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Biomedical subjects

A C Porter

Publications and source records attributed to A C Porter.

At least 19 recordsLinked to original sources
PubMed2001

Cannabinoid CB(1) receptor expression, activation and detection of endogenous ligand in trabecular meshwork and ciliary process tissues.

Elevated intraocular pressure is the primary risk factor for glaucoma. Cannabinoids interact with molecular targets in the eye and lower intraocular pressure by an unknown mechanism. The purpose of the present study was to examine eye tissues for functional cannabinoid receptors of the neuronal, CB(1) class, and an endogenous ligand, anandamide. The trabecular meshwork and ciliary processes are the primary structures of the eye that contribute to intraocular pressure and thus were our focus. Total RNA, frozen sections, cellular membranes and primary cultures of cells were prepared from both bovine and cadaveric human tissues. Using cannabinoid CB(1) receptor-specific oligodeoxynucleotide primers, cannabinoid CB(1) receptor antiserum, and cannabinoid-specific compounds (CP-55,940, WIN55,212-2 and SR-141716A), the presence of cannabinoid CB(1) receptors in ciliary processes and trabecular meshwork was determined. Using reverse transcription-polymerase chain reaction, we identified mRNA encoding cannabinoid CB(1) receptor protein in ciliary process and trabecular meshwork cells. Specific binding of anti-CB(1) immunoglobulin-G in tissue sections localized cannabinoid CB(1) receptor protein to the non-pigmented epithelial cells of the ciliary process and cells of the trabecular meshwork. While CP-55,940 and WIN55,212-2 failed to stimulate [(35)S]GTP gamma S binding in membrane preparations from trabecular meshwork and ciliary process, CP-55,940 significantly stimulated whole cell [(35)S]GTP gamma S binding by 51% over basal in ciliary process epithelial cells and 69% over basal in trabecular meshwork cells permeabilized with 5 microM digitonin (p<0.001). Specificity of agonist stimulation was verified by complete blockade with the specific cannabinoid CB(1) receptor antagonist, SR-141716A. Moreover, activation of cannabinoid CB(1) receptors by CP-55,940 resulted in a 2.3+/-0.3 and 1.7+/-0.3-fold stimulation of cAMP accumulation in trabecular meshwork and ciliary process cells, respectively (p<0.01). Lastly, anandamide was detected in human trabecular meshwork (3.08 pmol/g), ciliary process (49.42 pmol/g) and neurosensory retinal (4.48 pmol/g) tissues. These data, for the first time, demonstrate in a single study the presence of both CB(1) mRNA and protein in trabecular meshwork and ciliary processes from two different species. Activation of heterotrimeric G-proteins and stimulation of cAMP accumulation by cannabinoids in vitro suggest that their intraocular pressure-lowering effects in vivo result from activation of cannabinoid CB(1) receptors in the trabecular meshwork and increase aqueous outflow.

Animals↗
PubMed2001

Elucidating the role of muscarinic receptors in psychosis.

Muscarinic receptors have been implicated in the regulation of cognition and psychosis based on pharmacological evidence from pre-clinical and clinical studies. Muscarinic agonists have shown promise in the clinic in improving cognition and reducing psychotic episodes in Alzheimer's patients. However, lack of selective muscarinic ligands has limited their use due to troublesome side effects observed at higher doses. Without selective ligands, it has been difficult to assign a specific muscarinic receptor subtype to these high order mental processes. Recent development of muscarinic receptor knockout mice has provided additional tools to investigate cognition and psychosis in behavioral assays and to determine the receptor subtypes associated with parasympathomimetic physiology. Biochemical studies indicate that the M1 receptor plays a significant role in regulating G alpha q-mediated signal transduction in the hippocampus and cortex. Behavioral studies suggest that the M4 receptor is involved in movement regulation and prepulse inhibition of the startle reflex, a measure of attention. These findings support a role for the development of M1 and M4 receptor agonists for diseases in which symptoms include cognitive impairment and psychotic behaviors.

Alzheimer Disease↗
PubMed2001

Selective silencing of full-length CD80 but not IgV-CD80 leads to impaired clonal deletion of self-reactive T cells and altered regulation of immune responses.

Co-stimulation provided by the B7 family of proteins underpins the development of protective immunity. There are three identified members of this family: CD80, its splice variant IgV-CD80 and CD86. It has hitherto been difficult to analyze the expression and function of IgV-CD80 since there are no appropriate reagents capable of distinguishing it from CD80. We have generated mice, by gene targeting, the lack CD80 whilst maintaining expression of IgV-CD80. Mutant animals did not delete T cells bearing mammary tumor virus-reactive TCR as efficiently as wild-type animals. We also demonstrate the importance of IgV-CD80 in the responses of recently activated cells and reveal a role for CD80 in sustaining T cell responses. CD86, whilst critical to primary T cell activation, made only a minor contribution to re-activation of normal cells.

Animals↗
PubMed2001

The endocannabinoid nervous system: unique opportunities for therapeutic intervention.

The active principle in marijuana, Delta(9)-tetrahydrocannabinol (THC), has been shown to have wide therapeutic application for a number of important medical conditions, including pain, anxiety, glaucoma, nausea, emesis, muscle spasms, and wasting diseases. Delta(9)-THC binds to and activates two known cannabinoid receptors found in mammalian tissue, CB1 and CB2. The development of cannabinoid-based therapeutics has focused predominantly on the CB1 receptor, based on its predominant and abundant localization in the CNS. Like most of the known cannabinoid agonists, Delta(9)-THC is lipophilic and relatively nonselective for both receptor subtypes. Clinical studies show that nonselective cannabinoid agonists are relatively safe and provide therapeutic efficacy, but that they also induce psychotropic side effects. Recent studies of the biosynthesis, release, transport, and disposition of anandamide are beginning to provide an understanding of the role of lipid transmitters in the CNS. This review attempts to link current understanding of the basic biology of the endocannabinoid nervous system to novel opportunities for therapeutic intervention. This new knowledge may facilitate the development of cannabinoid receptor-targeted therapeutics with improved safety and efficacy profiles.

Amidohydrolases↗
PubMed2001

A 'select and swap' strategy for the isolation of clones with tightly regulated transgenes.

Increasing numbers of biological problems are being addressed by genetic approaches that rely on inducible expression of transgenes. It is desirable that expression of such a transgene is tightly regulated, from close to zero expression in the 'off' state, to appreciable (at least physiological) expression in the 'on' state. Although there are many examples where tight regulation has been achieved, certain factors, including chromosomal position effects due to random integration of the transgene, often cause suboptimal inducibility and make the isolation of tightly regulated clones difficult and/or laborious. Here we describe a 'select and swap' strategy for the isolation, from a population of stable transfectants, of clones with tightly regulated transgenes. In this approach, a positively and negatively selectable, inducible marker gene is used to select for clones with optimal transgene regulation. After isolation of such clones, the marker gene is swapped with a linked gene of interest by the use of site-specific recombination. To test this strategy we introduced into human cells a plasmid with a tetracycline-inducible bacterial gpt gene linked to a promoterless luciferase gene, isolated clones with tight gpt expression and used the Cre/loxP site-specific recombination system to swap the gpt gene with the luciferase gene. We discuss ways for refining and developing the system and widening its applicability.

Blotting, Southern↗
PubMed2000

Uracil incorporation into a gene targeting construct reduces the frequency of homologous and nonhomologous recombinants in human cells.

Gene targeting allows the introduction of specific modifications into the eukaryotic genome by homologous recombination, but its efficiency is low in many mammalian systems. We are exploring different ways to increase the efficiency of gene targeting and we report here the effect of uracil incorporation in the targeting construct. Plasmids containing uracil substituting for a fraction of thymine residues are hyperrecombinogenic in some bacterial systems. To test whether a similar stimulation of recombination occurs in mammalian cells, we have prepared a uracil-rich HPRT targeting construct and quantified its homologous and nonhomologous recombination frequencies compared to the same plasmid lacking uracil. The uracil-rich plasmid led to reductions in both homologous and nonhomologous recombination in human cells.

Analysis of Variance↗
PubMed2000

Pancreatic eukaryotic initiation factor-2alpha kinase (PEK) homologues in humans, Drosophila melanogaster and Caenorhabditis elegans that mediate translational control in response to endoplasmic reticulum stress.

In response to different cellular stresses, a family of protein kinases regulates translation by phosphorylation of the alpha subunit of eukaryotic initiation factor-2 (eIF-2alpha). Recently, we identified a new family member, pancreatic eIF-2alpha kinase (PEK) from rat pancreas. PEK, also referred to as RNA-dependent protein kinase (PKR)-like endoplasmic reticulum (ER) kinase (PERK) is a transmembrane protein implicated in translational control in response to stresses that impair protein folding in the ER. In this study, we identified and characterized PEK homologues from humans, Drosophila melanogaster and Caenorhabditis elegans. Expression of human PEK mRNA was found in over 50 different tissues examined, with highest levels in secretory tissues. In mammalian cells subjected to ER stress, we found that elevated eIF-2alpha phosphorylation was coincident with increased PEK autophosphorylation and eIF-2alpha kinase activity. Activation of PEK was abolished by deletion of PEK N-terminal sequences located in the ER lumen. To address the role of C. elegans PEK in translational control, we expressed this kinase in yeast and found that it inhibits growth by hyperphosphorylation of eIF-2alpha and inhibition of eIF-2B. Furthermore, we found that vaccinia virus K3L protein, an inhibitor of the eIF-2alpha kinase PKR involved in an anti-viral defence pathway, also reduced PEK activity. These results suggest that decreased translation initiation by PEK during ER stress may provide the cell with an opportunity to remedy the folding problem prior to introducing newly synthesized proteins into the secretory pathway.

Amino Acid Sequence↗
PubMed2000

A mammalian homologue of GCN2 protein kinase important for translational control by phosphorylation of eukaryotic initiation factor-2alpha.

A family of protein kinases regulates translation in response to different cellular stresses by phosphorylation of the alpha subunit of eukaryotic initiation factor-2 (eIF-2alpha). In yeast, an eIF-2alpha kinase, GCN2, functions in translational control in response to amino acid starvation. It is thought that uncharged tRNA that accumulates during amino acid limitation binds to sequences in GCN2 homologous to histidyl-tRNA synthetase (HisRS) enzymes, leading to enhanced kinase catalytic activity. Given that starvation for amino acids also stimulates phosphorylation of eIF-2alpha in mammalian cells, we searched for and identified a GCN2 homologue in mice. We cloned three different cDNAs encoding mouse GCN2 isoforms, derived from a single gene, that vary in their amino-terminal sequences. Like their yeast counterpart, the mouse GCN2 isoforms contain HisRS-related sequences juxtaposed to the kinase catalytic domain. While GCN2 mRNA was found in all mouse tissues examined, the isoforms appear to be differentially expressed. Mouse GCN2 expressed in yeast was found to inhibit growth by hyperphosphorylation of eIF-2alpha, requiring both the kinase catalytic domain and the HisRS-related sequences. Additionally, lysates prepared from yeast expressing mGCN2 were found to phosphorylate recombinant eIF-2alpha substrate. Mouse GCN2 activity in both the in vivo and in vitro assays required the presence of serine-51, the known regulatory phosphorylation site in eIF-2alpha. Together, our studies identify a new mammalian eIF-2alpha kinase, GCN2, that can mediate translational control.

Amino Acid Sequence↗
PubMed2000

Repression of CDK1 and other genes with CDE and CHR promoter elements during DNA damage-induced G(2)/M arrest in human cells.

Entry into mitosis is controlled by the cyclin-dependent kinase CDK1 and can be delayed in response to DNA damage. In some systems, such G(2)/M arrest has been shown to reflect the stabilization of inhibitory phosphorylation sites on CDK1. In human cells, full G(2) arrest appears to involve additional mechanisms. We describe here the prolonged (>6 day) downregulation of CDK1 protein and mRNA levels following DNA damage in human cells. This silencing of gene expression is observed in primary human fibroblasts and in two cell lines with functional p53 but not in HeLa cells, where p53 is inactive. Silencing is accompanied by the accumulation of cells in G(2), when CDK1 expression is normally maximal. The response is impaired by mutations in cis-acting elements (CDE and CHR) in the CDK1 promoter, indicating that silencing occurs at the transcriptional level. These elements have previously been implicated in the repression of transcription during G(1) that is normally lifted as cells progress into S and G(2). Interestingly, we find that other genes, including those for CDC25C, cyclin A2, cyclin B1, CENP-A, and topoisomerase IIalpha, that are normally expressed preferentially in G(2) and whose promoter regions include putative CDE and CHR elements are also downregulated in response to DNA damage. These data, together with those of other groups, support the existence of a p53-dependent, DNA damage-activated pathway leading to CHR- and CDE-mediated transcriptional repression of various G(2)-specific genes. This pathway may be required for sustained periods of G(2) arrest following DNA damage.

CDC2 Protein Kinase↗
PubMed1999

Signal transduction pathways regulated by arsenate and arsenite.

Arsenate and arsenite activate c-Jun N-terminal kinase (JNK), however, the mechanism by which this occurs is not known. By expressing inhibitory mutant small GTP-binding proteins, p21-activated kinase (PAK) and mitogen-activated protein kinase/extracellular signal-regulated kinase kinase kinases (MEKKs), we have identified specific proteins that are involved in arsenate- and arsenite-mediated activation of JNK. We observe a distinct difference between arsenate and arsenite signaling, which demonstrates that arsenate and arsenite are capable of activating unique proteins. Both arsenate and arsenite activation of JNK requires Rac and Rho. Neither arsenate nor arsenite signaling was inhibited by a dominant-negative mutant of Cdc42 or Ras. Arsenite stimulation of JNK requires PAK, whereas arsenate-mediated activation of JNK was unaffected by inhibitory mutant PAK. Of the four MEKKs tested, only MEKK3 and MEKK4 are involved in arsenate-mediated activation of JNK. In contrast, arsenite-mediated JNK activation requires MEKK2, MEKK3 and MEKK4. These results better define the mechanisms by which arsenate and arsenite activate JNK and demonstrate differences in the regulation of signal transduction pathways by these inorganic arsenic species.

Arsenates↗
PubMed1999

Forced MyHCIIB expression following targeted genetic manipulation of conditionally immortalized muscle precursor cells.

The ability to carry out gene targeting in somatic stem cells while maintaining their stem cell characteristics would have important implications for gene therapy and for the analysis of gene function. Using mouse myoblasts, we have explored this possibility by attempting to alter the promoter of a myosin heavy chain gene (MyHCIIB) characteristic of physiologically "fast" muscle so as to force its unscheduled expression in physiologically "slow" muscle fibers. Conditionally immortalized muscle precursor cells were transfected with a gene targeting construct designed to replace the MyHCIIB promoter with that for the carbonic anhydrase III gene (CAIII), which is highly expressed in slow muscle. A potentially targeted clone was isolated and differentiated in culture to form myotubes which expressed MyHCIIB. Cells from the same clone were injected into both slow and fast muscle of host mice, where they contributed to fiber formation. In slow muscle, the fibers derived from this clone did not express MyHCIIB; this may reflect an instability of the targeted MyHCIIB locus and/or a failure of the hybrid promoter to function in slow fibers in vivo. Nonetheless, we have demonstrated that a "promoter knock-in" gene targeting procedure can be used to generate unique MyHCIIB-expressing myotubes in culture and that conditionally immortalized myoblasts can be subjected to extensive passaging and genetic manipulation without losing their ability to form fibers in culture and in vivo.

Animals↗
PubMed1999

Influence of DNA delivery method on gene targeting frequencies in human cells.

Gene targeting can be used for genetic studies of human cell lines and has significant potential for somatic cell gene therapy. These applications are however restricted by the low frequency of homologous recombination in higher eukaryotes compared to the relatively efficient nonhomologous integration of transfected DNA into the genome. As part of our attempts to overcome this problem, we compared two widely used transfection methods for their efficiency in gene targeting. To our surprise we found that, for conditions that render similar frequencies of nonhomologous integrants, lipofection is much less efficient than electroporation in generating targeted clones. This suggests that nonhomologous and homologous recombination have different requirements for DNA delivery in human cells.

Cell Line↗
PubMed1999

Gene targeting is enhanced in human cells overexpressing hRAD51.

The ideal therapy for single gene disorders would be repair of the mutated disease genes. Homologous recombination is one of several cellular mechanisms for the repair of DNA damage. Recombination between exogenous DNA and homologous chromosomal loci (gene targeting) can be used to repair an endogenous gene, but the low efficiency of this process is a serious barrier to its therapeutic potential. Recent progress in the isolation and characterisation of mammalian genes and proteins involved in DNA recombination has raised the possibility that the cellular biochemistry of recombination can be manipulated to improve the efficiency of gene targeting. As an initial test of this approach, we have overexpressed the gene encoding hRAD51, a protein with homologous DNA pairing and strand exchange activities, in human cells and measured its effect on gene targeting. We report a two- to three-fold increase in gene targeting, and enhanced resistance to ionising radiation in hRAD51-overexpressing cells with no obvious detrimental effects. These observations provide valuable genetic evidence for the involvement of hRAD51 in both gene targeting and DNA repair in human cells. Our data also establish overexpression of recombination genes as a viable approach to improving gene targeting efficiencies.

Blotting, Western↗
PubMed1998

Tyrosine kinase receptor-activated signal transduction pathways which lead to oncogenesis.

Oncogenesis is a complicated process involving signal transduction pathways that mediate many different physiological events. Typically, oncogenes cause unregulated cell growth and this phenotype has been attributed to the growth-stimulating activity of oncogenes such as ras and src. In recent years, much research effort has focused on proteins that function downstream of Ras, leading to the identification of the Ras/Raf/MAPK pathway, because activation of this pathway leads to cellular proliferation. Activated receptor tyrosine kinases (RTKs) also utilize this pathway to mediate their growth-stimulating effects. However, RTKs activate many other signaling proteins that are not involved in the cellular proliferation process, per se and we are learning that these pathways also contribute to the oncogenic process. In fact, RTKs and many of the proteins involved in RTK-dependent signal transduction can also function as oncogenes. For example, the catalytic subunit of phosphoinositide 3-kinase (P13-K) was recently identified as an oncogenic protein. The scope of pathways that are activated by oncogenic RTKs is expanding. Thus, not only do RTKs activate Ras-dependent pathways that drive proliferation, RTKs activate P13-K-dependent pathways which also contribute to the oncogenic mechanism. P13-K can initiate changes in gene transcription, cytoskeletal changes through beta-catenin, changes in cell motility through the tumor suppressor, adenomatous polyposis coli (APC), and phosphorylation of BAD, a protein involved in apoptotic and antiapoptotic signaling. There is also cross-talk between RTKs and the oncostatin cytokine receptor which may positively and negatively influence oncogenesis. For this review, we will focus on oncogenic RTKs and the network of cellular proteins that are activated by RTKs because multiple, divergent pathways are responsible for oncogenesis.

Animals↗
PubMed1998

14-3-3 proteins interact with specific MEK kinases.

MEK (mitogen-activated protein kinase/extracellular signal-regulated kinase kinase) kinases (MEKKs) regulate c-Jun N-terminal kinase and extracellular response kinase pathways. The 14-3-3zeta and 14-3-3epsilon isoforms were isolated in a two-hybrid screen for proteins interacting with the N-terminal regulatory domain of MEKK3. 14-3-3 proteins bound both the N-terminal regulatory and C-terminal kinase domains of MEKK3. The binding affinity of 14-3-3 for the MEKK3 N terminus was 90 nM, demonstrating a high affinity interaction. 14-3-3 proteins also interacted with MEKK1 and MEKK2, but not MEKK4. Endogenous 14-3-3 protein and MEKK1 and MEKK2 were similarly distributed in the cell, consistent with their in vitro interactions. MEKK1 and 14-3-3 proteins colocalized using two-color digital confocal immunofluorescence. Binding of 14-3-3 proteins mapped to the N-terminal 393 residues of 196-kDa MEKK1. Unlike MEKK2 and MEKK3, the C-terminal kinase domain of MEKK1 demonstrated little or no ability to interact with 14-3-3 proteins. MEKK1, but not MEKK2, -3 or -4, is a caspase-3 substrate that when cleaved releases the kinase domain from the N-terminal regulatory domain. Functionally, caspase-3 cleavage of MEKK1 releases the kinase domain from the N-terminal 14-3-3-binding region, demonstrating that caspases can selectively alter protein kinase interactions with regulatory proteins. With regard to MEKK1, -2 and -3, 14-3-3 proteins do not appear to directly influence activity, but rather function as "scaffolds" for protein-protein interactions.

14-3-3 Proteins↗
PubMed1998

Therapeutic gene targeting.

Gene targeting is the use of homologous recombination to make defined alterations to the genome. One of the possible outcomes of gene targeting is the accurate correction of genetic defects, and this would make it the ideal method of gene therapy for single gene disorder. While gene targeting has been achieved both in human cell lines and in nontransformed, primary human cells, its low efficiency has been a major limitation to its therapeutic potential. Gene therapy in vivo gene targeting is there for impractical without dramatic improvements in targeting efficiency. Ex vivo approaches might more realistically be considered, but would benefit from progress in the isolation and growth of somatic stem cells and improvements in targeting efficiency. We provide here a brief review of the challenges of gene therapy by gene targeting. This is followed by a critical overview of recent developments in gene targeting techniques, and in our understanding of the underlying processes of homologous and nonhomologous recombination.

Adenoviridae↗