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The p75 Neurotrophin Receptor Activates Akt (Protein Kinase B) through a Phosphatidylinositol 3-Kinase-dependent Pathway*

  • Philippe P. Roux
    Footnotes
    Affiliations
    From the Centre for Neuronal Survival, Montreal Neurological Institute, and the Department of Neurology and Neurosurgery, McGill University, Montréal, Québec H3A 2B4, Canada
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  • Asha L. Bhakar
    Footnotes
    Affiliations
    From the Centre for Neuronal Survival, Montreal Neurological Institute, and the Department of Neurology and Neurosurgery, McGill University, Montréal, Québec H3A 2B4, Canada
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  • Timothy E. Kennedy
    Footnotes
    Affiliations
    From the Centre for Neuronal Survival, Montreal Neurological Institute, and the Department of Neurology and Neurosurgery, McGill University, Montréal, Québec H3A 2B4, Canada
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  • Philip A. Barker
    Correspondence
    To whom correspondence should be addressed
    Affiliations
    From the Centre for Neuronal Survival, Montreal Neurological Institute, and the Department of Neurology and Neurosurgery, McGill University, Montréal, Québec H3A 2B4, Canada
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  • Author Footnotes
    * This work was supported in part by operating grants from the Canadian Institutes of Health Research (to P. A. B.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
    ‡ Supported by the Neuroscience Foundation of Canada and by a Canadian Institutes of Health Research studentship.
    § Supported by a Canadian Institutes of Health Research studentship.
    ¶ Canadian Institutes of Health Research Scholar.
Open AccessPublished:June 22, 2001DOI:https://doi.org/10.1074/jbc.M011520200
      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.
      NGF
      nerve growth factor
      p75NTR
      p75 neurotrophin receptor
      PI3K
      phosphatidylinositol 3-kinase
      PIP3
      phosphatidylinositol 3,4,5-trisphosphate
      TNFR
      tumor necrosis factor receptor
      JNK
      c-Jun N-terminal kinase
      PTPase
      protein-tyrosine phosphatase
      BCS
      bovine calf serum
      PBS
      phosphate-buffered saline
      FACS
      fluorescence-activated cell sorter
      m.o.i.
      multiplicity of infection
      CREB
      cAMP-responsive element-binding protein
      MKK
      mitogen-activated protein kinase kinase
      MAPK
      mitogen-activated protein kinase
      FAP-1
      Fas-associated phosphatase-1
      TRAF
      tumor necrosis factor receptor-associated factor
      The neurotrophins are a family of growth factors involved in the survival, development, and death of specific populations of neurons and non-neuronal cells. Nerve growth factor (NGF),1 the prototypic neurotrophin, is the best characterized member of this family, which in mammals, also includes brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4/5 (
      • Lewin G.R.
      • Barde Y.A.
      ). The signal transduction systems that mediate the diverse biological functions of the neurotrophins are initiated by two categories of cell-surface receptors: the Trk receptors and the p75 neurotrophin receptor (p75NTR).
      One of the main survival pathways for neuronal cell survival is mediated by phosphatidylinositol 3-kinase (PI3K) and involves activation of the Akt serine/threonine kinase (
      • Dudek H.
      • Datta S.R.
      • Franke T.F.
      • Birnbaum M.J.
      • Yao R.
      • Cooper G.M.
      • Segal R.A.
      • Kaplan D.R.
      • Greenberg M.E.
      ). Increased phosphatidylinositol 3,4,5-trisphosphate (PIP3) production results primarily from relocalization of PI3K from the cytosol to a juxtamembrane location that provides access to PIP substrates. This redistribution of PI3K requires the association of the SH2 domain within the p85 regulatory subunit of PI3K with phosphorylated tyrosines present on activated cell-surface receptors or on receptor-associated adaptor proteins (reviewed in Ref.
      • Kaplan D.R.
      • Miller F.D.
      ). Accumulation of PIP3 and its phospholipid phosphatase product, phosphatidylinositol 3,4-bisphosphate, in the plasma membrane creates docking sites for the pleckstrin homology domains of phosphoinositide-dependent kinase-1 and Akt. Phosphorylation of Akt on threonine 308 by phosphoinositide-dependent kinase-1 followed by autophosphorylation on serine 473 activates Akt (
      • Bellacosa A.
      • Chan T.O.
      • Ahmed N.N.
      • Datta K.
      • Malstrom S.
      • Stokoe D.
      • McCormick F.
      • Feng J.
      • Tsichlis P.
      ,
      • Toker A.
      • Newton A.C.
      ) and allows the enzyme to facilitate survival by phosphorylation of downstream substrates that may include Bad, Caspase-9, Forkhead family members, IκB kinase, and glycogen synthase kinase-3 (
      • Brunet A.
      • Bonni A.
      • Zigmond M.J.
      • Lin M.Z.
      • Juo P.
      • Hu L.S.
      • Anderson M.J.
      • Arden K.C.
      • Blenis J.
      • Greenberg M.E.
      ,
      • Kennedy S.G.
      • Kandel E.S.
      • Cross T.K.
      • Hay N.
      ,
      • Fujita E.
      • Jinbo A.
      • Matuzaki H.
      • Konishi H.
      • Kikkawa U.
      • Momoi T.
      ,
      • Tang Y.
      • Zhou H.
      • Chen A.
      • Pittman R.N.
      • Field J.
      ,
      • Datta S.R.
      • Dudek H.
      • Tao X.
      • Masters S.
      • Fu H.
      • Gotoh Y.
      • Greenberg M.E.
      ,
      • Ozes O.N.
      • Mayo L.D.
      • Gustin J.A.
      • Pfeffer S.R.
      • Pfeffer L.M.
      • Donner D.B.
      ,
      • Romashkova J.A.
      • Makarov S.S.
      ).
      p75NTR binds all neurotrophins with similar affinity and is a member of the tumor necrosis factor receptor (TNFR) superfamily (
      • Barker P.
      ,
      • Barrett G.L.
      ). Current data suggest that the main physiological functions of p75NTR are to regulate Trk receptor activation and signaling (
      • Barker P.A.
      • Shooter E.M.
      ,
      • Verdi J.M.
      • Birren S.J.
      • Ibanez C.F.
      • Persson H.
      • Kaplan D.R.
      • Benedetti M.
      • Chao M.V.
      • Anderson D.J.
      ,
      • Ryden M.
      • Hempstead B.
      • Ibanez C.F.
      ,
      • Brennan C.
      • Rivas-Plata K.
      • Landis S.C.
      ,
      • Bibel M.
      • Hoppe E.
      • Barde Y.A.
      ) and to activate Trk-independent signal transduction cascades involving sphingomyelinase (
      • Dobrowsky R.T.
      • Werner M.H.
      • Castellino A.M.
      • Chao M.V.
      • Hannun Y.A.
      ,
      • Dobrowsky R.T.
      • Jenkins G.M.
      • Hannun Y.A.
      ,
      • Brann A.B.
      • Scott R.
      • Neuberger Y.
      • Abulafia D.
      • Boldin S.
      • Fainzilber M.
      • Futerman A.H.
      ), nuclear factor-κB (
      • Carter B.D.
      • Kaltschmidt C.
      • Kaltschmidt B.
      • Offenhauser N.
      • Bohm M.R.
      • Baeuerle P.A.
      • Barde Y.A.
      ,
      • Bhakar A.L.
      • Roux P.P.
      • Lachance C.
      • Kryl D.
      • Zeindler C.
      • Barker P.A.
      ,
      • Khursigara G.
      • Orlinick J.R.
      • Chao M.V.
      ), and JNK (
      • Aloyz R.S.
      • Bamji S.X.
      • Pozniak C.D.
      • Toma J.G.
      • Atwal J.
      • Kaplan D.R.
      • Miller F.D.
      ,
      • Bamji S.X.
      • Majdan M.
      • Pozniak C.D.
      • Belliveau D.J.
      • Aloyz R.
      • Kohn J.
      • Causing C.G.
      • Miller F.D.
      ,
      • Yoon S.O.
      • Casaccia-Bonnefil P.
      • Carter B.
      • Chao M.V.
      ). Several findings indicate that NGF binding to p75NTR can initiate a cell death cascade in some cell types. For example, NGF treatment of embryonic retinal cells or postnatal oligodendrocytes that express p75NTR, but not TrkA, increases cellular apoptosis (
      • Casaccia-Bonnefil P.
      • Carter B.D.
      • Dobrowsky R.T.
      • Chao M.V.
      ,
      • Frade J.M.
      • Rodriguez-Tebar A.
      • Barde Y.A.
      ,
      • Frade J.M.
      • Barde Y.A.
      ). The precise signaling pathway(s) used by p75NTR to activate cell autonomous death cascades remain unclear, but may involve activation of caspase-1, -2, and -3 (
      • Gu C.
      • Casaccia-Bonnefil P.
      • Srinivasan A.
      • Chao M.V.
      ) as well as cyclin-dependent kinases (
      • Frade J.M.
      ). A number of cytosolic proteins that interact directly with the p75NTR intracellular domain have been identified, including TRAF proteins (
      • Khursigara G.
      • Orlinick J.R.
      • Chao M.V.
      ,
      • Ye X.
      • Mehlen P.
      • Rabizadeh S.
      • Van Arsdale T.
      • Zhang H.
      • Shin H.
      • Wang J.J.
      • Leo E.
      • Zapata J.
      • Hauser C.A.
      • Reed J.C.
      • Bredesen D.E.
      ), caveolin (
      • Bilderback T.R.
      • Grigsby R.J.
      • Dobrowsky R.T.
      ), SC-1 (
      • Chittka A.
      • Chao M.V.
      ), NRIF (
      • Casademunt E.
      • Carter B.D.
      • Benzel I.
      • Frade J.M.
      • Dechant G.
      • Barde Y.A.
      ), FAP-1 (
      • Irie S.
      • Hachiya T.
      • Rabizadeh S.
      • Maruyama W.
      • Mukai J.
      • Li Y.
      • Reed J.C.
      • Bredesen D.E.
      • Sato T.A.
      ), NADE (
      • Mukai J.
      • Hachiya T.
      • Shoji-Hoshino S.
      • Kimura M.T.
      • Nadano D.
      • Suvanto P.
      • Hanaoka T.
      • Li Y.
      • Irie S.
      • Greene L.A.
      • Sato T.A.
      ), RhoA (
      • Yamashita T.
      • Tucker K.L.
      • Barde Y.A.
      ), and NRAGE (
      • Salehi A.H.
      • Roux P.P.
      • Kubu C.J.
      • Zeindler C.
      • Bhakar A.
      • Tannis L.L.
      • Verdi J.M.
      • Barker P.A.
      ), but linking each of these to precise p75NTR signaling cascades remains a major challenge.
      Activation of cell death cascades can result from suppression of signaling pathways that normally support survival. In some systems, sphingomyelinase activation results in a ceramide-dependent decrease in the generation of PIP3 and a subsequent reduction in Akt activity (
      • Zundel W.
      • Giaccia A.
      ,
      • Zhou H.
      • Summers S.A.
      • Birnbaum M.J.
      • Pittman R.N.
      ); and in others, ceramide reduces Akt activity through specific dephosphorylation of serine 473 (
      • Schubert K.M.
      • Scheid M.P.
      • Duronio V.
      ). Since p75NTR activates sphingomyelinase in a neurotrophin-dependent manner, we have determined if p75NTR activation can suppress Akt and thereby facilitate apoptosis. Our results show that p75NTR does indeed regulate Akt; but contrary to our expectations, we found that p75NTR increases Akt activation through a Trk-independent pathway that requires PI3K and show that p75NTR expression suppresses apoptosis. Although high levels of p75NTR will mediate cell death, the p75NTR expression level required to elicit Akt phosphorylation is much lower than that required to activate the JNK pathway or to mediate apoptosis. The effect of p75NTR on Akt correlates with increased tyrosine phosphorylation of the p85 regulatory subunit of PI3K and of Shc adaptor proteins, suggesting that PTPase inhibition may play a role in this effect. Consistent with this, p75NTR expression results in reduced cytosolic tyrosine phosphatase activity. These data indicate that a physiological role of p75NTR is to enhance cell survival through an Akt-dependent pathway.

      DISCUSSION

      We have demonstrated that p75NTR expression leads to Trk-independent, PI3K-dependent Akt phosphorylation. High levels of p75NTR expression increase JNK and c-Jun phosphorylation and promote apoptosis, yet lower p75NTR expression levels are associated with activation of Akt and suppression of apoptosis induced by distinct stressors. p75NTR expression levels that potentiate Akt phosphorylation and survival increase the phosphotyrosine content of several cellular proteins, including p85 and Shc, suggesting that p75NTR affects the activity of tyrosine kinases or PTPases. Consistent with this, we demonstrate that p75NTR expression is associated with a decrease in cytosolic PTPase activity.
      Many studies have demonstrated that p75NTR can facilitate apoptosis. Our earlier work (
      • Majdan M.
      • Lachance C.
      • Gloster A.
      • Aloyz R.
      • Zeindler C.
      • Bamji S.
      • Bhakar A.
      • Belliveau D.
      • Fawcett J.
      • Miller F.D.
      • Barker P.A.
      ) has shown that overexpression of the p75NTR intracellular domain within neurons of transgenic mice results in dramatic loss of peripheral and central neurons, and Barde and co-workers (
      • Frade J.M.
      • Rodriguez-Tebar A.
      • Barde Y.A.
      ,
      • Frade J.M.
      • Barde Y.A.
      ) has shown that embryonic retinal cells that express p75NTR undergo cell death that can be prevented by the application of antibodies against either NGF or the p75NTR extracellular domain. Genetically altered mice rendered null at p75NTR or NGF loci show deficits in developmental apoptosis within the retina and spinal cord (
      • Frade J.M.
      • Barde Y.A.
      ), and p75NTR can facilitate apoptosis of cultured rat oligodendrocytes (
      • Casaccia-Bonnefil P.
      • Carter B.D.
      • Dobrowsky R.T.
      • Chao M.V.
      ) and sympathetic neurons (
      • Bamji S.X.
      • Majdan M.
      • Pozniak C.D.
      • Belliveau D.J.
      • Aloyz R.
      • Kohn J.
      • Causing C.G.
      • Miller F.D.
      ). The precise pathways that p75NTR activates to induce apoptosis are unclear, but JNK, caspase activation, and increased p53 levels have been observed in some systems (
      • Aloyz R.S.
      • Bamji S.X.
      • Pozniak C.D.
      • Toma J.G.
      • Atwal J.
      • Kaplan D.R.
      • Miller F.D.
      ,
      • Bamji S.X.
      • Majdan M.
      • Pozniak C.D.
      • Belliveau D.J.
      • Aloyz R.
      • Kohn J.
      • Causing C.G.
      • Miller F.D.
      ,
      • Yoon S.O.
      • Casaccia-Bonnefil P.
      • Carter B.
      • Chao M.V.
      ,
      • Gu C.
      • Casaccia-Bonnefil P.
      • Srinivasan A.
      • Chao M.V.
      ). To reliably activate p75NTR signaling cascades, we created recombinant adenoviruses that encode either full-length p75NTR or the p75NTR intracellular domain. As expected from our earlier work in transgenic mice (
      • Majdan M.
      • Lachance C.
      • Gloster A.
      • Aloyz R.
      • Zeindler C.
      • Bamji S.
      • Bhakar A.
      • Belliveau D.
      • Fawcett J.
      • Miller F.D.
      • Barker P.A.
      ), adenovirus-mediated overexpression of p75ICD resulted in cellular apoptosis that was associated with increased JNK activity and c-Jun phosphorylation. Expression of p75NTR or myristoylated p75ICD gave similar results, with p75mICD proving a particularly potent apoptotic inducer. These reagents will be useful for studies designed to identify specific signaling events in the p75NTR apoptotic cascade.
      Neurotrophin binding to p75NTR results in the activation of sphingomyelinase and the production of ceramide (
      • Dobrowsky R.T.
      • Werner M.H.
      • Castellino A.M.
      • Chao M.V.
      • Hannun Y.A.
      ,
      • Dobrowsky R.T.
      • Jenkins G.M.
      • Hannun Y.A.
      ,
      • Brann A.B.
      • Scott R.
      • Neuberger Y.
      • Abulafia D.
      • Boldin S.
      • Fainzilber M.
      • Futerman A.H.
      ,
      • Casaccia-Bonnefil P.
      • Carter B.D.
      • Dobrowsky R.T.
      • Chao M.V.
      ,
      • Blochl A.
      • Sirrenberg C.
      ). Ceramide generated by p75NTR activation may inhibit Trk receptor activation (
      • MacPhee I.J.
      • Barker P.A.
      ), activate JNK (
      • Casaccia-Bonnefil P.
      • Carter B.D.
      • Dobrowsky R.T.
      • Chao M.V.
      ,
      • Westwick J.K.
      • Bielawska A.E.
      • Dbaibo G.
      • Hannun Y.A.
      • Brenner D.A.
      ,
      • Verheij M.
      • van Blitterswijk W.J.
      • Bartelink H.
      ), and affect neuronal differentiation (
      • Brann A.B.
      • Scott R.
      • Neuberger Y.
      • Abulafia D.
      • Boldin S.
      • Fainzilber M.
      • Futerman A.H.
      ). In some systems, sphingomyelinase activation results in a ceramide-dependent decrease in PIP3 production and a subsequent reduction in Akt activity (
      • Zundel W.
      • Giaccia A.
      ,
      • Zhou H.
      • Summers S.A.
      • Birnbaum M.J.
      • Pittman R.N.
      ), and our initial hypothesis was that p75NTR-dependent ceramide accumulation would suppress PI3K activity and thereby reduce Akt activation. However, overexpression of p75NTR resulted in ligand-independent activation of Akt in multiple cell types. Indeed, Akt was activated even at p75NTR expression levels that facilitate apoptosis, indicating that when the apoptotic pathway is activated, it can override the pro-survival effect of Akt.
      The activation of Akt by p75NTR requires active PI3K and correlates with increases in the phosphotyrosine content of several proteins, including the adaptor protein Shc, the p85 regulatory subunit of PI3K, and a 120-kDa cell-surface protein. The increased phosphotyrosine content of these proteins correlates with reduced cytosolic PTPase activity in the presence of p75NTR. p75NTR-mediated inhibition of a cytosolic PTPase may therefore be a proximal event in the signaling cascade that may allow a Shc·PI3K complex to associate with the plasmalemma and increase PIP3 production. Additional studies will be required to identify specific phosphatase(s) inhibited by p75NTR, but one candidate is FAP, which physically interacts with p75NTR when overexpressed in 293T cells (
      • Irie S.
      • Hachiya T.
      • Rabizadeh S.
      • Maruyama W.
      • Mukai J.
      • Li Y.
      • Reed J.C.
      • Bredesen D.E.
      • Sato T.A.
      ). FAP also binds human Fas (
      • Sato T.
      • Irie S.
      • Kitada S.
      • Reed J.C.
      ), and Fas-mediated apoptosis can be suppressed by FAP (
      • Li Y.
      • Kanki H.
      • Hachiya T.
      • Ohyama T.
      • Irie S.
      • Tang G.
      • Mukai J.
      • Sato T.
      ,
      • Yanagisawa J.
      • Takahashi M.
      • Kanki H.
      • Yano-Yanagisawa H.
      • Tazunoki T.
      • Sawa E.
      • Nishitoba T.
      • Kamishohara M.
      • Kobayashi E.
      • Kataoka S.
      • Sato T.
      ) through an unknown mechanism. An alternative mechanism that may account for p75NTR-mediated Akt activation involves a link between TRAF proteins and Src kinases. The TRANCE receptor is a member of the TNFR superfamily that activates survival pathways in osteoclasts in part by activating Akt, and a recent study has found that a complex of Src kinase and TRAF-6 is required for this effect (
      • Wong B.R.
      • Besser D.
      • Kim N.
      • Arron J.R.
      • Vologodskaia M.
      • Hanafusa H.
      • Choi Y.
      ). p75NTR interacts with several members of the TRAF family, including TRAF-6 (
      • Khursigara G.
      • Orlinick J.R.
      • Chao M.V.
      ,
      • Ye X.
      • Mehlen P.
      • Rabizadeh S.
      • Van Arsdale T.
      • Zhang H.
      • Shin H.
      • Wang J.J.
      • Leo E.
      • Zapata J.
      • Hauser C.A.
      • Reed J.C.
      • Bredesen D.E.
      ), raising the possibility that this signaling path may also contribute to p75NTR-mediated Akt activation. Future experiments specifically examining FAP and Src signaling will be required to reveal the relative contributions of each of these pathways to p75NTR-mediated Akt activation.
      The signaling mechanisms employed by p75NTR are not well understood, and the relationship of neurotrophin binding to p75NTR action remains unclear. All neurotrophins activate sphingomyelinase when bound to p75NTR (
      • Dobrowsky R.T.
      • Werner M.H.
      • Castellino A.M.
      • Chao M.V.
      • Hannun Y.A.
      ,
      • Dobrowsky R.T.
      • Jenkins G.M.
      • Hannun Y.A.
      ), but only NGF is capable of inducing apoptosis and nuclear factor-κB activation in most systems (
      • Carter B.D.
      • Kaltschmidt C.
      • Kaltschmidt B.
      • Offenhauser N.
      • Bohm M.R.
      • Baeuerle P.A.
      • Barde Y.A.
      ,
      • Casaccia-Bonnefil P.
      • Carter B.D.
      • Dobrowsky R.T.
      • Chao M.V.
      ,
      • Frade J.M.
      • Rodriguez-Tebar A.
      • Barde Y.A.
      ). Paradoxically, some studies suggest that the receptor signals apoptosis when free of ligand and that this function is suppressed by ligand binding to p75NTR (
      • Rabizadeh S.
      • Ye X.
      • Wang J.J.
      • Bredesen D.E.
      ,
      • Rabizadeh S.
      • Oh J.
      • Zhong L.T.
      • Yang J.
      • Bitler C.M.
      • Butcher L.L.
      • Bredesen D.E.
      ). For our studies, we produced a group of recombinant adenovirus that would constitutively activate p75NTR signaling and thereby allow us to identify p75NTR cascades irrespective of ligand binding. All of the p75NTR constructs employed specifically activate the JNK pathway and mediate apoptosis when expressed at high levels, indicating that they are capable of activating p75NTR signaling pathways. The high expression levels required to induce apoptosis presumably reflect forced formation of a receptor signaling complex normally obtained in the presence of appropriate ligand (
      • Majdan M.
      • Lachance C.
      • Gloster A.
      • Aloyz R.
      • Zeindler C.
      • Bamji S.
      • Bhakar A.
      • Belliveau D.
      • Fawcett J.
      • Miller F.D.
      • Barker P.A.
      ). It is noteworthy that proximity of the intracellular domain to the plasma membrane appears important for activation of apoptosis by p75NTR since the p75mICD fragment elicits stronger apoptotic signaling than either p75NTR or p75ICD.
      Cytotoxic effects are observed when p75NTR is highly overexpressed, but much lower expression levels of full-length p75NTR and the intracellular domain mutants elicit Akt phosphorylation and enhance survival. The canonical view of p75NTR action is that receptor signaling is activated by ligand binding, but recent studies on other TNFR superfamily members suggest an alternative paradigm for p75NTR signaling. Lenardo and co-workers (
      • Chan F.K.
      • Chun H.J.
      • Zheng L.
      • Siegel R.M.
      • Bui K.L.
      • Lenardo M.J.
      ,
      • Siegel R.M.
      • Frederiksen J.K.
      • Zacharias D.A.
      • Chan F.K.
      • Johnson M.
      • Lynch D.
      • Tsien R.Y.
      • Lenardo M.J.
      ) have recently shown that some TNFR superfamily members must pre-assemble into cell-surface oligomers before binding ligand. Many investigators have observed oligomeric p75NTR in the absence of ligand in a variety of preparations (for an early example, see Ref.
      • Grob P.M.
      • Ross A.H.
      • Koprowski H.
      • Bothwell M.
      ), consistent with the possibility that p75NTR may also pre-assemble into oligomers. With Fas and TNFR-1, ligand binding produces a conformational shift that enables specific signaling events, but does not alter the oligomeric nature of the receptor complex. Thus, the role of ligand is to shift the pre-assembled receptor complex to a different signaling mode. All three p75NTR constructs employed in our studies elicited Akt activation when produced at low expression levels, but much higher levels of expression were required for apoptotic signaling. We favor the hypothesis that these signaling events reflect distinct p75NTR signaling complexes and that oligomeric p75NTR exists in at least two distinct signaling complexes; in the ligand-free state, p75NTR will constitutively activate survival pathways that involve Akt, and when bound by ligand, it activates signaling pathways that result in cellular apoptosis.
      In conclusion, we have identified a novel p75NTR signaling pathway that results in phosphorylation of Akt and that enhances cell survival. These data suggest that the autonomous signaling role of p75NTR may be broader than previously considered, with p75NTR capable of signaling pathways that support survival and death under different cellular circumstances.

      Acknowledgments

      We are grateful to Farid Arab Said and Sandra MacPherson for technical assistance with adenoviruses; Alain Boudreault for providing phosphatase assay reagents; Jane McGlade and David Kaplan for providing the anti-Shc and anti-TrkA antibodies, respectively; and Wayne Sossin and Sylvie La Boissière for careful reading of the manuscript.

      REFERENCES

        • Lewin G.R.
        • Barde Y.A.
        Annu. Rev. Neurosci. 1996; 19: 289-317
        • Dudek H.
        • Datta S.R.
        • Franke T.F.
        • Birnbaum M.J.
        • Yao R.
        • Cooper G.M.
        • Segal R.A.
        • Kaplan D.R.
        • Greenberg M.E.
        Science. 1997; 275: 661-665
        • Kaplan D.R.
        • Miller F.D.
        Curr. Opin. Neurobiol. 2000; 10: 381-391
        • Bellacosa A.
        • Chan T.O.
        • Ahmed N.N.
        • Datta K.
        • Malstrom S.
        • Stokoe D.
        • McCormick F.
        • Feng J.
        • Tsichlis P.
        Oncogene. 1998; 17: 313-325
        • Toker A.
        • Newton A.C.
        J. Biol. Chem. 2000; 275: 8271-8274
        • Brunet A.
        • Bonni A.
        • Zigmond M.J.
        • Lin M.Z.
        • Juo P.
        • Hu L.S.
        • Anderson M.J.
        • Arden K.C.
        • Blenis J.
        • Greenberg M.E.
        Cell. 1999; 96: 857-868
        • Kennedy S.G.
        • Kandel E.S.
        • Cross T.K.
        • Hay N.
        Mol. Cell. Biol. 1999; 19: 5800-5810
        • Fujita E.
        • Jinbo A.
        • Matuzaki H.
        • Konishi H.
        • Kikkawa U.
        • Momoi T.
        Biochem. Biophys. Res. Commun. 1999; 264: 550-555
        • Tang Y.
        • Zhou H.
        • Chen A.
        • Pittman R.N.
        • Field J.
        J. Biol. Chem. 2000; 275: 9106-9109
        • Datta S.R.
        • Dudek H.
        • Tao X.
        • Masters S.
        • Fu H.
        • Gotoh Y.
        • Greenberg M.E.
        Cell. 1997; 91: 231-241
        • Ozes O.N.
        • Mayo L.D.
        • Gustin J.A.
        • Pfeffer S.R.
        • Pfeffer L.M.
        • Donner D.B.
        Nature. 1999; 401: 82-85
        • Romashkova J.A.
        • Makarov S.S.
        Nature. 1999; 401: 86-90
        • Barker P.
        Cell Death Differ. 1998; 5: 346-356
        • Barrett G.L.
        Prog. Neurobiol. 2000; 61: 205-229
        • Barker P.A.
        • Shooter E.M.
        Neuron. 1994; 13: 203-215
        • Verdi J.M.
        • Birren S.J.
        • Ibanez C.F.
        • Persson H.
        • Kaplan D.R.
        • Benedetti M.
        • Chao M.V.
        • Anderson D.J.
        Neuron. 1994; 12: 733-745
        • Ryden M.
        • Hempstead B.
        • Ibanez C.F.
        J. Biol. Chem. 1997; 272: 16322-16328
        • Brennan C.
        • Rivas-Plata K.
        • Landis S.C.
        Nat. Neurosci. 1999; 2: 699-705
        • Bibel M.
        • Hoppe E.
        • Barde Y.A.
        EMBO J. 1999; 18: 616-622
        • Dobrowsky R.T.
        • Werner M.H.
        • Castellino A.M.
        • Chao M.V.
        • Hannun Y.A.
        Science. 1994; 265: 1596-1598
        • Dobrowsky R.T.
        • Jenkins G.M.
        • Hannun Y.A.
        J. Biol. Chem. 1995; 270: 22135-22142
        • Brann A.B.
        • Scott R.
        • Neuberger Y.
        • Abulafia D.
        • Boldin S.
        • Fainzilber M.
        • Futerman A.H.
        J. Neurosci. 1999; 19: 8199-8206
        • Carter B.D.
        • Kaltschmidt C.
        • Kaltschmidt B.
        • Offenhauser N.
        • Bohm M.R.
        • Baeuerle P.A.
        • Barde Y.A.
        Science. 1996; 272: 542-545
        • Bhakar A.L.
        • Roux P.P.
        • Lachance C.
        • Kryl D.
        • Zeindler C.
        • Barker P.A.
        J. Biol. Chem. 1999; 274: 21443-21449
        • Khursigara G.
        • Orlinick J.R.
        • Chao M.V.
        J. Biol. Chem. 1999; 274: 2597-2600
        • Aloyz R.S.
        • Bamji S.X.
        • Pozniak C.D.
        • Toma J.G.
        • Atwal J.
        • Kaplan D.R.
        • Miller F.D.
        J. Cell Biol. 1998; 143: 1691-1703
        • Bamji S.X.
        • Majdan M.
        • Pozniak C.D.
        • Belliveau D.J.
        • Aloyz R.
        • Kohn J.
        • Causing C.G.
        • Miller F.D.
        J. Cell Biol. 1998; 140: 911-923
        • Yoon S.O.
        • Casaccia-Bonnefil P.
        • Carter B.
        • Chao M.V.
        J. Neurosci. 1998; 18: 3273-3281
        • Casaccia-Bonnefil P.
        • Carter B.D.
        • Dobrowsky R.T.
        • Chao M.V.
        Nature. 1996; 383: 716-719
        • Frade J.M.
        • Rodriguez-Tebar A.
        • Barde Y.A.
        Nature. 1996; 383: 166-168
        • Frade J.M.
        • Barde Y.A.
        Development. 1999; 126: 683-690
        • Gu C.
        • Casaccia-Bonnefil P.
        • Srinivasan A.
        • Chao M.V.
        J. Neurosci. 1999; 19: 3043-3049
        • Frade J.M.
        J. Cell Sci. 2000; 113: 1139-1148
        • Ye X.
        • Mehlen P.
        • Rabizadeh S.
        • Van Arsdale T.
        • Zhang H.
        • Shin H.
        • Wang J.J.
        • Leo E.
        • Zapata J.
        • Hauser C.A.
        • Reed J.C.
        • Bredesen D.E.
        J. Biol. Chem. 1999; 274: 30202-30208
        • Bilderback T.R.
        • Grigsby R.J.
        • Dobrowsky R.T.
        J. Biol. Chem. 1997; 272: 10922-10927
        • Chittka A.
        • Chao M.V.
        Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 10705-10710
        • Casademunt E.
        • Carter B.D.
        • Benzel I.
        • Frade J.M.
        • Dechant G.
        • Barde Y.A.
        EMBO J. 1999; 18: 6050-6061
        • Irie S.
        • Hachiya T.
        • Rabizadeh S.
        • Maruyama W.
        • Mukai J.
        • Li Y.
        • Reed J.C.
        • Bredesen D.E.
        • Sato T.A.
        FEBS Lett. 1999; 460: 191-198
        • Mukai J.
        • Hachiya T.
        • Shoji-Hoshino S.
        • Kimura M.T.
        • Nadano D.
        • Suvanto P.
        • Hanaoka T.
        • Li Y.
        • Irie S.
        • Greene L.A.
        • Sato T.A.
        J. Biol. Chem. 2000; 275: 17566-17570
        • Yamashita T.
        • Tucker K.L.
        • Barde Y.A.
        Neuron. 1999; 24: 585-593
        • Salehi A.H.
        • Roux P.P.
        • Kubu C.J.
        • Zeindler C.
        • Bhakar A.
        • Tannis L.L.
        • Verdi J.M.
        • Barker P.A.
        Neuron. 2000; 27: 279-288
        • Zundel W.
        • Giaccia A.
        Genes Dev. 1998; 12: 1941-1946
        • Zhou H.
        • Summers S.A.
        • Birnbaum M.J.
        • Pittman R.N.
        J. Biol. Chem. 1998; 273: 16568-16575
        • Schubert K.M.
        • Scheid M.P.
        • Duronio V.
        J. Biol. Chem. 2000; 275: 13330-13335
        • Radeke M.J.
        • Misko T.P.
        • Hsu C.
        • Herzenberg L.A.
        • Shooter E.M.
        Nature. 1987; 325: 593-597
        • Majdan M.
        • Lachance C.
        • Gloster A.
        • Aloyz R.
        • Zeindler C.
        • Bamji S.
        • Bhakar A.
        • Belliveau D.
        • Fawcett J.
        • Miller F.D.
        • Barker P.A.
        J. Neurosci. 1997; 17: 6988-6998
        • Robbins S.M.
        • Quintrell N.A.
        • Bishop J.M.
        Mol. Cell. Biol. 1995; 15: 3507-3515
        • Roux P.P.
        • Colicos M.A.
        • Barker P.A.
        • Kennedy T.E.
        J. Neurosci. 1999; 19: 6887-6896
        • Laemmli U.K.
        Nature. 1970; 227: 680-685
        • Boldin M.
        • Mett I.
        • Varfolomeev E.
        • Chumakov I.
        • Shemeravni Y.
        • Camonis J.
        • Wallach D.
        J. Biol. Chem. 1995; 270: 387-391
        • Cochran B.H.
        Natl. Inst. Drug Abuse Res. Monogr. 1993; 125: 3-24
        • Mahadeo D.
        • Kaplan L.
        • Chao M.V.
        • Hempstead B.L.
        J. Biol. Chem. 1994; 269: 6884-6891
        • Liu L.
        • Damen J.E.
        • Ware M.
        • Hughes M.
        • Krystal G.
        Leukemia ( Baltimore ). 1997; 11: 181-184
        • Grob P.M.
        • Ross A.H.
        • Koprowski H.
        • Bothwell M.
        J. Biol. Chem. 1985; 260: 8044-8049
        • Frade J.M.
        • Barde Y.A.
        Neuron. 1998; 20: 35-41
        • Blochl A.
        • Sirrenberg C.
        J. Biol. Chem. 1996; 271: 21100-21107
        • MacPhee I.J.
        • Barker P.A.
        J. Biol. Chem. 1997; 272: 23547-23551
        • Westwick J.K.
        • Bielawska A.E.
        • Dbaibo G.
        • Hannun Y.A.
        • Brenner D.A.
        J. Biol. Chem. 1995; 270: 22689-22692
        • Verheij M.
        • van Blitterswijk W.J.
        • Bartelink H.
        Acta Oncol. 1998; 37: 575-581
        • Sato T.
        • Irie S.
        • Kitada S.
        • Reed J.C.
        Science. 1995; 268: 411-415
        • Li Y.
        • Kanki H.
        • Hachiya T.
        • Ohyama T.
        • Irie S.
        • Tang G.
        • Mukai J.
        • Sato T.
        Int. J. Cancer. 2000; 87: 473-479
        • Yanagisawa J.
        • Takahashi M.
        • Kanki H.
        • Yano-Yanagisawa H.
        • Tazunoki T.
        • Sawa E.
        • Nishitoba T.
        • Kamishohara M.
        • Kobayashi E.
        • Kataoka S.
        • Sato T.
        J. Biol. Chem. 1997; 272: 8539-8545
        • Wong B.R.
        • Besser D.
        • Kim N.
        • Arron J.R.
        • Vologodskaia M.
        • Hanafusa H.
        • Choi Y.
        Mol. Cell. 1999; 4: 1041-1049
        • Rabizadeh S.
        • Ye X.
        • Wang J.J.
        • Bredesen D.E.
        Cell Death Differ. 1999; 6: 1222-1227
        • Rabizadeh S.
        • Oh J.
        • Zhong L.T.
        • Yang J.
        • Bitler C.M.
        • Butcher L.L.
        • Bredesen D.E.
        Science. 1993; 261: 345-348
        • Chan F.K.
        • Chun H.J.
        • Zheng L.
        • Siegel R.M.
        • Bui K.L.
        • Lenardo M.J.
        Science. 2000; 288: 2351-2354
        • Siegel R.M.
        • Frederiksen J.K.
        • Zacharias D.A.
        • Chan F.K.
        • Johnson M.
        • Lynch D.
        • Tsien R.Y.
        • Lenardo M.J.
        Science. 2000; 288: 2354-2357