The Effects of Pharmacological Inhibition of Histone Deacetylase 3 (HDAC3) in Huntington's Disease Mice

An important epigenetic modification in Huntington’s disease (HD) research is histone acetylation, which is regulated by histone acetyltransferase and histone deacetylase (HDAC) enzymes. HDAC inhibitors have proven effective in HD model systems, and recent work is now focused on functional dissection of the individual HDAC enzymes in these effects. Histone deacetylase 3 (HDAC3), a member of the class I subfamily of HDACs, has previously been implicated in neuronal toxicity and huntingtin-induced cell death. Hence, we tested the effects of RGFP966 ((E)-N-(2-amino-4-fluorophenyl)-3-(1-cinnamyl-1H-pyrazol-4-yl)acrylamide), a benzamide-type HDAC inhibitor that selectively targets HDAC3, in the N171-82Q transgenic mouse model of HD. We found that RGFP966 at doses of 10 and 25 mg/kg improves motor deficits on rotarod and in open field exploration, accompanied by neuroprotective effects on striatal volume. In light of previous studies implicating HDAC3 in immune function, we measured gene expression changes for 84 immune-related genes elicited by RGFP966 using quantitative PCR arrays. RGFP966 treatment did not cause widespread changes in cytokine/chemokine gene expression patterns, but did significantly alter the striatal expression of macrophage migration inhibitory factor (Mif), a hormone immune modulator associated with glial cell activation, in N171-82Q transgenic mice, but not WT mice. Accordingly, RGFP966-treated mice showed decreased glial fibrillary acidic protein (GFAP) immunoreactivity, a marker of astrocyte activation, in the striatum of N171-82Q transgenic mice compared to vehicle-treated mice. These findings suggest that the beneficial actions of HDAC3 inhibition could be related, in part, with lowered Mif levels and its associated downstream effects.

Mechanism of Action of 2-Aminobenzamide HDAC Inhibitors in Reversing Gene Silencing in Friedreich's Ataxia

The genetic defect in Friedreich’s ataxia (FRDA) is the hyperexpansion of a GAA•TTC triplet in the first intron of the FXN gene, encoding the essential mitochondrial protein frataxin. Histone post-translational modifications near the expanded repeats are consistent with heterochromatin formation and consequent FXN gene silencing. Using a newly developed human neuronal cell model, derived from patient-induced pluripotent stem cells, we find that 2-aminobenzamide histone deacetylase (HDAC) inhibitors increase FXN mRNA levels and frataxin protein in FRDA neuronal cells. However, only compounds targeting the class I HDACs 1 and 3 are active in increasing FXN mRNA in these cells. Structural analogs of the active HDAC inhibitors that selectively target either HDAC1 or HDAC3 do not show similar increases in FXN mRNA levels. To understand the mechanism of action of these compounds, we probed the kinetic properties of the active and inactive inhibitors, and found that only compounds that target HDACs 1 and 3 exhibited a slow-on/slow-off mechanism of action for the HDAC enzymes. HDAC1- and HDAC3-selective compounds did not show this activity. Using siRNA methods in the FRDA neuronal cells, we show increases in FXN mRNA upon silencing of either HDACs 1 or 3, suggesting the possibility that inhibition of each of these class I HDACs is necessary for activation of FXN mRNA synthesis, as there appears to be redundancy in the silencing mechanism caused by the GAA•TTC repeats. Moreover, inhibitors must have a long residence time on their target enzymes for this activity. By interrogating microarray data from neuronal cells treated with inhibitors of different specificity, we selected two genes encoding histone macroH2A (H2AFY2) and Polycomb group ring finger 2 (PCGF2) that were specifically down-regulated by the inhibitors targeting HDACs1 and 3 versus the more selective inhibitors for further investigation. Both genes are involved in transcriptional repression and we speculate their involvement in FXN gene silencing. Our results shed light on the mechanism whereby HDAC inhibitors increase FXN mRNA levels in FRDA neuronal cells.

Epigenetic therapy for Friedreich ataxia

OBJECTIVE

To investigate whether a histone deacetylase inhibitor (HDACi) would be effective in an in vitro model for the neurodegenerative disease Friedreich ataxia (FRDA) and to evaluate safety and surrogate markers of efficacy in a phase I clinical trial in patients.

METHODS

We used a human FRDA neuronal cell model, derived from patient induced pluripotent stem cells, to determine the efficacy of a 2-aminobenzamide HDACi (109) as a modulator of FXN gene expression and chromatin histone modifications. FRDA patients were dosed in 4 cohorts, ranging from 30mg/day to 240mg/day of the formulated drug product of HDACi 109, RG2833. Patients were monitored for adverse effects as well as for increases in FXN mRNA, frataxin protein, and chromatin modification in blood cells.

RESULTS

In the neuronal cell model, HDACi 109/RG2833 increases FXN mRNA levels and frataxin protein, with concomitant changes in the epigenetic state of the gene. Chromatin signatures indicate that histone H3 lysine 9 is a key residue for gene silencing through methylation and reactivation through acetylation, mediated by the HDACi. Drug treatment in FRDA patients demonstrated increased FXN mRNA and H3 lysine 9 acetylation in peripheral blood mononuclear cells. No safety issues were encountered.

INTERPRETATION

Drug exposure inducing epigenetic changes in neurons in vitro is comparable to the exposure required in patients to see epigenetic changes in circulating lymphoid cells and increases in gene expression. These findings provide a proof of concept for the development of an epigenetic therapy for this fatal neurological disease.

The DcpS inhibitor RG3039 improves motor function in SMA mice

Spinal muscular atrophy (SMA) is caused by mutations of the survival motor neuron 1 (SMN1) gene, retention of the survival motor neuron 2 (SMN2) gene and insufficient expression of full-length survival motor neuron (SMN) protein. Quinazolines increase SMN2 promoter activity and inhibit the ribonucleic acid scavenger enzyme DcpS. The quinazoline derivative RG3039 has advanced to early phase clinical trials. In preparation for efficacy studies in SMA patients, we investigated the effects of RG3039 in severe SMA mice. Here, we show that RG3039 distributed to central nervous system tissues where it robustly inhibited DcpS enzyme activity, but minimally activated SMN expression or the assembly of small nuclear ribonucleoproteins. Nonetheless, treated SMA mice showed a dose-dependent increase in survival, weight and motor function. This was associated with improved motor neuron somal and neuromuscular junction synaptic innervation and function and increased muscle size. RG3039 also enhanced survival of conditional SMA mice in which SMN had been genetically restored to motor neurons. As this systemically delivered drug may have therapeutic benefits that extend beyond motor neurons, it could act additively with SMN-restoring therapies delivered directly to the central nervous system such as antisense oligonucleotides or gene therapy.

Histone deacetylases 1 and 2 maintain S-phase chromatin and DNA replication fork progression

BACKGROUND

Histone deacetylases (HDACs) play a critical role in the maintenance of genome stability. Class I HDACs, histone deacetylase 1 and 2 (Hdac1 and Hdac2) are recruited to the replication fork by virtue of their interactions with the replication machinery. However, functions for Hdac1 and Hdac2 (Hdacs1,2) in DNA replication are not fully understood.

RESULTS

Using genetic knockdown systems and novel Hdacs1,2-selective inhibitors, we found that loss of Hdacs1,2 leads to a reduction in the replication fork velocity, and an increase in replication stress response culminating in DNA damage. These observed defects are due to a direct role for Hdacs1,2 in DNA replication, as transcription of genes involved in replication was not affected in the absence of Hdacs1,2. We found that loss of Hdacs1,2 functions increases histone acetylation (ac) on chromatin in S-phase cells and affects nascent chromatin structure, as evidenced by the altered sensitivity of newly synthesized DNA to nuclease digestion. Specifically, H4K16ac, a histone modification involved in chromatin decompaction, is increased on nascent chromatin upon abolishing Hdacs1,2 activities. It was previously shown that H4K16ac interferes with the functions of SMARCA5, an ATP-dependent ISWI family chromatin remodeler. We found SMARCA5 also associates with nascent DNA and loss of SMARCA5 decreases replication fork velocity similar to the loss or inhibition of Hdacs1,2.

CONCLUSIONS

Our studies reveal important roles for Hdacs1,2 in nascent chromatin structure maintenance and regulation of SMARCA5 chromatin-remodeler function, which together are required for proper replication fork progression and genome stability in S-phase.

Increasing frataxin gene expression with histone deacetylase inhibitors as a therapeutic approach for Friedreich's ataxia

The genetic defect in Friedreich's ataxia (FRDA) is the expansion of a GAA·TCC triplet in the first intron of the FXN gene, which encodes the mitochondrial protein frataxin. Previous studies have established that the repeats reduce transcription of this essential gene, with a concomitant decrease in frataxin protein in affected individuals. As the repeats do not alter the FXN protein coding sequence, one therapeutic approach would be to increase transcription of pathogenic FXN genes. Histone posttranslational modifications near the expanded repeats are consistent with heterochromatin formation and FXN gene silencing. In an effort to find small molecules that would reactivate this silent gene, histone deacetylase inhibitors were screened for their ability to up-regulate FXN gene expression in patient cells and members of the pimelic 2-aminobenzamide family of class I histone deacetylase inhibitors were identified as potent inducers of FXN gene expression and frataxin protein. Importantly, these molecules up-regulate FXN expression in human neuronal cells derived from patient-induced pluripotent stem cells and in two mouse models for the disease. Preclinical studies of safety and toxicity have been completed for one such compound and a phase I clinical trial in FRDA patients has been initiated. Furthermore, medicinal chemistry efforts have identified improved compounds with superior pharmacological properties.

Development of frataxin gene expression measures for the evaluation of experimental treatments in Friedreich's ataxia

BACKGROUND

Friedreich ataxia is a progressive neurodegenerative disorder caused by GAA triplet repeat expansions or point mutations in the FXN gene and, ultimately, a deficiency in the levels of functional frataxin protein. Heterozygous carriers of the expansion express approximately 50% of normal frataxin levels yet manifest no clinical symptoms, suggesting that therapeutic approaches that increase frataxin may be effective even if frataxin is raised only to carrier levels. Small molecule HDAC inhibitor compounds increase frataxin mRNA and protein levels, and have beneficial effects in animal models of FRDA.

METHODOLOGY/PRINCIPAL FINDINGS

To gather data supporting the use of frataxin as a therapeutic biomarker of drug response we characterized the intra-individual stability of frataxin over time, determined the contribution of frataxin from different components of blood, compared frataxin measures in different cell compartments, and demonstrated that frataxin increases are achieved in peripheral blood mononuclear cells. Frataxin mRNA and protein levels were stable with repeated sampling over four and 15 weeks. In the 15-week study, the average CV was 15.6% for protein and 18% for mRNA. Highest levels of frataxin in blood were in erythrocytes. As erythrocytes are not useful for frataxin assessment in many clinical trial situations, we confirmed that PBMCs and buccal swabs have frataxin levels equivalent to those of whole blood. In addition, a dose-dependent increase in frataxin was observed when PBMCs isolated from patient blood were treated with HDACi. Finally, higher frataxin levels predicted less severe neurological dysfunction and were associated with slower rates of neurological change.

CONCLUSIONS/SIGNIFICANCE

Our data support the use of frataxin as a biomarker of drug effect. Frataxin levels are stable over time and as such a 1.5 to 2-fold change would be detectable over normal biological fluctuations. Additionally, our data support buccal cells or PBMCs as sources for measuring frataxin protein in therapeutic trials.

HDAC3-selective inhibitor enhances extinction of cocaine-seeking behavior in a persistent manner

Nonspecific histone deacetylase (HDAC) inhibition has been shown to facilitate the extinction of drug-seeking behavior in a manner resistant to reinstatement. A key open question is which specific HDAC is involved in the extinction of drug-seeking behavior. Using the selective HDAC3 inhibitor RGFP966, we investigated the role of HDAC3 in extinction and found that systemic treatment with RGFP966 facilitates extinction in mice in a manner resistant to reinstatement. We also investigated whether the facilitated extinction is related to the enhancement of extinction consolidation during extinction learning or to negative effects on performance or reconsolidation. These are key distinctions with regard to any compound being used to modulate extinction, because a more rapid decrease in a defined behavior is interpreted as facilitated extinction. Using an innovative combination of behavioral paradigms, we found that a single treatment of RGFP966 enhances extinction of a previously established cocaine-conditioned place preference, while simultaneously enhancing long-term object-location memory within subjects. During extinction consolidation, HDAC3 inhibition promotes a distinct pattern of histone acetylation linked to gene expression within the infralimbic cortex, hippocampus, and nucleus accumbens. Thus, the facilitated extinction of drug-seeking cannot be explained by adverse effects on performance. These results demonstrate that HDAC3 inhibition enhances the memory processes involved in extinction of drug-seeking behavior.

Rationale for the development of 2-aminobenzamide histone deacetylase inhibitors as therapeutics for Friedreich ataxia

Numerous studies have pointed to histone deacetylase inhibitors as potential therapeutics for various neurodegenerative diseases, and clinical trials with several histone deacetylase inhibitors have been performed or are under way. However, histone deacetylase inhibitors tested to date either are highly cytotoxic or have very low specificities for different histone deacetylase enzymes. The authors' laboratories have identified a novel class of histone deacetylase inhibitors (2-aminobenzamides) that reverses heterochromatin-mediated silencing of the frataxin (FXN) gene in Friedreich ataxia. The authors have identified the histone deacetylase enzyme isotype target of these compounds and present evidence that compounds that target this enzyme selectively increase FXN expression from pathogenic alleles. Studies with model compounds show that these histone deacetylase inhibitors increase FXN messenger RNA levels in the brain in mouse models for Friedreich ataxia and relieve neurological symptoms observed in mouse models and support the notion that this class of molecules may serve as therapeutics for the human disease.

RGFP109, a histone deacetylase inhibitor attenuates L-DOPA-induced dyskinesia in the MPTP-lesioned marmoset: a proof-of-concept study

BACKGROUND

l-3,4-dihydroxyphenylalanine (l-DOPA)-induced dyskinesia (LID) are a complication of chronic dopamine replacement therapy in Parkinson's disease (PD). Recent studies have suggested that the mechanisms underlying development and expression of LID in PD may involve epigenetic changes that include deacetylation of striatal histone proteins. We hypothesised that inhibition of histone deacetylase, the enzyme responsible of histone deacetylation, would alleviate LID.

METHODS

Four female common marmoset (Callithrix jacchus) were rendered parkinsonian by administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Following stabilisation of the parkinsonian phenotype, marmosets were primed to exhibit dyskinesia with chronic administration of L-DOPA. We then investigated the effects of the brain-penetrant histone deacetylase inhibitor, RGFP109 (30 mg/kg p.o. once daily for 6 days), on LID and L-DOPA anti-parkinsonian efficacy.

RESULTS

RGFP109 had no acute effects on dyskinesia after single or 6 days once-daily treatment (both P > 0.05). However, one week following cessation of RGFP109, dyskinesia and duration of ON-time with disabling dyskinesia were reduced by 37% and 50%, respectively (both P < 0.05), compared to that seen previously with L-DOPA alone. There was no change in anti-parkinsonian actions of, or ON-time duration afforded by, L-DOPA (P > 0.05).

CONCLUSIONS

Histone deacetylation inhibition may represent a novel approach to reverse established LID in PD and improve quality of the anti-parkinsonian benefit provided by L-DOPA.

A gene expression phenotype in lymphocytes from Friedreich ataxia patients

OBJECTIVE

Gene expression studies in peripheral tissues from patients with neurodegenerative disorders can provide insights into disease pathogenesis, and identify potential biomarkers, an important goal of translational research in neurodegeneration. Friedreich Ataxia (FRDA) is a chronic neurodegenerative disease caused by reduced transcription of frataxin, a ubiquitously expressed protein. We studied in vitro lymphocytes from FRDA patients and carriers to identify a peripheral gene expression phenotype. Peripheral biomarkers related to disease status would be extremely valuable for assessing drug efficacy and could provide new pathophysiological insights.

METHODS

We characterized the gene expression profiles in peripheral blood mononuclear cells (PBMCs) from FRDA patients, compared with controls and related carriers. Cells were studied both before and after in vitro treatment with compounds that increase frataxin levels. Quantitative real-time polymerase chain reaction and additional microarrays were used to confirm a core set of genes in multiple independent series.

RESULTS

We identified a subset of genes changed in cells from patients with pathological frataxin deficiency, and a core set of these genes were confirmed in independent series. Changes in gene expression were related to the mitochondria, lipid metabolism, cell cycle, and DNA repair, consistent with FRDA's known pathophysiology. We evaluated the in vitro effect of multiple compounds (histone deacetylase inhibitors) on this putative biomarker set, and found that this biochemical phenotype was ameliorated in accordance with drug efficacy.

INTERPRETATION

Frataxin downregulation is associated with robust changes in gene expression in PBMCs, providing pathogenetic insights and a core subset of genes that, if verified in vivo, could be used as a peripheral biomarker.

Prolonged treatment with pimelic o-aminobenzamide HDAC inhibitors ameliorates the disease phenotype of a Friedreich ataxia mouse model

Friedreich ataxia (FRDA) is an inherited neurodegenerative disorder caused by GAA repeat expansion within the FXN gene, leading to epigenetic changes and heterochromatin-mediated gene silencing that result in a frataxin protein deficit. Histone deacetylase (HDAC) inhibitors, including pimelic o-aminobenzamide compounds 106, 109 and 136, have previously been shown to reverse FXN gene silencing in short-term studies of FRDA patient cells and a knock-in mouse model, but the functional consequences of such therapeutic intervention have thus far not been described. We have now investigated the long-term therapeutic effects of 106, 109 and 136 in our GAA repeat expansion mutation-containing YG8R FRDA mouse model. We show that there is no overt toxicity up to 5 months of treatment and there is amelioration of the FRDA-like disease phenotype. Thus, while the neurological deficits of this model are mild, 109 and 106 both produced an improvement of motor coordination, whereas 109 and 136 produced increased locomotor activity. All three compounds increased global histone H3 and H4 acetylation of brain tissue, but only 109 significantly increased acetylation of specific histone residues at the FXN locus. Effects on FXN mRNA expression in CNS tissues were modest, but 109 significantly increased frataxin protein expression in brain tissue. 109 also produced significant increases in brain aconitase enzyme activity, together with reduction of neuronal pathology of the dorsal root ganglia (DRG). Overall, these results support further assessment of HDAC inhibitors for treatment of Friedreich ataxia.

Evaluation of histone deacetylase inhibitors as therapeutics for neurodegenerative diseases

Various neurodegenerative diseases are associated with aberrant gene expression. We recently identified a novel class of pimelic o-aminobenzamide histone deacetylase (HDAC) inhibitors that show promise as therapeutics in the neurodegenerative diseases Friedreich's ataxia (FRDA) and Huntington's disease (HD). Here, we describe the various techniques used in our laboratories to dissect mechanisms of gene silencing in FRDA and HD, and to test our HDAC inhibitors for their ability to reverse changes in gene expression in cellular models.

Two new pimelic diphenylamide HDAC inhibitors induce sustained frataxin upregulation in cells from Friedreich's ataxia patients and in a mouse model

BACKGROUND

Friedreich's ataxia (FRDA), the most common recessive ataxia in Caucasians, is due to severely reduced levels of frataxin, a highly conserved protein, that result from a large GAA triplet repeat expansion within the first intron of the frataxin gene (FXN). Typical marks of heterochromatin are found near the expanded GAA repeat in FRDA patient cells and mouse models. Histone deacetylase inhibitors (HDACIs) with a pimelic diphenylamide structure and HDAC3 specificity can decondense the chromatin structure at the FXN gene and restore frataxin levels in cells from FRDA patients and in a GAA repeat based FRDA mouse model, KIKI, providing an appealing approach for FRDA therapeutics.

METHODOLOGY/PRINCIPAL FINDINGS

In an effort to further improve the pharmacological profile of pimelic diphenylamide HDACIs as potential therapeutics for FRDA, we synthesized additional compounds with this basic structure and screened them for HDAC3 specificity. We characterized two of these compounds, 136 and 109, in FRDA patients' peripheral blood lymphocytes and in the KIKI mouse model. We tested their ability to upregulate frataxin at a range of concentrations in order to determine a minimal effective dose. We then determined in both systems the duration of effect of these drugs on frataxin mRNA and protein, and on total and local histone acetylation. The effects of these compounds exceeded the time of direct exposure in both systems.

CONCLUSIONS/SIGNIFICANCE

Our results support the pre-clinical development of a therapeutic approach based on pimelic diphenylamide HDACIs for FRDA and provide information for the design of future human trials of these drugs, suggesting an intermittent administration of the drug.

Chemical probes identify a role for histone deacetylase 3 in Friedreich's ataxia gene silencing

We recently identified a class of pimelic diphenylamide histone deacetylase (HDAC) inhibitors that show promise as therapeutics in the neurodegenerative diseases Friedreich's ataxia (FRDA) and Huntington's disease. Here, we describe chemical approaches to identify the HDAC enzyme target of these inhibitors. Incubation of a trifunctional activity-based probe with a panel of class I and class II recombinant HDAC enzymes, followed by click chemistry addition of a fluorescent dye and gel electrophoresis, identifies HDAC3 as a unique high-affinity target of the probe. Photoaffinity labeling in a nuclear extract prepared from human lymphoblasts with the trifunctional probe, followed by biotin addition through click chemistry, streptavidin enrichment, and Western blotting also identifies HDAC3 as the preferred cellular target of the inhibitor. Additional inhibitors with different HDAC specificity profiles were synthesized, and results from transcription experiments in FRDA cells point to a unique role for HDAC3 in gene silencing in Friedreich's ataxia.

CTLA4Ig treatment in patients with multiple sclerosis: an open-label, phase 1 clinical trial

BACKGROUND

The modulation of costimulatory pathways represents an original therapeutic approach to regulate T cell-mediated autoimmune diseases by preventing or reducing autoantigen-driven T-cell activation in humans. Autoreactive CD4(+) T cells play a critical role in initiating the immune response leading to the chronic inflammation and demyelination characteristic of multiple sclerosis (MS).

METHODS

We used IV infusions of CTLA4Ig to block the CD28/B7 T-cell costimulatory pathway in a phase 1 dose-escalation study in MS. Sixteen patients with relapsing-remitting MS received a single CTLA4Ig infusion and were monitored for up to 3 months after treatment. In an extension study, four additional subjects received four doses of CTLA4Ig.

RESULTS

CTLA4Ig was well tolerated in patients with MS, and most adverse events were rated as mild. Immunologic assessment of the patients showed a reduction in myelin basic protein (MBP) proliferation within 2 months of infusion and decreased interferon-gamma production by MBP-specific lines.

CONCLUSIONS

Inhibiting costimulatory molecule interactions by using CTLA4Ig seems safe in multiple sclerosis (MS), and the immunologic effects suggest that it may be a promising approach to regulate the inflammatory process associated with MS.

Effects of uridine on kindling

The anticonvulsant effect of the nucleoside uridine has been studied for several decades with controversial results. One of its attractive properties is that as a natural endogenous molecule, it lacks the serious side effects of common antiepileptic drugs used today. In the current study, we examined the potential antiepileptogenic effect of uridine in the hippocampal kindling model, using once-daily stimulations. Uridine was administered once or three times daily; levetiracetam was administered as a positive control; and normal saline was used as a negative control. Rats receiving uridine or levetiracetam had slower kindling rates and shorter afterdischarge durations than the normal saline controls. These results are consistent with previous work using a rapid kindling model and suggest that uridine has antiepileptogenic properties. Because of its combination of low toxicity and efficacy, uridine is a possible candidate for the treatment of epilepsy.

Triacetyluridine (TAU) decreases depressive symptoms and increases brain pH in bipolar patients

Eleven patients with bipolar depression were given doses of up to 18 g per day of triacetyluridine (TAU) over 6 weeks to test the effect of uridine on symptoms of depression via Montgomery-Asberg Depression Rating Scale (MADRS; Asberg, Montgomery, Perris, Schalling, & Sedvall, 1978) scores and on cellular bioenergetics using phosphorus magnetic resonance spectroscopic imaging (31P-MRSI). All patients and comparison participants (n = 9) completed baseline 31P-MRSI scans, and 9 patients completed posttherapy scans. The percentage changes for MADRS scores (Week 2, -23.8; Week 3, -34.9; Week 4, -42.5) and the time effects of TAU on MADRS scores (Week 2, z = -2.07, p = .039; Week 3, z = -4.28, p < .001; Week 4, z = -4.54, p < .001) may reflect TAU effects on early symptom improvement. TAU responders (patients who had a 50% or greater reduction in MADRS scores from baseline at any time) demonstrated a significant difference from nonresponders in pH changes from baseline (effect size = 150). These results suggest that TAU treatment may decrease symptoms of depression and improve mitochondrial functioning.

Increased amygdala fMRI activation after secretin administration

It has recently been reported that secretin activates gene expression in the central nucleus of the amygdala in rats. To examine the neurophysiological effects of secretin on amygdalar activation in humans, the authors measured Blood Oxygen Level Dependent functional magnetic resonance imaging signal change during facial affect processing in a placebo-controlled double-blind study. The authors studied 12 healthy male subjects who were presented with three stimulus conditions: viewing happy, fearful, and neutral faces, before and after infusion with either secretin or placebo. To test whether treatment was associated with distinct patterns of activation, the two conditions (Pre and Post) were subjected to a subtraction analyses in SPM99 and hypotheses regarding the activation of the left and right amygdala were tested using a region-of-interest approach. Subtraction of treatment minus baseline activation during the fear condition yielded significant (p=.001) activation in the right amygdala and a nonsignificant increase in activation in the left amygdala. No significant differences were seen between the treatment conditions for the amygdala when viewing happy or neutral faces. These preliminary findings indicate that secretin may alter responsivity to affective stimuli. The presence of increased activation of the amygdala during the viewing of fearful faces is consistent with findings from animal studies and suggests a mechanism by which secretin may modulate social behavior.

Lack of effect of secretin on kindling and seizures

Secretin infused into rats activates neurons located in brain areas controlling autonomic function and emotion. The brain activity of secretin is mediated, at least in part, through vagal pathways. It is known that afferent stimulation of the vagus nerve results in considerable antiepileptic effects. Whether or not secretin has an effect on seizures is unknown. In this study, we evaluated the efficacy and safety of secretin as an antiepileptogenic agent in electrical kindling and as an anticonvulsant in fully kindled seizures. To assess antiepileptogenic effects, we administered secretin (10, 30, or 100 microg/kg/dose) or normal saline intravenously 5 min before twice-daily kindling stimulation. To assess the anticonvulsant effect of secretin, we administered either normal saline or secretin (100 microg/kg/dose) 5 min before the electrical stimulation to fully kindled rats. We observed no effect on kindling rate or afterdischarge duration. In fully kindled rats, secretin administration had no effect on kindling stage or afterdischarge duration. Thus, in the dose range used in this preliminary acute treatment study, secretin had no discernible antiepileptogenic or anticonvulsant effects. Secretin was very well tolerated in this multidose protocol.

Effects of uridine in models of epileptogenesis and seizures

Due to the limited efficacy and side effects of current antiepileptic drugs (AEDs), the search for new therapeutic agents is critical. Uridine, a possible endogenous antiepileptic modulator, has been demonstrated to have anticonvulsant effects in some models of epilepsy, but not others. In this study, we examined possible neuroprotective effects of uridine by administering the agent following lithium-pilocarpine induced status epilepticus. The effects of uridine were assessed on EEG patterns, visual-spatial memory in the water maze and histopathology. There was a trend for reduced EEG spike frequency, improved visual spatial memory and better histology score in rats receiving uridine. The antiepileptogenic and anticonvulsant effects of uridine were studied by administering uridine to rats undergoing rapid kindling or following full kindling. In the rapid kindling models, uridine had a moderate antiepileptogenic and anticonvulsant effect. These results suggest uridine may have potential to aid in the prevention and treatment of epilepsy.

A secretin i.v. infusion activates gene expression in the central amygdala of rats

For the last 100 years secretin has been extensively studied for its hormonal effects on digestion. Recent observations that the deficits in social reciprocity skills seen in young (3-4-year-old) autistic children are improved after secretin infusions suggest an additional influence on neuronal activity. We show here that i.v. administration of secretin in rats induces Fos protein expression in the neurons of the central amygdala as well as the area postrema, bed nucleus of the stria terminalis, external lateral parabrachial nucleus and supraoptic nucleus. However, secretin infusion did not induce Fos expression in the solitary tract nucleus or paraventricular nucleus, regions normally activated by related peptides such as cholecystokinin. The peak blood levels of secretin that induce Fos protein expression in rat brain are similar to the peak blood levels observed during i.v. treatment with secretin in humans. The amygdala is known to be critical for developing reciprocal social interaction skills and abnormalities in this brain region have been demonstrated in autistic children.

Differential transport of a secretin analog across the blood-brain and blood-cerebrospinal fluid barriers of the mouse

Secretin is a gastrointestinal peptide belonging to the vasoactive intestinal peptide (VIP)/glucagon/pituitary adenylate cyclase-activating polypeptide (PACAP) family recently suggested to have therapeutic effects in autism. A direct effect on brain would require secretin to cross the blood-brain barrier (BBB), an ability other members of the VIP/PACAP family have. Herein, we examined whether a secretin analog (SA) radioactively labeled with (131)I (I-SA) could cross the BBB of 4-week-old mice. We found I-SA was rapidly cleared from serum with fragments not precipitating with acid appearing in brain and serum. Levels of radioactivity were corrected to reflect only intact I-SA as estimated by acid precipitation. After i.v. injection, I-SA was taken up by brain at a modest rate of 0.9 to 1.5 microl/g-mm. Capillary depletion, brain perfusion, and high-performance liquid chromatography were used to confirm the passage of intact I-SA across the BBB. I-SA entered every brain region, with the highest uptake into the hypothalamus and cerebrospinal fluid (CSF). Unlabeled SA (10 microg/mouse) did not inhibit uptake by brain but did inhibit clearance from blood and uptake by the CSF, colon, kidney, and liver. The decreased clearance of I-SA from blood increased the percentage of the i.v. injected dose taken up per brain (%Inj/g) from about 0.118 to 0.295%Inj/g. In conclusion, SA crosses the vascular barrier by a nonsaturable process and the choroid plexus by a saturable process in amounts that for other members of its family produce central nervous system (CNS) effects. This passage provides a pathway through which peripherally administered SA could affect the CNS.

CTLA4Ig-induced linked regulation of allogeneic T cell responses

The mechanisms by which CTLA4Ig exerts its powerful immunomodulatory effects are not clear. We show here that CTLA4Ig can induce linked regulation of allogeneic porcine T cell responses in vitro. Naive miniature swine SLA(dd) T cells were rendered hyporesponsive to specific allogeneic Ag after coculturing with MHC-mismatched SLA(cc) stimulators in the presence of CTLA4Ig. These Ag-specific hyporesponsive T cells were subsequently able to actively inhibit the allogeneic responses of naive syngeneic T cells in an MHC-linked fashion, as the responses of naive SLA(dd) responders against specific SLA(cc) and (SLA(ac))F(1) stimulators were inhibited, but allogeneic responses against a 1:1 mixture of SLA(aa) (I(a), II(a)) and SLA(cc) (I(c), II(c)) were maintained. This inhibition could be generated against either class I or class II Ags, was radiosensitive, and required cell-cell contact. Furthermore, the mechanism of inhibition was not dependent upon a deletional, apoptotic pathway, but it was reversed by anti-IL-10 mAb. These data suggest that CTLA4Ig-induced inhibition of naive allogeneic T cell responses can be mediated through the generation of regulatory T cells via an IL-10-dependent mechanism.

Schwann cell heparan sulfate proteoglycans play a critical role in glial growth factor/neuregulin signaling

Glial growth factors are proteins encoded by the neuregulin gene and are thought to signal via receptor tyrosine kinases. Many neuregulin gene products bind heparin, and we hypothesize that affinity for heparin may implicate cell surface heparan sulfate proteoglycans (HeSPGs) as co-receptors for the soluble neuregulin gene product, recombinant human glial growth factor 2 (rhGGF2). Using primary rat Schwann cell cultures, we show that exogenous heparin and heparan sulfate block rhGGF2-induced phosphorylation of putative neuregulin receptors, and block subsequent DNA synthesis; other glycosaminoglycans show no such effect. Inhibition of Schwann cell HeSPG biosynthesis by administration of beta-xyloside also blocks responsiveness to rhGGF2. In cell-free binding assays, rhGGF2 binds heparin and heparan sulfate with high affinity, while suramin and suramin-like molecules block this binding. These suramin-like molecules reversibly block Schwann cell responsiveness to rhGGF2 with a rank order of potency identical to that in the cell-free binding assay. Thus we demonstrate high affinity and specificity in the interaction of rhGGF2 with heparin-like molecules, and show that three distinct perturbations of this interaction on Schwann cells (exogenous heparin/ heparan sulfate treatment, inhibition of HeSPG biosynthesis, and treatment with suramin-like molecules) result in a loss of responsiveness to rhGGF2. These results support a model in which HeSPGs are critical components that modulate extracellular rhGGF2 signaling interactions with appropriate receptor tyrosine kinases.

The amino terminal domain of HIV-1 Rev is required for discrimination of the RRE from nonspecific RNA

The ability of HIV-1 Rev to successfully discriminate between specific Rev-responsive elements (RRE) and nonspecific binding sites in the presence of excess nonspecific RNA was examined using filter binding, gel shift, and gel filtration techniques, using purified M4 Rev mutant protein and endoproteinase Lys-C cleaved wild-type Rev. The M4 Rev displayed a slightly reduced binding affinity to the RRE, as well as a tenfold decrease in its ability to discriminate the RRE from non-specific RNA compared to the wild-type Rev. Gel shift and gel filtration chromotography data also showed decreased ability of the mutant to multimerize in the absence or presence of the RRE. The Lys-C cleaved Rev, which lacks the amino-terminal 20 amino acids of the protein, displayed less ability to discriminate the RRE from nonspecific RNA compared to either the wild-type or the M4 mutant Rev and appeared unable to form protein-protein interactions, yet still bound sense and antisense RNA species with high affinity (Kd was in the nanomolar concentration range). A 40 amino acid peptide containing the arginine-rich RRE binding domain of Rev was also observed to interact with both the RRE and antisense RNA fragments with a binding constant of about 1 x 10(-9) M. However, the peptide displayed almost no ability to discriminate between the RRE and a comparably sized antisense RRE. The loss in ability to discriminate correct from incorrect binding sites correlates with overall decreases in the alpha-helical character of the protein and perturbations within the amino terminus. The amino terminus of Rev is likely to maintain the conformational integrity of the arginine rich RRE binding domain which is required for specific RNA binding site discrimination or stabilization of specific Rev-RRE interactions.

Differences in the binding of blocking anti-CD11b monoclonal antibodies to the A-domain of CD11b

CD11b/CD18 (Mac-1) is a leukocyte integrin that plays a critical role in neutrophil adhesion and the initiation of acute inflammatory responses. Several Mac-1 blocking mAbs bind to the A-domain of CD11b, a approximately 200 amino acid region in the N-terminal portion of the protein that is involved in ligand binding and Mac-1 functional activity. We examined several CD11b blocking mAbs for different patterns of binding to A-domain. We used human/murine chimeric CD11b expression constructs and deletions of the A-domain to examine binding. We describe the binding characteristics of mAbs 60.1, LM2/1, LPM19C, M170, 44, and 904. All of these mAbs, except for 60.1, bind to the C-terminal half of the human A-domain (CD11b181-316). mAb 60.1 was unique in that it required regions of the N- and C-terminal ends of the A-domain for binding. mAbs 60.1, LPM19C, 904, and 44 all required the A-domain to be intact for binding. This suggests that these CD11b mAbs recognize a conformational epitope. LM2/1 was capable of binding to a fragment of the A-domain, CD11b285-300. Inasmuch as this system has been used to define different mAb binding sites, it may be used to analyze specific ligand binding sites in the A-domain of CD11b.

Differences in the binding of blocking anti-CD11b monoclonal antibodies to the A-domain of CD11b

CD11b/CD18 (Mac-1) is a leukocyte integrin that plays a critical role in neutrophil adhesion and the initiation of acute inflammatory responses. Several Mac-1 blocking mAbs bind to the A-domain of CD11b, a approximately 200 amino acid region in the N-terminal portion of the protein that is involved in ligand binding and Mac-1 functional activity. We examined several CD11b blocking mAbs for different patterns of binding to A-domain. We used human/murine chimeric CD11b expression constructs and deletions of the A-domain to examine binding. We describe the binding characteristics of mAbs 60.1, LM2/1, LPM19C, M170, 44, and 904. All of these mAbs, except for 60.1, bind to the C-terminal half of the human A-domain (CD11b181-316). mAb 60.1 was unique in that it required regions of the N- and C-terminal ends of the A-domain for binding. mAbs 60.1, LPM19C, 904, and 44 all required the A-domain to be intact for binding. This suggests that these CD11b mAbs recognize a conformational epitope. LM2/1 was capable of binding to a fragment of the A-domain, CD11b285-300. Inasmuch as this system has been used to define different mAb binding sites, it may be used to analyze specific ligand binding sites in the A-domain of CD11b.

Anti-CD11b monoclonal antibody reduces ischemic cell damage after transient focal cerebral ischemia in rat

We investigated the effect of an anti-CD11b monoclonal antibody (1B6c) on ischemic cell damage after transient middle cerebral artery occlusion. We divided animals into three groups: MAb 1 group (n = 5)--rats were subjected to 2 hours of transient occlusion and 1B6c (1 mg/kg) was administered intravenously at 0 and 22 hours of reperfusion; MAb 2 group (n = 5)--same experimental protocol as MAb 1 group, except that the initial dose of 1B6c was increased to 2 mg/kg; and control group (n = 5)--same experimental protocol as MAb 2 group, except that an isotype-matched control antibody was administered. Animals were weighed and tested for neurological function before and after occlusion of the middle cerebral artery. Forty-six hours after reperfusion, brain sections were stained with hematoxylin and eosin for histology evaluation. We observed a significant reduction of weight loss and improvement in neurological function after ischemia in the MAb 2 animals compared to MAb 1 and vehicle-treated animals (p < 0.05). The lesion volume was significantly smaller in the MAb 2 group (19.5 +/- 1.9%) compared to MAb 1 (29.9 +/- 2.6%) and vehicle-treated (34.2 +/- 5.4%) groups (p < 0.01). Tissue polymorphonuclear cell numbers were reduced in both 1B6c-administered groups. Our data demonstrate that administration of anti-CD11b antibody results in a dose-dependent, significant functional improvement and reduction of ischemic cell damage after transient focal cerebral ischemia in the rat.

Postischemic administration of an anti-Mac-1 antibody reduces ischemic cell damage after transient middle cerebral artery occlusion in rats

BACKGROUND AND PURPOSE

Postischemic cerebral inflammation may contribute to ischemic cell damage. The CD11b/18 (Mac-1) integrin mediates stimulated neutrophil binding to endothelia. We therefore investigated the effect of administration of an anti-Mac-1 monoclonal antibody on cerebral ischemic cell damage in the rat.

METHODS

Rats (n = 10) were subjected to 2 hours of middle cerebral artery occlusion; the anti-Mac-1 antibody was administered at a dose of 2 mg/kg i.v. at 1 hour of reperfusion and 1 mg/kg i.v. at 22 hours of reperfusion or an isotype-matched control antibody (n = 10) was administered using the same experimental protocol. Rats were killed at 46 hours of reperfusion, and brain sections were stained with hematoxylin and eosin for histological evaluation. In a separate population of rats given either vehicle (n = 8) or anti-Mac-1 antibodies (n = 9), intraparenchymal neutrophils were measured by means of a myeloperoxidase assay.

RESULTS

The lesion volume was significantly smaller (28%) in the anti-Mac-1 antibody group compared with the vehicle control group (P < .01). Numbers of intraparenchymal polymorphonuclear cells were significantly reduced (P < .05) in the cortex of the anti-Mac-1 antibody group compared with the vehicle control group.

CONCLUSIONS

Our data demonstrate that administration of anti-Mac-1 antibody 1 hour after onset of reperfusion results in significant reductions of ischemic cell damage and intraparenchymal neutrophils after transient (2-hour) focal cerebral ischemia in the rat.

Molecular determinants of lipid mediator-induced leukocyte adherence and emigration in rat mesenteric venules

The objective of this study was to identify the molecular determinants of leukocyte rolling, adherence, and emigration elicited in postcapillary venules by the lipid mediators leukotriene B4 (LTB4) or platelet-activating factor (PAF). Leukocyte-endothelial cell adhesion and shear rate were monitored in rat mesenteric venules during superfusion with either LTB4 or PAF in the presence or absence of monoclonal antibodies (MAbs) directed against either leukocyte (CD18, CD11b) or endothelial cell [intercellular adhesion molecule 1 (ICAM-1), E-selectin, P-selectin] adhesion glycoproteins. In untreated animals and in animals receiving a nonbinding control MAb, LTB4 and PAF increased the number of both adherent (8- and 4-fold, respectively) and emigrated (14- and 8-fold, respectively) leukocytes, while reducing leukocyte rolling velocity (36 and 33%, respectively). The LTB4- and PAF-induced leukocyte adherence and emigration were significantly attenuated by pretreatment with MAbs directed against CD18, CD11b, ICAM-1, and E-selectin, but not P-selectin. The reduction in leukocyte rolling velocity induced by LTB4 was not affected by any of the MAbs; however, both P- and E-selectin MAbs significantly attenuated the reduction in leukocyte rolling velocity elicited by PAF. The results of this study indicate that the leukocyte adherence and emigration induced by both LTB4 and PAF are mediated by CD11b/CD18 on leukocytes and by ICAM-1 and E-selectin on endothelial cells. The molecular determinant of leukocyte rolling appears to be mediator specific, with the selectins mediating the rolling elicited by PAF.

Molecular determinants of reperfusion-induced leukocyte adhesion and vascular protein leakage

The adherence and emigration of leukocytes have been implicated as a rate-limiting step in the microvascular dysfunction associated with reperfusion of ischemic tissues. The objective of the present study was to define the relation between leukocyte-endothelial cell adhesion and albumin leakage in rat mesenteric venules exposed to ischemia and reperfusion (I/R). Leukocyte adherence and emigration as well as albumin extravasation were monitored in single post-capillary venules using intravital fluorescence microscopy. Ischemia (0, 10, 15, or 20 minutes) was induced by complete occlusion of the superior mesenteric artery, and all parameters were monitored for 30 minutes after reperfusion. The magnitude of the leukocyte adherence and emigration and albumin leakage elicited by I/R was positively correlated with the duration of ischemia. The albumin leakage response was also highly correlated with the number of adherent and emigrated leukocytes. Monoclonal antibodies against the adhesion glycoproteins CD18, CD11b, intercellular adhesion molecule-1 (ICAM-1) (at 10 and 30 minutes), and L-selectin (at 10 minutes), but not P- or E-selectin, reduced I/R-induced leukocyte adherence and emigration as well as albumin leakage. Platelet-leukocyte aggregates were formed in postischemic venules; the number of aggregates was reduced by antibodies against P-selectin, CD11b, CD18, and ICAM-1, but not E- or L-selectin. These results indicate that reperfusion-induced albumin leakage is tightly coupled to the adherence and emigration of leukocytes in postcapillary venules. This adhesion-dependent injury response is primarily mediated by CD11b/CD18 on activated neutrophils and ICAM-1 on venular endothelium and appears to require L-selectin-dependent leukocyte rolling.

Biochemical characterization of binding of multiple HIV-1 Rev monomeric proteins to the Rev responsive element

Recombinant HIV-1 Rev protein was overexpressed in Escherichia coli using translational coupling to the beta-glucuronidase gene and demonstrated to interact with high affinity and specificity with the Rev responsive element (RRE). A complex Rev-dependent binding pattern was observed using the gel shift assay which could be simplified to one or two primary bands in the presence of stoichiometric concentrations of RRE. Competition of these bands with a series of homopolymer RNA species demonstrated that Rev is essentially a poly-G binding protein, although poly-I was also shown to compete for specific RRE binding. The stoichiometry of the Rev-dependent gel shift complexes was determined using 125I-labeled Rev. The stable, lowest mobility complex was determined to possess a ratio of between 7 and 8 Rev molecules per RRE containing RNA fragment while the two fastest migrating complexes contained ratios of one and two Rev molecules per RRE, respectively. Using the Hill equation as a model for cooperative interactions, a Hill coefficient of n(app) = 2 was obtained from fitting of direct nitrocellulose filter binding assays, reflecting cooperatively bound Rev molecules on the RRE under equilibrium binding conditions. An increase in ionic strength from 0.0 to 0.3 M NaCl reduced cooperative Rev binding to the RRE, but specificity of Rev for the RRE relative to antisense RNA was increased 100,000-fold. At molar ratios of Rev to RRE above 2, Rev dissociated from the RRE with a T1/2 of approximately 20-25 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Perturbation of the carboxy terminus of HIV-1 Rev affects multimerization on the Rev responsive element

Perturbations within the transactivation and carboxy-terminal domains of HIV-1 Rev were examined for effects on Rev responsive element (RRE) binding activities in vitro and biological activity in vivo. Binding affinities, specificities, and multimerization of the transactivation mutants M10 and Rev/Rex M10-16 on the RRE were equivalent to wild-type Rev. Substitution of the Rex transactivation domain within Rev resulted in the incorporation of an internal methionine residue which, when cleaved with CNBr and subsequently purified, produced a protein species (CNBr-Rev) unable to fully multimerize on the RRE. Instead, two discrete protein-dependent species were generated in the gel shift assay. Furthermore, CNBr-Rev was observed to bind to the RRE with high specificity and an equilibrium binding constant of 6 x 10(-10) M. A C-terminal Rev deletion mutant (Rev M9 delta 14) lacking amino acids 68-112 displayed identical RRE binding characteristics to the CNBr-Rev protein. This protein, which lacks both the activation and the C-terminal domains, was biologically inactive but maintained the ability to discriminate the RRE from nonspecific RNA. Deletion of amino acids 92-112 resulted in a Rev mutant (Rev M11 delta 14) which bound to the RRE with wild-type affinity and high specificity. This purified mutant was observed to be aberrant in multimerization activity on the RRE with reduced multimerization apparent in the gel shift assay. However, Rev M11 delta 14 possessed biological activity equivalent to wild-type Rev in a cell-based p24 ELISA assay. These results suggest that polymerization on the RRE is dispensable for Rev activity and that two monomeric Rev proteins bound to the RRE are sufficient for biological activity. Furthermore, in vivo experiments using the Rev/Rex chimeric mutant and the M10 transdominant mutant as well as in vitro dissociation rate studies with Rev M11 delta 14 and Rev M9 delta 14 suggest that the M9 through M11 domain of the protein may be involved in RRE-dependent specific Rev dimerization.

Broadly neutralizing monoclonal antibodies to the V3 region of HIV-1 can be elicited by peptide immunization

The third hypervariable region (V3) of the HIV-1 envelope gp120 protein contains the principal neutralizing domain. Most neutralizing antibodies directed toward this region are very type-specific. Conserved sequences do exist within this region, however, and may prove useful in developing vaccines and therapeutic monoclonal antibodies (MABs) capable of targeting diverse HIV-1 isolates. We have used synthetic peptides containing conserved V3 sequences as immunogens to produce a panel of neutralizing MABs. The characterization of these MABs is described here. In addition, a series of in vitro assays has been developed that may be useful in predicting the neutralization potential of individual antibodies.

In vitro inhibition of a variety of human immunodeficiency virus isolates by a broadly reactive, V3-directed heteroconjugate antibody

The cytotoxic efficacy of a heteroconjugate antibody composed of OKT3 cross-linked to a broadly reactive antibody directed against the GPGRAF sequence of the gp120 V3 region has been characterized. The heteroconjugate antibody could completely inhibit viral replication of both the IIIB and MN isolates of human immunodeficiency virus (HIV) at concentrations as low as 0.5 ng/ml. At an antibody concentration of 1 micrograms/ml, heteroconjugate-mediated cytotoxicity occurred at effector-to-target ratios as low as 0.006:1. Eight different HIV isolates were tested for in vitro inhibition by the anti-V3-OKT3 conjugate, and all but one were completely inhibited for at least 7 days. These results indicate that heteroconjugate antibodies are a potent, effective means by which HIV-infected cells can be killed and viral replication suppressed.

Definition of the human immunodeficiency virus type 1 Rev and human T-cell leukemia virus type I Rex protein activation domain by functional exchange

The human retroviruses human immunodeficiency virus type 1 (HIV-1) and human T-cell leukemia virus type I (HTLV-I) are characterized by complex regulation of gene expression. Each virus encodes a posttranscriptional regulator, the 19-kDa HIV-1 Rev protein and the 27-kDa HTLV-I Rex protein, which is required for viral replication. Expression of these trans activators results in the cytoplasmic accumulation of unspliced or singly spliced viral mRNA which encode the gag, pol, and env gene products. The finding that the HTLV-I Rex protein is able to functionally substitute for the Rev protein of HIV-1 indicates that HIV-1 Rev and HTLV-I Rex may interact with the same component of a cellular pathway involved in either mRNA splicing or transport. In this study, we have generated functional Rev/Rex hybrid proteins by domain exchange. We have defined, using in vivo and in vitro analyses, the activation domains of Rev and Rex which are the putative targets of a common host cell factor(s) required for Rev and Rex function.

Characterization of HIV-1 REV protein: binding stoichiometry and minimal RNA substrate

The HIV-1 REV protein binds to the stem II region of the REV-responsive element (RNA). Studies to further define the RNA sequence and structure specifically bound by REV protein identify a minimal RNA element of 40 nucleotides. Analysis of RNA fragments by gel retardation and filter binding suggest that a core element composed of one particular stem with flanking sequences capable of forming a second double stranded region is essential for specific recognition by REV protein. Stable REV-RNA complexes are formed in a stoichiometry of 1 REV: 1 RNA. The minimal RNA element binds 1 REV molecule while the stem II saturates at 3 REV molecules per RNA. These results establish that REV recognizes a primary binding site within the RRE and support the notion that the initial viral transcript binding event involves a monomeric REV protein.

Circular dichroism studies of the HIV-1 Rev protein and its specific RNA binding site

The circular dichroism (CD) spectrum of the Rev protein from HIV-1 indicates that Rev contains about 50% alpha helix and 25% beta sheet at 5 degrees C in potassium phosphate buffer, pH 3, and 300 mM KF. The spectrum is independent of protein concentration over a 20-fold range. At neutral pH, Rev is relatively insoluble but can be brought into solution by binding to its specific RNA binding site, the Rev-responsive element (RRE), at a Rev:RNA ratio of about 3:1. Nonspecific binding to tRNA does not solubilize Rev. As judged by difference CD spectra, the conformation of Rev when bound to the RRE at neutral pH is similar to the conformation of unbound Rev at pH 3, although changes in the RNA may also contribute to the difference spectrum. Indeed, some difference is observed near 260 nm, consistent with a conformational change of the RRE upon Rev binding. Rev alone at pH 3 shows irreversible aggregation as the temperature is raised, while Rev bound to the RRE at neutral pH shows a reversible transition with a Tm of 68 degrees C.

C-terminal fragments of gp120 and synthetic peptides from five HTLV-III strains: prevalence of antibodies to the HTLV-III-MN isolate in infected individuals

The immunoreactivity of HTLV-III-infected individuals and virus-inoculated chimpanzees with gp120 synthetic peptides of the HTLV-III gp120 envelope principle neutralizing domain (amino acid 301-324 sequences), derived from the HTLV-III isolates 3B, RF, MN, WMJ2, and SC were determined. Sequential bleeds from an infected lab worker and chimpanzees, both infected with the HTLV-IIIB, were immunoreactive only with the 3B peptide. In contrast, 33 HTLV-III-infected individuals were immunoreactive with the HTLV-III(MN) peptide. Of these 33 individuals, 23 were also immunoreactive with the HTLV-III(SC) peptide, and 18 with the HTLV-III(WMJ2) peptide. The data suggest that HTLV-III strains related to MN are most prevalent among HTLV-III-infected individuals. The binding specificities of goat sera generated against either of these synthetic peptides or the C-terminal fragment of gp120 (PB-1, amino acid 287-467, derived from the HTLV-III isolates 3B, RF, MN, WMJ2, and SC) were also determined. Four different ELISA formats (peptide sera/peptide antigens, peptide sera/PB-1 antigens, PB-1 sera/PB-1 antigens, and PB-1 sera/peptide antigens) were utilized to determine the cross-reactivity patterns of goat sera with the antigens. Goat sera generated against MN and SC sequences (PB-1 proteins, as well as synthetic peptides) were highly cross reactive. Thus, patient sera cross reactivity to multiple strains of the principal neutralizing domain may reflect the antigenic relatedness of the virus isolates rather than multiple infection events or strains generated during disease progression.

HIV-1 structural gene expression requires binding of the Rev trans-activator to its RNA target sequence

Expression of human immunodeficiency virus type 1 structural proteins requires both the viral Rev trans-activator and its cis-acting RNA target sequence, the Rev response element (RRE). The RRE has been mapped to a conserved region of the HIV-1 env gene and is predicted to form a complex, highly stable RNA stem-loop structure. Site-directed mutagenesis was used to define a small subdomain of the RRE, termed stem-loop II, that is essential for biological activity. Gel retardation assays demonstrated that the Rev trans-activator is a sequence-specific RNA binding protein. The RRE stem-loop II subdomain was found to be both necessary and sufficient for the binding of Rev by the RRE. We propose that the HIV-1 Rev trans-activator belongs to a new class of sequence-specific RNA binding proteins characterized by the presence of an arginine-rich binding motif.

Specific binding of HIV-1 recombinant Rev protein to the Rev-responsive element in vitro

The human immunodeficiency virus type 1 (HIV-1) genome encodes the regulatory protein Rev, of relative molecular mass 13,000, which is synthesized from fully processed viral transcripts before synthesis of HIV-1 structural proteins. Rev has been postulated to exert control within the nucleus at the level of messenger RNA processing. The availability of Rev in the nucleus serves to increase the proportion of unspliced and singly spliced mRNA species relative to fully spliced mRNA molecules, resulting in an increased synthesis of viral structural proteins. A highly conserved cis-acting sequence termed the Rev-responsive element (RRE) has been identified in the envelope gene (env) of the viral transcript that seems to control mRNA processing in a Rev-dependent manner. Genetic studies have identified rev gene mutants with dominant phenotypes, supporting the hypothesis that Rev interacts directly with the RRE. Here we demonstrate that Rev protein, purified from Escherichia coli, binds in a sequence-specific manner to the RRE element in vitro.

Principal neutralizing domain of the human immunodeficiency virus type 1 envelope protein

The principal neutralizing determinant of human immunodeficiency virus type 1 (HIV-1) is located in the external envelope protein, gp120, and has previously been mapped to a 24-amino acid-long sequence (denoted RP135). We show here that deletion of this sequence renders the envelope unable to elicit neutralizing antibodies. In addition, using synthetic peptide fragments of RP135, we have mapped the neutralizing determinant to 8 amino acids and found that a peptide of this size elicits neutralizing antibodies. This sequence contains a central Gly-Pro-Gly that is generally conserved between different HIV-1 isolates and is flanked by amino acids that differ from isolate to isolate. Antibodies elicited by peptides from one isolate do not neutralize two different isolates, and a hybrid peptide, consisting of amino acid sequences from two isolates, elicits neutralizing antibodies to both isolates. By using a mixture of peptides of this domain or a mixture of such hybrid peptides the type-specificity of the neutralizing antibody response to this determinant can perhaps be overcome.

The human immunodeficiency virus type 2 (HIV-2) contains a novel gene encoding a 16 kD protein associated with mature virions

The HIV-2 genome contains an open reading frame (designated X-orf) that does not have a counterpart in HIV-1. To establish whether X-orf is a gene, we studied its expression in HIV-2-infected individuals and in infected cells in vitro. An HIV-2 proviral DNA fragment containing the X-orf was expressed in E. coli, and the recombinant protein was used in an immunoblot assay. The X-orf protein was recognized specifically by the sera of HIV-2-infected people but not by the sera of SIV-infected monkeys or HIV-1-infected humans. A rabbit antiserum raised against the recombinant X-orf protein recognized a 16 kD protein in HIV-2-infected cells. The native X-orf protein was not glycosylated or phosphorylated, was localized in the cytoplasm of HIV-2-infected cells, and appeared to be associated with mature virions.

Type-restricted neutralization of molecular clones of human immunodeficiency virus

In a study of the immunologic significance of the genetic diversity present within single isolates of human immunodeficiency virus type 1 (HIV-1), the neutralization of viruses derived from molecular clones of the HIV-1 strain HTLV-IIIB by an extensive panel of sera was compared. Sera from HIV-1-infected patients and from goats immunized with polyacrylamide gel-purified HIV-1 envelope glycoprotein (gp120), native gp120, or gp120-derived recombinant peptides, showed marked heterogeneity in neutralizing activity against these closely related viruses. The change of a single amino acid residue in gp120 may account for such "clonal restriction" of neutralizing activity.