Atrial muscarinic receptors: Effect of triton X-100 on guanine nucleotide and ammonium ion modulation

Membranes isolated from bovine atria were labeled with [³H]quinuclidinyl benzylate (³H-QNB), in control conditions and after 0.02% Triton X-100. This treatment inactivated abcut 20% of muscarinic receptor sites without loss of protein. The remaining 80% sites showed no changes in affinity, as determined by equilibrium or kinetic binding. Competition experiments with carbachol showed no differences in IC₅₀ and Hill number between the control and detergent-membranes, suggesting that the different populations of agonist binding sites are inactivated in equal proportions by the detergent. In binding experiments, done in the presence of carbachol and guanine nucleotides, the detergent treated membranes were slightly more sensitive to the enhancing action of the nucleotide. The inhibition caused by ammonium ions was also more marked in the Triton X-100 treated membranes. The decay of binding with thermal inactivation was faster in the detergent treated membranes and this effect was enhanced in the presence of ammonium ions. These results may be interpreted as an indication that the receptors, remaining after the mild Triton X-100 treatment, are equally sensitive to the inactivation. We suggest that, while maintaining the heterogeneity of sites, the detergent produces a perturbation that could affect the molecular interactions between the receptor and other components of the membrane.     

An [3H]oxotremorine binding method reveals regulatory changes by guanine nucleotides in cholinergic muscarinic receptors of cerebral cortex

A rapid, reliable filtration method for [³H]oxotremorine binding to membranes of the cerebral cortex that allows the direct study of regulation by guanine nucleotides of muscarinic receptors was developed. [³H]Oxotremorine binds to cerebral cortex membranes with high affinity (K _D, 1.9 nM) and low capacity (B _max, 187 pmol/g protein). These sites, which represent only about 18% of those labeled with [³H]quinuclidinyl benzilate, constitute a population of GTP-sensitive binding sites. Association and dissociation binding experiments revealed a similar value ofK _D (2.3 nM). Displacement studies with 1–4000 nM oxotremorine showed the existence of a second binding site of low affinity (K _D, 1.2 M) and large capacity (B _max, 1904 pmol/g protein). Gpp(NH)p, added in vitro, produced a striking inhibition of [³H]oxotremorine binding with an IC 50 of 0.3 M. Saturation assays, in the presence of 0.5 M Gpp(NH)p, revealed a non-competitive inhibition of the binding with little change in affinity. These results are discussed from the viewpoint of conflicting reports in the literature about guanine nucleotide regulation of muscarinic receptors in reconstituted systems and membranes from different tissues.     

Taurine Modulation of the Benzodiazepine-γ-Aminobutyric Acid Receptor Complex in Brain Membranes

In unwashed brain membranes taurine produced an inhibition of [3H]flunitrazepam [( 3H]FNZ) binding with IC50 ranging between 31.5 and 11.9 microM; the IC20 varied between 18 and 26 nM. This inhibitory effect was of a mixed type, with a reduction in Bmax and an increase in KD. Various precursors and metabolites of taurine have a less inhibitory effect. Taurine also has little inhibitory effect (IC50 above 500 microM) on the binding of [3H]ethyl-beta-carboline-3-carboxylate. In extensively washed membranes, 10(-5) M taurine produces a 16-21% increase in the binding of [3H]FNZ while 10(-5) M gamma-aminobutyric acid (GABA) increases it between 31 and 42%. However, if 10(-5) M GABA plus 10(-5) M taurine is included in the assay there is a dramatic inhibitory effect. Taurine causes an inhibition of the GABAergic enhancement of [3H]FNZ binding with an IC50 between 7.3 and 7.8 microM. Binding experiments with [3H]taurine done under different conditions failed to detect a Na+-independent and specific [3H]taurine receptor. These results suggest that endogenous taurine, the second most abundant free amino acid in brain, may play an important modulatory role in the GABA-benzodiazepine receptor complex.     

Binding of d-[dimethyl-14C]tubocurarine, acetylcholinesterase and proteolipids in subcellular membranes of cerebral cortex of the developing rat

Abstract— The cerebral cortex of rats at postnatal ages of 5,15,30 and 50 days was homogenized and fractionated to separate the crude mitochondrial fraction. This fraction was osmotically shocked and the Mi fraction and subfractions were separated. The variations with age in the morphological composition of subfractions M₁ 0·8, Mi 1·3 and M₁p were studied under the electron microscope. Because of the changes observed in the various fractions the need for such type of control is stressed. The changes in total protein and proteolipid-protein of the fractions at different ages, as well as the acetylcholinesterase activity and the binding of d -[dimethyl^?14C]tubocurarine were studied. The results obtained were interpreted on the basis of the important morphogenetic changes that the cerebral cortex undergoes postnatally. The progressive and parallel increase in acetylcholinesterase and in the binding of d -[dimethyl-¹⁴C]tubocurarine to proteolipids of fraction Mi 1·3, suggest a close relationship between these two events and the development of the cholinergic synapses.     

Human Sarcoma Growth Is Sensitive to Small-Molecule Mediated AXIN Stabilization

Sarcomas are mesenchymal tumors showing high molecular heterogeneity, reflected at the histological level by the existence of more than fifty different subtypes. Genetic and epigenetic evidences link aberrant activation of the Wnt signaling to growth and progression of human sarcomas. This phenomenon, mainly accomplished by autocrine loop activity, is sustained by gene amplification, over-expression of Wnt ligands and co-receptors or epigenetic silencing of endogenous Wnt antagonists. We previously showed that pharmacological inhibition of Wnt signaling mediated by Axin stabilization produced in vitro and in vivo antitumor activity in glioblastoma tumors. Here, we report that targeting different sarcoma cell lines with the Wnt inhibitor/Axin stabilizer SEN461 produces a less transformed phenotype, as supported by modulation of anchorage-independent growth in vitro. At the molecular level, SEN461 treatment enhanced the stability of the scaffold protein Axin1, a key negative regulator of the Wnt signaling with tumor suppressor function, resulting in downstream effects coherent with inhibition of canonical Wnt signaling. Genetic phenocopy of small molecule Axin stabilization, through Axin1 over-expression, coherently resulted in strong impairment of soft-agar growth. Importantly, sarcoma growth inhibition through pharmacological Axin stabilization was also observed in a xenograft model in vivo in female CD-1 nude mice. Our findings suggest the usefulness of Wnt inhibitors with Axin stabilization activity as a potentialyl clinical relevant strategy for certain types of sarcomas.     

Darmin is a novel secreted protein expressed during endoderm development in Xenopus

Endoderm development is an area of intense interest in developmental biology, but progress has been hampered by the lack of specific markers for differentiated endodermal cells. In an unbiased secretion cloning screen of Xenopus gastrula embryos we isolated a novel gene, designated Darmin. Darmin encodes a secreted protein of 56 kDa containing a peptidase M20 domain characteristic of the glutamate carboxypeptidase group of zinc metalloproteases. We also identified homologous Darmin genes in other eukaryotes and in prokaryotes suggesting that Darmin is the founding member of a family of evolutionarily conserved proteins. Xenopus Darmin showed zygotic expression in the early endoderm and later became restricted to the midgut. By secretion cloning of Xenopus cleavage-stage embryos we isolated another novel protein, designated Darmin-related (Darmin-r) due to its sequence similarity with Darmin. Darmin-r was maternally expressed and showed at later stages expression in the lens and pronephric glomus. The endoderm-specific expression of Darmin makes this gene a useful marker for the study of endoderm development.