination of actomyosin and microtubule networks. In this study, we show that LRAP35a potentiates microtubule stabilization by promoting CLASP2/EB1 interaction besides its complex formation with MRCK/MYO18A for retrograde actin flow. The alternate regulation of these two networks by LRAP35a is tightly regulated by a series of phosphorylation events that dictated its specificity. Sequential phosphorylation of LRAP35a by Protein Kinase A (PKA) and Glycogen Synthase Kinase-3β (GSK3β) initiates the association of LRAP35a with CLASP2, while subsequent binding and further phosphorylation by Casein Kinase 1δ (CK1δ) induce their dissociation, which facilitates LRAP35a/MRCK association in driving lamellar actomyosin flow. Importantly, microtubule dynamics is directly moderated by CK1δ activity on CLASP2 to regulate GSK3β phosphorylation of the SxIP motifs that blocks EB1 binding, an event countered by LRAP35a interaction and its competition for CK1δ activity. Overall this study reveals an essential role for LRAP35a in coordinating lamellar contractility and microtubule polarization in cell migration.

New twists in actin-microtubule interactions

Actin filaments and microtubules are cytoskeletal polymers that participate in many vital cell functions including division, morphogenesis, phagocytosis, and motility. Despite the persistent dogma that actin filament and microtubule networks are distinct in localization, structure, and function, a growing body of evidence shows that these elements are choreographed through intricate mechanisms sensitive to either polymer. Many proteins and cellular signals that mediate actin–microtubule interactions have already been identified. However, the impact of these regulators is typically assessed with actin filament or microtubule polymers alone, independent of the other system. Further, unconventional modes and regulators coordinating actin–microtubule interactions are still being discovered. Here we examine several methods of actin–microtubule crosstalk with an emphasis on the molecular links between both polymer systems and their higher-order interactions.

Wnt/β-catenin signaling pathway in uterine leiomyoma: role in tumor biology and targeting opportunities

Uterine leiomyoma is the most common tumor of the female reproductive system and originates from a single transformed myometrial smooth muscle cell. Despite the immense medical, psychosocial, and financial impact, the exact underlying mechanisms of leiomyoma pathobiology are poorly understood. Alterations of signaling pathways are thought to be instrumental in leiomyoma biology. Wnt/β-catenin pathway appears to be involved in several aspects of the genesis of leiomyomas. For example, Wnt5b is overexpressed in leiomyoma, and the Wnt/β-catenin pathway appears to mediate the role of MED12 mutations, the most common mutations in leiomyoma, in tumorigenesis. Moreover, Wnt/β-catenin pathway plays a paracrine role where estrogen/progesterone treatment of mature myometrial or leiomyoma cells leads to increased expression of Wnt11 and Wnt16, which induces proliferation of leiomyoma stem cells and tumor growth. Constitutive activation of β-catenin leads to myometrial hyperplasia and leiomyoma-like lesions in animal models. Wnt/β-catenin signaling is also closely involved in mechanotransduction and extracellular matrix regulation and relevant alterations in leiomyoma, and crosstalk is noted between Wnt/β-catenin signaling and other pathways known to regulate leiomyoma development and growth such as estrogen, progesterone, TGFβ, PI3K/Akt/mTOR, Ras/Raf/MEK/ERK, IGF, Hippo, and Notch signaling. Finally, evidence suggests that inhibition of the canonical Wnt pathway using β-catenin inhibitors inhibits leiomyoma cell proliferation. Understanding the molecular mechanisms of leiomyoma development is essential for effective treatment. The specific Wnt/β-catenin pathway molecules discussed in this review constitute compelling candidates for therapeutic targeting.

Branched actin networks are assembled on microtubules by adenomatous polyposis coli for targeted membrane protrusion

Cell migration is driven by pushing and pulling activities of the actin cytoskeleton, but migration directionality is largely controlled by microtubules. This function of microtubules is especially critical for neuron navigation. However, the underlying mechanisms are poorly understood. Here we show that branched actin filament networks, the main pushing machinery in cells, grow directly from microtubule tips toward the leading edge in growth cones of hippocampal neurons. Adenomatous polyposis coli (APC), a protein with both tumor suppressor and cytoskeletal functions, concentrates at the microtubule-branched network interface, whereas APC knockdown nearly eliminates branched actin in growth cones and prevents growth cone recovery after repellent-induced collapse. Conversely, encounters of dynamic APC-positive microtubule tips with the cell edge induce local actin-rich protrusions. Together, we reveal a novel mechanism of cell navigation involving APC-dependent assembly of branched actin networks on microtubule tips.

EB1 Restricts Breast Cancer Cell Invadopodia Formation and Matrix Proteolysis via FAK

Regulation of microtubule dynamics by plus-end tracking proteins (+TIPs) plays an essential role in cancer cell migration. However, the role of +TIPs in cancer cell invasion has been poorly addressed. Invadopodia, actin-rich protrusions specialized in extracellular matrix degradation, are essential for cancer cell invasion and metastasis, the leading cause of death in breast cancer. We, therefore, investigated the role of the End Binding protein, EB1, a major hub of the +TIP network, in invadopodia functions. EB1 silencing increased matrix degradation by breast cancer cells. This was recapitulated by depletion of two additional +TIPs and EB1 partners, APC and ACF7, but not by the knockdown of other +TIPs, such as CLASP1/2 or CLIP170. The knockdown of Focal Adhesion Kinase (FAK) was previously proposed to similarly promote invadopodia formation as a consequence of a switch of the Src kinase from focal adhesions to invadopodia. Interestingly, EB1-, APC-, or ACF7-depleted cells had decreased expression/activation of FAK. Remarkably, overexpression of wild type FAK, but not of FAK mutated to prevent Src recruitment, prevented the increased degradative activity induced by EB1 depletion. Overall, we propose that EB1 restricts invadopodia formation through the control of FAK and, consequently, the spatial regulation of Src activity.

Bil2 Is a Novel Inhibitor of the Yeast Formin Bnr1 Required for Proper Actin Cable Organization and Polarized Secretion

Cell growth in budding yeast depends on rapid and on-going assembly and turnover of polarized actin cables, which direct intracellular transport of post-Golgi vesicles to the bud tip. Saccharomyces cerevisiae actin cables are polymerized by two formins, Bni1 and Bnr1. Bni1 assembles cables in the bud, while Bnr1 is anchored to the bud neck and assembles cables that specifically extend filling the mother cell. Here, we report a formin regulatory role for YGL015c, a previously uncharacterized open reading frame, which we have named Bud6 Interacting Ligand 2 (BIL2). bil2Δ cells display defects in actin cable architecture and partially-impaired secretory vesicle transport. Bil2 inhibits Bnr1-mediated actin filament nucleation in vitro, yet has no effect on the rate of Bnr1-mediated filament elongation. This activity profile for Bil2 resembles that of another yeast formin regulator, the F-BAR protein Hof1, and we find that bil2Δ with hof1Δ are synthetic lethal. Unlike Hof1, which localizes exclusively to the bud neck, GFP-Bil2 localizes to the cytosol, secretory vesicles, and sites of polarized cell growth. Further, we provide evidence that Hof1 and Bil2 inhibitory effects on Bnr1 are overcome by distinct mechanisms. Together, our results suggest that Bil2 and Hof1 perform distinct yet genetically complementary roles in inhibiting the actin nucleation activity of Bnr1 to control actin cable assembly and polarized secretion.

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.

EB1 Directly Regulates APC-Mediated Actin Nucleation

EB1 was discovered 25 years ago as a binding partner of the tumor suppressor adenomatous polyposis coli (APC) [1]; however, the significance of EB1-APC interactions has remained poorly understood. EB1 functions at the center of a network of microtubule end-tracking proteins (+TIPs) [2-5], and APC binding to EB1 promotes EB1 association with microtubule ends and microtubule stabilization [6, 7]. Whether EB1 interactions govern functions of APC beyond microtubule regulation has not been explored. The C-terminal basic domain of APC (APC-B) directly nucleates actin assembly, and this activity is required in vivo for directed cell migration and for maintaining normal levels of F-actin [8-10]. Here, we show that EB1 binds APC-B and inhibits its actin nucleation function by blocking actin monomer recruitment. Consistent with these biochemical observations, knocking down EB1 increases F-actin levels in cells, and this can be rescued by disrupting APC-mediated actin nucleation. Conversely, overexpressing EB1 decreases F-actin levels and impairs directed cell migration without altering microtubule organization and independent of its direct binding interactions with microtubules. Overall, our results define a new function for EB1 in negatively regulating APC-mediated actin assembly. Combining these findings with other recent studies showing that APC interactions regulate EB1-dependent effects on microtubule dynamics [7], we propose that EB1-APC interactions govern bidirectional cytoskeletal crosstalk by coordinating microtubule and actin dynamics.

Profilin choreographs actin and microtubules in cells and cancer

Actin and microtubules play essential roles in aberrant cell processes that define and converge in cancer including: signaling, morphology, motility, and division. Actin and microtubules do not directly interact, however shared regulators coordinate these polymers. While many of the individual proteins important for regulating and choreographing actin and microtubule behaviors have been identified, the way these molecules collaborate or fail in normal or disease contexts is not fully understood. Decades of research focus on Profilin as a signaling molecule, lipid-binding protein, and canonical regulator of actin assembly. Recent reports demonstrate that Profilin also regulates microtubule dynamics and polymerization. Thus, Profilin can coordinate both actin and microtubule polymer systems. Here we reconsider the biochemical and cellular roles for Profilin with a focus on the essential cytoskeletal-based cell processes that go awry in cancer. We also explore how the use of model organisms has helped to elucidate mechanisms that underlie the regulatory essence of Profilin in vivo and in the context of disease.

A Support Vector Machine Model Predicting the Risk of Duodenal Cancer in Patients with Familial Adenomatous Polyposis at the Transcript Levels

Objective

Familial adenomatous polyposis (FAP) is one major type of inherited duodenal cancer. The estimate of duodenal cancer risk in patients with FAP is critical for selecting the optimal treatment strategy.

Methods

Microarray datasets related with FAP were retrieved from the Gene Expression Omnibus (GEO) database. Differentially expressed genes were identified by FAP vs. normal samples and FAP and duodenal cancer vs. normal samples. Furthermore, functional enrichment analyses of these differentially expressed genes were performed. A support vector machine (SVM) was performed to train and validate cancer risk prediction model.

Results

A total of 196 differentially expressed genes were identified between FAP compared with normal samples. 177 similarly expressed genes were identified both in FAP and duodenal cancer, which were mainly enriched in pathways in cancer and metabolic-related pathway, indicating that these genes in patients with FAP could contribute to duodenal cancer. Among them, Cyclin D1, SDF-1, AXIN, and TCF were significantly upregulated in FAP tissues using qRT-PCR. Based on the 177 genes, an SVM model was constructed for prediction of the risk of cancer in patients with FAP. After validation, the model can accurately distinguish FAP patients with high risk from those with low risk for duodenal cancer.

Conclusion

This study proposed a cancer risk prediction model based on an SVM at the transcript levels.

Glucocorticoids rapidly inhibit cell migration through a novel, non-transcriptional HDAC6 pathway

Glucocorticoids (GCs) act through the glucocorticoid receptor (GR, also known as NR3C1) to regulate immunity, energy metabolism and tissue repair. Upon ligand binding, activated GR mediates cellular effects by regulating gene expression, but some GR effects can occur rapidly without new transcription. Here, we show that GCs rapidly inhibit cell migration, in response to both GR agonist and antagonist ligand binding. The inhibitory effect on migration is prevented by GR knockdown with siRNA, confirming GR specificity, but not by actinomycin D treatment, suggesting a non-transcriptional mechanism. We identified a rapid onset increase in microtubule polymerisation following GC treatment, identifying cytoskeletal stabilisation as the likely mechanism of action. HDAC6 overexpression, but not knockdown of αTAT1, rescued the GC effect, implicating HDAC6 as the GR effector. Consistent with this hypothesis, ligand-dependent cytoplasmic interaction between GR and HDAC6 was demonstrated by quantitative imaging. Taken together, we propose that activated GR inhibits HDAC6 function, and thereby increases the stability of the microtubule network to reduce cell motility. We therefore report a novel, non-transcriptional mechanism whereby GCs impair cell motility through inhibition of HDAC6 and rapid reorganization of the cell architecture.This article has an associated First Person interview with the first author of the paper.

Microtubules in cell migration

Directed cell migration is critical for embryogenesis and organ development, wound healing and the immune response. Microtubules are dynamic polymers that control directional migration through a number of coordinated processes: microtubules are the tracks for long-distance intracellular transport, crucial for delivery of new membrane components and signalling molecules to the leading edge of a migrating cell and the recycling of adhesion receptors. Microtubules act as force generators and compressive elements to support sustained cell protrusions. The assembly and disassembly of microtubules is coupled to Rho GTPase signalling, thereby controlling actin polymerisation, myosin-driven contractility and the turnover of cellular adhesions locally. Cross-talk of actin and microtubule dynamics is mediated through a number of common binding proteins and regulators. Furthermore, cortical microtubule capture sites are physically linked to focal adhesions, facilitating the delivery of secretory vesicles and efficient cross-talk. Here we summarise the diverse functions of microtubules during cell migration, aiming to show how they contribute to the spatially and temporally coordinated sequence of events that permit efficient, directional and persistent migration.

PEBP1/RKIP behavior: a mirror of actin-membrane organization

Phosphatidylethanolamine-binding protein 1 (PEBP1), a small 21 kDa protein, is implicated in several key processes of the living cell. The deregulation of PEBP1, especially its downregulation, leads to major diseases such as cancer and Alzheimer's disease. PEBP1 was found to interact with numerous proteins, especially kinases and GTPases, generally inhibiting their activity. To understand the basic functionality of this amazing small protein, we have considered several known processes that it modulates and we have discussed the role of each molecular target in these processes. Here, we propose that cortical actin organization, associated with membrane changes, is involved in the majority of the processes modulated by PEBP1. Furthermore, based on recent data, we summarize some key PEBP1-interacting proteins, and we report their respective functions and focus on their relationships with actin organization. We suggest that, depending on the cell status and environment, PEBP1 is an organizer of the actin-membrane composite material.

Wnt signaling in cancer: therapeutic targeting of Wnt signaling beyond β-catenin and the destruction complex

Wnt/β-catenin signaling is implicated in many physiological processes, including development, tissue homeostasis, and tissue regeneration. In human cancers, Wnt/β-catenin signaling is highly activated, which has led to the development of various Wnt signaling inhibitors for cancer therapies. Nonetheless, the blockade of Wnt signaling causes side effects such as impairment of tissue homeostasis and regeneration. Recently, several studies have identified cancer-specific Wnt signaling regulators. In this review, we discuss the Wnt inhibitors currently being used in clinical trials and suggest how additional cancer-specific regulators could be utilized to treat Wnt signaling-associated cancer.

FOCAD loss impacts microtubule assembly, G2/M progression and patient survival in astrocytic gliomas

In search of novel genes associated with glioma pathogenesis, we have previously shown frequent deletions of the KIAA1797/FOCAD gene in malignant gliomas, and a tumor suppressor function of the encoded focadhesin impacting proliferation and migration of glioma cells in vitro and in vivo. Here, we examined an association of reduced FOCAD gene copy number with overall survival of patients with astrocytic gliomas, and addressed the molecular mechanisms that govern the suppressive effect of focadhesin on glioma growth. FOCAD loss was associated with inferior outcome in patients with isocitrate dehydrogenase 1 or 2 (IDH)-mutant astrocytic gliomas of WHO grades II-IV. Multivariate analysis considering age at diagnosis as well as IDH mutation, MGMT promoter methylation, and CDKN2A/B homozygous deletion status confirmed reduced FOCAD gene copy number as a prognostic factor for overall survival. Using a yeast two-hybrid screen and pull-down assays, tubulin beta-6 and other tubulin family members were identified as novel focadhesin-interacting partners. Tubulins and focadhesin co-localized to centrosomes where focadhesin was enriched in proximity to centrioles. Focadhesin was recruited to microtubules via its interaction partner SLAIN motif family member 2 and reduced microtubule assembly rates, possibly explaining the focadhesin-dependent decrease in cell migration. During the cell cycle, focadhesin levels peaked in G2/M phase and influenced time-dependent G2/M progression potentially via polo like kinase 1 phosphorylation, providing a possible explanation for focadhesin-dependent cell growth reduction. We conclude that FOCAD loss may promote biological aggressiveness and worsen clinical outcome of diffuse astrocytic gliomas by enhancing microtubule assembly and accelerating G2/M phase progression.

Varying crosslinking motifs drive the mesoscale mechanics of actin-microtubule composites

The cytoskeleton precisely tunes its mechanics by altering interactions between semiflexible actin filaments, rigid microtubules, and crosslinking proteins. We use optical tweezers microrheology and confocal microscopy to characterize how varying crosslinking motifs impact the mesoscale mechanics and mobility of actin-microtubule composites. We show that, upon subtle changes in crosslinking patterns, composites can exhibit two distinct classes of force response - primarily elastic versus more viscous. For example, a composite in which actin and microtubules are crosslinked to each other but not to themselves is markedly more elastic than one in which both filaments are independently crosslinked. Notably, this distinction only emerges at mesoscopic scales in response to nonlinear forcing, whereas varying crosslinking motifs have little impact on the microscale mechanics and mobility. Our unexpected scale-dependent results not only inform the physics underlying key cytoskeleton processes and structures, but, more generally, provide valuable perspective to materials engineering endeavors focused on polymer composites.

The role of APC-mediated actin assembly in microtubule capture and focal adhesion turnover

Actin assembly by APC maintains proper organization and dynamics of F-actin at focal adhesions. This, in turn, impacts the organization of other molecular components and the responsiveness of focal adhesions to microtubule capture and autophagosome-induced disassembly.

Focal adhesion (FA) turnover depends on microtubules and actin. Microtubule ends are captured at FAs, where they induce rapid FA disassembly. However, actin’s roles are less clear. Here, we use polarization-resolved microscopy, FRAP, live cell imaging, and a mutant of Adenomatous polyposis coli (APC-m4) defective in actin nucleation to investigate the role of actin assembly in FA turnover. We show that APC-mediated actin assembly is critical for maintaining normal F-actin levels, organization, and dynamics at FAs, along with organization of FA components. In WT cells, microtubules are captured repeatedly at FAs as they mature, but once a FA reaches peak maturity, the next microtubule capture event leads to delivery of an autophagosome, triggering FA disassembly. In APC-m4 cells, microtubule capture frequency and duration are altered, and there are long delays between autophagosome delivery and FA disassembly. Thus, APC-mediated actin assembly is required for normal feedback between microtubules and FAs, and maintaining FAs in a state “primed” for microtubule-induced turnover.

XMAP215 promotes microtubule-F-actin interactions to regulate growth cone microtubules during axon guidance in Xenopus laevis

It has long been established that neuronal growth cone navigation depends on changes in microtubule (MT) and F-actin architecture downstream of guidance cues. However, the mechanisms by which MTs and F-actin are dually coordinated remain a fundamentally unresolved question. Here, we report that the well-characterized MT polymerase, XMAP215 (also known as CKAP5), plays an important role in mediating MT-F-actin interaction within the growth cone. We demonstrate that XMAP215 regulates MT-F-actin alignment through its N-terminal TOG 1-5 domains. Additionally, we show that XMAP215 directly binds to F-actin in vitro and co-localizes with F-actin in the growth cone periphery. We also find that XMAP215 is required for regulation of growth cone morphology and response to the guidance cue, Ephrin A5. Our findings provide the first strong evidence that XMAP215 coordinates MT and F-actin interaction in vivo We suggest a model in which XMAP215 regulates MT extension along F-actin bundles into the growth cone periphery and that these interactions may be important to control cytoskeletal dynamics downstream of guidance cues. This article has an associated First Person interview with the first author of the paper.

The Cytoskeleton-A Complex Interacting Meshwork

The cytoskeleton of animal cells is one of the most complicated and functionally versatile structures, involved in processes such as endocytosis, cell division, intra-cellular transport, motility, force transmission, reaction to external forces, adhesion and preservation, and adaptation of cell shape. These functions are mediated by three classical cytoskeletal filament types, as follows: Actin, microtubules, and intermediate filaments. The named filaments form a network that is highly structured and dynamic, responding to external and internal cues with a quick reorganization that is orchestrated on the time scale of minutes and has to be tightly regulated. Especially in brain tumors, the cytoskeleton plays an important role in spreading and migration of tumor cells. As the cytoskeletal organization and regulation is complex and many-faceted, this review aims to summarize the findings about cytoskeletal filament types, including substructures formed by them, such as lamellipodia, stress fibers, and interactions between intermediate filaments, microtubules and actin. Additionally, crucial regulatory aspects of the cytoskeletal filaments and the formed substructures are discussed and integrated into the concepts of cell motility. Even though little is known about the impact of cytoskeletal alterations on the progress of glioma, a final point discussed will be the impact of established cytoskeletal alterations in the cellular behavior and invasion of glioma.

Adenomatous Polyposis Coli as a Scaffold for Microtubule End-Binding Proteins

End-binding proteins (EBs), referred to as the core components of the microtubule plus-end tracking protein network, interact with the C-terminus of the adenomatous polyposis coli (APC) tumor suppressor. This interaction is disrupted in colon cancers expressing truncated APC. APC and EBs act in synergy to regulate microtubule dynamics during spindle formation, chromosome segregation and cell migration. Since EBs autonomously end-track microtubules and partially co-localize with APC at microtubule tips in cells, EBs have been proposed to direct APC to microtubule ends. However, the interdependency of EB and APC localization on microtubules remains elusive. Here, using in vitro reconstitution and single-molecule imaging, we have investigated the interplay between EBs and the C-terminal domain of APC (APC-C) on dynamic microtubules. Our results show that APC-C binds along the microtubule wall but does not accumulate at microtubule tips, even when EB proteins are present. APC-C was also found to enhance EB binding at the extremity of growing microtubules and on the microtubule lattice: APC-C promotes EB end-tracking properties by increasing the time EBs spend at microtubule growing ends, whereas a pool of EBs with a fast turnover accumulates along the microtubule surface. Overall, our results suggest that APC is a promoter of EB interaction with microtubules, providing molecular determinants to reassess the relationship between APC and EBs.

Functions of the APC tumor suppressor protein dependent and independent of canonical WNT signaling: implications for therapeutic targeting

The acquisition of biallelic mutations in the APC gene is a rate-limiting step in the development of most colorectal cancers and occurs in the earliest lesions. APC encodes a 312-kDa protein that localizes to multiple subcellular compartments and performs diverse functions. APC participates in a cytoplasmic complex that promotes the destruction of the transcriptional licensing factor β-catenin; APC mutations that abolish this function trigger constitutive activation of the canonical WNT signaling pathway, a characteristic found in almost all colorectal cancers. By negatively regulating canonical WNT signaling, APC counteracts proliferation, promotes differentiation, facilitates apoptosis, and suppresses invasion and tumor progression. APC further antagonizes canonical WNT signaling by interacting with and counteracting β-catenin in the nucleus. APC also suppresses tumor initiation and progression in the colorectal epithelium through functions that are independent of canonical WNT signaling. APC regulates the mitotic spindle to facilitate proper chromosome segregation, localizes to the cell periphery and cell protrusions to establish cell polarity and appropriate directional migration, and inhibits DNA replication by interacting directly with DNA. Mutations in APC are often frameshifts, insertions, or deletions that introduce premature stop codons and lead to the production of truncated APC proteins that lack its normal functions and possess tumorigenic properties. Therapeutic approaches in development for the treatment of APC-deficient tumors are focused on the inhibition of canonical WNT signaling, especially through targets downstream of APC in the pathway, or on the restoration of wild-type APC expression.

Wnt/Beta-Catenin Signaling Regulation and a Role for Biomolecular Condensates

Wnt/β-Catenin signaling plays key roles in tissue homeostasis and cell fate decisions in embryonic and post-embryonic development across the animal kingdom. As a result, pathway mutations are associated with developmental disorders and many human cancers. The multiprotein destruction complex keeps signaling off in the absence of Wnt ligands and needs to be downregulated for pathway activation. We discuss new insights into destruction complex activity and regulation, highlighting parallels to the control of other cell biological processes by biomolecular condensates that form by phase separation to suggest that the destruction complex acts as a biomolecular condensate in Wnt pathway regulation.

EhFP10: A FYVE family GEF interacts with myosin IB to regulate cytoskeletal dynamics during endocytosis in Entamoeba histolytica

Motility and phagocytosis are key processes that are involved in invasive amoebiasis disease caused by intestinal parasite Entamoeba histolytica. Previous studies have reported unconventional myosins to play significant role in membrane based motility as well as endocytic processes. EhMyosin IB is the only unconventional myosin present in E. histolytica, is thought to be involved in both of these processes. Here, we report an interaction between the SH3 domain of EhMyosin IB and c-terminal domain of EhFP10, a Rho guanine nucleotide exchange factor. EhFP10 was found to be confined to Entamoeba species only, and to contain a c-terminal domain that binds and bundles actin filaments. EhFP10 was observed to localize in the membrane ruffles, phagocytic and macropinocytic cups of E. histolytica trophozoites. It was also found in early pinosomes but not early phagosomes. A crystal structure of the c-terminal SH3 domain of EhMyosin IB (EhMySH3) in complex with an EhFP10 peptide and co-localization studies established the interaction of EhMySH3 with EhFP10. This interaction was shown to lead to inhibition of actin bundling activity and to thereby regulate actin dynamics during endocytosis. We hypothesize that unique domain architecture of EhFP10 might be compensating the absence of Wasp and related proteins in Entamoeba, which are known partners of myosin SH3 domains in other eukaryotes. Our findings also highlights the role of actin bundling during endocytosis.

Low-dose eribulin reduces lung metastasis of osteosarcoma in vitro and in vivo

Lung metastasis markedly reduces the prognosis of osteosarcoma. Moreover, there is no effective treatment for lung metastasis, and a new treatment strategy for the treatment of osteosarcoma lung metastasis is required. Therefore, in this study, we investigated the suppressive effect of the microtubule inhibitor eribulin mesylate (eribulin) on lung metastasis of osteosarcoma. At concentrations >proliferation IC₅₀, eribulin induced cell cycle arrest and apoptosis in a metastatic osteosarcoma cell line, LM8. However, at concentrations <proliferation IC₅₀, (low dose), eribulin changed cell morphology and decreased LM8 migration. Low eribulin concentrations also reduced directionality during migration, peripheral localization of adenomatous polyposis coli protein, and turnover of focal adhesions. In a three-dimensional collagen culture system, low eribulin concentrations inhibited tumor cell proliferation and colony formation. Higher doses of eribulin administered on a standard schedule inhibited lung metastasis and primary tumor growth in a murine osteosarcoma metastasis model. Frequent low-dose eribulin administration (0.3 mg/kg every 4 days × 4) effectively inhibited lung metastasis but had little effect on primary tumor growth. Overall, our results indicate that eribulin could reduce osteosarcoma lung metastasis.

APC2 associates with the actin cortex through a multipart mechanism to regulate cortical actin organization and dynamics in the Drosophila ovary

The actin cortex that lines the plasma membrane of most eukaryotic cells resists external mechanical forces and plays critical roles in a variety of cellular processes including morphogenesis, cytokinesis, and cell migration. Despite its ubiquity and significance, we understand relatively little about the composition, dynamics, and structure of the actin cortex. Adenomatous polyposis coli (APC) proteins regulate the actin and microtubule cytoskeletons through a variety of mechanisms, and in some contexts, APC proteins are cortically enriched. Here we show that APC2 regulates cortical actin dynamics in the follicular epithelium and the nurse cells of the Drosophila ovary and in addition affects the distribution of cortical actin at the apical side of the follicular epithelium. To understand how APC2 influences these properties of the actin cortex, we investigated the mechanisms controlling the cortical localization of APC2 in S2 cultured cells. We previously showed that the N-terminal half of APC2 containing the Armadillo repeats and the C-terminal 30 amino acids (C30) are together necessary and sufficient for APC2's cortical localization. Our work presented here supports a model that cortical localization of APC2 is governed in part by self-association through the N-terminal APC Self-Association Domain (ASAD) and a highly conserved coiled-coil within the C30 domain.

Truncated Adenomatous Polyposis Coli Mutation Induces Asef-Activated Golgi Fragmentation

Adenomatous polyposis coli (APC) is a key molecule to maintain cellular homeostasis in colonic epithelium by regulating cell-cell adhesion, cell polarity, and cell migration through activating the APC-stimulated guanine nucleotide-exchange factor (Asef). The APC-activated Asef stimulates the small GTPase, which leads to decreased cell-cell adherence and cell polarity, and enhanced cell migration. In colorectal cancers, while truncated APC constitutively activates Asef and promotes cancer initiation and progression, regulation of Asef by full-length APC is still unclear. Here, we report the autoinhibition mechanism of full-length APC. We found that the armadillo repeats in full-length APC interact with the APC residues 1362 to 1540 (APC-2,3 repeats), and this interaction competes off and inhibits Asef. Deletion of APC-2,3 repeats permits Asef interactions leading to downstream signaling events, including the induction of Golgi fragmentation through the activation of the Asef-ROCK-MLC2. Truncated APC also disrupts protein trafficking and cholesterol homeostasis by inhibition of SREBP2 activity in a Golgi fragmentation-dependent manner. Our study thus uncovers the autoinhibition mechanism of full-length APC and a novel gain of function of truncated APC in regulating Golgi structure, as well as cholesterol homeostasis, which provides a potential target for pharmaceutical intervention against colon cancers.

The Rb tumor suppressor regulates epithelial cell migration and polarity

Altered cell polarity and migration are hallmarks of cancer and metastases. Here we show that inactivation of the retinoblastoma gene (Rb) tumor suppressor causes defects in tissue closure that reflect the inability of Rb null epithelial cells to efficiently migrate and polarize. These defects occur independently of pRB's anti-proliferative role and instead correlate with upregulation of RhoA signaling and mislocalization of apical-basal polarity proteins. Notably, concomitant inactivation of tp53 specifically overrides the motility defect, and not the aberrant polarity, thereby uncovering previously unappreciated mechanisms by which Rb and tp53 mutations cooperate to promote cancer development and metastases.

Integrated control of formin-mediated actin assembly by a stationary inhibitor and a mobile activator

This study shows that in vivo actin nucleation by the yeast formin Bnr1 is controlled through the coordinated effects of two distinct regulators, a stationary inhibitor (the F-BAR protein Hof1) and a mobile activator (Bud6), establishing a positive feedback loop for precise spatial and temporal control of actin assembly.

Formins are essential actin assembly factors whose activities are controlled by a diverse array of binding partners. Until now, most formin ligands have been studied on an individual basis, leaving open the question of how multiple inputs are integrated to regulate formins in vivo. Here, we show that the F-BAR domain of Saccharomyces cerevisiae Hof1 interacts with the FH2 domain of the formin Bnr1 and blocks actin nucleation. Electron microscopy of the Hof1–Bnr1 complex reveals a novel dumbbell-shaped structure, with the tips of the F-BAR holding two FH2 dimers apart. Deletion of Hof1’s F-BAR domain in vivo results in disorganized actin cables and secretory defects. The formin-binding protein Bud6 strongly alleviates Hof1 inhibition in vitro, and bud6Δ suppresses hof1Δ defects in vivo. Whereas Hof1 stably resides at the bud neck, we show that Bud6 is delivered to the neck on secretory vesicles. We propose that Hof1 and Bud6 functions are intertwined as a stationary inhibitor and a mobile activator, respectively.

Actin and microtubule cross talk mediates persistent polarized growth

How the actin and microtubule cytoskeletons work together during diverse cellular functions is unclear. Wu et al. describe an apical actin pool in plant cells organized by a microtubule template at the site of polarized growth. Disconnecting the two cytoskeletons by removing class VIII myosins alters both cytoskeletal structures and impairs polarized growth.

Coordination between actin and microtubules is important for numerous cellular processes in diverse eukaryotes. In plants, tip-growing cells require actin for cell expansion and microtubules for orientation of cell expansion, but how the two cytoskeletons are linked is an open question. In tip-growing cells of the moss Physcomitrella patens, we show that an actin cluster near the cell apex dictates the direction of rapid cell expansion. Formation of this structure depends on the convergence of microtubules near the cell tip. We discovered that microtubule convergence requires class VIII myosin function, and actin is necessary for myosin VIII–mediated focusing of microtubules. The loss of myosin VIII function affects both networks, indicating functional connections among the three cytoskeletal components. Our data suggest that microtubules direct localization of formins, actin nucleation factors, that generate actin filaments further focusing microtubules, thereby establishing a positive feedback loop ensuring that actin polymerization and cell expansion occur at a defined site resulting in persistent polarized growth.

Focal Adhesions Undergo Longitudinal Splitting into Fixed-Width Units

Focal adhesions (FAs) and stress fibers (SFs) act in concert during cell motility and in response to the extracellular environment. Although the structures of mature FAs and SFs are well studied, less is known about how they assemble and mature de novo during initial cell spreading. In this study using live-cell Airyscan microscopy, we find that FAs undergo "splitting" during their assembly, in which the FA divides along its longitudinal axis. Before splitting, FAs initially appear as assemblies of multiple linear units (FAUs) of 0.3-μm width. Splitting occurs between FAUs, resulting in mature FAs of either a single FAU or of a small number of FAUs that remain attached at their distal tips. Variations in splitting occur based on cell type and extracellular matrix. Depletion of adenomatous polyposis coli (APC) or vasodilator-stimulated phosphoprotein (VASP) results in reduced splitting. FA-associated tension increases progressively during splitting. Early in cell spreading, ventral SFs are detected first, with other SF sub-types (transverse arcs and dorsal SFs) being detected later. Our findings suggest that the fundamental unit of FAs is the fixed-width FAU, and that dynamic interactions between FAUs control adhesion morphology.

Cholesterol Depletion by TASIN-1 Induces Apoptotic Cell Death through the ER Stress/ROS/JNK Signaling in Colon Cancer Cells

Truncated APC selective inhibitor-1 (TASIN-1) is a recently identified small molecule that selectively kills colorectal cancer cells that express truncated adenomatous polyposis coli (APC) by reducing cellular cholesterol levels. However, the downstream mechanism responsible for its cytotoxicity is not well understood. In this study, we show that TASIN-1 leads to apoptotic cell death via inducing ER stress-dependent JNK activation in human truncated APC colon cancer cells, accompanied by production of reactive oxygen species (ROS). In addition, TASIN-1 inhibits AKT activity through a cholesterol-dependent manner. Human colon tumor xenografts in immunodeficient mice also show the same TASIN-1 induced molecular mechanisms of tumor cell death as observed in vitro Taken together, cholesterol depletion by TASIN-1 treatment induces apoptotic cell death through activating ER stress/ROS/JNK axis and inhibiting AKT pro-survival signaling in colon cancer cells with truncated APC both in vitro and in vivoMol Cancer Ther; 17(5); 943-51. ©2018 AACR.

All-Round Manipulation of the Actin Cytoskeleton by HIV

While significant progress has been made in terms of human immunodeficiency virus (HIV) therapy, treatment does not represent a cure and remains inaccessible to many people living with HIV. Continued mechanistic research into the viral life cycle and its intersection with many aspects of cellular biology are not only fundamental in the continued fight against HIV, but also provide many key observations of the workings of our immune system. Decades of HIV research have testified to the integral role of the actin cytoskeleton in both establishing and spreading the infection. Here, we review how the virus uses different strategies to manipulate cellular actin networks and increase the efficiency of various stages of its life cycle. While some HIV proteins seem able to bind to actin filaments directly, subversion of the cytoskeleton occurs indirectly by exploiting the power of actin regulatory proteins, which are corrupted at multiple levels. Furthermore, this manipulation is not restricted to a discrete class of proteins, but rather extends throughout all layers of the cytoskeleton. We discuss prominent examples of actin regulators that are exploited, neutralized or hijacked by the virus, and address how their coordinated deregulation can lead to changes in cellular behavior that promote viral spreading.