title: Detection of p75NTR Trimers: Implications for Receptor Stoichiometry and Activation.
abstract: The p75 neurotrophin receptor (p75(NTR)) is a multifunctional receptor that participates in many critical processes in the nervous system, ranging from apoptosis to synaptic plasticity and morphological events. It is a member of the tumor necrosis factor receptor (TNFR) superfamily, whose members undergo trimeric oligomerization. Interestingly, p75(NTR) interacts with dimeric ligands (i.e., proneurotrophins or mature neurotrophins), but several of the intracellular adaptors that mediate p75(NTR) signaling are trimeric (i.e., TNFR-associated factor 6 or TRAF6). Consequently, the active receptor signaling unit remains uncertain. To identify the functional receptor complex, we evaluated its oligomerization in vitro and in mice brain tissues using a combination of biochemical techniques. We found that the most abundant homotypic arrangement for p75(NTR) is a trimer and that monomers and trimers coexist at the cell surface. Interestingly, trimers are not required for ligand-independent or ligand-dependent p75(NTR) activation in a growth cone retraction functional assay. However, monomers are capable of inducing acute morphological effects in neurons. We propose that p75(NTR) activation is regulated by its oligomerization status and its levels of expression. These results indicate that the oligomeric state of p75(NTR) confers differential responses and offers an explanation for the diverse and contradictory actions of this receptor in the nervous system.
  
  
  CONCLUSIONS
  The p75 neurotrophin receptor (p75(NTR)) regulates a wide range of cellular functions, including apoptosis, neuronal processes remodeling, and synaptic plasticity. The goal of our work was to inquire whether oligomers of the receptor are required for function. Here we report that p75(NTR) predominantly assembles as a trimer, similar to other tumor necrosis factor receptors. Interestingly, monomers and trimers coexist at the cell surface, but trimers are not required for p75(NTR) activation in a functional assay. However, monomers are capable of inducing acute morphological effects in neurons. Identification of the oligomerization state of p75(NTR) begins to provide insights to the mechanisms of signal initiation of this noncatalytic receptor, as well as to develop therapeutic interventions to diminish its activity.
authors: Department of Medicine, Weill Cornell Medical College of Cornell University, New York, New York 10065, Instituto de Investigación Médica Mercedes y Martín Ferreyra, Universidad Nacional de Córdoba, Córdoba, Córdoba 5016, Argentina, aanastasia@immf.uncor.edu blhempst@med.cornell.edu.
authors: Agustin
authors: http://orcid.org/0000-0001-7238-9041
authors: A
authors: Anastasia
authors: Montreal Neurological Institute, McGill University, Montreal, Quebec H3A 2B4, Canada, and.
authors: Phillip A
authors: http://orcid.org/0000-0002-3444-7551
authors: PA
authors: Barker
authors: Skirball Institute, New York University School of Medicine, New York, New York 10016.
authors: Moses V
authors: MV
authors: Chao
authors: Department of Medicine, Weill Cornell Medical College of Cornell University, New York, New York 10065, aanastasia@immf.uncor.edu blhempst@med.cornell.edu.
authors: Barbara L
authors: BL
authors: Hempstead
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000378:metabolism; D005260:Female; D057809:HEK293 Cells; D006624:Hippocampus / Q000378:metabolism; D006801:Humans; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D009419:Nerve Tissue Proteins / Q000096:biosynthesis* / Q000737:chemistry* / Q000235:genetics; D016716:PC12 Cells; D051381:Rats; D017475:Receptors, Nerve Growth Factor / Q000096:biosynthesis* / Q000737:chemistry* / Q000235:genetics; D013237:Stereoisomerism
chemical_list: C484065:NGFR protein, human; D009419:Nerve Tissue Proteins; D017475:Receptors, Nerve Growth Factor
keywords: TNF receptor; p75NTR; proNGF; signaling unit; stoichiometry; trimerization