Overexpression of a functional NMDA receptor subunit (NMDAR1) in baculovirus-infected Trichoplusia ni insect cells

Identification and characterization of the NMDA receptor and its role in regulating reproduction in the cockroach Diploptera punctata

The NMDA receptor (NMDAR) plays important roles in excitatory neurotransmission and in the regulation of reproduction in mammals. NMDAR in insects comprises two subunits, NR1 and NR2. In this study, we identified two NR1 paralogs and eleven NR2 alternatively spliced variants in the cockroach Diploptera punctata. This is the first report of NR1 paralogs in insects. The tissue distributions and expression profiles of DpNR1A, DpNR1B and DpNR2 in different tissues were also investigated. Previous studies have demonstrated NMDA-stimulated biosynthesis of juvenile hormone (JH) in the corpora allata through the influx of extracellular Ca2+ in Diploptera punctata. However, our data show that the transcript levels of DpNR1A, DpNR1B and DpNR2 were low in the corpora allata. MK-801, a high-affinity antagonist of NMDAR, did not show any effect on JH biosynthesis in vitro. In addition, neither partial knockdown of DpNR2 nor in vivo treatment with a physiologically relevant dose of MK-801 resulted in any significant change in JH biosynthesis or basal oocyte growth. Injection of animals with a high dose of MK-801 (30 µg per animal per injection), which paralyzed the animals for 4–5 h, resulted in a significant decrease in JH biosynthesis on days 4 and 5. However, the reproductive events during the first gonadotrophic cycle in female D. punctata were unaffected. Thus, NMDAR does not appear to play important roles in the regulation of JH biosynthesis or mediate reproduction of female D. punctata.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

G protein-coupled receptor overexpression with the baculovirus-insect cell system: a tool for structural and functional studies

G protein-coupled receptors, whose topology shows seven transmembrane domains, form the largest known family of receptors involved in higher organism signal transduction. These receptors are generally of low natural abundance and overexpression is usually a prerequisite to their structural or functional characterisation. The baculovirus-insect cell system constitutes a versatile tool for the maximal production of receptors. This heterologous expression system also provides interesting alternatives for receptor functional studies in a well-controlled cellular context.

Pharmacological characterization of recombinant rat corticotropin releasing factor binding protein using different sauvagine analogs

Little is known on the structural ligand requirements for corticotropin-releasing factor binding protein (CRFBP) of the rat used as an important experimental animal. To obtain such information recombinant rat CRFBP was produced in stably transfected HEK 293 cells. The primary structure and posttranslational processing of purified rat CRFBP was established by peptide mapping using HPLC combined with mass spectrometric analysis. Rat CRFBP was pharmacologically characterized employing a competition binding assay with tritium-labeled rat urocortin. The rank order of declining affinity of various CRF analogs was urotensin-I, human/rat CRF (h/rCRF), rat urocortin, sauvagine (Svg), and ovine CRF in agreement with the rank order found for human CRFBP. In contrast to astressin, the CRF receptor 2-selective antagonist anti-sauvagine-30 did not show any detectable specific binding to rat CRFBP. The significance of residues 10 to 12 and 21 to 24 of Svg for its low affinity binding was established by changing these residues of Svg to those of h/rCRF. The corresponding residues 22 to 25 of h/rCRF represented the ARAE motif determined to be crucial for binding in agreement with reported data on human CRFBP. Residues 11 to 13 of CRF introduced into Svg also enhanced the affinity to rat CRFBP.

Characterization of native corticotropin-releasing factor receptor type 1 (CRFR1) in the rat and mouse central nervous system

Corticotropin releasing factor (CRF), the most important regulator of various responses to stress, acts through CRF receptors (CRFR). For their characterization in brain tissue of Sprague-Dawley rats and C57BL/6J mice, a recently described polyclonal antibody directed against the N-terminus of rat CRFR1 (rCRFR1) was used. The molecular weights of rat and mouse brain receptors were determined by Western blot analysis to be 80,000-76,000 and 83,000-79,000, respectively, whereas molecular weights of 72,000-59,000 were observed for CRFR1 from rat and mouse pituitary. Immunohistochemical analysis was performed with brain sections of naive rats and mice. Strong CRFR1 staining was detected in the cortex, cerebellum, mesencephalon and pons of both species, whereas weak staining was observed in amygdala and hippocampus. The striatum did not show immunoreactivity. The density of immunostaining was significantly lower in murine than in rat cortex. In contrast, in the pons and mesencephalon of mice, higher density of immunostaining was observed than in the same brain structures of rats. On the basis of the observed differences, it is suggested that CRFR1 is differentially processed in rats and mice. In addition, the density of CRFR1 staining differed between both species.

Structure-function relationship of different domains of the rat corticotropin-releasing factor receptor

The significance of different domains of corticotropin-releasing factor receptor, type 1, (CRFR1) for ligand binding and cAMP accumulation was investigated with C-terminally truncated forms of rat CRFR1 (rCRFR1) tagged by a sequence of six histidine residues (His-tag) to facilitate protein purification and identification. These different forms of the receptor were N-glycosylated and transported properly to the membranes of transfected mammalian cells as indicated by Western blot analysis and immunocytochemical staining with two polyclonal antibodies developed against the N- and C-terminus of rCRFR1. The N-terminal fragment, rCRFR1(23-121), expressed in Escherichia coli bound oCRF specifically, but with low affinity. Several mutants lacking transmembrane domain (TM) 7 and the C-terminus exhibited similarly low affinities to oCRF after expression in transfected mammalian cells. None of these cells produced significant amounts of cAMP after exposure to oCRF. Only mutants containing the N-terminus, all loops and TMs bound oCRF and produced cAMP with high affinity (Kd = 62 nM) and efficacy (EC50 = 0.8 nM). The additional presence of the C-terminus provided similar characteristics (Kd = 5 nM, EC50 = 0.3 nM) as known for the native receptor. It is suggested on the basis of these data that the last extracellular loop is involved in ligand binding.

Quantitative evaluation of neurotensin receptor purification by immobilized metal affinity chromatography

Immobilized metal affinity chromatography has recently been used for purification of histidine-tagged membrane proteins in the presence of detergents with varying success. Strong binding to the metal resin is essential for purification when expression levels are low. We have investigated the influence of tag length and type of detergent on the purification of a neurotensin receptor fusion protein expressed in Escherichia coli at a level of about 0.1% of membrane protein. Receptors with six C-terminal histidine residues did not bind to nickel resin in the presence of the anionic detergent sodium dodecyl sulfate. In contrast, partial purification assessed by densitometry of Coomassie-stained gels was achieved using the nonionic detergents dodecyl maltoside or Triton X-100 (53% pure), or a detergent mixture containing the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (46% pure). Linking a highly charged epitope tag to the histidine tail did not affect the nickel-binding properties of receptors. The level of purification was substantially improved (72% pure) by extending the histidine tail to 10 residues because this allowed stringent washes at high imidazole concentration to remove nonspecifically bound contaminants. This strategy not only resulted in efficient purification of receptors from crude membranes, but also worked particularly well for single-step purification from total cell lysates, resulting in 340-fold purification of functional neurotensin receptor.