Yasuto Tanabe

A family of metabotropic glutamate receptors.

Three cDNA clones, mGluR2, mGluR3, and mGluR4, were isolated from a rat brain cDNA library by cross-hybridization with the cDNA for a metabotropic glutamate receptor (mGluR1). The cloned receptors show considerable sequence similarity with mGluR1 and possess a large extracellular domain preceding the seven putative membrane-spanning segments. mGluR2 is expressed in some particular neuronal cells different from those expressing mGluR1 and mediates an efficient inhibition of forskolin-stimulated cAMP formation in cDNA-transfected cells. The mGluRs thus form a novel family of G protein-coupled receptors that differ in their signal transduction and expression patterns.

Generation of Reelin-Positive Marginal Zone Cells from the Caudomedial Wall of Telencephalic Vesicles

An early and fundamental step of the laminar organization of developing neocortex is controlled by the developmental programs that critically depend on the activities of reelin-positive cells in the marginal zone. However, the ontogeny of reelin-positive cells remained elusive. To gain insights into the spatial and temporal regulation of reelin-positive marginal zone cell development, we used a transgenic mouse line in which we defined the green fluorescent protein (GFP) transgene as a novel reliable molecular marker of reelin-positive marginal zone cells from the early stages of their development. We further used exo utero electroporation-mediated gene transfer that allows us to mark progenitor cells and monitor the descendants in the telencephalon in vivo. We show here the generation of reelin-positive marginal zone cells from the caudomedial wall of telencephalic vesicles, including the cortical hem, where the prominent expression of GFP is initially detected. These neurons tangentially migrate at the cortical marginal zone and are distributed throughout the entire neocortex in a caudomedial-high to rostrolateral-low gradient during the dynamic developmental period of corticogenesis. Therefore, our findings on reelin-positive marginal zone cells, in addition to the cortical interneurons, add to the emerging view that the neocortex consists of neuronal subtypes that originate from a focal source extrinsic to the neocortex, migrate tangentially into the neocortex, and thereby underlie neural organization of the neocortex.

Agonist analysis of 2-(carboxycyclopropyl)glycine isomers for cloned metabotropic glutamate receptor subtypes expressed in Chinese hamster ovary cells

1 2‐(Carboxycyclopropyl)glycines (CCGs) are conformationally restricted glutamate analogues and consist of eight isomers including l‐ and d‐forms. The agonist potencies and selectivities of these compounds for metabotropic glutamate receptors (mGluRs) were studied by examining their effects on the signal transduction of representative mGluR1, mGluR2 and mGluR4 subtypes in Chinese hamster ovary cells expressing the individual cloned receptors. 2 Two extended isomers of l‐CCG, l‐CCG‐I and l‐CCG‐II, effectively stimulated phosphatidylinositol hydrolysis in mGluR1‐expressing cells. The rank order of potencies of these compounds was l‐glutamate > l‐CCG‐I > l‐CCG‐II. 3 l‐CCG‐I and l‐CCG‐II were effective in inhibiting the forskolin‐stimulated adenosine 3′:5′‐cyclic monophosphate (cyclic AMP) accumulation in mGluR2‐expressing cells. Particularly, l‐CCG‐I was a potent agonist for mGluR2 with an EC50 value of 3 × 10−7 m, which was more than an order of potency greater than that of l‐glutamate. 4 l‐CCG‐I evoked an inhibition of the forskolin‐stimulated cyclic AMP production characteristic of mGluR4 with a potency comparable to l‐glutamate. 5 In contrast to the above compounds, the other CCG isomers showed no appreciable effects on the signal transduction involved in the three mGluR subtypes. 6 This investigation demonstrates not only the importance of a particular isomeric structure of CCGs in the interaction with the mGluRs but also a clear receptor subtype specificity for the CCG‐receptor interaction, and indicates that the CCG isomers would serve as useful agonists for investigation of functions of the mGluR family.

Immunohistochemical Study of a Rat Membrane Protein which Induces a Selective Potassium Permeation: Its Localization in the Apical Membrane Portion of Epithelial Cells

SummaryWe previously reported a novel rat membrane protein that exhibits a voltage-dependent potassium channel activity on the basis of molecular cloning combined with an electrophysiological assay. This protein, termedIsK protein, is small and different from the conventional potassium channel poroteins but induces selective permeation of potassium ions on its expression inXenopus oocytes. In this investigatiion, we examined cellular localization of ratIsK protein by preparing three different types of antibody that specifically reacts with a distinct part of ratIsK protein. Immunohistochemical analysis using these antibody preparations demonstrated that ratIsK protein is confined to the apical membrane portion of epithelial cells in the proximal tubule of the kidney, the submandibular duct and the uterine endometrium. The observed tissue distribution of ratIsK protein was consistent with that of theIsK protein mRNA determined by blot hybridization analysis. In epithelial cells the sodium, potassium-ATPase pump in the basolateral membrane generats a sodium gradient acrossthe epithelial cell and allows sodium ions to entere the cell through the apical membrane. Thus, taking into account the cellular localization of theIsK protein, together with its electrophysiological properties, we discussed a possible function of theIsK protein, namely that this protein is involved in potassium permeation in the apical membrane of epithelial cells through the depolarizing effect of sodium entry.