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Biomedical subjects

T Clackson

Publications and source records attributed to T Clackson.

At least 19 recordsLinked to original sources
PubMed2001

A Fas-based suicide switch in human T cells for the treatment of graft-versus-host disease.

Graft-versus-host disease (GVHD) is a major complication of allogeneic bone marrow transplantation. One strategy to treat GVHD is to equip donor T cells with a conditional suicide mechanism that can be triggered when GVHD occurs. The herpes simplex virus thymidine kinase (HSV-tk)/ganciclovir system used clinically has several limitations, including immunogenicity and cell cycle dependence. An alternative switch based on chemically inducible apoptosis was designed and evaluated. A chimeric human protein was expressed comprising an extracellular marker (DeltaLNGFR), the Fas intracellular domain, and 2 copies of an FK506-binding protein (FKBP). Primary human T lymphocytes retrovirally transduced with this construct could be purified to homogeneity using immunomagnetic beads. Genetic integrity of the construct was ensured by redesigning repetitive sequences. Transduced T cells behaved indistinguishably from untransduced cells, retaining the ability to mount a specific antiallogeneic immune response. However, they rapidly underwent apoptosis with the addition of subnanomolar concentrations of AP1903, a bivalent "dimerizer" drug that binds FKBP and induces Fas cross-linking. A single 2-hour treatment eliminated approximately 80% of T cells, and multiple exposures induced further apoptosis. T cells were eliminated regardless of their proliferation state, suggesting that the AP1903/Fas system, which contains only human components, is a promising alternative to HSV-tk for treating GVHD.

Apoptosis↗
PubMed2001

Intravenous safety and pharmacokinetics of a novel dimerizer drug, AP1903, in healthy volunteers.

AP1903 is a novel gene-targeted drug that is being developed for use in drug-regulated cell therapies. An intravenous, single-blind, placebo- and saline-controlled, ascending-dose study was performed to evaluate the safety, tolerability, and pharmacokinetics of AP1903. Twenty-eight normal healthy male volunteers were randomized into five dosage groups of AP1903 (0.01, 0.05, 0.1, 0.5, and 1 mg/kg). Within each group, 4 volunteers received a single dose of AP1903, 1 volunteer received an equal volume of placebo, and 1 received an equal volume of normal saline. The only exception was in the 0.5 mg/kg group, in which 4 volunteers were dosed: 3 received AP1903 and 1 received normal saline. All dosages were administered as intravenous infusions over 2 hours. Clinical safety parameters were monitored, and serial blood and urine samples were collected for analysis of AP1903. No drug-related adverse events were observed at any of the dose levels with the possible exception of facial flushing in 1 volunteer at the 1.0 mg/kg dose level. AP1903 plasma levels were directly proportional to the administered dose, with mean Cmax values ranging from approximately 10 to 1,275 ng/mL over the 0.01 to 1.0 mg/kg dose range. Following the infusion period, blood concentrations revealed a rapid distribution phase, with plasma levels being reduced to approximately 18%, 7%, and 1% of the maximal concentration at 0.5, 2, and 10 hours postdose, respectively. AP1903 was shown to be safe and well tolerated at all dose levels and demonstrated a favorable pharmacokinetic profile at doses well above the anticipated therapeutic dose.

Adult↗
PubMed2000

Delivery of a stringent dimerizer-regulated gene expression system in a single retroviral vector.

Small molecule-regulated transcription has broad utility and would benefit from an easily delivered self-contained regulatory cassette capable of robust, tightly controlled target gene expression. We describe the delivery of a modified dimerizer-regulated gene expression system to cells on a single retrovirus. A transcription factor cassette responsive to the natural product dimerizer rapamycin was optimized for retroviral delivery by fusing a highly potent chimeric activation domain to the rapamycin-binding domain of FKBP-rapamycin-associated protein (FRAP). This improvement led to an increase in both the potency and maximal levels of gene expression induced by rapamycin, or nonimmunosuppressive rapamycin analogs. The modified transcription factor cassette was incorporated along with a target gene into a single rapamycin-responsive retrovirus. Cell pools stably transduced with the single virus system displayed negligible basal expression and gave induction ratios of at least three orders of magnitude in the presence of rapamycin or a nonimmunosuppressive rapamycin analog. Levels of induced gene expression were comparable to those obtained with the constitutive retroviral long terminal repeat and the single virus system performed well in four different mammalian cell lines. Regulation with the dimerizer-responsive retrovirus was tight enough to allow the generation of cell lines displaying inducible expression of the highly toxic diphtheria toxin A chain gene. The ability to deliver the tightly inducible rapamycin system in a single retrovirus should facilitate its use in the study of gene function in a broad range of cell types.

3T3 Cells↗
PubMed2000

Megavesicles implicated in the rapid transport of intracisternal aggregates across the Golgi stack.

Engineered protein aggregates ranging up to 400 nm in diameter were selectively deposited within the cis-most cisternae of the Golgi stack following a 15 degrees C block. These aggregates are much larger than the standard volume of Golgi vesicles, yet they are transported across the stack within 10 min after warming the cells to 20 degrees C. Serial sectioning reveals that during the peak of anterograde transport, about 20% of the aggregates were enclosed in topologically free "megavesicles" which appear to pinch off from the rims of the cisternae. These megavesicles can explain the rapid transport of aggregates without cisternal progression on this time scale.

Biological Transport↗
PubMed2000

A ligand-reversible dimerization system for controlling protein-protein interactions.

Chemically induced dimerization provides a general way to gain control over intracellular processes. Typically, FK506-binding protein (FKBP) domains are fused to a signaling domain of interest, allowing crosslinking to be initiated by addition of a bivalent FKBP ligand. In the course of protein engineering studies on human FKBP, we discovered that a single point mutation in the ligand-binding site (Phe-36 --> Met) converts the normally monomeric protein into a ligand-reversible dimer. Two-hybrid, gel filtration, analytical ultracentrifugation, and x-ray crystallographic studies show that the mutant (F(M)) forms discrete homodimers with micromolar affinity that can be completely dissociated within minutes by addition of monomeric synthetic ligands. These unexpected properties form the basis for a "reverse dimerization" regulatory system involving F(M) fusion proteins, in which association is the ground state and addition of ligand abolishes interactions. We have used this strategy to rapidly and reversibly aggregate fusion proteins in different cellular compartments, and to provide an off switch for transcription. Reiterated F(M) domains should be generally useful as conditional aggregation domains (CADs) to control intracellular events where rapid, reversible dissolution of interactions is required. Our results also suggest that dimerization is a latent property of the FKBP fold: the crystal structure reveals a remarkably complementary interaction between the monomer binding sites, with only subtle changes in side-chain disposition accounting for the dramatic change in quaternary structure.

Dimerization↗
PubMed2000

Investigating protein-ligand interactions with a mutant FKBP possessing a designed specificity pocket.

Using structure-based design and protein mutagenesis we have remodeled the FKBP12 ligand binding site to include a sizable, hydrophobic specificity pocket. This mutant (F36V-FKBP) is capable of binding, with low or subnanomolar affinities, novel synthetic ligands possessing designed substituents that sterically prevent binding to the wild-type protein. Using binding and structural analysis of bumped compounds, we show here that the pocket is highly promiscuous-capable of binding a range of hydrophobic alkyl and aryl moieties with comparable affinity. Ligand affinity therefore appears largely insensitive to the degree of occupancy or quality of packing of the pocket. NMR spectroscopic analysis indicates that similar ligands can adopt radically different binding modes, thus complicating the interpretation of structure-activity relationships.

Acetamides↗
PubMed2000

Regulation of protein secretion through controlled aggregation in the endoplasmic reticulum.

A system for direct pharmacologic control of protein secretion was developed to allow rapid and pulsatile delivery of therapeutic proteins. A protein was engineered so that it accumulated as aggregates in the endoplasmic reticulum. Secretion was then stimulated by a synthetic small-molecule drug that induces protein disaggregation. Rapid and transient secretion of growth hormone and insulin was achieved in vitro and in vivo. A regulated pulse of insulin secretion resulted in a transient correction of serum glucose concentrations in a mouse model of hyperglycemia. This approach may make gene therapy a viable method for delivery of polypeptides that require rapid and regulated delivery.

Animals↗
PubMed2000

Regulated gene expression systems.

Most gene therapy research to date has focused on solving the delivery problem--getting genes efficiently and stably into target cells and tissues. Those working on systems for regulating the expression of genes once delivered have often been accused of trying to run before they can walk. Yet regulation is likely to be essential if gene therapy is to realize its full potential as a mainstream clinical option for delivering proteins. Dramatic progress has been made in designing and testing systems in which expression is controlled by orally active drugs. The next few years should see the first clinical trials of drug-regulated gene therapies.

Animals↗
PubMed1998

Redesigning an FKBP-ligand interface to generate chemical dimerizers with novel specificity.

FKBP ligand homodimers can be used to activate signaling events inside cells and animals that have been engineered to express fusions between appropriate signaling domains and FKBP. However, use of these dimerizers in vivo is potentially limited by ligand binding to endogenous FKBP. We have designed ligands that bind specifically to a mutated FKBP over the wild-type protein by remodeling an FKBP-ligand interface to introduce a specificity binding pocket. A compound bearing an ethyl substituent in place of a carbonyl group exhibited sub-nanomolar affinity and 1,000-fold selectivity for a mutant FKBP with a compensating truncation of a phenylalanine residue. Structural and functional analysis of the new pocket showed that recognition is surprisingly relaxed, with the modified ligand only partially filling the engineered cavity. We incorporated the specificity pocket into a fusion protein containing FKBP and the intracellular domain of the Fas receptor. Cells expressing this modified chimeric protein potently underwent apoptosis in response to AP1903, a homodimer of the modified ligand, both in culture and when implanted into mice. Remodeled dimerizers such as AP1903 are ideal reagents for controlling the activities of cells that have been modified by gene therapy procedures, without interference from endogenous FKBP.

Amino Acid Sequence↗
PubMed1998

Structural and functional analysis of the 1:1 growth hormone:receptor complex reveals the molecular basis for receptor affinity.

The designed G120R mutant of human growth hormone (hGH) is an antagonist and can bind only one molecule of the growth hormone receptor. We have determined the crystal structure of the 1:1 complex between this mutant and the receptor extracellular domain (hGHbp) at 2.6 A resolution, and used it to guide a detailed survey of the structural and functional basis for hormone-receptor recognition. The overall structure of the complex is very similar to the equivalent portion of the 1:2 complex, showing that formation of the active complex does not involve major conformational changes. However, a segment involved in receptor-receptor interactions in the 1:2 complex is disordered in this structure, suggesting that its productive conformation is stabilized by receptor dimerization. The hormone binding site of the receptor comprises a central hydrophobic patch dominated by Trp104 and Trp169, surrounded by a hydrophilic periphery containing several well-ordered water molecules. Previous alanine scanning showed that the hydrophobic "hot spot" confers most of the binding energy. The new structural data, coupled with binding and kinetic analysis of further mutants, indicate that the hot spot is assembled cooperatively and that many residues contribute indirectly to binding. Several hydrophobic residues serve to orient the key tryptophan residues; kinetic analysis suggests that Pro106 locks the Trp104 main-chain into a required conformation. The electrostatic contacts of Arg43 to hGH are less important than the intramolecular packing of its alkyl chain with Trp169. The true functional epitope that directly contributes binding energy may therefore comprise as few as six side-chains, participating mostly in alkyl-aromatic stacking interactions. Outside the functional epitope, multiple mutation of residues to alanine resulted in non-additive increases in affinity: up to tenfold for a hepta-alanine mutant. Contacts in the epitope periphery can therefore attenuate the affinity of the central hot spot, perhaps reflecting a role in conferring specificity to the interaction.

Binding Sites↗
PubMed1998

Redesigning small molecule-protein interfaces.

Several recent reports have described the rational redesign of small molecule-protein interfaces, generating modified ligands that interact only with suitably mutated proteins. These studies provide powerful reagents for specifically manipulating engineered proteins inside cells, as well as general insights into the factors underlying the binding affinity and specificity of small molecules in general. Progress this year includes the development of allele-specific inhibitors of Src-family tyrosine kinases and the crystal structure determination of a remodeled FKBP-ligand interface.

Adenosine Triphosphate↗
PubMed1998

Synthesis and activity of bivalent FKBP12 ligands for the regulated dimerization of proteins.

The total synthesis and in vitro activities of a series of chemical inducers of dimerization (CIDs) is described. The use of small-molecule CIDs to control the dimerization of engineered FKBP12-containing fusion proteins has been demonstrated to have broad utility in biological research as well as potential medical applications in gene and cell therapies. The facility and flexibility of preparation make this new class of wholly synthetic compounds exceptionally versatile tools for the study of intracellular signaling events mediated by protein-protein interactions or protein localization. While some congeners possess potency comparable to or better than the first generation natural product-derived CID, FK1012, structure-activity relationships are complex and underscore the need for application-specific compound optimizations.

Apoptosis↗
PubMed1997

A versatile synthetic dimerizer for the regulation of protein-protein interactions.

The use of low molecular weight organic compounds to induce dimerization or oligomerization of engineered proteins has wide-ranging utility in biological research as well as in gene and cell therapies. Chemically induced dimerization can be used to activate intracellular signal transduction pathways or to control the activity of a bipartite transcription factor. Dimerizer systems based on the natural products cyclosporin, FK506, rapamycin, and coumermycin have been described. However, owing to the complexity of these compounds, adjusting their binding or pharmacological properties by chemical modification is difficult. We have investigated several families of readily prepared, totally synthetic, cell-permeable dimerizers composed of ligands for human FKBP12. These molecules have significantly reduced complexity and greater adaptability than natural product dimers. We report here the efficacies of several of these new synthetic compounds in regulating two types of protein dimerization events inside engineered cells--induction of apoptosis through dimerization of engineered Fas proteins and regulation of transcription through dimerization of transcription factor fusion proteins. One dimerizer in particular, AP1510, proved to be exceptionally potent and versatile in all experimental contexts tested.

Apoptosis↗
PubMed1997

Controlling mammalian gene expression with small molecules.

Several systems are now available that enable transcription of a gene introduced into mammalian cells to be controlled using small molecules. Among the potential applications are the analysis of gene function through creation of inducible alleles in cell culture and transgenic animals, and, ultimately, the pharmacological control of therapeutic protein production in vivo in the context of gene therapy. Highlights of the past year include several demonstrations of regulated protein production in animal models of gene and cell therapy, and the development of a new approach to transcriptional regulation using chemical inducers of dimerization.

Ecdysone↗
PubMed1996

A humanized system for pharmacologic control of gene expression.

Gene therapy was originally conceived as a medical intervention to replace or correct defective genes in patients with inherited disorders. However, it may have much broader potential as an alternative delivery platform for protein therapeutics, such as cytokines, hormones, antibodies and novel engineered proteins. One key technical barrier to the widespread implementation of this form of therapy is the need for precise control over the level of protein production. A suitable system for pharmacologic control of therapeutic gene expression would permit precise titration of gene product dosage, intermittent or pulsatile treatment, and ready termination of therapy by withdrawal of the activating drug. We set out to design such a system with the following properties: (1) low baseline expression and high induction ratio; (2) positive control by an orally bioavailable small-molecule drug; (3) reduced potential for immune recognition through the exclusive use of human proteins; and (4) modularity to allow the independent optimization of each component using the tools of protein engineering. We report here the properties of this system and demonstrate its use to control circulating levels of human growth hormone in mice implanted with engineered human cells.

Animals↗
PubMed1995

Mutations of the growth hormone receptor in children with idiopathic short stature. The Growth Hormone Insensitivity Study Group.

BACKGROUND: Short stature in children who are not deficient in growth hormone (GH) is probably caused by a variety of defects. Some children with idiopathic short stature have low serum concentrations of GH-binding protein, which is derived from the GH receptor. The possibility that low serum concentrations of GH-binding protein might indicate partial insensitivity to GH led us to investigate possible defects in the gene for the GH receptor in children with idiopathic short stature and low serum concentrations of GH-binding protein. METHODS: We studied 14 children with idiopathic short stature who were selected on the basis of normal GH secretion and low serum concentrations of GH-binding protein. Analysis of single-strand conformation polymorphisms and DNA sequencing were both used to identify mutations in the GH-receptor gene. RESULTS: Mutations in the region of the GH-receptor gene that codes for the extracellular domain of the receptor were found in 4 of the 14 children, but in none of 24 normal subjects. One of the four children with mutations was a compound heterozygote, with one mutation that reduced the affinity of the receptor for GH and a second mutation that may affect a function other than ligand binding. The remaining three children had single mutations in one allele of the gene. One mutation introduced a premature termination codon, and two caused substitutions of single amino acids in a structurally conserved domain of the receptor. CONCLUSIONS: Some children with idiopathic short stature may have partial insensitivity to GH due to mutations in the GH-receptor gene.

Adolescent↗
PubMed1995

A hot spot of binding energy in a hormone-receptor interface.

The x-ray crystal structure of the complex between human growth hormone (hGH) and the extracellular domian of its first bound receptor (hGHbp) shows that about 30 side chains from each protein make contact. Individual replacement of contact residues in the hGHbp with alanine showed that a central hydrophobic region, dominated by two tryptophan residues, accounts for more than three-quarters of the binding free energy. This "functional epitope" is surrounded by less important contact residues that are generally hydrophilic and partially hydrated, so that the interface resembles a cross section through a globular protein. The functionally important residues on the hGHbp directly contact those on hGH. Thus, only a small and complementary set of contact residues maintains binding affinity, a property that may be general to protein-protein interfaces.

Carrier Proteins↗