Akt3 activation by R-Ras in an endothelial cell enforces quiescence and barrier stability of neighboring endothelial cells via Jagged1

Communication between adjacent endothelial cells is important for the homeostasis of blood vessels. We show that quiescent endothelial cells use Jagged1 to instruct neighboring endothelial cells to assume a quiescent phenotype and secure the endothelial barrier. This phenotype enforcement by neighboring cells is operated by R-Ras through activation of Akt3, which results in upregulation of a Notch ligand Jagged1 and consequential upregulation of Notch target genes, such as UNC5B, and VE-cadherin accumulation in the neighboring cells. These signaling events lead to the stable interaction between neighboring endothelial cells to continue to fortify juxtacrine signaling via Jagged1-Notch. This mode of intercellular signaling provides a positive feedback regulation of endothelial cell-cell interactions and cellular quiescence required for the stabilization of the endothelium.

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.

Increased Expression Levels of Netrin-1 in Visceral Adipose Tissue during Obesity Favour Colon Cancer Cell Migration

Netrin (NTN)-1, an extracellular matrix protein with a crucial role in inflammation, is dysregulated during obesity (OB) and influences colon cancer (CC) progression. To decipher the mechanisms underlying CC development during obesity, we examined the expression of NTN1 and its receptors in the visceral adipose tissue (VAT) of 74 (25 normal weight (NW)) (16 with CC) and 49 patients with OB (12 with CC). We also evaluated the effect of caloric restriction (CR) on the gene expression levels of Ntn1 and its receptors in the colon from a rat model fed a normal diet. The impact of adipocyte-conditioned media (ACM) from patients with OB and NTN-1 was assessed on the expression levels of neogenin 1(NEO1), deleted in colorectal carcinomas (DCC) and uncoordinated-5 homolog B (UNC5B) in Caco-2 and HT-29 human colorectal cell lines, as well as on Caco-2 cell migration. Increased NTN1 and NEO1 mRNA levels in VAT were due to OB (p < 0.05) and CC (p < 0.001). In addition, an upregulation in the expression levels of DCC and UNC5B in patients with CC (p < 0.01 and p < 0.05, respectively) was observed. Decreased (p < 0.01) Ntn1 levels in the colon from rats submitted to CR were found. In vitro experiments showed that ACM increased DCC (p < 0.05) and NEO1 (p < 0.01) mRNA levels in HT-29 and Caco-2 cell lines, respectively, while UNC5B decreased (p < 0.01) in HT-29. The treatment with NTN-1 increased (p < 0.05) NEO1 mRNA levels in HT-29 cells and DCC (p < 0.05) in both cell lines. Finally, we revealed a potent migratory effect of ACM and NTN-1 on Caco-2 cells. Collectively, these findings point to increased NTN-1 during OB and CC fuelling cancer progression and exerting a strong migratory effect on colon cancer cells.

GPC3-Unc5 receptor complex structure and role in cell migration

Neural migration is a critical step during brain development that requires the interactions of cell-surface guidance receptors. Cancer cells often hijack these mechanisms to disseminate. Here, we reveal crystal structures of Uncoordinated-5 receptor D (Unc5D) in complex with morphogen receptor glypican-3 (GPC3), forming an octameric glycoprotein complex. In the complex, four Unc5D molecules pack into an antiparallel bundle, flanked by four GPC3 molecules. Central glycan-glycan interactions are formed by N-linked glycans emanating from GPC3 (N241 in human) and C-mannosylated tryptophans of the Unc5D thrombospondin-like domains. MD simulations, mass spectrometry and structure-based mutants validate the crystallographic data. Anti-GPC3 nanobodies enhance or weaken Unc5-GPC3 binding and, together with mutant proteins, show that Unc5/GPC3 guide migrating pyramidal neurons in the mouse cortex, and cancer cells in an embryonic xenograft neuroblastoma model. The results demonstrate a conserved structural mechanism of cell guidance, where finely balanced Unc5-GPC3 interactions regulate cell migration.

UNC5B Overexpression Alleviates Peripheral Neuropathic Pain by Stimulating Netrin-1-Dependent Autophagic Flux in Schwann Cells

Lesions or diseases of the somatosensory system can cause neuropathic pain (NP). Schwann cell (SC) autophagy plays an important role in NP. Uncoordinated gene 5 homolog B (UNC5B), the canonical dependent receptor of netrin-1, is known to be exclusively expressed in SCs and involved in NP; however, the underlying mechanisms were unclear. A rat model of sciatic nerve chronic constriction injury (CCI) was used to induce peripheral neuropathic pain. Adeno-associated virus (AAV) overexpressing UNC5B was applied to the injured nerve, and an autophagy inhibitor, 3-mechyladenine (3-MA), was intraperitoneally injected in some animals. Behavioral tests were performed to evaluate NP, the morphology of the injured nerves was analyzed, and autophagy-related proteins were detected. A rat SC line (RSC96) undergoing oxygen and glucose deprivation (OGD) was used to mimic an ischemic setting to examine the role of UNC5B in autophagy. Local UNC5B overexpression alleviated CCI-induced NP and rescued myelin degeneration. Meanwhile, UNC5B overexpression improved CCI-induced impairment of autophagic flux, while the autophagy inhibitor 3-MA reversed the analgesic effect of UNC5B. In cultured SCs, UNC5B helped recruit netrin-1 to the cell membrane. UNC5B overexpression promoted autophagic flux while inhibiting apoptosis, which was further augmented with exogenous netrin-1 and reversed by netrin-1 knockdown. The enhanced phosphorylation of AMP-activated protein kinase (AMPK) and Unc51-like autophagy activating kinase 1 (ULK1) by UNC5B overexpression was also correlated with netrin-1. Our results suggest that UNC5B facilitates autophagic flux in SCs via phosphorylation of AMPK and ULK1, dependent on its ligand netrin-1, protecting myelin and partly preventing injury-induced NP.

Role of Neuronal Guidance Cues in the Pathophysiology of Obesity: A Peripheral and Central Overview

Obesity is associated with an exacerbated synthesis and secretion of several molecules, which culminates in chronic low-grade inflammation and insulin resistance. Such conditions affect molecular and physiological responses of several organs and, if not resolved, predispose the obese patients to other diseases such as Type 2 diabetes, atherosclerosis, cancer, neural injuries, and cognitive impairments. A microenvironment with an excess of pro-inflammatory cytokines released by different cells, including immune and adipose cells lead to metabolic and non-metabolic diseases during obesity. In this context, the role of neuronal guidance cues named netrin, semaphorin and ephrin is novel. Specifically, the available literature indicates that besides their classic role as molecules that guide the axon to its target site, the neuronal guidance cues exhibit immunomodulatory functions from adipose tissue to the neural environment. In the current narrative review, we discuss the participation of the neuronal guidance cues on the physiology and pathophysiology of obesity. We also discuss the feedback loop of obesity on the netrin, semaphorin and ephrin functions that impair the structure and function of the brain. The integrative view of the neuronal guidance cues can be relevant in designing new treatments focus on attenuating metabolic and immune disorders in obese patients and reduce the risk of acquiring diseases such as Type 2 diabetes, atherosclerosis, cancer, and neural injuries.

Long non-coding RNA PITPNA-AS1 regulates UNC5B expression in papillary thyroid cancer via sponging miR-129-5p

BACKGROUND

Long non-coding RNA (lncRNA) PITPNA antisense RNA 1 (PITPNA-AS1) expression characteristics, function, and mechanism in papillary thyroid cancer are unclear.

METHODS

Quantitative real-time polymerase chain reaction (qRT-PCR) was applied for detecting PITPNA-AS1, UNC-5 netrin receptor B (UNC5B) mRNA, and miR-129-5p expressions in papillary thyroid cancer tissues and cell lines. EdU assay, cell counting kit-8 (CCK-8) assay, wound healing assay, and flow cytometry analysis were performed to investigate the biological functions of PITPNA-AS1 in papillary thyroid cancer. Dual-luciferase reporter assay was utilized for determining whether PITPNA-AS1 and miR-129-5p, as well as UNC5B and miR-129-5p could directly bind to each other. Western blot assay was employed for measuring UNC5B protein expression level in papillary thyroid cancer cell lines.

RESULTS

PITPNA-AS1 and UNC5B expressions were markedly increased in papillary thyroid cancer tissues and cell lines while miR-129-5p expression was down-regulated. Knockdown of PITPNA-AS1 could significantly inhibit papillary thyroid cancer cell growth and migration and promote cell apoptosis while UNC5B overexpression plasmids or miR-129-5p inhibitors counteracted the knockdown effect of PITPNA-AS1 on papillary thyroid cancer cells. PITPNA-AS1 targeted miR-129-5p to repress its expression and miR-129-5p targeted UNC5B to repress its expression. Silencing PITPNA-AS1 reduced the expression of UNC5B via regulating miR-129-5p expression.

CONCLUSIONS

PITPNA-AS1 facilitated papillary thyroid cancer cell proliferation and migration, and suppressed apoptosis through miR-129-5p/UNC5B axis.

The intracellular domain of UNC5B facilities proliferation and metastasis of bladder cancer cells

The intracellular domain of UNC5B contains both death domain and caspase‐3 cleavage site, and is regarded as a functional domain that mediates apoptosis. However, in our previous studies, we found that the death domain of UNC5B in bladder cancer cells could not be activated to promote apoptosis. In this study, different UNC5B truncates (residue 399‐945, residue 412‐945) were created to explore whether the caspase‐3 cleavage site (site 412), as another potential functional domain of its intracellular portion, could be activated to induce apoptosis in bladder cancer cells. Using mass spectrometry, we acquired a comprehensive and detailed identification of differentially expressed proteins by overexpressing UNC5B and its truncates. Protein‐protein‐interaction (PPI) network analysis was also applied to investigate the aggregation of related proteins and predict the functional changes. EDU assay, apoptosis, xenograft tumour implantation, migration, invasion and tumour metastasis were performed to comprehensively identify the effects of UNC5B truncates on bladder cancer cells. We demonstrate that the intracellular domain of UNC5B promotes cell proliferation in vitro and tumour formation in vivo, by binding to a large number of ribosomal proteins. The overexpression of intracellular domain also facilitates cells to migrate, invade and metastasize by interacting with fibronectin, beta‐catenin and vimentin. In addition, we reveal that overexpressing the intracellular domain of UNC5B cannot bind or activate cleaved caspase‐3 to trigger apoptosis in bladder cancer cells.

High expression of UNC5B enhances tumor proliferation, increases metastasis, and worsens prognosis in breast cancer

UNC-5 Homolog B (UNC5B) is a member of the dependence receptor family that regulates cell survival and apoptosis in a ligand-dependent manner. UNC5B plays an important role in the development of multiple cancers, including colorectal, bladder, and thyroid cancer. However, the exact expression pattern and mechanism of UNC5B in breast cancer have not been well elucidated. Here, we showed that UNC5B expression was significantly upregulated in breast cancer using bioinformatics analysis and experimental validation. High UNC5B expression was correlated with poor overall survival in breast cancer patients. UNC5B knockdown inhibited breast cancer cell proliferation and metastasis and compromised PI3K/Akt signaling activation. In summary, UNC5B is a promising diagnostic and prognostic biomarker and targeting UNC5B is a potential strategy for individualized breast cancer treatment.

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.

HIF-1α-dependent miR-424 induction confers cisplatin resistance on bladder cancer cells through down-regulation of pro-apoptotic UNC5B and SIRT4

Background

Chemo-resistance of bladder cancer has been considered to be one of the serious issues to be solved. In this study, we revealed pivotal role of miR-424 in the regulation of CDDP sensitivity of bladder cancer cells.

Methods

The cytotoxicity of cisplatin and effect of miR-424 were assessed by flow cytometry and TUNEL. Transcriptional regulation of miR-424 by HIF-1α was assessed by Chromatin immunoprecipitation (ChIP). Effect of miR-424 on expression of UNC5B, SIRT4 (Sirtuin4) and apoptotic markers was measured by QRT-PCR and/or Western blot. The regulation of miR-424 for UNC5B and SIRT4 were tested by luciferase reporter assay. The 5637-inoculated nude mice xenograft model was used for the in vivo study. The clinical significance of miR-424 was demonstrated mainly through data mining and statistical analysis of TCGA.

Results

In this study, we have found for the first time that cisplatin (CDDP) induces the expression of miR-424 in a HIF-1α-dependent manner under normoxia, and miR-424 plays a vital role in the regulation of CDDP resistance of bladder cancer cells in vitro. Mechanistically, we have found that UNC5B and SIRT4 are the direct downstream target genes of miR-424. CDDP-mediated suppression of xenograft bladder tumor growth was prohibited by the addition of miR-424, whereas ectopic expression of UNC5B or SIRT4 partially restored miR-424-dependent decrease in CDDP sensitivity of bladder cancer 5637 and T24 cells. Moreover, knockdown of UNC5B or SIRT4 prohibited CDDP-mediated proteolytic cleavage of PARP and also decreased CDDP sensitivity of these cells. Consistently, the higher expression levels of miR-424 were closely associated with the poor clinical outcome of the bladder cancer patients. There existed a clear inverse relationship between the expression levels of miR-424 and pro-apoptotic UNC5B or SIRT4 in bladder cancer tissues.

Conclusions

Collectively, our current results strongly suggest that miR-424 tightly participates in the acquisition/maintenance of CDDP-resistant phenotype of bladder cancer cells through down-regulation of its targets UNC5B and SIRT4, and thus combination chemotherapy of CDDP plus HIF-1α/miR-424 inhibition might have a significant impact on hypoxic as well as normoxic bladder cancer cells.

Netrin-1 works with UNC5B to regulate angiogenesis in diabetic kidney disease

Netrin-1, an axon guidance factor, and its receptor UNC5B play important roles in axonal development and angiogenesis. This study examined netrin-1 and UNC5B expression in kidneys with diabetic kidney disease (DKD) and investigated their roles in angiogenesis. Netrin-1 and UNC5B were upregulated in streptozotocininduced DKD Wistar rats, and their expression was compared with that in healthy controls. However, exogenous netrin-1 in UNC5B-depleted human renal glomerular endothelial cells (HRGECs) inhibited cell migration and tubulogenesis. This effect was likely associated with SRC pathway deactivation. Netrin-1 treatment also eliminated the pro-angiogenic effects of exogenous VEGF-165 on UNC5B-silenced HRGECs. These results indicate that UNC5B antagonizes netrin-1 and that UNC5B upregulation contributes partly to enhancing angiogenesis in DKD. Therefore, introducing exogenous netrin-1 and depleting endogenous UNC5B are potential strategies for reducing the incidence of early angiogenesis and mitigating kidney injury in DKD.

Lack of Myosin X Enhances Osteoclastogenesis and Increases Cell Surface Unc5b in Osteoclast-Lineage Cells

Normal bone mass is maintained by balanced bone formation and resorption. Myosin X (Myo10), an unconventional "myosin tail homology 4-band 4.1, ezrin, radixin, moesin" (MyTH4-FERM) domain containing myosin, is implicated in regulating osteoclast (OC) adhesion, podosome positioning, and differentiation in vitro. However, evidence is lacking for Myo10 in vivo function. Here we show that mice with Myo10 loss of function, Myo10^m/m , exhibit osteoporotic deficits, which are likely due to the increased OC genesis and bone resorption because bone formation is unchanged. Similar deficits are detected in OC-selective Myo10 conditional knockout (cko) mice, indicating a cell autonomous function of Myo10. Further mechanistic studies suggest that Unc-5 Netrin receptor B (Unc5b) protein levels, in particular its cell surface level, are higher in the mutant OCs, but lower in RAW264.7 cells or HEK293 cells expressing Myo10. Suppressing Unc5b expression in bone marrow macrophages (BMMs) from Myo10^m/m mice by infection with lentivirus of Unc5b shRNA markedly impaired RANKL-induced OC genesis. Netrin-1, a ligand of Unc5b, increased RANKL-induced OC formation in BMMs from both wild-type and Myo10^m/m mice. Taken together, these results suggest that Myo10 plays a negative role in OC formation, likely by inhibiting Unc5b cell-surface targeting, and suppressing Netrin-1 promoted OC genesis. © 2019 American Society for Bone and Mineral Research.

LncRNA TNRC6C-AS1 regulates UNC5B in thyroid cancer to influence cell proliferation, migration, and invasion as a competing endogenous RNA of miR-129-5p

To investigate the biological functions and regulatory mechanism of lncRNA TNRC6C-AS1 in thyroid cancer (TC). TNRC6C-AS1, miR-129-5p, and UNC5B expression levels were investigated by qRT-PCR and Western blot. CCK-8 assay was conducted to determine cell proliferation, while transwell assay was for inspection of cell migration and invasion. Through bioinformatic analysis, the interactions among TNRC6C-AS1, miR-129-5p, and UNC5B were predicted. Dual luciferase reporter gene assay and RNA pull-down assay confirmed the predicted target relationships. Tumor xenograft assay was applied to inspect the effect of TNRC6C-AS1 downregulation on TC development in vivo. TNRC6C-AS1 and UNC5B were overexpressed, while miR-129-5p was underexpressed in TC tissues and cells. TNRC6C-AS1/UNC5B downregulation and miR-129-5p overexpression could suppress proliferation, migration, and invasion of TC cells as well as inhibit tumorigenesis in vivo. MiR-129-5p targeted TNRC6C-AS1 and UNC5B in TC cells; and UNC5B expression was downregulated by knocking down TNRC6C-AS1, which competitively bound with miR-129-5p. Downregulation of TNRC6C-AS1 restrained TC development by knocking down UNC5B through upregulating the expression of miR-129-5p.

Placental labyrinth formation in mice requires endothelial FLRT2/UNC5B signaling

The placental labyrinth is the interface for gas and nutrient exchange between the embryo and the mother; hence its proper development is essential for embryogenesis. However, the molecular mechanism underlying development of the placental labyrinth, particularly in terms of its endothelial organization, is not well understood. Here, we determined that fibronectin leucine-rich transmembrane protein 2 (FLRT2), a repulsive ligand of the UNC5 receptor family for neurons, is unexpectedly expressed in endothelial cells specifically in the placental labyrinth. Mice lacking FLRT2 in endothelial cells exhibited embryonic lethality at mid-gestation, with systemic congestion and hypoxia. Although they lacked apparent deformities in the embryonic vasculature and heart, the placental labyrinths of these embryos exhibited aberrant alignment of endothelial cells, which disturbed the feto-maternal circulation. Interestingly, this vascular deformity was related to endothelial repulsion through binding to the UNC5B receptor. Our results suggest that the proper organization of the placental labyrinth depends on coordinated inter-endothelial repulsion, which prevents uncontrolled layering of the endothelium.

Overexpression of UNC5B in bladder cancer cells inhibits proliferation and reduces the volume of transplantation tumors in nude mice

Background

The netrin-1 receptor UNC5B plays vital roles in angiogenesis, inflammation, embryonic development and carcinogenesis. However, the functional significance of UNC5B overexpression in bladder cancer remains unclear. In this study, we investigated the role of UNC5B in bladder cancer in vitro and in vivo.

Methods

Stable transfection of the human bladder cancer cell line 5637 with UNC5B (5637-U) was confirmed by real-time RT-PCR, western blot and immunofluorescence assays. UNC5B expression in 5637 and 5637-U cells and mice tumor specimens derived from these cell lines was analyzed by immunohistochemistryand western blotting. Changes in the levels of cell cycle proteins were evaluated by western blotting. Flow cytometry, CCK-8 and scratch tests were used to examine cell cycle distribution, proliferation and migration, respectively.

Results

UNC5B overexpression in 5637 cells inhibited cell multiplication and migration and induced cell cycle arrest at the G2/M phase, meanwhile exhibited changes in the expression of cell cycle-associated proteins, showing that UNC5B may inhibit metastatic behaviors in bladder cancer cells. In addition, tumors generated from 5637-U cells were smaller than tumors generated from control 5637 cells.

Conclusions

Our findings suggest that UNC5B is a potential anti-neoplastic target in bladder cancer progression.

Netrin-1 overexpression in bone marrow mesenchymal stem cells promotes functional recovery in a rat model of peripheral nerve injury

Transplantation of bone marrow mesenchymal stem cells (BMSCs) has been developed as a new method of treating diseases of the peripheral nervous system. While netrin-1 is a critical molecule for axonal path finding and nerve growth, it may also affect vascular network formation. Here, we investigated the effect of transplanting BMSCs that produce netrin-1 in a rat model of sciatic nerve crush injury. We introduced a sciatic nerve crush injury, and then injected 1×10⁶ BMSCs infected by a recombinant adenovirus expressing netrin-1 Ad5-Netrin-1-EGFP or culture medium into the injured part in the next day. At day 7, 14 and 28 after injection, we measured motor nerve conduction and detected mRNA expressions of netrin-1 receptors UNC5B and Deleted in Colorectal Cancer (DCC), and neurotrophic factors brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) by real-time PCR. We also detected protein expressions of BDNF and NGF by Western blotting assays and examined BMSCs that incorporated into myelin and vascellum. The results showed that BMSCs infected by Ad5-Netrin-1-EGFP significantly improved the function of the sciatic nerve, and led to increased expression of BDNF and NGF (P<0.05). Moreover, 28 days after injury, more Schwann cells were found in BMSCs infected by Ad5-Netrin-1-EGFP compared to control BMSCs. In conclusion, transplantation of BMSCs that produce netrin-1 improved the function of the sciatic nerve after injury. This method may be a new treatment of nerve injury.

Netrin-1 induces the migration of Schwann cells via p38 MAPK and PI3K-Akt signaling pathway mediated by the UNC5B receptor

Schwann cells (SCs) play an essentially supportive role in the regeneration of injured peripheral nerve system (PNS). As Netrin-1 is crucial for the normal development of nervous system (NS) and can direct the process of damaged PNS regeneration, our study was designed to determine the role of Netrin-1 in RSC96 Schwann cells (an immortalized rat Schwann cell line) proliferation and migration. Our studies demonstrated that Netrin-1 had no effect on RSC96 cells proliferation, while significantly promoted RSC96 cells migration. The Netrin-1-induced RSC96 cells migration was significantly attenuated by inhibition of p38 and PI3K through pretreatment with SB203580 and LY294002 respectively, but not inhibition of MEK1/2 and JNK by U0126-EtOH and SP600125 individually. Treatment with Netrin-1 enhanced the phosphorylation of p38 and Akt. QRT-PCR indicated that Netrin-1 and only its receptors Unc5a, Unc5b and Neogenin were expressed in RSC96 cells, among which Unc5b expressed the most. And UNC5B protein was significantly increased after stimulated by Netrin-1. In conclusion, we show here that Netrin-1-enhanced SCs migration is mediated by activating p38 MAPK and PI3K-Akt signal cascades via receptor UNC5B, which suggests that Netrin-1 could serve as a new therapeutic strategy and has potential application value for PNS regeneration.

Netrin-1 is a critical autocrine/paracrine factor for osteoclast differentiation

Bone metabolism is a vital process that involves resorption by osteoclasts and formation by osteoblasts, which is closely regulated by immune cells. The neuronal guidance protein Netrin-1 regulates immune cell migration and inflammatory reactions, but its role in bone metabolism is unknown. During osteoclast differentiation, osteoclast precursors increase expression of Netrin-1 and its receptor Unc5b. Netrin-1 binds, in an autocrine and paracrine manner, to Unc5b to promote osteoclast differentiation in vitro, and absence of Netrin-1 or antibody-mediated blockade of Netrin-1 or Unc5b prevents osteoclast differentiation of both murine and human precursors. We confirmed the functional relationship of Netrin-1 in osteoclast differentiation in vivo using Netrin-1-deficient (Ntn1(-/-) ) or wild-type (WT) bone marrow transplanted mice. Notably, Ntn1(-/-) chimeras have markedly diminished osteoclasts, as well as increased cortical and trabecular bone density and volume compared with WT mice. Mechanistic studies revealed that Netrin-1 regulates osteoclast differentiation by altering cytoskeletal assembly. Netrin-1 increases regulator of Rho-GEF subfamily (LARG) and repulsive guidance molecule (RGMa) association with Unc5b, which increases expression and activation of cytoskeletal regulators RhoA and focal adhesion kinase (FAK). Netrin-1 and its receptor Unc5b likely play a role in fusion of osteoclast precursors because Netrin-1 and DC-STAMP are tightly linked. These results identify Netrin-1 as a key regulator of osteoclast differentiation that may be a new target for bone therapies.

UNC5B receptor deletion exacerbates DSS-induced colitis in mice by increasing epithelial cell apoptosis

The netrin-1 administration or overexpression is known to protect colon from acute colitis. However, the receptor that mediates netrin-1 protective activities in the colon during colitis remains unknown. We tested the hypothesis that UNC5B receptor is a critical mediator of protective function of netrin-1 in dextran sodium sulfate (DSS)-induced colitis using mice with partial deletion of UNC5B receptor. DSS colitis was performed in mice with partial genetic UNC5B deficiency (UNC5B^+/− mice) or wild-type mice to examine the role of endogenous UNC5B. These studies were supported by in vitro models of DSS-induced apoptosis in human colon epithelial cells. WT mice developed colitis in response to DSS feeding as indicated by reduction in bw, reduction in colon length and increase in colon weight. These changes were exacerbated in heterozygous UNC5B knockout mice treated with DSS. Periodic Acid-Schiff stained section shows damages in colon epithelium and mononuclear cell infiltration in WT mice, which was further increased in UNC5B heterozygous knockout mice. This was associated with large increase in inflammatory mediators such as cytokine and chemokine expression and extensive apoptosis of epithelial cells in heterozygous knockout mice as compared to WT mice. Overexpression of UNC5B human colon epithelial cells suppressed DSS-induced apoptosis and caspase-3 activity. Moreover, DSS induced large amount of netrin-1 and shRNA mediated knockdown of netrin-1 induction exacerbated DSS-induced epithelial cell apoptosis. Our results suggest that UNC5B is a critical mediator of cell survival in response to stress in colon.