Sodium-dependent transport into astrocytes is critical for maintaining the extracellular concentrations of glutamate below toxic levels in the central nervous system. In this study, the expression of the glial glutamate transporters GLT-1 and GLAST was studied in primary cultures derived from cortical tissue. In primary astrocytes, GLAST protein levels were approximately one half of those observed in cortical tissue, but GLT-1 protein was present at very low levels compared with cortical tissue. Maintenance of these astrocytes in medium supplemented with dibutyryl-cAMP (dbcAMP) caused a dramatic change in cell morphology, increased GLT-1 and GLAST mRNA levels approximately 5-fold, increased GLAST protein approximately 2-fold, and increased GLT-1 protein >/=8-20-fold. These increases in protein expression were accompanied by 2-fold increases in the Vmax and Km values for Na+-dependent L-[3H]glutamate transport activity. Although GLT-1 is sensitive to inhibition by dihydrokainate in heterologous expression systems, no dihydrokainate sensitivity was observed in astrocyte cultures that expressed GLT-1. Biotinylation with a membrane-impermeant reagent, separation of the biotinylated/cell surface proteins, and subsequent Western blotting demonstrated that both GLT-1 and GLAST were present at the cell surface. Coculturing of astrocytes with neurons also induced expression of GLT-1, which colocalized with the glial specific marker, glial fibrillary acidic protein. Neurons induced a small increase in GLAST protein. Several studies were performed to examine the mechanism by which neurons regulate expression of the glial transporters. Three different protein kinase A (PKA) antagonists did not block the effect of neurons on glial expression of GLT-1 protein, but the addition of dbcAMP to mixed cultures of neurons and astrocytes did not cause GLT-1 protein to increase further. This suggests that neurons do not regulate GLT-1 by activation of PKA but that neurons and dbcAMP regulate GLT-1 protein through convergent pathways. As was observed with GLT-1, the increases in GLAST protein observed in cocultures were not blocked by PKA antagonists, but unlike GLT-1, the addition of dbcAMP to mixed cultures of neurons and astrocytes caused GLAST protein to increase approximately 2-fold. Neurons separated from astrocytes with a semipermeable membrane increased GLT-1 protein, indicating that the effect of neurons was mediated by a diffusible molecule. Treatment of cocultures with high concentrations of either N-methyl-D-aspartate or glutamate killed the neurons, caused GLT-1 protein to decrease, and caused GLAST protein to increase. These studies suggest that GLT-1 and GLAST protein are regulated independently in astrocyte cultures and that a diffusible molecule secreted by neurons induces expression of GLT-1 in astrocytes.

The glial glutamate transporter GLT-1 may be the predominant Na(+)-dependent glutamate transporter in forebrain. Expression of GLT-1 correlates with astrocyte maturation in vivo and increases during synaptogenesis. In astrocyte cultures, GLT-1 expression parallels differentiation induced by cAMP analogs or by coculturing with neurons. Molecule(s) secreted by neuronal cultures contribute to this induction of GLT-1, but little is known about the signaling pathways mediating this regulation. In the present study, we determined whether growth factors previously implicated in astrocyte differentiation regulate GLT-1 expression. Of the six growth factors tested, two [epidermal growth factor (EGF) and transforming growth factor-alpha] induced expression of GLT-1 protein in cultured astrocytes. Induction of GLT-1 protein was accompanied by an increase in mRNA and in the V(max) for Na(+)-dependent glutamate transport activity. The effects of dibutyryl-cAMP and EGF were additive but were independently blocked by inhibitors of protein kinase A or protein tyrosine kinases, respectively. The induction of GLT-1 in both EGF- and dibutyryl-cAMP-treated astrocytes was blocked by inhibitors targeting phosphatidylinositol 3-kinase (PI3K) or the nuclear transcription factor-kappaB. Furthermore, transient transfection of astrocyte cultures with a constitutively active PI3K construct was sufficient to induce expression of GLT-1. These data suggest that independent but converging pathways mediate expression of GLT-1. Although an EGF receptor-specific antagonist did not block the effects of neuron-conditioned medium, the induction of GLT-1 by neuron-conditioned medium was completely abolished by inhibition of PI3K or nuclear factor-kappaB. EGF also increased expression of GLT-1 in spinal cord organotypic cultures. Together, these data suggest that activation of specific signaling pathways with EGF-like molecules may provide a novel approach for limiting excitotoxic brain injury.

Agonist-induced internalization of G protein-coupled receptors (GPCRs) is a well characterized phenomenon believed to contribute to receptor desensitization. The 5-hydroxytryptamine (5-HT) 2C subtype of serotonin receptor is a GPCR that we have shown to internalize upon agonist incubation. In this study, we have examined the effects of 5-HT 2C on the internalization of a C-terminal green fluorescent protein (GFP)-tagged 5-HT 2C receptor (VSV isoform) expressed in transiently transfected human embryonic kidney cells. We detected internalization with an automated, cell-based fluorescence-imaging system (Arrayscan) and monitored function with intracellular Ca 2ϩ measurements (flourometric imaging plate reader). The 5-HT 2C -GFP construct exhibited appropriate pharmacology, and we observed that although all three agonists resulted in similar magnitudes of dose-dependent internalization, the partial agonists resulted in ϳ50% less internalization, and the inverse agonists and neutral antagonists failed to induce internalization. These results were confirmed by confocal microscopy. They demonstrate that the 5-HT 2C receptor is internalized by incubation with agonists and partial agonists but not with inverse agonists or neutral antagonists.Agonist induced internalization of G protein-coupled receptors (GPCRs) is a well characterized phenomenon believed to contribute to receptor desensitization. Upon agonist binding, GPCRs undergo conformational changes that result in activation of heterotrimeric G protein subunits, which activate or inhibit downstream targets such as nucleotide cyclases, phospholipases, and kinases. Subsequent to receptor-effector coupling, second messenger-dependent protein kinases and G protein receptor kinases phosphorylate the receptor, which promotes recruitment of ␤-arrestins, uncoupling of heterotrimeric G proteins, and internalization in clathrin-coated vesicles (for review, see Ferguson, 2001). Although such internalization is typically associated with agonist induction, antagonists have been shown to induce internalization of several GPCRs, including the 5-hydroxytryptamine (5-HT) 2A receptor (Willins et al., 1998), the cholecystokinin receptor (Roettger et al., 1997), the vasopressin V 2 receptor (Pfeiffer et al., 1998), and the A 1 adenosine receptor (Navarro et al., 1999), indicating that the ligand dependence of GPCR internalization is a receptor-specific property.Although agonist-and antagonist-induced internalization has been studied for the 5-HT 2A serotonin receptor, it has not been examined for the closely related 5-HT 2C receptor subtype. The 5-HT 2C receptor functions through the same mechanism as the other members of the 5-HT 2 family, activating phospholipase C, promoting the hydrolysis of membrane phospholipids, leading to increases in the intracellular levels of inositol phosphates and intracellular calcium (for review, see Boess and Martin, 1994). Both the 5-HT 2A and 5-HT 2C receptor subtypes have received considerable attention in pharmaceutical drug discovery. In...

Isogenic cell lines differing only in the expression of the protein of interest provide the ideal platform for cell-based screening. However, related natural lines differentially expressing the therapeutic target of choice are rare. Here the authors report a strategy for drug screening employing isogenic human cell lines in which the expression of the target protein is regulated by a gene-specific engineered zinc-finger protein (ZFP) transcription factor (TF). To demonstrate this approach, a ZFP TF activator of the human parathyroid hormone receptor 1 (PTHR1) gene was identified and introduced into HEK293 cells (negative for PTHR1). Following induction of ZFP TF expression, this cell line produced functional PTHR1 protein, resulting in a robust and ligand-specific cyclic adenosine monophosphate (cAMP) response. Reciprocally, the natural expression of PTHR1 observed in SAOS2 cells was dramatically reduced by the introduction of the appropriate PTHR1-specific ZFP TF repressor. Moreover, this ZFP-driven PTHR1 repression selectively eliminated the functional cAMP response invoked by known ligands of PTHR1. These data establish ZFP TF-generated isogenic lines as a general approach for the identification of therapeutic agents specific for the target gene of interest. (Journal of Biomolecular Screening 2005:304-313)