DIXDC1 isoform, l-DIXDC1, is a novel filamentous actin-binding protein

Quantitative trait loci on chromosomes 9 and 19 modulate AII amacrine cell number in the mouse retina

Sequence variants modulating gene function or expression affect various heritable traits, including the number of neurons within a population. The present study employed a forward-genetic approach to identify candidate causal genes and their sequence variants controlling the number of one type of retinal neuron, the AII amacrine cell. Data from twenty-six recombinant inbred (RI) strains of mice derived from the parental C57BL/6J (B6/J) and A/J laboratory strains were used to identify genomic loci regulating cell number. Large variation in cell number is present across the RI strains, from a low of ∼57,000 cells to a high of ∼87,000 cells. Quantitative trait locus (QTL) analysis revealed three prospective controlling genomic loci, on Chromosomes (Chrs) 9, 11, and 19, each contributing additive effects that together approach the range of variation observed. Composite interval mapping validated two of these loci, and chromosome substitution strains, in which the A/J genome for Chr 9 or 19 was introgressed on a B6/J genetic background, showed increased numbers of AII amacrine cells as predicted by those two QTL effects. Analysis of the respective genomic loci identified candidate controlling genes defined by their retinal expression, their established biological functions, and by the presence of sequence variants expected to modulate gene function or expression. Two candidate genes, Dtx4 on Chr 19, being a regulator of Notch signaling, and Dixdc1 on Chr 9, a modulator of the WNT-β-catenin signaling pathway, were explored in further detail. Postnatal overexpression of Dtx4 was found to reduce the frequency of amacrine cells, while Dixdc1 knockout retinas contained an excess of AII amacrine cells. Sequence variants in each gene were identified, being the likely sources of variation in gene expression, ultimately contributing to the final number of AII amacrine cells.

High-Efficiency, Selection-free Gene Repair in Airway Stem Cells from Cystic Fibrosis Patients Rescues CFTR Function in Differentiated Epithelia

Cystic fibrosis (CF) is a monogenic disorder caused by mutations in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene. Mortality in CF patients is mostly due to respiratory sequelae. Challenges with gene delivery have limited attempts to treat CF using in vivo gene therapy, and low correction levels have hindered ex vivo gene therapy efforts. We have used Cas9 and adeno-associated virus 6 to correct the ΔF508 mutation in readily accessible upper-airway basal stem cells (UABCs) obtained from CF patients. On average, we achieved 30%-50% allelic correction in UABCs and bronchial epithelial cells (HBECs) from 10 CF patients and observed 20%-50% CFTR function relative to non-CF controls in differentiated epithelia. Furthermore, we successfully embedded the corrected UABCs on an FDA-approved porcine small intestinal submucosal membrane (pSIS), and they retained differentiation capacity. This study supports further development of genetically corrected autologous airway stem cell transplant as a treatment for CF.

DIXDC1 promotes the growth of acute myeloid leukemia cells by upregulating the Wnt/β-catenin signaling pathway

Accumulating evidence suggests that dysregulation of Dishevelled-Axin domain-containing 1 (DIXDC1) is involved in the progression and development of various cancers. However, little is known about the relevance of DIXDC1 in acute myeloid leukemia (AML). In this study, we aimed to investigate the expression status and potential biological function of DIXDC1 in AML. Our results showed that DIXDC1 expression was highly upregulated in AML cell lines and primary AML blasts compared with normal blasts. Knockdown of DIXDC1 by siRNA-mediated gene silencing significantly inhibited proliferation, induced cell cycle arrest, and promoted apoptosis of AML cells in vitro. By contrast, DIXDC1 overexpression promoted proliferation, accelerated cell cycle progression, and reduced apoptosis of AML cells. Moreover, we found that DIXDC1 knockdown decreased the expression of β-catenin and restricted the activation of Wnt signaling. In addition, DIXDC1 knockdown decreased the expression of Wnt/β-catenin target genes, including cyclin D1 and c-myc, while DIXDC1 overexpression had the opposite effect. Notably, β-catenin knockdown partially reversed the oncogenic effect of DIXDC1 in AML cells. Taken together, these results demonstrate that DIXDC1 promotes the growth of AML cells, possibly through upregulating the Wnt/β-catenin signaling pathway. Our study suggests that DIXDC1 may serve as a potential therapeutic target for the treatment of AML.

DIXDC1 contributes to psychiatric susceptibility by regulating dendritic spine and glutamatergic synapse density via GSK3 and Wnt/β-catenin signaling

Mice lacking DIX domain containing-1 (DIXDC1), an intracellular Wnt/β-catenin signal pathway protein, have abnormal measures of anxiety, depression and social behavior. Pyramidal neurons in these animals' brains have reduced dendritic spines and glutamatergic synapses. Treatment with lithium or a glycogen synthase kinase-3 (GSK3) inhibitor corrects behavioral and neurodevelopmental phenotypes in these animals. Analysis of DIXDC1 in over 9000 cases of autism, bipolar disorder and schizophrenia reveals higher rates of rare inherited sequence-disrupting single-nucleotide variants (SNVs) in these individuals compared with psychiatrically unaffected controls. Many of these SNVs alter Wnt/β-catenin signaling activity of the neurally predominant DIXDC1 isoform; a subset that hyperactivate this pathway cause dominant neurodevelopmental effects. We propose that rare missense SNVs in DIXDC1 contribute to psychiatric pathogenesis by reducing spine and glutamatergic synapse density downstream of GSK3 in the Wnt/β-catenin pathway.

Expression of Dixdc1 and its Role in Astrocyte Proliferation after Traumatic Brain Injury

DIX domain containing 1 (Dixdc1), a positive regulator of Wnt signaling pathway, is recently reported to play a role in the neurogenesis. However, the distribution and function of Dixdc1 in the central nervous system (CNS) after brain injury are still unclear. We used an acute traumatic brain injury (TBI) model in adult rats to investigate whether Dixdc1 is involved in CNS injury and repair. Western blot analysis and immunohistochemistry showed a time-dependent up-regulation of Dixdc1 expression in ipsilateral cortex after TBI. Double immunofluorescent staining indicated a colocalization of Dixdc1 with astrocytes and neurons. Moreover, we detected a colocalization of Ki-67, a cell proliferation marker with GFAP and Dixdc1 after TBI. In primary cultured astrocytes stimulated with lipopolysaccharide, we found enhanced expression of Dixdc1 in parallel with up-regulation of Ki-67 and cyclin A, another cell proliferation marker. In addition, knockdown of Dixdc1 expression in primary astrocytes with Dixdc1-specific siRNA transfection induced G0/G1 arrest of cell cycle and significantly decreased cell proliferation. In conclusion, all these data suggest that up-regulation of Dixdc1 protein expression is potentially involved in astrocyte proliferation after traumatic brain injury in the rat.

Knockdown of DIXDC1 Inhibits the Proliferation and Migration of Human Glioma Cells

DIX domain containing 1 (DIXDC1), the human homolog of coiled-coil-DIX1 (Ccd1), is a positive regulator of Wnt signaling pathway. Recently, it was found to act as a candidate oncogene in colon cancer, non-small-cell lung cancer, and gastric cancer. In this study, we aimed to investigate the clinical significance of DIXDC1 expression in human glioma and its biological function in glioma cells. Western blot and immunohistochemistry analysis showed that DIXDC1 was overexpressed in glioma tissues and glioma cell lines. The expression level of DIXDC1 was evidently linked to glioma pathological grade and Ki-67 expression. Kaplan-Meier curve showed that high expression of DIXDC1 may lead to poor outcome of glioma patients. Serum starvation and refeeding assay indicated that the expression of DIXDC1 was associated with cell cycle. To determine whether DIXDC1 could regulate the proliferation and migration of glioma cells, we transfected glioma cells with interfering RNA-targeting DIXDC1; investigated cell proliferation with Cell Counting Kit (CCK)-8, flow cytometry assays, and colony formation analyses; and investigated cell migration with wound healing assays and transwell assays. According to our data, knockdown of DIXDC1 significantly inhibited proliferation and migration of glioma cells. These data implied that DIXDC1 might participate in the development of glioma, suggesting that DIXDC1 can become a potential therapeutic strategy for glioma.

Downregulated expression of DIXDC1 in hepatocellular carcinoma and its correlation with prognosis

Dishevelled-Axin domain containing 1 (DIXDC1) is a DIX (Dishevelled-Axin) domain possessing protein that acts as a positive regulator of the Wnt pathway. Although DIXDC1 has been investigated in several cancers, it has not yet been studied in human hepatocellular carcinoma (HCC). The purpose of the current study was to investigate the expression pattern of DIXDC1 and assess the clinical significance of DIXDC1 expression in HCC patients. Data containing three independent investigations from Oncomine database demonstrated that DIXDC1 mRNA was downregulated in HCC compared with matched non-cancerous tissues. Similar results were also obtained in 25 paired HCC tissues and corresponding non-cancerous tissues by qPCR and Western blot analysis. Additionally, another independent set of 140 pairs of HCC specimens was evaluated for DIXDC1 expression by IHC and demonstrated that reduced expression of DIXDC1 in 50.7 % (71/140) of HCC tissues was significantly correlated with tumor size (p = 0.024), tumor differentiation (p < 0.001), tumor thrombi (p = 0.019), TNM stage (p = 0.019), and BCLC stage (p = 0.008). Importantly, Kaplan-Meier survival and Cox regression analyses were executed to evaluate the prognosis of HCC patients and found that DIXDC1 protein expression was one of the independent prognostic factors for overall survival of HCC patients.

DIXDC1 promotes tumor proliferation and cell adhesion mediated drug resistance (CAM-DR) via enhancing p-Akt in Non-Hodgkin's lymphomas

DIX domain containing 1 (DIXDC1), is a human homolog of Ccd1, a DIX domain containing protein in zebrafish. The present study was undertaken to determine the expression and biologic function of DIXDC1 in Non-Hodgkin's lymphoma (NHL). Clinically, we detected that the expression of DIXDC1 was significantly lower in the indolent lymphomas compared with the progressive lymphomas by immunohistochemistry analysis. Functionally, we found that DIXDC1 could promote cell proliferation via modulating cell cycle progression and PI3K/AKT signaling pathway in NHLs. Moreover, we confirmed that DIXDC1 was involved in the process of lymphoma cell adhesion mediated drug resistance (CAM-DR). Adhesion to fibronectin (FN) or HS-5 up-regulated DIXDC1 expression, and up-regulation of DIXDC1 resulted in an increased expression of p-AKT, which promoted CAM-DR. Our finding supports the role of DIXDC1 in cell proliferation, cell cycle and CAM-DR in NHLs. We propose that inhibition of DIXDC1 expression may be a novel therapeutic approach for NHLs patients, and it may be a target for drug resistance.

Upregulated expression of DIXDC1 in intestinal-type gastric carcinoma: co-localization with β-catenin and correlation with poor prognosis

Background

DIXDC1 (Dishevelled-Axin domain containing 1) is a positive regulator of the Wnt pathway. In the field of cancer research, the role of DIXDC1 is unclear. Our previous in vitro study showed that DIXDC1 enhances β-catenin nuclear accumulation in gastric cancer cell lines. The aim of this study was to detect the expression of DIXDC1 in different histological subtypes of gastric carcinoma and to evaluate the correlation between the expression of DIXDC1 and β-catenin localization and clinicopathological parameters, including patients’ survival.

Methods

Immunohistochemical staining was performed to characterize the expression of DIXDC1 and β-catenin in archived materials from 259 cases of gastric carcinoma. The χ2 test and the Fisher’s test were used to analyze correlations between DIXDC1 expression, β-catenin localization, and clinicopathological parameters. Univariate analyses were performed using the Kaplan–Meier method, and the survival difference between groups was assessed by the log-rank test. Multivariate analysis was performed using the Cox proportional hazards regression model.

Results

Positive DIXDC1 staining was detected in tumor cells in 123 of 259 (47.5 %) cases. DIXDC1 expression in gastric carcinoma was significantly correlated with the histological intestinal-type (P < 0.001), the depth of tumor invasion (P < 0.001) and the lymph node metastasis (P = 0.006). In the intestinal-type, DIXDC1 was correlated with the nuclear and cytoplasmic β-catenin expression (P = 0.002). Kaplan–Meier analysis indicated that patients with high DIXDC1 expression had poor disease-specific survival (P < 0.001), especially in the intestinal-type. Moreover, multivariate regression analysis showed that positive expression of DIXDC1 was an independent prognostic predictor of intestinal-type gastric carcinoma.

Conclusion

Our study indicated that DIXDC1 is a significant independent prognostic indicator in intestinal-type gastric carcinoma that plays an important role in carcinogenesis and progression of gastric carcinoma through the Wnt signaling pathway.

DIXDC1 activates the Wnt signaling pathway and promotes gastric cancer cell invasion and metastasis

DIXDC1 (Dishevelled-Axin domain containing 1) is a DIX (Dishevelled-Axin) domain-possessing protein that promotes colon cancer cell proliferation and increases the invasion and migration ability of non-small-cell lung cancer via the PI3K pathway. As a positive regulator of the Wnt/β-catenin pathway, the biological role of DIXDC1 in human gastric cancer and the relationship between DIXDC1 and the Wnt pathway are unclear. In the current study, the upregulation of DIXDC1 was detected in gastric cancer and was associated with advanced TNM stage cancer, lymph node metastasis, and poor prognosis. We also found that the overexpression of DIXDC1 could promote the invasion and migration of gastric cancer cells. The upregulation of MMPs and the downregulation of E-cadherin were found to be involved in the process. DIXDC1 enhanced β-catenin nuclear accumulation, which activated the Wnt pathway. Additionally, the inhibition of β-catenin in DIXDC1-overexpressing cells reversed the metastasis promotion effects of DIXDC1. These results demonstrate that the expression of DIXDC1 is associated with poor prognosis of gastric cancer patients and that DIXDC1 promotes gastric cancer invasion and metastasis through the activation of the Wnt pathway; E-cadherin and MMPs are also involved in this process. © 2015 Wiley Periodicals, Inc.

Regulatory mechanisms underlying the differential growth of dendrites and axons

A typical neuron is comprised of an information input compartment, or the dendrites, and an output compartment, known as the axon. These two compartments are the structural basis for functional neural circuits. However, little is known about how dendritic and axonal growth are differentially regulated. Recent studies have uncovered two distinct types of regulatory mechanisms that differentiate dendritic and axonal growth: dedicated mechanisms and bimodal mechanisms. Dedicated mechanisms regulate either dendritespecific or axon-specific growth; in contrast, bimodal mechanisms direct dendritic and axonal development in opposite manners. Here, we review the dedicated and bimodal regulators identified by recent Drosophila and mammalian studies. The knowledge of these underlying molecular mechanisms not only expands our understanding about how neural circuits are wired, but also provides insights that will aid in the rational design of therapies for neurological diseases.

An AMPK-independent signaling pathway downstream of the LKB1 tumor suppressor controls Snail1 and metastatic potential

The serine/threonine kinase LKB1 is a tumor suppressor whose loss is associated with increased metastatic potential. In an effort to define biochemical signatures of metastasis associated with LKB1 loss, we discovered that the epithelial-to-mesenchymal transition transcription factor Snail1 was uniquely upregulated upon LKB1 deficiency across cell types. The ability of LKB1 to suppress Snail1 levels was independent of AMPK but required the related kinases MARK1 and MARK4. In a screen for substrates of these kinases involved in Snail regulation, we identified the scaffolding protein DIXDC1. Similar to loss of LKB1, DIXDC1 depletion results in upregulation of Snail1 in a FAK-dependent manner, leading to increased cell invasion. MARK1 phosphorylation of DIXDC1 is required for its localization to focal adhesions and ability to suppress metastasis in mice. DIXDC1 is frequently downregulated in human cancers, which correlates with poor survival. This study defines an AMPK-independent phosphorylation cascade essential for LKB1-dependent control of metastatic behavior.

DIXDC1 increases the invasion and migration ability of non-small-cell lung cancer cells via the PI3K-AKT/AP-1 pathway

DIX domain containing 1 (DIXDC1), is a human homolog of Ccd1, a recently identified DIX domain containing protein in zebrafish. DIXDC1 protein was detected in human colorectal adenocarcinoma tissues and was found to be correlated with a high cell proliferation index. We demonstrated DIXDC1 overexpression in 55% (92/167) of non-small cell lung cancer (NSCLC) cases, compared to adjacent noncancerous lung tissues (P < 0.01). Overexpression of DIXDC1 was associated with lymph node metastasis and more advanced TNM stage (P < 0.001 and P = 0.001, respectively). Kaplan-Meier survival curves and log-rank testing indicated that overexpression of DIXDC1 correlated with worse overall survival in NSCLC (P = 0.031). DIXDC1 was more abundant in seven NSCLC lines than the bronchial cell line HBE, and modulation of its expression regulated AP-1 activity; MMP2, MMP7, and MMP9 protein and mRNA; and invasion ability. Metalloproteinase induction was reversed by PI3K/AKT and AP-1 inhibition. These results suggest DIXDC1 is associated with stage and prognosis in NSCLC, and may promote invasion and migration through PI3K-AKT/AP-1-dependent activation of metalloproteinases.

Therapeutic effects of microRNA-582-5p and -3p on the inhibition of bladder cancer progression

Many reports have indicated that the abnormal expression of microRNAs (miRNAs) is associated with the progression of disease and have identified miRNAs as attractive targets for therapeutic intervention. However, the bifunctional mechanisms of miRNA guide and passenger strands in RNA interference (RNAi) therapy have not yet been clarified. Here, we show that miRNA (miR)-582-5p and -3p, which are strongly decreased in high-grade bladder cancer clinical samples, regulate tumor progression in vitro and in vivo. Significantly, the overexpression of miR-582-5p or -3p reduced the proliferation and invasion of UM-UC-3 human bladder cancer cells. Furthermore, transurethral injections of synthetic miR-582 molecule suppressed tumor growth and metastasis in an animal model of bladder cancer. Most interestingly, our study revealed that both strands of miR-582-5p and -3p suppressed the expression of the same set of target genes such as protein geranylgeranyltransferase type I beta subunit (PGGT1B), leucine-rich repeat kinase 2 (LRRK2) and DIX domain containing 1 (DIXDC1). Knockdown of these genes using small interfering RNA (siRNA) resulted in the inhibition of cell growth and invasiveness of UM-UC-3. These findings uncover the unique regulatory pathway involving tumor suppression by both strands of a single miRNA that is a potential therapeutic target in the treatment of invasive bladder cancer.

Transcriptional regulators that differentially control dendrite and axon development

Neurons are the basic units establishing connectivity in the nervous system. As a signature feature, neurons form polarized structures: dendrites and axons, which integrate either sensory stimuli or inputs from upstream neurons and send outputs to target cells, respectively. The separation of dendritic and axonal compartments is achieved in two steps during development: 1) dendrite and axon specification: how neurites are initially specified as dendrites and axons; and 2) dendrite and axon commitment: how dendrites and axons are committed to distinct compartmental fates and architectures. In order to understand neural circuit assembly and to correct erroneous dendrite or axon growth in a compartment-specific manner, it is essential to understand the regulatory mechanisms underlying dendrite and axon commitment. Compared to extensive studies on dendrite and axon specification, little is known about the molecular mechanisms exclusively dedicated to dendrite or axon commitment. Recent studies have uncovered the requirement of transcriptional regulation in this process. Here, we review the studies on transcriptional regulators: Dar1, p300-SnoN, NeuroD, which have been shown to separate dendrite- and axon-specific growth of the same neuron type after compartmental fates are specified.

Crystallographic characterization of the DIX domain of the Wnt signalling positive regulator Ccd1

Coiled-coil DIX1 (Ccd1) is a positive regulator that activates the canonical Wnt signalling pathway by inhibiting the degradation of the key signal transducer β-catenin. The C-terminal DIX domain of Ccd1 plays an important role in the regulation of signal transduction through homo-oligomerization and protein complex formation with other DIX domain-containing proteins, i.e. axin and dishevelled proteins. Here, the expression, purification, crystallization and X-ray data collection of the Ccd1 DIX domain are reported. The crystals of the Ccd1 DIX domain belonged to space group P2(1)2(1)2(1), with unit-cell parameters a=72.9, b=75.7, c=125.6 Å. An X-ray diffraction data set was collected at 3.0 Å resolution.

SnoN: bridging neurobiology and cancer biology

The transcriptional regulator SnoN has been the subject of growing interest due to its diverse functions in normal and pathological settings. A large body of evidence has established a fundamental role for SnoN as a modulator of signaling and responses by the transforming growth beta (TGFbeta) family of cytokines, though how SnoN regulates TGFbeta responses remains incompletely understood. In accordance with the critical and complex roles of TGFbeta in tumorigenesis and metastasis, SnoN may act as a tumor promoter or suppressor depending on the stage and type of cancer. Beyond its role in cancer, SnoN has also been implicated in the control of axon morphogenesis in postmitotic neurons in the mammalian brain. Remarkably, signaling pathways that control SnoN functions in the divergent cycling cells and postmitotic neurons appear to be conserved. Identification of novel SnoN regulatory and effector mechanisms holds the promise of advances at the interface of cancer biology and neurobiology.

Strategy for comprehensive identification of human N-myristoylated proteins using an insect cell-free protein synthesis system

To establish a strategy for the comprehensive identification of human N-myristoylated proteins, the susceptibility of human cDNA clones to protein N-myristoylation was evaluated by metabolic labeling and MS analyses of proteins expressed in an insect cell-free protein synthesis system. One-hundred-and-forty-one cDNA clones with N-terminal Met-Gly motifs were selected as potential candidates from approximately 2000 Kazusa ORFeome project human cDNA clones, and their susceptibility to protein N-myristoylation was evaluated using fusion proteins, in which the N-terminal ten amino acid residues were fused to an epitope-tagged model protein. As a result, the products of 29 out of 141 cDNA clones were found to be effectively N-myristoylated. The metabolic labeling experiments both in an insect cell-free protein synthesis system and in the transfected COS-1 cells using full-length cDNA revealed that 27 out of 29 proteins were in fact N-myristoylated. Database searches with these 27 cDNA clones revealed that 18 out of 27 proteins are novel N-myristoylated proteins that have not been reported previously to be N-myristoylated, indicating that this strategy is useful for the comprehensive identification of human N-myristoylated proteins from human cDNA resources.

Dixdc1 is a critical regulator of DISC1 and embryonic cortical development

The psychiatric illness risk gene Disrupted in Schizophrenia-1 (DISC1) plays an important role in brain development; however, it is unclear how DISC1 is regulated during cortical development. Here, we report that DISC1 is regulated during embryonic neural progenitor proliferation and neuronal migration through an interaction with DIX domain containing-1 (Dixdc1), the third mammalian gene discovered to contain a Disheveled-Axin (DIX) domain. We determined that Dixdc1 functionally interacts with DISC1 to regulate neural progenitor proliferation by co-modulating Wnt-GSK3beta/beta-catenin signaling. However, DISC1 and Dixdc1 do not regulate migration via this pathway. During neuronal migration, we discovered that phosphorylation of Dixdc1 by cyclin-dependent kinase 5 (Cdk5) facilitates its interaction with the DISC1-binding partner Ndel1. Furthermore, Dixdc1 phosphorylation and its interaction with DISC1/Ndel1 in vivo is required for neuronal migration. Together, these data reveal that Dixdc1 integrates DISC1 into Wnt-GSK3beta/beta-catenin-dependent and -independent signaling pathways during cortical development and further delineate how DISC1 contributes to neuropsychiatric disorders.

Wnt signaling stabilizes the DIXDC1 protein through decreased ubiquitin-dependent degradation

Wnt signaling plays key roles in development, cell growth, differentiation, polarity formation, neural development, and carcinogenesis. DIX Domain Containing 1 (DIXDC1), a novel component of the Wnt pathway, was recently cloned. DIXDC1 is the human homolog of Ccd1, a positive regulator of the Wnt signaling pathway during zebrafish neural patterning. Little has been known about DIXDC1 gene expression regulation. In the present study, we showed that the DIXDC1 protein was induced upon Wnt-3a stimulation, whereas the DIXDC1 mRNA level was not significantly increased after Wnt-3a treatment. Positive DIXDC1 staining was detected in colon cancer cells and was colocalized with beta-catenin staining. However, the DIXDC1 mRNA expression decreased in human colon cancer cells compared to the matched normal colon epithelial cells. Our further investigation showed that the DIXDC1 protein was degraded through the proteasome pathway, and the activation of canonical Wnt signaling decreased the ubiquitin-dependent degradation of both the ectopic and endogenous DIXDC1 protein. In order to explore the possible mechanism of the ubiquitination of DIXDC1, we found that the phosphorylation of DIXDC1 was inhibited by Wnt-3a. Collectively, these results indicate that canonical Wnt/beta-catenin pathway activation might upregulate DIXDC1 through a post-translational mechanism by inhibiting the ubiquitin-mediated degradation of the DIXDC1 protein.

DIXDC1 targets p21 and cyclin D1 via PI3K pathway activation to promote colon cancer cell proliferation

DIXDC1 is the human homolog of Ccd1, a recently identified DIX domain containing protein in zebrafish. It is a positive regulator in the Wnt signaling pathway functioning downstream of Wnt and upstream of Axin. Since Wnt pathway activation is correlated with human colon cancer formation and progression, the biological role of DIXDC1 in human colon cancer was examined. In the current study, up-regulation of DIXDC1 protein was detected in human colorectal adenocarcinoma tissues and was found to be correlated well with high cell proliferation index. Ectopic over-expression of DIXDC1 resulted in increased cell proliferation in vitro and accelerated tumorigenesis on nude mice in vivo. We also showed that DIXDC1 promoted G0/G1 to S phase transition concomitantly with up-regulation of cyclin D1 and down-regulation of p21 protein. DIXDC1 over-expression cells showed activation of the PI3K/AKT pathway. Both siRNA knockdown of DIXDC1 and blocking the PI3K pathway using a specific inhibitor caused G1/S phase arrest, as well as down-regulation of cyclin D1 and up-regulation of p21 in DIXDC1 over-expression colon cancer cells. Collectively, this study demonstrates that over-expression of DIXDC1 might target p21 and cyclin D1 to promote colon cancer cell proliferation and tumorigenesis at least partially through activation of the PI3K/Akt pathway.

A SnoN-Ccd1 pathway promotes axonal morphogenesis in the mammalian brain

The transcriptional corepressor SnoN is a critical regulator of axonal morphogenesis, but how SnoN drives axonal growth is unknown. Here, we report that gene-profiling analyses in cerebellar granule neurons reveal that the large majority of genes altered upon SnoN knockdown are surprisingly downregulated, suggesting that SnoN may activate transcription in neurons. Accordingly, we find that the transcriptional coactivator p300 interacts with SnoN, and p300 plays a critical role in SnoN-induced axon growth. We also identify the gene encoding the signaling scaffold protein Ccd1 as a critical target of SnoN in neurons. Ccd1 localizes to the actin cytoskeleton, is enriched at axon terminals in neurons, and activates the axon growth-promoting kinase JNK (c-Jun N-terminal protein kinase). Knockdown of Ccd1 in neurons reduces axonal length and suppresses the ability of SnoN to promote axonal growth. Importantly, Ccd1 knockdown in rat pups profoundly impairs the formation of granule neuron parallel fiber axons in the rat cerebellar cortex in vivo. These findings define a novel SnoN-Ccd1 link that promotes axonal growth in the mammalian brain, with important implications for axonal development and regeneration.

DIXDC1 promotes retinoic acid-induced neuronal differentiation and inhibits gliogenesis in P19 cells

Human DIXDC1 is a member of Dishevelled-Axin (DIX) domain containing gene family which plays important roles in Wnt signaling and neural development. In this report, we first confirmed that expression of Ccd1, a mouse homologous gene of DIXDC1, was up-regulated in embryonic developing nervous system. Further studies showed that Ccd1 was expressed specifically in neurons and colocalized with early neuronal marker Tuj1. During the aggregation induced by RA and neuronal differentiation of embryonic carcinoma P19 cells, expressions of Ccd1 as well as Wnt-1 and N-cadherin were dramatically increased. Stable overexpression of DIXDC1 in P19 cells promoted the neuronal differentiation. P19 cells overexpressing DIXDC1 but not the control P19 cells could differentiate into Tuj1 positive cells with RA induction for only 2 days. Meanwhile, we also found that overexpression of DIXDC1 facilitated the expression of Wnt1 and bHLHs during aggregation and differentiation, respectively, while inhibited gliogenesis by down-regulating the expression of GFAP in P19 cells. Thus, our finding suggested that DIXDC1 might play an important role during neurogenesis, overexpression of DIXDC1 in embryonic carcinoma P19 cells promoted neuronal differentiation, and inhibited gliogenesis induced by retinoic acid.