AXIN2 promotes degradation of AXIN1 through tankyrase in colorectal cancer cells.

AXIN1 and AXIN2 are homologous proteins that inhibit the Wnt/ß-catenin signaling pathway, which is frequently hyperactive in colorectal cancer. Stabilization of AXIN1 and AXIN2 by inhibiting their degradation through tankyrase (TNKS) allows the attenuation of Wnt signaling in cancer, attracting interest for potential targeted therapy. Here, we found that knockout or knockdown of AXIN2 in colorectal cancer cells increased the protein stability of AXIN1. The increase in AXIN1 overcompensated for the loss of AXIN2 with respect to protein levels; however, functionally it did not because loss of AXIN2 activated the pathway. Moreover, AXIN2 was highly essential in the context of TNKS inhibition because TNKS-targeting small-molecule inhibitors completely failed to inhibit Wnt signaling and to stabilize AXIN1 in AXIN2 knockout cells. The increased AXIN1 protein stability and the impaired stabilization by TNKS inhibitors indicated disrupted TNKS-AXIN1 regulation in AXIN2 knockout cells. Concordantly, mechanistic studies revealed that co-expression of AXIN2 recruited TNKS to AXIN1 and stimulated TNKS-mediated degradation of transiently expressed AXIN1 wild-type and AXIN1 mutants with impaired TNKS binding. Taken together, our data suggest that AXIN2 promotes degradation of AXIN1 through TNKS in colorectal cancer cells by directly linking the two proteins, and these findings may be relevant for TNKS inhibition-based colorectal cancer therapies.

Phosphorylation of axin within biomolecular condensates counteracts its tankyrase-mediated degradation.

Axin (also known as AXIN1) is a central negative regulator of the proto-oncogenic Wnt/ß-catenin signaling pathway, as axin condensates provide a scaffold for the assembly of a multiprotein complex degrading ß-catenin. Axin, in turn, is degraded through tankyrase. Consequently, tankyrase small-molecule inhibitors block Wnt signaling by stabilizing axin, revealing potential for cancer therapy. Here, we discovered that axin is phosphorylated by casein kinase 1 alpha 1 (CSNK1A1, also known as CK1α) at an N-terminal casein kinase 1 consensus motif, and that this phosphorylation is antagonized by the catalytic subunit alpha of protein phosphatase 1 (PPP1CA, hereafter referred to as PP1). Axin condensates promoted phosphorylation by enriching CK1α over PP1. Importantly, the phosphorylation took place within the tankyrase-binding site, electrostatically and/or sterically hindering axin-tankyrase interaction, and counteracting tankyrase-mediated degradation of axin. Thus, the presented data propose a novel mechanism regulating axin stability, with implications for Wnt signaling, cancer therapy and self-organization of biomolecular condensates.

Gαi2-induced conductin/axin2 condensates inhibit Wnt/ß-catenin signaling and suppress cancer growth.

Conductin/axin2 is a scaffold protein negatively regulating the pro-proliferative Wnt/ß-catenin signaling pathway. Accumulation of scaffold proteins in condensates frequently increases their activity, but whether condensation contributes to Wnt pathway inhibition by conductin remains unclear. Here, we show that the Gαi2 subunit of trimeric G-proteins induces conductin condensation by targeting a polymerization-inhibiting aggregon in its RGS domain, thereby promoting conductin-mediated ß-catenin degradation. Consistently, transient Gαi2 expression inhibited, whereas knockdown activated Wnt signaling via conductin. Colorectal cancers appear to evade Gαi2-induced Wnt pathway suppression by decreased Gαi2 expression and inactivating mutations, associated with shorter patient survival. Notably, the Gαi2-activating drug guanabenz inhibited Wnt signaling via conductin, consequently reducing colorectal cancer growth in vitro and in mouse models. In summary, we demonstrate Wnt pathway inhibition via Gαi2-triggered conductin condensation, suggesting a tumor suppressor function for Gαi2 in colorectal cancer, and pointing to the FDA-approved drug guanabenz for targeted cancer therapy.

SIRT6-CBP-dependent nuclear Tau accumulation and its role in protein synthesis.

Several neurodegenerative diseases present Tau accumulation as the main pathological marker. Tau post-translational modifications such as phosphorylation and acetylation are increased in neurodegeneration. Here, we show that Tau hyper-acetylation at residue 174 increases its own nuclear presence and is the result of DNA damage signaling or the lack of SIRT6, both causative of neurodegeneration. Tau-K174ac is deacetylated in the nucleus by SIRT6. However, lack of SIRT6 or chronic DNA damage results in nuclear Tau-K174ac accumulation. Once there, it induces global changes in gene expression, affecting protein translation, synthesis, and energy production. Concomitantly, Alzheimer's disease (AD) case subjects show increased nucleolin and a decrease in SIRT6 levels. AD case subjects present increased levels of nuclear Tau, particularly Tau-K174ac. Our results suggest that increased Tau-K174ac in AD case subjects is the result of DNA damage signaling and SIRT6 depletion. We propose that Tau-K174ac toxicity is due to its increased stability, nuclear accumulation, and nucleolar dysfunction.

Analysis of the Circular Transcriptome in the Synaptosomes of Aged Mice.

Recently, circular RNAs (circRNAs) have been revealed to be an important non-coding element of the transcriptome. The brain contains the most abundant and widespread expression of circRNA. There are also indications that the circular transcriptome undergoes dynamic changes as a result of brain ageing. Diminished cognitive function with increased age reflects the dysregulation of synaptic function and ineffective neurotransmission through alterations of the synaptic proteome. Here, we present changes in the circular transcriptome in ageing synapses using a mouse model. Specifically, we observed an accumulation of uniquely expressed circular transcripts in the synaptosomes of aged mice compared to young mice. Individual circRNA expression patterns were characterized by an increased abundance in the synaptosomes of young or aged mice, whereas the opposite expression was observed for the parental gene linear transcripts. These changes in expression were validated by RT-qPCR. We provide the first comprehensive survey of the circular transcriptome in mammalian synapses, thereby paving the way for future studies. Additionally, we present 16 genes that express solely circRNAs, without linear RNAs co-expression, exclusively in young and aged synaptosomes, suggesting a synaptic gene network that functions along canonical splicing activity.

An aggregon in conductin/axin2 regulates Wnt/ß-catenin signaling and holds potential for cancer therapy.

The paralogous scaffold proteins axin and conductin/axin2 are key factors in the negative regulation of the Wnt pathway transcription factor ß-catenin, thereby representing interesting targets for signaling regulation. Polymerization of axin proteins is essential for their activity in suppressing Wnt/ß-catenin signaling. Notably, conductin shows less polymerization and lower activity than axin. By domain swapping between axin and conductin we here identify an aggregation site in the conductin RGS domain which prevents conductin polymerization. Induction of conductin polymerization by point mutations of this aggregon results in enhanced inhibition of Wnt/ß-catenin signaling. Importantly, we identify a short peptide which induces conductin polymerization via masking the aggregon, thereby enhancing ß-catenin degradation, inhibiting ß-catenin-dependent transcription and repressing growth of colorectal cancer cells. Our study reveals a mechanism for regulating signaling pathways via the polymerization status of scaffold proteins and suggests a strategy for targeted colorectal cancer therapy.

Sulforaphane inhibits growth and blocks Wnt/ß-catenin signaling of colorectal cancer cells.

The naturally occurring isothiocyanate sulforaphane (SFN) from cruciferous vegetables is associated with growth inhibition of various cancer types, including colorectal cancer. Colorectal cancer is most frequently driven by hyperactive Wnt/ß-catenin signaling. Here, we show that SFN treatment reduced growth of three unrelated colorectal cancer cell lines (SW480, DLD1 and HCT116) via induction of cell death and inhibition of proliferation. Importantly, SFN inhibits Wnt/ß-catenin signaling in colorectal cancer cells as shown by inhibition of ß-catenin-dependent luciferase reporters and repression of ß-catenin target genes (AXIN2, LGR5). SFN inhibits Wnt signaling downstream of ß-catenin degradation and induces the formation of nuclear ß-catenin structures associated with closed chromatin. Co-expression of the transcription factors LEF1 or TCF4 prevented formation of these structures and rescued inhibition of Wnt/ß-catenin signaling by SFN. Our findings provide a molecular basis explaining SFN effects in colorectal cancer cells and underline its potential for prevention and therapy of colorectal cancer.

Pgam5 released from damaged mitochondria induces mitochondrial biogenesis via Wnt signaling.

Mitochondrial abundance is dynamically regulated and was previously shown to be increased by Wnt/ß-catenin signaling. Pgam5 is a mitochondrial phosphatase which is cleaved by the rhomboid protease presenilin-associated rhomboid-like protein (PARL) and released from membranes after mitochondrial stress. In this study, we show that Pgam5 interacts with the Wnt pathway component axin in the cytosol, blocks axin-mediated ß-catenin degradation, and increases ß-catenin levels and ß-catenin-dependent transcription. Pgam5 stabilized ß-catenin by inducing its dephosphorylation in an axin-dependent manner. Mitochondrial stress triggered by carbonyl cyanide m-chlorophenyl hydrazone (CCCP) treatment led to cytosolic release of endogenous Pgam5 and subsequent dephosphorylation of ß-catenin, which was strongly diminished in Pgam5 and PARL knockout cells. Similarly, hypoxic stress generated cytosolic Pgam5 and led to stabilization of ß-catenin, which was abolished by Pgam5 knockout. Cells stably expressing cytosolic Pgam5 exhibit elevated ß-catenin levels and increased mitochondrial numbers. Our study reveals a novel mechanism by which damaged mitochondria might induce replenishment of the mitochondrial pool by cell-intrinsic activation of Wnt signaling via the Pgam5-ß-catenin axis.

Transgenerational transmission of an anticholinergic endophenotype with memory dysfunction.

Impaired cholinergic neurotransmission associated with cognitive dysfunction occurs in various mental disorders of different etiologies including Alzheimer's disease and postalcoholic dementia and others. To address the question whether there exists a common endophenotype with a defined genetic and/or epigenetic signature causing mental dysfunction in these disorders, we investigated 2 generations of offspring born to alcohol-treated mothers. Here, we show that memory impairment and reduced synthesis of acetylcholine occurs in both F1 (exposed to ethanol in utero) and F2 generation (never been exposed to ethanol). Effects in the F2 generation are most likely consequences of transgenerationally transmitted epigenetic modifications in stem cells induced by alcohol. This clearly documents the role of ancestral history of drug abuse on the brain development of subsequent generations. The results further suggest an epigenetic trait for an anticholinergic endophenotype associated with cognitive dysfunction which might be relevant to our understanding of mental impairment in neurodegenerative disorders such as Alzheimer's disease and related disorders.

GSK3ß-dependent dysregulation of neurodevelopment in SPG11-patient induced pluripotent stem cell model.

OBJECTIVE: Mutations in the spastic paraplegia gene 11 (SPG11), encoding spatacsin, cause the most frequent form of autosomal-recessive complex hereditary spastic paraplegia (HSP) and juvenile-onset amyotrophic lateral sclerosis (ALS5). When SPG11 is mutated, patients frequently present with spastic paraparesis, a thin corpus callosum, and cognitive impairment. We previously delineated a neurodegenerative phenotype in neurons of these patients. In the current study, we recapitulated early developmental phenotypes of SPG11 and outlined their cellular and molecular mechanisms in patient-specific induced pluripotent stem cell (iPSC)-derived cortical neural progenitor cells (NPCs). METHODS: We generated and characterized iPSC-derived NPCs and neurons from 3 SPG11 patients and 2 age-matched controls. RESULTS: Gene expression profiling of SPG11-NPCs revealed widespread transcriptional alterations in neurodevelopmental pathways. These include changes in cell-cycle, neurogenesis, cortical development pathways, in addition to autophagic deficits. More important, the GSK3ß-signaling pathway was found to be dysregulated in SPG11-NPCs. Impaired proliferation of SPG11-NPCs resulted in a significant diminution in the number of neural cells. The decrease in mitotically active SPG11-NPCs was rescued by GSK3 modulation. INTERPRETATION: This iPSC-derived NPC model provides the first evidence for an early neurodevelopmental phenotype in SPG11, with GSK3ß as a potential novel target to reverse the disease phenotype. Ann Neurol 2016;79:826-840.

Regional mosaic genomic heterogeneity in the elderly and in Alzheimer's disease as a correlate of neuronal vulnerability.

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by fibrillary aggregates of Aß peptide and tau protein. The distribution of these pathological hallmarks throughout the brain is not random; it follows a predictive pattern that is used for pathological staging. However, most etiopathogenetic concepts, irrespective of whether they focus on Aß or tau pathology, leave a key question unanswered: what is the explanation for the different vulnerabilities of brain regions in AD? The pattern of regional progression of neurofibrillary degeneration in AD to some extent inversely recapitulates ontogenetic and phylogenetic brain development. Accordingly, degeneration preferentially affects brain areas that have recently been acquired or restructured during anthropoid evolution, which means that the involvement of a neurodevelopmental mechanism is highly likely. Since evolutionary expansion of the neocortex is based on a substantial extension of the mitotic activity of progenitor cells, we propose a conceptual link between neurogenesis in anthropoid primates and a higher risk of accumulating mitotic errors that give rise to genomic aberrations commonly referred to as DNA content variation (DCV). If increased rates of DCV make neurons more vulnerable to AD-related pathology, one might expect there to be a higher rate of DCV in areas that are affected very early during the course of AD, as compared to areas which are hardly affected or are affected only during the most advanced stages. Therefore, in the present study, we comparatively analyzed the DCV in five different cortical areas that are affected during the early stage (entorhinal cortex), the intermediate stage (temporal, frontal, and parietal association cortex), and the late stage (primary sensory occipital cortex) of AD in both normal elderly subjects and AD patients. On average, we observed about 10 % neuronal mosaic DCV in the normal elderly and a two- to threefold increase in DCV in AD patients. We were able to demonstrate, moreover, that the neuronal DCV in the cerebral cortex of the normal elderly as well as the increased neuronal DCV in AD patients are not randomly distributed but instead show systematic regional differences which correspond to differences in vulnerability. These findings provide additional evidence that mosaic genomic heterogeneity may play a key role in AD pathology.

Early neurone loss in Alzheimer's disease: cortical or subcortical?

Alzheimer's disease (AD) is a degenerative disorder where the distribution of pathology throughout the brain is not random but follows a predictive pattern used for pathological staging. While the involvement of defined functional systems is fairly well established for more advanced stages, the initial sites of degeneration are still ill defined. The prevailing concept suggests an origin within the transentorhinal and entorhinal cortex (EC) from where pathology spreads to other areas. Still, this concept has been challenged recently suggesting a potential origin of degeneration in nonthalamic subcortical nuclei giving rise to cortical innervation such as locus coeruleus (LC) and nucleus basalis of Meynert (NbM). To contribute to the identification of the early site of degeneration, here, we address the question whether cortical or subcortical degeneration occurs more early and develops more quickly during progression of AD. To this end, we stereologically assessed neurone counts in the NbM, LC and EC layer-II in the same AD patients ranging from preclinical stages to severe dementia. In all three areas, neurone loss becomes detectable already at preclinical stages and is clearly manifest at prodromal AD/MCI. At more advanced AD, cell loss is most pronounced in the NbM > LC > layer-II EC. During early AD, however, the extent of cell loss is fairly balanced between all three areas without clear indications for a preference of one area. We can thus not rule out that there is more than one way of spreading from its site of origin or that degeneration even occurs independently at several sites in parallel.

Pin1 promotes degradation of Smad proteins and their interaction with phosphorylated tau in Alzheimer's disease.

AIMS: Neurodegeneration in Alzheimer's disease (AD) is characterized by pathological protein aggregates and inadequate activation of cell cycle regulating proteins. Recently, Smad proteins were identified to control the expression of AD relevant proteins such as APP, CDK4 and CDK inhibitors, both critical regulators of cell cycle activation. This might indicate a central role for Smads in AD pathology where they show a substantial deficiency and disturbed subcellular distribution in neurones. Still, the mechanisms driving relocation and decrease of neuronal Smad in AD are not well understood. However, Pin1, a peptidyl-prolyl-cis/trans-isomerase, which allows isomerization of tau protein, was recently identified also controlling the fate of Smads. Here we analyse a possible role of Pin1 for Smad disturbances in AD. METHODS: Multiple immunofluorescence labelling and confocal laser-scanning microscopy were performed to examine the localization of Smad and Pin1 in human control and AD hippocampi. Ectopic Pin1 expression in neuronal cell cultures combined with Western blot analysis and immunoprecipitation allowed studying Smad level and subcellular distribution. Luciferase reporter assays, electromobility shift, RNAi-technique and qRT-PCR revealed a potential transcriptional impact of Smad on Pin1 promoter. RESULTS: We report on a colocalization of phosphorylated Smad in AD with Pin1. Pin1 does not only affect Smad phosphorylation and stability but also regulates subcellular localization of Smad2 and supports its binding to phosphorylated tau protein. Smads, in turn, exert a negative feed-back regulation on Pin1. CONCLUSION: Our data suggest both Smad proteins and Pin1 to be elements of a vicious circle with potential pathogenetic significance in AD.

Changes in neuronal DNA content variation in the human brain during aging.

The human brain has been proposed to represent a genetic mosaic, containing a small but constant number of neurons with an amount of DNA exceeding the diploid level that appear to be generated through various chromosome segregation defects initially. While a portion of these cells apparently die during development, neurons with abnormal chromosomal copy number have been identified in the mature brain. This genomic alteration might to lead to chromosomal instability affecting neuronal viability and could thus contribute to age-related mental disorders. Changes in the frequency of neurons with such structural genomic variation in the adult and aging brain, however, are unknown. Here, we quantified the frequency of neurons with a more than diploid DNA content in the cerebral cortex of normal human brain and analyzed its changes between the fourth and ninth decades of life. We applied a protocol of slide-based cytometry optimized for DNA quantification of single identified neurons, which allowed to analyze the DNA content of about 500 000 neurons for each brain. On average, 11.5% of cortical neurons showed DNA content above the diploid level. The frequency of neurons with this genomic alteration was highest at younger age and declined with age. Our results indicate that the genomic variation associated with DNA content exceeding the diploid level might compromise viability of these neurons in the aging brain and might thus contribute to susceptibilities for age-related CNS disorders. Alternatively, a potential selection bias of "healthy aging brains" needs to be considered, assuming that DNA content variation above a certain threshold associates with Alzheimer's disease.

Transcriptional control of cell cycle-dependent kinase 4 by Smad proteins--implications for Alzheimer's disease.

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by deregulation of neuronal cell cycle and differentiation control eventually resulting in cell death. During brain development, neuronal differentiation is regulated by Smad proteins, which are elements of the canonical transforming growth factor ß (TGF-ß) signaling pathway, linking receptor activation to gene expression. In the normal adult brain, Smad proteins are constitutively phosphorylated and predominantly localized in neuronal nuclei. Under neurodegenerative conditions such as AD, the subcellular localization of their phosphorylated forms is heavily disturbed, raising the question of whether a nuclear Smad deficiency in neurons might contribute to a loss of neuronal differentiation control and subsequent cell cycle re-entry. Here, we show by luciferase reporter assays, electromobility shift, and RNA interference (RNAi) technique a direct binding of Smad proteins to the CDK4 promoter inducing transcriptional inhibition of cell cycle-dependent kinase 4 (Cdk4). Mimicking the neuronal deficiency of Smad proteins observed in AD in cell culture by RNAi results in elevation of Cdk4 and retardation of neurite outgrowth. The results identify Smad proteins as direct transcriptional regulators of Cdk4 and add further evidence to a Smad-dependent deregulation of Cdk4 in AD, giving rise to neuronal dedifferentiation and cell death.

Cell cycle control of Wnt/ß-catenin signalling by conductin/axin2 through CDC20.

Wnt/ß-catenin signalling regulates cell proliferation by modulating the cell cycle and is negatively regulated by conductin/axin2/axil. We show that conductin levels peak at G2/M followed by a rapid decline during return to G1. In line with this, Wnt/ß-catenin target genes are low at G2/M and high at G1/S, and ß-catenin phosphorylation oscillates during the cell cycle in a conductin-dependent manner. Conductin is degraded by the anaphase-promoting complex/cyclosome cofactor CDC20. Knockdown of CDC20 blocks Wnt signalling through conductin. CDC20-resistant conductin inhibits Wnt signalling and attenuates colony formation of colorectal cancer cells. We propose that CDC20-mediated degradation of conductin regulates Wnt/ß-catenin signalling for maximal activity during G1/S.

Cerebral amyloid angiopathy in streptozotocin rat model of sporadic Alzheimer's disease: a long-term follow up study.

Cerebral amyloid angiopathy is manifested as accumulation of amyloid ß (Aß) peptide in the wall of meningeal and cerebral arteries, arterioles and capillaries and is frequently found postmortem in sporadic Alzheimer's disease (sAD) patients. It is difficult to assess when and how cerebral amyloid angiopathy develops and progresses in humans in vivo, which is why animal AD models are used. Streptozotocin-intracerebroventricularly (STZ-icv) treated rats have been recently proposed as the model of sAD which develops insulin resistant brain state preceding Aß pathology development. Vascular Aß deposits in the brain of STZ-icv-treated rats (3 months old at the time of icv treatment) were visualized by Thioflavine-S staining, Congo red staining and Aß immunohistochemistry. Thioflavine-S and Congo red staining revealed diffuse congophilic deposits in the wall of meningeal and cortical blood vessels both 6 and 9 months after the STZ-icv treatment. Preliminary Aß1-42 and Aß1-16 immunohistochemistry experiments showed positive staining in blood vessels 3 and 9 months after the STZ-icv treatment, respectively. Results suggest that cerebral amyloid angiopathy observed 6 and 9 months after the STZ-icv treatment seems to be a continuation and progression of the amyloid pathology observed already 3 months following the STZ-icv treatment in this non-transgenic sAD animal model.

The physiological link between metabolic rate depression and tau phosphorylation in mammalian hibernation.

Abnormal phosphorylation and aggregation of tau protein are hallmarks of a variety of neurological disorders, including Alzheimer's disease (AD). Increased tau phosphorylation is assumed to represent an early event in pathogenesis and a pivotal aspect for aggregation and formation of neurofibrillary tangles. However, the regulation of tau phosphorylation in vivo and the causes for its increased stage of phosphorylation in AD are still not well understood, a fact that is primarily based on the lack of adequate animal models. Recently we described the reversible formation of highly phosphorylated tau protein in hibernating European ground squirrels. Hence, mammalian hibernation represents a model system very well suited to study molecular mechanisms of both tau phosphorylation and dephosphorylation under in vivo physiological conditions. Here, we analysed the extent and kinetics of hibernation-state dependent tau phosphorylation in various brain regions of three species of hibernating mammals: arctic ground squirrels, Syrian hamsters and black bears. Overall, tau protein was highly phosphorylated in torpor states and phosphorylation levels decreased after arousal in all species. Differences between brain regions, hibernation-states and phosphosites were observed with respect to degree and kinetics of tau phosphorylation. Furthermore, we tested the phosphate net turnover of tau protein to analyse potential alterations in kinase and/or phosphatase activities during hibernation. Our results demonstrate that the hibernation-state dependent phosphorylation of tau protein is specifically regulated but involves, in addition, passive, temperature driven regulatory mechanisms. By determining the activity-state profile for key enzymes of tau phosphorylation we could identify kinases potentially involved in the differentially regulated, reversible tau phosphorylation that occurs during hibernation. We show that in black bears hibernation is associated with conformational changes of highly phosphorylated tau protein that are typically related to neuropathological alterations. The particular hibernation characteristics of black bears with a continuous torpor period and an only slightly decreased body temperature, therefore, potentially reflects the limitations of this adaptive reaction pattern and, thus, might indicate a transitional state of a physiological process.

Selective cell death of hyperploid neurons in Alzheimer's disease.

Aneuploidy, an abnormal number of copies of a genomic region, might be a significant source for neuronal complexity, intercellular diversity, and evolution. Genomic instability associated with aneuploidy, however, can also lead to developmental abnormalities and decreased cellular fitness. Here we show that neurons with a more-than-diploid content of DNA are increased in preclinical stages of Alzheimer's disease (AD) and are selectively affected by cell death during progression of the disease. Present findings show that neuronal hyperploidy in AD is associated with a decreased viability. Hyperploidy of neurons thus represents a direct molecular signature of cells prone to death in AD and indicates that a failure of neuronal differentiation is a critical pathogenetic event in AD.

Conductin/axin2 and Wnt signalling regulates centrosome cohesion.

Activated Wnt/beta-catenin signalling is a characteristic of many cancers and drives cell-cycle progression. Here, we report a mechanism linking Wnt/beta-catenin signalling to centrosome separation. We show that conductin/axin2, a negative regulator of beta-catenin, localizes at the centrosomes by binding to the centriole-associated component C-Nap1. Knockout or knockdown of conductin leads to premature centrosome separation--that is, splitting--which is abolished by knockdown of beta-catenin. Conductin promotes phosphorylation of the amino-terminal serine (Ser 33/37) and threonine (Thr 41) residues of centrosome-associated beta-catenin. Beta-catenin mutated at these residues causes centrosomal splitting, whereas a phospho-mimicking mutant of beta-catenin does not. Importantly, beta-catenin-induced splitting is not inhibited by blocking beta-catenin-dependent transcription. Treatment with Wnts and inhibition of glycogen synthase kinase 3 block beta-catenin phosphorylation and induce centrosomal splitting. These data indicate that Wnt/beta-catenin signalling and conductin regulate centrosomal cohesion by altering the phosphorylation status of beta-catenin at the centrosomes.