Dependence receptors: between life and death

Netrin-1: Key insights in neural development and disorders

Netrin-1, an essential extracellular protein, has gained significant attention due to its pivotal role in guiding axon and cell migration during embryonic development. The fundamental significance of netrin-1 in developmental biology is reflected in its high conservation across different species as a part of the netrin family. The bifunctional nature of netrin-1 demonstrates its functional versatility, as it can function as either a repellent or an attractant according to the context and the expressed receptors on the target cells including the deleted in colorectal cancer (DCC), the uncoordinated-5 (UNC5), DSCAM, Neogenin-1, Adenosine A2b and Draxin receptors. By directing axonal growth cones toward the appropriate targets, netrin-1 is a critical actor in the formation of the intricate architecture of the nervous system. Netrin-1 is believed to be involved in additional biological and pathological processes in addition to its traditional function in neural development. The behavior of a diverse array of cell types is influenced by controlling cell adhesion and movement, which is impacted by netrin-1. It is a molecule of interest in both developmental biology and clinical research because of its involvement in angiogenesis, tumorigenesis, inflammation, and tissue regeneration, as confirmed by recent studies. The therapeutic capability of netrin-1 in disorders such as cancer, neurodegenerative disorders, and cardiovascular diseases warrants significant attention.

DCC/netrin-1 regulates cell death in oligodendrocytes after brain injury

Hallmark pathological features of brain trauma are axonal degeneration and demyelination because myelin-producing oligodendrocytes (OLs) are particularly vulnerable to injury-induced death signals. To reveal mechanisms responsible for this OL loss, we examined a novel class of "death receptors" called dependence receptors (DepRs). DepRs initiate pro-death signals in the absence of their respective ligand(s), yet little is known about their role after injury. Here, we investigated whether the deleted in colorectal cancer (DCC) DepR contributes to OL loss after brain injury. We found that administration of its netrin-1 ligand is sufficient to block OL cell death. We also show that upon acute injury, DCC is upregulated while netrin-1 is downregulated in perilesional tissues. Moreover, after genetically silencing pro-death activity using DCCD1290N mutant mice, we observed greater OL survival, greater myelin integrity, and improved motor function. Our findings uncover a novel role for the netrin-1/DCC pathway in regulating OL loss in the traumatically injured brain.

Multiple Functions of Draxin/Netrin-1 Signaling in the Development of Neural Circuits in the Spinal Cord and the Brain

Axon guidance proteins play key roles in the formation of neural circuits during development. We previously identified an axon guidance cue, named draxin, that has no homology with other axon guidance proteins. Draxin is essential for the development of various neural circuits including the spinal cord commissure, corpus callosum, and thalamocortical projections. Draxin has been shown to not only control axon guidance through netrin-1 receptors, deleted in colorectal cancer (Dcc), and neogenin (Neo1) but also modulate netrin-1-mediated axon guidance and fasciculation. In this review, we summarize the multifaceted functions of draxin and netrin-1 signaling in neural circuit formation in the central nervous system. Furthermore, because recent studies suggest that the distributions and functions of axon guidance cues are highly regulated by glycoproteins such as Dystroglycan and Heparan sulfate proteoglycans, we discuss a possible function of glycoproteins in draxin/netrin-1-mediated axon guidance.

Protein Kinase-Mediated Decision Between the Life and Death

Protein kinases are intracellular signaling enzymes that catalyze the phosphorylation of specific residues in their target substrate proteins. They play important role for regulation of life and death decisions. The complexity of the relationship between death receptors and protein kinases' cell death decision-making mechanisms create many difficulties in the treatment of various diseases. The most of fifteen different cell death pathways, which are reported by Nomenclature Committee on Cell Death (NCCD) are protein kinase signal transduction-mediated negative or positive selections. Tumor necrosis factor (TNF) as a main player of death pathways is a dual-functioning molecule in that it can promote both cell survival or cell death. All apoptotic and necrotic signal transductions are conveyed through death domain-containing death receptors, which are expressed on the surface of nearly all human cells. In humans, eight members of the death receptor family have been identified. While the interaction of TNF with TNF Receptor 1 (TNFR1) activates various signal transduction pathways, different death receptors activate three main signal transduction pathways: nuclear factor kappa B (NF-ĸB)-mediated differentiation or pro-inflammatory cytokine synthesis, mitogen-activated protein kinase (MAPK)-mediated stress response and caspase-mediated apoptosis. The link between the NF-ĸB and the c-Jun NH₂-terminal kinase (JNK) pathways comprise another check-point to regulate cell death. TNF-α also promotes the "receptor-interacting serine/threonine protein kinase 1" (RIPK1)/RIPK3/ mixed lineage kinase domain-like pseudokinase (MLKL)-dependent necrosis. Thus, necrosome is mainly comprised of MLKL, RIPK3 and, in some cases, RIPK1. In fact, RIPK1 is at the crossroad between life and death, downstream of various receptors as a regulator of endoplasmic reticulum stress-induced death. TNFR1 signaling complex (TNF-RSC), which contains multiple kinase activities, promotes phosphorylation of transforming growth factor β-activated kinase 1 (TAK1), inhibitor of nuclear transcription factor κB (IκB) kinase (IKK) α/IKKβ, IκBα, and NF-κB. IKKs affect cell-survival pathways in NF-κB-independent manner. Toll-like receptor (TLR) stimulation triggers various signaling pathways dependent on myeloid differentiation factor-88 (MyD88), Interleukin-1 receptor (IL-1R)-associated kinase (IRAK1), IRAK2 and IRAK4, lead to post-translational activation of nucleotide and oligomerization domain (NLRP3). Thereby, cell fate decisions following TLR signaling is parallel with death receptor signaling. Inhibition of IKKα/IKKβ or its upstream activators sensitize cells to death by inducing RIPK1-dependent apoptosis or necroptosis. During apoptosis, several kinases of the NF-κB pathway, including IKK1 and NF-κB essential modulator (NEMO), are cleaved by cellular caspases. This event can terminate the NF-κB-derived survival signals. In both canonical and non-canonical pathways, IKK is key to NF-κB activation. Whereas, the activation process of IKK, the functions of NEMO ubiquitination, IKK-related non-canonical pathway and the nuclear transportation of NEMO and functions of IKKα are still debated in cell death. In addition, cluster of differentiation 95 (CD95)-mediated non-apoptotic signaling and CD95- death-inducing signaling complex (DISC) interactions are waiting for clarification.

Blocking SHH/Patched Interaction Triggers Tumor Growth Inhibition through Patched-Induced Apoptosis

The Sonic Hedgehog (SHH) pathway plays a key role in cancer. Alterations of SHH canonical signaling, causally linked to tumor progression, have become rational targets for cancer therapy. However, Smoothened (SMO) inhibitors have failed to show clinical benefit in patients with cancers displaying SHH autocrine/paracrine expression. We reported earlier that the SHH receptor Patched (PTCH) is a dependence receptor that triggers apoptosis in the absence of SHH through a pathway that differs from the canonical one, thus generating a state of dependence on SHH for survival. Here, we propose a dual function for SHH: its binding to PTCH not only activates the SHH canonical pathway but also blocks PTCH-induced apoptosis. Eighty percent, 64%, and 8% of human colon, pancreatic, and lung cancer cells, respectively, overexpressed SHH at transcriptional and protein levels. In addition, SHH-overexpressing cells expressed all the effectors of the PTCH-induced apoptotic pathway. Although the canonical pathway remained unchanged, autocrine SHH interference in colon, pancreatic, and lung cell lines triggered cell death through PTCH proapoptotic signaling. In vivo, SHH interference in colon cancer cell lines decreased primary tumor growth and metastasis. Therefore, the antitumor effect associated to SHH deprivation, usually thought to be a consequence of the inactivation of the canonical SHH pathway, is, at least in part, because of the engagement of PTCH proapoptotic activity. Together, these data strongly suggest that therapeutic strategies based on the disruption of SHH/PTCH interaction in SHH-overexpressing cancers should be explored. SIGNIFICANCE: Sonic Hedgehog-overexpressing tumors express PTCH-induced cell death effectors, suggesting that this death signaling could be activated as an antitumor strategy.

Fast-diffusing p75NTR monomers support apoptosis and growth cone collapse by neurotrophin ligands

Neurotrophins (NTs) are homodimeric growth factors displaying fundamental roles in the nervous system. Their activity stems from binding and activation of 3 different receptor types in nervous cell membranes. The p75 NT receptor (p75^NTR) was the first to be discovered in 1986; nevertheless, for the numerous structural and functional facets so far reported, its activation mechanisms have remained elusive. Here, we demonstrate that its pleiotropic functions are regulated by different redistributions of the receptors, which crucially depend on the available NT and on the involved subcellular compartment but are unrelated to its oligomerization state. Single-particle studies proved receptors to be monomers with a fast-diffusive behavior in the membrane with, at most, transient self-interactions on the millisecond time scale.

The p75 neurotrophin (NT) receptor (p75^NTR) plays a crucial role in balancing survival-versus-death decisions in the nervous system. Yet, despite 2 decades of structural and biochemical studies, a comprehensive, accepted model for p75^NTR activation by NT ligands is still missing. Here, we present a single-molecule study of membrane p75^NTR in living cells, demonstrating that the vast majority of receptors are monomers before and after NT activation. Interestingly, the stoichiometry and diffusion properties of the wild-type (wt) p75^NTR are almost identical to those of a receptor mutant lacking residues previously believed to induce oligomerization. The wt p75^NTR and mutated (mut) p75^NTR differ in their partitioning in cholesterol-rich membrane regions upon nerve growth factor (NGF) stimulation: We argue that this is the origin of the ability of wt p75^NTR , but not of mut p75^NTR, to mediate immature NT (proNT)-induced apoptosis. Both p75^NTR forms support proNT-induced growth cone retraction: We show that receptor surface accumulation is the driving force for cone collapse. Overall, our data unveil the multifaceted activity of the p75^NTR monomer and let us provide a coherent interpretative frame of existing conflicting data in the literature.

Inhibition of neogenin promotes neuronal survival and improved behavior recovery after spinal cord injury

Following spinal cord trauma, axonal regeneration in the mammalian spinal cord does not occur and functional recovery may be further impeded by retrograde neuronal death. By contrast, lampreys recover after spinal cord injury (SCI) and axons re-connected to their targets in spinal cord. However, the identified reticulospinal (RS) neurons located in the lamprey brain differ in their regenerative capacities - some are good regenerators, and others are bad regenerators - despite the fact that they have analogous projection pathways. Previously, we reported that axonal guidance receptor Neogenin involved in regulation of axonal regeneration after SCI and downregulation of Neogenin synthesis by morpholino oligonucleotides (MO) enhanced the regeneration of RS neurons. Incidentally, the bad regenerating RS neurons often undergo a late retrograde apoptosis after SCI. Here we report that, after SCI, expression of RGMa mRNA was upregulated around the transection site, while its receptor Neogenin continued to be synthesized almost inclusively in the "bad-regenerating" RS neurons. Inhibition of Neogenin by MO prohibited activation of caspases and improved the survival of RS neurons at 10 weeks after SCI. These data provide new evidence in vivo that Neogenin is involved in retrograde neuronal death and failure of axonal regeneration after SCI.

Targeting ALK in Cancer: Therapeutic Potential of Proapoptotic Peptides

ALK is a receptor tyrosine kinase, associated with many tumor types as diverse as anaplastic large cell lymphomas, inflammatory myofibroblastic tumors, breast and renal cell carcinomas, non-small cell lung cancer, neuroblastomas, and more. This makes ALK an attractive target for cancer therapy. Since ALK–driven tumors are dependent for their proliferation on the constitutively activated ALK kinase, a number of tyrosine kinase inhibitors have been developed to block tumor growth. While some inhibitors are under investigation in clinical trials, others are now approved for treatment, notably in ALK-positive lung cancer. Their efficacy is remarkable, however limited in time, as the tumors escape and become resistant to the treatment through different mechanisms. Hence, there is a pressing need to target ALK-dependent tumors by other therapeutic strategies, and possibly use them in combination with kinase inhibitors. In this review we will focus on the therapeutic potential of proapoptotic ALK-derived peptides based on the dependence receptor properties of ALK. We will also try to make a non-exhaustive list of several alternative treatments targeting ALK-dependent and independent signaling pathways.

Molecular genetics of syndromic and non-syndromic forms of parathyroid carcinoma

Parathyroid carcinoma (PC) may occur as part of a complex hereditary syndrome or an isolated (i.e., non-syndromic) non-hereditary (i.e., sporadic) endocrinopathy. Studies of hereditary and syndromic forms of PC, which include the hyperparathyroidism-jaw tumor syndrome (HPT-JT), multiple endocrine neoplasia types 1 and 2 (MEN1 and MEN2), and familial isolated primary hyperparathyroidism (FIHP), have revealed some genetic mechanisms underlying PC. Thus, cell division cycle 73 (CDC73) germline mutations cause HPT-JT, and CDC73 mutations occur in 70% of sporadic PC, but in only ∼2% of parathyroid adenomas. Moreover, CDC73 germline mutations occur in 20%-40% of patients with sporadic PC and may reveal unrecognized HPT-JT. This indicates that CDC73 mutations are major driver mutations in the etiology of PCs. However, there is no genotype-phenotype correlation and some CDC73 mutations (e.g., c.679_680insAG) have been reported in patients with sporadic PC, HPT-JT, or FIHP. Other genes involved in sporadic PC include germline MEN1 and rearranged during transfection (RET) mutations and somatic alterations of the retinoblastoma 1 (RB1) and tumor protein P53 (TP53) genes, as well as epigenetic modifications including DNA methylation and histone modifications, and microRNA misregulation. This review summarizes the genetics and epigenetics of the familial syndromic and non-syndromic (sporadic) forms of PC.

Pro-oxidant activity of sulforaphane and cisplatin potentiates apoptosis and simultaneously promotes autophagy in malignant mesothelioma cells

Sulforaphane (SFN) is an isothiocyanate compound derived from glucoraphanin, which is found in cruciferous vegetables, and has been heralded as a chemopreventive and/or chemotherapeutic agent. The present study investigated the effects of SFN on enhancing the anticancer role of cisplatin (cis-dichlorodiammineplatinum; CDDP) in H‑28 malignant mesothelioma cells. At concentrations demonstrating limited toxicity in MeT‑5A normal human mesothelial cells, combination treatment with the two compounds exhibited synergistic growth‑inhibiting and apoptosis‑promoting activities, as demonstrated by a series of proapoptotic events, including reactive oxygen species accumulation, loss of mitochondrial membrane potential, upregulation of p53 expression, increased B‑cell lymphoma 2 (Bcl‑2) associated X protein/Bcl‑2 ratio, activation of caspase‑3, the occurrence of a sub‑G0/G1 peak and an increase in cells with pyknotic and fragmented nuclei, Annexin V‑phycoerythrin‑positive staining and G2/M phase‑transition delay in the cell cycle. The phosphorylation levels of Akt and mammalian target of rapamycin were reduced by the combination treatment, which was accompanied by a significant increase in the level of autophagosomal marker protein microtubule‑associated protein 1 light chain 3B‑II and the accumulation of acidic vesicular organelles. Pretreatment with the antioxidant N‑acetylcysteine attenuated both apoptosis and autophagy, whereas inhibition of autophagy by bafilomycin A1 potentiated apoptotic cell death following the combination treatment with SFN and CDDP. Considering the pro‑oxidant‑based combinational approach, the results of the present study provide a rationale for targeting cytoprotective autophagy as a potential therapeutic strategy for malignant mesothelioma.

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

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

Peptides derived from the dependence receptor ALK are proapoptotic for ALK-positive tumors

ALK is a receptor tyrosine kinase with an oncogenic role in various types of human malignancies. Despite constitutive activation of the kinase through gene alterations, such as chromosomal translocation, gene amplification or mutation, treatments with kinase inhibitors invariably lead to the development of resistance. Aiming to develop new tools for ALK targeting, we took advantage of our previous demonstration identifying ALK as a dependence receptor, implying that in the absence of ligand the kinase-inactive ALK triggers or enhances apoptosis. Here, we synthesized peptides mimicking the proapoptotic domain of ALK and investigated their biological effects on tumor cells. We found that an ALK-derived peptide of 36 amino acids (P36) was cytotoxic for ALK-positive anaplastic large-cell lymphoma and neuroblastoma cell lines. In contrast, ALK-negative tumor cells and normal peripheral blood mononuclear cells were insensitive to P36. The cytotoxic effect was due to caspase-dependent apoptosis and required N-myristoylation of the peptide. Two P36-derived shorter peptides as well as a cyclic peptide also induced apoptosis. Surface plasmon resonance and mass spectrometry analysis of P36-interacting proteins from two responsive cell lines, Cost lymphoma and SH-SY5Y neuroblastoma, uncovered partners that could involve p53-dependent signaling and pre-mRNA splicing. Furthermore, siRNA-mediated knockdown of p53 rescued these cells from P36-induced apoptosis. Finally, we observed that a treatment combining P36 with the ALK-specific inhibitor crizotinib resulted in additive cytotoxicity. Therefore, ALK-derived peptides could represent a novel targeted therapy for ALK-positive tumors.

Sonic Hedgehog promotes the survival of neural crest cells by limiting apoptosis induced by the dependence receptor CDON during branchial arch development

Cell-adhesion molecule-related/Downregulated by Oncogenes (CDO or CDON) was identified as a receptor for the classic morphogen Sonic Hedgehog (SHH). It has been shown that, in cell culture, CDO also behaves as a SHH dependence receptor: CDO actively triggers apoptosis in absence of SHH via a proteolytic cleavage in CDO intracellular domain. We present evidence that CDO is also pro-apoptotic in the developing neural tube where SHH is known to act as a survival factor. SHH, produced by the ventral foregut endoderm, was shown to promote survival of facial neural crest cells (NCCs) that colonize the first branchial arch (BA1). We show here that the survival activity of SHH on neural crest cells is due to SHH-mediated inhibition of CDO pro-apoptotic activity. Silencing of CDO rescued NCCs from apoptosis observed upon SHH inhibition in the ventral foregut endoderm. Thus, the pair SHH/dependence receptor CDO may play an important role in neural crest cell survival during the formation of the first branchial arch.

EphrinB3 blocks EphB3 dependence receptor functions to prevent cell death following traumatic brain injury

Eph receptor tyrosine kinases and their membrane-bound ligands, ephrins, have a variety of roles in the developing and adult central nervous system that require direct cell-cell interactions; including regulating axon path finding, cell proliferation, migration and synaptic plasticity. Recently, we identified a novel pro-survival role for ephrins in the adult subventricular zone, where ephrinB3 blocks Eph-mediated cell death during adult neurogenesis. Here, we examined whether EphB3 mediates cell death in the adult forebrain following traumatic brain injury and whether ephrinB3 infusion could limit this effect. We show that EphB3 co-labels with microtubule-associated protein 2-positive neurons in the adult cortex and is closely associated with ephrinB3 ligand, which is reduced following controlled cortical impact (CCI) injury. In the complete absence of EphB3 (EphB3(-/-)), we observed reduced terminal deoxynucleotidyl transferase-dUTP nick end labeling (TUNEL), and functional improvements in motor deficits after CCI injury as compared with wild-type and ephrinB3(-/-) mice. We also demonstrated that EphB3 exhibits dependence receptor characteristics as it is cleaved by caspases and induces cell death, which is not observed in the presence of ephrinB3. Following trauma, infusion of pre-clustered ephrinB3-Fc molecules (eB3-Fc) into the contralateral ventricle reduced cortical infarct volume and TUNEL staining in the cortex, dentate gyrus and CA3 hippocampus of wild-type and ephrinB3(-/-) mice, but not EphB3(-/-) mice. Similarly, application of eB3-Fc improved motor functions after CCI injury. We conclude that EphB3 mediates cell death in the adult cortex through a novel dependence receptor-mediated cell death mechanism in the injured adult cortex and is attenuated following ephrinB3 stimulation.

MiR-18a regulates the proliferation, migration and invasion of human glioblastoma cell by targeting neogenin

MiR-17-92 cluster has recently been reported as an oncogene in some tumors. However, the association of miR-18a, an important member of this cluster, with glioblastoma remains unknown. Therefore, this study aims to investigate the expression of miR-18a in glioblastoma and its role in biological behavior of U87 and U251 human glioblastoma cell lines. Quantitative RT-PCR results showed that miR-18a was highly expressed in glioblastoma tissues and U87 and U251 cell lines compared with that in human brain tissues and primary normal human astrocytes, and the expression levels were increased along with the rising pathological grades of glioblastoma. Neogenin was identified as the target gene of miR-18a by dual-luciferase reporter assays. RT-PCR and western blot results showed that its expression levels were decreased along with the rising pathological grades of glioblastoma. Inhibition of miR-18a expression was established by transfecting exogenous miR-18a inhibitor into U87 and U251 cells, and its effects on the biological behavior of glioblastoma cells were studied using CCK-8 assay, transwell assay and flow cytometry. Inhibition of miR-18a expression in U87 and U251 cells significantly up-regulated neogenin, and dramatically suppressed the abilities of cell proliferation, migration and invasion, induced cell cycle arrest and promoted cellular apoptosis. Collectively, these results suggest that miR-18a may regulate biological behavior of human glioblastoma cells by targeting neogenin, and miR-18a can serve as a potential target in the treatment of glioblastoma.

Glioma cell migration and invasion as potential target for novel treatment strategies

Diffuse human gliomas constitute a group of most treatment-refractory tumors even if maximum treatment strategies including neurosurgical resection followed by combined radio-/chemotherapy are applied. In contrast to most other neoplasms, diffusely infiltrating gliomas invade the brain along pre-existing structures such as axonal tracts and perivascular spaces. Even in cases of early diagnosis single or small clusters of glioma cells are already encountered far away from the main tumor bulk. Complex interactions between glioma cells and the surrounding extracellular matrix and considerable changes in the cytoskeletal apparatus are prerequisites for the cellular movement of glioma cells through the brain thereby escaping from most current treatments. This review provides an overview about classical and current concepts of glioma cell migration/invasion and promising preclinical treatment approaches.

Augmented generation of protein fragments during wakefulness as the molecular cause of sleep: a hypothesis

Despite extensive understanding of sleep regulation, the molecular-level cause and function of sleep are unknown. I suggest that they originate in individual neurons and stem from increased production of protein fragments during wakefulness. These fragments are transient parts of protein complexes in which the fragments were generated. Neuronal Ca²⁺ fluxes are higher during wakefulness than during sleep. Subunits of transmembrane channels and other proteins are cleaved by Ca²⁺-activated calpains and by other nonprocessive proteases, including caspases and secretases. In the proposed concept, termed the fragment generation (FG) hypothesis, sleep is a state during which the production of fragments is decreased (owing to lower Ca²⁺ transients) while fragment-destroying pathways are upregulated. These changes facilitate the elimination of fragments and the remodeling of protein complexes in which the fragments resided. The FG hypothesis posits that a proteolytic cleavage, which produces two fragments, can have both deleterious effects and fitness-increasing functions. This (previously not considered) dichotomy can explain both the conservation of cleavage sites in proteins and the evolutionary persistence of sleep, because sleep would counteract deleterious aspects of protein fragments. The FG hypothesis leads to new explanations of sleep phenomena, including a longer sleep after sleep deprivation. Studies in the 1970s showed that ethanol-induced sleep in mice can be strikingly prolonged by intracerebroventricular injections of either Ca²⁺ alone or Ca²⁺ and its ionophore (Erickson et al., Science 1978;199:1219-1221; Harris, Pharmacol Biochem Behav 1979;10:527-534; Erickson et al., Pharmacol Biochem Behav 1980;12:651-656). These results, which were never interpreted in connection to protein fragments or the function of sleep, may be accounted for by the FG hypothesis about molecular causation of sleep.

The subcommissural organ and the development of the posterior commissure

Growing axons navigate through the developing brain by means of axon guidance molecules. Intermediate targets producing such signal molecules are used as guideposts to find distal targets. Glial, and sometimes neuronal, midline structures represent intermediate targets when axons cross the midline to reach the contralateral hemisphere. The subcommissural organ (SCO), a specialized neuroepithelium located at the dorsal midline underneath the posterior commissure, releases SCO-spondin, a large glycoprotein belonging to the thrombospondin superfamily that shares molecular domains with axonal pathfinding molecules. Several evidences suggest that the SCO could be involved in the development of the PC. First, both structures display a close spatiotemporal relationship. Second, certain mutants lacking an SCO present an abnormal PC. Third, some axonal guidance molecules are expressed by SCO cells. Finally, SCO cells, the Reissner's fiber (the aggregated form of SCO-spondin), or synthetic peptides from SCO-spondin affect the neurite outgrowth or neuronal aggregation in vitro.

Down-regulation of neogenin accelerated glioma progression through promoter Methylation and its overexpression in SHG-44 Induced Apoptosis

Background

Dependence receptors have been proved to act as tumor suppressors in tumorigenesis. Neogenin, a DCC homologue, well known for its fundamental role in axon guidance and cellular differentiation, is also a dependence receptor functioning to control apoptosis. However, loss of neogenin has been reported in several kinds of cancers, but its role in glioma remains to be further investigated.

Methodology/Principal Findings

Western blot analysis showed that neogenin level was lower in glioma tissues than in their matching surrounding non-neoplastic tissues (n = 13, p<0.01). By immunohistochemical analysis of 69 primary and 16 paired initial and recurrent glioma sections, we found that the loss of neogenin did not only correlate negatively with glioma malignancy (n = 69, p<0.01), but also glioma recurrence (n = 16, p<0.05). Kaplan-Meier plot and Cox proportional hazards modelling showed that over-expressive neogenin could prolong the tumor latency (n = 69, p<0.001, 1187.6±162.6 days versus 687.4±254.2 days) and restrain high-grade glioma development (n = 69, p<0.01, HR: 0.264, 95% CI: 0.102 to 0.687). By Methylation specific polymerase chain reaction (MSP), we reported that neogenin promoter was methylated in 31.0% (9/29) gliomas, but absent in 3 kinds of glioma cell lines. Interestingly, the prevalence of methylation in high-grade gliomas was higher than low-grade gliomas and non-neoplastic brain tissues (n = 33, p<0.05) and overall methylation rate increased as glioma malignancy advanced. Furthermore, when cells were over-expressed by neogenin, the apoptotic rate in SHG-44 was increased to 39.7% compared with 8.1% in the blank control (p<0.01) and 9.3% in the negative control (p<0.01).

Conclusions/Significance

These observations recapitulated the proposed role of neogenin as a tumor suppressor in gliomas and we suggest its down-regulation owing to promoter methylation is a selective advantage for glioma genesis, progression and recurrence. Furthermore, the induction of apoptosis in SHG-44 cells after overexpression of neogenin, indicated that neogenin could be a novel target for glioma therapy.

Noncanonical Hedgehog signaling

The notion of noncanonical hedgehog (Hh) signaling in mammals has started to receive support from numerous observations. By noncanonical, we refer to all those cellular and tissue responses to any of the Hh isoforms that are independent of transcriptional changes mediated by the Gli family of transcription factors. In this chapter, we discuss the most recent findings that suggest that Patched1 can regulate cell proliferation and apoptosis independently of Smoothened (Smo) and Gli and the reports that Smo modulates actin cytoskeleton-dependent processes such as fibroblast migration, endothelial cell tubulogenesis, axonal extension, and neurite formation by diverse mechanisms that exclude any involvement of Gli-dependent transcription. We also acknowledge the existence of less stronger evidence of noncanonical signaling in Drosophila.

Variants in the netrin-1 receptor UNC5C prevent apoptosis and increase risk of familial colorectal cancer

BACKGROUND & AIMS

Expression of the netrin-1 dependence receptor UNC5C is reduced in many colorectal tumors; mice with the UNC5C mutations have increased progression of intestinal tumors. We investigated whether specific variants in UNC5C increase risk of colorectal cancer (CRC).

METHODS

We analyzed the sequence of UNC5C in blood samples from 1801 patients with CRC and 4152 controls from 3 cohorts (France, United States, and Finland). Almost all cases from France and the United States had familial CRC; of the Finnish cases, 92 of 984 were familial. We analyzed whether CRC segregates with the UNC5C variant A628K in 3 families with histories of CRC. We also performed haplotype analysis to determine the origin of this variant.

RESULTS

Of 817 patients with familial CRC, 14 had 1 of 4 different, unreported missense variants in UNC5C. The variants p.Asp353Asn (encodes D353N), p.Arg603Cys (encodes R603C), and p.Gln630Glu (encodes Q630E) did not occur significantly more often in cases than controls. The variant p.Ala628Lys (A628K) was detected in 3 families in the French cohort (odds ratio, 8.8; Wald's 95% confidence interval, 1.47-52.93; P = .03) and in 2 families in the US cohort (odds ratio, 1.9; P = .6) but was not detected in the Finnish cohort; UNC5C A628K segregated with CRC in families. Three families with A628K had a 109-kilobase identical haplotype that spanned most of UNC5C, indicating recent origin of this variant in white subjects (14 generations; 95% confidence interval, 6-36 generations). Transfection of HEK293T cells with UNC5C-A628K significantly reduced apoptosis compared with wild-type UNC5C, measured in an assay of active caspase-3.

CONCLUSIONS

Inherited mutations in UNC5C prevent apoptosis and increase risk of CRC.

UNCovering the molecular machinery of dependence receptor signaling

Dependence receptors send opposite signals in the presence or absence of ligand, but the underlying mechanisms have been elusive. In this issue of Molecular Cell, Guenebeaud et al. (2010) elucidate the molecular signaling machinery of the dependence receptor UNC5B.

Dependence receptors: from basic research to drug development

The fourth meeting on dependence receptors featured descriptions of previously unknown dependence receptors. New mechanistic data were presented on the switch between the trophic, antiapoptotic response with the proapoptotic response that occurs with loss of trophic support. The possibility that the loss of trophic support may also involve the binding of an active antitrophin was also discussed. New in vivo data were presented on the roles of dependence receptors in development, angiogenesis, oncogenesis, and neurodegeneration, as well as new therapeutic approaches based on dependence receptor function. The next meeting on dependence receptors is scheduled for 2012.

EphB3 limits the expansion of neural progenitor cells in the subventricular zone by regulating p53 during homeostasis and following traumatic brain injury

Ephrins and Eph receptor(s) have recently been implicated in regulating neurogenesis in the adult subventricular zone (SVZ) and rostral migratory stream. Here, we examined the role of ephrinB3-EphB3 signaling in mediating the SVZ response to traumatic brain injury (TBI). Analysis of EphB3 expression showed colocalization with glial fibrillary acidic protein-positive neural stem progenitor cells (NSPCs) and doublecortin-positive neuroblasts, whereas ephrinB3 was expressed outside the neurogenic region. TBI resulted in a significant reduction in EphB3 expression, which coincided with enhanced NSPC survival and proliferation at 3 and 7 days postinjury. Analysis of mice lacking either ephrinB3 (ephrinB3(-/-)) or EphB3 (EphB3(-/-)) showed a significant increase in bromodeoxyuridine (BrdU) incorporation and Ki67 immunoreactivity in the SVZ. Interestingly, cell death was dissimilar between knockout mice, where cell death was reduced in EphB3(-/-) but increased in ephrinB3(-/-) mice. Lateral ventricle infusion of soluble preclustered ephrinB3-Fc reversed the proliferative and cell death defects in ephrinB3(-/-) but not EphB3(-/-) mice and prevented TBI-induced proliferation in wild-type NSPCs. Coincidently, tumor suppressor p53 expression was increased following EphB3 stimulation and is reduced in the absence of either EphB3 or ephrinB3. Furthermore, pharmacological inhibition and siRNA knockdown of p53-attenuated ephrinB3-Fc-mediated growth suppression while having no effect on cell death in cultured NSPCs. These data demonstrate that EphB3 signaling suppresses NSPC proliferation in a p53-dependent manner, induces cell death in the absence of ligand stimulation and is transiently reduced in the SVZ to initiate the expansion and survival of endogenous adult NSPCs following TBI.

Netrin-1 and its dependence receptors as original targets for cancer therapy

PURPOSE OF REVIEW

The dependence receptor notion has recently seen an interesting development. From a basic cell biology concept, which proposes that some transmembrane receptors can be active in the absence of their ligand and induce in the setting apoptosis, recent observations have provided new hope for the development of alternative targeted therapies. The purpose of this review is to show, with the example of netrin-1 dependence receptors, the path from cell biology to promising anticancer-targeted therapy.

RECENT FINDINGS

The dependence receptors Deleted in Colorectal Cancer and Unc-5 homolog that bind netrin-1 had been implicated in nervous system development as they participate in neuronal navigation. They were also implicated beyond the developing brain with roles in angiogenesis regulation and homeostasis of various tissues. However, these receptors were shown to trigger apoptosis in the absence of netrin-1 and, as such, act as tumor suppressors. Recent data support the view that Deleted in Colorectal Cancer/Unc-5 homolog proapoptotic signals are indeed a safeguard mechanism regulating tumor growth and metastasis.

SUMMARY

In this review, we will develop the different data supporting the view that a selective advantage for a tumor is to inactivate this dependence receptor's proapoptotic signal and will describe a putative therapeutic approach that is to reactivate this death signaling in tumor cells.

Neurotrophin-3 production promotes human neuroblastoma cell survival by inhibiting TrkC-induced apoptosis

Tropomyosin-related kinase receptor C (TrkC) is a neurotrophin receptor with tyrosine kinase activity that was expected to be oncogenic. However, it has several characteristics of a tumor suppressor: its expression in tumors has often been associated with good prognosis; and it was recently demonstrated to be a dependence receptor, transducing different positive signals in the presence of ligand but inducing apoptosis in the absence of ligand. Here we show that the TrkC ligand neurotrophin-3 (NT-3) is upregulated in a large fraction of aggressive human neuroblastomas (NBs) and that it blocks TrkC-induced apoptosis of human NB cell lines, consistent with the idea that TrkC is a dependence receptor. Functionally, both siRNA knockdown of NT-3 expression and incubation with a TrkC-specific blocking antibody triggered apoptosis in human NB cell lines. Importantly, disruption of the NT-3 autocrine loop in malignant human neuroblasts triggered in vitro NB cell death and inhibited tumor growth and metastasis in both a chick and a mouse xenograft model. Thus, we believe that our data suggest that NT-3/TrkC disruption is a putative alternative targeted therapeutic strategy for the treatment of NB.

Hedgehog signalling: emerging evidence for non-canonical pathways

Hedgehog (HH) signalling is involved in the development of numerous embryonic tissues. In humans,germline mutations in hedgehog pathway components cause congenital malformations and somatic mutations are associated with cancers. The basic framework of the HH pathway was elucidated in the fruitfly, Drosophila melanogaster, and this pathway is largely conserved in vertebrates, although some important differences have been noted. The current paradigm of the "canonical" pathway views HH signalling as a series of repressive interactions which culminates in GLI-mediated transcriptional regulation of a variety of cellular processes. Definitions of "non-canonical" signalling stem from examples where the response to HH morphogen deviates from this paradigm and, according to current reports, three general scenarios of noncanonical HH signalling can be defined: (1) Signalling that involves HH pathway components but which is independent of GLI-mediated transcription; (2) Direct interaction of HH signalling components with components of other molecular pathways; and (3) "Non-contiguous" or "atypical" interaction of core HH pathway components with one another. Currently, the evidence supporting non-canonical HH signalling is not conclusive. However, Sonic hedgehog (SHH) has been shown to regulate cell migration and axon guidance in several contexts, and some of these processes are independent of downstream components of the HH pathway, and presumably the transcriptional response to morphogen. Furthermore, biochemical studies have shown that the HH receptor, PTCH1, can directly interact both with Cyclin B1 and caspases, to inhibit cell proliferation and to promote apoptosis, respectively, and that these functions are inhibited in the presence of morphogen. Genetic analysis of orthologues of the HH pathway in nematode worms further supports the notion that PTCH1-related molecules can function independently of other components of the canonical HH pathway, and the phenotypes of mice with point mutations in the Ptch1 gene offer clues as to the processes that non-canonical HH signalling might regulate. While none of these evidences are conclusive,collectively they point to the existence of added complexity in the HH pathway in the form of non-canonical pathways. A major difficulty in studying this problem is that canonical and non-canonical pathways are likely to act in parallel, and so in many situations it will not be possible to implicate non-canonical responses in certain cellular processes simply by excluding a role for the canonical pathway-directed analyses of non-canonical HH signalling are therefore necessary. The aim of this review is to present the cumulative evidence supporting non-canonical HH signalling, with the hope of promoting further enquiry into this area.

Netrin-1 acts as a survival factor for aggressive neuroblastoma

Neuroblastoma (NB), the most frequent solid tumor of early childhood, is diagnosed as a disseminated disease in >60% of cases, and several lines of evidence support the resistance to apoptosis as a prerequisite for NB progression. We show that autocrine production of netrin-1, a multifunctional laminin-related molecule, conveys a selective advantage in tumor growth and dissemination in aggressive NB, as it blocks the proapoptotic activity of the UNC5H netrin-1 dependence receptors. We show that such netrin-1 up-regulation is a potential marker for poor prognosis in stage 4S and, more generally, in NB stage 4 diagnosed infants. Moreover, we propose that interference with the netrin-1 autocrine loop in malignant neuroblasts could represent an alternative therapeutic strategy, as disruption of this loop triggers in vitro NB cell death and inhibits NB metastasis in avian and mouse models.

Interference with netrin-1 and tumor cell death in non-small cell lung cancer

BACKGROUND

Netrin-1 may promote colorectal and breast tumorigenesis, by inhibiting apoptosis induced by its dependence receptors, deleted in colorectal cancer (DCC) and uncoordinated-5-homolog (UNC5H). The status of netrin-1 and its receptors in non-small cell lung cancer (NSCLC) was unknown.

METHODS

The levels of netrin-1 and its receptors were analyzed in a panel of 92 NSCLC and 25 human lung cancer cell lines by quantitative reverse transcription-polymerase chain reaction and immunohistochemistry. In lung cancer cell lines that express netrin-1, the expression of netrin-1 was inhibited by using small interfering RNA (siRNA), or interference with netrin-1 was performed by treatment with a decoy recombinant DCC ectodomain protein (DCC-5Fbn). Cell death was monitored with a trypan blue exclusion assay or by measuring caspase-3 activity. The effect of netrin-1 interference on tumor growth was analyzed by DCC-5Fbn intratumoral or netrin-1 siRNA intraperitoneal injection in mice engrafted with lung cancer cell lines. All statistical tests were two-sided.

RESULTS

High levels of netrin-1 were found in 43 of the 92 NSCLC tumor samples (47%). Interference with netrin-1 in human lung cancer cell lines was associated with UNC5H-mediated cell death in vitro (percentage of cell death in untreated and in DCC-5Fbn-treated cells = 8% and 26%, respectively, difference = 18%, 95% confidence interval [CI] = 10% to 26%; P = .049) and with lung tumor growth inhibition and/or regression in xenografted nude mice (12 mice in DCC-5Fbn-treated group and 13 mice in control group). Mean volume of control and DCC-5Fbn-treated tumors on day 46 was 489 and 84 mm(3), respectively (difference = 404 mm(3), 95% CI = 145 to 664 mm(3); P < .001).

CONCLUSIONS

Almost half of the NSCLC tissue samples examined expressed high levels of netrin-1. Extracellular targeting of the interaction between netrin-1 and UNC5H may be a promising therapeutic approach for NSCLCs that express netrin-1.

NF-kappaB regulates netrin-1 expression and affects the conditional tumor suppressive activity of the netrin-1 receptors

BACKGROUND & AIMS

Netrin-1 was recently proposed to play a crucial role during colorectal tumorigenesis by regulating apoptosis. Because netrin-1 receptors belong to the family of dependence receptors, a selective advantage for a tumor is either to lose netrin-1 receptors or to gain autocrine expression of netrin-1. We have investigated whether netrin-1 is up-regulated in colorectal cancer and have searched for a link between NF-kappaB activation and netrin-1 up-regulation.

METHODS

The level of netrin-1, netrin-1 receptors, ie, DCC, UNC5H1, UNC5H2, UNC5H3, and the proinflammatory markers cyclooxygenase-2 and inhibitor of nuclear factor-kappaB (IkappaB) alpha were analyzed in a panel of 59 primary sporadic colorectal carcinomas. Netrin-1 expression was investigated in tumor cells and in mouse colonic crypts in response to NF-kappaB activation but also in a mouse model of inflammation-induced colorectal cancer. Binding of NF-kappaB to netrin-1 promoter and effect of NF-kappaB activation to the proapoptotic activity of UNC5H2 were also analyzed.

RESULTS

We show that colorectal tumors with a gain of netrin-1 are tumors that display increased activation of the NF-kappaB pathway. Moreover, netrin-1 up-regulation, which is associated with tumor formation in mice, is observed in mouse colonic crypts in response to NF-kappaB activation but also in a mouse model of inflammation-induced colorectal cancer. We demonstrate that the netrin-1 gene is a direct transcriptional target of NF-kappaB. We show that NF-kappaB-induced netrin-1 expression inhibits proapoptotic activity of the netrin-1 receptors.

CONCLUSIONS

We propose that NF-kappaB activation that occurs in response to inflammation confers a selective advantage for tumor development through NF-kappaB-mediated netrin-1 up-regulation.

Netrin-1 and its receptors in tumour growth promotion

BACKGROUND

Netrin-1 belongs to a family of secreted proteins that act as migration and adhesion cues in the developing CNS and in a number of non-neural tissues. Netrin-1 is the ligand of deleted in colorectal cancer (DCC) and the uncoordinated family member 5 (UNC5) orthologues of the dependence receptor family. Over the past ten years, a novel mechanism has emerged, that a receptor unoccupied by its ligand is not necessarily inactive. Rather, such a receptor can mediate two signalling pathways, depending on whether it is bound to its ligand or not. In the absence of ligand, an active signalling pathway results in cell death through apoptosis.

OBJECTIVE

Coupled netrin-1 receptors have been shown to regulate diverse processes such as maintenance, integrity, migration and renewal of many tissues. We propose that netrin-1 receptors can regulate tumour development.

METHODS

We review the properties of netrin-1 and present netrin-1 receptors as regulators of tumourigenesis.

RESULTS/CONCLUSION

Netrin-1 and its receptors are unexplored critical targets in cancer.

Netrin-1 is a survival factor during commissural neuron navigation

DCC (Deleted in Colorectal Cancer) is a putative tumor suppressor whose expression is lost in numerous cancers and whose tumor suppressor activity appears to be dependent on its ability to trigger apoptosis when disengaged by its ligand netrin-1. In this sense, netrin-1 is a survival factor that controls tumorigenesis. However, netrin-1 is also the prototypical axon guidance cue and has been shown to orient many neurons or axons, especially commissural axons, during spinal cord development. Here we show that netrin-1 is not only an attractive cue for developing commissural axons but also promotes their survival. In primary neuronal culture, in mice or in chick embryos, netrin-1 inhibits the proapoptotic activity of DCC in developing commissural neurons. Thus, adequate commissural neurons navigation requires both the attractive activity of netrin-1 and the anti-apoptotic function of this cue.

The neogenin intracellular domain regulates gene transcription via nuclear translocation

Neogenin is a multifunctional receptor implicated in axon navigation, neuronal differentiation, morphogenesis, and cell death. Very little is known about signaling downstream of neogenin. Because we found that the neogenin intracellular domain (NeICD) interacts with nuclear proteins implicated in transcription regulation, we investigated further whether neogenin signals similarly to the Notch receptor. We show here that neogenin is cleaved by gamma-secretase, an event that releases the complete NeICD. We also describe that NeICD is located at the nucleus, a feature regulated through a balance between nuclear import and export. NeICD triggers gene reporter transactivation and associates with nuclear chromatin. Direct transcriptional targets of NeICD were determined and were shown to be up-regulated in the presence of neogenin ligand. Together, we reveal here a novel aspect of neogenin signaling that relies on the direct implication of its intracellular domain in transcriptional regulation.

Netrin-1 expression confers a selective advantage for tumor cell survival in metastatic breast cancer

Netrin-1, an axon navigation cue was proposed to play a crucial role during colorectal tumorigenesis by regulating apoptosis. The netrin-1 receptors DCC and UNC5H were shown to belong to the family of dependence receptors that share the ability to induce apoptosis in the absence of their ligands. Such a trait confers on these receptors a tumor suppressor activity. Expression of one of these dependence receptors at the surface of a tumor cell is indeed speculated to render this cell dependent on ligand availability for its survival, hence inhibiting uncontrolled cell proliferation or metastasis. Consequently, it is a selective advantage for a tumor cell to lose this dependence receptor activity, as previously described with losses of DCC and UNC5H expression in human cancers. However, the model predicts that a similar advantage may be obtained by gaining autocrine expression of the ligand. We describe here that, unlike human nonmetastatic breast tumors, a large fraction of metastatic breast cancers overexpress netrin-1. Moreover, we show that netrin-1-expressing mammary metastatic tumor cell lines undergo apoptosis when netrin-1 expression is experimentally decreased or when decoy soluble receptor ectodomains are added. Such treatments prevent metastasis formation both in a syngenic mouse model of lung colonization of a mammary cancer cell line and in a model of spontaneous lung metastasis of xenografted human breast tumor. Thus, netrin-1 expression observed in a large fraction of human metastatic breast tumors confers a selective advantage for tumor cell survival and potentially represents a promising target for alternative anticancer therapeutic strategies.

Purification and bioassay of hedgehog ligands for the study of cell death and survival

The Hedgehog (Hh) family of secreted ligands-composed of Sonic Hedgehog (Shh), Indian Hedgehog (Ihh), and Desert Hedgehog (Dhh)-possesses many roles during embryonic development, adult homeostasis, and cancer. The specific functions of the Hh proteins are intertwined with their requirement as survival factors in Hh-responsive cells. However, studies designed to dissect the anti-apoptotic role of Hhs have been hindered by the lack of simple approaches to purify large quantities of recombinant ligands in the average laboratory setting because of the natural modifications of these proteins with palmitic acid and cholesterol. In this chapter, we provide a comprehensive protocol for the expression of Shh, Ihh, and Dhh in Escherichia coli as fusion proteins with calmodulin-binding peptide to allow easy and rapid purification. The ligands are engineered with a new N-terminus containing two isoleucine residues to provide an essential hydrophobic interphase for achieving high biologic activity. The protocol includes a detailed description of a method for determination of the specific activity of the generated proteins by use of a cell culture-based luciferase approach.

Inactivation of the UNC5C Netrin-1 receptor is associated with tumor progression in colorectal malignancies

BACKGROUND & AIMS

The UNC5H netrin-1 receptors (UNC5H1-3 also called UNC5A-C) belong to the functional dependence receptors family, which share the ability to induce apoptosis in the absence of their ligands. Such a trait has been hypothesized to confer a tumor-suppressor activity. Indeed, cells harboring these receptors are thought to be dependent on ligand availability for their survival, thereby inhibiting uncontrolled tumor cell proliferation. We investigate here whether UNC5C acts as a tumor suppressor in colorectal malignancies.

METHODS

The level of UNC5C was analyzed in a panel of 86 primary sporadic colorectal carcinomas. Loss of heterozygosity in the UNC5C locus and epigenetic alterations in the UNC5C promoter were also analyzed. Intestinal tumor progression was monitored in mice bearing both UNC5C and APC1638N mutations, and apoptosis was measured in intestinal tumors developed in UNC5C/APC1638N mutant mice.

RESULTS

We show here that UNC5C expression is down-regulated in a large fraction of human colorectal cancers, mainly through promoter methylation. Moreover, in mice, inactivation of UNC5C is associated with increased intestinal tumor progression and a decrease in tumor cell apoptosis.

CONCLUSIONS

The loss of UNC5C expression observed in human colorectal cancer is a selective advantage for tumor progression, in agreement with the dependence receptor hypothesis. Thus, the UNC5C dependence receptor is a tumor suppressor that regulates sporadic colorectal cancer.

The role of apoptosis in respiratory diseases

PURPOSE

The purpose of this article is to define apoptosis and describe how this cellular pathway is relevant to the pathogenesis of different respiratory diseases. This will assist clinical nurse specialists in understanding how new drugs and therapies inhibit and stimulate apoptotic pathways.

BACKGROUND

Clinical nurse specialists need to expand their knowledge concerning the role of apoptosis so that they can better expand their spheres of influence. The 4 stages of apoptosis are discussed, as well as the various apoptotic pathways involved with asthma, emphysema, and acute respiratory distress syndrome that promote and inhibit apoptosis in patients.

CONCLUSION

It is crucial for clinical nurse specialists to know what apoptosis is and how it relates to different pathophysiologic states. The challenge facing clinical nurse specialists is how to be kept informed and current concerning molecular and cellular mechanisms that are important in the practice setting. Strategies needed to maintain expertise include acquiring new knowledge, developing new skills, and changing attitudes about molecular biology. Apoptosis must become a significant part of any health professionals' continuing educational program because it has been recognized as the pathway to most any disease. Clinical nurse specialists who understand apoptosis and its pathways can use this knowledge to aid in the prevention and treatment of respiratory diseases.

Repulsive guidance molecule plays multiple roles in neuronal differentiation and axon guidance

Repulsive guidance molecule (RGM) is a membrane-bound protein originally isolated as a guidance molecule for retinal axons. Three RGM isoforms (RGMa-RGMc) exist in vertebrates. We showed previously that RGMa is a cell-survival factor in the neuroepithelium of chick embryos that suppresses the proapoptotic activity of its receptor neogenin. In the present study, we performed gain- and loss-of-function analysis of RGMa in chick embryos to further investigate RGMa function. We found that RGMa overexpression promotes neuronal differentiation, whereas RGMa small interference RNA represses it. Similar experiments conducted at later developmental stages using retroviral vectors reveal that perturbation of RGMa expression disturbs the retinotectal projection. Our work provides the first evidence for a role for RGMs in axon guidance in vivo. In addition, these results suggest that RGMa exerts multiple functions during neural development.