Do cancer cells die because of Nogo-B?

Endoplasmic reticulum membrane remodeling by targeting reticulon-4 induces pyroptosis to facilitate antitumor immune

Pyroptosis is an identified programmed cell death that has been highly linked to endoplasmic reticulum (ER) dynamics. However, the crucial proteins for modulating dynamic ER membrane curvature change that trigger pyroptosis are currently not well understood. In this study, a biotin-labeled chemical probe of potent pyroptosis inducer α-mangostin (α-MG) was synthesized. Through protein microarray analysis, reticulon-4 (RTN4/Nogo), a crucial regulator of ER membrane curvature, was identified as a target of α-MG. We observed that chemically induced proteasome degradation of RTN4 by α-MG through recruiting E3 ligase UBR5 significantly enhances the pyroptosis phenotype in cancer cells. Interestingly, the downregulation of RTN4 expression significantly facilitated a dynamic remodeling of ER membrane curvature through a transition from tubules to sheets, consequently leading to rapid fusion of the ER with the cell plasma membrane. In particular, the ER-to-plasma membrane fusion process is supported by the observed translocation of several crucial ER markers to the "bubble" structures of pyroptotic cells. Furthermore, α-MG-induced RTN4 knockdown leads to pyruvate kinase M2 (PKM2)-dependent conventional caspase-3/gasdermin E (GSDME) cleavages for pyroptosis progression. In vivo, we observed that chemical or genetic RTN4 knockdown significantly inhibited cancer cells growth, which further exhibited an antitumor immune response with anti-programmed death-1 (anti-PD-1). In translational research, RTN4 high expression was closely correlated with the tumor metastasis and death of patients. Taken together, RTN4 plays a fundamental role in inducing pyroptosis through the modulation of ER membrane curvature remodeling, thus representing a prospective druggable target for anticancer immunotherapy.

The Reticulon-4 3-bp Deletion/Insertion Polymorphism Is Associated with Structural mRNA Changes and the Risk of Breast Cancer: A Population-Based Case-Control Study with Bioinformatics Analysis

Breast cancer (BC) is a complex disease caused by molecular events that disrupt cellular survival and death. Discovering novel biomarkers is still required to better understand and treat BC. The reticulon-4 (RTN4) gene, encoding Nogo proteins, plays a critical role in apoptosis and cancer development, with genetic variations affecting its function. We investigated the rs34917480 in RTN4 and its association with BC risk in an Iranian population sample. We also predicted the rs34917480 effect on RTN4 mRNA structure and explored the RTN4's protein-protein interaction network (PPIN) and related pathways. In this case-control study, 437 women (212 BC and 225 healthy) were recruited. The rs34917480 was genotyped using AS-PCR, mRNA secondary structure was predicted with RNAfold, and PPIN was constructed using the STRING database. Our findings revealed that this variant was associated with a decreased risk of BC in heterozygous (p = 0.012), dominant (p = 0.015), over-dominant (p = 0.017), and allelic (p = 0.035) models. Our prediction model showed that this variant could modify RTN4's mRNA thermodynamics and potentially its translation. RTN4's PPIN also revealed a strong association with apoptosis regulation and key signaling pathways highly implicated in BC. Consequently, our findings, for the first time, demonstrate that rs34917480 could be a protective factor against BC in our cohort, probably via preceding mechanisms.

Genome-wide methylation analyses identifies Non-coding RNA genes dysregulated in breast tumours that metastasise to the brain

Brain metastases comprise 40% of all metastatic tumours and breast tumours are among the tumours that most commonly metastasise to the brain, the role that epigenetic gene dysregulation plays in this process is not well understood. We carried out 450 K methylation array analysis to investigate epigenetically dysregulated genes in breast to brain metastases (BBM) compared to normal breast tissues (BN) and primary breast tumours (BP). For this, we referenced 450 K methylation data for BBM tumours prepared in our laboratory with BN and BP from The Cancer Genome Atlas. Experimental validation on our initially identified genes, in an independent cohort of BP and in BBM and their originating primary breast tumours using Combined Bisulphite and Restriction Analysis (CoBRA) and Methylation Specific PCR identified three genes (RP11-713P17.4, MIR124-2, NUS1P3) that are hypermethylated and three genes (MIR3193, CTD-2023M8.1 and MTND6P4) that are hypomethylated in breast to brain metastases. In addition, methylation differences in candidate genes between BBM tumours and originating primary tumours shows dysregulation of DNA methylation occurs either at an early stage of tumour evolution (in the primary tumour) or at a later evolutionary stage (where the epigenetic change is only observed in the brain metastasis). Epigentic changes identified could also be found when analysing tumour free circulating DNA (tfcDNA) in patient’s serum taken during BBM biopsies. Epigenetic dysregulation of RP11-713P17.4, MIR3193, MTND6P4 are early events suggesting a potential use for these genes as prognostic markers.

Nogo-B promotes tumor angiogenesis and provides a potential therapeutic target in hepatocellular carcinoma

Tumor angiogenesis is one of the hallmarks of cancer as well as an attractive target for cancer therapy. Characterization of novel pathways that act in parallel with the VEGF/VEGFR axis to promote tumor angiogenesis may provide insights into novel anti-angiogenic therapeutic targets. We found that the expression level of Nogo-B is positively correlated with tumor vessel density in hepatocellular carcinoma (HCC). While Nogo-B depletion inhibited tumor angiogenesis, Nogo-B overexpression promoted tumor angiogenesis in a tumor xenograft subcutaneous model of the human HCC cell line. Mechanically, Nogo-B regulates tumor angiogenesis based on its association with integrin αv β3 and activation of focal adhesion kinase. Moreover, Nogo-B antibody successfully abolished the function of Nogo-B in tumor angiogenesis in vitro and in vivo. Collectively, our results strongly suggest that Nogo-B is an important tumor angiogenic factor and blocking Nogo-B selectively inhibits tumor angiogenesis.

Cell fate regulation by reticulon-4 in human prostate cancers

Reticulon-4 (RTN4), a reticulon family protein localized in the endoplasmic reticulum, is reported to be involved in multiple physiological processes like neuroendocrine secretion and membrane trafficking in neuroendocrine cells. Previous studies have presented a great potential of RTN4 for the treatment of autoimmune-mediated demyelinating diseases and spinal cord injury regeneration. While interaction with Bcl-2 and Bcl-2-like family in apoptosis modulation implicated its possible role in various human cancers. However, the investigation of this gene in prostate cancer is mainly ignored. Here in our current study, we focused on its role in prostate cancer and found that RTN4 DNA copy numbers were higher in prostate cancer than normal prostate gland while its RNA and protein expressions were relatively lower. Chromosomal neighbor gene EML6 had similar expression patterns with RTN4 in prostate cancer tissues and cell lines, and further research found that they could be both targeted by miR-148a-3p. Lentivirus-mediated RTN4 overexpression potently inhibited DU145 and LNCaP cells proliferation. Cell cycle was blocked in G2/M phase and significant cell senescence was observed in RTN4 overexpressed prostate cancer cells. Finally, interaction networks in the normal prostate gland and cancer tissues further revealed that RTN4 maybe phosphorylated by MAPKAPK2 and FYN at tyrosine 591 and serine 107, respectively. All these results implied that RTN4 might somehow participate in prostate tumor progression, and this elicits possibility to develop or identify selective agents targeting RTN4 for prostate cancer therapy.

Nogo-B (Reticulon-4B) functions as a negative regulator of the apoptotic pathway through the interaction with c-FLIP in colorectal cancer cells

Nogo-B is a member of the Nogo/Reticulon-4 family and has been reported to be an inducer of apoptosis in certain types of cancer cells. However, the role of Nogo-B in human cancer remains less understood. Here, we demonstrated the functions of Nogo-B in colorectal cancer cells. In clinical colorectal cancer specimens, Nogo-B was obviously overexpressed, as determined by immunohistochemistry; and Western blot analysis showed its expression level to be significantly up-regulated. Furthermore, knockdown of Nogo-B in two colorectal cancer cell lines, SW480 and DLD-1, by transfection with si-RNA (siR) resulted in significantly reduced cell viability and a dramatic increase in apoptosis with insistent overexpression of cleaved caspase-8 and cleaved PARP. The transfection with Nogo-B plasmid cancelled that apoptosis induced by siRNogoB in SW480 cells. Besides, combinatory treatment with siR-Nogo-B/staurosporine (STS) or siR-Nogo-B/Fas ligand (FasL) synergistically reduced cell viability and increased the expression of apoptotic signaling proteins in colorectal cancer cells. These results strongly support our contention that Nogo-B most likely played an oncogenic role in colorectal cancer cells, mainly by negatively regulating the extrinsic apoptotic pathway in them. Finally, we revealed that suppression of Nogo-B caused down-regulation of c-FLIP, known as a major anti-apoptotic protein, and activation of caspase-8 in the death receptor pathway. Interaction between Nogo-B and c-FLIP was shown by immunoprecipitation and immunofluorescence studies. In conclusion, Nogo-B was shown to play an important negative role in apoptotic signaling through its interaction with c-FLIP in colorectal cancer cells, and may thus become a novel therapeutic target for colorectal cancer.

NOGO-A/RTN4A and NOGO-B/RTN4B are simultaneously expressed in epithelial, fibroblast and neuronal cells and maintain ER morphology

Reticulons (RTNs) are a large family of membrane associated proteins with various functions. NOGO-A/RTN4A has a well-known function in limiting neurite outgrowth and restricting the plasticity of the mammalian central nervous system. On the other hand, Reticulon 4 proteins were shown to be involved in forming and maintaining endoplasmic reticulum (ER) tubules. Using comparative transcriptome analysis and qPCR, we show here that NOGO-B/RTN4B and NOGO-A/RTN4A are simultaneously expressed in cultured epithelial, fibroblast and neuronal cells. Electron tomography combined with immunolabelling reveal that both isoforms localize preferably to curved membranes on ER tubules and sheet edges. Morphological analysis of cells with manipulated levels of NOGO-B/RTN4B revealed that it is required for maintenance of normal ER shape; over-expression changes the sheet/tubule balance strongly towards tubules and causes the deformation of the cell shape while depletion of the protein induces formation of large peripheral ER sheets.

Comprehensive proteome quantification reveals NgBR as a new regulator for epithelial-mesenchymal transition of breast tumor cells

Nogo-B receptor (NgBR) is a type I receptor and specifically binds to ligand Nogo-B. Our previous work has shown that NgBR is highly expressed in human breast invasive ductal carcinoma. Here, comprehensive proteome quantification was performed to examine the alteration of protein expression profile in MDA-MB-231 breast tumor cells after knocking down NgBR using lentivirus-mediated shRNA approach. Among a total of 1771 proteins feasibly quantified, 994 proteins were quantified in two biological replicates with RSD <50%. There are 122 proteins significantly down-regulated in NgBR knockdown MDA-MB-231 breast tumor cells, such as vimentin and S100A4, well-known markers for mesenchymal cells, and CD44, a stemness indicator. The decrease of vimentin, S100A4 and CD44 protein expression levels was further confirmed by Western blot analysis. MDA-MB-231 cells are typical breast invasive ductal carcinoma cells showing mesenchymal phenotype. Cell morphology analysis demonstrates NgBR knockdown in MDA-MB-231 cells results in reversibility of epithelial-mesenchymal transition (EMT), which is one of the major mechanisms involved in breast cancer metastasis. Furthermore, we demonstrated that NgBR knockdown in MCF-7 cells significantly prevented the TGF-β-induced EMT process as determined by the morphology change, and staining of E-cadherin intercellular junction as well as the decreased expression of vimentin.

BIOLOGICAL SIGNIFICANCE

Our previous publication showed that NgBR is highly expressed in human breast invasive ductal carcinoma. However, the roles of NgBR and NgBR-mediated signaling pathway in breast tumor cells are still unclear. Here, we not only demonstrated that the quantitative proteomics analysis is a powerful tool to investigate the global biological function of NgBR, but also revealed that NgBR is involved in the transition of breast epithelial cells to mesenchymal stem cells, which is one of the major mechanisms involved in breast cancer metastasis. These findings provide new insights for understanding the roles of NgBR in regulating breast epithelial cell transform during the pathogenesis of breast cancer.

The role of reticulons in neurodegenerative diseases

Reticulons (RTNs) are a group of membrane-associated proteins mainly responsible for shaping the tubular endoplasmic reticulum network, membrane trafficking, inhibition of axonal growth, and apoptosis. These proteins share a common sequence feature, the reticulon homology domain, which consists of paired hydrophobic stretches that are believed to induce membrane curvature by acting as a wedge in bilayer membranes. RTNs are ubiquitously expressed in all tissues, but each RTN member exhibits a unique expression pattern that prefers certain tissues or even cell types. Recently, accumulated evidence has suggested additional and unexpected roles for RTNs, including those on DNA binding, autophagy, and several inflammatory-related functions. These manifold actions of RTNs account for their ever-growing recognition of their involvement in neurodegenerative diseases like Alzheimer's disease, amyotrophic lateral sclerosis, multiple sclerosis, as well as hereditary spastic paraplegia. This review summarizes the latest discoveries on RTNs in human pathophysiology, and the engagement of these in neurodegeneration, along with the implications of these findings for a better understanding of the molecular events triggered by RTNs and their potential exploitation as next-generation therapeutics.

Expression of NgBR is highly associated with estrogen receptor alpha and survivin in breast cancer

NgBR is a type I receptor with a single transmembrane domain and was identified as a specific receptor for Nogo-B. Our recent findings demonstrated that NgBR binds farnesylated Ras and recruits Ras to the plasma membrane, which is a critical step required for the activation of Ras signaling in human breast cancer cells and tumorigenesis. Here, we first use immunohistochemistry and real-time PCR approaches to examine the expression patterns of Nogo-B and NgBR in both normal and breast tumor tissues. Then, we examine the relationship between NgBR expression and molecular subtypes of breast cancer, and the roles of NgBR in estrogen-dependent survivin signaling pathway. Results showed that NgBR and Nogo-B protein were detected in both normal and breast tumor tissues. However, the expression of Nogo-B and NgBR in breast tumor tissue was much stronger than in normal breast tissue. The statistical analysis demonstrated that NgBR is highly associated with ER-positive/HER2-negative breast cancer. We also found that the expression of NgBR has a strong correlation with the expression of survivin, which is a well-known apoptosis inhibitor. The correlation between NgBR and survivin gene expression was further confirmed by real-time PCR. In vitro results also demonstrated that estradiol induces the expression of survivin in ER-positive T47D breast tumor cells but not in ER-negative MDA-MB-468 breast tumor cells. NgBR knockdown with siRNA abolishes estradiol-induced survivin expression in ER-positive T47D cells but not in ER-negative MDA-MB-468 cells. In addition, estradiol increases the expression of survivin and cell growth in ER-positive MCF-7 and T47D cells whereas knockdown of NgBR with siRNA reduces estradiol-induced survivin expression and cell growth. In summary, these results indicate that NgBR is a new molecular marker for breast cancer. The data suggest that the expression of NgBR may be essential in promoting ER-positive tumor cell proliferation via survivin induction in breast cancer.

Nogo/RTN4 isoforms and RTN3 expression protect SH-SY5Y cells against multiple death insults

Among the members of the reticulon (RTN) family, Nogo-A/RTN4A, a prominent myelin-associated neurite growth inhibitory protein, and RTN3 are highly expressed in neurons. However, neuronal cell-autonomous functions of Nogo-A, as well as other members of the RTN family, are unclear. We show here that SH-SY5Y neuroblastoma cells stably over-expressing either two of the three major isoforms of Nogo/RTN4 (Nogo-A and Nogo-B) or a major isoform of RTN3 were protected against cell death induced by a battery of apoptosis-inducing agents (including serum deprivation, staurosporine, etoposide, and H2O2) compared to vector-transfected control cells. Nogo-A, -B, and RTN3 are particularly effective in terms of protection against H2O2-induced increase in intracellular reactive oxygen species levels and ensuing apoptotic and autophagic cell death. Expression of these RTNs upregulated basal levels of Bax, activated Bax, and activated caspase 3, but did not exhibit an enhanced ER stress response. The protective effect of RTNs is also not dependent on classical survival-promoting signaling pathways such as Akt and Erk kinase pathways. Neuron-enriched Nogo-A/Rtn4A and RTN3 may, therefore, exert a protective effect on neuronal cells against death stimuli, and elevation of their levels during injury may have a cell-autonomous survival-promoting function.

Nogo-A Represses Anatomical and Synaptic Plasticity in the Central Nervous System

Nogo-A was initially discovered as a myelin-associated growth inhibitory protein limiting axonal regeneration after central nervous system (CNS) injury. This review summarizes current knowledge on how myelin and neuronal Nogo-A and its receptors exert physiological functions ranging from the regulation of growth suppression to synaptic plasticity in the developing and adult intact CNS.

Absence of Nogo-B (reticulon 4B) facilitates hepatic stellate cell apoptosis and diminishes hepatic fibrosis in mice

Nogo-B (reticulon 4B) accentuates hepatic fibrosis and cirrhosis, but the mechanism remains unclear. The aim of this study was to identify the role of Nogo-B in hepatic stellate cell (HSC) apoptosis in cirrhotic livers. Cirrhosis was generated by carbon tetrachloride inhalation in wild-type (WT) and Nogo-A/B knockout (Nogo-B KO) mice. HSCs were isolated from WT and Nogo-B KO mice and cultured for activation and transformation to myofibroblasts (MF-HSCs). Human hepatic stellate cells (LX2 cells) were used to assess apoptotic responses of activated HSCs after silencing or overexpressing Nogo-B. Livers from cirrhotic Nogo-B KO mice showed significantly reduced fibrosis (P < 0.05) compared with WT mice. Apoptotic cells were more prominent in fibrotic areas of cirrhotic Nogo-B KO livers. Nogo-B KO MF-HSCs showed significantly increased levels of apoptotic markers, cleaved poly (ADP-ribose) polymerase, and caspase-3 and -8 (P < 0.05) compared with WT MF-HSCs in response to staurosporine. Treatment with tunicamycin, an endoplasmic reticulum stress inducer, increased cleaved caspase-3 and -8 levels in Nogo-B KO MF-HSCs compared with WT MF-HSCs (P < 0.01). In LX2 cells, Nogo-B knockdown enhanced apoptosis in response to staurosporine, whereas Nogo-B overexpression inhibited apoptosis. The absence of Nogo-B enhances apoptosis of HSCs in experimental cirrhosis. Selective blockade of Nogo-B in HSCs may represent a potential therapeutic strategy to mitigate liver fibrosis.

Integrase-deficient lentiviral vectors mediate efficient gene transfer to human vascular smooth muscle cells with minimal genotoxic risk

We have previously shown that injury-induced neointima formation was rescued by adenoviral-Nogo-B gene delivery. Integrase-competent lentiviral vectors (ICLV) are efficient at gene delivery to vascular cells but present a risk of insertional mutagenesis. Conversely, integrase-deficient lentiviral vectors (IDLV) offer additional benefits through reduced mutagenesis risk, but this has not been evaluated in the context of vascular gene transfer. Here, we have investigated the performance and genetic safety of both counterparts in primary human vascular smooth muscle cells (VSMC) and compared gene transfer efficiency and assessed the genotoxic potential of ICLVs and IDLVs based on their integration frequency and insertional profile in the human genome. Expression of enhanced green fluorescent protein (eGFP) mediated by IDLVs (IDLV-eGFP) demonstrated efficient transgene expression in VSMCs. IDLV gene transfer of Nogo-B mediated efficient overexpression of Nogo-B in VSMCs, leading to phenotypic effects on VSMC migration and proliferation, similar to its ICLV version and unlike its eGFP control and uninfected VSMCs. Large-scale integration site analyses in VSMCs indicated that IDLV-mediated gene transfer gave rise to a very low frequency of genomic integration compared to ICLVs, revealing a close-to-random genomic distribution in VSMCs. This study demonstrates for the first time the potential of IDLVs for safe and efficient vascular gene transfer.

Nogo-B promotes the epithelial-mesenchymal transition in HeLa cervical cancer cells via Fibulin-5

Cervical cancer is a common malignancy in women worldwide, and the occurrence of invasion and metastasis is the major cause for most cancer-related deaths. Epithelial-mesenchymal transition (EMT) has been implicated in the metastasis of primary tumors and provides molecular mechanisms for cervical cancer metastasis. We previously reported that Nogo-B mediates cell motility by binding Fibulin-5. Herein, we show that the increased expression of Nogo-B is correlated with the degree of cervical cancer metastasis. In HeLa cervical cancer cells, overexpression of Nogo-B induces the EMT and promotes cell migration and invasion, while inhibiting cell adhesion. Furthermore, we found that Nogo-B accumulates and co-localizes with Fibulin-5 in pseudopods, and the downstream effects of overexpression of Nogo-B on cell motility could be partially abolished by RNA interference against Fibulin-5. These results suggest that Nogo-B functions as an inducer of cervical cancer metastasis and that this effect is mediated, at least in part, through Fibulin-5.

The reticulons: guardians of the structure and function of the endoplasmic reticulum

The endoplasmic reticulum (ER) consists of the nuclear envelope and a peripheral network of tubules and membrane sheets. The tubules are shaped by a specific class of curvature stabilizing proteins, the reticulons and DP1; however it is still unclear how the sheets are assembled. The ER is the cellular compartment responsible for secretory and membrane protein synthesis. The reducing conditions of ER lead to the intra/inter-chain formation of new disulphide bonds into polypeptides during protein folding assessed by enzymatic or spontaneous reactions. Moreover, ER represents the main intracellular calcium storage site and it plays an important role in calcium signaling that impacts many cellular processes. Accordingly, the maintenance of ER function represents an essential condition for the cell, and ER morphology constitutes an important prerogative of it. Furthermore, it is well known that ER undergoes prominent shape transitions during events such as cell division and differentiation. Thus, maintaining the correct ER structure is an essential feature for cellular physiology. Now, it is known that proper ER-associated proteins play a fundamental role in ER tubules formation. Among these ER-shaping proteins are the reticulons (RTN), which are acquiring a relevant position. In fact, beyond the structural role of reticulons, in very recent years new and deeper functional implications of these proteins are emerging in relation to their involvement in several cellular processes.

Endoplasmic reticulum stress enters a Nogo zone

The debilitating disease pulmonary arterial hypertension (PAH) is characterized by an elevation in blood pressure in the lung arteries caused by vessel-blocking vascular-cell proliferation. This vascular remodeling is thought to result in part from defects in the endoplasmic reticulum stress response and mitochondrial dysfunction in pulmonary artery smooth muscle cells. In this issue of Science Translational Medicine, Sutendra et al. show that the vascular remodeling protein Nogo-B plays a role in the development of PAH in response to hypoxia-induced stress. The new findings finger Nogo-B as a possible therapeutic target for PAH.

Expression and functional validation of new p38α transcriptional targets in tumorigenesis

p38α MAPK (mitogen-activated protein kinase) plays an important tumour suppressor role, which is mediated by both its negative effect on cell proliferation and its pro-apoptotic activity. Surprisingly, most tumour suppressor mechanisms co-ordinated by p38α have been reported to occur at the post-translational level. This contrasts with the important role of p38α in the regulation of transcription and the profound changes in gene expression that normally occur during tumorigenesis. We have analysed whole-genome expression profiles of Ras-transformed wild-type and p38α-deficient cells and have identified 202 genes that are potentially regulated by p38α in transformed cells. Expression analysis has confirmed the regulation of these genes by p38α in tumours, and functional validation has identified several of them as probable mediators of the tumour suppressor effect of p38α on Ras-induced transformation. Interestingly, approx. 10% of the genes that are negatively regulated by p38α in transformed cells contribute to EGF (epidermal growth factor) receptor signalling. Our results suggest that inhibition of EGF receptor signalling by transcriptional targets of p38α is an important function of this signalling pathway in the context of tumour suppression.

Nogo-B regulates migration and contraction of airway smooth muscle cells by decreasing ARPC 2/3 and increasing MYL-9 expression

BACKGROUND

Abnormal proliferation, apoptosis, migration and contraction of airway smooth muscle (ASM) cells in airway remodeling in asthma are basically excessive repair responses to a network of inflammatory mediators such as PDGF, but the mechanisms of such responses remain unclear. Nogo-B, a member of the reticulum family 4(RTN4), is known to play a key role in arteriogenesis and tissue repair. Further studies are needed to elucidate the role of Nogo-B in airway smooth muscle abnormalities.

METHODS

A mouse model of chronic asthma was established by repeated OVA inhalation and subjected to Nogo-B expression analysis using immunohistochemistry and Western Blotting. Then, primary human bronchial smooth muscle cells (HBSMCs) were cultured in vitro and a siRNA interference was performed to knockdown the expression of Nogo-B in the cells. The effects of Nogo-B inhibition on PDGF-induced HBSMCs proliferation, migration and contraction were evaluated. Finally, a proteomic analysis was conducted to unveil the underlying mechanisms responsible for the function of Nogo-B.

RESULTS

Total Nogo-B expression was approximately 3.08-fold lower in chronic asthmatic mice compared to naïve mice, which was obvious in the smooth muscle layer of the airways. Interference of Nogo-B expression by siRNA resulted nearly 96% reduction in mRNA in cultured HBSMCs. In addition, knockdown of Nogo-B using specific siRNA significantly decreased PDGF-induced migration of HBSMCs by 2.3-fold, and increased the cellular contraction by 16% compared to negative controls, but had limited effects on PDGF-induced proliferation. Furthermore, using proteomic analysis, we demonstrate that the expression of actin related protein 2/3 complex subunit 5 (ARPC 2/3) decreased and, myosin regulatory light chain 9 isoform a (MYL-9) increased after Nogo-B knockdown.

CONCLUSIONS

These data define a novel role for Nogo-B in airway remodeling in chronic asthma. Endogenous Nogo-B, which may exert its effects through ARPC 2/3 and MYL-9, is necessary for the migration and contraction of airway smooth muscle cells.

The age- and amyloid-β-related increases in Nogo B contribute to microglial activation

The family of reticulons include three isoforms of the Nogo protein, Nogo A, Nogo B and Nogo C. Nogo A is expressed on neuronal tissue and its primary effect is widely acknowledged to be inhibition of neurite outgrowth. Although both Nogo B and Nogo C are also expressed in neuronal tissue, their roles in the CNS remain to be identified. In this study, we set out to assess whether expression of Nogo A or Nogo B was altered in tissue prepared from aged rats in which increased microglial activation is accompanied by decreased synaptic plasticity. The data indicate that Nogo B, but not Nogo A, was markedly increased in hippocampal tissue prepared from aged rats and that, at least in vitro, Nogo B increased several markers of microglial activation. In a striking parallel with the age-related changes, we demonstrate that intracerebroventricular delivery of amyloid-β (Aβ)(1-40)+Aβ(1-42) for 8 days was associated with a depression of long-term potentiation (LTP) and an increase in markers of microglial activation and Nogo B. In both models, evidence of cell stress was identified by increased activity of caspases 8 and 3 and importantly, incubation of cultured neurons in the presence of Nogo B increased activity of both enzymes. The data identify, for the first time, an effect of Nogo B in the brain and specifically show that its expression is increased in conditions where synaptic plasticity is compromised.

Functions of Nogo proteins and their receptors in the nervous system

The membrane protein Nogo-A was initially characterized as a CNS-specific inhibitor of axonal regeneration. Recent studies have uncovered regulatory roles of Nogo proteins and their receptors--in precursor migration, neurite growth and branching in the developing nervous system--as well as a growth-restricting function during CNS maturation. The function of Nogo in the adult CNS is now understood to be that of a negative regulator of neuronal growth, leading to stabilization of the CNS wiring at the expense of extensive plastic rearrangements and regeneration after injury. In addition, Nogo proteins interact with various intracellular components and may have roles in the regulation of endoplasmic reticulum (ER) structure, processing of amyloid precursor protein and cell survival.

Nogo-B mediates HeLa cell adhesion and motility through binding of Fibulin-5

Nogo-B is a known regulator of neural functions and plays an important role in cell adhesion and migration. To our knowledge, the molecular mechanism behind its regulation of cell motility is still unknown. Here, we identified Fibulin-5, a secreted extracellular matrix protein, as a binding partner of Nogo-B. Using HeLa cells as a model, we found that Nogo-B and Fibulin-5 co-localize in the cytoplasm and plasma membrane. Furthermore, in HeLa cells that overexpress Nogo-B, cell migration and invasion was promoted by the elevated secretion of Fibulin-5. Thus, identification of the Nogo-B binding protein Fibulin-5 may contribute to uncover the pathway in which Nogo-B regulates tumor cell movement.

Nogo-66 regulates nanog expression through stat3 pathway in murine embryonic stem cells

Homeodomain transcription factor Nanog plays a critical role in maintaining murine embryonic stem (ES) cells pluripotency. However, its expression regulation largely remains unknown. In this study we show that Nogo receptor (NgR) participates in the regulation of Nanog expression via Stat3 pathway. Activation of NgR results in the phosphorylation of Stat3 and increases expression levels of Nanog mRNA and protein, which inhibits differentiation of embryoid bodies. This up-regulation of Nanog can be abolished by NgR inhibitor PI-PLC and NEP1-40, or phospho-Stat3 inhibitor AG490 and rapamycin. Immunofluorescence assay demonstrates that NgR and its ligand Nogo-A/B exist on mouse blastocysts and cultured ES cells, suggesting NgR might play a role in early embryo development.

The natural history of the myelin-derived nerve growth inhibitor Nogo-A

Nogo-A is possibly the best characterized myelin-derived inhibitor of nerve growth in the adult central nervous system (CNS). It is a member of the ancient reticulon family of mainly endoplasmic reticulum resident proteins with representatives found throughout the eukaryotic domain. Orthologs of the nogo gene were identified in tetrapods and teleost fish but none have been detected in invertebrates. Evolution of the nogo gene has been non-homogeneous. The exon-intron arrangement is conserved from amphibians (Xenopus) to mammals, but partly deviates from that found in several teleost fish species, indicating that the recruitment of nogo exons proceeded along at least two independent lines during early vertebrate evolution. This might have far-reaching consequences. Tetrapod nogo orthologs encode two neurite growth inhibitory domains whereas in fish nogo only one of the inhibitory domains is present. These distinct paths in nogo evolution have potentially contributed to the regeneration permissive CNS in fish as opposed to the non-regenerating CNS in higher vertebrates.

Reticulon 4B (Nogo-B) is necessary for macrophage infiltration and tissue repair

Blood vessel formation during ischemia and wound healing requires coordination of the inflammatory response with genes that regulate blood vessel assembly. Here we show that the reticulon family member 4B, aka Nogo-B, is upregulated in response to ischemia and is necessary for blood flow recovery secondary to ischemia and wound healing. Mice lacking Nogo-B exhibit reduced arteriogenesis and angiogenesis that are linked to a decrease in macrophage infiltration and inflammatory gene expression in vivo. Bone marrow-derived macrophages isolated from Nogo knock-out mice have reduced spreading and chemotaxis due to impaired Rac activation. Bone marrow reconstitution experiments show that Nogo in myeloid cells is necessary to promote macrophage homing and functional recovery after limb ischemia. Thus, endogenous Nogo coordinates macrophage-mediated inflammation with arteriogenesis, wound healing, and blood flow control.

In vivo modulation of Nogo-B attenuates neointima formation

Nogo-B was recently identified as a novel vascular marker; the normally high vascular expression of Nogo-B is rapidly lost following vascular injury. Here we assess the potential therapeutic effects of Ad-Nogo-B delivery to injured vessels in vivo. Nogo-B overexpression following Ad-Ng-B infection of vascular smooth muscle cells (VSMCs) was shown to block proliferation and migration in a dose-dependent manner in vitro. We next assessed the effects of Ad-Ng-B treatment on neointima formation in two in vivo models of acute vascular injury. Adventitial delivery of Ad-Ng-B to wire-injured murine femoral arteries led to a significant decrease in the intimal area [0.014 mm(2) versus 0.030 mm(2) (P = 0.049)] and the intima:media ratio [0.78 versus 1.67 (P = 0.038)] as compared to the effects of Ad-beta-Gal control virus at 21 days after injury. Similarly, lumenal delivery of Ad-Ng-B to porcine saphenous veins prior to carotid artery grafting significantly reduced the intimal area [2.87 mm(2) versus 7.44 mm(2) (P = 0.0007)] and the intima:media ratio [0.32 versus 0.55 (P = 0.0044)] as compared to the effects following the delivery of Ad- beta-Gal, at 28 days after grafting. Intimal VSMC proliferation was significantly reduced in both the murine and porcine disease models. Gene delivery of Nogo-B exerts a positive effect on vascular injury-induced remodeling and reduces neointimal development in two arterial and venous models of vascular injury.

Nogo-a expression in glial CNS tumors: a tool to differentiate between oligodendrogliomas and other gliomas?

Gliomas are the most frequent primary brain tumors. In a minority of cases, the differentiation between astrocytomas and oligodendrogliomas based on morphologic characteristics alone can be difficult; though it is important, as patients with oligodendrogliomas follow a more favorable clinical course. Here we report on the immunohistochemical expression pattern of the oligodendrocytic marker Nogo-A in 113 central nervous system tumors including 28 oligodendrogliomas [15, World Health Organization (WHO) grade II; 13, grade WHO III], 50 astrocytomas [10, grade WHO II; 11, grade WHO III; 29 glioblastoma multiforme (GBM)], 11 ependymomas WHO grade II, 7 central neurocytomas, 2 dysembryoplastic neuroepithelial tumors (DNTs), 5 clear cell meningiomas, and 10 metastases to the brain. The oligodendrocytic marker Nogo-A was found to be strongly expressed in 71% of oligodendrogliomas, but in 0% of ependymomas WHO grade II, astrocytomas WHO grade II or III, DNTs, central neurocytomas, or clear cell meningiomas. In GBM, a subgroup of tumors (24%) showed strong expression of Nogo-A coincidently with Ki67 positivity but glial fibrillary acidic protein-negativity. However, neither in oligodendrogliomas nor GBM was a correlation between the loss of 1p19q and the extent of Nogo-A expression observed. Our findings indicate that Nogo-A is strongly expressed in the majority of oligodendrogliomas and might be a helpful marker to distinguish oligodendrogliomas from astrocytomas WHO grades II and III as well as ependymomas. They also support the hypothesis that GBM may be a heterogeneous group of tumors derived from different progenitor cells.

Cell autonomous function of Nogo and reticulons: The emerging story at the endoplasmic reticulum

The myelin-associated membrane protein reticulon-4 (RTN4)/Nogo has been extensively studied with regards to its neurite outgrowth inhibitory function, both in limiting plasticity in the healthy adult brain and regeneration during central nervous system injury. These activities are presumably associated with Nogo splice isoforms expressed on the cell surface and function largely in trans, exerting an influence as an intercellular membrane-bound ligand. Nogo, and other reticulon paralogues and orthologues, are however mainly localized to the endoplasmic reticulum (ER), and are likely to have cell autonomous functions that are not yet clear. Emerging evidence suggests that Nogo may have a role in modulating the morphology and functions of the ER. This role is apparently not essential for cell viability under normal growth conditions, but may be manifested under certain stress conditions.

Phosphorylation-regulated cleavage of the reticulon protein Nogo-B by caspase-7 at a noncanonical recognition site

Reticulons (RTNs) are a large family of transmembrane proteins present throughout the eukaryotic domain in virtually every cell type. Despite their wide distribution, their function is still mostly unknown. RTN4, also termed Nogo, comes in three isoforms, Nogo-A, -B, and -C. While Nogo-A has been described as potent inhibitor of nerve growth, Nogo-B has been implicated in vascular remodeling and regulation of apoptosis. We show here that Nogo-B gets cleaved by caspase-7, but not caspase-3, during apoptosis at a caspase nonconsensus site. By a combination of MS and site-directed mutagenesis we demonstrate that proteolytic processing of Nogo-B is regulated by phosphorylation of Ser(16) within the cleavage site. We present cyclin-dependent kinase (Cdk)1 and Cdk2 as kinases that phosphorylate Nogo-B at Ser(16) in vitro. In vivo, cleavage of Nogo-B is markedly increased in Schwann cells in a lesion model of the rat sciatic nerve. Taken together, we identified an RTN protein as one out of a selected number of caspase targets during apoptosis and as a novel substrate for Cdk1 and 2. Furthermore, our data support a functionality of caspase-7 that is distinct from closely related caspase-3.

Adaptor FADD is recruited by RTN3/HAP in ER-bound signaling complexes

It has been well established that FADD plays a critical role in the membrane bound death-inducing signaling complexes. Herein, we report that endogenous FADD could interact with ectopic or endogenous RTN3/HAP. ER-bound RTN3 protein recruited endogenous FADD to the ER membrane and subsequently initiated caspase-8 cascade, including activation of caspase-8, processing of Bid and release of cytochrome c from mitochondria. Furthermore, we demonstrated that endogenous FADD was recruited by ER-bound endogenous RTN3 to the ER membrane under the tunicamycin stimulation. The dominant negative form of FADD containing DD could abolish these RTN3 generated events in the caspase-8 cascade. Moreover, we found that RTN3 induced caspase-9 processing was only partially resulted from caspase-8 activation (data unshown), indicating that multiple caspase cascades participated in the apoptosis from RTN3 over-expression. Furthermore, NogoB/ASY, a homologue of RTN3 and a potential RTN3 interacting protein, also associated with FADD and induced cytochrome c release in a FADD dependent manner.

Negative correlation of Nogo-A with the malignancy of oligodendroglial tumor

OBJECTIVE

Nogo-A is an axon regeneration inhibitor, and its function in central nervous system (CNS) is still unknown. The present study is to explore the relationship between the expression of Nogo-A and the malignancy of oligodendroglial tumors in patients.

METHODS

Tumor tissue samples with different malignancy grade were obtained from the hospitals. The samples used for detection had been diagnosed as oligodendroglial tumors (oligodendroglioma or anaplastic oligodendroglioma). The expression of Nogo-A was detected by immunohistochemistry and western-blot analysis. The correlation test between the Nogo-A expression and the morphological changes (the percentages of atypical cells and mitotic cells in the tumors) related to the malignancy of tumor tissues was performed.

RESULTS

There was significant negative correlation between the Nogo-A expression and the morphological change of tumor tissues according to immunohistochemistry. Western-blot analysis also indicated that the gray value of Nogo-A protein band in the oligodendroglioma group was significantly higher than that in the anaplastic oligodendroglioma group.

CONCLUSION

Nogo-A expression was negatively correlated with the malignancy grade of oligodendroglial tumors.

Reticulon 3 mediates Bcl-2 accumulation in mitochondria in response to endoplasmic reticulum stress

Reticulon3 (RTN3), firstly isolated from the retina and widely expressed in human tissues with the highest expression in the brain, is presumed to play an important role in the developing axons through the transport of liquids and proteins. We have identified and characterized RTN3 as a RTN4B/ASY interaction protein. Here we demonstrated that ER-stress activated RTN3 expression. CHOP and ATF6 were sufficient to up-regulate the expression of RTN3. The down-regulation of RTN3 would induce apoptosis and attenuate the anti-apoptotic activity of Bcl-2, indicating RTN3 was required for the cellular survival and optimal anti-apoptotic activity of Bcl-2. Our present studies also indicated ER-stress induced RTN3 up-regulation could trigger Bcl-2 translocation from ER to mitochondria. Moreover, the previous studies showed that RTN4B was also a Bcl-2-interacted protein. We found that RTN3 and RTN4B could block the access of Bcl-2 to each other and thereafter determined the Bcl-2 subcellular distribution. Taken together, our findings indicate that RTN3 is directly involved in the ER-constituents trafficking events through dually acting as an essential and important ER-stress sensor, and a trigger for the Bcl-2 translocation.

Low levels of Nogo-B in human carotid atherosclerotic plaques are associated with an atheromatous phenotype, restenosis, and stenosis severity

OBJECTIVE

Reticulon-4/Nogo (Nogo-B) protects mouse arteries from lumen loss by reducing smooth muscle cell (SMC) migration and intimal thickening. Our goal was to determine plaque and circulating levels of Nogo-B in atherosclerotic and control subjects. Therefore, we studied the relationships between local Nogo-B, plaque characteristics, and clinical data in patients undergoing carotid endarterectomy.

METHODS AND RESULTS

Western blot analysis showed that endarterectomy specimens from the femoral (n=19) and carotid arteries (n=145) contained significantly less Nogo-B than nonatherosclerotic mammary arteries (n=8; P<0.003) and aortas (n=15; P=0.03). Immunohistochemistry revealed that in atherosclerotic lesions, Nogo-B was expressed by macrophage/foam cells, SMC rich, and neo-vascularized areas. Atheromatous plaques (>40% fat content) showed a significant reduction in Nogo-B expression (P=0.002). Nogo-B expression levels were significantly lower in patients with more than 90% of carotid stenosis (P=0.04) or restenotic lesions after prior carotid intervention (duplex; P=0.01). In contrast, plasmatic levels of Nogo-B (soluble Nogo-B) did not differ between atherosclerotic subjects (n=68) and risk-factor matched controls (n=63; P=0.5).

CONCLUSION

Our findings suggest that local reduction of Nogo-B in atherosclerotic tissue might contribute to plaque formation and/or instability triggering luminal narrowing. In contrast, plasma Nogo-B levels are not associated with clinically manifested atherosclerotic disease.

Nogo-A is a Reliable Oligodendroglial Marker in Adult Human and Mouse CNS and in Demyelinated Lesions

The unambiguous identification of oligodendrocytes in tissue sections, especially in myelinated tracts, is often difficult. Most of the antibodies used to identify oligodendrocytes label the myelin sheath as well. Originally described as an inhibitor of axonal outgrowth, Nogo-A is known to be strongly expressed in mature oligodendrocytes in vivo. In the present investigation we analyzed the expression patterns of Nogo-A in adult mouse and human CNS as well as in demyelinating animal models and multiple sclerosis lesions. Nogo-A expression was compared with that of other frequently used oligodendroglial markers such as CC1, CNP, and in situ hybridization for proteolipid protein mRNA. Nogo-A strongly and reliably labeled oligodendrocytes in the adult CNS as well as in demyelinating lesions and thus represents a valuable tool for the identification of oligodendrocytes in human and mouse CNS tissue.

The reticulons: a family of proteins with diverse functions

The reticulon family is a large and diverse group of membrane-associated proteins found throughout the eukaryotic kingdom. All of its members contain a carboxy-terminal reticulon homology domain that consists of two hydrophobic regions flanking a hydrophilic loop of 60-70 amino acids, but reticulon amino-terminal domains display little or no similarity to each other. Reticulons principally localize to the endoplasmic reticulum, and there is evidence that they influence endoplasmic reticulum-Golgi trafficking, vesicle formation and membrane morphogenesis. However, mammalian reticulons have also been found on the cell surface and mammalian reticulon 4 expressed on the surface of oligodendrocytes is an inhibitor of axon growth both in culture and in vivo. There is also growing evidence that reticulons may be important in neurodegenerative diseases such as Alzheimer's disease and amyotrophic lateral sclerosis. The diversity of structure, topology, localization and expression patterns of reticulons is reflected in their multiple, diverse functions in the cell.

Reticulons as markers of neurological diseases: focus on amyotrophic lateral sclerosis

Reticulons (RTNs) are a family of proteins that are primarily associated with the endoplasmic reticulum. In mammals, four genes have been identified and referred as to rtn1, 2, 3 and the neurite outgrowth inhibitor rtn4/nogo. These genes generate multiple isoforms that contain a common C-terminal reticulon homology domain of 150-200 amino-acid residues. The N-terminal regions of RTNs are highly variable, and result from alternative splicing or differential promoter usage. Although widely distributed, the functions of RTNs are still poorly understood. Much interest has been focused on rtn4/nogo because of its activity as a potent inhibitor of axonal growth and repair. In the present study, we update recent knowledge on mammalian RTNs paying special attention to the involvement of these proteins as markers of neurological diseases. We also present recent data concerning RTN expression in amyotrophic lateral sclerosis, a fatal degenerative disorder characterized by loss of upper and lower motor neurons, and muscle atrophy. The rearrangement of RTN expression is regulated not only in suffering skeletal muscle but also preceding the onset of symptoms, and may relate to the disease process.

Human Nogo-C overexpression induces HEK293 cell apoptosis via a mechanism that involves JNK-c-Jun pathway

The neurite outgrowth inhibitor protein Nogo-A has been identified as an inhibitor of axonal regeneration, and Nogo-B as a regulator of vasculature remodeling, but the additional roles of Nogo isoforms, especially Nogo-C, have obtained little attention. Nogo-C is weakly expressed in liver and kidney compared to the high expression in skeletal muscle. Here we detected the weak expression of Nogo-C in human embryonic kidney cell line HEK293, and found that Nogo-C expressed in HEK293 could induce cell apoptosis. Further experiments demonstrated the activation of JNK/SAPK and c-Jun, but not p38 in Nogo-C expressed cells. And JNK-specific inhibitor SP600125 could reduce cell apoptosis induced by Nogo-C. Furthermore, the activation of caspase-3 and PARP, the expression and phosphorylation of p53 were also detected. The data first revealed Nogo-C expressed in HEK293 confers apoptosis by inducing caspase-3 and p53 activation through the JNK-c-Jun-dependent pathway.

Nogo in multiple sclerosis: Growing roles of a growth inhibitor

In recent years, knowledge about the physiological functions of the Nogo-A protein has grown considerably, and this molecule has evolved from being one of the most important axonal regrowth inhibitors present in central nervous system (CNS) myelin, to several other potentially important roles in different areas such as nervous system development, epilepsy, vascular physiology, muscle pathology and CNS tumors. Therapeutically, targeting the Nogo-A protein by means of the immune response has been tried in an attempt to block neurite growth inhibition and promote regeneration in spinal cord injury models; the immune response to Nogo-A, however, has not been extensively studied. We propose to review recent evidence that Nogo-A may also play an important role in autoimmune demyelinating diseases such as experimental autoimmune encephalomyelitis and multiple sclerosis, including that Nogo-66 derived epitopes are encephalitogenic antigens in susceptible mouse strains, and that the immune response to Nogo-66 antigens includes both strong T cell and B cell activation, with epitope spreading of the antibody response to other myelin molecules. In CNS immunotherapy, careful targeting of neural self-antigens is a prerequisite in order to avoid unexpected deleterious effects, and increasing knowledge about the immune response to Nogo-A may provide a safe basis for the development of relevant therapeutic alternatives for several neurological conditions.

ER stress triggers apoptosis induced by Nogo-B/ASY overexpression

Nogo-B/ASY has been characterized as a novel human apoptosis-inducing protein without any known apoptosis-related motifs. However, the validity of Nogo-B/ASY as a physiological apoptotic protein was recently questioned. In present research, we demonstrate that ASY overexpression contributes to ER stress and induces apoptosis through ER Ca2+ depletion and ER-specific pathways. ER stress and the disorder of intracellular calcium trigger the apoptosis induced by ASY overexpression. At the same time, stable transfectants overexpressing high levels of ASY are resistant to ER-stress-associated stimuli, which implies that ASY overexpression activates protective response in response to ER stress. Our results provide a direct apoptotic pathway that ASY overexpression induces apoptosis through ER stress and ER-specific signal pathways.

Nogo-B is a new physiological substrate for MAPKAP-K2

The neurite outgrowth inhibitor protein Nogo is one of 300 proteins that contain a reticulon homology domain, which is responsible for their association with the endoplasmic reticulum. Here we have found that the Nogo-B spliceform becomes phosphorylated at Ser107 in response to lipopolysaccharide in RAW264 macrophages or anisomycin in HeLa cells. The phosphorylation is prevented by SB 203580, an inhibitor of SAPK2a (stress-activated protein kinase 2a)/p38a and SAPK2b/p38b, and does not occur in embryonic fibroblasts generated from SAPK2a/p38a-deficient mice. Nogo-B is phosphorylated at Ser107 in vitro by MAPKAP-K2 [MAPK (mitogen-activated protein kinase)-activated protein kinase-2] or MAPKAP-K3, but not by other protein kinases that are known to be activated by SAPK2a/p38a. The anisomycin-induced phosphorylation of Ser107 in HeLa cells can be prevented by 'knockdown' of MAPKAP-K2 using siRNA (small interfering RNA). Taken together, our results identify Nogo-B as a new physiological substrate of MAPKAP-K2.

Identification of a new RTN3 transcript, RTN3-A1, and its distribution in adult mouse brain

The Reticulon (RTN) family of proteins is thought to play important roles in the regulation of neuronal regeneration. In this study, we have identified a novel alternative splicing isoform of the RTN gene family, RTN3-A1, which contains an additional 2.3-kb exon. The transcripts of human and mouse RTN3-A1 (about 5.0 kb) were first discovered by database sequence mining and analysis, and verified by cloning and sequencing. Northern blot analysis of 16 human tissues with a common probe of RTN3 transcripts and a specific probe for RTN3-A1 demonstrated that human RTN3-A1 is expressed mainly in brain tissues with a weak expression in the skeletal muscle. With Western blot analysis, the expected 100-kDa RTN3-A1 protein was detected in mouse brain. In situ hybridization with a mouse RTN3-A1-specific cRNA probe revealed that the mouse RTN3-A1 mRNA was regionally expressed in the neurons of the cerebral cortex, hippocampus, hypothalamus, and cerebellum of the adult mouse brain. In contrast to the transcripts of RTN1 and RTN2, RTN3-A1 shares some significant similarity with RTN4-A in exon structure, tissue distribution, and brain expression profile. Since other reports have shown that RTN4-A inhibits neuronal outgrowth and restricts the plasticity of the central nervous system, we speculate that RTN3-A1 might play certain roles in the central nervous system.

No go for brain tumors?

The Nogo gene and its products are well known as adult central nervous system (CNS) myelin inhibitors of neuronal regeneration. We review here experimental findings that might link Nogo to CNS malignancy. These links are founded on two very different modes of cellular action by Nogo isoforms. Acting intracellularly and in conjunction with other molecules, cytoplasmic domains of Nogo might predispose cancer cells to apoptotic susceptibility. On the other hand, extracellular domains of Nogo might inhibit the migration and invasion of CNS tumors. Depending on the physiological context, Nogo isoforms might therefore be antitumorigenic or have tumor-suppressing activities.

ER Ca2+ depletion triggers apoptotic signals for endoplasmic reticulum (ER) overload response induced by overexpressed reticulon 3 (RTN3/HAP)

Perturbance of endoplasmic reticulum (ER) function, either by the mutant proteins not folding correctly, or by an excessive accumulation of proteins in the organelle, will lead to the unfolded protein response (UPR) or ER overload response (EOR). The signal-transducing pathways for UPR have been identified, whereas the pathway for EOR remains to be elucidated. Our previous study demonstrated that the overexpression of reticulon 3 (RTN3, also named HAP, homologue of ASY protein) caused apoptosis with the depletion of ER Ca(2+) stores. In present research, we characterized RTN3 as a novel EOR-induced protein, triggering the apoptotic signals through the release of ER Ca(2+) and the elevation of cytosolic Ca(2+). Our studies showed that overexpressed RTN3 induced EOR, eliciting ER-specific apoptosis with activation of caspase-12 and mitochondrial dysfunction through ER Ca(2+) depletion and the sustained elevation of cytosolic Ca(2+). Furthermore, we demonstrated that overexpressed RTN3 and stimuli that activate both EOR and UPR, not UPR only, were able to induce up-regulation of inducible nitric oxide synthase (iNOS) in HeLa cells through ER Ca(2+) release and reactive oxygen intermediates (ROIs), resulting in endogenous calcium-dependent nitric oxide protecting cells against ER specific apoptosis, which suggested that the nitric oxide and iNOS represented a likely protective response to EOR, not the UPR. These results supported that the release of ER Ca(2+) stores triggered the initial signal-transducing pathways for EOR induced by overexpressed RTN3.

Tissue specificity and regulation of the N-terminal diversity of reticulon 3

Over the last few years, the widely distributed family of reticulons (RTNs) is receiving renewed interest because of the implication of RTN4/Nogo in neurite regeneration. Four genes were identified in mammals and are referred to as RTN1, 2, 3 and the neurite outgrowth inhibitor RTN4/Nogo. In the present paper, we describe the existence of five new isoforms of RTN3 that differ in their N-termini, and analysed their tissue distribution and expression in neurons. We redefined the structure of human and murine rtn3 genes, and identified two supplementary exons that may generate up to seven putative isoforms arising by alternative splicing or differential promoter usage. We confirmed the presence of five of these isoforms at the mRNA and protein levels, and showed their preferential expression in the central nervous system. We analysed rtn3 expression in the cerebellum further, and observed increased levels of several of the RTN3 isoforms during cerebellum development and during in vitro maturation of cerebellar granule cells. This pattern of expression paralleled that shown by RTN4/Nogo isoforms. Specifically, RTN3A1 expression was down-regulated upon cell death of cerebellar granule neurons triggered by potassium deprivation. Altogether, our results demonstrate that the rtn3 gene generates multiple isoforms varying in their N-termini, and that their expression is tightly regulated in neurons. These findings suggest that RTN3 isoforms may contribute, by as yet unknown mechanisms, to neuronal survival and plasticity.

Nogo signaling and non-physical injury-induced nervous system pathology

The Nogo gene products were described first as myelin-associated inhibitors that prevent neuronal regeneration upon injury. Recent findings have also implicated Nogo in several neuronal pathologies that are not induced by physical injury. Nogo-A may be an important determinant of autoimmune demyelinating diseases, as active immunization with Nogo-A fragments attenuates the symptoms of experimental autoimmune encephalomyelitis (EAE). Nogo-A levels are elevated markedly in hippocampal neurons of patients with temporal lobe epilepsy (TLE), in brain and muscle of patients with amyotrophic lateral sclerosis (ALS), and in schizophrenic patients. Concrete evidence for a direct role of Nogo-A in the latter neuropathies is not yet available, but such a role is logically in line with new findings associated with localization of Nogo-A and Nogo-Nogo-66 receptor (NgR)-mediated signaling. We speculate on possible linkages between the effect of aberrant elevation of Nogo levels and the signaling consequences that could lead to nervous system pathology.

Analysis of the Reticulon Gene Family Demonstrates the Absence of the Neurite Growth Inhibitor Nogo-A in Fish

Reticulons (RTNs) are a family of evolutionary conserved proteins with four RTN paralogs (RTN1, RTN2, RTN3, and RTN4) present in land vertebrates. While the exact functions of RTN1 to RTN3 are unknown, mammalian RTN4-A/Nogo-A was shown to inhibit the regeneration of severed axons in the mammalian central nervous system (CNS). This inhibitory function is exerted via two distinct regions, one within the Nogo-A-specific N-terminus and the other in the conserved reticulon homology domain (RHD). In contrast to mammals, fish are capable of CNS axon regeneration. We performed detailed analyses of the fish rtn gene family to determine whether this regeneration ability correlates with the absence of the neurite growth inhibitory protein Nogo-A. A total of 7 rtn genes were identified in zebrafish, 6 in pufferfish, and 30 in eight additional fish species. Phylogenetic and syntenic relationships indicate that the identified fish rtn genes are orthologs of mammalian RTN1, RTN2, RTN3, and RTN4 and that several paralogous fish genes (e.g., rtn4 and rtn6) resulted from genome duplication events early in actinopterygian evolution. Accordingly, sequences homologous to the conserved RTN4/Nogo RHD are present in two fish genes, rtn4 and rtn6. However, sequences comparable to the first approximately 1,000 amino acids of mammalian Nogo-A including a major neurite growth inhibitory region are absent in zebrafish. This result is in accordance with functional data showing that axon growth inhibitory molecules are less prominent in fish oligodendrocytes and CNS myelin compared to mammals.

Reticulon 4 in chondrocytic cells: barosensitivity and intracellular localization

Members of the reticulon gene family are endoplasmic reticulum (ER)-related proteins expressed in various human tissues, but their molecular functions are not understood. The reticulon 4 subfamily consists of three members, reticulon 4/Nogo-A, -B and -C. Reticulon 4-A is under intense investigation because of its inhibitory effect on neurite outgrowth, and reticulon 4-B has been suggested to induce apoptosis. Reticulon 4-C, the shortest member of this subfamily, is the least characterized. Reticulons are presumably guided to endoplasmic reticulum by a putative N-terminal retention motif. In this study the expressions of reticulon 4 subtypes in human chondrosarcoma cell line and in primary bovine chondrocytes were analyzed on mRNA level. These cell types, exposed to strong mechanical forces in vivo, were subjected to high hydrostatic pressure and mechanical stretch to study the possible mechanosensitivity of reticulon 4 genes. In addition, a green fluorescent protein-tagged reticulon 4-C and a fusion protein with mutated endoplasmic reticulum retention signal were used to study the significance of the C-terminal translocation signal (the di-lysine motif). As the result, both cell types expressed the three main isoforms of reticulon 4 family. The steady-state level of reticulon 4-B mRNA was shown to be up-regulated by pressure, but not by mechanical stretch indicating transcriptional barosensitivity. The reticular distribution pattern of reticulon 4-C was observed indicating a close association with endoplasmic reticulum. Interestingly, this pattern was maintained despite of the disruption of the putative localization signal. This suggests the presence of another, yet unidentified endoplasmic reticulum retention mechanism.

Nogo A expression in the adult enteric nervous system

Neuronal plasticity plays an important role in physiological and pathological processes within the gastrointestinal (GI) tract. Nogo A is a major contributor to the negative effect central nervous system (CNS) myelin has on neurite outgrowth after injury and may also play a role in maintaining synaptic connections in the healthy CNS. Nogo A is highly expressed during neuronal development but in the CNS declines postnatally concomitantly with a loss of regenerative potential while ganglia of the Peripheral Nervous System (PNS) retain Nogo A. The enteric nervous system shares a number of features in common with the CNS, thus the peripheral distribution of factors affecting plasticity is of interest. We have investigated the distribution of Nogo in the adult mammalian gastrointestinal tract. Nogo A mRNA and protein are detectable in the adult rat GI tract. Nogo A is expressed heterogeneously in enteric neurons throughout the GI tract though expression levels appear not to be correlated with neuronal sub-type. The pattern of expression is maintained in cultured myenteric plexus from the guinea-pig small intestine. As is seen in developing neurons of the CNS, enteric Nogo A is present in both neuronal cell bodies and axons. Our results point to a hitherto unsuspected role for Nogo A in enteric neuronal physiology.

Regulation of Nogo and Nogo receptor during the development of the entorhino-hippocampal pathway and after adult hippocampal lesions

Axonal regeneration in the adult CNS is limited by the presence of several inhibitory proteins associated with myelin. Nogo-A, a myelin-associated inhibitor, is responsible for axonal outgrowth inhibition in vivo and in vitro. Here we study the onset and maturation of Nogo-A and Nogo receptor in the entorhino-hippocampal formation of developing and adult mice. We also provide evidence that Nogo-A does not inhibit embryonic hippocampal neurons, in contrast to other cell types such as cerebellar granule cells. Our results also show that Nogo and Nogo receptor mRNA are expressed in the adult by both principal and local-circuit hippocampal neurons, and that after lesion, Nogo-A is also transiently expressed by a subset of reactive astrocytes. Furthermore, we analyzed their regulation after kainic acid (KA) treatment and in response to the transection of the entorhino-hippocampal connection. We found that Nogo-A and Nogo receptor are differentially regulated after kainic acid or perforant pathway lesions. Lastly, we show that the regenerative potential of lesioned entorhino-hippocampal organotypic slice co-cultures is increased after blockage of Nogo-A with two IN-1 blocking antibodies. In conclusion, our results show that Nogo and its receptor might play key roles during development of hippocampal connections and that they are implicated in neuronal plasticity in the adult.