Neurotrophin dependence mediated by p75NTR: contrast between rescue by BDNF and NGF

STAU1 selectively regulates the expression of inflammatory and immune response genes and alternative splicing of the nerve growth factor receptor signaling pathway

Double‑stranded RNA‑binding protein Staufen homolog 1 (STAU1) is a highly conserved multifunctional double‑stranded RNA‑binding protein, and is a key factor in neuronal differentiation. RNA sequencing was used to analyze the overall transcriptional levels of the upregulated cells by STAU1 and control cells, and select alternative splicing (AS). It was determined that the high expression of STAU1 led to changes in the expression levels of a variety of inflammatory and immune response genes, including IFIT2, IFIT3, OASL, and CCL2. Furthermore, STAU1 was revealed to exert a significant regulatory effect on the AS of genes related to the 'nerve growth factor receptor signaling pathway'. This is of significant importance for neuronal survival, differentiation, growth, post‑damage repair, and regeneration. In conclusion, overexpression of STAU1 was associated with immune response and regulated AS of pathways related to neuronal growth and repair. In the present study, the whole transcriptome of STAU1 expression was first analyzed, which laid a foundation for further understanding the key functions of STAU1.

Spontaneous regression of neuroblastoma

Neuroblastomas are characterized by heterogeneous clinical behavior, from spontaneous regression or differentiation into a benign ganglioneuroma, to relentless progression despite aggressive, multimodality therapy. Indeed, neuroblastoma is unique among human cancers in terms of its propensity to undergo spontaneous regression. The strongest evidence for this comes from the mass screening studies conducted in Japan, North America and Europe and it is most evident in infants with stage 4S disease. This propensity is associated with a pattern of genomic change characterized by whole chromosome gains rather than segmental chromosome changes but the mechanism(s) underlying spontaneous regression are currently a matter of speculation. There is evidence to support several possible mechanisms of spontaneous regression in neuroblastomas: (1) neurotrophin deprivation, (2) loss of telomerase activity, (3) humoral or cellular immunity and (4) alterations in epigenetic regulation and possibly other mechanisms. It is likely that a better understanding of the mechanisms of spontaneous regression will help to identify targeted therapeutic approaches for these tumors. The most easily targeted mechanism is the delayed activation of developmentally programmed cell death regulated by the tropomyosin receptor kinase A (TrkA) pathway. Pan-Trk inhibitors are currently in clinical trials and so Trk inhibition might be used as the first line of therapy in infants with biologically favorable tumors that require treatment. Alternative approaches consist of breaking immune tolerance to tumor antigens but approaches to telomere shortening or epigenetic regulation are not easily druggable. The different mechanisms of spontaneous neuroblastoma regression are reviewed here, along with possible therapeutic approaches.

Mechanisms of neuroblastoma regression

Recent genomic and biological studies of neuroblastoma have shed light on the dramatic heterogeneity in the clinical behaviour of this disease, which spans from spontaneous regression or differentiation in some patients, to relentless disease progression in others, despite intensive multimodality therapy. This evidence also suggests several possible mechanisms to explain the phenomena of spontaneous regression in neuroblastomas, including neurotrophin deprivation, humoral or cellular immunity, loss of telomerase activity and alterations in epigenetic regulation. A better understanding of the mechanisms of spontaneous regression might help to identify optimal therapeutic approaches for patients with these tumours. Currently, the most druggable mechanism is the delayed activation of developmentally programmed cell death regulated by the tropomyosin receptor kinase A pathway. Indeed, targeted therapy aimed at inhibiting neurotrophin receptors might be used in lieu of conventional chemotherapy or radiation in infants with biologically favourable tumours that require treatment. Alternative approaches consist of breaking immune tolerance to tumour antigens or activating neurotrophin receptor pathways to induce neuronal differentiation. These approaches are likely to be most effective against biologically favourable tumours, but they might also provide insights into treatment of biologically unfavourable tumours. We describe the different mechanisms of spontaneous neuroblastoma regression and the consequent therapeutic approaches.

An agonistic mAb directed to the TrkC receptor juxtamembrane region defines a trophic hot spot and interactions with p75 coreceptors

The D5 domain of TrkC receptors is a docking site for Neurotrophin-3 (NT-3), but other domains may be relevant for function or harmonizing signals with p75(NTR) coreceptors. We report a monoclonal antibody (mAb) 2B7 targeting the juxtamembrane domain of TrkC. mAb 2B7 binds to murine and human TrkC receptors and is a functional agonist that affords activation of TrkC, AKT, and MAPK. These signals result in cell survival but not in cellular differentiation. Monomeric 2B7 Fabs also affords cell survival. Binding of 2B7 mAb and 2B7 Fabs to TrkC are blocked by NT-3 in a dose-dependent manner but not by pro-NT-3. Expression of p75(NTR) coreceptors on the cell surface block the binding and function of mAb 2B7, whereas NT-3 binding and function are enhanced. mAb 2B7 defines a previously unknown neurotrophin receptor functional hot spot; that exclusively generates survival signals; that can be activated by non-dimeric ligands; and potentially unmasks a site for p75-TrkC interactions.

Bivalent peptidomimetic ligands of TrkC are biased agonists and selectively induce neuritogenesis or potentiate neurotrophin-3 trophic signals

This study was initiated to find small molecule ligands that would induce a functional response when docked with neurotrophin Trk receptors. "Minimalist" mimics of beta-turns were designed for this purpose. These mimics are (i) rigid, yet easily folded into turn-like conformations, and (ii) readily accessible from amino acids bearing most of the natural side chains. Gram quantities of 16 of these turn mimics were prepared and then assembled into 152 fluorescein-labeled bivalent peptidomimetics via a solution-phase combinatorial method. Fluorescence-based screening of these molecules using cells transfected with the Trk receptors identified 10 potential ligands of TrkC, the receptor for neurotrophin-3. Analogues of these bivalent peptidomimetics with biotin replacing the fluorescein label were then prepared and tested to confirm that binding was not due to the fluorescein. Several assays were conducted to find the mode of action of these biotinylated compounds. Thus, direct binding, survival and neuritogenic, and biochemical signal transduction assays showed 8 of the original 10 hits were agonistic ligands binding to the ectodomain of TrkC. Remarkably, some peptidomimetics afford discrete signals leading to either cell survival or neuritogenic differentiation. The significance of this work is three-fold. First, we succeeded in finding small, selective, proteolytically stable ligands for the TrkC receptor; there are very few of these in the literature. Second, we show that it is possible to activate distinct and biased signaling pathways with ligands binding at the ectodomain of wild-type receptors. Third, the discovery that some peptidomimetics initiate different modes of cell signaling increases their potential as pharmacological probes and therapeutic leads.

The p75(NTR) tumor suppressor induces cell cycle arrest facilitating caspase mediated apoptosis in prostate tumor cells

The p75 neurotrophin receptor (p75(NTR)) is a death receptor which belongs to the tumor necrosis factor receptor super-family of membrane proteins. This study shows that p75(NTR) retarded cell cycle progression by induced accumulation of cells in G0/G1 and a reduction in the S phase of the cell cycle. The rescue of tumor cells from cell cycle progression by a death domain deleted (DeltaDD) dominant-negative antagonist of p75(NTR) showed that the death domain transduced anti-proliferative activity in a ligand-independent manner. Conversely, addition of NGF ligand rescued retardation of cell cycle progression with commensurate changes in components of the cyclin/cdk holoenzyme complex. In the absence of ligand, p75(NTR)-dependent cell cycle arrest facilitated an increase in apoptotic nuclear fragmentation of the prostate cancer cells. Apoptosis of p75(NTR) expressing cells occurred via the intrinsic mitochondrial pathway leading to a sequential caspase-9 and -7 cascade. Since the death domain deleted dominant-negative antagonist of p75(NTR) rescued intrinsic caspase associated apoptosis in PC-3 cells, this shows p75(NTR) was integral to ligand independent induction of apoptosis. Moreover, the ability of ligand to ameliorate the p75(NTR)-dependent intrinsic apoptotic cascade indicates that NGF functioned as a survival factor for p75(NTR) expressing prostate cancer cells.

Integrins as a distinct subtype of dependence receptors

In the absence of their cognate ligand, dependence receptors trigger programmed cell death. This function is the defining feature of dependence receptors, which include members of several different protein families. The integrins are a family of heterodimeric receptors for extracellular matrix (ECM) proteins, mediating cell anchorage and migration. Integrins share characteristics with dependence receptors, and integrin binding to substrate ECM ligands is essential for cell survival. Although integrins do not conform in all characteristics to the established definitions of dependence receptors, alterations in the expression of integrins and their ligands during physiological and pathological events, such as wound healing, angiogenesis and tumorigenesis, do regulate cell fate in a ligand-dependent manner. This biosensory function of integrins fits well with our current concept of dependence receptor action, and thus integrins may rightly be considered to comprise a distinct subclass of dependence receptor.

Receptors that mediate cellular dependence

Cells depend for their survival on stimulation by trophic factors and other prosurvival signals, the withdrawal of which induces apoptosis, both via the loss of antiapoptotic signaling and the activation of proapoptotic signaling via specific receptors. These receptors, dubbed dependence receptors, activate apoptotic pathways following the withdrawal of trophic factors and other supportive stimuli. Such receptors may feature in developmental cell death, carcinogenesis (including metastasis), neurodegeneration, and possibly subapoptotic events such as neurite retraction and somal atrophy. Mechanistic studies of dependence receptors suggest that these receptors form ligand-dependent complexes that include specific caspases. Complex formation in the absence of ligand leads to caspase activation by a mechanism that is typically dependent on caspase cleavage of the receptor itself, releasing proapoptotic peptides. Cellular dependence receptors, considered in the aggregate, may thus form a system of molecular integration, analogous to the electrical integration system provided by dendritic arbors in the nervous system.

Selective Small Molecule Peptidomimetic Ligands of TrkC and TrkA Receptors Afford Discrete or Complete Neurotrophic Activities

We designed a minilibrary of 55 small molecule peptidomimetics based on beta-turns of the neurotrophin growth factor polypeptides neurotrophin-3 (NT-3) and nerve growth factor (NGF). Direct binding, binding competition, and biological screens identified agonistic ligands of the ectodomain of the neurotrophin receptors TrkC and TrkA. Agonism is intrinsic to the peptidomimetic ligand (in the absence of neurotrophins), and/or can also be detected as potentiation of neurotrophin action. Remarkably, some peptidomimetics afford both neurotrophic activities of cell survival and neuronal differentiation, while others afford discrete signals leading to either survival or differentiation. The high rate of hits identified suggests that focused minilibraries may be desirable for developing bioactive ligands of cell surface receptors. Small, selective, proteolytically stable ligands with defined biological activity may have therapeutic potential.

Functional interaction between p75NTR and TrkA: the endocytic trafficking of p75NTR is driven by TrkA and regulates TrkA-mediated signalling

The topology and trafficking of receptors play a key role in their signalling capability. Indeed, receptor function is related to the microenvironment inside the cell, where specific signalling molecules are compartmentalized. The response to NGF (nerve growth factor) is strongly dependent on the trafficking of its receptor, TrkA. However, information is still scarce about the role of the cellular localization of the TrkA co-receptor, p75NTR (where NTR is neurotrophin receptor), following stimulation by NGF. It has been shown that these two receptors play a key role in epithelial tissue and in epithelial-derived tumours, where the microenvironment at the plasma membrane is defined by the presence of tight junctions. Indeed, in thyroid carcinomas, rearrangements of TrkA are frequently found, which produce TrkA mutants that are localized exclusively in the cytoplasm. We used a thyroid cellular model in which it was possible to dissect the trafficking of the two NGF receptors upon neurotrophin stimulation. In FRT (Fischer rat thyroid) cells, endogenous TrkA is localized exclusively on the basolateral surface, while transfected p75NTR is selectively distributed on the apical membrane. This cellular system enabled us to selectively stimulate either p75NTR or TrkA and to analyse the role of receptor trafficking in their signalling capability. We found that, after binding to NGF, p75NTR was co-immunoprecipitated with TrkA and was transcytosed at the basolateral membrane. We showed that the TrkA-p75NTR interaction is necessary for this relocation of p75NTR to the basolateral side. Interestingly, TrkA-specific stimulation by basolateral NGF loading also induced the TrkA-p75NTR interaction and subsequent p75NTR transcytosis at the basolateral surface. Moreover, specific stimulation of p75NTR by NGF activated TrkA and the MAPK (mitogen-activated protein kinase) pathway. Our data indicate that TrkA regulates the subcellular localization of p75NTR upon stimulation with neurotrophins, thus affecting the topology of the signal transduction molecules, driving the activation of a specific signal transduction pathway.

Long-lasting rescue of age-associated deficits in cognition and the CNS cholinergic phenotype by a partial agonist peptidomimetic ligand of TrkA

Previously, we developed a proteolytically stable small molecule peptidomimetic termed D3 as a selective ligand of the extracellular domain of the TrkA receptor for the NGF. Ex vivo D3 was defined as a selective, partial TrkA agonist. Here, the in vivo efficacy of D3 as a potential therapeutic for cholinergic neurons was tested in cognitively impaired aged rats, and we compared the consequence of partial TrkA activation (D3) versus full TrkA/p75 activation (NGF). We show that in vivo D3 binds to TrkA receptors and affords a significant and long-lived phenotypic rescue of the cholinergic phenotype both in the cortex and in the nucleus basalis. The cholinergic rescue was selective and correlates with a significant improvement of memory/learning in cognitively impaired aged rats. The effects of the synthetic ligand D3 and the natural ligand NGF were comparable. Small, proteolytically stable ligands with selective agonistic activity at a growth factor receptor may have therapeutic potential for neurodegenerative disorders.

The scaffold protein CNK1 interacts with the tumor suppressor RASSF1A and augments RASSF1A-induced cell death

The connector enhancer of KSR (CNK) is a multidomain scaffold protein discovered in Drosophila, where it is necessary for Ras activation of the Raf kinase. Recent studies have shown that CNK1 also interacts with RalA and Rho and participates in some aspects of signaling by these GTPases. Herein we demonstrate a novel aspect of CNK1 function, i.e. reexpression of CNK1 suppresses tumor cell growth and promotes apoptosis. As shown previously for apoptosis induced by Ki-Ras(G12V), CNK1-induced apoptosis is suppressed by a dominant inhibitor of the mammalian sterile 20 kinases 1 and (MST1/MST2). Immunoprecipitates of MST1 endogenous to LoVo colon cancer cells contain endogenous CNK1; however, no association of these two polypeptides can be detected in a yeast two-hybrid assay. CNK1 does, however, bind directly to the RASSF1A and RASSF1C polypeptides, constitutive binding partners of the MST1/2 kinases. Deletion of the MST1 carboxyl-terminal segment that mediates its binding to RASSF1A/C eliminates the association of MST1 with CNK1. Coexpression of CNK1 with the tumor suppressive isoform, RASSF1A, greatly augments CNK1-induced apoptosis, whereas the nonsuppressive RASSF1C isoform is without effect on CNK1-induced apoptosis. Overexpression of CNK1-(1-282), a fragment that binds RASSF1A but is not proapoptotic, blocks the apoptosis induced by CNK1 and by Ki-Ras(G12V). Thus, in addition to its positive role in the proliferative outputs of active Ras, the CNK1 scaffold protein, through its binding of a RASSF1A.MST complex, also participates in the proapoptotic signaling initiated by active Ras.

Apoptosis and Dependence Receptors: A Molecular Basis for Cellular Addiction

Bredesen, Dale E., Patrick Mehlen, and Shahrooz Rabizadeh. Apoptosis and Dependence Receptors: A Molecular Basis for Cellular Addiction. Physiol Rev 84: 411–430, 2004; 10.1152/physrev.00027.2003.—Classical signal transduction is initiated by ligand-receptor interactions. We have described an alternative form of signal transduction that is initiated by the withdrawal of ligands from specific receptors referred to as dependence receptors. This process is widespread, featuring in developmental cell death, carcinogenesis (especially metastasis), neurodegeneration, and possibly subapoptotic events such as neurite retraction and somal atrophy. Initial mechanistic studies of dependence receptors suggest that these receptors form complexes that include specific caspases. Complex formation appears to be a function of ligand-receptor interaction, and dependence receptors appear to exist in at least two conformational states. Complex formation in the absence of ligand leads to caspase activation by a mechanism that in at least some cases is dependent on caspase cleavage of the receptor itself, releasing proapoptotic peptides. Thus these receptors may serve in caspase amplification, and in so doing create cellular states of dependence on their respective ligands.

Meeting report: cellular dependence--old concept, new mechanisms

Programmed cell death occurs in response to both the presence of various extracellular factors and the lack of specific factors. Receptors that can mediate cell death in the absence of ligand binding are called dependence receptors, and they were the topic of the meeting held during the summer of 2003 in Fondation des Treilles, France. Not only is progress being made in the identification of new dependence receptors, but the partners that carry out this "negative" signal are also coming to light. With several of the receptors implicated in various human developmental disorders or disease states, gaining an understanding of the molecular mechanisms controlling dependence receptor-mediated cell death has clear clinical relevance.

Differential cross-regulation of TrkA and TrkC tyrosine kinase receptors with p75

The neurotrophins neurotrophin-3 (NT-3), brain-derived growth factor (BDNF) and nerve growth factor (NGF) bind to the p75 receptor, but each neurotrophin also binds a more selective Trk receptor (e.g. TrkA-NGF and TrkC-NT-3). The biochemical signals following engagement of either Trk or p75 with ligands are well understood, but long-term biological outcomes (trophic, proapoptotic or differentiative) remain unclear because they are cell/tissue specific. For example, Trk receptors are usually trophic but when overexpressed they can be proapoptotic in neuroblastomas and medulloblastomas. We hypothesized that coexpression of Trk and p75 receptors may lead to cross-regulation of signals and different biological outcomes; and used receptor-selective ligands to study cross-regulation by these receptors. We show that in the absence of Trk activation, expression of TrkC is permissive of p75 trophic and differentiation signals induced by p75 ligands, whereas expression of TrkA abolishes trophic and differentiation signals induced by p75 ligands. In contrast, in the presence of Trk activation, p75 ligands can regulate TrkA-mediated survival and TrkC-mediated differentiation. Therefore, a complex homeostasis of p75-selective and Trk-selective signals may determine the fate of cells expressing both receptors.

Ten years on: mediation of cell death by the common neurotrophin receptor p75(NTR)

The common neurotrophin receptor p75(NTR) remains one of the most enigmatic of the tumor necrosis factor receptor (TNFR) superfamily: on the one hand, it displays a death domain and has been shown to be capable of mediating programmed cell death (PCD) upon ligand binding; on the other hand, its death domain is of type II (unlike that of Fas or TNFR I), and it has also been shown to be capable of mediating cell death in response to the withdrawal of ligand. Thus, p75(NTR) may function as a death receptor-similar to Fas or TNFR I-or a dependence receptor-similar to deleted in colorectal cancer (DCC) or uncoordinated gene-5 homologues 1-3 (UNC5H1-3). Here, we review the data relating to the mediation of PCD by p75(NTR), and suggest that one reasonable model for the apparently paradoxical effects of p75(NTR) is that this receptor functions as a "quality control" in that it is capable of mediating PCD in at least four situations: (1). withdrawal of neurotrophins; (2). exposure to mismatched neurotrophins; (3). exposure to unprocessed neurotrophins; and (4). exposure of inappropriately immature cells to neurotrophins. Results to date suggest that these functions are mediated through different underlying mechanisms, and that their respective signaling pathways are cell type and co-receptor dependent.

Death-associated protein 4 binds MST1 and augments MST1-induced apoptosis

The protein kinase MST1 is proapoptotic when overexpressed in an active form, however, its physiologic regulation and cellular targets are unknown. An overexpressed inactive MST1 mutant associates in COS-7 cells with an endogenous 761-amino acid polypeptide known as "death-associated protein 4" (DAP4). The DAPs are a functionally heterogeneous array of polypeptides previously isolated by Kimchi and colleagues (Kimchi, A. (1998) Biochim. Biophys. Acta 1377, F13-F33 in a screen for elements involved in the interferon gamma-induced apoptosis of HeLa cells. DAP4, which is encoded by a member of a vertebrate-only gene family, contains no identifiable domains, but is identical over its amino-terminal 488 amino acids to p52(rIPK), a putative modulator of protein kinase R. DAP4 is a widely expressed, constitutively nuclear polypeptide that homodimerizes through its amino terminus and binds MST1 through its carboxyl-terminal segment. MST1 is predominantly cytoplasmic, but cycles continuously through the nucleus, as evidenced by its rapid accumulation in the nucleus after addition of the Crm1 inhibitor, leptomycin B. Overexpression of DAP4 does not cause apoptosis, however, coexpression of DAP4 with a submaximal amount of MST1 enhances MST1-induced apoptosis in a dose-dependent fashion. DAP4 is not significantly phosphorylated by MST1 nor does it alter MST1 kinase activity in vivo or in vitro. MST1-induced apoptosis is suppressed by a dominant interfering mutant of p53. MST1 is unable to directly phosphorylate p53, however, DAP4 binds endogenous and recombinant p53. DAP4 may promote MST1-induced apoptosis by enabling colocalization of MST with p53.

Gene expression profiling of mouse bladder inflammatory responses to LPS, substance P, and antigen-stimulation

Inflammatory bladder disorders such as interstitial cystitis (IC) deserve attention since a major problem of the disease is diagnosis. IC affects millions of women and is characterized by severe pain, increased frequency of micturition, and chronic inflammation. Characterizing the molecular fingerprint (gene profile) of IC will help elucidate the mechanisms involved and suggest further approaches for therapeutic intervention. Therefore, in the present study we used established animal models of cystitis to determine the time course of bladder inflammatory responses to antigen, Escherichia coli lipopolysaccharide (LPS), and substance P (SP) by morphological analysis and cDNA microarrays. The specific aim of the present study was to compare bladder inflammatory responses to antigen, LPS, and SP by morphological analysis and cDNA microarray profiling to determine whether bladder responses to inflammation elicit a specific universal gene expression response regardless of the stimulating agent. During acute bladder inflammation, there was a predominant infiltrate of polymorphonuclear neutrophils into the bladder. Time-course studies identified early, intermediate, and late genes that were commonly up-regulated by all three stimuli. These genes included: phosphodiesterase 1C, cAMP-dependent protein kinase, iNOS, beta-NGF, proenkephalin B and orphanin, corticotrophin-releasing factor (CRF) R, estrogen R, PAI2, and protease inhibitor 17, NFkB p105, c-fos, fos-B, basic transcription factors, and cytoskeleton and motility proteins. Another cluster indicated genes that were commonly down-regulated by all three stimuli and included HSF2, NF-kappa B p65, ICE, IGF-II and FGF-7, MMP2, MMP14, and presenilin 2. Furthermore, we determined gene profiles that identify the transition between acute and chronic inflammation. During chronic inflammation, the urinary bladder presented a predominance of monocyte/macrophage infiltrate and a concomitant increase in the expression of the following genes: 5-HT 1c, 5-HTR7, beta 2 adrenergic receptor, c-Fgr, collagen 10 alpha 1, mast cell factor, melanocyte-specific gene 2, neural cell adhesion molecule 2, potassium inwardly-rectifying channel, prostaglandin F receptor, and RXR-beta cis-11-retinoic acid receptor. We conclude that microarray analysis of genes expressed in the bladder during experimental inflammation may be predictive of outcome. Further characterization of the inflammation-induced gene expression profiles obtained here may identify novel biomarkers and shed light into the etiology of cystitis.

p75-nerve growth factor as an antiapoptotic complex: independence versus cooperativity in protection from enediyne chemotherapeutic agents

Growth factors, including nerve growth factor (NGF), have been hypothesized to play a role in resistance to chemotherapeutic agent-induced apoptosis. Induction by NGF of resistance to apoptosis is primarily thought to be the result of its binding to its high-affinity receptor, TrkA. The low-affinity NGF receptor, p75, has long been thought merely to facilitate NGF binding to TrkA. However, we have previously shown that the binding of NGF to its low-affinity receptor, p75, protects neuroblastoma cells that do not express TrkA against apoptosis induced by enediyne chemotherapeutic agents. In cells that express both receptors, it is not clear what determines which receptor is responsible for the protective effect of NGF. We now show that, in enediyne-treated SH-SY5Y neuroblastoma transfectants with native levels of p75 and a low TrkA/p75 ratio (1/100), the anti-apoptotic effect of NGF requires binding to p75. In contrast, in transfectants with native levels of p75 and a high TrkA/p75 ratio (100/100), NGF treatment prevents enediyne-induced apoptosis by a mechanism independent of p75 binding. Treatment of low TrkA/p75 ratio cells with NGF results in activation and nuclear translocation of NF-kappaB and tyrosine phosphorylation of TrkA. Analogous treatment of high TrkA/p75 ratio cells results only in phosphorylation of TrkA even though nuclear factor (NF)-kappaB signaling is not inactive and can be initiated by other ligands. The ratio of TrkA/p75 in cells that express both receptors probably contributes to the determination of which of the two known roles of p75 (i.e., TrkA independent or TrkA facilitatory) are responsible for NGF-mediated protection from enediyne-induced apoptosis.

Identification of a novel Ras-regulated proapoptotic pathway

BACKGROUND

The Ras-GTPase controls cell fate decisions through the binding of an array of effector molecules, such as Raf and PI 3-kinase, in a GTP-dependent manner. NORE1, a noncatalytic polypeptide, binds specifically to Ras-GTP and to several other Ras-like GTPases. NORE is homologous to the putative tumor suppressor RASSF1 and to the Caenorhabditis elegans polypeptide T24F1.3.

RESULTS

We find that all three NORE-related polypeptides bind selectively to the proapoptotic protein kinase MST1, a member of the Group II GC kinases. Endogenous NORE and MST1 occur in a constitutive complex in vivo that associates with endogenous Ras after serum stimulation. Targeting recombinant MST1 to the membrane, either through NORE or myristoylation, augments the apoptotic efficacy of MST1. Overexpression of constitutively active Ki-RasG12V promotes apoptosis in a variety of cell lines; Ha-RasG12V is a much less potent proapoptotic agent; however, a Ha-RasG12V effector loop mutant (E37G) that binds NORE, but not Raf or PI 3-kinase, exhibits proapoptotic efficacy approaching that of Ki-RasG12V. The apoptotic action of both Ki-RasG12V and Ha-RasG12V, E37G is suppressed by overexpression of the MST1 carboxy-terminal noncatalytic segment or by the NORE segment that binds MST1.

CONCLUSIONS

MST1 is a phylogenetically conserved partner of the NORE/RASSF polypeptide family, and the NORE-MST1 complex is a novel Ras effector unit that mediates the apoptotic effect of Ki-RasG12V.

Down-regulation of trkA mRNA within nucleus basalis neurons in individuals with mild cognitive impairment and Alzheimer's disease

Recent studies indicate that trkA expression is reduced in end-stage Alzheimer's disease (AD). However, understanding the neuropathologic correlates of early cognitive decline, as well as the changes that underlie the transition from nondemented mild cognitive impairment (MCI) to AD, are more critical neurobiological challenges. In these regards, the present study examined the expression of trkA mRNA in individuals diagnosed with MCI and AD from a cohort of people enrolled in a Religious Orders Study. Individuals with MCI and AD displayed significant reductions in trkA mRNA relative to aged-matched controls, indicating that alterations in trkA gene expression occur early in the disease process. The magnitude of change was similar in MCI and AD cases, suggesting that further loss of trkA mRNA is not necessarily associated with the transition of individuals from nondemented MCI to AD. The loss of trkA mRNA was not associated with education, apolipoprotein E allele status, gender, Braak score, global cognitive score or Mini-Mental Status Examination. In contrast, the loss of trkA mRNA in MCI and AD was significantly correlated with function on a variety of episodic memory tests.

Autocrine expression of neurotrophins and their receptors in prostate cancer

Previously, it has been demonstrated that the neurotrophins and their receptors are present in human prostate tissue, but neither their functional role nor localization is clearly understood. We studied the expression of neurotrophins and their receptors in prostate cancer. Between 1990 and 1999, 48 prostate cancer specimens were obtained from patients undergoing radical prostatectomy, of whom 25 received neoadjuvant hormonal therapy (NHT) and 23 were untreated. The specimens were analyzed immunohistochemically for neurotrophins (nerve growth factor, brain derived neurotrophic factor, neurotrophin 3, neurotrophin 4/5) and their receptors (TrkA, TrkB, TrkC, p75NTR). Immunohistochemical studies revealed that both benign and malignant prostate gland epithelial cells expressed the neurotrophins and their receptors to various degrees, but no obvious immunopositive reaction was observed in stromal cells. In benign epithelial cells, the neurotrophins were localized to secretory cells and the receptors were localized to basal cells. The neurotrophins, TrkA and TrkC were expressed to a similar extent in prostate cancer specimens obtained from patients both with and without NHT. In contrast, the expression of TrkB was down-regulated and the expression of p75NTR was up-regulated in prostate cancer after hormonal therapy. These findings suggest that neurotrophins are secreted by prostate cancer cells in an autocrine fashion. Neurotrophins may be involved, through their receptors, in the escape mechanism from cell death after androgen depletion found in prostate cancer.

The p75 neurotrophin receptor activates Akt (protein kinase B) through a phosphatidylinositol 3-kinase-dependent pathway

The Akt kinase plays a crucial role in supporting Trk-dependent cell survival, whereas the p75 neurotrophin receptor (p75NTR) facilitates cellular apoptosis. The precise mechanism that p75NTR uses to promote cell death is not certain, but one possibility is that p75NTR-dependent ceramide accumulation inhibits phosphatidylinositol 3-kinase-mediated Akt activation. To test this hypothesis, we developed a system for examining p75NTR-dependent apoptosis and determined the effect of p75NTR on Akt activation. Surprisingly, p75NTR increased, rather than decreased, Akt phosphorylation in a variety of cell types, including human Niemann-Pick fibroblasts, which lack acidic sphingomyelinase activity. The p75NTR expression level required to elicit Akt phosphorylation was much lower than that required to activate the JNK pathway or to mediate apoptosis. We show that p75NTR-dependent Akt phosphorylation was independent of TrkA signaling, required active phosphatidylinositol 3-kinase, and was associated with increased tyrosine phosphorylation of p85 and Shc and with reduced cytosolic tyrosine phosphatase activity. Finally, we show that p75NTR expression increased survival in cells exposed to staurosporine or subjected to serum withdrawal. These findings indicate that p75NTR facilitates cell survival through novel signaling cascades that result in Akt activation.

Nerve growth factor stimulates proliferation and survival of human breast cancer cells through two distinct signaling pathways

We show here that the neurotrophin nerve growth factor (NGF), which has been shown to be a mitogen for breast cancer cells, also stimulates cell survival through a distinct signaling pathway. Breast cancer cell lines (MCF-7, T47-D, BT-20, and MDA-MB-231) were found to express both types of NGF receptors: p140(trkA) and p75(NTR). The two other tyrosine kinase receptors for neurotrophins, TrkB and TrkC, were not expressed. The mitogenic effect of NGF on breast cancer cells required the tyrosine kinase activity of p140(trkA) as well as the mitogen-activated protein kinase (MAPK) cascade, but was independent of p75(NTR). In contrast, the anti-apoptotic effect of NGF (studied using the ceramide analogue C2) required p75(NTR) as well as the activation of the transcription factor NF-kB, but neither p140(trkA) nor MAPK was necessary. Other neurotrophins (BDNF, NT-3, NT-4/5) also induced cell survival, although not proliferation, emphasizing the importance of p75(NTR) in NGF-mediated survival. Both the pharmacological NF-kappaB inhibitor SN50, and cell transfection with IkBm, resulted in a diminution of NGF anti-apoptotic effect. These data show that two distinct signaling pathways are required for NGF activity and confirm the roles played by p75(NTR) and NF-kappaB in the activation of the survival pathway in breast cancer cells.

Phosphorylation of the common neurotrophin receptor p75 by p38beta2 kinase affects NF-kappaB and AP-1 activities

The signaling pathways invoked by ligand binding to the common neurotrophin receptor p75NTR are incompletely understood. Using the yeast two-hybrid system, we identified the mitogen-activated protein (MAP) kinase p38beta2 as a specific interactor with the 5th and 6th alpha helices of the p75NTR intracytoplasmic region. The consequences of this interaction were studied, using primary cultures of Schwann cells and the 293T cell line. Phosphorylation of p75NTR by p38beta2 was induced in vitro and in vivo by MAP kinase kinases (MKK) 6 activation. This pathway demonstrated feedback in that nerve growth factor (NGF) binding increased p38beta2 activity, causing an increase of nuclear factor-kappaB (NF-kappaB) activation and a decrease of AP-1 activation. The mechanisms described explain at least in part why NGF binding to p75NTR increases cell survival in certain circumstances.