A miniaturized cell-based fluorescence resonance energy transfer assay for insulin-receptor activation

This report describes the development, optimization, and implementation of a miniaturized cell-based assay for the identification of small-molecule insulin mimetics and potentiators. Cell-based assays are attractive formats for compound screening because they present the molecular targets in their cellular environment. A fluorescence resonance energy transfer (FRET) cell-based assay that measures the insulin-dependent colocalization of Akt2 fused with either cyan fluorescent protein or yellow fluorescent protein to the cellular membrane was developed. This ratiometric FRET assay was miniaturized into a robust, yet sensitive 3456-well nanoplate assay with Z' factors of approximately 0.6 despite a very small assay window (less than twofold full activation with insulin). The FRET assay was used for primary screening of a large compound collection for insulin-receptor agonists and potentiators. To prioritize compounds for further development, primary hits were tested in two additional assays, a biochemical time-resolved fluorescence resonance energy transfer assay to measure insulin-receptor phosphorylation and a translocation-based imaging assay. Results from the three assays were combined to yield 11 compounds as potential leads for the development of insulin mimetics or potentiators.

Rescue of ΔF508-CFTR trafficking and gating in human cystic fibrosis airway primary cultures by small molecules

Cystic fibrosis (CF) is a fatal genetic disease caused by mutations in cftr, a gene encoding a PKA-regulated Cl−channel. The most common mutation results in a deletion of phenylalanine at position 508 (ΔF508-CFTR) that impairs protein folding, trafficking, and channel gating in epithelial cells. In the airway, these defects alter salt and fluid transport, leading to chronic infection, inflammation, and loss of lung function. There are no drugs that specifically target mutant CFTR, and optimal treatment of CF may require repair of both the folding and gating defects. Here, we describe two classes of novel, potent small molecules identified from screening compound libraries that restore the function of ΔF508-CFTR in both recombinant cells and cultures of human bronchial epithelia isolated from CF patients. The first class partially corrects the trafficking defect by facilitating exit from the endoplasmic reticulum and restores ΔF508-CFTR-mediated Cl−transport to more than 10% of that observed in non-CF human bronchial epithelial cultures, a level expected to result in a clinical benefit in CF patients. The second class of compounds potentiates cAMP-mediated gating of ΔF508-CFTR and achieves single-channel activity similar to wild-type CFTR. The CFTR-activating effects of the two mechanisms are additive and support the rationale of a drug discovery strategy based on rescue of the basic genetic defect responsible for CF.

IL-converting enzyme/caspase-1 inhibitor VX-765 blocks the hypersensitive response to an inflammatory stimulus in monocytes from familial cold autoinflammatory syndrome patients

Familial cold autoinflammatory syndrome (FCAS) and the related autoinflammatory disorders, Muckle-Wells syndrome and neonatal onset multisystem inflammatory disease, are characterized by mutations in the CIAS1 gene that encodes cryopyrin, an adaptor protein involved in activation of IL-converting enzyme/caspase-1. Mutations in cryopyrin are hypothesized to result in abnormal secretion of caspase-1-dependent proinflammatory cytokines, IL-1beta and IL-18. In this study, we examined cytokine secretion in PBMCs from FCAS patients and found a marked hyperresponsiveness of both IL-1beta and IL-18 secretion to LPS stimulation, but no evidence of increased basal secretion of these cytokines, or alterations in basal or stimulated pro-IL-1beta levels. VX-765, an orally active IL-converting enzyme/caspase-1 inhibitor, blocked IL-1beta secretion with equal potency in LPS-stimulated cells from FCAS and control subjects. These results further link mutations in cryopyrin with abnormal caspase-1 activation, and support the clinical testing of caspase-1 inhibitors such as VX-765 in autoinflammatory disorders.

A FRET-based assay platform for ultra-high density drug screening of protein kinases and phosphatases

Protein phosphorylation is one of the major regulatory mechanisms involved in signal-induced cellular events, including cell proliferation, apoptosis, and metabolism. Because many facets of biology are regulated by protein phosphorylation, aberrant kinase and/or phosphatase activity forms the basis for many different types of pathology. The disease relevance of protein kinases and phosphatases has led many pharmaceutical and biotechnology companies to expend significant resources in lead discovery programs for these two target classes. The existence of >500 kinases and phosphatases encoded by the human genome necessitates development of methodologies for the rapid screening for novel and specific compound inhibitors. We describe here a fluorescence-based, molecular assay platform that is compatible with robotic, ultra-high throughput screening systems and can be applied to virtually all tyrosine and serine/threonine protein kinases and phosphatases. The assay has a coupled-enzyme format, utilizing the differential protease sensitivity of phosphorylated versus nonphosphorylated peptide substrates. In addition to screening individual kinases, the assay can be formatted such that kinase pathways are re-created in vitro to identify compounds that specifically interact with inactive kinases. Miniaturization of this assay format to the 1-microl scale allows for the rapid and accurate compound screening of a host of kinase and phosphatase targets, thereby facilitating the hunt for new leads for these target classes.

A collaborative screening program for the discovery of inhibitors of HCV NS2/3 cis-cleaving protease activity

This report describes the development of a cell-based assay for high-throughput screening and detection of small-molecule inhibitors for hepatitis C virus (HCV) NS2/3 protease. The HCV NS2/3 protease is essential for the normal infectious cycle of HCV. Generation of a cell-based assay for this cis-acting viral protease involved reporter constructs in which the NS2/3 protease sequence was inserted between the ,B-lactamase (BLA) reporter and a ubiquitin-based destabilization domain. In stable cell lines, NS2/3 cis cleavage of the NS2/3-BLA fusion protein resulted in differential stability of the cleaved versus uncleaved BLA reporter, providing a robust readout for protease activity. BLA reporter activity was shown to be a function of NS2/3-specific protease activity, by using genetic mutants of the NS2/3 sequence. In addition, the cell-based assay was validated and screened in a 384-well format on a fully automated robotic platform where small-molecule inhibitors of NS2/3 protease activity were identified.

A ubiquitin-based tagging system for controlled modulation of protein stability

Many biotechnology applications depend on the expression of exogenous proteins in a predictable and controllable manner. A key determinant of the intracellular concentration of a given protein is its stability or "half-life." We have developed a versatile and reliable system for producing short half-life forms of proteins expressed in mammalian cells. The system consists of a series of destabilization domains composed of varying numbers of a mutant form of ubiquitin (UbG76V) that cannot be cleaved by ubiquitin hydrolases. We show that increasing the number of UbG76V moieties within the destabilization domain results in a graded decrease in protein half-life and steady-state levels when fused to heterologous reporter proteins as well as cellular proteins. Cells expressing a destabilized beta-lactamase reporter act as a robust, high-throughput screening (HTS)-compatible assay for proteasome activity within cells.

Developmentally regulated activation of apoptosis early in Xenopus gastrulation results in cyclin A degradation during interphase of the cell cycle

Previous work identified a developmental timer that controls the stability of cyclin A protein in interphase-arrested Xenopus embryos. It was shown that cyclins A1 and A2 abruptly become unstable in hydroxyurea-treated embryos at the time that untreated embryos are beginning gastrulation (early gastrulation transition; EGT). We have demonstrated here that cyclins A1 and A2 are degraded at the equivalent of the EGT by the ICE-like caspases that are responsible for programmed cell death or apoptosis. Analysis of embryos treated with hydroxyurea or cycloheximide showed widespread cellular apoptosis coincident with cyclin A cleavage. Our data further indicate that the apoptotic pathway is present in Xenopus embryos prior to the EGT; however, it is maintained in an inactive state in early cleaving embryos by maternally encoded inhibitors. Characterization of the timing of the activation of apoptosis implicates the initiation of zygotic transcription at the mid-blastula transition (MBT) in the suppression of apoptosis in normal embryos. The decreased biosynthetic capacity of embryos treated with hydroxyurea or cycloheximide most likely interferes with the ability to maintain sufficient levels of apoptotic inhibitors and results in widespread apoptosis. Our results suggest a scenario whereby the apoptotic pathway is suppressed in the early cleaving embryo by maternally contributed inhibitors. Degradation at the EGT of maternal RNAs encoding these inhibitors is compensated for by new zygotic transcription beginning at the MBT. This indicates that the interval between the MBT and the EGT represents a critical developmental period during which the regulation of embryonic cellular processes is transferred from maternal to zygotic control.

Receptor-mediated protein sorting to the vacuole in yeast: roles for a protein kinase, a lipid kinase and GTP-binding proteins

In this review we summarize the structural and functional characteristics of the VPS (vacuolar protein sorting) gene products that have provided insight into the regulatory interactions and molecular mechanisms underlying protein sorting pathways in eukaryotic cells. Genetic selections in yeast have resulted in the identification of more than 40 genes required for the vesicle-mediated sorting of proteins to the lysosome-like vacuole. Molecular characterization of these VPS gene products has revealed a number of biochemical activities involved in this process. Analogous to the mannose-6-phosphate receptors in mammalian cells, the VPS10 gene encodes a transmembrane sorting receptor for the yeast vacuolar hydrolase carboxypeptidase Y. The VPS15 and VPS34 genes encode components of a novel signal transduction complex essential for the delivery of soluble vacuolar hydrolases. VPS15 and VPS34 encode a serine/ threonine protein kinase and a phosphatidylinositol 3-kinase, respectively, that interact at the cytoplasmic face of an intracellular membrane compartment, most likely corresponding to the late Golgi. Other VPS gene products have homologues that are involved in membrane trafficking pathways: The VPSI and VPS21 genes encode GTPases of the dynamin and rab families, respectively, and the products of the VPS33, VPS45, and PEP12/VPS6 genes are homologues of proteins involved in regulated synaptic vesicle exocytosis. The VPS gene products constitute components of a molecular apparatus responsible for the recognition, packaging, and vesicular transport of proteins to the vacuole in yeast.

Vesicle-mediated protein transport: regulatory interactions between the Vps15 protein kinase and the Vps34 PtdIns 3-kinase essential for protein sorting to the vacuole in yeast

A membrane-associated complex composed of the Vps15 protein kinase and the Vps34 phosphatidylinositol 3-kinase (PtdIns 3-kinase) is essential for the delivery of proteins to the yeast vacuole. An active Vps15p is required for the recruitment of Vps34p to the membrane and subsequent stimulation of Vps34p PtdIns 3-kinase activity. Consistent with this, mutations altering highly conserved residues in the lipid kinase domain of Vps34p lead to a dominant-negative phenotype resulting from titration of activating Vps15 proteins. In contrast, catalytically inactive Vps15p mutants do not produce a dominant mutant phenotype because they are unable to associate with Vps34p in a wild-type manner. These data indicate that an intact Vps15p protein kinase domain is necessary for the association with and activation of Vps34p, and they demonstrate that a functional Vps15p-Vps34p complex is absolutely required for the efficient delivery of proteins to the vacuole. Analysis of a temperature-conditional allele of VPS15, in which a shift to the nonpermissive temperature leads to a decrease in cellular PtdIns(3)P levels, indicates that the loss of Vps15p function leads to a defect in activation of Vps34p. In addition, characterization of a temperature-sensitive allele of VPS34 demonstrates that inactivation of Vps34p leads to the immediate missorting of soluble vacuolar proteins (e.g., carboxypeptidase Y) without an apparent defect in the sorting of the vacuolar membrane protein alkaline phosphatase. This rapid block in vacuolar protein sorting appears to be the result of loss of PtdIns 3-kinase activity since cellular PtdIns(3)P levels decrease dramatically in vps34 temperature-sensitive mutant cells that have been incubated at the nonpermissive temperature. Finally, analysis of the defects in cellular PtdIns(3)P levels in various vps15 and vsp34 mutant strains has led to additional insights into the importance of PtdIns(3)P intracellular localization, as well as the roles of Vps15p and Vps34p in vacuolar protein sorting.

Vps34p required for yeast vacuolar protein sorting is a multiple specificity kinase that exhibits both protein kinase and phosphatidylinositol-specific PI 3-kinase activities

The Vps15 protein kinase and the Vps34 phosphatidylinositol 3-kinase have been shown to function as a membrane-associated complex which facilitates the delivery of proteins to the vacuole in yeast. Biochemical characterization of the autophosphorylation reaction catalyzed by Vps15p demonstrates that it is a functional serine/threonine protein kinase. In addition, we show that the Vps34 phosphatidylinositol 3-kinase undergoes an autophosphorylation event both in vivo and in vitro, indicating that it represents a novel multiple specificity kinase capable of phosphorylating both protein and lipid substrates. Vps34p is phosphorylated predominately on serine in vivo and is able to phosphorylate serine, threonine, and tyrosine residues in vitro. Mutant Vps34 proteins containing alterations in conserved amino acids in the lipid kinase domain are severely defective for both PI 3-kinase activity and autophosphorylation. Characterization of the PI 3-kinase activity of Vps34p demonstrates that it, unlike the mammalian p110 PI 3-kinase, is highly resistant to the PI 3-kinase inhibitors wortmannin and LY294002. We also find that Vps34p is a phosphatidylinositol-specific 3-kinase, as it is able to utilize phosphatidylinositol (PtdIns) but not PtdIns(4)P or PtdIns(4,5)P2 as substrates in an in vitro PI kinase reaction. The substrate specificity, wortmannin resistance, and other biochemical characteristics of its PtdIns 3-kinase activity suggest that Vps34p is quite similar to a PtdIns-specific 3-kinase activity recently characterized from mammalian cells. These data indicate the existence of a family of PI 3-kinases composed of p110-like PI 3-kinases and Vps34p-like PtdIns-specific 3-kinases. On the basis of the role for Vps34p in vacuolar protein sorting, we propose that the production of a specific phosphoinositide, PtdIns(3)P, is involved in regulating intracellular protein sorting reactions in eukaryotic cells.

Genetic and biochemical studies of protein sorting to the yeast vacuole

Studies of vacuolar protein sorting in yeast are revealing important new insights into the mechanisms of intracellular membrane trafficking. Recent work has raised questions about the prevailing view concerning membrane protein transport to the vacuole, and has indicated roles in vacuolar protein sorting for GTP-binding proteins, clathrin, a serine/threonine protein kinase and a phosphatidylinositol 3-kinase.

A membrane-associated complex containing the Vps15 protein kinase and the Vps34 PI 3-kinase is essential for protein sorting to the yeast lysosome-like vacuole

The Vps15 protein kinase and the Vps34 phosphatidylinositol 3-kinase (PI 3-kinase) are required for the sorting of soluble hydrolases to the yeast vacuole. Over-production of Vps34p suppresses the growth and vacuolar protein sorting defects associated with vps15 kinase domain mutants, suggesting that Vps15p and Vps34p functionally interact. Subcellular fractionation and sucrose density gradients indicate that Vps15p is responsible for the association of Vps34p with an intracellular membrane fraction. Chemical cross-linking and native immunoprecipitation experiments demonstrate that Vps15p and Vps34p interact as components of a hetero-oligomeric protein complex. In addition, we show that an intact Vps15 protein kinase domain is required for activation of the Vps34 PI 3-kinase, suggesting that the Vps34 lipid kinase is regulated by a Vps15p-mediated protein phosphorylation event. We propose that Vps15p and Vps34p function together as components of a membrane-associated signal transduction complex that regulates intracellular protein trafficking decisions through protein and lipid phosphorylation events.

Phosphatidylinositol 3-kinase encoded by yeast VPS34 gene essential for protein sorting

The VPS34 gene product (Vps34p) is required for protein sorting to the lysosome-like vacuole of the yeast Saccharomyces cerevisiae. Vps34p shares significant sequence similarity with the catalytic subunit of bovine phosphatidylinositol (PI) 3-kinase [the 110-kilodalton (p110) subunit of PI 3-kinase], which is known to interact with activated cell surface receptor tyrosine kinases. Yeast strains deleted for the VPS34 gene or carrying vps34 point mutations lacked detectable PI 3-kinase activity and exhibited severe defects in vacuolar protein sorting. Overexpression of Vps34p resulted in an increase in PI 3-kinase activity, and this activity was specifically precipitated with antisera to Vps34p. VPS34 encodes a yeast PI 3-kinase, and this enzyme appears to regulate intracellular protein trafficking decisions.

An essential role for a protein and lipid kinase complex in secretory protein sorting

Yeast genetics has identified more than 40 genes involved in the biogenesis and maintenance of the yeast lysosome-like vacuole. Recent data on two of these genes, VPS15 and VPS34, are beginning to provide some fundamental insights into the mechanisms governing protein sorting within the eukaryotic secretory pathway. VPS15 and VPS34 encode a novel protein kinase and a phosphatidylinositol 3-kinase, respectively, that function together as components of a membrane-associated signal transduction complex. These studies of the VPS15-VPS34 complex indicate that intracellular protein trafficking decisions may be regulated by protein phosphorylation and phosphatidylinositol signalling events.

A genetic and structural analysis of the yeast Vps15 protein kinase: evidence for a direct role of Vps15p in vacuolar protein delivery

The yeast VPS15 gene encodes a novel protein kinase homolog that is required for the sorting of soluble hydrolases to the yeast vacuole. In this study, we extend our previous mutational analysis of the VPS15 gene and show that alterations of specific Gps15p residues, that are highly conserved among all protein kinase molecules, result in the biological inactivation of Vps15p. Furthermore, we demonstrate here that short C-terminal deletions of Vps15p result in a temperature-conditional defect in vacuolar protein sorting. Immediately following the temperature shift, soluble vacuolar hydrolases, such as carboxypeptidase Y and proteinase A, accumulate as Golgi-modified precursors within a saturable intracellular compartment distinct from the vacuole. This vacuolar protein sorting block is efficiently reversed when mutant cells are shifted back to the permissive temperature; the accumulated precursors are rapidly processed to their mature forms indicating that they have been delivered to the vacuole. This rapid and efficient reversal suggests that the accumulated vacuolar protein precursors were present within a normal transport intermediate in the vacuolar protein sorting pathway. In addition, this protein delivery block shows specificity for soluble vacuolar enzymes as the membrane protein, alkaline phosphatase, is efficiently delivered to the vacuole at the non-permissive temperature. Interestingly, the C-terminal Vps15p truncations are not phosphorylated in vivo suggesting that the phosphorylation of Vps15p may be critical for its biological activity at elevated temperatures. The rapid onset and high degree of specificity of the vacuolar protein delivery block in these mutants suggests that the primary role of Vps15p is to regulate the sorting of soluble hydrolases to the yeast vacuolar compartment.

A novel protein kinase homolog essential for protein sorting to the yeast lysosome-like vacuole

The VPS15 gene encodes a novel protein kinase homolog that is essential for the efficient delivery of soluble hydrolases to the yeast vacuole. Point mutations altering highly conserved residues within the Vps15p kinase domain result in the secretion of multiple vacuolar proteases. In addition, the in vivo phosphorylation of Vps15p is defective in these kinase domain mutants, suggesting that Vps15p may regulate specific protein phosphorylation reactions required for protein sorting to the yeast vacuole. Subcellular fractionation studies further demonstrate that the 1455 amino acid Vps15p is peripherally associated with the cytoplasmic face of a late Golgi or vesicle compartment. This association may be mediated by myristate as Vps15p contains a consensus signal for N-terminal myristoylation. We propose that protein phosphorylation may act as a molecular "switch" within intracellular protein sorting pathways by actively diverting proteins from a default transit pathway (e.g., secretion) to an alternative pathway (e.g., to the vacuole).

Erythroid anion transporter assembly is mediated by a developmentally regulated recruitment onto a preassembled membrane cytoskeleton

Analysis of the expression and assembly of the anion transporter by metabolic pulse-chase and steady-state protein and RNA measurements reveals that the extent of association of band 3 with the membrane cytoskeleton varies during chicken embryonic development. Pulse-chase studies have indicated that band 3 polypeptides do not associate with the membrane cytoskeleton until they have been transported to the plasma membrane. At this time, band 3 polypeptides are slowly recruited, over a period of hours, onto a preassembled membrane cytoskeletal network and the extent of this cytoskeletal assembly is developmentally regulated. Only 3% of the band 3 polypeptides are cytoskeletal-associated in 4-d erythroid cells vs. 93% in 10-d erythroid cells and 36% in 15-d erythroid cells. This observed variation appears to be regulated primarily at the level of recruitment onto the membrane cytoskeleton rather than by different transport kinetics to the membrane or differential turnover of the soluble and insoluble polypeptides and is not dependent upon the lineage or stage of differentiation of the erythroid cells. Steady-state protein and RNA analyses indicate that the low levels of cytoskeletal band 3 very early in development most likely result from limiting amounts of ankyrin and protein 4.1, the membrane cytoskeletal binding sites for band 3. As embryonic development proceeds, ankyrin and protein 4.1 levels increase with a concurrent rise in the level of cytoskeletal band 3 until, on day 10 of development, virtually all of the band 3 polypeptides are cytoskeletal bound. After day 10, the levels of total and cytoskeletal band 3 decline, whereas ankyrin and protein 4.1 continue to accumulate until day 18, indicating that the cytoskeletal association of band 3 is not regulated solely by the availability of membrane cytoskeletal binding sites at later stages of development. Thus, multiple mechanisms appear to regulate the recruitment of band 3 onto the erythroid membrane cytoskeleton during chicken embryonic development.

Regulated expression of multiple chicken erythroid membrane skeletal protein 4.1 variants is governed by differential RNA processing and translational control

Protein 4.1 is an extrinsic membrane protein that facilitates the interaction of spectrin and actin in the erythroid membrane skeleton and exists as several structurally related polypeptides in chickens. The ratio of protein 4.1 variants is developmentally regulated during terminal differentiation of chicken erythroid and lenticular cells. To examine the mechanisms by which multiple chicken protein 4.1 variants are differentially expressed, we have isolated cDNA clones specific for chicken erythroid protein 4.1. We show that a single protein 4.1 gene gives rise to multiple 6.6-kilobase mRNAs by differential RNA processing. Furthermore, the ratios of protein 4.1 mRNAs change during chicken embryonic erythropoiesis. We observe a quantitative difference in variant ratios when protein 4.1 is synthesized in vivo or in a rabbit reticulocyte lysate in vitro. Our results show that the expression of multiple protein 4.1 polypeptides is regulated at the levels of translation and RNA processing.