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B Tavitian

Publications and source records attributed to B Tavitian.

At least 19 recordsLinked to original sources
PubMed2006

[Aptamers: selection and scope of applications].

Aptamers are short oligonucleotides selected from large combinatorial pools of sequences for their capacity to bind to many different targets ranging from small molecules (amino acids, antibiotics...) to proteins or nucleic acid structures. Aptamers present the same high specificity and affinity for their targets as antibodies. In addition to efficient binding, aptamers have been shown in many cases to display an inhibitory activity against their targets. Many aptamers are now being developed against biomedical relevant targets, and one aptamer that inhibits the human VEGF165 already received approval for the treatment of age-related macular degeneration. Here we discuss the principles and the practical way of selecting aptamers (SELEX technology) as well as the structural basis for their performance as ligands. A wide scope of applications is described - aptamers have been used as tools for studying nucleic acids/proteins interactions, detecting, purifying or imaging target molecules, regulating gene expression - and includes recent developments of aptamers for therapy and diagnosis.

Combinatorial Chemistry Techniques↗
PubMed2006

Improved synthesis of the peripheral benzodiazepine receptor ligand [11C]DPA-713 using [11C]methyl triflate.

Recently, the pyrazolopyrimidine, [11C] N,N-Diethyl-2-[2-(4-methoxyphenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl]acetamide (DPA-713) has been reported as a new promising marker for the study of peripheral benzodiazepine receptors with positron emission tomography. In the present study, DPA-713 has been labelled from the corresponding nor-analogue using [11C]methyl triflate (CH3OTf). Conditions for HPLC were also modified to include physiological saline (aq. 0.9% NaCl)/ethanol:60/40 as mobile phase making it suitable for injection. The total time of radiosynthesis, including HPLC purification, was 18-20 min. This reported synthesis of [11C]DPA-713, using [11C]CH3OTf, resulted in an improved radiochemical yield (30-38%) compared to [11C]methyl iodide (CH3I) (9) with a simpler purification method. This ultimately enhances the potential of [11C]DPA-713 for further pharmacological and clinical evaluation. These improvements make this radioligand more suitable for automated synthesis which is of benefit where multi-dose preparations and repeated syntheses of radioligand are required.

Acetamides↗
PubMed2003

In vivo imaging with oligonucleotides for diagnosis and drug development.

Molecular imaging, the science that combines non-invasive in vivo imaging and molecular biology, has begun to use labelled oligonucleotides as radiotracers. Antisense oligonucleotides target gene expression at the RNA level, while aptamer oligonucleotides are designed to hit proteins of interest. Oligonucleotides for imaging cover a large range of applications, from the invention of new contrast agents for diagnosis to exquisite research tools for the development of new drugs.

Drug Design↗
PubMed2001

Quantitative measurement of cerebral acetylcholinesterase using.

[11C]physostigmine, an acetylcholinesterase inhibitor, has been shown to be a promising positron emission tomography ligand to quantify the cerebral concentration of the enzyme in animals and humans in vivo. Here, a quantitative and noninvasive method to measure the regional acetylcholinesterase concentration in the brain is presented. The method is based on the observation that the ratio between regions rich in acetylcholinesterase and white matter, a region almost entirely deprived of this enzyme, was found to become approximately constant after 20 to 30 minutes, suggesting that at late time points the uptake mainly contains information about the distribution volume. Taking the white matter as the reference region, a simplified reference tissue model, with effectively one reversible tissue compartment and three parameters, was found to give a good description of the data in baboons. One of these parameters, the ratio between the total distribution volumes in the target and reference regions, showed a satisfactory correlation with the acetylcholinesterase concentration measured postmortem in two baboon brains. Eight healthy male subjects were also analyzed and the regional enzyme concentrations obtained again showed a good correlation with the known acetylcholinesterase concentrations measured in postmortem studies of human brain.

Acetylcholinesterase↗
PubMed2000

In vivo antisense imaging.

Antisense oligonucleotides, in short antisense, are small chains of nucleic acids capable to bind to cellular ribonucleic acid (RNA) by a hybridization mechanism. In vitro, antisense are widely used as reagents to detect or block specific RNA sequences. The use of antisense as in vivo diagnostic agents is attractive because it would bring molecular imaging at the level of gene expression. However, oligonucleotides are non-canonical radiopharmaceuticals and much progress is needed to adapt them to in vivo imaging. The requirements to reach this goal include improvements in radiosynthesis, stability, targeting, and specific and non-specific binding. They will be examined in this review together with the current achievements in the applications of antisense as nuclear medicine radiopharmaceuticals.

Animals↗
PubMed1999

In vivo PET study of cerebral [11C] methyl- tetrahydroaminoacridine distribution and kinetics in healthy human subjects.

It is unclear whether the palliative effects of tetrahydroaminoacridine (THA) (tacrine, Cognex) on the clinical symptoms of patients affected by Alzheimer's disease (AD) are the result of its inhibitory activity on acetylcholinesterase or on other complex sites of action. In order to investigate the cerebral distribution and kinetics of THA in the human brain in vivo, we performed positron emission tomography (PET) imaging with [11C]N-methyl-tetrahydro-aminoacridine (MTHA) in healthy human volunteers. After intravenous injection, [11C]MTHA crossed the blood-brain barrier and reached its maximum uptake between 10 and 40 minutes, depending on the brain regions. Uptake was higher in the grey matter structures, and lower in the white matter. After this peak, the radioactivity remained quasi- constant until 60 minutes in all regions with a half-life varying from 2.44 hours in the thalamus to 3.42 hours in the cerebral cortex. The ratios of regional to whole cerebral cortex brain radioactivity calculated between 50 and 70 minutes after the tracer injection were 1.14 +/- 0.04, 1.07 +/- 0. 03 and 1.06 +/- 0.04 in the putamen, cerebellum and thalamus, respectively. Overall, these results show that: (1) [11C]MTHA crosses the blood-brain barrier easily and is highly concentrated in the brain; (2) the regional brain distribution of [11C]MTHA does not parallel that of in vivo acetylcholinesterase (AChE) concentrations; and (3) the cerebral kinetics of [11C]MTHA are consistent with known plasmatic pharmacokinetics of THA in AD patients. We conclude that PET imaging with [11C]MTHA is a useful method for assessing the cerebral distribution and kinetics of THA in vivo.

Aged↗
PubMed1997

Rab3A immunolocalization in the mammalian vestibular end-organs during development and comparison with synaptophysin expression.

Rab proteins are essential for membrane vesicle docking and fusion and for transport vesicle formation at the presynaptic membrane, a step in the release of neurotransmitters. The vestibular sensory epithelia contain three types of synapses: afferent terminals, efferent endings and possible synaptic contacts between the apex of the afferent nerve calyces and the sensory cells. We report an immunocytochemical codetection of rab3A and synaptophysin in the vestibular end-organs of mouse, between fetal day 14 and adult, and of rat during the postnatal development. During mouse fetal development, rab3A appeared in afferent neurites on F16, and in sensory cells on F19. This was respectively two and five days later than the appearance of synaptophysin-IR in the same compartments. During the late postnatal development and in the adult sensory epithelia, rab3A and synaptophysin were strongly detected in nerve terminals of efferent and possibly afferent nature and in the upper part of the nerve calyces. The presence of rab3A in the nerve calyces is consistent with the putative secretory function of the calyx. In addition, rab3A immunostaining was also present in the sensory cells together with a faint synaptophysin-IR, that had not been described in previous reports [Scarfone, E., Demêmes, D. and Sans, A. J. Neurosci., 11 (1991) 1173-1181.]. The presence of these two proteins in the sensory cells supports the existence of a synaptic vesicle cycle in these cells.

Animals↗
PubMed1997

Synthesis of [11C]RPR-72840A and its evaluation as a radioligand for the serotonin reuptake site in positron emission tomography.

RPR-72840A, an inhibitor of serotonin reuptake, was labelled with carbon-11. The synthesis of the nonradioactive precursor, which exhibited some unexpected chemistry, and its reaction with [11C]phosgene affording [11C]RPR-72840A are described. Biodistribution studies in rats and PET studies in baboons were conducted to evaluate [11C]RPR-72840A as a tracer for PET imaging of the serotonin reuptake system.

Animals↗
PubMed1997

p34(cdc2) and mitotic cyclin expression in the developing quail neuroretina.

After an initial proliferation phase, neurons of the central nervous system (CNS) of higher eukaryotes remain postmitotic during their entire lifespan. This requires that a very stringent control be exerted on the cell division apparatus, whose molecular mechanisms remain quite elusive. Here we have used quail neuroretina as a model to study the control of cell division in the developing CNS. In vertebrates, embryonic neuroretinal cells (NR cells) stop their proliferation at different times depending on the cell type. Most NR cells in the quail embryo become postmitotic between E7 and E8. To acquire a better understanding of the molecular events leading to quiescence in NR cells, we have analyzed the expression of cdc2 and of two activators of p34(cdc2): cyclin A and cyclin B2 in the developing neuroretina. We report that these three proteins are downregulated between E7 and E9, suggesting that a common mechanism could block their transcription in differentiating neurons. We also report, using an immunohistochemical approach, that p34(cdc2) downregulation is correlated with the appearance of the microtubule-associated protein tau. These results strongly suggest that inhibition of cdc2 gene expression is closely linked to the achievement of terminal differentiation in neurons. However, we also show that postmitotic ganglion cells precursors begin to synthesize the early neuronal differentiation marker beta3-tubulin while p34(cdc2) is still detectable in these cells, suggesting that p34(cdc2) or a closely related kinase could play a role in some "young" postmitotic neurons.

Animals↗
PubMed1996

Differential synaptic vesicle protein expression in the barrel field of developing cortex.

The distribution of four proteins associated with synaptic vesicles, SV2, synaptophysin, synapsin I, and rab3a, was investigated during postnatal development of the posteromedial barrel subfield (PMBSF) in the rat somatosensory cortex. A distinct progression in the appearance of the different synaptic vesicle proteins within the PMBSF was observed. SV2, synapsin I, and synaptophysin revealed the organization of the barrel field in the neonate. This early demarcation of the cortical representation of the vibrissal array coincides with the earliest known age for the emergence of the cytoarchitectonic organization of this region. In contrast, rab3a did not delimit the barrels until the end of the 1st postnatal week, coincident with the known onset of adult-like physiological activity and the loss of plasticity in afferents to this region. In addition, the appearance of the different synaptic vesicle proteins occurred earlier within the PMBSF than in the adjacent extra-barrel regions of the cortex. These results show that the molecular differentiation of synaptic fields across the cortex is not a homogeneous and synchronous process in terms of synaptic vesicle protein expression. Because these proteins act together in mature synapses to ensure the regulated release of neurotransmitters, our results suggest that this temporo-spatial asynchrony may underlie different potentials for synaptic activity and thus contribute to the development of cortical maps.

Animals↗
PubMed1996

In vivo imaging of human cerebral acetylcholinesterase.

We report here the first positron emission tomography (PET) images showing the in vivo regional distribution of acetylcholinesterase (AChE) in human brain. The study was carried out in eight healthy human volunteers using as a tracer [11C]-physostigmine ([11C]PHY), an inhibitor of AChE. After intravenous injection of [11C]PHY, radioactivity was rapidly taken up in brain tissue and reached maximal uptake within a few minutes, following a regional pattern mostly related to cerebral perfusion. After the peak, the cerebral radioactivity gradually decreased with a half-life varying from 20 to 35 min, depending on the brain structure. [11C] PHY retention was higher in regions rich in AChE, such as the striatum (half-life, 35 min), than in regions poor in AChE, such as the cerebral cortex (half-life, 20 min). At later times (25-35 min postinjection), the cerebral distribution of [11C]PHY was typical of AChE activity: putamen-caudate > cerebellum > brainstem > thalamus > cerebral cortex, with a striatal to cortex ratio of 2. These results suggest that PET studies with [11C]PHY can provide in vivo brain mapping of human AChE and are promising for the study of changes in AChE levels associated with neurodegenerative diseases.

Acetylcholinesterase↗
PubMed1996

Two distinct regulatory elements control quail cdc2 transcription: possible involvement in the control of retinoblast differentiation.

It is a characteristic of the central nervous system of higher eukaryotes that neurons, after an initial proliferation phase, remain postmitotic for their whole life span. In the developing quail neuroretina, most retinoblasts become postmitotic after 7-8 days of incubation. They also cease to express cdc2, which is presumably necessary to allow retinoblasts to definitively leave the cell cycle. The molecular mechanisms monitoring cdc2 expression during differentiation remain partly understood. To further study the control of cdc2 transcription in avian cells, we have cloned the quail cdc2 promoter. Two functional regulatory elements have been characterized. One of them contains an E2F-binding site. Human E2F-1 was found to transactivate the quail cdc2 promoter very efficiently in avian and human cells. Gel retardation experiments are presented, suggesting that E2F, in association with different partners, is a major regulatory of cdc2 transcription during the development of the neuroretina. Our data also indicate that another transcription factor binds to the octamer CAGGTGGC located 115 nucleotides above the main transcription start site. This motif is thus another important regulatory element participating in the control of cdc2 expression.

Amino Acid Sequence↗
PubMed1995

Differential effect of functional olfactory deprivation on synaptic vesicle proteins in rat olfactory bulb.

The effects of functional odour deprivation on three different proteins associated with the membrane of the synaptic vesicle were examined in the rat olfactory bulb. Six weeks after neonatal unilateral nostril closure, Rab3a, a ras-like GTPase, was down-regulated in the odour-deprived bulb in the same manner as tyrosine hydroxylase. In contrast, synaptophysin, a protein of the channel family, and SV2, a putative transporter protein, were not altered. These results suggest that afferent activity is a factor controlling the level of some, but not all, proteins associated with presynaptic vesicles.

Animals↗
PubMed1995

Double in situ hybridization reveals overlapping neuronal populations expressing the low molecular weight GTPases Rab3a and Rab3b in Rat brain.

The ras-related Rab3 gene subfamily codes for small GTP-binding proteins which control a late step of exocytosis during which vesicles become docked to the plasma membrane. Rab3a and Rab3b are the most abundant Rab3 isoforms expressed in the CNS of mammals. We have shown previously that the Rab3a protein was selectively distributed and expressed in various regions of the rat brain. Here we have determined the pattern of expression of Rab3b mRNA in the brain and compared it with that of Rab3a mRNA. In addition, we examined the co-expression of these two Rab within individual neurons. In general the Rab3b transcript was detected in many regions which also express Rab3a mRNA but at a lower level than Rab3a, except in the olfactory bulb and in the pituitary where the Rab3b hybridization signal was similar and higher respectively. Double in situ hybridization revealed that Rab3a and Rab3b mRNAs were co-localized in most neurons, in all brain areas examined. However, in each of these areas, subsets of neurons appeared to preferentially express either Rab3b or Rab3a, or some neurons did not express either Rab3 homologue at detectable levels. These data support the view of a functional specialization of Rab3a and Rab3b in the control of exocytosis in neuronal and neuroendocrine cells.

Animals↗
PubMed1994

Rat brain acetylcholinesterase visualized with [11C]physostigmine.

Physostigmine, a powerful cholinesterase inhibitor, has recently been labelled with 11C in view of its potential application for in vivo imaging of cerebral acetylcholinesterase (AChE) using positron emission tomography. Here we carried out autoradiography of the rat brain using [11C]physostigmine in order to characterize the cerebral targets of this ligand. Autoradiograms were obtained using phosphor storage plates which, compared to autoradiographic films, greatly improved the quality of 11C images. Following autoradiography, brain sections were stained for AChE activity, allowing a direct comparison of autoradiographic and histoenzymatic localizations. The distributions of 11C label and of AChE activity were found to be essentially super-imposable, both after in vivo injection of and after in vitro incubation with [11C]physostigmine. Densitometric analysis showed that radioactivity and enzymatic activity distributions were regionally correlated. The fixation of [11C]physostigmine to cerebral tissue was abolished after incubation of the rat brain sections with BW 284C51, a specific AChE inhibitor, but not after incubation with iso-OMPA, a specific inhibitor of butyrylcholinesterase. Unilateral excitotoxic lesions of the striatum that eliminated local AChE expression concomitantly reduced the binding of the ligand in the lesioned area. These results indicate that autoradiographic images of the rat brain obtained with [11C]physostigmine reflect AChE distribution, thus supporting the use of this radioligand to trace cerebral AChE activity in humans with positron emission tomography.

Acetylcholinesterase↗
PubMed1994

Developmental changes in the localization of the synaptic vesicle protein rab3A in rat brain.

Rab3A is a protein associated with the membrane of synaptic vesicles and is involved in the control of the targeting or docking of these vesicles at the presynaptic membrane for the release of neurotransmitters. Here, we have examined the expression and localization of this protein during the development of the rat brain. Relative to total protein, the concentration of rab3A greatly increased during brain development. Both the intracellular localization of the protein and its cerebral distribution showed an age-dependent shift. In contrast to other synaptic vesicle proteins, rab3A was heavily concentrated in cell bodies when immature neurons were migrating and during early differentiation. Later, the protein disappeared from perikarya and had a diffuse distribution in the neuropil, indicating a redistribution to nerve terminals, its exclusive localization in the adult. In the developing somatosensory cortex, rab3A delimited the modular organization of the barrels well after the afferents have arrived but just around the time that mature synaptic activity has been observed. In the hippocampus, rab3A defined a novel "blob-like" organization of the mossy fibre terminals and its appearance in terminal fields closely preceded the known onset of long-term potentiation. The appearance of rab3A in specific terminal fields during the period of increased physiological activity suggests that this small GTP-binding protein may be an important late element in the establishment of the mature characteristics of the presynaptic terminal.

Aging↗