SSRIs target prefrontal to raphe circuits during development modulating synaptic connectivity and emotional behavior.

Antidepressants that block the serotonin transporter, (Slc6a4/SERT), selective serotonin reuptake inhibitors (SSRIs) improve mood in adults but have paradoxical long-term effects when administered during perinatal periods, increasing the risk to develop anxiety and depression. The basis for this developmental effect is not known. Here, we show that during an early postnatal period in mice (P0-P10), Slc6a4/SERT is transiently expressed in a subset of layer 5-6 pyramidal neurons of the prefrontal cortex (PFC). PFC-SERT+ neurons establish glutamatergic synapses with subcortical targets, including the serotonin (5-HT) and GABA neurons of the dorsal raphe nucleus (DRN). PFC-to-DRN circuits develop postnatally, coinciding with the period of PFC Slc6a4/SERT expression. Complete or cortex-specific ablation of SERT increases the number of functional PFC glutamate synapses on both 5-HT and GABA neurons in the DRN. This PFC-to-DRN hyperinnervation is replicated by early-life exposure to the SSRI, fluoxetine (from P2 to P14), that also causes anxiety/depressive-like symptoms. We show that pharmacogenetic manipulation of PFC-SERT+ neuron activity bidirectionally modulates these symptoms, suggesting that PFC hypofunctionality has a causal role in these altered responses to stress. Overall, our data identify specific PFC descending circuits that are targets of antidepressant drugs during development. We demonstrate that developmental expression of SERT in this subset of PFC neurons controls synaptic maturation of PFC-to-DRN circuits, and that remodeling of these circuits in early life modulates behavioral responses to stress in adulthood.

Correction: SSRIs target prefrontal to raphe circuits during development modulating synaptic connectivity and emotional behavior.

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In vitro transcriptional activation by a metabolic intermediate: activation by Leu3 depends on alpha-isopropylmalate.

In the absence of the leucine biosynthetic precursor alpha-isopropylmalate (alpha-IPM), the yeast LEU3 protein (Leu3p) binds DNA and acts as a transcriptional repressor in an in vitro extract. Addition of alpha-IPM resulted in a dramatic increase in Leu3p-dependent transcription. The presence of alpha-IPM was also required for Leu3p to compete effectively with another transcriptional activator, GAL4/VP16, for limiting transcription factors. Therefore, the addition of alpha-IPM appears to convert a transcriptional repressor into an activator. This represents an example in eukaryotes of direct transcriptional regulation by a small effector molecule.

Transcriptional regulator Leu3 of Saccharomyces cerevisiae: separation of activator and repressor functions.

The Leu3 protein of Saccharomyces cerevisiae binds to specific DNA sequences present in the 5' noncoding region of at least five RNA polymerase II-transcribed genes. Leu3 functions as a transcriptional activator only when the metabolic intermediate alpha-isopropylmalate is also present. In the absence of alpha-isopropylmalate, Leu3 causes transcription to be repressed below basal levels. We show here that different portions of the Leu3 protein are responsible for activation and repression. Fusion of the 30 C-terminal residues of Leu3 to the DNA-binding domain of the Gal4 protein created a strong cross-species activator, demonstrating that the short C-terminal region is not only required but also sufficient for transcriptional activation. Using a recently developed Leu3-responsive in vitro transcription assay as a test system for repression (J. Sze, M. Woontner, J. Jaehning, and G. B. Kohlhaw, Science 258:1143-1145, 1992), we show that mutant forms of the Leu3 protein that lack the activation domain still function as repressors. The shortest repressor thus identified had only about 15% of the mass of the full-length Leu3 protein and was centered on the DNA-binding region of Leu3. Implications of this finding for the mechanism of repression are discussed.

Discovery of potent isoxazoline glycoprotein IIb/IIIa receptor antagonists.

Using the isoxazoline as a common structural feature, three series of glycoprotein IIb/IIIa receptor antagonists were evaluated, culminating in the discovery of XR299 (30). In an in vitro assay of platelet inhibition, XR299 had an IC50 of 0.24 microM and was a potent antiplatelet agent when dosed intravenously in a canine model. It was shown through X-ray studies of the cinchonidine salt 49 that the receptor required the 5(R)-stereochemistry for high potency. The ethyl ester prodrug of XR299, XR300 (29), was orally active in the dog.

Discovery of an orally active series of isoxazoline glycoprotein IIb/IIIa antagonists.

Using isoxazoline XR299 (1a) as a starting point for the design of highly potent, long-duration GPIIb/IIIa antagonists, the effect of placing lipophilic substituents at positions alpha and beta to the carboxylate moiety was evaluated. Of the compounds studied, it was found that the n-butyl carbamate 24u exhibited superior potency and duration of ex vivo antiplatelet effects in dogs. Replacement of the benzamidin-4-yl moiety with alternative basic groups, elimination of the isoxazoline stereocenter, and reversal of the orientation of the isoxazoline ring resulted in lowered potency and/or duration of action.

Purification and structural characterization of transcriptional regulator Leu3 of yeast.

The transcriptional regulatory protein Leu3 of Saccharomyces cerevisiae was enriched approximately 70-fold above wild type level in yeast cells carrying a pGAL1-LEU3 expression vector. Sustained overproduction of Leu3 following induction by galactose required elevated intracellular levels of alpha-isopropylmalate, a leucine pathway intermediate known to act as transcriptional co-activator. Starting with galactose-induced cells, the Leu3 protein was purified about 3,500-fold (i.e. 245,000-fold over wild type level) by a procedure that included treatment of the cell-free extract with polyethylenimine, fractionation with ammonium sulfate, heat treatment, and DNA affinity chromatography. Highly purified preparations still showed two protein bands when subjected to polyacrylamide electrophoresis under denaturing conditions. Their apparent molecular masses were about 104,000 and 110,000 kDa. The smaller of these values was very close to the maximum molecular weight obtained previously for Leu3 protein translated in vitro in a rabbit reticulocyte lysate. (The molecular weight deduced from the open reading frame of the LEU3 gene is 100,162.) Both protein bands reacted with antibodies raised against different portions of the Leu3 molecule and were, therefore, likely to represent two forms of Leu3. Treatment with calf intestinal phosphatase quantitatively converted the slower moving band into the faster moving one. Conversion was prevented by inorganic phosphate, a phosphatase inhibitor. These experiments showed that the two bands very likely correspond to phosphorylated and nonphosphorylated forms of Leu3. Phosphorylation did not appear to affect the DNA binding function of Leu3, but (indirect) effects on the activation function or effects on the modulation by alpha-isopropylmalate have not been ruled out. Electrophoretic mobility shift assays were used to estimate the apparent dissociation constants of the two specific Leu3-DNA complexes routinely seen in these assays. The values obtained were 1.1 and 2.6 nM. Finally, using size exclusion chromatography, native Leu3 protein was shown to have dimeric structure, irrespective of the state of phosphorylation.

VP16-activation of the C. elegans neural specification transcription factor UNC-86 suppresses mutations in downstream genes and causes defects in neural migration and axon outgrowth.

The POU homeobox gene unc-86 specifies many neuroblast and neural fates in the developing C. elegans nervous system. Genes regulated by unc-86 are mostly unknown. Here we describe a genetic strategy for the identification of downstream pathways regulated by unc-86. We activate UNC-86 transcription activity by inserting the VP16 activation domain into an unc-86 genomic clone that bears all regulatory sequences necessary for normal expression in C. elegans. unc-86/VP16 complements unc-86 mutations in the specification of neuroblast and neural cell fates, but displays novel genetic activities: it can suppress non-null mutations in the downstream genes mec-3 and mec-7 that are necessary for mechanosensory neuron differentiation and function. These data suggest that UNC-86/VP16 increases the expression of mec-3 and mec-7 to compensate for the decreased activities of mutant MEC-3 or MEC-7 proteins. The suppression of mutations in downstream genes by an activated upstream transcription factor should be a general strategy for the identification of genes in transcriptional cascades. unc-86/VP16 also causes neural migration and pathfinding defects and novel behavioral defects. Thus, increased or unregulated expression of genes downstream of unc-86 can confer novel neural phenotypes suggestive of roles for unc-86-regulated genes in neural pathfinding and function. Genetic suppression of these unc-86/VP16 phenotypes may identify the unc-86 downstream genes that mediate these events in neurogenesis.

Food and metabolic signalling defects in a Caenorhabditis elegans serotonin-synthesis mutant.

The functions of serotonin have been assigned through serotonin-receptor-specific drugs and mutants; however, because a constellation of receptors remains when a single receptor subtype is inhibited, the coordinate responses to modulation of serotonin levels may be missed. Here we report the analysis of behavioural and neuroendocrine defects caused by a complete lack of serotonin signalling. Analysis of the C. elegans genome sequence showed that there is a single tryptophan hydroxylase gene (tph-1)-the key enzyme for serotonin biosynthesis. Animals bearing a tph-1 deletion mutation do not synthesize serotonin but are fully viable. The tph-1 mutant shows abnormalities in behaviour and metabolism that are normally coupled with the sensation and ingestion of food: rates of feeding and egg laying are decreased; large amounts of fat are stored; reproductive lifespan is increased; and some animals arrest at the metabolically inactive dauer stage. This metabolic dysregulation is, in part, due to downregulation of transforming growth factor-beta and insulin-like neuroendocrine signals. The action of the C. elegans serotonergic system in metabolic control is similar to mammalian serotonergic input to metabolism and obesity.