Neuronal Pentraxin Receptor, a Novel Putative Integral Membrane Pentraxin That Interacts with Neuronal Pentraxin 1 and 2 and Taipoxin-associated Calcium-binding Protein 49

Abstract

We have identified the first putative integral membrane pentraxin and named it neuronal pentraxin receptor (NPR). NPR is enriched by affinity chromatography on columns of a snake venom toxin, taipoxin, and columns of the taipoxin-binding proteins neuronal pentraxin 1 (NP1), neuronal pentraxin 2 (NP2), and taipoxinassociated calcium-binding protein 49 (TCBP49). The predominant form of NPR contains an putative NH 2 -terminal transmembrane domain and all forms of NPR are glycosylated. NPR has 49 and 48% amino acid identity to NP1 and NP2, respectively, and NPR message is expressed in neuronal regions that express NP1 and NP2. We suggest that NPR, NP1, NP2, and TCBP49 are involved in a pathway responsible for the transport of taipoxin into synapses and that this may represent a novel neuronal uptake pathway involved in the clearance of synaptic debris.We identified two taipoxin binding proteins for a presynaptic-acting snake venom neurotoxin, taipoxin, that blocks recycling of synaptic vesicles (1, 2). Affinity chromatography of solubilized rat brain membranes on columns of immobilized taipoxin enriches two major proteins: (i) neuronal pentraxin 1 (NP1), 1 a neuronally secreted protein with homology to serum pentraxins (2), and (ii) taipoxin-associated calcium-binding protein 49 (TCBP49), a reticular calcium-binding protein (3). NP1 has homology to previously identified pentraxins, such as serum amyloid P protein and C-reactive protein, which are elevated in the serum during acute phase response. Although the exact functions of these previously identified pentraxins are not known, they have been shown to bind, in a calcium-dependent manner, a wide variety of ligands and have been proposed to mediate the uptake of bacteria, toxins, and extracellular debris (4, 5). Homology to serum pentraxins, as well as the presence of a cleaved signal peptide and N-linked glycosylation sites, suggests that NP1 is secreted. The abundance of NP1 mRNA and rarity of NP1 protein suggest that NP1 protein has a rapid turnover. We have proposed that NP1 has a role in uptake at the synapse and that NP1 mediates the uptake of taipoxin into neurons. By low stringency screening, we identified an additional neuronal pentraxin (NP2) in human that has 54% amino acid identity with NP1 and is expressed in brain and multiple other tissues (6). Potential homologs of NP2 have been identified in guinea pig as a sperm acrosomal protein, apexin/p50 (7,8), and in rat as a neural activity-regulated pentraxin, narp (9). The second taipoxin-binding protein, TCBP49, binds calcium via six EF-hand calcium binding motifs and is localized to the lumen of reticular membranes in neurons and glia (3). It contains the carboxyl-terminal sequence HDEL which has been shown to occasionally mediate endoplasmic reticulum retention in mammalian cells (10 -12). We have suggested that NP1 binds to synaptic material and is taken up into a compartment containing TCBP49 (2, 3). We have also suggested that NP1 allows the internalization of taipoxin or a taipoxin⅐NP1 complex and t...

Regulation of AMPA receptors and synaptic plasticity

“…This family consists of neuronal-activity-regulated pentraxin (Narp) and neuronal pentraxin 1 (NP1), both secreted proteins homologous to the serum pentraxins (Schlimgen et al, 1995;Tsui et al, 1996), and neuronal pentraxin receptor (NPR), an integral membrane protein (Dodds et al, 1997). Narp and NP1 coimmunoprecipitate with AMPARs in heterologous cells (Xu et al, 2003), and Narp-expressing HEK cells seeded on neurons recruit GluR1 to sites where they contact neuronal dendrites .…”

Biochemical Interactions of the Neuronal Pentraxins

“…NP1, NP2, NPR, and TCBP49 were isolated by affinity chromatography of solubilized brain membranes on columns of immobilized taipoxin. NP1, NP2, and NPR define a family of neuronal pentraxins that are 50% identical to each other (3). The Nterminal half of neuronal pentraxins are 20 -30% identical to previously identified pentraxins, such as serum amyloid P protein and C-reactive protein, which are elevated in the serum during acute phase response (5,6).…”

“…Immunofluorescence microscopy shows TCBP49 to be localized to an internal reticular compartment in neurons and glia. NP1, NP2, and NPR can all be isolated on taipoxin columns, NP1 and NP2 columns, and TCBP49 columns, suggesting that they interact in vivo (3). These interactions are likely to mediate aspects of taipoxin toxicity in neurons and are indicative of a normal neuronal pathway that may be involved in the uptake of extracellular material.…”

“…We identified four novel proteins, NP1, 1 NP2, NPR, and TCBP49, that bind to affinity columns of the presynaptic snake venom neurotoxin taipoxin (1)(2)(3)(4). The action of this toxin in blocking synaptic transmission and synaptic vesicle recycling led us to study the interactions between these taipoxin-binding proteins, their role in the action of taipoxin at synapses, and their potential role in general neuronal uptake.…”

“…Based on cDNA sequence, NP1 and NP2 are predicted to be secreted proteins (1,2), whereas NPR contains a putative membrane-spanning hydrophobic domain (3), raising the possibility that it is present on the neuronal cell surface. The three neuronal pentraxins are significantly larger than the classical pentraxins (Ͼ50 versus 30 kDa), suggesting that they may have additional novel functions.…”