Spiperone: a receptor ligand and/or a granular uptake tracer?

The accumulation of [³H]spiperone and [³H]dopamine was measured in striatum and pituitary gland slices of rat. Contrary to [³H]dopamine, [³H]spiperone storage was similar in striatum and pituitary gland. In addition, [³H]spiperone accumulation was not diminished by reserpine and tetrabenazine. These data show that spiperone is not subject to the granular uptake/storage mechanism and suggest that spiperone and its derivatives are specific ligands for dopamine receptors only.     

Immune privilege or inflammation? Insights into the Fas ligand enigma

Fas ligand (FasL) has become an enigmatic molecule: some evidence indicates that it contributes to immune privilege in tissues and tumors, whereas other data demonstrates that FasL can elicit inflammation. New findings may begin to reconcile the paradoxical effects of FasL.     

Fas ligand: Can it ever be tamed?

Fas ligand (FasL) was first described functionally as an inducible cell surface molecule used by cytotoxic T cells to induce apoptotic cell death in tumor cells and activated lymphocytes. With the identification of Fas as the Ipr gene product, FasL became recognized as a molecule involved in down-regulation of the immune system. While FasL can be used to efficiently kill Fas-expressing tumor cells as well as activated T and B lymphocytes in vitro, attempts to use FasL therapeutically to treat cancer or to prevent transplant     

NMDA receptor signaling: death or survival?

Glutamate-induced neuronal damage is mainly caused by overactivation of N-methyl-D-aspartate (NMDA) receptors.Conversely,normal physiological brain function and neuronal survival require adequate activ...     

NF-κB mediates the induction of Fas receptor and Fas ligand by microcystin-LR in HepG2 cells

Microcystin-LR (MC-LR) is the most frequent and most toxic microcystin identified. This natural toxin has multiple features, including inhibitor of protein phosphatases 1 and 2A, inducer of oxidative stress, as well as, tumor initiator and promoter. One unique character of MC-LR is this chemical can accumulate into liver after contacting and lead to severe damage to hepatocytes, such as apoptosis. Fas receptor (Fas) and Fas ligand (FasL) system is a critical signaling system initiating apoptosis. In current study, we explored whether MC-LR could induce Fas and FasL expression in HepG2 cells, a well used in vitro model for the study of human hepatocytes. The data showed MC-LR induced Fas and FasL expression, at both mRNA and protein levels. We also found MC-LR induced apoptosis at the same incubation condition at which it induced Fas and FasL expression. The data also revealed MC-LR promoted nuclear translocation and activation of p65 subunit of NF-κB. By applying siRNA to knock down p65 in HepG2 cells, we successfully impaired the activation of NF-κB by MC-LR. In these p65 knockdown cells, we also observed significant reduction of MC-LR-induced Fas expression, FasL expression, and apoptosis. These findings demonstrate that the NF-κB mediates the induction of Fas and FasL as well as cellular apoptosis by MC-LR in HepG2 cells. The results bring important information for understanding how MC-LR induces apoptosis in hepatocytes.     

The NAADP receptor: new receptors or new regulation?

Nicotinic acid adenine dinucleotide phosphate (NAADP) is the most potent calcium mobilizing messenger yet discovered. Its action has now been reported in a large number of cell types from a diverse array of organisms, and in some cases linked to the transduction of specific cellular stimuli. However, what is controversial is the nature of its target calcium release channel, as well as the subcellular localization of its receptor. Some have proposed that NAADP activates a novel calcium release channel distinct from the two major classes of channels known, the inositol trisphosphate receptors and ryanodine receptors. However, others have suggested that it acts in a novel way to regulate a known calcium release channel, the ryanodine receptor.     

The roles of interleukin-15 receptor alpha: trans-presentation, receptor component, or both?

Interleukin-15 receptor alpha (IL-15R alpha) is a high affinity IL-15 binding protein that is crucial for mediating IL-15 functions such as memory CD8 T cell proliferation and NK, NK/T cell, and intestinal intraepithelial lymphocyte development. However, the mechanism by which IL-15R alpha mediates IL-15 functions is unique among cytokines. Originally, IL-15R alpha was thought to be a component of a heterotrimeric receptor complex containing the IL-2/IL-15R beta and common gamma chains (gammaC) that were required for mediating signaling. Although IL-15R alpha may in some cases act as a component of this receptor complex, more recent evidence indicates that IL-15R alpha predominately functions by presenting IL-15 to opposing cells expressing the IL-15R betagamma signaling components. This theory is consistent with the broad, non-lymphoid expression pattern of IL-15R alpha and the evidence that IL-15R alpha expression by lymphocytes is dispensable for IL-15 action in vivo. This new concept of cytokine delivery will allow us to better understand the regulation and function IL-15.     

Polymeric Ig receptor: defender of the fort or Trojan horse?

The polymeric immunoglobulin receptor (pIgR) is important in host defense, transporting antibodies across mucosal epithelial cells. Recent work has shown that, using a protein that binds directly to the pIgR, Streptococcus pneumoniae can co-opt the transcytosis machinery and gain entry into airway epithelial cells.     

Neuropeptide Y receptor gene y6: multiple deaths or resurrections?

The neuropeptide Y family of G-protein-coupled receptors consists of five cloned members in mammals. Four genes give rise to functional receptors in all mammals investigated. The y6 gene is a pseudogene in human and pig and is absent in rat, but generates a functional receptor in rabbit and mouse and probably in the collared peccary (Pecari tajacu), a distant relative of the pig family. We report here that the guinea pig y6 gene has a highly distorted nucleotide sequence with multiple frame-shift mutations. One evolutionary scenario may suggest that y6 was inactivated before the divergence of the mammalian orders and subsequently resurrected in some lineages. However, the pseudogene mutations seem to be distinct in human, pig, and guinea pig, arguing for separate inactivation events. In either case, the y6 gene has a quite unusual evolutionary history with multiple independent deaths or resurrections.     

Spying on IgE receptor signaling: simply complex,or not?

Plasma membrane organization and the potential role, or not, of lipid raft microdomains in signal transduction is a controversial topic. Cross-correlation fluorescent correlation spectroscopy (CC-FCS) shows promise as a new approach to rapidly probe protein-protein interactions in living cells during signal transduction. CC-FCS data from studies of IgE receptor signaling challenge models of large stable lipid raft signaling domains and reveal a new complexity in the dynamic (re)organization of signaling complexes.     

Opioid and chemokine receptor heterodimers: arranged marriages or dangerous liaisons?

Adiponectin is one of the most effective adipokines in the context of correcting obesity-induced insulin resistance. However, adiponectin-deficient animal models show a relatively modest phenotype unless metabolically challenged. This suggests that potent compensatory mechanisms are in place. In this issue of the Biochemical Journal, Wong et al. characterize new members of the CTRPs [C1q-TNFalpha (tumour necrosis factor alpha)-related proteins]. They establish that some CTRPs are produced primarily in the stromal vascular fraction of adipose tissue, and that expression of CRTP1, in particular (like adiponectin), is induced by PPARgamma (peroxisome-proliferator-activated receptor gamma) agonists. Moreover, injection of recombinant CTRP1 displays glucose-lowering effects. These observations suggest that CTRP1 may have partially overlapping functions and, along with other paralogues, may effectively compensate for the chronic loss of adiponectin function.     

New endogenous benzodiazepine receptor ligand in human urine: Identity with endogenous monoamine oxidase inhibitor?

Normal human urine contains both monoamine oxidase-inhibiting and benzodiazepine receptor-binding material. Each was extracted into ethyl acetate at pH 1 and subjected to high performance liquid chromatography: they ran similarly, showing three major peaks. The correlation coefficient between the pattern of MAO inhibition and inhibition of 3H-flunitrazepam binding to benzodiazepine receptors in the second half of the elution process was 0.78 (p less than 0.001): most UV-absorbing material present was eluted earlier in the run. These results are compatible with, although they do not prove, the hypothesis that the endogenous MAO inhibitor, previously shown to be increased in stress, is also an endogenous inhibitor of 3H-flunitrazepam binding to the benzodiazepine receptor. This material is different from other putative endogenous ligands: it migrates more rapidly than the potent but artefactual beta-carboline-3-carboxylic acid ethyl ester previously isolated from human urine; nor can the effect we have identified derive from harmane, inosine, hypoxanthine or nicotinamide which fail to extract into ethyl acetate at pH 1.     

IL-6 type cytokine receptor complexes: hexamer, tetramer or both?

The typical protein fold of most cytokines is a bundle of four antiparallel helices. This 'four-helical bundle fold' seems to be unique to cytokines and has not been detected in other proteins. Cytokine receptors, however, can be classified as a subfamily of the immunoglobulin superfamily. Cytokines using the same receptor subunits are grouped into cytokine families. The interleukin-6 (IL-6) type cytokine family comprises six members. IL-6 type cytokines may interact with three receptor subunits instead of the usual two subunits. A tetramer would be the simplest model to describe such a receptor complex, but present orthodoxy describes the active complexes of IL-6 and ciliary neurotrophic factor (CNTF) as hexamers. Here, we summarize the structural and biochemical information on IL-6 type cytokines and discuss interactions between cytokine and individual receptor subunits at alternative positions. Contradictory results regarding the stoichiometry and assembly of signaling receptor complexes are rationalized by a new, unique model. The model stipulates that a ligand-induced transition from an active tetrameric to an inactive hexameric complex serves as a molecular switch that turns off cytokine signals in the presence of supraoptimal cytokine concentrations.     

Toll-like receptor 4-mediated signaling by epithelial surfaces: necessity or threat?

Recent data suggest that the lipopolysaccharide receptor Toll-like receptor (TLR) 4 is expressed by epithelial cells and might play a role in the mucosal host defense against Gram-negative bacteria. However, since many body surfaces are colonized by the physiological microflora, activation of epithelial TLRs must be tightly controlled to avoid unintended stimulation and mucosal inflammation. The present review summarizes the current understanding of TLR4-mediated recognition and addresses specific questions on microbial recognition on mucosal surfaces, with particular emphasis on the gastrointestinal and urinary tract.     

Does a targeting ligand influence nanoparticle tumor localization or uptake?

Inclusion of a tumor-targeting molecule in nanosized delivery systems increases their in vivo efficacy. However, the biodistribution and pharmacokinetics of the uptake of such particles have not yet been well addressed. Several recent papers have suggested that tumor-targeting ligands function primarily to increase intracellular uptake of the nanocomplex and do not influence tumor localization. However, other reports indicate that they do play a role in the accumulation in the tumor. One difference might be the presence or absence of poly-[ethylene glycol] (PEG) in the complex and its impact on the enhanced permeability and retention (EPR) effect. Further studies are clearly needed to more fully elucidate the influence of composition on tumor-targeted, systemic delivery of nanoparticles.     

Formation of acetylcholine receptor clusters in chick myotubes: migration or new insertion?

Experiments were performed to study the feasibility of two mechanisms of acetylcholine receptor (ACHR) accumulation in chick myotubes: diffusion and trapping of previously dispersed surface receptors and localized insertion of new receptors at accumulation sites. Fluorescence photobleaching recovery (FPR) measurements indicated that the majority of diffusely distributed ACHRs in chick myotube membranes were mobile whereas nearly all receptors within high density clusters were effectively immobile. Unlike previous reports, two rates of ACHR movement characterized the mobile population. Moreover, we found that the estimated diffusion coefficient depended critically on the objective (spot size) used to assay recovery from bleaching. Implications of this finding for mechanisms of receptor immobilization are discussed. Extracts of chick brain, known to increase the number of surface receptors, did not alter receptor mobility. Extracts of Torpedo electric organ that increase the number of receptor aggregates, decreased the mobile fraction of ACHRs. Simulations of the diffusion and trapping mechanism indicated that captured receptors should congregate around the periphery of a receptor patch during the first hour after they were inserted into the membrane. However, newly inserted ACHRs were found to be located centrally within receptor patches under neurites, and this was not consistent with an exclusive diffusion-trapping mechanism. We also studied the mobility of ACHRs near points of contact made by cholinergic growth cones. The rate of receptor movement was increased in the vicinity of growth cones, but the magnitude of this effect was small.