Rapid desensitization of the thyrotropin-releasing hormone receptor expressed in single human embryonal kidney 293 cells

Biochemical and physiological insights into TRH receptor-mediated signaling

Thyrotropin-releasing hormone (TRH) is an important endocrine agent that regulates the function of cells in the anterior pituitary and the central and peripheral nervous systems. By controlling the synthesis and release of thyroid hormones, TRH affects many physiological functions, including energy homeostasis. This hormone exerts its effects through G protein-coupled TRH receptors, which signal primarily through G_q/11 but may also utilize other G protein classes under certain conditions. Because of the potential therapeutic benefit, considerable attention has been devoted to the synthesis of new TRH analogs that may have some advantageous properties compared with TRH. In this context, it may be interesting to consider the phenomenon of biased agonism and signaling at the TRH receptor. This possibility is supported by some recent findings. Although knowledge about the mechanisms of TRH receptor-mediated signaling has increased steadily over the past decades, there are still many unanswered questions, particularly about the molecular details of post-receptor signaling. In this review, we summarize what has been learned to date about TRH receptor-mediated signaling, including some previously undiscussed information, and point to future directions in TRH research that may offer new insights into the molecular mechanisms of TRH receptor-triggered actions and possible ways to modulate TRH receptor-mediated signaling.

The SZT2 Interactome Unravels New Functions of the KICSTOR Complex

Seizure threshold 2 (SZT2) is a component of the KICSTOR complex which, under catabolic conditions, functions as a negative regulator in the amino acid-sensing branch of mTORC1. Mutations in this gene cause a severe neurodevelopmental and epileptic encephalopathy whose main symptoms include epilepsy, intellectual disability, and macrocephaly. As SZT2 remains one of the least characterized regulators of mTORC1, in this work we performed a systematic interactome analysis under catabolic and anabolic conditions. Besides numerous mTORC1 and AMPK signaling components, we identified clusters of proteins related to autophagy, ciliogenesis regulation, neurogenesis, and neurodegenerative processes. Moreover, analysis of SZT2 ablated cells revealed increased mTORC1 signaling activation that could be reversed by Rapamycin or Torin treatments. Strikingly, SZT2 KO cells also exhibited higher levels of autophagic components, independent of the physiological conditions tested. These results are consistent with our interactome data, in which we detected an enriched pool of selective autophagy receptors/regulators. Moreover, preliminary analyses indicated that SZT2 alters ciliogenesis. Overall, the data presented form the basis to comprehensively investigate the physiological functions of SZT2 that could explain major molecular events in the pathophysiology of developmental and epileptic encephalopathy in patients with SZT2 mutations.

Host gene expression modulated by Zika virus infection of human-293 cells

The HEK-293 cell line was created in 1977 by transformation of primary human embryonic kidney cells with sheared adenovirus type 5 DNA. A previous study determined that the HEK-293 cells have neuronal markers rather than kidney markers. In this study, we tested the hypothesis whether Zika virus (ZIKV), a neurotropic virus, is able to infect and replicate in the HEK-293 cells. We show that the HEK-293 cells infected with ZIKV support viral replication as shown by indirect immunofluorescence (IFA) and quantitative reverse transcriptase-PCR (qRT-PCR). We performed RNA-seq analysis on the ZIKV-infected and the control uninfected HEK-293 cells and find 659 genes that are differentially transcribed in ZIKV-infected HEK-293 cells as compared to uninfected cells. The results show that the top 10 differentially transcribed and upregulated genes are involved in antiviral and inflammatory responses. Seven upregulated genes, IFNL1, DDX58, CXCL10, ISG15, KCNJ15, IFNIH1, and IFIT2, were validated by qRT-PCR. Altogether, our findings show that ZIKV infection alters host gene expression by affecting their antiviral and inflammatory responses.

Ca2+ responses to thyrotropin-releasing hormone and angiotensin II: the role of plasma membrane integrity and effect of G11alpha protein overexpression on homologous and heterologous desensitization

The molecular mechanisms involved in GPCR-initiated signaling cascades where the two receptors share the same signaling cascade, such as thyrotropin-releasing hormone (TRH) and angiotensin II (ANG II), are still far from being understood. Here, we analyzed hormone-induced Ca(2+) responses and the process of desensitization in HEK-293 cells, which express endogenous ANG II receptors. These cells were transfected to express exogenously high levels of TRH receptors (clone E2) or both TRH receptors and G(11)alpha protein (clone E2M11). We observed that the characteristics of the Ca(2+) response, as well as the process of desensitization, were both strongly dependent on receptor number and G(11)alpha protein level. Whereas treatment of E2 cells with TRH or ANG II led to significant desensitization of the Ca(2+) response to subsequent addition of either hormone, the response was not desensitized in E2M11 cells expressing high levels of G(11)alpha. In addition, stimulation of both cell lines with THR elicited a clear heterologous desensitization to subsequent stimulation with ANG II. On the other hand, ANG II did not affect a subsequent response to TRH. ANG II-mediated signal transduction was strongly dependent on plasma membrane integrity modified by cholesterol depletion, but signaling through TRH receptors was altered only slightly under these conditions. It may be concluded that the level of expression of G-protein-coupled receptors and their cognate G-proteins strongly influences not only the magnitude of the Ca(2+) response but also the process of desensitization and resistance to subsequent hormone addition.

β-Arrestin Mediates Desensitization and Internalization but Does Not Affect Dephosphorylation of the Thyrotropin-releasing Hormone Receptor

The G protein-coupled thyrotropin-releasing hormone (TRH) receptor is phosphorylated and binds to beta-arrestin after agonist exposure. To define the importance of receptor phosphorylation and beta-arrestin binding in desensitization, and to determine whether beta-arrestin binding and receptor endocytosis are required for receptor dephosphorylation, we expressed TRH receptors in fibroblasts from mice lacking beta-arrestin-1 and/or beta-arrestin-2. Apparent affinity for [(3)H]MeTRH was increased 8-fold in cells expressing beta-arrestins, including a beta-arrestin mutant that did not permit receptor internalization. TRH caused extensive receptor endocytosis in the presence of beta-arrestins, but receptors remained primarily on the plasma membrane without beta-arrestin. beta-Arrestins strongly inhibited inositol 1,4,5-trisphosphate production within 10 s. At 30 min, endogenous beta-arrestins reduced TRH-stimulated inositol phosphate production by 48% (beta-arrestin-1), 71% (beta-arrestin-2), and 84% (beta-arrestins-1 and -2). In contrast, receptor phosphorylation, detected by the mobility shift of deglycosylated receptor, was unaffected by beta-arrestins. Receptors were fully phosphorylated within 15 s of TRH addition. Receptor dephosphorylation was identical with or without beta-arrestins and almost complete 20 min after TRH withdrawal. Blocking endocytosis with hypertonic sucrose did not alter the rate of receptor phosphorylation or dephosphorylation. Expressing receptors in cells lacking Galpha(q) and Galpha(11) or inhibiting protein kinase C pharmacologically did not prevent receptor phosphorylation or dephosphorylation. Overexpression of dominant negative G protein-coupled receptor kinase-2 (GRK2), however, retarded receptor phosphorylation. Receptor activation caused translocation of endogenous GRK2 to the plasma membrane. The results show conclusively that receptor dephosphorylation can take place on the plasma membrane and that beta-arrestin binding is critical for desensitization and internalization.

Constitutive activity of endogenous receptors by inducible Gq overexpression

We have developed an inducible cell line that transiently expresses Gq alpha G protein subunits in response to doxycycline. HEK293/Tet-On pBI(Gq alpha) cells worked consistently, achieving high and tightly regulated levels of Gq alpha overexpression (38-fold increase compared with non-induced cells). We investigated the possibility of using an inducible system to increase the proportion of constitutively active endogenously expressed G protein-coupled receptors (GPCRs) by overexpressing Gq alpha. Not only did we observe an increase in basal activity following doxycycline treatment, but also increased intrinsic activity of agonists such as carbachol, endothelin, lysophosphatidic acid (LPA), and bradykinin. Furthermore, carbachol and LPA potency increased following Gq alpha overexpression, as did the intrinsic activity of the partial agonist pilocarpine, observations indicative of constitutive activity. An inducible cell line, transiently expressing G proteins, can therefore be employed to induce constitutive activity of endogenously expressed GPCRs. This model system could be used to identify clinically important inverse agonists.

Homo- and hetero-oligomerization of thyrotropin-releasing hormone (TRH) receptor subtypes. Differential regulation of beta-arrestins 1 and 2

G-protein-coupled receptors (GPCRs) are regulated by a complex network of mechanisms such as oligomerization and internalization. Using the GPCR subtypes for thyrotropin-releasing hormone (TRHR1 and TRHR2), the aim of this study was to determine if subtype-specific differences exist in the trafficking process. If so, we wished to determine the impact of homo- and hetero-oligomerization on TRHR subtype trafficking as a potential mechanism for the differential cellular responses induced by TRH. Expression of either beta-arrestin 1 or 2 promoted TRHR1 internalization. In contrast, only beta-arrestin 2 could enhance TRHR2 internalization. The preference for beta-arrestin 2 by TRHR2 was supported by the impairment of TRHR2 trafficking in mouse embryonic fibroblasts (MEFs) from either a beta-arrestin 2 knockout or a beta-arrestin 1/2 knockout, while TRHR1 trafficking was only abolished in MEFs lacking both beta-arrestins. The differential beta-arrestin-dependence of TRHR2 was directly measured in live cells using bioluminescence resonance energy transfer (BRET). Both BRET and confocal microscopy were also used to demonstrate that TRHR subtypes form hetero-oligomers. In addition, these hetero-oligomers have altered internalization kinetics compared with the homo-oligomer. The formation of TRHR1/2 heteromeric complexes increased the interaction between TRHR2 and beta-arrestin 1. This may be due to conformational differences between TRHR1/2 hetero-oligomers versus TRHR2 homo-oligomers as a mutant TRHR1 (TRHR1 C335Stop) that does not interact with beta-arrestins, could also enhance TRHR2/beta-arrestin 1 interaction. This study demonstrates that TRHR subtypes are differentially regulated by the beta-arrestins and also provides the first evidence that the interactions of TRHRs with beta-arrestin may be altered by hetero-oligomer formation.

Preferential transformation of human neuronal cells by human adenoviruses and the origin of HEK 293 cells

The 293 cell line was derived by transformation of primary cultures of human embryonic kidney (HEK) cells with sheared adenovirus (Ad)5 DNA. A combination of immunostaining, immunoblot, and microarray analysis showed that 293 cells express the neurofilament (NF) subunits NF-L, NF-M, NF-H, and a-internexin as well as many other proteins typically found in neurons. Three other independently derived HEK lines, two transformed by Ad5 and one by Ad12, also expressed NFs, as did one human embryonic retinal cell line transformed with Ad5. Two rodent kidney lines transformed with Ad12 were also found to express NF proteins, although several rodent kidney cell lines transformed by Ad5 DNA and three HEK cell lines transformed by the SV40 early region did not express NFs. These results suggest that human Ads preferentially transform human neuronal lineage cells. We also demonstrate that the widely used HEK293 cells have an unexpected relationship to neurons, a finding that may require reinterpretation of many previous studies in which it was assumed that HEK293 cells resembled more typical kidney epithelial cells.

A diverse family of GPCRs expressed in specific subsets of nociceptive sensory neurons

In vertebrates, peripheral chemosensory neurons express large families of G protein-coupled receptors (GPCRs), reflecting the diversity and specificity of stimuli they detect. However, somatosensory neurons, which respond to chemical, thermal, or mechanical stimuli, are more broadly tuned. Here we describe a family of approximately 50 GPCRs related to Mas1, called mrgs, a subset of which is expressed in specific subpopulations of sensory neurons that detect painful stimuli. The expression patterns of mrgs thus reveal an unexpected degree of molecular diversity among nociceptive neurons. Some of these receptors can be specifically activated in heterologous cells by RFamide neuropeptides such as NPFF and NPAF, which are analgesic in vivo. Thus, mrgs may regulate nociceptor function and/or development, including the sensation or modulation of pain.

Selective regulation of endogenous G protein-coupled receptors by arrestins in HEK293 cells

Arrestins play an important role in regulating desensitization and trafficking of G protein-coupled receptors (GPCRs). However, limited insight into the specificity of arrestin-mediated regulation of GPCRs is currently available. Recently, we used an antisense strategy to reduce arrestin levels in HEK293 cells and characterize the role of arrestins on endogenous G(s)-coupled receptors (Mundell, S. J., Loudon, R. B., and Benovic, J. L. (1999) Biochemistry 38, 8723-8732). Here, we characterized GPCRs coupled to either G(q) (M(1) muscarinic acetylcholine receptor (M(1)AchR) and P2y(1) and P2y(2) purinergic receptors) or G(i) (somatostatin and AT1 angiotensin receptors) in wild type and arrestin antisense HEK293 cells. The agonist-specific desensitization of the M(1)Ach and somatostatin receptors was significantly attenuated in antisense-expressing cells, whereas desensitization of P2y(1) and P2y(2) purinergic and AT1 angiotensin receptors was unaffected by reduced arrestin levels. To further examine arrestin/GPCR specificity, we studied the effects of endogenous GPCR activation on the redistribution of arrestin-2 epitope tagged with the green fluorescent protein (arrestin-2-GFP). These studies revealed a receptor-specific movement of arrestin-2-GFP that mirrored the arrestin-receptor specificity observed in the antisense cells. Thus, agonist-induced activation of endogenous beta(2)-adrenergic, prostaglandin E(2), M(1)Ach, and somatostatin receptors induced arrestin-2-GFP redistribution to early endosomes, whereas P2y(1) and P2y(2) purinergic and AT1 angiotensin receptor activation did not. Thus, endogenous arrestins mediate the regulation of selective G(q)- and G(i)-coupled receptors in HEK293 cells.

Kinetic analysis of the internalization and recycling of [3H]TRH and C-terminal truncations of the long isoform of the rat thyrotropin-releasing hormone receptor-1

The C-terminal tail of the long splice variant of the rat thyrotropin-releasing hormone (TRH) receptor-1 (TRHR-1L) comprises around 93 amino acids. A series of C-terminal truncations was constructed and expressed transiently in HEK-293 cells. The extent of steady-state internalization of these in response to [(3)H]TRH was dependent upon the degree of truncation. Little effect was produced by deletion of the C-terminal to 50 amino acids, although there was a substantial decrease in the extent of internalization by deletion to 45-46 amino acids. The rate of internalization of TRHR-1L in response to ligand was substantially decreased by the acid-wash procedures often used in the analysis of cellular distribution of receptors with peptide ligands, and thus an alternative procedure using a Mes-containing buffer was employed in the present study. Apart from a truncation anticipated to eliminate post-translational acylation of the re-ceptor, which altered both the association and dissociation rates of [(3)H]TRH, the kinetics of ligand binding were unaffected by C-terminal truncation. Equally, the rate of recycling to the plasma membrane of internalized receptors was unaffected by C-terminal truncation. Although the extent of internalization of the full-length receptor was impaired by pre-exposure of cells to TRH, this was not true of C-terminal truncation mutants, which displayed limited steady-state internalization ratios. A mutant with a substantial C-terminal deletion also displayed decreased functional desensitization compared with the full-length receptor.

Functional characterization of corticotropin-releasing factor type 1 receptor endogenously expressed in human embryonic kidney 293 cells

The endogenous expression in human embryonic kidney 293 (HEK293) cells of corticotropin-releasing factor (CRF) receptors was detected. High-affinity binding sites for human CRF (K(i)=3.6 nM), ovine CRF (K(i)=4.6 nM), rat urocortin (K(i)=2.2 nM), sauvagine (K(i)=2.4 nM) and astressin (K(i)=4.3 nM) with the pharmacological characteristics for CRF type 1 (CRF(1)) receptors and B(max) values of approximately 30 fmol/mg protein were determined. The four CRF receptor agonists nonselectively stimulated cAMP production in HEK293 cells at low agonist concentrations, whereas the antagonist astressin shifted the dose-response curve for ovine CRF significantly rightward. Transfection of the pcDNA3 vector into HEK293 cells strongly reduced the expression of the endogenous CRF receptor. Northern blot analysis revealed the expression of a CRF(1) transcript in human neuronal tissues, HEK293, human NTera-2 (NT2) carcinoma, Y-79 retinoblastoma and African green monkey kidney (COS-7) cells. Neither by Northern blot analysis nor by reverse transcriptase PCR (RT-PCR), the expression of CRF(2) could be detected. In cAMP stimulation experiments, functional CRF receptors were detected in these cell lines. These data show that HEK293 and other cell lines endogenously express CRF(1) receptors.

The rat gonadotropin-releasing hormone receptor internalizes via a beta-arrestin-independent, but dynamin-dependent, pathway: addition of a carboxyl-terminal tail confers beta-arrestin dependency

This study examined the mechanism underlying the rat GnRH receptor (GnRH-R) internalization pathway by investigating the role of added/extended C-terminal tails and the effect of beta-arrestins and dynamin. The internalization of the wild-type (WT) rat GnRH-R, stop codon mutants, GnRH-R/TRH receptor (TRH-R) chimera, rat TRH-R, and catfish GnRH-R was examined using radioligand binding assay. Overexpression of beta-arrestin in COS-7 cells expressing each of the receptor constructs substantially increased endocytosis rate constants (k(e)) of the TRH-R, catfish GnRH-R, and GnRH-R/TRH-R chimera, but not of the WT rat GnRH-R and stop codon mutants. The beta-arrestin-promoted increase in the k(e) value was diminished by cotransfecting cells with the dominant negative beta-arrestin-(319-418) mutant, whereas WT GnRH-R and stop codon mutant internalization were unaffected. Additionally, confocal microscopy showed that activated GnRH-Rs failed to induce time-dependent redistribution of either beta-arrestin-1- or beta-arrestin-2-green fluorescent protein conjugate to the plasma membrane. However, the dominant negative dynamin (DynK44A) mutant impaired internalization of all of the receptors regardless of their beta-arrestin dependency, indicating that they internalize via a clathrin-mediated pathway. We conclude that the mammalian GnRH-R uses a beta-arrestin-independent, dynamin-dependent internalization mechanism distinct from that employed by the other receptors studied.

Overexpression of the G protein G11alpha prevents desensitization of Ca2+ response to thyrotropin-releasing hormone

Doubly transfected human embryonal kidney cells (clone E2M11 of the HEK 293 cell line) expressing both thyrotropin-releasing hormone (TRH) receptors and G11alpha protein in high amounts were used to analyze the desensitization phenomenon of the Ca2+-mobilizing pathway. Quite unexpectedly, we did not observe any significant desensitization of the [Ca2+]i response to TRH in these cells after repeated or prolonged incubation with the hormone (up to 5 h). Under the same conditions, the TRH-induced [Ca2+]i response was completely desensitized in the parent cell line (293-E2 cels) expressing TRH receptors alone. In both cell lines, inositol phosphate response was desensitized after TRH exposure, although basal levels of inositol phospates in TRH-pretreated cells were much higher than in "naive" TRH-unexposed cells. These data suggest a significant role of the G protein G11alpha in desensitization of the Ca2+-mobilizing pathway occuring after repeated or long-term exposure of target cells to TRH-receptor agonists.

Visualization of agonist-induced association and trafficking of green fluorescent protein-tagged forms of both beta-arrestin-1 and the thyrotropin-releasing hormone receptor-1

A fusion protein (beta-arrestin-1-green fluorescent protein (GFP)) was constructed between beta-arrestin-1 and a modified form of the green fluorescent protein from Aequorea victoria. Expression in HEK293 cells allowed immunological detection of an 82-kDa cytosolic polypeptide with antisera to both beta-arrestin-1 and GFP. Transient expression of this construct in HEK293 cells stably transfected to express the rat thyrotropin-releasing hormone receptor-1 (TRHR-1) followed by confocal microscopy allowed its visualization evenly distributed throughout the cytoplasm. Addition of thyrotropin-releasing hormone (TRH) caused a profound and rapid redistribution of beta-arrestin-1-GFP to the plasma membrane followed by internalization of beta-arrestin-1-GFP into distinct, punctate, intracellular vesicles. TRH did not alter the cellular distribution of GFP transiently transfected into these cells nor the distribution of beta-arrestin-1-GFP following expression in HEK293 cells lacking the receptor. To detect potential co-localization of the receptor and beta-arrestin-1 in response to agonist treatment, beta-arrestin-1-GFP was expressed stably in HEK293 cells. A vesicular stomatitis virus (VSV)-tagged TRHR-1 was then introduced transiently. Initially, the two proteins were fully resolved. Short term exposure to TRH resulted in their plasma membrane co-localization, and sustained exposure to TRH resulted in their co-localization in punctate, intracellular vesicles. In contrast, beta-arrestin-1-GFP did not relocate or adopt a punctate appearance in cells that did not express VSV-TRHR-1. Reciprocal experiments were performed, with equivalent results, following transient expression of beta-arrestin-1 into cells stably expressing VSVTRHR-1-GFP. These results demonstrate the capacity of beta-arrestin-1-GFP to interact with the rat TRHR-1 and directly visualizes their recruitment from cytoplasm and plasma membrane respectively into overlapping, intracellular vesicles in an agonist-dependent manner.

Neuroregulation of ProTRH biosynthesis and processing

This review presents an overview of the current knowledge on proTRH biosynthesis, its processing, its tissue distribution, and the role of known processing enzymes in proTRH maturation. The neuroendocrine regulation of TRH biosynthesis, the biological actions of its products, and the signal transduction and catabolic pathways used by those products are also reviewed. The widespread expression of proTRH, PC1, and PC2 rnRNAs in hypophysiotropic and extrahypophysiotropic areas of the brain, with their overlapping distribution in many areas, indicates the striking versatility provided by tissue-specific processing in generating quantitative and qualitative differences in nonTRH peptide products as well as TRH. Evidence is presented suggesting that differential processing for proTRH at the intracellular level is physiologically relevant. It is clear that control over the diverse range of proTRH-derived peptides within a specific cell is accomplished most from the regulation at the posttranslational level rather than the translational or transcriptional levels. Several examples supporting this hypothesis are presented in this review. A better understanding of proTRH-derived peptides role represents an exciting new frontier in proTRH research. These connecting sequences in between TRH molecules to form the precursor protein may function as structural or targeting elements that guide the folding and sorting of proTRH and its larger intermediates so that subsequent processing and secretion are properly regulated. The particular anatomical distribution of the proTRH end products, as well as regulation of their levels by neuroendocrine or pharmacological manipulations, supports a unique potential biologic role for these peptides.

Real time visualization of agonist-mediated redistribution and internalization of a green fluorescent protein-tagged form of the thyrotropin-releasing hormone receptor

The long isoform of the rat thyrotropin-releasing hormone receptor (TRHR) was modified by the addition of a vesicular stomatitis virus (VSV) epitope tag and green fluorescent protein (GFP). VSV-TRHR-GFP bound TRH with affinity similar to that of the unmodified receptor and stimulated [3H]inositol phosphate production. A clone stably expressing VSV-TRHR-GFP at some 120,000 copies/cell was selected to visualize this receptor during cellular exposure to TRH. Internalization was detected within 3-5 min after treatment with 1 x 10(-7) M TRH, with dramatic reductions in plasma membrane localization achieved within 10-15 min. The TRHR antagonist/inverse agonist chlordiazepoxide competitively inhibited internalization. Hyperosmotic sucrose inhibited internalization of VSV-TRHR-GFP, measured both by intact cell [3H]TRH binding studies and by confocal microscopy. Now TRH caused a redistribution of VSV-TRHR-GFP to highly punctate but plasma membrane-delineated foci. Pretreatment with the microtubule-disrupting agent nocodazole allowed internalization of the VSV-TRHR-GFP construct but only into vesicles that remained in close apposition to the plasma membrane. Covisualization of VSV-TRHR-GFP and Texas Red transferrin initially indicated entirely separate localizations. After exposure to TRH substantial amounts of VSV-TRHR-GFP were present in vesicles overlapping those containing Texas Red transferrin. Such results demonstrate the G protein-coupling capacity and provide real time visualization of the processes of internalization of a TRH-receptor-GFP construct in response to agonist.

Gonadotropin-releasing hormone receptors with intracellular carboxyl-terminal tails undergo acute desensitization of total inositol phosphate production and exhibit accelerated internalization kinetics

The mammalian gonadotropin-releasing hormone receptor (GnRH-R) is the only G-protein-coupled receptor (GPCR) in which the intracellular C-terminal tail is completely absent. In contrast to other GPCRs, the GnRH-R does not show rapid desensitization of total inositol (IP) production, and the rates of internalization are exceptionally slow. We investigated whether the incorporation of a cytoplasmic tail into the C terminus of the GnRH-R affects desensitization events and receptor internalization rates. A GnRH-R/TRH-R chimera was created where the intracellular tail of the rat thyrotropin-releasing hormone receptor (TRH-R) was engineered into the C terminus of the rat GnRH-R. Three different rat GnRH-R cDNA stop codon mutations (one for each reading frame) were also made. The GnRH-stimulated IP production of the wild-type rat GnRH-R expressed in either COS-7 or HEK 293 cells did not desensitize even after prolonged stimulation with GnRH. In contrast, the catfish GnRH-R (which does possess an intracellular tail) and the TRH-R rapidly (<10 min) desensitized following agonist stimulation. The GnRH-R/TRH-R chimera also desensitized following treatment with GnRH, resembling the pattern shown by the TRH-R and the catfish GnRH-R. Two of the stop codon mutants did not show desensitization of IP production, and the third mutant with the longest tail was not functional. Internalization experiments showed that the rat GnRH-R had the slowest endocytosis and recycling rates compared with the TRH-R, the catfish GnRH-R, and the chimeric GnRH/TRH-R. This study demonstrates that the addition of a functional intracellular C-terminal tail to the GnRH-R produces rapid desensitization of IP production and significantly increases internalization rates.

Comparison of the Ca2+ movement by activation of alpha1-adrenoceptor subtypes in HEK-293 cells

We studied the Ca2+ movement induced by activation of alpha1A-, alpha1B- and alpha1D-adrenoceptor subtypes in transfected HEK-293 cells with the fura-2 probe. All these alpha1-AR subtypes induced both Ca2+ release and Ca2+ entry. The effect on Ca2+ release in alpha1b transfected HEK-293 cells was bigger than that in alpha1a and alpha1d transfected HEK-293 cells, and the effects on Ca2+ entry were the same in alpha1a, alpha1b and alpha1d transfected HEK-293 cells. The Ca2+ entry was inhibited by 1 mM NiSO4, but not by nifedipine. Cyclopiazonic acid (CPA) produced a biphasic Ca2+ signal response in Ca2+ medium, and only induced a transient response in Ca2+-free medium. After depletion of CPA-sensitive Ca2+ pool by 10 microM CPA in Ca2+-free medium, 10 microM adrenaline (Adr) still transiently increased [Ca2+]i in three different alpha1-adrenoceptor subtype transfected HEK-293 cells. However, after depletion of adrenaline-sensitive Ca2+ pool by 10 microM Adr, CPA transiently elevated [Ca2+]i only in alpha1a and alpha1d transfected HEK-293 cells, not in alpha1b transfected HEK-293 cells. U73122, a phospholipase C (PLC) inhibitor, inhibited both Ca2+ release and Ca2+ entry induced by activation of alpha1A alpha1B and alpha1D subtypes in transfected HEK-293 cells. These results suggest that HEK-293 cell line contains two functionally separate intracellular Ca2+ pools, CPA-sensitive and Adr-sensitive pools. Activation of alpha1B-AR stimulates Ca2+ release from both CPA-sensitive and Adr-sensitive Ca2+ pools. Alpha1A and alpha1D subtypes induce Ca2+ release only from Adr-sensitive Ca2+ pool.

Desensitization of thyrotropin-releasing hormone receptor-mediated responses involves multiple steps

Desensitization and recovery of the inositol 1,4,5-trisphosphate (IP3) and intracellular free calcium concentration ([Ca2+]i) responses to thyrotropin-releasing hormone (TRH) were measured in HEK293 cells stably expressing the G protein-coupled TRH receptor. TRH caused a large, rapid, and transient increase in IP3 and a biphasic increase in [Ca2+]i. Desensitization of the TRH response was measured by exposing cells to TRH, washing, and then incubating the cells in hormone-free medium before reintroducing TRH and measuring IP3, [Ca2+]i, and intracellular Ca2+ pool size. When cells were incubated with 1 microM TRH for 10 s or 10 min and reexposed to TRH, there was almost no IP3 or [Ca2+]i increase. The IP3 response recovered first, followed by the [Ca2+]i response. The ionomycin-releasable intracellular Ca2+ pool was almost completely depleted by TRH, and pool refilling was slow. Thrombin, endothelin, and carbachol, when combined, stimulated large increases in IP3 and [Ca2+]i, but did not block the IP3 or [Ca2+]i responses to TRH measured 10 min later. In contrast, cells exposed to TRH first responded to combined agonists with a nearly normal increase in IP3, but no rise in [Ca2+]i. Thus, the IP3 response to TRH displays homologous desensitization, whereas the [Ca2+]i response displays heterologous desensitization because depletion of intracellular Ca2+ pools prevents responses to other hormones.

Does nitric oxide regulate capacitative Ca influx in HEK 293 cells?

It has been proposed that capacitative Ca influx into both pancreatic acinar cells and HT-29 colonic cells is regulated by stimulation of nitric oxide synthase (NOS). NO, in turn, controls cGMP levels through effects on guanylate cyclase. We tested this possibility by measuring Ca (and Ba) entry into human embryonic kidney 293 cells and into 293 cells that had been transfected with the neuronal NOS gene (293/NOS). 293 cells had undetectable levels of NOS, while 293/NOS cells exhibited very high levels [Bredt D.S., Ferris C.D., Snyder S.H. Nitric oxide synthase regulatory sites. J Biol Chem 1992; 267: 10976-10981]. Ca (or Ba) entry into single cells was measured as the rate of increase of the Fura-2 fluorescence ratio (digital imaging microscopy) during rapid changes from Ca-free (or Ba-free) to Ca- (or Ba-) containing solutions (using high K to depolarize the membrane potential). cGMP levels (EIA method) were measured to correlate to rates of Ca entry. 100 microM ATP caused release of Ca from internal stores, but no sustained plateau due to Ca entry in either 293 or 293/NOS cells. Cyclopiazonic acid (CPA, which inhibits the Ca pump of the internal store, allowing Ca to leak from the store) caused apparent Ca entry to increase 5-10-fold from similar, low levels in both 293 and 293/NOS cells. CPA-stimulated Ca entry was unaffected by the NOS inhibitor N-nitro-L-arginine (L-NA) in either 293 or 293/NOS cells. In 293 cells [cGMP] was low; ATP and CPA both increased [cGMP] by 2-fold, and the guanylate cyclase inhibitor LY83583 and L-NA decreased [cGMP] by 50-75%. [cGMP] was 20-fold higher in 293/NOS cells than in 293 cells; these [cGMP] were not affected by ATP and CPA, but were effectively decreased by 80-90% by L-NA and by LY83583. Thus, [cGMP] and Ca or Ba entry showed no relationship to each other: Ca entry was small into cells in which [cGMP] was either low (resting 293, CPA + L-NA or CPA + LY83583), intermediate (ATP-treated 293) or high (resting 293/NOS). Similarly, Ca entry was high into cells in which [cGMP] was low (CPA + L-NA- or CPA + LY83583-treated 293), intermediate (CPA-treated 293 and CPA + L-NA-treated 293/NOS) or high (CPA- or ATP-treated 293/NOS). We conclude that, as in most other non-excitable cells, Ca entry into 293 cells is stimulated by loss of Ca from the store but, unlike pancreatic and colonic cells, this capacitative Ca entry does not appear to be regulated by NO and cGMP. Therefore, although capacitative entry across the plasma membrane may be regulated by NO and cGMP in Gl epithelial cells, this regulation does not occur in all cells.