The cellular distribution of the adenomatous polyposis coli tumour suppressor protein in neuroblastoma cells is regulated by microtubule dynamics

Beyond Trophic Factors: Exploiting the Intrinsic Regenerative Properties of Adult Neurons

Injuries and diseases of the peripheral nervous system (PNS) are common but frequently irreversible. It is often but mistakenly assumed that peripheral neuron regeneration is robust without a need to be improved or supported. However, axonal lesions, especially those involving proximal nerves rarely recover fully and injuries generally are complicated by slow and incomplete regeneration. Strategies to enhance the intrinsic growth properties of reluctant adult neurons offer an alternative approach to consider during regeneration. Since axons rarely regrow without an intimately partnered Schwann cell (SC), approaches to enhance SC plasticity carry along benefits to their axon partners. Direct targeting of molecules that inhibit growth cone plasticity can inform important regenerative strategies. A newer approach, a focus of our laboratory, exploits tumor suppressor molecules that normally dampen unconstrained growth. However several are also prominently expressed in stable adult neurons. During regeneration their ongoing expression “brakes” growth, whereas their inhibition and knockdown may enhance regrowth. Examples have included phosphatase and tensin homolog deleted on chromosome ten (PTEN), a tumor suppressor that inhibits PI3K/pAkt signaling, Rb1, the protein involved in retinoblastoma development, and adenomatous polyposis coli (APC), a tumor suppressor that inhibits β-Catenin transcriptional signaling and its translocation to the nucleus. The identification of several new targets to manipulate the plasticity of regenerating adult peripheral neurons is exciting. How they fit with canonical regeneration strategies and their feasibility require additional work. Newer forms of nonviral siRNA delivery may be approaches for molecular manipulation to improve regeneration.

Kinase activity of endosomal kinase LMTK1A regulates its cellular localization and interactions with cytoskeletons

Neurite formation, a fundamental process in neuronal maturation, requires the coordinated regulation of cytoskeletal reorganization and membrane transport. Compared to the understanding of cytoskeletal functions, less is known about the supply of membranes to growing neurites. Lemur kinase 1A (LMTK1A) is an endosomal protein kinase that is highly expressed in neurons. We recently reported that LMTK1A regulates the trafficking of Rab11-positive recycling endosomes in growing axons and dendrites. Here, we used the kinase-negative (kn) mutant to investigate the role of the kinase activity of LMTK1A in its cellular localization and interactions with the cytoskeleton in Neuro2A and PC-12 cells. Kinase activity was required for the localization of LMTK1A in the perinuclear endocytic recycling compartment. Perinuclear accumulation was microtubule dependent, and LMTK1A wild type (wt) localized mainly on microtubules, whereas kn LMTK1A was found in the actin-rich cell periphery. In the neurites of PC-12 cells, LMTK1A showed contrasting distributions depending on the kinase activity, with wt being located in the microtubule-rich shaft and the kn form in the actin-rich tip. Taken together, these results suggest that the kinase activity of LMTK1A regulates the pathway for endosomal vesicles to transfer from microtubules to actin filaments at the tip of growing neurites.

Tumour Suppressor Adenomatous Polyposis Coli (APC) localisation is regulated by both Kinesin-1 and Kinesin-2

Microtubules and their associated proteins (MAPs) underpin the polarity of specialised cells. Adenomatous polyposis coli (APC) is one such MAP with a multifunctional agenda that requires precise intracellular localisations. Although APC has been found to associate with kinesin-2 subfamily members, the exact mechanism for the peripheral localization of APC remains unclear. Here we show that the heavy chain of kinesin-1 directly interacts with the APC C-terminus, contributing to the peripheral localisation of APC in fibroblasts. In rat hippocampal neurons the kinesin-1 binding domain of APC is required for its axon tip enrichment. Moreover, we demonstrate that APC requires interactions with both kinesin-2 and kinesin-1 for this localisation. Underlining the importance of the kinesin-1 association, neurons expressing APC lacking kinesin-1-binding domain have shorter axons. The identification of this novel kinesin-1-APC interaction highlights the complexity and significance of APC localisation in neurons.

Adenomatous polyposis coli is differentially distributed in growth cones and modulates their steering

Axonal steering reactions depend on the transformation of environmental information into internal, directed structures, which is achieved by differential modulation of the growth cone cytoskeleton; key elements are the microtubules, which are regulated in their dynamics by microtubule-associated proteins (MAPs). We investigated a potential role of the MAP adenomatous polyposis coli (APC) for growing axons, employing embryonic visual system as a model system. APC is concentrated in the distalmost (i.e., growing) region of retinal ganglion cell axons in vivo and in vitro. Within the growth cone, APC is enriched in the central domain; it only partially colocalizes with microtubules. When axons are induced to turn toward a cell or away from a substrate border, APC is present in the protruding and absent from the collapsing growth cone regions, thus indicating the future growth direction of the axon. To assess the functional role of the differential distribution of APC in navigating growth cones, the protein was inactivated via micro-scale chromophore-assisted laser inactivation in one half of the growth cone. If the N-terminal APC region (crucial for its oligomerization) is locally inactivated, the treated growth cone side collapses and the axon turns away. In contrast, if the 20 aa repeats in the middle region of APC (which can negatively regulate its microtubule association) are inactivated, protrusions are formed and the growth cone turns toward. Our data thus demonstrate a crucial role of APC for axon steering attributable to its multifunctional domain structure and differential distribution in the growth cone.

Adenomatous polyposis coli localization is both cell type and cell context dependent

The adenomatous polyposis coli (APC) tumor suppressor protein is mutated in most colorectal carcinomas. In addition to its role in WNT signaling it is proposed to be involved in both cell migration and mitosis. Although a variety of studies have shown an APC localization along lateral membranes of adjacent epithelial cells the existence of a cortical APC localization in mammalian cells remains controversial. To address this we have used matched rat epithelial (NRK-52E) and fibroblast (NRK-49F) cell lines to investigate the localization of APC. Subconfluent cultures of NRK-52E and -49F cells displayed microtubule-associated APC populations by immunostaining. However, confluent NRK-52E, but not -49F monolayers, exhibited a cortical APC distribution. Cortical APC localized in close proximity to a number of cell junction proteins in a microtubule-independent manner while calcium switch experiments suggested that APC was recruited to the cortex only when junction assembly was complete. Confluent NRK-49F and -52E cells also showed contrasting APC localizations in response to monolayer wounding. Our data suggests APC cortical localization is a feature of confluent epithelioid cells and that the subcellular distribution of APC is therefore dependent upon both cell type and context.

Examination of actin and microtubule dependent APC localisations in living mammalian cells

BACKGROUND

The trafficking of the adenomatous polyposis coli (APC) tumour suppressor protein in mammalian cells is a perennially controversial topic. Immunostaining evidence for an actin-associated APC localisation at intercellular junctions has been previously presented, though live imaging of mammalian junctional APC has not been documented.

RESULTS

Using live imaging of transfected COS-7 cells we observed intercellular junction-associated pools of GFP-APC in addition to previously documented microtubule-associated GFP-APC and a variety of minor localisations. Although both microtubule and junction-associated populations could co-exist within individual cells, they differed in their subcellular location, dynamic behaviour and sensitivity to cytoskeletal poisons. GFP-APC deletion mutant analysis indicated that a protein truncated immediately after the APC armadillo repeat domain retained the ability to localise to adhesive membranes in transfected cells. Supporting this, we also observed junctional APC immunostaining in cultures of human colorectal cancer cell line that express truncated forms of APC.

CONCLUSION

Our data indicate that APC can be found in two spatially separate populations at the cell periphery and these populations can co-exist in the same cell. The first localisation is highly dynamic and associated with microtubules near free edges and in cell vertices, while the second is comparatively static and is closely associated with actin at sites of cell-cell contact. Our imaging confirms that human GFP-APC possesses many of the localisations and behaviours previously seen by live imaging of Xenopus GFP-APC. However, we report the novel finding that GFP-APC puncta can remain associated with the ends of shrinking microtubules. Deletion analysis indicated that the N-terminal region of the APC protein mediated its junctional localisation, consistent with our observation that truncated APC proteins in colon cancer cell lines are still capable of localising to the cell cortex. This may have implications for the development of colorectal cancer.

Non-traditional roles for the Adenomatous Polyposis Coli (APC) tumor suppressor protein

The Adenomatous Polyposis Coli (APC) tumor suppressor is a multifunctional protein that is mutated in a majority of colon cancers. The role of APC as an antagonist of the Wnt signaling pathway is well known and it is widely accepted that inappropriate activation of this pathway through loss of APC function contributes to the progression of colon cancers. However, a body of evidence is growing to support the idea that APC plays non-traditional functions outside of the Wnt pathway with roles in cell migration, adhesion, chromosome segregation, spindle assembly, apoptosis, and neuronal differentiation. This review highlights the research into alternate functions for APC beyond its role in Wnt signaling and discusses the possible contributions for these non-traditional functions of APC in tumor formation.

Surfing, regulating and capturing: are all microtubule-tip-tracking proteins created equal?

A diverse group of microtubule-binding proteins has been linked through live-cell imaging of green fluorescent protein (GFP) fusion proteins. These proteins share the ability to associate with the plus ends of elongating microtubules and track with these tips as the microtubules grow, in a process known as "tip tracking". Several models have been proposed to explain the significance of this activity, including roles in delivering proteins to the cell periphery and in modulating microtubule dynamics. However, the recent observation that some of the tip trackers colocalize on structures undergoing search-capture suggests that tip tracking could be a fundamental aspect of the search-capture process. Focusing on the shared ability of these proteins to undergo tip tracking, this article is intended to place the search-capture model in the context of other proposed functions and to stimulate discussion in this area.

Microtubules and growth cone function

It has been recognized for a long time that the neuronal cytoskeleton plays an important part in neurite growth and growth cone pathfinding, the mechanism by which growing axons find an appropriate route through the developing embryo to their target cells. In the growth cone, many intracellular signaling pathways that are activated by guidance cues converge on the growth cone cytoskeleton and regulate its dynamics. Most of the research effort in this area has focussed on the actin, microfilament cytoskeleton of the growth cone, principally because it underlies growth cone motility, the extension and retraction of filopodia and lamellipodia, and these structures are the first to encounter guidance cues during growth cone advance. However, more recently, it has become apparent that the microtubule cytoskeleton also has a role in growth cone pathfinding and is also regulated by guidance cues operating through intracellular signaling pathways via engagement with cell membrane receptors. Furthermore, recent work has revealed an interaction between these two components of the growth cone cytoskeleton that is probably essential for growth cone turning, a fundamental growth cone behavior during pathfinding. In this short review I discuss recent experiments that uncover the function of microtubules in growth cones, how their behavior is regulated, and how they interact with the actin filaments.

Dynamic properties of APC-decorated microtubules in living cells

The adenomatous polyposis coli (APC) tumour suppressor protein is a component of the Wnt signalling pathway in which it plays a major role in controlling nuclear accumulation of beta-catenin and hence in the modulation of beta-catenin-regulated gene transcription. APC also associates with microtubules at the ends of cytoplasmic extensions in epithelial cells, a distribution that can be reproduced in COS cells ectopically expressing APC. To examine the effect of APC on microtubule properties, we monitored directly the behaviour of APC and of APC-decorated microtubules by time-lapse imaging of cytoplasmic extensions in live COS cells expressing APC tagged with a green fluorescent protein. On the proximal part of microtubules, APC was visualised as particulate material moving unidirectionally towards the plus end of microtubules. The distal parts of microtubules were uniformly decorated by APC and were animated by a motile behaviour in the form of aperiodic bending. This behaviour is likely to be the consequence of compression forces acting on microtubules encountering obstacles while elongating. The majority of APC-decorated microtubules in transfected COS cells was sensitive to depolymerisation by nocodazole, but they contained detyrosinated and acetylated alpha-tubulin, suggesting a reduction in the rate of subunit exchange at their growing end. Taken together, these results demonstrate that microtubule domains uniformly decorated by APC display dynamic and motile properties that may be significant for the postulated role of APC in targeting microtubules to specialised membrane sites.

Direct Raman imaging techniques for study of the subcellular distribution of a drug

Direct Raman imaging techniques are demonstrated to study the drug distribution in living cells. The advantage of Raman imaging is that no external markers are required, which simplifies the sample preparation and minimally disturbs the drug mechanism during imaging. The major challenge in Raman imaging is the weak Raman signal. In this study, we present a Raman image model to describe the degradation of Raman signals by imaging processes. Using this model, we demonstrate special-purpose image-processing algorithms to restore the Raman images. The processing techniques are then applied to visualize the anticancer agent paclitaxel in living MDA-435 breast cancer cells. Raman images were obtained from a cancer cell before, during, and after drug treatment. The paclitaxel distribution illustrated in these images is explained by means of the binding characteristics of the paclitaxel and its molecular target-the microtubules. This result demonstrates that direct Raman imaging is a promising tool to study the distribution of a drug in living cells.

Regulation of microtubule-associated proteins

Microtubule-associated proteins (MAPs) function to regulate the assembly dynamics and organization of microtubule polymers. Upstream regulation of MAP activities is the major mechanism used by cells to modify and control microtubule assembly and organization. This review summarizes the functional activities of MAPs found in animal cells and discusses how these MAPs are regulated. Mechanisms controlling gene expression, isoform-specific expression, protein localization, phosphorylation, and degradation are discussed. Additional regulatory mechanisms include synergy or competition between MAPs and the activities of cofactors or binding partners. For each MAP it is likely that regulation in vivo reflects a composite of multiple regulatory mechanisms.

EB1 identifies sites of microtubule polymerisation during neurite development

EB1 is a microtubule associated protein which interacts with the APC tumour suppressor protein and components of the cytoplasmic dynein/dynactin complex. EB1 is also a specific marker of growing microtubule tips. Here we demonstrate that EB1 protein levels are increased during axon but not dendrite formation in differentiated N2A neuroblastoma cells, and that EB1 localises to microtubule tips throughout extending neurites in these cells. In N2A axons, analysis of the ratio of EB1/beta-tubulin fluorescence demonstrated that the distal tip region contained the highest proportion of polymerising microtubules. Time-lapse confocal imaging of an EB1-GFP fusion protein in transfected N2A cells directly revealed the dynamics of microtubule extension in neurites, and demonstrated the existence of unusual, discrete knots of microtubule polymerisation at the periphery of non-process bearing cells which may represent an early event in neurite outgrowth. We conclude that EB1 localisation can be used to identify and analyse sites of microtubule polymerisation at a high resolution during neurite development, a process to which it may contribute.

Regulation of microtubule assembly by human EB1 family proteins

The EB1 family proteins are highly conserved microtubule-associated proteins. The EB1 protein in yeast has been shown to play an important role in regulating microtubule dynamics and chromosome segregation. Human EB1 family proteins include EB1, RP1 and EBF3. Although EB1 and RP1 have been shown to associate with microtubules, the subcellular localization of endogenous EBF3 had not been characterized. The function of human EB1 family proteins was also not clear. We therefore investigated the cellular localization of EBF3 and the regulation of microtubule organization by EB1 family proteins. As do EB1 and RP1, EBF3 was found to colocalize with microtubules, preferentially at their plus ends, throughout the cell cycle. Moreover, there was a very strong EBF3 signal at the centrosome in interphase cells and at the spindle poles in mitotic cells. When EB1 family proteins were overexpressed, they associated with the entire microtubule cytoskeleton. In addition, EB1 and EBF3 induced microtubule bundling in some cells overexpressing these proteins. These microtubule bundles were more resistant to nocodazole and were more acetylated than regular microtubules. Our results demonstrate for the first time that human EB1 family proteins could regulate microtubule assembly and stability.

Human homologue of the Drosophila discs large tumor suppressor protein forms an oligomer in solution. Identification of the self-association site

The human homologue of the Drosophila discs large tumor suppressor protein (hDlg), a member of the membrane-associated guanylate kinase (MAGUK) superfamily, interacts with K(+) channels, N-methyl-d-aspartate receptors, calcium ATPase, adenomatous polyposis coli, and PTEN tumor suppressor proteins, and several viral oncoproteins through its PDZ domains. MAGUKs play pivotal roles in the clustering and aggregation of receptors, ion channels, and cell adhesion molecules at the synapses. To investigate the physiological basis of hDlg interactions, we examined the self-association state of full-length hDlg as well as defined segments of hDlg expressed as recombinant proteins in bacteria and insect Sf9 cells. Gel permeation chromatography of full-length hDlg revealed that the purified protein migrates as a large particle of size >440 kDa. Similar measurements of defined domains of hDlg indicated that the anomalous mobility of hDlg originated from its amino-terminal domain. Ultrastructural analysis of hDlg by low angle rotary shadow electron microscopy revealed that the full-length hDlg protein as well as its amino-terminal domain exhibits a highly flexible irregular shape. Further evaluation of the self-association state of hDlg using sedimentation equilibrium centrifugation, matrix-assisted laser desorption/ionization mass spectrometry, and chemical cross-linking techniques confirmed that the oligomerization site of hDlg is contained within its amino-terminal domain. This unique amino-terminal domain mediates multimerization of hDlg into dimeric and tetrameric species in solution. Sedimentation velocity experiments demonstrated that the oligomerization domain exists as an elongated tetramer in solution. In vitro mutagenesis was used to demonstrate that a single cysteine residue present in the oligomerization domain of hDlg is not required for its self-association. Understanding the oligomerization status of hDlg may help to explicate the mechanism of hDlg association with multimeric K(+) channels and dimeric adenomatous polyposis coli tumor suppressor protein. Our findings, therefore, begin to rationalize the role of hDlg in the clustering of membrane channels and formation of multiprotein complexes necessary for signaling and cell proliferation pathways.

Regulation and function of the interaction between the APC tumour suppressor protein and EB1

The interaction between the adenomatous polyposis coli (APC) tumour suppressor and the microtubule-associated protein EB1 was examined. Immunoprecipitation suggested that APC and EB1 were not associated in cultures of HCT116 cells arrested in mitosis. The C-terminal 170 amino acids of APC, purified as a bacterial fusion protein, precipitated EB1 from cell extracts, significantly refining the location of the EB1 interaction domain in APC. In vitro phosphorylation of this fusion protein by either protein kinase A or p34cdc2 reduced its ability to bind to EB1. Expression of GFP fusions to C-terminal APC sequences lacking or including the APC basic domain but encompassing the EB1 binding region in SW480 cells revealed a microtubule tip association which co-localized with that of EB1. Expression of the basic domain alone revealed a non-specific microtubule localization. In vitro interaction studies confirmed that the APC basic domain did not contribute to EB1 binding. These findings strongly suggest that the interaction between APC and EB1 targets APC to microtubule tips, and that the interaction between the two proteins is down-regulated during mitosis by the previously described mitotic phosphorylation of APC.

The adenomatous polyposis coli protein

Mutations in the adenomatous polyposis coli (APC) gene are associated with most colorectal cancers. The APC protein has been implicated in many aspects of tumour development. This article will discuss recent data suggesting that APC may have multiple functions in the cell. First, APC is a component of the Wnt signalling pathway; second, APC may have a role in cell migration; finally, APC may regulate proliferation and apoptosis.

Lithium induces morphological differentiation of mouse neuroblastoma cells

Neuroblastoma cells are used as a model system to study neuronal differentiation. Here we describe the induction of morphological differentiation of mouse neuroblastoma Neuro 2a (N2a) cells by treatments with either chemical inhibitors of cyclin-dependent kinases or lithium, which inhibits glycogen synthase kinase-3. Cyclin-dependent kinase inhibitors cause a rapid cell cycle block as well as the extension of multiple neurites per cell. These multipolar differentiated cells then undergo a massive death. However, lithium promotes a delayed mitotic arrest and the extension of one or two long neurites per cell. This differentiation is maximal after 48 hours of lithium treatment and the differentiated cells remain viable for long periods of time. Neuronal differentiation in lithium-treated cells is preceded by the accumulation of beta-catenin, a protein which is efficiently proteolyzed when it is phosphorylated by glycogen synthase kinase-3. Both neuronal differentiation and beta-catenin accumulation are observed in lithium-treated cells either in the absence or in the presence of supraphysiological concentrations of inositol. The results are consistent with the hypothesis that inhibition of glycogen synthase kinase-3 by lithium triggers the differentiation of neuroblastoma N2a cells.

Apc1638T: a mouse model delineating critical domains of the adenomatous polyposis coli protein involved in tumorigenesis and development

The adenomatous polyposis coli (APC) gene is considered as the true gatekeeper of colonic epithelial proliferation: It is mutated in the majority of colorectal tumors, and mutations occur at early stages of tumor development in mouse and man. These mutant proteins lack most of the seven 20-amino-acid repeats and all SAMP motifs that have been associated with down-regulation of intracellular beta-catenin levels. In addition, they lack the carboxy-terminal domains that bind to DLG, EB1, and microtubulin. APC also appears to be essential in development because homozygosity for mouse Apc mutations invariably results in early embryonic lethality. Here, we describe the generation of a mouse model carrying a targeted mutation at codon 1638 of the mouse Apc gene, Apc1638T, resulting in a truncated Apc protein encompassing three of the seven 20 amino acid repeats and one SAMP motif, but missing all of the carboxy-terminal domains thought to be associated with tumorigenesis. Surprisingly, homozygosity for the Apc1638T mutation is compatible with postnatal life. However, homozygous mutant animals are characterized by growth retardation, a reduced postnatal viability on the B6 genetic background, the absence of preputial glands, and the formation of nipple-associated cysts. Most importantly, Apc1638T/1638T animals that survive to adulthood are tumor free. Although the full complement of Apc1638T is sufficient for proper beta-catenin signaling, dosage reductions of the truncated protein result in increasingly severe defects in beta-catenin regulation. The SAMP motif retained in Apc1638T also appears to be important for this function as shown by analysis of the Apc1572T protein in which its targeted deletion results in a further reduction in the ability of properly controlling beta-catenin/Tcf signaling. These results indicate that the association with DLG, EB1, and microtubulin is less critical for the maintenance of homeostasis by APC than has been suggested previously, and that proper beta-catenin regulation by APC appears to be required for normal embryonic development and tumor suppression.

Functional interaction of an axin homolog, conductin, with beta-catenin, APC, and GSK3beta

Control of stability of beta-catenin is central in the wnt signaling pathway. Here, the protein conductin was found to form a complex with both beta-catenin and the tumor suppressor gene product adenomatous polyposis coli (APC). Conductin induced beta-catenin degradation, whereas mutants of conductin that were deficient in complex formation stabilized beta-catenin. Fragments of APC that contained a conductin-binding domain also blocked beta-catenin degradation. Thus, conductin is a component of the multiprotein complex that directs beta-catenin to degradation and is located downstream of APC. In Xenopus embryos, conductin interfered with wnt-induced axis formation.

Expression of beta-catenin and the adenomatous polyposis coli tumour suppressor protein in mouse neocortical cells in vitro

Beta-catenin is known to associate with the tumour suppressor protein adenomatous polyposis coli (APC), which is highly expressed in developing brain. We have therefore investigated the distribution of beta-catenin and APC in primary cultures of mouse neocortex. Western blotting demonstrated the presence of a single beta-catenin species in our cultures. Immunocytochemistry showed that beta-catenin was plasma membrane associated and concentrated in growth cones in cultured neurons. The APC tumour suppressor protein was also concentrated in growth cones. In glial cells, beta-catenin was localised at cell-cell contacts in a manner similar to that previously described in other cell types. This data suggests a role for both APC and beta-catenin in neuronal growth cones, and for beta-catenin in the formation of cell to cell contacts between glia.