Netrin-1 in the developing enteric nervous system and colorectal cancer

Structural insights into the formation of repulsive netrin guidance complexes

Netrins dictate attractive and repulsive responses during axon growth and cell migration, where the presence of the receptor Uncoordinated-5 (UNC-5) on target cells results in repulsion. Here, we showed that UNC-5 is a heparin-binding protein, determined its structure bound to a heparin fragment, and could modulate UNC-5-heparin affinity using a directed evolution platform or structure-based rational design. We demonstrated that UNC-5 and UNC-6/netrin form a large, stable, and rigid complex in the presence of heparin, and heparin and UNC-5 exclude the attractive UNC-40/DCC receptor from binding to UNC-6/netrin to a large extent. Caenorhabditis elegans with a heparin-binding-deficient UNC-5 fail to establish proper gonad morphology due to abrogated cell migration, which relies on repulsive UNC-5 signaling in response to UNC-6. Combining UNC-5 mutations targeting heparin and UNC-6/netrin contacts results in complete cell migration and axon guidance defects. Our findings establish repulsive netrin responses to be mediated through a glycosaminoglycan-regulated macromolecular complex.

Netrin-1 inducing antiapoptotic effect of acute myeloid leukemia cells in a concentration-dependent manner through the Unc-5 netrin receptor B-focal adhesion kinase axis

Acute myeloid leukemia (AML) is a hematological malignancy that commonly occurs in children. The prognosis of pediatric AML is relatively poor, thus threatening the patient's survival. The aberrant expression of the axon guidance factor, netrin-1, is observed in various types of malignancies, and it participates in the proliferation and apoptosis of tumor cells. Herein, we aimed to explore the role of netrin-1 in AML cells. Netrin-1 is highly expressed in AML patients. Proliferation and anti-apoptosis were observed in AML cells treated with netrin-1. The interaction between netrin-1 and Unc-5 netrin receptor B (UNC5B) was detected through coimmunoprecipitation, and UNC5B ribonucleic acid interference restrained the influence of netrin-1 on the AML cells. The phosphorylation of focal adhesion kinase-protein kinase B (FAK-Akt) was upregulated in AML cells treated with netrin-1. Both FAK and Akt inhibitors abrogated the effects of netrin-1 on the proliferation and apoptosis of AML cells. In conclusion, netrin-1 could promote the growth and reduce the apoptosis of AML cells in a concentration-dependent manner, and that these effects were mediated by activating the FAK-Akt signaling pathway via the UNC5B.

Enhanced Neuron Growth and Electrical Activity by a Supramolecular Netrin-1 Mimetic Nanofiber

Neurotrophic factors are essential not only for guiding the organization of the developing nervous system but also for supporting the survival and growth of neurons after traumatic injury. In the central nervous system (CNS), inhibitory factors and the formation of a glial scar after injury hinder the functional recovery of neurons, requiring exogenous therapies to promote regeneration. Netrin-1, a neurotrophic factor, can initiate axon guidance, outgrowth, and branching, as well as synaptogenesis, through activation of deleted in colorectal cancer (DCC) receptors. We report here the development of a nanofiber-shaped supramolecular mimetic of netrin-1 with monomers that incorporate a cyclic peptide sequence as the bioactive component. The mimetic structure was found to activate the DCC receptor in primary cortical neurons using low molar ratios of the bioactive comonomer. The supramolecular nanofibers enhanced neurite outgrowth and upregulated maturation as well as pre- and postsynaptic markers over time, resulting in differences in electrical activity similar to neurons treated with the recombinant netrin-1 protein. The results suggest the possibility of using the supramolecular structure as a therapeutic to promote regenerative bioactivity in CNS injuries.

Differential expression of PLAC1 and Netrin-1 in liver metastasis of colorectal cancer and its predictive value

OBJECTIVE

To explore the differential expression of placental specific gene 1 (PLAC1) and neurite guidance factor 1 (netrin-1) in colorectal cancer (CRC) liver metastasis and its predictive value.

METHODS

Paraffin specimens of primary CRC were selected, including 60 simple colorectal cancer specimens and 47 liver metastasis specimens. At the same time, 40 cases of normal colorectal mucosa were taken as the control group. The expression of PLAC1 and Netrin-1 in tissue was detected by immunohistochemistry (IHC). The correlation between PLAC1 and Netrin-1 expression and clinicopathological characteristics of patients with CRC liver metastases was analyzed. Logistic analysis was adopted to analyze the influencing factors of liver metastasis in CRC. A prediction model was established and ROC curve was used to detect the discrimination of the prediction model. The clinical value of PLAC1 and netrin-1 in predicting liver metastasis of CRC was analyzed using ROC curve. The relationship between the expression of PLAC1 and netrin-1 and the prognosis of CRC patients with liver metastasis was analyzed using Kaplan Meier survival curve.

RESULTS

The positive staining of PLAC1 and netrin-1 was mainly located in the cytoplasm by IHC detection. Positive expression of PLAC1 and netrin-1 in CRC tissues was markedly higher than that in normal colorectal mucosal epithelium (P < 0.05). Positive expression of PLAC1 in metastatic group was higher than that in non-metastatic group without significant difference (P > 0.05). The metastasis group had much higher positive expression of netrin-1 than the non-metastasis group (P < 0.05). The content of PLAC1 in the tissues of CRC with liver metastasis had a close relationship with differentiation degree and lymph node metastasis (P < 0.05). The expression of Netrin-1 in the tissues of CRC with liver metastasis was associated with Dukes stage, differentiation degree and lymph node metastasis (P < 0.05). Logistic regression analysis showed that Dukes stage, differentiation, lymph node metastasis, CEA, Alb and D-dimer were the independent risk factors for liver metastasis of CRC (P < 0.05). The model was constructed according to the regression coefficients and constant terms, and the discrimination of the prediction model was evaluated using ROC curve, with the AUC of 0.903 (95% CI 0.831 ~ 0.975), the sensitivity of 93.80%, the specificity of 80.00%, and the Jordan index of 0.738. The AUC of PLAC1 and netrin-1 alone and combined detection to predict liver metastasis of CRC were 0.805, 0.793 and 0.921, respectively. The survival time of patients with positive PLAC1 and netrin-1 expression were sharply shorter than that of the patients with negative expression (P < 0.05).

CONCLUSIONS

The expression of PLAC1 and netrin-1 was strongly increased in CRC with liver metastasis, which had a certain clinical value in predicting liver metastasis of CRC. Dukes stage, differentiation degree, lymph node metastasis, CEA, Alb and D-dimer were independent risk factors for liver metastasis of CRC, and the model based on these indicators had good discrimination for effectively evaluating the risk of liver metastasis in CRC.

Tryptase activates enteric glial cells followed by affecting neuronal properties possibly via the stimuli-associated mediators

OBJECTIVES

Mast cell-derived tryptase causes neuronal elongation/sensitization leading to visceral hypersensitivity. However, effects of tryptase on enteric glial cells (EGCs) and subsequent interaction between EGCs and neurons remain unknown.

METHODS

We evaluated proteins and mRNA expressions in EGC (CRL-2690, ATCC) after tryptase stimulation: nerve growth factor (NGF), netrin-1, and glial cell-derived neurotrophic factor (GDNF). We examined morphological changes in neurons (PC12 cells, CRL-1721.1) by co-incubation with the conditioned medium of EGCs after tryptase stimulation.

RESULTS

EGC was activated by tryptase, and proliferated (by 1.8-fold) with cytoplasmic expansion and process elongation. Intercellular connections of EGC were more complexed. Tryptase induced mRNA expression (2.5-fold) and protein expression of NGF. Netrin-1 (3-fold) and GDNF (3-fold) mRNA expressions were increased at 30 min. Increase in netrin-1 continued until 6 h, whereas the latter decreased by 3 h. The conditioned medium of EGC after tryptase stimulation expanded neuronal cytoplasm (round or ramified shapes) and neurite outgrowth with elongation of cytoskeletal filaments in time-dependent and dose-dependent manners. These changes were similar to those after NGF stimulation. Growth cone proteins of neurons were also increased by the conditioned medium.

CONCLUSION

EGC activated by tryptase changes neuronal morphology (process elongation and cytoplasm expansion) possibly via the stimuli-associated mediators.

Neuronal differentiation pathways and compound-induced developmental neurotoxicity in the human neural progenitor cell test (hNPT) revealed by RNA-seq

There is an increased awareness that the use of animals for compound-induced developmental neurotoxicity (DNT) testing has limitations. Animal-free innovations, especially the ones based on human stem cell-based models are pivotal in studying DNT since they can mimic processes relevant to human brain development. Here we present the human neural progenitor test (hNPT), a 10-day protocol in which neural progenitor cells differentiate into a neuron-astrocyte co-culture. The study aimed to characterise differentiation over time and to find neurodevelopmental processes sensitive to compound exposure using transcriptomics. 3992 genes regulated in unexposed control cultures (p ≤ 0.001, log2FC ≥ 1) showed Gene Ontology (GO-) term enrichment for neuronal and glial differentiation, neurite extension, synaptogenesis, and synaptic transmission. Exposure to known or suspected DNT compounds (acrylamide, chlorpyrifos, fluoxetine, methyl mercury, or valproic acid) at concentrations resulting in 95% cell viability each regulated unique combinations of GO-terms relating to neural progenitor proliferation, neuronal and glial differentiation, axon development, synaptogenesis, synaptic transmission, and apoptosis. Investigation of the GO-terms 'neuron apoptotic process' and 'axon development' revealed common genes that were responsive across compounds, and might be used as biomarkers for DNT. The GO-term 'synaptic signalling', on the contrary, whilst also responsive to all compounds tested, showed little overlap in gene expression regulation patterns between the conditions. This GO-term may articulate compound-specific effects that may be relevant for revealing differences in mechanism of toxicity. Given its focus on neural progenitor cell to mature multilineage neuronal cell maturation and its detailed molecular readout based on gene expression analysis, hNPT might have added value as a tool for neurodevelopmental toxicity testing in vitro. Further assessment of DNT-specific biomarkers that represent these processes needs further studies.

Netrin-1 as a Multitarget Barrier Stabilizer in the Peripheral Nerve after Injury

The blood–nerve barrier and myelin barrier normally shield peripheral nerves from potentially harmful insults. They are broken down during nerve injury, which contributes to neuronal damage. Netrin-1 is a neuronal guidance protein with various established functions in the peripheral and central nervous systems; however, its role in regulating barrier integrity and pain processing after nerve injury is poorly understood. Here, we show that chronic constriction injury (CCI) in Wistar rats reduced netrin-1 protein and the netrin-1 receptor neogenin-1 (Neo1) in the sciatic nerve. Replacement of netrin-1 via systemic or local administration of the recombinant protein rescued injury-induced nociceptive hypersensitivity. This was prevented by siRNA-mediated knockdown of Neo1 in the sciatic nerve. Mechanistically, netrin-1 restored endothelial and myelin, but not perineural, barrier function as measured by fluorescent dye or fibrinogen penetration. Netrin-1 also reversed the decline in the tight junction proteins claudin-5 and claudin-19 in the sciatic nerve caused by CCI. Our findings emphasize the role of the endothelial and myelin barriers in pain processing after nerve damage and reveal that exogenous netrin-1 restores their function to mitigate CCI-induced hypersensitivity via Neo1. The netrin-1-neogenin-1 signaling pathway may thus represent a multi-target barrier protector for the treatment of neuropathic pain.

Genomic methylation variations predict the susceptibility of six chemotherapy related adverse effects and cancer development for Chinese colorectal cancer patients

Colorectal cancer (CRC) remains a major concern with high morbidity and mortality worldwide. Despite the positive influence of chemotherapy on the decline in CRC mortality, the negative influence of chemotherapy-related adverse effects (CRAEs) caused by capecitabine (Cap) remains a challenging problem. DNA methylation alteration plays a pivotal role in gene expression regulation. Here, we aimed to screen reliable and novel biomarkers for CRC diagnosis and CRAE prediction using the advanced Illumina Infinium MethylationEPIC (850 K) BeadChip. Paired tumor and normal tissues from 21 Chinese CRC patients who received Cap-based adjuvant chemotherapy were analyzed. CRC-related methylation was characterized by hypermethylated promoter islands and hypomethylated intragenic openseas; CRAE-related methylation was characterized by hyper- (or hypo-) methylated intragenic (or intergenic) regions. Based on three types of methylation profiles (differentially methylated probes, differentially methylated regions, and gene-function-differentially methylated regions), pathway enrichment analyses revealed that CRC-related genes were significantly enriched in the neuronal system, metabolism of RNA, and extracellular matrix organization; CRAE-related genes were abundantly enriched in pathways controlling regeneration functions and immune response. Finally, based on genes within the mostly related pathways and LASSO logistic regression selection, the integrated-methylation-marker systems developed here demonstrated high discriminative accuracy in both CRC diagnosis (AUROC = 1) and CRAE prediction (AUROC = 0.817-1). In conclusion, we conducted a comprehensive DNA methylation analysis of CRC patients with chemotherapy, which provided new insights into the formation of CRC and CRAEs. Most importantly, our findings identified potentially CRAE-related metabolic pathways and markers, providing a valuable reference for personalized medicine promising better safety. Trail registration:ClinicalTrials.gov,NCT03030508, Registered 25 January 2017,https://www.clinicaltrials.gov/ct2/show/NCT03030508?term=NCT03030508&draw=2&rank=1.

Polyomavirus Small T Antigen Induces Apoptosis in Mammalian Cells through the UNC5B Pathway in a PP2A-Dependent Manner

UNC5B is a dependence receptor that promotes survival in the presence of its ligand, netrin-1, while inducing cell death in its absence. The receptor has an important role in the development of the nervous and vascular systems. It is also involved in the normal turnover of intestinal epithelium. Netrin-1 and UNC5B are deregulated in multiple cancers, including colorectal, neuroblastoma, and breast tumors. However, the detailed mechanism of UNC5B function is not fully understood. We have utilized the murine polyomavirus small T antigen (PyST) as a tool to study UNC5B-mediated apoptosis. PyST is known to induce mitotic arrest followed by extensive cell death in mammalian cells. Our results show that the expression of PyST increases mRNA levels of UNC5B by approximately 3-fold in osteosarcoma cells (U2OS) and also stabilizes UNC5B at the posttranslational level. Furthermore, UNC5B is upregulated predominantly in those cells that undergo mitotic arrest upon PyST expression. Interestingly, although its expression was previously reported to be regulated by p53, our data show that the increase in UNC5B levels by PyST is p53 independent. The posttranslational stabilization of UNC5B by PyST is regulated by the interaction of PyST with PP2A. We also show that netrin-1 expression, which is known to inhibit UNC5B apoptotic activity, promotes survival of PyST-expressing cells. Our results thus suggest an important role of UNC5B in small-T antigen-induced mitotic catastrophe that also requires PP2A.IMPORTANCE UNC5B, PP2A, and netrin-1 are deregulated in a variety of cancers. UNC5B and PP2A are regarded as tumor suppressors, as they promote apoptosis and are deleted or mutated in many cancers. In contrast, netrin-1 promotes survival by inhibiting dependence receptors, including UNC5B, and is upregulated in many cancers. Here, we show that UNC5B-mediated apoptosis can occur independently of p53 but in a PP2A-dependent manner. A substantial percentage of cancers arise due to p53 mutations and are insensitive to chemotherapeutic treatments that activate p53. Unexpectedly, treatment of cancers having functional p53 with many conventional drugs leads to the upregulation of netrin-1 through activated p53, which is counterintuitive. Therefore, understanding the p53-independent mechanisms of the netrin-UNC5B axis, such as those involving PP2A, assumes greater clinical significance. Anticancer strategies utilizing anti-netrin-1 antibody treatment are already in clinical trials.

Serum netrin-1 as a biomarker for colorectal cancer detection

BACKGROUND

Recent evidence support that netrin-1 involves in colorectal carcinogenesis.

OBJECTIVE

This study was to evaluate the performance of serum netrin-1 for detection of colorectal cancer (CRC) in both clinical/screening sets.

METHODS

A total of 115 consecutive patients with CRC and matched healthy controls were included in Clinical Set. Fifty subjects with CRC, 50 subjects with advanced adenoma (AA), and 150 matched control participants free of neoplasia were included in Screening Set.

RESULTS

In Clinical set, subjects with CRC presented higher levels of serum netrin-1 (513.9 ± 22.6 pg/mL) than controls (347.8 ± 20.3 pg/mL, p< 0.0001). Similar in Screening set, serum netrin-1 was higher in CRC (644.5 ± 37.0 pg/mL, both p< 0.0001), compared with controls (407.7 ± 14.8 pg/mL) and AA (416.5 ± 18.5 pg/mL). However, there was no difference between controls and AA (p= 0.752). Compared with the low netrin-1 group, the high group presented increased risk of CRC (Clinical set: OR = 4.300, p< 0.001; Screening set: OR = 7.731, p< 0.001). ROC curve of netrin-1 was developed to detect CRC (Clinical set: AUC 0.703; Screening set: AUC 0.759).

CONCLUSIONS

It suggests netrin-1 as a potential biomarker for CRC detection.

Netrin-1 promotes naive pluripotency through Neo1 and Unc5b co-regulation of Wnt and MAPK signalling

In mouse embryonic stem cells (mESCs), chemical blockade of Gsk3α/β and Mek1/2 (2i) instructs a self-renewing ground state whose endogenous inducers are unknown. Here we show that the axon guidance cue Netrin-1 promotes naive pluripotency by triggering profound signalling, transcriptomic and epigenetic changes in mESCs. Furthermore, we demonstrate that Netrin-1 can substitute for blockade of Gsk3α/β and Mek1/2 to sustain self-renewal of mESCs in combination with leukaemia inhibitory factor and regulates the formation of the mouse pluripotent blastocyst. Mechanistically, we reveal how Netrin-1 and the balance of its receptors Neo1 and Unc5B co-regulate Wnt and MAPK pathways in both mouse and human ESCs. Netrin-1 induces Fak kinase to inactivate Gsk3α/β and stabilize β-catenin while increasing the phosphatase activity of a Ppp2r2c-containing Pp2a complex to reduce Erk1/2 activity. Collectively, this work identifies Netrin-1 as a regulator of pluripotency and reveals that it mediates different effects in mESCs depending on its receptor dosage, opening perspectives for balancing self-renewal and lineage commitment.

Netrin-1 promotes epithelium repair in corneal injury

AIM

To explore netrin-1 functions on corneal epithelium in vitro and in vivo.

METHODS

In vitro the human corneal epithelial (HCE) cells were treated with serum free DMEM-F12 basic media containing 0, 50, 100, 200, 300, 500, 800, and 1000 ng/mL of netrin-1, respectively. The cells viability was detected by cell counting kit-8 (CCK-8). The wound-healing assay was applied to assess the migration proficiency of HCE cells. Flow cytometry was used to analyze the cell-cycle distribution and apoptosis. In vivo, normal c57 (6wk) mice were demarcated with a trephine in the middle of the cornea to produce a 3-mm circular wound. Mice corneas were inflicted no epithelium with a 3-mm wound displayed, but remained the limbal epithelium intact. A blunt scalpel blade was used to remove the corneal epithelian cells, followed by topical netrin-1 application (200 ng/mL), and the group treated by PBS as control. The treated group was injected netrin-1 into the normal c57 mice inferior subconjunctival 4h before trauma. Mouse corneal inflammation and neovascularization were observed under slit lamp microscope. The apoptosis of corneal cells was determined by TUNEL staining.

RESLUTS

A concentration of 200 ng/mL netrin-1 enhanced 25% of the HCE viability. The relative migration rates were 76.3% and 100% in control and netrin-1 treated group after cultured 72h. Treated with netrin-1 (200 ng/mL) decreased the apoptosis of HCE cells, as well as decreased their percentage from 19.3%±0.57% to 12.7%±0.42% of the total. The remaining wound area was 1.22 mm² in control group but 0.22 mm² in the netrin-1 treated group. Exogenous Netrin-1 inhibits apoptosis of corneal epithelial cells of c57 mice. TUNEL-positive cells at the epithelial layer of the corneas of the control and netrin-1 treated c57 mice at 24h after wounding were 43.3% and 16.7% respectively.

CONCLUSION

Netrin-1 can reduce HCE apoptosis as well as promote its proliferation and migration. Topical application of netrin-1 promotes the injuryed cornea epithelial wound repair and inhibits apoptosis of corneal epithelial cells. These findings may offer potential therapies to repair the defects of corneal epithelial based on netrin-1.

The role of enteric neurons in the development and progression of colorectal cancer

The enteric nervous system (ENS) is the intrinsic neural network of the gastrointestinal tract, which is essential for regulating gut functions and intestinal homeostasis. The importance of the ENS is underscored by the existence of severe gastrointestinal diseases, such as Hirschsprung's disease and intestinal pseudo-obstruction, which arise when the ENS fails to develop normally or becomes dysregulated. Moreover, it is known that enteric neurons are involved in intestinal inflammation. However, the role of the ENS in colorectal cancer (CRC) carcinogenesis remains poorly understood, even though processes like perineural invasion and neoneurogenesis are important factors in CRC. Here we summarize how enteric neurons are affected during CRC and discuss the influence of enteric neurons, either direct or indirect, on the development and/or progression of CRC. Finally, we illustrate how the ENS could be targeted as a potential anti-cancer therapy, establishing the ENS as an integral part of the tumor microenvironment.

Serum netrin-1 in relation to gadolinium-enhanced magnetic resonance imaging in early multiple sclerosis

Background

Netrin-1, a secreted laminin-related protein, is known to regulate not only axonal guidance and neuronal cell migration, but also blood–brain barrier integrity and inflammation. Two preliminary studies reported altered serum netrin-1 levels in multiple sclerosis; however, associations with longitudinal clinical and magnetic resonance imaging activity have not been investigated.

Objectives

We aimed to assess serum netrin-1 in multiple sclerosis and controls with respect to disease activity and its temporal dynamics.

Methods

Serum netrin-1 was assessed by enzyme-linked immunosorbent assay in 79 patients with clinically isolated syndrome or multiple sclerosis, and 30 non-inflammatory neurological disease controls. In patients, serum samples were collected immediately prior to gadolinium-enhanced 3 T magnetic resonance imaging at two time points (initial contrast-enhancing gadolinium+ n = 47, non-enhancing gadolinium– n = 32; reference gadolinium– n = 70; median time-lag 1.4, interquartile range 1.0–2.3 years).

Results

Serum netrin-1 levels were similar in clinically isolated syndrome, multiple sclerosis and controls, and gadolinium+ and gadolinium– patients. Among gadolinium+ patients, serum netrin-1 was decreased in clinically active (n = 8) vs non-active patients (n = 39; p = 0.041). Serum netrin-1 showed no temporal dynamics in multiple sclerosis and was unrelated to clinical data.

Conclusions

Serum netrin-1 levels show no multiple sclerosis specific changes and are not sensitive for detection of subclinical disease activity. Netrin-1 changes during relapses may deserve further examination.

G9a/RelB regulates self-renewal and function of colon-cancer-initiating cells by silencing Let-7b and activating the K-RAS/β-catenin pathway

Epigenetic reprogramming has been associated with the functional plasticity of cancer-initiating cells (CICs); however, the regulatory pathway has yet to be elucidated. A siRNA screen targeting known epigenetic genes revealed that G9a profoundly impairs the chemo-resistance, self-renewal and metastasis of CICs obtained from patients with colorectal cancer (CRC). Patients with elevated G9a were shown to face a high risk of relapse and poor survival rates. From a mechanistic perspective, G9a binds with and stabilizes RelB, thereby recruiting DNA methyltransferase 3 on the Let-7b promoter and repressing its expression. This leads to the activation of the K-RAS/β-catenin pathway and regulates self-renewal and function of CICs. These findings indicate that the G9a/RelB/Let-7b axis acts as a critical regulator in the maintenance of CIC phenotypes and is strongly associated with negative clinical outcomes. Thus, these findings may have diagnostic as well as therapeutic implications for the treatment of chemotherapy-resistant or metastatic CRC.

Netrin-1 Peptide Is a Chemorepellent in Tetrahymena thermophila

Netrin-1 is a highly conserved, pleiotropic signaling molecule that can serve as a neuronal chemorepellent during vertebrate development. In vertebrates, chemorepellent signaling is mediated through the tyrosine kinase, src-1, and the tyrosine phosphatase, shp-2. Tetrahymena thermophila has been used as a model system for chemorepellent signaling because its avoidance response is easily characterized under a light microscope. Our experiments showed that netrin-1 peptide is a chemorepellent in T. thermophila at micromolar concentrations. T. thermophila adapts to netrin-1 over a time course of about 10 minutes. Netrin-adapted cells still avoid GTP, PACAP-38, and nociceptin, suggesting that netrin does not use the same signaling machinery as any of these other repellents. Avoidance of netrin-1 peptide was effectively eliminated by the addition of the tyrosine kinase inhibitor, genistein, to the assay buffer; however, immunostaining using an anti-phosphotyrosine antibody showed similar fluorescence levels in control and netrin-1 exposed cells, suggesting that tyrosine phosphorylation is not required for signaling to occur. In addition, ELISA indicates that a netrin-like peptide is present in both whole cell extract and secreted protein obtained from Tetrahymena thermophila. Further study will be required in order to fully elucidate the signaling mechanism of netrin-1 peptide in this organism.

Pharmacological targeting of AKAP-directed compartmentalized cAMP signalling

The second messenger cyclic adenosine monophosphate (cAMP) can bind and activate protein kinase A (PKA). The cAMP/PKA system is ubiquitous and involved in a wide array of biological processes and therefore requires tight spatial and temporal regulation. Important components of the safeguard system are the A-kinase anchoring proteins (AKAPs), a heterogeneous family of scaffolding proteins defined by its ability to directly bind PKA. AKAPs tether PKA to specific subcellular compartments, and they bind further interaction partners to create local signalling hubs. The recent discovery of new AKAPs and advances in the field that shed light on the relevance of these hubs for human disease highlight unique opportunities for pharmacological modulation. This review exemplifies how interference with signalling, particularly cAMP signalling, at such hubs can reshape signalling responses and discusses how this could lead to novel pharmacological concepts for the treatment of disease with an unmet medical need such as cardiovascular disease and cancer.

Netrin-1 as a potential target for metastatic cancer: focus on colorectal cancer

Despite advanced screening technology and cancer treatments available today, metastasis remains an ongoing major cause of cancer-related deaths worldwide. Typically, colorectal cancer is one of the cancers treatable by surgery in conjunction with chemotherapy when it is detected at an early stage. However, it still ranks as the second highest modality and mortality of cancer types in western countries, and this is mostly due to a recurrence of metastatic colorectal cancer post-resection of the primary malignancy. Colorectal cancer metastases predominantly occur in the liver and lung, and yet the molecular mechanisms that regulate these organ-specific colorectal cancer metastases are largely unknown. Therefore, the identification of any critical molecule, which triggers malignancy in colorectal cancer, would be an excellent target for treatment. Netrin-1 was initially discovered as a chemotropic neuronal guidance molecule, and has been marked as a regulator for many cancers including colorectal cancer. Here, we summarise key findings of the role of netrin-1 intrinsic to colorectal cancer cells, extrinsic to the tumour microenvironment and angiogenesis, and consequently, we evaluate netrin-1 as a potential target molecule for metastasis.

Aspirin-induced histone acetylation in endothelial cells enhances synthesis of the secreted isoform of netrin-1 thus inhibiting monocyte vascular infiltration

Background and Purpose

There are conflicting data regarding whether netrin-1 retards or accelerates atherosclerosis progression, as it can lead either to monocyte repulsion from or retention within plaques depending on its cellular source. We investigated the effect of aspirin, which is widely used in cardiovascular prophylaxis, on the synthesis of different isoforms of netrin-1 by endothelial cells under pro-inflammatory conditions, and defined the net effect of aspirin-dependent systemic modulation of netrin-1 on atherosclerosis progression.

Experimental Approach

Netrin-1 synthesis was studied in vitro using human endothelial cells stimulated with TNF-α, with or without aspirin treatment. In vivo experiments were conducted in ApoE^−/− mice fed with a high-fat diet (HFD), receiving either aspirin or clopidogrel.

Key Results

TNF-α-induced NF-κB activation up-regulated the nuclear isoform of netrin-1, while simultaneously reducing secreted netrin-1. Down-regulation of the secreted isoform compromised the chemorepellent action of the endothelium against monocyte chemotaxis. Aspirin counteracted TNF-α-mediated effects on netrin-1 synthesis by endothelial cells through COX-dependent inhibition of NF-κB and concomitant histone hyperacetylation. Administration of aspirin to ApoE^−/− mice on HFD increased blood and arterial wall levels of netrin-1 independently of its effects on platelets, accompanied by reduced plaque size and content of monocytes/macrophages, compared with untreated or clopidogrel-treated mice. In vivo blockade of netrin-1 enhanced monocyte plaque infiltration in aspirin-treated ApoE^−/− mice.

Conclusions and Implications

Aspirin counteracts down-regulation of secreted netrin-1 induced by pro-inflammatory stimuli in endothelial cells. The aspirin-dependent increase of netrin-1 in ApoE^−/− mice exerts anti-atherogenic effects by preventing arterial accumulation of monocytes.

Netrin-1 promoted pancreatic cancer cell proliferation by upregulation of Mdm2

Netrin-1 displays proto-oncogenic activity in several cancers, which is thought to result from the ability of netrin-1 secretions to stimulate survival when bound to associated receptors. The objective of this study was to determine the role of netrin-1 in pancreatic cancer cell proliferation in vitro. Our results revealed that netrin-1 overexpression promoted while its silence inhibited two pancreatic cancer cell lines. At the molecular level, we found that netrin-1 promoted cell proliferation by upregulation of murine double minute 2 (Mdm2). As a result, p53 protein contents were reduced in cells overexpressing netrin-1. Therefore, our data suggests the presence of a previously unknown network in the proliferation and progression of pancreatic cancer cells.

PKC α regulates netrin-1/UNC5B-mediated survival pathway in bladder cancer

BACKGROUND

Netrin-1 and its receptor UNC5B play important roles in angiogenesis, embryonic development, cancer and inflammation. However, their expression patttern and biological roles in bladder cancer have not been well characterized. The present study aims to investigating the clinical significance of PKC α, netrin-1 and UNC5B in bladder cancer as well as their association with malignant biological behavior of cancer cells.

METHODS

Netrin-1 and UNC5B expression was examined in 120 bladder cancer specimens using immunohistochemistry and in 40 fresh cancer tissues by western blot. Immunofluorescence was performed in cancer cell lines. PKC α agonist PMA and PKC siRNA was employed in bladder cancer cells. CCK-8, wound healing assays and flow cytometry analysis were used to examine cell proliferation, migration and cell cycle, respectively.

RESULTS

Netrin-1 expression was positively correlated with histological grade, T stage, metastasis and poor prognosis in bladder cancer tissues. Immunofluorescence showed elevated netrin-1 and decreased UNC5B expression in bladder cancer cells compared with normal bladder cell line. Furthermore, cell proliferation, migration and cell cycle progression were promoted with PMA treatment while inhibited by calphostin C. In addition, PMA treatment could induce while calphostin C reduce netrin-1 expression in bladder cancer cells.

CONCLUSIONS

The present study identified netrin-1/UNC5B, which could be regulated by PKC signaling, was important mediators of bladder cancer progression.

A fast growing spectrum of biological functions of γ-secretase in development and disease

γ-secretase, which assembles as a tetrameric complex, is an aspartyl protease that proteolytically cleaves substrate proteins within their membrane-spanning domain; a process also known as regulated intramembrane proteolysis (RIP). RIP regulates signaling pathways by abrogating or releasing signaling molecules. Since the discovery, already >15 years ago, of its catalytic component, presenilin, and even much earlier with the identification of amyloid precursor protein as its first substrate, γ-secretase has been commonly associated with Alzheimer's disease. However, starting with Notch and thereafter a continuously increasing number of novel substrates, γ-secretase is becoming linked to an equally broader range of biological processes. This review presents an updated overview of the current knowledge on the diverse molecular mechanisms and signaling pathways controlled by γ-secretase, with a focus on organ development, homeostasis and dysfunction. This article is part of a Special Issue entitled: Intramembrane Proteases.

Growth factors in synaptic function

Synapses are increasingly recognized as key structures that malfunction in disorders like schizophrenia, mental retardation, and neurodegenerative diseases. The importance and complexity of the synapse has fuelled research into the molecular mechanisms underlying synaptogenesis, synaptic transmission, and plasticity. In this regard, neurotrophic factors such as netrin, Wnt, transforming growth factor-β (TGF-β), tumor necrosis factor-α (TNF-α), and others have gained prominence for their ability to regulate synaptic function. Several of these factors were first implicated in neuroprotection, neuronal growth, and axon guidance. However, their roles in synaptic development and function have become increasingly clear, and the downstream signaling pathways employed by these factors have begun to be elucidated. In this review, we will address the role of these factors and their downstream effectors in synaptic function in vivo and in cultured neurons.

Control of male and female fertility by the netrin axon guidance genes

The netrin axon guidance genes have previously been implicated in fertility in C. elegans and in vertebrates. Here we show that adult Drosophila lacking both netrin genes, NetA and NetB, have fertility defects in both sexes together with an inability to fly and reduced viability. NetAB females produce fertilized eggs at a much lower rate than wild type. Oocyte development and ovarian innervation are unaffected in NetAB females, and the reproductive tract appears normal. A small gene, hog, that resides in an intron of NetB does not contribute to the NetAB phenotype. Restoring endogenous NetB expression rescues egg-laying, but additional genetic manipulations, such as restoration of netrin midline expression and inhibition of cell death have no effect on fertility. NetAB males induce reduced egg-laying in wild type females and display mirror movements of their wings during courtship. Measurement of courtship parameters revealed no difference compared to wild type males. Transgenic manipulations failed to rescue male fertility and mirror movements. Additional genetic manipulations, such as removal of the enabled gene, a known suppressor of the NetAB embryonic CNS phenotype, did not improve the behavioral defects. The ability to fly was rescued by inhibition of neuronal cell death and pan-neural NetA expression. Based on our results we hypothesize that the adult fertility defects of NetAB mutants are due to ovulation defects in females and a failure to properly transfer sperm proteins in males, and are likely to involve multiple neural circuits.

Human height genes and cancer

Body development requires the ability to control cell proliferation and metabolism, together with selective 'invasive' cell migration for organogenesis. These requirements are shared with cancer. Human height-associated loci have been recently identified by genome-wide SNP-association studies. Strikingly, most of the more than 100 genes found associated to height appear linked to neoplastic growth, and impose a higher risk for cancer. Height-associated genes drive the HH/PTCH and BMP/TGFβ pathways, with p53, c-Myc, ERα, HNF4A and SMADs as central network nodes. Genetic analysis of body-size-affecting diseases and evidence from genetically-modified animals support this model. The finding that cancer is deeply linked to normal, body-plan master genes may profoundly affect current paradigms on tumor development.